Yolk-Shell Ni@SnO2 Composites with a Designable Interspace To Improve the Electromagnetic Wave Absorption Properties

Functional Carbon Springs Enabled Dynamic Tunable Microwave Absorption and Thermal Insulation

Electromagnetic (EM) wave pollution and thermal damage pose serious hazards to delicate instruments. Functional aerogels offer a promising solution by mitigating EM interference and isolating heat. However, most of these materials struggle to balance thermal protection with microwave absorption (MA) efficiency due to a previously unidentified conflict between the optimizing strategies of the two properties. Herein, this study reports a solution involving the design of a carbon-based aerogel called functional carbon spring (FCS). Its unique long-range lamellar multi-arch microstructure enables tunable MA performance and excellent thermal insulation capability. Adjusting compression strain from 0% to 50%, the adjustable effective absorption bandwidth (EAB) spans up to 13.4 GHz, covering 84% of the measured frequency spectrum. Notably, at 75% strain, the EAB drops to 0 GHz, demonstrating a novel "on-off" switchability for MA performance. Its ultralow vertical thermal conductivity (12.7 mW m-1 K-1) and unique anisotropic heat transfer mechanism endow FCS with superior thermal protection effectiveness. Numerical simulations demonstrate that FCS outperforms common honeycomb structures and isotropic porous aerogels in thermal management. Furthermore, an "electromagnetic-thermal" dual-protection material database is established, which intuitively demonstrates the superiority of the solution. This work contributes to the advancement of multifunctional MA materials with significant potential for practical applications.

Design of a yolk-shell ternary metal sulfide with a tunable structure for high-performance electromagnetic wave absorption

Herein, we report a strategy for preparing a novel ternary metal sulfide (TMS) with a yolk-shell structure based on a "penetration-solidification-annealing" method. A series of FeCoNiSxGy with tunable structure, different morphology and composition was synthesized by adjusting the degree of sulfuration. Due to the synergistical manipulation of rich interface, defects, vacancies and electrical conductivity, as well as the unique yolk-shell structure, the appropriate sulfuration strategy achieved excellent impedance-matching and strong dielectric loss. The as-synthesized FeCoNiSxGy sulfides were blended with paraffin wax (under a filler loading of 30 wt%), which had the minimum reflection loss of -49.44 dB (2.20 mm) and the effective absorption bandwidth of 4.76 GHz (1.6 mm). In conclusion, this work could aid the design and regulation of a new type of ternary metal sulfides with high-performance electromagnetic wave absorption.

Metal Valence State Modulation Strategy to Design Core@shell Hollow Carbon Microspheres@MoSe2/MoOx Multicomponent Composites for Anti-Corrosion and Microwave Absorption

The exploitation of multicomponent composites (MCCs) has become the main pathway for obtaining advanced microwave absorption materials (MAMs). Herein, a metal valence state modulation strategy is proposed to tune the electromagnetic (EM) parameters and improve microwave absorption performances. Core@shell hollow carbon microspheres@MoSe2 and hollow carbon microspheres@MoSe2/MoOx MCCs with various mixed-valence states content are well-designed and produced by a simple hydrothermal reaction or/and heat treatment process. The results reveal that the thermal treatment of hollow carbon microspheres@MoSe2 in Ar and Ar/H2 leads to the in situ formation of MoOx and multivalence state, respectively, and the enhanced content of Mo4+ in the designed MCCs greatly boosts their impedance matching characteristics, polarization, and conduction loss capacities, which lead to their evidently improved EM wave absorption properties. Amongst, the as-prepared hollow carbon microspheres@MoSe2/MoOx MCCs achieve an effective absorption bandwidth of 5.80 GHz under a matching thickness of 1.97 mm and minimum reflection loss of -21.49 dB. Therefore, this work offers a simple and universal method to fabricate core@shell hollow carbon microspheres@MoSe2/MoOx MCCs, and a novel and feasible metal valence state modulation strategy is proposed to develop high-efficiency MAMs.

Pomegranate plasma heterostructure regulated 1D biomass derived microtube networks for lightweight broadband microwave absorber

The excessive aggregation of magnetic metal particles and the resulting skin effect tend to cause a serious imbalance in impedance matching, which hinders its application in aerospace and military wave absorption fields. Obviously, effective dispersion configuration and network construction are two practical measures to develop broadband lightweight absorbers. Based on the recycling theme, pomegranate plasma heterostructure regulated one-dimensional (1D) biomass derived microtube networks are achieved through the conversion and utilization of waste Platanus ball fibers. The metal-organic framework strategy successfully avoids the hard agglomeration of metal particles. The pomegranate seed-like heterostructure effectively modulated the impedance of carbon microtubes, resulting in coordinated dielectric and magnetic losses. Such composites exhibited an effective absorbing bandwidth of 6.08 GHz and a minimum reflection loss of -29.8 dB. This work provides a new approach for constructing sustainable ultralight electromagnetic wave absorbers using plasmon modification and a 1D built-up network structure.

