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A novel low temperature sintering process for PMnN-PZT ceramics
Materials Chemistry and Physics 99 (2006) 26–29
A novel low temperature sintering process for PMnN-PZT ceramics Chih-Yen Chen ∗ , Yi Hu, Hur-Lon Lin Department of Materials Engineering, Tatung University, Taipei 104, Taiwan, ROC Received 19 April 2005; received in revised form 22 July 2005; accepted 5 September 2005
Abstract Samples with the composition of 0.07Pb(Mn1/3 Nb2/3 )O3 –0.468PbZrO3 –0.462PbTiO3 was prepared in this study. A novel process was adapted to lower the sintering temperature without properties deterioration. The sintering process includes a vacuumed heating stage, a air-venting heating stage, and a final sintering at 1050 ◦ C with certain holding time without PbO compensating atmosphere. The influence of holding time at the final sintering stage on the characteristics of the ceramics was studied. The air-venting stage caused a large shrinkage (about 10%) and then slowing down the shrinking rate when the sample was heated up from 800 to 1050 ◦ C. The relationships between the crystalline characteristics and physical properties, such as microstructure, the mechanical quality factor (Qm ), and electromechanical coupling factor (kp ), are discussed in this paper. © 2006 Published by Elsevier B.V. Keywords: PMnN-PZT; Sintering; Air-venting
1. Introduction Since the lead zirconate–lead titanate system, Pb(Zrx – Ti1−x )O3 (PZT), has excellent piezoelectric properties and high dielectric properties, it has emerged as one of the most widely studied and developed ferroelectric oxides [1–4]. PZT is normally sintered at temperatures between 1200 and 1300 ◦ C to obtain complete densification. It is essential to control the PbO vapor pressure to avoid PbO weight loss and a change in composition [5–7]. Most investigations use the composition with excess PbO and PbO evaporation atmosphere to compensate the loss of PbO during firing. However, the excess PbO would form a liquidphase to enhance densification rate during the initial and intermediate stages of sintering and thus lower the final density [8,9]. A lot of efforts, therefore, have been done in the past to lower the sintering temperature of PZT while relating good piezoelectric properties. Among these investigations, liquid-phase sintering with additives and using ultra-fine powders has been applied [10–12]. Additives that result in a liquid-phase formation, at temperature well below the traditional sintering temperatures, make it possible to achieve a dense material at reduced temperatures by liquid-phase sintering. However, these additives would make segregation at grain boundary and deteriorate the properties of the samples. ∗
Corresponding author. E-mail address: [email protected] (C.-Y. Chen).
0254-0584/$ – see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.matchemphys.2005.09.009
In this study, a novel process was adapted to lower the sintering temperature without properties deterioration. The sintering process includes a vacuumed heating stage, a air-venting heating stage, and a final sintering with different holding time without PbO compensating atmosphere. In this work, sample with the composition of 0.07Pb(Mn1/3 Nb2/3 )O3 –0.468PbZrO3 – 0.462PbTiO3 was used according to our previous study [13]. 2. Experimental procedures Samples with the composition of 0.07Pb(Mn1/3 Nb2/3 )O3 –0.468PbZrO3 – 0.462PbTiO3 were prepared in this study. This formula is abbreviated as PMnNPZT. All chemicals used to prepare the samples are of reagent grade. First, columbite phase of manganese niobate (MnNb2 O6 ) was prepared by calcining the mixture of MnO and Nb3 O5 at 1100 ◦ C for 2 h. Then the columbite phase was pulverized and wet-mixed with the predetermined amounts of PbO, TiO2 and ZrO2 in a plastic can with YSZ balls. The mixtures were dried and calcined at 850 ◦ C for 3 h. In order to compensate the evaporation of PbO during sintering, 2 wt% of excessive PbO was added to the composition. The powders were pressed into pellets with 17 mm in diameter and 1.5 of thickness under 1250 MPa. The pellets were then sintered under a vacuum/air furnace system. The samples were heated in vacuum from room temperature to the intermediate 800 ◦ C with 10 ◦ C min−1 heating rate and under 1.3 × 10−2 mbar pressure. When the intermediate temperature was reached, air with ambient pressure was introduced into the furnace and the samples were continuously heated to the final sintering temperature of 1050 ◦ C with 5 ◦ C min−1 . The samples were held at 1050 ◦ C for various soaking time and then were cooled in the furnace. A conventional sintering process was also conducted at 1050 ◦ C for the comparison. After sintering, the surfaces of the pellets were polished and painted with silver-based glue and fired at 590 ◦ C for 12 min to make electrodes. The samples
