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Reduced Activation Energy for GrainGrowth in NanocrystallineYttria-Stabilized ZirconiaSatyajit Shukla, Sudipta Seal,*, Rashmi Vij, and Sri Bandyopadhyay

Ad Vanced Materials Processing and Analysis Center (AMPAC) and Mechanical Materials Aerospace Engineering Department (MMAE), Uni Versity of Central Florida,4000 Central Florida Bl Vd., Orlando, Florida 32816, and School of Materials Scienceand Engineering, Uni Versity of New South Wales, NSW 2052, Australia

Received December 11, 2002; Revised Manuscript Received January 20, 2003

ABSTRACT

Nanocrystalline ( 15 20 nm) 3 mol % yttria-stabilized zirconia (3YSZ) powder is synthesized via sol gel technique. A reduced activationenergy value of 13.0 0.9 kJ/mol is observed for grain growth in nano-3YSZ powder, within the calcination temperature range of 400 1200 C, which is much lower than that (580 kJ/mol) reported for submicron/micron-sized 3YSZ. This is attributed to large oxygen-ion vacancyconcentration in nano-3YSZ.

Introduction. Yttria-stabilized zirconia (YSZ) is a well-known structural ceramic material. It is recognized that thenanocrystalline YSZ exhibits superior mechanical, electrical,and thermal properties as compared to its conventionalcoarse-grained counterpart. Nanocrystalline YSZ is a super-plastic material, 1 which has made possible to form differentYSZ components using the conventional forming techniques

generally applied to metals and alloys. The nanocrystallinenature of YSZ has been reported to increase the deformationstrain rate by a factor of 4, 2 simultaneously decreasing theforming temperature by 200 C,3 when compared withstandard bulk YSZ. Moreover, nanocrystalline YSZ particlesare used as a dispersed phase in composite materials toincrease their fracture toughness by taking advantage of thevolume expansion associated with the phenomenologicaltetragonal-to-monoclinic transformation, which effectivelysuppresses the crack propagation in the composite materi-als. 4,5 Furthermore, nanocrystalline YSZ exhibits reducedthermal conductivity compared to its coarse-grained coun-terpart, due to reduction in the mean free path of phonons

and the presence of excess grain boundaries. 6,7 This behavior,combined with improved mechanical properties, makesnanostructured YSZ an excellent candidate for thermal barriercoatings. In addition to this, the specific grain boundary dcionic conductivities of nanocrystalline YSZ is shown to be1- 2 orders of magnitude higher than that of the micro-YSZ. 8

This makes nanocrystalline YSZ a promising choice as a

solid electrolyte material for conducting oxygen ions for solidoxide fuel cell and gas sensor applications. 9- 11 Thus,nanocrystalline ( < 100 nm) YSZ possesses a range of industrial applications; and as a result, not only is synthesiz-ing nanocrystalline YSZ important but also understandingits grain growth characteristics at the nanolevel is essential.

Bulk components of nanocrystalline YSZ are manufactured

by consolidating nanocrystalline YSZ powder using a suitableconsolidation technique. 12 Synthesis of nanocrystalline YSZparticles using various techniques such as hydrothermaltreatment, 13 combustion synthesis, 14 chemical vapor conden-sation, 15 chemical coprecipitation, 16 ultrasonic spray freeze-drying, 17 solvent extraction, 18 and decomposition of metalnitrates coated on carbon powder 19 have been reported inthe literature. In this investigation, we use a sol - geltechnique, 20 utilizing mixed alkoxide and nonalkoxide pre-cursors, to synthesize nanocrystalline 3YSZ powder. Interest-ingly, it is observed that nanocrystalline 3YSZ powderexhibits very low activation energy for the grain growth,relative to the bulk counterpart. The detail analysis revealsthat very low activation energy for the grain growth is relatedto the presence of a large concentration of oxygen vacanciesin the nanocrystalline 3YSZ, which may have a tremendousimpact on the various potential applications mentioned above.Hence, the objective of the present investigation is set toreport and discuss these new findings.

Experimental Section. Pure zirconium(IV) n-propoxide,yttrium nitrate hexahydrate (Y(NO 3)3 6H 2O), anhydrousethanol (200 proof), and the hydroxypropyl cellulose (HPC)polymer having molecular weight (MW HPC ) 80 000 g/mol

* Corresponding author: E-mail: [email protected]. University of Central Florida. University of New South Wales.

