Enhanced temperature stability and quality factor with Hf substitution for Sn and MnO₂ doping of (Ba_0.97Ca_0.03)(Ti_0.96Sn_0.04)O₃ lead-free piezoelectric ceramics with high Curie temperature

In this work, the process of two-stage modifications for (Ba_0.97Ca_0.03)(Ti_0.96Sn_0.04-xHf_x)O₃ (BCTS4-100xH100x) ceramics was studied. The trade-off composition was obtained by Hf substitution for Sn and MnO₂ doping (two-stage modification) which improves the temperature stability and piezoelectric properties. The phase structure ratio, microstructure, and dielectric, piezoelectric, ferroelectric, and temperature stability properties were systematically investigated. Results showed that BCTS4-100xH100x piezoelectric ceramics with x=0.035 had a relatively high Curie temperature (T_C) of about 112 °C, a piezoelectric charge constant (d₃₃) of 313 pC/N, an electromechanical coupling factor (k_p) of 0.49, a mechanical quality factor (Q_m) of 122, and a remnant polarization (P_r) of 19μC/cm2. In addition, the temperature stability of the resonant frequency (f_r), k_p, and aging d₃₃ could be tuned via Hf content. Good piezoelectric temperature stability (up to 110 °C) was found with x =0.035. BCTS0.5H3.5 + a mol% Mn (BCTSH + a Mn) piezoelectric ceramics with a = 2 had a high T_C of about 123 °C, k_p ∼ 0.39, d₃₃ ∼ 230 pC/N, Q_m ∼ 341, and high temperature stability due to the produced oxygen vacancies. This mechanism can be depicted using the complex impedance analysis associated with a valence compensation model on electric properties. Two-stage modification for lead-free (Ba_0.97Ca_0.03)(Ti_0.96Sn_0.04)O₃ ceramics suitably adjusts the compositions for applications in piezoelectric motors and actuators.