Investigation of liquid-liquid phase transitions in oxide melts by viscosity measurements

Results from viscosity measurements conducted both above and below the liquid-liquid phase transition of a series of molten oxide glasses are reported in order to analyze the effect of supercritical composition fluctuations on viscous flow, and to investigate the mechanisms of phase separation. Measurements of four oxide mixtures with similar high temperature structures and widely different critical temperatures, revealed an anomalous increase in viscosity at temperatures above the critical point. The anomalous increase occurs when large composition fluctuations characterizing the critical point are present. The effect is explained in terms of an interaction between viscous flow and the supercritical fluctuations through the structural relaxation process. An analysis of this interaction is presented.Measurements conducted at temperatures slightly below the critical point of one of these glasses indicate that the microstructure resulting from the phase separation is highly sensitive to the preceding heat-treatment. Phase separation by the formation of isolated spheres of the silica-rich component is identified a few degrees below the critical point. Further measurements of viscosity by a fibre elongation method, conducted far below the critical temperature, are reported in order to analyze the growth mechanisms occurring in the separated phases. In this case, the rearrangement stage of phase separation is characterized by a growing interconnected structure.