Narrow Results

Graphene/Ag composite aerogels (GA/Ag) with three-dimensional hierarchical porous microstructure were prepared by in-situ formation of silver nanoparticles obtained from AgNO₃ into the porous structure of graphene aerogel. When the used AgNO₃ concentration was 12wt.%, the specific surface area of the obtained GA/Ag was 128.6m²g${}^{-1}$, which was higher than that of GA. GA/Ag exhibited electrical resistivity of 0.63$\mathrm{\Omega }\cdot$cm and excellent microwave absorption property with the minimum reflection loss of −40dB at 10.9GHz with the corresponding effective absorption bandwidth of 4GHz (8.8–12.8GHz). The GA/Ag composite aerogels enabled multiple reflection loss and absorption of incident electromagnetic waves, realizing excellent microwave absorption effects based on impedance matching and efficient synergy. It could be predicted that GA/Ag with a hierarchical porous structure may show potential application prospects in the field of microwave absorption.

Microwave response of a two-dimensional (2D) electron stripe, supplied by two semi-infinite side contacts, is theoretically studied taking into account retardation and radiative effects. Absorption spectra of the stripe are calculated as a function of the microwave frequency ω, magnetic field B, parameters of the 2D electron gas and of the contacts. It is shown that the contacts substantially modify the position, the linewidth and the strength of the absorption resonances, leading for instance to the inversion of the ab-sorption maxima of the right and left circularly polarized radiation in magnetic fields.

A layered structure model is proposed for microwave dielectric properties of nonhomogeneous hydrogen plasma in carbon nanotubes (CNTs) film. Using the transfer matrix method for solving electromagnetic wave propagation equation, the microwave attenuation of the film is calculated in the range of 0–30 GHz under different conditions. It is found theoretically that with the increase of hydrogen plasma nonhomogeneity, the frequency bandwidth of strong microwave absorption by the film increases markedly. The application of a moderate static magnetic field can effectively improve microwave attenuation properties of hydrogen plasma in CNTs. The numerical results are in good agreement with the available experimental data.

The possibility of high-field Josephson magnetic resonance (HFJMR) in high-T_c superconductors (HTSCs) is theoretically demonstrated. The HFJMR is caused by the penetration of Abrikosov vortices into the grains in fields within H_c1≤H<H_x (where H_c1 is the first critical field and H_x is the field corresponding to vortex separation from pinning centers in the grains). The signals from microwave absorption is dependent on the ratio of grain sizes that produces junction, the anisotropy and the intergrain coupling strength. These properties can be used for investigations of HTSC macrostructures and a vortex matter.

Carbon nanofiber (CNF) films synthesized from linum usitatissimum seed oil at 850°C have been studied as a microwave absorber. Scanning electron microscopy, transmission electron microscopy, micro-Raman spectroscopy, and Fourier transform infrared spectroscopy reveal the graphitic nature of fibers, having uniform diameter ~ 600 nm. Results of microwave absorption study explore high absorption efficiency of CNF film, contributed by low reflection of about -20 to -29 dB. Shielding effectiveness for the film observed between -31 and -60 dB in 8–12 GHz range.

Carbon nanotubes have been observed to emit ultraviolet, visible and infrared radiation when exposed to microwaves. We have performed experiments in which both short (0.5 μm–2 μm) and long (5 μm–30 μm) single and double-walled carbon nanotubes were exposed to 2.46 GHz microwaves at a pressure of ~ 10^-6 Torr. Structural modifications of the carbon nanotubes due to microwave absorption have been studied using the Raman spectroscopy G-band and D-band intensities, which suggest that microwave irradiation at relatively low pressure results in an increase in nanotube defects, especially in the case of the long nanotubes. Furthermore, a comparison of the spectra of the radiation emitted from the nanotubes suggests that the longer nanotubes emitted radiation of much greater intensity than the shorter nanotubes. Based on the results of the experiments and results described in previous reports, the ultraviolet, visible and infrared radiation emitted as the result of microwave absorption by carbon nanotubes seems to be primarily blackbody radiation emitted due to Joule heating. However, the presence of several broad photopeaks in the spectra of the emitted radiation (which do not seem to be related to gases absorbed by the nanotubes or the presence of catalyst particles) suggest that emissions are not the result of Joule heating alone.

Functionally graded multiwalled carbon nanotube (MWCNT) reinforced epoxy matrix composites are fabricated using a centrifugal method. Aggregation of the MWCNTs during the epoxy curing process is prevented using a two-step aminosilane modification. Chemical interaction of the silane with the oxidized nanotube surface is confirmed using Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. Raman spectroscopy of acid-treated MWCNTs corroborates the formation of surface defects owing to the introduction of carboxyl groups. The mechanical and microwave absorption property gradients of the composites correspond with those produced via silane modification indicating potential application to microwave absorbing materials. The MWCNTs are better dispersed in the epoxy resin after the modification, making it possible for them to become efficiently graded in the epoxy matrix. We therefore show that it is possible to fabricate functionally graded nanofiller-reinforced materials using the centrifugal method by modifying the surface of the nanofiller.

