A Brief Review of Studies on the Mechanism of Helium Permeation through Silica Glass | Chapter 6 | Chemical and Materials Sciences: Developments and Innovations Vol. 6

 

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.

Author (s) Details

Shangcong Cheng
Molecular Foundry of Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.

 

Please see the book here:- https://doi.org/10.9734/bpi/cmsdi/v6/2707

Density Anomaly of Silica Glass: A Novel Approach | Chapter 8 | Current Innovations in Chemical and Materials Sciences Vol. 8

In this work, the nanoflake model is utilized here to provide a structural explanation for the anomalies of the density-temperature relation of silica glass. Silica glass is the most essential glass-forming material with several technologically important properties. The structure, formation and properties of silica glass have been studied for many decades. The creation of medium-range ordering structure during the glass transition process explains the abnormal density-temperature connection of vitreous silica with low hydroxyl content. The two layers of SiO4 tetrahedra in the organized medium-range structure, which resembles a "nanoflake," are joined by O atoms in the center of the formation. The nanoflakes use van der Waals bonds in addition to covalent chemical connections to interact with the surrounding structures. In the formation of the van der Waals bonds, the orientation of SiO4 tetrahedra can change, which results in an increase of distance between the nanoflakes and their surrounding structures. Thus, there is a slight volume enlargement associated with the formation of nanoflakes. Since the nanoflakes’ formation starts at a temperature near 1480°C, and the population of the nanoflakes grows continuously as temperature decreases until about 950°C, the bulk volume of silica glass increases in the temperature range from about 1480°C to 950°C. The density anomaly of silica glass can be explained as a byproduct of the formation of medium-range ordering structure in the glass transition and provides additional support for the new structural model.

Author(s) Details:

Shangcong Cheng,
Molecular Foundry of Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.

Please see the link here: https://stm.bookpi.org/CICMS-V8/article/view/14102