Preparation And Electromagnetic Wave Absorption Properties Of Carbon-coated Nanocrystalline Fe₉₀Si₇B₃ Composite Powders

Fe-based nanocrystalline soft magnetic alloys have broad application prospects in the field of electromagnetic wave absorption in the GHz band due to their high saturation magnetization(M_s)and high frequency permeability.However,it only has a single magnetic loss,and is dense and easily corroded.It is difficult to meet the comprehensive performance requirements of modern high-efficiency absorbing materials.Carbon materials with good dielectric loss characteristics have low density,high temperature resistance and corrosion resistance.The composite modification of Fe-based nanocrystalline alloys and carbon materials is expected to improve the absorbing performance while reducing the density of the absorbent and improving the stability.In this work,based on the research group's early exploration of Fe-based nanocrystalline alloy absorbing materials,the Fe₉₀Si₇B₃ alloy with high Fe content is compounded with carbon,and different morphologies are prepared by adjusting the presence or absence of process control agents in the ball milling process.The morphology,microstructure and electromagnetic properties of composite powders with a carbon content of 8?15 wt.%are studied,and the reflection loss of the composite powders in the frequency range of 2–18 GHz is calculated by the transmission line principle to evaluate its absorbing performance.In addition,the effect of annealing process on the microstructure and wave absorbing properties of the composite powder was also evaluated.The main results are as follows:(1)Without process control agent,a nearly spherical composite powder with an average particle size of 3.5?m can be obtained after ball milling,which has a core-shell structure.The core FeSiB alloy is an amorphous/?-Fe nanocrystalline composite structure,and the grain size is 7 nm,the shell part is amorphous carbon.The composite powder has both high M_s and high-frequency magnetic permeability.The M_s of the composite powders with a carbon content of 8 and 15 wt.%are 163.4 and 145.3 emu/g,respectively.The composite powder exhibits better electromagnetic wave absorbing performance than a single nanocrystalline powder.The coating made of composite powder with a carbon content of 8 wt.%and paraffin wax in a 3:2(wt.%)mixture has a peak reflection loss(RL_min)of-54.8 d B at 12.7 GHz,and an effective absorption bandwidth(?f_{RL<-10 d B})of 6.5(10.0-16.5)GHz,and the matching thickness(d_m)is1.9 mm.When the matching thickness is 1.7 mm,the effective absorption bandwidth can reach7.3 GHz,covering the X-band high-frequency region and the entire Ku-band.(2)Using absolute ethanol as a process control agent,a lamellar composite powder can be obtained after ball milling.The particle size of the flakes is different.The composite powder has an amorphous/?-Fe nanocrystalline dual-phase structure and a crystal grain size of14 nm.Carbon of the small platelet particles is amorphous,and is stacked and covered on the plate-shaped alloy particles.The composite powder also has both high M_s and high-frequency magnetic permeability.The composite powders with a carbon content of 8 and 15 wt.%have M_s of 158.6 and 147.0 emu/g,respectively.After flattening,compared with the near-spherical composite powder,the effective absorption bandwidth of the wafer composite powder with carbon content of 8 wt.%is larger in the range of 1.4?1.7 mm.When d_m is 1.6 mm,the RL_minand?f_{RL<-10 d B} of the flake samples are-29.7 d B and 6.5(11.5–18.0)GHz,respectively.And the RL_min and?f_{RL<-10 d B} of of the spherical samples are-31.7 d B and 5.7(12.3–18.0)GHz,respectively.The microwave absorbing properties of the flaky composite powder with 15wt.%carbon are better,when d_m is 1.5 mm,the RL_min and?f_{RL<-10 d B} of the flake samples are-29.6 d B and 5.1(12.9–18.0)GHz,respectively.And the RL_min and?f_{RL<-10 d B} of of the spherical samples are-19.2 d B and 2.9(15.1–18.0)GHz,respectively.(3)After the composite powder with a nearly spherical carbon content of 8 wt.%is heat-treated at 860 K for 90 min,the grain size of the?-Fe phase increases to 30 nm,the M_sincreases to 182.0 emu/g,and the RL_min can reach-36.6 d B at 17.1 GHz,the d_m is only 1.7 mm,and the?f_{RL<-10 d B} can be up to 8.2 GHz at 2.1 mm.After the flaky composite powder with a carbon content of 8 wt.%is heat-treated at 873 K for 60 min,the grain size is 36 nm,and the M_sis 167.8 emu/g.When d_m is 1.7 mm,the?f_{RL<-10 d B} can reach 6.1(11.6–17.7)GHz.(4)The excellent electromagnetic wave absorption of of nanocrystalline FeSiB/C composite powders come from the good impedance matching and the synergistic effect of electromagnetic provided by the magnetic-dielectric component.On the one hand,carbon can effectively adjust the complex permittivity of composite materials by introducing conductance loss and polarization relaxation.On the other hand,the aggregation of Fe-based alloy particles is interrupted,the eddy current and skin effect of the alloy particles are weakened,and strong magnetic loss is generated to make up for the magnetic loss loss caused by the introduction of non-magnetic materials.