  We investigated thermal properties of the epoxy-based composites with a high
loading fraction - up to f=45 vol.% - of the randomly oriented electrically
conductive graphene fillers and electrically insulating boron nitride fillers.
It was found that both types of the composites revealed a distinctive thermal
percolation threshold at the loading fraction f>20 vol.%. The graphene loading
required for achieving the thermal percolation was substantially higher than
the loading for the electrical percolation. Graphene fillers outperformed boron
nitride fillers in the thermal conductivity enhancement. It was established
that thermal transport in composites with the high filler loading, above the
thermal percolation threshold, is dominated by heat conduction via the network
of percolating fillers. Unexpectedly, we determined that the thermal transport
properties of the high loading composites were influenced strongly by the
cross-plane thermal conductivity of the quasi-two-dimensional fillers. The
obtained results shed light on the debated mechanism of the thermal
percolation, and facilitate the development of the next generation of the
efficient thermal interface materials for electronic applications.
