Partition of Hydrogen in the Modified Chemical Vapor Deposition Process

In the modified chemical vapor deposition (MCVD) process for making glass fibers, most of the hydrogen coming into the reaction from hydrogen‐bearing impurities in the starting materials is not incorporated in the glass. It is instead mostly converted to HCl, which is not absorbed by the newly formed silica particles, and passes out the exhaust stack. The residual partial pressure of H 2 O formed in equilibrium with HCl in the presence of O 2 and Cl 2 accounts quantitatively for the OH appearing in the glass when the known solubility of H 2 O in silica is considered. The H 2 O/HCl equilibrium is quenched at a temperature below that of the reaction zone at a value for which the rate of reaction approximates the transit time across the deposition zone. Quantitative agreement is obtained for published OH concentrations produced by SiHCl 3 doping.