A DUAL-LAYER COATING USING NANOPARTICLE-POLYMER HYBRID MATERIALS FOR DAYTIME PASSIVE RADIATIVE COOLING

Radiative cooling takes advantage of cold outer space as an ultimate heat sink to cool objects by spontaneously radiative heat loss in the mid-infrared wavelength range where the atmosphere is highly transparent. This renewable cooling strategy is considered as a sustainable alternative to save energy and reduce the adverse effect on the environment caused by traditional air-conditioning systems. However, it is still challenging to achieve a 24-hour continuous cooling which requires materials for broadband reflection in the solar spectrum from ultraviolet to near-infrared to reduce heat absorption from the sun during daytime operation. Recently, daytime passive radiative cooling has been achieved by designs using dielectric materials, optical structures, metal reflectors, etc. Although effective in optical properties, those designs are costly to fabricate and are difficult for scalable applications. In this work, we present a bi-layer radiative cooling paint (BRCP) using nanoparticle-polymer hybrid materials for daytime passive radiative cooling applications. The bottom layer, doping TiO₂ nanoparticles in PDMS polymer, selectively reflects sunlight from the visible to near-infrared range. An Al₂O₃-nanoparticle-doped PDMS layer is applied atop the bottom layer to enhance ultraviolet reflection. Consequently, the dual-layer coating with optimized thickness and particle concentration attains an overall solar reflection of 92.2% and a mid-infrared emittance (8~13 μm) of 95.3%. With the promising optical performance, a daytime radiative cooling power of 97.17 W/m² is theoretically expected under a clear and dry climate. Overall, the dual-layer coating promises an appealing solution for cooling while offering good applicability and scalability in paint format.