- Email: [email protected]
Effects of excess PbO on the preparation process of modified lead–calcium titanate ceramics
Microelectronic Engineering 66 (2003) 918–925 www.elsevier.com / locate / mee
Effects of excess PbO on the preparation process of modified lead–calcium titanate ceramics Yuanwei Zhang a,b , *, Anxiang Kuang b , Helen Lai-Wah Chan a a
Department of Applied Physics and Materials Research Centre, Hong Kong Polytechnic University, Hunghom, Kowloon, Hong Kong, China b Faculty of Physics and Electronic Technology, Hubei University, Wuhan 430062, China
Abstract The effects of excess PbO on the preparation process of modified lead–calcium titanate ceramics were studied. The sintering period were divided into two stages: the initial sintering stage and the grain growth stage, at which the temperatures were chosen to be 980 and 1000 8C, respectively. In this study, an excess amount of PbO was added before the calcining stage and the initial sintering stage. The changes in the ceramic performance due to the excess amount of PbO were investigated. The figures of merit were the electromechanical coupling coefficients and mechanical quality factors. We observed that 1 mol% excess of PbO added at the calcining stage and 2.5 mol% excess of PbO added at the sintering stage would lead to a better performance and a smaller weight loss. By using the XRD and SEM results, the effects caused by the excess of PbO in the material processing were discussed. 2002 Elsevier Science B.V. All rights reserved. Keywords: Modified lead–calcium titanate ceramics; Lead excess; Piezoelectric anisotropy; Mechanical quality factor
1. Introduction Modified lead titanate (PbTiO 3 ) ceramics have larger electromechanical anisotropy, higher Curie temperature, lower dielectric constant than PZT ceramics, so it is an attractive material for high temperature, high frequency applications [1–4]. As PbTiO 3 ceramics are fragile during sintering, it is necessary to add some cations in the A- or B-site to decrease its c /a ratio. Adding calcium to partially substitute lead in the A-site is one of the commonly used methods (lead–calcium titanate). Wersing et al. [5] reported a composition of lead–calcium titanate ceramics modified with some Pb(Ni 1 / 3 Nb 2 / 3 )O 3 and MnO 2 . Its sintering temperature was about 1150–1200 8C. Based on these
* Corresponding author. 0167-9317 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-9317(02)01021-3
Y. Zhang et al. / Microelectronic Engineering 66 (2003) 918–925
919
studies, by adding a small amount of Pb(Zn 1 / 2 W1 / 2 )O 3 in the composite, we have successfully lowered the sintering temperature to 1050–1100 8C. The optimized properties of the modified lead–calcium titanate ceramics were k p 50.04–0.05, k t 50.49–0.52, Q m ¯1000, T C $300 8C and ´33 /´0 ¯ 190 [6]. This material was chosen as the basic composition to study the influence of lead excess to the properties of the ceramics. In general, excess of lead is necessary in the fabrication of lead-based ceramics. It can improve the repeatability of the properties of the ceramics, as well as lower the sintering temperature and improve the bulk density [7–13], which are important in device applications of the ceramics. For example, the composition of the internal electrodes is an important factor in determining the cost of a multilayer device, if the ceramics could be sintered at lower temperature, an Ag–Pd electrode with a lower palladium content can be used which greatly reduced the cost of the devices. Excess PbO can reduce the sintering temperature [8]. Snow [9] reported the sintering behavior of lanthanum-modified PZT (PLZT) powder in the fabrication of electro-optic ceramics. The PLZT powder batched with lead excess formed a liquid phase during sintering at 1200 8C for 60 h. Though the liquid phase was not observed as a secondary phase in the grain boundaries, from the observed weight loss it was concluded that the excess PbO evaporated in the sintering process as the sintering temperature for PZT (|1200 8C) is higher than the melting point of PbO, which is about 880 8C [8]. Kingon and co-workers [10,11] investigated the sintering process of PZT ceramics buried in PZT1PbO powder to control the PbO activity, they observed particle rearrangement, as well as chemical rearrangement (solution–precipitation) in the sample containing a large amount of PbO-rich liquid phase. Swartz and Shrout [12,13] developed the columbite precursor method to eliminate the pyrochlore phases in PbMg 1 / 3 Nb 2 / 3 O 3 –PbTiO 3 system and excess PbO plays an important role in this method. In this article, we will focus on the ceramic properties influenced by lead excess and the role of excess PbO in the sintering process; a two-step sintering method was designed.
