Effects of Co and Mn Addition in ZnO Varistors- Approach from Solid State Chemistry -

Abstract

We analyzed the distribution of Co and Mn in the microstructure of ZnO varistor and discussed the effects of Co and Mn addition on electrical properties from the viewpoint that the influences of Co and Mn on the grain boundary layer, not on ZnO grains, are important. ZnO varistor showed excellent non-ohmic properties even when Mn was not incorporated into the ZnO grains. It was also found that the grain boundary layers consisted of thin intergranular layers between ZnO grains and grain boundary multiple points. The spinel particles remain at the grain boundary multiple points, and then react with the liquid phase, which is mainly composed of Bi₂O₃, to form ZnO with conductive properties during the cooling process. In ZnO varistor, the grain boundary multiple points are connected in a mesh-like structure, and if leakage currents occur at these points, the non-ohmic characteristics are degraded. Co and Mn are soluble in the spinel phase and act to increase chemical stability, and they suppress the reaction with the liquid phase, thereby maintaining the insulating properties of the grain boundary multiple points and contributing to the improvement of non-ohmic properties.