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Metal–organic framework-derived carbon materials have attracted considerable attention as efficient electromagnetic microwave absorption (EMA) materials due to their tunable structure and components. However, their poor electrical conductivity and restricted pore volume lead to poor impedance matching and high filler content, limiting further applications. Herein, an efficient strategy is proposed for designing hollow CoNi/carbon nanotube (CNT)/hollow carbon nanocage (HCN) composites through Ni2+ etching and subsequent thermal catalysis treatment. The CoNi alloy nanoparticle-encapsulated CNTs are decorated on the surface of HCNs and unique CoNi alloy nanoparticles on the top of the CNT. The introduction of CNTs not only enhances the attenuation capacity of the electromagnetic microwave (EMW) but also effectively maintains the hollow structure of the matrix to optimize impedance matching characteristics and construct abundant heterogeneous interfaces with the CoNi alloy, enhancing the interfacial polarization loss. Benefitting from the hollow structure and a large number of micropores which facilitate impedance matching, the strong conduction loss, and enhanced interfacial polarization loss of the CNT, the dielectric loss of CoNi/CNT/HCNs is effectively improved. CoNi/CNT/HCNs exhibit excellent EMA performance with a minimum reflection loss of −59.5 dB and achieve a maximum effective absorption bandwidth of 7.1 GHz at 1.8 mm. This study not only proposes an effective strategy to achieve an efficient EMW absorber with a strong attenuation and broadband but also provides insights into how to maintain the hollow structure of the substrate. [ABSTRACT FROM AUTHOR]

In this paper, one-dimensional Fe3O4/C core/shell nanorods (NRs) were successfully prepared through magnetic-field-induced (MFI) assembly approach (0.3 T) and subsequent high-temperature carbonization using phenol formaldehyde resin as a carbon source. A serial of examinations were performed thoroughly, such as X-ray diffraction, scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy. The results indicated that phenol formaldehyde resin (PFR) shell was transformed to carbon during the carbonization process and the thickness exhibited significant decrease. The Fe3O4/C core/shell nanoparticles (NPs) were prepared in the same way without magnetic-field-induced (0 T). The electromagnetic properties of Fe3O4/C core/shell composites were measured with coaxial reflection/transmission technique at 2.0–18.0 GHz. Electromagnetic performances indicated that the special structure played a crucial role in determining their dielectric behavior. Compared with the Fe3O4/C core/shell NPs, Fe3O4/C nanorods exhibited higher dielectric constant and higher reflection loss (RL) at a high frequency band with a thin absorber thickness. In detail, the optimal absorption peak value for Fe3O4/C core/shell NRs could reach a strong intensity of -44.1 dB at 1.6 mm, and the effective absorption bandwidth lower than −10 dB is 2.7 GHz (from 11.6 to 14.3 GHz). It is expected that our Fe3O4/C core/shell NRs synthesized via a magnetic field induced in this work could be used as a reference to design novel low-dimensional microwave absorption materials. [ABSTRACT FROM AUTHOR]

Fe3O4/carbon core/shell composites were fabricated via a two-step process. Fe3O4/phenol formaldehyde resin (PFR) core/shell composites were first obtained by hydrothermal method, and the Fe3O4/carbon core/shell composites were produced by annealing Fe3O4/PFR core/shell composites under nitrogen flow. The phase structures and morphologies of the composites had been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscope (TEM). The microwave absorption properties of the Fe3O4/carbon core/shell composites were measured by vector network analysis (VNA). When the ratio of Fe3O4 and carbon is 1:2 and the sample thickness is 2.4 mm, the Fe3O4/carbon core/shell composites have an optimal absorption peak value of about −45.3 dB at 9.7 GHz and its effective absorption bandwidth lower than −10 dB reaches 2.3 GHz (from 7.8 to 11.1 GHz). The reflection loss results indicate that the Fe3O4/carbon core/shell composites possess higher microwave absorption. The excellent electromagnetic wave absorption properties of the Fe3O4/carbon core/shell composites were attributed to effective complementarities between the dielectric loss and the magnetic loss. [ABSTRACT FROM AUTHOR]

