Preparation And Properties Of Carbon Based Electromagnetic Wave Absorbers And Their Applications

In recent years,with the explosive development of the electric information techniques,electromagnetic（EM）radiation from the surrounding electronic devices has become increasing severe,which disturbs the operation of electronic equipment and also threatens the health of living beings.Additionally,in military field,it is well known that the stealth performance of the weapons and equipments can greatly improve their strike capability and viability in the battlefield.It is believed that the most effective way to solve these thorny problems is to exploit EM wave absorbers which can convert the incident EM wave energy into thermal energy or other form of energy through the interactions of the EM field with the material’s molecular and electronic structure.Therefore,according to the urgent requirements from the national defense security,electronic safety as well as human healthcare,highly efficient and light weight EM wave absorbers are highly demanded to eliminate and absorb the adverse EM wave effectively in a wide EM wave frequency range.In this case,numerous candidates,such as nonmagnetic metal nanoparticles（Ag,Au）,magnetic nanoparticles（Fe,Co,Fe₃O₄,Co₃O₄,FeCo₂O₄,carbonyl iron）,conductive polymers（PANI,PPy）,semiconductor-based materials（ZnO,SiC and TiO2）,carbon materials（carbon fiber,carbon nanotube,carbon black,graphite,graphene）and their composites,have been developed to meet the versatile EM wave absorption application requirements.EM wave absorption materials usually have two main components,i.e.,absorber material and matrix material.The absorber materials can bring the composite material with EM wave attenuation ability,while the matrix materials function as adhesive.Among the materials exploited,carbon materials are one of the most promising absorber candidates for their great advantages of low density,superior mechanical and electric properties,and the excellent thermal stability.According to the transmission line theory,it is generally accepted that the EM wave absorption materials can be divided into two categories on the basis of their EM wave loss mechanism,likewise dielectric loss and magnetic loss.However,for unilateral dielectric loss or magnetic loss materials,it is of great difficulty to attain a desired impedance matching condition,which would correspondingly result in a poor EM wave absorption.Accordingly,the EM wave absorption capability of the carbon-based materials is not good enough,limiting their further application.Therefore,the combination of dielectric loss carbon materials with the magnetic loss materials would be a nice choice to exploit excellent EM wave absorbers.The synergistic effect on dissipation of the EM wave energy would contribute to achieving an excellent EM absorption.Moreover,polyaryletherketone（PAEK）,a superior engineering thermoplastices is one of the most promising candidates as matrix polymer for the synthesis of high performance electromagnetic wave absorption materials.Because of the distinctive aromatic structure,PAEK has outstanding properties,such as methancial properties,thermal stability and solvent resistance.However,PAEK is insoluble in common solvents which might restrict its applications in composite matericals field.In this work,three kinds of carbon based electromagnetic（EM）wave absorbers were designed and synthesized to meet the “strong,thin,broad and lightweight” requirements for EM wave absorbers.And one of the most excellent EM wave absorbers was selected to cooperate with the specially designed solvable amino-functionalized PAEK for the fabrication of high performance EM wave absorption materials.The structure and the properties of the carbon based absorbers and the PAEK based composite EM wave absorption material were detailed investigated.Firstly,in chapter 3,a chopped carbon fiber based three-phase heterostructures composite absorber(CF/Co_0.2Fe_2.8O₄/PANI)with a layer by layer（LBL）structure was designed and synthesized for achieving improvement on electromagnetic（EM）wave attenuation of carbon fiber（CF）.The structural and morphology analyses demonstrated the chemical structure and the LBL structure of the synthesized absorber.The Co_0.2Fe_2.8O₄ nanoparticles layer can bring the composite absorber with magnetic loss ability,while the PANI layer can serve as an efficient coating to prevent Co_0.2Fe_2.8O₄ nanoparticles from oxidation and enhance the dielectric loss ability at the same time.Moreover,with the increase of coating layer,the interaction between the EM wave and the EM wave absorber is strongly enhanced and more interfaces are introduced which can contribute to the absorption of EM wave inside the absorber.Through layer by layer coating,the EM