Artigo MAt. CHEm. Phys. v. 130

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Materials Chemistry and Physics 130 (2011) 84–89

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Materials Chemistry and Physics

journa l homepage: www.elsevier .com/locate /matchemphys

Nanostructured Nb2O5–natural hydroxyapatite formed by the mechanicalalloying method: A bulk composite

W.J. Nascimento∗, T.G.M. Bonadio, V.F. Freitas, W.R. Weinand, M.L. Baesso, W.M. LimaUniversidadeEstadual de Maringá, Departamento de Física, Av. Colombo, 5790, 87020-900Maringá, PR, Brazil

a r t i c l e i n f o

Article history:

Received 14 September 2010

Received in revised form 6 April 2011Accepted 26 May2011

Keywords:

BiomaterialsSinteringNanostructuresMechanical properties

a b s t r a c t

The aim of this study was to develop a nanostructured Nb2O5–natural hydroxyapatite bulk compositeto serve as an alternative biocompatible bulk material for implants. A set of samples of hydroxyapatitefrom fish bones with different concentrations of Nb2O5 were designed. They were prepared througha milling process, compacted under different pressures (350, 450, 550 and 650 MPa) and sintered inair atmosphere at 1000 ◦C for 1 h. The results revealed that the prepared composites presented stronginteractions between the two elements and showed improvement in the sinterability with significantdensification and microstructure changes, suggesting that they are promising for implants meant toreplace bone tissues.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

The search for biocompatible materials for implants in thehuman body is an important area of research. Up to now, the bioac-tive calcium phosphates, among them hydroxyapatite (HAp) withthe composition Ca10(PO4)6(OH)2, have been widely used for thereconstruction and/or replacement of tissues, allowing medicineand modern dentistry to offer new options for patients, thusimproving their quality of life with the partial or total replacementof fractured bones. It is well known that these materials presentosteoconductibility and intrinsically osteoinductibilitycharacteris-ticsthat are similar to thoseof mineral bone matrices andhavebeenassociated with their chemical and structural similarities [1–7].However, due to their low mechanical strength compared with nat-ural bones,particularly regarding fragility and low resistance,thereare restrictions for their use, especially in implants intended forhighmechanicalefforts.The mainchallenge,therefore,has beenthedevelopment of alternative compounds with improved mechanical

resistances and biocompatibilities.Because of its excellent biocompatibility and bioactivity, HAp

has been used as a coating on materials with high mechanicalstrength, such as titanium, niobium and their alloys, resulting inimproved bioactive properties, thus contributing to a higher levelof osteointegration between the metal and the bone. This has beendone to overcome the undesirable low biocompatibility of thosemetals when used in the restoration of anatomical structures. In

∗ Corresponding author.E-mail address: [email protected] (W.J. Nascimento).

other words, the metallic component contributes to improving themechanical resistance, and the HAp coating facilitates the biolog-ical osteointegration between the implant and the bone tissue. Inaddition, it is known that an ideal alloy for implants should have alow density, excellent mechanical strength, high resistance againstcorrosion, and it must be free of toxic elements. Among the suit-able metals, the most resistant in terms of corrosion is titaniumand its alloys, followed by niobium, tantalum and stainless steel.Several studies have focused on the development of biocompat-ible composites based on these metals aiming to improve theircharacteristics in terms of their use in implants [8–13].

The choice elementdependson various factors that enable theiruse. Sometimes this choice is focused on the material’s cost oravailability. Niobium is abundant in Brazil, and it is known thatniobium pentoxide (Nb2O5) is simple to obtain. Moreover, thisoxide is biocompatible and resistant to corrosion [14–18]. The lowdensity of this oxide associated with the biocompatibility and non-toxicity suggests its use to improve the biomechanical properties

of the intended composites. Thus, the combination of a naturalHAp with Nb2O5 is promising for the development of biomaterialswith improved figures of merits in terms of their use in implants.In addition, the possibility of developing a bulk and homogeneouscomposite with these two compounds may improve the biocom-patible characteristics compared with those of existing implantsthat are usually made with a biocompatible coating deposited on ametal surface.

Therefore, the aim of this study was to develop, through pow-der metallurgy, a nanostructured Nb2O5–natural hydroxyapatitecomposite, to serve as an alternative biocompatible bulk materialfor implants. The influence of compaction pressure and sinter-

0254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved.

doi:10.1016/j.matchemphys.2011.05.069

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W.J. Nascimento et al./ Materials Chemistryand Physics 130 (2011) 84–89 85

ing on the physico-mechanical and microstructural properties of the HAp–Nb2O5 composites is discussed, and their properties arecompared with those of existing biocompatible materials used forimplants.

