Facile synthesis of 3D Ni@C nanocomposites derived from two kinds of petal-like Ni-based MOFs towards lightweight and efficient microwave absorbers

Tunable Impedance of Cobalt Loaded Carbon for Wide-Range Electromagnetic Wave Absorption

Impedance matching modulation of the electromagnetic wave (EMW) absorbers toward broad effective absorption bandwidth (EAB) is the ultimate aim in EMW attenuation applications. Here, a Joule heating strategy is reported for preparation of the Co-loaded carbon (Co/C) absorber with tunable impedance characteristics. Typically, the size of the Co can be regulated to range from single-atoms to clusters, and to nanocrystals. The varied sizes of the Co combined with different graphitization degrees of carbon can result in different relative input impedances and electromagnetic loss, leading to the tunable EMW absorption properties of the Co/C absorber. By meticulously coalescing the different prepared Co/C, the working frequency can be easily tuned, covering Ku, X, and C bands. Furthermore, the Co/C demonstrates a high EMW attenuation due to its unique dielectric loss capability and magnetic loss characteristics. The abundant interfaces of Co/C can also contribute to the enhanced interfacial polarization for improving EMW attenuation. This work demonstrates the importance of optimizing the metal and carbon interaction to the impedance matching toward wide EAB of the EMW absorbers.

Effect of Reduced Graphene Oxide on Microwave Absorbing Properties of Al1.5Co4Fe2Cr High-Entropy Alloys

The microwave absorption performance of high-entropy alloys (HEAs) can be improved by reducing the reflection coefficient of electromagnetic waves and broadening the absorption frequency band. The present work prepared flaky irregular-shaped Al1.5Co4Fe2Cr and Al1.5Co4Fe2Cr@rGO alloy powders by mechanical alloying (MA) at different rotational speeds. It was found that the addition of trace amounts of reduced graphene oxide (rGO) had a favorable effect on the impedance matching, reflection loss (RL), and effective absorbing bandwidth (EAB) of the Al1.5Co4Fe2Cr@rGO HEA composite powders. The EAB of the alloy powders prepared at 300 rpm increased from 2.58 GHz to 4.62 GHz with the additive, and the RL increased by 2.56 dB. The results showed that the presence of rGO modified the complex dielectric constant of HEA powders, thereby enhancing their dielectric loss capability. Additionally, the presence of lamellar rGO intensified the interfacial reflections within the absorber, facilitating the dissipation of electromagnetic waves. The effect of the ball milling speed on the defect concentration of the alloy powders also affected its wave absorption performance. The samples prepared at 350 rpm had the best wave absorption performance, with an RL of -16.23 and -17.28 dB for a thickness of 1.6 mm and EAB of 5.77 GHz and 5.43 GHz, respectively.

Three-dimensional biotemplate-loaded nickel sulfide vacancies engineered to promote the absorption of electromagnetic waves

Vacancy engineering offers an appealing strategy for modifying the electronic structure of transition metals. Transition metals with abundant sulfur vacancies can significantly contribute to the microwave absorption capabilities of absorbers. In this study, an NixSy@De composite material was synthesized through a straightforward hydrothermal synthesis technique. The effective absorption bandwidth (EAB) of this composite material reached 9.86 GHz at 1.44 mm. A minimum reflection loss (RLmin) of -33.61 dB at 1 mm was achieved, and after mild etching, the RLmin further improved to -93.53 dB at 1.16 mm to achieve a high-attenuation microwave absorption. The exceptional performance of NixSy@De for the absorption of electromagnetic waves (EMWs) is based on its high dielectric loss, substantial magnetic loss, and excellent impedance matching. This work combines transition metal sulfides with three-dimensional biotemplated diatomite, providing valuable insights into the design of advanced EMW absorbing materials.

Controlled Synthesis of MOF-Derived Nano-Microstructure toward Lightweight and Wideband Microwave Absorption

Correlating metal-organic framework (MOF) synthesis processes and microwave absorption (MA) enhancement mechanisms is a pioneer project. Nevertheless, the correlation process still relies mainly on empirical doctrine, which hardly corresponds to the specific mechanism of the effect on the dielectric properties. Hereby, after the strategy of modulation of protonation engineering and solvothermal temperature in the synthesis route, the obtained sheet-like self-assembled nanoflowers were constructed. Porous structures with multiple heterointerfaces, abundant defects, and vacancies are obtained by controlled design of the synthesis procedure. The rearrangement of charges and enhanced polarization can be promoted. The designed electromagnetic properties and special nano-microstructures of functional materials have significant impact on their electromagnetic wave energy conversion effects. As a consequence, the MA performance of the samples has been enhanced toward broadband absorption (6.07 GHz), low thickness (2.0 mm), low filling (20%), and efficient loss (-25 dB), as well as being suitable for practical environmental applications. This work establishes the connection between the MOF-derived materials synthesis process and the MA enhancement mechanism, which provides insight into various microscopic microwave loss mechanisms.

