Study on the electromagnetic characteristics of composites with hollow magnetic fibers and their applications for the absorbers

Abstract

Ultrafine hollow magnetic fibers were prepared by electroless plating using hydrolyzed polyester fibers as a sacrificial substrate. These hollow fibers can be served for lightweight and efficient electromagnetic (EM) absorbing materials. In order to characterize the surface morphology and composition of the fibers, SEM and EDS analysis were carried out. Hollow structures consisting of Ni inner layer and Fe or Fe-Co outer layer are obtained. By introducing Co onto Fe, oxidation of the Fe layer was successfully prevented making it possible to enhance the magnetic property compared to a case in which only Fe was used. Polymeric composites containing the hollow fibers with different weight fractions and fiber lengths were prepared by a simple mixing process. The electromagnetic properties were investigated, and it was found that the hollow magnetic fibers show a clear resonance peak within the X-band range (5~12 GHz) and the resonance frequency depends strongly on the fiber concentration and length. A possible explanation for the unexpected resonance is that the hollow fibers possess relatively low electron conductivity and a long electron path due to their oxidized phase and hollow structure.

Electromagnetic properties of epoxy composites containing hollow Ni/Fe fibers with different weight fraction and 30 wt% of hollow Ni/Fe-Co fibers with different fiber lengths were measured. The simulation results showed the possibility that the composite of 4mm in thickness containing 20wt% of hollow Ni/Fe fiber could have a high reflection loss at 3 GHz. In case of hollow Ni/Fe-Co (7:3) fibers, the composite designed for 13 GHz exhibited multiple absorbance peaks. It is obvious that this multiple absorbance and broad band width are attributed to the resonance characteristic of the composite. The real reflection losses of the composites could be compared to the designed values by attaching a copper tape on the one side of the toroidal specimen and just calculating the S11 p...