Carbon nanotube or CNT is not a new term in the current scenario, it is actually the allotrope of carbon that shares a cylindrical nanostructure. The length-to-diameter ratio of nanotubes is between 132,000,000:1 and has fascinating properties for use in nanotechnology, optics, materials science, electronics, and other fields of science. Due to their extraordinary thermal conductivity, mechanical and electrical properties, carbon nanotubes are used as additives for various structural materials, for example in baseball bats, car parts and golf clubs, nanotubes form a very small fraction of the material. . Nanotubes are members of the fullerene family, which also includes buckyballs, and the ends of these nanotubes can be capped with the hemisphere of buckyballs. Its name derives from its long, hollow structure with walls made up of one-atom-thick sheets of carbon known as graphene. These sheets are then rolled at a specific, discrete angle and the combination of the rolling angle and radius decides the properties of these nanotubes. Nanotubes are either single-walled nanotubes (SWNTs) or multi-walled nanotubes (MWNTs). The nanotube particles are held together by van der Waals forces. Applied quantum chemistry, especially orbital hybridization, better describes the chemical bond in them. The chemical bonds are mainly made up of sp2 bonds similar to those found in graphite and are stronger than the sp3 bonds found in diamond and alkanes, thus they are responsible for the great strength of these structures.

Historical background

In 1952, LV Radushkevich and LM Lukyanovich published sharp images of 50nm tubes made of carbon in the Soviet Journal of Physical Chemistry but the published paper did not arouse the interest of Western scientists because it was in the Russian language and access was not open due to oh the cold war. The invention of the transmission electron microscope (TEM) made it possible to visualize these structures. An article published by Oberlin, Endo, and Koyama in 1976 reported on hollow carbon fibers with a nanometer-scale diameter by using the vapor growth technique. In 1979, John Abrahamson presented evidence for carbon nanotubes at the 14th Pennsylvania State University Biennial Conference on Carbon.

All credit for the current interest in carbon nanotubes is due to the discovery of buckminsterfullerene C60 and other allied fullerenes in 1985. The discovery that carbon can form stable structures other than graphite and diamond forced researchers to find new forms of carbon and the result came in the form of C60 which can be made available to all laboratories in a simple arc evaporation apparatus. Sumio Lijima, a Japanese scientist discovered the fullerene-related carbon nanotube using the single-arc evaporation apparatus in 1991. The tubes were made of two layers with a diameter ranging from 3 to 30 nm and were closed at both ends. Single-layer carbon nanotubes with a diameter of 1-2 nm and can be bent were discovered in 1993, but they did not arouse much interest among researchers because they were structurally imperfect, so now they are working to improve the catalytic properties of these nanotubes.

Single-walled nanotubes (SWNTs)

Most single-walled nanotubes share a diameter close to 1nm with a length a million times longer, and the structure can be imagined wrapping a one-atom-thick layer of graphite called graphene in a seamless cylinder. The way graphene is wrapped is represented by a pair of indices (n,m) and the integers n and m represent the unit vectors along the two directions in the honeycomb crystal lattice of graphene. If m = 0, the nanotubes are called zigzag nanotubes and if n = m, they are called armchair; otherwise they are chiral. SWNTs are a very important variety of nanotubes because their properties change with changing values ​​of n and m and they are widely used in the development of the first intermolecular field effect transistors. The price of these nanotubes has decreased in the current era.

Multi-Walled Nanotubes (MWNTs)

Consisting of multiple coiled layers of graphene, there are two layers that can better define the structure of these nanotubes. The Russian doll model says that the graphite layers are arranged in concentric cylinders, for example, a single-walled nanotube within a single-walled nanotube. The parchment model says that a single sheet of graphite is rolled up on itself like a rolled up newspaper. The distance between layers in these nanotubes is 3.4. The Russian doll model is generally considered when studying the structure of MWNTs. Double-walled nanotubes (DWNTs) are a special type of nanotube with morphology and properties similar to MWNTs with greatly improved resistance against chemicals.

