Microwave Absorption of Crystalline Fe/MnO@C Nanocapsules Embedded in Amorphous Carbon

Crystalline Fe/MnO@C core–shell nanocapsules inlaid in porous amorphous carbon matrix(FMCA)was synthesized successfully with a novel confinement strategy.The heterogeneous Fe/MnO nanocrystals are with approximate single-domain size which gives rise to natural resonance in 2–18 GHz.The addition of MnO2 confines degree of graphitization catalyzed by iron and contributes to the formation of amorphous carbon.The heterogeneous materials composed of crystalline–amorphous structures disperse evenly and its density is significantly reduced on account of porous properties.Meanwhile,adjustable dielectric loss is achieved by interrupting Fe core aggregation and stacking graphene conductive network.The dielectric loss synergistically with magnetic loss endows the FMCA enhanced absorption.The optimal reflection loss(RL)is up to−45 dB,and the effective bandwidth(RL<−10 dB)is 5.0 GHz with 2.0 mm thickness.The proposed confinement strategy not only lays the foundation for designing high-performance microwave absorber,but also offers a general duty synthesis method for heterogeneous crystalline–amorphous composites with tunable composition in other fields.     

The enhanced microwave absorption property of CoFe(2)O(4) nanoparticles coated with a Co(3)Fe(7)-Co nanoshell by thermal reduction

CoFe(2)O(4) nanoparticles were fabricated by a sol-gel method and then were coated with Co(3)Fe(7)-Co by means of a simple reduction process at different temperatures under 2% H(2) with the protection of argon to generate the dielectric-core/metallic-shell structure. The optimum reflection loss (RL) calculated from permittivity and permeability of the 80 wt% CoFe(2)O(4)/Co(3)Fe(7)-Co and 20 wt% epoxy resin composites reached - 34.4 dB, which was much lower than that of unreduced CoFe(2)O(4) and epoxy resin composites, at 2.4 GHz with a matching thickness of 4.0 mm. Moreover the RL exceeding - 10 dB in the maximum frequency range of 2.2-16 GHz was achieved for a thickness of composites of 1.0-4.5 mm with 600?°C thermal reduction process. The improved microwave absorption properties are a consequence of a proper electromagnetic match and the enhanced magnetic loss besides its dielectric loss due to the existence of the core/shell structure in CoFe(2)O(4) composites. Thus, the reductive CoFe(2)O(4) nanoparticles have great potential for being a highly efficient microwave absorber.     

Double layer microwave absorber based on Cu dispersed SiC composites

The present work has been focused on designing an efficient and cost-effective double layer microwave absorber in 8.2–12.4?GHz frequency range. For the same, Cu particles were dispersed in SiC to achieve enhanced microwave absorption by combining the excellent dielectric characteristics of SiC with highly conductive Cu. Cu dispersed SiC composites were prepared by dispersing various weight fractions of Cu particles in the SiC matrix using planetary ball mill. The Cu dispersion in SiC yielded excellent relative complex permittivity values translating into a decrease in the reflection loss (RL) values of dispersed composites as compared to the pristine counterpart. The minimum RL of ?17.18?dB has been observed for 2?wt% Cu dispersed SiC composite at 11.81?GHz with a thickness of 1.3?mm and bandwidth corresponding to ?10?dB is 1.77?GHz. Genetic algorithm approach has been implemented to design double layer microwave absorber to further enhance the microwave absorption of the prepared composites for realizing a cost-effective solution. The optimum double layer results show the RL of ?32.16?dB at 11.05?GHz with 1.67?mm total thickness and bandwidth corresponding to ?10?dB is 2.35?GHz.     

Microwave absorption of gamma'-Fe2.6 Ni1.4N nanoparticles derived from nitriding counterpart precursor

Gamma-Fe2.6Ni1.4 nanoparticles were prepared by the arc-discharge method as the precursor and its nitride counterpart of gamma'-Fe2.6Ni14N nanoparticles was synthesized directly through a thermal ammonolysis reaction at the temperature of 673 K for two hours. The resultant product was identified as a homogeneous ternary nitride with nearly spherical shape and average size of about 60.0 nm. The electromagnetic characteristics of gamma'-Fe2.6Ni1.4N derivant and gamma-Fe2.6Ni1.4 precursor have been studied in the frequency range of 2-18 GHz. Compared with the precursor, gamma'-Fe2.6Ni1.4N nanoparticles exhibits an enhanced electromagnetic absorption property resulted from the increased dielectric loss by nitriding process. The optimal reflection loss (RL) of gamma'-Fe2.6Ni1.4N nanoparticles/paraffin composite can reach -39.9 dB at 5.2 GHz in a thickness of 2.29 mm, and the frequency band corresponding RL < -10 dB is over 2.6-18 GHz in the thickness range of 0.78-4.20 mm.     

