Chemical Ni-C Bonding in Ni-Carbon Nanotube Composite by a Microwave Welding Method and Its Induced High-Frequency Radar Frequency Electromagnetic Wave Absorption

State-of-the-art in carbides/carbon composites for electromagnetic wave absorption

Electromagnetic wave absorbing materials (EWAMs) have made great progress in the past decades, and are playing an increasingly important role in radiation prevention and antiradar detection due to their essential attenuation toward incident EM wave. With the flourish of nanotechnology, the design of high-performance EWAMs is not just dependent on the intrinsic characteristics of single-component medium, but pays more attention to the synergistic effects from different components to generate rich loss mechanisms. Among various candidates, carbides and carbon materials are usually labeled with the features of chemical stability, low density, tunable dielectric property, and diversified morphology/microstructure, and thus the combination of carbides and carbon materials will be a promising way to acquire new EWAMs with good practical application prospects. In this review, we introduce EM loss mechanisms related to dielectric composites, and then highlight the state-of-the-art progress in carbides/carbon composites as high-performance EWAMs, including silicon carbide/carbon, MXene/carbon, molybdenum carbide/carbon, as well as some uncommon carbides/carbon composites and multicomponent composites. The critical information regarding composition optimization, structural engineering, performance reinforcement, and structure-function relationship are discussed in detail. In addition, some challenges and perspectives for the development of carbides/carbon composites are also proposed after comparing the performance of some representative composites.

The Improved Microwave Absorption Performance of the 3D Porous (Ni@NO-C)n/NO-C Composite Absorber

Microwave absorbers that are lightweight and have good stability and high efficiency have attracted much attention for their applications in many contemporary fields. In this work, a 3D porous (Ni@NO-C)n/NO-C composite absorber was prepared using a wet chemistry method with Ni chains and melamine as precursors, in which NO-C (N,O-doped carbon)-encapsulated Ni particles are homogenously dispersed in the 3D porous networks of NO-C in the form of (Ni@NO-C)n chains. The special microstructure of the as-prepared material is proven to be beneficial for the improvement of its microwave absorption performance. The as-synthesized (Ni@NO-C)n/NO-C composite absorber exhibited an effective absorption bandwidth of 4.1 GHz and an extremely large reflection loss of -72.3 dB. The excellent microwave-absorbing performances can be ascribed to the cooperative consequences of dielectric loss and magnetic loss, along with the balance between attenuation capability and impedance matching.

Polyvinylpyrrolidone induced uniform coating of nickel nanoparticles on carbon nanotubes for efficient microwave absorption

The impedance matching performance of carbon nanotubes (CNTs) can be effectively enhanced by developing a uniform magnetic impedance matching layer, which can take on critical significance in achieving the desirable microwave absorption (MA) performance. To obtain a uniform coating of Nickel (Ni) nanoparticles on CNTs, several methods have been developed (e.g., the γ-irradiation technique, electroless deposition, as well as microwave welding method). However, the intricate and complicated conditions of the above-mentioned methods limit their wide application. Therefore, controlling the distribution of Ni nanoparticles with the aid of a concise and effective method remains a great challenge. Herein, in view of the uniform dispersion effect of polyvinylpyrrolidone (PVP) on CNTs and its complexation with Ni ions, uniform coating of Ni nanoparticles on CNTs is well developed after it is introduced in the hydrothermal process. The prepared Ni/CNTs composites exhibited excellent MA performance in comparison with those of reported Ni/CNTs composites for the ideal impedance matching performance and microwave attenuation ability. When the filler content was only 15 wt%, the minimum reflection loss (RL_min) reached -39.5 dB, and the effective bandwidth (EB) with RL < -10 dB reached 5.2 GHz at the thickness of 1.15 mm. A scalable strategy of regulating the distribution of Ni nanoparticles and preparing a lightweight microwave absorber based on CNTs was developed in this study, which can serve as a vital guideline for preparing novel MA composite materials.

Highly Performant Electromagnetic Absorption at the X Band Based on Co@NCS/Ti3C2Tx Composites

Electromagnetic waves at the X band (8.2-12.4 GHz) play significant roles in military applications such as radar, satellite, and wireless communication. However, within this band range, the developed performance of electromagnetic absorption (EMA) is still unsatisfied, and it is hard to settle the corresponding problems on radar stealth and electromagnetic pollution. Herein, we demonstrate a state-of-the-art EMA property of -82.6 dB at 8.24 GHz with 2.57 mm thickness and 30 wt % paraffin filling ratio. For this purpose, an optimal Co@NCS/Ti₃C₂T_x composite is prepared by an electrostatic self-assembly approach through compelling Co-loading of nitrogen-doped carbon sheets (Co@NCS) derived from the pyrolysis of ZIF-67 (CoZn) with 2D Ti₃C₂T_x MXene nanosheets. Experimental results show that the highly efficient EMA performance of this Co@NCS/Ti₃C₂T_x composite originates from the large surface area for multiple reflection and electromagnetic wave scattering, from abundant defects sites for dipole and interfacial polarization, and from the optimizing impedance matching by the combination of Co magnetic nanoparticles and conductive NCS/Ti₃C₂T_x composite. These results confirm that the as-fabricated composites possess scientific and practical values for EMA applications at the X band, paving the way for developing highly performant electromagnetic absorbers toward specific microwave bands.

