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.     

Determining factors for high performance silicone rubber microwave absorbing materials

Silicone rubber microwave absorbing materials (RMAMs) based on ferrite as the major absorbent were prepared by the mechanical blending method. The determining factors for the complex permittivity, complex permeability, and reflectivity of RMAM were thoroughly investigated with various samples including different crystal structures of Ba-ferrite (M-type, W-type, and Y-type), the ferrite with doped elements (Ba, Sr), the materials' thickness, the combination ratio of ferrite and carbonyl iron. The effects of surface modification and loading amount of ferrite on the mechanical properties, processing performance, and absorbing property of RMAM were also assessed. The results show that W-type Ba-ferrite based RMAM exhibits better absorbing property at high frequencies (8-18 GHz) than the other two barium ferrites (M-type and Y-type) based ones, and the absorbing property of RMAM based on Sr-ferrite is best. As the thickness of RMAM and the amount of absorbents increase, the absorption peak moves toward low frequency, the absorption frequency bandwidth is narrowed, and the reflectivity first decreases and later increases. The optimum thickness is 1.5-1.7 mm, and the amount of ferrite is 450 parts per hundreds of rubber (phr). Surface modification of the absorbent with silane coupling agent could improve the mechanical properties and processing performance of RMAM. It is concluded that there will be a synergistic effect when carbonyl iron (CI) is used in combination with Sr-ferrite (Sr-W) in an appropriate proportion. When the total volume fraction of absorbents is 51%, the optimum ratio of Cl to Sr-W is 17:34, the absorption frequency bandwidth (<−10 dB) is about 8 GHz, and the absorption area is −99 dB.     

Electroless Ni—P—ferrite composite coatings for microwave applications

Electroless, EL coating technique is one of the elegant ways of coating by controlling the temperature and pH of the coating bath in which there is no usage of electric current. It is estimated that the market for this chemistry will increase at a rate of about 15% per year. Use of microwave energy for synthesis of material with novel microstructures is an exciting new field in material science with enormous application. In this investigation, nanograined BaZn_2−yCo_yFe₁₆O₂₇ y = 0.0, 0.4, 0.8, 1.2, 1.6 and 2.0) powders have been synthesized by citrate precursor method followed by heat treatment at various specified temperatures like 650, 750 and 850° C for 3 h in the furnace. In addition heat treatments are also carried out in the microwave oven of the power rating of 760 W. The powders thus produced have been characterized by SEM, EPMA, VSM, XRD and thermal analysis techniques. As a forward step towards EL nano-composite coatings, Ni-P-X (X = BaZn_2−y Co_yFe₁₆O₂₇) coatings with thickness less than ∼0.1 mm thick has been produced. Such coating exhibits absorption of microwave in the range of 12–18 GHz up to about 20 db depending upon the volume fraction of the ferrite particles embedded in the Ni-P matrix     

Enhanced microwave magnetic and attenuation properties of composites with free-standing spinel ferrite thick films as fillers

Free-standing thick films of spinel ferrite, Ni_0.89−xCu_0.11Zn_xFe₂O₄ with x=0.55 and 0.60, were prepared as fillers to fabricate electromagnetic composites. Compared to those made with conventional spherical fillers, the composites made with thick film fillers showed enhanced static permeability (μ′₀) and maximum imaginary permeability (μ″_max). At the same time, complex permittivity (ε′ and ε″) were almost unchanged. A relative bandwidth W_R of 7–8 was achieved, which is about 75% of the theoretical maximum relative bandwidth. These composites are potential candidates as electromagnetic attenuation materials with ultrabroad absorption bandwidth in L and S bands.     

