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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]

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]

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]

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]

• 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]

• 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]

Abstract: The water atomized Fe87.5Si7Al5.5 (Wt.%) alloy was processed by a high-energy planetary ball-milling. The characterization of morphology, microstructure, and electromagnetic properties were measured by scanning electron microscope (SEM), X-ray diffractometer, vibrating sample magnetometer (VSM), vector network analyzer and the first principle method. The analysis results showed that the powders shape became flaky from fusiform. The powders showed a reduction of the average grain size and the increase of the internal strain, and then presented an adverse variation trend after 55h milling. The powders that milled 10h had the largest saturation magnetization M S (131emu/g). The value μ′ of the powders decreased with increasing milling time at relatively lower frequency (2–8GHz), but opposite variation tendency happened at higher frequency (8–18GHz). Also, only short time milling can enhance the value of μ″ in the test frequency. The powders after 10h milling showed excellent microwave absorption (RL<−10dB) at the frequency 9.0–15.6GHz and the absorption peak shifted regularly to the high frequency as the increased milling time. Furthermore, the effect of charge exchange between the Fe and Si on the saturation magnetization in the ball-milling process was also investigated by using density functional theory (DFT) of first principle. [Copyright &y& Elsevier]

• FeCoNiCr 0.4 Cu Χ HEAs with large aspect ratio are successfully prepared by mechanical alloying method. • Relying on the "cocktail effect" of HEAs, the addition of appropriate Cu element can increase the aspect ratio of the particles. • It has been proven that the large aspect ratio is beneficial to enhance permeability in the megahertz frequency range. • The sample has stable electromagnetic wave absorption performance and superior oxidation resistance with increasing temperature. Electromagnetic wave absorption materials for megahertz (MHz) frequency range remains a key core issue that the poor effective attenuation performance and impedance mismatch. Based on previous research and theoretical analysis, an effective solution is to improve the real and imaginary parts of the complex permeability. In addition, the absorbents are also required to have excellent temperature stability and oxidation resistance due to exposure to harsh environments such as air and high temperature. Herein, the FeCoNiCr 0.4 Cu X high entropy alloys powders (HEAs) with planar anisotropy are prepared by mechanical alloying method. Relying on the "cocktail effect" of HEAs, the addition of appropriate Cu element can increase the aspect ratio of the particles and the shape-dependent anisotropy field to further improve the permeability. Experimentally, the real part of the permeability for the FeCoNiCr 0.4 Cu 0.2 HEAs ranges from 2.7 to 7.5 and the imaginary part of the permeability can reach 2.7 in the MHz frequency range. The maximum reflection loss can reach −33.6 dB at 484 MHz and the effective absorption bandwidth (reflection loss<−10 dB) can almost cover the entire MHz frequency range. Furthermore, the FeCoNiCr 0.4 Cu 0.2 HEAs have stable electromagnetic wave absorption performance and superior oxidation resistance with increasing temperature. This research exploits the characteristics of HEAs to further break the inherent magnetic limit by adjusting the morphology and provides a new pathway for solving the matter of MHz electromagnetic wave absorption. [ABSTRACT FROM AUTHOR]

The FeCoNiCu x Al (x = 0.2–0.7) high-entropy alloy powders were synthesized by ball milling and then the prepared FeCoNiCu 0.5 Al powders were annealed at different temperatures (673K, 773K and 873K). The characteristics of powders were discussed, including microstructure, morphology, particle sizes, magnetic and electromagnetic properties. The crystal structures of powders are simple FCC and BCC phases with flaky microscopic morphologies. The FeCoNiCu x Al alloys with various Cu contents are soft magnetic materials, which possess high saturation magnetization and low coercivity. High conductivity and improved surface polarization with large aspect ratio of flaky powders can enhance permittivity. High saturation magnetization of soft magnetic materials can reinforce permeability. These can promote the microwave absorbing capacity of FeCoNiCu x Al alloy powders, the reflection loss (RL) of FeCoNiCu x Al alloy powders all can reach −10dB or less and the minimum RL value is −28.04 dB at Cu x=0.5. Meanwhile, phases, morphology, conductivity and saturation magnetization are changed during different temperatures annealing treatment, which makes the RL values improved differently, and the minimum RL value is −40.05 dB at 673K annealing. These studies have shown that high-entropy alloys have great potential to become promising absorbers. • The content of Cu in FeCoNiCu x Al can adjust the ratio of FCC and BCC phase. • Annealing treatment can improve the microwave absorption performance of high entropy alloys. • The minimum RL value can achieve −40.05 dB. [ABSTRACT FROM AUTHOR]

