Silicon wafers coated with the gradient ENZ materials viewed through a thermal imaging camera. Image: Raman Laboratory/UCLA.
Silicon wafers coated with the gradient ENZ materials viewed through a thermal imaging camera. Image: Raman Laboratory/UCLA.

Materials scientists at the University of California, Los Angeles (UCLA) have developed a class of optical material that controls how heat radiation is directed from an object. Similar to the way overlapping blinds direct the angle of visible light coming through a window, their breakthrough involves utilizing a special class of materials that can manipulate the emission of thermal radiation.

This advance, reported in a paper in Science, could be used to improve the efficiency of energy-conversion systems and lead to more effective sensing and detection technologies.

"Our goal was to show that we could effectively beam thermal radiation – the heat all objects emanate as electromagnetic waves – over broad wavelengths to the same direction," said study leader Aaswath Raman, an assistant professor of materials science and engineering at the UCLA Samueli School of Engineering.

"This advance offers new capabilities for a range of technologies that depend on the ability to control the flows of heat in the form of thermal radiation. This includes imaging and sensing applications that rely on thermal sources or detecting them, as well as energy applications such as solar heating, waste heat recovery and radiative cooling, where restricting the directionality of heat flow can improve performance."

Every object emits heat as light, a phenomenon known as thermal radiation. Familiar examples include the filament in a light bulb, glowing coils in a toaster and even the natural light from the Sun. This phenomenon can also be detected on our skin and in common objects – from the clothes you're wearing to the walls around you.

On Earth, from objects at ambient temperatures to modestly hot objects, much of the emitted thermal radiation resides in the infrared part of the spectrum.

Previously, a fundamental challenge had prevented materials from directing their heat in specific directions over a broad spectrum to ensure a sufficient amount of heat is emitted. To solve this puzzle, the researchers created a new theoretical framework using nanophotonic materials. For the first time, the researchers demonstrated that this new class of effective materials allows broad bands of thermal radiation to disperse over predetermined angles.

"To demonstrate this concept, we layered several oxide materials, that each manipulate infrared light over different wavelengths, and beamed much of the emitted heat toward the same fixed angles, " said the study's first author Jin Xu, a UCLA materials science and engineering graduate student. "Additionally, the oxides we used are common, so supplies would not be a problem in the production of the material. "

The class of materials that directs heat is known as 'epsilon-near-zero' (ENZ) materials. The researchers call their new material a gradient ENZ material, as it is made up of layers of different oxide materials, such as silicon dioxide and aluminum oxide. They demonstrated two such material samples that can beam thermal radiation over broad bandwidths to narrow bands of angles – from 60° to 75° and 70° to 85° respectively.

These radiation angles could be seen with a thermal imaging camera when looking at silicon discs coated in the gradient ENZ materials. Viewed from most angles, the heated discs appeared to be cold, similar to how polished metals such as aluminum look under a thermal camera. However, when viewed at the designed specific angles, the higher heat signatures could be spotted on the discs.

This story is adapted from material from UCLA, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.