On the mechanism of helium permeation through silica glass

Abstract

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. 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.

Keywords

• permeation
• helium gas
• silica glass
• sodium silicate glass
• medium-range structure
• van der waals bond
• nano-fissures
• nanoflake model