ISSN 1063-7834, Physics of the Solid State, 2006, Vol. 48, No. 6, pp. 1169–1170. © Pleiades Publishing, Inc., 2006. Original Russian Text © S.N. Kallaev, G.G. Gadzhiev, I.K. Kamilov, Z.M. Omarov, S.A. Sadykov, L.A. Reznichenko, 2006, published in Fizika Tverdogo Tela, 2006, Vol. 48, No. 6, pp. 1099–1100.
FERROELECTRIC CERAMICS
Thermal Properties of PZT-Based Ferroelectric Ceramics S. N. Kallaeva, G. G. Gadzhieva, I. K. Kamilova, Z. M. Omarova, S. A. Sadykovb, and L. A. Reznichenkoc a Institute
of Physics, Dagestan Scientific Center, Russian Academy of Sciences, ul. 26 Bakinskikh Komissarov 94, Makhachkala, 367003 Russia b Dagestan State University, Makhachkala, 367045 Russia c Research Institute of Physics, Rostov State University, pr. Stachki 194, Rostov-on-Don, 344090 Russia e-mail:
[email protected] Abstract—The thermal properties of piezoelectric ceramics (PKR-8, PKR-7M) based on lead zirconate titanate solid solutions Pb(Ti, Zr)O3 were studied over the temperature range 300–800 K. The thermal conductivity and thermal expansion coefficients were found to exhibit an anomalous behavior in the region of the ferroelectric phase transition. PACS numbers: 77.22.–d, 77.84.Dy, 66.70.–f, 77.80.Bh DOI: 10.1134/S1063783406060473
was maintained with an accuracy of 0.2°C. The measurement error of the thermal conductivity coefficient was 4% at 500 K. The thermal expansion coefficient (TEC) was measured using a quartz capacitance dilatometer, which, together with a sample, was placed in an autoclave evacuated to a pressure of 10–2 Torr. The sensitivity of the setup to displacement was 10–9 m, and the measurement error was 3%. The temperature variation rate was 0.8 K/min. The measurement process and experimental data processing were controlled using a code for thermal study automation. Figures 1 and 2a show the temperature dependences of the thermal conductivity λ of PKR-8 and PKR-7M ferroelectric ceramic samples over a wide temperature range (290–800 K), including the ferroelectric phase transition region. We can see that these dependences are
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Ferroelectric ceramics that are based on lead zirconate titanate solid solutions Pb(Ti, Zr)O3 (PZT) with a perovskite structure are widely used due to their excellent properties and the possibility of varying them by changing the chemical composition. These piezoelectric ceramics belong to an interesting class of ferroelectric systems that have disordered structures and can exhibit diffuse phase transitions. The mechanism of phase transitions in such inhomogeneous multicomponent systems is complex and is not clearly understood. This paper is devoted to studying the thermal properties (thermal conductivity and thermal expansion) of multicomponent systems based on lead zirconate titanate solid solutions Pb(Ti, Zr)O3 (PKR-8, PKR-7M) over a wide temperature range of 300–700 K, including the ferroelectric transition region. It is known that the cubic-to-ferroelectric phase transition in PKR-8 and PKR-7M ceramics occurs via a temperature region where there exists a tetragonal– rhombohedral structure (a morphotropic region) [1]. In the phase diagram of concentration states at room temperature, PKR-8 belongs to the tetragonal region adjacent to the morphotropic region, while PKR-7M is in the region of the morphotropic phase transition on the side of the rhombohedral phase [1]. Structural transitions from the cubic to ferroelectric phase in PKR-8 and PKR-7M occur at temperatures of 600 and 460 K, respectively. According to [2], PKR-7M is a relaxor ferroelectric and undergoes a diffuse phase transition. The thermal conductivity was measured using the absolute compensation method in an argon atmosphere [3]. Purified argon was introduced into an autoclave evacuated to a pressure of 10–3 Torr. Measurements were performed at a temperature gradient of 5 K (or 2 K in the region of a structural transition). The temperature
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Fig. 1. Temperature dependence of the thermal conductivity coefficient λ and the thermal expansion coefficient α of PKR-8 ferroelectric ceramics.
KALLAEV et al.
