Absorbing properties and structural design of microwave absorbers based on W-type La-doped ferrite and carbon fiber composites

Sol–gel method was used to prepare W-type BaCo₂Fe₁₆O₂₇ hexaferrite and La-doped Ba_0.7La_0.3Co₂Fe₁₆O₂₇ hexaferrite. Electromagnetic parameters of the ferrites and short carbon fiber composites were measured, and reflectivity was calculated according to transmission-line theory in the range 12.4–18 GHz. The results show that reflection loss of the doped ferrite composite is higher as compared to the no doped ferrite composite. Based on the above calculation, double-layer absorbers containing La-doped ferrite and carbon fiber composites were designed, and reflectivity of the double-layer absorbers made of different thickness and composition was calculated. Finally, a kind of structural absorber having excellent absorbing properties was achieved, and the bandwidth of the reflection loss less than −10 dB can reach 5.2 GHz in the range of 12.4–18 GHz.     

Absorption properties of carbonyl-iron/carbon black double-layer microwave absorbers

Double-layer materials were devised in order to improve the absorbing properties of electromagnetic wave absorbing plates. The double-layer wave absorbing materials are composed of a matching layer and an absorption layer. The matching layer is the surface layer through which most of the incident waves can enter, and the absorption layer beneath it plays an important role in incident wave attenuation. The total thickness of the double layer is the sum of the thicknesses of these two layers. Carbonyl iron (CI) and carbon black (CB) were used as absorbents in the matching and absorption layers, respectively. The structures of the CI and CB particles were analyzed using scanning electron microscopy and transmission electron microscopy; the dielectric properties and absorption mechanisms were also studied. In the testing frequency range 2-18 GHz, the results show that the double-layer absorbers have two absorption peaks, and the positions and values of these peaks change with the content level of the absorbents. When the mass fraction of CI in the matching layer is 50% and the total thickness of the absorber is 4 mm, the effective absorption band (below −8 dB) reaches 5.5, 5.8, and 6.5 GHz. Where the mass fraction of CB is 50% or 60% and the mass fraction of CI is 70%, the bandwidth with reflection loss below −4 dB is larger than 10 GHz.     

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

W-type barium hexaferrites with compositions of Ba₁Co_0.9Zn_1.1Fe₁₆O₂₇ and Ba_0.8La_0.2Co_0.9Zn_1.1Fe₁₆O₂₇ 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 Co₂W 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 La³⁺ for Ba²⁺ in the W-type barium hexaferrites. The microwave absorption property of the La³⁺ 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.     

Effect of annealing temperature on the structural, photoluminescence and magnetic properties of sol-gel derived Magnetoplumbite-type (M-type) hexagonal strontium ferrite

Magnetoplumbite-type (M-type) hexagonal strontium ferrite particles were synthesized via sol-gel technique employing ethylene glycol as the gel precursor at two different calcination temperatures (800 and 1000 °C). Structural properties were systematically investigated via X-ray diffraction (XRD), field emission scanning electron microscopy, high resolution transmission electron microscopy (HRTEM), energy dispersive spectroscopy (EDS), thermogravimetric analysis (TGA), photoluminescence spectrophotometry and superconducting quantum interference device magnetometer. XRD results showed that the sample synthesized at 1000 °C was of single-phase with a space group of P6₃/mmc and lattice cell parameter values of a=5.882 Å and c=23.048 Å. EDS confirmed the composition of strontium ferrite calcined at 1000 °C being mainly of M-type SrFe₁₂O₁₉ with HRTEM micrographs confirming the ferrites exhibiting M-type long range ordering along the c-axis of the crystal structure. The photoluminescence (PL) property of strontium ferrite was examined at excitation wavelengths of 260 and 270 nm with significant PL emission peaks centered at 350 nm being detected. Strontium ferrite annealed at higher temperature (1000 °C) was found to have grown into larger particle size, having higher content of oxygen vacancies and exhibited 83-85% more intense PL. Both the as-prepared strontium ferrites exhibited significant oxygen vacancies defect structures, which were verified via TGA. Higher calcination temperature turned strontium ferrite into a softer ferrite.     

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.     

Magnetic nanocomposite thin films of BaFe12O19 and TiO2 prepared by sol-gel method

The composite films with different weight ratio of barium ferrite to titanium dioxide are successfully prepared using sol-gel method for the first time. The morphology, crystal structure and magnetic properties of composite films are investigated with atomic force microscopy, X-ray diffraction and vibrating sample magnetometry. The results show that the composite films are uniform with no microcracks. The grain diameters are less than 100 nm. With the increase of barium ferrite, the grain diameter decreases. The composite films are composed of M-type hexagonal barium ferrite and rutile titanium dioxide. The composite films possess the excellent magnetic properties. The specific saturation magnetization and coercivity reach 18.3 emu/g and 3350 Oe, respectively. The application of composite films in magnetic recording and electromagnetic absorption fields is promising.     

