Electromagnetic Matching Research Articles

3D conductive network wrapped CeO2-x Yolk@Shell hybrid microspheres for selective-frequency microwave absorption

Carbon nanotubes (CNTs) based materials have been popular as microwave absorbers owing to their tunable electrical conductivity. Here, a hierarchical CeO2-x yolk@shell microsphere encapsulated by the CeO2-x/CNTs hybrid three-dimensional (3D) conductive network (denoted as CMC) was successfully fabricated by a simple solvothermal method through a cooperative-assembly strategy owing to a synergistic effect of reactants. The well-designed hierarchical structure is beneficial to optimize the electrical conductivity for the dielectric-dominated microwave absorber, resulting in the enhanced electromagnetic impedance matching and energy dissipation ability. Importantly, the primary units made up of conductive CNTs and insulated CeO2-x nanoparticles simultaneously provide large room for resistivity adjustment and abundant of localized polarized sites. Moreover, the hierarchical dielectric structure is similar to a microwave trapping cage in micro-meter size can effectively dissipates microwave energy. Thus, the strong microwave absorption (−52.4 dB at 6.1 GHz) and selective-frequency response (shift from 5.2 GHz to 18 GHz) can be achieved through regulating the composite conductivity only by tailoring the input amount of oxidized CNTs. Considering the promising performance of the CMC yolk@shell@shell microspheres, this work is expected to enlighten the design and synthesis of the carbon-based materials in other related fields.

High performance and lightweight electromagnetic wave absorbers based on TiN/RGO flakes

In this study, titanium nitride (TiN)/reduced graphene oxide (RGO) composites were prepared via a facile hydrothermal and thermal nitridation process. Due to their favorable electromagnetic wave dissipation ability and appropriate electromagnetic (EM) impedance matching, the TiN/RGO flakes embodies distinct advantage as compared to single component RGO nanosheets and TiN nanoparticles. Moreover, the effective electromagnetic absorption frequency range of the TiN/RGO flakes could be designed by controlling the mass ratio of the precursor. When the mass ratio of nanosized titanic acid to graphene oxide is 1.4:1, the reflection loss of the TiN/RGO with only 2.0 wt% absorber loading reaches −42.85 dB at 8.88 GHz with a thickness of 4.0 mm and a broad effective absorption bandwidth (RL < −10 dB) of 6.7 GHz, which compete well with those observed in several RGO-based composites with much higher filler loadings. The combined excellent EM absorption property with simple production procedure and ultralow absorbent content requirement endow the TiN/RGO flakes as a promising lightweight and efficient EM absorption material suitable for harsh application conditions.

Dielectric relaxation and magnetic resonance in microwave absorption performance of CoxFe3-xO4 (0≤x≤1) nanocrystals

Cobalt ferrites (Co x Fe 3-x O 4 , 0 ≤ x ≤ 1) may possess large magnetocrystalline anisotropy and coercivity at certain cobalt/iron (Co/Fe) ratios, while further explorations on their microwave absorption mechanisms are not adequate so far. In this study, a series of Co x Fe 3–x O 4 nanocrystals were synthesized by a developed oxidation-precipitation method, and a combination of dielectric relaxation and magnetic resonance was revealed in electromagnetic studies. Dielectric relaxation peaks were originated from orientation polarization and affected by oxygen vacancy densities. Magnetic resonance peaks were shifted to higher frequency due to the increased magnetocrystalline anisotropy at higher Co/Fe ratios. The excellent microwave absorption performance of as-prepared Co x Fe 3–x O 4 were also obtained, which should be attributed to the electromagnetic matching of dielectric relaxation and magnetic resonance at higher frequency ranges.

