Controllable synthesis of porous Fe3O4@ZnO sphere decorated graphene for extraordinary electromagnetic wave absorption

Recent advances on outstanding microwave absorption and electromagnetic interference shielding nanocomposites of ZnO semiconductor

The electromagnetic interference shielding and microwave attenuation capabilities of ZnO semiconductor nanocomposites have recently been improved using a variety of approaches by correctly modifying their permittivity. To improve microwave attenuation, ZnO semiconductor nanostructures have been combined with graphene, multi-wall carbon nanotubes, metal nanoparticles and their alloys, two-dimensional MXene, spinel ferrite magnetic nanoparticles, polymer systems, and textiles. This paper covers the opportunities and constraints that these cutting-edge nanocomposites in the field of electromagnetic wave absorption encounter as well as the research progress of ZnO semiconductor-based nanocomposite. The structure-function relationship of electromagnetic wave absorption nanocomposites, design strategies, synthesis techniques, and various types of advanced nanocomposites based on ZnO semiconductor are also covered. In order to design and prepare high efficiency ZnO semiconductor based electromagnetic wave absorbing materials for use in applications of next-generation electronics and aerospace, this article can offer some useful ideas.

Flexible cobalt nanoparticles/carbon nanofibers with macroporous structures toward superior electromagnetic wave absorption

The increasing electromagnetic (EM) pollution that has seriously threatened human health and electronic devices urgently required high-performance absorbents toward attenuating EM wave (EMW). The combination of microstructure modulation and appropriate components regulation has proven to be a feasible strategy for improving the EMW absorption performance of absorbents. In this work, well-designed one-dimensional carbon nanofibers with macroporous structures and uniformly magnetic metal nanoparticles modification were prepared by the hard-template assisted electrospinning method followed by carbonization and template-elimination processes. The strong interfacial polarization loss and multireflection strengthened by the hollow structures and the magnetic loss induced by the introduced cobalt nanoparticles evidently enhanced the impedance matching level of the macroporous carbon nanofibers/cobalt nanoparticles (MCF/Co). As a result, MCF/Co composite offers broad absorption bandwidth (6.24 GHz) and strong electromagnetic wave absorption performance (-40.1 dB) at a thickness of 3.0 mm. This work inspires the rational one-dimensional macroporous carbon nanofibers design for new-generation EMW materials and provides an important research basis for the porous flexible EMW absorption materials.

Electromagnetic Absorption and Mechanical Properties of Natural Rubber Composites Based on Conductive Carbon Black and Fe3O4

In contemporary civilization, the electromagnetic radiation from electronic devices and communication systems has become a substantial pollutant. High-performance electromagnetic absorbers have become a solution for absorbing unwanted electromagnetic waves. This research proposed a lightweight and flexible electromagnetic absorber produced from natural rubber filled with conductive carbon black (CCB) and Fe₃O₄. The effect of CCB, Fe₃O₄, and a combination of CCB and Fe₃O₄ as a hybrid filler on foam morpholog, electromagnetic reflectivity, tensile strength, and compression set properties were investigated. In addition, the effect of the alternating layered structure of CCB and Fe₃O₄ on electromagnetic absorption was investigated. The results indicated that the composite foam exhibited an interconnected network structure that enhanced the electromagnetic attenuation in the absorber. CCB increased the electromagnetic absorption of the foam, whereas Fe₃O₄ had less of an effect. The foam filled with the hybrid filler at the CCB/Fe₃O₄ ratio of 8/2 exhibited excellent electromagnetic absorption. The composite foam had a higher tensile modulus and higher strength compared to neat foam. The addition of CCB decreased the compression set; however, the compression set was improved by the incorporation of Fe₃O₄. Composite foams filled with hybrid filler can serve as highly efficient electromagnetic absorbing materials.

Dimensional Design and Core-Shell Engineering of Nanomaterials for Electromagnetic Wave Absorption

Electromagnetic (EM) wave absorption materials possess exceptionally high EM energy loss efficiency. With vigorous developments in nanotechnology, such materials have exhibited numerous advanced EM functions, including radiation prevention and antiradar stealth. To achieve improved EM performance and multifunctionality, the elaborate control of microstructures has become an attractive research direction. By designing them as core-shell structures with different dimensions, the combined effects, such as interfacial polarization, conduction networks, magnetic coupling, and magnetic-dielectric synergy, can significantly enhance the EM wave absorption performance. Herein, the advances in low-dimensional core-shell EM wave absorption materials are outlined and a selection of the most remarkable examples is discussed. The derived key information regarding dimensional design, structural engineering, performance, and structure-function relationship are comprehensively summarized. Moreover, the investigation of the cutting-edge mechanisms is given particular attention. Additional applications, such as oxidation resistance and self-cleaning functions, are also introduced. Finally, insight into what may be expected from this rapidly expanding field and future challenges are presented.

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.