On the thermal stability of Co2Z hexagonal ferrites for low-temperature ceramic cofiring technologies

Co₂Z hexaferrite Ba₃Co₂Fe₂₄O₄₁ was prepared by a mixed oxalate co-precipitation route and the standard ceramic technology. XRD studies show that at T<1300 °C different ferrite phases coexist with the M-type hexaferrite as majority phase between 1000 and 1100 °C and the Y-type ferrite at 1230 °C. The Z-type material has its stability interval between 1300 and 1350 °C. Both synthesis routes result in almost single-phase Z-type ferrites after calcination at 1330 °C, intermediate grinding and sintering at 1330 °C. The permeability of Co₂Z-type ferrite of about μ=20 is stable up to several 100 MHz, with maximum losses μ′′ around 700 MHz. Addition of 3 wt% Bi₂O₃ as sintering aid shifts the temperature of maximum shrinkage down to 950 °C and enables sintering of Z-type ferrite powders at 950 °C. However, the permeability is reduced to μ=3. It is shown here for the first time that Co₂Z ferrite is not stable under these conditions; partial thermal decomposition into other hexagonal ferrites is found by XRD studies. This is accompanied by a significant decrease of permeability. This shows that Co₂Z hexagonal ferrite is not suitable for the fabrication of multilayer inductors for high-frequency applications via the low-temperature ceramic cofiring technology since the material is not compatible with the typical process cofiring temperature of 950 °C.     

Ni–Zn nanoferrite for radar-absorbing material

Nanoparticles of nickel–zinc ferrite have been prepared by using the citrate precursor method. According to scanning electron microscopy (SEM), the particle size is nanometric for the powder calcined at 350 °C/3.5 h. The phase formation has been studied by applying different calcining atmospheres, such as air and argon. Pure Ni–Zn ferrite has been observed when calcined in argon at the temperature of 350 °C. Hysteresis analyses have been done with magnetization of 53.01 emu/g at 350 °C and obtaining 84.62 emu/g at 1100 °C due to an optimization of domains formation at high temperature. Measures of reflectivity of Ni–Zn ferrite/epoxy composite have been obtained below 21% at 350 °C and above 96% at 1100 °C with a coercive field of 26.61 Oe. Low value of coercive field increased the mobilization of domains wall and increased the radiation absorption.     

Effect of doping MnO2 on magnetic properties for M-type barium ferrite

The crystalline structure and magnetic properties of M-type barium ferrite doped with small amounts of MnO₂ (0, 0.25, 0.5, 0.75, 1.0, 1.5, and 2.0 wt%, respectively) have been investigated by means of XRD, SEM and VSM. The results show that the crystalline structures of barium ferrite are still M-type hexagonal structure and Mn ions are distributed homogeneously in both the grains and the grain boundaries. The saturation magnetization and magnetocrystalline anisotropy constants both reach the highest values when x=0.75 wt%. The displacement of Fe ions from 4f₁ to 2b site is mainly responsible for the appearance of the maximum values.     

Low-temperature sintering of M-type barium ferrite with BaCu(B2O5) additive

The aim of this work is to lower the sintered temperature of M-type barium ferrite (BaM) by BaCu(B₂O₅) (BCB) additives. The effects of BCB additives on the sintering behavior, structure and magnetic properties of BaM were also discussed. It was found that the sintered density, saturation magnetization and initial permeability of BaM are modified obviously as small amount of BCB (1-4 wt%) is added. Especially, when BaM with 3 wt% BCB was sintered at 900 °C, the single-phase BaM was obtained and showed excellent properties with sintered density of 4.88 g/cm³, saturation magnetization of 61.4 emu/g and initial permeability of 3.15. In addition, the SEM result revealed that the sample can be co-fired well with the Ag electrode at 900 °C. The reason for this was attributed to be the formation of the BCB liquid phase. It suggests that this M-type barium ferrite can be used as LTCC substrate for millimeter wave circulator, filter and other magnetic microwave devices.     

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.     

M型钡铁氧体陶瓷微珠核壳腔结构吸波材料

利用自反应淬熄法制备了一种M型钡铁氧体空心陶瓷微珠材料,在此基础上,对其表面进行超声波化学镀Ni-Co复合层,从而形成了具有核/壳/腔结构的材料;通过扫描电子显微镜、能谱仪和X射线衍射仪分析表明,该材料具有中空结构,主要物相BaFe12O19为M型钡铁氧体,经过化学镀后,在其表面形成了一层Ni-Co复合层;经过吸波性能测试,化学镀Ni-Co复合层后,在2~18GHz范围内,当厚度为2.10mm时,最低反射率达到了-28.62dB,反射率小于-10dB的带宽为3.33GHz。     

Magnetic properties of La-Co substituted M-type strontium hexaferrites prepared by polymerizable complex method

Magnetic properties of La-Co substituted M-type strontium hexaferrites were studied. The samples were prepared by polymerizable complex method. Crystal structure of samples has been investigated by powder X-ray diffraction (XRD). Single-phase M-type strontium hexaferrites with chemical composition of Sr_1.05−xLa_xFe_12−xCo_xO₁₉ (x=0-0.4) were formed by heating at 1173 K for 24 h in air. Magnetic properties were discussed by measurements of M-H curves with vibrating sample magnetometer (VSM). La-Co substituted M-type strontium hexaferrites prepared by polymerizable complex method showed typical magnetic hysteresis of hard ferrite. The coercive force increased significantly by La-Co substitution with polymerizable complex method. Maximum coercive force achieved in this study is 8.0 kOe (640 kA/m). Scanning electron microscopy revealed that the prepared ferrite particles have plate-like shape of diameter range between 20 and 500 nm.     

