An experimental investigation of the interdiffusion behavior of gases in a low permeability graphite was performed by sweeping the opposite faces of a graphite septum with helium and argon at uniform pressure and measuring the diffusive flux of both gases. The objectives were to ascertain the diffusion mechanism, to verify the applicable equations and associated theories, and to determine the parameters required to use these equations. At all experimental pressures, contributions of both normal and Knudsen diffusion effects were detectable via the pressure dependence of the diffusion fluxes. It was found that a previously proposed dusty-gas model formed an excellent basis for correlating the results. The dusty-gas model yields flux equations which predict the diffusion behavior over a wide range of pressures for particular gas concentrations at the boundaries. Only two experimentally determined parameters (characteristic of the gases and graphite) are required. These are: an effective normal-diffusion coefficient obtained through interdiffusion experiments and a Knudsen coefficient obtained through single-gas (permeability) experiments. The procedures used to evaluate these parameters in terms of the experimental data are described in detail.
A previous experimental investigation of the interdiffusion behavior of two unlike gases within a porous medium at uniform pressure has been extended. The extended studies were focused on interdiffusion under the influence of pressure gradients which, in turn, give rise to both viscous and diffusive components of the experimentally determined fluxes. The fact that the diffusion processes investigated occurred within the region bounded by the normal and Knudsen regimes (the transition region), constituted one of the most important features of these studies. The experimentally observed flux phenomena could be readily explained within the framework of a ``dust-gas'' model. The flux equations exhibited only one major weakness and this involved the absence of viscous flow contributions resulting from the pressure gradients. However, it was possible to use these and related equations and, with associated parameters, to reconstruct the experimental curves via computations. It was found that the range of applicability of the equations was limited and that none of the parameters could be predicted beforehand. This was partially the result of the internal-pore geometry of the graphite. Nevertheless, the applicability of the existing dust-gas equations to nonuniform pressure conditions has been established for the transition region insofar as is possible with a medium such as graphite.
Rates of migration of heavy metals in carbons and graphites are of practical importance, though accurate determination of diffusion behavior in such systems presents formidable problems both in experiment and in theory. Diffusion coefficients . . h' 2 3 d ph'te 4 for uraOlum 10 grap Ites t an a pyro-gra 1 have been reported; we now report coefficients for diffusion of thorium recently obtained from specimens prepared by ion bombardment.The pyrolytic carbon used was prepared by the thermal decomposition of methane. Porosity determinations with helium indicated a poreless structure. This finding was consistent with measured crystalloo 0 graphic parameters (a = 2.46 A, c = 6.86 A) and density (p = 2.2 g/cm 3 ). X-ray determinations using a rocking-curve techniqueS with flat specimens revealed preferred-orientation indices 6 of 80-90 based on (QQ2) reflections. The average distance between coherently diffracti~ domain boundaries (in the c direction) was 150 A, determined by line broadening of the (QQ2) reflections.The effects of lattice strains were not accounted for in the index and broadening re suIts; additional characterization studies 7 are in progress. Nevertheless, it is clear that the pyrolytic carbon used possessed a high degree of orientation, a high density, and a columnal structure arising from the presence of conical grains perpendicular to the c direction of the material.The porous graphite was prepared by impregnating a fine-grained' graphite with an organic compound that was subsequently carbonized. The density of CONTENT ANALYSIS
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