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Possibility of one-step approach to 0.7PZN–0.3BT multiple ceramics from component constituent oxides
March 2002
Materials Letters 53 Ž2002. 205–210 www.elsevier.comrlocatermatlet
Possibility of one-step approach to 0.7PZN–0.3BT multiple ceramics from component constituent oxides L.B. Kong ) , J. Ma, R.F. Zhang School of Materials Engineering, Nanyang Technological UniÕersity, Block N4, B2 Nanyang AÕenue, Singapore, 639798, Singapore Received 28 March 2001; accepted 11 June 2001
Abstract 0.7PbŽZn 1r3 Nb 2r3 .O 3 –0.3BaTiO 3 ŽPZN–BT. ceramics were prepared directly from their constituent oxide mixture via a one-step sintering process without involving the calcination step. Reaction of the oxide mixture was observed to occur at about 800 8C through the expansion of the compact at that temperature. XRD results show that formation of single phase perovskite PZN–BT was achieved at temperatures from 1050 to 1125 8C while the pyrochlore phase appeared at 1150 8C. When sintered at 1125 8C for 1 h, the PZN–BT ceramics produced using the present technique were measured to have a density of ; 99% of the theoretical density, an average grain size of 6.2 mm, a dielectric constant of 2826 and a dielectric loss of 0.03. The present results indicate that it is possible to prepare PbŽZn 1r3 Nb 2r3 .O 3-based ceramics via a one-step process. q 2002 Elsevier Science B.V. All rights reserved. Keywords: Perovskite; Single phase ceramic; Direct high-temperature synthesis from constituent oxides
1. Introduction Lead zinc niobate wPbŽZn 1r3 Nb 2r3 .O 3 , or PZNx is a ferroelectric compound with perovskite structure showing a diffuse phase transition from rhombohedral phase at room temperature to cubic phase at ; 140 8C w1–3x. It has been reported that PZN-based single crystals with perovskite structure, exhibiting outstanding dielectric, electrostrictive and optical properties, can be readily prepared using the flux methods w4,5x. PZN-based solid-state solution systems also perform excellent ferroelectric and piezoelectric properties. For example, 0.91PZN–0.09PT, which is near the morphotropic phase boundary ) Corresponding author. Tel.: q65-7904-590; fax: q65-7934528. E-mail address: [email protected] ŽL.B. Kong..
ŽMPB. at room temperature, shows a surprisingly large dielectric and piezoelectric constant and higher electromechanical coupling coefficient than the PZT family of ferroelectrics. It is well known that single phase PZN is difficult to prepare via a conventional ceramic process at ambient pressure due to the instability of PZN over the temperatures ranging from 600 to 1400 8C w6–8x. However, perovskite PMN phase can be stabilized by the addition of BaTiO 3 ŽBT. w9x, SrTiO 3 ŽST. and PbTiO 3 ŽPT. w10x to form solid-state solution of PZN–BT, PZN–ST or PZN–PT. Among these additives, BT has been found to be the most effective additive to suppress the formation of the pyrochlore phase in PZN systems. PZN–BT ceramics were usually prepared by the conventional ceramic process, which involved several steps of mixing, such as calcination, crashing, compacting and sintering. It is
00167-577Xr02r$ - see front matter q 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 1 . 0 0 4 7 7 - 3
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L.B. Kong et al.r Materials Letters 53 (2002) 205–210
very time-consuming and the multi-step process is quite vulnerable to impurity contamination. In this paper, we present the possibility to prepare PZN-based ceramics with a composition of PZN– 0.3BT via a one-step sintering process, where the mixture of its corresponding oxides or carbonates was directly treated at high temperature without undergoing the calcination step. The results obtained have indicated that the electric properties of the one-step derived PZN–BT ceramics are comparable to those produced by the conventional solid-state ceramic process. Ceramics fabrication in this way will be of great advantage in product cost reduction.
