Transport Properties of Sodium Silicate Glasses with High Water Contents

Sodium disilicate and sodium tetrasilicate glasses with water contents ranging up to approximately eleven weight percent water were prepared by a hydrothermal method. They contain water as hydroxyls bonded to the silica network and molecular water located within the glass structure in non-network sites. Conduction and diffusion measurements were performed on these glasses to see the effects of the sample compositions on such properties as ionic conductivity, dielectric relaxation and sodium diffusion. Water hydration measurements were performed on sodium trisilicate glasses as well. It was found that ionic conductivity is due mainly to the motion of sodium ions. The conductivity, static dielectric constant, and sodium diffusion coefficient go through a minimum with increasing water content. This is attributed to the predominance of each of the two "types" of water in a part of the range of water contents studied. For any given water content, the conductivity and diffusion coefficient increase with increasing sodium oxide content. From the water hydration measurements, the diffusion coefficient of molecular water in the glass was estimated and was found to increase with increasing water content in the glasses studied. The hydration was found to be controlled by either of two mechanisms, the usual Fickian diffusion of molecular water due to the chemical potential gradient or Case II diffusion due to the stress gradient. Most compositions studied were predominantly controlled by one of these types, although there are expected to be compositions in which both types contribute significantly. It is believed that the hydroxyl content prevents ionic motion in these glasses. On the other hand, it is believed that the molecular water content contributes to the motion by a general loosening of the network, increasng conductivity and diffusion.