The increasing prevalence of electromagnetic wave technology has raised concerns regarding electromagnetic pollution and its potential hazards. Microwave absorbing materials (MAMs) have emerged as an effective solution for capturing and dissipating electromagnetic waves. This study presents a novel approach for developing flexible microwave absorbers by dispersing carbon-coated FeNi alloy nanoparticles (FeNi@C NPs) into a polydimethylsiloxane (PDMS) matrix. The unique contributions lie in the mass production of (FeNi)@C NPs through the arc plasma and controlled making of (FeNi)@C/PDMS nanocomposites for practical electromagnetic wave (EMW) absorption/shielding application. Simultaneously, the formation mechanism of (FeNi)@C NPs is illustrated in detail by real-time optical emission spectroscopy. The obtained (FeNi)@C/PDMS nanocomposites show multifunctionality such as hydrophobicity and mechanical performance, rendering them suitable for applications in lightweight flexible electronics, wearable devices, and stealth technology. Through adjustments in the contents of (FeNi)@C fillers, the (FeNi)@C/PDMS nanocomposites exhibit excellent microwave absorption capacity. At a mass ratio of 15% of (FeNi)@C NPs, the minimum reflection loss is −33.51 dB at a thickness of 2.3 mm with an effective absorption band of 4.96 GHz. This enhanced performance is attributed to the synergistic effects of magnetic and dielectric loss, multiple reflection/scattering, conduction loss, and suitable impedance matching. Furthermore, the (FeNi)@C/PDMS nanocomposites demonstrate remarkable corrosion resistance, exhibiting a positive shift in corrosion potential to −0.18 V compared to bare aluminum (−0.83 V), along with a corresponding protection efficiency of 98.78%. In conclusion, this study provides a validated strategy for the fabrication of EMW absorption nanocomposites consisting of magnetic nanoparticle fillers and polymer matrix, highlighting their potential for application in intricate environments. • The optical emission spectroscopy was employed to analyze the growth of FeNi@C NPs. • A large-sized, flexible FeNi@C/PDMS nanocomposites were obtained by mechanical mixing. • The minimum RL of FeNi@C/PDMS nanocomposites was −33.51 dB with an EAB of 4.96 GHz. • The FeNi@C/PDMS nanocomposites exhibited good mechanical properties, corrosion resistance, and hydrophobicity. [ABSTRACT FROM AUTHOR]

Abstract: The flake-shaped barium titanate (FBT) was prepared by a modified sol–gel method, and their microstructure and electromagnetic (EM) properties (2–18GHz) were investigated by means of differential thermal analysis (DTA), thermogravimetry analysis (TG), X-ray diffraction (XRD), scanning electronic microscope (SEM) and a network analyser. The reflection loss R of FBT (annealing at different temperatures from 700 to 1200°C)/paraffin composite samples with thickness of 1mm and FBT (annealing at 1000°C)/paraffin composite samples with different thickness from 1 to 5mm were calculated in 2–18GHz using the relative complex permittivity and permeability according to the transmit-line theory. It was found that the electromagnetic characteristics and the wave absorption properties of FBT were improved. The maximal reflection loss of FBT (annealing at 1000°C)/paraffin composite sample reached −29.6dB at 12GHz with a matching thickness of 4mm. [Copyright &y& Elsevier]

