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Electret state in PZT-based ceramics
Journal of
ELECTROSTATICS ELSEVIER
Journal of Electrostatics 42 (1997) 295 297
Electret state in PZT-based ceramics E.M. P a n c h e n k o lnstitule c?/Physics, Rostov State UnirersiO', Stachki Str. 194, 344090 Rostor-on-Don, Russia
Received 14 August 1996:received in revised form 19 May 1997: accepted 20 May 1997
Abstract Regularities have been studied that characterize the existence of the electret state in multicomponent solid solutions of the lead zirconate-titanate (PZT). When the unit cell displays rhombohedral distortion in these solutions, their electret state is more stable compared to the case of the tetragonal distortion, t" 1997 Elsevier Science B.V. Keywords: Electrets; Ferroelectricity; Relaxation time: PZT-based ceramics: Solid solutions:
Surface charge
1. Introduction The electret state (ES) in ferroelectric materials, on the one hand, may find practical applications [1], but on the other, the development of super-low relaxation polarizations may have a considerable influence on their properties [2]. A stable ES has been reported for PZT ceramics [3], however, no rationalizing studies have been conducted in that area. It should be emphasized in this connection that the existence of a stable ES in a PZT system is an interesting phenomenon. This interest stems from the fact that in PbZrO3-ceramic the electret effect is unstable while in PbTiO3 it is absent altogether [4]. The purpose of the present work is to reveal the regularities governing the existence of the ES in the PZT-based solid solutions.
2. Experimental results Solid solutions of the PZT-system with the tetragonal or rhombohedral distortion of the unit cell have been studied. The multicomponent solutions differ in their degree of ferrohardness, indicating the stability of the domain structure to external effects. The compositions (C) investigated are listed in Table 1. Synthesis of the solid solutions was effected via solid-phase reactions in two steps in the 1120-1170 K temperature range for periods of 1.4 x 104-3.6 x 104 s. Sintering of the samples was done by the 0304-3886/97/$17.00 © 1997Elsevier Science B.V. All rights reserved. PII S 0 3 0 4 - 3 8 8 6 ( 9 7 ) 0 0 1 53-8
E.I~L Panchenko/Journal of Electrostatics 42 (1997) 295 297
296
Table 1
Composition of multicomponent solid solutions of the PZT system (in m o l % ) Components
Index
PbTiO3 PbZrO3
PbW1/2GdII203 PbMnl/3Nb2/303 PbWl/2Mnl/203
C-I
C-2
C-3
C-4
C-5
45.0 53.0 2.0 -
43.7 37.3
47.0 51.0
30.0 61.6
41.7 42.0
4.4 -
-
5.5 2.9
PbLil/,~Sb3/403 PbSbl/2Bil/203 PbWl/zMgl/203 PbZnlj3Nb2/303 PbMnl/3Sb2/303
" -
2.0 -
14.6
3.6 9.4 3.3
-
hot-pressing technique [5] at temperatures from 1270 to 1470 K (depending on composition), at a pressure of 2 x 107 Pa with the time of isothermal exposure equal to 2.4 × 103 s. The porosity of hot-pressed samples (0.6-1.5%) is lower than that of samples obtained by traditional techniques. In ferroelectric materials, local elastic stresses arising during the domain switching depend upon the domain structure. At the same time, the domain structure is determined by the unit cell symmetry, which may be characterized by the uniform parameter of deformation 6 = ( d - do)/do. The unit cell dimension d was defined along the spontaneous polarization direction from X-ray diffraction, and do is the side length of the cube whose volume is equal to the unit cell volume considered. The smaller the value of 6, the more complicated is the domain structure and, consequently, the smaller the mobility of the domain walls and higher the ferrohardness of materials due to a reduction in the dielectric permittivity e. The parameters 6 and e for multicomponent piezo-electric materials are given in Table 2 where, within each symmetry group, the compositions are listed in the order of growing ferrohardness.
Table 2
The parameters of multicomponent solid solutions of the PZT systems Index
Symmetry
6
e
a × l04 C / m 2
r x 10- 7s
C-2 C-5 C-3 C-1 C-4
T T R R R
0.011 0.013 0.004 0.005 0.004
2300 2000 1250 700 290
31.0 23.0 13.0 2.8 0.1
0.2 0.l 11.6 8.3 11.8
Note: T denotes the tetragonal and R the rhombohedral distortion of the elementary cell.
