Carbon-based Composites For Highperformance Microwave Dielectric Application

Since the drastically increasing of the application of wireless communications,the electromagnetic interferences(EMI)are ever-increasing and attracting global attention.In order to reduce EMI issue,development of an electromagnetic(EM)absorber has been regarded as an efficiently way to harvest electromagnetic waves and convert their energy into heat by intrinsic magnetic and dielectric loss ability.Among the candidates,growning attentions have been focused on carbon materials,owing to their ultralow density,easyproduction,low costs etc.Among them,the amorphous carbon materials with light weight,low cost,and certain graphitization procedures are quite absorbing.It is generally believed that the performance of the electromagnetic absorber is closely related to its composition,microstructure,etc.,but the understanding of the relevant influence mechanism needs to be further improved.In order to obtain the wideband EM absorption ability,the progress in carbon material are generally included the component manipulation(such as decoration,element doping etc)and structure tuning.In this case,various loss mechanisms,such as conductive loss,dipole,interfacial polarization and magnetic loss etc can be co-existed in carbon absorber.Towards this purpose,we employed one step-by-step method to introduce various loss genes into the carbon material,aiming to the wideband absorption.The main research contents are as follow:(1)Structure-tuning to boost conductive lossFirst,the influences of structure on conductive loss(a kind of dielectric loss)has been investigated.By using in situ polymerization method combined with hard template method,solid and hollow graphitized carbon spheres were prepared respectively.Through comparative studies,it was found that the hollow carbon spheres have enhanced dielectric loss characteristics and enhanced graphitization.High graphitization also gives hollow carbon spheres a higher carrier concentration,which in turn increases their conductivity loss(a type of dielectric loss).The reason why the hollow carbon sphere has higher graphitization may be because its higher specific surface area is beneficial to its more complete carbonization reaction during the carbonization process.In order to verify this hypothesis,hollow graphitized carbon spheres of different sizes were prepared,and related experimental studies further confirmed that increasing the specific surface area did indeed help improve the conductivity loss capability of the material.Finally,by optimizing the size of the hollow graphitized carbon sphere,it was found that when the diameter of the hollow graphitized carbon sphere is 2.0 nm,it has the best conductivity loss capability and the absorption band width can reach 2.9 GHz.(2)Doping strategy to induce dipole polarizationIn secondly part,we employed the element doing method to incorporate the dipole polarization.In details,two types of element dotted graphitized carbon,knowing as N and S dotted carbon,have been made.The results confirm that the graphitized carbon dotted by S enables to form the various C-S bonds,such as C=S,C-S etc.Partial C-S bond existing in the graphitized areas would damage the original graphitized level,and then induce the dipole polarization,which attributes to the polarization relaxation loss.However,the S dotted amorphous carbon possesses a decreased conductive loss ability,owing to the weakly graphitized degree.On the contract,the N dotted graphitized carbon would make a contribution for the conductive loss,but weaken to the dipole polarization,which is due to the increased density of carriers.The best sample after doping has a absorption band width of 4.6 GHz at a thickness of 1.5 mm.(3)Interfacial strategy to incorporate interfacial polarizationIn addition to conductive and dipole polarization,the interfacial polarization as another form of dielectric loss,also benefits to the EM absorption.In this part,we designed a spherical-like hybrids,using the Co3O4 and amorphous carbon as the core and shell,respectively.Then,the existed Co3O4 core could be reduced by the carbon shell,resulting in Co/Co3O4@C ternary hybrids.To heat the Co3O4@C at various temperatures,Co3O4 would be partial or totally reduced to metallic Co,thus realization of controllable interfaces.The results revealed that interfacial types have a significantly influences on the interfacial polarization behavior.Due to the incorporation of interfacial polarization type,the EM attenuation ability can be boosted.Experimental results show that the introduction of interface polarization contributes to the EM absorption energy of the material at a low thickness,that is,at a thickness of 1.4 mm,the absorption band can reach 4.3 GHz.(4)Magnetic carbon-based composite with various loss mechanisms In the former sections,much progress has mainly focused on the dielectric loss ability but ignoring the magnetic loss.In order to introduce the magnetic loss and maintain a strong dielectric loss,herein,a component-manipulation has been confirmed as an efficiently way to achieve wideband,thin thickness absorber.Specifically,the acid-treated carbon nanotubes(CNTs)was synthesized which showed the coexisted dipole and conducive loss ability.On the basis of CNTs,the magnetic nanoparticles CNTs/Fe@Fe2O3 composite could be fabricated by another thermally-decomposition rout.The ultimately exposed various interfaces,thus was favoring for the interfacial relaxation interface.Additionally,the magnetic Fe nanoparticles was help for the magnetic loss.Due to the various loss forms,a desirable EM absorption performance can be realized for the ultimately CNTs/Fe@Fe2O3composite.The final sample confirmed the excellent broadband EM absorption capability,that is,the maximum absorption width reached 5.5 GHz.