Microwave Electromagnetic Parameters Research Articles

Enhanced Microwave Absorption Bandwidth in Graphene-Encapsulated Iron Nanoparticles with Core-Shell Structure.

Graphene-encapsulated iron nanoparticles (Fe(G)) hold great promise as microwave absorbers owing to the combined dielectric loss of the graphene shell and the magnetic loss of the ferromagnetic metal core. Transmission electron microscopy (TEM) revealed transition metal nanoparticles encapsulated by graphene layers. The microwave electromagnetic parameters and reflection loss (R) of the Fe(G) were investigated. Graphene provided Fe(G) with a distinctive dielectric behavior via interfacial polarizations taking place at the interface between the iron cores and the graphene shells. The R of Fe(G)/paraffin composites with different Fe(G) contents and coating thickness was simulated according to the transmit-line theory and the measured complex permittivity and permeability. The Fe(G)/paraffin composites showed an excellent microwave absorption with a minimum calculated R of −58 dB at 11 GHz and a 60 wt% Fe(G) loading. The composites showed a wide bandwidth (the bandwidth of less than −10 dB was about 11 GHz). The R of composites with 1–3 mm coating thickness was measured using the Arch method. The absorption position was in line with the calculated results, suggesting that the graphene-coated iron nanoparticles can generate a suitable electromagnetic match and provide an intense microwave absorption. Excellent Fe(G) microwave absorbers can be obtained by selecting optimum layer numbers and Fe(G) loadings in the composites.

Sandwich-Like Graphite–Fullerene Composites with Enhanced Electromagnetic Wave Absorption

Sandwich-like graphite–fullerene composites have been prepared via a simple solution mixing/evaporation method. The complex relative permittivity and permeability of the graphite–fullerene composites in the frequency range from 0.5 GHz to 18 GHz were measured using a vector network analyzer with the reflection/transmission technique. Additionally, the microwave reflection loss of the composites was calculated using the obtained complex microwave electromagnetic parameters. It was found that the microwave loss peaks in the Ku band were dependent on the concentration of fullerene nanoparticles in the composites. Maximum reflection loss of −30 dB was observed between 2 GHz and 8 GHz when the graphite composites were doped with 1 wt.% fullerene. This absorption loss dropped (−24 dB) when the composite contained 3 wt.% fullerene. In addition, the electrical properties of the graphite were independent of the presence of fullerene in the composites. The tunable microwave reflection loss indicates that these graphite–fullerene composites show promise as wideband electromagnetic wave absorption materials.

Theoretical calculation and experiment of microwave electromagnetic property of Ni(C) nanocapsules**Project supported by the Science and Technology Program of Guangdong Province, China (Grant Nos. 2014B010106005, 2013B051000077, and 2015A050502047) and the Science and Technology Program of Guangzhou City, China (Grant No. 201508030018).

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 dB at 12.7 GHz, and below −10 dB, 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.

Effect of Nd Substitution on Magnetic and Microwave Absorption Properties of Nanocrystalline Sr(MnSn)0.5Fe11O19

The preparation of Sr1-xNdx(MnSn)0.5Fe11O19 M-type hexaferrites powders by a citrate sol-gel method was investigated. The samples were determined by X-ray diffraction (XRD) and transmission electron microscope (TEM). Microwave absorption properties of hexaferrite- acrylic resin composites were measured using SWR (standing-wave-ratio) method in the range of 12 to 20 GHz. It was shown that small amounts of Nd+3 substitution could significantly modify the microwave electromagnetic parameters. Furthermore, the investigation indicated that substitution of Nd+3 ions for Sr+2 ions in Sr(MnSn)0.5Fe11O19 resonant frequency would move toward higher frequency. The results showed that the powder possesses excellent microwave absorption properties. A minimum reflection loss of –43 dB was observed for the composite, where x = 0.03. A broad bandwidth of microwave absorption (with a –10 dB reflection loss as a reference) is predicted in the composite samples containing the ferrite powders of x=0.02, and x=0.03 in Ku-band frequencies. Keywords: Chemical synthesis, magnetic materials, magnetic properties, nano crystalline materials.