Co doped mesoporous carbon composites (MC–Co) have been synthesized at different carbonization temperatures via evaporation-induced multicomponent co-assembly strategy. The nanostructures and chemical compositions of MC–Co composites were characterized by transmission electron microscope (TEM), scanning electron microscope (SEM) and X-ray diffraction analysis (XRD), etc. Their electromagnetic and microwave absorption properties were investigated in the frequency ranging from 2 GHz to 18 GHz. By the measurement of electromagnetic parameters and theoretical simulation of reflection loss (RL), the results showed that the minimum RL value of the optimum composite MC–Co-800 reached up to -41.9 dB at 13.5 GHz with a thickness of only 1.80 mm, and the effective absorption bandwidth (< -10 dB) is 4.4 GHz (from 11.4 GHz to 15.8 GHz). This work demonstrated that the optimum carbonization temperature of the obtained composites is 800°C, which is critical to its electromagnetic properties. The excellent microwave absorption properties can be attributed to dielectric loss, Ohmic loss, and characteristic impedance matching. Therefore, the as-synthesized composites could be acted as a candidate of microwave absorber, especially for the light weight demand.

Owing to the unique microstructure and the excellent dielectric properties, carbon nanotubes (CNTs) were decorated with CoFe₂O₄ nanoparticles to synthesize the CoFe₂O₄/CNTs nanocomposites by the solvothermal method. The phase structure, morphology, magnetic properties and microwave absorption performance of the as-prepared CoFe₂O₄/CNTs were characterized and discussed by X-ray diffraction (XRD), thermal gravity analysis (TGA), transmission electron microscope (TEM), vibrating sample magnetometer (VSM) and vector network analyzer (VNA). All results indicated that the diameter of CoFe₂O₄ nanoparticles decorating on the surface of CNTs increased with the solvothermal temperature. CoFe₂O₄/CNTs prepared at 180°C, 200°C and 220°C exhibited superparamagnetism, while the other samples presented ferromagnetism at room temperature. And with the increasing solvothermal temperature, the saturation magnetization and coercivity increased up to 72 emu/g and 2000 Oe for the sample prepared at 260°C (S-26). And the reflection loss of CoFe₂O₄/CNTs nanocomposites increased with the solvothermal temperature up to -15.7 dB for S-26 with the bandwidth of 2.5 GHz.

Low-density Fe-doped ordered mesoporous carbon (CMK-3)-silica (SBA-15) nanocomposites with different Fe contents have been prepared by a catalytic carbonization procedure followed by high-temperature calcination in N₂. From field emission-scanning electron microscope (FE-SEM) and high resolution-transmission electron microscope (HR-TEM) images, it can be concluded that CMK-3 particles are dispersed homogeneously into a silica matrix and form a novel, special and interesting composite nanostructure. The metal species ($\sim$18nm) are dispersed on the surface of frameworks during the catalytic carbonization procedure and endow a magnetic property to the carbon–silica nanocomposites. The optimal reflection loss (RL) calculated from the measured permittivity and permeability is $-$19dB at 17.2GHz for an absorber thickness of 2.00mm. Moreover, the electromagnetic (EM) wave absorption less than $-$10dB is found to exceed 5.76GHz as the layer thickness is 2.37 mm. The permittivity dispersion behaviors have been explained based on the Cole–Cole model and the conductivity contribution model. A new simple empirical model was also supposed to find the fitted curves of the multi-resonance imaginary permeability spectra of the composites. The EM wave can hardly be reflected on the absorber surface because of a better match between dielectric loss and magnetic loss, which originates from the combination of dielectric carbon–silica and magnetic Fe species.

In this work, Ni-doped ZnO/Al composites were prepared by a facile chemical co-precipitation method. The morphology and structure of the as-prepared composites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS), respectively. It was found that the flake-like Al powders were successfully coated by Ni-doped ZnO nanoparticles with slight aggregation and Ni${}^{2+}$ was successfully doped into the crystal lattice of ZnO. Moreover, the effects of ZnO concentration and doped Ni concentration on the infrared emissivity of ZnO/Al composites at the waveband range of 8–14$\mu$m were studied. The results showed that the ZnO/Al composites exhibited the lowest infrared emissivity of 0.34 with 50wt.% ZnO concentration. Meanwhile, the electromagnetic parameters and microwave absorbing properties of Ni-doped ZnO/Al composites in the frequency range of 2–18GHz were explored. Significantly, 12mol.% Ni-doped ZnO/Al composites presented the lowest infrared emissivity of 0.37 and the maximum reflection loss reached $-$32.5dB at 13.6GHz with a thickness of 4.5mm. The excellent microwave absorbing properties could be attributed to the good impedance match, crystal lattice defects and interfacial polarization. It was believed that the Ni-doped ZnO/Al composites could be used as potential infrared-microwave compatible stealth materials.

