Microwave absorbing properties of activated carbon-fiber felt dipole array/epoxy resin composites

Microwave absorbing properties of the composites containing activated carbon-fiber felt dipole arrays (ACFFDAs) were investigated. The results show that the absorbing performances of the composites containing ACFFDAs are affected greatly by the dimension parameters of the arrays, the resistance connecting the two arms and the materials of the dipoles. The absorption of the composites containing ACFFDAs presents anisotropy. When the dipoles are parallel to the incident electric field, the composites show better absorbing effect. The absorbing properties rise at first and then fall with increasing the resistance connecting arms or the space between dipoles. In this work, when the dipoles are parallel to the incident electric field, the composite obtains a reflection loss below −10 dB over 12.2 GHz and the minimum value reaches −32 dB. The bandwidth below −10 dB increases with increasing the length of the arms when the dipoles are parallel to the incident electric field. The bandwidth below −10 dB is 13.1 GHz when the length of the arms is 85 mm. Compared with copper plate, the dipole arrays whose arms are made of activated carbon-fiber felt exhibit better absorption properties.     

Dielectric constant, magnetic permeability and microwave absorption studies of hot-pressed Ba-CoTi hexaferrite composites in X-band

The microwave absorption, complex permittivity and complex permeability studies of hot-pressed hexaferrite composites prepared with Ba(CoTi)_xFe_12-2xO₁₉ (x = 0.0, 0.2, 0.4, 0.8, and 1.0) were made in the frequency range from 8.0 to 12.4 GHz. The hexaferrite composites with x > 0.0 exhibit significant dispersion in the complex permittivity (ε_r′-jε_r″). However the dispersion in complex permeability (μ_r′-jμ_r″) is not significant and is attributed to the shielding effect of polymer matrix over the ferrite crystallites. The reflection loss has been studied as function of frequency, composition and thickness of absorber. A comparison of reflection loss of hot-pressed ferrite composites with that of normal sintered ferrite composites was made and analyzed. A minimum reflection loss of—24.0 dB is obtained at 9.9 GHz for 2.8 mm thick sample of BaCo_0.4Ti_0.4Fe_11.2O₁₉ hot-pressed hexaferrite composite.     

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.     

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.     

Investigation of the microwave-absorbing properties of Fe-filled carbon nanotubes

Carbon nanotubes encapsulated Fe nanowire composites were synthesized via pyrolyzing of ferrocene. The reflection loss (R.L.), matching frequency (f_m) and matching thickness (d_m) were calculated using the theory of the absorbing wall. The electromagnetic properties and microwave-absorbing characteristic effects by the encapsulation of metal Fe were investigated in a frequency range of 8-18 GHz. With matching thickness of 3.5 mm, the maximum reflection loss is about − 22.73 dB for the absorber. The bandwidth corresponding to the reflection loss below − 10 dB is more than 4.22 GHz. With increasing thickness, the peak value of the reflection loss shifts to a lower frequency.     

Electromagnetic characteristic and microwave absorbing performance of different carbon-based hydrogenated acrylonitrile–butadiene rubber composites

Hydrogenated acrylonitrile–butadiene rubber (HNBR) was mixed with carbon fiber (CF), conductive carbon black (CCB) and multi-walled carbon nanotubes (MWCNT) to prepare microwave absorbing composites, their complex permittivity was measured in microwave frequencies (2–18 GHz), and their electromagnetic characteristics and microwave absorbing performance were studied. The real part and imaginary part of permittivity of the composites increased with increasing carbon filler loading, showing dependency on filler type. The microwave reflection loss of the composites also depended on the loading and type of fillers. The matching thickness of the absorber layer decreased with increasing permittivity, while the matching frequency decreased with increasing layer thickness. The minimum reflection loss was −49.3 dB for HNBR/MWCNT (100/10) composite, while −13.1 dB for HNBR/CCB (100/15) composite and −7.1 dB for HNBR/CF (100/30) composite. The efficient microwave absorption of HNBR/MWCNT composites is accounted from high conduction loss and dielectric relaxation of MWCNT, and strong interface scattering.     

