From: "mlbrown{at}everstrive.com" 
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http://www.newscientist.com/news/news.jsp?id=ns99993356 >

Researchers have created the blackest black ever made on Earth, by bubbling
a shiny metal plate in nitric acid for a few seconds.


 Ultimate darkness
This new super-black coating produced by Richard Brown and his colleagues
at the UK's National Physical Laboratory (NPL) in Teddington is designed to
revolutionise the manufacture of optical instruments.

It reflects 10 to 20 times less light than the black paint currently used
to reduce unwanted reflections in instruments. And this means the
super-black coating may one day help improve the vision of the Hubble Space
Telescope, says Nigel Fox, who heads the optics group at NPL.

The idea of making a blacker black by chemically etching a
nickel-phosphorus alloy goes back 20 years. But early attempts to come up
with a nickel-phosphorus formulation and etching process that was better at
absorbing light than black paint came to naught. Now using many different
etching processes, and varying the composition of the alloy in hundreds of
tests, NPL has made the crucial breakthrough.


Stalagmites and craters


By examining the surface of hundreds of alloy plates under an electron
microscope, NPL has discovered where previous researchers went wrong. It
has developed a two-stage technique that produces the blacker black New
Scientist saw emerge from the acid tank last week.

In the first stage, an object to be blackened is immersed for five hours in
a solution of nickel sulphate and sodium hypophosphite. This produces a
nickel and phosphorus coating containing between five and seven per cent
phosphorus. Then the surface is etched with nitric acid to produce the
super-black surface structure.

One of the crucial discoveries, says Brown, was how the percentage of
phosphorus in the nickel coating affected the surface after etching. An
electron micrograph of the surface of an alloy containing more than eight
per cent phosphorus (see graphic) looks like a collection of stalagmites.

But if the phosphorus content is around six per cent the surface becomes
pitted with craters. The curved craters reflect less light that the
straighter-sided stalagmites, so super-black reflects about half as much
light as the high-phosphorus surfaces.


Right Angle

Super-black is especially effective at absorbing light that hits it at an
angle. With the light source at right angles the super-black coating
reflects less than 0.35 per cent. Black paint, by comparison, reflects
about 2.5 per cent, or seven times as much. With the light source at an
angle of 45ø, black paint reflects 25 times as much light as the
super-black.

Brown says a wide range of materials can be coated with super-black,
including metals and ceramics. And unlike some black paints the coating
will not crack at cryogenic temperatures used on some space instruments,
such as the Earth radiation monitors used to keep a check on global
warming. But the super-black alloy is quite fragile and can easily be
scratched.

One of the early applications of the coating, says Fox, is likely to be on
star trackers, which help spacecraft maintain their position by fixing on
the pinpricks of light from the heavens. These black-painted tubes often
have to be at least 50 centimetres long to allow the walls to absorb any
stray light. But with a super-black coating the tube can be one-third
shorter and correspondingly lighter yet still absorb the same amount of
light.

One of the more interesting applications for the new black outside space
and optical applications may be in fine art. NPL says that several artists
are keen to use the new material. "When you look at the black, it is
an incredibly beautiful surface. It's like black velvet."


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