• A facile two-step calcinating method is developed to prepare holey carbon absorbent. • The appropriate holey structure is beneficial for microwave absorption. • HCDC-30 exhibits a wide bandwidth (5.5 GHz) at a low filler loading (8 wt%). Lightweight, inexpensiveness, easy synthesis, associated with high and broadband absorption are the intensive pursuit for advanced microwave absorbents. Herein, we employ a facile preparation to create holey structures on the wasted biomass-derived carbon materials by air-calcination. All of the carbon absorbents derived from wasted corn cobs, bagasse and grapefruit peels exhibit controlled holey structures on their surfaces, demonstrating the universality of this strategy. The obtained metal-free products are fabricated from bare wasted biomass, without any additional reagents addition. Benefiting from the holey structure generation, more incident microwaves can be consumed due to the increased surface areas and polarization centers for the microwave radiation. The performance measurements suggest the achieved sample could reach a minimum reflection loss value of −47.0 dB at 14.1 GHz and an effective absorption bandwidth (lower than −10 dB) of 5.5 GHz. It is anticipated that this facile holey generation method can be extended to more wasted biomass-derived carbon materials for more sustainable applications. [ABSTRACT FROM AUTHOR]

Metal organic frameworks (MOFs) derived carbon materials have attracted much attention as hopeful electromagnetic microwave absorbents. However, the poor impedance matching and high filler content severely limited its electromagnetic absorption application. The introduction of hollow structural engineering is beneficial to improving microwave absorption performance. Herein, we designed and constructed Ni-incorporated hollow N-doped carbon nanocages (Ni/NCNs) to reveal interfacial charge transfer in electromagnetic wave attenuation. The hollow carbon structure facilitates impedance matching, highly dispersive Ni nanoparticles not only build a dense magnetic coupling network but also generate a series of Ohmic contact heterogeneous interfaces with hollow NCNs, extremely accelerating the charge transfer and enhancing conduction loss. Thanks to hollow structure, optimized impedance matching, and abundant Ohmic contact heterogeneous interfaces, the Ni 50 /NCNs exhibit a minimum reflection loss of −57.3 dB. The results demonstrate that Ni 50 /NCNs composites have great potential to be considered efficient electromagnetic wave materials, and the designed Ohmic contact heterogeneous interfaces pave the way in the study of electromagnetic wave absorption mechanisms. Metal organic frameworks (MOFs) derived carbon materials have attracted much attention as hopeful electromagnetic microwave absorbents. However, the poor impedance matching and high filler content severely limited its electromagnetic absorption application. The introduction of hollow structural engineering is beneficial to improve microwave absorption performance. Herein, we designed and constructed Ni-incorporated hollow N-doped carbon nanocages (Ni/NCNs) to reveal interfacial charge transfer in electromagnetic wave attenuation. The hollow carbon structure facilitates impedance matching, highly dispersive Ni nanoparticles not only build dense magnetic coupling network, but also generate a series of Ohmic contact heterogeneous interfaces with hollow NCNs, extremely accelerated the charge transfer and enhanced conduction loss. Thanks to hollow structure, optimized impedance matching and abundant ohmic contact heterogeneous interfaces, the Ni 50 /NCNs exhibit a minimum reflection loss of −57.3 dB. The results demonstrate that Ni 50 /NCNs composites have great potential to be considered as the efficient electromagnetic wave materials, and the designed ohmic contact heterogeneous interfaces pave the way in the study of electromagnetic wave absorption mechanism. [Display omitted] • The microwave absorber of Ni-incorporated hollow N-doped carbon nanocages were prepared. • The RL min reaches −57.3 dB and the effective absorption bandwidth is 5.8 GHz. • Highly dispersive Ni nanoparticles generate abundant Ohmic contact heterogeneous interfaces with hollow NCNs, extremely accelerated the charge transfer and enhanced the conduction loss. [ABSTRACT FROM AUTHOR]

Here, we synthesized LaNiO 3 microspheres by a simple hydrothermal synthesis and subsequent annealing process. The LaNiO 3 microspheres are characterized by TG-DSC, XRD, FESEM, TEM and VAN. Noticeably, owing to the effective synergism between significant dielectric loss and optimum impedance match, LaNiO 3 microspheres exhibit significantly remarkable attenuation capability and wide-band respond. Therefore, the possible mechanism of microwave absorption enhancement is carefully investigated. A minimum reflection loss intensity −30.6 dB of LaNiO 3 microspheres can be achieved and a broad effective absorbing bandwidth (reflection loss reaches −10.0 dB) of 4.72 GHz (8.16–12.88 GHz) at the thickness of 2.0 mm. Furthermore, the overall effective bandwidth is obtained up to 14.5 GHz from 3.5 GHz to 18 GHz, covering more than 90% of the measured frequency range. It is expected that the LaNiO 3 microspheres could be used to design efficient microwave absorption materials. • The LaNiO 3 microspheres were obtained by a hydrothermal synthesis and subsequent annealing process and firstly tried for microwave absorption. • Enhanced dielectric loss and promoted impedance matching are achieved. • The minimum reflection loss reaches −30.6 dB, with a broad effective absorbing bandwidth of 4.72 GHz (8.16-12.88 GHz) at the thickness of 2.0 mm. [ABSTRACT FROM AUTHOR]