Effects of nano- and micro-filler mixture on electrical insulation properties of epoxy based composites

This paper focuses on the electrical insulation properties of a newly prepared composite material by nano- and micro-filler mixture. Nano- and micro-filler mixture composites were made by dispersing nano-scale layered silicate fillers and micro-scale silica fillers in epoxy resin. To investigate the effects of nano- and micro-filler mixture, the thermal expansion coefficient and insulation breakdown properties by a needle-plate electrode method were measured for the filler mixture composite and the conventional filled epoxy. The filler mixture composite had almost the same thermal expansion coefficient as the conventional filled epoxy. In a continuous voltage rising test, the filler mixture composite had 7% higher insulation breakdown strength than the conventional filled epoxy. Moreover, under constant AC voltage (10 kV at 1 kHz), the filler mixture composite had an insulation breakdown time of more than 20,000 minutes whereas the conventional filled epoxy had a breakdown time of 830 minutes. Electron microscope observation showed that the area surrounded by dispersed micro-scale silica fillers were also filled with the nano-scale layered silicate fillers. Furthermore, the estimate of spacing between the fillers and the filler/epoxy interface area showed a more densely-packed structure of the filler mixture composite than the conventional filled epoxy. The morphological feature of the filler mixture composite seems to improve its insulation breakdown strength and time.

I. Introduction

Epoxy resin is an indispensable material for insulation systems of solid insulated switchgear (SIS) and gas insulated switchgear (GIS). for example, SIS requires many epoxy castings to mold main circuits and GIS requires many insulating spacers to support of the inside conductor in GIS tanks [1]–[3]. The epoxy resin is usually filled with a large amount of micro-scale fillers (e.g. silica or alumina) to achieve the same low thermal expansion as aluminum or copper conductors. The conventional filled epoxy with low thermal expansion avoids exfoliating between the epoxy casting part and the conductor caused by heat-cycle.