DRYING AND SINTERING OF SOL-GEL DERIVED LARGE SIO2 MONOLITHS

This review article summarizes the development of drying and sintering techniques for the production of sol-gel derived, large silica glass components. Gels may be synthesized using particulate or metal alkoxid e precursors, or both in combination. Rapid fracture-free drying has b een achieved easily with particulate gels because of their large pore size (100-6000 Angstrom). Alkoxide Is, which generally have small pore s (<200 Angstrom), were initially difficult to dry without cracking. H owever, recent studies have shown that large alkoxide gel monoliths ca n also be dried in reasonably short times (<10 days). During subsequen t heat treatment, alkoxide gels tend to have high shrinkage rates, whi ch may cause trapping of hydroxyl ions or organic groups remaining on the gel surface. Although the removal of these species is easier for p articulate gels, their large pore size necessitates heating above 1400 degrees C to achieve full consolidation. Sintering at such temperatur es was observed to deteriorate glass quality, through crystallization, warping, and/or sagging. Extensive optimization of the entire process has shown that on a laboratory scale, high-optical-quality glass can be produced from both alkoxide and particulate gels. It remains to be seen whether sol-gel process will be feasible for the manufacture of h igh-quality glass products on a commercial scale.