2. Experimental Fig. 2. Sintering behavior of the oxide mixture.
Commercial oxide powders of PbO Ž99.9 q % purity, Aldrich Chemical, USA., Nb 2 O5 Ž99 q % purity, Alfa Aesar, USA., ZnO Ž99 q % purity, Aldrich Chemical., BaCO 3 Ž99 q % purity, Aldrich Chemical. and TiO 2 Ž99.9 q % purity, Aldrich Chemical. were used as starting materials with the nominal composition of 0.7PbŽZn 1r3 Nb 2r3 .O 3 –0.3 BaTiO 3 . The oxide mixture was milled for 4 h using alcohol and ZrO 2 balls as milling media. The slurry was dried at 80 8C in a box oven, and then uniaxially
Fig. 1. XRD pattern for the oxide mixture with a nominal composition of 0.7PbŽZn 1r 3 Nb 2r3 .O 3 –0.3BaTiO 3 .
pressed into green pellets of about 70% of the theoretical density with a diameter of 10 mm and thickness of 1 mm, using a hardened stainless steel die at
Fig. 3. XRD patterns for the oxide mixture sintered for 1 h at different temperatures: Ža. 1050, Žb. 1075, Žc. 1100, Žd. 1125 and Že. 1150 8C.
L.B. Kong et al.r Materials Letters 53 (2002) 205–210
a pressure of 50 MPa. PVA aqueous solution of 5 wt.% was also added to the mixture powder as binder. Sintering of the pellets was carried out using a Carbolite RHF 1600 type furnace in air at temperatures ranging from 1050 to 1125 8C for a fixed duration of 1 h, with both heating and cooling rates of 5 8C. Before reaching soaking temperature, the samples were kept at 600 8C for 2 h to burn out the PVA binder. Lead atmosphere was employed to
207
eliminate the evaporation of PbO during sintering by using compact of PbO and ZrO mixture. The sintering behavior of the mixture was conducted using a Setaram Setsys 16r18 type dilatometer at a heating rate of 10 8Crmin. X-ray diffraction analysis of the oxide mixture and the sintered ceramics was performed using a Rigaku ultimaq type diffractometer ŽXRD. with Cu K a radiation. The density of the PZT ceramics was measured using a
Fig. 4. Microstructures of the cross-sectional surfaces for the PMN–BT ceramics sintered at different temperatures: Ža. 1050, Žb. 1075, Žc. 1100, Žd. 1125 and Že. 1150 8C.
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Mirage MD-200S type electronic densimeter. The microstructure development for the sintered samples was characterized using a JEOL JSM-6340F type field emission scanning electronic microscope ŽFESEM.. All the sintered samples were then polished and covered with silver paste as the electrode for electrical and dielectric measurement. The dielectric and ferroelectric properties of the sintered samples were measured using HP 4194A impedance analyzer and Radiant Technology RT6000HVA type standard ferroelectric tester, respectively.
3. Results and discussions Fig. 1 shows the XRD pattern for the oxide mixture ball milled for 4 h. All the diffraction peaks correspond to the constituent oxides, indicating that no reaction takes place in the mixing process. Fig. 2 shows linear shrinkage and sintering rate of the green pellet of the oxide mixture. The pellet begins expansion at about 600 8C and exhibits a maximum at about 790 8C. This expansion is attributed to the reaction of the constituent oxidesrcarbonate with the PZN–BT perovskite phase because the volume of the PZN–BT is larger than the total volume of the components. Maximum sintering rate was observed at about 805 8C, which indicates the completion of the reaction. In general, the present technique is a one-step processing technique applying the reactive sintering approach. Reactive sintering is a promising fabrication technique for multiple component ceramics, in which the reactions between constituent phases take place during sintering process at high temperatures. It is advantageous over other methods because of its simplicity of experimental operation and enhanced densification progress and has been successfully applied to several multiple component ceramic systems w11,12x. Since most densification takes place above 1000 8C, the sintering temperature of the PZN–BT ceramics via the one-step process was chosen from 1050 to 1150 8C. To check the phase composition of the sintered PZN–BT ceramics, X-ray diffraction on all the sintered samples was performed. Fig. 3 shows the XRD patterns for the samples sintered at temperatures from 1050 to 1125 8C. It can be seen that single phase perovskite 0.7PbŽZn 1r3 Nb 2r3 .O 3 –0.3Ba
TiO 3 has been formed as the mixture was subjected to sintering at 1050 8C for 1 h. The perovskite phase is also noted to be stable up to 1125 8C. As the sintering temperature increases to 1150 8C, the presence of a slight amount of pyrochlore phase is observed. The appearance of the pyrochlore phase in the 1150 8C-sintered samples may be related to the loss of PbO. Fig. 4 shows the cross-sectional surface microstructure of the PZN–BT ceramics sintered at different temperatures. The grain size of the PZN–BT ceramics was estimated from the SEM observations and is plotted as a function of sintering temperature as shown in Fig. 5. Almost fully dense Ž) 96% of the theoretical density., PZN–BT ceramics with an average grain size of 2.4 mm were obtained when the mixture was sintered at 1050 8C for 1 h. There is no significant change observed in the density of the samples as a function of sintering temperature. The average grain size increases almost linearly with the sintering temperature Žfrom about 3.2 mm at 1075 8C to about 8.5 mm at 1150 8C.. Finally, the 1125 8C-sintered PZN–BT ceramics were noted to exhibit a density of ; 99% with an average grain size of 6.2 mm.
