Page 123 - Carbon Nanotube Fibres and Yarns

Post-spinning treatments to carbon nanotube fibers 115

The purification effect of the acid treatment is consistent with the reduc-
tion of the mass density of the CNT fibers after the treatment. Specifically,
3
the mass density of the 15-min acidized fibers was about 1.53 g/cm , 15%
3
lower than that of the as-spun fibers (1.80 g/cm ). Since the amorphous
carbon impurities are reactive and invariably exist in the as-spun CNT
fibers, the acid treatment may oxidize them to form oxidized debris before
they are removed by subsequent aqueous washing [53, 54]. However, pro-
longed treatment beyond 15 min cannot further improve the mechanical
performance of the CNT fibers, as presented in Fig. 6.4D. This result can
be explained by the fact that the CNT structures are possibly damaged at
the prolonged treatment [50, 55] although denser fiber structure can be
obtained. The competition between improved interfacial shearing strength
and damaged CNT structures would finally determine the mechanical per-
formance of the CNT fibers. Therefore, the acidization parameters, such as
acid concentration and treatment time need to be carefully controlled to
optimize the acidization process.

6.5 Mechanical densification and stretching
Among all the densification methods, mechanical densification is one of the
best approaches to produce highly dense CNT structures since the den-
sifying forces are applied directly to the fibers [26, 28]. Several traditional
textile twistless methods based on mechanical densification have been used
to densify the CNT fibers [25, 26]. Miao [25] used a rubbing roller system
(Fig. 6.6A) to densify the CNT web drawn from a vertically aligned CNT
array into compact twistless fibers. The resulting densified fibers consisted
of a high packing density sheath with CNTs lying straight and parallel to
the fiber axis and a low density core with microscopic voids. Due to the
improved contact length between CNTs in the dense sheath, the core-
sheath structured, twistless CNT fibers exhibited high specific modulus up
to 59 N/tex and specific strength up to 75 cN/tex.
Wang et al. [26] used a pressurized rolling system to densify the CNT
fibers spun by the floating catalyst method to a high density structure
(Fig. 6.6B). With a densification factor up to 10, their densified fibers showed
significant improvement in strength from 0.3 to 4.3 GPa. This result was the
highest strength reported for the CNT fibers at the gage length of 10 mm
in literature while their Young’s modulus remained at 90 GPa, as shown in
Fig. 6.6C. This impressive enhancement was due to improved load transfer
between the nanotubes and bundles after the treatments. Badaire et al. [14]