Constructing magnetic/dielectric dual loss and phonon/electron thermal carriers γ-Al2O3-based yolk-shell microspheres to collaboratively advance microwave absorption and heat conduction

The severe electromagnetic (EM) interference and overheating issues in 5G/6G electric devices increasingly heighten the need for developing multifunctional materials with large heat conduction (HC) and high EM wave (EMW) absorption. Here, a series of γ-Al2O3-based yolk-shell microspheres (γ-AlOOH, γ-Al2O3, γ-Al2O3@C, γ-Al2O3@Fe3O4@C, and γ-Al2O3@FeAl2O4@Fe@C YSMSs) as multifunctional fillers are investigated for the simultaneous improvement in the HC and EMW absorption of γ-Al2O3-based composites. Using γ-AlOOH YSMSs as precursors produced from a hydrothermal method, the γ-Al2O3-based YSMSs were synthesized via an annealing route or soaking-annealing route; their phases, textures, and compositions were finely adjusted by changing the Al3+/Fe3+ molar ratio (β) and annealing temperature (Ta). Results show that the thermal transfers in the γ-Al2O3-based YSMSs are promoted by the synergic effect of phonons and electrons when they are utilized as thermally conductive fillers. Comparatively, the γ-Al2O3@FeAl2O4@Fe@C YSMSs formed at β = 8 : 2 and Ta = 700 °C exhibit a high HC of 1.84-3.29 W m-1 K-1 in a loading amount of 5-40%, exceeding those of not merely γ-Al2O3, γ-AlOOH, γ-Al2O3@C, and γ-Al2O3@Fe3O4@C YSMSs but also most previously reported fillers. Furthermore, the γ-Al2O3@Fe3O4@C YSMSs exhibit prominent EMW absorption properties with a large ABW/d of 4.49 GHz mm-1 (just 30% loading), superior to most other Al2O3-based absorbers. Such excellent EMW absorption could be explained by magnetic/dielectric dual loss and significant cavity and interfacial effects caused by yolk-shell structures. In conclusion, this work inspires the development of yolk-shell structures with magnetic/dielectric dual loss and phonon/electron thermal carriers as high-performance bifunctional materials with exceptional heat conduction and EMW absorption.

High-Quality Ultrathin Gd2O2S Nanosheets with Oxygen Vacancy-Decorated rGO for Enhanced Electromagnetic Wave Absorption

The development of extreme performance and multifunctional electromagnetic (EM) wave absorption materials is essential to eliminating undesirable frequency EM pollution. As a promising rare-earth compound, gadolinium oxysulfide (Gd2O2S) has become a significant field of study among nanomaterials with multidisciplinary applications. Herein, the ultrathin Gd2O2S nanosheets with 1 nm thickness were fabricated via a facile hot injection method and then mixed with reduced graphene oxide (rGO) through coassemble and carbonization methods to form Gd2O2S/rGO composites. As a new kind of multifunction EM-wave absorption materials, Gd2O2S/rGO composites exhibited excellent EM-wave absorption performance with an absorption capacity of -65 dB (2.1 mm) and an adequate absorption bandwidth of 5.6 GHz at 1.9 mm. Additionally, their EM-wave absorption mechanisms have been unveiled for the first time. The outstanding EM-wave absorption performance of Gd2O2S/rGO composites could be attributed to the ultrathin Gd2O2S nanosheets with oxygen vacancy and rGO layers with high conductivity and large specific surface area, which will also facilitate the polarization loss, conductivity loss, and multiple reflection and scattering of EM waves between the rGO layer and Gd2O2S nanosheets. Overall, compared to previously reported rGO-based EM-wave absorption materials, this work provides a promising approach for the exploitation and synthesis of Gd2O2S/rGO composites with lightweight and high-performance microwave attenuation.

Dual Resonance Behavior and Enhanced Microwave Absorption Performance of Fe3O4@C@MoS2 Composites with Shape Magnetic Anisotropy

Ternary hierarchical Fe3O4@C@MoS2 composites and binary hierarchical Fe3O4@C composites were successfully fabricated by a modified mixed solvothermal method, a self-oxidation polymerization method, and a hydrothermal process. Their magnetic properties and microwave absorption performance were investigated. Dual resonance behavior was observed in the Fe3O4@C@MoS2 composites. One of the resonances was attributed to natural resonance with a resonance frequency of 2.58 GHz, which was much higher than that for Fe3O4 bulk (1.5 GHz). The other originated from the superparamagnetic/ferromagnetic relaxation with a resonance frequency of 12.45 GHz. The minimum reflection loss (RLmin) reached -64.30 dB with a matched thickness of 2.24 mm at 11.64 GHz, and the maximum effective absorption bandwidth (EABmax) covered 6.39 GHz with a matched thickness of 1.89 mm. In addition, the maximum Radar cross section (RCS) reduction value reached 31.90 dB m2 at a scattering angle of 0°. Electron holography analysis confirmed a dense magnetic absorption network in the Fe3O4@C@MoS2 composites. The boost in microwave absorption performance was caused by the synergistic effects of magnetic and dielectric properties owing to the ternary hierarchical structure, shape magnetic anisotropy, and incorporation of 1T/2H MoS2. Besides, the binary hierarchical Fe3O4@C composites also exhibited good absorbing performance caused by natural resonance, with an RLmin of -52.90 dB at 5.80 mm, an EABmax of 5.98 GHz at 3.38 mm, and a relatively high RCS reduction value of 13.04 dB m2 at θ = 20°. This work paves the way for designing multicomponent hierarchical absorbers with broadband and intensive microwave absorption.

