Shell thickness-dependent microwave absorption of core-shell Fe ₃O₄@C composites

Core-shell composites, Fe₃O₄@C, with 500 nm Fe ₃O₄ microspheres as cores have been successfully prepared through in situ polymerization of phenolic resin on the Fe₃O ₄ surface and subsequent high-temperature carbonization. The thickness of carbon shell, from 20 to 70 nm, can be well controlled by modulating the weight ratio of resorcinol and Fe₃O₄ microspheres. Carbothermic reduction has not been triggered at present conditions, thus the crystalline phase and magnetic property of Fe ₃O₄ micropsheres can be well preserved during the carbonization process. Although carbon shells display amorphous nature, Raman spectra reveal that the presence of Fe₃O₄ micropsheres can promote their graphitization degree to a certain extent. Coating Fe ₃O₄ microspheres with carbon shells will not only increase the complex permittivity but also improve characteristic impedance, leading to multiple relaxation processes in these composites, thus the microwave absorption properties of these composites are greatly enhanced. Very interestingly, a critical thickness of carbon shells leads to an unusual dielectric behavior of the core-shell structure, which endows these composites with strong reflection loss, especially in the high frequency range. By considering good chemical homogeneity and microwave absorption, we believe the as-fabricated Fe ₃O₄@C composites can be promising candidates as highly effective microwave absorbers.