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Dielectric and field-induced strain behavior of modified lead zirconate titanate ceramics
Pergamon
Materials Research Bulletin 36 (2001) 171–179
Dielectric and field-induced strain behavior of modified lead zirconate titanate ceramics Yun-Woo Nam, Ki Hyun Yoon* Department of Ceramic Engineering, Yonsei University, Seoul 120 –140 South Korea (Refereed) Received 28 March 2000; accepted 31 May 2000
Abstract Dielectric and field-induced strain behavior of the Pb0.97Y0.02[(Zr0.6Sn0.4)1-yTiy]O3 ceramics (PYZST) has been investigated as a function of Ti4⫹ content (0.07ⱕyⱕ0.20) and temperature. A structural change from the orthorhombic to rhombohedral phase was observed with increasing Ti4⫹ concentration, accompanied by the antiferroelectric to ferroelectric phase transition. As the temperature increases from the room temperature, the specimens of the composition with yⱕ0.12 undergo phase transitions from the ferroelectric rhombohedral to paraelectric cubic through antiferroelectric orthorhombic. The specimens with compositions near the antiferroelectric-ferroelectric phase boundary exhibited digital-type strain curve with the shape memory effect due to the antiferroelectric to ferroelectric phase transition. In the specimen of y⫽0.085, the electric field for the antiferroelectric to ferroelectric phase transition was 32kV/cm and the field-induced strain accompanied by the phase transition was 3x10-3. In addition, a temperature-composition phase diagram was determined from the study of crystal structure and the dielectric and field-induced strain properties. © 2001 Elsevier Science Ltd. All rights reserved. Keywords: A. Ceramics; A. Electronic materials; D. Dielectric properties; D. Ferroelectricity
1. Introduction Pb(Zr,Ti)O3 is widely used in sensor and actuator applications as a representative piezoelectric material. Especially important are compositions in the range of the morphotropic
* Corresponding author. Tel.: ⫹82-2-2123-2847; fax: ⫹82-2-362-9199. E-mail address: [email protected] (K.H. Yoon). 0025-5408/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 5 - 5 4 0 8 ( 0 0 ) 0 0 4 8 1 - 5
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phase boundary (MPB) from the rhombohedral to tetragonal phase due to their excellent piezoelectric properties [1– 4]. After Berlincourt et al. reported energy conversion due to the antiferroelectric to ferroelectric phase transition in Pb(Nb,Zr,Sn,Ti)O3 [5– 8], there have been continuous studies on the antiferroelectricity of modified Pb(Zr,Ti)O3 for use in many types of transducers [9 –13]. This transition is accompanied by a large volume change due to the difference in the unit cell parameters between two phases [7], therefore, the modified Pb(Zr,Sn,Ti)O3 has been also investigated for various actuator applications [11,14]. The field-induced antiferroelectric to ferroelectric phase transition in PbZrO3 and modified PbZrO3 has been restricted to a very limited temperature range just under a dielectricconstant maximum [15]. However, the range of modified Pb(Zr,Ti)O3 has been extended to lower temperature [10 –13]. On the other hand, the field-induced phase transition properties are very sensitive to the composition in modified Pb(Zr,Ti)O3 [10 –11]. The energy required for the antiferroelectric to ferroelectric phase transition is also very sensitive to composition of modified Pb(Zr,Ti)O3. Therefore, the precise control of the composition is very important to determine the phase transition properties. In this study, the structure of Pb0.97Y0.02[(Zr0.6Sn0.4)1-yTiy]O3 ceramics (0.07ⱕyⱕ0.20), and their dielectric and strain properties have been investigated with increasing Ti4⫹ concentration and temperature. A temperature-composition phase diagram of the Pb0.97Y0.02[(Zr0.6Sn0.4)1-yTiy]O3 ceramics has also been determined. 