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Rapid mechanochemical synthesis of fine barium titanate nanoparticles
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Materials Letters 62 (2008) 2957 – 2959 www.elsevier.com/locate/matlet
Rapid mechanochemical synthesis of fine barium titanate nanoparticles Satoshi Ohara ⁎, Akira Kondo, Hirofumi Shimoda, Kazuyoshi Sato, Hiroya Abe, Makio Naito Joining and Welding Research Institute, Osaka University, 11-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan Received 13 July 2007; accepted 28 January 2008 Available online 5 February 2008
Abstract A rapid mechanochemical synthesis of fine barium titanate (BaTiO3) nanoparticles is reported. Mechanical forces such as compression and shear stress were repeatedly applied on a mixture of BaCO3 and TiO2, using an attrition type milling apparatus. The formation of BaTiO3 was observed in the subsequent milling, and its synthesis was completed after only 12 min. The particle size of the synthesized BaTiO3 was mostly of several 10 nm. The amount of hydroxide ions incorporated into the BaTiO3 nanoparticles was less than 0.3 wt.%. © 2008 Elsevier B.V. All rights reserved. Keywords: Barium titanate nanoparticles
1. Introduction Barium titanate (BaTiO3) particles have been widely used as a raw material for the dielectric layer of multi-layer ceramic capacitors (MLCCs). Recently, MLCCs with thin dielectric layers are required due to the miniaturization of advanced electronic devices. It has been predicted that the thickness of the dielectric layer will reach thinner than 500 nm in the near future. Therefore, fine BaTiO3 nanoparticles smaller than 100 nm have been eagerly desired. Liquid phase reaction has provided an exciting possibility for high purity, homogeneous, and fine BaTiO3 nanoparticles [1]. BaTiO3 nanoparticles have been synthesized by using a hydrothermal method [2–10], sol–gel processing [11–13], the oxalate route [14], microwave heating [15], a micro-emulsion process [16,17], and a polymeric precursor method [18]. Although the BaTiO3 nanoparticles produced by these methods are smaller than 100 nm, they contain residual hydroxide ions [9,19]. It has been reported that the hydroxide ions in BaTiO3 nanoparticles result in the formation of intergranular pores in MLCCs during annealing [9,19,20].
⁎ Corresponding author. Tel./fax: +81 6 6879 4370. E-mail address: [email protected] (S. Ohara). 0167-577X/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.01.083
A conventional solid state reaction at high temperature is a suitable method to prevent the presence of the hydroxide ions in the oxygen lattice of BaTiO3, however, it leads to large particle size and limited degree of chemical homogeneity. A satisfactory choice may be a mechanochemical method. The mechanochemical route has several advantages over both conventional solid state reaction and liquid phase processes including the use of low cost raw materials, simplicity of the process and the ability to obtain fine particles [21]. The mechanochemical method is characterized by the repeated welding, deformation and fracture of the constituent powder materials [22]. Chemical reactions occur at the interfaces of the particles that are continuously regenerated during milling [23]. As a consequence, the solid state reactions can promote in milling apparatus without any need for external heating. Recently, some groups have synthesized fine BaTiO3 particles by an intensive ball milling such as a planetary ball milling with long period of time [24–27]. However, this may result in serious drawbacks of high contamination due to wear of the balls and the vial. In this paper, we demonstrate a rapid mechanochemical synthesis of fine BaTiO3 nanoparticles, starting from a mixture of BaCO3 and TiO2. For the synthesis, mechanical forces such as compression and shear stress were repeatedly applied on the powder mixture using an attrition type milling apparatus [28,29]. No media were employed in the milling. It was found that
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BaTiO3 was synthesized after only 12 min. The obtained fine BaTiO3 nanoparticles possess the particle size of about several 10 nm and the amount of hydroxide ions of less than 0.3 wt.%. 2. Experimental Raw materials were BaCO3 (BW-KS20, Sakai Kagaku, Japan) and TiO2 (ST-01, Ishihara Sangyo, Japan). The mean particle sizes calculated from the specific surface area (SSA) were 50 nm and 7 nm for the BaCO3 and TiO2, respectively. These two powders were mixed with equimolar ratio, and the powder mixture of 60 g placed into the chamber of an attrition type apparatus. This apparatus was previously employed for the mechanochemical synthesis of LaMnO3 [28,29]. Its main components are a fixed chamber and a rotor set with a certain clearance against the inside wall of the chamber. Both the chamber and the rotor were made from stainless steel. When the rotor rotates, the powder mixture is compressed into the clearance (1 mm in gap) and receives various kinds of mechanical forces including compression and shearing. No media balls were used in this milling, and the ambient atmosphere in the chamber was not controlled (i.e., the milling was conducted in air). The rotating speed was 4000 rpm in the present set of experiments. The milled powders were characterized by X-ray diffraction (XRD; JDX-3530M, JEOL, Japan) using Ni filtered Cu-Kα radiation. SSA of the samples was measured by a nitrogen gas adsorption instrument (Micromeritics ASAP 2010, Shimadzu, Japan) based on the BET method. Particle morphology was observed with a SEM (ERA-8800FE, Elionics, Japan). The hydroxide ion concentration incorporated into the lattice was evaluated by thermogravimetry (TG; EXSTAR6000, SII NanoTechnology, Japan) analysis.
