Compositional and morphological design of heterojunction modified by Schottky junction as highly efficient microwave absorbers

Synthesis and excellent electromagnetic absorption properties of reduced graphene oxide/PANI/BaNd0.2Sm0.2Fe11.6O19 nanocomposites

Electromagnetic synergy and porous characteristics are two dominant factors in realizing light-weight and high-efficient microwave absorption performance. In this paper, a formaldehyde-assisted metal-ligand crosslinking strategy and a subsequent pyrolysis process are employed to synthesize magnetic porous carbon spheres with the electromagnetic synergy and porous characteristics, in which metal ions are tightly anchored in poly-(tannin acid) spheres because of the strong chelation coordination between them. The chemical composition of magnetic particles and the microwave absorption performance of the derived magnetic porous carbon spheres can be manipulated by adjusting the metal ions. Benefiting from the cooperative effects of porous structure, matched impedance, the electromagnetic synergistic enhancement between magnetic particles and carbon matrix, as well as the improved interfacial polarization caused by the large number of hetero-interfaces, both the microwave absorption intensity and the effective absorption bandwidths are significantly enhanced for magnetic porous carbon spheres, such as Co-PCSs and CoNi-PCSs, compared with PCSs. With 15 wt.% filler loading, the maximum reflection loss of CoNi-PCSs is −51 dB at 2.2 mm and the effective bandwidth is 7.2 GHz at 2.9 mm. Furthermore, this study provides the theoretical theory for the design and development of light-weight and highly efficient microwave absorption materials.

The exploration for the potential lightweight and high efficiency microwave absorbers with thermal stability is a great challenge for researchers. More importantly, the electromagnetic parameters of absorbers at elevated temperatures are seldom studied. In this work, sandwich MXene‐Ti3C2Tx@flake carbonyl iron (MF) composites with tunable and efficient microwave absorption at elevated temperatures are successfully fabricated by an electrostatic self‐assembly method. Both the complex permittivity and permeability of the MF are strongly temperature dependent in the temperature range of 298–473 K and X band (8.2–12.4 GHz). The efficient microwave absorption of MF at different temperatures can be achieved by changing the ratio of raw materials. Specifically, at room temperature, reflection loss (RL) ≤−10 dB of MF composite covers the whole X band at 2.12 mm, while the optimum RL value is −26.3 dB at 11.81 GHz. At 373 K, the optimum microwave absorption value of the MF composite reaches up to −63 dB at 10.27 GHz and RL ≤−10 dB can cover a whole X band with a thickness of 1.52 mm. Therefore, MF composites are expected to be promising candidates as ultrathin, highly efficient, and broadband absorbing materials at elevated temperatures.

To construct effective electromagnetic parameters of reduced graphene oxide (RGO)-based composites for lightweight and high efficiency microwave absorption, Ni/RGO composites were prepared via a facile in-situ hydrothermal process. Due to the strong high-frequency polarization effect related to the large surface contact area of the obtained Ni/RGO hybrids, the composites presented strong electromagnetic wave attenuation ability even when the filler content is only 1.6 wt. %. This work might deepen the understanding of the effective dielectric loss mechanism and impedance matching and provide an effective strategy for the practical application of RGO as a lightweight and high-performance microwave absorption material.

Cellular-like sericin-derived carbon decorated reduced graphene oxide for tunable microwave absorption

The exploration of high-efficiency microwave absorption materials with lightweight and hydrophobic features is highly expected to reduce or eliminate the electromagnetic pollution. Graphene-based nanocomposites are universally acknowledged as promising candidates for absorbing microwaves due to their remarkable dielectric properties and lightweight characteristic. However, the hydrophilicity of graphene may reduce their stability and restrict the applications in moist environment. Herein, a well-designed heterostructure composed of crystalline permalloy core and amorphous iron oxide shell was uniformly adhered on oleylamine-modified graphene nanosheets by a one-pot thermal decomposition method. Compared with the recognized hydrophilic graphene-based hybrid materials, the permalloy@iron oxide/graphene nanocomposites show excellent hydrophobic and water-resistant features with a water contact angle of 136.5°. Besides, the nanocomposites show high-efficiency microwave absorption performance, benefiting from the tunneling effect, polarization, interface interaction, impedance matching condition, and synergistic effect between core-shell permalloy@iron oxide nanoparticles and graphene nanosheets. A broad effective absorption bandwidth with reflection loss (RL) value exceeding -10 dB can be obtained from 4.25 to 18 GHz, covering about 86% measured frequency range when the absorber thickness is 2.0-5.0 mm. Also, the microwave absorption performance of nanocomposites can be tuned by changing the amount of graphene. More importantly, a greatly improved microwave absorption effectiveness of -71.1 dB can be achieved for the nanocomposites in comparison with the bare permalloy@iron oxide nanoparticles (-5.6 dB) and oleylamine-modified GO nanosheets (-3.56 dB). The lightweight and hydrophobic permalloy@iron oxide/graphene nanocomposites with high-efficiency microwave absorption performance are highly promising to improve the environmental adaptability of electric devices, especially in the wet environment.

