oxide

MIT has been showcasing what it describes as “the blackest black material to date”, at a time when BMW has unveiled a new car coated with what the automaker also claims is “the world’s blackest black” material. 

However, MIT claims that its material is “10 times blacker than anything that has previously been reported”.

Although the university does not specifically mention “Vantablack” – which is the name of the black nano-material used by BMW for its X6 prototype – its emphatic claim would suggest that Vantablack is included in its considerations.

The MIT material is made from vertically aligned carbon nanotubes, or CNTs – microscopic filaments of carbon, like a fuzzy forest of tiny trees, that the team grew on a surface of chlorine-etched aluminum foil.

The foil captures at least 99.995 percent of any incoming light, making it the blackest material on record.

Brian Wardle, professor of aeronautics and astronautics at MIT, says: “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.”

Wardle collaborated with Diemut Strebe, an artist-in-residence at the MIT Center for Art, Science, and Technology, on an exhibition to demonstrate the features of the new black nano-material.

The exhibition – entitled The Redemption of Vanity – features a 16.78-carat natural yellow diamond from LJ West Diamonds, estimated to be worth $2 million, which the team coated with the new, ultrablack CNT material.

The effect is that gem, normally brilliantly faceted, appears as a flat, black void. (See main picture.)

Wardle’s co-author on the paper about the MIT black nano-material is Kehang Cui, who says the team had some difficulties in developing the new black.

The group was using salt and other pantry products, such as baking soda and detergent, to grow carbon nanotubes.

In their tests with salt, Cui noticed that chloride ions were eating away at aluminum’s surface and dissolving its oxide layer.

Cui says: “This etching process is common for many metals.

“For instance, ships suffer from corrosion of chlorine-based ocean water. Now we’re using this process to our advantage.”

Cui found that if he soaked aluminum foil in saltwater, he could remove the oxide layer.

He then transferred the foil to an oxygen-free environment to prevent reoxidation, and finally, placed the etched aluminum in an oven, where the group carried out techniques to grow carbon nanotubes via a process called chemical vapor deposition.

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

They also saw that the combination of CNTs on aluminum significantly enhanced the material’s thermal and electrical properties – a finding that they expected.

What surprised them was 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,” Cui recalls. “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,” says Wardle.

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.

Main picture: The Redemption of Vanity… (left) 16.78 carat natural yellow diamond; (right) the diamond covered with “the blackest black on Earth”. Exclusive image copyright : Diemut Strebe