Temperature dependence of hydroxyl formation in the reaction of hydrogen with silica glass

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Abstract
The formation of hydroxyl due to reaction of a silica glass made by the modified chemical vapor deposition process with hydrogen has been studied for temperatures between 200 and 900 °C. The glass is treated under 1 atm of hydrogen, which diffuses into and reacts with the glass. For treatment temperatures between 900 and 500 °C, spatial hydroxyl concentration profiles are measured. For treatment at 400 °C and below, only equilibrium OH concentrations are measured. Three temperature regimes are identified for the reaction: T>700 °C, 700 °C>T>400 °C, and T2 diffusion with chemical reaction explains the profile data for temperatures above 500 °C. The model assumes a finite number of preexisting reactive sites in the glass, which does not depend on treatment temperature. Assuming the same net reaction over the entire temperature range, enthalpies and entropies of reaction are derived from the data. The reaction at 800 °C of H2 with the glass Suprasil W has also been studied. For this glass the reaction is extremely rapid compared to H2 diffusion, and the kinetics are described by a tarnishing model. These results demonstrate that OH formation in silica glass due to reaction with H2 depends on preexisting defects in the glass, and therefore depends strongly on the nature and thermal history of the glass.