Shell Thickness-Dependent Microwave Absorption of
Core–Shell Fe<sub>3</sub>O<sub>4</sub>@C Composites
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Abstract
Core–shell composites, Fe<sub>3</sub>O<sub>4</sub>@C, with
500 nm Fe<sub>3</sub>O<sub>4</sub> microspheres as cores have been
successfully prepared through in situ polymerization of phenolic resin
on the Fe<sub>3</sub>O<sub>4</sub> 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
Fe<sub>3</sub>O<sub>4</sub> microspheres. Carbothermic reduction has
not been triggered at present conditions, thus the crystalline phase
and magnetic property of Fe<sub>3</sub>O<sub>4</sub> micropsheres
can be well preserved during the carbonization process. Although carbon
shells display amorphous nature, Raman spectra reveal that the presence
of Fe<sub>3</sub>O<sub>4</sub> micropsheres can promote their graphitization
degree to a certain extent. Coating Fe<sub>3</sub>O<sub>4</sub> 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<sub>3</sub>O<sub>4</sub>@C composites
can be promising candidates as highly effective microwave absorbers