Investigation of electrical inhomogeneity in ZnO varistor ceramics based on electronic relaxations

Spatially non-uniform electrical performance in ZnO varistor ceramics, which restricts application for higher energy absorption capability, is investigated in terms of electronic relaxations corresponding to intrinsic point defects. Decreased breakdown field and nonlinear coefficient are observed from 135.5 V/mm and 53 in central area to 122.0 V/mm and 20 in peripheral area as well as significantly increased leakage current density from 3.4 μA/cm2 to 18.3 μA/cm2. From microstructure analysis, grain size distribution is found relatively homogeneous, suggesting that electrically non-uniform grain boundaries should be responsible for divergent electrical properties. Investigation via energy dispersive spectrometer (EDS) shows higher Zn/O ratio in peripheral area, indicating higher possibility of generating intrinsic point defects like zinc interstitials and oxygen vacancies. This is further verified via dielectric spectroscopy that the densities of zinc interstitials and oxygen vacancies are both larger in peripheral area than those in central area. Oxygen is easier to be removed from ZnO lattice and diffuse into atmosphere during high-temperature sintering process, resulting in larger amount of oxygen vacancies in peripheral area. Meanwhile, more zinc atoms are removed from lattice and form interstitials as a result of distortion of ZnO lattice in peripheral area. Increased donor density in peripheral area attributed by oxygen vacancies and zinc interstitials results in decreased Schottky barrier height. Non-uniform distribution of Schottky barrier height is the origin of overall electrical inhomogeneity.