The sol-gel glass transformation of silica

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Abstract
Large monolithic xerogel silica glasses were successfully made from tetramethyl-orthosilicate and distilled water using the combination of an acidic drying control chemical additive (DCCA) and a specially designed drying chamber. The acidic DCCA increases the gel strength by formation of a fibrillar ultrastructure, and the drying chamber reduces the catastrophic capillary forces inside the wet gel body. Partially densified monolithic gels up to 850°C were routinely made for physical property tests and compared to commercial fused silicas. Although the mechanical properties of the porous gel-silica monoliths such as microhardness, Young's modulus, toughness, flexural strength, density are relatively lower than fused silica, the optically transparent porous gel silica has a UV cut-off ranging from 250–300 nm. Such a porous gel with excellent optical transmission and a highly uniform pore radius of 10–50 Å offers a unique, chemically stable matrix for impregnation with a second phase of optically active organic or inorganic compounds. The processing and properties of Types I and II fused quartz optics and Types III and IV synthetic fused silica optics are compared with the new organometallic sol-gel derived gel-silica optics. Fully dehydrated and densified gel-silica has excellent transmission from 165 nm to 4400 nm with no OH absorption peaks. This optical transmission is equivalent to the best Type IV fused silica. The other physical properties and structural characteristics of the dehydrated dense gel-silica are similar to fused quartz and fused silica. However, the dense gel-silica has a lower coefficient of thermal expansion of 2.0 × 10−7 cm/cm compared with 5.5 × 10−7 cm/cm for standard vitreous silicas. The CTE value is temperature independent from 80 K to 500 K. Sol-gel silica optics can be made as complex shapes by casting of the sol into inexpensive plastic molds.