New magnetic fillers for EM attenuation composites at microwave frequencies

This project aims at developing electromagnetic (EM) composite materials consisting of magnetic or dielectric fillers and epoxy, which have strong EM energy attenuations.

(1) Ferrite plate fillers (Fig.1)
Conventional ferrite fillers for EM composites are fine spherical powders with sizes of 3-5 μm. The permeability of composites with the fillers is rather small. This intrinsic drawback has been overcome by using newly developed ferrite plate fillers. The most important characteristic of the plate fillers is their very small ratio of thickness to plane dimensions (<1/10). Consequently, μ’ and μ’’max increase greatly by more than 10-fold, and therefore the attenuation properties can be significantly improved. At the same time, the bandwidth can be greatly expanded, covering from 0.13 to 0.34 GHz for RL<-20 dB. The thickness is reduced to 1.3 cm. A minimum reflectivity RLmin<-35 dB has been achieved.

(2) Meta-structure with greatly enhanced permeability (Fig.2)
A new metamaterial has been prepared by using a coil wound on a ferrite core, whose real and imaginary permeabilities are both greatly enhanced. The maximum μ’ and μ’’ are only 13 and 8 for the core material, while they greatly increase to 60 and 120, respectively, when] greatly increase to 60 and 120, respectively, when 16 turns of coil is wound. More importantly, a large negative permeability can also be obtained at certain frequency ranges. The metamaterials might be useful as: (a) left-hand materials due to the negative permeability, (b) frequency tunable devices by changing the turns of coil, and (c) ultra-thin absorbers due to the large imaginary permeability.

(3) Barium ferrite fillers with c-axis anisotropy (Fig.3)
A series of barium ferrites with c-axis anisotropy, instead of normal c-plane anisotropy, has been developed. With these powders have been used to fabricate composites with two important characteristics: high resonance frequency (6-13 GHz) and large ratio of μ’’max/(μ’-1)>1, which can be used to make materials with high attenuation and small thickness. Specifically, for x=0.6 and 0.5, the bandwidth for RL<-20 dB covers over 8.3-12.4 GHz (Red) and 12.5->16.5 GHz (Black), at thickness of 0.22 and 0.16 cm, respectively. They are good composites with low EM reflectivity and broad bandwidth at X and Ku bands.