Shear viscosity of glass-forming melts in the liquid-glass transition region
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AL, NONLINEAR, AND SOFT MATTER PHYSICS
Shear Viscosity of GlassForming Melts in the Liquid–Glass Transition Region D. S. Sanditov Buryat State University, ul. Smolina 24a, UlanUde, 670000 Buryat Republic, Russia email: [email protected] Department of Physical Problems, Buryat Scientific Center, Siberian Branch, Russian Academy of Sciences, ul. Sakh’yanovoі 8, UlanUde, 670047 Buryat Republic, Russia Received September 2, 2009
Abstract—A new approach to interpreting the holeactivation model of a viscous flow of glassforming liq uids 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 com plete agreement with available experimental data. The “formation of a fluctuation hole” in highheat glass forming melts is considered as a smallscale lowactivation local deformation of a structural network, i.e., the quasilattice 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 inde pendent of temperature. The holeactivation model is closely connected with a number of wellknown mod els describing the viscous flow of glassforming liquids (the Avramov–Milchev, Nemilov, Ojovan, and other models). DOI: 10.1134/S106377611004014X
1. INTRODUCTION Viscosity is a property of fundamental importance, which determines the specific features of the glassy state of matter. Glassforming liquids are character ized by a high viscosity that decreases the rate of crys tallization. The concept of glass as a complex melt with a high viscosity was long ago formulated by Men deleev. However, no generally accepted explanation has so far been offered for either the drastic increase in the viscosity within the glass transition region of liq uids or the nature of the liquid–glass transition itself [1–24]. The atomic mechanism of viscous flow of inor ganic glasses and their melts is described in [1–15]. The hole theory of liquids in its simplest version [23] cannot be applied to the flow of highheat glassform ing melts because of the high energies of valence bonds in their structure [8]. Myuller [8] was the first to develop the concept that the mechanism of viscous flow of glasses and their melts lies in the activation switching of valence bonds between atoms. This idea was further developed in
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