**1.4. Conclusions in this part**

Longitudinal and transverse wave velocities in PZT, lead titanate, and lead-free ceramics were measured by an ultrasonic precision thickness gauge with high-frequency pulse generation to calculate elastic constants such as Young's modulus, Poisson's ratio, and so forth. Since the ceramic bulk density was focused on to improve piezoelectricity in lead-free ceramics, the candidates of lead-free ceramic compositions with high piezoelectricity were proposed. It was confirmed that our evaluation method was an effective tool for R&D on piezoelectric material. Furthermore, the origin of piezoelectricity in piezoelectric ceramics could be explained by the elastic constants before and after DC poling.
