Effects of Multi-walled Carbon Nanotubes on the Electromagnetic Absorbing Characteristics of Composites Filled with Carbonyl Iron Particles

The electromagnetic(EM) wave absorbing property of silicone rubber filled with carbonyl iron particles(CIPs) and multi-walled carbon nanotubes(MWCNTs) was examined.Absorbents including MWCNTs and spherical/flaky CIPs were added to silicone rubber using a two-roll mixer.The complex permittivity and complex permeability were measured over the frequency range of 1-18 GHz.The two EM parameters were verified and the uniform dispersion of MWCNTs and CIPs was confirmed by comparing the measured reflection loss(RL) with the calculated one.As the MWCNT weight percent increased,the RL of the spherical CIPs/silicone rubber composites changed insignificantly.It was attributed to the random distribution of spherical CIPs and less content of MWCNTs.On the contrary,for composites filled with flaky CIPs the absorption bandwidth increased at thickness 0.5 mm(RL value lower than-5 dB in 8-18 GHz) and the absorption ratio increased at lower frequency(minimum-35 dB at 3.5 GHz).This effect was attributed to the oriented distribution of flaky CIPs caused by interactions between the two absorbents.Therefore,mixing MWCNTs and flaky CIPs could achieve wider-band and higher-absorption ratio absorbing materials.     

Microwave Absorption and Shielding Property of Composites with FeSiAl and Carbonous Materials as Filler

Silicone rubber composites filled with FeSiAl alloys and multi-walled carbon nanotubes(MWCNT)/graphite have been prepared for the first time by a coating process.The complex permittivity and permeability of the composites were measured with a vector network analyzer in a 1-4 GHz frequency range,and the DC electric conductivity was measured by a standard four-point contact method.These parameters were then used to calculate the reflection loss(RL) and shielding effectiveness(SE) of the composites.The results showed that the added MWCNT increased the permittivity and permeability of composites in the L-band,while the added graphite increased only the permittivity.The variation lies in the interactions between two carbonous absorbents.Addition of 1 wt% MWCNT enhanced the RL in the L-band(minimum 5.7 dB at 1 mm,7.3 dB at 1.5 mm),while the addition of graphite did not.Addition of MWCNT as well as graphite reinforced the shielding property of the composites(maximum SE 13.3 dB at 1 mm,18.3 dB at 1.5 mm) owing to the increase of conductivity.The addition of these carbonous materials could hold the promise of enforcing the absorption and shielding property of the absorbers.     

Facile synthesis of ultra-long α-MnO2 nanowires and their microwave absorption properties

Well crystallized α-MnO₂ nanowires (NWs) with an average diameter of about 40 nm and an average length of about 30 μm were successfully synthesized by hydrothermal method. The complex permittivity and permeability of α-MnO₂ NWs/paraffin composites with 20 vol.% α-MnO₂ NWs were measured in a frequency region from 0.1 to 13 GHz. The value of maximum reflection loss of the composites with 20 vol.% α-MnO₂ NWs is approximately − 35 dB at 3.13 GHz with a thickness of 3.6 mm, and the bandwidth corresponding to reflection loss below − 10 dB is higher than 1.8 GHz with a lower thickness of 1.2 mm.     

Preparation and electromagnetic wave absorption properties of CoNi@SiO<Subscript>2</Subscript> microspheres decorated graphene–polyaniline nanosheets

In this work, we successfully parepared the quaternary composites of CoNi@SiO₂@graphene@PANI via a four-step method. The structures, chemical composition and morphologies of obtained composites are analyzed in detail. The electron microscopy results show spherical CoNi@SiO₂ particles evenly dispersed into the surface of graphene@polyaniline nanosheets. The electromagnetic parameters indicate that CoNi@SiO₂@graphene@PANI exhibits enhanced electromagnetic absorption properties compared to CoNi@SiO₂, which can be mainly attributed to the improved impedance matching and multi-interfacial polarization. The maximum reflection loss of CoNi@SiO₂@graphene@PANI can reach ??43 dB at 15.4 GHz and the absorption bandwidth with the reflection loss exceeding ??10 dB is 5.7 GHz (from 12.3 to 18 GHz) with the thickness of 2 mm. Our results demonstrate the quaternary composites composed of CoNi@SiO₂ microparticles and rGO–PANI nanocomposites can serve as light weight and high-performance EM absorbing material.     