2. Experimental procedure The basic composition of materials studied in this paper is: Pb 0.76 Ca 0.24 [(Zn 1 / 2 W1 / 2 ) 0.03 (Ni 1 / 3 Nb 2 / 3 ) 0.03 Ti 0.94 ]O 3 10.013 mol% MnO 2. Oxide powders of PbO, CaCO 3 , NiO, WO 3 , ZnO, Nb 2 O 5 , MnCO 3 and TiO 2 were used as raw materials. The first step, excessive lead oxide of 0, 1, 2, and 3 mol% were added and calcined at 850 8C for 2 h in an un-covered Al 2 O 3 crucible. Some calcined powders were pressed in a mold and sintered at 1050 8C for 1 h. After poled under an electric field of |5 kV/ mm, the electromechanical coupling coefficients and mechanical quality factors were measured. The second step, calcined powder with 1 mol% excessive lead was chosen as the basic material, excessive lead oxide of 0.61, 1.25, 2.50, 4.36, 6.22, and 8.08 mol% were added. The samples were fired at 980 8C for 30 min, 2 and 4 h separately in an open air, in order to investigated the microstructures at this step, then these samples were sintered at 1000 8C for 2 h in a covered crucible. The properties were measured after sample poling. The phases of the ceramic surface and the calcined powders were determined by X-ray diffraction (Philips X’pert XRD system). The cross-sectional microstructures of the ceramics were observed by
920
Y. Zhang et al. / Microelectronic Engineering 66 (2003) 918–925
scanning electron microscopy (SEM, stereoscan 440) and the piezoelectric parameters were measured by an HP4292 impedance analyzer.
3. Results and discussion
3.1. Effects of lead excess after calcinations at 850 8 C and then sintered at 1050 8 C Figs. 1 and 2 show the piezoelectric properties of the samples after being calcined at 850 8C for 2 h and then sintered at 1050 8C for 1 h. As it is desirable for the samples to have larger electromechanical coupling coefficient anisotropy k t /k p and higher mechanical quality factors Q m , it is seen from the figures that the sample with lead excess of 1 mol% has the best properties. Hence, this composition was chosen for subsequent experiments.
3.2. The ceramic characteristics after firing at 980 8 c Various PbO excess were added in the calcined powders (with 1 mol% lead excess added before calcinations). Fig. 3 shows the weight loss after firing at 980 8C with different firing time. Fig. 3 indicates that the longer firing time gives rise to higher weight loses, which is reasonable because more lead would be evaporated with the longer firing time. It is interesting to note that each curve had a valley, indicating that the weight loss does not increase monotonously with the lead excess. This can be understood by supposing there exist two main processes during firing. One process is the evaporation of the lead oxide, which causes the weight loss. The other process is the reaction of lead oxide with the elements in the material, which leads to the Pb being combined into base material.
Fig. 1. Electromechanical coupling coefficients k t (%) and k p (%) versus different amount of lead excess.
Y. Zhang et al. / Microelectronic Engineering 66 (2003) 918–925
921
Fig. 2. The mechanical quality factor Q m and the dielectric loss tan d versus different amount of lead excess.
These combinations may be more active by the increase of the lead excess. So at 0.61 mol% lead excess, PbO evaporation was almost the only process and the weight loss were relatively high. Further increase in the lead excess would provide more lead ions to react with the elements in the material. As less lead oxide would evaporate than the sample with 0.61 mol% lead excess, a minimum value occurred at about 1.25 or 2.50 mol% depending on the firing time. When the lead excess was over
Fig. 3. The weight losses after firing at 980 8C for 30 min, 2 and 4 h versus lead excess.
922
Y. Zhang et al. / Microelectronic Engineering 66 (2003) 918–925
2.50 mol%, more PbO will evaporate then the lead ions combined with the elements in the material, which made the weight loss increased.
