Thermal Expansion, Composition and Structure of Glass—PART II

Abstract

Co-efficient of linear thermal expansion of the glass-forming oxides has been calculated from the following equation derived earlier¹. where Cp is the heat capacity, E the crystal energy, n the Born constant and Z is the average ionic valency. The calculated values of α for the different glass-forming oxides are in agreement with the empirical additive factors for calculating thermal expansion of glass, suggested earlier².

In vitreous silica, having an open network structure the transverse modes of vibration of oxygen, perpendicular to the Si-O-Si line, do not contribute towards thermal expansion but they do so with progressive saturation of the network by the network-modifying ions. The three factors for silica calculated on this basis are in agreement with the empirical values.

For boric oxide, thermal expansion is as high as 16 × 10^-6 compared to the theoretical value 4.3 × 10^-6 possibly because it has a molecular structure. With addition of alkalies the structure is progressively transformed to a saturated network. In the alkali borates containing upto about 84 mol % B₂O₃ the additive relation is valid and the empirical factors due to the alkalies and boric oxide are in f air agreement with the theoretical values.