Single and double-layered Tri-band Microwave absorbing materials

Absorbers at microwave frequencies with multiple frequency-band response are particularly important for use in military for stealth technology. Specially, ferrite based absorbing materials are significant for electromagnetic shielding and signal attenuation. The enhancement of reflection loss of ferrites along with carbonaceous materials are even more beneficial. Recently double-layer absorbers have extensively studied to meet the requirements of advanced absorbing materials in multiple frequency-band response. It still remains a challenge how to determine the type and thickness to couple the impedance-matching-layer to the absorption-layers for a double-layer absorber. We applied hydrothermal method to prepare Fe₃O₄ nanoparticle and combine them with either graphene oxide (GO) or reduced graphene oxide (rGO) to prepare a composite of specific quality to obtain Fe₃O₄@GO and Fe₃O₄@rGO nanocomposite. We studied microwave attenuation capabilities of single and double-layer absorbers containing these two materials. We have demonstrated that with a thin impedance matching layer as a first layer and an absorbing layer behind this layer for the double-layered absorber has much higher reflection loss (RL) than a single-layer. The Fe₃O₄@rGO composite as a single-layer absorber shows the best microwave absorption performance with RL close to −30 dB in all three microwave bands (X, Ku and K bands). The use of a double-layer structure as Fe₃O₄@GO as impedance matching layer and Fe₃O₄@rGO as absorbing layer exhibits the best absorption of −50 dB. This is much larger than the single-layered absorbers at all three frequency-bands. Such a performance is superior to many reported ferrite-based carbonaceous composites. Therefore, a double-layer absorber is best suited to coat the whole body of the aircraft or missiles to evade satellite detection, a preparation towards new-generation weapons for future warfare. Before performing the absorption studies we have characterized the ferrites, GO and rGO materials with various microstructural and magnetic characterizations.     

Electromagnetic wave absorbing properties of In–Sn/rGO composites supported by impurity defect

With the fast development of E-communication technology, effective electromagnetic wave absorbing materials are highly needed to address the growing electromagnetic pollution. Herein, Indium doped tin microsphere/reduced graphene oxide (In–Sn/rGO) composites with rich impurity defects were synthesized via the sol-gel and hydrothermal method. The excellent microwave absorption of In–Sn/rGO composites can be attributed to the modifications of electronics status and Fermi energy level after In doping. This can significantly increase the carrier mobility between In–Sn microspheres and rGO sheets to strike a superior interfacial polarization loss. As a result, the maximum absorptivity can reach ?51.16 dB at 8.73 GHz (thickness: 3.5 mm) with a lower filler loading of 10 wt%. Meanwhile, the synthesized In–Sn/rGO composites also exhibit an ultra-wide absorbing frequency range of 13.84 GHz (within the X band, Ku band, and most of the C band). This research provides a new idea for the synthesis of effective microwave absorbing material by introducing impurity defects.     

Synthesis and strengthened microwave absorption properties of three-dimensional porous Fe3O4/graphene composite foam

The three-dimensional porous Fe₃O₄/graphene composite foam as a new kind of absorbing composite with electrical loss and magnetic loss was successfully synthesized by a facile method. Fe₃O₄ was evenly attached on structure of graphene sheets which overlapped with each other to form three-dimensional porous graphene foam. The results revealed that when the mass ratio of graphene oxide (GO) and Fe₃O₄ was 1:1, the Fe₃O₄/graphene composite foam possessed the best absorption properties: the minimum reflection loss was up to ??45.08?dB when the thickness was 2.5?mm and the bandwidth below ??10?dB was 6.7?GHz when the content of the composite foam absorbents was just 8%. The micron-sized three-dimensional porous structure provided more propagation paths, enhancing the energy conversion of incident electromagnetic waves. The addition of Fe₃O₄ contributed to improving the impedance matching performance and magnetic loss. The three-dimensional porous Fe₃O₄/graphene composite foam was a kind of high-efficiency wave absorber, providing a new idea for the development of microwave absorbing materials.     

