Page 112 - Carbon Nanotube Fibres and Yarns

104 Carbon Nanotube Fibers and Yarns

significantly. This chapter reviews the effects of several posttreatments on
the mechanical performance of the CNT fibers.

6.1 Twist insertion

The first CNT fiber was successfully prepared through spinning a CNT ho-
mogeneous dispersion into a polyvinyl alcohol (PVA) coagulation bath [32].
This approach was modified by Baughman’s group to make single-walled
CNT composite fibers with very high strength [33, 34]. The major issues
with this approach include a relatively high fraction of remaining polymer
volume and short individual CNTs, which limits the fiber strength, and
electrical and thermal conductivity [35].
The load transfer between CNTs in a fiber depends on both
the contact areas and the inter-tube spaces between the CNTs [17].
Twisting is an effective posttreatment to densify the CNT fibers by
reducing the inter-tube spaces. At high twist angle, the CNTs in the
fibers are in closer contact with each other, enhancing the van der
Waals force and friction between the bundles and, hence, increasing the
fiber strength. Zhang et al. [20] investigated the effect of fiber twist on
the mechanical properties of the CNT fibers spun from 650-μm-long
arrays. CNT fibers were spun from a CNT array using a spindle made
of a microprobe. In the post-spin twisting process, a proper weight was
hung on one end of a fiber to provide tension in the axial direction,
while the other end of the fiber was attached to a rotator. The extent of
post-spin twisting depends on the twisting speed and duration. In their
study, a CNT fiber 5 cm long was typically twisted at a rotation rate
of 500 rpm for 2 min, which gave 20,000 turns/m of twist. The tensile
strength of the CNT fibers significantly increased from 0.85 to 1.9 GPa
after post-spin twisting. Moreover, the reduction in the diameter of the
fibers from 4 to 3 μm after twisting evidenced the densification effect
of the treatment.
However, research conducted by Zhao et al. showed that twist also
had a negative contribution to the fiber strength if fiber was over-twisted
[3, 21]. This could be explained by the fact that at a higher twisting angle,
the fibers are more misaligned with the fiber axis, therefore lowering the
fiber strength, although twisting reduces the inter-tube spaces and mini-
mizes the contact resistance between the CNTs [15, 18]. Miao reported
that the CNT fibers with higher density (i.e., lower fiber porosity) could
be achieved by increasing their twisting angle [18]. Interestingly, he
found that the electrical conductivity of the CNT fibers decreased with