Two kinds of epoxy/silica nanocomposites, i.e. Aerosil and Nanopox specimens, were prepared to clarify the effect of nano-filler dispersion and of coupling agents on electric field endurance. According to SEM observation, nano-fillers were well dispersed in the former specimens, while some agglomeration was found in the latter specimens. This difference caused different characteristics for PD resistance and treeing resistance. PD resistance was improved only by adding nano-fillers, and treeing resistance was improved not only by adding nano-fillers but must by making nano-fillers uniformly dispersed. Coupling agents were also effective in treeing resistance in both kinds of specimens. It was then clarified that nano-fillers and coupling agents could improve PD and treeing resistance, but that treeing resistance could be upgraded or degraded depending on the level of nano-filler dispersion.
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