Polyurethane Composite Foams in High-Performance Applications: A Review

Polyurethane foam is a polymeric material having cellular structure. Multifunctional polyurethane foams reinforced with nanofiller have combined enhanced specific properties with density reduction. This article primarily considers important aspects of various foam processing techniques. Numerous nanofillers such as graphite, graphene, graphene oxide, carbon black, carbon nanotube, nanoclay, and inorganic nanoparticle have been reinforced in polyurethane foam. Particular attention is given to various categories of polymer/carbon nanofiller and polymer/inorganic nanofiller composite foams. Applications of polyurethane composite foams have been focused with relevance to aerospace and automotive industry, radar absorbing and electromagnetic interference shielding, oil absorbants, sensors, fire proof, shape memory, and biomedical materials.     

Development of Low Density,Heat Resistant and Broadband Microwave Absorbing Materials (MAMs) for Stealth Applications

The development of low density and broadband microwave absorbers are the need of the hour to cater for the needs of all military platforms for stealth technology. The low density and broadband properties can be inculcated in microwave absorbers using dielectric lossy materials (e.g. carbon fibres, carbon nanotubes, carbon black, fullerene, graphite, graphene and silicon carbide fibre). Therefore, we designed low density and heat resistant microwave absorbing materials (MAMs) using a novel approach of ceramic fibre board manufacturing technology. The microwave absorbing composites were prepared with varying percentage of milled carbon fibres, discontinuous aluminosilicate fibres and silicone resin as the matrix. The physico-mechanical properties of microwave absorbing composites were determined. Reflection loss of microwave absorbing composites was measured in the frequency range 2–18 GHz by unique single horn interferometry technique. The electromagnetic properties were measured in X-band using free space measurement system. Based on these properties the effect of thickness on the microwave absorbing properties in X-band was simulated The effect of weight % variation of milled carbon fibres on the microwave absorbing properties of composites have been studied in the frequency range 2–18 GHz.     

Electromagnetic loss mechanisms in antimony doped tin oxide and reduced graphene oxide multilayer films

In this paper, the antimony doped tin oxide (ATO) and reduced graphene oxide (rGO) were prepared by coprecipitation method and modified Hummers’ method, respectively. Both were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and vector network analyzer (VNA). The as-prepared rGO showed typical sheets-like structure and the as-prepared ATO showed typical nano-particle shape. Then, the ATO and rGO multilayer films were designed and the microwave absorption performances were numerically evaluated using finite element methods. The results showed that synergetic effects of interfacial polarization resonance and 1/4λ elimination were stimulated in 10-layerd 1.8 mm multilayer films to give a minimal reflection loss of −45.2 dB @ 16.1 GHz, and −10 dB bandwidth of 5.4 GHz, which was enhanced more than −30 dB in ATO or rGO monophase absorbers with the same thickness. This work provides a novel technical route to realize high-performance microwave absorbers in terms of simultaneously stronger absorption, broader absorption bandwidth and smaller thickness to facilitate higher flexibility and stability for practical applications.     

Incorporate boron and nitrogen into graphene to make BCN hybrid nanosheets with enhanced microwave absorbing properties

This work demonstrates the conversion of graphene oxide into BCN hybrid nanosheets by reaction with boric acid and urea at 900 °C, during which boron and nitrogen atoms are incorporated into the graphene atomic sheets. X-ray diffraction pattern and X-ray photoelectron spectroscopy reveal the existence of h-BN. High-resolution electron microscopy and Raman spectrum indicate the presence of graphene-like layers with h-BN nanodomains. The content of h-BN in the BCN nanosheets can also be tuned by further heat-treatment in an ammonia environment, which in turn affects the band gap of these nanosheets. The electromagnetic parameters suggest that these samples can be used as good microwave absorbing materials at G band (5.6–8.2 GHz) and X band (8.2–12.4 GHz). This study provides a simple route to BCN hybrid nanosheets with tunable band gap and adjustable conductivity for microwave absorbing applications.     