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Fig. 2. Temperature dependence of (a) the thermal conductivity coefficient λ and (b) the thermal expansion coefficient α of PKR-7M ferroelectric ceramics under (1) cooling and (2) heating. Arrows indicate the direction of temperature variation.
typical of disordered and vitreous materials, for which the thermal conductivity coefficient increases with temperature. Previously, a similar temperature dependence was also observed in PZT-19 and PZT-23 ceramics [4]. According to [3], such a behavior of λ can be due to the interaction of longitudinal acoustic phonons with the soft vibrational mode, which can cause a decrease in the mean free path as the temperature is lowered. From the viewpoint of the dynamic theory, the interaction of soft-mode phonons with lattice acoustic vibrations near the phase transition temperature Tc significantly increases since their energies become closer. The latter factor can cause an increase in the number of scattering events involving optical phonons, which can result in an appreciable decrease in the thermal conductivity during the ferroelectric phase transition [5, 6]. This scattering mechanism is also confirmed by studies performed on SrTiO3 crystals [3]. Figures 1 and 2b show the temperature dependences of the TEC α of PKR-8 and PKR-7M piezoceramic samples. We can see that these dependences exhibit an anomaly in the ferroelectric phase transition region upon cooling and heating. Below Tc, in the ferroelectric phase, the TEC increases as the temperature is lowered; i.e., the linear expansion coefficient is negative. Then, upon further cooling, the TEC varies gradually and corresponds to ordinary thermal expansion. The TEC variation below Tc indicates not only significant changes in
the local environment of atoms but also a significant change in their interaction. In the region of the diffuse phase transition in PKR-7M (at T > 420 K), the TEC decreases sharply to negative values and reaches a minimum at the temperature Tc = 460 K of the phase transition to the nonpolar phase. This means that the polycrystalline ferroelectric sample decreases sharply in volume as the temperature increases in the region of the diffuse phase transition. From the thermodynamic viewpoint, the TEC becomes negative if the effective Grüneisen coefficient γ takes on a negative value at a certain temperature. The γ values corresponding to different branches of the phonon spectrum can have opposite signs. The main contribution to the effective coefficient γ comes from longitudinal and transverse waves. It is likely that the negative value of γ of transverse waves can become larger in magnitude than the positive value of γ for longitudinal acoustic waves. As can be seen in Fig. 2b, in a wide temperature range below Tc, there is a thermal hysteresis, which is generally typical of incommensurate structures. We note that x-ray diffractometry studies of (NH4)2SO4 and (Rb0.1(NH4)0.9)SO4 crystals performed in [7] showed that the TEC exhibits an anomalous behavior and that there is a global hysteresis below the temperature of the ferroelectric phase transition. This situation can occur if atoms form bound nonequivalent sublattices. As the temperature is lowered, these sublattices can move apart not due to anharmonic atomic vibrations but rather due to the strong dipole–dipole interaction of the sublattice building blocks in the region of the transition to the polar state. Thus, our study has shown that, first, the temperature dependence of the thermal conductivity of PZT-based piezoelectric ceramics is similar in character to that for vitreous materials and, second, in the temperature region of the ferroelectric phase with a tetragonal–rhombohedral structure, the thermal expansion coefficient exhibits an anomalous behavior; namely, it decreases as the temperature increases. In the region of the diffuse phase transition, the TEC decreases sharply to negative values. REFERENCES 1. E. G. Fesenko, A. Ya. Dantsiger, and O. N. Razumovskaya, New Piezoelectric Ceramic Materials (Rostov-on-Don State University, Rostov-on-Don, 1983) [in Russian]. 2. Ya. B. Bogosova, G. M. Konstantinov, and M. F. Kupriyanov, Izv. Akad. Nauk, Ser. Fiz. 57 (6), 89 (1993). 3. E. F. Steigmeier, Phys. Rev. 168, 523 (1968). 4. S. N. Kallaev, G. G. Gadzhiev, I. K. Kamilov, Z. M. Omarov, and S. A. Sadykov, Izv. Akad. Nauk, Ser. Fiz. 68 (7), 978 (2004). 5. A. J. Mante and J. Volger, Phys. Lett. A 24, 139 (1967). 6. M. Jnoue, J. Phys. Soc. Jpn. 25, 288 (1968). 7. I. M. Shmyt’ko, N. S. Afonikova, and V. I. Torgashev, Fiz. Tverd. Tela (St. Petersburg) 44 (12), 2204 (2002) [Phys. Solid State 44 (12), 2309 (2002)].
Translated by A. Kazantsev PHYSICS OF THE SOLID STATE
Vol. 48
No. 6
2006