Hierarchical and Orderly Surface Conductive Networks in Yolk-Shell Fe3 O4 @C@Co/N-Doped C Microspheres for Enhanced Microwave Absorption

Constructing the adjustable surface conductive networks is an innovation that can achieve a balance between enhanced attenuation and impedance mismatch according to the microwave absorption mechanism. However, the traditional design strategies remain significant challenges in terms of rational selection and controlled growth of conductive components. Herein, a hierarchical construction strategy and quantitative construction technique are employed to introduce conductive metal-organic frameworks (MOFs) derivatives in the classic yolk-shell structure composed of electromagnetic components and the cavity for remarkable optimized performance. Specifically, the surface conductive networks obtained by carbonized ZIF-67 quantitative construction, together with the Fe3 O4 magnetic core and dielectric carbon layer linked by the cavity, achieve the cooperative enhancement of impedance matching optimization and synergistic attenuation in the Fe3 O4 @C@Co/N-Doped C (FCCNC) absorber. This interesting design is further verified by experimental results and simulation calculations. The products FCCNC-2 yield a distinguished minimum reflection loss of -66.39 dB and an exceptional effective absorption bandwidth of 6.49 GHz, indicating that moderate conduction excited via hierarchical and quantitative design can maximize the absorption capability. Furthermore, the proposed versatile methodology of surface assembly paves a new avenue to maximize beneficial conduction effect and manipulate microwave attenuation in MOFs derivatives.

Symbiotic strategy of Cu on CuFe2O4 realizing high-efficiency electromagnetic wave absorption

Low complex permittivity and easy magnetic agglomeration prevent ferrites from achieving high-efficiency electromagnetic wave (EMW) absorption owing to the resultant narrow absorption bandwidth. Existing composition- and morphology-controlled strategies have made limited progress in fundamentally improving the intrinsic complex permittivity and absorption performance of pure ferrite. In this study, Cu/CuFe₂O₄ composites were synthesized using a facile and low-energy sol-gel self-propagating combustion, and the metallic Cu content was adjusted by changing the ratio of the reductant (citric acid) to the oxidant (ferric nitrate). The symbiosis and coexistence of metallic Cu with ferritic CuFe₂O₄ increases the intrinsic complex permittivity of CuFe₂O₄, which can be regulated by changing the metallic Cu content_. Moreover, the unique ant-nest-like microstructure overcomes the issue of magnetic agglomeration. Because of the favorable impedance matching and strong dielectric loss (interfacial polarization and conduction loss) provided by the moderate metallic Cu content, S0.5 concurrently displays broadband absorption with an effective absorption bandwidth (EAB) of 6.32 GHz at an ultrathin thickness of 1.7 mm and strong absorption relying on minimum reflection loss (RL_min) of -48.81 dB at 4.08 GHz and 4.0 mm. This study provides a new perspective for improving the EMW absorption performance of ferrites.

Manipulation of Impedance Matching toward 3D-Printed Lightweight and Stiff MXene-Based Aerogels for Consecutive Multiband Tunable Electromagnetic Wave Absorption

Highly conductive MXene material exhibits outstanding dissipation capability of electromagnetic (EM) waves. However, the interfacial impedance mismatch due to high reflectivity restricts the application of MXene-based EM wave absorbing materials. Herein, a direct ink writing (DIW) 3D printing strategy to construct lightweight and stiff MXene/graphene oxide aerogels (SMGAs) with controllable fret architecture is demonstrated, exhibiting tunable EM wave absorption properties by manipulating impedance matching. Noteworthy, the maximum reflection loss variation value (ΔRL) of SMGAs is -61.2 dB by accurately modulating the width of the fret architecture. The effective absorption region (f_E) of SMGAs exhibits consecutive multiband tunability, and the broadest tunable f_E (Δf) is 14.05 GHz, which could be continuously tuned in the whole C- (4-8 GHz), X- (8-12 GHz), and Ku-bands (12-18 GHz). Importantly, the hierarchical structures and the orderly stacking of filaments endow lightweight SMGAs (0.024 g cm^-3) with a surprising compression resistance, which can withstand 36 000 times its own weight without obvious deformation. Finite element analysis (FEA) further indicates that the hierarchical structure facilitates stress dispersion. The strategy developed here provides a method for fabricating tunable MXene-based EM wave absorbers that are lightweight and stiff.

Absorption-Dominant mmWave EMI Shielding Films with Ultralow Reflection using Ferromagnetic Resonance Frequency Tunable M-Type Ferrites

Although there is a high demand for absorption-dominant electromagnetic interference (EMI) shielding materials for 5G millimeter-wave (mmWave) frequencies, most current shielding materials are based on reflection-dominant conductive materials. While there are few absorption-dominant shielding materials proposed with magnetic materials, their working frequencies are usually limited to under 30 GHz. In this study, a novel multi-band absorption-dominant EMI shielding film with M-type strontium ferrites and a conductive grid is proposed. This film shows ultralow EMI reflection of less than 5% in multiple mmWave frequency bands with sub-millimeter thicknesses, while shielding more than 99.9% of EMI. The ultralow reflection frequency bands are controllable by tuning the ferromagnetic resonance frequency of M-type strontium ferrites and composite layer geometries. Two examples of shielding films with ultralow reflection frequencies, one for 39 and 52 GHz 5G telecommunication bands and the other for 60 and 77 GHz autonomous radar bands, are presented. The remarkably low reflectance and thinness of the proposed films provide an important advancement toward the commercialization of EMI shielding materials for 5G mmWave applications.

