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1916-2016

The volatilization of constituents from borosilicate glass at elevated temperatures GT_1962_059-068

The volatilization of constituents from borosilicate glass at elevated temperatures GT_1962_059-068

The kinetics of volatilization of constituents from the surface of a standard commercial borosilicate glass have been studied in the laboratory using a thermal balance for the temperature range 1200 to 1500°C and for periods of time up to 600 h. The techniques and apparatus used are described. A preliminary investigation of the kinetics of volatilization from a standard soda-lime glass and from some alumina-rich borosilicate glasses has also been made for comparative purposes. In the case of the standard borosilicate glass it is shown that the loss rates show a simple temperature dependence only in the early stages of volatilization. At later stages, after 30 h for the laboratory melts, and for temperatures greater than 1400°C, the volatilization rates are influenced by a silica-rich surface layer which may crystallize as a cristobalite scum layer and which covers the glass surface to a greater or lesser extent. It is probable that two volatilization processes are occurring either in parallel or in sequence depending on both temperature and time. One process is that of volatilization from a 'free' surface and the other process is from a silica-rich surface resulting from the volatilization losses. The kinetics of volatilization from the standard soda- lime glass and the alumina-rich borosilicate glasses do not appear to be influenced by the presence of a surface scum layer. The weight loss in each case is directly proportional to the square root of the time during which the glass surface is exposed at the elevated temperature. This fact implies that for each of these glasses the rate of volatilization is controlled by the rate of diffusion of the volatilizing constituents through the glass to the surface. The data for the soda-lime glass can be used to calculate the activation energy for the diffusion process in the temperature range 1300 to 150O°C. A value of 2·25 eV±0·25 eV has been obtained which is similar in magnitude to that obtained by Isard & Douglas for high temperature sulphuring of the same glass, and which they attribute to oxygen ion diffusion

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