Research On The Preparation And Properties Of Radiation Cooling Coatings And Fabrics Based On Silicon Dioxide

Radiative cooling has great potential for energy conservation and environmental protection,but there are strict requirements on the spectral properties of materials to realize radiative cooling,that is,radiative cooling materials should have high reflectivity in the visible to near-infrared band（i.e.,solar band）,where most of the solar irradiation energy is concentrated,to reduce energy absorption by the materials,at the same time,radiative cooling materials should have high emissivity in the mid-infrared band,especially in the atmospheric window,to emit heat to the cold outer space through the window.However,this is difficult to be achieved,which will leads to the fact that radiative cooling materials are usually very expensive and the preparation process is always complicated,so it is not suitable to large-scale promotion.Therefore,it is necessary to develop new material system and preparation process of radiative cooling materials.According to previous studies,silicon dioxide（SiO₂）has good spectral selectivity,and its infrared absorption/emission is mainly concentrated inside the atmospheric window,in addition,SiO₂is a very common substance with wide distribution on earth and low price,so it is an excellent candidate for radiative cooling materials.However,there are many gaps in the specific application form and performance optimization of SiO₂in radiation refrigeration,here in this paper,a series of studies on SiO₂is carried out,radiative cooling coatings and fabrics based on SiO₂is developed,and the microstructure,phase composition,spectral characteristics and other properties of the as-prepared radiative cooling materials are studied by SEM,XRD,FTIR,XPS and so on,also the cooling performance of these material are tested.The main work in this paper are as follows:Using SiO₂as the emissive raw material,radiative coating is prepared by selecting suitable raw materials in the coating through infrared spectroscopy.In this dissertation,liquid silicone is selected as film forming materials,titanium dioxide and aluminum oxide as the fillers,and a double-layer radiative cooling coating is prepared.The two coatings have high reflectivity in the solar band and high emissivity in the mid-infrared band,respectively,thus endowing the coatings with radiative cooling performance.The coating is a mechanical mixture of the raw materials,and there is no chemical reaction between raw materials.The coating has a high reflectivity in the solar band,with the maximum value of 0.9 at a wavelength of about 0.42μm,and a high emissivity in the mid-infrared band,where the emissivity in the atmospheric window exceeds 0.9,so excellent radiative cooling effect is achieved.The on-site temperature measurement shows that this double layer radiative cooling coating can drop down the temperature of up to 14.1°C compared with the ambient temperature.Using common commercial fabric（e.g.polyester fabric）as raw material,and tetramethyl orthosilicate（TEOS）as the silicon source,a preparation process of in-situ growth is developed,by which TEOS can be hydrolyzed into SiO₂nanoshperes and the common fabric can be prepared into a fabric with radiative cooling ability,By controlling the reaction conditions,the morphology and particle size of sio₂microspheres can be adjusted,and then the cooling effect of the fabric can be controlled.The SiO₂microparticles in this fabric can adhere to the fibers firmly and is not easy to fall off,thus solving the problem of powder loss caused by ordinary impregnation modification of ordinary fabric.The PET@SiO₂fabric is characterized and it can be found that the microstructure and cooling performance of the fabric would vary with TEOS and NH₃·H₂O,with a maximum temperature drop of 13.5°C comparing with the ambient temperature,and its weighted average emissivity in the solar band is 0.7,while the weighted average emissivity in the atmospheric window is 0.96.Radiative cooling fabrics is prepared by electrospinning using PVDF-HFP and tetramethyl orthosilicate as raw materials and the cooling fabric samples is denoted as PVDF-HFP@SiO₂fabric.The basic preparation process of this method is to pre-hydrolyze tetramethyl orthosilicate under acidic conditions to form a sol solution of SiO₂precursor,and dissolve PVDF-HFP in DMF to make a transparent solution,then the SiO₂precursor solution is added to the PVDF-HFP solution and stirred evenly,obtaining the electrospinning solution,and the elctrospinning solution can be spun into PVDF-HFP@SiO₂ fabrics.The electrospinning solution prepared by this method can keep uniform for a long period,which is helpful for long time electrospinning process and avoids the precipitation problem caused by the direct mixing of SiO₂particles into PVDF-HFP solution,which can lead to instability in the electrospinning process.The PVDF-HFP@SiO₂fabrics are composed of PVDF-HFP fibers and SiO₂nanospheres.Changing the adding amount of tetraethyl orthosilicate can change the quantity and morphology of SiO₂microspheres,and then affect the cooling performance of the fabric,and the PVDF-HFP@SiO₂fabric with the best cooling effect can be 18.3°C lower than the ambient temperature under direct sunlight,the weighted average emissivity between the atmospheric window is 0.94,and the weighted average reflectivity at solar band is 0.98,which results in the excellent radiative cooling properties for the PVDF-HFP@SiO₂ fabric.