Substituted nickel ferrite coated MWCNT/PVDF based epoxy nanocomposite for microwave absorption

Metal-substituted spinel ferrite-based microwave absorbing materials (MAMs) are attracting significant attention due to their varied soft magnetic behaviour, and ease of synthesis. This work established the efficiency of Cd²⁺ substituted nickel ferrite coated MWCNT (with varied MWCNT loading) for X-band microwave absorption, in the presence of PVDF. The substituted ferrite was prepared with a facile solvothermal process. X-ray diffraction and vibrating sample magnetometer analysis confirmed the spinel structure of substituted ferrites and tuned magnetic behaviour of Cd²⁺ substituted ferrite structure, respectively. FESEM revealed a uniform coating of substituted ferrite on MWCNT and XPS confirmed Cd²⁺ substitution. Hybrid nanocomposites of ferrite coated MWCNT with PVDF, in the epoxy matrix, revealed superior microwave absorption for NiF-150_10 PV and CdNiF-150_10 PV with an absorber thickness of 3 mm in the X band. The absorption bandwidth with the RL below ?10 dB reached 3.9 GHz and 3.6 GHz for NiF-150_10 PV and CdNiF-150_10 PV, respectively. The microwave absorption mechanism was discussed in detail. The developed NiF-150_10 PV and CdNiF-150_10 PV composites can be used as lightweight, low thickness microwave absorbers in the defense and telecommunication industries.     

Fabrication of hexagonal cerium oxide nanoparticles decorated nitrogen-doped reduced graphene oxide hybrid nanocomposite as high-performance microwave absorbers in the Ku band

With the aim to obtain microwave absorbers simultaneously possessing broad absorption bandwidth, strong absorption intensity and thin matching thickness, nitrogen-doped reduced graphene oxide decorated by cerium oxide particles (NRGO/CeO₂) hybrid nanocomposite was prepared through a hydrothermal and calcination two-step route. Results of micromorphology analysis showed that numerous hexagonal CeO₂ nanoparticles were evenly anchored on the crumpled surfaces of NRGO. Moreover, both nitrogen doping and hybridization with RGO could notably strengthen the microwave absorption capacity of CeO₂. Remarkably, the NRGO/CeO₂ hybrid nanocomposite exhibited the minimum reflection loss of ?57.2 dB at 13.4 GHz (Ku band) under a matching thickness of 1.66 mm and maximum absorption bandwidth of 4.6 GHz (from 13.2 to 17.8 GHz) at an ultrathin thickness of only 1.5 mm. Meanwhile, the hybrid nanocomposites displayed strong absorption intensity (≤-20 dB, 99% absorption) in almost the whole measured thicknesses range. Furthermore, the relationship between absorption intensity and filler loadings was uncovered. The potential microwave absorption mechanisms were further revealed. Therefore, this work opened a novel idea for designing RGO-based hybrid nanocomposites as high-performance microwave absorbers.     

Highly conductive interconnected graphene foam based polymer composite

In this study, the impact of graphene sheet size on the electrical conductivity of interconnected graphene foam polymer composite is thoroughly investigated. Graphene oxide solution is produced from small flake graphite (SFG) (2–15 μm) and large flake graphite (LFG) (>100 μm), respectively. Each solution is used to produce three-dimensional GO foam, which is subsequently heat-treated to produce reduced graphene oxide (RGO) foam. The RGO foams are then infiltrated with poly(dimethylsiloxane) (PDMS) to produce graphene-PDMS (G-PDMS) composites. The in-plane electrical conductivity of the G-PDMS composite (0.4 wt%) from LFG reaches ∼3.2 S/m, which is more than two orders of magnitude greater than that of G-PDMS (1.9 wt%, 1.4 × 10⁻² S/m) from SFG. This value is also four orders of magnitude higher than that of the G-PDMS composite prepared from mechanical mixing of 4 wt% RGO powder made from SFG with PDMS (4.2 × 10⁻⁵ S/m). The though-plane electrical conductivity followed the same trend for SFG and LFG. This reveals that the interconnected graphene foam supplies more efficient paths for electron transfer inside the polymer than conventional graphene powder and the use of large sized graphene sheets can significantly improve the electrical properties of G-PDMS.     