Reduced Graphene Oxide Functionalized Strontium Ferrite in Poly(3,4‐ethylenedioxythiophene) Conducting Network: A High‐Performance EMI Shielding Material

AbstractThe advent of nanotechnology leading to high‐frequency device realization has resulted in a rapid increase in radiation pollution. The synthesis of core–shell morphology based poly(3,4‐ethylenedioxythiophene) (PEDOT)/ reduced graphene oxide (RGO) nanocomposites incorporated with SrFe12O19 nanoparticles via in situ emulsion polymerization is a step to control ever‐increasing radiation pollution. For electromagnetic (EM) shielding, impedance matching, and absorption of EM wave being the two key parameters. RGO and PEDOT establish an interconnected conducting network inside the PEDOT matrix and the resulting conductive pathway provides better impedance matching for the incident radiation. The RGO sheets decorated with ferrite nanoparticles strengthen the mechanism of the shielding by absorbing incoming EM radiation. The synergic coupling of the magneto‐dielectric characteristics of nanocomposites result in high electromagnetic shielding effectiveness (SE) of 42.29 dB at 12.4 GHz for 2.5 mm thick sample. The SE is mainly dominated by absorption and also measured as a function of thickness resulting in SE(max) value of 62 dB at a thickness of 4.66 mm. The present set of nanocomposites are found to exhibit attenuation of more than 99.999% and have the potential for commercial application in EM shielding.

Tailoring the shape and size of Fe3O4 nanocrystals by oxidation–precipitation processes for microwave absorption enhancement

Microwave absorbing materials refined into nanoscale are known to have fascinating electromagnetic properties, whereas investigations on their shape and size dependence to microwave absorption performance are insufficient. In this study, single-crystal Fe3O4 with various shapes of nanoblocks, nanowires and nanospheres were prepared by surfactant-assisted oxidation–precipitation processes in oxygen-free environment without autoclaves. In microstructural characterizations, a broad size distribution of 30–200 nm for Fe3O4 nanoblocks and a uniform size around 50 nm for Fe3O4 nanospheres were obtained respectively, while Fe3O4 nanowires exhibited non-uniform length-to-diameter ratios. In electromagnetic analysis, Fe3O4 nanoblocks have higher saturation magnetization and increased coercivity in contrast to Fe3O4 nanowires and nanospheres. Both permeability and permittivity of Fe3O4 nanospheres are limited, while an increased attenuation constant is obtained at higher frequency due to the effective electromagnetic matching. As the absorbent thickness reaches 3.5 mm, the reflection loss of Fe3O4 nanowires achieves the minimum value of − 29.7 dB around 7.3 GHz, while the effective absorbing bandwidth (reflection loss ≤ − 10 dB) of Fe3O4 nanoblocks covers 5.6 GHz, demonstrating their potential applications in electromagnetic devices.

The rambutan-like C@NiCo2O4 composites for enhanced microwave absorption performance

To obtain outstanding electromagnetic microwave absorption (EMWA) properties, the rambutan-like dielectric–magnetic C@NiCo2O4 material was successfully prepared by a simple hydrothermal method, followed by a carbonization process. Benefiting from the unique rambutan-like structure, the dielectric–magnetic C@NiCo2O4 composites showed excellent microwave attenuation ability: minimum reflection loss (RLmin) value of − 39.0 dB at 17.4 GHz and wide effective absorption bandwidth (EAB, reflection loss exceeding − 10 dB) of 4.16 GHz (> 13.84 GHz) with a matching thickness of only 1.5 mm, which were much better than those of pure C and NiCo2O4. The superior properties might be due to multiple synergistic effects: magnetic loss (NiCo2O4), dielectric loss (C, NiCo2O4), the multi-reflections, scattering and interface relaxation resulting from mesoporous rambutan-like structures, and the dipole polarization to get good electromagnetic matching and high attenuation efficiency.

