Retention of Fluorine in Silica Gels and Glass*

Abstract

F− 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 ∼1 wt% F can be preserved in gels heated to 1000 °C. Introduction of SiF4 into the sintering atmosphere can increase the amount of retained fluoride in the silica glass up to ∼1.8%.