Page 202 - Carbon Nanotube Fibres and Yarns

192 Carbon Nanotube Fibers and Yarns

multiwalled yarns produced by Rice University (Rice yarns). Besides the
accurate prediction of yarn strength, the model also revealed the different
dominant failure mechanisms for the two yarns. For the NU yarn, the first
bundle rupture occurred very early, at a stress of 0.2 GPa and the rupture
rate accelerated gradually until the yarn failed, while the rupture in the
Rice yarn occurred much later, at a high stress of 1.0 GPa.
Although these models work well to some extent, it is still difficult to
use them to describe the phenomena taking place at the microscopic and
mesoscopic scales, which are of great importance to reveal the underlying
mechanics for the mechanical properties of CNT yarns.

8.3 Load transfer between nanotubes
Similar to conventional textile yarns, the rich intertube contacts govern
the stiffness and strength of CNT yarns. This is because the intertube slid-
ing determines the tensile behaviors of CNT bundles [92]. Based on MM
approach using Lennard-Jones potential, Qian et al. [93] predicted that a
contact length up to ≈4 μm was required to realize a strength of 11 GPa for
a (10,10) CNT bundle. Suekane et al. [94] measured the static friction force
between the CNTs and found that the friction force increased with increas-
ing overlap length for low-crystallinity CNTs while the static friction force
of highly crystalline CNTs was nearly independent of the overlap length.
Paci et al. [95] performed a detailed density functional theory analysis of the
factors that determine the intertube friction. The intertube shear strength
for pristine CNTs is estimated to be <0.24 MPa, that is, extremely small.
Instead, it is pinning due to the presence of defects and functional groups at
the tube ends that primarily cause resistance to shear. Defects such as holes
not only increase the intertube friction but also weaken the tubes signifi-
cantly. The key factor for producing strong CNT yarns is enhancing the
friction force by designing the intertube contacts or CNT arrangements,
rather than relying on the simple vdW force to bundle them together.
Chemical functionalization can increase tube-tube friction which has a mi-
nor impact on the strengths of the tubes, but push-pull effects can lead to
substantial force cancellation. Covalent cross-links are the most promising
route to the production of strong CNT-based yarns [95].
Flattened CNT surface can enhance the intertube sliding friction.
Zhang et al. [96] considered CNTs with larger diameters, namely a bundle
of (23,0) tubes, (30,0) tubes, and CNTs with random chiralities (RC), and
found that above a certain pressure these tubes could collapse and maintain