DRYING AND SINTERING OF BULK SILICA-GELS

Over the last few years, the feasibility of fabricating near net shape silica glass components, using a sub-critical drying process for pure alkoxide gels, has been demonstrated in our laboratories. Cracking du ring drying, due to capillary forces generated in the gel body, was ov ercome through two particular innovations. The first was the developme nt and optimization of a dual-catalyzed high strength gel. The second was a controlled atmosphere drying process that allowed the gel to dry utilizing a newly observed phenomenology, postulated to be due to cav itation of the pore fluid. Contrary to conventional wisdom, in this dr ying approach the smallest pore size gels are the easiest to dry. Deta ils of the types of gels and the drying process are reported. Gels of small size were sintered into crack-free glasses, utilizing convention al sintering approaches. However, large size gels always developed vis ible surface cracks that formed above 800 degrees C. To successfully d ry and sinter large monolithic gels then required re-optimization of t he entire process. A great number of micron-range defects were initial ly detected in these sintered bodies. After analyzing the defects, fur ther steps were taken to improve glass quality to the level of optical glass produced by vapor deposition processes. This included mixing an d filtering of sols in a clean room, varying the pore size distributio n, and optimizing the pre-sintering and sintering processes. Data of r elevant glass quality parameters attained so far in the laboratory are reported.