| Magnetic metals are potential heat-resistant microwave absorbers,but the problem of oxidation at high temperatures severely restricts their absorption performance.Austenitic alloys have very good oxidation and corrosion resistance properties,and have been widely used in the chemical and power generation industries.However,due to their paramagnetic properties,they have poor magnetic loss ability and have not been reported in the field of wave absorption.The performance of magnetic metal absorbents deteriorates in high temperature environment,which is related to the difficulty of maintaining the grain size in nanometers in addition to oxidation.In this paper,starting from Fe42.5Co42.5Cr15 alloy powder with austenitic structure,the feasibility of austenitic alloy as a heat-resistant and microwave absorber was explored by stirring ball milling.Fe/SiBa was prepared by ball milling-stirring ball milling "two-step method" the effect and principle of SiBa alloy doping on stabilizing the grain size and electromagnetic properties of magnetic metal nanocrystals were studied.The main conclusions of this paper are as follows:(1)During the stirring ball milling process,the Fe42.5Co42.5Cr15 alloy particles gradually transitioned from spherical particles to flaky particles,and a significant ferromagnetic transition occurred,and the Ms gradually increased from 1 emu/g to about 110 emu/g.The reason for its ferromagnetism is the deformation-induced martensitic transformation,that is,the transformation from paramagnetic γ-austenite to ferromagnetic α’-martensite,during the milling process.The magnetic permeability of Fe42.5Co42.5Cr15 alloy was greatly improved after ball milling treatment,μ’ increased from 1.13 to 2.54,and μ" increased from 0.15 to 0.92.The simulated reflection at a coating thickness of 2 mm can be improved from-4.57 dB for the original powder to36.94 dB.The ferromagnetism of Fe42.5Co42.5Cr15 alloy powder obtained by ball milling can be further improved after heat treatment,and the increase of Ms after heat treatment of alloy particles with longer milling time is also greater.After vacuum heat treatment,Ms can increase to 181.58 emu/g;however,martensite begins to reversely transform to austenite after heat treatment temperature exceeds 500℃,and all samples return to austenite phase after heat treatment at 900℃.The coating prepared by mixing Fe42.5Co42.5Cr15 alloy with silicone resin can still maintain relatively stable absorption performance after heat treatment at 300℃ for 200 h.(2)During the preparation process of the "two-step method",the grains of the composite particles are gradually refined and the internal strain is continuously accumulated.The final obtained flake Fe/SiBa composite particles have a grain size of about 9 nm and an internal strain of around 0.9%.With the increase of doping amount,the Ms of the composite particles decreases gradually.When the SiBa alloy content is 0 and 15 wt.%,the Ms is 190.43 emu/g and 147.13 emu/g,respectively.With the increase of doping content,the effect of composite particles on inhibiting grain growth at high temperature continues to improve.After heat treatment at 500℃,600℃ and 700℃,the grain sizes of carbonyl iron(Fe)particles reach 46.2 nm,85.1 nm and 158.1 nm respectively,while the grain sizes of composite particles containing 15 wt.%SiBa alloy are 19.5 nm,32.0 nm and 40.9 nm respectively.The doping of SiBa alloy significantly changed the change trend of the complex permittivity of the composite particles with the heat treatment temperature,from the gradual increase of Fe to the first increase and then the decrease;the decrease of complex permeability with the increase of heat treatment temperature decreases with the addition of SiBa alloy.The excellent grain size stability enables the composite particles to have a moderate dielectric constant,thereby maintaining relatively good impedance matching performance,and thus having stable temperature resistance and absorption performance. |
