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With apologies to "Spinal Tap,"It appears that black can indeed become darker.
MIT engineers announced today that they have prepared a material 10 times blacker than anything previously reported. The material consists of vertically aligned carbon nanotubes, or CNTs – microscopic carbon filaments, resembling a fuzzy forest of tiny trees, which the team developed on a surface of chlorine-etched aluminum foil. The sheet captures over 99.96% of all incoming light, making it the darkest material ever recorded.
The researchers published their findings today in the journal Materials and interfaces applied to the ACS. They also present the cloak-like material as part of a new exhibition presented today on the New York Stock Exchange, titled "The Redemption of Vanity."
The work, a collaboration between Brian Wardle, professor of aeronautics and astronautics at MIT, and his group, and MIT artist-in-residence, Diemut Strebe, presents a natural yellow diamond of 16.78 carats, valued at $ 2 million. covered with the new CNT ultrablack material. The effect is striking: the gemstone, usually bright, appears as a black and flat void.
Wardle says the CNT material, in addition to making an artistic statement, can also be useful, for example in the area of optical blinkers that reduce unwanted glare, to help space telescopes to spot exoplanets in orbit. .
"There are applications in optics and space science for very dark materials, and of course, artists were interested in black, long before the Renaissance," says Wardle. "Our material is 10 times darker than anything that has ever been reported, but I think the darkest black is a constantly moving target. Someone will find a darker material and eventually we will understand all the underlying mechanisms and we will be able to correctly design the ultimate black. "
Wardle's co-author on paper is the former MIT post-doc, Kehang Cui, now a professor at Shanghai Jiao Tong University.
In the void
Wardle and Cui did not intend to create an ultrablack material. Instead, they were experimenting with ways to grow carbon nanotubes on conductive materials such as aluminum, in order to improve their electrical and thermal properties.
But when trying to grow NTCs on aluminum, Cui 's is hit with a barrier, literally: an ever present oxide layer that covers aluminum when it' s exposed in the air. This layer of oxide acts as an insulator, blocking rather than driving electricity and heat. As he was looking for ways to remove the aluminum oxide layer, he found a solution of salt or sodium chloride.
At the time, the Wardle Group used salt and other pantry products, such as baking soda and detergents, to grow carbon nanotubes. In their tests with salt, Cui noticed that the chloride ions gnawed the surface of the aluminum and dissolved the oxide layer.
"This engraving process is common for many metals," says Cui. "For example, ships suffer from corrosion of chlorine-based seawater. We are now using this process to our advantage. "
Cui discovered that if he dipped aluminum foil in salt water, he could remove the oxide layer. He then transferred the sheet to an oxygen-free environment to avoid re-oxidation, and then placed the etched aluminum in an oven where the group developed techniques to grow carbon nanotubes via a process called chemical vapor deposition.
By eliminating the oxide layer, the researchers were able to grow carbon nanotubes on aluminum, at temperatures much lower than they would normally have, from about 100 degrees Celsius. They also found that the combination of NTC on aluminum significantly improved the thermal and electrical properties of the material – a conclusion they expected.
What surprised them was the color of the material.
"I remember seeing how black it was before growing carbon nanotubes, and then after growing, it looked even darker," recalls Cui. "So I thought I should measure the optical reflectance of the sample.
"Our group does not usually focus on the optical properties of materials, but this work was happening at the same time as our art-science collaborations with Diemut, so that art has influenced science in this case," says Wardle.
Wardle and Cui, who have filed a patent for this technology, make the new CNT process freely available to any artist who can be used for a non-commercial art project.
"Built to support abuse"
Cui measured the amount of light reflected by the material, not only directly above the head, but from all possible angles. The results showed that the material absorbed more than 99.995% of incoming light from all angles. In essence, if the material contained bumps or ridges, or features of all kinds, regardless of their angle of observation, these features would be invisible, obscured in a void of black.
Researchers are not entirely sure of the mechanism that contributes to the opacity of the material, but they suspect that this may have something to do with the combination of etched aluminum, which is a bit blackened, with nanotubes carbon. Scientists believe that carbon nanotube forests can trap and convert most of the incoming light into heat, reflecting very little back light, giving CNTs a particularly dark shade.
"We know that the forests of CNT of different varieties are extremely black, but it is difficult to understand why this material is the blackest. This requires further study, "says Wardle.
The material is already starting to interest the aerospace community. John Mather, astrophysicist and Nobel laureate of research, who did not participate in research, investigates the possibility of using Wardle's material as a basis for a star tint – a massive black hue that would protect a space telescope stray light.
"Optical instruments like cameras and telescopes need to get rid of unwanted reflections so you can see what you want to see," says Mather. "Would you like to see the Earth orbiting another star? We need something very dark. … and this black must be hard to resist a rocket launch. Older versions were fragile fur forests, but they look more like pot washers – designed to resist abuse. "
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