Gas Permeation and Diffusion

The temperature dependence of gas permeability in glasses has been the subject of some controversy. The temperature dependence of the permeability, K, can be described by an expression similar to Eq. 8.9  

Gas permeability varies linearly with the partial pressure of the diffusing gas for all gases, for pressures up to many atmospheres. The diffusivity of the gas is independent of pressure for modest pressures. At high pressures, non-linear effects occur due to the thermodynamic requirement that gas fugacity be considered in place of gas pressure, and due to the onset of saturation effects due to filling of the interstices in the vitreous network by dissolved gas atoms or molecules. 

Alkaline earth ions are also very effective in blocking the diffusion of helium atoms, with a similar trend toward decreasing helium permeability in the order Ba Sr Ca Mg for glasses having otherwise identical compositions. The least permeable traditional commercial glasses are based on alkaline earth aluminosilicate compositions. 

Phase separation has a major effect on helium permeation in oxide glasses. The permeabilities of the two phases often differ by several 

Helium permeability at 200 °C in potassium silicate and potassium germanate glasses 

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Kanezashi, M. and lin, Y.S. (2009) Gas permeation and diffusion characteristics of MFI-type zeolite membranes at high temperatures. J. Chem. Phys. C, 113, 3767-3774. 

Lin, H., and Freeman, B. D. (2006a). Gas permeation and diffusion in crosslinked poly(ethylene glycol) diacrylate. Macromolecules 39, 3568. 

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