Molecular basis for coke strength: Stacking-fault structure of wrinkled carbon layers

Abstract

A macromolecular model with formula C60468H2193O527N468S49 was constructed for Pingyao coke by using microstructural characteristics and theoretical calculations. The coke model geometry was obtained by simulated annealing algorithm in molecular dynamics simulations with reactive force field (ReaxFF). ReaxFF parameters were modified iteratively until the model geometry agreed with X-ray photoelectron spectroscopy and X-ray diffraction data. The model indicates that the carbon matrix in coke is composed of wrinkled carbon layers, which agrees with reported high-resolution transmission electron microscope results. Neighboring carbon layers are not synclastic and have some stacking faults (such as arch-shaped moieties and screw dislocation), which result in increased layer spacing or non-parallel layers. To investigate the molecular basis for coke strength, ReaxFF simulations of the coke compression process were performed at 300 K and 3500 K, respectively. Results showed that the stacking-fault structure of wrinkled carbon layers was the main microscopic cause of coke strength. Wrinkled carbon layers can disperse the external force, and stacking-fault moieties restrained the interface slippage of carbon layers. Coke structural changes that were caused by the external force presented different characteristics at different temperatures. High temperatures aid the coke matrix reconstruction and prompt the formation of large and planar carbon layers.