3.3. The microstructures of ceramics fired at 980 8 C for 2 h Figs. 4 and 5 are the XRD patterns and SEM micrographs of the samples fired at 980 8C for 2 h. The XRD patterns show that no pyrochlore phase is observed. It can be concluded that excess PbO limited the occurrences of the pyrochlore phase efficiently. This is favorable in obtaining homogeneous and reliable ceramics. SEM micrographs show that grain size grows gradually with the increase of the lead excess, except the sample with 1.25 mol% PbO excess. In this sample, as we discussed above, PbO may be, was combined into the ceramics, therefore less liquid phases existed in the grain boundaries, which hinders the grain growth.
3.4. The properties of ceramics sintered at 1000 8 C for 2 h Figs. 6 and 7 show the electromechanical coupling coefficients, mechanical quality factors Q m and dielectric losses tan d of the samples fired at 980 8C for 2 h and sintered at 1000 8C for 2 h. With this special two-step sintering process, better properties were obtained in samples with 2.50 mol% of PbO excess.
Fig. 4. The XRD patterns with lead excess from 0.00 to 8.08 ml%.
Y. Zhang et al. / Microelectronic Engineering 66 (2003) 918–925
Fig. 5. SEM micrographs with lead excess from 0.61 to 8.08 mol%.
923
Y. Zhang et al. / Microelectronic Engineering 66 (2003) 918–925
924
Fig. 6. Electromechanical coupling coefficients k t (%), k p (%) and k t /k p versus lead excess.
4. Conclusion Modified lead–calcium titanate ceramics with Pb(Zn 1 / 2 W1 / 2 )O 3 addition were fabricated. The influences of lead excess on the properties of the ceramics were studied. Ceramics with 1 mol% excess lead showed better performances. Using the composition with 1 mol% lead excess as the base materials, various amounts of PbO excess were added in the calcined powders. With the PbO addition of about 2.50 mol% in the calcined
Fig. 7. The mechanical quality factors Q m and the dielectric loss tan d versus lead excess.
Y. Zhang et al. / Microelectronic Engineering 66 (2003) 918–925
925
powders, when fired at about 980 8C for 2 h and sintered at 1000 8C for 2 h, the samples had better performances. After firing at 980 8C, the weight losses versus the lead excess showed a minimum value at about 1.25 or 2.50 mol% depending on the firing time. The causes may depend on two main processes that related to lead oxide. One is the lead evaporation during firing and another is the reaction of lead with elements in the ceramics. The ceramics with lead excess of 2.50 mol% had better performances. The microstructures observed by XRD and SEM show that excess lead can eliminate the pyrochlore phase efficiently, the ceramic grain sizes grew larger with the increase in the lead excess except for the sample with lead excess of 1.25 mol%.
Acknowledgements The authors would like to thank Professor R. E. Newnham for his suggestions and discussions. This work was supported by the Centre for Smart Materials of the Hong Kong Polytechnic University
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
Y. Yamashita, K. Yokoyama, H. Honda et al., Jpn. J. Appl. Phys. 20 (Suppl.) (1981) 183. Y. Ito, H. Takuechi, K. Nagatsuma, S. Jyomura, S. Ashida, J. Appl. Phys. 52 (1981) 3223. N. Ichinose, Am. Ceram. Soc. Bull. 64 (1985) 1581. H. Takeuchi, S. Jyomura, E. Yamamoto et al., J. Acoust. Soc. Am. 72 (1982) 1114. W. Wersing, K. Lubitz, J. Mohaupt, IEEE Trans. Ultrasonic Ferroelectr. Freq. Control 36 (1989) 1424. Y.W. Zhang, X.Z. Shang, T.S. Zhou, A.X. Kuang, R.Z. Yuan, Ferroelectrics 263 (2001) 309. W.H. Rhodes, Phase Diagrams in Advanced Ceramics, Academic Press, New York, 1995, p. 32. K. Masao, K. Kazuaki, J. Am. Ceram. Soc. 84 (2001) 2469. G.S. Snow, J. Am. Ceram. Soc. 56 (1973) 91. A. Kingon, J.B. Clark, J. Am. Ceram. Soc. 66 (1983) 253. A. Kingon, J.B. Clark, J. Am. Ceram. Soc. 66 (1983) 256. S.L. Swartz, T.R. Shrout, Mater. Res. Bull. 17 (1982) 1245. C.A. Randall, A.D. Hilton, D.J. Barber, T.R. Shrout, J. Mater. Res. 8 (1990) 880.
