Electrical properties and depolarization temperature of (Bi1/2Na1/2)TiO3-(Bi1/2K1/2)TiO3 lead-free piezoelectric ceramics

The piezoelectric properties of a solid solution of the binary system, x(Bi1/2Na1/2)TiO3-(1-x)(Bi1/2K1/2)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, k(33), and piezoelectric constant, d(33), were 0.56 for BNKT84 and 157 pC/N for BNKT80, respectively. However, the T-d of BNKT80 was low (174 degrees 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, d(33) (126 pC/N), dynamic d(33) (214 pm/V), and high depolarization temperature, T-d (206 degrees C). In this study, we determined depolarization temperature, T-d, from the temperature dependence of dielectric and piezoelectric properties.