Upload
magdamnr
View
139
Download
4
Embed Size (px)
DESCRIPTION
SILICA AEROGELS: SYNTHESIS AND CHARACTERIZATION
Citation preview
UNIVERSITAT DE BARCELONA
DEPARTAMENT DE FÍSICA APLICADA I ÒPTICA
SILICA AEROGELS: SYNTHESIS
AND CHARACTERIZATION
Magda Moner i Gerona
INSTITUT DE CIÈNCIA DE MATERIALS DE BARCELONA-CSIC
ii
UNIVERSITAT DE BARCELONA
DEPARTAMENT DE FÍSICA APLICADA I ÒPTICA
SILICA AEROGELS: SYNTHESIS
AND CHARACTERIZATION
Magda Moner i Gerona
INSTITUT DE CIÈNCIA DE MATERIALS DE BARCELONA-CSIC
Programa de doctorat: Física i Tecnologia de Materials
Bienni: 1997-1999
Tutor: Joan Esteve i Pujol Directors: Elies Molins i Grau Anna Roig i Serra
Memòria presentada per optar al grau de Doctor en Ciències Físiques
Barcelona, Juny 2002
iii
Aquesta tesi ha estat realitzada a l‟Institut de Ciència de Materials de
Barcelona-CSIC en el departament de Cristal.lografia i Química de
l‟Estat Sòlid, sota la direcció del doctor Elies Molins i la doctora Anna
Roig. El treball ha esta realitzat dins el marc dels projectes d‟investigació
de la CICYT MAT97-0688 i MAT2000-2016, i de varis projectes R+D
entre l‟empresa Carburos Metálicos S.A i l‟Institut de Ciència de
Materials de Barcelona. Durant els dos últims anys, la doctorant ha
gaudit d‟una beca 2001TDOC011 de la Direcció General de Recerca de
la Generalitat de Catalunya.
iv
v
AGRAÏMENTS
He d’agrair al Dr. Elies Molins, cap del grup d’aerogels de l’ICMAB i co-director d’aquesta tesi,
l‘oportunitat que em va donar d’iniciar la tesi amb un material tan sorprenent i atractiu com és
l’aerogel. Per descomptat agrair-li les seves constants noves idees, la il·lusió que m’ha transmès per
està obert a qualsevol proposta, i la seva accessibilitat i respecte pel meu treball.
De manera especial, m’agradaria transmetre el meu agra ïment a la Dra. Anna Roig, co-directora
d’aquesta tesi, per oferir-me un continu i ferm suport tant científic com personal en la realització
d’aquesta tesi. Agreixo la seva dedicació i interès constant per la meva feina, i sobretot pel seu tracte
com a persona.
La meva gratitud al Dr. Carles Miravitlles director de l’Institut de Ciència de Materials de Barcelona
per posar a la meva disposició els mitjans necessaris per dur a terme aquesta tesi i al Dr. José-Luis
Morenza, director del departament de Física Aplicada i Òptica pels seus consells en la logística de
presentació de la memòria.
Durant el temps de realització d’aquesta tesi he tingut la sort de treballar en un grup que sempre
m'ha ofert la seva ajuda i amb moltes ganes per treballar en equip tant dins com fora de l'institut. A
tots els companys que heu participat en la realització d'aquesta tesis gràcies pel vostre ajut i amistat: a
l’Ignasi (pel seu constant ajut i per l’esforç de millorar la situació dels becaris), l’Elisenda (per la seva
amistat i la seva voluntat de donar sempre un cop de mà), en Lluís (com no, per fer-nos compartir la
seva passió pel món occità), el Raul (sense el seu treball constant i la seva paciència no hauria estat
possible l’obtenció dels aerogels presentats en aquesta tesi), la Joana (el seu ajut ha estat
imprescindible alhora d’organitzar aquesta tesi), en Martí, la Mònica, la Mihaela, l’Agnes, la
Stephanie, en Fabian i en Ramon (a tots ells per haver aportat part d’aquest treball) .
A molts d’altres companys de l’ICMAB, la Karina (por su original y apasionante visión de la ciencia),
el Ramon, l’Oriol, l’Andrea, l’Imma, el Manu, en Jerôme, en Felip, la Sílvia, per ajudar-me a passar
molt bons moments a l’Institut. I especialment en Lluís Balcells per tots els seus consells i les
discussions que hem mantingut. To Sari for introducing me in the IR analysis and to share a very nice
time in Gràcia.
Aquesta tesi ha estat realitzada gràcies a un conctracte d’investigació per part de Carburos Metalicos,
i al suport científic de Joan Llibre, Joaquim Torras, i d’Emili de la Serna
He d’agrair tot l’equip d’infrastructura de l’Institut, administració, documentació, servei informàtic i
manteniment per la seva ajuda en aquella part del treball que no queda reflectit en la tesi però que és
imprescindible alhora de desenvolupar-la.
vi
Així mateix, el fet de treballar amb un material poc conegut i amb unes propietats tan sorprenents
com les de l’aerogel ajuda a que molt sovint m’hagi trobat amb l’entusiasme d’altres investigadors
per a ‘provar’ noves iniciatives i ‘jugar’ amb la seva peculiar estructura. Principalment he tingut la
sort de treballar conjuntament amb l’Elena Martínez i en Joan Esteve en la caracterització mecànica
dels aerogels. Ha estat una experiència molt positiva de treballar en col·laboració d'un equip que em
rebien amb les portes ben obertes cada vegada que els portava noves mostres per caracteritzar.
Gràcies Elena per l'ajuda i per l’afecte que sempre m’has mostrat i a en Joan per aquella energia que
transmets per gaudir amb la investigació.
Sense l'ajuda de Yves Maniette i Joaquim Portillo (i la visualització dels nostres ‘núvols’ per TEM),
Núria Ferrer i Pau Gorostiza, tots ells dels serveis científico-tècnics de la UB, no hauria estat possible
la caracterització dels aerogels. Agraeixo en Miquel-Àngel Cuevas per haver-me acollit en el seu grup
i per haver-me donat suport al llarg dels anys de la realització d’aquesta tesi.
I would like to kindly express my gratitude to Arlon Hunt for his guidance during the realization of
the stages in his Microstructure Group at the Lawrence National Berkeley Laboratory. I have learned
a lot not only about these surprising aerogels but also about the sweet manner to appreciate science.
Of course to Mike Ayers, thanks a lot to teach me so many fruitful ‘tricks’ about how to handle the
aerogels, for his interest and nice suggestions. And in particular, this wonderful decision to bike
always to everywhere and under any kind of weather, and to repair my ‘vintage’ bike so many
times…To Ian Shepherd for his interest and pleasant discussions and to try (without much results) to
improve my English, and to Paul Berdahl and Gary for his valuable contributions to the development
of the nephelometer set-up and analysis of the data. I nicely thank to Giovanni Dietler, Cynthia
Lopez and Steven Grover to advice and help me so many times.
D’altres persones que no han col·laborat directament en la meva feina però han estat imprescindibles
perquè fos complerta: l’Àlex, per la seva paciència constant durant el temps de la realització de la tesi,
i per haver-me donat suport sempre que l’he necessitat. Els meus avis Núria, Antonio i Roser per
encoratjar-me sempre tan dolçament i pel vostre suport incondicional. L'Alba, l'Enric, el Josep-Lluís i
la Mercè per aguantar a la tipàtica força sovint. I com no, per ensenyar-me uns valors tan importants
com són el respecte a la natura i la passió per la muntanya. A totes/ts les jugadores/rs i companyes
del Sitari i del Bonanova que sense saber-ho hi han col·laborat ajudant-me a desconnectar totalment
dels aerogels. A tot aquest cercle d’amics que m’han encoratjat i que han tingut la paciència d'escoltar
el procés d'aquesta tesi: A l'Aracel.li, la Marta Pérez (per desenboirar-me amb les correlacions i per
mantenir la nostre amistat a distància), la Maria Serra, la Marta Janeras, l’Oriol, la Betlem, l’Albert, i el
Fidel.
INDEX
PRÒLEG ....................................................................................................................................................................... 1
GLOSSARY OF TERMS ......................................................................................................................................... 4
C h a p t e r I . AEROGELS: AN INTRODUCTION
1. GENERAL OVERVIEW ON AEROGELS .....................................................................................12
1.1. COMMERCIAL SILICA AEROGEL .................................................................................. 14
2. SOL-GEL METHOD.............................................................................................................................15
2.1. ANTECEDENTS OF GEL SYNTHESIS ......................................................................... 16
2.2. PREPARATION OF SILICA GELS.................................................................................... 16
3. DRYING PROCEDURE OF GELS.................................................................................................. 20
4. AEROGEL APPLICATIONS ............................................................................................................. 22
4.1. POROSITY AND SURFACE AREA APPLICATIONS ................................................. 22
Electrodes in capacitors ................................................................................. 22
Capacitive deionization ................................................................................. 23
Studies of superf luid transitions and phase separation of 3He-4He...................... 23
4.2. OPTICAL PROPERTY APPLICATIONS ........................................................................ 23
Çerenkov (particle detection and counters)........................................................ 23
4.3. THERMAL INSULATOR APPLICATIONS .................................................................. .24
4.4. ACOUSTICAL AND MECHANICAL APPLICATIONS ............................................. 24
Shock compression experiments ...................................................................... 25
4.5. ELECTRICAL AND ELECTRONIC APPLICATIONS .............................................. 25
4.6. SPACE APPLICATIONS........................................................................................................ 25
5. REFERENCES....................................................................................................................................... 25
C h a p t e r I I : SYNTHESIS OF SILICA AEROGELS
1. INTRODUCTION ................................................................................................................................ 32
2. PREPARATION OF AEROGELS .................................................................................................... 33
2.1. USED REACTIVES ................................................................................................................. 33
2.2. SYNTHESIS PROCEDURE ................................................................................................. 34
2.3. DRYING PROCEDURE ........................................................................................................ 36
Index ii
2.3.1 Supercritical drying at high temperature ................................................................ 36
2.3.2 CO2 supercritical drying ................................................................................................ 38
3. SYNTHESIS ROUTES: EFFECT OF DIFFERENT ALKOXIDE PRECURSORS ........... 39
3.1. TETRAMETHOXYSILANE: TMOS AEROGELS ........................................................ 40
3.1.1 The effect of the TMOS concentration...................................................................... 45
3.1.2 The effect of the nature of the solvent ...................................................................... 46
3.1.3 The effect of the hydrolysis solution .......................................................................... 47
Water amount.............................................................................................. 47
3.2. TETRAETHOXYSILANE: TEOS AEROGELS ............................................................. 49
3.2.1 The effect of the TEOS concentration ................................................................... 53
Neutral ....................................................................................................... 53
Acid catalyst ................................................................................................ 54
Base-catalyst ................................................................................................ 56
3.2.2 The effect of hydrolysis solution ............................................................................... 58
Water amount.............................................................................................. 58
Influence of the amount of the catalyst ............................................................. 61
Influence of the nature of the catalyst ............................................................... 64
3.3. ‘TWO-STEP’ SYNTHESIS .................................................................................................... 66
Effect of precursor concentration...................................................................... 67
Effect of water amount .................................................................................. 69
Influence of the amount of the catalyst ............................................................. 70
4. SUMMARY AND CONCLUSIONS...................................................................................................71
5. REFERENCES....................................................................................................................................... 72
C h a p t e r I I I : BULK SILICA AEROGEL CHARACTERIZATION
1. MONOLITHICITY, BULK SHRINKAGE, DENSITY AND POROSITY............................ 75
1.1. TMOS AEROGELS .................................................................................................................. 76
Skeletal density ............................................................................................ 76
Bulk density ................................................................................................ 76
1.1.1 Supercritical drying at CO2 conditions.................................................................... 80
1.2. TEOS AEROGELS................................................................................................................... 81
1.2.1 TEOS aerogels without presence of catalyst.......................................................... 82
1.2.2 Base-catalyst ................................................................................................................. 84
1.2.3 Acid-catalyst .................................................................................................................. 87
Fluorhydric acid ........................................................................................... 88
Citric acid.................................................................................................... 88
1.3. TWO-STEP METHOD H5 AEROGELS .......................................................................... 91
Index iii
2. SURFACE AREA MEASURENTS BY BET (BRUNAUER, EMMET AND TELLER).... 94
3. INFRARED SPECTROPHOTOMETRY, IR ................................................................................ 99
3.1. METHANOL SERIES .......................................................................................................... 100
3.2. ACETONE SERIES ............................................................................................................... 102
4. ULTRA VIOLET-VISIBLE SPECTROSCOPY ............................................................................103
4.1. AEROGEL TRANSPARENCY .......................................................................................... 103
4.2. RAYLEIGH SCATTERING................................................................................................ 107
4.2.1 A model to interpret the porous aerogel structure using Rayleigh scattering110
5. LIGHT SCATTERING MEASUREMENTS OF AEROGELS BY A POLARIZATION-
MODULATED NEPHELOMETER .............................................................................................. 113
5.1. INTRODUCTION TO LIGHT SCATTERING VERSUS ANGLE
EXPERIMENTS......................................................................................................................113
5.1.1 Description of the polarization-modulated nephelometer ................................. 114
5.2. EXPERIMENTAL RESULTS .............................................................................................115
5.3. STRUCTURAL INFORMATION FROM THE LIGHT SCATTERING
MEASUREMENTS .................................................................................................................118
5.3.1 Inhomogeneous media .............................................................................................. 118
Short range correlations: Rayleigh scattering................................................... 120
Long range correlations: departures f rom Rayleigh scattering ............................ 121
5.4. COMPARATIVE STUDY BETWEEN EXPERIMENTAL MEASUREMENTS
AND THEORY ....................................................................................................................... 122
5.5. CONCLUSIONS AND FUTURE WORK ....................................................................... 125
6. DIRECT METHODS: ELECTRON MICROSCOPY ...............................................................127
6.1. STRUCTURAL STUDIES BY SCANNING ELECTRON MICROSCOPY ........ 127
6.1.1 Acetone series ..............................................................................................................127
6.1.2 Effect of the solvent .................................................................................................... 131
6.1.3 Drying procedure ........................................................................................................132
6.1.4 TMOS aerogels in CO2 as solvent ...........................................................................133
6.2. TRANSMISSION ELECTRON MICROSCOPY .......................................................... 134
Sample preparation..................................................................................... 134
TEM set-up .............................................................................................. 134
6.2.1 Imaging the acetone-series silica aerogels ...........................................................135
6.2.2 Methanol series............................................................................................................135
6.2.3 Replicas visualization.................................................................................................138
Replicas visualization.................................................................................. 138
7. REFERENCES...................................................................................................................................... 141
Index iv
C h a p t e r I V : MECHANICAL CHARACTERIZATION OF SILICA AEROGELS
1. INTRODUCTION ...............................................................................................................................146
1.1. MICROINDENTER DESCRIPTION............................................................................. 147
1.2. MECHANICAL CHARACTERIZATION...................................................................... 149
2. MECHANICAL PROPERTIES OF SILICA AEROGELS AS A FUNCTION OF
DENSITY................................................................................................................................................152
2.1. SAMPLE PREPARATION .................................................................................................. 152
2.2. EFFECT OF THE ALKOXIDE......................................................................................... 152
3. INFLUENCE OF SOLVENT AND SUPERCRITICAL DRYING METHOD ON THE
MECHANICAL PROPERTIES .......................................................................................................156
3.1. EFFECT OF THE DRYING PROCEDURE................................................................. 156
3.2. EFFECTS OF THE SOLVENT......................................................................................... 158
4. MICROMECHANICAL PROPERTIES OF CARBON-SILICA AEROGEL
COMPOSITES ......................................................................................................................................159
4.1. INTRODUCTION ................................................................................................................. 159
4.2. EFFECT OF CARBON ADDITION................................................................................ 160
5. VISCOELASTICITY OF SILICA AEROGELS AT ULTRASONIC FREQUENCIES .....169
5.1. INTRODUCTION ................................................................................................................. 166
5.2. EXPERIMENTAL SET-UP: AIR-COUPLED BROAD-BAND
PIEZOELECTRIC TRANSDUCERS ............................................................................. 167
5.3. DETERMINATION OF THE VISCOELASTICITY OF SILICA AEROGELS AT
ULTRASONIC FREQUENCIES ..................................................................................... 169
5.4. DISCUSSION............................................................................................................................171
6. CONCLUSIONS ...................................................................................................................................173
7. REFERENCES......................................................................................................................................175
C h a p t e r V : SILICA AEROGEL MICROPARTICLES
1. SOL-GEL ROUTE TO DIRECT FORMATION OF SILICA AEROGEL
MICROPARTICLES USING SUPERCRITICAL ETHANOL/ACETONE........................ 181
1.1. ‘IN SITU’ PARTICLE PROCESSING ............................................................................. 182
1.2. AEROGEL MICROPARTICLE CHARACTERIZATION ........................................ 184
1.2.1 Scanning Electron Microscopy ................................................................................185
Independent solutions .................................................................................. 189
Index v
1.2.2 Transmission Electron Microscopy ........................................................................190
1.2.3 Atomic Force Microscopy ......................................................................................... 191
2. SOL-GEL ROUTE TO DIRECT FORMATION OF SILICA AEROGEL
MICROPARTICLES USING SUPERCRITICAL CARBON DIOXIDE ...............................195
2.1. ‘IN SITU LOW TEMPERATURE MICROPARTICLES: TEOS, HCOOH, AND
SUPERCRITICAL CO2 AS SOLVENT ............................................................................196
2.2. ‘PRECURSOR DIRECTLY INJECTED IN CO2 SUPERCRITICAL
CONDITIONS AT LOW TEMPERATURE.................................................................. 198
2.2.1 Injection of hydrolysis and precursor solution independently ..........................199
2.2.2 Injection of sol ............................................................................................................ 203
2.3. IN SITU’ INJECTION IN LIQUID CO2........................................................................ 203
2.4. INJECTION OF PREPOLYMERIZED PRECURSOR IN SUPERCRITICAL
CO2 .............................................................................................................................................. 205
3. CONCLUSIONS .................................................................................................................................. 208
4. REFERENCES..................................................................................................................................... 208
C h a p t e r V. SILICA AEROGEL FILMS
1. APPLICATIONS OF AEROGEL FILMS.......................................................................................212
1.1. ELECTRONIC........................................................................................................................ 212
1.2. OPTICAL .................................................................................................................................. 213
1.3. THERMAL ............................................................................................................................... 214
1.4. ACOUSTIC ............................................................................................................................... 214
1.5. ENVIRONMENT AND OTHERS................................................................................... 214
2. SOL-GEL COATING METHODS...................................................................................................215
2.1. DIP COATING........................................................................................................................ 215
2.2. SPIN COATING ..................................................................................................................... 216
2.3. SPRAY COATING.................................................................................................................. 217
2.4. SURFACE TENSION COATING..................................................................................... 217
2.5. SUBCRITICAL DRYING BY SURFACE DERIVATION ......................................... 217
3. REFERENCE EXPERIMENTAL RESULTS..............................................................................217
3.1. DIP COATING........................................................................................................................ 217
a) Low-temperature dip coating .................................................................... 218
b) High-temperature dip coating ................................................................... 219
3.2. SPIN COATING ..................................................................................................................... 221
a) Spin coating with High-Temperature drying .............................................. 222
Index vi
4. PROPOSED NEW METHODS: ‘IN SITU’ PREPARATION AT HIGH PRESSURE
AND INJECTION AT SUPERCRITICAL CONDITIONS..................................................... 224
4.1. ‘IN SITU’ PREPARATION AT HIGH PRESSURE.................................................... 224
4.1.2 ‘In situ’ high temperature......................................................................................... 225
4.1.3 ‘In Situ’ low-temperature coating method............................................................ 225
4.2. SPRAY COATING BY DIRECT INJECTION IN SUPERCRITICAL CO2 AT
LOW TEMPERATURE........................................................................................................ 227
5. CONCLUSIONS .................................................................................................................................. 230
6. REFERENCES......................................................................................................................................231
CONCLUSIONS ....................................................................................................................................................233
ARTICLES PUBLISHED RELATED TO THIS THESIS ........................................................................241
ANNEX I: SUPERCRITICAL FLUIDS..........................................................................................................243
ANNEX II: TECHNICAL DESCRIPTION OF THE JOIN ICMAB-CM HIGH PRESSURE-HIGH TEMPERATURE LABORATORY ....................................................................................................247
ANNEX III: INTRODUCTION TO ADSORPTION ANALYSIS AND THE BET MODEL ...257
REsum de la tesis. AEROGELS de sílice: SÍNTESIS i caracterització 265
AGRAÏMENTS ............................................................................................................................................... 297
PRÒLEG
Durant el temps de realització d‟aquesta tesi he tingut la sort de treballar amb un material tan
sorprenent i atractiu com és l‟aerogel. El resultat ha estat aquesta tesi estructurada en 5
capítols:
El primer capítol („Aerogels: An Introduction’), és una introducció amb apunts històrics
dels aerogels i de les seves aplicacions. El capítol està estructurat en 5 apartats: 1) mètode sol-gel,
una recapitulació bàsica de la tècnica sol-gel, 2) assecat supercrític dels gels, on s‟expliquen els
conceptes bàsics dels fluids supercrítics i els diferents mètodes d‟assecat supercrític, 3)
tècniques de caracterització, breu introducció a les tècniques aplicades per la caracterització dels
aerogels, 4) aplicacions, un resum de les aplicacions més comunes dels aerogels i, finalment 5)
bibliografia, on es presenta un ampli llistat de referències.
La motivació per escriure aquesta llarga introducció és el fet de que aquesta tesis doctoral és
„la primera‟ del nostre grup, i probablement de l‟estat espanyol, centrada en l‟estudi,
caracterització i síntesi d‟aerogels de sílice. La idea principal és que serveixi d‟ajut a futurs
projectes que impliquin aquest material.
El segon capítol, (Synthesis and optimization of silica aerogels: Influence of molar
ratios of precursor, solvent and water) és bàsicament descriptiu i el seu objectiu es centra
en fer un seguiment del procés usat per a l‟optimització de la síntesi i del cicle d‟assecat dels
aerogels. En aquest capítol s‟estableix quin és el camí de síntesi més adequat per obtenir les
característiques requerides per cada tipus d‟aerogel. Aquesta secció també conté una
bibliografia àmplia per a que es faciliti l‟oportunitat d‟obtenir més informació d‟una
determinada síntesi. En definitiva el segon capítol inclou bàsicament informació per a la
reproducció de les síntesis que he seguit en el nostre laboratori. Aquesta informació la trobem
esbossada en taules que contenen la majoria de paràmetres importants de la preparació de les
mostres.
En l‟últim apartat d‟aquest capítol s‟inclouen les conclusions on es resumeix les síntesis més
adequades per a cada procés, per tant la lectura completa d‟aquest capítol serà essencialment
necessària només en el cas que es vulgui reproduir algun dels resultats.
Prefaci 2
En el tercer capítol (Aerogel Characterization) s‟exposa la caracterització realitzada en les
diverses sèries d‟aerogels obtingudes amb diferent transparència, densitat i porositat, com a
conseqüència de la seva microestructura. Un dels aspectes més importants en la preparació
d‟aerogels és la possibilitat de control de les propietats físiques del material: densitat, porositat
i àrea superficial. Aquestes propietats estan directament relacionades amb l‟estructura del
material: la distribució i mida de porus, la distribució i mida de partícules, la mida de
„clusters‟,...
Per determinar la microestructura porosa d‟aquest material s‟ha desenvolupat un model que
utilitza els resultats de varies tècniques ja que l‟ús d‟una sola no permet la caracterització
completa de tot el rang de porositats i de mida de partícules. Les tècniques utilitzades en
aquest treball per a obtenir informació estructural dels aerogels són les tècniques de BET,
microscopia electrònica (SEM, TEM) o microscopia de forces atòmiques AFM). Per a
obtenir informació complementaria de la microestructura d‟aquests materials tan altament
porosos -determinació de la mida de porus i partícula- és necessària la utilització d‟altres
tècniques (l‟anàlisi de la dispersió de la llum).
Un dels inconvenients dels aerogels és la seva fragilitat, és per aquest motiu que un dels
objectius d‟aquesta tesi s‟ha centrat en la caracterització mecànica d‟aquests materials
mitjançant una tècnica no destructiva: la microindentació. El quart capítol („Mechanical
properties of silica aerogels’) mostra els resultats obtinguts explicant els resultats publicats
en una sèrie de quatre articles, el primer relaciona la dependència de les propietats
mecàniques dels aerogels de sílice amb la seva densitat, el segon article analitza l‟influencia de
varis paràmetres de síntesi (solvent utilitzat i procediment de assecat supercrític) amb les
propietats mecàniques dels aerogels de sílice, finalment el tercer article descriu com, amb la
finalitat de millorar les propietats mecàniques dels aerogels es varen sintetitzar nous aerogels
compostos amb carbó actiu. Aquests composites presenten un augment molt accentuat de
l‟elasticitat de l‟aerogel de sílice. En l‟últim article es caracteritzen les propietats
viscoelàstiques dels aerogels de sílice.
En el cinquè capítol (Aerogel Microparticles) es descriu el procés d‟obtenció de
micropartícules d‟aerogels mitjançant una nova tècnica. També s‟hi descriu la caracterització
de les diferents micropartícules obtingudes.
Prefaci 3
En el sisè capítol (Aerogel Films) es desenvolupa un estudi d‟obtenció de capes d‟aerogels
mitjançant una nova tècnica. També es porta a terme la caracterització de les diferents capes
obtingudes.
GLOSSARY OF TERMS1:
Aerogel
Defined as a group of extremely light and porous solid materials; the lightest is less
than four times as dense as dry air. Aerogels are derived via a sol-gel process in
combination with a subsequent drying step (most often achieved by supercritical
extraction) the result are monolithic, open porous materials with a backbone
morphology that can be modeled in terms of three dimensionally interconnected
strings of microscopic pearls.
Aerosol
A colloidal suspension of particles in a gas (the suspension may be called a fog if the
particles are liquid and a smoke if the are solid).
Aging
The term aging is applied to the process of change in structure and properties after
gelation. Bond formation does not stop at the gel point. In the first place, the
network is initially compliant, so segments of the gel network can still move close
enough together to allow further condensation (or other bond-forming processes).
Moreover, there is still a sol within the gel network, and then those smaller polymers
or particles continue to attach themselves to the network.
Alcohol
A molecule formed by adding a hydroxyl (OH) group to an alkyl molecule, as in
methanol (CH3OH) or ethanol (C2H5OH).
1 Their literal definitions were taken from http://eande.lbl.gov/ECS/aerogels/satoc.htm, Academic Press Dictionary of
Science and Technology and from C.J. Brinker, G.W. Sherer, Sol-Gel Science. Physics and Chemistry of Sol-Gel
Processing, Academic Press, New York, 1990.
Alkane
A molecule containing only carbon and hydrogen linked exclusively by single bonds,
as in methane (CH4) and ethane (C2H6); the general formula is CnH2n+2.
Glossary of terms 5
Alkoxy
A ligand formed by removing a proton from the hydroxyl on an alcohol, as in methoxy
( OCH3) or ethoxy ( OC2H5). Where the dot, , indicates an electron that is available
to form a bond.
Alkyl
A ligand formed by removing one hydrogen (proton) from an alkane molecule
producing, for example, methyl ( CH3) or ethyl ( C2H5)
Brownian motion
In a colloid, the inertia of the dispersed phase is small enough to exhibit Brownian
motion (or Brownian Diffusion), a random walk driven by momentum imparted by
collisions with molecules of the suspending medium.
Ceramic
A nonmetallic and inorganic material; included all metal oxides, nitrides, and carbides,
both crystalline and noncrystalline.
Colloid
A mixture in which one substance is divided into minute particles (called colloidal
particles) and dispersed throughout a second substance. The dispersed phase is so
small ( 1–1000 nm) that gravitational forces are negligible and interactions are
dominated by short-range forces, such as van der Waals attraction and surface
charges. The mixture is also called a colloidal system, colloidal solution, or colloidal
dispersion. Familiar colloids include fog, smoke, and homogenized milk.
Glossary of terms 6
Condensation
A condensation reaction occurs when two metal hydroxides (M-OH + HO-M)
combine to give a metal oxide species (M-O-M). The reaction forms one water
molecule.
(OR)3Si-OH + HO-Si(OR)3 (OR)3Si-O-Si(OR)3 + H2O
or
(OR)3Si-OR + HO-Si(OR)3 (OR)3Si-O-Si(OR)3 + ROH.
By definition, condensation liberates a small molecule, such as water or alcohol. The
R represents a proton or other ligand (if R is an alkyl, then OR is an alkoxy group),
and ROH is an alcohol; the bar (-) is sometimes used to indicate a chemical bond.
Critical point
State at which two phases of a substance first become indistinguishable. For example,
at pressures higher than 217.6 atm and temperatures above 374°C, the meniscus
between steam and liquid water will vanish; the two phases become indistinguishable
and called supercritical fluid.
Crosslinkage/branching
If a polyfunctional unit with 2 is present, the chains can be joined by crosslinks to
form a three-dimensional structure. Polymerization of silicon alkoxide, for instance,
can lead to complex branching of the polymer because a fully hydrolyzed monomer
Si(OH)4 is tetrafunctional. On the other hand, under certain conditions (e.g., low
water concentration) fewer than four ligands will be capable of condensation, so
relatively little branching will occur.
Emulsion
Is a suspension of liquid droplets in another liquid. These types of colloids can be
used to generate polymers or particles from which ceramic materials can be made.
Glossary of terms 7
Functionality
The number of bonds that a monomer can form is called its functionality, . Typical
oxide monomers are bifunctional ( = 2), trifunctional ( = 3), or tetrafunctional ( = 4),
any of which may be called polyfunctional ( arbitrary).
Gel Point
If a monomer can make more than two bonds, then there is no limit on dimensions
so that it extends throughout the solution. The point in time at which the network of
linked oxide particles spans the container holding the sol. At the gel point, the sol
becomes a gel. The gel point corresponds to the percolation threshold, when a single
cluster (called the spanning cluster) appears that extends throughout the sol; the
spanning cluster coexists with a sol phase containing many smaller clusters, which
gradually become attached to the network
Gel
A gel consists of two parts, a solid part and a liquid part. The solid part is formed by
the three-dimensional network of linked oxide particles. The liquid part (the original
solvent of the sol) fills the free space surrounding the solid part. The liquid and solid
parts of a gel occupy the same apparent volume. Thus, a gel is a substance that
contains a continuous solid skeleton enclosing a continuous liquid phase. The gel can
be removed from its original container and can stand on its own. The continuity solid
structure gives elasticity to the gel (as in the familiar gelatin dessert).
Hydrolysis
The reaction of a metal alkoxide (M-OR) with water forms a metal hydroxide (M-
OH). A hydroxyl ion becomes attached to the metal atom, as in the following
reaction:
Si(OR)4 + H2O HO-Si(OR)3 + ROH
Glossary of terms 8
Depending on the amount of water and catalyst present, hydrolysis may go to
completion (so that all of the OR groups are replaced by OH).
Si(OR)4 + 4H2O H2 O-Si(OH)4 + 4ROH
or stop while the metal is only partially hydrolyzed, Si(OR)4-n(OH)n.
Meniscus
A phase boundary that is curved because of the surface tension.
Miscible
Two liquids are considered miscible or mixable if shaking them together results in a
single liquid phase, with no meniscus visible between layers of liquid.
Metal alkoxides
Are members of the family of metalorganic compounds, which have an organic
ligand attached to a metal o metalloid atom. The most thoroughly studied example is
silicon tetraethoxide (or tetraethoxy-silane, or tetraethyl orthosilicate, TEOS),
Si(OC2H5)4. Organometallic compounds are defined as having direct metal-carbon
bonds, not metal-oxygen-carbon linkages as in metal alkoxides; thus, alkoxides are
not organometallic compounds. Metal alkoxides are popular precursors because they
react readily with water.
Monolith
Gelation can occur after a sol is cast into a mold, in which case it is possible to make
objects of a desired shape. If the smallest dimension of the gel is greater than a few
millimeters, the object is generally called a monolith.
Polymer
A polymer (“many member”) is a huge molecule (also called a macromolecule) formed
from hundreds o thousands of units called monomers that are capable of forming at
least two bonds.
Glossary of terms 9
Porosity:
Microporosity
IUPAC definition: pores with mean diameter lower than 2 nm.
Mesoporosity
Pores with mean diameter between 2 nm and 50 nm.
Macroporosity
Pores with mean diameter larger than 50 nm.
Precursors
In the sol-gel process, the precursors (starting compounds) for preparation of a
colloid consist of a metal or metalloid element surrounded by various ligands
(appendages not including another metal or metalloid atom). The latter is an example
of an alkoxide, the class of precursors most widely used in sol-gel research.
Shrinkage
Shrinkage of a gel, either during syneresis or as liquid evaporates during drying,
involves deformation of the network and transport of liquid through the pores.
Sol
A colloidal suspension of solid particles in a liquid. A solution of various reactants
that are undergoing hydrolysis and condensation reactions. The molecular weight of
the oxide species produced increases continuously. As these species grow, they may
begin to link together in a three-dimensional network.
Supercritical fluid
A substance that is above its critical pressure and critical temperature. A supercritical
fluid possesses some properties in common with liquids (density, thermal
Glossary of terms 10
conductivity) and some in common with gases (fills its container, does not have
surface tension).
Syneresis
Some gels exhibit spontaneous shrinkage; called syneresis, as bond formation or
attraction between particles induces contraction of the network and expulsion of
liquid from the pores.
Xerogel
Drying by evaporation under normal conditions by evaporation gives rise to capillary
pressure that causes shrinkage of the gel network. The resulting dried gel is named
"xerogel", a word issued from the Greek word "xeros" and which means dry. The
shrinkage during drying is often extreme (~90%) for xerogels. The no collapse
requirement distinguishes aerogels from xerogels.
C h a p t e r I
AEROGELS: AN INTRODUCTION
SECTION OUTLINE
1. GENERAL OVERVIEW ON AEROGELS .....................................................................................12
1.1. COMMERCIAL SILICA AEROGEL...................................................................................... 14
2. SOL-GEL METHOD.............................................................................................................................15
2.1. ANTECEDENTS OF GEL SYNTHESIS .............................................................................. 16
2.2. PREPARATION OF SILICA GELS ....................................................................................... 16
3. DRYING PROCEDURE OF GELS.................................................................................................. 20
4. AEROGEL APPLICATIONS ............................................................................................................. 22
4.1. POROSITY AND SURFACE AREA APPLICATIONS...................................................... 22
Electrodes in capacitors ................................................................................. 22
Capacitive deionization ................................................................................. 23
Studies of superf luid transitions and phase separation of 3He-4He...................... 23
4.2. OPTICAL PROPERTY APPLICATIONS ............................................................................. 23
Çerenkov (particle detection and counters)........................................................ 23
4.3. THERMAL INSULATOR APPLICATIONS........................................................................ .24
4.4. ACOUSTICAL AND MECHANICAL APPLICATIONS ................................................... 24
Shock compression experiments ...................................................................... 25
4.5. ELECTRICAL AND ELECTRONIC APPLICATIONS..................................................... 25
4.6. SPACE APPLICATIONS .......................................................................................................... 25
5. REFERENCES....................................................................................................................................... 25
Chapter I. Silica aerogel: An Introduction 12
1. GENERAL OVERVIEW ON AEROGELS
This section surveys the literature and summarizes the historical background of aerogels
development, their production by the sol-gel process, several drying methods, and various
structural investigations.
Aerogel is defined as a group of extremely light and porous solid materials. Silica-based
aerogels are among the lightest ones, can be less than four times as dense as dry air, and
some are nearly transparent, its nickname is “solid smoke” or “frozen smoke”.
Since this definition is good for most porous materials, the term aerogels became reserved
for the porous gels obtained by removing solvent from highly swollen gels at the conditions
that no or minimal collapse occurs, which causes the liquid in the gel to become supercritical
(in a state between a liquid and a gas) and lose its surface tension. The result is an open
porous material with a backbone morphology that can be modeled in terms of three
dimensionally interconnected strings of nanoscopic pearls. The length scale of both the
“pearls” as well as the interconnected voids can be independently tailored over a wide range,
i.e. from a few nanometers to several microns.
Figure I.1 The basic blocks of the structure of silica aerogel are spherical primary particles with five nanometers in diameter and with the same density as bulk silica. These spheres cluster into secondary particles that are linked in chains to create the porous aerogel skeleton (on the range 20-50 nm).
One of the striking advantages of aerogels compared to other porous materials is that both
porosity and inner surface area can be tuned independently. Porosities of up to 99.9 % are
achievable; when microporosity is present, the specific surface area can exceed 1500 m2/g.
Chapter I. Silica aerogel: An Introduction 13
Because of their unique properties, i.e., large surface area, very small pores and very low bulk
density, aerogels are potentially important candidates for a wide range of applications. Table
I.1 gathers some of the remarkable properties of silica aerogels.
Table I.1 Physical properties of silica aerogels.
Property Value Comments
Apparent density 0.003-0.5 g/cm3 Most common density is 0.1g/cm3 ( air = 0.001g/cm3)
Inner surface area 500-1500 m2/g As determined by nitrogen adsorption/desorption A cubic centimeter of an aerogel has about the same surface area as one soccer field)
Solid percentage in volume
0.13-15 % Typically 5 % (95 % free space)
Mean pore diameter 20-150 nm As determined by nitrogen adsorption/desorption (varies with density)
Primary particle diameter
2-5 nm Determined by transmission electron microscopy
Index of refraction 1.007-1.24 Very low for solid material (nair= 1.004)
Thermal tolerance Up to 500 C Shrinkage begins slowly at 500 C, increases with increasing temperature. Melting point is ~1200ºC
Poisson’s ratio 0.2 Independent of density, similar to dense silica. Determined using ultrasonic methods.
Young’s modulus 0.1-300 MPa Very small (<104) compared to dense silica
Tensile strength 16 kPa For density of 0.1 g/cm3
Fracture toughness 0.8 kPa.m1/2 For density of 0.1 g/cm3. Determined by 3-point bending
Dielectric constant 1.1 For density of 0.1 g/cm3, very low for a solid material (kair= 1)
Acoustic impedance 104 Kg/m2.s Determined using ultrasonic methods al KHz frequency.
Sound velocity through the medium
20-800m/s 100 m/s for density of 0.07 g/cm3, one of the lowest velocities for a solid material
Optical property Transmittance>90%
(630nm) Transparent-blue haze
Thermal conductivity 0.02 W/mK (20 C) Very low thermal conductivity. 2 cm slab provides the same insulation as 30 panes of glass
Aerogel is an extremely adaptable material: aerogels have been prepared from many metal
oxides, including tin, tungsten and iron and also, alumina, zirconia, titanic, and magnesia , as
well as from organic gels: carbon, gelatin, organic polymers, proteins, and cellulose.
Chapter I. Silica aerogel: An Introduction 14
The laboratory of the Aerogel Research Group at the Material Science Institute of Barcelona
(ICMAB) is equipped to produce aerogels in a variety of shapes and configurations (from
bulk monoliths to thin films or microspheres), in small to medium-sized batches.
1.1 COMMERCIAL SILICA AEROGEL
A sign of growth of the technology is the increasing number of companies producing
aerogels and the increasing number of patents involving aerogels. Some works have been
published about aerogel commercialization, technology, markets and costs [29]. In 1930, S.S.
Kistler invented the aerogels by supercritically drying of gels, and Montsanto Corp. produced
thousands of tons of silica aerogels during 1940s and 1950s, using the substance as an
additive in cosmetics and toothpaste. Aerogel research was largely abandoned for the next 50
years. Then, in the 1980s, newer safer production processes to create aerogels were
developed under the leadership of A.J. Hunt at the Lawrence Berkeley Laboratory
eetd.lbl.goc/ecs/aerogels/kistler/inde.htm, and J. Phalippou at the University of Montpellier,
leading to the identification of applications for aerogels as insulators for rocket fuel storage
and later as cosmic dust collectors on two shuttle missions. Airglass, in Sweden,
(www.airglass.se) is the only current large producer of aerogel for thermal insulation, but
Cabot Corp. (www.cabot-corp.com/cabot.nsf) and Aspen Systems Inc.
(www.aspensystems.com/aerogel.html) will reach the market in the next months. Nanopore
(www.nanopore.com) explores other applications for nanoporous solids.
Silica aerogels, attracted international attention early in the 1990s after Livermore Lab.
Scientist created a silica aerogel 10 time less dense than the lightest precious version.
(http://www.llnl.gov/).
Aerogels are also the best thermal insulators ever discovered. NASA used aerogels to insulate
the electronics on the intrepid Sojourner from the cold of the Martian night
www.science.nasa.gov/aerogel. Aerogels have amazing thermal dissipation properties.
Aerogels can also be either electrically conductive (i.e. carbon aerogels) or insulators (silica
aerogels). Electrical insulators fabricated with aerogels may double the actual computer
speeds. Contemporary circuits boards have dielectric constants, k, between 2.5 and 4 (air has
k=1). Decreasing k of the insulating film can increase the speed of the computer by allowing
engineers to place components closer together. Researchers have already successfully created
aerogel films -made mostly of air- with dielectric values ranging between 2.3 and 2.01.
Other aerogels are organic, made of carbon and hydrogen atoms. Organic aerogels are stiffer
and stronger than silica aerogels and are measurably better insulators. Organic aerogels have
Chapter I. Silica aerogel: An Introduction 15
extremely high thermal resistance (six times higher than fiberglass) and can be converted to
pure carbon aerogels with still retaining many properties of the original aerogel, and at the
same time becoming electrically conductive. Carbon aerogels have been used as electrode in
energy storage devices known as double-layer capacitors. Such devices are able to deliver
power faster than conventional batteries and thus have potential application in electric
vehicles, “pure power” stations, telecommunications, and microelectronics. Carbon aerogels
capacitors are already in the electronics shops by Cooper Electronics Technologies
(www.cooperelectech.com/power/indexIntro.htm).
One of the applications uses aerogels as catalysts to reduce nitrous oxide emissions from cars
exhaust systems. Some scientists expect aerogels will be used as catalysts within a few years
because of their high surface area (a cubic centimeter of an aerogel has about the same
surface area as one soccer field). In addition, metallic atoms or metal-oxide particles can be
placed in aerogels to cause reactions.
Lots of information about aerogels can be obtained from Internet. A good starting point can
be the NASA aerogel Web site at the http://www.science.nasa.gov/aerogel. A nice and
documented work on the history of aerogels has been prepared by Mike Ayers (LBNL) and
can be navigated at eetd.lbl.goc/ecs/aerogels/kistler/index.htm.
2. SOL-GEL METHOD
The formation of aerogels, in general, involves two major steps, the formation of a wet gel,
and the drying of the wet gel, avoiding major shrinkage, to form an aerogel.
2.1 ANTECEDENTS OF GEL SYNTHESIS
Kistler [1, 2] was the first researcher who formulated the idea of replacing the liquid phase by
a gas with only a slight shrinkage of the gel back in the 1930s. Eventually, he obtained silica
aerogels by a technique known as the „water-glass process‟ outlined below:
Chapter I. Silica aerogel: An Introduction 16
Figure I.2 Left recipient contains a sol /transparent at visible range). Right recipient contains a gel (blue shading).
1. Preparation of a hydrogel (gels with water as a solvent) in reaction of sodium silicate
with hydrochloric acid.
2. Removal of sodium and chlorine ions. This step involves a long and tedious soaking of
the gel.
3. Converts the hydrogel into alcogel by replacing water with ethyl alcohol in a lengthy
process of solvent replacement.
4. Drying at above critical conditions for ethyl alcohol.
When these steps were followed, an extremely light solid remained; however, the work on
solid aerogels was mostly forgotten until 1970s.
An improved method of preparing gels took place in the Teichner and Nicholaon group [3]
[4] at the Claude Bernard University in Lyon. The procedure was substantially simplified by
carrying out the sol¯gel transition directly in the solvent (that was removed at supercritical
conditions) through the use of relatively new class of compounds called metal alkoxides.
Alkoxide-based sol-gel chemistry avoids the formation of undesirable salt by-products, and
allows a much greater degree of control over the final product.
2.2 PREPARATION OF SILICA GELS
A common way to synthesize gels at room temperature corresponds to a chemical reaction
implying metal alkoxides and water in an alcoholic solvent. The majority of silica aerogels
prepared utilizes silicon alkoxide precursors. The most common of these are tetramethyl
orthosilicate (TMOS or Si(OCH3)4), and tetraethyl orthosilicate (TEOS or Si(OCH2CH3)4).
However, many other alkoxides, containing various organic functional groups, can be used to
Chapter I. Silica aerogel: An Introduction 17
impart different properties to the gel [5, 6]. The first reaction is a hydrolysis which induces
the substitution of OR groups linked to silicon by silanol Si-OH groups. A condensation
reaction occurs when two silanol groups (Si-OH + HO-Si) react together to form Si-O-Si
(siloxane) bonds, which lead to the silica network formation.
Three reactions are generally used to describe the sol̄ gel process [7]:
(Eq. I.1)
(Eq. I.2)
(Eq. I.3)
where R is an alkyl group, CxH2x+1
The hydrolysis reaction (Eq. I.1) replaces alkoxide groups (OR) with hydroxyl groups (OH).
Subsequent condensation reactions (Eq. I.2, Eq. I.3) involving the silanol groups produce
siloxane bonds (Si¯O¯Si) plus the by-products alcohol (ROH) or water.
Because water and alkoxysilanes are immiscible, a mutual solvent such as alcohol is normally
used as a homogenizing agent. The final density of the aerogel depends on the concentration
of silicon alkoxide monomers in the solution.
The balanced chemical equation for the formation of a silica gel is:
Si(OR)4 (liq.) + 2H2O (liq.) = SiO2 (solid)+ 4HOR (liq.) (Eq. I.4)
The stoichiometry of the reaction requires two moles of water per mole of alkoxysilane. In
practice, this amount of water leads to incomplete reaction, and weak, cloudy aerogels.
Therefore, most aerogel recipes use a higher water ratio than is required by the balanced
equation (anywhere from 4-30 equivalents).
The gel point is the time at which the network of linked oxide particles spans the container
holding the sol. At the gel point, the sol becomes a gel. This two-phase material, a solid part
and a liquid part, consists of shaped solid exhibiting specific properties.
The solid part is formed by the three-dimensional network of linked oxide particles. The
liquid part (the original solvent of the sol and a small amount of water) fills the free space
Chapter I. Silica aerogel: An Introduction 18
surrounding the solid part. The liquid and solid parts of a gel occupy the same apparent
volume.
Figure I.3 Scheme of silica aerogel sol-gel synthesis by condensation of silica alkoxide precursor on alcohols.
As condensation reactions progress the gel will gain rigidity. At this point, the gel is usually
removed from its mould. However, the gel must be kept covered by alcohol to prevent
evaporation of the liquid contained in the pores of the gel. Evaporation causes severe damage
to the gel and will lead to poor quality aerogels.
Catalysts
The kinetics of the above reaction is impractically slow at room temperature, often requiring
several days to reach completion. For this reason, acid or base catalysts are added to the
formulation. The amount and type of catalyst used play key roles in the microstructural,
physical and optical properties of the final aerogel product (sections 3.5, 3.5).
Acid catalysts can be any protic acid, such as HCl. Base-catalysis usually uses ammonia, or
ammonia buffered with ammonium fluoride. Aerogels prepared with acid catalysts often
Alcohol TMOS/ TEOS
Catalyst + Water
Start of reaction
Sol
Alcogel
Gelation
Skeleton
Solvent
Chapter I. Silica aerogel: An Introduction 19
show more shrinkage during supercritical drying and may be less transparent than base
catalyzed aerogels. The microstructural effects of various catalysts are harder to describe
accurately, as the substructure of the primary particles of aerogels can be difficult to image
with electron microscopy (section 3.7). All have small (2-5 nm diameter) particles that are
generally spherical or egg-shaped. With acid catalysis, however, these particles may appear
less defined than those in base-catalyzed gels.
Two-Step Aerogels
Typical acid or base catalyzed gels are often classified as "single-step" gels, referring to the
"one-pot" nature of this reaction. A more recently developed approach uses pre-polymerized
TEOS as the silica source. Pre-polymerized TEOS is prepared by heating an ethanol solution
of TEOS with a sub-stoichiometric amount of water and an acid catalyst. The solvent is
removed by distillation, leaving a viscous fluid containing higher molecular weight silicon
alkoxy-oxides. In a second step, this material is redissolved in ethanol and reacted with
additional water under basic conditions until gelation occurs. Gels prepared in this way are
known as "two-step" acid-base catalyzed gels. Pre-polymerized TEOS is available
commercially from Silbond Corp. (Silbond H-5).
These slightly different processing conditions impart important changes to the final aerogel
product. Single-step base catalyzed aerogels are typically more brittle than two-step aerogels.
Moreover, two-step aerogels have a smaller and narrower pore size distribution and are often
optically clearer than single-step aerogels (Chapter II, section 3).
Aging and Soaking
At the gel point the silica backbone of the gel contains a significant number of unreacted
alkoxide groups. Sufficient time must be given for the strengthening of the silica network.
This can be enhanced by controlling the pH and water content of the covering solution.
Common aging procedures for base catalyzed gels typically involve soaking the gel in an
alcohol/water mixture of equal proportions to the original sol. The gels are soaked in this
solution for up to 24 hours. This step, and all subsequent processing steps, is diffusion
controlled. Diffusion is affected by the thickness of the gel. Then, the time required for each
processing step increases dramatically as the thickness of the gel increases.
Chapter I. Silica aerogel: An Introduction 20
After aging the gel, all water still contained within its pores must be removed prior to drying.
This is simply accomplished by soaking the gel in pure alcohol several times until all the water
is removed. Again, the length of time required for this process is dependent on the thickness
of the gel. Any water left in the gel will not be removed by supercritical drying, and will lead
to an opaque, white, and dense aerogel.
Chapter II shows that variations in synthesis conditions (for example, ratio H2O/Si, the
catalyst type and concentrations, the type of solvent, temperature and pressure of
supercritical drying) cause modifications in the structure and properties of the obtained silica
aerogels [8]. Thus, porous structure of silica aerogels strongly depends on preparation and
drying parameters [9 -18].
3. DRYING PROCEDURE OF GELS
An aerogel results from a supercritical drying process. This is where the liquid within the gel
is removed, leaving only the linked silica network. The difference between classical drying
and supercritical drying is shown in Figure I.4. From point 1 to 5, (purple arrow) the liquid is
depressurized isothermally (classical drying). Consequently, we can say that xerogels refer to
gels dried at temperature close to room temperature and under atmospheric pressure. The
supercritical drying is performed inside an autoclave, which allows to overpass the critical
point (PC, TC) of the solvent, as shown in Figure I.4 (Path 1-2-3-4-5); or by prior solvent
exchange with liquid CO2 followed by supercritical CO2 venting (lower temperature drying).
The supercritical fluid is a substance that is above its critical pressure and critical
temperature; it possesses some properties in common with liquid (density, thermal
conductivity) and some in common with gas (fills its container, does not have surface
tension). A more detailed description of supercritical fluids can be found in Annex I). Strong
inorganic solids are commonly dried using alcohol (or acetone) as solvent or dried using CO2
as solvent.
Chapter I. Silica aerogel: An Introduction 21
Figure I.4 Scheme of the pressure and temperature variation on the solvent phase diagram during a gel supercritical drying process. The shaded area represents the supercritical region (SCF), where C is the critical point, Tr represents the triple point, and 1 to 5 are random points in the phase diagram.
Under ambient conditions, during the evaporation of the solvent, a liquid-vapor interface is
formed within the pores of the gel. The surface tension of the liquid creates a concave
meniscus in each capillary. By evaporation, the meniscus recedes and the compressive force
on the wall of the pores produces the collapse of the initial gel framework, or shrinkage
(Figure I.5). A liquid-vapor interface or the presence of a liquid in equilibrium with the vapor
is only observed below the critical temperature and pressure of the solvent. Above the critical
point (Figure I.4) the liquid no longer existed. Supercritical fluid is a gaseous like-phase, so
the liquid meniscus and its interfacial tension would not form in these conditions.
r
cos2p , capillary pressure
is the surface tension of the liquid
r is the pore radius
Figure I.5 The liquid-vapor interface formed in the gel capillary during drying.
Using the high temperature drying procedure some problems may arise from the
combination of high pressures and high temperatures (methanol critical parameters: Pc=81
bar, Tc=240 C) , i.e. flammability of the solvents. Alternatively, supercritical drying with CO2
has been developed by substituting, under pressure, alcohol in the gel by liquid carbon
Liquid
Meniscus
r, pore radius
Vapor
Chapter I. Silica aerogel: An Introduction 22
dioxide and then drying the aerogel with carbon dioxide at supercritical conditions. CO2 is of
particular interest due to its low critical temperature (31 C), non-flammability, and non-
toxicity. The process results in a reduction of the temperature and pressure required for
drying aerogels.
4. AEROGEL APPLICATIONS*
Aerogels were discovered more than 70 years ago. During these years, many potential
applications were described [19-22] and more new applications are mentioned in recent
reviews [23-28]. This section reviews some of the aerogels applications.
In addition to being the best thermal insulators ever discovered, aerogels have the lowest
dielectric constant, and the lowest sound velocity of any known solid material, sound
propagates more slowly through aerogels than through air. Other possible applications
include, substrates for chemical catalysis, ultrafilters, seawater desalinization, battery
electrodes, solar collector covers, acoustic delay lines, refrigerator insulation, replacements for
the air between the panes of double-glazed windows, to detect high-energy subatomic
particles emitted by particle accelerators, micrometeoroid collectors, and supercapacitors,
adsorbents, sensors and fuel cells, and even safe insecticides.