Double-layer microwave absorber of nanocrystalline strontium ferrite and iron microfibers

Microwave absorption properties of the nanocrystalline strontium ferrite (SrFe₁₂O₁₉) and iron (α-Fe) microfibers for single-layer and double-layer structures are investigated in a frequency range of 2 GHz-18 GHz. For the single-layer absorbers, the nanocrystalline SrFe₁₂O₁₉ microfibers show some microwave absorptions at 6 GHz-18 GHz, with a minimum reflection loss (RL) value of -11.9 dB at 14.1 GHz for a specimen thickness of 3.0 mm, while for the nanocrystalline α-Fe microfibers, their absorptions largely take place at 15 GHz-18 GHz with the RL value exceeding -10 dB, with a minimum RL value of about -24 dB at 17.5 GHz for a specimen thickness of 0.7 mm. For the double-layer absorber with an absorbing layer of α-Fe microfibers with a thickness of 0.7 mm and matching layer of SrFe₁₂O₁₉ microfibers with a thickness of 1.3 mm, the minimum RL value is about -63 dB at 16.4 GHz and the absorption band width is about 6.7 GHz ranging from 11.3 GHz to 18 GHz with the RL value exceeding -10 dB which covers the whole K_u-band (12.4 GHz-18 GHz) and 27% of X-band (8.2 GHz-12.4 GHz).     

Templated synthesis of highly ordered mesoporous cobalt ferrite and its microwave absorption properties

Based on the nanocasting strategy, highly ordered mesoporous CoFe2O4 is synthesized via the ‘two-solvent’ impregnation method using a mesoporous SBA-15 template. An ordered two-dimensional(P6mm) structure is preserved for the CoFe2O4/SBA-15 composite after the nanocasting. After the SBA-15 template is dissolved by NaOH solution, a mesoporous structure composed of aligned nanoparticles can be obtained, and the P6 mm structure of the parent SBA-15is preserved. With a high specific surface area(above 90 m2/g) and ferromagnetic behavior, the obtained material shows potential in light weight microwave absorption application. The minimum reflection loss(RL) can reach-18 dB at about16 GHz with a thickness of 2 mm and the corresponding absorption bandwidth is 4.5 GHz.     

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.     

Heat treatment effects on microstructure and magnetic properties of Mn-Zn ferrite powders

Mn-Zn ferrite powders (Mn_0.5Zn_0.5Fe₂O₄) were prepared by the nitrate-citrate auto-combustion method and subsequently annealed in air or argon. The effects of heat treatment temperature on crystalline phases formation, microstructure and magnetic properties of Mn-Zn ferrite were investigated by X-ray diffraction, thermogravimetric and differential thermal analysis, scanning electron microscopy and vibrating sample magnetometer. Ferrites decomposed to Fe₂O₃ and Mn₂O₃ after annealing above 550 °C in air, and had poor magnetic properties. However, Fe₂O₃ and Mn₂O₃ were dissolved after ferrites annealing above 1100 °C. Moreover, the 1200 °C annealed sample showed pure ferrite phase, larger saturation magnetization (M_s=48.15 emu g⁻¹) and lower coercivity (H_c=51 Oe) compared with the auto-combusted ferrite powder (M_s=44.32 emu g⁻¹, H_c=70 Oe). The 600 °C air annealed sample had the largest saturation magnetization (M_s=56.37 emu g⁻¹) and the lowest coercivity (H_c=32 Oe) due to the presence of pure ferrite spinel phase, its microstructure and crystalline size.     

Tunable broadband metamaterial absorber consisting of ferrite slabs and a copper wire

A tunable broadband metamaterial absorber is demonstrated at microwave frequencies in this paper.The metamaterial absorber is composed of ferrite slabs with large resonance beamwidths and a copper wire.The theoretical analysis for the effective media parameters is presented to show the mechanism for achieving the perfect absorptivity characteristic.The numerical results of transmission,reflectance,and absorptivity indicate that the metamaterial absorber exhibits a near perfect impedance-match to free space and a high absorptivity of 98.2% for one layer and 99.97% for two layers at 9.9 GHz.The bandwidth with the absorptivity above 90% is about 2.3 GHz.Moreover,the absorption band can be shifted linearly in a wide frequency range by adjusting the magnetic bias.This metamaterial absorber opens a way to prepare perfectly matched layers for engineering applications.     

