Phase Transition Temperatures and Piezoelectric Properties of (Bi1/2Na1/2)TiO3-and (Bi1/2K1/2)TiO3-Based Bismuth Perovskite Lead-Free Ferroelectric Ceramics

The phase transition temperatures and the dielectric, ferroelectric, and piezoelectric properties of bismuth perovskite lead-free ferroelectric ceramics such as (Bi_1/2Na_1/2)TiO₃ (BNT)- and (Bi_1/2Na_1/2)TiO₃ (BKT)-based solid solutions have been reviewed. According to the results obtained by our group, these ceramics can be considered as superior lead-free piezoelectric materials for reducing environmental damage. Perovskite-type ceramics appear to be suitable for actuator and high-power applications that require a large piezoelectric constant d₃₃ and a high Curie temperature T_C or a high depolarization temperature T_d (> 200degC). In this paper, we summarize the relationship between phase transition temperatures and piezoelectric properties. In the case of the BNT-based solid solutions, the highest piezoelectric properties were obtained at the morphotropic phase boundary (MPB) between rhombohedral and tetragonal phases. However, d₃₃ and T_d were shown to have a tradeoff relationship. Considering the high T_d and high d₃₃, the tetragonal side of the MPB composition is suitable for piezoelectric actuator application. Meanwhile, the Q_m values on the rhombohedral side of the MPB composition were better than those on the tetragonal side, and excellent high-power characteristics were obtained for Mn-doped BNT-(Bi_1/2Na_1/2)TiO₃-BKT ternary systems with rhombohedral symmetry. BKT ceramics were prepared by the hot-pressing (HP) method, and their ferroelectric and piezoelectric properties were clarified. BKT ceramics doped with a small amount of Bi have a relatively high remanent polarization of P_r = 27.6 muC/cm² and high piezoelectric properties (k₃₃ = 0.40 and d₃₃ = 101 pC/N). In addition, it was clarified that BKT ceramics have a high T_{d of approximately 300degC. The solid solution (1-x)BKT-xBaTiO3 (BKT-BT100x) exhibited a high Td of approximately 300degC at x > 0.4.}