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• Yolk-shell MnSe@C is fabricated via a thermally-driven contraction strategy. • Yolk-shell MnSe@C have tunable impedance matching and dielectric loss capability. • The RL min of Yolk-shell MnSe@C is −54.4 dB, and EAB max covers 5.1 GHz. The conventional selective template etching method to fabricate yolk-shell microwave absorbers is inconvenient and inefficient, so the thermally-driven contraction strategy was used to prepare asymmetric yolk-shell MnSe@C microsphere microwave absorbers via self-template directed transformation. The self-templated oriented transformation enables compositional customization and enhances the template utilization. The confinement effect of the carbon shell is crucial for realizing the thermally-driven contraction strategy and contributes to the strong conduction loss to MnSe@C. On the other hand, the enhanced polarization loss benefits from the abundant heterogeneous interfaces and defects in the asymmetric yolk-shell MnSe@C microspheres. The rich cavities in the yolk-shell structure not only facilitate optimal impedance matching, but also promote the enhancement of multiple reflection loss (RL). As a result, the asymmetric yolk-shell MnSe@C microspheres obtain excellent microwave absorption performance, the minimum RL (RL min) and the maximum effective absorption bandwidth (EAB) reaching –54.4 dB and 5.1 GHz, respectively, at a thickness of 1.9 mm. The successful obtainment of the asymmetric yolk-shell MnSe@C microspheres paves the way for the convenient synthesis of the yolk-shell transition-metal selenides (TMSs) microwave absorbers. Thermally-driven contraction strategy was used as the guideline to fabricate asymmetric yolk-shell MnSe@C microsphere microwave absorbers via self-templated oriented transformation. The enriched cavities of the yolk shell optimize the impedance matching of MnSe@C, and the enhanced dielectric loss comes from the conduction loss and polarization loss. Therefore, the asymmetric yolk-shell MnSe@C microspheres obtain excellent microwave absorption performance, the RLmin and EABmax reach -54.4 dB and 5.1 GHz at 1.9 mm, respectively. [Display omitted] [ABSTRACT FROM AUTHOR]

• The proposed UWMA has excellent electromagnetic stealth performance and is suitable for various complex applications. • The perfect combination of cut-wire metasurface with local-chiral characteristics and magnetic mediums enhances the intrinsic attenuation while also reduces the deterioration of absorption property caused by constructive interference. • The effective absorption bandwidth of UWMA can reach 15.1 GHz with a thickness of only 3.9 mm. Metamaterials with artificial designability and perfect absorption provide a novel design method for the manipulation and attenuation of microwaves. However, owing to the limitations of meta-atomic resonance effect and the lossless medium, it is difficult to achieve effective microwave absorption in a broadband frequency range. Hence, an ultra-wideband metamaterial absorber (UWMA) composed of two magnetic media and a metasurface was designed, and the absorption bandwidth of 2.9 to 18 GHz for 90% absorptivity under normal incidence was achieved, covering almost the entire S, C, X, and Ku bands. Through detailed investigation of equivalent circuit, equivalent medium theory, electromagnetic field distribution and front-back interface interference, the internal physical working mechanism was clarified essentially. In addition, the multi-environment adaptabilities of the proposed absorber, including polarization sensitivity, large angle incidence, dual-station radar stealth, bearing capacity and corrosion resistance, were analyzed. The proposed metamaterial absorber presents an effective way to achieve ultra-wideband absorption and compatibility with environmental adaptability, and provides a new way of thinking for intelligent full-band stealth technology. [Display omitted] [ABSTRACT FROM AUTHOR]

ABSTRACT 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 [...]