含有吸收介质的突变结构腔体场匹配分析

采用分区求解场及边界场匹配方法推导出含有吸收介质波导的色散方程及突变结构高频腔体混合模式的场匹配方程。将解析分析与数值计算结合，对回旋速调管放大器高频腔体进行了数值计算，研究了吸收层对波导传播、衰减特性及谐振腔的谐振特性、损耗特性、Q值、场分布的影响。给出了数值模拟主要结果。 关键词： 吸收介质 场匹配 混合模 谐振腔 高功率微波     

Bio-silica incorporated barium ferrite composites: Evaluation of structure,morphology, magnetic and microwave absorption traits

A series of bio-silica incorporated barium-ferrite-composites with the composition of (x)Bio-SiO₂:(80-x)γ-Fe₂O₃:(20)BaO, where x = 0, 1, 2, and 3 wt% were prepared using the modified solid-state reaction method. The influence of different bio-silica (extricated from sintered rice husk) contents on the surface morphologies, structures, and magnetic characteristics of these composites were assessed. The relative complex permittivity and permeability were resolved using the Nicholson-Ross-Weir strategy in the frequency range of 8–13 GHz. Meanwhile, the reflection loss was estimated through the transmission/reflection line theory to assess the MW absorption properties of the composites. Incorporation of the bio-silica in the barium ferrite composites generated a new hexagonal phase (Ba₃Fe₃₂O₅₁) and a tetragonal phase (BaFeSi₄O₁₀) which led to a decrease in the saturation magnetization and significant shift in the MW frequency absorption peak positions.     

Electromagnetic properties and microwave absorbing characteristics of doped barium hexaferrite

M-type barium hexaferrite BaFe_12−x(Mn_0.5Cu_0.5Ti)_x/2O₁₉ (x varying from 0 to 3 in steps of 1) have been synthesized by the usual ceramic sintering method. The ferrite powders possess hexagonal shape and are well separated from one another. The powder of these ferrites were mixed with polyvinylchloride plasticizer to be converted in to a microwave absorbing composite. X-ray diffraction (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, the magnetoplumbite structures for all the samples have been formed. The sample having higher magnetic susceptibility and coercivity exhibits a larger microwave absorbing ability. Also, the present investigation demonstrates that microwave absorber using BaFe_12−x (Mn_0.5Cu_0.5Ti)_x/2O₁₉ (x=2$x=2$ and 3)/polyvinylchloride can be fabricated for the applications over 15 GHz, with reflection loss more than −25 dB for specific frequencies, by controlling the molar ratio of the substituted ions.     

Carbon fiber/epoxy composite materials cured thermally and with microwave irradiation

In this paper, composite materials of short carbon fibers (CFs) and a thermosetting epoxy were prepared in three different ways: without curing, thermal curing, and thermal curing followed by microwave irradiation. Mechanical properties of the three kinds of CF reinforced plastic (CFRP) composites were studied to explore the effect of microwave irradiation. Microscopic study with the aid of a scanning electron microscope (SEM) was performed on fractured composite surfaces to identify the principle features of failure. Degree of polymerization of the epoxy resin in the three CFRP composites was evaluated by infrared (IR) spectroscopy. The microwave irradiated CFRP exhibited mechanically ductile behavior even though its highest degree of polymerization. Use of microwaves and resultant stronger physico-chemical linkage at the interface between CF and epoxy resin are the main feature of this study.     

Two-dimensional materials: Emerging toolkit for construction of ultrathin high-efficiency microwave shield and absorber

Two-dimensional (2D) materials generally have unusual physical and chemical properties owing to the confined electro-strong interaction in a plane and can exhibit obvious anisotropy and a significant quantum-confinement effect, thus showing great promise in many fields. Some 2D materials, such as graphene and MXenes, have recently exhibited extraordinary electromagnetic-wave shielding and absorbing performance, which is attributed to their special electrical behavior, large specific surface area, and low mass density. Compared with traditional microwave attenuating materials, 2D materials have several obvious inherent advantages. First, similar to other nanomaterials, 2D materials have a very large specific surface area and can provide numerous interfaces for the enhanced interfacial polarization as well as the reflection and scattering of electromagnetic waves. Second, 2D materials have a particular 2D morphology with ultrasmall thickness, which is not only beneficial for the penetration and dissipation of electromagnetic waves through the 2D nanosheets, giving rise to multiple reflections and the dissipation of electromagnetic energy, but is also conducive to the design and fabrication of various well-defined structures, such as layer-by-layer assemblies, core–shell particles, and porous foam, for broadband attenuation of electromagnetic waves. Third, owing to their good processability, 2D materials can be integrated into various multifunctional composites for multimode attenuation of electromagnetic energy. In addition to behaving as microwave reflectors and absorbers, 2D materials can act as impedance regulators and provide structural support for good impedance matching and setup of the optimal structure. Numerous studies indicate that 2D materials are among the most promising microwave attenuation materials. In view of the rapid development and enormous advancement of 2D materials in shielding and absorbing electromagnetic wave, there is a strong need to summarize the recent research results in this field for presenting a comprehensive view and providing helpful suggestions for future development.     