Microstructure optimization strategy of ZnIn2S4/rGO composites toward enhanced and tunable electromagnetic wave absorption properties

Although microstructure optimization is an effective strategy to improve and regulate electromagnetic wave (EMW) absorption properties, preparing microwave absorbents with enhanced EMW absorbing performance and tuned absorption band by a simple method remains challenging. Herein, ZnIn2S4/reduced graphene oxide (rGO) composites with flower-like and cloud-like morphologies were fabricated by a convenient hydrothermal method. The ZnIn2S4/rGO composites with different morphologies realize efficient EMW absorption and tunable absorption bands, covering a wide frequency range. The flower-like structure has an optimal reflection loss (RL) of up to -49.2 dB with a maximum effective absorption bandwidth (EAB) of 5.7 GHz, and its main absorption peaks are concentrated in the C and Ku bands. The minimal RL of the cloud-like structure can reach -36.3 dB, and the absorption peak shifts to the junction of X and Ku bands. The distinguished EMW absorption capacity originates from the uniquely optimized microstructure, complementary effect of ZnIn2S4 and rGO in dielectric constant, and synergy of various loss mechanisms, such as interfacial polarization, dipole polarization, conductive loss, and multiple reflections. This study proposes a guide for the structural optimization of an ideal EMW absorber to achieve efficient and tunable EMW absorption performance.

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.

A review on composite strategy of MOF derivatives for improving electromagnetic wave absorption

To address the electromagnetic wave (EMW) pollution issues caused by the development of electronics and wireless communication technology, it is urgent to develop efficient EMW-absorbing materials. With controllable composition, diverse structure, high porosity, and large specific surface area, metal-organic framework (MOF) derivatives have sparked the infinite passion and creativity of researchers in the electromagnetic field. Against the challenges of poor inherent impedance matching and insufficient attenuation capability of pure MOF derivative, designing and developing MOF derivative-based composites by compounding MOF with other materials, such as graphene, CNTs, MXene, and so on, has been an effective strategy for constructing high-efficiency EMW absorbing materials. This review systematically expounds the research progress of MOF derivative-based composite strategies, and discusses the challenges and opportunities faced by MOF derivatives in the field of EMW absorption. This work can provide some good ideas for researchers to design and prepare high-efficiency MOF-based EMW absorbing materials in applications of next-generation electronics and aerospace.

Controlled formation of multiple core-shell structures in metal-organic frame materials for efficient microwave absorption

The design of metal-organic frameworks (MOF) derived composites with multiple loss mechanisms and multi-scale micro/nano structures is an important research direction of microwave absorbing materials. Herein, multi-scale bayberry-like Ni-MOF@N-doped carbon composites (Ni-MOF@NC) are obtained by a MOF assisted strategy. By utilizing the special structure of MOF and regulating its composition, the effective improvement of Ni-MOF@NC's microwave absorption performance has been achieved. The nanostructure on the surface of core-shell Ni-MOF@NC can be regulated and N doping on carbon skeleton by adjusting the annealing temperature. The optimal reflection loss of Ni-MOF@NC is -69.6 dB at 3 mm, and the widest effective absorption bandwidth is 6.8 GHz. This excellent performance can be attributed to the strong interface polarization caused by multiple core-shell structures, the defect and dipole polarization caused by N doping, and the magnetic loss caused by Ni. Meanwhile, the coupling of magnetic and dielectric properties enhances the impedance matching of Ni-MOF@NC. The work proposes a particular idea of designing and synthesizing an applicable microwave absorption material that possesses excellent microwave absorption performance and promising application potential.

