Report Nikhil

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EffectofBufferGasandCathodeshape

ontheSynthesisofCarbonNanotubesusingArc-Dischargemethod

April 2010

Performed atInternational Advanced Research Centre for Powder Metallurgyand New Materials (ARCI)

Balapur PO, Hyderabad 500005, Andhra Pradesh, IndiaBy

RangaNikhil

VinayakYellakara

Mahatma Gandhi Institute of TechnologyJawaharlal Nehru Technological University, Hyderabad, India


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MAHATMAGANDHIINSTITUTEOFTECHNOLOGY

(AffiliatedtoJawaharlalNehruTechnologicalUniversity)

SponsoredbyChaitanya

BharathiEducationalSociety

GANDIPET,HYDERABAD500075.A.P.(INDIA)

Mr. P.K. Subramanian

B.E.(Met.Engg.),REC/JM.Tech.,IIT/KanpurM.I.S.T.E.,M.I.I.M.HeadoftheDepartmentDept.ofMetallurgyandMaterialsTechnology

Ph:

0091-8413-233355


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(Off)Ph:0091-8413233672(Off)Ph

:0091-0-9885236053(Cell)Ph009104023515369

CERTIFICATE

This is to certify that the project titled EffectofBufferGasandCathodeshapeontheSynthesisof

CarbonNanotubesusing Arc-Dischargemethod is a bonafide work done by RangaNikhilVinayakYellakara, bearing Roll no. 06261A1835during the academic year 2009-2010 as a part ofFinal year industrial project.

The report of this project work is submitted to the department/college in the fulfillment of therequirements for the award of BachelorofTechnologyin Metallurgy andMaterialsTechnologyby Jawaharlal Nehru Technological University.

P. K. SUBRAMANIANInternal GuideHead of the DepartmentMetallurgy and Materials Technology3


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Acknowledgements

I would like to express the deepest gratitude to all those who had been instrumental in the successfulcompletion of my project work at International Advanced Research Centre for Powd

er Metallurgyand New Materials (ARCI), Hyderabad, India.My heartfelt gratitude to Mr. K. V. Phani Prabhakar, Scientist D, ARCI, who continually andconvincingly conveyed a spirit of adventure in regard to project. Without his guidance and persistenthelp, this dissertation would not have been possible.I am deeply indebted to Dr. Pavan Kumar Jain, Team Leader, Scientist E, ARCI, without whosesupport I could not have accomplished the project.I would like to thank Mr. Balaji Padya, Scientist B, Mr. G. Venkata Ramana, Post Graduate

Trainee, ARCI, for having shared their time and knowledge with me.I would also like to acknowledge Mr. Venkat Reddy and Mr. Subba Rao, Technicians, ARCI, forhelping me with the technical aspects.In addition, I would like to thank Dr. J. Viplava Kumar and Mr. P. K. Subramanian (Internal Guide),who introduced me to Metallurgy and Material Sciences, and whose enthusiasm forthe field haslasting effect.I would like to acknowledge Mr. K. Ramanjaneyulu, Mr. P. Venkata Ramana, Associate Professors,MGIT, Mr. Bhomik. K. Deogade, Mr. P. V. S. Lakshmi Narayana, Mrs. Jhansi Jadav and Mr. R. V.

S. M. Ramakrishna, Assistant Professors, MGIT, who has always been a great source of inspirationand a driving force in all my endeavors.My sincere thanks to the department of Metallurgy and Materials Technology, Mahatma GandhiInstitute of Technology and the Jawaharlal Nehru Technological University, Hyderabad, for givingme the permission to do my project work in ARCI.I would like to give my special thanks to my parents, sister and friends whose love and supportenabled me to complete this work.Ranga Nikhil Vinayak Yellakara

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TableofContents

Abstract 6

.Introduction 7.Theory.8.CharacterizationTechniques.28.Experiment 31

.ResultsandDiscussion.34.Conclusions 40References..41

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Abstract

We have investigated the effect of the buffer gas, its partial pressure and theshape of the cathode onthe synthesis of multi-walled carbon nanotubes using Arc-Discharge method. We ha

ve synthesizedMWCNTs in two different gas atmospheres i.e. in helium and hydrogen at differentpartial pressures,separately. We observed that the time taken for the synthesis process drastically decreased inHydrogen atmosphere. Observations using field emission scanning electron microscopy indicatethe presence of MWCNTs in hydrogen atmosphere at 350 Torr in a large number compared to thatof helium atmosphere at 300 Torr. Characterization using Thermo Gravimetric Analysis (TGA)confirmed the SEM results. This suggests that hydrogen at 350 Torr partial press

ure is muchfavorable for the synthesis of MWCNTs using arc-discharge method in order to gethigh yield ofMWCNTs.MWCNTs were also synthesized using a cup shaped cathode instead of a flat cathode in hydrogenatmosphere. FE-SEM analysis and TGA suggests that MWCNTs are more in the depositobtained onflat cathode compared to that on cup shaped cathode.

