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High-contrast gratings (HCGs) based spoof surface plasmon polaritons (SSPPs), as a new kind of SSPPs, have been proposed recently. Here, we study their particular mechanism and characteristics by comparing them with traditional metal-type SSPPs. The different fundamental dielectric resonator modes decided by the height–length ratio R of the dielectric element are found to be the key factor of the mechanism, leading to new characteristics and providing more working modes. In low R mode, HCG-based SSPPs can work under deep-subwavelength thickness and can be easily excited by free space wave and, thus, are considered to be applied to all-dielectric metamaterial absorbers (ADMMAs). Simulations and analysis confirm the microwave absorption effect of HCG-based SSPPs as a new mechanism of ADMMAs. Based on this, a carbon black based ADMMA is designed and the simulation and experimental results verify the important role of HCG-based SSPPs in broadband absorption. It is expected that this work can give rise to further and wider studies on the characteristics and applications of HCG-based SSPPs. [ABSTRACT FROM AUTHOR]

FeSiAl flakes were fabricated by vibrating ball milling the FeSiAl ribbons. And the microwave absorption properties of FeSiAl flakes were improved by doping the multi-wall carbon nanotubes (MWCNTs) with different mass concentrations. The results show that the FeSiAl/MWCNT composites exhibit significantly improved microwave absorption performance with advantages of strong and broadband absorption in the L-band and S-band. In particular, the reflection loss (RL) of the FeSiAl/MWCNT2 composite reaches −7.4 dB at 1.0 GHz, whereupon, through the electromagnetic simulation software CST Microwave Studio, FeSiAl/MWCNT2 all-dielectric metamaterial absorbers (ADMMAs) were macroscopically designed, achieving an ultra-wideband absorption (RL ≤ −10 dB) of 14.4 GHz (3.6~18.0 GHz). It is recognized that the standing wavelength resonance and diffraction effect are responsible for absorbing electromagnetic waves, and the broadband absorption is improved via dielectric dispersion; their synergistic effect makes the ADMMAs exhibit good microwave absorption performance. This work provides a useful method for designing microwave absorption materials with broadband absorption. [ABSTRACT FROM AUTHOR]

Heterogeneous interfaces are of particular interest for developing highly efficient microwave absorbing materials. In this work, nanocarbon layer was in situ grown on the surface of Ni particles by plasma-enhanced chemical vapor deposition (PECVD), which created heterogeneous interfaces between nanocarbon materials and Ni particles via physical interactions. The defects of nanocarbon layer could be controlled by changing the mixture of reaction gas, resulting in tunable permittivity. Significant enhancement of microwave absorbing performance was obtained for the nanocarbon-coated Ni composites due to the enhanced impedance matching and polarization relaxations. The percentage bandwidths with reflection loss ≤ − 10 dB for the nanocarbon-coated composites were considerably larger than that for the original composite at any specified thickness in the range of 1.50 ∼ 5.00 mm. Quantitative loss analysis revealed that the energy dissipations of nanocarbon-coated composites originated from synergistic magnetic and dielectric losses, where dielectric loss was even predominate in high-frequency region. It is expected that the current method would provide an effective way for the optimal design of microwave absorbing materials. [ABSTRACT FROM AUTHOR]

In this paper, we propose and demonstrate a broadband microwave absorber based on magnetic metamaterial structure elements. We deploy FeCo soft magnetic composite with droplet shape as the primary resonant element. By investigating the resonant modes and the coupling between periodic unit-cells and the incident microwave, it is found the intrinsic magnetic susceptibility of the magnetic elements could make the magnetic field confined in the magnetic element, and thus results in a broadband microwave absorption. Our work provides a new approach to realize broadband microwave absorbers based on hybrid absorption mechanisms, including dielectric resonator effect, one quarter wavelength resonance effect, and grating effect. [ABSTRACT FROM AUTHOR]

In this article, we theoretically investigate properties of normally incident microwave propagation in dilute metallic ferromagnetic wire array (DMFWA) slabs with gyrotropic tensors. It is found that DMFWA may become left-handed materials (LHM) in a narrow frequency range below the plasma frequency, even the real part of the corresponding inversion permeability element is positive. Comparing with the isotropic materials having same diagonal permeability elements, due to existence of off-diagonal elements of the gyrotropic permeability tensor, the DMFWA becomes a LHM in a higher frequency range depended on its geometric structure. In addition, E, H, and wave vector Re(k) of microwave propagating in the DMFWA form an approximately left-handed triplet of vectors. [ABSTRACT FROM AUTHOR]

