Microstructure and Piezoelectric Properties of [(K_0.5Na _0.5)_1-xLi_x](Nb_1-yTa _y)O₃ Lead-free Piezoelectric Ceramics

Li⁺ and Ta⁵⁺ modified lead-free piezoelectric ceramics [(K_0.5Na_0.5)_1-xLi_x](Nb _1-yTa_y)O₃ have been prepared by an ordinary sintering technique. Effect of Li+ and Ta5+ on microstructure and piezoelectric properties of the ceramics was systematically studied. A morphotropic phase boundary between orthorhombic and tetragonal phases is identified in the composition range of (0.02≤x≤0.04, 0.15≤y≤0.25), which can enhance electrical properties of [(K_0.5Na_0.5)_1-xLi _x](Nb_1-yTa_y)O₃ ceramics. The Curie temperature TC of these ceramics is lower than that of pure (K _0.5Na_0.5)NbO₃ ceramics, but the temperature of orthorhombic to tetragonal phase transition T_O-T of the former is higher than that of the latter. T_C and T_O-T both decrease as the content of Ta⁵⁺ increases. With the addition of Li⁺ increasing, T_C increases. The optimal piezoelectric properties are obtained at (x, y)=(0.03, 0.20): piezoelectric constant d₃₃ is 192pC/N; the electromechanical coefficient of the planar mode k_p is 44%; room temperature dielectric constant ε_r is 1049, and the corresponding mechanical quality factor Q_m is 49.