Preparation And Performance Characterization Of Composite Microwave Absorption Materials Based On Iron Group Element

With the rapid development of modern science and technology,communication technology has been rapidly developed.However,the progress of technology not only facilitated human production and life,but also accompanied by electromagnetic radiation and electromagnetic pollution would affect the human health and the function of precision devices.Therefore,the concept of electromagnetic protection by microwave absorption materials(MAMs)has been proposed.MAMs are a class of materials that can reduce electromagnetic interference by absorbing the electromagnetic wave energy projected onto its surface and converting it into thermal energy to attenuate it,and are currently being widely developed for electromagnetic shielding applications.In addition,in the military field,with the continuous development of radar detection technology,MAMs play an important role in maintaining national defense security as an important basis for stealth technology.Therefore,it is of great importance to explore efficient,lightweight and high performance MAMs.At present,single types of MAMs are difficult to meet the requirements of applications due to the disadvantages of low microwave absorption performance and difficult to adjust impedance matching.Therefore,in order to solve these problems,the synthesis,microstructure characterization,and microwave absorption performance of iron group elemental compounds,alloys,and composites with graphene are investigated in this thesis.The aim is to optimize the electromagnetic properties by modulating the material components and microstructure design,explore the electromagnetic absorption mechanism,and finally achieve improvements in the absorption strength,effective absorption bandwidth,and matching thickness of the materials.The main contents are as follows:1.Ni Fe-oxide,nitrides,alloy and three-phase heterostructured composites were prepared by calcination at different temperatures under ammonia and argon atmospheres using Ni Fe-layered double hydroxide(LDH)as precursors.The X-ray diffraction(XRD)results showed that the sample obtained by heat treatment at 330?for 2 h under argon atmosphere(O330)contained only Ni Fe-ferrite;the samples obtained by heat treatment at 370?(N370)and 400?(A400)for 2 h under ammonia gas yielded Ni Fe-nitride and alloy,respectively;the sample(ONA380)obtained by heat treatment at 380?for 2 h under ammonia gas was a three-phase heterogeneous structure consisting of Ni Fe-ferrite,nitride,and alloy.The XRD refinement results confirmed that the relative contents of oxide phase(Ni O and Ni Fe₂O₄),nitride phase(Fe Ni₃N)and alloy phase(Fe Ni₃)in ONA380 were 33.34,44.74 and 21.92 wt%,respectively,with the relative contents of Ni O and Ni Fe₂O₄ being 12.01 and 21.33 wt%,respectively.The samples obtained by heat treatment of Ni Fe-LDH retained the plate morphology derived from LDH as observed by scanning electron microscopy(SEM),transmission electron microscopy(TEM)and atomic force microscopy(AFM).The microwave absorption performance measurements show that the ONA380 outperforms the O300,N370,and A400.The minimum reflection loss value(RL_min)of ONA380reaches-59.29 d B at 9.68 GHz,while the matched thickness is only 2.02 mm and the effective absorption bandwidth(EAB,RL<-10 d B)is 2.44 GHz(8.68-11.12 GHz).The analysis of electromagnetic parameters indicates that the three-phase heterojunction composite prepared by Ni Fe-LDH nanosheets maintains the plate-like morphology by heat treatment at this temperature,which facilitates the magnetic material to break the Snoek's limit in the GHz range,and the high anisotropy can strengthen the natural resonance of the material and enhance the natural resonance frequency.The first principles calculations demonstrate that the electron transport between heterogeneous interfaces facilitates the increase of the conductivity loss capacity of the material.Thus,the synergistic effects of the above-mentioned consumption mechanisms jointly contribute to the attenuation of electromagnetic waves.2.The Co₃O₄/N-doped carbon(NC)@Co Ni_x(x=1,1.5,2)yolk-shell structural composites