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The golden secret behind spinning carbon-nanotube fibers

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5:44pm, May 30, 2004
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In 1991, while analyzing a sample of soot under an electron microscope at his NEC laboratory in Tokyo, chemist Sumio Iijima made the discovery of a lifetime. He identified needle-shape formations that became the first examples of the structures now widely appreciated as carbon nanotubes. Among high-tech materials, carbon nanotubes have ever since been on a steadily accelerating ascent to superstardom. Their physical talents warrant such status: They're lightweight, stronger than steel, and as stiff as diamonds, and they blow away most of the competition when it comes to conducting heat and electricity. In the past few years, carbon nanotubes have been making their way into prototype miniature devices, among them flat-panel displays, fuel cells, and electronic circuits.

Resembling a microscopic version of chicken wire rolled into cylinders, carbon nanotubes have an average diameter of a nanometer or two, on par with the width of a DNA molecule. Scientists have come a long way in understanding what gives these molecular structures their unique mechanical and electrical properties. One of the next grand challenges is to figure out how to coerce these molecules—each one only several micrometers long—into fibers, yarn, ropes, and other macroscale structures without losing the superlative properties of the individual nanotubes.

For comparison, consider polyethylene. It took chemists a half-century to figure out how to exploit this simple carbon-based polymer that's now used in vast quantities worldwide to make products ranging from garbage bags to bulletproof vests, says Satish Kumar of the Georgia Institute of Technology in Atlanta. He suspects it will take less time to turn nanotubes into functional fibers for making macroscopic materials. But, when that happens, he says, "we will have fibers with some fantastic potential and properties."

Materials made from carbon-nanotube fibers could lead to jet airliners much lighter than today's aircraft, power-transmission cables that can carry more current over longer distances than traditional cable materials do and also have less risk of failure, or especially efficient light bulbs (see "Tiny Tubes Brighten Bulbs," in this week's issue: Tiny Tubes Brighten Bulbs: Nanotubes beat tungsten in lightbulb test—maybe). Beyond those goals, some scientists are even toying with a truly far-out vision: an elevator from Earth to space using cables made from carbon nanotubes (SN: 10/5/02, p. 218: Ribbon to the Stars).

"The question is, 'Can we produce a high-strength fiber out of carbon nanotubes?'" says polymer chemist R. Byron Pipes of the University of Akron in Ohio. "No one has done that yet."

Going for a spin

Research groups around the world have been pursuing myriad strategies for creating nanotube fibers. One lead contender resembles the spinning process used in making textiles. In 2000, Philippe Poulin of the Paul Pascal Research Laboratory at the University of Bordeaux in France reported a spinning process that could generate continuous fibers out of carbon nanotubes. Using high-frequency sound waves, he and his colleagues dispersed billions of carbon nanotubes in a detergent solution. When the researchers squeezed the solution through a syringe needle into a rotating bath, the nanotubes aligned with each other and coagulated into ribbons. Slowly pulling the ribbons out of the bath yielded dense fibers tens of centimeters long.

Although Poulin's fibers were far weaker than existing high-strength materials, the technique was the first to demonstrate the feasibility of scaling up carbon nanotubes into fibers.

Other groups followed suit. Ray Baughman at the University of Texas at Dallas in Richardson, for one, modified the French team's spinning process. In the June 12, 2003 Nature, Baughman's group reported spinning nanotube fibers that are not only tougher and stronger than Poulin's fibers but also an improvement on spider silk and Kevlar (SN: 6/14/03, p. 372: Super Fibers: Nanotubes make tough threads).

However, none of these methods is capable of producing 100 percent pure carbon-nanotube fibers. Some detergent molecules remain in each fiber, and the scientists replace them with polymers.

Moreover, the high-frequency sound waves can damage the nanotubes and result in weaker fibers, says chemical engineer Matteo Pasquali at Rice University in Houston.

To overcome those limitations, Pasquali, with Rice chemist Richard Smalley and their colleagues, devised an alternative approach. They disperse the materials in sulfuric acid instead of detergent. No high-frequency sound waves are required because, in the acid, positive charges accumulate on the surfaces of the individual nanotubes and make the tubes gently repel one another. As the researchers gradually increase the concentration of the nanotubes, the tiny structures orient themselves in the same direction, forming a liquid crystal.