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Introduction

Since their discovery in 1991 by Iijima, carbon nanotubes have been of great interest, both from afundamental point of view and for future applications. The most eye-catching fea

tures of thesestructures are their electronic, mechanical, optical and chemical characteristics, which open a way tofuture applications. These properties can even be measured on single nanotubes.For commercialapplication, large quantities of purified nanotubes are needed.Different types of carbon nanotubes can be produced in various ways. The most common techniquesused nowadays are: arc discharge, laser ablation, chemical vapour deposition, HiPCO and flamesynthesis. Purification of the tubes can be divided into a couple of main techniques: oxidation, acid

treatment, annealing, sonication, filtering and fictionalization techniques. Economically feasiblelarge-scale production and purification techniques still have to be developed.Fundamental and practical nanotube researches have shown possible applications in the fields ofenergy storage, molecular electronics, nanomechanic devices, and composite materials. Realapplications are still under development.Arc discharge, which is one of main methods of preparing carbon nanotubes (CNTs), has beenexplored extensively since this process has a very high temperature (about 4000K) and it can yieldthe most highly graphitized tubes. For example, much work has been done to inves

tigate the effect ofbuffer gas, arc current and electrode temperature on the quality and output of CNTs in arc discharge,and then optimal conditions for preparing high density and loosely entangled CNTs have beenobtained.MWCNTs synthesized by arc discharge always coexist with carbonaceous nanoparticles, possessundefined morphologies, and have wide size and shape distributions, which have become one ofmain obstacles to the applications of MWCNTs.Thermo Gravimetric Analysis (TGA) is commonly employed in research and testing to determinecharacteristics of materials such as MWCNTs, to determine degradation temperatures, absorbedmoisture content of materials, the level of inorganic and organic components inmaterials,decomposition points of explosives, and solvent residues. It is also often usedto estimate thecorrosion kinetics in high temperature oxidation.Microscopy techniques such as Field Emission Scanning Electron Microscopy (FE-SEM) are usedto produce real space magnified images of CNTs showing what it looks like. In general, microscopyinformation concerns surface crystallography, surface morphology and surface com

position.

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Theory

Before dealing with the experiment, it is very important to know what exactly are carbon nanotubes,their growth mechanism, their properties, and their applications for which MWCNT

s are creating ahuge impact on the field of advanced materials. The name Carbon Nanotubes clearly indicates thattheir source material is carbon.

Carbon

Carbon is a Group 14 element and is distributed very widely in nature. Its atomic number is 6 andatomic weight is 12.0107 g/mol. It is a non-metallic solid which comes under p-block elements of

the periodic table. It is a tetravalent compound having four valence electrons to form covalentchemical bonds. Carbons electronic configuration is 1s2 2s2 2p2.All forms of carbon are highly stable, requiring high temperature to react evenwith oxygen. Themost common oxidation state of carbon in inorganic compounds is +4, while +2 isfound in carbonmonoxide and other transition metal carbonyl complexes. It has an affinity for bonding with othersmall atoms, including other carbon atoms, and is capable of forming multiple stable covalent bondswith such atoms. As a result, carbon is known to form almost ten million different compounds, the

large majority of all chemical compounds1. Carbon also has the highest melting and sublimationpoint of all elements2 under inert conditions.Carbon sublimes in a carbon arc which has a temperature of about 5800 K. Thus, irrespective of itsallotropic form, carbon remains solid at higher temperatures in the absence of oxygen than thehighest melting point metals such as tungsten or rhenium. Although thermodynamically prone tooxidation, carbon resists oxidation more effectively than elements such as ironand copper which areweaker reducing agents at room temperature.Hybridization in Carbon:Hybridization is the concept of mixing atomic orbitals to form new hybrid orbitals suitable for thequalitative description of atomic bonding properties. Hybridised orbitals are very useful in theexplanation of the shape of molecular orbitals for molecules. Three types of hybridized bonds arepossible for carbon. They are sp3, sp2 and sp. In Methane (CH4), in which 1 electron from 2s orbitalis migrated into 2pz orbital to attain partial stability, these four sp3 hybridised orbitals are overlappedby hydrogen's 1s orbital, yielding four s (sigma) bonds (that is, four single covalent bonds). The four

bonds are of the same length and strength. In the case of Ethane (C2H4), carbonwill sp2 hybridise,


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because hybrid orbitals will form only s bonds and one p (pi) bond is required for the double bondbetween the carbons. The hydrogen-carbon bonds are all of equal strength and length. The chemicalbonding in compounds such as alkynes with triple bonds is explained by sp hybridization. The 2s

orbital mixes with only one of the three p-orbitals resulting in two sp orbitalsand two remainingunchanged p orbitals. The chemical bonding in acetylene (ethyne) (C2H2) consistsof spsp overlapbetween the two carbon atoms forming a s bond and two additional p bonds formedby pp overlapresulting in triple bond3.It is found in abundance in the sun, stars, comets, and atmospheres of most planets. Carbon is the15th most abundant elements in the Earth's crust, and the fourth most abundant element in theuniverse by mass after hydrogen, helium, and oxygen. It is present in all known

life forms, and in thehuman body carbon is the second most abundant element by mass (about 18.5%) after oxygen.Carbon is found free in nature in three allotropic forms: amorphous, graphite, and diamond. Thephysical properties of carbon vary widely with the allotropic form. For example,diamond is highlytransparent, while graphite is opaque and black. Graphite is one of the softestknown materials whilediamond is one of the hardest. Diamond has a very low electrical conductivity, while graphite is avery good conductor.

Allotropes of Carbon:

The three relatively well-known allotropes of carbon are amorphous carbon, graphite, and diamond.Once considered exotic, fullerenes are nowadays commonly synthesized and used inresearch; theyinclude buckyballs (C60), carbon nanotubes, carbon nanobuds (C60 attached to Carbon nanotubewall) and nanofibers. Several other exotic allotropes have also been discovered,such as lonsdaleite,glassy carbon, carbon nanofoam and linear acetylenic carbon.AmorphousCarbon: The amorphous form is a collection of carbon atoms in a non-crystalline,irregular, glassy state, which is generally graphite but not held in a crystalline macrostructure. It ispresent as a powder, and is the main constituent of substances such as charcoal,lampblack (soot) andactivated carbon. In a crystallographic sense, however, these materials are nottruly amorphous, butare polycrystalline or nanocrystalline materials of graphite or diamond within an amorphous carbonmatrix.The properties of amorphous carbon films vary depending on the parameters used d

uring deposition.One of the most common ways to characterize amorphous carbon is through the ratio of sp2 to sp3


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hybridized bonds present in the material. Graphite consists purely of sp2 hybridized bonds, whereas

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diamond consists purely of sp3 hybridized bonds. Materials that are high in sp3hybridized bonds arereferred to as tetrahedral amorphous carbon or as diamond-like carbon4.Amorphous carbon materials may also be stabilized by terminating dangling-p bonds with hydrogen.These materials are then called hydrogenated amorphous carbon.