were prepared by sequential etching of Zeolitic imidazolate framework material-67(ZIF-67)templates and subsequent heat treatment under ammonia gas.Combining the results of XRD and elemental analysis(Mapping),it can be seen that the shell layer of the yolk-shell structure is a Co Ni_x alloy and the core is a combination of Co₃O₄ and NC.SEM and TEM results show that controlling the Ni content can control the size of the core and the thickness of the shell layer of the yolk-shell structure.Nitrogen adsorption and desorption experiments(BET)confirmed the mesoporous nature and abundant specific surface area of the structure.The absorption performance test shows that the Co₃O₄/NC@Co Ni_1.5 sample obtained with a controlled Ni content of 1.5 has the best absorption performance with an RL_min value of-71.15 d B at 15.2GHz and a thickness of only 1.71 mm.when the thickness is 1.78 mm,the EAB reaches5 GHz(13-18 GHz),covering more than 83%of the Ku-band(12-18 GHz).When the thickness increases to 2.57 mm,the EAB reaches 3.48 GHz(8-11.48 GHz),covering more than 87%of the X-band(8-12 GHz).Thus,adjusting the thickness between 1.78and 2.57 mm allows to achieve a complete absorption of Ku-band or X-band.The analysis of the electromagnetic parameters showed that the improved wave absorption performance is mainly attributed to the following aspects:(1)the combination of Co₃O₄/NC and Co Ni alloys has a dual loss capability,improving the complex permittivity and complex permeability,which regulates the impedance matching of the yolk-shell structure;(2)the abundant heterogeneous interfaces in the Co₃O₄/NC@Co Ni_1.5 composite lead to more interfacial polarization and dipole polarization,and the cavity between the core and shell structures enhances the incident electromagnetic wave multiple reflections and scattering,leading to further enhancement of the attenuation properties of the material;(3)the shell layer composed of alloy nanoparticles forms a conductive network that facilitates the movement of hopping electrons,thus promoting conduction losses;(4)the natural and exchange resonances cause an increase in the magnetic loss capability of the Co₃O₄/NC@Co Ni_1.5.Co₃O₄/NC@Co Ni_x(x=1,1.5 and 2)yolk-shell composites contribute to the attenuation of electromagnetic waves through a combination of good impedance matching and synergistic effects from interfacial polarization,dipole polarization,conductivity loss and magnetic loss.3.Volume-shrinkage free graphene aerogel(GA)materials with precisely controllable density were prepared by a solvothermal method using ethanol as the solvent and adding 3-aminopropyltriethoxysilane(APTES)as the surface modifier and cross-linking agent.Adjusting the concentration of graphene colloid enables precise control of GA density and pore size.Fe₃O₄@C/GA composites were prepared by adding carbon-coated ferric tetroxide(Fe₃O₄@C)microspheres during the solvent heat process.The results of electron microscopy analysis revealed that the Fe₃O₄@C microspheres were uniformly adsorbed on the graphene aerogel skeleton.The microwave absorption performance tests showed that when the density of GA was controlled at 4.5 mg·cm^-3,the GA material exhibited excellent wave absorption performance with an RL_min value of-50.3 d B at 24.29 GHz,a matched thickness of only 1.14 mm,and an EAB of 5.27GHz.while the Fe₃O₄@C microspheres loaded with 10 wt%of Fe₃O₄@C microspheres in this density GA obtained The matching thickness and effective absorption bandwidth of the Fe₃O₄@C/GA composite are further improved with a RL_min value of-54.0 d B at19.87 GHz,a matching thickness reduced to 0.99 mm,and an EAB of 6.5 GHz,covering 80%of the entire K-band.The analysis of electromagnetic parameters indicates that the impedance matching and conductivity loss can be adjusted by changing the density of GA,thus optimizing the electromagnetic parameters to achieve the maximum absorption performance;the introduction of magnetic Fe₃O₄@C microspheres in the GA matrix not only realizes the interfacial polarization relaxation and eddy current loss,but also can further meet the impedance matching,which finally improves the magnetic loss and dielectric loss capability of the material.