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BIJU PATNAIK UNIVERSITY OF TECHNOLOGY ROURKELA ORISSA
A Seminar Report on
“CARBON NANOTUBES”
Submitted by
Ranjit Kumar Biswal Branch:-MECHANICAL ENGG. Regd. No.:-0901217157
Under the Guidance of
Er.Santosh Panda
DEPARTMENT OF MECHANICAL ENGINEERING PADMASHREE KRUTARTHA ACHARYA COLLEGE OF ENGINEERING BARGARH ORISSA 2012
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BIJU PATNAIK UNIVERSITY OF TECHNOLOGY ROURKELA ORISSA
PADMASHREE KRUTARTHA ACHARYA COLLEGE OF ENGINEERING BARGARH ORISSA
CERTIFICATE
Certified that the seminar report entitled “Carbon Nanotubes”is a bonafied work carried out by Ranjit Kumar Biswal,Regd. No.-0901217157,in partial fulfillment for the award of Bachelor in Technology in Mechanical Engineering prescribed by BijuPatnaik University Of Technology,Rourkeladuring 2012-13.It is also certified that all the corrections/suggestions indicated for the seminar have been incorporated in the report.This seminar report has been approved as it satisfies the academic requirements in respect of the Seminar prescribed for the 7thSemistarofBachelor in Technology.
H.O.DSeminar Guide(ASST.PROF.B.B.SATPATHY)(ER.SANTOSH
PANDA)
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ACKNOWLEDGEMENT
?The acts of few specific people are influence of many?. Determination and inspiration in many minds are the reflection of these people. I am greatly thankful to our respected Head of the Department Asst.Prof.B.B.Satpathywho are the backbone for the success of this seminar. My sincere thanks to the Seminar Guide Er.SantoshPandafor his guidance and suggestions which helped in overcoming the hurdles in the completion of this seminar report. My sincere thanks to all the staff members of our department for their immense support during the seminar work. I also thank the non-teaching staff members of Mechanical Engineering Department for their kind support and help in carrying out the seminar work. And last but not the least, I also thank my parents and friends for their cooperation and encouragement in successfully completing the seminar work.
Signature of the student
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CONTENTS
1. 2. 3. 4. 5. 6. 7. 8. Introduction Types of Carbon Nanotubes And Related Structures Classification Properties Synthesis Applications Advantages Disadvantages
9. Conclusion 10. References
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List of figuresPage no.
Fig.1.1(Spinning Carbon Nanotube)…………………………………………6 Fig.2.1[Armchair (n,n)]……………………………………………………...7 Fig.2.2[Vector (Ch)]…………………………………………………………7 Fig.2.3(The translation vector is bent, while the chiral vector stays straight)8 Fig.2.4(Graphenenanoribbon)………………………………………………8 Fig.2.5(The chiral vector is bent, while the translation vector stays straight)8 Fig.2.6[Zigzag (n,0)]………………………………………………………...9 Fig.2.7[Chiral (n,m)]………………………………………………………...9 Fig.2.8(n and m can be counted at the end of the tube)…………………….9 Fig.2.9(Graphenenanoribbon)……………………………………………...10 Fig.3.1.1(Single-walled Carbon Nanotubes)……………………………….10 Fig.3.1.2(Single-walled Carbon Nanotubes)……………………………….11 Fig.3.1.3(Single-walled Carbon Nanotubes)……………………………….12 Fig.4.4.1(Electrical properties)……………………………………………..15 Fig.5.3.1(Chemical vapor deposition)……………………………………...17
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Carbon Nanotube
1.INTRODUCTION:Carbon comes from a latin word ?carbo?,which is derived from a Franch word ?charbon?,meaning charcoal.It is the fourth most abundant chemical element in the universe by mass,afterhydrogen,helium, and oxygen.The allotropes of carbon are the different molecular configurations that pure carbon can take.AllotropesofcarbonincludeDiamond,Graphite,Amorphouscarbon,carbonNanotubes. Carbon nanotubes (CNTs) are allotropes of carbon with a cylindrical nanostructure. Nanotubes have been constructed with length-to-diameter ratio of up to 132,000,000:1, significantly larger than for any other material. These cylindrical carbon molecules have unusual properties, which are valuable for nanotechnology, electronics, optics and other fields of materials science and technology. In particular, owing to their extraordinary thermal conductivity and mechanical and electrical properties, carbon nanotubes find applications as additives to various structural materials. For instance, nanotubes form only a tiny portion of the material(s) in (primarily carbon fiber) baseball bats, golf clubs, or car parts.Nanotubes are members of the fullerene structural family. Their name is derived from their long, hollow structure with the walls formed by one-atom-thick sheets of carbon, called graphene. These sheets are rolled at specific and discrete ("chiral") angles, and the combination of the rolling angle and radius decides the nanotube properties; for example, whether the individual nanotube shell is a metal or semiconductor. Nanotubes are categorized as single-walled nanotubes (SWNTs) and multi-walled nanotubes (MWNTs). Individual nanotubes naturally align themselves into "ropes" held together by van der Waals forces, more specifically, pi-stacking.
Fig.1.1(Spinning Carbon Nanotube)
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Applied quantum chemistry, specifically, orbital hybridization best describes chemical bonding in nanotubes. The chemical bonding of nanotubes is composed entirely of sp2 bonds, similar to those of graphite. These bonds, which are stronger than the sp3 bonds found in alkanesanddiamond, provide nanotubes with their unique strength.
2. TYPES OF CARBON NANOTUBES AND RELATED STRUCTURES:Terminology There is no consensus on some terms describing carbon nanotubes in scientific literature: both "-wall" and "-walled" are being used in combination with "single", "double", "triple" or "multi", and the letter C is often omitted in the abbreviation; for example, multi-walled carbon nanotube (MWNT). Single-walled
?
Fig.2.1[Armchair (n,n)]
Fig.2.2[Vector (Ch)]
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?
Fig.2.3(The translation vector is bent, while the chiral vector stays straight)
?
Fig.2.4(Graphenenanoribbon)
?
Fig.2.5(The chiral vector is bent, while the translation vector stays straight)
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?
Fig.2.6[Zigzag (n,0)]
?
Fig.2.7[Chiral (n,m)]
?
Fig.2.8(n and m can be counted at the end of the tube)
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?
Fig.2.9(Graphenenanoribbon)
3.CLASSIFICATION:Carbon-based nanomaterials has revealed a new promising form of carbon carbon nanotubes (CNTs) - with huge potential in the developing field of nanotechnology. Unique properties and novel applications of CNTs make them potentially useful in wide range of technological applications. Characterization of basic properties of nanotubes is indispensable to the further scientific and industrial development. Scanning electron microscopy (SEM) is one of the most powerful tools in nanotechnology and plays an important role in the research of nanowires and CNTs. 3.1.CLASSIFICATION BASED ON LAYER:Single-walled:-
Fig.3.1.1(Single-walled Carbon Nanotubes) Most single-walled nanotubes (SWNT) have a diameter of close to 1 nanometer, with a tube length that can be many millions of times longer. The structure of a SWNT can be conceptualized by wrapping a one-atom-thick layer of graphite called graphene into a seamless cylinder. The way the graphene sheet is wrapped is represented by a pair of indices (n,m). The
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integers n and m denote the number of unit vectors along two directions in the honeycomb crystal latticeofgraphene. If m = 0, the nanotubes are called zigzag nanotubes, and if n = m, the nanotubes are called armchair nanotubes. Otherwise, they are called chiral. SWNTs are an important variety of carbon nanotube because most of their properties change significantly with the (n,m) values, and this dependence is non-monotonic (see Kataura plot). In particular, their band gap can vary from zero to about 2 eV and their electrical conductivity can show metallic or semiconducting behavior. Single-walled nanotubes are likely candidates for miniaturizing electronics. The most basic building block of these systems is the electric wire, and SWNTs with diameters of an order of a nanometer can be excellent conductors. One useful application of SWNTs is in the development of the first intermolecular field-effect transistors (FET). The first intermolecular logic gate using SWCNT FETs was made in 2001.A logic gate requires both a p-FET and an n-FET. Because SWNTs are p-FETs when exposed to oxygen and n-FETs otherwise, it is possible to protect half of an SWNT from oxygen exposure, while exposing the other half to oxygen. This results in a single SWNT that acts as a NOT logic gate with both p and n-type FETs within the same molecule. Single-walled nanotubes are dropping precipitously in price, from around $1500 per gram as of 2000 to retail prices of around $50 per gram of asproduced 40–60% by weight SWNTs as of March 2010. Multi-walled
Fig.3.1.2(Single-walled Carbon Nanotubes)
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A scanning electron microscopy image of carbon nanotubes bundles
Fig.3.1.3(Single-walled Carbon Nanotubes)
Multi-walled nanotubes (MWNT) consist of multiple rolled layers (concentric tubes) of graphene. There are two models that can be used to describe the structures of multi-walled nanotubes. In the Russian Doll model, sheets of graphite are arranged in concentric cylinders, e.g., a (0,8) single-walled nanotube (SWNT) within a larger (0,17) single-walled nanotube. In the Parchment model, a single sheet of graphite is rolled in around itself, resembling a scroll of parchment or a rolled newspaper. The interlayer distance in multi-walled nanotubes is close to the distance between graphene layers in graphite, approximately 3.4 Å. The Russian Doll structure is observed more commonly. Its individual shells can be described as SWNTs, which can be metallic or semiconducting. Because of statistical probability and restrictions on the relative diameters of the individual tubes, one of the shells, and thus the whole MWNT, is usually a zero-gap metal. Double-walled carbon nanotubes (DWNT) form a special class of nanotubes because their morphology and properties are similar to those of SWNT but their resistance to chemicals is significantly improved. This is especially important when functionalization is required (this means grafting of chemical functions at the surface of the nanotubes) to add new properties to the CNT. In the case of SWNT, covalent functionalization will break some C=C double bonds, leaving "holes" in the structure on the nanotube and, thus, modifying both its mechanical and electrical properties. In the case of DWNT, only the outer wall is modified. DWNT synthesis on the gram-scale was first proposed in 2003 by the CCVD technique, from the selective reduction of oxide solutions in methane and hydrogen.
