In Situ Growth of Core-Sheath Heterostructural SiC Nanowire Arrays on Carbon Fibers and Enhanced Electromagnetic Wave Absorption Performance

Advantageous Effects of Phase Transition-Modulated Electric Polarization of Hollow CuSx for Enhanced Electromagnetic Wave Absorption

Scrutinizing the electromagnetic wave absorption mechanism of sulfides remains a challenge due to the variability of the modulation of the crystal structure of the sulfides. To take advantage of this variability, nanosheet-assembled Cu9S5/CN composites with sulfur vacancies were prepared in this study by self-assembly synthesis and subsequent high-temperature heat treatment. Systematic studies show the phase transition-dependent induced decrease in the conductivity, the defect site-induced difference in the charge density, the weakened vacancy formation of defect polarization loss, and the influence of valence state on electric dipole polarization loss and interfacial polarization loss, making the optimization of the dielectric constant a significant positive effect on the improvement of impedance matching. This work provides a reliable example and theoretical guidance for the crystal structure design for the preparation of a new generation of efficient sulfide-based wave-absorbing materials.

Multifunctional Carbon Fiber Reinforced C/SiOC Aerogel Composites for Efficient Electromagnetic Wave Absorption, Thermal Insulation, and Flame Retardancy

Carbon fiber composites have great application prospects as a potential electromagnetic (EM) wave-absorbing material, yet it remains extremely challenging to integrate multiple functions of EM wave absorption, mechanical strength, thermal insulation, and flame retardancy. Herein, a novel carbon fiber reinforced C/SiOC aerogel (CF/CS) composite is successfully prepared by sol-gel impregnation combined with an ambient drying process for the first time. The density of the obtained CF/CS composites can be controlled just by changing sol-gel impregnation cycles (original carbon fiber felt (S0), and samples with one (S1) and two (S2) impregnation cycles are 0.249, 0.324, and 0.402 g cm-3, respectively), allowing for efficient tuning of their properties. Remarkably, S2 displays excellent microwave absorption properties, with an optimal reflection loss of -65.45 dB, which is significantly improved than S0 (-10.90 dB). Simultaneously, compared with S0 (0.75 and 0.30 MPa in the x/y and z directions), the mechanical performance of S2 is dramatically improved with a maximum compressive strength of 10.37 and 4.93 MPa in the x/y and z directions, respectively. Moreover, CF/CS composites show superior thermal insulation capability than S0 and obtain good flame-retardant properties. This work provides valuable guidance and inspiration for the development of multifunctional EM wave absorbers.

Facile constructing Ti3C2Tx/TiO2@C heterostructures for excellent microwave absorption properties

Optimizing and enhancing the performance of electromagnetic wave (EMW) absorption materials relies on the modification of their composition and structure through heterogeneous interface engineering. Ti3C2Tx's high conductivity results in an impedance mismatch, which hinders efficient EMW absorption. Herein, a one-step catalytic chemical vapor deposition (CCVD) method is used to construct the Ti3C2Tx/TiO2@C heterogeneous structure. Upon annealing at 500 °C, amorphous carbon is uniformly deposited on the Ti3C2Tx surface, thereby incorporating the scale-like TiO2 generated during the process. The inclusion of the amorphous carbon layer and TiO2 reduces the substrate's conductivity, achieving optimized impedance matching. Additionally, building heterogeneous interfaces between Ti3C2Tx, TiO2, and C enriches multiple loss mechanisms involving dipole and interfacial polarization, ultimately enabling optimal EMW absorption performance. The minimum reflection loss (RLmin) value of Ti3C2Tx/TiO2@C-500 is -53.12 dB when its thickness and frequency are 1.15 mm and 13.80 GHz, respectively. Moreover, thermal infrared imaging confirms that coatings fabricated using Ti3C2Tx/TiO2@C-500 demonstrate a favorable heat dissipation rate, validating its effectiveness in addressing the challenge of efficient heat dissipation in electronic devices. This study significantly contributes to the progress of two-dimensional (2D) materials, enabling high-performance EMW absorption and expanding their applications in complex scenarios.

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

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

Layered polymer composite foams for broadband ultra-low reflectance EMI shielding: a computationally guided fabrication approach

The development of layered polymer composites and foams offers a promising solution for achieving effective electromagnetic interference (EMI) shielding while minimizing secondary electromagnetic pollution. However, the current fabrication process is largely based on trial and error, with limited focus on optimizing geometry and microstructure. This often results in suboptimal electromagnetic wave reflection and the use of unnecessarily thick samples. In this study, an input impedance model was employed to guide the fabrication of layered PVDF composite foams. This approach optimized the void fraction (VF) and the thickness of each layer to achieve broadband low reflection. Moreover, hybrid heterostructures of SiCnw@MXene were incorporated into the PVDF composite foams as an absorption layer, while the conductive PVDF/CNT composite foams served as a shielding layer. Directed by theoretical computations, we found that combining 2.2 mm of PVDF/SiCnw@MXene composite foam (50% VF) and 1.6 mm of PVDF/CNT composite yielded EMI shielding effectiveness of 45 dB, with an average reflectivity (R) of 0.03 and an effective absorption bandwidth of 5.54 GHz (for R < 0.1) over the Ku-band (12.4-18 GHz). Importantly, the corresponding peak R was only 0.000017. Our work showcases a theoretically guided approach for developing absorption-dominant EMI shielding materials with broadband ultra-low reflection, paving the way for cutting-edge applications.