High-performance electromagnetic wave absorption of NiCoFe/N-doped carbon composites with a Prussian blue analog (PBA) core at 2-18 GHz

Structure design and assembly control are the two key factors in designing new microwave absorbing materials and improving their electromagnetic wave absorption (EMWA) performance; however, balancing the coordination between these factors remains a great challenge. In this manuscript, a coprecipitation method and an in-situ polymerization method were used to construct nitrogen-carbon-doped popcorn-like porous nanocomposites (NiCoFe/NC). The metallic particles were encapsulated in approximately 10 layers of graphite carbon shells, and a NiCoFe/NC core-shell structure was formed. The EMWA properties of the NiCoFe/NC composites were adjusted by varying the divinylbenzene (DVB) to acrylonitrile (AN) content. The optimized NiCoFe/NC composite showed a minimum reflection loss of -57.5 dB and a maximum effective absorption bandwidth (EAB) of 5.44 GHz. The excellent EMWA properties of the NiCoFe/NC composites can be attributed to the synergistic effect among the core-shell structure, popcorn-like structure, magnetic metal, carbon and nitrogen. This effect leads to enhanced impedance matching, interface polarization, dipole polarization, multiple reflection and scattering in the composites. In this paper, an effective strategy for the preparation of high-performance magnetic/dielectric composites is provided by carefully designing a new microstructure.

Synthesis of hybrid biosorbent based on 1,2-cyclohexylenedinitrilotetraacetic acid modified crosslinked chitosan and organo-functionalized calcium alginate for adsorptive removal of Cu(II)

The present study is based on the synthesis of a novel hybrid biosorbent using 1,2-cyclohexylenedinitrilotetraacetic acid modified crosslinked chitosan and amino-thiocarbamate moiety functionalized sodium alginate (CDTA-CS/TSC-CA). The fabricated sorbent was employed to investigate the efficient recovery of Cu(II) from aqueous media. CDTA-CS/TSC-CA was characterized using Fourier Transform Infrared Spectroscopy (FTIR) and Scanning Electron Microscopy (SEM). Analysis confirmed the successful modification of both biopolymers and subsequent loading of Cu(II) ions. CDTA-CS/TSC-CA was casted in the form of hydrogel beads having different CDTA-CS to TSC-CA mass ratios i.e., 10.0-40.0% by mass. The hydrogel beads 4CDTA-CS/TSC-CA with CDTA-CS/TSC-CA mass ratio of 40.0% was found most effective for copper sorption. Equilibrium sorption results showed that initial concentration of copper, medium pH, contact time, sorbent dosage and temperature influenced the sorption capacity (q_e). Rate of sorption data was interpreted using different kinetic models and found best fitted with pseudo second order rate expression (R² ≈ 0.99), illustrating that the rate determining step includes the electron density transfer from sorbent coordination sites to central copper ions. Crank's RIDE equation and Elovich chemisorption model (ECM) revealed the presence of two sorption phases, initially rapid sorption followed by comparatively a slow uptake. Equilibrium sorption data was well depicted by Langmuir model and maximum monolayer adsorption capacity (q_m) was computed as 276.53 mg·g^-1 at 298 K. Standard Gibbs free energy change, ∆G° (-19.99, -20.18 and -20.36 kJ/ mol), standard enthalpy change, ∆H° (-8.95 kJmol) and standard entropy change, ∆S° (0.04 kJ/mol K^-1) values suggested that the adsorption process is spontaneous and exothermic. Hence, 4CDTA-CS/TSC-CA was found efficient biosorbent for copper removal from its dilute effluents.

