A mixing rule for predicting frequency dependence of material parameters in magnetic composites

A number of mixing rules are proposed in the literature to predict the dependence of effective material parameters (permittivity and permeability) of composites on frequency and concentration. However, the existing mixing rules for frequency dependence of permeability in magnetic composites typically do not provide satisfactory agreement with measured data. Herein, a simple mixing rule is proposed. Its derivation is based on the Bergman-Milton spectral theory. Both the Bruggeman effective medium theory and the Maxwell Garnett approximation are included as particular cases of the proposed mixing rule. The derived mixing rule is shown to predict accurately the frequency dependence of permeability in magnetic composites, which contain nearly spherical inclusions.     

Theoretical calculation and experiment of microwave electromagnetic property of Ni(C) nanocapsules

With the combination of the dielectric loss of the carbon layer with the magnetic loss of the ferromagnetic metal core,carbon-coated nickel Ni(C) nanoparticles are expected to be the promising microwave absorbers. Microwave electromagnetic parameters and reflection loss in a frequency range of 2 GHz–18 GHz for paraffin-Ni(C) composites are investigated.The values of relative complex permittivity and permeability, the dielectric and magnetic loss tangent of paraffin-Ni(C) composites are measured, respectively, when the weight ratios of Ni(C) nanoparticles are equal to 10 wt%, 40 wt%, 50 wt%,70 wt%, and 80 wt% in paraffin-Ni(C) composites. The results reveal that Ni(C) nanoparticles exhibit a peak of magnetic loss at about 13 GHz, suggesting that magnetic loss and a natural resonance could be found at that frequency. Based on the measured complex permittivity and permeability, the reflection losses of paraffin-Ni(C) composites with different weight ratios of Ni(C) nanoparticles and coating thickness values are simulated according to the transmission line theory. An excellent microwave absorption is obtained. To be proved by the experimental results, the reflection loss of composite with a coating thickness of 2 mm is measured by the Arch method. The results indicate that the maximum reflection loss reaches-26.73 d B at 12.7 GHz, and below-10 d B, the bandwidth is about 4 GHz. The fact that the measured absorption position is consistent with the calculated results suggests that a good electromagnetic match and a strong microwave absorption can be established in Ni(C) nanoparticles. The excellent Ni(C) microwave absorber is prepared by choosing an optimum layer number and the weight ratio of Ni(C) nanoparticles in paraffin-Ni(C) composites.     

Extrinsic permeability of Fe-based flake composites from intrinsic parameters: A comparison between the aligned and random cases

Composites consisting of aligned ferromagnetic flakes embedded in a nonmagnetic matrix are the focus of attention as a kind of promising microwave material. In this paper, the effective permeability for composites consisting of nanocrystalline Fe-based flakes with aligned and random spatial orientation distribution of inclusions for comparison is measured in gigahertz. Flakes were coated with thin silica layer to reduce permittivity and improve impedance match. It is found that the magnetic loss in flakes is mainly caused by the natural ferromagnetic resonance and exchange resonance. To explore the unsolved problem of calculating effective permeability for the aligned case, the combination of Landau-Lifshitz-Gilbert equation and Maxwell-Garnett mixing rule with important modification is used to predict the effective permeability spectrum of composites. An excellent agreement with the experiment results indicates the validity of proposed theoretical approaches.     

Effective electromagnetic properties of honeycomb substrate coated with dielectric or magnetic layer

Effective electromagnetic properties of aramid honeycomb board coated with a layer of multi-wall carbon nanotube or iron flakes composites were measured with waveguide method from 4 to 12 GHz. It was proved that homogenization theory could predict the effective permittivity or permeability of the honeycomb composites with good accuracy. The coated honeycomb composites of relatively high permittivity and permeability could potentially be used to develop dielectric or magnetic substrate for shielding layer or absorbing structures working at microwave frequencies.     

高浓度纤维增强材料介电特性计算方法

针对复合材料的微观结构非均匀和各向异性特点带来的数值方法计算慢、内存消耗大的问题,利用均匀化方法计算纤维增强复合材料的等效电磁参数.采用了纤维低体积添加比至高体积添加比的迭代方法,同时提出了一个描述材料微观结构的修正的特征长度,将现有的均匀化方法推广至非准静态(微波频段)条件下高纤维浓度情况.提出的修正的均匀化模型可直接用于反射系数、屏蔽效能等计算,其屏蔽效能与实际微观结构复合材料的数值仿真结果进行了对比,验证了提出的等效电磁参数计算公式的有效性和频率适用范围.     