Comparison in dielectric and microwave absorption properties of SiC coated carbon fibers with PyC and BN interphases

SiC coated carbon fibers with two different types of interphases, pyrolytic carbon and boron nitride (Cf-PyCi-SiC and Cf-BNi-SiC), were respectively prepared and their dielectric and microwave absorption properties were investigated. Results showed that both real (ε′) and imaginary part (ε″) of permittivity of these two SiC coated carbon fibers decrease significantly, in comparison to carbon fibers. The decreased permittivity is mainly due to the large decrease in electrical conductivity. Furthermore, electromagnetic impedance match is improved with the decreasing permittivity, leading to much better microwave absorption properties. Compared to PyC, BN with higher electrical resistivity and lower permittivity endues better impedance match and stronger microwave attenuation for Cf-BNi-SiC. Both Cf-PyCi-SiC and Cf-BNi-SiC with superior microwave attenuation performance exhibit promising prospects as microwave absorbing materials.

Electromagnetic matching and microwave absorption abilities of Ti3SiC2 encapsulated with Ni0.5Zn0.5Fe2O4 shell

Abstract Ti3SiC2 particles encapsulated by Ni0.5Zn0.5Fe2O4 shell were in-situ synthesized via a facile sol-gel method. Effects of Ni0.5Zn0.5Fe2O4 shell on electromagnetic properties of Ti3SiC2 were investigated in 2–18 GHz. Moderated complex permittivity and desirable higher complex permeability presented for the Ni0.5Zn0.5Fe2O4/Ti3SiC2 composite, resulting in strong microwave attenuation ability and good impedance matching property. The measured effective absorbing bandwidth (RL

The synergetic electromagnetic properties and enhanced microwave absorption of BiFeO3/BaFe7(MnTi)2.5O19 composite

The effective microwave absorption materials could contribute to alleviating the electromagnetic wave pollution. However, conventional microwave absorption materials usually suffer from insufficient absorption intensity and the narrow effective absorption bandwidth. Herein, BiFeO3/BaFe7(MnTi)2.5O19 composites are proposed to address these issues through offering synergetic electromagnetic properties and proper electromagnetic properties. BiFeO3 combined with BaFe7(MnTi)2.5O19 exhibits dielectric multiple relaxation behaviors, strong ferromagnetic resonance and electromagnetic matching, ensuring increased multi-band microwave absorption. Accordingly, the minimum reflection loss (RL) of the composite with volume ratio of 1.5:1 reaches − 48 dB, and the bandwidth less than − 10 dB covers multi-frequencies at C, X and Ku band. These results suggest that BiFeO3/BaFe7(MnTi)2.5O19 composite could be a promising microwave absorption material in imaging, healthcare, information safety and military fields.

Synthesis and enhanced microwave absorption properties of urchin-like polyaniline/Ni0.4Zn0.4Co0.2Fe2O4 composites

A novel polyaniline/Ni0.4Zn0.4Co0.2Fe2O4 (PANI/NZCF) composite was synthesized by in situ polymerization of aniline in the presence of NZCF. The studies of structural, morphological and surface chemical bonding states were performed by X-ray diffraction, scanning electron microscopy and Fourier transform-infrared spectrometry, respectively. It was found that greater reflection loss and wider absorption bandwidths were possible by adjusting the mass ratio of NZCF to PANI on account of electromagnetic loss and impedance matching. The highest reflection loss (RL) was − 40 dB at the frequency 15.8 GHz for the PANI/NZCF composite when the effective absorption frequency at which RL < − 10 dB was in the range of 13.5–18 GHz with an absorber thickness of 1.7 mm at the mass ratio of 2:1. These effects are due to the urchin-like structure when compared to traditional encapsulation structures. Therefore, the above findings show that the PANI/NZCF composite with negligible thickness and strong absorption properties has great potential in the application of electromagnetic shielding and microwave absorbing.

