A thermal camera demonstrates, through varying shades of color, the intensity of heat emanating from an object.
A new coating developed by engineers from the University of Wisconsin employs some temperature trickery to disguise the thermal changes.
By using a quantum material called samarium nickel oxide, the UW–Madison team is challenging a conventional idea in thermal imaging: The hotter the object, the brighter it glows.
The new coating breaks the relationship between temperature and thermal radiation, says Mikhail Kats, a UW–Madison professor of electrical and computer engineering.
“Essentially, there is a temperature range within which the power of the thermal radiation emitted by our coating stays the same,” said Prof. Kats .
With the help of samarium nickel oxide (SmNiO3), the coating emits the same amount of thermal radiation irrespective of temperature, within a temperature range of about 30 °C (from approximately 105 °C to 135 °C). The team is currently researching way to shift the thermal-shielding to a central temperature of 30°C to 60 °C.
Think of the coating as a potential privacy shield, said Alireza Shahsafi, a doctoral student in Kats’ lab and one of the lead authors of the study.
“The thermal radiation of the surface is dramatically changed and can confuse an infrared camera,” said Shahsafi.
What can be done with a confused infrared camera exactly?
Infrared devices are designed to see objects not visible with the naked eye or regular cameras. If you could cover the outside of clothing or a vehicle with this type of coating, the camera would have a more difficult time determining what is underneath.
To demonstrate the coating’s efficacy, Shahsafi and fellow members of Kats’ group suspended two samples — a coated piece of sapphire and a reference piece with no coating — from a heater, leaving part of the sample suspended in much cooler air. The researchers saw a distinct temperature gradient on the reference sapphire, from deep blue to pink, red, orange and almost white, while the coated sapphire’s thermal image remained largely uniform.
A group at Purdue University led by Shriram Ramanathan synthesized the samarium nickel oxide and performed detailed materials characterization. Colleagues at MIT and at Brookhaven National Laboratory used the bright light of a particle-accelerating synchrotron to study the coating’s atomic-level behavior.
The thermal control provided by the coating can be used to support thermal stability in space, where thermal radiation is the sole mode of heat transfer.
“More generally, our work is about controlling thermal radiation using temperature-responsive materials,” Kats told Tech Briefs. “We believe that similar concepts can be applied to passive control of temperature, for example, to maintain a relatively constant temperature for spacecraft.”
In an edited interview below, Prof. Kats tells Tech Briefs what else is possible when you confuse a thermal camera.
Tech Briefs: What inspired you to work on this kind of thermal coating?
Mikhail Kats: For a while now, I have been interested in engineering of thermal radiation to do unconventional things. Thermal radiation is a very well-known concept — for example, it is the mechanism by which the sun emits light. It is also very important historically: The development of the modern theory that describes thermal radiation was perhaps the first major success of quantum mechanics about a century ago. It is fun to try to find new and interesting phenomena and applications related to concepts that have been well-established so long ago.
Tech Briefs: Why is this kind of thermal camouflage important? What is possible with this kind of thermal invisibility?
Mikhail Kats: Our group has been thinking about the privacy implications of this technology. Infrared imaging is becoming more common and less expensive — one can now buy very decent mid-infrared, or “thermal,” cameras for hundreds of dollars. And that comes with potential privacy implications, since infrared cameras can be used to see things that are not visible with the naked eye or regular cameras.
To be clear, our technology is not about “invisibility” — it does not make objects invisible to infrared cameras. Instead, it masks differences in temperature. So you can imagine coating the side of a vehicle so you can't use an infrared camera to tell where the engine is, or coating the outside of a coat to conceal a phone or insulin pump, or a pair of pants to make it harder to see a prosthetic.
More generally, our work is about controlling thermal radiation using temperature-responsive materials. We believe that similar concepts can be applied to passive control of temperature, for example, to maintain a relatively constant temperature for spacecraft. In space, thermal radiation is the only mode of heat transfer, since there is no air.
Tech Briefs: How is the coating able to block temperature changes?
Mikhail Kats: For any given object, there are two parameters that determine how much thermal radiation is emitted: its temperature, and its propensity to emit light, which is called the “emissivity.” The emissivity in general depends on various material parameters — for example, how many electrons can flow in the material. Our coating is based on a rather exotic material — samarium nickel oxide — which changes dramatically as a function of temperature. We designed the coating in such a way that the emissivity decreases when the temperature increases, so that the natural increase of thermal radiation with increasing temperature is canceled out.
Tech Briefs: What kinds of tests did you do to demonstrate the material’s capabilities?
Mikhail Kats: The most-straightforward experiment we performed involved heating up a coated object asymmetrically (so that one corner was hotter than the other), and imaged it with an infrared camera. The temperature gradient disappeared for the coated object, but was very visible for an uncoated object.
We did a lot of other testing, including detailed materials characterization at a synchrotron, but the infrared imaging was both the simplest and most direct demonstration.
Tech Briefs: What’s most exciting to you about this work?
Mikhail Kats: In addition to the prospective applications, I think the demonstration of temperature-independent thermal radiation is going to make for an excellent textbook problem and for students studying thermal radiation or radiative heat transfer. Honestly, I'm quite excited about that!
Tech Briefs: And what’s next regarding your research?
Mikhail Kats: Well, there are several obvious directions that we must follow before this technology becomes useful for applications. I think the main one is to vary the temperature range of the effect. Right now our demonstration works at temperatures over 100 degrees °C — way too high for most applications. So we're going to look to decrease that operating temperature, and we think we know where to start.
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