  We report on the thermal and electrical properties of hybrid epoxy composites
with graphene and boron nitride fillers. The thicknesses, lateral dimensions,
and aspect ratios of each filler material were intentionally selected for
geometric similarity to one another, in contrast to prior studies that utilized
dissimilar filler geometries to achieve a "synergistic" effect. We demonstrate
that the electrically-conductive graphene and electrically-insulating boron
nitride fillers allow one to effectively engineer the thermal and electrical
conductivities of their resulting composites. By varying the constituent
fraction of boron nitride to graphene in a composite with ~44% total filler
loading, one can tune the thermal conductivity enhancement from a factor of x15
to x35 and increase the electrical conductivity by many orders of magnitude.
The obtained results are important for the development of next-generation
thermal interface materials with controllable electrical properties necessary
for applications requiring either electrical grounding or insulation.
