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.     

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.     

Structural and magnetic properties of sol-gel Co2xNi0.5−x Zn0.5−xFe2O4 thin films

Nanocrystalline Co_2xNi_0.5−xZn_0.5−xFe₂O₄ (x=0−0.5) thin films have been synthesized with various grain sizes by a sol-gel method on polycrystalline silicon substrates. The morphology as well as magnetic and microwave absorption properties of the films calcined at 1073 K were studied using X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, and vibrating sample magnetometry. All films were uniform without microcracks. The Co content in the Co-Ni-Zn films resulted in a grain size ranging from 15 to 32 nm while it ranged from 33 to 49 nm in the corresponding powders. Saturation and remnant magnetization increased with increase in grain size, while coercivity demonstrated a drop due to multidomain behavior of crystallites for a given value of x. Saturation magnetization increased and remnant magnetization had a maximum as a function of grain size independent of x. In turn, coercivity increased with x independent of grain size. Complex permittivity of the Co-Ni-Zn ferrite films was measured in the frequency range 2-15 GHz. The highest hysteretic heating rate in the temperature range 315-355 K was observed in CoFe₂O₄. The maximum absorption band shifted from 13 to 11 GHz as cobalt content increased from x=0.1 to 0.2.     

Microwave absorption properties of Sr2FeMoO6 nanoparticles

The microwave absorption properties of nanosized double perovskite Sr₂FeMoO₆ and epoxy resin composites were investigated in the frequency range of 2-18 GHz using the coaxial method. The Sr₂FeMoO₆ composites with an optimal 20 wt% epoxy resin showed a strong electromagnetic attenuation of −49.3 dB at 8.58 GHz with a matching thickness of 2.15 mm. Moreover the optimum absorption frequency at which the reflection loss is less than −20 dB, which corresponds to 99% reflection loss of the incident microwave, is from 5.7 to 13.2 GHz with the matching thickness ranging from 3.0 to 1.5 mm. The excellent microwave-absorption properties are a consequence of a proper electromagnetic match due to the existence of the insulating matrix of anti-site defects and anti-phase domains, which not only contribute to the dielectric loss but also to the reduced eddy current loss.     

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.     

Only Ku-band microwave absorption by Fe3O4/ferrocenyl-CuPc hybrid nanospheres

A novel kind of hybrid nanospheres made of Fe₃O₄ and ferrocenyl-CuPc (FCP) was prepared via effective solvothermal method and performed microwave absorptivity only in Ku-band with minimum reflection loss of −25 dB at 16.0 GHz corresponding to absorbing about 99.7% content of microwave. Scanning electron microscopy images indicated that the nanospheres with uniform particle size distribution have the average diameter of 135 nm. Due to the synergistic reaction between magnetic ferrocenyl-CuPc and Fe₃O₄, the hybrid nanospheres showed novel electromagnetic properties. The real part of complex permittivity of hybrid nanospheres remains stable in the range of 0.5–12.0 GHz and has a large fluctuation at 16.5 GHz. Moreover, the dielectric loss of hybrid nanospheres also appeared a sharp peak at 16.3 GHz with the value of 2.7. The specific gravity of hybrid nanospheres is about 2.08. On the basis of these results, the novel hybrids are believed to have potential applications in the microwave absorbing area in Ku-band.     

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.     

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.     

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.     

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.     

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.     

Model design on calculations of microwave permeability and permittivity of Fe/SiO2 particles with core/shell structure

Fe/SiO₂ particles with core/shell structure were prepared by coating silica on the surface of a commercial spherical carbonyl iron via the hydrolysis process of tetraethyl orthosilicate (TEOS). The electromagnetic performance of commercial carbonyl iron and as-prepared Fe/SiO₂ particles was studied theoretically and experimentally. As predicted by the theoretical calculation based on the Bruggeman formula and the Landau–Lifshitz–Gilbert (LLG) theory, the insulating surface layer of silica was effective to reduce the permittivity parameters of pure carbonyl iron. The measured results showed good agreement with the theoretical prediction. Although there was a little decrease in the permeability of the Fe/SiO₂ core/shell particles, a better impedance match especially at higher frequency range was obtained when used as a microwave absorber. The reflection loss (RL) curves show that the lowest reflection loss of Fe/Epoxy composite (−20.5 GHz) was obtained corresponding to the frequency of 8.5 GHz when the thickness of the absorber was 3 mm. A different trend was observed in Fe/SiO₂/Epoxy composite. The reflection loss value got lower by decreasing the thickness of absorbers. At the thickness of 2.2 mm, a relative low reflection loss (−17 GHz) corresponding to the frequency of 13.6 GHz was obtained. Compared with the Fe/Epoxy composite, the improvement on shifting the reflection loss peak to higher frequency and on reducing the optimal thickness of absorbers was made by Fe/SiO₂/Epoxy composite.     

