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Phase transformation of ZrO2 nanocrystals induced by Li+
Materials Letters 79 (2012) 75–77
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Phase transformation of ZrO2 nanocrystals induced by Li + Lu Liu a,⁎, Chunlei Li b, Yujin Chen a, Xinlu Zhang a, Li Li a, Yuxiao Wang c a b c
School of Science, Harbin Engineering University, Harbin 150001, China Department of Physics, Northeast Forestry University, Harbin 150040, China Department of Physics, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 22 February 2012 Accepted 28 March 2012 Available online 4 April 2012 Keywords: Nanoparticles Crystal structure Phase transformation ZrO2 Li+
a b s t r a c t It is well known that monoclinic ZrO2 nanocrystals can be synthesized at ~1000 °C by sol–gel method. In this letter, we demonstrate a novel method to synthesize monoclinic ZrO2 nanocrystal through sol–gel technique under relatively lower calcined temperature. It is found that composition of monoclinic ZrO2 can be significantly enhanced by doping Li +, and pure monoclinic phase ZrO2 nanocrystals appear at 900 °C, 800 °C and 700 °C for samples doped with 0.5, 2, and 5 mol% Li+, respectively. This phase transformation of ZrO2 is dependent on the crystallite size and there is a range of around 11–35 nm of crystallite size in which tetragonal and monoclinic ZrO2 coexist. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved.
1. Introduction Zirconium oxide, with a phase-dependent potential application in a number of technologies, has three polymorph forms including monoclinic, tetragonal, and cubic (m-, t-, and c-, respectively). M-ZrO2 is important for catalysis, gate dielectrics, and bioactive coatings on bone implants, while t- and c-ZrO2 are promising candidates for fuel cell electrolytes, oxygen sensors, and phase-transformation-toughened structural materials [1]. Garvie advanced the hypothesis that the t-phase form had a lower surface free energy than the m-phase, thereby accounting for the spontaneous occurrence of the m-phase structure at a critical crystallite size rc at room temperature [2,3]. It is well known that crystallite size is dependent on the calcined temperature, which increases with increasing temperature. Sol–gel methods are often used to produce zirconia powders. The initial product is a mixture of amorphous ZrO2 (a-ZrO2) and t-ZrO2. Calcination of the initial product at progressively higher temperatures leads to the conversion of all of the a-ZrO2 to t-ZrO2, and at higher temperatures to the conversion of t-ZrO2 to m-ZrO2. However, high processing temperature means high cost and more difficulty in the synthesized procedure. Lots of efforts were paid for the synthesis of m-ZrO2 nanocrystals (NCs) under lower temperature. Murase et al. [4,5] reported that at temperatures above 673 K, the presence of water vapor was found to promote the t- to m-phase transformation of ZrO2. Xie et al. [6] found that exposure of t-ZrO2 to
⁎ Corresponding author. Tel.: + 86 451 82519754; fax: + 86 451 86417072. E-mail address: [email protected] (L. Liu).
3 kPa of H2O or immersion in liquid water at 298 K results in extensive (~80%) transformation of t-ZrO2 → m-ZrO2. In the present work, to the best of our knowledge, we for the first time synthesized pure m-ZrO2 NCs by introducing Li + under a traditional sol–gel method at lower calcined temperature. 2. Experimental Nanocrystallite ZrO2 powders with various Li+ were prepared by a sol–gel process [7], the dried gels were calcined at different temperatures (600, 700, 800, and 900 °C). The structures of the samples in powder form were identified by X-ray diffraction (XRD) using an X'Pert Pro diffractometer. The 2θ angle of the XRD spectra were recorded from 20° to 70° at a scanning rate of 8°/min. 3. Results and discussions The powder XRD patterns (Fig. 1) show the presence of both t- and m-phase of ZrO2 NCs with different dopants and calcined temperatures. All of the diffraction peaks from ZrO2 are in good agreement with the standard values for the highly crystalline ZrO2 crystal. Two strongest lines are at ~28 ° (−111) and ~31 ° (111) for the m-phase (JCPDS No. 37–1484) and the strongest line is at ~ 30 ° (101) for the t-phase (JCPDS No. 79–1771). The phase compositions were estimated I m ð−111Þþ Im ð111Þ using the expression C m ¼ Im ð−111 Þþ Im ð111Þþ I t ð101Þ and Ct = 1 − Cm [8],
where m and t stand for the m- and t-phases of the host and I stands for the integrated intensity of each peak. As shown in Fig. 2, t-phase is dominant in ZrO2 under 600 °C sintering, while it diminishes with the increasing calcined temperature,
0167-577X/$ – see front matter. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2012.03.112
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L. Liu et al. / Materials Letters 79 (2012) 75–77
Fig. 3. Crystallite sizes of t-ZrO2 of samples doped with various concentrations of Li+ under different calcined temperatures, inset is the sizes of m-ZrO2. Fig. 1. XRD patterns of ZrO2 NCs doped with 0, 0.5, 1, 2, 5, and 10 mol% Li+ calcined at a) 600 °C, b) 700 °C, c) 800 °C, and d) 900 °C.
