Journal of Alloys and Compounds 592 (2014) 288–295

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Journal of Alloys and Compounds

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Growth kinetics of tetragonal and monoclinic ZrO2 crystallites in 3 mol%yttria partially stabilized ZrO2 (3Y-PSZ) precursor powder

0925-8388/$ - see front matter � 2014 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.jallcom.2014.01.019

⇑ Corresponding authors. Tel.: +886 7 3121101x2028; fax: +886 7 3210683(F.-L. Yen). Tel.: +886 7 3121101x2366; fax: +886 7 3210683 (M.C. Wang).

E-mail addresses: flyen@kmu.edu.tw (F.-L. Yen), mcwang@kmu.edu.tw(M.-C. Wang).

Chih-Wei Kuo a, Kuen-Chan Lee b, Feng-Lin Yen b,⇑, Yun-Hwei Shen a, Huey-Er Lee c,d, Shaw-Bing Wen e,Moo-Chin Wang b,⇑, Margaret Mary Stack f

a Department of Resources Engineering, National Cheng Kung University, 1 Ta-Hsueh Road, Tainan 70101, Taiwanb Department of Fragrance and Cosmetic Science, Kaohsiung Medical University, 100 Shih-Chuan 1st Road, Kaohsiung 807, Taiwanc Department of Dentistry, Kaohsiung Medical University, 100 Shih-Chuan 1st Road, Kaohsiung 807, Taiwand Department of Dentistry, Kaohsiung Medical University, Chung Ho Memorial Hospital, 100 Tzyou 1st Road, Kaohsiung 807, Taiwane General Education Center, Meiho Institute of Technology, 23 Pingguang Road, Neipu, Pingtung 91202, Taiwanf Department of Mechanical and Aerospace Engineering, University of Strathclyde, 75 Montrose Street, Glasgow, UK

a r t i c l e i n f o a b s t r a c t

Article history:Received 10 November 2013Received in revised form 2 January 2014Accepted 4 January 2014Available online 11 January 2014

Keywords:Crystallite growthTetragonal ZrO2

Monoclinic ZrO2

3 mol% yttria partially stabilized ZrO2

(3Y-PSZ)Activation energy

The growth kinetics of tetragonal and monoclinic ZrO2 crystallites in 3 mol% yttria partiallystabilized ZrO2 (3Y-PSZ) precursor powder has been investigated using X-ray diffraction (XRD),Brunauer–Emmett–Teller (BET) specific surface area analysis, transmission electron microscopy (TEM)and high resolution TEM (HRTEM). After calcination of the 3Y-PSZ precursor powder between 773 and1073 K for 2 h, the crystalline structures were composed of tetragonal and monoclinic ZrO2 as theprimary and secondary phases, respectively. When the 3Y-PSZ precursor powder was calcined at 773 Kfor 2 h, the BET specific surface area was 97.13 m2/g, which is equivalent to a particle size of 10.30 nm.The crystallite sizes determined via XRD and BET agreed well, indicating that the powder was virtuallynon-agglomerated. The growth kinetics of tetragonal and monoclinic ZrO2 crystallite isothermal growth

in the 3Y-PSZ precursor powder are described by: D2te ¼ ð4:57� 0:55Þt0:12�0:02 expð� ð24:79�0:38Þ�103

RT Þ and

D2m ¼ ð4:40� 1:63Þt0:17�0:08 expð� ð66:47�3:97Þ�103

RT Þ; respectively, for 773 K � T � 1073 K. Dte and Dm denotethe crystallite size of tetragonal and monoclinic ZrO2 at time t and temperature T, respectively.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

The three polymorphic forms of zirconia (ZrO2) are monoclinic,tetragonal and cubic; which one forms is dependent on the tem-perature of the environment [1]. Monoclinic ZrO2 is stable at tem-peratures below 1443 K. When heated from 1443 to 2643 K, ZrO2 isin the tetragonal phase. However, when the temperature is higherthan 2643 K, the phase of the ZrO2 transforms from tetragonal tocubic [2]. Furthermore, when the ZrO2 contains both tetragonaland monoclinic phases in the matrix it is called partially stabilizedzirconia (PSZ). Whereas the component phase of ZrO2 is almosttetragonal, so called tetragonal zirconia polycrystals (TZP).