C.-Y. Chen et al. / Materials Chemistry and Physics 99 (2006) 26–29
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Fig. 1. TMA profile of the sample as a function of the vacuumed-air-venting process. 120 ◦ C
were poled in silicone oil at for 30 min under a static electrical field of 4 kV mm−1 . The sintering process was simulated with Thermal Mechanical Analysis (SETARAM SETSYS-1750) to study the densification behavior of the samples. The crystallographic structures of the samples were examined by powder X-ray diffraction (XRD, MAC Science M18XHF) with Cu K␣ irradiation. The density of the sintered samples was measured with Archimedes’ method and the relative density was obtained by dividing the measured density with theoretical density (8.0 g cm−3 ). The microstructures of the fractured surfaces were examined by scanning electron microscope (JEOL-5600 SEM). The mechanical quality factor (Qm ) and electromechanical coupling factor (kp ) of the samples were measured using HP 4194A impedance analyzer.
3. Results and discussion Fig. 1 shows the TMA profile of the sample under the programmed sintering process. It was found that the sample started to shrink when the air was being introduced into the chamber at 800 ◦ C. The displacement of the sample was large initially at the air-venting stage under heating from 800 to 1050 ◦ C. The displacement of the sample then slowed down when the temperature was held at 1050 ◦ C. The large displacement at the air-venting stage may be explained by the effect of air pressure difference. The air-venting sintering process would cause a large air pressure difference between the external and interior sites of the sample, which has been vacuumed. About 10% shrinkage of the sample was obtained at the air-venting stage. Fig. 2 shows the XRD patterns of the samples sintered at 1050 ◦ C for various holding time. It was found that the predominated phase of the sample was rhombohedral and perovskite phase. This was evidenced form the split of the reflection peaks (1 0 0), (2 0 0) and (2 1 0) as seen in the XRD patterns [14]. The relative amount of rhombohedral phase decreased with the increase of the heating duration. The variation of relative amount of each phase as a function of sintering duration could be explained by the homogenization of the composition with the enhancement of diffusion process.
Fig. 2. XRD of the samples sintered at 1050 ◦ C for: (a) 0.5, (b) 1, (c) 2 and (d) 3 h under the vacuumed-air-venting process.
Fig. 3 shows the relative density of the samples heated at 1050 ◦ C versus the holding time. All samples show relative densities higher than 96%. Slight difference in the relative density was observed for these samples. Maximum relative density was observed for the sample at the heating duration of 1 h. Then the relative density of the sample decreased with higher heating duration. This was thought from the formation of the pore agglomeration and closing or the evaporation of PbO. This sintering process shows to obtain the samples with high relative density in a short sintering time (97.4% for 1 h) whereas the relative density of sample can only be 93.8% sintered at 1050 ◦ C for 3 h with conventional sintering process (in Fig. 3). This shows the great improvement on densification with novel sintering process.
Fig. 3. Relative density of the samples as a function of holding time at 1050 ◦ C under the vacuumed-air-venting process.
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C.-Y. Chen et al. / Materials Chemistry and Physics 99 (2006) 26–29
Fig. 4. SEM micrographs of fracture surface for the samples heated at 1050 ◦ C for: (a) 0.5, (b) 1, (c) 2 and (d) 3 h.
