Achieving well-balanced performance of d₃₃ and T_d in unmodified morphotropic phase boundary BNKT20 ceramics by optimizing phase structure

The incompatibility of the large piezoelectric response (d₃₃) and high depolarization temperature (T_d) in lead-free piezoelectric ceramics limited their practical applications. The most used modification methods for improving d₃₃ or T_d commonly required a delicate control of complex compositions. Here, a strategy for simply changing the sintering temperature is used to defer the T_d and increase d₃₃ simultaneously. The T_d and d₃₃ of unmodified Bi_0.5Na_0.5TiO₃-0.2Bi_0.5K_0.5TiO₃ (BNKT20) ceramics increase with increasing sintering temperature until both of them reach the maximum (d₃₃ ~ 174pC/N, T_d ~ 177 °C) at the sintering temperature of 1100 °C. In-depth analysis unveils the critical role of multiphase coexistence and oxygen vacancies in enhancing T_d by optimizing sintering temperature. Meanwhile, increasing sintering temperature promotes large grain size with easy domain orientation, thus, yielding the improved d₃₃. This work provides a simple and effective method to broaden the working range of Bi_0.5Na_0.5TiO₃-based (BNT) piezoelectric ceramics at high temperature under the premise of realizing high piezoelectric properties.