Investigation of MnO₂-doped (Ba, Ca)TiO₃ lead-free ceramics for high power piezoelectric applications

x% mol MnO₂-doped Ba_0.925Ca_0.075TiO₃ ceramics (abbreviated as BCT-Mnx, x=0-1.5) were synthesized by conventional solid-state reaction method. The effects of MnO₂ addition and (Ba+Ca)/Ti mole ratio (A/B ratio) on the microstructure and electrical properties of the ceramics were investigated. The internal bias filed E_i was determined from the asymmetrical polarization hysteresis loops and found to increase with the doping concentration of MnO₂. High mechanical quality factors (Q_m>1200) and low dielectric loss (tanδ<0.5%) were found in the BCT-Mn0.75 and BCT-Mn1.0 ceramics with E_i>3 kV/cm, meanwhile, the piezoelectric and electromechanical properties were found to decrease compared with the pure BCT, exhibiting a typical characteristic of “hard” behavior. Of particular interest is that the microstructure of BCT-Mn0.75 ceramics could be controlled by changing the A/B ratio, where enhanced piezoelectric coefficient d₃₃ on the order of 190 pC/N was obtained in the BCT-Mn0.75 ceramics with A/B=1.01 due to its fine-grained microstructure, with yet high Q_m, being on the order of 1000. The high d₃₃ and Q_m in MnO₂-doped BCT ceramics make it a promising candidate for high power piezoelectric applications.