Microwave Reflection Loss Research Articles

Preparation, characterization and microwave absorption properties of barium-ferrite-coated fly-ash cenospheres

Magnetic composites of barium ferrite coated on fly-ash cenospheres (BFACs) were prepared by sol–gel auto-combustion method. To promote surface activity, we modified fly-ash cenospheres (FACs) surfaces using γ-aminopropyltriethoxysilane (APS) as coupling agent and silver nitrate as activating agent before coating barium ferrite films on FACs. The morphology, composition, crystal structure, magnetic and microwave absorption properties of these composite powders were characterized by scanning electron microscope, energy dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy, X-ray diffraction, vibrating sample magnetometer, and vector network analyzer. Continuous and uniform coatings of barium ferrite were found on the surfaces of the FACs. The BFACs powders-epoxy composite possesses excellent microwave absorption properties in the 2–18GHz frequency range. The maximum microwave reflection loss reaches −15.4dB at 8.4GHz with a thickness of 3.0mm, and the widest bandwidth less than −12dB is 6.2GHz with a sample thickness of 2.0mm. The intrinsic reasons for microwave absorption were also investigated. Applications of this composite material in magnetic recording, electromagnetic wave shielding, and lightweight microwave-absorbing fields are promising.

Effects of graphite additive on dielectric properties and microwave absorption properties of zinc-containing foam glass

This paper reports effects of graphite additive on dielectric properties and microwave absorption performances of zinc-containing foam glasses. Changes of pore structure of porous glass are analyzed. One-dimensional zinc oxide crystal whiskers on walls of foam glass are observed. With the graphite additive increases, both real (ε′) and imaginary (ε″) parts of complex permittivity increase from ~3.25 and ~0.2 to ~7.5 and ~2.8, respectively. Microwave reflection loss of porous glass was calculated based on the permittivity in the range of 8.2–12.4GHz(X-band). Reflection loss of foam glass decreases as graphite additive and the sample with 2.0wt% graphite additive achieve reflection loss less than −10dB in whole X-band with thickness ranging from 2.3 to 3.5mm. Linear relation between reduction of reflectivity and increase of graphite content is achieved. The results show that changing foaming agent additive is an efficient way to design dielectric and microwave absorption properties of foam glass. Foam glass with 1.0wt% graphite additive may find application in photocatalytic degradation of many environmental pollutants due to its considerable zinc oxide whiskers.

Effect of Ag substitution on the electromagnetic property and microwave absorption of LaMnO3

La1−xAgxMnO3 perovskites with different doping Ag-content were prepared by the sol–gel method. The electromagnetic characteristics and microwave loss behavior of these ion-doped rare-earth manganites were studied in the 2–18GHz frequency range. The microstructure and morphology of the samples were characterized by X-ray diffraction (XRD), and scanning electron microscopy (SEM) techniques. The complex permittivity spectra, the complex permeability spectra and microwave reflection loss were measured by a microwave vector network analyzer system. The XRD patterns show that the crystalline perovskite main phase ABO3 is formed and impurity phases disappear when calcined at 1100°C, and Ag metal as an impurity phase appears when excessive Ag+ is doped. The SEM image indicates that many of the La0.85Ag0.15MnO3 particles are fiber-like or ellipsoidal. Magnetic loss and dielectric loss coexist and cooperate in microwave attenuation by moderate substitution of Ag+ for La3+. The microwave absorption property of the La0.85Ag0.15MnO3 sample is enhanced with the bandwidth below −10dB at about 6GHz and the peak value of reflection loss is near −25.0dB at the layer thickness of 2mm.

Microwave absorption characteristics of substituted Ba0.5Sr0.5MxFe(12−2x)O19 (M = Co2+Zr4+ AND Co2+Ti4+) sintered ferrite at X‐band

AbstractThe microwave reflection loss (RL) of four series of synthesized ferrites, Ba0.5Sr0.5CoxZrxFe(12 − 2x)O19 and Ba0.5Sr0.5CoxTixFe(12 − 2x)O19, have been measured using network analyzer in the frequency range from 8.2 to 12.4 GHz. The comprehensive analysis of RL is carried out as a function of substitution, frequency, and thickness. The various models contributing to large RL are described. The composition accompanying maximum microwave absorption is suggested. © 2012 Wiley Periodicals, Inc. Microwave Opt Technol Lett 54:1661–1665, 2012; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26912

