  The thermal properties of an epoxy-based binary composites comprised of
graphene and copper nanoparticles are reported. It is found that the
"synergistic" filler effect, revealed as a strong enhancement of the thermal
conductivity of composites with the size-dissimilar fillers, has a well-defined
filler loading threshold. The thermal conductivity of composites with a
moderate graphene concentration of ~15 wt% exhibits an abrupt increase as the
loading of copper nanoparticles approaches ~40 wt%, followed by saturation. The
effect is attributed to intercalation of spherical copper nanoparticles between
the large graphene flakes, resulting in formation of the highly thermally
conductive percolation network. In contrast, in composites with a high graphene
concentration, ~40 wt%, the thermal conductivity increases linearly with
addition of copper nanoparticles. The electrical percolation is observed at low
graphene loading, less than 7 wt.%, owing to the large aspect ratio of
graphene. At all concentrations of the fillers, below and above the electrical
percolation threshold, the thermal transport is dominated by phonons. The
obtained results shed light on the interaction between graphene fillers and
copper nanoparticles in the composites and demonstrate potential of such hybrid
epoxy composites for practical applications in thermal interface materials and
adhesives.