During cooling from the elaboration temperature to room tem-perature, a spontaneous stress-induced martensitic transformationfrom t to m occurs, which limits the ability to use pure ZrO2 as anadvanced structural material. To avoid this phase transformation

occurring during heating and cooling cycles, 3–5 mol% yttria(Y2O3) can be added to ZrO2 to form the fine crystallites PSZ andTZP; these have excellent strength and fracture properties becausea stress-induced martensitic transformation from t to m takesplace [3].

There have been major advances in the application of yttria par-tially stabilized zirconia (Y-PSZ) ceramics for restoration, s includ-ing in fixed partial dentures (FPDs), implant abutments,endodontic posts and resin-bonded FPDs and full-coverage crowns[4,5]. This is due to the unique combination of mechanical proper-ties in ZrO2 ceramics such as high wear resistance, low coefficientof fraction, greater crack resistance than alumina and very goodbiocompatibility [6,7]. The increase in crack resistance is ac-counted for by its ability to undergo phase transformation [8,9]from the tetragonal to monoclinic phase at temperatures from1443 K to room temperature, accompanied by a volume increaseof about 4% but also with microcracking [7].

It is essential to increase the fracture toughness and fracturestrength of partially stabilized zirconia by increasing the retentionof the tetragonal phase at room temperature. Due to its highstrength and toughness compared with other ceramics, tetragonal

Fig. 1. XRD patterns of 3Y-PSZ precipitates after calcination at differenttemperatures for 2 h: (a) in the dried state, (b) 773 K, (c) 873 K, (d) 973 K and(e) 1073 K.

C.-W. Kuo et al. / Journal of Alloys and Compounds 592 (2014) 288–295 289

PSZ is regarded as an important engineering ceramic. Moreover, ithas been reported that the tetragonal phase on the surface of par-tially stabilized zirconia rapidly transforms to the monoclinicphase due to low-temperature annealing at 473–573 K in air, andboth fracture strength and fracture toughness are greatly reduced[10].

The performance of ZrO2 in practical uses is strongly influencedby the properties of the constituents and the crystallite size. There-fore, controlling crystallite size has become very important. Shuklaet al. [11] obtained a lower activation energy of about 13 kJ/mol fortetragonal ZrO2 crystallite growth from 3 mol% Y2O3 PSZ andpointed out that the vacancy defects existing in the ZrO2 latticecause the surface energy of ZrO2 to increase, resulting in the loweractivation energy for crystallite growth. Moreover, an activationenergy of about 34 kJ/mol for 2Y-PSZ crystallite growth was ob-tained by Chen et al. [12], who proposed that this crystallitegrowth resulted from a mechanism of grain-rotation-induced graincoalescence. In addition, an activation energy of 11.4 kJ/mol fortetragonal ZrO2 crystallite growth from freeze-dried 3Y-PSZ pre-cursor powder was obtained by Hsu et al. [13]. They proposed thatthe lower activation energy of tetragonal ZrO2 crystallite growthresulted from lattice-plane-rotation-induced crystallite growth,forming aggregated crystallites into a larger crystallite. However,the growth behavior of tetragonal and monoclinic ZrO2 crystallitesin a 3 mol% yttria partially stabilized ZrO2 (3Y-PSZ) precursor pow-der has not been discussed in detail.

In the present study, zirconyl chloride (ZrOCl2�8H2O) andyttrium nitrate (Y(NO3)3�6H2O) were used as the initial materialsfor synthesis of a 3Y-PSZ precursor powder using a coprecipitationprocess. X-ray diffractometry (XRD), Brunauer–Emmett–Teller(BET) specific surface area analysis, transmission electronmicroscopy (TEM), selected area electron diffraction (SAED),nanobeam electron diffraction (NBED) and high resolution TEM(HRTEM) were utilized to characterize the behavior of tetragonaland monoclinic ZrO2 crystallite growth in the 3Y-PSZ precursorpowder.

The purposes of this study are focused on: (i) investigating thebehavior of phase formation for the 3Y-PSZ precursor powder aftercalcination, (ii) studying the growth behavior of tetragonal andmonoclinic ZrO2 crystallites, (iii) exploring the kinetics of tetrago-nal and monoclinic ZrO2 crystallites growth and (iv) Examinationthe microstructure of tetragonal and monoclinic ZrO2 crystallitesby TEM.

