Three-dimensional conductive network constructed by in-situ preparation of sea urchin-like NiFe2O4 in expanded graphite for efficient microwave absorption.

Expanded graphite (EG) is a modified conductive lamellar carbon that has been widely studied in the field of electromagnetic wave absorption due to its low density, good electrical conductivity, and unique structure. However, its application is limited because the interlayer gap cannot match microwave wavelength, and its single composition has less microwave loss. In this study, sea urchin-like NiFe2O4/EG composites are prepared in situ between expanded graphite layers by microwave treatment. The sea urchin-like NiFe2O4 grows between the expanded graphite to form a three-dimensional conductive network structure, which enhances conductive loss of composites and further increases the interlayer distance of EG. The extended interlayer distance promotes multiple reflections and scattering of electromagnetic waves in composites and improves dielectric properties. In addition, EG with a large specific surface area provides many active sites, further promoting interface and dipole polarization. Benefiting from synergistic effect of NiFe2O4 and EG, magnetic loss and dielectric loss of NiFe2O4/EG composites have been improved and impedance matching is further enhanced. The results indicate that the minimal reflection loss of NiFe2O4/EG-4 reaches -53.47 dB at 2.69 mm, and the effective absorption bandwidth reaches 2.97 GHz. In addition, based on the computer simulation technology results, NiFe2O4/EG can attenuate microwave energy under experimental conditions. This work provides a strategy for synthesizing carbon matrix composites with adjustable dielectric parameters and electromagnetic wave properties.

Controlled formation of multiple core-shell structures in metal-organic frame materials for efficient microwave absorption.

The design of metal-organic frameworks (MOF) derived composites with multiple loss mechanisms and multi-scale micro/nano structures is an important research direction of microwave absorbing materials. Herein, multi-scale bayberry-like Ni-MOF@N-doped carbon composites (Ni-MOF@NC) are obtained by a MOF assisted strategy. By utilizing the special structure of MOF and regulating its composition, the effective improvement of Ni-MOF@NC's microwave absorption performance has been achieved. The nanostructure on the surface of core-shell Ni-MOF@NC can be regulated and N doping on carbon skeleton by adjusting the annealing temperature. The optimal reflection loss of Ni-MOF@NC is -69.6 dB at 3 mm, and the widest effective absorption bandwidth is 6.8 GHz. This excellent performance can be attributed to the strong interface polarization caused by multiple core-shell structures, the defect and dipole polarization caused by N doping, and the magnetic loss caused by Ni. Meanwhile, the coupling of magnetic and dielectric properties enhances the impedance matching of Ni-MOF@NC. The work proposes a particular idea of designing and synthesizing an applicable microwave absorption material that possesses excellent microwave absorption performance and promising application potential.

Construction of heterointerfaces and honeycomb-like structure for ultrabroad microwave absorption.

Heterointerface design is an effective strategy to improve the effective absorption bandwidth in electromagnetic wave EMW absorbing materials. In this paper, honeycomb-like Fe-doped tremella carbide composites (FCT) with a large number of heterogeneous interfaces were obtained by in-situ construction of multiphase composite particles (Fe3C, Fe3O4, and a-Fe) during the carbonization process. The effects of Fe doping on the phase, structure, morphology, and absorption properties of FCT were investigated. The results show that the porous structure and the heterogeneous interface can significantly improve the electromagnetic wave absorption performance of FCT. Iron doping introduces a heterogeneous multiphase structure into FCT, which increases the interfacial loss and magnetic loss of the material, thereby improving the overall impedance matching of the material. FCT-4 composite exhibited excellent microwave attenuation capability with a reflection loss of -34.6 dB. Simultaneously, the widest effective absorption bandwidth is up to 8.84 GHz (9.16-18 GHz) with a matching thickness of 2.8 mm, which covers almost the entire X (8-12 GHz) and Ku (12-18 GHz) bands. Thus, this paper provides an effective strategy for the preparation of excellent electromagnetic wave absorbing materials by in situ construction of heterointerfaces.

In-situ growth of core-shell ZnFe2O4 @ porous hollow carbon microspheres as an efficient microwave absorber.

The special structure and composition are the important factors that determine the microwave absorption properties. In this study, the porous hollow carbon microsphere (PHCMS) is synthesized by the self-assembly technology, and ZnFe2O4 particles are synthesized inside the carbon sphere by in-situ preparation with taking advantage of the porous and hollow characteristics of the carbon sphere, which prepares ZnFe2O4@PHCMS composite material. The composite shows good performance in terms of minimum reflection loss and absorption bandwidth. The results show that the maximum adsorption capacity of the composite is -51.43 dB at 7.2 GHz. When the thickness is 4.8 mm, the effective absorption bandwidth of RL ≤ 10 dB electromagnetic wave is 3.52 GHz. Such enhanced electromagnetic wave absorption properties of ZnFe2O4@PHCMS are ascribed to the suitable impedance characteristic, the dipole polarization and interfacial polarization, the multiple Debye relaxation process and strong natural resonance, multiple reflection and scattering. This work provides an approach to design effective microwave absorbers having a unique structure to enhance the microwave absorption properties.