Fabrication and magnetic property analysis of monodisperse manganese-zinc ferrite nanospheres

Monodisperse Mn-Zn ferrite (Mn_1−xZn_xFe₂O₄) nanospheres have been prepared via a simple solvothermal method. The as-synthesized samples were characterized in detail by X-ray diffraction pattern (XRD), transmission electron microscope (TEM), high-solution transmission electron microscope (HRTEM), select area electron diffraction pattern (SAED), scanning electron microscope (SEM), and vibrating sample magnetometer (VSM). The results show that a large number of the high-purity Mn_1−xZn_xFe₂O₄ nanocrystallites were synthesized and these nanocrystallites oriented aggregated to nanospheres. The dependence of magnetic properties of Mn_1−xZn_xFe₂O₄ nanospheres on the composition content x of Zn was studied. The maximum saturation magnetization value of the as-prepared sample (Mn_0.6Zn_0.4Fe₂O₄) reached 52.4 emu g⁻¹.     

Complex permittivity,complex permeability and microwave absorption properties of ferrite–polymer composites

The complex permittivity (ε′–jε″), complex permeability (μ′–jμ″) and microwave absorption properties of ferrite–polymer composites prepared with different ferrite ratios of 50%, 60%, 70% and 80% in polyurethane (PU) matrix have been investigated in X-band (8.2–12.4 GHz) frequency range. The M-type hexaferrite composition BaCo⁺²_0.9Fe⁺²_0.05Si⁺⁴_0.95Fe⁺³_10.1O₁₉ was prepared by solid-state reaction technique, whereas commercial PU was used to prepare the composites. At higher GHz frequencies, ferrite's permeabilities are drastically reduced, however, the forced conversion of Fe⁺³ to Fe⁺² ions that involves electron hopping, could have increased the dielectric losses in the chosen composition. We have measured complex permittivity and permeability using a vector network analyzer (HP/Agilent model PNA E8364B) and software module 85071. All the parameters ε′, ε″, μ′ and μ″ are found to increase with increased ferrite contents. Measured values of these parameters were used to determine the reflection loss at various sample thicknesses, based on a model of a single-layered plane wave absorber backed by a perfect conductor. The composite with 80% ferrite content has shown a minimum reflection loss of −24.5 dB (>99% power absorption) at 12 GHz with the −20 dB bandwidth over the extended frequency range of 11–13 GHz for an absorber thickness of 1.6 mm. The prepared composites can fruitfully be utilized for suppression of electromagnetic interference (EMI) and reduction of radar signatures (stealth technology).     

CoFe_2O_4/空心微球复合体的制备与吸波性能

采用化学共沉淀法,在空心微球上包覆一层CoFe2O4,得到一种低密度的空心磁性微球.磁测量结果表明,磁场下退火制备的CoFe2O4样品反位缺陷减少,从而导致饱和磁化强度随退火磁场的增强而增大.吸波性能测试结果表明,包覆结构的CoFe2O4/空心球样品是一种轻质的微波吸收材料.     

CoFe2O4/空心微球复合体的制备与吸波性能

采用化学共沉淀法,在空心微球上包覆一层CoFe2O4,得到一种低密度的空心磁性微球．磁测量结果表明,磁场下退火制备的CoFe2O4样品反位缺陷减少,从而导致饱和磁化强度随退火磁场的增强而增大．吸波性能测试结果表明,包覆结构的CoFe2O4/空心球样品是一种轻质的微波吸收材料.