Baoshan Zhang

Coin-like α-Fe2O3@CoFe2O4 Core–Shell Composites with Excellent Electromagnetic Absorption Performance

In this paper, we designed a novel core-shell composite for microwave absorption application in which the α-Fe2O3 and the porous CoFe2O4 nanospheres served as the core and shell, respectively. Interestingly, during the solvothermal process, the solvent ratio (V) of PEG-200 to distilled water played a key role in the morphology of α-Fe2O3 for which irregular flake, coin-like, and thinner coin-like forms of α-Fe2O3 can be produced with the ratios of 1:7, 1:3, and 1:1, respectively. The porous 70 nm diameter CoFe2O4 nanospheres were generated as the shell of α-Fe2O3. It should be noted that the CoFe2O4 coating layer did not damage the original shape of α-Fe2O3. As compared with the uncoated α-Fe2O3, the Fe2O3@CoFe2O4 composites exhibited improved microwave absorption performance over the tested frequency range (2-18 GHz). In particular, the optimal reflection loss value of the flake-like composite can reach -60 dB at 16.5 GHz with a thin coating thickness of 2 mm. Furthermore, the frequency bandwidth corresponding to the RLmin value below -10 dB was up to 5 GHz (13-18 GHz). The enhanced microwave absorption properties of these composites may originate from the strong electron polarization effect (i.e., the electron polarization between Fe and Co) and the electromagnetic wave scattering on this special porous core-shell structure. In addition, the synergy effect between α-Fe2O3 and CoFe2O4 also favored balancing the electromagnetic parameters. Our results provided a promising approach for preparing an absorbent with good absorption intensity and a broad frequency that was lightweight.

A brief introduction to the fabrication and synthesis of graphene based composites for the realization of electromagnetic absorbing materials

Owing to the fast development of wireless information technologies at the high-frequency range, the electromagnetic interference problem has been of increasing significance and attracting global attention. One key solution for this problem is to develop materials that are able to attenuate the unwanted electromagnetic waves. The desired properties of these materials include low reflection loss value, wide attenuation band, light weight, and low cost. This review gives a brief introduction to graphene-based composites and their electromagnetic absorption properties. The ultimate goal of these graphene absorbers is to achieve a broader effective absorption frequency bandwidth (fE) at a thin coating thickness (d). Representative and popular composite designs, synthesis methods, and electromagnetic energy attenuation mechanisms are summarized in detail. The two key factors, impedance matching behavior and attenuation ability, that determine the electromagnetic behavior of graphene-based materials are given particular attention in this article.

A simple hydrothermal process to grow MoS2 nanosheets with excellent dielectric loss and microwave absorption performance

In this study, two-dimensional MoS2 nanosheets synthesized by a hydrothermal method were firstly investigated for microwave absorbing performance. The obtained MoS2 nanosheets are highly desirable as an electromagnetic wave (EM) absorber because of its larger interfacial polarization and high dielectric loss. Our results show that the real and imaginary parts of permittivity of MoS2 prepared at 180 °C are higher than those of other samples. A broad bandwidth absorption at a thin thickness can be obtained between 2 and 18 GHz. The microwave reflection loss (RL) of MoS2 nanosheets prepared at 180 °C reaches as high as −47.8 dB at 12.8 GHz due to its high electrical conductivity and the polarization effect. It can also be found that MoS2 exhibits an effective electromagnetic wave absorption bandwidth of 5.2 GHz (<−10 dB) at the thicknesses of 1.9 and 2.0 mm. The results showed that the MoS2 nanosheets can be a candidate for microwave absorption with a broad effective absorption bandwidth at thin thicknesses.

CoxFey@C Composites with Tunable Atomic Ratios for Excellent Electromagnetic Absorption Properties.

The shell on the nano-magnetic absorber can prevent oxidation, which is very important for its practical utilization. Generally, the nonmagnetic shell will decrease the integral magnetic loss and thus weaken the electromagnetic absorption. However, maintaining the original absorption properties of the magnetic core is a major challenge. Here, we designed novel and facile CoxFey@C composites by reducing CoxFe3−xO4@phenolic resin (x = 1, 0.5 and 0.25). High saturation magnetization value (Ms) of CoxFey particle, as a core, shows the interesting magnetic loss ability. Meanwhile, the carbon shell may increase the integral dielectric loss. The resulting composite shows excellent electromagnetic absorption properties. For example, at a coating thickness of 2 mm, the RLmin value can reach to −23 dB with an effective frequency range of 7 GHz (11–18 GHz). The mechanisms of the improved microwave absorption properties are discussed.

Facile synthesis of porous coin-like iron and its excellent electromagnetic absorption performance

In this paper a novel electromagnetic absorbent, porous coin-like iron with a diameter of ∼10 μm and a thickness of 2 μm, was fabricated using a hydrogen gas reduction process. This special porous coin-like structure was attributed to a decrease in density and exceeded the Snoek limitation. It was observed that these coin-like iron structures exhibit excellent microwave absorption properties. An optimal reflection loss value of −53.2 dB was obtained at 16 GHz, moreover, the effective frequency bandwidth could be up to 6.3 GHz (11.7–18 GHz) at a thickness of 1.4 mm. The microwave absorption mechanism may have originated from the following factors: firstly, these coin-like irons were favorable for obtaining a lower real part of permittivity value and thus gained the improvement of impedance matching behavior, as compared with other reported irons. Secondly, the coin-like morphology exhibited a strong magnetic loss ability. Further analysis revealed that the magnetic loss mechanism may rely mainly on the resonance. In addition, the porous feature of the coin-like iron offered a rough surface on the large size of the coin-like structure, which was beneficial for electromagnetic wave scattering and further enhanced their microwave absorption properties.

