Synthesis And Microwave Absorption Properties Of Carbon-Manganese Oxide Nanocomposites

The explosive advance of information technology and radar detection that has improved communication and the economy has also increased electromagnetic interference(EMI)pollution and threatened national defense security.Hence improved electromagnetic wave absorbing materials are required in both civil and military spaces.Since traditional microwave absorbing materials cannot meet all of the requirements,hybrid nanomaterials consisting of a dielectric medium and magnetic metal oxides that are improved microwave absorbers are attractive substitutes.Nanocarbon materials,because of their strong electrical dissipation.low density and high surface area,are popular substrate materials on which to grow other nanomaterials to produce hybrid microwave absorbing nanomaterials.Manganese oxide nanoparticles have been treated as an efficient microwave absorbing materials due to their high dielectric loss,variable crystal and tunnel structure.Carbon-based manganese oxide,with better impedance matching than carbon materials and lower density than manganese oxide,is fabricated as high-performance microwave absorbers with strong absorption over a broad frequency range.In this work,the carbon-based manganese oxide nanocomposites are synthesized and its microwave absorption performance in 2-18 GHz is investigated.Firstly,the carbon-based manganese oxide nanocomposites are fabricated through coprecipitation method,and the influences of preparation process on the product,composition,morphology are described.Secondly,the Fe magnetic particles are introduced to enhance the magnetic loss and microwave absorption.The relations between composition,morphology,loss mechanisms,microwave absorbing properties are also investigated.Thirdly,to enhance the microwave absorption,the microstructure is optimized through synthesizing double-layered CNT-MFe2O4-MnO2 composites.The microwave absorption performance of different mixed metal oxide composite is analyzed.The XRD,XPS,EDX,SEM,TEM,DLS,VNA,four point probe instrument are performed to analyze the phase,chemical state,elements distribution,morphology,particle size,electromagnetic parameters and electrical conductivity.The main research achievements are as follows:(1)The GN-Mn3O4 composite is prepared by hydrothermal method which makes the synthesis possible at a far lower temperature than calcination.High temperature and long reaction time give rise to lattice defects,particles size,which leads to the increase of dielectric loss and conductivity loss,the decrease of magnetic loss.The GN-Mn3O4 composite synthesized in 140? for 4 h shows best RL of-18.5 dB,as well as an effective absorption bandwidth of more than 5 dB corresponding to RL<-10 dB.The interfacia]polarization,relaxation polarization,conductive and magnetic loss contribute to the microwave absorption.(2)To improve the microwave absorbing properties,CNT-based manganese oxide nanocomposite is synthesized instead of graphene composites.The oxide phase can be altered by changing the pH of the solution during the chemical reaction.The morphologies,particle size,microwave absorbing performance vary.Trough reasonable designing on the thickness of absorber,the effective absorption of CNT-MnO2 and CNT-Mn3O4 covers over 3-11 GHz and 4.5-16.5 GHz,respectively.The reflection loss of a nanocomposite is?3 times larger than for the corresponding pure manganese oxide.There is a minimum reflection loss of-34 dB at 4 GHz for a CNT-MnO2 nanocomposite.The main loss mechanisms are conductivity loss and dielectric loss for CNT-MnOx composite.The polarization,conductive loss,impedance matching are much enhanced after the addition of CNT into manganese oxide.By adding MnO,to CNT,conductivity and permittivity decrease,and permeability increase,which give rise to the microwave absorption.(3)The microwace absorbing properties of carbon-based manganese oxide is further optimized by introducing magnetic particles Fe into carbon-based manganese oxide.Crystalline manganese iron oxide(MnFe2O4)is deposited over CNT through hydrothermal method.Adjusting the content of MnFe2O4 in composites and the hydrothermal chemical reaction time allows control over the microstructure and microwave absorption.The introduction of Fe enhances the magnetic loss which consist of eddy current and nature resonance.The increase of CNT ratio helps increase the dielectric loss and conductivity loss,but decrease the magnetic loss and the RL peaks shift to higher frequency.The dielectric polariziton and conductivity loss trend to decrease along with the increase of reaction time,while the magnetic loss increases.The lowest RL of-41 dB is observed for the nanomaterial with 10 wt.%CNT and 90 wt.%MnFe2O4.(4)CNT-NiFe2O4-MnO2 and CNT-NiFe2O4-MnO2 show enhanced microwave absorption attributing to the optimized double layer structures and magnetic middle layer.The morphology,microwave absorption,optimal absorption frequency of double-layered nanocomposites with different middle layer are analyzed.It is concluded that the exist of CoFe2O4 and NiFe2O4 middle layer increases the magnetic loss,dielectric polarization,multiple scattering and refletion,and eventually rise up to the microwave absorption.The minimum RL of CNT-CoFe2O4-MnO2 reaches up to-31 dB at 16.5 GHz which enhanced the absorbing property in 15-18 GHz of carbon-base manganese oxide.