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Anirudh Krishna1 Juyoung Leem2 Michael Cai Wang2 SungWoo Nam2 Jaeho Lee1

1, University of California, Irvine, Irvine, California, United States
2, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States

Radiative cooling is an attractive method of passive thermal management that utilizes spectral radiation characteristics in the ambient environment. While the solar spectrum heats up surface areas facing the sun in the wavelength range of 200 nm to 2500 nm, the atmospheric transmission window allows reemission of the heat (i.e., cooling) from the ambient surface to the outer space in the wavelength range of 8 μm to 14 μm. Because of the distinct spectral ranges allowed for heating and cooling phenomena, conventional approaches relying on a simply high or low emissivity material do not offer optimal solutions for temperature control. By selectively controlling the emissivity spectrum, we can govern the thermal energy exchange in the ambient environment and optimally control the surface temperature without running electricity or any active components such as bulky heat exchangers. A novel surface design can lead to breakthroughs in thermal management, energy harvesting, and thermal imaging, particularly for surface systems where radiative heat transfer is critical to performance and reliability.
Here we show reconfigurable modulations of the surface topography of graphene via mechanical straining-induced crumpling and demonstrate how the crumpled graphene offers an innovative approach to control emissivity and temperature. The crumpled graphene maximizes radiative cooling by keeping the emissivity high in the mid-infrared range while keeping the thermal absorption low in the solar spectrum. The crumpled surface design has been inspired by Saharan silver ants, where a dense array of triangular-shaped hairs is found to be highly emissive in the mid-infrared range, which helps them stay cool in the desert. Our emissivity computation based on the rigorous coupled-wave analysis predicts that the crumpled graphene can keep the emissivity below 0.3 in the range of 100 nm and 6000 nm, which minimizes the solar spectrum heating, while keeping the emissivity above 0.7 in the spectral range beyond 8 μm, which maximizes atmospheric cooling. The reconfigurabiliy of the crumpling effect on emissivity has been demonstrated on a stretchable polymer substrate by characterizing the reflectance spectra before and after stretching the substrate by mechanical forces. Our thermal analysis shows that the optimally crumped graphene will achieve a net radiative cooling power of 110 W/m2 and a surface temperature reduction 10 K below the ambient air. This work shows the straining-induced crumpling (or uncrumpling) in two-dimensional (2D) materials leads to significant changes in the emissivity and enables a novel temperature control mechanism. These findings advance our knowledge of surface topography-driven radiative properties in 2D materials and enable developments of dynamic and self-regulating temperature control systems.

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