Page 54 - Carbon Nanotube Fibres and Yarns

Carbon nanotube fibers spun directly from furnace 47

temperature gradients and responsible for keeping CNTs away from the re-
actor wall. Small particles under aerodynamic inertial migration could seg-
regate into annular regions, much similar to the shell of the CNT sock [53].
Other effects might also provide the bonding force, such as van der Waals
attraction [24, 27, 37]. Gspann et al. [11] suggested that due to the slow
Poiseuille flow, the catalyst particles close to the reactor move slower and
could grow larger through collision. It could be possible that the catalyst
particle close to the wall has longer residence so CNTs around that regions
will grow longer. The velocity gradient will help partial CNT connection
and accumulation, preferentially close to the reactor wall. Some useful in-
formation could be learned from substrate-grown CNTs. Blakrishnan
et al. found that the mechanical coupling between CNTs is critical for the
self-organization of forest and it also introduces deformation and defects in
CNT walls [54]. We have investigated the sock dynamics by controlling the
feedstock type, injection rate, and carrier gas flow rate. A convection vortex
has been identified, and a new convection vortex-driven model [55, 56] is
proposed to explain the sock formation (Fig. 3.8A). We have also proposed
a web-shell structure model (Fig. 3.8B) for the study of sock dynamics. The
proposed model correlates well with experimental results.

3.3.2 CNT fibers

The aerogel-like CNT sock in floating catalyst method can be trans-
formed into a CNT fiber by direct-spinning [12, 50], bath-spinning [57],
or rotating-anchor spinning method [26, 58, 59], as illustrated in Fig. 3.9.
Since water does not wet or penetrate CNT [31], it is used in the bath for
densifying the sock into a fiber. Alternatively, CNT sheets can be collected
by directly winding the sock on a spool.

3.4 Structure and properties of CNT fibers

CNT fibers collected from the direct-spinning technique have better align-
ment, smaller diameter, and linear density (0.02–0.5 tex) due to fast winding.
CNT threads from the rotating-anchor method have the largest and a broad
range of diameter and linear density (1–40 tex) [60]. The bath- spinning
method provides CNT fibers with intermediate diameter and linear density
(0.1–1.0 tex). Representative images of CNT fibers from the three spinning
methods are shown in Fig. 3.10.
Measuring CNT fibers by their diameter can introduce errors up to a factor
of five [22] due to their noncircular cross-sectional shape. Cross-sectional area