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 (Bi1/2Na1/2) TiO3 (BNT)- and (Bi1/2K1/2)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 d(33) and a high Curie temperature T-C or a high depolarization temperature T-d (> 200 degrees C). 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, d(33) and T-d were shown to have a tradeoff relationship. Considering the high T-d and high d(33), 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-(Bi1/2Li1/2)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,. = 27.6 mu C/cm(2) and high piezoelectric properties (A.,33 = 0.40 and (133 = 101 pC/N). In addition, it was clarified that BKT ceramics have a high Td of approximately 300 degrees C. The solid solution (1-x)BKT-xBaTiO(3) (BKT-BT100x) exhibited a high T-d of approximately 300 degrees C at x > 0.4.