The Diffusion of Helium and of Hydrogen Through Pyrex Chemically Resistant Glass

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Abstract
The effects of temperature and of heat treatment on Pyrex chemically resistant glass have been studied by measuring the rate of diffusion of helium and of hydrogen through a tube of about one millimeter wall thickness. Measurements were made with helium between 180°C and 590°C, and with hydrogen at 512°C. The influence of gas pressure was determined and the diffusion rate was found to be directly proportional to the pressure for both helium and hydrogen. The diffusion data throw light on the processes of molecular rearrangement involved in stabilization of the glass. Below 548°C the rates of diffusion (R) were increased about 10 percent by giving the glass sufficient heat treatment to assure ``stabilization,'' i.e., equilibrium in the processes of dissociation and association. This increase occurred despite the fact that such heat treatment normally increases the density of the glass and therefore points to a chemical theory rather than a simple theory of holes governing the diffusion process. The data also showed a continuous linear dependence for the stabilized glass of log R on 1/T from 440°C—590°C. This range extends into the region of a true viscous liquid and it follows that the properties of a stabilized glass are continuous with that of a viscous liquid and that there is no need to postulate a separate glassy state. The ``transformation point'' mentioned by some investigators is merely the temperature of rapid strain release of an unannealed sample. At or near 440°C the log R versus 1/T curve takes a lesser slope (for lower temperatures) and this change is attributed to the loss of rotational or vibrational freedom in the silicate complex, analogous to that frequently found in dielectric studies on solids. Activation energies have been calculated for the two temperature ranges. At 512°C the rate of helium diffusion is approximately 45 times that of hydrogen, and it is believed that chemical forces delay the progress of the hydrogen molecule through the silicate network. The general conclusion is drawn that gas diffusion furnishes a highly sensitive method for studying the molecular condition of silicate glasses and similar materials.