The following applications for aerogels are associated with certain properties of aerogel
materials. In many cases, the application is associated with a single property even if the
aerogels have a combination of properties appropriate to the given application.
4.1 POROSITY AND SURFACE AREA APPLICATIONS
Due to their high porosity, their very large inner surface area (easily accessible because of the
open porosity), and the controllable dispersion of the active component, they are especially
active catalysts or catalytic substrates [30-36]. There are numerous references of this
application for various aerogels and doped aerogels [37-43].
Moreover, the high porosity and large surface areas lead to applications as filters [44],
absorbing media for desiccation [45,46,47], filters, reinforcement agents, pigments, gellifying
agents [48], waste containment [49], encapsulation media [50], and pesticides [23].
* Lawrence W. Hrubesh April 1998 Journal of Non-Crystalline Solids Volume 225, Issue 1 Pages 335-342 and from C.J.
Brinker, G.W. Sherer, Sol-Gel Science. Physics and Chemistry of Sol-Gel Processing, Academic Press, New York, 1990.
Chapter I. Silica aerogel: An Introduction 23
Electrodes in capacitors
The carbon aerogels have been used as electrodes capacitors in energy storage devices known
as double layer capacitor because they are electrically conductive with a very large surface area
[51-53]. The stored energy in these devices can be released faster than conventional batteries
with high power densities. Thus, have potential application in electric vehicles,
microelectronics, and hydrogen fuel storage [54].
Capacitive deionization
One of the promising new applications for aerogels is in a cost-effective purification process
[55]. The carbon aerogel capacitive deionization process works by sending solutions with
various positively and negatively charged ions through an electrochemical cell consisting of
numerous electrodes containing carbon aerogels in the form of sheets. The aerogel process
can have a variety of uses ranging from extracting harmful contaminants from industrial
waste water [56] to desalinizing seawater.
Studies of superfluid transitions and phase separation of 3He¯4He
Low-density silica aerogels are used to study the superfluid transition of 4He and phase
separation of the 3He¯4He mixture. The aerogels provide a random disordered structure that
modifies the normal superfluid and phase separation behavior with helium. [57- 62].
4.2 OPTICAL PROPERTY APPLICATIONS
Aerogel is transparent when its microstructural components are very small compared with
the wavelength of light. Transparent aerogels, together with their exceptional thermal
insulation ability, have been considered for use as super-insulating sheets in double walled
window systems because help considerably to reduce thermal losses in windows [63- 69].
Translucent aerogels have been proposed to improve the efficiency of solar thermal energy
storage devices [69-75]. Moreover, the ultra-low density aerogels can be used as lightweight
mirror backings [76].
Aerogels have been used to prepare ultra-pure, full-density silica glass by sintering at
temperatures below the melting temperature of silica [77- 80].
Silica aerogels with silicon exhibits strong photo-luminescence (luminescence stimulated by
visible or ultraviolet radiation). Silica aerogel, doped with radioactive tritium and phosphor,
makes an efficient radio-luminescent light source [81]. There is also evidence for quantum
confinement in nanoparticle-loaded silica aerogels [82] for producing blue light emission.
Chapter I. Silica aerogel: An Introduction 24
Çerenkov (particle detection and counters)
The first modern application, in the early 1980s, involved the use the silica aerogels in
detectors called Çerenkov counters. The Çerenkov detector measures the velocity of
elementary particles and cosmic rays [83-84]. Since the speed of light depends on the
refraction index of the medium, media with different refraction indices were searched. Silica
aerogels with refraction indices between 1.007 and 1.024 offer specific refractive indexes for
range threshold detectors. [86-92]. Aerogel are used for particle detectors and counters
continue in space, at accelerators around the world [93]. A low-density silica aerogel was used
in radiation detection vacuum tubes to support the high voltage wire [94].
4.3 THERMAL INSULATOR APPLICATIONS
Aerogel materials, known for exhibit the lowest thermal conductivity of any of the solid or
porous materials, are excellent for applications requiring thermal insulation [95]. These
include development of electric automobiles equipped with batteries that operate at high
temperatures and that need heat storage [95- 97], and insulation for architectural purposes
[98]. A layer of transparent aerogel allows the penetration of the sun radiation to the wall,
but not the escape of the heat generated [99]. Then, aerogels can be used in heat and cold
storage devices [100], automotive exhaust pipes, transport vehicles, and vessels.
4.4 ACOUSTICAL AND MECHANICAL APPLICATIONS
Aerogels may also have acoustic and mechanical applications. Because of their unusual
structure, aerogels have low sound velocities, as low as 30 meters per second.
Another important acoustic property of aerogel is its mechanical impedance. The impedance
is the product of density and the sound velocity of the material. Since both are low, silica
aerogel has the lowest impedance of all solid material. This allows the aerogels to be used for
coupling sound waves in air to a transducer (device that converts energy from one form to
another), this may be useful either to generating or detecting sound. Therefore, they should
be efficient ultrasonic devices as acoustic impedance matching [101-103], and sound
absorption (anechoic chambers) [104, 105].
in Ardon (Switzerland) a house was built using aerogel granulate as a translucent insulating material
Chapter I. Silica aerogel: An Introduction 25
Shock compression experiments
Aerogels have also been proposed as a shock absorbing material. One of the earliest
experiments was to measure shock compression in silica aerogels [106]. The low density of
the silica aerogel allowed more internal energy could be deposited in it.
4.5 ELECTRICAL AND ELECTRONIC APPLICATIONS
Silica aerogel is an electrical insulator with a low dielectric constant, k (k is the measure of the
ability of a material to store electrical potential energy under the influence of an electric field).
The velocity of signal propagation in a chip is dependent on the dielectric constant of the
surrounding electrical insulation. The lower the dielectric constant, the higher the velocity.
Therefore, thin aerogel films are almost ideal dielectrics for ultra-fast integrated circuits [107-
109].
The bulk aerogels can be used for microwave electronics and high voltage insulators [110].
The pure carbon aerogels are quite electrically conductive, so they have applications as
electrodes for batteries, fuel cells, and capacitors [111]. Other metal oxide aerogels have been
made, which exhibit super-conducting behavior [112], thermoelectric behavior [113], and
piezoelectric properties [114].
4.6 SPACE APPLICATIONS
Aerogels have already captured cosmic dust while on the European Retrieval Carrier
(EURECA) satellite and in Space Shuttle experiments [115], and will capture cometary‟s dust
in NASA's STARDUST project. Lightweight silica aerogels have also been proposed as a
contaminant collector, to protect space mirrors from volatile organics [116]. Aerogels were
used to insulate the Mars Rover, where its lightness and strength were established as ideal.
5. REFERENCES
1. S.S. Kistler. Nature 127 (1931), p. 741.
2. S.S. Kistler. J. Phys. Chem. 36 (1932), p. 52.
3. G.A. Nicholaon and S.J. Teichner. Bull.
Soc. Chim. Fr. 5 (1968), p. 1906.
4. J. Fricke, R. Caps, D. Buttner, V.
Heinemann, E. Himmer, G. Reichenamer,
Structural, elasto-mechanical and thermal
properties of silica aerogels, in: K.K.
Kruger et al. (Eds.), Characterization of
Porous Structure, vol. 629, Elsevier,
Amsterdam, 1988
5. J. Fricke, in: J. Fricke (Ed.), Aerogels, vol.
2, Springer, Berlin, 1986
6. M. Gronauer, A. Kadur, J. Fricke, in: J.
Fricke (Ed.), Aerogels, vol. 167, Springer,
Berlin, 1986
Chapter I. Silica aerogel: An Introduction 26
7. C.J. Brinker, G.W. Sherer, Sol¯Gel
Science. Physics and Chemistry of Sol¯Gel
Processing, Academic Press, New York,
1990
8. J. Livage and C. Sanchez. J. Non-Cryst.
Solids 11 (1992), p. 145.
9. D.R. Uhlmann, B.J. Zeliñski, L. Silverman,
S.B. Warner, B.D. Fabes, W.F. Doyle,
Kinetic processes in sol-gel processing, in:
L.L. Hench, D.R. Ulrich (Eds.), Science of
Ceramic Processing, vol. 173, Wiley, New
York, 1986
10. J.D. Mackenzie, Applications of the
sol¯gel method: some aspects of initial
processing, in: L.L. Hench, D.R. Ulrich
(Eds.), Science of Ceramic Processing, vol.
113, Wiley, New York, 1986
11. M. Prassar, J. Phalippou, J. Zarzycki,
Sintering of monolithic silica aerogel, in:
L.L. Hench, D.R. Ulrich (Eds.), Science of
Ceramic Processing, vol. 156, Wiley, New
York, 1986
12. S.J. Teichner, in: J. Fricke (Ed.), Aerogels,
vol. 22, Springer, Berlin, 1986
13. J. Zarzycki, T. Wognier, in: J. Fricke (Ed.),
Aerogels, vol. 42, Springer, Berlin, 1986
14. G.M. Pajonk. Rev. Phys. Appl. 24 (1989),
pp. C4¯13.
15. S.Y. Chang, T.A. Ring, J. Non-Cryst.
Solids 147¯148, 56 (1992)
16. G.M. Pajonk, M. Repelin-Lacroix, S.
Abouarnadasse, J. Chaouki and D. Klvana.
J. Non-Cryst. Solids 121 (1990), p. 66.
17. J. Walendziewski, M. Stolarski, M.
Steininger and B. Pniak. React. Kinet. Catal.
Lett. 58 1 (1996), p. 85.
18. C.W. Turner, K.J. Franklin, in: L.L.
Hench, D.R. Ulrich (Eds.), Science of
Ceramic Chemical Processing, New York,
1986, p. 81
19. J. Fricke, Aerogels, Springer Proceedings
in Physics, Vol. 6, Springer, Heidelberg,
1986.
20. R. Vacher, J. Phalippou, J. Pelous, T.
Woignier, Proc. 2nd Int. Symp. on
Aerogels, J. Phys. C 4 (1989).
21. J. Fricke, Proc. 3rd Int. Symp. on
Aerogels, J. Non-Cryst. Solids 145 (1992).
22. R.W. Pekala, L.W. Hrubesh, Proc. 4th Int.
Symp. on Aerogels, J. Non-Cryst. Solids
186 (1995).
23. J. Fricke and T. Tillotson, Aerogels:
production, characterization, and
applications Thin Solid Films Volume 297,
Issues 1-2 1997 Pages 212-223
24. J. Fricke, and; Emmerling, Journal of the
American Ceramic Society Volume 75,
Issue 8 1992 Pages 2027-2036
25. Gesser, Hyman D.; Goswami, Prabhat C.
Chemical Reviews Volume 89, Issue 4
June 1989 Pages 765-788
26. J. Fricke, A. Emmerling, in: R. Reisfeld,
C.K. Jorgensen (Eds.), Chemistry,
Spectroscopy and Applications of Sol-Gel
Glasses, Springer Series in Structure and
Bonding, Vol. 77, Springer, Heidelberg,
1991, p. 37.
27. J. Fricke and A. Emmerling. J. Am. Ceram.
Soc. 75 (1992), p. 2027.
28. J. Fricke and T. Tillotson. Thin Solid Films
297 (1997), p. 212.
29. J. Richardson, G. Carlson, K. McKinley,
D. Lewis and T. Tillotson Journal of Non-
Chapter I. Silica aerogel: An Introduction 27
Crystalline Solids Volume 186 June 1995
Pages 372-387
30. S.J. Teichner, in: J. Fricke (Ed.), Aerogels,
Springer Proceedings in Physics, Vol. 6,
Springer, Heidelberg, 1986, p. 22.
31. F. Blanchard, J.P. Reymond, B. Pommier
and S.J. Teichner. J. Mul. Catal. 17 (1982),
p. 171.
32. J.N. Armor and E. Carlson. Appl. Catal. 19
(1985), p. 32.
33. L. Wang, K. Eguchi and H. Arai. Appl.
Catal. 33 (1987), p. 107
34. G.C. Bond and S. Flamerz. Appl. Catal. 33
(1987), p. 219
35. D. Kalvana, J. Chaouki, D. Kusohorsky,
C. Chavarje and G.M. Pajonk. Appl. Catal.
42 (1988), p. 121.
36. G.M. Pajonk. Appl. Catal. 72 (1991), p.
217
37. G.M. Pajonk, S.J. Teichner, in: J. Fricke
(Ed.), Aerogels, Springer Proceedings in
Physics, Vol. 6, Springer, Heidelberg,
1986, p. 193.
38. S.J. Teichner, in: R. Vacher, J. Phalippou,
J. Pelous, T. Woignier (Eds.), Proc. 2nd
Int. Symp. on Aerogels, J. Phys. C 4
(1989) 1.
39. G.M. Pajonk. Appl. Catal. 72 (1991), p.
217.
40. Y. Mizushima and M. Hori. Appl. Catal. A:
General 88 (1992), p. 137.
41. E.I. Ko, Chemtech (1993) 31.
42. R.J. Willey, C.-T. Wang, J.B. Peri, in: R.W.
Pekala, L.W. Hrubesh (Eds.), Proc. 4th
Int. Symp. on Aerogels, J. Non-Cryst.
Solids 186 (1995) 408.
43. M. Moner Girona, E. Martínez, J. Esteve,
A. Roig, R. Solanas, and E. Molins Applied
Physics A 74 1 (2002) 119-122 (rapid
communication).
44. D.W. Cooper. Part. Sci. Technol. 7 (1989), p.
371.
45. H.D. Gesser and P.C. Goswami. Chem.
Rev. 89 (1989), pp. 765¯788.
46. C. Liu, S. Komarneni, in: S. Komarneni,
D.M. Smith, J.S. Beck (Eds.), Advances in
Porous Materials, Mater. Res. Soc. Symp.
Proc. 371, Materials Research Society,
Pittsburgh, 1995, p. 217.
47. S. Komarneni, R. Roy, U. Selvaraj, P.B.
Malla and E. Breval. J. Mater. Res. 8 (1993),
p. 3163.
48. H.D. Gesser and P.C. Goswami. Chem.
Rev. 89 (1989), p. 765.
49. Y.A. Attia, M.S. Ahmed, M. Zhu, in: Y.A.
Attia (Ed.), Sol¯Gel Processing and
Applications, Plenum, New York, 1994, p.
311.
50. B.C. Dave, B. Dunn, J.S. Valentine, J.I.
Zink, in: R.F. Lobo, J.S. Beck, S.L. Suib,
D.R. Corbin, M.E. Davis, L.E. Iton, S.I.
Zones (Eds.), Microporous and
Macroporous Materials, Mater. Res. Soc.
Symp. Proc. 431, Materials Research
Society, Pittsburgh, 1996, p. 285.
51. R.W. Pekala, S.T. Mayer, J.L. Kaschmitter,
F.M. Kong, in: Y.A. Attia (Ed.), Sol¯Gel
Processing and Applications, Plenum,
New York, 1994, p. 369.
52. R.W. Pekala. J. Mater. Sci. 24 (1989), p.
3221.
Chapter I. Silica aerogel: An Introduction 28
53. S.T. Mayer, R.W. Pekala and J.L.
Kaschmitter. J. Electrochem. Soc. 140 (1993),
p. 446.
54. S.T. Teichner, M. Khalfallah, D. Bianchi,
J.-L. Gass, in: Y.A. Attia (Ed.), Sol¯Gel
Processing and Applications, Plenum,
New York, 1994, p. 323.
55. J.C. Farmer, D.V. Fix, G.V. Mack, R.W.
Pekala and J.F. Poco. J. Electrochem. Soc.
143 (1996), p. 159.
56. M.S. Ahmed and Y.A. Attia. J. Non-Cryst.
Solids 186 (1995), p. 402
57. J. Ma, S.B. Kim, L.W. Hrubesh and
M.H.W. Chan. J. Low-Temp. Phys. 93
(1993), p. 945.
58. M.H.W. Chan, K.J. Blum, S.Q. Murphy,
G.K.S. Wong and J.D. Reppy. Phys. Rev.
Lett. 61 (1988), p. 1950.
59. R. Maynard and G. Deutscher. Europhys.
Lett. 10 (1989), p. 257
60. W.M. Snow and P.E. Sokol. J. Low-Temp.
Phys. 80 (1990), p. 197.
61. J. Ma, S.B. Kim, L.W. Hrubesh and
M.H.W. Chan. J. Low-Temp. Phys. 93
(1993), p. 945.
62. J. Yoon, N. Mulders, L.W. Hrubesh and
M.H.W. Chan. Czech. J. Phys. 46 (1996), p.
157.
63. P.H. Tewari, A.J. Hunt, K.D. Lofftus, in:
J. Fricke (Ed.), Aerogels, Springer
Proceedings in Physics, Vol. 6, Springer,
Heidelberg, 1986, p. 31.
64. J. Fricke, in: J. Fricke (Ed.), Aerogels,
Springer Proceedings in Physics, Vol. 6,
Springer, Heidelberg, 1986, p. 94.
65. E. Schreiber, E. Boy, K. Bertsch, in: J.
Fricke (Ed.), Aerogels, Springer
Proceedings in Physics, Vol. 6, Springer,
Heidelberg, 1986, p. 133.
66. E. Boy, M. Munding, V. Wittwer, in: R.
Vacher, J. Phalippou, J. Pelous, T.
Woignier (Eds.), Proc. 2nd Int. Symp. on
Aerogels, J. Phys. C 4 (1989) 99.
67. V. Wittwer. J. Non-Cryst. Solids 145 (1992),
p. 240.
68. P.H. Tewari, A.J. Hunt, K.D. Lofftus, in:
J. Fricke (Ed.), Aerogels, Springer
Proceedings in Physics, Vol. 6, Springer,
Heidelberg, 1986, p. 31.
69. M. Rubin and C.M. Lampert. Solar Energy
Mater. 7,1 (1983), p. 393.
70. D. Buttner and J. Fricke. Int. J. Sol. Energy
3 (1985), p. 89.
71. P.H. Tewari and A.J. Hunt, 1986 US Pat
4610863. Chem. Abstr. 105 (1986), p.
214727C.
72. R. Caps and J. Fricke. J. Sol. Energy 36
(1986), p. 316.
73. J. Fricke, R. Caps, D. Buttner, U.
Heinemann and E. Hummer. Sol. Energy
Mater. 16 (1987), p. 267
74. K.I. Jensen. J. Non-Cryst. Solids 145 (1992),
p. 237.
75. S. Svendson. J. Non-Cryst. Solids 145 (1992),
pp. 240¯243.
76. S.P. Hotaling. J. Mater. Res. 8 (1993), p.
352.
77. C. Mulder, J.G. van Lierop Aerogels, in: J.
Fricke (Ed.), Springer Proceedings in
Physics, Vol. 6, Springer, Heidelberg,
1986, p. 68.
78. R. Sempere, D. Bourret, J. Bouaziz, A.
Sivade, in: R. Vacher, J. Phalippou, J.
Pelous, T. Woignier (Eds.), Proc. 2nd Int.
Chapter I. Silica aerogel: An Introduction 29
Symp. on Aerogels, J. Phys. C 4 (1989)
227.
79. B. Dunn and J.I. Zink. J. Mater. Chem. 1
(1991), p. 903.
80. T.M. Tillotson, W.E. Sunderland, I.M.
Thomas and L.W. Hrubesh. J. Sol¯Gel Sci.
Technol. 1 (1994), p. 241.
81. C.S. Ashley, S.T. Reed, C.J. Brinker, R.J.
Walko, R.E. Ellefson, J.T. Gill, in: L.L.
Hench, J.K. West (Eds.), Chemical
Processing of Advanced Materials, Wiley,
New York, 1992 p. 989.
82. T.J. Goodwin, V.J. Leppert, C.A. Smith,
S.H. Risbud, M. Niemeyer, P.P. Power,
H.W.H. Lee, L.W. Hrubesh, in: J.S. Beck
(Ed.), Microporous and Mesoporous
Materials, Materials Research Society
Symposium Proceedings, Pittsburgh,
1996.
83. D.E. Fields, H. van Hecke, J. Boissevain,
B.V. Jacak, W.E. Sondheim, J.P. Sullivan,
W.J. Willis, K. Wolf and E. Noteboom.
Nucl. Instrum. Methods A 349 (1994), p. 431.
84. R.M. Bionta, H.S. Park, E. Abels, T.E.
Cowan, F.S. Dietrich, E.P. Hartouni, K.A.
Van Bibber, Am. Nucl. Soc. Proc.,
Accelerat. Appl., Nov 1997.
85. M. Cantin, M. Casse, L. Koch, R. Jouan,
P. Mestran, D. Roussel, F. Bonnin, J.
Moutel and S.J. Teichner. Nucl. Instrum.
Methods 118 (1974), p. 177.
86. S. Henning and L. Svensson. Phys. Scr. 23
(1981), p. 697.
87. G. Poelz and R. Riethmuller. Nucl. Instrum.
Methods 195 (1981), p. 491.
88. G. Poelz, in: J. Fricke (Ed.), Aerogels,
Springer Proceedings in Physics, Vol. 6,
Springer, Heidelberg, 1986, p. 176.
89. P. Carlson. Nucl. Instrum. Methods A 248
(1986), p. 110.
90. G. Poelz. Nucl. Instrum. Methods A 248
(1986), p. 118.
91. I.L. Rasmussen, in: R. Vacher, J.
Phalippou, J. Pelous, T. Woignier (Eds.),
Proc. 2nd Int. Symp. on Aerogels, J. Phys.
C 4 (1989) 221.
92. S. Henning, in: J. Fricke (Ed.), Aerogels,
Springer Proceedings in Physics, Vol. 6,
Springer, Heidelberg, 1986, p. 38.
93. L. Koch-Miramond, in: J. Fricke (Ed.),
Aerogels, Springer Proceedings in Physics,
Vol. 6, Springer, Heidelberg, 1986, p. 188.
94. C. Weust, T. Tillotson, US patent
5,416,376 (1992).
95. G. Herrmann, R. Iden, M. Mielke, F.
Teich, B. Ziegler, in: R.W Pekala, L.W.
Hrubesh (Eds.), Proc. 4th Int. Symp. on
Aerogels, J. Non-Cryst. Solids 186 (1995)
380.
96. J. Fricke. Phys. Unserer Zeit. 20 (1989), p.
189
97. J. Fricke, M. Arduini, D. Buttner, U.
Heinemann and E. Hummer. Thermal
Cond.. 21 (1990), p. 235
98. G.M. Pajonk, E. Elaloui, M. Durant, J.L.
Chevalier, B. Chevalier, P. Achard, in:
Y.A. Attia (Ed.), Sol¯Gel Processing and
Applications, Plenum, New York, 1994, p.
267.
99. B. Wolff and G. Seybold. Chem. Abstr. 112
(1990), p. 828294
Chapter I. Silica aerogel: An Introduction 30
100. J. Fricke, X. Lu, P. Wang, D. Buttner and
U. Heineman. Int. J. Heat Mass Transfer 35
(1992), pp. 2305¯2309.
101. M. Gronauer and J. Fricke. Acoustica 59
(1986), p. 177.
102. (T. E. Gómez Álvarez, F. R. Montero, M.
Moner-Girona, E. Rodríguez, A. Roig and
E. Molins. Viscoelasticity of silica aerogels
at ultrasonic frequencies, Applied Physics
Letters (accepted)
103. R. Gerlach, O. Kraus, J. Fricke, P.-Ch.
Eccardt, N. Kroemer and V. Magori. J.
Non-Cryst. Solids 145 (1992), p. 227.
104. A. Zimmermann, J. Gross, J. Fricke, in:
R.W. Pekala, L.W. Hrubesh (Eds.), Proc.
4th Int. Symp. on Aerogels, J. Non-Cryst.
Solids 186 (1995) 238.
105. V. Gibiat, O. Lefeuvre, T. Woignier, J.
Pelous, J. Phalippou, in: R.W Pekala, L.W.
Hrubesh (Eds.), Proc. 4th Int. Symp. on
Aerogels, J. Non-Cryst. Solids 186 (1995)
244.
106. N.C. Holmes, H.B. Radousky, M.J. Moss,
W.J. Nellis and S. Henning. Appl. Phys.
Lett. 45 (1984), p. 626.
107. L.W. Hrubesh, in: T.M. Lu, S.P. Murarka,
T.-S. Kuan, C.H. Ting (Eds.), Low-
Dielectric Constant Materials¯¯Synthesis
and Applications in Microelectronics,
Mater. Res. Soc. Symp. Proc. 381 (1995)
267¯272.
108. D.M. Smith, J. Anderson, C.C. Cho, B.E.
Gnade, in: S. Komarneni, D.M. Smith, J.S.
Beck (Eds.), Advances in Porous
Materials, Mater. Res. Soc. Symp. Proc.
371, Materials Research Society,
Pittsburgh, 1995, p. 261.
109. D.M. Smith, J. Anderson, C.C. Cho, G.P.
Johnston, S.P. Jeng, in: T.M. Lu, S.P.
Murarka, T.-S. Kuan, C.H. Ting (Eds.),
Low-Dielectric Constant
Materials¯¯Synthesis and Applications in
Microelectronics, Mater. Res. Soc. Symp.
Proc. 381, Materials Research Society,
Pittsburgh, 1995, p. 261.
110. L.W. Hrubesh, R.W. Pekala, in: Y.A. Attia
(Ed.), Sol¯Gel Processing and
Applications, Plenum, New York, 1994, p.
363.
111. R.W. Pekala, S.T. Mayer, J.L. Kaschmitter,
F.M. Kong, in: Y.A. Attia (Ed.), Sol¯Gel
Processing and Applications, Plenum,
New York, 1994, p. 369.
112. B. Pommier, S.J. Teichner, P. Lejay, A.
Sulpice, R. Tournier, in: R. Vacher, J.
Phalippou, J. Pelous, T. Woignier (Eds.),
Proc. 2nd Int. Symp. on Aerogels, J. Phys.
C 4 (1989) 41.
113. K.E. Swider, C.I. Merzbacher, P.L.
Hagans and D.R. Rolison. Chem. Mater. 9
(1997), p. 1248.
114. P. Lobmann, W. Glaubitt, J. Gross, J.
Fricke, in: R.W Pekala, L.W. Hrubesh
(Eds.), Proc. 4th Int. Symp. on Aerogels, J.
Non-Cryst. Solids 186 (1995) 59.
115. P. Tsou. J. Non-Cryst. Solids 186 (1995), p.
415.
116. S.P. Hotaling, Rome Laboratory Report
RL-TR-93-148, July 1993.
C h a p t e r I I
SYNTHESIS OF SILICA AEROGELS
SECTION OUTLINE
1. INTRODUCTION ................................................................................................................................ 32
2. PREPARATION OF AEROGELS .................................................................................................... 33
2.1. USED REACTIVES ................................................................................................................. 33
2.2. SYNTHESIS PROCEDURE ................................................................................................. 34
2.3. DRYING PROCEDURE ........................................................................................................ 36
2.3.1 Supercritical drying at high temperature................................................................... 36
2.3.2 CO2 supercritical drying ................................................................................................ 38
3. SYNTHESIS ROUTES: EFFECT OF DIFFERENT ALKOXIDE PRECURSORS ..... 39
3.1. TETRAMETHOXYSILANE: TMOS AEROGELS ........................................................ 40
3.1.1 The effect of the TMOS concentration...................................................................... 45
3.1.2 The effect of the nature of the solvent ...................................................................... 46
3.1.3 The effect of the hydrolysis solution .......................................................................... 47
Water amount.............................................................................................. 47
3.2. TETRAETHOXYSILANE: TEOS AEROGELS ............................................................. 49
3.2.1 The effect of the TEOS concentration ...................................................................... 53
Neutral ....................................................................................................... 53
Acid catalyst ................................................................................................ 54
Base-catalyst ................................................................................................ 56
3.2.2 The effect of hydrolysis solution ................................................................................. 58
Water amount.............................................................................................. 58
Influence of the amount of the catalyst ............................................................. 61
Influence of the nature of the catalyst .............................................................. 64
3.3. ‘TWO-STEP’ SYNTHESIS .................................................................................................... 66
Effect of precursor concentration...................................................................... 67
Effect of water amount .................................................................................. 69
Influence of the amount of the catalyst ............................................................. 70
4. SUMMARY AND CONCLUSIONS.............................................................................................71
5. REFERENCES................................................................................................................................. 72
Chapter II. Synthesis of silica aerogels 32
1. INTRODUCTION
The aim of this section is to present detailed description of the conditions at which silica
aerogels were prepared through out the thesis and the influence of the synthesis routes on
some of the silica aerogel final properties. Hydrolysis and polycondensation of a silicon
alkoxide is a well-known route to prepare silica gels [1]. Hydrolysis and condensation
reactions compete with each other during all the stages of the sol-gel process, and are
additionally influenced to a different degree by many parameters. The influence of the
different parameters on the network formation is very complex, since many parameters
change progressively as polycondensation proceeds. Synthesis of silica gels has been carried
out by hydrolysis of several silicon alkoxide precursors in different solvent solutions.
To tailor the structure and properties of the final material the influence of different
parameters have been studied. The varied parameters were:
1. The type of metal alkoxide(s)
2. The type of solvent
3. The relative and absolute concentration of the metal alkoxide and the solvent
4. The concentration of alkoxy group to water ratio
5. The type of catalyst
6. The pH of hydrolysis
A data–base computer file containing all the information since 1995 to these days was used
with the aim to classify synthesis and sample information of all products prepared in our
laboratory. Twenty characterization parameters were used together with a code name to
identify each sample. The Database allowed us to develop a statistical study of different
parameters of all the synthesis prepared until now.
A simple code name was used in order to name the samples from different experimental
conditions. As an example TE01AA00 means:
TE TEOS precursor, (TM when TMOS was used, H5 when Prepolymerized TEOS)
01 2001 year,
AA name of the synthesis process, in a chronological order
01 number of the sample with AA synthesis.
Another example: H501AB97 meaning H5 Prepolymerized TEOS precursor, 97 1997
year, AB name of the synthesis process, 02 number 02 of the sample with AB synthesis.
Chapter II. Synthesis of silica aerogels 33
Few aerogels that have been thoroughly characterized and that will appear ‘recurrently’ on
the thesis are labeled in a short form as acetone series A1, A2, A3, and A4 and methanol as
M. These syntheses are described in section 4.
2. PREPARATION OF AEROGELS
2.1 USED REACTIVES
Sols were prepared by using three different silicon alkoxide precursors: (a) tetraethoxysilane
(TEOS) (b) tetramethoxysilane (TMOS), and (c) prepolymerized tetraethoxysilane (H5).
Their purity was above 98% (GC) and they were used as supplied. The solvents used were
ethanol, methanol, and acetone. The hydrolysis and catalyst solutions were done using one of
the following catalysts: HCl, C6H8O7, HNO3, NH3, CH3COOH, KOH, C2H2O4+ NH4OH,
NH4F+NH3, NH4OH+CH3COOH in the form of solutions in demineralized H 2O. Table
II.1 summarizes the properties of the reactives used in the sol-gel synthesis, molar density, M,
density , purity, melting and boiling points (mp and bp) and commercial trade.
Table II.1 Properties of the synthesis reactive: molar density, M,
density , purity, melting and boiling points (mp and bp) and commercial trade.
Reactive M
(g/mol)
(g/cm3) Purity
(%) mp-bp
( C) Trade
Tetraethoxysilane, TEOS
Si(OCH2CH3)4 208.33 0.933 98+ 163-167 Fluka
Prepolymerized
Tetraethoxysilane, H5 98+ Silbond
Tetramethoxysilane, TMOS
Si(OCH3)4 152.22 1.027 98+ 118-122 Fluka
Acetone
(CH3)2CO 58.08 0.789 99.5 - Panreac
Citric Acid
C6H8O7 192.12 - 99.5 - Fluka
Clorhidric Acid
HCl
36.46
1.18
1.017
35
0.9834N -
Panreac
Aldrich
Formic Acid
HCOOH 46.03 1.220 95-97 100-101 Aldrich
Phosphoric Acid
H3PO4 98.00 1.685 85 - Aldrich
Nitric Acid
HNO3 63.01 1.38 60 - Panreac
Ammonia
NH3 17.03 0.88 32 - Merck
Ethanol
CH3CH2OH 46.070 0.789 99.8 v/v 78.5 Carlo Erba
Hydroxide sodic
NaOH 39.996 - 97+ - Carlo Erba
Chapter II. Synthesis of silica aerogels 34
CH3COOH
Acetic acid 60.05 1.05 99 Panreac
Methanol
CH3OH 32.04 0.790 99+ - Aldrich
2.2 SYNTHESIS PROCEDURE
The sol-gel syntheses of the silica aerogels comprises three steps: (i) hydrolysis and
condensation of alkoxide precursor, (ii) gelation and aging, and (iii) drying. The gels were cast
into several sized and shaped recipients (summarized in Table II.2): Pyrex® test tubes with a
different inner diameter and height, and disks with different inner diameter and height were
used in order to obtain aerogels with several size and shape. The specific shape (rod, plate or
square) of the gels was needed for some of the measurements of the aerogel properties [2, 3]
(Chapter III. Bulk characterization).
Table II.2 Characteristics of the recipients
Recipient
Material
Volume (cm3)
High (mm)
Diameter (mm)
Glass tube Pyrex® 50 10 30
Glass tube, g Pyrex® 25 140 15
Glass tube, p Pyrex® 15 12
Quimiboro tube Pyrex® 750 60 32
Culture tube Polystyrene 5 75 12
Petri Dish Polystyrene 38.5 10 35
The following steps were common to all routes. A solution was firstly prepared by adding the
solvent to metal precursor drop-wise while stirring. After five minutes, the water was added,
eventually containing the catalyst. The solutions of precursors in alcohol were brought to
desired pH by introducing the catalyst with a precision micropipette (Gilson P1000). pH of
reaction mixture during hydrolysis was monitored in some of the cases and values ranged
from 2.3 to 7.8. After stirring for fifteen additional minutes, the solution was distributed in
recipients. The recipients were tightly closed and kept either at room temperature or in a
constant temperature chamber (40°C) until gelification took place, then the gels were covered
with their respective solvents and left to age until supercritical drying. Gelation time depends
strongly on the initial conditions, it ranges from a few minutes to months. Some of the
obtained alcogels were aged at 40°C to accelerate the aging rate. In some synthesis, before the
supercritical drying, the alcogel was washed 3 times for 24 h, at room temperature, in a fresh
pure alcohol bath to remove water and oligomers that were still present in the liquid phase
Chapter II. Synthesis of silica aerogels 35
[4]. In order to control the reproducibility, at least two samples were prepared at identical
conditions. Next figure shows a photo of the sample holder for those samples that were
soaked in the washing solution. This holder configuration allowed to facilitate the washing
process.
Figure II.1 Sample holder formed by six independent shelves. The net will allow that the gels follow a washing process.
Table II.3 gathers the experimental runs for gel synthesis with TMOS as alkoxide precursor
and Table II.4 with TEOS:
Table II.3 Synthesis experimental conditions: TMOS as alkoxide precursor, two different kind of solvent (methanol and acetone), with several acid and base catalysts
Solvent Catalyst
Methanol HCl
Methanol KOH
Methanol NH4OH+CH3COOH
Methanol NH4OH
Acetone NH4OH Acetone KOH Acetone -
- HCOOH
Sample-holder
Gel samples
Etanol bath
Chapter II. Synthesis of silica aerogels 36
Table II.4 Synthesis experimental conditions: TEOS and H5 as precursor, three different kind of solvent, with several acid and base catalysts
Precursor Solvent Catalyst
TEOS Ethanol - TEOS Ethanol C6H8O7 TEOS Ethanol HCl TEOS Ethanol HNO3 TEOS Ethanol NH4OH TEOS Ethanol C2H2O4+NH4OH
TEOS Ethanol CH3COOH TEOS Acetone - TEOS Methanol HNO3
H5 Ethanol NH3
2.3 DRYING PROCEDURE
2.3.1 Supercritical drying at high temperature
Supercritical extraction took place in a high-temperature and high-pressure plant [5,6]. See
Annex II for the technical details of the equipment. Silica gels were placed inside the vessel
filled with the corresponding solvent. It allowed to maintain the solid part of the gel covered
with solvent until the two phases (gas and liquid) were no longer distinguishable. Supercritical
drying with alcohol requires a cautious manipulation of the autoclave because the alcohol is
flammable and has a high critical temperature. Alcohol supercritical drying has some
advantages in producing silica aerogels such as, good transparency and hydrophobicity
(characterization, Chapter II). As an example, Figure II.2 shows a process of a supercritical
drying cycle for a real experiment of the methanol aerogels. Each step of the process is
labeled. The methanol liquid-gas interface has been depicted (Pc=77bar, T c=240°C). Every
point of the graph corresponds to an elapsed time of five minutes.
Chapter II. Synthesis of silica aerogels 37
0 50 100 150 200 250 300 350 400
0
50
100
150
200
Supercritical Cycle
Temperature (oC)
Pre
ssur
e (a
tm)
Methanol
Liquid
Gas
Supercritical Region
Figure II.2 Supercritical drying cycle used for a set of alcogels with methanol as a solvent.
Before starting the drying sequence, the reactor was sealed and flushed for a few minutes with
CO2 gas at 20¯25°C. Then, the pressure was raised up until the pressure reached 150 bars (100
bar for the acetone series) (step i). The pressurization was accomplished with compressed CO2
and the pressure was monitored and kept stable with great accuracy. Once the final pressure
was reached, the chamber was slowly heated from room temperature to a final temperature
to avoid thermal shock, varying between 230°C and 280°C (which is above the critical
temperature of alcohol) (step ii). Care must be taken not to reach too high temperatures to
avoid solvent decomposition or even its autoignition. The rate of heating was about 100°C/h.
The autoclave was flushed with CO2 for 2 hours after reaching the predetermined conditions
(step iii) keeping the high temperature and high pressure. The samples were kept in the
reactor for one more hour. Then, the solvent was removed by slow depressurization over a
period of 2h at a constant rate of 10bar/h (flow about 6ml/min) while the temperature was
maintained (step iv). After the pressure inside the autoclave reached atmospheric pressure,
autoclave was cooled down to room temperature at 40 °C /h (step v), before that, the reactor
was again flushed with CO2 for a few minutes. Finally, the vessel was opened and the aerogels
were removed [8- 10].
Figure II.3 shows a photograph of the sample holder for those samples that were dried at
high temperature without solvent washing. In that case, the gels were not subtracted from its
respective recipients.
(i)
(ii) (iii)
(iv) (v)
Chapter II. Synthesis of silica aerogels 38
Figure II.3 Sample holder for the gels dried under, ethanol, methanol or acetone supercritical conditions without washing process.
Table II.5 shows the critical temperature and critical pressure of the used solvents, ethanol,
methanol, acetone, CO2, and other supercritical fluids frequently used.
Table II.5 Critical pressure, Pc, and critical temperature, Tc, of various solvents used as pore liquid
Solvent
Pc (atm)
Tc (°C)
H20 216 374
NH3 110 132
CO2 70 31
CH3CH2OH 62 243
CH3OH 77 240
CH3CN 47 275
Few experiments were not successful due to that the drying pressure was below the threshold
pressure (below which the gel network begins to shrink) [14] and consequently the dried gels
showed a significant shrinkage, even up to 70%.
2.3.2 CO2 supercritical drying
Supercritical drying with CO2 is widely used because it is a safer and cheaper method than
alcohol supercritical drying: CO2 is non-flammable and has a low critical temperature [15, 16,
17]. The gels were placed in a 2000 ml vessel and covered with the corresponding solvent.
Sample-holder
Pyrex tubes
Gel
Chapter II. Synthesis of silica aerogels 39
The vessel was sealed and liquid CO2 was pumped inside at room temperature until the
pressure reached about 100 bars. Then the micrometer valve at the autoclave bottom outlet
was opened to allow the ethanol to flow out. The flow was about 6ml/min. The
displacement of ethanol, methanol or acetone by liquid CO2 took from about 4h to 12h,
which allowed the liquid CO2 to diffuse into the gel pores displacing the solvent. The solvent
was collected in another reactor for evaluation. After a complete substitution of the solvent
for liquid CO2, the outlet valve was closed, the pump was turned off, and the temperature
was raised up to 45°C (above the critical temperature of CO2). The ramping took about
30min, after which the vessel remained closed for another hour. Then the micrometer valve
was opened, the pump was turned on, and the system was flushed for 30min. The CO2 flow
was stopped and the system was slowly depressurized, over a period of 1 hour at a constant
rate of 10bar/h while the temperature was maintained at 45°C. After the autoclave reached
atmospheric pressure, the vessel was opened and the aerogels were removed [18]. The
characterization of the aerogels is shown in Chapter III (Bulk characterization).
3. SYNTHESIS ROUTES: EFFECT OF DIFFERENT ALKOXIDE
PRECURSORS
The effect of various precursors such as (a) tetraethoxysilane (TEOS) [19- 23], (b)
tetramethoxysilane (TMOS) [24- 32], and (c) prepolymerized tetraethoxysilane (H5) [33- 35]
will be presented and discussed in this section. For each type of alkoxide precursor, sol -gel
reactions were performed under acidic, neutral or basic conditions, in alcohols (methanol or
ethanol) or acetone. The use of these precursors allowed synthesis of a very large variety of
monolithic transparent, translucent, and opaque silica aerogels. A systematic and detailed
study was undertaken regarding the comparison of physical properties such as monolithicity,
bulk density, optical transmission, total surface area, porosity, pore size distribution, and total
pore volume (see chapter III: Bulk characterization). It was shown that these values are
strongly correlated.
The wet gels were also characterized by means of the gelification time, tg. The gelification
time is a parameter that characterizes the sol gelification velocity, and at the same time
determines the aerogel microstructure, resulting in a more polymeric aerogel for longer
gelification times.
Chapter II. Synthesis of silica aerogels 40
3.1 TETRAMETHOXYSILANE: TMOS AEROGELS
The first series of silica gels were prepared from TMOS (tetramethoxysilane) in different
concentrations, different combinations of solvent (methanol and acetone) and catalyst (none,
HCl, potassium hydroxide, ammonium hydroxide, and NH4OH+CH3COOH). Tables II.6,
and II.8 gather the synthesis experimental conditions for the TMOS aerogels, when the used
solvent were methanol and acetone, respectively.
Table II.6 Synthesis experimental conditions, TMOS as precursor, methanol as solvent and different type of catalyst, several TMOS molar concentration (m=solvent/TMOS) versus water concentration (h=H2O/TMOS), and several catalyst concentrations (c).
Catalyst Label m h c
HCl TM99AQ 8 4 0.05
KOH TM99AR 8 4 0.05
NH4OH+CH3COOH TM96C-D 12.25 4 0.065
NH4OH M TM96B-E-F-H-97F-O-
98A-H-I-99AU 12.25 4 0.065
NH4OH TM00AR 3 2 NH4OH TM00AS 4 2
NH4OH TM00AU 4 4
The gels synthesized with TMOS and methanol as solvent under NH4OH catalyst
TMOS/EtOH/H2O= 1/12.25/4 were labeled as M gels. Those samples offered a good
reproducibility and very good quality.
Chapter II. Synthesis of silica aerogels 41
Moreover, the gels series synthesized with TMOS with H2O/TMOS fixed at 4 and a variable
concentration of acetone as solvent was labeled as A-series gel. The gels were labeled as A1,
A2, A3, A4, when m= EtOH/TMOS= 1.22, 0.54, 0.32, 0.20. Those samples offered a good
reproducibility and good quality. Next Table II.7 lists these experiments.
Table II.7 Synthesis experimental conditions, TMOS as precursor, acetone as solvent and different type of catalyst, several precursors (m) versus water concentration (h), and several catalyst concentrations (c).
Catalyst Label m h c
NH4OH TM97E 12.25 4 0.065 KOH TM96A-I-J 12.25 4 0.065
- TM00AB-AW 0.1 4 -
- A05TM98F-G 3.87 4 -
- TM98O 3.87 2 -
- A1TM96N-97A-N-98E-99AO 1.22 4 -
- A2TM96K-R-97B-G-H-I-J-K-
M-P-Q-98A-B-99AJ-AN-AP-D-
00AC-AE
0.54 4 -
- A3TM96M-Q-97C-99AL-AQ 0.32 4 -
- A4TM96L-97D-L-P-98C-N-S-
AC-99Q-AM-AR-AS-00AD 0.20 4 -
- TM99O 0.54 2 - - TM98J-Q 0.20 2 - - TM98L-T 0.20 6 -
- TM98M-U 0.20 8 - - TM98V 0.20 16 - - TM98K-R 0.20 3 -
Another type of sol-gel syntheses route was performed at low temperature without needing
to use water. In these syntheses, supercritical CO2 was used as solvent, and HCOOH as the
condensation agent. Because the use of supercritical CO2 as solvent, it was necessary to
synthesize the gels directly inside the reactor. Next Figure II.4 shows a scheme of this
process.
Chapter II. Synthesis of silica aerogels 42
Si (OR)4 + HCOOH 4SiO2 + 4HCOOR
Figure II.4 Aerogels produced with TMOS as alkoxide precursor, supercritical CO2 as solvent, and HCOOH as the condensation agent.
In parallel to this synthesis, some experiments were performed out of the reactor in order to
have an estimation of the gelification rate inside the reactor. Table II.8 lists several
HCOOH/TMOS molar ratios used.
Table II.8 Synthesis experimental conditions, TMOS as precursor and with HCOOH as condensation agent
Gel label HCOOH/TMOS
TM99F 6.67
TM99G 6 TM99H 4 TM99I 3
Two TMOS one-step routes were selected because of the quality and reproducibility of the
resulting products. The first set consisted of methanol as solvent and ammonium hydroxide
(NH4OH at 32%) as catalyst, M labeled samples. The total molar ratio of the reagents was
kept constant precursor/solvent/water/catalyst = 1/12.25/4/6.5·10 -2 [36]. In the second set,
no catalyst was added, being acetone the solvent [37]. In this case, different concentrations of
solvent were used. The water molar ratio, TMOS/water, was kept at four: A-series. The
scheme below shows these two synthesis routes:
Chapter II. Synthesis of silica aerogels 43
Figure II.5 The two selected synthesis routes for the TMOS aerogels, M-series and A-series.
Moreover, a second route was prepared to compare two different drying cycles: some of the
gel samples were submitted to supercritical drying in acetone while the others were dried by
CO2 exchange (see supercritical drying section).
The two selected synthesis routes and a list of the gelation times is collected in Table II.9, the
given values were averaged for a number of synthesis processes.
Table II.9 Effect of synthesis conditions on gelation time for M and A-series: type of solvent, molar ratio, volume ratio and gelation time.
Label Solvent Solvent
Precursor
V
V
TMOS
(TMOS+solvent)
Gelation time (Mean value)
M Methanol 12.25 0.08 5 minutes A05 Acetone 3.874 0.05 180 days
A1 Acetone 1.224 0.10 90 days A2 Acetone 0.544 0.20 16 days
A3 Acetone 0.317 0.30 7 days A4 Acetone 0.204 0.40 2 days
TMOS
+
MetOH
NH 4 OH
+
H 2 O
Hydrolisis Polymerization
Gelling
Supercritical drying
Evaporation
TMOS
+ Acetone
H 2 O
Hydrolisis Polymerization
Gelling
Supercritical drying
Evaporation
M A-series
Chapter II. Synthesis of silica aerogels 44
Figure II.6 shows gelification times for all the gels synthesized following the same A2 route.
Two groups can be differentiated. The first one shows a longer gelification time
corresponding to the gels with larger volume, 750 ml. Its mean gelification time was 30 days
and the standard deviation was of 4 days. The other group corresponding to 50 and 25 ml
presented a mean gelification time of 16 days and its standard deviation was of 4 days.
0 10 20 300
5
10
15
20
25
30
35
40
45
50
Small tubes
Large tubes
16 days
30 days
4 days
4 days
t g
elif
ication (
da
ys)
A2 samples
Figure II.6 Gelification time for the samples with 0.2 and h=4. The mean value for the gelification time was of 20 days for large tubes and 18 days for small tubes.
These results show that the size of the recipient it is an important factor influencing the
gelification time. The gel point was defined as the point in time at which the network of
linked silica oxide particles spans the container holding the sol. Then, it can be observed that,
larger recipient implied longer gelification time.
A similar study was done comparing samples that remained either at 40 C or at room
temperature. At 40 C, the mean gelification time was reduced for the 50 ml gels from 18 to 7
days (with a standard deviation of 30 hours) verifying that increasing temperature accelerates
the gelification rate.
The sol-gel reactions lead to a progressive decrease of the solution pH with time because the
hydrolysis reaction is generally more rapid and complete under acidic conditions, [ 38]
whereas the average condensation rate is generally maximized near pH=4. Figure II.7 shows
a decrease of pH during the gelation process in A1 gels.
Chapter II. Synthesis of silica aerogels 45
Figure II.7 pH versus time during gelation process of TMOS at room temperature in acetone solution (A1 type gel).
3.1.1 The effect of the TMOS concentration
In the TMOS set, when acetone was used as solvent, different concentrations of acetone
were used. The water molar ratio, TMOS/water, was kept at value 4. The concentration of
the alkoxide was given by ,
acetoneTMOS
TMOS
VV
Vv (Eq. II.1)
The value was varied from 0.05 to 0.4. The samples were labeled as A05, A1, A2, A3 and
A4 corresponding to = 0.05, 0.1, 0.2, 0.3 and 0.4, respectively. In turn, these values
correspond to molar ratios of Acetone/TMOS of 3.874, 1.224, 0.5434, 0.317 and 0.204.
The dependency of the gelification time with the volume concentration, , was studied.
Figure II.8 shows the gelification time versus the concentration of TMOS in acetone. The
gelification times ranged from an average of three months for A1 to three days for A4, longer
times as expected, for more diluted sols [39]. The gelification time decreases exponentially as
a function of the TMOS/Acetone volume concentration. Table II.10 gathered several
characteristics of the different samples. The obtained values were averaged for a number of
synthesis processes.
0 20 40 60 80 100
5,4
5,6
5,8
6,0
6,2
6,4
6,6
6,8
7,0
7,2
7,4
A1 gel
pH
time (hours)
Chapter II. Synthesis of silica aerogels 46
Figure II.8 Gelification time versus concentration, v for TMOS/acetone gels. The exponential fitting results in tgeli fication=180.e(-v/0.06).
The influence of the TMOS/acetone concentration of the sol on the aerogels can be assume
that when value decreases, i.e. decreasing the concentration of the precursor, the excess of
acetone increases the distance between the reacting species and hinder the progress of cross-
linkage of the siloxane chains (Si-O-Si) leading to the separation of SiO2 clusters in the sol.
This process leads to a decrease in the reaction rate and hinders the formation and growth of
the gel network particles, resulting in smaller particles and larger pore sizes [40] (see chapter
III: Silica aerogel characterization).
3.1.2 The effect of the nature of the solvent
The TMOS series of silica gels were prepared under different conditions by using various
solvents as pore liquids: methanol, ethanol, acetone, and CO2. Table II.5 showed their critical
pressure and temperature: Pc, and T c [41- 44].
Because water and alkoxysilanes are immiscible, a mutual solvent is normally used as a
homogenizing agent. The alcohol is not simply a solvent since, as indicated in equations I.2
and I.3, it can participates in esterification or alcholysis reactions.
The gelification times for methanol were of few minutes, for the acetone series they ranged
from an average of three months for A1 to three days for A4, longer times as expected for
longer solvent chains. The effects of the solvent, methanol or acetone, on the differences in
gelation time of the two solvents can be explained by considering two retarding factors: i) the
0,0 0,1 0,2 0,3 0,4 0,5
0
30
60
90
120
150
180
Fit y0+Ae
(-x/t)
y0=0, A=180, t=0.06
t g
elif
icat
ion
(d
ays)
v
Chapter II. Synthesis of silica aerogels 47
hydrogen bonding and ii) the steric hindrence [36]. Steric (spatial) factors exert a great effect
on the hydrolytic stability of organoxylanes [45]. Any obstacle of the alkoxy group retards the
hydrolysis of alkoxysilanes, but the hydrolysis rate is mainly lowered by branched alkoxy
groups. If one compares the gel time under the same conditions, the retarding effect of the
ethoxide group in TEOS is evident. In the case of methanol, these two factors have very little
effect and hence the shortest gelation time was observed. Complexes are formed between
silicic acid and esters through hydrogen bonding [46] which retard the polymerization process.
The relative effectiveness of hydrogen bonding activity is the shortest for methanol, and the
highest for acetone, therefore the gelation time using methanol is shorter than for acetone.
Steric hindrence increases when acetone is used as solvent which will lead to larger pores and
hence a decrease in transparency of the aerogels (see chapter III: characterization).
With respect to supercritical drying, an amount of solvent was added into the autoclave. This
additional solvent can take part in the further dissolution of the silica gel [47].
Summarizing the effect of the solvent in TMOS gels:
i) Gelification time for methanol samples is shorter than for acetone samples due to the
effect of the hydrogen bonding and the steric hindrence of the solvents.
iii) Methanol gels are very transparent while acetone aerogels had white shading; their
opacity decreases with increasing TMOS content.
iv) When ethanol was used as a solvent (using TMOS as a precursor) cracked gels were
obtained.
v) Two good quality routes were selected for obtaining acetone monolithic structures
without cracks. The M gels were very transparent and strong. The A1 gels were especially
fragile.
