Preparation And Electromagnetic Properties Of Flake-shaped Fe-Si Particles

Due to the high value of permeability in target bands, ferromagnetic materialsare of interest among various candidates of electromagnetic absorbing (EMA)materials. Particularly, the ferromagnetic metallic particles with a flake shape havedrawn much attention. In this paper, commercially available Fe-Si alloy particleswere milled under controlled conditions to fabricate flake-shaped particles. This way,flake-shaped Fe–Si particles with various shape and size were successfully preparedby changing the milling parameters. The influence of milling time, rotation speedand ball to powder ratio on the phase structure and morphology were studied.Meantime, the conductivity and static magnetism properties were investigated. Andthe electromagnetic and microwave absorbing properties of the prepared Fe-Siparticles were also investigated in0.1-18GHz.The study on the milling parameters suggested that as the milling proceeded,most particles were squashed into thin flakes and the thickness of the particledecreased and the aspect ratios increased. When the ball milling time was furtherelongated, large flakes would eventually be crushed into fine pieces when theybecame very thin in the thickness. Compared with500rpm, the Fe-Si particlesmilled at the rotation speed of450rpm had smaller size and they tend to be crushedearlier. Next changing the ball to powder ratio, we fabricate the flake-shapedparticles with the highest aspect ratio.The static magnetic properties were measured. Both the coercivity and theremanent magnetization were found to increase with the milling time, which isconsidered to originate from the disordered crystal structure and the increasedinternal strain. The saturation magnetization of the particle did not change evidentlyin the beginning, but was found to decrease slightly afterwards. The formation ofhigher density of defects, due to the severe deformation in Fe–Si alloy particles, wasbelieved to contribute to the decrease in the saturation magnetization.The study of electromagnetic properties suggested that flake-shaped Fe-Siparticles possessed higher permeability than the raw powder. We founded that flakeparticle with high aspect ratio has high shape anisotropy, which makes a significantcontribution in exceeding the Snoek's limit. Moreover, the flake particles' smallthickness successfully suppressed the negative effect of eddy current. Meanwhile,the flake-shaped Fe-Si particles also have a higher level of permittivity, which isbecause of the intense dielectric relaxations and enhanced polarization.Base on the results of the electromagnetic properties. EMA performances ofcoatings using Fe-Si particles as fillers were evaluated by calculating the reflection loss using the transmission line theory. To compare with the raw powder, the RLpeaks of coatings using milled Fe–Si particles as fillers shifted dramatically towardslower frequency range. According to a quarter-wavelength cancellation theory, theshift of RL peaks is attributed to the increase of the permittivity and the permeability.At last, a minimum reflection loss of-16dB was achieved at around2GHz incoatings containing milled particles as fillers, suggesting a high EMA efficiency inL-and S-band.