Effect of thermal arc spray technique on varistor electric properties

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.Since the ceramic ZnO material has very poor contacting properties, the contact surfaces of the varistors are coated by a metallic electrode, predominantly aluminum. The function of the electrode is twofold: 1) Distribute the current evenly over the contact surface.2) Inject the current into the varistor material with low power losses.This deposition method is fast and costefficient. 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.This paper will discuss the overall framework of experiments to justify the performance of thermal arc spray aluminium coating. In this research 99.5% aluminium was used as the coating material in wire form of 2.3 mm diameter using wire arc spray method. Six samples were prepared with different thickness of coating from 20 to 400 microns and pre heating from 50̊c to 450̊c. Mechanical tests such as Pull-off Test and Bend Test were performed to evaluate the coating performance.Hardness Test and Surface Morphology Analysis were also to evaluate the coating properties. The hardness test showed an average value of 209.7 HV. Surface morphology analysis using FE-SEM indicated low porosity structures and high content of oxide.

Keywords:

varistor , surge arrester , zinc oxide , metallization , thermal spraying , electrical contact , electrode , energy withstand capability