Super‐Large‐Scale Hierarchically Porous Films Based on Self‐Assembled Eye‐Like Air Pores for High‐Performance Daytime Radiative Cooling

• Published in: Small (Weinheim an der Bergstrasse, Germany) Germany), 2022-12, Vol.18 (51), p.e2205091-n/a
• Main Authors: Tian, Qing, Tu, Xitao, Yang, Liu, Liu, Haibo, Zhou, Yubo, Xing, Yuxin, Chen, Zhe, Fan, Shanhui, Evans, Julian, He, Sailing
• Format: Article
• Language: English
• Subjects: Absorptivity; Air pores; Air temperature; Chemical etching; Cooling; Cushions; Daytime; daytime radiative cooling; Electromagnetic wave emission; Emissivity; Etching; Eye-like air pore; eye-like air pores; film-stretching; Hemispheres; Hierarchically porous; Hierarchically porous film; hierarchically porous films; Large-scales; Nanotechnology; Optical properties; Polymethyl methacrylate; Polymethyl methacrylates; Porous film; Radiative Cooling; super large scale; Thermodynamic properties; Tropical environments
• ISSN: 1613-6810; 1613-6829; 1613-6829
• DOI: 10.1002/smll.202205091

Metal‐free polymer daytime radiative cooling coatings with hierarchical eye‐like air pores are proposed and fabricated with a super‐large‐scale film‐stretching method. The hierarchically porous film (HPF) can be further coated with polymethyl methacrylate (PMMA) micro‐hemispheres, forming coated HPF (cHPF), which do not dramatically change the optical or thermal properties. The cHPF is slightly better with a lower solar absorptivity (2.4%) and a higher thermal emissivity over the atmospheric transparency window (90.1%). The low solar absorptivity is due to the strong scattering of the hierarchical eye‐like air pores, while the molecular vibrations and the focusing effect of the PMMA micro‐hemispheres contribute to the high emissivity. An average mid‐day temperature reduction of 7.92 °C is achieved relative to the air temperature, and the average cooling power reaches 116.0 W m−2, which are much better than the cooling performances of the commercial cooling cushion. During the day, the cHPF‐covered simulated building is up to 6.47 and 4.84 °C cooler than the ambient and the white painted counterpart, respectively. The film is durable and resistant to chemical etching, and very promising to use globally, especially in warm and tropical regions. Metal‐free polymer daytime radiative cooling coatings with hierarchical eye‐like air pores are proposed and fabricated first with a super‐large‐scale and high‐throughput film‐stretching method. Low solar absorptivity and high thermal emissivity are achieved, enabling excellent daytime radiative cooling performances. The coatings have outstanding durability and corrosion resistance, and are very promising to use globally, especially in warm and tropical regions.