Functional Chameleon

Could a colour changing nano coating provide changing properties and functionality to non-metal parts?

Researchers at the Fraunhofer Institute have developed a colour changing nano coating that is reportedly able to change its emissivity depending on ambient temperature. It's all part of the Institutes work on thermochromism, the study of substances that change colour with temperature. It's found that using a thermochromic coating on metal strips allows the material to absorb heat to one range of temperatures, before reflecting it away at another.

As nano particles possess especially large surface to volume ratio it allows them to be extremely efficient and reactive. Even a tiny amount finely distributed in a coating can produce a significant effect.

The coating incorporates an 'active nano material' in various polymer systems, which can then be applied in the same way as a paint or varnish. At temperatures above 30°C the coating is transparent and allows the metal underneath to reflect heat away.

"Metal strips possess very special properties when coated in this way," says Helmut Schmid from Fraunhofer Institute for Chemical Technology. "If temperatures are below 30°C, the black coating absorbs heat, if it is warmer the colour changes. The varnish, which has now become transparent, allows the infrared radiation to be reflected."

Strips and wires coated in this material could be used in a number of applications from allowing engine components to warm up more quickly and stop them from overheating, to being used on exterior self-regulating thermal cladded walls to help cool buildings.

Researchers at Fraunhofer are continuing to work on nano based coatings with other more exotic material such as luminescence. The hope is that this type of coating could be used for safety markings and signage, or to differentiate branded products from pirated copies.

In all reality, it is likely that engineers will come up a plethora of applications that were never foreseen during development. And this, indeed, is part and parcel of the bluesky research and development portfolio of the Fraunhofer Institute. While it can hint of applications – it is up to industry to really apply these innovations.

The facility is also designing carbon and metallic nano coatings that allow insulators to become electrically conductive, or increase conductivity of metallics. If fully developed, it could be used to enhance the performance of lithium-ion batteries.

Researchers have found applying titanium nitride or copper to stainless steel particles, for example, means the substance no longer agglomerates. A powder treated in this way can therefore be used to 'metalise' temperature-sensitive materials like plastic or paper. The scientists employ plasma spraying to homogeneously apply it to thin pieces of metal. Nano shaped particles of stainless steel are thermally activated using an atmospheric pressure plasma process and are deposited as a metal film.

Thermochromic coatings are also being used by RUD Chains. It is using a pink powder coating to act as an overheating indicator to show the chains can be safely used at a given temperature. This is particularly important for those chains working in very hot environments, where temperatures above 300°C are deemed to be excessively high. If the chain reaches temperatures it will turn from pink to brown and then black, which indicates that the chain is unsafe for use and must be taken out of service.

Justin Cunningham

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