structural and morphologic characterization of zirconia–silica nanocomposites prepared by a...

Structural and morphologic characterization of zirconia–silicananocomposites prepared by a modified sol–gel method

Marcela Stoia a, Paul Barvinschi b, Floricica Barvinschi c,n

a Faculty of Industrial Chemistry and Environmental Engineering, “Politehnica” University of Timisoara, P. Victoriei No. 2, Timisoara 300006, Romaniab Faculty of Physics, West University of Timisoara, Bv. V. Parvan, No. 4, Timisoara 300223, Romaniac Department of Physical Foundation of Engineering, “Politehnica” University of Timisoara, Bv. V. Parvan, No. 2, Timisoara 300223, Romania

a r t i c l e i n f o

Keywords:A1. PolyalcoholsA1. Sol–gelB1. CompositesB1. NanomaterialsB1. Zirconia

a b s t r a c t

In this study, we have synthesized zirconia–silica nanocomposites with different ZrO2 content (50% and30%) using a modified sol–gel method, based on TEOS–Zr(IV)–NO3

�–polyalcohols–H2O sols that success-fully gelified to homogenous gels. The gels obtained at room temperature have been dried and thenxxxthermally treated at up to 150 1C when a redox reaction took place between the organic polyalcoholand zirconia nitrate, evidenced by DTA analysis, leading to the formation of zirconia nanoparticlesprecursors in the pores of silica matrix. The obtained precursors have been characterized by thermalanalysis and FTIR spectroscopy, and further annealed at higher temperature in the range 500–1200 1C.XRD analysis of the resulted composites have evidenced the presence of nanocrystalline tetragonalzirconia as single crystalline phase up to 1200 1C in the case of 30% ZrO2 samples, and up to 1000 1C forthe 50% ZrO2 samples. In the later samples, after being annealed at 1200 1C, several crystalline phaseshave been present. For all nanocomposites we have obtained a homogenous dispersion of zirconia withinthe silica matrix, evidenced by Zr and Si elemental maps, performed by SEM-EDX analysis.

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1. Introduction

The interest devoted to binary zirconia–silica oxides can betraced back to their enhanced catalytic, mechanical, electronic,thermal properties. These materials can be used as coatings, fibers,catalysts and high-refractive-index glasses [1].

The addition of zirconium in silica significantly improves theirmechanic, electric and chemical properties, more particularlytowards alkaline attacks [2]. The structure of mixed zirconia–silicaoxides is very interesting because zirconium cannot substitutesilicon, due to the different atomic radius and coordination numberbetween these two elements. Thus, zirconia will develop as distinctphase in the nanocomposites. The formation of only monocliniczirconia [3], at temperatures lower than 1000 1C, has been reportedin literature. The presence of SiO2 in small amounts was reportedas a stabilizer for tetragonal zirconia [4]. It has also been reportedthat the insertion of zirconium atoms in the silicate matrixincreases the thermal stability of tetragonal zirconia. It is impor-tant to find optimal conditions of synthesis that lead to a pure

tetragonal phase in the silica matrix, for a larger temperatureinterval, in order to promote catalytic and sorption activity of thecomposite materials in comparison with the individual oxides orxerogels [5].

The sol–gel process allows us to synthesize oxide-silica nano-composites for various oxide contents at relative low tempera-tures, compared with the ceramic method [6]. The reactivity ofzirconium alkoxides towards hydrolysis being higher than that ofsilicon alkoxides, mixtures of zirconium oxides and alkoxidesprecipitate during hydrolysis instead of forming a homogeneousgel. It has been reported in literature that the presence of someorganic additives, for example acetylacetone, slows the hydrolysisrate of zirconium alkoxide leading to more homogenous gels [7].

The aim of this work is to study the local structure of ZrO2–SiO2

mixed oxides obtained from hybrid inorganic–organic materialswith different Si/Zr atomic ratios. In this study, we have synthe-sized zirconia–silica nanocomposites using a modified sol–gelmethod, based on TEOS–Zr(IV)–NO3

�–polyol–H2O sols that succes-fully gelified to homogenous gels. We have studied the gels Zr(IV)–NO3

�–polyol (1,3-propanediol and poly(vinyl alcohol), and we haveevidenced during gels heating that a redox reaction takes placebetween zirconium nitrate and polyol with formation of somecarboxylate type compounds of Zr, that were used as precursor for

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Journal of Crystal Growth

http://dx.doi.org/10.1016/j.jcrysgro.2014.02.0080022-0248 & 2014 Elsevier B.V. All rights reserved.

n Corresponding author.E-mail address: [email protected] (F. Barvinschi).

