Carbon nanotubes (CNTs) are allotropes of carbonwith a cylindrical nanostructure. Nanotubes havebeen constructed with length-to-diameter ratioof up to 132,000,000:1,[1] significantly largerthan for any other material. These cylindricalcarbon molecules have unusual properties, whichare valuable for nanotechnology, electronics,optics and other fields of materials science andtechnology. In particular, owing to theirextraordinary thermal conductivity andmechanical and electrical properties, carbonnanotubes find applications as additives tovarious structural materials. For instance,nanotubes form a tiny portion of the material(s)in some (primarily carbon fiber) baseball bats,golf clubs, or car parts.[2]

Nanotubes are members of the fullerenestructural family. Their name is derived fromtheir long, hollow structure with the wallsformed by one-atom-thick sheets of carbon,called graphene. These sheets are rolled atspecific and discrete ("chiral") angles, and thecombination of the rolling angle and radiusdecides the nanotube properties; for example,whether the individual nanotube shell is a metalor semiconductor. Nanotubes are categorized assingle-walled nanotubes (SWNTs) and multi-wallednanotubes (MWNTs). Individual nanotubesnaturally align themselves into "ropes" held

together by van der Waals forces, morespecifically, pi-stacking.Applied quantum chemistry, specifically, orbitalhybridization best describes chemical bonding innanotubes. The chemical bonding of nanotubes iscomposed entirely of sp 2 bonds , similar to thoseof graphite. These bonds, which are strongerthan the sp 3 bonds found in alkanes and diamond,provide nanotubes with their unique strength

Case Study: Carbon nanotube paper batteries

In July 2010, Fraser Cottington, CEO of VendumBatteries (www.vendumbatteries.com) approachedDr Bojan Boskovic from Cambridge NanomaterialsTechnology Ltd with request for help inunderstanding full potential of carbon nanotubepaper based technology for batteries thatcompany was looking to commercialise.

The CNT Ltd prepared initial patent landscapingreport and followed by additional technologydevelopment strategy reports identifyingtechnology threats and compatible technologiesthat could strengthen and enhanced a value of

the technology that Vendum Batteries wasdeveloping.

In order to fully support the company intechnology strategy development Dr BojanBoskovic, became a member of the VendumBatteries Inc. Advisory Board providing advicewith commercialisation of carbon nanotube basedelectrochemical devices and help withcollaborative R&D efforts and further IPdevelopment and product commercialisation.

Fraser Cottington, CEO of Vendum Batteries, saidregarding the Advisory Board appointment of DrBoskovic: “We are very excited about workingwith Dr. Boskovic, he understands new materialsand how to deliver solutions in multipleindustry applications, his expertise will helpdrive the company forward towards new andexciting frontiers in a number of clean energymarket sectors.”

This appointment of Dr Bojan Boskovic into theAdvisory Board has been reported by a number ofthe business news website in March 2011. Pleasefollow the link to see at the Reuters website:

http://www.reuters.com/article/2011/03/29/idUS135993+29-Mar-2011+MW20110329

 

Carbon Nanomaterials for Transport

Carbon nanotubes – the wondernanomaterial - has important implications forall kinds of transport. Dr Bojan Boskovic ofCambridge Nanomaterials Technology Ltd,discusses. Carbon nanomaterials, such as carbonnanotubes, nanofibres and graphene are becomingincreasingly important in many applicationsincluding aerospace, automotive, marine and railtransport due to their fascinating properties.Carbon nanotubes can behave like metals orsemiconductors, they can conduct electricitybetter than copper and transmit heat better thandiamond, and they rank among the strongestmaterials known. Applications of carbonnanomaterials are now attracting considerableinterest from both the academic and theindustrial communities. It is already proventhat carbon nanomaterials are good fieldemitters for flat screens, conducting fillers inpolymer composite materials, and electrodes infuel cells. They can be used in nanoelectronicsas diodes and transistors and in supercapacitors

as electromechanical actuators and as chemicalsensors. A complex electromechanical system usedin cars, aeroplanes or trains could include manyof these applications.

