SILICA GLASS - PREPARATION AND CHARACTERIZATION

Silica glass with good optical quality and low OH content has been prepared by a sol-gel process. The sols have been prepared by mixing TMOS, methanol and water in acid condition, and 0.15 μm SiO2 Stöber microspheres or CabOSil M5 have been added to the sols in order to reinforce mechanically the gels and obtain monolithic samples. The gels were then dried at 60°C during 15 to 20 days. The gelation time and BET surface area have been determined as a function of the concentration of these additions and sol pH. The BET area shows an anomalous behavior with a narrow dip down to 300 m2/g for concentration of SiO2 powder of the order of 1 wt%. The gelation time exhibits a maximum at a pH of 2.5 and a minimum at a pH of 4.5. Sintering into clear and bubble free amorphous silica has been obtained by heating under controlled atmospheres (CCL4, O2 and vacuum) up to 1070°C. The OH content was measured by IR spectroscopy and found to decrease with the chlorination time. © 1990.