This study was focused on the evaluation of electromagnetic wave shielding and absorption characteristics of composites by using magnetic lossy materials and conductive networking effect at far-field and near-field range.
In order to enhance the EM absorption and shield capability, the magnetic composite sheet is required the high conductivity for shield and high magnetic loss for absorption, simultaneously. To design the magnetic composites filled with magnetic fillers, the magnetic properties, the dimension of magnetic filler such as an aspect ratio should be considered. The conductive magnetic fillers with high aspect ratio have an advantage of the conductive path even at a low contents in polymer matrix, Therefore, FeCoNi was coated on continuous glass fiber using by electroless plating and then they were filled in polymer matrix. To evaluate the EM absorption and shielding characteristics, the measurement system based on IEC 62333 standard evaluation methods were employed for the near-field absorption and shielding effects. The properties of the home-made fixtures for measurement were verified by the EM simulation. Power absorption of FeCoNi filled in composite sheet was drastically increased up to 95% with the increment of frequency from 0.1 GHz to 10 GHz. Inter-decoupling showed maximum 30 dB at around 5.3 GHz, which is comparable to that of a conductive Cu foil. Shielding effectiveness (SE) was measured by using rectangular waveguide method. SE of the 0.5 mm-thick composite sheet obtained about 37 dB at X-band frequency region.
Second, to enhance the EM shielding and absorption effect, FeCoNi coated glass fabric (MGF)/polycarbonate composite sheets were prepared. The composite sheets were composed of the laminated structure which has one or two ply-FeCoNi coated glass fabrics with or without Ni grid in polymer matrix. The conductive paths in the fabrics was better aligned with the current directions than fiber type which minimize the conductive loss. In addition, the insertion of Ni grid in composite sheet can be enhanced the shielding due to the high conductive and controlled the absorption frequency. The power absorptions were up to about 86 % at 10 GHz. The inter-decoupling effect for FeCoNi coated glass fabric with Ni grid in with 0.5 mm-thick composite sheet exhibited about 45 dB at around 1.3 GHz, which is comparable to that of a conductive Cu foil. The shielding effectiveness (SE) was obtained over 70 dB at X-band region.
Third, in an application of magnetic composite on electronic circuits and devices, the EM absorber requires heat dissipation of the thermogenesis with noise reduction simultaneously in limited dimension and a complicated structure. The Al2O3 coated Fe-Al-Si flakes in 150 um-thick dielectric matrix were prepared. The power absorptions for Al2O3 coated Fe-Al-Si flakes composites were decreased from 62.7 % to 57.4 % (at 5 GHz) with the increment of coating thickness in comparison with that of uncoated Fe-Al-Si flakes composite (80% at 5 GHz). The thermal conductivities of Al2O3 coated composites were increased up to 0.249 W/mK (260 %) in comparison with that of uncoated composite of 0.096 W/mK.
Fourth, to measure the permeability and ferromagnetic resonance up to GHz frequency region, the shorted microstripline fixture was produced and extracted complex permeability. From the different distribution of electric and magnetic field in coaxial airline and shorted microstripline, the effect of filler alignment direction were studied. The measured complex permeability of sendust flakes composite sheet by coaxial airline were compared with that of shorted microstripline.
Finally, to understand the domain wall movement and spin resonance phenomena observed in complex permeability. The parameters were obtained from broadband VNA-FMR measurement system. FMR signal were measured on FeCoN thin film with uniaxial anisotropy to in plane direction. The resonance frequency and gilbert damping parameters were determined by analyzing measured resonance signal. The FeCoN thin film with PN2 5.5%, low damping constant of 0.0046 were extracted from fitting process.