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Carbon nanotubes (CNTs ) are allotropes of carbon witha cylindrical nanostructure . Nanotubes have been constructedwith length-to-diameter ratio of up to132,000,000:1 ,[1] significantly larger than for any other material.These cylindrical carbon molecules have unusual properties,which are valuable for nanotechnology , electronics, optics andother fields of materials science and technology. In particular,owing to their extraordinary thermal conductivity andmechanical and electrical properties, carbon nanotubes findapplications as additives to various structural materials. For instance, nanotubes form a tiny portion of the material(s) insome (primarily carbon fiber ) baseball bats, golf clubs, or car parts .[2]

Nanotubes are members of the fullerene structural family. Their name is derived from their long, hollow structure with the wallsformed 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 radiusdecides the nanotube properties; for example, whether theindividual 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.

Applied quantum chemistry , specifically, orbitalhybridization best describes chemical bonding in nanotubes.

The chemical bonding of nanotubes is composed entirely

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of sp 2 bonds , similar to those of graphite. These bonds, whichare stronger than the sp 3 bonds found in alkanes and diamond,provide nanotubes with their unique strength.

Types of carbon nanotubes and relatedstructuresSingle-walled

Armchair (n ,n) i.e.: m=n

The translation vector is bent, while the chiral vector stays straight

Graphene nanoribbon

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The chiral vector is bent, while the translation vector stays straight

Zigzag (n ,0)

Chiral (n ,m)

n and m can be counted at the end of the tube

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Graphene nanoribbon

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 graphenesheet to make the nanotube. Tdenotes the tube axis, and a 1 and a 2 are theunit vectors of graphene in real space.

A scanning tunneling microscopy image of single-walled carbon nanotube

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A transmission electron microscopy image of a single-walled carbonnanotube

Most single-walled nanotubes (SWNT) have a diameter of close to 1 nanometer, with a tube length that can be manymillions of times longer. The structure of a SWNT can beconceptualized by wrapping a one-atom-thick layer of graphitecalled graphene into a seamless cylinder. The way thegraphene sheet is wrapped is represented by a pair of indices(n ,m). The integers n and m denote the number of

unit vectors along two directions in the honeycomb crystallattice of graphene. If m = 0, the nanotubes are called zigzagnanotubes, and if n = m , the nanotubes are called armchair nanotubes. Otherwise, they are called chiral. The diameter of an ideal nanotube can be calculated from its (n,m) indices asfollows

where a = 0.246 nm.

SWNTs are an important variety of carbon nanotubebecause most of their properties change significantly withthe (n ,m) values, and this dependence is non-monotonic(see Kataura plot). In particular, their band gap can varyfrom zero to about 2 eV and their electrical conductivity canshow metallic or semiconducting behavior. Single-wallednanotubes are likely candidates for miniaturizing electronics.The most basic building block of these systems is theelectric wire, and SWNTs with diameters of an order of ananometer can be excellent conductors .[3][4] One useful

application of SWNTs is in the development of the first

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intermolecular field-effect transistors (FET). The firstintermolecular logic gate using SWCNT FETs was made in2001.[5] A logic gate requires both a p-FET and an n-FET.Because SWNTs are p-FETs when exposed to oxygen andn-FETs otherwise, it is possible to protect half of an SWNTfrom oxygen exposure, while exposing the other half tooxygen. This results in a single SWNT that acts as a NOTlogic gate with both p and n-type FETs within the samemolecule.

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 as-produced 40 60% by weightSWNTs as of March 2010. [citation needed ]

Multi-walled [edit source | editbeta]

A scanning electron microscopy image of carbon nanotubes bundles

Triple-walled armchair carbon nanotube

Multi-walled nanotubes (MWNT) consist of multiple rolledlayers (concentric tubes) of graphene. There are two models

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that can be used to describe the structures of multi-wallednanotubes. In the Russian Doll model, sheets of graphite arearranged in concentric cylinders, e.g., a (0,8) single-wallednanotube (SWNT) within a larger (0,17) single-wallednanotube. 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 inmulti-walled nanotubes is close to the distance betweengraphene layers in graphite, approximately 3.4 . The

Russian Doll structure is observed more commonly. Itsindividual shells can be described as SWNTs, which can bemetallic or semiconducting. Because of statistical probabilityand restrictions on the relative diameters of the individualtubes, one of the shells, and thus the whole MWNT, isusually a zero-gap metal.