2. Materials andmethods

Natural HAp powder was obtained via the calcinations of fish bone at 900 ◦Cfor8 h. This calcination procedure was shown to be capable of inducing nanoparticlesin the powder [19–21]. Subsequently, this natural HAp powder and Nb2O5 were

mixed and milled for 3 h with the mechanical alloying technique. The milling wasperformed on a Fritsch Pulverisette6 planetaryball mill.The processwas madewitha stainless steel crucible and balls with mass ratios of 6/1 and a rotational speed of 300 rpm. The composites were designed in five compositions defined in (vol.%) by(100− x) HAp+ x Nb2O5, in which x variedfrom 50 to 90%. Samples compacted intocylindrical forms of 10 mm in diameter and 2 mm thickness were obtained with auniaxial press at 350,450, 550 and 650MPawithout using lubrication. The sinteringwas performed in air atmosphere at 1000 ◦C for 1 h. The (Nb2O5–HAp) powder wasexamined using CuK radiation on Shimadzu 6000 X-ray diffractometersource at ascan speed of 0.5◦ min−1 and step scan of 0.02◦ , performed at the final stage of themilling. The crystallite size (Lc) of composites after milling was estimated from theScherrer’s equation [22,23]:

Lc =k

ˇhkl cosÂ(1)

in whichLc is the averagecrystallitesize (nm),ˇhk l is the full widthof the peakat half of the maximum intensity, is the wavelength of X-rayradiationand k is the shape

coefficient (value between 0.9 and 1). The instrumental corrected broadening ˇh k l

corresponding to the diffraction peak was estimated using the following equation[24]:

ˇhkl =

ˇ2exp − ˇ2

inst (2)

in whichˇexp correspondsto the experimental half width andˇinst the instrumentalhalf widthrelated to the powder standard LaB6 (SRM 660-NIST). The crystallitesizeis a measure of the size of coherently diffraction domain and is not the same asparticle size as whole due to polycrystalline aggregates [25].

The mass densities were determined by the Archimedes method and followingthe standard normMPIF 42 [26]. The porositywas evaluatedby comparingthe massdensity of the sintered material (S) with the theoretical mass density (T ) of theobtained composite. The linear dimensional change was evaluated through com-parison between the dimensions (diameter) of the parts in green with the sinteredones. The measures were done with a micrometer with a precision of 0.01 mm.

The morphological evolution of the porosity and the microstructure of the sin-tered samples were analysed by employing techniques of metallography and a

scanning electron microscope (SEM) using a Shimatzu-SS-Supercan system. Trans-mission electron microscopy (TEM) observation was performed on a JEM – 1400(JEOL) instrument operating at 120 kV. For TEM analysis, the powder sample wasultrasonically dispersed in ethanol and drops were deposited on 300 carbon filmsquare mesh copper grids.

The Vickers hardness was evaluated by employing the techniques of indenta-tion on a microdurometer, Display Microhardeness Tester – HVS – 1000, followingthe 384–89 ASTM norm [27]. Average hardness value was determined from 10indentations in each sampleat a loadof 1000 g for 15 s.

20 30 40 50 60 70 80

I n t e n s i t y ( a . u . )

2θ (degree)

Composite: Cx

= (100 - x) HAp + x Nb2O

5

x = 50, 60, 70, 80 and 90 (vol. %)

HAp

Nb2O

5

C90

C80

C70

C50

C60

Fig.1. X-raydiffractionpatterns of HApandNb2O5 precursors,and of the composite

powders (100− x) HAp+ x Nb2O5, milled for 3 h. x=50,60, 70, 80 and 90 (vol.%).

50 60 70 80 90

20

25

30

35

40

45

50

55

C r y s t a l l i t e s i z e ( n m )

Nb2O

5content (vol. %)

(100 - x) HAp + x Nb2O

5

hkl = 110 hkl = 211

Crystallite size

Pure - HAp: 61.4 ± 0.7 nm

Pure - Nb2O

5: 51.7 ± 0.9 nm

Fig. 2. Variation of crystallite sizes with Nb2O5 quantity’s (vol.%) for lines(hk l) = (1 1 0) () and (h k l) = (2 1 1) () of composite powders (100− x) HAp+ xNb2O5, milled for 3 h. x=50,60, 70, 80 and 90%.

