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 (<i>T</i>_C). Hard piezoelectric material used in high-power transducer applications must exhibit a high mechanical quality factor (<i>Q</i>_m), high coercive field (<i>E</i>_C), high <i>T</i>_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-<i>x</i>Mn_<i>x</i>)O₃-0.2BaTiO₃ (<i>x</i> = 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 (<i>d</i>₃₃), <i>Q</i>_m, <i>T</i>_C, planar piezoelectric coupling factor (<i>k</i>_p), <i>E</i>_C, and internal bias field (<i>E</i>_i), reaching their maximum values at <i>x</i> = 0.01 (<i>d</i>₃₃ = 74 pC/N, <i>Q</i>_m = 757, <i>T</i>_C = 652 °C, <i>k</i>_p = 0.26, <i>E</i>_C = 65 kV/cm, and <i>E</i>_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.