Graphite:In graphite, sp2 hybridized bonds are present between two carbon atoms, in whicheachatom is bonded trigonally to three others in a plane composed of fused hexagonalrings withseparation of 0.142 nm, and the distance between planes is 0.335 nm. The resulting network is 2dimensional,and the resulting flat sheets are stacked and loosely bonded through weak van derWaals forces. Its specific gravity is 2.3, which makes it lighter than diamond.At high temperaturesand pressures, it can be transformed into diamond. At about 700 C, it burns in ox

ygen formingcarbon dioxide5. The two known forms of graphite, alpha (hexagonal) and beta (rhombohedral), havevery similar physical properties. The hexagonal graphite may be either flat or buckled. Naturalgraphite contain up to 30% of the beta form, when synthetically-produced graphite only contains thealpha form. Synthetic graphite is used as electrode.

Fig 1:Graphite sheetsGraphite holds the distinction of being the most stable form of carbon under sta

ndard conditions.Therefore, it is used in thermo chemistry as the standard state for defining theheat of formation ofcarbon compounds. Graphite is able to conduct electricity, due to delocalizationof the pi bondelectrons above and below the planes of the carbon atoms. These electrons are free to move, so areable to conduct electricity. In graphite, each carbon atom uses only 3 of its 4outer energy levelelectrons in covalently bonding to three other carbon atoms in a plane. Each carbon atom contributesone electron to a delocalised system of electrons that is also a part of the chemical bonding. Thedelocalised electrons are free to move throughout the plane. For this reason, graphite conducts

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electricity along the planes of carbon atoms, but does not conduct in a direction at right angles to theplane.Graphite powder is used as a dry lubricant. This industrially important propertyis due to the looseinterlamellar coupling between sheets in the structure and also adsorbed air and

water between thelayers, unlike other layered dry lubricants6.Diamond:At very high pressures carbon forms the more compact allotrope diamond, havingnearly twice the density of graphite. Here, each atom is bonded tetrahedrally (sp3) to four others,thus making a 3-dimensional network of puckered six-membered rings of atoms. Thecarbon atomsare arranged in a variation of the face-centered cubic crystal structure calleda diamond lattice.Diamond is renowned as a material with superlative physical qualities, most of which originate from

the strong covalent bonding between its atoms. In particular, diamond has the highest hardness(Mohr scale: 10) and thermal conductivity (9002,320 Wm-1K-1) of any bulk material.Naturallyoccurring diamonds have a density ranging from 3.153.53 g/cm3, with pure diamondclose to

3.52 g/cm3. Those properties determine the major industrial application of diamond in cutting andpolishing tools7. Diamond has remarkable optical characteristics. Its refractiveindex is 2.418 (at500 nm). Diamond also has relatively high optical dispersion, which is ability to disperse light of

different colors, which results in its characteristic lustre8.Fig 2: Diamond Structure

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Fullerene:A fullerene is any molecule composed entirely of carbon, in the form of a hollowsphere,ellipsoid, or tube. Spherical fullerenes are also called buckyballs, and cylindrical ones are calledcarbon nanotubes or buckytubes. Buckminsterfullerene is the smallest fullerene m

olecule in whichno two pentagons share an edge. It is also the most common in terms of natural occurrence, as it canoften be found in soot.The structure of C60 is a truncated icosahedron, which resembles a soccer ball of the type made oftwenty hexagons and twelve pentagons, with a carbon atom at the vertices of eachpolygon and abond along each polygon edge.9 The van der Waals diameter of a C60 molecule is about 1 nm. Thenucleus to nucleus diameter of a C60 molecule is about 0.7 nm. Fullerenes are used in various fields

such as in armor, medicine, nanotechnology due to their excellent heat resistance andsuperconductivity.10

Fig 3: C60BuckminsterFullerene structure

Carbon Nanotubes:

Carbon Nanotubes (MWCNTs) were discovered by Sumio Iijima in 1991 in his study of arc-discharge products. MWCNTs are allotropes of carbon with a cylindrical nanostructure. Their largelength (up to several microns) and small diameter (a few nanometres) result in alarge aspect ratio.They can be seen as the nearly one-dimensional form of fullerenes. They exhibitextraordinarystrength and unique electrical properties, and are efficient thermal conductors.Nanotubes arecategorized as single-walled nanotubes (SWNTs) and multi-walled nanotubes (MWNTs)11 .

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SWNTs:Most single-walled nanotubes (SWNT) have a diameter of close to 1 nanometre, with atube length that can be many millions of times longer. The structure of a SWNT can beconceptualized by wrapping a one-atom-thick layer of graphite called graphene in

to a seamlesscylinder. The way the graphene sheet is wrapped is represented by a pair of indices (n, m) called thechiral vector. The integers n and m denote the number of unit vectors along twodirections in thehoneycomb crystal lattice of graphene. If m = 0, the nanotubes are called "zigzag". If n = m, thenanotubes are called "armchair". Otherwise, they are called "chiral".12

Fig 4: Structure of a SWNT (a) Armchair (n, n), (b) Zigzag (n, 0), (c)Chiral (n, m)

SWNTs with different chiral vectors have dissimilar properties such as optical activity, mechanicalstrength and electrical conductivity.Single-walled nanotubes are an important variety of carbon nanotube because theyexhibit electricproperties. The most basic building block of these systems is the electric wire,and SWNTs can be

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excellent conductors. One useful application of SWNTs is in the development of the firstintramolecular field effect transistors (FET)13 ..MWNTs:Multi-walled nanotubes (MWNT) consist of multiple rolled layers (concentric tube

s) ofgraphite. MWNTs can also be considered as a collection of concentric SWNTs withdifferentdiameters. The length and diameter of these structures differ a lot from those of SWNTs and, ofcourse, their properties are also very different.

