The thermal properties of VO2 mean it could be used by the military

'Chameleon' coating tricks thermal cameras

US scientists have developed an ‘active camouflage’ coating that presents a false temperature reading to thermal cameras. The researchers from the Harvard School of Engineering and Applied Sciences believe their surprising discovery could have useful military and everyday applications.

The team used an infra-red camera to monitor a thin film of vanadium oxide (VO2) on a sapphire substrate after it was placed on a hot plate. As the temperature rose, it initially behaved as expected: at 60°C it appeared blue-green; by 70°C it was red and yellow. At 74°C it turned a deep red – but then the thermal radiation dropped sharply. At 80°C it looked blue, as if it could be 60°C, and at 85°C it looked even colder.

Principal investigator Professor Federico Capasso predicts that the coating could be used as a thermal camouflage or as a kind of encrypted beacon to allow soldiers to covertly communicate their locations.

VO2 is a phase-change material that is electrically insulating at room temperature, but undergoes a transition at approximately 70°C to a metallic, electrically conductive state. During that transition, the optical properties also change, giving rise to the temperature-dependent camouflage effect. However, it has been seen as a difficult material to work with.
Recent advances in materials synthesis and characterisation, especially by co-author Shriram Ramanathan, have allowed the creation of extremely pure samples of thin-film VO2, opening up new opportunities.

“Thanks to these very stable samples, we now know that if we introduce small changes to the material, we can dramatically change the optical phenomena we observe,” explained lead author Mikhail Kats, a graduate student at Harvard SEAS. “By introducing defects in a controlled way, it is possible to create a wide range of behaviours.”
The researchers say a vehicle coated in VO2 could potentially mimic its environment like a chameleon, appearing invisible to an infra-red camera with only very slight adjustments to the tiles’ actual temperature.

Tuned differently, the material could become a component of a secret beacon, displaying a particular thermal signature on cue to an infrared surveillance camera.

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