Structural Design And Microwave Absorption Properties Of Graphene-based Composite Materials

Reduced Graphene Oxide?RGO?,which has excellent properties,such as ultrathin,low density,large interface,electrical and thermal conductivity,high specific surface area and strong designability,shows great potential in microwave absorption.It has been widely studied that the nanophase absorbing agent is introduced into RGO to control the micro-components and optimize the microwave absorption property of materials.However,there are still problems waiting to be solved urgently.On the one hand,nanophases tend to agglomerate and disperse unevenly in the RGO,which can not make fully use of a large number of heterogeneous interfaces,thus,making effective absorption bandwidth?EAB?narrow.On the other hand,nanophases are mostly dielectric materials and the ability to regulate the effective dielectric constant is limited,causing that the material can not meet the requirements of thin thickness and broadband absorption simultaneously.In addition,the research of RGO and RGO-based composites focuses on the nano,micro-scale absorber,which limits their application in a large scale.Therefore,it is necessary to carry out a design of macro-structure and fabricate large-sized microwave absorbing materials with excellent performance.Based on the principle of optimizing the effective dielectric constant and increasing the heterogeneous interfaces,the micro-components are regulated and the micro and macro structure are designed to improve the impedance matching and increase microwave absorption mechanism,thereby making composites with excellent microwave absorbing properties.The main research contents and results are as follows:1.RGO foam modified with in situ oriented ZnO_nws is sythethsized by freeze-drying and hydrothermal reaction,forming a three-dimensional network structure ranging from micrometer to nanometer scale.By studying the growth mechanism of ZnO_nws,the problem of agglomeration of nanocomposites is solved and heterogeneous interfaces between RGO and ZnO_nws are effectively increased,achieving impedance matching and optimization of interfaces.?1?The preparation of ZnO nanocrystal seed layer ensures the uniform growth of ZnO_nws on the RGO foam and the addition of polyethyleneimine?PEI?guarantees that ZnO_nws has a large aspect ratio.?2?By introducing ZnO_nws and adjusting the concentration of RGO foam,composites with adjustable dilectric properties are obtained.When the concentration of RGO foam is 0.8 mg/ml,EAB covers the whole X band when the thickness ranges from4.55 mm to 5 mm.The minimum value of RC(RC_min)of the composite is-31.1 dB and the content of absorbing agent is very low with only 3.3 wt%.?3?There are four aspects mainly contributing to the excellent property of microwave absorption of ZnO_nws/RGO foam/PDMS,which are as follows:multiple scattering caused by the three-dimensional network structure;well-matched impedance characteristics;enhanced interfacial and dipolar polarization caused by heterogeneous interface,defects and residual functional groups and stronger conductivity loss caused by RGO conductive network structure.2.Using PMMA as a template,the three-dimensional structure of Ti₃C₂T_X hollow spheres coated by RGO is designed and prepared by ultrasonic centrifugation and liquid nitrogen directional freeze-drying technique.The relationship between microstructure and dielectric properties is also analyzed and the microwave absorption mechanism is revealed.?1?The obtained Ti₃C₂T_X hollow sphere has a diameter of 4?m approximately,and the wall of the sphere is very thin and in a transparent state.Ti₃C₂T_X hollow spheres are evenly distributed in the foam structure and are enveloped by RGO layers,forming a multi-phase and multi-scale network structure with many heterogeneous interfaces.?2?The introduction of Ti₃C₂T_X hollow sphere with high conductivity gives help to light weight,broadband and thinness of the composite.When the material is 3.2 mm,RC is less than-10 dB in the band of 8.2-12.4 GHz and the thickness of the composite is 1.4 mm thinner compared to that of RGO foam.?3?By the micro-structure design and micro-component regulation,the density of Ti₃C₂T_X hollow sphere/RGO foam is reduced and the electromagnetic wave is effectively absorbed by the polarization loss resulting from various functional groups,defects and heterogeneous interfaces,and the conduction loss induced by the conductive network composed of Ti₃C₂T_X and RGO.3.Combining frequency selective surface?FSS?with sandwich structure,a broadband resin matrix composite with great macroscopic heterogeneous interfaces is optimized and fabricated.The mechanism of microwave absorption is analyzed and it provides a guidance for the design and preparation of ceramic matrix composites with microwave absorption.?1?Through the solvothermal reaction and filtration,the RGO film is obtained and has a thickness of 2 to 3 microns with good flexibility.Through the optimization and analysis of simulation,the RGO film with heat treatment at 800°C is selected as the unit cell of FSS.When the upper and lower dielectric layer are 1.0 mm and 2.2 mm,respectively,the EAB of the composite covers the whole band from 8 to 18 GHz and the average RC in this band is-22.8 dB.?2?RGO films as FSS optimize the effective dielectric constant and increase the polarization and the conduction loss of the electromagnetic wave.The design of sandwich structure improves the impedance match and increases macro heterogeneous interfaces and the destructive interference between electromagnetic waves.The synergistic effect of these factors contributes to the excellent performance of microwave absorption of the composite.