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members of Mikhail Kats’ UW-Madison research team
December 17, 2019

Optical illusion: New coating looks cool, even when it’s not

Written By: Renee Meiller

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.

thermal imaged Kats group
Taken with a long-wave infrared camera that’s sensitive to thermal radiation emitted by room-temperature objects, this image of co-authors from Kats’ group shows distinct color variations across areas that are warmer (faces and bodies) vs. cooler (the table). Image courtesy of the Kats group.

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,” says 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 degrees Celsius. 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.

Kats, his group members, and their collaborators at UW-Madison, Purdue, Harvard, MIT and Brookhaven National Laboratory published details of the advance in the Dec. 17, 2019, issue of the journal PNAS (the Proceedings of the National Academy of Sciences).

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, says Alireza Shahsafi, a PhD student in Kats’ lab and one of the lead authors of the study. “We can imagine a future where infrared imaging is much more common, negatively impacting personal privacy,” he says. “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.”

Infrared images
Infrared images that show how temperature differences in conventional materials (top three rows) are hidden by the special coatings in this work (bottom two rows). Image courtesy of Patrick Roney, Alireza Shahsafi and Mikhail Kats.

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.

A team effort was critical to the project’s success. 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.

And Shahsafi and Patrick Roney (whose employer, Sandia National Laboratory, funded his master’s degree under Kats) 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 says that this long list of contributors reflects his collaborative and inclusive approach to not only advancing technology, but to developing future scientific leaders.

Kats is the Dugald C. Jackson Faculty Scholar in Electrical and Computer Engineering at UW-Madison. In addition to Shahsafi and Roney, UW-Madison authors on the PNAS paper include Yuzhe Xiao, Chenghao Wan, Raymond Wambold, Jad Salman and Zhaoning Yu. In addition to Ramanathan, other authors on the paper include You Zhou of Harvard University, Zhen Zhang of Purdue University, Jiarui Li and Riccardo Comin of MIT, and Jerzy Sadowski of Brookhaven National Laboratory.

Grants from the Office of Naval Research (N00014-16-1-2556) and the National Science Foundation (ECCS-1750341) supported the UW-Madison portion of this research.