Abstract
Zinc Oxide (ZnO) varistor elements which have non-linear current-voltage characteristic have a number of failure mechanism when ZnO varistor elements absorb surge energy. Failure mode at the edge of the aluminum electrode formed by the thermal spraying process on the ZnO varistor element surface is quite common failure mechanism by high current surges. Such failures take the form of a melt pinhole from the electrode edge to the counter electrode or the edge of the disk element. As a result of the transient finite element analysis for the electric and thermal field distribution and the measurement of the resistively-temperature characteristic at high current region, it was made clear that the failure at the electrode edge was caused by the partial melting by the current concentration and the local thermal runaway at the electrode edge. Moreover, the current concentration was reduced by decreasing of the distance between the electrode edge and the sintered ZnO body edge.
We developed the aluminum sprayed electrode manufacturing process which could control the edge margin with accuracy. And using this electrode process, the energy absorption capability of the ZnO elements was improved largely by reducing the current concentration at the electrode edge.
