Processing of gel-silica monoliths for optics: Drying behavior of small pore gels

Abstract

Processing and optical properties of fully dense sol-gel derived silica, termed Type V Gel-Silica, are described. Drying of large optically transparent acid-catalyzed alkoxide derived monolithic silica xerogels is emphasized. Changes in the weight, dimensions and optical properties are monitored during drying which allows a quantitative analysis of the drying process to be made. Results show three stages of drying. Stage 1. Where the greatest changes in volume, weight and structure occur. This stage is controlled by evaporation from the surface of the body and ends when shrinkage ceases. There is no constant rate period of drying but a progressive reduction in loss rate as the average pore radius decreases from 3.9 nm to 2.3 nm. Stage 2. Where changes in weight and volume are small but large changes in optical characteristics occur. This opaque stage is controlled by flow through small pores and liquid layers over the pore surfaces. Most of the pore emptying occurs during this stage. Stage 3. Where there are no further dimensional changes, but a progressive loss of weight occurs until equilibrium is reached. This stage starts when ∼ 94% of the liquid has been removed. Liquid pathway to the surface become discontinuous, and the remaining liquid can only be removed by evaporation within the pores and diffusion of the vapor to the surface. After stabilization and densification the silica xerogel monoliths have a degree of homogeneity, purity and optical and thermal properties superior to other commercial optical silicas.