Soot nanostructure evolution in premixed flames by high resolution electron transmission microscopy (HRTEM)

HRTEM fringe analysis by means of our "Analyse Plan" (AP) tool and a new HRTEM image analysis procedure, based on mathematical morphological analysis (MA), was applied to soot sampled along the axis of sooting premixed benzene and ethylene flames. The layer length obtained by means of MA approach was found to be higher in respect to the layer length derived from AP method, as it better accounts for their local curvature and distortion. The mean layer length obtained by the MA method was then proved to agree well with the data evaluated by Raman spectroscopy. Moreover, the MA method was useful for extracting other soot nanostructural parameters related to the geometry of individual fringes as layer length, tortuosity, and local curvature radii, in turn correlated with the defective nature of soot structure. Some parameters provided by the MA method appeared to be more sensitive and informative about structural changes occurring as soot is formed. In particular, the percentage of fringes with high tortuosity and the percentage of nearly-straight fragments were found to decrease and increase, respectively, throughout the soot formation region. These trends suggested a better stacking of longer and more planar layers testifying the structural order improvement with soot aging. It was also observed that the soot nanostructural ordering occurred faster for benzene with respect to ethylene soot in agreement with H/C and absorption coefficients trends. The distributions of carbon in differently-sized aromatic structures were evaluated on the basis of the fringe length distribution to follow the aromatic growth. The method assigned aromatic sizes to the extracted HRTEM fringes, assuming that the fringes are in the shape of parallelograms. The distributions were found to evolve toward larger size aromatic systems demonstrating the occurrence of aromatic growth during soot formation. © 2014.