Fig. 5. Grain size of the PMN–BT ceramics with sintering temperature.
L.B. Kong et al.r Materials Letters 53 (2002) 205–210
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4. Conclusions
Fig. 6. Dielectric constant of the PMN–BT ceramics varying with sintering temperature.
Fig. 6 illustrates the dielectric constant measured at 1 kHz and room temperature as a function of sintering temperature. The dielectric constant increases rapidly from 1050 to 1100 8C and then gradually from 1100 to 1125 8C. The increase in dielectric constant of the PZN–BT ceramics as a function of sintering temperature from 1050 to 1125 8C is related to the grain size and density of the samples varying with temperature. For ferroelectric ceramic materials, it is well accepted that the coupling effect between grain boundaries constrains the domain reorientation and domain wall mobility w13,14x. When the grain size increases, this effect will be reduced because the grain boundary fraction is inversely proportional to the grain size. As a result, the dielectric constant increases with the increase of grain size of the ferroelectric ceramics. The decrease in dielectric constant of the 1150 8C-sintered sample can be attributed to the presence of the pyrochlore phase although its grain size is larger. In addition, the dielectric properties Ždielectric constant of 2826 and dielectric loss of 0.03. of the PZN–BT ceramics sintered at 1125 8C for 1 h in the present work are in good agreement with the literature value via the conventional process w9x, implying the reliability of the one-step process.
Our results demonstrate that it is possible to prepare PZN–BT ceramics via a one-step sintering of their corresponding oxide mixture. Reaction of the constituent oxides into the perovskite phase occurred at about 800 8C, after which densification took place. Single phase PZN–BT with perovskite structure was achieved at 1050 8C and kept stable up to 1125 8C, while pyrochlore phase was observed at sintering temperature of 1150 8C. Almost fully dense ceramics are achieved from all the sintered samples. The dielectric constant of the PZN–BT ceramics increases with sintering temperature from 950 to 1125 8C and maximizes at 1125 8C, which may be attributed to the increased grain size as a result of increasing sintering temperature. The decrease in dielectric constant at 1150 8C is mainly due to the presence of the pyrochlore phase.
References w1x Y. Yokomizo, T. Takahashi, S. Nomura, Ferroelectric properties of PbŽZn 1r 3 Nb 2r3 .O 3 , J. Phys. Soc. Jpn. 28 Ž950. Ž1970. 1278–1284. w2x S. Nomura, J. Kuwata, Electrostriction in PbŽZn 1r 3 Nb 2r3 .O 3 , Mater. Res. Bull. 14 Ž1979. 769–774. w3x J. Kuwata, K. Uchino, S. Nomura, Diffuse phase transitions in lead zinc niobate, Ferroelectrics 22 Ž1979. 863–867. w4x J. Kuwata, K. Uchino, S. Nomura, Phase relations in the PbŽZn 1r 3 Nb 2r3 .O 3 –PbTiO 3 system, Ferroelectrics 37 Ž1981. 579–582. w5x M. Ozgul, K. Tekemura, S. Trolier-Mckinstry, C.A. Randall, Polarization fatigue in PbŽZn 1r 3 Nb 2r3 .O 3 –PbTiO 3 ferroelectric single crystal, J. Appl. Phys. 89 Ž9. Ž2001. 5100– 5106. w6x H.M. Jan, S.H. Oh, J.H. Moon, Thermodynamic stability and mechanisms of formation and decomposition of perovskite PbŽZn 1r 3 Nb 2r3 .O 3 prepared by the PbO flux method, J. Am. Ceram. Soc. 75 Ž1992. 82–88. w7x T.R. Shrout, A. Halliyal, Preparation of lead-based ferroelectric relaxors for capacitors, Am. Ceram. Soc. Bull. 66 Ž1987. 704–711. w8x N. Wakiya, K. Shinozaki, N. Mizutani, Estimation of phase stability in PbŽMg 1r 3 Nb 2r3 .O 3 and PbŽZn 1r3 Nb 2r3 .O 3 using the bond valence approach, J. Am. Ceram. Soc. 80 Ž1997. 3217–3220. w9x A. Halliyal, U. Kumar, R.E. Newnham, L.E. Cross, Stabilization of the perovskite phase and dielectric properties of ceramics in the PbŽZn 1r 3 Nb 2r3 .O 3 –PbTiO 3 system, Am. Ceram. Soc. Bull. 66 Ž1987. 671–676.
