NONUNIFORM HEATING IN ZINC-OXIDE VARISTORS STUDIED BY INFRARED IMAGING AND COMPUTER-SIMULATION

State-of-the-art infrared (IR) thermal imaging was used to monitor the heating of ZnO varistors by electrical transients. On a macroscopic s cale (e.g., 10 mm), heating in large varistor blocks (i.e., diameter o f 42 mm) was found to be the greatest near the block edges and to be a pproximately radially symmetric in blocks fabricated at a low aspect r atio. In blocks fabricated at a higher aspect ratio, the heating was l ess symmetric, presumably because uniform properties are more difficul t to achieve. Nonuniform heating in large blocks can be attributed to processing-induced variations in the electrical properties of the bloc ks. On an intermediate size scale (e.g., 1 mm), the heating in small v aristor disks (e.g., diameter of 10 mm) was observed to be most intens e along localized electrical paths. The high electrical conductivity o f these paths originates from the statistical fluctuations in properti es that inevitably occur in polycrystalline materials. On a microscopi c scale (e.g., 10 pm), the heating in thin varistor slices (e.g., thic kness of 100 pm) was observed to be localized in strings of tiny hot s pots. The hot spots occur at the grain boundaries in a conducting path , where the potential is decreased across Schottky-type barriers and t he heat is generated. The experimentally observed heating is interpret ed by applying transport theory and using computer simulations. It is shown that, on the scale of the grain size, the heat transfer is too f ast to permit temperature differences that could cause a varistor fail ure. Current localization and nonuniform heating on an intermediate si ze scale can have a microstructural origin (e.g,, statistical fluctuat ions of grain sizes and grain-boundary properties). However, these are shown to be significant only in small varistors, whereas destructive failures (puncture and cracking) of large varistor blocks can be cause d only by nonuniform heating on a macroscopic scale.