carbon nano tubes RRS feed

  • General discussion


































    Carbon Nano-tubes:
     An Overview





    •Manufacturing Techniques

    •Current Applications

    • Defects

    • Future Applications























    This paper throws an insight on the facets of a technology that is fast emerging and in latest use…nano carbon tubes an advent of nano technology. This paper holds views on basic ideals of nano carbon tubes, history and structure. This includes the manufacturing techniques of nano tubes and an analysis on it along with the perspective applications.


    The current applications include a vast field and hold large future prospects…also discussed are the defects of nano carbon tubes. Besides the minimal defects, nano carbon tubes hold its place in future…as a boon for the humankind.














    Important History

    •1991 Discovery of multi-wall carbon nanotubes

    •1992 Conductivity of carbon nanotubes      

    •1993 Structural rigidity of carbon nanotubes

    •1993 Synthesis of single-wall nanotubes        •1995 Nanotubes as field emitters

    •1997 Hydrogen storage in nanotubes             •1998 Synthesis of nanotube peapods

    •2000 Thermal conductivity of nanotubes

    •2001 Integration of carbon nanotubes for logic circuits

    •2001 Intrinsic superconductivity of carbon nanotubes








    Carbon nanotubes are molecular-scale tubes of graphitic carbon with outstanding properties. They are among the stiffest and strongest fibres known, and have remarkable electronic properties and many other unique characteristics. For these reasons they have attracted huge academic and industrial interest.

    Nano-tubes are generally hollow cylindrical structures made up of carbon atoms having high strength and low weight so that they can be used for manufacturing of satellites and spacecrafts. They exist in various shapes - cylinders, spheres, cones, tubes and also complicated shapes


    Single-wall carbon nanotubes are a new form of carbon made by rolling up a single graphite sheet to a narrow but long tube closed at both sides by fullerene-like end caps

            Single Walled Nano-tubes (SWNT) can be considered as long graphene sheets. Nano-tubes generally have a length to diameter ratio of 1000 so they can be approximated as one-dimensional structures. Multi Walled Nano-tubes (MWNT) can be considered as a collection of concentric SWNTs with different diameters, lengths, and properties






      Metallic conductivity (e.g. the salts A3C60 (A=alkali metals))

      Superconductivity of up to 33K (e.g. the salts A3C60 (A=alkali                metals)) with Tc's

    Ferromagnetism (in (TDAE)C60 - without the presence of d-electrons)

    Non-linear optical activity

         Polymerization to form a variety of 1-,2-, and 3D polymer structures




    •These one-dimensional fibers exhibit electrical conductivity as high as copper, thermal conductivity as high as diamond,

    •Nanotubes can be either electrically conductive or semiconductive, depending on their helicity.

    •Strength 100 times greater than steel at one sixth the weight, and high strain to failure.

    •Current length limits are about one millimeter




    The bonding in carbon nanotubes is sp², with each atom joined to three neighbours, as in graphite. The tubes can therefore be considered as rolled-up graphene sheets (graphene is an individual graphite layer). There are three distinct ways in which a graphene sheet can be rolled into a tube


    The structure of a nanotube can be specified by a vector, (n,m), which defines how the graphene sheet is rolled up. This can be understood with reference to figure . To produce a nanotube with the indices (6,3), say, the sheet is rolled up so that the atom labelled (0,0) is superimposed on the one labelled (6,3). It can be seen from the figure that m = 0 for all zigzag tubes, while n = m for all armchair tubes.



    Carbon nano-tube synthesis Carbon nano-tubes are generally produced by three main techniques:  

    1. Arc discharge        

    In this synthesis technique, vapor is created by an arc      discharge between two carbon electrodes with or without catalyst. Nano-tubes self-assemble from the resulting carbon vapor. This method generally produces large quantities of impure material.


    2. Laser ablation

    In the Laser ablation technique, a pulsed or continuous Nd YAG laser impinges on a graphite target placed in an oven at 1200º C and 500 Torr. The graphite target gets vaporized to form small carbon molecules and atoms which condense to form single walled nano-tubes held together by van der Waals forces. SWNT are produced if the target consists of graphite mixed with cobalt, nickel or iron. A pure graphite target yields MWNT.

