Nowadays, low-frequency electromagnetic interference (<2.0 GHz) remains a key core issue that plagues the effective attenuation performance of conventional absorption devices prepared via the component-morphology method (Strategy I). According to theoretical calculations, one fundamental solution is to develop a material that possesses a high e' but lower e″. Thus, it is attempted to control the dielectric values via applying an external electrical field, which inducts changes in the macrostructure toward a performance improvement (Strategy II). A sandwich-structured flexible electronic absorption device is designed using a carbon film electrode to conduct an external current. Simultaneously, an absorption layer that is highly responsive to an external voltage is selected via Strategy I. Relying on the synergistic effects from Strategies I and II, this device demonstrates an absorption value of more than 85% at 1.5-2.0 GHz with an applied voltage of 16 V while reducing the thickness to ≈5 mm. In addition, the device also shows a good absorption property at 25-150 °C. The method of utilizing an external voltage to break the intrinsic dielectric feature by modifying a traditional electronic absorption device is demonstrated for the first time and has great significance in solving the low-frequency electromagnetic interference issue.

A Voltage-Boosting Strategy Enabling a Low-Frequency, Flexible Electromagnetic Wave Absorption Device.

With the rapid arising of information technology, microwave absorbing materials (MAMs) are playing an increasingly significant role in electronic reliability, healthcare, and national defense security. Hence, development of high performance MAMs with thin thickness, low density, wide bandwidth, and strong absorption has attracted great interests. Recently, taking graphene as MAMs for high-performance electromagnetic (EM) wave attenuation has grabbed considerable attention, owing to their low density, high specific surface area, strong dielectric loss, and high electronic conductivity. Furthermore, in order to address the interfacial impedance mismatching of the sole graphene materials, incorporation of other lossy materials has been widely studied as the imperative solution to improve its MA performance. In this review, we introduce the theory of microwave absorption and summarize recent advances in the fabrication of graphene-based MAMs, including rational design of the microstructure of pure graphene and tunable chemical integrations with polymers, magnetic metals, ferrites, ceramics, and multicomponents composites. The key point of enhancing MA in graphene-based MAMs is to regulate their EM properties, improve of impedance matching, and create diversified loss mechanisms. Furthermore, the shortcomings, challenges, and prospects of graphene-based MAMs are also put forward, which will be helpful to people working in the related fields.

Graphene-based microwave absorbing composites: A review and prospective

Graphene has been long sought-after over the past few decades because of its multiple functions and broad applications in various fields, such as energy, information, medicine, military equipments, and aerospace. In particular, it has been significantly reported in electromagnetic wave absorbing and shielding fields, promoting it as the most cutting-edge topic. Herein, we systematically comb the structures and electromagnetic functions of graphene hybrids. We demonstrate the advances in the microwave response mechanism, including relaxation, charge transport, magnetic resonance, and eddy currents, as well as magnetic-dielectric synergistic effects. Furthermore, our review mainly focuses on graphene-dispersed systems, flexible graphene papers, graphene hybrids, and 3D graphene architectures.

Graphene nanohybrids: excellent electromagnetic properties for the absorbing and shielding of electromagnetic waves

Lightweight microwave absorbing materials have drawn tremendous attention. Herein, nano-porous biomass carbon materials have been prepared by carbonization with a subsequent potassium hydroxide activation of walnut shells and the microwave absorption properties have also been investigated. The obtained samples have large specific surface areas with numerous micropores and nanopores. The sample activated at 600 °C with a specific surface area of 736.2 m2 g-1 exhibits the most enhanced microwave absorption performance. It has the maximum reflection loss of -42.4 dB at 8.88 GHz and the effective absorption bandwidth (reflection loss below -10 dB) is 1.76 GHz (from 8.08 GHz to 9.84 GHz), corresponding to a thickness of 2 mm. Additionally, the effective absorption bandwidth can reach 2.24 GHz (from 10.48 GHz to 12.72 GHz) when the absorber thickness is 1.5 mm. Three-dimensional porous architecture, interfacial polarization relaxation loss, and the dipolar relaxation loss make a great contribution to the excellent microwave absorption performance. In contrast, the non-activated sample with lower specific surface area (435.3 m2 g-1) has poor microwave absorption performance due to a poor dielectric loss capacity. This comparison highlights the role of micropores and nanopores in improving the dielectric loss property of porous carbon materials. To sum up, porous biomass carbon has great potential to become lightweight microwave absorbers. Moreover, KOH is an efficient activation agent in the fabrication of carbonaceous materials.