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w10x J.R. Belsick, A. Halliyal, U. Kumar, R.E. Newnham, Phase relations and dielectric properties of ceramics in the system PbŽZn 1r3 Nb 2r3 .O 3 –SrTiO 3 –PbTiO 3 , Am. Ceram. Soc. Bull. 66 Ž1987. 664–667. w11x T.R. Shrout, P. Patet, S. Kim, G.S. Lee, Conventionally prepared submicrometer lead-based perovskite powders by reactive calcinations, J. Am. Ceram. Soc. 73 Ž1990. 1862– 1867.
w12x M.N. Rahaman, L.C. De Jonghe, Reaction sintering of zinc ferrite during constant rates of heating, J. Am. Ceram. Soc. 76 Ž1993. 1739–1744. w13x H.D. Chen, K.R. Udayakumar, C.J. Gaskey, L.E. Cross, J.J. Bernstein, L.C. Niles, J. Am. Ceram. Soc. 79 Ž1996. 2189. w14x C.A. Randall, N. Kim, J.P. Kucera, W. Cao, T.R. Shrout, J. Am. Ceram. Soc. 81 Ž1998. 677.
Materials Letters 53 Ž2002. 205–210 www.elsevier.comrlocatermatlet
Possibility of one-step approach to 0.7PZN–0.3BT multiple ceramics from component constituent oxides L.B. Kong ) , J. Ma, R.F. Zhang School of Materials Engineering, Nanyang Technological UniÕersity, Block N4, B2 Nanyang AÕenue, Singapore, 639798, Singapore Received 28 March 2001; accepted 11 June 2001
Abstract 0.7PbŽZn 1r3 Nb 2r3 .O 3 –0.3BaTiO 3 ŽPZN–BT. ceramics were prepared directly from their constituent oxide mixture via a one-step sintering process without involving the calcination step. Reaction of the oxide mixture was observed to occur at about 800 8C through the expansion of the compact at that temperature. XRD results show that formation of single phase perovskite PZN–BT was achieved at temperatures from 1050 to 1125 8C while the pyrochlore phase appeared at 1150 8C. When sintered at 1125 8C for 1 h, the PZN–BT ceramics produced using the present technique were measured to have a density of ; 99% of the theoretical density, an average grain size of 6.2 mm, a dielectric constant of 2826 and a dielectric loss of 0.03. The present results indicate that it is possible to prepare PbŽZn 1r3 Nb 2r3 .O 3-based ceramics via a one-step process. q 2002 Elsevier Science B.V. All rights reserved. Keywords: Perovskite; Single phase ceramic; Direct high-temperature synthesis from constituent oxides
1. Introduction Lead zinc niobate wPbŽZn 1r3 Nb 2r3 .O 3 , or PZNx is a ferroelectric compound with perovskite structure showing a diffuse phase transition from rhombohedral phase at room temperature to cubic phase at ; 140 8C w1–3x. It has been reported that PZN-based single crystals with perovskite structure, exhibiting outstanding dielectric, electrostrictive and optical properties, can be readily prepared using the flux methods w4,5x. PZN-based solid-state solution systems also perform excellent ferroelectric and piezoelectric properties. For example, 0.91PZN–0.09PT, which is near the morphotropic phase boundary ) Corresponding author. Tel.: q65-7904-590; fax: q65-7934528. E-mail address: [email protected] ŽL.B. Kong..
