Acicular or octahedral Fe₃O₄/rice husk-based activated carbon composites through graphitization synthesis as superior electromagnetic wave absorbers

We designed unique acicular or octahedral Fe₃O₄/rice husk-based activated carbon (AC) composites in electromagnetic wave (EMW) absorption. The key factors included the carbon layer structure and the regular metal oxide (Fe₃O₄) morphology. Particularly, the experiments involved a thorough ball milling method on biological substrates, which would significantly improve the absorber performance than conventional treated steps. The microstructures were controlled flexibly by adjusting the ratio of activator and magnetizing agent. The formed pores and the regular structures enhanced multiple interface polarizations. The minimum reflectance (R_Lmin) values of sample H4 reached –52.137, –41.918, –51.733, and –47.614 dB at 1.669, 2.002, 2.393, and 3.000 mm, respectively. The R_Lmin values were mainly concentrated in C, X and Kμ bands, and the reflectance (R_L ≤ –10 dB) values covered a wide frequency range of approximately 13 GHz. In summary, the composites had the characteristics of interface engineering, high specific surface area, strong dielectric, and magnetic loss.