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

The microstructure of soot at larger (∼1 s) residence times within a propane–air diffusion flame is here investigated using Transmission Electron Microscopy. Three classes of nanoparticles are observed. The first is recognised as the well-known class of primary particles (20–50 nm) that usually are reported to be the sub-units that make up the chain-like and fractal soot aggregates. The other two categories of carbon nanoparticles here shown are termed sub-primary graphitical particles (6–9 nm), and elementary particles (<5 nm). Sub-primaries appear to be graphite-like, ellipsoidal in shape and clustered together to form small chains of less than 10 units. Elementary particles appear to be made up of very regularly sized transparent shells (∼4 nm), amorphous carbon and 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 about 15 nm external dimension 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. The major result can be summarised as evidence of a strongly polydispersed agglomeration mechanism occurring under three different scales of particle size. The implications of such a result in terms of coagulation times for each of the observed nanoparticle types are also discussed.