Kinetics of concentration fluctuations in a binary alkali–silicate system

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Abstract
Laser light scattering was applied to a potassium–silicate system to observe the growth of supercritical concentration fluctuations. The Rayleigh and Brillouin spectra of our glass samples were measured as a function of heat treatment time duration and temperature. From the spectra we obtained the Landau–Placzek ratios and Brillouin shifts for each heat treatment interval and temperature. In glass forming melts, critical points may occur in regions of high shear viscosity; here because of the low mobility, relaxation times for concentration fluctuations are on the order of many hours. Thus, heat treatment followed by rapid quenching ’’freezes in’’ the fluctuations characteristic of the treatment temperature, and consequently the light scattering measurements detect the high temperature structure. The measurements reported here provide the first direct evidence for a phase transition in a potassium–silicate system. From the equilibrium Landau–Placzek ratios, the spinodal temperatures were calculated. We also discuss the relaxation times for the system and the critical exponents.