Dielectric Loss Mechanism in Electromagnetic Wave Absorbing Materials

Electromagnetic (EM) wave absorbing materials play an increasingly important role in modern society for their multi-functional in military stealth and incoming 5G smart era. Dielectric loss EM wave absorbers and underlying loss mechanism investigation are of great significance to unveil EM wave attenuation behaviors of materials and guide novel dielectric loss materials design. However, current researches focus more on materials synthesis rather than in-depth mechanism study. Herein, comprehensive views toward dielectric loss mechanisms including interfacial polarization, dipolar polarization, conductive loss, and defect-induced polarization are provided. Particularly, some misunderstandings and ambiguous concepts for each mechanism are highlighted. Besides, in-depth dielectric loss study and novel dielectric loss mechanisms are emphasized. Moreover, new dielectric loss mechanism regulation strategies instead of regular components compositing are summarized to provide inspiring thoughts toward simple and effective EM wave attenuation behavior modulation.

Enhanced catalytic performance of spinel-type Cu-Mn oxides for benzene oxidation under microwave irradiation

Microwave-assisted heterogeneous catalytic oxidation of benzene was investigated over Cu-Mn spinel oxides. The spinel oxides were synthesized by a coprecipitation method from metal nitrate hydrolysis in a solution using tetramethylammonium hydroxide (TMAH) as a precipitation reagent. The catalysts were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, X-ray absorption fine structure, scanning electron microscopy, transmission electron microscope and H₂-temperature-programmed reduction studies. Microwave absorption by the Cu-Mn spinel oxide is mainly driven by dielectric losses (dielectric heating). Cu-Mn spinel oxide with a Cu/Mn ratio of 1 exhibited superior activity to single oxides under microwave heating, demonstrating lower apparent activation energy than that obtained under conventional heating. Microwave irradiation lowered the reaction temperature required for benzene oxidation compared with conventional heating. Transient tests were used to investigate the reactivity of oxygen species in the catalytic reaction, and the high reactivity of Cu-Mn spinel oxides was related to the high reactivity of lattice oxygen on the catalyst surface. The reactivity of the oxygen species was enhanced under microwave heating, leading to an enhanced benzene oxidation reaction. The combination of adsorption and catalytic oxidation processes using Cu-Mn spinel oxides and zeolites efficiently decomposed benzene at low concentrations.

Modified alginate-chitosan-TiO2 composites for adsorptive removal of Ni(II) ions from aqueous medium

In this study, organo-funtionalization of sodium-alginate has been carried out using phenylsemicarbazide as modifier to graft N, O-donor atoms containing functional groups (amino-carbamate moieties) to offer novel support for TiO₂ immobilization. Hybrid composite made of aminocarbamated alginate, carboxymethyl chitosan (CMC) and titanium oxide TiO₂ (MCA-TiO₂) was prepared for the promising adsorptive remediation of Ni(II). FT-IR, SEM-EDX were employed to characterize MCA-TiO₂. The optimization of TiO₂ to modified alginate mass ratio was carried out and hydrogel beads with TiO₂/MCA mass ratio of 10.0% (2MCA-TiO₂) revealed highest sorption efficiency. The produced sorbents were adapted in the form of hydrogel beads for operation. Organic functionalization based on aminocarbamate (OCONHNH₂) moieties on linear chains of alginate embedded additional chelating functional sites which enhanced sorption and selectivity. Batch mode experiments were conducted for optimization of pH and sorbent dose. Equilibrium sorption, kinetic and thermodynamic studies were performed to pattern the nature of sorption. Kinetic data was found in close agreement with pseudo-second order rate expression (PSORE). Isothermal equilibrium sorption data was well fitted with Langmuir adsorption model. Maximum sorption capacity was evaluated as 229 mg/g at 298 K and pH = 6.0.

Enhancement in microwave absorption properties by adjusting the sintering conditions and carbon shell thickness of Ni@C submicrospheres

The core–shell Ni@C submicrospheres were fabricated by sintering method and combined with PVDF to prepare Ni@C/PVDF composite, which possessed optimal wave absorption property with RL min of −71.32 dB and EAB of 5.62 GHz at 1.95 mm.

Composition Optimization and Microstructure Design in MOFs-Derived Magnetic Carbon-Based Microwave Absorbers: A Review

Magnetic carbon-based composites are the most attractive candidates for electromagnetic (EM) absorption because they can terminate the propagation of surplus EM waves in space by interacting with both electric and magnetic branches. Metal-organic frameworks (MOFs) have demonstrated their great potential as sacrificing precursors of magnetic metals/carbon composites, because they provide a good platform to achieve high dispersion of magnetic nanoparticles in carbon matrix. Nevertheless, the chemical composition and microstructure of these composites are always highly dependent on their precursors and cannot promise an optimal EM state favorable for EM absorption, which more or less discount the superiority of MOFs-derived strategy. It is hence of great importance to develop some accompanied methods that can regulate EM properties of MOFs-derived magnetic carbon-based composites effectively. This review comprehensively introduces recent advancements on EM absorption enhancement in MOFs-derived magnetic carbon-based composites and some available strategies therein. In addition, some challenges and prospects are also proposed to indicate the pending issues on performance breakthrough and mechanism exploration in the related field.

