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Derived from the increasing demand for radar and infrared camouflage integration, a two‐layer hierarchical structure that combines coating and a square patch‐based metasurface is designed. Under deep subwavelength thickness, the infrared stealth layer (IRSL) can markedly reinforce microwave absorption layer (MAL) performance. More than 90% of the electromagnetic wave energy in the 3.7–5.1 GHz and 12.8–18 GHz bands can be absorbed. The hierarchical structure expands the effective absorption bandwidth of the exposed coatings from 0 to 6.6 GHz, and the maximum reflection loss increases from −8 to −40 dB. Moreover, the emissivity in the midinfrared range is maintained at an extremely low value of 0.281. The delay of magnetic resonance is found in the simulation model, indicating that strong magnetic resonance is formed in coatings after covering the IRSL. Meanwhile, the absorption peaks of the coatings are controlled by low‐pass filter metasurfaces. These two factors contribute to the enhancement effect of IRSL on MAL and break the boundary between radar and infrared stealth. Herein, an original solution for tuning multispectral electromagnetic (EM) waves is provided and it has broad application prospects in advanced compatible camouflage material. [ABSTRACT FROM AUTHOR]

Carbon-coated Cu nanocapsules (Cu@C NCs) with controllable graphite-like shells were designed and synthesized for applications in wireless communication, military stealth or electronic devices. A modified arc plasma method was employed to prepare the NCs, where a solid carbon source was used to provide a carbon-saturated atmosphere, resulting in the constitution of core–shell nanostructures with thickness-controllable carbon shells. Formation mechanisms were involved in the nucleation and growth, adsorption–diffusion or secondary growth process. The Cu@C NCs with thicker graphite-like shells demonstrated the enhanced oxidation resistance, thermal stability and excellent electromagnetic absorption performance ranging between 2 and 18 GHz. A mass of hetero-interfaces and defects inside them strengthened dielectric polarizations and achieved impedance matching. The Cu@C NCs exhibited a minimum reflection loss of − 38.4 dB at 4.88 GHz. Benefiting from the structurally controllable core–shell nanostructure, the Cu@C NCs would become a potential candidate material for electromagnetic wave absorption applications. [ABSTRACT FROM AUTHOR]

Chirality, in which electromagnetic characteristic promotes asymmetric polarization, is expected to realize broadband absorption for microwave absorption materials (MAMs). Herein, inspired by the microstructure of nepenthes, a scalable approach is reported for fabricating hierarchical‐chiral helical carbon fibers with broadband microwave absorption as well as multispectral chiral manipulation. The chiral potential barriers are established by applying helically distributed stress to the fiber, which induce helical electric dipoles via positive and negative charges accumulated at the potential barriers. The states of polarization loss by Debye relaxation are 2, 3, and 4 in the samples with achirality, single‐chirality, and dual‐chirality, respectively; the polar vector of achiral electric dipoles is transformed into pseudovectors through chiral potential barriers to improve the synergistically magnetic loss, and the grade of chirality is quantified based on symmetry breaking theory. The single‐chiral fibers reach strongest absorption peak value of −30.67 dB at 2.96 GHz (carbon content is 50 wt%), and the dual‐chiral fibers achieve effective absorption bandwidth (RL ≤ −10 dB) of 8.8–18 GHz. In addition to microwaves, the chiral photonic manipulation plays a role in near‐ultraviolet and visible spectra. The present discovery points to a pathway for using chiral model to open up new microwave absorption mechanism and integrated application. [ABSTRACT FROM AUTHOR]

Despite great efforts in material design and modification, there are still obstacles to improve the effective microwave absorption, which hinders the development of stealth materials. By introducing a magnetically induced Huygens' metasurface with Fibonacci spiral meta‐atom, a hierarchical absorber with carbonyl iron particle coating is designed and prepared, of which the total effective absorption bandwidth (EAB, reflection loss ≤−10 dB) amazingly reaches 9.41 GHz, exceeding 199% of the original coatings', and the absorption peak value declines from −15.91 to −32.12 dB. The overall thickness of the absorber is 7 mm, only 0.17 over the minimum thickness dictated by the causality limit. A conceptual basis for the proposed design is provided, that is, the balance of effective surface current and magnetic current induced simultaneously will enhance the electromagnetic (EM) wave transmission, which contributes to broadening the absorption bandwidth with promising impedance matching. The system is validated numerically and experimentally, suggesting a novel route to obtain high‐performance microwave absorption materials for the EM stealth technology. [ABSTRACT FROM AUTHOR]

Microwave absorbing materials with stable-wide effective absorbing bandwidth (EAB) are regarded as desirable candidates for eliminating electromagnetic interference at high temperatures. However, it still remains a challenge to simultaneously regulate the dielectric and magnetic losses in wide temperature ranges for the above excellent microwave absorption performance. Herein, a novel magnetic-dielectric composite of carbonyl iron (CI)/carbon foam (CF) with three-dimensional (3D) reticulated structure is fabricated for the first time by carbonization and vacuum impregnation. 3D reticulated structure not only provides well-defined conductive paths and controlled electrical conductivity-temperature characteristics for optimizing dielectric loss, but also induces structural magnetic loss through induced currents to retard the attenuation of magnetic loss at high temperatures. Benefiting from the synergistic regulation of dielectric and magnetic losses, CI/CF composites show the ultra-stable wide EABs at a thickness of only 1 mm, maximum EAB of 8.48 GHz and minimum EAB of 8.32 GHz in the temperature range of 298–573 K. This work provides inspirations for construction of high-temperature microwave absorbing materials with tunable magnetic and dielectric losses. [Display omitted] [ABSTRACT FROM AUTHOR]