Fig 5: Structure of MWNT

Properties of MWCNTs:

(a)Chemicalreactivity: The chemical reactivity of a CNT is, compared with a graphene sheet,enhanced as a direct result of the curvature of the CNT surface. Carbon nanotubereactivity isdirectly related to the pi-orbital mismatch caused by an increased curvature. Therefore, a distinctionmust be made between the sidewall and the end caps of a nanotube. For the same reason, a smallernanotube diameter results in increased reactivity. Covalent chemical modificatio

n of either sidewallsor end caps has shown to be possible. For example, the solubility of MWCNTs in different solventscan be controlled this way. Though, direct investigation of chemical modifications on nanotubebehaviour is difficult as the crude nanotube samples are still not pure enough.14(b)Electricalconductivity: Depending on their chiral vector, carbon nanotubes with a smalldiameter are either semi-conducting or metallic. The differences in conducting properties are causedby the molecular structure that results in a different band structure and thus adifferent band gap. The14


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differences in conductivity can easily be derived from the graphene sheet properties.15 It was shownthat a (n, m) nanotube is metallic as accounts that: n = m or (n -m) = 3i, wherei is an integer and nand m are defining the nanotube. The resistance to conduction is determined by quantum mechanical

aspects and was proved to be independent of the nanotube length.16

(c)Opticalactivity:Spectroscopic methods offer the possibility of quick and non-destructivecharacterization of relatively large amounts of carbon nanotubes. There is a strong demand for suchcharacterization from the industrial point of view: numerous parameters of the nanotube synthesiscan be changed, intentionally or unintentionally, to alter the nanotube quality.Optical absorption,

photoluminescence and Raman spectroscopy allow quick and reliable characterization of this"nanotube quality" in terms of non-tubular carbon content, structure (chirality)of the producednanotubes, and structural defects. Use of the optical activity might result in optical devices in whichMWCNTs play an important role.17(d)Mechanicalstrength:Carbon nanotubes are the strongest and stiffest materials yet discoveredin terms of tensile strength and elastic modulus respectively. This strength results from the covalent

sp bonds formed between the individual carbon atoms. Since carbon nanotubes havea low densityfor a solid of 1.3 to 1.4 gcm-3, its specific strength of up to 48,000 kNmkg-1 is the best of knownmaterials, compared to high-carbon steel's 154 kNmkg-1. One study succeeded in thesynthesis of asuper-hard material by compressing SWNTs to above 24 GPa at room temperature.Synthesisof Carbon Nanotubes:

Techniques have been developed to produce nanotubes in sizeable quantities, including arcdischarge, laser ablation, high pressure carbon monoxide (HiPCO), and chemical vapour deposition(CVD) and flame synthesis. First, the growth mechanism is explained, as it is almost general for alltechniques.

Growth Mechanism:

The way in which nanotubes are formed is not exactly known. The growth mechanismis still asubject of controversy, and more than one mechanism might be operative during th

e formation ofMWCNTs. One of the mechanisms consists out of three steps. First a precursor tothe formation of


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nanotubes and fullerenes, C2, is formed on the surface of the metal catalyst particle. From thismetastable carbide particle, a rod-like carbon is formed rapidly. Secondly thereis a slowgraphitisation of its wall. This mechanism is based on in-situ TEM observations.18

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There are several theories on the exact growth mechanism for nanotubes. One theory19 postulatesthat metal catalyst particles are floating or are supported on graphite or another substrate. Itpresumes that the catalyst particles are spherical or pear-shaped, in which casethe deposition will

take place on only one half of the surface. The carbon diffuses along the concentration gradient andprecipitates on the opposite half, around and below the bisecting diameter. However, it does notprecipitate from the apex of the hemisphere, which accounts for the hollow corethat is characteristicof these filaments. For supported metals, filaments can form either by "base-growth in which thenanotube grows upwards from the metal particles that remain attached to the substrate, or theparticles detach and move at the head of the growing nanotube, labelled "tip-growth". Depending on

the size of the catalyst particles, SWNT or MWNT are grown. In arc discharge, ifno catalyst ispresent in the graphite, MWNT will be grown on the C2-particles that are formedin the plasma.

Fig 6:Possible CNT growth mechanism in CVD method

Chemical Vapour Deposition (CVD):

The catalytic vapour phase deposition of carbon was first reported in 1959, butit was not until 1993that carbon nanotubes were formed by this process. Chemical vapour deposition (CVD) synthesis isachieved by putting a carbon source in the gas phase and using an energy source,such as plasma or aresistively heated coil, to transfer energy to a gaseous carbon molecule. Commonly used gaseouscarbon sources include methane, carbon monoxide and acetylene. The energy sourceis used to

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"crack" the molecule into reactive atomic carbon. Then, the carbon diffuses towards the substrate,which is heated and coated with a catalyst, usually a first row transition metalsuch as Ni, Fe or Co,where it will bind. Carbon nanotubes will be formed if the proper parameters aremaintained.

Excellent alignment20, as well as positional control on nanometre scale21, can be achieved by usingCVD. Control over the diameter, as well as the growth rate of the nanotubes canalso be maintained.CVD carbon nanotube synthesis is essentially a two-step process consisting of acatalyst preparationstep followed by the actual synthesis of the nanotube. The catalyst is generallyprepared bysputtering a transition metal onto a substrate and then using either chemical etching or thermalannealing to induce catalyst particle nucleation. Thermal annealing results in cluster formation on

the substrate, from which the nanotubes will grow22. The temperatures for the synthesis of nanotubesby CVD are generally within the 650-900 oC range. Typical yields for CVD are approximately 30%.