Carbon nanotubes, what they are and how they aremade

A carbon nanotube (CNT) is a tubular carbonstructure with hollow cylindrical graphene wallscapped by fullerene-type hemispheres. A carbonnanotube comprising a singular graphene tube(Fig. 1) is called a 'single wall carbonnanotube' (SWCNT), and concentric graphene tubesnested within each other like "Russian dolls"are called 'multi-wall carbon nanotubes',(MWCNT). Other carbon structures withherringbone or cup-stacked graphene layers,which form an angle with the longitudinal axis,

are often called carbon nanofibres (CNFs). Thecarbon nanotube diameter can vary from only fewnanometers for a SWCNT up to few tens ofnanometers for MWCNTs, and more than hundreds ofnanometers for CNFs; and tube length can varyfrom microns up to millimeters and evencentimeters.

Carbon nanotubes and nanofibres (filaments) havebeen made successfully for more than two decadesusing a method known as chemical vapourdeposition (CVD). In the CVD of carbonnanomaterials, a hydrocarbon gas is passed overa heated catalyst. The actions of the catalystcause the hydrocarbon to decompose into hydrogenand carbon atoms, which provide the "feedstock"for carbon nanofibre/nanotube growth. Carbonnanofibres and nanotubes grown by the CVD methodusually have catalyst particles attached to oneend. Multi-walled nanotubes grown this way tendto have a large number of structural defects.

 

Carbon nanotubes – a brief background

 

The discovery of the C60 molecule in 1985 (alsoknown as fullerene or buckminsterfullerene) by

Sir Harry Kroto from University of Sussex,Brighton, with a team led by Richard Smalleyfrom Rice University, Houston, marked thebeginning of a new era in carbon science. TheNobel Prize for Chemistry in 1996 was awardedfor this research to Kroto, Smalley and RopbertCurl (also from Rice). In 1991, Sumio Iijima,using high-resolution transmission electronmicroscopy at the NEC in Tsukuba Japan, reportedthe first observation of structures thatconsisted of several concentric tubes of carbonnested inside each other like "Russian dolls".He called them microtubules of graphitic carbon,but from 1992 Iijima and other researchers beganto call them carbon nanotubes. Iijima observedMWCNTs in the soot produced by the electric arcdischarge between graphite electrodes in ahelium atmosphere.

 

Carbon Nanotubes – Exploiting their Properties

Due to the high aspect ratio, the quasi-one-dimensional structure, and the graphite-likearrangement of the carbon atoms in the shells,nanotubes exhibit a very broad range of uniquechemical, mechanical and electronic properties.The properties of nanotubes can change depending

on their different structures and quality. Largeincreases in strength, toughness, and superiorelectrical and thermal properties, are some ofthe potential benefits of using nanotubes as thefiller material in polymer-based composites,when compared with traditional carbon, glass ormetal fibres.

The remarkable electrical and mechanicalproperties of carbon nanotubes make themexcellent candidates for a range of electrical,mechanical and electro-mechanical applications.Carbon fibres have already been used tostrengthen a wide range of materials, and thespecial properties of carbon nanotubes mean thatthey could be the ultimate-strength fibre. Sincethey are composed entirely of carbon, nanotubesalso have a low specific weight.

Three-dimensional (3D) nano-carbon structuresthat can transfer the exceptional properties ofcarbon nanomaterials to meso- and micro-scaleengineering materials are essential for thedevelopment of many applications. Well knownengineering materials like carbon, ceramic orglass fibre could be exploited as a support forthe formation of 3D nano-structures (Fig. 2).Carbon fibre bundles, woven and non-woven carbonfibre cloth can be used as three-dimensionalscaffolds for carbon nanotube synthesis on the

surface of fibres and in the empty spacesbetween them, to improve the mechanical,electrical and thermal properties of thecomposite material. Growing CNTs and CNFs on thesurface of carbon fibres could also improvecomposite shear strength and load transfer atthe fibre/matrix interface.

