Large Piezoelectric Constant and High Curie Temperature of Lead-Free Piezoelectric Ceramic Ternary System Based on Bismuth Sodium Titanate-Bismuth Potassium Titanate-Barium Titanate near the Morphotropic Phase Boundary

Abstract

A lead-free piezoelectric ceramic ternary system based on bismuth sodium titanate, (Bi_1/2Na_1/2)TiO₃ (BNT) - bismuth potassium titanate (Bi_1/2K_1/2)TiO₃ (BKT) - barium titanate BaTiO₃ (BT) near the morphotropic phase boundary (MPB) between the tetragonal and rhombohedral phases has been investigated. In the case of a(Bi_1/2Na_1/2)TiO₃–bBaTiO₃–c(Bi_1/2K_1/2)TiO₃ [BNBK(100a/100b/100c)] solid solution ceramics, the highest piezoelectric constant d₃₃=191 pC/N, Curie temperature, T_c=301°C, electromechanical coupling factor, k₃₃=0.56 and dielectric constant, ε₃₃^T/ε₀=1141 are observed for the BNBK(85.2/2.8/12) composition which has a tetragonal phase near the MPB. The d₃₃ value is the highest so far reported for all lead-free piezoelectric ceramics with T_c>300°C. The BNT-BKT-BT ternary ceramics system sintered at 1200°C for 2 h in air has a pure perovskite structure and a high density more than 95% of the theoretical density.