Preparation and characterization of bifunctional wolfsbane-like magnetic Fe3O4 nanoparticles-decorated lignin-based carbon nanofibers composites for electromagnetic wave absorption and electrochemical energy storage

Recently, with the pursuit of high-efficiency electromagnetic wave absorption (EMWA) and electrochemical energy storage (EES) materials, multifunctional lignin-based composites have attracted significant interest due to their low cost, vast availability, and sustainability. In this work, lignin-based carbon nanofibers (LCNFs) was first prepared by electrospinning, pre-oxidation and carbonization processes. Then, different content of magnetic Fe3O4 nanoparticles were deposited on the surface of LCNFs via the facile hydrothermal way to produce a series of bifunctional wolfsbane-like LCNFs/Fe3O4 composites. Among them, the synthesized optimal sample (using 12 mmol of FeCl3·6H2O named as LCNFs/Fe3O4-2) displayed excellent EMWA ability. When the minimum reflection loss (RL) value achieved -44.98 dB at 6.01 GHz with an thickness of 1.5 mm, and the effective absorption bandwidth (EAB) was up to 4.19 GHz ranging from 5.10 to 7.21 GHz. For supercapacitor electrode, the highest specific capacitance of LCNFs/Fe3O4-2 reached 538.7 F/g at the current density of 1 A/g, and the capacitance retention remained at 80.3 %. Moreover, an electric double layer capacitor of LCNFs/Fe3O4-2//LCNFs/Fe3O4-2 also showed a remarkable power density of 7755.29 W/kg, outstanding energy density of 36.62 Wh/kg and high cycle stability (96.89 % after 5000 cycles). In short, the construction of this multifunctional lignin-based composites has potential applications in electromagnetic wave (EMW) absorbers and supercapacitor electrodes.

Gradient Hierarchical Hollow Heterostructures of Ti3C2Tx@rGO@MoS2 for Efficient Microwave Absorption

Heterostructure engineering has emerged as a promising approach for creating high-performance microwave absorption materials in various applications such as advanced communications, portable devices, and military fields. However, achieving strong electromagnetic wave attenuation, good impedance matching, and low density in a single heterostructure remains a significant challenge. Herein, a unique structural design strategy that employs a hollow structure coupled with gradient hierarchical heterostructures to achieve high-performance microwave absorption is proposed. MoS₂ nanosheets are uniformly grown onto the double-layered Ti₃C₂T_x MXene@rGO hollow microspheres through self-assembly and sacrificial template techniques. Notably, the gradient hierarchical heterostructures, comprising a MoS₂ impedance matching layer, a reduced graphene oxide (rGO) lossy layer, and a Ti₃C₂T_x MXene reflective layer, have demonstrated significant improvements in impedance matching and attenuation capabilities. Additionally, the incorporation of a hollow structure can further improve microwave absorption while reducing the overall composite density. The distinctive gradient hollow heterostructures enable Ti₃C₂T_x@rGO@MoS₂ hollow microspheres with exceptional microwave absorption properties. The reflection loss value reaches as strong as -54.2 dB at a thin thickness of 1.8 mm, and the effective absorption bandwidth covers the whole Ku-band, up to 6.04 GHz. This work provides an exquisite perspective on heterostructure engineering design for developing next-generation microwave absorbers.

ZnO-decorated SiC@C hybrids with strong electromagnetic absorption

A novel strategy is provided to improve the absorption of SiC nanomaterials through surface carbonization of SiC nanowires and hydrolysis. SiC@C-ZnO composites were synthesized with different dosages of ZnNO3·6H2O. Composition, microstructure, and electromagnetic properties of the composites were characterized and analyzed. Results from TEM and XRD show that crystalline ZnO particles adhere to the surface of amorphous carbon, and the ZnO content increases as a function of a dosage of ZnNO3·6H2O. The as-prepared SiC@C-ZnO hybrids exhibit effective electromagnetic absorption, which is related to a synergy effect of different dielectric loss processes. The minimum reflection loss reached -65.4 dB at 11 GHz at a sample thickness of 3.1 mm, while the effective absorption bandwidth (EAB) reached 7 GHz at a sample thickness of 2.56 mm. Furthermore, the EAB of the samples can also cover the whole X band and Ku band at small sample thicknesses (2.09-3.47 mm). The excellent properties of the materials suggest great prospect as electromagnetic absorbers.

Nanoscale pulverization effect in double-layered MOF-derived hierarchical G/Co@C composites for boosting electromagnetic wave dissipation

Metal-organic frameworks (MOFs) have drawn a lot of interest as prospective starting points for highly effective electromagnetic wave (EMW) absorbers. However, the inevitable shrinkage and probable densification that occur during pyrolysis significantly reduce the microwave-loss capacity. A dual-layer MOF, ZIF-8@ZIF-67, is created and effectively decorated on graphene sheets as a solution to this problem. The shrinkage and densification were then suppressed by the subsequent pulverization effect between the two MOFs. Due to suitable compositions and specialized microstructures, G/Co@C exhibits excellent impedance matching and dissipates EMW by combining magnetic and dielectric loss. The maximum reflection loss of G/Co@C-7/paraffin is -55.0 dB at 5.8 GHz with just 7% filler. Therefore, the preparation of high-efficiency MOF-derived microwave absorbers by the pulverization effect is demonstrated to be an efficient strategy.