E.M. Panchenko/Journal of Electrostatics 42 (1997) 295-297
297
The ES was formed by poling the samples at 320-440 K in an electric field strength of 3 x 105-7 x 105 V/m depending on composition. The effective surface charge densities a of electrets and the relaxation times r of a for the multicomponent solid solutions of the P Z T system are given in Table 2 to a precision of the measuring error. The values of a were determined by the compensation method [6] which ensures the determination of macroscopic charge Q = aS where S is the sample area. The time dependence of a is well described by the expression a = ao e x p ( - t/r). The ES field direction shows that a is formed due to the external electric field but it is not related to the remanent polarization.
3. Discussion Analysis of the experimental results listed in the tables enables us to draw the following conclusions. The electret state in multicomponent solid solutions of a PZT system with a rhombohedral distortion of the unit cell is more stable compared to the case of the tetragonal distortion. The experimental investigations of the electret charge value dependence, upon the proper point structure defect concentration fulfilled for the defective single-phase solid solutions Nal _~Nb3_ x/203 and on subjecting to thermovacuum processing, ceramic CaTiO3 shows the dependence of the electret state upon the ionic vacancy concentration [-7]. Evidently, in the case of the P Z T system compositions considered, the ferrosoft materials are near the optimum, ensuring the development of space-charge polarization due to the ionic vacancy migration (that is one from the necessary components of the ES origin). In ferrohardness materials produced by poling, the more complicated domain structure prevents the vacancy displacement. As a result, the space-charge polarization develops to a smaller extent, the relaxation time increases, the effective surface charge density decreases, and the electret effect is weaker in such materials.
References [1] E.Zh. Freidenfeld, E.M. Panchenko, R.Z. Kleine, Ferroelectrics 68 (1986) 141-143. [2] M.E. Lines, A.M. Glass, Principles and Application of Ferroelectrics and Related Materials, Clarendon Press, Oxford, 1977. [3] Y.A. Vusevker, O.P. Kramarov, P.S. Nesterenko, Ferroelectrics 6 (1973) 107-109. I-4] E.M. Panchenko, O.I. Prokopalo, V.A. Zagoruiko, J. Phys. D: Appl. Phys. 22 (1989) 1372 1374. I-5] K. Okazaki, Ceramic Engineering for Dielectrics, Gakkensha, Tokyo, 1969. 1-6] G.M. Sessler (ed), Electrets, Springer, Berlin, 1980. 1-7] S.O. Lisitsina, E.M. Panchenko, I.P. Raevski, J. Electrostatics 24 (1990) 295-300.
ELECTROSTATICS ELSEVIER
Journal of Electrostatics 42 (1997) 295 297
Electret state in PZT-based ceramics E.M. P a n c h e n k o lnstitule c?/Physics, Rostov State UnirersiO', Stachki Str. 194, 344090 Rostor-on-Don, Russia
Received 14 August 1996:received in revised form 19 May 1997: accepted 20 May 1997
Abstract Regularities have been studied that characterize the existence of the electret state in multicomponent solid solutions of the lead zirconate-titanate (PZT). When the unit cell displays rhombohedral distortion in these solutions, their electret state is more stable compared to the case of the tetragonal distortion, t" 1997 Elsevier Science B.V. Keywords: Electrets; Ferroelectricity; Relaxation time: PZT-based ceramics: Solid solutions:
Surface charge
1. Introduction The electret state (ES) in ferroelectric materials, on the one hand, may find practical applications [1], but on the other, the development of super-low relaxation polarizations may have a considerable influence on their properties [2]. A stable ES has been reported for PZT ceramics [3], however, no rationalizing studies have been conducted in that area. It should be emphasized in this connection that the existence of a stable ES in a PZT system is an interesting phenomenon. This interest stems from the fact that in PbZrO3-ceramic the electret effect is unstable while in PbTiO3 it is absent altogether [4]. The purpose of the present work is to reveal the regularities governing the existence of the ES in the PZT-based solid solutions.
