A Study of the Variables Influencing the Durability and Stability of Zinc-Oxide Based Varistors
Date of Award
Dissertation - Restricted
Doctor of Philosophy (PhD)
Martin A Set
Robert N. Blumenthal
Zinc Oxide based metal oxide varistors are voltage dependent resistors, with nonlinear current-voltage characteristics similar to back to back zener diodes. These devices have an extremely high resistance at voltages less than me critical value. Here the current-voltage relationships can be described as being in a number of low conduction states. Above this critical voltage, these behaviors are no longer obeyed, and the current increases very rapidly with voltage. Such characteristics are of particular interest to the electrical and electronic industry where over voltage protection are needed.
The heterogeneous structure that is responsible for the non-destructive breakdown in the devices, is generally composed of relative conductive ZnO grains in a matrix of thin, relative insulating, dielectric layers. When these layers are stressed with a relative high field, deterioration in the device performances are experienced. This degradation in the electrical performance of the MOV consist of specific types of charged species diffusing and/or reacting within a specific type of physical region, within the heterogeneous system. The response of physiochemical reactions and diffusional processes to temperature provides a method by which these devices can be tested in an accelerated fashion.
The long term thermal and bias stability of the varistor are dependent on their ability to withstand conditions that leads to increases in their leakage current and subsequent power dissipation. A number of factors are found to influence this ability, including composition, heat treatment, AC and/or DC bias level, and temperature and time of usage. Therefore it is important to characterize the devices in terms of these controlling variables.
In this study, the deterioration kinetics of MOV's were characterized over an extended range of temperatures and applied bias levels, along with composition and process variable variations. The aforementioned characteristics provides means to improve and optimize the general performance of individual specimens. These studies should rapidly extend to big blocks and prototype arrestors.