Dielectric properties of Boron Nitride and silica epoxy composites

Epoxies are widely used materials, especially in high voltage insulation for its mechanical and dielectric properties. Boron Nitride is an attractive filler because of its high insulating properties associated with a high thermal conductivity. Nanometric Silica used as filler is actually widely studied in the literature and enhancement of dielectric breakdown and insulating properties has been shown. Using a combination of these two fillers in an epoxy matrix may produce a new material with improved properties. Samples containing only micrometric filler (Boron Nitride), only nanometric filler (Silica) and both fillers were prepared by gravity moulding. A pure epoxy sample was also prepared to quantify the enhancement properties. Broadband Dielectric Spectroscopy was used to study the influence of nanoparticles on the relaxation mechanisms in the composites. An additional relaxation peak was highlighted for samples containing nanosilica between the local β relaxation, associated with crankshaft motions of the hydroxylether groups, and the main α relaxation, corresponding to glass transition cooperative movements. This relaxation phenomenon was found to be associated with the water bounded at the surface of the silica particles. Further relaxation investigation will be performed to clarify the influence of water, its state and its role.