Controlling the Direction of Thermal Radiation Using a Micro-Optical Structure

2023-07-03 Research

As thermal radiation occurs evenly in all directions, steering the thermal radiation in a specific direction has been regarded as a great technical challenge

Professor Sun-Kyung Kim's research team at the Department of Applied Physics attempted to solve the challenge based on the understanding of micro-optical multidimensional oxide films and molecular vibration characteristics of emissivity-changing materials. The research team controlled the emission angle of thermal radiation through a hollow micro-optical structure surrounded by a typical oxide silica and alumina shell layer. The research results were published online on May 18 in ACS Nano, an international academic journal in the field of nanoscience published by the American Chemical Society. Student Jin-Woo Cho of the Department of Applied Physics participated as the first author.

Designing a heat radiation structure that disappears from the front and is visible from the side··· Applicable as military camouflage technology
In our daily lives, we are affected by thermal radiation emitted from objects around us. When using a smartphone, for instance, the radiant light emitted from the smartphone screen is absorbed by our face, which may impair the user's thermal comfort. Using this as a point of departure, the research team began their research.

Most research on thermal radiation focuses on wavelength-selective emission that controls transmission and reflection in a specific wavelength band, but Professor Kim's research team selected angle-selective thermal emission. This method requires physically more complex concepts and design techniques than wavelength control. The research team designed and fabricated a micro-optical structure coated with oxide with a thickness of 100 nm (nanometer, 1 billionth of a meter), and observed a phenomenon in which front radiation was significantly reduced while side radiation was increased.

Professor Kim said, “Since the micro-optical structure we designed emits thermal radiation energy in the lateral direction, the structure may appear or disappear depending on the viewing angle of the thermal imaging camera. A new concept of ‘thermal radiation camouflage technology’ through angular thermal emission has now been demonstrated, which has potential applications in the military. In addition, building an angular thermal radiation structure into the screen of a smartphone, for instance, can improve the user thermal comfort by radiating the heat from the device sideways away from the user while holding it close to the face.”

Thermal comfort experiments. (a) Schematic of the experimental setup. (b) Temporal change of ΔT values (upper panel) and infrared thermography images (lower panel).

Follow-up research on using the technology to cool vehicles and buildings
To demonstrate the practical value of the technology, the research team conducted experiments on thermal radiation camouflage and thermal comfort. In the thermal radiation camouflage experiment, a blackbody (an ideal object that completely absorbs light and radiates heat in return in all directions) and a micro-optical structure designed and manufactured by the research team were photographed with a thermal imaging camera while rotating from a vertical angle. In the experiment, the blackbody was clearly visible at all angles, but the micro-optical structure disappeared at the vertical angle (rotation angle 0 degrees) and was only clearly observable when the rotation angle was 60 degrees or more. In other words, the micro-optical structure thermally disappears from the front and is visible only from the side.

The thermal comfort test was conducted by comparing the surface temperatures of objects that shaped-like human faces. When a blackbody and a micro-optical structure were placed in front of the facial feature, the latter lowered the surface temperature of the faces by more than three degrees Celsius.

This principle can lead to radiative cooling, which is attracting much attention as an alternative cooling method that does not require additional energy, as the global energy demand for cooling is rapidly increasing due to climate change. Radiant cooling can dissipate heat and lower the temperature without external power and can be manufactured in the form of an ultralight microminiature cooling system.

The research team plans to pursue commercial applications and opportunities through follow-up research. Professor Kim said, “Manufacturing the heat radiation structure developed in this research would require a high degree of advanced semiconductor process technology. Therefore, it is not easy to apply it to large structures such as large TVs, cars, and buildings. Through follow-up research, if a selective emission structure can be fabricated in large scale using micro-printing technology, it will become a key component in cooling cars or buildings.”

This research was conducted with the support from the Nano Material Technology Development Project (Future Lab), which is managed by the Ministry of Science and ICT and the National Research Foundation of Korea.

※ View related information
Nano Photonics Lab
Professor Sun-Kyung Kim

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