Porous carbon/graphite nanosheet/ferromagnetic nanoparticle composite absorbents with adjustable electromagnetic properties

With the rapid development of electronic devices and wireless communication tools, it is urgent to design and fabricate low-cost, lightweight and effective electromagnetic absorption materials to solve interference of electromagnetic waves. Herein, a new strategy toward porous carbon/graphite nanosheet/ferromagnetic nanoparticle (PC/GNS/Fe) composites was designed to investigate the influence of crystalline carbon on electromagnetic wave absorption. To begin with, graphite nanosheets (GNSs) were incorporated into the porous polyimide by in situ polymerization, and Fe were added as a magnetic particle source and an agent to regulate the pore size. A series of PC/GNS/Fe composite absorbents were obtained. The direct carbonization of porous polymer precursors was beneficial to the design of the pore structure of materials. A hierarchically porous structure derived from the phase separation process was well maintained in the polyimide pyrolysis process. The results demonstrated that the presence of crystalline carbon could influence the reflection loss value and the frequency range. Hence, the absorbing performance can be optimized by adjusting the pore structure and the content of crystalline carbon in materials, which is conducive to obtaining electromagnetic wave absorption materials with excellent comprehensive performance.

Compressible and flexible PPy@MoS2/C microwave absorption foam with strong dielectric polarization from 2D semiconductor intermediate sandwich structure

Structural engineering represents a major trend in the field of two-dimensional (2D) materials regarding microscopic interfacial electric/dielectric properties and macroscopic device strategies. 2D molybdenum disulfide (MoS2) with semiconductive features and lamellar architecture has been widely applied in the microwave absorption (MA) field. However, due to its limitations of weak dielectric loss capacity and poor intrinsic mechanical property, MoS2-based MA devices are a considerable design challenge for practical applications with the peculiarities of light weight, high absorption performance, flexibility, and compressibility. Herein, 2D MoS2 was riveted on carbonized melamine foam (CMF) templated from a commercial foam skeleton, which was cladded with the conductive polymer polypyrrole (PPy). The as-prepared PPy@MoS2/CMF was integrated to simultaneously achieve an excellent MA performance including a maximum reflection loss (RL) value of -45.40 dB and a wide absorption bandwidth of 3.8 GHz, together with mechanical practicability including a high compression ratio of over 45.6% in volume and a bending angle of over 43.2°. This excellent MA performance is attributed to the synergetic effect from its sandwiched multi-layered skeleton, consisting of a conductive/semiconductive/conductive ternary conductive network, and multiple polarizations from the 2D MoS2 interlayer. Our strategy sheds novel insight into the construction of advanced carbon-supported composites and 2D materials for use in devices, which can be further extended to energy storage and conversion applications.

Recent progress of microwave absorption microspheres by magnetic-dielectric synergy

Designing and developing high-performance microwave absorption (MA) materials for electromagnetic protection and radar detection have received widespread attention. Recently, magnetic-dielectric MA materials have become a research hotspot due to their unique complementary functions and synergy loss mechanism. Herein, we review important research progress of excellent MA systems combining strong magnetic components and dielectric substrates. The functional materials involve magnetic materials, carbon components, semiconductors, polymer and so on. For a comprehensive analysis, current development and challenges are firstly introduced in the background. Modern requirements for microwave energy conversion are elaborated in the following part. To highlight the key points, more attention has been paid to the magnetic-dielectric synergy microsphere: (i) core/yolk-shell structure, (ii) multi-component assembly and (iii) MOF-derived synergy composites. Meanwhile, classical and typical high-performance MA composites with a multi-loss mechanism are also mentioned in this review paper. Finally, the design principles, electromagnetic synergy, future mechanism exploration and device application are presented, which provides guidance for understanding MA materials.

Three-Dimensional Bi2Fe4O9 Nanocubes Loaded on Reduced Graphene Oxide for Enhanced Electromagnetic Absorbing Properties

Bi₂Fe₄O₉(BFO) nanocubes were prepared in proportion using a simple and easy hydrothermal method, and were then assembled on reduced graphene oxide (rGO) multilayered sheets. The excellent microwave absorption properties of Bi₂Fe₄O₉/rGO nanohybrids were achieved by properly adjusting the impedance matching and getting a high attenuation capability contributed from different ratios of the BFO and rGO. A minimum reflection loss value of −61.5 dB at 12.8 GHz was obtained with a Bi₂Fe₄O₉/rGO ratio of 2:1, and the broadest bandwidth below −10 dB was up to 5.0 GHz (from 10.8 to 15.8 GHz) with a thickness of 2.4 mm. Additionally, the elementary mechanism of wave absorption performance is also investigated.

