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To address the increasing demand for high microwave‐absorbing performance, new polarization‐response wave‐absorbing metamaterials are developed herein inspired by the microstructure of locust compound eyes by employing bionics and optimization principles. The materials overcome a common limitation of wave‐absorbing metamaterials, wherein the performance substantially decreases with changes in the polarization mode, enabling the broadband absorption for x and y polarizations. The designed bioinspired materials have various shapes of structural units, triggering a multifrequency point response mechanism that effectively improves the wave‐absorbing performance. Experimental and simulation results show that the designed materials can form different equivalent circuits based on structural and polarization differences, influencing the overall wave‐absorbing performance. They exhibit an effective absorption bandwidth of ≈12 GHz (reflection loss < −10 dB) for x or y polarizations at a thickness of 2 mm. This study provides a typical example for future research on wave‐absorbing metamaterials with polarization‐response characteristics. [ABSTRACT FROM AUTHOR]

There is an urgent need to develop microwave absorbing materials with ultra‐broadband absorption and multiband stealth compatibility, but this is difficult to achieve due to the difficulty of perfect electromagnetic impedance matching. Therefore, inspired by the anti‐reflection moth eye model, this article prepares carbonyl iron (CIP)/polyurethane (PU) absorbing materials into three biomimetic structures of cylinder, circular cone, and semi‐ellipsoid structures to optimize the absorption performance, which realizes the effective absorption of 5.13–18 GHz. The simulation models proved that the biomimetic structures can improve the anti‐reflection performance by building gradient impedance inside the materials. Finally, under the synergistic action of multiple reflection and gradient impedance mechanisms, the effective absorption bandwidth (EAB) of the semi‐ellipsoid moth eye biomimetic structure material with a total thickness of 3 mm can reach 6.94 GHz. The material can still achieve the EAB of 6.84 GHz with a low infrared emissivity of 0.242 by constructing the double‐layer structure. This work effectively solves the problem that the EAB of traditional microwave absorbing materials is difficult to be broadened due to impedance mismatch, and puts forward a new idea for optimizing the performance of microwave absorbing materials and compatible stealth. [ABSTRACT FROM AUTHOR]

Objectives: Respiratory syncytial virus (RSV) is a leading cause of lower respiratory tract infection in young children. We aimed to analyze the factors affecting the estimation of RSV‐related disease burden, and to provide evidence to help establish a surveillance system. Methods: We searched the English‐ and Chinese‐language databases for articles published between January 1, 2010 and June 2, 2022. The quality of the included articles was assessed using the Agency for Healthcare Research and Quality scale. Random‐effects models were used for data synthesis and subgroup analyses. This review was registered in the Prospective Register of Systematic Reviews (PROSPERO: CRD42022372972). Results: We included 44 studies (149,321,171 participants), all of which were of medium or high quality. The pooled RSV‐related disease incidence, hospitalization rate, in‐hospital mortality, and overall mortality rates in children aged 5 years and younger were 9.0 per 100 children per year (95% confidence interval [CI]: 7.0–11.0), 1.7 per 100 children per year (95% CI: 1.3–2.1), 0.5 per 100 children per year (95% CI: 0.4–0.5), and 0.05 per 100 children per year (95% CI: 0.04–0.06), respectively. Age, economics, surveillance types, case definition, and data source were all recognized as influencing factors. Conclusions: A standardized and unified RSV surveillance system is required. Case definition and surveillance types should be fully considered for surveillance of different age groups. [ABSTRACT FROM AUTHOR]

[Display omitted] The compound eyes of the dragonfly, Pantala flavescens Fabricius, are covered by micro-scaled ocelli capable of sensing polarized light, an attractive property for radar stealth and counterreconnaissance. In this work, we fabricated biomimetic electromagnetic wave absorption materials (EAMs) by analyzing the covert information identifications of biological systems and focusing on the design of metastructures and microstructures. Several bionic metasurfaces with anisotropic double-V meta atoms made up of (FeCoNiSi 8.9 Al 8.9)C 0.2 high-entropy-alloy (HEA) ribbons for multiple-degrees-of-freedom recognition and broadband absorption are presented. The covert phase, amplitude, and angular momentum of electromagnetic waves were controlled and recognized as information by manipulating the rotation angle θ of meta atoms. A vortex wave with a topological charge of 1 was generated to recognize linearly polarization and left- and right-handed circular polarization. In addition, the polarization conversion enhanced absorption. The hierarchical nanostructures of HEA ribbons give rise to suitable electromagnetic loss and a superior impedance match. Finally, inspired by the structure of compound eyes, the designed multilayer metamaterials realized effective absorption (reflection loss (RL) ≤ − 10 dB) within the 4.5–18 GHz regime under 2.8 mm thickness. These materials provide evidence for a new way for integrated EAMs and metamaterials. [ABSTRACT FROM AUTHOR]

Highlights: Nanocrystalline high-entropy alloy powders (HEAs) with the unprecedented combination of superior manufacturability, large aspect ratios, and thin intergranular amorphous layers are constructed via a multistage mechanical alloying strategy. FeCoNiCr0.4Cu0.2 HEAs for 20 h of dry milling and 5 h of wet milling (D20) exhibit excellent corrosion resistance, high Curie temperature, and temperature-stable broadband megahertz electromagnetic wave absorption.Developing megahertz (MHz) electromagnetic wave (EMW) absorption materials with broadband absorption, multi-temperature adaptability, and facile preparation method remains a challenge. Herein, nanocrystalline FeCoNiCr0.4Cu0.2 high-entropy alloy powders (HEAs) with both large aspect ratios and thin intergranular amorphous layers are constructed by a multistage mechanical alloying strategy, aiming to achieve excellent and temperature-stable permeability and EMW absorption. A single-phase face-centered cubic structure with good ductility and high crystallinity is obtained as wet milling precursors, via precisely controlling dry milling time. Then, HEAs are flattened to improve aspect ratios by synergistically regulating wet milling time. FeCoNiCr0.4Cu0.2 HEAs with dry milling 20 h and wet milling 5 h (D20) exhibit higher and more stable permeability because of larger aspect ratios and thinner intergranular amorphous layers. The maximum reflection loss (RL) of D20/SiO2 composites is greater than − 7 dB with 5 mm thickness, and EMW absorption bandwidth (RL < − 7 dB) can maintain between 523 and 600 MHz from − 50 to 150 °C. Furthermore, relying on the “cocktail effect” of HEAs, D20 sample also exhibits excellent corrosion resistance and high Curie temperature. This work provides a facile and tunable strategy to design MHz electromagnetic absorbers with temperature stability, broadband, and resistance to harsh environments. [ABSTRACT FROM AUTHOR]