The Effect of Core–Shell Structure on Microwave Absorption Properties of Graphite-Coated Magnetic Nanocapsules

Interface polarization is one important factor in producing a good electromagnetic impedance match and enhancing the microwave-absorption properties of core/shell nanocomposites. However, the effect of the core–shell nanocapsules having insulating shells, and the polarization effect at interfaces of graphite-coated magnetic metallic nanoparticles, on the microwave absorption properties, are not clear. In this work, the microwave-absorption properties of magnetic nanocapsules with α-Fe/γ-Fe(C)/Fe3C as cores and graphite as shells have been investigated in the frequency range 2–18 GHz. Partial magnetic cores were removed by a ∼ 19 wt.% HCl solution from the as-prepared nanocapsules (A-nanocapsules), but the carbon shells were kept constant. Transmission electron microscopy confirms that hollow carbon nanocages are almost the same as the as-prepared shells, except for slight change in shape due to removing the cores. As a result, the complex permeability drops slightly, while the complex permittivity has an obvious increase, which can be ascribed to percolation effects due to the hollow carbon nanocages, rather than the interface polarization originating from the core/shell interfaces and interfaces between the core components of α-Fe, γ-Fe(C) and Fe3C nanocrystals. The optimal reflection loss (RL) value of the A-nanocapsules reaches − 27.6 dB at 16.2 GHz for a thickness of 2 mm. This study clarifies that the interface polarization effect in the A-nanocapsules is negligible in enhancing the complex permittivity and the synergetic effect of the magnetic loss of cores, and the dielectric loss of graphite shells is the dominant mechanism in attenuating microwaves.

In-situ growth of Fe/Fe3O4/C hierarchical architectures with wide-band electromagnetic wave absorption

Carbon materials have aroused extensive interests for their remarkable electrical properties as electromagnetic wave (EMW) absorption. However, the synthesis of the wide-band electromagnetic wave absorbent is an insuperable challenge for attaining effective refection loss and electromagnetic matching. Herein, a facile method for large-scale synthesis of monodispersed Fe/Fe3O4/C composite microspheres is proposed. The carbon microspheres (1–2 µm in diameter) imbedded with nanosized Fe/Fe3O4 particles (10–20 nm) are uniformly produced by polymerization and carbothermic reduction processes. The products exhibit the minimum refection loss −45.5 dB at 9.4 GHz and a broad bandwidth of 4.1 GHz below −10 dB from 7.8 GHz to 11.9 GHz with a thickness of 3.0 mm. The absorption mechanism indicates a unique deviated Debye dipolar relaxation effect in the absorbing bandwidth.

Tunable and weakly negative permittivity at radio frequency range based on titanium nitride/polyethylene terephthalate composites

Metacomposites have been induced widespread concern in the realization of negative permittivity. In this paper, composites with titanium nitride (TiN) particles homogeneously dispersed in polyethylene terephthalate (PET) resin were prepared by high energy ball-milling and adhesive hot-pressing method. The influences of TiN networks in composites on the electric and dielectric properties were investigated in detail. With the formation of conductive TiN networks, the conductance mechanism changed from hopping conduction to metal-like conduction. The tunable and weakly negative permittivity in the radio frequency range was obtained in TiN/PET composites by adjusting the frequency range and volume fraction (v) of TiN in composites. Negative permittivity behavior raises from the low-frequency plasma oscillation of free electrons in TiN networks, which could be analyzed by the Drude model. The impedance of TiN/PET composites were investigated by the equivalent circuit analysis, demonstrating the capacitive or inductive of the composites. This paper shows an effective way toward the tunable and weakly negative permittivity, which will promote practical applications of metacomposites in electromagnetic shielding and impedance matching fields.

Improved microwave absorbing properties by designing heterogeneous interfaces in Mo@2D-MoS2

Microwave absorbing materials are usually designed to solve electromagnetic impedance matching by compositing the magnetic and dielectric loss materials. Herein we demonstrated that the Mo@2D-MoS2 nanocomposites synthesized by arc-discharge method exhibit excellent microwave absorbility at gigahertz. Based on the transmission line theory, we prove quantitatively that more than 90% of the electromagnetic power can be attenuated at 5.6–18 GHz. The experimental results and theoretical analyses evidence that the heterogeneous interfaces and the few layers of the two-dimension graphene-like structures of Molybdenum disulfide (MoS2) contribute to the enhancement of the microwave absorption properties. The present study has important implications in understanding the microwave absorbing properties of the 2D materials and provides a new strategy for the design of variety microwave absorbers.