Facile Synthesis of Ultralight and Porous Melamine-Formaldehyde (MF) Resin-Derived Magnetic Graphite-Like C3N4/Carbon Foam with Electromagnetic Wave Absorption Behavior

Society demands effective electromagnetic wave (EMW) absorbers that are lightweight, with a broad absorption band and strong absorption, to solve excessive electromagnetic radiation. Herein, ultralight magnetic graphite-like C3N4/carbon foam (MCMF) was fabricated via impregnating polymerized melamine formaldehyde (MF) foams in Fe3O4 nanoparticle solution, followed by in situ pyrolysis at 1000 °C. MCMF possesses porous architectures consisting of graphitic C3N4/carbon and CFe15.1. The magnetic particles (α-Fe, Fe3O4 and Fe3C) were formed and modified on the internal skeleton surface. The EMW absorption capacity of MCMF is better than the that of carbonized MF foam without Fe3O4 (CMF), possessing excellent absorption behavior, with a minimum RL value of −47.38 dB and a matching thickness as thin as 3.90 mm. The corresponding effective absorbing bandwidth is as broad as 13.32 GHz. Maxwell–Wagner–Sillars (MWS) polarization and the residual loss are proved to be beneficial for such superior absorption behavior. Besides, graphitic C3N4 enriches the interface polarization effect and the electromagnetic matching effect. The microporous structures are beneficial for increasing EMW propagation, resulting in internal multiple reflections and scatterings, which are also beneficial for EMW attenuation.

Hydrophobic SiC@C Nanowire Foam with Broad-Band and Mechanically Controlled Electromagnetic Wave Absorption

With the booming of modern information technology, electromagnetic wave (EMW) absorption materials are playing more and more crucial roles in applications ranging from wearable smart electronics to national defense security. However, the application of present EMW absorption materials is severely hindered by their drawbacks, such as narrow absorption bandwidth and low absorption intensity. In this work, a series of highly porous and well-interconnected SiC@C nanowire foams (SCNFs) are rationally designed to exhibit modified impedance match and multiscale EMW energy dissipation mechanisms. The SCNF with a density of 108 mg cm^-3 realizes a broad absorption bandwidth covering the whole X and Ku bands with an intensity of -52.5 dB. The SCNF with a density of 36 mg cm^-3 and a thickness of 9.6 mm exhibits a mechanically controlled absorption band ranging from 2.9 to 18 GHz (covering over 93% of the entire radar band, 2-18 GHz) with a minimum intensity of -46 dB by simply applying a reversible compressive strain from 0 to 66.7%. Moreover, the special microstructure of SCNF also endows it with excellent hydrophobicity, which enables its good self-cleaning property. These encouraging achievements pave the way to the development of the continuous network microstructure of absorbents with a broad-band and tunable EMW absorption property.

A multi-dimensional and level-by-level assembly strategy for constructing flexible and sandwich-type nanoheterostructures for high-performance electromagnetic interference shielding

To shield against massive electromagnetic pollution and meet increasing demand in portable electronics, the development of flexible, lightweight and high-performance electromagnetic interference (EMI) shielding materials with good environmental friendliness is an urgent but still challenging need. Herein, a creative multi-dimensional and level-by-level assembly strategy is proposed to construct free-standing and sandwich-type nanoheterostructures consisting of flexible cotton-derived carbon fibers (CFs), magnetic and conductive nickel nanoparticles (Ni NPs) and highly conductive and large-surface-area dandelion-like graphene (DLG), via a high-precision combination technology of magnetron sputtering-plasma enhanced chemical vapor deposition. The multiple spatial-scale DLG/Ni NPs/CF composites achieve a remarkable conductivity of 625 S m^-1 and an outstanding EMI shielding effectiveness of ∼50.6 dB in the X-band (8.2-12.4 GHz) which can be classified as attenuation levels of "AAAA" for professional use. The dielectric loss from multiple polarizations is principally responsible for the electromagnetic loss of the composites. Besides, the large surface area of heterogeneous interfaces and defects in DLG contribute to enhancing the amount of polarization. In addition, the ultrathin and ultralight composites (d = 0.65 mm, ρ = 113 mg cm^-3) can be bent, twisted and folded, revealing their excellent processability for commercial uses. More importanly, this novel structural design concept opens up an interesting promising research field of novel next-generation EMI shielding materials.

Enhanced and selective photocatalytic reduction of CO2 by H2O over strategically doped Fe and Cr into porous boron carbon nitride

Strategic doping of metals and non-metals into a photocatalyst can help in tuning the band gap and alignment of band structure.

Controlling the electric permittivity of honeycomb-like core–shell Ni/CuSiO3 composite nanospheres to enhance microwave absorption properties

Controlling impedance matching and enhancing absorption properties are crucial for developing next-generation microwave absorbing materials.

Nonsiliceous Mesoporous Materials: Design and Applications in Energy Conversion and Storage

In this work, the progress in the design of nonsiliceous mesoporous materials (nonSiMPMs) over the last five years from the perspectives of the chemical composition, morphology, loading, and surface modification is summarized. Carbon, metal, and metal oxide are in focus, which are the most promising compositions. Then, representative applications of nonSiMPMs are demonstrated in energy conversion and storage, including recent technical advances in dye-sensitized solar cells, perovskite solar cells, photocatalysts, electrocatalysts, fuel cells, storage batteries, supercapacitors, and hydrogen storage systems. Finally, the requirements and challenges of the design and application of nonSiMPMs are outlined.