Fig. 1. XRD patterns of the powder mixture before and after milling.
Fig. 2. SEM image of the synthesized BaTiO3 nanoparticles.
3. Results and discussion Fig. 1 shows the XRD patterns of the staring powder mixture and the milled powder mixture for time period of 12 min. Only peaks corresponding to BaCO3 and TiO2 were observed for the powder mixture before milling. These peak intensities drastically decreased with an increase in milling time. Concomitant with the decrease, the peaks corresponding to BaTiO3 appeared. After 10 min, the peaks corresponding to BaCO3 and TiO2 almost disappeared, and only the peak intensity for BaTiO3 increased. The BaTiO3 particles size of 22 nm is calculated using the Scherrer equation. It was shown that the single phase BaTiO 3 powder was mechanochemically synthesized from the constituent powder mixture in 12 min. The SSA significantly decreased with an increase in milling time. The mechanical treatment at the early stage of milling resulted particle bonding or aggregation of the powder mixture. Therefore, it is concluded that the following steps proceeded: the aggregation with amorphization of the raw material in the early stage of the milling, and solid state reaction, led to the formation of BaTiO3 phase. Localized heating and pressure at regions of contact between the reactant grains may be a contributing factor for the phase formation in the activated matrix [30]. Our results indicated considerable activation took place in the powder mixture. The phase formation among nanosized grains occurred in the present mechanical milling without any media balls, because the BaTiO3 phase was not realized in 12 min when the larger
Fig. 3. Thermogravimetry analysis of the synthesized BaTiO3.
S. Ohara et al. / Materials Letters 62 (2008) 2957–2959
TiO2 (70 nm) was mixed with the same BaCO3 powder, emphasizing the importance of TiO2 size in the present mechanochemical reaction. Fig. 2 shows the SEM image of the BaTiO3 particles synthesized by the mechanochemical milling for 12 min. The sizes of the observed particles were mostly of several tens of nanometers. These values are in good agreement with those calculated from XRD peaks. The mean particles size calculated from the SSA was about 100 nm. Fig. 3 shows the result of the TG analysis of the synthesized BaTiO3 nanoparticles. The weight loss from 300 to 500 °C is due to the removal of hydroxyl defects, whereas the weight loss at higher temperature is the removal of BaCO3. On the other hand, the weight loss around 800 °C may be considered to be CO2 which is due to the presence of residual BaCO3 [32], because very small amount of BaCO3 cannot be detected by XRD peaks. It can be assumed that the hydroxide ions incorporated into the BaTiO3 lattice were expelled in the temperature range of below 500 °C. It was measured that the amount of hydroxide ions incorporated into the BaTiO3 nanoparticles was less than 0.3 wt.%. The hydroxyl defect concentration of the commercial hydrothermal BaTiO3 particles has been reported to be 0.75 wt.% and more [31]. Therefore, it is concluded that the mechanochemical process is suitable method to prevent the presence of the hydroxide ions in the oxygen lattice of BaTiO3 nanoparticles.
4. Conclusions Fine BaTiO3 nanoparticles were mechanochemically synthesized from the constituent powder mixture in short times. The particle size of the obtained BaTiO3 was about several 10 nm. The amount of hydroxide ions incorporated into the BaTiO3 nanoparticles was less than 0.3 wt.%. We believe the BaTiO3 nanoparticles prepared by mechanochemical synthesis could be used as the starting materials of the miniaturization of advanced MLCCs. Acknowledgment This work was partly supported by Grant-in-Aid for Cooperative Research Project of Nationwide Joint-Use Research Institute on Development Base of Joining Technology for New Metallic Glasses and Inorganic Materials from The Ministry of Education, Culture, Sport, Science and Technology, Japan. References [1] P.P. Phule, S.H. Risbud, J. Mater. Sci. 25 (1990) 1169–1183. [2] C.T. Xia, E.W. Shi, W.E. Zhong, J.K. Guo, J. Eur. Ceram. Soc. 15 (1995) 1171–1176.
