Made
from carbon nanotubes, the new coating is 10 times darker than other very black
materials
Massachusetts Institute of Technology –
September 13, 2019 -- With apologies to "Spinal Tap," it appears that
black can, indeed, get more black.
MIT engineers report today that they
have cooked up a material that is 10 times blacker than anything that has
previously been reported. The 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 more than 99.96 percent of any incoming light, making it the
blackest material on record.
The researchers have published their
findings today in the journal ACS-Applied Materials and Interfaces. They
are also showcasing the cloak-like material as part of a new exhibit today at
the New York Stock Exchange, titled "The Redemption of Vanity."
The artwork, a collaboration between
Brian Wardle, professor of aeronautics and astronautics at MIT, and his group,
and MIT artist-in-residence Diemut Strebe, features a 16.78-carat natural
yellow diamond, estimated to be worth $2 million, which the team coated with
the new, ultrablack CNT material. The effect is arresting: The gem, normally
brilliantly faceted, appears as a flat, black void.
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 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's co-author on the paper is
former MIT postdoc Kehang Cui, now a professor at Shanghai Jiao Tong
University.
Into the void
Wardle and Cui didn't intend to engineer
an ultrablack material. Instead, they were experimenting with ways to grow
carbon nanotubes on electrically conducting materials such as aluminum, to
boost their electrical and thermal properties.
But in attempting to grow CNTs on
aluminum, Cui ran up against a barrier, literally: an ever-present layer of
oxide that coats aluminum when it is exposed to air. This oxide layer acts as
an insulator, blocking rather than conducting electricity and heat. As he cast
about for ways to remove aluminum's oxide layer, Cui found a solution in salt,
or sodium chloride.
At the time, Wardle's 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.
"This etching process is common for
many metals," Cui says. "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.
"Built to take abuse"
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 greater
than 99.995 percent of incoming light, from every angle. In essence, 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 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 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, but there is a lack of mechanistic understanding as to why this material is the blackest. That needs further
study," Wardle says.
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