Polyimide aerogel-derived amorphous porous carbon/crystalline carbon composites for high-performance microwave absorption

High-performance polyimide-based porous carbon/crystalline composite absorbers (PIC/rGO and PIC/CNT) were prepared by vacuum freeze-drying and high-temperature pyrolysis. The excellent heat resistance of polyimides (PIs) ensured the integrity of their pore structure during high-temperature pyrolysis. The complete porous structure improves the interfacial polarization and impedance-matching conditions. Furthermore, adding appropriate rGO or CNT can improve the dielectric losses and obtain good impedance-matching conditions. The stable porous structure and strong dielectric loss enable fast attenuation of electromagnetic waves (EMWs) inside PIC/rGO and PIC/CNT. The minimum reflection loss (RLmin) for PIC/rGO is -57.22 dB at 4.36 mm thickness. The effective absorption bandwidth (EABW, RL below -10 dB) for PIC/rGO is 3.12 GHz at 2.0 mm thickness. The RLmin for PIC/CNT is -51.20 dB at 2.02 mm thickness. The EABW for PIC/CNT is 4.08 GHz at 2.4 mm thickness. The PIC/rGO and PIC/CNT absorbers designed in this work have simple preparations and excellent EMW absorption performances. Therefore, they can be used as candidate materials in EMW absorbing materials.

Bird-Nest-Like Multi-Interfacial MXene@SiCNWs@Co/C Hybrids with Enhanced Electromagnetic Wave Absorption

Newly emerged two-dimensional transition metal carbides and/or nitrides (MXenes) have attracted considerable interest in the field of electromagnetic wave absorption, but their excessive conductivity and single loss mechanism limit their applicability. Herein, an MXene decorated with SiC_NWs@Co/C was prepared by in situ growth and carbonization processes, followed by electrostatic self-assembly. The electromagnetic wave absorption performance of MXene@SiC_NWs@Co/C with a bird-nest-like structure could be effectively regulated and optimized by changing the proportion of MXene and SiC_NWs@Co/C. The prepared MXene@SiC_NWs@Co/C hybrid absorbers reveal superior impedance matching, complementary dissipation mechanism, and plentiful heterointerfaces. Profiting from the synergy of abovementioned factors, the resultant MXene@SiC_NWs@Co/C absorber exhibits an optimum reflection loss (RL) value of -76.5 dB at 6.36 GHz under a thickness of 3.9 mm and broad effective absorption bandwidth (EAB, RL ≤ -10 dB) of 6.2 GHz (11.8-18.0 GHz) with a thickness of only 2.0 mm, covering the entire Ku band. This work offers new insights into designing and fabricating highly efficient MXene-based electromagnetic absorbers.

Nacre-Mimetic Hierarchical Architecture in Polyborosiloxane Composites for Synergistically Enhanced Impact Resistance and Ultra-Efficient Electromagnetic Interference Shielding

Pervasive mechanical impact is growing requirement for advanced high-performance protective materials, while the electromagnetic interference (EMI) confers severe risk to human health and equipment operation. Bioinspired structural composites achieving outstanding safeguards against a single threat have been developed, whereas the synergistic implementation of impact/EMI coupling protection remains a challenge. This work proposes the concept of nacre-mimetic hierarchical composite duplicating the "brick-and-mortar" arrangement, which consists of freeze-drying constructed chitosan/MXene lamellar architecture skeleton embedded in a shear stiffening polyborosiloxane matrix. The resulting composite effectively attenuates the impact force of 85.9%-92.8% with extraordinary energy dissipation capacity, in the coordinative manner of strain-rate enhancement, structural densification, lamella dislocation and crack propagation. Attributed to the alternate laminated structure promoting the reflection loss of electromagnetic waves, it demonstrates an ultraefficient EMI shielding effectiveness of 47.2-71.8 dB within extremely low MXene loadings of 1.1-1.3 wt %. Furthermore, it serves favorably in impact monitoring and wireless alarm systems and accomplishes performance optimization through the combination of multiple biomimetic strategies. In conclusion, this function-integrated structural composite is shown to be a competitive candidate for sophisticated environments by resisting impact damage and EMI hazards.

Ion-Exchange Strategy for Metal-Organic Frameworks-Derived Composites with Tunable Hollow Porous and Microwave Absorption

Hollow metal-organic frameworks (MOFs) with careful phase engineering have been considered to be suitable candidates for high-performance microwave absorbents. However, there has been a lack of direct methods tailored to MOFs in this area. Here, a facile and safe Ni²⁺ -exchange strategy is proposed to synthesize graphite/CoNi alloy hollow porous composites from Ni²⁺ concentration-dependent etching of Zeolite imidazole frame-67 (ZIF-67) MOF and subsequent thermal field regulation. Such a special combination of hollow structure and carefully selected hybrid phase are with optimized impedance matching and electromagnetic attenuation. Especially, the suitable carrier transport model and the rich polarization site enhance the dielectric loss, while more significant hysteresis loss and more natural resonance increase the magnetic loss. As a result, excellent microwave absorbing (MA) performances of both broadband absorption (7.63 GHz) and high-efficiency loss (-63.79 dB) are obtained. Moreover, the applicability and practicability of the strategy are demonstrated. This work illustrates the unique advantages of ion-exchange strategy in structure design, component optimization, and electromagnetic regulation, providing a new reference for the 5G cause and MA field.