    3. Chemical vapor deposition

    Chemical vapor deposition (CVD) results in MWNTs or poor quality SWNTs. The SWNTs produced with CVD have a large diameter range, which can be poorly controlled





    Nanostructures of carbon and boron nitride

     Properties and perspectives of applications


    Nano-structures provide interesting perspectives of applications because of their unique properties. Many possible developments have been suggested (mainly for carbon nano-tubes), although none of them is at a stage of commercialisation yet. One practical difficulty is, obviously, the manipulation of individual structures on the nanometre range (as required for nano-electronics, for instance). This is possible in laboratory conditions, at the tip of a scanning microscope for instance. However a large-scale manipulation of individual nano-objets is not achieved. Another difficulty is that, synthesis methods are still little efficient. Similar results can now be obtained for BN and carbon tubes. But, production in kg quantities at low cost is still not achieved. Also, the technical ability in synthesising selected structures is still limited, because most synthesis methods produce particles with a large structural


    diversity. There is a need to develop post synthesis techniques for particle segregation.
    Note: POSSIBLE TOXICITY. It should be stressed that it is not yet clear if nano-tube-based material have some toxicity. As a volatile fibrous material, nano-tubes may cause damage lung cells, as it is famously the case of asbestos fibres

    . Some mechanical applications

    Composite materials

    The strength of a composite material is linked to the strength of the fibres embedded in the matrix. As both carbon and BN nano-tubes  have an exceptional elastic modulus, using them as reinforcement fibres is a possible way to obtain ultra resistant materials. For its good chemical inertness, especially to oxygen, BN is the best candidate.
    The Typical problem encountered practically is the adherence at contact surface between tubes and matrix material. Tubes may actually slip along
    the matrix material. This may be worse for ropes, in which tubes can slip on each other. (Tubes with a spherical extremity, as synthesised by the present method, could lower this problem, because spherical extremities may fix as anchor in the matrix.)

    Solid lubricants

    Solid lubricants are used when conditions do not allow the usage of standard lubrication oil. This is typically under vacuum or in oxidising atmosphere. h-BN, graphite and WS2 are already intensively used as a solid lubricant in industry. h-BN is especially interesting for having both a very low friction coefficient and a high range of suitable temperature in air (up to 900 °C). [1.1.5] Nano-onions powders are exceptional solid lubricants, because onions act like nanometric ball bearings. This was demonstrated for powders of WS2 nano-onions , but it is probably true for the other materials.

    Filters, Tissues, insulator materials

    As any fibres, tubes and ropes could be applied to filters, tissues, thermal or acoustic insulator, or any other fibrous material. The exceptional porosity of such a material, due to the high aspect ration of nano-tubes, enables a filtering of much smaller particles, and/or a higher crossing flux (because the efficient collision surface is very small). For instance, this could be applied on a car, to filter gases in the exhaustive pipe.
    Other properties of nano-tube tissues are not obvious, like thermal conductance, or resistance to tearing. Further studies are clearly needed, but this first requires an improvement of mass production

    . Some chemical applications


    Nano-tubes and onions of carbon have the ability to shell many materials inside their structure. It has been confirmed for many simple elements (Y, Bi, Gd, Ti, Cr, Fe, Zn, Mo, Pd, Sn, Ta, W, Gd, Dy, Yb, Pb, Mn, Co, Ni, Cu, Si, Ge...) and for some compounds. [6.7.6]. This may be of some interest, to protect nano-material from their environment, especially from oxidation. For instance, magnetic particles for data storage could be protected from air. It also offers a possibility to synthesise diverse hybrid nano-object, like metallic nano-rods, inside a tube cavity.

    Hydrogen containers

    Industrial age is causing an exponential growth of CO2 concentration in the atmosphere, mainly due to extensive use of fossil energy sources. The harmfulness of such a change on the ecosystem, first expected as a "global warming effect", makes research on non-polluting energy sources a priority. Hydrogen is the ideal candidate, because its combustion produces no other release than water. Its energy per mass is higher than usual hydrocarbons. Furthermore, it is present in high quantity on earth.
    Practically, the main limit to the commercialisation of hydrogen motor is the difficulty for a safe way to store hydrogen. Nano-tubes and nano-onions are thought to be a safe storage, because their cavity can absorb hydrogen molecules. Such storage was measured with variable success, between 0 and 10 wt % in carbon tubes

    . Some electronic applications


    The conductivity of a carbon SWNT depends on its diameter and helicity, hence on its structure . Different carbon nano-tubes can theoretically form a nano-sized junction, which is a first step toward a "nano-electronic". A nano-transistor was realised through the body of a carbon nano-tube.
    One practical difficulty is the manipulation on nanometre scale. A focused ion beam (FIB) can be used to make the contact electrodes, but not on a large production scale. Another problem is to synthesise carbon tubes with a specific structure. This is not affordable with present synthesis methods.
    The case of BN is different, conductivity is little dependent on structure. But on the other hand, it is an insulator of large band gap (~ 5eV, like diamond). To be used as a conductor, it should be efficiently doped. Diverse possibility of hetero-structures, like C/BN or C/Si are studied at present.