when the processing temperature reaches 900 °C, m-ZrO2 becomes the dominant phase. This kind of phase transformation aroused by the increased sintering temperature is consistent with the previous report [9]. In addition, phase compositions of ZrO2 NCs hardly change by introducing Li + under 600 °C sintering except 10 mol% Li + doped sample, where obvious m-ZrO2 shows. Under other calcined temperatures, Li + evidently increases the compositions of m-ZrO2, samples transform to pure m-ZrO2 with Li+ concentrations of 5, 2, and 0.5 mol% for 700, 800, and 900 °C calcinations, respectively, which clearly confirms that doping Li+ into ZrO2 NCs arouses the t-ZrO2 → m-ZrO2 transformation. The crystallite sizes of the NCs as shown in Fig. 3 are calculated by the Scherrer's equation, almost all the average sizes of crystallites are increased by doping Li +, and sizes of t-ZrO2 are usually smaller than that of m-ZrO2. Interestingly, when crystallite sizes reach ~ 35 nm, samples calcined at 700, 800, and 900 °C wholly transform to pure m-ZrO2. Garvie proposed that the lower surface energy of the tetragonal ZrO2 was the cause for this phase to be present in nanocrystalline form at or below room temperature [2,3]. He predicted that particles below about 10 nm in diameter are stabilized in the tetragonal form,
and those that are above this critical particle size are subject to the t-ZrO2 → m-ZrO2 transformation. Chraska et al. [10] confirm the sizedependent phase transformation of ZrO2 nanocrystals, the critical size is around 18 nm by comparison between sample calcined at 500 and 900 °C, where the sizes of t-ZrO2 are substantially unchanged by increasing the calcined temperatures. In the present work, far from the results as reported by Chraska et al. [10], the crystallite size of t-ZrO2 increases evidently with increasing calcined temperature, which might be caused by the differences of the preparation methods. Yet it is clear that the phase transformation of ZrO2 NCs is dependent on the crystallite size. Based on the XRD data, it can be concluded that there is a range of crystallite size in which t- and m-ZrO2 coexist rather than a precise critical size which demarcates the two phases. As shown in Figs. 2 and 3, average crystallite size of samples with almost pure t-ZrO2 is 11 nm (t-phase > 90% in ZrO2 doped with 0, 0.5, 1, 2, and 5 mol% Li+), and that of pure m-ZrO2 is 35 nm, thus the range aforementioned is around 11 to 35 nm. There are two types of sites occupied by Li+, namely, in the bulk lattice and outside the lattice (i.e. in the grain boundaries and surface). The amount of Li+ in these two sites is significantly different. Straumal et al. [11,12] performed systematic studies on the location of doping impurities in oxide matrix. It is found that the impurities are mostly distributed in grain boundaries and surface in the small size polycrystals since the solubility limit is fairly low in bulk matrix and is much more
Fig. 2. Phase compositions of m-ZrO2 of samples under different calcined temperatures.
Fig. 4. Comparison of XRD patterns of pure ZrO2 and ZrO2 doped with Na+ and K+ calcined at 700 °C.