Further study on the effects of the supercritical drying media on aerogel properties and
microstructures is essential for developing means of quality control in creating aerogels.
3.1.3 The effect of hydrolysis solution
Water amount
The amount of water used at the hydrolysis step was found to have a significant effect on the
texture of aerogels. The effect of water concentrations was examined for the samples
prepared at acetone/TMOS molar concentrations of 0.204 (v=0.4).
Chapter II. Synthesis of silica aerogels 48
The dependency of the gelification time with the molar ratio h, TMOS
OHh 2 , was studied.
The reaction time increases very quickly with the water molar ratio, h. With an increase from
h=2 to a water excess over the stoichiometric amount (h=16), the time of gelation was
reduced from 3 days to 56 min. This effect is reflected in the exponential decreases of the
gelification time as a function of the water molar ratio (Figure II.9).
Figure II.9 Gelification time as a function of the hydrolysis constant, h, for gels with acetone/TMOS molar ratio of 0.204. tgel decreases exponentially as a function of water molar ratio.
Figure II.10 shows the variation of pH in a gel with acetone/TMOS molar ratio of 0.204
when water molar ratio, h, was increased from 2 to 16. The pH variation will be controlled by
the variation in hydrolysis and condensation rates.
0 2 4 6 8 10 12 14 16 18
0,0
2,0x103
4,0x103
6,0x103
8,0x103
1,0x104
1,2x104
1,4x104
1,6x104
Chi2 = 248014
y0= 398.1±309.0
A= 15770±1493
t = 3.1±0.38
t gelif
icació
n (
min
)
[H2O]/[TMOS]
tgelification=400+16.103.e-h/3.1.
Chapter II. Synthesis of silica aerogels 49
Figure II.10 pH versus water amount for gels with acetone/TMOS molar ratio fixed at 0.204
3.2 TETRAETHOXYSILANE: TEOS AEROGELS
As the fumes from TMOS are toxic and may cause blindness, other esters of orthosilicic acid
like TEOS were used to obtain silica aerogels. TEOS is not only less toxic when compared to
TMOS but it is cheaper too. Hence, TEOS is a suitable precursor for the commercial
production of silica aerogels.
The experimental results on the influence of molar ratios of precursor, solvent and water of
TEOS silica aerogels are reported in this section. Alcogels were prepared by hydrolysis and
polycondensation of tetraethoxysilane (TEOS) in ethanol using either C6H8O7, HCl,
NH4OH, or NH4F+ NH4OH as a variable catalyst solution [48 - 50]. In order to identify the
optimal condition for producing the best quality TEOS silica aerogels in terms of
monolithicity, density and transparency, the molar ratios of EtOH/TEOS (0.15 m 9) and
H2O/TEOS (0.23 h 10) were systematically varied. The synthesis conditions are
summarized in Table II.10, 11, 12, and 13.
0 2 4 6 8 10 12 14 16 18
5,4
5,6
5,8
6,0
6,2
6,4
6,6
6,8
7,0
7,2
Acetone/TMOS 0.204/1
pH
h (H2O/TMOS)
Chapter II. Synthesis of silica aerogels 50
Table II.10 Synthesis experimental conditions, type of precursor TEOS, as solvent EtOH, without presence of catalyst, precursor (m) and water concentration (h)
Label m h
TE99M07-N17 7 8
TE00AA-AF-AK-N5 5 7
TE99BS 9 6
TE99BG-M05-N13 7 6
TE99BU-M-N 5 6
TE99BW 4 6
TE99BV 3 6
TE00AA-W-CD-BZ-99M03-N7 7 5
TE00AM5b 6.9 5
TE99BI 7.6 5.4
TE00AMa 3.8 3.3
TE99C1 4 3.17
TE99C2 3.45 2.72
TE99C3 2.87 2.26
TE98W 2.9 2.2
TE99B 2.8 2.2
TE99C6 2.3 1.81
TE99C7 0.15 0.91
TE99C8 0.57 0.45
TE99C8 0.29 0.23
Table II.10 shows m and h values of TEOS gels produced without catalyst. m (EtOH/TEOS
molar ratio) has been changed from 0.3 to 7.6 and h (H2O/TEOS molar ratio) from 0.23 to
8. The most reproducible samples were TEOS/EtOH/ H2O molar ratio equal to 1/7/5 and
1/5/7.
Chapter II. Synthesis of silica aerogels 51
Table II.11 Synthesis experimental conditions when using TEOS as alkoxide precursor, EtOH as solvent, several type of acid-catalyst, with a variable precursor and water concentration (m/h), and catalyst concentration (c).
Catalyst Label m/h c
C6H8O7 TE98X07bb 3/5 0.0001 C6H8O7 TE98X07 5/5 0.0009 C6H8O7 TE98X07bc 6/5 0.0001
C6H8O7 TE98X07bd 9/5 0.0001 C6H8O7 TE98X09-12 5/6 0.0001 C6H8O7 TE98X13-18 5/7 0.0001 C6H8O7 TE98X19-20 5/8 0.0001 C6H8O7 TE00AT 7/5 0.003 C6H8O7 TE00AV-99AM 7/5 0.0005
C6H8O7 TE98X01-99S1 7/5 0.0009 C6H8O7 TE98X01-99S2 7/5 0.0004 C6H8O7 TE99AO 7/5 0.01 C6H8O7 TE99AR 7/6 0.01 C6H8O7 TE00AA-99AN-AQ 7/6 0.003 C6H8O7 TE99AP 7/6 0.0005
C6H8O7 TE98X03 7/6 0.0001 C6H8O7 TE98X03bb 7/7 0.0001 C6H8O7 TE98X05 7/8 0.0001 C6H8O7 TE99AS 7/8 0.0005 C6H8O7 TE99AT 7/8 0.003 C6H8O7 TE99AU 7/8 0.01
HCl TE99AB 6.4/7.5 0.51 HCl TE99AF 6.8/4.6 0.058 HCl TE99BE 6.9/4 0.13 HCl TE99BF 6.9/4 0.2 HCl TE99AI00-02 6.9/5.2 0.21 HCl TE99AI03-04 6.9/5.7 0.23
HCl TE99AI05-06 6.9/6.2 0.25 HCl TE99BK 6.9/6.2 0.51 HCl TE99AI07-08 6.9/6.7 0.27 HCl TE99AI09-10 6.9/7.23 0.3 HCl TE99AI11-12 6.9/7.75 0.32 HCl TE99BK 2.3/6.5 0.53
HCl TE99BH 7/4.8 0.1 HNO3 TE99AG 6.8/5.5 0.03 HNO3 TE99AH01-02 2.7/0.4 0.23 HNO3 TE99AH03-04 2.7/0.5 0.23 HNO3 TE99AH05-06 2.7/0.6 0.23 HNO3 TE99AH07-08 2.7/0.7 0.23
HNO3 TE99AH09-10 2.7/0.9 0.23 HNO3 TE99AH11-12 2.7/1 0.23 HNO3 TE99AJ 3/10 0.3 HNO3 TE99AK 3/9.6 0.5
Chapter II. Synthesis of silica aerogels 52
Table II.12 shows TEOS gels synthesized with ethanol as solvent and three types of acid-
catalysts: C6H8O7, HCl, and HNO3. When C6H8O7 was used, m (EtOH/TEOS) was ranged
from 3 to 9 and h (H2O/TEOS) from 5 to 8, the concentration of C6H8O7 was ranged from
1.10-3 to 1.10-2..
Table II.12 Synthesis experimental conditions when using TEOS as alkoxide precursor, EtOH as solvent, several base-catalyst, and a variable precursor and water concentration (m, h), and catalyst concentration (c).
Catalyst Label m/h c
NH4OH TE99AV 5/4 0.01 C2H2O4+NH4OH TE99AW 5/4 0.01 C2H2O4+NH4OH TE99BA 5/5 0.05
C2H2O4+NH4OH TE99AY 5/5 0.01 C2H2O4+NH4OH TE99AZ-BB 5/5 0.005
CH3COOH TE99AX 5/4 0.01
Table II.13 Synthesis experimental conditions when using TEOS as alkoxide precursor, acetone or methanol as solvent, several catalyst, precursor and water concentration (m, h), and catalyst concentration (c).
Solvent Catalyst Label m/h c
Acetone - TE97A0 0.317/4 - Acetone - TE97B0 0.204/4 - Methanol HNO3 TE00AM'-AN-AÑ-AP 4.3/ 5 0.05
The solutions were kept either at room temperature, 25°C or at 40°C for gelation. In some
cases, to accelerate the aging time in base-catalyzed gels, the gels were first soaked in an aging
solution, that is produced by water and catalyst diluted in ethanol (of equal proportions to the
original sol), for 24 hours at room temperature. In addition, in a second step were soaked in a
washing bath of ethanol for 3 times from 6 to 72 hours at room temperature. This step is
diffusion controlled, then the time required for washing increases strongly whit the gel
thickness. Generally, the volume of the washing and aging solution was approximately three
times the volume of the gels. It is observed that during aging, the wet gels gain weight.
A statistical study was performed on the variation of the gelation time on the aerogels
obtained using the same synthesizing route, TEOS/EtOH/H2O=1/7/5, to ensure
reproducibility of the series.
Chapter II. Synthesis of silica aerogels 53
3.2.1 The effect of the TEOS concentration
Neutral
A study was done using the EtOH/TEOS series without presence of catalyst. The molar
ratio water/TEOS, h, was fixed at 6 and the EtOH/TEOS molar ratio was varied from 3 to
9. Monolicithy and homogeneity of the gels obtained were good except for samples with too
high TEOS concentration, which had cloudy parts in the aerogel center as reported in
chapter III. Next Figure studies the dependency of gelation time with the molar ratio
EtOH/TEOS when h was fixed at 6. The gelation time was large compare to those gels
synthesized when using catalyst.
Figure II.11 Gelation time versus EtOH/TEOS molar ratio for water/TEOS=6 and without using catalyst.
The gelation time showed an exponential dependency versus EtOH/TEOS molar ratio.
Summarizing the effect of ethanol content on the TEOS gels, it can be concluded, that an
increase in solvent content reduces the probability of mutual collisions of hydrolyzed
alkoxides molecules (Si-OH), resulting in a decrease in the rate of polymerization reaction.
Moreover, although ethanol acts as homogenizing agent to promote hydrolysis of TEOS, an
increase of the ethanol content reversed the hydrolysis and polymerization processes and
promoted esterification (Eq. II.2 and Eq. II.3):
Si-(OH) + C2H5OH Si-(OC2H5) + H2O Esterification ( Eq. II.2)
Si-O-Si + C2H5OH Si-(OC2H5) + (HO)-Si Alcholysis (Eq. II.3)
3 4 5 6 7 8 9
0
48
96
144
192
240
288
336
y0
0 0
A 0.37 0.03
t0
1.33 0.02
y = y0 + Ae
x/t0
Water/TEOS= 6
no catalyst
Gel
atio
n t
ime
(ho
urs
)
molar ratio EtOH/TEOS
Chapter II. Synthesis of silica aerogels 54
Acid catalyst
To establish the effect of precursor concentration in alcohol the gels were prepared by fixing
the molar ratio of H2O/TEOS ,h, constant at h=5 (excess over the stoichiometric amount)
and the catalyst (citric acid) concentration at 0.0001M. The molar ratio of EtOH/TEOS (m)
was changed from 3 to 9. All samples were treated in the same way.
As the ethanol content increased, the gelling process was significantly retarded, as shown in
Figure II.12. This was due to the fact that excess of ethanol separates the molecular species
formed and hinders the progress of cross-linkage of the siloxane bond chains (Si-O-Si),
leading to the separation of sol clusters.
Figure II.12 Gelation time versus EtOH/TEOS molar ratio at h=5 and 0.0001M of citric acid. The gelation time increased exponentially with the EtOH/TEOS molar ratio.
The time of gelation increased from 30 to 60 hours when increasing the EtOH/TEOS molar
ratio from 3 to 9. The same study was repeated adding water concentration at 6, 7, 8, and 10
or at lower values from 3 to 4.
Figure II.13 shows the tg dependency on EtOH/TEOS molar ratio when h (water/TEOS
molar ratio) was fixed at 8.
3 4 5 6 7
35
40
45
50
55
60
y0
36.02 ±0.16
A1
0.44 ±0.03
t1 1.88 ±0.03
y = y0 + A
1e
x/t1
Water/TEOS= 5
0.0001M
Gel
atio
n t
ime
(ho
urs
)
molar ratio EtOH/TEOS
Chapter II. Synthesis of silica aerogels 55
Figure II.13 Gelation time versus EtOH/TEOS molar ratio for samples with water/TEOS ratio of 8 and acid citric 0.0001M as catalyst
When h was fixed at 8 the rate of gelation when increasing TEOS concentration increased
faster than for h=5, so increasing the water concentration the gel rate of reactions are more
sensitive to the rise of ethanol concentration.
The acid-catalyst used in the following series was 52%HF. The acid TEOS series was
obtained by first synthesizing the more concentrated solution, AHF1, and then diluting it in
different volumes of ethanol, AHF2, to AHF5. Next table shows the acid-catalyzed gels (HF)
with water/TEOS fixed at 25.
Table II.14 Experimental parameters of TEOS gels with acid-catalyst (HF), with water/TEOS molar ratio fixed at 25.
Label EtOH/TEOS Optical
transparency
AHF1 12.23 Transparent
AHF2 18.85 Transparent
AHF3 26.25 Transparent
AHF 33.65 Transparent
AHF5 41.05 Transparent
Label TEOS H2O EtOH 52% HF
A1 25ml 50 ml 80ml 0.4ml
3 4 5 6 7
12
16
20
24
28
y0 13.25 ±0.41
A1
0.23 ±0.08
t1
1.83 ±0.16
y = y0 + Ae
x/t
Water/TEOS= 8
0.0001M
Gel
atio
n t
ime
(hours
)
molar ratio EtOH/TEOS
Chapter II. Synthesis of silica aerogels 56
All the series of acid aerogels obtained with TEOS, acid-catalyst (HF), and with water/TEOS
molar ratio fixed at 25 were monoliths very transparent and without cracks.
Figure II.14 Synthesis scheme for the TEOS gels with acid-catalyst (HF). In this case, the alkoxide solution was dissolved in ethanol and then mixed with the hydrolysis solution dissolved in ethanol.
Base-catalyst
The base-catalyzed gels were obtained using a base-catalyst of 30%NH3 +NH4F 0.5M. The
series was obtained by first synthesizing the more concentrated solution, B1, and then
diluting it in different concentrations.
Figure II.15 Synthesis scheme for the TEOS gels with base-catalyst (NH3+NH4F)
TEOS + EtOH EtOH + H2O
+52% HF
SOL
Gelification
time
GEL
TEOS + EtOH EtOH + H2O +
30%NH3+ 0.5M NH4F
SOL
Gelification
time
GEL
Chapter II. Synthesis of silica aerogels 57
Table II.15 summarizes the basic TEOS gels produced at several dilutions with a fixed water
ratio (h= 10) and with h=25.
Table II.15 Experimental parameters of TEOS gels with water/TEOS molar ratio fixed at 10 and 25 and base-catalyst (30%NH3+0.5MNH4F) concentration of 3.3.10-3.
Label EtOH/TEOS Optical
transparency
B1 17 Transparent
B2 25 Transparent
B3 66 Translucent
B4 91 Translucent
B5 116 Opaque
B1’ 12.23 Transparent
B2’ 18.85 Transparent
B3’ 26.25 Transparent
B4’ 33.65 Transparent
B5’ 41.05 Transparent
Label TEOS H2O EtOH 30%NH3 0.5MNH4F
B1’ 25ml 50 ml 80ml 0.055ml 0.055ml
B1 25ml 20 ml 110ml 0.055ml 0.055ml
The obtained samples presented different shrinkage and transparency. Some of the aerogels
with highest density were cracked (B1 and B2). In order to obtain more transparent aerogels
the water/TEOS molar ratio was increased up to 25. When h=25, the gels were slightly more
transparent than when water amount was 10. All gels were monolithic.
In some samples, acetone was used as a solvent and non-monolithic gels were obtained. The
acetone amount was fixed at two and the water amount was varied from 0.017 to 0.2
(TE97A). The initial solution appeared translucent and, in the cases with lowest h, divided in
two phases. Upper phase was more opaque and lower phase was transparent. For the largest
h, appeared as translucent solution including a white precipitated (indicating that the amount
of water was too large). Therefore, no monolithic gels were produced when synthesizing
TEOS gels with acetone as solvent.
In conclusion, the aerogels had better quality when TEOS was diluted with i ts respective
parent alcohol (EtOH) because it avoided trans-esterification.
Chapter II. Synthesis of silica aerogels 58
3.2.2 The effect of hydrolysis solution
Water amount
The amount of water used at hydrolysis step was found to have a significant effect on the
texture of aerogels. Water directly participates in the hydrolysis reaction, and can be described
by three successive steps:
(OH)nSi(C2H5O)4-n+4H2O (OH)n+1Si(C2H5O)3-n +C2H5OH (Eq. II.4)
where n=0, 1, 2, 3, and 4.
From Equation II.4, an increased value for h is expected to promote hydrolysis reaction.
Depending on the water/TEOS molar ratio, the following two condensation reactions
occurred after the initial hydrolysis:
(Eq. II.5)
(Eq. II.6)
Then, the initial relative amount of water determined the distribution and number of the
hydrolyzed monomers formed. The amount of water formed during the condensation
reactions (Eq. II.6) is half to the water amount used for the formation of silanols (Si-OH)
(Eq. II.5).
The effect of water concentrations was examined by keeping the EtOH/TEOS molar
concentration constant at 7. At the same time, the catalyst concentration was kept constant at
0.001M. The dependency of the gelification time with the molar ratio h, H 2O/TEOS, was
studied (Figure II.16).
Chapter II. Synthesis of silica aerogels 59
Figure II.16 Gelation time versus water/TEOS molar ratio for gels with EtOH/TEOS molar ratio fixed at 7 and acid catalyst 0.0001M.
The same dependency was observed for gels with acid citric as a catalyst but with smaller
gelation time. The most obvious effect of increasing h value is the acceleration of hydrolysis
reactions (Eq. II. 4), the reaction time increased very quickly with the water molar ratio, h.
The same study was performed in EtOH/TEOS fixed at 7 without the presence of catalyst.
The tg dependency is more pronounced than with acid catalyst but the gelation time was
much slower.
4 5 6 7 8 9
24
30
36
42
48
54
60
66
72
78
84
y=y0+Ae
-x/t0
y0
22.77 0.66
A 5623.9 1661.0
t0
0.97 0.05
EtOH/TEOS = 7
acid citric 0.0001MG
elat
ion t
ime
(hours
)
molar ratio water/TEOS
Chapter II. Synthesis of silica aerogels 60
Figure II.17 Gelation time versus water/TEOS molar ratio for gels with EtOH/TEOS molar ratio fixed at 7 and no use of catalyst
In Figures II.16 and 17, gelation time, tg, was shown as a function of water/TEOS molar
ratio, h. As the h value increase, gelation time first decreased sharply, then for h values
greater than 6 there was a gradual decrease in gelation. This means that for a complete
hydrolysis, h should be at least 6. This higher value over the theoretic stoichiometric value is
because of the steric hindrance of Si-OC2H5 groups. Then, adding water for ratios h=6, the
corresponding value for the complete consumption of the initial water, will not decreases
very strongly the gelification time. It was found that at low h values (h 4) the tg was very
large. For h values 4, the hydrolysis reaction proceeded rapidly to the complete
consumption of the initial water. This observation indicates that the rate of hydrolysis
reaction (Eq. II.1) was much faster than the sum of the rates of the water and ethanol
producing condensation reactions (Eq. II.2)+ ( Eq. II.3). Then, the t g was very large because
the low number of condensation reactions. Also, in this h range, was observed that gelation
time decreased when h increased. With h 4 an increase of tg is more pronounced because
the hydrolysis and the resulting condensation reactions were not completed, leading to a very
limited cross-linking of the silanols.
For larger h values (4<h<6) it was observed a general decreased in tg. This indicated that
when adequate amount of water is present, tg was probably governed primarily by
condensation. For 4 h 6, the ethanol producing reaction (Eq. II.2) dominated, while water-
producing condensation (Eq.II.3) dominated for h 6 [51]. The consequence of these
4 5 6 7 8 9
24
48
72
96
120
144
168
192
y0
46.67
A 157478.31
t0
0.64
y = y0 + Ae
-x/t0
EtOH/TEOS= 7
no catalyst
Gel
atio
n t
ime
(ho
urs
)
molar ratio water/TEOS
Chapter II. Synthesis of silica aerogels 61
reactions was a decrease in the concentration of monomeric molecules and an increase in the
concentration of oligomers.
On the contrary, at h 6, the rate of hydrolysis increased, enhancing the gelation process that
increased the cross-linkage of siloxane bonds chains (Si-O-Si) leading to a three-dimensional
gel network. This explains the general decrease in tg at larger h values.
Influence of the amount of the catalyst
The influence of the amount and the type of the catalyst on the properties of aerogels was
studied. Tetraethoxysilane easily hydrolyzes in the presence of water, the rate of hydrolysis
and condensation reactions depends on pH of the solution [52, 53].
The conditions that were considered the optimal concentrations of reagents were for 7 initial
molar concentration of TEOS in ethanol, and with excess of stoichiometric amount of water
5, 6, and 8. Therefore, the influence of catalyst on these three ratios was studied. The
following range of acid citric ratio was tested:
0.0005 < Ccitr ic acid < 0.1
TEOS/EtOH/H2O
1 / 7 / 5, 6, 8
The variation of citric acid concentration leaded to change in pH as shown in Figure II.18
and then, induced changes in the rate of reaction (Figure II.19).
Chapter II. Synthesis of silica aerogels 62
Figure II.18 pH of the sol versus the citric acid concentration in TEOS/EtOH/H2O gels 1/7/8.
Increasing the citric acid concentration from 0.0005 to 0.1, the pH was changed from 5 to
3.5.
Table II.16 Synthesis experimental conditions for TEOS as precursor, ethanol as solvent, citric acid as catalyst, and several precursor and water concentration (EtOH/H2O), and catalyst concentration (Ccitr ic acid). Gelation time (tg) and transparency of the resulting gels.
EtOH/H2O Ccitric acid tg (h) transparency
7/5
0.0005 44-141 translucent
0.003 47 transparent 0.01 138 very transparent
7/6
0.0005 44 translucent
0.003 41 transparent
0.01 69 transparent
7/8 0.0005 23 opaque 0.003 23 opaque
0.01 46 very transparent
1E-3 0,01 0,1
3,4
3,6
3,8
4,0
4,2
4,4
4,6
4,8
5,0
pH 1/7/8
pH
log10
(catalyst concentration)
Chapter II. Synthesis of silica aerogels 63
Gelification was quite rapid for all the set samples mostly for 7/8. Between 0.01 and 0.0005
M, gels were monolithic. The gels obtained at EtOH/H2O = 7/5, 7/6, 7/8 have no cracks
and give a good quality aerogels.
Figure II.19 Gelation time as a function of the citric acid concentration for 1/7/6 TEOS/EtOH/water molar ratio
It was found that the role of catalyst concentration was very important in the formation of
silica alcogels for a mixed molar ratio of TEOS/EtOH/H2O at 1/7/6 (Figure II.19). In the
case of lower catalyst, ( 0.005M) only turbid gels were obtained. This may be due to the
formation of silica precipitates. On the other hand, catalyst concentrations 0.001M lead to
the formation of transparent alcogels. Figure II.20 shows the gelation time as a function of
catalyst concentration. It is clear that a maximum gelation time was obtained for 0.001 M
citric acid whereas at lower and higher concentrations of the citric acid the gelation time
decreased.
Hydrolysis (substitution of OR for OH) or condensation (substitution of OSi for OR or OH)
decreases the electron density on silicon (see Figure II.21). This line of reasoning leads to the
hypothesis that under acidic conditions, the hydrolysis rates decreases with each subsequent
hydrolysis step (electron withdrawing), whereas under basic conditions each subsequent
hydrolysis step occurs more quickly as hydrolysis and condensation proceed [54]. Then, the
decrease of tg at lower concentrations of acid catalyst may be due to the fact that at lower
catalyst concentrations the rate of condensation overcompensates the increased surface
charge of all ions in the sol and provided for a faster gelation [55].
10-4
10-3
10-2
30
40
50
60
70
80
G
elat
ion
tim
e (h
ours
)
log10
(concentration of the catalyst (citric acid))
Chapter II. Synthesis of silica aerogels 64
On the other hand, at higher catalyst concentrations hydrolysis and condensation rates are
high which lead to decrease surface charge of all ions in the sol and hence a faster gelation
[56].
Figure II.20 Inductive effects of substitution attached to silicon, R, OR, OH or OSi [56].
Figure II.21 Gelation time versus water/TEOS molar ratio for several catalyst concentrations. Gels with EtOH/TEOS fixed at 7 and citric acid as catalyst
Influence of the nature of the catalyst
Although the hydrolysis and condensation can proceed without the involving catalyst, their
use is quite useful. The understanding of catalytic effects is often complicated. There are two
interlinked effects that play an important role: the acidity of silanol groups (Si-OH) increases
with the extent of the hydrolysis and polymerization, and the effects of reverse reactions that
become increasingly important with greater concentration of water and /or base. Hence, to
4,5 5,0 5,5 6,0 6,5 7,0 7,5 8,0
20
40
60
80
100
120
140
0.0001M
0.0005M
0.001M
0.003M
EtOH/TEOS = 7
acid citric
Gel
atio
n t
ime
(hours
)
molar ratio water/TEOS
-OSi
Si−OH
-OR
- R
Increasing acidity
(electron withdrawn) OR
RO
Chapter II. Synthesis of silica aerogels 65
facilitate the study of the catalyst in the synthesis, sol-gel parameters were kept constant and
then the type of catalyst was varied.
Previous work indicated that using base-catalyzed in the synthesis with large water ratio (h)
produced highly condensed ‘particulate’ aerogels whereas acid catalyzed with low h produced
branched ‘polymeric’ sols. Intermediate conditions produce structures intermediate to these
extremes. See Figure II.22.
Figure II.22 Scheme of gel syntheses routes either under basic or acid conditions
The understanding of catalytic effect is complicated because with continue hydrolysis and
condensation the acidity of silanol groups (Si-OH) increased, and because the effects of
reverse reactions become increasingly important with greater concentrations of water or base.
In this section, numerous acid or base catalysts were used. The basic catalysts used were
NH4OH (weak basic catalyst), and NH3+NH4F. The acid catalyst used were HF, HCl, HNO3
and C6H8O7 and the mixture of some of these acids with NH4OH, NH4OH+C2H2O4. Their
syntheses have been described in the previous sections of this chapter. Table II.18 gathers
the gel time (tg) and transparency of the experimental synthesis that resulted in monolithic
gels.
monomer
dimer
cycle
particle
.
Base
conditions
Sols
10 nm
5 nm
Acid
conditions
100
nm
1 nm
Gel: three
dimensional
network
30 nm
monomer
dimer
cycle
particle
.
Base
conditions
Sols
10 nm
5 nm
Acid
conditions
100
nm
1 nm
Gel: three
dimensional
network
30 nm
Chapter II. Synthesis of silica aerogels 66
Table II.17 Synthesis experimental conditions for TEOS as precursor, ethanol as solvent, and several kind of catalyst, gel time (tg) and transparency of the resulting gels.
Gel label Catalyst tg Gel
TE99AV
(1/5/4) NH4OH 0,01M 2 days
Transparent monolith,
opaque gel
TE99AW (1/5/4)-
AY(1/5/5)
C2H2O4+NH4OH
0.01M 6 days
Transparent, monolith
transversal crack.
TE99AZ-BA-BB
(1/5/5)
C2H2O4+NH4OH
0.005M 23 hour Turbid monolith
TE99AZ? NH3+NH4F
2 days
Transparent monolith,
opaque gel
TE99AW?
C2H2O4
1 day
Transparent monolith,
and opaque gel
0.03M TE99AG HNO3 (1/7/5) 19 days White gel
0.01M TE99AX CH3COOH (1/5/4) 69 hours Monolith, transparent
TE99BL,TE99BJ HCl (1/7/5) 12days+2 h Cracked gel
For all the catalysts when the gels were set with low catalyst concentration (c<10 -4 M), gels
were turbid, with c > 1M they were transparent. Turbid alcogels were obtained for the
mixtures of acids and bases at lower concentrations (c<10 -2 M).
1. Strong acid + weak basic mixture used as catalysts gives transparent but cracked gels.
It is reported that the hydrolysis rate is faster under strongly acidic conditions [57].
2. Weak acid + weak basic mixture gives shrunk and semi-transparent gels.
3. Weak basic gives the best gel.
3.3 ‘TWO-STEP’ SYNTHESIS
This section deals with the achievement in the optimization of the two-steps preparation
process roughly corresponding to previous reported recipes [42, 58, 59]. This process was
performed using prepolymerized TEOS obtained by the reaction of tetraethoxysilane
(TEOS) and sub-stoichiometric amount of water in the presence of acid catalyst. This acid
step involves the formation of partially hydrolyzed and partially condensed silica mixture in
ethanol, leaving a viscous fluid containing higher molecular weight silicon alkoxy-oxides. In a
second part, this viscous mixture is re-dissolved in ethanol with additional water under basic
conditions [60]. Gels prepared in this way are known as ‘two-step’ acid-base catalyzed gels.
The chemical reactions carried out by this two-step process can be summarized as:
Chapter II. Synthesis of silica aerogels 67
(1) The first step under acid conditions, which operated in concentrated medium, enhanced
hydrolysis and formation of dimmers and oligomers.
(2) The second step under basic conditions that enhanced polycondensation and allowed
dimmers and oligomers to stick in order to form large clusters involving very large pores.
This route permitted us to obtain uncracked transparent aerogels with low densities. Specific
surface area values were in the range of usual aerogels. (See chapter III, bulk
characterization). To establish the effect of several synthesis routes all gels were achieved
following the same receipt. The first step was replaced for using the commercially available
Prehydrolyzed Ethyl Silicate (H5) as prepolymerized precursor from Silbond Corp. (Silbond,
H5).
The second step was performed mixing:
i) The silica solution containing a variable volume of precondensed silica (v H5) diluted in
ethanol.
ii) With the catalyst solution, that was containing EtOH/H2O/NH3 or NH4F+NH3 with
variable amounts according to EtOH/H2O ratio. Nevertheless, the volume ratio vNH3/vH5
was varied from 0.1 to 0.02.
Then after gelification, for enhancing the aging process, the gels were soaked in an
alcohol/water/catalyst mixture of equal proportions to those of the original sol. The gels
were maintained in this solution up to 24 hours.
Figure II.23 Aging bath including H2O, EtOH and catalyst
After aging the gels, all water still contained within the pores was removed prior drying by
soaking the gels in pure ethanol several times.
Effect of precursor concentration
The effect of precursor concentrations was examined by keeping the H 2O/H5 volume
concentration constant at 1.5 and the catalyst concentration was kept constant at 7.10 -3 M.
H2O+EtOH+ NH3
18ml 20ml 0.08ml
Chapter II. Synthesis of silica aerogels 68
In order to study the dependency of the gel time with the volume ratio the vEtOH/ vH5
concentration was changed from 0.8 to 2.5.EtOH/H5. (see Figure II.22). Table II.19
summarizes the two-steps conditions: volume amounts and volume ratios for precursor,
ethanol, and NH3
Table II.18 Volume amounts and volume ratios for two-steps aerogels with H5 as precursor, ethanol as solvent, and NH3 as base-catalyst
H5 H2O EtOH NH3
Volume
(ml) 12 18 VETOH 0.08
Volume
ratio 1 1.5
Variable
VETOH/VTEOS 7.10-3
Table II.20 gathers the gelation time and transparency of a series of ‘two-steps’ aerogels
synthesized at vH5/vH2O/vNH3 1/1.5/7.10-3 with a variable amount of ethanol, VETOH/VTEOS
Table II.19 Gelation time and transparency of a series of ‘two-steps’ aerogels synthesized at vH5/vH2O/vNH3 1/1.5/7.10-3 with a variable amount of ethanol, VETOH/VTEOS
Label VETOH/VH5 tgel Observations
H501AF00-05 0.8 - White precipitate
H501AG00-05 1.25 30’ Translucent gels
H501AA00-05 1.7 1h Transparent gels
H501AH00-05 2.1 1h30’ Transparent gels
H501AI00-05 2.5 2h45’ Transparent and very soft gels
H501AJ00-05 3.3 4h45’ Transparent gels
The 36 gels (six samples for each molar ratio) were soaked in an aging solution for 24 hours,
and after soaked 4 times in pure ethanol for 10 days. It should be pointed out that the
washing process increased strongly the quality of the aerogels and decreased the shrinkage
during supercritical drying.
Chapter II. Synthesis of silica aerogels 69
Figure II.24 shows the exponential dependency of gelation time versus volume ratio
EtOH/H5 for the two-steps gels with EtOH/H5 molar ratio fixed at 1.5 and base-catalyst
(NH3).
Figure II.24 Gel time versus water/H5 volume ratio for gels with EtOH/H5 molar ratio fixed at 1.5 and base-catalyst (NH3).
It was observed the same gel time dependency versus EtOH/TEOS volume ratio that one-
step gels with TEOS and acid citric as a catalyst. The reaction time increased sharply with the
ethanol/H5 volume ratio. For volume ratios lower than one a white precipitate was
produced, gels were not formed.
Effect of water amount
As the same effect that one-step gels, the amount of water used at hydrolysis step was found
to have a significant effect on the texture of aerogels. The effect of water concentrations was
examined by keeping the catalyst concentration constant at 8.7.10 -5.
1,0 1,5 2,0 2,5 3,0
20
40
60
80
100
120
140
160
180
200
y = y0 + A
1e
x/t1
y0
9.12367
A1
6.11881 4.9821-144
t1
0.65685
vH2O
/vH5
= 1.5
G
elat
ion
tim
e (m
in)
volume ratio EtOH/H5
Chapter II. Synthesis of silica aerogels 70
Table II.20 Synthesis experimental conditions for H5 as precursor, ethanol as solvent, and NH3 catalyst.
H5 H2O EtOH NH3
Volume (ml) 14 32 14 0.01
Volume ratio 1 2.3 1 7x10-4
molar ratio - 1 0.13 8.7x10-5
molar ratio - 7.4 1 6.5x10-4
The gel time was too short because water was added in excess, then the amount of water was
decreased and homogeneous clear and gels were produced.
Table II.21 Synthesis experimental conditions for H5 as precursor, ethanol as solvent, and NH3 catalyst
H5 H2O EtOH NH3
Volume (ml) 14 21 24 0.01
Volume ratio 1 1.5 1.7 7.10-4
In this synthesis, the gels were not diluted because the quality of the alcogels was very good
resulting in non-cracked and transparent samples.
Influence of the amount of the catalyst
The influence of the amount of the catalyst on the properties of aerogels was studied. The
conditions that were considered the optimal concentrations of reagents were for
vH5/vEtOH/vH2O 1/1.5/1.7 initial volume concentration of H5 in ethanol with presence of
water. Therefore, the influence of the amount of catalyst was studied. The following range of
NH3 was tested:
0.002 < xNH3 < 0.1 vH5/vEtOH/vH2O
1/1.5/1.7
Table II.22 Synthesis experimental conditions for H5 as precursor, ethanol as solvent, and a variable amount of NH3 catalyst, c.
H5 H2O EtOH NH3
Volume(ml) 14 21 24 x (volume ratio)
Volume ratio 1 1.5 1.7 c (molar ratio)
Chapter II. Synthesis of silica aerogels 71
Table II.23 H5 aerogels synthesized with vH5/vEtOH/vH2O fixed at 1/1.7/1.5 and with a variable amount of NH3 catalyst, XNH3.
Label XNH3 tgel
H501AA00-05 0.08 1 h
H501AB00-05 0.04 1h 30’
H501AC00-05 0.06 1 h
H501AD00-05 0.10 <1h
H501AE00-05 0.02 12h 10’
All the 30 gels (6 for each catalyst concentration) obtained in the series of two-steps aerogels
synthesized with vH5/vEtOH/vH2O fixed at 1/1.7/1.5 were monolithic and very transparent.
4. SUMMARY AND CONCLUSIONS
For aerogels using TMOS as alkoxide precursor
To obtain transparent aerogels the best synthesis was by using methanol as solvent with the
presence of base catalyst at low concentrations.
For aerogels using TEOS as alkoxide precursor
The best quality TEOS aerogels, in terms of monolithicity and transparency, without much
shrinkage were obtained by using weak base or acid catalyst:
1. Tetraethoxysilane concentration in alcohol: 10¯40 volume%,
2. Presence of low concentration catalyst.
3. Excess of stoichiometric amount of water (1mol/mol of oxyethylene group).
It was found that the gel time varied widely from few minutes to several days, depending on
the type of solvent and catalyst combinations, with shortest tg being for methanol solvent and
sodium hydroxide catalyst. This effect is a result of the shortest chain length and branching
of the solvent, and the effect of the catalyst. Strong acidic catalyst gave transparent but
cracked aerogels, whereas weak acids yielded monolithic, transparent aerogels.
For all the precursors, it was observed a decrease of gel time either increasing the amount of
water or decreasing the concentration of the precursor. With the presence of low
concentration of catalyst, the reaction was more easily controlled.
The conditions that were considered optimal concentration of reactive were for 1/5/7 and
1/7/5, 6 or 8 with low concentration of acid citric catalyst (0.01<c<0.03). But, summarizing
one can state that from the point of view of the texture, the most favorable condition of the
preparation were when the ‘two-step’ method was followed and water was removed by
Chapter II. Synthesis of silica aerogels 72
soaking the gels in an ethanol solution. In that case, non-cracked and very transparent
aerogels were obtained with a high surface area and a varied density (depends on the
vEtOH/vH5 from 0.03 to 0.1).
5. REFERENCES
1. N. Husing, U. Schubert, Aerogels-Airy
Materials: Chemistry, Structure, and Properties,
Review in Angew. Chem. Int. Ed. (1998), 37, p.22.
2 J. Fricke, R. Caps, D. Buttner, V. Heinemann,
E. Himmer, G. Reichenamer, Structural, elasto-
mechanical and thermal properties of silica
aerogels, in: K.K. Kruger et al. (Eds.),
Characterization of Porous Structure, vol. 629,
Elsevier, Amsterdam, 1988
3 T. Woignier, G. Scherer and A. Alaoui. J.
Sol¯Gel Sci. Tech. 3 (1994), p. 141.
4 S. Yoda, S. Ohshima and F. Ikazaki. J. Non-
Cryst. Solids 231 (1998), p. 41
5 S.Y. Chang, T.A. Ring, J. Non-Cryst. Solids
147¯148, 56 (1992)
6 G.M. Pajonk, M. Repelin-Lacroix, S.
Abouarnadasse, J. Chaouki and D. Klvana. J.
Non-Cryst. Solids 121 (1990), p. 66.
7 S.S. Kistler. Nature 127 (1931), p. 741.
8 J. Walendziewski, M. Stolarski, M. Steininger
and B. Pniak. React. Kinet. Catal. Lett. 58 1
(1996), p. 85.
9 J. Phalippou, T. Woignier and M. Prassas. J.
Mater. Sci. 25 (1990), p. 3111.
10 T. Woignier, G. Scherer and A. Alaoui. J.
Sol¯Gel Sci. Tech. 3 (1994), p. 141.
11 Transformation of nanostructure of silica
gels during drying Journal of Non-Crystalline
Solids Volume 262, Issues 1-3 February 2000
Pages 155-161
12 M. Prassar, J. Phalippon, J. Zarzycki,
Sintering of monolithic silica aerogel, in: L.L.
Hench, D.R. Ulrich (Eds.), Science of Ceramic
Processing, vol. 156, Wiley, New York, 1986
13 S.J. Teichner, in: J. Fricke (Ed.), Aerogels,
vol. 22, Springer, Berlin, 1986
14 F. Kirkbir, H. Murata, D. Meyers, S. Ray
Chaudhuri, in 9th Int. Workshop on Glasses,
Ceramics, Hybrids and Nanocomposites from
Gels, Sheffield, England (1997)
15 K. Yokota, S. Ohmori and S. Takishima. H.
Masuoka, Kagaku Kogaku Symposium Series 35
(1992), p. 149
16 Supercritical drying media modification for
silica aerogel preparation
Journal of Non-Crystalline Solids Volume 248,
Issues 2-3 2 June 1999 Pages 224-234
Satoshi Yoda
17 Ambient-temperature supercritical drying of
transparent silica aerogels Tewari, Param H.;
Hunt, Arlon J.; Lofftus, Kevin D. Materials
Letters Volume 3, Issue 9-10 July 1985 Pages
363-367
18 Drying of silica aerogel with supercritical
carbon dioxide M. J. Van Bommela, and A. B.
De Haanb,Journal of Non-Crystalline Solids
Volume 186 June1995 Pages 78-82
19 Comparison of some physical properties of
silica aerogel monoliths synthesized by different
precursors Materials Chemistry and Physics
Volume 57, Issue 3 25 January 1999 Pages 214-
218 P. B.Wagh
Chapter II. Synthesis of silica aerogels 73
20 A.H. Boonstra and C.A.M. Mulder. J. Non-
Cryst. Solids 105 (1988), p. 201.
21 J.G. van Lierop, A. Huizing, W.C.P.M.
Meeram and C.A.M. Mulder. J. Non-Cryst. Solids
82 (1986), p. 265.
22 L.C. Klein. Ann. Rev. Mater. Sci. 15 (1985), p.
227.
23 R.A. Laudise and D.W. Johnson, Jr.. J. Non-
Cryst. Solids 79 (1986), p. 155.
24 Physical properties of silica gels and aerogels
prepared with new polymeric precursors
Journal of Non-Crystalline Solids Volume 186
June 1995 Pages 1-8
25 S. Henning and L. Svensson. Phys. Scr. 23
(1981), p. 697.
26 P.H. Tewari, A.J. Hunt, K. Lofftus, in: J.
Fricke (Ed.), Aerogels, Springer, Berlin, 1986, p.
31
27 G. Poelz and R. Riethmuller. Nucl. Instrum.
Methods 195 (1982), p. 491.
28 R.A. Assink and B.D. Kay. J. Non-Cryst.
Solids 99 (1988), p. 359
29 M. Yamane, S. Inove and A. Yasumori. J.
Non-Cryst. Solids 63 (1984), p. 12.
30 S.J. Teichner, G.A. Nicolaon, M.A. Vicarini
and G.E.E. Gardes. Adv. Coll. Interface Sci. 5
(1976), p. 245.
31 R. Winter, D.W. Hau, D. Thiyagarajan and J.
Jonas. J. Non-Cryst. Solids 108 (1989), p. 137.
32 M. Prassas, J. Phalippou and Z. Zarzycki. J.
Mater. Sci. 19 (1984), p. 1665
33 Physical properties of silica gels and aerogels
prepared with new polymeric precursors
Journal of Non-Crystalline Solids Volume 186
June 1995 Pages 1-8
34 Ultralow density silica aerogels by alcohol
supercritical drying L. Kocon Journal of Non-
Crystalline Solids Volume 225, Issue 1 April
1998 Pages 96-100
35 T.M. Tillotson and L.W. Hrubesh. J. Non-
Cryst. Solids 186 (1995), p. 209
36 D.J. Stein, A. Maskara, S. Hæreid, J.
Anderson, D.M. Smith, in: A.K. Cheetham, C.J.
Brinker, M.A. Mecartney, C. Sanchez (Eds.),
Better Ceramics Through Chemistry VI, Mater.
Res. Soc. Proceed., Vol. 346, Materials Research
Society, Pittsburgh, PA, 1994, p. 643.
37 J. Zarzycki, T. Wognier, in: J. Fricke (Ed.),
Aerogels, vol. 42, Springer, Berlin, 1986
38 Dependence of monolithicity and physical
properties of TMOS silica aerogels on gel aging
and drying conditions G. M. Pajonk Journal of
Non-Crystalline Solids Volume 209, Issues 1-2
January 1997 Pages 40-50
39 D.R. Uhlmann, B.J. Zeliñski, L. Silverman,
S.B. Warner, B.D. Fabes, W.F. Doyle, Kinetic
processes in sol¯gel processing, in: L.L. Hench,
D.R. Ulrich (Eds.), Science of Ceramic
Processing, vol. 173, Wiley, New York, 1986
40 G.M. Pajonk. Rev. Phys. Appl. 24 (1989), pp.
C4¯13.
41 Ultralow density silica aerogels by alcohol
supercritical drying L. Kocon Journal of Non-
Crystalline Solids Volume 225, Issue 1 April
1998 Pages 96-100
42 T.M. Tillotson and L.W. Hrubesh. J. Non-
Cryst. Solids 186 (1995), p. 209.
43. S.S. Kistler. J. Phys. Chem. 36 (1932), p. 52.
44 M. Pauthe and J. Phalippou. Rev. Phys.
Appl. C 4 (1989), p. 215.
45 M.G. Vronkov, The siloxane bond
(Consultants Bureau, New York, 1978).
Chapter II. Synthesis of silica aerogels 74
46 Structural development of silica gels aged in
TEOS Journal of Non-Crystalline Solids
Volume 231, Issues 1-2 1 July 1998 Pages 10-16
47 Transformation of nanostructure of silica
gels during drying Journal of Non-Crystalline
Solids Volume 262, Issues 1-3 February 2000
Pages 155-161
48. S. Wang, S. Raychaudhuri, A. Sarkar, US
patent 5,264,197 (1993).
50 Influence of molar ratios of precursor,
solvent and water on physical properties of
citric acid catalyzed TEOS silica aerogels.
Materials Chemistry and Physics Volume 53,
Issue 1 April 1998 Pages 41-47 P. B. Wagha, A.
Venkateswara Raoa, and D. Haranatha
51 G.W. Scherer, J. Non-Cryst. Solids 108 (1989),
p. 18 and p. 28.
52 C.J. Brinker, G.W. Sherer, Sol¯Gel Science.
Physics and Chemistry of Sol¯Gel Processing,
Academic Press, New York, 1990
53 Influence of temperature on the physical
properties of citric acid catalyzed TEOS silica
aerogels P. B. Wagha, D. Haranatha, A.
Venkateswara Raoa, and G. M. Pajonkb
Materials Chemistry and Physics Volume 50,
Issue 1 August 1997 Pages 76-81
54 W. Y. Shih, J, Chem. Phys. 86 (1997), p.
5127
55 C. Okkerse, in: Physical and Chemical
Aspects of Adsorbents and Catalyst, edited by
B.G. Linsen (Academic Press, 1990), page 214.
56 C.J. Brinker, G.W. Sherer, Sol¯Gel Science.
Physics and Chemistry of Sol¯Gel Processing,
Academic Press, New York, 1990, page 131.
57 I. A. Aksay, in: L.L. Hench, D.R. Ulrich
(Eds.), Science of Ceramic Processing, vol. 156,
Wiley, New York, 1986, pp513-521.
58. A.H. Boonstra and T.N.M. Bernards. J.
Non-Cryst. Solids 105 (1988), p. 207.
59 Transparent ultralow-density silica aerogels
prepared by a two-step sol-gel process
Tillotson, T.M.; Hrubesh, L.W. Journal of Non-
Crystalline Solids Volume 145, Issue 1-3 1
August 1992 Pages 44-50]
60 The dependence of the gelation time on the
hydrolysis time in a two-step SiO2 sol-gel
process Journal of Non-Crystalline Solids
Volume 105, Issue 3 1988 Pages 207-213
Chapter III. Bulk silica aerogel characterization 75
C h a p t e r I I I
BULK SILICA AEROGEL CHARACTERIZATION
SECTION OUTLINE
1. MONOLITHICITY, BULK SHRINKAGE, DENSITY AND POROSITY .................................... 75
1.1 TMOS AEROGELS ........................................................................................................................ 76
Skeletal density......................................................................................................................... 76
Bulk density.............................................................................................................................. 76
1.1.1 Supercritical drying at CO2 conditions ............................................................................ 80
1.2 TEOS AEROGELS ......................................................................................................................... 81
1.2.1 TEOS aerogels synthesized without presence of catalyst ............................................ 82
1.2.2 Base-catalyst .......................................................................................................................... 84
1.2.3 Acid catalyst ........................................................................................................................... 87
Fluorhydric acid ....................................................................................................................... 88
Citric acid ................................................................................................................................. 90
1.3 TWO-STEPS SYNTHESIS ........................................................................................................... 91
2. SURFACE AREA MEASUREMENTS BY BET (BRUNAUER, EMMET AND TELLER) ..... 94
3. INFRARED SPECTROPHOTOMETRY, IR......................................................................................... 99
3.1 METHANOL SERIES ................................................................................................................. 100
3.2 ACETONE SERIES ..................................................................................................................... 102
4. ULTRAVIOLET-VISIBLE (UV-VIS) SPECTROSCOPY ...................................................................103
4.1 AEROGEL TRANSPARENCY ................................................................................................. 103
4.2 RAYLEIGH SCATTERING ...................................................................................................... 107
4.2.1 A model to interpret the porous aerogel structure using Rayleigh scattering ........ 110
5. LIGHT SCATTERING MEASUREMENTS OF AEROGELS BY A POLARIZATION-
MODULATED NEPHELOMETER. ................................................................................................................ 113
5.1 INTRODUCTION TO LIGHT SCATTERING VS ANGLE EXPERIMENTS .........113
5.1.1 Description of the polarization-modulated nephelometer.......................................... 114
5.2 EXPERIMENTAL RESULTS ....................................................................................................115
5.3 STRUCTURAL INFORMATION FROM THE LIGHT SCATTERING
MEASUREMENTS .......................................................................................................................118
5.3.1 Inhomogeneous media....................................................................................................... 118
Short range correlations: Rayleigh scattering....................................................................... 120
Long range correlations: departures from Rayleigh scattering ........................................... 121
5.4 COMPARATIVE STUDY BETWEEN EXPERIMENTAL MEASUREMENTS AND
THEORY ......................................................................................................................................... 122
5.5 CONCLUSIONS AND FUTURE WORK.............................................................................. 125
6. DIRECT METHODS: ELECTRON MICROSCOPY ........................................................................127
6.1 STRUCTURAL STUDIES BY SCANNING ELECTRON MICROSCOPY .............. .127
6.1.1 Acetone series .......................................................................................................................127
6.1.2 Effect of the solvent............................................................................................................. 131
6.1.3 Drying procedure.................................................................................................................132
6.1.4 TMOS aerogels in CO2 as solvent....................................................................................133
6.2 TRANSMISSION ELECTRON MICROSCOPY ................................................................ 134
Sample preparation .................................................................................................................... 134
TEM set-up.............................................................................................................................. 134
6.2.1 Imaging the acetone-series silica aerogels.....................................................................135
Direct visualization .................................................................................................................... 135
6.2.2 Imaging the methanol silica aerogels ........................................................................... .138
Replicas visualization ................................................................................................................. 138
7. REFERENCES ............................................................................................................................................. 141
Chapter III. Bulk silica aerogel characterization 75
Previous studies have shown that it is possible to control such physical properties as the
porosity and transparency of the aerogels by adjusting the so-called sol-gel parameters. These
parameters include the type and concentration of the alkoxide precursor, acid or base catalyst,
and water content [1-3]. These parameters affect the structure of the initial gel and, in turn, the
properties of the resulting aerogel. In this chapter, it has been taken up a detailed study
regarding the comparison of physical properties of silica aerogel monoliths synthesized by
different recipes (see chapter II synthesis optimization). Then, each section containing the
characterization of a different physical property is arranged in subsections with the different
series of silica aerogels. The studied physical properties were:
i) Monolithicity: monolithic structure or presence of cracks (Section 1),
ii) Shrinkage: linear, diametric and volumetric (Section 1)
iii) Density of the bulk aerogel (Section 1) and density of the skeleton (Section 1)
iv) Volume porosity (Section 1)
v) Specific surface area and pore size distribution (Section 2)
vi) Optical transmission and transparency (Section 3)
vii) Silica nanoparticle size distribution (Section 6)
It will be shown that all these properties are strongly correlated.
1. MONOLITHICITY, BULK SHRINKAGE, DENSITY AND POROSITY
Depending on the synthesis and drying conditions of the gels, the aerogels samples were
completely cracked, monolith with presences of cracks, or entirely monolithic. Table III.1
summarizes the experimental observations.
In order to measure the shrinkage ( V/Vgel) produced during supercritical extraction (see
Table III.1) the aerogels and the wet gel dimensions should be compared.
Volume shrinkage is defined as: gel
aerogelgel
gel V
VV
V
V, (Eq. III.1)
Longitudinal shrinkage: gel
aerogelgel
gel L
LL
L
L, (Eq. III.2)
Diametric shrinkage: gel
aerogelgel
gel
(Eq. III.3)
Chapter III. Bulk silica aerogel characterization 76
Bulk aerogel density was determined by weighing samples of known dimensions and by
dividing the aerogel mass by its volume. A maximum error in the density of about 10% was
estimated.
The volume porosity was calculated from the density using the equation:
100*(%)
2
2
SiO
aerogelSiOP (Eq. III.4)
where: SiO2 = 2.19 g/cm3 is the density of pure silica
aerogel is the bulk aerogel density
A more accurate value of the aerogel porosity can be obtained using the aerogel skeletal
density, S, instead of SiO2. For few samples, s was measured by helium pycnometry at room
temperature (Micrometrics, AccuyPyc 1330) [4, 5]. For each aerogel, at least four
measurements of porosity and aerogel density were carried out.
