3. Results and discussion
The high-resolution transmission electron micrographs are illustrated in
Figure 1 for the catalytically-grown multi-walled carbon nanotube
material. A typical particle consists of loosely-agglomerated
multi-walled carbon nanotubes, most of which are about 30-40 nm in
diameter and more than one micron in length. Therefore, the multi-walled
carbon nanotubes have high aspect ratios. In several cases, the
multi-walled carbon nanotubes have white-contrasting tips, which are
transition-metal catalyst particles. Hence, it is apparent that the
multi-walled carbon nanotubes are produced by the particle-at-the-tip
growth mechanism. Generally, multi-walled carbon nanotubes are used for
applications in plastic composites. Multi-walled carbon nanotubes,
however, can have inferior electrical properties due to amorphous carbon
or defects on the surface formed during synthesis. Accordingly, a
composite including the same may not have the desired electrical
property. Thus, it can be important to improve the crystallinity of the
surface of the carbon nanotubes. One method re-aligns the hexagon
structure of carbon atoms using a high temperature treatment to improve
surface crystallinity of carbon nanotubes. This method, however,
increases manufacturing cost because an additional process is required
after carbon nanotube synthesis. It can also be difficult to select
synthesis conditions for the high heat treatment. In addition,
productivity can be impaired. Carbon nanotubes are nanomaterials that
individually include properties of high modulus, tensile strength,
aspect ratio, and electrical and thermal conductivity. The fabrication
of carbon nanotube filaments can be based on wet spinning methods, which
require dispersing carbon nanotubes in a solution for further spinning
processes. The chemical dispersion process generally leads to a low
usage of carbon nanotubes, and the spun filament usually contains
surfactants or polymer molecules which reduce the strength and the
thermal and electrical properties of the filament. Dry spinning methods
can be used to prepare carbon nanotube filaments composed of pure carbon
nanotubes from as-grown super-aligned carbon nanotube arrays. The dry
spinning method that directly spins carbon nanotube filaments from
super-aligned carbon nanotube arrays has attracted attention because the
dry spinning method is simple and controllable to produce continuous
carbon nanotube filaments. The dry spinning method is enabled by the van
der Waals forces between carbon nanotubes that provide a cohesive force
that enables the carbon nanotubes to readily stick to one another. While
the van der Waals forces may be disadvantageous to applications such as
composite films where a high degree of dispersion is preferred, the van
der Waals forces can prove useful in the preparation of carbon nanotube
filaments, threads, and ribbons. However, while the van der Waals forces
are relatively strong in an orientation that adheres the carbon
nanotubes to one another the van der Waals forces are comparatively weak
in an orientation of carbon nanotube alignment. Therefore, filaments
made from carbon nanotubes typically exhibit tensile strength that is
orders of magnitude less than the tensile strength of the individual
carbon nanotubes. Accordingly, improvements in the strength of thread
made from carbon nanotubes is desired. It should now be understood that
carbon nanotube filaments that are braided into a thread exhibit tensile
strength properties that exceed the tensile strength properties of the
carbon nanotube filaments themselves. The improvement in tensile
strength properties is attributed to the frequency of intersections per
length of the individual carbon nanotubes that are formed into the
carbon nanotube filament. To increase the frequency of intersections in
a thread, thinner carbon nanotube filaments may be braided with a high
pick frequency. Further, to prevent breakage of the thinner carbon
nanotube filaments in a machine braiding operation, low-tension carriers
that pay out the carbon nanotube filaments may be used.