The epoxy resin was filled with nano-BN, surface treated, and untreated nano-SiO 2. Measurements of dielectric spectroscopy cover the range of frequency from 10 −2 to 10 5 Hz and will relate to the hydration of samples. It was observed from the results that permittivity of pure epoxy was strongly affected by the water absorptions and a bit of temperature. Then it will allow us to extract the influence of water absorption on dielectric properties and proposed a relatively reliable method by using Monte-Carlo simulation to estimate the average thickness of water shell or related relaxation peak of epoxy nanocomposites with spherical particles. At the end, the authors experimentally demonstrated the existence of two layer structure of water shell and concluded that surface treatment is able to reduce the water uptake, however, no obvious impact on modifying its effects on dielectric properties due to the limitation of thickness of tightly bonded layer. The 'hydrophobic' performance of BN nanocomposites is much better than silica ones, especially the no formation of water shell around the particles, and may be more suitable for application under environment with humidity.

The ability to suppress space charge accumulation at high electric fields makes nanocomposites attract significant research interest as potential insulation materials in high-voltage direct current cable development. At present, the deep trap introduced by nanoparticles is frequently applied to be responsible for the observed space charge suppression in nanocomposites. However, the experimental results that support deep-trap formation have not been rigorously examined. We therefore propose herein a simple and more direct approach based on isothermal surface potential decay combined with pulsed electro-acoustic measurements to verify the presence of deep traps in silicabased blend polyethylene nanocomposites. The results indicate that the deep traps are indeed introduced by filling nanosilica and the space charge suppression observed in the nanocomposite with a low loading ratio is caused by deeply trapped charges in the sub-surface region of specimens.

Nanocomposites those exhibit good insulation properties have already attracted numbers 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 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 silicabased 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 term of 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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