Highlights: A multifunctional microwave modulator is developed for electromagnetic protection, infrared stealth and operating temperature monitoring over wide temperature ranges for the first time. Microwave modulator achieves the integration of two electromagnetic protection mechanisms, microwave absorption and radiation deflection. Microwave modulator demonstrates the maximum effective bandwidth of 5.2 GHz with a thickness of only 1.5 mm in the temperature range 298–673 K. High-temperature electromagnetic (EM) protection materials integrated of multiple EM protection mechanisms and functions are regarded as desirable candidates for solving EM interference over a wide temperature range. In this work, a novel microwave modulator is fabricated by introducing carbonyl iron particles (CIP)/resin into channels of carbonized wood (C-wood). Innovatively, the spaced arrangement of two microwave absorbents not only achieves a synergistic enhancement of magnetic and dielectric losses, but also breaks the translational invariance of EM characteristics in the horizontal direction to obtain multiple phase discontinuities in the frequency range of 8.2–18.0 GHz achieving modulation of reflected wave radiation direction. Accordingly, CIP/C-wood microwave modulator demonstrates the maximum effective bandwidth of 5.2 GHz and the maximum EM protection efficiency over 97% with a thickness of only 1.5 mm in the temperature range 298–673 K. Besides, CIP/C-wood microwave modulator shows stable and low thermal conductivities, as well as monotonic electrical conductivity-temperature characteristics, therefore it can also achieve thermal infrared stealth and working temperature monitoring in wide temperature ranges. This work provides an inspiration for the design of high-temperature EM protection materials with multiple EM protection mechanisms and functions. [ABSTRACT FROM AUTHOR]

The microscopic morphology of the composites has a great influence on their microwave absorbing (MA) performances. However, it remains a challenge to achieve morphology control through a facile way when preparing composites of carbon nanotubes and zinc oxide. In this work, a simple and room-temperature approach is proposed to synthesize zinc oxide/carbon nanotubes (CNT/ZnO) nanocomposites with different microstructures. Changing the type of solvent can lead to different microstructures of CNT/ZnO nanocomposites. This is mainly because of the different ion molar ratio to Zn2+ and OH- in the reaction solution. Besides by adjusting the content of CNT, the microwave absorbing properties of the obtained CNT/ZnO nanocomposites can be controlled. When the CNT content is 10% mass fraction, the minimum reflection loss (RL) of CNT/ZnO-E nanocomposite can reach − 55.42 dB at a thickness of 3 mm. With the increasing weight ratio of CNT, the degree of disorder in the material and the number of the defects increase, 3D conductive networks are easier to form, resulting in the enhanced polarization loss and conductive loss. However, excessive CNTs will cause excessive current induced which is not conducive to electromagnetic wave absorption. [ABSTRACT FROM AUTHOR]

[Display omitted] • Transparent basalt is covered with Co/CoNi sequential configurations. • CoNi nanolayers have a net-like morphology at the higher Co ratio. • The effective absorption bandwidth reaches 4.665 GHz at 1.47 mm. • Sequential nanolayers boost the charge transfer and magnetic coupling. The typical design for electromagnetic wave absorbing materials is the composites made of high dielectric components and magnetic materials, which is a single type. This article presents a unique paradigm for high microwave absorption (MA), whereby the transparent basalt substrates are evenly covered by the sequential configurations formed by the entangled Co/CoNi nano-units. The sequential nanostructure consistently displays a net-like morphology at a 5/10 min reaction for the higher Co content in CoNi, exhibiting the regular variety for complex permittivity. On the other hand, it is constructed by the spherical nano-units with a higher Ni ratio in CoNi. After 5 min of reaction, the nanospheres are arranged with the nanogaps, or they are continuously arranged on basalt after 10 min. Correspondingly, the basalt/CoNi composites exhibit unfavorable 'mutation' characteristics with the too-small or too-large complex permittivity, detrimental for optimized MA. As expected, with the specific Co/Ni ratio, the net-like basalt/CoNi obtained at 5 min displays outstanding MA due to appropriate dielectric loss and strong magnetic coupling (−61.98 dB at 2.03 mm, 4.665 GHz at only 1.47 mm). Especially, the nano-scale magnetic coupling is confirmed by micromagnetic simulation. This work breaks the limitations of traditional absorbers and provides new design ideas about magnetic-dielectric components. [ABSTRACT FROM AUTHOR]

Dielectric composites based on polybasic components with hierarchical structures have promising potentials for electromagnetic absorber. However, strategy to strengthen coupling of transition metal oxides still demands to be optimized to enhance electromagnetic loss. Herein, inspired by the hierarchical structure of balsam pear, ultrafine ZnO is uniformly distributed and tightly capped in MnO2 by self‐assembly hydrothermal method. The unique hierarchical structure provides abundant polarization centers with large specific surface area. The compositions promote attenuation performance of electromagnetic waves, enhance relative permittivity and satisfy impedance matching as compared with that of pure ZnO or MnO2. The effective bandwidth (RL< −10 dB) reaches 5.93 GHz obtained with thin thickness of 1.8 mm. The effective bandwidth reaches 15 GHz via regulating their thickness. The fabricated MnO2/ZnO composites can achieve great adaptability in microwave. MnO2/ZnO composites keep stability even heated 700 °C in air. The phase and weight remain unchanged. The proposed interfacial modulation strategy can provide new opportunities for the design of efficient electromagnetic absorber with thermal stability. [ABSTRACT FROM AUTHOR]