Opportunities and challenges in microwave absorption of nickel-carbon composites

In recent years, the problem of electromagnetic wave (EMW) pollution has attracted more and more attention with the development of science and technology. In order to solve this complex problem, the research and development of EMW-absorbing materials is crucial. The new absorbing materials should have the characteristics of light weight, high efficiency, wide bandwidth, environmental protection, oxidation resistance, and other characteristics. Traditional single-phase Ni materials exhibit remarkable ferromagnetic behavior and double-loss mechanisms (dielectric loss and magnetic loss), and are considered as efficient EMW absorbers. However, under the action of EMWs, especially in the GHz frequency band, Ni materials tend to produce an eddy current effect, which limits their application prospects. For Ni-based materials, there is much interest in modifying the composite materials by designing a hierarchical structure for their preparation. Traditional, single-phase, carbon-based materials have been widely used in related fields because of their light weight and good conductivity. However, a single-loss mechanism will affect the impedance matching of carbon materials, thus affecting their application in the field of absorbing waves. For carbon materials, people use them as a filler or matrix material to fabricate composites with metals, metal oxides, or polymer materials to obtain carbon-containing absorbing materials. This paper reviews the evaluation and design principles of the absorbing properties of EMW-absorbing materials. Then, the progress of modified single-phase Ni-based materials (designed materials with 0D, 1D, 2D, and 3D structures), the development of carbon materials (carbon black, carbon nanotubes, carbon fiber, graphite oxide, reduced graphene oxide, and biomedical carbon), and the research progress of Ni-C composite materials (the composite material formed by nickel and carbon) are reviewed. The ultimate goal is to obtain absorbers with light weight, strong absorbing ability, and a wide frequency band. In particular, Ni-MXene, Ni-biomedical carbon, and Ni-multiphase carbon composites are the target direction for designing new and high efficiency EMW absorbers. Finally, the basic challenges and opportunities in this field are discussed.

One-dimensional Ni@Co/C@PPy composites for superior electromagnetic wave absorption

The conductive networks for electron hopping and migration constructed by one-dimensional (1D) composite absorbers are highly desirable to improve the electromagnetic (EM) wave attenuation capacity. Herein, the Ni@Co/C@polypyrrole (PPy) composites integrating the advantages of component and microstructure were fabricated. The addition of Co/C and PPy effectively optimized the impedance matching and improved the EM attenuation. Under the comprehensive impacts of multiple reflections/scattering, conduction loss and interface polarization, the Ni@Co/C@PPy composites showed superior EM wave absorption with the reflection loss (RL) value of -48.76 dB and the effective absorption bandwidth (EAB) of 5.10 GHz at a corresponding thickness of 2.0 mm. The largest EAB could reach 5.54 GHz (7.24-12.78 GHz) at the thickness of 2.2 mm. This work provides a great reference for fabricating 1D novel EM wave absorption materials.

Facile synthesis of nickel/carbon nanotubes hybrid derived from metal organic framework as a lightweight, strong and efficient microwave absorber

Transition metal carbon composites derived from metal organic frameworks (MOFs) attract increasing attention in microwave absorption field. However, finding a relatively facile and green method to prepare MOFs precursor is still a challenge. Besides, it is also difficult to obtain carbon nanotubes based compounds by only using MOF as sacrificed template. Herein, nickel (Ni) MOF is fabricated at room temperature with water as solvent. Afterwards, nickel/carbon nanotubes composite (Ni/CN) is prepared via only in-situ pyrolysis of Ni MOF. The pyrolysis temperature greatly affects nitrogen (N) dopant state for Ni/CN composites. The Ni/CN composite prepared at 700 °C (Ni/CN-700) exhibits the maximum reflection loss (RL) of -65 dB with the effective absorbing bandwidth (EAB) about 4.6 GHz at 1.9 mm, when the filling loading is only 10 wt% in the matrix. Remarkably, the Ni/CN composite is excellent microwave absorber with lightweight and strong absorption. The magnetic metal/carbon nanotubes derived from MOF prepared in green solvent offers a facile, environmentally friendly and designable strategy for exploring excellent microwave absorber.

