Excellent heat conduction properties of graphene and the progress in the large-scale few-layer graphene exfoliation make the prospects of graphene composite applications particularly promising [1-2]. 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 above 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 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. It was also found that composites with the certain types of few-layer graphene fillers reveal an efficient total electromagnetic interference shielding in the important X-band frequency range, while simultaneously providing the high thermal conductivity . The efficiency of the dual functional application depends on the filler characteristics: thickness, lateral dimensions, aspect ratio and concentration. Graphene loading fractions above the electrical and thermal percolation thresholds allow for strong enhancement of both the electromagnetic interference shielding and heat conduction properties. Interestingly, graphene composites can block the electromagnetic energy even below the electrical percolation threshold, remaining electrically insulating. The dual functionality of the graphene composites can substantially improve the electromagnetic shielding and thermal management of the airborne systems while simultaneously reducing their weight and cost.
This work was supported, in part, by the National Science Foundation (NSF) through the Emerging Frontiers of Research Initiative (EFRI) 2-DARE award 1433395, and by the University of California – National Laboratory Collaborative Research and Training Program LFR-17-477237.
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