Microwave reflection properties of planar anisotropy FesoNi50 powder/paraffin composites

The reflection properties of planar anisotropy Fe50Ni50 powder/paraffin composites have been studied in the microwave frequency range.The permeability of Fe50Ni50 powder/paraffin composites is greatly enhanced by introducing the planar anisotropy,and can be further enhanced by using a rotational orientation method.The complex permeability can be considered as the superposition of two types of magnetic resonance.The resonance peak at high frequency is attributed to the natural resonance,while the peak at low frequency is attributed to the domain-wall resonance.The simulated results of the microwave reflectivity show that the matching thickness,peak frequency,permeability,and permittivity are closely related to the quarter wavelength matching condition.The Fe50Ni50 powder/paraffin composites can be attractive candidates for thinner microwave absorbers in the L-band(1-2 GHz).     

Modelling of isotropic double negative media for microwave applications

A composite medium consisting of two sublattices of dielectric spherical particles of high permittivity and different radii embedded in a dielectric matrix of smaller permittivity are considered. It has been shown that such a composite medium reveals properties of an isotropic double negative media (DNG) in a limited frequency range, when resonance oscillations of H_III mode in one kind of particles and E_III mode in another kind of particles are excited simultaneously. The E_III resonance and the H_III resonance give rise to the magnetic dipole momentum and the electric dipole momentum correspondingly. Averaging the magnetic momentum and the electric momentum over the cells belonging to the appropriate spherical particles gives the negative permittivity and permeability. The model of diffraction of a plane electromagnetic wave on a dielectric sphere is presented and compared with the mixing rule based consideration. The results obtained are rather close. Distribution of the electromagnetic wave outside the sphere is calculated. Influence of the dispersion of the sphere size and the dielectric permittivity on the effective parameters of the DNG material is estimated.     

具有横向磁晶各向异性的钴纳米线的微波吸收性能

使用电化学脉冲沉积法制备了磁晶各向异性易磁化方向(c轴)垂直纳米线长轴方向的钴纳米线.受到磁晶各向异性、静磁相互作用等因素与形状各向异性相互竞争的结果,纳米线阵列的磁滞回线显示出较弱的磁各向异性.此外,在2—18 GHz频率范围内,纳米线/石蜡复合材料的介电色散谱的虚部在5 GHz处有一个主峰,在10 GHz附近有一个较弱的峰;德拜弛豫特性和材料的电导率对这两个峰的形成均有贡献.同时,其磁导率色散谱的虚部在频率为6.1 GHz处有一个主峰,在10 GHz以上有两个较微弱的峰. 前一个峰源于自 关键词： 钴纳米线 介电色散谱 磁导率色散谱 微波吸收剂     

基于等效介质原理的宽角超材料吸波体的理论分析

目前,很少有文章就如何实现宽角度吸波材料进行详细的理论分析和设计指导,设计宽角度吸波材料仍然是一件很困难的事情.本文基于等效介质理论对带有反射地板的单层介质超材料吸波体进行较为详细的理论分析.从基础电磁理论出发,推导TE波(横电波,电场方向与入射面垂直的平面电磁波)和TM波(横磁波,磁场方向与入射面垂直的平面电磁波)照射下吸波体的反射系数,分析实现宽角度吸波效果所需的等效电磁参数,为宽角度超材料吸波体的设计提供了理论基础.此外,论文还理论分析了实现宽带宽角吸波等效电磁参数所要满足的条件,并做了计算检验.结果表明,当介质等效电磁参数按照特殊曲线随频率发生变化时,理论上能实现宽带宽角的吸波效果.     

Experimental demonstration of isotropic negative permeability in a three-dimensional dielectric composite

Isotropic negative permeability resulting from Mie resonance is demonstrated in a three-dimensional (3D) dielectric composite consisting of an array of dielectric cubes. A strong subwavelength magnetic resonance, corresponding to the first Mie resonance, was excited in dielectric cubes by electromagnetic wave. Negative permeability is verified in the magnetic resonance area via microwave measurement and the dispersion properties. The resonance relies on the size and permittivity of the cubes. It is promising for construction of novel isotropic 3D left-handed materials with a simple structure.     