Bull

A nanotorus is a carbon nanotube bent into a torus shape and has many unique properties such as a 1000 times larger magnetic moment. Thermal stability and magnetic moment depend on the radius of the torus as well as the radius of the tube.

nanobud

Nanobuds are newly created materials by joining two allotropes of carbon, namely carbon nanotubes and fullerenes. In this material, fullerene-like buds are covalently bonded to the outer sidewalls of the underlying nanotube. This new material shares the properties of both fullerenes and carbon nanotubes. They are supposed to be good field emitters.

graphenized carbon nanotubes

They are relatively recently developed hybrid materials that combine graphitic foliates grown along the sidewalls of a multiwalled nanotube. Stoner and his collaborators reported that these hybrid materials have improved the capacity of supercapacitors.

Pea

Carbon peapod is a new hybrid material composed of a network of fullerene trapped inside a carbon nanotube. It has interesting magnetic, calorific and radiating properties.

cup stacked carbon nanotubes

They differ from other quasi-1D carbon materials in that they behave as quasi-metallic conductors of electrons. The semiconductor behavior of these structures is due to the presence of a stacking microstructure of graphene layers.

extreme carbon nanotubes

The longest carbon nanotube was reported in 2009, measuring 18.5 cm grown on Si substrates by chemical vapor deposition method and represent electrically uniform arrays of single-walled carbon nanotubes. Cycloparaphenylene was the shortest carbon nanotube reported in 2009. The thinnest carbon nanotube is armchair with a diameter of 3.

Properties

1.Strength

Carbon nanotubes have the strongest tensile strength and elastic modulus among all materials discovered so far. The tensile strength is due to the presence of sp2 hybridization between the individual carbon atoms. The tensile strength of multi-walled tube was reported to be 63 gigapascals (GPa) in 2000. Additional studies in 2008 have found the shell of these tubes to have a strength of 100 gigapascals, which is in good agreement with models. quantum. Since these tubes have a low density, their strength is high. If excessive tensile stress is applied to these tubes, they will plastically deform, meaning permanently altered. Although the strength of the individual tubes is very high, the weak shear interactions between the shells and the adjacent tubes result in the weakening of the strength of the multi-walled tubes. They are also not strong when compressed. Due to their hollow structure and high aspect ratio, they exhibit buckling when held under torsional or bending stress.

2. Hardness

Standard single-walled nanotubes can tolerate a pressure of around 24 GPa without deformation and can be transformed into superhard phase nanotubes. The maximum pressure tolerated with current experimental techniques is 55 GPa. But these superhard nanotubes can collapse at pressures greater than 55 GPa. The bulk modulus of these nanotubes is 462-546 GPa, much higher than that of diamond.

3.Kinetic properties

Multi-walled nanotubes are multiple concentric nanotubes folded into each other and endowed with an amazing teleoscopic property in which the inner tube can slide without friction within its outer shell, thus creating a rotating bearing. This is perhaps the first true example of molecular nanotechnology useful in machine building. This property has already been used to make the smallest rotary motor in the world.

4.Electrical properties

The symmetry and unique electronic structure of graphene are responsible for giving carbon naotubes their amazing electrical properties. Intrinsic superconductivity has been observed in nanotubes, but it is a controversial topic in the current context.

5. Catching waves

The most recently worked out property of multi-walled carbon nanotubes is their efficiency in displaying microwave absorption and is the current area of ​​research by researchers for radar absorbent materials (RAM) to provide better resistance to aircraft and vehicles. military. . Research is in progress where researchers are trying to fill MWNTs with metals such as iron, nickel or cobalt to increase the effectiveness of these tubes for the microwave regime and results have shown an improvement in maximum absorption and bandwidth. adequate absorption.

6.Thermal properties

In general, all nanotubes are believed to be good thermal conductors exhibiting the property of ballistic conduction.

Defects

Crystallographic defect affects the material property of any material and the defect is due to the presence of atomic vacancies and such defects can reduce the tensile strength of the material to around 85%. The Strong Wales Flaw creates a pentagon and a heptagon by rearranging the bonds. The tensile strength of carbon nanotubes depends on the weakest segment. The crystallographic defect also affects the electrical properties of the tubes by lowering the conductivity. The crystallographic defect also affects the thermal conductivity of the tubes, resulting in phonon scattering which reduces the mean free path.

Applications

Nanotubes are widely used in the manufacture of atomic force microscopic probe tips. They are also used in tissue engineering by acting as a scaffold for bone growth. Their potential strength helps them be used as filler material to increase the tensile strength of other nanotubes. Their mechanical property helps them to be used in the manufacture of clothing, sports jackets and space elevators. They are also used in the manufacture of electrical circuits, cables and wires.