Ferroelectrics, 293: 283–290, 2003 c Taylor & Francis Inc. Copyright ISSN: 0015-0193 print / 1563-5112 online DOI: 10.1080/00150190390238612
Interferometric Measurement of the Temperature Dependence of Piezoelectric Coefficients for PZN-8%PT Single Crystals ˇ M. SULC, J. ERHART, and J. NOSEK International Center for Piezoelectric Research, Technical University of Liberec, H´alkova 6, CZ-461 17 Liberec 1, Czech Republic, E-mail:
[email protected] (Received September 4, 2002; In final form December 15, 2002) Piezoelectrically induced displacements were measured by a laser interferometric method in the temperature range 150 K–333 K for two domain-engineered 0.92Pb(Zn1/3 Nb2/3 )-0.08PbTiO3 single crystals poled in the [001]-direction. Assuming an effective tetragonal symmetry, we determined the piezoelectric d31 coefficient by measuring the displacement on (100) and (110) crystal faces. Coefficient d31 = −1280 pC/N was calculated for the [100]-direction at room temperature. Its value is reduced to −550 pC/N at 210 K. Piezoelectric coefficient d31 measured on a second sample in the [110]-direction exhibited a smaller value than the previous one, only about –555 pC/N at room temperature and –360 pC/N at 190 K. Different domain structures in both samples might be the possible reason for this difference. Temperature dependence of piezoelectric coefficient d33 was also investigated. The effect of aging is also discussed. The domain configuration in crystals was observed by optical microscopy. Keywords: Piezoelectric single crystals; relaxor ferroelectrics; temperature dependence of piezoelectric coefficients; interferometry PACS: 77.84, 77.65, 42.87
1. INTRODUCTION Piezoelectric single crystals Pb(Zn1/3 Nb 2/3 )O3 -PbTiO3 (PZNT) have been investigated for their excellent piezoelectric properties. Piezoelectric coefficient d33 exceedes 2500 pC/N [1, 2] and electromechanical coupling factor k33 is greater than 90% in [001] orientated crystals PZN-8%PT, poled along [001] axis. Single crystals of PZNT are considered as one of the most promising materials that can be used in various advanced applications [3], as actuators, sonar transducers, and medical ultrasound imaging systems. Most of the devices use these relaxors at about room temperature, near morphotropic 283
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phase boundaries. Same special application requires piezoelectric materials for cryogenic temperature, for example as sensors for strain measurement in the superconductor magnet [4], actuators for space telescopes, etc. In addition, fast response is one of the desirable features of piezoactuators. Tests at cryogenic temperatures must be conducted so that the fact that this reliability is also true at low temperatures is verified. In some applications both high displacement and large dimension of piezoelectric materials are required. But it is problematic to grow large crystals only in the single domain state. Therefore it is important to know how much the multidomain crystal properties are reproducible and age free because only single domain crystals or precisely engineered domain state samples exhibit stable properties [5].
2. EXPERIMENTAL Samples Four samples of single crystal 0.92 Pb(Zn1/3 Nb2/3 )O3 -0.08 PbTiO3 were tested. Crystals were grown by the method of spontaneous nucleation and slow cooling and were provided by Crystal Associates, Inc., East Hanover, NJ. In this method, the solution PbO flux and PZN-PT are heated until the entire solution is molten and then cooled through the saturation temperature to create nuclei which grow as the solution is further cooled. The amount of PT in solid solution is always very close to that in the starting mixture. The samples were approximately cubic in shape, with dimensions approximately 4,9 × 4,9 × 4,9 mm3 . The samples were orientated along [001] axe; the axe z of cube had direction [001]. The first two samples A, B had axes x and y identical with axes [100] and [010] respectively, and for the second two samples C, D axes x and y were parallel with [−110] and [110]. The sample surfaces were mirror polished. Samples were poled 20 minutes along [001] by applied electric field 3 kV/cm at room temperature. Coefficients d33 were measured after polling by d33 meter ZJ-3C (Institute of Acousctic, Academica Sinica, China). The coefficients d33 for samples A, B were higher than 2000 pC/N; the coefficients for C, D were d33 = 1910 pC/N and 1750 pC/N respectively. The structure of the domains was observed by using a polarized light microscope (Olympus BX60) under crossed polarizers. Stripe domains were visible only in samples A, B, in direction of observation [010]. The typical width of domains was 0.1 mm; the domain wall direction was [−101], see Fig. 1. The crystals were not homogeneous. In crystals C, D were observed
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Figure 1. Stripe domains in PZN-8%PT single crystal.
randomly oriented domains (about 0.01 mm) and in all crystals were noticed dark places, maybe inclusions of PbO.
