An ultrathin coating engineered from samarium nickel oxide (SmNiO3) has been found to emit the same amount of thermal radiation irrespective of temperature, within a range of about 30 °C. The coating, which was developed by engineers at the University of Wisconsin-Madison and exhibits temperature-independent thermal radiation, could someday be used to control the visibility of objects to infrared (IR) cameras. The thermally emitted power of most solids increases monotonically with temperature in a one-to-one relationship. However, the researchers found that the use of SmNiO3, a quantum material that undergoes a reversible, temperature-driven solid-state phase transition as an ultrathin thermal emitter, alters this one-to-one relationship. This insulator-to-metal phase transition enabled the researchers to engineer the temperature dependence of emissivity to cancel out the intrinsic blackbody profile described by the Stefan-Boltzmann law, for both heating and cooling. Infrared images show how conventional materials (top three rows) appear to an IR camera as they heat up. Special coatings developed by UW-Madison engineers hide the temperature changes of the objects in the bottom two rows. Courtesy of Patrick Roney, Alireza Shahsafi, and Mikhail Kats. “We built a coating that ‘breaks’ the relationship between temperature and thermal radiation in a very particular way,” professor Mikhail Kats said. “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. Kats believes, however, that with further development, the coating could have applications in heat transfer, in camouflage, and, as IR cameras become more widely available to consumers, in clothing to protect people’s personal privacy. Taken with a long-wave infrared camera, this image of researchers in Mikhail Kats' lab shows distinct color variations across areas that are warmer (faces and bodies) and cooler (the table). Courtesy of the Kats group. “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,” researcher Alireza Shahsafi said. The new coating design provides temperature-independent thermally emitted power within the long-wave atmospheric transparency window (wavelengths of 8 to 14 µm), across a temperature range of about 30 °C, centered around 120 °C. Members of Kats’ UW-Madison research team who contributed to this work include postdoctoral scholar Yuzhe Xiao, and graduate students Alireza Shahsafi, Zhaoning (April) Yu, Jad Salman, Chenghao Wan, and Ray Wambold. Courtesy of Renee Meiller. To demonstrate the coating’s efficacy, the researchers suspended two samples — a coated piece of sapphire and a reference piece with no coating — from a heater. A portion of each sample was put in contact with the heater while the remaining portions were suspended in cool air. When the researchers viewed each sample with an IR 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. Kats and his group collaborated with scientists at Purdue University, Harvard University, MIT, and Brookhaven National Laboratory on the research. The research was published in the Proceedings of the National Academy of Sciences (www.doi.org/10.1073/pnas.1911244116).