2. Experimental procedure The Pb0.97Y0.02[(Zr0.6Sn0.4)1-yTiy]O3 ceramics with Ti4⫹ concentration of 0.07ⱕyⱕ0.20 were prepared by a conventional ceramic fabrication technique using high purity (⬎99.9%) PbO, Y2O3, ZrO2, SnO2, and TiO2 as raw materials. In preparing the specimens, the materials were first weighed for given compositions and ball milled for 48h in ethyl alcohol. The mixed slurries were dried and calcined at 9000C for 3h with a heating rate of 300oC/h. Green disks mixed with small amount of PVA as a binder were cold pressed. The pellets were sintered at 1250oC for 2 h with a heating rate of 200oC/h in a lead rich environment [16 –18]. The sintered specimens were cut into thin plates with thickness ranging from 0.2 to 0.5mm, and silver electrodes were coated onto the polished surfaces by screen printing for the electric tests. The crystal structures after sintering were verified with an X-ray diffraction. Dimensional change was studied using a dilatometer. Dielectric constant vs. temperature was evaluated by measuring the temperature dependence of capacitance at 1 kHz using an LCR meter. Polarization hysteresis loops were determined using a Sawyer-Tower circuit at 60Hz. Longitudinal field-induced strains were evaluated by measuring output voltage from a contact sensor driven by a lock-in amplifier.
3. Results and discussion Figure 1 shows the XRD patterns for sintered specimens with compositions of 0.07⬍y⬍0.20. All compositions show a perovskite structure. As the Ti4⫹ concentration
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Fig. 1. XRD patterns of Pb0.97Y0.02[(Zr0.6Sn0.4)1-yTiy]O3 ceramics sintered at 1250oC for 2h.
increases from y⫽0.08, the crystal structure changes from an orthorhombic into a rhombohedral at a composition of y⫽0.09 Figure 2 shows the polarization hysteresis loops of the specimens with compositions in the range of 0.08⬍y⬍0.20. The specimen of y⫽0.08 shows a typical antiferroelectric double hysteresis loop. The polarization hysteresis loop changes into a typical ferroelectric hysteresis loop at y⫽0.10, passing through an intermediate form which displays both antiferroelectric and ferroelectric characteristics at y⫽0.085⬃0.09. The composition of y⫽0.09, which corresponds to the transitional point in the shape of the hysteresis loop from the
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Fig. 2. Polarization hysteresis loops of Pb0.97Y0.02[(Zr0.6Sn0.4)1-yTiy]O3 with an increase of Ti4⫹ concentration. [x axis: electric field (14 kV/cm/div), y axis: polarization (13.25 C/cm2/div)].
antiferroelectric to the ferroelectric, agrees well with the structural transition point from the orthorhombic to rhombohedral in XRD analysis. Figure 3 shows the curves of dielectric constant versus temperature for specimens of various compositions. The specimen with y⫽0.07 shows a broad transition peak corresponding to a maximum dielectric constant, while the peak becomes sharper with increasing Ti4⫹
Fig. 3. Dielectric constant versus temperature of Pb0.97Y0.02[(Zr0.6Sn0.4)1-yTiy]O3 ceramics with the compositions of y⫽0.07– 0.20.
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Fig. 4. Polarization hysteresis loops of Pb0.97Y0.02[(Zr0.6Sn0.4)1-yTiy]O3 ceramics of y⫽0.085, y⫽0.12, and y⫽0.15 with an increase of temperature. [x axis: electric field (10 kV/cm/div), y axis: polarization (10 C/cm2/div)].
