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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]

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 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]

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]

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]

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]

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

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

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

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

With the increase of electromagnetic pollution and the wide use of commercial and military products, there is an increasing interest in electromagnetic interference (EMI) shielding. This article mainly aims at the effect of high magnetic field (HMF) on the grain shape and electrical conductivity of HCl-doped polyaniline (Pani-HCl), and the electrical conductivity and EMI shielding effectiveness (SE) of the conducting composites made from silicone rubber (SR) with different loading levels of Pani-HCl (without HMF) in the low frequency range from 3 to 1500 MHz. The result indicates that SE of the composites containing Pani-HCl (without HMF) increases and the volume resistivity decreases with increasing mass ratio loading of Pani-HCl in the SR. The measured SE of the composites are from 16 to 19.3 dB at 100 mass ratio loading of Pani-HCl and the volume resistivity decreases nine orders of magnitude compared with that of the emeraldine base form of Pani (Pani-EB) composites. The influence of HMF on electrical conductivity and grain shape of Pani-HCl is great. The grain shape of the in situ polymerization of PAN in 10T is an open-ramified rod-like pattern with a diameter of 50 nm and that of secondary doped PAN in 10T change to spherical structures of diameter of about 20-30nm, which are obviously different from the irregular reef-like particle shape of doped PAN in 0T. It is thought that the reason for the different grain shape is anisotropic diamagnetic susceptibility of doped PAN and orientations of the HMF. [ABSTRACT FROM AUTHOR]

Abstract: With the increasing of electromagnetic pollution and the widely use of commercial and military products, there is an increasing interest in electromagnetic interference (EMI) shielding. This paper mainly aims at electrical conductivity and EMI shielding effectiveness (SE) of the conducting composites made from silicone rubber (SR) with different loading levels of HCl-doped polyaniline (PAN-HCl) in the low frequency range from 3 to 1500MHz. The result indicates that SE of the composites increase and the volume resistivity decrease with increasing mass ratio loading of PAN-HCl in the SR. The measured SE of the composites are from 16 to 19.3dB at 100 mass ratio loading of the PAN-HCl and the volume resistivity decrease nine orders of magnitude compared with that of the emeraldine base form of PAN (PAN-EB) composites. [Copyright &y& Elsevier]

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: 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]