The exceptionally high thermal conductivity of graphene has driven interest toward its applications in thermal interface materials (TIMs) [1-3]. In addition to its unique heat conduction properties, graphene is also a strong conductor of electricity, which is problematic for certain TIM applications where electrical insulation is paramount. A common strategy for the optimization of composite materials is to combine two or more different thermally conductive fillers. Typically, work along this vein employs fillers of disparate size, shape, and aspect ratios with the larger-sized fillers providing the greatest contribution of overall thermal transport and the smaller fillers improving the interstitial thermal coupling between the larger fillers. Unlike much of the previous work into binary fillers, we report on polymer composites filled with few-layer graphene and hexagonal boron nitride (h-BN) of similar lateral dimensions, thicknesses, and aspect ratios. In each filler material, phonons are the primary heat carrier, necessitating the selection of lateral dimensions larger than the “gray” phonon mean free path (MFP) in the micrometer distance range. The composition and structure of the resulting epoxy-based composites were verified using scanning electron microscopy (SEM), Raman, and Brillouin spectroscopy. Thermal measurements were conducted using the “laser flash” techniques. It was found that the use of electrically conductive graphene and electrically insulating h-BN fillers of similar physical dimensions can be complementarily leveraged to achieve an independent control of the thermal and electrical conductivity of the TIM. Varying the constituent fraction of graphene in composites with ~44% total filler loading can tune the thermal conductivity enhancement from a factor of ×15 to ×34 while changing the electrical resistivity from 3×108 Ω-mm to 102 Ω-mm, i.e. spanning the resistivity range from an insulator to a conductor. We offer an analytical model that describes the experimental thermal conductivity data of the binary filler composites. The obtained results are illustrative of a promising strategy for the development of next-generation thermal interface materials with specific control of electrical properties, allowing for the expression of electrically insulating or electrically grounding behaviors.
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 UC – National Laboratory Collaborative Research and Training Program LFR-17-477237.
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 F. Kargar, Z. Barani, R. Salgado, B. Debnath, J.S. Lewis, E. Aytan, R.K. Lake, A.A. Balandin, Thermal percolation threshold and thermal properties of composites with high loading of graphene and boron nitride fillers, ACS Applied Materials and Interfaces, (2018) doi:10.1021/acsami.8b16616.
5:00 PM–7:00 PM Apr 23, 2019 (US - Arizona)
PCC North, 300 Level, Exhibit Hall C-E