Effects of excess PbO on the preparation process of modified lead–calcium titanate ceramics Yuanwei Zhang a,b , *, Anxiang Kuang b , Helen Lai-Wah Chan a a
Department of Applied Physics and Materials Research Centre, Hong Kong Polytechnic University, Hunghom, Kowloon, Hong Kong, China b Faculty of Physics and Electronic Technology, Hubei University, Wuhan 430062, China
Abstract The effects of excess PbO on the preparation process of modified lead–calcium titanate ceramics were studied. The sintering period were divided into two stages: the initial sintering stage and the grain growth stage, at which the temperatures were chosen to be 980 and 1000 8C, respectively. In this study, an excess amount of PbO was added before the calcining stage and the initial sintering stage. The changes in the ceramic performance due to the excess amount of PbO were investigated. The figures of merit were the electromechanical coupling coefficients and mechanical quality factors. We observed that 1 mol% excess of PbO added at the calcining stage and 2.5 mol% excess of PbO added at the sintering stage would lead to a better performance and a smaller weight loss. By using the XRD and SEM results, the effects caused by the excess of PbO in the material processing were discussed. 2002 Elsevier Science B.V. All rights reserved. Keywords: Modified lead–calcium titanate ceramics; Lead excess; Piezoelectric anisotropy; Mechanical quality factor
1. Introduction Modified lead titanate (PbTiO 3 ) ceramics have larger electromechanical anisotropy, higher Curie temperature, lower dielectric constant than PZT ceramics, so it is an attractive material for high temperature, high frequency applications [1–4]. As PbTiO 3 ceramics are fragile during sintering, it is necessary to add some cations in the A- or B-site to decrease its c /a ratio. Adding calcium to partially substitute lead in the A-site is one of the commonly used methods (lead–calcium titanate). Wersing et al. [5] reported a composition of lead–calcium titanate ceramics modified with some Pb(Ni 1 / 3 Nb 2 / 3 )O 3 and MnO 2 . Its sintering temperature was about 1150–1200 8C. Based on these
* Corresponding author. 0167-9317 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-9317(02)01021-3
Y. Zhang et al. / Microelectronic Engineering 66 (2003) 918–925
919
studies, by adding a small amount of Pb(Zn 1 / 2 W1 / 2 )O 3 in the composite, we have successfully lowered the sintering temperature to 1050–1100 8C. The optimized properties of the modified lead–calcium titanate ceramics were k p 50.04–0.05, k t 50.49–0.52, Q m ¯1000, T C $300 8C and ´33 /´0 ¯ 190 [6]. This material was chosen as the basic composition to study the influence of lead excess to the properties of the ceramics. In general, excess of lead is necessary in the fabrication of lead-based ceramics. It can improve the repeatability of the properties of the ceramics, as well as lower the sintering temperature and improve the bulk density [7–13], which are important in device applications of the ceramics. For example, the composition of the internal electrodes is an important factor in determining the cost of a multilayer device, if the ceramics could be sintered at lower temperature, an Ag–Pd electrode with a lower palladium content can be used which greatly reduced the cost of the devices. Excess PbO can reduce the sintering temperature [8]. Snow [9] reported the sintering behavior of lanthanum-modified PZT (PLZT) powder in the fabrication of electro-optic ceramics. The PLZT powder batched with lead excess formed a liquid phase during sintering at 1200 8C for 60 h. Though the liquid phase was not observed as a secondary phase in the grain boundaries, from the observed weight loss it was concluded that the excess PbO evaporated in the sintering process as the sintering temperature for PZT (|1200 8C) is higher than the melting point of PbO, which is about 880 8C [8]. Kingon and co-workers [10,11] investigated the sintering process of PZT ceramics buried in PZT1PbO powder to control the PbO activity, they observed particle rearrangement, as well as chemical rearrangement (solution–precipitation) in the sample containing a large amount of PbO-rich liquid phase. Swartz and Shrout [12,13] developed the columbite precursor method to eliminate the pyrochlore phases in PbMg 1 / 3 Nb 2 / 3 O 3 –PbTiO 3 system and excess PbO plays an important role in this method. In this article, we will focus on the ceramic properties influenced by lead excess and the role of excess PbO in the sintering process; a two-step sintering method was designed.
