Preparation Of Fe₄N-based Composites And Their Electromagnetic Wave Absorption Properties

In recent years,while the continuous advancement of electronic information technology has brought convenience to human life,it has also caused increasingly serious problems of electromagnetic interference and electromagnetic pollution,which seriously threaten human health.In the military field,the development of various detection technologies puts forward new requirements for the electromagnetic wave absorption ability of weapons and equipment.Electromagnetic wave absorbing materials can convert the energy of incident electromagnetic waves into heat energy and lose them,thus becoming a hot spot of current research.On the one hand,Fe₄N has gradually become the focus of research on electromagnetic wave absorbing materials due to its excellent magnetic properties and good environmental stability.However,due to many shortcomings such as high density,low impedance matching level,and narrow absorption frequency band,Fe₄N is difficult to meet the requirements of ideal electromagnetic wave absorbing materials "thin thickness,low density,wide effective absorption frequency band,and strong absorption intensity".On the other hand,carbon materials have been widely concerned by researchers as lightweight absorbing materials.However,a single carbon material lacks magnetism and poor impedance matching level,which limits its further application.To overcome the insufficient electromagnetic wave absorption performance of single-phase Fe₄N and carbon materials,this study adopted the optimized gas nitriding process,and prepared composite wave absorbers of Fe₄N and different forms of carbon through strategies such as morphology control and composition compounding,including hollow microsphere carbon,multi-walled carbon nanotubes and metal-organic framework compounds（MOFs）derived porous carbon;the formation mechanism,structure and morphology of the above composite absorbing materials were studied;by comparison with solid spherical single-phase Fe₄N powder,Revealed the influence of morphology,phase,heterogeneous interface and other factors on the electromagnetic wave absorption performance;realized the synergistic effect of various loss mechanisms,optimized the impedance matching of composite materials,and finally achieved"thin,light,wide and strong" electromagnetic wave absorption effect.The main research work of this paper is as follows:（1）The solid spherical Fe₄N was prepared,and the microwave absorption performance of spherical Fe was compared with that of spherical Fe.The results showed that solid spherical Fe₄N showed stronger electromagnetic wave absorption than Fe with the same shape and size.When the matching thickness is 1.5 mm,the minimum reflection loss reaches-22.3 dB.When the matching thickness increases to 2 mm,the effective absorption bandwidth reaches 4.5 GHz.Through the characterization of the crystal structure,microscopic morphology,static magnetic properties and electromagnetic wave absorption properties,the reasons for the improvement of electromagnetic wave absorption properties in spherical Fe₄N were explored,and it was found that the dielectric loss mechanism of spherical Fe₄N including interfacial polarization and conductance loss has been greatly improved.The electromagnetic wave absorption capability is also largely derived from magnetic loss mechanisms such as eddy current loss,natural resonance,exchange resonance,and hysteresis loss.At the same time,spherical Fe₄N has a higher attenuation constant than spherical Fe,which is due to the synergistic effect of dielectric loss and magnetic loss,which optimizes impedance matching.In addition,the interference loss plays an important role in the electromagnetic wave absorption process of spherical Fe₄N.（2）Porous Fe3O4/C and Fe₄N/C hollow microspheres were obtained by heat treatment and gas nitriding treatment of iron-glycerol hollow microspheres,and their formation mechanism was studied.The presence of porous and hollow microsphere features was demonstrated through structural and morphology analyses.By comparison,it can be found that Fe₄N/C hollow microspheres have better electromagnetic wave absorption ability than Fe3O4/C hollow microspheres.When the matching thickness is only 1.4 mm,the reflection loss of porous Fe₄N/C hollow microspheres reaches-42.2 dB,and the effective absorption bandwidth is 4.5 GHz,while the reflection loss of porous Fe3O4/C hollow microspheres in the frequency range of 2-18 GHz all above-10 dB.By analyzing the internal reasons for the excellent electromagnetic wave absorption performance of Fe₄N/C hollow microspheres,it is found that magnetic Fe₄N has high complex permittivity and complex permeability,and the porous hollow microsphere structure increases the multiple scattering and reflection of electromagnetic waves.Meanwhile,through the synergistic effect of dielectric loss and magnetic loss,impedance matching and attenuation constant are co-optimized.This research can provide new ideas for preparing electromagnetic wave absorbing materials with thin matching thickness.（3）The Fe₄N@MWCNTs composite was successfully prepared by heat treatment and gas nitriding process of melamine and ferric chloride hexahydrate,and its formation mechanism was studied.The results show that when the heat treatment temperature is 800℃ or 900℃,the efficiency of Fe catalyzing the crystallization of carbon atoms to form carbon nanotubes is higher,so the content of multi-walled carbon nanotubes in the prepared samples is higher.The Fe₄N in the sample Fe₄N@CNTs-900 is uniformly distributed in the carbon nanotubes,and it also exhibits a high specific saturation magnetization of 140.9 emu/g at room temperature.Calculating the reflection loss of the Fe₄N@MWCNTs composite shows that when the sample mass fraction is 29%,Fe₄N@CNTs-900 has the best reflection loss performance,and when the thickness is 1.5 mm,the minimum reflection loss is-39.3 dB,while the effective absorption bandwidth is 4.2 GHz（12.8-17.0 GHz）.This is mainly due to the synergy between the magnetic loss capability of Fe₄N and the dielectric loss capability of multi-walled carbon nanotubes.The simultaneous existence of two phases of Fe₄N and multi-walled carbon nanotubes enriches the loss mechanism and optimizes the impedance matching,thus obtaining strong absorption,low density Fe₄N@MWCNTs composite absorber.（4）The iron-based metal-organic framework compound precursor is prepared by solvothermal method,and then the precursor is subjected to heat treatment and gas nitriding process to obtain a composite material of magnetic particles and porous carbon.The structure and morphology characterizations show that the magnetic particles are evenly distributed in the porous carbon matrix and form a core-shell structure;the heat treatment temperature and nitriding process have a significant impact on the phase of the magnetic particles in the composite material and the degree of graphitization of the porous carbon.Phase and morphology jointly affect the electromagnetic wave absorption performance of materials.When the heat treatment temperature is 700℃,the composite material of Fe₄N and porous carbon obtained by the gas nitriding process achieves the best electromagnetic wave absorption performance,and the minimum reflection loss reaches-56 dB.When the matching thickness is 2.5 mm,the reflection loss reaches-42 dB at 13.4 GHz,and the effective absorption bandwidth reaches 6.7 GHz.The excellent electromagnetic wave absorption performance comes from the synergy between Fe₄N and porous carbon.There are various magnetic loss mechanisms such as eddy current loss,natural resonance,exchange resonance and hysteresis loss in Fe₄N.Multiple scattering and reflection of electromagnetic waves occur inside the porous carbon matrix,which increased possibility of being wasted.