Exploration of lightweight and thin electromagnetic (EM) wave absorption materials is still urgent because of the issues related to EM pollution. In this work, one-dimensional core–shell nanowires with Co nanoparticles embedded in N doped porous carbon (Co@NPC NWs) have been successfully fabricated for microwave absorption. The obtained Co@NPC composites with a pea-like structure have high loading of Co nanoparticles (68 wt%) encapsulated by the defective carbon shell. Thanks to these special characteristics, the Co@NPC NWs exhibit superb microwave absorption properties in comparison with the pristine nano-Co powder and NPC. The strong reflection loss (RL) of −48 dB could be achieved at a thickness of only 1.45 mm. In addition, the effective absorption bandwidth is up to 5.2 GHz. These superb absorption properties coupled with thin thickness endow the Co@NPC NWs with great potential for application in lightweight EM wave absorption. [ABSTRACT FROM AUTHOR]

W-hexaferrite particles (Ba(Co0.5Zn0.5)2Fe16O27) have been successfully synthesized using a combination of co-precipitation and mechanical milling. These particles possess excellent hexagonal shape and are well separated from one another. The complex permeability and complex permittivity of ferrite–epoxy composite have been measured in the frequency range of 2–18 GHz. Based on the theoretical calculation of reflection loss, the microwave absorption properties of the ferrite–epoxy composite have been investigated. The result indicates that these ferrite particles have significant potential for microwave absorption applications in the future. [Copyright &y& Elsevier]

Effectively broadening microwave absorbing frequency of pure magnetic substances remains a huge challenge. Herein, micro-perspective structures can be controlled through a calcination route. Satisfactorily, the composites prepared at the calcination temperature of 900 °C exhibit excellent microwave attenuation performance with a broad working frequency and appropriate paraffin filling ratio. Remarkably, the composites can reach an extremely high reflection loss (RL) value of − 49.79 dB, and the extended effective working frequency range (RL < − 10 dB) of 6.84 GHz can also be obtained. Superb magnetic loss, admirable dielectric loss, sufficient dipole polarization, as well as superior impedance matching should be band together for obtaining ideal microwave absorbers. The CoNi hydroxides derived bimatallic alloy composites were fabricated via a cost-effective and facile synthesis process, and this work aroused inspirations of designing high-performance microwave absorbers for mataining the sustainable development. [ABSTRACT FROM AUTHOR]

With the continuous progress of science and technology, the traditional magnetic material is no longer able to meet the new complex electromagnetic (EM) environment due to its high bulk density. Therefore, the novel excellent EM absorber with the feature of thin thickness, low density, broad absorption bandwidth and strong absorption intensity is highly desired. Herein, we fabricated a porous carbon with ultrahigh porosity through a facile KOH activation from biomass waste pumpkin seed shell for lightweight EM wave absorption application. By optimizing the porous structures, the strong absorption intensity of −50.55 dB is achieved at thin thickness of 1.85 mm under low filler content of only 10 wt %. More interestingly, a broad frequency bandwidth of 7.4 GHz could cover the whole Ku band. These outstanding microwave absorption performances, couple with low cost ingredients and ease of fabrication process enable the porous carbon framework as the next generation promising candidate for lightweight and remarkable EM absorber. [ABSTRACT FROM AUTHOR]

Abstract: Based upon Landau–Lifshitz equation and Maxwell''s equations, we theoretically investigated properties of normally incident microwave propagation in interlayer exchange-coupled trilayer ferromagnetic film. It is found that, near resonance frequency of optic mode, imaginary part of permeability of one ferromagnetic layer is smaller than zero unusually, i.e., the ferromagnetic layer may be taken as an active medium. Thus a number of unique electromagnetic properties are presented, such as, the ferromagnetic layer becomes a left-handed material near low side of the resonance frequency of optic mode, and both phase velocity and time-averaged Poynting flow of the usually defined forward wave are negative simultaneously near high side of the resonance frequency. This work provides a feasible active medium to demonstrate the unique microwave properties. [Copyright &y& Elsevier]

