Investigation of the reactions of SiCl4 and O2 at elevated temperatures by infra-red spectroscopy

Abstract

The modified chemical vapour deposition (MCVD) process for making fibre optic preforms uses direct oxidation of SiCl4, together with dopant chlorides to produce core glass. This reaction has been studied by direct infra-red absorption spectrophotometry of the flowing gas system. The spectrum of the SiCl4 reactant is recognizable, its decrease with increasing temperature was measured, and the absorption spectra of SiO2 and of silicon oxychloride reaction products were recognized in the flowing gas stream. The oxychlorides began to appear in the spectra at 900° C and increased with temperature to 1110° C. Their concentration decreased drastically between 1110° C and 1160° C above which only the spectrum of SiO2 was observed. The identities of the oxychloride molecular species produced are uncertain, but small amounts of Si2OCl6 and the cyclic Si4O4Cl8 were present together with several unidentified higher molecular weight compounds. The particulate matter formed in the MCVD process shows infra-red spectra varying with conditions of formation. At temperatures below 1160° C the particulate matter shows absorption bands attributable to silicon oxychlorides. At higher temperatures, the particulate matter is free of oxychloride bands and shows only the proper SiO2 spectrum. If moisture is present, these spectra are modified.