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3)O3 ceramics produced by the reaction-sintering process
Materials Letters 58 (2004) 944 – 947 www.elsevier.com/locate/matlet
Effect of heating rate on properties of Pb(Mg1/3Nb2/3)O3 ceramics produced by the reaction-sintering process Yi-Cheng Liou * Department of Electronic Engineering, Kun-Shan University of Technology, Tainan Hsien, Taiwan, ROC Received 6 January 2003; received in revised form 21 July 2003; accepted 24 July 2003
Abstract Effects of heating rate on properties of stoichiometric Pb(Mg1/3Nb2/3)O3 (PMN) perovskite ceramics produced by the reaction-sintering process are investigated. Without calcination, a mixture of PbO, Nb2O5 and Mg(NO3)2 was pressed and heated to 1270 jC directly at various rates. Stoichiometric PMN ceramics of 100% perovskite phase are obtained at 5, 10, 20 and 30 jC/min heating rates. Density increases as heating rate increased from 5 to 10 jC/min and reaches a maximum 8.06 g/cm3 at 10 jC/min. Grains 5.4 – 7.5 Am in size are obtained at heating rates from 5 to 30 jC/min. Kmax increases as the rate increases from 5 to 10 jC/min and decreases at higher rates. D 2003 Elsevier B.V. All rights reserved. PACS: 77.22.-d; 77.84.Dy; 81.20.Ev; 81.40.Tv; 84.32.Tt Keywords: PMN; Reaction-sintering; Multilayer capacitor
1. Introduction Lead magnesium niobate, Pb(Mg1/3Nb2/3)O3 (PMN), has been widely investigated in the area of electronic ceramics because of its high dielectric constant and excellent voltage stability [1 – 5]. The main problem in producing PMN ceramics has been the formation of unwanted pyrochlore phase. Cubic pyrochlore Pb3Nb4O13 phase always formed in perovskite PMN ceramics produced by the conventional mixed oxide method. In order to avoid the formation of pyrochlore, Swartz and Shrout [6] proposed a columbite route. It consists of two calcination steps: Columbite is formed first, followed by the formation of perovskite. Two calcination and pulverization stages were carried out before sintering the PMN ceramics. Liou et al. [7] proposed an effective and simplified method to produce pyrochlore-free PMN ceramics with dielectric constant higher than 17,000 under 1 kHz. A mixture of MgNb2O6 and PbO was pressed to pellets and sintered into PMN ceramics. The second calcination and pulverization stages in columbite route were skipped in the simplified columbite route. Han and Kim [8] proposed a method to prepare PMN powder with >99% perovskite phase by adding an aqueous Mg(NO3)2 solution
* Fax: +886-6-2731863. E-mail address: [email protected] (Y.-C. Liou). 0167-577X/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2003.07.041
rather than MgO to the alcoholic slurry of PbO and Nb2O5, followed by calcination at 950 jC for 2 h. Liou et al. [9,10] reported a reaction-sintering process to produce PMN and PFN ceramics. Without calcination, a mixture of PbO, Nb2O5 and Mg(NO3)2 (Fe(NO3)3) was pressed and sintered directly. Lejeune and Boilot [11] showed that the percentage of PMN phase decreased from 79% to 49% as the heating rate increased from 5 to 170 jC/min. Wu and Liou [12] reported that increasing heating rate resulted in high perovskite content, large grain size and dense PMN ceramics by columbite route. The maximum dielectric constant also increased with increasing heating rate. Liou and Wu [5] reported that increasing heating rate resulted in high perovskite content and dense 0.9PMN – 0.1PT ceramics. The maximum dielectric constant was optimized with a heating rate of 10 jC/min. In this study, the effects of heating rate on properties of stoichiometric Pb(Mg1/3Nb2/3)O3 perovskite ceramics produced by the reaction-sintering process are investigated.
2. Experimental procedure Stoichiometric Pb(Mg1/3Nb2/3)O3 ceramics were prepared from reagent-grade oxides: PbO (>99%, E. Merck, Darmstadt, Germany), Mg(NO3)26H2O (>99%, Showa
Y.-C. Liou / Materials Letters 58 (2004) 944–947
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Fig. 3. Variation of density with heating rate for PMN ceramics produced by the reaction-sintering process. Fig. 1. XRD profile of PMN ceramic sintered at 1270 jC/2 h with a heating rate of 10 jC/min.
