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Domain reorientation by poling of PZT ceramics in the morphotropic phase boundary region
Solid State Communications, Vol. 90, No. 6, pp. 383-385, 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0038-1098/94 $7.00 + .00
Pergamon
0038-1098(94)E0084-0 DOMAIN REORIENTATION BY P O L I N G OF PZT CERAMICS IN THE M O R P H O T R O P I C PHASE BOUNDARY REGION W.L. Zhong, Y.G. Wang, S.B. Yue and P.L. Zhang Physics Department, Shandong University, Jinan, 250100, P.R. China
(Received 8 November 1993 by D.J. Lockwood) PZT ceramics of composition in the morphotropic phase boundary region are prepared and poled at d.c. fields of different strength. The domain reorientation by poling is studied by X-ray diffraction. A method to calculate the non-180 ° domain switching percentage in the ceramics of two-phase coexistence is proposed.
TO M A K E a ferroelectric ceramic piezoelectric or pyroelectric, poling is necessary whereby the randomly distributed domains are oriented in the direction closest to the applied d.c. field. The percentage of domain switching is a measure of the effectiveness of poling. It depends on crystal symmetry, composition, and imperfection as well as the field. Much work has been done devoted to the determination of domain switching percentage [1-7]. Basically, two techniques can be used. One is based on the diffraction intensity changes resulting from domain switching [3-7]. The other is to measure the switching induced strain [1, 2]. The principal piezoelectric ceramics are PbZrxTil_xO3(PZT), which may have tetragonal or rhombohedral structure depending on x. Because the diffraction pattern is relatively simple for the tetragonal structure, the formula proposed in the literature are solely for the tetragonal phase. However, PZT ceramics with strongest piezoelectric effect and most widely used are those near the morphotropic phase boundary (MPB), which in fact is a region where tetragonal and rhombohedral phases coexist [8]. Although the width of this region depends on grain size etc. [9], in some cases it can be as wide as from x = 0.49 to x = 0.64 [10, 11]. Therefore, it is worthwhile to determine the domain switching percentage of ceramics in the MPB region. The following is our work in this aspect. The composition chosen in this work was PbZro.saTio.4603 + 0.1 wt% MnO2 + 0.1 wt% CeO2. Ceramic disks were prepared by traditional technique. The ceramics are subjected to lead loss at high temperature, and to prevent it and make it up, firing
was carried out in a closed crucible and the disks were covered with ceramic powders of the same composition, besides, an excess of about 0.1 wt% Pb304 was added in the raw materials. Under proper firing conditions, the ceramics usually have a density not less than 98% of the theoretical. The grain size was about 3 - 4 # m . X-ray diffraction patterns were obtained from the flat, as fired faces of seven disks with a Rigaku DMAX-IIIA diffractometer using Cu K a radiation. The diffraction patterns obtained for each of the disks were identical. A conductive paste was provided on both faces as electrodes. After drying, the disks were poled at d.c. fields of different strength for 20min. The piezoelectric strain constant d33, planar electromechanical coupling factor k v and the dielectric constant Er3/~0 were measured one day after poling. The temporary electrodes were then dissolved in an organic solvent. X-ray diffraction patterns were taken from the same face of each disk. At each stage, the samples were not ground or etched, because that will seriously change the domain structure at the surface [12]. The electromechanical properties as a function of poling field are listed in Table 1. It is seen that a saturation is obtained when the field is above 2 k V m m -1. Some of the diffraction patterns are shown in Fig. 1. It is seen that each reflection splits into three peaks, showing the coexistence of both tetragonal and rhombohedral phases [10, 11, 13]. In this figure, reflections indexed with T are assigned to the tetragonal phase and those with R are assigned to the rhombohedral phase. The lattice parameters of the two coexisting phases were calculated from the
383
384
Vol. 90, No. 6
DOMAIN R E O R I E N T A T I O N BY P O L I N G O F PZT CERAMICS relation [8]
