Thermal Percolation Threshold and Synergistic Effects in Graphene Enhanced Thermal Interface Materials

First, we investigated the 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_T>20 vol.%. The graphene loading required for achieving thermal percolation, f_T, was substantially higher than the loading, f_E, for 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, f ≥ f_T, 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. In the second part, the thermal properties of 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 f_g=15 wt% exhibits an abrupt increase as the loading of copper nanoparticles approaches f_Cu~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, f_g=40 wt%, the thermal conductivity increases linearly with addition of copper nanoparticles. The thermal conductivity of 13.5±1.6 Wm^(-1) K^(-1) is achieved in composites with binary fillers of f_g=40 wt% and f_cu=35 wt%. The electrical percolation in composites with graphene fillers only is observed at low loading, f_g<7 wt.%. It has also been demonstrated that the thermal percolation can occur prior to electrical percolation even in composites with electrically conductive 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