Enhanced microwave absorption of Fe3O4 nanocrystals after heterogeneously growing with ZnO nanoshell

To enhance the microwave absorption of Fe3O4 nanocrystals, ZnO nanoshells with a thickness of 2 nm were grown on Fe3O4 nanocrystals by heterogeneous nucleation. After being coated with ZnO nanoshells, the material possesses a far more improved ability for microwave absorption. A minimum reflection loss of −3.31 dB for Fe3O4 nanocrystals alone was improved to a minimum reflection loss of −22.69 dB and with an effective absorption band (RL < −10 dB) covering a frequency range of 10.08–15.97 GHz. The reasons for the enhanced microwave absorption were studied by the use of X-ray absorption near-edge structures at O K-edge, electron spin resonance analysis and microwave electromagnetic parameters mapping. The results indicate that the decoration of the dielectric ZnO shell had varied the dielectric property as well as the oxidization environment and distribution of Fe ions on the surface of the Fe3O4. This well balances the permeability and the permittivity of the nanomaterials and decreases the difficulty of impedance matching the microwave absorber within the free space. This leads to the Fe3O4@ZnO nanohybrids possessing vastly improved microwave absorption as compared to Fe3O4 nanocrystals alone.

PIC-MCC Simulation for HPM Multipactor Discharge on Dielectric Surface in Vacuum

In order to understand the physical mechanism of multipactor discharge on dielectric window surface under high power microwave (HPM) excitation in vacuum, an electron movement simulation model based on the particle-in-cell (PIC) Monte Carlo (MC) is built in this paper. The influences of microwave electromagnetic field and electrostatic field from dielectric surface charging are simultaneously considered in this model. During the simulation, the emission velocity and angle distribution of secondary electrons from the dielectric surface are taken into account. The movement trajectories of electron clusters under complex field excitation are obtained. The influences of emergence angle and microwave electromagnetic parameters on the electron movement are analyzed. It is found that the emergence angle of electrons from the surface has significant effect on its movement, and both the impact energy and return time of electrons oscillate periodically with the phase of microwave field. The number of secondary electrons and induced electrostatic field from multipactoring are also investigated. The results reveal that both values oscillate periodically at twice the microwave frequency, which is due to the electron impact energy oscillating with microwave period. A schematic diagram is proposed to explain the periodical oscillation phenomena.

Microwave electromagnetic and absorbing properties of Dy 3+ doped MnZn ferrites

Dy 3+ doped Mn-Zn ferrites Mn 0.3Zn 0.7Fe 2– x Dy x O 4 x=0, 0.01, 0.02, 0.03, 0.04were prepared by the conventional solid-state reaction. The crystal structure, surface morphology and electromagnetic properties of the calcined samples were characterized by X-ray diffraction analysis(XRD), scanning electron microscopy(SEM) and network analyzer (Agilent 8722ET). All the XRD patterns showed the single phase of the spinel-type ferrite without other intermediate when x≤0.03. The average crystallite size was about 44–56 nm. The microwave electromagnetic properties of the samples were studied at the frequency range from 2 GHz to 18 GHz. It was shown that small amounts of Dy 3+ substitution could adjust microwave electromagnetic parameters magically. The tan δ exhibited a maximal peak when x=0.03, and the peak value was 0.56. It indicated that the microwave electromagnetic loss properties were excellent when x=0.03. Furthermore, the reasons were also discussed using electromagnetic theory. The reflection loss (RL) increased with the Dy 3+ content when x<0.04. The Mn 0.3Zn 0.7Fe 1.97Dy 0.03O 4 ferrite displayed excellent microwave absorption properties. The frequency (with respect to −10 dB) started from 11.9 GHz, and the bandwidth reached about 3.5 GHz. The peak value of RL was about −20.5 dB at a matching thickness of 2.7 mm.