Novel nanoporous carbon derived from metal–organic frameworks with tunable electromagnetic wave absorption capabilities

Nanoporous carbon materials derived from metal organic frameworks (MOFs) have attracted considerable attention due to their low density for microwave absorption. Nevertheless, their poor impedance matching has reduced the absorber performance. The design and fabrication of complex nanocarbon materials with outstanding impedance matching is still a challenge. Here, we prepared a core–shell structured ZIF-8@ZIF-67 crystal through a new seed-mediated growth method. After the thermal treatment of ZIF-8@ZIF-67 crystals, we obtained selectively nanoporous carbon materials consisting of ZnO/NPC as the cores and highly graphitic Co/NPC as the shells. The shell thicknesses of ZIF-67 can be tuned simply by varying the feeding molar ratios of Co2+/Zn2+. The composites exhibited excellent impedance matching and strong absorption. The composite ZnO/NPC@Co/NPC-0.5 samples filling with 50 wt% of paraffin show a maximum reflection loss (RL) of −28.8 dB at a thickness of 1.9 mm. In addition, a broad absorption bandwidth for RL <−10 dB which covers from 13.8–18 GHz can be obtained. Our study not only bridges diverse carbon-based materials with infinite metal–organic frameworks but also opens a new avenue for artificially designed nano-architectures with target functionalities.

Preparation and electromagnetic characteristics of a novel iron-coated carbon fiber

Abstract A novel iron-coated carbon fiber was fabricated electrochemically. The magnetic properties, complex permeability, and complex permittivity were measured. It was found that iron-coated fiber–epoxy composites with different mass loadings have different magnetic resonance frequencies. The resonance frequency of the 75 wt% sample is higher than that of the 80 wt% sample due to the influence of the demagnetization field. The e e ′ value of both composite samples exhibits a high value in the frequency range of 2 to 18 GHz, which is due to the longer conducting length of the fiber and the good conductivity of the iron. The values of e e ″ are also high, which is attributed to the intrinsic electric property of the carbon fiber.

A proposed electron transmission mechanism between Fe3+/Co2+ and Fe3+/Fe3+ in the spinel structure and its practical evidence in quaternary Fe0.5Ni0.5Co2S4

This study is focused on the spinel structure of metal oxides and sulfides, from which the ternary (NiCo2O4/NiCo2S4) and quaternary (Fe0.5Ni0.5Co2O4/Fe0.5Ni0.5Co2S4) samples with hollow sphere structures were prepared. Among these samples, Fe0.5Ni0.5Co2S4 was highly effective in its ability to attenuate electromagnetic waves, wherein a broader absorption bandwidth of 6.2 GHz could be achieved with a thinner coating layer of 1.3 mm. The cation distribution rule for Fe, Co and Ni ions in the spinel structure is given according to hybrid orbital theory to support the excellent electromagnetic absorption properties. Relying on the distribution of Fe, Co and Ni cations, the probable electron transmission and coupling between Fe3+/Co2+ and Fe3+/Fe3+ adjacent cation ion pairs could occur at the octahedral site (B site), which reflects the enhanced dielectric loss.

Strong electric wave response derived from the hybrid of lotus roots-like composites with tunable permittivity

Lotus roots-like NiO/NiCo2O4 hybrids derived from Metal-organic frameworks (MOFs) are fabricated for the first time by using flake NiCo-MOF precursors as reactant templates. It was found that a thin sample consisting of 60 wt % NiO/NiCo2O4 hybrids in the wax matrix exhibited an effective microwave absorption bandwidth of 4.2 GHz at the thickness of 1.6 mm. The highest reflection loss of −47 dB was observed at 13.4 GHz for a sample with a thickness of 1.7 mm. Results obtained in this study indicate that hybrids of NiO and NiCo2O4 are promising microwave absorbing materials with adjustable permittivity, which can exhibit broad effective absorption bandwidth at low filler loading and thin thickness.

The flaky porous Fe3O4 with tunable dimensions for enhanced microwave absorption performance in X and C bands

Special electric and magnetic characteristics make Fe3O4 widely applied in the electromagnetic (EM) wave absorption region. However, for pure Fe3O4, it is still a challenge to simultaneously obtain high absorption intensity and broadband absorption at a low thickness, owing to its low dielectric property. As we realized, flake configuration and the porous structure have obviously promote the EM wave absorption property. Because the former can lead to multi-reflection between flakes and the latter is conductive to interface polarization, flaky Fe3O4 with a porous and coarse surface was designed to overcome the deficiency of traditional Fe3O4 particles. The experimental results demonstrate that the flaky configuration is conductive to enhancing the dielectric coefficient and optimizing impedance matching. Moreover, the complex permittivity rises with the aspect ratio of the sheet. Under a suitable dimension, the flaky Fe3O4 could acquire targeted EM wave absorption capacity in the X band (8-12 GHz). In detail, the maximum reflection loss (RL) could reach a strong intensity of -49 dB at 2.05 mm. The effective absorption bandwidth (EAB) with RL below -10 dB is 4.32 (7.52-11.84) GHz, which is almost equivalent to the whole X band (8-12 GHz). Even more exciting, when regulating the thickness between 2.05 and 3.05 mm, the EAB could cover the entire C and X bands (4-12 GHz). This study provides a good reference for the future development of other ferromagnetic materials toward specific microwave bands.