| Passive daytime radiative cooling technology offers a way to cool objects by reflecting sunlight in the solar spectrum(0.3～2.5 μm)and simultaneously radiating excessive heat through atmospheric transparency window(8～13 μm)to the cold outer space.High solar reflectance and strong mid-infrared emissivity in the atmospheric transparency window are two essentials to guarantee the radiative cooling performance.However,long-time exposure of radiative cooling materials to complex environments such as rain flushing,dust settling,sun burning and bacterial breeding would damage the microstructure of the materials,thereby causing attenuated optical property and degradation of cooling capacity.In addition,the white appearance of radiative cooling materials faced challenges for pursing aesthetic of architectures as well as visual comfort.Based on these,this research aims to fabricate superhydrophobic organic/inorganic radiative cooling composites with porous and rough structure by utilizing high infrared emissivity polymers and inorganic particles or polymers via non-solvent induced phase separation process.The excellent cooling performance was attributed to the synergistic effects of high solar reflectance generated by porous rough structure along with the strong mid-infrared emissivity of high infrared emissivity and hydrophobic polymers simultaneously.The superhydrophobic self-cleaning property plays an important role in defending the materials from contamination,which is favorable for maintaining the cooling performance for a long-term outdoor use.Additionally,the applications of radiative cooling materials are further expanded by incorporation of selective visible absorption pigments to endow the materials with desired colors.The main contents are as follows:(1)Superhydrophobic radiative cooling poly(ethylene-vinyl acetate)/silicon dioxide(EVA/SiO2)composite film was constructed,and the relationships between the morphology and superhydrophobicity as well as the cooling performance of the material were investigated.EVA/SiO2 film was fabricated by firstly mixing high emissive EVA in xylene with hydrophobic SiO2 particles into composite dispersion,followed by blading the dispersion on the substrate and finally immersing into non-solvent ethanol bath to undergo phase separation.The effects of VA content of EVA,EVA concentration,SiO2 content,solvent exchange time and film thickness on the morphology,optical properties,cooling performance and superhydrophobic properties of the composite films were investigated.The effect of superhydrophobicity on the stability of radiative cooling performance of the film was investigated.The effects of acid,alkall,salt solution immersion,UV irradiation and sandpaper abrasion on the superhydrophobicity and optical properties of the films were studied.The results showed that the average solar reflectance of the EVA/SiO2 film reached 96.0%with an average emissivity of 94.0%in the atmospheric transparent window.The obtained film exhibited superhydrophobic self-cleaning property with a water contact angle(CA)of 160.4° and sliding angle(SA)of 2.6°,and realized a sub-ambient temperature drop of 12.0℃ in sealed space and 7.0℃ in the open air.Superhydrophobic self-cleaning properties defended the film from contamination to sustain stable cooling performance.In addition,the obtained composite film showed stable optical and superhydrophobic property after immersion in acid,alkali,salt solution and UV irradiation.(2)The flexible superhydrophobic radiative cooling ethylenepropylene-diene copolymer(EPDM)/SiO2 composite film was established,and the stability of radiative cooling performance of the material was investigated.The porous structure of the materials was regulated by non-solvent tert-butyl alcohol.The effects of the EPDM concentrations,SiO2 content,solvent exchange time and film thickness on the morphology,optical properties,cooling performance and superhydrophobic properties of the films were investigated.The effect of superhydrophobicity on the stability of radiative cooling performance of the film was studied.Chemical and mechanical durability of the superhydrophobicity and cooling performance of the film were investigated.The average solar reflectance and average emissivity of the EPDM/SiO2 film was 96.0%and 94.0%,respectively.The CA of the film reached 162.0° with SA of 1.2°.The synergistic effects of the effective solar reflection and thermal-infrared emission enabled the film to yield sub-ambient temperature drop up to 12.0℃ in a sealed space and 7.0℃ in the open air.The self-cleaning properties helped prevent the film from contamination,maintaining well the radiative cooling performance.Moreover,the film showed stable optical and superhydrophobic property after 7 days immersion in acid,alkali and salt solution for 7 days UV irradiation,and 3 m 1000 mesh sandpaper abrasion.In addition,EPDM/SiO2 film had good flexibility with a breaking elongation of 284%.