Advances in ZnO–Bi2O3 based varistors
In this review paper detailed development of the ZnO–Bi2O3 based polycrystalline varistors are presented as the technological importance emerged with the rapid applications of the gapless surge protectors superseding traditional gapped SiC designs. The excellent symmetric nonlinear current–voltage (I–V) characteristic of these varistors results from the type of back-to-back electrical potential barriers that are formed between the successive ZnO grains during the step-wise processing cycles. The role of microstructures and the defect structures in conjunction with the processing parameters paved the road to the understanding of the degradation and failure mechanisms of the electrical potential barriers for acceptable protective characteristics. The nature of the processing variables influencing eventual performance of varistors is discussed.