Polarization conversion of chiral ordered structures can achieve effective electromagnetic attenuation through flexible field rotation. However, the mechanism of near‐field coupling generation on which it depends remains unclear, hindering the development of chiral wave‐absorbing materials. In this work, a beetle‐inspired ordered structure, which is a kind of chiral metamaterials (CMMs) consisting of high entropy alloy (HEA) electromagnetic dissipative unit and media‐layer, is rationally designed. Such CMMs show that the coupling and decoupling effects can achieve dynamic conversion between linearly polarized waves and circularly polarized waves, and the mechanisms analyzed and verified by simulation models confirm that as a universal method. The fabricated biomimetic metamaterials show desirable absorption with effective absorption bandwidth of 4.48 GHz at 2 mm interlayer medium thickness (≥75% absorptivity, reflection loss ≤ −6 dB) and minimum reflection loss (RLmin) of −53.6 dB. Moreover, pasting the flexible CMMs conformally on conventional coating surfaces can boost microwave‐invisible capacity, whose absorption bandwidth with RL lower than −6 dB increases to 5.18 GHz over HEA coating. In short, this work elucidates the mechanism of chiral near‐field coupling and provides a novel solution principle and methodology for using CMMs in wave‐absorbing materials. [ABSTRACT FROM AUTHOR]

Potassium-ion-doped MnO2 has been successfully synthesized using the hydrothermal method, and the influence of the doped potassium ions on the electrical conductivity and permittivity is studied. X-ray powder diffraction, scanning electron microscopy, electron-probe micro-analysis, and a vector network analyzer are used to perform characterization. The densities of states of doped and undoped MnO2 tunnel structures are also discussed based on first-principles calculations. Results show that the conductivity and dielectric resonance of MnO2 can be elevated by means of K+ doping. The conductivity of K+-doped MnO2 prepared at different reaction times shows a decreasing trend and is generally 1 order of magnitude higher than that of pure MnO2. The electrical conductivity of K+-doped MnO2 (R3) shows the highest value of 3.33 × 10−2 S/cm at the reaction time of 24 h, while that of pure MnO2 is 8.50 × 10−4 S/cm. When treated with acid, the conductivity of samples remains basically stable along with the increase of treatment time. In addition, acid treatment plays a very significant role in controlling the amount of K+ ions in crystals. The K+ contents of acid-treated samples are 5 times lower than that of the untreated R1. The dielectric losses of the samples with different reaction times are enhanced markedly with frequency increment. The complex permittivity of pure MnO2 only exhibits a resonance at ∼12 GHz, while K+-doped MnO2 exhibits another resonance behavior at ∼9 GHz. The capacity of the dielectric property in the net structure is enhanced by the interfacial polarization, dielectric relaxation, multiple internal reflections, and multiple scattering benefiting. [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]

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]

The meta-structural design of flat absorbers demonstrates significant potential for improving microwave attenuation performance. However, the single mechanism of amplitude attenuation through resonance-enhanced effect necessitates considerable material thickness, restricting its application in electromagnetic (EM) defense. Utilizing the synergistic effect of multiple mechanisms is expected to break through the performance limits of microwave stealth metamaterials at the sub-wavelength scale. Herein, absorption-diffusion integration stacked metamaterials (ADISM) were designed using a multi-compound strategy, which comprises bionic porous spherical carbonyl iron powder (SCIP) coatings and stacked vortex metasurfaces (VMs). Such a metamaterial integrates EM scattering modulation with microwave absorption, enabling the creation of a broadband, high-efficiency, low-reflection design, providing an effective absorption bandwidth (EAB) reaching 10.42 GHz (7.58–18 GHz) at a thickness of 2.48 mm. Manipulating the orbital angular momentum (OAM) to effect wavefront transformation, the dissipation of EM wave energy is achieved. The absorption peak frequency modulation and absorption bandwidth broadening are achieved by the intrinsic loss capability of the porous coating, and corresponding mechanisms are demonstrated by simulation models. In short, the synergistic effects of microwave absorption and scattering modulation significantly enhance the return loss capability and facilitate broadband microwave attenuation. This research offers new insights into the multifunctional integration of EM stealth metamaterials. [Display omitted] [ABSTRACT FROM AUTHOR]

Chiral wave-absorbing metamaterials can enhance microwave attenuation performance by converting linearly polarized waves into circularly polarized waves. However, the mechanism for transforming incident waves into vortex waves to overcome the angular momentum mismatch to satisfy broadband stealth is unclear. This work proposes a chiral-assisted geometrically phase (CAGP) metasurfaces that utilizes the Pancharatnam-Berry (PB) phase concept to convert incident plane waves into reflected vortex waves. Such metamaterials demonstrate adaptive transformation capabilities under the incidence of electromagnetic (EM) waves with multi-polarization modes, providing an effective absorption bandwidth (≥90 % absorptivity, reflection loss ≤ −10 dB) reaches 6.92 GHz (9.16–16.08 GHz) at a thickness of 2.48 mm. Based on an orbital angular momentum (OAM) phase and amplitude analysis, the mode purity of the generated vortex waves is discussed in detail, and the mechanisms are analyzed and verified by simulation models to confirm their effectiveness and practicability. Additionally, the metamaterials exhibit excellent radar cross-section (RCS) reduction properties and stable polarization insensitivity properties. In short, this work presents a tailorable metasurface to manipulate the OAM spectrum and achieve broadband microwave attenuation, which holds great potential for various applications in chiral wave-absorbing metamaterials. [Display omitted] [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]

