Study On The Electromagnetic Properties And Mechanism Of FeSiAl Alloy Micropowder Composites

FeSiAl alloy flakes have low magnetic anisotropy and high magnetic permeability,which have excellent electromagnetic absorption at the Gigahertz frequency band by controlling the shape of the particle.The flakes has been widely used in military stealth and against electromagnetic interference.In order to satisfy the microwave absorption in the L and S bands,a higher demand for high magnetic permeability and low dielectric constant of the FeSiAl alloy flakes is proposed.The study on the mechanism of the relevant electromagnetic parameters is particularly important,which can provide a theoretical guidance for the material design and preparation.For the flake-shaped Fe SiAl alloy powders,the shape factor and the internal fine structure are the main factors regulating the microwave magnetic properties in Gigahertz frequency band.For the former,the brittleness of FeSiAl alloys will limit the further improvement of the shape factor.For the latter,the detailed study of superlattice transformation and corresponding magnetic properties inside the alloy is lack.On the other hand,the high dielectric constant of flake-shaped FeSiAl alloy micropowders will affect the impedance matching.The research on this problem is still focused on solving the problem by coating a layer of high resistance layer.The mechanism of high dielectric constant is less studied.At the same time,the common surface coating or oxidation treatment not only reduces the dielectric constant,but also reduces the magnetic properties.It is necessary to further study and analyze the mechanism of high permittivity and explore the modification technology which can reduce the permittivity and maintain high permeability.The research work of this dissertation is mainly based on the electromagnetic parameter characteristics of flake-shaped FeSiAl alloy powders and the requirement of electromagnetic properties of microwave absorbing materials in L and S bands.The effects of the composition,fine structure,surface structure and composition on the microwave electromagnetic properties of alloy micropowders are systematically studied.The goal of the research is to control the FeSiAl alloy microwave electromagnetic parameters and obtain better microwave absorbing performance by controlling the shape factor,crystal structure,superlattice and surface composition of FeSiAl alloy.The main research work and innovation are divided into the following four aspects:1.Flake particles with high shape factor and small demagnetization factor in the plane are prepared by using Cr element in formula optimization.The saturation magnetization of Cr substituted Fe samples is obviously decreased,which is not conducive to the increase of microwave permeability.M?ssbauer spectroscopy was used to analyze the fine structure of Cr substituted Fe samples.It was found that the reasons for the decrease in saturation magnetization were the decrease of nearest neighboring Fe atoms atom around the central Fe atom and the weakened of exchange between adjacent Fe atoms.This phenomenon is not observed in Cr substituted Si samples,so the samples can maintaine high saturation magnetization.When the atomic ratio of Cr to Si increases from 0 at%to at6%,the resonant frequency of the imaginary part of the magnetic permeability increases from 1.7 GHz to 2.5 GHz,and the peak value increases from 3.7to 5.2.Combined with the LLG equation?Laudau-Lifshitz-Gilbert?,the change of the magnetic spectrum was mainly due to the increase of the demagnetization factor.2.It is proposed that as annealing temperature increasing,a transitional phase with weak magnetic properties,which crystal structure is approximate to B2 phase,exists in the process of transition from disorder phase A2 to ordered phase D0₃ of FeSiAl alloy.The characteristics of the three superlattice crystal structures and the corresponding magnetic properties of Fe₁₆Si₂Al₂ alloy were analyzed by First-principles calculation.It was found that the alloy with D0₃ structure has low magnetocrystalline anisotropy?K₁?and high saturation magnetization?Ms?,suitable for the use in the microwave absorption,and the approximate-B2 phase has low Ms and high K₁,which should be avoided.3.The crystal structure of Fe₇₅Si₁₃Al₁₀Cr₂ alloy flakes and their effects on magnetic properties were studied after annealing at different temperatures.Combining with the superlattice structure and characteristics in Fe₇₅Si₁₅Al₁₀0 alloy,it is found that there is a approximate-B2 phase in the superlattice transformation process of the alloy crystal structure with the increase of annealing temperature.M?ssbauer spectroscopy was used to analyze the fine structure of samples under different crystal structures.It was found that the main reason for the lower magnetic moment of the sample with approximate-B2phase was that the hyperfine field of the Fe?8?and Fe?6?sites was small.The results of the test are consistent with the those obtained by the first-principles calculations.After annealing at suitable temperature,the microwave complex permeability of the sample is obviously improved.4.According to the actual situation and the polarization model of the dielectric constant,the main factor of the high dielectric constant of the FeSiAl alloy flake composites is that the particle conductivity is too strong and the conductivity of the composite is improved.The selective surface oxidation of FeSiAl alloy flake particles was achieved by annealing in H₂/N₂ atmosphere.A high-resistance layer of alumina oxides and silica oxides was formed on the surface of the particles,and oxidation of iron was avoided at the same time.Selective surface oxidation process can reduce the dielectric constant while maintaining the high permeability of the FeSiAl alloy flakes.The research provide a guidence the preparation of FeSiAl alloy flake micropowders.The proposed fine structure transformation and selective surface oxidation method provide design ideas for the preparation of FeSiAl alloy powders with high permeability and low dielectric constant.