Another Milestone for Optics Engineer’s Superblack Material
GREENBELT, Md., July 18, 2013 — A superblack carbon-nanotube technology developed five years ago has achieved another milestone, and this time it promises to make spacecraft instruments more sensitive without enlarging their size.
Since beginning his R&D efforts, optics engineer John Hagopian and colleagues at NASA’s Goddard Space Flight Center have made significant strides in applying the carbon-nanotube technology to a number of spaceflight applications, including, among other things, the suppression of stray light that can overwhelm faint signals that sensitive detectors are supposed to retrieve.
Optics engineer John Hagopian works with a nanotube material sample. Courtesy of NASA Goddard/Chris Gunn.
During this research, Hagopian tuned the nano-based superblack material, making it suitable for this application, absorbing on average more than 99 percent of the UV, visible, IR and far-IR light that strikes it — a never-before-achieved milestone that now promises to open new frontiers in scientific discovery (See: Superblack Material Absorbs Multiwavelengths of Light). The material consists of a thin coating of multiwalled carbon nanotubes.
Now, Hagopian’s team has demonstrated that it can grow a uniform layer of carbon nanotubes using atomic layer deposition (ALD). The marriage of the two technologies means that NASA can grow nanotubes on 3-D components, such as complex baffles and tubes commonly used in optical instruments.
“The significance of this is that we have new tools that can make NASA instruments more sensitive without making our telescopes bigger and bigger,” Hagopian said. “This demonstrates the power of nanoscale technology, which is particularly applicable to a new class of less-expensive tiny satellites called CubeSats that NASA is developing to reduce the cost of space missions.”
Once a laboratory novelty grown only on silicon, the NASA team now grows these forests of vertical carbon tubes on commonly used spacecraft materials, such as titanium, copper and stainless steel. Tiny gaps between the tubes collect and trap light, while the carbon absorbs the photons, preventing them 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.
Before growing the nanotube forest on instrument parts, however, materials scientists must first deposit a highly uniform foundation or catalyst layer of iron oxide that supports the nanotube growth. For ALD, technicians do this by placing a component or some other substrate material inside a reactor chamber and sequentially pulsing different types of gases to create an ultrathin film whose layers are no thicker than a single atom. Once applied, the scientists can grow the nanotubes, heating the component to about 1832 ºF. While it heats, the component is bathed in carbon-containing feedstock gas.
Lachlan Hyde, an expert in atomic layer deposition at Australia’s Melbourne Centre for Nanofabrication, works with one of the organization’s two ALD systems. Courtesy of MCN.
“The samples we’ve grown to date are flat in shape,” Hagopian said. “But given the complex shapes of some instrument components, we wanted to find a way to grow carbon nanotubes on three-dimensional parts, like tubes and baffles. The tough part is laying down a uniform catalyst layer. That’s why we looked to atomic layer deposition instead of other techniques, which only can apply coverage in the same way you would spray something with paint from a fixed angle.”
ALD — first described in the 1980s and later adopted by the semiconductor industry — offers several advantages over competing techniques for applying thin films. Technicians can accurately control the thickness and composition of the deposited films, even deep inside pores and cavities, giving ALD the ability to coat in and around 3-D objects.
NASA’s Vivek Dwivedi is now customizing the ALD reactor technology for spaceflight applications through a partnership with the University of Maryland. While Dwivedi built the ALD reactor, Hagopian worked with researchers through the Science Exchange services of the Melbourne Centre for Nanofabrication (MCN) to determine the technique’s viability for creating the catalyst layer.
Through the collaboration, the NASA team delivered a number of components, including an intricately shaped occulter used in a new NASA-developed instrument for observing planets around other stars. The Australian investigators fine-tuned the recipe for laying down the catalyst layer, detailing the type of precursor gas, the reactor temperature and pressure needed to deposit a uniform foundation.
Australia’s Melbourne Centre for Nanofabrication applied a catalyst layer using atomic layer deposition to this occulter mask. Courtesy of NASA.
“The iron films that we deposited initially were not as uniform as other coatings we have worked with, so we needed a methodical development process to achieve the outcomes that NASA needed for the next step,” said Lachlan Hyde, MCN’s expert in ALD.
The work was a success, Hagopian said.
“We have successfully grown carbon nanotubes on the samples we provided to MCN and they demonstrate properties very similar to those we’ve grown using other techniques for applying the catalyst layer,” he said. “This has really opened up the possibilities for us. Our goal of ultimately applying a carbon-nanotube coating to complex instrument parts is nearly realized.”
For more information, visit: www.nasa.gov
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