Morphological Evolution of Nanoparticles in Diffusion Flames: Measurements and Modeling


The morphological evolution of flame-generated -œprimary- spherules and inorganic aggregates was studied at low particle volume fractions [O(10−1 ppm)] in a welldefined/characterized laminar nonpremixed combustion environment which produces particle heating rates of 104 K/s. Pure Al2O3 particles synthesized in an Al(CH3)3 (TMA-) seeded atmospheric pressure laminar counterflow diffusion flame -œfueled- with CH4/O2/N2 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 (-œgrain-) 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 -œcollapse- 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 -œunit- 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.


Mechanical and Aerospace Engineering

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Article - Journal

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© 1997 Wiley-Blackwell, All rights reserved.

Publication Date

01 Jan 1997