Microwave reflection properties of planar anisotropy Fe50Ni50 powder/paraffin composites

The reflection properties of planar anisotropy Fe₅₀Ni₅₀ powder/paraffin composites have been studied in the microwave frequency range. The permeability of Fe₅₀Ni₅₀ powder/paraffin composites is greatly enhanced by introducing the planar anisotropy, and can be further enhanced by using a rotational orientation method. The complex permeability can be considered as the superposition of two types of magnetic resonance. The resonance peak at high frequency is attributed to the natural resonance, while the peak at low frequency is attributed to the domain-wall resonance. The simulated results of the microwave reflectivity show that the matching thickness, peak frequency, permeability, and permittivity are closely related to the quarter wavelength matching condition. The Fe₅₀Ni₅₀ powder/paraffin composites can be attractive candidates for thinner microwave absorbers in the L-band (1-2 GHz).     

Effect of nickel nanofillers on the dielectric and magnetic properties of composites based on rubber in the X-band

Nickel–rubber nanocomposites were synthesized by incorporating ferromagnetic nickel nanoparticles in a natural rubber as well as neoprene rubber matrix. Complex dielectric permittivity and magnetic permeability of these composites were evaluated in the X-band microwave frequencies at room temperature using cavity perturbation technique. The dielectric loss in natural rubber is smaller compared to neoprene rubber. A steady increase in the dielectric permittivity is observed with increase in the content of nickel in both the composites. The magnetic permeability exhibits a steady decrease with increase in frequency and magnetic loss shows a relaxation at 8 GHz. The suitability of these composites as microwave absorbers is modeled based on the reflection loss which is dependant on the real and imaginary components of the complex dielectric permittivity and magnetic permeability.     

Microwave properties of composites with chromium dioxide

The permittivity and permeability of composites filled with CrO₂ powder are measured within the frequency range from 0.05 to 12 GHz. A sharp line of magnetic absorption is detected near 8 GHz. The effects of magnetic bias and remanence on the permittivity and permeability spectra are analyzed. The hysteretic behavior of dynamic permeability is observed for both parallel and perpendicular bias orientations relative to the microwave magnetic field. The effect is due to switching of the magnetic texture under bias equal to coercive field. At 50 MHz the parallel bias close to coercive field affects permeability much stronger than the perpendicular one. At 10 GHz the effect of perpendicular bias is higher than that of the parallel one. The effect of remanence on the microwave permeability is negligible. The 3 kOe parallel bias suppresses the line of magnetic absorption and decreases the conductivity of the composite and its microwave permittivity. This can be attributed to the magnetostatic interaction of inclusions in the vicinity of the percolation threshold.     

CoFeB/MgO不连续多层纳米软磁薄膜微波电磁特性

采用直流/射频磁控溅射方法和不连续多层交替沉积工艺,制备了CoFeB/MgO系列纳米不连续多层薄膜,研究了微波频段下的电磁性能.结果表明,通过调整CoFeB合金相和MgO介质相的相对含量可有效调控薄膜的微结构和电磁性能,且在［Co₆₄Fe₂₄B₁₂(0.7nm)/MgO(0.4nm)］₄₀薄膜中获得了优良的微波软磁性能和高电阻率,其饱和磁化强度1.3 T,难轴矫顽力130 A/m,电阻率3.4mΩ·cm,且共振频率高达2.1 GHz,磁导率实部μ′和虚部μ″在1.59 GHz处均高于240,并且在0.9—2 GHz宽频带范围同时大于100.该薄膜可用于微波吸收材料和电磁兼容的设计中. 关键词： 纳米磁性颗粒膜 磁导率频谱 电磁性能 吸波材料     

Acoustic wave generation by microwaves and applications to nondestructive evaluation

Although acoustic wave generation by electromagnetic waves has been widely studied in the case of laser-generated ultrasounds, the literature on acoustic wave generation by thermal effects due to electromagnetic microwaves is very sparse. Several mechanisms have been suggested to explain the phenomenon of microwave generation, i.e. radiation pressure, electrostriction or thermal expansion. Now it is known that the main cause is the thermal expansion due to the microwave absorption. This paper will review the recent advances in the theory and experiments that introduce a new way to generate ultrasonic waves without contact for the purpose of nondestructive evaluation and control. The unidirectional theory based on Maxwell's equations, heat equation and thermoviscoelasticity predicts the generation of acoustic waves at interfaces and inside stratified materials. Acoustic waves are generated by a pulsed electromagnetic wave or a burst at a chosen frequency such that materials can be excited with a broad or narrow frequency range. Experiments show the generation of acoustic waves in water, viscoelastic polymers and composite materials shaped as rod and plates. From the computed and measured accelerations at interfaces, the viscoelastic and electromagnetic properties of materials such as polymers and composites can be evaluated (NDE). Preliminary examples of non-destructive testing applications are presented.     

