(Bi_1/2K_1/2)TiO₃–SrTiO₃ solid-solution ceramics for high-temperature capacitor applications

The relaxor state of (Bi_1/2K_1/2)TiO₃ exhibits promising dielectric properties for high-temperature capacitor applications, but the spontaneous phase transition into the low-temperature ferroelectric state and the excessively high dielectric maximum temperature (T_m) at around 360 °C are the main drawback to this material. In this study we examined solid solutions of (Bi_1/2K_1/2)TiO₃ with SrTiO₃ to improve the temperature stability of the dielectric properties. As precursors to fabricate the sold-solution ceramics, fine powders of (Bi_1/2K_1/2)TiO₃ and SrTiO₃ were both synthesized by the hydrothermal method. Dense (1 − x)(Bi_1/2K_1/2)TiO₃–xSrTiO₃ ceramics with x up to 0.5 were then obtained by reaction sintering of the powders. A crystal structure analysis revealed that the average symmetry of the solid-solution ceramics changes from tetragonal to cubic with increasing the SrTiO₃ content. Dielectric measurements showed that the incorporation of SrTiO₃ into (Bi_1/2K_1/2)TiO₃ stabilizes the relaxor state to shift T_m largely toward lower temperatures. As a result, the sample with x = 0.5 exhibited a temperature-stable dielectric permittivity of 1700 ± 15% over a wide temperature range from room temperature up to 260 °C. The electric-field and temperature dependences of the energy-storage properties of the sample were also investigated.