Page 138 - Carbon Nanotube Fibres and Yarns

130 Carbon Nanotube Fibers and Yarns

electrical conductivity of 0.38 GPa, 20.66 GPa, and 4679 S/cm, respectively,
corresponding to 120%, 250%, and 200% of their as-spun fibers.
The hybrid treatment consisting of acidization and epoxy infiltration
treatments, on the other hand, showed a much higher improvement factor
in mechanical properties of the CNT fibers. After the treatment, the CNT
fibers exhibited a 2.8 times increase in strength and 4.2 times increase in
Young’s modulus, reaching 1.1 GPa and 68.78 GPa, respectively. The CNT
fibers treated by the hybrid treatment of mechanical densification and epoxy
infiltration possessed excellent strength of 3.6 GPa and Young’s modulus of
266 GPa, representing enhancement factors for strength and Young’s mod-
ulus of 13.5 times and 62 times, which are the highest reported in the lit-
erature [28]. Their performance was comparable to many commercial high
strength fibers, such as PAN-based carbon fibers, suggesting great potentials
for the mass production of high-performance CNT fibers.
Although the experimental setups for the hybrid-posttreatments can
be only used to produce short CNT fibers, the posttreatments can be
scaled up to synchronize with the synthesis process for online fabrica-
tion of high-performance posttreated CNT fibers with unlimited length.
Only CNT fibers spun from methane source were treated with the hybrid
posttreatments so far. Fibers spun from toluene source and their puri-
fied counterparts have lower impurities and less defective structures. The
potential effects of the posttreatments such as acidization, mechanical
densification, and epoxy infiltration on their performance need to be in-
vestigated in the future.
Although the oxidative purification could significantly remove impuri-
ties in the CNT fibers spun from the floating catalyst method, the process
might damage the CNTs by introducing defects. These defects were unde-
sirable since they lowered the fiber performance and, therefore, limited the
efficiency of the purification method. Physical methods, such as centrifugation
[70] high-temperature annealing [71], filtration [72], and multistep purifica-
tion methods, such as microfiltration in combination with oxidation [62, 73],
high-temperature annealing in combination with extraction [74], and sonica-
tion in combination with oxidation [75] are known not to severely damage the
CNT structures [42] These methods should be investigated for purification of
the as-spun CNT fibers to improve fiber performance.
Finally, as the performance of the posttreated CNT fibers was com-
parable to many commercial high-strength fibers, such as carbon fibers
T300, Dyneema, and Twaron, they can be used as reinforcement for
advanced composites. Nanotube-based composites fabricated from un-
structured CNT powders have been widely used for a broad range of