Evaluation of Elastic Constants in Piezoelectric Ceramics by Measuring Acoustic Wave Velocities

Abstract

Material research and development on piezoelectric ceramics, especially lead-free ceramics, was proposed from a viewpoint of relationships between electromechanical couplig factor and elastic constants such as Young's modulus, Poisson's ratio, modulus of rigidity and bulk modulus. We developed a method to be convenient to measure acoustic wave velocities by an ultrasonic thickness gauge with high-frequency. From the values and changes in longitudinal and transverse wave velocities before and after DC poling, it was clarified that the relationships between coupling factor and elastic constants, and furthermore, the ceramic bulk density was important to improve the coupling factor in lead-free ceramics. In the case, the ratio of transverse wave velocity to longitudinal wave velocity was utilized to estimate compositions with higher coupling factor. As a result, the candidates of lead-free ceramic compositions with higher coupling factor were proposed. The measurement of sound velocities was an effective tool for researching and developing piezoelectric materials, and it was possible to design the material compositions of lead-free piezoelectric ceramics as well as lead-containing ceramics by the novel measuring method.