Monolithic dried gels and silica glass prepared by the sol-gel process

Porous Materials

“…The change in pore size and pore volume was followed by BET measurements which showed that the pore size did not change greatly up to 1000oe, although the pore volume decreased with the increasing temperature ( High purity, bubble-free silica glass was synthesized [162] using a procedure shown in Figure 4.72. The use of low density was found to be the key [160] as was also shown by Kawaguchi et al [159]. Crack-free monolithic gels of large size (85 mm in diameter and 200 mm in length) for use as optical fiber preforms were fabricated [163] by adding fluorinated silicon ethoxide and Si02 powder of low specific surface area into sol solution to improve mechanical strength (Figure 4.73).…”

“…This silica has been commercialized for optics and consists of interconnected porosity with a density of 1.2 glem 3 , an average pore diameter of 2.8 nm and a specific surface area in the range 700-800 m 2 /g. Lower bulk densities of monolithic gels led to monolithic glass [159]. Dried gels of bulk density lower than 0.9 g/cm 3 were successfully converted into silica glass at HOOoe in He atmosphere [159].…”

“…Lower bulk densities of monolithic gels led to monolithic glass [159]. Dried gels of bulk density lower than 0.9 g/cm 3 were successfully converted into silica glass at HOOoe in He atmosphere [159]. Gels with higher bulk density led to fracture and bloating as a result of apparently capillary stresses due to small pores.…”

Microstructural and physical properties of the ambient pressure dried hydrophobic silica aerogels with various solvent mixtures

“…The density of the aerogel is closely related to the PSD of the gel. Gel with small pores tends to fracture or bloat at high temperatures, whereas those containing large pores can be sintered without such problems [36][37][38]. It was found from the figure that there is a small pore size distribution in the HB aerogels, that is because of low molecular weight and high vapor pressure of hexane and benzene whereas the HpT and HT aerogels have larger pores with broad distribution due to low vapor pressure of heptane and toulene.…”

Further insights into the porous structure of TEOS derived silica gels

“…These monolithic gels have been converted to glass by heating at temperatures close to 1000~ [3]. During heat treatment, the high specific surface area and porosity decrease abruptly, and the density increases reaching a value close to that of vitreous silica [4]. At the same time, H20 and alcohol are eliminated from the gel structure, and serious problems are encountered due to crack formation and fracture of the silica gel [5].…”