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]

Optimizing impedance matching is crucial to achieve high-efficiency electromagnetic wave (EMW) absorption performance. In this work, Hollow CoSe 2 @N-doped Carbon/Carbon Nanotube-x (HCSC/CNTs-x) with unique three-dimensional conduction network was synthesized by using protection etching and high-temperature selenization strategies. The conductivity of HCSC/CNTs-x was regulated by varying the content of CNTs, and the extended three-dimensional electron transport pathway resulted in enhanced conduction and multiple reflection loss, while also optimizing impedance matching. Hollow CoSe 2 @N-doped carbon (HCSC) further facilitates the optimization of impedance matching. The abundant heterogeneous interfaces and defects induce strong interface polarization and dipole polarization, and the multiple loss mechanisms work in concert to confer excellent EMW absorption performance to HCSC/CNTs-x. The RL min of HCSC/CNTs-30 reaches −41.68 dB, and the EAB max is 4.92 GHz at only 2.0 mm. The design strategy of three-dimensional conduction network of HCSC/CNTs-x provides an effective way to achieve the impedance matching optimization of CoSe 2 -based EMW absorption materials. The unique three-dimensional conduction network of HCSC/CNTs-x was successfully synthesized by protected etching and high-temperature selenization. The extended three-dimensional electron transport path not only enhances the conduction loss and multiple reflection loss, but also optimizes the impedance matching to achieve the harmony between impedance matching and electromagnetic loss capability. Attributed to the synergistic effect of multiple loss mechanisms, HCSC/CNTs-30 has excellent electromagnetic wave absorption performance with the RL min up to −41.68 dB and the EAB max is 4.92 GHz only 2.0 mm. [Display omitted] [ABSTRACT FROM AUTHOR]

Electromagnetic wave loss caused by rotating polarization of chiral structures has long been considered as a disadvantage in transmission, detection, etc., however, it can be turned to benefit in microwave absorbing materials to enhance absorption. Herein, inspired by the chiral structure of jeweled beetles, gyrotropic metamaterials (MMs) are designed and prepared, and experimentally demonstrate the enhanced microwave absorption by rotating polarization; the bandwidth of absorptivity greater than 80% (reflection loss, RL ≤ −7 dB) covers the entire 4–18 GHz frequency range under an ultralow surface density of 1.45 kg m−2; the mechanism analyzed and verified by simulation models demonstrates this a universal method. A new paradigm of MMs fabricated by high entropy alloy (HEA) is offered in this work, which increases the freedom of MM design and promotes the integration of material science and physics. Multifarious wave adaptability and multifunctionality achieved by integrated HEA unit cell and foamed polyurethane realize ultrasonic and water wave absorption, as well as impact and corrosion resistance. These designs hold great promise for facilitating functionally integrated materials for stealth technology, antiterrorist, encryption, sensing, and photon detection. [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]

Hollow microstructures assembled of tetragonal MnO2 nano-columns have been synthesized through a facile hydrothermal method with the introduction of iron ions. The obtained samples were characterized by XRD, SEM, and vector network analysis. Results reveal that the presence of Fe3+ ions leads to transformation of phase structure from α-MnO2 to a mixture of &eh;-MnO2 and α-MnO2, and it is essential to include Fe3+ ions for the formation of the hollow MnO2 microstructures. The formation mechanism was investigated and proposed in detail. Fe-doped MnO2 exhibits decreased dielectric loss, but increased magnetic loss compared with the pure one. [ABSTRACT FROM AUTHOR]

The inability of existing electromagnetic wave absorbing materials (EAMs) to manipulate electromagnetic waves in multiple dimensions leads to the failure to satisfy the demands placed on electromagnetic (EM) defense technology by the current complex EM environment. To break this dilemma, this study focuses on the impedance properties of EAMs, conferring additional ability to disperse and deflect reflected waves by encoding EAMs with opposite phase responses in different impedance mismatch modes. Due to the synergy of both absorption and scattering mechanisms, the developed scattering metamaterial absorber exhibits excellent anti-reflection performance, with reflectivity below 0.1 in the 7.8–16.7 GHz at 2 mm thickness. Furthermore, a genetic algorithm is employed to tailor the desired scattering field to meet the stealth requirements of specific environments, allowing for the directional transmission of EM energy in the one-dimensional or homogeneous distribution in the three-dimensional. The proposed scattering metamaterial absorber constructed by coded EAMs exhibited excellent anti-reflection properties and environmental stealth adaptability, opening up new possibilities for the development of advanced EM defense technology. [ABSTRACT FROM AUTHOR]

Abstract Alloy powders with dual-phase nanocrystal (NC) and nanoglass (NG) have received much attention due to its small coercive force and high permeability, but the preparation of dual-phase high entropy alloy powders (HEAPs) simultaneously with larger aspect ratio constitute a great difficulty in microwave absorbing domain. Here, a new concept has been devised to tune the crystallinity of microwave absorber by regulating the phase proportion of milling precursor. A combination technology containing melting, strip casting, and milling has been adopted to prepare a serious of FeCoNiSi x Al 0.4 HEAPs to verify its correctness. Selective phase transition occurs in dry-milling process, generating various unique microstructures - NCs, NGs, and NGs embedding in NC grain. Besides, BCC phase content of HEAPs can be controlled by the amount of added ethanol. A less coercive force, increasing from 14.05 Oe (C 0.1) to 23.43 Oe (C 0.5), and a large magnetic permeability, varying from 2.41 (C 0.5) to 2.80 (C 0.3) at 1 GHz, can be obtained. Highlights • The crystallinity of powders relate to FCC phase proportion of milling precursor. • The phase structure of nanocrystal alloy ribbons can be tuned by element content. • Co-existed phase transitions promote the formation of dual-phase microstructure. • The variety of crystallinity lead to the change of magnetic properties. [ABSTRACT FROM AUTHOR]