Monolithic Fe2O3/graphene hybrid for highly efficient lithium storage and arsenic removal

We fabricated a monolithic Fe₂O₃/graphene hybrid directly by hydrothermal reaction of ferrous oxalate dihydrate and graphene oxide without using a reducing agent. The reduced graphene oxide formed an interconnected network structure that can be used as a support for homogeneous distribution of active Fe₂O₃ nanoparticles. The graphene network and the pore channels in the hybrid facilitate fast electron transfer and ion transport. This hybrid can be directly used as a free-standing anode for lithium ion batteries, which simplifies the fabrication procedure of electrodes, and also exhibited a high capacity of 1062 mA h g⁻¹ at 100 mA g⁻¹, high rate capability and excellent cyclic stability over 100 cycles. Furthermore, as a self-supported adsorbent, it provides a new idea on loading active materials to the suitable substrate, which can be used as a promising material for water purification due to its easy collection and excellent capability in removing As(V) from water. The results demonstrate the promising applications of bulk reduced assembly of graphene with functional metal oxides, which will be helpful for future development of graphene-based multifunctional materials.     

High electromagnetic interference shielding effectiveness achieved by multiple internal reflection and absorption in polybenzoxazine/graphene foams

Electromagnetic interference (EMI) is an increasingly severe issue in modern life and high-performance EMI shielding materials are in desperate need. To achieve high EMI shielding effectiveness (EMI SE), a series of polybenzoxazine/graphene composites foams are developed using a simple sol–gel method. When the graphene loading increases from 1 to 20 wt%, the density of the composites foams drops from 0.4143 g/cm³ to 0.1654 g/cm³. Meanwhile, an electrically conductive path is formed at around 7 wt% of graphene. Below the percolation threshold, the dielectric constant increases with graphene content and composite foam with 5 wt% graphene shows dielectric constant of 10.8 (1 MHz). At the highest graphene content of 20 wt%, the electric conductivity reaches 0.02 S/cm, 10 orders of magnitude higher than pure polybenzoxazine foam. Benefiting from the high electrical conductivity and lightweight porous structure, the composite foam PF/20G delivers an EMI SE of 85 dB and a specific SE of 513.9 dB·cm³/g. Importantly, the EMI shielding is dominated by absorption attenuation, with PF/20G shows absorption ratio higher than 98% in the range of 8.4–11.0 GHz, which is believed to be caused by multiple internal reflection and absorption inside the conductive foam.     

石墨烯/聚合物纳米复合材料制备与微波吸收性能研究进展

二维片状的石墨烯不仅具有优异的力学、热学和电学性能,而且还具有较好的微波吸收特性。自它被发现以来,一直受到科学界的广泛关注,目前已有学者将其与聚合物复合,制备了石墨烯/聚合物纳米复合材料,这种新型微波吸收材料不仅吸波效果好而且密度小、易加工。目前石墨烯/聚合物纳米复合材料用于微波吸收的报道还比较少,该研究基本处于起步阶段。本文首先概述了石墨烯独特的物理结构和优异的力学、热学、电学性能,然后综述了石墨烯/聚合物纳米复合材料的制备方法,并分析了其微波吸收机理,最后结合国内外研究现状展望了石墨烯/聚合物纳米复合材料制备与微波吸收性能研究的发展方向,指出调控复合材料的微观形貌,对石墨烯进行磁性掺杂,探索石墨烯与聚合物微波吸收的协同效应将成为今后研究的重点和热点。     

Effect of filler loading and thickness parameters on the microwave absorption characteristic of double-layered absorber based on MWCNT/BaTiO3/pitted carbonyl iron composite