Electromagnetic characteristic and microwave absorbing performance of different carbon-based hydrogenated acrylonitrile–butadiene rubber composites

Hydrogenated acrylonitrile–butadiene rubber (HNBR) was mixed with carbon fiber (CF), conductive carbon black (CCB) and multi-walled carbon nanotubes (MWCNT) to prepare microwave absorbing composites, their complex permittivity was measured in microwave frequencies (2–18GHz), and their electromagnetic characteristics and microwave absorbing performance were studied. The real part and imaginary part of permittivity of the composites increased with increasing carbon filler loading, showing dependency on filler type. The microwave reflection loss of the composites also depended on the loading and type of fillers. The matching thickness of the absorber layer decreased with increasing permittivity, while the matching frequency decreased with increasing layer thickness. The minimum reflection loss was −49.3dB for HNBR/MWCNT (100/10) composite, while −13.1dB for HNBR/CCB (100/15) composite and −7.1dB for HNBR/CF (100/30) composite. The efficient microwave absorption of HNBR/MWCNT composites is accounted from high conduction loss and dielectric relaxation of MWCNT, and strong interface scattering.

Fabrication of Fe/Fe3C-functionalized carbon nanotubes and their electromagnetic and microwave absorbing properties

Fe/Fe3C-functionalized carbon nanotubes (CNTs) have been prepared by the floating catalyst chemical vapor-deposition method. It is demonstrated that the Fe and Fe3C nanostructures are both encapsulated in the CNTs or decorated on the surface of CNTs. The Fe/Fe3C content in the composites can easily be adjusted by changing the ferrocene concentration in the preparation. The electromagnetic properties of the CNTs have been evaluated in the frequency range of 2–18 GHz, and the nanocomposites exhibit excellent microwave absorbing performance. The CNT composites with higher Fe/Fe3C content show enhanced microwave reflection losses. The significant influence of the Fe/Fe3C nanostructures on the microwave absorption is realized by tuning the characteristic impedance of the nanocomposites. With increasing thickness, the maximum reflection loss peak shifts to lower frequency. The microwave absorbing performance of the composites is mainly caused by dielectric loss, resulting from the continuous CNT networks with excellent electrical conductivity.

Analyses on multiple resonance behaviors and microwave reflection loss in magnetic Co microflowers

AbstractTriple dielectric and magnetic resonance behaviors have been observed and studied in Co microflowers within a frequency range of 0.1–18.0 GHz. To characterize such triple‐resonance behaviors, the dielectric resonance was resolved by fitting the Lorentzian dispersion law, magnetic resonance spectra was fitted by the Landau–Lifshitz–Gilbert equation. It is demonstrated that our fitted and calculated resonance frequencies agree well with the experimental data. Dielectric resonance in permittivity was due to the dipole polarization. Magnetic resonance in permeability was due to the natural/exchange resonance. Both the permittivity and permeability determined and contributed to the microwave reflection loss.

Influence of the interface reflections on the microwave reflection loss for carbonyl iron/paraffin composite backed by a perfect conduction plate

In this work, the influence of interface reflections on the microwave reflection loss (RL) for carbonyl iron/paraffin composite backed by a perfect conduction plate with 30vol% concentration at various thicknesses was investigated in the 0.1–18GHz. Using a vector network analyzer, the scattering parameters (S11 and S21) were measured in two different ways. Based on the quarter-wavelength matching model, the results of measurement were analyzed. The experiment shows that there are many minimum values (dips) in RL at various thicknesses when the reflective wave, which is reflected from the absorbing layer and the emerging wave, which is reflected from the backed metal plate are out of phase by 180°, and the peak intensity of the RL is directly affected by the intensity of the reflective wave and the emerging wave. Furthermore, the experiment and numerical calculation demonstrates that the modulus of the normalized input impedance |Zin/Z0| equals approximately 1, but the ratio between the modulus of permittivity and permeability |ε/μ| is far from unity at the minimal reflection point.

