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The role of structural point defects in the type formation of electret state in perovskite-type oxides
Journal of Electrostatics, 24 (1990) 295-300 Elsevier
295
THE ROLE OF S T R U C T U R A L P O I N T D E F E C T S IN THE F O R M A T I O N OF E L E C T R E T STATE IN P E R O V S K I T E - T Y P E OXIDES
S.O. LISITSINA, E.M. PANCHENKO, I.P. RAEVSKII, Yu.A. TRUSOV and E.G. FESENKO
Rostov State University, 344006 Rostov-on-Don (U.S.S.R.) (Received July 22, 1988; accepted in revised form May 14, 1989)
Summary In oxides with a perovskite-type structure an essential role in the formation of an electret state is played by point defects (anion and cation vacancies). The concentration of the anion vacancies, rather than the area of intercrystallite boundaries, predominantly affects the magnitude of the effective surface charge density. A qualitative model of the electret effect in ABO3 oxides, which agrees with the results obtained, is proposed.
1. Formulation of the problem
Among the substances of the oxygen-octahedral type, in which a stable electret state can be induced, oxides with the perovskite structure (OPS) take a prominent place [ 1-4 ]. However, the nature of the electret state in these oxides has not been studied well enough. The electrical properties of OPS in constant and low-frequency fields have been shown [5,6] to depend to a considerable degree on the point defects of the structure, i.e., the ion vacancies. Thus, the study of the influence the point and extended defects of the structure have on the electret state in OPS is clearly deserving of attention. The most likely point defects in the OPS of the ABO3 type are the oxygen vacancies Vo and the A-ion vacancies VA, whose concentrations can greatly vary in different oxides of this family, while the probability of B-ion vacancy formation is extremely low [5,6]. As regards the extended defects, their most probable types in the OPS crystals are the linear, surface and bulk dislocations. In ceramics, which consists of numerous crystallites divided by intercrystallite boundaries at which there often exist intercrystallite phase regions, an important type of defects is the intercrystallite layer (comprising both the surface layers of crystallites and any intercrystallite phase). Depending on the composition and the depth of this layer, it may be regarded as a surface or a volume defect, although, considering its small volume concentration, it appears preferable to view it as a surface defect. 0304-3886/90/$03.50
© 1990 Elsevier Science Publishers B.V.
296 Attempts to determine individual roles of the vacancies and of the intercrystallite layer in the formation of the electret properties in OPS met with certain difficulties. For example, in Ref. [ 7 ] a change was detected in the electret properties of CaTiO3 due to its modification with the oxides of rare-earth elements, but, at the same time, the Vo concentration (No) rose and the area of the intercrystallite boundaries was enlarged. In the present work, we have continued the attempts directed at the OPS from two subfamilies, AIIB~VO3 {CaTiO3 ) and AIBVO3 (NaNbO3). These were subjected to actions giving rise each time to only one of the factors: either a change in the vacancy concentration or a change in the area of intercrystallite boundaries. 2. Experimental
In order to induce the electret state, the samples were polarized under the following conditions: for CaTi03 the polarization temperature was 420 K and the strength of the electric field was 106 V/m; for solid solutions of NaNbO3 these values were 370 K and 106 V/m, respectively. {These conditions were chosen such that the current across the samples amounted to 10-7-10 -6 A.) The magnitude of the surface charge density a in the samples after polarization was determined using the compensation method [8]. In CaTiO3 the predominant type of point defect is Vo, whereas the VA concentration (NA) is small [5 ]. The change in No was effected through reduction of the ceramic in vacuum (10 -5 torr for 30 min at different temperatures of annealing, T,), with the area of the intercrystallite boundaries remaining unchanged. Reversible emergence and disappearance of