Retention of Fluorine in Silica Gels and Glass.

Fluoride ions enter silica gels as Si-F bonds and free and hydrogen-bonded HF. During drying, heating and sintering all HF is removed, and a significant portion of the structural fluoride (Si-F) is also eliminated through reaction with Si- OH which results in formation of siloxane bonds. Substitution of Si-Cl bonds for Si-OH bonds assists in retention of fluoride, and up to ~l wt.% F could be preserved in gels heated up to 1000C. Introduction of SiF sub 4 into the sintering atmosphere can increase the amount of retained fluoride in the silica glass up to ~1.8%.