1. Carbon Nanomaterials for Transport
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Carbon nanotubes – the wonder nanomaterial - has importantimplications for all kinds of
transport.Dr Bojan Boskovic of Cambridge Nanomaterials TechnologyLtd,discusses.Carbon nanomaterials,such as carbon
nanotubes,nanofibres and graphene are becoming increasinglyimportantin manyapplications including aerospace,
automotive,marine and rail transportdue to their fascinating properties.Carbon nanotubes can behave like metals or
semiconductors,they can conduct electricity better than copper and transmitheatbetter than diamond,and they rank amo ng
the strongestmaterials known.Applications ofcarbon nanomaterials are now attracting considerable interestfrom both the
academic and the industrial communities.It is alreadyproven that carbon nanomaterials are good field emitters for flat
screens,conducting fillers in polymer composite materials,and electrodes in fuel cells.They can be used in nanoelectronics
as diodes and transistors and in supercapacitors as electromechanical actuators and as chemical sensors.A complex
electromechanical system used in cars,aeroplanes or trains could include manyof these applications.
2. Carbon nanotubes, what they are and how they are made
A carbon nanotube (CNT) is a tubular carbon structure with hollow cylindrical graphene walls capped byfullerene -type
hemispheres.A carbon nanotube comprising a singular graphene tube (Fig.1) is called a 'single wall carbon nanotube'
(SWCNT), and concentric graphene tubes nested within each other like "Russian dolls"are called 'multi -wall carbon
nanotubes',(MWCNT). Other carbon structures with herringbone or cup-stacked graphene layers,which form an angle with
the longitudinal axis,are often called carbon nanofibres (CNFs).The carbon nanotube diameter can vary from only few
nanometers for a SWCNT up to few tens of nanometers for MWCNTs, and more than hundreds ofnanometers for CNFs;and
tube length can vary from microns up to millimeters and even centimeters.
Carbon nanotubes and nanofibres (filaments) have been made successfullyfor more than two decades using a metho d
known as chemical vapour deposition (CVD).In the CVD of carbon nanomaterials,a hydrocarbon gas is passed over a
heated catalyst. The actions of the catalyst cause the hydrocarbon to decompose into hydrogen and carbon atoms,which
provide the "feedstock"for carbon nanofibre/nanotube growth.Carbon nanofibres and nanotubes grown bythe CVD method
usuallyhave catalyst particles attached to one end. Multi-walled nanotubes grown this waytend to have a large number of
structural defects.
Carbon nanotubes – a brief background
The discovery of the C60 molecule in 1985 (also known as fullerene or buckminsterfullerene) by Sir Harry Kroto from
University of Sussex,Brighton, with a team led by Richard Smalleyfrom Rice University, Houston,marked the begin ning ofa
new era in carbon science.The Nobel Prize for Chemistryin 1996 was awarded for this research to Kroto, Smalleyand
RopbertCurl (also from Rice). In 1991, Sumio Iijima,using high-resolution transmission electron microscopyat the NEC in
Tsukuba Japan,reported the first observation ofstructures that consisted ofseveral concentric tubes ofcarbon nested inside
each other like "Russian dolls".He called them microtubules ofgraphitic carbon,butfrom 1992 Iijima and other researchers
began to call them carbon nanotubes.Iijima observed MWCNTs in the sootproduced by the electric arc discharge between
graphite electrodes in a helium atmosphere.
Carbon Nanotubes – Exploiting their Properties
3. Due to the high aspectratio, the quasi-one-dimensional structure,and the graphite-like arrangementofthe carbon atoms in
the shells,nanotubes exhibita very broad range of unique chemical,mechanical and electronic properties.The properties of
nanotubes can change depending on their differentstructures and quality. Large increases in strength,toughness,and
superior electrical and thermal properties,are some ofthe potential benefits ofusing nanotubes as the filler material in
polymer-based composites,when compared with traditional carbon,glass or metal fibres.
The remarkable electrical and mechanical properties ofcarbon nanotubes make them excellentcandidates for a range of
electrical,mechanical and electro-mechanical applications.Carbon fibres have alreadybeen used to strengthen a wide range
of materials,and the special properties ofcarbon nanotubes mean thatthey could be the ultimate-strength fibre.Since they
are composed entirelyof carbon,nanotubes also have a low specific weight.