"When we force the liquid through a syringe, we get nicely aligned tubes," the best reported so far, says Pasquali. Such alignment is crucial for making strong fibers.

Once the thin streams of liquid from the syringe hit the coagulation bath, the individual nanotubes stick together, forming fibers about as thick as a hair and meters long, says Pasquali.

The Rice team described these fibers in the January 13 Macromolecules. The researchers are currently in the process of measuring the fibers' mechanical properties, while their colleagues at the University of Pennsylvania are testing the structures' thermal and electrical conductivities.

Pasquali says that if the Rice group succeeds in making fibers with only, say, 10 percent of the stiffness of a single metallic carbon nanotube and with a conductivity comparable to that of copper, certain real-world applications would be within reach.

With an eye to the world's soaring energy demands, Smalley has been promoting the idea that carbon-nanotube fibers could become the stuff of next-generation power transmission cables, making them far more efficient than today's aluminum-steel lines. Electrons move ballistically down nanotubes and don't scatter about as they do in other conducting materials, such as copper and aluminum cables. Because scattering increases the metals' resistance, it causes today's power lines to heat up, expand, and then sag. Sagging lines are notorious for knocking down trees and causing blackouts.

Compared with traditional cable materials, a carbon-nanotube fiber could carry more current without heating up and do so over longer distances, says Howard Schmidt, director of the carbon-nanotechnology lab at Rice University. "If you can eliminate thermal failures [in power cables], you basically save $100 billion a year in blackout costs."

Congested urban centers where high real estate prices drive up the cost of purchasing land for adding new power lines would also benefit, he says. If electric companies could push power through carbon-nanotube fibers to meet increasing demand, they would need to build fewer new lines.

The first carbon-nanotube cables, measuring a few meters or so in length, could be ready for testing in the next 5 years, says Schmidt. In a decade, he predicts, several-kilometer-long cables could reach the testing stage.

A bundle of strength

Manufacturing nanotube fibers that are thousands of kilometers long would require a healthy supply of relatively inexpensive carbon nanotubes. More than a half-dozen companies are developing plants with the goal of supplying large amounts of carbon-nanotube feedstock, and Houston-based Carbon Nanotechnologies has emerged as an early leader. Cofounded by Smalley, the company is building a pilot plant to produce about 50 kilograms of the raw material in a day.

If a full-size plant comes on line next year as expected, the company will be able to produce nearly 500 kg per day, says Ken McElrath, vice president of product development. At that production rate, the vision of building long-distance power-transmission cables could start becoming a reality, he says.

Even if adequate supplies of raw nanotubes become available, it would still take a practical industrial-scale spinning process to produce fibers and cables in quantity. For now, Carbon Nanotechnologies is leaving that step to others.

However, Alan Windle of the University of Cambridge in England and his coworkers have developed an integrated, furnace-based method that combines the production of raw material and the making of fibers into one simple step.

As described in the April 9 Science, the Cambridge researchers inject ethanol, ferrocene, and thiophene into a stream of hydrogen gas flowing through a furnace. Ethanol provides the carbon atoms for making the nanotubes, while ferrocene and thiophene help assemble the tubular structures. When the furnace is heated to more than 1,000°C, a tangled mass of carbon nanotubes forms, which Windle describes as "very fine cobwebs." At the end of the furnace, a rotating rod captures the continuously forming nanotube mass and winds it into long, twisted fibers.

Although Windle expects this one-step process for making carbon-nanotube fibers to be more efficient and less expensive than other methods, the resulting fibers are still relatively weak. Whether spinning fibers from a solution or twisting threads inside a furnace, no one has yet to produce fibers that match the strength and stiffness of individual nanotubes.

The fibers' weakness stems from a lack of cohesion among the nanotubes, which slip and slide alongside each other. It would take approximately 1.6 trillion carbon nanotubes to make a meter-long fiber, Pipes notes. If the nanotubes aren't strongly oriented, the fibers won't show the nanotubes' impressive strength.