Chem., Japan) and Nb2O5 (99.9%, E. Merck). Appropriate amounts of PbO, Mg(NO3)2 and Nb2O5 for stoichiometric PMN were milled in acetone with alumina balls for 22 h. After the slurry was dried, the powder was pressed to pellets 12 mm in diameter and 2 mm thick. The pellets were then heated at various rates: 5, 10, 20 and 30– 1270 jC for 2 h sintering in air atmosphere. After sintering, the pellets were cooled with a rate 10 jC/min. The density of sintered PMN pellets was measured by water immersion method. Microstructures were analyzed by
scanning electron microscopy (SEM). The sintered PMN ceramics were analyzed by X-ray diffraction (XRD) at 2h between 25j and 35j. The relative amounts of perovskite and pyrochlore phases were determined from XRD patterns of the samples by measuring the major peak intensities for the perovskite (110) and pyrochlore (222) phases. The following qualitative equation was used. %perovskite ¼ 100 Iperov: =ðIperov: þ Ipyro: Þ After polishing, the dimensions were measured before silver electrodes were formed on the pellets. Dielectric
Fig. 2. XRD profiles of PMN ceramics sintered at 1270 jC/2 h with heating rates from 5 to 30 jC/min.
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Y.-C. Liou / Materials Letters 58 (2004) 944–947
Table 1 The weight loss and average grain sizes of PMN ceramics sintered at 1270 jC/2 h with various heating rates Heating rate, jC/min
Weight loss, % Grain size, Am
5
10
20
30
22.9 5.4
19.0 7.5
20.1 7.2
21.7 5.8
properties were measured with an HP4192A impedance analyzer in a temperature-controlled chamber from 30 to 50 jC.
3. Results and discussion The XRD profile of PMN ceramic sintered at 1270 jC for 2 h with a heating rate 10 jC/min is shown in Fig. 1. The
major peak (222) of Pb 3Nb4O 13 pyrochlore phase at 2h = 29.2j is not found in the pattern. PMN ceramics sintered with the rates 5– 30 jC/min are shown in Fig. 2. These results indicate that the reaction-sintering process is simple and effective in producing pyrochlore-free PMN ceramics at various heating rates. Fig. 3 shows the variation of density with heating rate for PMN ceramics produced by the reaction-sintering process. Density increases as heating rate increased from 5 to 10 jC/min and reaches a maximum of 8.06 g/cm3 at 10 jC/min. It decreases at heating rates 20 and 30 jC/min. This is different from PMN ceramics by columbite route [12]. In that study, the density of PMN ceramic increases with increased heating rate due to the decreased evaporation of PbO. Weight loss data are listed in Table 1. These data include the 18.13% loss of nitrate and water in Mg(NO3)26H2O. As in the reaction-sintering process, pellets of mixture of PbO, Nb2O5 and Mg(NO3)2
Fig. 4. SEM photographs of the as-fired PMN ceramics after 1270 jC/2 h sintering at (A) 5 jC/min, (B) 10 jC/min, (C) 20 jC/min and (D) 30 jC/min.
Y.-C. Liou / Materials Letters 58 (2004) 944–947 Table 2 Maximum dielectric constant (Kmax) and tan d under 1 kHz of PMN ceramics sintered at 1270 jC/2 h for various heating rates Heating rate, jC/min
Kmax tan d
5
10
20
30
14,800 ( 12 jC) 0.033
15,300 ( 11 jC) 0.041
14,100 ( 12 jC) 0.046
13,700 ( 12 jC) 0.037
were pressed and heated to 1270 jC directly. The calcination reaction was carried out during the heating up period. In the study of Han and Kim [8], the powder calcined at 950 jC/10 min showed 96.4% perovskite phase. At heating rates of 5 and 10 jC/min, there are 74 and 37 min for the calcination reaction under 900 – 1270 jC. Therefore, the mixture was calcined completely during the heating up period. While at heating rates of 20 and 30 jC/min, the mixture was not calcined completely during the heating up period. At these heating rates, there are only 18.5 and 12.3 min for the calcination reaction under 900– 1270 jC. This resulted in further calcination and evaporation of PbO at the fore period of 1270 jC. Therefore, the period for sintering at 1270 jC is less than 2 h. The density