**2.1. Introduction**

Recently, we have developed a method to evaluate elastic constants, such as Young's modulus and Poisson's ratio, by measuring longitudinal and transverse wave velocities using an ultrasonic thickness gauge with high-frequency generation in comparison with a conventional method [28]. This method has been confirmed to be useful for measuring ceramic disks with diameters of 10-20 mm and thicknesses of 0.5-2.0 mm. In addition, it has been clarified that this method is suitable for evaluating cases involving (1) firing process analysis, such as the analysis of the effect of lead oxide (PbO) atmosphere during firing on the dielectric and piezoelectric properties of lead zirconate titanate (PZT) ceramics, and the oxygen atmosphere firing of PZT ceramics to realize pore-free ceramics, (2) DC poling process analysis, such as the analysis of the DC poling field dependence in as-fired (before poling) ceramics and relaxor single crystals, and (3) piezoelectric materials research and development (R&D) for lead-free ceramics with high piezoelectricity from the viewpoints of elastic constants [18-20].

In this part, to clarify the effects of firing temperature and DC poling on barium titanate (BT) piezoelectric ceramics, we studied the firing temperature and DC poling dependences on acoustic wave velocities and dielectric and piezoelectric properties of BT ceramics. Here, we report the relationships between firing temperature and DC poling effect vs. acoustic wave velocities, Young's modulus, Poisson's ratio, modulus of rigidity, and bulk modulus.