Heterointerface construction for permalloy microparticles through the surface modification of bilayer metallic organic frameworks: Toward microwave absorption enhancement

Reasonable heterointerface modification can effectively regulate and enhance the microwave absorption of electromagnetic materials. The surface of magnetic permalloy (PM) microparticles is modified herein by coating double-layer metal organic frameworks (MOF), which are composed of a 2-methylimidazole cobalt salt (ZIF-67) layer and a 2-methylimidazole zinc salt (ZIF-8) layer. A stable heterointerface structure with cobalt/carbon (Co/C) and zinc/carbon (Zn/C) layers is formed on the surface of PM microparticles after pyrolysis. These particles include two types of composite particles of PM solely encapsulated by ZIF-67 or ZIF-8, PM@ZIF67 and PM@ZIF8, respectively, and two types of composite PM particles with a double-layered MOF outer shell structure obtained by exchanging the coating sequence (PM@ZIF8@ZIF67 and PM@ZIF67@ZIF8). Furthermore, the thermal decomposition temperature has a significant impact on the surface morphology and magnetic properties of the composite particles. After pyrolyzing at 500 °C, the PM@ZIF67@ZIF8 samples exhibit the highest microwave absorption performance among these samples. Specifically, the minimum reflection loss and effective absorption bandwidth of PM@ZIF67@ZIF8 after pyrolyzing at 500 °C can reach -47.3 dB at a matching thickness of 3.8 mm and 5.3 GHz at a matching thickness of 2.5 mm, respectively. A heterointerface with an electrical field orientation is created in the PM@ZIF67@ZIF8 particles, which effectively enhances the interface polarization and dipole polarization. Furthermore, the formation of a three-dimensional carbon network after pyrolysis is also useful for optimizing impedance matching and enhancing magneto-electric synergism.

Fabrication of CuS/Fe3O4@polypyrrole flower-like composites for excellent electromagnetic wave absorption

Recently, electromagnetic radiation is a serious threat to equipment accuracy, military safety and human health. The combination with different materials to fabricate absorber composites with well-designed morphology is expected to ameliorate this issue. In here, CuS/Fe₃O₄@polypyrrole (CuS/Fe₃O₄@PPy) flower-like composites are constructed by the combination of hydrothermal method, solvothermal method and in-situ polymerization. CuS with flower-like structure consisting of nanosheets can provide a conductive backbone and large specific surface area. Hollow Fe₃O₄ microspheres play a key role in deciding magnetic loss, and electromagnetic waves can penetrate their hollow structure, result in multiple reflection and refraction. PPy coating can enhance the combined strength of composite, and effectively consume microwaves by scattering and multiple refraction in the intercalated structure. As expected, the minimum reflection loss (RL_min) of CuS/Fe₃O₄@PPy composites is -74.12 dB at 8.16 GHz with a thickness of 2.96 mm, and the effective absorption bandwidth (EAB) is 4.6 GHz (13.4-18.0 GHz) at 1.68 mm. The excellent electromagnetic wave absorption performances are attributed to the synergy effect of different components. This work provides a unique strategy for the structural design of flower-like microspheres in the field of electromagnetic wave absorption.

Texture Regulation of Metal-Organic Frameworks, Microwave Absorption Mechanism-Oriented Structural Optimization and Design Perspectives

Texture regulation of metal-organic frameworks (MOFs) is essential for controlling their electromagnetic wave (EMW) absorption properties. This review systematically summarizes the recent advancements in texture regulation strategies for MOFs, including etching and exchange of central ions, etching and exchange of ligands, chemically induced self-assembly, and MOF-on-MOF heterostructure design. Additionally, the EMW absorption mechanisms in approaches based on structure-function dependencies, including nano-micro topological engineering, defect engineering, interface engineering, and hybrid engineering, are comprehensively explored. Finally, current challenges and future research orientation are proposed. This review aims to provide new perspectives for designing MOF-derived EMW-absorption materials to achieve essential breakthroughs in mechanistic investigations in this promising field.

Ligand-Assisted Controllable Growth of Self-Supporting Ultrathin Two-Dimensional Metal-Organic Framework Nanosheet Electrodes for an Efficient Oxygen Evolution Reaction

