Impact of Fe content of coal fly ash magnetospheres and the grinding size upon microstructure and microwave absorption properties of Fe₃C@C-CNTs nanocomposites

Nano-structured iron/carbon composites can be prepared by chemical vapor deposition (CVD) using coal fly ash magnetospheres as raw materials, showing good microwave absorption properties. However, there are problems such as uneven properties of magnetospheres and difficulty in structural regulation. In this paper, the magnetospheres were separated by the shaking bed method, and then were grinded. The effects of the magnetospheres Fe content and the grinding particle size on the CVD products were investigated. The results show that the CVD product of Fe-rich magnetospheres is Fe₃C@C-CNTs, and the composite exhibits a porous cluster spherical structure. With the increase of magnetic bead Fe content, the relative carbon deposition (C/Fe value) of the composite decreases, and the graphitization degree decreases (Higher D/G peak intensity ratio I_D/I_G value increases), resulting in the increase of impedance matching value and the improvement of wave absorption performance. When the Fe content is 71.43wt%, the effective absorption band of the composite reaches 4.5 GHz, and the minimum reflection loss (RL_min) reaches −16.1 dB. After grinding the magnetospheres, the C/Fe value of CVD products is unchanged, but the carbon deposition rate increases, the I_D/I_G value increases, and the electromagnetic wave attenuation constant decreases, but the impedance matching is significantly improved, and the microwave absorption performance is greatly improved. When the grinding particle size is 18.23 μm, the effective absorption band of the composite is 4.8 GHz, and the RL_min can reach −34.7 dB. The excellent microwave absorption properties of the composites benefit from the synergistic absorption of CNTs and Fe₃C@C. Multiple reflections of microwave are supposed to be enhances in the porous cluster aggregated spheres, and the promoted interface polarization is also attributed to the excellent microwave absorption properties.