    AFM / STM Tips

    Carbon nano-tubes are very thin. They can be used as a nano-tip in both Atomic force microscopy (AFM) and tunnelling microscopy, to improve the resolution of the image. Furthermore, the exceptional elasticity of the tip avoids the damage from contact surface. It also enables an improved resolution of surface irregularities, because the tip can enter small cavities. Such a carbon nano-tube tip was proved to be feasible [2.7].
    One problem is the reactivity of the tip with surface material. Carbon forms bounds with many materials. BN can be used instead to avoid this problem, in the case of AFM. (BN is probably not suitable for tunnel microscope, because of its electric resistance.)

    Electron field emission

    Electron sources are essential for screens or electron microscopes. Carbon nano-tubes can emit a high electron field emission current from their tip, when submitted to a bias voltage. The threshold voltage is exceptionally low because of the tip curvature. Emitting surfaces were realised by different post-synthesis methods. A prototype display and a lighting element were already produced. Hence, this application may seem the closest to commercialisation at present. However, many technical problems are still to be solved, regarding emission surface fabrication and, the understanding of the emission phenomenon.
    Carbon nano-tubes are also interesting for electron microscope emitters, because the emission form a single nano-tube is intense and very coherent. The
    energy dispersion of such a beam is of the order 0.2 eV, and the life length of a single tube was found in the order of 2 months, in emission conditions.


    Kuznetsov inthe journal Carbon[2] has described the interesting and often misstated origin of the carbon nanotube. A large percentage of academic and popular literature attributes the discovery of hollow, nanometer sized tubes composed of graphitic carbon to Sumio Iijima of NEC in 1991. However, the history of nanometer sized graphitic carbon tubes dates as far back as 1952. In that year, Radushkevich and Lukyanovich published clear images of 50 nanometer diameter tubes made of carbon in the Russian Journal of Physical Chemistry. It is possible that carbon nanotubes were produced before this date, but the invention of the transmission electron microscope allowed the direct visualization of these structures. This discovery was largely unnoticed in the West because of limited information exchange during the Cold War and because the article was pubished in the Russian language.

    Iijima's discovery of carbon nanotubes in the insoluble material of arc-burned graphite rods[3] created the buzz that is now associated with carbon nanotubes. The arc discharge technique was well-known to produce the famed Buckminster fullerene on a preparative scale,[4] and this was just one more accidental discovery relating to fullerenes. The original observation of fullerenes in mass spectrometry was not anticipated,[5] and the first mass-production technique by Kratchmer and Huffman was used for several years before realising that it produced fullerenes.Devil

    It seemed fitting that nanotubes were also serendipitously discovered. However, a paper by Oberlin, Endo, and Koyama published in 1976 clearly showed hollow carbon fibres with nanometer-scale diameters using a vapour-growth technique.[7] Also, in 1987, George Tennent of Hyperion Catalysis was issued a U.S. patent for the production of "cylindrical discrete carbon fibrils" with a "constant diameter between about 3.5 and about 70 nanometers…, length 102 times the diameter, and an outer region of multiple essentially continuous layers of ordered carbon atoms and a distinct inner core….Music" More recently, Endo has been credited with discovering CNTs, and Iijima has been credited for elucidating the structure of NTs. One aspect of the structure of carbon nanotubes is their one-dimensional structure and hollow interior. The one dimensional structure of nanotubes is of great interest to physicists because it permits experiments in one-dimensional quantum physics. The hollow core of carbon nanotubes is also of interest to chemists because it permits encapsulation of molecules, reactions in confined spaces, and controlled release of molecules for applications such as drug delivery.







    The (n,m) nanotube naming scheme can be thought of as a vector (Ch) in an infinite graphene sheet that describes how to "roll up" the graphene sheet to make the nanotube. T denotes the tube axis, and a1 and a2 are the unit vectors of graphene in real space.

