Microwave reflection properties of planar anisotropy FesoNi50 powder/paraffin composites

The reflection properties of planar anisotropy FesoNiso powder/paraffin composites have been studied in the microwave frequency range. The permeability of Fe50Ni50 powder/paraffin composites is greatly enhanced by introducing the planar anisotropy, and can be further enhanced by using a rotational orientation method. The complex permeability can be considered as the superposition of two types of magnetic resonance. The resonance peak at high frequency is attributed to the natural resonance, while the peak at low frequency is attributed to the domain-wall resonance. The simulated results of the microwave reflectivity show that the matching thickness, peak frequency, permeability, and permittivity are closely related to the quarter wavelength matching condition. The Fe50Ni50 powder/paraffin composites can be attractive candidates for thinner microwave absorbers in the L-band （1-2 GHz）.

Microwave reflection properties of planar anisotropy Fe_(50)Ni_(50) powder/paraffin composites

The reflection properties of planar anisotropy Fe50Ni50 powder/paraffin composites have been studied in the microwave frequency range.The permeability of Fe50Ni50 powder/paraffin composites is greatly enhanced by introducing the planar anisotropy,and can be further enhanced by using a rotational orientation method.The complex permeability can be considered as the superposition of two types of magnetic resonance.The resonance peak at high frequency is attributed to the natural resonance,while the peak at low frequency is attributed to the domain-wall resonance.The simulated results of the microwave reflectivity show that the matching thickness,peak frequency,permeability,and permittivity are closely related to the quarter wavelength matching condition.The Fe50Ni50 powder/paraffin composites can be attractive candidates for thinner microwave absorbers in the L-band(1-2 GHz).

Microwave absorption of magnesium/hydrogen-treated titanium dioxide nanoparticles

Interactions between materials and electromagnetic irradiations in the microwave frequency are critical for many civil and military applications, such as radar detection, communications, information processing and transport et al. Dipole rotations or magnetic domain resonance are the mainly traditional mechanisms for microwave absorption. The recent finding of the excellent microwave absorption from hydrogenated TiO2 nanoparticles provides us an alternative approach for achieving such absorption, by manipulating the structural defects inside nanoparticles through hydrogenation. In this study, we demonstrate that the microwave absorption can be not only achieved but fine-tuned with TiO2 nanoparticles thermally treated in a Mg/H2 environment. Their position and efficiency can be effectively controlled by the treating temperature. Specifically, the microwave absorption position shifts to the lower frequency region as the treating temperature increases, and there seems to exist an optimal treating temperature to obtain the maximum efficiency, as the absorbing efficiency first increases, and then decreases, with the increase in treatment temperature. Therefore, this study enriches our knowledge and understanding microwave absorption from TiO2-based nanomaterials which may inspire new ideas on other systems to enhance their performance as well.

Effects of Sm-doping on microstructure,magnetic and microwave absorption properties of BiFeO_(3)

In this paper,polycrystalline samples of Bi_(1-x)Sm_(x)FeO_(3)(x=0,0.05,0.1,0.15) were successfully synthesized by sol-gel method.The effects of Sm concentration on the crystal structure,morphology,chemical states,magnetic properties and microwave absorption performance were studied by X-ray diffraction(XRD),scanning electron microscopy(SEM),transmission electron microscopy(TEM),X-ray photoelectron spectroscopy(XPS),a vibrating sample magnetometer(VSM) and a Vector network analyzer(VNA),respectively.The results show that the rare earth Sm doping causes the crystal structure to change.When x≤0.1,Bi_(1-x)Sm_(x)FeO_(3) is the distorted rhombohedral structure with space group R3c.With the increase of Sm doping amount to x=0.15,the phase structure of Bi_(1-x)Sm_(x)FeO_(3) changes from rhombohedral structure to cubic structure with the space group Pm3 m.The particle size decreases with the increase of the Sm doping amount.The analysis results show that Sm doping can effectively reduce the oxygen vacancies and significantly improve its magnetic properties.The results exhibit that moderately doped rare earth Sm element can effectively improve microwave absorption properties of Bi_(1-x)Sm_(x)FeO_(3) powders.When Sm doping amount of x is 0.1,the Bi_(0.9)Sm_(0.1)FeO_(3) compound has good microwave absorption performance,and the minimum reflection loss value of Bi_(0.9)Sm_(0.1)FeO_(3) powder reaches about-32.9 dB at11.7 GHz,and its effective absorption bandwidth(RL <-10 dB) is 2.6 GHz with the optimal matching thickness of 2.0 mm.

