Enhancement of microwave absorption properties using spinodally decomposed Fe–Cr–Co flakes

Abstract

The magnetic loss of a microwave absorber strongly depends on its complex permeability (μr), and it is necessary to have a high imaginary part of the complex permeability (μr″) at the target frequency range. In our previous work, the peak μr″ for Fe–Cr–Co powder and resin composites was successfully shifted to higher frequency ranges due to microstructural changes resulting from spinodal decomposition of Fe–Cr–Co alloy powder. However, its μr was not high, and the matching frequency was limited to around 1.6 GHz. In this study, we aimed to improve μr for spinodally decomposed Fe–Cr–Co alloy powders by flattening the powders to flakes using the ball milling process. A change in shape from spherical to flake-like occurred not only for the Fe–Cr–Co powders but also the spinodally decomposed microstructure in the powder, in which FeCo-rich (α1) particles surrounded by a Cr-rich matrix (α2) phase was obtained by spinodal decomposition. Based on measurements of electromagnetic properties for the resin composites of Fe–Cr–Co flakes, we found that both μr′ and μr″ were significantly increased after flattening the powder. Furthermore, excellent microwave absorption with a matching frequency of around 3 GHz was achieved for the resin composites of Fe–Cr–Co flakes. The peak shift was ascribed to the enhanced μr″ at high frequencies induced by flake-shaped powders and spinodally decomposed microstructures. Noise suppression capability (ΔPloss/Pin) was also investigated for the resin composite sheets using spinodally decomposed Fe–Cr–Co flakes. ΔPloss/Pin of the sheet using flake-shaped powders was found to be higher than that of the precursor spherical powders in the frequency range of 15–40 GHz. Based on microstructure observations, these improvements were ascribed to enhancement of permeability in the high-frequency range due to flake-shaped Fe–Cr–Co powder containing spinodally decomposed α1 particles, which resulted in the formation of a magnetic circuit along the flake plane. These results showed that Fe–Cr–Co alloy flakes may be a promising material for both microwave absorbers and noise suppression sheets in the GHz frequency range.