Thermal expansion of silicate glasses: physical basis for the additivity relations

The thermal expansion coefficient of silicate glasses has been correlated with glass structure. It can be expressed in terms of factors for the structural units. The results show that each structural unit contributes to the expansion coefficient with a specific factor; the ‘differential expansion’ of unit. The factor for a structural unit remains the same over the composition region where it appears. It depends on the type but not the concentration of modifier ion. The factor for Q₄ in any silicate glass is the same as that obtained for vitreous SiO₂. The factors for structural units in binary silicate glasses are mostly the same in mixed modifier glasses.