A review of exfoliated graphite

Exfoliated graphite (EG) refers to graphite that has a degree of separation of a substantial portion of the carbon layers in the graphite. Graphite nanoplatelet (GNP) is commonly prepared by mechanical agitation of EG. The EG exhibits clinginess, due to its cellular structure, but GNP does not. The clinginess allows the formation of EG compacts and flexible graphite sheet without a binder. The exfoliation typically involves intercalation, followed by heating. Upon heating, the intercalate vaporizes and/or decomposes into smaller molecules, thus causing expansion and cell formation. The sliding of the carbon layers relative to one another enables the cell wall to stretch. The exfoliation process is accompanied by intercalate desorption, so that only a small portion of the intercalate remains after exfoliation. The most widely used intercalate is sulfuric acid. The higher concentration of residue in unwashed EG causes the relative dielectric constant (50 Hz) of the EG to be 360 (higher than 120 for KOH-activated GNP), compared to the value of 38 for the water-washed case. An EG compact is obtained by the compression of EG at a pressure lower than that used for the fabrication of flexible graphite. Compared to flexible graphite, EG compacts are mechanically weak, but they exhibit viscous character, out-of-plane electrical/thermal conductivity and liquid permeability. The viscous character (flexural loss tangent up to 35 for the solid part of the compact) stems from the sliding of the carbon layers relative to one another, with the ease of the sliding enhanced by the exfoliation process.