Density Anomaly of Silica Glass: A Novel Approach

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.