A Sustainable and Low-Cost Route to Design NiFe2O4 Nanoparticles/Biomass-Based Carbon Fibers with Broadband Microwave Absorption

Carbon-based microwave-absorbing materials with a low cost, simple preparation process, and excellent microwave absorption performance have important application value. In this paper, biomass-based carbon fibers were prepared using cotton fiber, hemp fiber, and bamboo fiber as carbon sources. Then, the precise loading of NiFe₂O₄ nanoparticles on biomass-based carbon fibers with the loading amount in a wide range was successfully realized through a sustainable and low-cost route. The effects of the composition and structure of NiFe₂O₄/biomass-based carbon fibers on electromagnetic parameters and electromagnetic absorption properties were systematically studied. The results show that the impedance matching is optimized, and the microwave absorption performance is improved after loading NiFe₂O₄ nanoparticles on biomass-based carbon fibers. In particular, when the weight percentage of NiFe₂O₄ nanoparticles in NiFe₂O₄/carbonized cotton fibers is 42.3%, the effective bandwidth of NiFe₂O₄/carbonized cotton fibers can reach 6.5 GHz with a minimum reflection loss of -45.3 dB. The enhancement of microwave absorption performance is mainly attributed to the appropriate electromagnetic parameters with the ε' ranging from 9.2 to 4.8, and the balance of impedance matching and electromagnetic loss. Given the simple synthesis method, low cost, high output, and excellent microwave absorption performance, the NiFe₂O₄/biomass-based carbon fibers have broad application prospects as an economic and broadband microwave absorbent.

Electromagnetic absorption enhancing mechanisms by modified biochar derived from Enteromorpha prolifera: a combined experimental and simulation study

Although the rapid advances of wireless technologies and electronic devices largely improve the quality of life, electromagnetic (EM) pollution increases the risk of exposure to EM radiation. Developing high-efficiency absorbers with a rational structure and wideband characteristics is of great significance to eliminate radiation pollution. Herein, Enteromorpha prolifera derived biochar which would provide a suitable surface and multiple polarizations has been prepared as the supporter to anchor nanoparticles. In addition, theoretical simulation results further confirm that radar wave scattering could be largely inhibited after coating with absorbing materials. As a result, the hybrid absorbers achieve remarkable EM absorption properties attributed to the synergistic magnetic-dielectric loss. Elaborate compositional and structural characterization studies indicate that the absorber has a large specific area and numerous polarization centers, which would make full use of waste biomass as light weight and broadband high-performance EM absorption materials.

SO3 decomposition over silica-modified β-SiC supported CuFe2O4 catalyst: characterization, performance, and atomistic insights

The sulfur-iodine (S-I) thermochemical water-splitting cycle is one of the potential ways to produce hydrogen on a large scale. CuFe₂O₄ was dispersed over modified silica or treated β-SiC and untreated β-SiC using the wet impregnation method for SO₃ decomposition, which is the most endothermic reaction of the S-I cycle. Various state-of-the-art techniques such as XRD, FT-IR, BET, XPS, TEM, HR-TEM, FESEM-EDS and elemental mapping were employed to characterize both the synthesized catalysts. CuFe₂O₄ catalyst supported on silica-modified β-SiC resulted in enhanced catalytic activity and stability due to better metal-support interaction. In order to get a better insight into the reaction mechanism over this bimetallic catalyst, the first principles based simulation under the framework of density functional theory was performed. We have found that the presence of Cu gives rise to an improved charge localization at the O-vacancy site alongside favourable reaction kinetics, which results in an enhanced catalytic activity for the CuFe₂O₄ nano-cluster compared to that of a single metallic catalyst containing Fe₂O₃ nano-cluster.

Controllable Synthesis of Mo3C2 Encapsulated by N-Doped Carbon Microspheres to Achieve Highly Efficient Microwave Absorption at Full Wavebands: From Lemon-like to Fig-like Morphologies

Mo₃C₂@N-doped carbon microspheres (Mo₃C₂@NC) have been discovered to be a family of superior microwave absorbing materials. Herein, Mo₃C₂@NC was synthesized through a simple high-temperature carbonization process by evaporating a graphite anode and Mo wire in Ar and N₂ atmospheres with an N-doping content of 6.4 at. %. Attributing to the self-assembly mechanism, the number of Mo wires inserted into the graphite anode determined the morphologies of Mo₃C₂@NC, which were the unique lemon-like (1- and 2-Mo₃C₂@NC) and fig-like (3-, 4-, and 5-Mo₃C₂@NC) microstructures. 1- and 2-Mo₃C₂@NC exhibited powerful reflection losses (RLs) of -45.60, -45.59, and -47.11 dB at the S, C and X bands, respectively, which corresponded to thinner thicknesses. 3-, 4-, and 5-Mo₃C₂@NC showed outstanding absorption performance at the C, X, and Ku bands, respectively, with each value of a minimum RL less than -43.00 dB. In particular, the strongest RL (-43.56 dB) for 5-Mo₃C₂@NC corresponded to an ultrathin thickness of 1.3 mm. In addition, the maximum effective absorption bandwidth was 6.3 GHz for 4-Mo₃C₂@NC. After analysis, all Mo₃C₂@NC samples showed well-matched impedance due to the enhanced dielectric loss caused by the unique carbon structure and moderate magnetic loss derived from the weak magnetic property of Mo₃C₂. More importantly, the unique lemon-like and fig-like microstructures created sufficient interfaces and differentiated multiple reflection paths, which greatly contributed to the strong microwave absorptions at full wavebands. In full consideration of the simple preparation method and tunable absorption properties, Mo₃C₂@NC composites can be regarded as excellent electromagnetic wave absorption materials.