One-pot synthesis of propargylamines using magnetic mesoporous polymelamine formaldehyde/zinc oxide nanocomposite as highly efficient, eco-friendly and durable nanocatalyst: optimization by DOE approach

Magnetic mesoporous polymelamine formaldehyde nanocomposite-incorporating ZnO nanoparticles were successfully synthesized using solvothermal and sol-gel methods. Fourier-transform infrared spectrometry (FT-IR), X-ray diffraction, Brunauer-Emmett-Teller, vibrating sample magnetometer, thermogravimetric analysis, elemental analysis, transmission electron microscopy and field emission scanning electron microscopy techniques were then utilized for evaluation of nanocomposites. The as-prepared nanocomposite can be used as heterogeneous nanocatalyst with remarkable performance for A³ coupling reaction toward one-pot synthesis of propargylamine and its derivatives under solvent-less condition. In order to maximize the product yield, the variables, i.e., reaction time, temperature and catalyst amount, were optimized by using a statistical approach. The synthesized nanocomposite can be easily separated from the reaction medium and reused over and over, without significant changes in its catalytic activity.

Core-Shell CoNi@Graphitic Carbon Decorated on B,N-Codoped Hollow Carbon Polyhedrons toward Lightweight and High-Efficiency Microwave Attenuation

Lightweight and high-efficiency microwave attenuation are two major challenges in the exploration of carbon-based absorbers, which can be achieved simultaneously by manipulating their chemical composition, microstructure, or impedance matching. In this work, core-shell CoNi@graphitic carbon decorated on B,N-codoped hollow carbon polyhedrons has been constructed by a facile pyrolysis process using metal-organic frameworks as precursors. The B,N-codoped hollow carbon polyhedrons, originated from the calcination of Co-Ni-ZIF-67, are composed of carbon nanocages and BN domains, and CoNi alloy is encapsulated by graphitic carbon layers. With a filling loading of 30 wt %, the absorber exhibits a maximum R_L of -62.8 dB at 7.2 GHz with 3 mm and the effective absorption bandwidth below -10 dB remarkably reaches as strong as 8 GHz when the thickness is only 2 mm. The outstanding microwave absorption performance stems from the hollow carbon polyhedrons and carbon nanocages with interior cavities, the synergistic coupling effect between the abundant B-C-N heteroatoms, the strong dipolar/interfacial polarizations, the multiple scatterings, and the improved impedance matching. This study demonstrates that the codoped strategy provides a new way for the rational design of carbon-based absorbers with lightweight and superior microwave attenuation.

Fabrication of Three-Dimensional Flower-like Heterogeneous Fe3O4/Fe Particles with Tunable Chemical Composition and Microwave Absorption Performance

Heterogeneous Fe₃O₄ and Fe composites are highly desirable for microwave absorption application because of their complementary electromagnetic (EM) properties. With three-dimensional (3D) Fe₂O₃ as a sacrificing template, we realize the construction of Fe₃O₄/Fe composites with tunable chemical composition, and more importantly, these composites inherit the unique 3D microstructure from their precursor. The change in chemical composition produces significant impacts on the EM functions of these composites. On the one hand, dielectric loss can be improved greatly through positive interfacial polarization and reach the peak when the mass contents of Fe₃O₄ and Fe are 72.1 and 27.9 wt %, respectively. On the other hand, high Fe content slightly pulls down magnetic loss in the low-frequency range but favors strong magnetic loss in the high-frequency range because of the breakthrough of Snoek's limitation. The attenuation constant reveals that dielectric loss dominates overall consumption of incident EM waves. As a result, the optimized composite, F-350 (the reduction of Fe₂O₃ is conducted at 350 °C), shows the best microwave absorption performance, whose strongest reflection loss is -56.0 dB at 17.5 GHz and the effective bandwidth can cover the frequency range of 12.0-15.5 GHz with the thickness of 1.5 mm. Furthermore, an ultrawide effective bandwidth of 15.3 GHz can be achieved with the integrated thickness of 1.0-5.0 mm. Such a performance is superior to those of many reported Fe₃O₄/Fe composites, and a comparative analysis manifests that good microwave absorption of F-350 is also benefited from its unique 3D architecture.

Review of electromagnetic interference shielding materials fabricated by iron ingredients

Iron (Fe) and its counterparts, such as Fe2O3, Fe3O4, carbonyl iron and FeO, have attracted the attention of researchers during the past few years due to their bio-compatibility, bio-degradability and diverse applications in the field of medicines, electronics and energy; including water treatment, catalysis and electromagnetic wave interference shielding etc. In this review paper, we aimed to explore iron based materials for the prevention of electromagnetic interference (EMI) by means of both reflection and absorption processes, including the standard methods of synthesis of Fe-based materials along with the determination of EMI performance. It is customary that a proper combination of two dielectric-losses, i.e. electrical and magnetic losses, give excellent microwave absorption properties. Therefore, we focused on the different strategies of preparation of these iron based composites with dielectric carbon materials, polymers etc. Additionally, we explained their positive and negative aspects.

