Current Localization, Non-Uniform Heating, and Failures of ZnO Varistors
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ABSTRACT Non-uniform heating of ZnO varistors by electrical pulses occurs on three different spatial scales: (1) microscopic (sub-micron), (2) intermediate (sub-millimiter), 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. INTRODUCTION Metal oxide varistors have highly nonlinear electrical characteristics and are widely used as devices for over-voltage protection [1, 2]. 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. Specifically, varistors provide protection against voltage surges due to lightning strikes, switching transients, and similar disturbances. While varistors have a large capacity to absorb energy (e.g., 500 J/cma), they are, in fact, subject to occasional failure. The significant varistor-failure mechanisms include: electrical puncture, thermal cracking, and thermal runaway -- all resulting from excessive heating, in particular, from non-uniform heating. Temperature and current flow in a varistor are highly correlated, since the heat is generated by electrical conduction which varies strongly with temperature. Non-uniform Joule heating occurs in varistors as a result of inhomogeneous electrical properties that originate in either the varistor fabrication process or in the statistical fluctuations in properties that generally occur in polycrystalline materials. The power dissipation characteristics, Joule heating, and energy-handling capability of
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ZnO varistors have attracted attention because of their relevance to device failure and service life [3]-[13]. Measurements have been made that directly connect varistor failure to heating, and the various modes of varistor failure have been mod
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