Theoretical calculation and experiment of microwave electromagnetic property of Ni(C) nanocapsules

With the combination of the dielectric loss of the carbon layer with the magnetic loss of the ferromagnetic metal core, carbon-coated nickel （Ni（C）） nanoparticles are expected to be the promising microwave absorbers. Microwave electromag- netic parameters and reflection loss in a frequency range of 2 GHz-18 GHz for paraffin-Ni（C） composites are investigated. The values of relative complex permittivity and permeability, the dielectric and magnetic loss tangent of paraffin-Ni（C） com- posites are measured, respectively, when the weight ratios of Ni（C） nanoparticles are equal to 10 wt%, 40 wt%, 50 wt%, 70 wt%, and 80 wt% in paraffin-Ni（C） composites. The results reveal that Ni（C） nanoparticles exhibit a peak of magnetic loss at about 13 GHz, suggesting that magnetic loss and a natural resonance could be found at that frequency. Based on the measured complex permittivity and permeability, the reflection losses of paraffin-Ni（C） composites with different weight ratios of Ni（C） nanoparticles and coating thickness values are simulated according to the transmission line theory. An ex- cellent microwave absorption is obtained. To be proved by the experimental results, the reflection loss of composite with a coating thickness of 2 mm is measured by the Arch method. The results indicate that the maximum reflection loss reaches -26.73 dB at 12.7 GHz, and below -10 dB, the bandwidth is about 4 GHz. The fact that the measured absorption position is consistent with the calculated results suggests that a good electromagnetic match and a strong microwave absorption can be established in Ni（C） nanoparticles. The excellent Ni（C） microwave absorber is prepared by choosing an optimum layer number and the weight ratio of Ni（C） nanoparticles in paraffin-Ni（C） composites.

Characteristics of permittivity and permeability spectra in range of 2?18 GHz microwave frequency for La_(1-x)SrxMn_(1-y)B_yO_3 (B=Fe, Co, Ni)

Doped LaMnO3 has unusual electromagnetic properties, which makes it possible for this material to be used for absorbing microwave. LaMnO3 systems doped by Sr at site A and Fe or Co, Ni at site B were prepared by sol-gel as an microwave absorption material and their permittivity and permeability spectra were measured by microwave vector network analyzer in the frequency range of 2-18 GHz. A novel phenomenon is discovered that the complex permittivity, complex permeability and electromagnetic loss tangent have suddenly a step change at a certain frequency and the step-change frequency is relevant to content of Sr and Fe or Co, Ni. The samples show mainly dielectric loss when microwave frequency is smaller than the step-change frequency, and mainly magnetic loss when larger than that frequency. It is indicated that anti-ferromagnetic clusters in the material can absorb energy quantum of microwave electromagnetic field to change into ferromagnetic clusters because they can overcome higher energy barrier when the frequency of incident microwave reaches a certain value.

球磨钢珠配比对片状羰基铁粉吸波性能影响的研究

片状羰基铁具有高的饱和磁化强度和一定的平面各向异性,可突破Snoek限制,在微波吸收领域具有很好的应用前景。本文以球状羰基铁粉为原料,机械球磨制备了片状羰基铁,并根据球磨钢珠配比的改变获得形貌和电磁性能各异的片状羰基铁:随着钢球平均直径的减小,羰基铁片状结构逐渐增多,片状尺寸增大,片层厚度变薄,吸波性能提高;当采用4 mm钢球进行球磨时,片状羰基铁具有最佳吸波性能,其复磁导率的实部和虚部在2 GHz时可达到2.8773和0.9493;涂层厚度为2 mm时,反射率损耗在9.1 GHz处达到最大值-35 dB,小于-10 dB频带宽为7.429 GHz。

羰基铁/三元乙丙橡胶复合材料的吸波性能

为了探明吸波材料的厚度、吸收剂的含量与吸波性能之间的关系,以羰基铁为吸收剂、三元乙丙橡胶为基体制备了复合橡胶吸波材料,采用矢量网络分析仪研究了在2.6-18 GHz范围内羰基铁含量和吸波材料厚度对吸波性能的影响,并利用电磁理论分析了吸波材料的吸波性能.对于3 mm吸波材料,当羰基铁体积分数为45%时,在3.5 GHz处其反射率的最小值达-21.7 dB;在羰基铁含量一定的条件下,微波吸收性能与吸波材料的厚度并不成正比关系,当厚度〈2 mm时,吸波材料的吸波效果较差;当厚度〉2 mm时,随着吸波材料厚度的增加,最大吸收峰的位置向低频移动,并且最大吸收峰的峰值和指定反射率的频率带宽也呈减小的趋势.在相同厚度下,随着羰基铁含量的增加,吸波材料在电磁波吸收峰处的反射率不断减小,而且吸波材料吸收峰的位置也向低频移动;输入阻抗与空气阻抗越接近,吸波材料的吸波性能越好.

