Good Microwave Absorption Research Articles

Static magnetic and microwave absorption properties of FeCo/Al2O3 composites synthesized by high-energy ball milling method

FeCo/Al2O3 composites were prepared by mechanochemical synthesis route using a mixture of Fe2O3, Co3O4 and Al as the starting materials. FeCo alloy can be obtained in a short time in contrast to the traditional method by means of milling Fe and Co powders directly. Most FeCo/Al2O3 composites can be formed when ball milling time has achieved 15 min. As the milling time increases, the saturation magnetization (Ms: magnetic moment per unit mass) and coercive force (Hc) of FeCo/Al2O3 composites increase gradually. The FeCo/Al2O3 composites exhibit a value of Ms of 98.5 emu g−1 and Hc of 350 Oe finally. The products with ball milling time 15 min and 30 min provided better microwave absorption performances in the frequency ranges of 8.6–18 GHz and 2.5–6.5 GHz for an absorber thickness of 1.2–2.2 mm and 2.1–4.9 mm, respectively. An optimal reflection loss of −43.1 dB can be obtained at 4.1 GHz with an absorber thickness of 3.1 mm. The good microwave absorption performances result from an appropriate impedance matching due to the insulator Al2O3 dispersed around FeCo alloy.

Dielectric and microwave absorption properties of polymer derived SiCN ceramics annealed in N2 atmosphere

Abstract Porous SiCN ceramics were successfully fabricated by pyrolysis of a kind of polysilazane. The effects of annealing temperature on the microstructure evolution, direct-current electrical conductivity, dielectric properties, and microwave absorption properties of SiCN in the frequency range 8.2–12.4 GHz (X-band) were investigated. With the increase of annealing temperature, SiC, Si3N4 and free carbon nanodomains are gradually formed in the SiCN. Both the SiC and free carbon nanodomains lead to the increases of the complex relative permittivity and loss tangent of SiCN. With the increase of the annealing temperature, the average real permittivity, imaginary permittivity and loss tangent increase from 4.4, 0.2 and 0.05 to 13.8, 6.3 and 0.46, respectively. The minimum reflection coefficient and the frequency bandwidth below −10 dB for SiCN annealed at 1500 °C are −53 dB and 3.02 GHz, indicating good microwave absorption properties.

Microwave absorbing ceramic coatings with multi-walled carbon nanotubes and ceramic powder by polymer pyrolysis route

Glass powder, talcum powder and multi-walled carbon nanotubes (MWCNTs) filled silicone resins were prepared for microwave absorbing coatings which can be used at elevated temperature. The impact strength and adhesive power, complex permittivity and reflection loss of the coatings at room temperature and after heat treatment were studied. Both the adhesive power and impact strength of the coatings can be adjusted by the glass powder content, and enhanced after heat treatment. The real part (ε′) of permittivity of the coatings decreased slightly while the imaginary part (ε″) kept almost constant after heat treatment. The reflection loss values below −10 dB can be obtained in the frequency range of 12.7–18 GHz at room temperature and 12.4–15.6 GHz at 600 °C for the coating after heat treatment. These results indicated such coatings containing glass powder, talcum powder and MWCNTs can be a promising microwave absorbing ceramic coating with both good mechanical properties and microwave absorption.

Microwave absorption and mechanical properties of β-SiC–C structure-function composites prepared by liquid phase sintering

β-SiC–C composites with good microwave absorption were prepared using the liquid phase sintering method. The effects of C content and sintering temperature on microwave absorption and fracture strength of the composites were investigated. The results were compared with those of the composites prepared by solid phase sintering. When C content was 3wt% and the sintering temperature was 1800°C, the composite 1-SAYC3 presented a good combination of microwave absorption and fracture strength, and almost all the attenuations of 1-SAYC3 were over −30dB in the whole X band. It might play an ideal role as an integrated structure-function material. The reason why microwave absorption of the composites prepared by liquid sintering was improved was mainly related to restriction of β–α SiC transformation.