Fig. 4 shows the SEM microstructures of the fractures surfaces of the samples. The distributions in the grain shape and size of the samples are rather uniform. The samples with sintering duration of 0.5 and 1 h showed an intergranular fracture morphology indicating that the grain boundaries are mechanically weaker than the grains (Fig. 4(a and b)). It was also noted that the fracture model transformed from intergranular to transgranular as seen in Fig. 4(c and d) when the heating duration increased. The grain boundaries became stronger indicating that the phases at the grain boundaries changed due to segregation or precipitation, which is usually observed from liquid-phase sintering process. The sealing grain boundaries also confirm that the pore agglomeration and closing at intersect of grain boundaries with higher heating duration (Fig. 4(d)). Fig. 5 shows the average grain size of the samples versus sintering duration at 1050 ◦ C. The average grain size increased with the increase of the holding time. It was found that when grain size (d) is plotted versus holding time (t) in logarithm, a straight line is obtained. The slope of curve is about 0.5 indicating that the grain growth is controlled by boundary diffusion. This is well in agreement with the results that the densification of PZT sintering at temperature above 800 ◦ C is ascribed to the volume diffusion through grain boundary as proposed [5]. In addition, the grain size of the sample sintered with the vacuumed-air-venting process (1.21 m for 3 h) is larger than that with conventional sintering process (0.87 m for 3 h). This indicates that the novel sintering process can enhance the diffusion mechanism and, therefore, promote the densification of the sample with higher relative density. Fig. 6 shows the mechanical quality factor, Qm , and electromechanical planar coupling factor (kp ) of the samples versus
the holding time. It was found that the change of Qm factor follows the change of relative density. This can be explained by the friction loss between the domain walls. Smaller grain size with larger interfacial area would increase the friction between domains and thus has lower value of Qm factor. On the other hand, it was found that the kp factor increased with the increase of holding time. The variation of kp factor as a function of holding time could be explained by the homogenization of the composition with the enhancement of diffusion process and the increase of grain size. It has been reported that variations in Qm and kp are possibly explained by density and/or grain size [15,16]. In addition, the porosity usually has a marked effect on the dielectric constant and coupling coefficient [17]. In our work, the coupling
Fig. 5. Variation of the average grain size vs. holding times.
C.-Y. Chen et al. / Materials Chemistry and Physics 99 (2006) 26–29
29
uumed and air-venting sintering process. The air-venting stage caused a large shrinkage (about 10%) initially and the shrinking rate slowed down when the sample was heated up from 800 to 1050 ◦ C. The predominated phase of the sample was rhombohedral and perovskite phase. The average grain size increased with the increase of sintering duration at 1050 ◦ C. The kp factor increased with the increase of holding time. Acknowledgement Financial support of this research by National Science Council, Taiwan, ROC, under the grant NSC 89-2218-E-036-017 is gratefully acknowledged. References
Fig. 6. Variation of mechanical quality factor (Qm ) and electromechanical coupling factor (kp ) of the samples as a function of holding time at 1050 ◦ C under the vacuumed-air-venting process.