Mxenes Derived Laminated and Magnetic Composites with Excellent Microwave Absorbing Performance

Two dimensional materials have been widely identified as promising microwave absorbers, owing to their large surface area and abundant interfaces. Here, a novel laminated and magnetic composite derived from Mxene was designed and successfully synthesized via facile hydrothermal oxidization of nickel ion intercalated Ti3C2. Highly disordered carbon sheets were obtained by low temperature hydrothermal oxidization, and the in-situ produced TiO2 and NiO nanoparticles embedded closely between them. This layered hybrid exhibits excellent microwave absorbing performance with an effective absorbing bandwidth as high as 11.1 GHz (6.9-18 GHz) and 9 GHz (9-18 GHz) when the thickness is 3 and 2 mm, respectively. Besides the high dielectric loss, magnetic loss and ohmic loss of the composite, the amorphous nature of obtained carbon sheets and multi-reflections between them are believed to play a decisive role in achieving such superior microwave absorbing performance.

Zinc oxide/nanoporous carbon hybrid materials derived from metal–organic frameworks with different dielectric and absorption performances

An outstanding microwave attenuation ability of ZnO/nanoporous carbon composite was obtained with conductivity and a nanoporous structure.

Functionalized Carbon Nanofibers Enabling Stable and Flexible Absorbers with Effective Microwave Response at Low Thickness

Lots of work has been done to develop microwave absorbing materials (MAM) utilized as flexible electronic devices and communication instruments. Conventionally developed powder MAM are often limited in practical applications because of the bad stability and poor durability, which is out of the scope for exploiting flexible and long-term microwave absorbers. To overcome such limitations, a facile and binder-free technique from a Co-based zeolitic imidazolate framework (ZIF-67, a member of metal-organic frameworks)-coated carbon fiber precursor is developed for the in situ horizontal growth of Co₃O₄ nanoparticles, which embedded nitrogen-doped carbon array (triangular nanoplates) on the surface of carbon fibers in the carbon paper (NC-Co₃O₄/CP) as low-thickness MAM. The maximum reflection loss (RL) values reaches -16.12 and -34.34 dB when the thickness is 1.1 and 1.5 mm, respectively. As the thickness increases, the absorbing performance at low frequency performs well (RL < -20 dB). The hierarchical architecture is facilely originated from a metal-organic framework precursor. In view of the simple preparation technique, NC-Co₃O₄/CP exhibit huge potential in large-scale production of portable microwave absorbing electronic devices with strong microwave response at low thickness.

Enhanced electromagnetic wave absorption induced by void spaces in hollow nanoparticles

We developed a facile method for the growth of hollow structured NiCo₂O₄ nanoparticles on a graphene sheet (NiCo₂O₄-h/G). The hollow NiCo₂O₄ nanoparticles have an average diameter of approximately 10.0 nm and a shell thickness of merely 2.5 nm. The NiCo₂O₄-h/G hybrid exhibited excellent electromagnetic wave absorption with minimal reflection loss below -20 dB at absorber thickness ranging from 2.0 to 5.0 mm, outperforming the solid NiCo₂O₄ nanoparticles on the graphene sheet. Remarkably, even for a thickness as small as 1.5 mm, the efficient absorption bandwidth and the minimal reflection loss of the hybrid can reach 2.6 GHz and -20.3 dB, respectively. Experimental results and theoretical calculations indicate that the void space in the hollow NiCo₂O₄ nanoparticles plays a crucial role in the excellent electromagnetic wave absorption property, which greatly increases the dielectric loss and impedance matching characteristics. Our results demonstrate that growing the hollow nanoparticles on a graphene sheet is an efficient way to produce high-performance electromagnetic wave absorbers.

Highly Cuboid-Shaped Heterobimetallic Metal-Organic Frameworks Derived from Porous Co/ZnO/C Microrods with Improved Electromagnetic Wave Absorption Capabilities

Metal-organic framework (MOF)-derived porous metal/C composites have drawn considerable attention from the microwave absorption field owing to their large pore volumes and surface areas. Exploring single-MOF-derived materials with high intensity and broadband absorption is largely needed but remains a challenge. Here, porous Co/ZnO/C (CZC) microrods were fabricated easily from cuboid-shaped heterobimetallic MOFs. CZC provides an efficient platform for integrating different semiconductors (ZnO), magnetic metal (Co), and carbon sources into one particle, which enhances the electromagnetic (EM) wave-absorbing ability. The carbonization temperature which is critical for EM parameters was studied in detail. CZC annealed at 700 °C outperformed those obtained at 600 or 800 °C in terms of microwave wave-absorbing properties. The reflection loss (RL) was optimized to -52.6 (or -20.6) dB at 12.1 (or 14.8) GHz with an effective bandwidth (RL ≤ -10 dB) of 4.9 (or 5.8) GHz at the coating thickness of 3.0 (or 2.5) mm. Such enhancement of EM wave-absorbing capabilities is ascribed to the well-built porous structure, dielectric loss, and magnetic loss. This work offers a new way to prepare porous magnetic metal/C composites with excellent microwave-absorbing properties starting from heterobimetallic MOFs.

