Piezoelectric properties and depolarization temperature of the lead-free piezoelectric ceramics of BNT-BKT-BiCrO₃

A (Bi_0.5Na_0.5)TiO₃-based lead-free piezoelectric ceramic, (1-x-y)Bi_0.5Na_0.5TiO₃-xBi_0.5K_0.5TiO₃-yBiCrO₃(BNT-BKT-BC-x/y), was prepared by a conventional ceramic sintering technique. Its crystal structure, and piezoelectric properties and depolarization temperature were investigated. The morphotropic phase boundary (MPB) of the system between rhombohedral and tetragonal locates in the range of x=0.18 to 0.21 and y=0 to 0.02. Moreover, an obvious second phase was observed in the samples with y=0. 025 and x=0.18. The optimum piezoelectric properties were obtained near the MPB composition, and the piezoelectric constant d₃₃ and the electromechanical coupling factor k_p of the ceramics attain the maximum values of 168 pC·N^-1 and 0. 326 at x=0.18, y=0.015 and at x=0.18, y=0.01, respectively. The depolarization temperatures confirmed by k_p and θ_max are almost identical. The depolarization temperature keeps decreasing with an increase content of BC and first increases and then decreases with an increase content of BKT.