Engineers at the Massachusetts Institute of Technology (MIT) have created a material that is ten times blacker than anything that has previously been reported, the university said on 12 September.
The material is made from vertically aligned carbon nanotubes (CNTs), microscope filaments of carbon that the team grew on a surface of chlorine-etched aluminium foil, which captures around 99.995 percent of any incoming light – making it the blackest material on record.
“There are optical and space science applications for very black materials, and of course, artists have been interested in black, going back well before the Renaissance,” Brian Wardle, professor of aeronautics and astronautics at MIT, said in a statement.
“I think the blackest black is a constantly moving target. Someone will find a blacker material, and eventually we’ll understand all the underlying mechanisms, and will be able to properly engineer the ultimate black,” he added.
The CNT material could have practical as well as artistic use (the engineers used it to create artwork in collaboration with Diemut Strebe, an artist-in-residence at the MIT Center for Art, Science, and Technology), such as “optical blinders that reduce unwanted glare, to help space telescopes spot orbiting exoplanets”, Wardle said.
Wardle’s co-author on the paper – published the journal ACS-Applied Materials and Interfaces – is former MIT postdoc Kehang Cui, now a professor at Shanghai Jiao Tong University.
They weren’t seeking to create an ultrablack material, but were instead experimenting with ways to grow carbon nanotubes on electrically conducting materials such as aluminium, to boost their electrical and thermal properties.
But while trying to grow CNTs on aluminium, Cui literally ran up against a barrier: an every-present layer of oxide that coats aluminium when it’s exposed to air. The oxide acts as an insulator to block, rather than conduct, electricity and heat.
Cui found that salt – or sodium chloride – could be used to remove the oxide layer. At the same time, Wardle’s group was using salt to grow carbon nanotubes, and in their tests with salt, Cui noticed that chloride ions were eating away at aluminum’s surface and dissolving its oxide layer.
“This etching process is common for many metals,” Cui said. “For instance, ships suffer from corrosion of chlorine-based ocean water. Now we’re using this process to our advantage.”
He found that soaking aluminium foil in saltwater allowed him to remove the oxide layer. Cui transferred the foil to an oxygen-free environment to prevent re-oxidation, then placed the etched aluminium in an oven, where the group used a process called chemical vapor deposition to grow carbon nanotubes.
By removing the oxide layer, the researchers were able to grow carbon nanotubes on aluminate at much lower temperatures than they otherwise would. They also saw that the CNTs significantly enhanced the material’s thermal and electrical properties – this was expected, but the colour of the material was not.
“I remember noticing how black it was before growing carbon nanotubes on it, and then after growth, it looked even darker,” Cui said. “So I thought I should measure the optical reflectance of the sample.
“Our group does not usually focus on optical properties of materials, but this work was going on at the same time as our art-science collaborations with Diemut, so art influenced science in this case,” Wardle added.
Wardle and Cui, who have applied for a patent on the technology, are making the new CNT process freely available to any artist to use for a non-commercial art project.
According to MIT, the research are not entirely sure what processes made the material quite so black, but they suspect that it has something to do with the combination of etched aluminium – which is somewhat blackened already – with the carbon nanotubes.
Scientists believe that forests of carbon nanotubes can trap and convert most incoming light to heat, reflecting very little of it back out as light, thereby giving CNTs a particularly black shade.
“CNT forests of different varieties are known to be extremely black, but there is a lack of mechanistic understanding as to why this material is the blackest. That needs further study,” Wardle said.
The material is already gaining interest in the aerospace community. Astrophysicist and Nobel laureate John Mather, who was not involved in the research, is exploring the possibility of using Wardle’s material as the basis for a star shade — a massive black shade that would shield a space telescope from stray light.
“Optical instruments like cameras and telescopes have to get rid of unwanted glare, so you can see what you want to see,” Mather commented. “Would you like to see an Earth orbiting another star? We need something very black. … And this black has to be tough to withstand a rocket launch. Old versions were fragile forests of fur, but these are more like pot scrubbers — built to take abuse.”
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