The Science of Vantablack
The name Vantablack® stands for Vertically Aligned Nanotube Array black.
Vantablack is a free-space coating consisting of a 'forest' of aligned and equally spaced, high aspect-ratio carbon nanotubes (CNTs).
The CNT array is patterned and spaced to allow photons to enter. Most of the light, or radiation arriving at the surface enters the space between the CNTs, and is repeatedly reflected between tubes until it is absorbed and converted to heat. This heat (largely undetectable in most applications) is conducted to the substrate and dissipated. The Vantablack array is very largly free-space; the volume of CNTs only makes up about 0.05% of the coating. Consequently, only a miniscule proportion of the incident radiation is able to hit the tip of a CNT, explaining why such a small amount is reflected back to the observer.
CNTs are hollow structures with one-or-more walls formed from atom-thick sheets of carbon. Each nanotube is around one fiftieth of one millionth (!) of a metre in diameter, making it an appropriately-sized building-block for engineering structures that exhibit low-reflectivity and high-emissivity across a wide-range of frequencies. In addition to incredible light absorption, the CNT array also has many other highly attractive properties:
- The high proportion of free-space within Vantablack (>99%) makes it extremely light. The height of a Vantablack coating is typically around 20 to 30 microns. One square metre of coating weighs around 2.5g (for a typical coating - growth parameters are varied to suit the application).
- The CNTs have an exceptionally high modulus of elasticity and will flex and bend, making them very robust in environments subject to extreme shock and vibration.
- The strength of the CNTs' bond to the substrate is high, making it difficult to remove the forest through thermal cycling, shock or vibration, even though the coating's characteristic's can be compromised through direct abrasion.
- Outgassing is virtually zero.
- The structures are incredibly tolerant to thermal cycling as a result of the intrinsic properties of the CNT building blocks, and their ability to adhere to a substrate.
These characteristics are, of course, highly-desirable in space applications, where components are subjected to extremes of thermal-cycling, mechanical shock, vibration and exposure to high-vacuum. In addition to withstanding the physical rigours of space-flight, the coating must also be suitable for application to light-weight engineering alloys of the type with proven aerospace heritage. The extremely-low reflectance exhibited by Vantablack across both the visible and IR spectra, and its ability to be directly applied to aluminium alloys makes it no-surprise that space-borne instrumentation was its initial application-focus.