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4.PROPERTIES:4.1.Strength Carbon nanotubes are the strongest and stiffest materials yet discovered in terms of tensile strength and elastic modulus respectively. This strength results from the covalent sp2bonds formed between the individual carbon atoms. In 2000, a multi-walled carbon nanotube was tested to have a tensile strength of 63 gigapascals (GPa). (For illustration, this translates into the ability to endure tension of a weight equivalent to 6422 kg on a cable with cross-section of 1 mm2.) Further studies, conducted in 2008, revealed that individual CNT shells have strengths of up to ~100 GPa, which is in agreement with quantum/atomistic models.[30] Since carbon nanotubes have a low density for a solid of 1.3 to 1.4 g/cm3,[31] its specific strength of up to 48,000 kN•m•kg?1 is the best of known materials, compared to high-carbon steel's 154 kN•m•kg?1. Comparison of mechanical properties
Material Young's modulus (TPa) Tensile strength (GPa) 13–53 Elongation at break (%)
SWNTE
~1 (from 1 to 5)
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Armchair SWNTT
0.94
126.2
23.1
Zigzag SWNTT 0.94
94.5
15.6–17.5
Chiral SWNT MWNTE Stainless steelE
0.92
0.2–0.8–0.95
11–63–150
0.186–0.214
0.38–1.55
15–50
Kevlar– 29&149E
0.06–0.18
3.6–3.8
~2
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4.2.Kinetic properties Multi-walled nanotubes are multiple concentric nanotubes precisely nested within one another. These exhibit a striking telescoping property whereby an inner nanotube core may slide, almost without friction, within its outer nanotube shell, thus creating an atomically perfect linear or rotational bearing. This is one of the first true examples of molecular nanotechnology, the precise positioning of atoms to create useful machines. Already, this property has been utilized to create the world's smallest rotational motor. Future applications such as a gigahertz mechanical oscillator are also envisaged. 4.3.Hardness Standard single-walled carbon nanotubes can withstand a pressure up to 24GPa without deformation. They then undergo a transformation to superhard phase nanotubes. Maximum pressures measured using current experimental techniques are around 55GPa. However, these new superhard phase nanotubes collapse at an even higher, albeit unknown, pressure. The bulk modulus of superhard phase nanotubes is 462 to 546 GPa, even higher than that of diamond(420 GPa for single diamond crystal). 4.4.Electrical properties Band structures computed using tight binding approximation for (6,0) CNT (zigzag, metallic) (10,2) CNT (semiconducting) and (10,10) CNT (armchair, metallic). Because of the symmetry and unique electronic structure of graphene, the structure of a nanotube strongly affects its electrical properties. For a given (n,m) nanotube, if n = m, the nanotube is metallic; if n ? m is a multiple of 3, then the nanotube is semiconducting with a very small band gap, otherwise the nanotube is a moderate semiconductor. Thus all armchair (n = m) nanotubes are metallic, and nanotubes (6,4), (9,1), etc. are semiconducting. However, this rule has exceptions, because curvature effects in small diameter carbon nanotubes can strongly influence electrical properties. Thus, a (5,0) SWCNT that should be semiconducting in fact is metallic according to the calculations. Likewise, vice versa—zigzag and chiral SWCNTs with small diameters that should be metallic have finite gap (armchair nanotubes remain metallic).In theory, metallic nanotubes can carry an electric current density of 4
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× 109 A/cm2, which is more than 1,000 times greater than those of metals such as copper where for copper interconnects current densities are limited by electromigration. Because of their nanoscale cross-section, electrons propagate only along the tube's axis and electron transport involves quantum effects. As a result, carbon nanotubes are frequently referred to as one-dimensional conductors. The maximum electrical conductance of a single-walled carbon nanotube is 2G0, where G0 = 2e2/h is the conductance of a single ballistic quantum channel. There have been reports of intrinsic superconductivity in carbon nanotubes. Many other experiments, however, found no evidence of superconductivity, and the validity of these claims of intrinsic superconductivity remains a subject of debate.
Fig.4.4.1(Electrical properties) 4.5. Thermal properties All nanotubes are expected to be very good thermal conductors along the tube, exhibiting a property known as "ballistic conduction", but good insulators laterally to the tube axis. Measurements show that a SWNT has a roomtemperature thermal conductivity along its axis of about 3500 W·m?1·K?1 compare this to copper, a metal well known for its goodthermal conductivity, which transmits 385 W·m?1·K?1. A SWNT has a room15
temperature thermal conductivity across its axis (in the radial direction) of about 1.52 W·m?1·K?1 which is about as thermally conductive as soil. The temperature stability of carbon nanotubes is estimated to be up to 2800 °C in vacuum and about 750 °C in air. 4.6. Defects As with any material, the existence of a crystallographic defect affects the material properties. Defects can occur in the form of atomic vacancies. High levels of such defects can lower the tensile strength by up to 85%. An important example is the Stone Wales defect, which creates a pentagon and heptagon pair by rearrangement of the bonds. Because of the very small structure of CNTs, the tensile strength of the tube is dependent on its weakest segment in a similar manner to a chain, where the strength of the weakest link becomes the maximum strength of the chain. Crystallographic defects also affect the tube's electrical properties. A common result is lowered conductivity through the defective region of the tube. A defect in armchair-type tubes (which can conduct electricity) can cause the surrounding region to become semiconducting, and single monoatomic vacancies induce magnetic properties. Crystallographic defects strongly affect the tube's thermal properties. Such defects lead to phonon scattering, which in turn increases the relaxation rate of the phonons. This reduces the mean free path and reduces the thermal conductivity of nanotube structures. Phonon transport simulations indicate that substitutional defects such as nitrogen or boron will primarily lead to scattering of high-frequency optical phonons. However, larger-scale defects such as Stone Wales defects cause phonon scattering over a wide range of frequencies, leading to a greater reduction in thermal conductivity.
