Coalescence, melting, and mechanical characteristics of carbon nanotube junctions

This study employs classical molecular dynamics (MD) simulations to investigate the formation of single-walled carbon nanotube (SWCNT) T junctions via the melting and coalescence of two individual nanotubes. The simulations focus primarily on the synthesized multiterminals of (5, 5)-(9, 0)-(5, 5) and (5, 5)-(5, 5)-(5, 5) T junctions since these particular T junctions represent two extreme cases. The numerical results indicate that most of the cap-to-wall coalescence pathways identified for the nanotubes consist exclusively of Stone-Wales bond rotations. The thermal stability and melting behavior of the two T junctions are studied. It is found that for thermal treatment at high temperature, the (5, 5)-(9, 0)-(5, 5) T junction is more thermally stable than its (5, 5)-(5, 5)-(5, 5) counterpart since its structural dislocations and topological defects accelerate the onset of melting. The effects of the nanotube diameter and chirality on the mechanical responses of the T junctions under tensile and bending loads are also studied. The bending tests reveal an unexpected nanoplasticity mechanism in the T junction subjected to large bending deformation. This nanoplasticity effect causes the bonding geometry to transform from a graphitic $\left(s{p}^2\right)$ structure to a localized diamondlike $\left(s{p}^3\right)$ structure.

Figure 1

(Color online) Schematic of MD simulation model for cap-to-wall coalescence. (Note that $D_{\mathit{ave}}$ is the separation distance between the centers of mass of the designated nanotubes.)

Figure 2

Internal energy of (5,5)-(9,0)-(5,5) and (5,5)-(5,5)-(5,5) T junctions as a function of various temperatures between 300 and $4000\mathrm{K}$. [Labels (A)–(E) and (a)–(e) denote the different thermal structures of the (6,6)-(9,0)-(6,6) and (6,6)-(6,6)-(6,6) T junctions, respectively, during coalescence and melting. Note that these labels correspond to the snapshots presented in Fig. 3.]

Figure 3

(Color online) Typical snapshots of (5,5)-(9,0)-(5,5) and (5,5)-(5,5)-(5,5) T junctions during coalescence and melting. (Note that the brighter color denotes Stone-Wales bond rotation.)

Figure 4

(Color online) Variation of average distance $\left(D_{\mathrm{av}}\right)$ at various temperatures between 300 and $4000\mathrm{K}$ for (5,5)-(5,5)-(5,5) and (5,5)-(9,0)-(5,5) T junctions. Note that $D_{\mathit{ave}}$ is defined as the separation distance between the centers of mass of the designated nanotubes. [Legends (I)–(III) and (i)–(iii) denote the different structural transformation stages leading up to the cap-to-wall coalescence of the (5,5)-(5,5)-(5,5) and (5,5)-(9,0)-(5,5) T junctions, respectively.]

Figure 5

(Color online) Variation of formation energy of Stone-Wales rotation with SW sequence for (5,5)-(5,5)-(5,5) and (5,5)-(9,0)-(5,5) T junctions, respectively.

Figure 6

Variation in coalescence temperature of cap-to-wall coalescence with chiral angle for families of $(n,n)$ armchair and virtually identical radius $(n,m)$ chiral nanotubes.

Figure 7

(Color online) (a) Schematic of MD simulation model for T junctions under tensile deformation. $L_0$ denotes the initial equilibrium length of the cantilevered CNT and $\Delta l$ is the relative elongation. The two ends of the upright nanotube (labeled as $A$) are assumed to be fixed, while the end of the cantilevered specimen (labeled as $B$) is in free motion under the effect of a tension loading applied in the $Y$ direction. (b) Strain energy of various T junctions as a function of the axial tension.

Figure 8

(Color online) (a) Schematic of MD simulation model for silicon atomic force microscope probe and T junction under bending deformation. $L_0$ denotes the initial equilibrium length of the cantilevered CNT, $D$ is the downward displacement of the end of the cantilevered CNT, and ${\theta }_c$ is the tip deformation angle. (b) Strain energy of various T junctions as a function of the bending deformation.