Permittivity and loss tangent of unpoled LaSr-doped PZT under compressive loading

Abstract

Piezoelectric ceramics are commonly applied under a compressive pre-stress in order to avoid failure under tensile stresses. This is the case for piezoelectric actuators, for instance. Under such stress states, the piezoelectric large signal behaviour is strongly affected [1–4]. More recent results on various ferroelectric materials and single crystals are reported by Viehland [5]. But also small signal parameters are influenced by compressive pre-stresses [6, 7]. In order to reveal the effect of material composition, measurements were performed for systematically changed Zr/Ti ratios, resulting in materials located on both sides of the morphotropic phase boundary (MPB). In this communication the effects of compressive load on the dielectric properties of LaSr doped soft PZT ceramics are investigated. Lanthanum is a typical donor dopant in PZT. The La ion occupies the Pb (A) site in the ABO3 perovskite structure. The charge compensation occurs by formation of lead vacancies resulting in soft ferroelectric behaviour. Sr also substitutes for the Pb. The isovalent Sr doping reduces the Curie temperature Tc and enhances dielectric properties [8]. Measurements of the relative dielectric permittivity e and dielectric loss tand parallel and perpendicular to a compressive loading were performed for three PZT ceramics in the unpoled state and analysed as effects resulting from domain switching and lattice deformation. Three compositions of 1 mol% La-2 mol% Sr-doped morphotropic PZT with Zr/Ti ratios of 53/47, 54/46 and 55/45, respectively, were prepared by the mixed-oxide route. The powders were prepressed by uniaxial die pressing at low pressure and, after that, isostatically pressed at 400 MPa and sintered in closed alumina crucibles at 1225 C in an oxygen atmosphere [9]. From the sintered specimens, bars with dimensions of 20 · 4.5 · 4.5 mm were fabricated by sawing and grinding. These samples were equipped with sputtering Au electrodes on two opposite 4.5 · 20 mm surfaces. X-ray diffraction patterns showed after sintering phase assemblage of both tetragonal and rhombohedral perovskite. Volume fractions of these phases were estimated from integral peak intensity ratios of {200} and {111} reflections. The tetragonal volume fraction of the PZT 53/ 47 is about 65% and of the PZT 54/46 about 37%. The PZT 55/45 is a predominantly rhombohedral phase material with a tetragonal volume fraction of about 16% [10]. For the dielectric measurements a sinusoidal voltage of 1 V amplitude and a frequency of f = 1 kHz was used [7]. The measurements were carried out with an LCR meter (HP-4263A). Compressive loads were applied to the 4.5 · 20 mm surfaces (Fig. 1a). For the dielectric measurements in compression direction, the specimens (1) with electrodes (2) were put on a steel block (3) and loaded by a second steel block (4). Their cross sections were identical with those of the test specimens. This yields a nearly homogeneous z-stress in the test piece with rz 1⁄4 rz (PZT) ffi rz (steel). Superimposed xand y-stress components are very small and proportional to the difference in Poisson ratios, rx, ry / rz (mPZT mSteel), mSteel ffi 0.3, mPZT ffi 0.35 0.5. This ensures maximum xand y-stresses <10% T. Fett (&) H. Kungl M. J. Hoffmann Institut fur Keramik im Maschinenbau, Universitat Karlsruhe, Karlsruhe, Germany e-mail: theo.fett@ikm.uni-karlsruhe.de