Temperature stability and electrical properties of MnO-doped KNN-based ceramics sintered in reducing atmosphere

Lead-free MnO-doped 0.955K(0.5)Na(0.5)NbO(3)-0.045Bi(0.5)Na(0.5)ZrO(3) (abbreviate as KNN-0.045BNZ) ceramics have been prepared by a conventional solid-state sintering method in reducing atmosphere. The MnO addition can suppress the emergence of the liquid phase and improve the homogenization of grain size. All ceramics sintered in reducing atmosphere show a two-phase coexistence zone composed of rhombohedral (R) and tetragonal (T) phase. MnO dopant results in the content increase in R phase and slight increase in Curie temperature T-C. For KNN-0.045BNZ ceramics, Mn2+ ions preferentially occupy the cation vacancies in A-site to decrease oxygen vacancy concentration for 0.2%-0.4% MnO content, whereas Mn2+ ions substitute for Zr4+ ions in B-site to form oxygen vacancies at x0.5. The defect dipole (MnZr-Vo) is formed at the moderate concentration from 0.5 to 0.6, which can provide a preserve force to improve the temperature stability of piezoelectric properties for k(p) and d33. The Mn0.4 ceramics show excellent electrical properties with quasistatic piezoelectric constant d(33)=300 pC/N, electromechanical coupling coefficient k(p)=51.2%, high field piezoelectric constant d33=430pm/V (at E-max=25 kV/cm) and T-C=similar to 345 degrees C, insulation resistivity = 6.13x10(11) cm.