Phase transition, microstructure and electrical properties of K1-xNaxNbO3-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 K1-xNxN-0.045BNZ+0.4Mn) were prepared by a conventional solid-state sintering method in a reducing atmosphere (oxygen partial pressure pO2: 1x10(-11) MPa). All K1-xNxN-0.045BNZ+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 d33 and its temperature stability in K1-xNxN-0.045BNZ+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. K1-xNxN-0.045BNZ+0.4Mn ceramics with excellent piezoelectric properties (quasi-static piezoelectric coefficient d(33)=326pC/N, and d33=472pm/V at E-max=25kV/cm) were obtained at x=0.52.