Growth of hexagonal boron nitride from molten nickel solutions: a reactive molecular dynamics study
Summary¶
ReaxFF molecular dynamics in LAMMPS (GPU-accelerated) is used to simulate hBN nucleation and growth from liquid nickel solvent exposed to N\(_2\) gas, over ~20–30 ns trajectories and complementary temperature replica exchange equilibration of Ni–B melts. The work argues that chemistry localizes at the metal–gas interface, progresses through N–N–B and B–N–B intermediates, and that boron concentration and temperature strongly modulate hBN yield on accessible time scales.
Methods¶
- Engine and parameters: LAMMPS molecular dynamics with reactive ReaxFF for B/N/Ni (parameters aligned with the authors’ prior work; parameter files and Zenodo bundle referenced in the article). 0.25 fs integration timestep. NVT with Langevin thermostat (200 ps damping) for most constant-volume runs; NVE for diffusion estimates. NPT (later section) uses Nosé–Hoover barostat/thermostat as described (Shinoda-style barostat in z only for some cells).
- Nickel slab / melt: fcc Ni slab (12×12 close-packed planes, 1520 atoms), 3D PBC periodic 27.89 × 32.20 × 220 Å\(^3\), Ni(111) surfaces, melted at 1800 K in NVT to obtain a liquid slab.
- Boron loading: A β-rhombohedral boron nanocrystal (208 B) dissolves into molten Ni at 1900 K (13 ns segment), then temperature replica exchange (20 replicas, 1800–2700 K) equilibrates Ni–B melts, yielding among others a ~12 at.% B solution (2.5 wt%) used as a starting point for growth studies.
- N\(_2\) + Ni: Pure liquid Ni slabs with 60 / 120 / 240 N\(_2\) molecules at 84 / 168 / 336 atm effective pressures (~200 Å z extent), 5 ns runs to survey N\(_2\) interaction with molten nickel.
- hBN growth runs: From equilibrated Ni–B melts, 88 N\(_2\) molecules are placed >12 Å from the surface; simulations 20–30 ns at 1750–2700 K in the fixed 27.89 × 32.20 × 220 Å\(^3\) cell.
- Constant-pressure variants: 104 B atoms, 208 N\(_2\), 1750 K, 15 ns, 25 / 50 / 100 atm targets with z-only barostat and large N\(_2\) reservoirs (initial z chosen from ideal gas law).
Slot summary: N/A — electric field; N/A — umbrella sampling; replica exchange (temperature) used for Ni–B melt equilibration as above.
Findings¶
- Reaction localization: Conversion to BN occurs primarily at the liquid Ni surface because N solubility in bulk Ni is low and intermediates favor the interface.
- Mechanism: N\(_2\) attacks Ni-solvated B, forming N–N–B species, then B–N–B motifs; species ripen between hBN crystallites (Ostwald-like transport of nitrogen).
- Processing trends: hBN production on 10² ns scales is strongly sensitive to B concentration and mildly sensitive to N\(_2\) pressure in the 2.5–10 MPa-class regime explored for some setups (6–12 at.% B family in the abstract framing). 1750 K (just above Ni melting) gives the highest growth rate in the studied window; ≥2000 K yields no extended hBN sheets within the simulated trajectories.
- Kinetics: Path sampling indicates incorporation of small B–N motifs into large sheets is often rate-limiting; at >1900 K, sheet breakup competes with growth, explaining loss of large sheets despite persistent intermediates.
Limitations¶
- Force-field fidelity for molten alloy–gas chemistry and long-range transport is finite; experimental metal-flux reactors include impurities and convective mixing not modeled here.
Relevance to group¶
Complements other hBN and ReaxFF corpus entries by focusing on flux growth from molten Ni with explicit N\(_2\) gas environments.