羰基铁/NiLa_(0.02)Fe_(1.98)O_4双层宽频吸波体的构建及实现

根据电磁波传输线理论和阻抗匹配原理,设计羰基铁(CIP)/NiLa_(0.02)Fe_(1.98)O_4双层复合吸波材料,并以环氧树脂为基体制备相应的性能优异的吸波涂层。使用XDR和SEM对粉末和涂层的结构进行表征。通过同轴法测定CIP和NiLa_(0.02)Fe_(1.98)O_4的电磁参数,借助MATLAB软件,计算面层与底层不同厚度和不同次序条件下的反射率。结果表明:Ni La_(0.02)Fe_(1.98)O_4作为面层,CIP作为底层时可以极大地拓展涂层的吸收带宽和减小反射率峰值。以NiLa_(0.02)Fe_(1.98)O_4为面层,CIP为底层,厚度分别为1.5和0.6 mm时,涂层的吸波效果最好,反射率峰值为-38.2 dB,小于-10 dB频宽为13.4 GHz。制备的涂层中吸波剂总体分散良好,无明显缺陷。通过实验值和理论值的比较,验证优化结果的准确性,实现在较小厚度和较宽频带下,吸波涂层有较强吸收的要求。     

基于超材料设计的羰基铁低频复合吸波涂层研究

为获得低频宽带吸波材料,本文采用共沉淀和原位聚合技术制备了羰基铁/CoFe₂O₄/PANI三元复合材料,并以此为介质层,借鉴超材料思想,设计了一种基于超材料结构的羰基铁复合吸波涂层,改善了低频吸波性能。分析了超材料的结构设计对羰基铁/CoFe₂O₄/PANI涂层吸波性能的影响,并对赋予超材料结构后的复合涂层的吸波机理进行了研究和讨论。通过仿真优化发现,在电阻膜方阻值为10mΩ/□和镂空十字电阻膜图案尺寸达到最佳时,在相同厚度下赋予超材料结构后的复合涂层具有比单一羰基铁涂层更宽的吸收频带以及更低的吸收频率,在3.8-6.9GHz频段内反射率均小于-10dB。研究表明,将超材料结构融入到羰基铁涂层性能改进中,能够有效提升其低频吸波性能。     

环氧基复合吸波涂层的制备与性能研究

目的 制备综合吸波性能良好的复合吸波涂层.方法 选择炭黑和羰基铁粉进行机械混合,作为吸波剂加入到环氧树脂中进行吸波材料制备,利用扫描电子显微镜对炭黑和羰基铁粉分别进行微观形貌的观察.利用矢量网络分析仪在2～18 GHz内测试其电磁性能,研究炭黑和羰基铁粉含量及涂层厚度对吸波性能的影响规律.结果 通过观察微观形貌发现,炭...     

基于双磁介质层复合的超材料低频吸波体研究

本研究将磁性吸波涂层融入至超材料的结构设计中,得到了一种新型低频复合超材料吸波体,吸波体由环形电阻膜、双层磁性吸波涂层和金属背板组成。采用CST仿真软件计算了超材料吸波体的吸收性能,研究了吸波体各个结构参数对吸收性能的影响。仿真结果表明,设计的超材料吸波体厚度为2.5 mm时,在1.9和4 GHz处存在2个吸收峰,在1.59～6.59GHz频率范围内反射损耗低于-8dB,吸收带宽达到5GHz。通过吸波体电磁场分布对吸波机理进行了讨论。结果表明,吸波体低频吸收带宽的增加是由于表面的电阻膜图案改变了超材料吸波体的电场分布和磁场分布,促进了磁介质层的损耗。最后制备了吸波体样品并进行了反射率测试,实物测试结果与仿真结果基本一致,说明设计制备的吸波体具有优异的低频吸波性能,吸波带宽相比磁性吸波涂层大幅提高。     

