Chapter 9 - The viscosity of glass

Abstract

Viscosity is a liquid-state property. The mass can be thought of as a plastic substance with zero yield strength. The time rate of deformation (flow rate) is proportional to the applied shear stress; the constant of proportionality is called (Newtonian) viscosity. With the application of heat, glass gradually softens and converts to a liquid. Viscosity ranges between 1016Pas (solid-like) and 1Pas (liquid-like). The temperature dependence is described by several different models, including the Vogel-Fulcher-Tammann, Avramov-Milchev (AM), and Mauro-Yue-Ellison-Gupta-Allan (MYEGA) equations. The Adam-Gibbs model considers viscous flow as originating from cooperatively rearranging regions in the liquid. Glass-forming liquids can be classified as either “strong” or “fragile”; the more fragile liquids deviate more from an Arrhenius-type temperature dependence of viscosity because of the availability of large number of configurations. Viscosity is measured by the falling sphere or rotating cylinder methods in the liquid range; parallel plate, fiber elongation, and beam bending methods in the higher viscosity ranges. When high shearing rates are applied, it is possible for the viscosity of molten glass to become non-Newtonian; for example, shear thinning may occur.