Study On Preparation And Microwave Absorption Properties Of Fe₃Al@Al₂O₃ Magnetic Loss Absorbent

In recent years,with the rapid development of stealth technology,the ability of radar detection and tracking continue to be improved.Electromagnetic wave absorbents have become a hot research topic in the field of weapons and equipment.Fe₃Al magnetic powder is a typical magnetic loss absorbent that has a high permeability,high Curie temperature,low conductivity,and low density,which is currently a hot research direction that worth concerning.However,with the increase of service temperature,the Fe₃Al magnetic powder exhibits poor stability,and tends to be oxidized,which leads to the gradual attenuation of microwave absorption performance.Therefore,in this thesis,flake Fe₃Al magnetic powder absorbents were prepared by mechanical alloying,followed by ordering treatment.Then,anα-Al₂O₃ shell with a thickness in nanometer scale was constructed on the surface of Fe₃Al micron powder by a controlled oxidation technology.Finally,Fe₃Al@Al₂O₃ absorbents with antioxidant properties were prepared,and their electromagnetic wave absorption performances under different service temperatures were investigated.Fe₃Al magnetic powders were prepared by mechanical alloying and ordering treatment.Firstly,the ball milling time and rotation speed in the mechanical alloying process were explored.The flake Fe（Al）solid solution precursors powder with large aspect ratios were prepared.The increase of the anisotropy of the material shape is conducive to the occurrence of interfacial polarization,which can break the limitation of the Snoek’s limit and improve the electromagnetic wave absorption performance.The Fe₃Al magnetic micro powder was obtained by ordering treatment of the precursor powder prepared under the ideal high-energy ball milling conditions.The effects of Al content and ordering temperature on the morphology,phase structure,magnetic properties and electromagnetic wave absorption properties of the Fe₃Al magnetic powders were studied.It was found that when the content of Al is 30 at%and the ordered calcination temperature is 500℃,the VSM curve of the Fe₃Al magnetic powder shows a typical S curve,and the saturation magnetization is the highest.In the electromagnetic parameters,the imaginary part of the complex permeability has a more obvious resonance peak,and the tangent of magnetic loss（tanδ_ε）indicates the strongest magnetic loss performance.It can absorb and lose electromagnetic waves through interface polarization,space charge polarization,and natural resonance with suitable impedance matching characteristics and excellent attenuation characteristics.At the matching thickness of 1.25 mm,the reflection loss RL_min is-18.2 d B,and the effective absorption band width f _E can reach 6.5 GHz.The Fe₃Al@Al₂O₃ absorbent was prepared by a controlled oxidation technique,where a uniform and compactα-Al₂O₃ protective film was in situ grown on the surface of the micrometer Fe₃Al magnetic powder.The effects of oxygen content and calcination time on the thickness,compactness and uniformity of the shell were studied.The Fe₃Al@Al₂O₃ absorbents with a nanoscale core-shell structure composed of Fe₃Al as the core andα-Al₂O₃ as the shell were obtained.It was found that when the oxygen flux and calcination time are too low,the obtainedα-Al₂O₃ particles are dispersed,which cannot form a coating layer for Fe₃Al.However,when the oxygen flux and calcination time are too high,theα-Al₂O₃ particles grow abnormally large and agglomeration occurs.The shell layer is too thick,which reduces the soft magnetic properties of the material and affects its electromagnetic properties and electromagnetic wave absorbing properties.When the oxygen content is 5 m L and the calcination time is 2 h,the prepared Fe₃Al@Al₂O₃ absorbent has a continuous,uniform and denseα-Al₂O₃ coating layer,and the shell thickness is in nanometer scale.The magnetic loss performance in the electromagnetic parameters of this sample is the best.The values of the dielectric loss tangent（tanδ_ε）and the magnetic loss tangent（tanδ_μ）are similar,indicating the excellent impedance matching ability.Thus,the incident electromagnetic waves can enter the absorbent without being reflected.In addition,the value of the attenuation coefficient reflects its good electromagnetic wave attenuation performance.When the matching thickness is 1.25 mm,the reflection loss RL_min is-18.6 d B,and the effective absorption band width f _E is 6.0 GHz.Fe₃Al and Fe₃Al@Al₂O₃ microwave absorbents with the best electromagnetic wave absorption performance were served for 2h at different temperatures（25℃,200℃,300℃,and400℃）.It was found thatγ-Al₂O₃ and Fe₂O₃ phases appeared in the Fe₃Al absorbent when the temperature reached 400℃,In contrast,the phase composition of the Fe₃Al@Al₂O₃ did not change at different temperatures,indicating that the Fe₃Al@Al₂O₃ had better oxidation resistance.The magnetic loss tangent（tanδ_μ）values of the Fe₃Al and Fe₃Al@Al₂O₃ were compared at 400℃.It can be found that the attenuation amplitude of the magnetic loss performance of the Fe₃Al is much higher than that of Fe₃Al@Al₂O₃,which indicates that the Al₂O₃ nanoshell can effectively prevent the invasion of oxygen atoms and then avoid the oxidation of the internal Fe₃Al,ensuring the magnetic properties of the absorbent to keep the good electromagnetic wave absorption performance.At the matching thickness of 1.50 mm,the reflection loss diagrams at different temperatures were compared.It can be found that the maximum effective absorption bandwidth f _E the of Fe₃Al absorbent decreases by 46.8%at 400℃compared with that of 25℃at room temperature.In comparison,the reduced amplitude of the Fe₃Al@Al₂O₃ absorbent is 19.2%.The Fe₃Al@Al₂O₃ shows a larger effective absorption bandwidth at 400℃,indicating that the Al₂O₃nanoshell improves the high-temperature oxidation resistance of the Fe₃Al,enabling it to work at the temperature above 400℃for a long time,thus widening the service temperature range.