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NASA
engineers have produced a material that absorbs on average more than 99% of the ultraviolet, visible, infrared, and far-infrared light
that hits it—a development that promises to open new frontiers in space
technology.
The
team of engineers at NASA’s Goddard Space Flight Center in Greenbelt,
Md., reported their findings recently at the SPIE Optics and Photonics
conference, the largest interdisciplinary technical meeting in this
discipline. The team has since reconfirmed the material’s absorption
capabilities in additional testing, said John Hagopian, who is leading
the effort involving 10 Goddard technologists.
“The
reflectance tests showed that our team had extended by 50 times the
range of the material’s absorption capabilities. Though other
researchers are reporting near-perfect absorption levels mainly in the
ultraviolet and visible, our material is darn near perfect across
multiple wavelength bands, from the ultraviolet to the far infrared,”
Hagopian said. “No one else has achieved this milestone yet.”
The
nanotech-based coating is a thin layer of multi-walled carbon
nanotubes, tiny hollow tubes made of pure carbon about 10,000 times
thinner than a strand of human hair. They are positioned vertically on
various substrate materials much like a shag rug. The team has grown the
nanotubes on silicon, silicon nitride, titanium, and stainless steel,
materials commonly used in space-based scientific instruments. (To grow
carbon nanotubes, Goddard technologist Stephanie Getty applies a
catalyst layer of iron to an underlayer on silicon, titanium, and other
materials. She then heats the material in an oven to about 1,382 F.
While heating, the material is bathed in carbon-containing feedstock
gas.)
The
tests indicate that the nanotube material is especially useful for a
variety of spaceflight applications where observing in multiple
wavelength bands is important to scientific discovery. One such
application is stray-light suppression. The tiny gaps between the tubes
collect and trap background light to prevent it from reflecting off
surfaces and interfering with the light that scientists actually want to
measure. Because only a small fraction of light reflects off the
coating, the human eye and sensitive detectors see the material as
black.
In
particular, the team found that the material absorbs 99.5% of the light
in the ultraviolet and visible, dipping to 98% in the longer or
far-infrared bands. “The advantage over other materials is that our
material is from 10 to 100 times more absorbent, depending on the
specific wavelength band,” Hagopian said.
“We
were a little surprised by the results,” said Goddard engineer Manuel
Quijada, who co-authored the SPIE paper and carried out the reflectance
tests. “We knew it was absorbent. We just didn’t think it would be this
absorbent from the ultraviolet to the far infrared.”
If
used in detectors and other instrument components, the technology would
allow scientists to gather hard-to-obtain measurements of objects so
distant in the universe that astronomers no longer can see them in
visible light or those in high-contrast areas, including planets in
orbit around other stars, Hagopian said. Earth scientists studying the
oceans and atmosphere also would benefit. More than 90% of the
light Earth-monitoring instruments gather comes from the atmosphere,
overwhelming the faint signal they are trying to retrieve.
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Currently,
instrument developers apply black paint to baffles and other components
to help prevent stray light from ricocheting off surfaces. However,
black paints absorb only 90% of the light that strikes it. The
effect of multiple bounces makes the coating’s overall advantage even
larger, potentially resulting in hundreds of times less stray light.
In
addition, black paints do not remain black when exposed to cryogenic
temperatures. They take on a shiny, slightly silver quality, said
Goddard scientist Ed Wollack, who is evaluating the carbon-nanotube
material for use as a calibrator on far-infrared-sensing instruments
that must operate in super-cold conditions to gather faint far-infrared
signals emanating from objects in the very distant universe. If these
instruments are not cold, thermal heat generated by the instrument and
observatory, will swamp the faint infrared they are designed to collect.
Black
materials also serve another important function on spacecraft
instruments, particularly infrared-sensing instruments, added Goddard
engineer Jim Tuttle. The blacker the material, the more heat it radiates
away. In other words, super-black materials, like the carbon nanotube
coating, can be used on devices that remove heat from instruments and
radiate it away to deep space. This cools the instruments to lower
temperatures, where they are more sensitive to faint signals.
To
prevent the black paints from losing their absorption and radiative
properties at long wavelengths, instrument developers currently use
epoxies loaded with conductive metals to create a black coating.
However, the mixture adds weight, always a concern for instrument
developers. With the carbon-nanotube coating, however, the material is
less dense and remains black without additives, and therefore is
effective at absorbing light and removing heat. “This is a very
promising material,” Wollack said. “It’s robust, lightweight, and
extremely black. It is better than black paint by a long shot.”