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[3] M. Wu, R. Xu, S.H. Feng, L. Li, D. Chen, Y.J. Luo, J. Mater. Sci. 31 (1996) 6201–6205. [4] J.O. Eckert Jr., C.C. Hung-Houston, B.L. Gersten, M.M. Lencka, R.E. Riman, J. Am. Ceram. Soc. 79 (1996) 2929–2939. [5] E.B. Slamovich, I.A. Aksay, J. Am. Ceram. Soc. 79 (1996) 239–247. [6] L. Zhao, A.T. Chien, F.F. Lapse, J.S. Speck, J. Mater. Res. 11 (1996) 1325–1328. [7] T. Noma, S. Wada, M. Yano, T. Suzuki, J. Appl. Phys. 80 (1996) 5223–5233. [8] R. Asiaie, W. Zhu, S.A. Akbar, P.K. Dutta, Chem. Mater. 8 (1996) 226–234. [9] I.J. Clark, T. Takeuchi, N. Ohtori, D.C. Sinclair, J. Mater. Chem. 9 (1999) 83–91. [10] E.W. Shi, C.T. Xia, W.E. Zhong, B.G. Wang, C.D. Feng, J. Am. Ceram. Soc. 80 (1997) 1567–1572. [11] M.H. Frey, D.A. Payne, Phys. Rev. B 54 (1996) 3158–3168. [12] H. Shimooka, M. Kuwabara, J. Am. Ceram. Soc. 79 (1996) 2983–2985. [13] T. Takeuchi, M. Tabuchi, K. Ado, K. Honjo, O. Nakamura, H. Kageyama, Y. Suyama, N. Ohtori, M. Nagasawa, J. Mater. Sci. 32 (1997) 4053–4060. [14] H.I. Hsiang, F.S. Yen, J. Am. Ceram. Soc. 79 (1996) 1053–1060. [15] Y. Ma, E. Vileno, S. Suib, P.K. Dutta, Chem. Mater. 9 (1997) 3023–3031. [16] C. Beck, W. Hartl, R. Hempelmann, J. Mater. Res. 13 (1998) 3174–3180. [17] J. Wang, J. Fang, S.C. Ng, l.M. Gan, C.H. Chew, X. Wang, Z. Shen, J. Am. Ceram. Soc. 82 (1999) 873–881. [18] W.S. Cho, J. Phys. Chem. Solid 59 (1998) 659–666. [19] S.W. Lu, B.I. Lee, Z.L. Wang, W.D., J. Cryst. Growth 219 (2000) 269–276. [20] D.F.K. Hennings, C. Metzmacher, B.S. Schreinemacher, J. Am. Ceram. Soc. 84 (2001) 179–182. [21] B.D. Stojanovic, J. Mater. Process. Technol. 143/144 (2003) 78–81. [22] T. Tsuzuki, P.G. McCormick, J. Mater. Sci. 39 (2004) 5143–5146. [23] C.C. Koch, Mater. Sci. Technol. 15 (1991) 193–245. [24] J. Xue, J. Wang, D. Wan, J. Am. Ceram Soc. 83 (2000) 232–234. [25] C. Gomez-Yanez, C. Benitez, H. Balmori-Ramirez, Ceram. Int. 26 (2000) 271–277. [26] H.A.M. van Hal, W.A. Groen, S. Maassen, W.C. Keur, J. Eur. Ceram. Soc. 21 (2001) 1689–1692. [27] V. Berbenni, A. Marini, G. Bruni, Thermochim. Acta 374 (2001) 151–158. [28] K. Sato, J. Chaichanawong, H. Abe, M. Naito, Mater. Lett. 60 (2006) 1399–1402. [29] S. Ohara, H. Abe, K. Sato, A. Kondo, M. Naito, J. Eur, Ceram. Soc. (in press). [30] M.L. Trudeau, R. Schulz, R. Dussault, A.V. Neste, Phys. Rev. Lett. 65 (1990) 99–102. [31] T.J. Yosenick, D.V. Miller, R. Kumar, J.A. Nelson, C.A. Randall, J.H. Adair, J. Mater. Res. 20 (2005) 837–843. [32] D. Hennings, S. Schreinemacher, J. Eur. Ceram. Soc. 9 (1992) 41–46.