L. Liu et al. / Materials Letters 79 (2012) 75–77
increased in polycrystals with small size. Further, by comparing the solubility limit between hosts with similar small grain and particle size, it is found that more impurities are located at the grain boundaries than that at the surface. Thus in the present work, Li+ should also be located mostly at the grain boundaries and surface. We propose that the two sites of Li + arouse different influences on the structure of ZrO2. One hand, small amount of Li + doped into the bulk lattice is as a flux which decreases the calcined temperature of ZrO2. It is well known that TiO2 as a “substitution-type” flux decreases the calcined temperature of Al2O3 because the ionic radius of Ti 4+ (0.61 Å) is close to that of Al 3+ (0.54 Å) which makes Ti 4+ easily substitute the Al 3+, following this substitution, vacancies with positive charge are generated and thus cause the structural distortion which finally decrease the calcined temperature. Herein, Li + (0.76 Å) with similar ionic radius and different charge with Zr 4+ (0.72 Å) should also benefit the calcination. Thus structures of Li +:ZrO2 at lower calcined temperature are similar to pure ZrO2 calcined at higher temperature, in particular, ZrO2 doped with Li + have larger sizes than that of pure ZrO2, and the increased size arouse the t- to m-ZrO2 transformation. The structural distortion induced by Li + can be confirmed by the XRD data. Angles of diffraction peaks are investigated (data not shown), from where it can be seen that codoping Li + slightly alters the angles of the main diffraction peaks. These shifts illustrate that Li + slightly alters the structural parameters of ZrO2. Further, as shown in Fig. 4, Na + (1.02 Å) and K + (1.38 Å) with similar physical and chemical natures to Li + but larger sizes were doped into ZrO2 under 700 °C calcination. Li + as a “substitution-type” flux can be verified by the fact that Na + and K + cannot increase the crystallite size and m-ZrO2 composition due to their larger sizes than Zr 4+ which make the substitution hardly happen. On the other hand, large amount of Li+ doped in the grain boundaries and surface also has influence on the phase transformation of ZrO2. It is reasonable to consider that Li+ in grain boundaries and surface also arouse the structural distortion as the situation in the bulk lattice. We propose that this distortion generates larger stress, which further promotes the complete wetting of grain boundaries and surface, that is, the phase transformation [13]. So Li+ in grain boundaries and surface also benefit the t- to m-ZrO2 transformation. 4. Conclusion In conclusion, ZrO2 NCs doped with various concentrations of Li + were synthesized. T-ZrO2 is dominant under 600 °C calcination, by
77
introducing Li +, however, m-ZrO2 was generated, and composition of m-ZrO2 increases with increasing Li + content. A size range of around 11 to 35 nm where t- and m-ZrO2 coexist was found. The mechanism of phase transformation was attributed to the fact that Li + alters the structure of the ZrO2 in both the bulk lattice and grain boundaries or surface. This work provides practical value on applications of m-ZrO2 NCs.
Acknowledgment This work is supported by the Fundamental Research Funds for the Central Universities.
References [1] Sato K, Abe H, Ohara S. Selective growth of monoclinic and tetragonal zirconia nanocrystals. J Am Chem Soc 2010;132:2538–9. [2] Garvie RC. The occurrence of metastable tetragonal zirconia as a crystallite size effect. J Phys Chem 1965;69:1238–43. [3] Garvie RC. Stabilization of the tetragonal structure in zirconia microcrystals. J Phys Chem 1978;82:218–24. [4] Murase Y, Kato E. Phase transformation of zirconia by ball-milling. J Am Ceram Soc 1979;62:527. [5] Murase Y, Kato E. Role of water vapor in crystallite growth and tetragonalmonoclinic phase transformation of ZrO2. J Am Ceram Soc 1983;66:196–200. [6] Xie SB, Iglesia E, Bell AT. Water-assisted tetragonal-to-monoclinic phase transformation of ZrO2 at low temperatures. Chem Mater 2000;12:2442–7. [7] Liu L, Wang YX, Zhang XR, Yang K, Bai YF, Huang CH, et al. Efficient two-color luminescence of Er3+/Yb3+/Li+:ZrO2 nanocrystals. Opt Mater 2011;33:1234–8. [8] Solís D, López-Luke T, Rosa ED, Salas P, Angeles-Chavez C. Surfactant effect on the upconversion emission and decay time of ZrO2:Yb–Er nanocrystals. J Lumin 2009;129:449–55. [9] Patra A, Friend CS, Kapoor R, Prasad PN. Effect of crystal nature on upconversion luminescence in Er3+:ZrO2 nanocrystals. Appl Phys Lett 2003;83:284–6. [10] Chraska T, King AH, Berndt CC. On the size-dependent phase transformation in nanoparticulate zirconia. Mater Sci Eng A 2000;286:169–78. [11] Straumal BB, Mazilkin AA, Protasova SG, Myatiev AA, Straumal PB, Baretzky B. Increase of Co solubility with decreasing grain size in ZnO. Acta Mater 2008;56: 6246–56. [12] Straumal B, Baretzky B, Mazilkin A, Protasova S, Myatiev A, Straumal P. Increase of Mn solubility with decreasing grain size in ZnO. J Eur Ceram Soc 2009;29: 1963–70. [13] Cahn JW. Critical point wetting. J Chem Phys 1977;66:3667–72.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Phase transformation of ZrO2 nanocrystals induced by Li + Lu Liu a,⁎, Chunlei Li b, Yujin Chen a, Xinlu Zhang a, Li Li a, Yuxiao Wang c a b c
School of Science, Harbin Engineering University, Harbin 150001, China Department of Physics, Northeast Forestry University, Harbin 150040, China Department of Physics, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 22 February 2012 Accepted 28 March 2012 Available online 4 April 2012 Keywords: Nanoparticles Crystal structure Phase transformation ZrO2 Li+
a b s t r a c t It is well known that monoclinic ZrO2 nanocrystals can be synthesized at ~1000 °C by sol–gel method. In this letter, we demonstrate a novel method to synthesize monoclinic ZrO2 nanocrystal through sol–gel technique under relatively lower calcined temperature. It is found that composition of monoclinic ZrO2 can be significantly enhanced by doping Li +, and pure monoclinic phase ZrO2 nanocrystals appear at 900 °C, 800 °C and 700 °C for samples doped with 0.5, 2, and 5 mol% Li+, respectively. This phase transformation of ZrO2 is dependent on the crystallite size and there is a range of around 11–35 nm of crystallite size in which tetragonal and monoclinic ZrO2 coexist. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved.