Fig 7: Schematic representation of a CVD process

Laser Ablation Method:

In 1995, Smalley's group23 at Rice University reported the synthesis of carbon n

anotubes by laservaporisation. A pulsed24 or continuous25 laser is used to vaporise a graphite target in an oven at1200 C. The main difference between continuous and pulsed laser, is that the pulsed laser demandsa much higher light intensity (100 kW/cm2 compared with 12 kW/cm2). The oven isfilled withhelium or argon gas in order to keep the pressure at 500 Torr. A very hot vapourplume forms, then

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expands and cools rapidly. As the vaporised species cool, small carbon moleculesand atoms quicklycondense to form larger clusters, possibly including fullerenes. The catalysts also begin to condense,but more slowly at first, and attach to carbon clusters and prevent their closing into cage structures.

From these initial clusters, tubular molecules grow into single-wall carbon nanotubes until thecatalyst particles become too large, or until conditions have cooled sufficiently that carbon no longercan diffuse through or over the surface of the catalyst particles. The SWNTs formed in this case arebundled together by van der Waals forces.26Laser ablation is almost similar to arc discharge, since the optimum backgroundgas and catalyst mixis the same as in the arc discharge process. This might be due to very similar reaction conditionsneeded, and the reactions probably occur with the same mechanism.

Fig 8: Schematic representation of Laser Ablation method

High PressureCarbon Monoxide Disproportionation process:

The High pressure CO Disproportionation process (HiPCO) is a technique for catalytic production ofSWNTs in a continuous-flow gas phase using CO as the carbon feedstock and Fe (CO)5 as the iron-

containing catalyst precursor. SWNTs are produced by flowing CO, mixed with a small amount ofFe (CO)5 through a heated reactor. Size and diameter distribution of the nanotubes can be roughly

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selected by controlling the pressure of CO. This process is promising for bulk production of carbonnanotubes.

Fig 9: Schematic representation of HiPCO process27

Nanotubes as small as 0.7 nm in diameter, which are expected to be the smallestachievablechemically stable SWNTs28, have been produced by this method. The average diameter of HiPCOSWNTs is approximately 1.1 nm. The yield that could be achieved is approximately70%. Thehighest yields and narrowest tubes can be produced at the highest accessible temperature andpressure. SWNT material with 97% purity can be produced at rates of up to 450 mg/h.29

Arc Dischargemethod:

The carbon arc discharge method, initially used for producing C60 fullerenes, isthe most commonand perhaps easiest way to produce carbon nanotubes as it is rather simple to undertake. However, itis a technique that produces a mixture of components and requires separating nanotubes from thesoot and the catalytic metals present in the crude product.This method creates nanotubes through arc-vaporisation of two carbon rods placed

end to end,separated by approximately 1mm, in an enclosure that is usually filled with inert gas (helium, argon)at low pressure (between 50 and 700 mbar). Recent investigations have shown thatit is also possibleto create nanotubes with the arc method in liquid nitrogen30. A direct current of 50 to 100 A drivenby approximately 20 V creates a high temperature discharge between the two electrodes. The

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discharge vaporises one of the carbon rods and forms a small rod shaped depositon the other rod.Producing nanotubes in high yield depends on the uniformity of the plasma arc and the temperatureof the deposit form on the carbon electrode31 .Insight in the growth mechanism is increasing and measurements have shown that d

ifferent diameterdistributions have been found depending on the mixture of helium and argon. These mixtures havedifferent diffusions coefficients and thermal conductivities. These properties affect the speed withwhich the carbon and catalyst molecules diffuse and cool, affecting nanotube diameter in the arcprocess. This implies that single-layer tubules nucleate and grow on metal particles in different sizesdepending on the quenching rate in the plasma and it suggests that temperature and carbon and metalcatalyst densities affect the diameter distribution of nanotubes29 .

Depending on the exact technique, it is possible to selectively grow SWNTs or MWNTs, which isshown in Figure 10. Two distinct methods of synthesis can be performed with thearc dischargeapparatus.

Fig 10: Experimental set-up of an arc discharge apparatus.

Synthesis of SWNT

If SWNTs are preferable, the anode has to be doped with metal catalyst, such as

Fe, Co, Ni, Y orMo. A lot of elements and mixtures of elements have been tested by various authors32 and it is noted

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that the results vary a lot, even though they use the same elements. This is notsurprising asexperimental conditions differ. The quantity and quality of the nanotubes obtained depend onvarious parameters such as the metal concentration, inert gas pressure, kind ofgas, the current and

system geometry. Usually the diameter is in the range of 1.2 to 1.4 nm. A coupleof ways to improvethe process of arc discharge are stated below.

a) Inert gas

The most common problems with SWNT synthesis are that the product contains a lotof metalcatalyst, SWNTs have defects and purification is hard to perform. On the other hand, an advantage isthat the diameter can slightly be controlled by changing thermal transfer and di

ffusion, and hencecondensation of atomic carbon and metals between the plasma and the vicinity ofthe cathode cancontrol nanotube diameter in the arc process. This was shown in an experiment inwhich differentmixtures of inert gases were used33 .It appeared that argon, with a lower thermal conductivity and diffusion coefficient, gave SWNTswith a smaller diameter of approximately 1.2 nm. A linear fit of the average nanotube diametershowed a 0.2 nm diameter decrease per 10 % increase in argon helium ratio, whennickel/yttriumwas used (C/Ni/Y was 94.8 : 4.2 : 1) as catalyst.

