Technical Report 205, c4e-Preprint Series, Cambridge

Authors: Laura Pascazio, Jacob W. Martin, Maria Botero, Mariano Sirignano, Andrea D'Anna, and Markus Kraft*

Reference: Technical Report 205, c4e-Preprint Series, Cambridge, 2018

Associated Themes:
 

• Mechanical properties of crosslinked polycyclic aromatic hydrocarbons (PAHs) are investigated, making use of reactive molecular dynamics simulations.
• The degree of crosslinking of ethylene and diesel soot is between 2.25-2.75 and 2.85-3.15 respectively to have a comparable value of the hardness found experimentally.
• Electron microscopy image simulations demonstrate that kinked fringes in high resolution transmission electron microscopy (HRTEM) images of soot primary particles can be interpreted as crosslinked PAHs.
• HRTEM experiments of soot primary particles with radial fringe statistics give further evidence of the presence of a 3D network of crosslinked PAHs in soot.

In this paper, we estimate the degree of crosslinking within soot particles and carbon blacks making use of reactive force field molecular dynamics simulations of the mechanical properties of crosslinked polycyclic aromatic hydrocarbons (PAHs). Representative systems of PAHs with a density similar to nascent soot and with varying degrees of crosslinking were built. The yield stress and hardness of each sample were calculated and the results compared with nanoindentation experiments of soot particles. The results indicate that the ethylene and diesel soot degree of crosslinking is between 2.25-2.75 and 2.85-3.15 respectively. Further evidence of a three-dimensional network of crosslinks between PAHs in soot structure has been found from electron microscopy experiments. Electron microscopy image simulations demonstrate that kinked fringes observed in the electron micrographs can be interpreted as crosslinked PAHs. High resolution transmission electron micrographs of soot primary particles with fringe statistics were also analysed as a function of the radius for the first time, giving quantitative information about the soot core-shell structure and soot maturation. All the results indicate that a crosslinking process with subsequent carbonisation is taking place during soot formation, giving rise to a structure in which the majority of aromatics are aliphatically-linked with some ring condensations.

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