Self-Limiting Growth of Single-Layer N-Doped Graphene Encapsulating Nickel Nanoparticles for Efficient Hydrogen Production

Effective nonprecious metal catalysts are urgently needed for hydrogen evolution reaction (HER). The hybridization of N-doped graphene and a cost-effective metal is expected to be a promising approach for enhanced HER performance but faces bottlenecks in controllable fabrication. Herein, a silica medium-assisted method is developed for the high-efficient synthesis of single-layer N-doped graphene encapsulating nickel nanoparticles (Ni@SNG), where silica nanosheets molecule sieves tactfully assist the self-limiting growth of single-layer graphene over Ni nanoparticles by depressing the diffusion of gaseous carbon radical reactants. The Ni@SNG sample synthesized at 800 °C shows excellent activity for HER in alkaline medium with a low overpotential of 99.8 mV at 10 mA cm^-2, which is close to that of the state-of-the-art Pt/C catalyst. Significantly, the Ni@SNG catalyst is also developed as a binder-free electrode in magnetic field, exhibiting much improved performance than the common Nafion binder-based electrode. Therefore, the magnetism adsorption technique will be a greatly promising approach to overcome the high electron resistance and poor adhesive stability of polymer binder-based electrodes in practical applications.

Advances of 3D graphene and its composites in the field of microwave absorption

The intensive progress of information technology increases the demand for urgent development of practical materials for microwave absorption (MA), meeting the general requirement "thin, wide, light and strong". In the past 6 years, graphene is of great interest for MA performance due to its unique properties such as high specific surface area, high electrical conductivity, strong dielectric loss, and low density. Taking in account that the structure of absorber plays a key role in MA performance, the attempts to produce an efficient microwave absorbing materials (MAMs) have led to 3D graphene - aerogels and foams - due to their extremely high porosity, large specific surface area, excellent mechanical properties with ability of compression and further maintaining the original shape, lightweight, reduced agglomeration of graphene sheets. All listed parameters enhance the impedance matching of MAMs, generate the synergistic loss effects, thereby improving the MA properties. The review describes the bases of MA theory and summarizes the recent achievements in the fabrication of pure 3D graphene networks and their composites with magnetic, ceramic nanoparticles and nanowires, polymers, MXenes, and multicomponent systems, directed to improve the impedance matching and generate loss mechanisms for the overall improvement of their performance as MAMs.

Bimetallic Metal-Organic Framework-Derived Pomegranate-like Nanoclusters Coupled with CoNi-Doped Graphene for Strong Wideband Microwave Absorption

Metal-organic frameworks (MOFs) featuring high porosity and tunable structure make them become promising candidates to fabricate carbon-based microwave absorption (MA) materials to meet the requirements of electronic reliability and defense security. However, it is challenging to rationally design a well-organized micro-nanostructure to simultaneously achieve strong and wideband MA performance. Herein, a three-dimensional (3D) hierarchical nanoarchitecture (CoNi@NC/rGO-600) comprising pomegranate-like CoNi@NC nanoclusters and ultrasmall CoNi-decorated graphene has been successfully fabricated to broaden the absorption bandwidth and enhance the absorption intensity. The results confirm that the bimetallic MOF CoNi-BTC-derived pomegranate-like CoNi@NC nanoclusters with porous carbon shell as "peel" and sub-5 nm CoNi nanoparticles as "seeds" favor multiple polarization, magnetic loss, and impedance matching. Moreover, the interconnected 3D CoNi-doped graphene acts not only as a bridge to connect pomegranate-like CoNi@NC nanoclusters but also as a conductive network to supply multiple electron transportation paths. Consequently, the optimized CoNi@NC/rGO-600 exhibits extraordinary MA performance in terms of wide bandwidth (6.7 GHz) and strong absorption (-68.0 dB). As an effective strategy, this work provides a new insight into fabricating hierarchical composite structures for advancing MA performances and other applications.

Enhanced Electromagnetic Absorption Properties of Commercial Ni/MWCNTs Composites by Adjusting Dielectric Properties

In this manuscript, we constructed a Ni/MWCNTs absorber and properly adjusted the permittivity resulted from absorber content in the PVDF to optimize impedance matching properties. Both ε' and ε″ increase obviously with the increasing content of Ni/MWCNTs in PVDF, demonstrating that dielectric properties are dependent on the conductivity. Moderate dielectric properties and excellent impedance matching can be obtained for the filler content of 20 wt% Ni/MWCNTs. Reasonable impedance matching allows electromagnetic waves to propagate into the materials and finally realize energy dissipation through dielectric loss and interfacial polarization. As expected, the minimum reflection loss (RL) of -46.85 dB at 6.56 GHz with a low filler loading (20 wt%) and wide effective bandwidth (RL<-10 dB) of 14.0 GHz in the thickness range of 1.5-5.0 mm was obtained for the commercial Ni/MWCNTs composites, which is promising for mass production in industrial applications. Our findings offer an effective and industrialized way to design high-performance material to facilitate the research in microwave absorption.