In this work, single- and double-layer electromagnetic wave absorbers were prepared by as-prepared MWCNTs/BaTiO₃/pitted carbonyl iron composites. MWCNT/BaTiO₃ (MW/BTO) was prepared via sol-gel method whereas the carbonyl iron particles (CI) were corroded via pitting corrosion method. The structural, microstructural, magnetic and microwave absorption properties of the composites were evaluated via X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), vibrating sample magnetometer (VSM) and vector network analyzer (VNA) methods. CST studio software was employed to simulate the microwave absorption characteristics of double-layer absorbers. Moreover, the effects of changing matching and absorbing layer thickness (3 mm in total) and filler loading (10, 20 and 30 wt%) of the as-prepared composite on the microwave absorption properties were investigated. According to the results, maximum RL value for single layer absorber with 20 wt% filler loading can reach ?11.5 dB at 9.7 GHz with 3 mm thickness and 0.4 GHz bandwidth. In contrast, double-layered absorber using 10 wt% of the composite in the upper layer (as matching layer) and 30 wt% of the composite in lower layer (as absorbing layer) can increase the reflection loss and absorption bandwidth values to ?15.5 dB and 1 GHz respectively. Improving in absorption characteristics can be attributed to coupling interactions, impedance matching and multiple scattering. The main advantages of the prepared double layer absorber than single layer absorber are tuning the intensity and effective absorption bandwidth by adjusting the layer order, thickness and filler loading of each layer which shown good potential for practical application.     

High-Performance and Biobased Polyamide/Functionalized Graphene Oxide Nanocomposites through In Situ Polymerization for Engineering Applications

In this study, biobased polyamide/functionalized graphene oxide (PA-FGO) nanocomposite is developed using sustainable resources. Renewable PA is synthesized via polycondensation of hexamethylenediamine (HMDA) and biobased tetradecanedioic acid. Furthermore, GO is functionalized with HMDA to improve its compatibility with biobased PA and in situ polymerization is employed to obtain homogeneous PA-FGO nanocomposites. Compatibility improvement provides simultaneous increases in the tensile strength, storage modulus, and conductivity of PA by adding only 2 wt% FGO (PA-FGO2). The tensile strength and storage modulus of PA-FGO2 nanocomposite are enhanced dramatically by ≈50% and 30%, respectively, and the electrical conductivity reached 3.80 × 10^–3 S m⁻¹. In addition, rheology testing confirms a shear-thinning trend for all samples as well as a significant enhancement in the storage modulus upon increasing the FGO content due to a rigid network formation and strong polymer-filler interactions. All these improvements strongly support the excellent compatibility and enhanced interfacial interactions between organic–inorganic phases resulting from GO surface functionalization. It is expected that the biobased PA-FGO nanocomposites with remarkable thermomechanical properties developed here can be used to design high-performance structures for demanded engineering applications.     

Ultra-broadband electromagnetic wave absorbent: In-situ generated silica modified conductive carbon fiber aerogels

The research and development of dielectric microwave absorbing materials with broad electromagnetic (EM) response is a significant project in EM wave absorption field. To achieve high-performance absorption and strong interfacial bonding at the same time, thermal-assisted in-situ bonding technology was applied to fabricating the dielectric composite absorbing materials. Thanks to the combination of vacuum filtration and in-situ hydrothermal reaction, the as-prepared binary composite aerogel shows both strong interface contacting and good mechanical stability. In addition, the carbon nanofibers/silica composite aerogel (CSA) exhibits ultra-broad effective bandwidth covering from S to Ku band, originated from the uniform dispersed silica aerogel in conductive carbon fiber network. In details, for CSA1 sample, the maximum reflection loss (RL) values and effective absorption bandwidth reach −46.2 dB (1.8 mm) and 5.2 GHz (1.5 mm). Meanwhile, the optimum RCS reduction values reaches 16.2 dB m² when the detection theta was set as 0°. For CSA2 sample, the effective absorption bandwidth reaches 8.64 GHz at 1.5 mm, and tends to possess lower frequency EM response covering the S-band. This work exhibits a kind of broad-bandwidth aerogel absorbers at low thickness, which shows huge potential in large-scale production of microwave absorbing devices.     

Carbon–ceramic nanofiber embedded with LDHs-derived composites for enhanced electromagnetic wave absorption

Hydrotalcite-like layered double hydroxides (LDHs) exhibit distinct microstructures and atomic compositions, and thus, they are expected to be superior microwave absorbing materials. However, preparation of excellent absorbers from LDHs is still a challenging task. Herein, NiCo-LDHs@SiC/C heterostructure composites were designed and then heat-treated to obtain NiCo@SiC/C nanofibers. Finally, with a synergistic effect of dielectric and magnetic losses, the minimum reflection loss value of the sample could reach −56.3 dB with a thickness of 2.7 mm, and the maximum effective bandwidth absorption reached 6.8 GHz at 2.3 mm. Moreover, the simulated radar cross-section indicates that the NiCo@SiC/C nanofibers well suppress the electromagnetic scattering from metal trench structure. This result further indicates the outstanding microwave absorption properties of the nanofibers. This study contributes to the development of LDH materials in the field of wave-absorbing materials, and is an important guideline for exploring efficient microwave absorbers with fibrous structure.     

