When it comes to gathering measurements of objects so distant in the universe that they can no longer be seen in visible light, the smallest amount of stray light can play havoc with the sensitive detectors and other instrument components used by astronomers. Currently, instrument developers use black paint on baffles and other components to help prevent stray light ricocheting off surfaces, but the paint absorbs only 90 percent of the light that strikes it. NASA engineers have now developed a nanotech-based coating that absorbs on average more than 99 percent of the ultraviolet, visible, infrared, and far-infrared light that hits it, making it promising for a variety of space- and Earth-bound applications.
The new super-black material developed at NASA's Goddard Space Flight Center is a coating made up of a thin layer of multi-walled carbon nanotubes that are positioned vertically on various substrate materials, much like a shag rug. Using a catalyst layer of iron, the team has grown the nanotubes on underlayers of materials commonly used in space-based scientific experiments, including silicon, silicon nitride, titanium, and stainless steel.
The team found that the material absorbs 99.5 percent of ultraviolet and visible light and dips to 98 percent in the longer or far-infrared bands, making it ideal for stray-light suppression applications. This is because the tiny gaps between the nanotubes collect and trap background light to prevent it from reflecting off surfaces and interfering with the light that scientists actually want to measure.
"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," said John Hagopian, who is leading the 10 strong Goddard team. "No one else has achieved this milestone yet."
In addition to providing benefits to astronomers observing distant objects or those in high-contrast areas - planets orbiting other stars, for example - NASA says scientists studying Earth's oceans and atmosphere would also benefit. This is because more than 90 percent of the light Earth-monitoring instruments gather comes from the atmosphere, which overwhelms the faint signal they are trying to retrieve.
Another downside of the black paints currently used is that they don't remain black when exposed to cryogenic temperatures. Instead they take on a shiny, slightly silver quality, says 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.
In addition to absorbing light, black materials are also used to radiate heat away from spacecraft instruments, such as infrared-sensing instruments. Since the blacker the material, the more heat it radiates away, the new super-black material could be used to remove heat from instruments and radiate it away to deep space. This allows the instruments to be cooled to lower temperatures so they are more sensitive to faint signals.
And unlike black paints that require epoxies loaded with conductive metals to prevent them from losing their absorption and radiative properties, the nanotube-based coating is less dense and remains black without additives.
"This is a very promising material," said Wollack. "It's robust, lightweight, and extremely black. It is better than black paint by a long shot."
The NASA Goddard Space Flight Center team recently reported their findings at the SPIE Optics and Photonics conference.