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.