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

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

Hierarchical Ti3C2Tx@ZnO Hollow Spheres with Excellent Microwave Absorption Inspired by the Visual Phenomenon of Eyeless Urchins

Ingenious microstructure design and rational composition selection are effective approaches to realize high-performance microwave absorbers, and the advancement of biomimetic manufacturing provides a new strategy. In nature, urchins are the animals without eyes but can "see", because their special structure composed of regular spines and spherical photosensitive bodies "amplifies" the light-receiving ability. Herein, inspired by the above phenomenon, the biomimetic urchin-like Ti3C2Tx@ZnO hollow microspheres are rationally designed and fabricated, in which ZnO nanoarrays (length: ~ 2.3 μm, diameter: ~ 100 nm) as the urchin spines are evenly grafted onto the surface of the Ti3C2Tx hollow spheres (diameter: ~ 4.2 μm) as the urchin spherical photosensitive bodies. The construction of gradient impedance and hierarchical heterostructures enhance the attenuation of incident electromagnetic waves. And the EMW loss behavior is further revealed by limited integral simulation calculations, which fully highlights the advantages of the urchin-like architecture. As a result, the Ti3C2Tx@ZnO hollow spheres deliver a strong reflection loss of - 57.4 dB and broad effective absorption bandwidth of 6.56 GHz, superior to similar absorbents. This work provides a new biomimetic strategy for the design and manufacturing of advanced microwave absorbers.

MoS2 /MXene Aerogel with Conformal Heterogeneous Interfaces Tailored by Atomic Layer Deposition for Tunable Microwave Absorption

In the design of electromagnetic (EM) wave absorbing materials, it is still a great challenge to optimize the relationship between the attenuation capability and impedance matching synergistically. Herein, a 3D porous MoS2 /MXene hybrid aerogel architecture with conformal heterogeneous interface has been built by atomic layer deposition (ALD) based on specific porous templates to optimize the microwave absorption (MA) performance comprehensively. The original porous structure of pristine Ti3 C2 Tx aerogel used as templates can be preserved well during ALD fabrication, which prolongs the reflection and scattering path and ameliorates the dielectric loss. Meanwhile, plenty of heterointerfaces between MoS2 and Ti3 C2 Tx have been fabricated based on conformally ALD-deposited MoS2 with controlled thickness on the porous surfaces of the templates, which can effectively optimize the impedance matching and transform its response to EM waves from shielding into absorbing. Moreover, the interaction between the attenuation capability and impedance matching can also be modulated by the number of ALD cycle in MoS2 fabrication. After optimization, MoS2 /MXene hybrid aerogel obtained under 300 ALD cycles shows a minimum reflection loss of -61.65 dB at the thickness of 4.53 mm. In addition, its preferable lightweight, high surface area, mechanical, and hydrophobicity properties will also be conducive to further practical applications.

Multifunction lignin-based carbon nanofibers with enhanced electromagnetic wave absorption and surpercapacitive energy storage capabilities

It is difficult for green sustainable lignin-based materials to simultaneously obtain efficient electromagnetic wave absorption (EMWA) and supercapacitive energy storage (SCES), which has not yet been reported. Herein, the light-weight lignin-based carbon nanofibers (LCNFs) with proper pore size, well graphitization degree, and heteroatom doping were tailored through electrospinning and carbonization processes. Interestingly, the graphitization degree and porous structure of LCNFs could be easily adjusted by changing the activating temperature, and the higher conductivity was achieved for preparing LCNFs at higher activating temperature due to the differences in the crystal size and activating degree of LCNFs. As a result, in the field of EMWA, the LCNFs-950 exhibited the minimum reflection loss (RL) value was -41.4 dB and the absorbing frequency was 9.05 GHz at 2.5 mm thickness, which meant this absorbent could absorb and/or dissipate more than 99.9% of incident electromagnetic wave (EMW). Furthermore, the LCNFs-950 also exhibited excellent SCES ability. In two-electrode system, the optimal LCNFs-950 symmetric supercapacitor specific capacitance reached 139.4 F/g at a current density of 0.5 A/g, meanwhile, the energy density was 41.4 Wh/kg at a power density of 3500 W/Kg. These multifunctional features of LCNFs will be highly promising for the next-generation environmental remediating materials.