5.SYNTHESIS:5.1..Arcdischarge Nanotubes were observed in 1991 in the carbon soot of graphite electrodes during an arc discharge, by using a current of 100 amps, that was intended to produce fullerenes. However the first macroscopic production of carbon nanotubes was made in 1992 by two researchers at NEC's Fundamental Research Laboratory. The method used was the same as in 1991. During this process, the carbon contained in the negative electrode sublimates
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because of the high-discharge temperatures. Because nanotubes were initially discovered using this technique, it has been the most widely used method of nanotube synthesis. The yield for this method is up to 30% by weight and it produces both singleand multi-walled nanotubes with lengths of up to 50 micrometers with few structural defects. 5.2.Laser ablation In the laser ablation process, a pulsed laser vaporizes a graphite target in a hightemperature reactor while an inert gas is bled into the chamber. Nanotubes develop on the cooler surfaces of the reactor as the vaporized carbon condenses. A water-cooled surface may be included in the system to collect the nanotubes. This process was developed by Dr. Richard Smalley and co-workers at Rice University, who at the time of the discovery of carbon nanotubes, were blasting metals with a laser to produce various metal molecules. When they heard of the existence of nanotubes they replaced the metals with graphite to create multiwalled carbon nanotubes. Later that year the team used a composite of graphite and metal catalyst particles (the best yield was from a cobalt and nickel mixture) to synthesize single-walled carbon nanotubes. 5.3.Chemical vapor deposition (CVD)
Fig.5.3.1(Chemical vapor deposition)
Nanotubes being grown by plasma enhanced chemical vapor deposition
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The catalytic vapor phase deposition of carbon was reported in 1952 and 1959, but it was not until 1993 that carbon nanotubes were formed by this process. In 2007, researchers at the University of Cincinnati (UC) developed a process to grow aligned carbon nanotube arrays of 18 mm length on a FirstNano ET3000 carbon nanotube growth system. During CVD, a substrate is prepared with a layer of metal catalyst particles, most commonly nickel, cobalt, iron, or a combination.The metal nanoparticles can also be produced by other ways, including reduction of oxides or oxides solid solutions. The diameters of the nanotubes that are to be grown are related to the size of the metal particles. This can be controlled by patterned (or masked) deposition of the metal, annealing, or by plasma etching of a metal layer. The substrate is heated to approximately 700°C. To initiate the growth of nanotubes, two gases are bled into the reactor: a process gas (such as ammonia, nitrogen or hydrogen) and a carbon-containing gas (such as acetylene, ethylene, ethanol or methane). Nanotubes grow at the sites of the metal catalyst; the carbon-containing gas is broken apart at the surface of the catalyst particle, and the carbon is transported to the edges of the particle, where it forms the nanotubes. This mechanism is still being studied. The catalyst particles can stay at the tips of the growing nanotube during the growth process, or remain at the nanotube base, depending on the adhesion between the catalyst particle and the substrate.Thermal catalytic decomposition of hydrocarbon has become an active area of research and can be a promising route for the bulk production of CNTs. Fluidised bed reactor is the most widely used reactor for CNT preparation. Scale-up of the reactor is the major challenge.
CVD is a common method for the commercial production of carbon nanotubes. For this purpose, the metal nanoparticles are mixed with a catalyst support such as MgO or Al2O3 to increase the surface area for higher yield of the catalytic reaction of the carbon feedstock with the metal particles. One issue in this synthesis route is the removal of the catalyst support via an acid treatment, which sometimes could destroy the original structure of the carbon nanotubes. However, alternative catalyst supports that are soluble in water have proven effective for nanotube growth. If a plasma is generated by the application of a strong electric field during the growth process (plasma enhanced chemical vapor deposition), then the nanotube growth will follow the direction of the electric field. By adjusting the geometry of the reactor it is possible to synthesize vertically aligned carbon
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nanotubes(i.e., perpendicular to the substrate), a morphology that has been of interest to researchers interested in the electron emission from nanotubes. Without the plasma, the resulting nanotubes are often randomly oriented. Under certain reaction conditions, even in the absence of a plasma, closely spaced nanotubes will maintain a vertical growth direction resulting in a dense array of tubes resembling a carpet or forest. Of the various means for nanotube synthesis, CVD shows the most promise for industrial-scale deposition, because of its price/unit ratio, and because CVD is capable of growing nanotubes directly on a desired substrate, whereas the nanotubes must be collected in the other growth techniques. The growth sites are controllable by careful deposition of the catalyst. In 2007, a team from Meijo University demonstrated a high-efficiency CVD technique for growing carbon nanotubes from camphor. Researchers at Rice University, until recently led by the late Richard Smalley, have concentrated upon finding methods to produce large, pure amounts of particular types of nanotubes. Their approach grows long fibers from many small seeds cut from a single nanotube; all of the resulting fibers were found to be of the same diameter as the original nanotube and are expected to be of the same type as the original nanotube. 5.3.Natural, incidental, and controlled flame environments Fullerenes and carbon nanotubes are not necessarily products of high-tech laboratories; they are commonly formed in such mundane places as ordinary flames, produced by burning methane, ethylene, and benzene, and they have been found in soot from both indoor and outdoor air. However, these naturally occurring varieties can be highly irregular in size and quality because the environment in which they are produced is often highly uncontrolled. Thus, although they can be used in some applications, they can lack in the high degree of uniformity necessary to satisfy the many needs of both research and industry. Recent efforts have focused on producing more uniform carbon nanotubes in controlled flame environments. Such methods have promise for large-scale, low-cost nanotube synthesis based on theoretical models, though they must compete with rapidly developing large scale CVD production.
6.APPLICATION:Application of nanotubes has mostly been limited to the use of bulk nanotubes, which is a mass of rather unorganized fragments of nanotubes. Bulk nanotube
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materials may never achieve a tensile strength similar to that of individual tubes, but such composites may, nevertheless, yield strengths sufficient for many applications. Bulk carbon nanotubes have already been used as composite fibers in polymers to improve the mechanical, thermal and electrical properties of the bulk product. ? Easton-Bell Sports, Inc. have been in partnership with Zyvex Performance Materials, using CNT technology in a number of their bicycle components including flat and riser handlebars, cranks, forks, seatposts, stems and aero bars. ? Zyvex Technologies has also built a 54' maritime vessel, the Piranha Unmanned Surface Vessel, as a technology demonstrator for what is possible using CNT technology. CNTs help improve the structural performance of the vessel, resulting in a lightweight 8,000 lb boat that can carry a payload of 15,000 lb over a range of 2,500 miles. ? Amroy Europe Oy manufactures Hybtonite carbon nanoepoxy resins where carbon nanotubes have been chemically activated to bond to epoxy, resulting in a composite material that is 20% to 30% stronger than other composite materials. It has been used for wind turbines, marine paints and variety of sports gear such as skis, ice hockey sticks, baseball bats, hunting arrows, and surfboards. 6.1. Hydrogen storage In addition to being able to store electrical energy, there has been some research in using carbon nanotubes to store hydrogen to be used as a fuel source. By taking advantage of the capillary effects of the small carbon nanotubes, it is possible to condense gases in high density inside single-walled nanotubes. This allows for gases, most notably hydrogen (H2), to be stored at high densities without being condensed into a liquid. Potentially, this storage method could be used on vehicles in place of gas fuel tanks for a hydrogen-powered car. A current issue regarding hydrogen-powered vehicles is the onboard storage of the fuel. Current storage methods involve cooling and condensing the H 2 gas to a liquid state for storage which causes a loss of potential energy (25 –45%) when compared to the energy associated with the gaseous state. Storage using SWNTs would allow one to keep the H2 in its gaseous state, thereby increasing the storage effciency. This method allows for a volume to energy ratio slightly smaller to that of current gas powered vehicles, allowing for a slightly lower but comparable range.