concentration. In the specimens with composition of y⬎0.09, another transition peak appears between the room temperature and the maximum dielectric constant temperature, and the sharpness of the peak increases with Ti4⫹ concentration. However the peak disappears again in the specimens of yⱖ0.15. This phenomenon is also observed in the PbZrO3-PbTiO3 and PbZrO3-BaZrO3 systems for which the weak antiferroelectric to ferroelectric phase transition peak appears below the maximum dielectric constant temperature as the concentration of ferroelectric PbTiO3 and BaZrO3 increases [15,19,20]. Figure 4 illustrates the shape change of the polarization hysteresis loop with an increase of temperature for the specimens of y⫽0.085, y⫽0.12, and y⫽0.15. The antiferroelectric double hysteresis loop of the specimen of y⫽0.085 changes into a paraelectric linear loop as the temperature increases. On the other hand, the ferroelectric hysteresis loop of the specimen of y⫽0.12 changes into the antiferroelectric double hysteresis loop and then to the paraelectric linear loop. However, the ferroelectric hysteresis loop of the specimen of y⫽0.15 changes directly into the paraelectric shape. In addition, the transition temperatures agree well with phase transition temperatures in the dielectric constant vs. temperature curves. The specimens of y⫽0.085 and y⫽0.12 were subjected to XRD analysis near 2⫽ 45o to investigate the crystal structure of each phase with increasing temperature. The results are shown in Fig. 5. In the specimen of y⫽0.085, the split peaks of orthorhombic (042)o and (400)o are shown at room temperature and a single peak of cubic (200)c appears above the antiferroelectric to paraelectric phase transition temperature of 175oC. In the specimen of y⫽0.12, with the ferroelectric to antiferroelectric to paraelectric phase transition upon heating, the rhombohedral (200)r peak, the split orthorhombic (042)o and (400)o peaks, and the cubic (200)c peak are observed in the ferroelectric phase below 130oC, in the antiferro-
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Fig. 5. XRD patterns of Pb0.97Y0.02[(Zr0.6Sn0.4)1-yTiy]O3 ceramics of y⫽0.085 and y⫽0.12 with an increase of temperature.
electric phase between 130oC and 150oC, and above the antiferroelectric to paraelectric transition at 155oC, respectively. In the system with the phase transition of the ferroelectric to paraelectric through antiferroelectric as temperature increases, the field-induced antiferroelectric to ferroelectric phase transition occurs at relatively lower electric field than the system which shows the antiferroelectric to paraelectric phase transition through ferroelectric. This is because the free energy between the two phases is almost equal in the system [10,11]. Figure 6 shows the thermal expansion for the specimen of y⫽0.12, which indicates the linear change by the phase transition. The transition point is well established for the ferroelectric to antiferroelectric phase transition temperature of 120oC. Additionally, the linear dimension change of
Fig. 6. Linear thermal expansion of Pb0.97Y0.02[(Zr0.6Sn0.4)1-yTiy]O3 ceramics with the composition of y⫽0.12.
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Fig. 7. Longitudinal strain versus electric field of Pb0.97Y0.02[(Zr0.6Sn0.4)1-yTiy]3 ceramics. [x axis: electric field (10 kV/cm/div), y axis: strain (1x10-3/div)].
the ferroelectrics appears to be larger than that of the antiferroelectrics. Therefore, positive strain is expected when the ferroelectric phase is induced from the antiferroelectric phase via the electric field. Figure 7 illustrates the longitudinal strain vs. electric field of the specimens with various compositions. The specimen of y⫽0.08 shows a typical antiferroelectric strain curve in which the strain increases abruptly above the antiferroelectric to ferroelectric phase transition field (EA-F ⫽ 45kV/cm). The field-induced strain is maintained for a while upon removing the field, and the specimen recovers its initial antiferroelectric state again below the ferroelectric to antiferroelectric phase transition field (EF-A ⫽ 16kV/cm). In the specimen of y⫽0.085, the strain of 3x10-3 is induced at the field of 32kV/cm by the antiferroelectric to ferroelectric phase transition and the field-induced strain is maintained after removing the field. An inverse electric field turns the field-induced ferroelectric phase into the original antiferroelectric state. However, the field-induced strain in the specimen of y⫽0.09 does not recover its original strain state even though the inverse field is applied. Above the composition of y⫽0.09, the strain curve approaches the typical ferroelectric strain loop. This transition composition of y⫽0.09 for the shape change of strain curve with an increase of Ti4⫹ concentration agrees well with the structural transition composition from the orthorhombic to the rhombohedral as shown in Fig. 1. Therefore, it can be concluded that the ferroelectricity increases with an increase of Ti4⫹ concentration due to an increase of ferroelectric spontaneous polarization caused by the structural change from the orthorhombic to the rhombohedral. Figure 8 shows the temperature-composition phase diagram obtained from studies of the phase transition, and the dielectric and field-induced strain properties with an increase of Ti4⫹ concentration and temperature. It differs strongly from the unmodified Pb(Zr,Ti)O3 ceramic system [15] in that the stable range of the antiferroelectric orthorhombic phase extends to a higher concentration of Ti4⫹, and the paraelectric cubic phase becomes more stable at a lower temperature than that of the unmodified Pb(Zr,Ti)O3. As the Ti4⫹ concen-
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Fig. 8. Temperature-composition phase diagram of Pb0.97Y0.02[(Zr0.6Sn0.4)1-yTiy]O3 ceramic system.
