Ultralow Electrical Percolation in Graphene Aerogel/Epoxy\nComposites

Abstract

Graphene aerogels\n(GAs) with a highly aligned, porous structure\nare prepared using a novel unidirectional freeze casting method, followed\nby thermal reduction. The unique graphene orientation in a preferred\ndirection is achieved due to the large temperature gradient generated\nduring freeze casting, in which graphene oxide (GO) sheets are expelled\nby the rapidly advancing ice front to assemble between the aligned\nice crystals. The resulting unidirectional GAs (UGAs) possess ultralow\ndensities, high porosities, and large surface areas, as well as excellent\nelectrical conductivities. The solid UGA/epoxy composites fabricated\nby vacuum-assisted infiltration of liquid epoxy present an extremely\nlow percolation threshold of 0.007 vol %, which is the lowest value\nfor all graphene/polymer composites reported in the literature. Besides,\nthe anisotropic structure of UGAs gives rise to significant anisotropic\nelectrical conductivities of UGA/epoxy composites, a potentially useful\nattribute for many important applications. A new analytical model\nis formulated on the basis of the interparticle distance concept to\nexplain the percolation behaviors of composites with aligned anisotropic\nnanofillers. The prediction agrees well with experimental data, and\nthe model validates the importance of aspect ratio and orientation\nstate of nanofillers in controlling the percolation threshold of composites.