SOOT OPTICAL BAND GAP EVALUATED THROUGH IN-SITU AND EX-SITU MEASUREMENTS AS TRACER OF SOOT EVOLUTION IN PREMIXED FLAMES

The optical band gap concept for amorphous semiconductors proposed by Tauc 1 and further developed for amorphous carbon by Robertson and O'Reilly 2 has been transferred to the study of soot formation through in-situ measurements 3,4. The principle is that the optical band gap of carbons can provide information about the crystalline character of the soot, in terms of length of aromatic layers, La (often called the polyaromatic unit size), constituting the carbon nanostructure. UV-Visible spectroscopic measurements on carbon particulate matter (suspended in suitable solvents after filtration in sampling line or deposition on plates) 5,6 could allow for a deeper analysis of optical properties of soot by applying further chemical and spectroscopic tools on the carbon material sampled. However, in comparison to in-situ measurements, the intrusiveness of sampling methods (deposition or filtration) may affect carbon properties as ex situ spectroscopic measurements are performed at conditions far away from the real flame conditions. To this regard, the influence of sampling and analytical measurement conditions could be responsible for the very low band gap measured on spectra of soot sampled after inception 7 . By comparing results from in-situ and ex-situ techniques some light might be shed on this inference. Within this framework, the present study reports the crosscheck of optical band gaps evaluated from in-situ and ex-situ spectroscopic measurements performed in a fuel-rich premixed ethylene/air flame. The principal aim is to extract the optical band gap for organic carbon and soot separately, and verifying the usefulness of the optical band gap approach for investigating the change of soot properties during soot formation