Preparation And Electromagnetic Microwave Absorption Properties Of SiO₂/Carbon-based Composite Materials

The rapid development of electronic technology is accompanied by the generation of electromagnetic wave pollution,which can cause great harm to the environment and human health.In order to reduce these pollutions,it is urgent to prepare electromagnetic wave absorbing materials with excellent performance.In this dissertation,silica and carbon-based materials are the research object.Through the sol-gel method,a SiO₂ coating layer is formed on the surface of carbon-based materials(carbon nanotubes,graphene,carbon nano onion)to prepare composite powders.The introduction of SiO₂ can improve the impedance matching of carbon-based materials,and retain the advantages of low density and corrosion resistance as a wave absorbing material.This paper mainly prepared silica-coated carbon nanotubes(NW-CNT@SiO₂),porous silica layer-coated graphene nanosheets(PSiO₂@GNs)and silica-coated carbon nano onion(SiO₂@CNO).For the three composite powders,the effect of the preparation process on the morphology,structure and wave absorbing properties of the composite powders is studied,and the wave absorbing mechanisms of different composite powders are discussed.The obtained powder has many advantages such as light weight,strong absorption,and thin application thickness.Carbon nanotubes(CNT)have good physical and chemical properties,have been widely used in the field of electromagnetic wave absorption.However,CNT impedance mismatch results in poor electromagnetic wave absorption performance.Therefore,improving CNT impedance matching is an urgent problem to be solved.In this dissertation,a series of 3D SiO₂-coated carbon nanotube(3D NW-CNT@SiO₂)composite powders with three-dimensional network structure were prepared by an improved sol-gel process.The prepared 3D NW-CNT@SiO₂ has a low density of1.6±0.2 g/cm³,and the coating structure improves the impedance matching of the CNT and enhances the polarization loss.The analysis results show that the 3D NW-CNT@SiO₂ coating layer is uniformly distributed and the thickness is between 8.725-20.257 nm.When the filling amount is 10wt.%and the thickness is 1.56 mm,the optimal reflection loss(RL)of 3D NW-CNT@SiO₂ reaches-62.27 d B,which is 15.3times as large as uncoated CNT.Graphene nanosheets(GNs)have a strong electromagnetic wave attenuation ability,but high electrical conductivity will cause impedance mismatch,leading strong reflection,and reduce the wave absorption performance.Therefore,it is a challenging task to improve impedance matching and make GNs highly effective electromagnetic wave absorbers.This work uses cetyltrimethylammonium bromide(CTAB)as a template to construct porous silica-coated GNs composite powder(PSiO₂@GNs)via sol-gel reaction.The particle size and specific surface area of the composite powder can be adjusted by changing the thickness of the coating layer.When the coating layer becomes thicker,the particle size and specific surface area increase(particle size:5.16?m increased to 9.57?m;specific surface area:27.053m²/g increased to 231.39 m²/g).The test results of wave absorption performance show that the porous SiO₂ layer can make the input impedance of GNs closer to 1,which improves the impedance matching performance and greatly reduces the reflection of incident electromagnetic waves.At the same time,the formation of the SiO₂/GNs heterogeneous interface enhances the interface polarization,which is beneficial to the polarization loss of the composite powder.When the filling ratio is25wt.%and the thickness is 3.22 mm,the optimal RL value of PSiO₂@GNs is-59.835d B,and the effective absorption bandwidth(EAB)is 3.36 GHz.These results show that PSiO₂@GNs has potential application value in military radar,electronic communications,weapons,aerospace and other fields.The use of CTAB can generate micelles on the surface of CNO to provide growth sites for SiO₂.Using TEOS as the Si source,SiO₂@CNO composite powders were prepared.The influence of TEOS addition amount,powder filling ratio and wave absorbing performance of SiO₂@CNO composite powder was systematically explored.The results show that the final product presents a core-shell structure with CNO as the core and SiO₂ layer as the outer shell.Its thickness is in the range of 50-150 nm.The formation of SiO₂ optimizes the impedance matching of CNO,so that more electromagnetic waves can enter the inside of the absorbing material from the air.Moreover,due to the formation of the core-shell structure,electromagnetic waves can be reflected and refracted multiple times inside the material,which further enhances the absorbing performance of SiO₂@CNO.The RL peaks of all samples are located in the C-band(4-8 GHz).The best RL of SiO₂@CNO can reach-40.09 d B at5.68 GHz,which is 5.33 times larger than uncoated CNO.