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

Lead-free BiFeO 3 –BaTiO 3 (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 Mn 2+ are investigated to enhance their hard piezoelectric properties. The 0.8Bi 1.02 (Fe 1– x Mn x )O 3 –0.2BaTiO 3 ( 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 Mn 2+ content in BF–BT ceramics boosts the piezoelectric charge sensor coefficient ( d 33 ), 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 ( d 33 = 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 Mn 2+ in the BF–BT ceramics improves the performance of lead-free ceramics for high-power piezoelectric transducers in high-temperature environments.