of PMN ceramic is 7.75 g/cm3 after sintering at 1270 jC for 1 h, with a rate of 10 jC/min by the reaction-sintering process [9]. Therefore, the density values of 7.96 and 7.85 g/cm3 for PMN sintered at heating rates of 20 and 30 jC/min are acceptable. As compared with PMN produced by columbite route [12] and simplified columbite route [7], the density of PMN by the reaction-sintering process reaches 99% of the theoretical density. While in the other two studies, it is about 96% and 93% of the theoretical density for columbite route and simplified columbite route, respectively. SEM photographs of the as-fired PMN ceramics after 1270 jC/2 h sintering at various heating rates are illustrated in Fig. 4. No pyrochlore phase is found in these photos. The average grain sizes of PMN ceramics after 1270 jC/2 h sintering at various heating rates are also listed in Table 1. Grains 5.4 – 7.5 Am in size are obtained by the reactionsintering process at heating rates from 5 to 30 jC/min after 1270 jC/2 h sintering. As compared with PMN produced by simplified columbite route [7], 7.1 Am grain size was obtained at 1250 jC/2 h sintering with a heating rate of 10 jC/min. For PMN by the reaction-sintering process, 7.5 Am grain size was obtained at 1270 jC/2 h sintering with a heating rate of 10 jC/min. A higher sintering temperature is necessary in the reaction-sintering process than in simplified columbite route for PMN ceramics of the same grain size. No calcination stage before sintering is the major reason for this result. The grain size of PMN ceramic is 4.9 Am after sintering with a rate of 10 jC/min at 1270 jC for 1 h by the
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reaction-sintering process [9]. Therefore, the grain sizes of 7.2 and 5.8 Am for PMN sintered at heating rates of 20 and 30 jC/min are acceptable, because the period for sintering at 1270 jC is less than 2 h at these heating rates as discussed in the former paragraph. Maximum dielectric constant (Kmax) and tan d under 1 kHz for PMN ceramics after 1270 jC/2 h sintering at various heating rates are listed in Table 2. Kmax increases as the rate increases from 5 to 10 jC/min, and decreases at higher rates. As all pellets are 100% of perovskite phase, this resulted from increased density and grain size as shown in Fig. 3 and Table 1. As heating rate increases from 5 to 30 jC/min, tan d varies between 0.033 and 0.046.
4. Conclusion Stoichiometric PMN ceramics of 100% perovskite phase were obtained after 1270 jC/2 h of sintering at heating rates of 5, 10, 20 and 30 jC/min. Density increases as heating rate increased from 5 to 10 jC/min and reaches a maximum 8.06 g/cm3 at 10 jC/min. It decreases at heating rates of 20 and 30 jC/min. Grains 5.4– 7.5 Am in size are obtained by the reaction-sintering process at heating rates from 5 to 30 jC/min. Kmax increases as the rate increases from 5 to 10 jC/min, and decreases at higher rates.
Acknowledgements The author is grateful to Mr. Ko-Hao Tseng for his help in preparing the samples and Miss Shi-Yuea Hsu for her help in obtaining the SEM photos.
References [1] L.E. Cross, S.J. Jang, R.E. Newnham, K. Uchino, Ferroelectrics 23 (1980) 187. [2] J.P. Guha, H.U. Anderson, J. Am. Ceram. Soc. 69 (11) (1986) c-287. [3] F. Chaput, J.P. Boilot, M. Lejuene, R. Papiernik, L.G. Hubert-Pfalzgraf, J. Am. Ceram. Soc. 72 (8) (1989) 1355. [4] S.M. Landin, W.A. Schulze, J. Am. Ceram. Soc. 73 (4) (1990) 909. [5] Y.C. Liou, L. Wu, J. Am. Ceram. Soc. 77 (12) (1994) 3255. [6] S.L. Swartz, T.R. Shrout, Mater. Res. Bull. 17 (1982) 1245. [7] Y.C. Liou, L. Wu, S.S. Liou, Jpn. J. Appl. Phys. 33 (Part 2 (9B)) (1994) L1320. [8] K.R. Han, S. Kim, J. Mater. Sci. 35 (2000) 2055. [9] J.H. Chen, Y.C. Liou, K.H. Tseng, Jpn. J. Appl. Phys. 42 (1A) (2003) 175. [10] Y.C. Liou, C.Y. Shih, C.H. Yu, Mater. Lett. 57 (2003) 1977. [11] M. Lejeune, J.P. Boilot, Ceram. Int. 8 (3) (1982) 99. [12] L. Wu, Y.C. Liou, Ceram. Int. 21 (1995) 335.
