Ultralow Electrical Percolation in Graphene Aerogel/Epoxy Composites

Graphene aerogels (GAs) with a highly aligned, porous structure are prepared using a novel unidirectional freeze casting method, followed by thermal reduction. The unique graphene orientation in a preferred direction is achieved due to the large temperature gradient generated during freeze casting, in which graphene oxide (GO) sheets are expelled by the rapidly advancing ice front to assemble between the aligned ice crystals. The resulting unidirectional GAs (UGAs) possess ultralow densities, high porosities, and large surface areas, as well as excellent electrical conductivities. The solid UGA/epoxy composites fabricated by vacuum-assisted infiltration of liquid epoxy present an extremely low percolation threshold of (407 vol %, which is the lowest value for all graphene/polymer composites reported in the literature. Besides, the anisotropic structure of UGAs gives rise to significant anisotropic electrical conductivities of UGA/epoxy composites, a potentially useful attribute for many important applications. A new analytical model is formulated on the basis of the interparticle distance concept to explain the percolation behaviors of composites with aligned anisotropic nanofillers. The prediction agrees well with experimental data, and the model validates the importance of aspect ratio and orientation state of nanofillers in controlling the percolation threshold of composites.