b) Optical plasma control

A second way of control is plasma control by changing the anode to cathode distance (ACD). TheACD is adjusted in order to obtain strong visible vortices around the cathode. This enhances anodevaporisation, which improves nanotubes formation. Combined with controlling theargon-heliummixture, one can simultaneously control the macroscopic and microscopic parameters of thenanotubes formed34 .With a nickel and yttrium catalyst (C/Ni/Y is 94.8 : 4.2 : 1) the optimum nanotube yield was foundat a pressure of 660 mbar for pure helium and 100 mbar for pure argon. The nanotube diameterranges from 1.27 to 1.37 nanometre.

c) Catalyst

Knowing that chemical vapour deposition (CVD) could give SWNTs with a diameter of 0.61.2 nm;researchers tried the same catalyst as used in CVD on arc discharge. Not all of

the catalysts usedappeared to result in nanotubes for both methods. But there seemed to be a correlation of diameter of


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SWNTs synthesised by CVD and arc discharge.As a result, the diameter can be controllably lowered to a range of 0.6-1.2 nm with arc-discharge.Using a mixture of Co and Mo in high concentrations as catalyst resulted in thisresult. These

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diameters are considerably smaller than 1.2-1.4 nm32, which is the usual size gained from arcdischarge35.

d) Improvement of oxidation resistance

There is also progress in developing methods to improve the oxidation resistanceof the SWNTs,which is a consequence of the defects present in nanotubes. A strong oxidation resistance is neededfor the nanotubes if they have to be used for applications such as field emission displays. Recentresearch has indicated that a modified arc-discharge method using a bowl-like cathode, decreases thedefects and gives cleaner nanotubes, and thus improves the oxidation resistance36 .

Fig 11: Schematic drawings of the electrode set-ups for (a) the conventional and(b) the newarc dischargeelectrodes.

The Raman spectrum of the newly synthesised nanotubes shows that the nanotubes formed arecleaner and less defective compared with those synthesised by conventional methods. The anode rodcontained Ni and Y catalyst (C /Ni/Y is 94.8:4.2:1). No information is given about the diameter size.

e) Open air synthesis with welding arc torch

Only a couple of years ago, researchers discovered that it was possible to formMWNTs in openair37. A welding arc torch was operated in open air and the process was shieldedwith an argon gasflow. The anode and cathode were made of graphite containing Ni and Y (Ni/Y is 4.2:1 at. %).

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Fig 12: Experimental set-up of the torch arcmethod in open air.

This method was modified for preparing SWNTs38. A plate target made of graphitecontaining metalcatalyst Ni and Y (Ni/Y is 3.6:0.8 at. per cent), was fixed at the sidewall of a

watercooled, steelbased electrode. The torch arc aimed at the edge of the target and the soot wasdeposited on thesubstrate behind the target (see Figure 2-4). The arc was operated at a direct current of 100 A. andshielding argon gas flowed through the torch, enhancing the arc jet formation beyond the target.In the soot, carbon nanohorns (CNHs) and bundles of SWNT with an average diameter of 1.32 nmwere found. However, the yield was much lower than for the conventional low-pressure arcdischarge method. There are two reasons for this fact. At first, because of the

open air, the lightersoot will escape into the atmosphere. Secondly, the carbon vapour might be oxidised and emitted ascarbon dioxide gas. In order to improve the yield in this method, contrivances for collecting soot anddevelopment of an appropriate target are required.This method promises to be convenient and inexpensive once the conditions for higher yield areoptimised. With a Ni/Y catalyst (Ni/Y is 3.6:0.8), SWNT bundles and MWCNTs are formed. In thiscasethe SWNTs have a diameter of approximately 1.32 nm.

Synthesis of MWNT

If both electrodes are graphite, the main product will be MWNTs. But next to MWNTs a lot of sideproducts are formed such as fullerenes, amorphous carbon, and some graphite sheets. Purifying theMWNTs results in loss of structure and disordering of the walls. However scientists are developingways to gain pure MWNTs in a large-scale process without purification.

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Typical sizes for MWNTs are an inner diameter of 1-3 nm and an outer diameter ofapproximately10 nm. Because no catalyst is involved in this process, there is no need for a heavy acidicpurification step. This means, the MWNT, can be synthesised with a low amount ofdefects.

a) Synthesis in liquid nitrogen

A first, possibly economical route to highly crystalline MWNTs is the arc-discharge method inliquid nitrogen, with this route mass production is also possible. For this option low pressures andexpensive inert gasses are not needed.

Fig 13: Schematic drawings of the arc discharge apparatus in liquid nitrogen.

The content of the MWNTs can be as high as 70 % of the reaction product. Analysis with Augerspectroscopy revealed that no nitrogen was incorporated in the MWNTs. There is astrong possibilitythat SWNTs can be produced with the same apparatus under different conditions.

b) Magnetic field synthesis

Synthesis of MWNTs in a magnetic field39 gives defect-free and high purity MWNTsthat can be

applied as nanosized electric wires for device fabrication. In this case, the arc discharge synthesiswas controlled by a magnetic field around the arc plasma.

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Fig 14: Schematic diagram of the synthesis system for MWNTs in a magnetic field.

Extremely pure graphite rods (purity > 99.999 %) were used as electrodes. Highlypure MWNTs

(purity > 95 %) were obtained without further purification, which disorders walls of MWNTs.

Fig 15: SEM images ofMWNTs synthesised with (a) and without (b) the magnetic field.

c)Plasma rotating arcdischarge

A second possibly economical route to mass production of MWNTs is synthesis by plasma rotatingarc discharge technique40 . The centrifugal force caused by the rotation generates turbulence andaccelerates the carbon vapour perpendicular to the anode. In addition, the rotation distributes themicro discharges uniformly and generates a stable plasma. Consequently, it increases the plasmavolume and raises the plasma temperature.

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Fig 16: Schematic diagram of plasma rotating electrode system.

At a rotation speed of 5000 rpm a yield of 60 % was found at a formation temperature of 1025 Cwithout the use of any catalyst. The yield increases up to 90% after purification if the rotation speed

is increased and the temperature is enlarged to 1150 C.

