Permeability and formation factor in compressed expanded graphite

The permeabilities and the formation factors of an anisotropic block of compressed expanded graphite are measured in gas permeation and ion diffusion experiments, respectively. The critical pore diameter of the material is obtained from mercury-injection experiments carried out on the material, according to the work of Katz and Thompson (Katz A J and Thompson A H 1986 Phys. Rev B 34 8179, 1987 J. Geophys. Res. 92 599). Other length scales related to transport are obtained from the theory of Johnson ef al (Johnson D L, Koplik J and Schwartz L M 1986 Phys. Rev. Lett. 57 2564) and are calculated on the basis of several models of pore structure available from the literature. All of these different data are compared, and only the model for which the pores are cylinders of lengths l and diameters delta such that l = delta is shown to work. This finding is confirmed by the formation factor derived from the experimental values of the permeabilities and the kind of pore structure considered: again, only that for which l = delta leads to consistent results. Finally, the latter model and the theory of Katz and Thompson are used to estimate both the permeability and the formation factor of the blocks of compressed expanded graphite. A very good agreement is found between the estimate and the experimental results.