The superior properties of carbon fiber (CF) have long been widely recognized. However, industrially, CF production processes are still far from being optimized, leading to the high price of carbon fiber (~ $10 /pound). One obstacle is the lack of linkage between microscale (precursor molecular structure and chemistry; CF microstructure) and macroscale (processing conditions; CF morphology, and mechanical properties). In this work, we attempted to reduce this gap by employing full atomistic simulations based on reactive force field (ReaxFF), where bond energy is related to bond order and further to bond distance to allow bond dissociation and formation without examining subatomic details. We validated the applicability of ReaxFF by investigating the synthesis based on pitch derived small molecules (e.g. naphthalene, and methyl-naphthalene), for which relevant activation energies and reaction pathways are well documented in literature. Based on these benchmarking studies, we focused on the carbonization/graphitization stage of CF synthesis. Different precursor molecules were first aligned with their aromatic core areas parallel with one another, and then carbonized under a range of temperatures. It was found that for all precursors, increasing temperatures increase reaction kinetics, as well as CF yields quantified by the weight percentage of solid product. While the precursor consumption rate can be increased by both the presence of peripheral aliphatic side chains and the inclusion of sp3 carbons in the aromatic cores, the effect of sp3 carbons, presented in the core areas, seem to be more significant, evidenced by the larger number/rate of ring opening reactions observed. In addition, aliphatic side chains can facilitate the generation of methane as the major gas product; on the other hand, ethylene emerged as the most abundant species with the inclusion of sp3 carbons in the core area. During the graphitization stage, stacking of graphene layers requires high pressure to be applied to increase carbon atom density. Besides, the final stacking order of CF (out-of-plane arrangement of graphene layers) was found to be dependent on precursor molecular structure, carbonization temperature, as well as compositions of intermediate products piped into graphitization stage. These preliminary studies demonstrate the potential of ReaxFF based full atomistic simulations in helping with screening precursors/processing conditions during industrial CF productions.

Synthesis of pitch-based carbon fiber: Insights from ReaxFF based molecular dynamics simulations

2019 Spring National Meeting

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