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Introduction

Owing to their unique physical properties,carbon nanotubes (CNTs) are consideredone of the most important materials of the21st century. They consist of graphite-likecarbon, have a diameter of ca. 1 to 100nanometres (nm) and a length of up to sev-eral micrometres or even millimetres. Car-bon nanotubes can be single-walled (SW-CNT) or multi-walled (MWCNT), and theirunusual mechanical and electrical prop-erties make them suitable for numerous ap-plications. Because of the high current den-sity achievable in CNTs, researchers are al-ready exploring the next generation of mi-crochips, high-speed and low-loss transis-tors for highly integrated circuits and newelectronic components. Other groups areworking on the development of chemicaland biological sensors on a CNT basis, ontransparent electrodes for solar cells andlight-emitting diodes, membranes for fuelcells or CNT applications on circuit boards,to name but a few. CNTs not only raise hopesfor innovative applications in a range offields from technology to medicine, they al-so harbour the promise of considerableeconomic benefits. This dossier provides anoverview of fundamental facts, productionprocesses and spheres for the applicationof CNTs.

Carbon and Its Structural Forms

Carbon is the basic element of life on Earth,since every living tissue is composed of or-ganic carbon compounds. As carbon atomshave the ability of combining with other car-bon atoms as well as atoms of other ele-ments to form complex molecules, carbonboasts a greater diversity of compoundsthan any other chemical element. Even el-ementary carbon exists in various very dif-ferent structural forms (allotrope modifica-tions, see Fig. 1).

• One well-known form of pure carbon inwhich the carbon atoms combine to forma crystal structure is diamond. Formedin the Earth’s crust by high pressure andhigh temperatures, diamonds are thehardest material found in nature.

• Hypothetically calculated in 1970 by thechemist Eiji Osawa, a new carbon allo-trope, the spheroid fullerene, was dis-covered just 15 years later, in 1985. Thebest known fullerenes, the so-calledC60-fullerenes (“Buckyballs”), consist of60 carbon atoms arranged in a latticeof hexagons and pentagons. While thestructure of the C60-fullerenes is remi-niscent of a soccer ball, they owe theirname to their similarity with the domesdesigned by the US architect RichardBuckminster Fuller. The discovery of ful-lerenes earned Robert Curl jr. (USA), SirHarald Kroto (UK) and Richard Smalley(USA) the 1996 Nobel Prize for Chem-istry. The use of fullerenes as lubricants,catalysts or for medical purposes is cur-rently being researched.

• Another form of carbon is graphite,which consists of several superimposedlayers of carbon with a lattice structure.Rarely found in nature, graphite is usedas a sealant or lubricant because of itshigh temperature resistance. Its mostwell-known use, however, is for pencilleads.

Summary

As the basic element of life on Earth, car-bon boasts a greater diversity of com-pounds than any other chemical ele-ment. Even elementary carbon occurs inseveral structural forms, including dia-monds, graphite, fullerenes and carbonnanotubes (CNTs). The latter are wellknown and promising nanomaterials.CNTs have exceptional properties, com-bining high resistance, tensile strengthand electrical conductivity with a verylow weight. Carbon nanotubes are pro-duced in numerous industries worldwide,with CVD (chemical vapour deposition)currently being the most relevant pro-cessing technology. CNTs are used asadditives to various plastics in electron-ics, car manufacturing and shipbuildingor for the production of sports equip-ment. In the future, CNTs are expectedto be used particularly in environmen-tal and energy engineering, possible ap-plications including enhanced batteries,solar and fuel cells, as well as in the con-struction industry for high-performanceconcrete, but also in medicine for drugdelivery.

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Carbon Nanotubes – Part I:Introduction, Production, Areas of Application

Sabine Greßler, René Fries,Myrtill Simkó*

* Corresponding author

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• A graphite film consisting of only a single-atom-thick lattice-shaped layer of carbonis called graphene. Although graphenestructures have been known since the1960s, it was not possible to isolate sucha monolayer until a few years ago. Thefeat was first achieved in 2004 by AndreGeim and Konstantin Novoselov from theUniversity of Manchester (UK), who usedadhesive tape to peel thin layers off agraphite crystal4. In 2010, Geim and No-voselov were awarded the Nobel Prize inPhysics for “groundbreaking experimentsregarding the two-dimensional materialgraphene”. The methods have meanwhile been tech-nologically refined and allow for produc-ing graphene sheets up to a width of 70cm. Graphene has astounding properties.It is transparent, 100 times stronger thanthe strongest steel and has a very highelectrical and thermal conductivity. Theseunique properties make graphene an in-teresting candidate for a number of ap-plications currently under development, asfor instance transparent touch screens,light panels or solar cells. Combined withsynthetic materials, graphene opens upnew vistas for materials that are both high-ly resistant and very light-weight, to beused, for instance, in satellite or aircraftconstruction.