Please cite this article as: M. Stoia, et al., Journal of Crystal Growth (2014), http://dx.doi.org/10.1016/j.jcrysgro.2014.02.008i

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zirconia powders [8]. The gels obtained at room temperature havebeen dried and then thermally treated at 150 1C, when the redoxreaction between the organic polyol and zirconia nitrate tookplace completely, leading to the formation of zirconia precursorsin the pores of silica matrix. These xerogels obtained at 150 1Chave been annealed at higher temperature in the range 400–1100 1C. The obtained composites have been characterized by FTIRspectroscopy, X-ray diffraction, and SEM-EDX analysis.

2. Experimental

2.1. Synthesis method

We have used reagents (provided by Merck) without furtherpurification: tetraetylortosilicate Si(OC2H5)4 (TEOS) (min. 99%),zirconyl chloride ZrOCl2 �8H2O (min. 99.5%), NaOH (min. 98%),1,3-propanediol (1,3PG) (min. 99%) and poly(vinyl alcohol) (PVA)(M¼60,000 g/mol; hydrolysis degree498%).

We have obtained first an aqueous solution of zirconium nitrate(�0.15 M) by adding 100 mL solution of NaOH (1 mol/L) in 100 mLof ZrOCl2 solution when the precipitation of Zr(OH)4 took place,followed (after the filtering and washing of the precipitate) bydrop wise addition of concentrated HNO3, under stirring, tocomplete dissolution of the precipitate. The final zirconium nitratesolution was then evaporated at 100 mL under magnetic stirring,at 90 1C. At this solution, we have added the corresponding diol:PVA (in form of a 5% solution) or 1,3PG corresponding to a molarratio polyol:NO3

�¼2:1. The obtained clear solution was heated at70 1C, followed by the addition of an ethanolic TEOS–watersolution, at a molar ratio TEOS:H2O¼1:4, with addition of ethanolin order to obtain a clear solution. The sol obtained was kept undermagnetic stirring at 70 1C until it became clear. In case of the solwith PVA, at addition of too much ethanol, opalescence wasobserved due to the limited solubility of PVA in ethanol. In orderto evidence the effect of the polyol on the zirconia formation, wehave prepared in the same way a sol without diol as reference.

The sols were left for gelation at room temperature, in Petridishes. The obtained gels have been named as: ZrPVSi (the gelwith PVA), ZrPGSi (the gel with 1,3PG) and ZrSi (the gel withoutpolyol). After gelation, the obtained gels have been dried at 60 1Cin an oven. The obtained dried gels have been grinded and thenheated up to 150 1C, when a redox reaction with emission ofnitrogen oxides took place in the gels ZrPVSi and ZrPGSi (whichbecame brownish). The ZrSi gel was white after thermal treatmentat this temperature. The obtained powders have been furthercalcined at different temperatures, in order to study the formationof zirconia and evolution of its crystalline phases.

The syntheses have been performed for two different composi-tions: (1) 50%ZrO2–50%SiO2 and (2) 30%ZrO2–70%SiO2.

2.2. Characterization techniques

Thermal analysis was performed on a 1500D MOM BudapestDerivatograph. The heating was achieved in static air, up to 600 1C,with a heating rate of 5 1C min�1, on Pt plates using α-Al2O3 asinert material. The synthesized powders were characterized byFT-IR spectrometry with a Shimadzu Prestige FT-IR spectrometer,in KBr pellets, in the range 4000–400 cm�1. Phase analysis wasachieved at ambient temperature with a Bruker D8 Advancediffractometer, using the MoKα radiation (Zr filter). SEM imagesand elemental maps have been recorded on a Quanta 3D FEG (FEI)microscope.

3. Results and discussion

The gels obtained at room temperature have been dried at60 1C. At this temperature, the gels with polyalcohol turnedbrownish as a result of the beginning redox reaction betweennitrate ions and polyalcohol. However, at that low temperature theredox reaction will not be completed. These dried gels have beenstudied by means of differential thermal analysis, in order toevidence the redox reaction, in comparison with the gel withoutpolyalcohol.