1. Introduction Zirconium oxide, with a phase-dependent potential application in a number of technologies, has three polymorph forms including monoclinic, tetragonal, and cubic (m-, t-, and c-, respectively). M-ZrO2 is important for catalysis, gate dielectrics, and bioactive coatings on bone implants, while t- and c-ZrO2 are promising candidates for fuel cell electrolytes, oxygen sensors, and phase-transformation-toughened structural materials [1]. Garvie advanced the hypothesis that the t-phase form had a lower surface free energy than the m-phase, thereby accounting for the spontaneous occurrence of the m-phase structure at a critical crystallite size rc at room temperature [2,3]. It is well known that crystallite size is dependent on the calcined temperature, which increases with increasing temperature. Sol–gel methods are often used to produce zirconia powders. The initial product is a mixture of amorphous ZrO2 (a-ZrO2) and t-ZrO2. Calcination of the initial product at progressively higher temperatures leads to the conversion of all of the a-ZrO2 to t-ZrO2, and at higher temperatures to the conversion of t-ZrO2 to m-ZrO2. However, high processing temperature means high cost and more difficulty in the synthesized procedure. Lots of efforts were paid for the synthesis of m-ZrO2 nanocrystals (NCs) under lower temperature. Murase et al. [4,5] reported that at temperatures above 673 K, the presence of water vapor was found to promote the t- to m-phase transformation of ZrO2. Xie et al. [6] found that exposure of t-ZrO2 to
⁎ Corresponding author. Tel.: + 86 451 82519754; fax: + 86 451 86417072. E-mail address: [email protected] (L. Liu).
3 kPa of H2O or immersion in liquid water at 298 K results in extensive (~80%) transformation of t-ZrO2 → m-ZrO2. In the present work, to the best of our knowledge, we for the first time synthesized pure m-ZrO2 NCs by introducing Li + under a traditional sol–gel method at lower calcined temperature. 2. Experimental Nanocrystallite ZrO2 powders with various Li+ were prepared by a sol–gel process [7], the dried gels were calcined at different temperatures (600, 700, 800, and 900 °C). The structures of the samples in powder form were identified by X-ray diffraction (XRD) using an X'Pert Pro diffractometer. The 2θ angle of the XRD spectra were recorded from 20° to 70° at a scanning rate of 8°/min. 3. Results and discussions The powder XRD patterns (Fig. 1) show the presence of both t- and m-phase of ZrO2 NCs with different dopants and calcined temperatures. All of the diffraction peaks from ZrO2 are in good agreement with the standard values for the highly crystalline ZrO2 crystal. Two strongest lines are at ~28 ° (−111) and ~31 ° (111) for the m-phase (JCPDS No. 37–1484) and the strongest line is at ~ 30 ° (101) for the t-phase (JCPDS No. 79–1771). The phase compositions were estimated I m ð−111Þþ Im ð111Þ using the expression C m ¼ Im ð−111 Þþ Im ð111Þþ I t ð101Þ and Ct = 1 − Cm [8],
where m and t stand for the m- and t-phases of the host and I stands for the integrated intensity of each peak. As shown in Fig. 2, t-phase is dominant in ZrO2 under 600 °C sintering, while it diminishes with the increasing calcined temperature,
0167-577X/$ – see front matter. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2012.03.112
76
L. Liu et al. / Materials Letters 79 (2012) 75–77
Fig. 3. Crystallite sizes of t-ZrO2 of samples doped with various concentrations of Li+ under different calcined temperatures, inset is the sizes of m-ZrO2. Fig. 1. XRD patterns of ZrO2 NCs doped with 0, 0.5, 1, 2, 5, and 10 mol% Li+ calcined at a) 600 °C, b) 700 °C, c) 800 °C, and d) 900 °C.
