ChemMatters, FEBRUARY 2006 11

magine you’re a soldier of the future. Asyou scan the horizon for possible threatsto your platoon, the day heats up, but youstay cool. Tiny air conditioners placedstrategically within your shirt turn up the

juice before sweat makes its first appearance.But that’s just the start of the laundry

list of things your new standard-issue attiremight do.

For example, sensors integrated into thefabric could take minute-by-minute healthreadings to make sure you’re fit, calm, andwell hydrated. Radios woven right into thecloth might communicate your position andstatus back to the home base. Your camou-flage could instantly morph to hide you in anybackground, not just the usual forest greensor desert browns. All of these powerful appli-cations might be fueled by light-weight capac-itors, or batteries, twisted right into thethreads. To top it all off, your shirts and pantsmight stop projectiles on their own, withoutthe help of extra bullet proof materials.

Think these revolutionary duds are “mis-sion impossible”? Think again. Scientists arecurrently working on developing super fibersmade of powerful carbon nanotubes. Theseultra-thin threads can be less than one-ten

thousandth the width of a human hair, andthey come with a bevy of interesting chemicalproperties such as super strength, and superelectrical and thermal conductivity. Woveninto a fabric, these fibers could turn any articleof clothing into extraordinary wired attire.

The research still has a long way to gobefore you'll be able to pick up your ownsuper shirt at the army surplus store—chemists need to work out some toughkinks in manufacturing the rightkind of nanotubes at the rightlength for these applications. Butwith the science marching swiftlyahead, an army of super-tailoredsoldiers won’t be long behind.

Strong characterDespite scientists’ long-range

scheming over possible applica-tions, carbon nanotubes are still arelatively new technology. Theywere discovered by chance in 1991by researcher Sumio Iijima atJapan-based NEC Laboratorieswhile he was examining other carbonstructures for unrelated applications.

Iijima and his colleagues knew they hadsomething special. They immediately saw howthe structure of the tubes, which look likerolled-up chicken wire at the molecular level,gives them unique chemical features that sci-entists haven’t seen in any other material.

Carbon nanotubescould be the key to spinning the future’shottest threads.

COURTESY OF OAK RIDGE NATIONAL LABORATORY

By Christen Brownlee

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According to MatteoPasquali, a chemical engineerat Rice University in Houston,TX, like every covalent net-work solid, every atom in acarbon nanotube shares elec-trons with its neighbors.Sometimes, this propertygives them the ability to con-duct electricity extremely effi-ciently. The tubes’ honeycomblattice and cylindrical structurealso allow them to channelheat effectively and retain theirshape.

Carbon nanotubes con-duct heat better than anyknown material and are manytimes stronger than any knownfiber. Plus, they are extremelylightweight, making them per-fect for adding these specialqualities to other materialswithout adding extra pounds.

“To some extent,” saysPasquali, “they’re the HolyGrail of fibers.” But much likethe original Holy Grail, headds, good carbon nanotubesare extremely tricky to find.

When Iijima’s group dis-covered nanotubes, theynoticed that the tubes sponta-neously arise from a varietyof combustion reactions.For example, nano-tubes emerge everytime you light a candle.But this run-of-the-millproduction makes nano-tubes that aren’t suitable foranything useful, says Pasquali.

First, many combustionreactions produce a randommishmash of different nan-otube structures—the hexa-gons in the tubes’chicken-wire structure may berolled up at different angles,for example, giving them dif-ferent properties. This slip-shod production also spewsout various assortments ofnanotube types. Single-wallednanotubes, the hose-likestructures that seem to have

the most benefits, are often mixed in withmultiwall nanotubes, which look like tubesrolled up within tubes. To get the most reli-able properties, scientists need to work witha uniform batch of tubes that perfectly matcheach other.

Second, the manufacturing techniquesavailable today—such as knocking carbonoff of a surface with a laser, for example, ordischarging bits of carbon by zapping car-bon rods with electricity—can only producenanotubes that are usually only a fewmicrometers long. “If you interrupt the nan-otube, then you interrupt their properties,”says Ray Baughman of the University ofTexas at Dallas. The most bang for chemists’buck lies in learning how to manufactureindividual, extra-long nanotubes. However,many scientists agree that it will be a bigstretch to produce single nanotubes muchlonger than the current limits with today’stechnology.