Rational design of metal-organic frameworks (MOFs) into ultrathin two-dimensional (2D) nanosheets with controllable thickness is significantly attractive but is also a significant challenge. Herein, the authors report, for the first time, the synthesis of ultrathin 2D nickel-based MOF nanosheets with a thickness of only about 2 nm via a ligand-assisted controllable growth strategy, which cannot be acquired from the exfoliation of their bulky counterparts or the conventional hydrothermal method. The correlation between 2D nanosheets and crystal growth preference was demonstrated through a judicious choice of a specific [Ni(BIP)(p-BDC)(H₂O)₂]_n framework (BIP = (3,5-bis(1-imidazoly)pyridine), p-H₂BDC = terephthalic acid) to underlie the geometry of the resultant morphology. Under the modulation by the dosage of terephthalic acid through a corrosion-dissolution-coordination process, the nanosheets of Ni-MOFs with a controllable thickness can be tuned to 50 and 100 nm. Ultrathin 2D Ni-MOF nanosheet-derived N-doped Ni@carbon exhibits a satisfactory electrocatalytic performance with a small overpotential of 170 mV to achieve a current density of 10 mA cm^-2, much outperforming the bulk Ni-MOF and the most reported non-noble-metal oxygen evolution reaction electrocatalysts to date. It is believed that this ligand-assisted controllable growth strategy represents a novel and simple path to prepare high-performance MOF-based electrocatalysts for wide applications.

Lightweight electromagnetic wave absorbent composites with Fe3O4 nanocrystals uniformly decorated on the surface of carbon spheres

The application of electromagnetic waves has reached every aspect of human life, but the search for superior electromagnetic wave absorbent materials has been a constant quest of researchers. The application of heterogeneous structures has been favored by researchers of electromagnetic wave absorbent materials and the quest for simple preparation methods and homogeneous distribution of heterogeneous structures is continuing. In this study, we synthesized carbon sphere/Fe₃O₄ nanocrystal (CS/Fe₃O₄) composites by uniformly decorating Fe₃O₄ nanoparticles on the surface of carbon spheres through a simple strategy of expanding the heterogeneous structured interface. The heterogeneous interface formed by graphite and amorphous carbon in the carbon spheres is a boundary-type defect and combined with the magnetic loss capability of the Fe₃O₄ nanocrystals, this composite material has excellent electromagnetic wave absorption properties. The composite material synthesized with 0.05 M solution of iron nitrate has the best electromagnetic wave absorption performance of all samples due to the synergistic effect of interfacial polarization, eddy current loss, defect engineering, and magnetic energy attenuation capability. Reflection losses of -50.932 dB and -49.143 dB were achieved at 4.65 GHz and 10.6 GHz respectively, corresponding to thicknesses of 3.74 mm and 1.74 mm. In addition, the widest effective absorption bandwidth (EAB) at 1.27 mm was 4.5 GHz (13.50-18 GHz). This study enhances the electromagnetic wave absorption performance of carbon spheres by surface-decorating Fe₃O₄ nanoparticles, solves the problem of homogeneity of decorative magnetic oxides on the surface of carbon-based materials, and provides new ideas for the design of controllable, lightweight, ultra-thin composites of carbon-based electromagnetic wave absorbent materials that possess strong electromagnetic wave absorption capability.

Facile synthesis of ZIF-67 derived dodecahedral C/NiCO2S4 with broadband microwave absorption performance

The increasing hazard of electromagnetic radiation prompts people to pursue absorbing materials with better performance. However, absorbing materials with a single loss mechanism usually is unable to obtain better absorbing performance due to low impedance matching or high filling ratio. Therefore, this work proposes a C/NiCo₂S₄ (CNCS) material with both dielectric loss/magnetic loss to achieve efficient absorption of electromagnetic waves. The simple preparation of CNCS materials was achieved through the etching of the ZIF-67 template by nickel nitrate and the subsequent hydrothermal vulcanization process. Its unique prismatic dodecahedron hollow structure promotes multiple scattering of electromagnetic waves. The attachment of the magnetic NiCo₂S₄ particles on the surface of the carbon template further promotes the interface polarization and dipole polarization, which is equivalent to the formation of a resistance-rich microcircuit and enhances the effect of the conductance loss on electromagnetic waves. At 2-18 GHz, the CNCS-2 with 30% paraffin addition achieves an effective bandwidth of 5.54 GHz at a matching thickness of 1.7 mm, and has a maximum reflection loss of -36.44 dB at 1.5 mm. By adjusting the thickness of the material matching layer (1-3 mm), an effective bandwidth of up to 13.48 GHz can be achieved, perfectly covering the X-band and Ku-band. Based on the simple preparation process of the material, the special hollow structure and the multiple loss mechanisms for electromagnetic waves, we believe that CNCS can become a strong competitor for high-efficiency broadband absorbers.