Fabricating lignin-based carbon nanofibers as versatile supercapacitors from food wastes

Recently, the high-value utilization of food wastes has attracted great interest in sustainable development. Focusing on the major application of electrochemical energy storage (ECES), light-weight lignin-based carbon nanofibers (LCNFs) were controllably fabricated as supercapacitors from melon seed shells (MSS) and peanut shells (PS) through electrospinning and carbonizing processes. As a result, the optimal specific capacitance of 533.7 F/g in three-electrode system, energy density of 69.7 Wh/kg and power density of 780 W/Kg in two-electrode system were achieved. Surprisingly, the LCNFs also presented a satisfied electromagnetic absorption property: The minimum reflection loss (RL) value reached -37.2 dB at an absorbing frequency of 7.98 GHz with an effective frequency (RL < 10 dB) of 2.24 GHz (6.88 to 9.12 GHz) at a thickness of 3.0 mm. These features make the multifunctional LCNFs highly attractive for light-weight supercapacitor electrodes and electromagnetic wave absorbers applications.

Layered Foam/Film Polymer Nanocomposites with Highly Efficient EMI Shielding Properties and Ultralow Reflection

Lightweight, high-efficiency and low reflection electromagnetic interference (EMI) shielding polymer composites are greatly desired for addressing the challenge of ever-increasing electromagnetic pollution. Lightweight layered foam/film PVDF nanocomposites with efficient EMI shielding effectiveness and ultralow reflection power were fabricated by physical foaming. The unique layered foam/film structure was composed of PVDF/SiCnw/MXene (Ti₃C₂T_x) composite foam as absorption layer and highly conductive PVDF/MWCNT/GnPs composite film as a reflection layer. The foam layer with numerous heterogeneous interfaces developed between the SiC nanowires (SiCnw) and 2D MXene nanosheets imparted superior EM wave attenuation capability. Furthermore, the microcellular structure effectively tuned the impedance matching and prolonged the wave propagating path by internal scattering and multiple reflections. Meanwhile, the highly conductive PVDF/MWCNT/GnPs composite (~ 220 S m^-1) exhibited superior reflectivity (R) of 0.95. The tailored structure in the layered foam/film PVDF nanocomposite exhibited an EMI SE of 32.6 dB and a low reflection bandwidth of 4 GHz (R < 0.1) over the Ku-band (12.4 - 18.0 GHz) at a thickness of 1.95 mm. A peak SE_R of 3.1 × 10^-4 dB was obtained which corresponds to only 0.0022% reflection efficiency. In consequence, this study introduces a feasible approach to develop lightweight, high-efficiency EMI shielding materials with ultralow reflection for emerging applications.

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.

Design of conical hollow ZnS arrays vertically grown on carbon fibers for lightweight and broadband flexible absorbers

High-performance electromagnetic (EM) absorbers are necessary for military and industry application in view of the extensive utilization of EM devices. Carbon fibers (CFs) have been considered as promising candidates in electromagnetic wave (EMW) absorption materials, while the single carbon fiber material cannot achieve satisfactory EMW absorption performance because of its limited impedance matching. Herein, electrodeposition and hydrothermal methods were used to fabricate vertical hollow ZnS nanoarrays on carbon cloth (CC) substrate, and then one kind of novel flexible EM composite absorbers with excellent performance was obtained through adjusting morphology of hollow ZnS nanoarrays by easily changing the synthesis parameters of the precursor. Noteworthy, the miniaturized cone-shaped hollow ZnS nanoarray composite absorber shows excellent EMW absorption performance of strong absorption and wide absorption band. The maximum reflection loss value is -52.5 dB and the effective absorption bandwidth reaches 5.1 GHz when the thickness is only 1.9 mm. At the same time, the composite possesses the characteristics of light weight and thin thickness. The excellent properties of the composite absorbers are mainly attributed to their morphological structure. The unique hollow ZnS nanoarray structure enhances the interface polarization and multiple reflections, meanwhile also giving it the properties of metamaterials with resonant absorption. Furthermore, the adjustment of the ZnS nanoarray morphology can not only change the transmission behavior of EMW but also affect the resonance frequency and intensity of the ZnS nanoarray unit. This study obtains high-performance absorbing materials with flexible characteristics as well as highlights the importance of the adjustment of the morphological structure to improve the EMW absorption performance.

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.

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.

Multi-Path Electron Transfer in 1D Double-Shelled Sn@Mo2 C/C Tubes with Enhanced Dielectric Loss for Boosting Microwave Absorption Performance

1D tubular micro-nano structural materials have been attracting extensive attention in the microwave absorption (MA) field for their anisotropy feature, outstanding impedance matching, and electromagnetic energy loss capability. Herein, unique double-shelled Sn@Mo₂ C/C tubes with porous Sn inner layer and 2D Mo₂ C/C outer layer are successfully designed and synthesized via a dual-template method. The composites possess favorable MA performance with an effective absorption bandwidth of 6.76 GHz and a maximum reflection loss value of -52.1 dB. Specifically, the rational and appropriate construction of Sn@Mo₂ C/C tubes promotes the multi-path electron transfer in the composites to optimize the dielectric constant and consequently to enhance the capacity of electromagnetic wave energy dissipation. Three mechanisms dominate the MA process: i) the conductive loss resulted from the rapid electron transmission due to the novel 1D hollow coaxial multi-shelled structure, especially the metallic Sn inner layer; ii) the polarization loss caused by abundant heterogeneous interfaces of Sn-Mo₂ C/C and Mo₂ CC from the precise double-shelled structure; iii) the capacitor-like loss by the potential difference between Mo₂ C/C nanosheets. This work hereby sheds light on the design of the 1D hierarchical structure and lays out a profound insight into the MA mechanism.