Light-weight, low-loading and large-sheet reduced graphene oxide for high-efficiency microwave absorber

An atomic layer deposition (ALD) method has been employed to synthesize Fe3O4/graphene and Ni/graphene composites. The structure and microwave absorbing properties of the as-prepared composites are investigated. The surfaces of graphene are densely covered by Fe3O4 or Ni nanoparticles with a narrow size distribution, and the magnetic nanoparticles are well distributed on each graphene sheet without significant conglomeration or large vacancies. The coated graphene materials exhibit remarkably improved electromagnetic (EM) absorption properties compared to the pristine graphene. The optimal reflection loss (RL) reaches −46.4 dB at 15.6 GHz with a thickness of only 1.4 mm for the Fe3O4/graphene composites obtained by applying 100 cycles of Fe2O3 deposition followed by a hydrogen reduction. The enhanced absorption ability arises from the effective impedance matching, multiple interfacial polarization and increased magnetic loss from the added magnetic constituents. Moreover, compared with other recently reported materials, the composites have a lower filling ratio and smaller coating thickness resulting in significantly increased EM absorption properties. This demonstrates that nanoscale surface modification of magnetic particles on graphene by ALD is a very promising way to design lightweight and high-efficiency microwave absorbers.

Strong microwave absorption performance of simply grinding FAPbI3/CNTs composite absorbers

Graphene foams with three-dimensional (3D) network structure, high porosity, and ultralow density have been regarded as lightweight microwave absorption materials. Herein, nitrogen-doped reduced graphene oxide/multi-walled carbon nanotube composite foams were prepared through a two-step strategy of hydrothermal self-assembly and subsequent high-temperature calcination. Morphology analysis indicated that the 3D networks were composed of overlapped flaky reduced graphene oxide. In addition, the influences of nitrogen doping, calcination temperature, and filler ratios on microwave absorption of composite foams were explored. Results manifested that the microwave absorption of composite foams was remarkably improved with the calcination temperature increased. Dramatically, it was noteworthy that the composite foam obtained under 600 °C calcination (bulk density of ∼10.8 mg/cm3) with an 8 wt % mass filler ratio presented the strongest microwave absorption of -69.6 dB at 12.5 GHz and broadest absorption bandwidth achieved 4.3 GHz (13.2-17.5 GHz) at an extremely low matching thickness equal to 1.5 mm. Moreover, the microwave absorption performance could be conveniently adjusted through modifying the thicknesses, filler ratios, and calcination temperature. The excellent microwave absorption performance of as-prepared composite foams was greatly derived from a well-constructed 3D network structure, significant nitrogen doping, enhanced polarization relaxation, and improved conduction loss. This work proposed a new strategy for fabricating graphene-based composites with a 3D network structure as high-efficiency microwave absorbers.

Bimetallic nanoarrays embedded in three-dimensional carbon foam as lightweight and efficient microwave absorbers

In this study, hollow three-dimensional CuS hierarchical microspheres were prepared via a facile galvanic replacement reaction. The hollow flower-like CuS microspheres were characterized by XRD, Raman, XPS, SEM and TEM techniques, which revealed that numerous nanoflakes were self-assembled to construct hollow flowers. Based on time-dependent experiments, a plausible formation mechanism (galvanic replacement reaction) was proposed. Paraffin-based composites, containing 50 wt% hollow CuS, exhibited outstanding microwave absorption capabilities, which were attributed to the suitable impedance match and dielectric loss. The minimal reflection loss was −17.5 dB and the effective bandwidth was 3.0 GHz with thin absorber thickness of 1.1 mm. In addition, we put forward a novel solution to evaluate the microwave absorption efficiency. The high efficiency microwave absorption properties originate from the electric/dielectric polarization and unique hollow flower-like structure. The hollow structures can adjust the dielectric properties to obtain good impedance matching. Moreover, the two-dimensional flakes and hollow flowers can induce more multiple reflection and scattering, which consumes more microwave energy. This study has led to a novel useful method for the design of hollow structures used as high efficiency microwave absorbers.

Magnetic Ni/graphene connected with conductive carbon nano-onions or nanotubes by atomic layer deposition for lightweight and low-frequency microwave absorption

Microwave absorbing materials have attracted much attention in solving electromagnetic interference and pollution problems. Hierarchical cobalt selenides have been obtained through a facile selenization annealing process. The as-prepared samples exhibit distinct reflection losses (RL) and frequency responses via tailoring their crystalline configurations, with excellent absorption in Ku, X, or C band. All of the samples show RL greater than or near -50 dB with effective bandwidths more than 4 GHz, indicating that they may serve as high-efficient and frequency-tunable microwave absorbers. Especially, the sample annealed at 400 °C shows a competitive RL of -62.04 dB at 9.92 GHz with a thickness of 2.25 mm; meanwhile, its effective absorption bandwidth reaches 5.36 GHz with a thickness as small as 1.56 mm. The cobalt selenides as microwave absorbers exhibit a promising prospect applied in complex electromagnetic environments.

3D porous coral-like Co1.29Ni1.71O4 microspheres embedded into reduced graphene oxide aerogels with lightweight and broadband microwave absorption