Preparation and microwave absorption properties of loose nanoscale Fe 3O 4 spheres

Magnetite nanoparticles are found to assemble into randomly dispersed loose nanoscale spheres with diameters ∼300 nm in ethylene glycol in the presence of polyethylene and a small quantity of polyethyleneimine. Modern analysis methods are employed to provide structure information of the magnetic loose spheres. The ferromagnetic saturation magnetization is ∼80.0 emu g −1, and the coercive force is 209 Oe. The microwave electromagnetic parameters are measured by a vector network analyzer. The synthesized loose spheres exhibit novel microwave properties compared with the conventional Fe 3O 4 nanoparticles. An additional microwave loss peak appears in the Ku band, which is attributed to the loose structure.

Morphology evolution, magnetic and microwave absorption properties of nano/submicrometre iron particles obtained at different reduced temperatures

Nano/submicrometre iron particles were prepared by a hydrogen reduction method in a fluidized bed furnace using α-FeOOH nanorods as precursors. The effect of the reducing temperature (T) on the microstructure, static magnetic properties, microwave electromagnetic parameters and microwave absorption properties of the resultant iron particles was investigated. When T increases from 450 to 650 °C, the as-obtained iron particles show an obvious morphology evolution from anisotropic nanorods to isotropic submicrometre polyhedra. As a result, the saturation magnetization, the complex permittivity and the real permeability all increase, while the coercivity and the imaginary permeability decrease due to the reducing surface effect and shape anisotropy. Nanocomposites containing 30 wt% iron nanorods obtained at 450 °C show a minimal reflection loss (RL) as low as −36.8 dB at 14.1 GHz and an absorption band with RL under −10 dB from 11.6 to 17.0 GHz when the thickness is 1.5 mm, suggesting that they are promising as a strong absorption, thin and lightweight microwave absorber.

Microwave permeability change of FeCo nanocrystalline during high energy ball milling

The microwave magnetic properties of the ball milled FeCo particles were investigated as functions of ball milling time (t) using microwave electromagnetic parameters analysis techniques. The results show that the imaginary part of intrinsic dynamic permeability (μi) of the ball-milled particles is much bigger than that of raw powders. μi strongly depends on t and exhibits several slightly damped ferromagnetic resonances. These phenomena are in qualitative agreement with the formation of the corresponding microstructure or the Aharoni s model of non-uniform exchange resonance modes. The present microwave permeability behavior indicates that nanocrystalline materials with the same grain size may exhibit different properties that depend upon the microstructure, which provides a possibility for manufacturing high performance microwave absorber.

One-Step Synthesis of Cobalt−Phthalocyanine/Iron Nanocomposite Particles with High Magnetic Susceptibility

Low-density cobalt−phthalocyanine (Co−Pc)/Fe nanocomposite particles with both adjustable electric and magnetic properties were synthesized using one-step thermolysis. The nanocomposite particles were fully characterized by Fourier transform infrared spectroscopy, X-ray diffraction analysis, scanning electron microscopy, high-resolution electron microscopy, and thermogravimetric−differential thermal analysis. The magnetic hysteresis loop and microwave electromagnetic parameters of the nanocomposite particles were measured. The magnetorheological (MR) properties of the MR suspensions based on the nanocomposite particles dispersed in methyl silicone oil were investigated as functions of weight percent of particles, magnetic field strength (H), temperature (T), and shear rate (γ). The results indicated that the Co−Pc/Fe nanocomposite consisted of micrometer-sized regular spheroids with hundreds of thousands of Co−Pc-coated α-Fe nanoparticles on the inside and Co−Pc layers on the surface of the spheroids. It ...

Study of the Anisotropy of Microwave Electromagnetic Parameters of Polycrystalline Iron Fiber Absorber

Based on Maxwell equations, the theoretical formulas of microwave electromagnetic parameters of polycrystalline iron fiber absorber are dervied. The anisotropy of microwave electromagnetic parameters of polycrystalline iron fiber absorber is explained by use of numerical calculations. Some samples of fiber orientation have been produced and their electromagnetic parameters have been tested, the results confirm the anisotropy of electromagnetic parameters. Both of the theoretical and experimental results demonstrate that the axial permeability of polycrystalline iron fiber is greater than its radial permeability, and the axial permittivity of polycrystalline iron filber is greater than its radial permittivity.