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.     

螺旋形炭纤维的吸波性能

通过气相催化裂解法分别制得了螺径约为4μm、螺距为0.5μm～0.8μm的炭纤维（简称为coils-A）和螺径为20μm左右、螺距为1μm～4μm的炭纤维（简称为coils-B）.以coils-A和coils-B为掺杂体与石蜡制成复合材料在8.2 GHz～124 GHz范围内通过反射传输系统测量其电磁参数,结果表明该等微米级螺旋形炭纤维磁损耗为零,其中coils-B的介电参数的虚部及其损耗正切值tanδε较coils-A的高.分别以coils-A和coils-B为手性掺杂体制得填充有手性材料的夹芯蜂窝板复合材料,研究发现coils-A的吸波效果较好,在10 GHz～15 GHz的范围内对电磁波的反射衰减量大于10 dB,在4.6 GHz～18 GHz 的范围内对电磁波的反射衰减量均大于5 dB,在12.4 GHz时最大的反射衰减量为18 dB,其结果与藉由电磁参数所预测的结果相反.经计算,coils-A的手性参数ξ较大.因此,手性参数ξ对于提高吸波性能的影响大于介电参数ε的影响.     

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。     

Controllable synthesis of Ni/NiO@porous carbon hybrid composites towards remarkable electromagnetic wave absorption and wide absorption bandwidth

The reasonable design of the composition of the composite materials is of great significance to optimized the electromagnetic (EM) wave absorption performance.Herein,the Ni/NiO@C hybrid composites with tunable Ni proportion were successfully synthesized through a two-step process.With the assistance of X-ray diffraction with refinement treatment,the specific proportion of Ni of as-obtained hybrid com-posites could be obtained.Employing controlling calcination time to adjust the Ni content of Ni/NiO@C hybrid composites,it has been found that the composite carbonized at 500 ℃ exhibited remarkable EM wave absorption with the minimum reflection loss (RLmin) of-49.1 dB at 4.9 mm and the widest effective absorption bandwidth (EABmax) of 4.56 GHz at 2.1 mm.Moreover,by adjusting the Ni source,the optimal EM wave absorption performance could be achieved.Results illustrated that the N3PC with the Ni pro-portion of 13.17 ％ showed the RLmin as low as-51.1 d B at 2.4 mm and the EABmax was 5.12 GHz at 2.7 mm.It is worth noting that this work demonstrates the relevance of the composition and EM wave absorption performance of hybrid composites,which offers a feasible reference for the absorption mechanism of 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 response of nickel/graphene nanocomposites prepared by electrodeposition

In this study, nanostructures of nickel have been successfully deposited on graphene nanosheet by direct electrochemical deposition. The morphology, nickel content, and magnetic properties of the graphene as well as composites were examined by scanning electron microscopy, transmission electron microscopic, elemental analysis, and vibrating sample magnetometer, respectively. Their relative complex permeability and permittivity were also measured, and reflection loss values were calculated at given thickness layer according to transmit line theory in the range 2–18 GHz. The results reveal that with the increasing of the thickness of the samples, the matching frequency tends to shift to the lower frequency region, and theoretical reflection loss becomes less at the matching frequency. When the absorbing thickness is 1 mm, the maximum absorption value of graphene is ?6.5 dB at about 7 GHz. After decorating graphene sheet with magnetic nickel nanoparticles, the composites were shown to efficiently promote microwave absorbability. When the thickness is 1.5 mm, the absorption value of the composites exceeds ?10 dB in the 5 GHz absorbing bandwidth and the maximum absorption value is ?16.0 dB at 9.15 GHz.     

