Synthesis of dendritic cobalt with flower-like structure by a facile wet chemistry method as an excellent electromagnetic wave absorber

In this study, a three-dimensional (3D) floral dendritic cobalt (FDC) consisting of layered flakes was effectively synthesized using a facile wet chemistry method. The impact of the molar amount of NaOH on the microscopic morphology, magnetic characteristics, and electromagnetic wave (EMW) absorption properties of the FDC magnetic materials was comprehensively investigated. The results revealed that the prepared FDC features primary, secondary, and multi-level branches, with the majority of secondary branches being parallel to one another. The dendrites grew closely towards the flower's center at one end, while the tips extend in various directions, forming a dendritic flower cluster. The optimal reflection loss (RL) of S3 at 9.3 GHz was -56.34 dB with a thickness of 1.89 mm, and the maximum effective absorption bandwidth (EAB, RL < -10 dB) reached 6.0 GHz (12.0-18.0 GHz) at a thickness of 1.30 mm. Consequently, the FDC magnetic materials produced in this study presented a method for fabricating high-performance electromagnetic wave absorption (EMWA) materials.

Silica-Modified Ordered Mesoporous Carbon for Optimized Impedance-Matching Characteristic Enabling Lightweight and Effective Microwave Absorbers

Ordered mesoporous carbon (OMC) is considered to be a prospective carbon-based material for microwave absorption because of its abundant well-ordered mesoporous structures, high specific surface area, numerous active sites, and facile preparation process. However, its development has been seriously hindered by its poor impedance-matching characteristic. Herein, silica-modified OMC composites with a designable impedance-matching transition layer are successfully fabricated via a self-assembly method and succeeding calcination treatment. In addition, the silica in OMC@SiO₂ composites can maintain the mesoporous structure, which facilitates the scattering and reflection of microwaves in the tunnel structure. The as-prepared sample OMC-5@SiO₂ exhibits a minimum reflection loss (RL) value of -40.7 dB at 10.8 GHz with 2 mm and an effective absorption bandwidth (RL ≤ -10 dB) of 4.8 GHz with a thinner absorber thickness of 1.5 mm. We believe that the as-prepared OMC@SiO₂ composites can be prospective candidates as high-efficiency and lightweight microwave absorbers.

Dandelion-like carbon nanotube assembly embedded with closely separated Co nanoparticles for high-performance microwave absorption materials

Enhancing the magnetic loss capacity by microstructure design remains a considerable challenge in the microwave absorption field. Herein, a high-performance microwave absorbent is developed by dispersing a considerable amount of magnetic nanoparticles within the dandelion-like carbon nanotube assembly. A controllable fabrication method is further exploited to adjust the distribution feature of these embedded nanomagnets. In such a hierarchical composite, parts of the interaction network among the coupled closely spaced nanomagnets can be frequently broken and rebuilt to intensively dissipate the microwave energy, which is confirmed by electron holography and micromagnetic simulation for the first time. By virtue of this dynamic magnetic coupling network mechanism, the hierarchical C/Co composite acquires the first-rate microwave absorption performance. The maximum reflection loss value reaches as much as -52.9 dB (absorbance >0.99999) and the effective absorption bandwidth (absorbance >0.9) occupies the entire X band. It is believed that the above insightful mechanism provides a new opportunity to lower the density of the magnet-based microwave absorbent as much as possible. Besides, the unique method for dispersing magnetic nanoparticles also broadens the pathway to assemble the hierarchical architecture.

High-aspect-ratio iron nanowires: magnetic field-assisted in situ reduction synthesis and extensive parametric study

High-performance iron nanowires have attracted wide attention from researchers due to their 'controllable' arrangement distribution by magnetic fields. In this paper, a simple magnetic field assisted in situ reduction method was proposed to synthesize Fe NWs with high aspect ratio, small-diameter, and good dispersion. A detailed parametric study determining the relationship among the final morphologies of the products and magnetic field, injection sequence of sodium borohydride that was injected into ferrous sulfate heptahydrate, reactant concentration, and injection rate is presented. The as-synthesized Fe NWs were analyzed by scanning electron microscopy, transmission electron microscopy, x-ray diffraction, x-ray photoelectron spectroscopy and vibrating sample magnetometry. A plausible mechanism for the formation of high-aspect-ratio Fe NWs is proposed. The SEM images showed the dependence of the NWs morphology and aspect ratio on synthesis parameters. Magnetic field and injection sequence showed considerable influences on the synthesis of high-aspect-ratio Fe NWs. In the absence of magnetic field or with the changes in injection sequence, only the Fe flakes were obtained. The NWs diameter decreased, and the aspect ratio increased with the increase in injection rate. The FeSO₄·7H₂O and NaBH₄ concentration considerably influenced the aspect ratio of the product, which increased first, decreased, and then increased again with the increase in FeSO₄·7H₂O concentration. Meanwhile, the product aspect ratio increased and then became saturated with the increase in NaBH₄ concentration Thus, an optimum synthesis process was obtained, with the average aspect ratio of 350, and the average diameter of 60 nm. The results reported in this paper provide a basis for optimizing the growth of Fe NWs by magnetic field-assisted method.

