Transport in porous graphite: gas permeation and ion diffusion experiments

Permeabilities k and formation factors F of a number of monoliths made of compressed expanded graphite (CEG) having different porosities are measured from gas permeation and ion diffusion experiments, respectively. The results are analyzed and discussed in the framework of a model, according to which the pores are cylinders of lengths l and diameters δ such that l=δ. Formation factors are calculated using this model and from simple assumptions concerning the packing of expanded graphite particles. For each porosity of the monoliths, a characteristic pore size Λ is derived from the latter hypotheses. All these results are shown to be accurate basing on three statements. First, the expected and well-known relationship k∼F−2 is recovered. Secondly, the calculated values are in good agreement with the experimental ones. Finally, the characteristic pore sizes Λ are obtained from both surface area and helium pycnometry measurements, and shown to support the former calculations of Λ.