To design smart microwave-absorbing materials (MAMs), it is essential to adjust the corresponding electrical conductivity and dielectric parameters according to variable conditions. However, it is still challenging to concurrently adjust the effective absorbing intensity and frequency range in MAMs due to their interdependent constraints. Here, we developed intelligent MAMs by incorporating core-shell structure vanadium dioxide @ polydopamine (VO2@PDA) powders as polarization loss units, while the subwavelength-sized reduced graphene oxide microspheres (RGOms) were used as conduction loss units. When the temperature is higher than the metal-insulator phase transition temperature of the insulator state VO2 (M), the corresponding metal state VO2 (R) could be produced, which, therefore, contributes to an enhanced interfacial polarization loss due to the significant electrical performance differences between the VO2 (R) and the PDA shell. As an optimized result, the changes of the effective absorption frequency band (Delta EAF) and reflection loss (Delta RL) of the RV3 composite could be approximately 1.5 GHz and 24 dB, respectively, attributable to the phase transition of VO2. This study provides a novel approach for the adjustment of electromagnetic responses based on dynamic interfacial polarization performance, which offers broader prospects for developing next-generation smart electromagnetic absorption devices with both reversible microwave absorption frequency range and intensities.

Benefitting from stable chemical and physical properties, and high hardness and acid/alkali resistance, titanium nitride (TiN) is used as a high-performance functional ceramic material. Recently, it has been found that the unique conductivity and dielectric characters of TiN-based materials have a significant effect on the versatile electromagnetic wave absorption (EMWA) properties in the visible-infrared (Vis-IR) and microwave frequencies bands. This review summarizes the multispectral EMWA characters of TiN-based materials and the corresponding selective absorption performances in the Vis-IR bands, which are unique advantages with a wide temperature range in the field of microwave absorption. Finally, the improvement strategies of EMWA properties for TiN-based materials are prospected.

Magnetic/dielectric composites offer good impedance matching for electromagnetic wave absorption, though achieving both strong absorption and broadband absorption in such composites is challenging. Herein polypyrrole-Maghemite-graphitic carbon nitride (PPy/gamma-Fe2O3/g-C3N4) composites were successfully fabricated via the one-step chemical synthesis of PPy/gamma-Fe2O3 nanospheres in the presence g-C3N4 nanosheets. The PPy/gamma-Fe2O3/g-C3N4 composites demonstrated outstanding electromagnetic wave (EMW) absorption properties at microwave frequencies. A compositionally optimized PPy/gamma-Fe2O3/g-C3N4 composite delivered a minimum reflection loss (RLmin) value of 53.66 dB at 11.92 GHz and the effective absorption bandwidth (EAB, RL < -10 dB) of 5.13 GHz (10.46-15.59 GHz) at a thickness of 2.5 mm. The excellent EMW absorption properties of the composite could be attributed to the introduction of g-C3N4 nanosheets with a lamella structure, which enhanced interfacial polarization losses, multi-reflections, scattering, and impedance matching within the composite. This work offers a simply and effective strategy for the preparation of high-performance EMW absorbers.

An FeNi/nitrogen, sulfur-codoped carbon (FeNix/NS-C) composite was synthesized by a microwave-assisted method, polymerization of polypyrrole (PPy) and thermal decomposition. The FeNix/NS-C composite consisted of bimetallic FeNi, heteroatomic dopants (N, S) and (Fe/Ni)-N-x bonds. The FeNix/NSC composites showed superior electromagnetic wave absorption (EMWA) performances, e.g., a strong reflection loss (RL) of similar to 55.3 dB (21.4 GHz) and a broad effective absorption bandwidth of 32.46 GHz (3.92 -17.08 and 18-37.3 GHz) by controlling the layer thickness (1.0-5.5 mm). The exceptional EMWA performances are attributed to the synergistic effect of magnetic loss that originated from the FeNi component, dielectric loss derived from the conduction loss (graphitic carbon), dipole polarization (heteroatoms (N and S), defective carbon, abundant functional groups (C-O, C-N, C-S, C=O, etc.) and interfacial polarization (hierarchically porous features, numerous small core-shell configurations and heterojunctions among FeNi, "double-layer" carbon shells). (C) 2020 Elsevier Ltd. All rights reserved.