Ultrabroad Microwave Absorption Ability and Infrared Stealth Property of Nano-Micro CuS@rGO Lightweight Aerogels

Developing ultrabroad radar-infrared compatible stealth materials has turned into a research hotspot, which is still a problem to be solved. Herein, the copper sulfide wrapped by reduced graphene oxide to obtain three-dimensional (3D) porous network composite aerogels (CuS@rGO) were synthesized via thermal reduction ways (hydrothermal, ascorbic acid reduction) and freeze-drying strategy. It was discovered that the phase components (rGO and CuS phases) and micro/nano structure (microporous and nanosheet) were well-modified by modulating the additive amounts of CuS and changing the reduction ways, which resulted in the variation of the pore structure, defects, complex permittivity, microwave absorption, radar cross section (RCS) reduction value and infrared (IR) emissivity. Notably, the obtained CuS@rGO aerogels with a single dielectric loss type can achieve an ultrabroad bandwidth of 8.44 GHz at 2.8 mm with the low filler content of 6 wt% by a hydrothermal method. Besides, the composite aerogel via the ascorbic acid reduction realizes the minimum reflection loss (RL_min) of - 60.3 dB with the lower filler content of 2 wt%. The RCS reduction value can reach 53.3 dB m², which effectively reduces the probability of the target being detected by the radar detector. Furthermore, the laminated porous architecture and multicomponent endowed composite aerogels with thermal insulation and IR stealth versatility. Thus, this work offers a facile method to design and develop porous rGO-based composite aerogel absorbers with radar-IR compatible stealth.

Flower-like bimetal-organic framework derived composites with tunable structures for high-efficiency electromagnetic wave absorption

Recently, high-performance functional composites for electromagnetic wave absorption (EWA) with tunable nano/micro-structures have attracted extensive attention. Herein, the flower-like electrically conductive and magnetic cobalt-nickel@carbon (CoNi@C) composites derived from bimetallic metal-organic frameworks (MOFs) were fabricated via solvothermal method and pyrolysis. By adjusting the ratios of different precursors, different morphological features of composites were formed. When the molar ratio of Co and Ni was 1:2, the CoNi@C composites exhibited the optimal minimum reflection loss (RL_min) of -56.89 dB at 6.7 GHz with an effective absorption bandwidth of 4.7 GHz, due to the coordinated dielectric and magnetic loss caused by the electromagnetic properties of each component as well as the interactions between the unique three-dimensional (3D) interfaces of flower-like structures that promoted the absorption and dissipation of composites for microwaves. The composites are expected to become promising candidates as high-efficiency absorbers in the electromagnetic protection field.

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.

A General Way to Fabricate Chain-like Ferrite with Ultralow Conductive Percolation Threshold and Wideband Absorbing Ability

The magnetic nanochain-like material has been regards as one of the most promising electromagnetic (EM) absorbing material but remains a challenging. Herein, magnetic chain-like ferrite (included Fe3O4, CoFe2O4 and NiFe2O4) are successfully produced through a general solvothermal method, using PVP as the structural-liking agent. Experimental results confirm the ultimate sample possess a 3-dimensional chain-like structure which are constructed by numerous ferrite's nanoparticles with ~60 nm in diameter. Their electromagnetic parameters can be also manipulated by such a chain structure, especially the dielectric loss, where a sharply increases can be observed on within a lower filling ratio. It greatly benefits to the EM absorbing property. In this article, the electromagnetic absorption layer made with a lower content of ferrite possess the excellent electromagnetic absorption ability, where the optimized effective absorption band was nearly 6.4 GHz under a thickness of 1.8 mm. Moreover, the filling ratio is only 30 wt%. Our method for designing of chain-like magnetic material can be helpful for producing wideband electromagnetic absorption in a low filling ratio.

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.

Facile fabrication of metal-organic framework derived Fe/Fe3O4/FeN/N-doped carbon composites coated with PPy for superior microwave absorption

At present, the magnetic metal/carbon composites have been widely explored for microwave absorption (MA), which effectively integrate the characteristics of magnetic and dielectric materials. As a typical material, metal-organic framework (MOF) shows tremendous potential as a precursor or template. However, its development is limited by the inferior impedance matching. Herein, a novel rod-like Fe/Fe₃O₄/FeN/N-doped carbon (FON/NC) composite was synthesized via dual-ligand strategy and following calcination. The outer polypyrrole (PPy) shell, obtained by a facile polymerization method, effectively optimized the impedance matching and observably enhanced the MA capacity. Both the multi-component loss mechanism and unique porous core-shell structures of MOF-derived composites were beneficial for microwave attenuation. The effects of filler loadings (20 wt%, 25 wt%, 30 wt% and 35 wt%) on electromagnetic (EM) properties of FON/NC@PPy composites were discussed. Remarkably, as-obtained composites exhibited a minimum reflection loss (RL) value of -60.08 dB at the layer thickness of merely 1.44 mm and the widest effective absorption bandwidth (EAB, RL ≤ -10 dB) of 5.06 GHz at 1.64 mm with 30 wt% filler loading. This work provides a great reference for designing MOF-derived absorbers with high MA performance.

MoS2 decorated on one-dimensional MgFe2O4/MgO/C composites for high-performance microwave absorption

Advanced microwave absorption (MA) materials have attracted widespread attention to meet the challenges of electromagnetic (EM) pollution. Herein, MgFe₂O₄/MgO/C fibers were successfully prepared via electrospinning technology and carbonization, and their surfaces were coated by MoS₂ via hydrothermal method. The EM wave absorption performance of composites was enhanced due to the introduction of MoS₂. The results showed that the EM wave absorption performance of MgFe₂O₄/MgO/C could not meet the requirements due to low dielectric loss and poor impedance matching. The performance of the composites was improved after coating of MoS₂, which showed the strong wave absorption capability and the broad absorption bandwidth. The optimal reflection loss (RL) is -56.94 dB at 9.5 GHz and the effective absorption bandwidth is 3.9 GHz (8.08-11.98 GHz) with a thickness of 2.7 mm. The excellent MA performance can be mainly attributed to excellent synergistic effect between MgFe₂O₄/MgO/C and MoS₂. Furthermore, MoS₂ also contributes to dielectric loss and ideal impedance matching. MgFe₂O₄/MgO/C@MoS₂ composites may be utilized for lightweight and high-efficient MA materials.

