Dielectric properties of graphite-based epoxy composites

Composite materials based on epoxy resin filled with various kinds of graphite particles: exfoliated graphite, natural graphite, and coarse, medium and fine artificial graphites have been prepared. Results of broadband dielectric investigations of such materials in wide temperature (25-450 K) and frequency (20 Hz-3 THz) ranges are presented. The dielectric permittivity strongly increases with graphite particle size. The graphite particle size and shape also have a strong impact on freezing temperature, conductivity activation energy and composite electromagnetic absorption properties at room temperature. The lowest percolation threshold is observed for exfoliated graphite (EG)-based composites. At low temperatures (below glass transition temperature of pure polymer matrix), the electrical conductivity in composites above the percolation threshold is mainly governed by electron tunnelling between graphite particles. At higher temperatures, electrical conductivity due to finite electrical conductivity of polymer matrix and by electron tunnelling from polymer matrix to graphite particles occurs in all composites. Microwave experiments show that EG is the only really effective additive, out of all investigated graphite particles, for producing electromagnetic interference shielding composite materials: 2 wt% of EG in epoxy is indeed not transparent for the electromagnetic radiation at 30 GHz. (C) 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim