Page 35 - Carbon Nanotube Fibres and Yarns

28 Carbon Nanotube Fibers and Yarns

twist in the same zone cancels the opposite directional twist introduced
during the previous half cycle of reciprocation. So the CNTs in a rub den-
sified CNT yarn are substantially straight and parallel to each other and are
aligned in the direction of the yarn axis (Fig. 2.11F).
Because of the small yarn diameter, twisted CNT yarns require many
thousands of twist turns per meter. The large number of twists per meter limits
the rate of yarn production and demands for highly engineered spinning ma-
chinery. In the rubbing machine, the yarn rotation is frictionally driven by the
rubbing rollers so a small yarn diameter means that the yarn can be rolled at a
high rotational speed by relatively low-speed rubbing rollers. Therefore a high
production rate can be achieved without requiring high-speed machine parts.

2.3.3 Die drawing
Die-drawing method was first used to produce a CNT fiber from single-walled
carbon nanotubes (SWNTs) in the form of a film that was peeled off from
the interior wall of a quartz tube used in floating catalytic CVD [43] (see
Chapter 3). A series of diamond wire drawing dies were used. The drawing was
carried out through 18 diamond dies with decreasing diameter from 1.2 to
0.2 mm. During drawing, the SWNT film formed a strand with the same di-
ameter as that of the die used. Initially the as-grown nanotube fiber was black.
The final SWNT fiber became gray with metallic luster. The obtained SWNT
fibers were highly dense and aligned. The die-drawing method was adapted
to densify webs drawn from CNT forests [44]. The nanotubes in the yarn are
compressed together by the inner wall of the die, resulting in densification. The
yarn diameter springs back by about 10% after coming out from the die. In the
final yarn, the nanotubes are held together by van der Waals forces that arise
from the intimacy between the tubes.
Fig. 2.12 illustrates the CNT yarn production process and the design
of the die. For the same CNT web (constant weight/unit length of web),
a smaller diameter die results in a higher density yarn. The yarn produced
using 35 μm die had the highest tensile strength over and above the yarn
3
produced using 30 μm die. Although having a higher density (1.15 g/cm )
3
than typical twisted yarns (<1.0 g/cm ), the CNT yarn produced using 35
μm die showed a high tensile strength (1 GPa) and a high elastic modulus
(79 GPa) similar to the rub-densificated yarns described earlier.

2.3.4 Liquid densification

Wet CNT arrays can reorganize into cellular structures upon drying [45].
The liquid-driven CNT densification achieved is attributed to the tube-tube
separation in CNT bundles ranging from 10 to 100 nm, which results in a