"Matryoshka Doll"-Like CeO2 Microspheres with Hierarchical Structure To Achieve Significantly Enhanced Microwave Absorption Performance

Recently, it is still a great challenge to develop a new type of absorber that possesses special advantages of low cost, ultrawide bandwidth, and strong absorption intensity. Herein, the unique "Matryoshka doll"-like CeO₂ microspheres with tunable interspaces were successfully synthesized by a facile and template-free method. The as-synthesized hierarchical yolk-shell CeO₂ microspheres were constructed by a layer of outer shell and multiple inner cores. The interspace gap of the microspheres can be simply adjusted only by altering the solvothermal reaction time. Simultaneously, Ostwald ripening, Kirkendall effect, and self-etching process contribute a synergetic growth mechanism responsible for this amazing hierarchical architecture. Importantly, the "Matryoshka doll"-like CeO₂ microspheres exhibited significantly strong microwave absorption in the frequency range of 2-18 GHz, with a reflection loss of -71.3 dB at 14.5 GHz and an effective absorption bandwidth of 5.4 GHz (<-10 dB), which is superior to the multicomponent absorbers. Such an outstanding microwave absorption performance stems from the unique hierarchical yolk-shell structure and the designable interspaces, leading to the multiple scattering, interfacial polarization, and plasma dielectric oscillation from the abundant interfaces and curved surfaces, which can be illustrated by the related results from electron holography and electron energy loss spectroscopy. To the best of our knowledge, the "Matryoshka doll"-like CeO₂ microspheres with a facile synthesis process, low cost, and excellent microwave absorption performance are believed to be an optimal candidate of single-component absorbers and helpful in the study of absorption mechanism.

An ultra-small NiFe2O4 hollow particle/graphene hybrid: fabrication and electromagnetic wave absorption property

Herein, ultra-small NiFe₂O₄ hollow particles, with the diameter and wall thickness of only 6 and 1.8 nm, respectively, were anchored on a graphene surface based on the nanoscale Kirkendall effect. The hybrid exhibits an excellent electromagnetic wave absorption property, comparable or superior to that of most reported absorbers. Our strategy may open a way to grow ultra-small hollow particles on graphene for applications in many fields such as eletromagnetic wave absorption and energy storage and conversion.

Nickel Nanoparticle Encapsulated in Few-Layer Nitrogen-Doped Graphene Supported by Nitrogen-Doped Graphite Sheets as a High-Performance Electromagnetic Wave Absorbing Material

Herein we develop a facile strategy for fabricating nickel particle encapsulated in few-layer nitrogen-doped graphene supported by graphite carbon sheets as a high-performance electromagnetic wave (EMW) absorbing material. The obtained material exhibits sheetlike morphology with a lateral length ranging from a hundred nanometers to 2 μm and a thickness of about 23 nm. Nickel nanoparticles with a diameter of approximately 20 nm were encapsulated in about six layers of nitrogen-doped graphene. As applied for electromagnetic absorbing material, the heteronanostructures exhibit excellent electromagnetic wave absorption property, comparable to most EMW absorbing materials previously reported. Typically, the effective absorption bandwidth (the frequency region falls within the reflection loss below -10 dB) is up to 8.5 GHz at the thicknesses of 3.0 mm for the heteronanostructures with the optimized Ni content. Furthermore, two processes, carbonization at a high temperature and subsequent treatment in hot acid solution, were involved in the preparation of the heteronanostructures, and thus, mass production was achieved easily, facilitating their practical applications.

Ti3C2MXene: a promising microwave absorbing material

A novel microwave absorbing material with Ti₃C₂MXene nanosheets.

Facile synthesis of porous Fe3O4@C core/shell nanorod/graphene for improving microwave absorption properties

Porous Fe₃O₄@C core/shell nanorods decorated with reduced graphene oxide were synthesized by a facile one-pot method, and exhibit high microwave absorption performance: maximum reflection loss reaches −48.6 dB.

Microporous Co@C Nanoparticles Prepared by Dealloying CoAl@C Precursors: Achieving Strong Wideband Microwave Absorption via Controlling Carbon Shell Thickness

Excellent magnetic features make Co-based materials promising candidates as high-performance microwave absorbers. However, it is still a significant challenge for Co-based absorbers to possess high-intensity and broadband absorption simultaneously, owing to the lack of dielectric loss and impedance matching. Herein, microporous Co@C nanoparticles (NPs) with carbon shell thicknesses ranging from 1.8-4.9 nm have been successfully synthesized by dealloying CoAl@C precursors. All of the samples exhibit high microwave absorption performance. The microporous Co@C sample possessing a carbon shell of 1.8 nm exhibits the highest absorption intensity among these samples with a minimum reflection loss (RL) of -141.1 dB, whose absorption bandwidth for RL ≤ -10 dB is 7.3 GHz. As the thickness of the carbon shell increases, the absorption bandwidth of the NPs becomes wider. For the sample with the carbon shell thickness of 4.9 nm, the absorption bandwidth for RL ≤ -10 dB reaches a record high of 13.2 GHz. The outstanding microwave attenuation properties are attributed to the dielectric loss of the carbon shell, the magnetic loss of the Co core, and the cooperation of the core-shell structure and microporous morphology. The strong wideband microwave absorption of the carbon-coated microporous Co NPs highlights their potential applications in microwave absorbing systems.

