Research On Microwave Absorption Properties Of Magnetic FeO_x/C-Based Composites

Due to the rigorous electromagnetic pollution and the demand of military stealth materials,great efforts are still on the way to design excellent microwave absorbing materials with the characteristics of strong absorption,wide bandwidth,thin thickness and light weight.In addition,the materials should have stable physical and chemical properties and can be obtained through a low-cost and facile procedure.Compositing magnetic absorber with carbon-based material is one of the important methods to design absorbing materials.At present,a large number of researches based on carbon materials are focusing on carbon fiber,carbon nanotubes,graphene and graphene like composites.However,magnetic microwave absorbing materials based on these carbon materials generally have high cost or complicated preparation process.In addition,this kind of carbon material has high conductivity,so the microwave absorption properties are very sensitive to the contents of carbon material.This sensitivity leads to poor reproducibility of the experimental results.In this paper,different kinds of magnetic carbon-based composites were prepared with low-cost,environment-friendly graphite,glucose and biomass as the caron resources.The electromagnetic parameters of composites were adjusted by designing and controlling the composition,morphology and crystallization degree of the materials,so as to control the microwave absorbing properties of the composites.The main research work and conclusions are as follows.（1）Fe₃O₄/graphite composites（Fe₃O₄/FG）were prepared through one-step solvothermal method.The crystal structure,surface chemical state and morphology of the composites were characterized with XRD,IR,SEM and TEM.The magnetic properties,electromagnetic parameters and microwave absorption properties were tested through SQUID and VNA.The results show that Fe₃O₄ nanoparticles are spherical and distributed on the surface of graphite sheet uniformly.The absorption properties of Fe₃O₄/FG are improved after Fe₃O₄ nanoparticles are compounded with graphite.When the molar ratio of iron source to graphite is 3:10,the minimum reflection loss(RL_min)are lower than–10d B in high frequency band（13.2-17.8 GHz）and in low frequency band（3.6-8.5 GHz）by adjusting the thickness from 3 to 5 mm.The strong absorption peak in low frequency band is very meaningful for its application in C-band（4-8 GHz）.（2）Using Na Cl as activator,ferromagnetic nanoparticles/glucose carbon composites（Fe OX/GC）were prepared by one-step pyrolysis method.The phase structure,morphology,specific surface area,magnetic properties and microwave absorption properties of Fe OX/GC composites were analyzed and discussed.The results show that the structure of Fe OX/GC is porous lamellar flower-like.The addition of Na Cl promoted the crystallization of iron oxide and glucose carbon in the reaction process,supporting the collapse of pores in the process of thermal shrinkage,increasing the specific surface area of Fe OX/GC,and thus finally affects the microwave absorption performance.When the matching thickness is 2 mm,the RL_min of Fe OX/GC composite can reach–51.3 d B.When the matching thickness is adjusted to 2.3 mm,the effective adsoption bandwidth（EABW）is 5.7 GHz（12.3-18.0 GHz）.By adjusting the matching thickness from 2 to 6.5mm,absorbing materials with wide EABW can be obtained in the frequency range of 4-18 GHz.Its absorbing bandwidth covers the Ku band（12-18 GHz）,X-band（8-12 GHz）and C-band（4-8 GHz）,which has a strong application prospect.The excellent microwave absorption properties of Fe OX/GC are closely related to their composition and morphology.Compositing dielectric loss component with the magnetic loss component improves the impedance matching characteristics of the material.The porous structure reduces the material density,increasing the specific surface area and the interface between the two phases,and finally improves the electromagnetic loss.（3）Using jute,sawdust and coconut shell as carbon sources and Na Cl as activator,the multi porous structure of Fe OX/biomass carbon composites（Fe OX/BC）were prepared.The phase structure,morphology,specific surface area,magnetic properties and microwave absorption properties were analyzed and discussed.Three kinds of biomass carbon composites all exhibit excellent microwave absorption properties,which exceeds other many microwave absorbing materials.When the matching thickness is 2.7 mm,the RL_min of the composite obtained with the 1.5 g dosage jute is–28.7 d B,and its EABW reaches 6.2 GHz（11.8-18 GHz）.When the matching thickness is 2.5 mm,3 mm and 3.5mm,the EABWs of the composite obtained with the 2.0 g dosage jute reach 4.9 GHz,5.2GHz and 5.4 GHz,respectively,and the RL_min is also lower than–20 d B.When the matching thickness is 2.7 mm,the RL_min of the composite obtained with the 1.0 g dosage sad-dust is–29.5 d B,and its EABW reaches 6.2 GHz.When the matching thickness is2.5 mm,the EABW of the composite obtained with the 1.5 g dosage coconut shell reaches6.5 GHz.The EABW and RL_min of the composite obtained with the 2.5 g dosage coconut shell also reach 6.4 GHz and–26.1 d B when matching thickness is 2.5 mm.（4）The microwave absorption properties of Fe OX/BC are insensitive to the biomass dosages in a certain range.The results show that the Fe OX/BCs all exhibit good microwave absorption properties when the amount of jute,saw-dust and coconut shell are adjusted from 1.5 g to 3.0 g,from 0.8 g to 2.0 g,and from 1.2 g to 2.5 g,respectively.On the one hand,this characteristic avoids the influence of the weak difference of biomass properties or structure caused by climate,place of origin and growth time on the microwave absorption performance.On the other hand,it eliminates the serious dependence on the amount of carbon source for carbon-based composite absorbing materials,which is beneficial to the large-scale production of microwave absorbing material preparation process and the improvement of the reproducibility of the experimental results.