    Most SWNTs have a diameter of close to 1 nm, with a tube length that can be many thousands of times larger. SWNTs with length up to orders of centimeters have been produced (Zhu, et al., 2002). 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) called the chiral vector. The integers n and m denote the number of unit vectors along two directions in the honeycomb crystal lattice of graphene. If m=0, the nanotubes are called "zigzag". If n=m, the nanotubes are called "armchair". Otherwise, they are called "chiral".

    SWNTs are a very important variety of carbon nanotube because they exhibit important electric properties that are not shared by the multi-walled carbon nanotube (MWNT) variants. SWNTs are the most likely candidate for miniaturizing electronics past the microelectromechanical scale that is currently the basis of modern electronics. The most basic building block of these systems is the electric wire, and SWNTs can be excellent conductors (Dekker, et al., 1999). One useful application of SWNTs is in the development of the first intramolecular field effect transistors (FETs). The production of the first intramolecular logic gate using SWNT FETs has recently become possible as well (Derycke, et al., 2001). To create a logic gate you must have both a p-FET and an n-FET. Because SWNTs are p-FETs when exposed to air and n-FETs when unexposed to oxygen, they were able to protect half of a SWNT from oxygen exposure, while exposing the other half to oxygen. The result was a single SWNT that acted as a NOT logic gate with both p and n-type FETs within the same molecule.

    SWNTs are still very expensive to produce, and the development of more affordable synthesis techniques is vital to the future of carbon nanotechnology. If cheaper means of synthesis cannot be discovered, it would make it financially impossible to apply this technology to commercial-scale applications.



    Multiwalled nanotubes (MWNT) consist of multiple layers of graphite rolled in on themselves to form a tube shape. There are two models which can be used to describe the structures of multiwalled nanotubes. In the Russian Doll model, sheets of graphite are arranged in concentric cylinders, eg a (0,8) SWNT within a larger (0,10) SWNT. In the Parchment model, a single sheet of graphite is rolled in around itself , resembling a scroll of parchment or a rolled up newspaper.


    Fullerite is a highly incompressible nanotube form. Polymerized single walled nanotubes (P-SWNT) are a class of fullerites and are comparable to diamond in terms of hardness (see also ultrahard fullerite).



    A nanotorus is a carbon nanotube bent into a torus (donut shape). Nanotori have many unique properties, such as magnetic moments 1000 times larger than previously expected for certain specific radii. Many properties such as magnetic moment, thermal stability, etc. vary widely depending on radius of the torus and radius of the tube.




    Carbon nanotubes are one of thestrongest materials known to man, both in terms of tensile strength and elastic modulus. This strength results from the covalent sp2 bonds formed between the individual carbon atoms. In 2000, an MWNT was tested to have a tensile strength of 63 GPa [9]. In comparison, high-carbon steel has a tensile strength of approximately 1.2 GPa. CNTs also have very high elastic modulus, on the order of 1 TPa [10]. Since carbon nanotubes have a low density for a solid of 1.3-1.4 g/cm3, its specific strength is the best of known materials.

    Under excessive tensile strain, the tubes will undergo plastic deformation, which means the deformation is permanent. This deformation begins at strains of approximately 5% [Qian et al, 2002] and can increase the maximum strain the tube undergoes before fracture by releasing strain energy.

    CNTs are not nearly as strong under compression. Because of their hollow structure, they tend to undergo buckling when placed under compressive, torsional or bending stress.



    Multiwalled carbon nanotubes, multiple concentric nanotubes precisely nested within one another, 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[11][12]. 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[13] and a nanorheostat[14]. Future applications such as a gigahertz mechanical oscillator are envisioned[15]. Synthesis

    Techniques have been developed to produce nanotubes in sizeable quantities, including arc discharge, laser ablation, high pressure carbon monoxide (HiPco), and chemical vapor deposition (CVD). Most of these processes take place in vacuum or with process gases. CVD growth of CNTs can take place in vacuum or at atmospheric pressure. Large quantities of nanotubes can be synthesized by these methods; advances in catalysis and continuous growth processes are making CNTs more commercially viable.


