Synchrotron SAXS 〈in situ〉 identification of three different size modes for soot nanoparticles in a diffusion flame

The high photon flux (10¹⁴ photons/s) and high spatial resolution (∼100 μm) of synchrotron radiation available at ID09b/ESRF facility has been exploited in an in situ investigation of the early stages of soot formation in an ethylene-air diffusion flame. SAXS data demonstrate in situ evidence that the size distribution of soot nanoparticles within the flame exhibits three distinct modes at different heights above the burner z’s. In particular, at z ∼ 3 mm, small particles (4-6 nm size), called sub-primary particles, are observed to come into existence. The corresponding monomodal distribution is observed to evolve to bimodal one at z > 5 mm, with the sub-primary particle mode being progressively depleted in favor of the growth of a mode corresponding to larger primary particles (10-12 nm size). The sub-primary peak vanishes completely at about z ∼ 20 mm where distribution is again single mode about 12 nm diameter. Porod’s plots show that the sub-primaries are born as configurational flat entities similar to discs or lamellae with a small aspect ratio (Porod’s exponent about 2), and upon going to higher z’s, they approach a spherical form with a smooth surface (Porod’s exponent about 4). Moreover, the careful use of Kratky plots has allowed to demonstrate the presence within the flame of very small nuclei, sized about 1.5-2 nm, which have a nucleation burst at about z ∼ 5 mm, whose number concentration progressively decreases at larger z, finally disappearing around z ∼ 15 mm. The relative increase of primary particles (size larger than 12 nm) is found to correspond to the progressive decrease of these very small nuclei (∼2 nm) concentration. At heights larger than 15 mm a strong ionization signal is observed that increases with height. These findings are in agreement with previous experimental works in the literature performed by Transmission Electron Microscopes and Differential Mobility Analyzers as well as with theoretical studies of dimer-dominated stochastic coagulation.