Ultralight and High-Strength SiCnw@SiC Foam with Highly Efficient Microwave Absorption and Heat Insulation Properties

An ultralight and high-strength SiC_nw@SiC foam with highly efficient microwave absorption and heat insulation properties was successfully synthesized using the template sacrifice method and chemical vapor deposition process. The microstructure is a novel double network structure, which is formed by the coupling of the morphology-controlled SiC_nw and the SiC skeleton. The introduction of SiC_nw can not only provide more interface polarization and dielectric loss to the SiC foam, which greatly enhances the microwave absorption capacity of the composite foam, but also can enable it to act as an excellent radiation absorbent, which can effectively reduce the thermal conductivity of the foam, especially at high temperatures. In this study, a minimum reflection loss (RL_min) of -52.49 dB was achieved at 2.82 mm thickness with an effective absorption bandwidth of 5.6 GHz. As the length/diameter ratio of SiC_nw decreases, the composite foam exhibits excellent high-temperature thermal insulation and mechanical properties. For the SiC_nw@SiC foam, the thermal conductivity is only 0.304 W/mK at 1200 °C and the compressive strength reaches 1.53 MPa. This multifunctional SiC_nw@SiC foam is an outstanding material, which has potential applications in microwave absorption and high-temperature heat insulation in harsh environments.

Ultra-broadband and covalently linked core-shell CoFe2O4@PPy nanoparticles with reduced graphene oxide for microwave absorption

Covalent bond usually ensures a stable connection between nonmetallic atoms. However, the traditional reflux method usually requires the construction of complex instruments and equipment with tedious steps to ensure airtightness and reaction stability. In this work, an advanced method is adopted to bind core-shell CoFe₂O₄@PPy and rGO tightly via the aid of 2-(1H-pyrrol-1-yl)ethanamine (PyEA), dispense with a high-temperature environment or protective gas. Cobalt ferrite core and polypyrrole shell collaborate to approach suitable magnetic and conduction loss, while reduced graphene oxide usually provides a stable sheet structure for interface multiple reflections, and replenish the insufficient dielectric loss. The filled biscuit-shaped covalently bond CoFe₂O₄@PPy-rGO has a fantastically broad absorption bandwidth of 13.12 GHz under the thickness of 3.6 mm, together with a minimum reflection loss of -50.1 dB at 6.56 GHz, achieving both impedance matching and attenuation matching, and effectively responding to all electromagnetic waves in the X and Ku bands. Thus, the covalently bonded CoFe₂O₄@PPy-rGO has potential application in broadband absorption.

Metal-organic framework derived hollow CoFe@C composites by the tunable chemical composition for efficient microwave absorption

Controlling the composition and microstructure of nanomaterials is still a significant challenge in developing high-performance microwave absorption (MA) materials. Herein, metal-organic framework (MOF)-derived hollow CoFe@C nanoboxes are designed and prepared through the facile regulating the mass ratios of ZIF-67/PFC and a thermal annealing treatment. The CoFe@C composite can achieve an excellent MA performance, which have two high reflection loss (RL) values at different thickness. A RL value of -31.0 dB is obtained at 11.84 GHz with a matching thickness of 2.4 mm, and a RL value can reach -44.1 dB (4.08 GHz) at a matching thickness of 5.8 mm, and a correspondingly wide absorbing bandwidth (5.20 GHz, from 9.7 to 14.9 GHz) is simultaneously obtained at a matching thickness of 2.3 mm. The magnetic loss, interfacial polarization and hollow structure are the main reasons for their excellent MA capability. This work provides a research idea for the development of the efficient MOF-based MA materials in practical application.

Strong and thermostable hydrothermal carbon coated 3D needled carbon fiber reinforced silicon-boron carbonitride composites with broadband and tunable high-performance microwave absorption

Excellent electromagnetic wave (EMW) absorbing materials with high-temperature stable and superior mechanical properties are among the most promising candidates for practical application. Here, novel hydrothermal carbon coated three-dimensional (3D) needled carbon fiber reinforced silicon-boron carbonitride (HC-CF/SiBCN) composites with a hierarchical A (CF)/B (HC)/C (SiBCN) structure were constructed and prepared for the first time by combining hydrothermal transformation and precursor infiltration and pyrolysis (PIP) process. The thickness of the HC coating controlled by the glucose concentration played a crucial role in tailoring the EMW capacity of the composite. The incorporation of SiBCN could not only effectively improve the oxidation resistance but also actively enhance the mechanical properties of the HC coated CF structure. Compared to the weak high-temperature oxidation resistance and mechanical properties of pristine 3D needled CF felt, the composites after the introduction of HC and SiBCN were thermostable in air atmosphere beyond 1000 °C to about above 70% weight retention, and the maximum flexural and compression strength of the composites could reach to 23.51 ± 1.37 and 12.22 ± 1.12 MPa, respectively. A substantial enhancement of EMW absorption ability was achieved through incorporation of HC and SiBCN, which could be attributed to the matched characteristic impedance and enhanced loss ability, whose optimization EMW absorption performance was the minimum reflection loss (RL_min) of -52.08 dB and effective absorption bandwidth (EAB) of 7.64 GHz for the composite obtained by two PIP cycles with 24 wt% glucose solution, demonstrating that the HC-CF/SiBCN composites with high-temperature stable, excellent mechanical and superior EMW absorption properties could be considered as a promising candidate for the applications in harsh environments.