A facile and simple chemical route has been used to synthesize novel three-dimensional (3D) architectures of nickel-doped ε-MnO2 without the addition of any surfactant or organic template. Nickel salt is used directly as the reagent rather than as an additive to produce doped manganese dioxide, which is different from the overwhelming majority of previous synthetic methods for doped MnO2 for use as an electromagnetic wave absorption material. This method overcomes the shortcomings of the previously reported approaches of doping with a slight amount of metallic ion, which is sometimes hard to detect. The chemical composition of the samples is analyzed by electron-probe micro-analysis (EPMA) and energy dispersive spectroscopy (EDS). The chemical state of the elements in the composites is demonstrated with X-ray photoelectron spectroscopy (XPS). The structures of the micro-spheres are detected by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results show that the self-organized crystals are made up of walnut-like spheres and are arrays of polycrystals. The nickel ion is certified to have been successfully doped into the crystal based on the results of EPMA, EDS, and XPS as well as dark field scanning TEM. Thus, a multiple heterojunction structure is constructed. After nickel doping, the crystalline phase remains ε type and the morphology turns into a walnut-like structure. Electromagnetic performances also exhibit significant variation with the introduction of nickel ions. Nickel-doped MnO2 has a decreased dielectric constant compared with that of commercial MnO2, while the nickel-doped MnO2 appears to have fascinating magnetic properties with a maximum magnetic loss tangent value of 0.37, which is 7 times greater than that of the dielectric loss tangent. Likewise, it is further presented that the optimized electromagnetic capacities are related to the mass fraction of the walnut-like MnO2 spheres in the composite. When the mass fraction is as high as 50%, the magnetic loss tangent goes up with a distinct increase in mutations as well as in relaxation times and in the real part and the imaginary part of the relative complex permeability. Furthermore, the mechanisms of the highlighted electromagnetic attenuation are explored in detail. [ABSTRACT FROM AUTHOR]

Abstract The FeCoNiCuAl high-entropy alloy powders are prepared in equimolar ratio by mechanical alloying in different milling time and then annealing at 773 K. The microstructure, morphology, particle size, magnetic and electromagnetic performance of powders were studied by X-ray diffraction (XRD), scanning electron microscope (SEM), laser scattering particle size distribution analyzer, vibrating sample magnetometer (VSM) and vector network analyzer. The results indicate that flaky particles, which formed in the process of ball milling, show a soft magnetic character, as well as, the ranges of coercivity (H c) are 67.6–105.1 O e and the values of saturation magnetization (M s) range from 69.1 emu/g to 94.2 emu/g. As the increasing of the milling time, the real parts of complex permittivity (ε′) increase firstly and then decrease, however, the imaginary parts of complex permittivity (ε″) close to zero for all the samples. Meanwhile, the real parts of complex permeability (μ′) are 1.26–1.56 and the imaginary parts of complex permeability (μ″) are 0.28–0.36 with change of milling time. The reflection loss (RL) of powders can reach −10 dB or less and 30 h-milling sample shows the maximum RL is −19.17 dB at 7.84 GHz with the thickness of 2 mm. After annealing, there appear two new phases, CoFe 2 O 4 and AlFe 3, that improve the values of M s. The increased particles sizes and the enhanced values of Ms of powders all can promote complex permittivity and complex permeability. In addition, the RL also can get increased. These encouraging findings provide the potential advantages of high-entropy alloys for microwave absorption application. Graphical abstract Image 1 Highlights • The mechanical milling method was designed to prepare flaky FeCoNiCuAl HEA powders. • The Cu elements increase the ductility and conductivity of the FeCoNiCuAl HEA powders. • Variety of aspect radio and phases lead to the change of electromagnetic properties for milling and annealing HEA powders. • The maximum reflection loss (RL) can reach −19.17 dB at 7.84 GHz with the thickness of 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]

Abstract Polyaniline (PANI)/graphene oxide (GO) sandwich-like composite was synthesized for achieving higher microwave absorption performance using a one-step intercalation polymerization of aniline under acidic condition in the presence of GO layers with ammonium persulfate as an oxidant at various GO contents. Long-time mechanical stirring was adopted to obtain perfect sandwich-like structure. The samples were characterized by Scanning electron microscopy, Transition electron microscopy, X-ray diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy, 4-point probes resistivity measurements system and vector network analysis. The results revealed that the PANI-GO-PANI sandwich-like microstructures were successfully synthesized. The electromagnetic absorption properties were enhanced versus pristine PANI. The maximum reflection loss (RL) value reached −41.29 dB at 6.8 GHz with a thickness of 5 mm. The bandwidth (RL < −10 dB) attained an amazing 13.1 GHz from 4.9 to 18 GHz with suitable thickness. By controlling the content of GO, the average conjugation length of PANI nanorods on GO layers can be modulated, resulting in controllable wave-absorbing properties and various frequency bands. Graphical abstract By controlling the content of GO, the average conjugation length of PANI nanorods on GO layers can be modulated, resulting in controllable wave-absorbing properties and various frequency bands. Image 1 Highlights • Synthesize sandwich-like structured PANI/GO composite through mechanical stirring for enough long time. • Content of GO modulated the average conjugation length of PANI in sandwich-like composite. • Average conjugation length of PANI modulated conductive and microwave properties of sandwich-like composite. • Sandwich-like PANI/GO achieved promising EM wave absorption properties combined dielectric loss and impedance matching. [ABSTRACT FROM AUTHOR]