ŽMPB. at room temperature, shows a surprisingly large dielectric and piezoelectric constant and higher electromechanical coupling coefficient than the PZT family of ferroelectrics. It is well known that single phase PZN is difficult to prepare via a conventional ceramic process at ambient pressure due to the instability of PZN over the temperatures ranging from 600 to 1400 8C w6–8x. However, perovskite PMN phase can be stabilized by the addition of BaTiO 3 ŽBT. w9x, SrTiO 3 ŽST. and PbTiO 3 ŽPT. w10x to form solid-state solution of PZN–BT, PZN–ST or PZN–PT. Among these additives, BT has been found to be the most effective additive to suppress the formation of the pyrochlore phase in PZN systems. PZN–BT ceramics were usually prepared by the conventional ceramic process, which involved several steps of mixing, such as calcination, crashing, compacting and sintering. It is
00167-577Xr02r$ - see front matter q 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 1 . 0 0 4 7 7 - 3
206
L.B. Kong et al.r Materials Letters 53 (2002) 205–210
very time-consuming and the multi-step process is quite vulnerable to impurity contamination. In this paper, we present the possibility to prepare PZN-based ceramics with a composition of PZN– 0.3BT via a one-step sintering process, where the mixture of its corresponding oxides or carbonates was directly treated at high temperature without undergoing the calcination step. The results obtained have indicated that the electric properties of the one-step derived PZN–BT ceramics are comparable to those produced by the conventional solid-state ceramic process. Ceramics fabrication in this way will be of great advantage in product cost reduction.
2. Experimental Fig. 2. Sintering behavior of the oxide mixture.
Commercial oxide powders of PbO Ž99.9 q % purity, Aldrich Chemical, USA., Nb 2 O5 Ž99 q % purity, Alfa Aesar, USA., ZnO Ž99 q % purity, Aldrich Chemical., BaCO 3 Ž99 q % purity, Aldrich Chemical. and TiO 2 Ž99.9 q % purity, Aldrich Chemical. were used as starting materials with the nominal composition of 0.7PbŽZn 1r3 Nb 2r3 .O 3 –0.3 BaTiO 3 . The oxide mixture was milled for 4 h using alcohol and ZrO 2 balls as milling media. The slurry was dried at 80 8C in a box oven, and then uniaxially
Fig. 1. XRD pattern for the oxide mixture with a nominal composition of 0.7PbŽZn 1r 3 Nb 2r3 .O 3 –0.3BaTiO 3 .
pressed into green pellets of about 70% of the theoretical density with a diameter of 10 mm and thickness of 1 mm, using a hardened stainless steel die at
Fig. 3. XRD patterns for the oxide mixture sintered for 1 h at different temperatures: Ža. 1050, Žb. 1075, Žc. 1100, Žd. 1125 and Že. 1150 8C.
L.B. Kong et al.r Materials Letters 53 (2002) 205–210
a pressure of 50 MPa. PVA aqueous solution of 5 wt.% was also added to the mixture powder as binder. Sintering of the pellets was carried out using a Carbolite RHF 1600 type furnace in air at temperatures ranging from 1050 to 1125 8C for a fixed duration of 1 h, with both heating and cooling rates of 5 8C. Before reaching soaking temperature, the samples were kept at 600 8C for 2 h to burn out the PVA binder. Lead atmosphere was employed to
207
eliminate the evaporation of PbO during sintering by using compact of PbO and ZrO mixture. The sintering behavior of the mixture was conducted using a Setaram Setsys 16r18 type dilatometer at a heating rate of 10 8Crmin. X-ray diffraction analysis of the oxide mixture and the sintered ceramics was performed using a Rigaku ultimaq type diffractometer ŽXRD. with Cu K a radiation. The density of the PZT ceramics was measured using a
Fig. 4. Microstructures of the cross-sectional surfaces for the PMN–BT ceramics sintered at different temperatures: Ža. 1050, Žb. 1075, Žc. 1100, Žd. 1125 and Že. 1150 8C.
