THE SIZE AND SHAPE OF SILICA PARTICLES PRODUCED IN FLAMES OF H_2/O_2/N_2 WITH A SILICON-CONTAINING ADDITIVE

摘要

Well-defined, premixed and also diffusion flames of H_2/O_2/N_2 have been burned with silicon added. Tiny particles are produced. Initially, Si forms molecules of SiO and SiO_2 in a flame, but with [SiO_2] [SiO]. If the flame is hotter than 2000 K, only SiO_2 condenses out to form particles, but if the flame is cooler than ～2000 K, SiO can also condense. In addition, it appears that species like OSiOH and OSi(OH)_2 are stable in a flame and can condense out too, but well below 2000 K. The sizes and shapes of particles collected thermophoretically from various flames have been measured in an electron microscope; the particles are remarkably similar to carbonaceous soot particles in a rich hydrocarbon flame. The rate at which particles are produced is confirmed by modeling their growth and particle size distributions along a one-dimensional, hot, premixed flame. Here, the chemistry of the different species formed by Si is important. Larger amounts of solids, collected by burning a flame against a water-cooled plate, were examined by nuclear magnetic resonance, which revealed that the oxygen atoms were in SiO_4 tetrahedra; a substantial fraction of these O atoms were joined to atoms of hydrogen. Also, IR measurements on the deposited solids indicated OH groups, some of which were hydrogen bonded into the structure. This is in line with species like OSi(OH)_2 existing in the flames.