Abstract
The electric charge density accumulated at the interface of a double layer consisting of a liquid and a solid insulator was obtained from the electric field strength in the liquid insulator as a function of time during application of DC stress (24h) and after short-circuiting positive and negative electrodes. The field strength was measured using the high sensitive Kerr electro-optic effect whose minimum measurble electric field intensity was 100V/cm in silicone oil with 6-cm parallel electrodes. The double-layer system consisted of a liquid insulator (Si-oil; pure, water-containing and thermally aged) and a solid insulator (polypropylene (PP), normex and polyester nonwoven fabric). According to simple calculations, the initially applied voltage (3, 000V) was divided in the ratio of 1.5:98.5 (PP, Si-oil, respectively). However, after 24h, the ratio changed to 80:20. This indicates that 80 to 90nC/cm2 accumulated at the interface between PP and Si-oil. By observing the time course of the electric field in the Sioil for 24h after short-circuiting the two electrodes, we found two rates of decay indicating two decay mechanisms: the fast rate is related to the charge leakage from the interface and the slower rate is related to the leakage of charges trapped in the PP.