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L.B. Kong et al.r Materials Letters 53 (2002) 205–210
Mirage MD-200S type electronic densimeter. The microstructure development for the sintered samples was characterized using a JEOL JSM-6340F type field emission scanning electronic microscope ŽFESEM.. All the sintered samples were then polished and covered with silver paste as the electrode for electrical and dielectric measurement. The dielectric and ferroelectric properties of the sintered samples were measured using HP 4194A impedance analyzer and Radiant Technology RT6000HVA type standard ferroelectric tester, respectively.
3. Results and discussions Fig. 1 shows the XRD pattern for the oxide mixture ball milled for 4 h. All the diffraction peaks correspond to the constituent oxides, indicating that no reaction takes place in the mixing process. Fig. 2 shows linear shrinkage and sintering rate of the green pellet of the oxide mixture. The pellet begins expansion at about 600 8C and exhibits a maximum at about 790 8C. This expansion is attributed to the reaction of the constituent oxidesrcarbonate with the PZN–BT perovskite phase because the volume of the PZN–BT is larger than the total volume of the components. Maximum sintering rate was observed at about 805 8C, which indicates the completion of the reaction. In general, the present technique is a one-step processing technique applying the reactive sintering approach. Reactive sintering is a promising fabrication technique for multiple component ceramics, in which the reactions between constituent phases take place during sintering process at high temperatures. It is advantageous over other methods because of its simplicity of experimental operation and enhanced densification progress and has been successfully applied to several multiple component ceramic systems w11,12x. Since most densification takes place above 1000 8C, the sintering temperature of the PZN–BT ceramics via the one-step process was chosen from 1050 to 1150 8C. To check the phase composition of the sintered PZN–BT ceramics, X-ray diffraction on all the sintered samples was performed. Fig. 3 shows the XRD patterns for the samples sintered at temperatures from 1050 to 1125 8C. It can be seen that single phase perovskite 0.7PbŽZn 1r3 Nb 2r3 .O 3 –0.3Ba
TiO 3 has been formed as the mixture was subjected to sintering at 1050 8C for 1 h. The perovskite phase is also noted to be stable up to 1125 8C. As the sintering temperature increases to 1150 8C, the presence of a slight amount of pyrochlore phase is observed. The appearance of the pyrochlore phase in the 1150 8C-sintered samples may be related to the loss of PbO. Fig. 4 shows the cross-sectional surface microstructure of the PZN–BT ceramics sintered at different temperatures. The grain size of the PZN–BT ceramics was estimated from the SEM observations and is plotted as a function of sintering temperature as shown in Fig. 5. Almost fully dense Ž) 96% of the theoretical density., PZN–BT ceramics with an average grain size of 2.4 mm were obtained when the mixture was sintered at 1050 8C for 1 h. There is no significant change observed in the density of the samples as a function of sintering temperature. The average grain size increases almost linearly with the sintering temperature Žfrom about 3.2 mm at 1075 8C to about 8.5 mm at 1150 8C.. Finally, the 1125 8C-sintered PZN–BT ceramics were noted to exhibit a density of ; 99% with an average grain size of 6.2 mm.
Fig. 5. Grain size of the PMN–BT ceramics with sintering temperature.
L.B. Kong et al.r Materials Letters 53 (2002) 205–210
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4. Conclusions
Fig. 6. Dielectric constant of the PMN–BT ceramics varying with sintering temperature.
Fig. 6 illustrates the dielectric constant measured at 1 kHz and room temperature as a function of sintering temperature. The dielectric constant increases rapidly from 1050 to 1100 8C and then gradually from 1100 to 1125 8C. The increase in dielectric constant of the PZN–BT ceramics as a function of sintering temperature from 1050 to 1125 8C is related to the grain size and density of the samples varying with temperature. For ferroelectric ceramic materials, it is well accepted that the coupling effect between grain boundaries constrains the domain reorientation and domain wall mobility w13,14x. When the grain size increases, this effect will be reduced because the grain boundary fraction is inversely proportional to the grain size. As a result, the dielectric constant increases with the increase of grain size of the ferroelectric ceramics. The decrease in dielectric constant of the 1150 8C-sintered sample can be attributed to the presence of the pyrochlore phase although its grain size is larger. In addition, the dielectric properties Ždielectric constant of 2826 and dielectric loss of 0.03. of the PZN–BT ceramics sintered at 1125 8C for 1 h in the present work are in good agreement with the literature value via the conventional process w9x, implying the reliability of the one-step process.
