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.     

γ-MnO2/polyaniline composites: Preparation, characterization, and applications in microwave absorption

MnO₂/doped polyaniline (PANI) is prepared by an in situ polymerization method using γ-MnO₂ as the addition agent and hydrochloric acid as the doping agent. Products are characterized by FT-IR, UV-vis, XRD, and TEM. Conductivity, electromagnetic properties, and microwave absorption properties are first discussed on the basis of structural characterization. The as-prepared products of MnO₂/PANI are partially crystalline in nature and spherical in pattern with grain sizes of 50-70 nm. MnO₂ particles are successfully decorated with doped PANI. MnO₂/PANI displays moderate electric conduction, excellent dielectric losses, and microwave absorption capabilities. Compared to pure MnO₂, the dielectric and reflection loss properties of MnO₂/PANI composites exhibit significant improvements, with an effective absorption band at 5 GHz under −10 dB and maximum reflection loss of −21 dB at 13.56 GHz. Pure MnO₂ shows an effective absorption band of 3 GHz under −10 dB and a maximum reflection loss of −14.20 dB at 11.5 GHz. Thus, MnO₂/PANI composites are found to be a promising microwave absorption material.     

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.     

Optimization of electromagnetic matching of carbonyl iron/BaTiO3 composites for microwave absorption

Microwave absorbing materials filled with BaTiO₃ and carbonyl iron (CI) particles with various weight fractions (BaTiO₃/CI particles=100/0 to 0/100) are investigated. The dielectric and magnetic properties of the absorbers can be tuned by changing the weight ratio of BaTiO₃/CI particles in the frequency range of 2-18 GHz. Numerical simulations are also performed to design a single-layer and double-layer absorber. The minimum reflection loss of the composite filled with 20 wt% BaTiO₃ and 60 wt% CI particles at 2.0 mm thickness can be reached to −42 dB at 4.1 GHz. With the weight ratio of CI particles in the composite increased, the microwave absorption peak shifted to the lower frequency region. By using a double-layer absorber structure, the microwave absorption performance of the absorber is enhanced. The result shows that the total thickness of the absorber can be reduced below 1.4 mm by using a matching layer filled with 50 wt% BaTiO₃, and an absorption layer filled with 60 wt% BaTiO₃ and 20 wt% CI particles, whereas the reflection loss below −10 dB can be obtained in the frequency range of 10.8-14.8 GHz and the minimum reflection loss of −59 dB can be obtained at 12.5 GHz.     

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.     

Preparation and microwave absorption properties of Fe-phthalocyanine oligomer/Fe3O4 hybrid microspheres

The novel nano-scale Fe-phthalocyanine oligomer/Fe₃O₄ hybrid microspheres were synthesized from bis-phthalonitrile and FeCl₃·6H₂O through a simple solvent-thermal route. The morphology and structure of the hybrid microspheres were characterized by FTIR, XRD, SEM and TEM. These results showed that the hybrids were monodispersed solid microspheres and the morphology can be adjusted by controlling the addition of bis-phthalonitrile. On the basis of these results, the formation process was discussed. Magnetization measurement indicated that saturation magnetizations decreased linearly with increasing the addition of bis-phthalonitrile, while coercivities increased. The microwave absorption properties were measured by a vector network analyzer. The dielectric loss of the hybrid microspheres was larger and a new magnetic loss peak appeared at high frequency. The microwave absorbing properties enhanced with increasing the addition of bis-phthalonitrile and a maximum reflection loss of −31.1 dB was obtained at 8.6 GHz with 1 g bis-phthalonitrile when the matching thickness was 3.0 mm. The novel hybrid materials are believed to have potential applications in the microwave absorbing performances.     

The influence of temperature on magnetic and microwave absorption properties of Fe/graphite oxide nanocomposites

Fe/graphite oxide nanocomposites were prepared by inserting Fe³⁺ into layers of graphite oxide and then reducing Fe³⁺/graphite oxide compound at different reduced reaction temperatures in H₂. The composition, crystal structure, magnetic and microwave absorption properties of Fe/graphite oxide nanocomposites were investigated using elemental analysis, transmission electron microscope (TEM), X-ray diffraction (XRD), magnetic hysteresis curve and electromagnetic parameter analysis. The results show that the densities of samples are 2.43–2.47 g/cm³ and the nanocomposites are soft magnetic materials. The optimum reduced reaction temperature for preparing Fe/graphite oxide nanocomposites is 600 °C. With the increase of the thickness of the sample, the matching frequency tends to shift to the lower frequency region, and theoretical reflection loss becomes less at the matching frequency. Microwave absorption property of Fe/graphite oxide nanocomposites prepared at 600  °C (FeGO600) is the best. When the thickness is 1 mm, the maximum theoretical reflection loss of FeGO600 is −9 dB and the frequency region in which the maximum reflection loss is more than −6.0 dB is 11–18 GHz. In conclusion, FeGO600 is a good candidate for microwave absorbent due to its low density, wide frequency region for microwave absorption and large reflection loss.     