NANOLETTERS

2003Vol. 3, No. 3

397 - 401

10.1021/nl0259380 CCC: $25.00 2003 American Chemical SocietyPublished on Web 02/05/2003


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and 100 000 g/mol were obtained from Aldrich and wereused as received.

Nanosized 3YSZ powder was synthesized in the presentinvestigation via hydrolysis of zirconium(IV) n-propoxidein an alcohol solution. Y(NO 3)3 6H 2O was used as a sourceof Y 3+ ions. The beakers used in the experiments werecleaned, washed with deionized water, dried completely, andrinsed with anhydrous ethanol. During synthesis, two dif-ferent but equal parts of alcohol solutions were prepared. Inthe first part, deionized water was dissolved into anhydrousethanol using R-values, which is the ratio of molar concen-tration of water to that of zirconium(IV) n-propoxide, withinthe range of 5 - 60. HPC polymer ([HPC] ) 2.0 g/L, MW HPC) 100 000 g/mol) was then added to this part and wasdissolved completely by stirring the solution overnight usinga magnetic stirrer. An appropriate amount (0.0062 M) of Y(NO 3)36H 2O was added to this part in order to synthesizenanocrystalline 3YSZ. However, no attempt was made todetermine the exact powder composition after the processing.The second part of the alcohol solution was then preparedby completely dissolving zirconium(IV) n-propoxide (0.1 M)in an anhydrous ethanol under atmospheric conditions andhomogenized using magnetic stirring for 2 - 3 minutes only.Longer stirring time may result in the partial hydrolysis of zirconium(IV) n-propoxide, due to atmospheric moisture,before the water part is added. After preparing each solution,both the beakers were sealed immediately with paraffin tapeand were kept in this condition until mixing. Hydrolysis of zirconium(IV) n-propoxide was then carried out underatmospheric conditions by rapidly mixing the two sealedsolutions under vigorous stirring for 2 - 3 min. The sol wasthen stirred very slowly for 4 h and then held under staticconditions for 24 h to ensure completion of the hydrolysisand the condensation reactions. The sol was subsequently

dried at 80 - 100 C using Petri dishes in order to removethe solvent completely. The small gel pieces obtained werethen crushed using a mortar and pestle to obtain thenanocrystalline 3YSZ powder, which was calcined, as a free-standing powder, at different temperatures (400, 800, and1200 C) in air for the phase evolution study. The sampleswere heated at a rate of 30 C/min up to the calcinationtemperature, held at that temperature for 2 h, and then slowlycooled to room temperature in the furnace. Nanosizedreference-ZrO 2 powder was also synthesized for comparingthe phase evolution behavior of nanosized 3YSZ with thatof the former as a function of calcination temperature. TheHPC polymer having molecular weight of 80 000 g/mol was

used for the synthesis of nanosized reference-ZrO 2 powder;the experimental values of remaining synthesis parametersremained identical to those used for synthesizing nanosized3YSZ powder.

The as-synthesized and calcined nanosized 3YSZ powderwas then examined using Phillips EM400 transmissionelectron microscope (TEM) at 120 kV. The crystalline phasespresent in the as-synthesized and in the calcined powderswere determined using a Rigaku X-ray diffractometer (XRD).Line traces were obtained over 2 values ranging from 10 to 80 . Narrow scan analysis was conducted in the 2 range

of 29.5 to 31.5 as it contained the strongest line for thetetragonal (111) t phase. This intense peak was then curve-fitted using the peak-fit software (peak-fit, version-4, SPSSInc.). The average tetragonal ( Dt) crystallite size wascalculated from the (111) t diffraction peak using Scherrersequation, 21

where, D t is the average crystallite size in nm, the radiationwavelength (0.154 nm), the corrected half-width at half-intensity (fwhm), and is the diffraction peak angle.

