Microstructure control to reduce leakage current of medium and high voltage ceramic varistors based on doped ZnO

The leakage current and the non-linearity coefficient (α) are crucial parameters in varistors. This work, deals with the optimization of the electrical characteristics of medium and high voltage varistors based on the ZnO–Bi2O3–Sb2O3 system. First, the aim was to allow the formation and the stabilization of the spinel phase by reducing the heating rate. To do so, the right sintering temperature and dwell time have to be chosen to obtain a breakdown field suitable for industrial requirements. During cooling, the spinel phase can return to a pyrochlore phase, which contributes to the increase of leakage current. In a second part, this reaction was prevented by faster cooling in an appropriate temperature range. The fine-tuning of both the heating and cooling phases leads to a significant decrease of the leakage current. Moreover, the value of the non-linearity coefficient was increased by 80%, due to better and more homogeneous wetting of the ZnO grains by the Bi-rich phase.