Adjustable microwave absorption properties of flake shaped (Ni0.5Zn0.5)Fe2O4/Co nanocomposites with stress induced orientation

Flake shaped (Ni_0.5Zn_0.5)Fe₂O₄/Co nanocomposites were successfully fabricated by co-precipitating of Ni-Zn ferrite on the surface of cobalt nanoflakes. The electromagnetic characteristics of the samples were studied at the frequency of 0.1–14 GHz. The results showed that the cobalt nanoflakes in compacted nanocomposites were well orientated, and the nanocomposites were characterized with low optimal reflection loss (RL) of −33.8 dB at 11.5 GHz and broad RL bandwidth for <−20 dB in the frequency range of 7.6–12.1 GHz. At the same time, the position of the absorptive band can be adjusted by changing the mass ratio of ferrite to cobalt in the nanocomposites. It is proposed that the excellent microwave absorption properties are related to the combination of strong shape anisotropy of cobalt nanoflakes and adjustable dielectric loss.     

Microwave absorption properties of one thin sheet employing carbonyl-iron powder and chlorinated polyethylene

To solve more and more serious electromagnetic interference problem, one thin microwave absorbing sheet employing carbonyl-iron powder (CIP) and chlorinated polyethylene (CPE) was prepared. The pattern, static magnetic properties and phase of CIP were characterized by scanning electron microscope (SEM), vibrating sample magnetometer (VSM) and X-ray diffraction (XRD), respectively. The electromagnetic parameters of CIP were measured in the frequency range of 2-18 GHz, and the electromagnetic loss mechanisms of the powder were discussed. The microwave absorption properties of composite sheets with different thicknesses and CIP ratios in matrix were investigated by measuring reflection loss (RL) in 2-18 GHz frequency range using the arch method. The results showed that appropriate CIP content and thickness could greatly improve microwave absorption properties in lower frequency range. For the sample with the weight ratio (CIP:CPE) of 16:1 and 1.5 mm thickness, the bandwidth (RL below −10 dB) achieved 1.1 GHz (2-3.1 GHz), and the minimum reflection loss value was obtained −13.2 dB at 2.2 GHz. This suggested that CIP/CPE composites could be applied as thin microwave absorbers in S-band (2-4 GHz).     

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

La_1−xAg_xMnO₃ 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 2–18 GHz 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 La_0.85Ag_0.15MnO₃ 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 La_0.85Ag_0.15MnO₃ sample is enhanced with the bandwidth below −10 dB at about 6 GHz and the peak value of reflection loss is near −25.0 dB at the layer thickness of 2 mm.     

Electromagnetic wave absorption properties of flaky Fe-Ti-Si-Al nanocrystalline composites

Nanocrystalline Fe-Ti-Si-Al flaky powders with large aspect ratio have been fabricated by high-energy planetary milling melt-spun ribbons. Magnetic composites made of thermally annealed Fe-Ti-Si-Al flaky powders embedded in epoxy resin exhibit excellent electromagnetic wave absorption properties in a wide frequency range from 100 MHz to 2.65 GHz. Ti additions can enhance the electrical resistivity of Fe-Si-Al alloy without changing the crystallographic structure, resulting in the reduction of eddy current loss at high frequencies. Moreover, the substitution of Ti for Fe enhances the cut-off frequency of flaky powder-polymer composites. Meanwhile, compared to the Ti-free composites, reflection loss of the Ti-doped composites (3 mm in thickness) is lower than −10 dB in the frequency range from 593 MHz to 1.83 GHz, indicating that the composites can effectively work in a wider frequency bandwidth.     

Synthesis, characterization and electromagnetic properties of Fe1−xCox alloy flower-like microparticles

Fe_1−xCo_x alloy microparticles with size 3-5 μm and novel flower-like shapes were prepared by a simple low temperature reduction method. The electromagnetic properties for the paraffin matrix composites containing Fe_1−xCo_x alloy microparticles were measured using a vector network analyzer in the 2-18 GHz frequency range. As a consequence of large surface- and shape-anisotropy energy for the flower-like shaped 3D microstructures, the strong natural resonance around 8-12 GHz and remarkable dielectric relaxation were observed in the complex permittivity and permeability spectrum, which are dominant in the enhanced electromagnetic wave absorption (EMA) performance. It was found that both the electromagnetic parameters of complex permittivity and permeability and the intensity and location of absorption band were remarkably dependent on the Co/Fe molar ratio. The enhanced EMA performance was obtained in these Fe_1−xCo_x-paraffin (x=0.4, 0.5, and 0.6) composites system. For the Fe_0.5Co_0.5 alloy, the reflection loss (RL) exceeding −20 dB was obtained in the broad frequency range of 5.4-18 GHz with a thin sample thickness of between 1.0 and 2.9 mm. In particular, an optimal RL of −59 dB was obtained at 3.61 GHz with a thin thickness of 3.6 mm for the Fe_0.4Co_0.6 sample. The Fe_1−xCo_x alloy microparticles may be attractive candidates for applications of microwave absorption materials with a wide frequency range and strong absorption in the high frequency region.     

The microwave absorption properties of La0.8Sr0.2Mn1−yFeyO3 nanocrystalline powders in the frequency range 2-18 GHz

La_1−xSr_xMn_1−yFe_yO₃ nanocrystalline powders were prepared by the sol-gel method as a microwave absorption material. The reflectance, the dielectric loss tan δ_e and the magnetic loss tan δ_m of the samples were calculated according to the data of electromagnetism parameters measured by a microwave vector network analyzer in the frequency range 2-18 GHz. The dielectric loss tan δ_e and the magnetic loss tan δ_m had a step-change at a certain frequency so that the superiority of dielectric loss change into the superiority of magnetic loss, which indicated that anti-ferromagnetic clusters in the material change into ferromagnetic clusters by absorbing quantum of microwave electromagnetic field when the frequency of incident microwave reaches a certain value. The effective absorption bandwidth higher than 10 dB reached 6.2 GHz. As a result, the La_0.8Sr_0.2Mn_1−yFe_yO₃ has shown useful applications as a microwave absorption material.     

Microwave absorbing properties of rare-earth elements substituted W-type barium ferrite

W-type barium ferrites Ba(MnZn)_0.3Co_1.4R_0.01Fe_15.99O₂₇ with R=Dy, Nd and Pr were prepared by chemical coprecipitation method. Effects of rare-earth elements (RE) substitution on microstructural and electromagnetic properties were analyzed. The results show that a small amount of RE³⁺ ions can replace Fe³⁺ ions and adjust hyperfine parameters. An obvious increase in natural resonance frequency and high frequency relaxation, and a sharp decrease for complex permittivity have been observed. Furthermore, the matching thickness and the reflection loss (RL) of one-layer ferrite absorber were calculated. It reveals that thin and broad-band can be obtained by RE-substitution. But only when the magnetic moment of RE³⁺ is higher than that of Fe³⁺, can substitution be effective for higher RL. Dy-substituted ferrite composite has excellent microwave absorption properties. The frequency (with respect to −10 dB RL) begins from 9.9 GHz, and the bandwidth reaches far more than 8.16 GHz. The peak value is −51.92 dB at a matching thickness of 2.1 mm.     

Electromagnetic and microwave absorption properties of Fe–Sr0.8La0.2Fe11.8Co0.2O19 shell-core composites

Shell-core Fe–Sr_0.8La_0.2Fe_11.8Co_0.2O₁₉ composites are prepared by chemical vapor deposition (CVD) for use as microwave absorbing materials. Scanning electron microscopy and X-ray diffraction analyses show that the CVD method yields Sr_0.8La_0.2Fe_11.8Co_0.2O₁₉ powders with a uniform coating of Fe. Compared with Sr_0.8La_0.2Fe_11.8Co_0.2O₁₉, Fe–Sr_0.8La_0.2Fe_11.8Co_0.2O₁₉ composites have higher electrical conductivity, permittivity, and dielectric loss, which gradually increase with increasing Fe content. When Sr_0.8La_0.2Fe_11.8Co_0.2O₁₉/Fe=7:3, a reflection loss (RL) exceeding −10 dB is obtained in the frequency range of 10–14 GHz at a coating thickness of 2.0 mm. A minimum RL of −30 dB was found at 8.0 GHz, corresponding to a matching thickness of 2.8 mm.     

Enhanced microwave absorption in ZnO/carbonyl iron nano-composites by coating dielectric material

The microwave absorption properties of zinc oxide/carbonyl iron composite nanoparticles fabricated by high energy ball milling were studied at 0-20 GHz. Experiments showed that ZnO as a kind of dielectric material coating carbonyl iron particles made the bandwidth of reflection loss (RL)<−5 dB expanding to the low frequency, and enhanced absorption effect obviously. For a 3 mm thickness absorber of ZnO/carbonyl iron after 30 h milling, the values of RL<−5 dB and RL<−8 dB were obtained in the frequency range from 7.0 GHz to 17.8 GHz and from 9.8 dB to 14.9 dB, respectively, and its strongest RL peak was −29.34 dB at 13.59 GHz. The magnetic loss of carbonyl iron particles and the dielectric loss of ZnO particles were the main mechanisms of microwave absorption for the composites.     