SiC(N)纳米粉体的吸波性能研究

采用CVD法合成了SiC(N)纳米粉体.在NH3流量为0～480ml/min范围内,合成了氮原子百分含量随NH3流量逐渐增大的一系列SiC(N)纳米粉体.研究了SiC(N)纳米粉体的介电常数和介电损耗角正切与粉体组成的关系.发现介电常数的实部、虚部和介电损耗角正切均随粉体中氮原子摩尔分数的升高而降低.依据粉体的介电常数设计了双层吸波涂层,涂层的吸波效果随粉体氮含量的升高而降低.N原子取代SiC晶格中C产生的带电缺陷在电磁场作用下的极化弛豫是SiC(N)纳米粉体吸波的主要机理.

Nd掺杂对钛酸钡电磁及微波吸收特性影响研究

采用溶胶-凝胶法制备稀土Nd掺杂钛酸钡纳米粉体。采用X射线衍射(XRD)、矢量网络分析仪等手段分别分析表征了样品的微结构和高频电磁特性。结果表明,Nd离子掺杂的钛酸钡样品在750℃温度下退火1h,结晶良好。随着Nd掺杂量的提高,晶粒细化。Nd离子掺杂后,在高频下钛酸钡纳米粉体的介电常数的实部略有减小,虚部峰值得到提高,复介电常数和复磁导率的峰位均发生蓝移,粉体材料在2～18GHz范围内的反射损耗得到提高,吸收频带拓宽明显。

六角晶系钡铁氧体与锶铁氧体吸波性能的比较

采用柠檬酸盐溶胶-凝胶法制备了六角晶系钡铁氧体BaFe12O19与锶铁氧体SrFe12O19。采用XRD、AFM、TEM研究了产物的粒形、粒径与矿物组成。结果表明,铁氧体的平均粒径为50nm。采用网络分析仪分别测试了0.1～6GHz内铁氧体的介电常数、磁导率与12～18GHz内吸波涂层的微波损耗。结果表明,钡、锶铁氧体的介电常数虚部形态与损耗角正切形态基本相同,但钡铁氧体在较宽的频率范围内具有较多的损耗角正切峰值,钡铁氧体的吸波性能优于锶铁氧体。

包覆钡铁氧体的多孔玻璃微珠吸波材料制备与性能

以硝酸钡、硝酸铁、柠檬酸及多孔玻璃微珠制备前驱体,在60℃／h升温速率下加热至850℃,保温2h．研究了产物的结构组成、表面形貌、复介电常数与磁导率．实验表明,多孔空心玻璃微珠表面形成厚度<1μm六角磁铅型钡铁氧体BaFe12O19包覆层．包覆钡铁氧体的多孔玻璃微珠吸波粉体吸波性能良好．采用聚合物乳液与复合粉体制备了1．8mm厚的涂层．在5—18GHz内,涂层的微波反射损失>-8dB．在6GHz处最大为-15dB．

铁镍合金与碳纳米管复合物的制备及吸波性能

以采用溶胶凝胶结合氢气还原法制备出的铁镍合金纳米颗粒为催化剂,原位裂解苯,成功合成铁镍合金与纳米碳管的复合物。场发射扫描电镜和高分辨透射电镜结果显示复合物为一维纳米管状结构。微波电磁参数测量结果表明:随着试样厚度的增加,匹配频率处的理论反射损耗量呈下降趋势;当厚度为1.5mm时,FeNi22@C复合物的微波吸收效果最好,在频率11.35GHz处反射损耗量为-4.74dB;可以通过改变铁镍合金中各成分的比例、复合物吸收厚度,调整复合物的复数介电常数和复数磁导率,优化复合物的微波吸收性能。

Co替代Y型六角铁氧体粉末在X波段的吸波特性

采用固相反应方法制备了Ba2Zn1.6-xCoxCu0.4Fe12O22(x=0,0.2,0.4,0.6)铁氧体粉末,将粉末在1050℃下退火4小时。利用X射线衍射(XRD)、振动样品磁强计(VSM)以及矢量网络分析仪研究了退火样品的晶体结构及其在8.2～12.5GHz波段的微波吸收特性。XRD分析表明样品为Y型六角铁氧体。VSM及微波矢量网络测量表明,掺Co样品的介电常数实部ε′的平均值由不掺时的6.5降为5.0。随着Co替代量的增加,样品的饱和磁化强度和剩磁不断增大;矫顽力先维持在62～70Oe之间,至Co含量x=0.6时迅速增为315.5Oe。在磁导率虚部μ″谱中观察到样品的自然共振吸收峰,其共振频率和峰值与样品中的Co含量有关,Co含量x=0.2的样品的μ″峰值达6.579,相应的反射功率损耗达41.796dB,吸波性能良好,基本达到在共振频率附近提高μ′、μ″和ε″值,同时降低ε′值的目的。