tration increases from y⫽0.07 to y⫽0.20, the antiferroelectric orthorhombic phase transforms into the ferroelectric rhombohedral phase at y⫽0.10, passing through the intermediate phase between the antiferroelectric orthorhombic and the ferroelectric rhombohedral. With increasing temperature, the ferroelectric rhombohedral phase of the specimen with Ti4⫹ concentration of yⱕ0.12 transforms into the antiferroelectric orthorhombic and then to the paraelectric cubic. However, the ferroelectric rhombohedral phase of the specimen with Ti4⫹ concentration of yⱖ0.15 transforms directly into the paraelectric cubic. 4. Conclusion A structural change from the orthorhombic to rhombohedral phase was observed with increasing Ti4⫹ concentration, accompanied by the antiferroelectric to ferroelectric phase transition. As the temperature increased, the ferroelectric rhombohedral phase of the specimens with compositions of yⱕ0.12 changed into the paraelectric cubic, passing through the antiferroelectric orthorhombic. However, the ferroelectric rhombohedral phase of the specimens with compositions of y⬎0.12 changed directly into the paraelectric cubic phase. The specimen of y⫽0.085 shows a digital-type strain curve, in which the field-induced strain was 3x10-3 at the antiferroelectric to ferroelectric phase transition field of 32kV/cm, and the strain value at the shape memory state was 2.8x10-3. The composition-temperature phase diagram was obtained, demonstrating that the stable composition range of the antiferroelectric orthorhombic phase extended to y⫽0.09 in the room temperature, and the stable temperature range of the antiferroelectric and the paraelectric phase expands to a temperature lower than that of the unmodified Pb(Zr,Ti)O3 system
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Acknowledgments The authors wish to acknowledge the financial support of the Korea Research Foundation made in the program year of 1999, under Grant No. KRF-97– 005-E00129.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
J.S. Kim, K.H. Yoon, J. Mater. Sci. 29 (1994) 809. I. Kanno, S. Fujii, T. Kamada, R. Takayama, Appl. Phys. Lett. 7 (1997) 1378. B. Xu, Y. Ye, L.E. Cross, Appl. Phys. Lett. 74 (1999) 3549. B.A. Scott, G. Burns, J. Am. Ceram. Soc. 55 (1972) 331. D. Berlincourt, H. Jaffe, H.H.A. Krueger, B. Jaffe, Appl. Phys. Lett. 3 (1963) 90. D.A. Berilincourt, H.H.A. Krueger, Annual Progress Rep., Sandia Corp., P.O., 51–9689A:1963. D. Berlincourt, H.H. Krueger, B. Jaffe, J. Phys. Chem. Solids 25 (1964) 659. D. Berlincourt, IEEE transactions on Sonics and Ultrasonics, su-13 (1966) 116. I.J. Fritz, J.D. Keck, J. Phys. Chem. Solids, 39 (1978) 1163. K. Uchino, MRS Int’l., Mtg. on Adv. Mats. 9 (1989) 489. K. Uchino, S. Nomura, Ferroelectrics 50 (1983) 191. W.Y. Pan, C.Q. Dam, Q.M. Zhang, L.E. Cross, J. Appl. Phys. 66 (1989) 6014. P. Yang, D.A. Payne, J. Appl. Phys. 71 (1992) 1361. W. Pan, Q. Zhang, A. Bhalla, L.E. Cross, J. Am. Ceram. Soc. 72 (1989) 571. E. Sawaguchi, J. Phys. Soc. Jpn. 8 (1953) 615. A.I. Kingon, J. Brian Clark, J. Am. Ceram. Soc. 66 (1983) 253. J.J. Dih, R.M. Fulrath, J. Am. Ceram. Soc. 61 (1978) 448. E.K. W. Gook, R.K. Mishra, G. Thomas, J. Am. Ceram. Soc. 64 (1987) 517. K.H. Yoon, S.C. Whang, D.H. Kang, J. Mater. Sci. 32 (1997) 17. B.P. Pokharel and D. Pandey, J. Appl. Phys. 86 (1999) 3327.
