Polymorphic phase transition and excellent piezoelectric performance of (K0.55Na0.45)0.965Li0.035Nb0.80Ta0.20O3 lead-free ceramics

A lead-free ceramic with the composition (K0.55Na0.45)(0.965)Li0.035Nb0.80Ta0.20O3 was found having an outstanding piezoelectric performance. It possesses high piezoelectric properties of d(33)=262 pC/N, k(p)=0.53, k(33)=0.63, and k(31)=0.31 with epsilon(')=1290 and tan delta=0.019 at room temperature. In spite of the orthorhombic-tetragonal polymorphic phase transition around 30 degrees C, temperature stability of the electromechanical coupling coefficients is very good over the common usage temperature interval between -40 and 85 degrees C. Furthermore, the piezoelectric properties remain almost unchanged in the severe thermal aging test down to -150 degrees C and up to about 300 degrees C. It is suggested that the outstanding piezoelectric performance of this ceramic can be largely ascribed to the phase coexistence in a wide temperature range and the Ta-rich composition.