Synthesis and restructuring of inorganic nano-particles in counterflow diffusion flames

Abstract

The formation/growth/coagulation/sintering of flame-generated inorganic aggregates at low particle volume fractions ( O(10 −1ppm)) was investigated. Al 2O 3 particles synthesized in a Al(CH 3) 3 (TMA)-seeded atmospheric pressure counterflow diffusion flame (CDF) fueled with CH 4/O 2/N 2 were used as the model material/combustion system. Experimental techniques included thermocouple, laser light scattering (LLS) and thermophoretic sampling/Transmission Electron Microscopy (TEM). Local aggregate morphology evolution was characterized in terms of “primary” particle size, aggregate size, and fractal structure. Additionally, the effects of temperature and TMA concentrations on morphology and size were also investigated systematically in the CDF. Light scattering signals as well as TEM analysis clearly illustrated particle/aggregate size and morphology evolution as a result of two competing processes, with coagulation increasing aggregate sizes, and sintering reducing aggregate surface areas. Mean “primary” particle diameters were found to be in the range of 13–47 nm, increasing with TMA concentration and sampling position (increasing residence time). On the other hand, mean aggregate sizes reached a maximum at about 4 mm above the bottom fuel duct (corresponding to a local temperature of only 1250 K) and increased with TMA seed level. Fractal dimension and fractal prefactor of alumina aggregates with negligible sintering rates were found to be 1.52 and 2.4, respectively. The final products were larger spherical particles with up to 60 nm diameter, resulting from complete “collapse” of the aggregates. These observations were shown to be compatible with our independent evaluation of the characteristic times associated with the participating rate processes in this class of two-phase CDFs. Systematic modification of these characteristic times can be used to control the size and morphology of flame-synthesized particles.