Our results demonstrate that it is possible to prepare PZN–BT ceramics via a one-step sintering of their corresponding oxide mixture. Reaction of the constituent oxides into the perovskite phase occurred at about 800 8C, after which densification took place. Single phase PZN–BT with perovskite structure was achieved at 1050 8C and kept stable up to 1125 8C, while pyrochlore phase was observed at sintering temperature of 1150 8C. Almost fully dense ceramics are achieved from all the sintered samples. The dielectric constant of the PZN–BT ceramics increases with sintering temperature from 950 to 1125 8C and maximizes at 1125 8C, which may be attributed to the increased grain size as a result of increasing sintering temperature. The decrease in dielectric constant at 1150 8C is mainly due to the presence of the pyrochlore phase.
References w1x Y. Yokomizo, T. Takahashi, S. Nomura, Ferroelectric properties of PbŽZn 1r 3 Nb 2r3 .O 3 , J. Phys. Soc. Jpn. 28 Ž950. Ž1970. 1278–1284. w2x S. Nomura, J. Kuwata, Electrostriction in PbŽZn 1r 3 Nb 2r3 .O 3 , Mater. Res. Bull. 14 Ž1979. 769–774. w3x J. Kuwata, K. Uchino, S. Nomura, Diffuse phase transitions in lead zinc niobate, Ferroelectrics 22 Ž1979. 863–867. w4x J. Kuwata, K. Uchino, S. Nomura, Phase relations in the PbŽZn 1r 3 Nb 2r3 .O 3 –PbTiO 3 system, Ferroelectrics 37 Ž1981. 579–582. w5x M. Ozgul, K. Tekemura, S. Trolier-Mckinstry, C.A. Randall, Polarization fatigue in PbŽZn 1r 3 Nb 2r3 .O 3 –PbTiO 3 ferroelectric single crystal, J. Appl. Phys. 89 Ž9. Ž2001. 5100– 5106. w6x H.M. Jan, S.H. Oh, J.H. Moon, Thermodynamic stability and mechanisms of formation and decomposition of perovskite PbŽZn 1r 3 Nb 2r3 .O 3 prepared by the PbO flux method, J. Am. Ceram. Soc. 75 Ž1992. 82–88. w7x T.R. Shrout, A. Halliyal, Preparation of lead-based ferroelectric relaxors for capacitors, Am. Ceram. Soc. Bull. 66 Ž1987. 704–711. w8x N. Wakiya, K. Shinozaki, N. Mizutani, Estimation of phase stability in PbŽMg 1r 3 Nb 2r3 .O 3 and PbŽZn 1r3 Nb 2r3 .O 3 using the bond valence approach, J. Am. Ceram. Soc. 80 Ž1997. 3217–3220. w9x A. Halliyal, U. Kumar, R.E. Newnham, L.E. Cross, Stabilization of the perovskite phase and dielectric properties of ceramics in the PbŽZn 1r 3 Nb 2r3 .O 3 –PbTiO 3 system, Am. Ceram. Soc. Bull. 66 Ž1987. 671–676.
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L.B. Kong et al.r Materials Letters 53 (2002) 205–210
w10x J.R. Belsick, A. Halliyal, U. Kumar, R.E. Newnham, Phase relations and dielectric properties of ceramics in the system PbŽZn 1r3 Nb 2r3 .O 3 –SrTiO 3 –PbTiO 3 , Am. Ceram. Soc. Bull. 66 Ž1987. 664–667. w11x T.R. Shrout, P. Patet, S. Kim, G.S. Lee, Conventionally prepared submicrometer lead-based perovskite powders by reactive calcinations, J. Am. Ceram. Soc. 73 Ž1990. 1862– 1867.
w12x M.N. Rahaman, L.C. De Jonghe, Reaction sintering of zinc ferrite during constant rates of heating, J. Am. Ceram. Soc. 76 Ž1993. 1739–1744. w13x H.D. Chen, K.R. Udayakumar, C.J. Gaskey, L.E. Cross, J.J. Bernstein, L.C. Niles, J. Am. Ceram. Soc. 79 Ž1996. 2189. w14x C.A. Randall, N. Kim, J.P. Kucera, W. Cao, T.R. Shrout, J. Am. Ceram. Soc. 81 Ž1998. 677.