The formation of SiO2 from hexamethyldisiloxane combustion in counterflow methane-air flames

Abstract

Silica formation from hexamethyldisiloxane (HMDS) oxidation was studied by means of a CH 4 −N 2 /air opposed diffusion flame technique to which vaporized HMDS was added to the fuel flow. The CH 4 −N 2 /air flame changed color from a pale-blue flame to whitish pink color when small amounts of HMDS were introduced in the flame. Increasing concentrations (1.3 mol %) made the flame more luminous, and a second thin-flame zone, orange in color, appeared on the fuel side. Emission spectroscopy revealed the existence of Si−H and Si−O species. SiO 2 particles were observed only in the postcombustion gases, and the analysis of solid materials suggested the formation of fused silica particles, which were initially about 10 nm in size, forming outside the flame zone in these experiments. The overall mechanism in addition to that for methane oxidation is suggested as follows: C 6 H 18 Si 2 O+OH=2C 3 H 9 SiO+H C 6 H 18 Si 2 O+O 2 =2C 3 H 9 SiO+O C 6 H 18 Si 2 O+HO 2 =2C 3 H 9 SiO+OH C 3 H 9 SiO+M=3CH 3 +SiO+M SiO+HO 2 =HSiO+O 2 SiO+OH=SiO 2 +H SiO+O+M=SiO 2 +M SiO+O 2 =SiO 2 +O together with the formation of SiOH and SiO(OH) species. The agreement between the model predictions using Sandia code OPPDIF and the experimental data was found to be satisfactory. The model appears to be a useful tool in elaboration of chemistry of formation of SiO 2 in flames used to synthesize pure silica in this way.