Hierarchical Fe3O4/C (h-Fe3O4/C) with a flower-like morphology was successfully prepared using a facile hydrothermal method, followed by annealing at 400 °C under N2. The h-Fe3O4/C product consisted of radially assembled porous nanosheets (thickness ∼53 nm) and exhibited improved adsorption ability for cationic rhodamine B (RhB) over a wide pH range compared with other forms of Fe3O4/C. The excellent dye adsorption performance of h-Fe3O4/C could be attributed to its high specific surface area (113.6 m2 g−1) and abundance of accessible adsorption sites for dye molecules on the porous nanosheets. In addition, h-Fe3O4/C effectively adsorbed anionic and neutral dyes, suggesting both electrostatic and van der waals interactions underpinned its adsorption performance. Importantly, h-Fe3O4/C could be easily recovered from dye solutions by magnetic separation, and then regenerated by washing and heat treatment with no loss in adsorption capacity. Due to its low cost, ease of fabrication, excellent dye adsorption properties, rapid magnetic recovery and regeneration, h-Fe3O4/C represents a very promising adsorbent for large scale removal of organic dyes and other organic pollutants from water.

Based on the super-composite effect of clay minerals consisting of silicate layers on the polymer, nano-hybrid organic–inorganic composites have attracted great interest. In this paper, a novel organoclay with reactive hydroxyl group was prepared by incorporation into a hydroxyl-terminated cationic polyurethane oligomer (HTCPU) via interlamination. Sodium montmorillonite was organically modified with HTCPU oligomers in the water medium. The modified organo-montmorillonite was characterized by Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction, and thermogravimetric analysis. After sequential poly-addition reactions, the hydroxyl-terminated polyurethane hybrid resins with nano-silica layers were prepared and then the resin mixed with blocked hexamethylene diisocyanate trimer (crosslinking agent) in advance and then was emulsified with deionized water to produce the nano-hybrid electrophoresis coating. A series of PU films were prepared by the electrophoresis process. The optimized baking temperature was determined by in situ FTIR, and the effect of the organo-montmorillonite content on the acid resistance of film was studied. When the mass fraction of organo-montmorillonite was about 0.35%, the PU film showed the best acid resistance. It was found that the desirable appearance and properties could be obtained by a prebake of a temperature of 80°C to remove water in the wet film to avoid the pinholes in the paint film for 20 min, followed by a baking temperature of 135°C for 60 min.

Hollow gamma-Fe2O3@poly (3,4-ethylenedioxythiophene) (PEDOT) core-shell nanocomposites have been successfully constructed via the reaction of hydrofluoric acid and gamma-Fe2O3@SiO2@ @PEDOT core-shell nanocomposites. To evaluate the microwave absorption properties, the electromagnetic parameters of the absorber and the wax composite were measured at 2-18 GHz. Investigations of the microwave absorbing properties indicate that the hollow gamma-Fe2O3@PEDOT core-shell nanocomposites exhibit remarkably improved microwave absorption properties compared to the gamma-Fe2O3@SiO2@PEDOT core-shell nanocomposites. For hollow gamma-Fe2O3@PEDOT coreshell nanocomposites, a minimum reflection loss (RL) of -44.7 dB at 12.9 GHz with a matching layer thickness of 2.0 mm, and an effective absorption bandwidth (RL < -10 dB) of 4.3 GHz (10.8-15.1 GHz) can be obtained. The enhanced absorption ability may benefit from the unique structure and the synergistic effect between the magnetic and dielectric components. It is expected that the proposed strategy can be extended to the fabrication of other lightweight and high-performance microwave absorbing materials.