The effects of size and shape of iron particles on the microwave absorbing properties of composite absorbers

High values of magnetic permeability and dielectric permittivity of polymer composite containing flake-shaped iron particles were prepared by the mechanical milling of two batches of iron powders [Fe(A) and Fe(B)] under controlled conditions. The frequency-dependent complex permittivity (ε′-jε″) and permeability (μ′-jμ″) of resin composites with 40 wt.% of flake-shaped iron particles are investigated in 2-18 GHz. The results show that all ε′, ε″, μ′, μ″ substantially increase due to the increase of milling time and the size of the flake-shaped particles. The enhancement of space-charge polarization increases the permittivity of the composite absorbers. The frequency dispersion of μ″ of flake-shaped Fe/epoxy composites show a higher resonance frequency (around 3 GHz) which exceeds the Snoek's limit in the gigahertz range due to their low eddy current loss coming from the particle shape effects. For 40 wt.% composite with 2 mm thickness, the predicted reflection loss reaches - 19.4 dB at 14.9 GHz for Fe(A)/2 hr, -28.0 dB at 8.96 GHz for Fe(A)/4 hr, -31.6 dB at 11.6 GHz and -42.9 dB for Fe(B)/2 hr at 7.84 GHz for Fe(B)/4 hr as magnetic fillers.