Mechanical Properties of Carbon Nanotubes Bundles

Molecular dynamics (MD) simulations were used to calculate the tensile and compressive properties of carbon nanotube (CNT) bundles, with the atomic interactions modeled by the short-range Brenner potential coupled with the long-range van der Waals potential.

Fig. 1 shows the plot of strain energy, which is determined as the difference in total energy of the strained and unstrained single-walled carbon nanotube (SWCNT) in a bundle, as a function of strain. The solid dots were calculated with the short-range Brenner's "second generation" REBO potential energy coupled with the intratube long-range van der Waals interactions, and the solid curve was obtained only with the short-range Brenner's "second generation" REBO potential energy. From the force and strain energy evaluations, it is obvious that there was no significant difference whether or not the intratube long-range van der Waals interactions is included in the calculations. As suggested by other research work, the SWCNT should fail at around 12 ps, which translates to a force of about 175 nN. In our work, as shown in Fig. 1, the SWCNT failed at 154 nN, which is close to the 175 nN predicted by other research work, at strain=0.23, which falls in the widely reported ability of SWCNTs to sustain elongations as large as 20–30% without breaking. Hence, through the results shown in the MD simulations, we conclude that the intratube van der Waals interactions are not crucial for single SWCNTs.

During tensile loading, the CNT bundles undergo structural deformation as shown in Fig. 2. Fig. 2(a) shows that there was no structural deformation in both CNT bundles of three and seven (10,10) SWCNTs at strain=0.0. When the CNT bundles are further subjected to tensile force as seen in Fig. 4(b), and at strain=0.130, the CNT bundles tend to shrink their overall diameter along their lengths. At strain=0.266, both CNT bundles begin to fracture before they completely disintegrate.

When compressed, the CNT bundles will undergo structural deformation. Fig. 3 shows the morphological changes at various stages of the compression process of CNT bundles of three and seven (10,10) SWCNTs. Fig. 3(a) shows the CNT bundles at strain e = 0.0, where there is no deformation. Further compression until strain=0.04435 will cause the CNT bundle of three to spontaneously collapse into a three-fin pattern, while the CNT bundle of seven is bent slightly outwards from the core SWCNT. At strain=0.05010, the CNT bundle of three continues to deform. However, the CNT bundle of seven begins to buckle forming three-fin patterns in each of its SWCNTs. Finally at strain 0.1536, the CNT bundle of three buckles sideways at the same direction, while the CNT bundle of seven buckles outwards from its core SWCNT forming a 'birdcage' pattern.

When a twisted CNT bundle is under axial compression, the six surrounding SWCNTs will expand and contract radially around the midlength along the center axis of the CNT bundle. Figure 4 shows the morphological changes of a 60°-twisted CNT bundle at different deformations. In the initial stage when there is no deformation, the whole CNT bundle is relaxed, and the helical angle that is formed due to the twisting of the six surrounding SWCNTs is about 77°. Upon further compression, up to when the deformation=0.048 [see Fig. 4(b)], the six surrounding SWCNTs buckle outwards from the center axis fractionally earlier and form a 'bird-cage' pattern, while the core SWCNT remains unbuckled. The fully buckled CNT at deformation=0.06 is shown in Fig. 4(c).

In contrast to the theory of wire ropes in a continuum domain, twisted CNT bundles do not exhibit better tensile properties. When the twisting angle is greater than 75°, the failure load decreases sharply. An investigation into the cause of this was carried out, and it was found that when a CNT bundle with a twisting angle that is greater than 75° is subjected to axial tension, the intertube distance between the SWCNTs decreases to less than 0.2 nm. This results in an increase in the repulsive energy between the SWCNTs since the contact distance is less than 0.34 nm. Consequently, these CNT bundles tend to fail at the junctions at lower loads and strains. This is in qualitative agreement with previous work, in which it was found that the dangling C-C bonds that were formed between SWCNTs and which were brought close together were not strong enough to withstand bending and axial forces.

More information about this research work can be found in the following publications:

• K. M. Liew, C. H. Wong, M. J. Tan, and P. D. Chuang, “Non-twisted and twisted CNT bundles under axial tensile and compressive loads,” Solid State Phenomena 121-123, pp. 1415, 2007.
• K. M. Liew, C. H. Wong, and M. J. Tan, “Twisted carbon nanotube bundles under axial compression and tension,” Journal of Applied Physics 99, pp. 114312, 2006.
• K. M. Liew, C. H. Wong, and M. J. Tan, “Tensile and compressive properties of carbon nanotube bundles,” Acta Materialia 54, pp. 225, 2006.
• K. M. Liew, C. H. Wong, and M. J. Tan, “Buckling properties of carbon nanotube bundles,” Applied Physics Letters 87, pp. 041901, 2005.