LAUSR

Abstract Two kinds of ternary KNN-based lead-free piezoceramics: (0.985- x )K 0.5 Na 0.5 NbO 3 -0.015Bi 0.5 Na 0.5 TiO 3 - x BiScO 3 and (0.985- y )K 0.5 Na 0.5 NbO 3 -0.015Bi 0.5 Na 0.5 TiO 3 - y BiFeO 3 have been prepared by a conventional solid-state sintering. For these KNN-based ceramics, the increasing doping of BiScO 3 or BiFeO 3 changed the phase structure from orthorhombic symmetry to tetragonal symmetry and then to pseudocubic symmetry, while a typical bimodal grain size distribution was observed in their rectangular grains at x = 0– 0.015 or y = 0.06–0.012. With increasing x or y , T O-T gradually decreases to room temperature, and an orthorhombic-tetragonal (O-T) morphotropic phase boundary was formed at 0.009 ≤  x  ≤ 0.015 or 0.009 ≤  y  ≤ 0.012. Around the phase boundary, both a good piezoelectric activity and a high Curie temperature were obtained, such as d 33 –320pC/N, k p –0.45,  T C  = 327 °C at x  = 0.015, and d 33 –304pC/N, k p –0.42,  T C  = 350 °C at y  = 0.012. In addition, the test for the piezoelectric constant following the annealing temperature indicate that these two compositions also have a good thermal stability.