Laszlo Forro of the Swiss Federal Institute of Technology in Lausanne and his colleagues are trying to get nanotubes to work together as though they were one continuous material. The researchers irradiate loose bundles of about 100 carbon nanotubes. When exposed to high-energy electrons, some of the chemical bonds within each nanotube break and form new bonds with adjacent nanotubes. These cross-links essentially glue the tubes together, preventing them from sliding past each other.

"The trick is finding the right dose of radiation," says Forro. "If we damage the nanotubes too much, we lose all their strength." Using computer simulations, the researchers estimate that about one cross-link every 1.2 nm along the length of the nanotube produces the best results.

With just the right amount of radiation, the exposed nanotube bundles were 30 times as stiff as those that weren't irradiated. In some cases, the stiffness of the exposed bundles was close to three-quarters that of a single carbon nanotube. According to Forro, these measurements, reported in the March Nature Materials, demonstrate that it's possible to retain much of the mechanical properties of individual carbon nanotubes when scaling up to make larger structures.

Mix master

While researchers such as Forro have laid much of the groundwork for getting tiny carbon nanotubes to assemble into macroscale fibers, there's still no way to produce large batches of a single type of carbon nanotube.

Georgia Tech's Kumar laments that there are 400 types of carbon nanotubes, each characterized by its diameter and chirality—the angle at which the chicken wire–like sheet of carbon atoms is rolled into a cylinder. Chirality determines, for instance, whether a specific nanotube behaves as a semiconductor or a metal.

A given production run can contain anywhere between 10 and 50 types of nanotubes, but the strongest fibers will consist of only one type of carbon nanotube, Kumar predicts. "Once you can do that, then the potential for industrial applications will be huge," he says.

While researchers race to figure out how to make pure batches of a single type of carbon nanotube, less refined feedstocks of carbon nanotubes could find their way into textiles and construction materials for which the purity of the starting materials isn't essential. Kumar and his colleagues have shown that carbon nanotubes make excellent reinforcement additives in the production of other fibers, for example. Adding small amounts of nanotubes to a polymer base can yield lightweight composite fibers that have extraordinary strength.

In one experiment, which the researchers described in the Jan. 5 Advanced Materials, Kumar's group mixed carbon nanotubes with polyacrylonitrile—a precursor used today for making conventional carbon fibers found in such items as tennis rackets and aircraft. The team then spun the mixture into hair-thick fibers. With carbon nanotubes contributing just 10 percent of the weight of the fiber, the researchers boosted the fiber's strength by 50 percent and its stiffness by 100 percent.

In another experiment, the Georgia Tech researchers, in collaboration with the Air Force Research Laboratory in Dayton, Ohio, added small amounts of carbon nanotubes to the chemical precursor of Zylon fibers. Kumar describes Zylon as the "strongest polymeric fiber in the world, stronger than Kevlar." The simple addition of nanotubes increased the resulting fibers' strength by 50 percent. The potential payoff could be the strongest and lightest-weight commercial fiber yet, says Kumar.

Composite fibers that don't include nanotubes are already finding their way into automotive and aircraft parts. For instance, composite materials will account for 50 percent of the weight of the new Boeing 7E7s that are due to enter service in 2008. Because the composites are only a fraction of the weight of aluminum and other traditional aircraft materials, the new planes are expected to use 20 percent less fuel than today's comparably sized aircraft.

Many researchers predict that superstrong, pure carbon-nanotube fibers will ultimately prove to be the strongest materials on the planet. However, according to Pipes, "the jury is still out on that one."

Luckily, strength isn't everything. Their electrical properties give nanotube-based materials other advantages. "These are intelligent fibers," Pipes says. "You can think about weaving carbon nanotube fibers into a structure where they would carry out multiple functions."

For example, construction materials embedded with carbon-nanotube fibers could sense cracks in a structure or changes in temperature and relay that information to a central monitoring system. Weaving nanotube fibers into textiles could support clothing with embedded electronic gadgets, say, a cell phone or a computing device. In fact, many scientists predict that the first carbon-nanotube fibers to reach the market are likely to be in applications where a high-strength fiber isn't essential.

"It's the multifunctional character of the carbon-nanotube fiber that is going to be really exciting," says Pipes. "That's where we're heading."

All in all, not bad for a little bit of soot.

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