Optimization of calcination temperature of lead-free BiFeO₃–BaTiO₃ high-temperature piezoceramics

0.7BiFeO₃-0.3BaTiO₃+1mol% MnO₂(0.7BFO-0.3BTO) ceramics were synthesized by conventional solid-state powder method under different calcination temperatures (T_cal) between 770 and 830 °C. The phase structure, microstructure, and ferroelectric and piezoelectric properties changed greatly depending on the applied T_cal. Benefitting from the formation of low defect levels and large grain size and an appropriate morphotropic phase boundary (MPB) with the rhombohedral-to-pseudocubic phase ratio = 49.1 : 50.9, BFO-BTO ceramics calcined at 785 °C showed the best ferroelectric, piezoelectric, and insulating properties (P_r = 23.1 μC/cm², E_C = 25.8 kV/cm, d₃₃ = 167.8 pC/N, k_p = 0.342%). Above T_cal = 800 °C, however, the ferroelectric and piezoelectric properties deteriorated because volatilization of Bi and reduction of Fe caused a poor insulating property and high degree of chemical inhomogeneity. Moreover, the ceramics calcined at 785 °C showed a high Curie temperature (T_C) of 509.2 °C and excellent thermal aging resistance of d₃₃ up to 450 °C, demonstrating great potential for use in high-temperature applications.