Results. Typical TEM images of 3YSZ nanoparticles inthe as-synthesized condition, processed under the conditionsof R ) 5 and [HPC] ) 2.0 g/L are shown in Figures 1a and1b, at low and high magnifications, respectively. TEM imageof the nanocrystalline 3YSZ powder after calcination at 1200 C is shown in Figure 2. In Figure 1, the average 3YSZnanoparticle size of 15 - 20 nm is observed, which corre-sponds to the as-synthesized condition. The 3YSZ nano-particles are observed to form hard aggregates of size lessthan 100 nm. Few nonagglomerated 3YSZ nanoparticlesare also visible in the TEM images, Figure 1. After calcining

Figure 1. Typical TEM images of sol-gel derived nanosized 3YSZpowder in the as-synthesized condition at low (a) and high (b)magnifications. [HPC] ) 2.0 g/L and MW HPC ) 100 000 g/mol.

Figure 2. Typical TEM image of sol - gel synthesized nanosized3YSZ powder after calcination at 1200 C for 2 h. [HPC] ) 2.0g/L and MW HPC ) 100 000 g/mol.

Dt ) (0.9 )/( cos ) (1)

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the as-synthesized nanosized 3YSZ powder at 1200 C, theaverage 3YSZ nanoparticle size is observed to increase to 40 - 60 nm, Figure 2. This suggests that the growth in theaverage nanocrystallite size is due to calcination of free-standing powder at higher temperature, which in turnindicates the presence of hard aggregates of size 40 - 60nm in the as-synthesized nanosized 3YSZ powder.

Typical broad-scan XRD patterns obtained for the nano-sized reference-ZrO 2 and 3YSZ powders, in the as-synthesized and calcined conditions, are presented in Figures3 and 4, respectively. The XRD patterns of the former are

shown for comparison only. The as-synthesized nanosizedreference-ZrO 2 and 3YSZ powders are X-ray amorphous andremain amorphous up to the calcination temperature of 300 C. Both powders crystallize into tetragonal crystal structureafter calcination at 400 C. Using eq 1, the nanocrystallitesize in the reference-ZrO 2 powder is calculated to be 30nm. Calcination at higher temperatures (800 - 1200 C)results in the tetragonal-to-monoclinic phase transformationin the nanosized reference-ZrO 2, Figure 3. In contrary to thephase evolution behavior of nanosized reference ZrO 2, thenanocrystalline 3YSZ powder exhibits stabilized tetragonalcrystal structure, at room temperature, after calcination at800 C and 1200 C, Figure 4. The decrease in the full widthat half-maximum intensity (fwhm) of the (111) t peak withincreasing calcination temperature indicates an increase inthe 3YSZ nanocrystallite size as a result of calcinationtreatment.

The effect of calcination temperature on the average 3YSZnanocrystallite size (calculated using eq 1) is presented inFigure 5a for R values within the range of 5 to 60. Theaverage 3YSZ nanocrystallite size is observed to increasewith increasing calcination temperature. The average 3YSZnanocrystallite size increases from 15 - 20 nm to 55 - 70nm as the calcination temperature increases within the range

of 400 - 1200 C. The growth in the average 3YSZ nano-crystallite size is observed to be independent of R values.However, at the largest calcination temperature of 1200 C,marginal difference in the average 3YSZ nanocrystallite sizeis noted, where the lowest average nanocrystallite size is

Figure 3. Typical broad scan XRD patterns, obtained for sol - gelderived reference-ZrO 2 powder in as-synthesized condition and aftercalcination at different temperatures. [HPC] ) 2.0 g/L and MW HPC) 80 000 g/mol: t - tetragonal (111) peak and m - monoclinic(- 111) and (111) peaks.

Figure 4. Typical broad scan XRD patterns obtained for sol - gelderived nanosized YSZ powder in as-synthesized condition and aftercalcination at different temperatures. [HPC] ) 2.0 g/L and MW HPC) 100 000 g/mol: t - tetragonal (111) peak.

Figure 5. (a) Effect of calcination temperature on the averagenanocrystallite size for nanocrystalline 3YSZ powder synthesizedunder different R values. [HPC] ) 2.0 g/L and MW HPC ) 100 000g/mol. (b) Activation energy plot for nanocrystalline 3YSZ atdifferent R values.

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observed for R value of 5. This may be due to the variationin the average size of the hard aggregates in the as-synthesized condition for different R values within the range5- 60. The limited growth in the average 3YSZ nanocrys-tallite size with increasing calcination temperature indicatesthat, under the given processing conditions, the as-synthe-sized nanosized 3YSZ powder contains hard aggregates of size 55 - 70 nm, in agreement with the TEM analysis.