The new paradigm was introduced for achieving excellent electromagnetic (EM) waves absorption through multi-layer superimposed quasiperiodic-metamaterials, which were generated in rotary high entropy alloys ribbon. With the help of asymmetry, the designed anisotropic metamaterials with excellent absorption (Reflection Loss ≤ − 10 dB) covered from 3.8 GHz to 18 GHz on the scale of sub-wavelength. High entropy alloys ribbon increased soft magnetic properties that design cascaded configuration metamaterials displayed distinct phase differences with increased rotation angle, which significantly focus on microwave attenuation. Meanwhile, the direction of the wave vector accompanied by rotation can realize polarized conversion, from linear polarizer to circular polarizer which is demonstrated in simulation, highly desirable in controlling EM waves and shrinking the size of traditional absorbing coating. Our study provides a fresh perspective to developing high-performance absorbers by combining soft magnetic and irregular phase patterns as possible as need for complex EM waves manipulations, arbitrary phase encoding, and vortex field. [Display omitted] • Quasiperiodic structure, without translation symmetry but possess rotation symmetries, can regard as a novelty paradigm for achieving excellent electromagnetic wave absorption. • Based on alloying elements, the electromagnetic parameters of high entropy alloys ribbon used to design cascaded configuration metamaterials for rotating wavefront. • With the help of asymmetry, the effective absorption bandwidth (Reflection Loss ≤ − 10 dB) covered from 3.8 GHz to 18 GHz on the scale of sub-wavelength. • The high entropy alloy ribbons can realize polarized conversion, from linear polarizer to circular polarizer in simulation, and adjust the position of absorption peak when combined with traditional wave absorbing coating. [ABSTRACT FROM AUTHOR]

Mechanical milling and melting-strip casting-milling have been used to prepare FeCoNiSi 0.4 Al 0.4 high entropy alloy (M-HEA and C-HEA respectively) powders. Both techniques have different solution and phase formation rules. The M-HEA powders have more FCC, and the opposite is true for C-HEA powders. In addition, the C-HEA powders have better elemental uniformity and a larger aspect ratio. Because of this, the C-HEA powders have a smaller M s and a larger H c ; the M s range from 93.6 emu/g to 104.4 emu/g, and the H c vary from 81.5 Oe to 159.6 Oe. As the milling time increases, the variety of aspect ratios and uniformity give rise to the change of electromagnetic (EM) parameters for both HEA powders. Versus the EM parameters of M-HEA powders (ranging from 7.5 to 16 and from 0 to 1.4 for ε’ and ε” , respectively), these C-HEA powders have larger values (ranging from 11 to 26 and from 0 to 8 for ε’ and ε” , respectively). At the same time, the μ’ (varying from 1.78 to 1.90 at 2 GHz) and the μ″ (up to 0.52) of C-HEA powders are larger than the M-HEA powders. [ABSTRACT FROM AUTHOR]

The cement-based composites made from graphene nano-platelets (GN) and hollow glass microspheres (HGM) were prepared and its electromagnetic waves absorbing properties were researched in this work. Results show that the absorbing properties were improved after the combination of GN and HGM. As the filling ratio of glass microspheres increases, the value of absorption peak and the bandwidth below −5 dB increases at first and decreases afterwards. In addition, several sharp peaks were obtained and the values tend to appear at high frequency. With further increase of GN, the value at absorbing peak decreases and the curves become relatively flatter. When GN is 0.2%, HGM is 40% (vol/vol) and the thickness is 20 mm, materials have the excellent absorbing properties with the average reflectivity loss being −8.2 dB in the range of 2–18 GHz and the bandwidth was 4.4 GHz below −5 dB. The thickness of sample has a significant influence on the absorbing properties. The optimal thickness is 20–30 mm with 40% (vol/vol) HGM and 0.2% (wt/vol) GN combine together. [ABSTRACT FROM AUTHOR]

The δ-MnO 2 has a layered structure and is expected to be a good absorbing material. Here, CASTEP was used for a theoretical study of the microwave absorption properties of δ-MnO 2 with defects (interstitial and substitutional cobalt atoms, vacancies). The results show that by analyzing the density of states (DOS) and partial density of states (PDOS), the defects change the charge distribution and increase the magnetic moment (increased by about 19 orders of magnitude). The bond length increased from 1.979 Å to 1.980 Å after substitutional defects formed to enhance the displacement polarizability. The changes in the charge distribution increase the atomic polarizability. The presence of crystal defects enhances both the magnetic loss and dielectric loss. In addition, the calculated defect formation energies show that the Co atoms tend to form interstitial atoms in MnO 2 (−16.90 eV), and the oxygen vacancy defects (−0.77 eV) are more easily formed than the manganese vacancy (33.14 eV). [ABSTRACT FROM AUTHOR]

[Display omitted] • Complex spinel ferrimagnets (CSFMs) are constructed in situ in multi-elemental alloy powders (MEAs) by annealing treatment. • The microstructure and composition of the CSFMs are regulated to modulate the natural resonance frequency and improve permeability via adjusting the annealing temperature. • The MEAs/CSFMs composites not only exhibit efficient broadband megahertz electromagnetic wave absorption, but also have multi-temperature adaptability, oxidation and corrosion resistance. The contradiction between the demand for highly efficient low-frequency electromagnetic wave (EMW) absorption materials and the relatively poor permeability of existing absorbers has become increasingly prominent. However, the intrinsic natural resonance in magnetic material hinders the modulation and enhancement of permeability. Herein, (FeCoNiCu) x Cr 3−x O 4 complex spinel ferrimagnets (CSFMs) are constructed in situ in multi-elemental alloy powders (MEAs) for the first time, via annealing treatment. The microstructure and composition of the CSFMs are regulated by adjusting the annealing temperature, aiming to modulate the natural resonance frequency and achieve highly efficient megahertz (MHz) electromagnetic absorption. Simultaneously, annealing treatment improves the crystal integrity, which is conducive to enhancing intergranular magnetic coupling to elevate permeability temperature stability and the Curie temperature. The minimum reflection loss value of the MEAs/CSFMs composites is less than −7 dB at 3 mm and the effective EMW absorption bandwidth maintains between 425 and 470 MHz at 5 mm from −50 to 150 °C. In addition, the MEAs/CSFMs composites also exhibit excellent oxidation resistance and corrosion resistance due to the oxide layer on the surface. This work offers a new and tunable strategy toward modulating the natural resonance frequency for efficient temperature-stable broadband EMW absorption at specific frequency bands. [ABSTRACT FROM AUTHOR]

