Simultaneously improving piezoelectric properties and temperature stability of Na_0.5K_0.5NbO₃ (KNN)-based ceramics sintered in reducing atmosphere

It is a very difficult work to sinter K_0.5Na_0.5NbO₃ (KNN)-based materials with good reduction resistance in strong reducing atmosphere. 0.945K_0.48Na_0.52Nb_0.96Ta_0.04O₃-0.055BaZrO₃ + 0.03ZrO₂ + y mol%MnO (KNNT-0.055BZ + 0.03Zr + yMn) ceramics sintered in reducing atmosphere were prepared successfully by conventional solid-state reaction methods. MnO dopant increases grain size at y = 5-8 due to strong lattice distortion and then decreases grain size at y = 9 due to much Mn₄Nb₂O₉ accumulated at the grain boundary. MnO dopant as an excellent sintering aid can effectively reduce volatilization of alkali metal by decreasing the sintering temperature (T_sinter). Reducing alkali metal volatilization can greatly reduce oxygen vacancies and improve piezoelectric properties. MnO dopant can improve the anti-reduction properties. The KNNT-0.055BZ + 0.03Zr + yMn ceramics at y = 6-9 show outstanding anti-fatigue of unipolar piezoelectric strain under the synergistic effect of reduced oxygen vacancies due to reduced volatilization and increased grain size. Piezoelectric properties and temperature stability of KNNT-0.055BZ + 0.03Zr ceramics sintered in reducing atmosphere are improved simultaneously by MnO dopant. Optimum inverse piezoelectric coefficient (d^*₃₃) of ceramics at y = 8 reaches up to 480 pm/V under low driving electric field E = 20 kV/cm at room temperature, and its temperature stability of d^*₃₃ reaches 158 ℃. It will be an excellent lead-free material candidate for the preparation of multilayer piezoelectric actuators co-fired with nickel electrode.