Large-Scale Synthesis of Three-Dimensional Reduced Graphene Oxide/Nitrogen-Doped Carbon Nanotube Heteronanostructures as Highly Efficient Electromagnetic Wave Absorbing Materials

Herein, we use reduced graphene oxide as a substrate and NiFe as a catalyst to fabricate three-dimensional (3D) nitrogen-doped carbon nanotube (NCNT)/reduced graphene oxide heteronanostructures (3D NiFe/N-GCTs). The 3D NiFe/N-GCTs are composed of two-dimensional (2D) reduced graphene oxide-supported one-dimensional (1D) NiFe nanoparticle-encapsulated NCNT arrays. The NCNTs exhibit bamboo-like shapes with the length and diameter of 3-10 μm and 15-45 nm, respectively. Besides integration of advantages of 1D and 2D nanomaterials, the 3D NiFe/N-GCT heteronanostructure possesses interconnected network structures, sufficient interfaces, numerous defects, hundreds of void spaces enclosed by bamboo joints and the walls of the NCNT in an individual carbon nanotube, and large surface areas, which can improve their dielectric losses toward electromagnetic wave. Thus, the 3D NiFe/N-GCTs show satisfied property toward electromagnetic wave absorption. Typically, the optimized 3D NiFe/N-GCT displays excellent minimal reflection loss (-40.3 dB) and outstanding efficient absorption bandwidth (4.5 GHz), outperforming most of the reported absorbers. Remarkably, the synthesis of 3D NiFe/N-GCTs only involves vacuum freeze-drying and subsequent thermal treatment process at a high temperature, and thus, the large-scale production of 3D NiFe/N-GCTs can be achieved in each batch, affording the possibility of the practical applications of the 3D NiFe/N-GCTs.

Cactus-Inspired Bimetallic Metal-Organic Framework-Derived 1D-2D Hierarchical Co/N-Decorated Carbon Architecture toward Enhanced Electromagnetic Wave Absorbing Performance

Metal-organic framework (MOF)-derived magnetic metal/carbon nanocomposites have shown tremendous potential for lightweight electromagnetic wave (EMW) absorption. However, it is a challenge but highly significant to design and construct mixed-dimensional hierarchical architectures with synergistically integrated characteristics from individual MOFs for advancing the EMW absorption performance. Inspired by the structure of cactus, a novel hierarchical one-dimensional (1D)-two-dimensional (2D) mixed-dimensional Co/N-decorated carbon architecture comprising carbon nanotubes grafted on carbon flakes (abbreviated as CoNC/CNTs) has been fabricated by the pyrolysis of bimetallic CoZn-ZIF-L. The CoNC/CNTs integrate the advantages of 1D nanotubes for the extra polarization of EMW and 2D nanoflakes with an interconnected porous structure for multiple reflection losses of EMW and optimization of impedance matching. The resultant CoNC/CNTs demonstrate excellent EMW absorbing performance. For the optimal EMW absorbing material of CoNC/CNT-3/1, minimum reflection loss reaches -44.6 dB at 5.20 GHz with a low filler loading of 15 wt %. Moreover, the largest effective bandwidth range achieves 4.5 GHz with a thickness of 1.5 mm and a filled ratio of 20 wt %. These findings indicate that such a mixed 1D-2D hierarchical architecture synergistically enhances EMW absorbing performance. This work sheds light on the rational design of a mixed-dimensional carbon architecture derived from MOFs for desirable functionalities.

Crystalline-Amorphous Permalloy@Iron Oxide Core-Shell Nanoparticles Decorated on Graphene as High-Efficiency, Lightweight, and Hydrophobic Microwave Absorbents

The exploration of high-efficiency microwave absorption materials with lightweight and hydrophobic features is highly expected to reduce or eliminate the electromagnetic pollution. Graphene-based nanocomposites are universally acknowledged as promising candidates for absorbing microwaves due to their remarkable dielectric properties and lightweight characteristic. However, the hydrophilicity of graphene may reduce their stability and restrict the applications in moist environment. Herein, a well-designed heterostructure composed of crystalline permalloy core and amorphous iron oxide shell was uniformly adhered on oleylamine-modified graphene nanosheets by a one-pot thermal decomposition method. Compared with the recognized hydrophilic graphene-based hybrid materials, the permalloy@iron oxide/graphene nanocomposites show excellent hydrophobic and water-resistant features with a water contact angle of 136.5°. Besides, the nanocomposites show high-efficiency microwave absorption performance, benefiting from the tunneling effect, polarization, interface interaction, impedance matching condition, and synergistic effect between core-shell permalloy@iron oxide nanoparticles and graphene nanosheets. A broad effective absorption bandwidth with reflection loss (RL) value exceeding -10 dB can be obtained from 4.25 to 18 GHz, covering about 86% measured frequency range when the absorber thickness is 2.0-5.0 mm. Also, the microwave absorption performance of nanocomposites can be tuned by changing the amount of graphene. More importantly, a greatly improved microwave absorption effectiveness of -71.1 dB can be achieved for the nanocomposites in comparison with the bare permalloy@iron oxide nanoparticles (-5.6 dB) and oleylamine-modified GO nanosheets (-3.56 dB). The lightweight and hydrophobic permalloy@iron oxide/graphene nanocomposites with high-efficiency microwave absorption performance are highly promising to improve the environmental adaptability of electric devices, especially in the wet environment.

