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 appear ance and microstructure of Li2O-ZnO-Al2O2-SiO2 glasses with varying ZnO/Li2 O. The base glasses coloured yellowish, become violet at temperatures betwe en 700-800 degrees C. The glass remains transparent and the obtained colour is stable during cooling. When heated at temperatures >800 degrees C the s amples become opaque violet and finally opaque white at 900 degrees C. The effect of the heat treatment on the physical and chemical properties of the se materials is explained on the basis of changes in composition and morpho logy of the residual amorphous and crystalline phases. Electron microscopic observations individuated an initial glass nucleation induced by phase sep aration, which occurs at lower temperatures, followed at intermediate tempe ratures by the crystallisation of beta-quartz (ss) which at higher temperat ures converts to beta-spodumene as shown by x-ray diffraction and thermal a nalysis. Mie light scattering as a consequence of phase separation have bee n proved to be responsible for bulk glass colouring. Besides microwave irra diation effects on the crystalline phases evolution has been explained on t he basis of ionic diffusion.