2. Experimental procedure

2.1. Sample preparation

The initial materials were ZrOCl2�8H2O (purity =99.5%, supplied by Alfa Acesar,USA) and Y(NO3)3�6H2O (purity =99.5%, supplied by Riedel-de Haën, Germany).ZrOCl2�8H2O and Y(NO3)3�6H2O were dissolved in a deionized water and ethanolsolution at a ratio of 1:5 vol/vol. A ratio of Y2O3 to (Y2O3 + ZrO2) equal to 3% in solu-tion was prepared and labeled as 3Y-PSZ. To the mixed solution was then added1.0 wt% polyethylenglycol (PEG, extra pure reagent, molecular weight 2000 Da, sup-plied by Nippon Shyiaku, Kogyo K.K., Japan) as a dispersant because an appropriateamount of PEG can greatly decrease agglomeration. The mixed solution was stirredand heated in a thermostatic bath and held at 348 K for 2 h to obtain a white pre-cipitate. NH4OH was then added to the solution until pH 9 was attained. After pre-cipitation, the precipitate was repeatedly rinsed and filtered with a large amount ofdeionized water and tested with AgNO3 solution to make sure no AgCl precipitationoccurred. Subsequently, precipitates were at 218 K in a vacuum. Finally, the powderwas heated in a furnace at various temperatures for different durations.

2.2. Sample characterization

The crystalline phase was identified by X-ray diffraction (XRD, Model Rad II A,Rigaku, Tokyo). XRD was performed using an X-ray diffractometer with Cu Ka radi-ation and an Ni filter, operating at 30 kV, 20 mA with a scanning rate (2h) of 0.25�/min. The line broadening method was used to determine the crystalline size of thepowder using the Scherrer equation [14]:

dXRD ¼0:89k

Bhkl cos hð1Þ

where dXRD is the crystallite size, Bhkl is the calibrated width of the diffraction peakmeasured at half maximum intensity, k is the wavelength of the X-ray radiationand h is the Bragg angle.

The specific surface area of the powder was obtained by the BET method, andthe measured surface area was converted to the equivalent particle size using thefollowing equation [15]:

dBET ¼ j=ðqSBET Þ ð2Þ

where dBET is the average particle size, j is the shape coefficient (close to a sphericalshape, j ¼ 6), SBET is the specific surface area expressed in m2/g, and q is the theo-retical density of the YSZ in g/cm3.

The microstructure of the calcined 3Y-PSZ nanocrystallite powder was exam-ined by transmission electron microscopy (TEM, JEM 2100F, JEOL, Japan) operatingat 200 kV. Selected area electron diffraction (SAED) and nanobeam electron diffrac-tion (NBED) were utilized to identify the structure of the 3Y-PSZ crystallites, and ahigh resolution TEM (HRTEM) examination of the calcined sample was also made.

3. Results and discussion

3.1. Phase formation when 3Y-PSZ precursor powder after calcination

XRD patterns of the 3Y-PSZ precursor powder calcined atdifferent temperatures for 2 h are shown in Fig. 1. Fig. 1(a) showsthe 3Y-PSZ precursor powder maintained its amorphous state be-fore calcination. The XRD pattern of the 3Y-PSZ dried precursorpowder calcined at 773 K for 2 h is illustrated in Fig. 1(b). It revealsthat the crystalline phase characteristic of tetragonal ZrO2

appeared. In addition, the broadening reflection peaks in Fig. 1(b)reveal either the poor crystallinity of the tetragonal ZrO2 and/or itis composed of small crystallites in the submicron to nanometricrange [16,17]. Moreover, Fig. 1(b) also reveals that the monoclinic(m) peak of ð111Þm appeared initially. The reflection peak of(111)m is very weak and broad, indicating poor crystallinity and/or small crystallite size [16,17]. The 3Y-PSZ precursor powder aftercalcination between 873 and 1073 K for 2 h is shown in Fig. 1(c)–(e). It was found that the monoclinic and tetragonal phases coexistand the intensity of the reflection peaks is greater than the

Fig. 2. The crystallite sizes of (a) tetragonal (t-) and (b) monoclinic (m-) ZrO2 aftercalcination at various temperatures for different times.

