Facile synthesis of FeCo–MnO2 core–shell nanoparticles as high-frequency microwave absorbers using a two-step method

Abstract

The absorption performance of Fe80Co20 (FeCo) nanoparticles for electromagnetic waves is diminished by their low complex permittivity. The complex permittivity of FeCo nanoparticles can be easily tuned by combining the nanoparticles with dielectric materials. A MnO2 shell has a porous structure, and its pore size increases with increasing thickness, resulting in enhanced impedance matching and multiple scattering within pores. Herein, FeCo–MnO2 core–shell nanoparticles with different MnO2 shell thicknesses were synthesized using a polyol-coating method. Their imaginary permeability was almost constant regardless of the MnO2 shell thickness, whereas their complex permittivity increased with increasing MnO2 shell thickness. The FeCo–MnO2 (71 nm) nanoparticles showed Lorentz-type dielectric resonance due to a displacement current lag at a core–shell interface. The FeCo–MnO2 (71 nm) nanoparticles showed a high reflection loss of –36 dB at 17 GHz, wide bandwidth of 4.9 GHz, and microwave absorber thickness of 1.9 mm. The mechanisms responsible for microwave absorption were magnetic coupling, natural resonance, interfacial polarization, Lorentz type resonance, hopping, and multiple scattering. This strategy enables the easy and practical design of magnetic core–dielectric shell-based microwave absorbers.