Carbides/carbon
composites are becoming a new kind of microwave
absorption (MA) materials with great potential in chemical stability
and light weight, as well as enhanced performance. Herein, we design
a series of Mo2C/C composites through a simple pyrolysis
of Mo-substituted ZIF-8 (Mo/ZIF-8). It is found that the transformation
of zeolitic imidazolate frameworks into carbon polyhedrons is accompanied
by the in situ formation of ultrasmall Mo2C nanoparticles
(Mo2C NPs) less than 5.0 nm. The molar ratio of 2-methylimidazole
to molybdic acid (MIM/Mo) presents a significant effect on the relative
content of Mo2C NPs but not on their average size. The
uniform distribution of Mo2C NPs in carbon polyhedrons
overcomes the poor chemical homogeneity in conventional carbides/carbon
composites. More importantly, Mo2C NPs modulate the dielectric
loss of these composites effectively. On one hand, they moderately
weaken the contribution from conductivity loss and dipole orientation
polarization; on the other hand, they create considerable interfacial
polarization. As a result, these Mo2C/C composites display
much better impedance matching than individual carbon polyhedrons.
When the MIM/Mo ratio reaches 6.0, the optimized composite, S-Mo2C/C-6.0, displays good MA performance in the frequency range
of 2.0–18.0 GHz, including powerful reflection loss and broad
qualified bandwidth. Its performance is actually superior to those
conventional carbides/carbon composites in previous studies, demonstrating
that Mo2C/C composite from this novel strategy may be a
promising candidate for high-performance MA materials in the future.
Carbon materials with multilevel structural features are showing great potentials in electromagnetic (EM) pollution precaution. With ZIF‐67 microcubes as a self‐sacrificing precursor, hierarchical carbon microcubes with micro/mesoporous shells and hollow cavities have been successfully fabricated with the assistance of rigid SiO2 coating layers. It is found that the SiO2 layer can effectively counteract the inward shrinkage of organic frameworks during high‐temperature pyrolysis due to intensive interfacial interaction. The obtained hollow porous carbon microcubes (HPCMCs) exhibit larger Brunauer–Emmett–Teller surface area and pore volume than porous carbon microcubes (PCMCs) directly derived from ZIF‐67 microcubes. The unique microstructure is confirmed to be favorable for conductive loss and interfacial polarization, thus boosting the overall dielectric loss capability of carbon materials. Besides, hollow cavity will also promote multiple reflection of incident EM waves and intensify the dissipation of EM energy. As expected, HPCMCs harvest better microwave absorption performance, including strong reflection loss intensity and broad response bandwidth, than many traditional microporous/mesoporous carbon materials. This study provides a new strategy for the construction of hierarchical carbon materials and may inspire the design of carbon‐based composites with excellent EM functions.
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