Density and thermal expansion of borate and borosilicate glass-forming melts

Density, molar volume and thermal expansion of borate and borosilicate melts are properties that significantly affect the process of forming composite coatings by slip-firing technology. Based on experimental data on the density of glass-forming melts, adequate generalizing mathematical models have been developed by using the method of multiple correlation; these models describe the dependence of the molar volume (VT) of alkaline borate, alkaline silicate and multicomponent melts on their oxide composition and temperature with a standard deviation of (0.22–0.27) cm3 mol–1. The regression equation has the following general form: VT=A(xi)–B(xi)/T, where xi is the content of an oxide, T is the temperature, subscript "i" stands for the number of an oxide concerned. The changes in volumetric coefficient of thermal expansion (CTE) of the molten glasses are analyzed as functions of both temperature and chemical composition. It should be noted that borate melts in comparison with silicate ones have much higher values of CTE. The highest value of CTE of experimental melts is observed at glass transition temperatures (Tg). An increase in the temperature above Tg contributes to a significant decrease in CTE. An increase in the content of alkali metal oxides in most glass-forming melts results in an increase in the values of CTE.