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An interdisciplinary group of scientists and engineers has synthesised a hexagonal boron nitride (hBN) monolayer that shows an excellent corrosion resistance, with the potential to be employed in the future as an atomically thin layer anti-corrosion coating.
Boron nitride comes in many structural forms. Hexagonal boron nitride is an sp2 hybridised 2-dimensional sheet, similar to graphene, that adopts a covalently bound Kekule-type structure with alternating boron and nitrogen atoms (in a hexagonal array).
Hexagonal boron nitride is a useful material with a high band gap, thermal stability, chemical stability and is often employed as a dielectric (insulating) material alongside its carbon counterpart, graphene- a highly conductive material. It can withstand temperatures up to 3000 °C before melting.
Hexagonal boron nitride has found to be impermeable to oxygen and water, even at high temperatures and in oxidising atmospheres. Not only that, as a coating material, hexagonal boron nitride can increase the open circuit potential and supress the oxidation of the material that it is coating, namely copper.
This is not the only example where a 2D material can be used to provide a protective barrier. Everybody’s favourite monolayer, graphene, can act as an electrochemical barrier to water and inhibit corrosion. However, graphene is a quasi-metal in nature and the protection it provides is limited for long-term use. Hexagonal boron nitride on the other hand, has the potential for long-term protection as its insulating nature suppresses electron transfer processes for a long period of time. Electron transfer is the basis of corrosion and as such becomes limited when the facilitation of electrons is hindered.
Similar to graphene, the boron nitride monolayers are fabricated by chemical vapour deposition (CVD), under a low pressure and high temperature. The monolayers are uniform in nature and only have a maximum thickness of 0.45 nm which allows this material to form an atomically thin coating.
The researchers identified by cyclic voltammetry that both cathodic and anodic currents are unable to penetrate the coating and are supressed by three orders-of-magnitude.
Hexagonal boron nitride does have some potential coating limits. Where many coatings can be applied multiple times with strong interactions between each coat, this is not the case for hBN. Whilst the intramolecular forces in a single layer are strong, multi-layer hBN is held together by weak intermolecular forces, such as Van der Waals and induced dipole forces.
As a consensus, multi-layer hBN acts as a lubricant where the layers slide over each- a similar relationship that is present with graphene and graphite. The fluidity does not provide adequate protection as aqueous molecules can penetrate to the layer directly above the metal. This renders more than one layer pointless and it also allows water to intercalate and build up on the surface of the coating, which produces a greater chance of coating penetration than isolated water molecules do.
This limits the potential hBN to a single-layer coating, but a chemically strong one that can easily protect against oxygen and moisture. The single-layer also acts as a conformal coating which effectively coats the whole surface.
A layer of hBN on the surface of copper provides a lower amount of corrosion than uncoated copper, by one order of magnitude. This is more effective than graphene and any other corrosion inhibiting layers that have previously been tested.
Whilst hBN still requires some work to be brought to a commercial level as an atomically thin corrosion coating, it is a novel discovery in its infancy, that carries great potential for long term, atomically thin, corrosion protection, for a variety of copper (and potentially other metal) devices.
Sources
Mahvash F., Eissa S., Bordjiba T., Tavares A. C., Szkopek T., Siaj M., Corrosion resistance of monolayer
hexagonal boron nitride on copper, Scientific Reports, 2017, 7, 42139
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