Synthesis and microwave absorption properties of carbon coil–carbon fiber hybrid materials

Uniform growth of carbon coils on the surface of carbon fibers was achieved by acetylene decomposition at 680°C with Ni nanoparticles as the catalyst. The homogeneous precipitation-reduction method was used to coat the carbon fibers with Ni nanoparticles. The scanning electron microscope images reveal that these carbon coils are mainly composed of spring-like microcoils, and include a few twist-shaped nanocoils. The carbon yield calculated from carbon yield=(mass of product/mass of Ni)×100% is about 34,000% in a run of 1h. By incorporating 10wt% of the carbon coil–carbon fiber hybrid in a paraffin matrix, a material with good microwave absorption is produced, despite the low mass percentage of filler.

ELECTROMAGNETIC MICROWAVE PROPERTIES OF Fe82B17Cu1 BALL MILLED ALLOY

High saturation magnetization and magnetic anisotropy are helpful for getting a high frequency electromagnetic microwave absorption performance. The α- Fe possesses a high saturation magnetization. Fe - B phases exhibit a relatively higher magnetic anisotropy and higher resistivity than α- Fe simultaneously. In this work, we made nanocrystalline powders of Fe 82 B 17 Cu 1, mainly consisting of α- Fe and Fe 2 B phases, by ball milling and post-annealing. Electromagnetic microwave characterization shows that Fe 82 B 17 Cu 1 powders possess a relative high permeability and considerable permittivity. Due to a good electromagnetic impedance matching, a good electromagnetic microwave absorption property (RL < -35 dB) has been achieved at 3.6 GHz. The experimental frequency and the matching thickness are coincident with the quarter wavelength matching condition.

Microwave Absorption of Substituted M-type Lead Hexaferrite Nanopowders

M-type lead hexaferrites with compositions of PbFe12O19 and PbSnZnFe10O19 were synthesized by the citrate sol–gel method. The structural and magnetic properties of resultant particles were investigated by X-ray diffraction (XRD), transmission electron microscope (TEM), vibrating sample magnetometer (VSM) and vector network analyzer. The magnetic properties such as saturation magnetization (Ms) and coercivity (Hc) were calculated from the hysteresis loops. Microwave absorption properties of ferrite–polymer composites prepared with different ferrite ratios of 50%, 60%, 70% and 80% have been investigated in the frequency range of 12–20 GHz. The composite powder with 80 wt% lead ferrite possesses good microwave absorption properties. The minimum reflection loss of the composite powder reaches –30 dB at the thickness of 2 mm for composite powder with 80 wt% ferrite. The composite with 80% ferrite content has shown a minimum reflection loss of -24.5 dB at 18.6GHz with the -15 dB bandwidth over the extended frequency range of 16.3– 19.4GHz for an absorber thickness of 1.8 mm. The prepared composites can fruitfully be utilized for suppression of electromagnetic interference (EMI) and reduction of radar signatures (stealth technology). Keywords: Lead hexaferrite, nano-crystalline materials, microwave absorption, magnetic properties.

Microwave absorption properties of polymer composites with amorphous Fe-B and Ni-Zn-Co ferrite nanoparticles

Nowadays, many communication devices use GHz-range microwaves, which cause serious issues, such as electromagnetic interference. For the prevention of these problems, microwave absorbers consisting of polymer composites with magnetic particles have received attention. With the current trend being to miniaturize devices, thin microwave absorber are now required. This paper reports that polymer composites with amorphous Fe-B submicrometer particles and Ni-Zn-Co ferrite nanoparticles showed a permeability of µ r ′ = 8.0–9.0 at 1.0 GHz and a resonant frequency of f r = 1.8–2.4 GHz. They also exhibited good microwave absorption properties at 0.7–1.4 GHz for thicknesses of 2.5–3.9 mm.