Electromagnetic wave absorption properties of cement-based composites filled with porous materials

To solve more and more serious electromagnetic radiations, cement-based composites were prepared by introducing porous materials into cement. The reflection losses were studied using arched testing method in the frequency range of 1.7–18 GHz. The results showed that the absorption properties were improved obviously. The mechanisms of wave attenuation of the composites were discussed, which indicated that the scattering and multi-scattering in porous beads played an important role. The filling ratio of porous beads, the bead geometries as well as the conformation of cement all had noticeably influence on the absorption properties. The lowest reflection loss of −22 dB was obtained at 5.6 GHz when the specimen was filled with 50 vol.% expanded polystyrene, and the effective absorption bandwidth (less than −10 dB) reached 10.6 GHz when the specimen was filled with 50 vol.% expanded polystyrene and 2 vol.% carbon black.     

Synthesis and microwave absorption enhancement of Fe-doped NiO@SiO2@graphene nanocomposites

Fe-doped NiO@SiO₂@graphene nanocomposites have been successfully fabricated for the first time, in which Fe-doped NiO nanoparticles are about 3 nm in diameter. In order to measure their electromagnetic properties, Fe-doped NiO@SiO₂@graphene (25 wt%) wax composites were then prepared. The experimental results show that Fe-doped NiO@SiO₂@graphene nanocomposites exhibit significantly enhanced microwave absorption performance in terms of both the maximum reflection loss value and the absorption bandwidth in comparison with NiO@SiO₂@graphene. The maximum reflection loss of Fe-doped NiO@SiO₂@graphene nanocomposites can reach −51.2 dB at 8.6 GHz with a thickness of 4 mm, and the absorption bandwidth with the reflection loss below −10 dB is 4 GHz (from 7 to 11 GHz). Therefore, this kind of nanocomposites may have the potential as high-efficient absorbers for microwave absorption applications.     

Synthesis,characterization and evaluation of reflectivity of nanosized CaTiO<Subscript>3</Subscript>/epoxy resin composites in microwave bands

Microwave absorbing materials play a major role in electromagnetic interference and compatibility measurements in anechoic chambers. Nanocrystalline calcium titanate (CT) was synthesized by hydrothermal method and further composites of CT/epoxy resin were fabricated as thin solid slabs of four different weight ratios. The composite material was analyzed by X-ray diffraction (XRD) and transmission electron microscopy (TEM), which reveals that CT was observed to be in the monoclinic phase with an average crystallite size of 24 nm. The reflectivity measurement of the composite materials was carried out by the transmission/reflection method using a vector network analyzer R&S: ZVA40, in the X- and Ku-bands. The effective permittivity and permeability of the samples was also computed with the help of measured transmission and reflection coefficients. The results show that CT with equal weight of epoxy resin provides −30 dB at 8.5 GHz in the X-band and −19.5 dB at 18.0 GHz in the Ku-band. Reflectivity was found to be better than −10 dB for 2.2 GHz and 1.9 GHz for X-band and Ku-band, respectively and encourages use of it as potential microwave absorber material.     

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.     

Microwave-absorbing properties of Co-filled carbon nanotubes

Co-filled carbon nanotubes composites were synthesized via using a simple and efficient wet chemistry solution method. The samples were characterized by transmission electron microscopy. Microwave-absorbing properties were investigated by measuring complex permittivity and complex permeability of the absorber in a frequency range of 2-18 GHz. The reflection loss (R.L.), matching frequency (f_m) and matching thickness (d_m) were calculated using the theory of the absorbing wall. The electromagnetic properties and microwave-absorbing characteristics effects of the modified carbon nanotubes by the encapsulation of metal Co were investigated. A matching thickness is found corresponding to a matching frequency. The maximum reflection loss is about −39.32 dB and the bandwidth corresponding to the reflection loss below −10 dB is 3.47 GHz. With increasing thickness, the maximum reflection loss shifts to lower frequency.     

