4.8 Article

Soot-particle core-shell and fractal structures from small-angle X-ray scattering measurements in a flame

Journal

CARBON
Volume 196, Issue -, Pages 440-456

Publisher

PERGAMON-ELSEVIER SCIENCE LTD
DOI: 10.1016/j.carbon.2022.05.009

Keywords

SAXS; Soot; Surface-fractal; Mass-fractal; Core-shell; Flame

Funding

  1. DOE under a sub-contract from Sandia National Laboratories [FWP 21-022187]
  2. College of Engineering and Applied Sciences at the University of Colorado Boulder
  3. Sandia Laboratory Directed Research and Development (LDRD) Program
  4. Lawrence Livermore National Laboratory (LLNL) [DE-AC52-07NA27344]
  5. LDRD Program at LLNL [14-ERD-067]
  6. DOE Office of Science User Facility [DE-AC02-05CH11231]
  7. Division of Chemical Sciences, Geosciences, and Biosciences, the Office of Basic Energy Sciences, the U.S. Department of Energy (DOE)
  8. DOE's National Nuclear Security Administration [DE-NA0003525]

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In this study, soot particles in a laminar co-flow ethylene-air diffusion flame were characterized using small-angle X-ray scattering (SAXS). The results showed that the soot particles in the middle of the flame have an established core-shell structure, while the particles higher and lower in the flame have smoother surfaces. SAXS measurements were more sensitive than laser-induced incandescence (LII) in detecting the incipient and young soot particles, especially in the low flame regions where the LII signal was negligible.
We have characterized soot particles measured in situ in a laminar co-flow ethylene-air diffusion flame using small-angle X-ray scattering (SAXS). The analysis includes temperature measurements made with coherent antiStokes Raman spectroscopy (CARS) and complements soot volume-fraction and maturity measurements made with laser-induced incandescence (LII). We compared the results of fits to the SAXS measurements using a unified model and a fractal core-shell model. Power-law parameters yielded by the unified model indicate that aggregates of primary particles are in the mass-fractal regime, whereas the primary particles are in the surface-fractal regime in the middle of the flame. Higher and lower in the flame, the primary-particle power-law parameter approaches 4, suggesting smooth primary particles. These trends are consistent with fits using the fractal coreshell model, which indicate that particles have an established core-shell structure in the middle of the flame and are internally homogeneous at higher and lower heights in the flame. Primary-particle size distributions derived using the fractal core-shell model demonstrate excellent agreement with distributions inferred from transmission electron microscopy (TEM) images in the middle of the flame. Higher in the flame, a second small mode appears in the size distributions, suggesting particle fragmentation during oxidation. Surface oxidation would explain (1) aggregate fragmentation and (2) loss of core-shell structure leading to smoother primary-particle surfaces by removal of carbon overlayers. SAXS measurements are much more sensitive to incipient and young soot particles than LII and demonstrate significant volume fraction from particles low in the flame where the LII signal is negligible.

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