Magnetic ferrite/carbonized cotton fiber composites for improving electromagnetic absorption properties at gigahertz frequencies

Ferrite/carbon composited materials,especially the bio-derived composited materials possessing both environmental friendliness and outstanding microwave absorption performance,attract numerous attentions for solving the"electromagnetic problem"in the Gigahertz frequency range.In this work,we demonstrate a bio-derived ferrite/carbon material by compositing functional carbonized cotton fibers(CCFs)and Fe3O4 nanoparticles with optimized microwave-absorption properties.By adjusting the carbonization conditions systematically,the Fe3O4 loading contents and the microwave absorption properties can be varied simultaneously-and,indeed,optimized and tuned.The CCFs-Fe3O4 composites exhibited a minimum reflection-loss capacity RL(dB)of-56.8 dB at 10.9 GHz with a thickness of 1.67 mm,and its effective absorption bandwidth(RL(dB)＜-20 dB)was found to broaden to 7.1 GHz.Electromagnetic characterizations,coupled with microstructure analyses,revealed that the enhancement in microwave absorption was triggered by the different microstructures of CCFs-Fe3O4 composites-attributable to the different carbonization processes.These different conditions result in different amounts of Fe3O4 attachment sites and lead to the enhancement of dielectric polarization at localized microstructures.The present work of bio-derived ferrite/carbon materials has important implications in understanding structure-performance relationships in dielectric-magnetic materials,and,meanwhile,could well be extended to a microwave-absorber design approach.     

Microwave absorption performance of hierarchical structured composites of greigite and reduced graphene oxide

The greigite (Fe₃S₄)/reduced graphene oxide (RGO) hierarchical structural composites (F–R) with the Fe₃S₄ nanoparticles attached to the RGO layers were successfully prepared via a simple one-pot solvothermal method. The microwave absorption properties were evaluated by calculating the reflection loss (RL) values. The results show that the RGO content and the filler loading of composites in paraffin mixture are very critical to the microwave absorption properties because they can improve the electromagnetic parameters. Sample F–R-3 presents the best microwave absorption capacity, in which an optimum RL value of ??62.3 dB and an effective absorption bandwidth (EAB, RL value?<???10 dB) of 3.04 GHz (14.96–18 GHz) can be obtained when the matching thickness is only 1.29 mm. Meanwhile, the widest EAB reaches 4.08 GHz (13.92–18 GHz) at the matching thickness of only 1.37 mm. Impressively, when the matching thickness is in the range of 1.2–5.5 mm, all RL peaks are below ??20 dB, and the EAB can be 14.98 GHz (3.02–18 GHz), covering the whole C, X and Ku bands. The distinguished absorption property is mainly ascribed to the combined effect of strong loss ability and good impedance matching. Apparently, the F–R composite with strong absorption ability, thin thickness and wide EAB is suitable for the efficient microwave absorber.

Graphical abstract

     

碳纳米管/平面各向异性羰基铁复合材料的液相共混法制备及其电磁性能

采用机械球磨法制备了平面各向异性羰基铁（Planar Anisotropic Carbonyl Iron,PACI）,然后通过液相共混法制备了碳纳米管（CNTs）/PACI复合材料。采用同轴法测定CNTs/PACI复合材料在2~18 GHz频段内的复介电常数和复磁导率,研究了CNTs掺杂量对复合材料电磁性能的影响。结果表明：CNTs/PACI复合材料相对于PACI具有更高的复介电常数和衰减常数,随着CNTs质量分数的提高,复合材料的复介电常数和衰减常数逐渐增大,特征阻抗则逐渐减小。CNTs掺杂能够有效提高CNTs/PACI复合材料的吸波性能,通过调整厚度和CNTs掺杂量可以对复合材料的吸波性能进行有效调控。厚度为1.2 mm、CNTs质量分数为2wt%和厚度为1.6 mm、CNTs质量分数为0.5wt%的CNTs/PACI复合材料在Ku波段（12~18 GHz）的反射率均小于-10 dB;厚度为2.0 mm、CNTs质量分数为0.5wt%和1wt%的复合材料反射率小于-10 dB的频带宽分别为5.28 GHz（8.24~13.52 GHz）和5.04 GHz（7.52~12.56 GHz）,覆盖整个X波段（8~12 GHz）。     