Development of Magnetic Nanoparticles as Microwave-Specific Catalysts for the Rapid, Low-Temperature Synthesis of Formalin Solutions

A series of heterogeneous catalyst materials possessing good microwave absorption properties were investigated for their activity as oxidation catalysts under microwave irradiation. These catalysts, a series of nanoscale magnetic spinel oxides of the composition MCr2O4 (M = Cu, Co, Fe), were irradiated in aqueous methanol solution (1:1 MeOH:H2O v:v). This resulted in rapid conversion of methanol to formaldehyde, directly generating aqueous formalin solutions. The catalytic reaction occurred under relatively mild conditions (1 atm O2, 60 °C), with irradiation times of 80 min converting 24.5%, 17.7%, and 13.2% of the available methanol to formaldehyde by the Cu, Fe, and Co chromite spinel catalysts, respectively. Importantly, reactions run under identical conditions of concentration, time, and temperature using traditional convective heating yielded dramatically lower amounts of conversions; specifically, 1.0% and 0.21% conversions were observed with Cu and Co spinels, and no observable thermal products wer...

Microwave-assisted conversion of ethane to ethylene

A novel microwave reactor has been constructed for the conversion of ethane to ethylene. The bench-top, pilot-scale facility is capable of conventional furnace heating and microwave-assisted heating, thus allowing direct comparative studies between the conventional industry-wide process and the novel microwave-assisted conversion process. The latter makes use of chemical substances called susceptors that are good microwave absorbers and can transfer heat energy to the surrounding environment. The ethane feed stream is thus heated to achieve thermal conversion to ethylene. Data validating the apparatus and preliminary data on the feasibility of conversion are presented.

Structural, infrared, magnetic and microwave absorption properties of rare earth doped X-type hexagonal nanoferrites

A series of Sr2−xNdxNi2 Fe28−yCoyO46 (x=0.02, 0.04, 0.06, 0.08, 0.10 and y=0.1, 0.2, 0.3, 0.4, 0.5) X-type hexagonal ferrites has been prepared by sol–gel method at 1250°C for 6h. XRD analysis shows the single phase of the prepared ferrites material. The lattice constant values and c/a ratio shows that the present assembled material belongs to the family X-type hexagonal ferrites. The peaks in the FTIR spectra are the characteristics of the hexagonal ferrites. The crystallite size measured by Scherer formula is in good agreement with the crystallite size measured by TEM and HRTEM. The enhancement in the magnetic properties can easily be observed with the doping of rare earth element Nd3+ and divalent Co2+. Due to high saturation and low coercivity values the present material can be good microwave absorber. The temperature dependent saturation and remanance magnetization decreases monotonically with temperature. The minimum value of reflection loss enhances with the substituent and the attenuation constant results are in agreement with reflection loss results.

Synthesis and enhanced microwave absorption properties of Ni@Ni2O3 core–shell particles

In this paper, we reported on the enhanced microwave absorbing properties of the Ni@Ni2O3 core–shell soft magnetic particles which were synthesized by a simple hydrogen peroxide oxidation method. The chemical state and the morphology of the Ni@Ni2O3 particle surface were analyzed by X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HRTEM), respectively. Complex electromagnetic parameters (permittivity and permeability) of the paraffin mixed composite were measured in the frequency range of 0.1–18GHz. Compared with the uncoated Ni/paraffin composite, the permittivity of Ni@Ni2O3/paraffin composite decreased dramatically. The reflection loss (RL) of the composite exceeding −20dB in the frequency range of 4.0–12.6GHz were obtained for the absorber thicknesses of 1.8–4.3mm, while an optimal RL of −53.5dB was achieved at 7.9GHz for the thickness of 2.5mm. The good microwave absorption properties resulted from a proper impedance match as a consequence of the insulator Ni2O3 shell around Ni particles. Quantitative calculation demonstrated that not only the RL peak position but also the number of the peaks were determined by the quarter-wavelength matching model.

