Chapter 14 - Permeation, diffusion, and ionic conduction in glass

Abstract

Atoms are able to jump from one site to the other due to thermal vibrations. The probability of a successful jump increases with temperature. The flux of a diffusing species is related to the driving force, the concentration gradient of the species, through Fick's first law. The application of conservation of mass yields Fick's second law, also called the diffusion equation, which can be solved for a variety of initial and boundary conditions. The transport of inert gases through glass without reacting with the network is called permeation. Permeability is the product of diffusivity and solubility. Large inert gas atoms permeate through more open network easier. Silica glass, hence, is more permeable to helium than is a soda lime silicate. When ions diffuse, charge transport occurs. Alkali ions are the most mobile. The ion diffusivity and electrical conductivity increase exponentially with temperature in accordance with Arrhenius relationship involving energy for thermally activated motion. Both the diffusion coefficient and electrical conductivity decrease with ionic size and valence. Electrical conductivity drops dramatically, as much as 4–7 orders of magnitude, when mixed alkalis are present. This phenomenon is called the mixed-alkali effect and is one of the most interesting unsolved mysteries of glass science. Alkali containing glasses can be ion exchange-strengthened by immersing in a bath of molten salt containing larger alkali ions at temperatures lower than the glass transition.