1.1 TMOS AEROGELS
During gel formation, hydrolyzed TMOS forms the silica gel network and the solvent fills the
gel pores. In this section, depending on the solvent used in the gel synthesis, two TMOS
routes were characterized: methanol, labeled as M, [6] and acetone [7] labeled as A-series (the
corresponding sol-gel syntheses are described in chapter II). The A-series consisted in four
aerogels, A1, A2, A3, and A4 with an increasing density.
Skeletal density
The skeletal density for each of A-series was obtained by helium pycnometry. All the samples
showed a similar s (around 2.45 g/cm3) slightly larger than that of vitreous silica ( SiO2 =2.19
g/cm3).
Bulk density
In order to study the batch-to-batch reproducibility a statistical study was performed for a
series of aerogels samples obtained using the identical synthesis and drying process for
methanol synthesis. Figure III.1 shows the density for each produced M aerogel, mean
density value, standard deviation and a density distribution histogram.
Chapter III. Bulk silica aerogel characterization 77
Figure III.1 Study of the reproducibility of the aerogel densities obtained using methanol as solvent. Density histogram (left part of the figure) the mean density value was
of =0.122 g/cm3 with a standard deviation of 0.010 g/cm3.
To study the dispersion of the aerogel density the same reproducibility studies were
performed on the acetone series. Figure III.2 shows the densities values for each acetone
aerogel sample, the density mean value, the standard deviation and the histogram of the
density distribution.
0 10 20 30
0,09
0,10
0,11
0,12
0,13
0,14
0,15
0,16
0.010
0.122
Den
sity
(g/
cm3 )
TMOS aerogels, methanol as solvent
0,09
0,10
0,11
0,12
0,13
0,14
0,15
0,16
0 2 4 6 8
Density (g/cm3)Number of samples
Chapter III. Bulk silica aerogel characterization 78
Figure III.2 Aerogel densities for each series obtained when using acetone as gel solvent. 3/01.006.0 cmg for the A1C
series, 3/01.008.0 cmg for the A1 series, 3/02.015.0 cmg for the A2 series, 3/02.024.0 cmg for
the A3 series, 3/02.025.0 cmg for the A2CO2 series, and 3/03.027.0 cmg for the A4 series. For each series, the
density histogram is shown at the right of the figure.
It was observed in the Figure III.2 that the samples classified themselves in six different
densities groups depending on the different followed synthesis: The first four groups A1, A2,
A3 and A4, correspond to the different precursor dilution. The fifth density group is the A2
dried under carbon dioxide supercritical conditions labeled as A2CO 2. It can be observed that
A2CO2 samples have the same density than of A3 aerogels. Finally, the group with the lowest
density corresponds to A1C samples that are the A1 samples with added activated carbon.
The A-series was chosen to study the dependency of the bulk aerogel density and volume
porosity with the concentration of precursor on the initial synthesis solution. As it is shown in
Figure III.3, the densities and porosities of the acetone series aerogels presented a linear
relationship versus volume concentration of the precursor, acetoneTMOS
TMOS
VV
Vv .
=0.0099 + 0.7229 v (Eq. III.5)
0,0 0,1 0,2 0,3 0,40
2A4
A1C
0,0 0,1 0,2 0,3 0,4
0
2
4
A1
0,0 0,1 0,2 0,3 0,4
0
2
A2
0,0 0,1 0,2 0,3 0,4
0
2
4
6
8
A2CO2
0,0 0,1 0,2 0,3 0,4
0
2
0,0 0,1 0,2 0,3 0,4
0
2
A3
A4
0 10 20 30 40 50
0,05
0,10
0,15
0,20
0,25
0,30
0,35
A4
A3
A2CO2
A2
A1
A1C
A1C 0,06g/cm3 0,007
A1 0,08g/cm3 0,009
A2 0,15g/cm3 0,010
A2CO2 0,25g/cm3 0,002
A3 0,24g/cm3 0,017
A4 0,29g/cm3 0,026
den
sity
(g/
cm3 )
Acetona seriesAcetone series
Chapter III. Bulk silica aerogel characterization 79
Figure III.3 Porosity and apparent silica aerogel density versus v (precursor volume ratio) for the acetone samples. A
linear fitting gives, =0.0099 + 0.7229 v.
Varying the TMOS concentration in acetone gels proves to be a very easy way for controlling
density and porosity of the resulting aerogels. Following table gathers the mean values of
bulk density aerogels, porosity and volume shrinkage of the different TMOS samples.
Table III.1 Volume concentration of TMOS,
acetoneTMOS
TMOS
VV
Vv
,
apparent density, , porosity and volume shrinkage, V/Vgel, of the different TMOS aerogels.
Sample
(g/cm3)
Porosity
(%) V/Vgel
M 0.23 0.13 0.01 93 0.27
A1 0.10 0.08 0.01 96 0.43
A2 0.20 0.15 0.02 93 0.42
A3 0.30 0.25 0.02 88 0.47
A4 0.40 0.27 0.03 86 0.46
It was observed that the methanol-synthesized aerogels, M, presented a smaller shrinkage
( 27 %) than the acetone-series, A, ( 45 %) in which, volume shrinkage was independent on
the precursor concentration within the experimental error. Shrinkage differences could be
0,0 0,1 0,2 0,3 0,4 0,5
0,05
0,10
0,15
0,20
0,25
0,30
0,35
Porosity
v
Den
sity
(g
/cm
3)
82
84
86
88
90
92
94
96
98
100
DensityP
oro
sity (%
)
Chapter III. Bulk silica aerogel characterization 80
related with the use of a basic catalyst for the methanol aerogel while for the acetone series
no catalyst was used (sol polymerization conditions of pH between 5 and 6). It is interesting
to note that the pH values for both sols are in the acidic side. This is due to the fact that the
pH values of TMOS and methanol are on the acidic side (pH 4) and hence the pH of
NH4OH incorporated sol has a value typical of weak-acidic conditions. It is also possible that
a number of micro- and/or macropores in the acetone synthesized gels collapsed during
supercritical extraction and this resulted in a larger shrinkage.
1.1.1 Supercritical drying at CO2 conditions
To observe the effect of supercritical drying process at low-temperature on the physical
properties of the TMOS silica aerogels the acetone gels were used since liquid CO 2 is highly
miscible in acetone. The gels used were the A2 synthesis [7, 8] and the resulting aerogels were
called A2CO2.
In this first low-temperature drying set, monolithic aerogels free of cracks were obtained. The
aerogels obtained had different characteristics that those dried under conventional drying by
hypercritical evacuation of the solvent. The main differences between the two drying
procedure were:
i. The mean aerogel density for the acetone dried gels was = 0.141 g/cm3 while for the
CO2 dried aerogels was = 0.245 g/cm3. The aerogels with CO2 exchange were denser,
then, lower porosity: 85% for aerogels with exchange and 93% for acetone dried
aerogels.
ii. The mean diametric shrinkage was larger for CO 2 aerogels / =0.30 than for
acetone dried aerogels / =0.21.
iii. The low- temperature silica aerogels were more opaque.
iv. The A2CO2 aerogels were less fragile.
Is important to remark that in the synthesis of the A2CO2-series the gels were not washed
with ethanol after gelation. The soaking solvent was not exchanged, and then, the remaining
water and catalyst were still present in the liquid when A2 were supercritcally dried. It may be
the main reason for the dramatic shrinkage observed, and consequently the higher density
and opacity.
Chapter III. Bulk silica aerogel characterization 81
Previous work [9] reported that the silica aerogels dried at low temperature were hydrophilic
while the dried ones at high temperature were hydrophobic. Because the different drying
temperatures the composition of the aerogel surface was different. The CO 2 dried aerogels
presented OH groups very polar that can react with the water. In the surface of conventional
aerogels, there exists alkoxy groups that are not polar and do not react with water. However,
in the synthesized aerogels samples presented in this section, the water penetrated in the
porous of the aerogels dried conventionally, whiles the CO 2 dried aerogels are hydrophobic.
More extended work is necessary to understand this behavior.
Conclusions for TMOS aerogels
The properties of aerogels are modified according to the conditions of their preparation.
i) An easy way for controlling density and porosity of the resulting aerogels is by the
variation of TMOS concentration in acetone gels.
ii) Methanol aerogels are very transparent while acetone aerogels had a white shading;
their opacity decreases with increasing TMOS content.
iii) All the aerogels have monolithic structure without cracks. The A1 aerogels were
especially fragile.
iv) Volume shrinkage for all acetone aerogels is about 45 %, independently of the
reactive concentrations. This shrinkage is much larger than that observed for
methanol aerogels 27 %.
v) Methanol aerogels presented densities and porosities similar to those for A2. For
acetone aerogels, it has been observed that density increases, and porosity is reduced,
when increasing TMOS content (Figure III.3), the variation could be adjusted to a
straight line. This is the behavior that one should expect: denser material needs more
precursors, leading to reduced porosities.
These results will be supported and discussed by considering the particle and pore sizes
observed by scanning electron microscopy (section SEM), by transmission electron
microscopy (section TEM) and the optical transmission of aerogels (section UV-VIS-NIR
spectroscopy) [10].
1.2 TEOS AEROGELS
A large number of silica aerogels were prepared following, neutral, basic, and acid synthesis.
The samples were dried either under ethanol [11-14] or CO2 supercritical conditions [15, 16].
Chapter III. Bulk silica aerogel characterization 82
The bulk density, porosity and shrinkage of optimized TEOS aerogels (see chapter II for
synthesis optimization) are determined in this section.
1.2.1 TEOS aerogels synthesized without presence of catalyst
A TEOS-series of aerogels was synthesized without catalyst. Several EtOH/TEOS molar
ratio, m, and several H2O/TEOS molar ratio, h, were tested. Two different series of TEOS
aerogels were prepared under the same synthesis conditions. After gelling, the gels of the first
series were not washed with an ethanol bath, Table III.2, while the gels of the second series
were washed and aged in an ethanol soaking (Table III.3). Table III.2 gathers the shrinkages
for all the TEOS samples produced without catalyst and at different TEOS/EtOH/H2O
molar ratios.
Table III.2 Ethanol/TEOS and water/TEOS molar ratios,
density, porosity, volume shrinkage, V
V , diametric shrinkage,
and linear shrinkage, L
L for a series of TEOS aerogels without
ethanol washing.
EtOH/
TEOS
H2O/
TEOS
(g/cm3)
Porosity
(%) V/V / L/L Observations
5 5 0.2515 88.5 0.545 0.22 0.26 Cracks
5 6 0.2333 89.3 0.515 0.21 0.22 Cracks
5 7 0.1932 91. 2 0.444 0.18 0.17 Cracks
5 8 0.2215 89.9 0.535 0.23 0.23 Cracks
7 5 0.1625 92.6 0.41 0.15 0.18 Monolithic
7 6 0.1620 92.6 0.425 0.16 0.18 Monolithic
7 8 0.1645 92.5 0.480 0.19 0.21 Monolithic
The quality of the set of samples without ethanol exchange was not very good because these
aerogels presented some cracks. The shrinkage is similar in all the samples with different
TEOS concentration: the volume shrinkage ranged from 0.42 to 0.53, the diametric
shrinkage from 0.17 to 0.23, and the linear shrinkage from 0.16 to 0.23.
At fixed H2O/TEOS ratio, when increasing the EtOH/TEOS ratio the densities appeared to
be smaller.
Chapter III. Bulk silica aerogel characterization 83
Table III.3 EtOH/H2O molar ratio, density, volume shrinkage, and linear shrinkage for a series of TEOS aerogels with ethanol washing.
EtOH/
TEOS
H2O/
TEOS (g/cm3) V/V / L/L Observations
5 6 0.123 0.1192 0.365 0.058 Monolithic
5 7 0.121 0.1481 0.502 0.056 Monolithic
7 5 0.116 0.1321 0.360 0.0673 Monolithic
7 6 0.122 0.1191 0.161 0.0826 Monolithic
7 8 0.113 0.1552 0.530 0.0569 Monolithic
For the washed gels, all the produced silica aerogel were monolithic and with similar smaller
volume shrinkage (0.11-0.12). No significant differences of density (0.133 g/cm3< 0.123
g/cm3) were obtained when varying the TEOS/EtOH/ H2O molar ratio.
There was a significant difference between the gels not washed and those washed in an
ethanol solution [16]. For the same synthesis and drying conditions, gels washed in ethanol
exhibited a lower bulk density and the incidence of cracking or fracture of aerogels was
significantly lower. This difference in bulk density is attributed to the presence of water
during the drying that caused a larger shrinkage giving a denser bulk structure.
It was found that the optimum aerogels were produced in the range of 5>h>8, lower (h< 5)
and higher (h> 8) values resulted in opaque and cracked aerogels. On the contrary taking the
m precursor concentration lower than 5 (m< 5) and higher than 9 (m> 9) values resulted in
opaque, high density as well as cracked aerogels. Moreover, the density of aerogels decreases
with an increase in m values up to 9. Further increases in m values (m > 9) lead to an increase
in the bulk density of the aerogels (because the bad quality of the aerogels). Summarizing,
monolithic, low density and transparent TEOS silica aerogels were found for h values
between 5 and 8 (5<h<8) and m values between 5 and 9 (5<m<9) when no catalyst was
Chapter III. Bulk silica aerogel characterization 84
used. As a final point, the effect of washing solution was found to be a very important factor
to control the quality of the aerogels.
As mentioned before, some of the aerogels dried conventionally were hydrophobic (see
Figure III.4), one possible explanation of this effect may be that in the surface of
conventional aerogels, there exists alkoxy groups that are not polar and do not react with
water. However, in the synthesized aerogels samples presented in this section, the water
penetrated in the porous of some aerogels indistinctly of the followed drying process. Figure
III.4 shows a picture of a hydrophobic silica aerogel floating on water.
Figure III.4 Photograph of a hydrophobic aerogel produced by using TEOS as metal alkoxide precursor with a molar ratio of TEOS/EtOH/H2O=1/5/6 and dried under ethanol supercritical conditions.
1.2.2 Base-catalyst
One-step gels were prepared from TEOS under base-catalyzed conditions. In this case, a
small amount of ammonium with ammonium fluoride solution (0.03M NH3 + 0.5M NH4F)
was added as a supplementary catalyst to accelerate the gel formation. Gels were poured in
Petri disks ( =30mm, h= 5mm) and dried under CO2 conditions.
Two series of TEOS base-catalyzed aerogels with a variable ethanol concentration were
prepared, the first series with H2O/TEOS fixed at 10, and the second series with
H2O/TEOS fixed at 25. The obtained samples presented different shrinkage and
transparency. The aerogel densities for the two series are gathered in Tables III.4 (h=10) and
III.5 (h=25). From the aerogels listed in Table III.4, those ones with highest density
presented some cracks and those with lowest density presented an opaque appearance. A
Chapter III. Bulk silica aerogel characterization 85
large variation in aerogel density was produced in that series from 0.0190 g/cm3 up to 0.124
g/cm3.
Table III.4 Bulk density and porosity of TEOS aerogels with base-catalyst (NH3+NH4F) and H2O/catalyst/TEOS fixed at 10/3.3.10-3/1.
EtOH/TEOS (g/cm3)
Porosity
(%)
16.82 0.1244 94.3
24.73 0.0602 97.2
66.29 0.0408 98.1
91.27 0.0295 98.6
115.75 0.0190 99.1
The most important factor to obtain more transparent aerogels in the base-catalyzed aerogels
is to increases the H2O/TEOS molar ratio [17]. Then, in order to improve the transparency
of the base silica aerogels the H2O/TEOS molar ratio was increased at 25.
Table III.5 Bulk density and porosity of TEOS aerogels with base-catalyst (NH3+NH4F) and H2O/catalyst/TEOS =25/3.3.10-3/1.
EtOH/TEOS (g/cm3)
Porosity
(%)
12.23 0.053 97.6
18.85 0.047 97.8
26.25 0.037 98.3
33.65 0.036 98.4
When H2O/TEOS molar ratio was fixed at 25 (Table III.5) all the aerogel samples were
monolithic without presence of cracks and more transparent than when H2O/TEOS molar
ratio was fixed at 10. In this series, the dependency of density and porosity versus
EtOH/TEOS molar ratio was studied. Figure III.4 shows the linear dependency of density
and porosity versus EtOH/TEOS molar ratio when H2O/TEOS=25.
Chapter III. Bulk silica aerogel characterization 86
Figure III.5 Porosity and apparent silica aerogel density versus molar EtOH/TEOS ratio for the TEOS aerogels with base-catalyst 3.3.10-3 (NH3+NH4F) and TEOS/H2O fixed at 25. Solid lines are the linear fitting.
If the two density-molar ratio dependencies are compared, one can observe that for larger
H2O/TEOS the linear variation of the density versus the EtOH/TEOS was faster. See
Figure III.6.
10 15 20 25 30 35 40
0,035
0,040
0,045
0,050
0,055
Density
EtOH/TEOS
den
sity
(g/
cm3 )
97,4
97,6
97,8
98,0
98,2
98,4
98,6
water/TEOS = 25
base-catalyst (NH3+NH
4F)
PorosityP
oro
sity (%)
Chapter III. Bulk silica aerogel characterization 87
Figure III.6 Comparison of the dependency of density versus EtOH/TEOS ratio for two different samples, one with H2O/TEOS fixed at 10 at the other fixed at 25. Linear fittings.
It should be pointed out that the gels with base-catalyst were soaked in water, catalyst, and
ethanol solution in the same proportions than the initial sol in order to accelerate the aging of
the gels. There was a significant difference between the gels soaked only in ethanol and those
soaked in an aging solution. Gels washed only in ethanol exhibit a more fragile skeleton. This
difference is attributed to the presence of water during the aging solution. The presence of
water causes solution/reprecipitation giving a smoother network. In addition, and probably
more important, continued hydrolysis and condensation reactions occur giving a stronger
network [18, 19].
1.2.3 Acid catalyst
The influence of two acid-catalysts (acid fluorhydric and acid citric) on the density, porosity,
and shrinkage is studied in this section. All samples were dried under CO 2 supercritical
conditions. Several EtOH/H2O/TEOS ratios were used in order to optimize the quality of
the acid-catalyzed aerogels.
20 40 60 80 100 120
0,01
0,02
0,03
0,04
0,05
0,06
0,07
water/TEOS = 25
base-catalyst (NH3+NH
4F)
water/TEOS = 10
base-catalyst (NH3+NH
4F)
den
sity
(g
/cm
3)
molar concentration EtOH/TEOS
Chapter III. Bulk silica aerogel characterization 88
Fluorhydric acid
The acid catalyst used was 52%HF. The recipe followed was fixing the H2O/TEOS ratio at
10 and then varying EtOH/TEOS concentration from 12 to 42. Gels were washed with
ethanol for several days; Table III.6 shows some of the obtained aerogel densities.
Table III.6 Density and porosity of TEOS aerogels with acid-catalyst (52%HF) and H2O/TEOS fixed at 25.
EtOH/TEOS Density
g/cm3
Porosity
(%)
12.23 0.102 95.3
18.85 0.068 96.9
26.25 0.043 98.0
33.65 0.024 98.9
41.05 0.018 99. 2
In that case, the produced TEOS aerogels with HF catalyst were very transparent samples.
Figure III.7 shows a linear decrease in the bulk density and linear increase in the bulk
porosity with an increase of EtOH/TEOS ratio, which is due to the smaller SiO 2
concentration in the gels when larger ethanol concentrations.
15 20 25 30 35 40 45
0,01
0,02
0,03
0,04
0,05
0,06
0,07
Density
EtOH/TEOS
den
sity
(g/
cm3 )
96,5
97,0
97,5
98,0
98,5
99,0
99,5 Porosity
Po
rosity (%
)
Figure III.7 Porosity and apparent silica aerogel density versus EtOH/TEOS ratio for the TEOS aerogels acid-catalyst (HF) and H2O/TEOS fixed at 25. Linear fitting Porosity(%)=95.1+0.1m with R=0.993.
Chapter III. Bulk silica aerogel characterization 89
Another synthesis using HF was tried with a lower H2O/TEOS molar ratio (fixed at 12) and
with a variable EtOH/TEOS molar ratio (in polystyrene tubes of =16mm). Table III.7
gathers the densities obtained.
Table III.7 TEOS aerogel densities and porosities of acid series with a variable EtOH/TEOS molar ratio. Acid-catalyst (52% HF) and H2O/TEOS fixed at 12.
EtOH/TEOS Density
(g/cm3)
Porosity
(%) Transparency
8.55 0.0192 99.1 Most opaque
6.67 0.0294 98.7 Opaque
6.71 0.0410 98.2 Translucent
5.43 0.0602 97.2 Most transparent
4.52 0.1244 94.3 Transparent but opaque inside
In that case, it was found that EtOH/TEOS molar ratio strongly affects the transparency due
to the structural change of pore and particle size of the aerogel. A larger density results in
more transparent aerogel. Next photography compares a TEOS silica aerogel with an
ambient evaporated gel (xerogel). In the case of xerogel sample, it was observed a transparent
sample but with a very large shrinkage.
Figure III.8 Photograph comparing two citric acid TEOS silica dried gels samples, on the top a CO2 supercritically dried sample, and on the bottom, an ambient evaporated gel.
Chapter III. Bulk silica aerogel characterization 90
Citric acid
As a comparison, gels were prepared from TEOS under acid-catalyzed conditions at molar
ratios of the starting components of the sol: TEOS/EtOH/H2O=1/7/5. In this case, a
variable amount of acid citric (from 0.0005 to 0.1) was added as a supplementary catalyst to
speed up the gel formation. The gels were dried under ethanol supercritical conditions.
Table III.8 TEOS aerogel, density, shrinkage, and monolicithy of acid series with a variable H2O/EtOH/TEOS molar ratio. Acid-catalyst (0,0001M acid citric).
Label m h
(g/cm3)
Porosity
(%) V/V
d/d
(%) L/L Monolithicity
TE98X01-02b 7 5 0,160 92,7 0,40 14,52 0,18 Monolithic
TE98X03-04b 7 6 0,162 92,6 0,43 16,94 0,18 Monolithic
TE98X05-06b 7 8 0,164 92,5 0,48 19,2 0,21 Monolithic
TE98X07-08b 5 5 0,249 88,6 0,54 21,77 0,23 Monolithic with crack
TE98X09-12b 5 6 0,233 89,4 0,52 21,77 0,22 Monolithic with crack
TE98X13-18b 5 7 0,189 91,4 0,43 16,94 0,18 Monolithic
TE98X19-20b 5 8 0,222 89,9 0,54 22,58 0,23 Monolithic with crack
At fixed water concentration, the bulk density is larger when larger TEOS concentrations.
The gels experienced a typical linear shrinkage of 0.18-0.23 during the supercritical drying.
The density as a function of water molar ratio is shown in Figure III.9.
Chapter III. Bulk silica aerogel characterization 91
4,5 5,0 5,5 6,0 6,5 7,0 7,5 8,0 8,5
0,18
0,20
0,22
0,24
0,26
H2O/TEOS
den
sity
(g/
cm3 )
Figure III.9 Variation of density from silica aerogel versus H2O/TEOS molar ratio of acid series at fixed concentration of citric acid (0,0001M).
It was found that for h values lower than seven an increase in h value leads to a decrease of
the density of the aerogels. Moreover, for higher h values, h>7, the density increases with h.
Lowest (h<5) and highest values (h<8) lead to the cracking due to the high shrinkage.
1.3 TWO-STEPS SYNTHESIS
As described in chapter II, in this thesis the two-steps synthesis was achieved by replacing the
first acid-step for the commercially partially hydrolyzed silica precursor, H5 obtained by
Silbond corp. Then, it will be necessary to identify the desirable processing conditions in the
second step. In that second step, the prepared precursor, mixed with appropriate amounts of
H2O, EtOH and NH4OH underwent further hydrolysis and condensation reactions formed a
clear gel usually within few hours. Then after aging for 24 h in a solution with the same
amount of water and catalyst, they were soaked for several days in a pure ethanol bath for
removing the interstitial water. They were dried using the CO 2 substitution method. All of
the aerogels obtained were hydrophobic.
Chapter III. Bulk silica aerogel characterization 92
Figure III.10 Photograph comparing the quality of two silica aerogel samples obtained with TEOS as alkoxide precursor. The aerogel on the top is a one-step silica aerogel and on the bottom a two-step aerogel. The two-step aerogel shows a higher transparency and lower density.
A first series of two-steps silica aerogels were prepared with different amounts of water and
ethanol. A second series was prepared to show the effect of the amount of catalyst used in
the gel preparation [20]. Table III.9 gathers some of the densities and transparencies of the
two-step series obtained by changing the H5 dilution and the catalyst amount.
Table III.9 Density and transparency of the two-step aerogel series obtained with NH3 catalyst in the second step.
Label Catalyst
amount
VETOH/
VH5
VH2O/
VH5
Density
(g/cm3)
Porosity
(%) Transparency
H501AE 0.02 1.7 1.5 0.0299 98.6 Opaque
H501AB 0.04 1.7 1.5 0.0330 98.5 Opaque
H501AC 0.06 1.7 1.5 0.0466 97.9 Opaque
H501AA 0.08 1.7 1.5 0.0550 97.5 Transparent
H501AD00 0.1 1.7 1.5 0.0823 96.2 Translucent
H501AD01 0.1 1.7 1.5 0.0800 96.3 Translucent
H501AH01 0.08 2.1 1.5 0.0734 96.6 Translucent
H501AG01 0.08 1.25 1.5 0.0841 96.2 Translucent
H501AI01 0.08 2.5 1.5 0.0748 96.6 Translucent
H501AJ01 0.08 3.3 1.5 0.0672 96.9 Translucent
The two-step aerogel densities ranged from 0.03 to 0.08 g/cm3. For samples with the same
water and catalyst concentration increasing EtOH/H5 ratio from 1 to 1.7, the aerogel density
Chapter III. Bulk silica aerogel characterization 93
decreases from 0.0786 to 0.0550 g/cm3. In addition, it was observed that the most
transparent and with low-density samples were produced when a catalyst amount of 0.08 and
water/EtOH/H5=1.5/1.7/1. The dependence of silica aerogel density on the base-catalyst
amount for a fixed molar ratio of TEOS/EtOH/H2O is plotted in Figure III.11.
Figure III.11 Density of two-step silica aerogel versus the concentration of base catalyst
It is seen from the Figure III.11 that the aerogel density increases as the concentration of
catalyst increases. This is due to the fact that at higher concentrations the colloidal particles
and pores are smaller and therefore the gels tends to shrink and become denser [23, 24]. It
was observed that a very important factor to produce non-cracked two-step aerogels is to
soak the gels in a solution with water and basic catalyst dissolved in ethanol. This solution
enhances the aging of the gels. When skipping this step, highly cracked aerogels were
obtained. In conclusion, two-step process appears to be the best synthesis method to obtain
low-density, non-cracked, and transparent aerogels.
Figure III.12 Transparent ‘two-step’ silica aerogels.
10-2
10-1
0,02
0,03
0,04
0,05
0,06
0,07
0,08
den
sity
(g/
cm3 )
log10
(concentration of the catalyst (NH3))
Chapter III. Bulk silica aerogel characterization 94
2. SURFACE AREA MEASUREMENTS BY BET (BRUNAUER, EMMET
AND TELLER)
One important aspect of the aerogel pore network is its open nature. In a closed-pore
material, gases or liquids cannot enter the pore without breaking the pore walls. Instead, with
an open-pore structure, gases or liquids flow through the entire material. The IUPAC
classification for the pore size is given by: Macropores, when pore diameter, pore, is larger than
50 nm (0.05 m). Mesopores: when 2 nm pore 50 nm, and Micropores when pore is smaller
than 2 nm.
Silica aerogels possess a distribution of pores sizes in the micro, meso and macropores
regimes. However, the majority of the pores fall in the mesopore range, with relatively few
micropores.
To get an inside knowledge of the pore structure of aerogels several techniques are used:
Small angle X-ray scattering, SAXS, maybe the most useful one for such characteristics and a
lot of articles deal with it [25, 26]. Unfortunately, we were not able to have access to this
technique.
BET (Brunauer, Emmet and Teller) is the most widely available and utilized method for
determining aerogel porosity [27]. A detailed description of this method can be found in
annex III. This technique successfully accounts for pores below about 100 nm. Thus, in
aerogels with typical pore sizes in the 1-1000 nm range, only a fraction of the total available
pore space is detected. However, microporosity information can be obtained through
mathematical analyses of BET technique such as t-plots or the Dubinin-Radushevich method
(Annex III). Gas adsorption can not effectively determines macroporosity.
In this technique, nitrogen at its boiling point is adsorbed on the solid sample. The amount
of gas adsorbed depends of the size of the pores within the sample and on the partial
pressure of the gas relative to its saturation pressure. By measuring the volume of gas
adsorbed at a particular partial pressure, the Brunauer, Emmet and Teller equation gives the
specific surface area of the material and the pore size distribution of the sample. The pore
size distribution used in this work was determined using the Kelvin equation in the analysis
of the nitrogen adsorption/desorption curves and the parameter C is obtained by analyzing
the standard 2 parameter BET isotherm gives the amount of gas adsorbed as a function of
the relative pressure of the adsorbing gas:
Chapter III. Bulk silica aerogel characterization 95
)P/P()1c(1)P/P1(
)P/P(c
V
V
00
0
m
(Eq. III.6)
where:
V = Volume of gas adsorbed at pressure P, Vm = Volume of gas covering the surface with a
monomolecular layer, n/nmono is the ratio of the moles adsorbed to the moles adsorbed in a
single monolayer. Po = Saturation pressure of the gas (vapor pressure), i.e. the pressure of the
gas in equilibrium with bulk liquid at the temperature of the measurement and C is a constant
for the gas/solid combination, C=(slope+Yintercept)/Yintercept.
The isotherm can be converted to a linear form for ease of extracting the values of Vm and C.
The constant C represents the relative strengths of adsorption to the surface and
condensation of the pure adsorbate (see annex III).
BET porosity characterization of the aerogel samples was performed with a Micromeritics
ASAP 2000 instrument. The BET surface area, SBET, pore volume, VBET, mean pore diameter,
< pore>BET, and C parameter were obtained from the adsorption isotherm of N2 at 77K. The
samples were outgassed overnight at 300 C.
The porosity of several TMOS aerogels, A1, A2, A2CO2, A3, A4, M, and M+2%C was
characterized by BET method following the protocol described in Annex III. Surface area,
SBET, total pore volume, VBET, and mean pore diameter, < pore>BET, measured by BET, are
presented in Table III.10. It can be observed that all samples show surface area between 410
and 630 m2g-1.
Table III.10 BET measurements of A-series and M aerogels. Apparent density, porosity, BET surface area, SBET, total volume of
pores, VBET, mean pore diameter measured by BET, < pore>BET =
4VBET / SBET, and C parameter.
Sample
(g/cm3)
Porosity
(%)
SBET
[m2/g]
VBET
[cm3/g]
< pore>BET
(nm)
C
A1 0.08 0.01 96 470 1.0 10 57
A2 0.15 0.02 93 420 2.7 30 46
A2CO2 0.24 0.02 89 551 2.3 17 113
A3 0.25 0.02 88 560 2.1 20 51/58
A4 0.27 0.03 86 414 3.2 20 56
M 0.13 0.01 93 589 5.7 50 37
M+2%C 0.12 0.01 94 632 5.7 34 40
Chapter III. Bulk silica aerogel characterization 96
The C value showed in Table III.10 gives a quantitative method to evaluate the
hydrophobicity of the aerogel samples, smaller C values means larger hydrophobicity.
The pore size distributions of the aerogels are estimated by applying the Pierce method [32]
to the measured absorption isotherms. Some examples of the pore size distributions obtained
by BET are shown in Figure III.13.
Figure III.13 Incremental pore volume for A-series, A1, A2, A3, A4 and M aerogels obtained by BET.
From the pore size distribution of Figure III.13 it was observed that for the samples A1
(lowest density, largest opacity), no peak was observed in the mesopore region. In the aerogel
prepared at the composition of A4 (largest density, most transparent from A-series), a large
fraction of pores has a size in the range of 10 - 100 nm. A2 and A3 aerogels shows that only a
fraction of the pores is accounted by BET. For the methanol aerogels, a strong and narrow
increment in the range of 40 - 90 nm was observed. It should be noticed that the mean pore
diameter values shown in Table III.10 were calculated using de VBET, which only measures
100 1000
0
A1
A2
A3
A4
M
Incr
emen
tal
Po
re V
olu
me
(cm
3/g
)
Mean Pore Diameter (Å)
10
Chapter III. Bulk silica aerogel characterization 97
the pores in the mesopore range, so in the case of BET measurements the mean value
< pore>BET will be working successfully only for aerogels with a porosity included exclusively
below 100nm [34, 35].
By plotting the incremental surface area versus pores diameter, we can obtain more
information about microporosity. A clear presence of micropores is shown in the incremental
surface area plot (Figure III.14). For example, in A1 aerogel surface area are slightly increases
when we are close to the micropore range. In this sample, there was no presence of peak in
the mesopore range and increased very quickly when close to micropore range, meaning that
some pores are out of the measurable BET range.
100 1000
0
50
100
150
200
250
Incr
emen
tal su
rfac
e ar
ea (
m2/
g)
Mean Pore Diameter (A)
Figure III.14 Surface area increases for A1 aerogel. The value when close to micropore range was increasing.
If the pore volume is calculated based in the silica aerogel density:
2SiOAerogel
pore
11V (Eq. III.6)
Most of the aerogels showed larger pore volume values than that obtained by BET. Then the
V= Vpore -VBET will indicate approximately the volume of porous not measured by BET
(pores diameter smaller than 2 nm and larger than 50nm), as well as the percentage of micro
and/or macropores. The obtained results are also shown in Table III.11. In addition, using
the pore volume obtained by density, the mean pore diameter can be calculated by:
BET
pore
densityporeS
V4 (Eq. III.7)
Chapter III. Bulk silica aerogel characterization 98
Table III.11 Comparison of pore volume and mean pore diameter obtained by BET analysis and by density measurements for the A-series silica aerogels (TMOS).
Sample Vpores
[cm3/g]
VBET
[cm3/g]
Volume of
micro+macropores*
< pore>density
(nm)
< pore>BET
(nm)
A1 12.0 1.0 90% 110 10
A2 6.2 2.7 60% 60 30
A3 3.5 2.1 40% 30 20
A4 3.2 3.2 0% 20 20
M 7.2 5.7 20% 60 50
* Excluding mesopores, pore range being measured by the BET method.
Main discrepancies were observed when the obtained mean pore diameters are compared
with the real one obtained from the own density of each sample. For A1 and A2 aerogels the
differences are associated at the presence of macropore because the opaque aspect of the
samples (Rayleigh effect). This phenomenon appears when the scattering center, the pore,
has a diameter close to the visible wavelength, around 500nm.
From these data, one might conclude the following:
i) The opacity in A1 could be explained by the large value of V which would correspond
approximately to the macropores not measured by BET (~90%).
ii) A2 and A3 have a pore size distribution with an important number of micropores and/or
macropores, but the mean pore diameter obtained is within the mesopore range.
iii) A4 presents a mesopores distribution, which is totally accounted by the BET technique,
and agrees with the mean pore value obtained by optical measurements.
vi) M have a pore size distribution that is mostly accounted by BET (~80%).
On the other hand, the clear transparency of monolithic aerogels gives an upper limit for their
pore size of about 100 nm [33]. The translucence of some aerogel samples indicates the
presence of macropores, although they are not accounted by the BET technique. In order to
evaluate the effect of the macropores on the transparency, light transmission (LT) experiments
were performed (see section Rayleigh scattering).
The porosity of several silica aerogels produced with TEOS as metal alkoxide precursor, by
one and two-step synthesis was characterized by BET method following the protocol
Chapter III. Bulk silica aerogel characterization 99
described in Annex III. Next Table gathers the surface area; total pore volume, and mean
pore diameter measured by BET. It can be observed that all samples show larger surface area
than TMOS aerogels, between 700 and 1012 m2g-1. These differences can be originated by the
addition of the washing step in those TEOS aerogels.
Table III.12 BET measurements of TEOS aerogels by one and two-step synthesis. Apparent density, porosity, BET surface area, SBET, volume of pores, VBET, means pore diameter and C parameter measured by BET.
Sample
(g/cm3)
Porosity
(%)
SBET
[m2/g]
VBET
[cm3/g]
< pore>BET
(nm) C
TE00AA
(1/7/5)
0.115 94.7 780 4.8 22 48
TE00AF
(1/7/5)
0.107 95.1 812 4.8 18 60
H501AI
(0.08)
0.075 96.9 946 5.7 24 90.8
H501AD
(0.1)
0.080 96.2 899 4.9 21 95.2
It is observed that two-step aerogels shows a larger surface area (between 890 and 950 m2g-1)
than one-step aerogels.
3. INFRARED SPECTROPHOTOMETRY, IR
The infrared spectroscopy is generally used for the characterization and identification of
organic compounds or functional groups of those. Infrared spectroscopic methods have
provided considerable information about the surface and structure of silica [36-39].
In this section, the acetone series (A1, A2, A3 and A4) and the methanol aerogel series (M
synthesis) were characterized using IR spectrophotometry, a Fourier Transform Infrared
Spectrometer Nicolet 710 was used. The IR spectrometer settings were 50 scans, 16.0 cm-1
resolution, and ratio mode. The spectra were registered in a wavelength range of 400-4000
cm-1. A beam path background spectrum was recorded without sample. Then, the
background was subtracted for each IR spectrum to eliminate the vibrations due to air. The
IR samples were prepared by making a KBr tablet, approximately 1% weight mass of aerogel
mixed with KBr powder were placed inside a cylindrical stainless steel die. The tablet (wafer)
was compacted with a pressure of 1000 Kg/cm2, for 20 s in a Carver press. The wafer was
inspected for cracks or holes before fit the tablet in the IR holder.
Chapter III. Bulk silica aerogel characterization 100
Figures III.15, III.17 and III.18 show the IR spectra of M, A1 and A2 aerogels. The main
common features of these spectra are: two bands around 3500 cm-1 and 1600 cm-1
representing adsorbed water, and the bands around 2950 cm-1 associated to C-H groups. The
1080, 800, and 460 cm-1 bands are attributed to different modes of Si-O or O-Si-O vibrations
and the band around 960 cm-1 correspond to stretching vibrations of Si-OH.
3.1 METHANOL SERIES
Figure III.15 shows IR spectrum for the methanol aerogels, M, synthesized using TMOS, Si
(OCH2)4, water and base-catalyst (NH4OH).
Si-O-Si symmetric
464.8
559.3
808.1 958.6
1633.62854.4
2950.9
3438.8
Si-OH
Si-O-Si bending
C-O
OH- fromH2O
CH- frommethanol
M
20
40
60
%T
500100015002000300040001/cm
1091.6O-Si-O asymmetric
Figure III.15 IR spectrum of methanol aerogel (M).
The curve in Figure III.15 is the infrared spectrum of aerogel M. Several important bands
were observed. The adsorption bands identified characterizing the SiO2 aerogel were:
1200-1000 (1092) cm-1 : O-Si-O asymmetric stretching mode.
808 cm-1 :Si-O-Si symmetric stretching mode.
465 cm-1 : Si-O-Si bending mode.
These three IR adsorption peaks correspond to the different modes of silica [40].
959 cm-1 : Si-OH: stretching mode. Si-OH is a quite visible peak, indicating the
hydrophilic nature of the M silica aerogel. This result can be confirmed by the
C value obtained for this sample by BET.
Other identified adsorption bands are:
atmospheric CO2
atmospheric H2O
Chapter III. Bulk silica aerogel characterization 101
3439 cm-1: A broad band appears corresponding to water
1634 cm-1: Atmospheric H2O band that overlaps with the SiO-H band.
Both of these bands are related to adsorbed molecular water, indicating
again the hydrophilic nature of the sample.
2850 -2950 cm-1: C-H (O-CH2) methanol bonds. The bands of adsorbed
methanol are assigned to the symmetric and antisymmetric stretching
vibrations of the C-H bonds of residual methanol.
An IR spectrum complement is obtained using UV-VIS- near IR spectroscopy technique. In
a similar way of IR spectroscopy, absorption bands were obtained in the range of visible,
ultraviolet and near infrared. Figure III.16 shows a complete UV-VIS- near IR spectrum
from M aerogel.
500 1000 1500 2000 2500 3000
0
20
40
60
80
100
Tra
nsm
issio
n (
%)
Wavelength (nm)
Figure III.16 UV – VIS - Near IR Transmission spectrum of base-catalyzed TMOS aerogel M (sample 1 cm thick)
The intrinsic absorbance of silica is low in the visible region. It can be observed the low
intrinsic adsorption in the visible range (300-900 nm) resulting in a 100% of transmission in
the visible range. As wavelengths become progressively shorter, scattering increased,
eventually cutting off transmission near 300 nm. Weak absorbance begins to appear in the
near infrared, and again cut off transmission around 2700-3200 nm. There is then a "visible
UV VIS Near-IR IR
Chapter III. Bulk silica aerogel characterization 102
window" of transmission through silica aerogel that is an attractive feature of this material for
day lighting applications. As the spectrum moves into the infrared, scattering becomes less
important, and standard molecular vibrations account for the spectral structure. The
observed bands in the frequency range of the infrared and near infrared were:
vibrations of the ‘stretching’ modes of the SiO-H bonds at 7390 cm-1 (2700-
3200 nm).
hydrogen bond of the water at 5476 cm-1
SiO-H stretching and bending modes 4598 cm-1. The analysis of the UV-VIS
spectrum shows that the formation of SiO2 is not completed because the
presence of SiO-H groups.
C-H stretching and bending modes at 4233 cm-1
There is a region of high infrared transparency between 3300 and 2000 cm-1.
This allows a certain amount of thermal radiation to pass through silica aerogel
and lower its thermal insulative performance. Addition of additives that absorb
radiation in this region can remedy this problem
3.2 ACETONE SERIES
Figure III.17 shows IR spectrum of the A2 aerogel:
Si-O-Si symmetric
Si-OH
O-Si-O asymmetric
0
25
50
75
100
%T
50010001500200030004000
1 / c m
470.6
804.3
968.2
1103.2
1639.42358.8
3425.3
A2
OH- fromH2O
contamination C-O
Si-O-Si bending
Figure III.17 IR spectrum of A2 acetone aerogel.
atmospheric CO2
atmospheric H2O
Chapter III. Bulk silica aerogel characterization 103
No large differences were observed compared to M aerogels meaning that the A-series silica
skeleton composition is not much different from that one from M aerogels. The C-H peak at
2950 cm-1for A2 sample, present at M samples, did not appear indicating that no acetone
remains in the A2 aerogels.
Figure III.18 compares the spectrum for each of the acetone-series aerogel.
0
25
50
75
100
%T
500100015002000300040001/cm
A2
A1
A4
A3
Figure III.18 IR spectra for all the aerogels from the acetone-series, A1, A2, A3 and A4.
The four spectra did not present remarkable qualitative differences. The analysis shows that
the formation of SiO2 is not completed because the presence of SiO-H groups at 3820 cm-1.
The CO2 atmospheric band at 2360cm-1 (A2 and A4) depends on the quality of the
background extraction.
4. ULTRAVIOLET-VISIBLE (UV-VIS) SPECTROSCOPY
4.1 AEROGEL TRANSPARENCY
Depending on the preparation conditions silica aerogels may appear transparent, translucent
and opaque. They are transparent when the sizes of pore and particle are smaller than the
wavelength of light, and they are homogeneously distributed. The key to control the
inhomogenities (implying pore and particle size) of the aerogels lies in the sol-gel stage and in
the supercritical extraction process. The amount of light scattered from an aerogel depends
mainly on structural inhomogeneities smaller than the wavelength of visible light [41].
Therefore, transparent aerogel transmits rather than scatter light.
Chapter III. Bulk silica aerogel characterization 104
Figure III.19 shows a photograph of the aerogels produced in our laboratory, ordered from
the most opaque to the most transparent ones depending on the different sol compositions.
These aerogels have been optically characterized in this section by using UV-VIS
spectroscopy.
Figure III.19 Photograph of silica aerogels obtained under different sol-gel conditions. The acetone series from A1 to A4, and the methanol sample, M. Its degree of transparency is usually related with the presence of inhomogeneities, amount and size of macropores (section Rayleigh scattering).
In this section, with a view to understand the opacity and transparency of the aerogels, the
light transmission in the ultraviolet-visible (UV-VIS) wavelength was measured using a
Shimadzu UV/VIS UV-2102 spectrometer. The apparatus was equipped with light sources
covering the ultraviolet-visible wavelength range from 300-900 nm. The light transmission
(LT) is the amount of light with a fixed wavelength that is transmitted through the aerogel
without being scattered in other directions. Then, the maximum of LT corresponds to a
completely transparent aerogel where all the incident light is transmitted in the same direction
that was incidented. See Figure III.20
Figure III.20 UV/VIS spectrometer scheme used to measure light transmittance.
Sample
Detector
Lens Lens
Ligth
source
Chapter III. Bulk silica aerogel characterization 105
Aerogel tiles were carefully cut into samples with thickness of 1 cm, and with parallel sides.
Then, the incident, Ii, and transmitted, It, intensity of a monochromatic beam with a fixed
wavelength were measured. The optical transmittance, T, is defined as the ratio between
transmitted and incident intensities (with a fixed wavelength):
T = It/ Ii (Eq. III.8)
The transmittance, T, is related to the aerogel thickness, x, as:
Where the constant k is related to the sample structure. Therefore, from the k value obtained
it may be possible to extract some structural information. The optical absorbance, A, is
defined as:
Figure III.21 shows the experimental absorbance obtained for different synthesized samples.
Figure III.21 Absorbance versus wavelength at UV-VIS range for four acetone aerogels, A-series, and a methanol aerogel, M.
)10.III.Eq(kxTlnA
)9.III.Eq(eT kx
0
1
2
3
4
5
375 475 575 675 775 875
wavelength (nm)
Ab
so
rvan
ce
A1
A4
A3
A2
M
Ab
sorb
ance
(a.
u)
Chapter III. Bulk silica aerogel characterization 106
For all aerogel samples it was observed that the absorbance was larger at blue wavelength
(358nm) than at red (635nm). It may be used to quantify the red shading of aerogels as the
transmitted light is passed through a silica aerogel and the slight bluish haze when an
illuminated piece is viewed against a dark background. The optical transmission of the
aerogels was measured at a wavelength of 900 nm in order to compare quantitatively the
degree of transparency of the samples at the visible range. The results are presented in Table
III.13.
Table III.13 Optical transmission at 900nm for 1 cm thick aerogels.
Sample Transmission
at 900nm [%] Transparency
A1 23 Opaque
A2 28 Translucent
A3 30 Translucent
A4 40 Transparent
M 70 Very transparent
It was observed that the aerogels prepared using methanol as a solvent and under basic
conditions are more transparent than other solvents. This may be due to the fact that as size
of the alkoxy group increases, steric hindrance occurs, then when acetone was used as a
solvent instead of methanol leaded to larger pores, less homogeneity and hence a decrease in
transparency of the aerogels.
In Figure III.22 was observed that the amount of precursor strongly affects the optical
transmission due to the structural change of pore and particles of the aerogel. The lowest
transmission was obtained when more diluted sol conditions (A1), the percentage of
transmission was of 23% for a sample with a thickness of 1 cm. In this plot the thickness of
the aerogels are not normalized. A1 (x=0.75cm) thickness is smaller than A2, A3, A4 and M
(x=1cm). The large size pores may be responsible for the opaque nature of A1 aerogel.
Chapter III. Bulk silica aerogel characterization 107
Figure III.22 Percentage of optical transmission (at 900 nm) vs. TMOS volume ratio, v, for the acetone series with H2O/TMOS ratio fixed at 4.
In next section, the percentage of transmitted light will be related to the diameter size of the
porous, larger porous leads to lower transmission.
4.2 RAYLEIGH SCATTERING
Most of the light that our eyes receive comes not directly from the light source but comes
through the scattered light (light that reaches our eyes in an indirect way). The phenomenon
of scattering leads to several well-known natural effects, such as blue skies, red sunsets, the
white (or gray) color of clouds, and poor visibility on foggy days [42, 43]. Mie developed a
very complete theory for scattering spheres of arbitrary size using electromagnetic theory
[44].
As mentioned before, in silica aerogels the Rayleigh effect is observe by the reddening of
transmitted light (red light has a longer wavelength, and is scattered less by the fine structure
of aerogels) and the blue appearance of the reflected light of silica aerogels.
Scattering results from the interaction of light with inhomogeneities in aerogel structure. The
actual entity that causes scattering, called the scattering center, can be as small as a single large
molecule (with an inherent inhomogeneity) or clusters of small molecules arranged in a non-
uniform way. However, scattering becomes more effective when the size of the scattering
center is similar to the wavelength of the incident light. This occurs in small particles (~400-
0,1 0,2 0,3 0,4
20
25
30
35
40
vTMOS
/vTMOS+acetone
Tra
nsm
issi
on
at
900n
m [
%]
A1
A2
A3
A4
Chapter III. Bulk silica aerogel characterization 108
700 nm in diameter for visible light) that are separated from another, or by larger,
macroscopic, particles with inherent irregularities. When scattering centers are smaller in size
than the wavelength of the incident light, scattering is much less effective. In silica aerogels,
the primary particles have a diameter of ~2-5 nm, and do not contribute significantly to the
observed scattering. However, scattering does not necessarily arise from solid structures.
There is in silica aerogels, a network of pores, which can act, themselves, as scattering
centers. The majority of these are much smaller (~20 nm) than the wavelength of visible light
(see section 1 on Porosity). There are, however, invariably a certain number of larger pores
that scatter visible light. Control of the number and size of these larger pores is, to a certain
degree, possible by modifying the sol-gel chemistry used to prepare the aerogel. As scattering
efficiency is dependent on the size of the scattering center, different wavelengths will scatter
with varying magnitudes.
In this section, a method is proposed to quantitatively measure the relative contributions of
Rayleigh scattering (21-23) and the wavelength-independent transmission factor (due to
surface damage and imperfections) for silica aerogels prepared with different recipes and/or
drying procedures.
The transmission spectrum of an aerogel slab of known thickness is given by:
4
Cx
AeT (Eq. III.11)
Where:
T = transmittance
A = wavelength independent transmission factor,
C = intensity of Rayleigh scattering,
x =sample thickness,
= wavelength of the incident light
Then, the transmission is plotted against the inverse fourth power of the wavelength and A
and C parameters can be determined by fitting to the equation III.11. Aerogels with a high
value of A and a low value of C will be the most transparent.
Chapter III. Bulk silica aerogel characterization 109
Figure III.23 Light UV/VIS wavelength transmission and Rayleigh scattering fitting. The data are fitted by lnT=lnA-
C.x/ 4.
Table III.14 lists A and C values of A-series and M aerogels obtained from the optic
transmission at ultraviolet-visible range, wavelength between 300 and 900 nm.
Table III.14 A and C parameters from the analysis of light transmission
Sample A C (nm-3)
A1 3.62 1.70 1011
A2 3.81 4.41 1010
A3 3.96 3.73 1010
A4 3.67 3.51 1010
M 4.17 1.27 1010
1,0x10-11
2,0x10-11
3,0x10-11
4,0x10-11
0
2
4M
A4
A3
A2
A1
A1
A2
A3
A4
M
ln (
Tra
nsm
issi
on
)
nm4
Chapter III. Bulk silica aerogel characterization 110
The fit of the absorbance data in Figure III.23 confirms that Rayleigh scattering contributes to
the extinction of the light through an aerogel. The fitting parameter C is a measure of the
transparency of the aerogel, for lower values of C, more transparent are the aerogels. The
most transparent aerogel, M, has a C value of 1.27 1010 while the most opaque aerogel, A1, has a C
value of 1.7 1011. A is the wavelength-independent transmission factor that gives the
contribution of the surface effects on the optical transmission of visible light (due to surface
damage and imperfections). Besides the wavelength factor, the scattering intensity depends of
the pore size distribution. It is the absence of macropores, which is primarily responsible for
the lower light scattering and therefore higher visible transmission for M sample. However,
the light scattering measurements do not provide much information about the size of primary
particles, which are too small compared to the light wavelength. Section 5 will use the
dependency of visible light versus scattering angle to get further structural information.
4.2.1 A model to interpret the porous aerogel structure using
Rayleigh scattering
In this section, the correlation of scattering measurements with other structural investigations
(BET) has been attempted. The principal idea is to correlate the pore information obtained
by using the BJH pore size distribution (that determines the mesoporosity), with the Rayleigh
scattering measurements (that determines the macroporosity). Making use of both results,
one can obtain information on a wider range of pore sizes.
Silica absorbs only slightly in the visible and near ultraviolet, so most of the optical
attenuation results from Rayleigh scattering. For a material with pore structures in the 1 to
100nm size range, the strong Rayleigh scattering is expected towards the blue and ultraviolet
spectral region. The amount of scattered light in aerogels mainly depends on the number and
size of the pores. Thus, the optical transmission curve (see Figure III.21) is analyzed for
Rayleigh scattering and the data are fitted to the equation:
x).(eAT (Eq. III.12)
Where:
T = transmittance at the wavelength ,
A = wavelength independent transmission factor,
= wavelength of the incident light
( ) is the volumetric coefficient of scattering.
Chapter III. Bulk silica aerogel characterization 111
One might assume independent Rayleigh scattering from spheres with index of refraction n.
