Role of inhomogeneous intrinsic point defect distribution on electrical properties in ZnO varistor ceramics

Abstract

Inhomogeneous electrical performance in ZnO varistor ceramics are investigated from distribution of intrinsic point defects using thermally stimulated depolarization current (TSDC). The nonlinear coefficient is 50 in central area while it is only 25 at the brim. Meanwhile, the leakage current density increases from 2.9 μA/cm2 in the center to 14.7 μA/cm2 at the brim. From X-ray detection, phase compositions in the center and at the brim show no difference. At the same time, grain size distribution is relatively uniform among the ZnO sample, indicating non-uniform grain boundary characteristics should be responsible for the electrical inhomogeneity. From analysis of TSDC, it is found that intrinsic point defect distributions are inhomogeneous. The densities of zinc interstitials and oxygen vacancies are much higher at the brim than those in the center. The higher intrinsic defect densities at the brim might induce higher donor density in the depletion layers at Schottky barriers resulting in lowered Schottky barrier, which is in accordance with the result of Schottky barrier height calculated from dc current. Therefore, the electrical inhomogeneity in ZnO varistor ceramics is understood from the aspect of intrinsic point defect distribution.