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Very White and Very Black

November 23, 2011

Many physical quantities need to be referenced to standards. Such standards were the original mission of the US National Bureau of Standards, now known as the National Institute of Standards and Technology.

When Fahrenheit was devising his temperature scale, he needed reference points at high and low temperature. His high temperature reference was chosen to be body temperature, and his low temperature reference was chosen to be the triple point of water (essentially, 0 °C, or 32 °F).

Fahrenheit would likely have preferred to reference to absolute zero (-459.67 °F), but that's impossible in principle. Today, we've come to within 100 picoKelvin of absolute zero. That's 10^-10 K. The uppermost temperature is likewise problematic. In theory, it's the Planck temperature, about 10³² Kelvin.

The reflectance of light from an object, more technically the diffuse reflectivity, is known as its albedo (from the Latin, alba, "white."). Standards are not needed for albedo, since it's a ratio, the ratio of reflected to incident light. The albedo of an object will depend on the wavelength of light; so, something that's black at visible wavelengths might be white at other wavelengths.

The albedo of the Moon is about 0.135. Here, on Earth, we have a wide range of albedo for objects, ranging from about 0.02 for fresh asphalt, to about 0.85 for fresh snow. Pure as the driven snow is still about 15% not so pure. Although the purest white (albedo = 1) and the purest black (albedo =0) are not needed to make an albedo measurement, such objects are technologically useful. For example, an albedo = 1 material would be an excellent roofing material in warmer climates.

"Pure as the driven snow" is actually not that pure.

Snow has an albedo of just 0.85 at visible wavelengths, so 15% of light is absorbed.

That's all for the best, or snow would never melt.

(Via Wikimedia Commons))

Photometric and radiometric measurements commonly make use of an integrating sphere, which is a hollow sphere that's coated on the inside with a material of high diffuse reflectance. Barium sulfate, which has an albedo of better than 97% at visible wavelengths, is often used in this application. Teflon (PTFE), with an albedo better than 98% is used, also. These two materials are much whiter than the driven snow.

One the other side of the whiteness spectrum, there's pure black. Soot, formed in the smoke of incompletely combusted hydrocarbons, is also black. An aerosol particle of soot has a single-scatter albedo at visible wavelengths of 0.226.[1] When soot is applied as a layer to a surface, the albedo is diminished because of multiple scatters. Two scatters reduce the albedo to 0.05, and three scatters will reduce the albedo to about 0.01. This would be an interesting simulation problem. "Consider a spherical soot particle...;"[2] or, more properly, consider fractal soot particles.

A technologically useful black material is black gold, which I wrote about in a previous article (Black Gold, November 29, 2006). When gold is evaporated under an argon or nitrogen pressure of about 750 millitorr, the gold atoms cool and cluster before attaching to a substrate, forming a black layer.[3-4] The process works for other metals, as well.[5] Black gold has been formed also by irradiating gold with intense bursts of femtosecond laser light, a process that transforms the surface into a convoluted nanoscale topography that is optically absorbent.[6] Anodization is another technique to produce dark surfaces.

Soot, of course, is mostly carbon, so this leads one to wonder how effective carbon nanotubes can be as a constituent of a super-black material. In 2008, a team of scientists from Rensselaer Polytechnic Institute, in work sponsored by the U.S. Department of Energy, created a carbon nanotube material with a lower reflectance than any other material to date. Their super-black, a low-density array of vertically aligned carbon nanotubes, absorbed more than 99.955% of incident light at visible wavelengths. The supposed mechanism is that the nanotube forest has a multitude of holes and gaps to trap light; that is, incident light must make many multiple reflections to exit.[7-8]

In 2009, scientists from the Japanese National Institute of Advanced Industrial Science and Technology and Nagoya University prepared a similar array of vertically-aligned carbon nanotubes. They found their material to be an extremely broadband blackbody, absorbing light almost perfectly from 0.2 - 200 μm.[9-10]

Two images of NASA's super-black carbon nanotube composition.

Upper image, with portion removed to show vertical alignment of the nanotubes.

Lower image, a higher resolution image of the surface. The white bar is a 50 μm marker.

Images: Stephanie Getty, NASA Goddard)

The Japanese study used single-walled carbon nanotubes, but a recent NASA study looked at the super-black properties of vertically-aligned multi-walled carbon nanotubes, as shown in the photographs, above. The NASA team prepared these arrays on stainless steel, titanium, silicon and silicon nitride substrates. The nanotube growth process involves putting a thin iron layer, which acts as a catalyst for the nanotube growth process, on such substrates, and then heating them at 750 °C in the presence of carbon feedstock gases.[11]

The NASA material absorbs 99.5% of visible and ultraviolet light, but only 98% of far-infrared light. NASA is concerned not only with the optical absorption property, but with the complementary emissivity. Super-black materials will efficiently radiate heat energy to keep spacecraft sensors and electronic components cool.[11]

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Linked Keywords: Standard; metrology; reference; National Bureau of Standards; National Institute of Standards and Technology; Daniel Gabriel Fahrenheit; Fahrenheit; Fahrenheit temperature scale; body temperature; triple point of water; absolute zero; K; Planck temperature; Kelvin; reflectivity; reflectance of light; diffuse reflectivity; albedo; wavelength; black; white; Moon; Earth; asphalt; snow; pure as the driven snow; Wikimedia Commons; photometric; radiometric; integrating sphere; sphere; barium sulfate; polytetrafluoroethylene; PTFE; Teflon; soot; smoke; combustion; hydrocarbon; aerosol; spherical cow; fractal; black gold; gold; argon; nitrogen; millitorr; femtosecond laser; nanoscale; topography; anodization; carbon nanotube; Rensselaer Polytechnic Institute; U.S. Department of Energy; Japanese National Institute of Advanced Industrial Science and Technology; Nagoya University; single-walled carbon nanotubes; NASA; multi-walled carbon nanotubes; stainless steel; titanium; silicon; silicon nitride; iron; catalyst; ultraviolet; infrared; emissivity; thermal radiation; spacecraft; sensor; electronic component.

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