Structure and electrical properties of MnO₂-doped Sr _2-xCa_xNaNb₅O₁₅ lead-free piezoelectric ceramics

Lead-free piezoelectric ceramics Sr_2-xCa_xNaNb ₅O₁₅ + y wt% MnO₂have been prepared by the conventional solid state reaction method. Our results reveal that Ca ²⁺and Mn ions have entered into the Sr₂NaNb ₅O₁₅ lattices to form a solid solution with tungsten-bronze structure. The substitution of Ca²⁺ induces a decrease in piezoelectric coefficient d₃₃, electromechanical coupling factors k_p and k_t, while, the addition of Mn ions decreases the sintering temperature and effectively promotes the densification of the ceramics. The effect of substitution of Ca²⁺and Mn ions on the structure, electrical properties and diffused phase changing were investigated systematically. For the ceramic with x = 0.05 and y = 0.5, the piezoelectric, dielectric and ferroelectric properties become optimum, giving a piezoelectric coefficient d₃₃ = 190 pC/N, electromechanical coupling factors k _p = 13.4% and k_t = 36.5%, ε_r = 2123, loss tangent tan δ = 0.038, remanent polarization P_r = 4.76 μC/cm², coercive field E_c = 12.68 kV/cm, and Curie temperature T_c = 260 °C.