Carbon Nanotubes

Whereas diamond has a rigid and unchange-able crystal structure, graphene is flexibleand can be bent and even shaped into cylin-drical form. (Fig. 2). Such “rolled-up” struc-tures are called carbon nanotubes (CNTs)5.One distinguishes single-walled CNTs (SW-CNTs), consisting of only one rolled-up lay-er of graphite with a diameter of up to 3 nm,and multi-walled CNTs (MWCNTs), consist-ing of several concentrically interlinked graph-ite tubes6 with a diameter of up to 100 nm.The rolling-up direction (rolling-up or chi-ral vector) determines different modificationsof the structures which will then have differ-ent electrical properties. Depending on therolling-up vector, SWCNTs can behave likea metal and be electrically conducting, dis-play the properties of a semi-conductor orbe non-conducting. MWCNTs will achieve atleast the same level of conductivity as met-als. CNTs were discovered in 1991 by Su-mio Iijima7. Like the fullerenes, CNTs occurin nature as by-products of combustion proc-esses. Hence, they are not a human “inven-tion”. These materials have even been de-tected in 10,000 year-old ice core samples8.

Apart from their electrical properties, CNTsalso have special thermal and mechani-cal properties that make them an interest-ing option for the development of new ma-terials5; 6; 9:

• CNTs are electrically conducting(MWCNTs always; SWCNTs as afunction of their rolling-up direction)

• their mechanical tensile strength canbe 400 times that of steel

• CNTs are very light-weight – theirdensity is one sixth of that of steel

• their thermal conductivity is betterthan that of diamond – the bestthermal conductor known so far

• CNTs have a very high aspect ratio, i.e.in relation to their length they are extreme-ly thin

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Figure 2:Like fullerenes, carbon nanotubes consistbasically of rolled-up layers of graphene1

Figure 1: Examples of the various structural forms of carbon. a.) diamond; b.) graphite; c.) Lonsdaleite (hexagonal diamond);

d.) Buckminster-Fullerene (C60); e. C540, f. C70; g.) amorphous carbon, h.) Nanotube2

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• just like graphite they are highly chemi-cally stable and resist virtually any chem-ical impact unless they are simultaneous-ly exposed to high temperatures andoxygen. Hence, CNTs are extremely resist-ant to corrosion.

• the hollow interior of CNTs can be filledwith various nano-scale materials, thusseparating and shielding them from thesurrounding environment.

Carbon nanotubes must be distinguishedfrom carbon nanofibres (CNFs), even if theterms are often used interchangeably in litera-ture. CNFs are usually several micrometerslong and have a diameter of about 200 nm.Carbon fibres have been used for decadesto strengthen compound materials (e.g. inboat-building, aeronautics and astronautics,sports equipment, etc.). CNFs do not havethe same lattice structure as CNTs, but con-sist of a combination of several forms of car-bon and/or several layers of graphite whichare stacked at various angles on amorphouscarbon (where atoms do not arrange them-selves in ordered structures). CNFs have sim-ilar properties as CNTs, but their tensilestrength is lower owing to their variable struc-ture and they are not hollow inside.