Sintering temperature dependence of room temperature magnetic and dielectric properties of Co0.5Zn0.5Fe2O4 prepared using mechanically alloyed nanoparticles

Co_0.5Zn_0.5Fe₂O₄ nanoparticles were prepared using mechanical alloying (MA) and sintering. The crystallite size, coercivity, retentivity and saturation magnetization were also measured. The frequency dependence of dielectric and the magnetic parameters, namely, real permittivity ε′, loss tanget tan δ, real permeability μ′ and loss factor μ″ were measured at room temperature for samples sintered from 600 to 1000 °C, in the frequency range 10 MHz to 1.0 GHz. The results show that the crystallite size of the resulting products ranges between 16 and 67 nm for as-milled sample and the sample sintered at 1000 °C, respectively. The sample sintered at 1000 °C, measured at room temperature exhibited a saturation magnetization of 37 emu g⁻¹. The values of permittivity remain constant within the measured frequency, but vary with sintering temperature. The permeability values, on the other hand however vary with both the sintering temperature and the frequency, thus, the absolute value of the permeability decreased after the natural resonance frequency.     

Microwave dielectric and magnetic properties of superparamagnetic 8-nm Fe3O4 nanoparticles

The superparamagnetic 8-nm Fe₃O₄ nanoparticles were successfully prepared by chemical oxidation process. For the complex permittivity, the dual dielectric relaxation processes have been proved by two overlapped Cole–Cole semicircles, and the natural resonance frequency is 3.03 GHz for the complex permeability. The maximum reflection loss value reaches −55.5 dB at 6.11 GHz with 3.85 mm in the thickness of the absorbers for the superparamagnetic 8-nm Fe₃O₄ nanoparticles which is better than that of 150 nm and 30 nm Fe₃O₄ nanoparticles. It is believed that the superparamagnetic 8-nm Fe₃O₄ nanoparticles can be used as a kind of candidate for microwave absorber.     

Post-sintering annealing effect on the diffuse phase transition in (Pb1−xBax)(Yb0.5Ta0.5)O3 ceramics

The effect of post sintering annealing on the dielectric response of (Pb_1−xBa_x)(Yb_0.5Ta_0.5)O₃ ceramics in the diffuse phase transition range (x=0.2) has been investigated. The samples are prepared by conventional solid-state reaction method. The samples are sintered at 1300 °C for 2 h and annealed at different temperatures (800, 900 and 1000 °C) for 8 h and at 800 °C for different time durations (8, 12 and 24 h). A significant change in the dielectric response has been observed in all the samples. The dielectric constant increases remarkably and the dielectric loss tangent decreases. The dielectric peaks of the annealed samples are observed to be more diffused with noticeable frequency dispersion compared to the as sintered sample.     

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.     

Electrical properties of SrBi2(Nb0.5Ta0.5)2O9 ceramics

Aurivillius SrBi₂(Nb_0.5Ta_0.5)₂O₉ (SBNT 50/50) ceramics were prepared using the conventional solid-state reaction method. Scanning electron microscopy was applied to investigate the grain structure. The XRD studies revealed an orthorhombic structure in the SBNT 50/50 with lattice parameters a=5.522 Å, b=5.511 Å and c=25.114 Å. The dielectric properties were determined by impedance spectroscopy measurements. A strong low frequency dielectric dispersion was found to exist in this material. Its occurrence was ascribed to the presence of ionized space charge carriers such as oxygen vacancies. The dielectric relaxation was defined on the basis of an equivalent circuit. The temperature dependence of various electrical properties was determined and discussed. The thermal activation energy for the grain electric conductivity was lower in the high temperature region (T>303.6 °C, E_a−ht=0.47 eV) and higher in the low temperature region (T<303.6 °C, E_a−lt=1.18 eV).     

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.     

Sol-gel preparation and characterization of CoFe2O4-SiO2 nanocomposites

Magnetic nanocomposites formed by cobalt ferrite particles dispersed in a silica matrix were prepared by a sol-gel process. The effects of the thermal treatment temperature and the salt concentration on the structural and magnetic properties of the composites were investigated. By controlling these parameters, CoFe₂O₄/SiO₂ nanocomposites with different crystallite size and magnetic properties were obtained. By increasing the annealing temperature and salt concentration, composites with a progressive increase in the coercive field and of the density of magnetization were produced. In particular, a nanocomposite, with a Fe/Si molar concentration of 21%, obtained by drying the gel at 150 °C and further annealing at 800 °C, has a coercivity of 2000 Oe, which is more than twice higher than the coercivity of bulk cobalt ferrite.     

Plasma sprayed Al2O3/FeCrAl composite coatings for electromagnetic wave absorption application

Al₂O₃/FeCrAl composite coatings were fabricated by atmosphere plasma spraying technique. Microstructure and dielectric properties in the frequency range from 8.2 to 12.4 GHz were investigated. The microstructure of composite coatings shows a uniform dispersion of metal particles with litter pores and microcracks in the composite coatings. The relaxation polarization and interfacial polarization in the coatings would contribute to enhance ?′ with rising FeCrAl content, and the associated loss could be considered as a dominating factor enhancing ?″. By calculating the microwave-absorption as a single-layer absorber, for the composite coatings with 41 wt.% FeCrAl content, the reflection loss values exceeding −10 dB are achieved in the frequency range of 9.1-10.6 GHz when the coating thickness is 1.3 mm.