空气层匹配碳纤维吸波涂层的吸波性能

目的探究空气层匹配厚度及碳纤维含量、长度对碳纤维吸波涂层吸波性能的影响。方法以短切碳纤维为吸收剂,水性聚氨酯为基体树脂,制备雷达吸波涂层。采用扫描电子显微镜和金相显微镜对短切碳纤维和涂层的微观形貌进行分析表征,将碳纤维涂层与空气层进行匹配,并采用矢量网络分析仪测试分析涂层的吸波性能。结果当空气匹配层厚度由1 mm逐渐增加到3.5 mm时,复合涂层的最大吸收峰由高频逐渐向低频移动。匹配厚度为3 mm时,反射率峰值最低(-41 d B)。匹配厚度为2.5 mm时,有效吸收频段(反射率-10 d B)最宽,为8.6~18 GHz。随着碳纤维含量的增加,涂层的最大吸收峰频率均呈下降趋势,有效吸收频段向低频移动。碳纤维含量(质量分数)低于0.1%时,只有碳纤维长度达到3 mm,涂层才具备有效吸波性能。碳纤维含量为0.1%~0.2%,碳纤维长度为2 mm时,涂层吸波性能最好。碳纤维含量超出0.2%,碳纤维长度为1 mm时,涂层已经具备较好的吸波性能。结论通过调节空气层匹配厚度及碳纤维含量、长度,空气层匹配碳纤维吸波涂层在不同频段均能实现对电磁波的有效吸收。     

一种热喷涂雷达吸波涂层制备技术

采用耐温的磷酸盐玻璃和改性的β–SiC吸收剂的热喷涂材料体系,使用火焰喷涂工艺制备了热喷涂雷达吸波涂层,对粉末、涂层制备过程及性能进行了研究,结果表明:喷雾干燥造粒是制备热喷涂雷达吸波涂层粉末材料体系的合理方法,使用火焰喷涂制备的涂层,吸收剂含量为20%时,涂层性能最佳,当涂层厚度为1 mm时,在1.2×1010～1.8×1010 Hz范围内,涂层反射率均低于-8 dB。     

纳米铁纤维与羰基铁粉共混制备轻质宽带吸波涂层材料

为降低羰基铁粉吸波剂的密度、提高介电常数,采用轻质、高介电常数的纳米铁纤维与羰基铁粉共混,制备了轻质宽带吸波涂层.研究了纳米铁纤维含量对复合吸波剂微波电磁与吸收特性的影响.结果表明,随纳米铁纤维含量的提高,复合吸波剂的介电常数和磁导率增大.当复合吸波剂中纳米铁纤维含量(质量分数)为2.2%-4.4%时,吸波涂层有更低的面密度和更宽的有效带宽,这是由于在一定范围内提高介电常数,可以改善吸波涂层的匹配吸收特性.     

羰基铁粉复合多孔吸波涂层的优化设计

目的 解决吸波剂羰基铁粉颗粒(CIP)构成的吸波涂层存在密度较大、涂层厚度过大的问题.方法 利用三维多孔结构降低复合吸波涂层的密度并改善阻抗失配,从而构筑轻质宽频羰基铁粉复合吸波涂层.利用有限元分析软件建立了羰基铁粉/石蜡复合多孔吸波涂层的仿真模型,通过仿真研究了三维多孔结构的孔隙率、孔径和孔隙分布方式对复合吸波涂层性能(最小反射损耗、有效吸收带宽、峰值吸收频率和密度)的影响规律,揭示了羰基铁粉多孔结构的吸波机理,并确定了具有最佳综合性能的羰基铁粉三维多孔复合吸波涂层的结构参数.结果 随着孔隙率的增加,涂层密度减小且峰值吸收频率向高频移动;而随着孔径的减小,涂层除峰值吸收频率向高频移动外,最小反射损耗和有效吸收带宽分别呈减低和增加的趋势,吸波性能得到有效改善.孔隙分布方面,在随机、有序、梯度递减和梯度递增4种分布方式中,梯度递减分布表现出最佳的吸波性能.相较于无孔结构,羰基铁粉质量分数为75％、孔隙率为16％、孔径为0.325 mm、孔隙呈梯度递减分布的三维多孔涂层,其有效吸收带宽(RL<–10 dB)拓展了49.3％(从4.10 GHz增加到6.12 GHz),密度降低了4％(从2.71 g/cm3降低到2.6 g/cm3),而最小反射损耗仅仅损失0.7％.结论 多孔结构的引入可以实现羰基铁粉涂层轻质、宽频吸波的目的.     