Table 1. Variation of the properties with poling field Poling field (kVmm -I)
eT3/e0
0 0.5 1.0 1.5 2.0 2.5 3.0
702 650 640 622 617 620 615
d33 (10 -12 C/N)
kv
0 98 128 141 162 167 165
0 33 45 49 52 52 52
fr
lr
(1)
(10 -2) The results are listed in Table 2. The amount of the rhombohedral phase increases with increasing poling field, indicating that an irreversible phase transition T - R takes place during poling [10, 14]. To determine the 90 ° domain switching percentage in the tetragonal phase, the formula of [4] is used R-R' n r = R(1 + IFoo212R'/iF200i2) '
triplets 200 and 102. The results are: a r = 0.404 nm, cr = 0.413 nm, ag = 0.406 rim, c~ = 89°12 '. In order to get the required intensities, we recorded the reflections (2 0 0)r, (2 0 0)R and (0 0 2)r in a step manner and fitted each experimental line with the superposition of Lorentzian lines as in [10] and [11]. F r o m the intensity ratio Ir/IR, we obtained the fraction amount of each phase by the E
I1"-
glz
It
15
where IF0o2i2 and [F2oo[2 are the squared structure factor amplitudes for reflections 002 and 200 respectively, R is the intensity ratio I(2oo)/I(2oo) before poling, and R' that after poling. In the calculation of Ifoo212 and IF20012, we use the following fractional atomic coordinates [15] of a modified tetragonal PZT: Pb at (0,0,0), 0.54Zr+ 0.46Ti at (0.5,0.5,0.547), O(1) at (0.5,0.5,0.108), 0(2) at (0.5, 0, 0.606). n r as a function of the poling field is shown in Table 2. Because the X-ray diffraction pattern of the rhombohedral phase is rather complicated compared with that of the tetragonal phase, a simple formula to calculate the domain switching from the diffraction intensity change is difficult to derive. Our poling experiments with ceramics on the rhombohedral side showed that their coercive field is much lower than that on the tetragonal side. This can be explained by the much smaller rhombohedral distortion [8]. Judging from the variation of d33 with poling field, saturated poling is reached for a field not less than I k V m m -I at room temperature. Using the strain measurement, Berlincourt [2] obtained the 71 ° and 109 ° switching percentage of a saturately poled rhombohedral PZT to be 65°/,. Because the rhombobedral distortion in the whole range on the R side is to a good approximation constant [8], we assume that
Table 2. Relative amount and non-180 ° switching percentage of the two phases
(¢)
lO
(2)
t
-'=
30
Fig. 1. X-ray diffraction patterns of the ceramics before poling (A), poled at 1 k V m m -t (B), and 3 kV ram-' (C).
Poling field (kV mm - t )
fy
fe
ny
ne
n
0 0.5 1.0 1.5 2.0 2.5 3.0
0.603 0.583 0.574 0.567 0.545 0.548 0.545
0.397 0.417 0.426 0.433 0.455 0.452 0.455
-0.22 0.30 0.37 0.40 0.42 0.41
--0.65 0.65 0.65 0.65 0.65
--0.45 0.49 0.51 0.52 0.52
Vol. 90, No. 6
DOMAIN REORIENTATION BY POLING OF PZT CERAMICS
the switching percentage (0.65) and saturate field (1 kV mm -I) can be applied to all the ceramics on the R side. Thus, we take the switching percentage nR of the samples to be 0.65 for poling field not less than 1 kV mm- i in Table 2. Just as the spontaneous polarizations of the coexisting two phases have additivity, we assume that the total percentage of non-180 ° switching can be expressed as
n = f r n r +fRnR.
(3)
The results are listed in Table 2. It is seen that the percentage becomes nearly constant when poling field is above 2kVmm -1, which is consistent with the saturation of the properties as shown in Table 1. In summary, a method to calculate the non-180 ° domain switching during poling of PZT ceramics in the MPB region is proposed. First, the relative amount of the T phase and R phase is obtained from the intensity ratio of some characteristic reflections. The 90 ° switching percentage of the T phase nr is then calculated by the change in I(200)/I(002). The 71 ° and 109° switching percentage of the R phase nR is taken from the literature. The total percentage of non-180 ° switching is finally obtained as the weighted sum of nr and nR. It is worth mentioning that the percentage of domain switching tends to decrease (depoling) during aging. The slow rearrangements of domains during aging can be examined by XRD, TEM or piezoelectric measurement. Our XRD and piezoelectric measurement were carried out one day after poling.