Lightweight multi-walled carbon nanotube buckypaper/glass fiber–epoxy composites for strong electromagnetic interference shielding and efficient microwave absorption

Multi-walled carbon nanotube buckypaper (BP) reinforced glass fiber–epoxy (GF/EP) composites were selected to fabricate electromagnetic interference (EMI) shielding and microwave absorbing materials. Six different composite configurations with 3.0 mm thick have been conceived and tested over the X-band (8.2–12.4 GHz). Flexible and low-density (0.29 g/cm³) BP provided a high specific EMI SE of 76 dB with controlled electrical conductivity. GF/EP/BP₁₁₁ and GF/EP/BP₁₀₁ composites possess EMI SE as high as of 50–60 dB, which can be attributed to the number of BP inserted and variation in the wave-transmitting layer of the laminates. Furthermore, the shielding mechanism was discussed and suggested that the absorption was the dominant contribution to EMI SE. GF/EP/BP₁₁₀ laminate demonstrated suitable EMI performance (~?20 dB), whereas GF/EP/BP₀₁₁ composite revealed excellent microwave performance, achieving an effective ? 10 dB bandwidth of 3.04 GHz and minimum reflection loss (RL) value of ? 21.16 dB at 10.37 GHz. On the basis of these results, GF/EP/BP composites prepared in this work have potential applications as both EMI shielding and microwave absorber materials given their facile preparation and lightweight use.

     

Flexible and Ultrathin Waterproof Cellular Membranes Based on High-Conjunction Metal-Wrapped Polymer Nanofibers for Electromagnetic Interference Shielding

Ultrathin, lightweight, and flexible electromagnetic interference (EMI) shielding materials are urgently demanded to address EM radiation pollution. Efficient design to utilize the shields' microstructures is crucial yet remains highly challenging for maximum EMI shielding effectiveness (SE) while minimizing material consumption. Herein, novel cellular membranes are designed based on a facile polydopamine-assisted metal (copper or silver) deposition on electrospun polymer nanofibers. The membranes can efficiently exploit the high-conjunction cellular structures of metal and polymer nanofibers, and their interactions for excellent electrical conductivity, mechanical flexibility, and ultrahigh EMI shielding performance. EMI SE reaches more than 53 dB in an ultra-broadband frequency range at a membrane thickness of merely 2.5 µm and a density of 1.6 g cm⁻³, and an SE of 44.7 dB is accomplished at the lowest thickness of 1.2 µm. The normalized specific SE is up to 232 860 dB cm² g⁻¹, significantly surpassing that of other shielding materials ever reported. More, integrated functionalities are discovered in the membrane, such as antibacterial, waterproof properties, excellent air permeability, high resistance to mechanical deformations and low-voltage uniform heating performance, offering strong potential for applications in aerospace and portable and wearable smart electronics.     

A carbonyl iron/carbon fiber material for electromagnetic wave absorption

A carbonyl iron/carbon fiber material consisting of carbon fibers grown on micrometer-sized carbonyl iron sphere, was synthesized by chemical vapor deposition using a mixture of C2H2 and H2. The hollow-core carbon fibers (outer diameter: 140 nm and inner diameter: 40 nm) were composed of well-ordered graphene layers which were almost parallel to the long axis of the fibers. A composite (2 mm thick) consisting of the carbonyl iron/carbon fibers and epoxy resin demonstrated excellent electromagnetic (EM) wave absorption. Minimum reflection losses of -36 dB (99.95% of EM wave absorption) at 7.6 GHz and -32 dB (99.92% of EM wave absorption) at 34.1 GHz were achieved. The well-dispersed and network-like carbon fibers in the resin matrix affected the dielectric loss of the EM wave while the carbonyl iron affected the magnetic loss.     

Broadband electromagnetic shielding and dielectric properties of polyaniline-stannous oxide composites

Conducting polyaniline-stannous oxide (PAni-SnO) composites were synthesized by the in situ polymerization of aniline in the presence of SnO. The composites formed were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As there is a greater need for materials with electromagnetic interference (EMI) shielding properties over a large operating frequency band, the present study highlights the dielectric and EMI shielding response of PAni-SnO composites in the microwave frequency range from 8 to 18 GHz (X and Ku bands). All the computations were based on microwave scattering parameters measured by transmission line waveguide technique. The EMI shielding effectiveness (EMI SE), return loss, microwave absorption and dielectric properties of the PAni-SnO composites were evaluated for various wt% of SnO (10, 20, 30, 40 and 50 wt%) in PAni. In X-band, the composites exhibits EMI SE in the range ?18 to ?23 dB, with microwave absorbance of 70–83 % and in the Ku-band, the composites exhibits EMI SE values of ?17.5 to ?22.5 dB with 67–85 % absorbance. Our investigations reveal that the PAni-SnO composites are potential candidates for EMI shielding applications for both the X and Ku bands.     