In Situ Stringing of Metal Organic Frameworks by SiC Nanowires for High-Performance Electromagnetic Radiation Elimination

The design of novel hybrid nanostructures has been seen as an effective route to tune the properties of materials. Herein, we provide an in situ growth strategy to efficiently construct kebab-like hybrids, which are composed of one-dimensional SiC nanowires stringing polyhedral metal organic frameworks (MOFs). Through a heat-treatment process regardless of under air or argon, these hybrids generate an excellent electromagnetic absorption (EMA) ability. We comprehensively explored the growth and calcination process of these hybrids as well as their EMA enhanced mechanism. The results indicate that the MOFs kept as shrunken polyhedrons under air but decomposed to small particles under argon, due to the different calcination mechanism. In addition, the enhanced EMA ability should be attributed to the combined influences of the reduced dielectric constant, enlarged aspect ratio, and enhanced interface polarization. This research opens up the rational designs and applications of novel materials by the hybridizing of nanomaterials in multidimensions.

A facile self-template strategy for synthesizing 1D porous Ni@C nanorods towards efficient microwave absorption

Ni@C composites, which simultaneously possess porous, core-shell and 1D nanostructures have been synthesized with a facile self-template strategy. The precursors were obtained by a hydrothermal process using NiCl₂ · 6H₂O and nitrilotriacetic acid as the starting material and then annealed at 400 °C, 500 °C, and 600 °C. The Ni@C composites annealed at 500 °C display a nanorod feature with a length of ∼3 μm and diameter of 230-500 nm. In addition, about 3 nm carbon shells and 4 nm Ni cores can be found in Ni@C nanorods. Attributed to the interconnected mesoporous texture in nanorods, strengthened interfacial polarization from core-shell structure, and better impedance matching benefiting from a great deal of pores, Ni@C nanorod composites exhibit perfect microwave absorption performance. The minimum reflection loss (RL) value of -26.3 dB can be gained at 10.8 GHz with a thickness of 2.3 mm. Moreover, the effective bandwidth (RL ≤ -10 dB) can be achieved, 5.2 GHz (12.24-17.4 GHz) under an absorber thickness of 1.8 mm, indicating its great potential in the microwave absorption field. Considering this technique is facile and effective, our study may provide a good reference for the synthesis of 1D carbon-based microwave absorbers with core-shell nanostructure.

Enhanced Microwave Absorption Properties by Tuning Cation Deficiency of Perovskite Oxides of Two-Dimensional LaFeO3/C Composite in X-Band

Development of microwave absorption materials with tunable thickness and bandwidth is particularly urgent for practical applications but remains a great challenge. Here, two-dimensional nanocomposites consisting of perovskite oxides (LaFeO₃) and amorphous carbon were successfully obtained through a one pot with heating treatment using sodium chloride as a hard template. The tunable absorption properties were realized by introducing A-site cation deficiency in LaFeO₃ perovskite. Among the A-site cation-deficient perovskites, La_0.62FeO₃/C (L_0.62FOC) has the best microwave absorption properties in which the maximum absorption is -26.6 dB at 9.8 GHz with a thickness of 2.94 mm and the bandwidth range almost covers all X-band. The main reason affecting the microwave absorption performance was derived from the A-site cation deficiency which induced more dipoles polarization loss. This work proposes a promising method to tune the microwave absorption performance via introducing deficiency in a crystal lattice.

Carbon Hollow Microspheres with a Designable Mesoporous Shell for High-Performance Electromagnetic Wave Absorption

In this work, mesoporous carbon hollow microspheres (PCHMs) with designable mesoporous shell and interior void are constructed by a facile in situ stöber templating approach and a pyrolysis-etching process. The PCHMs are characterized by scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectra, Raman spectroscopy, and nitrogen adsorption and desorption system. A uniform mesoporous shell (pore size 4.7 nm) with a thickness of 55 nm and a cavity size of 345 nm is realized. The composite of paraffin mixed with 20 wt % PCHMs exhibits a minimum reflection coefficient (RC_min) of -84 dB at 8.2 GHz with a sample thickness of 3.9 mm and an effective absorption bandwidth (EAB) of 4.8 GHz below -10 dB (>90% electromagnetic wave is attenuated). Moreover, the composite of phenolic resin mixed with 20 wt % PCHMs exhibits an ultrawide EAB of 8 GHz below -10 dB with a thinner thickness of 2.15 mm. Such excellent electromagnetic wave absorption properties are ascribed to the large carbon-air interface in the mesoporous shell and interior void, which is favorable for the matching of characteristic impedance as compared with carbon hollow microspheres and carbon solid microspheres. Considering the excellent performance of PCHMs, we believe the as-fabricated PCHMs can be promising candidates as highly effective microwave absorbers, and the design philosophy can be extended to other spherical absorbers.