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corresponding peaks shown in Fig. 1(b). The intensity of the mono-clinic peaks ð111Þm and (111)m increased when the calcinationtemperature rose from 873 to 973 K. In addition, the crystallinityand crystallite size of the tetragonal phase were also remarkablyimproved. This result was because the crystallinity and size of thet-ZrO2 crystallites increases with the calcination temperature rosefrom 873 to 973 K.

The phase equilibrium in the zirconia-rich part of the systemZrO2–Yb2O3–Y2O3 was determined at 1473 K, 1673 K and 1923 Kby Corman and Stubican [18]. They demonstrated that the stabiliz-ing effects of Yb2O3 and Y2O3 on ZrO2 were found to be quite sim-ilar. When pure ZrO2 dopant either for about 9.2 mol% and sinteredat 1473 K, then the fully stabe cubic fluorite-structure ZrO2 was ob-tained. The two binary ordered phases, Zr3Yb4O4 and Zr3Y4O12,were completely miscible at 1473 K. In the present study, theY2O3 supplementation and maximum calcination temperatureare only 3 mol% and 1073 K, respectively. Hence, the cubic ZrO2

and ordered phase Zr3Y4O12 are not found.Moreover, in a ZrO2–Y2O3 system, spontaneous transformation

from the metastable cubic phase to the tetragonal phase can betriggered at room temperature by a mechanical force. The fact thattetragonality decreases with the addition of Y2O3 and vanishes at9 mol% Y2O3 was also demonstrated by Sheu et al. [19]. In addition,the phase is maintained in the glassy state when the 3Y-PSZ pre-cipitates are calcined at 623 K for 2 h [17]. The tetragonality de-creased as the calcination temperature increased. This is due tothe fact that the mechanical force in the 3Y-PSZ precursor powderis very low and is not triggered by the phase transformation. Onthe other hand, when the 3Y-PSZ precursor powder was calcinedat 773 to 1073 K for 2 h, the phase transformation can be attrib-uted to the characteristic local structure in the zirconia solutionZr, which is dopant-independent but phase-dependent [20].

3.2. Growth behavior of tetragonal and monoclinic ZrO2 in 3Y-PSZprecursor powder after calcination

The average crystallite sizes of the tetragonal ZrO2 in the 3Y-PSZprecursor powder after calcination at various temperatures for dif-ferent times are plotted in Fig. 2(a). When the 3Y-PSZ precursorpowder was calcined at 773 K for 10 min to 120 min, the crystallitesize of the tetragonal ZrO2 only increased slightly, from 9.05 to10.73 nm. When the 3Y-PSZ precursor powder was calcined at873 K for 10–120 min, the crystallite size of the tetragonal ZrO2 in-creased from 11.12 to 12.85 nm. Moreover, the crystallite size ofthe tetragonal ZrO2 increased from 13.57 to 14.01 nm when the3Y-PSZ precursor powder was calcined at 973 K for 10 to 30 min.However, it only increased from 14.92 to 15.36 nm as the calcina-tion time was increased from 60 to 120 min. On the other hand,when calcined at 1073 K for 10–30 min, the crystallite size oftetragonal ZrO2 increased from 15.54 to 16.27 nm. The crystallitesize abruptly increased from 16.27 to 18.30 nm as the calcinationtime was increased from 30 to 120 min.

Fig. 2(b) shows the average crystallite size of the m-ZrO2 for the3Y-PSZ precursor powder after calcination at different tempera-tures for various times. It can be seen that when calcined at773 K for 10–120 min, the crystallite size of m-ZrO2 only increasedslightly, from 5.23 to 7.07 nm. Whereas, when calcined at 773–1073 K for 10 min, the crystallite size of m-ZrO2 increased rapidlyfrom 5.23 to 22.77 nm.

Fig. 2 also shows that the crystallite sizes of the t- and m-ZrO2

are 10.73 nm and 7.07 nm, respectively, after the 3Y-PSZ precursorpowder was calcined at 773 K for 120 min. These results revealthat the crystallite growth of t- and m-ZrO2 are very slow after cal-cination between 773 and 1073 K.