50 60 70 80 90

3.3

3.4

3.5

3.6

3.7

3.8

3.9

4.0

M a s s d e n s i t y ( g / c m

3 )

Nb2O

5content (vol. %)

Compacting pressure:

350 MPa 450 MPa

550 MPA 650 MPa

a

50 60 70 80 90

6

8

10

12

14

16

18

20

22

24

P o r o s i t y ( % )

Nb2O

5content (vol. %)

Compacting pressure:

350 MPa 450 MPa

550 MPa 650 MPa

b

Fig. 3. Effect of Nb2O5 quantities (vol.%) and compacting pressures on (a) massden-sity and(b) porosity of the sintered composites (100− x) HAp+ x Nb2O5, sintered in

air atmosphere at 1000◦

C for 1 h. x=50,60, 70, 80 and 90%.



86 W.J. Nascimento et al. / Materials Chemistry andPhysics 130 (2011) 84–89

-6.4

-5.6

-4.8

-4.0

-3.2

-2.4

-1.6

-0.8

0.0

350 450 550 650

Compacting pressure (MPa)

D i m e n s i o n a l c h a n g e ( % )

Nb2O

5content (vol.%):

x = 90% x = 80% x = 70%

x = 60% x = 50%

Fig. 4. Effect of Nb2O5 quantities (vol.%) and compacting pressures on linear dimen-sional change of the sintered composites (100− x) HAp+ x Nb2O5, sintered in airatmosphere at 1000 ◦C for 1 h. x=50,60,70, 80 and 90%.

3. Results and discussion

To verify the formation of composites, X-ray diffraction spectrawere obtained. The results for both the starting materials and thecompositesare presentedin Fig.1. Despite the fact that the programX-Pert High Score permitted only the identification of the Nb2O5

(JCPDSN◦ 037-1468 andN◦ 027-1313) andHAp (JCPDSN◦ 09-0432)phases, the diffractionspectra afterthe milling process showedthatthe peaks (3 1 2), (41 1) and (−3 1 5) of the Nb2O5 powder overlap

350 400 450 500 550 600 650

0.4

0.6

0.8

1.0

1.2

1.4

M i c r o h

a r d n e s s ( G P a )

Compacting pressure (MPa)

Nb2O

5content (vol. %):

50% 60% 70%

80% 90%

Fig. 5. Vickers hardness values as a function of compacting pressures and Nb2O5

quantities (vol.%) of sintered composites (100− x) HAp+ x Nb2O5, sintered in airatmosphere at 1000 ◦C for 1 h. x=50, 60, 70, 80 and 90%. Average hardness valuewas determined from 10 indentations in each sampleat a loadof 1000 g for 15 s.

with peaks (21 1), (11 2) and (30 0) of the hydroxyapatite, whichsuggests the formationof nanostructured composites.These resultsindicate that, during the mechanical alloying process, the powderparticles of HAp and Nb2O5 are repeatedly flattened, cold-welded,fracturedandre-welded.As describedbefore[22], the impact forcesplastically deform the powder particles leading to work hardeningand fracture.

Fig. 6. TEM photomicrographs of the powder particles from the composite 50 HAp+50 Nb2O5 (vol.%), after milling for 3 h.




Table1

Vickers hardness values of the HAp–Nb2O5 composites pressed at 650 MPa andsinteredat 1000 ◦C. Results reported in the literature for HA,HAp, BHA, BHA–Ti and EHA–Ti,all of them also sintered at 1000 ◦C,are also presented.

T (◦C) Material Hardness (GPa) Material Hardness (GPa)

1000 HAp–Nb2O5 (50) 1.27±0.14 BHA [36] 0.3111000 HAp–Nb2O5 (60) 1.06±0.07 BHA [37] 0.41±0.021000 HAp–Nb2O5 (70) 1.05±0.10 BHA [38] 0.43±0.051000 HAp–Nb2O5 (80) 0.8±0.1 BHA–Ti (5) [39] 0.141000 HAp–Nb2O5 (90) 0.6±0.1 BHA–Ti (10) [39] 0.13

1000 HA [33] 1.17±0.07 EHA–Ti (5) [39] 0.391000 HA [34] 1.16 EHA–Ti (10) [39] 0.391000 HAp [35] 0.9±0.1 – –

HAp–Nb2O5 (50), (60), (70), (80) and (90) (vol.%), composites in this work; HA, hydroxyapatite commercial [33]; HA, hydroxyapatite synthetic [34]; HAp, hydroxyapatitefrom fish bones [35]; BHA, hydroxyapatite from bovine bones [36–38]; EHA–Ti (5) and (10) (vol.%), composites [39]; and BHA–Ti (5) and (10) (vol.%), composites [39].