2. Experimental procedure The basic composition of materials studied in this paper is: Pb 0.76 Ca 0.24 [(Zn 1 / 2 W1 / 2 ) 0.03 (Ni 1 / 3 Nb 2 / 3 ) 0.03 Ti 0.94 ]O 3 10.013 mol% MnO 2. Oxide powders of PbO, CaCO 3 , NiO, WO 3 , ZnO, Nb 2 O 5 , MnCO 3 and TiO 2 were used as raw materials. The first step, excessive lead oxide of 0, 1, 2, and 3 mol% were added and calcined at 850 8C for 2 h in an un-covered Al 2 O 3 crucible. Some calcined powders were pressed in a mold and sintered at 1050 8C for 1 h. After poled under an electric field of |5 kV/ mm, the electromechanical coupling coefficients and mechanical quality factors were measured. The second step, calcined powder with 1 mol% excessive lead was chosen as the basic material, excessive lead oxide of 0.61, 1.25, 2.50, 4.36, 6.22, and 8.08 mol% were added. The samples were fired at 980 8C for 30 min, 2 and 4 h separately in an open air, in order to investigated the microstructures at this step, then these samples were sintered at 1000 8C for 2 h in a covered crucible. The properties were measured after sample poling. The phases of the ceramic surface and the calcined powders were determined by X-ray diffraction (Philips X’pert XRD system). The cross-sectional microstructures of the ceramics were observed by
920
Y. Zhang et al. / Microelectronic Engineering 66 (2003) 918–925
scanning electron microscopy (SEM, stereoscan 440) and the piezoelectric parameters were measured by an HP4292 impedance analyzer.
3. Results and discussion
3.1. Effects of lead excess after calcinations at 850 8 C and then sintered at 1050 8 C Figs. 1 and 2 show the piezoelectric properties of the samples after being calcined at 850 8C for 2 h and then sintered at 1050 8C for 1 h. As it is desirable for the samples to have larger electromechanical coupling coefficient anisotropy k t /k p and higher mechanical quality factors Q m , it is seen from the figures that the sample with lead excess of 1 mol% has the best properties. Hence, this composition was chosen for subsequent experiments.
3.2. The ceramic characteristics after firing at 980 8 c Various PbO excess were added in the calcined powders (with 1 mol% lead excess added before calcinations). Fig. 3 shows the weight loss after firing at 980 8C with different firing time. Fig. 3 indicates that the longer firing time gives rise to higher weight loses, which is reasonable because more lead would be evaporated with the longer firing time. It is interesting to note that each curve had a valley, indicating that the weight loss does not increase monotonously with the lead excess. This can be understood by supposing there exist two main processes during firing. One process is the evaporation of the lead oxide, which causes the weight loss. The other process is the reaction of lead oxide with the elements in the material, which leads to the Pb being combined into base material.
Fig. 1. Electromechanical coupling coefficients k t (%) and k p (%) versus different amount of lead excess.
Y. Zhang et al. / Microelectronic Engineering 66 (2003) 918–925
921
Fig. 2. The mechanical quality factor Q m and the dielectric loss tan d versus different amount of lead excess.
These combinations may be more active by the increase of the lead excess. So at 0.61 mol% lead excess, PbO evaporation was almost the only process and the weight loss were relatively high. Further increase in the lead excess would provide more lead ions to react with the elements in the material. As less lead oxide would evaporate than the sample with 0.61 mol% lead excess, a minimum value occurred at about 1.25 or 2.50 mol% depending on the firing time. When the lead excess was over
Fig. 3. The weight losses after firing at 980 8C for 30 min, 2 and 4 h versus lead excess.
922
Y. Zhang et al. / Microelectronic Engineering 66 (2003) 918–925
2.50 mol%, more PbO will evaporate then the lead ions combined with the elements in the material, which made the weight loss increased.