Self-reducing coal-derived carbon/Ni3Fe magnetic composites with frequency-dependent microwave absorption performance

Coal-derived carbon/Ni₃Fe magnetic composites with frequency dependent microwave absorption performance were prepared at low temperatures (750–850 °C) using coal as the carbon source. The Ni₃Fe alloy was successfully formed due to the carbothermal reaction and reducing gas. SEM images indicate the surface becomes rougher and the number of interlayer of the composites increases with increasing reaction temperature. Consistently, high degree of graphitization of the coal-derived carbon was confirmed by using Raman spectroscopy. Specifically, coal-derived carbon/Ni₃Fe magnetic composites exhibit frequency-dependent microwave absorption characteristics at 2–18 GHz, that is, as the reaction temperature rises from 750 °C to 850 °C, the minimum reflection loss gradually shifts to low frequencies. Among them, CC/Ni₃Fe_(8 0 0)-0.4 exhibits a minimum reflection loss of ?60.76 dB at 16.64 GHz, while the thickness is only 1.28 mm. Such a clean strategy provides experience for the environmental application of coal and microwave absorption. Meanwhile, a lightweight, stable and efficient microwave absorber has been developed.     

Microwave transmission,reflection and dielectric properties of conducting and semiconducting polypyrrole films and powders

Microwave transmission, reflection and some dielectric properties of the conducting polymer, polypyrrole, are presented. Methods are discussed for determining microwave transmission and reflection of electrochemically synthesized and doped polypyrrole films with conductivities ranging from 0.1–5000 S m⁻¹. Polypyrrole films were placed between waveguides and irradiated with microwaves centred at frequencies 2.45 and 10 GHz with 0.1 GHz span. The results indicate that the conductivity of doped polypyrrole films has a significant effect on both transmission and reflection. Microwave opacity of polypyrrole varied with the synthesis conditions of the polymer. Samples with low conductivity exhibited high transmission whereas low transmission readings were observed with highly conducting films. Dielectric properties are also presented for frequencies from 100 to 10⁶ Hz in a temperature range of 90–280 K and at microwave frequencies of 2.45 and 10 GHz. These measurements indicate that the real and imaginary parts of the dielectric constant increase in magnitude with increasing doping level.     

Study on microwave absorption properties of metal-containing foam glass

Materials with the properties of electromagnetic (EM) wave absorption are attractive topics. In this work, we report that EM wave absorption composites, consisting of foam glass, zinc and zinc oxide, were prepared by sintering mixture of foam glass raw material and zinc powder. Microwave reflection loss of composite was calculated based on the permittivity in the range of 8.2-12.4 GHz. The results show that zinc-containing foam glass absorbs efficiently microwaves. The sample with zinc filler to foam glass mass ratio of 3/18 had a reflection loss below −10 dB in the range of 11.3-12.4 GHz, and the minimum reflectivity was −15.6 dB at both 12.0 and 12.4 GHz. Microwave absorption performances of specimens can be controlled by changing the ratio between zinc powder and foam glass mass. The detailed mechanism of the control was investigated through X-ray diffraction (XRD) analysis and scanning electrical microscopy (SEM) observations.     

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.     

Synthesis of nanosized barium titanate/epoxy resin composites and measurement of microwave absorption

Barium titanate/epoxy resin composites have been synthesized and tested for microwave absorption/transmission. Nanocrystalline barium titanate (BaTiO₃ or BT) was synthesized by the hydrothermal method and the composites of BT/epoxy resin were fabricated as thin solid slabs of four different weight ratios. BT was obtained in the cubic phase with an average particle size of 21 nm, deduced from the X-ray diffraction data. The reflection loss (RL) and transmission loss (TL) of the composite materials were measured by the reflection/transmission method using a vector network analyser R&S: ZVA40, in the frequency range 8·0–18·5 GHz (X and Ku-bands). The RL was found to be better than −10 dB over wide frequency bands. The higher RL for lower concentration of BT could be due to increase in impedance matching effects. Low TL values indicate that the absorption by BT is quite low. This could be due to formation of BT in the cubic paraelectric phase.     