Si含量对NdFe材料微波吸收特性的影响

采用高能球磨和晶化热处理方法制备钕铁吸波粉体,研究Si含量对钕铁粉体吸波性能的影响。结果发现:在高能球磨和微氧化气氛下进行700℃,1h晶化处理后,Nd8Fe92、Nd8Fe89Si3和Nd8Fe87S5粉体组织均由α-Fe、Nd2O3和Fe17Nd2相组成,加入Si后,粉体的Fe17Nd2相对量增加;Nd8Fe92粉体的反射率最小值约为-4.5dB,吸收峰频率约为4.4GHz,加入Si后,粉体的反射率最小值降低,吸收峰频率升高,吸波带宽变宽;Nd8Fe92粉体以介电损耗为主,Nd8Fe89Si3和Nd8Fe87Si5粉体以磁损耗为主。     

等离子喷涂制备直流接地极钢基体NiFe2O4防腐涂层

为提高常规碳钢直流接地极本体的耐蚀性能,采用固相反应法结合团聚-破碎法造粒合成出适合等离子喷涂的NiFe2O4粉体,利用等离子喷涂工艺在钢基体上制备NiFe2O4铁氧体涂层,并利用场发射扫描电镜、金相显微镜、X射线衍射仪以及涂层结合强度、电阻率、直流电解等测试方法,对NiFe2O4涂层的微观组织结构、物理电气和腐蚀性能进行了研究。结果表明:NiFe2O4涂层与基体的平均结合强度为21.8MPa,涂层平均电阻率约为3.60×10-2Ω.cm,且涂层耐电解腐蚀性能良好,腐蚀率约为1.86g/(A.a),可较好满足直流输电系统中接地极材料的服役要求,初步显示出等离子喷涂制备直流接地极用铁氧体环保防腐涂层的工艺可行性。     

Microwave-assisted synthesis and characterization of Nd1.5Mg17Ni0.5-Fe3O4 hydrogen storage composite

This work reports the preparation of Nd_1.5Mg₁₇Ni_0.5-Fe₃O₄ hydrogen storage composite in a single mode 2.45 GHz microwave cavity. The physicochemical properties (thermodynamic and kinetic characteristics, hydrogen absorption/desorption properties, thermal behavior, phase composition and morphology) were characterized by pressure-composition isotherms, differential scanning calorimetry, X-ray diffraction, scanning electron microscope with an energy dispersive X-ray spectrometer, transmission electron microscopy, and laser granulometry. The proposed microwave synthesis, in contrast with conventional sintering method, offers rapid heating, makes homogenous composition and hence improves the hydrogen storage properties of the composite.     

Synthesis and microwave absorbing properties of polyaniline/MnFe2O4 nanocomposite

Conductive polyaniline (PANi)-manganese ferrite (MnFe₂O₄) nanocomposites with core-shell structure were synthesized by in situ polymerization in the presence of dodecyl benzene sulfonic acid (DBSA) as the surfactant and dopant and ammonium persulfate (APS) as the oxidant. The structure and magnetic properties of manganese ferrite nanoparticles were measured by using powder X-ray diffraction (XRD) and vibrating sample magnetometer (VSM), respectively. Its morphology, microstructure and DC conductivity of the nanocomposite were characterized by scanning electron microscopy (SEM), fourier transform infrared spectroscopy (FTIR) and four-wire-technique, respectively. The microwave absorbing properties of the nanocomposite powders dispersing in resin acrylic coating with the coating thickness of 1_.4 mm were investigated by using vector network analyzers in the frequency range of 8-12 GHz. A minimum reflection loss of −15.3 dB was observed at 10.4 GHz.     

