SYNTHESIS AND RESTRUCTURING OF INORGANIC NANO-PARTICLES IN COUNTERFLOW DIFFUSION FLAMES

The formation/growth/coagulation/sintering of flame-generated inorgani c aggregates at low particle volume fractions (O(10(-1) ppm)) was inve stigated. Al2O3 particles synthesized in a Al(CH3)(3) (TMA)-seeded atm ospheric pressure counterflow diffusion flame (CDF) fueled with CH4/O- 2/N-2 were used as the model material/combustion system. Experimental techniques included thermocouple, laser light scattering (LLS) and the rmophoretic sampling/Transmission Electron Microscopy (TEM). Local agg regate morphology evolution was characterized in terms of ''primary'' particle size, aggregate size, and fractal structure. Additionally, th e effects of temperature and TMA concentrations on morphology and size were also investigated systematically in the CDF. Light scattering si gnals as well as TEM analysis dearly illustrated particle/aggregate si ze and morphology evolution as a result of two competing processes, wi th coagulation increasing aggregate sizes, and sintering reducing aggr egate surface areas. Mean ''primary'' particle diameters were found To be in the range of 13-47 nm, increasing with TMA concentration and sa mpling position (increasing residence time). On the other hand, mean a ggregate sizes reached a maximum at about 4 mm above the bottom fuel d uct (corresponding to a local temperature of only 1250 K) and increase d with TMA seed level. Fractal dimension and fractal prefactor of alum ina aggregates with negligible sintering rates were found to be 1.52 a nd 2.4, respectively. The final products were larger spherical particl es with up to 60 nm diameter, resulting from complete ''collapse'' of the aggregates. These observations were shown to be compatible with ou r independent evaluation of the characteristic times associated with t he participating rate processes in this class of two-phase CDFs. Syste matic modification of these characteristic times can be used to contro l the size and morphology of flame-synthesized particles.