Annealing of hydrogen-impregnated and irradiated vitreous silica

The effects of isochronal thermal annealing are reported for a type I commercial vitreous silica which had been saturated with hydrogen at 45MPa and irradiated to 56MGy. It has been shown previously that several hundred wtppm of Si–OH and Si–H are formed in this glass by an identical treatment, accompanied by a decrease in density of 520±20ppm. Infrared spectra of this glass indicate that it contains free Si–OH, hydrogen-bonded Si–OH, molecular H2O, and Si–H. Annealing of this glass initially results in a large change in the shape of the hydroxyl band, with a shift in intensity from one region to another. The initial change in shape is followed by a gradual decrease in band intensity with a return to the pre-irradiated shape. These changes are interpreted as indicating that the H2O molecules react with the network to form additional Si–OH during the initial stages of annealing. At higher temperatures Si–OH and Si–H begin to react to produce H2, which diffuses out of the glass. Hydrogen-bonded Si–OH is converted to free Si–OH at even higher temperatures. The infrared band due to Si–H does not change in shape, but decreases in intensity during annealing. Sample density, as measured using a sink-float method, increases by 480±50ppm during thermal annealing.