Preparation And Performance Of Colored Porous Composites Materials For Passive Daytime Radiative Cooling

With rapid economic development,the urban heat island effect and the greenhouse effect are becoming increasingly serious,especially in the hot summer months,and the use of electrical facilities such as air conditioners is exacerbating energy consumption.Passive Daytime Radiative Cooling（PDRC）technology can produce cooling effects without additional energy input and has great potential for applications in areas such as personal thermal management and building energy efficiency.PDRC mainly works by reflecting sunlight（0.3～2.5μm）and emitting heat as infrared radiation from an atmospheric window（8～13μm）to outer space and cooling the surface of the object spontaneously.However,the current PDRC material still suffers from poor cooling performance and is mostly white or silver in colour.To address the above problems,this paper prepares daytime radiative cooling materials with excellent cooling effect and aesthetic appearance by a simple phase inversion method,and applies them in the fields of personal thermal management and building energy conservation,with the following main studies:（1）Using polyacrylonitrile（PAN）and modified silica particles as the main raw materials,a daytime radiative cooling porous film was prepared by the phase inversion method,and good cooling effect was achieved without adding a metal reflective layer.The reflectance,mid-infrared emissivity and cooling performance of the porous film were investigated under different process conditions by adjusting the particle size,mass concentration and thickness of the porous film.The solar reflectance and mid-infrared emissivity of the porous films can be controlled by varying the particle size,mass concentration and thickness of the film.The results show that the average reflectance of the porous film can reach 94.22%and the average emissivity can reach 97.75%.Under direct sunlight conditions,the maximum cooling effect can be 5.4℃compared to the ambient temperature inside the test device,and the cooling effect can be 8.9℃and 7.4℃when applied to the car model and the house model.（2）In order to meet the demand for aesthetics and multi-functional applications,this paper adds inorganic pigment particles with near-infrared reflectivity to the porous cooling film to produce a colourful porous film with daytime radiative cooling performance.The combination of pigments,silica and polymers gives the film a high infrared emissivity for daytime cooling.The colour porous film has a maximum average reflectance of 69.9%and an average emissivity of 97.95%in the visible range.Under direct sunlight conditions,the colour porous film can achieve an ambient temperature drop of 2～7.8℃compared to the ambient temperature inside the box.When the coloured porous film was covered over the kiosk model,the yellow,red and blue refrigeration porous films were able to reduce the surface temperature of the kiosk by4.2℃,3.4℃and 2℃.（3）To further improve the cooling performance of the coloured porous film,a coloured radiative cooling bilayer film consisting of a thin colouring layer on top and a thick reflective layer on the bottom was prepared by a two-step method.The thin colouring layer consists of a large number of filled Si O₂ particles and highly reflective inorganic pigments that absorb specific wavelengths to present the desired colour,while the presence of porous structures and silica particles in the bottom reflective layer can significantly enhance backscattering and maximise.The presence of porous structures and silica particles in the underlying reflective layer significantly enhances backscattering and maximises the reflection of sunlight from the coloured layer,thereby increasing the overall reflectivity of the film.The spectral performance of the coloured bilayers was regulated by controlling the polymer concentration of the upper and lower layers,the amount of inorganic pigment particles added and the thickness of the bilayers,resulting in highly reflective coloured films.The results show that the coloured bilayer film has an average reflectance of up to 84%and an average emissivity of up to98%,and is able to achieve an ambient cooling of 6.6～9.2℃under direct sunlight conditions,which can reduce its surface temperature by 2.0～11.1℃compared to commercial films.（4）The coloured cooling films can be used in different areas such as coloured cooling fabrics and cooling laminates,which are applied to polyester fabrics to obtain coloured polyester cooling fabrics.The results show that the coloured polyester cooling fabric can reduce the temperature by 2.1℃compared to the original polyester fabric when applied on the human skin surface.Applying the coloured radiant cooling double-layer membrane to the roof of a model house can reduce the roof surface temperature by 2.1～4℃.To further explore the potential of coloured daytime radiant cooling double-layer membranes in the field of building energy efficiency,energy consumption simulations were conducted on buildings covered with coloured double-layer membranes,and the results showed that buildings covered with double-layer membranes can significantly reduce energy consumption and CO₂ emissions,saving approximately 655 MJ/m² of energy per year in summer nationwide.