Double-walled carbon nanotubes (DWNT) form a specialclass of nanotubes because their morphology and propertiesare similar to those of SWNT but their resistance tochemicals is significantly improved. This is especiallyimportant when functionalization is required (this meansgrafting 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 someC=C double bonds , leaving "holes" in the structure on thenanotube and, thus, modifying both its mechanical andelectrical properties. In the case of DWNT, only the outer wall is modified. DWNT synthesis on the gram-scale was

first proposed in 2003[6]

by the CCVD technique, from the

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selective reduction of oxide solutions in methane andhydrogen.

The telescopic motion ability of inner shells[7]

and their unique mechanical properties [8] will permit the use of multi-walled nanotubes as main movable arms in comingnanomechanical devices. Retraction force that occurs totelescopic motion caused by the Lennard-Jonesinteraction between shells and its value is about 1.5 nN .[9]

Torus[edit source | editbeta]

A stable nanobud structure

In theory, a nanotorus is a carbon nanotube bent intoa torus (doughnut shape). Nanotori are predicted to havemany unique properties, such as magnetic moments 1000times larger than previously expected for certain specificradii.[10] Properties such as magnetic moment , thermal

stability, etc. vary widely depending on radius of the torusand radius of the tube .[10][11]

Nanobud [edit source | editbeta] Carbon nanobuds are a newly created material combiningtwo previously discovered allotropes of carbon: carbonnanotubes and fullerenes. In this new material, fullerene-like

"buds" are covalently bonded to the outer sidewalls of the

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underlying carbon nanotube. This hybrid material has usefulproperties of both fullerenes and carbon nanotubes. Inparticular, they have been found to be exceptionally goodfield emitters. In composite materials, the attached fullerenemolecules may function as molecular anchors preventingslipping of the nanotubes, thus improving the compositesmechanical properties.

Graphenated carbon nanotubes (g-CNTs) [edit source | editbeta]

SEM series of graphenated CNTs with varying foliate density

Graphenated CNTs are a relatively new hybrid thatcombines graphitic foliates grown along the sidewalls of multiwalled or bamboo style CNTs. Yu et al .[12]r eported on"chemically bonded graphene leaves" growing along the

sidewalls of CNTs. Stoner et al .[13] described thesestructures as "graphenated CNTs" and reported in their usefor enhanced supercapacitor performance. Hsu et al. further reported on similar structures formed on carbon fiber paper,also for use in supercapacitor applications.[14] The foliatedensity can vary as a function of deposition conditions (e.g.

temperature and time) with their structure ranging from fewlayers of graphene (< 10) to thicker, more graphite-like.[15]

The fundamental advantage of an integrated graphene -CNTstructure is the high surface area three-dimensionalframework of the CNTs coupled with the high edge densityof graphene. Graphene edges provide significantly higher

charge density and reactivity than the basal plane, but they

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are difficult to arrange in a three-dimensional, high volume-density geometry. CNTs are readily aligned in a high densitygeometry (i.e., a vertically aligned forest) [16]but lack highcharge density surfaces the sidewalls of the CNTs aresimilar to the basal plane of graphene and exhibit low chargedensity except where edge defects exist. Depositing a highdensity of graphene foliates along the length of alignedCNTs can significantly increase the total charge capacity per unit of nominal area as compared to other carbon

nanostructures .[17] Nitrogen Doped Carbon Nanotubes [editsource | editbeta] Nitrogen doped carbon nanotubes (N-CNT's), can beproduced through 5 main methods, Chemical Vapor Deposition,[18][19] high-temperature and high-pressurereactions, gas-solid reaction of amorphous carbon withNH3 at high temperature ,[20] solid reaction,[21] andsolvothermal synthesis .[22]

N-CNTs can also be prepared by a CVD method of pyrolysizing melamine under Ar at elevated temperatures of 800oC - 980oC. However synthesis via CVD and melamineresults in the formation of bamboo structured CNTs. XPSspectra of grown N-CNT's reveals nitrogen in five maincomponents, pyridinic nitrogen, pyrrolic nitrogen, quaternarynitrogen, and nitrogen oxides. Furthermore synthesistemperature affects the type of nitrogen configuration .[23]

Nitrogen doping plays a pivotal role in Lithium storage. N-doping provides defects in the walls of CNT's allowing for Li

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ions to diffuse into interwall space. It also increases capacityby providing more favorable bind of N-doped sites. N-CNT'sare also much more reactive to metal oxide nanoparticledeposition which can further enhance storage capacity,especially in anode materials for Li-ionbatteries .[24] However Boron doped nanotubes have beenshown to make batteries with triple capacity .[25]

Peapod [edit source | editbeta] A Carbon peapod [26][27] is a novel hybrid carbon materialwhich traps fullerene inside a carbon nanotube. It canpossess interesting magnetic properties with heating andirradiating. It can also be applied as an oscillator duringtheoretical investigations and predictions .[28][29]