    Arc discharge

    Nanotubes were observed in 1991 in the carbon soot of graphite electrodes during an arc discharge that was intended to produce fullerenes. During this process, the carbon contained in the negative electrode sublimates because of the high temperatures caused by the discharge. Because nanotubes were initially discovered using this technique, it has been perhaps the most widely used method of nanotube synthesis.


    Laser ablation

    In the laser ablation process, a pulsed laser vaporizes a graphite target in a high temperature reactor while an inert gas is bled into the chamber. The 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.


    Chemical vapor deposition (CVD)

    Nanotubes being grown by plasma enhanced chemical vapor deposition

    The catalytic vapor phase deposition of carbon was first reported in 1959[17], but it was not until 1993[18] that carbon nanotubes could be formed by this process.

    During CVD, a substrate is prepared with a layer of metal catalyst particles, most commonly nickel, cobalt, iron, or a combination. 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, hydrogen, etc.) and a carbon-containing gas (such as acetylene, ethylene, ethanol, etc.). 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. The catalyst particles generally stay at the tips of the growing nanotube during the growth process, although in some cases they remain at the nanotube base, depending on the adhesion between the catalyst particle and the substrate.

    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[19]. By properly adjusting the geometry of the reactor it is possible to synthesize vertically aligned carbon 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 randomly oriented, resembling a bowl of spaghetti.

    Of the various means for nanotube synthesis, CVD shows the most promise for industrial scale deposition in terms of its price/unit ratio. There are additional advantages to the CVD synthesis of nanotubes. Unlike the above methods, 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. Additionally, no other growth methods have been developed to produce vertically aligned nanotubes.

    clothes: waterproof tear-resistant cloth fibers

    combat jackets: MIT is working on combat jackets that use carbon nanotubes as ultrastrong fibers and to monitor the condition of the wearer.[citation needed]

    concrete: In concrete, they increase the tensile strength, and halt crack propagation.

    polyethylene: Researchers have found that adding them to polyethylene increases the polymer's elastic modulus by 30%.

    sports equipment: Stronger and lighter tennis rackets, bike parts, golf balls, golf clubs, and baseball bats.

    space elevator: If tensile strengths of more than about 70 GPa can be achieved. Monoatomic oxygen in the Earth's upper atmosphere would erode carbon nanotubes at some altitudes, so a space elevator constructed of nanotubes would need to be protected (by some kind of coating). Carbon nanotubes in other applications would generally not need such surface protection.

    ultrahigh-speed flywheels: The high strength/weight ratio enables very high speeds to be achieved.


    artificial muscles [25]

    buckypaper - a thin sheet made from nanotubes that are 250 times stronger than steel and 10 times lighter that could be used as a heat sink for chipboards, a backlight for LCD screens or as a faraday cage to protect electrical devices/aeroplanes.

    chemical nanowires: Carbon nanotubes additionally can also be used to produce nanowires of other chemicals, such as gold or zinc oxide. These nanowires in turn can be used to cast nanotubes of other chemicals, such as gallium nitride. These can have very different properties from CNTs - for example, gallium nitride nanotubes are hydrophilic, while CNTs are hydrophobic, giving them possible uses in organic chemistry that CNTs could not be used for.

    computer circuits: A nanotube formed by joining nanotubes of two different diameters end to end can act as a diode, suggesting the possibility of constructing electronic computer circuits entirely out of nanotubes. Because of their good thermal properties, CNTs can also be used to dissipate heat from tiny computer chips. The longest electricity conducting circuit is a fraction of an inch long.(Source: June 2006 National Geographic).

    conductive films: A 2005 paper in Science notes that drawing transparent high strength swathes of SWNT is a functional production technique (Zhang et. al., vol. 309, p. 1215). Additionally, Eikos Inc.[26] of Franklin, Massachusetts is developing transparent, electrically conductive films of carbon nanotubes to replace indium tin oxide (ITO) in LCDs, touch screens, and photovoltaic devices. Carbon nanotube films are substantially more mechanically robust than ITO films, making them ideal for high reliability touch screens and flexible displays. Nanotube films show promise for use in displays for computers, cell phones, PDAs, and ATMs.

    electric motor brushes: Conductive carbon nanotubes have been used for several years in brushes for commercial electric motors. They replace traditional carbon black, which is mostly impure spherical carbon fullerenes. The nanotubes improve electrical and thermal conductivity because they stretch through the plastic matrix of the brush. This permits the carbon filler to be reduced from 30% down to 3.6%, so that more matrix is present in the brush. Nanotube composite motor brushes are better-lubricated (from the matrix), cooler-running (both from better lubrication and superior thermal conductivity), less brittle (more matrix, and fiber reinforcement), stronger and more accurately moldable (more matrix). Since brushes are a critical failure point in electric motors, and also don't need much material, they became economical before almost any other application.

