Structure: a sheet rolled into a tube
A carbon nanotube is a graphene sheet rolled into a seamless cylinder. The tube is typically a few nanometers across or somewhat larger for multi-wall types, but can be thousands of times longer than it is wide. The atoms are bonded in the same honeycomb pattern found in graphite, and the strong bonds run around and along the tube.
Two features define a nanotube:
- Number of walls. Single-wall nanotubes have one cylinder. Multi-wall nanotubes are nested like tree rings, with several cylinders inside each other.
- Chirality. This describes the angle at which the sheet is rolled. It decides whether a single-wall tube behaves as a metal or as a semiconductor, a point that matters for electronics but is hard to control during synthesis.
How they are made
Common methods include arc discharge, laser ablation, and chemical vapor deposition, in which a carbon-containing gas decomposes over metal catalyst particles and the tubes grow from them. The vapor route is the most scalable and is the basis for most commercial supply. As-made material usually contains leftover catalyst and a mix of tube sizes and quality, so purification and sorting are often needed.
Properties that attract engineers
Measurements on individual, well-formed tubes indicate very high strength and stiffness along the axis, good electrical conduction in metallic tubes, and good heat conduction along the length. In principle this combination is hard to match. The very high aspect ratio, meaning length divided by width, is also useful because a small amount of tube can form a conducting or reinforcing network.
The gap between one tube and a real material
Single-tube numbers are impressive, but a bulk material is a different matter. Several problems separate laboratory values from engineering ones:
- Dispersion. Tubes attract each other and clump into bundles. Getting them spread evenly through a polymer or metal is a major practical challenge.
- Load transfer. For a tube to reinforce a polymer, forces must pass between the matrix and the tube. Smooth, chemically inert walls can bond poorly, so the fiber-matrix interface issues known from larger composites appear here at a smaller scale.
- Defects and length. Real tubes have defects and are often short, which reduces performance.
- Cost and consistency. Quality varies between batches and suppliers.
Because of these factors, nanotube-containing materials generally show more modest improvements than the best single-tube figures suggest.
Where they are used
Conductive additives. This is the most established role. A small loading of multi-wall nanotubes can make a plastic electrically conductive, useful for static dissipation, electromagnetic shielding and conductive coatings. Battery makers also add nanotubes to electrodes to improve electrical contact, which links to energy storage.
Reinforcement in polymers. In polymer matrix composites, nanotubes may improve stiffness, toughness or fatigue resistance. They are sometimes added to the resin of a carbon fiber laminate to improve properties between plies, although gains depend on processing.
Coatings and films. Thin nanotube networks can form transparent conductive films or add wear and conductivity properties to a coating.
Thermal materials. Nanotubes have been investigated for heat spreading and thermal interface materials, but practical gains over other fillers are mixed.
Yarns, sheets and fibers. Nanotubes can be spun or drawn into yarns and sheets. These are active research and specialty products rather than mainstream structural materials.
Sensors and electronics. Individual tubes have been demonstrated as transistors and sensing elements. Large-scale use has been limited by the difficulty of making identical tubes in the right places.
Health and handling
Some forms of nanotubes, particularly long, rigid, needle-like ones, have raised concerns about inhalation, since they can resemble certain hazardous fibers in shape. Research is ongoing and results depend on the specific tube type. Manufacturers and users typically follow occupational exposure guidance and treat loose nanotube powders with care. Anyone planning to work with them should consult current safety data and regulations for the material in question.
How they compare with relatives
Nanotubes are close cousins of graphene sheets, which are flat rather than rolled, and of fullerenes, which are closed cages. Unlike conventional carbon fiber, nanotubes are far smaller and are mainly used as additives rather than as the main load-bearing fiber. Their role complements rather than replaces larger reinforcements. Identification and quality checks often rely on Raman spectroscopy, which gives a characteristic signal for nanotubes and indicates their quality.
For the overall map of how nanotubes relate to diamond, graphite and other forms, see the article on carbon allotropes.