when the processing temperature reaches 900 °C, m-ZrO2 becomes the dominant phase. This kind of phase transformation aroused by the increased sintering temperature is consistent with the previous report [9]. In addition, phase compositions of ZrO2 NCs hardly change by introducing Li + under 600 °C sintering except 10 mol% Li + doped sample, where obvious m-ZrO2 shows. Under other calcined temperatures, Li + evidently increases the compositions of m-ZrO2, samples transform to pure m-ZrO2 with Li+ concentrations of 5, 2, and 0.5 mol% for 700, 800, and 900 °C calcinations, respectively, which clearly confirms that doping Li+ into ZrO2 NCs arouses the t-ZrO2 → m-ZrO2 transformation. The crystallite sizes of the NCs as shown in Fig. 3 are calculated by the Scherrer's equation, almost all the average sizes of crystallites are increased by doping Li +, and sizes of t-ZrO2 are usually smaller than that of m-ZrO2. Interestingly, when crystallite sizes reach ~ 35 nm, samples calcined at 700, 800, and 900 °C wholly transform to pure m-ZrO2. Garvie proposed that the lower surface energy of the tetragonal ZrO2 was the cause for this phase to be present in nanocrystalline form at or below room temperature [2,3]. He predicted that particles below about 10 nm in diameter are stabilized in the tetragonal form,
and those that are above this critical particle size are subject to the t-ZrO2 → m-ZrO2 transformation. Chraska et al. [10] confirm the sizedependent phase transformation of ZrO2 nanocrystals, the critical size is around 18 nm by comparison between sample calcined at 500 and 900 °C, where the sizes of t-ZrO2 are substantially unchanged by increasing the calcined temperatures. In the present work, far from the results as reported by Chraska et al. [10], the crystallite size of t-ZrO2 increases evidently with increasing calcined temperature, which might be caused by the differences of the preparation methods. Yet it is clear that the phase transformation of ZrO2 NCs is dependent on the crystallite size. Based on the XRD data, it can be concluded that there is a range of crystallite size in which t- and m-ZrO2 coexist rather than a precise critical size which demarcates the two phases. As shown in Figs. 2 and 3, average crystallite size of samples with almost pure t-ZrO2 is 11 nm (t-phase > 90% in ZrO2 doped with 0, 0.5, 1, 2, and 5 mol% Li+), and that of pure m-ZrO2 is 35 nm, thus the range aforementioned is around 11 to 35 nm. There are two types of sites occupied by Li+, namely, in the bulk lattice and outside the lattice (i.e. in the grain boundaries and surface). The amount of Li+ in these two sites is significantly different. Straumal et al. [11,12] performed systematic studies on the location of doping impurities in oxide matrix. It is found that the impurities are mostly distributed in grain boundaries and surface in the small size polycrystals since the solubility limit is fairly low in bulk matrix and is much more
Fig. 2. Phase compositions of m-ZrO2 of samples under different calcined temperatures.
Fig. 4. Comparison of XRD patterns of pure ZrO2 and ZrO2 doped with Na+ and K+ calcined at 700 °C.
