Current localization, non-uniform heating, and failures of ZnO varistors

摘要

Metal oxide varistors have highly nonlinear electrical characteristics and are widely used as devices for over-voltage protection. Varistor applications range from the use of small varistors to protect delicate electronic components to the use of much larger varistors for the protection of electrical-power-distribution systems. Non-uniform heating of ZnO varistors by electrical pulses occurs on three different spatial scales: (1) microscopic (sub-micron), (2) intermediate (sub-millimeter), and (3) macroscopic (of order of millimeters or centimeters). Heating on these scales has different origins and different consequences for device failure in large and small varistors. On the microscopic scale, the heating localizes in strings of tiny hot spots. They occur at the grain boundaries in a conducting path where the potential is dropped across Schottky-type barriers. These observations are interpreted by applying transport theory and using computer simulations. It is shown that the heat transfer on a scale of the grain size is too fast to permit temperature differences that could cause a varistor failure. On an intermediate size scale, the heating is most intense along localized electrical paths. The high electrical conductivity of these paths has microstructural origin, i.e., it derives from the statistical fluctuations of grain sizes and grain boundary properties. Current localization on the intermediate size scale appears to be significant only in small varistors. On the macroscopic scale, current localization in large blocks can be attributed to inhomogeneities in the electrical properties which originate during ceramic processing. The resulting non-uniform heating is shown to cause destructive failures of large varistor blocks.