Effect of heating rate on properties of Pb(Mg1/3Nb2/3)O3 ceramics produced by the reaction-sintering process Yi-Cheng Liou * Department of Electronic Engineering, Kun-Shan University of Technology, Tainan Hsien, Taiwan, ROC Received 6 January 2003; received in revised form 21 July 2003; accepted 24 July 2003
Abstract Effects of heating rate on properties of stoichiometric Pb(Mg1/3Nb2/3)O3 (PMN) perovskite ceramics produced by the reaction-sintering process are investigated. Without calcination, a mixture of PbO, Nb2O5 and Mg(NO3)2 was pressed and heated to 1270 jC directly at various rates. Stoichiometric PMN ceramics of 100% perovskite phase are obtained at 5, 10, 20 and 30 jC/min heating rates. Density increases as heating rate increased from 5 to 10 jC/min and reaches a maximum 8.06 g/cm3 at 10 jC/min. Grains 5.4 – 7.5 Am in size are obtained at heating rates from 5 to 30 jC/min. Kmax increases as the rate increases from 5 to 10 jC/min and decreases at higher rates. D 2003 Elsevier B.V. All rights reserved. PACS: 77.22.-d; 77.84.Dy; 81.20.Ev; 81.40.Tv; 84.32.Tt Keywords: PMN; Reaction-sintering; Multilayer capacitor
1. Introduction Lead magnesium niobate, Pb(Mg1/3Nb2/3)O3 (PMN), has been widely investigated in the area of electronic ceramics because of its high dielectric constant and excellent voltage stability [1 – 5]. The main problem in producing PMN ceramics has been the formation of unwanted pyrochlore phase. Cubic pyrochlore Pb3Nb4O13 phase always formed in perovskite PMN ceramics produced by the conventional mixed oxide method. In order to avoid the formation of pyrochlore, Swartz and Shrout [6] proposed a columbite route. It consists of two calcination steps: Columbite is formed first, followed by the formation of perovskite. Two calcination and pulverization stages were carried out before sintering the PMN ceramics. Liou et al. [7] proposed an effective and simplified method to produce pyrochlore-free PMN ceramics with dielectric constant higher than 17,000 under 1 kHz. A mixture of MgNb2O6 and PbO was pressed to pellets and sintered into PMN ceramics. The second calcination and pulverization stages in columbite route were skipped in the simplified columbite route. Han and Kim [8] proposed a method to prepare PMN powder with >99% perovskite phase by adding an aqueous Mg(NO3)2 solution
* Fax: +886-6-2731863. E-mail address: [email protected] (Y.-C. Liou). 0167-577X/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2003.07.041
rather than MgO to the alcoholic slurry of PbO and Nb2O5, followed by calcination at 950 jC for 2 h. Liou et al. [9,10] reported a reaction-sintering process to produce PMN and PFN ceramics. Without calcination, a mixture of PbO, Nb2O5 and Mg(NO3)2 (Fe(NO3)3) was pressed and sintered directly. Lejeune and Boilot [11] showed that the percentage of PMN phase decreased from 79% to 49% as the heating rate increased from 5 to 170 jC/min. Wu and Liou [12] reported that increasing heating rate resulted in high perovskite content, large grain size and dense PMN ceramics by columbite route. The maximum dielectric constant also increased with increasing heating rate. Liou and Wu [5] reported that increasing heating rate resulted in high perovskite content and dense 0.9PMN – 0.1PT ceramics. The maximum dielectric constant was optimized with a heating rate of 10 jC/min. In this study, the effects of heating rate on properties of stoichiometric Pb(Mg1/3Nb2/3)O3 perovskite ceramics produced by the reaction-sintering process are investigated.
2. Experimental procedure Stoichiometric Pb(Mg1/3Nb2/3)O3 ceramics were prepared from reagent-grade oxides: PbO (>99%, E. Merck, Darmstadt, Germany), Mg(NO3)26H2O (>99%, Showa
Y.-C. Liou / Materials Letters 58 (2004) 944–947
945
Fig. 3. Variation of density with heating rate for PMN ceramics produced by the reaction-sintering process. Fig. 1. XRD profile of PMN ceramic sintered at 1270 jC/2 h with a heating rate of 10 jC/min.