However, the crystallite size of t- and m-ZrO2 increased rapidlyto 18.30 nm and 25.54 nm, respectively, when the 3Y-PSZ

precursor powder calcined at 1073 K for 2 h. Continuous grainboundary networks were formed at calcination tempera-tures P973 K. These networks lead to the bridging of fine crystal-lites, thereby increasing the crystallite size was. These results wereattributed to the fact that the tetragonal and monoclinic ZrO2 canbe obtained enough energy for crystallite growth when calcinationtemperatures P973 K [21].

Fig. 3 shows the N2 adsorption/desorption isotherms of the 3Y-PSZ precursor powder calcined at 973 K for 2 h. It is seen that thiscurve at a relative pressure (P/P0) of about 0.60–0.98, has a hyster-esis loop shows the H3-type was due to the small plate-like parti-cles interconnect created the pores of interparticles [22]. The BETspecific surface area of the 3Y-PSZ precursor powder calcined at773 K is 97.13 m2/g, it is equivalent to an average particle size of10.30 nm.

Fig. 4 shows the BET surface area of the 3Y-PSZ precursor pow-der calcined at different temperatures for 2 h, which reveals thatthe BET surface area decreased from 97.13 to 35.29 m2/g whenthe calcination temperature increased from 773 to 1073 K. This re-sult is attributable to the increase in crystallite and aggregategrowth with increasing calcination temperature, resulting in theBET surface area decreasing. In addition, the crystallize sizes deter-mined using XRD and particle sizes determined by BET surface areaagree well when the calcination temperature is less than 1073 K.This is attributable to the size of the tetragonal ZrO2, while the

Fig. 3. The N2 adsorption/desorption isotherms of freeze-dried 3Y-PSZ precursorpowder after calcination at 773 K for 2 h.

Fig. 5. The dependence of (a) ln D2te and (b) ln D2

m on 1/T for tetragonal andmonoclinic ZrO2 crystallite growth at various temperatures and times.

Fig. 4. The BET surface area and equivalent particle size of freeze-dried 3Y-PSZprecursor powder after calcination at different temperatures for 2 h.

C.-W. Kuo et al. / Journal of Alloys and Compounds 592 (2014) 288–295 291

phase of monoclinic ZrO2 is quite small, as determined, and so maybe omitted [17,23]. Moreover, after the 3Y-PSZ precursor powder iscalcinated between 773 and 1073 K for 2 h, one particle containedapproximately a single crystallized tetragonal ZrO2. This result alsoreveals that the powder is virtually non-agglomerated.

The crystallite sizes calculated from the BET specific surfacearea of the 3Y-PSZ precursor powder calcined at different temper-atures for 2 h are also shown in Fig. 4.

3.3. Growth kinetics of tetragonal and monoclinic ZrO2 crystallites

In order to determine the activation energies for tetragonal andmonoclinic ZrO2 crystallites growth when the 3Y-PSZ precursorpowder was calcined at various temperatures for different dura-tions, the general equation for the kinetics of isothermal crystallitegrowth may be described as follows [24]:

Dnt � Dn

0 ¼ k0tm exp � Ea

RT

� �ð3Þ

where Dt is the crystallite size at time t and temperature T, D0 is theinitial crystallite size at time t0 = 0 and room temperature (i.e. as theprecursor powder), m is the time exponent, k0 denotes a preexpo-nential constant, Ea is the activation energy for crystallite growth,and R is the gas constant.

The result of Fig. 1(a) shows that the 3Y-PSZ precursor powderstill maintained its amorphous state, thus, D0 = 0. In addition, whencrystallite growth occurred by an isothermal process, the crystal-

lite size, D, was solely a function of time t. Therefore, Eq. (3) canbe rewritten as follows:

D2 ¼ k0tm exp � Ea

RT

� �ð4Þ

Eq. (5) is obtained from Eq. (4) by taking the natural log of bothsides:

ln D2 ¼ ln k0 þm ln t � DEa

RTð5Þ

Taking the relation between ln D2te and ln D2

m as 1/T for thetetragonal ZrO2 and monoclinic ZrO2 crystallites, respectively,growth at various temperatures for different times are shown inFig. 5(a) and (b). Dte and Dm denote the crystallite size of tetragonaland monoclinic ZrO2 at time t and temperature T, respectively. Theactivation energy values obtained are 24.79 ± 0.38 kJ/mol and66.47 ± 3.97 kJ/mol for t-ZrO2 and m-ZrO2 crystallite growth,respectively.

