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Tiny tubes, big riddles

Carbon cylinders’ odd traits continue to stump scientists

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11:51am, November 19, 2010
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Moses didn’t need a physics degree to know something was afoot when that woody bush burned and burned but was not consumed. Set fire to carbon — whether shrubbery, paper or charcoal briquettes — and it burns until nothing’s left but carbon dioxide and water vapor. That’s a fundamental of carbon chemistry.

Yet in the tiny world of nano, where objects and distances measure mere billionths of a meter, rules of chemistry and physics that operate at ordinary scales often don’t apply. Scientists recently discovered, for example, that slathering a minuscule tube of carbon in fuel and lighting one end doesn’t destroy the carbon. Flames course down the nanotube, and it gets scorching hot. But the tube remains intact.

“The carbon doesn’t burn up,” says chemist Michael Strano of MIT, who led the research. “What really should happen is oxidation. They should catch fire; there should be nothing left. But the carbon seems to be unscathed. It’s kind of a walking-across-hot-coals type of thing, where you would expect to be scorched but you’re not.”

Fire without burning is just one of the newer oddities to emerge from the nanoworld. In this landscape, molecules take on new personalities — it’s like discovering that your mild-mannered uncle is a world-renowned salsa dancer and moonlights as a bounty hunter. Some of these eccentricities have been known for decades or longer. When nanosized, calm, inert gold, for example, becomes a reactive molecule that has a different melting point than ordinary gold. It no longer looks gold, in fact, but red — a property that stained glass artisans exploited centuries ago.

For novel exotic properties, you needn’t look further than good old carbon. An ordinary working stiff in the macroworld, carbon in its ultratiny form is like a one-man circus: stronger than steel, extremely elastic and light as a feather. And carbon nanotubes keep excelling in unexpected ways. In addition to resisting burning when engulfed in flames, one new study suggests that these little tubes shuttle heat with a fierceness that generates a lot of electricity, a find that has physicists scratching their heads. Other researchers working with carbon nanotubes have created the darkest material ever made, a perplexing blackness not explained by standard optics. And carbon nanotubes’ elastic properties have allowed scientists to create surprisingly stretchy muscles.

Many of these experiments, done in the last few years, are expected to lead to life-improving technologies. But perhaps just as marvelous, the bizarreness of nanoland continually pushes the envelope of scientific understanding.

“There are fundamental physical properties that we still don’t understand at the nanometer scale,” Strano says. “In terms of carbon nanotube research, from my perspective, these materials continue to teach us new and interesting science.”

Hot mess

Carbon nanotubes consist of carbon atoms bonded to carbon atoms, bonded to carbon atoms and so on, all connecting to form a cylindrical lattice, like a piece of rolled-up chicken wire. While their diameter varies, the cylinders are typically a few nanometers across (about the width of a DNA molecule). Nanotubes can be single-walled, standing alone, or multiwalled, stacked inside each other like Russian nesting dolls. The tiny tubes are typically made by taking a hydrocarbon gas, such as carbon monoxide or methane, and busting its molecules apart with something such as heat, a laser or a metal catalyst. Then the carbons find each other and bond to become nanotubes.

Scientists have long known that carbon nanotubes conduct heat extraordinarily well. This is a property usually ascribed to metals (which explains why potholders were a good invention; grab the metal handle of a pot on a hot stove and you’ve experienced metal’s superior heat conduction). But new research suggests that carbon nanotubes’ heat-­conducting abilities defy explanation.

When talking about thermal conductivity, scientists often refer to heat as phonons, little packets of vibration that can move through a material. In ordinary bulk materials, such as a piece of wood, these heat packets quickly bump into things. These collisions scatter the phonons, slowing them down.

“It’s almost like trying to run through a crowded field that’s populated with people,” Strano says. “You can’t run your fastest, even if you are an Olympic runner.”

