Aerospace and aircraft companies as well as the military have been challenged to find ways of effectively shielding sensitive electronic equipment such as radar and radios from electromagnetic interference (EMI) without adding a lot of weight to aircraft and satellites (the more massive they are, the more fuel they need to stay in the air or achieve orbit, respectively). Whereas EMI can lead to headaches like erased data and loss of connectivity for casual computer and cell phone users, the problem is far more serious in aircraft, where interference can jam cockpit radio and radar signals, preventing pilots from sending and receiving crucial information.



Looking to solve this problem on its manned and unmanned aircraft, the U.S. Air Force will by the end of the year to kick off an 18-month study of the use of carbon nanotube sheets to create a shielding layer on the surfaces of lightweight composites. Nanocomp Technologies, Inc., a Concord, N.H.–based maker of carbon nanotube materials, said last this week the Air Force selected the company as prime contractor for the second phase of a larger program intended to find a substitute for the nickel-based conductors currently used for electrostatic discharge (ESD) and EMI shielding.



EMI is the result of electromagnetic radiation given off by electronic devices interrupting an electrical circuit's performance, whereas ESD is the sudden and rapid transfer of an electrostatic charge. Nanocomp's carbon nanotube sheets are designed to act as a "Faraday cage" that can block out external static electrical fields from sensitive circuitry. Because the material can carry electricity, it redirects energy along its conducting plane rather than allowing electronic emissions to penetrate the protected area, the company says.



Although the Air Force will not formally award the contract to Nanocomp for a few weeks, the company, which successfully completed the program's initial phase earlier this year, is working to scale up volume and decrease the cost of its carbon nanotube–impregnated mats, which can be made as large as 1.2 by 2.4 meters, says Nanocomp CEO Peter Antoinette. Larger sheets allow for more surface area coverage, reducing the need for seaming or joining and other manufacturing-intensive assembly steps, he adds. Northrop Grumman Aerospace Systems and Cytec Engineered Materials are also expected to participate in phase two of this program.



The Air Force's testing of Nanocomp's sheets is just the latest example of its work with carbon nanotubes. The U.S. Air Force Research Laboratory's Materials & Manufacturing Directorate (ARFL/RX), based at Wright–Patterson Air Force Base in Dayton, Ohio, has been investigating applications for carbon nanotubes for at least the past decade, mixing them with polymers in search of stronger, lighter materials that could replace aluminum and even copper wiring, which can represent as much as a third of the aircraft's total weight.



Sheets of paper or fabric made from carbon nanotubes could prove useful for allowing satellites to safely manage static electricity while in space, particularly because there is no way to provide electrical grounding once they're in orbit, says Karla Strong, a materials engineer for the Thermal Sciences & Materials branch within the ARFL/RX. The branch's engineers research, develop and apply new technologies in an effort to prevent the large amounts of heat produced by increasingly powerful electrical and propulsion systems from damaging aircraft.



The problems with carbon nanotubes have been making them into usable "macromaterials," says Jennifer Chase Fielding, a materials research engineer with another ARFL/RX branch—Composites & Hybrid Materials—which studies fibers and paper made from carbon nanotubes and nanofibers for ways to leverage their highly thermally and electrically conductive properties.



In addition to studying Nanocomp's technology, Wright–Patterson researchers spent the past three years evaluating carbon nanotube materials from NanoTechLabs, Inc., in Yadkinville, N.C., for possible use as electromagnetic protection films as well as to provide a small amount of electrical conductivity, Strong says. The Air Force has also taken a close look at "buckypaper" created by researchers at Florida State University's High-Performance Materials Institute (HPMI). Buckypaper sheets are made using densely packed single-walled carbon nanotubes.



Carbon nanotubes have specific mechanical properties that make them good candidates for the Air Force's work, says Ryne Raffaelle, director of the National Center for Photovoltaics at the U.S. Department of Energy's National Renewable Energy Laboratory. "In terms of strength-to-weight ratio, carbon nanotubes are stronger than steel but infinitely more flexible," he says.



A single carbon nanotube is typically about one nanometer in diameter (the approximate length of 10 atoms) and millions of times longer than it is wide, says Raffaelle, who previously served as director of the NanoPower Research Laboratory at the Rochester Institute of Technology (R.I.T.) in New York State, where he and his team conducted purity assessments of carbon nanotube products, including those made by Nanocomp.



Although Nanocomp and others developing new nanomaterials are on the right track, one of their main challenges will be mass-producing a product with consistent properties. This won't be easy, because each individual carbon nanotube's conductive properties are dependent on how they are formed, Raffaelle says. "It would be nice to have every tube the same," he adds, "but no one can do this."



There is a tremendous upside for success, however. Whereas it is possible to poke an ice pick through a piece of Kevlar body armor, Raffaelle says, you could not do this with a piece of fabric made from carbon nanotubes.