The high surface area of carbon and ceramicfibres coated with nanotubes and nanofibres isimportant for use in electrochemicalapplications. The high thermal conductivity ofthese materials may also be of use in automotiveand aerospace applications, and for heatdistribution or hot spot control. The firstattempts to develop carbon-carbon compositescontaining carbon nanotubes have already beenreported. The high electrical conductivity ofthese materials could be used, for example, inelectronic components packaging, as gasdiffusion layers in fuel cells or inelectromagnetic shielding. Carbon fabricimpregnated with carbon nanotubes could be usedfor 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 inapplications, such as aircraft bodies with in-situ 'health' monitoring and self healingproperties; superior brakes with carbon-carboncomposite discs that could dissipate a heat moreefficiently; strong and interactive windscreenswith de-icing properties (Fig. 3). Even a fewpercentage loading of carbon nanotubes in apolymer matrix could make non-conductivepolymers conductive, solving many problems withstatic electricity that could spark a firewithin a vehicle. In aircraft wings, theconductivity of carbon nanotubes could providede-icing and lighting strike protection,combined with weight reduction. Importantly,they could improve the strength of vehiclebodies, decrease their weight and make armyvehicles or military airplaneselectromagnetically invisible. Carbon nanotubesand nanofibres could be added to metals in orderto improve their properties and make lighterengines, they could be used in tyres instead ofcarbon black to improve wear properties - andprovide in situ pressure sensing!

 

 

Realising the promise of CNTs

There are a number of problems to overcome inthe development process of carbon nanomaterialsapplication. The properties of nanotubes need tobe optimised for specific applications. Thenanotubes must be efficiently dispersed andbonded to the material they are reinforcing (thematrix), in order to maximize load transfer.Many companies manage to solve some of theseproblems, with more or less success, and havedeveloped new composite materials from variouspolymers and carbon nanotubes with improvedmechanical, thermal or electrical properties.There are still many opportunities for theimprovement of properties of traditionalengineering materials and their replacement withuse of carbon nanomaterials in transport and awide range of other applications.

 

Looking to the future

Carbon nanotube and nanofibre related researchand development has led to new production routesand the suggestion and realisation of many newexciting applications, steadily growing in

number around the world. Carbon nanotube andnanofibre composites have already reached themarket with the first sports applicationsincluding tennis rackets, baseball bats, andracing bikes. It is expected that carbonnanotube composite materials will have many moreapplications including automotive and aerospacecomponents, structures and brakes.

Chemical vapour deposition has proven to be aneffective technique for synthesis of carbonnanomaterials and nanostructures. It offerscontrolled carbon nanomaterials andnanostructures fabrication that is easilyscalable and can be adopted for mass productionat an industrial level. Similar temperature andgas composition environments needed for carbonnanomaterials synthesis exist in vehicleengines, and catalysts have already been used inmodern vehicles in order to reduce green housegases emission. We can imagine a possibilitywhere vehicles could be used to produce valuablecarbon nanomaterials and further reduce greenhouse gas emissions

Structure

One concept for the space elevator has ittethered to a mobile seagoing platform.

There are a variety of space elevator designs.Almost every design includes a base station, acable, climbers, and a counterweight. Earth'srotation creates upward centrifugal force on thecounterweight. The counterweight is held down bythe cable while the cable is held up and taut bythe counterweight. The base station anchors thewhole system to the surface of the Earth.Climbers climb up and down the cable with cargo.