Production

Three methods are currently available for theproduction of CNTs: arc discharge, laser ab-lation of graphite and chemical vapour dep-osition (CVD). In the first two processes,graphite is combusted electrically or bymeans of a laser, and the CNTs developingin the gaseous phase are separated. All threemethods require the use of metals (e.g. iron,cobalt, nickel) as catalysts. The CVD processcurrently holds the greatest promise, sinceit allows the production of larger quantitiesof CNTs under more easily controllable con-ditions and at lower cost3. During CVD, CNTsform on the surface of a catalyst that is ex-posed to carbon-containing gas (e.g. car-bon monoxide or ethylene) (Fig. 3). First,small secondary catalyst particles of the sizeof a CNT diameter develop, on which thenanotubes start growing. The catalyst par-ticle is either at the top or at the bottom ofthe emerging nanotube. Growth will stop ifthe catalyst particle is deactivated throughthe development of a carbon envelope. Onedistinguishes between purely catalytic andplasma-supported CVD. The latter requiresslightly lower temperatures (200-500°C) thanthe catalytic process (up to 750°C) and aimsat producing “lawn-like” CNT growth5; 6.Structural defects and impurities, such as re-sidues of the metal catalysts, alter the physi-co-chemical behaviour of CNTs, which is whythey need to be cleaned with the help of var-ious methods (e.g. acid treatment or ultra-sound) at the end of the production process3.

Areas of Application

CNTs are credited with the potential to helpproduce completely new materials andproducts with properties that were hithertoimpossible to realise with available technol-ogy. Some even call them a “megatrend” inmaterial technology10. A great deal of effortis being invested worldwide in harnessing thespecial properties of CNTs for new materi-als and products, and these aspirations havealready been fulfilled in several domains.Table 1 provides an overview description andexamples of available applications andthose currently under research and develop-ment. Currently, CNTs are mainly used asadditives to synthetics. CNTs are commer-cially available as a powder, i.e. in a high-ly tangled-up and agglomerated form (Fig.4). For CNTs to unfold their particular prop-erties they need to be untangled and spreadevenly in the substrate. This major technicalchallenge is met by special process engineer-ing methods11. Another requirement is thatCNTs need to be chemically bonded with thesubstrate, e.g. a plastic material. For thatpurpose, CNTs are functionalised, i.e. theirsurface is chemically adapted for optimal in-corporation into different materials and forthe specific application in question.

Carbon nanotubes can also be spun into fi-bres12, which not only promise interestingpossibilities for specialty textiles but may al-so help realise a particularly utopian proj-ect – the space elevator13.

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Figure 3:Schematic view of CNT growth on catalyst particles during CVD3

Figure 4:Commercial CNTs

(Bayer Baytubes, Bayer AG, Germany)

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Notes and References1 www.kva.se/en/pressroom/Press-releases-

2010/The-Nobel-Prize-in-Physics-2010/.2 de.wikipedia.org/wiki/Kohlenstoff.3 Endo, M., Hayashi, T., Kim, Y. A., Terrones, M.

and Dresselhaus, M. S., 2004, Applications ofcarbon nanotubes in the twenty-first century,Phil. Trans. R. Soc. Lond. A 362, 2223-2238.

4 Novoselov, K. S., Geim, A. K., Morozov, S. V.,Jiang, D., Zhang, Y., Dubonos, S. V., Griogrie-va, I. V. and Firsov, A. A., 2004, Electric FieldEffect in Atomically Thin Carbon Films, Science306, 666-669.

5 Klingeler, R., Kramberger, C., Müller, C., Pich-ler, T., Leonhardt, A. and Büchner, B., 2007,Funktionalisierte Kohlenstoffnanoröhren: Ma-terialforschung in der Nanowelt, Wissenschaft-liche Zeitschrift der Technischen UniversitätDresden 56(1-2), 105-110.

6 Jess, A., Kern, C., Schrögel, K., Jung, A. andSchütz, W., 2006, Herstellung von Kohlenstoff-Nanotubes und -fasern durch Gasphasenab-scheidung, Chemie Ingenieur Technik 78(1-2),94-100.

7 Iijima, S., 1991, Helical microtubules of graph-itic carbon, Nature 354, 56-58.

8 Murr, L. E., Soto, K. F., Esquivel, E. V., Bang,J. J., Guerrero, P. A., Lopez, D. A. and Ramirez,D. A., 2004, Carbon Nanotubes and OtherFullerene-Related Nanocrystals in the Environ-ment: A TEM Study, Journal of Minerals, Met-als and Materials Society 56(6), 28-31.

9 Holister, P., Harper, T. E. and Román Vas, C.,2003, Nanotubes White Paper: CMP Cienti-fica.

10 Inno.CNT, 2009, Innovationsallianz CNT –Kohlenstoffnanomaterialien erobern Märkte, imAuftrag von: Bundesministerium für Bildungund Forschung, Januar 2009, Düsseldorf: In-novationsallianz Carbon Nanotubes.