Preparation of porous CoNi/N-doped carbon microspheres based on magnetoelectric coupling strategy: A new choice against electromagnetic pollution

Magnetoelectric coupling is a key strategy to obtain high-performance microwave absorption materials. Especially for carbon matrix composites, the absorbing capacity can be optimized via the tuning of the graphitization degree and the content ratio of the magnetic and dielectric components. Based on this theory, a simple strategy, consisting of the solvothermal method and annealing in an inert atmosphere, is adopted in this study to combine CoNi magnetic alloys with graphitized carbon into micron-scale composite spherical particles. Additionally, special attention is paid to the correlation among the graphitization degree of carbon matrix, component proportion, and dielectric response ability, so as to achieve a flexible micromorphology design and a tunable microwave absorption performance. When the pyrolysis temperature is offset to the best of 700 ℃, a broadband absorption of 6.61 GHz (reflection loss <  - 10 dB) is achieved at an ultrathin matching thickness of 1.9 mm. Adjusting the carbon content can further optimize the impedance matching and realize a high-intensity absorption with a reflection loss of - 72.7 dB. Our work proposes a useful strategy to realize the effective combination of the magnetic and dielectric loss mechanisms and boost the microwave absorption capacity toward achieving the desired broadband and a high-efficiency absorption performance.

Enhanced Electromagnetic-Wave Absorbing Performances and Corrosion Resistance via Tuning Ti Contents in FeCoNiCuTix High-Entropy Alloys

Efficient and stable electromagnetic-wave (EMW) absorption materials have attracted great attention in the field of reducing microwave pollution. Herein, FeCoNiCuTi_x high-entropy alloys (HEAs) as electromagnetic-wave absorbing materials were prepared by a high-energy ball-milling method. The as-milled HEA powders presented a flaky shape with a high aspect ratio. Impedance matching was efficiently optimized by severe lattice distortion, which was caused by Ti incorporation. The introduced plentiful defects in FeCoNiCuTi_x HEAs provided abundant polarization sites for dielectric loss. By tuning Ti contents, FeCoNiCuTi_0.2 HEAs delivered excellent EMW absorption performances. The maximal reflection loss (RL_max) values reached -47.8 dB at 10.86 GHz as thin as 2.16 mm, and the widest bandwidth was 4.76 GHz (5.97-10.73 GHz). Furthermore, the introduction of Ti enhanced corrosion resistance via increasing the charge transfer resistance of a passivated film. Those characteristics of FeCoNiCuTi_x HEAs made these materials a practical gigahertz-range EMW absorber. Additionally, our findings provided a facile and tunable strategy for designing EMW absorbing materials, which was aimed at lightweight, highly efficient absorption, and resistance to harsh environments.

Liquid metal derived MOF functionalized nanoarrays with ultra-wideband electromagnetic absorption

Low melting point liquid metal alloys are progressively utilized in different research fields due to their unique physicochemical properties. Among them, EGaIn is liquid at room temperature with an excellent solubility for reactive metal atoms such as Al. Combined with their characteristic flexible surface, large area and atomically flat interfaces, a library of two-dimensional materials can be generated. Liquid metal synthesis routes provide a highly reproducible thickness of nanosheets with fast, simple, scalable, inexpensive, high yield and non-toxic methods, especially for Al oxides and hydroxides. At the same time, Al-based heterojunction structure also shows a good application prospect in the field of electromagnetic wave absorption, therefore, the use of liquid metal synthesis methods to find the synthesis methods of Al-based layered double hydroxide (LDH) and its derivatives remains to be explored. In this work, EGaIn was used as an aluminum reservoir to prepare LDH and metal organic framework (MOFs) nano-arrays. The prepared CoAl-LDH@ZIF 67 can be transformed into CoAl-LDO@Co-C in the subsequent annealing process performed under nitrogen environments. Interestingly, a series of samples with different morphologies can be obtained by changing the synthesis parameters. The excellent electromagnetic wave interactions are fully characterized. It has an effective absorption bandwidth of 8.48 GHz at 2.6 mm. The findings demonstrated in this work pave the way for the application of lightwave and ductile complex nanoarrays obtained from liquid metals.

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.

Sulfur-doped wood-derived porous carbon for optimizing electromagnetic response performance

Bio-mass materials have been selected as one of the advanced electromagnetic (EM) functional materials due to their natural porous framework for dynamically and flexibly optimizing the EM response property. Herein, we demonstrate sulfur-doped wood-derived porous carbon EM materials (SPC) for optimizing the EM response performance via the coupling between doped heterostructures and the original 3D microchannels. The experimental results reveal that both the dielectric loss capacity and interfacial impedance matching could be increased by the sulfur-doped heterostructures. By tailoring the sulfur content, the microwave absorption (normalized RL_min) of SPC could be optimized to -15.90 dB mm^-1, while the effective absorption bandwidth (EAB_{RL≤-10 dB}) could cover the K band. Moreover, the shielding effectiveness of SPC can be enhanced from 10 dB to 30 dB with the assistance of water, ascribed to the super-wettability performance. This present study provides a novel strategy to further optimize the EM response performance of wood-derived materials, and meanwhile could be widely extended to other bio-mass absorbers.

