Capacitance changes in degraded metal oxide varistors

Abstract

ZnO based ceramics show a highly nonlinear current-voltage (I-V) characteristic due to intergranular layers at the boundaries of ZnO grains. This research indicates that the junctions formed on the border between ZnO grains and the intergranular phase are responsible for the creation of potential barriers which control the flow of current through the bulk of such a ceramic. Having polycrystalline nature and a large number of intergranular barriers, and so a high energy absorption capability, ZnO ceramics are used as active components in overvoltage protection devices. But as a result of charge transport the height of the barriers decreases and varistor degradation occurs. The degradation seems to affect mainly the prebreakdown region of the I-V curve and it results in increased leakage current. This paper presents a part of research aimed at assessing the effect of ageing on the capacitance characteristic of ZnO varistors.

Stability of ZnO-Pr₆O₁₁-Cr₂O₃ Varistor Ceramics against Electrical Degradation

“…To certain extent, a degraded varistor could experience thermal runaway resulting in overheating or potentially explosion. Some of degraded varistors become asymmetrical in polarity and exhibit increment in terms of its capacitance component [4,5]. Degradation is an unintended process as it could cause many implications during operation such as device interruption, shortening of device's lifetime and compromising the safety of the device.…”

Application of Infrared Thermography to Non-Contact Testing of Varistors

“…The latter practically eliminates the varistor from its application as a protective element, due to the change of its impedance and resulting changes in I-V characteristics. The issues were published in a conference paper [8].…”