| Daytime radiative cooling is a new cooling technology that realizes energy-free cooling of objects under direct sunlight conditions.An ideal daytime radiative cooling material is expected to have low absorption in the solar radiation range,high emission in the atmospheric window,and low non-radiative heat transfer properties.Therefore,by combining the respective advantages of selective radiators and thermally insulating materials,new-type daytime radiative cooling materials with high cooling power are expected to be obtained.Aerogels are a class of nanoscale porous solid materials formed by the sol-gel method and by replacing the liquid in the gel with air by certain drying methods.Aerogels usually have low thermal conductivity to significantly suppress the non-radiative heat gain of the external environment,and can achieve efficient reflection and low absorption of the solar spectrum through Mie’s scattering,showing unique advantages in the field of daytime radiative cooling.Cellulose nanofiber aerogels（CNFA）are formed by the assembly of cellulose nanofibers（CNF）,which have the advantages of low cost,simple preparation,and biodegradability.However,at present,CNFA as daytime radiative cooling materials face bottlenecks of insufficient mechanical properties,hard regulation of pore structures,poor stability due to the abundance of hydrophilic groups,and easy combustion due to high carbon/hydrogen contents,which greatly limit their application in the field of daytime radiative cooling.In this thesis,a series of new-type CNF composite aerogels with good mechanical strength,thermal insulation/flame retardant properties and hydrophobic properties are designed and prepared,and the daytime radiative cooling performance of the prepared CNF composite aerogel is investigated.The main findings of this thesis are as follows:（1）The MTMS cross-linked CNF composite aerogels with good mechanical elasticity and thermal-insulating properties were prepared by using CNF and montmorillonite（MMT）as the building blocks,using unidirectional freeze casting and chemical cross-linking processes of methyltrimethoxysilane（MTMS）.The resultant M-CNFA has a unique oriented pore structure,i.e.,a distinct honeycombed pore structure in the radial direction and a highly oriented pore structure in the axial direction.By adjusting the mass fraction of MMT and the chemical cross-linking time of MTMS,the M-CNFA has good compressional resilience in the radial direction and can withstand 90%of the compressional strain,and its mechanical properties basically have no decay after 105 cycles of compressional resilience.The theoretical cooling power is 79.5 W m-2 under the daytime condition,which can achieve a cooling of 6-10 oC below the ambient temperature under direct sunlight conditions.（2）Silanized CNF aerogels（S-CNFA）were obtained by freeze-assisted 3D printing and freeze-drying processes using the aqueous dispersions of CNF and polymethylsilsesquioxane（PMSQ）as functional inks.Compared with the pure CNF ink,the apparent viscosity and shear thinning properties of the composite ink were enhanced due to the formation of chemical cross-linkage between the CNF and PMSQ,and the custom structured S-CNFA samples could be obtained by direct-ink-write 3D printing.The prepared S-CNFA has good compressive resilience properties compared with the CNFA,which has a compressive resilience even under the strain larger than 50%.Due to the introduction of an oriented porous structure with high solar scattering by the freeze-casting process and the introduction of functional groups with high emission in the atmospheric transparent window by the chemical cross-linking of PMSQ,the S-CNFA has an average reflectance of>90.0%in the solar spectral range and an average emission of>95.0%in the mid-infrared atmospheric window.The S-CNFA shows the theoretical daytime cooling power of 72.0 W m-2.The cooling results show that the S-CNFA can be cooled by 5-6 oC below the ambient temperature under direct sunlight conditions. |
PDF Full Text Request