Vo in redox processes in the oxides of the ATiO3 type ( A = B a , St, Ca) have been confirmed by the results of gravimetric studies [ 9-11 ]. In the process of losing oxygen, the structure of an OPS remains stable, while the emerging Vo substantially affect the value of electroconductivity 7. Under the assumption that the formation of one Vo corresponds to the appearance of one conduction electron, the change in the mass of the sample correlates quite well with the change in its value of 7. This enables information to be obtained on the change of No in a sample from the change in its y. In the present work, the change in No was monitored by the change in ~. Figure 1 shows the dependence of 7 in the samples on Ta. The measured values of a for the reduced CaTiO3 ceramics are also given in Fig. 1. In order to elucidate the role of intercrystallite boundaries in inducing the electret state, we investigated ceramic samples of CaTiO3 having different average grain sizes D. The samples were prepared either by usual sintering or by hot pressing. The microstructure of the ceramic was studied with a light microscope (Neophot 21B). The average size of a grain was estimated by the method of chords of Spector [ 12 ]. The results of measurements of a are given
297
2F.~O'°,(Ohm.m)~
O-.~04Cou[.m "2
31-
I/
I
45
12 11-
l
500
~
j2
700
go0
~
"11
flO0
Fig. 1. Dependence of the effective surface charge density a (curves 1 a n d 2) and electroconductivity y (curve 3) of the CaTi03 ceramics on the temperature of vacuum annealing (curve 1 measured 9 × 102 s after polarization, curve 2 - 2.7 X 10 ° s after polarization ). TABLE 1
Dependence of the effective surface charge density on the average grain size of the CaTiO3 ceramics D×IO 5 (m)
a × 1 0 4 (C m -2)
1.9 2.5 3.0
3.5 3.0 2.75
in Table 1. In the ceramic with grains of a smaller size (i.e.with increased intercrystallite boundary area), the density of the electret charge slightly increases. Unlike CaTiO3, in the stoichiometric ceramic NaNbO3, regardless of whether it was prepared by usual sintering (D--20-30 pro) or by hot pressing (D = 5-10 ~m), no electret effect was observed. In order to create the Vo and VA vacancies, excess Nb205 was introduced into the starting mixture of Na2CO3
298
~.~0 5, Coue.m -2
4
! 03 0,2 0,5 0,4 0,5 0,6 0,7 × Fig. 2. Dependence of the effective surface charge density on the composition of the ceramics sintered from the starting material Nal_xNbQ_=/2.
and Nb205 in accordance with the formula Nal_xNbO3_x/2. The formation of defective solid solutions Nai_ xNbO3_ x/2 can be controlled by the change in the parameter of the unit cell when a one-phase structure of the perovskite type is preserved [13]. With an increase in x in the range 0 < x < 0.1, the values of No and NA ( A = N a ) must grow. In the range x>0.1, the ceramic becomes twophase and a change in the parameter of the unit cell can no longer be determined exclusively by the changes in No and NA [13]. Using X-ray measurements, we have found an analogous variation in the parameter of the unit cell. Thus, the conclusion may be drawn that No and NA grow in the ceramic Nal_xNbO3_x/2with increasing x. As x increased from 0.005 to 0.1, for the ceramics obtained by usual sintering, the values of a grew monotonically, while the average grain size in the samples under study changed insignificantly, equalling 5-7 pm. In the region x> 0.1, where the ceramics is two-phase, the value of a decreased with increasing x (Fig. 2 ). 3. D i s c u s s i o n
It has been found that a crucial role in the formation of an electret state in NaNbO3 is played by point defects of the structure. In CaTiO3, the magnitude of a depends both on No and on the average size of a grain. It is evident from Fig. 1 that in the range 700 K < Ta < 900 K the change in a, measured 9 × 102 s
299