Chem., Japan) and Nb2O5 (99.9%, E. Merck). Appropriate amounts of PbO, Mg(NO3)2 and Nb2O5 for stoichiometric PMN were milled in acetone with alumina balls for 22 h. After the slurry was dried, the powder was pressed to pellets 12 mm in diameter and 2 mm thick. The pellets were then heated at various rates: 5, 10, 20 and 30– 1270 jC for 2 h sintering in air atmosphere. After sintering, the pellets were cooled with a rate 10 jC/min. The density of sintered PMN pellets was measured by water immersion method. Microstructures were analyzed by
scanning electron microscopy (SEM). The sintered PMN ceramics were analyzed by X-ray diffraction (XRD) at 2h between 25j and 35j. The relative amounts of perovskite and pyrochlore phases were determined from XRD patterns of the samples by measuring the major peak intensities for the perovskite (110) and pyrochlore (222) phases. The following qualitative equation was used. %perovskite ¼ 100 Iperov: =ðIperov: þ Ipyro: Þ After polishing, the dimensions were measured before silver electrodes were formed on the pellets. Dielectric
Fig. 2. XRD profiles of PMN ceramics sintered at 1270 jC/2 h with heating rates from 5 to 30 jC/min.
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Y.-C. Liou / Materials Letters 58 (2004) 944–947
Table 1 The weight loss and average grain sizes of PMN ceramics sintered at 1270 jC/2 h with various heating rates Heating rate, jC/min
Weight loss, % Grain size, Am
5
10
20
30
22.9 5.4
19.0 7.5
20.1 7.2
21.7 5.8
properties were measured with an HP4192A impedance analyzer in a temperature-controlled chamber from 30 to 50 jC.
3. Results and discussion The XRD profile of PMN ceramic sintered at 1270 jC for 2 h with a heating rate 10 jC/min is shown in Fig. 1. The
major peak (222) of Pb 3Nb4O 13 pyrochlore phase at 2h = 29.2j is not found in the pattern. PMN ceramics sintered with the rates 5– 30 jC/min are shown in Fig. 2. These results indicate that the reaction-sintering process is simple and effective in producing pyrochlore-free PMN ceramics at various heating rates. Fig. 3 shows the variation of density with heating rate for PMN ceramics produced by the reaction-sintering process. Density increases as heating rate increased from 5 to 10 jC/min and reaches a maximum of 8.06 g/cm3 at 10 jC/min. It decreases at heating rates 20 and 30 jC/min. This is different from PMN ceramics by columbite route [12]. In that study, the density of PMN ceramic increases with increased heating rate due to the decreased evaporation of PbO. Weight loss data are listed in Table 1. These data include the 18.13% loss of nitrate and water in Mg(NO3)26H2O. As in the reaction-sintering process, pellets of mixture of PbO, Nb2O5 and Mg(NO3)2
Fig. 4. SEM photographs of the as-fired PMN ceramics after 1270 jC/2 h sintering at (A) 5 jC/min, (B) 10 jC/min, (C) 20 jC/min and (D) 30 jC/min.
Y.-C. Liou / Materials Letters 58 (2004) 944–947 Table 2 Maximum dielectric constant (Kmax) and tan d under 1 kHz of PMN ceramics sintered at 1270 jC/2 h for various heating rates Heating rate, jC/min
Kmax tan d
5
10
20
30
14,800 ( 12 jC) 0.033
15,300 ( 11 jC) 0.041
14,100 ( 12 jC) 0.046
13,700 ( 12 jC) 0.037
were pressed and heated to 1270 jC directly. The calcination reaction was carried out during the heating up period. In the study of Han and Kim [8], the powder calcined at 950 jC/10 min showed 96.4% perovskite phase. At heating rates of 5 and 10 jC/min, there are 74 and 37 min for the calcination reaction under 900 – 1270 jC. Therefore, the mixture was calcined completely during the heating up period. While at heating rates of 20 and 30 jC/min, the mixture was not calcined completely during the heating up period. At these heating rates, there are only 18.5 and 12.3 min for the calcination reaction under 900– 1270 jC. This resulted in further calcination and evaporation of PbO at the fore period of 1270 jC. Therefore, the period for sintering at 1270 jC is