组分对FeCoB纳米晶微波电磁参数及吸波性能的影响

利用两步机械球磨法制备了薄片状的Fe-CoB纳米晶材料,并研究了B含量对材料物相结构、静态磁性能、微波电磁参数和吸波性能的影响。结果表明,适当地加入非晶B会加强晶相与非晶相的交换耦合作用,从而使FeCoB的磁导率随着B含量的增加先增大后减小,复介电常数则随着B含量的增加单调降低,显著地改善了吸波材料的阻抗匹配特性。通过计算机模拟表明,与未添加非晶B的样品相比,FeCoB纳米晶材料在低频端有优异的吸波性能。当B含量从3%变化到15%时,在1.8～6.7GHz范围内的吸收强度均<-6dB,可应用于拓展吸波材料吸收带宽的设计中。

Cr代替部分Si对Sendust微波吸收特性的影响

利用X射线衍射(XRD)和扫描电镜(SEM)研究了Cr代替Sendust部分Si后对其微波吸收特性的影响。结果表明:Cr代替部分Si后,使FeSiAl的脆性得到改善,颗粒的纵横比增大,从而使FeSiAl磁导率提高;Cr的加入虽对合金的复介电常数实部变化影响不大,但虚部增加较大;且微波吸收带宽通过添加Cr后改善较大。

Co添加对Fe-Si基合金微波吸收性能影响研究

为研制低频效应和宽频特性较好的吸波材料,以高纯块状Co、Fe、Si为原料,采用电弧熔炼和高能球磨工艺制备CoxFe80-xSi20(x=0,10,20,30)合金粉体,使用SEM、XRD和矢量网络分析仪等测试手段,研究了合金粉体形貌、结构及Co添加对合金吸波性能的影响。结果表明:制备的合金粉体主要由α-Fe相组成;Co的添加使Co-Fe-Si合金的反射率峰值变小,并随α-Fe晶粒尺寸增大而向低频移动;在涂层厚度为1.8mm的条件下,x=20的合金吸收峰频率和峰值分别为4.8GHz和-13.4dB,低频效应较好,x=10的合金吸波峰频率和峰值分别为6.2GHz和-14.8dB,低于-5dB的吸波带宽达14GHz,具有良好的吸波宽频效应。系统研究了Co10Fe70Si20合金的反射率随涂层厚度变化的规律,结果表明随着涂层厚度的增加,合金的反射率峰值向低频移动,当涂层厚度为3mm时,反射率峰值达到极值,即在3.7GHz处达到-14.8dB。

Fe基玻璃包覆微丝的吸波性能研究

实验分析了微丝的组织结构，表明微丝是非晶态的，内部芯丝表面光滑，在长度方向上厚度均匀。用矢量网络分析仪测定了微丝的电磁参数，并进行了不同填充率不同厚度的单层吸波涂层设计，结果显示微丝在微波波段10～18GHz频段范围内具有较好的衰减特性，反射率最小值为--32dB，吸波效果好。

GF/ACF电路屏复合材料吸波性能研究

将一定尺寸的玻璃纤维片(GF-sheet)填充至矩形活性碳纤维(Activated Carbon Fiber,ACF)电路屏空穴中,制备了GF/ACF电路屏复合材料,研究了GF-sheet尺寸对ACF电路屏复合材料电磁波反射及吸收特性的影响。结果表明,在均匀分布的ACF电路屏空穴中填充相应尺寸的GF-sheet能有效降低复合材料对电磁波的反射特性,增强吸波能力;GF水平排布的反射性能优于垂直排布;当在正方形ACF电路屏空穴中填充边长l=2 cm的GF-sheet时,最低反射值达到-29.8 d B,其吸波性能较未填充GF-sheet的复合材料,其-10 d B以下的最低反射衰减提高了60.4%,达到-21.5 d B,有效带宽有所拓宽;当在长方形ACF电路屏空穴中填充l=2 cm,w=1 cm的GF-sheet,反射衰减最低,最低反射衰减为-16.1 d B,有效带宽为12.1 GHz。

单层高磁导率吸波薄膜的设计

以高磁导率薄膜作为现代电子系统内的吸波材料具有薄、轻的优点,但传统观点认为单层吸波材料的厚度取1/4个介质内波长时,会产生谐振损耗或干涉相消而致使功率反射系数最小。而本文的计算分析表明,要使目标频点的电磁波吸收最强,单层高磁损耗吸波材料的厚度一般小于1/4个介质内波长。且薄膜的磁导率实部的增加能够扩展吸波的带宽,而虚部的减小则有助于缩减吸波的带宽。