Dopamine-derived cavities/Fe3O4 nanoparticles-encapsulated carbonaceous composites with self-generated three-dimensional network structure as an excellent microwave absorber

Dopamine-derived cavities/Fe₃O₄ nanoparticles-encapsulated carbonaceous composites with self-generating three-dimensional (3D) network structure were successfully fabricated by a facile synthetic method, in which sodium alginate provided carbon matrix pores and excellent microwave absorption performance was established. The hollow cavities derived from the core–shell-like CaCO₃@polydopamine were creatively introduced into the 3D absorber to significantly improve the absorption performance. The sample calcined at 700 °C exhibited the most outstanding microwave absorption performance, with minimal reflection loss up to −50.80 dB at 17.52 GHz with a rare thickness of only 1.5 mm when filler loading was 35% in paraffin matrix. The effective absorption bandwidth of reflection loss < −10 dB reached 3.52 GHz from 14.48 GHz to 18 GHz, corresponding to the same thickness of 1.5 mm. In contrast, the sample without hollow dopamine-derived cavities showed poor performance due to poor impedance matching, and this highlights the role of hollow cavities brought into the 3D structure, which led to a difference in interfacial polarization, multiple reflections and scattering. The novel dopamine-derived cavities/Fe₃O₄ nanoparticles-encapsulated carbonaceous composites with 3D network structure can be regarded as a promising candidate for application as a microwave absorber with strong absorption.

Hollow dopamine-derived cavities/Fe₃O₄ nanoparticles-encapsulated carbonaceous composites with self-generating 3D network structure were fabricated for potential application as excellent microwave absorbers.

Fabrication of 3D Hierarchical Byttneria Aspera-Like Ni@Graphitic Carbon Yolk-Shell Microspheres as Bifunctional Catalysts for Ultraefficient Oxidation/Reduction of Organic Contaminants

The search for earth-abundant, low-cost, recyclable, multifunctional as well as highly active catalysts remains the most pressing demand for heterogeneous catalytic elimination of pollutants in water environment remediation. Herein, a porous graphitic carbon-encapsulated Ni nanoparticles (NPs) hybrid (Ni@GC) is designed/constructed by direct pyrolysis of a Ni-based metal-organic framework (MOF) in N₂ . The resulting Ni@GC exhibits a unique 3D hierarchical byttneria aspera-like yolk-shell structure with a high surface area, abundant active sites as well as good microwave (MW)-absorbing performance. The outstanding MW-driven oxidation of norfloxacin to inorganic molecules and direct catalytic reduction of 4-nitrophenol to less toxic and more useful chemical raw material (4-aminophenol) can originate from the synergistic effects, including the presence of both zero-valent Ni NPs as the active center and graphitic carbon as the protective layer/electron acceptor as well as unique porous yolk-void-shell structure, which facilitates the MW energy-harvesting and/or rapid mass transfer of the reactant/product in the channels and cavities. Accordingly, the localized surface plasmon resonance-excitation and electronic relay mechanism are proposed to account for the catalytic oxidation/reduction, respectively. This work provides a new strategy for the design/assembly of multifunctional metal@GC hybrids with a unique architecture and elucidates new opportunities for remediation of environmental water.

Solid and macroporous Fe3C/N-C nanofibers with enhanced electromagnetic wave absorbability

A series of solid and macroporous N-doped carbon nanofibers composed of Fe₃C nanoparticles (named as solid Fe₃C/N-C NFs, solid Fe₃C/N-C NFs-1, solid Fe₃C/N-C NFs-2, macroporous Fe₃C/N-C NFs, macroporous Fe₃C/N-C NFs-1 and macroporous Fe₃C/N-C NFs-2, respectively) were prepared through carbonization of as-electrospun nanofiber precursors. The results show that the magnetic Fe₃C nanoparticles (NPs) dispersed homogeneously on the N-doped carbon fibers; as-prepared six materials exhibit excellent microwave absorption with a lower filler content in comparison with other magnetic carbon hybrid nanocomposites in related literatures. Particularly, the solid Fe₃C/N-C NFs have an optimal reflection loss value (RL) of −33.4 dB at 7.6 GHz. For solid Fe₃C/N-C NFs-2, the effective absorption bandwidth (EAB) at RL value below −10 dB can be up to 6.2 GHz at 2 mm. The macroporous Fe₃C/N-C NFs have a broadband absorption area of 4.8 GHz at 3 mm. The EAB can be obtained in the 3.6–18.0 GHz frequency for the thickness of absorber layer between 2 and 6 mm. These Fe₃C–based nanocomposites can be promising as lightweight, effective and low-metal content microwave absorption materials in 1–18 GHz.

