Preparation And Properties Of Flexible Films For Daytime Radiation Cooling

Radiant cooling technology can efficiently use outer space as a renewable and stable energy source to dissipate heat without consuming energy,which is the most promising method to alleviate current global warming,energy crisis and urban heat island effect.In view of the shortcomings of photonic crystal and metamaterial coolers with the best performance of radiation refrigeration,such as complex structure,high manufacturing difficulty,high cost and difficult large-scale production,a simple and effective preparation strategy is proposed in this paper.High performance flexible composite films of radiation refrigeration are prepared by electrostatic spinning process which has the potential of textile industrial production.The selective spectral response and low-cost preparation of radiation cooling materials are realized,which has the potential for scalable industrialization.The research idea of this paper can further expand the outdoor application range of radiant cooling flexible film,which is of great significance in energy saving and emission reduction and mitigating urban heat island effect.The main contents of this paper are as follows:（1）Preparation of PVDF-HFP/Al₂O₃ flexible composite film:PVDF-HFP/Al₂O₃ flexible composite films were prepared by electrospinning technology using PVDF-HFP and Al₂O₃ as raw materials,and the effects of Al₂O₃ content,PVDF-HFP content and sample film thickness on the radiation refrigeration performance of the composite films were investigated.The results show that the 0.30 mm thick sample film of P15A6-ES prepared by spinning liquid formula with15 wt%PVDF-HFP content and 6 wt%Al₂O₃ content（relative to PVDF-HFP）has excellent radiation cooling performance.（2）Radiation cooling performance characterization of PVDF-HFP/Al₂O₃ flexible composite film:The micromorphology and element distribution of P15A6-ES sample film were characterized by FESEM and EDS analysis,which proved that the micro and nano fibers doped with Al₂O₃ were successfully prepared and stacked into films.The reflectance,emissivity and group of P15A6-ES sample film were analyzed by UV-VIS-NIR,FTIR and AT-FTIR.The results show that the reflectivity of the film is as high as 0.98 due to the strong scattering of light by microfiber,fiber pore and Al₂O₃.At the same time,due to the strong absorption of C-F bond and C-H bond of PVDF-HFP in the fingerprint region and the phonon polarization resonance of Al₂O₃,the emissivity of the membrane is 0.932.The surface contact Angle of P15A6-ES was 140°,the tensile strength was 4.8 Mpa,and the self-cleaning ability was good when analyzed by CAA,two-arm universal material testing machine and simple decontamination test method.The outdoor radiation cooling performance of P15A6-ES film was analyzed by a self-made device,which showed that the film showed good cooling performance under different weather conditions.In ideal conditions,the net radiative cooling power is calculated to be 138.5 W/m²,and it has the potential for application in building and automotive cooling.（3）Preparation of TPU/TiO₂ flexible composite film:Using TPU and TiO₂ as raw materials,TPU/TiO₂ flexible composite film was prepared by electrostatic spinning technology,and the effects of TiO₂ content,TPU content and sample film thickness on the radiation refrigeration performance of the composite film were studied.The results show that the T20T8-ES 0.40 mm thick sample film prepared by the spinning solution formula with TPU content of 20 wt%and TiO₂ content of 8 wt%（relative to TPU）has excellent radiation refrigeration performance.（4）Radiation refrigeration performance characterization of TPU/TiO₂ flexible composite films:The test results showed that the microscopic morphology and element distribution of T20T8-ES sample films were characterized by FESEM and EDS analysis,proving that TiO₂doped micro-nano fibers were successfully prepared and stacked into each other into films.The reflectance,emissivity and groups of the T20T8-ES sample film were analyzed by UV-VIS-NIR,FTIR and AT-FTIR.The results showed that the reflectance of the film was 0.976,which was attributed to the diffuse reflection effect of microfibers and fiber pores and the high refractive index of TiO₂ to light.At the same time,due to the bending vibration of C-O bond and C-H bond of TPU and the stretching vibration of Ti-O bond in TiO₂ particles,the emissivity of the film is0.931.The tensile strength of the membrane was 3.8 Mpa by double-arm universal material testing machine.The outdoor radiation cooling performance and application potential of T20T8-ES sample films were analyzed with a homemade device.The results show that the film has good cooling performance under different weather conditions,and the ideal net radiation power is134.2 W/m²,and has the application potential of building,intelligent equipment and automobile cooling.