Abstract
Synthetic fused silica for optical fibres is currently produced by vapour phase oxidation of silicon tetrachloride in a plasma torch, according to the equation: SiC1 4( g) + O 2( glass) +2 C1 2( g). The resulting soot particles form a deposit which is immediately vitrified onto the edge of a rotating substrate to give a cylindrical preform. We have made spontaneous Raman scattering suitable for a punctual determination of very low OH concentrations inside samples cut out of the preform, the limit of detection being estimated at 0.3 ppm (weight of OH groups/weight of silica). A thermodynamic study of the OH content at the heart of the preform has shown that the surrounding air enters the reaction zone. On the other hand, the outer zone is deposited in thin successive layers which makes possible the incorporation of OH groups by diffusion of atmospheric damp. Thermodynamic and diffusional calculations allow a good prediction to be made of the OH concentration profiles inside the preforms.
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