impedance matching condition and the EM wave dissipation ability are improved.CF/Co_0.2Fe_2.8O₄/PANI has a strong EM wave attenuation ability in EM wave Ku band.With a thickness of 4.1 mm,an optimal RL of-38.2 dB（>99.9 % attenuation）is observed at 12.7 GHz.Moreover,with the thickness from 3.1 to 4.1 mm,the minimum RL curves in Ku band are all lower than-20 d B.It can serve as a promising EM wave absorber in the Ku band of EM wave.However,the EM wave bandwidth with RL below-10 dB for CF/Co_0.2Fe_2.8O₄/PANI at each thickness is relatively narrow which might not be good enough for further application in the filed of EM wave absorption.Therefore,in chapter 4,the structure and the composition of the absorber are further optimized to broaden the absorption bandwidth.Two kinds of rice husk based porous carbon and magnetic particles composite absorbers（RHPC/Fe and RHPC/Co）were specially designed and synthesized through a pre-modification method and a post-modification method.The porous structure can not only contribute to the purpose of being lightweight but also enhance the EM wave attenuation ability in the interior by means of the inside multiple reflection and scattering.And the soft metallic magnets are potential materials for EM wave absorption which can bring stronger magnetic loss than superparamagnetic materials.Moreover,the fine EM impedance matching condition and the strong EM wave dissipation ability derived from the synergistic effect between the dielectric loss and magnetic loss components are beneficial for EM wave absorption.For RHPC/Fe,at a thickness of 1.4 mm,RL value of-21.8 dB can be achieved with the effective absorption bandwidth（RL≤-10 dB）of 5.6 GHz,while for RHPC/Co,at a thickness of 1.8 mm,RL value of-40.1 dB can be achieved with the effective absorption bandwidth（RL≤-10 dB）of 2.7 GHz.Thus,RHPC/Fe and RHPC/Co have great advantages in the field of EM wave absorption.Although RHPC/Fe and RHPC/Co can attenuate the incident EM wave in a thin thickness,the absorption ability for RHPC/Fe is not strong enough and the absorption bandwidth for RHPC/Co is not wide enough.Thus,RHPC/Fe and RHPC/Co can not well match the “thin,broad,strong and lightweight” requirements for EM wave absorbers.Accordingly,in chapter 5,we draw on the experiences of chapter 4 to further optimize the structure and the composite composition.A method combining liquid-liquid phase separation and pyrolysis process has been developed to fabricate the wormhole-like porous carbon/magnetic nanoparticles composite absorber（WPC/MNPs-80）.The porous structure was designed to enhance the interaction between the electromagnetic（EM）wave and the absorber,while the magnetic nanoparticles were used to bring magnetic loss ability.The Co and Fe nanoparticles derived from the chemical reduction of Co_0.2Fe_2.8O₄ can enhance the graphitization process of carbon and thus bring enhancement in the dielectric loss ability.Polarizations existed in the nanocomposite absorber also play an important role in the EM wave absorption.Thus,EM wave can be effectively attenuated by dielectric loss and magnetic loss through multiple reflections and scattering in the porous structure.WPC/MNPs-80 has an excellent EM wave absorbency with wide absorption band at a relatively low loading and thin absorber thickness.At the absorber thickness of 1.5 and 2.0 mm,minimum RL values of-29.2 and-47.9 dB are achieved with the RL below-10 dB in 12.8¹8 and 9.2¹3.3 GHz,respectively.Therefore,WPC/MNPs-80 can be an excellent absorber for the absorption characteristics of “thin,broad,strong and lightweight”.As WPC/MNPs-80 has the most excellent EM wave absorption property,it is selected as the absorber to fabricate EM wave absorption material.In chapter 6,in order to improve the solubility of the polymer matrix and solve the deposition problem of the absorber particles during the membrane-forming process,PAEK with trifluoromethyl and amino groups was designed and synthesized（6F-PAEK-NH2）.6F-PAEK-NH2 and WPC/MNPs-80 were used to fabricate PAEK based EM wave absorbing material（6FPAEK-Crosslinking@WPC/MNPs-80）through a solution blend method.The crosslinked structure of the matrix can not only solve the deposition problem but also bring the composite material with excellent properties,such as thermal property,solvent resistance and mechanical property.6F-PAEK-Crosslinking@WPC/MNPs-80 shows fine EM absorbency.The minimum RL value of-33 dB is obtained with the bandwidth of 4.8 GHz（12.7¹7.5）at a thickness of 1.4 mm,while a RL value of-44.5 dB is obtained with the bandwidth of 3.1 GHz at a thickness of 1.8 mm.Therefore,the synthesized PAEK based EM wave absorbing material（6F-PAEK-Crosslinking@WPC/MNPs-80）is a promising candidate for EM wave attenuation because of its excellent comprehensive properties.