选择表面工艺改性的CIPs涂层及其氧化物的吸波性能

针对吸波涂层氧化腐蚀现象,提出一种基于选择表面的周期结构涂层维修工艺。利用腐蚀法,在以羰基铁粉(CIPs)为吸收剂的吸波涂层表面制备CIPs及其氧化物涂层。用扫描电子显微镜(SEM)对腐蚀后颗粒的形貌进行了分析。测试了8~18GHz下混合颗粒的复介电常数和复磁导率,并用等效介质理论计算氧化涂层的参数。分析腐蚀和维修工艺对反射损耗(RL)的影响。结果表明:当CIPs涂层表面被氧化时,随着氧化涂层厚度的增加,涂层的吸波性能减弱,反射损耗增量值约为2dB。当涂层厚度为0.8mm时,维修效果不理想,若涂层厚度增加至1mm,且氧化涂层厚度为0.1mm时,维修效果较好,体现为10~18GHz频率范围内的吸波带宽增加,反射损耗相应减小约为2dB。     

Enhanced microwave absorption of ZnO-coated planar anisotropy carbonyl-iron particles in quasimicrowave frequency band

For the aim of thin electromagnetic wave absorbers used in quasimicrowave frequency band, planar anisotropy carbonyl-iron (PACI) particles coated with ZnO nanoshells were prepared by ball milling technique and chemical precipitation method. Compared with the as-milled PACI/paraffin composite, lower dielectric constant was obtained for the composite containing PACI at ZnO particles, and hence a dramatic enhancement of reflection loss (RL) was obtained. The minimum RL of PACI at ZnO composite reaches −31.93 dB at 1.96 GHz with the matching thickness of 2.5 mm. Furthermore, the absorbing property is further improved after the PACI at ZnO composite was rotationally oriented in an external magnetic field. The minimum RL of the oriented PACI at ZnO composite reaches −40.06 dB and the matching thickness reduces to 2.2 mm with a slight variation of matching frequency. The PACI at ZnO core-shell particles exhibit great potential in application of the thin absorber in the 1–4 GHz frequency range.     

Effect of ball milling and moderate surface oxidization on the microwave absorption properties of FeSiAl composites

Commercially available irregular FeSiAl alloys were used as raw materials. The microwave absorption properties of FeSiAl/paraffin composites were improved by ball milling the alloys and moderately oxidizing their surfaces. Permittivity and permeability of the as-milled composites distinctly increased compared with those before milling. Moderate surface oxidization significantly reduced permittivity whereas permeability almost maintained its initial value compared with those of as-milled composites. Consequently, the microwave absorption properties were significantly improved. The minimum reflection loss (RL) of the absorber with 35 vol% surface-oxidated flake reached −39.67 dB at 1.40 GHz at a thickness of 4 mm. Effective microwave absorption (RL < −10 dB) was achieved within the range of 0.73–3.94 GHz at the thickness of 2–5 mm, which may be applied to the L- and S-bands.     

Hierarchical composite of biomass derived magnetic carbon framework and phytic acid doped polyanilne with prominent electromagnetic wave absorption capacity

To solve the electromagnetic pollution,herein,a CoFe₂O₄/C/PANI composite was developed by a green route,which was constructed with spinel of metal oxide,graphitized carbon and conductive polymer composites.Benefiting from the designable interfaces and increased dipoles,the microwave dielectric response capability can be boosted significantly and resulted in the enhanced microwave absorbing performance.As revealed by the reflection loss curve,the minimum reflection loss(RLmin) reached-51.81 dB at 12.4 GHz under a matched thickness of 2.57 mm.At 2.5 mm,the effective absorbing band covered 8.88 GHz,suggesting the desirable wideband feature.In our case,the method of utilization of a novel green way to fabricate multiple-component EM absorber can be a promising candidate for high-performance EM absorber.     