Electromagnetic wave absorption properties of mechanically mixed Nd2Fe14B/C microparticles

Nd₂Fe₁₄B/C microparticles were prepared by a mechanical mixing technique using a weight ratio of 2:1. Paraffin-bonded Nd₂Fe₁₄B/C composites were fabricated using 40 wt% microparticles, and their electromagnetic wave absorption properties were studied and compared with those of the paraffin-bonded Nd₂Fe₁₄B composites in the 2-18 GHz frequency range and for 1-5 mm thickness. The Nd₂Fe₁₄B/C-paraffin composites exhibit dual dielectric resonance in complex relative permittivity (?_r) and essentially flat response in complex relative permeability (μ_r) rather than showing an abrupt change in both ?_r and μ_r as in the Nd₂Fe₁₄B-paraffin composites. The results are ascribed to the increased electrical resistivity in the Nd₂Fe₁₄B/C-paraffin composites and the protection on the magnetic properties of the Nd₂Fe₁₄B microparticles at 2-18 GHz by the presence of the C phase. Large reflection loss (RL) exceeding −10 dB and an optimal RL of −13.2 dB are achieved in the Nd₂Fe₁₄B/C-paraffin composites from 9.6 to 18 GHz at a thickness of 1.4-2.6 mm and at 18 GHz at a thickness of 1.4 mm, respectively.     

Preparation and microwave absorbency of Fe/epoxy and FeNi3/epoxy composites

The focus of this study was placed on the lightness of microwave absorbing effective metal/epoxy composites. For such a focus, high aspect ratio of flake iron powder and high absorbing FeNi₃ were prepared. The iron powder particle size was reduced significantly through wet milling, comparing to dry milling. The FeNi₃ alloy powders were synthesized by mechanical alloying (MA); then, the particle size was reduced through wet milling. The iron powder and FeNi₃ alloy were characterized by scanning electron microscopy (SEM) and X-ray diffraction. SEM of the metal particles showed the flake and small structure by wet milling. The microwave absorbing effectiveness of metal/epoxy composite was affected by the structure, loading and dispersion of metal materials. The polyvinylpyrrolidone (PVP) plays an important role in suspending metal powders in wet milling to reduce powder size. Besides, the PVP will be a coupling agent in inhibiting the aggregation and enhancing the interfacial interaction between metal and epoxy. Results suggested that after the above manufacturing process, the microwave absorbency was enhanced substantially. Composite films of Fe/epoxy and FeNi₃/epoxy 1.6 mm in thickness possessed a microwave absorbency above 10 dB at 9.2-15.2 GHz and 13.1-16.2 GHz, respectively.     

Design of mesostructured γ-Fe2O3/carbon nanocomposites for electromagnetic wave absorption applications

We propose and demonstrate a scheme to enhance microwave absorption property through mesoporous structure with high regularity. The mesostructured nanocomposites, embedding γ-Fe₂O₃ within carbon matrix, exhibit a strong and broadband attenuation of microwave in the frequency range of 0.5-18 GHz, mainly due to the better impedance matching. C-2Fe-900 exhibits strong absorption characteristics with an absorption peak of −32.0 dB at 6.4 GHz. The absorption peak intensity and position can be adjusted by changing the matching thickness of the coating. Reflection loss below −10 dB (i.e., absorption above 90%) is obtained in the frequency range of 1.9-10.7 GHz.     

Facile synthesis, magnetic and microwave absorption properties of Fe3O4/polypyrrole core/shell nanocomposite

Fe₃O₄/polypyrrole (PPy) core/shell nanocomposite, with Fe₃O₄ nanoparticle as core and PPy as shell, could be facilely synthesized via in situ chemical oxidative polymerization of pyrrole monomers on the surface of Fe₃O₄ nanoparticles. The results indicate that core/shell nanocomposite consists of Fe₃O₄ core with the mean diameter of 100 nm and adjacent PPy shell with a thickness of about 70 nm. The as-prepared Fe₃O₄/PPy core/shell nanocomposite exhibits a saturated magnetization of 20.1 emu/g and coercivity value of 368.3 Oe, respectively. The electromagnetic characteristics of Fe₃O₄/PPy core/shell nanocomposite were also investigated with a vector network analyzer in the 2-18 GHz range. The absorbing peak position moves to lower frequency with increasing the thicknesses of samples. The value of the minimum reflection loss is −22.4 dB at 12.9 GHz for Fe₃O₄/PPy core/shell nanocomposite with a thickness of 2.3 mm, and a broad peak with a bandwidth lower than −10 dB is about 5 GHz. Such strong absorption is attributed to better electromagnetic matching due to the existence of PPy and the special core/shell structure.     

