Structure and properties of glassy substances in the oxide system Li2O–B2O3–SiO2

The degree of connectivity of the spatial structural network of boron, silicate, and boron-silicate glass, depending on the chemical composition, is proposed to be estimated using the values of the relative fraction of free volume in their structure. These values are calculated using developed formulas as a function of the molar volume of the glass and the content of B2O3, SiO2, and Li2O oxides in the glass. For glassy substances in the oxide systems Li2O–B2O3, Li2O–SiO2, and Li2O–B2O3–SiO2, the dependence of the calculated values of the relative fraction of free volume in their structure on their chemical composition was established, as well as a strong correlation between the amount of free volume in experimental glassy substances and the values of their properties. This provides reason to believe that the proposed formulas are sufficiently justified and suitable for the quantitative assessment of the degree of connectivity of the spatial structural network of boron, silicate, or boron-silicate glassed. The differences in the values of the coefficient of linear thermal expansion and glass transition temperature for glassy boric anhydride, in comparison with quartz glass, are due to a significantly higher content of free volume in its structure than in glassy SiO2. In this regard, the addition of Li2O to the composition of boron glass in an amount of up to approximately 37 mol.% contributes to a decrease in the proportion of free volume in the glass structure and a corresponding increase in the degree of connectivity of its anionic network, as well as the formation of [BO4]5– tetrahedra. In contrast, an increase in the Li2O content in the glass composition above approximately 37 mol.% leads to an increase in the proportion of free volume in the glass structure and a decrease in the degree of connectivity of its anionic network.