11 Nanotechnologie in Kunststoff. Innovationsmo-tor für Kunststoffe, ihre Verarbeitung und An-wendung, Nr. Band 15 der Schriftenreihe derAktionslinie Hessen-Nanotech des HessischenMinisteriums f. Wirtschaft, Verkehr und Landes-entwicklung.

12 Li, Y.-L., Kinloch, I. A. and Windle, A. H., 2004,Direct Spinning of Carbon Nanotube Fibresfrom Chemical Vapor Deposition Synthesis, Sci-ence 304, 276-278.

13 www.liftport.com.14 www.nanocyl, www.baytubes.com and

www.amroy.fi.15 www.inno-cnt.de.16 Liu, Z., Tabakman, S., Welsher, K. and Dai, H.,

2009, Carbon Nanotubes in Biology and Med-icine: in vitro and in vivo Detection, Imagingand Drug Delivery, Nano Res. 2(2), 85-120.

17 Siegfried, B., 2007, NanoTextiles: Functions,nanoparticles and commercial applications,December 2007: EMPA.

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Table 1: Examples of CNT applications – accomplished and in R&D

Materials and properties14: Areas of application:

Accomplished:

Plastic masterbatches with CNT additives;improved electrostatic dissipation properties ofthermoplastic synthetics; weight reduction;increased corrosion resistance

Electronics: Packaging material for integrated-circuit transport and storage (IC trays);test sockets for microchips;transport and storage containers for semi-conductor discs;

Automotive: fuel system components,mudguards, mirror housings, door handles;

Silicone resins with CNT additives for paintsand varnishes; increased fire and scratchresistance; surface treatment of metal,concrete, wood, brick and fibreboard;

Fire safety: flame arrester for foam material;coating of cables and wires;

Non-stick coating for ships to preventaccumulation of marine organisms (such asbarnacles)

Epoxy resins (duroplastics) with CNTs;increased wear resistance and breakingstrength, antistatic properties; weight reduction

Sports equipment: bicycle frames, tennisrackets, hockey sticks, golf clubs, skis, kayaks;sports arrows:Yachting: masts and other parts of sailboats;

Aeronautics: no detailed information available

Energy engineering: coating of wind-turbinerotor blades

Industrial engineering: industrial robot arms

Research & Development15:

Electrically conducting inks; improvedconductivity and mechanical resistance

Energy engineering: solar cells

Bipolar plates and gas diffusion layers Energy engineering: fuel cells

Electrodes made from CNTs Energy engineering: lithium-ion batteries withincreased storage capacity

Membranes with CNT; higher energyefficiency and productivity

Environmental engineering: seawaterdesalination, CO2 separation

Ultra-light composite materials Aeronautics and astronautics, automotive

High-strength CNT-based particle foams;improved absorption of deformation energy

Automotive: improved safety of body parts

Plastic parts and sealants on the basis ofCNT-elastomers; improved friction, lubricationand wear properties

Construction industry

Metals with CNTs Automotive: parts exposed to high mechanicalloads

Ultra-high strength CNT-based concrete;improved stability and elasticity

Construction industry: e.g. high-risebuildings, bridges

Surface-functionalised, biocompatible CNTsfor targeted drug delivery in the human body16

Medicine: e.g. cancer therapy

CNT-enhanced polymer composite fibresand fabrics; CNT-based fibres; improvedresistance; conductivity, hydrophobic andflame retarding properties17

Textiles: antistatic and electrically conductingtextiles (“smart clothes”); bullet-proof vests,water-resistant and flame-retardant textiles

CNT-based transistors: faster and morepowerful circuits

Electronics: computer chips

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Mode of publication: The NanoTrust Dossiers are published irregularly and contain the research results of the Institute of Technology Assessment in the framework of its research project NanoTrust. The Dossiers are made available to the public exclusively via the Internet portal “epub.oeaw” :epub.oeaw.ac.at/ita/nanotrust-dossiers

NanoTrust-Dossier No. 022en, February 2012: epub.oeaw.ac.at/ita/nanotrust-dossiers/dossier022en.pdf

ISSN: 1998-7293

This Dossier is published under the Creative Commons (Attribution-NonCommercial-NoDerivs 2.0 Austria) licence: creativecommons.org/licenses/by-nc-nd/2.0/at/deed.en