Shear viscosity of glass-forming melts in the liquid-glass transition region |
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Authors: | D S Sanditov |
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Institution: | (1) Department of Physics, Graduate School of Science and Technology, Kumamoto University, Kumamoto 860-8555, Japan |
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Abstract: | A new approach to interpreting the hole-activation model of a viscous flow of glass-forming liquids is proposed. This model
underlies the development of the concept on the exponential temperature dependence of the free energy of activation of a flow
within the range of the liquid-glass transition in complete agreement with available experimental data. The “formation of
a fluctuation hole” in high-heat glass-forming melts is considered as a small-scale low-activation local deformation of a
structural network, i.e., the quasi-lattice necessary for the switching of the valence bond, which is the main elementary
event of viscous flow of glasses and their melts. In this sense, the hole formation is a conditioned process. A drastic increase
in the activation free energy of viscous flow in the liquid-glass transition region is explained by a structural transformation
that is reduced to a limiting local elastic deformation of the structural network, which, in turn, originates from the excitation
(critical displacement) of a bridging atom like the oxygen atom in the Si-O-Si bridge. At elevated temperatures, as a rule,
a necessary amount of excited bridging atoms (locally deformed regions of the structural network) always exists, and the activation
free energy of viscous flow is almost independent of temperature. The hole-activation model is closely connected with a number
of well-known models describing the viscous flow of glass-forming liquids (the Avramov-Milchev, Nemilov, Ojovan, and other
models). |
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