Apply heat to one end of a carbon nanotube, though, and it zips to the other end 100 times faster than heat traveling through the best metals. Scientists have tried to explain this speediness in terms of a phonon’s unobstructed path: In the nearly one-dimensional environment of a carbon nanotube, the little heat packets whiz along, presumably because they travel a long time without collisions. But there should be some limit to that thermal path, scientists surmised. And bending the carbon cylinders creates the perfect test tube for examining that limit, says Chih-Wei Chang of the Center for Condensed Matter Sciences at National Taiwan University in Taipei.

While at the University of California, Berkeley, Chang and his colleagues contorted their nanotubes, introducing defects that should have tripped up any traveling heat packets. The team thought the ensuing stumbling and scattering would affect the nanotubes’ heat-conducting powers. But the heat packets kept right on trucking.

“To our surprise, our experiments show that the thermal conductivity remains intact,” Chang says. “The result is far beyond our expectations.” The discovery, published in Physical Review Letters in 2007, suggests that nanotubes could be used to transmit information the way optical fibers do. Yet three years on, scientists still don’t completely understand why the heat doesn’t travel slower in the contorted tubes.

“It’s just completely against all thinking that the thermal conductivity remains the same,” Strano says. “It’s really unprecedented.”

Strano has also been investigating how heat courses through a nanotube. In the experiments where the flaming carbon did not burn, the nanotubes reached temperatures as high as 2,800 kelvins (or 2,500º Celsius).

“Clearly science tells us that it shouldn’t take long for carbon at a thousand degrees kelvin to turn to carbon dioxide and water,” Strano says. “It should completely burn up.”

A clue to the nanotubes’ durability may lie in the exceptional speed of the heat wave, he speculates. In the macroworld, if you were to pour a line of gasoline on the ground, lay a stick of the same length next to it and light both at one end, the flame would travel far faster through the liquid fuel than through the stick of wood. But in the lab experiments, the heat wave created by igniting one end of the fuel-drenched nanotube moved 10,000 times faster than it did through the bulk fuel alone.

This superfast wave turns out to be self-propagating, Strano says. As the fuel burns, it releases heat, which goes into the nanotube. The heat wave moves faster than the flame and heat leaks back out ahead of the burnt fuel. This ignites more fuel, and the overall effect is of a heat wave moving so quickly that, perhaps, the oxidation reactions that would combust the carbon can’t even get started.

Not only does the heat wave move at rocket speed, but as it surges forward, it excites the nanotubes’ electrons, propelling them forward as well, Strano and his colleagues reported in Nature Materials in March. This thermopower wave, as Strano calls it, generates electricity at an astounding rate, a discovery that could lead to new sources of energy.

“The faster the wave goes, the more power you get out,” he says. “In fact, no one could have predicted that it would be this much power. From a practical standpoint, the power density is already higher than a lithium-ion battery, and we’re not even really trying.”

Dark matter

While some researchers probe the bright and fiery side of carbon nanotubes, others focus on the material’s dark side. Grow millions of the tubes en masse in a forest and they can become black as a starless night. Working with such a forest, physicist Shawn-Yu Lin of the Rensselaer Polytechnic Institute in Troy, N.Y., recently created the darkest material ever made.

More than 99.95 percent of the light striking the nanotube forest is absorbed, Lin, Pulickel Ajayan, now at Rice University in Houston, and colleagues reported in Nano Letters in 2008. This darkest of materials literally holds a Guinness World Record; the previous record absorbed a mere 99.84 percent of incoming light.

Plenty of researchers work with nanotube forests, but they are typically densely planted, thick with trees. Lin and his team took the sparse route, growing a thin woodland rather than a Grimm Brothers’ forest. Yet almost all the light that enters this forest is never seen again. (“Sparse” is relative, though. A patch of this light-absorbing forest the size of a quarter still contains tens of millions of nanotubes.)