less than 2 h. The density of PMN ceramic is 7.75 g/cm3 after sintering at 1270 jC for 1 h, with a rate of 10 jC/min by the reaction-sintering process [9]. Therefore, the density values of 7.96 and 7.85 g/cm3 for PMN sintered at heating rates of 20 and 30 jC/min are acceptable. As compared with PMN produced by columbite route [12] and simplified columbite route [7], the density of PMN by the reaction-sintering process reaches 99% of the theoretical density. While in the other two studies, it is about 96% and 93% of the theoretical density for columbite route and simplified columbite route, respectively. SEM photographs of the as-fired PMN ceramics after 1270 jC/2 h sintering at various heating rates are illustrated in Fig. 4. No pyrochlore phase is found in these photos. The average grain sizes of PMN ceramics after 1270 jC/2 h sintering at various heating rates are also listed in Table 1. Grains 5.4 – 7.5 Am in size are obtained by the reactionsintering process at heating rates from 5 to 30 jC/min after 1270 jC/2 h sintering. As compared with PMN produced by simplified columbite route [7], 7.1 Am grain size was obtained at 1250 jC/2 h sintering with a heating rate of 10 jC/min. For PMN by the reaction-sintering process, 7.5 Am grain size was obtained at 1270 jC/2 h sintering with a heating rate of 10 jC/min. A higher sintering temperature is necessary in the reaction-sintering process than in simplified columbite route for PMN ceramics of the same grain size. No calcination stage before sintering is the major reason for this result. The grain size of PMN ceramic is 4.9 Am after sintering with a rate of 10 jC/min at 1270 jC for 1 h by the
947
reaction-sintering process [9]. Therefore, the grain sizes of 7.2 and 5.8 Am for PMN sintered at heating rates of 20 and 30 jC/min are acceptable, because the period for sintering at 1270 jC is less than 2 h at these heating rates as discussed in the former paragraph. Maximum dielectric constant (Kmax) and tan d under 1 kHz for PMN ceramics after 1270 jC/2 h sintering at various heating rates are listed in Table 2. Kmax increases as the rate increases from 5 to 10 jC/min, and decreases at higher rates. As all pellets are 100% of perovskite phase, this resulted from increased density and grain size as shown in Fig. 3 and Table 1. As heating rate increases from 5 to 30 jC/min, tan d varies between 0.033 and 0.046.
4. Conclusion Stoichiometric PMN ceramics of 100% perovskite phase were obtained after 1270 jC/2 h of sintering at heating rates of 5, 10, 20 and 30 jC/min. Density increases as heating rate increased from 5 to 10 jC/min and reaches a maximum 8.06 g/cm3 at 10 jC/min. It decreases at heating rates of 20 and 30 jC/min. Grains 5.4– 7.5 Am in size are obtained by the reaction-sintering process at heating rates from 5 to 30 jC/min. Kmax increases as the rate increases from 5 to 10 jC/min, and decreases at higher rates.
Acknowledgements The author is grateful to Mr. Ko-Hao Tseng for his help in preparing the samples and Miss Shi-Yuea Hsu for her help in obtaining the SEM photos.
References [1] L.E. Cross, S.J. Jang, R.E. Newnham, K. Uchino, Ferroelectrics 23 (1980) 187. [2] J.P. Guha, H.U. Anderson, J. Am. Ceram. Soc. 69 (11) (1986) c-287. [3] F. Chaput, J.P. Boilot, M. Lejuene, R. Papiernik, L.G. Hubert-Pfalzgraf, J. Am. Ceram. Soc. 72 (8) (1989) 1355. [4] S.M. Landin, W.A. Schulze, J. Am. Ceram. Soc. 73 (4) (1990) 909. [5] Y.C. Liou, L. Wu, J. Am. Ceram. Soc. 77 (12) (1994) 3255. [6] S.L. Swartz, T.R. Shrout, Mater. Res. Bull. 17 (1982) 1245. [7] Y.C. Liou, L. Wu, S.S. Liou, Jpn. J. Appl. Phys. 33 (Part 2 (9B)) (1994) L1320. [8] K.R. Han, S. Kim, J. Mater. Sci. 35 (2000) 2055. [9] J.H. Chen, Y.C. Liou, K.H. Tseng, Jpn. J. Appl. Phys. 42 (1A) (2003) 175. [10] Y.C. Liou, C.Y. Shih, C.H. Yu, Mater. Lett. 57 (2003) 1977. [11] M. Lejeune, J.P. Boilot, Ceram. Int. 8 (3) (1982) 99. [12] L. Wu, Y.C. Liou, Ceram. Int. 21 (1995) 335.