For all types of spaceflight applications, an ultra-dark coating comprising nearly invisible shag rug-like strands composed of pure carbon proves to be very versatile, says NASA.
John Hagopian, an optical engineer who founded Lanham—a Maryland-based Advanced Nanophotonics, and contractor at NASA’s Goddard Space Flight Centre in Greenbelt, Maryland, and Lucy Li, a scientist at Goddard, are expanding a series of miniscule, button-shaped bumps of multi-walled nanotubes on a silicon wafer—amidst the most recent application of carbon-nanotube coating.
Each dot, which measures only 100 microns in diameter—approximately the thickness of a human hair—can serve as an “ammunition” source for a mini-electron probe. This variety of instrument is used to analyse chemical properties of rocks and soil on airless surfaces, such as the moon or an asteroid.
While this technology development is still in its infancy, it appears to be promising, Lim—who is utilising funding from NASA’s Planetary Instrument Concepts for the Advancement of Solar System Observations Programme to improve the concept—stated.
Nanotech-Sized Electron Gun
Carbon nanotubes—ideal electron emitters—form the basis of Lim’s instrument. Discovered in 1991, they also have useful electronic, magnetic, and mechanical properties. To produce them, technicians place a silicon wafer or other substrate to be heated in a furnace. During which, the substrate is bathed with a carbon feedstock gas to form the thin coating of almost invisible hair-like structures.
Hagopian and Lim are using this technique to grow tiny, circular dots of carbon nanotubes in a grid pattern—that the Goddard’s detector branch designed using photolithography, for the electron emitter.
Silicon wires or traces and a grid that produce two different voltages are placed above and below the lattice of dots. An electrical field is created with these voltages, activating the release of electrons from within the carbon-nanotube bumps or forests.
With Lim’s instrument concept, the electron beams pass through a stack of electrostatic lenses to increase their speed and to help focus them on an extra-terrestrial target. As the electrons hit the sample, the impact excites the elements in the sample—producing X-ray which would be analysed with a spectrometer to identify the sample’s chemical make-up.
The greatest difference about the new carbon-nanotube electron field emitter is its small size and that it is fully addressable. “We would be able to choose which bump to activate. We would be able to analyse different spots on the sample individually,” said Lim.
If the instrument had only one electron source, it is only possible to analyse one portion of the sample, according to Lim. “We want to obtain compositional maps. Without the addressable emitter, we might not discover all the minerals contained within a sample, how big they are, or their relationship to each other,” Lim added.
Through testing, Lim demonstrated that the bumps emit electrons to excite a sample. Hagopian has also proven that the technology can survive an excursion into outer space. However, existing challenges include packaging the nanotube-based grid into a tiny package, and then connecting it to the instrument’s electronics.
During testing, carbon-nanotube coatings prove to be highly effective in absorbing light—99.8 percent that strikes them—and is the reason for their very black appearance. As light enter the nanotube forest, tiny gaps between the tubes stop the light from bouncing.
Light is not absorbed by the gaps, however. Their electric field instead excites the electrons in the carbon nanotubes, which turns light to heat and effectively absorbing it, said Hagopian.
Hagopian is also in the process of developing intricately patterned nanotubes onto a component that changes the pattern of light that is diffracted off the edges of telescope structures by using coronagraphic masks—which block starlight. The effort is funded by NASA’s Small Business Innovative Research Programme.
Collaborating with Antonio Mannino, principal investigator of the research team at the Space Telescope Science Institute in Baltimore, Maryland, he is creating a coating to prevent straylight from contaminating measurements collected by a new instrument called the Coastal Ocean Ecosystem Dynamics Imager (COEDI).
This hyperspectral spectrometer is designed to monitor ocean colour from a geostationary orbit—measurements used to assess and mange coastal resources. “I started working with Hagopian two years ago when I discovered in testing that straylight was going to be a problem with COEDI,” said Mannino.