Light absorbing coating set to enable more powerful space instruments
A team from NASA has successfully grown uniform layers of carbon nanotubes using another emerging technology called Atomic Layer Deposition (ALD). The marriage of the two technologies means that nanotubes can now be grown on three-dimensional components such as complex baffles and tubes.
"The significance of this is that now we have a tool that can make instruments more sensitive without making them bigger," says lead engineer on the project, John Hagopian.
"This demonstrates the power of nanoscale technology, which is particularly applicable to a new class of less expensive small satellites called CubeSats that are being developed to reduce the cost of space missions."
The resultant material absorbs more than 99% of the ultraviolet, visible, infrared and far-infrared light that strikes it, which has never been achieved to this level before. It is the culmination of five years work by Hagopian who has been able to 'tune' the nano-based super-black material for practical applications that require the suppression of stray light.
An example is a part used within the sensitive detector of a powerful telescope known as an occulter. This is an intricately-shaped small disk used to block bright objects in order to allow the observation of fainter ones.
The material consists of a thin coating of multi-walled carbon nanotubes 10,000 times thinner than human hair. The team is able to grow 'forests' of vertical carbon tubes on common materials such as titanium, copper and stainless steel. Tiny gaps between the tubes collect and trap light, while the carbon absorbs photons preventing any reflection off the surface.
One of the major challenges before growing the nanotubes is the need to deposit a highly uniform catalyst layer of iron oxide to support the nanotube growth. Using ALD, technicians place the substrate material inside a reactor chamber and pulse different types of gases to create layers of ultra-thin film, each no thicker than a single atom. ALD allows technicians to accurately control the thickness and composition of the deposited films, even inside pores and cavities. This gives ALD its unique ability to coat inside and around 3D objects.
Once applied, scientists can then grow the carbon nanotubes by placing the component in another oven and heating the part to about 750°C. As it heats, the component is 'bathed' in a feedstock gas containing carbon.
"The samples we've grown to date are flat in shape," says Hagopian. "But given the complex shapes of some instrument components, we wanted to find a way to grow carbon nanotubes on 3D parts, like tubes and baffles.
"The tough part is laying down that 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 as spraying paint from a fixed angle."
Collaboration with the Melbourne Centre for Nanofabrication (MCN) was able to fine tune the process for laying down the catalyst layer further, with precise methodology being developed to enable the reliable and repeatable deposit of a uniform coating.
Lachlan Hyde, an expert on ALD from MCN says: "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."
Now this has been achieved, Hagopian concludes: "This has really opened up the possibilities for us. Our goal of ultimately applying carbon-nanotube coatings to complex instrument parts is nearly realised."
