Magnetic FeOX/biomass carbon composites with broadband microwave absorption properties

•Magnetic porous biomass carbon-based composites are fabricated by a simple pre-modification.•Magnetic FeOX/biomass carbon composites exhibit excellent microwave absorption performances.•Absorption performances of FeOx/CSBC is insensitive to amount of coconut shells from 1.2 g to 2.5 g.•Electromagnetic properties and loss mechanism are objectively reviewed. [Display omitted] Coconut shell-based biomass carbon (CSBC) composites loaded with magnetic FeOx nanoparticles (FeOX/CSBC) were successfully prepared by one-step pyrolysis method using sodium chloride as an additive at 600 °C. The crystal structures, surface morphologies, saturation magnetizations, and microwave absorption properties of FeOX/CSBC composites were investigated. FeOX/CSBC composites showed broadband microwave absorption properties and insensitivity to the amount of coconut shell in a certain dosage range. When the amount of coconut shell was 2.5 g, 2.0 g, 1.5 g and 1.0 g respectively, the obtained composites of CSBC-2.5, CSBC-2.0, CSBC-1.5 and CSBC-1.2 showed that the effective absorption bandwidth (EABW, RL≤−10 dB) covered 6.5 GHz, 6.4 GHz, 6.4 GHz and 6.0 GHz with 30 wt% filler loading, respectively. Moreover, RLmin values of CSBC-2.5, CSBC-2.0, CSBC-1.5 and CSBC-1.2 composite were all lower than − 20 dB in the thickness range of 2–4 mm. The broadband microwave absorption properties of FeOX/CSBC composites could be attributed to the superb impedance matching, multi-polarization relaxation, magnetic-dielectric synergetic effect, and multiple scattering. The insensitivity to a certain dosage range of coconut shell was beneficial to practical operation of the preparation process, and improved reproducibility of the experimental results of microwave absorption properties. This work provides a good foundation for practical application in the absorbing field because of broadband microwave absorption properties and high reproducibility of experimental results.