Phase transition, microstructure, and dielectric properties of Li/Ta/Sb co-doped (K, Na)NbO3 lead-free ceramics

Li/Ta/Sb co-doped lead-free (K0.4425Na0.52Li0.0375)(Nb0.93-xTaxSb0.07)O-3 (abbreviated KNLNSTx) piezoelectric ceramics, with Ta-doping ratio of x ranging from 0.0275 to 0.0675, were synthesized using the conventional solid-state reaction method at the sintering temperature of 1130 degrees C. The effects of Ta content on the microstructure, dielectric properties, and phase transition behavior of the prepared ceramics were systematically investigated. The X-ray diffraction results show that all KNLNSTx ceramics formed a secondary phase, which is assigned to the tetragonal tungsten-bronze type (TUB) structure phase, and showed a phase transition from an orthorhombic symmetry to a tetragonal symmetry across a composition region of 0.0375 < x < 0.0475. The grain shape and size that correspond to the phase structure transformations can be clearly observed in the scanning electron microscopy images. As x increased to 0.0475, the KNLNST0.0475 ceramics changed from orthorhombic to tetragonal structure and showed excellent piezoelectric properties of d(33)=313 pC/N, k(p)=47%, and epsilon(r)=1825. By contrast, samples of x=0.0375 with orthorhombic symmetry exhibited poor piezoelectric properties, with d(33)=200 pC/N and epsilon(r)=1015. These results indicate that phase structure is vital in the piezoelectric properties of KNN lead-free ceramics. (C) 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved.