Phase transition, microstructure and electrical properties of K 1‐ x N a x NbO 3 ‐based ceramic sintered in reducing atmosphere

Lead‐free 0.955K 1‐ x Na x NbO 3 ‐0.045Bi 0.5 Na 0.5 ZrO 3 +0.4% mol MnO ceramics (Abbreviated as K 1‐ x N x N ‐0.045 BNZ +0.4Mn) were prepared by a conventional solid‐state sintering method in a reducing atmosphere (oxygen partial pressure : 1 × 10 −11 MP a). All K 1‐ x N x N ‐0.045 BNZ +0.4Mn samples show a pure perovskite structure with a polymorphic phase boundary ( PPB ) composed of rhombohedral (R) and tetragonal (T) phases. A high Na/K ratio and a low Na/K ratio can both induce an increase in the rhombohedral phase. The reverse piezoelectric coefficient and its temperature stability in K 1‐ x N x N ‐0.045 BNZ +0.4Mn ceramics can be improved by controlling the Na/K ratio. The increase in the Na/K ratio from x = 0.46 to x = 0.56 can decrease the A‐site cation vacancies. The activation energy of the grain is higher than that of the grain boundary due to the accumulation of oxygen vacancies at the grain boundary. K 1‐ x N x N ‐0.045 BNZ +0.4Mn ceramics with excellent piezoelectric properties (quasi‐static piezoelectric coefficient d 33 = 326 pC /N, and = 472 pm/V at E max = 25 kV /cm) were obtained at x = 0.52.