Density and structure of stabilised sodium borate glass at temperatures below the glass transition point

This paper considers the density of sodium borate glass, 20Na2O.80B2O3 (mol%), stabilised at various temperatures over the region from the glass transition point Tg=450°C down to 390°C. It was found that the density, which was measured after the glass sample air cooled to room temperature, increased with decreasing stabilisation temperature. Since the density stopped changing over the interval from 402 to 390°С, it was assumed that the glass structure became as similar to that of the corresponding crystalline compound Na2O.4B2O3 as was possible under the chosen experimental conditions. The structural changes that proceeded in stabilised glass with decreasing temperature are analysed quantitatively in terms of the short-range order and intermediate-range order in the glass structure. This analysis is based on the concept of the chemical structure, which enables both structural levels to be calculated. The concept applies to systems formed from oxides with different chemical natures, and it takes into account the set of crystalline compounds that form in a given system.