In this work, reduced graphene oxide/CeO₂ nanocomposites (RGO/CeO₂) with two different RGO contents were synthesized using a facile one-step hydrothermal method, and the microwave absorption properties of RGO/CeO₂ were investigated for the first time. Morphology and structure analysis shows that the CeO₂ nanoparticles are uniformly dispersed on the RGO sheets with average size of 15nm. The as-prepared RGO/CeO₂ exhibits excellent microwave absorbability. An optimal reflection loss (RL) of $-$32dB is found at 17GHz with a coating layer thickness of 1.5mm. The product with a coating layer thickness of only 2.0mm shows a bandwidth of 4.3GHz, corresponding to RL at $-$10dB (90% of electromagnetic wave absorption). Compared with pristine RGO or pure CeO₂ nanoparticles, the reported nanocomposites achieved both wider and stronger wave absorption in the frequency range of 2–18GHz. The enhanced microwave absorption properties are attributed to the conductive loss and dielectric loss mainly caused by the higher oxygen vacancy concentration of CeO₂ in RGO/CeO₂, which is demonstrated by X-ray photoelectron spectroscopy. Moreover, multiple interfacial polarizations occurring in the multi-interfaces between CeO₂ and RGO sheets may be beneficial to microwave absorption. RGO/CeO₂ could be used as an attractive candidate for the new type of microwave absorptive materials.

In this work, the Fe-doped ZnO powders have been synthesized by a facile chemical coprecipitation method. The structure, morphology and magnetic properties of the as-prepared powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectrum (EDS), X-ray photoelectron spectroscopy (XPS) and vibrating sample magnetometer (VSM). The results showed that the Fe ions were well incorporated into the crystal lattice of ZnO and had a valence state of $+3$. The magnetization curves indicated the Fe-doped ZnO presented the ferromagnetic behavior at room temperature. Moreover, the electromagnetic (EM) parameters and microwave absorption properties of Fe-doped ZnO/paraffin wax in the frequency range of 2–18GHz were explored. The minimum reflection loss reached $-38.4$ dB at 6.6 GHz, and the reflection loss less than $-10.0$ dB was 4.0 GHz (from 11.0GHz to 15.0GHz) with a thickness of only 2.5 mm. Significantly, the enhanced microwave absorption of the as-prepared powders could be achieved by doping with Fe${}^{3+}$ ions or varying the thickness of the absorbers. The mechanism of microwave absorption were attributed to the good impedance matching, the dielectric loss resulted from the crystal lattice defects and the magnetic loss originated from the natural resonance. It is believed that the Fe-doped ZnO powders could be used as potential microwave absorbers.

ZnO nanocrystals were introduced into Fe₃O₄/MWCNTs composites to improve the impedance matching and electromagnetic (EM) wave attenuation of the system. The as-synthesized ZnO/Fe₃O₄/MWCNTs composites were characterized by X-ray diffraction, vibrating sample magnetometer, field-emission scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy. SEM and TEM images showed that Fe₃O₄ microspheres 100–200nm in size connected MWCNTs. Analysis of EM parameters revealed that the impedance matching of the ZnO/Fe₃O₄/MWCNTs composites was considerably improved after ZnO nanocrystals were introduced. The ZnO/Fe₃O₄/MWCNTs composites exhibited a highly efficient microwave absorption (MA) capacity within the tested frequency range of 2–18GHz. The optimal reflection loss of EM waves was $-38.2$dB at 6.08GHz with an absorber thickness of 3.5mm. The excellent MA properties of the composites could be attributed to the improved impedance matching, interfacial polarization, and combined effects of dielectric and magnetic losses.

Ni@onion-like carbon (OLC)/reduced graphene oxide (RGO) nanocomposites were synthesized, and their multicomponent microstructure was confirmed by X-ray diffraction, transmission electron microscopy, Raman spectra, the thermal gravimetric analysis and magnetic hysteresis loops. The obtained nanocomposite possesses a unique structure, in which core–shell Ni@OLC nanocapsules are decorated on the surface of RGOs. The synergistic effect of the dielectric loss of RGO and OLC and the magnetic loss of Ni nanoparticles can be constructed. The RGO can provide tremendous electric dipoles. Multi-interface among RGO, OLC and Ni nanoparticles can enhance dielectric performance and cause multiple reflections. The combination of these merits makes the nanocomposite a promising candidate material for electromagnetic absorber. The 20wt.% nanocomposite-paraffin composite can possess an optimal reflection loss (RL) of $-47.5$dB at 9.75GHz with a thickness of 3.1mm. When the thickness is 2.0mm, the RL of composite can reach $-32.6$dB at 17.4GHz. The effective frequency is 6.54GHz (11.16–17.7GHz) for 2.4mm thickness layer.

