Studying the porosity of graphite foil with different densities: pore space model and gas permeability

The study is aimed to deepen the understanding of the interrelation between density, open and closed porosity as well as gas permeability of compressed graphite foils with a density ranging from 0.5 to 1.8 g·cm −3 designed especially for sealing applications. The pore structure of the graphite foil samples is experimentally measured by several complementary methods: low-temperature N 2 adsorption, method of saturation with liquids (hydrostatic weighting), mercury porosimetry, and helium leak detection for gas permeability measurement. A comparative study of the porosity obtained by mercury porosimetry and by saturation with water and isopropanol, made it possible to propose a reliable express method for determining and controlling the porosity of dense graphite foil. It was found that the characteristic pore size and open porosity of graphite foil decreases with increasing its density from 0.5 to 1.8 g·cm −3 leading to a decrease in helium gas permeability of the foils. An average capillary diameter, the number of capillaries and their effective cross-sectional area was calculated on the basis of the dependence of helium gas permeability on foil density and gas pressure. The obtained data were applied for describing the pore structure of the graphite foils with low density and explanation of their low gas permeability.