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‘Super Black’ Coating Flies at NASA

Thursday, November 10, 2011

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Think of it as the Darth Vader of coatings—absorbing all, reflecting nothing, and blacker than a Dark Side soul.

NASA engineers have reportedly produced a “super-black” material that absorbs more than 99 percent of the ultraviolet, visible, infrared and far-infrared light that hits it.

 Close-up view of internal structure of carbon-nanotube coating

 Images: Stephanie Getty / NASA Goddard

A close-up view (about 0.03 inches wide) shows the internal structure of NASA’s new carbon-nanotube coating. A section of the coating, which was grown on smooth silicon, was removed to show the tubes’ vertical alignment.

The development promises to open new frontiers in space technology, says the team at NASA’s Goddard Space Flight Center in Greenbelt, MD, which recently reported its findings at the SPIE Optics and Photonics conference.

Additional testing since the conference has reaffirmed the material’s capabilities, said John Hagopian, who is leading the project with 10 Goddard technologists.

‘Darn Near Perfect’

Reflectance tests showed that the team had extended the range of the material’s absorption capabilities by 50 times, 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,” he said. “No one else has achieved this milestone yet.”

1/10,000 of a Hair

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, much like a shag rug, on various substrates. The team has grown the nanotubes on silicon, silicon nitride, titanium and stainless steel—all materials commonly used in space-based scientific instruments.

 High-magnification image of hollow carbon nanotubes
A high-magnification image, taken with an electron microscope, shows the hollow carbon nanotubes. The human eye and sensitive detectors see the coating made of this material as black because tiny gaps between the tubes collect and trap light, preventing reflection.

Goddard technologist Stephanie Getty grows the nanotubes by applying 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 degrees Fahrenheit. While heating, the material is bathed in carbon-containing feedstock gas, according to NASA.

Spaceflight Applications

Tests indicate that the material is 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 want to measure. With so little light reflecting off the coating, the human eye and sensitive detectors see the material as black.

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, including planets in orbit around other stars, Hagopian said.

Earth scientists studying the oceans and atmosphere also would benefit. More than 90 percent of the light gathered by Earth-monitoring instruments comes from the atmosphere, overwhelming the faint signal they are trying to retrieve.

The Problem with Paint

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 percent of the light that strikes it. Multiple bounces can exacerbate the problem.

In addition, black paints do not remain black when exposed to cryogenic temperatures. They take on a shiny, slightly silver quality—a significant drawback for far-infrared-sensing instruments that must operate in super-cold conditions to gather signals from the very distant universe. (If the instruments are not cold, they generate thermal heat that will swamp the faint infrared they are designed to collect.)

Super-black materials can also be used on devices that remove heat from instruments and radiate it away to deep space, said Goddard engineer Jim Tuttle. The blacker the material, the more heat it radiates away. This cools the instruments, making them more sensitive to faint signals.

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, NASA said. The new coating is less dense and remains black without additives.

‘Better by a Long Shot’

NASA’s material absorbs 99.5 percent of the light in the ultraviolet and visible bands and about 98 percent in the longer or far-infrared bands.

“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.”

Added Wollack: “This is a very promising material. It’s robust, lightweight and extremely black. It is better than black paint by a long shot.”

   

Tagged categories: Coatings technology; Nano and hybrid coatings; Nanotechnology; Research

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