Study On The Preparation Of CNF/rGO Composite Aerogels And Their Microwave Absorption Properties

The increasingly severe situation of electromagnetic pollution has put forward higher requirements for microwave absorbing materials.Graphene,as a new type of carbon material,has great application potential in the field of microwave absorption.However,when used alone,there are also some problems,such as,high conductivity,impedance mismatch,easy agglomeration,high cost,and so on.On the other hand,with the increasing popularity of green chemistry and the concept of sustainable development,people have begun to shift their attention to the research of natural renewable biomass resources.Cellulose is the most abundant natural organic polymer on the earth.As a one-dimensional product of cellulose,nanocellulose has a relatively ideal geometric structure and many excellent properties.However,as a kind of insulating non-magnetic material,nanocellulose cannot be directly applied to the microwave absorbing material.Therefore,building nanocellulose/graphene composite microwave absorbing materials with nanocellulose as a green substrate may be a good choice.In this way,graphene,an effective microwave absorbing medium,can be introduced on the one hand,and on the other hand,the structural characteristics of nanocellulose matrix can be fully utilized to avoid self-stacking of graphene sheets,so as to effectively play the performance of graphene,further reduce the amounts of graphene and reduce costs.They can also be used as structural materials to further load magnetic nanoparticles,endows them with a magnetic loss mechanism,and improves their microwave absorption performance.In this thesis,we prepared nanocellulose?CNF?and graphene oxide?GO?as two substrates first,then studied their physical and chemical properties,and further explored the reduction process of GO.Subsequently,CNF/rGO composite aerogels were prepared through liquid phase compounding,chemical reduction,gelation reaction,solvent replacement and lyophilization processes.The effects of the initial GO ratio on their morphologies,structures,thermal properties,and microwave absorption properties were studied.On this basis,two types of CNF/rGO magnetic composite aerogels were prepared by introducing magnetic nanoparticles through in-situ generation method and physical blending method,respectively.And the effects of magnetic components on their structures and properties were studied.The main research contents and results are as follows:?1?CNF was successfully prepared by stripping cotton fibers using the method of TEMPO oxidation assisted high-intensity ultrasonic treatment.Its morphology,surface chemical structure,crystal structure,thermal properties,and rheological properties were studied.The results show that TEMPO oxidation introduces carboxyl groups at the C₆position of cellulose molecules,promotes fiber swelling,and gives the surface electronegativity,which is helpful for stripping microfibril bundles into CNF by the cavitation of high-intensity ultrasonic.The crystallinity of the obtained CNF is as high as76.05%,and the crystal form of cellulose is still type I.The thermal decomposition point is 267.711°C.The diameter is mainly distributed between 10?100 nm and the length can reach several microns.It is stably dispersed in aqueous solution,has high light transmittance.Meanwhile,its suspended liquid exhibits viscoelastic solid state characteristics,which has typical shear thinning behavior characteristics.?2?GO was prepared by the modified Hummers method,and then,it was reduced by L-ascorbic acid?LAA?to obtain rGO.The preparation mechanism and structural properties of GO and rGO were discussed.Results show that the surface of GO contains a large number of oxygen-containing functional groups and defects,and it is a free-folded sheet.And rGO was obtained by reducing GO with LAA,it showed that most of the oxygen-containing groups were effectively removed,the conjugate structure was rebuilt,and thermal stability was improved.However,the reduction process also caused the stacking of graphene sheets to some extent.Considering the reduction degree,stacking degree,energy consumption and other factors,85?was selected as the reduction reaction temperature for subsequent experiments.?3?On the basis of the previous two chapters,CNF/rGO composite aerogels were obtained by using the mixed dispersion of CNF and GO as precursors,through LAA reduction,gelation,solvent replacement and lyophilization processes.The structures,thermal properties,and microwave absorption properties were mainly studied.And the microwave absorption mechanism was further discussed.The results show that the CNF/rGO composite aerogels have a three-dimensional mesh structure with extremely low density,and have the potential to be an ideal lightweight microwave-absorbing material.The insertion of CNF into rGO sheet through entanglements and hydrogen bondings can effectively inhibit self-stacking of rGO sheets and construct a three-dimensional porous structure.Controlling the initial GO ratio can effectively regulate the structure and properties of the composite aerogels.As the initial GO ratio increases,the lamellar structure becomes more obvious,the porosity decreases,the density increases,the thermal stability improves,and the microwave absorption enhance first and then weaken.When the thickness of A30 is 3.7 mm,the RL_min value is-53.66 dB,and the effective absorption bandwidth is 6.64 GHz.?4?To endow the material with more microwave loss mechanism,magnetic nanoparticles were introduced by in-situ generation method and physical blending method,respectively,based on the above CNF/rGO composite aerogels.And the CNF/rGO magnetic composite aerogels were obtained.Their structures,thermal properties,microwave absorption properties were studied and analyzed.The microwave absorption mechanism was further discussed.SEM,XRD,and TG results show that Fe₃O₄nanoparticles are dispersed inside the material.They can not only increase the densities of magnetic composite aerogels,but also affect their structures and reduce their thermal stabilities.At the same time,the analysis of microwave absorption performance shows that the introduction of magnetic components is conducive to realize the synergistic effect of dielectric loss and magnetic loss,as well as the regulation of the impedance matching of the material,so as to endue the composite aerogels with better microwave absorption loss capacities.Among them,when the thickness of B20 is 3.65 mm,the RL_min value is-66.13 dB and the effective absorption bandwidth is 7.12 GHz.When the thickness of C30is 3.5 mm,the RL_min value is-57.4 dB and the effective absorption bandwidth is 5.2 GHz.