Searching for ultralight microwave absorber with good heat-insulating property is highly desirable to fulfill the technical requirement of modern society. Herein, the two-dimension (2D) graphene-like porous carbon nanosheets (GPCN) derived from the waste pomelo peel were developed. The preparation is performed by a facile hydrothermal process with synergistic usage of HAc and H 2 O 2. Interestingly, the thickness and porosity of GPCN can be readily regulated by adjusting addition amount of the reagents. The related synthetic mechanism is investigated in depth. Compared with traditional graphene, the fabrication strategy of GPCN possesses many advantages, such as low cost, high yield and ease of production. The results show that GPCN presents superior thermal insulation and microwave absorption properties. The heat-insulating property is comparable to commercial products, e.g. polyurethanes (PU), nickel foam (NF), carbon foam (CF). The strong microwave absorption intensity of −56.4 dB and bandwidth of 6.4 GHz is achieved under ultralow filling ratio of 4 wt%. Moreover, the density of assembled 3D macroscopically product is merely 0.01 g/cm−3. The excellent performance should be attributed to the unique 2D morphology and high porosity of GPCN. Our work paves a new way for developing porous carbon nanosheet from sustainable biomass as heat-insulating ultralight microwave absorber. Image 1 • A graphene-like porous carbon nanosheet was prepared from waste pomelo peel. • The relating synthetic mechanism has been deeply investigated. • The heat-insulating property is comparable to commercial products. • Under the ultralow filler content of 4 wt%, the broad effective bandwidth can reach 6.4 GHz. [ABSTRACT FROM AUTHOR]

Carbon fibers exhibit the potential as lightweight absorbers, however, their high conductive capability often results in much reflection waves on the absorber surfaces. Here, novel ZnO/carbon porous nanofibers were derived from zinc-based organic-metal frameworks (MOFs, ZIF-8) by electro-spinning method. The porous structures make a great contribution to dielectric loss. In addition, the conductive networks can also provide more pathways for electrons transferring. With the good impedance matching and attenuation capability, ZnO/carbon porous nanofibers display good microwave response ability. The minimum reflection loss (RL) is −22.89 dB at 16.2 GHz with 1.5 mm, the effective absorption bandwidth (f E) reach 5.64 GHz. In this work, the influences of microstructure on electromagnetic parameters and absorbing properties are revealed by designing the novel porous carbon fibers. Thanks to the special porous structure, dipole polarization and conductive network, the novel ZnO/C nanofibers fabricated by electrospinning method exhibit excellent microwave absorption. Image 1 • Novel ZnO/carbon porous nanofibers derived from MOFs have been fabricated by electrospinning processes. • The ZnO/carbon porous nanofiber showed excellent microwave attenuation capability under 1.5 mm. • Porous structure, dipole polarization and conductive network are adopted to enhance microwave absorption. [ABSTRACT FROM AUTHOR]

Composite microwave absorbers with multiple loss mechanism have attracted more and more attention. Herein, we report a strategy of step-by-step synthesis of polyaniline (PANI)/FeCo@C composite microspheres by hydrothermal, calcination and in-situ polymerization. The multilayer heterogeneous interfaces of microspheres and the multi dipole center of polyaniline molecular chain significantly improve the polarization relaxation loss. Polyaniline on the surface connects the microspheres to form a conductive network, which effectively enhances the conduction loss. FeCo alloy particles also provide a certain magnetic loss. Their synergistic effect makes the composite microspheres exhibit good microwave absorption performance. When the thickness of P2 is only 1.7 mm, it could exhibit an effective absorption bandwidth (EAB) of 5.31 GHz and maximum reflection loss (RL) of −33.86 dB. Moreover, we optimized the macro structure of the absorber through gradient design, and successfully extended the effective absorption bandwidth to 11.81 GHz by introducing multiple interference. This work provides a useful reference for the comprehensive optimization of micro regulation and macro design of composite microwave absorbers. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract: In order to investigate microwave damping mechanisms of ferromagnetic films with a structure of [ferromagnetic (FM)/non-magnetic insulator (NMI)] n , a serial of [Fe65Co35/SiO2] n multilayer films was prepared by a magnetron sputtering method on the glass substrate. As-prepared films show excellent soft magnetic properties with tiny coercivities along the easy and hard axis respectively, and remarkable in-plane unixial anisotropies were observed too. The microwave properties were characterized by the ferromagnetic resonance (FMR) and a broadband permeameter. The corresponding theoretical fitting was conducted based on the Landau–Lifshitz–Gilbert (LLG) and two magnon scattering (TMS) theory. Fitting results show the TMS linewidth has a value of 36 times bigger than Landau–Lifshitz intrinsic linewidth, which gives a quantitative and explicit physical picture of microwave damping mechanisms in [Fe65Co35/SiO2] n multilayer films and may be important for the understanding of damping in [FM/NMI] n multilayer films. [Copyright &y& Elsevier]