Experimental Method Laser interferometry was used for d33 and d31 measurements. A single beam Michelson microinterferometer, with sample crystal in one arm, was situated in the cryogenic cooling system (Oxford Instruments), which was based on closed circuit of helium gas. Since the interferometer was small and compact, the situation inside the cryostat suppressed the amplitude of vibration of optical elements in the interferometer, caused by the compressor in the cryostat system. Motion of these elements only shifted the interference pattern, which remained stable. This technical solution also helped to solve the problem of temperature stability. Both arms of the interferometer were at the same temperature. The thermal shift of optical elements (due to the unhomogeneity of temperature field), and the change of sample length were compensated by feedback. For details see [6].
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The sample in the cryostat was cooled (or warmed up) to the selected temperature and stabilized at this temperature for 10 minutes. The cryostat system was switched off so that the sample vibration was eliminated than d31 and d33 coefficients were measured in the frequency range 1 Hz–20 kHz. The useful temperature range of measurement depended on the accuracy of the interferometer arrangement at room temperature. During measurements at cryogenic temperatures, it was not possible to adjust the specimen again because the interferometer was in the closed evacuated cryostat chamber. Cryostat vibration and unhomogeneous dilatation of parts of the interferometer (there are elements of brass, quartz, glue, samples) caused deviation of the beams in the interferometer and cancelled interference pattern. This is why the lower limit of the temperature range at present is approximately 150 K. The upper temperature was 330 K.
3. RESULTS AND DISCUSSION Coefficients d31 of samples were measured by laser interferometry 30 days after poling. They were similar to coefficient published for single domain crystal. Coefficient d31 = −1280 pC/N was calculated for sample A, and d31 = −1150 pC/N for sample B, displacement in [100] direction at room temperature. These values were reduced to −550 pC/N at 210 K. A surprising result was obtained for displacement measurement of the same sample in the [010] direction when values of d31 were significantly lowered, see Fig. 2. Piezoelectric coefficient d31 measured on the second sample C in the [110]-direction exhibited a smaller value than the previous one, only about −555 pC/N at room temperature and −360 pC/N at 190 K, see Fig. 2. Single crystal PZN-8PT has a rhombohedral structure below the morphotropic phase boundary and there is no phase transition while cooling from room temperature. A similar decrease in d31 was measured by standard IEEE method (d31 = 1200 pC/N at room temperature, d31 =800 pC/N at 150 K and d31 = 300 pC/N at 30 K), published by D.S. Paik at all [7]. The frequency dependence of the d31 coefficient shows that it asymptotically approaches a constant value at high frequencies, see Fig. 3. The dispersion appears at higher temperatures. The magnitude of the piezoelectric coefficients decreases with decreasing temperature as given in Fig. 4. The measurements were performed at 3 different time periods after poling. Aging is presented in the inset of Fig. 4 and it seems that aging had exponential character. Similar results were observed for the d33 coefficient. The frequency dependence of the d33 coefficient for
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Figure 2. Temperature dependence of d31 coefficient for PZN-8%PT crystal, frequency of applied field 1000 Hz, 30 days after poling. Curve a is obtained from measurement of induced displacement of (100) face, curve b of (010) face, both for crystal A; curve c for crystal C, (110) face.
Figure 3. Frequency dependence of d31 coefficient for PZN-8%PT crystal A at selected temperatures, 630 days after poling, (010) face.
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Figure 4. Temperature dependence of d31 coefficient for PZN-8%PT crystal, frequency of applied field 1000 Hz, at time 30, 160 and 630 day after poling, (010) face. Aging of d31 coefficient, frequency 1000 Hz, is presented in the inset.
Figure 5. Frequency dependence of d33 coefficient for PZN-8%PT crystal B at selected temperatures, 630 days after poling.
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Figure 6. Temperature dependence of d33 coefficient for PZN-8%PT crystal B, frequency of applied field 1000 Hz, 630 days after poling.
crystal B at selected temperatures, 630 days after poling is presented in Fig. 5. The temperature dependence of d33 coefficient is given in Fig. 6. The reason of piezoelectric constant dispersion seems to be in domain wall motion. Velocity of domain motion is low in crystals; therefore the relaxation frequency is low too. Near the morphotropic phase boundary domain motion increases, and higher values of d31 and d33 are measured. At higher frequencies, there is no domain contribution and only intrinsic piezoelectric properties are measured.
4. CONCLUSIONS The temperature dependencies of piezoelectric coefficients d33 and d31 were measured in the temperature range 150 K–330 K for multidomain single crystals PZN-8%PT. High values of coefficients were observed at temperature near the morphotropic phase boundary. The coefficients were still significant (>400 pC/N) at low temperatures (150 K) and crystals can be used for possible cryogenic applications. However the aging of samples, and the variation in piezoelectric properties for multidomain single crystals, make them problematic in practical applications. Different domain structures in samples might be a possible reason for this variation of piezoelectric properties.
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ACKNOWLEDGEMENTS This work was supported by the grant CEZ J11/98, 242200002 of Ministry of Education of the Czech Republic, and by the Grant Agency of the Czech Republic, projects no. 202/02/1006 and 202/00/1245.
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