Flaky carbonyl iron particles/nano silicon dioxide/Polyurethane (FCIP/SiO2/PU) composites coatings were fabricated. And FCIP were fabricated from sphere carbonyl iron particles (SCIP) by ball milling. The electromagnetic parameters of absorbers (SCIP, FCIP and SiO2) and the reflection loss (RL) of coatings were measured in the frequency range of 2–18 GHz. The results showed that the complex permeability and absorbing properties of FCIP are significantly enhanced compared with SCIP because of its smaller thickness and easy magnetization plane. In addition, as a low dielectric material, SiO2 can build more wave-transmission channels and optimize the impedance matching, making more electromagnetic wave transmitting into the coatings and being attenuated. Due to the addition of SiO2, the absorbing properties and effective absorption bandwidth of coatings are further enhanced. The reflection loss values exceeding − 10 dB can be obtained in the frequency range of 8.5–18 GHz with a thickness of 1 mm. [ABSTRACT FROM AUTHOR]

To design a single-component absorber with optimizing reflections loss and tunable absorption frequency, the unique lightweight MnO 2 microspheres are successfully synthesized by a template-free method. The as-synthesized MnO 2 microspheres are of mesoporous in diverse specific surface area. By controlling the pH value of the reacting solution, the specific surface area of MnO 2 microspheres is adjustable. The maximum specific surface area of MnO 2 microspheres with mesoporous nature being 90.52 m2/g for specimens is obtained from solution of pH = 5. The complex permittivity and permeability of manganese dioxide are tailored to obtain proper impedance matching. Thus, the resultant MnO 2 microspheres exhibit remarkable microwave characteristic in a wax matrix. The minimum reflection loss reaches −31.79 dB at 14.60 GHz, with the effective absorbing bandwidth below −10 dB being 5.78 GHz with a 2 mm thickness. The effective absorption broadband also changes from 10.27 to 13.75 GHz to 12.56–18.00 GHz as the increasing pH value. Such an outstanding microwave absorption performance ascribes to the unique mesoporous structure with designable specific surface area, giving rise to the multiple scattering and reflecting, interfacial polarization, and dielectric relaxation from the abundant interfaces and curved surfaces and impedance matching. Unlabelled Image • Mesoporous MnO 2 microspheres are facilely synthesized instead of tedious process. • Diverse specific surface area is obtained through adjusting pH of reaction system. • Specific surface area controls property and tunable absorption frequency response. • Maximum specific surface area is 90.52 m2/g, pore size concentrates on 4.909 nm. • Minimum reflection loss reaches −31.79 dB with effective bandwidth 5.78 GHz at 2 mm. [ABSTRACT FROM AUTHOR]

Highlights • The carbonyl iron/graphene sheets/polyurethane ternary coatings were prepared. • The graphene sheets can tune microwave absorption frequencies and enhance the absorption properties of composites coating. • The effect bandwidth (RL<−10 dB) is in the frequency range of 10.9 −18 GHz with a thickness of 1 mm. • The minimum reflection loss is −30.1 dB at 15.6 GHz. Abstract Waterborne polyurethane matrix coatings using the carbonyl iron particles (CIP) and graphene sheets (GNs) as electromagnetic wave absorbing fillers were fabricated. The electromagnetic parameters of CIP and GNs were measured in 2–18 GHz by transmission/reflection technology. The reflection loss (RL) of the coatings was measured by the arch method. The electromagnetic loss mechanisms and the effect of GNs addition on absorption properties and of composites were discussed. And the morphology of fillers and the micrographs of coatings fracture surface were observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results showed that, the unique electromagnetic properties of GNs can make up the shortage of CIP, and the synergistic effects of dielectric/magnetic composites can improve the electromagnetic wave attenuation. In addition, the electromagnetic wave absorption properties can be adjusted by simply changing the content of GNs and CIP particles to obtain the broadband microwave absorption coatings with small thickness. When the mass ratio is CIP:PU:GNs = 2.5:1:0.01, reflection loss values exceeding -10 dB can be obtained in the frequency range of 10.9−18 GHz with a thickness of 1 mm. The models of equivalent circuit and synergistic effect among CIP/GNs/PU composite coating were built to analyze the improvement mechanisms of absorption properties. [ABSTRACT FROM AUTHOR]

High-entropy alloys (HEAs) with nanoscale crystal structure have supposed to be good microwave absorbers due to their excellent soft magnetic properties. Annealing is typically employed to optimize microwave absorption, however, grain coarsening motivated by temperature will lead to serious deterioration of soft magnetic properties. Herein, we introduce the rare earth (RE) element Gd into FeCoCrAlGd x (x = 0, 0.1, 0.2, 0.3) HEAs, which is expecting to inhibit the nanograin coarsening through dispersing Gd 2 O 3. The incorporation of Gd results in a low coercivity (H c), high saturation magnetization (M s), and high complex permeability (μ′ and μ″) of the HEAs. Especially, the H c for as-annealed FeCoCrAlGd 0.2 (114 Oe) is only 60.6 % of that for as-annealed FeCoCrAl (188 Oe). Owing to the remarkable magnetic properties, all Gd-doped HEAs have a larger effective absorption bandwidth at lower thicknesses. The reflection loss (RL) of as-annealed FeCoCrAlGd 0.1 reaches a maximum absorption of −45 dB and an effective absorption bandwidth of 5 GHz (thickness is 1.5 mm). In addition, DFT (density functional theory) calculations are performed to analyze the energies and magnetic moments of the systems. This article provides guidance for design of high performance soft magnetic microwave absorbers. • Coarsening behavior of nanograins during annealing is suppressed by dispersed Gd 2 O 3 particles. • Gd-doped high-entropy alloys have significantly lower coercivity and higher saturation magnetization. • Static magnetic properties induce higher complex permeability and excellent microwave absorption. • The energies and magnetic moments of the systems were calculated by density functional theory (DFT) calculations. [ABSTRACT FROM AUTHOR]

