The carbon nanotube sheet shows tantalizing properties for the aerospace industry. Research at HTMI aims to hold it to its promise
It’s real, it’s as good as you’ve heard, and it’s coming soon—if not quite yet. What we’re talking about is the aerospace material of tomorrow, carbon nanotube (CNT) sheets with awe-inspiring properties and the less-than-awe-inspiring name of “buckypaper.” That name comes from the discovery by Robert Curl, Harold Kroto and Richard Smalley in 1985 of a molecule made up of 60 carbon atoms shaped like a soccer ball. To Kroto, it also looked like the geodesic domes promoted by architect and futurist Buckminster Fuller and so the C60 molecule was named buckminsterfullerene, or “buckyball” for short. Curl, Kroto and Smalley were awarded the 1996 Nobel Prize for Chemistry for their work, and the development of buckypaper is just one outgrowth of their discovery.
Buckypaper is about 25-µm thick and made from CNT fibers about 1/50,000th the diameter of a human hair. The nanotubes have atomic bonds powerful enough to make them twice as hard as diamond. When sheets of buckypaper are stacked, the resulting composite material is 10 times lighter than steel—but 250 times stronger. The implications for aerostructures—increased payloads and improved fuel efficiency—are tantalizing.
Also, unlike conventional composite materials, buckypaper conducts electricity about as well as silicon and disperses heat like steel. Instead of the metal mesh currently used in the structure of the composite aircraft to disperse lightning strikes, buckypaper, with its high current-carrying capacity, would allow lightning’s electrical charge to flow around the plane and dissipate without causing damage.
Furthermore, buckypaper is flame retardant and could one day help prevent fires on aircraft, ships and other structures. Its strength-to-weight ratio might also prove ideal when making protective gear, including helmets and body armor, for the military and police, as well as create improved, more comfortable prosthetics for wounded veterans. Such features explain why the US Air Force and companies including Raytheon and Lockheed Martin have been investing in R&D efforts aimed at getting the material to fulfill its promise.
Florida State University’s High-Performance Materials Institute (HPMI; Tallahassee), where C60 co-discoverer and Nobel Laureate Harold Kroto serves as senior science advisor—is paving the way when it comes to narrowing the gap between research and the practical use of buckypaper. Nobel Laureate Richard Smalley first produced buckypaper during the 1990s by filtering a nanotube suspension in order to prepare samples for various tests. HPMI has spent the past several years building upon this work, making buckypapers larger and more multifunctional for composite fabrication and achieving several patents for its efforts.
It’s taking longer than they might have hoped: There was a flurry of interest in buckypaper in 2008 and 2009—it was named as one of SME’s “Innovations That Could Change the Way You Manufacture” in 2009—and media reports from that time made it sound like the material would be ready for use in production imminently. However, right now HPMI is producing buckypaper at only a fraction of its potential strength, in small quantities and at a high price. Buckypaper may be the aerospace material of tomorrow, but tomorrow, to paraphrase Broadway’s “Annie,” is still a day away.
As HPMI notes on its Web site (hpmi.net), “Nanotube reinforced composite materials have yet to demonstrate their much-anticipated success due to non-uniform dispersion, lack of nanotube orientation, and weak interface between nanotubes and the matrix.” The institute is currently researching an innovative processing method to improve nanotube-reinforced polymer composites using buckypaper/resin infusion techniques. The goals of the project are to prepare and test nanotube reinforced composites using a high magnetic field to realize nanotube alignment and establish a comprehensive database for producing and deploying buckypaper/resin infusion composite materials.
Situated inside a new $20 million, 45,000-ft2 (4185-m2) building that houses 13 laboratories, HPMI brings together highly technical researchers and state-of-the-art equipment to continually explore and realize the astounding potential of NOLES, or nanotubes optimized for lightweight exceptional strength. Researchers go from idea to concept to prototype and beyond, working one-on-one with private businesses and the government with complete confidentiality to meet the specific needs of each partner.
Both researchers and students have access to the facility’s equipment, which is valued at more than $10 million. They can watch the tiniest particles come alive under a scanning electron microscope that has 100,000 times more magnification than what is found in a high school laboratory. And strong composite materials that once seemed impossible to cut, researchers and students are easily slicing away thanks to a powerful abrasive waterjet machine. HPMI has been successfully using an Omax 55100 JetMachining Center for the past six years to quickly and accurately cut a wide variety of components, some less than a quarter of an inch in size.
“The composite materials we work with are really strong, and I can’t think of a material we haven’t cut on the Omax,” said Jerry Horne, an HPMI engineer who cuts materials for students and researchers. “It doesn’t matter how strong the material is because the Omax will cut through anything.” The machine is designed to accommodate the cutting of large, complex parts and can handle sheet materials of up to 5 × 10′ (1.524 × 3.048 m) in dimension. Furthermore, its user-friendly controls simplify the programming of traditionally complex techniques. In fact, Horne can machine student components directly from a CAD drawing or DXF file.
The era of buckypaper use in production is getting closer, insist the scientists and engineers at HPMI. According to Frank Allen, HPMI operations director, the facility was producing buckypaper at the size of a quarter back when he joined the institute in 2001, and now it is making much larger sheets using a batch production process.
Eleven years ago, the price of the best quality single-walled nanotubes was approximately $500 a gram. Today, single-walled nanotubes are about $200 a gram and the price continues to decline. Furthermore, multi-walled nanotubes are available for purchase by the pound and nanofibers by the barrel. Allen noted that the facility tailors which form of nanotubes it uses based on properties required for the buckypaper.
“We don’t always need the more expensive single-wall nanotubes to get the properties we want,” Allen said. “The quality of the nanotubes we use makes a difference, and right now we are trying to make it cheaper by improving the manufacturing process of the buckypaper.”
In an attempt to make buckypaper more commercially feasible, the High-Performance Materials Institute is looking to scale up its production by working on a prototype that would produce buckypaper strips at 5 fpm (1.5 m/min).
While HPMI research on buckypaper is at the forefront of a technological revolution, it is still in its infancy stage. But despite the hurdles that come with trying to produce buckypaper for practical use, the researchers at Florida State University are not about to give up. And why should they? Even Thomas Edison’s improvement of the light bulb did not happen overnight. He relied on existing research to invent a total of seven system elements before the practical application of electric light was possible. Buckypaper use in aerospace production will likewise eventually see the light of day.