Microstructure, ferroelectric and piezoelectric properties of MnO₂-modified Ba_0.70Ca_0.30TiO₃ lead-free ceramics

Lead-free Ba_0.70Ca_0.30TiO₃ + x mol% MnO₂ (abbreviated as BCTMx, x = 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 1.0 and 1.2) piezoelectric ceramics were synthesized by the solid-state reaction method. The effects of MnO₂ additives on the phase transition, microstructure, dielectric, ferroelectric, strain behaviors and piezoelectric properties are investigated. BCTMx ceramics (x = 0–1.2) exhibit diphasic tetragonal and orthorhombic phases. The tetragonal phase is gradually suppressed by increasing x. As x increases from 0 to 1.2, the Curie temperature (T_C) decreases monotonically from 128 to 45 °C, while the grain size, dielectric and ferroelectric properties increase and reach the maximum near x = 0.6. On the other hand, the piezoelectric coefficient (d₃₃) and the electromechanical coupling coefficient (k_p%) decrease simultaneously, whereas the mechanical quality factor (Q_m), bipolar and unipolar strain increase and reach the maximum values of 684, 0.28% and 0.24%, respectively, near x = 0.6. The optimum electrical performance with ε_r = 1323, tan δ = 0.08, T_C = 66 °C, P_max = 12.2 μC/cm², P_r = 6.9 μC/cm², E_C = 13.3 kV/cm, d₃₃ = 116 pC/N, k_p% = 0.22%, Q_m = 684 and the normalized strain d33∗ = S_max/E_max as high as 480 pm/V were observed for x = 0.6. The high strain and Q_m suggested that the MnO₂-modified Ba_0.70Ca_0.30TiO₃ ceramics are promising candidate for high power applications.