The counterflow diffusion flame burner: A new tool for the study of the nucleation of refractory compounds

This paper describes the development of the counterflow diffusion flame burner into a tool which provides great promise as a technique for the study of the nucleation from the vapor phase of refractory compounds such as SiO₂, Al₂O₃, and other metal oxides. This burner produces a flame which is flat, stable, and rectangular in shape, with temperature and species-concentration distributions which are uniform in the horizontal plane. Thus, concentrations and temperatures can be measured spectroscopically. Hydrogen (diluted in argon) and oxygen (diluted in argon) have been used as the primary fuels and oxidants. Small amounts of appropriate gases, e.g., silane, are added to the fuel stream. Light scattering measurements indicate that an avalanche of nucleation occurs when the silane flow rate is increased beyond a critical value (e.g., 2.3 cm³/min). The temperatures and the partial pressures of SiO and O₂ were measured. Assuming chemical equilibrium in the vapor phase, the SiO₂ partial pressure and the SiO₂ supersaturation were calculated. The H₂ O₂ flame changes color from pale-blue to a whitish pale-yellow when small amounts of silane are introduced into the flame. Increasing the silane flow rates makes this flame brighter and brighter. When the silane flow rate is increased sufficiently (e.g., to 9.3 cm³/min), a second flame, thin and orange in color, appears on the H₂-side of the H₂ O₂ flame zone. Between the two flames there is always a dark space. The peak in light scattering intensity always occurs in the middle of this dark space.