Efficient Thin Polymer Coating as a Selective Thermal Emitter for Passive Daytime Radiative Cooling.

Area of Science:

• Materials Science
• Nanotechnology
• Thermodynamics

Background:

• Passive daytime radiative cooling (PDRC) offers a sustainable method for subambient temperature control.
• Current PDRC technologies often rely on complex and costly nanoengineered structures.
• Spectral selectivity in the 8-13 μm atmospheric transparency window is crucial for efficient radiative cooling.

Purpose of the Study:

• To demonstrate a simple, cost-effective, and highly selective radiative cooling emitter.
• To investigate the potential of polysilazane-derived materials for PDRC applications.
• To achieve significant subambient cooling under direct sunlight using a planar polymer structure.

Main Methods:

• Fabrication of a thin (5 μm) polysilazane-derived silicon oxycarbonitride coating via dip-coating.
• Characterization of the spectral emissivity of the coating, particularly within the 8-13 μm window.
• Integration of the coating with a silver mirror to enhance solar reflectance.
• Experimental measurement of temperature reduction and cooling power under direct sunlight.

Main Results:

• The polysilazane coating exhibited a high emissivity of 0.86 in the 8-13 μm atmospheric transparency window.
• The structure suppressed atmospheric heat absorption due to low emissivity outside the target spectral range.
• The complete structure reflected 97% of incoming solar irradiation.
• A temperature reduction of 6.8 °C below ambient temperature was achieved, with a cooling power of 93.7 W m^-2.

Conclusions:

• Polysilazane-derived materials offer a simple and effective solution for passive daytime radiative cooling.
• The demonstrated planar polymer emitter exhibits high spectral selectivity and durability.
• This approach presents a promising pathway for scalable and efficient radiative cooling technologies.