Constructing and optimizing core–shell structured Co@TiO2 as highly efficient electromagnetic wave absorber

To achieve highly efficient electromagnetic wave absorber, elaborately designing magnetic–dielectric Co@TiO₂ microspheres with core–shell configuration are successfully constructed through a facile sequential process of liquid phase reduction–sol–gel–annealing. Owing to the core–shell configuration and the synergistic effect among magnetic and dielectric components, the annealed sample shows outstanding electromagnetic wave absorption (EMA) in X and K_u band. Impressively, a maximum reflection loss (RL_max) is reached?–56.6 dB at a coating thickness of 2.3 mm with corresponding effective absorption bandwidth (EAB₁₀) of 7.2 GHz (including 65% of K_u band and 82.5% of X band), much stronger than those of as-prepared Co (EAB₁₀ of 1.6 GHz, RL_max of 14.6 dB) and pristine Co@TiO₂ (EAB₁₀ of 2.9 GHz, RL_max of 16.3 dB). An EAB₁₀ covering completely the whole X and K_u band could be obtained by controlling the thickness only from 2.0 mm to 2.5 mm. The composites have both outstanding RL and wide EAB₁₀ with the thin coating thickness, reinforcing that fabricating core–shell configuration composites is an efficient strategy to boost the EMA efficiency.

     

镀镍碳纳米管的微波吸收性能研究

用竖式炉流动法制备了碳纳米管,碳纳米管的外径40nm～70nm,内径7nm～10nm,长度50μm～1000μm,呈直线型,用化学镀法在碳纳米管表面镀上了一层均匀的金属镍。碳纳米管吸波涂层在厚度为0.97mm时,在8GHz～18GHz,最大吸收峰在11.4GHz(R=-22.89dB),R<-10dB的频宽为3.0Hz,R<-5dB的频宽为4.7GHz。镀镍碳纳米管吸波涂层在相同厚度下,最大吸收峰在14GHz(R=-11.85dB),R<-10dB的频宽为2.23Hz,R<-5dB的频宽为4.6GHz。碳纳米管表面镀镍后虽然吸收峰值变小,但吸收峰有宽化的趋势,这种趋势对提高材料的吸波性能是有利的。碳纳米管作为偶极子在电磁场的作用下,会产生耗散电流,在周围基体作用下,耗散电流被衰减,从而雷达波能量被转换为其它形式的能量。     

Ferromagnetic and microwave absorption properties of copper oxide/cobalt/carbon fiber multilayer film composites

The copper oxide/cobalt/carbon fiber multilayer film composites were synthesized by thermal oxidation route. In order to investigate the intrinsic reasons for microwave absorption properties of absorbers, the complex permittivity, complex permeability and the microwave absorption properties of composites were studied in the 1-18 GHz range. The strongest reflectivity loss (RL) of microwave absorber was further enhanced to − 42.7 dB (microwave absorption rate > 99.9%) at 10.8 GHz for a layer of 2.0 mm thickness, and the strong absorption (RL < − 10 dB) was obtained between 8.72 and 18 GHz for the thickness of 1.3-2.2 mm. The results indicated that the dielectric loss and magnetic loss led to the excellent microwave absorption property of CuO/Co/CF composites. It is believed to be ideal for making a lightweight, strong absorption and wide-frequency microwave absorbing material.     

Synthesis and microwave absorption properties of magnetite nanoparticles

Nanoparticles of Fe3O4 with various sizes were synthesized from FeCl3 x 6H2O, FeCl2 x 4H2O and NaOH by coprecipitation process. The crystal structure, morphology, particle size and magnetic property of the products were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM). It was found that the molar ratio of ferrous to ferric played an important role in the formation of Fe3O4 nanoparticles. The particle mean diameter swelled from approximately 10 to approximately 20 nm with the molar ratio range from 1:2 to 6:1. The saturation magnetization and the coercivity increased correspondingly. The complex permittivity epsilon(r) and permeability mu(r) of the Fe3O4 mixture with paraffin were measured using vector network analysis. Values of epsilon(r), and mu(r) were used to determine the reflection loss at various sample thicknesses, based on a model of microwave absorbing layer backed by a metal plate. The minimal reflection loss or the dip shifts to a lower frequency region with increasing thickness. When the thickness is 5 mm, the minimal reflection loss of Fe3O4 synthesized with the molar ratio of 6:1 and paraffin wax composites reaches -35.1 dB at 5.2 GHz and -30.2 dB at 17.6 GHz, respectively. The minimal reflection loss is attributed to the thickness of the absorber approximates an odd number multiple of a quarter of the propagation wavelength.     

