Electrical Properties and Depolarization Temperature of (Bi_1/2Na_1/2)TiO₃–(Bi_1/2K_1/2)TiO₃ Lead-free Piezoelectric Ceramics

The piezoelectric properties of a solid solution of the binary system, x(Bi_1/2Na_1/2)TiO₃–(1-x)(Bi_1/2K_1/2)TiO₃ [BNKT100x; x=0.50–0.98] were investigated, focusing on depolarization temperature, T_d. Fine piezoelectric properties in lead-free piezoelectric ceramics were obtained near the morphotropic phase boundary (MPB) composition between the rhombohedral and tetragonal structures, and the highest electromechanical coupling factor, k₃₃, and piezoelectric constant, d₃₃, were 0.56 for BNKT84 and 157 pC/N for BNKT80, respectively. However, the T_d of BNKT80 was low (174 °C). The T_d of the MPB composition was low, and the T_d near the MPB composition was sharply decreased. It is thought that BNKT70 is a candidate composition for lead-free actuator applications owing to its relatively large piezoelectric constant, d₃₃ (126 pC/N), dynamic d₃₃ (214 pm/V), and high depolarization temperature, T_d (206 °C). In this study, we determined depolarization temperature, T_d, from the temperature dependence of dielectric and piezoelectric properties.