Solvent-Free Synthesis of Ultrafine Tungsten Carbide Nanoparticles-Decorated Carbon Nanosheets for Microwave Absorption

Carbides/carbon composites are emerging as a new kind of binary dielectric systems with good microwave absorption performance. Herein, we obtain a series of tungsten carbide/carbon composites through a simple solvent-free strategy, where the solid mixture of dicyandiamide (DCA) and ammonium metatungstate (AM) is employed as the precursor. Ultrafine cubic WC1-x nanoparticles (3-4 nm) are in situ generated and uniformly dispersed on carbon nanosheets. This configuration overcomes some disadvantages of conventional carbides/carbon composites and is greatly helpful for electromagnetic dissipation. It is found that the weight ratio of DCA to AM can regulate chemical composition of these composites, while less impact on the average size of WC1-x nanoparticles. With the increase in carbon nanosheets, the relative complex permittivity and dielectric loss ability are constantly enhanced through conductive loss and polarization relaxation. The different dielectric properties endow these composites with distinguishable attenuation ability and impedance matching. When DCA/AM weight ratio is 6.0, the optimized composite can produce good microwave absorption performance, whose strongest reflection loss intensity reaches up to - 55.6 dB at 17.5 GHz and qualified absorption bandwidth covers 3.6-18.0 GHz by manipulating the thickness from 1.0 to 5.0 mm. Such a performance is superior to many conventional carbides/carbon composites.

Microwave-induced release and degradation of airborne antibiotic resistance genes (ARGs) from Escherichia coli bioaerosol based on microwave absorbing material

Antibiotic resistance genes (ARGs) have been detected in the atmosphere. Airborne ARGs transmission threatens human health. In the present study, we investigated the release and degradation of airborne ARGs from Escherichia coli bioaerosol through microwave (MW) irradiation. In this study, a new MW absorbing material (Fe₃O₄@SiC ceramic foam) that contributed to its stronger MW absorption is presented. When the MW input energy density was 7.4 × 10³ kJ/m³, the concentration of airborne Escherichia coli decreased by 4.4 log. Different DNA forms were found in the air because MW irradiation ruptured cell membranes. The bound particles provide more protection for bound DNA in the degradation process than free DNA. After the self-degradation of the released airborne free ARGs, some of them would remain and continue to spread in the atmosphere. The released airborne free ARGs cannot be ignored. Total ARGs concentrations decrease rapidly with increased temperature. The inactivation rate constant of ARGs through MW irradiation is higher than that through the Fenton and UV, however, the energy efficiency per order of MW irradiation is lower. Therefore, MW irradiation with Fe₃O₄@SiC ceramic foam could efficiently degrade the distribution of ARGs in the atmosphere.

High-Performance Electromagnetic Wave Absorbing CNT/SiCf Composites: Synthesis, Tuning, and Mechanism

High-performance electromagnetic (EM) wave absorbing materials are strongly desired in many fields like portable devices and aircraft. Introducing carbon nanotubes (CNTs) to certain materials has been proved to be an effective method leading to good EM wave absorption capability. In this work, CNTs are successfully synthesized on SiC fibers with high speed by using a newly developed method which is far more efficient than the commonly used one. The obtained CNT/SiC_f composites exhibit high-performance EM wave absorption capability. With 0.72 wt % CNTs, the reflection loss of the 4 mm composite with only 20 wt % filler loading reaches -62.5 dB with the broad effective absorption bandwidth of 8.8 GHz, covering almost the entire Ku band and three-quarters X band. Moreover, the composites can be added to varying matrices so as to modify their EM wave absorption and other properties. The EM wave absorption performance can be easily tuned in a wide range by varying the CNT content, thickness, and filler loading. This work offers a new route for efficiently synthesizing CNTs but, more importantly, for designing high-performance and multifunctional EM wave absorbing materials.

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

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

Ultrafine FeNi3 Nanocrystals Embedded in 3D Honeycomb-Like Carbon Matrix for High-Performance Microwave Absorption

The reasonable design of magnetic carbon-based composites is of great significance to improving the microwave absorption (MA) performance of the absorber. In this work, ultrafine FeNi₃ nanocrystals (5-7 nm) embedded in a 3D honeycomb-like carbon matrix (FeNi₃@C) were synthesized via a facile strategy that included a drying and carbonization process. Because of the soft magnetic property of the FeNi₃ nanocrystals and their unique 3D honeycomb-like structure, the FeNi₃@C composites exhibit excellent MA abilities. When the filler loading ratio of FeNi₃@C/paraffin composites is only 30 wt%, the maximum reflection loss (RL) value is -40.6 dB at 10.04 GHz. Meanwhile, an ultra-wide absorption frequency bandwidth of 13.0 GHz (5.0-18.0 GHz over -10 dB) can be obtained in the thickness range of 2.0-4.5 mm, and this means that the absorber can consume 90% of the incident waves. It benefits from the dual loss components, multiple polarizations, and multiple reflections for improving MA performances of FeNi₃@C composites. These observations suggest that the 3D honeycomb-like FeNi₃@C composites have broad application prospects in exploring new MA materials that have a wide frequency bandwidth and strong absorption.