Materials Research Bulletin 36 (2001) 171–179
Dielectric and field-induced strain behavior of modified lead zirconate titanate ceramics Yun-Woo Nam, Ki Hyun Yoon* Department of Ceramic Engineering, Yonsei University, Seoul 120 –140 South Korea (Refereed) Received 28 March 2000; accepted 31 May 2000
Abstract Dielectric and field-induced strain behavior of the Pb0.97Y0.02[(Zr0.6Sn0.4)1-yTiy]O3 ceramics (PYZST) has been investigated as a function of Ti4⫹ content (0.07ⱕyⱕ0.20) and temperature. A structural change from the orthorhombic to rhombohedral phase was observed with increasing Ti4⫹ concentration, accompanied by the antiferroelectric to ferroelectric phase transition. As the temperature increases from the room temperature, the specimens of the composition with yⱕ0.12 undergo phase transitions from the ferroelectric rhombohedral to paraelectric cubic through antiferroelectric orthorhombic. The specimens with compositions near the antiferroelectric-ferroelectric phase boundary exhibited digital-type strain curve with the shape memory effect due to the antiferroelectric to ferroelectric phase transition. In the specimen of y⫽0.085, the electric field for the antiferroelectric to ferroelectric phase transition was 32kV/cm and the field-induced strain accompanied by the phase transition was 3x10-3. In addition, a temperature-composition phase diagram was determined from the study of crystal structure and the dielectric and field-induced strain properties. © 2001 Elsevier Science Ltd. All rights reserved. Keywords: A. Ceramics; A. Electronic materials; D. Dielectric properties; D. Ferroelectricity
1. Introduction Pb(Zr,Ti)O3 is widely used in sensor and actuator applications as a representative piezoelectric material. Especially important are compositions in the range of the morphotropic
* Corresponding author. Tel.: ⫹82-2-2123-2847; fax: ⫹82-2-362-9199. E-mail address: [email protected] (K.H. Yoon). 0025-5408/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 5 - 5 4 0 8 ( 0 0 ) 0 0 4 8 1 - 5
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phase boundary (MPB) from the rhombohedral to tetragonal phase due to their excellent piezoelectric properties [1– 4]. After Berlincourt et al. reported energy conversion due to the antiferroelectric to ferroelectric phase transition in Pb(Nb,Zr,Sn,Ti)O3 [5– 8], there have been continuous studies on the antiferroelectricity of modified Pb(Zr,Ti)O3 for use in many types of transducers [9 –13]. This transition is accompanied by a large volume change due to the difference in the unit cell parameters between two phases [7], therefore, the modified Pb(Zr,Sn,Ti)O3 has been also investigated for various actuator applications [11,14]. The field-induced antiferroelectric to ferroelectric phase transition in PbZrO3 and modified PbZrO3 has been restricted to a very limited temperature range just under a dielectricconstant maximum [15]. However, the range of modified Pb(Zr,Ti)O3 has been extended to lower temperature [10 –13]. On the other hand, the field-induced phase transition properties are very sensitive to the composition in modified Pb(Zr,Ti)O3 [10 –11]. The energy required for the antiferroelectric to ferroelectric phase transition is also very sensitive to composition of modified Pb(Zr,Ti)O3. Therefore, the precise control of the composition is very important to determine the phase transition properties. In this study, the structure of Pb0.97Y0.02[(Zr0.6Sn0.4)1-yTiy]O3 ceramics (0.07ⱕyⱕ0.20), and their dielectric and strain properties have been investigated with increasing Ti4⫹ concentration and temperature. A temperature-composition phase diagram of the Pb0.97Y0.02[(Zr0.6Sn0.4)1-yTiy]O3 ceramics has also been determined. 