To determine the activation energy ( Q , kJ/mol) for the

grain growth within an agglomerated nanosized 3YSZpowder containing hard aggregates of nanocrystallites, wereplot Figure 5a in Figure 5b, assuming that the grain growthin the nanosized 3YSZ, being a thermally activated process,is dependent on the calcination temperature according to eq2:

where Do and D t are the initial and final nanocrystallite sizes(nm), R the gas constant (kJ/degree mol), T the calcinationtemperature (K). Hence, the activation energy for the grain

growth can be obtained from the average slope of the graphsin Figure 5b and is calculated to be 13.0 ( 0.9 kJ/mol.

Discussion. Nanocrystalline reference-ZrO 2 and 3YSZpowders are synthesized, in the present investigation via sol -gel technique utilizing mixed alkoxide and nonalkoxideprecursors. Both the powders are X-ray amorphous at roomtemperature, but crystallize into tetragonal crystal structureafter calcination at 400 C. The stabilization of tetragonalphase in the nanocrystalline reference-ZrO 2 powder appearsto be due to the nanoparticle size effect as proposed byGarvie. 22 With the reference to the nanosized undoped ZrO 2powder, the stabilization of tetragonal phase in the nano-crystalline 3YSZ powder, within the calcination temperature

range of 400 - 1200 C, is a result of doping the ZrO 2 latticewith Y 3+ cations. 23

The strong covalent nature of the Zr - O bond within theZrO 2 lattice favors a 7-fold coordination number, which isoffered by the monoclinic crystal structure. As a result,monoclinic is the most stable crystal structure at roomtemperature for undoped ZrO 2 (assuming no particle sizeeffect). When a low valency dopant cation, such as Y 3+ , isintroduced into the ZrO 2 lattice, oxygen vacancies are createdfor the charge balance. Due to the large size of Y 3+ cationsrelative to Zr 4+ cations, the generated oxygen vacancies tendto be associated with Zr 4+ cations. This oxygen vacancyassociation with Zr 4+ cations reduces the effective coordina-tion number of Zr 4+ cations below 7. To maintain its effectivecoordination number close to 7, as dictated by the covalentnature of the Zr - O bond, the ZrO 2 lattice assumes a crystalstructure, which offers 8-fold (higher than 7) coordinationnumber (typically tetragonal or cubic structures) and simul-taneously incorporates the generated oxygen vacancies intothe lattice as the nearest neighbors to Zr 4+ cations. Hence,although in the tetragonal or cubic lattices the ideal coordina-tion number for Zr 4+ cations is 8, the association of theoxygen vacancies with Zr 4+ cations reduces the effectivecoordination number of the latter below 8 (that is, it tends

to approach the coordination number of 7). Further, thecrystal chemistry model also postulates that the dopant Y 3+

cations would also favor this 8-fold coordination withoxygen. 24 Hence, the overall effect of this is reflected in the

stabilization of the tetragonal phase in 3YSZ. Thus, thestabilization of the tetragonal phase in 3YSZ at roomtemperature, after calcination within the temperature range400 - 1200 C, is due to the creation of oxygen vacanciesand their association with Zr 4+ cations. 23

The as-synthesized 3YSZ nanoparticles are further ob-served to form hard aggregates of size less than 100 nm.The relatively small size of hard aggregates is attributed tothe adsorption of the HPC polymer over the 3YSZ nano-particle surface. The adsorption of the polymer over the3YSZ nanoparticle surface provides a steric hindrance forparticle - particle aggregation, resulting in reduced aggrega-tion formation tendency. 25 The concept of hard aggregates

is schematically shown in Figure 6, 12 where the grain growthprocess in the free-standing nanocrystalline powder is alsoschematically described. In Figure 6, the hard aggregates areindicated by the dotted circles and are shown to form weaklyagglomerated powder. During the calcination treatment of such a free-standing powder, the increase in the particle sizeis mainly due to the elimination of the boundaries betweenthe nanocrystallites within the hard aggregates. As a result,the average nanoparticle size tends to approach the averagesize of the hard aggregates. The very low activation energyvalue of 13.0 ( 0.9 kJ/mol, observed in this investigation,appears to be related to this type of grain growth behavior.