Hierarchical Carbon Nanotube-Coated Carbon Fiber: Ultra Lightweight, Thin, and Highly Efficient Microwave Absorber

Strong EM wave absorption and lightweight are the foremost important factors that drive the real-world applications of the modern microwave absorbers. This work mainly deals with the design of highly efficient microwave absorbers, where a hierarchical carbon nanotube (CNT) forest is first grown on the carbon fiber (CF) through the catalytic chemical vapor deposition method. The hierarchical carbon nanotube grown on the carbon fiber (CNTCF) is then embedded in the epoxy matrix to synthesize lightweight nanocomposites for their use as efficient microwave absorbers. The morphological study shows that carbon nanotubes (CNTs) self-assemble to form a trapping center on the carbon fiber. The electromagnetic characteristics of resultant nanocomposites are investigated exclusively in the X-band (8.2-12.4 GHz) using the network analyzer. The synthesized nanocomposites, containing 0.35 and 0.50 wt % CNTCFs, exhibit excellent microwave absorption properties, which could be attributed to the better impedance matching conditions and high dielectric losses. The reflection loss (RL) of -42.0 dB (99.99% absorption) with -10 dB (90% absorption) and -20 dB (99% absorption) bandwidths of 2.7 and 1.16 GHz, respectively, is achieved for 0.35 wt % CNTCF loading at 2.5 mm thickness. The composite with 0.50 wt % CNTCF loading illustrates substantial absorption efficiency with the RL reaching -24.5 dB (99.65% absorption) at 9.8 GHz and -10 dB bandwidth comprising 84.5% of the entire X band. The excellent microwave properties obtained here are primarily due to the electric dipole polarization, interfacial polarization, and unique trapping center. These trapping centers basically induce multiple reflections and scatterings, which attenuate more microwave energy. This investigation opens a new approach for the development of extremely lightweight, small-thickness, and highly efficient microwave absorbers for X-band applications.

In Situ Preparation of Cobalt Nanoparticles Decorated in N-Doped Carbon Nanofibers as Excellent Electromagnetic Wave Absorbers

Electrospinning and annealing methods are applied to prepare cobalt nanoparticles decorated in N-doped carbon nanofibers (Co/N-C NFs) with solid and macroporous structures. In detail, the nanocomposites are synthesized by carbonization of as-electrospun polyacrylonitrile/cobalt acetylacetonate nanofibers in an argon atmosphere. The solid Co/N-C NFs have lengths up to dozens of microns with an average diameter of ca. 500 nm and possess abundant cobalt nanoparticles on both the surface and within the fibers, and the cobalt nanoparticle size is about 20 nm. The macroporous Co/N-C NFs possess a hierarchical pore structure, and there are macropores (500 nm) and mesopores (2-50 nm) existing in this material. The saturation magnetization ( M_s) and coercivity ( H_c) of the solid Co/N-C NFs are 28.4 emu g^-1 and 661 Oe, respectively, and those of the macroporous Co/N-C NFs are 23.3 emu g^-1 and 580 Oe, respectively. The solid Co/N-C NFs exhibit excellent electromagnetic wave absorbability, and a minimum reflection loss (RL) value of -25.7 dB is achieved with a matching thickness of 2 mm for solid Co/N-C NFs when the filler loading is 5 wt %, and the effective bandwidth (RL ≤ -10 dB) is 4.3 GHz. Moreover, the effective microwave absorption can be achieved in the whole range of 1-18 GHz by adjusting the thickness of the sample layer and content of the dopant sample.

Rational design of CNTs with encapsulated Co nanospheres as superior acid- and base-resistant microwave absorbers

A Co@CNT material with a specific coating structure displays good EM wave absorption, even after treatment with concentrated acid or base.

A novel sponge-like 2D Ni/derivative heterostructure to strengthen microwave absorption performance

One of the major hurdles of Ni-based microwave absorbing materials is the preparation of two-dimensional (2D) Ni flakes that can improve magnetic anisotropy to tune complex permeability.