A Brief Review of Studies on the Mechanism of Helium Permeation through Silica Glass

Since the early twentieth century, there have been many investigations on the penetration of silica glass by light gases. In 1926, H. Elsey reported that silica glass has the property of passing helium but stopping air at room temperature when the pressure is about 100 atmospheres. Although the densities of crystal quartz and vitreous silica differ only by about 17% (2.65 and 2.20 g/cm3, respectively), the helium permeability of silica glass is six orders more elevated than that of crystal quartz. This vast difference has puzzled researchers for decades considering that silica glass and quartz crystal have the same chemical composition. This work discusses the mechanism of high helium permeation through silica glass. It briefly reviews the experimental data and its contradictions with the continuous random network theory. The research not only sheds light on the strength of silica glass but also suggests potential implications for other abnormal properties of silica glass. A recently proposed nanoflake model for silica glass structure is utilized to explain the origin of glass permeation to helium. According to the nanoflake model, the formation of nanoflakes not only brings a one-dimensional medium-range ordering structure into silica glass but simultaneously creates regions where van der Waals bonds replace the oxygen-silicon covalent bonds. It is the weakness of van der Waals bonds that causes the helium mobility in these areas to increase. Accordingly, the permeation rate of helium through silica glass with fast cooling is expected to be lower than that of slow-cooling glass. This prediction should be experimentally verified in the near future.