PURPOSE: To reduce the defect by using a dried gel having bulk density in a prescribed average value range and difference between the max. and min. values of the bulk density below a specific value as a starting material before firing.
CONSTITUTION: A silica sol is made by dissolving a Si alkoxide such as polymethylsiloxane and adequate quantity of an organic polymer such as PVAc into an organic solvent such as tetrahydrofurfuryl alcohol and hydrolyzing in the presence of a catalyst by dropping water of a prescribed quantity. The sol is fed into a polyfluoroethylene coating vessel hermetically sealed with a PVC film and is kept at 30-60C for a fixed period to form a gel having ≤20% nonvolatile matter. Next, the gel is heated and dried to obtain a dried gel having 0.2-0.4g/cm3 average bulk density and ≤0.1g/cm3 four point average value of the difference between the max. and min. value of the bulk density. The dried gel is fired in an inert gas atmosphere such as He at 1000-1400°C to obtain the silica glass free from crack.
MACHII YOICHI
SHIMAZAKI TOSHIKATSU
TERASAKI HIROKI