The dependence of ln D2te and ln D2

m on ln t for tetragonal ZrO2

and monoclinic ZrO2 crystallite growth, respectively, is shown inFig. 6(a) and (b). Based on the calculations of the slopes inFig. 6(a) and (b), the values of the average time exponentmte = 0.12 ± 0.02 and mm = 0.17 ± 0.08 for tetragonal and mono-clinic ZrO2 crystallites, respectively, are obtained. If we letln t ? 0, then average values of kte = 4.57 ± 0.55 and

Fig. 6. The dependence of (a) ln D2te and (b) ln D2

m on ln t for tetragonal andmonoclinic ZrO2 crystallite growth, respectively.

292 C.-W. Kuo et al. / Journal of Alloys and Compounds 592 (2014) 288–295

km = 4.40 ± 1.63 for tetragonal and monoclinic ZrO2 crystallites,respectively, are also obtained.

Consequently, the crystallite growth kinetics for tetragonal andmonoclinic ZrO2 may be described as follows using the full growthkinetics equations:

D2te ¼ ð4:57� 0:55Þt0:12�0:02 exp �ð24:79� 0:38Þ � 103

RT

" #ð6Þ

D2m ¼ ð4:40� 1:63Þt0:17�0:08 exp �ð66:47� 3:97Þ � 103

RT

" #ð7Þ

Eqs. (6) and (7) are important because they help us betterunderstand the crystallite growth behavior of tetragonal andmonoclinic ZrO2 in 3Y-PSZ precursor powder after calcination.

From the slope of the plots of log D2 versus 1/T (Fig. 5(a) and(b)), activation energies for t-ZrO2 and m-ZrO2 crystallite growthof 24.79 and 66.47 kJ/mol are obtained, respectively. The value of24.79 kJ/mol is larger than the value of 5.22 kJ/mol for 8YSZ pre-pared by a sol–gel process [25], the value of 11.4 kJ/mol for tetrag-onal ZrO2 crystallite growth [13] or the 21.8 kJ/mol for tetragonalZrO2 crystallite growth in the 2Y-PSZ precursor powder [23]. Onthe other hand, the 24.79 kJ/mol is less than the value of34.03 kJ/mol for nano-sized 2YSZ [12]. The activation energies ofvarious types of ZrO2 crystallite growth are listed in Table 1[12,13,23,25,26].

The very low activation energies for tetragonal and monoclinicZrO2 crystallite growth in the 3Y-PSZ powder is a characteristicfeature of the observed behavior. Bogush and Zukoski [27] haveproposed a nucleation and aggregation model as follows: Althoughnegatively charged, the initial primary particles are unstable due totheir small size, causing aggregation. The colloidal stable aggre-gates then sweep through the suspension, picking up freshlyformed primary particles and small aggregates. Mono-dispersedparticles are achieved through size-dependent aggregation rates[28]. During calcination, the most common type of aggregation ina conventional powder is due to solid state bonds formed betweennanoparticles and precipitates [25]. On the other hand, thepresence of a large number of oxygen vacancies on the crystallitesurface, which mainly come from the doping of Y3+ [11] and thenano-size effect [29], makes the surface energy increase drasticallyand decreases the growth activation energy of the nanoparticles[12].

Hsu et al. [13] have used a mechanism of lattice-plane-rotation-induced crystallite growth (LPRICG) to explain the lower activationof tetragonal ZrO2 crystallite growth. They described the LPRICGprocess for two crystallites with different d-spacing merging intoa larger crystallite. The two different d-spacings of the nanosizedcrystallites aggregated together, and then the lattice plane of oneof them began to rotate when enough energy was obtained asthe initial step. At the end, the crystallites were gradually mergedinto a larger crystallite by discontinuous lattice planes rotating atan angle to adjust the d-spacings until they were the same.

Moreover, it can also be assumed that the presence of vacancydefects must have an influence on the growth mechanism ofgrains. A grain-rotation-induced grain coalescence mechanismmust be considered during the growth of nano-crystalline colloidaloxides [30,31]. The coherence of the grain–grain interface formedby the rotation of neighboring grains leads to the coalescence ofneighboring grains via the elimination of common grain bound-aries, thereby creating a single larger grain [12]. During calcinationof such a freestanding powder at low temperature, the increase inthe particle size is mainly due to the elimination of boundaries be-tween the nano-crystallites within the hard aggregates [32]. As aresult, the very low activation energy values of tetragonal ZrO2

and the growth of monoclinic ZrO2 crystals are obtained.