Abstract: The structural and microwave properties of melt spun Fe100− x Si x (x =10, 20, 30) nanocomposites were investigated. The phases varied with Si content in FeSi alloys. It is found that the Fe3Si and FeSi phases could be obtained with Si content up to 20at.%. The X-ray absorption fine structure (XAFS) spectra of Fe K-edges show that the local structures around Fe atoms in melt spun Fe–Si alloys become more disordered with increasing Si content when compared with that of -Fe. The complex permittivity–frequency and permeability–frequency properties were determined in the microwave frequency regime of 2–18GHz by vector network analysis. It is found that flake-like FeSi powder composite has the largest values of μ′ and μ″ at 2GHz. The reflection loss shifts to the higher frequency with the Si content increasing for melt spun FeSi alloys. A minimum reflection loss of −16.5dB is obtained at 13.9GHz for composition Fe70Si30 with the thickness of 1.5mm. However, for composition Fe70Si30, the minimum reflection loss shifts to lower frequency and larger value with the thickness increasing. The results suggest a new design for microwave absorbers based on electromagnetic wave-absorbing materials. [Copyright &y& Elsevier]

Abstract: [NiO/Fe65Co35]10 exchange-coupled multilayer films for high frequency applications are fabricated, and their static magnetic property and microwave permeability are studied systematically. Both exchange bias field and ferromagnetic resonance frequency of the multilayers increase with decreasing Fe65Co35 thickness, which means that the microwave properties such as permeability and FMR frequency can be controlled by changing Fe65Co35 thickness in the exchange-coupled films. Ferromagnetic resonance frequencies beyond 7GHz of the films are measured and reported for the first time. [Copyright &y& Elsevier]

In this work, a series of cellulose-chitosan/PANI aerogel were synthesized through a simple procedure. Those specimens not only achieved desired microwave absorption property which was compatible with the thermal insulation, but also had an improved mechanical ability. • Employing cellulose as the framework to fabricate a novel sustainable microwave absorber. • The cellulose framework was enhanced by the blending with chitosan to achieve a more flexible bulk. • The loading of polyaniline (PANI) significantly improved the bulk's thermal insulation ability. • The broad bandwidth of 6.04 GHz at thickness of 1.9 mm was obtained. Traditional infrared radiation control materials, such as metallic silver, have low infrared emissivity, but they are expensive, dense, and not compatible with absorbing performance in microwave section. Meanwhile, the study of traditional absorbing materials is also limited to the synthesis and property investigation of powder materials, without considering the mechanical properties, thermal insulation properties, corrosion resistance and other requirements required by the actual application conditions. To solve the above problems, a cellulose skeleton with three-dimensional (3D) structure was synthesized with polyaniline (PANI) conductive polymer cladding on its surface. This aerogel could not only effectively consume the electromagnetic wave but also has great potential in thermal insulation application. The three-dimensional skeleton of cellulose-chitosan framework may greatly improve the conduction loss ability, while polyaniline itself has a higher infrared reflectance and a lower microwave reflectance, so the composite material has strong microwave loss performance and is compatible with thermal insulation. Remarkably, the microwave absorption (MA) performance of the aerogel with broad f e of 6.04 GHz and strong absorption intensity of −54.76 dB can be achieved. Benefiting from its special 3D structure, the aerogel can possess enhanced conductive loss ability. Notably, the as-prepared aerogel can also show impressive heat insulation performance, which means it could have the potential in the infrared stealth application. Above all, those multifunctional properties greatly inherit the positive feature of the raw materials. At the same time, the structural optimization further exploits its best potentialities, giving it possibilities in future application. [ABSTRACT FROM AUTHOR]

Rationally engineering on nanostructure of electromagnetic absorbers provides massive potential for eliminating the pollution caused by electromagnetic radiation. In this work, a series of core-shell Co@C nanotubes were produced via facile pyrolysis of cobalt carbonate hydroxide hydrate/dopamine precursor. Thanks to the well introduction of core-shell and nanotube structure, Co@C composite shows more superior microwave absorption than pristine Co nanoparticles. The unique microstructure provides abundant heterostructures and conductive paths to induce interfacial polarization and conductive loss for boosting dielectric loss. Through regulating the graphitization layer of C shell, the dielectric property could be further enhanced. Coupled with the favorable magnetic loss from the embedded Co nanoparticles, the products exhibit an optimal absorption intensity of −48 dB under low filler content of 30%. And effective absorption frequency bandwidth is up to 5.2 GHz at small thickness of 1.8 mm. Such excellent achievements demonstrate that the core-shell Co@C nanotube composites can be applied as a promising candidate for lightweight and thin electromagnetic absorption. This work presents a core shell structural Co@C composite for lightweight and high-performance microwave absorption. The remarkable broad absorption frequency bandwidth of 5.2 GHz could be obtained at 1.8 mm under low filler content of 30 wt%. Image 1 [ABSTRACT FROM AUTHOR]