(3)The polystyrene(PS)/polydimethylsiloxane(PDMS)/poly(αcyanoacrylate)(PECA)superhydrophobic radiative cooling coating was fabricated for scale-up production,and the relationshin between the morphology and superhydrophobic as well as cooling performance of the material was investigated.Superhydrophobic radiative cooling PS/PDMS/PECA coating was fabricated by first adding non-solvent n-propanol into the PS/PDMS/ECA solution to obtain PS/PDMS/ECA precursor solution,followed by spraying it onto different ECA-coated substrate to undergo phase separation based on the principle of non-solvent induced phase separation.The effects of the proportion of polymer,ECA concentration,the amount of nonsolvent,the thickness of the coating and polymer concentration on the morphology,optical properties,cooling performance and superhydrophobic properties of the composite coating were studied.The effect of superhydrophobic properties on stability of the radiative cooling coating was investigated.The effects of sandpaper abrasion,sand impact and water impact on the wettability and optical properties of the coating were studied.The results showed that the average solar reflectance of the PS/PDMS/PECA coating reached 96.0%with an average emissivity of 95.0%in the atmospheric transparent window.The obtained coating exhibited a water contact angle of 160.00 with sliding angle of 2.0°.The coating sprayed on glass,wood and cotton fabric achieved a temperature drop of 12.9℃,11.2℃ and 5.9℃ in a sealed space,as well as 7.0℃,7.4℃ and 6.6℃ in the open air.The self-cleaning properties of the coating prevent it from pollution to maintain steady cooling performance.The coating maintained optical and superhydrophobicity unchanged after immersion in acid,alkali and salt solution for 7 days,UV irradiation for 7 days,1000 mesh sandpaper abrasion for 2.5 m,200 g water flow impaction for 30 times and 20 g sand grains impaction for 7 times.In addition,the PS/PDMS/PECA-coated house produced a sub-ambient temperature drop of 7.2℃,which made it simple to implement large-scale production.(4)Colored superhydrophobic radiative cooling PS/PDMS/PECA coating was constructed,and the relationship between the morphology,chroma,optical performance as well as superhydrophobic properties of the material was investigated.In order to reduce visual discomfort and enhance aesthetic enjoyments of radiative cooling materials,colored superhydrophobic radiative cooling PS/PDMS/PECA coating was obtained by introducing selectively absorbing visible light pigments into PS/PDMS/PECA system via spraying method.The effects of the amount of non-solvent,the concentration of ECA,the thickness of the coating and the concentration of pigment on the morphology,optical properties,cooling performance and superhydrophobic properties of the composite coating were investigated.The effect of superhydrophobicity on the stability of the radiative cooling capacity of the coating was studied.The effect of external damage on superhydrophobicity and optical properties of the coating was studied.The results showed that the solar reflectance of yellow,red and green PS/PDMS/PECA coatings are 92.8%,89.8%and 86.6%with the average emissivity of 95.4%,95.3%and 96.3%.The obtained colored coating exhibited a water contact angle greater than 150.0°.The yellow,red and green coating achieved a sub-ambient temperature drop of 5.3℃,3.5℃ and 2.5℃ respectively in a sealed space,as well as 3.2℃,2.3℃ and 2.2℃in an open air.The superhydrophobicity endowed the coating with stable cooling performance.The colored coating showed stable optical and superhydrophobic property after immersion in acid,alkali or salt solution for 7 days,UV lamp irradiation for one week,1000 mesh sandpaper abrasion for 2.5 m,200 g water flow impaction for 30 times and 20 g sand grains impaction for 6 times.In summary,the composites integrating superhydrophobicity and radiative cooling functions together into one material were prepared by utilizing the high thermal emissivity and hydrophobic polymers and nanoparticles,which combined with the non-solvent induced phase separation characteristic of the polymers to make micro/nano porous structure of the material,thereby greatly enhanced the solar reflectance and thermal emittance of the materials.The superhydrophobic self-cleaning property played an important role in defending the surface from contamination,which is favorable for sustaining the optical properties and cooling performance of the composite materials.This study provided a new scientific approach for the development of long-time radiative cooling materials.At the same time,the construction method of colored superhydrophobic radiative cooling materials has been established,which improved the applicability of the radiative cooling materials and might promote the cooling materials towards large-area production and practical application. |
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