Flame temperature effect on sp2 bonds on nascent carbon nanoparticles formed in premixed flames (Tf,max > 2100 K): A Raman spectroscopy and particle mobility sizing study

The evolution in carbon particle size and carbon bonds was observed with increasing flame temperature for a fixed growth time (t(p) similar to 13 ms) and equivalence ratio (Phi = 2.5) in set of sooting premixed stagnation flames. For carbon formed in each flame, detailed particle size distribution functions (PSDF) and Raman spectra (excitation energy of 2.33 eV) were measured as the maximum flame temperature increased from 1911 K < T-f,T-max < 2263 K. The PSDF steadily decreased in size and narrowed in width as the flame temperature increased, a trend which reversibility in precursors is expected to cause for carbon particles formed at elevated temperatures. Several features of the Raman spectra were used to analyze carbon bonds on the flame-formed carbon with increasing flame temperature. Typical features of Raman spectra corresponding to soot were observed for carbon products formed with T-f,T-max = 1911 K. The widths of the overall G (sp(2) in ideal graphite) and D (sp(2) defects) Raman bands narrowed significantly in spectra from elevated temperature flame carbon. The intensity of the amorphous band (D3) relative the G band decreased. For carbon products of the lower temperature flames, a relatively wide band in the vicinity of 1600 cm(-1) encompassing the G band was observed. As the flame temperature increased, two separate peaks were observed in this region; the G band and a separate band in the vicinity of 1620 cm(-1) The characteristic distance between defects was estimated to grow from 1 nm to 2 nm for carbon products formed at T-f,T-max = 1911 K and 2260 K, respectively. The corresponding surface area observed at these conditions was on the order of 600 nm(2) and 200 nm(2) per particle, respectively, which indicates the relative area of ordered structures on the particle increases. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.