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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]

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]

Abstract: The crystalline structure and magnetic properties of M-type barium ferrite doped with small amounts of MnO2 (0, 0.25, 0.5, 0.75, 1.0, 1.5, and 2.0wt%, respectively) have been investigated by means of XRD, SEM and VSM. The results show that the crystalline structures of barium ferrite are still M-type hexagonal structure and Mn ions are distributed homogeneously in both the grains and the grain boundaries. The saturation magnetization and magnetocrystalline anisotropy constants both reach the highest values when x=0.75wt%. The displacement of Fe ions from 4f1 to 2b site is mainly responsible for the appearance of the maximum values. [Copyright &y& Elsevier]

• Mechanical alloying is applied to prepare FeCoNiAlCr x high entropy alloy powders. • The effect of Cr content and annealing on electromagnetic properties is explained. • Changes in morphology and composition usefully improve electromagnetic properties. • The Cr element and annealing are of positive role to improving reflection loss. High entropy alloy powders are promising candidates for new microwave absorbents due to its advantages in corrosion resistance, heat resistance and soft magnetic properties. In this work, flake-shaped FeCoNiAlCr x (x = 0.1, 0.3, 0.5, 0.7, and 0.9, in molar ratios) alloy powders are prepared by mechanical alloying, and then annealed at 500 °C. The experimental results show that the milled and annealed FeCoNiAlCr x powders contain similar FCC and BCC phase structure. The addition of Cr element results in more flattened powder particles and more polarization sites, which enhances the surface polarization and ultimately increases the real and imaginary parts of complex permittivity (ε′ and ε″). The real and imaginary parts of complex permeability (μ′ and μ″) decrease first and then increase as the proportion of Cr increases. The minimum reflection loss (RL min) is improved and moves to the low frequency region (11.20 GHz to 8.23 GHz) as the proportion of Cr increases. After annealing, the enhanced saturation magnetization (M s) increased μ r , and the improvement of crystallinity also increased ε r. For FeCoNiAlCr 0.9 samples, annealing treatment directly improves the minimum reflection loss from −26.88 dB (8.23 GHz) to −47.55 dB (7.375 GHz), suggesting the positive effects of annealing treatment on reflection loss. All the above findings provide experimental and theoretical basis for FeCoNiAlCr x high entropy alloy powders as absorbing material. [ABSTRACT FROM AUTHOR]

• Fe 80.7 Si 4 B 13 Cu 2.3 powders has a bcc-Fe nanocrystalline/amorphous composite structure. • The powders possess high M s , high μ and matchable ε in high frequency. • The powders exhibit superior electromagnetic wave absorption properties in X-band. Fe 80.7 Si 4 B 13 Cu 2.3 nanocrystalline soft magnetic alloy powders with outstanding electromagnetic wave absorption properties in X-band (8–12 GHz) have been successfully synthesized by a ball-milling method. As milling time increases from 40 to 160 h, the mean particle size of the near spherical powders with a bcc-Fe nanocrystalline/amorphous composite structure decreases from 9 to 6 μm, while the average nanocrystalline grain size stabilizes at around 10 nm. The alloy powders possess high saturation magnetization of 178.2–183.4 emu/g. The alloy powders/paraffin (3:1 in mass ratio) composites exhibit real permeability and permittivity of 2.18–2.42 and 7.3–9.7 at 2 GHz, respectively, and imaginary permeability of 0.68–0.77 at resonance frequencies of around 12.5 GHz. The composites with thicknesses of 2.1–2.3 mm exhibit large reflection losses (RL) of −39.1 to −50.5 dB with effective absorption bandwidths (RL < −10 dB) of 4.5–5.0 GHz covering the X-band. The excellent absorption properties are benefited from the high permeability and matchable permittivity of the nanocrystalline alloy powders in GHz range. [ABSTRACT FROM AUTHOR]