Recently, multilevel structural carbon aerogels are deemed as attractive candidates for microwave absorbing materials. Nevertheless, excessive stack and agglomeration for low-dimension carbon nanomaterials inducing impedance mismatch are significant challenges. Herein, the delicate “3D helix–2D sheet–1D fiber–0D dot” hierarchical aerogels have been successfully synthesized, for the first time, by sequential processes of hydrothermal self-assembly and in-situ chemical vapor deposition method. Particularly, the graphene sheets are uniformly intercalated by 3D helical carbon nanocoils, which give a feasible solution to the mentioned problem and endows the as-obtained aerogel with abundant porous structures and better dielectric properties. Moreover, by adjusting the content of 0D core–shell structured particles and the parameters for growth of the 1D carbon nanofibers, tunable electromagnetic properties and excellent impedance matching are achieved, which plays a vital role in the microwave absorption performance. As expected, the optimized aerogels harvest excellent performance, including broad effective bandwidth and strong reflection loss at low filling ratio and thin thickness. This work gives valuable guidance and inspiration for the design of hierarchical materials comprised of dimensional gradient structures, which holds great application potential for electromagnetic wave attenuation. "Image missing"
Electromagnetic
(EM) absorbers serving in the megahertz (MHz) band
and a wide temperature range (from −50 to 150 °C) require
high and temperature-stable permeability for outstanding EM absorption
performance. Herein, FeCoNiCr0.4Cu
X
high-entropy alloy (HEA) powders with a unique nanocrystalline
structure separated by a thin amorphous layer (NTA) are designed to
improve permeability and enhance intergranular coupling. Simultaneously,
the long-range anisotropy is introduced via devising the preparation
process and tuning the chemical composition, such that the intergranular
exchange interaction is further strengthened for stable permeability
and EM wave absorption in a wide temperature range. FeCoNiCr0.4Cu0.2 HEAs exhibit a near-zero permeability temperature
coefficient (5.7 × 10–7 °C–1) a in wide temperature range. The maximum reflection loss (RL) of
FeCoNiCr0.4Cu0.2 HEAs is higher than −7
dB with 5 mm thickness at −50–150 °C, and the absorption
bandwidth (RL < −7 dB) can almost cover 400–1000
MHz. Furthermore, FeCoNiCr0.4Cu0.2 HEAs also
have a high Curie temperature (770 °C) and distinguished oxidation
resistance. The permeability temperature dependence of FeCoNiCr0.4Cu
X
HEAs is investigated in-depth
in light of the microstructural change induced by tuning the chemical
composition, and a new inspiration is provided for the design of magnetic
applications serving in wide temperature, such as transformers, sensors,
and EM absorbers.
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