Effect of MnO₂ addition on microstructure and piezoelectric properties of 0.95(Na_0.5K_0.5)NbO₃-0.05CaTiO₃ piezoelectric ceramics

MnO₂ was added to the 0.95(Na_0.5K_0.5)NbO₃-0.05CaTiO₃ (NKN-CT) ceramics in order to promote the densification and improve the poling efficiency by increasing the resistance of the specimens. Densification and abnormal grain growth occurred in the MnO₂-added NKN-CT ceramics sintered at 1020°C, indicating that MnO₂ assisted the liquid-phase sintering of these materials. Mn³⁺ ions were considered to enter the A-site of the matrix, thereby producing the free electrons, which interacted with the holes resulting from the evaporation of alkali ions. This interaction results in an increase in the resistance of the specimens. The increased resistance improved the poling efficiency and, hence, the dielectric and piezoelectric properties of the NKN-CT ceramics. A few of the Mn³⁺ ions that entered the B-site of the NKN-CT matrix led to a slight increase in the mechanical quality factor.