Phase transition and electrical properties of (K0.5Na0.5)(Nb1-xTax)O3 lead-free piezoelectric ceramics

(K0.5Na0.5)(Nb1-xTax)O-3 lead-free piezoelectric ceramics have been prepared by an ordinary sintering technique. The results of X-ray diffraction reveal that Ta5+ diffuses into the K0.5Na0.5NbO3 lattices to form a solid solution with an orthorhombic perovskite structure. Because of the high melting temperature of KTaO3, the (K0.5Na0.5)(Nb1-xTax)O-3 ceramics can be sintered at higher temperatures. The partial substitution of Ta5+ for the B-site ion Nb5+ decreases both paraelectric/cubic-ferroelectric/tetragonal and ferroelectric/tetragonal-ferroelectric/orthorhombic phase transition temperatures, T-C and TO-T. It also induces a relaxor phase transition and weakens the ferroelectricity of the ceramics. The ceramics become 'softened', leading to improvements in d(33), k(p), k(t) and epsilon(r) and a decease in E-c, Q(m) and N-p. The ceramics with x=0.075-0.15 become optimum, having d(33)=127-151 pC/N, k(p)=0.43-0.44, k(t)=0.43-0.44, epsilon(r)=541-712, tan delta=1.75-2.48% and T-C=378-329 degrees C.