Carbon nanotube or CNT is not a new term in the present scenario, in fact it is the allotrope of carbon sharing a cylindrical nanostructure. The length to diameter ratio of nanotubes is between 132,000,000:1 and they have many fascinating properties to be used in nanotechnology, optics, materials science, electronics and other fields of science. Due to their exceptional 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 part 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. Their name is derived from their long, hollow structure with walls formed from one-atom-thick sheets of carbon known as graphene. These sheets are then rolled up at a certain and specific angle, and the combination of roll angle and radius determines 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, best describes chemical bonding in them. The chemical bonds are mainly composed of sp2 bonds, similar to those found in graphite, and stronger than the sp3 bonds found in diamond and alkanes, and thus are responsible for the great strength of these structures.
In 1952, L. V. Radushkevich and L. M. Lukyanovich published clear images of 50 nm tubes composed of carbon in the Soviet Journal of Physical Chemistry, but the paper failed to arouse interest among Western scientists because it was published in Russian language and access is not open due to cold war. The invention of the transmission electron microscope (TEM) made it possible to visualize these structures. A paper published by Oberlin, Endo, and Koyama in 1976 reported hollow carbon fibers with nanometer diameters using a vapor growth technique. In 1979, John Abrahamson presented evidence of carbon nanotubes at the Pennsylvania State University’s 14th Biennial Conference on Carbon.
All the credit for the current interest in carbon nanotubes goes to the discovery of buckminsterfullerene C60 and other related 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 under form of C60, which can be provided in all laboratories in a simple arc evaporation apparatus. Sumio Lijima, a Japanese scientist, discovered the fullerene-bonded carbon nanotube using a simple arc evaporation apparatus in 1991. The tubes were composed of two layers with diameters in the range of 3-30 nm and closed at both ends. In 1993, single-walled carbon nanotubes with a diameter of 1-2 nm and can be bent were discovered, but they failed to generate much interest among researchers because they were structurally imperfect, so researchers are now working to improve the catalytic properties of these nanotubes.
Single-Walled Nanotubes (SWNTs)
Most single-walled nanotubes are close to 1 nm in diameter with a length millions of times longer, and the structure can be imagined by wrapping a one-atom-thick layer of graphite, called graphene, into a seamless cylinder. The way graphene wraps is represented by a pair of subscripts (n, m), and the integers n and m represent the unit vectors along both directions in the graphene honeycomb crystal lattice. If m=0 then the nanotubes are called zigzag nanotubes and if n=m then 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 were widely used in the development of the first intermolecular field-effect transistors. The cost of these nanotubes has come down in the current era.
Multiwalled Nanotubes (MWNTs)
They consist 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 states that a sheet of graphite coils around itself, resembling a rolled-up newspaper. The interlayer spacing in these nanotubes is 3.4. The Russian doll model is commonly considered when studying MWNT structure. Double-walled nanotubes (DWNTs) are a special type of nanotubes with morphology and properties similar to MWNTs, with greatly improved resistance against chemicals.
The nanotorus is a carbon nanotube bent into the shape of a torus and has very unique properties such as a magnetic moment 1000 times more. Thermal stability and magnetic moment depend on the torus radius as well as the tube radius.
Nanobuds are newly created materials made by joining two allotropes of carbon, namely carbon nanotubes and fullerenes. In this material, the fullerene-like buds are covalently bonded to the outer sidewalls of the main nanotube. This new material shares the properties of both fullerenes and carbon nanotubes. They are supposed to be good field emitters.
Graphed carbon nanotubes
They are relatively newly developed hybrid materials combining graphite sheets grown on the sidewalls of a multi-walled nanotube. Stoner and co-workers reported that these hybrid materials have improved supercapacitor capability.
Carbon peapod is a new hybrid material composed of a network of fullerenes trapped in a carbon nanotube. It has interesting magnetic, heating and radiating properties.
Cupped carbon nanotubes
They differ from other quasi 1D carbon materials that behave as quasi metallic electron conductors. The semiconducting behavior of these structures is due to the presence of an ordering microstructure of graphene layers.
Extreme carbon nanotubes
The longest carbon nanotube was reported in 2009 with dimensions of 18.5 cm, grown on Si substrates by chemical vapor deposition, and is 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.
Carbon nanotubes have the highest tensile strength and modulus of elasticity of any material discovered so far. The tensile strength is due to the presence of sp2 hybridization between the individual carbon atoms. The tensile strength of a multiwalled tube was reported to be 63 gigapascals (GPa) in 2000. Additional studies conducted in 2008 found that the shell of these tubes had a strength of 100 gigapascals, which is in good agreement with quantum models . Since these pipes have a low density, their strength is high. If these tubes are given excessive tensile stress, they undergo plastic deformation, meaning they change permanently. Although the strength of individual tubes is very high, the weak shear interaction between adjacent shells and tubes leads to a weakening of the strength of multiwalled tubes. They are also not strong when compressed. Due to their hollow structure and high aspect ratio, they show buckling when subjected to torsional or bending stress.
Standard single-walled nanotubes can withstand pressures of about 24GPa without deforming and can undergo transformation into superhard phase nanotubes. The maximum allowable pressure under current experimental techniques is 55 GPa. But these super-stiff nanotubes can collapse at pressures higher than 55 GPa. The bulk modulus of these nanotubes is 462-546 GPa much higher than that of diamond.
3. Kinetic properties
Multiwalled nanotubes are concentric arrays of nanotubes folded into one another and endowed with an amazing telescoping property where the inner tube can slide frictionlessly in its outer shell, creating a rotational bearing. This is perhaps the first real examples of molecular nanotechnology useful in machine manufacturing. This property has already been used in making the world’s smallest rotary engine.
4. Electrical properties
Graphene’s symmetry and unique electronic structure are responsible for giving carbon shavings their amazing electrical properties. Intrinsic superconductivity has been observed in nanotubes, but this is a controversial issue in the present context.
5. Absorption of waves
The most recently studied properties of multi-walled carbon nanotubes are their effectiveness in exhibiting microwave absorption and the current area of research by researchers for radar absorbing materials (RAMs) so as to provide better strength to aircraft and military vehicles. Research is in progress, with researchers trying to fill MWNTs with metals such as iron, nickel or cobalt to increase the efficiency of these tubes for microwave mode, and the results show an improvement in the maximum absorption and the bandwidth of adequate absorption.
6. Thermal properties
All nanotubes are generally considered to be good thermal conductors, exhibiting the property of ballistic conduction.
Crystallographic defect affects the material properties 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 about 85%. The Strong Wells defect 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 reducing the conductivity. The crystallographic defect also affects the thermal conductivity of the tubes, leading to phonon scattering that reduces the mean free path.
Nanotubes are widely used to make the tips of atomic force microscope probes. They are also used in tissue engineering, acting as a scaffold for bone growth. Their potential strength helps them be used as a filler to increase the tensile strength of other nanotubes. Their mechanical property helps them to be used in the production of clothing, sports jackets and space elevators. They are also used in making electric circuits, cables and wires.