Effect of heat treatment on the electromagnetic absorption performance of spherical, flower-like and thorny Co@Fe composite particles

Abstract

Spherical, flower-like, and thorny Co@Fe composite particles were prepared by liquid-phase reduction, and the effect of heat treatment on the micromorphology, magnetostatic properties and electromagnetic wave absorption properties of the Co@Fe composite particles were investigated. Studies showed that the heat-treated spherical, flower-like and thorny Co@Fe composite particles retained morphologies similar to those before annealing without too much variations. Furthermore, FCC-Co, HCP-Co, CoFe, Co₇Fe₃, γ-Fe, Fe₃O₄, and Co₂O₃ were generated via heat treatment. Fe underwent a phase transformation from the body-centered cubic α-Fe to the face-centered cubic γ-Fe. The saturation magnetization and coercivity of the spherical, flower-like, and thorny Co@Fe composite particles increased, with the thorny Co@Fe particles exhibiting higher coercivity than the spherical and flower-like Co@Fe particles. With heat treatment, the ε′ values of the spherical and thorny Co@Fe particles decreased, while the ε′ value of the flower-like Co@Fe particles increased. Within the frequency range of 2–9 GHz, the ε″ values of the spherical and flower-like Co@Fe particles showed an increasing trend, while in the 9–18 GHz range, the ε″ values of these particles significantly decreased. The µ′ values of the spherical, flower-like and thorny Co@Fe composite particle samples exhibited minor fluctuations before and after heat treatment, but the changes were not significant. The µ″ value of the heat-treated spherical Co@Fe particles showed a slight increase, while the µ″ value of the flower-like Co@Fe particles increased within the 13–18 GHz range. The µ″ value of the thorny Co@Fe particles showed a slight increase in the 15–18 GHz range, but a decrease was observed in the 2–15 GHz range. Heat treatment improved the tan δ_m values of the spherical and thorny Co@Fe particle samples while decreasing the tan δ_m value of the flower-like Co@Fe particle samples. The relaxation polarization effects of the heat-treated spherical, flower-like, and thorny Co@Fe composite particle samples were diminished. The C₀ values of the heat-treated spherical, flower-like, and thorny Co@Fe composite particle samples exhibited a downward trend with increasing electromagnetic wave frequency. In the 2–6 GHz range, magnetic losses were primarily due to natural resonance, while in the 6–18 GHz range, magnetic losses resulted from the combined effects of eddy current loss and natural resonance. The maximum attenuation constants of the heat-treated spherical, flower-like, and thorny Co@Fe composite particle samples showed a slight increase, although the value of the thorny Co@Fe particle sample decreased. Heat treatment improved the impedance matching of the spherical Co@Fe particle samples, while the impedance matching performance of the flower-like and thorny Co@Fe composite particle samples deteriorated. Heat treatment enhanced the electromagnetic wave absorption performance of the spherical and flower-like Co@Fe particles to some degree, while for the thorny Co@Fe particles, the electromagnetic wave absorption performance did not improve after heat treatment. The heat-treated spherical Co@Fe particle achieved a minimum reflection loss of −15.36 dB at a thickness of 2.0 mm, with an effective absorption bandwidth of 4.96 GHz.