New contacting solutions for high voltage ZnO varistors

Overvoltage protection systems are today predominantly based on zinc oxide varistor technology. A varistor is basically a resistive device with an extremely nonlinear V-I-characteristic, which allows it to switch between an insulating and a conducting behavior within a narrow voltage interval. The varistor is made from a sintered ceramic material consisting mainly of zinc oxide, together with a number of other oxides and additives. The ceramic material is normally contacted via an aluminum electrode which is coated onto the surface by thermal spraying. This deposition method is fast and cost-efficient. However, the quality of the coating and its bond to the ceramic do not allow for the full utilization of the performance of the varistor. In order to improve the quality and performance of the electrode, alternative coating methods were considered, and a new concept based on Physical Vapor Deposition (PVD) has been evaluated. The new contacting technology has shown superior electrical and mechanical performance, manifested by a substantial increase in bond strength and energy withstand capability. The present paper describes the new solution and compares it with the present one.