September 2019 brings the world the blackest black ever created! Developed by MIT engineers, it is said to be 10 times blacker than anything previously reported. It captures over 99.995% of any incoming light. That makes it darker than Vantablack, now the second darkest black material on record, which captures up to 99.965% of any incoming light.
This new material is made from vertically aligned carbon nanotubes (CNTs). CNTs are microscopic filaments of carbon. The engineers arrayed the CNTs, like a fuzzy forest of microscopic trees, across the surface of chlorine-etched aluminum foil. The researchers have published their findings in the journal ACS-Applied Materials and Interfaces.
Engineering And Art Combine
They are showcasing the effects of the material as part of a new exhibit at the New York Stock Exchange, titled “The Redemption of Vanity,” in which they collaborated with artists to make a 2-million-dollar diamond “disappear.” Brian Wardle, professor of aeronautics and astronautics at MIT, and his group, and MIT artist-in-residence Diemut Strebe, took a 16.78-carat natural yellow diamond and coated it with the new, ultra-black CNT material. It is meant to be the ultimate example of how black this material is because the gem is normally brilliantly faceted, but with the coating, it appears as a flat black void.
Artistic statements aside, Wardle says the CNT material has practical uses. For example, it could be used in optical blinders that reduce unwanted glare to help space telescopes spot orbiting exoplanets.
Wardle said:
“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. 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.”
Astrophysicist and Nobel laureate John Mather, not involved in the study, said:
“Optical instruments like cameras and telescopes have to get rid of unwanted glare, 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.”
Unintended Discovery
Wardle and former MIT postdoc Kehang Cui, co-author of the paper and currently a professor at Shanghai Jiao Tong University, didn’t intend to engineer an ultra-black material. They were only trying to boost the electrical and thermal properties of electrically conducting materials such as aluminum by growing carbon nanotubes on them. They were experimenting with different ways to achieve this.
In the midst of these experiments, while attempting to grow CNTs on aluminum, Cui ran into a problem. When the aluminum is exposed to air, it develops a coating – an ever-present layer of oxide that acts as an insulator, blocking rather than conducting electricity and heat. After trying many different things, Cui came up with a solution: salt or sodium chloride.
Salt just happened to be one of the many materials, like baking soda and detergent, that Wardle’s group was using to grow carbon nanotubes. When Cui put salt on the surface of aluminum, the chloride ions ate away at it and dissolved its oxide layer.
Cui said:
“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 he could remove the oxide layer if he soaked aluminum foil in saltwater. Then, to ensure reoxidation wouldn’t occur, he transferred the foil to an oxygen-free environment. Lastly, he placed the etched aluminum in an oven, where the group carried out techniques to grow carbon nanotubes via a process called chemical vapor deposition.
Only once they removed the oxide layer were they able to grow carbon nanotubes on the aluminum and at about 100 degrees Celsius less than they otherwise would. They found that the combination of CNTs on aluminum significantly enhanced the material’s thermal and electrical properties. This result, they expected…but the color they did not.
Cui recalled:
“I remember noticing how black it was before growing carbon nanotubes on it, and then after growth, it looked even darker. So, I thought I should measure the optical reflectance of the sample.”
Wardle added:
“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.”
To see how black it was, Cui measured the amount of light reflected by the material, not just from directly overhead but also from every other angle possible. The results revealed the material had a light-absorbing capability greater than 99.995% from any angle. Meaning, no matter what angle it was viewed from, or if the surface contained bumps or ridges or features of any kind – all of it would be invisible, obscured in a void of black.
