Characteristics of segmental dynamics in EP/TiO2 nanocomposites and its effect on dielectric breakdown

Abstract

Epoxy nanocomposites with excellent mechanical, thermal and dielectric properties have potential applications in electrical and power electronic devices. However, due to the complex structure of the interfacial region in nanocomposites, molecular dynamic characteristics and how they relate to macro-dielectric performances still needs to be investigated. In this study, several kinds of epoxy nanocomposites were prepared. Segmental dynamics and charge migration parameters are characterised by a dielectric spectroscopy using the Havriliak–Negami equation. The DC breakdown strength of EP/TiO2 nanocomposites was tested for temperature values between 403 and 433 K, and the influence of segmental dynamic characteristics on DC breakdown was discussed. The results indicate that EP/TiO2 nanocomposites with optimised filler content (0.5 wt%) have lower values of dielectric relaxation strength, characteristic relaxation time and thermal expansion coefficients than other samples and that segment dynamics and charge transport processes are suppressed by nanoparticle incorporation. The DC breakdown strength of EP/TiO2 nanocomposites with filler content initially increases and then decreases at a certain temperature, while it significantly decreases with temperature. Correlation analysis indicates that DC breakdown above the glass transition temperature is dominated by segmental dynamics rather than carrier transport. Owing to the tight bond between the nanoparticle and the epoxy matrix, segmental dynamics are impeded when the proper contents of nanoparticles is incorporated, resulting in a decrease in the free-volume fraction and an increase in DC breakdown strength. However, when the temperature rises, the free volume in the intersection of segments expands due to thermal stimulation and free electrons gain sufficient energy in the free length to trigger DC breakdown, hence DC breakdown strength decreases with temperature.