Steve Scotti, Chief Engineer for Structures and Materials at NASA’s Langley Research Center, borrowed the Boeing mantra “atoms to airplanes” when explaining the wide range of capabilities his area possesses to a small group of media touring the Hampton, VA-based facilities.
“We actually do that,” he said. “We’ve got people looking at atoms in microscopes, and we can go up to the very large-scale structure, and all the things in between.” Capabilities include durability and damage tolerance assessment, materials synthesis and processing, and computational materials design—and, of course, the necessary facilities to perform these and other diverse tasks. The nearly 800-acre NASA Langley campus includes 240 buildings and 71 labs. As Scotti noted, “a cook needs a kitchen.”
One of those “cooks” is Mia Siochi, Senior Materials Scientist at NASA Langley. And one of the main “ingredients” she and other researchers at Langley are examining, particularly in their efforts to reduce the weight of structural elements, is carbon nanotubes.
“For those of us who are working in this area, state of the art is carbon-fiber composites,” Siochi said. “So the question is, ‘What is next generation beyond that? And how much more weight can we save?’”
Carbon nanotubes’ properties make them a promising candidate, according to Siochi. “What’s novel about them is that because their diameter is so small and even though they’re only a few microns, you have high aspect ratio, which allows you to carry a lot of weight. Also, the way it’s bonded together is conducive for electro-conductivity.”
Carbon nanotubes are 1000 times stronger and about 50 times stiffer than aluminum. They are more than 10 times stronger and about five times stiffer than IM7 carbon fiber. Their current-carrying capacity exceeds copper, and thermal conductivity is about 20 times better than Al and nine times better than Cu. And, significant to Langley’s goal of lighter weight, the nanotubes are almost half the density of Al and about 15% that of Cu.
Dispersion of carbon nanotubes for structural apps
“The challenge, however, is that those properties are measured at the nanoscale,” Siochi said. “When we started working with these materials back in around 2000, these cost about $500/gram. At that time, you were lucky to get some. So we were trying to understand how you take advantage of these properties in a structure.”
The researchers first concentrated on how to disperse the “very little” bit of the material they had in a polymer matrix. They employed molecular modeling, synthesis, and characterization tools “that allow us to visualize what we make, so when we talk about dispersion we can actually see it.” That progress took about five to seven years, Siochi said.
“One of the big lessons learned is that if we are going to compete with [carbon-fiber] composites, doping a matrix with a small amount of this material is not going to get us there,” she stressed.
Carbon nanotube-based composites have generated a lot of interest mainly related to conductivity because that’s the “low hanging fruit,” Siochi said: a small amount of this material can greatly boost the electrical conductivity of a matrix. About two years ago, however, NASA Langley researchers determined it was a good time to “redirect attention back to structural properties—which is probably the most challenging piece,” said Siochi—because larger quantities of carbon nanotubes, from companies such as New Hampshire-based NanoComp Technologies, had become available.
“They’re now available in the volumes that we can consider for making larger parts, and so that is what we’re focusing on right now. We are working to understand how we take a material like this, in sheets or yarns, and make it into structures,” Siochi explained. “The problem is the focus has been [on electrical and thermal conductivity] and structural properties have been [largely] neglected. We’re saying get the structural [part of the equation] and the electrical and thermal will be a bonus because they already do it anyway.”
The researchers are exploring the use of conventional pre-pregging processes to determine how much of what they already know about carbon-fiber composites can be translated to carbon nanotube composites for structures. “What is very different is surface area; it’s got a very high surface area,” said Siochi. “It’s soaking up all that resin so you can get it to wet but the adhesion is not there, so that interface is not optimal at this point.”
Collaborating to compress time to market
Another focus of NASA Langley researchers is to compress the development and insertion cycles of advanced technologies so they make it to market faster. As a point of reference, it has taken about 60 years from the discovery of carbon fibers to where carbon-fiber composites are now—comprising 50% by weight of the Boeing 787. Carbon nanotubes have been on the radar for about 20 years now, Siochi said.
The big question, however, is how to speed up the process. “The other experiment we’re doing here is vertical integration—instead of taking a material and developing it in isolation, [then] passing it to the structures people and trying to convince them to use this for design, and then getting this inserted into industry, we actually are using a multidisciplinary approach,” Siochi explained. “On the team, we have people going from computational modeling, to synthesis, processing, characterization, structural design, structural analysis, systems analysis, and we’re tied into manufacturers so that we can provide guidance on what can be tweaked to make materials to the necessary requirements.
“And we’re also tied into industry, like Boeing and Lockheed, so we know where we can feed this to. The hope is that by having everybody on board from the very beginning, we compress that maturation in the insertion cycle and we can get these emerging technologies into structural elements much sooner than 60 years.”
Siochi and other NASA Langley researchers are confident that this collaborative parallel approach, rather than the traditional sequential approach, will be successful in cutting time out of the process. “Because when you do it [sequentially], you’re always convincing the next level that this is worth looking at,” she said.
These processes and development tools are also relevant for other nanostructured materials, such as boron nitride nanotubes, which NASA Langley is also pursuing. “Besides carbon nanotubes, which are more mature, we also have the capability here to make boron nitride nanotubes,” said Siochi. “This is experimental at this point, but the nice thing about boron nitride nanotubes is they have similar properties [as carbon nanotubes]; they’re insulating instead of conducting, but they are also better at higher operating temperatures than carbon nanotubes”—800°C compared to 400 or 500°C.
With carbon nanotubes as “a good working example,” NASA Langley has the capability to start promoting “all sorts of nanostructured materials,” said Rob Bryant, Advanced Materials and Processing Branch Head. “Some of them NASA might need, some of them NASA might not need, but we have the ability with the teaming efforts to transfer those into other industries,” he said, calling out the automotive, medical, and tooling industries, among others.