Fig. 1 presents the DTA curves of the gels dried at 60 1C: ZrPVSi,ZrPGSi and ZrSi for the two compositions of the final materials(1) 50%ZrO2–50%SiO2 (Fig. 1a) and (2) 30%ZrO2–70%SiO2 (Fig. 1b).The shape of DTA curves for the gels with polyalcohol (curves(1) and (2)) are completely different than the ones withoutpolyalcohol (curve (3)). Thus in the range 70–120 1C all gels withpolyalcohol (ZrPVSi(1), ZrPGSi(1), ZrPVSi(2) and ZrPGSi(2)) pre-sent an exothermic effect generated by the redox reaction betweenthe nitrate ions and the C–OH groups from the alcohol (verystrong in case of ZrPVSi(1) due to the high content of PVA) [6,8].It has been reported very early in the literature that zirconiumforms with different carboxylate anions (oxalate, malonate, malate,succinate, adipate and phthalate) compounds of the type ZrO �C2O4 �2H2O, ZrO �C3H2O4, ZrO �C4H4O5, ZrO �C4H4O4, ZrO �C6H8O4;K2[ZrO(C2O4)2], K2[ZrO(C3H2O4)2] and K2[ZrO(C4H4O5)2] [9]. In ourcase the carboxylate anions are generated in situ by the oxidation

Fig. 1. DTA curves of the gels dried at 60 1C: (1) ZrPVSi; (2) ZrPGSi; (3) ZrSi fordifferent final compositions: (a) 50%ZrO2–50%SiO2; (b) 30%ZrO2–70%SiO2.

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of 1,3-propanediol and poly(vinyl alcohol). Both blank gels ZrSi(1) and ZrSi(2) exhibit in this temperature range an endothermiceffect, due to the elimination of volatiles generated from thecondensing reaction. These blank gels do not exhibit any other

significant thermal effect up to 550 1C, while the gels withpolyalcohol exhibit a large exothermic effect in the range 300–450 1C. These exothermic effects can be attributed to the thermaldecomposition of the products of the redox reaction, which aremost probably hydroxyl-carboxylates of Zr(IV) [6,8]. The samethermal behavior was reported previously for gels obtained fromzirconium propoxide and tetraethylorthosilicate in the presence offormamide [2]. In the case of the gels with a lower content ofnitrate ions and polyalcohol (Fig. 1b) the behavior of the gels issimilar, but the thermal effects are smaller and located at highertemperatures as an effect of the dispersion of the reacting speciesinside the silica matrix.

In order to confirm the evolution of the gels during the thermaltreatment, we have characterized all gels by FTIR spectroscopy,heated at different temperatures: 60 1C, 150 1C and 500 1C. Fig. 2apresents the FT-IR spectra of the ZrSi(1) gel thermally treated atdifferent temperatures. The FT-IR spectrum (1) of the ZrSi(1) gel,dried at 60 1C, presents bands characteristic to the silica matrix(456 cm�1, 800 cm�1, 1068 cm�1 [5]), to the associated –OH andH–O–H bands (1635 cm�1, 3412 cm �1 [10]) and to the nitrate ions(823 cm�1, 1384 cm�1, 1550 cm�1 [11,12]). Spectrum (2) of the gelheated at 150 1C presents the same absorption bands. There are nosignificant changes. In case of the gel heated at 500 1C (spectrum(3)), the bands characteristic to nitrate ions are missing, due totheir thermal decomposition up to this temperature. In thisspectrum only the bands characteristic to the silica matrix andadsorbed water are present.

In case of the ZrPVSi(1) gel (Fig. 2b), the evolution withtemperature is different. Thus, spectrum (1) of the gel driedat 60 1C, presents, beside the absorption band of silica matrix(462 cm�1, 804 cm�1, 1045 cm�1), H–O bonds (3431 cm�1) andnitrate ions (1384 cm�1), some bands characteristic to PVA(1095 cm�1)[13]. It is to be noticed that a strong band appears at1685 cm�1, which can be assigned to the vibrations of thecarboxylate groups formed by the oxidation of PVA by the nitrateions, due to the redox reaction that already starts at this tempera-ture. The completing of the redox reaction up to 150 1C is provedby the disappearance of the strong band located at 1384 cm�1,characteristic to the nitrate ions, and by the appearance of the twobands located at 1680 and 1409 cm�1, characteristic to thecarboxylate groups, coordinated to metal cations (Zr(IV)) [14].Some unreacted PVA is still present in the silica matrix pores,confirmed by the presence of the band located at 1101 cm�1. Bycalcinations at 500 1C (Fig. 2b spectrum (3)) the organic part(carboxylate ligands and unreacted PVA) completely decomposes,thus the FT-IR spectrum is almost identical with that of the ZrSi(1) gel (Fig. 2a spectrum (3)).