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.     

Microwave absorption characteristics of CH<Subscript>3</Subscript>NH<Subscript>3</Subscript>PbI<Subscript>3</Subscript> perovskite/carbon nanotube composites

The CH₃NH₃PbI₃ (MAPbI₃) and CH₃NH₃PbI₃/carbon nanotube (MC) composite have been successfully synthesized by a facile in situ solution method, which are investigated as the microwave absorption materials. For the MAPbI₃ particles, the minimum reflection loss is only ?4.9 dB around 16.4 GHz due to the poor relative complex permittivity. Then, the relative complex permittivity of MC composites could be adjusted by changing the mass fraction of CNTs in composite, which is a vital role for the dielectric loss. The reflection loss of MC-5 composite (MAPbI₃/CNT, 5:1 wt%) can be improved to ?35.7 dB with thickness of 1.3 mm at 13.1 GHz. When the thickness is <3.0 mm, the microwave absorption bandwidth of MC-5 is 11.8 GHz (5.0–16.8 GHz) under the reflection loss lower than ?20 dB. The quarter-wavelength (λ/4) matching model is used to discuss the microwave absorption mechanism of MC composites. These results indicate that MC-5 composite could be used as the microwave absorption materials with strong reflection loss, lightweight and broad bandwidth.     

Controllable synthesis and morphology-dependent microwave absorption properties of iron nanocrystals

Iron nanospheres, nanoflakes and nanofibers were synthesized via a simple pyrolysis method. When the pyrolysis temperature increased from 523 to 623 K and the flow rate of Ar carrier gas maintained at 100 sccm, the as-prepared iron nanocrystals showed a morphology evolution from isotropic nanospheres to isotropic nanofibers. The phase structures and morphologies of the composite were characterized by X-ray diffraction and scanning electron microscopy. The complex permittivity (ε′ − jε″) and permeability (μ′ − jμ″) of these composites were measured using the transmission/refection coaxial line method in the frequency range of 1–18 GHz by a vector network analyzer. The iron nanofibers exhibited superior microwave absorbing properties compared to iron nanoparticles and nanoflakes. The optimal reflection loss (RL) reached −17.8 dB at 9.9 GHz with a layer thickness of 2.0 mm. The RL below −10 dB can be obtained in the frequency range of 7.3–11.7 GHz. Considering the low cost and high efficiency; the iron nanofibers are favorable for application as microwave absorber.     

单壁碳纳米管-CoFe2O4双层复合材料的微波吸收特性

通过直流电弧放电法制备了高结晶性单壁碳纳米管（SWCNTs）,采用溶胶凝胶自燃法制备CoFe₂O₄,并将两种材料复合制成SWCNTs-CoFe₂O₄双层吸波材料。使用Raman光谱、XRD、SEM、TEM和矢量网络分析仪对SWCNTs和CoFe₂O₄的形貌、结构和电磁性能进行了表征,并利用传输线理论分析了SWCNTs-CoFe₂O₄双层吸波材料在2~18 GHz频带内的微波吸收性能。结果表明,相对于单一材料,SWCNTs-CoFe₂O₄双层复合材料的吸波性能得到了极大提高。当CoFe₂O₄作为匹配层、SWCNTs作为吸收层时,通过调节匹配层和吸收层的厚度,SWCNTs-CoFe₂O₄双层复合材料的最强反射损耗可以达到-61.13 dB,低于-10 dB的吸收带宽达到7 GHz （8~15 GHz）。因此,SWCNTs-CoFe₂O₄双层复合材料是一种新型的有应用前景的高吸收宽频带吸波材料。