Cubic-like Co/NC composites derived from ZIF-67 with a dual control strategy of size and graphitization degree for microwave absorption

MOFs with high tunability are considered ideal candidates as microwave-absorbing materials. Strict experimental conditions can ensure the repeatability and maximize the potential of such materials. In this study, cubic ZIF-67 carbides synthesized at different solution temperatures showed an adjustable average size, and then by adjusting the calcination temperature we could control the degree of graphitization, so as to explore the synergistic effect of these two aspects to achieve an in-depth understanding of the electromagnetic properties and microwave absorption properties. The results showed that sample 30-600 (with the former number referring to the synthesis temperature and the latter to the calcination temperature) showed the widest effective absorption bandwidth (5.75 GHz, 1.8 mm) and the optimal reflection loss (-56.92 dB, 2.1 mm). The best matching electromagnetic parameters were obtained under the synergistic action of a smaller particle size and appropriate degree of graphitization, so as to achieve strong attenuation characteristics under low electromagnetic wave reflection. The microwave loss mechanism of the sample mainly involved dielectric losses, such as from conductance loss, dipole polarization, and interface polarization. Starting from the experimental details, this work proposes a dual control strategy for developing microwave-absorbing materials with both simplicity and practicability, which provides a useful reference for other microwave absorbents synthesized at room temperature.

3D core-shell Fe3O4@SiO2@MoS2 composites with enhanced microwave absorption performance

In this work, a 3D ternary core-shell Fe₃O₄@SiO₂@MoS₂ composite is synthesized by a hydrothermal technique and a modified Stöber method, where magnetic Fe₃O₄@SiO₂ microsphere with the core of raspberry-like Fe₃O₄ nanoparticles is completely coated by the flower-like MoS₂. Herein, the electromagnetic parameters of the composites are effectively tuned by the combination of magnetic Fe₃O₄ with dielectric SiO₂ and MoS₂. The obtained ternary composites exhibit remarkable enhancement of microwave absorption. The measurement results indicate that the minimum reflection loss (RL) of Fe₃O₄@SiO₂@MoS₂ composites reaches -62.98 dB at 1.83 mm with the effective absorption bandwidth (RL < -10 dB) of 5.76 GHz (from 11.28 to 17.04 GHz) at 1.92 mm, much higher than those of pure Fe₃O₄ particles and Fe₃O₄@SiO₂ microsphere. It is believed that the improved performances come from the specific structural design and the plentiful interfacial construction. Further, the synergistic effect of the dielectric and magnetic loss as well as the promoted impedance matching also help to enhance the microwave absorption of the composites. The microwave absorption behavior of the composites conforms to the quarter-wavelength cancellation theory. Our study offers an effective and promising strategy in the structural design and interfacial construction of the novel magnetic/dielectric composites with high-efficiency microwave absorption.

Constructing a nitrogen-doped carbon and nickel composite derived from a mixed ligand nickel-based a metal-organic framework toward adjustable microwave absorption

The rational design of nanostructures for absorbers has great potential in the microwave absorption field. In this work, a mixed ligand nickel metal-organic framework (ML-Ni MOF) was first prepared by the self-assembly of pyrazine and 1,3,5-benzenetricarboxylic acid with nickel ions. Then, the as-prepared ML-Ni MOF was used as a precursor to fabricate a nitrogen-doped carbon and nickel composite (ML-Ni/C). With the molar ratio of pyrazine and 1,3,5-benzenetricarboxylic acid of 1 : 1, the flower-like ML-Ni MOF was obtained. After pyrolysis, the ML-Ni MOF-derived ML-Ni/C composite showed an optimal reflection loss value of -65.33 dB with a thickness of 2.4 mm and a corresponding effective absorbing bandwidth (EAB, RL ≤ -10 dB) of 5.1 GHz. Besides, the broadest EAB of 7.6 GHz was achieved when the thickness was about 2.8 mm. This strategy paves a new way to design novel MOFs as precursors for fabricating absorbers.