• Square frustum metastructure composites are fabricated and optimized based on genetic algorithms. • The hierarchical strategy and asymmetric infrared unit create a nonuniform distribution of the electromagnetic field in the near local field, contributing to broadening the absorption bandwidth. • The designed metamaterial with a subwavelength thickness of 0.049 λ max achieves excellent radar-infrared compatible camouflage that microwave absorption bandwidth is 12.2 GHz and infrared emissivity is 0.29. Metamaterials are widely used in electromagnetic radiation and camouflage for their flexible wavefront manipulation and polarization over a broad spectrum ranging from microwaves to optics. However, multispectral compatible camouflage faces significant challenges due to tremendous scale differences of unit cells and desired radiative properties in various spectral regimes. This study assembles a micron-scale infrared emitter, a millimeter-scale microwave absorber, and a metal reflector to propose a hierarchical metamaterial that reduces microwave scattering and reflects low-infrared waves. As a proof of concept, laser etching micro-manufactures an upper infrared shielding layer with a periodic metal pattern. At the same time, bottom square frustum metastructure composites are fabricated and optimized based on genetic algorithms. Under the normal incidence transverse electromagnetic wave with a 90° azimuth angle, the hierarchical strategy and infrared unit create an asymmetric electric field distribution of local near-field coupling, which is conducive to generating additional resonance for broadening the absorption bandwidth. Experiments verify the multispectral camouflage, which shows a high absorption efficiency of more than 90%, ranging from 3.6 to 6.2 and from 8.4 to 18 GHz with a total thickness of 4.05 mm (0.049 λ max). Due to the non-reflection of surrounding thermal signals in the infrared 2-22 μm region, low-infrared emissivity (0.29) metamaterials can adapt to various thermal backgrounds. This methodology can provide a novel route for fabricating multispectral camouflage devices. [ABSTRACT FROM AUTHOR]

• Design quasiperiodic absorbers with long-range order but no periodicity. • Explored the relationship between electromagnetic parameters and frequency by structural design and component selection in depth. • The effective absorption bandwidth of Penrose absorber can realize 9.32 GHz in 2 mm thickness. • The multilayer metamaterials perfectly unify radar-infrared compatible capability. In the electronic information and intelligent era, microwave absorption materials have been focused on the issue of electromagnetic (EM) pollution, but high-performance absorbers with thin thickness are highly desirable. The positive action between structural design and component selection can be regarded as a function to optimize the microwave loss capacity, which contributes to understanding the response mechanisms from the micro and macro. Therefore, we designed the absorber with quasiperiodic structures of Penrose tiling that has finely engineered the location of maximum absorption peak, the spherical carbonyl iron in thin rhombus, and flakey carbonyl iron in fat rhombus, the effective absorption bandwidth (Reflection Loss ≤ –10 dB) was realized from 6.46 GHz to 15.78 GHz in the 2 mm thickness. The unique arrangement of Penrose tiling can respond alternatingly to EM waves at large scales, and multicomponent coupling is distinctly in the neighboring unit cells. In addition, spraying aluminum pigment as the upper layer, the infrared emissivity is less than 0.35 to meet the complex application environment. This work opens up novel ideas on quasiperiodic structure combined multicomponent and injects infinite vitality to provide prospects for broadband responsive and high-efficiency EM absorbers. [ABSTRACT FROM AUTHOR]