[Display omitted] • T-ZnO w can enable FCIPs to show the vertically oriented distribution. • The vertical distribution can significantly improve impedance matching. • The effective bandwidth of T-ZnO w /FCIPs covers the entire X-band over 30–350 °C. Currently, most magnetic high-temperature microwave absorbing materials (MAMs) suffer from the problem of impedance mismatch caused by sudden increase in conductivity along with the changing temperature. Herein, the tetra-needle zinc oxide whisker (T-ZnO w) is used as a structural template to manage the spatial distribution of flake carbonyl iron powders (FCIPs), achieving temperature-stable broadband microwave absorption in the range of 30–350 °C. By adjusting the filling content of T-ZnO w makes FCIPs change from random distribution to a vertically oriented distribution. This distribution provides more stable conductive paths and facilitates low temperature-sensitivity conductance loss. At the same time it provides more transmission channels for the incidence of electromagnetic waves (EMW) and achieves excellent impedance matching. Compatibility of conductance loss and impedance matching is thus achieved. The effective absorption bandwidth (reflection loss (RL) < −10 dB) of T-ZnO w /FCIPs in the range of 30–350 °C could cover entire X-band (8.2–12.4 GHz). This work proposes a simple method to adjust the spatial distribution of absorbers and provides a valuable inspiration for the preparation of temperature-stable broadband MAMs. [ABSTRACT FROM AUTHOR]

The polyaniline water hydrogen-bonded complex was studied by first-principles calculation. The density functional theory method was used to calculate the structure characters, natural bond orbital charge distribution, infrared spectra and the frontier molecular orbital. Results showed that the H-O···H-N and C-N···H-O type intermolecular hydrogen bonds were formed. The bonds involved in the intermolecular H-bond were all influenced by the hydrogen bonding interaction. During the hydrogen bond formation, the polymer chains in the complexes were all charged, which can be an important factor contributing to the increase of electrical conductivity. The N1-H vibration was strongly influenced, and the locations as well as the intensities of N1-H absorption bands were all changed in the complexes. In the orbital transition of HOMO to LUMO, the electron density transferred from benzenoid ring to quinoid ring. [ABSTRACT FROM AUTHOR]

We fabricated flaky FeSiAl (FFSA) absorption coatings and flaky FeSiAl (FFSA)/carbon black (CB) composite absorption coatings, and studied the effects of a discrete structure on the absorption performance of both coatings. The electromagnetic parameters of FFSA and CB were measured in the frequency range of 2–18 GHz using transmission/reflection technology. We also investigated the electromagnetic loss mechanisms of FFSA and CB. The microwave absorption properties of the coatings were determined by measuring the reflection loss (RL) using the arch method. We then analyzed the effects of the contents and the discrete structure on absorption properties. The results showed that, for the FFSA coatings, discrete structures markedly improved absorption properties. When the discrete unit sizes decreased, the absorption properties increased. The addition of CB further improved the absorption performance of the FFSA absorption coating. And when FFSA/CB composite coatings were cut into discrete structures, the absorption peak values decreased and the absorption peak frequencies shifted toward higher frequency ranges, which were consistent with those of the FFSA coating. Because of the conductivity differences between FFSA and CB, the FFSA/CB composite coatings with discrete structures had significant improvements in absorption. The improved absorption weakened when the CB content achieved a certain value. The equivalent circuit models were built to analyze the improvement mechanisms of absorption properties when the coating was cut into a discrete structure. [ABSTRACT FROM AUTHOR]

A theoretical study was carried out to study the excited-state of hydrogen-bonding characteristics of polyaniline (PANI) in aqueous environment. The hydrogen-bonded PANI-H 2 O complexes were studied using first-principles calculations based on density functional theory (DFT). The electronic excitation energies and the corresponding oscillator strengths of the low-lying electronically excited states for hydrogen-bonded complexes were calculated by time-dependent density functional theory (TDDFT). The ground-state geometric structures were optimized, and it is observed that the intermolecular hydrogen bonds C N ⋯ H O and N H ⋯ O H were formed in PANI-H 2 O complexes. The formed hydrogen bonds influenced the bond lengths, the charge distribution, as well as the spectral characters of the groups involved. It was concluded that all the hydrogen-bonded PANI-H 2 O complexes were primarily excited to the S1 states with the largest oscillator strength. In addition, the orbital transition from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) involved intramolecular charge redistribution resulting to increase the electron density of the quinonoid rings. [ABSTRACT FROM AUTHOR]

A method of improving the electromagnetic (EM) absorption property of conventional microwave absorber (CMA) is proposed here. The structural design process was mainly concerned with systematic analysis and research into the impedance matching characteristic and induced current. By processing a CMA-carbonyl-iron powder (CIP) coating into many isolated regions, the discrete-structure microwave absorber (DMA) had a much better absorption property than the corresponding CMA. When the thickness was only 2.0 mm and the component content was 33 wt%, the loss of reflection was less than −10 dB shifted from 6–7 GHz to 7–13 GHz and the loss of minimum reflection decreased from 12.5 dB lost to 32 dB lost through a discrete-structure process. The microwave absorption properties of coatings with different component contents and thicknesses were investigated. The minimum reflection peaks tended to shift towards the lower frequency region as CIP content or coating thickness increased. By adjusting these three factors, a high-performance broadband absorber was produced. [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]

