Dielectric properties of epoxy based nanocomposites filled with nano SiO₂ and BN and moisture effects

Abstract

Epoxy resin is one of the most commonly used thermosetting macromolecular synthetic materials, and is currently used in many areas such as casting in transformers and motors. To meet the broader usage demand for epoxy resins, many modification methods have been applied, including being filled with various nanoparticles. This has received increasing attention over recent years, because of the ability to obtain enhanced properties in the resultant nanocomposites. The investigation of this study used a range of techniques to analyse chemically and electrically the properties of epoxy resin and its nanocomposites. The first aim of the study is to develop a method of describing the morphology of nanofillers in the nanocomposites more precisely. Moreover, it will be investigated how the presence of these fillers influences the physical and chemical properties of epoxy resins by employing a number of techniques. Second, an investigation will be conducted into the consequences of the addition of nanofillers and resultant interfacial regions on the dielectric and electrical properties of epoxy nanocomposites. Third, an in-depth study into the influences of water absorption on the dielectric/electrical properties of epoxy and its nanocomposites is considered a worthwhile subject to investigate. Consequently, a number of conclusions have been made, some of which are listed below.

According to the proposed quantification method, epoxy nanocomposites with surface-treated particles achieved superior overall dispersion and distribution of particles/aggregates than those with untreated particles. Moreover, poor mixing resulted in an adverse impact on the morphology of the epoxy matrix by creating some physical defects (such as cracks and cavities). In hBN-based samples, particles were more likely to exist solely, which resulted in many more impacts on the base material. The presence of nano-SiO2 and hBN particles both resulted in a decrease in cross-linking density during the curing of epoxy resins, and further resulted in poorer thermal stability than with pure samples. Surface treatment modified the cross-linking density in the interfacial areas of SiO2-based samples by removing some surface groups, and the morphology of hBN-based nanocomposites had a significant impact on their thermal properties.

The addition of nanoparticles physically and chemically affected the base material and influenced the dielectric responses; thus, it is a solution for modifying the dielectric properties. The presence of nanoparticles resulted in morphology changes (mostly in the form of physical defects), while the surface treatment introduced deeper traps at the interface, and there were more shallow traps presented in the bulk of materials. Therefore, this hindered the charge injection and reduced the mobility of charge carriers, which consequently reduced the DC conductivity. However, with increased filler loadings, there was a greater effect on polymer structures and thus a higher density of traps. Thus, charge carriers are more easily transported by hopping or via the quantum tunnelling mechanism. This supported the concept of reduction in DC breakdown strength with the growth of filler loadings. In addition, the presence of plate-like nano-hBN created complex microstructures in epoxy resins and acted as barriers for charge injection and movement, which resulted in increased DC breakdown strength as filler loadings increased.

Studies on moisture effects have experimentally demonstrated the existence of a two-layer water shell structure, by proposing a method of estimating the average thickness of the water shell. According to the analysis of moisture uptake, surface treatment can reduce water absorption; however, there was no clear impact of modifying effects on the dielectric. The “hydrophobic” performance of BN nanocomposites is superior to silica samples, especially because of the lack of formation of water shells around the particles. According to the quantitative results from SEM and dielectric spectroscopy, the presence of water caused an increase in charge injection, higher mobility of charge carriers (both in base materials and within traps/particles,) and a clear reduction in DC breakdown results. Moreover, water shells around spherical particles contributed to higher probability of the quantum tunnelling process and the formation of conductive percolation channels.

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