Study on microwave absorption properties of metal-containing foam glass

Abstract Materials with the properties of electromagnetic (EM) wave absorption are attractive topics. In this work, we report that EM wave absorption composites, consisting of foam glass, zinc and zinc oxide, were prepared by sintering mixture of foam glass raw material and zinc powder. Microwave reflection loss of composite was calculated based on the permittivity in the range of 8.2–12.4 GHz. The results show that zinc-containing foam glass absorbs efficiently microwaves. The sample with zinc filler to foam glass mass ratio of 3/18 had a reflection loss below −10 dB in the range of 11.3–12.4 GHz, and the minimum reflectivity was −15.6 dB at both 12.0 and 12.4 GHz. Microwave absorption performances of specimens can be controlled by changing the ratio between zinc powder and foam glass mass. The detailed mechanism of the control was investigated through X-ray diffraction (XRD) analysis and scanning electrical microscopy (SEM) observations.

Electromagnetic properties and microwave absorption of W-type hexagonal ferrites doped with La 3+

W-type barium hexaferrites with compositions of Ba 1Co 0.9Zn 1.1Fe 16O 27 and Ba 0.8La 0.2Co 0.9Zn 1.1Fe 16O 27 were synthesized by the sol–gel method. The electromagnetic properties and microwave absorption behavior of these two ferrites were studied in the 2–18 GHz frequency range. The microstructure and morphology of the ferrites were characterized by X-ray diffraction (XRD), and scanning electron microscopy (SEM) techniques. The complex permittivity spectra, the complex permeability spectra and microwave reflection loss were measured by a microwave vector network analyzer. The XRD patterns show that the main phase of the Co 2W ferrite forms without other intermediate phases when calcined at 1200 °C. The SEM images indicate that flake-like hexagonal crystals distribute uniformly in the materials. Both the magnetic and dielectric losses are significantly enhanced by partial substitution of La 3+ for Ba 2+ in the W-type barium hexaferrites. The microwave absorption property of the La 3+ doping W-type hexaferrite sample is enhanced with the bandwidth below −10 dB around 8 GHz and the peak value of reflection loss about −39.6 dB at the layer thickness of 2 mm.

Microwave absorption properties of rod-shaped Co–Ni–P shells prepared by metallizing Bacillus

The rod-shaped Co–Ni–P shells were prepared by metalling Bacillus. The microstructures and composition of the shells were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive analysis (EDS). The electromagnetic parameters were measured by the coaxial line method in the frequency of 2–18 GHz. It was found that the Bacillus were successfully coated with Co–Ni–P, and the inner structure of the shells are hollow in structure. The shells exhibit excellent microwave absorption properties in 5–17 GHz frequency. The microwave reflection loss is above −10 dB in 5.38–16.6 GHz frequency. The maximum microwave reflection loss reaches −35.83 dB at 9.12 GHz for samples thickness 2.4 mm, and the widest bandwidth for microwave reflection loss above −10 dB is about ∼5.32 GHz for samples thickness 2.0 mm. These results confirm the feasibility of applying Bacillus as biotemplates for fabrication of the metallic shells as lightweight microwave absorption materials are very promising for applications.

Microwave Absorbing Property and Modification of Ferrite-encapsulated Cenosphere Powders

Ferrite-encapsulated cenosphere powders were prepared by citrate sol-gel method. The additive was used to improve the performance of ferrite coating. Microstructure of the products was tested by SEM and EDS. It was found that the coating was independent of the type of ferrite. Adding glycol or polyethylene glycol into the precursors can help the ferrite encapsulation. Microwave reflection loss of the coatings with thickness of 1.8mm made from ferrite-encapsulated cenosphere powders was tested in the frequency range from 5GHz to 18GHz. It was found that the ferrite-encapsulated cenosphere coating exhibited better microwave absorbing property than the pure ferrite. More,adding glycol can improve the microwave absorbing property of the coatings.

Electromagnetic properties of FeNi alloy nanoparticles prepared by hydrogen-thermal reduction method

FeNi alloy nanoparticles were prepared by hydrogen-thermal reduction in nickel ferrite nanoparticles at 400 °C for 1 h. FeNi alloy nanoparticles with average particle size of 150 nm in diameter are composed of Fe, Ni, and FeNi solid solution. The effective complex permittivity and complex permeability of FeNi alloy nanoparticles in the frequency range of 2–18 GHz were measured, and the microwave reflection loss of FeNi/wax with different assumed coating thicknesses was calculated according to the transmission line theory. The hydrogen-thermal reduced FeNi alloy nanoparticles were found to possess high permeability and superior microwave absorption performance in the microwave band.