Synthesis and characterization of polyaniline nanoparticles with enhanced microwave absorption

A series of polyaniline (PANI) materials have been prepared by a reverse dropping method with the assistance of polyvinylpyrrolidone (PVP). It can be found that PVP and the dropping rate play critical roles in determining the morphology evolution of PANI, and the restriction of PVP is highly dependent on the dropping rate. Well dispersed PANI nanoparticles can only be obtained under moderate conditions. Thanks to the unique preparative process, these samples show significant changes in the length and oxidation state of conjugated chains, as proved by the results of UV/vis absorption spectra and FT-IR spectra, which result in their distinguishable conductivity and microwave absorption. Very interestingly, PANI nanoparticles exhibit substantially enhanced microwave absorption properties. In particular the optimum one, PANI-NP2, presents very strong reflection loss (−40.5 dB at 5.8 GHz) and wide response bandwidth (3.2–18 GHz over −10 dB), which are indeed comparable to those composites of PANI with various magnetic particles, implying promising applications as a kind of light-weight and highly effective microwave absorber. By systematically investigating the electromagnetic parameters, it can be concluded that suitable complex permittivity, improved characteristic impedance and multiple relaxation processes in PANI nanoparticles should be responsible for their good microwave absorption.

碳包覆镍纳米微粒的吸波性能研究

用直流碳弧法，在氩气氛下烧制碳包覆镍纳米微粒。使用矢量网络分析仪AV3618测得碳包覆镍纳米微粒在1~18 GHz的电磁参数，分析材料电磁性能。结果显示，由于碳包覆镍纳米微粒的碳层与镍核的特殊结构，使得阻抗匹配良好，而测试的磁导率参数表明，碳包覆镍纳米微粒复合材料在5~10 GHz和13 GHz存在损耗峰，这可能是由纳米颗粒表面各向异性，表面能的增加导致的自然共振引起的。模拟测试表明2 mm厚碳包覆镍纳米微粒复合材料在13 Ghz时的微波吸收效果可以达到−30 dB，当厚度增加到3和4 mm厚时，峰值增加到−63 dB，但是对应频段却向低频转移，分别为5.2和8.2 GHz。模拟计算显示，碳包覆镍纳米微粒复合材料具有很好的微波吸收性能，尤其对高频段微波具有更好的吸收性能。 The carbon-coated nickel (C@Ni) was prepared by direct current arc method in argon atmosphere. The permittivity and permeability of carbon-coated nickel composite were practically tested in 1 - 18 GHz range using AV3618 machine. The impedance between carbon and nickel matched well as a result of the special structure of C@Ni, while the permeability showed that there were loss peaks within the frequency of 5 - 10 GHz and 13 GHz owning to the natural resonance. The maximum analog reflection losses of 2 mm thickness C@Ni composite layer can reach −30 dB at 13 GHz, while the reflection losses of C@Ni became −63 dB with both 3 mm thickness at 5.2 GHz and 4 mm thickness at 8.2 GHz. There was a good microwave absorption performance of carbon-coated nickel composite through analog computation, especially better microwave absorption performance within high microwave frequency.

Mechanochemical Carboaluminothermic Reduction of V2O5 to Produce VC-Al2O3 Nanocomposite

The aim of this investigation is to produce VC–Al2O3 nanocomposite by reducing V2O5 with Aluminum and black carbon powders via mechanochemical process. The effect of milling time on this process was investigated. Milling process was done powder mixture at a rotation speed of 250 rpm for different times. Results showed that VCx has been synthesized after 1 hour of milling. The characterization of phase formation, crystallite size, strain percentage and lattice parameter was done by XRD analysis. To study the morphological evolution and determination of particle size of nanocomposite powders, Field Emission Scanning Electron Microscope (FESEM) was used. The crystallite size and lattice strain were determined by Williamson-Hall method. XRD study showed that for 6 h milling, the width of V4C3 peaks increased while the crystallite size of these phases decreased to about 27nm. In order to formVC–Al2O3 nanocomposite, the mixture was heat treated with the aid of microwave oven. The composite revealed good microwave absorption and heated up to 1150°C.

