| Radiative cooling material remains a temperature below that of the ambient air without energy input even under direct sunlight,thus becoming a hot topic in the field of energy and environmental cooling materials.However,it still faces many challenges in terms of potential functionalization,durability and commercialization.On the one hand,the surface of the material is easily contaminated by dust,microorganisms and/or rainwater in outdoor applications,resulting in a significant decrease in its cooling effect.On the other hand,the optical properties of most current radiative cooling systems are static,with fixed solar reflectivity and constant thermal infrared emissivity,thus resulting in over-cooling at low temperatures.This study proposed to endow radiative cooling materials with superhydrophobicity,and utilize the superhydrohobic self-cleaning effect to guarantee the stability of the optical properties of the materials to improve the durability of the radiative cooling function;to combine thermochromic fillers and thermal infrared high emission polymers to construct asymmetric three-dimensional porous superhydrophobic polymer composite with micro/nano pores inside and rough structure at the surface.Utilizing the thermochromic property of the filler to modulate the optical properties of the material to obtain temperature-adaptive radiative cooling functions.At the same time,the low surface energy characteristic and the surface micro-roughness of the material are used to give the superhydrophobic functions.The influences of the microstructure and properties of the material on the superhydrophobicity and radiative cooling functions and the synergistic mechanism were investigated,and the mechanism of spectral modulation and temperature regulation of the materials in response to the air temperature changes was explored.The details are as follows:(1)Fabrication of hierarchically micro/nano-porous superhydrophobic radiative cooling composite coating with embedded inorganic microspheres of phonon-enhanced Frohlich resonance.The superhydrophobic radiative cooling composite coating with inorganic particles was prepared with designed micro-nano porous structure and micro/nano rough surface using poly(methyl methacrylate)(PMMA),poly(vinylidene fluoride-co-hexafluoropropylene)[P(VDF-HFP)]with high infrared emissivity,and hydrophobic microscale silica(HM-SiO2)with midinfrared emission enhanced by phonon-polarization resonance and low surface energy property via solvent-nonsolvent induced phase separation.Investigations were conducted of the effects of the mass ratio of P(VDF-HFP)/PMMA,amount of non-solvent water,amount of HM-SiO2 and thickness on the surface morphology,wettability and optical properties of the composite coating.A temperature testing device was built and the radiative cooling performance of the coating were investigated.The influence of self-cleaning performance on the radiative cooling stability,and stabilities of the radiative cooling performance and superhydrophobicity of the coating were investigated.It was found that the prepared coating has high solar reflectivity and strong thermal infrared emissivity of 97.1%and 96.0%,so that the coatings can achieve an average daytime radiative cooling of 12.5℃.At the same time,the coating has excellent self-cleaning performance with contact angle of 160.1°and sliding angle of 2.8°,which can effectively prevent dust pollution and maintain good radiative cooling performance.In addition,the coating remains stable superhydrophobicity and radiative cooling after mechanical wear,acid and alkali immersion,and UV aging.(2)Fabrication of superhydrophobic radiative cooling all polymer coating with hierarchically m i cro/nano-porous structure inside with synergistic roughening surface.The superhydrophobic radiative cooling P(VDF-HFP)/PDMS composite coating with rich pores inside and rough structures at the surface was fabricated by adding water as non-solvent into the composite solution of P(VDF-HFP)and polydimethylsiloxane(PDMS)to induce microphase separation to form P(VDF-HFP)/PDMS sol,followed by casting the sol onto substrate.The effects of the mass ratio of the composite polymer and microphase separation conditions on the microstructure of the coating and its relationship with superhydrophobicity and radiative cooling properties were investigated;The effects of self-cleaning properties on the stability of the radiative cooling coating were studied,and the effects of the external environment on the stability of the coating superhydrophobicity and radiative cooling were investigated.The results show that the surface contact angle and sliding angle of the P(VDF-HFP)/PDMS coating reach 162.3°and 2.3°,respectively.The solar reflectance and mid-infrared emissivity are 96.5%and 94.3%,respectively.Outdoor tests show that the coating achieves an average daytime radiative cooling of 13.8℃.In addition,the superhydrophobic self-cleaning property of the P(VDF-HFP)/PDMS coating effectively prevents the optical properties of the material from being damaged,and imparts the surface with excellent acid/alkali resistance and UV stability to guarantee durable radiative cooling.(3)Construction of temperature-adaptive superhydrophobic radiative cooling coating and investigation of its dynamic spectral regulation and thermoregulation mechanisms.The temperature-adaptive superhydrophobic radiative cooling coating with tunable optical properties,asymmetric microporous structure and rough surface was constructed by compounding P(VDF-HFP),thermochromic microcapsules(TCMCs)and hydrophobic silica nanoparticles(H-SiO2)in a onestep phase separation method.The relationship between H-SiO2,phase separation conditions and the microstructure of the coating,and their effects on superhydrophobic performance were explored.The relationship between TCMCs content and solar reflectance of the coating were investigated.Further the law and mechanism of dynamic regulation of the optical properties of the coatings in response to air temperature changes,and the mechanism of the temperature regulation were investigated.The effect of self-cleaning performance on the stability of radiative cooling with temperature-adaptivity were studied.The results show that the prepared TRSRC coating can adjust the solar reflectance in response to the ambient temperature change.When the ambient temperature is higher than the critical transition temperature of the TCMCs,the TRSRC coating reflects sunlight to the maximum extent and keeps white with high solar reflectance of 94.6%and strong thermal infrared emissivity of 95.8%for radiative cooling.When the ambient temperature is lower than the critical transition temperature of the TCMCs,the coating shows a dark color with a maximum visible light absorption rate of 53.0%and solar reflectivity of 83.1%for absorbing solar energy and warms up.Outdoor tests show that TRSRC coating can achieve 4.5-7.4℃radiative cooling in hot summer and 2.2-4.6℃ solar heating in cold winter.In addition,the coating has good anti-fouling and self-cleaning properties,which greatly guarantees continuous cooling/heating applications in outdoor environments.In summary,the superhydrophobic self-cleaning radiative cooling coatings with hierarchically micro/nano-porous structures were prepared by using the excellent low surface energy properties and high mid-infrared emission properties of P(VDF-HFP)and PDMS polymers and hydrophobic micro-nanoparticles to construct three-dimensional micro-nano porous structures and apparent micronano rough surface.Temperature-adaptive radiative cooling coating with asymmetric micro-nano porous structures mosaicked with TCMCs was achieved by using the spectral modulation triggered by the free exchange of electrons or the cessation of exchange of electrons between the chromogenic and cryptogenic agents of the TCMCs at high or low temperature.The spectral modulation properties of the TCMCs,together with the surface and internal microscopic porous structures,synergistically help realize the reversible temperature modulation and surface superhydrophobicity,which guarantee the weatherability and service life of the superhydrophobic self-cleaning temperatureadaptive radiative cooling coatings. |
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