Carbon-nanotube ribbons developed by researchers at the University of Texas at Dallas are stronger than steel, as stretchy as rubber, and as light as air. The ribbons, which are made of long, entangled 11-nanometer-thick nanotubes, can stretch to more than three times their normal width but are stiffer and stronger than steel or Mylar lengthways. They can expand and contract thousands of times and withstand temperatures ranging from -190 to over 1,600 °C. What's more, they are almost as light as air, and are transparent, conductive, and flexible.
The material, presented in the journal Science this week, was developed by Ray Baughman, director of the Nanotech Institute at UT Dallas, who is developing various kinds of carbon-nanotube-based "artificial muscles" for prosthetics and robotics. These materials change shape and size in response to electrical or chemical signals; some expand by up to 1 percent and exert 100 times more force than natural human muscle over the same area.
The new actuators, on the other hand, expand by up to 200 percent but generate small forces per unit area, making them less than ideal for many applications, including robotics. However, their novel properties, especially their temperature range, could open up exciting new applications. "No other actuator technology can provide actuation at these extreme temperatures," Baughman says. "And these actuation rates are giant."
Qibing Pei, a materials-science and engineering professor at the University of California, Los Angeles, believes that the material could be a good candidate for shape-changing aircraft wings. Pei has developed polymer actuators that expand by up to 400 percent and work between -40 and 200 °C.
But for now, Baughman and his colleagues are focusing on optical applications for the material. Because carbon nanotubes are highly conductive, the flexible sheets could perhaps be used to make electrodes for solar cells and organic light-emitting diodes with controllable transparency and conductivity. "For that application, you want to tune the density of carbon nanotubes per unit area," Baughman says. "That determines how much transparency the sheet has." In the Science paper, the researchers show that the ribbons can be deposited on a silicon substrate in their expanded, more transparent state. The ribbons also diffract light so that they could perhaps prove useful in optical communications. Changing their dimensions sends different wavelengths of light in different directions.