Numerical analysis, experimental verification and criterion establishment of non-magnetic microwave absorbing material

Non-magnetic materials show a great potentiality in microwave absorption with the advantages of low-density, wideband, and thin thickness. Even so, it is still difficult to accurately analyze the connection between performance and parameters. To reveal what electromagnetic parameters could lead to excellent absorbing performance, we simplify and derive the formulae based on Transmission-Reflection-Line theory (TRL) and computer programs. Based on the relation of ε' and ε'', a criterion is established to decide what parameters have the possibility of absorbing performance. Using a new fitting method, the relationship between dielectric constant and absorber content is established. Further, an instruction derived from the relation between ε' and p is used to screen thicknesses. The optimum permittivity of ultra-low reflectivity and ultra-wide band is obtained by combining the numerical analysis results. To verify the accuracy and reliability of results and deductions, the permittivity of four prepared materials and fifty published papers are investigated.

Ferromagnetic Ti3CNCl2-decorated RGO aerogel: From 3D interconnecting conductive network construction to ultra-broadband microwave absorber with thermal insulation property

It remains urgent challenges to adopt suitable strategies to consume unwanted microwave pollution emitted by high-tech electronic devices satisfactorily. Confronted with narrow effective absorption bandwidth (EAB) and high filler loading bottlenecks of MXene-Based microwave absorber, herein, we employ Lewis molten salt etching approach to both exfoliate Ti₃AlCN powders into Ti₃CNCl₂ suspension and intercalate ferromagnetic composition into interlamination simultaneously. By utilizing the crosslinking effect of dopamine, the Ti₃CNCl₂ are anchored on the surfaces of graphene oxide (GO) nanosheets, constructing interconnecting microstructure. Both the 3D conductive network and the modification of MXene manifest crucial impacts on enhancing microwave absorption performance of the resulting ultra-lightweight reduced GO (RGO)-based aerogel. The minimum intensity of reflection loss achieves -62.62 dB with the absorber mass loading of 0.7 wt%. Remarkably, more than 90% of the incident microwave is qualified to be absorbed over the whole Ku band. The EAB is broadened while tailoring the thickness to 3 mm, ranging from 10.2 to 18 GHz. Besides, the aerogel presents valuable thermal insulation properties. Our methodology of synthesizing MXene/RGO aerogel not only provides promising insights into microstructural construction but also endows the possibility for integrating thermal insulation property towards next-generation high-performance microwave absorption devices.

Electromagnetic Properties of Carbon Nanotube/BaFe12-xGaxO19/Epoxy Composites with Random and Oriented Filler Distributions

The microwave properties of epoxy composites filled with 30 wt.% of BaFe_12–xGa_xO₁₉ (0.1 ≤ x ≤ 1.2) and with 1 wt.% of multi-walled carbon nanotubes (CNTs) were investigated in the frequency range 36–55 GHz. A sufficient increase in the microwave shielding efficiency was found for ternary 1 wt.%CNT/30 wt.% BaFe_12–xGa_xO₁₉/epoxy composites compared with binary 1% CNT/epoxy and 30 wt.% BaFe_12–xGa_xO₁₉/epoxy due to the complementary contributions of dielectric and magnetic losses. Thus, the addition of only 1 wt.% of CNTs along with 30 wt.% of barium hexaferrite into epoxy resin increased the frequency range where electromagnetic radiation is intensely attenuated. A correlation between the cation Ga³⁺ concentration in the BaFe_12–xGa_xO₁₉ filler and amplitude–frequency characteristics of the natural ferromagnetic resonance (NFMR) in 1 wt.%CNT/30 wt.% BaFe_12–xGa_xO₁₉/epoxy composites was determined. Higher values of the resonance frequency fres (51.8–52.4 GHz) and weaker dependence of fres on the Ga³⁺ concentration were observed compared with pressed polycrystalline BaFe_12–xGa_xO₁₉ (fres = 49.6–50.4 GHz). An increase in the NFMR amplitude on the applied magnetic field for both random and aligned 1 wt.% CNT/30 wt.% BaFe_12–xGa_xO₁₉/epoxy composites was found. The frequency of NFMR was approximately constant in the range of the applied magnetic field, H = 0–5 kOe, for the random 1 wt.% CNT/30 wt.% BaFe_12–xGa_xO₁₉/epoxy composite, and it slightly increased for the aligned 1 wt.% CNT/30 wt.% BaFe_12–xGa_xO₁₉/epoxy composite.

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.

Core-shell structured nanocomposites formed by silicon coated carbon nanotubes with anti-oxidation and electromagnetic wave absorption

The silicon coated Carbon nanotubes (CNTs) nanocomposite (CNTs@Si) with a shell structure was successfully synthesized by a simple chemical vapor deposition (CVD) method. In this work, the CNTs@Si is not only introduced as a structural material providing oxidation performance, but also as an extremely effective electromagnetic wave (EMW) absorption nanocomposite. Dielectric characteristics EMW absorption properties within the frequency range of 2-18 GHz of CNTs@Si were studied, and the oxidation resistance of CNTs@Si was characterized. Due to the dense space conductive network formed by CNTs, the EMW absorbing properties of CNTs@Si nanocomposite features excellent electromagnetic wave absorption capacity at a filling amount of 1%. The maximum reflection loss (RL) reaches -61.57 dB at the thickness of 1.8 mm, and a wide effective absorption bandwidth (EAB, RL < -10 dB) of 2.88 GHz is achieved. The obtained CNTs@Si core-shell nanocomposites exhibit excellent antioxidant performance and absorbing performance due to silicon bridging. Efficient electromagnetic wave absorption and excellent oxidation resistance of CNTs@Si can be regarded as a brand-new competitive candidate for EMW absorption materials in harsh environment.

