‘Blackest black’ material created at MIT
The material is made from vertically aligned carbon nanotubes (CNTs) – microscopic filaments of carbon – grown on a surface of chlorine-etched aluminum foil. The foil, say the researchers, captures at least 99.995% of any incoming light, making it the blackest material on record.
The material is showcased as part of an art exhibit featuring a 16.78-carat natural yellow diamond – estimated to be worth $2 million – coated with the new, ultrablack CNT material (see image). Practical applications, say the researchers, could include use as optical blinders that reduce unwanted glare, such as to help space telescopes spot orbiting exoplanets.
“There are optical and space science applications for very black materials,” says Brian Wardle, professor of aeronautics and astronautics at MIT, “and of course, artists have been interested in black, going back well before the Renaissance. Our material is ten times blacker than anything that’s ever been reported, but 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.”
The material was found by accident while the researchers were experimenting with ways to grow carbon nanotubes on electrically conducting materials, such as etched aluminum, to boost their electrical and thermal properties. While the combination of CNTs on aluminum significantly enhanced the material’s thermal and electrical properties as expected, the researchers say they were surprised by the material’s color.
“I remember noticing how black it was before growing carbon nanotubes on it, and then after growth, it looked even darker,” says former MIT postdoc Kehang Cui and co-author of a paper on the research. “So I thought I should measure the optical reflectance of the sample.”
Cui measured the amount of light reflected by the material – not just from directly overhead, but also from every other possible angle. The results showed that the material absorbed at least 99.995% of incoming light, from every angle.
In other words, say the researchers, it reflected ten times less light than all other superblack materials, including Vantablack. If the material contained bumps or ridges, or features of any kind, no matter what angle it was viewed from, these features would be invisible, obscured in a void of black.
The researchers say they suspect that the mechanism contributing to the material’s opacity may have something to do with the combination of etched aluminum, which is somewhat blackened, 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,” says Wardle, “but there is a lack of mechanistic understanding as to why this material is the blackest. That needs further study.”
The material, say the researchers, 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 the 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,” says Mather, “so you can see what you want to see. 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.”
For more, see “Breakdown of Native Oxide Enables Multifunctional, Free-Form Carbon Nanotube–Metal Hierarchical Architectures.”
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