MOF-derived yolk–shell Ni/C architectures assembled with Ni@C core–shell nanoparticles for lightweight microwave absorbents

The yolk–shell Ni/C microspheres assembled by Ni@C core–shell nanoparticles with excellent microwave absorption performance can be simply fabricated by decomposition of a Ni-based metal–organic framework (Ni-MOF).

Microwave plasma assisted reduction synthesis of hexagonal cobalt nanosheets with enhanced electromagnetic performances

In this study, we employed a microwave plasma assisted reduction (MPAR) method to prepare metallic nanoparticles with desirable morphology. Compared with the hydrogen thermal reduction technique, the MPAR technique could greatly maintain the original morphology of self-sacrificing precursors, as well as proving to be highly efficient, energy-saving and pollution-free. Taking ferromagnetic metallic Co as a forerunner, Co nanosheets with inerratic hexagonal morphology were successfully synthesized on a large scale uniformly. The lateral dimension of the achieved Co nanosheets is in the range of 3∼5 μm with tens of nanometers in thickness. The intact hexagonal flaky shape of Co nanosheets is beneficial for improving dielectric loss by increasing electric channels and interfacial polarization. Consequently, the minimum reflection loss could reach up to -71 dB at a thin thickness of 1.2 mm. Furthermore, the effective bandwidth (RL < -10 dB) could be achieved in a wide range of 2.8∼18 GHz by integrating the thickness from 5.0∼1.0 mm, which provides the possibility for applications in electromagnetic shielding and radar stealth fields. It is believed that the MPAR technique is suitable for designing and preparing novel microwave absorbers on the basis of appropriate precursors, providing new opportunities to acquire high-performance microwave absorbers in the future.

Preparation and high-performance microwave absorption of hierarchical dendrite-like Co superstructures self-assembly of nanoflakes

Dendritic-like Co superstructures based on the self-assembly of nanoflakes that could efficiently suppress the eddy current were successfully synthesized via a facile, rapid, and energy-saving chemical reduction method. Since crystal structure, size, and special geometrical morphology, magnetism have a vital influence on microwave absorption properties, the as-obtained products were characterized by x-ray diffraction, scanning electron microscopy, vibrating sample magnetometry, and vector network analysis. The prepared dendritic Co possesses abundant secondary branches that extend to the 3D space. Their dimensions, spacing, sheet-like blocks, and high-ordering microstructures all contribute to the penetration, scattering, and attenuation of EM waves. The composites present attractive microwave absorption performances in the X band, as well as in the whole S band (2-4 GHz). This work investigates the mechanism of absorption for the as-obtained Co, offers a promising strategy for the fabrication of hierarchical Co microstructure assemblies by multi-leaf flakes and introduces the application of dendritic-like Co as a highly efficient absorber in the S band and X band.

Three-dimensional reduced graphene oxide powder for efficient microwave absorption in the S-band (2–4 GHz)

Three-dimensional reduced graphene oxide (3D-rGO) powders are synthesized and demonstrate remarkably enhanced microwave absorption in the S-band (2–4 GHz).

Decomposition of MOFs for the preparation of nanoporous Co3O4 fibres and sheets with excellent microwave absorption and photocatalytic properties

Unique nanoporous Co₃O₄ fibres and sheets were successfully fabricated via a facile hydrothermal route (150 °C) and subsequent annealing process at 500 °C in air.

Preparation of broccoli-like ferromagnetic cobalt microstructures with superior coercivity via an aqueous reduction strategy

Controlled synthesis of novel hierarchical cobalt (Co) microstructures with extraordinary magnetic performances is a promising strategy for the development of magnetic metals for industrial purposes.