    light bulb filament: alternative to tungsten filaments in incandescent lamps.

    magnets: MWNTs coated with magnetite

    optical ignition: A layer of 29% iron enriched SWNT is placed on top of a layer of explosive material such as PETN, and can be ignited with a regular camera flash.

    solar cells: GE's carbon nanotube diode has a photovoltaic effect. Nanotubes can replace ITO in some solar cells to act as a transparent conductive film in solar cells to allow light to pass to the active layers and generate photocurrent.

    superconductor: Nanotubes have been shown to be superconducting at low temperatures.

    ultracapacitors: MIT is researching the use of nanotubes bound to the charge plates of capacitors in order to dramatically increase the surface area and therefore energy storage ability [27].

    displays: One use for nanotubes that has already been developed is as extremely fine electron guns, which could be used as miniature cathode ray tubes in thin high-brightness low-energy low-weight displays. This type of display would consist of a group of many tiny CRTs, each providing the electrons to hit the phosphor of one pixel, instead of having one giant CRT whose electrons are aimed using electric and magnetic fields. These displays are known as field emission displays (FEDs).

    transistor: developed at Delft, IBM, and NEC.



    air pollution filter: Future applications of nanotube membranes include filteringcarbon dioxid from power plant emissions [28].

    biotech container: Nanotubes can be opened and filled with materials such as biological molecules, raising the possibility of applications in biotechnology.

    water filter: Recently nanotube membranes have been developed for use in filtration. This technique can purportedly reduce desalination costs by 75%. The tubes are so thin that small particles (like water molecules) can pass through them, while larger particles (such as the chloride ions in salt) are blocked.



    oscillator: fastest known oscillators (> 50 GHz).

    liquid flow array: Liquid flows up to five orders of magnitude faster than predicted through array.

    slick surface: slicker than Teflon and waterproof.



    Natural and incidental

    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,[20] ethylene,[21] and benzene,[22] and they have been found in soot from both indoor and outdoor air.[23] However, these naturally occurring varieties, because of the highly uncontrolled environment in which they are produced, are highly irregular in size and quality, lacking the high degree of uniformity necessary to meet the needs of both research and industry.


    oscillator: fastest known oscillators (>Carbon Nano-tubes

    Carbon nano-tube structure

    Nano-tubes are generally hollow cylindrical structures made up of carbon atoms having high strength and low weight so that they can be used for manufacturing of satellites and spacecrafts. They exist in various shapes - cylinders, spheres, cones, tubes and also complicated shapes.

    Single Walled Nano-tubes (SWNT) can be considered as long graphene sheets. Nano-tubes generally have a length to diameter ratio of 1000 so they can be approximated as one-dimensional structures.

    VRML model of Single Walled Nano-tubes created at VEEL

    Multi Walled Nano-tubes (MWNT) can be considered as a collection of concentric SWNTs with different diameters, lengths, and properties.

    VRML model of Multi Walled Nano-tube created at VEEL

    Carbon nano-tube synthesis

    Carbon nano-tubes are generally produced by three main techniques:

    1. Arc discharge

    In this synthesis technique, vapor is created by an arc discharge between two carbon electrodes with or without catalyst. Nano-tubes self-assemble from the resulting carbon vapor. This method generally produces large quantities of impure material.

    2. Laser ablation

    The model below shows the Laser ablation technique, in which a pulsed or continuous Nd YAG laser impinges on a graphite target placed in an oven at 1200º C and 500 Torr. The graphite target gets vaporized to form small carbon molecules and atoms which condense to form single walled nano-tubes held together by van der Waals forces. SWNT are produced if the target consists of graphite mixed with cobalt, nickel or iron. A pure graphite target yields MWNT.

    VRML animation of Laser ablation (before self assembly) created at VEEL


    VRML animation of Laser ablation (after self assembly) created at VEEL


    3. Chemical vapor deposition

    Chemical vapor deposition (CVD) results in MWNTs or poor quality SWNTs. The SWNTs produced with CVD have a large diameter range, which can be poorly controlled.