Base station

Modern concepts for the base station/anchor aretypically mobile stations, large oceangoingvessels or other mobile platforms. Mobile basestations have the advantage over the earlierstationary concepts (with land-based anchors) bybeing able to maneuver to avoid high winds,storms, and space debris. Oceanic anchor pointsare also typically in international waters,

simplifying and reducing cost of negotiatingterritory use for the base station.[2]

Stationary land based platforms have simpler andless costly logistical access to the base. Theyalso have an advantage of being able to be athigh altitude, such as on top of mountains, oreven potentially on high towers. This reducesinfluence from the atmosphere and how deep downinto the Earth's gravity field the cable needsto extend, and so reduces the critical strength-to-density requirements for the cable material abit (with all other design factors being equal).[6]

Cable

Carbon nanotubes are one of the candidates for acable material

A space elevator cable must carry its own weightas well as the additional weight of climbers.The required strength of the cable will varyalong its length. This is because at variouspoints it has to carry the weight of the cablebelow, or provide a downward force to retain thecable and counterweight above. Maximum tensionon a space elevator cable is at geosynchronousaltitude so the cable must be thickest there andtaper carefully as it approaches Earth. Anypotential cable design may be characterized bythe taper factor – the ratio between the cable'sradius at geosynchronous altitude and at theEarth's surface.[31]

The cable must be made of a material with alarge tensile strength/density ratio. Forexample, the Edwards space elevator designassumes a cable material with a specificstrength of at least 100,000 kN/(kg/m).[2] Thisvalue takes into consideration the entire weightof the space elevator. An untapered spaceelevator cable would need a material capable ofsustaining a length of 4,960 kilometers(3,080 mi) of its own weight at sea level to reach ageostationary altitude of 35,786 km (22,236 mi)without yielding.[32] Therefore, a material withvery high strength and lightness is needed.

For comparison, metals like titanium, steel oraluminium alloys have breaking lengths of only20–30 km. Modern fibre materials such as kevlar,fibreglass and carbon/graphite fibre havebreaking lengths of 100–400 km. Quartz fibershave an advantage that they can be drawn to alength of hundreds of kilometers[33] even with thepresent-day technology. Nanoengineered materialssuch as carbon nanotubes and, more recentlydiscovered, graphene ribbons (perfect two-dimensional sheets of carbon) are expected tohave breaking lengths of 5000–6000 km at sealevel, and also are able to conduct electricalpower.

For high specific strength, carbon hasadvantages because it is only the 6th element inthe periodic table. Carbon has comparatively fewof the protons and neutrons which contributemost of the dead weight of any material. Most ofthe interatomic bonding forces of any elementare contributed by only the outer few electrons.For carbon, the strength and stability of thosebonds is high compared to the mass of the atom.The challenge in using carbon remains to extendto macroscopic sizes the production of suchmaterial that are still perfect on themicroscopic scale (as microscopic defects aremost responsible for material weakness). The

current (2009) carbon nanotube technology allowsgrowing tubes up to a few tens of centimeters.[34]

A seagoing anchor station would incidentally actas a deep-water seaport.

Climbers

A conceptual drawing of a space elevator climberascending through the clouds.

A space elevator cannot be an elevator in thetypical sense (with moving cables) due to theneed for the cable to be significantly wider atthe center than at the tips. While variousdesigns employing moving cables have beenproposed, most cable designs call for the"elevator" to climb up a stationary cable.

Climbers cover a wide range of designs. Onelevator designs whose cables are planarribbons, most propose to use pairs of rollers tohold the cable with friction.

Climbers must be paced at optimal timings so asto minimize cable stress and oscillations and tomaximize throughput. Lighter climbers can besent up more often, with several going up at thesame time. This increases throughput somewhat,but lowers the mass of each individual payload.[35]

As the car climbs, the cable takes on a slightlean due to the Coriolis force. The top of thecable travels faster than the bottom. Theclimber is accelerated horizontally as itascends by the Coriolis force which is impartedby angles of the cable. The lean-angle shown isexaggerated.

