Electrical conductivity of anisotropic expanded graphite-based monoliths

The electrical conductivity of cubic samples (monoliths) made of moderately compressed expanded graphite has been measured along two orthogonal directions, namely parallel and perpendicular to the pressing force. It is found that these highly porous materials exhibit a percolation behaviour, with a transition from insulator to conductor occurring at a very low critical density. Besides, the monoliths are found to be quite isotropic as their porosity is high; conversely, increasing their apparent density makes them become more and more anisotropic thanks to the orientation of their constitutive graphite particles. Both percolation and effective media theories are shown to accurately fit the conductivity data as far as isotropic materials are concerned. Microstructural parameters derived from the fits are discussed and found to be in fair agreement with the expected description of compressed expanded graphite, i.e. insulating voids surrounded by thin conducting flakes.