This electron microscope image shows how silicon carbide nanotubes attached to separate silicon carbide fibers become entangled with each other. Image: Ajayan Research Group/Rice University.
This electron microscope image shows how silicon carbide nanotubes attached to separate silicon carbide fibers become entangled with each other. Image: Ajayan Research Group/Rice University.

The laboratory of materials scientist Pulickel Ajayan at Rice University, in collaboration with researchers at NASA, has developed ‘fuzzy fibers’ of silicon carbide that act like Velcro and can stand up to the punishment that materials experience in aerospace applications.

In a paper in Applied Materials and Interfaces, the researchers report that the fibers can strengthen composites used in advanced rocket engines that have to withstand temperatures up to 1600°C (2912°F). Ceramic composites in rockets now being developed use silicon carbide fibers to strengthen the material, but they can crack or become brittle when exposed to oxygen.

The Rice lab embedded silicon carbide nanotubes and nanowires onto the surface of NASA's fibers. The nanotube and nanowires are curly and act like the hooks and loops that make Velcro so valuable – but on the nanoscale.

The result, according to lead researchers Amelia Hart, a Rice graduate student, and Chandra Sekhar Tiwary, a Rice postdoctoral associate, is very strong interlocking connections where the fibers tangle. This not only makes the composite less prone to cracking but also seals it to prevent oxygen from changing the fiber's chemical composition.

This research began when Hart, who had been studying the growth of carbon nanotubes on ceramic wool, met Michael Meador, then a scientist at NASA's Glenn Research Center in Cleveland at the kick-off reception for Rice's Materials Science and NanoEngineering Department. (Meador is now nanotechnology project manager at NASA's Game Changing Technologies program.)

That led to a fellowship in Cleveland and the chance to combine her ideas with those of NASA research engineer and paper co-author Janet Hurst. "She was partially converting silicon carbide from carbon nanotubes," Hart said. "We used her formulation and my ability to grow nanotubes and figured out how to make the new composite."

Back at Rice, Hart and her colleagues grew their hooks and loops by first bathing silicon carbide fiber in an iron catalyst and then using water-assisted chemical vapor deposition, a process developed in part at Rice, to embed a carpet of carbon nanotubes directly onto the fiber surface. The fibers were then heated in silicon nanopowder at high temperatures to convert the carbon nanotubes into silicon carbide ‘fuzz’.

The researchers hope their fuzzy fibers will upgrade the strong, light and heat-resistant silicon carbide fibers that, when incorporated into ceramic composites, are being tested for robust nozzles and other parts in rocket engines. "The silicon carbide fiber they already use is stable to 1600°C," Tiwary said. "So we're confident that attaching silicon carbide nanotubes and wires to add strength will make it even more cutting-edge."

The new materials should also make entire turbo engines significantly lighter. "Before they used silicon carbide composites, many engine parts were made of nickel superalloys that had to incorporate a cooling system, which added weight to the whole thing," Hart said. "By switching to ceramic matrix composites, they could take out the cooling system and go to higher temperatures. Our material will allow the creation of larger, longer-lasting turbo jet engines that go to higher temperatures than ever before."

Friction and compression testing showed that the lateral force needed to move silicon carbide nanotubes and wires over each other was much greater than the force needed to slide past either plain nanotubes or unenhanced fibers. The fuzzy fibers were also able to easily bounce back from high compression applied with a nano-indenter, showing their ability to resist breaking down for longer amounts of time.

Tests to see how well the fibers handled heat showed that while plain carbon nanotubes burned away from the fibers, the silicon carbide nanotubes easily resisted temperatures of up to 1000°C. Hart said the next step will be to apply her conversion techniques to other carbon nanomaterials to create unique three-dimensional materials for additional applications.

This story is adapted from material from Rice University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.