coefficient is not affected much with the relative density. Therefore, grain size is thought as the dominant factor for influencing coupling coefficient. From the results of the present study, dense products with high quality can be obtained by the vacuumed-air-venting sintering process at relative lower temperature without compensating PbO atmosphere. Other processing factors influencing the microstructure and properties of the samples with the vacuumedair-venting sintering process are under investigation. 4. Conclusion The ceramics with the composition of 0.07Pb(Mn1/3 Nb2/3 ) O3 –0.468PbZrO3 –0.462PbTiO3 was prepared through the vac-
[1] K.W. Tang, H.L.W. Chana, Y.M. Cheungb, P.C.K. Liu, Mater. Chem. Phys. 75 (2002) 196. [2] E. Quandt, A. Ludwig, Sens. Actuators 81 (2000) 275. [3] R. Zuo, L. Li, X. Hu, Z. Gui, Mater. Lett. 54 (2002) 185. [4] K. Nagata, J. Thongrueng, J. Korea Appl. Soc. 32 (1998) S1278. [5] R.B. Atkin, R.M. Fulrath, J. Am. Ceram. Soc. 54 (1971) 265–270. [6] G.H. Haertling, Am. Ceram. Soc. Bull. 43 (1964) 113–118. [7] B. Jaffe, Piezoelectric Ceramics, Academic Press, London, UK, 1971. [8] A.I. Kingon, J.B. Clark, J. Am. Ceram. Soc. 66 (1983) 256–260. [9] S.S. Chiang, M. Nishioka, R.M. Fulrath, J.A. Pask, Am. Ceram. Soc. Bull. 60 (1981) 484–489. [10] G. Tomandl, A. Stiegelschmitt, R. Bohner, Sci. Ceram. 14 (1988) 305–308. [11] R.P. Tandon, V. Singh, N. Narayanaswami, V.K. Hans, Ferroelectrics 196 (1997) 117–120. [12] K. Murakaimi, D. Mabuchi, D. Kurita, Y. Niwa, S. Kaneko, Jpn. J. Appl. Phys. 35 (1996) 5188–5191. [13] C.Y. Chen, Yi Hu, H.L. Lin, W.Y. Wei, Ceramics International, in press. [14] J.C. Fernandes, D.A. Hall, M.R. Cockburn, G.N. Greaves, Nuclear instruments and method, Phys. Res. B97 (1995) 137–141. [15] K. Okazaki, N. Nagata, J. Am. Ceram. Soc. 56 (1972) 82–86. [16] K. Okazaki, N. Nagata, Electron. Commun. Jpn. 53C (1970) 147– 154. [17] M.L. Dunn, M. Taya, J. Am. Ceram. Soc. 76 (1993) 1697–1706.
A novel low temperature sintering process for PMnN-PZT ceramics Chih-Yen Chen ∗ , Yi Hu, Hur-Lon Lin Department of Materials Engineering, Tatung University, Taipei 104, Taiwan, ROC Received 19 April 2005; received in revised form 22 July 2005; accepted 5 September 2005
Abstract Samples with the composition of 0.07Pb(Mn1/3 Nb2/3 )O3 –0.468PbZrO3 –0.462PbTiO3 was prepared in this study. A novel process was adapted to lower the sintering temperature without properties deterioration. The sintering process includes a vacuumed heating stage, a air-venting heating stage, and a final sintering at 1050 ◦ C with certain holding time without PbO compensating atmosphere. The influence of holding time at the final sintering stage on the characteristics of the ceramics was studied. The air-venting stage caused a large shrinkage (about 10%) and then slowing down the shrinking rate when the sample was heated up from 800 to 1050 ◦ C. The relationships between the crystalline characteristics and physical properties, such as microstructure, the mechanical quality factor (Qm ), and electromechanical coupling factor (kp ), are discussed in this paper. © 2006 Published by Elsevier B.V. Keywords: PMnN-PZT; Sintering; Air-venting
1. Introduction Since the lead zirconate–lead titanate system, Pb(Zrx – Ti1−x )O3 (PZT), has excellent piezoelectric properties and high dielectric properties, it has emerged as one of the most widely studied and developed ferroelectric oxides [1–4]. PZT is normally sintered at temperatures between 1200 and 1300 ◦ C to obtain complete densification. It is essential to control the PbO vapor pressure to avoid PbO weight loss and a change in composition [5–7]. Most investigations use the composition with excess PbO and PbO evaporation atmosphere to compensate the loss of PbO during firing. However, the excess PbO would form a liquidphase to enhance densification rate during the initial and intermediate stages of sintering and thus lower the final density [8,9]. A lot of efforts, therefore, have been done in the past to lower the sintering temperature of PZT while relating good piezoelectric properties. Among these investigations, liquid-phase sintering with additives and using ultra-fine powders has been applied [10–12]. Additives that result in a liquid-phase formation, at temperature well below the traditional sintering temperatures, make it possible to achieve a dense material at reduced temperatures by liquid-phase sintering. However, these additives would make segregation at grain boundary and deteriorate the properties of the samples. ∗
Corresponding author. E-mail address: [email protected] (C.-Y. Chen).
