Investigation on Microstructure Design and Electromagnetic Properties of Two-dimensional Microwave Absorption Materials

Author:Han Mei Kang

Supervisor:yin xiao zuo


Degree Year:2018





Electromagnetic absorbing and shielding materials are attracting extensive attentions,as the rapid development of communication and electronic equipment that leads to the increasing electromagnetic pollutions.Strong electromagnetic absorption capability is required for both microwave absorption materials and electromagnetic interference(EMI)shielding materials.Furthermore,electromagnetic materials with a single function of strong absorption can no longer meet the needs,as the increasing requirements for portability and application environments of the electronic devices.Novel electromagnetic materials with multiple functions are desired,including lightweight,flexibility,thermostability and so on.Two-dimensional materials with a single atom layer or few atoms layers,which have unique two-dimensional structure,large specific surface area and surface modification spaces,exhibit the great potential for interface design and performance optimization.Reduced graphene oxide(RGO)and two-dimensional transition metal carbides(MXene)have a similar two-dimensional layer structure.Both of them have abundant native defects and functional groups on the surface,and high electrical conductivity.These characteristics are of benefit to the active surface modification and the dielectric loss of electromagnetic absorbing materials.Herein,the heterogeneous interfaces of absorbers were constructed through surface modification and microstructure design(core-shell structure,hierarchical structure,laminated structure and aligned layer struture)of RGO and MXenes.The effects of lattice defects,multi-scale interfaces and pores on the electromagnetic wave properties were investigated.More than90%of incident electromagnetic wave were absorbed in the whole X(8.212.4 GHz)and Ku(12.418 GHz)bands for the as-prepared materials.The major research contents and results are as follows:(1)The core-shell structure design,electromagnetic wave absorption properties optimization,and the mechanisms of core-shell structure on electromagnetic wave absorption were investigated.The results show that the wrapping of graphene oxide on the surface of ZnO hollow microsphere was achieved by electrostatic interaction.The core-shell structure of ZnO@RGO microsphere avoids the re-stacking and agglomeration of graphene materials,increases heterogeneous interfaces,and optimizes impedance match.The hollow structure increases the multiple scattering of the incident electromagnetic wave,leading to the enhanced electromagnetic wave attenuation capability.Meanwhile,the density of absorber is decreased.The minimum reflection coefficient of ZnO@RGO hollow microspheres reaches-45.05 dB,indicating more than 99.99%of electromagnetic wave are absorbed.The effective absorption bandwidth reachs 3.3GHz.(2)The multi-dimensional hierarchical structure design,electromagnetic wave absorption properties optimization,and the mechanisms of lattice defects and multi-scale heterogeneous interfaces on electromagnetic wave attenuation were investigated.The results show that the in situ growth of one-dimensional SiC nanowires in three-dimensional flexible RGO aerogels enhances the thermostability of aerogels,maintains the flexibility of the material,constructs multi-dimensional conductive network,increases the surface defects of RGO,and forms the bridged heterogeneous interfaces between RGO and SiC nanowires.The hierarchical structure enhances the polarization relaxation behaviors of the foams,and increases the dielectric loss capability.RGO/SiC nanowires foams with a 22%decrease of thickness achieved effective absorption in the whole X band.Based on the construction of three-dimensional RGO/SiC nanowires structure,graphene oxide was added into the precursor of polymer-derived ceramics.RGO/SiOC multiphase ceramics were prepared after the pyrolysis and sintering processes.The electromagnetic properties and dielectric loss mechanisms at high temperature were investigated.The results show that the heterogeneous nucleation temperature of SiC nanowires in SiOC ceramic is decreased by the adding of graphene oxide.The bridged SiC nanowires and RGO formed a unique three-dimensional hierachical network in SiOC ceramic matrix.The bridged SiC nanowires and RGO enhance the electromagnetic wave absorption capability of SiOC ceramic.The minimum reflection coefficient of SiOC ceramics with GO at room temperature reaches-69.3dB(>99.9999%electromagnetic wave absorption).At high temperature(673K),the effective absorption covers 93%of X band when the mass ratio of GO is 2 wt%.(3)The layered structure design of MXene,electromagnetic wave absorption properties optimization,and the mechanisms of laminated structure on electromagnetic wave absorption were investigated.Multi-layered MXene(Ti3C2Tx)was prepared by the etching of Al layers in MAX phase(Ti3AlC2).As-prepared MXene was heated in argon and carbon dioxide,respectively.The effects of heat treatment on the surface structure evolution and dielectric properties were investigated.The results show that the oxidation occurred between the surface groups and Ti layers in argon atmosphere.The regional sandwich structure of TiO2-C-Ti3C2Tx was formed,which enhances the dielectric loss capability.When MXene was heated in CO2atmosphere,the carbon layer in MXene can be completely exfoliated.This process leads to the formation of laminated structure consisting of aligned two-dimensional carbon layers and TiO2particles with an interphase distribution.The unique laminated structure is of benefit to take full advantage of two-dimensional surface,and enhance the electromagnetic wave dissipation.The effective absorption in the whole Ku band was achieved.(4)The preparation and electromagnetic properties of aligned MXene(Ti3C2Tx and Ti2CTx)aerogels,and the mechanisms of layered porous structure on the absorption and reflection of electromagnetic wave were investigated.MXene with few layers was synthesized using wet-chemical etching process.Aligned and ordered MXene aerogels were prepared by bidirectional freeze-drying process.The results show that MXene aerogels can be compressed with recovery.MXene aerogels show absorption-dominated EMI shielding performance,owing to the native defects,surface functional groups and the unique layered structure of MXene aerogels.EMI shielding effectiveness of Ti2CTx aerogel reachs 73.7 dB,and the absorption shielding effectiveness reach 64 dB.The compression process decreases the resistance and conductivity of MXene aerogel,and increases the reflection ratio of incident electromagnetic wave.