Current Flow and Structural Inhomogeneities in Nonlinear Materials
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computer-modelling of simple or random networks of nonlinear resistors. The results provided an interesting insight in how current paths develop and are affected by the disorder on the grain boundary level and with respect to the grain size distribution [2,3,14]. The filamentary current flow in the highly nonlinear part of the I-V-characteristics is well reproduced and predicts a high sensitivity of the breakdown voltage to the structural or electrical disorder.
Figure 1: Infrared thermal images of PTC-resistor and ZnO-varistor in their nonlinear current- or voltage limiting state. Left: Polymer-composite PTC [4] with hot plane caused by current limitation. Right: Hot channel in ZnO-ceramic varistor caused by (surface) contacts with protrusion. Existing simulation work, however, does not include spatially distributed second phases. Depending on the overall composition of the varistor material and the ceramic process quality, these insulating clusters can significantly reduce the available cross section for current conduction and lengthen the current paths. Also, the computer modelling has to be limited to small 2dimensional networks (
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