Graphene nanosheets/epoxy resin (GN/EP) composites with enhanced microwave absorption properties were prepared. The influence of GNs content on the electromagnetic and microwave-absorbing properties of single-layer GN/EP composites was investigated in the frequency range of 2–18GHz. A novel double-layer microwave absorber by combining absorbers with different contents of graphene was designed to achieve absorbers with wide absorption band and optimal microwave absorption. The calculated microwave absorption properties indicated that broader absorption band was obtained by the double-layer GN/EP composites. Reflection loss (RL) values below $-$5dB can be obtained in the frequency range of 4.4–18GHz by the double-layer GN/EP composites at thickness of 3.9mm.

In this work, reduced graphene oxide (RGO)/ferroferric oxide (RGO/Fe₃O₄) hybrid composite was successfully fabricated by a facile one-step solvothermal method. The structure, chemical composition, morphology and magnetic properties of the samples were investigated in detail. Fe₃O₄ microspheres with an average diameter of 250nm were uniformly anchored on the surface of the RGO sheets without large aggregation. Moreover, the results demonstrated that the hybrid composite exhibited obviously enhanced microwave absorption properties compared with the pure Fe₃O₄ microspheres. The minimum reflection loss (RL) of the hybrid composite reached $-$36.8dB at 17.2GHz with a thickness of 5.0mm and its effective absorption bandwidth (lower than $-$10dB) was 3.9GHz with a thickness of 2.5mm. Significantly, the hybrid composite exhibited a dual-waveband absorption characteristic covering the C and Ku bands. Besides, the relationship between the matching thickness and peak frequency was reasonably explained according to the quarter-wavelength matching theory. Therefore, the obtained composite was a promising candidate for application in microwave absorption.

Porous flower nickel oxide@polyaniline (NiO@PANI) composites as excellent microwave absorption (MA) materials in the X-band were synthesized via a two-step strategy in this work. The porous NiO flower is uniformly dispersed and homogeneous in particle size after Ostwald ripening process. Coating conductive PANI on the surface of porous NiO microspheres could improve interfacial polarization and dielectric loss property that will lead to a great improvement of MA properties. Electromagnetic (EM) parameters of NiO@PANI composites with different NiO contents were investigated by a vector network analyzer and the reflection loss (RL) values with varied thickness were also calculated. The results showed that the effective absorption bandwidths ($\mathrm{\text{RL}}<-10$dB) of all NiO@PANI composites can cover the whole X-band. Especially, the NiO@PANI${}_{0.1}$ composite is able to attenuate microwave energy in the X-band with the thickness of 2.5mm. The NiO@PANI${}_{0.2}$ has a maximum RL of $-32.8$dB at 10.1GHz and the effective absorption bandwidths cover 4.64GHz (11.12–15.76GHz) at 2.0mm. The excellent MA absorption performance may be ascribed to the polarization effect, dielectric loss and structure of porous flower-like NiO@PANI. Our work confirms that the synthesized NiO@PANI composite is an attractive candidate as a highly efficient MA material in the X-band.

Polyaniline (PANI)/Zn ferrite composites were fabricated by simple two-step method. The crystal phase, particle size, morphology, thermal stability and conductivity were characterized. Electromagnetic parameters of PANI/Zn ferrite composites were measured at room temperature in the frequency of 2–18GHz. The prepared composite had an amorphous fluffy structure, Zn ferrite nanoparticles with diameters ranging from 20nm to 30nm are encapsulated in PANI or on PANI surface, and the thermal stability of the composite is poor. But Zn ferrite content plays a key role in influencing this structure and regulating microwave attenuation capability. The PANI/Zn ferrite composites showed an enhanced microwave absorption performance in Ku band at thin coating thickness which corresponds to RL value below $-10$dB and the minimum reflection loss (RL) is $-54.4$dB at 17.6GHz with the coating thickness of 1.4mm.

Core–shell structured Fe/C have been successfully derived from a metal-organic framework for microwave absorbing. Based on the measured electromagnetic parameters, it is found that the maximum reflection loss (RL) of Fe/C reaches $-40$dB at 5.8GHz with a thickness of 3.0mm and the broadest absorption bandwidth ($\mathrm{\text{RL}}<-10$dB) is up to 6.0GHz (from 11.2 to 17.2GHz) with a thickness of 1.5 mm. The excellent microwave absorption is mainly ascribed to the multiple reflections, good impedance matching, dielectric loss and interface polarization originating from the core–shell structure. It is believed that Fe/C can be a promising microwave absorbing material.