Preparation And Properties Of Wood-based Reversible Switching Radiative Thermal Management Materials

As a sustainable biomass material,wood has developed a delicate cellular structure in nature,with hierarchical and complexly arranged hierarchical porous structure.The energy consumption for cooling and heating（thermal management）of buildings reaches 70.67 k Wh m^-2each year,resulting in an increasingly serious energy crisis.In response to the energy crisis,radiative cooling/daylight harvesting technologies have emerged in recent years,which can effectively reduce（by～19.2%）the energy consumption for thermal management of buildings.Wood has high scattering ratio of sunlight due to its porous structure,and its main component cellulose has low absorption ratio in the whole solar spectrum as well as high radiation ratio in the infrared spectral range.All these features make wood and cellulose appropriate materials for radiative cooling.At the same time,the sufficient pore structure of wood provides the feasibility of winter heating through controllable regulation of light transmittance by heterogeneous composite,and thus achieving effective reduction of building energy consumption,resulting in all-season controlled thermal management.The main components of this thesis can be summarized as follows:（1）Lignin（containing a large number of heat/light absorbing functional groups）in poplar wood was removed through Na Cl O₂ oxidation to obtain a structural wood with enhanced layers and pores with three-dimensional ordered porosity,providing more scattering interfaces for visible light.The structure can be adjusted by an oxidation process to achieve 95.9%reflectance in the solar spectrum and 93%IR emissivity in the long-wave IR band.The as-prepared radiative cooling wood can achieve a radiative cooling effect of 4.5°C and a radiative cooling power of81.4 W m^-2 during daytime in summer.（2）The as-prepared radiative cooling poplar wood has excellent cooling power,but it is difficult to fulfill the demand for heating in winter or cold weather.Therefore,by eliminating the scattering interface by impregnation with phenylethanol（which has the same refractive index as cellulose）,the reflectivity was reduced from 95.9%to 30.4%,and transmittance of 68.4%was achieved,resulting in a transparent daylight harvesting heating wood.A temperature rise of 5.6°C and a winter heating power of 229.5 W m^-2 were achieved.The switch of daylight harvesting wood and radiative cooling wood was achieved by alternate impregnation with anhydrous ethanol and phenylethanol,and the cycling stability capability was demonstrated.（3）The radiative thermal management wood cannot be used in some circumstances due to defects and shape limitation.To solve this problem,regenerated cellulose films were prepared by ionic liquids,and the characteristics of the different optical states of regenerated cellulose within different solvents were explored.Thermal management films with switchable optical properties were prepared,and the switching conditions and mechanisms of the regenerated cellulose films were also investigated.The regenerated cellulose films prepared by this method possess the effect of switching between highly reflective state（reflectivity=78.5%）and highly transmissive state（transparency=98.6%）to achieve temperature modulation of 14°C in summer and temperature rise of 13.9°C in winter.（4）A thermal management gel with critical temperature at 26°C was prepared from methyl cellulose,methyl cellulose,by adjusting the gel temperature of the material by regulating factors（Na Cl and Na₂SO₄）.The gel can achieve the effect of automatically switching between radiative cooling（reflectivity=92.6%and infrared emissivity=92%）and daylight harvesting heating（transmittance=89%）in different environments at high and low temperatures,achieving a radiative cooling effect of 3.9°C below the ambient air temperature in summer and a heating effect of 12.2°C in winter.