Structural Relaxations in the GHz Frequency Range in Glass Forming Silicate Melts
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STRUCTURAL RELAXATIONS IN THE GHz FREQUENCY RANGE IN GLASS FORMING SILICATE MELTS JOHN E. MASNIK*, J. KIEFFER* AND J.D. BASS** *Department of Materials Science & Engineering, University of Illinois, Urbana IL 61801 **Department of Geology, University of Illinois, Urbana IL 61801
ABSTRACT The Brillouin light scattering technique is used for the investigation of structural relaxations in glass-forming liquids at high temperatures. From the analysis of the line shapes of Rayleigh and Brillouin peaks, the friction coefficients, which are associated with the atomic scale mechanisms of the structural relaxations in these systems, are determined. Results for a series of K20-SiO2 compositions, which were chosen as model substances, are reported. As a function of temperature, maxima in the Brillouin line widths were observed, which reflect resonant conditions of molecular scale structural motions, where the relaxation time is of the order of the reciprocal Brillouin frequency shift. Due to thermal activation of the component mobilities, different relaxation mechanisms couple at different temperatures. Typically, at least one strongly absorbing regime is observed in between the glass transition and the equilibrium melting temperatures. The prominence of this regime decreases with increasing silica concentration. The friction coefficients approach the hydrodynamic viscosity in the high temperature limit, when the relaxation times become short in comparison with the time scale of the Brillouin shift. INTRODUCTION The predominant method for the production of glassy materials is the melt processing. There is much incentive for a better understanding of the visco-elastic behavior of glass forming substances at stages just before the structural constituents become trapped in their final structural state. The formation of amorphous solids is the result of a kinetic arrest of structural relaxations. As the mobilities of molecular particles decrease, their motion becomes correlated with that of an increasing number of neighboring particles. The structural changes in the glass forming liquids and the dynamic response of their molecular constituents are closely coupled. Relaxational spectroscopy has long been recognized as a powerful method for the investigation of complex structures.O) The actuation of structural components into forced vibration can be done by direct mechanical coupling,( 2 -4) or through the intermediate of dielectric polarization.( 5-7) In the latter case, one takes advantage of the polarized nature of atomic constituents of the structure and induces small scale deformation by applying an alternating electric field. This field can be established between electrically conducting electrodes or in the form of electromagnetic radiation. By changing the frequency of the applied force, one can create resonant conditions for various relaxation mechanisms and determine the characteristic relaxation times of these processes. At temperatures above the glass transition, the size of structural features, that are likely to form
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