after poling, is proportional to the increase in 7 of the samples. Thus, in the samples reduced at 870 K (7 and consequently No were increased 2.4 times) the values of a turned out to be 2.6 times greater than in the unreduced samples, with the size of the grains remaining unchanged. The relaxation time of a in the samples reduced at Ta ~<780 K is approximately the same as in unreduced samples. For T , > 780 K, the relaxation time of a is sharply decreased though being still much greater than the Maxwell relaxation time (~m = eeo/7~ 10 s). The appearance of a maximum on the curve a (Ta) is due to the competition of two processes, i.e., the growth in the absolute value of a resulting from an increase in No and the reduction in the relaxation time of abecause of an increase in 7. Along with the data obtained for NaNb03, the data of these experiments support the conclusion as to the decisive role of Vo in the formation of an electret state in the OPS. The relative increase in the value of a due to an enlargement of the area of intercrystallite boundaries (in the model ceramic having the form of cubic crystallites, the area of the intercrystallite boundaries is inversely proportional to the average size of the crystallite) is approximately 1.2 times less than the relative change in a brought about by the thermal vacuum treatment of the samples. This confirms the conclusion as to the predominant effect the concentration of Vo, rather than the area of intercrystallite boundaries, has on the value of a. An analysis of the results obtained in the present work as well as of the data in literature on the important role of the point defects in the processes of polarization and electrical conduction in OPS [14-16] makes it possible to suggest a qualitative model of the electret state in the OPS. This model is based on the conclusion as to the crucial role in the formation of an electret state of the structural point defects, namely, the vacancies Vo and VA. Some vacancies have a charge (positive in the case of Vo and negative for VA) and form complexes. Under the action of a poling field these complexes break up and the charged vacancies displaced toward the electrodes give rise to a heterocharge. The accumulation of the charged vacancies near the electrodes increases the strength of the electric field. This leads to injection of charge carriers from the electrodes, thus producing a homocharge. During the polarization of the electret the injected charge is trapped by ion vacancies. Owing to the interaction between the injected and the volume charges, the relaxation time of the total charge is much greater than that of the respective charges taken separately. After removal of the field, the captured charge carriers are released from traps and compensated due to conductivity of the sample. In the case of a high conductivity (e.g., in OPS when a high value of No is reached), the available free charge carriers rapidly screen the electret charge which becomes unstable or cannot be registered at all.
300
References 1 2 3 4 5 6 7
8 9 10 11 12 13 14 15 16
G. Wiseman and G. Feaster, J. Chem. Phys., 26 (3) (1957) 521-527. A.N. Gubkin, Elektret, Nauka, Moscow, U.S.S.R., 1978, 190 pp. (in Russian). J. Vusevker, O. Kramarov, P. Nesterenko and A. Sokallo, Ferroelectrics, 6 (1973) 107-109. E. Freudenfeld, E. Panchenko, R. Kleine and V. Zagoruiko, Ferroelectrics, 68 (1-4) (1986) 141-143. O.I. Prokopalo and I.P. Raevskii, Elektrofizicheskiye svoistva oksidov semeystva perovskita, Rostov University Publishing House, Rostov-on-Don, U.S.S.R., 1985, 104 pp. (in Russian ). O. Prokopalo and A. Turik, Ferroelectrics, 22 (1-2) (1978) 749-751. A.N. Gubkin and A.G. Tochenaya, O structure i elektretnykh svoistvakh keramicheskogo titanata kaltsiya s dobavkami ionov khroma i redkozemelnykh elementov, in: Fizika dielektrikov i perspektivy yeyo razvitiya, Leningrad, U.S.S.R., 1973, pp. 128-129 (in Russian). C. Reedyk and M. Perlman, J. Electrochem. Soc., 115 (1) (1968) 49-51. N.M. Tallan (Ed.), Electrical Conductivity in Ceramics and Glass, Marcel Dekker, New York, 1974. R.J. Paulener and R.N. Blumenthal, J. Am. Ceram. Soc., 54 (1971) 610. T.V. Boys and N.A. Mikhailova, Kinetika vosstanovleniya titanata bariya vodorodom, Voprosy radioelektroniki, 2 ( 1962 ) 35-41. S.M. Saltykov, Stereometricheskaya metallografiya, Moscow, 1970, 376 pp. (in Russian). Y. Bouilland, Bull. Soc. Fr. Mineral. Crystallogr., 92 (4) (1969) 347. O. Prokopalo, Ferroelectrics, 14 (1976) 683-686. Y. Saito and S. Yamanaka, Commun. Electron., 41 (1959) 70-76. T. Fukami, M. Kusunoki and H. Tsuchiva, Jpn. J. Appl. Phys., 1987, Pt 2, 26, Suppl., 46-49.