The structural destructive resonance caused by the single resonant thickness of flat microwave absorbers results in narrow absorption, hindering the progress of stealth materials. Multi-scale optimization and metastructure design are effective in the improvement of multiple resonance thickness as well as wavefront transmission, allowing for greater freedom in the realization of broadband. Herein, hierarchical metamaterials constructed from pyramid metastructure composites and Huygens' metasurface on the basis of Fibonacci spiral element are reported here, with effective absorption bandwidth (reflection loss ≤ −10 dB) ranging from 4.8 to 18 GHz in terms of subwavelength thickness. The hybrid absorber also exhibits robustness with incident angle across 0°–50° for transverse magnetic polarization. The gradient impedance model based on pyramid composites comprising spherical carbonyl iron particles and graphene sheets will serve as a driver for the enhancement of the electromagnetic wave transmission and the booster of impedance matching. Huygens' resonance of HMS induces the magnetic field convergence effect due to the simultaneous balance of effective surface current and magnetic current, which facilitates the absorption of lossy composites for further broadening absorption bandwidth. This integrated design could be widely applied to other absorbent materials of choice. [Display omitted] • In the case of the electrical and magnetic response are properly balanced, the field excited by the meta-atom composed of centrosymmetric Fibonacci spirals shows similar dispersion of electric and magnetic polarizabilities, resulting in a negligible reflection of the proposed Huygens' metasurface in the frequency range of 2–18 GHz. • A comprehensive design of hierarchical metamaterials from four perspectives: microscale, mesoscale, macroscale and metamaterials, which provides a rigorous and scientific route for the broadband directional design of microwave absorbing materials. • The effective absorption bandwidth of proposed hierarchical metamaterial ranges from 4.8 to 18 GHz across C, X and Ku-band in terms of subwavelength thickness (0.096 λ 0) which merely 7% over theoretical minimum thickness dictated by the causality limit, and exhibits robustness with incident angle across 0°–50° for transverse magnetic polarization. [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]

Although various absorbers have been developed for electromagnetic-wave absorption materials (EAMs), the uniform composition and microstructure limit the design freedom of the soft magnetic properties, thus resulting in a ceiling of the absorption properties. Herein, we propose a revolutionary concept of C-doped FeCoNiSiAl high-entropy-alloy (HEA) ribbons with hierarchical nanostructures composed of ultrafine grains (400 nm), B2 nanoprecipitates (80 nm), and C@FeNi core–shell nanoparticles (15 nm) to induce superior soft magnetization and absorption. The hierarchical nanostructures resulted in a low coercivity (H C , 589.04 A/m) and a low conductivity (294.1 S/cm) to support the loss effect and impedance matching, and 87% of the saturation magnetization (M S , 135.13 emu/g) was maintained, even at the high temperature of 973 K, corresponding to ultra-stable magnetization. Furthermore, the Curie-temperature was greater than 1000 K. The B2 nanoprecipitates and corresponding interfaces created strong stray magnetic flux lines and magnetic domain vortices, and thus, the metasurfaces innovatively fabricated with HEA ribbons induced a strong absorption peak (−50.88 dB) and effective broadband absorption (reflection loss (RL) ≤ −10 dB within 5.86–18 GHz) due to the strong magnetic coupling. The hierarchical nanostructure is a promising strategy to tailor magnetic HEAs with a new degree of freedom for advanced EAMs. [Display omitted] [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]

Quaternion representation of color image has attracted great attention due to its capability to treat holistically the three color channels. In a more general way, it has successfully been used in multi-channel signal processing applications over the past few decades. In this study, a joint encryption/watermarking system with more security based on double random phase encoding (DRPE) in quaternion gyrator transform domain is addressed. In the proposed scheme, an RGB-scale watermark image together with a grayscale watermark image or not is encoded into a quaternion matrix and encrypted through the DRPE, the encrypted data is then fused into the middle coefficients of the quaternion gyrator-transformed host image. In the process of extracting watermarks, it is impossible to retrieve them without authorized keys. Compared with the three channels independently processing approach implemented in fractional Fourier domain, the proposed algorithm achieves lower complexity by reason of avoiding repetitive operations. Experimental results have demonstrated the feasibility of the proposed algorithm and its superior performance in terms of noise robustness. [ABSTRACT FROM AUTHOR]

Co 2+ , Ni 2+ and Co 2+ /Ni 2+ co-doped β-MnO 2 were synthesized by hydrothermal reaction to study the influence of crystal defects on the microwave dielectric response. The samples were characterized by X-ray powder diffraction (XRD), X-ray fluorescence spectrometry (XRF), transmission electron microscopy (TEM), and vector network analyzer. The construction of defective β-MnO 2 , with doping of Co 2+ and Mn/O vacancies, was also discussed based on first-principles calculation. Results showed that the crystallographic defects played important role in modifying the microwave dielectric performance. The dielectric loss capacity at 2–18 GHz was enhanced after doping of Ni 2+ and Co 2+ , which was ascribed to the ionic polarization process. In the deficient configurations, the appearance of Mn vacancy narrowed the band gap, forming weak bound electrons and weak contact ions. Then the electronic and ionic relaxation polarization processes took place among these charged particles in the applied field. Mn vacancy was the most important factor in controlling the microwave polarization process, followed by O vacancy and extrinsic Co 2+ . [ABSTRACT FROM AUTHOR]

Hydrothermal synthesis between (NH )S O and MnSO was carried out to investigate the structure, morphology, and electromagnetic properties of Ni-doped MnO with different doping amounts. The samples were characterized by X-ray powder diffraction (XRD), X-ray fluorescence spectrometry (XRF), scanning electron microscopy (SEM), and vector network analysis. The analysis results revealed that all the final products were α-MnO and few amounts of γ-MnO , and the morphology changed obviously with the increase of the Ni doping amount. The doping of Ni had a certain effect on the electromagnetic properties, and some other properties derived from electromagnetic performances were also investigated, such as dielectric and magnetic loss tangent and microwave-absorbing properties. All the reflection loss (RL) peaks of the final products were moving toward lower frequency with increasing thickness, and with the thickness of 2 mm, the final products all had the maximum effective absorption bandwidth (RL < −10 dB), and the Ni-doped MnO had better reflection loss performance than the undoped MnO in the range of high frequency. [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]

