Results of broadband dielectric spectroscopy of epoxy resin composites containing graphene nanoplatelets (GNP) are presented in a wide temperature range (25-500 K). The as-produced composites were heated at temperatures above the epoxy glass transition and subsequently cooled down to room temperature. This annealing was proved to be a simple but powerful process to improve significantly the electromagnetic properties of the GNP-based composites. The dc conductivity of epoxy filled with 4 wt% GNP is 68 times higher after annealing. Another benefit of the annealing is to lower substantially the percolation threshold, from 2.3 wt% for as-produced samples to 1.4 wt%. In composites above the percolation threshold, the electrical conductivity is the result of tunneling between GNP clusters. For a given GNP concentration, the tunnel barrier decreases after annealing. © 2016 Elsevier Ltd.

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