Drying and sintering of bulk silica gels

• Published in: Journal of sol-gel science and technology 1997-02, Vol.8 (1-3), p.397-402
• Main Authors: Murata, H., Meyers, D. E., Kirkbir, F., Chaudhuri, S. Ray, Sarkar, A.
• Format: Article
• Language: English
• Subjects: Cavitation; Cleanrooms; Defects; Drying; Glass & glassware industry; Laboratories; Near net shaping; Optical glass; Optimization; Particle size distribution; Phenomenology; Pore size; Pore size distribution; Porosity; Silica gel; Silica glass; Silicon dioxide; Sintering; Sintering (powder metallurgy); Surface cracks; Vapor deposition
• ISSN: 0928-0707; 1573-4846
• DOI: 10.1007/BF02436872

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 during drying, due to capillary forces generated in the gel body, was overcome through two particular innovations. The first was the development 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 cavitation of the pore fluid. Contrary to conventional wisdom, in this drying approach the smallest pore size gels are the easiest to dry. Details of the types of gels and the drying process are reported. Gels of small size were sintered into crack-free glasses, utilizing conventional sintering approaches. However, large size gels always developed visible surface cracks that formed above 800°C. To successfully dry and sinter large monolithic gels then required re-optimization of the entire process. A great number of micron-range defects were initially detected in these sintered bodies. After analyzing the defects, further steps were taken to improve glass quality to the level of optical glass produced by vapor deposition processes. This included mixing and filtering of sols in a clean room, varying the pore size distribution, and optimizing the pre-sintering and sintering processes. Data of relevant glass quality parameters attained so far in the laboratory are reported.