Formation and growth of SiO2 particles in low pressure H-2/O-2/Ar flames doped with SiH4

Low pressure premixed H-2/O-2/Ar flames (H-2/O-2 = 1.69, 1.04 less than or equal to Ar/(H-2 + O-2) less than or equal to 1.36) were doped with small amounts of SiH4 (131 ppm less than or equal to SiH4/(H-2 + O-2 + Ar) less than or equal to 524 ppm) and were stabilized on a fiat, water-cooled sintered bronze plate al pressures 27.5 mbar less than or equal to p less than or equal to 30 mbar. Particles formed during the flame reaction process were extracted at different heights From the flame zone by a molecular beam sampling technique, which is part of a particle mass spectrometer, PMS. This instrument, which was developed earlier in this laboratory, generates a particle beam in a high vacuum containment. From direct measurements of both, the kinetic energy and the velocity of charged particles, their mass was directly determined. As the amount of naturally charged particles in the SiH4-doped H-2/O-2/Ar flame was nearly zero, small amounts of C2H2 (about 5 ppm) was added to the flame, which changes the charging situation drastically. The PMS measurements indicate a strong formation rate of charged particles at flame co-ordinate x greater than or equal to 20 mm, which were in the mass range 3.7.10(-20) g less than or equal to (m) over bar(p) less than or equal to 58.3.10(-20) g. The equivalent particle size determined with a material density of rho = 2.65 g cm(-3) was 3.0 nm less than or equal to d(p) less than or equal to 7.6 nm. The absolute particle mass depends on the amount of SiH4, on the dilution ratio, and is a Function of the flame co-ordinate x. An increasing amount of C2H2 added to the H-2/O-2/Ar + SiH4 flame results in an increase in the amount of charged particles by more than one order of magnitude. The experimental results on SiO2 particle formation were verified by a theoretical model. It includes the full homogeneous H-2/O-2 and SiH4/O-2 kinetics as well as transport properties of burner stabilised flames. The particle formation was described based on homogeneous nucleation of SiO2-vapour and Brownian coagulation, simulated both by a monodisperse as well as by a sectional model. Considering the size dependent charging probability of neutral particles by chemo-ions, good agreement in particle size and size distribution between the experimental results and the computer simulations was obtained.