Electron microscopy evidence of aggregation under three different size scales for soot nanoparticles in flame

The microstructure of soot at larger (~1 s) residence times within a propane-air diffusion flame is here investigated by Transmission Electron Microscopy. Three classes of nanoparticles are observed. The first of them is recognised as the well-known class of primary particles (20-50 nm) that usually are reported to be the sub-units constituting the chain-like and fractal soot aggregates. The other two categories of carbon nanoparticles here evidenced are distinguished in sub-primary graphitical particles (6-9 nm) and elementary particles, (less than 5 nm). Sub-primaries appear to be graphite-like, ellipsoidal in shape and clustered together to form small chains of less than ten units. Elementary particles appear to be constituted by very regularly sized transparent shells (~ 4 nm) of amorphous carbon and by inner graphitic nuclei (~0.5-2 nm). Elementary particles are often observed to be closely packed together and to fill all the available space between graphitic layers and larger particles. Aggregation of sub-primary with primary particles is also observed to occur. Vesicle-like structures of external dimension about 15 nm with a shell about 5 nm thick, and short tubular structures of length 5-10 nm and diameter 0.5-1 nm are episodically individuated in TEM micrographs. Major result can be summarised in the evidence of a strongly polydispersed agglomeration mechanism occurring under three different scales of particle size. Implications of such a result, in terms of coagulation times for each of the observed nanoparticle typology are also discussed.