Effects of Dopants and Processing Parameters on the Properties of ZnO-VO-Based Varistors Prepared by Powder Metallurgy: A Review.

This article reviews the progress in developing ZnO-VO-based metal oxide varistors (MOVs) using powder metallurgy (PM) techniques. The aim is to create new, advanced ceramic materials for MOVs with comparable or superior functional properties to ZnO-BiO varistors using fewer dopants. The survey emphasizes the importance of a homogeneous microstructure and desirable varistor properties, such as high nonlinearity (α), low leakage current density (J), high energy absorption capability, reduced power loss, and stability for reliable MOVs. This study investigates the effect of VO and MO additives on the microstructure, electrical and dielectric properties, and aging behavior of ZnO-based varistors. The findings show that MOVs with 0.25-2 mol.% VO and MO additives sintered in air over 800 °C contain a primary phase of ZnO with a hexagonal wurtzite structure and several secondary phases that impact the MOV performance. The MO additives, such as BiO, InO, SbO, transition element oxides, and rare earth oxides, act as ZnO grain growth inhibitors and enhance the density, microstructure homogeneity, and nonlinearity. Refinement of the microstructure of MOVs and consolidation under appropriate PM conditions improve their electrical properties (J ≤ 0.2 mA/cm, α of 22-153) and stability. The review recommends further developing and investigating large-sized MOVs from the ZnO-VO systems using these techniques.