Materials Letters 62 (2008) 2957 – 2959 www.elsevier.com/locate/matlet
Rapid mechanochemical synthesis of fine barium titanate nanoparticles Satoshi Ohara ⁎, Akira Kondo, Hirofumi Shimoda, Kazuyoshi Sato, Hiroya Abe, Makio Naito Joining and Welding Research Institute, Osaka University, 11-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan Received 13 July 2007; accepted 28 January 2008 Available online 5 February 2008
Abstract A rapid mechanochemical synthesis of fine barium titanate (BaTiO3) nanoparticles is reported. Mechanical forces such as compression and shear stress were repeatedly applied on a mixture of BaCO3 and TiO2, using an attrition type milling apparatus. The formation of BaTiO3 was observed in the subsequent milling, and its synthesis was completed after only 12 min. The particle size of the synthesized BaTiO3 was mostly of several 10 nm. The amount of hydroxide ions incorporated into the BaTiO3 nanoparticles was less than 0.3 wt.%. © 2008 Elsevier B.V. All rights reserved. Keywords: Barium titanate nanoparticles
1. Introduction Barium titanate (BaTiO3) particles have been widely used as a raw material for the dielectric layer of multi-layer ceramic capacitors (MLCCs). Recently, MLCCs with thin dielectric layers are required due to the miniaturization of advanced electronic devices. It has been predicted that the thickness of the dielectric layer will reach thinner than 500 nm in the near future. Therefore, fine BaTiO3 nanoparticles smaller than 100 nm have been eagerly desired. Liquid phase reaction has provided an exciting possibility for high purity, homogeneous, and fine BaTiO3 nanoparticles [1]. BaTiO3 nanoparticles have been synthesized by using a hydrothermal method [2–10], sol–gel processing [11–13], the oxalate route [14], microwave heating [15], a micro-emulsion process [16,17], and a polymeric precursor method [18]. Although the BaTiO3 nanoparticles produced by these methods are smaller than 100 nm, they contain residual hydroxide ions [9,19]. It has been reported that the hydroxide ions in BaTiO3 nanoparticles result in the formation of intergranular pores in MLCCs during annealing [9,19,20].
⁎ Corresponding author. Tel./fax: +81 6 6879 4370. E-mail address: [email protected] (S. Ohara). 0167-577X/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.01.083
A conventional solid state reaction at high temperature is a suitable method to prevent the presence of the hydroxide ions in the oxygen lattice of BaTiO3, however, it leads to large particle size and limited degree of chemical homogeneity. A satisfactory choice may be a mechanochemical method. The mechanochemical route has several advantages over both conventional solid state reaction and liquid phase processes including the use of low cost raw materials, simplicity of the process and the ability to obtain fine particles [21]. The mechanochemical method is characterized by the repeated welding, deformation and fracture of the constituent powder materials [22]. Chemical reactions occur at the interfaces of the particles that are continuously regenerated during milling [23]. As a consequence, the solid state reactions can promote in milling apparatus without any need for external heating. Recently, some groups have synthesized fine BaTiO3 particles by an intensive ball milling such as a planetary ball milling with long period of time [24–27]. However, this may result in serious drawbacks of high contamination due to wear of the balls and the vial. In this paper, we demonstrate a rapid mechanochemical synthesis of fine BaTiO3 nanoparticles, starting from a mixture of BaCO3 and TiO2. For the synthesis, mechanical forces such as compression and shear stress were repeatedly applied on the powder mixture using an attrition type milling apparatus [28,29]. No media were employed in the milling. It was found that
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BaTiO3 was synthesized after only 12 min. The obtained fine BaTiO3 nanoparticles possess the particle size of about several 10 nm and the amount of hydroxide ions of less than 0.3 wt.%. 2. Experimental Raw materials were BaCO3 (BW-KS20, Sakai Kagaku, Japan) and TiO2 (ST-01, Ishihara Sangyo, Japan). The mean particle sizes calculated from the specific surface area (SSA) were 50 nm and 7 nm for the BaCO3 and TiO2, respectively. These two powders were mixed with equimolar ratio, and the powder mixture of 60 g placed into the chamber of an attrition type apparatus. This apparatus was previously employed for the mechanochemical synthesis of LaMnO3 [28,29]. Its main components are a fixed chamber and a rotor set with a certain clearance against the inside wall of the chamber. Both the chamber and the rotor were made from stainless steel. When the rotor rotates, the powder mixture is compressed into the clearance (1 mm in gap) and receives various kinds of mechanical forces including compression and shearing. No media balls were used in this milling, and the ambient atmosphere in the chamber was not controlled (i.e., the milling was conducted in air). The rotating speed was 4000 rpm in the present set of experiments. The milled powders were characterized by X-ray diffraction (XRD; JDX-3530M, JEOL, Japan) using Ni filtered Cu-Kα radiation. SSA of the samples was measured by a nitrogen gas adsorption instrument (Micromeritics ASAP 2010, Shimadzu, Japan) based on the BET method. Particle morphology was observed with a SEM (ERA-8800FE, Elionics, Japan). The hydroxide ion concentration incorporated into the lattice was evaluated by thermogravimetry (TG; EXSTAR6000, SII NanoTechnology, Japan) analysis.
