Synthesis and electromagnetic wave absorption characteristics of magnetic γ-Fe₂O₃ modified Co@NC composite particles

Co@NC particles were prepared with liquid-phase deposition combined high-temperature pyrolysis, while nano γ-Fe₂O₃ was synthesized using the sol-gel method. Different ratios of nano γ-Fe₂O₃/Co@NC composite particles were produced through ball milling. The microstructure, crystalline properties, static magnetic characteristics, and electromagnetic wave absorption properties of the materials were systematically investigated with scanning electron microscopy (SEM), X-ray diffraction (XRD), vibrating sample magnetometry (VSM), and vector network analyzer techniques. The results indicate that the Co@NC particles retained the characteristics of the nearly dodecahedral structure derived from ZIF-67, with an approximate particle size of 700 nm. The γ-Fe₂O₃ synthesized via the sol-gel method exhibited a spherical morphology with a particle size of approximately 80-100 nm. Following ball milling, a tight composite structure was formed between γ-Fe₂O₃ and Co@NC, with γ-Fe₂O₃ covering the surface of Co@NC, and particle agglomeration intensifying with increased γ-Fe₂O₃ content. XRD analysis confirmed the coexistence of characteristic diffraction peaks for both Co@NC and γ-Fe₂O₃ in the composite samples. Analysis of the electromagnetic parameters showed that the real part of the complex permittivity of the composites decreased with increasing frequency, while the real part of the complex permeability remained stable within the frequency range of 2-18 GHz. As the γ-Fe₂O₃ content increased, the dielectric loss tangent (tan δ_E) gradually decreased, whereas the magnetic loss tangent (tan δ_M) increased in the low-frequency range. The dielectric loss mechanisms varied across different samples, involving conductive loss, polarization relaxation, and multiple polarizations. In the low-frequency range, magnetic loss was primarily dominated by natural resonance, while eddy current loss became predominant at higher frequencies. The attenuation constant increased with frequency but exhibited an overall decline with increasing γ-Fe₂O₃ content. A weight ratio of 3 : 2 of γ-Fe₂O₃ to Co@NC exhibited optimal impedance matching and attenuation characteristics, achieving an effective absorption bandwidth (EAB) of 5.6 GHz (12.4-18 GHz) at a thickness of 1.9 mm, and a minimum reflection loss of -41.731 dB at a thickness of 1.6 mm.