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black carbon nanotubes

Blackest material known to man created accidentally using carbon nanotubes

Image credit: Dreamstime

A material that is 10 times blacker than anything made before has been created by MIT engineers using carbon nanotubes.

The material is made from vertically aligned carbon nanotubes (CNTs), which are microscopic filaments of carbon.

The tubes were grown on a surface of chlorine-etched aluminum foil which captures more than 99.96 per cent of any incoming light, making it the blackest material on record.

The material is being exhibited as part of an artistic collaboration with MIT artist-in-residence Diemut Strebe and features a 16.78-carat natural yellow diamond, estimated to be worth $2m, which the team coated with the new, ultrablack material.

MIT aeronautics professor Brian Wardle says the CNT material, aside from making an artistic statement, may also be of practical use, for instance in optical blinders that reduce unwanted glare, to help space telescopes spot orbiting exoplanets.

“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,” Wardle said.

“Our material is 10 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 actually created accidentally while experimenting with ways to grow carbon nanotubes on electrically conducting materials such as aluminium, to boost their electrical and thermal properties.

An ever-present layer of oxide that coats aluminium when it is exposed to air acts as an insulator, blocking rather than conducting electricity and heat and the team looked for ways to remove this layer.

By removing it, the researchers were able to grow carbon nanotubes on aluminium at much lower temperatures than they otherwise would, by about 100 degrees Celsius.

The combination of CNTs on aluminium significantly enhanced the material’s thermal and electrical properties - a finding that they expected, but they were surprised by its strong black colour.

“I remember noticing how black it was before growing carbon nanotubes on it and then after growth it looked even darker,” said MIT postdoc Kehang Cui. “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 said.

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 noncommercial art project.

The material is so dark that even if it 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.

This is a similar effect to the Vantablack material that was first demonstrated in 2014 at the Farnborough Airshow.

The researchers aren’t entirely sure of the mechanism contributing to the material’s opacity, but they suspect that it may have something to do with the combination of etched aluminium, 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.

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