Sodium bismuth titanate-based ceramics

Abstract

Dielectric, ferroelectric, and piezoelectric properties including phase transition temperatures of perovskite-type bismuth sodium titanate, (Bi1/2Na1/2)TiO3 [BNT]-based lead-free piezoelectric ceramics have been reviewed from the results obtained by our group being superior candidates for lead-free piezoelectric materials to reduce environmental damage. Perovskite-type ceramics seem 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 depolarization temperature, T d (>200°C). The x wt.% Bi-excess BNT-x and hot-pressed (HP) BNT ceramics were prepared and investigated to estimate piezoelectric properties of the BNT ceramic. In the case of x(Bi1/2Na1/2)TiO3-y(Bi1/2K1/2)TiO3-zBaTiO3, [x + y + z = 1, y:z = 2:1, BNBK2:1(x)], the d 33 are 126, 181, and 80 pC/N for x = 0.78, 0.88, and 0.98, respectively. The depolarization temperature T d, rhombohedral-tetragonal phase transition temperature T R-T and the temperature of the maximum dielectric constant T m were determined from the temperature dependence of the dielectric and piezoelectric properties. (1 - x)(Bi0.5Na0.5)TiO3-xSrTiO3 [BNST100x] forms a morphotropic phase boundary (MPB) of rhombohedral ferroelectric and pseudocubic (tetragonal) paraelectric at x = 0.26-0.28 for BNST100x, and demonstrates a very large strain and a normalized strain of 0.29% and 488 pm/V, respectively, for x = 0.28. In addition, it was clarified that the intermediate phase between T R-T (≥T d) and T m shows relaxor behavior. The T d of (1 - x)(Bi1/2Na1/2)TiO3-x(Bi1/2K1/2)TiO3 (BNKT100x) and (1 - x)(Bi1/2Na1/2)TiO3-x(Bi1/2Li1/2)TiO3 (BNLT100x) increased to 199 and 209°C, respectively, at the rhombohedral composition. The T d is related to the magnitude of the rhombohedrality 90°-α and the tetragonality c/a. Moreover, we discussed the ferroelectricity of the middle phases of T d-T R-T and T R-T-T m. The X-ray powder diffraction patterns of a(Bi1/2Na1/2)TiO3-b(Bi1/2Li1/2)TiO3-c(Bi1/2K1/2)TiO3 [a + b + c = 1] (BNLKT100b-100c) show the MPB between rhombohedral and tetragonal phases. The k 33, the d 33 and the T d of BNLKT4-20 and BNLKT8-20 were 0.603, 176 pC/N and 171°C, and 0.590, 190 pC/N, and 115°C, respectively. On the other hand, the d 33 and T d of BNLKT4-28 were 135 pC/N and 218°C, respectively. Considering both high T d and high d 33, the tetragonal compositions of BNLKT4-100c are thought to be the superior candidate. Also the effects of Mn doping on the variations in the T d and piezoelectric properties including high-power characteristics were investigated using rhombohedral BNLKT4-8. The Q m of w wt.% MnCO3-doped BNLKT4-8 (BNLKT4-8Mnw) markedly increased with increasing Mn concentration w, while T d, coupling factor k 33, and d 33 slightly decreased. The high-power characteristics of BNLKT4-8Mn0.6 were superior to those of hard PZT at a vibration velocity v 0-p > 0.6 m/s. Therefore, a Mn-doped BNT-based solid solution with rhombohedral symmetry is a promising candidate for lead-free high-power applications. Solid solution, (1 - x)(Bi1/2K1/2)TiO3-xBaTiO3 [BKT-BT100x], seems to be lead-free piezoelectric ceramics with wide working temperatures. The BKT-BT80 + MnCO3 (0.1 wt.%) shows the higher T c than 200°C and the coupling factor of k 33 = 0.35. The BKT-BT100x ceramics (x = 0-0.4) indicated high depolarization temperatures, T d, around 300°C. From these results, BKT-BT system is considered the superior candidate of lead-free piezoelectric materials for high-power and/or high-temperature applications.