The effect of filler loading ratios and moisture on DC conductivity and space charge behaviour of SiO 2 and hBN filled epoxy nanocomposites

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Abstract
Nanocomposites that exhibit good insulation properties have already attracted a variety of research and their electrical properties are believed to be related to charge dynamics in bulk of materials. However, it is still unclear on how nanofiller loading ratios, surface treatment and resultant changes in morphology will influence the charge dynamics of nanocomposites. In this paper, we have clearly mentioned the influence of adding nanoparticles into epoxy resins and the characteristics of the movement of charges in the materials based on combining analysis on morphology, DC conductivity and space charge measurements. The presence of spherical nanoparticles (SiO 2 ) introduced additional traps in bulk, which impaired the charge injection and reduced the mobility of charge carriers in samples of low filler loading ratios (e.g. 0.5 wt%). However, in silica-based samples of higher filler loadings, more nanoparticles further caused a higher density of traps, which resulted in lower average distance between arbitrary traps/inter-particle surface distances and thus charge carriers required less energy when moving from one to another by hopping or the quantum tunnelling mechanism. The surface treatment of SiO 2 particles introduced deep traps which helped the separation of particles or related traps, and to some extent restricted the transport of charge carriers. In addition, hBN particles seem to act as barriers to charge injection and movement due to the layered structures and large numbers of resultant shallow traps in bulk. In terms of the moisture effect, the presence of water led to an obvious increase in charge injection and mobility, and resulted in the higher mobility of charge carriers in both base materials and within traps/particles of nanocomposites. The existence of water shells around spherical particles could contribute to a higher probability of the quantum tunnelling process and the formation of conductive percolation channels.

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