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6.2. Medical In the Kanzius cancer therapy, single-walled carbon nanotubes are inserted around cancerous cells, then excited with radio waves, which causes them to heat up and kill the surrounding cells. Researchers at Rice University, Radboud University Nijmegen Medical Centre and University of California, Riverside have shown that carbon nanotubes and their polymer nanocomposites are suitable scaffold materials for bone cell proliferation and bone formation. 6.3. Textile The previous studies on the use of CNTs for textile functionalization were focused on fiber spinning for improving physical and mechanical properties. Recently a great deal of attention has been focused on coating CNTs on textile fabrics. Various methods have been employed for modifying fabrics using CNTs. Shim et al. produced intelligent e-textiles for Human Biomonitoring using a polyelectrolyte-based coating with CNTs. Additionally, Panhuis et al. dyed textile material by immersion in either a poly (2-methoxy aniline-5sulfonic acid) PMAS polymer solution or PMAS-SWNT dispersion with enhanced conductivity and capacitance with a durable behavior.In another study, Hu and coworkers coated single-walled carbon nanotubes with a simple ?dipping and drying? process for wearable electronics and energy storage applications.CNTs have an aligned nanotube structure and a negative surface charge. Therefore, they have similar structures to direct dyes, so the exhaustion method is applied for coating and absorbing CNTs on the fiber surface for preparing multifunctional fabric including antibacterial, electric conductive, flame retardant and electromagnetic absorbance properties. 6.4.Toxicity The toxicity of carbon nanotubes has been an important question in nanotechnology. Such research has just begun. The data are still fragmentary and subject to criticism. Preliminary results highlight the difficulties in evaluating the toxicity of this heterogeneous material. Parameters such as structure, size distribution, surface area, surface chemistry,surface charge, and agglomeration state as well as purity of the samples, have considerable impact
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on the reactivity of carbon nanotubes. However, available data clearly show that, under some conditions, nanotubes can cross membrane barriers, which suggests that, if raw materials reach the organs, they can induce harmful effects such as inflammatory and fibrotic reactions. A study led by Alexandra Porter from the University of Cambridge shows that CNTs can enter human cells and accumulate in the cytoplasm, causing cell death. Results of rodent studies collectively show that regardless of the process by which CNTs were synthesized and the types and amounts of metals they contained, CNTs were capable of producing inflammation, epithelioid granulomas (microscopic nodules), fibrosis, and biochemical/toxicological changes in the lungs. Comparative toxicity studies in which mice were given equal weights of test materials showed that SWCNTs were more toxic than quartz, which is considered a serious occupational health hazard when chronically inhaled. As a control, ultrafine carbon black was shown to produce minimal lung responses. The needle-like fiber shape of CNTs is similar to asbestos fibers. This raises the idea that widespread use of carbon nanotubes may lead to pleural mesothelioma, a cancer of the lining of the lungs or peritoneal mesothelioma, a cancer of the lining of the abdomen (both caused by exposure to asbestos). A recently published pilot study supports this prediction.Scientists exposed the mesothelial lining of the body cavity of mice to long multiwalled carbon nanotubes and observed asbestos-like, length-dependent, pathogenic behavior that included inflammation and formation of lesions known as granulomas. Authors of the study conclude. According to co-author Dr. Andrew Maynard: This study is exactly the kind of strategic, highly focused research needed to ensure the safe and responsible development of nanotechnology. It looks at a specific nanoscale material expected to have widespread commercial applications and asks specific questions about a specific health hazard. Even though scientists have been raising concerns about the safety of long, thin carbon nanotubes for over a decade, none of the research needs in the current U.S. federal nanotechnology environment, health and safety risk research strategy address this question.
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7.ADVANTAGES:Composite materials reinforced with carbon nanotube "stitching" are not only ten times stronger than those that go without, but they are also one million times more electrically conductive, according to a report soon to be published in the Journal of Composite Materials. Benefits to aircraft structures therefore include increased strength and lightning protection along with decreased weight for composite aircraft. Carbon nanotubes are not new, and have been flown aboard aircraft, including a Giles G-200 high-performance homebuilt aerobatic aircraft in 2008, as part of that aircraft's cowling. But researchers at MIT say that through a process called "nanostitching," they can add nanotubes, "the strongest fibers known to humankind," to the composite's weakest part -- between its layers -- adding strength, without adding weight. And that's not nearly all they can do. Composite structures are strengthened by adding layers or plies of composite fiber on top of one another, bonded by resin. Nanostitching aligns rows of carbon nanotubes perpendicular to the layered fibers of a carbon composite structure, effectively filling the spaces between them and stitching the layers together without adding weight. Nanostitching does not add weight to the structure, because the nanotubes exist in space formerly taken up solely by heavier, weaker resin. The process actually increases strength and saves weight (as it was in the case of the G-200). • Extremely small and lightweight, making them excellent replacements for metallic wires • Resources required to produce them are plentiful, and many can be made with only a small amount of material • Are resistant to temperature changes, meaning they function almost just as well in extreme cold as they do in extreme heat • Have been in the R&D phase for a long time now, meaning most of the kinks have been worked out • As a new technology, investors have been piling into these R&D companies, which will boost the economy.
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8.DISADVANTAGES:The carbon nanotube degrades in a few days when exposed to oxygen. There has been several works done on passivating the nanotubes with different polymers and increasing their lifetime. Carbon nanotubes have shown reliability issues when operated under high electric field or temperature gradients. Avalanche breakdown occurs in semiconducting CNT and joule breakdown in metallic CNT. Unlike avalanche behavior in silicon, avalanche in CNTs is negligibly temperature-dependent. Applying high voltages beyond avalanche point results in Joule heating and eventual breakdown in CNTs. This reliability issue has been studied, and it is noticed that the multi-channeld structure can improve the reliability of the CNTFET. The multi-channeled CNTFETs can keep a stable performance after several months, while the single-channeled CNTFETs are usually out of work after a few weeks in the ambient atmosphere. The multi-channeled CNTFETs keep operating when some channels break down, this won’t happ en in the single-channeled ones.Although CNTs have unique properties such as stiffness, strength, and tenacity compared to other materials especially to silicon, we are still experiencing lack of technology for mass production and high production cost. To overcome the fabrication difficulties, several methods have been studied such as direct growth, solution dropping, and various transfer printing techniques. • Despite all the research, scientists still don't understand exactly how they work • • Extremely small, so are difficult to work with Currently, the process is relatively expensive to produce the nanotubes
• Would be expensive to implement this new technology in and replace the older technology in all the places that we could • At the rate our technology has been becoming obsolete, it may be a gamble to bet on this technology.
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CONCLUSION
As carbon-nanotube technology progresses, it becomes clearer each day that these materials will most likely constitute the foundation of tomorrow's society. They will be used in just about everything, from sports equipment to computer processors and next-generation electronic devices, but experts are still concerned about what effects they may have on the human body. Some studies proposed that the brain could defend itself against this type of intruders, but a new research suggests that the lungs may not be able to do this. The investigation revealed that CNT could adversely affect the outer lining of the lungs. The team behind the study, composed of scientists from the North Carolina State University (NC State), the Hamner Institutes for Health Sciences, and the National Institute of Environmental Health Sciences, admits, however, that more studies are required to confirm its finds. It adds that the long-term effects of exposure are still a mystery. Unsuspecting mice were used for the new experiments, which sought to test whether the new materials could be used without harm to humans. In the study, mouse models were subjected to a single dose of carbon nanotubes, and then their evolution was followed. The scientists discovered that an immune-system response began to form within one day from the inhalation stage, with clusters of lymphocytes and monocytes, two types of immunesystem cells, forming on the surface of the pleura, the outer lining of the lungs. They also discovered that some regions of the pleura had been scarred, a phenomenon known as fibrosis. This is common in people working in mines or with asbestos, when fine particles deposit themselves in the lungs, and reduce the organs' ability to process oxygen and carbon dioxide. Within thee months of exposure, all signs of the scarring and immune response in the mice disappeared, the team reports. It adds, however, that mice were subjected to only one exposure to the CNT, whereas, in real life, many more ?doses? would be inhaled by a single person. ?It remains unclear whether the pleura could recover from chronic, or repeated, exposures. More work needs to be done in that area and it is completely unknown at this point whether inhaled carbon nanotubes will prove to be carcinogenic in the lungs or in the pleural lining,? the senior author of the new study, NC State Associate Professor of Environmental and Molecular Toxicology Dr. James Bonner, explains.
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REFERENCES
1. Physics of Carbon-nanotubes(by M.S.Dresselhaus and R.Saito .M.I.T) 2. Nanotubes for Electronics (by Philip G.Collins and PhaedonAvouris) 3. Carbon Nanotube Applications in Microelectronics(by G.S.Dusberg) 4. Carbon Nanotubes (by Anthony Kendall and Elizabeth Pfaff) 5. Physical Properties of Carbon Nanotubes (by Thomas A.Adams) 6.www.wikipedia.org/wiki/Allotropes_of_carbon. 7.www.wikipedia.org/wiki/Nanotechnology. 8.www.wikipedia.org/wiki/Thermal_conductivity. 9.www.wikipedia.org/wiki/fullerene. 10.www.wikipedia.org/wiki/Graphene. 11.www.wikipedia.org/wiki/File:carbon_nanotube_zigzag_povray.PNG. 12.www.wikipedia.org/wiki/Torus. 13.www.wikipedia.org/wiki/Laser_ablation. 14.www.wikipedia.org/wiki/Kevlar. 15.www.wikipedia.org/wiki/Tensile_strength.