Synthetic fractals have a large potential to achieve tunability and broadband serviceability for multimodal microwave devices owing to their scale-invariant property that generate strongly enhanced local fields with multiscale spatial distributions over multi-spectral ranges. Herein, we demonstrate a microwave metasurface absorber consist of periodic supercells of Fibonacci spiral capable of achieving highly efficient absorptions in a certain bandwidth and several discrete frequencies. Multiple absorption modes are achieved through the synergistic effect of multiple LC-resonances and cross-coupling of the patterned elements, and a broadband operation is completed by adjusting the thickness of dielectric layer based on interface interference theory. Experimental microwave average absorptivity over 82.9% (reflection loss, RL ≤ −7 dB) covering the 10.82–14.18 GHz region is obtained and the maximum absorptivity exceeds 99.8% with sub-wavelength thickness (0.039 λ 0). These results demonstrate that the synthetic fractal metasurface can be a good candidate absorber for microwave applications like sensing, multiband detecting and filtering. • A compact multiband fractal metamaterial absorber is realized by frequency shift and multi-resonance response. • Multi-narrow-band absorption with high Q factor and relative wide-band absorption (∼ 3.36GHz) are achieved simultaneously. • The inversely proportional logarithmic relationship between resonance frequency and initial arc radius is confirmed. [ABSTRACT FROM AUTHOR]

β-MnO 2 microncube is a novel morphology of microncube with rectangular pyramid. It has been synthesized for the first time via a simple aqueous chemical route under hydrothermal conditions without any templates. When the reaction time is increased from 1 h to 10 h, the crystal type changed from γ-MnO 2 to β-MnO 2 and the morphology changed from an urchin-like shape to a microncube with rectangular pyramid. We propose a possible growth mechanism for intrinsic structural growth mechanism, the evolution process of the β-MnO 2 structure is proposed to involve two main stages. Besides, the electromagnetic properties of the synthesized β-MnO 2 were investigated. It shows that β-MnO 2 has certain dielectric loss properties and an effective absorption bandwidth (reflection loss lower than − 10 dB) of 3.5 GHz was achieved from the β-MnO 2 /paraffin wax composite. [ABSTRACT FROM AUTHOR]

Microsized MnN powders have been prepared via a convenient route by the reaction of pure electrolytic manganese powder and ammonia in a tube furnace at 900 °C. The microstructure, morphology, thermal stability, magnetism as well as electromagnetic properties were characterized by powder X-ray diffraction, scanning electron microscopy, thermogravimetric analysis, vibrating sample magnetometer and vector network analyzer. The enhanced relative complex permeability, complex permittivity and static magnetic were achieved when compared with pure manganese. Moreover, MnN had good thermal stability and oxidation resistance below 300 °C in air. A possible mechanism of the improved properties of MnN was discussed based on first-principles calculations. The results revealed that MnN could be a potential microwave absorbent. [ABSTRACT FROM AUTHOR]

• Fe/MnO 2 @C composites with unique Schottky heterojunction have been designed by a simple modified arc-discharge method. • Compared to binary composite, ternary Fe/MnO 2 @C composite show attractive advantages in microwave absorption performs. • A maximum reflection loss of − 53.6 dB is observed at 2 mm thickness. And the effective bandwidth reaches 6.35 GHz. The ubiquitous and further deterioration of electromagnetic environment has put forward higher demand for microwave absorption materials with the rapid advancement of 5 G communication and artificial intelligence. Rational manipulation of materials with unique heterojunctions and adjustable electromagnetic properties as lightweight and high-efficiency microwave absorption is the focus and difficulty of research at home and abroad. Schottky junction is designed and fabricated via implanting Fe/MnO 2 nanocrystal confined in carbon matrix, wherein core nanoclusters are tailored. Manganese oxide coupling magnetic metallic iron lamellar nanocrystal benefits the improvement of static magnetization and tuning graphitization degree. The fascinating heterojunction by modifying a Schottky junction can not only enable balance among loss capacity and impedance matching, but also can accelerate the carrier migration and promote abundant interfaces. The efficient microwave absorption is optimized yielding maximum reflection loss of − 53.60 dB and broadening the effective bandwidth of 6.35 GHz. Furthermore, tailoring the morphology of implanted core nanoclusters successfully realizes selective-frequency of microwave absorption. The ternary Fe/MnO 2 @C composite formed by the self-assembly of Fe/MnO 2 nanocrystal core and carbon shell show superior advantages compared to binary composite (e.g., Fe@C, Fe@MnO 2 , MnO 2 @C) in microwave absorption performance. Thus, the performances of appropriately tailored nanoclusters with unique architecture endow optimal synergistic effects between magnetic and dielectric loss. [ABSTRACT FROM AUTHOR]

