The effect of oxidation pressure on the equilibrium nanostructure of soot particles

The notion of equilibrium soot nanostructure was introduced by Hurt et al., who argued that the peculiar turbostratic carbon structure is an equilibrium arrangement of lamellar, graphene-like molecules. It was proposed that the typical, onion-like internal structure of primary soot particles can be satisfactorily described by thermodynamic principles. There are two main objectives of this paper. First, the effects of oxidation pressures above atmospheric pressure on soot nanostructure are investigated experimentally. The analyzed soot was generated in premixed flames of liquid fuels: n-dodecane, m-xylene and n-butanol and further oxidized in a thermogravimetric analyzer under atmospheric, 10 atm and 40 atm pressures. Nanostructure is described by utilizing high-resolution transmission electron microscopy and recently developed image analysis techniques. Second, empirical observations are compared against behavior that is semi-quantitatively predicted by the thermodynamic model. The utilization of the novel analysis technique made direct comparison between observed and computed properties possible. Reasonable consistency was found between experimental and computational results. The results suggest that the known thermodynamic model can be used to predict equilibrium structure even when soot is oxidized under pressurized conditions. Since diesel and jet engines operate at elevated pressures, the conclusions drawn in this paper may find their use in predicting soot nanostructure in the limiting case of equilibrium conditions.