295
THE ROLE OF S T R U C T U R A L P O I N T D E F E C T S IN THE F O R M A T I O N OF E L E C T R E T STATE IN P E R O V S K I T E - T Y P E OXIDES
S.O. LISITSINA, E.M. PANCHENKO, I.P. RAEVSKII, Yu.A. TRUSOV and E.G. FESENKO
Rostov State University, 344006 Rostov-on-Don (U.S.S.R.) (Received July 22, 1988; accepted in revised form May 14, 1989)
Summary In oxides with a perovskite-type structure an essential role in the formation of an electret state is played by point defects (anion and cation vacancies). The concentration of the anion vacancies, rather than the area of intercrystallite boundaries, predominantly affects the magnitude of the effective surface charge density. A qualitative model of the electret effect in ABO3 oxides, which agrees with the results obtained, is proposed.
1. Formulation of the problem
Among the substances of the oxygen-octahedral type, in which a stable electret state can be induced, oxides with the perovskite structure (OPS) take a prominent place [ 1-4 ]. However, the nature of the electret state in these oxides has not been studied well enough. The electrical properties of OPS in constant and low-frequency fields have been shown [5,6] to depend to a considerable degree on the point defects of the structure, i.e., the ion vacancies. Thus, the study of the influence the point and extended defects of the structure have on the electret state in OPS is clearly deserving of attention. The most likely point defects in the OPS of the ABO3 type are the oxygen vacancies Vo and the A-ion vacancies VA, whose concentrations can greatly vary in different oxides of this family, while the probability of B-ion vacancy formation is extremely low [5,6]. As regards the extended defects, their most probable types in the OPS crystals are the linear, surface and bulk dislocations. In ceramics, which consists of numerous crystallites divided by intercrystallite boundaries at which there often exist intercrystallite phase regions, an important type of defects is the intercrystallite layer (comprising both the surface layers of crystallites and any intercrystallite phase). Depending on the composition and the depth of this layer, it may be regarded as a surface or a volume defect, although, considering its small volume concentration, it appears preferable to view it as a surface defect. 0304-3886/90/$03.50
© 1990 Elsevier Science Publishers B.V.
296 Attempts to determine individual roles of the vacancies and of the intercrystallite layer in the formation of the electret properties in OPS met with certain difficulties. For example, in Ref. [ 7 ] a change was detected in the electret properties of CaTiO3 due to its modification with the oxides of rare-earth elements, but, at the same time, the Vo concentration (No) rose and the area of the intercrystallite boundaries was enlarged. In the present work, we have continued the attempts directed at the OPS from two subfamilies, AIIB~VO3 {CaTiO3 ) and AIBVO3 (NaNbO3). These were subjected to actions giving rise each time to only one of the factors: either a change in the vacancy concentration or a change in the area of intercrystallite boundaries. 2. Experimental
In order to induce the electret state, the samples were polarized under the following conditions: for CaTi03 the polarization temperature was 420 K and the strength of the electric field was 106 V/m; for solid solutions of NaNbO3 these values were 370 K and 106 V/m, respectively. {These conditions were chosen such that the current across the samples amounted to 10-7-10 -6 A.) The magnitude of the surface charge density a in the samples after polarization was determined using the compensation method [8]. In CaTiO3 the predominant type of point defect is Vo, whereas the VA concentration (NA) is small [5 ]. The change in No was effected through reduction of the ceramic in vacuum (10 -5 torr for 30 min at different temperatures of annealing, T,), with the area of the intercrystallite boundaries remaining unchanged. Reversible emergence and disappearance of Vo in redox processes in the oxides of the ATiO3 type ( A = B a , St, Ca) have been confirmed by the results of gravimetric studies [ 9-11 ]. In the process of losing oxygen, the structure of an OPS remains stable, while the emerging Vo substantially affect the value of electroconductivity 7. Under the assumption that the formation of one Vo corresponds to the appearance of one conduction electron, the change in the mass of the sample correlates quite well with the change in its value of 7. This enables information to be obtained on the change of No in a sample from the change in its y. In the present work, the change in No was monitored by the change in ~. Figure 1 shows the dependence of 7 in the samples on Ta. The measured values of a for the reduced CaTiO3 ceramics are also given in Fig. 1. In order to elucidate the role of intercrystallite boundaries in inducing the electret state, we investigated ceramic samples of CaTiO3 having different average grain sizes D. The samples were prepared either by usual sintering or by hot pressing. The microstructure of the ceramic was studied with a light microscope (Neophot 21B). The average size of a grain was estimated by the method of chords of Spector [ 12 ]. The results of measurements of a are given
297
2F.~O'°,(Ohm.m)~
O-.~04Cou[.m "2
31-
I/
I
45
12 11-
l
500
~
j2
700
go0
~
"11
flO0
Fig. 1. Dependence of the effective surface charge density a (curves 1 a n d 2) and electroconductivity y (curve 3) of the CaTi03 ceramics on the temperature of vacuum annealing (curve 1 measured 9 × 102 s after polarization, curve 2 - 2.7 X 10 ° s after polarization ). TABLE 1
Dependence of the effective surface charge density on the average grain size of the CaTiO3 ceramics D×IO 5 (m)
a × 1 0 4 (C m -2)
1.9 2.5 3.0
3.5 3.0 2.75
in Table 1. In the ceramic with grains of a smaller size (i.e.with increased intercrystallite boundary area), the density of the electret charge slightly increases. Unlike CaTiO3, in the stoichiometric ceramic NaNbO3, regardless of whether it was prepared by usual sintering (D--20-30 pro) or by hot pressing (D = 5-10 ~m), no electret effect was observed. In order to create the Vo and VA vacancies, excess Nb205 was introduced into the starting mixture of Na2CO3
298
~.~0 5, Coue.m -2
4
! 03 0,2 0,5 0,4 0,5 0,6 0,7 × Fig. 2. Dependence of the effective surface charge density on the composition of the ceramics sintered from the starting material Nal_xNbQ_=/2.
and Nb205 in accordance with the formula Nal_xNbO3_x/2. The formation of defective solid solutions Nai_ xNbO3_ x/2 can be controlled by the change in the parameter of the unit cell when a one-phase structure of the perovskite type is preserved [13]. With an increase in x in the range 0 < x < 0.1, the values of No and NA ( A = N a ) must grow. In the range x>0.1, the ceramic becomes twophase and a change in the parameter of the unit cell can no longer be determined exclusively by the changes in No and NA [13]. Using X-ray measurements, we have found an analogous variation in the parameter of the unit cell. Thus, the conclusion may be drawn that No and NA grow in the ceramic Nal_xNbO3_x/2with increasing x. As x increased from 0.005 to 0.1, for the ceramics obtained by usual sintering, the values of a grew monotonically, while the average grain size in the samples under study changed insignificantly, equalling 5-7 pm. In the region x> 0.1, where the ceramics is two-phase, the value of a decreased with increasing x (Fig. 2 ). 3. D i s c u s s i o n
It has been found that a crucial role in the formation of an electret state in NaNbO3 is played by point defects of the structure. In CaTiO3, the magnitude of a depends both on No and on the average size of a grain. It is evident from Fig. 1 that in the range 700 K < Ta < 900 K the change in a, measured 9 × 102 s
299
