Page 24 - Carbon Nanotube Fibres and Yarns

Yarn production from carbon nanotube forests 17

identified three factors involved in the bundle drawing process: end knots,
entanglement in the middle of the CNT forest, and the interaction force
between adjacent CNTs in the middle of the forest. High-density CNTs
in the forest encourages the formation of entanglement at the top of the
forest. Although the end knots are responsible for preserving the fiber con-
tinuity at the ends of the forest (top and bottom), the entanglement and the
inter-tube interaction force in the middle of the forest cause the formation
of new bundles. Entanglement in the middle of the forest plays the same
role, but to a lesser extent. A high degree of drawability occurs when all
these factors act simultaneously.
While all the above studies placed emphases on the CNT entanglement
at the top of the forest, Zhu et al. [17] demonstrated that drawable CNT
forests remain to be drawable after removing the entangled layer at the top
of the forest by reactive ion etching. They observed the formation of en-
tangled structures when the pulling process approaches the bottom and top
ends of the CNT arrays, as depicted in Fig. 2.3. These entangled structures
were considered to be responsible for maintaining the continuity of the
drawing process.
CNT entanglements which are crucial to drawability require nanotube
crossovers (loose entanglement) originated from the morphology of CNT
forest. Manchard et al. [23] observed that nanotubes rotated during syn-
thesis. A group of rotating CNTs could therefore form a loosely entangled
bundle during synthesis in a similar way as the formation of self-twist yarn
from two or more twisted strands of conventional textile fibers [24]. When
the loosely entangled bundle is pulled apart into two or more parts during
web drawing, the twists or entanglements in the bundle are pushed to one
end, forming a tight knot [17].

2.2.2 Improving drawability
It is generally agreed that high level of CNT alignment in the forest is a pre-
requisite for drawability [8–10, 12, 20, 21, 25, 26]. Fig. 2.4A–C show SEM
(scanning electron microscope) images of undrawable, marginally drawable,
and fully drawable CNT forests. The SEM images of CNT forests and
webs are often used to demonstrate CNT alignment. Only images taken
at the same magnification should be compared for nanotube alignment.
Fig. 2.4D–F [28] are SEM images of the same CNT forest taken at different
magnifications, but the smaller magnification image (Fig. 2.4D) gives an
impression of very high degree of CNT alignment while the larger mag-
nification image (Fig. 2.5F) gives an impression of low CNT alignment.