In order to obtain the xerogels used as precursor for thecomposites ZrO2/SiO2 we have heated all gels at 150 1C for 6 h,until no emission of nitrogen oxides was observed. The obtainedxerogels have been characterized by thermal analysis. The thermalbehavior of the gels without polyalcohol (ZrSi(1) and ZrSi(2)) wassimilar and exhibited an endothermic process with mass loss of�20%, up to 150 1C, due to the eliminations of the volatilesresulted in the condensation reaction and to the adsorbed water.A second process with �20% mass loss and a weak endothermiceffect was registered in the range 200–350 1C, which can beattributed to the decomposition of zirconium nitrate in the poresof the silica matrix. Fig. 3a presents the TG and DTA curves of theZrSi(1) gel.

The thermal curves TG and DTA of the xerogels obtained withpolyalcohol (ZrPVSi(1), ZrPGSi(1), ZrPVSi(2), ZrPGSi(2)) are similar,presenting an endothermic effect in the range 70–120 1C due tothe elimination of the volatiles and a large exothermic effect in therange 280–450 1C, because of the burning of Zr(IV) hydroxyl-carboxylates embedded in silica matrices pores. Fig. 3b presents

Fig. 2. FT-IR spectra of the gels (a) ZrSi(1) and (b) ZrPVSi(1) heated at differenttemperatures: (1) 60 1C; (2) 150 1C and (3) 500 1C.

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the thermal curves of the ZrPGSi(1) gel. As resulted from thethermal analysis, the thermal decomposition of zirconia precur-sors inside the silica matrix completes up to 450 1C, thus we havecalcinated the xerogels at temperatures higher than 500 1C inorder to obtain the ZrO2/SiO2 composites.

In the case of the composites obtained at 500 1C, no crystallinephases have been evidenced in their XRD patterns. Thus, the ZrO2

Fig. 3. TG and DTA curves of the gels (a) ZrSi(1) and (b) ZrPGSi(1), dried at 60 1C.

Fig. 4. XRD patterns of the composites ZrO2/SiO2 obtained at 700 1C.








has not crystallized yet, at this temperature. Fig. 4 presentsthe XRD patterns of the composites obtained at 700 1C. The XRDpatterns of the samples with 50% ZrO2 present some weakdiffraction peaks corresponding to the tetragonal phase of ZrO2,better crystallized in the case of the ZrPVSi(1) sample. The XRDpatterns of the samples with 30% ZrO2 are characteristic to anamorphous sample, probably due to the smaller sizes of the ZrO2

particles embedded in the amorphous silica matrix. After calcina-tions at 1000 1C, the tetragonal phase of zirconia crystallizes inall samples, as results from their XRD patterns presented in Fig. 5.For both compositions the samples obtained with PVA presents ahigher crystallization degree (ZrPVSi(1) and ZrPVSi(2)).

After calcinations at 1200 1C, the samples with 30% ZrO2 (ZrSi(2), ZrPGSi(2), ZrPVSi(2)) present a better crystallized tetragonalZrO2 in the amorphous silica matrix. The samples with 50%ZrO2/50%SiO2 exhibit several crystalline phases: ZrO2-tetragonal,

ZrO2-monoclinic, ZrSiO4 and SiO2–cristobalite [15]. Thus, in allthree XRD patterns (ZrSi(1), ZrPGSi(1), ZrPVSi(1)) a diffractionpeak located at about 2θ¼101 is present, characteristic to crystal-line SiO2 in the form of cristobalite. In the case of the ZrPVSi(1) sample, the ZrSiO4 phase is almost absent; ZrO2 does notinteract with SiO2 at this temperature, probably due to the smalleramounts of Zr–O–Si bonds in this case due to the presence of theresidual carbon from the thermal decomposition of PVA, thatmight surround zirconia’s particles (Fig. 6).

