Elimination of Cl sub 2 and H sub 2 O in Gel Glasses

Chlorine is commonly used to remove H sub 2 O and OH from silica gel glasses by treatment of the material prior to sintering. However, chlorine remains in the sintered material after treatment and may form gaseous species when the glass is further heated. Fluorine reduces the residual Cl concentration, and thus reduces the tendency to form vapors. Fluorine also reduces the H sub 2 O content of dried gels, thus requiring less extensive dehydration to form OH-free glass. It is thought that the action of fluorine occurs through the formation of stronger Si-F bonds in place of weaker Si-Cl bonds. Effective methods of introducing F sup (-) ions include the use of suitable compounds in the starting mix from which the gel is formed and vapor treatment before or during sintering.