Three-dimensional (3D) nano-carbon structures thatcan transfer the exceptional properties of carbon nanomaterials to meso-
and micro-scale engineering materials are essential for the developmentofmany applications.Well known engineering
materials like carbon,ceramic or glass fibre could be exploited as a supportfor the formation of 3D nano-structures (Fig.2).
Carbon fibre bundles,woven and non-woven carbon fibre cloth can be used as three-dimensional scaffolds for carbon
nanotube synthesis on the surface of fibres and in the empty spaces between them,to improve the mechanical,electrical and
thermal properties ofthe composite material.Growing CNTs and CNFs on the surface of carbon fibres could also improve
composite shear strength and load transfer atthe fibre/matrix interface.
The high surface area of carbon and ceramic fibres coated with nanotubes and nanofibres is importantfor use in
electrochemical applications.The high thermal conductivity of these materials mayalso be of use in automotive a nd
aerospace applications,and for heat distribution or hotspotcontrol. The first attempts to develop carbon -carbon composites
containing carbon nanotubes have alreadybeen reported.The high electrical conductivity of these materials could be used,
for example,in electronic components packaging,as gas diffusion layers in fuel cells or in electromagnetic shielding.Carbon
fabric impregnated with carbon nanotubes could be used for applications as varied as lightweight-yet-strong structures,brake
discs and bullet-proofvests.
Carbon nanotubes in cars and planes
The mechanical and electrical properties ofcarbon nanotubes can be exploited in applications,such as aircraftbodies with i n-
situ 'health' monitoring and selfhealing properties;superior brakes with carbon-carbon composite discs thatcould dissipate a
heat more efficiently; strong and interactive windscreens with de-icing properties (Fig.3). Even a few percentage loading of
carbon nanotubes in a polymer matrix could make non-conductive polymers conductive,solving manyproblems with static
electricity that could spark a fire within a vehicle. In aircraft wings,the conductivity of carbon nanotubes could provide d e-icing
and lighting strike protection,combined with weightreduction.Importantly,they could improve the strength of vehicle bodies,
decrease their weightand make army vehicles or military airplanes electromagneticallyinvisible.Carbon nanotubes and
nanofibres could be added to metals in order to improve their properties and make lighter engines,they could be used in tyres
instead ofcarbon black to improve wear properties - and provide in situ pressure sensing!
Realising the promise of CNTs
There are a number of problems to overcome in the developmentprocess ofcarbon nanomaterials application.The properties
of nanotubes need to be optimised for specific applications.The nanotubes mustbe efficiently dispersed and bonded to the
material they are reinforcing (the matrix), in order to maximize load transfer.Many companies manage to solve some ofthese
problems,with more or less success,and have developed new composite materials from various polymers and carbon
nanotubes with improved mechanical,thermal or electrical properties.There are still manyopportunities for the improvement
of properties oftraditional engineering materials and their replacementwith use ofcarbon nanomaterials in transportand a
wide range of other applications.
Looking to the future
4. Carbon nanotube and nanofibre related research and developmenthas led to new production routes and the suggestion and
realisation ofmanynew exciting applications,steadilygrowing in number around the world.Carbon nanotube and nanofibre
composites have alreadyreached the marketwith the firstsports applications including tennis rackets,baseball bats,and
racing bikes.It is expected that carbon nanotube composite materials will have manymore applications including automotive
and aerospace components,structures and brakes.
Chemical vapour deposition has proven to be an effective technique for synthesis ofcarbon nanomaterials and
nanostructures.It offers controlled carbon nanomaterials and nanostructures fabrication thatis easilyscalable and can be
adopted for mass production atan industrial level.Similar temperature and gas composition environments needed for carbon
nanomaterials synthesis existin vehicle engines,and catalysts have already been used in modern vehicles in order to reduce
green house gases emission.We can imagine a possibilitywhere vehicles could be used to produce valuable carbon
nanomaterials and further reduce green house gas emissions.
Intensive research in recent decades has alreadysetup solid foundations for exploitation of the many properties thatcarbon
nanomaterials have for the benefit of humanity. However, significantadvances in understanding,synthesis and applications of
carbon nanomaterials are still ahead ofus.
Dr Bojan Boskovic , Cambridge Nanomaterials Technology Ltd, www.CNT-Ltd.co.uk; Bojan.Boskovic@CNT-Ltd.co.uk