0254-0584/$ – see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.matchemphys.2005.09.009
In this study, a novel process was adapted to lower the sintering temperature without properties deterioration. The sintering process includes a vacuumed heating stage, a air-venting heating stage, and a final sintering with different holding time without PbO compensating atmosphere. In this work, sample with the composition of 0.07Pb(Mn1/3 Nb2/3 )O3 –0.468PbZrO3 – 0.462PbTiO3 was used according to our previous study [13]. 2. Experimental procedures Samples with the composition of 0.07Pb(Mn1/3 Nb2/3 )O3 –0.468PbZrO3 – 0.462PbTiO3 were prepared in this study. This formula is abbreviated as PMnNPZT. All chemicals used to prepare the samples are of reagent grade. First, columbite phase of manganese niobate (MnNb2 O6 ) was prepared by calcining the mixture of MnO and Nb3 O5 at 1100 ◦ C for 2 h. Then the columbite phase was pulverized and wet-mixed with the predetermined amounts of PbO, TiO2 and ZrO2 in a plastic can with YSZ balls. The mixtures were dried and calcined at 850 ◦ C for 3 h. In order to compensate the evaporation of PbO during sintering, 2 wt% of excessive PbO was added to the composition. The powders were pressed into pellets with 17 mm in diameter and 1.5 of thickness under 1250 MPa. The pellets were then sintered under a vacuum/air furnace system. The samples were heated in vacuum from room temperature to the intermediate 800 ◦ C with 10 ◦ C min−1 heating rate and under 1.3 × 10−2 mbar pressure. When the intermediate temperature was reached, air with ambient pressure was introduced into the furnace and the samples were continuously heated to the final sintering temperature of 1050 ◦ C with 5 ◦ C min−1 . The samples were held at 1050 ◦ C for various soaking time and then were cooled in the furnace. A conventional sintering process was also conducted at 1050 ◦ C for the comparison. After sintering, the surfaces of the pellets were polished and painted with silver-based glue and fired at 590 ◦ C for 12 min to make electrodes. The samples
C.-Y. Chen et al. / Materials Chemistry and Physics 99 (2006) 26–29
27
Fig. 1. TMA profile of the sample as a function of the vacuumed-air-venting process. 120 ◦ C
were poled in silicone oil at for 30 min under a static electrical field of 4 kV mm−1 . The sintering process was simulated with Thermal Mechanical Analysis (SETARAM SETSYS-1750) to study the densification behavior of the samples. The crystallographic structures of the samples were examined by powder X-ray diffraction (XRD, MAC Science M18XHF) with Cu K␣ irradiation. The density of the sintered samples was measured with Archimedes’ method and the relative density was obtained by dividing the measured density with theoretical density (8.0 g cm−3 ). The microstructures of the fractured surfaces were examined by scanning electron microscope (JEOL-5600 SEM). The mechanical quality factor (Qm ) and electromechanical coupling factor (kp ) of the samples were measured using HP 4194A impedance analyzer.
