Tuning the Phase Structure and Electrical Properties of BNKT-Based Ceramics via Li⁺ Doping

Bi_0.5[Na_0.84-xK_0.16Li_x]_0.5TiO₃ (BNKT-xLi) lead-free piezoelectric ceramics were successfully prepared. The phase structure and domain engineering synergistic method were used to improve the piezoelectric properties, and the influence of Li⁺ doping on its phase structure and electrical properties was studied. The ceramics are located at the MPB, in the process of introducing Li⁺, altering both the phase ratio and tetragonality (c_T/a_T), as well as influencing grain size and leakage current. These factors together make ferroelectric and piezoelectric properties stronger. However, with the increase of Li⁺ content, the long-range ordered structure of ferroelectrics is disrupted, and the material gradually becomes a relaxation type, resulting in stronger electrostrain. Specifically, BNKT-0Li exhibited a high depolarization temperature (T_d = 143.5°C), while BNKT-0.045Li demonstrated optimal piezoelectric properties (d₃₃ = 173 pC/N, T_d = 111.8°C), the Curie temperature T_m and dielectric constant ε_max are 332.9°C and 4675°C, respectively. The variable temperature ferroelectric test found that the material gradually changed from a ferroelectric state to a relaxation state during temperature increase. The maximum electrostrain of BNKT-0.045Li near T_d can reach more than 0.3%, which maintains the high piezoelectric coefficient d₃₃, and makes T_d high enough, so that the operating temperature range of BNT ceramics is widened.