Mn doped with various loadings of nitrogen was prepared by nitridation of a micron α-Mn powder in a flowing ammonia atmosphere at a range of temperatures from 400 to 900 °C. X-ray diffraction (XRD) studies reveal that doping nitrogen content increases as the temperature rising from 400 to 600 °C and then decreases from 600 to 900 °C. A mass of ε-MnN started to form during the heat treatment of α-Mn powder at 900 °C. The structural morphology and grain size of the products were carried out using scanning electron microscope (SEM). Vibration sample magnetometer (VSM) and vector network analyzer (VNA) were employed to investigate the effect of nitrogen doping on the magnetic and electromagnetic performance of the as-synthesized samples. Odd circles were found in the $\varepsilon ^{\prime } - \varepsilon ^{\prime \prime }$ figure, which may come from the interaction between electron charge and applied field. In addition, sphere Landau-Lifshitz-Gilbert (LLG) equation fitting approach is adopted to explain the magnetic resonance of the 900 °C sample. The composite of the 900 °C sample exhibits a wider frequency range for microwave absorption applications than other composites. The sample with a thickness of 7 mm shows two absorption peaks at about 3.6 and 14.48 GHz with the maximum reflection loss of −28.71 dB at 11.18 GHz and the absorption range below −5 dB from 2 to 18 GHz. [ABSTRACT FROM AUTHOR]

[Display omitted] • Basalt coated by NiO is synthesized by water bath method followed by sintering process. • The enhanced microwave absorption for basalt@NiO heterostructure has been achieved when sintered at 300 °C. • An amount of vacancies defects and large heterogeneous interface in basalt@NiO-300 °C improve the dielectric loss. • Basalt@NiO heterostructure also has low density, high temperature resistance and excellent anticorrosion. The heterostructure has attracted widespread attention in improving microwave absorption due to the unique composition and structure. Herein, a novel two-dimensional (2D) plate@shell basalt@NiO heterostructure is synthesized through a sequential process of water bath method followed by a sintered operation. The sintering temperature helps regulate the microwave absorption performance by controlling the heterogeneous interface and vacancy defects of basalt@NiO to optimize the dielectric loss. The RL of the prepared basalt@NiO with the sintering temperature of 300 °C reaches up to −15.93 dB at 16.13 GHz, which is attributed to the polarization loss enhanced by vacancy defects (O, Ni vacancies) introduced by low-temperature sintering and large heterogeneous contacts constructed by basalt and NiO. Meanwhile, the high concentration of vacancy defects also leads to a large electrical conductivity that positively contributes to dielectric loss. Furthermore, the basalt@NiO composite possesses relatively low density, high thermal stability and anticorrosion. This finding provides a way for designing the light microwave absorption materials with excellent environmental stability. [ABSTRACT FROM AUTHOR]

Chemically synthesized α-MnO2 powder was annealed at different temperatures with (NH4)2S2O8 as the oxidant and MnSO4 as the manganese source. The obtained samples were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and vector network analysis. Results indicated that the structures and morphologies of the products did not change remarkably, mainly consisting of α-MnO2 with a sea urchin-like shape, when the annealing temperature was below 500 °C. However, it was observed that the powder changed from α-MnO2 to Mn2O3 at 500 °C and 600 °C. The scanning electron microscopy images showed that the morphologies of the resulted Mn2O3 powder were microspheres in various sizes. Microwave absorption properties of the as-synthesized samples were also discussed, which showed that the dielectric loss tangent, magnetic loss tangent and reflection loss could be improved significantly by increasing the annealing temperature appropriately (<500 °C). [ABSTRACT FROM PUBLISHER]

Abstract: The electromagnetic and microwave absorption properties of the composites employing FeSi alloy powders with different particle sizes as absorbent and paraffin as matrix were investigated. The results showed that the particle size had significant influence on the electromagnetic and microwave absorption properties of the composites in the 2–7GHz frequency range. By decreasing the particle size of FeSi alloy powders, both the complex permittivity and permeability of the composites increased to a certain extent. In addition, the microwave absorption properties were improved, and the frequency of absorption peak shifted towards lower frequency range. In other words, the micron-grade FeSi alloy powders with smaller particle size were more suitable to be used as absorbent in measured frequency region. [Copyright &y& Elsevier]

Abstract: To prevent serious electromagnetic interference, a single-layer wave-absorbing coating employing complex absorbents composed of carbonyl-iron powder (CIP) and carbon black (CB) with epoxy resin as matrix was prepared. The morphologies of CIP and CB were characterized by scanning electron microscope (SEM) and transmission electron microscope (TEM), respectively. The electromagnetic parameters of CIP and CB were measured in the frequency range of 2–18GHz by transmission/reflection technology, and the electromagnetic loss mechanisms of the two particles were discussed, respectively. The microwave absorption properties of the coatings were investigated by measuring reflection loss (RL) using arch method. The effects of CIP ratio, CB content and thickness on the microwave absorption properties were discussed, respectively. The results showed that the higher thickness, CIP or CB content could make the absorption band shift towards the lower frequency range. Significantly, the wave-absorbing coating could be applied in different frequency ranges according to actual demand by controlling the content of CIP or CB in composites. [Copyright &y& Elsevier]

The complex permittivity, permeability and microwave-absorbing properties of rubber composites filled with carbonyl iron are measured at frequencies from 2-18 GHz. The results indicate that the reflection loss peak shifts towards low frequency region with increasing layer thickness or increasing weight concentration. The minimum reflection loss value of −23.06 dB was obtained at 3.3 GHz for the composites with 80% wt. These results show that the composites possess good microwave absorbing ability in both low- and highfrequency bands. [ABSTRACT FROM AUTHOR]