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doc_614311420.docx
what is nct
BIJU PATNAIK UNIVERSITY OF TECHNOLOGY ROURKELA ORISSA
A Seminar Report on
“CARBON NANOTUBES”
Submitted by
Ranjit Kumar Biswal Branch:-MECHANICAL ENGG. Regd. No.:-0901217157
Under the Guidance of
Er.Santosh Panda
DEPARTMENT OF MECHANICAL ENGINEERING PADMASHREE KRUTARTHA ACHARYA COLLEGE OF ENGINEERING BARGARH ORISSA 2012
1
BIJU PATNAIK UNIVERSITY OF TECHNOLOGY ROURKELA ORISSA
PADMASHREE KRUTARTHA ACHARYA COLLEGE OF ENGINEERING BARGARH ORISSA
CERTIFICATE
Certified that the seminar report entitled “Carbon Nanotubes”is a bonafied work carried out by Ranjit Kumar Biswal,Regd. No.-0901217157,in partial fulfillment for the award of Bachelor in Technology in Mechanical Engineering prescribed by BijuPatnaik University Of Technology,Rourkeladuring 2012-13.It is also certified that all the corrections/suggestions indicated for the seminar have been incorporated in the report.This seminar report has been approved as it satisfies the academic requirements in respect of the Seminar prescribed for the 7thSemistarofBachelor in Technology.
H.O.DSeminar Guide(ASST.PROF.B.B.SATPATHY)(ER.SANTOSH
PANDA)
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ACKNOWLEDGEMENT
?The acts of few specific people are influence of many?. Determination and inspiration in many minds are the reflection of these people. I am greatly thankful to our respected Head of the Department Asst.Prof.B.B.Satpathywho are the backbone for the success of this seminar. My sincere thanks to the Seminar Guide Er.SantoshPandafor his guidance and suggestions which helped in overcoming the hurdles in the completion of this seminar report. My sincere thanks to all the staff members of our department for their immense support during the seminar work. I also thank the non-teaching staff members of Mechanical Engineering Department for their kind support and help in carrying out the seminar work. And last but not the least, I also thank my parents and friends for their cooperation and encouragement in successfully completing the seminar work.
Signature of the student
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CONTENTS
1. 2. 3. 4. 5. 6. 7. 8. Introduction Types of Carbon Nanotubes And Related Structures Classification Properties Synthesis Applications Advantages Disadvantages
9. Conclusion 10. References
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List of figuresPage no.
Fig.1.1(Spinning Carbon Nanotube)…………………………………………6 Fig.2.1[Armchair (n,n)]……………………………………………………...7 Fig.2.2[Vector (Ch)]…………………………………………………………7 Fig.2.3(The translation vector is bent, while the chiral vector stays straight)8 Fig.2.4(Graphenenanoribbon)………………………………………………8 Fig.2.5(The chiral vector is bent, while the translation vector stays straight)8 Fig.2.6[Zigzag (n,0)]………………………………………………………...9 Fig.2.7[Chiral (n,m)]………………………………………………………...9 Fig.2.8(n and m can be counted at the end of the tube)…………………….9 Fig.2.9(Graphenenanoribbon)……………………………………………...10 Fig.3.1.1(Single-walled Carbon Nanotubes)……………………………….10 Fig.3.1.2(Single-walled Carbon Nanotubes)……………………………….11 Fig.3.1.3(Single-walled Carbon Nanotubes)……………………………….12 Fig.4.4.1(Electrical properties)……………………………………………..15 Fig.5.3.1(Chemical vapor deposition)……………………………………...17
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Carbon Nanotube
1.INTRODUCTION:Carbon comes from a latin word ?carbo?,which is derived from a Franch word ?charbon?,meaning charcoal.It is the fourth most abundant chemical element in the universe by mass,afterhydrogen,helium, and oxygen.The allotropes of carbon are the different molecular configurations that pure carbon can take.AllotropesofcarbonincludeDiamond,Graphite,Amorphouscarbon,carbonNanotubes. Carbon nanotubes (CNTs) are allotropes of carbon with a cylindrical nanostructure. Nanotubes have been constructed with length-to-diameter ratio of up to 132,000,000:1, significantly larger than for any other material. These cylindrical carbon molecules have unusual properties, which are valuable for nanotechnology, electronics, optics and other fields of materials science and technology. In particular, owing to their extraordinary thermal conductivity and mechanical and electrical properties, carbon nanotubes find applications as additives to various structural materials. For instance, nanotubes form only a tiny portion of the material(s) in (primarily carbon fiber) baseball bats, golf clubs, or car parts.Nanotubes are members of the fullerene structural family. Their name is derived from their long, hollow structure with the walls formed by one-atom-thick sheets of carbon, called graphene. These sheets are rolled at specific and discrete ("chiral") angles, and the combination of the rolling angle and radius decides the nanotube properties; for example, whether the individual nanotube shell is a metal or semiconductor. Nanotubes are categorized as single-walled nanotubes (SWNTs) and multi-walled nanotubes (MWNTs). Individual nanotubes naturally align themselves into "ropes" held together by van der Waals forces, more specifically, pi-stacking.
Fig.1.1(Spinning Carbon Nanotube)
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Applied quantum chemistry, specifically, orbital hybridization best describes chemical bonding in nanotubes. The chemical bonding of nanotubes is composed entirely of sp2 bonds, similar to those of graphite. These bonds, which are stronger than the sp3 bonds found in alkanesanddiamond, provide nanotubes with their unique strength.
2. TYPES OF CARBON NANOTUBES AND RELATED STRUCTURES:Terminology There is no consensus on some terms describing carbon nanotubes in scientific literature: both "-wall" and "-walled" are being used in combination with "single", "double", "triple" or "multi", and the letter C is often omitted in the abbreviation; for example, multi-walled carbon nanotube (MWNT). Single-walled
?
Fig.2.1[Armchair (n,n)]
Fig.2.2[Vector (Ch)]
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?
Fig.2.3(The translation vector is bent, while the chiral vector stays straight)
?
Fig.2.4(Graphenenanoribbon)
?
Fig.2.5(The chiral vector is bent, while the translation vector stays straight)
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?
Fig.2.6[Zigzag (n,0)]
?
Fig.2.7[Chiral (n,m)]
?
Fig.2.8(n and m can be counted at the end of the tube)
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?
Fig.2.9(Graphenenanoribbon)
3.CLASSIFICATION:Carbon-based nanomaterials has revealed a new promising form of carbon carbon nanotubes (CNTs) - with huge potential in the developing field of nanotechnology. Unique properties and novel applications of CNTs make them potentially useful in wide range of technological applications. Characterization of basic properties of nanotubes is indispensable to the further scientific and industrial development. Scanning electron microscopy (SEM) is one of the most powerful tools in nanotechnology and plays an important role in the research of nanowires and CNTs. 3.1.CLASSIFICATION BASED ON LAYER:Single-walled:-
Fig.3.1.1(Single-walled Carbon Nanotubes) Most single-walled nanotubes (SWNT) have a diameter of close to 1 nanometer, with a tube length that can be many millions of times longer. The structure of a SWNT can be conceptualized by wrapping a one-atom-thick layer of graphite called graphene into a seamless cylinder. The way the graphene sheet is wrapped is represented by a pair of indices (n,m). The
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integers n and m denote the number of unit vectors along two directions in the honeycomb crystal latticeofgraphene. If m = 0, the nanotubes are called zigzag nanotubes, and if n = m, the nanotubes are called armchair nanotubes. Otherwise, they are called chiral. SWNTs are an important variety of carbon nanotube because most of their properties change significantly with the (n,m) values, and this dependence is non-monotonic (see Kataura plot). In particular, their band gap can vary from zero to about 2 eV and their electrical conductivity can show metallic or semiconducting behavior. Single-walled nanotubes are likely candidates for miniaturizing electronics. The most basic building block of these systems is the electric wire, and SWNTs with diameters of an order of a nanometer can be excellent conductors. One useful application of SWNTs is in the development of the first intermolecular field-effect transistors (FET). The first intermolecular logic gate using SWCNT FETs was made in 2001.A logic gate requires both a p-FET and an n-FET. Because SWNTs are p-FETs when exposed to oxygen and n-FETs otherwise, it is possible to protect half of an SWNT from oxygen exposure, while exposing the other half to oxygen. This results in a single SWNT that acts as a NOT logic gate with both p and n-type FETs within the same molecule. Single-walled nanotubes are dropping precipitously in price, from around $1500 per gram as of 2000 to retail prices of around $50 per gram of asproduced 40–60% by weight SWNTs as of March 2010. Multi-walled
Fig.3.1.2(Single-walled Carbon Nanotubes)
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A scanning electron microscopy image of carbon nanotubes bundles
Fig.3.1.3(Single-walled Carbon Nanotubes)
Multi-walled nanotubes (MWNT) consist of multiple rolled layers (concentric tubes) of graphene. There are two models that can be used to describe the structures of multi-walled nanotubes. In the Russian Doll model, sheets of graphite are arranged in concentric cylinders, e.g., a (0,8) single-walled nanotube (SWNT) within a larger (0,17) single-walled nanotube. In the Parchment model, a single sheet of graphite is rolled in around itself, resembling a scroll of parchment or a rolled newspaper. The interlayer distance in multi-walled nanotubes is close to the distance between graphene layers in graphite, approximately 3.4 Å. The Russian Doll structure is observed more commonly. Its individual shells can be described as SWNTs, which can be metallic or semiconducting. Because of statistical probability and restrictions on the relative diameters of the individual tubes, one of the shells, and thus the whole MWNT, is usually a zero-gap metal. Double-walled carbon nanotubes (DWNT) form a special class of nanotubes because their morphology and properties are similar to those of SWNT but their resistance to chemicals is significantly improved. This is especially important when functionalization is required (this means grafting of chemical functions at the surface of the nanotubes) to add new properties to the CNT. In the case of SWNT, covalent functionalization will break some C=C double bonds, leaving "holes" in the structure on the nanotube and, thus, modifying both its mechanical and electrical properties. In the case of DWNT, only the outer wall is modified. DWNT synthesis on the gram-scale was first proposed in 2003 by the CCVD technique, from the selective reduction of oxide solutions in methane and hydrogen.
