Preparation and Electromagnetic Wave Absorption Properties of Magnetic Nanocomposites
Author:Wu Zuo Zuo
Supervisor:wang wei min liu jiu rong
In recent years,the rapid development of electronic technologies has brought great conveniences to human being’s life.However,it has also caused serious electromagnetic(EM)wave intereference and EM wave pollutions problems,which created great threats to people’s health.In the military field,with the development of various detective technologies,it is quite necessary to develop the stealth capabilities of weapons and equipmens.EM wave absorption materials have drawn great interests due to their ability of converting incident EM wave energy into thermal energy.Magnetic materials,such as Fe,Co,Ni etc,are considered as excellent EM wave absorbers owing to their large saturate magnetization,strong magnetic loss,and low cost.However,their drawbacks of large density,poor impedance matching,easy to be oxidized,narrow absorption bandwidth make them unable to satisfy the demands of ideal absorbers.In recent years,carbon materials have become attractive as lightweight absorbers.But individual carbon materials always suffer from unbalanced impedance matching because they have no magnetisims,which limited their further applications in the EM wave absorption field.In order to overcome the above shortcomings and optimize the EM wave absorption properties of pure magnetic materials and carbon materials,in this paper we achieved the synergy of multiple loss mechanisms,greatly improved their impedance matching level and decreased the influences of eddy current effect through regulating the microstructures,constructing composites and designing nanomaterials by using a simple and low-cost synthetic method.Finally,the excellent EM wave absorption properties with strong absorption,lightweight,broad absorption bandwidth and low thickness are obtained.The main contents in this paper are shown as follows:(1)The microstructures of materials have a significant influence on the EM wave absorption properties.The hollow structures possess the advantages of lightweight and excellent impedance matching due to the existence of abundant air.In this paper,the Fe3O4 hollow spheres were firstly synthesized through a combination of facile solvothermal method and subsequently calcination.The porous Fe hollow spheres were obtained through reducing the as-obtained Fe3O4 hollow spheres under hydrogen gas at 350 0C.Then the EM wave absorption properties of the porous Fe3O4 hollow spheres and Fe hollow spheres were investigated in 2-18 GHz.The results showed that compared to the porous Fe3O4 hollow spheres and previously reported Fe absborbers,the porous Fe hollow spheres exhibited more superior EM wave absorption performances.The minimum reflection loss(RL)for porous Fe hollow spheres reached-47.1 dB at 15.2 GHz and the matching thickness was as thin as 1.69 mm.The effective bandwidth(RL<-10 dB)from 13.2-18.0 GHz at 1.69 mm almost covered the whole Ku band.Above all,designing hollow structural magnetic materials with strong magnetisims can be an effective way to improve the EM wave absorption properties.(2)As a new kind of materials,CNTs possess lots of unique properties,such as low density,excellent conductive properties,and large specific surface area etc.However,pure CNTs are non-magnetic materials,which have poor impedance matching level.In order to improve their EM wave absorption properties,in this paper the Co nanoparticles decorated CNTs composites(Co/CNTs)were prepared through combining the solvothermal method with carbon reduction process.Compared with pure CNTs,the Co/CNTs composites showed improved impedance matching and greatly enhanced EM wave absorption performances due to the synergistic effect of Co and CNTs.Besides,the filling ratio of the Co/CNTs is only 20 wt%,showing the features of low density.The results indicated that constructing the composites of magnetic metallic particles and CNTs could achieve both lightweight and highly efficient absorption.(3)The three-dimensional(3D)porous flower-like Co/C composites were synthesized through combining solvothermal method with carbon reduction.SEM and TEM observations showed that the Co/C composites displayed 3D crabapple-like structure composing of core-shell Co@C nanoparticles and 2D amorphous carbon sheets.The room-tempurature magnetic hysteresis loops showed the saturate magnetization of the Co/C crabapples could reach as high as 149.6 emu g-1.The EM wave absorption results showed that the 3D porous Co/C crabapples exhibited minimum RL of-56.9 dB at 9.3 GHz with thickness of 1.92 mm.When the thickness was decreased to 1.4 mm,the effective absorption bandwidth was as broad as 5.9 GHz.A broad bandwidth of 5.8 GHz at 2.0 mm thickness was still achieved at a low filling ratio of 30 wt%filler content.The 3D porous Co/C crabapples displayed superior EM wave absorption as compared to many previously reported Co and Co/C composites.The mechanisms affecting their EM wave absorption were investigated considering the synergistic effects between multiple components and their structural characteristics in detail.(4)Metal-organic frameworks(MOFs),which consisted of organic ligands and metal ions,can be transformed into metal or metal oxide composited with porous carbon through controllable treatments.In this paper,the rod-shape composites with Fe-containing magnetic nanoparticles(Fe3O4,Fe3C and Fe NPs)embedded into nano-porous carbon(NPC)were obtained through calcination of Fe-based MOFs(Fe-MIL-88A)in the temperature range of 500-800℃.XRD and Raman results showed the compositions and the graphitization degree of the magnetic NPC nanorods can be influenced greatly with the variation of calcination tempuratures.The graphitization degree was found to have a significant on the permittivity level,thereby tunable EM wave absorption properties can be obtained.At calcination temperature of 600℃,the magnetic porous carbon nanorods performed the most outstanding EM wave absorption performances with a strong reflection loss of-52.9 dB at 4.64 GHz with absorber thickness of 3.07 mm.With a thickness of 3.5 mm,the magnetic porous carbon nanorods achieved effective bandwidth of 7.92-12.48 GHz,which almost covered the whole X band.What’s more,this MOFs derived method to obtain the composites of magnetic particles and porous carbon is quite simple and efficient,which provides new thoughts for the design of novel EM wave absorption materials.