Do the twistTo get enough material for

usable super fibers, scientistshave a few tricks up theirsleeves. The bestidea for now,says R.Byron

Pipes ofPurdue Uni-

versity in WestLafayette, IN, is to take

many short nanotubes andbundle them into yarns. Although theresulting yarn has less than 1% of the theo-retical strength, heat conductivity, and elec-trical conductivity, the end product still hassome intriguing possibilities.

“For many centuries, man had only dis-continuous fibers at his disposal, like flaxand cotton. But we’ve made yarn, rope, andcloth, twisting fibers so they become

COURTESY OF NATIONAL NANOTECHNOLOGY IN

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Carbon nanotube fibers are 4 times stronger than spider silk and 17 times stronger than the Kevlar used in bullet-proof vests.

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strong,” says Pipes. “Most people think thatif the elements aren't continuous then it’snot strong, but that's not true.”

For example, Baughman and hiscoworkers have manufactured carbon nan-otube yarns by distributing billions of nan-otubes into a detergent solution. Thescientists keep the tubes from bunchingtogether by blasting the solution with high-frequency sound waves. Feeding a thinstream of the solution into a whirling bath,the scientists have twisted yarns up to 200meters long and as thickas a human hair,but muchstronger.These

super-fibers are 17

times as tough asthe Kevlar used in bullet

proof vests, and 4 times astough as spider silk—the

strongest known natural fiber.Pasquali and his colleagues are using

another solution-based way to make theirown nanotube yarns. Dumping their individ-ual nanotubes into sulfuric acid, theresearchers found that electrical chargeswithin the acid distributed the tubes evenlywithout the high-frequency sound waves.Pasquali’s team simply pressed the nan-otube solution through a syringe into acoagulating bath, pushing out meters ofsuper-strong nanotube cables.

Next big thingResearchers are now testing the new

fibers to see exactly what kind of physicalproperties they possess. If they can tweakthe properties of these yarns to even one-tenth of an individual nanotube, then scien-tists will be in business for a number ofopportunities.

Clothes for a futuristic soldier will beonly the tip of the iceberg, says Baughman.

They could play a majorpart in building strongervests for police officers andartificial muscles that twitchwith electricity. These aretwo super fiber possibilitiesthat he and his team aretalking about.

Many scientists aredreaming much bigger; forexample, some researchersbelieve that an extra-longcable spun of carbon superfibers might somedaystretch all of the way tospace, tethering an orbitingspacecraft to the earth.Once the initial thin cord isestablished, a small climb-ing machine could

strengthen and thickenthe line, eventuallycreating a space ele-

vator that couldcheaply carry people andequipment into space.

On a more practicalnote, Pasquali thinks thatstretching electrical cablesmade of super fibers fromcoast to coast—or evencontinent to continent—might save an enormousamount of electricityaround the world. “One ofthe big problems withpower is that, for the mostpart, you can’t store it,” saysPasquali. “You continuouslyhave to produce the power that’s needed.”

Right now, loads of electricity arewasted as power plants churn away throughthe night, supplying electricity for just a fewnight owls. But at night in one part of theworld, it’s daytime in another. If scientistscould move electricity instantly from onehalf of the world to another, energy produc-tion’s efficiency would dramaticallyimprove.

Traditional cables won’t work for suchan expansive application—right now, theylose about 50% of the energy that’s pro-duced through resistance while just movingit around. However, a cable woven of car-bon super fibers could be a perfect fit forthe job, notes Pasquali.

Although the phenomenon has beenonly demonstrated over a distance of amicrometer, it’s an exciting result. He esti-mates that some of the applications of car-bon super fibers could be here in a mere 10to 20 years.

In the meantime, he and other carbonnanotube chemists will keep spinning longerand longer superfibers, weaving theirdreams toward reality.

Christen Brownlee is a contributing editor toChemMatters. Her article “Flaking Away” alsoappears in this issue.

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Some researchers believe that carbon superfibers may somedaystretch to space.

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