Toward Ultralight High Strength Structural Materials via Collapsed Carbon Nanotube Bonding

Journal pre-proof

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Summary¶

LAMMPS ReaxFF (C-2013) simulations pressurize parallel and perpendicular arrangements of double-walled CNT stacks (~120k atoms, ~8 nm inner diameter, mixed armchair/zigzag chirality) to induce covalent bonding across collapsed tubes—mirroring spark plasma sintering experiments on CNT sheets. Pressure–structure snapshots inform processing conditions for ultralight high-strength solids. The NASA/aerospace motivation is structural nanocarbon preforms where vdW cohesion alone limits strength; pressure-induced crosslinking is explored as a processing route analogous to SPS densification observed in buckypaper-like samples.

Methods¶

Force field (A)¶

ReaxFF C-2013 parameterization for carbon condensed-phase chemistry (LAMMPS implementation); used to capture pressure-induced C–C bond formation between CNT walls without a predefined bond topology.

Reactive MD and processing mimic (B)¶

Engine / analysis: LAMMPS; OVITO for visualization; coordination / diamond identification with ~0.18 nm cutoff.

Systems: Periodic cells with parallel bundles (4 tubes) vs perpendicular junction (2 tubes); double-walled stack geometry and dimensions as in Carbon Methods.

Thermodynamics / loading: Isothermal pressure ramps at 300 K, 1000 K, 2000 K, 3000 K (default 50 GPa/ns; portions at 500 GPa/ns where response is flat). Ensemble: NPT-style anisotropic stress control: Langevin thermostat with Nose–Hoover anisotropic barostat; parallel geometry fixes axial stress 0 and loads transverse; perpendicular geometry loads normal to junction (overlap-area correction ~17% as stated).

Integration: 0.2 fs timestep (some high-T repeats at 0.1 fs).

Post-processing: snapshots each 5 GPa; optional quench (10 ps to 300 K, hold 10 ps) to interrogate bonding snapshots.

Electric field: N/A — not applied. Replica / enhanced sampling: N/A — not used (direct MD only).

DFT (C)¶

Not reported as primary—ReaxFF drives the large-scale welding study.

Findings¶

Mechanisms¶

Reactive C–C crosslinking under pressure yields load-bearing networks beyond vdW sheet cohesion; parallel collapse vs perpendicular junction welding produce distinct microstructures. Pre-strained experimental sheets show larger gains after SPS, consistent with improved tube–tube contact.

Processing guidance¶

Simulations bracket P–T windows where bonding activates without excessive damage—useful for textile preform design.

Limitations and future constraints¶

C-2013 omits catalyst chemistry (Fe in experiments); disorder, sp/sp² mixing, oxygen functionality, solvent/binder effects, and non-ideal contacts can shift real yields vs idealized cells.

Limitations¶

ReaxFF C-2013 omits explicit metallic catalyst chemistry (Fe particles in experimental sheets are not fully modeled). Turbostratic disorder, sp/sp² mixing, and oxygen functionalities on real CNT surfaces may shift crosslink yields relative to the idealized bundles simulated. Residual solvent and polymer binders in experimental preforms can change effective contact area and local heating during SPS compared to pristine simulation cells.

Relevance to group¶

Demonstrates large-scale ReaxFF CNT mechanochemistry tied to processing of nanocarbon preforms.

Citations and evidence anchors¶

papers/ReaxFF_others/Jensen_Carbon_2019_CNT_pressure_preprint.pdf

The preprint filename in pdf_path signals Elsevier pre-proof status—confirm pagination against the final Carbon issue when citing figure numbers.