Ruey‐Bin Yang

The one-pot process, which combines the polymerization of polyaniline (i.e., PANI) with subsequent reduction of iron nanowire (i.e., Fe NW) under a magnetic field, was developed to produce Fe@PANI core–shell nanowires. The synthesized nanowires with various PANI additions (0–30 wt.%) were characterized and used as microwave absorbers. Epoxy composites with 10 wt.% absorbers were prepared and examined using the coaxial method to reveal their microwave absorbing performance. Experimental results showed that the Fe NWs with PANI additions (0–30 wt.%) had average diameters ranging from 124.72 to 309.73 nm. As PANI addition increases, the α-Fe phase content and the grain size decrease, while the specific surface area increases. The nanowire-added composites exhibited superior microwave absorption performance with wide effective absorption bandwidths. Among them, Fe@PANI-90/10 exhibits the best overall microwave absorption performance. With a thickness of 2.3 mm, effective absorption bandwidth was the widest and reached 3.73 GHz, ranging from 9.73 to 13.46 GHz. Whereas with a thickness of 5.4 mm, Fe@PANI-90/10 reached the best reflection loss of −31.87 dB at 4.53 GHz.

Controlling material structure and its electromagnetic properties, including complex permittivity and permeability, could enhance the microwave absorption performance of the material in terms of reflection loss and effective absorption bandwidth. In this study, La‐substituted barium hexaferrite, Ba _{3− x} La _x Co ₂ Fe ₂₄ O ₄₁ ( x = 0, 0.1, 0.3, and 0.5) compounds were successfully prepared using the solid‐state reaction method, and their corresponding microstructures, static magnetic properties, and electromagnetic features in 2–18 GHz were investigated. The doping of La content increased saturation magnetization, coercivity, and remnant magnetization. The Ba _2.7 La _0.3 Co ₂ Fe ₂₄ O ₄₁ epoxied sample with 3.5 mm thickness possessed an excellent microwave absorption of −47.3 dB at 3.52 GHz, and its corresponding effective absorption bandwidths were 3.75 GHz (2.25–6 GHz) and 0.57 GHz (17.43–18 GHz). It is shown that doping with various La concentrations on Ba ₃ Co ₂ Fe ₂₄ O ₄₁ can be used as an effective technique to tune the performance of microwave absorbers based on barium hexaferrite.

A broadband and thin microwave absorber composed of metal-base planar metamaterials (MMs) sandwiched between two layers of flaked-shaped carbonyl iron (FCI) composites is presented. FCI powder was prepared by ball milling spherical carbonyl iron powder and then added to epoxy resin to prepare FCI composite (FCIC) samples with different weight fractions. With these electromagnetic characteristics, a broadband, polarization-insensitive and thin metamaterial absorber comprising of arrays of square spirals embedded between two different layers of FCIC is optimally designed through a genetic algorithm. The reflection loss of the measured square spiral MM absorber with a thickness of 3.4 mm can reach –26 dB at matching frequency 4.4 GHz and the absorbing bandwidth below –9.3 dB is covered from 3.6 to 18.0 GHz which almost 4 fourfold bandwidth compared to that of the FCIC without the incorporation of MM layers.

In this study, carbon nanotube papers were employed in fabricating thin and broadband radar absorbing structures (RAS). Different concentrations of the CNT papers have been made by using a vacuum filtration method, with 20 × 20 cm in size and 21‐27 μm in thickness. An epoxy resin was added into the CNT paper and then cured to become a composite with 1‐5 wt.% of CNTs and 83‐309 μm in thickness. The complex permittivity and permeability ( ε ′, ε ″, μ ′, μ ″) of the CNT paper composites were measured using the transmission/reflection method in the frequency range of 2‐18 GHz. The results reveal that the real ( ε ′) and imaginary ( ε ″) parts of the complex permittivity are increased with the CNT concentration. The ε ′ of 5 wt.% CNT sample reaches 323 at 2 GHz and then decreases to 49.0 at 18 GHz. The ε ″ reaches 321 at 2 GHz and decreases to 26.0 at 18 GHz. The CNT paper composite combined with a glass fabric composite used for a dielectric spacer is fabricated for an innovative RAS and the reflection loss is measured using the arch method in a microwave anechoic chamber. The results show that the 5 wt.% CNT paper composite/glass fabric composite absorbers attain maximum reflection loss of −13.3 dB at 12.0 GHz, −13.8 dB at 10.0 GHz, and −16.0 dB at 7.5 GHz for spacer thickness of 1.5, 2.0, and 3.0 mm, respectively.