Cup-stacked carbon nanotubes [edit source | editbeta]

Cup-stacked carbon nanotubes (CSCNTs) differ from other quasi-1D carbon structures, which normally behave asquasi-metallic conductors of electrons. CSCNTs exhibitsemiconducting behaviors due to the stacking microstructureof graphene layers .[30]

Extreme carbon nanotubes [edit source | editbeta]

Cycloparaphenylene

The observation of the longest carbon nanotubes (18.5 cmlong) was reported in 2009. These nanotubes were grown onSi substrates using an improved chemical vapor

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deposition (CVD) method and represent electrically uniformarrays of single-walled carbon nanotubes .[1]

The shortest carbon nanotube is the organic compoundcycloparaphenylene, which was synthesized in early2009.[31][32][33]

The thinnest carbon nanotube is armchair (2,2) CNT with adiameter of 3 . This nanotube was grown inside a multi-walled carbon nanotube. Assigning of carbon nanotube type

was done by combination of high-resolution transmissionelectron microscopy (HRTEM), Ramanspectroscopy and density functional theory (DFT)calculations.[34]

The thinnest freestanding single-walled carbon nanotube isabout 4.3 in diameter. Researchers suggested that it can

be either (5,1) or (4,2) SWCNT, but exact type of carbonnanotube remains questionable .[35] (3,3), (4,3) and (5,1)carbon nanotubes (all about 4 in diameter) wereunambiguously identified using more precise aberration-corrected high-resolution transmission electron microscopy . However, they were found inside of double-walled carbonnanotubes. [36]

Properties [edit source | editbeta] Strength [edit source | editbeta] See also: Mechanical properties of carbon nanotubes

Carbon nanotubes are the strongest and stiffest materialsyet discovered in terms of tensile strength and elasticmodulus respectively. This strength results from the covalent

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sp2 bonds formed between the individual carbon atoms. In2000, a multi-walled carbon nanotube was tested to have atensile strength of 63 gigapascals (GPa).[37] (For illustration,this translates into the ability to endure tension of a weightequivalent to 6422 kg (14,158 lbs) on a cable with cross-section of 1 mm2.) Further studies, such as one conducted in2008, revealed that individual CNT shells have strengths of up to ~100 GPa, which is in agreement withquantum/atomistic models .[38] Since carbon nanotubes have

a low density for a solid of 1.3 to 1.4 g/cm3,[39] its specificstrength of up to 48,000 kNmkg1 is the best of knownmaterials, compared to high-carbon steel's 154 kNmkg 1 .

Under excessive tensile strain, the tubes will undergo plasticdeformation, which means the deformation is permanent.This deformation begins at strains of approximately 5% and

can increase the maximum strain the tubes undergo beforefracture by releasing strain energy.

Although the strength of individual CNT shells is extremelyhigh, weak shear interactions between adjacent shells andtubes leads to significant reductions in the effective strengthof multi-walled carbon nanotubes and carbon nanotubebundles down to only a few GPas .[40] This limitation hasbeen recently addressed by applying high-energy electronirradiation, which crosslinks inner shells and tubes, andeffectively increases the strength of these materials to ~60GPa for multi-walled carbon nanotubes [38] and ~17 GPa for double-walled carbon nanotube bundles .[40]

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CNTs are not nearly as strong under compression. Becauseof their hollow structure and high aspect ratio, they tend toundergo buckling when placed under compressive, torsional,or bending stress .[41]

Comparison of mechanical properties [42][43][44][45]

Material Young's

modulus (TPa)

Tensile

strength (GPa)

Elongation at

break (%)

SWNTE ~1 (from 1 to 5) 13 53 16

Armchair SWNTT

0.94 126.2 23.1

ZigzagSWNTT

0.94 94.5 15.6 17.5

ChiralSWNT

0.92

MWNTE 0.2[37] 0.8[46]

0.95[37] 11 [37] 63[37]

150[46]

Stainlesssteel E

0.186 [47] 0.214 [48] 0.38[47] 1.55[48] 15 50

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Kevlar 29&149E

0.06 0.18[49] 3.6 3.8[49] ~2

EExperimental observation; TTheoretical prediction

The above discussion referred to axial properties of thenanotube, whereas simple geometrical considerationssuggest that carbon nanotubes should be much softer in theradial direction than along the tube axis.

Indeed, TEM observation of radial elasticity suggested thateven the van der Waals forces can deform two adjacentnanotubes .[50] Nanoindentation experiments, performed byseveral groups on multiwalled carbon nanotubes [51][52] andtapping/contact mode atomic forcemicroscope measurement performed on single-walled

carbon nanotube ,[53]

indicated Young's modulus of the order of several GPa confirming that CNTs are indeed rather softin the radial direction.

