Page 23 - Carbon Nanotube Fibres and Yarns

16 Carbon Nanotube Fibers and Yarns

Kuznetsov et al. [22] proposed a structural model for the drawing of a
continuous web from a vertically oriented MWNT (multi-walled nano-
tube) forest (Fig. 2.2). The original forest consists of vertically oriented
forest trees (big CNT bundles) interconnected by smaller bundles or indi-
vidual nanotubes (called connects). The CNT web formation involves two
principal processes: (1) unzipping by preferentially peeling off interconnec-
tions between the bundles in the forest and (2) self-strengthening of these
interconnections by densification at the top and bottom of the forest during
draw-induced reorientation of the bundles.
Zhang et al. [8] considered that CNTs from highly aligned arrays of
clean surface CNTs form tight bundles due to strong van der Waals inter-
actions between the tubes. When pulling the CNTs from a super-aligned
array, van der Waals force makes the CNTs join end to end, thus, forming a
continuous yarn. These end-to-end joints are enhanced by bridging CNT
bundles to form a continuous ribbon web.
Fallah Gilvaei et al. [12] measured the force required to separate CNTs
from parent drawable and undrawable forests. The drawable forest showed
high separation force at the top of the forest and low separation force
in the middle of the forest height, while in the undrawable forests the
separation force showed no marked difference at different locations. They

1 1
2
2
1 1
3 3
2 2
4 4 3 3 1
4 4 2 3
5 5 4
5 5
5
4 4
3 3

1 2 2
2
3 1
4 1
5
Fig. 2.2 Scheme of the process of pulling out a forest tree from the forest. Blue and red
bars represent forest trees or bundles of CNTs, and the separation distance between
them is increased for clarity. Red arrows show the external force applied to the first bun-
dle. Small black arrows show the direction of the net movement of the interconnec-
tions [22]. (Reprinted with permission from A.A. Kuznetsov, A.F. Fonseca, R.H. Baughman,
A.A. Zakhidov, Structural model for dry-drawing of sheets and yarns from carbon nanotube
forests, ACS Nano 5 (2) (2011) 985–993.)