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Carbon Nanotubes: Pros and Cons
Carbon nanotube or CNT is not a new term in the current scenario actually it is an allotrope of carbon sharing a cylindrical nanostructure. Nanotubes have a length-to-diameter ratio of 132,000,000:1 and have many attractive properties for use 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, nanotubes make up a very small fraction of the material in baseball bats, car parts and golf clubs. Nanotubes are members of the fullerene family that also includes buckyballs, and the ends of these nanotubes can be capped with hemispheres of buckyballs. Their name comes from a long, hollow structure with walls made of one-atom thick sheets of carbon known as graphene. These sheets are then rolled at specific and discrete angles and the combination of rolling angle and radius determines the properties of these nanotubes. Nanotubes are either single-walled nanotubes (SWNTs) or multi-walled nanotubes (MWNTs). Nanotube particles are held together by van der Waals forces. Applied quantum chemistry, particularly orbital hybridization, best describes chemical bonding in them. The chemical bonds are mainly composed of sp2 bonds similar to those in graphite and stronger than the sp3 bonds found in diamond and alkanes and are therefore responsible for the greater strength of these structures.
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 failed to arouse the interest of Western scientists because it was published in Russian and was not open access. For the Cold War. The invention of the transmission electron microscope (TEM) made visualization of these structures possible. A paper published by Oberlin, Endo and Koyama in 1976 indicated hollow carbon fibers with nanometer scale diameters using the vapor growth technique. In 1979, John Abrahamson presented evidence of carbon nanotubes at the 14th Biennial Conference on Carbon at Pennsylvania State University.
Much of 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 look for new forms of carbon. The result came out as C60 which can be made available in all laboratories in simple arc evaporation equipment. Japanese scientist Sumio Lijima discovered carbon nanotubes related to fullerenes in 1991 using simple pressure evaporation equipment. The tubes were composed of bilayers 3–30 nm in diameter and closed at both ends. Single layered carbon nanotubes with a diameter of 1-2 nm were discovered in 1993 and could be twisted but failed to generate much interest among researchers because they were structurally incomplete, so researchers are now 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 by wrapping a single atom-thick layer of graphite, called graphene, into a seamless cylinder. The index (n, m) and the integers n and m represent the unit vectors in two directions in the honeycomb crystal lattice of graphene as the graphene is wrapped. If m=0 then 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 changes in n and m values and were widely used in the development of the first intermolecular field effect transistors. In the present era, the price of these nanotubes has decreased.
Multi Weld Nanotubes (MWNTs)
They consist of multiple rolled layers of graphene. There are two layers that can better define the structure of these nanotubes. The Russian butterfly model states that layers of graphite are arranged in single-walled nanotubes within single-walled nanotubes, for example dark cylinders. The parchment model states that a sheet of graphite is rolled around itself like a rolled newspaper. The interlayer distance in these nanotubes is 3.4. The Russian doll model is commonly considered when studying the structure of MWNTs. Double-walled nanotubes (DWNTs) are a special type of nanotubes with morphology and properties similar to MWNTs with highly improved resistance against chemicals.
A nanotorus is a carbon nanotube bent into a torus and has many unique properties such as a magnetic moment 1000 times greater. The thermal stability and magnetic moment depend on the radius of the torus and the radius of the tube.
Nanobeads are new materials made by combining two allotropes of carbon called fullerenes and carbon nanotubes. In this material, fullerene-like buds are covalently attached to the outer sidewalls of the underlying nanotubes. This new material shares the properties of both fullerenes and carbon nanotubes. They are considered to be good field emitters.
Graphenetized carbon nanotubes
They are relatively recently developed hybrid materials that combine graphitic foliates grown on the sidewalls of multi-walled nanotubes. Stoner and co-workers have reported that these hybrid materials have increased supercapacitor capacity.
Carbon Peapod is a new hybrid material made of a network of fullerenes trapped inside carbon nanotubes. It has interesting magnetic, heating and radiative properties.
Cup-stacked carbon nanotubes
They differ from other semi-1D carbon materials in that they behave as semi-metallic conductors of electrons. The semiconducting behavior of these structures is due to the presence of a stacking microstructure of graphene layers.
Extreme carbon nanotubes
The longest carbon nanotube reported in 2009 was grown on an 18.5 cm Si substrate by chemical vapor deposition and represented an electrically uniform array of single-walled carbon nanotubes. Cycloparaphenylene was the shortest carbon nanotube reported in 2009. The thinnest carbon nanotube is an armchair with a diameter of 3.
Carbon nanotubes have the strongest tensile strength and elastic modulus of all materials discovered so far. Tensile strength is due to the presence of sp2 hybridization between individual carbon atoms. The tensile strength of the multi-walled tube was reported to be 63 gigapascals (GPa) in 2000. Further studies in 2008 revealed that the shell of these tubes has a force of 100 gigapascals, which is in good agreement with quantum models. Since these tubes have low density, their strength is high. If excessive tensile stress is applied to these tubes they undergo plastic deformation which means they are permanently deformed. Although the strength of individual tubes is very high, the strength of multi-walled tubes is weakened by the weak shear interaction between adjacent shells and 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 deformation and transform into superhard phase nanotubes. The maximum pressure tolerated under current experimental techniques is 55 GPa. But these superhard nanotubes can collapse at pressures above 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 concentric multiple nanotubes folded inside each other and are gifted with an attractive telescopic property where the inner tube can slide without friction within its outer shell, thus creating a spiral effect. This is the first true example of molecular nanotechnology useful for making machines. This property has been used to make the world’s smallest rotating motor.
4. Electrical properties
Graphene’s symmetry and unique electronic structure are responsible for giving carbon nanotubes their amazing electrical properties. Intrinsic superconductivity has been observed in nanotubes but is currently a controversial issue.
5. Wave absorption
One of the most recently worked properties of multi-walled carbon nanotubes is their efficiency to exhibit microwave absorption and is a current area of research by researchers for radar absorbing materials (RAM) to provide better strength to aircraft and military vehicles. 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 a substantial improvement in maximum absorption and bandwidth.
6. Thermal properties
All nanotubes are generally considered to be good thermal conductors that exhibit the property of ballistic conduction.
Crystallographic defects affect the physical properties of any material and the defect is caused by the presence of atomic vacancies and such defects can reduce the tensile strength of the material by about 85%. Strong Wells defects create pentagons and heptagons by rearrangement of bonds. The tensile strength of carbon nanotubes depends on the weakest section. Crystallographic defects also affect the electrical properties of tubes by reducing conductivity. Crystallographic defects also affect the thermal conductivity of the tubes resulting in phonon scattering that reduces the mean free path.
Nanotubes are widely used to make the tips of atomic force microscopic probes. They are also used in tissue engineering acting as scaffolds for bone growth. Their potential strength allows them to be used as fillers to increase the tensile strength of other nanotubes. Their mechanical properties enable them to be used to make clothes, sports jackets and space elevators. They are also used to make electrical circuits, cables ad wires.
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