Abstract: To solve more and more serious electromagnetic interference problem, one thin microwave absorbing sheet employing carbonyl-iron powder (CIP) and chlorinated polyethylene (CPE) was prepared. The pattern, static magnetic properties and phase of CIP were characterized by scanning electron microscope (SEM), vibrating sample magnetometer (VSM) and X-ray diffraction (XRD), respectively. The electromagnetic parameters of CIP were measured in the frequency range of 2–18GHz, and the electromagnetic loss mechanisms of the powder were discussed. The microwave absorption properties of composite sheets with different thicknesses and CIP ratios in matrix were investigated by measuring reflection loss (RL) in 2–18GHz frequency range using the arch method. The results showed that appropriate CIP content and thickness could greatly improve microwave absorption properties in lower frequency range. For the sample with the weight ratio (CIP:CPE) of 16:1 and 1.5mm thickness, the bandwidth (RL below −10dB) achieved 1.1GHz (2–3.1GHz), and the minimum reflection loss value was obtained −13.2dB at 2.2GHz. This suggested that CIP/CPE composites could be applied as thin microwave absorbers in S-band (2–4GHz). [Copyright &y& Elsevier]

Perfect stealth has attracted humanity for decades owing to the advent of metamaterials, however, despite long-standing efforts in design and fabrication, it has been challenging to achieve ultra-broadband absorption under deep-subwavelength thickness, the fundamental hurdle is Plank-Rozanov limit, which hindering the development of stealth materials. Herein, inspired by the absorption model of Pachliopta aristolochiae, present works demonstrate an approach with composition concept to ideal stealth, through the synergy of microwave melanin and bionic metastructure, the fabricated biomimetic metamaterials exhibit effective absorption over 2–18 GHz range under 0.04 λ max thickness for radar stealth, which breakthrough the Plank-Rozanov limit by utilizing the enhanced resonance of bionic metastructure in absorption, corresponding mechanisms are demonstrated by simulation models. The metamaterials also robustness for arrangement and polarizations, compatible infrared emissivity is reduced to 0.43, as well as flexibility and impact resistance during minus temperature. The principle behind here is universal and can be extended to many metamaterials and a broad range of absorption. Image 1 [ABSTRACT FROM AUTHOR]

Building core@shell nanostructures and doping ions are two important strategies to optimize the performance for the microwave impedance matching and attenuation. In this research, the Ag-doped α-MnO 2 @δ-MnO 2 was successfully synthesized by combining hydrothermal reaction and chemical deposition reaction. Driven by the lowest surface energy principle, all the δ-MnO 2 nanosheets nucleate on the surface of α-MnO 2 nanowires. During this process, the edge of nanosheets attach on the α-MnO 2 nanowire with low lattice mismatch of δ (201)//α (211) (nearly 0) and vertically grow on the nanowires surfaces, then the assembled core-shell composites can be prepared. The core-shell composites exhibit synergistic effect and improved the impedance matching, so that the absorbing performance has been obtained. The composite shows the minimum reflection loss (RL) of −53.67 dB at 13.28 GHz for 1.8 mm thickness, and the broadest effective absorbing bandwidth (RL < −10 dB) is 5.44 GHz as the thickness was 1.9 mm. The excellent performance is come from the preferable impedance matching, the large heterogeneous existence of the interfaces and the numerous transmission paths for microwave. Image 1 [ABSTRACT FROM AUTHOR]

Highlights: The crystalline Fe/MnO@C core–shell nanocapsules embedded in porous amorphous carbon matrix (FMCA) was prepared by a novel confinement strategy of modified arc-discharge method. The heterogeneous crystalline–amorphous nanocrystals disperse evenly and exhibit improvement of static magnetization and excellent electromagnetic absorption properties. The adding MnO2 confines degree of graphitization and contributes to form amorphous carbon. Dielectric loss and microwave absorption are achieved adjustable. Crystalline Fe/MnO@C core–shell nanocapsules inlaid in porous amorphous carbon matrix (FMCA) was synthesized successfully with a novel confinement strategy. The heterogeneous Fe/MnO nanocrystals are with approximate single-domain size which gives rise to natural resonance in 2–18 GHz. The addition of MnO2 confines degree of graphitization catalyzed by iron and contributes to the formation of amorphous carbon. The heterogeneous materials composed of crystalline–amorphous structures disperse evenly and its density is significantly reduced on account of porous properties. Meanwhile, adjustable dielectric loss is achieved by interrupting Fe core aggregation and stacking graphene conductive network. The dielectric loss synergistically with magnetic loss endows the FMCA enhanced absorption. The optimal reflection loss (RL) is up to − 45 dB, and the effective bandwidth (RL < − 10 dB) is 5.0 GHz with 2.0 mm thickness. The proposed confinement strategy not only lays the foundation for designing high-performance microwave absorber, but also offers a general duty synthesis method for heterogeneous crystalline–amorphous composites with tunable composition in other fields. [ABSTRACT FROM AUTHOR]

The microwave absorbers developed so far encounter a bottleneck for the constraint of absorption by deep sub-wavelength thickness. In this work, we report enhanced and tunable structural composite metamaterials, which were fabricated by parallel and orthogonal arrayed Fe-Si alloy ribbons sandwiched between two layers of carbonyl iron powder/polyurethane matrix. It demonstrated a minimum resonance peak value −66.90 dB under deep sub-wavelength thickness (1.25 mm), an extraordinary result that indicated almost complete absorption. The absorption of 90% (reflection loss (RL) < −10 dB) bandwidth covered 10.2 to 18 GHz, almost the entire band of test. The absorbing mechanisms were explored by the simulation models that verified the coupling in the ribbons and the matrix. The composite metamaterials have adaptability during various bending, curling and impact conditions, and the radar-infrared compatible stealth property could also be reached. These results demonstrated that the composite metastructure provides new possibilities for the manufacture of high-performance microwave absorbing materials. [ABSTRACT FROM AUTHOR]

