Transparent glass-ceramics and interaction with alkali metal vapours
The aims of the project were to investigate the fundamental processes involved when a glass is exposed to sodium vapour, and to examine the different ways of improving the resistance towards sodium attack. With regard to the interaction of glasses with sodium, the silicates were studied in most detail. The discolouration of the silicate glasses by sodium was treated as a surface phenomenon and techniques such as ESCA, MRS and ESR were used to investigate the interaction. The results from, these various techniques all point to the same conclusion, that the mechanism of sodium attack is the progressive breaking of Si-O bonds to form Si-0- bonds. The reaction between borate glasses and sodium showed certain similarities with the silicates. This led to the suggestion that the same mechanism of attack might also apply to the borates. In attempting to improve the resistance of transparent materials towards sodium attack, two methods were considered. The first approach was to change the chemical composition by selecting components of a glass which are inert to sodium. The chemical compatibilities of the components were assessed using an equilibrium thermodynamic argument to consider the possible reactions with sodium, and a number of materials have been suggested to be inert in this way. The second method of improving the resistance was to rearrange the structure of a glass by crystallisation. The crystallisation behaviour and the microstructures of two glass compositions - MgBAl (3-2) and CAl (6) were studied in detail, and transparent glass-ceramics were derived from them. It was found that the improvement in resistance towards sodium attack was only marginal in the case of crystallising the glasses in bulk. However, substantial improvements were observed in the case of surface crystallisation.