Fig. 1. XRD patterns of the powder mixture before and after milling.
Fig. 2. SEM image of the synthesized BaTiO3 nanoparticles.
3. Results and discussion Fig. 1 shows the XRD patterns of the staring powder mixture and the milled powder mixture for time period of 12 min. Only peaks corresponding to BaCO3 and TiO2 were observed for the powder mixture before milling. These peak intensities drastically decreased with an increase in milling time. Concomitant with the decrease, the peaks corresponding to BaTiO3 appeared. After 10 min, the peaks corresponding to BaCO3 and TiO2 almost disappeared, and only the peak intensity for BaTiO3 increased. The BaTiO3 particles size of 22 nm is calculated using the Scherrer equation. It was shown that the single phase BaTiO 3 powder was mechanochemically synthesized from the constituent powder mixture in 12 min. The SSA significantly decreased with an increase in milling time. The mechanical treatment at the early stage of milling resulted particle bonding or aggregation of the powder mixture. Therefore, it is concluded that the following steps proceeded: the aggregation with amorphization of the raw material in the early stage of the milling, and solid state reaction, led to the formation of BaTiO3 phase. Localized heating and pressure at regions of contact between the reactant grains may be a contributing factor for the phase formation in the activated matrix [30]. Our results indicated considerable activation took place in the powder mixture. The phase formation among nanosized grains occurred in the present mechanical milling without any media balls, because the BaTiO3 phase was not realized in 12 min when the larger
Fig. 3. Thermogravimetry analysis of the synthesized BaTiO3.
S. Ohara et al. / Materials Letters 62 (2008) 2957–2959
TiO2 (70 nm) was mixed with the same BaCO3 powder, emphasizing the importance of TiO2 size in the present mechanochemical reaction. Fig. 2 shows the SEM image of the BaTiO3 particles synthesized by the mechanochemical milling for 12 min. The sizes of the observed particles were mostly of several tens of nanometers. These values are in good agreement with those calculated from XRD peaks. The mean particles size calculated from the SSA was about 100 nm. Fig. 3 shows the result of the TG analysis of the synthesized BaTiO3 nanoparticles. The weight loss from 300 to 500 °C is due to the removal of hydroxyl defects, whereas the weight loss at higher temperature is the removal of BaCO3. On the other hand, the weight loss around 800 °C may be considered to be CO2 which is due to the presence of residual BaCO3 [32], because very small amount of BaCO3 cannot be detected by XRD peaks. It can be assumed that the hydroxide ions incorporated into the BaTiO3 lattice were expelled in the temperature range of below 500 °C. It was measured that the amount of hydroxide ions incorporated into the BaTiO3 nanoparticles was less than 0.3 wt.%. The hydroxyl defect concentration of the commercial hydrothermal BaTiO3 particles has been reported to be 0.75 wt.% and more [31]. Therefore, it is concluded that the mechanochemical process is suitable method to prevent the presence of the hydroxide ions in the oxygen lattice of BaTiO3 nanoparticles.
4. Conclusions Fine BaTiO3 nanoparticles were mechanochemically synthesized from the constituent powder mixture in short times. The particle size of the obtained BaTiO3 was about several 10 nm. The amount of hydroxide ions incorporated into the BaTiO3 nanoparticles was less than 0.3 wt.%. We believe the BaTiO3 nanoparticles prepared by mechanochemical synthesis could be used as the starting materials of the miniaturization of advanced MLCCs. Acknowledgment This work was partly supported by Grant-in-Aid for Cooperative Research Project of Nationwide Joint-Use Research Institute on Development Base of Joining Technology for New Metallic Glasses and Inorganic Materials from The Ministry of Education, Culture, Sport, Science and Technology, Japan. References [1] P.P. Phule, S.H. Risbud, J. Mater. Sci. 25 (1990) 1169–1183. [2] C.T. Xia, E.W. Shi, W.E. Zhong, J.K. Guo, J. Eur. Ceram. Soc. 15 (1995) 1171–1176.
