Thermal effects on domain orientation of tetragonal piezoelectrics

Wonyoung Chang, Purdue University

Abstract

Thermal effects on electrical poling or mechanical grinding induced texture in tetragonal lead zirconate titanate (PZT) and lead titanate (PT) have been investigated using ex situ and in situ X-ray diffraction (XRD) with an area detector. According to previous results using ex situ XRD, domain configurations of poled samples after heat-treatment at or higher than the Curie temperature (TC) are similar to that of unpoled samples showing random domain distributions. The texture parameter called multiples of a random distribution (MRD) gradually decreases with increasing depoling temperature. On the other hand, using in situ XRD measurements, it was found that the MRD maximum for soft PZT initially increases with temperature up to approximately 100°C and then falls to unity at temperatures approaching the TC, whereas the MRD of hard PZT and PT initially undergoes a smaller increase or no change. Mechanical strain energy has an apparent effect on domain wall mobility. In contrast with previous results on electrical poling, mechanically-ground PT and soft PZT materials retained strong ferroelastic textures during thermal cycling, even after excursions to temperatures slightly above the TC . For the ground PT, it was found that repeated cycling above T C results in changes in both peak intensity and peak position, whereas the ground soft PZT undergoes the decrease in intensity of the (002) reflection after the first cycle of heating. Residual stresses in the surface region from grinding resulted in domain wall motion and the retention of textures in annealed samples. The research in this thesis demonstrates that the magnitude of loading applied to the sample surface, the speed used for grinding, or the grit size, can greatly affect the grinding induced damage zone and the depoling behavior of piezoelectric ceramics. Among the possible effects of grinding conditions on surface textures, one of particular interest is the effect of mechanical stresses produced during grinding on the texture intensity in the ground surface region. Inhibited depoling of ground PT materials under different loading conditions investigated by in situ texture measurements between room temperature and approximately 100°C above the Curie temperature demonstrates the effects of residual stresses. For all the ground PT and soft PZT samples, there was little or no evidence of time dependence for domain reorientation at the whole annealing temperature range. In addition, both ground PT and soft PZT materials under lower loading conditions showed a gradual depoling behavior with increasing heat treatment temperature, whereas ground materials under higher loading conditions retained relatively strong ferroelastic texture up to temperatures around their respective TC, and then underwent a drastic change in MRD at temperatures above TC. Compared to the ground soft PZT materials, all the ground PT samples still maintained high MRD values after heat treatment above TC.

Degree

Ph.D.

Advisors

Bowman, Purdue University.

Subject Area

Materials science

COinS