385
Although no TEM or further XRD was done, piezoelectric properties were measured repeatedly during a one-year shelf aging. The variation was very small, implying that the ceramics have a good time stability. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.
D. Berlincourt & H.H.A. Krueger, J. Appl. Phys. 30, 1804 (1959). D. Berlincourt, Sandia Corp. Tech. Rept., SC4443 (RR), U.S. Dept. Commerce (1960). E.C. Subbara, M.C. McQuarrie & W.R. Buessum, J. Appl. Phys. 28, 1194 (1957). Zhang Xiao-Wen, Ye Chu-Cai & Li Cheng-Cui, Acta Physica Sinica 28, 524 (1979) (in Chinese). Li Cheng-Cui, Zhang Xiao-Wen & Xia YiJiang, Ferroelectrics 37, 623 (1981). Y.S. Ng & A.D. McDonaled, Ferroelectrics 62, 167 (1985). J. Mendiola & L. Pardo, Ferroelectrics 54, 199 (1984). B. Jaffe, W.R. Cook & H. Jaffe, Piezoelectric Ceramics, Academic Press, London (1971). Wenwu Cao & L.E. Cross, Phys. Rev. 1347, 4825 (1993). P. Ari-Gur & L. Benquigui, J. Phys. D8, 1856 (1975). P. Ari-Gur & L. Benquigui, Solid State Commun. 15, 1077 (1974). V.K. Seth, G.I. Gatins & W.A. Schulze, Ferroelectrics 87, 243 (1988). P.Gr. Lucata, FL. Constantinescu & D. Barb, J. Am. Ceram. Soc. 68, 533 (1985). H. Thomann, Ferroelectrics 4, 141 (1972). Y.S. Ng, Ferroelectrics Lett. 3, 141 (1985).
Pergamon
0038-1098(94)E0084-0 DOMAIN REORIENTATION BY P O L I N G OF PZT CERAMICS IN THE M O R P H O T R O P I C PHASE BOUNDARY REGION W.L. Zhong, Y.G. Wang, S.B. Yue and P.L. Zhang Physics Department, Shandong University, Jinan, 250100, P.R. China
(Received 8 November 1993 by D.J. Lockwood) PZT ceramics of composition in the morphotropic phase boundary region are prepared and poled at d.c. fields of different strength. The domain reorientation by poling is studied by X-ray diffraction. A method to calculate the non-180 ° domain switching percentage in the ceramics of two-phase coexistence is proposed.
TO M A K E a ferroelectric ceramic piezoelectric or pyroelectric, poling is necessary whereby the randomly distributed domains are oriented in the direction closest to the applied d.c. field. The percentage of domain switching is a measure of the effectiveness of poling. It depends on crystal symmetry, composition, and imperfection as well as the field. Much work has been done devoted to the determination of domain switching percentage [1-7]. Basically, two techniques can be used. One is based on the diffraction intensity changes resulting from domain switching [3-7]. The other is to measure the switching induced strain [1, 2]. The principal piezoelectric ceramics are PbZrxTil_xO3(PZT), which may have tetragonal or rhombohedral structure depending on x. Because the diffraction pattern is relatively simple for the tetragonal structure, the formula proposed in the literature are solely for the tetragonal phase. However, PZT ceramics with strongest piezoelectric effect and most widely used are those near the morphotropic phase boundary (MPB), which in fact is a region where tetragonal and rhombohedral phases coexist [8]. Although the width of this region depends on grain size etc. [9], in some cases it can be as wide as from x = 0.49 to x = 0.64 [10, 11]. Therefore, it is worthwhile to determine the domain switching percentage of ceramics in the MPB region. The following is our work in this aspect. The composition chosen in this work was PbZro.saTio.4603 + 0.1 wt% MnO2 + 0.1 wt% CeO2. Ceramic disks were prepared by traditional technique. The ceramics are subjected to lead loss at high temperature, and to prevent it and make it up, firing