Magnetic and microwave absorption properties of electrospun Co0.5Ni0.5Fe2O4 nanofibers

Co0.5Ni0.5Fe2O4 nanofibers with 70–100nm in diameter were fabricated by the electrospinning and calcination process. Their saturation magnetization gradually enhances from 37.5 to 66.2emu/g with increasing calcination temperature from 450 to 800°C, while the corresponding coercivity increases initially, achieving a maximum value of 78.3kA/m at 600°C and then decreases with further increase in calcination temperature due to the transformation of magnetic structure from single-domain to multi-domain. By dispersing the nanofibers into a silicon rubber matrix homogeneously, the electromagnetic parameters of them were measured in the frequency range of 2–18GHz and the microwave reflection loss was calculated according to the transmission line theory. The reflection loss exceeding −5dB is obtained between 11 and 18GHz for the silicon rubber composites containing 15vol% Co0.5Ni0.5Fe2O4 nanofibers with coating thicknesses of 2.5–3.2mm. These Co0.5Ni0.5Fe2O4 nanofibers possess a good microwave absorption performance in the Ku band and have a great potential as microwave absorber for practical applications.

Enhanced microwave absorption of Fe flakes with magnesium ferrite cladding

Surface cladding of Magnesium ferrite (MgFe2O4) was found to be helpful for effectively decreasing the permittivity of Fe flakes. So the electromagnetic impedance matching was improved, resulting in a good microwave absorption. Careful characterizations showed that the giant decreases of permittivity were ascribed to the high resistivity of surface due to the MgFe2O4 cladding. A high frequency microwave absorption property with thin absorber thickness was obtained for the Fe flakes with 10wt% MgFe2O4.

Dielectric and Microwave-absorbing Properties of the Carbon/Alumina/Silica Coating

The permittivity and microwave-absorbing properties of carbon black or carbon fiber filled alumina/ silica coating were investigated in X-band. The results indicate that both the real and imaginary part of the complex permittivity increase with the increasing content of carbon absorber. With the same content of carbon absorber, the coatings filled with carbon fiber show higher complex permittivity than the coatings filled with carbon black. As the absorber contents increase to 2wt%, the complex permittivity of the coatings decrease with the increase of fre- quency, which is so-called frequency-dependence dielectric response. The minimum reflection loss of the coatings shifts to low frequency region as their thickness increasing. The results of the microwave absorbing properties show that good microwave absorption ability (below −10 dB) can be obtained in the frequency range from 9.2 GHz to 12.4 GHz for the coating with 2wt% carbon black and 2 mm in thickness.

Relationship of mechanical properties and temperature of carbon fiber-reinforced plastics under microwave irradiation

One of the problems with the use of carbon fiber-reinforced plastics (CFRP) is the difficulty of recycling or disposing of them. Hence, it is necessary to find processes to treat CFRP wastes faster and more effectively. Because carbon fibers are good microwave absorbers, there is significant potential for the use of microwave irradiation for the processing of CFRP wastes. In this study, the relationship of mechanical properties and radiation temperature during microwave irradiation of unidirectional and cloth-type CFRP were studied. Experimental results indicate that microwave irradiation degrades CFRP’s mechanical properties by breaking down the bond between the polymer matrix and the carbon fibers.

Microwave absorption properties of in situ grown CNTs/SiC composites

CNTs/SiC composites were synthesized by pyrolysis of methane over SiC supported nickel catalysts. The composites show good microwave absorption ability in the frequency range of 2–18GHz. The pyrolysis temperature has profound influences on the microwave absorption characteristic, and 700°C is the most suitable temperature for preparation of the composites. The microwave absorption characteristic is also influenced by the Ni loading. By varying the Ni loading from 5wt.% to 7.5wt.%, the bandwidth of reflection loss below −10dB can be broadened by 1.5GHz under the same conditions. For the composite obtained from 7.5wt.% Ni loading and the pyrolysis temperature of 700°C, a minimum reflection loss of −37.6dB can be obtained at 14.5GHz with a thickness of 1.9mm and the bandwidth of reflection loss below −10dB can reach 5.1GHz (from 12.4 to 17.5GHz) with a thickness of 2mm.