Consideration on Filamentary Thermal Breakdown by Measuring Pre-Breakdown Current in Solid Dielectrics

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Abstract
Filamentary thermal breakdown is discussed on the basis of experimental results on the electrical conduction just prior to breakdown in polyimide (PI) and polyethylene (PE) with a dc ramp voltage applied at 90°C. In PI, the pre-breakdown current was almost proportional to the electrode area S till about 100 µs before breakdown, but a sharp current increase then appeared at smaller values of S. In PE, this sharp current increase could be detected only after about 25 ns before breakdown, using a new measuring device with higher time resolution. Assuming that this sharp current increase is due to the temperature rise of a small spot in the sample leading to filamentary thermal breakdown, the maximum radius of the filament spot was estimated at about 35 µm for PI and about 2000 Å for PE.

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