The evolution mechanism of defect dipoles and high strain in MnO₂-doped KNN lead-free ceramics

Defect dipoles in acceptor-doped (K_0.5Na_0.5)NbO₃-based ceramics have a significant influence on their electrical properties. The present study examined the influence of the sintering atmosphere on the electrical properties of MnO₂-doped (K_0.5Na_0.5)NbO₃. The poled and aged samples sintered in the Ar atmosphere depicted unusual behavior related to the formation of defect dipoles $(Mn2+Nb‴−VO··)′$⁠, aligned in the poling direction having larger polarity. The S-E loop of the poled and aged MnO₂-doped (K_0.5Na_0.5)NbO₃ ceramics sintered in the Ar atmosphere revealed larger strains in the poling direction and restrained strains in the opposite direction. Furthermore, it is observed that the unipolar electro-strain could reach 0.28% (d₃₃^*= 800 pm/V) at 3.5 kV/mm, a value nearly 5.6-fold higher than those obtained in the air atmosphere (0.05%). This method based on the sintering atmosphere and process control provides a promising way to obtain substantial electro-strain values suitable for applications in high-displacement actuators.