3. Results and discussion Fig. 1 shows the TMA profile of the sample under the programmed sintering process. It was found that the sample started to shrink when the air was being introduced into the chamber at 800 ◦ C. The displacement of the sample was large initially at the air-venting stage under heating from 800 to 1050 ◦ C. The displacement of the sample then slowed down when the temperature was held at 1050 ◦ C. The large displacement at the air-venting stage may be explained by the effect of air pressure difference. The air-venting sintering process would cause a large air pressure difference between the external and interior sites of the sample, which has been vacuumed. About 10% shrinkage of the sample was obtained at the air-venting stage. Fig. 2 shows the XRD patterns of the samples sintered at 1050 ◦ C for various holding time. It was found that the predominated phase of the sample was rhombohedral and perovskite phase. This was evidenced form the split of the reflection peaks (1 0 0), (2 0 0) and (2 1 0) as seen in the XRD patterns [14]. The relative amount of rhombohedral phase decreased with the increase of the heating duration. The variation of relative amount of each phase as a function of sintering duration could be explained by the homogenization of the composition with the enhancement of diffusion process.
Fig. 2. XRD of the samples sintered at 1050 ◦ C for: (a) 0.5, (b) 1, (c) 2 and (d) 3 h under the vacuumed-air-venting process.
Fig. 3 shows the relative density of the samples heated at 1050 ◦ C versus the holding time. All samples show relative densities higher than 96%. Slight difference in the relative density was observed for these samples. Maximum relative density was observed for the sample at the heating duration of 1 h. Then the relative density of the sample decreased with higher heating duration. This was thought from the formation of the pore agglomeration and closing or the evaporation of PbO. This sintering process shows to obtain the samples with high relative density in a short sintering time (97.4% for 1 h) whereas the relative density of sample can only be 93.8% sintered at 1050 ◦ C for 3 h with conventional sintering process (in Fig. 3). This shows the great improvement on densification with novel sintering process.
Fig. 3. Relative density of the samples as a function of holding time at 1050 ◦ C under the vacuumed-air-venting process.
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C.-Y. Chen et al. / Materials Chemistry and Physics 99 (2006) 26–29
Fig. 4. SEM micrographs of fracture surface for the samples heated at 1050 ◦ C for: (a) 0.5, (b) 1, (c) 2 and (d) 3 h.
Fig. 4 shows the SEM microstructures of the fractures surfaces of the samples. The distributions in the grain shape and size of the samples are rather uniform. The samples with sintering duration of 0.5 and 1 h showed an intergranular fracture morphology indicating that the grain boundaries are mechanically weaker than the grains (Fig. 4(a and b)). It was also noted that the fracture model transformed from intergranular to transgranular as seen in Fig. 4(c and d) when the heating duration increased. The grain boundaries became stronger indicating that the phases at the grain boundaries changed due to segregation or precipitation, which is usually observed from liquid-phase sintering process. The sealing grain boundaries also confirm that the pore agglomeration and closing at intersect of grain boundaries with higher heating duration (Fig. 4(d)). Fig. 5 shows the average grain size of the samples versus sintering duration at 1050 ◦ C. The average grain size increased with the increase of the holding time. It was found that when grain size (d) is plotted versus holding time (t) in logarithm, a straight line is obtained. The slope of curve is about 0.5 indicating that the grain growth is controlled by boundary diffusion. This is well in agreement with the results that the densification of PZT sintering at temperature above 800 ◦ C is ascribed to the volume diffusion through grain boundary as proposed [5]. In addition, the grain size of the sample sintered with the vacuumed-air-venting process (1.21 m for 3 h) is larger than that with conventional sintering process (0.87 m for 3 h). This indicates that the novel sintering process can enhance the diffusion mechanism and, therefore, promote the densification of the sample with higher relative density. Fig. 6 shows the mechanical quality factor, Qm , and electromechanical planar coupling factor (kp ) of the samples versus
the holding time. It was found that the change of Qm factor follows the change of relative density. This can be explained by the friction loss between the domain walls. Smaller grain size with larger interfacial area would increase the friction between domains and thus has lower value of Qm factor. On the other hand, it was found that the kp factor increased with the increase of holding time. The variation of kp factor as a function of holding time could be explained by the homogenization of the composition with the enhancement of diffusion process and the increase of grain size. It has been reported that variations in Qm and kp are possibly explained by density and/or grain size [15,16]. In addition, the porosity usually has a marked effect on the dielectric constant and coupling coefficient [17]. In our work, the coupling
Fig. 5. Variation of the average grain size vs. holding times.