3.3. The microstructures of ceramics fired at 980 8 C for 2 h Figs. 4 and 5 are the XRD patterns and SEM micrographs of the samples fired at 980 8C for 2 h. The XRD patterns show that no pyrochlore phase is observed. It can be concluded that excess PbO limited the occurrences of the pyrochlore phase efficiently. This is favorable in obtaining homogeneous and reliable ceramics. SEM micrographs show that grain size grows gradually with the increase of the lead excess, except the sample with 1.25 mol% PbO excess. In this sample, as we discussed above, PbO may be, was combined into the ceramics, therefore less liquid phases existed in the grain boundaries, which hinders the grain growth.
3.4. The properties of ceramics sintered at 1000 8 C for 2 h Figs. 6 and 7 show the electromechanical coupling coefficients, mechanical quality factors Q m and dielectric losses tan d of the samples fired at 980 8C for 2 h and sintered at 1000 8C for 2 h. With this special two-step sintering process, better properties were obtained in samples with 2.50 mol% of PbO excess.
Fig. 4. The XRD patterns with lead excess from 0.00 to 8.08 ml%.
Y. Zhang et al. / Microelectronic Engineering 66 (2003) 918–925
Fig. 5. SEM micrographs with lead excess from 0.61 to 8.08 mol%.
923
Y. Zhang et al. / Microelectronic Engineering 66 (2003) 918–925
924
Fig. 6. Electromechanical coupling coefficients k t (%), k p (%) and k t /k p versus lead excess.
4. Conclusion Modified lead–calcium titanate ceramics with Pb(Zn 1 / 2 W1 / 2 )O 3 addition were fabricated. The influences of lead excess on the properties of the ceramics were studied. Ceramics with 1 mol% excess lead showed better performances. Using the composition with 1 mol% lead excess as the base materials, various amounts of PbO excess were added in the calcined powders. With the PbO addition of about 2.50 mol% in the calcined
Fig. 7. The mechanical quality factors Q m and the dielectric loss tan d versus lead excess.
Y. Zhang et al. / Microelectronic Engineering 66 (2003) 918–925
925
powders, when fired at about 980 8C for 2 h and sintered at 1000 8C for 2 h, the samples had better performances. After firing at 980 8C, the weight losses versus the lead excess showed a minimum value at about 1.25 or 2.50 mol% depending on the firing time. The causes may depend on two main processes that related to lead oxide. One is the lead evaporation during firing and another is the reaction of lead with elements in the ceramics. The ceramics with lead excess of 2.50 mol% had better performances. The microstructures observed by XRD and SEM show that excess lead can eliminate the pyrochlore phase efficiently, the ceramic grain sizes grew larger with the increase in the lead excess except for the sample with lead excess of 1.25 mol%.
Acknowledgements The authors would like to thank Professor R. E. Newnham for his suggestions and discussions. This work was supported by the Centre for Smart Materials of the Hong Kong Polytechnic University
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
Y. Yamashita, K. Yokoyama, H. Honda et al., Jpn. J. Appl. Phys. 20 (Suppl.) (1981) 183. Y. Ito, H. Takuechi, K. Nagatsuma, S. Jyomura, S. Ashida, J. Appl. Phys. 52 (1981) 3223. N. Ichinose, Am. Ceram. Soc. Bull. 64 (1985) 1581. H. Takeuchi, S. Jyomura, E. Yamamoto et al., J. Acoust. Soc. Am. 72 (1982) 1114. W. Wersing, K. Lubitz, J. Mohaupt, IEEE Trans. Ultrasonic Ferroelectr. Freq. Control 36 (1989) 1424. Y.W. Zhang, X.Z. Shang, T.S. Zhou, A.X. Kuang, R.Z. Yuan, Ferroelectrics 263 (2001) 309. W.H. Rhodes, Phase Diagrams in Advanced Ceramics, Academic Press, New York, 1995, p. 32. K. Masao, K. Kazuaki, J. Am. Ceram. Soc. 84 (2001) 2469. G.S. Snow, J. Am. Ceram. Soc. 56 (1973) 91. A. Kingon, J.B. Clark, J. Am. Ceram. Soc. 66 (1983) 253. A. Kingon, J.B. Clark, J. Am. Ceram. Soc. 66 (1983) 256. S.L. Swartz, T.R. Shrout, Mater. Res. Bull. 17 (1982) 1245. C.A. Randall, A.D. Hilton, D.J. Barber, T.R. Shrout, J. Mater. Res. 8 (1990) 880.