was carried out in a closed crucible and the disks were covered with ceramic powders of the same composition, besides, an excess of about 0.1 wt% Pb304 was added in the raw materials. Under proper firing conditions, the ceramics usually have a density not less than 98% of the theoretical. The grain size was about 3 - 4 # m . X-ray diffraction patterns were obtained from the flat, as fired faces of seven disks with a Rigaku DMAX-IIIA diffractometer using Cu K a radiation. The diffraction patterns obtained for each of the disks were identical. A conductive paste was provided on both faces as electrodes. After drying, the disks were poled at d.c. fields of different strength for 20min. The piezoelectric strain constant d33, planar electromechanical coupling factor k v and the dielectric constant Er3/~0 were measured one day after poling. The temporary electrodes were then dissolved in an organic solvent. X-ray diffraction patterns were taken from the same face of each disk. At each stage, the samples were not ground or etched, because that will seriously change the domain structure at the surface [12]. The electromechanical properties as a function of poling field are listed in Table 1. It is seen that a saturation is obtained when the field is above 2 k V m m -1. Some of the diffraction patterns are shown in Fig. 1. It is seen that each reflection splits into three peaks, showing the coexistence of both tetragonal and rhombohedral phases [10, 11, 13]. In this figure, reflections indexed with T are assigned to the tetragonal phase and those with R are assigned to the rhombohedral phase. The lattice parameters of the two coexisting phases were calculated from the
383
384
Vol. 90, No. 6
DOMAIN R E O R I E N T A T I O N BY P O L I N G O F PZT CERAMICS relation [8]
Table 1. Variation of the properties with poling field Poling field (kVmm -I)
eT3/e0
0 0.5 1.0 1.5 2.0 2.5 3.0
702 650 640 622 617 620 615
d33 (10 -12 C/N)
kv
0 98 128 141 162 167 165
0 33 45 49 52 52 52
fr
lr
(1)
(10 -2) The results are listed in Table 2. The amount of the rhombohedral phase increases with increasing poling field, indicating that an irreversible phase transition T - R takes place during poling [10, 14]. To determine the 90 ° domain switching percentage in the tetragonal phase, the formula of [4] is used R-R' n r = R(1 + IFoo212R'/iF200i2) '
triplets 200 and 102. The results are: a r = 0.404 nm, cr = 0.413 nm, ag = 0.406 rim, c~ = 89°12 '. In order to get the required intensities, we recorded the reflections (2 0 0)r, (2 0 0)R and (0 0 2)r in a step manner and fitted each experimental line with the superposition of Lorentzian lines as in [10] and [11]. F r o m the intensity ratio Ir/IR, we obtained the fraction amount of each phase by the E
I1"-
glz
It
15
where IF0o2i2 and [F2oo[2 are the squared structure factor amplitudes for reflections 002 and 200 respectively, R is the intensity ratio I(2oo)/I(2oo) before poling, and R' that after poling. In the calculation of Ifoo212 and IF20012, we use the following fractional atomic coordinates [15] of a modified tetragonal PZT: Pb at (0,0,0), 0.54Zr+ 0.46Ti at (0.5,0.5,0.547), O(1) at (0.5,0.5,0.108), 0(2) at (0.5, 0, 0.606). n r as a function of the poling field is shown in Table 2. Because the X-ray diffraction pattern of the rhombohedral phase is rather complicated compared with that of the tetragonal phase, a simple formula to calculate the domain switching from the diffraction intensity change is difficult to derive. Our poling experiments with ceramics on the rhombohedral side showed that their coercive field is much lower than that on the tetragonal side. This can be explained by the much smaller rhombohedral distortion [8]. Judging from the variation of d33 with poling field, saturated poling is reached for a field not less than I k V m m -I at room temperature. Using the strain measurement, Berlincourt [2] obtained the 71 ° and 109 ° switching percentage of a saturately poled rhombohedral PZT to be 65°/,. Because the rhombobedral distortion in the whole range on the R side is to a good approximation constant [8], we assume that
Table 2. Relative amount and non-180 ° switching percentage of the two phases
(¢)
lO
(2)
t
-'=
30
Fig. 1. X-ray diffraction patterns of the ceramics before poling (A), poled at 1 k V m m -t (B), and 3 kV ram-' (C).