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4.PROPERTIES:4.1.Strength Carbon nanotubes are the strongest and stiffest materials yet discovered in terms of tensile strength and elastic modulus respectively. This strength results from the covalent sp2bonds formed between the individual carbon atoms. In 2000, a multi-walled carbon nanotube was tested to have a tensile strength of 63 gigapascals (GPa). (For illustration, this translates into the ability to endure tension of a weight equivalent to 6422 kg on a cable with cross-section of 1 mm2.) Further studies, conducted in 2008, revealed that individual CNT shells have strengths of up to ~100 GPa, which is in agreement with quantum/atomistic models.[30] Since carbon nanotubes have a low density for a solid of 1.3 to 1.4 g/cm3,[31] its specific strength of up to 48,000 kN•m•kg?1 is the best of known materials, compared to high-carbon steel's 154 kN•m•kg?1. Comparison of mechanical properties
Material Young's modulus (TPa) Tensile strength (GPa) 13–53 Elongation at break (%)
SWNTE
~1 (from 1 to 5)
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Armchair SWNTT
0.94
126.2
23.1
Zigzag SWNTT 0.94
94.5
15.6–17.5
Chiral SWNT MWNTE Stainless steelE
0.92
0.2–0.8–0.95
11–63–150
0.186–0.214
0.38–1.55
15–50
Kevlar– 29&149E
0.06–0.18
3.6–3.8
~2
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4.2.Kinetic properties Multi-walled nanotubes are multiple concentric nanotubes precisely nested within one another. These exhibit a striking telescoping property whereby an inner nanotube core may slide, almost without friction, within its outer nanotube shell, thus creating an atomically perfect linear or rotational bearing. This is one of the first true examples of molecular nanotechnology, the precise positioning of atoms to create useful machines. Already, this property has been utilized to create the world's smallest rotational motor. Future applications such as a gigahertz mechanical oscillator are also envisaged. 4.3.Hardness Standard single-walled carbon nanotubes can withstand a pressure up to 24GPa without deformation. They then undergo a transformation to superhard phase nanotubes. Maximum pressures measured using current experimental techniques are around 55GPa. However, these new superhard phase nanotubes collapse at an even higher, albeit unknown, pressure. The bulk modulus of superhard phase nanotubes is 462 to 546 GPa, even higher than that of diamond(420 GPa for single diamond crystal). 4.4.Electrical properties Band structures computed using tight binding approximation for (6,0) CNT (zigzag, metallic) (10,2) CNT (semiconducting) and (10,10) CNT (armchair, metallic). Because of the symmetry and unique electronic structure of graphene, the structure of a nanotube strongly affects its electrical properties. For a given (n,m) nanotube, if n = m, the nanotube is metallic; if n ? m is a multiple of 3, then the nanotube is semiconducting with a very small band gap, otherwise the nanotube is a moderate semiconductor. Thus all armchair (n = m) nanotubes are metallic, and nanotubes (6,4), (9,1), etc. are semiconducting. However, this rule has exceptions, because curvature effects in small diameter carbon nanotubes can strongly influence electrical properties. Thus, a (5,0) SWCNT that should be semiconducting in fact is metallic according to the calculations. Likewise, vice versa—zigzag and chiral SWCNTs with small diameters that should be metallic have finite gap (armchair nanotubes remain metallic).In theory, metallic nanotubes can carry an electric current density of 4
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× 109 A/cm2, which is more than 1,000 times greater than those of metals such as copper where for copper interconnects current densities are limited by electromigration. Because of their nanoscale cross-section, electrons propagate only along the tube's axis and electron transport involves quantum effects. As a result, carbon nanotubes are frequently referred to as one-dimensional conductors. The maximum electrical conductance of a single-walled carbon nanotube is 2G0, where G0 = 2e2/h is the conductance of a single ballistic quantum channel. There have been reports of intrinsic superconductivity in carbon nanotubes. Many other experiments, however, found no evidence of superconductivity, and the validity of these claims of intrinsic superconductivity remains a subject of debate.
Fig.4.4.1(Electrical properties) 4.5. Thermal properties All nanotubes are expected to be very good thermal conductors along the tube, exhibiting a property known as "ballistic conduction", but good insulators laterally to the tube axis. Measurements show that a SWNT has a roomtemperature thermal conductivity along its axis of about 3500 W·m?1·K?1 compare this to copper, a metal well known for its goodthermal conductivity, which transmits 385 W·m?1·K?1. A SWNT has a room15
temperature thermal conductivity across its axis (in the radial direction) of about 1.52 W·m?1·K?1 which is about as thermally conductive as soil. The temperature stability of carbon nanotubes is estimated to be up to 2800 °C in vacuum and about 750 °C in air. 4.6. Defects As with any material, the existence of a crystallographic defect affects the material properties. Defects can occur in the form of atomic vacancies. High levels of such defects can lower the tensile strength by up to 85%. An important example is the Stone Wales defect, which creates a pentagon and heptagon pair by rearrangement of the bonds. Because of the very small structure of CNTs, the tensile strength of the tube is dependent on its weakest segment in a similar manner to a chain, where the strength of the weakest link becomes the maximum strength of the chain. Crystallographic defects also affect the tube's electrical properties. A common result is lowered conductivity through the defective region of the tube. A defect in armchair-type tubes (which can conduct electricity) can cause the surrounding region to become semiconducting, and single monoatomic vacancies induce magnetic properties. Crystallographic defects strongly affect the tube's thermal properties. Such defects lead to phonon scattering, which in turn increases the relaxation rate of the phonons. This reduces the mean free path and reduces the thermal conductivity of nanotube structures. Phonon transport simulations indicate that substitutional defects such as nitrogen or boron will primarily lead to scattering of high-frequency optical phonons. However, larger-scale defects such as Stone Wales defects cause phonon scattering over a wide range of frequencies, leading to a greater reduction in thermal conductivity.