The peak broadening of the XRD reflection can be used to esti-mate the crystallite sizes along the direction perpendicular tothe crystallographic plane, what was performed using the Scher-rer’s equation [23–25,28]. For this purpose the most intense peakswere selected, near 23.80◦ and 31.84◦, corresponding to the peaks(hk l) = (1 1 0) and (hk l) = (2 1 1) of the phases Nb2O5 and HAp,respectively. Fig. 2 shows the variation in crystallite size withquantity (vol.%) of Nb2O5, after milling by 3 h, and also the val-

ues of the starting powders, using the mentioned reflections. Itcan be observed that increasing Nb2O5 content (vol.%) the crys-tallite sizes for the reflection (1 1 0) remains virtually constant,between42.1±0.6 and43.9±0.5 nm,whilefor the reflection(2 1 1)theyprogressivelydecreasesfrom51.7±0.9 to 22.5±1.2 nm.Thesereductions may be caused by the impact forces that plasticallydeform the powder particles leading to work hardening and frac-tures. Fragments generated by this mechanism may continue

Fig. 7. Microstructural fracture analysis of the composites (100− x) HAp+ x Nb2O5 (vol.%), pressured at 650 MPa andsinteredin air atmosphere at 1000 ◦C for 1 h.(a) x=50%,

(b) x=60%, (c) x=70%, (d) x=80%, (e) x=90%.



88 W.J. Nascimento et al. / Materials Chemistry andPhysics 130 (2011) 84–89

reducing the particle sizes and, consequently, can reduce the crys-tallite sizes [22]. The shape of the peaks of X-ray diffraction of Nb2O5–HAp composites indicates that the milling process may nothave changed the crystalline structure, despite reducing the parti-cle size and crystallite size.

The effects of the composition and compaction pressure on thedensity and porosity of the composites are shown in Fig. 3(a) and(b), respectively. The solid and dashed lines are only visual guides.It can be observed that, after sintering, the density of the samplesincreased with compaction pressure for all compositions. For a sin-gle compaction pressure, the density shows significant growth upto a composition of 70 (vol.%) of Nb2O5, but for higher amountsof the added reinforcement, the compression tends to minimisethe changes thus reaching a region of stabilisation. The maximumvariation was on the order of 7% and occurred for the compositecontaining 70 (vol.%) of Nb2O5 and compressed at 650 MPa. Thehighest porosity values as a function of compacting pressure wereobservedfor the samplewith 90 (vol.%) of Nb2O5 .Again,as occurredfor the mass density results, the maximum variation in porosityfor a specific compaction pressure was around 8%. These resultsindicated that the processing procedure via mechanical milling inthe high energy regime induced a reduction in particle sizes, thusincreasing the areaof contact betweenthem, a factor that produceda considerable decrease in the porosity, which resulted in bettersintering [29–31]. It is important to mention that the porosity of a material is one of the factors that control the osteointegrationprocess [32].

Fig. 4 shows the sample’s linear dimensional variation, indi-cating that the composites undergo greater contraction when theconcentration of Nb2O5 is reduced. For example, the sample with90 (vol.%) Nb2O5 presented a maximum contraction variation onthe order of 1.0%, while that with 50 (vol.%) Nb2O5 presented acontraction variation of about 5.5%. This comparison was madefor a pressure of 550 MPa. In relation to the compaction pressureitself, it was noted that no substantial change in contraction wasobserved, indicating that the effect of the composition of the sam-

ples on their contraction was more significant than the appliedpressure.