While scientists don’t fully understand how the thicket of nanotubes swallows all light, the dilute packing of the tubes seems to be crucial. The trees in this particular carbon nanotube forest are so tall, thin and sparsely planted that there’s no real surface for light to strike. “It’s almost like light has a soft landing on the structure,” Lin says.

The carbon nanotube trees are different heights and tangle together at the tips. And while the tubes are a somewhat hefty 10 nanometers across, Lin and his colleagues made them very, very tall, about 500 micrometers. (A similarly proportioned No. 2 pencil would be more than three times as tall as the Statue of Liberty). This extreme skinniness, uneven height and sparse packing transforms the forest into a sponge that soaks up light.

“For light, it is almost like nothing. It is like the empty sky,” Lin says. “Why is the empty sky so dark? Because it almost has nothing. It is so dilute, nothing ever comes back. Material is what reflects.”

When light strikes ordinary materials, it bounces off in a predictable manner related to the angle at which it came in. But the nanotube forest doesn’t care about angles. The small amount of reflection that does occur is totally angle independent, says Lin, which makes no sense optically.

“There is no classical theory to explain this new type of surface,” Lin says. “There is no theory,” he laughs. “That’s my theory.”

Scientists at the National Institute of Standards and Technology and Stony Brook University in New York have already put this new dark material to use (for good, not evil). They grew a similar nanotube forest as a coating for a contraption that accurately detects the power of lasers shined into it. This dark detector might also help improve measurements of the temperature of the Earth and sun, the team reports in the September Nano Letters.

Carbon flex

Fire and light are surely captivating, but no big top is complete without feats of strength. Scientists exploiting carbon nanotubes’ stretchy properties recently created giant artificial muscles.

Many materials when stretched one way, will contract in another way, says Ray Baughman, director of the NanoTech Institute at the University of Texas at Dallas. Think of yanking on a rubber band — as it lengthens, its width shrinks. The relationship between the amount of stretching and contraction is known as Poisson’s ratio. Rubber, for example, has a very high Poisson’s ratio, nearly 0.5. Stretch it one way, and it contracts in the other direction by quite a bit. Cork, on the other hand, doesn’t bulge out much when pushed. It has a Poisson’s ratio near zero, making it easy to wedge back into a wine bottle.

While exploring the push-and-pull of various materials in the lab, Baughman and his colleagues spun carbon nanotubes into airy sheets. These sheets “represent a strange state of matter,” he says, with fantastic elastic properties that correspond to Poisson’s ratios as high as 15.

Taking advantage of these bizarrely large Poisson’s ratios, Baughman, colleague Ali Aliev and others turned their sheets into giant muscles that contract like crazy when pulled just a tiny bit. Stretch these sheets just 1 percent in one direction and their volume shrinks by 23.5 percent, the team reported in Science in 2009.

When natural muscles contract and expand, their length typically changes by less than 40 percent. But the team found that the nanotube muscles can change their length by more than 230 percent in a fraction of the time, giving them some serious punch. And since they can still flex their stuff at extremely high and low temperatures, the artificial muscles may be ideal for robots exploring hostile environments, such as Mars.

“Ordinary muscles of course are wonderful,” Baughman says. “They are self-repairing, they can last a lifetime. But these artificial muscles based on carbon nanotubes are much faster in terms of response than natural muscle. And they can operate in extreme environments where no other artificial muscle will survive.”

The artificial muscles’ massive contractions can now be nicely described with theory and numbers, but it took mucking about in the lab to discover the strange behavior. And so it goes with science. Eventually, researchers will probably gain some clarity concerning the other unusual properties exhibited by carbon nanotubes. Those new theories will lead to more experimental work and then to additional mysteries. In the scientific volley between theory and experimentation, surprises can spring like a sudden backhand. 

Strano says he has complete faith that theory will soon explain why a carbon nanotube can behave so strangely. “But observation and discovery will still play a role,” he says. “Making one and being able to manipulate it in the lab and do strange things to it has taught us quite a bit.”

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