Crystallogenesis of particles formed in hydrocarbon combustion

The formation of crystallinity in combustion generated soots is explored by three traditional methods that are based on their sensitivity to the Bragg diffraction produced by multilayer atomic structures. These techniques are high resolution electron microscopy (HRTEM), X-Ray diffraction (XRD), and dark field transmission electron microscopy (DFTEM). These methods provide complementary information on the nature of the crystallinity in combustion generated soots. The lattice parameters of both flame generated soots and d iesel soots are consistent with the structure of disordered carbons with gr aphitic basal planes. The initial evidence of crystallinity in flame genera ted particles is detected by XRD in the precursor particles that previously have been found to contain multiring polycyclic aromatic hydrocarbons (PAH s). These results are consistent with the hypothesis of Oberlin (1984) that PAHs in carbonizing hydrocarbon pitches are assembled in parallel layers t o produce diffraction peaks. The more intense display of diffraction maxima is evident in DFTEM when the particles sampled from flames undergo the tra nsformation from isolated precursor spheroids to clustered aggregates durin g the carbonization process. These results support the view that the PAHs i nitially formed in the gas phase combustion processes undergo a series of t ransformations in which the hexagonal geometry is preserved and form the ba sal planes of the crystallites found carbonaceous soot particles. This desc ription evidently applies to the formation of soot from a wide variety of h ydrocarbon fuels burned in various combustion devices ranging from gaseous- fueled laboratory burners to diesel engines.