Tunable properties of microwire composites at microwave frequency

Tunable electromagnetic properties (permeability and permittivity) of composites filled with glass-coated microwires under external magnetic field were investigated experimentally. The microwire composites have been measured with 7 mm coaxial airline fixture and vector network analyzer from 0.01 to 18 GHz. The ferromagnetic resonance may contribute to the tunability of permeability of microwire composites. The tunability of effective dielectric permittivity may be attributed to the magneto-impedance effect. The microwire composites are promising candidates for smart or tunable applications at microwave frequency.     

Electromagnetic and microwave absorption properties of Fe3O4 magnetic films plated on hollow glass spheres

Magnetic hollow spheres of low density were prepared by plating Fe₃O₄ magnetic films on hollow glass spheres using ferrite plating. The complex permeability and permittivity of spheres–wax composites were measured in the range of 2–18 GHz. The complex permeability and permittivity increased, and the dielectric and magnetic losses were improved as the volume fraction of the magnetic spheres in the composites increased from 60% to 80%, which also resulted in a great improvement of microwave absorption properties. For composites with volume fraction 80%, its magnetic resonance frequency was at about 13 GHz and it appeared three loss peaks in the calculated reflection loss curves; the bandwidth less than −10 dB was almost 4 GHz which was just in the Ku-band frequencies (12–18 GHz) and a minimum reflection loss of −20 dB was obtained when the thickness was 2.6 mm; the microwave absorbing properties were mainly due to the magnetic loss. The results showed that the magnetic spheres composites were good and light microwave absorbers in the Ku-band frequencies.     

Influence of particle size on electromagnetic behavior and microwave absorption properties of Z-type Ba-ferrite/polymer composites

The electromagnetic and microwave absorption properties of the Z-type Ba-ferrite/polymer composites were investigated. The results showed that particle size of the Ba-ferrite fillers has a significant influence on the effective properties of the two-phase composites. The relative dielectric constant and initial permeability of such composites are about 95 and 5.2 at high frequency under certain combination of ferrite fillers with different particle size, respectively. Microwave absorption properties of the composites are simultaneously influenced due to the strong correlation between reflection loss and electromagnetic parameters of the ferrite/polymer composites.     

Electromagnetic and microwave absorption properties of carbonyl iron/La0.6Sr0.4MnO3 composites

In this work carbonyl iron/La_0.6Sr_0.4MnO₃ composites were prepared to develop super-thin microwave absorbing materials. The complex permittivity, permeability and microwave absorption properties are investigated in the frequency range of 8-12 GHz. An optimal reflection loss of −12.4 dB is reached at 10.5 GHz with a matching thickness of 0.8 mm. The thickness of carbonyl iron/La_0.6Sr_0.4MnO₃ absorber is thinner, compared with conventional carbonyl iron powders with the same absorption properties. The bandwidth with a reflection loss exceeding −7.4 dB is obtained in the whole measured frequency range with the thickness of 0.8 mm. The excellent microwave absorption properties are attributed to a better electromagnetic matching established by the combination of the enhanced dielectric loss and nearly invariable magnetic loss with the addition of La_0.6Sr_0.4MnO₃ nanoparticles in the composites. Our work indicates that carbonyl iron/La_0.6Sr_0.4MnO₃ composites may have an important application in wide-band and super-thin electromagnetic absorbers in the frequency range of 8−12 GHz.     

Synthesis and microwave absorption properties of graphene-oxide(GO)/polyaniline nanocomposite with gold nanoparticles

A composite of graphene/PANI/GAunano is synthesized using the co-blend method. The morphologies and microstructures of samples are examined by transition electron microscopy(TEM) and Fourier transform infrared spectroscopy(FTIR). Moreover, the microwave absorption properties of both graphene/PANI and GO/PANI/ GAunano composites are investigated in a microwave frequency band from 1 GHz to 18 GHz. The maximum reflection loss(RL) of GO/PANI/GAunano with a thickness of 2 mm is up to-24.61 d B at 15.45 GHz, and the bandwidth corresponding to RL at-10 d B can reach 4.08 GHz(from 13.92 GHz to 18.00 GHz) for a 2-mm-thick layer. The electromagnetic data demonstrate that GO/PANI/GAunano can be used as an attractive candidate for microwave absorbers.