Interface Strategy To Achieve Tunable High Frequency Attenuation

Among all polarizations, the interface polarization effect is the most effective, especially at high frequency. The design of various ferrite/iron interfaces can significantly enhance the materials’ dielectric loss ability at high frequency. This paper presents a simple method to generate ferrite/iron interfaces to enhance the microwave attenuation at high frequency. The ferrites were coated onto carbonyl iron and could be varied to ZnFe₂O₄, CoFe₂O₄, Fe₃O₄, and NiFe₂O₄. Due to the ferrite/iron interface inducing a stronger dielectric loss effect, all of these materials achieved broad effective frequency width at a coating layer as thin as 1.5 mm. In particular, an effective frequency width of 6.2 GHz could be gained from the Fe@NiFe₂O₄ composite

Constructing Large Interconnect Conductive Networks: An Effective Approach for Excellent Electromagnetic Wave Absorption at Gigahertz

In recent years, with the development of electronic equipment, fabricating a lightweight and effective absorber to prevent electromagnetic (EM) wave pollution has been an imperative mission. Among diverse innovative strategies, constructing a large interconnected conductive network has been an effective approach to dissipate the EM wave. Herein, Co/C hybrids with twisty carbon nanotubes (CNTs) were prepared from melamine-formaldehyde resin through a polymerization method together with a subsequent calcination process. The resultant Co/C hybrids were strung into an interconnected framework by long and twist CNTs, which exhibited remarkable microwave characteristic at low loading content of 20 wt % in a wax matrix. The reflection loss (RL) intensity of −43 dB was achieved with an effective frequency bandwidth (RL < −10 dB) of 4.45 GHz at 1.85 mm. Such excellent absorbing properties at lower filler loading and thin thickness endow the Co/C hybrids with significant potential for application in attenuation EM wave energy

Cross-Linking-Derived Synthesis of Porous Co_<i>x</i>Ni_<i>y</i>/C Nanocomposites for Excellent Electromagnetic Behaviors

The magnet/dielectric composites with tunable structure and composition have drawn much attention because of their particular merits in magnetoelectric properties compared with the sole dielectric or magnetic composites. In addition, porous materials at the nanoscale can satisfy the growing requirements in many industries. Therefore, constructing porous metal alloy/carbon nanocomposites is to be an admirable option. Unfortunately, traditional synthesis methods involve multistep routes and complicated insert-and-remove templates approaches. Here we report a facile process to synthesize Co_<i>x</i>Ni_<i>y</i>/C composites via a spontaneous cross-linking reaction and subsequent calcination process, during which multiple processes, including reducing polyvalent metal ions, forming alloy, and encapsulating alloy nanoparticles into porous carbon matrix, are achieved almost simultaneously. By adjusting the feed ratio of Co²⁺ to Ni²⁺ ions, controllable composition of Co_<i>x</i>Ni_<i>y</i>/C composites can be gained. It should be noted that the Co_<i>x</i>Ni_<i>y</i>/C composites are demonstrated to be excellent microwave absorbers from every aspect of assessment criteria including reflection loss, effective bandwidth, thickness, and weight of absorber. Our study opens up a promising technique for the synthesis of alloy/carbon composites with porous nanostructures with target functionalities

A Versatile Route toward the Electromagnetic Functionalization of Metal–Organic Framework-Derived Three-Dimensional Nanoporous Carbon Composites

Designable electromagnetic parameters accompanied by a low density of metal–organic framework (MOF)-derived metal/carbon composites are essential prerequisites for excellent microwave-absorbing materials. However, the conventional route is confined to slight modification of the physicochemical properties of metal species and carbon, which also restricts the functionalization of MOF-derived materials. Here, a facile technique has been improved by making full use of highly porous structure to uniformly introduce metallic Co nanoparticles into carbon matrix derived from Cu₃(btc)₂. Through changing the starting amount of Co sources, the composition of the final products can be tuned, offering an effective route to control electromagnetic properties. Multiple attenuation mechanisms are employed to realize excellent reflection loss performance, which can be clarified by modified equivalent circuit mode. Effective frequency bandwidth (<i>f</i>_e) over the whole X band can be obtained by optimizing interfacial polarization through changing interface area and electrical conductivity. Broad <i>f</i>_e covering almost the whole K_u band from 12.3 to 18 GHz with a thin thickness of 1.85 mm can be gained through improving impedance matching and enhancing conduction loss. The present work not only sheds light on the easy fabrication of high-performance lightweight microwave-absorbing materials but also paves the way for extending functionalities of MOF-derived carbon composites

Hydrogenated Anatase TiO₂ as Lithium-Ion Battery Anode: Size–Reactivity Correlation

An improved hydrogenation strategy for controllable synthesis of oxygen-deficient anatase TiO₂ (H-TiO₂) is performed via adjusting the particle size of starting rectangular anatase TiO₂ nanosheets from 90 to 30 nm. The morphology and structure characterizations obviously demonstrate that the starting materials of TiO₂ nanosheets are transformed into nanoparticles with distinct size reduction; meanwhile, the concentration of oxygen vacancy is gradually increased with the decreasing particle size of starting TiO₂. As a result, the Li-storage performance of H-TiO₂ is not only much better than that of the pure TiO₂ but also elevated stage by stage with the decreasing particle size of starting TiO₂; especially the H-TiO₂ with highest concentration of oxygen vacancy from smallest TiO₂ nanosheets shows the best Li-storage performance with a stable discharge capacity 266 mAh g^–1 after 100 cycles at 1 C. Such excellent performance should be attributed to the joint action from oxygen vacancy and size effect, which promises significant enhancement of high electronic conductivity without weakening Li⁺ diffusion via hydrogenation strategy

