If you’re a fan of the color black, then you’re in luck! The world’s blackest material, Vantablack, no longer holds its title. Engineers from MIT developed a material with higher optical absorption, and thus a darker color. While Vantablack absorbs almost all light to which it is exposed (99.96%), the new material is even closer to complete absorption, at more than 99.995%.
Like its predecessor, this new creation has a structure based on a “forest” of carbon nanotubes- tiny structures smaller than one-ten-millionth of a meter made of rolled sheets of carbon atoms. However, the engineers were not seeking out a new blackest material. Instead, they sought to tackle a problem experienced when growing these nanotube structures on the surface of metals like aluminum or iron. Since carbon nanotubes are known to have low electrical resistance (they let electrical currents, made of electrons, pass through them easily), they can be useful in electronic applications. However, when exposed to air, materials like aluminum undergo oxidation, where a small layer of the surface changes its chemical composition by reacting with oxygen. The formation of this oxide layer increases the resistance to the flow of current between the nanotubes and the underlying aluminum, thereby causing the electronics to work less efficiently.
To counteract this oxidation problem, the authors took standard aluminum and placed it in a sodium chloride (salt) solution. The chloride attacks the oxide layer and forms a rough surface of exposed aluminum. After soaking for ten hours and rinsing, carbon nanotubes were grown on top of the exposed surface, forming a final product with an electrical resistance five times lower than when the oxide layer was present and a higher stability to changes in temperature. As a bonus, the reflectivity of light was ten times lower than Vantablack, making it a candidate for applications that require maximal absorption. For example, employing highly absorbant materials in solar panels which produce heat improves the efficiency of this conversion over conventional materials. Additionally, in astronomic telescopes used to spot far away planets, light entering from bright nearby objects (like the Sun) can contaminate the signal, making identification of these celestial objects more challenging. Superblack materials can be used to ‘collect’ this stray light before it reaches the light sensor, reducing the effects of this unwanted light.
A remaining question is the underlying mechanism of this absorption – what is it about the combination of the nanotubes and the aluminum that improves its absorption? And since the health effects of carbon nanostructures are still not entirely understood (there is evidence that they may be carcinogenic and have other negative effects; for example see this article comparing carbon nanotubes to asbestos), fans of black clothing are not likely to see this in their nearest fabric store any time soon.
Managing Correspondent: Andrew T. Sullivan
Original Journal Article: “Breakdown of Native Oxide Enables Multifunctional, Free-Form
Carbon Nanotube−Metal Hierarchical Architectures”, ACS Applied Materials & Interfaces
Image Credit: Pixabay