The broadband and tunable high-performance microwave absorption properties of an ultralight and highly compressible graphene foam (GF) are investigated. Simply via physical compression, the microwave absorption performance can be tuned. The qualified bandwidth coverage of 93.8% (60.5 GHz/64.5 GHz) is achieved for the GF under 90% compressive strain (1.0 mm thickness). This mainly because of the 3D conductive network.

Broadband and Tunable High‐Performance Microwave Absorption of an Ultralight and Highly Compressible Graphene Foam

The synthesis of CoNi@SiO2 @TiO2 core-shell and CoNi@Air@TiO2 yolk-shell microspheres is reported for the first time. Owing to the magnetic-dielectric synergistic effect, the obtained CoNi@SiO2 @TiO2 microspheres exhibit outstanding microwave absorption performance with a maximum reflection loss of -58.2 dB and wide bandwidth of 8.1 GHz (8.0-16.1 GHz, < -10 dB).

CoNi@SiO2@TiO2 and CoNi@Air@TiO2 Microspheres with Strong Wideband Microwave Absorption

Multifunctional composite microspheres with spinel Fe(3)O(4) cores and anatase TiO(2) shells (Fe(3)O(4)@TiO(2)) are synthesized by combining a solvothermal reaction and calcination process. The size, morphology, microstructure, phase purity, and magnetic properties are characterized by scanning electron microscopy, transmission electron microscopy (TEM), high-resolution TEM, selected-area electron diffraction, electron energy loss spectroscopy, powder X-ray diffraction, and superconducting quantum interference device magnetometry. The results show that the as-synthesized microspheres have a unique morphology, uniform size, good crystallinity, favorable superparamagnetism, and high magnetization. By varying the experimental conditions such as Fe(3)O(4) size and concentration, microspheres with different core sizes and shell thickneses can be readily synthesized. Furthermore, the microwave absorption properties of these microspheres are investigated in terms of complex permittivity and permeability. By integration of the chemical composition and unique structure, the Fe(3)O(4)@TiO(2) microspheres possess lower reflection loss and a wider absorption frequency range than pure Fe(3)O(4). Moreover, the electromagnetic data demonstrate that Fe(3)O(4@TiO(2) microspheres with thicker TiO(2) shells exhibit significantly enhanced microwave absorption properties compared to those with thinner TiO(2) shells, which may result from effective complementarities between dielectric loss and magnetic loss. All the results indicate that these Fe(3)O(4)@TiO(2) microspheres may be attractive candidate materials for microwave absorption applications.

Microwave Absorption Enhancement of Multifunctional Composite Microspheres with Spinel Fe3O4 Cores and Anatase TiO2 Shells

Graphene is highly desirable as an electromagnetic wave absorber because of its high dielectric loss and low density. Nevertheless, pure graphene is found to be non-magnetic and contributes to microwave energy absorption mostly because of its dielectric loss, and the electromagnetic parameters of pure graphene, which are out of balance, result in a bad impedance matching characteristic. In this paper, we report a facile solvothermal route to synthesize laminated magnetic graphene. The results show that there have been significant changes in the electromagnetic properties of magnetic graphene when compared with pure graphene. Especially the dielectric Cole–Cole semicircle suggests that there are Debye relaxation processes in the laminated magnetic graphene, which prove beneficial to enhance the dielectric loss. We also proposed an electromagnetic complementary theory to explain how laminated magnetic graphene, with the combined advantages of graphene and magnetic particles, helps to improve the standard of impedance matching for electromagnetic wave absorbing materials. Besides, microwave absorption properties indicate that the reflection loss of the as-prepared composite is below −10 dB (90% absorption) at 10.4–13.2 GHz with a coating layer thickness of 2.0 mm. This further confirms that the nanoscale surface modification of magnetic particles on graphene makes graphene-based composites have a certain research value in electromagnetic wave absorption.

Laminated magnetic graphene with enhanced electromagnetic wave absorption properties

Electromagnetic Response and Energy Conversion for Functions and Devices in Low‐Dimensional Materials

Iron-based microstructured or nanostructured materials, including Fe, γ-Fe2O3, and Fe3O4, are highly desirable for magnetic applications because of their high magnetization and a wide range of magnetic anisotropy. An important application of these materials is use as an electromagnetic wave absorber to absorb radar waves in the centimeter wave (2−18 GHz). Dendrite-like microstructures were achieved with the phase transformation from dendritic α-Fe2O3 to Fe3O4, Fe by partial and full reduction, and γ-Fe2O3 by a reduction−oxidation process, while still preserving the dendritic morphology. The investigation of the magnetic properties and microwave absorbability reveals that the three hierarchical microstructures are typical ferromagnets and exhibit excellent microwave absorbability. In addition, this also confirms that the microwave absorption properties are ascribed to the dielectric loss for Fe and the combination of dielectric loss and magnetic loss for Fe3O4 and γ-Fe2O3.

Bingxiang Dong

Papers

Hierarchical Dendrite-Like Magnetic Materials of Fe3O4, γ-Fe2O3, and Fe with High Performance of Microwave Absorption

Rationally engineering hierarchical porous carbon via oxidation-induced strategy for a high-performance flexible quasi-solid-state supercapacitor