Current concentration at defects in ZnO varistor material
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We numerically simulated the current density distribution in electrically nonlinear varistor material containing geometrically simple inclusion defects. Nonconductive spheres and disks, which resemble inclusion shapes observed in chemically prepared varistor material, were investigated. Current densities near perfectly conductive spheres and rods were also computed to gain insight into observed electrical degradation phenomena. These defects were assumed to be much larger than the characteristic size of the zinc oxide (ZnO) grain structure, and our computational method treated the varistor material as electrically isotropic. Results showed strong, localized concentrations of current with either perfectly conductive or nonconductive inclusions, and a dependence on the density of the conductive defects. The small spatial extent of strong current intensification may help to explain the stepwise electrical degradation we have observed when a failing ZnO varistor is subjected to high-power electrical pulses.
I. INTRODUCTION
Varistors are electrical devices that are widely used to regulate voltage or to protect power lines and sensitive electrical equipment from transient high-voltage surges. Below a fairly well defined threshold (or “switching”) voltage a varistor behaves as an insulator, but at higher voltages the current through the varistor rises very steeply with increasing voltage. This property allows a varistor to regulate voltage over many decades of current. Most commercial applications are based on polycrystalline, semiconducting zinc oxide (ZnO) that has been modified by a variety of low concentration additives. Varistors sometimes fail to regulate at their original switching voltage after they are repeatedly subjected to high-power electrical pulses. The typical electrical degradation we have observed in failing ZnO varistors is evidenced by a progressive, permanent reduction in the regulation voltage at a specified current level when a varistor is stressed with multiple, brief electrical pulses.1 Individual test specimens vary greatly in the number of pulses that are withstood before the onset of degradation, but once the process starts there is a consistent downward step in regulation voltage with each additional pulse, and these steps vary little between the specimens. After the
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voltage degrades more than 5%, a hollow filament of approximately 20 m diameter, with glassy or recrystallized ZnO on the surface, is frequently found extending from one electrode partway through the specimen. This indicates an intense localized heating that exceeds the melting temperature of ZnO. Similar remnants of electrical breakdown have been shown by Eda2 and by Vojta and Clarke3
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