Preparation Of Nanocellulose-based Composite Films And Their Radiation Cooling Performance Study

Cooling is essential to human life,especially in hot regions.Population growth and industrialization have exacerbated global warming and made cooling increasingly necessary to provide a comfortable living environment.However,traditional cooling technologies,such as air conditioning,not only consume huge amounts of energy,but also emit greenhouse gases and harmful emissions that cause environmental problems.The ability of radiation cooling to cool objects below ambient temperature without consuming energy makes radiation cooling one of the promising cooling technologies.Radiation cooling materials currently need to address the following issues:high reflectivity(or low absorption)in the 0.4 to 2.5?m band and high infrared emissivity in the8 to 13?m band so that passive radiation cooling can be achieved in direct sunlight.Compared with conventional cooling systems,the zero-energy property enables effective reduction of negative environmental issues.In this thesis,based on the advantages and characteristics of nanocellulose in radiation cooling,CNF-g-Si O₂ transparent composite films and CNF-g-Ba SO₄ composite films were prepared through the functionalization method of nanocellulose and the design of radiation cooling materials,and their radiation cooling performance as well as application scope were also explored,with the following results:(1)Based on the green concept and also to obtain a stable source of nanocellulose,different pretreatment methods were adopted for waste paper,and then prepared into cellulose nanofiber(CNF)solution by TEMPO/Na Br/Na Cl O system and ultrasonic treatment,and various properties(microscopic morphology,particle size distribution,Zeta potential,optical properties and yield,etc.)of the prepared CNF were investigated,while using the CNF prepared from bleached paddleboard as a control.All samples were successfully prepared by scanning electron microscopy and transmission electron microscopy with good dispersion stability(Zeta potential less than-29.58 m V),light transmission(higher than 86.5%),and low scattering rate,and it was also found that the bleaching treatment was beneficial to the original fibrillation treatment,making the resulting CNF more fibrillated.(2)In this thesis,CNF-g-Si O₂ films with different Si O₂ contents were prepared by surface modification of CNF with 3-Glycidyloxypropyltrimethoxysilane(GPS),and then CNF-g-Si O₂ films with different Si O₂ contents were prepared by combining the covalent silane network formed on the surface of CNF with nanospherical Si O₂.Meanwhile,CNF/Si O₂ films with different Si O₂ contents were prepared by physical mixing method as a control group.The results showed that the CNF-g-Si O₂ films were easier to form stable,high transmittance and low scattering films than the physically mixed CNF/Si O₂ films.The successful modification of CNF by GPS was confirmed by IR spectra with XPS spectra.From the microscopic morphology,the mutual entanglement phenomenon of CNF and Si O₂in CNF-g-Si O₂ films can be observed,which indicates the presence of covalent silane network.Combining the UV and IR spectra,it is known that the CNF-g-Si O₂ films have high transmittance(Tt>86.6%)and high IR emissivity(>93.3%)in the solar spectrum,and the average haze parameter((?))in the visible range increases with the increase of Si O₂content.Thus,the average daytime cooling of CNF-GPS and CNF-g-Si O₂(1%)films was10.11?and 9.80?,respectively.In addition,CNF-GPS and CNF-g-Si O₂ samples have good adhesion,which makes this selective radiative cooling film more suitable for solar cell use in hot and humid regions.The solar panel application test illustrates that the CNF-g-Si O₂(1%)film does have a daytime cooling effect on the solar panel with an average temperature reduction of 2.54?,but its power generation decreases slightly.(3)The wide bandgap(6 e V)Ba SO₄ particles were selected as fillers,and it was observed from the microscopic morphology that CNF was bound to Ba SO₄ particles through a covalent silane network and dispersed uniformly inside the films.It is known from UV and IR spectra that the CNF-g-Ba SO₄ films have low absorption within the solar spectrum and high IR emissivity(>93.6%)in the 8-13?m band.With the increase of Ba SO₄ content,the reflectance and transmittance of CNF-g-Ba SO₄ films gradually increased and decreased,however,their absorbance and IR emissivity did not change significantly.Through the radiative cooling performance test,the addition of wide-bandgap Ba SO₄particles can reflect part of the sunlight,which further enhances the daytime radiative cooling performance,and the average daytime cooling reaches 7.33?,which is better than CNF-GPS(6.75?).In addition,the CNF-g-Ba SO₄ film with broadband radiative cooling performance has inferior nighttime cooling effect(1.63?)than CNF-GPS film(2.25?)in Guangdong region,which confirms that CNF-g-Ba SO₄ film is more suitable for application in hot desert regions.