2. Experimental procedure The Pb0.97Y0.02[(Zr0.6Sn0.4)1-yTiy]O3 ceramics with Ti4⫹ concentration of 0.07ⱕyⱕ0.20 were prepared by a conventional ceramic fabrication technique using high purity (⬎99.9%) PbO, Y2O3, ZrO2, SnO2, and TiO2 as raw materials. In preparing the specimens, the materials were first weighed for given compositions and ball milled for 48h in ethyl alcohol. The mixed slurries were dried and calcined at 9000C for 3h with a heating rate of 300oC/h. Green disks mixed with small amount of PVA as a binder were cold pressed. The pellets were sintered at 1250oC for 2 h with a heating rate of 200oC/h in a lead rich environment [16 –18]. The sintered specimens were cut into thin plates with thickness ranging from 0.2 to 0.5mm, and silver electrodes were coated onto the polished surfaces by screen printing for the electric tests. The crystal structures after sintering were verified with an X-ray diffraction. Dimensional change was studied using a dilatometer. Dielectric constant vs. temperature was evaluated by measuring the temperature dependence of capacitance at 1 kHz using an LCR meter. Polarization hysteresis loops were determined using a Sawyer-Tower circuit at 60Hz. Longitudinal field-induced strains were evaluated by measuring output voltage from a contact sensor driven by a lock-in amplifier.
3. Results and discussion Figure 1 shows the XRD patterns for sintered specimens with compositions of 0.07⬍y⬍0.20. All compositions show a perovskite structure. As the Ti4⫹ concentration
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Fig. 1. XRD patterns of Pb0.97Y0.02[(Zr0.6Sn0.4)1-yTiy]O3 ceramics sintered at 1250oC for 2h.
increases from y⫽0.08, the crystal structure changes from an orthorhombic into a rhombohedral at a composition of y⫽0.09 Figure 2 shows the polarization hysteresis loops of the specimens with compositions in the range of 0.08⬍y⬍0.20. The specimen of y⫽0.08 shows a typical antiferroelectric double hysteresis loop. The polarization hysteresis loop changes into a typical ferroelectric hysteresis loop at y⫽0.10, passing through an intermediate form which displays both antiferroelectric and ferroelectric characteristics at y⫽0.085⬃0.09. The composition of y⫽0.09, which corresponds to the transitional point in the shape of the hysteresis loop from the
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Fig. 2. Polarization hysteresis loops of Pb0.97Y0.02[(Zr0.6Sn0.4)1-yTiy]O3 with an increase of Ti4⫹ concentration. [x axis: electric field (14 kV/cm/div), y axis: polarization (13.25 C/cm2/div)].
antiferroelectric to the ferroelectric, agrees well with the structural transition point from the orthorhombic to rhombohedral in XRD analysis. Figure 3 shows the curves of dielectric constant versus temperature for specimens of various compositions. The specimen with y⫽0.07 shows a broad transition peak corresponding to a maximum dielectric constant, while the peak becomes sharper with increasing Ti4⫹
Fig. 3. Dielectric constant versus temperature of Pb0.97Y0.02[(Zr0.6Sn0.4)1-yTiy]O3 ceramics with the compositions of y⫽0.07– 0.20.