Iron phthalocyanine oligomer/Fe3O4 hybrid microspheres and their microwave absorption property

The novel nano-scale iron phthalocyanine oligomer/Fe₃O₄ (FePc/Fe₃O₄) hybrid microspheres were synthesized from iron phthalocyanine oligomer and FeCl₃·6H₂O via a solvent-thermal crystallization route. The morphology and structure of the hybrid microspheres were characterized by Fourier transform infrared spectrophotometer, X-ray diffraction, scanning electron microscopy and transmission electron microscopy. These results showed that the hybrids were monodisperse microspheres and the morphology can be adjusted by controlling pre-polymerization time. The saturation magnetization increased with increase in the pre-polymerization time, while the coercivities decreased. The FePc/Fe₃O₄ hybrid microspheres exhibited novel microwave electromagnetic properties: the dielectric loss was enhanced when the pre-polymerization time increased and a new microwave loss peak appeared at high frequency. The microwave absorbing properties enhanced with increase in the pre-polymerization time and a maximum reflection loss of −29.7 dB was obtained at 11.7 GHz with 6 h of pre-polymerization time when the matching thickness was 3.0 mm. The novel hybrid materials are believed to have potential applications as microwave absorbing materials.     

The electrochemical preparation and microwave absorption properties of magnetic carbon fibers coated with Fe3O4 films

Electrodeposition was employed to fabricate magnetite (Fe₃O₄) coated carbon fibers (MCCFs). Temperature and fiber surface pretreatment had a significant influence on the composition and morphology of Fe₃O₄ films. Uniform and compact Fe₃O₄ films were fabricated at 75 °C on both nitric acid treated and untreated carbon fibers, while the films prepared at 60 °C were continuous and rough. Microwave measurements of MCCF/paraffin composites (50 wt.% of MCCFs, pretreated carbon fibers as deposition substrates) were carried out in the 2-18 GHz frequency range. MCCFs prepared at 60 °C obtained a much higher loss factor than that prepared at 75 °C. However, the calculation results of reflection loss were very abnormal that MCCFs prepared at 60 °C almost had no absorption property. While MCCFs prepared at 75 °C exhibited a good absorption property and obtained −10 dB and −20 dB refection loss in wide matching thickness ranges (1.0-6.0 mm and 1.7-6.0 mm range, respectively). A secondary attenuation peak could also be observed when the thickness of MCCF/paraffin composite exceeded 4.0 mm. The minimum reflection loss was lower.     

Magnetic and microwave absorption properties of BaFe12−x (Mn0.5Cu0.5Zr)x/2O19 synthesized by sol–gel processing

BaFe_12−x (Mn_0.5Cu_0.5Zr)_x/2O₁₉ hexaferrites with x=1, 2 and 3 were prepared by sol–gel process. The ferrite powders possess hexagonal shape and are well separated from one another. The powders of these ferrites were mixed with polyvinylchloride (PVC) plasticizer to be converted into a microwave absorbing composite ferrite with a thickness of 1.8 mm. X-ray diffractometer (XRD), scanning electron microscope (SEM), ac susceptometer, vibrating sample magnetometer and vector network analyzer were used to analyze its structure, electromagnetic and microwave absorption properties. The results showed that magnetoplumbite structures for all samples were formed. The sample with higher magnetic susceptibility and coercivity exhibits a larger microwave absorbing ability. Also the present investigation demonstrates that a microwave absorber using BaFe_12−x(Mn_0.5Cu_0.5Zr)_x/2O₁₉ (x=2 and 3)/PVC with a matching thickness of 1.8 mm can be fabricated for applications over 15 GHz, with reflection loss more than −25 dB for specific frequencies, by controlling the molar ratio of the substituted ions.     

Preparation and microwave-absorbing properties of NiFe2O4-polystyrene composites

NiFe₂O₄ nanoparticles were synthesized by the polyacrylamide gel method with acrylamide as the monomer and N,N′-methylenediacrylamide as lattice agent. The average crystallite sizes of the nickel ferrites annealed at 500, 600 and 800 °C are about 10, 30 and 50 nm, respectively. Ferrite-polystyrene composites were made by hot pressing, and microwave-absorbing properties of the composites with different contents of 35, 45, 55 and 65 wt% ferrite were investigated by testing complex permeability and complex permittivity in the X-band (8.2-12.4 GHz) frequency range. All the parameters, ε′, ε″, μ′ and μ″, increase with increasing ferrite content. The reflection losses were calculated based on a model of a single-layered plane wave absorber backed by a perfect conductor. The composite with 65 wt% ferrite content shows a minimum reflection loss of −13 dB at 11.5 GHz with a −10 dB bandwidth over the extended frequency range of 10.3-13 GHz for an absorber thickness of 2 mm.