Hardness [edit source | editbeta] Standard single-walled carbon nanotubes can withstand apressure up to 25 GPa without deformation. They then

undergo a transformation to superhard phase nanotubes.Maximum pressures measured using current experimentaltechniques are around 55 GPa. However, these newsuperhard phase nanotubes collapse at an even higher,albeit unknown, pressure.

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The bulk modulus of superhard phase nanotubes is 462 to546 GPa, even higher than that of diamond (420 GPa for single diamond crystal). [54]

Kinetic properties [edit source | editbeta] Multi-walled nanotubes are multiple concentric nanotubesprecisely nested within one another. These exhibit a strikingtelescoping property whereby an inner nanotube core mayslide, almost without friction, within its outer nanotube shell,thus creating an atomically perfect linear or rotationalbearing. This is one of the first true examples of molecular nanotechnology , the precise positioning of atoms to createuseful machines. Already, this property has been utilized tocreate the world's smallest rotational motor .[55] Futureapplications such as a gigahertz mechanical oscillator arealso envisaged.

Electrical properties [edit source | editbeta]

Band structures computed using tight binding approximation for (6,0)CNT (zigzag, metallic) (10,2) CNT (semiconducting) and (10,10) CNT(armchair, metallic).

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Because of the symmetry and unique electronic structure of graphene, the structure of a nanotube strongly affects itselectrical properties. For a given ( n ,m) nanotube, if n = m ,the nanotube is metallic; if n m is a multiple of 3, then thenanotube is semiconducting with a very small band gap,otherwise the nanotube is a moderate semiconductor . Thusall armchair (n = m) nanotubes are metallic, and nanotubes(6,4), (9,1), etc. are semiconducting .[56]

However, this rule has exceptions, because curvature effectsin small diameter carbon nanotubes can strongly influenceelectrical properties. Thus, a (5,0) SWCNT that should besemiconducting in fact is metallic according to thecalculations. Likewise, vice versa zigzag and chiralSWCNTs with small diameters that should be metallic havefinite gap (armchair nanotubes remain metallic) .[56] In theory,

metallic nanotubes can carry an electric current density of 4 109 A/cm2, which is more than 1,000 times greater thanthose of metals such as copper ,[57] where for copper interconnects current densities are limitedby electromigration.

Because of their nanoscale cross-section, electrons

propagate only along the tube's axis and electron transportinvolves quantum effects. As a result, carbon nanotubes arefrequently referred to as one-dimensional conductors. Themaximum electrical conductance of a single-walled carbonnanotube is 2 G0, where G0 = 2e 2/h is the conductance of asingle ballistic quantum channel .[58]

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There have been reports of intrinsic superconductivity incarbon nanotubes .[59][60][61] Many other experiments,however, found no evidence of superconductivity, and thevalidity of these claims of intrinsic superconductivity remainsa subject of debate .[62]

Optical properties [edit source | editbeta] Main article: Optical properties of carbon nanotubes

EM Wave absorption [edit source | editbeta]

One of the more recently researched properties of multi-walled carbon nanotubes (MWNTs) is their wave absorptioncharacteristics, specifically microwave absorption. Interest inthis research is due to the current military push for radar absorbing materials (RAM) to better the stealthcharacteristics of aircraft and other military vehicles. There

has been some research on filling MWNTs with metals, suchas Fe, Ni, Co, etc., to increase the absorption effectivenessof MWNTs in the microwave regime. Thus far, this researchhas shown improvements in both maximum absorption andbandwidth of adequate absorption. The reason theabsorptive properties changed when filled is that thecomplex permeability ( r ) and complex permitivity ( r ), shownin the equations below, have been shown to vary dependingon how the MWNTs are called and what medium they aresuspended in. The direct relationship between r , r , and theother system parameters that affect the absorption samplethickness, d, and frequency, f, is shown in the equationsbelow, where Z in is the normalized input impedance. Asshown in the equation below, these characteristics vary by

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frequency. Because of this, it is convenient to set a baselinereflection loss (R.L.) that is deemed effective and determinethe bandwidth within a given frequency that produces thedesired reflection loss. A common R.L. to use for thisbandwidth determination is 10 dB, which corresponds to aloss of over 90% of the incoming wave. This bandwidth isusually maximized at the same time as the absorption is.This is done by satisfying the impedance matching condition,getting Zin = 1. In the work done at Beijing Jiaotong

University it was found that Fe filled MWNTs exhibited amaximum reflection loss of 22.73 dB and had a bandwidthof 4.22 GHz for a reflection loss of 10

dB.

