A new nanostructured coating could be used to make paints for stealth aircraft that can’t be seen at night and that are undetectable by radar at any time of day. The coating, made of carbon nanotubes, can be used to cloak an object in utter darkness, making it indistinguishable from the night sky.
Carbon nanotubes have many superlative properties, including excellent strength and electrical conductivity. They are also the blackest known material. The long straws of pure carbon, each just a few nanometers in diameter, absorb a broad spectrum of light—from radio waves through visible light through the ultraviolet—almost perfectly. Researchers are taking advantage of this perfect absorbance in highly sensitive imaging sensors and other prototype devices.
L. Jay Guo, professor of electrical engineering and computer science at the University of Michigan, realized it could be useful as a kind of camouflage. Stealth aircraft, he notes, are often painted black or dark blue to hide them from view.
Guo’s group grew sparse forests of vertical carbon nanotubes on the surface of various three-dimensional objects, including a silicon wafer patterned with the shape of a tiny tank. The nanotubes make the objects appear completely flat and black, and they disappear against a black background. The nanotube-coated objects neither reflect nor scatter light.
This effect works, Guo says, because the nanotubes are perfectly absorbing, and because when they are grown with some space between them, as in his experiments, their index of refraction is nearly identical to that of the surrounding air. This means that light won’t scatter out of the nanotubes without being absorbed. The work is described in the journal Applied Physics Letters.
Guo says if an airplane painted with the nanotube coating were hit with a radar beam, nothing at all would bounce back, and it would appear as if nothing were there.
“This type of cloaking is very interesting, especially since they have demonstrated operation in air,” says Ray Baughman, director of the MacDiarmid NanoTech Institute at the University of Texas at Dallas. Baughman recently demonstrated that nanotubes can form an invisibility cloak when they’re heated up under water. The heat from a sheet of nanotubes affects the optical properties of the surrounding water, creating the illusion of invisibility.
Invisibility cloaks shield objects by manipulating incident light so that it simply flows around them. Materials that can achieve this must be made very painstakingly and typically only work with a very narrow spectrum of light—say, microwaves, or red or green light. Nanotubes are relatively easy to make, and work across a broad spectrum.
However, it’s not yet practical to grow forests of nanotubes on the surface of an airplane directly—growing such forests is a high-temperature, high-pressure process done in chambers much smaller than an airplane. But Guo says it should be possible to grow the nanotubes on the surface of tiny particles which can then be suspended in paint.
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