5.SYNTHESIS:5.1..Arcdischarge Nanotubes were observed in 1991 in the carbon soot of graphite electrodes during an arc discharge, by using a current of 100 amps, that was intended to produce fullerenes. However the first macroscopic production of carbon nanotubes was made in 1992 by two researchers at NEC's Fundamental Research Laboratory. The method used was the same as in 1991. During this process, the carbon contained in the negative electrode sublimates
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because of the high-discharge temperatures. Because nanotubes were initially discovered using this technique, it has been the most widely used method of nanotube synthesis. The yield for this method is up to 30% by weight and it produces both singleand multi-walled nanotubes with lengths of up to 50 micrometers with few structural defects. 5.2.Laser ablation In the laser ablation process, a pulsed laser vaporizes a graphite target in a hightemperature reactor while an inert gas is bled into the chamber. Nanotubes develop on the cooler surfaces of the reactor as the vaporized carbon condenses. A water-cooled surface may be included in the system to collect the nanotubes. This process was developed by Dr. Richard Smalley and co-workers at Rice University, who at the time of the discovery of carbon nanotubes, were blasting metals with a laser to produce various metal molecules. When they heard of the existence of nanotubes they replaced the metals with graphite to create multiwalled carbon nanotubes. Later that year the team used a composite of graphite and metal catalyst particles (the best yield was from a cobalt and nickel mixture) to synthesize single-walled carbon nanotubes. 5.3.Chemical vapor deposition (CVD)
Fig.5.3.1(Chemical vapor deposition)
Nanotubes being grown by plasma enhanced chemical vapor deposition
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The catalytic vapor phase deposition of carbon was reported in 1952 and 1959, but it was not until 1993 that carbon nanotubes were formed by this process. In 2007, researchers at the University of Cincinnati (UC) developed a process to grow aligned carbon nanotube arrays of 18 mm length on a FirstNano ET3000 carbon nanotube growth system. During CVD, a substrate is prepared with a layer of metal catalyst particles, most commonly nickel, cobalt, iron, or a combination.The metal nanoparticles can also be produced by other ways, including reduction of oxides or oxides solid solutions. The diameters of the nanotubes that are to be grown are related to the size of the metal particles. This can be controlled by patterned (or masked) deposition of the metal, annealing, or by plasma etching of a metal layer. The substrate is heated to approximately 700°C. To initiate the growth of nanotubes, two gases are bled into the reactor: a process gas (such as ammonia, nitrogen or hydrogen) and a carbon-containing gas (such as acetylene, ethylene, ethanol or methane). Nanotubes grow at the sites of the metal catalyst; the carbon-containing gas is broken apart at the surface of the catalyst particle, and the carbon is transported to the edges of the particle, where it forms the nanotubes. This mechanism is still being studied. The catalyst particles can stay at the tips of the growing nanotube during the growth process, or remain at the nanotube base, depending on the adhesion between the catalyst particle and the substrate.Thermal catalytic decomposition of hydrocarbon has become an active area of research and can be a promising route for the bulk production of CNTs. Fluidised bed reactor is the most widely used reactor for CNT preparation. Scale-up of the reactor is the major challenge.
CVD is a common method for the commercial production of carbon nanotubes. For this purpose, the metal nanoparticles are mixed with a catalyst support such as MgO or Al2O3 to increase the surface area for higher yield of the catalytic reaction of the carbon feedstock with the metal particles. One issue in this synthesis route is the removal of the catalyst support via an acid treatment, which sometimes could destroy the original structure of the carbon nanotubes. However, alternative catalyst supports that are soluble in water have proven effective for nanotube growth. If a plasma is generated by the application of a strong electric field during the growth process (plasma enhanced chemical vapor deposition), then the nanotube growth will follow the direction of the electric field. By adjusting the geometry of the reactor it is possible to synthesize vertically aligned carbon
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nanotubes(i.e., perpendicular to the substrate), a morphology that has been of interest to researchers interested in the electron emission from nanotubes. Without the plasma, the resulting nanotubes are often randomly oriented. Under certain reaction conditions, even in the absence of a plasma, closely spaced nanotubes will maintain a vertical growth direction resulting in a dense array of tubes resembling a carpet or forest. Of the various means for nanotube synthesis, CVD shows the most promise for industrial-scale deposition, because of its price/unit ratio, and because CVD is capable of growing nanotubes directly on a desired substrate, whereas the nanotubes must be collected in the other growth techniques. The growth sites are controllable by careful deposition of the catalyst. In 2007, a team from Meijo University demonstrated a high-efficiency CVD technique for growing carbon nanotubes from camphor. Researchers at Rice University, until recently led by the late Richard Smalley, have concentrated upon finding methods to produce large, pure amounts of particular types of nanotubes. Their approach grows long fibers from many small seeds cut from a single nanotube; all of the resulting fibers were found to be of the same diameter as the original nanotube and are expected to be of the same type as the original nanotube. 5.3.Natural, incidental, and controlled flame environments Fullerenes and carbon nanotubes are not necessarily products of high-tech laboratories; they are commonly formed in such mundane places as ordinary flames, produced by burning methane, ethylene, and benzene, and they have been found in soot from both indoor and outdoor air. However, these naturally occurring varieties can be highly irregular in size and quality because the environment in which they are produced is often highly uncontrolled. Thus, although they can be used in some applications, they can lack in the high degree of uniformity necessary to satisfy the many needs of both research and industry. Recent efforts have focused on producing more uniform carbon nanotubes in controlled flame environments. Such methods have promise for large-scale, low-cost nanotube synthesis based on theoretical models, though they must compete with rapidly developing large scale CVD production.
6.APPLICATION:Application of nanotubes has mostly been limited to the use of bulk nanotubes, which is a mass of rather unorganized fragments of nanotubes. Bulk nanotube
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materials may never achieve a tensile strength similar to that of individual tubes, but such composites may, nevertheless, yield strengths sufficient for many applications. Bulk carbon nanotubes have already been used as composite fibers in polymers to improve the mechanical, thermal and electrical properties of the bulk product. ? Easton-Bell Sports, Inc. have been in partnership with Zyvex Performance Materials, using CNT technology in a number of their bicycle components including flat and riser handlebars, cranks, forks, seatposts, stems and aero bars. ? Zyvex Technologies has also built a 54' maritime vessel, the Piranha Unmanned Surface Vessel, as a technology demonstrator for what is possible using CNT technology. CNTs help improve the structural performance of the vessel, resulting in a lightweight 8,000 lb boat that can carry a payload of 15,000 lb over a range of 2,500 miles. ? Amroy Europe Oy manufactures Hybtonite carbon nanoepoxy resins where carbon nanotubes have been chemically activated to bond to epoxy, resulting in a composite material that is 20% to 30% stronger than other composite materials. It has been used for wind turbines, marine paints and variety of sports gear such as skis, ice hockey sticks, baseball bats, hunting arrows, and surfboards. 6.1. Hydrogen storage In addition to being able to store electrical energy, there has been some research in using carbon nanotubes to store hydrogen to be used as a fuel source. By taking advantage of the capillary effects of the small carbon nanotubes, it is possible to condense gases in high density inside single-walled nanotubes. This allows for gases, most notably hydrogen (H2), to be stored at high densities without being condensed into a liquid. Potentially, this storage method could be used on vehicles in place of gas fuel tanks for a hydrogen-powered car. A current issue regarding hydrogen-powered vehicles is the onboard storage of the fuel. Current storage methods involve cooling and condensing the H 2 gas to a liquid state for storage which causes a loss of potential energy (25 –45%) when compared to the energy associated with the gaseous state. Storage using SWNTs would allow one to keep the H2 in its gaseous state, thereby increasing the storage effciency. This method allows for a volume to energy ratio slightly smaller to that of current gas powered vehicles, allowing for a slightly lower but comparable range.
