# Carbon nanotubes: what are they?

The carbon nanotubes are another carbon allotropic form, they have many interesting properties and promise many applications.

## What are carbon nanotubes?

They are cylindrical molecules made of carbon atoms with a diameter on nanometric scale and length on micrometric or millimetric scale. They consist of a rolled graphene sheet, linking opposite extremities. Exist carbon nanotubes with many walls, consisting of many concentric cylinders. Source: Electrónica. Some nanotubes can have a spherical bottom. Source: physicsworld.

## Properties

The carbon nanotube’s properties depend on if it has single or multiple walls and how it’s rolled.

### Graphene winding A graphene sheet can be rolled in 3 configurations: armchair, zigzag and chiral. The configurations differ from each other by the borders. Source: (Monea et al., 2019).

The two numbers in parentheses (n,m) form the chiral vector ($\vec{C}_{h}$).

$\vec{C}_{h}=n\vec{a}_{1}+m\vec{a}_{2}=(n,m)$

Where $\vec{a}_{1}$ and $\vec{a}_{2}$ are unitary vectors, whose modules are equal to 1. The T vector is perpendicular to chiral vector and parallel to nanotube’s axis. Source: (Zhou et al., 2019).

Obtaining the chiral vector $\phi$ formula.

$\vec{C}_{h}\cdot \vec{a}_{1}=\left| \vec{C}_{h}\right|\cdot \left| \vec{a}_{1}\right|\cdot cos\phi$

$\phi=cos^{-1}\left ( \frac{2n+m}{2\sqrt{\left ( m^{2}+n^{2}+mn\right )}} \right )$

The tube’s diameter ($d$).

$d=\frac{a\sqrt{3(n^{2}+mn+m^{2})}}{\pi }$

Where $a$ is the length of covalent bond between two carbon atoms, which is 1.42 Å (angstrom).

1 Å = $1\times 10^{-10}m$

The integral numbers m and n determine if a nanotube is a conductor or a semiconductor.

### Other properties of carbon nanotubes (CNT)

• All nanotubes have great tensile strength, they are 100 times stronger than steel with only 1/6 of density.
• Are excellent heat conductors. Thermal conductivity can reach 6000 $W/m\cdot K$ (watt per meter times Kelvin).
• Since they are made of graphene, Young module (measures resistance to elastic deformation) is close to 1000 GPa, it read gigapascals.
• Carbon nanotubes kill the living cells when they come into contact with the latter.

## Some application examples

• Composite materials with nanotubes of multiple walls can have higher resistance to weariness and break, with less weight.
• The conductors CNT have better conductivity than copper. They can one day be used in electric energy transmission.
• It’s possible to put semiconductor nanotubes in field effect transistors, to make the connection between drain and source terminals. Creating high-performance smaller circuits. Transistor with a carbon nanotube (SWCNT). Source: (Kreupl, 2012). Exist thin film transistor projects with a carbon nanotubes network, instead of only one. This circuit is a digital door NOT or inverter. Source: gelonghui.
• Can be used in electrochemical and biological sensors. A flexible hydrogen sensor with single-walled carbon nanotubes. Source: nanowerk.

Carbon nanotubes still are very expensive to mass manufacture. 