Novel bimetallic MOF derived hierarchical Co@C composites modified with carbon nanotubes and its excellent electromagnetic wave absorption properties

In the severe electromagnetic wave pollution situation, the absorbers must meet the requirements of lightweight, strong absorption, thin thickness and wide band. Under such a circumstance, it is of great significance to construct reasonable structure and composition for excellent electromagnetic wave absorption. Therefore, the Co/C composites anchored with carbon nanotubes (Co@CNTs) have been successfully prepared by rationally regulating the growth of bimetallic MOF and subsequent pyrolysis process. It is revealed that the conduction loss and polarization loss caused by the carbon nanotubes with different lengths and densities and the porosity of the composites are together responsible for the attenuation of electromagnetic wave. As expected, the hierarchical Co@CNTs composites showed a strong reflection loss of -76.6 dB and a broad effective absorption bandwidth of 6.2 GHz through the improvement of impedance matching and electromagnetic wave absorption ability. Herein, this work presents a strategy for the development of composites as promising electromagnetic wave absorbent.

Polymer-Derived Lightweight SiBCN Ceramic Nanofibers with High Microwave Absorption Performance

Lightweight SiBCN ceramic nanofibers were prepared by a combination of electrostatic spinning and high-temperature annealing techniques, showing tunable electromagnetic wave absorption. By controlling the annealing temperature, the nanoscale architectures and atomic bonding structures of as-prepared nanofibers could be well regulated. The resulting SiBCN nanofibers ∼300 nm in diameter, which were composed of an amorphous matrix, β-SiC, and free carbon nanocrystals, were defect-free after annealing at 1600 °C. SiBCN nanofibers annealed at 1600 °C exhibited good microwave absorption, obtaining a minimum reflection coefficient of -56.9 dB at 10.56 GHz, a sample thickness of 2.6 mm with a maximum effective absorption bandwidth of 3.45 GHz, and a maximum dielectric constant of 0.44. Owing to the optimized A + B + C microstructure, SiBCN ceramic nanofibers with satisfying microwave absorption properties endowed the nanofibers with the potential to be used as lightweight, ultrastrong radar wave absorbers applied in military and the commercial market.

MOF-derived Co@C nanoparticle anchored aramid nanofiber (ANF) aerogel for superior microwave absorption capacity

High-efficiency, porous and renewable magnetic microwave absorbing (MA) materials have been enthusiastically pursued due to their suitable impedance matching, light weight, strong multiple scattering and the synergy effect of dielectric and magnetic loss. Herein, a three-dimensional (3D) Co@C/ANF aerogel, composed of magnetic MOF derivatives embedded in biomass aramid nanofiber (ANF), was prepared for the first time through a directional-freezing method followed by an annealing process. To evaluate their MA attenuation performance, the electromagnetic parameters of Co@C/ANF composites with different component ratios were measured at 2-18 GHz. Profiting from the preserved porous structure of MOF derivatives, the construction of multiple heterogeneous interfaces and suitable electromagnetic parameters, Co@C/ANF 2 : 1 exhibited a good MA performance of RL_min = -64.3 dB (indicating more than 99.99996% microwaves were absorbed) and EAB_max = 6.8 GHz. Considering the admirable overall performance, the Co@C/ANF aerogel is deemed to be a promising candidate for the next-generation of lightweight, reproducible, and high-performance MA materials.

Metal-Organic Framework-Derived Carbon/Carbon Nanotubes Mediate Impedance Matching for Strong Microwave Absorption at Fairly Low Temperatures

The use of magnetic particles and carbon materials, particularly those with compatible dielectric and magnetic loss, is crucial in managing microwave pollution. However, the mismatched impedance of currently available absorbers constrains their practical applications. Herein, we demonstrate the potential of a metal-organic framework (MOF)-derived composite whose impedance matching is optimized through spraying and immersion of MOF precursors in carbon nanotube socks followed by carbonization. The composite presents extremely strong microwave absorption with a reflection loss of -30 dB, a thin thickness of 1.5 mm, and a wide frequency bandwidth of 7.8 GHz. The excellent absorption can still be maintained even at a fairly low temperature of -40 °C. Such results are primarily attributed to the synergistic effect between the hierarchical architecture and multiple components that greatly optimizes the impedance matching. We believe that the composite is a promising microwave absorber that can help to solve the critical electromagnetic wave pollution.

