2 research outputs found
Shell Thickness-Dependent Microwave Absorption of Core–Shell Fe<sub>3</sub>O<sub>4</sub>@C Composites
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
Constructing Uniform Core–Shell PPy@PANI Composites with Tunable Shell Thickness toward Enhancement in Microwave Absorption
Highly uniform core–shell
composites, polypyrrole@polyaniline
(PPy@PANI), have been successfully constructed by directing the polymerization
of aniline on the surface of PPy microspheres. The thickness of PANI
shells, from 30 to 120 nm, can be well controlled by modulating the
weight ratio of aniline and PPy microspheres. PPy microspheres with
abundant carbonyl groups have very strong affinity to the conjugated
chains of PANI, which is responsible for the spontaneous formation
of uniform core–shell microstructures. However, the strong
affinity between PPy microspheres and PANI shells does not promote
the diffusion or reassembly of two kinds of conjugated chains. Coating
PPy microspheres with PANI shells increases the complex permittivity
and creates the mechanism of interfacial polarization, where the latter
plays an important role in increasing the dielectric loss of PPy@PANI
composites. With a proper thickness of PANI shells, the moderate dielectric
loss will produce well matched characteristic impedance, so that the
microwave absorption properties of these composites can be greatly
enhanced. Although PPy@PANI composites herein consume the incident
electromagnetic wave by absolute dielectric loss, their performances
are still superior or comparable to most PANI-based composites ever
reported, indicating that they can be taken as a new kind of promising
lightweight microwave absorbers. More importantly, microwave absorption
of PPy@PANI composites can be simply modulated not only by the thickness
of the absorbers, but also the shell thickness to satisfy the applications
in different frequency bands