Hot corrosion of a novel NiO/NiFe2O4 composite coating thermally converted from the electroplated Ni–Fe alloy

A novel NiO/NiFe₂O₄ composite coating thermally converted from an electroplated Ni–Fe alloy was successfully fabricated. The composite coating consisted of a NiO matrix and homogeneously distributed criss-cross intragranular and intergranular NiFe₂O₄ precipitates, with a very dense and flat structure. The composite, compared to bare Ni metal, exhibited increased hot corrosion resistance under an atmosphere of Na₃AlF₆–AlF₃–CaF molten salts and air at 960 °C, mainly because of the dense structure and well-adhered, homogeneously dispersed intragranular and intergranular NiFe₂O₄ precipitates.     

Enhanced electromagnetic characteristics of carbon nanotubes/carbonyl iron powders complex absorbers in 2-18 GHz ranges

The electromagnetic (EM) characteristics of the carbon nanotubes/carbonyl iron powders (CNTs/CIPs) complex absorbers synthesized by mixing CNTs with CIPs were studied at 2-18 GHz, for the aim of the absorbing coating with thinness, lightness, width, and strength. Compared with CIPs, the CNTs/CIPs composites had higher electrical conductivity, permittivity, and dielectric loss, which gradually increased with the increasing CNTs content (W_CNTs). Among them, with W_CNTs = 2.2 %, a reflection loss (R_L) exceeding −20 dB was obtained in the frequency range of 6.4-14.8 GHz for a coating thickness of 1.2-2.5 mm. Particularly, a minimum R_L of −33.3 dB was found at 11.2 GHz corresponding to a matching thickness of 1.5 mm. The excellent EM-wave absorption properties are a consequence of a proper EM matching and enhanced absorption abilities resulting from the addition of a small quantity of CNTs with high electrical conductivity, permittivity, and dielectric loss. Thus, CNTs/CIPs complex absorbers may be promising candidates for EM-wave-absorption materials with strong-absorption, thin-thickness, light-weight, and low-cost.     

Enhanced microwave absorption properties of Fe73.5Cu1Nb3Si15.5B7 nanoflakes by moderate surface oxidization and rotational orientation in composites

Fe_73.5Cu₁Nb₃Si_15.5B₇ nanoflakes were fabricated by ball milling the annealed ribbons. The microwave absorption properties of Fe_73.5Cu₁Nb₃Si_15.5B₇ nanoflakes were improved by moderate surface oxidization and rotational orientation in composites. As for the oxide-coated nanoflakes composite, the permittivity decreased distinctly and the permeability maintained the initial value compared with the as-milled nanoflakes composite. Through rotational orientation, the lower permittivity and higher permeability were obtained, and consequently the microwave absorption properties were improved obviously. The minimum reflectivity for the absorber of the oriented composites with 35 vol% oxide-coated nanoflakes could reach −46.4 dB at 1.31 GHz with the thickness of 4.2 mm.     

Structural and microwave absorption properties of Ni(1−x)Co(x)Fe2O4 (0.0 ≤ x ≤ 0.5) nanoferrites synthesized via co-precipitation route

Ni-Co nanoferrites show excellent magneto-dielectric properties and these materials can be used to miniaturize the size of the high frequency devices which is the order of the day. Nanocrystalline Ni-Co ferrites having general formula Ni_1−xCo_xFe₂O₄ (x = 0.0, 0.1, 0.2, 0.3, 0.4, and 0.5) were prepared by co-precipitation method. The structural morphology of the prepared samples was carried out using scanning electron microscopy. The results showed the spherical shaped nanoparticles varying in the range of 16-40 nm. The complex relative permittivity (?_r) and complex relative permeability (μ_r) were measured using vector network analyzer for all the samples in the frequency range of 1 MHz to 3 GHz. The variation of complex relative permittivity (?_r) as a function of frequency is explained in accordance with Maxwell-Wagner model and Koop's phenomenological theory. The effect of frequency and cobalt concentration on permeability are reported. The reflectivity (R) of nanoferrites is also calculated. The value of minimum reflection loss (RL) is about −18 dB at 2.45 GHz with a thickness of 2.1 ± 0.1 mm. The results indicate that Ni_1−xCo_xFe₂O₄ nanoparticles have excellent microwave absorbing properties and have a great potential for military use.