A characteristic size for the scatter centers (porous) can be derived from the C value using
the expression for the volumetric coefficient of scattering valid for an isolated spherical
scatter center with diameter < pore>Rayleigh:
2
2
2
4
43
Rayleigh
2n
1n)(8)1()( (Eq. III.13)
where is the volume fraction of air. The index of refraction for low-density aerogels is close
to one and was estimated with [22]:
)/(21.01 3cmgn (Eq. III.14)
Mean porous diameter derived from this calculation, < pore>Rayleigh, for the different samples
are shown in Table III.15. In order to show the discrepancies between mean pore diameters
obtained by the different analysis, the obtained mean pore diameters obtained by Rayleigh
scattering (Eq. III.13) were compared with the one obtained from the density of each sample
and from the BET analysis.
Table III.15 Mean pore values evaluated by three different
methods are compared, by BET, < pore>BET, by Rayleigh scattering
analysis, < pore> Rayleigh, and by bulk aerogel density, < pore>density.
Sample < pore>BET
(nm)
< pore>Rayleigh
(nm)
< pore>density
(nm)
A1 10 150 110
A2 30 50 60
A3 20 15 30
A4 20 20 20
M 50 40 60
The most astonishing feature is the difference between the mean pore diameters for the A1
sample as being determined by BET and LT techniques. For the most opaque sample, A1,
the mean pore diameter, < pore>Rayleigh, derived from this calculation was of 150 nm. This
value was very different of that obtained by BET, < pore>BET =10 nm. The reason of these
discrepancies may be due that BET accounts only in mesopore range, and A1 aerogel have
most of their pores in the macropore range (calculated by Rayleigh scattering). From
Chapter III. Bulk silica aerogel characterization 112
analyzing the differences in the data obtained using the different techniques one might obtain
the mean pore size distribution proposed in Figure III.24.
Figure III.24 Proposed pore size distribution for A1, A2, A3, A4 and M aerogels obtained by BET, Rayleigh scattering and density analysis.
The model accounts for the observed differences in the optical transparency of the aerogels:
the opacity in A1 is explained by the pore size distribution at macropore range (confirmed by
UV-VIS measurements). In addition, that clarifies why the A1 porosity was not measured by
BET. A2 and A3 have a bimodal pore size distribution with an important number of
micropores and/or macropores, but the mean pore diameter obtained is within the mesopore
range. That explains that BET measures only a part of the porosity. A4 presents a mesopores
distribution, which is totally accounted by the BET technique, and agrees with the mean pore
value obtained by optical measurements. M, completely transparent, have a pore size
M
A4
A3
A2
A1
Pore size (nm)
BET range
Mesopore range
1 10 100 1000
Chapter III. Bulk silica aerogel characterization 113
distribution that is mostly accounted by BET (~80%). Its mean value is similar for both
techniques, BET and optical measurements.
5. LIGHT SCATTERING MEASUREMENTS OF AEROGELS BY A
POLARIZATION-MODULATED NEPHELOMETER.
5.1 INTRODUCTION TO LIGHT SCATTERING VS ANGLE
EXPERIMENTS
Light scattering is a noninvasive and remote indirect method to derive structural information.
The experimental and theoretical development was carried out at the ‘Microstructured
Materials Group’ of the Lawrence Berkeley National Laboratory under the supervision of
Prof. Arlon Hunt and Dr. Michel Ayers. The objective of this work was to find the
correlation of the angular dependence of light scattering at visible regions with the structural
nature of the silica aerogel medium. In the previous section (Rayleigh scattering), it has been
proved that intensity of light measurements at fixed angle provide information about the size
of scattering centers. Few previous works on angular measurements at different wavelength
have been carried out for aerogels characterization. This section will demonstrate the
necessity of performing light scattering vs. angle measurements to obtain information outside
of the Rayleigh regime and to extract information about the inhomogeneities of the aerogel
microstructure. Since the nephelometer set up was not prepared to measure aerogels, and
very few articles about this field are published, the work in this section presented on
nephelometer characterization shows preliminary results indicating the advantage of using
this technique on supplementary structural information. Further studies should be
undertaken in order to optimize the use of this technique.
This section was structured in four sections:
1. Synthesis and drying of several transparent aerogel samples by the two-steps method
(explained in detail in chapter I and II).
2. Light scattering measurements of synthesized aerogels by a polarization-modulated
nephelometer (section 5.2)
3. Development of a model that gives structural information from the dependency of the
scattered light versus angle and wavelength (section 5.3).
4. And, finally the comparatives study between experimental measurements and theoretical
model (section 5.4).
Chapter III. Bulk silica aerogel characterization 114
Optical characterization of the aerogels was performed using scattering apparatus in which a
laser beam interacts with the specimen and a detector is rotated around the irradiated volume
section. Structural units comparable to the wavelength of the laser light cause a strong
isotropic scattering, and entities, which are smaller than the wavelength, cause more or less
isotropic scattering depending of the homogeneity of the microstructure.
5.1.1 Description of the polarization-modulated nephelometer
The polarization-modulated angle-scanning nephelometer apparatus, illustrated in Figure
III.25, was constructed to measure the wavelength, angular, and polarization dependence of
the scattered light intensity at visible regions. Three lasers were used, a first one was tunable
from 450 to 540 nm (from green to violet), a second one provides a fixed 635 nm red beam,
and the last one a ultra-violet beam at 355 nm. The wavelengths used for the scattering
measurements were of 355 nm, 458 nm, 488 nm, 514 nm and 635 nm provided the source of
light.
Figure III.25 Photograph of the nephelometer apparatus when silica aerogel light scattering is measured at blue wavelength
It was critical for correct measurements, the alignment of the detection optics and to ensure
that the scattering volume seen by the detector was at the center of rotation.
A cylindrical aerogel sample was correctly placed in the nephelometer taken attention to the
correct alignment with the incoming laser beam in order to diminish the shape and
geometrical effects on the detected light scatter intensity. Then, the incoming beam polarized
at an angle of 45 to the scattering plane, was focused into the aerogel, with plano-convex
aerogel sample
polarizer
laser beam
laser
Photomultiplier tube
Chapter III. Bulk silica aerogel characterization 115
lens. A rotatory detector was used to pick up the intensity of the scattered light at each angle
from 10 to 170 . The rotating arm carries collimation optics, a photomultimeter, and
polarizers oriented either parallel or perpendicular to the scattering plane. The rotating stage
is controlled by a personal computer. A scheme of the experimental apparatus of this set-up
is shown in Figure III.26.
Figure III.26 Schematic diagram of the angle-scanning nephelometer.
The formalism used to describe the polarization states of scattered light is based on the
Stokes vector [48]. The Stokes vectors describing incident and scattered light are connected
by a 4x4-element Mueller matrix, Si,j [48, 49]. I’=MI where the M is a 16-element Mueller
matrix, I is the Stokes vector of the incident light and I’ is the Stokes vector of the scattered
light. The information that can be gleaned from each element is dependent on the scattering
system. Depending on the polarizer’s configuration different elements of the Mueller matrix
can be obtained. In our experiments only S11 were analyzed because are those related with
the scattering intensity [48].
5.2 EXPERIMENTAL RESULTS
Scattering measurements were performed on aerogels following the above-described
conditions. In this section, it is proposed that a fraction of the aerogel scattering might be
due to the density fluctuations in the material on a scale range larger than the pore size [50,
51]. The predicted scattering from inhomogeneous two-phase materials will be described by
following the proposed hypothesis. Next figure shows the intensity of scattered light versus
scattering angle from a methanol aerogel sample, M, at different wavelength.
Chapter III. Bulk silica aerogel characterization 116
Figure III.27 The total intensity S11 on a log10 scale as a function of scattering angle for M aerogels at 4 different wavelengths. In each measure, the scattering angle varies from
10 to 170 . Intensity was arbitrary normalized at =90 and the geometrical factor for horizontally polarized incident light
was subtracted, 2
11
cos1
s, for each intensity.
The results given in Figure III.27 are the experimental results of the dependency of scattered
intensity versus scattering angle, , for a M aerogel. Each curve is the average of three
reproducible experimental data. The laser beam have different incident intensity for each
different wavelength, in the case of methanol sample was arbitrary normalized at 90 . The
angular dependence of the scattered intensity varied when using different wavelength. This
dependency is the most fundamental measurement for sizing and will be used in section 5.3
to obtain structural information of the aerogels.
The measurements of intensity versus h, )2/(sin4h , are called optical structure factor
measurement and leads to remark the variations in scattered intensity and quantify the
analysis. Figure III.28 shows the experimental s11 data for M aerogel plotted as a function of
h.
0 20 40 60 80 100 120 140 160 180
100
1000
10000
M aerogel
nm514
nm488
nm635
nm458
s 11/(1
+co
s2)
Chapter III. Bulk silica aerogel characterization 117
Figure III.28 Scattered intensity versus h, )2/(sin4h ,
from M silica aerogel for four different wavelength.
This experimental dependency will be used in next sections to calculate the density-density
correlation function. The intensity versus angle was also studied in samples with different
density and porosity using a fixed wavelength. Figure III.29 shows the dependency of the
scattered light for M, A4 and A3 for 635 nm wavelengths. The intensity of the scattered light
for A1, A2 aerogels was too large to get any useful data.
Figure III.29 Scattered intensity versus h for M, A3, and A4 silica aerogels at fixed wavelength: 635 nm. The intensities are not normalized.
0,0000 0,0005 0,0010 0,0015 0,0020 0,0025
1E-3
0,01
0,1
M
A3
A4
s 11
h
0,0000 0,0005 0,0010 0,0015 0,0020 0,0025 0,0030
100
1000
10000
M aerogel
nm458
nm488
nm514
nm635
s 1
1
h=4 sin( /2)
Chapter III. Bulk silica aerogel characterization 118
As treated in the Rayleigh scattering section, the intensity versus wavelength gave us some
information about pore size distribution, but if we want to get additional characterization of
the density inhomogeneities in the aerogels, the angular Rayleigh effect caused by the scatter
centers should be normalized, sin
1 .
5.3 STRUCTURAL INFORMATION FROM THE LIGHT SCATTERING
MEASUREMENTS
A model to interpret the experimental results may allow extracting structural information of
the aerogels.
5.3.1 Inhomogeneous media
In treating the scattering by a particle, it is assumed that the medium where the particle is
comprised is optically homogeneous. However, extended continuous media scatters light,
indicating that the solid material contains inhomogeneities. The problem of characterizing
inhomogeneities in solids and of relating these to the scattered intensity has been treated by
Debye using the Rayleigh-Debye theory [52].
In this section, the Rayleigh effect caused by the presence of ‘discrete’ scatter centers will be
normalized. By the scattering, we will obtain information about the inhomogeneities of the
aerogel media by treating the case assuming that the inhomogeneity of the medium is due to
a continuous variation of the dielectric constant. The macroscopic property of the solids
appears to be uniform. However, the dielectric constant varies from point to point, then the
dielectric constant at point A is given by [53]:
AA (Eq. III.15)
Where is the average values and A a local variation at the point A.
The study of the local variation A is related to the determination of the inhomogeneities of
the media. The correlation distance, BA
, is the average extension of the inhomogeneities.
In order to visualize the correlation distance, one can consider the product of the fluctuations
A and B at two points A and B separated by a distance r. The average of this product for all
points of the solid will depend upon the distance r. For r=0, BA is obviously equal to the
mean square value of the fluctuation 2 . If the scattering medium is statistically uniform and
Chapter III. Bulk silica aerogel characterization 119
isotropic, the correlation function will depend only upon the magnitude r, and will vanish for
sufficiently large r, 0BA.
The correlation function is defined by:
2
BA)r( (Eq. III.16)
Figure III.30 shows a scheme that may help to understand the behavior of the correlation
function in a two-different medium as aerogels are. Aerogels can be considered a physical
mixture of two media: the silica solid skeleton, medium A, and the pore network: medium B.
Figure III.30 Sketch describing the two aerogels media: the silica solid skeleton, medium A, and the pore network: medium B. To understand the variation on density-density correlation function the two figures may be progressively superposed, then density-density correlation function may account for the ‘self-similitude’ of the aerogel microstructure.
The density-density correlation function can be visualized with the aid of Figure III.30 that
represents the variations in density when left and right figures were superposed giving a
scheme about the ‘self-similitude’ of the microstructure. Then, an idea of the average
extension of the inhomogeneities is the steepness of the correlation function from 1 (r=0) to
0 (large r). A narrow shape (continuous line in Figure III.32) means short correlations: large
porous and more irregular structure. A broad shape (discontinuous line in Figure III.31)
means long correlation: smaller pores and structure that is more regular. If the pore size
distribution is very wide, then, the correlation density of the pore structure is too small to
contribute.
r
r
'
Medium A:
Silica skeleton
Medium B: Pore
Chapter III. Bulk silica aerogel characterization 120
Figure III.31 Density-density correlation function for two types of aerogel microstructure: the discontinuous line means long correlation (smaller pores and regular structure) and the continuous line means short correlations (large porous and more irregular structure).
The presence of the correlation function (r) in the scattered intensity equations permits one
to use the angular distribution of the scattered light to determine density-density correlation
in the aerogels medium and then, when analyzing this correlation function, allows to obtain
structural information. Following the Rayleigh-Debye mode the scattered light is defined by
[52]:
I 4
0
rr2
r( )sin h r( )
h rd
h 4
sin2
(Eq. III.16)
Where:
is the scattering angle.
is the wavelength of the laser beam.
(r) is the density-density correlation function.
Short range correlations: Rayleigh scattering
If the medium contains only short-range correlations in comparison with the wavelength of
the light (discrete scatter centers with a size similar of the wavelength). Then (r) vanishes as r
increases, in this case, the angular and wavelength dependence of the scatter intensity is the
described for the Rayleigh scattering (section 4), symmetrical about 90 and proportional to
-4.
r 5nm 200nm
Chapter III. Bulk silica aerogel characterization 121
Long range correlations: departures from Rayleigh scattering
When correlation (r) does not vanish for values of r comparable to the wavelength then, if
the correlation function (r) is known, the scattering can be predicted. For example, P.
Berdhald and A. Hunt assumed that the correlation function consists of a short-range
exponential part, 1(r), and a long-range gaussian part, 2(r):
22
2
1 a
r
a
r
wee)w1()r(2)r(1)r( (Eq. III.17)
Figure III.32 Density-density correlation used in the predictions for silica aerogels, consisting of a short-range exponential and a long-range gaussian part function
In aerogels, it can be assumed that short-range correlations will be related to the silica
nanoparticles, a1, and large-range correlation to the pore structure, a2. The parameter w is the
fraction of short to range correlation.
Substitution of the proposed correlation function (Eq. III.16) into the intensity equation (Eq.
III.17) and performing the Fourier transform of the intensity dependency on and , the
intensity resulted in the two components:
i i1 i2 (Eq. III.17)
Where:
i1 8 1 w( )a1
3
1 162
sin1
2
2
2a1
2
2
0 1 104
2 104
3 104
0
1.1
0.0
1
r( )
300000 r
a1 4000 a1
part icle radius
a2 porous radiusa2 500
22
2
1 a
r
a
r
wee)w1()r(
Chapter III. Bulk silica aerogel characterization 122
i2
3
2w a2 a2
2exp 4
2sin
1
2
2
2a2
2
Figure III.33 illustrates the dependency of the predicted scattered intensity for the proposed
model versus the scattering angle for each of the used laser. Various wavelengths will cause
different scattering intensity in the same aerogel.
Figure III.33 Theoretical angle dependency of the scattered
light intensity, i( , ), for each of the 5 laser wavelength.
Where: 1 = 355 nm ultraviolet wavelength, 2 = 458 nm
violet wavelength, 3 = 488 nm purple wavelength, 4 = 514
nm green wavelength, and 5 = 635 nm red wavelength.
5.4 COMPARATIVE STUDY BETWEEN EXPERIMENTAL
MEASUREMENTS AND THEORY
The experimental measurements of the angular distribution of the scattered light compared
with the theoretically predicted values of intensity. The experimental data have been fitted to
the obtained theoretical intensity function, i( , ), by the variation of the parameters a1, a2
and w. Rayleigh angular dependence and a wavelength-dependent intensity have already been
removed from both the experimental and theoretical expressions. The following procedure
was used to fit the proposed model to the experimental data:
0 50 100 1500
1 1010
2 1010
i 30 4580( ) 1.2
0
i 4880( )
i 5145( )
i 6350( )
i 4580( )
i 3550( )
1800
1
2
3
4
5
Chapter III. Bulk silica aerogel characterization 123
i) The experimental intensity/angle output file was appended into a matrix for posterior
fitting process. In the fitting process, the function readprint reads a data matrix into the
document from i( , ) file for each of the aerogel samples measured.
Figure III.34 Experimental data file for =514.5 nm of a M aerogel sample.
ii) Then, The MathCAD program has been used to find a linear combination of i1 and i2
functions that best fits to the experimental data. The proposed functions i1 and i2 were
entered in the vector F ion order to be fitted:
iii) For each wavelength, an independent file is obtained, so the same procedure to fit a1, a2,
and w parameters was repeated in for each of the wavelength experimentally used.
Figure III.35 compares the experimental curve with the curves fitted from the model
proposed for a M silica aerogel at 635 nm with a final values of a 1=50 nm, and a2 =400 nm.
The measured scattered intensity is fairy well predicted by the calculations, even is incorrectly
predicted for angles close to 10º. This disagreement cannot be remedied by calculations
0 60 120 180
0.02
0.04
.051
1.61 103
int1
1800 int( )0
Scat
tere
d in
tensi
ty
F
3
2w a2 a2
2exp 4
2sind
1
2
2
2a2
2
1 w( ) 8a1
3
1 162
sind1
2
2
2a1
2
2
Chapter III. Bulk silica aerogel characterization 124
assuming any reasonable a1, a2 and w values. A "better" fit could be obtained ignoring
several of the data points near = 0º and 180º. Further measurements of s11 may be
performed in order to diminish this effect.
Figure III.35 Intensity of scattered light versus angle for a M sample with red wavelength. Solid line shows fitted curve and cross curve gives experimental measurements. The correlation parameters obtained were: a1=50nm, a2=400nm, w=310-6
For the same aerogel sample, the intensity function was fitted for each of the wavelength
resulting in a similar a1, a2, and w values for the different wavelength. The fitted values are in
agreement with those values obtained by TEM (for particle size) and pore size (150 nm by
the model presented in previous section).
Figure III.36 compares the experimental curve with the curves fitted from the model
proposed for a two-step silica aerogel at 635 nm with a final values of a 1=150 nm, and a2
=2000 nm. The residual plot, q
jmin X( ) j
max X( ) min X( )( )
npoints , shows the differences
between theoretical and measured angle distribution of the scattering.
0
1
2
3
4
5
6
7
0 20 40 60 80 100 120 140 160 180
Serie1
Serie2
Serie3
Aerogel
H5
red
purple
green
M
i/
Chapter III. Bulk silica aerogel characterization 125
Figure III.36 Intensity of scattered light versus angle for a H5 sample with red wavelength. Solid line shows fitted curve and cross curve gives experimental measurements. The residual plot, qj, shows the differences between theoretical and measured angle distribution of the scattering. The correlation parameters obtained were: a1=50nm, a2=400nm, w=310-6
The total intensity results of Figure III.36 shows that the predicted calculations are quite
good approximations to the experimentally measured data. Moreover, the fitted parameters:
a1=50nm, a2=400nm shows poor agreement with the pore and particle values obtained by
other techniques. A possible reason is that the two-step analyzed sample was extremely
transparent so the effects that caused scattering are exclusively the surface damage
(microcracks in the range of 2 microns, a2 value). In this case, the nephelometer technique
does not give any further information.
Further studies may be developed in order to improve the fitting of the data with the aim of
extract more accurate structural information from the scattered intensity. Also to normalize
the intensity for each laser beam.
5.5 CONCLUSIONS AND FUTURE WORK
The nephelometer has been used to measure the angular and polarization dependence of
light scattered at visible regions. It has been proved that intensity measurements at variable
angle provide structural information outside the Rayleigh scattering regime. The model
proposed for a correlation function 22
2
1 a
r
a
r
wee)w1()r( has fitted the experimental data
50 100 1500
0.02
0.04
X-Y data
Least-squares fit
.051
0.001
Yi
fit q j
max X( ) 1min X( ) 1 Xi q j
i
50 100 150
0
Residual Plot
qj
Chapter III. Bulk silica aerogel characterization 126
by the variation of the a1, a2 and w parameters. The model has also been fitted with five or
four different wavelengths. New correlation functions should be proposed trying to provide a
more accurate fits to the experimental scattering data.
The normalization of the experimental curves should be changed from 90º to 20º. Further
studies may find a better physical interpretation. The useful part of the curves should be
normalized with different scale factors because the incident intensity was different for each
wavelength.
For the short range e -r/a1 correlation function, since a1<<1/h, the Fourier transform can be
taken independent of h. In addition, in some places it may be possible to ignore w as small
compared to one.
Assuming that the present correlation function (r) could be improved to fit the data well, we
should plot i( , ) as a function of h alone. It should be possible to get the data for all three
wavelengths to plot on the same curve, by adjusting the normalization. Having constructed
such a function, it will be needed to extrapolate it to h = infinity (there may be more than
one way to do this).
Attention should be taken in the interpretation of the analyzed curves. Strong backscattering
was observed for values close to 10º, this should account the strong inhomogeneities of the
surface. Therefore, when these curves were fitted large a2 values were obtained explaining the
larger pores of the surface.
Chapter III Bulk silica aerogel characterization 127
6 DIRECT METHODS: ELECTRON MICROSCOPY
Scanning and Electron microscopy (SEM and TEM) techniques yield direct images of the
aerogel structure. Thus, morphological features, such as particle shapes and particle
arrangements, can be recognized. An estimation of the particle size can also be obtained,
although the acquisition of enough data as to evaluate the particle size distribution is rather
tedious.
6.1 STRUCTURAL STUDIES BY SCANNING ELECTRON MICROSCOPY
A microstructural investigation has been taken up using scanning electron microscopy (Leica
360 scanning electron microscope).
Preparation of the samples
A special problem with the aerogels is the image degradation due to the charging of the
samples during the sample exposure to the electron beam. This charge is caused by the
strong electrical isolating nature of the silica aerogels. This problem is solved by depositing a
thin, conductive 20 nm gold coating by sputtering, thus apparently the gold penetrates in the
pores, making the aerogel more conductive avoiding electrostatic charge during the SEM
observations. In such case, the tenuous aerogel skeleton may be slightly distorted by the gold
coating. A second coating with silver contacts was done to facility the electrical contact
between sample and holder. Thus, SEM pictures must be interpreted with caution [54].
6.1.1 Acetone series
When silica aerogel samples were observed by SEM at low amplification, a porous structure
was observed. Figure III.37 compare, at the same scale, the morphology of two different
types of aerogels, A3 and A4. In both cases, it was observed a cracked surface, probably
caused by the preparation process of the samples: samples were carefully cut but it did not
avoid the formation of some cracks of some microns in size.
Chapter III Bulk silica aerogel characterization 128
Figure III.37 Morphology of A3, and A4 silica aerogels imaged by SEM.
SEM micrographs of A-series aerogels were compared at the same enlargement in order to
know the effect of TMOS/acetone molar ratio (directly related with the aerogel density) on
the microstructures of silica aerogels. Figure III.38 shows the A1, A2 microstructure, and
Figure III.39 the A3 and A4 microstructure at the same magnification.
Figure III.38 SEM micrograph of A1, the lowest density aerogel from A-series, and A2 silica aerogel. Parallel arrows
mark the constituting particles, particle, and pores, pore.
5.0m 5.0m
A3 A4
1.0m
A2
1.0m
A1
pore
pore
particle particle
Chapter III Bulk silica aerogel characterization 129
Figure III.39 SEM micrograph of A3, and A4 silica aerogels.
Parallel arrows mark the constituting particles, particle, and the
pores, pore.
In all the SEM images at 1-micron range, the aerogels had a granular appearance composed
by spherical particles and some pores. SEM images show that the A1 sample is built by
smaller interconnected particles than the denser A4 sample. The A4 particles showed the
smallest pores, although particles were larger than those of A1. The constituting particle size
distributions of 300 particles for each of the aerogel samples calculated from the
corresponding micrographs are shown in Figures III.40, III.41, III.42 and III.43,
corresponding to A1, A2, A3 and A4, respectively. The slash indicates the corresponding
magnification.
SEM images show that A1 sample is built by smaller interconnected particles (with a mean
size of 48 nm) than the denser A4 sample (with a mean particle size around 90 nm).
Figure III.40 SEM micrographs of A1 silica aerogel and particle-size distribution of 300 particles seen in micrographs.
20 40 60 80
Øp=48 nm
σ=8 nm
500 nm
500 nm Particle size (nm)
1 m
A3
1 m
A4
pore
pore
particle
particle
Chapter III Bulk silica aerogel characterization 130
Figure III.41 SEM micrographs of A4 silica aerogel and particle-size distribution of 300 particles seen in micrographs.
Figure III.42 SEM micrographs of A2 silica aerogel and particle-size distribution of 300 particles seen in micrographs.
A4
500 nm 20 40 60 80 100 120
Mean 88,1
sd 11,4
Øp =88 nm
σ=11 nm
Particle size (nm)
Particle size
(
μm) 20 40 60 80 0
Ø=55 nm σ=8 nm
Particle size (nm) 500 nm
Chapter III Bulk silica aerogel characterization 131
Figure III.43 SEM micrographs of A3 silica aerogel and particle-size distribution of 300 particles seen in micrographs: samples.
The average diameter for 300 particles calculated from a similar SEM micrograph is
summarized in Table III.16. The A1 sample showed the smallest particle size. Considering
mean particle size and density, one could assume that in A1 aerogel there are more particles
per unit volume and a larger number of contacts among them.
Section 2.3 of chapter IV shows a model that correlates mechanical properties and
microstructure.
6.1.2 Effect of the solvent
To investigate the effect of the solvent; SEM measurements have also been performed on two
samples with similar density but different solvent, acetone (A2) and methanol (M). Figure
III.45 compares the SEM micrographs from A2 and M aerogels.
A variation of particle size is observed; the more opaque aerogel (A2) contains closely bound
spherical particles of 30-80 nm in diameter and larger interstitial pores whilst the particle size
of the transparent aerogel (M) is smaller. M aerogel shows an interconnected band structure
with smaller particle size (less than 50 nm). These microstructures are consistent with the
observed differences in optical transparency of the obtained samples.
20 40 60 80 100
Øp=62 nm σ=11 nm
200 nm Particle size (nm)
Chapter III Bulk silica aerogel characterization 132
Figure III.44 SEM micrograph of M aerogel shows interconnected band microstructure.
Figure III.45 SEM micrographs comparing microstructures of a) A2, and b) M silica aerogels.
6.1.3 Drying procedure
A2 has been dried in two different manners. First, by a supercritical extraction of the solvent
and secondly by exchanging the acetone by liquid carbon dioxide, samples are labeled as A2
and A2CO2 respectively. The resulting aerogel microstructures have been compared. Figures
III.46a) and III.46b) compare the morphology of A2 and A2CO2 aerogels.
Figure III.46 SEM micrographs comparing microstructures from A2, and A2CO2 aerogels.
500 nm 500 nm
A2 M
1 m 1 m
A2 A2CO2
Chapter III Bulk silica aerogel characterization 133
A2CO2 presents a slightly more cracked structure and larger particles. See TableIII.16.
Shrinkage during drying is larger in A2CO2 than in A2 samples and their final density is similar
to A3 sample.
6.1.4 TMOS aerogels in carbon dioxide as solvent
The microstructure of the aerogels obtained at low temperature using supercritical carbon
dioxide as solvent was compared to the aerogel obtained by the classical method.
Figure III.47 SEM of HCOOH aerogel obtained at low temperature.
The aerogel microstructure for samples obtained without presence of water was more
polymeric than a sample with similar density obtained by one –step method. Moreover, the
presence of porous was hardly detected in HCOOH aerogels compare to one-step aerogels.
Table III.16 Mean constituting particle diameter, p of silica
aerogels measured by SEM.
Sample
(g/cm3)
p
(nm)
A1 0.080.01 483
A2 0.150.02 554
A3 0.230.02 622
A4 0.260.03 886
M 0.140.01 401
A1 + C 0.060.01 463
A2CO2 0.240.03 604
Chapter III Bulk silica aerogel characterization 134
It should be pointed at that this values are slightly larger than those expected for constituting
particles. Probably, this effect is caused by the agglomeration of the nanoparticles by the gold
treatment. Although this coating distortion, this technique was useful in order to account for
the differences in microstructure, i.e. in A-series increasing density implies increasing particle
size.
Further studies may be done in order to study the dependency of the microstructure with the
variation of the so-called sol-gel parameters.
6.2 TRANSMISSION ELECTRON MICROSCOPY
In general, the counting of particles for obtaining a distribution of particle sizes is tedious
and, usually only two-dimensional projections are available. In this section, some three-
dimensional images were obtained by the acquisition of stereographic micrographs. The main
effect of the electron beam is to charge the small particles because are made of so strong
insulating material. Therefore, some instability occurred (spatial drift) which affect the
resolution. Other effect of the radiation was an overall deformation of the structure after
long irradiation time so special attention was taken to control this effect.
Sample preparation:
The specimen preparation is important if one wants to preserve initial structure of the dried
aerogels. Small pieces of material were directly produced by crushing the solid in the agate
mortar with the presence of some small pieces of glass to improve the crushing effect. The
fine powder was directly deposited on the grid. In the aerogels with replicas visualization, the
replicas were obtained loading the aerogels specimens onto the stage of a freezing
microtome, evacuated to 1.10 -6 Pa, and cooled to –185C. The aerogels were platinum-
carbon replicated at an almost vertical angle (80) and backed with a rotary deposited carbon
film at a 100 angle. The silica replica was removed from the aerogel with diluted acid. The
replicas were deposed on copper grids. It should be account that the Pt-C film increases the
average of particle size (approx. 0.5nm).
TEM set-up
Electron microscope observations were performed either, directly on the aerogel samples at
100kV, 200 kV or 400 kV, or imaging the aerogel replicas at 20k-80k magnifications. All
replicas samples were examined in stereo.
Low voltages give high contrast, but the resolution is limited to 5 Å. High voltage gives low
contrast but very good resolution (aprox. 2 Å at 400kV). However, this theoretical resolution
level has never been obtained due to the charging effect. Practically the best resolution at
Chapter III Bulk silica aerogel characterization 135
400kV was of the order of 4 to 5 Å. Stereopairs were taken to study and visualize the three-
dimensional arrangement: a tilt angle of ±5º is usually sufficient. Stereopairs were examined
to give an idea about the three-dimensional arrangement and the number of chains at the
crossing points. The effect of radiation damage by the electron beam was studied in some
aerogel samples.
6.2.1 Imaging the acetone-series silica aerogels
Direct visualization
The structure of A-series was confirmed to be a three-dimensional network with an open
structure of small chains of diameter a, interconnected at average distances, d. Figure III.48
shows an example of A4 microstructure. The structure is defined by chains with connecting
points with a number of departing branches (either 2 or 3). The optical diffractographs
exhibit the usual rings due to an amorphous phase indicating no sign of crystallinity.
Figure III.48 TEM image of A4 silica aerogel shows length chains of 70nm, marked with arrows, with average particle
diameter of a=14.0 3.19nm, marked with parallel arrows, and
distance of d=30.8 3.9nm.
a
0 5 10 15 20 25 300
20
40
60
A46
Gaussian fit
Mean SD
--------------
14,0 3,1
a d
Chapter III Bulk silica aerogel characterization 136
Figure III.49 TEM image of a detail of the particles of A4
silica aerogel showing a mean particle diameter of 16.8 2.9nm.
The comparison of several aerogels with different densities, A1 and A4 from series-A,
(Figures III.50-53) indicated that the basic elements of the 3D network are dependent on the
aerogel density.
Figure III.50 TEM image of A4 silica aerogel shows length
chains of 70 nm with average particle diameter of 2.1 0.6 nm
and distance of 30.8 3.9 nm
8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 260
10
20
A4151 (A4+15%C)
Gaussian fit
Mean SD
------------
16,8 2,9
1,0 1,5 2,0 2,5 3,0 3,5 4,0 0
20
40
60
80
Mean 2,10109 SD 0,59944
Particle size (nm)
Ø p =10.1nm σ=0.6nm
5 7.5 10 12.5 15 17.5 20 Particle size (nm)
Chapter III Bulk silica aerogel characterization 137
Figure III.51 TEM image of A4 81K zoom silica aerogel.
Figure III.52 TEM image (x160K) of a detail of the particles of A1 silica aerogel showing a mean particle diameter of 8 nm.
Figure III.53 Zoom of A1 silica aerogel TEM image.
0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 0
20
40
60
Particle size (nm)
Mean = 0,00104 SD = 1,61623
Ø p = 8.0 nm σ= 0.1 nm
2.5 5 7.5 10 12.5 14.5 Particle size (nm)
Chapter III Bulk silica aerogel characterization 138
The chain diameter is smaller by 30% for a density variation of 30% and the average distance
increased of 15%. At very low magnification, the organization of the different densities was
hardly observed. The structure of A4 was shown more compact. The chain diameter is
smaller and the network was much tighter.
6.2.2 Imaging the methanol silica aerogels
Transmission electron microscopy was used to study the morphology of the most
transparent one-step silica aerogel. The samples were fractured, vertically replicated with 0.95
nm Pt-C and backed with approximately 12 nm of rotary evaporated carbon. The silica
aerogel was then removed from the replica with dilute acid and the replicas were studied by
TEM. The stereoscopic TEM images reveal that particles in methanol aerogels are slightly
smaller than acetone aerogels and that their structure is markedly more polymeric. This
morphology results from side-chain formation on a nearly linear structure. For M, the
particles have diameters ranging from 1.7 to 14.2 nm with an average of 6.4 ± 0.5 nm and the
chains lengths averaged 62 ± 21 nm with some as long as 132 nm. Pore sizes ranged from 12
to 277 nm with an averaged 61 ± 56 nm. The pores were slightly larger than pores in A-
series, which ranged from 13 to 240 nm with an average of 74 ± 43 nm.
Replicas visualization
In Figure III.54, the surface of a M silica aerogel with TMOS/acetone/water 1/12.25/4 is
shown at 80k magnification. If this image is visualized with a 10x magnifier, the silica chains
can be observed. In Figure III.55, it is possible to observe Figure III.54 in three dimensions
using stereo glasses. This aerogel, when viewed in stereo, demonstrates that the „airy‟
appearance of the aerogel structure. It can be observed that the chains contain many small
spherical particles. The range in size is from 5 to 15 nm with an average diameter of 10±1nm.
Chapter III Bulk silica aerogel characterization 139
Figure III.54 TEM micrograph of base-catalyzed silica aerogel. 50z M-aerogel: low temperature replica. In order to see the detail of the structure of the aerogel it should be viewed with an eye loop (x10)
Figure III.55 Stereoscopic images (tilt angle of 10 ). Aerogel skeleton is formed by interconnected particles in a three-
Chapter III Bulk silica aerogel characterization 140
dimensional structure (average diameter of 6.4±1nm). Low temperature M-aerogel replica.
Figure III.56 Stereoscopic image of detail M-aerogel replicas at low temperature.
Figure III.57 Stereoscopic image of M-aerogel replicas. On the right, a stereoscopic image of M replicas is shown.
Summarizing TEM was used to examine a series of acetone silica aerogel (A-series) and a
base catalyzed aerogel with methanol as solvent (M). This technique provided molecular
information about the aerogels and enables to distinguish the different parts of the structure
of the aerogel: individual chains and crosslinking junctions were visualized. TEM microscopy
was used to examine either directly the silica aerogel or the surface replicas. The stereoscopic
images with a tilt series at 20k-80k magnifications have made possible a three-dimensional
visualization of the aerogel structure.
Chapter III Bulk silica aerogel characterization 141
7 REFERENCES
1. Schmidt, H., and Scholze, H.,
"Aerogels" edited by J.Fricke Springer-
Verlag (1986), p.49.
2. Brinker, C.J., and Scherer, G.W., "Sol-
gel science: the physics and chemistry
of sol-gel processing" Academic Press,
N.Y; (1990).
3. M. Prassar, J. Phalippou, J. Zarzycki,
Sintering of monolithic silica aerogel,
in: L.L. Hench, D.R. Ulrich (Eds.),
Science of Ceramic Processing, vol.
156, Wiley, New York, 1986
4. Skeletal density of silica aerogels T.
Woignier, J. Phalippou, Journal of
Non-Crystalline Solids 93, 1987, 17-21
5. A. Ayral, J. Phalippou, T. Woignier J.
Mater. Sci. 27 (1992), p. 1166
6. Dependence of monolithicity and
physical properties of TMOS silica
aerogels on gel aging and drying
conditions G. M. Pajonk Journal of
Non-Crystalline Solids Volume 209,
Issues 1-2 January 1997 Pages 40-50
7. Comparison of some physical
properties of silica aerogel monoliths
synthesized by different precursors
Materials Chemistry and Physics
Volume 57, Issue 3 25 January 1999
Pages 214-218 P. B.Wagh
8. Drying of aerogels in different solvents
between atmospheric and supercritical
pressures
9. Fikret Kirkbir Journal of Non-
Crystalline Solids Volume 225, Issue 1
April 1998 Pages 14-18
10. S.S. Kistler. J. Phys. Chem. 36 (1928),
p. 52.
11. J. Phalippou, T. Woignier and M.
Prassas. J. Mater. Sci. 25 (1990), p.
3111
12. G.M. Pajonk, A.V. Rao, B.M. Sawant
and N.N. Paravathy. J. Non-Cryst.
Solids 209 (1997), p. 40
13. K. Tajiri, K. Igarashi and T. Nishio. J.
Non-Cryst. Solids 186 (1995), p. 83
14. A. Emmerling and J. Fricke. J. Non-
Cryst. Solids 145 (1992), p. 113
15. Supercritical drying media modification
for silica aerogel preparation. Satoshi
Yoda Journal of Non-Crystalline Solids
Volume 248, Issues 2-3 2 June 1999
Pages 224-234
16. Influence of molar ratios of precursor,
solvent and water on physical
properties of citric acid catalyzed
TEOS silica aerogels. P. B. Wagha, A.
Venkateswara Rao, and D. Haranatha
Materials Chemistry and Physics
Volume 53, Issue 1 April 1998 Pages
41-47
17. Structural development of silica gels
aged in TEOS Journal of Non-
Crystalline Solids Volume 231, Issues
1-2 1 July 1998 Pages 10-16
Chapter III Bulk silica aerogel characterization 142
18. W,. Cao, and A.. J. Hunt, J. Non-Cryst.
Solids 176 (1994) 18
19. Physical properties of silica gels and
aerogels prepared with new polymeric
precursors Journal of Non-Crystalline
Solids Volume 186 June 1995 1-8
20. G.M. Pajonk, A.V. Rao, B.M. Sawant
and N.N. Paravathy. J. Non-Cryst.
Solids 209 (1997), p. 40
21. G.W. Scherer, J. Non-Cryst. Solids 145,
33 (1992).
22. J.D. Mackenzie, Applications of the
sol¯gel method: some aspects of initial
processing, in: L.L. Hench, D.R. Ulrich
(Eds.), Science of Ceramic Processing,
vol. 113, Wiley, New York, 1986
23. Smith D.M, Hua D.W. and EarlW.L.,
MRS Bulletin, 44 - 48 (1994).
24. S. Lowell, J.E. Shields, Powder Surface
Area and Porosity, Chapman and Hall,
London, 1984.
25. J. Fricke, R. Caps, D. Buttner, V.
Heinemann, E. Himmer, G.
Reichenamer, Structural, elasto-
mechanical and thermal properties of
silica aerogels, in: K.K. Kruger et al.
(Eds.), Characterization of Porous
Structure, vol. 629, Elsevier,
Amsterdam, 1988 ll
26. S. Brunauer, P.H. Emmet and E.
Teller. J. Am. Chem. Soc. 60 (1938), p.
309.
27. Mercury porosimetry: applicability of
the buckling intrusion mechanism to
low-density xerogels, Journal of Non-
Crystalline Solids, Volume 292, Issues
1-3, November 2001, Pages 138-149
Christelle Alié, René Pirard and Jean-
Paul Pirard
28. C. Pierce , J. Phys. Chem. 57 (1953) 149.
29. G.W. Scherer, S. Haereid, E. Nilsen,
M.A. Einarsrud, J. Non-Cryst. Solids, 202
(1996) 42-52.
30. S. Yoda, S. Ohshima and F. Ikazaki. J.
Non-Cryst. Solids 231 (1998), p. 41
31. Pierce C., J. Phys. Chem. 57 (1953) 149
32. Fricke J., and Reichenaver G., in
"Better ceramics through chemistry II"
Ed. Brinker, C.J., Clark, D.E., and
Ulrich, D.R., MRS, 1986 , p775.
33. Nitrogen sorption in aerogels, Journal
of Non-Crystalline Solids, Volume 285,
Issues 1-3, June 2001, Pages 167-174
G. Reichenauer and G. W. Scherer
34. Extracting the pore size distribution of
compliant materials from nitrogen
adsorption, Colloids and Surfaces A:
Physicochemical and Engineering
Aspects, Volumes 187-188, 31 August
2001, Pages 41-50
35. J.B. Peri, J. Phys. Chem. 70 (1966)
2937.
36. J. Kratochvila and M. Gheirghiu, J.
Non-Cryst. Solids, 116 (1990) 93.
37. B.A. Morrow and A.J. Mc Farlan , J.
Non-Cryst. Solids, 120 (1990) 61.
38. M.L. Hair J. Phys. Chem. 73 (1969)
2372
39. B.E. Yoldas, J. Non-Cryst. Solids 63
(1984), 145.
40. .Fricke and T. Tillotson, Thin Solid
Films 297 (1997) 212-223.
41. H. C. Van de Hulst, Light scattering by
small particles, Wiley, New York, 1957.
Chapter III Bulk silica aerogel characterization 143
42. M. Kerker, The scattering of the light
and other electromagnetic radiation,
Academic Press, New York, 1969.
43. G. Mie, Ann. Physik 25 (1908) 377.
44. Rubin M. and Lampert C.M., Solar
Energy Materials 7 (1983) 393-400.
45. Born M. and Wolf E., “Principles of
optics” 6th ed.
46. Kistler S.S and Caldwell A.G., Ind. Eng.
Chem., 26 (1953) 658.
47. H. Mueller J.Opt. Soc. Am. 38 661
(1948).
48. A.J. Hunt, Proceedings of the intern.
conf. on ultrastructure processing of
ceramics, glasses, and composites,
(1983)
49. A. J. Hunt and P. Berdahl Mat. Res.
Soc. Symp. Proc. 32 (1984), 275
50. P. Debye, Ann. Phys. 30, 59 (1909).
51. Sorense C.M., Aerosol Science and
technology 35 (2001) 648-687. A
52. M. Kerker, The scattering of light,
Acacemic Press, New York, 1969
53. Bourret, A. Europhys. Lett. 6, 731
(1988)
54. Schaefer, D. MRS Bulletin, 49 (1994)
55. G. C. Ruben J. of Non-Crystalline
Solids 186, 209 (1995)
Chapter III Bulk silica aerogel characterization 144
SD = 1,61623 Mean = 0,00104 Particle size (nm) 60 40 20 0 4,0 3,5 3,0 2,5 2,0 1,5 1,0 0,5
C h a p t e r I V
MECHANICAL CHARACTERIZATION OF SILICA AEROGELS
SECTION OUTLINE
1. INTRODUCTION ............................................................................................................................... 146
1.1 MICROINDENTER DESCRIPTION .................................................................................................147
1.2 MECHANICAL CHARACTERIZATION ...........................................................................................149
2. MECHANICAL PROPERTIES OF SILICA AEROGELS AS A FUNCTION OF
DENSITY ............................................................................................................................................................ 151
2.1 SAMPLE PREPARATION ................................................................................................................151
2.2 EFFECT OF THE ALKOXIDE CONCENTRATION..........................................................................152
2.2.1 Relationship between silica aerogel microstructure and mechanical
properties ....................................................................................................................................... 154
3. INFLUENCE OF SOLVENT AND SUPERCRITICAL DRYING METHOD ON THE
MECHANICAL PROPERTIES .................................................................................................................... 156
3.1 EFFECT OF THE DRYING PROCEDURE........................................................................................156
3.2 EFFECT OF THE SOLVENT IN MECHANICAL PROPERTIES .........................................................158
4. MICROMECHANICAL PROPERTIES OF CARBON-SILICA AEROGEL
COMPOSITES ................................................................................................................................................... 159
4.1 INTRODUCTION............................................................................................................................159
4.2 EFFECT OF THE CARBON ADDITION ..........................................................................................160
5. VISCOELASTICITY OF SILICA AEROGELS AT ULTRASONIC FREQUENCIES ...... 166
5.1 INTRODUCTION .....................................................................................................................166
5.2 EXPERIMENTAL SET-UP: AIR-COUPLED BROAD-BAND PIEZOELECTRIC TRANSDUCERS ........167
5.3 DETERMINATION OF THE VISCOELASTICITY OF SILICA AEROGELS AT ULTRASONIC
FREQUENCIES. ..............................................................................................................................169
5.3.1 Normal incidence ...................................................................................................... 169
5.3.2 Oblique incidence ..................................................................................................... 171
6. MECHANICAL CHARACTERIZATION CONCLUSIONS ................................................... 173
7. REFERENCES...................................................................................................................................... 175
Chapter IV Mechanical characterization of silica aerogels 146
1. INTRODUCTION
The mechanical properties of the aerogels are crucial for almost any application because the
need of handling and machining the material, as well as its durability in rough environments.
In addition, there is fundamental interest in understanding the variation in elastic properties
with density, and to compare experimental results with theoretical models. The aim of this
chapter is to present a description of the mechanical characterization of the silica aerogels
which were prepared through out the thesis (chapter II). The mechanical measurements of
such samples were carried out by Elena Martínez at the „Departament de Física Aplicada i
Òptica‟ of the Universitat de Barcelona.
The mechanical characterization of silica aerogels presents some difficulties. Aerogel is a
fragile material, difficult to handle and brittle (very small loads are sufficient to crack them).
Efforts are being made on improving their mechanical properties and on overcoming the
difficulty of measuring such properties. Several testing methods for mechanical
characterization have been previously reported, namely; Young‟s modulus and toughness are
usually obtained by using ultrasonic sound velocity, [1-2-3], direct longitudinal compression
methods, [2, 4], and measurements by the three-point bending beam technique [5,6]. It has
been reported that tensile and bend specimens can give extraneous deformations under
estimating of the true modulus [7]. AFM in contact mode has also been used to measure
Young‟s modulus of aerogels [8]. Mechanical studies of reinforced aerogels using Vickers and
Knoop indentation techniques have also been published [1, 9] but are not sensitive enough to
measure the mechanical response of such materials. On the other hand, the Young‟s modulus
of aerogels is a factor 102 to 104 smaller than that of silica glass [10] and, therefore, they can be
easily compressed. It is a major challenge to improve the mechanical properties of such
materials without sacrificing others and also, to find a suitable non-destructive method for
their mechanical characterization.
Dynamical microindentation is a powerful technique, initially developed for thin film
mechanical characterization, where low applied loads and small penetration depths are required
[11-12-13]. When it is applied to aerogels, it presents the added advantage of recording the
continuous measurement of load-penetration curves, which makes the direct observation of the
indentation marks unnecessary. An asset of this technique is that the specimens do not need any
Chapter IV Mechanical characterization of silica aerogels 147
machining for the test to be performed, since the machining process may alter the sample
microstructure. Young‟s modulus (E), elastic parameter (EP) and indentation hardness (H)
can be measured by applying very small loads (~1mN), small enough to slightly deform the
composite aerogels while preventing crack formation in these brittle materials. This technique
allows elastic and plastic behavior to be differentiated.
The results are presented in five different sections corresponding to the mechanical
characterization of different aerogel series that were synthesized by a variety of the sol-gel
processes previously described in chapter II.
i) First section is an introduction to the mechanical characterization of silica aerogels
and a description of the microindenter.
ii) Second section shows the results obtained in a series of samples with different
densities. Results were analyzed as functions of density, , morphology and pore size
distribution. A relation of the type, E with ~2.9 was found for the acetone-
synthesized series. As a function of aerogel density, two different regimes of
mechanical behavior were observed. The lowest density aerogels are elastic but the
denser aerogels are elasto-plastic materials.
iii) Third section presents the mechanical characterization of different series of silica
aerogels using different supercritical drying procedure and different synthesis solvent.
iv) Fourth section presents the results obtained in a series of silica-carbon composite
aerogel obtained with a variable amount of activated carbon (Annex articles: article
III).
v) Fifth section reports the relationship between mechanical properties and the
microstructure of the silica aerogels and carbon-silica aerogel composites.
1.1 MICROINDENTER DESCRIPTION
The microindentation measurements were carried out on a Nanotest 550 (Micro Materials
Ltd., U.K.) provided with a Berkovich diamond indentation tip [14].
Chapter IV Mechanical characterization of silica aerogels 148
Figure IV.1 Photography of Nanotest 550 (Micro Materials Ldt.)
The Nanotest apparatus consists in two pendulums (1), and (2), and an indenter (3)
integrated in one platform (on the right of the photo). A second platform integrates the
motor (4), which allowed moving the sample (on the left of the photo). This system is placed
in an isolating chamber. The measured signal (load and penetration depth) is processed by a
control unit connected to a computer. The computer allows to control the process of
indentation, order the displacement of the motors and record the data. Figure IV.2 shows a
schematic design of the system.
Figure IV.2 Schematic design of Nanotest 550 system
weight to balance
the pendulum
Scheme of
Nanotest 550
weight to balance
the pendulum
weight to balance
the pendulum
Scheme of
Nanotest 550
•Non-destructive technique
• Doesn’t need sample preparation•
Dynamic microindentation
• Extremely small applied loads
• Diamond Berkovich tip
• Maximum loads up to 500 mN
• Maximum penetration
up to 10 mm
• Resolution: 0.1 nm, 0.1 mN
(1)
(2)
(3) (4)
Chapter IV Mechanical characterization of silica aerogels 149
The Nanotest consists in a diamond tip attached to a ceramic pendulum. When a current is
applied along the coil, it is attracted towards the magnet moving the arm until the diamond
tip penetrates on the sample surface. The displacement of the tip is measured using a
variable capacitor. The limit stop defines which will be the maximum penetration for each
type of sample. Continuous loads and penetrations are automatically recorded.
1.2 MECHANICAL CHARACTERIZATION
The microindentation system allows load and depth resolutions better than 1 N and 1 nm,
respectively [11]. The maximum applied load was 1 mN or 0.5mN for the most elastic
aerogels. These maximum loads led to maximum penetration depth in the range of 2 -7 m.
Each loading-unloading cycle was repeated at least 10 times at different points in each
sample to check the consistency of the results. Before measuring the unloading branch of the
cycle, the hold time at maximum load was varied between 0 to 600 seconds to determine if
aerogels were prone to stress relaxation or creep. Figure IV.3 illustrates a typical behavior for
the sample A2. It can be seen from Figure IV.3 that there is creep penetration when the
diamond tip is being held during some time at the maximum load.
Figure IV.3 Penetration creep as a function of time for the A2
sample under maximum indentation load.
0 200 400 600
Creep time (s)
2
4
6
8
Cre
ep p
ene
tratio
n (
m
)
Chapter IV Mechanical characterization of silica aerogels 150
The creep rate strongly increases for creep times less than 200 s and becomes almost
constant for longer creep times. This feature, if not considered, could lead to apparent higher
hardness values [15].
Figure IV.4 Scheme of penetration of the Berkovich indenter in
the sample surface.
Hardness values, H, were obtained from:
H = Pmax/A (Eq. IV. 1)
where Pmax is the maximum load and A is the projected indentation area for the maximum
penetration depth. For a Berkovich diamond tip, A is given by the expression A=24.5h t2,
where ht is the maximum penetration depth measured from the load-penetration curves (see
Figure IV.4).
Hardness values calculated by this method correspond to the total elasto-plastic
deformation. It should be pointed out that this is not the most usual hardness definition,
although is convenient in the case of very elastic materials, as aerogels are.
Young‟s modulus, E, is obtained from the analysis of the unloading branch of the
penetration vs. load curve by applying the method proposed by Oliver and Pharr [16] and
using Sneddon‟s relationship:
AE
dh
dP21
2, (Eq. IV. 2)
where is the Poisson‟s ratio ( =0.2 for a silica aerogel) [17,18] and dP/dh is obtained from
a polynomial fit of the unloading curve (see Figure IV.5).
P=0ht
hp
he
P
Pmax
P=0ht
hp
he
P
Pmax
P=0htht
hp
he
hp
he
PP
Pmax
ht: total penetration
depth
hp: plastic depth
he: elastic depth
Pmax: maximum load
Chapter IV Mechanical characterization of silica aerogels 151
Figure IV.5 Hysteresis curve penetration depth versus load
Another interesting parameter that can be derived from these measurements is the elastic
parameter, EP, which is similar to the one previously defined [19]. EP allows quantitative
comparison of the elastic behavior of the samples. EP [15], is defined as:
t
pt
h
hhEP (Eq. IV. 3)
where ht is the total penetration depth during the indentation process and hp is the non-recovery
depth (plastic depth) of the diamond tip inside the aerogel sample.
