Preparation And Microwave Absorption Properties Of Graphene-Based/ceramic Composites

With the rapid development of information technology,absorbing materials play an increasingly important role in electronic reliability,medical care and national security.Therefore,the development of high performance absorbing materials with thin thickness,low density,wide bandwidth and strong absorption has aroused great interest.In recent years,graphene has attracted extensive attention as a high-performance absorbing material due to its advantages of low density,high specific surface area,strong dielectric loss and high electronic conductivity.However,due to poor impedance matching and single absorption mechanism of pure graphene,its further application is limited.In order to optimize the microwave absorption properties of graphene,different graphene derivative and ceramic-based materials were combined in this study to finally prepare an absorber with strong comprehensive properties.The main research contents of this thesis are as follows:(1)Graphene nanosheets(GNS)/Zinc oxide(ZnO)/alumina(Al2O3)composite coatings with different GNS content were prepared by air plasma spraying technique.The microstructure and phase composition of the coating were systematically studied.The influence of different GNS content on complex permittivity and microwave absorption properties have been systematically investigated at the frequency range from 8.2 to 12.4 GHz.The results show that graphene was evenly dispersed between the powder and the coating.Due to the high conductivity and polarization loss of GNS,the complex permittivity of the coating is improved with the increasing of GNS content.The GNS/ZnO/Al2O3 coating with 15 wt.%GNS exhibits the most excellent microwave absorption properties,which shows strong absorption(RLmin of-45 dB)and the effective absorption bandwidth(EAB,RL≤-10 dB)covers 11.2～12.0 GHz when the thickness of 1.82 mm.Meanwhile,the hardness of the coating increased from 914.2 HV0.2 to 1057.3 HV0.2 with the introduce 15 wt.%GNS.(2)3D porous ZnO/RGO composites having various morphologies and mass ratios were synthesized using the hydrothermal and freeze-drying processes.The free Zn2+present in the composites controlled their morphology by regulating the pH value,and the Zn2+ions adsorbed onto the RGO sheets to form ZnO nanocrystals.The RGO sheets were dispersed by growing ZnO nanocrystals on them,effectively preventing their agglomeration.The effects of the ZnO morphology and ZnO:RGO mass ratio on the complex permittivity and microwave absorption properties of the composites were systematically studied.The results reveal that the composite with small hexagonal cylindrical ZnO and the ZnO:RGO mass ratio of 8:1(pH=8.5)shows excellent microwave absorption properties,and the EAB reaches 8.7-11.1 and 11.5-12.4 GHz at the thickness of 3.4 mm.(3)3D porous thermally expansion reduced graphene oxide(TERGO)/ZnO composites were fabricated via a simple heat treatment and hydrolyzation process.After the thermal expansion,GO deoxidized and rapidly expanded to form a 3D porous TERGO structure.Then,ZnO particles were uniformly dispersed on the surface or in the layer spacing of TERGO.In addition,3D porous expanded graphite(EG)/ZnO composites were prepared for comparison.The TERGO/ZnO composite shows excellent microwave absorption,and the EAB coves the X-band at a thickness of 3.9 mm.Due to the enhancement of multiple reflection and dipole polarization,the EAB width of TERGO/ZnO composites is twice that of EG/ZnO composites.(4)The TERGO and EG were coated with a layer of Al2O3 or ZnO by hydrolysis and heat treatment.Subsequently,the Al2O3 substrate was added and prepared three kinds of coatings by plasma spraying.The microstructure,microwave absorption properties and tribological properties of different coatings were systematically studied.The results show that Al2O3-based coating has a good melting state,while ZnO-based coating powder serious.In addition,TERGO and EG are homogeneously dispersed in the coating.Due to the enhanced impedance matching and dipole polarization,the TERGO/Al2O3 coating shows excellent microwave absorption.When the thickness is 2.2 mm,the EAB of TERGO/Al2O3 coating covers 2.8 GHz,the RLmin reaches-35.3 dB at the frequency of 9.8 GHz,and the bandwidth of RL≤-20 dB(99%absorption)covers 0.7 GHz.In addition,due to the self-lubrication of TERGO and the melting state of Al2O3 coating,TERGO/Al2O3 coating also shows excellent wear resistance and friction reduction performance.The coefficient of friction and wear rate decreased by 41%and 90%respectively compared with pure Al2O3 coating.