3.4. Microstructure of tetragonal and monoclinic ZrO2 produced in3Y-PSZ precursor powder after calcination

When the 3Y-PSZ precursor powder was calcined at 773 K for5 min, TEM micrographs, SAED pattern and HRTEM images weregenerated and are shown in Fig. 7. Fig. 7(a) shows the BF image,which reveals that the size of the ZrO2 crystallites is about8.82 nm. Fig. 7(b) and (c) shows DF images within the circles de-noted ‘‘01df’’ and ‘‘02df’’ in Fig. 7(d), respectively. Fig. 7(d) showsthe SAED pattern for the area denoted by ‘‘01sadp’’ in Fig. 7(a),which as indexed corresponds to coexisting tetragonal and mono-clinic ZrO2. Fig. 7(e) shows an HRTEM image of the area denoted as‘‘07hrtem’’ in Fig. 7(a), which reveals that the d-spacings of mono-clinic ZrO2 (111) and (002) reflections are 2.778 Å and 2.632 Å,respectively, and the d-spacing of tetragonal ZrO2 (101) reflectionsis 2.941 Å. The result in Fig. 7(e) also provides evidence forthe coexistence of monoclinic ZrO2 and tetragonal ZrO2 when the3Y-PSZ precursor powder is calcined at 773 K for 5 min.

When the 3Y-PSZ precursor powder was calcined at 773 K for2 h, TEM micrographs, SAED and NBED patterns and HRTEMimages were generated and are shown in Fig. 8. Fig. 8(a) showsthe BF image, which indicates that the fine particles had been grad-ually incorporated into large particles as the calcination time wasincreased. Fig. 8 (b) and (c) shows DF images of areas denoted bythe circles labelled ‘‘03df’’ and ‘‘04df’’, respectively, in Fig. 8(d).

Fig. 7. TEM micrographs, SAED patterns, and HRTEM images of freeze-dried 3Y-PSZ precursor powder after calcination at 773 K for 5 min: (a) BF image, (b) and (c) DF imageswithin the circles denoted as ‘‘01df’’and ‘‘02df’’, respectively, in (d), (d) SAED pattern of the area denoted as ‘‘01sadp’’ in (a), which was indexed as corresponding to thecoexistence of t-ZrO2 and m-ZrO2 and (e) HRTEM image showing d-spacings of monoclinic ZrO2 (111) and (002), and tetragonal ZrO2 (101) reflections of 2.778 Å, 2.632 Å,and 2.941 Å, respectively.

Table 1Activation energy for various ZrO2 crystallites growth.

Type Process Materials Method Activation energy (kJ/mol) Ref.

Powders Precipitation 3 mol% yttria-doped zirconia Isothermal �34.03 [12]Powders Co-precipitation 3 mol% yttria-doped zirconia Isothermal 11.4 (t-ZrO2) [13]Powders Co-precipitation 2 mol% yttria-doped zirconia Isothermal 21.8 (t-ZrO2) [23]Powders Co-precipitation 2 mol% yttria-doped zirconia Isothermal 7.26 [25]Pellet Precipitation and Sintering 15 wt% Ceria-doped zirconia (ZC1) Sintering �125 ± 9 (<1573 K) [26]

28 wt% ceria-doped zirconia (ZC2)ZC1 Sintering �560 ± 45 (>1573 K) [26]ZC2 Sintering �240 ± 17 (>1573 K) [26]

Powders Co-precipitation 3 mol% yttria-doped zirconia Isothermal 66.47 ± 3.97 (m-ZrO2) This study24.79 ± 0.38 (t-ZrO2)

C.-W. Kuo et al. / Journal of Alloys and Compounds 592 (2014) 288–295 293

Fig. 8. TEM micrographs, SAED and NBED patterns, and HRTEM images of freeze-dried 3Y-PSZ precursor powder after calcination at 773 K for 2 h: (a) BF image, (b) and (c) DFimages of areas in circles denoted as ‘‘03df’’and ‘‘04df’’, respectively in (d), (d) SAED pattern of the area denoted ‘‘02sadp’’ in (a), which was indexed as corresponding to thecoexistence of tetragonal ZrO2 and monoclinic ZrO2 phases, (e) NBED pattern of the area denoted ‘‘nbed1’’ in (a), which was indexed as corresponding to tetragonal ZrO2 withZA ¼ ½132� and (f) HRTEM image showing d-spacings for monoclinic ZrO2 (002) and tetragonal ZrO2 (112) reflections of 1.843 Å and 1.810 Å, respectively.