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Summary in a Tabular form:

METHODARC-DISCHARGECHEMICAL VAPOR

DEPOSITION (CVD)LASER ABLATIONInventionEbbesen and Ajayan,NEC, Japan, 1992Endo, Shinshu University,Nagano, JapanSmalley, RiceUniversity, 1995ProcedureConnect two graphiterods to a power supply,

place them a fewmillimetres apart, andthrow the switch. At100 amps, carbonvaporises and forms ahot plasma.Place substrate in oven,heat to 600 oC, and slowlyadd a carbon-bearing gassuch as methane. As gasdecomposes it frees upcarbon atoms, whichrecombine in the form of

MWCNTsBlast graphite withintense laser pulses; usethe laser pulses ratherthan electricity togenerate carbon gasfrom which theMWCNTs form; tryvarious conditions untilhit on one that producesprodigious amounts ofSWNTsApproximateyield30 to 90% 20 to 100 % Up to 70%SWNTShort tubes withdiameters of 0.6 -1.4nmLong tubes with diametersranging from 0.6-4 nmLong bundles of tubes(5-20 microns), withindividual diameterfrom 1-2 nm

MWNTShort tubes with innerdiameter of 1-3 nm and


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outer diameter ofapproximately 10 nmLong tubes with diameterranging from 10-240 nmNot very much interestin this technique, as it istoo expensive, but

MWNT synthesis ispossible.AdvantagesCan easily produceSWNT, MWNTs.SWNTs have fewstructural defects;MWNTs withoutcatalyst, not tooexpensive, open airsynthesis possibleEasiest to scale up to

industrial production; longlength, simple process,SWNT diametercontrollable, quite purePrimarily SWNTs, withgood diameter controland few defects. Thereaction product is quitepure.LimitationsTubes tend to be shortwith random sizes anddirections; often needs a

lot of purificationMWCNTs are usuallyMWNTs and often riddledwith defectsCostly technique,because it requiresexpensive lasers andhigh power requirement,but is improving

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CharacterizationTechniques

Field Emission Scanning Electron Microscope (FE-SEM)

Under vacuum, electrons generated by a Field Emission Source are accelerated ina field gradient.The beam passes through Electromagnetic Lenses, focusing onto the specimen. As result of thisbombardment different types of electrons are emitted from the specimen. A detector catches thesecondary electrons and an image of the sample surface is constructed by comparing the intensity ofthese secondary electrons to the scanning primary electron beam. Finally the image is displayed on amonitor41 .

Fig 17: Schematic diagram ofFE-SEM

The investigation of the surfaces and the distribution of the nanotubes in the deposit had beenperformed by FESEM using S4300 SE/N HITACHI Japan at 10 kV. The samples were cutand weremounted on copper stubs before the test.

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Fig 18:FE-SEMmodel: S4300SE/N at ARCI

Thermo Gravimetric Analysis (TGA)

TGA is a sample analytical technique that measures the weight loss (or weight gain) of a material asa function of temperature. As materials are heated, they can loose or gain weight by reacting with theatmosphere in the testing environment. Since weight loss and gain are disruptiveprocess to thesample material or batch, knowledge of the magnitude and temperature of those reactions arenecessary in order to design adequate thermal ramps and holds during those critical reaction periods.

The gas environment is pre-selected for either thermal decomposition (inert gases such as N2, He,Ar), oxidative decompositions (air or pure O2), or thermaloxidation. A peak of the temperature

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derivative of the weight dm/dt is indicate of the oxidation temperature weigh isshifted to lowertemperature for chemically modified MWCNTs.

Fig 19: TGA set up at ARCI

Sample preparation

Thermo gravimetric analysis was conducted with NETZCH STA 449 Jupiter model thatwas fitted toan argon purge gas from ambient temperature to 1400 oC at 10 oC/ min heating rate. Before the test,all the samples were carefully ground to powders to ensure sufficient diffusionof heat. Themeasurements were conducted using 100 mg samples and weight retention/ temperatu

re curves wererecorded.

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Experiment

In our initial experiments, a graphite bar (18x18x150 mm3) weighing 50 g and 75mm diametergraphite disc (20 mm thick) were installed vertically as the anode and cathode,

respectively. Bothelectrodes were further water-cooled independently. An accurate positioning system is employed tomaintain a steady feed rate of the anode, which can be instantly adjusted in therange of 0-3 cm min

1. The arc current and voltage are monitored automatically and can be preciselycontrolled byadjusting the output of the arc power supply and the gap between the two discharging surfaces. Aself-sustained stabilized arc-discharge is established by carefully adjusting th

e arc-dischargeparameters. This means that the anode rod is continuously consumed and the cylindrical cathodedeposit grows steadily while the gap between the two surfaces remains constant (about 1 mm). Forthe purpose of comparison, dc arc-discharge evaporation of graphite was carriedout in buffer gas ofhelium (300 Torr) and hydrogen (200-500 Torr) respectively under the same arc-dischargeparameters of 40 V, 250 A, at water cooling pressure of 6.5 kg/cm2.Fig 20: Arc Discharge set up at ARCI

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(a) (b)(c) (d) (e)Fig 21:(a)Graphite anode ba (b) Flat graphite cathode

(c) plasma arcing inside the chamber(d) Cathode depositin H2 (e) Cathode deposit in HeThe cathode deposits obtained in helium and hydrogen atmospheres were cylindrical. The weights ofthe deposits were 26.7 g and 17.8 g respectively.