Radar absorbing materials (RAMs) also known as microwave absorbers, which can absorb and dissipate incident electromagnetic wave, are widely used in the fields of radar-cross section reduction, electromagnetic interference (EMI) reduction and human health protection. In this study, the synthesis of functionally graded material (FGM) (CI/Polyurethane composites), which is fabricated with semi-sequentially varied composition along the thickness, is implemented with a genetic algorithm (GA) to optimize the microwave absorption efficiency and bandwidth of FGM. For impedance matching and broad-band design, the original 8-layered FGM was obtained by the GA method to calculate the thickness of each layer for a sequential stacking of FGM from 20, 30, 40, 50, 60, 65, 70 and 75 wt% of CI fillers. The reflection loss of the original 8-layered FGM below –10 dB can be obtained in the frequency range of 5.12∼18 GHz with a total thickness of 9.66 mm. Further optimization reduces the number of the layers and the stacking sequence of the optimized 4-layered FGM is 20, 30, 65, 75 wt% with thickness of 0.8, 1.6, 0.6 and 1.0 mm, respectively. The synthesis and measurement of the optimized 4-layered FGM with a thickness of 4 mm reveal a minimum reflection loss of –25.2 dB at 6.64 GHz and its bandwidth below – 10 dB is larger than 12.8 GHz.

The effect of dispersion processes on complex permittivity and microwave absorption in 2−18 GHz ranges is presented for nanocomposites loaded with different amounts of carbon nanotubes (CNTs) ranging from 1 to 5 wt %. The ultrasonic sonication (US) method and the three‐roll mill (TRM) method were performed for the manufacturing of the CNT/epoxy absorbers with different dispersing levels. Microscopic observations revealed that the CNT agglomerates were reduced after the TRM process, and individual CNTs were uniformly dispersed in the epoxy resin. For the same weight content of CNT fillers, the percentage increase of between US and TRM samples varies from 35.5% to 101.7% while the corresponding increment of (dielectric loss) varies from 79.6% to 248.8%. A minimum reflection loss for the US sample with 2 wt % CNTs is only −7.8 dB at 11.3 GHz while the corresponding TRM sample is greatly improved to reach −37.4 dB at 7.76 GHz for the same matching thickness of 3 mm. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40963.

Water atomized Fe16Ni82Mo2 (wt. %) alloy powder with spherical-like form was processed in a planetary mill for 2 h and 4 h, respectively. The complex permittivity (ε′−jε″) and permeability (μ′−jμ″) are measured using the transmission/reflection method in the frequency range of 2–18 GHz for FeNiMo/epoxy composites with 70 wt. % of magnetic fillers. The results show that all ε′, ε″, μ′, μ″ substantially increase due to the increase of aspect ratios. The magnetic susceptibility (μ′−1) of FeNiMo/epoxy absorber is negative above 5 GHz due to the natural magnetic resonance. The frequency dispersion of μ″ of flake-shaped FeNiMo composite shows a higher resonance frequency which exceeds the Snoek's limit in the gigahertz range as a result of large shape anisotropy. For composite absorbers with 2 mm thickness, the calculated reflection loss reaches −12.1 dB at 14.6 GHz without milling, −20.4 dB at 13.0 GHz for 2 h milling, and −13.1 dB at 11.0 GHz for 4 h milling.

In the present study, iron/iron oxide (with a ratio of 4/1, 1/1, and 1/4) composite particles were prepared by precipitation. The morphology and phase composition were examined by scanning electron microscopy and X-ray diffraction techniques. The as-prepared composite particles were mixed with epoxy resin to prepare microwave absorbers and the corresponding electromagnetic properties were evaluated. The experimental results showed that the composite powder exhibited a mixture of α-Fe (∼4 μm) and Fe₃O₄ (∼30 nm) phases. The amount of iron oxide plays an important role in the electromagnetic properties of microwave absorbers. In general, complex permittivity (ϵ’ and ϵ”) and permeability (μ’ and μ”) increased with increasing amount of iron oxide additions. The minimum reflection loss can reach −16.7 dB at 9.0 GHz for 50 wt% of Fe/Fe₃O₄ = 1/4 composite absorber with 3 mm in thickness.

Wave propagations in an inhomogeneous medium (e.g., voids, particles, defects, inclusions) undergo multiple scattering which results in a frequency-dependent velocity and attenuation of coherent wave. The aim of this study is to analyses multiple scattering of plane compressional and shear waves in a composite containing randomly distributed spherical inclusions in a homogenous isotropic medium. To calculate effective wave numbers of ultrasonic waves propagating in the heterogeneous material, a generalized self-consistent multiple scattering model is used in this study. Numerical results for the effective phase velocity and attenuation of both P and SV waves are calculated for a wide range of frequencies and concentrations. The proposed dynamic generalized self-consistent model for particulate composites recovers both well-known static effective moduli in the static limit and the results at higher frequencies and concentrations agree well with published experimental data.