Colour development during devitrification in Li2O-ZnO-Al2O3SiO2 glasses under conventional and microwave heating

The paper examines the effects of a series of heat treatments on the appearance and microstructure of Li2O-ZnO-Al2O2-SiO2 glasses with varying ZnO/Li2O. The base glasses coloured yellowish, become violet at temperatures between 700-800 degrees C. The glass remains transparent and the obtained colour is stable during cooling. When heated at temperatures >800 degrees C the samples become opaque violet and finally opaque white at 900 degrees C. The effect of the heat treatment on the physical and chemical properties of these materials is explained on the basis of changes in composition and morphology of the residual amorphous and crystalline phases. Electron microscopic observations individuated an initial glass nucleation induced by phase separation, which occurs at lower temperatures, followed at intermediate temperatures by the crystallisation of beta-quartz (ss) which at higher temperatures converts to beta-spodumene as shown by x-ray diffraction and thermal analysis. Mie light scattering as a consequence of phase separation have been proved to be responsible for bulk glass colouring. Besides microwave irradiation effects on the crystalline phases evolution has been explained on the basis of ionic diffusion.