Page 120 - Carbon Nanotube Fibres and Yarns

112 Carbon Nanotube Fibers and Yarns

bundles. Due to the volume shrinkage after the impurity removal, the CNT
fibers were strongly condensed inward, leading to a slightly wrinkled sur-
face morphology as shown in Fig. 6.3D.
The as-spun CNT fibers had an average tensile strength and Young’s
modulus of 0.32 and 8.41 GPa, respectively (Fig. 6.3E and F). Although the
CNTs may be damaged due to defect introduction during the purification
process, the significant reduction of the CNT fiber diameter after purifica-
tion contributed positively to the properties of the fibers, resulting in their
improved mechanical and electrical performance. Since the cross-sectional
area was reducted by a factor of 1.5 after purification, both mechanical and
electrical properties of the CNT fiber are expected to change by the same
ratio. In fact, the strength and Young’s modulus of the purified CNT fibers
were 0.38 and 20.66 GPa, corresponding to 119% and 246% of those of
the as-spun CNT fibers, respectively. However, the maximum tensile load
before fiber breakage decreased slightly from 1.6 to 1.2 cN due to the re-
duction of fiber diameter.
More importantly, the CNT fibers exhibited a nearly two-fold increase
in electrical conductivity after the purification treatment, from about 2400
to 4600 S/cm. The impressive enhancements in the properties of CNT fi-
bers might be explained by the improved inter-tube interactions and in-
creased CNT contact areas from the denser and cleaner structure of the
CNT fibers after the purification [30]. Therefore, effective purification of
the CNT fibers can result in significant improvements in the electrical and
mechanical properties. These findings are in good agreement with the im-
provements of morphology and properties of CNT films and twisted CNT
fibers purified by air oxidation methods [43, 51].
The as-spun CNT fibers produced by the floating catalyst method were
immersed in concentrated HNO 3 for purification. Fig. 6.4 shows SEM
images, diameters, and mechanical properties of the CNT fibers before and
after the acidization. As can be seen, the diameter of the acidized CNT fi-
bers became slightly smaller than that of the as-spun fibers (Fig. 6.4B). This
diameter reduction was due to the densification effects of the acid treatment
on the CNT fibers [50, 52]. Additionally, the CNT bundle size increased
and the inter-tube space reduced after the treatment, suggesting that the
acidized CNT fibers possessed slightly denser structures and better CNT
alignment than those of the as-spun CNT fibers.
Fig. 6.4D presents the tensile strength and Young’s modulus of the CNT
fibers at different acidization times. Acidization could considerably im-
prove the mechanical performance of the CNT fibers. The as-spun CNT