Small-angle X-ray studies of soot inception and growth

The high spectral intensity of X-rays produced by the undulator at the Basic Energy Sciences Synchrotron Radiation Center of Argonne's Advanced Photon Source has allowed us to perform small-angle X-ray scattering (SAXS) studies of the initial distribution of soot particles formed by various fuels. SAXS provides an in situ probe of the morphology of soot in the region between 1 and 100 nm and complements the ex situ technique of electron microscopy. The basic aspects of SAXS and its potential are illustrated with measurement on a laminar flame of acetylene in air. The more complex fuel toluene has been studied in a flat-flame burner that supports a CH4/H-2/air or CO/H-2/air diffusion flame stabilized by N-2 co-flow. This burner produces a nearly constant temperature region above the flame where the pyrolysis and combustion of the heavier fuels occurs. Kinetic information is obtained by performing measurements of the scattered intensity profile as a function of the height above the burner. These profiles have been reduced to give the mean radius and dispersion of a distribution of spherical particles. Mean radii between 0.8 and 18 nm have been observed. The smallest of these is a factor of ten smaller than previously detected with Lorentz-Mie scattering. Near 1550 K, the soot distribution found in toluene shows a distinct step behavior that is consistent with model calculations.