For a basic study for the development of polymer-insulated DC cables, we examined the effect of the morphology on the interface between two insulation layers and that of between a semi-conductor layer and an insulation layer, on the DC characteristics, particularly focusing the attention on the space charge accumulation and the breakdown stress. The investigation led to the following conclusions: (1)In case of a sample bonded with two insulation plates at a temperature of 130°C, a vertical column-like crystal structure was formed in the region at a depth of about 10-20 μ m from the interface between two insulation layers. However, for a sample bonded at a temperature of 180°C, spherulites could be observed in all regions with no traces of the interface. In case of a sample bonded with a semi-conductor and an insulation, a vertical column-like crystal structure was observed in the insulation region at a depth of about 20-30 μ m from the interface. (2)For a sample formed a vertical column-like crystal structure around the interface, the hetero charge accumulation was observed on the interface. The number of accumulated charges increases, if the temperature of a sample during a voltage application is higher. (3)The DC breakdown strength was almost similar regardless of the difference of the morphology on the interface.
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