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Fig. 4. Polarization hysteresis loops of Pb0.97Y0.02[(Zr0.6Sn0.4)1-yTiy]O3 ceramics of y⫽0.085, y⫽0.12, and y⫽0.15 with an increase of temperature. [x axis: electric field (10 kV/cm/div), y axis: polarization (10 C/cm2/div)].
concentration. In the specimens with composition of y⬎0.09, another transition peak appears between the room temperature and the maximum dielectric constant temperature, and the sharpness of the peak increases with Ti4⫹ concentration. However the peak disappears again in the specimens of yⱖ0.15. This phenomenon is also observed in the PbZrO3-PbTiO3 and PbZrO3-BaZrO3 systems for which the weak antiferroelectric to ferroelectric phase transition peak appears below the maximum dielectric constant temperature as the concentration of ferroelectric PbTiO3 and BaZrO3 increases [15,19,20]. Figure 4 illustrates the shape change of the polarization hysteresis loop with an increase of temperature for the specimens of y⫽0.085, y⫽0.12, and y⫽0.15. The antiferroelectric double hysteresis loop of the specimen of y⫽0.085 changes into a paraelectric linear loop as the temperature increases. On the other hand, the ferroelectric hysteresis loop of the specimen of y⫽0.12 changes into the antiferroelectric double hysteresis loop and then to the paraelectric linear loop. However, the ferroelectric hysteresis loop of the specimen of y⫽0.15 changes directly into the paraelectric shape. In addition, the transition temperatures agree well with phase transition temperatures in the dielectric constant vs. temperature curves. The specimens of y⫽0.085 and y⫽0.12 were subjected to XRD analysis near 2⫽ 45o to investigate the crystal structure of each phase with increasing temperature. The results are shown in Fig. 5. In the specimen of y⫽0.085, the split peaks of orthorhombic (042)o and (400)o are shown at room temperature and a single peak of cubic (200)c appears above the antiferroelectric to paraelectric phase transition temperature of 175oC. In the specimen of y⫽0.12, with the ferroelectric to antiferroelectric to paraelectric phase transition upon heating, the rhombohedral (200)r peak, the split orthorhombic (042)o and (400)o peaks, and the cubic (200)c peak are observed in the ferroelectric phase below 130oC, in the antiferro-
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Fig. 5. XRD patterns of Pb0.97Y0.02[(Zr0.6Sn0.4)1-yTiy]O3 ceramics of y⫽0.085 and y⫽0.12 with an increase of temperature.
electric phase between 130oC and 150oC, and above the antiferroelectric to paraelectric transition at 155oC, respectively. In the system with the phase transition of the ferroelectric to paraelectric through antiferroelectric as temperature increases, the field-induced antiferroelectric to ferroelectric phase transition occurs at relatively lower electric field than the system which shows the antiferroelectric to paraelectric phase transition through ferroelectric. This is because the free energy between the two phases is almost equal in the system [10,11]. Figure 6 shows the thermal expansion for the specimen of y⫽0.12, which indicates the linear change by the phase transition. The transition point is well established for the ferroelectric to antiferroelectric phase transition temperature of 120oC. Additionally, the linear dimension change of
Fig. 6. Linear thermal expansion of Pb0.97Y0.02[(Zr0.6Sn0.4)1-yTiy]O3 ceramics with the composition of y⫽0.12.