294 C.-W. Kuo et al. / Journal of Alloys and Compounds 592 (2014) 288–295

Fig. 8(d) shows the SAED pattern in the area denoted ‘‘02sadp’’ inFig. 8(a), which indexed as corresponding to coexisting tetragonalZrO2 and monoclinic ZrO2. The NBED pattern in the area denoted‘‘nbed1’’ in Fig. 8(a) is shown in Fig. 8(e), which indexed as corre-sponding to tetragonal ZrO2 with ZA ¼ ½132�. Fig. 8(f) shows anHRTEM image of the location denoted as ‘‘05hrtem’’ in Fig. 8(a).It reveals that the calcined products exhibited good crystallinityand clear lattice fringes. Further, Fig. 8(f) also shows the d-spacingsfor tetragonal ZrO2 (112) and monoclinic ZrO2 (022) reflectionsare 1.810 Å and 1.843 Å, respectively.

From the results in Figs. 7 and 8, it can be seen that preexistingmonoclinic domains may well act as heterogeneous nuclei for thetetragonal to monoclinic transformation. Therefore, if tetragonalZrO2 nanocrystallites are only metastable, thermal treatment ofthe ground powder should lead to complete transformation into

the m-structure. On the other hand, if the unstrained tetragonalZrO2 nanocrystallites are indeed thermodynamically more stablethan monoclinic ZrO2, one would expect the crystal structure tobe restored back to the tetragonal ZrO2 once sufficient thermal en-ergy is supplied to remove the stress and strain [33].

4. Conclusion

The growth kinetics of tetragonal and monoclinic ZrO2 crystal-lites in 3 mol% yttria partially stabilized ZrO2 (3Y-PSZ) precursorpowders have been investigated. The conclusions are summarizedas follows:

(i) When the 3Y-PSZ precursor powders calcined at 773 K for2 h, the BET surface area is 97.13 m2/g, which is equivalent

C.-W. Kuo et al. / Journal of Alloys and Compounds 592 (2014) 288–295 295

to a crystallite size of 10.30 nm. The crystallite sizes deter-mined via XRD and BET are in good agreement, indicatingthat the powder is essentially non-agglomerated.

(ii) When the calcinations temperatures of the 3Y-PSZ precursorpowders between 673 and 1073 K, the crystallite size andcrystallinity of each phase increase with increasing calcina-tion temperature and time. The tetragonal to monoclinicphase transformation for the 3Y-PSZ precursor powderswas confirmed by XRD and SAED. The crystallinity and aver-age crystallite size increased with increasing calcinationtemperature and time.

(iii) The activation energies for tetragonal ZrO2 and monoclinicZrO2 crystallite growth are 24.79 ± 0.38 and66.47 ± 3.97 kJ/mol, respectively.

(iv) When the 3Y-PSZ precursor powders calcined between773 K and 1273 K, the growth kinetics of tetragonaland monoclinic ZrO2 are described as follows:

D2te ¼ ð4:57 � 0:55Þt0:12�0:02 exp � ð24:79�0:38Þ�103

RT

h iand

D2m ¼ ð4:40 � 1:63Þt0:17�0:08 exp � ð66:47�3:97Þ�103

RT

h i; respec-

tively, for 773 K � T � 1073 K. Dte and Dm denote thecrystallite size of tetragonal and monoclinic ZrO2 at time tand temperature T, respectively.

Acknowledgments

The authors gratefully acknowledge the financial support of theMinistry of Economic Affairs, Taiwan, Republic of China, underGrant 102-EC-17-A-08-S1-142, and Prof. M.H. Hon and Mr. S.Y.Yau for offering valuable advice and suggestions on the experi-ments and analyses. We are also deeply thankful to the reviewersfor suggestions and help with the manuscript revision.

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