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6.2. Medical In the Kanzius cancer therapy, single-walled carbon nanotubes are inserted around cancerous cells, then excited with radio waves, which causes them to heat up and kill the surrounding cells. Researchers at Rice University, Radboud University Nijmegen Medical Centre and University of California, Riverside have shown that carbon nanotubes and their polymer nanocomposites are suitable scaffold materials for bone cell proliferation and bone formation. 6.3. Textile The previous studies on the use of CNTs for textile functionalization were focused on fiber spinning for improving physical and mechanical properties. Recently a great deal of attention has been focused on coating CNTs on textile fabrics. Various methods have been employed for modifying fabrics using CNTs. Shim et al. produced intelligent e-textiles for Human Biomonitoring using a polyelectrolyte-based coating with CNTs. Additionally, Panhuis et al. dyed textile material by immersion in either a poly (2-methoxy aniline-5sulfonic acid) PMAS polymer solution or PMAS-SWNT dispersion with enhanced conductivity and capacitance with a durable behavior.In another study, Hu and coworkers coated single-walled carbon nanotubes with a simple ?dipping and drying? process for wearable electronics and energy storage applications.CNTs have an aligned nanotube structure and a negative surface charge. Therefore, they have similar structures to direct dyes, so the exhaustion method is applied for coating and absorbing CNTs on the fiber surface for preparing multifunctional fabric including antibacterial, electric conductive, flame retardant and electromagnetic absorbance properties. 6.4.Toxicity The toxicity of carbon nanotubes has been an important question in nanotechnology. Such research has just begun. The data are still fragmentary and subject to criticism. Preliminary results highlight the difficulties in evaluating the toxicity of this heterogeneous material. Parameters such as structure, size distribution, surface area, surface chemistry,surface charge, and agglomeration state as well as purity of the samples, have considerable impact
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on the reactivity of carbon nanotubes. However, available data clearly show that, under some conditions, nanotubes can cross membrane barriers, which suggests that, if raw materials reach the organs, they can induce harmful effects such as inflammatory and fibrotic reactions. A study led by Alexandra Porter from the University of Cambridge shows that CNTs can enter human cells and accumulate in the cytoplasm, causing cell death. Results of rodent studies collectively show that regardless of the process by which CNTs were synthesized and the types and amounts of metals they contained, CNTs were capable of producing inflammation, epithelioid granulomas (microscopic nodules), fibrosis, and biochemical/toxicological changes in the lungs. Comparative toxicity studies in which mice were given equal weights of test materials showed that SWCNTs were more toxic than quartz, which is considered a serious occupational health hazard when chronically inhaled. As a control, ultrafine carbon black was shown to produce minimal lung responses. The needle-like fiber shape of CNTs is similar to asbestos fibers. This raises the idea that widespread use of carbon nanotubes may lead to pleural mesothelioma, a cancer of the lining of the lungs or peritoneal mesothelioma, a cancer of the lining of the abdomen (both caused by exposure to asbestos). A recently published pilot study supports this prediction.Scientists exposed the mesothelial lining of the body cavity of mice to long multiwalled carbon nanotubes and observed asbestos-like, length-dependent, pathogenic behavior that included inflammation and formation of lesions known as granulomas. Authors of the study conclude. According to co-author Dr. Andrew Maynard: This study is exactly the kind of strategic, highly focused research needed to ensure the safe and responsible development of nanotechnology. It looks at a specific nanoscale material expected to have widespread commercial applications and asks specific questions about a specific health hazard. Even though scientists have been raising concerns about the safety of long, thin carbon nanotubes for over a decade, none of the research needs in the current U.S. federal nanotechnology environment, health and safety risk research strategy address this question.
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7.ADVANTAGES:Composite materials reinforced with carbon nanotube "stitching" are not only ten times stronger than those that go without, but they are also one million times more electrically conductive, according to a report soon to be published in the Journal of Composite Materials. Benefits to aircraft structures therefore include increased strength and lightning protection along with decreased weight for composite aircraft. Carbon nanotubes are not new, and have been flown aboard aircraft, including a Giles G-200 high-performance homebuilt aerobatic aircraft in 2008, as part of that aircraft's cowling. But researchers at MIT say that through a process called "nanostitching," they can add nanotubes, "the strongest fibers known to humankind," to the composite's weakest part -- between its layers -- adding strength, without adding weight. And that's not nearly all they can do. Composite structures are strengthened by adding layers or plies of composite fiber on top of one another, bonded by resin. Nanostitching aligns rows of carbon nanotubes perpendicular to the layered fibers of a carbon composite structure, effectively filling the spaces between them and stitching the layers together without adding weight. Nanostitching does not add weight to the structure, because the nanotubes exist in space formerly taken up solely by heavier, weaker resin. The process actually increases strength and saves weight (as it was in the case of the G-200). • Extremely small and lightweight, making them excellent replacements for metallic wires • Resources required to produce them are plentiful, and many can be made with only a small amount of material • Are resistant to temperature changes, meaning they function almost just as well in extreme cold as they do in extreme heat • Have been in the R&D phase for a long time now, meaning most of the kinks have been worked out • As a new technology, investors have been piling into these R&D companies, which will boost the economy.
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8.DISADVANTAGES:The carbon nanotube degrades in a few days when exposed to oxygen. There has been several works done on passivating the nanotubes with different polymers and increasing their lifetime. Carbon nanotubes have shown reliability issues when operated under high electric field or temperature gradients. Avalanche breakdown occurs in semiconducting CNT and joule breakdown in metallic CNT. Unlike avalanche behavior in silicon, avalanche in CNTs is negligibly temperature-dependent. Applying high voltages beyond avalanche point results in Joule heating and eventual breakdown in CNTs. This reliability issue has been studied, and it is noticed that the multi-channeld structure can improve the reliability of the CNTFET. The multi-channeled CNTFETs can keep a stable performance after several months, while the single-channeled CNTFETs are usually out of work after a few weeks in the ambient atmosphere. The multi-channeled CNTFETs keep operating when some channels break down, this won’t happ en in the single-channeled ones.Although CNTs have unique properties such as stiffness, strength, and tenacity compared to other materials especially to silicon, we are still experiencing lack of technology for mass production and high production cost. To overcome the fabrication difficulties, several methods have been studied such as direct growth, solution dropping, and various transfer printing techniques. • Despite all the research, scientists still don't understand exactly how they work • • Extremely small, so are difficult to work with Currently, the process is relatively expensive to produce the nanotubes
• Would be expensive to implement this new technology in and replace the older technology in all the places that we could • At the rate our technology has been becoming obsolete, it may be a gamble to bet on this technology.
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CONCLUSION
As carbon-nanotube technology progresses, it becomes clearer each day that these materials will most likely constitute the foundation of tomorrow's society. They will be used in just about everything, from sports equipment to computer processors and next-generation electronic devices, but experts are still concerned about what effects they may have on the human body. Some studies proposed that the brain could defend itself against this type of intruders, but a new research suggests that the lungs may not be able to do this. The investigation revealed that CNT could adversely affect the outer lining of the lungs. The team behind the study, composed of scientists from the North Carolina State University (NC State), the Hamner Institutes for Health Sciences, and the National Institute of Environmental Health Sciences, admits, however, that more studies are required to confirm its finds. It adds that the long-term effects of exposure are still a mystery. Unsuspecting mice were used for the new experiments, which sought to test whether the new materials could be used without harm to humans. In the study, mouse models were subjected to a single dose of carbon nanotubes, and then their evolution was followed. The scientists discovered that an immune-system response began to form within one day from the inhalation stage, with clusters of lymphocytes and monocytes, two types of immunesystem cells, forming on the surface of the pleura, the outer lining of the lungs. They also discovered that some regions of the pleura had been scarred, a phenomenon known as fibrosis. This is common in people working in mines or with asbestos, when fine particles deposit themselves in the lungs, and reduce the organs' ability to process oxygen and carbon dioxide. Within thee months of exposure, all signs of the scarring and immune response in the mice disappeared, the team reports. It adds, however, that mice were subjected to only one exposure to the CNT, whereas, in real life, many more ?doses? would be inhaled by a single person. ?It remains unclear whether the pleura could recover from chronic, or repeated, exposures. More work needs to be done in that area and it is completely unknown at this point whether inhaled carbon nanotubes will prove to be carcinogenic in the lungs or in the pleural lining,? the senior author of the new study, NC State Associate Professor of Environmental and Molecular Toxicology Dr. James Bonner, explains.
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REFERENCES
1. Physics of Carbon-nanotubes(by M.S.Dresselhaus and R.Saito .M.I.T) 2. Nanotubes for Electronics (by Philip G.Collins and PhaedonAvouris) 3. Carbon Nanotube Applications in Microelectronics(by G.S.Dusberg) 4. Carbon Nanotubes (by Anthony Kendall and Elizabeth Pfaff) 5. Physical Properties of Carbon Nanotubes (by Thomas A.Adams) 6.www.wikipedia.org/wiki/Allotropes_of_carbon. 7.www.wikipedia.org/wiki/Nanotechnology. 8.www.wikipedia.org/wiki/Thermal_conductivity. 9.www.wikipedia.org/wiki/fullerene. 10.www.wikipedia.org/wiki/Graphene. 11.www.wikipedia.org/wiki/File:carbon_nanotube_zigzag_povray.PNG. 12.www.wikipedia.org/wiki/Torus. 13.www.wikipedia.org/wiki/Laser_ablation. 14.www.wikipedia.org/wiki/Kevlar. 15.www.wikipedia.org/wiki/Tensile_strength.
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