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[3] M. Wu, R. Xu, S.H. Feng, L. Li, D. Chen, Y.J. Luo, J. Mater. Sci. 31 (1996) 6201–6205. [4] J.O. Eckert Jr., C.C. Hung-Houston, B.L. Gersten, M.M. Lencka, R.E. Riman, J. Am. Ceram. Soc. 79 (1996) 2929–2939. [5] E.B. Slamovich, I.A. Aksay, J. Am. Ceram. Soc. 79 (1996) 239–247. [6] L. Zhao, A.T. Chien, F.F. Lapse, J.S. Speck, J. Mater. Res. 11 (1996) 1325–1328. [7] T. Noma, S. Wada, M. Yano, T. Suzuki, J. Appl. Phys. 80 (1996) 5223–5233. [8] R. Asiaie, W. Zhu, S.A. Akbar, P.K. Dutta, Chem. Mater. 8 (1996) 226–234. [9] I.J. Clark, T. Takeuchi, N. Ohtori, D.C. Sinclair, J. Mater. Chem. 9 (1999) 83–91. [10] E.W. Shi, C.T. Xia, W.E. Zhong, B.G. Wang, C.D. Feng, J. Am. Ceram. Soc. 80 (1997) 1567–1572. [11] M.H. Frey, D.A. Payne, Phys. Rev. B 54 (1996) 3158–3168. [12] H. Shimooka, M. Kuwabara, J. Am. Ceram. Soc. 79 (1996) 2983–2985. [13] T. Takeuchi, M. Tabuchi, K. Ado, K. Honjo, O. Nakamura, H. Kageyama, Y. Suyama, N. Ohtori, M. Nagasawa, J. Mater. Sci. 32 (1997) 4053–4060. [14] H.I. Hsiang, F.S. Yen, J. Am. Ceram. Soc. 79 (1996) 1053–1060. [15] Y. Ma, E. Vileno, S. Suib, P.K. Dutta, Chem. Mater. 9 (1997) 3023–3031. [16] C. Beck, W. Hartl, R. Hempelmann, J. Mater. Res. 13 (1998) 3174–3180. [17] J. Wang, J. Fang, S.C. Ng, l.M. Gan, C.H. Chew, X. Wang, Z. Shen, J. Am. Ceram. Soc. 82 (1999) 873–881. [18] W.S. Cho, J. Phys. Chem. Solid 59 (1998) 659–666. [19] S.W. Lu, B.I. Lee, Z.L. Wang, W.D., J. Cryst. Growth 219 (2000) 269–276. [20] D.F.K. Hennings, C. Metzmacher, B.S. Schreinemacher, J. Am. Ceram. Soc. 84 (2001) 179–182. [21] B.D. Stojanovic, J. Mater. Process. Technol. 143/144 (2003) 78–81. [22] T. Tsuzuki, P.G. McCormick, J. Mater. Sci. 39 (2004) 5143–5146. [23] C.C. Koch, Mater. Sci. Technol. 15 (1991) 193–245. [24] J. Xue, J. Wang, D. Wan, J. Am. Ceram Soc. 83 (2000) 232–234. [25] C. Gomez-Yanez, C. Benitez, H. Balmori-Ramirez, Ceram. Int. 26 (2000) 271–277. [26] H.A.M. van Hal, W.A. Groen, S. Maassen, W.C. Keur, J. Eur. Ceram. Soc. 21 (2001) 1689–1692. [27] V. Berbenni, A. Marini, G. Bruni, Thermochim. Acta 374 (2001) 151–158. [28] K. Sato, J. Chaichanawong, H. Abe, M. Naito, Mater. Lett. 60 (2006) 1399–1402. [29] S. Ohara, H. Abe, K. Sato, A. Kondo, M. Naito, J. Eur, Ceram. Soc. (in press). [30] M.L. Trudeau, R. Schulz, R. Dussault, A.V. Neste, Phys. Rev. Lett. 65 (1990) 99–102. [31] T.J. Yosenick, D.V. Miller, R. Kumar, J.A. Nelson, C.A. Randall, J.H. Adair, J. Mater. Res. 20 (2005) 837–843. [32] D. Hennings, S. Schreinemacher, J. Eur. Ceram. Soc. 9 (1992) 41–46.