Improving Piezoelectric Strain, Temperature Stability, and Strain Symmetry under a Low Driving Electric Field on KNN-Based Ceramics Sintered in a Reducing Atmosphere

The KNN-SZ + Zr + xMn ceramics prepared successfully in a reducing atmosphere show a high piezoelectric strain coefficient (d33*), high temperature stability of d33*, good bipolar strain symmetry, and high insulation resistance (1.42 × 1011 Ω·cm). Due to the occupying evolution of Mn ions, the concentration of defect dipoles (MnNa/K·−VNa/K′, MnNb′−VO••, and MnNb″−VO••) increases at x = 5–8 and the concentration of defect dipoles (MnNb‴−VO••) increases at x = 6–8. The optimum piezoelectric strain coefficient (d33* = 623 pm/V at 10 kV/cm) of ceramics (x = 6) is attributed to the synergistic effect of enhanced spontaneous polarization (PS) extension and strong reversibility of rhombohedral─monoclinic phase transitions. The reduction in the oxygen vacancy (VO••) concentration and formation of stable defect dipoles enable the ceramic (x = 6) to achieve a superior temperature stability in d33* (T90* = 160 °C). This study presents an important design to enhance synchronous piezoelectric strain, temperature stability, and bipolar strain symmetry of KNN-based ceramics sintered in a reducing atmosphere.