Synthesis of core–shell FeCo@SiO<Subscript>2</Subscript> particles coated with the reduced graphene oxide as an efficient broadband electromagnetic wave absorber

The FeCo@SiO₂@RGO composites were prepared by combining liquid-phase reduction reaction in Argon atmosphere with hydrothermal reaction. The crystal structure, chemical composition and morphology of the as-prepared composites have been investigated in detail. SEM and TEM results illustrate that the FeCo@SiO₂ composites are of core–shell structure with a diameter of about 150–200 nm. Compared with FeCo@SiO₂ and FeCo@RGO composites, the as-prepared FeCo@SiO₂@RGO composites exhibit excellent electromagnetic (EM) wave absorption properties. As an EM wave absorber, the maximum R_L reaches ?52.9 dB at 9.12 GHz with a thickness of 3.0 mm, and the absorption bandwidth with the reflection loss below ?10 dB was up to 5.36 GHz (from 8.8 to 14.16 GHz) with a thickness of 2.5 mm. It is believed that the FeCo@SiO₂@RGO composites can serve as an excellent microwave absorbent and can be widely used in the microwave absorbing area.     

Synthesis and Characterization of Nanocapsules of a-Fe(NiCoAl) Solid-solution

α-Fe(NiCoAl) solid-solution nanocapsules were prepared with pure powders of Fe, Ni, Co and Al by the plasma arc-discharging using a copper crucible. The shapes of the nanocapsules are in polyhedrons with the core/shell structure. The body centered cubic (BCC) phase is formed in the core. The size of the nanocapsules is in the range of 10~120 nm and the thickness of the shell is 4~11 nm. Saturation magnetization Js=150 Am2/kg and coercivity iHC=24.3 kA/m are achieved for the nanocapsules.     

Synthesis and Characterization of Nanocapsules of α-Fe(NiCoAl) Solid-solution

α-Fe(NiCoAl) solid-solution nanocapsules were prepared with pure powders of Fe, Ni, Co and Al by the plasma arc-discharging using a copper crucible. The shapes of the nanocapsules are in polyhedrons with the core/shell structure. The body centered cubic (BCC) phase is formed in the core. The size of the nanocapsules is in the range of 10～120 nm and the thickness of the shell is 4～11 nm. Saturation magnetization Js=150 Am2/kg and coercivity iHC=24.3 kA/m are achieved for the nanocapsules.     

Low Temperature Solution Synthesis and Microwave Absorption Properties of Multiwalled Carbon Nanotubes/Fe3O4 Composites

Multiwalled carbon nanotubes (MWCNTs)/Fe₃O₄ nanocomposites were synthesized via a simple low temperature solution method. The phase structures and morphologies of the composite were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results showed that the Fe₃O₄ spheres of about 150 nm were linked with MWCNTs. The microwave absorption properties of the MWCNTs/Fe₃O₄ nanocomposites were measured by vector network analysis (VNA). A wide region of microwave absorption was achieved due to dual magnetic and dielectric losses. When the matching thickness is 2 mm, the reflection loss (RL) of the sample exceeding ?10 dB was obtained at the frequency range of 9.9–12.4 GHz, with an optimal RL of ?29.8 dB at 11.04 GHz. A possible mechanism of the improved microwave absorption properties of the composites was discussed.     

Synthesis of covalently bonded reduced graphene oxide-Fe3O4 nanocomposites for efficient electromagnetic wave absorption

High-performance electromagnetic (EM) wave absorbers,covalently bonded reduced graphene oxide-Fe3O4 nanocomposites (rGO-Fe3O4),are synthesized via hydrothermal reaction,amidation reaction and reduction process.The microstructure,surface element composition and morphology of rGO-Fe3O4 nanocomposites are characterized and corresponding EM wave absorption properties are analyzed in great detail.It demonstrates that Fe3O4 nanoparticles are successfully covalently grafted onto graphene by amide bonds.When the mass ratio of rGO and Fe3O4 is 2∶1 (sample S2),the absorber exhibits the excellent EM wave absorption performance that the maximum reflection loss (RL) reaches up to-48.6 dB at 14.4 GHz,while the effective absorption bandwidth (RL＜-10 dB) is 6.32 GHz (11.68-18.0 GHz) with a matching thickness of 2.1 mm.Furthermore,radar cross section (RCS) simulation calculation is also adopted to evaluate the ability of absorbers to scatter EM waves,which proves again that the absorption performance of absorber S2 is optimal.The outstanding EM wave absorption performance is attributed to the synergistic effect between dielectric and magnetic loss,good attenuation ability and excellent impedance matching.Moreover,covalent bonds considered to be carrier channels can facilitate electron migration,adjust EM parameters and then enhance EM wave absorption performance.This work provides a possible method for preparing efficient EM wave absorbers.