Viscosity Behavior of Halide Glasses and Melts

Abstract

Viscosity is one of the most important properties of glass-forming systems. At temperatures above the melting point or the liquidus, viscosity and the activation energy for viscous flow provide indirect information on the structure of melts. They also furnish information on the ease of glass-formation and, obviously, the ease of crystallization. At temperatures between the melting point or liquidus and the glass transition temperature T_g, knowledge of viscosity and viscoelasticity is critical to the fabrication of the molten glass into various geometries such as plates and fibers. At temperatures below T_g, the deformation of glass under stress is of vital importance to the use of glass objects at elevated temperatures. Viscosity is of course governed by the structure of the melt and the glass which in turn is influenced by chemical composition. The variations of viscosity of glass-forming systems with temperature and with chemical composition have been well-studied for oxides (1). This is not so for halides. The objectives of this report are to review the status of our knowledge and understanding of halide glasses and melts. Arbitrarily, this review is divided into three sections separated by the three temperature zones as described above.