    Nano-gears below are modeled using VRML 2.0 to demonstrate that gears constructed from nano-tubes may replace the current manufacturing processes.



    VRML model of Nano Gears created at VEEL

    Researchers sometimes resort to manipulating nanotubes one-by-one with the tip of an atomic force microscope in a painstaking, time-consuming process. Perhaps the best hope is that carbon nanotubes can be grown through a chemical vapor deposition process from patterned catalyst material on a wafer, which serve as growth sites and allow designers to position one end of the nanotube. During the deposition process, an electric field can be applied to direct the growth of the nanotubes, which tend to grow along the field lines from negative to positive polarity. Another way for the self assembly of the carbon nanotube transistors consist in using chemical or biological techniques to place the nanotubes from solution to determinate place on a substrate.

    Even if nanotubes could be precisely positioned, there remains the problem that, to this date, engineers have been unable to control the types of nanotubes—metallic, semiconducting, single-walled, multi-walled—produced. A chemical engineering solution is needed if nanotubes are to become feasible for commercial circuits.


    As fiber and film

    One application for nanotubes that is currently being researched is high tensile strength fibers. Two methods are currently being tested for the manufacture of such fibers. A French team has developed a liquid spun system that involves pulling a fiber of nanotubes from a bath which yields a product that is approximately 60% nanotubes[citation needed]. The other method, which is simpler but produces weaker fibers uses traditional melt-drawn polymer fiber techniques with nanotubes mixed in the polymer. After drawing, the fibers can have the polymer component burned out of them leaving only the nanotube or they can be left as they are.

    Ray Baughman's group from the NanoTech Institute at University of Texas at Dallas produced the current toughest material known in mid-2003 by spinning fibers of single wall carbon nanotubes with polyvinyl alcohol. Beating the previous contender, spider silk, by a factor of four, the fibers require 600 J/g to break [29] In comparison, the bullet-resistant fiber Kevlar is 27–33 J/g. In mid-2005 Baughman and co-workers from Australia's Commonwealth Scientific and Industrial Research Organization developed a method for producing transparent carbon nanotube sheets 1/1000th the thickness of a human hair capable of supporting 50,000 times their own mass. In August 2005, Ray Baughman's team managed to develop a fast method to manufacture up to seven meters per minute of nanotube tape [30]. Once washed with ethanol, the ribbon is only 50 nanometers thick; a square kilometer of the material would only weigh 30 kilograms.

    In 2004 Alan Windle's group of scientists at the Cambridge-MIT Institute developed a way to make carbon nanotube fiber continuously at the speed of several centimetres per second just as nanotubes are produced. One thread of carbon nanotubes was more than 100 metres long. The resulting fibers are electrically conductive and as strong as ordinary textile threads.[31] [32]Electrical

    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 2n + m=3q (where q is an integer), then the nanotube is metallic, otherwise the nanotube is a semiconductor. Thus all armchair (n=m) nanotubes are metallic, and nanotubes (5,0), (6,4), (9,1), etc. are semiconducting. In theory, metallic nanotubes can have an electrical current density more than 1,000 times greater than metals such as silver and copper.



    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.



    Another well-known form of defect that occurs in carbon nanotubes is known as the Stone Wales defect, which creates a pentagon and heptagon pair by rearrangement of the bonds. because of the almost one-dimensional structure of CNTs, the tensile strength of the tube is on the weakest segment of it in a similar manner to a chain, where a defect in a single link diminishes the strength of the entire chain.

    The tube's electrical properties are also affected by the presence of defects. A common result is the lowered conductivity through the defective region of the tube. Some defect formation in armchair-type tubes (which are metallic) can cause the region surrounding that defect to become semiconducting. Furthermore single monoatomic vacancies induce magnetic properties.

    The tube's thermal properties are heavily affected by defects. 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.







    ·       properties—fro

    Future Applications



    Molecular transistors.

    Field emitters.

    Building blocks for bottom-up electronics.

    Smaller, lighter weight components for next generation spacecraft.

    Enable large quantities of hydrogen to be stored in small low-pressure tanks.

    Space elevator, Instead of blasting off for the heavens astronauts could reach the ISS as easily as they would a department store: Next floor, mars, watch your step please!







                                        Thus  nano carbon tubes are discussed in brief  …… commenting upon its magnanimous use and feasibilities.

    Saturday, February 10, 2007 4:18 PM