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New Camo Coating Tricks IR Cameras

Friday, October 25, 2013

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A new coating that can hide its internal temperature from thermal sensors holds hot potential for a new class of military materials that protect soldiers in the field, say developers at Harvard University.

Above a certain temperature, the hotter the material gets, the colder it appears on thermal imaging.

The secret: a very thin film of vanadium oxide, "an unusual material that undergoes dramatic electronic changes when it reaches a particular temperature," according to a research announcement from Harvard's School of Engineering and Applied Sciences (SEAS), where the work is underway.

'Something Strange Happened'

In a laboratory test, the team of applied physicists placed the device on a hot plate and watched it through an infrared camera as the temperature rose.

Infrared readings from a vanadium oxide sample
Courtesy of Mikhail Kats / Harvard SEAS

Infrared readings from a thin-film vanadium oxide sample show a cool blue-green reading at 85°C (185°F).

Initially, they said, the coating behaved as expected, giving off more infrared light as the sample was heated: At 60°C (140°F), the coating appeared a cool blue-green to the camera; by 70°C (158°F), it was red and yellow.

At 74°C (165.2°F), the coating registered as a deep red—"and then something strange happened," the team said. "The thermal radiation plummeted."

At 80°C (176°F), "it looked blue, as if it could be 60°; and at 85[°C, or 185°F], it looked even colder."

"Moreover," the team announced, "the effect was reversible and repeatable, many times over."


Pure vanadium oxide is electrically insulating at room temperature, but "transitions to a metallic, electrically conductive state" at slightly higher temperatures, the team explained.

Certain optical properties also change during that process, allowing for changes in some temperature-dependent effects, including infrared camouflage, the scientists said.

In other words, rather than our everyday experience of an object glowing brighter as it heats up, the glow can be manipulated independently of the internal heat. Not only that, but the team learned how to tweak the material to trigger the transition effect at different temperatures.

Federico Capasso Shiram Ramanathan
Photos: Harvard SEAS

Principal investigator Federico Capasso (left) says the coating could provide "a new type of thermal camouflage." Co-investigator Shriram Ramanathan is credited with developing a pure, stable sample of the material, known as thin-film vanadium oxide.

The tipping point at which the structural and electronic phase transition occurs naturally is about 70°C, the team said in its research, published Monday (Oct. 21) in the journal Physical Review X.

The effect is so pronounced that the team's sample emitted "half of the thermal radiation at 100°C that it does at 75°C—a remarkable contrast to the behavior of conventional thermal emitters," the article says.

From Camouflage to 'Encrypted Beacon'

The finding holds the potential for a new class of engineered materials for a range of military and everyday applications, including infrared camouflage, tagging and labeling as well as "smart" thermal devices that keep in heat when cold.

With small adjustments, the coating could be used as "a new type of thermal camouflage or as a kind of encrypted beacon to allow soldiers to covertly communicate their locations in the field," according to principal investigator Federico Capasso, Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering at Harvard SEAS.

TARDEC Ultra Light Vehicle

A military vehicle vehicle coated in thin-film vanadium oxide tiles could potentially mimic its environment like a chameleon, appearing invisible to an infrared camera with only slight adjustments to the tiles' temperature.

A critical piece of the puzzle was solved by coauthor Shriram Ramanathan, whose work in materials synthesis allowed the development of extremely pure—and very stable—samples of thin-film vanadium oxide.

The material's temperature-transition quirk has been known since 1959, but the material has been difficult to work with, typically cracking and shattering in the process, Harvard SEAS reports.

Tuning, Tweaking and Doping

Not only can the team now work with the material, but it has figured out how to modify it for various purposes.

"[W]e now know that if we introduce small changes to the material, we can dramatically change the optical phenomena we observe,” explains lead author Mikhail Kats, a graduate student in Capasso's group.

“By introducing impurities or defects in a controlled way via processes known as doping, modifying, or straining the material, it is possible to create a wide range of interesting, important, and predictable behaviors.”

By doping vanadium oxide with tungsten, for example, the transition temperature can be brought down to room temperature.

The researchers say a vehicle coated in vanadium oxide tiles could potentially mimic its environment like a chameleon, appearing invisible to an infrared camera with only very slight adjustments to the tiles’ actual temperature—a far more efficient system than the approaches in use today.

New coating intrinsically conceals its own temperature
Courtesy of Mikhail Kats / Harvard SEAS

A new coating intrinsically conceals its own temperature to thermal cameras. Researchers foresee a range of applications.

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

It even suggests that the material could be "engineered to operate at specific wavelengths, enabling simultaneous use by many individually identifiable soldiers."

And, because thermal radiation carries heat, the researchers believe a similar effect could be employed to deliberately speed up or slow down the cooling of structures ranging from houses to satellites.



Tagged categories: Coating properties; Military; Research; Temperature

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