Abstract
The morphological evolution of flame-generated primary spherules and inorganic aggregates was studied at low particle volume fractions [O(10 -1 ppm] in a well-defined/characterized laminar nonpremixed combustion environment which produces particle heating rates of 10 4 K/s. Pure Al 2 O 3 particles synthesized in an Al(CH 3 ) 3 (TMA-) seeded atmospheric pressure laminar counterflow diffusion flame fueled with CH 4 /O 2 /N 2 were used as the model material/combustion system. Experimental techniques included spatially resolved laser light scattering (LLS) and thermophoretic sampling/transmission electron microscopy. Local aggregate morphology was characterized in terms of spherule (grain') size, aggregate size, aggregate shape and fractal structure. Effects of flame temperature and TMA concentrations on particle inception location, sizes and morphology studied systematically were interpreted based on parallel theoretical studies. LLS signals and TEM images show particle/aggregate size and morphology evolution as a result of two competing rate processes. Mean spherule diameters prior to high-temperature coalescence are explained in terms of the strong size dependence of nanoparticle restructuring kinetics due to surface melting, even at 500 K Mean fractal aggregate sizes reached only 15-27 spherules near a local temperature of only 1,250 K Final particulate products were isolated spherical particles resulting from complete collapse of the aggregates in an interval of only 24 ms immediately upstream of the maximum gas temperature (2,280 K). Experimental results are compatible with the characteristic times governing each participating unit rate process. Some of these methods can be applied in controlling the larger-scale synthesis of valuable nanopowders and guide rational extensions into the domain of turbulent nonpremixed combustors generating ultrafine particles of tailored composition and morphology at high mass loadings.
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