C.-Y. Chen et al. / Materials Chemistry and Physics 99 (2006) 26–29
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uumed and air-venting sintering process. The air-venting stage caused a large shrinkage (about 10%) initially and the shrinking rate slowed down when the sample was heated up from 800 to 1050 ◦ C. The predominated phase of the sample was rhombohedral and perovskite phase. The average grain size increased with the increase of sintering duration at 1050 ◦ C. The kp factor increased with the increase of holding time. Acknowledgement Financial support of this research by National Science Council, Taiwan, ROC, under the grant NSC 89-2218-E-036-017 is gratefully acknowledged. References
Fig. 6. Variation of mechanical quality factor (Qm ) and electromechanical coupling factor (kp ) of the samples as a function of holding time at 1050 ◦ C under the vacuumed-air-venting process.
coefficient is not affected much with the relative density. Therefore, grain size is thought as the dominant factor for influencing coupling coefficient. From the results of the present study, dense products with high quality can be obtained by the vacuumed-air-venting sintering process at relative lower temperature without compensating PbO atmosphere. Other processing factors influencing the microstructure and properties of the samples with the vacuumedair-venting sintering process are under investigation. 4. Conclusion The ceramics with the composition of 0.07Pb(Mn1/3 Nb2/3 ) O3 –0.468PbZrO3 –0.462PbTiO3 was prepared through the vac-
[1] K.W. Tang, H.L.W. Chana, Y.M. Cheungb, P.C.K. Liu, Mater. Chem. Phys. 75 (2002) 196. [2] E. Quandt, A. Ludwig, Sens. Actuators 81 (2000) 275. [3] R. Zuo, L. Li, X. Hu, Z. Gui, Mater. Lett. 54 (2002) 185. [4] K. Nagata, J. Thongrueng, J. Korea Appl. Soc. 32 (1998) S1278. [5] R.B. Atkin, R.M. Fulrath, J. Am. Ceram. Soc. 54 (1971) 265–270. [6] G.H. Haertling, Am. Ceram. Soc. Bull. 43 (1964) 113–118. [7] B. Jaffe, Piezoelectric Ceramics, Academic Press, London, UK, 1971. [8] A.I. Kingon, J.B. Clark, J. Am. Ceram. Soc. 66 (1983) 256–260. [9] S.S. Chiang, M. Nishioka, R.M. Fulrath, J.A. Pask, Am. Ceram. Soc. Bull. 60 (1981) 484–489. [10] G. Tomandl, A. Stiegelschmitt, R. Bohner, Sci. Ceram. 14 (1988) 305–308. [11] R.P. Tandon, V. Singh, N. Narayanaswami, V.K. Hans, Ferroelectrics 196 (1997) 117–120. [12] K. Murakaimi, D. Mabuchi, D. Kurita, Y. Niwa, S. Kaneko, Jpn. J. Appl. Phys. 35 (1996) 5188–5191. [13] C.Y. Chen, Yi Hu, H.L. Lin, W.Y. Wei, Ceramics International, in press. [14] J.C. Fernandes, D.A. Hall, M.R. Cockburn, G.N. Greaves, Nuclear instruments and method, Phys. Res. B97 (1995) 137–141. [15] K. Okazaki, N. Nagata, J. Am. Ceram. Soc. 56 (1972) 82–86. [16] K. Okazaki, N. Nagata, Electron. Commun. Jpn. 53C (1970) 147– 154. [17] M.L. Dunn, M. Taya, J. Am. Ceram. Soc. 76 (1993) 1697–1706.