Page 61 - Carbon Nanotube Fibres and Yarns

Carbon nanotube fibers spun directly from furnace 53

By using a roller stretcher with N-methyl-2-pyrrolidone (NMP) as lubri-
cant, Schauer et al. [26] showed that the CNT fiber strength could be im-
proved from 1.2 to 2.2 GPa. Wang et al. reported improvement of mechanical
strength and electrical conductivity by more than one order of magnitude
with pressurized rolling [57]. Large strain stretching (~10%–20%) is often
used for processing CNT fibers produced by floating catalyst method [29,
30, 48], although less improvements (2%–5%) were reported [9, 22, 52, 62].
Stronger fibers are often associated with lower breaking strains, thus a com-
promise can lead to optimum fiber toughness [48, 63]. This seemingly con-
tradictory relationship between strength and strain could be explained by the
density and compactness of the fiber.
For electrical properties, the existence of impurities and defects in CNT
macro entities causes electron scattering and contact resistance while in-
sufficient CNT alignment leads to increased junction resistance. The CNT
inter-bundle junctions are a key component of the conductive path in the
CNT thread [31], whose separation and breakage will cause a decrease in
conductivity. Impurities have an adverse effect on the electrical conductiv-
ity. Schauer et al. found that acid purification treatment could improve the
4
4
CNT fiber conductivity from 0.36 × 10 to 0.8 × 10 S/cm [26]. Some
CNT fibers from floating catalyst method have a metal-like behavior, where
the electrical resistance increases with temperature [32] at a positive tem-
−4
perature coefficient of resistance of 4 × 10 /K. In comparison, conven-
tional carbon fiber shows a decrease in resistance with temperature at a
−4
negative coefficient of 4 × 10 /K. Terrones et al. [31] passed a current
through CNT fiber immersed in polar liquids. They found that the electro-
static force due to charge accumulation at open junctions brings the bun-
dles closer, or even closes the junctions, which increases the conductivity.
Typical properties of CNT fibers produced by the floating catalyst
method are listed in Table 3.1.

3.5 Conclusions and future directions
In this chapter, the method of spinning CNT fibers from floating cata-
lyst furnace has been reviewed. To produce a high-performance fiber, it
is critical to produce high-quality CNTs by optimizing the synthesis pa-
rameters. These individual CNTs serve as the building block of the final
fiber. There is a balance between quality and yield in the current processes.
Optimization in the parameter space and further understanding of the pro-
cessing mechanism will benefit the upscaling and future commercialization