Microstructure, phase transition, and electrical properties of (K0.5 Na0.5) 1-x Lix (Nb1-y Tay) O3 lead-free piezoelectric ceramics

Lead-free ceramics (K0.5 Na0.5) 1-x Lix (Nb1-y Tay) O3 have been prepared by an ordinary sintering technique. Our results reveal that Li+ and Ta5+ diffuse into the K0.5 Na0.5 Nb O3 lattices to form a solid solution with a perovskite structure. The substitution of Li+ induces an increase in the Curie temperature (TC) and a decrease in the ferroelectric tetragonal-ferroelectric orthorhombic phase transition temperature (TO-T). On the other hand, both TC and TO-T decrease after the substitution of Ta5+. A coexistence of the orthorhombic and tetragonal phases is formed at 0.03<x<0.06 and 0.10<y<0.25 near room temperature, leading to significant enhancements of the piezoelectric properties. For the ceramic with x=0.04 and y=0.225, the piezoelectric properties become optimum, giving a piezoelectric coefficient d33 =208 pCN, electromechanical coupling factors kP =48% and kt =49%, remanent polarization Pr =14.2 μC cm2, coercive field Ec =1.21 kVmm, and Curie temperature TC =320 °C.