Mass Reduction
U.S. Air Force Pursues Promise of Carbon Nanotubes
They have been touted as the key ingredient for making much stronger body armor and for healing broken bones. They have been fashioned into a molecule-sized radio and proclaimed the successor to silicon for building ever-smaller electronic components.
And now carbon nanotubes promise to reduce the weight of satellites and increase the payload capacity of UAVs by eliminating copper wiring.
Copper, though an excellent conductor, weighs nearly 9 grams per cubic centimeter. It's not light.
Aluminum weighs less, 2.8 grams per cubic centimeter, but wire made from carbon nanotubes promises to be lighter still - less than half a gram per cubic centimeter, said Peter Antoinette, president of Nanocomp Technologies. Antoinette's small New Hampshire company received a research contract from the U.S. Air Force in March to try to turn carbon nanotubes into electrical wiring.
The benefits to the U.S. Air Force are obvious.
Copper wiring makes up as much as one-third of the weight of a 15-ton satellite, Antoinette said. If copper can be replaced by carbon nanotube (CNT) wire, satellites will weigh a lot less and will be less expensive to launch. Launches now cost about $10,000 a pound.
Or the weight saved by eliminating copper can make way for additional satellite components, or additional fuel to extend the satellite's life.
Similarly, reducing the weight of wiring in UAVs would enable them to fly longer before refueling or carry more sensors and weapons.
CNT wiring would yield the same sort of savings for commercial aircraft, Antoinette said. A Boeing 747 uses about 135 miles of copper wire that weighs 4,000 pounds. Replacing that with 600 or 700 pounds of nanotube wire would save substantial amounts of fuel, he said.
In addition, CNT wires do not corrode or oxidize, and are not susceptible to vibration fatigue, Antoinette said.
The question is, can nanotube wire be manufactured in sufficient quantity and quality? So far, that's been elusive, say Wade Adams and Howard Schmidt, nanotube experts at Rice University in Houston.
Antoinette, however, thinks it can be done. Nanocomp has already produced "research-scale products," he said.
But Adams and Schmidt say some daunting problems stand in the way of producing usable carbon nanotube wire in bulk. One is producing the right kind of carbon nanotubes, or developing a practical way to sort the desirable ones from the others.
Cylinders Within Cylinders
Nanotubes can be thought of as a sheet of carbon atoms bonded together in hexagon patterns and rolled up into cylinders. They can be single-walled tubes - that is, a single layer of atoms - or multiwalled tubes composed of cylinders within cylinders.
Single-walled tubes are more desirable, Adams said. Each nanotube is about 50,000 times smaller in diameter than a human hair.
"They're about the diameter of your DNA," Antoinette said.
Strong bonds between the carbon atoms make carbon nanotubes many times stronger than steel. If they can be manufactured properly, "they should be the strongest material ever available to us," said Adams, who is director of the Smalley Institute for Nanoscale Science and Technology at Rice.
When nanotubes are created, they come in three types, depending on the angle at which the lattice of carbon atoms lines up along the tube.
Only one type, called metallic nanotubes, is a good conductor of electricity. The other two types are semiconductors and do not conduct electricity well.
Producing only the metallic nanotubes has proved "fiendishly difficult," said Adams and Schmidt, who is executive director of Rice's Carbon Nanotechnology Lab.
Researchers have developed ways to weed out the nonconducting nanotubes, but not efficiently enough for mass production purposes, they said.
A second problem is producing "crystalline fiber," Schmidt said. Those are long, highly ordered, single-wall nanotubes. They are the most perfect, hence the strongest and most conductive nanotubes.
What can be produced today is mostly a mixture of high-quality and mediocre single-wall and multiwall nanotubes, Adams said. Researchers are still working to produce uniform batches of "quality molecules," he said.
Antoinette said Nanocomp has made progress in that regard.
Instead of producing one-third conductors and two-thirds semiconductors, "we have almost a 50-50 mix," he said. But so far, he agreed, "nobody has been able to come up with pure metallic nanotubes."
Nanocomp has succeeded at producing nanotubes "distinguished by their long length - up to one millimeter," Antoinette said.
Only a few other nanotube makers have managed that, Adams said.
In theory, it should be possible to make nanotubes of unlimited length, said Air Force Capt. John Bulmer, who is overseeing Nano-comp's work for the Air Force. But so far, no one has been able to do that - and no one really understands why not, he said.
Bulk Manufacturing
Another Nanocomp feat: "They can make nanotube materials in bulk," Bulmer said. The company has produced commercially useful lengths of nanotube wire and "massive amounts" of nanotube cloth, he said.
Nanocomp announced in February it had succeeded in producing 3-foot-by-6-foot nanotube sheets.
The black, paperlike sheets are as strong as stainless steel, but many times less dense, the company said. The material might be used for shielding electronics from interference and for making ultra-strong composites for building aircraft, ships and vehicles that are lighter yet stronger than those being built today.
As for the company's nanotube wire, in Air Force tests so far, it has not proved to be more conductive than copper, Bulmer said. "In theory, it should be real conductive. In real life, we have a ways to go."
Nanocomp says its own tests show that at high electrical frequencies, its nanotube wire has been more conductive than copper.
If conductivity can be increased by factors of five to 10, Bulmer said, the lightweight wire will be very attractive for uses as varied as wiring in aircraft to building lightweight motors. Bulmer heads the Advanced Power Team of the Propulsion Directorate at the Air Force Research Laboratory at Wright-Patterson Air Force Base in Ohio.
Highly conductive nanotube wire would, indeed, be valuable across the military, said Adams and Schmidt. The Navy would want it for its all-electric ships and the Army could use it in rail guns and microwave weapons.
Nanotube wire promises new types of lightweight generators, better batteries, more efficient photo-electric technology and electric transmission lines that lose far less power.
"No other material has the theoretical potential" of carbon nanotubes, said Adams and Schmidt.
The challenges that remain in synthesis and manufacturing "are tremendous," they said, "but the payoffs are huge."
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