2. MECHANICAL PROPERTIES OF SILICA AEROGELS AS A FUNCTION
OF DENSITY
2.1 SAMPLE PREPARATION
In order to investigate the changes in the mechanical properties of the aerogels when varying
the concentration of the silicon alkoxide in the initial sol a series of acetone aerogels (samples
A1 to A4) was synthesized and characterized. Aerogels with different density, particle and pore
size were obtained using different molar ratio of TMOS/acetone. The molar ratio of TMOS
to water was always kept to four. Several volume ratios, v, defined as acetoneTMOS
TMOS
VV
Vv ,
were used. For v = 0.1, 0.2, 0.3 and 0.4, samples were labeled as A1, A2, A3 and A4 (chapter
II). All the obtained aerogels were monolithic, in the form of cylindrical rods of 1cm diameter
and lengths from 5 to 12 cm.
ht
h
Pmax
Loading
he
Unloading
P
hp
ht
h
Pmax
Loading
he
Unloading
P
hp
ht
h
Pmax
Loading
he
Unloading
P
hp
Pmax
Loading
he
Unloading
P
hp
Chapter IV Mechanical characterization of silica aerogels 152
2.2 EFFECT OF THE ALKOXIDE CONCENTRATION
Figure IV.6 shows the load-penetration hysteresis curves for each sample „as measured‟ without
any data smoothing or averaging.
Figure IV.6 Hysteresis loading-unloading curve penetration depth
versus load for the samples: A1 (+), A2 (∆), A3 (○) and A4 (□).
From those curves, we have obtained the three characteristic mechanical parameters,
Hardness, H, Young‟s modulus, E, and elastic parameter, EP. The results are shown in Table
IV.1, which also includes dilution and density for each sample preparation.
Table IV.1 Several volume ratios, v, were used to prepare the
aerogel samples, their densities, , mean particle diameter, p ,
and mean pore diameter, 0 , are listed below together with their mechanical properties: Hardness, H, Young‟s modulus, E, and elastic parameter, EP. Standard deviations are shown in parenthesis
and refer to the last digit. p / 0 is related with Young‟s modulus (see text).
Sample
label v
(g/cm3)
p
(nm)
0
(nm)
p
0
H
(MPa)
E
(MPa)
EP
(%)
A1 0.1 0.08 (1) 48 (7) 110 0.44 0.5 (1) 7.0 (9) 78 (1)
Load (mN)
.2 .4 .6 .8 1
A4
A1
A3
A2
0
1
2.
3
4
5
6
7
A1
Pen
etr
ati
on
depth
(m
)
Load (mN)
.2 .4 .6 .8 1
A4
A1
A3
A2
0
1
2.
3
4
5
6
7
A1
Pen
etr
ati
on
depth
(m
)
Load (mN)Load (mN)
.2 .4 .6 .8 1
A4
A1
A3
A2A2
0
1
2.
3
4
5
6
7
A1
Pen
etr
ati
on
depth
(m
)
Chapter IV Mechanical characterization of silica aerogels 153
A2 0.2 0.15 (2) 55 (8) 60 0.92 2.4 (3) 50 (2) 30 (1)
A3 0.3 0.23 (2) 70 (9) 30 2.3 4.8 (3) 153 (3) 10 (2)
A4 0.4 0.26 (3) 88 (11) 20 4.4 5.7 (3) 346 (6) 6 (2)
It can be observed that as the dilution of the sol changes from 0.1 to 0.4, the density of the
aerogels increases, so it does the hardness and Young‟s modulus. The EP values show a
decrease, as the samples become more plastic.
In order to analyze the evolution of Young‟s modulus, hardness and elastic parame ter as a
function of the aerogel densities, the experimental points were fitted to power law functions.
The resulting scaling exponent for the Young‟s modulus is = 3.0 0.2 (E ). This is in
agreement with other previous results reporting values within 2.5 and 3.8 [20, 21, 22, 23].
The scaling exponents for hardness, H, and elastic parameter, EP, were 2.0 0.1 and -1.7
0,4 respectively.
Figure IV.7 Plots of Young‟s modulus (a), hardness (b) and EP (c)
as a function of the aerogels density. Fittings are shown as solid
-3.0 -2.5 -2.0 -1.5 -1.0
log(density (g/cm-3))
-1
0
1
2
log
(H (
MP
a))
-3.0 -2.5 -2.0 -1.5 -1.0
log(density (g/cm-3))
0
2
4
6
log
(E
(M
Pa)
)
0.05 0.10 0.15 0.20 0.25 0.30
density (g/cm-3))
0.0
0.5
1.0
EP
H
E
EP
Chapter IV Mechanical characterization of silica aerogels 154
lines. The graphs of H and E are presented with the logarithm
values of the data.
From the load-displacement curves of Figure IV.6 and the results given on Table IV.1 it can
be observed that the aerogel samples investigated cover a wide range of mechanical
responses. In particular, lowest density samples are almost completely elastic materials, this is
seen because the loading and unloading branches do not show hysteresis, and they have a
very low Young‟s modulus and high EP value. While for larger density aerogels deformation
is almost entirely plastic and presents a much higher Young‟s modulus value, high modulus
values could be expected in a plastic material because Young‟s modulus accounts solely for
the elastic behavior of materials.
Hardness and Young‟s modulus increase and EP decrease when increasing the density of the
samples. Notice that hardness, as defined in Equation IV.1, is derived from the total elasto-
plastic penetration depth. If we had used the more common hardness definition, which
involves only the plastic penetration depth, then we would have obtained an unrealistically
high hardness value for the very elastic samples such as A1.
2.2.1 Relationship between silica aerogel microstructure and
mechanical properties
Chapter IV Mechanical characterization of silica aerogels 155
Microstructural observations were performed with a Leica 360 scanning electron
microscope. The analysis of such micrographs allowed us to assess the particle size
distribution for all the samples. Brunauer-Emmet-Teller (BET) porosity characterization of the
aerogel samples was performed with a Micrometrics ASAP 2000 instrument. The BET surface
area, SBET, was obtained from the adsorption isotherm of N2 at 77K. The mean pore diameter
was calculated by 0 = 4Vpore / SBET where Vpore = (1/ aerogel - 1/ Si lica). The results are shown
in Table IV.3.
Figure IV.8 Scanning Electron Micrographs and particle size
histograms for (a) A1 and (b) A4 aerogels. < p> mean particle
diameter, and σ standard deviation, calculated by SEM histogram
analysis.
SEM images (see Figures IV.8a) and b)), show that the A1 sample is built by smaller
interconnected particles than the denser A4 sample. The mean particle diameter obtained, p ,
for all samples were shown in Table IV.1. Assuming a similar fractal dimension between the
sample series [24] Young‟s modulus is related to the degree of necking among contiguous
particles. Necking contact increases with increasing particle size, p, but its density decreases
with pore size, 0. In fact, Young‟s modulus behaves linearly with the p / 0 quotient,
following E=k p / 0 , where k=3.4MPa.
20 40 60 80 100 120
Mean 88,1
sd 11,4 Ø =88nm σ=11nm
500 nm
20 40 60 80 100
48.5 nm
7.6 nm Ø
p
=48nm σ=7.6nm
a)
b)
Chapter IV Mechanical characterization of silica aerogels 156
3. INFLUENCE OF SOLVENT AND SUPERCRITICAL DRYING
METHOD ON THE MECHANICAL PROPERTIES
Silica gels were prepared by hydrolysis of TMOS under different conditions to evaluate the
different responses of the aerogels to indentation. The A2 sample ( 0.15 g/cm3) was used as
reference to compare the different syntheses conditions. A2 was compared in the previous
section to an aerogel series with different densities (A1, A3, and A4). In this section, A2 sample
will be compared with different drying procedure, A2CO2, and with a sample with the similar
density but different solvent, M. The solvent and the drying procedure used may cause
variations in the aerogel hardness and Young‟s modulus.
3.1 EFFECT OF THE DRYING PROCEDURE
The first sample was dry with a conventional high temperature supercritical extraction of
acetone while the second sample by exchanging the acetone for liquid carbon dioxide. The
corresponding samples were labeled as A2 and A2CO2 respectively. The microindentation
responses of both aerogels can be compared in Figure IV.9.
Chapter IV Mechanical characterization of silica aerogels 157
Figure IV.9 Penetration depth versus load loading and unloading
curves for A2 (∆) and A2CO2 (●) samples.
In the case of differently dried samples, differences can be due to sample shrinkage during
drying, which is more important in A2CO2 samples than in A2 samples. It may be remarked that
A2CO2 gels were not washed in an ethanol bath and then the presence of remaining water in the
gel solvent causes the noticeable shrinkage. As a result, the density of the A2 CO2 samples is
higher and similar to that of A3 samples. Therefore, the A2CO2 samples show a mechanical
behavior similar to A3 samples. This shrinkage might have shortened the length of the bonding
necks and increased connectivity between the primary silica particles, resulting in a slight
increase in the density of the silica aerogels, and consequently an increase in hardness and
Young‟s Modulus.
It can be pointed out that A2CO2 stills follow the scaling laws mentioned earlier in the previous
section. It is found that the A2 sample presents a much more plastic response than the A2CO2
sample, as it can be observed from the elastic recovery in Figure IV.9. Moreover, the A2CO2 is
harder than A2 and presents a much higher Young‟s modulus (values are shown in Table IV.2).
0 .2 .4 .6 .8 1.0
Load (mN)
0
1
2
3
4
5
6
A2 + CO2
A2
Pen
etr
ati
on
depth
(m
)
0 .2 .4 .6 .8 1.0
Load (mN)
0
1
2
3
4
5
6
A2 + CO2
A2
Pen
etr
ati
on
depth
(m
)
Label(g/cm3)
H(MPa)
E(MPa)
EP(%)
A2 0.15 2.4 50 30
A2CO2 0.24 4.9 197 8
Chapter IV Mechanical characterization of silica aerogels 158
Table IV.2 Several solvent volume ratios, v, and different solvent and drying procedures were used to prepare the aerogel samples, their densities are listed below together with their mechanical properties: Hardness, H, Young‟s modulus, E, and elastic parameter, EP. Standard deviations are shown in parenthesis and refer to the last digit.
Sample v
(g/cm3) H
(MPa) E
(MPa) EP (%)
A2 0.2 0.15 (2) 2.4 (3) 50 (2) 30 (1)
A3 0.3 0.23 (2) 4.8 (3) 153 (3) 10 (2)
M 0.23 0.14 (1) 1.4 (1) 29 (2) 49 (1)
A2CO2 0.2 0.24 (3) 4.9 (3) 197 (3) 8 (1)
v = VTMOS / (VTMOS + Vsolvent)
3.2 EFFECT OF THE SOLVENT IN MECHANICAL PROPERTIES
To investigate the effect of the solvent, measurements were performed on two different solvent
samples of similar density, acetone (A2) and methanol (M), see Table IV.2. When using
methanol as a solvent, samples labeled as M, the molar ratio of the reactives were:
TMOS:CH3OH:H2O:NH4OH = 1:12.25:4:6.5·10-2 [25]. For the syntheses with acetone [26], no
catalyst was used. Figure IV.10 shows the load-penetration curves obtained from A2 and M
aerogels.
Figure IV.10 Penetration depth versus load loading and unloading
curves for A2 (∆) and M (●) samples.
0 .2 .4 .6 .8 1.0
Load (mN)
0
1
2
3
4
5
6
M
A2
Pen
etr
ati
on
depth
(m
)
0 .2 .4 .6 .8 1.0
Load (mN)
0
1
2
3
4
5
6
M
A2
Pen
etr
ati
on
depth
(m
)
Label(g/cm
3)
H(MPa)
E(MPa)
EP(%)
M 0.14 1.4 29 49
A2 0.15 2.4 50 30
Chapter IV Mechanical characterization of silica aerogels 159
It can be observed that at equal maximum load, the maximum penetration depth is lower for
A2 than for M aerogel. However, both samples show similar residual penetration depths after
indenter unloading, so the M sample has a higher elasticity than the A2 sample while exhibiting
the same density. Consequently, the elastic parameter values increase and the Young‟s modulus
values decrease for the M sample. Thus, the A2 sample is harder than the M sample as can be
seen Table IV.2.
It is known that initial conditions for the TMOS sol-gel polymerization process, such as the
solvent, have a pronounced influence on the microstructure of the aerogels [18]. The
differences between elastic and plastic behavior in M and R samples must have their origin in
the differences in the aerogel microstructure. Base catalysts such as NH4OH, as in the M
sample, lead to polymeric samples with high cross-linking therefore increasing the elasticity
of the sample. Nevertheless, further structural and modeling efforts are needed for a
complete understanding of these dependencies.
In conclusion, the effects of the solvent, being methanol or acetone, on the silica aerogels have
been studied. It was found that for samples with similar density values, the ones obtained using
acetone have higher hardness and Young‟s modulus values than those obtained from methanol,
but with less elastic recovery. Moreover, the effect of the drying process has been studied. It has
been shown that the process with CO2-acetone exchange causes a slight improvement in
hardness and a relevant increase in the elastic modulus, mostly due to larger shrinking effects.
This shrinkage might have shortened the length of the bonding necks and increased
connectivity between the primary silica particles, resulting in a slight increase in the density, and
consequently an increase in hardness and Young‟s Modulus.
4. MICROMECHANICAL PROPERTIES OF CARBON-SILICA AEROGEL
COMPOSITES
4.1 INTRODUCTION
To fit engineering needs, some potential aerogel applications may require materials with a
higher elastic recovery than that inherent to pure silica aerogels. As will be shown, this can
be done by incorporating activated carbon particles within the silica framework. Contrary to
Chapter IV Mechanical characterization of silica aerogels 160
conventional composites, in the resulting carbon-silica aerogel the most rigid phase
corresponds to the silica aerogel matrix. Such systems are called inverted composites [7].
Carbon-silica aerogel composites may have a broad range of applications [27]. For instance,
the thermal conductivity of silica aerogels can be reduced by absorbing infrared radiative
component of the heat transfer. Carbon-loaded silica aerogels increase the thermal resistance
due to carbon broad-band absorption. [28, 29].Another possible application for carbon-silica
aerogel composites is as low temperature-black materials, for example, as opaque or low-
reflectivity monoliths and coatings. Sometimes, the addition of carbon creates an electronic
path through the aerogel network [30].Carbon-silica aerogel composites can be used as a
catalyst support, which facilitates access of reagents to the electrocatalyst. 12 These examples
illustrate the versatility of carbon-silica aerogel composites in a wide range of applications
where their unique combination of mechanical, thermal, electrical, microstructural, and
chemical properties have opened up new possibilities.
4.2 EFFECT OF THE CARBON ADDITION
It has been previously reported that by incorporating active carbon powder in the silica
network, the aerogel elasticity increases [[31]. Carbon-silica aerogel composites were obtained
by adding powdered activated carbon (supplied by Norit, Darco KB) to the sol in some A1
and A4 samples just before gelation. The colloidal mixture was stirred for some minutes,
poured into cylindrical molds and covered with Parafilm. Activated carbon powders are
produced from carbonized graphite-like plates, followed by the steam activation process
where the carbonized material is reacted with steam at 1000 C. The carbon particle
aggregates have a diameter ranging from 10 to 45 m. This carbon shows a tamped bulk
density of 0.45g/cm3, high specific surface area (1500m2/g), and large elastic recovery. The
mass fraction of carbon in the aerogel,
aerogelcomposite
carbon
m
mx , was varied from 0 to 0.5. The A1
carbon-silica samples were labeled as A1c. In the composite A4 samples, the mass fraction of
carbon,
aerogelcomposite
carbon
m
mx , was 0.02, 0.15 and 0.5. Samples were accordingly labeled as C2, C15 and
C50, respectively. Supercritical extraction of the solvent took place in a high-temperature
process. All aerogel samples obtained were monolithic, in the shape of 5 - to 12-cm-long
cylindrical rods and 1cm in diameter. The apparent density was determined by accura tely
weighing samples of well-defined dimensions. The final densities of carbon-silica aerogel
Chapter IV Mechanical characterization of silica aerogels 161
composites are significantly lower than those corresponding to conventional silica aerogels.
The volume shrinkage, gel
gelaerogel
gel V
VV
V
V, produced during supercritical extraction was
determined by comparing the dimensions of the dry aerogels and the dimensions of the original
gel. Shrinkage during drying decreases dramatically as the mass fraction of carbon increases.
The cause of the lower densities is a consequence of the lower shrinkage observed for the
aerogel composites with high carbon content.
Figure IV.11 Photography comparing a silica aerogel sample with
a carbon-silica aerogel composite sample.
The most plastic aerogel sample (A4) and the most elastic sample (A1) have been used to
analyze this effect. In both cases, identical samples but with added carbon showed a much more
indentation elastic response than pure silica samples. This feature is illustrated in Figure IV.12,
which shows the load vs. penetration depth curves for the A4 sample and for samples with
carbon mass fractions of 0.02, 0.15 and 0.5. It can be seen that the addition of small amounts
of activated carbon to pure silica aerogel induces great changes in the shape of the indentation
curves. They change from curves characteristic of elastoplastic materials to those corresponding
to highly elastic materials, as reported in [31]. This increase in elasticity results, in higher EP
values of the composite aerogel samples, as it can be observed from the values listed in Table
IV.3, which also includes the carbon mass fraction, the density and the shrinkage corresponding
to all the listed samples. In particular, A1 and A1C are almost totally elastic materials, so they
have a very low Young‟s modulus and a high EP value. It should also be noticed that
experimental values of Young‟s modulus strongly decrease with slightly increasing carbon
mass fraction (Table IV.3).
Chapter IV Mechanical characterization of silica aerogels 162
Table IV.3 Several carbon mass fractions were used to prepare
the composite aerogel samples. Their densities, , and shrinkage,
V/V are listed below together with their mechanical properties: Hardness, H, Young‟s modulus, E, and elastic parameter, EP. Standard deviations are shown in parenthesis and refer to the last digit.
Sample label
Carbon percentage (mass %)
(g/cm3)
V/V (%)
H (MPa)
E (MPa)
EP (%)
A4 0 0.24 (1) 34.3 5.1 (2) 190 (5) 6 (1)
C02 2 0.21 (2) 29.8 3.4 (2) 52 (3) 29 (1)
C15 15 0.17 (2) 19.8 2.1 (2) 28 (2) 45 (2)
C50 50 0.15 (3) 7.2 1.7 (2) 23 (2) 54 (2)
A1 0 0.08 (1) - 0.5 (1) 7.0 (9) 78 (1)
A1C - 0.06(1) - 0.4(1) 9.0(8) 89(1)
The maximum applied load was 1 mN. For the A1 and A1C samples, the maximum load was
lowered to 0.5 mN due to the materials extreme softness.
Figure IV.12 shows applied load vs. penetration depth indentation curves „as measured‟ for each
sample for a loading–unloading cycle.
Chapter IV Mechanical characterization of silica aerogels 163
Figure IV.12 Penetration depth versus load indentation curves for
a pure SiO2 sample AC0 (▲) and composite aerogels with 2% of
carbon, AC2 (●), 15% of carbon AC15 (◊), and 50% of carbon
AC50 (□). The edge values are indicated for the non-recovery
penetration depth, hp, the elastic penetration depth, he, and the total
penetration depth, ht for the pure SiO2 sample (AC0).
It can be seen that the addition of small amounts of activated carbon to pure silica aerogel
induces great changes in the shape of the indentation curves. They change from those
characteristics of elastoplastic materials to those corresponding to highly elastic materials. From
these curves we have calculated three mechanical parameters; Young‟s modulus, E, the elastic
parameter, EP, and the hardness, H. Results are listed in Table IV.3, which also includes the
carbon mass fraction, the density and the shrinkage corresponding to all the studied samples.
Experimental values of Young‟s modulus strongly decrease with slightly increasing carbon
mass fraction (Table IV.3).
The dependence of the experimental Young‟s modulus on the carbon content of our aerogel
composites will be discussed by using the regular model for composite materials [32, 33].
The mechanical properties of composite materials depend on the mechanical properties of
the components, on the morphology of the phases and on the nature of the interface
0,0 0,2 0,4 0,6 0,8 1,0
0
1
2
3
4
5
Pe
ne
tra
tio
n d
ep
th (m
)
Load (mN)
AC0
AC50
AC2
AC15
0,0 0,2 0,4 0,6 0,8 1,0
0
1
2
3
4
5
Pe
ne
tra
tio
n d
ep
th (m
)
Load (mN)
AC0
AC50
AC2
AC15
Chapter IV Mechanical characterization of silica aerogels 164
between the phases. Figure IV.13 shows a schematic picture that illustrates two extreme
phase distribution models that can be considered to estimate Young‟s modulus of composite
materials.
(b)
rigid phase
elastic phase
0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0
0
20
40
60
80
100
120
140
160
180
200
E
(M
Pa)
x
(a)
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Figure IV.13 Composite Young‟s modulus values obtained for
various mass fractions of silica, showing the upper and lower limits
for the two extreme layered ideal composites. Hollow arrows
indicate the direction of the external stress
The maximum possible value for the composite modulus is described by a linear
combination of the Young‟s modulus for the filler, E filler, and the matrix, Ematrix. This is
expected when the two components of the composite are distributed in parallel, E‖, in the
way schematically shown in Figure IV.13(a). In this case, the Young‟s modulus value, E‖, is
described by equation (4) as a function of the carbon mass fraction, x, see Figure IV.13 [31].
matrixfiller ExxEE )1( (Eq. IV.4)
The lowest possible modulus for a composite material results when the two phases of the
composite are connected in series Figure IV.13 (b). Then, the Young‟s modulus E is
described by equation (5).
matrixfiller E
x
E
x
E
11 (Eq. IV.5)
This model also predicts that when the composite is particulate, then the expected values for
the Young‟s modulus will be close to the equation IV.5, but do not follow the exact
Chapter IV Mechanical characterization of silica aerogels 165
hyperbolic function. Experimental Young‟s modulus values listed in Table IV.3 are also
included in Figure IV.12. It is found that the incorporation of small amounts of carbon into
the silica aerogels causes a large decrease in the modulus of the composite. Modulus values
for higher concentrations of carbon do not follow Equation IV.5, but lie between equations
IV.4 and IV.5 as predicted for a particulate composite. The measured modulus values are
closer to the lowest curve of Figure IV.12. We understand it in the following way: for low
carbon concentration, the carbon particles could act as anchorage centers for silica branches
and then be incorporated within the silica network. This may be the cause of the strong
diminution in Young‟s modulus when a small amount of carbon is added. When the carbon
mass fraction increases, the percentage of the carbon particles not incorporated into the
silica network rises, forming a particulate composite with Young‟s modulus values that differ
from the hyperbolic behavior of equation IV.5.
The elastic parameter values listed in Table IV.3 allow us to quantify the improvement in the
elastic response to indentation previously illustrated in Figure IV.11. In particular, the EP values
are dramatically increased by incorporating low carbon amounts in the silica aerogel and they
become saturated for higher carbon concentrations. A possible explanation for the important
changes observed in the elastic behavior of the carbon silica aerogel composites is that the filler
can also disrupt the cross-linking of the silica aerogel network and lead to a less rigid structure.
This consequently reduces the brittle failure typical of fragile materials such as silica aerogels and
thus, the indentation plastic deformation.
The measured hardness values of carbon-silica composites show a slight decrease when carbon
content increases (Table IV.3). These experimental hardness values are close to those previously
reported for silica aerogels [15]. Hardness as a function of aerogel density can be fitted to a
power law function: H with a scaling exponent = 2.3 0.1, very similar to the one
expected for pure silica aerogels [15]. Therefore, hardness of the composite is mainly due to the
silica phase and is much less affected by the addition of carbon than the elastic parameter or
Young‟s modulus values are.
Chapter IV Mechanical characterization of silica aerogels 166
5. VISCOELASTICITY OF SILICA AEROGELS AT ULTRASONIC
FREQUENCIES
This section is included for completeness and shows the measurements of viscoelastic
properties using a technique based on the analysis of thickness resonance of air-surrounded
aerogel plates at ultrasonic frequencies. The measurements and analysis of the results were
performed by T. E. Gómez Álvarez-Arenas and F. R. Montero de Espinosa from the
„Instituto de Acústica-CSIC‟ [34]. The aerogels samples were prepared at the ‟Institut de
Ciència dels Materials de Barcelona‟. This technique allow us to measure the density, Young
Modulus and Poisson ratio of the samples, these results can be compared to those ones
obtained by microindentation.
5.1 INTRODUCTION
Specially designed air-coupled, high-sensitivity and broad-band piezoelectric transducers [35,
36, 37], operating in the frequency range of 0.3-2 MHz, were used in order to excite and
sense the resonance by airborne ultrasonic waves. This technique allows us to obtain precise
and simultaneous measurements of velocity and attenuation of longitudinal and shear waves
in silica aerogels at different frequencies as well as aerogel density. From these results, it has
been achieved a full characterization of the viscoelastic properties of silica aerogels [38,15],
over a significant frequency range, i.e. to obtain complex-valued and frequency-dependent
data for all the elastic constants.
Figure IV.14 Specially designed air-coupled, high-sensitivity and
broad-band piezoelectric transducers.
The technique is based on a silica aerogel plate embedded in a continuum medium (air),
which is insonificated by a broad-band airborne ultrasonic pulse. The frequency spectrum of
the ultrasonic pulse comprises several eigenfrequencies of the plate [39]. Eigenvibrations of
the aerogel plate excited by the incident wave are recorded and analyzed. The theoretical
air
Aerogel
plate
Piezoelectric
transducers
Chapter IV Mechanical characterization of silica aerogels 167
approaches to this problem is to impose boundary conditions to stress and strains fields at
the surfaces of the aerogel plate and solve the system of equations for field amplitudes [40].
By this procedure, overlapping resonance can be discerned. The viscoelastic nature of the
silica aerogel plate is introduced into the theoretical analysis by means of the correspondence
principle (i.e. introducing complex and frequency dependent elastic constants) [41].
The use of aerogels as acoustic impedance matching layers for air-coupled piezoelectric
transducers in the kilohertz frequency range has previously been reported [42, 43, 44].
Operating over 0.5 MHz, air-coupled transducers could be used for particular applications
that require airborne signals and high spatial resolution: non-destructive testing [45], medical
applications [46], surface characterization [47], etc. Therefore, new techniques for
generation and detection of airborne ultrasonic signals are under very active research (e.g.
micromachined capacitive transducers) [48]. Recently, it has been analyzed the possibility of
using aerogel matching layers for the frequency range of 0.5-2 MHz. [49]. The use of
piezoceramics together with a stack of matching layers where the outer layer is made of
aerogel may give rise to a transducer whose performance may be better to that achieved
from other techniques. [50]. For this purpose, velocity and attenuation of ultrasonic waves in
the aerogel must be accurately measured. While methods to determine ultrasonic longitudinal
velocity are well established, attenuation data are limited. Attenuation was measured by
Debye-Sears diffraction for the frequency range of 0.8-8 MHz [51]. At lower frequencies (20-
200 kHz) a reverberation method was used [52]. Experimental data about shear wave
propagation are very limited.
5.2 EXPERIMENTAL SET-UP: AIR-COUPLED BROAD-BAND
PIEZOELECTRIC TRANSDUCERS
The synthesis of the aerogel used in this work was described in more detail in chapter II
(TE00AT page 49, 60, and 62.). The molar ratios between reagents were TEOS/EtOH/H2O
= 1/7/5 and 0.003M citric acid. The gel was washed several times in pure ethanol. The silica
aerogel slab (2.4 cm diameter and 0.3 cm height) had a density of 0.160 g/cm3. Before the
measurements the sample was placed in a vacuum chamber and heated to 100 ºC to remove
any absorbed moisture
A pair of specially designed air-coupled piezoelectric transducers facing each other was used
for the experimental set-up. The Insertion Loss (IL) for the aerogel plate is defined as:
refsample AAIL 10log20 (Eq. IV.6)
Chapter IV Mechanical characterization of silica aerogels 168
where:
sampleA is the amplitude of the FFT (Fast Fourier Transform) of recorded ultrasonic
waves with aerogel plate in between the transducers
refA is the amplitude of the FFT of recorded ultrasonic waves without the aerogel.
In addition, the transmission coefficient, T, is defined by the ratio transmitted to incident
energy fluxes:
ref
sample21
A
AT (Eq. IV.7)
Details of the technique can be found elsewhere [53, 54]. Two cases were analyzed: normal
and oblique incidence. At normal incidence (angle of 19 ), shear waves are not generated in
the aerogel. In this particular case, and following the quantum-mechanical theory of
resonance scattering [55], a simple analytical expression for T can be derived:
122
41
~sin11 tkmmT l (Eq. IV.8)
where:
m is the ratio between complex acoustic impedances of aerogel and air complex
acoustic impedance.
t is the thickness of the plate.
lk~
is defined as llllll cicikk ~~, the complex wave vector in the
aerogel, where:
lk is the wave vector,
l the longitudinal wave attenuation,
lc the longitudinal phase-velocity
is the angular frequency
lc~ the complex longitudinal velocity of sound.
Resonance of the plate appear at maximum values of transmission coefficient, T, which take
place at nkt , n = 0, 1, 2… On the contrary, no simple analytical expression for T can be
derived for oblique incidence.
Chapter IV Mechanical characterization of silica aerogels 169
5.3 DETERMINATION OF THE VISCOELASTICITY OF SILICA
AEROGELS AT ULTRASONIC FREQUENCIES.
5.3.1 Normal incidence
Figure IV.15 shows the measured insertion loss, IL (dots), for normal incidence in a
frequency range from 0.3 to 1.3 MHz. Separation between resonance is almost constant and
equal to 59 1 kHz. First order resonance might be located at 59 kHz and the first peak in
Figure IV.15 (354.5 kHz) should then correspond to the sixth order peak (n=6).
Figure IV.15 Insertion loss versus frequency for a silica aerogel
plate. Dots: experimental measurements. Solid line: theoretical
results. Normal incidence.
Longitudinal velocity, lc , longitudinal attenuation, l, and density, are used as fitting
parameters to match the theoretical calculations of IL to the measured values. The results for
the fitted IL (solid line) are compared with experimental values (dots) in Figure IV.16).
Figure IV.16 shows the obtained values of lc and l at different resonance.
0.3 0.5 0.7 0.9 1.1 1.3frequency (MHz)
-45
-35
-25
-15
insert
ion loss (
dB
) (a)
n=6
59 KHz
Chapter IV Mechanical characterization of silica aerogels 170
Figure IV.16 Attenuation versus frequency of longitudinal ( ) and
shear () waves. Solid lines: power fitting. Velocity versus
frequency of longitudinal () and shear (▲) waves. (50‟).
The density, , obtained by IL fitting is 0.220 0.020 g/cm3. While longitudinal phase
velocity, lc , is rather constant (324 m/s), longitudinal attenuation, l follows a frequency (f)
power law:
,y1ll fαfα (Eq. IV.9)
where: y = 1.1 0.05 and )./(1016.3 5 y
1lHzmNpα
It is of interest to compare these measurements with results obtained from different
techniques. Up to now, lc and l were also measured following a conventional “through
transmission” technique. High-frequency (1.4 MHz) and broad-band transducers were
employed in order to be able to separate, in the time domain, the different echoes due to
multiple reverberations within the sample [56]. Measured values at transducer center
frequency (1.4 MHz) are: lc = (325 3) m/s and l = (210 20) Np/m. These results are
consistent with those obtained from the resonance technique.
In addition, results obtained here are consistent with data reported by other authors.
Attenuation data shown in References 10 and 11 for the frequency range 0-200 kHz and 2.5-
8 MHz follow a linear frequency law with: Hz.m
Np102.1
Hz.m
Np101.2 4
1l
5 .
0.0 0.4 0.8 1.2 1.6frequency (MHz)
0
75
150
225
300
375
attenuation (
Np/m
)
185
210
235
260
285
310
335
velo
city
(m/s
)
Longitudinal
wave
Shear wave
Shear wave
Chapter IV Mechanical characterization of silica aerogels 171
Measurements for different aerogel samples having density values close to 0.2 g/cm3 provide
values of lc in the range 200-500 m/s [57, 58].
5.3.2 Oblique incidence
Figure IV.17 shows the measured IL (dots) for oblique incidence (19º). Interferences due to
the overlap of longitudinal and shear resonance are clearly appreciated.
Figure IV.17 Insertion loss versus frequency for a silica aerogel
plate. Dots: experimental measurements. Solid line: theoretical
results for incidence angle of 19º.
Using the values of llc and obtained in the analysis of IL for normal incidence, shear
wave phase-velocity ( tc ) and shear wave attenuation ( t ) were used as fitting parameters to
match theoretical calculations of IL to experimental values. The higher accuracy for
ttc and are obtained at frequencies were interferences between longitudinal and shear
resonance appear. The result for IL is shown in Figure IV.17 (solid line). Obtained results
for ttc and are shown in Figure IV.17. While tc is rather constant (197 m/s), results for
t can be adjusted by a power law:
5.0
1t
y
1tt Hz/m/Np226.0,15.05.0ywhere,ffα (Eq. IV.10)
For frequencies higher than 0.9 MHz, the uncertainty in the determination of t becomes
very high. Therefore, the error in the determination of the y exponent is also very high.
Using thinner samples and/or larger incidence angles might solve this problem.
0.3 0.5 0.7 0.9 1.1 1.3frequency (MHz)
-45
-35
-25
-15
insert
ion loss (
dB
)
(b)
Interaction longitudinal share
waves
Chapter IV Mechanical characterization of silica aerogels 172
Complex elastic constants and related viscoelastic parameters can be calculated from the
analysis of the exponential curves. Results are gathered in Table IV.4.
The complex shear modulus G is obtained from: 2~tcG . The complex elastic modulus M
is defined as: M=K+4/3G 2~lc , where K is the bulk modulus, and tc
~ is the complex shear
wave velocity. E is the Young Modulus defined by GK3
GK9E , with real values of the
same order as the ones found using microindentation [15]. Poisson coefficient, which is
difficult to obtain from other techniques, is calculated from: 1r2
1r2, where
2
l
2
t
c
cr .
Table IV.4 Poisson coefficient, , and the three complex elastic
modulus: complex shear modulus, G, shear loss, Gtan , complex
elastic modulus, M, and Young Modulus E are measured at different frequencies, f, from 0.44 MHz to 1.2 MHz.
f
(MHz)
G
(MPa) real(G)
imag(G)
Gδtan 1 M
(MPa) real(M )
imag(M )
M
1δtan E
(MPa) real(E)
imag(E)
E
1δtan
0.44 0.214 8.36+0.18i 0.0212 22.98+0.32i 0.0140 20.30+0.38i 0.0185
0.63 0.209 8.49+0.15i 0.0179 23.01+0.28i 0.0124 20.53+0.32i 0.0158
0.82 0.207 8.54+0.14i 0.0164 23.01+0.31i 0.0132 20.60+0.31i 0.0151
1.0 0.208 8.54+0.14i 0.0166 23.16+0.36i 0.0155 20.62+0.33i 0.0162
1.2 0.204 8.58+0.13i 0.0151 23.09+0.38i 0.0165 20.67+0.32i 0.0156
A detailed analysis of data shown in Table IV.4 reveals that none of the calculated elastic
moduli follows the frequency dependence predicted by any of the simple and basic
viscoelastic single relaxation models (Voigt and Maxwell). On the contrary, bearing in mind
that attenuations follow a frequency power laws (Equations. IV.9 and IV.10), a possible
alternative to single relaxation models could be a time causal model [59, 60].
The real value of E ranged from 20.3 to 20.7 Mpa, this values are of the same order as the
ones found using microindentation where a sample with a density similar to that one
(0.23g/cm3) was of 30Mpa (M aerogel). Moreover, Poisson ratio ranged from 0.204 to 2.214,
this values confirms that one used in microindentation, =0.2. In some aspects, the behavior
of the aerogel observed here resembles that of some well investigated materials. These
similarities permit to gain an insight into the underlying physics. For example, the shear
Chapter IV Mechanical characterization of silica aerogels 173
modulus (G) and shear loss ( Gtan ) exhibit a similar behavior to those reported for some
kind of polymers. For such polymers, the interpretation given is based on non-local
cooperative interactions of large molecules, which could also be applied for aerogels [60, 61].
Another interesting feature is provided by the fact that fctetanQ lM
1
M .
This relationship has also been observed, for some kind of aerogels, at higher and lower
frequencies [51, 52]. A similar behavior has been found in marine sediments (water-saturated
and dry sediments). Theoretical predictions of l in fluid-filled porous media (as sediments)
are based on the interaction between the fluid in the pores and the solid skeleton. This
provides a dependence of the attenuation with the frequency that varies at
or , 212 fff depending on the frequency range involved [62]. On the contrary,
experimental measurements over a wide frequency range suggest a linear frequency
dependence for l. This is the object of a long lasting argument between experimentalists
and theoreticians [62]. To explain this experimental behavior, different sources of dissipation
leading to an attenuation proportional to ƒ (e.g. friction at the contact area between particles
of the frame) were introduced into the theoretical modeling. The understanding of sound
attenuation in aerogels may benefit from this. In its turn, low-density aerogels exhibit
2fl , and some authors consider that this behavior is due to the influence of the fluid in
the pores [51, 63].
6. MECHANICAL CHARACTERIZATION CONCLUSIONS
In conclusion, the microindentation technique has proved to be a non-destructive, suitable
method to assess the parameters that characterize the mechanical behavior of extremely
porous materials such as aerogels, despite their brittleness and softness.
Silica aerogels of different mechanical responses have been obtained by varying the initial
parameters in the TMOS sol-gel polymerization process, such as alkoxide concentration,
solvent, drying process, as well as the carbon addition. Creep effects are very important for
these samples and should be taken into account during measurements and in the interpretation
of the microindentation results. Initial conditions have a pronounced influence on the density
of the aerogels and also on their microstructure, which is reflected by their mechanical behavior.
The A-series samples show two different types of mechanical behaviors; the low-density
aerogels are elastic, while the denser aerogels behave as elasto-plastic materials. Young‟s
Chapter IV Mechanical characterization of silica aerogels 174
modulus, hardness and the elastic parameter have been evaluated for these aerogel samples.
It is shown that the evolution of the parameters describing the mechanical behavior as a
function of the bulk density follows power-scaling laws. The evaluated exponents are 3.0, 2.0
and -3.2 for Young‟s modulus, hardness, and Elastic Parameter, respectively .
The large dependency of Young‟s modulus on the density as well as the change from elastic to
plastic behavior has its origin on the aerogel microstructure. Further structural and modelization
efforts are needed to a deeper understanding of these dependencies.
The effects of the solvent, being methanol or acetone, on the silica aerogels have also been
studied. It was found that for samples with similar density values, the ones obtained using
acetone have higher hardness and Young‟s modulus values than those obtained from
methanol, but with less elastic recovery. Moreover, the effect of the drying process has been
studied. It has been shown that the process with CO 2-acetone exchange causes a slight
improvement in hardness and a relevant increase in the elastic modulus, mostly due to larger
shrinking effects. From the results presented in the previous section, we conclude that the
addition of small amounts of powdered carbon as filler in silica aerogels increases the
elasticity of the composite and keeps the hardness similar to silica matrix values. It has been
found that including small amounts of activated carbon inside the fragile silica network
dramatically increases its elastic indentation recovery. This is reflected in the shape of the
indentation curves as well as in the increase of the elastic parameter value, which evaluates
the percentage of elasticity versus plasticity. Young‟s modulus values obtained for carbon-
reinforced aerogels show a similar variation to the carbon mass fraction as that predicted by
a commonly used model for composite materials. The measured hardness values
corresponding to the total elastoplastic deformation do not show such a prominent
dependency on the carbon mass fraction as the elastic parameter and Young‟s modulus do
and they are similar to those measured for the pure silica aerogel.
Moreover, we present an experimental technique that do not require any sample machining
to simultaneously measure velocity and attenuation of longitudinal and shear waves in
aerogels. A fully viscoelastic characterization of the aerogel is obtained and a deeper insight
on aerogel basic properties is gained.
Chapter IV Mechanical characterization of silica aerogels 175
7. REFERENCES
1 K. Parmenter, and F. Milstein, J. Non-Cryst.
Solids 223, 179 (1998).
2 J. Gross, G. Reichenauer, and J. Fricke, J.
Phys. D 21, 1447 (1988).
3 M. Gronauer, A. Kadur, and J. Fricke in:
J.Fricke (Ed), Aerogels, Springer, Berlin 167
(1986).
4 R.W. Pekala, L.W.Hrubesh, T.M. Tillotson,
C.T. Alviso, J.F. Poco, and J.D. Le May, Mat.
Res. Soc. Symp. Proc. 207, 197 (1991).
5 T. Woignier, J. Phalippou, H. Hdach, and
G.W. Scherer, Mat. Res. Soc. Symp. Proc. 180,
1087 (1990).
6 T. Woignier, and J. Phalippou, Rev. Phys.
App 24, C4 179 (1989).
7 L.E. Nielsen, and R.F. Landel, Mechanical
properties of polymers and composite, (Marcel
Decker Inc., New York, 1994), p. 392.
8 C.A. Rutiser, S. Komarneni, and R. Roy,
Mat. Res. Soc. Symp. Proc. 371, 223 (1995).
9 A.Venkateswara Rao, G.M. Pajonk, B.
Haranath, and P.B. Wagh, Microporous
Materials 12, 63, (1997).
10 M. Gronauer, and J. Fricke, Acustica 59,
177 (1986).
11 J. Pethica, R. Hutching, and W.C. Oliver,
„Hardness measurements at penetration depths
as small as 20nm‟ Philos. Mag. A48, 593
(1983).
12 W.C. Oliver, R. Hutching and J. Pethica,
„Measurements of hardness at indentation
depths as low as 20nanomters‟
Microindentation Techniques in Material
Science, P.J Blau and B. Lawn (Eds.) ASTM
Philadelphia (1986) 90-108.
13 M. F. Doerner, and W.D. Nix, „A method
for interprating the data from depth-sensing
indentation instruments‟, J. Mater. Res 1, 601
(1986).
14 J. Loubet, G. Georges, „Vickers indentation
curves of elastoplastic materials‟
Microindentation Techniques in Material
Science, P.J Blau and B. Lawn (Eds.) ASTM
Philadelphia (1986) 72-89.
15 M. Moner-Girona, A. Roig, E. Molins, E.
Martínez, and J. Esteve, Appl. Phys. Let. 75,
653 (1999)
16 W.C. Oliver, and G.M. Pharr, J. Mater. Res.
7, 1564 (1992).
17 Gross, G. Reichenauer, J. Fricke, J. Phys.
D: Appl. Phys 21 (1988) 1447.
18 A.V. Rao, G.M. Pajonk, N.N. Parvathy,, J.
Mat. Sci 29 (1994) 1807.
19 M. F. Doerner, and W.D. Nix, J. Mater. Res
1, 601 (1986).
20 R.W. Pekala, L.W.Hrubesh, T.M.
Tillotson, C.T. Alviso, J.F. Poco, and J.D. Le
May, Mat. Res. Soc. Symp. Proc. 207, 197
(1991).
21 T. Woignier, J. Phalippou, H. Hdach, and
G.W. Scherer, Mat. Res. Soc. Symp. Proc. 180,
1087 (1990).
22 T. Woignier, and J. Phalippou, Rev. Phys.
App 24, C4 179 (1989).
23 J. Gross, and J. Fricke, J. Non-Cryst. Solids
95/96,1197 (1987).
24 W. Schaefer, and K.D. Keefer, Phys. Rev.
Lett. 56, 2199 (1986).
25 G.M. Pajonk, A.V. Rao, N.N. Parvathy, E.
Elaloui , J. Mat. Sci 31 (1996) 5683
26 M. Pauthe, J. Phalippou J. Rev. Phys. App.
24 (1989) 215
Chapter IV Mechanical characterization of silica aerogels 176
27 C.A. Morris, M.L. Anderson, R.M. Stroud,
C.I. Merzbacher, and D.R. Rolison, Science
284, 622 (1999).
28 X. Lu, P. Wang, D. Büttner, U.
Heinemann, O. Nilsson, J. Kuhn, and J.
Fricke, High Temp. - High Press. 23, 431
(1991).
29 D. Lee, P. Stevens, S. Q. Zeng, and A.
Hunt, J. Non-Cryst. Solids 186, 285 (1995).
30 Th. Rettelbach, J. Säuberlich, S. Koreder,
and J. Fricke, J. Non-Cryst. Solids 186, 278
(1995).
31 M. F. Ashby, and D.R.H. Jones,
Engineering Materials (Butterworth
Heinemann, London, 1996), Vol 1, p. 63.
32 Z. Hashing, S. Shtrikman; J. Mech. Phys.
Solids 10, 335 (1962).
33 Z. Hashing, S. Shtrikman; J. Mech. Phys.
Solids 11, 127 (1963).
34 T. E. Gómez Álvarez, F. R. Montero, M.
Moner-Girona, E. Rodríguez, A. Roig and E.
MolinsViscoelasticity of silica aerogels at
ultrasonic frequencies, Applied Physics
Letters (accepted for publication).
35 . T.E. Gómez Álvarez-Arenas and F.
Montero de Espinosa. Bol. Soc. Esp. Cerám.
Vidrio 41(1), 16 (2002).
36 F. Montero, T. E. Gómez, A. Albareda, R.
Pérez, J. A. Casals. 2000 IEEE Ultrasonics
Symp. Proceedings, 1073, (2000).
37 S. P. Kelly, G. Hayward and T.E. Gómez.
2001 IEEE Ultrasonics Symp. Proceedings
(2001).
38 Y. Xie, J. R. Beamish. Phys. Rev. B 57(6),
3406, (1998).
39 L. Flax, G. C. Gaunaurd and H. Überall.
in Physical Acoustics vol. XV, edited by W. P.
Mason and R. N. Thurston (Academic Press,
1981).
40 L. M. Brekhovskikh. Waves in layered media
(Academic Press, New York, 1960)
41 . D.R. Bland. The Theory of linear
viscoelasticity (Pergamon Press, 1960).
42 R. Gerlach, O. Kraus, J. Fricke. J. of Non-
Cryst. Solids 145, 227 (1992).
43 O. Kraus, R. Gerlach and J. Fricke,
Ultrasonics 32 (3), 217 (1994).
44 V. Gibbiat, O. Lefeuvre, T. Woignier, J.
Pelous, J. Phalippou. J. of Non-Cryst. Solids
186, 244 (1995).
45 S. P. Kelly, R. Farlow, G. Hayward. IEEE
Trans. Ultrason., Ferroelect., Freq. Contr.,
43(4), 581. (1996).
46 . J. P. Jones, D. Lee, M. Bardwaj, V.
Vanderkam, B. Achauer. Acoustical Imaging.
23, 89, (1997).
47 T. E. Gómez y F. Montero. 2001 IEEE
Ultrasonics Symposium Proceedings, (2001).
48 I. Ladabaum, B. T. Khuri-Yakub, D.
Spoliansky, M.I. Haller. 1995 IEEE
Ultrasonics Symposium Proceedings, 501
(1995).
49 T. E. Gómez, F. Montero, M. Moner-
Girona, E. Rodriguez, A. Roig, E. Molins,
J.R. Rodríguez, S. Vargas, M. Esteves. 2001
IEEE Ultrasonics Symp Proceedings (Atlanta
7-10), (2001).
50 T.E. Gómez Álvarez-Arenas and F.
Montero de Espinosa. Bol. Soc. Esp. Cerám.
Vidrio 41(1), 16 (2002).
51 T. Schlief, J. Gross and J. Fricke J. Non-
Crys. Solids 145, 223, (1992).
52 A. Zimmerman, J. Gross and J. Fricke. J.
Non-Cryst. Solids 186, 238 (1995).
Chapter IV Mechanical characterization of silica aerogels 177
53 S. P. Kelly, G. Hayward and T.E. Gómez.
2001 IEEE Ultrasonics Symp. Proceedings
(2001).
54 . T. E. Gómez, F. Montero, 2000 IEEE
Ultrasonics Symp. Proceedings,1069 (2000).
55 L. Flax, G. C. Gaunaurd and H. Überall.
in Physical Acoustics vol. XV, edited by W. P.
Mason and R. N. Thurston (Academic Press,
1981).
56 W. Sachse and H. Y. Pao. J. Appl. Phys.
49 (8), 4320, (1978).
57 . J. Gross, J. Fricke J. Acoust. Soc. AM.
91(4), 2004, (1992).
58 J. Gross, G. Reichenauer and J. Fricke, J.
Phys. D 21, 1447 (1988).
59 T. L. Szabo and J. Wu. J. Acoust. Soc.
Am. 107 (5), 2437, (2000).
60 T. L. Szabo. J. Acoust. Soc. Am. 97 (1),
14, (1995).
61 J. D. Ferry. Viscoelastic properties of polymers
(John Wiley & Sons, 1980).
62 A. C. Kibblewhite. J. Acoust. Soc. Am. 86
(2), 716, (1989).
63 J. Fricke and T. Tillotson. Thin Solid
Films 297, 212 (1997).
C h a p t e r V
SILICA AEROGEL MICROPARTICLES
SECTION OUTLINE
1. SOL-GEL ROUTE TO DIRECT FORMATION OF SILICA AEROGEL
MICROPARTICLES USING SUPERCRITICAL ETHANOL/ACETONE........................ 181
1.1. ‘IN SITU’ PARTICLE PROCESSING .................................................................................. 182
1.2. AEROGEL MICROPARTICLE CHARACTERIZATION ................................................ 184
1.2.1 Scanning Electron Microscopy ............................................................................185
Independent solutions .................................................................................. 189
1.2.2 Transmission Electron Microscopy ....................................................................190
1.2.3 Atomic Force Microscopy ..................................................................................... 191
2. SOL-GEL ROUTE TO DIRECT FORMATION OF SILICA AEROGEL
MICROPARTICLES USING SUPERCRITICAL CARBON DIOXIDE ...............................195
2.1. ‘IN SITU’ LOW-TEMPERATURE MICROPARTICLES: TEOS, HCOOH, AND
SUPERCRITICAL CO2 AS SOLVENT.................................................................................. 196
2.2. PRECURSOR DIRECTLY INJECTED IN CO2 SUPERCRITICAL CONDITIONS
AT LOW TEMPERATURE ..................................................................................................... 198
2.2.1 Injection of hydrolysis and precursor solution independently ......................199
2.2.2 Injection of sol ........................................................................................................ 203
2.3. IN SITU’ INJECTION IN LIQUID CO2............................................................................... 203
2.4. INJECTION OF PREPOLYMERIZED PRECURSOR IN SUPERCRITICAL CO2 ..... 205
3. CONCLUSIONS...................................................................................................................... 208
4. REFERENCES ........................................................................................................................ 208
Chapter V Silica aerogel microparticles 180
This chapter focuses on the preparation of silica porous microspheres directly in the high-
pressure reactor. This process allows the use of supercritical fluids in manufacturing fine
particles with high porosity.
Different approaches were undertaken to obtain silica aerogel microparticles by this method
and two routes were used depending on the supercritical fluid chosen. High temperature
method when acetone or ethanol were used as solvents, and low temperature method if using
carbon dioxide.
Silica in the form of microspheres is a material of interest in several fields. The Stöber [1]
method for the synthesis of nonporous silica spheres via the hydrolysis and condensation of
tetralkoxysilane has been well recognized for producing particles with very narrow
distributions. Moreover, several methods have been attempted for obtaining silica
microparticles. These include nozzle-reactor systems (spray drying or pyrolysis) [2-4], and
emulsion/phase separation techniques with sol-gel processing [1-2, 5-10].
Aerogel microspheres offer an attractive alternative for microparticle applications. In general,
porous microparticles provides an advantage over nonporous materials when large surface
areas can be an advantage. The resulting silica aerogel particles would exhibit much of the
surface area characteristic of a silica particle, while maintaining the narrow size distribution of
Stöber-type particles. Silica aerogel microparticles can be used for a range of applications,
including: as additives for conventional foaming operations, controlled release agents,
encapsulation of products, gas separations, advanced thermal and acoustic insulation,
microelectronics, optics, catalysis, high surface-area adsorbents [2], and chromatography
materials. The use of small (i.e., <5.0 μm) diameter particles in ultra -high pressure liquid
chromatography provides both an increase in resolution and a decrease in analysis time over
more traditional column packing materials [11-12].
Supercritical fluids have received considerable attention as solvents or reaction media for the
processing of powders, particles, fibers and coatings [13-17], since they offer a combination
of gas like properties (viscosity, diffusion coefficient) and liquid like properties (density).
Moreover, the synthesis of aerogel microparticle process at low temperature uses supercritical
carbon dioxide. CO2 is an attractive medium because, is a non-toxic, non-flammable and
Chapter V Silica aerogel microparticles 181
inexpensive fluid with a low critical temperature. A simple and versatile method to obtain
silica aerogel particles based on the hydrolysis and subsequent condensation of silicon
alkoxides in several supercritical fluids is developed in this chapter. This microparticle
production route reduces the number of steps of traditional microparticle sol-gel processing.
The proposed method is based on two factors: the solvent power of the supercritical fluid
and the sol-gel reaction under these specific conditions.
Finally, efforts were aimed on understanding the relationship between the structure and the
synthesis conditions of these types of materials, the particle formation mechanisms and on
the conditions to tailor the particle morphology, size and porosity.
1. SOL-GEL ROUTE TO DIRECT FORMATION OF SILICA AEROGEL
MICROPARTICLES USING SUPERCRITICAL ETHANOL/ACETONE
In this section monodisperse, microspherical aerogel and fiber silica morphologies powders
were produced inside an autoclave by sol-gel process by using supercritical ethanol or
acetone as solvent. The influence of different synthesis parameters and the conditions of the
synthesis media on the structure and morphology of the obtained aerogel silica particles were
investigated.
