Researchers develop coating that uses thermal trickery for detection
An ultrathin coating developed by University of Wisconsin-Madison engineers upends a ubiquitous physics phenomenon of materials related to thermal radiation: The hotter an object gets, the brighter it glows.
The new coating—engineered from samarium nickel oxide, a unique tunable material—employs a bit of temperature trickery. “This is the first time temperature and thermal light emission have been decoupled in a solid object. We built a coating that ‘breaks’ the relationship between temperature and thermal radiation in a very particular way,” said Mikhail Kats, an associate professor of electrical and computer engineering at UW-Madison. “Essentially, there is a temperature range within which the power of the thermal radiation emitted by our coating stays the same.”
Currently, that temperature range is fairly small—between approximately 105 and 135 °C. With further development, however, Kats says the coating could have applications in heat transfer, camouflage and, as infrared cameras become widely available to consumers, even in clothing to protect people’s personal privacy.
The coating itself emits a fixed amount of thermal radiation regardless of its temperature. That’s because its emissivity — the degree to which a given material will emit light at a given temperature — actually goes down with temperature and cancels out its intrinsic blackbody radiation, said Alireza Shahsafi, a PhD student in Kats’ lab. “We can imagine a future where infrared imaging is much more common, negatively impacting personal privacy. If we could cover the outside of clothing or even a vehicle with a coating of this type, an infrared camera would have a harder time distinguishing what is underneath. View it as an infrared privacy shield. The effect relies on changes in the optical properties of our coating due to a change in temperature. Thus, the thermal radiation of the surface is dramatically changed and can confuse an infrared camera.
In the lab, he and fellow members of Kats’ group demonstrated the coating’s efficacy. They suspended two samples — a coated piece of sapphire and a reference piece with no coating — from a heater so that part of each sample was touching the heater and the rest was suspended in much cooler air. When they viewed each sample with an infrared camera, they 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.
Purdue University collaborator Shriram Ramanathan’s group synthesized the samarium nickel oxide and performed detailed materials characterization. Colleagues at MIT and at Brookhaven National Laboratory used synchrotron analysis to study the coating’s atomic-level behavior.
Shahsafi and Patrick Roney led the experimental work, which also led Kats’ postdoctoral researcher Yuzhe Xiao to author additional papers describing their very precise measurement techniques. Several other students in Kats’ group characterized the coating through microscopy and other methods. Kats said this long list of contributors reflects his collaborative and inclusive approach to not only advancing technology, but to developing future scientific leaders.