Graphitization of single-wall nanotube bundles at extreme conditions: Collapse or coalescence route

We determine the reaction phase diagram and the transformation mechanism of (5,5) and (10,10) single-walled carbon nanotube bundles up to 20 GPa and 4000 K. We use Monte Carlo simulations, based on the state-of-the-art reactive potential LCBOPII, that incorporates both covalent and van der Waals interactions among the tubes. At low temperature, upon increasing pressure, large (10,10) nanotubes first collapse and then coalesce, yielding almost perfect graphitic structures. In contrast, small (5,5) nanotubes do not collapse, but coalesce and transform to graphite via a mixed graphite-tube structure. At high temperature (above $\sim 2000$ K), for both (10,10) and (5,5) nanotubes, coalescence dominates the transformation to graphitic structures. We argue that the $s{p}^3$-interlinking defects appearing at coalescence can act as seed and facilitate the transformation to diamond structures.

Figure 1

Lower panel: volume per particle for the (5,5) and (10,10) SWCNT bundles at $T=300$ K, indicated by (blue) circles and (red) diamonds, respectively. Solid and dashed lines are a guide to the eye. Statistical errors are comparable to the size of the symbols. Upper panel: typical configurations for the indicated pressures show the change in shape of the cross section of the SWCNT. For clarity, the periodic cell is replicated twice in both the $x$ and $y$ direction, orthogonal to the tube axis.

Figure 2

Volume per particle of SWCNT bundles as a function of pressure for several temperatures, compared to the LCBOPII equaton of state for graphite.[30] Upper panel: (5,5) SWCNT. Lower panel: (10,10) SWCNT.

Figure 3

Reaction diagram for the (5,5) and (10,10) bundles, indicated by (blue) squares and (red) circles, respectively. The stars indicated state points where only partial graphitization ($T=1500$, 1000 K) or interlinking ($T=500$ K) is observed. Solid lines are guides to the eye. The graphite coexistence lines with graphite and diamond are indicated with (dark green) triangles connected by dashed lines. All results were obtained with the LCBOPII potential, except the graphite-diamond coexistence that has been obtained with LCBOPI${}^{+}$ by Ghiringhelli et al.[31]

Figure 4

Graphitization path of the (10-10) SWCNT bundle at $T=1500$ and $p=3$ GPa. Upper panel: a series of typical snapshots of the transformation process is reported. Lower panel: the (blue) dotted line is the volume per particle during the simulations (left $y$ axis), while the red solid line represents the fraction of fourfold ($s{p}^3$) coordinated atoms in the sample (right $y$ axes).

Figure 5

Graphitization path of the (10-10) SWCNT bundle at $T=3000$ and $p=2$ GPa. Upper panel: a series of typical snapshots of the transformation process is reported. Lower panel: the (blue) dotted line is the volume per particle during the simulations (left $y$ axis), while the red solid line represents the fraction of fourfold ($s{p}^3$) coordinated atoms in the sample (right $y$ axes).

Figure 6

Top row: SWCTN (5-5) transforming to a jammed graphitic structure, $T=1500$ K, and $p=2.0$ GPa. Middle row: SWCTN (5-5) transforming to graphite, $T=2000$ K, and $p=1.5$ GPa. Bottom row: SWCTN (5-5) transforming to (defected) graphite, $T=3000$ K, and $p=1.5$ GPa.