• Lightweight 3D structural nanocomposites based on PPy aerogel adopted with Co and SiO 2 nanoparticles are prepared. • A maximum reflection loss (RL) of −65.31 dB at 11.12 GHz with only 3 mm. The effective absorbing bandwidth covers the entire x band (8–12 GHz). • The improved impedance matching, high interfacial polarization and electro-magnetic synergistic loss mechanism in composites give rise to the higher efficient absorption. Lightweight nanocomposites consisting of magnetic and dielectric units aroused intensive interest as potential high performance electromagnetic wave absorbing materials. In this work, we report a facile and efficient method to fabricate (Co, SiO 2)/PPy composites with tunable electromagnetic properties. The absorbing properties and effective absorbing bandwidth can be regulated by controlling the content of SiO 2 in composites. The composite shows a maximum reflection loss (RL) of -65.31 dB at 11.12 GHz with a thickness of 3.002 mm when SiO 2 being 22 wt.%. The effective absorbing bandwidth reaches up to 5.1 GHz (8.91–14.01 GHz), which covers the entire X band (8–12 GHz). The improved impedance matching, high interfacial polarization and complex electromagnetic synergy in the composites are the key factors giving rise to the higher efficient absorption. The PPy aerogel-based nanocomposites with controllable absorption performance, lower density and strong environmental adaptability will become attractive candidates as advanced microwave absorbing materials. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] • Honeycomb structural materials are developed to obtain broad bandwidth absorption without weight increasing. • The composition is regulated to optimize the absorbing performance. • Magnetic resonation is found for the first time in honeycomb structure, which is beneficial to the microwave attenuation. • Simulation is performed to analyze the electromagnetic response when structure converses from solid to honeycomb. • Effective absorption bandwidth of 9.8 GHz is obtained in the fabricated honeycomb structural materials. The undesirable absorption bandwidth is the main limitation for the application of microwave absorbing materials. Noting that the design of structure and composition of absorbing coating allows it to obtain a satisfying absorption bandwidth and to avoid weight increasing, honeycomb and its sandwich structures coated by various absorbant have been developed and measured using an arch measurement system over 2−18 GHz. To further understand the absorption mechanisms of honeycomb structural materials, simulation based on finite element method and transmission line theory is performed to analyze the different responses to the applied microwave between honeycomb structural materials and uniform-medium slab materials. The proper composition design allows control over the microwave loss mechanisms, which optimize the coating to possess both of the dielectric and magnetic advantages. For the honeycomb structural materials, the magnetic resonation is found for the first time, which is resulting from the periodic honeycomb walls on both sides of the dielectric cavity (consisting of the aramid paper interlayer and vacuum hexagonal prisms). In addition, the conversion of structure from solid to honeycomb brings about changes in impedance, propagating path, effective wavelength, effective attenuation area, microwave phases, and inductive coupling effect, etc., which result in a better microwave absorbing performance. The honeycomb sandwich structure coated by CIP/CB/EP with the weight ratio of 4:0.03:1 shows the absorption bandwidth of ∼9.8 GHz for reflection loss (RL) lower than -10 dB which covers part of the S and C bands and almost the whole X and Ku bands. [ABSTRACT FROM AUTHOR]

[Display omitted] • FeCoNiAlCr 0.9 high entropy alloy (HEA) powders are prepared for efficient microwave absorbing materials. • Anhydrous ethanol is used as the process control agents in the wet ball milling process to prepare HEA. • PCA affects crystallinity, phase, morphology, interior stress, and thus affects microwave absorption. • For HEA absorbant, an excellent effective absorption bandwidth of 4.28 GHz is obtained. Different amounts of absolute ethanol (0−50 mL) are used as process control agents (PCA) to prepare FeCoNiAlCr 0.9 high entropy alloy (HEA) powders via 90 h ball milling. The results show that the increased amount of PCA plays an active role in the crystallinity of powders, and regulate the thickness and size distribution of flake particles. As the volume of PCA increases, the real and imaginary parts (ε′ and ε″) of complex permittivity get increased by the enhancement of the interface polarization and surface polarization, while the increase in the real and imaginary parts (μ′ and μ″) of complex permeability arises from the increased anisotropic energy. The addition of PCA not only promotes the reflection loss but also extends the effective bandwidth (up to 4.28 GHz). Here, the performance adjustment of HEA electromagnetic absorber is realized by forthrightly changing the process parameters of ball milling. [ABSTRACT FROM AUTHOR]

We design high entropy alloys (HEAs) with different induction elements (Si/Al/Sn). In order to keep the crystal structure invariant and to investigate how the increment in saturation magnetization (M s) is caused only by the change of electron spin state, each set of HEAs contains a different amount of Mn. Synergistic effects among induction elements that induce the magnetic transformation of Mn from anti-ferromagnetism to ferromagnetism are found. M s of added Mn reduces when a particular induction element (Si 0.4 /Al 0.4 /Sn 0.4) exists, while a larger increment of M s appears when two induction elements coexist, Si 0.4 Al 0.4 (25.79 emu/g) and Sn 0.4 Al 0.4 (15.43 emu/g). This is reflected in the microcosmic magnetic structure for the emergence of closed domains due to large demagnetization energy, which is confirmed by the Lorentz transmission electron microscope (LTEM) data. The calculated magnetic moments and the exchange integral constants from density functional theory based on the Exact Muffin-Tin Orbits formalism reveal that the magnetic state and the strength of ferromagnetic and anti-ferromagnetic coupling determine the variation of M s in different chemical environments. The difference in energy levels of coexisting multiple induction elements also leads to a larger increment of M s , Si 0.4 Al 0.4 Sn 0.4 (29.78 emu/g), and Si 0.4 Al 0.4 Ge 0.4 Sn 0.4 (31.00 emu/g). [ABSTRACT FROM AUTHOR]