In this paper, polyaniline/graphene oxide composites were prepared by a one-step intercalation polymerization of aniline in the presence of GO layers under different mechanical agitation duration period, and the formation mechanism and microwave absorbing mechanism were investigated for an entire hierarchical sandwich-like structured polyaniline/graphene oxide composites. The morphology and structure were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy, Brunauer-Emmett-Teller (BET) nitrogen sorption-desorption measurement and vector network analysis (VNA). The results revealed that mechanical agitation duration promoted the heterogeneous nucleation of aniline on graphene oxide layers and induced the formation of PANI-GO-PANI sandwich-like microstructure. The minimum reflection loss of entire sandwich-like structure reaches −28.12 dB at 5.675 GHz and an ultra-wide effective absorbing bandwidth (RL < −10 dB) over 13.175 GHz from 4.825 GHz to 18 GHz with the thickness varies from 2 mm to 5.5 mm, performing much better than pristine PANI and mixtures of PANI nanorods and sandwich-like structure. This paper reveals that constructing a hierarchical microstructure can be a promising method for developing novel microwave absorbing materials. [ABSTRACT FROM AUTHOR]

Carbon nanotubes (CNT) coated with magnetic nanoparticles are promising microwave absorbing materials with high performance. Despite several previous investigations, the relationship between intrinsic composition and preparation conditions of this composite material to its microwave loss mechanisms and absorbing properties have not yet been fully understood. We present a convenient and cost-effective hydrothermal method to deposit crystalline manganese iron oxide (MnFe 2 O 4) over CNT. Adjusting the hydrothermal reaction time and CNT content allows control over the microstructure of the resulting CNT-MnFe 2 O 4 nanocomposite and its microwave absorbing properties, resulting in a better microwave absorber than pure MnFe 2 O 4. Composites synthesized at 150 °C for 10 h with 10 wt% CNT reveal a reflection loss as high as −38 dB at 5.7 GHz for an absorber thickness of 5 mm and frequency bandwidth (RL < −10 dB) up to 3.59 GHz. As the hydrothermal reaction time is reduced to 1 h, the minimum reflection loss increases significantly to −41 dB at 11.8 GHz for a 2 mm coating thickness. Defects and functional groups on the surfaces of CNT act as nucleation sites for MnFe 2 O 4 and help reduce agglomeration, which decreases overall particle size. Microwave absorbing mechanisms are also discussed. Coating CNTs with manganese iron oxide increases microwave dissipation by enhancing conduction loss, interface polarization, relaxation polarization, and magnetic loss, including the contributions of eddy current and natural resonance, which are all related to the reaction time and CNT content. Consequently, the synthesis-property relationship could potentially be used to design CNT-based nanomaterials that have more effective microwave absorption. Unlabelled Image • A convenient and cost-effective method to deposit crystalline MnFe 2 O 4 over CNT • The minimum reflection loss obtained is −41 dB at 11.8 GHz. • Absorption bandwith of ~3.7 GHz with RL < −10 dB is observed at multiple frequencies. • The synthesis-property relationship of CNT-based nanomaterials is discussed. [ABSTRACT FROM AUTHOR]

• Microspheres composed of MnO 2 nanofibers are synthesized via a facile one-step method. • Nickel doping introduces defects in the MnO 2 , which enhanced microwave absorption. • The microspheres show tunable microwave absorption in the X and Ku bands. • A maximum reflection loss of −40 dB is observed at 8.31 GHz. Pure dielectric absorbers with a tunable wide-band response and strong attenuation capability for electromagnetic wave (EMW) absorption are challenging to synthesize. Herein, a series of hierarchical MnO 2 microspheres composed of MnO 2 nanofibers are synthesized via a simple one-step hydrothermal method. By varying the hydrothermal synthesis conditions, the surface architectures of the MnO 2 microspheres could be adjusted, thereby tuning their EMW absorption performance. In addition, nickel doping causes defects in the MnO 2 nanofibers, which further boosts microwave absorption performance. Compared to 2D MnO 2 nanorods, the hierarchical microspheres formed by the self-assembly of MnO 2 nanofibers offer tunable microwave absorption in the X and Ku bands. The Ni-doped microspheres with the optimized morphology show a maximum reflection loss of −40 dB is observed at 8.31 GHz, and an effective absorption bandwidth of 6.1 GHz at 2 mm thickness. [ABSTRACT FROM AUTHOR]

Due to the growing harm of electromagnetic radiation, it is extremely urgent to develop high-efficiency microwave absorbers with tunable electromagnetic properties. Herein, a simple strategy for synthesizing hetero-atoms (sulfur) doped carbon-coated FeNi alloy nanocapsules (FeNi@C(S x) NCs) by DC arc-discharge plasma was demonstrated. The hetero-interface between the FeNi alloy core and the graphitic shell induce interfacial polarization was found. Meantime, many defects produced by substitutional sulfur atoms in the graphitic shell facilitated dipole polarization and conduction loss. Besides, the multiple magnetic resonance produced by FeNi magnetic alloys worked together to optimize impedance matching. The excellent microwave absorption performance was attributed to the core-shell structure, dipole and interface polarizations, as well as favorable impedance matching. Importantly, the first-principles calculation was applied to prove the charge transfer and polarization process. The minimum reflection loss (RL) can reach up to −50.3 dB at a thickness of 1.6 mm. [Display omitted] • S-doped graphitic shells coated FeNi obtained by one-step arc discharge method. • S doping into the graphitic shells enhance dipole polarization. • S-doped graphitic shells can improve the conduction loss. • The DFT is applied to prove the charge transfer process. • The RL value reaches up to −50.3 dB with a thickness of 1.6 mm. [ABSTRACT FROM AUTHOR]

