Influence of filler content on trap and space charge properties of epoxy resin nanocomposites

Epoxy resin nanocomposites have excellent properties such as the suppression of space charge accumulation, high resistivity, and high electrical breakdown strength, which play an important role in developing the direct current power equipment. However, the influencing mechanisms of filler content on trap, conductivity, and space charge of nanocomposites have not been clear to date. In the present paper, a method to calculate the densities of shallow traps and deep traps in interaction zones is proposed based on the multi-region structure model of interaction zones, and the dependence of shallow traps and deep traps on filler content is obtained. It is found that the shallow trap density increases with the increase of filler content, while the deep trap density first increases and then decreases with increasing the filler content, which is caused by the overlap of interaction zones. Then, the relation between the shallow trap controlled carrier mobility and the filler content is investigated. With the filler content increasing, the density of shallow traps increases and their mean distance decreases, leading to an increase in the shallow trap controlled carrier mobility. Considering the charge injection from cathode into dielectrics, carrier hopping in shallow traps, charge trapping into and detrapping from deep traps, a unipolar charge transport model is established to study the conductivity and distributions of space charges and electric field in epoxy resin nanocomposites. At relatively low filler content, the charge transport is dominated by deep traps in interaction zones and the conductivity decreases with the increase of filler content. However, the charge transport is determined by shallow traps at relatively high filler content and the conductivity increases.