2. Experimental results Solid solutions of the PZT-system with the tetragonal or rhombohedral distortion of the unit cell have been studied. The multicomponent solutions differ in their degree of ferrohardness, indicating the stability of the domain structure to external effects. The compositions (C) investigated are listed in Table 1. Synthesis of the solid solutions was effected via solid-phase reactions in two steps in the 1120-1170 K temperature range for periods of 1.4 x 104-3.6 x 104 s. Sintering of the samples was done by the 0304-3886/97/$17.00 © 1997Elsevier Science B.V. All rights reserved. PII S 0 3 0 4 - 3 8 8 6 ( 9 7 ) 0 0 1 53-8
E.I~L Panchenko/Journal of Electrostatics 42 (1997) 295 297
296
Table 1
Composition of multicomponent solid solutions of the PZT system (in m o l % ) Components
Index
PbTiO3 PbZrO3
PbW1/2GdII203 PbMnl/3Nb2/303 PbWl/2Mnl/203
C-I
C-2
C-3
C-4
C-5
45.0 53.0 2.0 -
43.7 37.3
47.0 51.0
30.0 61.6
41.7 42.0
4.4 -
-
5.5 2.9
PbLil/,~Sb3/403 PbSbl/2Bil/203 PbWl/zMgl/203 PbZnlj3Nb2/303 PbMnl/3Sb2/303
" -
2.0 -
14.6
3.6 9.4 3.3
-
hot-pressing technique [5] at temperatures from 1270 to 1470 K (depending on composition), at a pressure of 2 x 107 Pa with the time of isothermal exposure equal to 2.4 × 103 s. The porosity of hot-pressed samples (0.6-1.5%) is lower than that of samples obtained by traditional techniques. In ferroelectric materials, local elastic stresses arising during the domain switching depend upon the domain structure. At the same time, the domain structure is determined by the unit cell symmetry, which may be characterized by the uniform parameter of deformation 6 = ( d - do)/do. The unit cell dimension d was defined along the spontaneous polarization direction from X-ray diffraction, and do is the side length of the cube whose volume is equal to the unit cell volume considered. The smaller the value of 6, the more complicated is the domain structure and, consequently, the smaller the mobility of the domain walls and higher the ferrohardness of materials due to a reduction in the dielectric permittivity e. The parameters 6 and e for multicomponent piezo-electric materials are given in Table 2 where, within each symmetry group, the compositions are listed in the order of growing ferrohardness.
Table 2
The parameters of multicomponent solid solutions of the PZT systems Index
Symmetry
6
e
a × l04 C / m 2
r x 10- 7s
C-2 C-5 C-3 C-1 C-4
T T R R R
0.011 0.013 0.004 0.005 0.004
2300 2000 1250 700 290
31.0 23.0 13.0 2.8 0.1
0.2 0.l 11.6 8.3 11.8
Note: T denotes the tetragonal and R the rhombohedral distortion of the elementary cell.
E.M. Panchenko/Journal of Electrostatics 42 (1997) 295-297
297
The ES was formed by poling the samples at 320-440 K in an electric field strength of 3 x 105-7 x 105 V/m depending on composition. The effective surface charge densities a of electrets and the relaxation times r of a for the multicomponent solid solutions of the P Z T system are given in Table 2 to a precision of the measuring error. The values of a were determined by the compensation method [6] which ensures the determination of macroscopic charge Q = aS where S is the sample area. The time dependence of a is well described by the expression a = ao e x p ( - t/r). The ES field direction shows that a is formed due to the external electric field but it is not related to the remanent polarization.
3. Discussion Analysis of the experimental results listed in the tables enables us to draw the following conclusions. The electret state in multicomponent solid solutions of a PZT system with a rhombohedral distortion of the unit cell is more stable compared to the case of the tetragonal distortion. The experimental investigations of the electret charge value dependence, upon the proper point structure defect concentration fulfilled for the defective single-phase solid solutions Nal _~Nb3_ x/203 and on subjecting to thermovacuum processing, ceramic CaTiO3 shows the dependence of the electret state upon the ionic vacancy concentration [-7]. Evidently, in the case of the P Z T system compositions considered, the ferrosoft materials are near the optimum, ensuring the development of space-charge polarization due to the ionic vacancy migration (that is one from the necessary components of the ES origin). In ferrohardness materials produced by poling, the more complicated domain structure prevents the vacancy displacement. As a result, the space-charge polarization develops to a smaller extent, the relaxation time increases, the effective surface charge density decreases, and the electret effect is weaker in such materials.
References [1] E.Zh. Freidenfeld, E.M. Panchenko, R.Z. Kleine, Ferroelectrics 68 (1986) 141-143. [2] M.E. Lines, A.M. Glass, Principles and Application of Ferroelectrics and Related Materials, Clarendon Press, Oxford, 1977. [3] Y.A. Vusevker, O.P. Kramarov, P.S. Nesterenko, Ferroelectrics 6 (1973) 107-109. I-4] E.M. Panchenko, O.I. Prokopalo, V.A. Zagoruiko, J. Phys. D: Appl. Phys. 22 (1989) 1372 1374. I-5] K. Okazaki, Ceramic Engineering for Dielectrics, Gakkensha, Tokyo, 1969. 1-6] G.M. Sessler (ed), Electrets, Springer, Berlin, 1980. 1-7] S.O. Lisitsina, E.M. Panchenko, I.P. Raevski, J. Electrostatics 24 (1990) 295-300.