Microstructures Modulation of Carbon Nanocomposites and Their Microwave Absorption Properties

Author:Feng Wei

Supervisor:wang ya ming


Degree Year:2019





The electromagnetic wave absorbing material is a kind of functional material which can absorb electromagnetic waves incident and convert incident electromagnetic waves into heat energy and other forms of energy.It can be applied to military aircraft and weaponry to reduce the radar wave scattering intensity of the target to achieve the purpose of weakening and reducing the exposure symptoms;it can also be applied to microwave communication to protect electromagnetic information from leakage;it can also be used in civil field to protect electronic equipment from electromagnetic interference.Carbon nanomaterials,including graphene,carbon nanotubes and porous carbon,have become hotspots in absorbing materials field in recent years due to their excellent electrical conductivity,easily regulated dielectric properties and their attribute of being carriers for other materials.However,due to the impedance mismatch of the carbon nanomaterials,the absorbing properties can not meet the application requirements,thus it is necessary to improve the microwave absorbing properties of the carbon nanomaterials by various methods.In order to improve the microwave absorption performance of carbon materials,ZnO/graphene composites with different microstructures,Co3ZnC nanoparticles/porous carbon composites,ZnO@porous carbon core-shell structures as well as carbon nanotube materials with different surface decorations were constructed from the perspective of improving electromagnetic loss and improving impedance matching.The effects of composition ratio,degree of graphitization,interface area,microstructure and surface functional groups on the dielectric properties,impedance matching properties and microwave absorption properties of carbon nanomaterials were studied.Fisrtly in order to compare the difference of impedance matching and microwave absorption characteristics of carbon nanomaterials and their composite materials,ZnO nanocrystals/graphene composites were synthesized by in-situ growth and heat treatment in organic solvents.The dielectric properties and microwave absorption properties of ZnO nanocomposites/graphene composites with different mass fractions(5,10,15,20 wt.%)were investigated.Experiments show that the composite has the best microwave absorption performance at a mass fraction of 15 wt.%:the minimum reflection loss reaches-54.2 dB at 15.2 GHz,the effective absorption bandwidth reaches 6.7 GHz,and the optimized sample thickness is only 2.4 mm.Its absorbing properties are greatly improved compared to pure ZnO nanocrystals and pure graphene.The excellent microwave absorption properties of the composites are mainly attributed to the interfacial polarization at the graphene/ZnO interface,the improvement of the impedance matching of the graphene caused by the surface-modified ZnO nanocrystals,the formation of residual functional groups of the reduced graphene oxide and the formation of lattice defects in ZnO.To further solve the problem of graphene agglomeration and study the effect of ZnO and graphene ratio on the properties of the material,ZnO micro-flower/ZnO nanocrystals-modified reduced graphene oxide was synthesized by in-situ growth and heat treatment in aqueous solution.The ZnO nanocrystals of the product were uniformly dispersed on the graphene sheet.Modified graphene sheets encapsulate ZnO micro-flowers or inserte into the gap.By changing the concentration of the reactants,the mass ratio of ZnO and graphene in the composite can be arbitrarily adjusted(in this study,the ratio of ZnO and graphene are 9:1,8:1,5.5:1,2:1).The dielectric properties and microwave absorption properties of different ZnO/graphene mass ratio samples were investigated in the paraffin wax with a mass fraction of 25 wt.%.The results show that the real and imaginary parts of the dielectric constant of the material increase with the decrease of the mass fraction of ZnO.Samples with a mass ratio of 8:1 showed the most outstanding microwave absorption characteristics,with a minimum reflection loss of-77.5 dB at 4.5 mm thickness and a maximum effective bandwidth of 6.9 GHz at a thickness of 3.5 mm.Secondly in order to study the degree of graphitization of nano-carbon materials and the effect of interfacial area on microwave absorption properties,CoZn alloy nanoparticles/porous carbon composites were synthesized by pyrolysis of bimetallic zeolitic imidazolate skeleton compounds(ZIFs).The graphitization,porosity and specific surface area of the composite were adjusted by adjusting the molar ratio of Zn to Co in the reactant.The dielectric constant of a composite material varies significantly with changes in graphitization,porosity,and specific surface area.The BM 0.2 sample having a suitable specific surface area and degree of graphitization showed the most superior electromagnetic wave absorption characteristics.When the sample thickness is only 1.5mm,the minimum RL can reach-53.2 GHz.The minimum RL is-59.7 dB at 4.5 mm,the maximum effective bandwidth is 5.3 GHz,and the sample thickness is 2.0 mm.The electromagnetic wave absorption ability is significantly improved compared to the single metal ZIFs derived carbon material,which can be attributed to the enhanced carbon-air interface enhanced interfacial polarization in porous carbon and the improved impedance matching due to composite magnetic metal ions.Thirdly an heterogeneous core-shell structure composed of a porous carbon shell with dispersed Co3ZnC nanoparticles and a ZnO microsphere core were constructed to study the effect of microstructure on the electromagnetic wave absorption properties of the materials.ZnO colloidal nuclei and double-layered ZIF shell microspheres were prepared as precursors by in-situ crystal growth.The concentration of the reactants was investigated to obtain a uniform core-shell structure.After annealing,the two-layer ZIF coating is pyrolyzed into an N-doped porous carbon shell with dispersed Co3ZnC nanoparticles.The obtained ZnO@porous carbon/Co3ZnC core-shell microspheres have better electromagnetic wave absorption capability than pure ZnO and ZIFs derived carbon samples due to their unique structural characteristics.When the sample thickness is 2.2mm,the reflection loss can reach-62.9 dB and the effective bandwidth is 5.5 GHz.The effect of microstructure on the electromagnetic absorption properties of the material can be confirmed by the experiments.Lastly,in order to study the effects of surface functional groups and magnetic material modification of carbon materials on their microwave absorption properties,functional groups and defects were introduced on the surface of multi-walled carbon nanotubes by concentrated acid treatment,and Fe3O4 was introduced into the material by hydrothermal treatment to modify the surface of carbon nanotubes.The electromagnetic parameters and microwave absorption properties of carbon nanotubes with different surface functionalizations were compared.The results show that the presence of Fe3O4can adjust the dielectric constant of materials,improve the comprehensive microwave absorption properties of materials.The reflection loss can reach-53.3 dB at 20 wt%and the maximum absorption bandwidth can reach 5.2 GHz,which is superior to the optimal performance of unmodified carbon nanotubes.