Effects of AETiO₃ additions on phase structure, microstructure and electrical properties of (K_0.5Na_0.5)NbO₃ ceramics

Lead-free piezoelectric ceramics with the composition of (K_0.5Na_0.5)NbO₃ (KNN) and 0.995(K_0.5Na_0.5)NbO₃-0.005AETiO₃ (AE = Mg, Ca, Sr, Ba) were successfully prepared by conventional ceramic processing. Single perovskite phase with orthorhombic syngony can be obtained in pure KNN, KNN-CT and KNN-ST ceramics. The trace amounts of secondary phase(s) can be detected in KNN-MT and KNN-BT ceramics. Ceramics of KNN-CT and KNN-ST have higher cell parameters and higher densities than those of pure KNN ceramics. Good electrical properties (d₃₃ = 122 pC/N, K_p = 0.38, ε_r = 421, tan δ = 0.033 and T_C = 406 °C) can be obtained in pure KNN ceramics. The KNN-CT ceramics have optimized electrical properties, which are d₃₃ = 116 pC/N, K_p = 0.39, ε_r = 411, tan δ = 0.024 and T_C = 397 °C. Besides, the P_r is 18.8 μC/cm² and the E_c is 9.65 kV/cm for pure KNN ceramics. By adding AET to KNN, the P_r values decrease with increasing the AE ions radii. The air-sintered pure KNN and KNN-CT ceramics are promising lead-free piezoelectric materials for transducer applications.