双层吸波材料吸波特性研究

依据阻抗匹配原理与电磁波传播规律,设计了具有阻抗渐变结构的双层吸波材料.实验表明,匹配层对提高吸收率起着重要作用;需精确控制其吸波剂含量,以实现吸波效果.经测试:4#试样厚度为6mm,测试频段为8-18GHz,最大吸收峰值在14.1GHz(R=-28.14dB),R＜-10dB的频宽为6.7GHz;7#试样厚度为5.5mm,最大吸收峰值在9.6GHz(R=-27.48dB),R＜10dB的频宽为8.6GHz,R＜-15dB的频宽为7.6GHz;8#试样厚度为6mm,最大吸收峰值在16.8GHz(R=-24.24dB),R＜-10dB的频宽为8.6Hz.该结果具有一定工程应用价值.     

Development of high performance EMI shielding material from EVA,NBR, and their blends: effect of carbon black structure

The conductive composites were prepared using two different types of conductive black (Conductex and Printex XE2) filled in matrices like EVA and NBR and their different blends. The electromagnetic interference shielding effectiveness (EMI SE) of all composites was measured in the X band frequency range 8–12 GHz. Both conductivity and EMI SE increase with filler loading. However, Printex black shows higher conductivity and better EMI SE at the same loading compared to Conductex black, and this can be used as a material having high EMI shielding effectiveness value. The conductivity of different blends with same filler loading generally found to increase slightly with the increase in NBR concentration. However, EMI SE has some dependency on blend composition. EMI SE increases linearly with thickness of the sample. EMI SE versus conductivity yields two master curves for two different fillers. EMI SE depends on formation of closed packed conductive network in insulating matrix, and Printex black is better than Conductex black in this respect. Some of the composites show appreciably high EMI SE (>45 dB).     

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.     

Novel optimization method of single square FSS impinged and cascaded radar absorbing composites

It is well known that radar absorbing potentiality of existing magneto-dielectric composites can be significantly enhanced by the application of frequency selective surface (FSS) and cascaded electromagnetic (EM) structures. But the optimization of such complex EM structures and validation of the adopted optimization strategy is still a very challenging task for the researchers. Therefore, in this study, an effective effort has been made for the optimization and the corresponding validation for Single Square FSS (SS-FSS) impinged and cascaded radar wave absorbers using advanced computational EM software’s like FEldberechnung fur Korper mit beliebiger Oberflache – a German acronym (FEKO) and high frequency structure simulator (HFSS). In addition, a critical analysis of dielectric constant (ε′) has been carried out to select the best combination of composites for the development of efficient radar wave absorbers. A comparison between optimized and simulated results have been carried out to examine the effect of advanced EM approaches over reflection loss (RL) characteristics of composite radar absorbing materials (CRAMs). A rapid change in radar absorption properties of composites has been observed after the application of SSFSS and cascading. A SS-FSS impinged composite has been found to provide a wide absorption bandwidth of 3.6 GHz at X-band. A cascaded absorber having layer thickness 1.8 mm provides a peak RL of ?42.6 dB at 10.6 GHz with an absorption bandwidth of 2.5 GHz. The strong agreement between mathematical model, HFSS and FEKO results clearly reflects the efficiency of adopted approach for distinct practical EM applications.     

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.     

Electrical conductivity and shielding effectiveness of poly(trimethylene terephthalate)/multiwalled carbon nanotube composites

Poly(trimethylene terephthalate) (PTT)/multiwalled carbon nanotube (MWCNT) composites have been fabricated to evaluate the potential of PTT composites as electromagnetic interference (EMI) shielding material. The room temperature electrical conductivity, complex permittivity, and shielding effectiveness (SE) of PTT/MWCNT composites were studied in the frequency range of 8.2–12.4 GHz (X-band). The dc conductivity (σ) of composites increased with increasing MWCNT loading and a typical percolation behavior was observed at 0.48 vol% MWCNT loading. The highest EMI SE of PTT/MWCNT composites was ~23 decibel (dB) at 4.76 vol% MWCNT loading which suggest that these composites can be used as light weight EMI shielding materials. The correlation among the SE, complex permittivity, and electrical conductivity was also studied. The EMI shielding mechanism of PTT/MWCNT composites was studied by resolving the total EMI SE into absorption and reflection loss.