Page 114 - Carbon Nanotube Fibres and Yarns

106 Carbon Nanotube Fibers and Yarns

spinning method that combines twisting and shrinking processes to pro-
duce CNT yarns. In their method, a yarn freshly spun from a super-aligned
CNT array is first twisted and then passes through a volatile solvent for
shrinking. The as-produced yarn consists of densely packed CNTs and
thus has a tensile strength up to about 1 GPa. The tensile strength depends
on the diameter and the twisting angle of the yarn. Different kinds of sol-
vents, such as water, ethanol, and acetone, are used to shrink the twisted
yarns, and acetone shows the best shrinking effect. They compared the
mechanical properties of the CNT fibers spun from vertically aligned
CNT arrays before and after liquid densification treatment. Their stress-
strain curves show that the maximum strain of the fibers was unchanged
while their strength and Young’s modulus enhanced significantly after the
application of liquid densification.
The fiber diameters decreased from 11.5 to 9.7 μm after the treatments,
indicating a denser structure of the fibers obtained [23]. The percentage
of load increase and diameter reduction of the CNT fibers after acetone
shrinking ranged from 15% to 40% and from 15% to 24%, respectively. The
method is suitable for continuous mass production of high strength CNT
yarns with a wide range of diameters, especially ultrathin yarns.

6.3 Coating and doping

The goal of CNT fiber production and treatment is to translate the excel-
lent properties of individual CNTs to large CNT assemblies. Among these,
a macroscopic cable that would replace metals as a universal conductive
wire would have large volume applications in electricity transmission, aero-
space, and automobile industry. Several methods have been developed for
creating multi-walled, double-walled, and single-walled CNT to generate
nanotube fibers with good mechanical properties. The electrical resistivities
−6
−3
reported are over a large range between 7.1 × 10 and 2 × 10 Ω m. The
CNT resistivity values reported up to now are 2–3 orders higher than that
−8
for oxygen-free Cu (1.68 × 10 Ω m), one of the most conducting metals
widely used in current carrying applications.
The electrical conductivity of the CNT fibers can be significantly in-
creased by nanoparticle coating or doping [36]. Randeniya et al. [36] com-
pared the electrical performance of the pure CNT fibers and the fibers
doped with metal nanoparticles, including Cu, Au, Pd, and Pt. They re-
ported that the CNT fibers doped with Cu and Au possessed an electri-
2
cal conductivity of up to 3 × 10 S/cm at room temperature, which was