Thermal properties [edit source | editbeta] Main article: Thermal properties of nanostructures

All nanotubes are expected to be very good thermalconductors 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 room-temperature thermal conductivity along its axis of about 3500Wm1 K1;[63] compare this to copper, a metal well known for its good thermal conductivity, which transmits 385Wm1 K1 . A SWNT has a room-temperature thermalconductivity across its axis (in the radial direction) of about1.52 Wm1 K1 ,[64] which is about as thermally conductive assoil. The temperature stability of carbon nanotubes is

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estimated to be up to 2800 C in vacuum and about 750 Cin air .[65]

Defects[edit source | editbeta]

As with any material, the existence of a crystallographicdefect affects the material properties. Defects can occur inthe form of atomic vacancies . High levels of such defectscan lower the tensile strength by up to 85%. An importantexample is the Stone Wales defect , which creates apentagon and heptagon pair by rearrangement of the bonds.Because of the very small structure of CNTs, the tensilestrength of the tube is dependent on its weakest segment ina similar manner to a chain, where the strength of theweakest link becomes the maximum strength of the chain.

Crystallographic defects also affect the tube's electricalproperties. A common result is lowered conductivity throughthe defective region of the tube. A defect in armchair-typetubes (which can conduct electricity) can cause thesurrounding region to become semiconducting, and singlemonoatomic vacancies induce magnetic properties .[66]

Crystallographic defects strongly affect the tube's thermal

properties. Such defects lead to phonon scattering, which inturn increases the relaxation rate of the phonons. Thisreduces the mean free path and reduces the thermalconductivity of nanotube structures. Phonon transportsimulations indicate that substitutional defects such asnitrogen or boron will primarily lead to scattering of high-frequency optical phonons. However, larger-scale defectssuch as Stone Wales defects cause phonon scattering over

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a wide range of frequencies, leading to a greater reduction inthermal conductivity.[67]

Toxicity[edit source | editbeta]

The toxicity of carbon nanotubes has been an importantquestion in nanotechnology. Such research has just begun.The data are still fragmentary and subject to criticism.Preliminary results highlight the difficulties in evaluating thetoxicity of this heterogeneous material. Parameters such asstructure, size distribution, surface area , surfacechemistry, surface charge , andagglomeration state as wellas purity of the samples, have considerable impact onthe reactivity of carbon nanotubes. However, available dataclearly show that, under some conditions, nanotubes cancross membrane barriers, which suggests that, if rawmaterials reach the organs, they can induce harmful effectssuch as inflammatory and fibrotic reactions .[68]

Under certain conditions CNTs can enter human cells andaccumulate in the cytoplasm, causing cell death .[69]

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 capableof producinginflammation, epithelioidgranulomas (microscopic nodules), fibrosis, andbiochemical/toxicological changes in thelungs.[70] Comparative toxicity studies in which mice weregiven equal weights of test materials showed that SWCNTswere more toxic than quartz, which is considered a seriousoccupational health hazard when chronically inhaled. As a

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control, ultrafine carbon black was shown to produceminimal lung responses .[71]

The needle-like fiber shape of CNTs is similar to asbestosfibers. This raises the idea that widespread use of carbonnanotubes 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 toasbestos). A recently published pilot study supports thisprediction.[72] Scientists exposed the mesothelial lining of thebody cavity of mice to long multiwalled carbon nanotubesand observed asbestos-like, length-dependent, pathogenicbehavior that included inflammation and formation of lesionsknown as granulomas . Authors of the study conclude:

This is of considerable importance, because researchand business communities continue to invest heavily incarbon nanotubes for a wide range of products under the assumption that they are no more hazardous thangraphite. Our results suggest the need for further research and great caution before introducing suchproducts into the market if long-term harm is to beavoided.[72]

Although further research is required, the available datasuggests that under certain conditions, especially thoseinvolving chronic exposure, carbon nanotubes can pose aserious risk to human health .[68][69][71][72]

Synthesis [edit source | editbeta]

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Powder of carbon nanotubes

Techniques have been developed to produce nanotubes insizeable quantities, including arc discharge , laser ablation , high-pressure carbon monoxide disproportionation ( HiPco),and chemical vapor deposition (CVD). Most of these

processes take place in vacuum or with process gases. CVDgrowth of CNTs can occur in vacuum or at atmosphericpressure. Large quantities of nanotubes can be synthesizedby these methods; advances in catalysis and continuousgrowth processes are making CNTs more commerciallyviable.

Arc discharge [edit source