  Carbon nanofibres (CNF) were dispersed into an epoxy resin using a
combination of ultrasonication and mechanical mixing. The electronic transport
properties of the resulting composites were investigated by means of impedance
spectroscopy. It was found that a very low critical weight fraction (pc = 0.064
wt %) which may be taken to correspond to the formation of a tunneling
conductive network inside the matrix. The insulator-to-conductor transition
region spanned about one order of magnitude from 0.1 to 1 wt %. Far from the
transition, the conductivity increased by two orders of magnitude. This
increase and the low value of the conductivity were explained in terms of the
presence of an epoxy film at the contact between CNF. A simple model based on
the CNF-CNF contact network inside the matrix was proposed in order to evaluate
the thickness of that film.
