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Carbon nanotubes are rolled-up graphene layers that are, ideally, hollow on the inside. They can be open at the ends or can have a fullerene-like cap. In principle, a differentiation is drawn between single-wall carbon nanotubes (SWCNTs) and multi-wall carbon nanotubes (MWCNTs). In between these, there are also double-wall carbon nanotubes (DWCNTs), as well as few-wall carbon nanotubes (FWCNTs), which cover the range between two-wall and thin multi-wall carbon nanotubes.
In practice, between the ideal tubular structures, one often finds bamboo-like structures that have connecting bars at irregular intervals, as well as multi-walled tubes in which the carbon layers are not aligned completely parallel to one another, but rather are present as nested funnels.
Typical impurities in carbon nanotubes include residues of the catalysts used (metal oxides) and catalyst supports such as aluminium oxide and silicon dioxide. Depending on the manufacturing process, small quantities of amorphous carbon may also be present.
Single-wall carbon nanotubes usually have a diameter of 0.7 to 4 nm. Due to their energy-rich surfaces, they often occur as a bundle. SWCNTs can vary in their crystal structure depending on the chirality with which the graphene layer is rolled. In addition to the “chiral” SWCNTs with any chirality, a differentiation is also drawn between the “zigzag” and “armchair” structures, which correspond to chiral angles of 0° and 30°, respectively, in the arrangement of the carbon atoms along the CNT axis. This differentiation between SWCNTs is important, as only the armchair structure exhibits metallic conductivity. All other chiralities can be electrically conductive or semiconductive, depending on the chiral angle and diameter of the CNTs. Various processes are used for the industrial manufacture of SWCNTs. In addition to catalytic deposition from the gas phase (catalytic chemical vapour deposition, CCVD), the arc discharge process and laser ablation are well known; all of these processes lead to a mixture of different chiralities.
Multi-wall carbon nanotubes can be imagined can be considered as as tubes nested within one another (Russian doll type) or as a graphene layer rolled up multiple times (scroll type). They usually have a diameter in the range of 5–20 nm. However, large MWCNTs with diameters > 100 nm are also known. Because of their structure, MWCNTs are always electrically conductive.
From a technical perspective, MWCNTs are now almost exclusively produced using the cost-effective CCVD process with high catalyst yields on an industrial scale. Various grades with different sizes and purities of the nanotubes are commercially available, like SWCNTs, MWCNTs have a very high modulus of elasticity, as well as very high electrical and thermal conductivity. Since they are commercially available in large quantities, have a high length-to-diameter ratio, and therefore produce good electrical conductivity and high mechanical strength even in small amounts, they are frequently used today for electrically conductive coatings and in composite materials.
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