Tikalon Blog by Dev Gualtieri

Ultra-Hyper-Super Black

November 4, 2019

Scientists secretly write superlatives into the scientific literature through use of metric prefixes. It isn't just technology, it's nanotechnology; and, it isn't just computing, it's exascale computing. That's easy to do, since physical quantities are involved - the meter in the first case, and floating point operations per second in the later.

Things move from the scientific to the advertising realm when we express qualities, such as flavor, rather than quantities. In that case, the Mad Men use intensifier words like super (from the Latin adverb, super, above), ultra (from the Latin adverb, ultra, beyond) and hyper (from the Greek prefix, ῾υπερ, over/beyond).

Colors are particularly hard to describe, and that leads to expressions such as snow white and black as night. One look in the paint section of your local home goods store demonstrates the colorful names invented for different shades of white. As for black, some nights are blacker than others, and some black paints reflect less light than others. Many technologies are aided by black coatings that absorb, rather than reflect, light. One example of this is the black coating used for fixtures that hold lenses, filters, mirrors, and other optical components, on laboratory optical tables (see photograph).

Manual rotary optical bench fixture (NRC)

An older model of a Newport Corporation manual rotary optical tables fixture with inserted polarizing filter.

This fixture is 3-1/2 inches square, and it was used in experiments on magneto-optical garnets.

The black coating prevents stray light reflections from causing errors in sensitive optical experiments.

(Photo by the author, also posted at Wikimedia Commons. Click for larger image.)

Aluminum is a convenient material for making small mechanical components, such as the optical fixture shown above. It's fortunate that there's an easy way to coat aluminum in a durable black layer; namely, anodization. In anodization, the aluminum piece is used as the anode (positive electrode) in an electrochemical cell where it's opposed by a cathode (negative electrode, usually lead) in a dilute sulfuric acid solution. Application of a current of a few tens of milliamps per square inch produces an aluminum oxide layer a few micrometers thick on the piece through migration of the anion in the electrolyte, oxygen in this case, to the aluminum anode.

The aluminum oxide layer produced by anodization is porous with pore size of the order of about 50 nanometers. The pores enable growth of a thick oxide layer, since a uniform oxide would be insulating. The pores can also be infiltrated with dye, and this allows the bright coloring of things such as carabiner, keychains, and cases for electronic devices. One dye color can be black, so black anodization followed by a process to give the surface a matte finish for diffuse reflection will give a good optical black on an aluminum piece.

When physicists in the 19th century needed to blacken an object for creation of a black body, they would form layers of soot on glass. Soot is prepared by condensing from the smoke from the incomplete combustion of hydrocarbons, as from a candle. Soot deposited carefully in this manner will reflect less than 1% of visible light, making the glass slide 99% black.

Soot is carbon, and when carbon nanotubes were discovered an obvious early experiment was to determine the blackness of carbon nanotube Arrays. Early experiments demonstrated that the multitude of holes and gaps in vertically-aligned carbon nanotubes arrays trap light by forcing light rays to make many multiple reflections to exit the material. As a result, more than 99.955% of incident light is absorbed at visible wavelengths.[1]

Science is a competition, so this initial result in 2008 started a race for blacker, and blacker, layers. The next year, another research group created a similar array of single-walled carbon nanotubes and found the material to be an extremely broadband black body from 0.2 - 200 μm.[2] The researchers theorized that this enhanced blackness arose from the sparseness and imperfect alignment of the vertical single-walled carbon nanotubes.[2] Single-walled carbon nanotubes seem to be preferable, since a NASA showed that arrays of vertically-aligned multi-walled carbon nanotubes absorb only 99.5% of visible and ultraviolet light, and just 98% of far-infrared light (see figure).

NASA vertically-aligned carbon nanotube absorber

An array of vertically-aligned multi-walled carbon nanotubes created as an optical absorber and emitter by NASA.

The upper image has a portion removed to show the vertical alignment of the nanotubes, and the lower image shows the surface at high resolution. The scale bar is a 50 micrometer marker.

This 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. Such black body materials will efficiently radiate heat energy to keep spacecraft sensors and electronic components cool.[5]

(Images by Stephanie Getty, NASA Goddard Space Flight Center)

In the race to blacker black, scientists from Shanghai Jiao Tong University (Shanghai, China) and the Massachusetts Institute of Technology (Cambridge, Massachusetts) have developed a vertically aligned carbon nanotubes material that's an order of magnitude blacker than previous materials. The nanotubes are grown on chlorine-etched aluminum foil, so the process is capable of producing commercially viable sheets of large area. Their foil material absorbs about 99.995% of incident light over a wide range of wavelengths.[4-5]

This research team had been investigating the growth of carbon nanotubes on aluminum for their electrical and thermal conductivity properties.[5] In their process, chlorine is used to strip the native oxide from the aluminum to provide a proper surface for nanotube growth.[5-6] Says paper author, Kehang Cui, formerly with MIT and now a professor at Shanghai Jiao Tong University, "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."[5] The aluminum is first soaked in salt water, transferred to an oxygen-free environment to prevent reoxidation, and carbon nanotubes are then deposited by chemical vapor deposition.[5] The process occurs at the very low temperature of 100 °C.[5]

The researchers were principally interested in the material's electrical and thermal properties, and they found a 5-fold decrease in interfacial electrical resistance and a 66% increase in specific surface area as compared with nanotube layers grown without the chlorine process.[4] But they also noticed that their layers were especially black. Says Cui, "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."[5]

The nanotube surface absorbed at least 99.995% of incoming light from every angle.[5] That means that the aluminum foil substrate can have bends and kinks and still be completely black.[5] This blackness might be helped by the fact that the etched aluminum on which the nanotubes are grown is black at the start.[5] Says co-author, Brian Wardle of MIT, "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."[5]

Reflectance of the MIT ultrablack material as a function of wavelength compared with the previous blackest material.

The red line is a moving average to smooth the data.

(Created using Inkscape from data in ref. 4.[4] Click for larger image.)

There has been an historical link between technology and art, as in the rapid development of video technology for entertainment, so it might not be surprising that this carbon nanotube black has already been used in art. Diemut Strebe, an MIT artist-in-residence, created a work of art entitled, "The Redemption of Vanity," in collaboration with the nanotube research team. This was a 16.78-carat natural yellow diamond coated with the ultrablack carbon nanotube material and exhibited at the New York Stock Exchange.[5] While there's a pending patent on the ultrablack material, the inventors have allowed a free license for noncommercial art projects.[5]

One practical application would be as an anti-glare coating for such things as space telescopes.[5] It would be a useful material for a star shade, the light shield that blocks a star's illuminance to allow the search for exoplanets.[5] The robustness of this aluminum backed material is important, since such a shade would need to survive being rocketed into space.[5]

In looking to the future, Wardle notes that "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."[5]

Ultra-Hyper-Super Black

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