Yolk-shell structured Co@SiO2@Void@C nanocomposite with tunable cavity prepared by etching of SiO2 as high-efficiency microwave absorber

Nowadays, high-performance microwave absorption materials with light weight, strong absorbing intensity and wide absorption bandwidth are urgently demanded to solve the electromagnetic pollution issues. In this work, the yolk-shell structured Co@SiO₂@Void@C nanocomposites with tunable cavity are obtained by etching SiO₂ in the Co@SiO₂@C nanoparticles. They exhibit better microwave absorption properties than the unetched counterpart. When the etching time is 6 h, the Co@SiO₂@Void@C nanocomposite shows high absorption efficiency with a minimum reflection loss (RL) value of -44.5 dB at 8.8 GHz. Notably, its effective absorption bandwidth (RL < -10 dB) is as wide as 8.0 GHz (9.7-17.7 GHz) at a thin thickness of only 1.7 mm. The excellent microwave absorbing performances are attributed to the abundant heterointerfaces, well-controlled cavity, synergistic effects between magnetic and dielectric loss, and optimal impedance matching. Owing to the characteristics of strong absorbing capacity, ultrabroad absorption bandwidth and thin matching thickness, the yolk-shell structured Co@SiO₂@Void@C nanocomposites are promising candidates as highly effective microwave absorbers.

One-dimensional Ag-CoNi nanocomposites modified with amorphous Sn(OH)2/SnO2 shells for broadband microwave absorption

High-performance microwave absorption absorbers play important roles in the fields of radar stealth, electromagnetic protection, and antenna technology. In this work, high aspect-ratio Ag nanowires were decorated with magnetic CoNi nanoparticles via a PVP-induced solvothermal method, and then amorphous Sn(OH)₂/SnO₂ shells were introduced through an in-situ oxidative hydrolysis method, successfully preparing Ag-CoNi@Sn(OH)₂/SnO₂ composites. The morphology and ingredient of composites were ascertained by SEM, TEM, XRD, EDX, and XPS. As Ag-CoNi nanocomposites are coated by Sn(OH)₂/SnO₂ shells, the minimum reflection loss value is decreased from -31.7 dB (10.1 GHz) to -37.8 dB (6.4 GHz), and the maximum effective absorption bandwidth is extended from 3.9 GHz (10.3-14.2 GHz) to 5.8 GHz (10.7-16.5 GHz). Analyses of electromagnetic parameters reveal the possible mechanisms, involving surface plasma resonance, conductive loss, interfacial polarization, dipole polarization, exchange resonance, eddy current effect, multiple reflection and scattering. Thus, Ag nanowires modified with CoNi nanoparticles and amorphous Sn(OH)₂/SnO₂ shells can effectively balance the impedance matching and attenuation capability. It is a new strategy to achieve broadband microwave absorbers.

NiCo alloy/C nanocomposites derived from a Ni-doped ZIF-67 for lightweight microwave absorbers

In this work, we prepared NiCo alloy/C with rhombic dodecahedron structure and superior microwave absorption performance by using ZIF-67 as the raw material. The rhombic dodecahedron NiCo alloy/C was with rough particles on the surface was photographed by field emission scanning electron microscopy. By adjusting the doping amount of Ni and the temperature of pyrolysis, improved the impedance matching of NiCo alloy/C. Specifically, NiCo alloy/C exhibits a minimum reflection loss of -65.48 dB at 13.48 GHz, while the thickness is 1.63 mm. Defects introduced in the Ni doping process and the special rhombic dodecahedral structure can cause multiple loss mechanisms. Therefore, this NiCo alloy/C composite has the potential to be a potential microwave absorber material with lightweight and high microwave absorption properties.

Confined Magnetic-Dielectric Balance Boosted Electromagnetic Wave Absorption

Magnetic-dielectric property plays a critical significance for the functional expression toward advanced materials. Within nanoscale, the simultaneous regulation of the electrical and magnetic properties of electromagnetic (EM) wave absorption materials faces huge challenges. Herein, using the metal-organic frameworks (MOF) as templates, highly-dispersed ZnO and Co nanoparticles are uniformly confined inside graphited N-doped carbon skeleton, constructing the balanced EM property in the Co@NC-ZnO absorbers. Meanwhile, a dynamics and symmetrical morphology optimization of MOF-derived Co@NC-ZnO are dependent on the Co/Zn mass ratio and adjusting MOF frameworks, which evolves from the cube, truncated cube, dodecahedron, and to the final microsphere. Simultaneously, both the electronic conduction network and magnetic coupling network are compatible together in the in situ transformed Co@NC-ZnO system. Boosted magnetic responding ability and unique magnetic coupling are verified by the off-axis electronic holography. Plentiful heterojunction interfaces and special electronic conduction paths can be built in this Co-Zn-MOF derivatives, facilitating the dielectric loss behaviors. As expected, MOF-derived Co@NC-ZnO absorber displays outstanding EM wave absorption ability with strongest reflection loss value of -69.6 dB at only 1.9 mm thickness and wideband absorption covering 6.8 GHz at 2.4 mm. Confined EM balance provides new design strategy toward MOF-derived excellent MA materials and functional devices.

Carbon-Based MOF Derivatives: Emerging Efficient Electromagnetic Wave Absorption Agents

To tackle the aggravating electromagnetic wave (EMW) pollution issues, high-efficiency EMW absorption materials are urgently explored. Metal-organic framework (MOF) derivatives have been intensively investigated for EMW absorption due to the distinctive components and structures, which is expected to satisfy diverse application requirements. The extensive developments on MOF derivatives demonstrate its significantly important role in this research area. Particularly, MOF derivatives deliver huge performance superiorities in light weight, broad bandwidth, and robust loss capacity, which are attributed to the outstanding impedance matching, multiple attenuation mechanisms, and destructive interference effect. Herein, we summarized the relevant theories and evaluation methods, and categorized the state-of-the-art research progresses on MOF derivatives in EMW absorption field. In spite of lots of challenges to face, MOF derivatives have illuminated infinite potentials for further development as EMW absorption materials.