It may be possible that the size of hard aggregates canlimit the grain growth during the calcination treatment.However, advantage of this fact has been taken to limit thegrowth within the nanorange (that is, < 100 nm). As theaverage nanocrystallite size increases from 15 - 20 nm to 55- 70 nm (as determined by XRD), within the calcinationtemperature range 400 - 1200 C, the activation energy valuedetermined in this investigation truly corresponds to theactivation energy for the growth process in the nanocrystal-line grains. Any excessive grain growth beyond 100 nm,would produce submicron sized particles and the determinedactivation energy cannot then be correlated with nanograins.

Dt ) Do e(- Q / RT ) (2)

Figure 6. Schematic presentation of the growth process innanosized agglomerated powders containing hard aggregates of nanocrystallites.

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Moreover, in a ceramic system, such as 3YSZ, the diffusionkinetics are limited. From Figure 5a, it is observed that theaverage crystallite size after calcination at 1200 C is muchhigher than that after calcination at 400 C and 800 C. Thissuggests that the grain growth during the calcination treat-ment at 400 C and 800 C for 2 h is certainly not limitedby the size of the hard aggregates. Due to the unavailabilityof grain-size data for the calcination treatment at temperatureshigher than 1200 C, the restriction or the nonrestriction of

the grain growth at 1200 C by the size of the hard aggregatescannot be fully justified. However, as the present data doindicate a sufficient increase in the nanocrystallite size within2 h time interval, after the calcination treatment at 1200 C,we believe that the grain growth within the hard aggregatesis not limited by their size.

The activation energy for the grain growth in bulk 3YSZis reported to be 580 kJ/mol, 26 which is much larger thanthat observed for nanocrystalline 3YSZ ( 13.0 ( 0.9 kJ/ mol). Similar observation has been reported recently for otherceramics, namely ZnO, 27 where the activation energy for thegrowth in the nanocrystalline grains is observed to be 20kJ/mol, while that for microcrystalline grains is observed tobe 275 kJ/mol. Hence, very low activation energy for thegrain growth appears to be a characteristic feature of nanocrystalline ceramic oxides. We attribute the drasticreduction in the activation energy value in nanocrystalline3YSZ to the possible presence of large amount of oxygenvacancies within nanocrystalline 3YSZ. There are twodifferent mechanisms by which the large amount of oxygenvacancies could be generated in the nanocrystalline 3YSZ.First, doping of Y 3+ cations into the ZrO 2 lattice results inthe generation of a large number of oxygen vacancies asdiscussed earlier. Second, as reported for nanocrystallineCeO 2 particles, 28 nanocrystalline ceramic particles show anincrease in the concentration of oxygen vacancies withdecrease in the nanoparticle size below 20 nm. Hence, itappears that, due to the combined effect of doping of Y 3+

cations and small 3YSZ nanoparticle size, large concentrationof oxygen vacancies may exist within the nanocrystalline3YSZ particles, which can drastically reduce the activationenergy for the grain growth.

A very large concentration of oxygen vacancies canenhance the oxygen - ion conductivity of nanocrystalline3YSZ relative to that of the bulk counterpart. Hence,nanocrystalline 3YSZ having large concentration of oxygenvacancies is the best candidate material for solid electrolytesas an oxygen - ion conductor in applications such as solid

oxide fuel cells and oxygen gas sensors.Conclusions. (1) Nanocrystalline 3YSZ powder havingaverage nanocrystallite size of 15 to 20 nm is synthesizedvia sol - gel technique, using mixed alkoxide and nonalkoxideprecursors, under the processing conditions of R ) 5 and[HPC] ) 2.0 g/L, MW HPC ) 100 000 g/L. (2) A very low

activation energy of 13.0 ( 0.9 kJ/mol is observed for thegrain growth in the free-standing nanocrystalline 3YSZpowder relative to that (580 kJ/mol) exhibited by submicron/ micron-sized 3YSZ and is attributed to the possible presenceof large oxygen - ion vacancy concentration in nanocrystal-line 3YSZ. This may be the combined effect of small 3YSZnanoparticle size and doping the ZrO 2 lattice with Y 3+

cations.

Acknowledgment. The authors take this opportunity tothank Energy Strategy Inc. (USA), Coal Resources Inc.(USA), and the National Science Foundation EEC 0136710,0085639, for financial support. The authors are also thankfulto the Materials Characterization Facility at University of Central Florida.

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