Strong and Thermostable Boron-Containing Phenolic Resin-Derived Carbon Modified Three-Dimensional Needled Carbon Fiber Reinforced Silicon Oxycarbide Composites with Tunable High-Performance Microwave Absorption Properties

This paper focuses on the preparation of boron-containing phenolic resin (BPR)-derived carbon modified three-dimensional (3D) needled carbon fiber reinforced silicon oxycarbide (SiOC) composites through a simple precursor infiltration and pyrolysis process (PIP), and the influence of PIP cycle numbers on the microstructure, mechanical, high-temperature oxidation resistance. The electromagnetic wave (EMW) absorption properties of the composites were investigated for the first time. The pyrolysis temperature played an important role in the structural evolution of the SiOC precursor, as temperatures above 1400 °C would cause phase separation of the SiOC and the formation of silicon carbide (SiC), silica (SiO2), and carbon. The density and compressive strength of the composites increased as the PIP cycle number increased: the value for the sample with 3 PIP cycles was 0.77 g/cm3, 7.18 ± 1.92 MPa in XY direction and 9.01 ± 1.25 MPa in Z direction, respectively. This composite presented excellent high-temperature oxidation resistance and thermal stability properties with weight retention above 95% up to 1000 °C both under air and Ar atmosphere. The minimal reflection loss (RLmin) value and the widest effective absorption bandwidth (EAB) value of as-prepared composites was −24.31 dB and 4.9 GHz under the optimization condition for the sample with 3 PIP cycles. The above results indicate that our BPR-derived carbon modified 3D needled carbon fiber reinforced SiOC composites could be considered as a promising material for practical applications.

Excellent lightweight carbon-based microwave absorbers derived from metal–organic frameworks with tunable electromagnetic properties

Upon introducing Gibbs free energy to effectively control the final products, the as-prepared carbon-based composites show excellent absorbing ability at 1.4 mm.

Ultrathin and Light-Weight Graphene Aerogel with Precisely Tunable Density for Highly Efficient Microwave Absorbing

Graphene aerogel (GA) possessing good electrical conductivity and low weight has been widely considered as a promising candidate for high-performance microwave-absorbing (MA) materials. However, simultaneous realization of high reflection loss (RL), low thickness, and light weight remains very challenging for GA because of the trade-off between impedance match and attenuation ability. Herein, through use of (3-aminopropyl)triethoxysilane as a surface modifier and cross-linker, the GA materials with precisely controlled density are fabricated via a unique solvothermal protocol of zero-volume shrinkage. The density-controlled GA (4.5 mg·cm^-3) exhibits a remarkable minimum RL (RL_min) of -50 dB at a thickness of 1.14 mm in the K-band, owing to the optimized dielectric properties. Moreover, even higher attenuation ability without sacrificed impedance match is obtained by incorporating magnetic Fe₃O₄@C microspheres into the density-controlled GA. Superior MA performance involving unprecedented RL_min of -54.0 dB and qualified bandwidth covering 80% of the K-band has been achieved in the superlight Fe₃O₄@C/GA composite at a thickness less than 1 mm, which is highly desirable for MA material applied in mobile devices.

Wide-Band Tunable Microwave-Absorbing Ceramic Composites Made of Polymer-Derived SiOC Ceramic and in Situ Partially Surface-Oxidized Ultra-High-Temperature Ceramics

Microwave-absorbing materials in a high-temperature harsh environment are highly desired for electronics and aerospace applications. This study reports a novel high-temperature microwave-absorbing ceramic composites made of polymer-derived SiOC ceramic and in situ partially surface-oxidized ultra-high-temperature ceramic (UHTC) ZrB₂ nanoparticles. The fabricated composites with a normalized weight fraction of ZrB₂ nanoparticles at 40% has a significantly wide microwave absorption bandwidth of 13.5 GHz (26.5-40 GHz) covering the entire K_a-band. This is attributed to the extensive nanointerfaces introduced in the composites, attenuation induced by the interference of electromagnetic wave, attenuation from the formed current loops, and the electronic conduction loss provided by the partially surface-oxidized ZrB₂ nanoparticles. The minimum reflection coefficient (RC) was -29.30 dB at 29.47 GHz for a thickness of 1.26 mm for the composites with a normalized weight fraction of ZrB₂ nanoparticles at 32.5%. The direct current (dc) conductivity of the nanocomposites showed a clear percolation phenomenon as the normalized weight fraction of ZrB₂ nanoparticles increases to 30.49%. The results provide new insights in designing microwave-absorbing materials with a wide absorption frequency range and strong absorption loss for high-temperature harsh environment applications.