Goddard Technologist Achieves Milestone
Super-Black Material Eyed for Multiple Spaceflight Applications
A new super-black material has shown in testing to absorb on average more than 99 percent of the ultraviolet, visible, infrared, and far-infrared light that strikes it — a never-before-achieved milestone that promises to open new frontiers in scientific discovery.
Due in part to test results, the technology now is being applied to a variety of spaceflight applications that could make it easier to gather hard-to-obtain measurements of objects so distant in the universe that astronomers no longer can see them in visible light. Efforts also are underway to apply the technology to an ocean-monitoring instrument and one that would study the outer planets, said Principal Investigator John Hagopian, who is leading a team funded under Goddard’s Internal Research and Development (IRAD) program.
The nano-based material is a thin coating of multi-walled carbon nanotubes made of pure carbon about 10,000 times thinner than a strand of human hair. These tiny hollow tubes are grown vertically on silicon, silicon nitride, titanium, and stainless steel — materials commonly used in space-based scientific instruments. Tiny gaps between the tubes collect and trap light and prevent it from reflecting off surfaces. Because only a small fraction of light reflects off the coating, the human eye and sensitive detectors see the material as black.
Effective Over Multiple Wavelength Bands
Reflectance tests, recently reported at the SPIE Optics and Photonics conference, have confirmed the material is blacker than originally expected across multiple wavelength bands.
Principal Investigator John Hagopian shows samples of the new highly absorbent material he and his team are developing for a wide variety of space applications. (Photo Credit: Chris Gunn)
“The reflectance tests showed that our team had extended by 50 times the range of the material’s absorption capabilities,” Hagopian said. “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. No one else has achieved this
In particular, the team found that the material absorbs 99.5 percent of the light in the ultraviolet and visible, dipping to 99 percent 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,” added 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.”
Its near-perfect blackness makes it an important new technology for suppressing stray light, calibrating infrared-sensitive instruments, radiating away heat, and absorbing infrared light for highly precise temperature measurements.
Better Than Black Paint
Currently, instrument developers apply black paint to baffles and other components to help prevent stray light from ricocheting off surfaces and contaminating measurements. However, black paints absorb only 90 percent 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. Furthermore black paints don’t remain black in the mid to far infrared or when exposed to cryogenic temperatures. They take on a shiny, slightly silver quality.
As a result, developers of the proposed Ocean Radiometer for Carbon Assessment (ORCA), a next-generation instrument that would measure marine photosynthesis, are evaluating its use on components to prevent errant light coming from the atmosphere from swamping the faint signal they want ORCA to retrieve.
Because the material remains black under cryogenic conditions, Goddard scientist Ed Wollack is evaluating the carbon-nanotube coating for use as a calibrator on far infrared-sensing instruments. These instruments must operate in super-cold temperatures to gather faint far-infrared signals emanating from objects in the very distant universe. If they aren’t cold, thermal heat generated by the instrument and observatory swamp the faint infrared they’re designed to collect.
Goddard engineer Jim Tuttle, meanwhile, believes the coating would make an ideal radiator — especially for instruments measuring faint, far-infrared light. 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.
In another application, Hagopian’s team is developing a 1 X 4 detector array to demonstrate the material’s use as an absorber for a thermopile, an electronic device that converts thermal energy — specifically, mid- to far-infrared light — into electrical energy. The carbon nanotubes absorb the light and heat up an ultra-sensitive thermometer, which changes the output voltage. “This makes the entire assembly exquisitely sensitive to minute variations in the infrared radiation, thereby enabling more sensitive measurements,” Hagopian said. This particular effort supports a possible follow-on to the Goddard-developed Composite Infrared Spectrometer, which flew on the Cassini mission.
“When we began growing carbon nanotubes four years ago, we knew the material could be widely used as a light-suppression technique,” Hagopian said. “We’re pleased that we can develop the technology for other uses to support the scientific community.”
This close-up view shows the internal structure of a carbon-nanotube coating that absorbs more than 99 percent of the ultraviolet, visible, infrared, and far-infrared light that strikes it. A section of the coating, which was grown on smooth silicon, was purposely removed to show the tubes’ vertical alignment. (Credit: Stephanie Getty)
“This is a very promising material,” Wollack added. “It’s robust, lightweight, and extremely black. It is better than black paint by a long shot.”
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