Passive daytime radiative cooling (PDRC) radiates heat into the ultracold universe and reflects sunlight simultaneously, offering significant potential for sustainable thermal management, renewable power generation, atmospheric water harvesting, and agricultural preservation. However, current radiative cooling materials cannot achieve multiple forms with the same formula. This results in considerable difficulties during retrofitting or removal operations, and substantially increased construction costs and maintenance burdens. To address this disadvantage, morphologically controllable daytime radiative cooling materials are developed. A dual-band selective spectrum regulation strategy is proposed to simultaneously optimize spectral performance of the developed materials across both solar and infrared bands through systematically characterizing the wavelength-selective properties of functional components.
Material morphology is controlled by adjusting the polydimethylsiloxane (PDMS) content in a solution of polyvinylidene fluoride (PVDF) in acetone. When PDMS-to-acetone mass ratio is 1∶5 or higher, the mixture forms a coating adhering firmly to the substrate upon drying. When the ratio is 1∶6, the adhesion is weak, and the dried paint can be peeled off as a free-standing film. The spectral properties of both coatings and free-standing films are characterized by ultraviolet-visible-near infrared (UV-Vis-NIR) spectrophotometry and Fourier transform infrared (FTIR) spectroscopy.
Metal compounds are also used to significantly reduce the reliance on the high substrate reflectivity and improve the cooling performance. Experiments on cooling performances are carried out by applying the developed coatings and free-standing films onto aluminum plates. By adding 20wt.% aluminum oxide (Al2O3) and 10wt.% polytetrafluoroethylene (PTFE) powder into PDMS-PVDF paint as solar scatters, the solar reflectivity of the coatings on aluminum substrates increases substantially from 0.614 to 0.910. Notably, the addition of PTFE powder reduces the required concentration of metallic solar scatters, thereby lowering the coating density and production costs. For comparison, barium sulfate (BaSO4), titanium dioxide (TiO2), and Alumina (Al2O3) are evaluated as solar scatters by preparing coatings and free-standing films using identical PVDF-PDMS methodologies. The solar reflectivity of the TiO2-based and Al2O3-based coatings on aluminum substrates is 0.909 and 0.910, respectively, while that of the corresponding free-standing films is 0.908 and 0.891, respectively.
The TiO2-based coating exhibits exceptional spectral independence on the underlying substrate due to the characteristically high refractive index of TiO2, enabling saturated optical scattering even at low mass fractions. Conversely, Al2O3 has a comparatively lower refractive index, resulting in incomplete spectral decoupling of the Al2O3-based coating. This is evidenced by the average solar reflectivity of the corresponding Al2O3-based free-standing film being 0.02 lower than that of the Al2O3-based coating on the aluminum plate. Therefore, a higher concentration of Al2O3 scatters is required to enhance the scattering cross section and maximize solar reflectivity. At an optimized concentration of 40wt.%, the Al2O3-based coating achieves a solar reflectivity of 0.93, corresponding to a theoretical cooling power gain of 22%, while the TiO2-based coating maintains a reflectivity of 0.911 at the same concentration. Crucially, the infrared emissivity remains virtually unaffected by the addition of these metal compounds.
Field experiments are conducted on a 40wt.% Al2O3-based coating, a 40wt.% TiO2-based coating, and a 20wt.% Al2O3-based coating/free-standing film to quantify practical cooling performance. When the ambient temperature exceeds 36 ℃, the 40wt.% Al2O3-based coating achieves the highest average temperature reduction of 2.9 ℃, and the 40wt.% TiO2-based coating achieves an average reduction of 2.3 ℃. Temperature reductions for the 20wt.% Al2O3-based coating and its corresponding free-standing film are 2.3 ℃ and 1.9 ℃, respectively.
In conclusion, through tuning solar scatters and infrared-emissive polymer matrices, this work successfully realizes both morphological control and dual-band selective spectrum regulation. The developed material system offers a feasible solution for scalable daytime radiative cooling applications, and has advantages of cost-effectiveness, high thermal performance, and controllable physical morphology adaptable to diverse implementation scenarios.
Key words
daytime radiative cooling /
coating /
film /
dual-band selective spectrum regulation /
flexible morphology /
thermal radiation
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Funding
Science and Technology Project of State Grid Jiangsu Electric Power Co., Ltd (J2025077)
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