Interface Polarization Strategy to Solve Electromagnetic Wave Interference Issue

Design of an interface to arouse interface polarization is an efficient route to attenuate high-frequency electromagnetic waves. The attenuation intensity is highly related to the contact area. To achieve stronger interface polarization, growing metal oxide granular film on graphene with a larger surface area seems to be an efficient strategy due to the high charge carrier concentration of graphene. This study is devoted to fabricating the filmlike composite by a facile thermal decomposition method and investigating the relationship among contact area, polarization intensity, and the type of metal oxide. Because of the high-frequency polarization effect, the composites presented excellent electromagnetic wave attenuation ability. It is shown that the optimal effective frequency bandwidth of graphene/metal oxide was close to 7.0 GHz at a thin coating layer of 2.0 mm. The corresponding reflection loss value was nearly −22.1 dB. Considering the attenuation mechanism, interface polarization may play a key role in the microwave-absorbing ability

Composition Design and Structural Characterization of MOF-Derived Composites with Controllable Electromagnetic Properties

Simply and effectively achieving the tunability of the composition and chemical state of each component remains a challenge for modifying the electromagnetic performance of metal–organic-framework-derived (MOF-derived) composites. In this work, quaternary ZnO/Fe/Fe₃C/carbon composites have been successfully synthesized by thermal decomposition of Fe^III-MOF-5. The composition and chemical state of each component can be effectively controlled by changing the heating temperature. In detail, with increasing temperature, the Fe element would be transformed from Fe³⁺ to Fe₃C and Fe, which also leads to the graphitization and weight loss of carbon. The effects on electromagnetic properties are also investigated, and the ZFC-700 sample possesses optimized reflection-loss (RL) performance with an RL value of −30.4 dB and a broad effective frequency bandwidth of 4.96 GHz at a thin thickness of only 1.5 mm. Conduction loss, interfacial polarization, ferromagnetic resonance, and interference cancelation should be responsible for ideal electromagnetic absorption. The porous quaternary composites not only convert incident electromagnetic energy to heat rather than reflect it back which is in favor of solving electromagnetic pollution, but also reduce the consumption of the metal source and poisonous raw materials for traditional microwave-absorbing materials

Strong Electromagnetic Wave Response Derived from the Construction of Dielectric/Magnetic Media Heterostructure and Multiple Interfaces

A novel yolk–shell structure of cobalt nanoparticle embedded nanoporous carbon@carbonyl iron (Co/NPC@Void@CI) was synthesized via metal organic chemical vapor deposition (MOCVD) and subsequent calcination treatment. The <i>in situ</i> generation of void layer, which originated from the shrink of a Co-based zeolitic imidazolate framework (ZIF-67) during carbonization, embodies distinct advantage compared to the conventional template method. Thanks to the introduction of custom-designed dielectric/magnetic media heterostructure and multiple interfaces, the composites filled with 40 wt % of Co/NPC@Void@CI samples in paraffin exhibit a maximum reflection loss of −49.2 dB at 2.2 mm; importantly, a broad absorption bandwidth (RL < −10 dB) of 6.72 GHz can be obtained, which covers more than one-third of the whole frequency region from 10.56 to 17.28 GHz. This study not only develops the application of carbonyl iron as a high-efficiency light absorber but also initiates a fire-new avenue for artificially designed heterostructures with target functionalities

Gram-Scale Synthesis of Graphene Quantum Dots from Single Carbon Atoms Growth via Energetic Material Deflagration

Graphene quantum dots (GQDs) with quantum confinement and size effect are proposed to be applicable in photovoltaic, nanodevices, and so on, due to extraordinary electronic and optical properties. Here we report a facile approach to synthesize gram-scale GQDs from active carbon atoms, which are obtained via the deflagration reaction of polytetrafluoroethylene (PTFE) and Si, growing from high- to low-temperature zones when traveling through the deflagration flame in a short time with releasing gas as the carrier medium. The prepared GQDs were aggregated into carbon nanospheres; thus, Hummer’s method was utilized to exfoliate the GQD aggregations into individual GQDs. We show that the length of GQDs is ∼10 nm and the exfoliated GQDs solution presents an obvious fluorescence effect with a strong emission peak at 570 at 460 nm excitation. And these GQDs are demonstrated to be excellent probes for cellular imaging. Furthermore, we propose a growth mechanism based on computer simulation, which is well verified by experimental reproduction. Our study opens up a promising route for high-yield and high-quality GQDs, as well as other various quantum dots

Fabrication of Hierarchical Macroporous/Mesoporous Carbons via the Dual-Template Method and the Restriction Effect of Hard Template on Shrinkage of Mesoporous Polymers

A series of hierarchically ordered macro-<b>/</b>mesoporous polymer resins and macro-<b>/</b>mesoporous carbon monoliths were synthesized using SiO₂ opal as a hard template for the macropore, amphiphilic triblock copolymer PEO–PPO–PEO as a soft template for the mesopore, and phenolic resin as a precursor for the polymer or carbon. The obtained hierarchical macro-<b>/</b>mesoporous frameworks had highly periodic arrays of uniform macropores that were surrounded by walls containing the mesoporous structures. The mesoporous structure of the walls was adjusted using different precursors for the synthesis of FDU-14, FDU-15, and FDU-16. Results of the N₂ adsorption–desorption analysis showed that the Brunauer–Emmett–Teller surface areas, the pore volumes, and the mesopore sizes of the macro-<b>/</b>mesoporous carbons were much larger than those of the FDU-14, FDU-15, and FDU-16 carbon materials. The mesopore size of the samples clearly increased with the increasing heat-treatment temperature when the temperature was below 700 °C. The results indicate that the SiO₂ hard template successfully restricted the shrinkage of the framework during the thermosetting and carbonization process