Microwave absorbing property optimization of starlike ZnO/reduced graphene oxide doped by ZnO nanocrystal composites

The microwave absorption properties of a nanocomposite containing star-like ZnO and reduced graphene oxide doped by ZnO nanocrystals are optimized.

High performance asymmetric supercapacitors with ultrahigh energy density based on hierarchical carbon nanotubes@NiO core–shell nanosheets and defect-introduced graphene sheets with hole structure

In this work, we report a fast and cost-effective cobalt catalyzed gasification strategy to obtain defect-introduced graphene sheets (DGNs) with hole structure.

Preparation and electromagnetic wave absorbing properties of 3D graphene/pine needle-like iron nano-acicular whisker composites

The improvement of high reflection loss and broad frequency bandwidth for electromagnetic wave absorption materials is a long-term effort.

Unusual continuous dual absorption peaks in Ca-doped BiFeO3 nanostructures for broadened microwave absorption

Electromagnetic absorption materials have received increasing attention owing to their wide applications in aerospace, communication and the electronics industry, and multiferroic materials with both polarization and magnetic properties are considered promising ceramics for microwave absorption application. However, the insufficient absorption intensity coupled with the narrow effective absorption bandwidth has limited the development of high-performance multiferroic materials for practical microwave absorption. To address such issues, in the present work, we utilize interfacial engineering in BiFeO3 nanoparticles via Ca doping, with the purpose of tailoring the phase boundary. Upon Ca-substitution, the co-existence of both R3c and P4mm phases has been confirmed to massively enhance both dielectric and magnetic properties via manipulating the phase boundary and the destruction of the spiral spin structure. Unlike the commonly reported magnetic/dielectric hybrid microwave absorption composites, Bi0.95Ca0.05FeO3 has been found to deliver unusual continuous dual absorption peaks at a small thickness (1.56 mm), which has remarkably broadened the effective absorption bandwidth (8.7-12.1 GHz). The fundamental mechanisms based on the phase boundary engineering have been discussed, suggesting a novel platform for designing advanced multiferroic materials with wide applications.

Enhanced high-frequency absorption of anisotropic Fe3O4/graphene nanocomposites

Anisotropic Fe₃O₄ nanoparticle and a series of its graphene composites have been successfully prepared as high-frequency absorbers. The crystal structure, morphology and magnetic property of the samples were detailed characterized through X-ray diffractometer (XRD), transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM). The high-frequency absorbing performance of the composites is evaluated within 2.0–18.0 GHz. Combining reduced graphene oxide (RGO) to Fe₃O₄ helps to adjust the permittivity and permeability of the composite, balance the dielectric loss and magnetic loss, consequently improve the absorbing performance in view of the impedance matching characteristic. The optimal reflection loss of the pure Fe₃O₄ sample reaches −38.1 dB with a thickness of 1.7 mm, and it increases to −65.1 dB for the sample grafted with 3 wt.% RGO. The addition of proper content of RGO both improves the reflection loss and expands the absorbing bandwidth. This work not only opens a new method and an idea for tuning the electromagnetic properties and enhancing the capacity of high-efficient absorbers, but also broadens the application of such kinds of lightweight absorbing materials frameworks.

A wormhole-like porous carbon/magnetic particles composite as an efficient broadband electromagnetic wave absorber

A method combining liquid-liquid phase separation and the pyrolysis process has been developed to fabricate the wormhole-like porous carbon/magnetic nanoparticles composite with a pore size of about 80 nm (WPC/MNPs-80). In this work, the porous structure was designed to enhance interaction between the electromagnetic (EM) wave and the absorber, while the magnetic nanoparticles were used to bring about magnetic loss ability. The structure, morphology, porosity and magnetic properties of WPC/MNPs-80 were investigated in detail. To evaluate its EM wave attenuation performance, the EM parameters of the absorber and wax composite were measured at 2-18 GHz. WPC/MNPs-80 has an excellent EM wave absorbency with a wide absorption band at a relatively low loading and thin absorber thickness. At the absorber thickness of 1.5 and 2.0 mm, minimum RL values of -29.2 and -47.9 dB were achieved with the RL below -10 dB in 12.8-18 and 9.2-13.3 GHz, respectively. The Co and Fe nanoparticles derived from the chemical reduction of Co0.2Fe2.8O4 can enhance the graphitization process of carbon and thus improve dielectric loss ability. Polarizations in the nanocomposite absorber also play an important role in EM wave absorption. Thus, EM waves can be effectively attenuated by dielectric loss and magnetic loss through multiple reflections and absorption in the porous structure. WPC/MNPs-80 could be an excellent absorber for EM wave attenuation; and the design strategy could be extended as a general method to synthesize other high-performance absorbers.