Laser-induced structural modifications of differently aged soot investigated by HRTEM

Laser heating was applied to soot probed in the first inception region (nascent soot) and downstream (mature soot) of premixed methane and ethylene flames, burning in similar temperature and equivalence ratio conditions. Different properties of nascent and mature soot were investigated by TEM/HRTEM and EELS along with elemental analysis, UV-visible and Raman spectroscopy. Structural modifications were followed by HRTEM imaging applied to the soot before and after laser heating. Laser heating produced varied modifications of the lamellae organization dependent upon the morphology of pristine soot, in turn dependent on soot aging and fuel type (ethylene and methane). Specifically, structures in form of void shells versus multifaceted rosette structures were observed in the case of young nascent and mature soot, respectively, showing a relationship with their different nanostructures, namely amorphous versus disordered turbostratic structures. Additionally, the differentiation between void shells or multifaceted rosette structures can be used as signature for the occurrence of laser heating effects, which have to be avoided when the laser is instead employed for diagnostic purposes.