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 diesel soots are consistent with the structure of disordered carbons with graphitic basal planes. The initial evidence of crystallinity in flame generated particles is detected by XRD in the precursor particles that previously have been found to contain multiring polycyclic aromatic hydrocarbons (PAHs). These results are consistent with the hypothesis of Oberlin (1984) that PAHs in carbonizing hydrocarbon pitches are assembled in parallel layers to produce diffraction peaks. The more intense display of diffraction maxima is evident in DFTEM when the particles sampled from flames undergo the transformation from isolated precursor spheroids to clustered aggregates during the carbonization process. These results support the view that the PAHs initially formed in the gas phase combustion processes undergo a series of transformations in which the hexagonal geometry is preserved and form the basal planes of the crystallites found carbonaceous soot particles. This description evidently applies to the formation of soot from a wide variety of hydrocarbon fuels burned in various combustion devices ranging from gaseous-fueled laboratory burners to diesel engines.