The variation of Vickers hardness of composites HAp–Nb2O5

with compaction pressured and quantity (vol.%) of Nb2O5 is shownin Fig. 5. For comparison purposes, in Table 1 are summarizedthe results obtained in this work for the samples compressed at650 MPa with Nb2O5 contents (vol.%) between 50 and90%,andalso,results reported in the literature for hydroxyapatite [33–38] andBHA–Ti and EHA–Ticomposites [39], all of themsinteredat1000 ◦C.It is observedin Fig.5, that the compactionpressurehasasignificantbearing on the microhardness, independently of the composition.So, the hardness of the material is greater for higher compactionpressures. Furthermore, the lowest hardness (0.43±0.07 GPa) isobserved for the composition with 90 (vol.%) of Nb2O5 and pressedat 350 MPa. These results are consistent with those of the porosityshown in Fig. 3 (b). It can be noted that the best response to micro-hardness occurs for the composition of 50 (vol.%) Nb2O5, and thiseffect is evident in the composite compressedat 650 MPa,for whichthe hardness reached value 1.27±0.14 GPa. These results suggestthat the formation of a ceramic composite with a more homoge-neous structure occurred as a function of the HAp concentration.This behaviour was evident in the analysis of the microstructure of the composite performed with the SEM data and is related to thecontinuous evolution of the densification of the material, as will beshown in the photomicrographs latter on.

For TEM analysis was selected the sample powder with 50HAp+50 Nb2O5 (vol.%). Micrographs of four regions of this com-posite are presented in Fig. 6(a)–(d). The diagonal image in Fig. 6(a)corresponds to the carbon film. The statistical analysis providedaverage particle size of 61±9 nm and average particle aggregatesize of 107±29 nm. It is observed that the particles of the compos-ite in the powder form have an irregular shaped morphology as aconsequence of the milling process. Comparing with the crystallitesize estimated by Scherrer’s equation (Fig. 2), the average particlesize observed in Fig. 6 indicates that these particles consist of oneor more crystallites [25,40].

Fig. 8. Microstructure and porosity of the composite 30 HAp+70 Nb2O5 (vol.%), sintered in air atmosphere at 1000 ◦C for 1 h,as a function of the compacting pressures: (a)

350MPa, (b) 450 MPa, (c) 550 MPa, (d) 650 MPa.




The photomicrographs in Fig. 7(a)–(c) show the analysis of thefracture of composites at the compressed pressure of 650 MPa.Thesecompositeswerechosenbecausetheypresentedbetter phys-ical and mechanical responses than the others. It can be seen thatthe studied samples have significant intergranular fractures, sug-gesting the occurrence of fractures along the grain boundaries,which indicates that the cohesive forces that hold the atoms con-nected to the surface of each particle that forms the necks arenot fully effective. Moreover, this analysis shows that the porosi-ties within the sintered samples are interconnected. This was notquantified, but it was observed that some compositions exhibitinterconnected porosities that are more pronounced than oth-ers. For example, the composites with 50 HAp+50 Nb2O5 and 10HAp+90 Nb2O5 (vol.%) have interconnected pores with sizes rang-ing from 2 to 4m.

The porous morphological evolution was carried out with SEM.The results indicated that the compositions did not affect the shapeand distribution of the pores. Then we choose the sample with 30HAp+70 Nb2O5 (vol.%) to show the behaviour of this feature, whichis shown in Fig. 8(a)–(d). The observed decrease in porosity maybe related to the high energy milling process used to obtain thecomposites, which reduces the particle sizes and increases the areaof contact between them, thus facilitating the occurrence of bothchemical and/or solid state reactions that may speed up the pro-cesses of mass diffusion during the thermal treatment. This is adesirable condition for the formation of composites. It can be seenthat the porous shape and distribution confirm the densification of the composites, as shown in Fig. 3.

4. Conclusion

In conclusion, the addition of Nb2O5 to natural hydroxyapatiteusing the mechanical alloying method favoured the formation of nanostructured composites and improved the sinterability, provid-ing a changein the physical and mechanical properties. The resultsshowed that the milling process reduced the particle sizes, sug-gesting that the energy of the successive shocks induced chemical

and/or solid state reactions that may have facilitated the forma-tion of composites. The low melting point and reactivity of Nb2O5

enabled the samples to be sintered at temperatures around 1000 ◦Cand under an air atmosphere. These sintering conditions and theuse of furnaces without atmospheric control capable of produc-ing composites with interconnected porosities and lower densitiesthanthoseobtainedwithtitanium, niobium and others, reduces thecost production, indicating that this bulk composite is a promisingcandidate to be used in the development of implants substitutingbone tissues. As a final remark, it is important to remember thatNb2O5 and HAp are non-toxic compounds.

Acknowledgements

The authors would like to thank the Brazilian agencies Finep,Capes, Fundac ão Araucária and CNPq for their financial support of this work.

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Artigo MAt. CHEm. Phys. v. 130