In our second phase of experiments, a graphite rod of 10mm diameter and 150 mm length weighing

18.5 g, and a cup shaped graphite of outer diameter 40 mm, inner diameter 30mm a

nd the depth ofthe cup 15mm were installed vertically as anode and cathode, respectively. The entire set up is assimilar to that of our previous experiments. For the purpose of studying the effect of cathode shapeon the synthesis of MWMWCNTs, the evaporation of the graphite rod was carried out in hydrogenbuffer gas at 350 Torr under arc discharge parameters of 30 V, 150 A and water cooling pressureof 6.5 kg/cm2. The time taken for each experiment was 6 min.32


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(a)(b)(c) (d)Fig 22: (a)Flatgraphite cathode (b) Cup shaped

graphite cathode

(c)Graphite anode rod (d) Cathode depositThe cathode deposits obtained on the flat cathode and cup shaped cathode were cylindrical withsimilar dimensions, approximately 12 mm diameter and 46 mm height. The weights of the depositswere 7.9 g and 7.8 g respectively. All the cathode deposits were investigated byfield emission scanningelectron microscopy (FE-SEM, S4300SE/N) and Thermo Gravimetric Analysis (TGA,

NETZCH STA 449 Jupiter).

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ResultsandDiscussionEffect of partial pressure of hydrogengas

Fig 23: TGA of cathode deposits obtained underdifferent pressures of hydrogen

Figure 23 shows the thermo gravimetric analysis of the cathode deposits obtainedin hydrogenatmosphere at 200 Torr, 350 Torr and 500 Torr respectively. It is seen that, theloss of deposit weightwithin the 650-750 oC range, which is the oxidizing temperature range of MWCNTs,is maximum

(~50%) for the deposit obtained at 350 Torr compared to deposits obtained at 500Torr (~29%) and200 Torr (~18%). Other particles such as amorphous carbon (750 oC) are also obtained in the cathode deposits.

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Fig 24: SEM image ofthe deposit obtained at 200 Torr of Hydrogen

Fig 25: SEM image of

the deposit obtained at 350 Torr (left) of Hydrogen and 500 Torr (right)

Figure 24-25 shows the SEM images of all the three deposits. Maximum amount of MWCNTs areseen in the deposit obtained at 350 Torr. The amount of MWCNTs is slightly on the lower side in thedeposit obtained at 500 Torr. Maximum amount of graphite nanoparticles are seenin the depositobtained at 200 Torr. Hence, partial pressure of 350 Torr with hydrogen can be considered asoptimum conditions for better yield of multi-walled carbon nanotubes.

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Effect of Buffer Gas

Fig 26: TGA of the deposits obtained under Helium and hydrogen atmospheresFigure 26 shows the thermo gravimetric analysis of the deposits obtained in helium and hydrogen

atmospheres at partial pressures of 300 Torr and 350 Torr respectively. The twocurves suggests thatmore number of MWCNTs are present in the deposit obtained in hydrogen atmosphere(~50%)compared to the deposit obtained in helium atmosphere (~37%).Figure 27-28 shows the SEM images of the deposits obtained in helium and hydrogen atmospheresrespectively. From the images, it is understood that MWCNTs are more and amorphous carbon isless in hydrogen atmosphere compared to helium atmosphere. Also, the MWCNTs obtained inhelium gas are bent and randomly oriented. Where as, the MWCNTs have linear orie

ntation and arewell distributed under hydrogen gas conditions.

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Fig 27: SEM image of the deposit obtained at 300 Torr of Helium

Fig 28: SEM image of the deposit obtained at 350 Torr of HydrogenSynthesis of MWCNTs is favourable in hydrogen gas because the ionization energy

of hydrogen(1312 kJ.mol-1) is very less compared to that of helium gas (2372.3 kJ.mol-1). Under such conditions,the plasma arc generated is of higher quality and the erosion of graphite rod becomes quick that isalmost ten times faster than in helium. (Synthesis time for helium which was 80minutes has comedown drastically to synthesis time of 8 minutes in hydrogen). In addition to that, the thermalconductivity of hydrogen (0.1805 W.m-1.K-1) is higher than that of helium (0.1513 W.m-1.K-1). Thisresults in generation of higher amount of heat energy which is much favourable f

or the sublimationof graphite rod.

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Effect Of cathode shape

We initially assumed that the cup shaped cathode can confine the carbon vapour,sublimated fromthe graphite anode which may facilitate in effective condensation of the carbon

vapour therebyincreasing the amount of cathode deposit, thus the amount of fullerenes obtainedon the chamberwalls may be reduced.

Fig29: TGA of the deposits obtained on cupshaped cathode and flat cathodein hydrogen

But the TGA result shows that the amount of MWCNTs in the deposits obtained on cup shapedcathode and on flat cathode are almost the same in hydrogen atmosphere. We can see that both theTGA curves are overlapped in the above graph indicating almost 70 % MWCNTs in the deposits.

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Fig 30: SEM image on the cathode deposit obtained onflat cathode

Fig 31: SEM image of the cathode deposit obtained on cup shaped cathode

However, from the SEM images, it appears more number of MWCNTs on flat cathode than cupshaped cathode. This may be due to adequate exposure of hydrogen near the vicinity of plasma arcon a flat cathode as compared to cup shaped cathode.

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Conclusions

We have investigated the effect of the buffer gas, its partial pressure and theshape of the cathode onthe synthesis of multi-walled carbon nanotubes using Arc-Discharge method. Obser

vations usingFE-SEM and TGA indicate maximum amount of MWCNTs in the deposit obtained in hydrogen gasat partial pressure of 350 Torr, while helium gas having partial pressure of 300Torr is effective.Hydrogen has more thermal conductivity and very less ionization energy than helium. Hence,hydrogen is more effective as buffer gas in the synthesis of MWCNTs.MWCNTs were also synthesized using a cup shaped cathode instead of a flat cathode in hydrogenatmosphere at 350 Torr. Since adequate amount of hydrogen was available for thesynthesis process

using flat cathode, MWCNTs obtained on flat cathode were more compared to that on cup shapedcathode. FE-SEM analysis and TGA confirmed the results.

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