Preparation And Properties Of Porous PVDF-based Highefficiency Daytime Radiant Cooling Materials

Radiation cooling（RC）technology has been frequently mentioned as a green and low-carbon cooling method,especially in the global focus on carbon emission reduction.It can reduce the temperature of an object without consuming any energy,heat is sent to outer space,and greenhouse gases are not emitted throughout the process.Compared with traditional refrigeration systems,environmentally friendly,zero-energy-consumption radiation cooling devices have achieved initial results in thermal management.Efficient radiation cooling devices need to meet both high reflectivity in the solar band and high infrared emissivity in the atmospheric band,which requires good material properties and device structure to be beneficial to reflection or emission energy.Poly（vinylidene fluoride）（PVDF）shows good radiation characteristics in the mid-infrared band and excellent weather resistance outdoors.Given this,it is proposed to select it as the substrate of radiation cooling devices.PVDF porous films were prepared by phase inversion method and electrospinning method,respectively.Firstly,the morphology difference,crystal form difference,radiation cooling performance and self-cleaning property of the phase inversion film under different conditions were studied.It was proposed that the desired porous structure was obtained by instantaneous film formation at room temperature,and the dense structure was obtained under vacuum.Subsequently,the morphology,radiation cooling performance and application range of the films obtained at room temperature with different casting solution ratios were prepared and investigated.It was found that the properties of the materials prepared when the polymer/good solvent/non-solvent ratio was 10:87:3 were excellent.Secondly,in order to further improve the solar band reflectivity of PVDF phase conversion film,inorganic particles with high refractive index were designed.After using silane coupling agent to modify the agglomeration of particles,the differences of phase inversion films of physical blending silica（Si O₂）and titanium dioxide（Ti O₂）were compared respectively.The results show that the film obtained by adding Ti O₂ has high porosity,excellent self-cleaning property and is expected to be used as piezoelectric material to expand the scope of application.By changing the content of inorganic particles and the thickness of the film,the effects of inorganic particles on the solar band reflectivity and atmospheric band emissivity were explored,and a series of composite porous films with good performance were obtained.It was found that the reflectivity and emissivity were positively correlated with the amount of Ti O₂ added and the thickness of the film,and the crystallinity was just the opposite.It is speculated that the casting solution of 2%Ti O₂ can be comparable to the same type of material when the film thickness reaches 300μm,which will be used outdoors for a long time and obtain good dielectric properties.Finally,starting from the controllable regulation of the reflectivity and emissivity of the radiation energy through the medium transition in the micro-nano structure,the nano-scale PVDF fiber membrane was constructed by electrospinning.The effects of spinning process（spinning voltage,solvent ratio,polymer concentration）on the properties of radiation cooling materials were investigated.The contribution of fiber structure and molecular crystal form in optimizing device performance was analyzed.It was found that when the filamentous structure increased,it was effective to improve the reflectivity and emissivity.The increase in the average diameter of the fiber meant moreβ-phase PVDF formation.The fiber membrane obtained by spinning the spinning solution with a solvent ratio of 8:2and a PVDF content of 15%at 35 k V is optimal.In terms of comprehensive performance,the electrospinning film is better and shows broad application prospects.