Dependence of microwave absorption properties on ferrite volume fraction in MnZn ferrite/rubber radar absorbing materials

We report the analysis of measurements of the complex magnetic permeability (μ_r) and dielectric permittivity (ε_r) spectra of a rubber radar absorbing material (RAM) with various MnZn ferrite volume fractions. The transmission/reflection measurements were carried out in a vector network analyzer. Optimum conditions for the maximum microwave absorption were determined by substituting the complex permeability and permittivity in the impedance matching equation. Both the MnZn ferrite content and the RAM thickness effects on the microwave absorption properties, in the frequency range of 2-18 GHz, were evaluated. The results show that the complex permeability and permittivity spectra of the RAM increase directly with the ferrite volume fraction. Reflection loss calculations by the impedance matching degree (reflection coefficient) show the dependence of this parameter on both thickness and composition of RAM.     

Synthesis and microwave magnetic properties of a-Fe/ferrite composites with sandwich structure

A new kind of a-Fe/ferrite composites with sandwich structure was realized by chemical reduction method, where the as-prepared W-type barium hexaferrite flake particles were subjected to a reduction treatment in hydrogen atmosphere at different temperatures. X-ray diffractometer reveals that a-Fe/Co particles precipitate in the ferrite matrix, when the reduction temperature is higher than 230 °C. With the temperature increased, the particles morphology changed into sandwich structure in hexagonal flake particles and the barium hexaferrite phase was decomposed gradually, when were completely decomposed at T=450 °C. Results show that the composites particles with sandwich structure (T=270 °C) have higher microwave complex permeability than the others.     

Microwave-absorbing properties of shape-optimized carbonyl iron particles with maximum microwave permeability

The microwave-absorbing properties for different shapes of carbonyl-iron particles prepared by the high-energy planetary ball milling with 40 vol% in epoxy resin matrix have been investigated. Higher value of magnetic permeability and permittivity can be obtained in the composites for thin flake carbonyl iron than spherical powders. The results are attributed to reduction of eddy current loss, orientation of magnetic moment and space-charge polarization with the shape change from spherical powders to thin flake particles. As the iron flakes with 0.4 μm in thickness as the absorbent fillers, the minimum RL value of −6.20 dB was observed at 4.57 GHz with thickness of 1 mm. The minimum reflection loss (RL) shifts to lower frequency and the value declines with change from spherical powders to thin flakes. It results from the considerable dielectric loss in the absorbing materials.     

Build-up processes of an optical cavity enclosing an absorbent thin film: computational study by the CIP method

Propagation of monochromatic light is analyzed by numerical calculations for an optical cavity enclosing a thin absorbent film. The computational study is performed by the CIP (constrained interpolation profile) method, which is shown to be able to solve temporal evolution of Maxwell’s equations even in a strongly absorbing medium. Simulations on build-up processes reveal that such a cavity exhibits transmittance even higher than the intrinsic transmissivity of the absorbent, i.e., the absorbent is virtually transparent, when the following requirements are satisfied: the film is much thinner than the wavelength, it is located at a node of a standing wave, and the cavity consists of a pair of mirrors with optimal reflectivity depending on the film thickness. Optimal conditions are discussed as well for maximizing absorption in the film.     

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.     

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.     

Electromagnetic wave absorption properties of carbonyl iron-ferrite/PMMA composites fabricated by hybridization method

The surface of carbonyl iron powder or a mixture of carbonyl iron and ferrite was coated with polymethylmethacrylate (PMMA) microspheres by a hybridization method to make hybrid powders, and then electromagnetic wave absorption properties of the hybrid composites prepared with these hybrid powders have been investigated. As for the carbonyl iron/PMMA hybrid composite, the reflection loss less than −20 dB could be achieved in a frequency range of 1.7–5.0 GHz when the composite thickness was below 5.00 mm. In the case of the carbonyl iron-ferrite/PMMA hybrid composite, a similar reflection loss was observed in a frequency range of 4.3–13.0 GHz. Thus, the addition of ferrite was found to be useful for achieving a large absorption in a wide frequency range, especially for higher frequency values. Simulated values for the minimum reflection loss are well agreed with actually measured ones, because of homogeneous distribution of carbonyl iron and/or ferrite in these hybrid composites.