Construction of Graphite Shells on Ferromanganese Oxide for Electromagnetic Wave Absorption

Ferromanganese oxide (FMO), a by-product of steelmaking industry, was coated with polyacrylonitrile (PAN) to construct an electromagnetic wave absorber (FMO@C) with a core–shell structure. The effect of heat treatment from 600 to 1000 °C on the phase transformation of FMO and carbonization of PAN was studied. Upon the heat treatment at 1000 °C, the reflection loss and effective bandwidth of the FMO@C reached −18.20 dB and 3.08 GHz at a thickness of 1.6 mm, presenting a significant improvement over FMO which only exhibited a reflection loss of −2.31 dB at 10 mm. Boric acid was infiltrated into the PAN shells to catalyze the carbonization process and adjust the impedance matching, which further improved the reflection loss to a minimum value of −28.25 dB. Via varying the concentration of boric acid, the reflection loss of −22.01 dB with an effective bandwidth of 3.36 GHz at a thickness of 1.3 mm was achieved. The enhanced EMW absorption performance was attributed to multiple reflections and polarization caused by the core–shell structure, magnetic loss from the phase transformation of FMO, dielectric loss from carbon shells, as well as the tunable impedance matching by boron-catalyzed carbonization. The construction of the core–shell structure could be a promising downstream processing of FMO and could extend the application of the solid wastes.