Synthesis of nitrogen-doped graphene-based binary composite aerogels for ultralightweight and broadband electromagnetic absorption

The development of ultralightweight and broadband electromagnetic wave (EMW) absorbing materials remains a big challenge. In this work, porous magnesium ferrite microspheres decorated nitrogen-doped reduced graphene oxide (NRGO/MgFe₂O₄) composite aerogels were prepared by a two-step route of solvothermal synthesis and hydrothermal self-assembly. Results of microscopic morphology characterization showed that NRGO/MgFe₂O₄ composite aerogels had a unique hierarchical porous structure. Moreover, the influence of additive amounts of graphene oxide on the electromagnetic parameters and EMW absorption properties of NRGO/MgFe₂O₄ composite aerogels was explored. Remarkably, the attained binary composite aerogel with the content of NRGO of 70.21 wt% exhibited the best EMW absorption performance. The minimum reflection loss reached up to −55.7 dB, and the corresponding effective absorption bandwidth was as large as 5.36 GHz at a thin matching thickness of 1.98 mm. Furthermore, when the matching thickness was slightly increased to 2.29 mm, the widest effective absorption bandwidth was enlarged to 7.1 GHz, covering the entire Ku-band. The magnetodielectric synergy and unique hierarchical porous structure in NRGO/MgFe₂O₄ composite aerogels not only improved the impedance matching, but also greatly enhanced the EMW absorption capacity. It was believed that the results of this work could be helpful for the preparation of graphene-based magnetic composites as broadband and efficient EMW absorbers.     

Conducting polyaniline composites as microwave absorbers

Conducting polymers are excellent microwave absorbers and they show technological advantage when compared with inorganic electromagnetic absorbing materials, being light weight, easily processable, and the ability of changing the electromagnetic properties with nature and amount of dopants, synthesis conditions, etc. In this paper we report the synthesis, dielectric properties, and expected application of conducting composites based on polyaniline (PAN). Cyclohexanone soluble conducting PAN composites of microwave conductivity 12.5 S/m was synthesized by the in situ polymerization of aniline in the presence of emulsion grade polyvinyl chloride. The dielectric properties of the composites, especially the dielectric loss, conductivity, dielectric heating coefficient, absorption coefficient, and penetration depth, were studied using a HP8510 vector network analyzer. The microwave absorption of the composites were studied at different frequency bands i.e, S, C, and X bands (2–12 GHz). The absorption coefficient was found to be higher than 200 m⁻¹ and it can be used for making microwave absorbers in space applications. POLYM. COMPOS., 28:588–592, 2007. © 2007 Society of Plastics Engineers     

Functionalized three-dimensional graphene networks for high performance supercapacitors

Three-dimensional (3D) thermal reduced graphene network (TRGN) deposition on Ni foam without any conductive agents and polymer binders was successfully synthesized by dipping Ni foam into graphene oxide (GO) suspension and subsequent thermal reduction process. The direct and close contact between thermal reduced graphene and Ni foam is beneficial to the enhanced conductivity of the electrode, as well as the improvement of ion diffusion/transport into the electrode. Additionally, low-temperature reduction of GO possesses a large amount of stable oxygen-containing groups that can provide high pseudocapacitance. As a result, the TRGN electrode delivers a high specific capacitance of 442.8 F g⁻¹ at 2 mV s⁻¹ in 6 mol L⁻¹ KOH. Moreover, symmetric supercapacitor based on TRGN exhibits a maximum energy density of 30.4 Wh kg⁻¹ based on the total mass of the two electrodes in 1 mol L⁻¹ Na₂SO₄ electrolyte, as well as excellent cycling stability with 118% of its initial capacitance after 5000 cycles.