after poling, is proportional to the increase in 7 of the samples. Thus, in the samples reduced at 870 K (7 and consequently No were increased 2.4 times) the values of a turned out to be 2.6 times greater than in the unreduced samples, with the size of the grains remaining unchanged. The relaxation time of a in the samples reduced at Ta ~<780 K is approximately the same as in unreduced samples. For T , > 780 K, the relaxation time of a is sharply decreased though being still much greater than the Maxwell relaxation time (~m = eeo/7~ 10 s). The appearance of a maximum on the curve a (Ta) is due to the competition of two processes, i.e., the growth in the absolute value of a resulting from an increase in No and the reduction in the relaxation time of abecause of an increase in 7. Along with the data obtained for NaNb03, the data of these experiments support the conclusion as to the decisive role of Vo in the formation of an electret state in the OPS. The relative increase in the value of a due to an enlargement of the area of intercrystallite boundaries (in the model ceramic having the form of cubic crystallites, the area of the intercrystallite boundaries is inversely proportional to the average size of the crystallite) is approximately 1.2 times less than the relative change in a brought about by the thermal vacuum treatment of the samples. This confirms the conclusion as to the predominant effect the concentration of Vo, rather than the area of intercrystallite boundaries, has on the value of a. An analysis of the results obtained in the present work as well as of the data in literature on the important role of the point defects in the processes of polarization and electrical conduction in OPS [14-16] makes it possible to suggest a qualitative model of the electret state in the OPS. This model is based on the conclusion as to the crucial role in the formation of an electret state of the structural point defects, namely, the vacancies Vo and VA. Some vacancies have a charge (positive in the case of Vo and negative for VA) and form complexes. Under the action of a poling field these complexes break up and the charged vacancies displaced toward the electrodes give rise to a heterocharge. The accumulation of the charged vacancies near the electrodes increases the strength of the electric field. This leads to injection of charge carriers from the electrodes, thus producing a homocharge. During the polarization of the electret the injected charge is trapped by ion vacancies. Owing to the interaction between the injected and the volume charges, the relaxation time of the total charge is much greater than that of the respective charges taken separately. After removal of the field, the captured charge carriers are released from traps and compensated due to conductivity of the sample. In the case of a high conductivity (e.g., in OPS when a high value of No is reached), the available free charge carriers rapidly screen the electret charge which becomes unstable or cannot be registered at all.
300
References 1 2 3 4 5 6 7
8 9 10 11 12 13 14 15 16
G. Wiseman and G. Feaster, J. Chem. Phys., 26 (3) (1957) 521-527. A.N. Gubkin, Elektret, Nauka, Moscow, U.S.S.R., 1978, 190 pp. (in Russian). J. Vusevker, O. Kramarov, P. Nesterenko and A. Sokallo, Ferroelectrics, 6 (1973) 107-109. E. Freudenfeld, E. Panchenko, R. Kleine and V. Zagoruiko, Ferroelectrics, 68 (1-4) (1986) 141-143. O.I. Prokopalo and I.P. Raevskii, Elektrofizicheskiye svoistva oksidov semeystva perovskita, Rostov University Publishing House, Rostov-on-Don, U.S.S.R., 1985, 104 pp. (in Russian ). O. Prokopalo and A. Turik, Ferroelectrics, 22 (1-2) (1978) 749-751. A.N. Gubkin and A.G. Tochenaya, O structure i elektretnykh svoistvakh keramicheskogo titanata kaltsiya s dobavkami ionov khroma i redkozemelnykh elementov, in: Fizika dielektrikov i perspektivy yeyo razvitiya, Leningrad, U.S.S.R., 1973, pp. 128-129 (in Russian). C. Reedyk and M. Perlman, J. Electrochem. Soc., 115 (1) (1968) 49-51. N.M. Tallan (Ed.), Electrical Conductivity in Ceramics and Glass, Marcel Dekker, New York, 1974. R.J. Paulener and R.N. Blumenthal, J. Am. Ceram. Soc., 54 (1971) 610. T.V. Boys and N.A. Mikhailova, Kinetika vosstanovleniya titanata bariya vodorodom, Voprosy radioelektroniki, 2 ( 1962 ) 35-41. S.M. Saltykov, Stereometricheskaya metallografiya, Moscow, 1970, 376 pp. (in Russian). Y. Bouilland, Bull. Soc. Fr. Mineral. Crystallogr., 92 (4) (1969) 347. O. Prokopalo, Ferroelectrics, 14 (1976) 683-686. Y. Saito and S. Yamanaka, Commun. Electron., 41 (1959) 70-76. T. Fukami, M. Kusunoki and H. Tsuchiva, Jpn. J. Appl. Phys., 1987, Pt 2, 26, Suppl., 46-49.