Figure IV.1 Scheme of the silica aerogel particle production by performing the sol-gel process under supercritical conditions.
Particle size was mainly controlled by varying the relative amounts of alkoxysilane, water and
acetone. Other important parameters in order to control the particle size are the relative time,
t/tgelification, and the order of introducing the reactants in the autoclave. It has been observed
that a relevant factor to control the particle morphology is the venting rate of the supercritical
Sol in process of
gelification
Solvent at
supercritical conditions
Colloidal particles
Chapter V Silica aerogel microparticles 182
fluid. The morphology of the particles was characterized by electron microscopy (SEM and
TEM) and Atomic Force Microscopy (AFM). Coulter technique was also used to account for
the particle size distribution. In addition, BET surface area, pore volume and mean pore
diameter, < pore>BET, of the particles were obtained from nitrogen sorption isotherms.
Reagent grade alkoxides and solvents were used without further purification. Reactions were
carried out either in a specially designed high-pressure 300-ml cell or in a 2000-ml reactor.
(See annex II: Facilities). The basic component of the system is a syringe pump that is able of
pumping fluids inside the autoclave through a nozzle.
1.1 ‘IN SITU’ PARTICLE PROCESSING
The ‘in situ’ process consisted in the following steps:
i) A sol in a colloidal initial stage (containing acetone as a solvent, tetramethoxysilane
as a precursor and water) was placed inside the reactor.
ii) The reactor was then driven above supercritical values of the acetone, Pc = 47 Bar,
Tc = 235 .
iii) The sol was solubilized in the supercritical acetone distributing it over the entire
reactor leading to the formation of gel particles.
iv) Finally, after a remaining time the solvent was extracted from the pores of the gel
particles at supercritical conditions.
Two different experiments were tested: in the first experiment, a sol (formed by acetone as a
solvent, tetramethoxysilane (TMOS) as a precursor and water) was placed inside the reactor:
Samples A, B, and C, (See Table IV.1). In the second experiment, the alkoxide precursor,
the solvent and water were introduced in two independent vessels. One with the hydrolysis
solution (water dissolved in acetone) and the precursor solution (TMOS dissolved in acetone)
in the other vessel: Sample D (See Table IV.2). Figure IV.2 shows a simplified scheme of the
‘in situ’ process for the two set-ups either as single vessel with containing the sol (1a path in
Figure IV.2) or two independent solutions (1b path in Figure IV.2).
Chapter V Silica aerogel microparticles 183
Figure IV.2 Illustration of the synthesis scheme for aerogel porous silica spheres by 'in situ' process either with sol (1b path): sample D or with independent solutions (1a path): samples A, B, and C.
In all the experiments the v volume ratio, acetoneTMOS
TMOS v , was fixed at 0.05. This
highly diluted sol was used to avoid the gelification of the sol before the supercritical
conditions were reached. The hydrolysis parameter, h, is defined, as in previous chapters, as
the molar ratio TMOS
OHh 2 . The h value was varied from 2 to 8 in order to observe the
effect of water concentration in the morphology of the aerogel microparticles. The obtained
samples were: A (h=2), B (h=4) and C (h=8).
H2O
+ acetone
silica
particles
TMOS
+
acetone
autoclave
sol
supercritical
fluid
Acetone
supercritical conditions a
)
b
)
Venting the
autoclave
1a)
2)
3)
4)
Sample D
Samples A, B, C
Chapter V Silica aerogel microparticles 184
Table IV.1 Mean particle size obtained by varying the hydrolysis parameter, h.
Sample h Mean Particle Size Distribution width
A 2 1.2 m 0.2 m
B 4 1.7 m 0.7 m
C 8 2.2 m 0.6 m
In a second set of the experiments the same ratio were reproduced but the solvent and water
were placed in the reactor independently in two vessels, one with the hydrolysis solution
(water + acetone) and in the other the precursor solution (TMOS + acetone), avoiding the
premature gelification because the temperature effect. See process in Figure IV.2. A bimodal
distribution in particle sizes was found as seen in Table IV.2.
Table IV.2 Mean particle size obtained by independent solutions and v volume ratio 0.05
Sample h Particle Size
D 4 630nm 1.2 m
1.2 AEROGEL MICROPARTICLE CHARACTERIZATION
Particles were collected and characterized by a Leica 360 Scanning Electronic Microscope
(SEM), a JEM 100CX Transmission Electron Microscope (TEM), a Nanoscope III Digital
Instruments Atomic Force Microscope (AFM), N2 adsorption-desorption (Micromeritics
ASAP 2000 instrument), and He-pycnometry.
In all the ‘in situ’ experiments, a white dry powder was uniformly distributed all over the walls
and base of the reactor with a deposition thickness from 10 to 100 m depending on the
initial parameters. No liquid was found, indicating a complete reaction of the initial reactives
of the sol.
a
)
Chapter V Silica aerogel microparticles 185
Figure IV.3 Several microparticles were collected depending on the initial conditions
An approximate value of the apparent density of the powder was obtained by measuring the
mass of a known volume of powder. The apparent powder density was between 0.06g/cm3
and 0.08g/cm3 depending on the initial parameters. The surface area of the particles was
characterized by nitrogen absorption (Brunauer, Emmet and Teller method). The technique
allows us to obtain the surface area of a material using the adsorption and desorption of an
inert gas, usually nitrogen. Gas adsorption measurements reveal the microspheres to have
surface areas from 400 to 600 m2/g, similar values to those found on their monolithic
counterparts. By pycnometry technique, it was checked that the skeleton density has the same
value to that of the silica (2.2 g/cm3) [18]. The particle surface microstructure was
characterized by several microscopy techniques (SEM, TEM and AFM) and it is shown in
the following sections.
1.2.1 Scanning Electron Microscopy
An example of a SEM micrograph from A experiment (h=2) is shown in Figure IV.4. The
formation of two kinds of morphologies: interlinked fibers and isometric spherical particles
are evidenced when a zoom is performed in this image (see Figure IV.5).
Initial
conditions
=400nm
=75nm
=1.8 m
=2.2 m
Chapter V Silica aerogel microparticles 186
Figure IV.4 Porous deposition of silica aerogel microparticles
for A sample with 100 m of thickness.
As already mentioned, the type of morphology depends on the rate of depressurization a
high venting rate implied the presence of interlinked fibbers.
Figure IV.5 a. Referring to sample A: Scanning electron micrograph of aerogel spherical shape silica particles narrowly
distributed in size, particle diameter between 0.50 and 2 m. b. Fiber-like structure with a diameter of 75 nm and a length of some microns.
100 m100 m100 m
7.5 m7.5 m7.5 m 750 nm750 nm750 nm
Chapter V Silica aerogel microparticles 187
The mean particle diameter and the particle size distribution could be modified by changing
the values of the h parameter. See from Figure IV.6) to 8). Table IV.1 shows the variation for
the high temperature experiments of the mean particle diameter , and the width of the
distribution 2 , when the TMOS dilution in acetone was fixed at 0.05 and the hydrolysis
parameter, h, was changed.
Figure IV.6 SEM micrograph and particle size distribution for sample A: v = 0.05, h=2.
Figure IV.7 SEM micrograph of the spherical particles and its particle size distribution for sample B: v = 0.05, h=4.
0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 2,2 2,4 2,60
5
10
15
Gaussian f it (0.3-2.6 0.1)
MeanSDAr ea
---------------------------------------------
1,182930,246498,4838
=1.2 m
2 =0.2 m
m
m
Particle diameter ( m)
0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 2,2 2,4 2,60
10
20
30Gaussian f it
Mean SD
-----------------------
1,676 0,359
Particle diameter ( m)
=1.7 m
2 = 0.7
0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 2,2 2,4 2,60
10
20
30Gaussian f it
Mean SD
-----------------------
1,676 0,359
Particle diameter ( m)
0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 2,2 2,4 2,60
10
20
30Gaussian f it
Mean SD
-----------------------
1,676 0,359
Particle diameter ( m)
0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 2,2 2,4 2,60
10
20
30Gaussian f it
Mean SD
-----------------------
1,676 0,359
0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 2,2 2,4 2,60
10
20
30Gaussian f it
Mean SD
-----------------------
1,676 0,359
Particle diameter ( m)
=1.7 m
2 = 0.7
m
m
Chapter V Silica aerogel microparticles 188
Figure IV.8 SEM micrograph and particle size distribution for Sample C. Where v = 0.05 and h=8.
The size of the collected particles was very sensitive to the water/alkoxide concentration
ratio: the higher the ratio, the smaller the particles, maintaining the width of dispersion
constant.
The size distribution and mean size value were measured by Coulter in order to compare the
results by those obtained by SEM. Figure IV.9a-b compares the Coulter technique and image
analysis size distribution for A sample =0.05, h=2.
Figure IV.9 a) Coulter size distribution
for A sample = 0.05, h=2.
Figure IV.9 b) SEM size distribution for
A sample = 0.05, h=2.
Both techniques show a similar particle size distribution.
Nu
mb
er
part
icle
s (
%)
2
4
6
8
SEM
Particle
Distribution
0,0 0,5 1,0 1,5 2,0 2,5 3,0
Particle diameter (um)
0,0 0,5 1,0 1,5 2,0 2,5
0
1
2
3
4
5
6
Particle diameter (um)
Coulter
Particle
Distribution
Nu
mb
er
part
icle
s (
%)
Mean particle diameter
= 1.2 ± 0.4 m
0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 2,2 2,4 2,6 2,80
10
20
30
40
Gaussian f it
MeanSD
------------------------------
2,176 0,309
Particle diameter ( m)
=2.2 m
2 = 0.6 m
0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 2,2 2,4 2,6 2,80
10
20
30
40
Gaussian f it
MeanSD
------------------------------
2,176 0,309
Particle diameter ( m)
=2.2 m
2 = 0.6 m
Chapter V Silica aerogel microparticles 189
Independent solutions
In the case of independent solutions, sample D, a differentiated morphology was observed.
This experiment allows us to observe the effect of water in the interlinkage of the
microstructure. Two different structures were observed, one placed inside the water recipient,
it is structured as a spider's web. This feature was formed by interlinkage of nanometric
spheres giving place to a fibrilar structure (Figure IV.10).
Figure IV.10 Spider’s web with a fiber diameter of 60 nm collected from the recipient where initially the water was dissolved in acetone.
The second structure was found overall the reactor as a 'snow' powder formed by
micrometric spheres, distributed as a bimodal function.
Figure IV.11 Bimodal particle size distribution for D sample.
0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 2,2 2,4 2,60
2
4
6
8
10
Gauss(2)
Peak Center Width Height
-----------------------------------------------------------
1 0,6320,352 6,34
2 1,244 0,182 3,72
-------------------------------------------------------
No
mb
re p
art
ícu
les
Diàmet re par t ícula (um)
1 =1.2 m
2 =0.6 m
2 1=0.2 m
2 2=0.1 m
Particle diameter
( m)
Chapter V Silica aerogel microparticles 190
1.2.2 Transmission Electron Microscopy
The preparation method that was employed was by grounding a small amount of silica
aerogel into a fine powder in a mortar. The powder was ultrasonically suspended in inert
liquid (methyl alcohol) and the finest particles are selected after decantation. A droplet of the
suspension is then put on the usual electron microscope copper grid.
TEM analysis was useful as to visualize with more detail the contact region between the
microparticles as can be seen in Figure IV.12, although the resolution was not good enough
as to visualize the fine nanostructure of the microspheres.
Figure IV.12 Sample A, TEM microphotograph shows the contact area between particles.
The degree of necking between particles was mainly related to the remaining time of the
dispersed sol at supercritical conditions. Further studies should be undertaken in order to
better determine this phenomena.
1.2.3 Atomic Force Microscopy
To visualize the roughness and microstructure of the particle’s surface an Atomic Force
Microscope (AFM) in non-contact mode was used. The AFM probes the surface of a sample
with a sharp tip, a couple of microns long and less than 100Å in diameter. The tip is located
at the end of a cantilever that is 100 to 200µm long. Forces between the tip and the sample
surface cause the cantilever to deflect. A detector measures the cantilever deflection as the tip
is scanned over the sample, or the sample is scanned under the tip. The measured cantilever
deflections allow a computer to generate a map of surface topography. AFM can be used to
250 nm
Chapter V Silica aerogel microparticles 191
study insulators. Non-contact AFM is one of several vibrating cantilever techniques in which
an AFM cantilever is vibrated near the surface of a sample. The spacing between the tip and
the sample for non-contact AFM is on the order of tens to hundreds of angstroms. In non-
contact mode, the system vibrates with amplitude of a few tens of angstroms for a stiff
cantilever near its resonant frequency (typically from 100 to 400 kHz). Changes in the
resonant frequency or vibration amplitude as the tip comes near the sample surface are
detected. There exists a relationship between the resonant frequency of the cantilever and
variations in sample topography. Thus, changes in the resonant frequency of a cantilever
reflect changes in the tip-to-sample spacing so allows to image sample topography.
In order to visualize the microstructure of the A particle’s surface an AFM was used. Before
the observation the particles were dispersed. A small quantity of powder (0.5 mg) was
dispersed in distilled water (50 ml) using ultrasounds (5 min). AFM was used in a high
amplitude resonant mode (‘tapping’ mode). The scale rate was around 1Hz. Squared images
were acquired by using a digitalization rate of 256 points per line.
The set of AFM images (Figures IV.13, 14, and 15) show the process of the high-resolution
images taken at the superior part of the curvature of the sphere. Figure IV.13 (14 m x 14
m) shows two localized microspheres.
Figure IV.13 AFM micrograph (14 m x 14 m) for A silica aerogel powder
These images are represented in dual mode. The left microphotography shows a height image
(z range from 0 m to 3 m). At the right part is a phase image; therefore it is possible to
obtain some differences in the material (in this case only from substrate to microparticle).
Chapter V Silica aerogel microparticles 192
When the feature is sharper than the tip, the shape of the tip will dominate the image, in
these images this effect is reflected in the ‘shadow’ on the spheres.
Figure IV.14 Sample A, Surface of a microsphere by AFM.
Lateral dimensions: 1 m x 1 m. The colored scale corresponds to vertical amplitude of 15 nm.
The images were taken at the upper part of the microsphere (2-3 m of diameter). Figure
IV.14 (1 m x 1 m). Because many samples have features with steep sides, tip imaging is a
common occurrence in images, in this case because the surface convexity the roughness is
not clearly observed. A plane fitting was applied (Nanoscope III; Digital Instruments) on the
data type images. The plane fit calculates a single polynomial of a selectable order for our
selected image and subtracts it from the image. A second order has been used to correct the
distortion. The image’s distortion was removed almost entirely reflecting ‘a flat surface’ A
flatten portion of one particle were taken at the superior part of the curvature of one sphere
as shown in Figure IV.15.
Chapter V Silica aerogel microparticles 193
Figure IV.15 Sample A, Surface of a microsphere by AFM. Lateral dimensions: (500nm x 500nm). The colored scale corresponds to vertical amplitude of 15 nm.
Figure IV.16 shows the roughness of the surface microspheres at high resolution (500nm x
500nm). The scale mark in z is of 0-15 nm. From those micrographs was possible to make
the image treatment for the surface analysis and roughness study. Figure IV.16 shows the
surface analysis study:
Figure IV.16 Micrographs of the microspheres surface by AFM: Roughness analysis.
It was observed that the surface of the microspheres had a micro-roughness only detectable
using this technique. The z range controls the vertical range of the image, corresponding to
the full extent of the color table and the scanning length of the 500 nm. It was also possible
to observe the existence of interlinked particles of few nanometers of diameter (5-30 nm) and
Chapter V Silica aerogel microparticles 194
the presence of pores of few nanometers (5-50 nm) of diameter. This structure is very similar
to that of an aerogel [19-20].
Using the AFM technique, with the ’contact mode’, a trial to calculate the density of the
particle and determining if the spheres can be assimilated to an aerogel or xerogel was
attempted. By the roughness analysis of the surface the Rms parameter can be obtained, it is
defined by:
22
izzRms zi is la height in each point and z is the mean height value. In our sample
the value was of 1 nm. This value is considered micro-roughness, that was the reason that it
was not observed neither by SEM nor TEM. Figure IV.17 shows the surface of the sphere in
three dimensions that will help us for interpreting the obtained images.
Figure IV.17 Three-dimensional AFM micrograph.
Figure IV.18 shows the results of a cross-section analysis of one of the microspheres.
Chapter V Silica aerogel microparticles 195
Figure IV.18 Sample A. Cross-section analysis of the surface of one microsphere by AFM. Lateral dimensions (500nmx500nm). The colored scale corresponds to vertical amplitude of 15nm.
2. SOL-GEL ROUTE TO DIRECT FORMATION OF SILICA AEROGEL
MICROPARTICLES USING SUPERCRITICAL CARBON DIOXIDE
Alternatively, a low-temperature synthesis was performed by using supercritical carbon
dioxide as solvent and formic acid as condensation agent. In previous works, sol -gel
reactions were directly performed in supercritical carbon dioxide as a reactive medium by
using formic acid as condensation agent to obtain aerogel monoliths [21].
We named this approach as low temperature synthesis to differentiate it from the first
method where it was needed to reach the critical temperature of the solvent >200ºC
(ethanol) while the critical temperature of CO2 is 32ºC.
Four types of low temperature experiments were performed and are described in the four
sections:
Horitzontal distance: 20 nm Horitzontal distance: 50 nm
Chapter V Silica aerogel microparticles 196
Section 2.1: ‘In situ’ low-temperature microparticles. The metal-organic precursor and the
hydrolysis solution (HCOOH) were placed inside the reactor in different ways either as a sol
or as independent solutions.
Section 2.2: Injection of precursor and hydrolysis solution at supercritica l conditions. This
approach consisted in injecting the precursor (TEOS/TMOS) and condensation solutions
(formic acid) in supercritical CO2, under several critical conditions. The injection of liquids
was achieved through a very fine nozzle using a syringe pump.
Section 2.3: Injection of precursor and hydrolysis solution in liquid CO 2: An alternative route
to this one was to inject the reactive in liquid CO2 and dissolving the remaining water and
alcohol from the sol reaction in a continuous flow of liquid CO2, then reach the CO2
supercritical parameters and vent the autoclave.
Section 2.4: Injection of prepolymerized precursor in supercritical CO 2. The prepolymerized
reactive was inject in supercritical CO2, then the temperature was reduced until liquid CO 2
and the remaining water and alcohol from the sol reaction were dissolved in a continuous
flow of liquid CO2, then reach the CO2 supercritical parameters and vent the autoclave.
2.1 ‘IN SITU’ LOW-TEMPERATURE MICROPARTICLES: TEOS, HCOOH,
AND SUPERCRITICAL CO 2 AS A SOLVENT
A process for making monolithic silica gels at low temperature is by reaction of tetralkoxy
orthosilicates (TEOS), with a strong carboxylic acid (p.e HCOOH) [22]. In this process, the
water does not need to be present as a reactant because the hydrolysis reaction is caused by
the carboxylic acid. In that case, supercritical CO2 can be used as solvent for the sol-gel
reactions. The supercritical extraction of the silica gel is performed under supercritical CO 2
conditions (temperature and pressure of 31 C and 71 Bar, respectively). Figure IV.19 shows a
sketch of an ‘in situ’ low-temperature process. The metal-organic precursor and the
hydrolysis solution (HCOOH) were placed inside the reactor in different ways either as a sol
or as independent solutions. Then, the temperature and pressure were raised over CO2
supercritical conditions waiting a certain period of time. In this case, supercritical CO2 was
expected to dissolve the precursor and formic acid (formic acid is miscible in supercritical
CO2 and water is immiscible) leading to the formation of a fine and uniformly aerogel
powder dispersed overall the reactor. Figure IV.19 shows a scheme of one of the ‘in situ’ low
temperature microparticles processing.
Chapter V Silica aerogel microparticles 197
Figure IV.19 Sketch of an 'in situ’ low-temperature microparticle production process.
Several experiments were developed in order to optimize the quality of the silica aerogel
microparticles. It is important to point out that these experiments were changed mainly by
varying the following four parameters:
TABLE IV.3 Experimental parameters to obtain microparticles by 'in-situ' low-temperature process
Parameter Range
HCOOH/precursor ratio From 1 up to 24
P and T of the reactor during
the dissolving step
P from 90 to
200 bar
T from 40 to
55 C
P and T of the reactor during
the drying step
P from 90 to
200 bar
T from 40 to
55 C
Time at supercritical conditions From 1 to 24 hours
Figure IV.20 shows an experiment where HCOOH/TMOS was fixed at four. This solution
when is synthesized outside of the reactor gels in 4 hours.
a
)
SCCO2
HCOOH
TMOS
+ EtOH
Chapter V Silica aerogel microparticles 198
Figure IV.20 SEM micrograph of microparticles obtained by ‘in situ’ low-temperature process in two vessels, one containing EtOH+TEOS the second one with water+NH3+NH4F.
Again, as in the ‘in situ’ at high temperature process, two kinds of morphologies were
collected, spherical particles and microfibers. In this case, the shape of the microparticles was
not as defined as in the high temperature method. Moreover, the diameter of the fibers was
much larger (a diameter of 750 nm and a length of 5 microns).
2.2 PRECURSOR DIRECTLY INJECTED IN CO2 SUPERCRITICAL
CONDITIONS AT LOW TEMPERATURE
Our approach for preparing aerogel powders at low temperature by direct injection of the
precursor at CO2 supercritical conditions mainly concerned the dissolution of the injected
precursor and formic acid in supercritical CO2 (the sol or the individual solutions), followed
by the sol-gel process and drying under supercritical conditions. The injection of liquids was
achieved through a very fine nozzle using a syringe pump. The reactor conditions were in the
temperature range of 313-368 K and pressure range of 100-150 Bar. The contact time
between precursor and hydrolysis solution at supercritical CO 2 conditions was varied from 1
to 24 hours and the duration of the pressure release in all the experiments was of 30 min.
Chapter V Silica aerogel microparticles 199
2.2.1 Injection of hydrolysis and precursor solutions independently
This section describes the results of the process for the processing of silica aerogel particles
by the independent injection of the sol components in the reactor using supercritical CO2 as
solvent. To avoid clogging of the nozzles the TEOS was diluted in ethanol. Next figure
shows the steps followed for one of the experimental procedures:
Figure IV.21 Schematic design of the injection process. Step i) Drive the reactor at CO2 supercritical conditions in order to obtain supercritical fluid before the injection of the sol. Step ii) Inject HCOOH in the reactor using a nozzle. It acts as hydrolysis reactive in the sol-gel reactions. Step iii) Injection of TEOS/TMOS. Step iv) Remaining certain time at supercritical conditions until gel microparticles were formed. Step v) Slow depressurization in order to avoid the damage of the aerogel microstructure
Several experiments were performed under different injected ratios of HCOOH/precursor.
Figure IV.22 shows the SEM characterization for one of the processing particles process with
HCOOH/TEOS=12.
Particles of SiO2
HCOOH
SC-CO2
SC-CO2 TEOS
Injection of HCOOH and TEOS/TMOS in supercritical CO2
Step i) +ii)
Step iii)
Step iv)+v)
Chapter V Silica aerogel microparticles 200
Figure IV.22 SEM micrograph obtained from the powder at low temperature process. Injection of TEOS and HCOOH with molar ratio=12. The injected TEOS was diluted in EtOH
with vTEOS/vethano=1. Remaining 3 hour at 150Bar and 85 C.
The powder was formed by microspheres aggregates of spheres with diameters between
100nm and 1 micron. In this case, the particle size distribution was not as narrow as those
obtained at high temperature. The resulting particles are less spherical in shape due to neck
forming between two of more particles.
In order to obtain better-defined shape microspheres by reducing the necking between
particles, the HCOOH/precursor molar ratio was reduced from 12 to 6 and the remaining
time at supercritical conditions was reduced from 3 hours to one hour.
Next SEM micrograph shows a closer up of the particle powder collected under these last
conditions.
Chapter V Silica aerogel microparticles 201
Figure IV.23 SEM micrograph obtained from the aerogel powder following a low temperature process. Injection of TMOS and HCOOH with 1/6 molar ratio and vTMOS/vacetone=1.
Remaining 1 hour at 150Bar and 55 C
The microparticle had a better-defined spherical shape (smaller necking) but still a very wide
size distribution, the particle size range from 100nm to 1 m. To better understand the
process that occurs inside the reactor a cartoon is shown in Figure IV.24.
Figure IV.24 Scheme of what happens inside the reactor while the formation of silica aerogel microparticles by injection at supercritical conditions.
CO2HCOOH CO2CO2CO2HCOOHHCOOH
TEOSEtOH TEOSEtOH TEOSEtOH TEOSEtOH TEOSEtOH
Chapter V Silica aerogel microparticles 202
When the autoclave was full of CO2 in supercritical conditions (marked in blue) the HCOOH
(marked in yellow) was injected (volume of the autoclave is 2 liters). The amount of the
injected HCOOH was of the order of 10 ml; depending on the number of CO 2 molecules
outnumber the HCOOH molecules by few order of magnitude. Then, each HCOOH
molecule was surrounded by CO2. Gel microparticles scattered all over the reaction. At
supercritical conditions the gel porous was filled with supercritical CO 2, therefore no capillary
forces collapsed the pores when the fluid was vented. In the reactor, there also were the
molecules resulting from the reaction, water and alcohol in very small quanti ties marked in
orange.
2.2.2 Injection of sol
Aerogel powder was obtained by a new approach injecting the sol at carbon dioxide
supercritical conditions. It is important to remark that the sol was constituted by HCOOH
since it is miscible in supercritical CO2. Next figure shows a scheme of the aerogel powder
processing by injection of a sol at supercritical conditions.
Figure IV.25 Scheme of ‘in situ’ injection of the sol in supercritical carbon dioxide
Next SEM images show interlinked microspheres with neck diameters between 100 and
500nm and particle diameters between 1 and 3 m.
SiO2 particles
P=150Bar
T= 80C
3 h at SC- CO2 conditions
Depressurization 30min
SC-CO2
Sol 50ml
TEOS : EtOH : HCOOH
60ml : 60ml: 12ml
Chapter V Silica aerogel microparticles 203
Figure IV.26 SEM image of the silica aerogel powder collected
with in jection of a sol with HCOOH/TEOS=5, and vTEOS/vEtOH=1.
Remaining three hours at supercritical conditions 150Bar and 80 C.
The particles obtained by the injection of the sol did not show large differences with those
obtained by the injection of the precursors separately. The later one presents the advantage
that avoids the obstruction of the nozzle.
2.3 ‘IN SITU’ INJECTION IN LIQUID CO2
An alternative method to the ones injecting at CO2 supercritical conditions (described in
previous sections), was to inject the reactants in liquid CO2 and dissolve the remained water
and alcohols formed during the reactions by continuous flow of liquid CO 2. Then, reach the
CO2 critical parameters and after a given time, vent the autoclave. The experiment were
performed following these steps:
i) Preparation of the sol with variable HCOOH/TEOS ratios
ii) Reactor full with liquid CO2, that acts as solvent media.
iii) Inject HCOOH in the reactor using one nozzle.
iv) Injection of TMOS by using a second nozzle.
v) Increase of P, T until supercritical CO2 conditions.
vi) Slow depressurization.
Chapter V Silica aerogel microparticles 204
Step ii) and iv) can be substituted by one injection step spraying directly the sol. Next figure
shows the steps followed by an 'in situ’ injection in liquid CO2 experiment.
Figure IV.27 Scheme of the route pursued for the processing of
silica aerogel microparticles using liquid CO2 as solvent. Step ii)
Reactor full with liquid CO2. Step iii)+iv) Injection of the sol. Step v)
autoclave under supercritical condit ions. Step vi) Vent the autoclave
and the silica aerogel microparticles are collected.
Depending on the experimental concentration ratios of TEOS/HCOOH/SCCO 2, diverse
particle agglomerations were obtained.
Figure IV.28 show the SEM micrographs of aerogel microparticles collected when the
injection of TEOS and HCOOH onto liquid carbon dioxide.
SC-CO2 liquid CO2
Sol TEOS + HCOOH
Aerogel SiO2
microparticles
Step ii)
Step iii)+iv)
Step v)
Step vi)
Chapter V Silica aerogel microparticles 205
Figure IV.28 Scanning electron micrograph of aerogel silica
particles obtained drying at supercritical carbon dioxide at 45 C, 100 bar when injecting the reactive at liquid carbon dioxide: HCOOH/TEOS=6, and TEOS dissolved in ethanol at vTEOS /vEtOH =1.
In this picture, the neck forming between particles is evidenced compared to the method by
injection at supercritical CO2. A closer up of a broken neck allowed to observe the porous
structure inside the microspheres. The shape of the particles was much better defined and the
size distribution was very narrow. The particle size distribution is also narrow and, in general,
by this method the mean particle size is smaller (<1 m) than in the previous explained one
where the particles were between one and two microns.
2.4 INJECTION OF PREPOLYMERIZED PRECURSOR IN
SUPERCRITICAL CO2
Finally, in order to obtain particles with larger surface area, prepolymerized reactive was
injected in supercritical CO2 After the injection of the sol, the temperature was reduced to
15 C maintaining the pressure at 100 Bar meaning that liquid CO 2 filled the reactor. The
liquid CO2 dissolved the remaining water and alcohol from the sol reaction. An exchange of
liquid CO2 was done in a continuous flow, until all the water and alcohol was substituted.
Then the CO2 supercritical parameters were reached and after a while the autoclave was
vented.
Chapter V Silica aerogel microparticles 206
Figure IV.29 Scheme of silica aerogel microparticles at low temperature by the injection of prepolymerized precursor and by addition of liquid CO2 exchange. Step i) Drive the reactor at CO2
supercritical conditions in order to obtain supercritical fluid before the injection of the sol. Step ii) Injection of H5. Step iii) Filling the reactor with liquid CO2. Step iv) Remaining certain time at supercritical conditions until gel microparticles were dried. Step v) Slow depressurization in order to avoid the damage of the aerogel microstructure.
Figure IV.30 shows some of the particles obtained following the above described process.
Step i)
Step ii)
Step iii)
Step iv)
Step v)
Chapter V Silica aerogel microparticles 207
Figure IV.30 Alkoxide precursor injected at supercritical CO2.
One hour at 45 C, 100Bar. 1 hour of CO2 liquid exchange.
A more porous surface was observed by SEM compared to the previous injection methods.
That may be caused by the CO2 liquid - ethanol exchange process that facilitated to reduce
the shrinkage of the solid skeleton in the gel microparticles. The size of the collected particles
ranged from 200nm to 600nm. BET method was used to determine the surface area of those
particles, SBET= 520 m2/g. It is a large value compare to that of pure silica (SBET=35 m2/g) but
not very large compare to H5 aerogels (1000 m2/g).
Figure IV.31 Two-steps precursor injected at supercritical CO2 One hour at 45C, 100Bar. 1hour CO2 liquid exchange.
Chapter V Silica aerogel microparticles 208
4. CONCLUSIONS
Reactions inside an autoclave open a whole range of possibilities for one step synthesis of
microparticles. The sol was solubilized in supercritical acetone to obtain powders of small
particles with narrow size distribution. Indeed, these microspheres syntheses were based on
hydrolysis and condensation in the supercritical fluid. The size of the collected particles was
very sensitive to the water/alkoxide concentration ratio: the higher the ratio, the smaller the
particles. Moreover, small changes on the depressurization rates results on very differentiated
morphologies, a slow and controlled venting of the autoclave leads to the formation of
monodispersed particles, spherical in shape, and narrowly distributed in the range of 0.5 to 2
m, otherwise fiber-like structures are present.
In the case of using supercritical carbon dioxide as a solvent, the resulting particles are less
spherical in shape due to necking formation between two or more particles. The particle size
distribution is also narrow and, in general, the mean particle size is smaller (<1 micron) than
in the previously described method.
Classical ways for obtaining aerogel particles involve high-temperature and high-pressure
reactions. However there are other ways to obtain aerogel particles based on the use of
supercritical CO2 medium and a condensation agent sufficiently soluble in CO2 (formic acid
has been proved to be a suitable one). In this case, the working temperature is lower. Aerogel
particle formation in the supercritical phase under the conditions employed is dependent on
the solubility of the reactive. The most porous microparticles were obtained when using two-
steps synthesis by injection of the precursors in CO 2 supercritical conditions and adding a
solvent exchange step ethanol- liquid CO2.
5. REFERENCES
1. W. Stöber, A. Fink, and E. Bohn, J.
Colloid Interface Science 26, 62
(1968).
2. D. L. Wilcox, M. Berg, T. Bernat, D.
Kellerman, J.K. Cochran, in Hollow
and Solids Spheres and
Microspheres: Science and
Technology Associated with their
fabrication and applications (Material
Research Society Proceedings,
Pittsburg, 1995), p. 372
3. M. Nogami, J. Mater. Sci. 17, 2845
(1982).
4. H. T. Blair, R.B. Mattews, Annual
Meeting Abstracts. Am. Ceram. Soc.
, 355 (1991)
Chapter V Silica aerogel microparticles 209
5. C. J. Brinker, and G. Scherer, in Sol-
Gel Science (Academic Press, New
York, 1990).
6. A. Van Blaader, and A. P. M.
Kentgens, J. Non-Cryst. Solids 149,
161 (1992).
7. J. G. Liu, D. L. Wilcox, J. Mater. Res.
10, 84 (1995).
8. K. J.Pekarek, J. S. Jacob, E.
Mathiowitz, Nature 367, 258 (1994).
9. Unger, K., et al., US Patent No.
4,775,520 (1988)
10. Unger, K., Kaiser, C. German Patent
DE 195 30 031 A1
11. MacNair, J.E., Patel, K.D., Jorgens,
J.W. Anal. Chem., 71, 700 (1999)
12. MacNair, J.E., Lewis, K.C., Jorgens,
J.W. Anal. Chem., 69, 983 (1997)
13. M. Barj, J.F. Bocquet, K. Chhor, and
C. Pommier, J. Mater. Sci. 27, 2187
(1992).
14. K. Chhor, J.F. Bocquet, and C.
Pommier, Mater. Chem. Phys. 32,
249 (1992).
15. X. Y . Zeng, Y. Arai, and T. Furuya,
Trends Chem. Eng. 3, 205 (1996).
16. D. W. Matson, R. D. Smith, J. Am.
Ceram. Soc. 72, 871 (1989).
17. P.G. Debenedetti, in 3·rd
International symposium on
supercritical fluids, (Proceedings,
Strasburg, 1994), p. 213
18. N. Tohge, G.S. Moore, and J.D.
Mackenzie, J. Non-Cryst. Solids 63,
95 (1984).
19. R. w. Stark, T. Drobek, M. Weth, J.
Fricke, and W.M. Heckel,
Ultramicroscopy 75, 161 (1998).
20. C. Marlière, F. Despetis, P. Etienne,
T. Woignier, P. Dieudonné, and J.
Phalippou, J. Non-Cryst. Solids 285,
148 (2001).
21. ‘Process for making inorganic gels’
Patent US5558849, issued 1996-09-
24 Sharp Kenneth G
22. D. A. Loy, E. M. Russick, S. A.
Yamanaka, and B.M. Baugher,
Chem. Mater 4, 749 (1997).
C h a p t e r V I
SILICA AEROGEL FILMS
SECTION OUTLINE
1. APPLICATIONS OF AEROGEL FILMS ...................................................................................... 212
1.1 ELECTRONIC .............................................................................................................................. 212
1.2 OPTICAL ...................................................................................................................................... 213
1.3 THERMAL.................................................................................................................................... 214
1.4 ACOUSTIC.................................................................................................................................... 214
1.5 ENVIRONMENT AND OTHERS................................................................................................. 214
2. SOL-GEL COATING METHODS .................................................................................................. 215
2.1 DIP COATING ............................................................................................................................. 215
2.2 SPIN COATING............................................................................................................................ 216
2.3 SPRAY COATING......................................................................................................................... 217
2.4 SURFACE TENSION COATING ................................................................................................. 217
2.5 SUBCRITICAL DRYING BY SURFACE DERIVATION................................................................. 217
3. REFERENCE EXPERIMENTAL RESULTS ............................................................................. 217
3.1 DIP COATING ............................................................................................................................. 217
a) Low-Temperature dip coating .................................................................. 218
b) High-Temperature drying ........................................................................ 219
3.2 SPIN COATING............................................................................................................................ 221
a) Spin coating with High-Temperature drying .............................................. 222
4. PROPOSED NEW METHODS: ‘IN SITU’ PREPARATION AT HIGH PRESSURE AND
INJECTION AT SUPERCRITICAL CONDITIONS ............................................................................. 224
4.1 ‘IN SITU’ PREPARATION AT HIGH PRESSURE......................................................................... 224
4.1.1 ‘In situ’ high temperature .......................................................................................... 225
4.1.2 ‘In Situ’ low temperature coating method ............................................................. 225
4.2 SPRAY COATING BY DIRECT INJECTION IN SUPERCRITICAL CO2 AT LOW TEMPERATURE
227
5. CONCLUSIONS .................................................................................................................................. 230
6. REFERENCES ..................................................................................................................................... 231
Chapter VI Silica aerogel films 212
Sol-gel processing has proved to be an important method for producing amorphous porous
silica films. A variety of processing techniques has been developed to minimize shrinkage and
prevent cracking of the films. One of the most successful methods is to avoid altogether the
capillary forces by drying the gel at temperature and pressure above the critical point of the
solvent. The resulting aerogel film retains most of the original volume of the wet gel and is
potentially useful as a wide range of applications due to its low density, high surface area and
low thermal conductivity.
This chapter is organized in four sections. Section 1 summarizes some of the aerogel film
applications. The most used techniques for obtaining films in the sol-gel processes and its
most used characteristics are summarized in section 2. In addition, dip coating and spin
coating were used as a pattern methods to be compared with the results from the „in-situ‟
method developed in our laboratory. Section 3 shows some of the silica aerogels films
obtained by spin and dip coating methods. Section 4 shows the proposed „one-pot‟ methods
to process silica aerogel films and the achieved experimental results. The acquired
„Laboratory Scale Plant‟ provided the possibility to better rationalize the number of free
parameters in the experiments and make simpler the realization of those „one-pot‟ tests.
1. APPLICATIONS OF AEROGEL FILMS
The most used applications for aerogel film either in thick or thin type are described in this
section.
1.1 ELECTRONIC
Thick organic aerogel films ( 0.5mm) are formed by capillary fill, and then pyrolyzed to
carbon aerogel films for use in aerocapacitors.
Thin aerogel films ( 2 m) are formed on silicon wafers to provide a low dielectric constant
in integrated circuits. SiO2 aerogel films are a promising material because it has a low
dielectric constant due to its inherent high porosity that is controllable in the fabrication
process. The dielectric constant for aerogels is around 2.0 and its value depends on the
porosity. The basic technological trend in ultra large-scale integration of electronic
circuits is the realization of higher speed devices with closer packing density, which
results in a multilevel interconnection structure. Improving a chip's insulation is one way
Chapter VI Silica aerogel films 213
to avoid the problem. Good insulators let chip designers place interconnects close
together without slowing down the electrical signals. Air, the perfect insulator, has a
dielectric constant of 1.0 but it is not possible to hold chips together with air. Silicon
dioxide, the material now used on most chips, rates at about four. Therefore, aerogel‟s
dielectric constant may be as low as 2.5. The aerogel chip insulator could more than
double computing speeds 1, 16 .
Figure VI. 1 Aerogel film as a chip insulator.
1.2 OPTICAL
Thick aerogel films ( 0.2mm) are used as cover slips on solar cells and on optical fibers. A
method for fabricating a lightweight solar cell 2 is provided by preparing a low density
silica aerogel substrate at densities between 0.01-0.1 g/cm3, then a dielectric planarization
layer of SiO2 is applied to the substrate surface and one or more photovoltaic thin film
layers are deposited on the planarization. These solar cells, with their low-density aerogel
substrates, are much lighter than prior solar cells. This is advantageous in satellite
applications since the solar array weight is substantial. Such lightweight solar cells of the
invention also find further advantage on the ground, e.g. for solar-powered vehicles in
which weight is a primary concern. Silica aerogel films are also used in optical fiber 3 .
The problem to be solved is to enable transmission of more light at a larger light
receiving angle by fixing a silica aerogel via a resin to the surface of a film-like base
material.
Thin aerogel films ( 30 m) are coated on the outside of glass laser pump tubes, on solar energy
collectors 4 , and on thin film detectors as low mass optical coating. The aerogel acts as a
refractive index match between the pump tube glass and air, to minimize reflection of
internally generated light. Some thermal detector have an optical coating comprising an
aerogel film with greater porosity than 80% 5 . An optional optical impedance matching
layer may be deposited over the porous film.
Chapter VI Silica aerogel films 214
1.3 THERMAL
Thick sheets ( 1mm) of silica aerogels are coated with thin film layers of metal, and then
laminated together to form a super-insulating thermal heat shield block 6 .
The aerogel layer may also contain infra-red (IR) opacifier and/or fibers. It is sandwiched
between two films.
Thin aerogel films ( 25 m) have been formed on glass substrates for use in „cool‟ infrared
(IR) detectors. The aerogel film serves as a thermal barrier to shield the IR detector elements
from the heat radiated by the substrate materials 7 .
1.4 ACOUSTIC
Thick aerogel films ( 0.5mm) are formed on the surface of ceramic transducers to serve as
acoustic impedance matching layers 8-12 . Thin films of silica aerogel on silicon have
acoustic waveguide properties. The large mismatch between the acoustic properties of the
silica aerogel film and substrate in creates pronounced dispersion in the velocities and leads
to unusual acoustic behavior: over a relatively large range of wavelengths, the group velocities
of certain modes are less than 50 m/s-nearly ten times slower than the intrinsic velocities of
the nanoporous silica and more than one hundred times slower than those of silicon.
Acoustic impedance matching between a transducer and the irradiated medium can be
achieved using quarterwave or impedance gradient layers 13-14 . SiO2 aerogels are suitable
materials for both purposes. To investigate matched transducers for air applications, sound
intensity measurements were performed with and without aerogel quarterwave layer for
different types of piezoceramic transducers. Ways to employ aerogel layers for the acoustic
matching of piezoceramics are discussed.
1.5 ENVIRONMENT AND OTHERS
Thick aerogel films ( 0.5mm) are formed on the surface a metal mesh matrix for getters in
decontamination. A porous lightweight getter that collects particulate and molecular
contaminates. Such composite getters are useful in decontamination in semiconductor
manufacturing processes and storage and in decontaminating optical systems including a
space-based telescope. In other embodiments, the getter can be mounted in air conducts to
Chapter VI Silica aerogel films 215
serve as a filter therefore, can be mounted in a photocopier for capturing of toner fog, it can
be mounted in areas of semiconductor manufacturing for collecting contaminates 15 .
2. SOL-GEL COATING METHODS
Some important applications require the aerogels to be as films. To do that, certain
experimental details should be taken in account. Since there is a large surface area in contact
with the atmosphere, the solvent evaporation rate should be slowed down. Thus, the coating
is usually done in a saturated solvent vapor chamber. A second step is required to
supercritcally dry the coating. Another important requirement is the substrate surface
preparation previous to coating because gels do not stick well to unprepared surfaces and any
kind of grease must be eliminated. Bonding to the surface is enhanced by etching the
substrate surface with a mild alkaline solution and then raising it with an alcohol immediately
prior to film deposition.
Next subsections summarize the most used techniques for obtaining silica aerogel films in
the sol-gel processes (2.1 to 2.5) together with two new methods developed in our group.
These „one-pot‟ techniques present the great advantage to perform the whole process inside
the autoclave. In addition, dip and spin coating were used as pattern methods to compare the
results with those of the „one-pot‟ methods.
2.1 DIP COATING
Substrates are dipped into a precursor solution and slowly withdrawn from it. They are then
placed edgewise and vertical in a holder that is located within the solvent vapor saturated
chamber. The disadvantage of this method is the low homogeneity of the obtained films,
since the solvent is evaporating and gravidity draining the film acquires a wedge-like shape.
The main characteristics of dip coating films are: i) thickness lower than few microns
depending on the viscosity of the precursor and withdrawal rate. ii) Gelation time of the film
took only a few minutes because the rapid evaporation of the solvent. iii) The coated
substrates can be either Pyrex glass or silicon wafers. iv) The coated films are immersed in
the corresponding solvent until ready for supercritical drying. Some of the resulting films are
shown in section 3.
Chapter VI Silica aerogel films 216
FigureVI. 2 shows the main steps of the dip-coating process.
Figure VI. 2 Stages of the dip coating process: (a-e) batch; (f)
continuous. (17)
2.2 SPIN COATING
The most used procedure to form gel films is by dropping the precursor solution onto a
spinning substrate while its spin rate increases. Spinner is then turned off and stopped with a
brake (causing minimal loss of solvent during gelation). The main problem is the low
homogeneity of the films and the need to work in a solvent saturated chamber, also it is
necessary to find the balance between the two main forces, centrifugal and viscous (friction).
The main characteristics of spin coating films are: i) thickness lower than two microns. ii)
The gel is formed within a few minutes. iii) The substrates used are silicon wafers up to 1 cm
of diameter. iv) The substrate coated with the gel film is immersed in solvent to age until the
supercritical drying.
Figure VI. 3 Stages of the spin-coating process 17 .
Chapter VI Silica aerogel films 217
Different type of catalyst and several TEOS/EtOH ratios were attempted with the purpose
of processing spin-coating films with a wide variety of porosity. Some of the results are
shown in section 3.
2.3 SPRAY COATING
Usually this technique is used to coat thicker single layer. films as thick as 80 m In this
method, the solution precursor is directly sprayed onto the substrate (supported in a vertical
position within the chamber). Excess solution drains by gravity leaving a thick film that gels
within a few minutes. The surface of the gel is smooth and continuous. In the same manner
that in the previously mentioned methods, the coated substrate is immersed in solvent after
gelation until ready for supercritical drying.
2.4 SURFACE TENSION COATING
In this coating method, surface tension is used to draw the liquid onto the solid surfaces. A
feature is made by forming a space between the substrate to be coated and another flat
surface (treated to prevent bonding to the gel so that should be removable after drying
process). Liquid precursor fills the available volume by capillarity when the element is dipped
into the solution. The spacers are used to separate the substrates by the desired film
thickness. Common film thicknesses are from 2 to 50 m.
2.5 SUBCRITICAL DRYING BY SURFACE DERIVATION
In this technique, aerogels are prepared by dip-coating at ambient pressure 18-19 without
needing of supercritical drying. To achieve that, surface groups (organosilyl-terminated
surfaces) are added to the gel, making drying shrinkage reversible: as the solvent is
withdrawn, the gel springs back to a porous state. Final pore volume of the ambient pressure
aerogel is a result of competing effects: the capillary stress induced collapse, and
condensation /polymerization reactions which tend to stiffen the matrix resisting the
collapse. The surface organic groups reduce capillary stresses due to contact angle
modification and offer resistance to pore collapse.
3. REFERENCE EXPERIMENTAL RESULTS
3.1 DIP COATING
This technique has been used as a guide to compare with the „one-pot‟ experiments. Film
thicknesses were of few microns depending on the initial molar ratios of the precursor sol.
Chapter VI Silica aerogel films 218
The gel film can be supercritically dried at CO2 conditions (Section 3.1.a) or under the
supercritical conditions of the gel solvent (Section 3.1.b).
a) Low-Temperature dip coating
A low temperature test comparing dip coating and „in situ method‟ (see section 4) has been
performed under the same supercritical drying experiment. The sol-gel synthesis is the same
for the two type of coating process: equivalent molar ratio,
TEOS/EtOH/H2O=1/12.6/34.7 and equivalent catalyst concentration (citric acid 0.03M).
Acid catalyst has been used because in acid conditions the contact surfaces of necks in the gel
increases and may facilitate the film formation.
Figure VI. 4 Scheme of comparat ive experiment, the d ip coated
samples were dried under the same experimental conditions that the ‘low
temperature’ coated samples.
In the case of dip-coating, the sol is prepared outside of the reactor and the substrates are
dipped in the sol before gelling. They are immediately placed in a saturated atmosphere
avoiding the evaporation of the solvent. For the in situ process, the two independent
solutions were placed inside the autoclave. When the substrates were ready for the
supercritical extraction P and T were increased until supercritical CO 2 conditions 150bar
(Pc=73bar) and 80 C (Tc=31 C). The autoclave remains 5 hours under these conditions. In
this step is when the sol-gel process occurs; in addition, some longer time was required to age
the film. Figure VI.5 shows the inhomogeneous but comparable films that were obtained by
the dip-coating and on the „in situ‟ method.
SCCO 2
2mlTEOS+20ml EtOH 1ml H 2 O+
catalitzador Dip coated substrate
In situ + Dip coating SCCO 2
2mlTEOS+20ml EtOH 1ml H 2 O+
ac cítric
In situ + Dip
Chapter VI Silica aerogel films 219
Figure VI. 5 SEM of the inhomogeneous films, dip coating at the left
and ‘in situ’ experiment at the right.
It should be remarked that the surfaces of the substrates were untreated so the homogeneity
of the films could be improved making a surface treatment on the substrates. Comparing
both experimental results, the morphology of the films seems to be very similar indicating
that in the „in situ‟ method the sol-gel process has been preceded in a similar way than for the
classical method.
b) High-Temperature drying
In order to optimize the synthesis parameters of the aerogels films several dip-coating tests
have been attempted. The sol-gel dip coatings following a high temperature route were dried
under supercritical ethanol conditions at 100bar (Pc=63bar) and 255 C (Tc=235 C). The
film morphology is modified by varying the aging time of the gel films that remains under
saturated atmosphere conditions. Figure VI.6 shows some of the experiments when using a
sol-gel process with a very short gelation time (few minutes). The reactive ratios are
TMOS/MetOH/H2O/NH4OH=1/12/4/6,5.10-4. Several types of morphologies were
produced by changing the aging times under saturated solvent (1 or 2 minutes).
Chapter VI Silica aerogel films 220
Figure VI.6 shows four SEM images obtained in two different aging times (one and two
minutes) and in different saturated conditions, left side for non-saturated and right side for
methanol saturated atmosphere.
Figure VI. 6 Dip coating silica aerogels with reactives ratios of
TMOS/MetOH/H2O/NH4OH=1/12/4/6,5.10-4
. Figure VI.a) 1 min of aging
at room conditions. Figure VI.b) 1 min of aging under methanol saturated
atmosphere Figure VI.c) 2 min aging under room conditions Figure VI.d)
2 min aging under methanol saturated atmosphere.
It can be observed that in the two experiments without saturated atmosphere (a and c) small
particles have been formed on the film surface. The presence of particles increases when
increasing the aging time. On the other side, in presence of saturated atmosphere no particles
are found (b and d). The films are more homogeneous and without cracks.
Figures VI.7 and 8 show a detail of the porous microstructure of the aerogel coating under
different aging time, one and two minutes, respectively.
a)
b)
c)
d)
Chapter VI Silica aerogel films 221
Figure VI. 7 Dip-coated substrate 1 min of aging time in methanol
Figure VI. 8 Dip-coated substrate 2 min of aging time in methanol
When the aging time is increased, the structure becomes more colloidal and less polymeric.
The film morphology seems to be more porous when decreasing aging time.
3.2 SPIN COATING
This technique has also been used in order to optimize the sol-gel procedure for film
formation and to use the obtained films as evaluation of the quality of the „in situ‟
experiments. A wide study has been performed using different catalysts and different ratio of
TEOS/ethanol. The film thickness obtained by this process is usually lower than 2 m. The
Chapter VI Silica aerogel films 222
drying procedure has been performed at solvent supercritical conditions (section 3.2.a) and
with CO2 liquid exchange (section 3.2.b).
a) Spin coating with High-Temperature drying
In order to optimize the synthesis parameters of the aerogels films several spin-coating
experiments have been tested. The sol-gel spin coatings following a high temperature route
were dried under supercritical ethanol conditions at 100bar (Pc=63bar) and 255 C
(Tc=235 C). The reactive ratios in the methanol gel spin coating are
TMOS/MetOH/H2O/NH4OH=1/12/4/6,5.10-4. The film morphology and coating
thickness is modified by varying the aging time of the coated substrates that remains under
saturated atmosphere conditions. Figure VI.9 shows four SEM images for one of the
experiments when using a sol-gel process with a very short gelation time (few minutes). The
reactive ratios are TMOS/MetOH/H2O/NH4OH=1/12/4/6,5.10-4.
Chapter VI Silica aerogel films 223
a) b)
c) d)
Figure VI. 9 a) An homogeneous film is observed with particles
deposited over the coating. Figure VI.b) shows a closer up of those
particles. In Figure VI.c) shows a cross -section of the film. It is possible
to observe that the thickness of the spin coating is of the order of 2 m.
The film shows some cracks as observed in Figure VI.d ).
Since the films are so thin, the gelification speed is accelerated because evaporation rate is
high. Then special attention must be taken to avoid the premature drying of the films at
ambient conditions. In view of the fact that in this experiment it was not possible to work
under a completed saturated atmosphere the coating presented some cracks (Figure VI.9b).
The microstructure of the spin-coating films was similar to that obtained by „one-pot‟
method.
Chapter VI Silica aerogel films 224
4. PROPOSED NEW METHODS: ‘IN SITU’ PREPARATION AT HIGH
PRESSURE AND INJECTION AT SUPERCRITICAL CONDITIONS
A new technique based on supercritical fluid technology for the production and processing
of aerogel film has been developed in our group. Two alternative methods are shown. „In
situ‟ preparation will be described in section 4.1 and „spray in supercritical CO2 method‟ in
section 4.2.
