| Electromagnetic wave absorbing materials working in 0.1-18 GHz are widely used in the communication field to reduce the interference caused by radiated electromagnetic waves to communication devices.An excellent absorbing material should not only have strong absorption and wide frequency band,but also have a thin thickness.The dielectric/adhesive absorbers have been widely reported at present.Although they have strong absorption,their absorber thicknesses are too thick,especially in the C band(4-8 GHz),S band(2-4 GHz)and L band(1-2 GHz).Compared to dielectric absorbers,magnetic powder/adhesive absorbers generally have a thinner thickness due to their high permeability,and increasing permeability can further decrease the thickness of absorber.In this paper,the easy-plane anisotropic R2Me17N3-x alloy(R=Pr,Ce;Me=Fe,Co;0?x?3)was selected as our research object,and R2Me17N3-x microflakes were prepared.The biggest feature of the micro-flake is that the easy-magnetization crystal plane-(006)plane coincides with the flake plane.Through a magnetic field-induced orientation,these flake planes will be parallel to each other,which makes the out-of-plane anisotropic field increasing and makes the magnetic moments to be distributed in the flake planes.As a result,the permeability in the flake plane increases significantly,and the thickness of absorber is decreased.First,we selected Pr2Co17 with easy-magnetization crystal plane-(006)plane as the research object,and prepared the Pr2Co17 microflakes fractured along the(006)crystal plane by a high-energy ball milling method with surfactant.The microflakes were then oxidized using hydrogen peroxide,which not only reduces the permittivity but also suppresses the eddy currents of Pr2Co17 microflakes.As a result,the oriented Pr2Co17 microflakes/epoxy composite obtained two strong absorptions in the C and S bands at two thin thicknesses.In order to further reduce the thickness of absorber,the permeability of material needs to be further increased.As we known,the permeability of Fe-based alloys is usually higher than that of Co-based alloys.So,we studied the effect of Fe substitution on the permeability and microwave absorption properties of Pr2Co17-xFex microflakes/epoxy composites.It was found that the permeability of the samples of x=11 and x=13 was significantly higher than that of the oriented Pr2Co17 microflakes/epoxy composite.Due to the higher permeability of the two samples,the thickness of absorber in the L band is also very thin.However,the permittivity of the two samples is too high,resulting in poor matching performance and weak absorption peaks.In order to improve the matching performance and enhance the absorption,we prepared Pr2Fe17N3 microflakes fractured along the(006)crystal plane.Compared with the Pr2Co17-xFex microflakes,the permeability of Pr2Fe17N3 microflakes is higher,the resistivity is larger and the permittivity is lower due to the presence of N atoms.Thus,a strong peak of-28.5 dB is obtained near the upper limit(2 GHz)of the L-band at a thickness of 2.4 mm.From the permeability spectrum of Pr2Fe17N3/PVB composite,it was found that the imaginary part(?")of permeability at 100 MHz is very low.If the real part(?')of permeability is increased,this material can be used as the magnetic powder core of power transformer.Thus,we selected Ce2Fe17N3 with higher saturation magnetization and smaller in-plane anisotropy field as the research object,and prepared Ce2Fe17N3 microflakes fractured along(006)plane.For a Ce2Fe17N3 microflake/polyurethane composite with a volume fraction of 65%,the ?' at 100 MHz is 9.3,while the ?" at 100 MHz is only 0.56.Thus,the magnetic loss tangent(tan?m)at 100 MHz is only 0.06,this makes it possible to manufacture power transformers operating at 100 MHz with high output power and low loss.It is more worth mentioning that the Ce2Fe17N3 sample with separated ?-Fe has a ?' of 13.5 and a ?" of 0.5 at 100 MHz,and the tan?m at 100 MHz is only 0.04. |
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