# Effect of Fuel Composition on Carbon Black Formation Pathways
## 3 Results and Discussion
### Laser Heat Treatment  
CT- and FCC-CB are laser heated with one 1064 nm pulse of 170 mJ/cm\({}^{2}\) of the Nd:YAG laser in an inert (N\({}_{2}\)) atmosphere. Laser heat treatment at extremely short (\(\sim\) 8 ns) timescales followed by microseconds long rapid cooling results in divergent nanostructure for the two carbon blacks. This is premised on source differentiation using laser derivatization [9], where subtle non-obvious differences in the materials' nascent nanostructure and chemistry may be enhanced by laser annealing. Given differences in their nascent material nanostructure, CT- and FCC-CB show a significant divergence in their annealed nanostructures, shown by the following TEM micrographs. Figure 9 shows micrographs with an overview of the aggregate morphology and structure of the two carbons, followed by Figure 10 showing HRTEM images of the particles after laser annealing. CT-CB shows hollow particles with intact shells, while FCC shows the formation of multiple connected internal ribbons of carbon lamellae. CT-CB shows a disordered core with a higher H/C ratio. Laser heat treatment of this carbon results in a greater extent of carbon vaporization and fragmentation of lamellae within the particle interior. Many smaller lamellae are created in the process, coupled with  
Figure 8: HRTEM micrographs of (**A**,**B**) CT- and (**C**,**D**) FCC-CB black after isothermal treatment in a TGA. greater mobility that leads to a shell formation as the mobile lamellae move towards the outer perimeter and template of the perimeter shell. FCC-CB has longer lamellae within a structured particle interior and overall, less radial variation from shell to core. It has a lower core H/C ratio and thus, laser heat treatment leads to lesser fragmentation of the longer lamellae. Coupled with lower mobility leads to internal ribbon formation and compartmentalization as seen in Figure 10.  
While differences in shell integrity versus crumbling were evident between the FCC-CB and CT-CB materials, respectively, both forms exhibited internal burning during oxidation--indicative in each case of more reactive and disordered interior carbon. However, though not substantial, initial, and later stage oxidation rates were consistently different-- reflecting differences in interior carbon. In contrast, laser annealing accentuates differences in nanostructure and chemistry yielding starkly different particle structures: a hollow void versus ribbon compartmentalization, as observed in Figure 9. At higher magnification Figure 10 reveals the full differentiation between the two annealed forms. For this reason of differentiation based on differences in formation conditions, the action of the laser may be considered a form of (nano)structural derivatization. Figure 9: Laser heated aggregates of (**A**,**B**) CT- and (**C**,**D**) FCC-CB blacks. These qualitative inferences are supported by numerical simulations for model compounds. For example, molecular dynamics revealed 1,2,4 Trimethylbenzene (124 TMB) to have earlier soot inception and PAH addition reactions than n-propyl benzene, (PBZ) [22]. Consistent with both earlier soot inception and PAH addition, the pyrolysis model for 124 TMB predicts earlier pyrene formation, suggesting 124 TMB has alternative reaction pathways for pyrene formation. As shown by [PERSON] et al., analysis of the reaction pathway suggested that pyrene is formed via an aromatic radical recombination route, as opposed to the conventional HACA mechanism, a slower route to 2 nd ring formation. These results are consistent with the resonance-stabilized hydrocarbon-radical chain reaction mechanism recently advanced for soot inception and growth [5,6]. Radical chain reaction pathways can lead to covalently bound clusters of PAHs and other hydrocarbons that otherwise would be too small to condense at high temperatures, thereby providing a mechanism for rapid particle formation and surface growth [4].