Abundant resources, light weight as well as stable physicochemical properties enable carbon nanomaterials promising candidates toward microwave absorption. However, it is still a great challenge for carbon-based absorbers to achieve broad frequency bandwidth and strong absorption, which fundamentally ascribes to the poor impedance matching resulted by their comparatively high electrical conductivities. Herein, a feasible and high-yield method has been employed to the in-situ synthesis of N-doped graphene nanoflakes which can effectively address interfacial impedance mismatching, and realize the majority of microwaves penetration into the interior of absorber. The incorporation of substitutional N atoms into graphene lattices markedly weaken crystallization degree and introduce masses of defects, directly leading to the decline of electric conductivity, meanwhile benefiting the improvement of static magnetization. Our findings indicate that compared with pure graphene nanoflakes, the counterpart containing 4.6 at.% of nitrogen exhibits an excellent absorption capability, in which more than 99% of microwave energy can be quantitatively attenuated at 5–18 GHz. Experimental results coupled with theory calculations further elucidate that such high performance essentially originates from the proper impedance matching constructed in the N-doped graphene nanoflakes. Image 1091 [ABSTRACT FROM AUTHOR]

Influences of the arrayed carbon fibers (CFs) and gradiently dispersed Fe nanoparticles (NPs) inside epoxy resin (ER) matrix on the microwave absorption and mechanical properties were investigated. As a microwave absorbent, the Fe NPs were synthesized by an arc discharge plasma method and subsequently surface-modified by using of silane coupling agent (KH-550) to improve their dispersion in organic ER matrix. To measure the naturalistic electromagnetic and mechanical properties of such nanocomposite plates, a series of square plates (20 × 20 cm 2 ) were fabricated by filling the modified Fe NPs (30 wt.%) with/without orientated CFs at different mass percentages (0, 1.38 wt.%, 2.76 wt.% and 5.52 wt.%) into the ER matrix. The excellent microwave absorption of nanocomposite plate occurred as the direction of CFs was vertical to that of incident microwave, in such case the multi-reflections of microwave were caused by CFs and favors to create a great absorption probability to Fe NPs. A well matched input impedance of the plate to air is necessary for the effective entrance of incident microwave and to be attenuated by the structural resonance and electromagnetic losses. The input impedance of nanocomposite plate is mainly determined by the content of absorbent (Fe NPs with a gradient dispersion), CFs (with orientations to incident microwave, i . e . vertical, parallel or perpendicularly cross-linked to each other) and the geometric configuration (sizes of the plate, distribution of each components, etc.). It was figured out that the nanocomposite plate (30 wt.% of Fe NPs, 5.52 wt.% of CFs vertical to incident microwave) exhibits a higher reflection loss (RL) of −26.8 dB at 4.9 GHz, in which the structural resonances, appropriate conductivity, dielectric polarization and impedance matching were involved. The effects of gradiently dispersed Fe NPs and the arrayed CFs with orientations on the electromagnetic and mechanical properties were emphasized and investigated. [ABSTRACT FROM AUTHOR]

[Display omitted] • A hierarchical multi-layered CNT-NiFe 2 O 4 @MnO 2 is produced though a two-step wet chemistry method followed by calcination. • CNTs-NiFe 2 O 4 nanoparticles are uniformly covered by in situ deposited MnO 2 layer. • The differences between the constant and alternating magnetic properties are attributed to the rearrangement of nanoparticles on CNT skeleton. • The structure effect increases dielectric loss and natural resonance by increasing phase interface, constructing conductive mesh and inducing magnetic coupling. The assembly of magnetic and dielectric materials around carbon nanotubes (CNTs) can create specific microstructures, leading to materials with relatively high microwave absorbing properties. Here, through a facile process, nickel iron oxide (NiFe 2 O 4) is synthesized and placed on the surface of CNTs and this material is covered with manganese oxide (MnO 2). In this hierarchical multi-layered nanomaterial (CNT-NiFe 2 O 4 @MnO 2) produced though a two-step wet chemistry method followed by calcination at 200 ℃, CNTs are uniformly covered by NiFe 2 O 4 nanoparticles formed in situ over which the MnO 2 layer is deposited. Although CNT-NiFe 2 O 4 @MnO 2 exhibits weaker soft magnetic performance, its magnetic attenuation is higher than for CNT-NiFe 2 O 4. Structural effects produce differences between the constant and alternating magnetic properties of these materials. With a reflection loss (RL) of −17 dB and effective absorption bandwidth of 3.8 GHz, the microwave absorption of CNT-NiFe 2 O 4 @MnO 2 is superior to that of CNTs and CNT-NiFe 2 O 4. The hierarchical multi-layered structure increases the phase interface area and rearranges magnetic particles, leading respectively to higher interface polarization and natural resonance. The conductive mesh structure induces magnetic coupling which attenuates the electromagnetic field. This material employs a new approach to design and synthesize carbon-based hierarchical multi-layered microstructures with high microwave absorption. [ABSTRACT FROM AUTHOR]