Origin of the OH impurity in silica preforms obtained by oxidation of SiCl4in a plasma torch

Abstract

Industrial silica preforms can be obtained by oxidation of silicon tetrachloride in a high-frequency plasma torch operated in air. In this manufacturing process, silica particles are formed in the flame and vitrified onto a part of the edge of the cylindrical preform, which is rotated around its axis. The OH groups are the most important impurity of the final preform. A part of these OH groups was initially present in the silica particles on account of the pollution of the gaseous phase by atmospheric damp; another part is brought by diffusion and reaction of H2O inside the thin layers which are successively deposited to constitute the outer zone of the preform. Corresponding thermodynamic and kinetic calculations can explain the experimental OH group concentration profiles, which are determined inside samples cut out of the preform. By using a special experimental procedure, the spontaneous Raman scattering technique becomes suitable for a punctual determination of very low OH concentrations. The detection limit is about 0.3 wt.-p.p.m.