Improved microwave absorption properties by atomic-scale substitutions

To solve the electromagnetic impedance matching issue, microwave absorption materials are usually composed of magnetic and dielectric components with heterogeneous interfaces at micro/nanoscales. Herein we demonstrate an arc-discharging approach to optimize microwave absorption properties of magnetic@dielectric Fe@C nanocapsules by in-situ substituting nitrogen heteroatoms in graphitic layers. By increasing the nitrogen content, we find that the electromagnetic properties can be effectively tuned, presenting the decreased transmission, the increased absorbance and the slight change for the reflection efficiencies. Experimental and theoretical results reveal that nitrogen dopants result in the atomic-scale symmetry breaking, inducing the separation of space charge at nitrogen-substituted sites, which play a role of electric dipole for the electromagnetic polarization. The present study has important significance in understanding the structural origin of microwave absorption, and meanwhile provides an effective way for designing microwave absorbents at atomic-scale.

Enhanced microwave electromagnetic properties of core/shell/shell-structured Ni/SiO2/polyaniline hexagonal nanoflake composites with preferred magnetization and polarization orientations

Core/shell/shell-structured Ni/SiO2/polyaniline hexagonal nanoflakes possessing in-plane [111] easy magnetization (M) and out-of-plane interfacial polarization (P) are synthesized by a three-step liquid chemical method, and their physicochemical properties and growth mechanism are investigated. Three characteristic types of paraffin-bonded ring-shaped nanoflake composites having random (R), vertical–horizontal (V–H), and horizontal–vertical (H–V) orientations of the orthogonal M and P to their two major surfaces are prepared by randomly, vertically, and horizontally aligning the nanoflakes in the paraffin matrix under a magnetic alignment and thermal curing process. The composites are evaluated experimentally and theoretically in the L–Ku (1–18 GHz) bands of microwaves in order to investigate the orientation effect of the orthogonal M and P on their microwave electromagnetic properties. The in-plane M in the H–V-oriented composite and the out-of-plane P in the V–H-oriented composite, which are parallel to the effective magnetic and electric field vectors of incident microwaves, result in a significant enhancement in permeability with multiple magnetic natural resonances and an obvious improvement in permittivity in comparison with other composites, respectively. The observations agree with the theoretical predictions based on the Landau–Lifshitz–Gilbert equation and Bruggeman's effective medium theory for permeability and the Debye's polarization theory for permittivity. As a result, the H–V-oriented composite achieves the best microwave electromagnetic impedance matching and absorption with a broad absorbing bandwidth of 4 GHz, a wide thickness range of 7–10 mm, and a minimal reflection loss of −41.5 dB in the Ku (12–18 GHz) band.

Tuning microwave absorption properties by hybriding heterogeneous components for core@shell structural Fe@SiC flakes

Flake-like Fe@SiC composites were synthesized by a combined approach consisting of ball-milling and heat-assisted surface adhesion processes. The experimental results indicate that the Fe micropowders with an initial size of 30–40 μm can be completely milled into a flake-like shape with the diameter of 35 μm and the thickness of 10 μm after 5 h ball-milling, and then the flakes would be further fractured to smaller pieces as increasing the milling time. The as-made Fe flakes were subsequently encapsulated within SiC nanopowders by high-speed mixing at 120 °C for 2 h under nitrogen atmosphere. By turning the ball-milling time and the Fe/SiC ratios, the reflection loss (RL) could be optimized to −10 dB in the frequency ranges of 21.5–22.3 GHz and 20.1–26.5 GHz for the absorber thicknesses of 0.3 mm and 0.5 mm, respectively. In particular, the maximum RL peaks for the flake-like Fe@SiC composites shifted to higher frequencies compared with the spherical Fe@SiC composite due to the increased magnetic anisotropy. The enhanced microwave absorption properties are a synergistic effect of magnetic loss and dielectric loss, resulting from the heterogeneous components, and their proper electromagnetic impedance matching.

