Characteristics of initial trees of 30 to 60 μm length in epoxy/silica nanocomposite

Many studies were carried out on breakdown (BD) V-t_b characteristics to bring about 1 to 3 mm long trees in the past. Instead, tree initiation V-t_i characteristics are intensively investigated in this paper in order to investigate interactions of trees with nano-filler in their early growth period. Initial trees are defined as tiny trees of 30 to 60 μm length that can be detected by a recently developed detection method. It was shown that tree initiation V-t_i characteristics show the same trends with voltage and treated and untreated nano-filler as the BD V-t_b characteristics though they are much shorter in time. From V-t_i characteristics the following four results are obtained. (1) Tree initiation time is prolonged when epoxy is filled with 5 wt% nano silica of any type in general. (2) It is longer for 40 nm silica than for 100 nm silica. (3) Surface treatment of filler by a coupling agent helps contribute to the further increase in initiation time. (4) Fumed silica gives longer time for tree initiation than fused silica. From SEM observation it is indicated that filler agglomeration is larger in epoxy with surface-treated fused silica than in epoxy with original silica, and that it is larger in epoxy with untreated fumed silica than in epoxy with untreated fused silica. Adhesion of nano-filler with epoxy must be more effective to increase tree initiation time than filler agglomeration. One of the most important findings is a positional relation between initial trees and nano-filler particles. SEM images support that most of initial trees are untouched with nano-filler particles, while some of initial trees are touched with them. Some discussion is made on how initial trees will interact with nano-filler in the path of their growth.