• FeCoNiMn x Al 1−x HEAs with similar proportion dual phase structure(FCC and BCC) are prepared by mechanical alloying. • The induction effect of Al promotes the ferromagnetic transformation of Mn and then improve the magnetism of HEAs. • A compensation mechanism is proposed to explain the magnetism changes of HEAs with different Mn contents. • A method for predicting magnetism and high frequency permeability of HEAs by first principles calculation is provided. Fe-Co-Ni-Al high entropy alloys (HEAs) with lower coercivity (H c) and high permeability are considered as a new generation of candidates for microwave absorbing materials. Expecting to obtain higher saturation magnetization (M s) and high frequency permeability, we designed a series of FeCoNiMn x Al 1−x (x = 0, 0.2, 0.4, 0.5, 0.6, 0.8, and 1) HEAs, and first principles calculations were used to explore the spin state and magnetic changes of Mn with different Al content under the ferromagnetic transition of Mn. Firstly, the face-centered-cubic (FCC) and body-centered-cubic (BCC) phases with almost equal content were determined by experiments. The atomic models of HEAs were successfully established using the obtained crystal structure information and the spin states and magnetic moments of elements in HEAs with different Mn content were calculated. Due to the magnetic moments of ferromagnetic elements and the anti-ferromagnetic to ferromagnetic transformation of Mn elements, when Mn content is 0.2, the average magnetic moment reached 0.85/atom. The experimental results authenticate that FeCoNiMn 0.2 Al 0.8 has the highest M s of 95.0 emu/g. Higher M s is favorable for HEAs to exhibit favorable high frequency magnetic properties, the maximum value of μ′ is 2.341, and that of μ″ is 0.823 in GHz frequency. This work illustrates the importance of the ferromagnetic transformation of Mn in HEAs for regulating high frequency magnetic properties and shows the guiding significance for predicting the magnetic properties of absorbing materials via using first principles calculations. [ABSTRACT FROM AUTHOR]

• We propose a multi-level structural loss function for small organ segmentation, which aggregates the region, boundary and pixel-wise information to supervise feature fusion and realize precise segmentation. • We further develop a multi-branch network, which incorporates three branches used to learn different features and uses a saliency guidance module to leverage the multi-scale information for small organ segmentation. • Both multi-level structural loss and multi-level structural network have been evaluated on three public datasets for pancreas and spleen segmentation. Compared to the state-of-the-art 2D and 3D methods, our models achieve the best accuracy by producing 0.5% ∼ 2% higher DSCs than the well-established nnU-Net, but without any pre-processing and post-processing procedure. Deep neural networks have achieved great success in medical image segmentation problems such as liver, kidney, the accuracy of which already exceeds the human level. However, small organ segmentation (e.g., pancreas) is still a challenging task. To tackle such problems, extracting and aggregating multi-scale robust features become essentially important. In this paper, we develop a multi-level structural loss by integrating the region, boundary, and pixel-wise information to supervise feature fusion and precise segmentation. The novel pixel-wise term can provide information complementary to the region and boundary loss, which helps to discover more local information from the image. We further develop a multi-branch network with a saliency guidance module to better aggregate the three levels of features. The coarse-to-fine segmentation architecture is adopted to use the prediction on the coarse stage to obtain the bounding box for the fine stage. Comprehensive evaluations are performed on three benchmark datasets, i.e., the NIH pancreas, ISICDM pancreas, and MSD spleen dataset, showing that our models can achieve significant increases in segmentation accuracy compared to several state-of-the-art pancreas and spleen segmentation methods. Furthermore, the ablation study demonstrates the multi-level structural features help both the training stability and the convergence of the coarse-to-fine approach. [ABSTRACT FROM AUTHOR]

Quasicrystals can enhance wave transmission and boost nonlinear phenomena due to rotational symmetry forbidden in conventional periodic crystals. As a projection of higher-dimensional quasicrystal lattice, quasiperiodic structures represent a novel platform that exhibits robust microwave-absorbing capacity in thin thickness and digs out the influence of structural configurations on electromagnetic wave absorption from local and global aspects. Due to the synergy effect of composites and structure, quasiperiodic metamaterials with Penrose tiling achieve 12.2 GHz in effective absorption bandwidth (reflection loss < −10 dB), from 5.78 GHz to 18.00 GHz with 2 mm thickness. It has obvious advantages in absorbing electromagnetic wave compared with corresponding periodic absorbers according to the characteristic of local anisotropy and higher order translational symmetry in Brillouin zone. By enhancing multiple scattering and nonlinear coupling effects, Penrose tiling provides new insights to design absorbers with outstanding electromagnetic performance and higher flexibility and tunability. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] • Hierarchical 2D N-doped graphene@amorphous carbon are designed. • Phase mutation, group delay and resonance response are certified. • Almost perfect impedance-matching (SWR approaching 1) is achieved. • An alternative route for polymers and graphene as promising MAMs is provided. Two dimensional (2D) nitrogen-doped graphene@amorphous carbon was constructed by in-situ polymerization of sandwich-like structured polyaniline/graphene oxide composites then carbonization at 650 °C in vacuum. The results revealed that the as-prepared 2D N-doped graphene@amorphous carbon and N-doped amorphous carbon nanorods have almost the same decomposition degree and crystal structure. However larger defective degree and more polar bonds are conducive to enhance the polarization loss capacity, and larger dielectric constant can contribute to prominent conductive loss potential. The standing-wave ratio at 13.325 GHz of 2D N-doped graphene@amorphous carbon is 1.08426 approaching to 1, almost perfect impedance-matching level achieving. HFSS simulations certify phase mutation, group delay and the resonance response between N-doped graphene@amorphous carbon and testing microwave, as well as the stronger microwave loss capacity and better impedance matching. Both of the above make 2D N-doped graphene@amorphous carbon a promising microwave absorbing material. The minimum reflection loss (RL) of nitrogen-doped graphene@amorphous carbon can reach −29.5 dB and the effective absorbing bandwidth (EAB, RL < −10 dB) can reach 5.355 GHz ranging from 12.56 to 17.915 GHz only at a thickness of 2.25 mm. [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]

Highlights: [•] We presented an efficient fixed-point augmented Lagrangian method (FPALM) for total variation minimization problems. [•] We proposed fast alternative algorithms to solve the minimization problems. [•] The proof for the convergence of the FPALM was provided under some mild assumptions. [•] The wide applicability of the proposed algorithms were demonstrated on several well-known image denoing models. [ABSTRACT FROM AUTHOR]