Carbon nanotubes: pros and cons
The 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 of nanotubes is between 132,000,000: 1 and they have very 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 that also includes buckyballs, and the ends of these nanotubes may be covered with the hemisphere of buckyballs. Its name is derived from its long, hollow structure with walls made up of one-atom-thick sheets of carbon known as graphene. These sheets are then rolled into a specific and dictated angle and the combination of rolling angle and radius decides the properties of these nanotubes. Nanotubes are single-walled nanotubes (SWNT) or multi-walled nanotubes (MWNT). Nanotube particles are held together by van der Waals forces. Applied quantum chemistry, especially orbital hybridization, best describes the chemical bonds in them. 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, so they are responsible for the great strength of these structures.
Historical background
In 1952, LV Radushkevich and LM Lukyanovich published clear images of 50 nm tubes made of carbon in the Soviet Journal of Physical Chemistry, but the article did not arouse the interest of Western scientists because it was published in the Russian language and access was not open. due to the cold war. The invention of the transmission electron microscope (TEM) made visualization of these structures possible. An article published by Oberlin, Endo, and Koyama in 1976 indicated hollow carbon fibers with a nano-scale diameter by using the vapor growth technique. In 1979, John Abrahamson presented carbon nanotube tests at the 14th Biennial Carbon Conference at Pennsylvania State University.
All the credit for the current interest in the carbon nanotube is due to the discovery of buckminsterfullerene C60 and other allied fullerenes in 1985. The discovery that carbon can form other stable structures in addition to graphite and diamond forced researchers to find new forms. carbon and the result was presented in the form of C60 which can be available in 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 consisted of two layers with a diameter ranging from 3 to 30 nm and were closed at both ends. In 1993, single-layer carbon nanotubes with a diameter of 1 to 2 nm and can be bent were discovered, but they failed to generate much interest among researchers as they were structurally imperfect, so the researchers are now working to improve the catalytic properties of these nanotubes.
Single Wall Nanotubes (SWNT)
Most single-walled nanotubes share a diameter close to 1 nm with a length a million times longer, and the structure can be imagined by wrapping a one-atom-thick layer of graphite called graphene in a seamless cylinder. The way the graphene is enveloped represented by a pair of indices (n, m) and the integers n and m represent the unit vectors along the two directions in the crystal lattice of graphene in the form of a honeycomb. 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 change in n and m values 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-gallery nanotubes (MWNT)
They consist of multiple rolled layers of graphene where 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 rolls 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 nanotubes with similar morphology and properties 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 1000 times more magnetic moment. Thermal stability and magnetic moment depend on the radius of the torus and the radius of the tube.
Nanobud
Nanobuds are newly created materials that are made by joining two allotropes of carbon, namely carbon nanotubes and fullerenes. In this material, the fullerene-shaped buds are covalently attached to the outer side walls of the underlying nanotube. This new material shares the properties of fullerenes and carbon nanotubes. They are supposed to be good field emitters.
Graffined carbon nanotubes
They are relatively recently developed hybrid materials that combine graphite sheets that grow along the side walls of a multi-walled nanotube. Stoner and his coworkers have reported that these hybrid materials have improved supercapacitor capacity.
Peapod
Carbon peapod is a new hybrid material composed of a network of fullerene trapped within a carbon nanotube. It has interesting magnetic, heating and irradiation properties.
Carbon nanotubes stacked in glasses
They differ from other quasi 1D carbon materials that behave as quasi-metallic conductors of electrons. The semiconductor behavior of these structures is due to the presence of graphene layer stacking microstructure.
Extreme carbon nanotubes
The longest carbon nanotube was reported in 2009 to measure 18.5 cm grown on Si substrates using the chemical vapor deposition method and represents electrically uniform matrices of single-walled carbon nanotubes. Cycloparaphenylene was the shortest carbon nantube reported in 2009. The thinnest carbon nanotube is the chair with a diameter of 3.
Properties
1. Force
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. Other studies conducted in 2008 have found that the casing of these tubes has a resistance of 100 gigapascals, which is consistent with quantum models. . Since these tubes have a low density, their resistance is high. If these tubes are subjected to excessive tensile stress, they undergo plastic deformation, which means that they are permanently altered. Although the strength of the individual tubes is very high, weak shear interactions between the shells and adjacent tubes result in a 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 show buckling when held under torsional or bending stress.
2. Hardness
Standard single-walled nanotubes can tolerate a pressure of approximately 24 GPa without deforming and can undergo transformation into super-hard phase nanotubes. The maximum pressure tolerated under current experimental techniques is 55 GPa. But these super-hard nanotubes can collapse at pressures in excess of 55 GPa. The modulus of volume of these nanotubes is 462-546 GPa much higher than that of diamond.
3. Kinetic properties
Multi-walled nanotubes are multiple concentric nanotubes bent over each other and endowed with an amazing teleoscopic property in which the inner tube can slide without friction within its outer shell, creating a rotational bearing. This is perhaps the first true example of molecular nanotechnology useful in machine building. This property has already been used to make the world’s smallest rotary motor.
4. Electrical properties
The symmetry and unique electronic structure of graphene is responsible for giving carbon naotubes their amazing electrical properties. Intrinsic superconductivity has been observed in nanotubes, but it is a controversial issue in the current context.
5. Wave absorption
The most recently worked properties of multi-walled carbon nanotubes is their efficiency in showing microwave absorption and it is the current research area of researchers for radar absorbing 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 the 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
The 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 about 85%. The Welsh Strong Defect creates a pentagon and a heptagon by rearranging the links. The tensile strength of carbon nanotubes depends on the weakest segment. The crystallographic defect also affects the electrical properties of the tubes by reducing conductivity. The crystallographic defect also affects the thermal conductivity of the tubes resulting in phonon scattering that reduces the mean free path.
Applications
Nanotubes are widely used to make microscopic atomic force probe tips. They are also used in tissue engineering that act as a scaffold for bone growth. Their potential strength helps them to be used as fillers 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.