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Fig. 7. Longitudinal strain versus electric field of Pb0.97Y0.02[(Zr0.6Sn0.4)1-yTiy]3 ceramics. [x axis: electric field (10 kV/cm/div), y axis: strain (1x10-3/div)].
the ferroelectrics appears to be larger than that of the antiferroelectrics. Therefore, positive strain is expected when the ferroelectric phase is induced from the antiferroelectric phase via the electric field. Figure 7 illustrates the longitudinal strain vs. electric field of the specimens with various compositions. The specimen of y⫽0.08 shows a typical antiferroelectric strain curve in which the strain increases abruptly above the antiferroelectric to ferroelectric phase transition field (EA-F ⫽ 45kV/cm). The field-induced strain is maintained for a while upon removing the field, and the specimen recovers its initial antiferroelectric state again below the ferroelectric to antiferroelectric phase transition field (EF-A ⫽ 16kV/cm). In the specimen of y⫽0.085, the strain of 3x10-3 is induced at the field of 32kV/cm by the antiferroelectric to ferroelectric phase transition and the field-induced strain is maintained after removing the field. An inverse electric field turns the field-induced ferroelectric phase into the original antiferroelectric state. However, the field-induced strain in the specimen of y⫽0.09 does not recover its original strain state even though the inverse field is applied. Above the composition of y⫽0.09, the strain curve approaches the typical ferroelectric strain loop. This transition composition of y⫽0.09 for the shape change of strain curve with an increase of Ti4⫹ concentration agrees well with the structural transition composition from the orthorhombic to the rhombohedral as shown in Fig. 1. Therefore, it can be concluded that the ferroelectricity increases with an increase of Ti4⫹ concentration due to an increase of ferroelectric spontaneous polarization caused by the structural change from the orthorhombic to the rhombohedral. Figure 8 shows the temperature-composition phase diagram obtained from studies of the phase transition, and the dielectric and field-induced strain properties with an increase of Ti4⫹ concentration and temperature. It differs strongly from the unmodified Pb(Zr,Ti)O3 ceramic system [15] in that the stable range of the antiferroelectric orthorhombic phase extends to a higher concentration of Ti4⫹, and the paraelectric cubic phase becomes more stable at a lower temperature than that of the unmodified Pb(Zr,Ti)O3. As the Ti4⫹ concen-
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Fig. 8. Temperature-composition phase diagram of Pb0.97Y0.02[(Zr0.6Sn0.4)1-yTiy]O3 ceramic system.
tration increases from y⫽0.07 to y⫽0.20, the antiferroelectric orthorhombic phase transforms into the ferroelectric rhombohedral phase at y⫽0.10, passing through the intermediate phase between the antiferroelectric orthorhombic and the ferroelectric rhombohedral. With increasing temperature, the ferroelectric rhombohedral phase of the specimen with Ti4⫹ concentration of yⱕ0.12 transforms into the antiferroelectric orthorhombic and then to the paraelectric cubic. However, the ferroelectric rhombohedral phase of the specimen with Ti4⫹ concentration of yⱖ0.15 transforms directly into the paraelectric cubic. 4. Conclusion A structural change from the orthorhombic to rhombohedral phase was observed with increasing Ti4⫹ concentration, accompanied by the antiferroelectric to ferroelectric phase transition. As the temperature increased, the ferroelectric rhombohedral phase of the specimens with compositions of yⱕ0.12 changed into the paraelectric cubic, passing through the antiferroelectric orthorhombic. However, the ferroelectric rhombohedral phase of the specimens with compositions of y⬎0.12 changed directly into the paraelectric cubic phase. The specimen of y⫽0.085 shows a digital-type strain curve, in which the field-induced strain was 3x10-3 at the antiferroelectric to ferroelectric phase transition field of 32kV/cm, and the strain value at the shape memory state was 2.8x10-3. The composition-temperature phase diagram was obtained, demonstrating that the stable composition range of the antiferroelectric orthorhombic phase extended to y⫽0.09 in the room temperature, and the stable temperature range of the antiferroelectric and the paraelectric phase expands to a temperature lower than that of the unmodified Pb(Zr,Ti)O3 system
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Acknowledgments The authors wish to acknowledge the financial support of the Korea Research Foundation made in the program year of 1999, under Grant No. KRF-97– 005-E00129.
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