L. Liu et al. / Materials Letters 79 (2012) 75–77
increased in polycrystals with small size. Further, by comparing the solubility limit between hosts with similar small grain and particle size, it is found that more impurities are located at the grain boundaries than that at the surface. Thus in the present work, Li+ should also be located mostly at the grain boundaries and surface. We propose that the two sites of Li + arouse different influences on the structure of ZrO2. One hand, small amount of Li + doped into the bulk lattice is as a flux which decreases the calcined temperature of ZrO2. It is well known that TiO2 as a “substitution-type” flux decreases the calcined temperature of Al2O3 because the ionic radius of Ti 4+ (0.61 Å) is close to that of Al 3+ (0.54 Å) which makes Ti 4+ easily substitute the Al 3+, following this substitution, vacancies with positive charge are generated and thus cause the structural distortion which finally decrease the calcined temperature. Herein, Li + (0.76 Å) with similar ionic radius and different charge with Zr 4+ (0.72 Å) should also benefit the calcination. Thus structures of Li +:ZrO2 at lower calcined temperature are similar to pure ZrO2 calcined at higher temperature, in particular, ZrO2 doped with Li + have larger sizes than that of pure ZrO2, and the increased size arouse the t- to m-ZrO2 transformation. The structural distortion induced by Li + can be confirmed by the XRD data. Angles of diffraction peaks are investigated (data not shown), from where it can be seen that codoping Li + slightly alters the angles of the main diffraction peaks. These shifts illustrate that Li + slightly alters the structural parameters of ZrO2. Further, as shown in Fig. 4, Na + (1.02 Å) and K + (1.38 Å) with similar physical and chemical natures to Li + but larger sizes were doped into ZrO2 under 700 °C calcination. Li + as a “substitution-type” flux can be verified by the fact that Na + and K + cannot increase the crystallite size and m-ZrO2 composition due to their larger sizes than Zr 4+ which make the substitution hardly happen. On the other hand, large amount of Li+ doped in the grain boundaries and surface also has influence on the phase transformation of ZrO2. It is reasonable to consider that Li+ in grain boundaries and surface also arouse the structural distortion as the situation in the bulk lattice. We propose that this distortion generates larger stress, which further promotes the complete wetting of grain boundaries and surface, that is, the phase transformation [13]. So Li+ in grain boundaries and surface also benefit the t- to m-ZrO2 transformation. 4. Conclusion In conclusion, ZrO2 NCs doped with various concentrations of Li + were synthesized. T-ZrO2 is dominant under 600 °C calcination, by
77
introducing Li +, however, m-ZrO2 was generated, and composition of m-ZrO2 increases with increasing Li + content. A size range of around 11 to 35 nm where t- and m-ZrO2 coexist was found. The mechanism of phase transformation was attributed to the fact that Li + alters the structure of the ZrO2 in both the bulk lattice and grain boundaries or surface. This work provides practical value on applications of m-ZrO2 NCs.
Acknowledgment This work is supported by the Fundamental Research Funds for the Central Universities.
References [1] Sato K, Abe H, Ohara S. Selective growth of monoclinic and tetragonal zirconia nanocrystals. J Am Chem Soc 2010;132:2538–9. [2] Garvie RC. The occurrence of metastable tetragonal zirconia as a crystallite size effect. J Phys Chem 1965;69:1238–43. [3] Garvie RC. Stabilization of the tetragonal structure in zirconia microcrystals. J Phys Chem 1978;82:218–24. [4] Murase Y, Kato E. Phase transformation of zirconia by ball-milling. J Am Ceram Soc 1979;62:527. [5] Murase Y, Kato E. Role of water vapor in crystallite growth and tetragonalmonoclinic phase transformation of ZrO2. J Am Ceram Soc 1983;66:196–200. [6] Xie SB, Iglesia E, Bell AT. Water-assisted tetragonal-to-monoclinic phase transformation of ZrO2 at low temperatures. Chem Mater 2000;12:2442–7. [7] Liu L, Wang YX, Zhang XR, Yang K, Bai YF, Huang CH, et al. Efficient two-color luminescence of Er3+/Yb3+/Li+:ZrO2 nanocrystals. Opt Mater 2011;33:1234–8. [8] Solís D, López-Luke T, Rosa ED, Salas P, Angeles-Chavez C. Surfactant effect on the upconversion emission and decay time of ZrO2:Yb–Er nanocrystals. J Lumin 2009;129:449–55. [9] Patra A, Friend CS, Kapoor R, Prasad PN. Effect of crystal nature on upconversion luminescence in Er3+:ZrO2 nanocrystals. Appl Phys Lett 2003;83:284–6. [10] Chraska T, King AH, Berndt CC. On the size-dependent phase transformation in nanoparticulate zirconia. Mater Sci Eng A 2000;286:169–78. [11] Straumal BB, Mazilkin AA, Protasova SG, Myatiev AA, Straumal PB, Baretzky B. Increase of Co solubility with decreasing grain size in ZnO. Acta Mater 2008;56: 6246–56. [12] Straumal B, Baretzky B, Mazilkin A, Protasova S, Myatiev A, Straumal P. Increase of Mn solubility with decreasing grain size in ZnO. J Eur Ceram Soc 2009;29: 1963–70. [13] Cahn JW. Critical point wetting. J Chem Phys 1977;66:3667–72.