Mapping the structural–mechanical landscape of amorphous carbon with ReaxFF molecular dynamics
Corpus note
The primary PDF is an AIP author-query / proof galley (Dernov_Kowalik_amorphous_carbon_JAP2025_galley.pdf); pagination in the extract may read 000000 pending final issue assignment.
Summary¶
The paper uses ReaxFF molecular dynamics with C-2013 and CHON-2019 parameter sets to generate liquid-quenched models of amorphous carbon (a-C) spanning 0.96–3.29 g cm\(^{-3}\), then performs uniaxial tensile simulations to relate ring statistics and hybridization to elasticity, plasticity, and fracture, including oxidative scenarios with added O\(_2\) and selected nanostructures (slabs, a-CNTs, partially amorphous CNTs).
Methods¶
- Force fields: ReaxFF C-2013 and CHON-2019 (same carbon parameters; differs in angle/dihedral terms; CHON-2019 used where oxygen chemistry in fractured gaps is relevant).
- a-C generation (liquid quench): 8000 C atoms in an orthorhombic 3D PBC periodic cell; target density via box volume; heat 300→3000 K at 200 K ps\(^{-1}\); hold 3000 K for 0.5–1 ns; cool to 300 K at 5 K ps\(^{-1}\) under NVT with Berendsen thermostat (100 fs damping); Δt = 0.1 fs during melt/quench. Post-quench NPT equilibration at 300 K, 1 atm isotropic pressure, Δt = 0.25 fs, Berendsen barostat (100 fs damping). Ensembles of three lowest-energy replicas per target density were analyzed.
- Derived structures: Slabs, rolled a-C nanotubes, and a partially amorphous tube on a crystalline double-walled template built from sliced/rolled a-C models with additional heating/annealing as described.
- Mechanical loading: Room-temperature, 1 bar, NPT integration (Martyna–Tobias–Klein), uniaxial tensile strain typically 0.5% ps\(^{-1}\). Selected runs paused to insert 100 O\(_2\) molecules in opened cracks, then resumed. AMS and LAMMPS used for MD; ISAACS for structural analysis; ring detection with King shortest-path rules (≤36 rings, neighbor cutoff 1.9 Å). N/A — electric field; N/A — umbrella / metadynamics (tensile MD is the primary rare-event tool here).
Findings¶
- Structure: Low-density a-C shows bimodal ring statistics (small 6-rings plus medium ~12-rings); ta-C at higher density shows unimodal small-ring dominance; g® and angle distributions shift from graphite-like toward diamond-like as density increases.
- Elasticity: Reported Young’s moduli rise strongly with density (e.g., a-C ~34–156 GPa and ta-C up to ~535 GPa depending on model/density in the article’s Fig. 3). Elastic isotropy in bulk responses is emphasized.
- Plasticity and fracture: Post-yield behavior is tied to ring-size expansion and C–C bond reactivity; ta-C shows much larger plastic strain before failure, associated with sp\(^3\)→sp\(^2\) conversion during loading. Ultimate failure involves reactive cracks with long sp chains; O\(_2\) in the crack reduces stress-bearing capacity by attacking chains (Fig. 7 scenario).
- Nanostructures: a-CNT fracture is circumferential; pa-CNT fails from the outer a-C wall inward, while crystalline walls exhibit helical cracking patterns as described in the text and figures.
Limitations¶
ReaxFF C/H/O models may miss some defect and oxidation channels; quench protocol sets a-C topology; proof/galley PDF may differ from final pagination. Tensile rates and O\(_2\) insertion are idealized crack scenarios.
Relevance to group¶
Dernov, Kowalik, van Duin, Dumitrică: a-C structure–mechanics and oxidative fracture with ReaxFF—relevant to carbon tribology and mechanics threads.