Poling field (kV mm - t )
fy
fe
ny
ne
n
0 0.5 1.0 1.5 2.0 2.5 3.0
0.603 0.583 0.574 0.567 0.545 0.548 0.545
0.397 0.417 0.426 0.433 0.455 0.452 0.455
-0.22 0.30 0.37 0.40 0.42 0.41
--0.65 0.65 0.65 0.65 0.65
--0.45 0.49 0.51 0.52 0.52
Vol. 90, No. 6
DOMAIN REORIENTATION BY POLING OF PZT CERAMICS
the switching percentage (0.65) and saturate field (1 kV mm -I) can be applied to all the ceramics on the R side. Thus, we take the switching percentage nR of the samples to be 0.65 for poling field not less than 1 kV mm- i in Table 2. Just as the spontaneous polarizations of the coexisting two phases have additivity, we assume that the total percentage of non-180 ° switching can be expressed as
n = f r n r +fRnR.
(3)
The results are listed in Table 2. It is seen that the percentage becomes nearly constant when poling field is above 2kVmm -1, which is consistent with the saturation of the properties as shown in Table 1. In summary, a method to calculate the non-180 ° domain switching during poling of PZT ceramics in the MPB region is proposed. First, the relative amount of the T phase and R phase is obtained from the intensity ratio of some characteristic reflections. The 90 ° switching percentage of the T phase nr is then calculated by the change in I(200)/I(002). The 71 ° and 109° switching percentage of the R phase nR is taken from the literature. The total percentage of non-180 ° switching is finally obtained as the weighted sum of nr and nR. It is worth mentioning that the percentage of domain switching tends to decrease (depoling) during aging. The slow rearrangements of domains during aging can be examined by XRD, TEM or piezoelectric measurement. Our XRD and piezoelectric measurement were carried out one day after poling.
385
Although no TEM or further XRD was done, piezoelectric properties were measured repeatedly during a one-year shelf aging. The variation was very small, implying that the ceramics have a good time stability. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.
D. Berlincourt & H.H.A. Krueger, J. Appl. Phys. 30, 1804 (1959). D. Berlincourt, Sandia Corp. Tech. Rept., SC4443 (RR), U.S. Dept. Commerce (1960). E.C. Subbara, M.C. McQuarrie & W.R. Buessum, J. Appl. Phys. 28, 1194 (1957). Zhang Xiao-Wen, Ye Chu-Cai & Li Cheng-Cui, Acta Physica Sinica 28, 524 (1979) (in Chinese). Li Cheng-Cui, Zhang Xiao-Wen & Xia YiJiang, Ferroelectrics 37, 623 (1981). Y.S. Ng & A.D. McDonaled, Ferroelectrics 62, 167 (1985). J. Mendiola & L. Pardo, Ferroelectrics 54, 199 (1984). B. Jaffe, W.R. Cook & H. Jaffe, Piezoelectric Ceramics, Academic Press, London (1971). Wenwu Cao & L.E. Cross, Phys. Rev. 1347, 4825 (1993). P. Ari-Gur & L. Benquigui, J. Phys. D8, 1856 (1975). P. Ari-Gur & L. Benquigui, Solid State Commun. 15, 1077 (1974). V.K. Seth, G.I. Gatins & W.A. Schulze, Ferroelectrics 87, 243 (1988). P.Gr. Lucata, FL. Constantinescu & D. Barb, J. Am. Ceram. Soc. 68, 533 (1985). H. Thomann, Ferroelectrics 4, 141 (1972). Y.S. Ng, Ferroelectrics Lett. 3, 141 (1985).