Abstract: Nanocrystalline carbon black/barium titanate compound particle (CP) was synthesized by sol–gel method. The phase structure and morphology of compound particle were investigated by X-ray diffraction (XRD), transmission electron microscope (TEM) and Raman spectrum measurements, the electroconductivity was test by trielectrode arrangement and the precursor powder was followed by differential scanning calorimetric measurements (DSC) and thermal gravimetric analysis (TGA). In addition, the complex relative permittivity and permeability of compound particle were investigated by reflection method. The compound particle/epoxide resin composite (CP/EP) with different contents of CP were measured. The results show barium titanate crystal is tetragonal phase and its grain is oval shape with 80–100nm which was coated by carbon black film. As electromagnetic (EM) complex permittivity, permeability and reflection loss (RL) shown that the compound particle is mainly a kind of electric and dielectric lossy materials and exhibits excellent microwave absorption performance in the X- and Ku-bands. [Copyright &y& Elsevier]

Abstract: Using a reflection/transmission technique the complex relative permittivity and permeability of the barium titanate (BT) powder from sol–gel method were measured in the X and Ku bands. The BT/epoxide resin composite with different contents were investigated. It was found that the higher content of BT in composite yielded the good reflection loss in the X and Ku bands. [Copyright &y& Elsevier]

Abstract To effectively optimize electromagnetic (EM) wave absorption performance and reduce the reflection at the absorbent-air interface, the absorbents coupling magnetic/dielectric loss components to construct heterogeneous interfaces are urgent. Herein, we report a facile method for preparing a unique (TiC/Ni)@C ternary composite. The incorporation of Ni into TiC/C matrix to form heterogeneous interface not only ameliorates impedance matching, but also introduces the additional interface polarization for tuning dielectric loss capacity. Comparing with the weak absorption performance of TiC@C composite, the minimum reflection loss (RL) value of (TiC/Ni)@C composite can reach up to −43.4 dB at 7.0 GHz with the 3.4 mm thickness, and the effective bandwidth of RL below −10 dB can be up to 5.1 GHz at the 1.8 mm thickness. The results reveal that the efficient synergistic effect between magnetic Ni and dielectric TiC/C plays a more important role for the absorption performance, comparing to the merely strong dielectric loss of TiC@C composite. Highlights • (TiC/Ni)@C ternary composite was firstly synthesized by an arc discharging approach. • Dual dielectric loss components (TiC/C) coupling with magnetic Ni is beneficial to achieve a good impedance matching. • The minimum RL values reaches to −43.4 dB at 7.0 GHz with the absorbent thickness of 3.4 mm. [ABSTRACT FROM AUTHOR]

Dielectric ceramics composites are potential high-temperature microwave absorption materials (MAMs), and the critical challenge is to achieve high-efficiency and temperature insensitive absorption characteristics suitable for wide temperature and frequency range. Herein, the electromagnetic attenuation properties of TiN/Fe 2 Ni 2 N/SiO 2 composites are studied in detail. The magnetic ceramic composites proposed exhibited stable attenuation behaviors at a wide temperature range (293–873 K). Notably, the optimal reflection loss (RL) value had reached −32.4 dB with a thickness of 2.8 mm at 373 K, and the effective absorption bandwidth (EAB) covered 3.89 GHz at 873 K. The excellent high-temperature attenuation capacity is attributed to the balancing strategy of dielectric and magnetic losses, as well as multiple polarization loss behaviors. The results showed that introducing magnetic components to form magnetic ceramic composites might shed light on the development of the TiN ceramic family for promising MAMs at a wide temperature range. [Display omitted] • Magnetic TiN/Fe 2 Ni 2 N nanocomposites were fabricated via an in-situ ammonia calcination process. • The introduction of Fe 2 Ni 2 N can compensate for the issue of impedance mismatch at elevated temperatures. • The magnetic-dielectric balance strategy achieves stable and efficient high-temperature microwave attenuation behavior. [ABSTRACT FROM AUTHOR]