It is interesting that in the case of the samples with 30% ZrO2

(ZrSi(2), ZrPGSi(2), ZrPVSi(2)) the silica matrix does not crystallizeat 1200 1C, and for all these samples the only crystalline phaseis the tetragonal zirconia. The absence of the ZrSiO4 phase in thiscase may be explained through the mechanism of ZrSiO4 forma-tion from α-SiO2 and tetragonal ZrO2, reported previously byVeytizou et al. [16]. According to these authors, the system isdescribed as either a series of spherical tetragonal zirconia grainssurrounded by a shell of zircon (ZrSiO4), which is covered byamorphous silica, or as spherical tetragonal zirconia grains sur-rounded by a shell of zircon, which is in contact with sphericalamorphous SiO2 particles. In the latter case, fast superficialdiffusion of Si4þ and electrons originating from silica grains onthe external surface of the zircon layer produce a homogenizationof the silicon concentration. The case of the samples with higher

Table 1Mean crystallite size D (nm) of tetragonal zirconia calculated from XRD data.

ZrPVSi(1) ZrPGSi(1) ZrSi(1) ZrPVSi(2) ZrPGSi(2) ZrSi(2)

10001C 12.0 8.6 8.3 6.8 4.5 4.312001C 18.0 21.0 20.5 12.0 9.0 10.0

Fig. 7. SEM images of the composites with 50%ZrO2 obtained at 1000 1C: (a) ZrPVSi (1), (b) ZrPGSi(1), and (c) ZrSi(1).






content of zirconia (50%) approaches more the favorable casepresented above. In addition, the crystallization of SiO2 as cristo-balite, with a tetragonal structure, probably promotes the interac-tion with tetragonal zirconia, to form tetragonal ZrSiO4.

The crystallite size D of tetragonal zirconia (Table 1) wasestimated using the Scherrer equation, D¼ 0:9λ=ðβU cos θÞ, whereλ is the X-ray wavelength, θ is the Bragg angle, and β is the purefull width of the (101) diffraction line at half of its maximumintensity (FWHM); the values of β were found by applying acorrection for the instrumental broadening that was calculatedusing the line (112) of an α-SiO2 etalon and following theprocedure given in [17]. From the data presented in Table 1 itresults that the prepared materials contain zirconia nanocrystal-lites, with diameters that increase with the increasing of theannealing temperatures and with zirconia content percentage.For both compositions, the dimensions of the zirconia crystallitesare larger in the samples synthesized with PVA (ZrPVSi(1) andZrPVSi(2)) than for the other samples.

Fig. 7 presents the SEM images of the composites with 50%ZrO2 obtained at 1000 1C. When studying the morphology of thesecomposites we can observe that all samples consist of more or lesscompact agglomerations of spherical nanoparticles (up to 50 nm).We can therefor consider our products as being nanocomposites.

In order to evidence the dispersion of zirconia nanocrystallitesinside the silica matrix we have performed elemental mapping ofZr and Si for all synthesized nanocomposites. When analyzingthese maps it is clear that zirconia crystallites are homogenouslydispersed within the silica matrix. Fig. 8 shows the elementalmaps for the ZrPVSi(1) (Fig. 8a) and ZrPVSi(2) samples (Fig. 8b).

4. Conclusions

This study has introduced a modified sol–gel method for theobtaining of ZrO2/SiO2 nanocomposites with different zirconiacontent (30% and 50%) starting from zirconyl chloride, nitric acid,

TEOS and polyalcohol (PVA and 1,3PG). The proposed methodleads to pure nanocrystalline tetragonal zirconia in the amorphoussilica matrix up to 1200 1C for a content of 30% ZrO2, and up to1000 1C for a content of 50% ZrO2. The use of PVA allowed for theobtaining of tetragonal zirconia as a major crystalline phase up to1200 1C. With the increase of calcination temperatures, the zirco-nia crystallite sizes increases, up to about 21 nm at 1200 1C. Theuse of PVA leads to larger zirconia crystallites within the silicamatrix. The increase of zirconia content leads to an increase inzirconia crystallite size. SEM-EDX analysis evidenced, by elementalmapping, that zirconia nanocrystallites are uniformly dispersedwithin the silica matrix.

Acknowledgments

The work of Floricica Barvinschi was partially supported by thenational grant CEEX-05-D11-63/10.10.2005.

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Fig. 8. Zr and Si maps for the nanocomposites obtained at 1000 1C: (a) ZrPVSi(1), and (b) ZrPVSi(2).



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