Tailoring the input impedance of FeCo/C composites with efficient broadband absorption.

Proper impedance matching and strong attenuation capabilities are crucial factors for an excellent microwave absorbent. Significant attention and effort have been focused on the attenuation capabilities, whereas little attention has been paid to impedance matching, which is particularly important to design broadband absorbing materials. In this study, coin-like porous FeCo/C composites were successfully prepared by a simple carbon thermal reduction method. The Zin/Zo values of the FeCo/C composites were calculated to investigate the effects of the Fe/Co molar ratio and structure on the impedance matching. The results show that the coin-like porous samples were equipped with optimal impedance matching (Zin/Zo ≈ 1) and broad frequency bandwidth. The coin-like porous structure can induce multiple scattering and extend the travel path of the waves, which is in favour of electromagnetic loss. In this way, the effective frequency bandwidth (RL < -10 dB) as large as 6 GHz (from 11.36 to 17.36 GHz) has been achieved at a thickness of 2 mm when the Fe/Co molar ratio is 4 : 6. In addition, the average frequency broadband reached 5.57 GHz in the thickness range of 2-2.6 mm. We believe that this study may provide a new strategy for tuning the impedance matching for optimal broadband absorbers.

Achieving the interfacial polarization on C/Fe3C heterojunction structures for highly efficient lightweight microwave absorption.

Design of dielectric/magnetic heterostructure and multiple interfaces is a challenge for the microwave absorption. Thus, in this study, a novel C/Fe3C nanocomposites have been fabricated by annealing the precursors obtained by the facile chemical blowing of polyvinyl pyrrolidone (PVP) and Fe(NO3)3·9H2O. By changing the content of Fe(NO3)3·9H2O, the honeycomb-like structure with scads of pores and electromagnetic parameters could be successfully tailored. When the addition of Fe(NO3)3·9H2O is ranging from 1 to 2g, honeycomb-structured nanocomposites possess high performance microwave absorption when mixed with 90wt% paraffin. The minimal reflection loss is -37.4dB at 13.6GHz and effective bandwidth can reach to 5.6GHz when the thickness is 2.0mm, indicating its great potential in microwave absorbing field. Its outstanding microwave performance is tightly related to the porous structure and substantial interface such as carbon/air and carbon/Fe3C, which are in favor of the impedance matching and interfacial polarization. Thus, our study may provide a good reference for the facile synthesis of light-weight carbon-based nanocomposites with effective interfacial polarization.

Adjustable 3-D structure with enhanced interfaces and junctions towards microwave response using FeCo/C core-shell nanocomposites.

In this work, the 3-D honeycomb-like FeCo/C nanocomposites were synthesized through the carbon thermal reduction under an inert atmosphere. The enhanced microwave absorption properties of the composites were mainly attributed to the unique three dimensional structure of the FeCo/C nanocomposites, abundant interfaces and junctions, and the appropriate impedance matching. The Cole-Cole semicircles proved the sufficient dielectric relaxation process. The sample calcinated at 600°C for 4h showed the best microwave absorption properties. A maximum reflection loss of -54.6dB was achieved at 10.8GHz with a thickness of 2.3mm and the frequency bandwidth was as large as 5.3GHz. The results showed that the as-prepared FeCo/C nanocomposite could be a potential candidate for microwave absorption.

Incorporation of dielectric constituents to construct ternary heterojunction structures for high-efficiency electromagnetic response.

To satisfy the diverse requirements of low reflection and high absorption of microwave attenuation, the construction of multiple heterojunction structure is imperative. On the one hand, the impedance mismatching could be ameliorated via the addition of new component; on the other hand, the multiple interface polarizations derived from the architecture of heterojunction make for the dissipation of microwave. In this work, the ternary TiO2/RGO/Fe2O3 composites exhibit tremendous superiority compared with single TiO2 or RGO no matter the absorption coefficient or effective bandwidth. The maximum absorption value of the TiO2/RGO/Fe2O3 composites is -44.05dB at 14.48GHz with a low thickness of 2.0mm. In addition, the effective bandwidth (RL<-10dB) reaches 5.6GHz from 11.96 to 17.56GHz. The superior electromagnetic wave absorbing performance of the TiO2/RGO/Fe2O3 composites derived from the appropriate impedance matching as well as the multiple polarization effect. The results adequately demonstrate the accessibility of the prepared TiO2/RGO/Fe2O3 composites as a preeminent absorber.