The reasonable design of multiple interfaces and heteroatom doping can substantially improve the waveabsorbing performance of electromagnetic wave (EMW) absorbing materials. As the carbon source of biomass, the carbon material with abundant oxygen-containing groups on the surface can be obtained by hydrothermal method, which is conducive to modification and design of the morphology. In addition, N-hydroxymethyl acrylamide contains rich N atoms and has good water solubility, which can be well combined with sugar to prepare high-performance wave-absorbing materials. In this study, D-xylose and glucose were used as carbon sources, water was used as solvent, N-hydroxymethyl acrylamide was added, and nitrogen-doped lychee-like saccharide-based carbon microspheres (LSCMs) were obtained by hydrothermal synthesis and high-temperature carbonization. LSCMs-2 with good EMW attenuation capability and impedance matching performance was obtained by controlling the proportion of different raw materials. Under the optimal filling conditions, the EMW absorber is only 2 mm thick and achieves a minimum reflection loss (RL min) of -47.44 dB at 13.84 GHz with an effective absorption bandwidth (EAB) of 12.24-17.36 GHz (5.12 GHz). The excellent EMW attenuation performance is mainly attributed to dipole polarization and interface polarization in the LSCMs. This research offers a new green synthesis route for the preparation of EMW absorbers with wider bandwidth, thinner thickness and better absorption performance.

In this study, we designed a simple green preparation method and synthesized polymer-derived N,O-doped bowl -like hollow carbon microspheres (BHCMs) to improve their electromagnetic wave absorption performance. Using copolymerized microspheres of allyl methacrylate and acrylonitrile (P(AMA-AN)) synthesized through soap-free emulsion polymerization as a template and polydextrose as the primary carbon source, BHCMs were fabricated by a hydrothermal method and high-temperature carbonization pyrolysis. Moreover, the P(AMA-AN) micro -spheres could directly provide nitrogen (N) and oxygen (O) heteroatoms, achieving heteroatom doping. The morphology and absorption properties of BHCMs can be adjusted by controlling the amount of P(AMA-AN) microspheres. The unique structure gives BHCMs-2 excellent electromagnetic wave attenuation and impedance-matching characteristics. At a low filling ratio (11.1%), the minimum reflection loss (RLmin) is-40.3 dB at 13.82 GHz with a 2.5 mm thickness. Surprisingly, a broad effective absorption bandwidth (EAB) is maintained over an extensive thickness range. The average EAB in the 2.5-3.5 mm thickness range reaches 5.52 GHz. This study proposes a new and green preparation method for carbon materials to improve their absorption properties.

The novel microstructure of magnetic-carbonaceous composites has become a prevalent route to improve electromagnetic wave absorption (EMWA) performance. Flower-like Ni/nitrogen-doped carbon (Ni/NC-X) composites with core-shell synergistic structure, N-doping and Ni-N bonds were rationally constructed and fabricated by hydrothermal methods and thermal decomposition with carbon reduction. The Ni cores were encapsulated within approximately 15 layers of the graphene shell, leading to the generation of a Ni/NC-X core-shell configuration. The EMWA performances of Ni/NC-X composites could be adjusted by modulating the acrylonitrile (AN) content. Benefiting from the synergistic effects of the core-shell configuration, a hierarchically flower-like architecture and the components (Ni and C), the flower-like Ni/NC-0.80 composite showed remarkable EMWA performance with a reflection loss (RL) of - 40.1 dB and a broad effective absorption bandwidth (EAB) of 10.05 GHz. The result of this study establishes a new strategy to prepare magnetic-carbonaceous composites by carefully designing the microstructure.

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.