The horizontal speed, i.e. due to orbitalrotation, of each part of the cable increaseswith altitude, proportional to distance from thecenter of the Earth, reaching low orbital speedat a point approximately 66 percent of theheight between the surface and geostationaryorbit (a height of about 23,400 km). A payloadreleased at this point will go into a highlyeccentric elliptical orbit, staying just barelyclear from atmospheric reentry, with theperiapsis at the same altitude as LEO and theapoapsis at the release height. With increasingrelease height the orbit becomes less eccentricas both periapsis and apoapsis increase,becoming circular at geostationary level.[36][37]

When the payload has reached GEO, the horizontalspeed is exactly the speed of a circular orbitat that level, so that if released, it wouldremain adjacent to that point on the cable. Thepayload can also continue climbing further upthe cable beyond GEO, allowing it to obtainhigher speed at jettison. If released from100,000 km, the payload would have enough speedto reach the asteroid belt.[38]

As a payload is lifted up a space elevator, itgains not only altitude, but horizontal speed(angular momentum) as well. This angularmomentum is taken from the Earth's own rotation.As the climber ascends, it is initially moving

slightly more slowly than each successive partof cable it is moving on to. This is thecoriolis force, the climber "drags" (Westward)on the cable as it climbs, slightly decreasingthe Earth's rotation speed. The opposite processwould occur for descending payloads, tilting thecable eastwards, and slightly increasing Earth'srotation speed.

The overall effect of the centrifugal forceacting on the cable causes it to constantly tryto return to the energetically favorablevertical orientation, so after an object hasbeen lifted on the cable the counterweight willswing back towards the vertical like an invertedpendulum.[35] Space elevators and their loads willbe designed so that the center of mass is alwayswell-enough above the level of geostationaryorbit[39] to hold up the whole system. Lift anddescent operations must be carefully planned soas to keep the pendulum-like motion of thecounterweight around the tether point undercontrol.[40]

Climber speed is limited by the Coriolis force,available power, and by the need to ensure theclimber's accelerating force does not break thecable. Climbers also need to maintain a minimumaverage speed in order to move material up anddown economically and expeditiously.[citation needed] At

the speed of a very fast car or train of300 km/h (190 mph) it will take about 5 days toclimb to geosynchronous orbit.[41]

Powering climbers

Both power and energy are significant issues forclimbers—the climbers need to gain a largeamount of potential energy as quickly aspossible to clear the cable for the nextpayload.

Various methods have been proposed to get thatenergy to the climber:

Transfer the energy to the climber throughwireless energy transfer while it isclimbing.

Transfer the energy to the climber throughsome material structure while it is climbing.

Store the energy in the climber before itstarts – requires an extremely high specificenergy such as nuclear energy.

Solar power – power compared to the weight ofpanels limits the speed of climb.[42]

Wireless energy transfer such as laser powerbeaming is currently considered the most likelymethod. Using megawatt powered free electron orsolid state lasers in combination with adaptivemirrors approximately 10 m (33 ft) wide and a

photovoltaic array on the climber tuned to thelaser frequency for efficiency.[2] For climberdesigns powered by power beaming, thisefficiency is an important design goal. Unusedenergy must be re-radiated away with heat-dissipation systems, which add to weight.

Yoshio Aoki, a professor of precision machineryengineering at Nihon University and director ofthe Japan Space Elevator Association, suggestedincluding a second cable and using theconductivity of carbon nanotubes to providepower.[22]

Counterweight

Several solutions have been proposed to act as acounterweight:

a heavy, captured asteroid;[5]

a space dock, space station or spaceportpositioned past geostationary orbit; or

a further upward extension of the cableitself so that the net upward pull is thesame as an equivalent counterweight;

parked spent climbers that had been used tothicken the cable during construction, otherjunk, and material lifted up the cable forthe purpose of increasing the counterweight.[38]

Extending the cable has the advantage of somesimplicity of the task and the fact that apayload that went to the end of thecounterweight-cable would acquire considerablevelocity relative to the Earth, allowing it tobe launched into interplanetary space. Itsdisadvantage is the need to produce greateramounts of cable material as opposed to usinganything that has mass.