Hard Piezoelectric Properties of MnCO₃-Doped Lead-Free BiFeO₃-BaTiO₃ Ceramics with High Curie Temperature

Lead-free BiFeO₃-BaTiO₃ (BF-BT) ceramics show high potential for piezoelectric sensors, actuators, and high-power transducers owing to their excellent piezoelectric properties and high Curie temperatures (T_C). Hard piezoelectric material used in high-power transducer applications must exhibit a high mechanical quality factor (Q_m), high coercive field (E_C), high T_C, and low dielectric loss (tan δ). Herein, the effects of doping in BF-BT ceramics with the hard acceptor dopant Mn²⁺ are investigated to enhance their hard piezoelectric properties. The 0.8Bi_1.02(Fe_1-Mn)O₃-0.2BaTiO₃ (x = 0, 0.005, 0.01, 0.015, and 0.02) ceramics are prepared using solid-state reaction and water-quenching processes. The increasing of the Mn²⁺ content in BF-BT ceramics boosts the piezoelectric charge sensor coefficient (d₃₃), Q_m, T_C, planar piezoelectric coupling factor (k_p), E_C, and internal bias field (E_i), reaching their maximum values at x = 0.01 (d₃₃ = 74 pC/N, Q_m = 757, T_C = 652 °C, k_p = 0.26, E_C = 65 kV/cm, and E_i = 6.2 kV/cm) with the lowest loss (tan δ = 0.009). Then, the hard piezoelectric properties deteriorate gradually with an increasing Mn content. The ferroelectric domain structure and dynamics are compared to defect-related ferroelectric and piezoelectric properties. These results show that the hard acceptor Mn²⁺ in the BF-BT ceramics improves the performance of lead-free ceramics for high-power piezoelectric transducers in high-temperature environments.