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Daytime radiative cooling outdoors is a passive cooling method that emits thermal radiation toward the sky while reflecting sunlight. Many different daytime radiative coolers have been developed, and some have been commercialized. Coatings offer advantages in ease of application and versatility across different surfaces. Typical daytime radiative cooling coatings are mixtures of powders or particles in polymer hosts. As these paintings reflect sunlight diffusively, the coating thicknesses are sub-millimeters or thicker. Thick coatings result in high thermal resistance, which is undesirable for cooling the objects below the coating. To address this problem, boron nitride (BN) powder has been used as a material with high thermal conductivity to reduce thermal resistance. However, the high refractive index of BN in the mid-infrared regions prevents the mid-infrared emissivity of BN-containing coatings from achieving values above 0.9 if the concentration is high. In the current work, we demonstrate a high average emissivity reaching 0.93 and solar reflectance of 0.99 by adding a layer containing silica powder where the silica layer is instrumental in enhancing the emissivity without deteriorating the solar reflectance. The double-layer coatings exhibit subambient outdoor temperatures in Japan. Introducing a silica layer for thermal emission on top of a BN-containing base layer presents a straightforward method to enhance the daytime radiative cooling performance of the BN-containing layer.

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• In Copyright
  

  This document is the Accepted Manuscript version of a Published Work that appeared in final form in ACS Applied Optical Materials, copyright © 2025 American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see https://doi.org/10.1021/acsaom.4c00519

Keyword: Daytime radiative cooling, Boron nitride, Thermal radiation, Double-layer coating

Date published: 2025-03-28

Publisher: American Chemical Society (ACS)

Journal:

• ACS Applied Optical Materials (ISSN: 27719855) vol. 3 issue. 3 p. 720-726

Funding:

• Precursory Research for Embryonic Science and Technology JPMJPR19I2
• Japan Society for the Promotion of Science 22H01917
• Fusion Oriented REsearch for disruptive Science and Technology JPMJFR2139

Manuscript type: Author's version (Accepted manuscript)

MDR DOI: https://doi.org/10.48505/nims.5463

First published URL: https://doi.org/10.1021/acsaom.4c00519

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Updated at: 2026-02-21 16:30:20 +0900

Published on MDR: 2026-02-21 13:38:38 +0900