Effects of Fe and Al co-doping on the leakage current density and clamp voltage ratio of ZnO varistor

Abstract

The effects of ferrum and aluminum ions co-doping on the phase evolution, microstructure, current-voltage characteristics and clamp characteristic of ZnO-based varistors were investigated. X-ray diffraction (XRD) analyses verify that Fe-Al co-doping in ceramic doesn't induce the presence of additional phase and diffraction peak shift. And the energy dispersive spectrometer (EDS) analyses demonstrate that most of the added Fe3+ and Al3+ penetrate into ZnO grains. This permeation significantly increases the sizes of ZnO grains, resulting in the noticeable decrease of the breakdown field. Additionally, the nonlinear coefficient and leakage current density were remarkably improved via the co-doping of Fe3+ and Al3+ in comparison to the single Al3+ doped sample. In particular, when the additive amount of Fe3+ and Al3+ reach to 0.014 mol% and 0.028 mol% respectively, the raised rate of nonlinear coefficient and the attenuation rate of leakage current density reach to the optimum. Meanwhile, this sample possesses the optimal donor density Nd, which is mainly responsible for the result that the clamp voltage ratio of sample obtains the lowest value under pulse current surge. This work reveals that the coexistence of an optimal Fe/Al ratio contributes to preparing the excellent ZnO varistor with the excellent electrical properties, low clamping voltage and high protective effect.