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  1 CARBON NANOTUBESNikhil Goyal , Gurvardaan Singh, Annie CocilaKhurana, VitastaDhar Dr. S.S.BUniversity Institute of Chemical Engineering & Technology, Panjab University, Chandigarh, India INTRODUCTION  Nanotechnologyis the field of science which is set to revolutionize the technological landscapeof our worldand shape ourfuture. This field covers many technologies which arebeing developedand one such important development is the discovery of carbon nanotubes.In 1993 Iijima and Donald Bethune found single walled nanotubes and Iijima was credited with the first visualisation of nanotubes.Carbon nanotubes are the members of the fullerene family(allotrope of carbon) in the form of long hollow cylindrical structures with the walls formed by thick layer of carbon atoms andthe ends sealed with fullerene caps,usually hemisphericalin shape.There are two types of carbon nanotubes:1)Single walled carbon nanotubes (SWCNTs)-They involve single layer of graphite rolled into cylinders. 2)Multi walled carbon tubes (MWCNTs)-Multilayer graphene sheets are rolled over one another in form of nested cylinders held by van-derWaal’sforces. MWCNTs are more stable than SWCNTs and are less prone to defects during functioningand purification. STRUCTURE OF CNTs Atomic structure of SWCNTs is explainedon the basis of chirality of nanotube,chiral vector ‘C h ‘ and chiral angle ‘θ‘ defines chirality on nanotubes on which electrical properties depend. METHODS OF PRODUCTION OF CARBON NANOTUBES Laser Ablation It uses a higher energy laser beam to dissolvethe molecule of a carbon based feed stock gas. The decomposed carbonmolecule reforms into nanotube. Chemical Vapour Deposition It is based on feeding in gas phase material which reconstructs to form the tubes. Chemical vapour deposition is the best known method to produce carbon nanotubes due to simpleand economic techniques for synthesisingCNT’s at low temperature and ambient pressure. Flame Synthesis In this method catalysing nano particles of Ni arecreated by laser ablation method, which react with CO/C 2 H 2 gas mixtures to produce nanotubes and nano fibres. Ball Milling Method The ball milling–annealing method, consisting of a pre- ball milling and a subsequent thermal annealing process, can produce much larger quantities of nanotubes due to a solid-state process without any vapour phase and the largeMilling capability. Arc Discharge Method Arc discharge method was found by Ebbesenand Ajayanat NEC Japan. It is based on creating carbon vapour  between two carbon electrodes by introducing an arc discharge between them. Nanotubes are formedfrom the resulting vapour. In this method twographite rods are connected to the power supply. The rods are placed afew mm apart and current of100Ais supplied. Carbon vaporises and forms hot plasma.The arcing process can  be optimized such that major portion of carbon anode gets deposited on the cathode in form of CNT and graphite particle. Acurrent of 60-100 A of current across a potential drop of 25 V gives high yield of CNT’s. Synthesis in a magnetic field can give defect free and high purity(>95%)MWCNT’s that can be used as nano-sized electric wires for devicefabrication. PROPERTIES OF CARBON NANOTUBESPhysical Properties  2 The apparent colourof MWCNTs is black. But qualities of colour,or reflectance with respect to wavelength, depend on topology and electronic structure. Strength Carbon nanotubes are the strongest and stiffest materials on earth, interms of tensile strength and elastic modulus respectively.MWCNT are electrically conductive, independent of synthesis method and have extremely high strength to weight ratio offering the potential to improve the stiffness of the polymer matrix. Electrical Properties Multiwall carbonnanotubes (MWCNT) are metallic. The measured electrical conductivity of metallic carbon nanotubes is in the order of 104 S/cm. The thermal conductivity of carbon nanotubes at room temperature can be as high as 6600 W/mK. Thermal Stability The thermal stability is directly attributed to the aromatic bonding withinthe MWCNT structure,  but can be influenced by the number ofwalls, the presence and composition of catalyst, the defectswithin the tubes etc. Tensile Strength MWCNT’s have maximum tensile strength of nearly 30 GPa. The measured specific tensile strength of a single layer of a multi-walled carbon nanotube can be as high as 100 times that of steel, and the graphene sheet (in-plane) is as stiff as diamond at low strain. Thedensity of MWCNT is 2.6 g/cm 3 . APPLICATIONS Electrical Applications  Nanotubes areused in Lithium-air batteries, which can store large energy. CNTs also have applications in fuel cells. For structural applications where electrical conductivity is required, plastics are filled with CNTs having a high aspect ratio. They have an ideal conductivity for multiple types of touch screens having applications including point-of-sale terminals, games, computers, etc. due to their ability to form transparent, porous, thin films. Carbon nanotubes are the best field emitters; hence find use in field-emission flat-panel displays, high-resolution x-ray sources andelectron microscope cathodes. Medical Applications They also have biomedical applications due to their biocompatibility and no toxic effect. Theyare also used to build artificial muscles that exert 100 times force per unit area than a natural muscle. Dental implants can be improved by addingnanotubes to the surface of the implant material. Industrial Applications Their size, surface area and adsorption  properties give them application in water  purifiers and cleaning filters for many industrial  processes. Ceramic materials reinforced with carbon are tougher than conventional ceramics. The high mechanical strength, durability makes them ideal in scanning probe microscopy. Structural Properties Because of the carbon nanotube's superior mechanical properties, many structures have  been proposed ranging from everyday items like clothes and sports gear to combat jackets. Hydrogen storage Carbon nanotubes can be used to store hydrogen to be used as a fuel source.This allows for gases, most notably hydrogen (H 2 ), to be stored at high densities without being condensed into a liquid.
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