4.1 ‘IN SITU’ PREPARATION AT HIGH PRESSURE
A substrate was placed in the autoclave directly over the recipient containing a diluted sol,
which was previously prepared out of the reactor. As a second option, the substrate was
placed over two independent vessels containing the sol-gel solutions: the precursor solution
(precursor dissolved by the solvent) and the water solution (water and in some cases a
catalyst).
The method consisted of four main steps:
i) The substrates -previously treated to enhance adherence-, the precursor, the
solvent, and some water were introduced in the autoclave before sol gellifies. See
Figure VI.10.
ii) The reactor was driven above solvent supercritical values. At supercritical
conditions, the sol components (the metal-organic precursor, the solvent and water)
were dragged by the supercritical fluid and the components were distributed over the
entire reactor.
iii) The sol gellifies and then, temperature is increased above the supercritical value.
iv) Finally, the solvent is extracted under supercritical conditions of the solvent.
Figure VI. 10 ‘In situ’ preparation at h igh pressure: The substrate is
placed over the recipient containing the sol that is dragged by the
supercritical flu id and deposited on the substrate surface.
sol
sol in process of
gelification
solvent
supercritical conditions
substrate
Chapter VI Silica aerogel films 225
Some examples will be more extensively explained, in section 4.1.1 when using ethanol in
supercritical conditions and section 4.1.2 for supercritical CO 2.
4.1.1 ‘In situ’ high temperature
Some experiments were performed placing two vessels inside the autoclave one with the
dissolved precursor in ethanol solution and the other one with hydrolyzing solution, and then
the autoclave was driven until supercritical ethanol conditions following the above described
method. In all the attempted high temperature tests a white powder was found spread over
the inside walls of the autoclave vessel (see Chapter V: Silica aerogel particles). For that
reason, further studies were needed in order to improve the „in situ‟ method. To get a better
route two changes were proposed. The first change was to vary the sol-gel conditions by
varying the TEOS/EtOH/H2O ratios with the intention to increase the contact of the neck
surfaces between particles leading to film morphology. The second change is to cool the
substrate whiles remaining at supercritical conditions (during step iii) allowing the
condensation of droplets on the surface.
4.1.2 ‘In Situ’ low temperature coating method
Aerogels films were obtained at low temperature (without needing to use water) by using
CO2 as solvent in the synthesis of the gels (see more details of the synthesis on section 2.1.b).
As an example, Figure VI.11 illustrates one scheme of these experiments.
Chapter VI Silica aerogel films 226
Figure VI. 11 ‘In situ’ Low-T experiment. (A) The precursor and the
hydrolysis solution were placed inside the autoclave as independent
solutions placing two vessels. The first vessel contains the precursor
solution (TEOS dissolved in ethanol) and the second vessel contains the
hydrolysis solution (HCOOH dissolved in ethanol). (B) Then, the two
solutions were dragged at low temperature by supercritical CO2. (C) After
a while, the sol gellify and supercritical CO2 extraction was processed.
The autoclave was leaded to supercritical CO2 (200Bar, 80 C) during 24 hours. The
experiment failed, since it was observed that the amount of the liquid in both vessels
diminished but no coating was obtained. Instead, few particles were found on the substrate
surface (described in Chapter V: Silica aerogel particles). In order to optimize the film
formation a series of „in situ‟ low temperature experiments were tried changing the following
parameters:
-Ratio of HCOOH/precursor.
-Pressure and temperature of the supercritical fluid.
-Time of permanence in supercritical conditions.
A B
C
Silica aerogel
Chapter VI Silica aerogel films 227
One of the experiments performed trying to improve the results, was decreasing the TEOS
concentration in ethanol (EtOH/TEOS =12) and the substrates were treated by an alkaline
solution (NaOH) and dipped in a solution with water and acid catalyst (1M HF). The idea is
to avoid the formation of particles by using the immiscibility of water in CO2. In this case, a
thick porous film was obtained on the substrate.
4.2 SPRAY COATING BY DIRECT INJECTION IN SUPERCRITICAL CO2
AT LOW TEMPERATURE
A new initiative was seek to obtain silica aerogel films in a „one-pot‟ process. The idea was to
spray the sol-gel reactives directly in the autoclave at supercritical conditions.
This method was based on the injection (by using a nozzle) of the precursor (TEOS or
TMOS dissolved in EtOH) at supercritical CO2 conditions. After that, the injected precursor
reacts with the HCOOH that covers the substrate, in these reactions the supercritical CO 2
acts as the solvent of the sol. Subsequently, after the gelification of the sol, the supercritical
extraction of the solvent from the gel film was performed on supercritical CO 2 conditions
(temperature and pressure of 31 C and 71 bar, respectively). As a final result, a silica aerogel
film was achieved at low-temperature without the presence of water. Figure VI.12 shows the
scheme of the experiments for the injection of TEOS (at supercritical CO 2) over a substrate
covered with HCOOH. The scheme exemplifies two different routes marked as 1) and 2)
corresponding to two different ratios of the injected solutions.
Chapter VI Silica aerogel films 228
Figure VI. 12 Injection of TEOS dissolved in EtOH (at supercritical
CO2 conditions) over a HCOOH covered substrate. For process 1), the
molar rat io was HCOOH/TEOS=6 and the volume ratio was
vEtOH/vTEOS=10. For p rocess 2), the molar rat io was HCOOH/TEOS=12
and the volume ratio was vEtOH/vTEOS=1.
For process 1), the molar ratio was HCOOH/TEOS=6 and the volume ratio was
vEtOH/vTEOS=10. A porous but cracked film was obtained, and no microparticles were found
on the substrate and inside the autoclave. Next Figure shows the SEM image of the film
under the above-described conditions.
Figure VI. 13 SEM image of a film obtained by spraying (in
supercritical CO2) TEOS+EtOH on the HCOOH wet substrate.
Chapter VI Silica aerogel films 229
It was observed that this film was porous and reasonably homogeneous. Figure VI.14 shows
a closer up of the film morphology, its microstructure was very similar to that of a bulk
aerogel.
Figure VI. 14 Micrograph of a detail of the film microstructure.
For the process number 2), see Figure VI.8, the substrate was initially covered just by
HCOOH (not dissolved in ethanol) and the ratios were increased to HCOOH/TEOS =12,
and vEtOH/vTEOS =1 in order to accelerate the gelification process. A porous and non-
homogeneous 100 m thick film was obtained. Some microparticles were on the substrate
but no powder was found inside the autoclave.
In order to better understanding the process that occurs inside the reactor a scheme is shown
in Figure VI.15.
CO2
TEOSHCOOH
On the substrate
EtOH
Figure VI. 15 Cartoon picture of what may happen inside the
autoclave when TEOS (marked in violet) dissolved in EtOH (marked in
green) is injected on the reactor while the substrates are dipped in a
HCOOH solution (marked in yellow).
Chapter VI Silica aerogel films 230
This cartoon gives an idea about what may happen inside the autoclave: When the autoclave
is full of CO2 molecules in supercritical conditions (marked in blue), the TEOS (marked in
violet) dissolved in EtOH (marked in green) is injected on the reactor while the substrates are
dipped in a HCOOH solution (marked in yellow). Then, supercritical CO 2 (marked in blue)
acts as the reactive media of the sol and contributes that the sol-gel process occurs on the
surface of the substrate.
5. CONCLUSIONS
1) The main objective of this chapter has been to synthesize aerogels as homogeneous
films and with good adherence to the substrate. The coated substrates were Pyrex
glass slides and silicon wafers.
2) Four methods were tried: dip and spin coating were used to evaluate the results of the
two „one-pot‟ proposed methods. Spin coating processed films were more
homogeneous and thinner than dip coated films. By varying the aging time and the
viscosity of the coating gel, we were able to obtain a variety of film porosities. The
processed films by spin coating were useful to evaluate the quality of the „in situ‟ and
spray coating experiments.
3) The „In situ‟ methods (at low or high temperature) give a wide variety of
morphologies. When the aging time was not long enough microparticles were
obtained. The presence of catalyst on the hydrolysis solution accelerates the sol-gel
process and then allows a better control of the homogeneity of the films. Another
important factor is the ratio between the sol-gel reactive a too large concentration of
hydrolysis solution results in particulate powder.
4) Spraying the sol-gel components directly in supercritical conditions seems to be the
most appropriate and simple method to obtain aerogel films by „one-pot‟ processing.
The substrate was covered by HCOOH and the precursor was sprayed through a
nozzle, the CO2 at supercritical conditions acts as the reactive medium. When the
aging time was too short, the sol-gel process was not finished and then microparticles
were found spread over the reactor. Under the optimized conditions, homogeneous
and porous films were manufactured. An important factor to control the porosity and
morphology of the coatings was the precursor/HCOOH molar ratio.
5) Further essays may be performed in order to find a technique to measure the porosity
of the films in order to ensure the presence of silica aerogel structure. One possible
technique is by elipsometry, which may allow measuring the density of the films.
Chapter VI Silica aerogel films 231
6) Further characterization of the films should be carried out (density, thickness,
adherence to different substrates, etc.).
7) Accurate theories of in situ and spray directly in supercritical conditions would permit
better control of these processes and would allow the design of specific coating
protocols for specific applications.
6. REFERENCES
1. M. Bunzendahl, "Development of a
Low-Dielectric Constant Thin-Film
Composite Material." M.S. thesis,
Department of Mechanical,
Aerospace, and Nuclear Engineering
University of Virginia, August 1998
2. “Lightweight solar cell” Patent
US5221364, issued1993-06-22,
Hotaling Steven P
3. “Silica aerogel film and its
production, optical fiber“ Tsubaki
Kenji; Yokoyama Masaru; Yokogawa
Hiroshi; Sonoda Kenji, JP10300995,
issued 1998-11-13
4. “Developed sol-gel spray coatings for
solar energy collectors”. Brinker C
Jeffrey; Fraval Hanafi R, U.S
Patent:WO0010044, issued 2000-02-
24.
5. “Low mass optical coating for thin
film detectors”, Cho Chih-Chen;
Beratan Howard R US Patent
US5929441, issued 1999-07-27.
6. “Coated film with aerogel layer with
low thermal conductivity and good
mechanical stability” Patent
DE19606114, issued 1997-08-21
Frank Dierk; Schwertfeger Fritz;
Zimmermann Andreas.
7. “Coated film used for thermal
insulation, electronic applications,
noise absorption or membranes”
Patent DE19537821, issued 1997-04-
17 Frank Dierk, Schwertfeger Fritz,
Zimmermann Andreas
8. “Acoustic waveguide properties of a
thin film of nanoporous silica on
silicon”. Rogers, John A.; Case,
Carlye, Appl. Phys. Lett., 75(6), 865-
867, 1999
9. “Acoustic properties and potential
applications of silica aerogels”. Gibiat,
V.; Lefeuvre, O.; Woignier, T.;
Pelous, J.; Phalippou, J. Non-Cryst.
Solids, 186, 244-55 1995
10. “Applications for silica-based aerogel
products on an industrial scale” M.
Scmidt, F. Schwertfeger. Mat. Res.
Soc. Symp. Proc. Vol 521 1998
11. “Evaluation of the acoustic properties
of silica aerogels” Conroy, John F. T.;
Hosticka, Bouvard; Davis, Scott C.;
Norris, Pamela M. 82 Porous, Cellular
and Microcellular Materials, 25-33
1998
12. “Localization of acoustic vibrations in
aerogels” Vacher, R.; Courtens, E.
Chapter VI Silica aerogel films 232
Ultrason. Symp. Proc., (2), 1237-9
(English) 1989.
13. “Modified silica aerogels as acoustic
impedance matching layers in
ultrasonic devices” Gerlach, R.;
Kraus, O.; Fricke, J.; Eccardt, P. C.;
Kroemer, N.; Magori, V. J. Non-
Cryst. Solids, 145(1-3), 227-32 1992.
14. “Flexible aerogel composite for
mechanical stability and process of
fabrication” Patent WO 9938610,
issued 1999-08-05 Coronado, Paul R;
Poco, John F.
15. „Aerogel mesh getter„, Patent
US5470612 , issued 1995-11-28
Hotaling Steven P, Dykeman Deidra
A .
16. “Rapid aging technique for aerogel
thin films” US5753305, issued 1998-
05-19, Jeng Shin-Puu; Smith Douglas
M; Ackerman William C
17. .D.E Bornside et al. J. Appl. Phys. 66
(1989) 5185.
18. "Preparation of High Porosity
Xerogels by Chemical Surface
Modification", Brinker, C.J.,
Desphande, R.,Smith, D.M., U.S.
Patent No. 5,565,142, issued October
15, 1996.
19. "Ambient Pressure Process for
Preparing Aerogel Thin Films",
Brinker, C.J., Prakash, S. U.S. Patent
No.5,948,482, issued September 7,
1999
CONCLUSIONS
This section provides an overview of the conclusions of each of the chapters presented in
this thesis.
The preparation of silica aerogels deals with the combination of the silica solid matrix
(formed by interlinked silica nanoparticles) and nanometer-sized pores (filled with air). Such
combination results in the unique optical, thermal, acoustic, and mechanical properties of
aerogels. In this work it has been shown that there is a direct connection between the
chemistry of the sol-gel process and the structure of the gels and consequently, on the
microstructure of the aerogels. The choice of the precursors and the chemical reaction
parameters determines the physical properties of the final silica aerogels. It has been shown
that it is possible to control porosity and transparency by adjusting the so-called sol-gel
parameters, which include the type and concentration of alkoxide precursor, acid or base
catalyst, and water content.
Tetramethoxysilane as metal alkoxide precursor (TMOS):
To obtain transparent aerogels the best synthesis is obtained by using methanol as solvent
with the presence of base catalyst at low concentrations. This synthesis has been labeled as
M-series.
Varying the TMOS concentration in acetone gels proves to be an easy way for controlling
density and porosity of the resulting aerogels. The study of the influence of the
TMOS/acetone concentration of the sol on the density of the aerogels was performed by
using the so-called A-series. Methanol gels are very transparent while acetone aerogels have a
white shading and their opacity decreases with increasing TMOS content. For all the samples
of A-series, monolithic structures without cracks were obtained. The A1 aerogels were
especially fragile. Cracked gels were produced when ethanol was used as a solvent.
It was found that the gel time, tg , widely expands from few minutes to several days,
depending on the type of solvent and catalyst combinations, with shortest t g being for
methanol solvent and sodium hydroxide catalyst. This effect is a result of the shortest chain
length and branching of the methanol compared to the other solvents.
Conclusions 234
Tetraethoxysilane as metal alkoxide precursor (TEOS):
Aside from TMOS, other esters of orthosilicic acid like TEOS were used to obtain silica
aerogels. TEOS is not only less toxic when compared to TMOS but it is cheaper too. Hence,
TEOS is a more suitable precursor for the commercial production of silica aerogels.
The best quality TEOS aerogels, in terms of monolithicity and transparency, without much
shrinkage were obtained by using weak base or acid catalyst, TEOS molar concentration in
alcohol: between 5 and 7, and excess of stoichiometric amount of water with values between
5 and 8. Strong acidic catalyst gave transparent but cracked aerogels, whereas weak acids
yielded monolithic and transparent aerogels. The conditions considered optimal
concentration of reactive were for TEOS/EtOH/H2O =1/5/7, 1/7/5, 1/7/6, and 1/7/8.
The reaction was more easily controlled with the presence of low concentration of acid citric
catalyst (0.01<c<0.03). On the contrary, taking the m precursor concentration (m=
TEOS/EtOH) lower than 5 (m< 5) and higher than 9 (m> 9) values resulted in opaque, high
density as well as cracked aerogels. The effect of ethanol content over the TEOS gels allowed
aerogel density to decrease. In addition, it can be concluded, that an increase in solvent
content reduces the probability of mutual collisions of hydrolyzed alkoxides molecules (Si -
OH), resulting in a decrease in the rate of polymerization reaction.
One can state that to produced a good quality aerogels, the most favorable condition of the
preparation were when the ‘two-step method’ was followed, a first step in acidic conditions
and a second one in basic conditions. The two-steps aerogel density increases as the
concentration of catalyst increases, due to the fact that at too large catalyst concentrations,
the colloidal particles and pores are smaller and therefore the gels tend to shrink and become
denser. In all two-steps obtained aerogels to enhance the aging process, the gels were soaked
in an alcohol/water/catalyst mixture of equal proportions to the original sol. The gels were
maintained in this solution up to 24 hours. After that, water was removed by soaking the gels
in an ethanol solution. In conclusion, although a slightly more laboriously synthesis, the two-
step process appears to be the best synthesis method to obtain low-density (from 0.03 to
0.1g/cm3), non-cracked, and very transparent aerogels with a high surface area (800-
1000m2/g).
For all the metal alkoxide precursors, it was observed a decrease of gel time by either
increasing the amount of water or decreasing the concentration of the precursor. It should be
pointed out that for the same synthesis and drying conditions, gels washed in ethanol
Conclusions 235
exhibited a lower bulk density and the incidence of cracking or fracture of aerogels was
significantly lower. This difference in bulk density is attributed to the presence of water
during drying process that caused a larger shrinkage giving a denser bulk structure.
Physical characterization of silica aerogels has been widely investigated. The aerogels were
characterized by BET, IR, UV-VIS spectroscopy, light scattering, SEM, and TEM techniques.
Aerogels from diverse sets presented clear differences in shrinkage, transparency and porosity.
Initial parameters, such as solvent, catalyst and water content, have a pronounced influence in
the pore structure and optical transparency of the final aerogel. Different microstructures,
from macroporosity to mesoporosity, have been obtained by varying the initial sol parameters.
The surface area and porosity of the resulting aerogels has been discussed using the BET
technique, M and A-series showed similar surface areas (from 400 to 600 m2/g). The
translucence of the aerogel samples indicates the presence of macropores, although they are
not accounted by BET. In order to evaluate the effect of the macropores on the transparency,
light transmission experiments have been performed. The optical transmission of the aerogels
was measured at a wavelength of 900 nm in order to compare quantitatively the degree of
transparency of the samples at the visible range. The lowest transmission was obtained for
more diluted sol conditions (A1), the percentage of transmission was of 23% for a sample
with a thickness of 1 cm and the highest transmission for denser aerogel (A4), the percentage
of transmission increased up to 40%. Aerogel transparency increases with the TMOS
concentration due to lower macropores content. The percentage of light absorbance
depending on the intensity of the Rayleigh scattering has been related to the diameter size of
the scatter centers, < pore>Rayleigh, larger porous leads to lower transmission.
It is important to point out that pore size distribution has been described by a proposed
model using BET, optical transparency and density measurements. From that model, it has
been concluded that:
The opacity in A1 is explained by the large value of V, where V= Vpore -VBET, which
corresponds approximately to the macropores not measured by BET (~90% for A1). UV-
VIS measurements allowed to confirm this hypothesis giving a A1 pore diameter,
< pore>Rayleigh, of 150 nm. A2 and A3 have a pore size distribution with an important number
of micropores and/or macropores, but the mean pore diameter obtained is within the
Conclusions 236
mesopore range. A4 presented a mesopores distribution, which is totally accounted by the
BET technique, and agrees with the mean pore value obtained by optical measurements. M
have a pore size distribution that is mostly accounted by BET (~80%). Its mean value is
similar for both techniques, BET and optical measurements.
The nephelometer measured the angular and polarization dependence of light scattered at
visible regions. It has been proved that intensity measurements at fixed angle provide
information about the size of scattering centers. Angular measurements at different
wavelength have demonstrated the necessity of performing scattering measurements to
obtain information outside of the Rayleigh regime. The presence of the correlation function
(r) in the scattered intensity equations has permitted to use light scattering measurements to
determine density-density correlation in the aerogels medium, and to extract information
about the inhomogeneities of the aerogel microstructure. It was assumed that the correlation
function consists of a short-range exponential part, 1(r), and a long-range gaussian part,
2(r). The model proposed was able to fit the experimental data by the variation of the a1, a2
and w parameters. The model was fitted with four different wavelengths.
Further studies should be undertaken in order to improve the fitting of the data with the aim
of extracting structural information from the scattered intensity and to normalize the
intensity for each laser beam. New correlation functions are proposed to fit the experimental
data.
Since bulk material properties are a function of aerogel microstructure, then efforts to direct
imaging aerogels at molecular level have been taken by using SEM and TEM. The
morphological features, such as particle shapes and particle arrangements, of the series of
acetone silica aerogel A-series, and the base catalyzed aerogel with methanol as solvent (M)
has been examined by Scanning and Electron microscopy (SEM and TEM). An estimation of
the particle size has also been evaluated. SEM images show that the aerogels have a granular
appearance composed by spherical particles. A1 sample is built by smaller interconnected
particles than the denser A4 sample. A4 showed the smallest pores, although particles were
larger than those of A1. M aerogel shows an interconnected structure with smaller particle
size. TEM technique has provided molecular information about the aerogels and to
distinguish the different parts of the structure of the aerogel. Individual chains and
Conclusions 237
crosslinking junctions have been visualized. TEM microscopy was used to examine the
surface replicas in stereo with a tilt series at 20k-80k magnifications. The stereoscopic images
have made possible a three-dimensional visualization of the aerogel structure.
The microindentation technique has proved to be a non-destructive, suitable dynamical
method to assess the parameters that characterize the mechanical behavior of extremely
porous materials such as aerogels, despite their brittleness and softness. Silica aerogels of
different mechanical responses have been obtained by varying the initial parameters in the
TMOS sol-gel polymerization process, such as alkoxide concentration, solvent, drying process,
as well as the carbon addition. Young’s modulus, hardness and the elastic parameter, that
measures the percentage of elasticity versus plasticity, have been evaluated for these aerogel
samples. It has been shown that the evolution of the parameters describing the mechanical
behavior as a function of the bulk density follows power-scaling laws. A relation of the type,
E with ~2.9 was found for the A-series. The evaluated exponents are 2.0 and -3.2 for
hardness, and elastic parameter, respectively. As a function of aerogel density, two different
regimes of mechanical behavior are observed. The lowest density aerogels are elastic but the
denser aerogels are elasto-plastic materials. The large dependency of Young’s modulus on the
density as well as the change from elastic to plastic behavior has its origin on the aerogel
microstructure. Further structural and modelisation efforts are needed for a deeper
understanding of these dependencies.
The effects of the solvent, being methanol or acetone, on the silica aerogels have also been
studied. It was found that for samples with similar density values, the ones obtained using
acetone have higher hardness and Young’s modulus values than those obtained from
methanol, but with less elastic recovery. Moreover, the effect of the drying process has been
studied. It has been shown that the process with CO 2-acetone exchange causes a slight
improvement in hardness and a relevant increase in the elastic modulus, mostly due to larger
shrinking effects.
An improvement of the material’s elasticity is needed for some applications. Carbon-
reinforced aerogels present a more elastic response to indentation compared to silica
aerogels. The addition of small amounts of powdered carbon as filler in silica aerogels
increases the elasticity of the composite and keeps the hardness similar to silica matrix values.
Conclusions 238
It has been found that including small amounts of activated carbon inside the fragile silica
network dramatically increases its elastic indentation recovery. This is reflected in the shape
of the indentation curves as well as in the increase of the elastic parameter value. Young’s
modulus values obtained for carbon-reinforced aerogels show a similar variation to the
carbon mass fraction as that predicted by a commonly used model for composite materials.
The measured hardness values corresponding to the total elastoplastic deformation do not
show such a prominent dependency on the carbon mass fraction as the elastic parameter and
Young’s modulus do and values of hardness are similar to those measured for the pure silica
aerogel.
A simple and versatile method to obtain silica aerogel particles based on the hydrolysis and
subsequent condensation of silicon alkoxides (TMOS/TEOS) in several supercritical f luids
has been proposed. The sol-gel route at supercritical conditions reduces the number of steps
of the traditional microparticle sol-gel processing. To obtain aerogel silica powders at low
temperature the synthesis has been performed using supercritical carbon dioxide as solvent.
Following the ‘in situ’ high-temperature method, spherical and fiber silica particles have been
produced by ‘one-pot’ method using the sol-gel process with supercritical acetone as a
solvent. The spherical particles showed a very narrow size distribution. The particle size is
controlled by varying the relative amounts of alkoxysilane, water and acetone. Other
important parameters in order to control the particle size are the relative time, t/tgelification,
and the way of introducing the reactants in the autoclave. Silica aerogel microparticles have
also been produced at low-temperature by ‘one-pot’ process by injecting the reactive under
CO2 supercritical conditions. In this case, less spherical in shape particles were obtained
because the existence of necking between particles. The most porous aerogel microparticles
have been produced when using two-steps synthesis and introducing a liquid CO2 exchange.
Major focus was given on the understanding of the particle formation mechanisms and on
the conditions to tailor the particle morphology, size and porosity. The morphology of the
particles has been characterized by electron microscopy (SEM and TEM) and Atomic Force
Microscopy (AFM). The microstructure observed on the silica aerogel surface was similar to
that one of bulk silica aerogels. Coulter technique has also been used to account for the
particle size distribution. In addition, BET surface area, pore volume and mean pore diameter
of the aerogel microparticles have been obtained from nitrogen sorption isotherms.
Conclusions 239
Aerogel films have a wide range of applications. The main objective of film chapter has been
to synthesize aerogels as homogeneous films. Two ‘one-pot’ methods have been described. A
recently set up ‘laboratory scale plant’ has provided us with the possibility to better rationalize
the number of free parameters in the experiments and simplify the realization of such. We
have also performed some dip coating and spin coating experiments to compare the results
of those with our methods. When dealing with dip and spin coating methods to produce
aerogel films, the solvent evaporation rate should be slowed down since there is a large
contact area of the forming gel with the atmosphere. Thus, the coating is usually done in a
saturated solvent vapor chamber. A second step is required to supercritically dry the coating.
Further characterization of the films should be done (density, thickness, adherence to
different substrates, etc.).
A N N E X I
SUPERCRITICAL FLUIDS
A supercritical fluid is defined as a fluid above its critical temperature (TC) and its critical
pressure (PC). The critical point represents the highest temperature and pressure at which the
substance can exist as a vapor and liquid in equilibrium.
A ‘physical’ way to explain what is happening in a supercritical fluid is that when two
molecules approach each other in a fluid, at a temperature where their relative speed is likely
to be low, their mutually attractive forces will bring a temporary association between them. If
there is a sufficient density of molecules, there is the possibility of condensation to a liquid.
On the other hand, if the temperature and the probable relative speeds are high, the attractive
force will be too weak to have more than a slight effect on the molecular velocities, and
condensation cannot occur however high the molecular density. It is therefore reasonable to
expect, on the basis of molecular behavior, that for every substance there is a temperature
below, which condensation to a liquid (and evaporation to a gas) is possible, but above which
these processes cannot occur. That there is a critical temperature above, which a single
substance can only exist as a fluid and not as either a liquid or gas.
The phenomenon can be easily explained using the phase diagram for pure carbon dioxide
(Figure AI.1).
Annex I. Supercritical f luids 244
Figure AI. 1 Phase diagram of a single substance, carbon dioxide.
The phase-diagram is schematic, and the pressure axis is non-linear. It shows the areas where
carbon dioxide exists as a single gas, liquid, solid phase or as a supercritical fluid. The curves
represent the temperatures and pressures where two phases coexist in equilibrium (at the
triple point, the three phases coexist). Moving along the gas-liquid curve, increasing both
temperature and pressure, then the liquid becomes less dense due to thermal expansion and
the gas becomes denser as the pressure rises. Eventually, the densities of the two phases
converge and become identical, the distinction between gas and liquid disappears, and the
end of the coexistence curve is defined as the critical point. The substance is now described
as a fluid. The critical point has pressure and temperature coordinates on the phase diagram,
which are referred to as the critical temperature, Tc, and the critical pressure, pc, and which
have particular values for particular substances, some examples are shown in the table below.
Annex I. Supercritical f luids 245
Table AI. 1 Substances useful as supercritical fluids, with critical
parameters
Solvent
Critical pressure Pc (atm)
Critical temperature Tc (°C)
H20 216 374
NH3 110 132
CO2 70 31
CH3CH2OH 62 243
CH3OH 77 240
CH3CN 47 275
Hexane 32 235
It was 170 years ago that Baron Charles Cagniard de la Tour showed experimentally the
disappearance of the distinction between the liquid and gas phases. In the experiment, it was
observed through a view cell how the meniscus between a liquid and a gas disappeared at the
critical temperature.
Figure AI. 2 Disappearance of the meniscus at the critical point. The
meniscus separating a liquid (bottom) from its vapor (top) disappears at
the critical point. The liquid state does not exist above the critical
temperature, regardless of the pressure that might be applied to the
substance.
Annex I. Supercritical f luids 246
Supercritical fluids exhibit important characteristics such as compressibility, homogeneity,
and a continuous change from gas-like to liquid-like properties. These properties are
characteristic of conditions inside supercritical fluid region.
The more fundamental interest in supercritical fluids arises because they can have properties
intermediate between those of typical gases and liquids. Compared with liquids, they have
lower densities and viscosities and greater diffusivities. The conditions may be optimum for a
particular process or experiment. Furthermore, properties are controllable by both pressure
and temperature and this characteristic compared with a liquid, leads to that more than one
property can be optimized. The main disadvantages are their cost and inconvenience of the
higher pressures needed. Consequently, supercritical fluids are exploited only in particular
areas.
The critical point for carbon dioxide occurs at a pressure of 73.8 bar and a temperature of
31.1°C. These parameters make equipment design relatively simple. Carbon dioxide is
available as a convenient supercritical fluid substance. Carbon dioxide has so far been the
most widely used, because of its convenient critical temperature, low price, chemical stability,
non-flammability, stability in radioactive applications and non-toxicity. Large amounts of
CO2 released accidentally could constitute a working hazard, but hazard detectors are
available. It is an environmentally friendly substitute for other organic solvents. Its polar
character as a solvent is intermediate between a truly non-polar solvent and weak polar
solvents.
BIBLIOGRAPHY
This annex is adapted from the introduction to Fundamentals of Supercritical Fluids by T.
Clifford, published by the Oxford University Press in 1998.
A N N E X I I
TECHNICAL DESCRIPTION OF THE JOIN ICMAB-CM HIGH PRESSURE-HIGH
TEMPERATURE LABORATORY*
The laboratory is composed by two main equipments (a laboratory scale plant and pilot plant).
The high pressure-high temperature lab was created by ICMAB and Carburos Metalicos in
1995 to investigate develop and promote new supercritical fluid applications. The lab was
designed to allow the maximum possible types of experiments, proposed by research groups or
industries.
The high pressure-high temperature facility is able to work up to 400 ºC and 500 bars with
CO2, as a supercritical fluid. However, others supercritical fluid can be used, if necessary. The
flexibility is one of the more important aspects of the ICMAB pilot plant. Temperature,
pressure and flow rate can be easily tuned to work at different conditions. This flexibility
allows planning different operations and to control pressurization, depressurization and
heating rates with great accuracy. The collected pressures have an uncertainty of 0.7%.
CO2 was chosen as main supercritical fluid due to the advantages that offer (Table AI.1).
Although its safety, some preventive measures must be taken when CO 2 is used. It may
produce immediately hazardous and asphyxiating atmospheres. Fast liquid CO 2
depressurization produce solid CO2 and it can block pipes. CO2 can be recycled as much as
needed and product solubility can be tuned by controlling pressure and temperature
*This section was taken from Supercritical Training Course. J. Torres and R. Solanas
Table A.I. 1 Advantages of carbon dioxide as supercritical fluid
Advantages
Non-toxic Highly selective Non-flammable Low viscosity
Cheap High diffusivity Readily available
Annex II. Technical description of the join ICMAB-CM High pressure-high temperature Laboratory 248
1. DESCRIPTION OF THE HIGH PRESSURE-HIGH TEMPERATURE
PLANT
The experimental apparatus used for drying is shown in Figure A.I.1.
Figure A.I.1 Pilot plant and its three unites: 1) CO2 supplier and
liquefier (blue square), 2) reactors – pressure, temperature and flow rate
control (violet square), 3) and conditioning before releasing (green
square).
Table AI.2 gathers the technical description of the pilot plant.
Table A.I. 2 Pilot plant technical data
Maximum pressure: 500 bar Maximum temperature: 400 ºC Number of reactors: 3 Reactors volume: 1-2 liter Heating Power: 14.5 kW (5 heaters)
CO2 maximum flow rate: 40 Kg/h Cosolvent maximum flow rate: 1,8 l/h Computer controlled plant
The chamber pressures given have an uncertainty of ±0.7percentage.
Additional features:
Work with liquid and solid samples
Recycling CO2
Feed cosolvent
Annex II. Technical description of the join ICMAB-CM High pressure-high temperature Laboratory 249
Spray liquids inside reactors
Data recording for temperature, pressure and CO2 flow rate
Different working conditions at each reactor
Adsorption tower before venting
Alternative gases (nitrogen,) can be fed
SCF sampling
2-litre agitated reactor with quartz viewer
Different types of filters are available
As it is shown in figure 1 the plant is divided in three unites: 1) CO 2 cylinders and its liquefier,
2) 3 reactors of 2 liters– pressure, temperature and flow rate control, and 3) the control
releasing facilities (a safety vessel of 51 liters). The first and third units are located outdoors
and the second inside the lab.
Unit 1: CO2 supplying and liquefier
Composed by:
- Two CO2 cylinders set each one with four cylinders
- 85 liters liquefier with its temperature and level control
- Two chillers
- Glycol cooling closed loops (with a pump included) to maintain the CO 2 in liquid phase
- Five pressure transmitters
- Six relief valves
- Three thermocouples, to control temperature of the cylinders
- Seven on/off valves, to control feeding of the CO2
- Several manual valves, to control the feeding in case of electronic failure
Annex II. Technical description of the join ICMAB-CM High pressure-high temperature Laboratory 250
Figure A.I.2 Outdoors view: high-pressure pump, CO2 liquefier,
chillers, and cylinders.
Unit 2: Reactors with its corresponding pressure, temperature and flow rate control
Composed by:
High pressure pump (to feed the CO2) outdoor
Five control valves for pressure and flow rate control
Seven on/off valves to control CO2 feeding
Three reactors (pressure maximum: 500 bar temperature maximum: 400 ºC): two 2-liter
capacity reactor and one 1-liter capacity extractors.
Five heaters (one for CO2 preheating, one for each reactor and piping)
Two mass flow meters
Sampling loop
Two heat exchangers for CO2
Four pressure transmitters
Four-safety pack composed by one relief valve and one pressure switch.
Annex II. Technical description of the join ICMAB-CM High pressure-high temperature Laboratory 251
Fourteen k thermocouples to control the temperature in different parts of the plant and
turn off the heating in case of safety
Several manual valves in case of automatic valves failure
Figure A.I.3 High pressure - high temperature vessel
Unit 3: System of releasing
Figure A.I.4 High pressure - high temperature pilot plant
Composed by:
Safety vessel (51 liters), designed to collect toxic or dangerous supercritical solutions,
avoiding releasing them to the atmosphere, during overpressures and emergency
shutdowns.
Annex II. Technical description of the join ICMAB-CM High pressure-high temperature Laboratory 252
Adsorption tower filled with activated carbon
Two pressure transmitter
Two-safety pack composed by one relief valve and one pressure switch
One control valve
One on/off valve
Several manual valves
Safety system (safeguards)
The most dangerous hazard is a rupture of the reactor, vessels or pipes under pressure, due to
an overpressure or high temperature.
A safety 10 mm polycarbonate screen protects the plant.
Overpressure safeguard: At each reactor, safeguards are redundant in four levels: i) Software
does not allow pressure set point higher than the maximum pressure. ii) Presence of a pressure
transmitter control– if measured pressure is higher than safety set point, the plant stops. iii)
Pressure switch, and iv) level is controlled by relief, then, the overpressure is released.
High temperature safeguard: At each reactor, safeguards are redundant in two levels: i)
Software does not allow temperature set point higher than the maximum temperature of the
plant. ii) Thermocouple – if measured temperature is higher than safety set point, heater is
turned off.
CO2 high concentration safeguard: In the laboratory, there are two O2 sensors and a fan to
renew air.
Personal protection safeguards: high temperature gloves, safety glasses, small masks for
organic solvent and powder.
Wrong operation safeguard: special software was designed to avoid wrong operation.
Switches and position detectors avoid wrong operation, too.
Safety vessel.
Water hammer appears after suddenly opening or closing an On/off valve, when the
pressure difference between valve inlet and outlet is high. It could be dangerous and it must be
avoided or diminished.
1.1.1 Description of the laboratory scale plant
Lab scale plant has an agitated 300 ml vessel, able to work up to 414 bars at 20 ºC or 227 bars
at 454 ºC. Samples can be liquids and/or solids and co-solvents can be fed.
Annex II. Technical description of the join ICMAB-CM High pressure-high temperature Laboratory 253
Figure A.I.5 Description of the laboratory scale plant and its two
unites: 1) CO2 supplier and liquefier (A square), 2) reactor– pressure,
temperature and flow rate control (B square),
esquema planta petita.dwgEsquema
Planta Petita
Laboratori Supercritic
Esquema Planta Petita
Laboratori Supercritic
B
A
Annex II. Technical description of the join ICMAB-CM High pressure-high temperature Laboratory 254
Figure A.I.6 Picture of lab scale plant. It is composed by: (1) reactor
(red line), (2) chiller (g reen line), (4) pressure valves (black lines), (5)
nitrogen and carbon dioxide gas connection (white line), (6) CO2
liquid pump (yellow line), and (9) pressure, temperature, and flow rate
control system.
A ‘zoom’ up of the reactor allows distinguishing the pressure valves and the realizing system
and the connections for the cooling/heating system.
5
9
1
6
2
Annex II. Technical description of the join ICMAB-CM High pressure-high temperature Laboratory 255
Figure A.I.7 Detail of the s mall of lab scale p lant
Analogical
manometer
CO2
Gas pipe
Magnetic
stirring drive
Pressure
valves
Rupture
disk
Control
panel
Thermocouple
Pre-heater
Valve
control
A n n e x I I I
INTRODUCTION TO ADSORPTION ANALYSIS AND TO THE BET
MODEL
1. INTRODUCTION TO ADSORPTION ANALYSIS
When a gas or vapour is brought into contact with a solid, the solid takes up part of it. The molecules
that disappear from the gas either enter the inside of the solid, or remain on the outside attached to
the surface. The former phenomenon is termed absorption (or dissolution) and the latter
adsorption. When the phenomena occur simultaneously, the process is termed sorption. The
phenomenon of adsorption was discovered over two centuries ago.
The solid that takes up the gas is called the adsorbent, and the gas or vapour taken up on the surface
is called the adsorbate. It is not always easy to tell whether the gas is inside the solid or merely at the
surface because most practical absorbents are very porous bodies with large internal surfaces. It is not
possible to determine the surface areas of such materials by optical or electron microscopy because of
the size and complexity of the pores and channels of the material. The gas adsorption itself, however,
can be used to determine the accessible surface area of most absorbents.
In this section the adsorption of N2 at cryogenic temperatures on silica aerogels and silica
microparticles was investigated
1.1 THEORY OF ADSORPTION
Molecules and atoms can attach themselves onto surfaces in two ways. In physisorption (physical
adsorption), there is a weak van der Waals attraction of the adsorbate to the surface. The attraction to
the surface is weak but long ranged and the energy released upon accommodation to the surface is of
the same order of magnitude as an enthalpy of condensation. During the process of physisorption,
the chemical identity of the adsorbate remains intact, i.e. no breakage of the covalent structure of the
adsorbate takes place. In chemisorption (chemical adsorption), the adsorbate sticks to the solid by the
formation of a chemical bond with the surface. This interaction is much stronger than physisorption,
and, in general, chemisorption has more stringent requirements for the compatibility of adsorbate and
surface site than physisorption. The physisorption classification for the pore size is given
by:Macropores: when pore diameter ( 0) is 50nm (0.05 m), Mesopores: when 2nm 0 50nm,
Micropores: when 0 2nm.
Annex III.Introduction to adsorption analysis and to the BET model 258
This definition is not exact because the filling of the pore depends on the shape of them and is
influenced by the adsorbate properties and for the adsorbent/adsorbate interactions.
The total accessible volume present in the micropores is considered as adsorption space, this process
is called micropores filling; different at the superficial coverage occurred on the open macropores -
mesopores walls. The mesopores physisorption occurs in two steps: monolayer adsorption and
capillary adsorption.
In the monolayer adsorption, all adsorbate molecules are in contact with the adsorbent surface. In the
multilayer adsorption not all the molecules adsorbate are in direct contact with the adsorbent surface
because the adsorption space is covered for multilayer molecules. In the capillary adsorption the
residual space of the pore that is empty after the multilayer adsorption is filled by a condensed,
separate of gaseous phase by meniscus. The capillary condensation is frequently followed by hysteresis.
This process does not exist in the micropores filling. In physisorption, the monolayer capacity, nmono,
is defined as the adsorbate quantity needed to cover all the absorbent surface with a complete
molecule monolayer and Vm as the volume of the maximum gas absorbed usually taken to be a
monolayer. The energy of adsorption depends on the extent to which the available surface is covered
with adsorbate molecules. This is because the adsorbate can interact with each other when they lie
upon the surface (in general they would be expected to repel each other). The fractional coverage of a
surface is defined by the quantity :
sitespossibleofnumberTotal
sitesadsorptionoccupiedofNumber (1)
At any temperature, the adsorbate and the surface come to a dynamic equilibrium, that is, the
chemical potentials of the free adsorbate and the surface bound adsorbate are equal. The chemical
potential of the free adsorbate depends on the pressure of the gas, and the chemical potential of the
bound adsorbate depends on the coverage, . Thus, the coverage at a given temperature is a function
of the applied adsorbate pressure. The variation of with P at a given T is called an adsorption
isotherm. Adsorption hysteresis is obtained when adsorption and desorption curves are different. The
specific surface area adsorbent (As) is calculated by:
mnNA monoAs
Where: NA is Avogadro's number, is the molecular area occupied per absorbate N2: 16.2Ų, m is
the mass of the sample
1.2 THE LANGMUIR MODEL
Langmuir suggested the earliest model of gas adsorption. The model is limited to monolayer
adsorption. If one assumes that all adsorption sites are equivalent then the adsorption and desorption
rate is independent of the population of neighbouring sites. Then one can derive a simple formula for
an adsorption isotherm.
Annex III.Introduction to adsorption analysis and to the BET model 259
Consider the equilibrium:
A + S A.S (2)
Where: A is the free adsorbate, S is the free surface, A.S is the substrate bound to the surface.
The rate of adsorption will be proportional to the pressure of the gas and the number of vacant sites
for adsorption. If the total number of sites on the surface is N, then the rate of change of the surface
coverage due to adsorption is:
)(1NPkdt
da
(3)
The rate of change of the coverage due to the adsorbate leaving the surface (desorption) is
proportional to the number of adsorbed species:
NPkdt
dd (4)
In these equations, ka and kd are the rate constants for adsorption and desorption respectively and p
is the pressure of the adsorbate gas. At equilibrium, the coverage is independent of time and thus the
adsorption and desorption rates are equal. The solution to this condition gives us a relation for :
mV
V
Pk
Pk
1 (5)
where :k = ka / kd, .V= volume of gas absorbed at pressure P
Note that because k is equilibrium constant, the value of k at various temperatures determined from
the Langmuir isotherm allows for the evaluation of the enthalpy of adsorption, ads, through the
Van't Hoff equation:
2
ln
RT
H
dT
kd ads (6)
As the strength of the interaction between the adsorbent and the adsorbate increases the value of p
increases and the surface coverage increases faster as the pressure is increased.
To use the Langmuir model adsorption data are plotted in the form:
mm V
P
kVV
P 1 (7)
Vm is calculated plotting P/V against P and find Vm from 1/slope of the line.
In practice, it has been found that the Langmuir model is rarely a useful model to fit gas adsorption
data and hence calculate the surface area. The Langmuir model is only applicable when adsorption at
low coverage. The Langmuir isotherm is found to be useful only at very small coverage (sub-
monolayer) but is generally applied to all cases involving chemisorption.
Annex III.Introduction to adsorption analysis and to the BET model 260
2. THE BET MODEL
Brunauer, Emmett and Teller developed several models of gas adsorption on solids, which have
become the effective standard for surface area measurements.
The models were generalisations of Langmuir theory monolayer adsorption to multilayer adsorption.
The BET isotherm is useful in cases where multilayer adsorption must be considered.
Nitrogen is the most commonly used BET adsorption gas because of its inertness to chemical
interaction with most materials and the ready availability of liquid nitrogen to control the temperature
of the adsorption process. The standard 2 parameter BET isotherm gives the amount of gas adsorbed
as a function of the relative pressure of the adsorbing gas:
xcx
xc
PPcPP
PPc
V
V
n
n
mmono )1(1)1()/()1(1)/1(
)/(
00
0 (8)
where
V = Volume of gas adsorbed at pressure P, Vm = Volume of gas covering the surface with a
monomolecular layer, n/nmono is the ratio of the moles adsorbed to the moles adsorbed in a single
monolayer.Po = Saturation pressure of the gas (vapour pressure), i.e. the pressure of the gas in
equilibrium with bulk liquid at the temperature of the measurement., x = P/Po = Relative pressure.
c = a constant for the gas/solid combination.
The isotherm can be converted to a linear form for ease of extracting the values of Vm and c. The
constant c represents the relative strengths of adsorption to the surface and condensation of the pure
adsorbate. Simple theory predicts an approximate value of this constant as:
RTH
RTH
vap
ads
e
ec
/
/
(9)
The constant c is related to the difference between the heat of adsorption of the first layer (H1) and
the heat of liquefaction (HL), Where H1-HL= H, is also known as the net heat of adsorption. R = gas
constant (8.31447 J K-1 mol-1) T = temperature (K)
The BET model is used to measure the surface areas of several porous materials. The BET isotherm
is found to describe adequately the physisorption at intermediate coverage ( = 0.8 - 2.0) but fails to
represent observations at low or high coverage. The BET isotherm is reasonably valid around =1.0,
however, and this is useful in characterising the area of the absorbent. If one can determine
experimentally the number of moles of adsorbate required to give , =1.0 (i.e. a monolayer), one can
determine the specific surface area of the absorbent.
gabsorbentofmass
mabsorbentofareasurfaceA
2
(11)
Practically, one measures the number of moles adsorbed as a function of equilibrium pressure, i.e. one
does not directly measure . Algebraic rearrangement of the BET isotherm to produce a linear
Annex III.Introduction to adsorption analysis and to the BET model 261
equation is usually applied to experimental data. For surface area measurements the BET equation is
used in the form:
xcn
c
cnxn
x
monomono
)1(1
)1(
Over the range where the BET isotherm is valid (in the range P/Po = 0.05-0.30), a plot of x/n(1-x) vs
x will be linear. The slope and intercept of this line will allow the determination of nmono (or Vm) and c.
erceptYSlopeVm
int
1 from the best fit straight line.
1int erceptY
Slopec from the best fit straight line.
Finally, the BET surface area is then calculated from:
mnNA monoAgm //2 (13)
Where: is the molecular area occupied per absorbate N2: 15.8 (16.2)Ų NA is Avogadro's number m
is the mass of the sample Vm is in cm³ at STP/g.
The adsorption process is generally taken as completely reversible, but, under some conditions the
isotherm may exhibit different shapes upon desorption as compared to absorption. This is called
hysteresis. Sometimes hysteresis data can be used to determine the structure and size of pores in the
absorbent. We will therefore need to generate an isotherm for both absorption and desorption.
2.1 EXPERIMENTAL PROCEDURE
We will perform the adsorption measurements in a commercial vacuum manifold called the ASAP
2000, manufactured by Micromeritics Instrument Corporation. The ASAP 2ooo system consists of
one analyser and a multi-function control module. The analyser it is designed for completely
automatic operation. Two separate internal vacuum systems are included-one for sample analysis and
one for sample preparation. It contains one sample analysis port and two sample preparation ports.
The sample P0 (saturation pressure) tube is located next to the sample analysis port. Since the analysis
results are expressed in units of surface area per gram of sample, the true weight of the sample must
be known, is necessary to determine the weight of the sample before degassed: Must solid materials
absorb moisture and other contaminants when exposed to the atmosphere. The sample must be clean
when analysis is performed. The sample is heated and placed under vacuum to remove moisture and
other contaminants. This process is referred to as degassing the sample. The approach to equilibrium
is perceptibly slow, especially at high coverage. It would in principle take an infinite time for
equilibrium to be exactly established. The nature of the isotherm and the required precision of the
measurement suggest that equilibrium pressures need be known only to a few percent. Recordding of
the equilibrium pressure generates the data needed to determine the number of moles adsorbed on
the solid as a function of (equilibrium) pressure and to generate the adsorption isotherm.
Annex III.Introduction to adsorption analysis and to the BET model 262
The desorption measurement is performed to see if there is any hysteresis or non-equilibrium effects
in the adsorption/desorption cycle. This is basically performed in reverse of the procedure above. The
value of the constant c in the BET equation affects the shape of the isotherm mainly at low relative
pressures (P/Po) as is shown below.
2.2 BJH PORE VOLUME AND AREA DISTRIBUTION CALCULATION
For adsorption data, the relative pressure and volume adsorbed data points pairs collected during an
analysis must be arranged in reverse order from which the points were collected during analysis. All
calculations are performed based on a desorption model, regardless of whether adsorption or
desorption data is being used. The data used in these calculations must be in order of strictly
decreasing numerical value. The data set is composed of relative pressure (Pr), volume adsorbed (Va)
pairs from (Pr1, Va1) to (PrN, VaN) where (PrN=0, VaN=0) is assumed as a final point. Generally, the
desorption branch of an isotherm is used to relate the amount of adsorbate lost in a desorption step
to the average size of pores emptied in the step. A pore loses its condensed liquid adsorbate, known
as the core of the pore, at a particular relative pressure related to the core radius by the Kelvin
equation (16). After the core has evaporated, a layer of adsorbate remains on the wall of the pore. The
thickness of this layer is calculated for a particular relative pressure from the thickness equation. This
layer becomes thinner with the successive decreases in pressure, so that the measured quantity of gas
desorbed in a step is composed of a quantity equivalent to the liquid cores evaporated in that step plus
the quantity desorbed from the pore walls of pores whose cores have been evaporated in that and
previous steps. Barret, Joyner, and Halenda developed the method (known as BJH method).
A pore filled with condensed N2 liquid has three zones:
The core: evaporates all at once when the critical pressure for the radius is reached; the relationship
between the core radius and the critical pressure is defined by the Kelvin equation.(16)
The adsorbed layer: composed of adsorbed gas that is stripped off a bit at a time with each pressure
step; the relationship between the thickness of the layer and the relative pressure is defined by the
thickness equation.
The walls of the cylindrical pore itself: the diameter of the empty pore is required to determine the
pore volume and the pore area. End area is neglected.
The total pore volume, Vp, is often derived from the amount of vapour adsorbed at a relative
pressure close to unity by assuming that the pores are then filled with condensed adsorptive in the
normal liquid state.
The pore size distribution is the distribution of pore volume respect to pore size. The computation
of the pore size distribution involves a number of assumptions (pore shape, mechanism of pore
filling, validity of Kelvin equation, etc.).
Application of the Kelvin equation
Mesopore size calculation are usually made with the aid of the Kelvin equation in the form
Annex III.Introduction to adsorption analysis and to the BET model 263
og p
pIn
v
RT
rr 11
21
11
Which relates the principal radii, r1 and r2, of curvature of the liquid meniscus in the pore to the
relative pressure, p/po, at which condensation occurs; here is the surface tension of the liquid
condensate and v1 is its molar volume. It is generally assumed that this equation can be applied locally
to each element of liquid surface.In using this approach to obtain the pore radius or pore width, it is
necessary to assume: (i) a model for the pore shape and (ii) that the curvature of the meniscus is
directly related to the pore width.
The use of the physisorption method for the determination of mesopore size distribution is subject to
a number of uncertainties arising from the assumptions made and the complexities of most real pore
structures. It should be recognised that derived pore size distribution curves may often give a
misleading picture of the pore structure. On the other hand, there are certain features of physisorption
isotherms (and hence of the derived pore distribution curves) which are highly characteristic of
particular types of pore structures. Physisorption is one of the few non-destructive methods available
for investigating mesoporosity, and it is to be hoped that future work will lead to refinements in the
application of the method - especially through the study of model pore system and the application of
modern computer techniques.
3. REFERENCES
S. Brunauer, "Physical Adsorption" (Princeton
University Press, Princeton, N. J., 1945)
S. Brunauer, P. H. Emmett and E. Teller, J.
Amer. Chem. Soc., 60, 309-319 (1938)
I.Langmuir, J. Am. Chem Soc., 38,
2219,(1916); 40, 1368, (1918)
Langmuir, J. Amer. Chem. Soc., 40, 1361
(1918);
Langmuir, J. Amer. Chem. Soc., 54, 2798
(1932);
Langmuir, Nobel Lecture, 1932].
P. Atkins, "Physical Chemistry" 5th ed
(Freeman, New York, 1994)
G. A. Somorjai, "Principles of Surface
Chemistry (Prentice-Hall, Englewood Cliffs,
N. J. 1972)
Drake ,J.M.; Nitrogen Adsorption on Porous
Silica: Model-Dependent Analysis; Journal of
Physical Chemistry; v.98, 1994, p. 380-2