Facile synthesis and enhanced microwave absorption properties of multiferroic Ni0.4Co0.2Zn0.4Fe2O4/BaTiO3 composite fibers

Multiferroic composite fibers composed of Ni0.4Co0.2Zn0.4Fe2O4 (NCZFO) spinel ferrite and BaTiO3 (BTO) have been successfully synthesized by a simple electrospinning and subsequent heat treatment. The effects of the BTO content on the structure, static magnetic performance, electromagnetic and microwave absorption properties of the as-prepared samples are thoroughly investigated. The electromagnetic parameters and microwave absorption performances can be readily and effectively tuned through controlling the molar ratio of the two phases. The sample of NCZFO/(40 mol%)BTO fibers shows the enhanced complex permittivity making it better in microwave absorption capability than the composite fibers with other compositions and the mixture of NCZFO fibers and BTO fibers. The microwave absorption values less than −10 dB can cover the entire 2–18 GHz frequency range over the absorber thickness of 2–7.5 mm, and the minimum reflection loss value reaches −65.6 dB at 15.7 GHz with a broad effective absorption bandwidth below −10 dB of 7.8 GHz ranging from 2.6 to 6.8 GHz and 13.7 to 17.3 GHz for a layer thickness of 5 mm. The enhanced absorbing ability for the optimal NCZFO/(40 mol%)BTO fibers arises from the good synergistic effect between magnetic and dielectric components within the quasi-one-dimensional space, the improved interfacial effect as well as the proper electromagnetic impedance matching.

Carbon spheres@MnO2 core-shell nanocomposites with enhanced dielectric properties for electromagnetic shielding

Carbon spheres (CS)@MnO2 core-shell nanocomposites, with MnO2 nanoflakes uniformly coating at the surface of CS cores, were successfully synthesized by a facile water-bathing method. MnO2 amounts is estimated to be 24.7 wt% in CS@MnO2 nanocomposites. A high dielectric loss value and an electromagnetic shielding effectiveness of 16‒23 dB were observed for the CS@MnO2 in the frequency range of 8‒18 GHz, which is mainly attributed to the enhanced absorption loss. The incorporation of the CS with MnO2 improves the electrical conductivity. Meanwhile, the electromagnetic impendence matching has been significantly ameliorated. Moreover, the increasing interfaces between the CS and MnO2 facilitate the microwave attenuation as well. Thus, the electromagnetic shielding performances were greatly enhanced. Our findings provide an effective methodology for the synthesis of the CS@MnO2 core-shell nanocomposite for potential electromagnetic applications.

3D graphene-Ni microspheres with excellent microwave absorption and corrosion resistance properties

In this study, we report a simple and efficient two-step method consisting of water-in-oil (W/O) emulsion technique and subsequent annealing process for synthesizing the hollow reduced graphene oxide microspheres embedded with Ni nanoparticles (Air@rGO€Ni). The microspheres showed good electromagnetic properties because of the coexistence of magnetic loss and dielectric loss to microwaves. The minimum reflection loss (RLmin) value of Air@rGO€Ni microsphere reaches up to − 59.7 dB at 11.6 GHz with a thickness of 2.7 mm and an absorption bandwidth (lower than − 10 dB) is 4.8 GHz (9.4–14.2 GHz). Their resistance to chemical corrosion is amazingly, after treatment in hydrochloric acid for 5 days, the saturation magnetization (Ms) values of Air@rGO€Ni microsphere has fallen only six percentage points. More interestingly, we can easily controll the microwave absorbing properties of the microspheres by changing the ratio of the two components in the composites. The unique 3D structure and excellent electromagnetic match at the corresponding resonance peaks for dielectric and magnetic loss play an important role in improving microwave absorption property. Our study provides a good potential method for preparation of lightweight microwave absorbing materials.