Microwave properties of high-aspect-ratio carbonyl iron/epoxy absorbers

Flake-shaped carbonyl iron (FCI) powder was prepared by ball milling spherical carbonyl iron (SCI) powder for 20 h. The thicknesses of the FCI particles were ∼0.1-0.3 μm with a diameter of ∼5-10 μm. The milling process increased the aspect ratio from ∼1 for spherical powder to an aspect ratio of ∼20-100 for flake one. The complex permittivity ( ɛ′ − jɛ″ ) and permeability ( μ′ − jμ″ ) were measured by the transmission/reflection method in the frequency range of 2-18 GHz for composites with various weight ratios of SCI and FCI. The results show that all ɛ′ , ɛ″ , μ′ and μ″ substantially increased after the milling treatment. Compared to SCI particles, the FCI particles exhibit higher permeability, higher permittivity, and better absorbing properties within the frequency test range. The magnetic loss tangent of FCI composites shows that FCI, with their large shape anisotropy, may have a higher resonance frequency and exceed the Snoek's limit in the gigahertz frequency range. For the 50 wt.% FCI composite with a thickness of 3 mm, the reflection loss at 5.5 GHz reaches −23.0 dB.