Satellites and spacecraft are at constant risk of colliding with destructive projectiles such as micrometeorites and orbital detritus. To minimize the damage caused to these structures by potential impacts from high-velocity particles, engineers are looking for lightweight, flexible materials as alternatives to the metals and composites currently used in the aerospace industry.
A study performed by researchers at Rice University in Houston, Texas (USA), in collaboration with Brazilian colleagues at the University of Campinas (UNICAMP), the Federal University of Rio Grande do Norte (UFRN) and the Federal University of the ABC (UFABC), has shown that carbon nanotubes may be a solution for making aerospace structures stronger.
A carbon nanotube is a single sheet of graphene rolled into a tight cylinder that resembles a drinking straw, but with a diameter of 1 nanometer (one-billionth of a meter).
The study, which was published in the journal Applied Materials & Interfaces, resulted from postdoctoral research supported by ascholarship from FAPESP and conducted at the Center for Computational Engineering and Sciences (CCES), one of theResearch, Innovation and Dissemination Centers (RIDCs) funded by FAPESP.
“We succeeded in analyzing how carbon nanotubes fracture at the nanometric scale, and based on that, we developed a process to make them even more resilient for applications in aerospace structures,” said Douglas Soares Galvão, a professor at UNICAMP’s Gleb Wataghin Physics Institute and one of the authors of the study, in an interview with Agência FAPESP.
The researchers studied how carbon nanotubes fractured when impacted at different velocities.
To do this, they used a light gas gun (LGG), a sort of hypersonic ballistic cannon, installed at Rice by NASA, the United States space agency. The device fires minute particles at over 20 times the speed of sound.
To evaluate how carbon nanotubes respond to high-velocity impact, the researchers molded pellets of the material and fired them from the LGG at an aluminum target at three different velocities.
Electron microscope analysis of the results showed that at 3.9 kilometers per second, a velocity that the researchers considered low, a large number of nanotubes remained intact upon colliding with the target and with each other. Some withstood a velocity of 5.2 km/s, but very few were found among samples smashed at a hypervelocity of 6.9 km/s.
When they analyzed the structure of the nanotubes, the researchers found that high-velocity impact caused atomic bonds in the nanotubes to break, sometimes followed by recombination into different structures.
Many of the nanotubes shattered into nanoribbons, while others displayed crack propagation along the tube axis, resulting in “unzipping” of the tube.
“We expected that the carbon nanotubes would splinter, rather than crack open, during high-velocity impact. In our simulations and tests, we observed that depending on the conditions of the impact, the material comes under such huge instantaneous pressure that there’s no time for it to fracture, and it splits open longitudinally,” Galvão said.
The researchers also found that the temperature and pressure at the site of fracture were sometimes so high that the tubes formed nanodiamonds.
Transmission electron microscopy showed that the few tubes and ribbons that had survived high-velocity impact were often welded together.
“This observation led us to develop a technique to fuse nanotubes locally by rapidly subjecting them to a very strong electric current. This produces a much more resilient material in terms of mechanical strength. If confirmed, it would have aerospace applications,” Galvão said. “We’re now using the knowledge gained from the fracturing of carbon nanotubes to reinforce the material.”
High cost and scant knowledge of the ways in which composites interact at the nanometric scale are among the factors that have limited the use of carbon nanotubes, even by the aircraft industry.
“An aircraft manufacturer that wanted to use composites based on carbon nanotubes in the aircraft structure to reduce weight and moderately increase cabin conductivity faced many problems in the design stage and took heavy losses,” Galvão said. “There’s still a great deal to learn about the application of composites in aircraft.”
Videos of the simulations performed by the researchers to see how nanotubes are deformed when fired at a metal target and of two carbon nanotubes interacting during impact can be watched at youtu.be/aOYHWaD27xE and youtu.be/9H3DOmIzoCI.