Lightweight and efficient microwave absorbing materials based on walnut shell-derived nano-porous carbon

We calculate the double resonant (DR) Raman spectrum of graphene, and determine the lines associated to both phonon-defect processes, and two-phonons ones. Phonon and electronic dispersions reproduce calculations based on density functional theory corrected with GW. Electron-light, -phonon, and -defect scattering matrix elements and the electronic linewidth are explicitly calculated. Defect-induced processes are simulated by considering different kind of idealized defects. For an excitation energy of $\epsilon_L=2.4$ eV, the agreement with measurements is very good and calculations reproduce: the relative intensities among phonon-defect or among two-phonon lines; the measured small widths of the D, $D'$, 2D and $2D'$ lines; the line shapes; the presence of small intensity lines in the 1800, 2000 cm$^{-1}$ range. We determine how the spectra depend on the excitation energy, on the light polarization, on the electronic linewidth, on the kind of defects and on their concentration. According to the present findings, the intensity ratio between the $2D'$ and 2D lines can be used to determine experimentally the electronic linewidth. The intensity ratio between the $D$ and $D'$ lines depends on the kind of model defect, suggesting that this ratio could possibly be used to identify the kind of defects present in actual samples. Charged impurities outside the graphene plane provide an almost undetectable contribution to the Raman signal.

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Theory of double-resonant Raman spectra in graphene: intensity and line shape of defect-induced and two-phonon bands

Constructing and optimizing core@shell structure CNTs@MoS2 nanocomposites as outstanding microwave absorbers

It is well-known that suitable optimization of electromagnetic parameters is a feature to achieve outstanding electromagnetic wave absorption performances (EMWAPs). Accordingly, an effective way to develop high-performance microwave absorbers is to construct dielectric–magnetic core/shell structure nanocomposites with a controllable microstructure and composition parameters. Herein, we propose a simple route to synthesize core/shell structure nanocomposites consisting of carbon nanotubes (CNTs) as the core and magnetic nanoparticles as the shell via a one-step hydrothermal method. By adjusting the experimental parameters, core/shell structure CNTs/CoxFe3−xO4 nanocomposites with different morphologies, contents of CNTs, Co : Fe molar ratios and types of CNTs were selectively synthesized. Also, the electromagnetic properties and EMWAPs of the obtained samples were highly dependent on their microstructure and composition parameters. Thus, by optimizing these parameters, the designed nanocomposites simultaneously exhibited very small values of minimum reflection loss and broad absorption bandwidth with very thin matching thicknesses. Additionally, the outstanding comprehensive EMWAPs of samples were proven to be attributed to their improved impedance matching, attenuation loss characteristics, dielectric loss abilities and excellent synergetic effect. Generally, considering the simple process, environment-friendly, low cost, controllable production and high stability of the nanocomposites, our findings provide an effective way to optimize electromagnetic properties and design high-performance microwave absorbers.

Optimization, selective and efficient production of CNTs/CoxFe3−xO4 core/shell nanocomposites as outstanding microwave absorbers

Lots of research have focused on the core/shell structure nanocomposites owing to their synergistic effect that were considered as the most promising high-performance microwave absorbers (MAs). How...

Positive and Reverse Core/Shell Structure CoxFe3–xO4/MoS2 and MoS2/CoxFe3–xO4 Nanocomposites: Selective Production and Outstanding Electromagnetic Absorption Comprehensive Performance

To effectively utilize the unique properties of layered transition metal dichalcogenide and the attractive morphology of hierarchical flower for the attenuation of electromagnetic wave, herein, high-efficiency flower-like core@shell structure FeSe2@MoSe2 nanocomposites were firstly synthesized through a simple in situ hydrothermal reaction on the surfaces of Fe3O4 nanoparticles with the adequate amounts of Mo and Se sources. The obtained results indicated that the designed flower-like core@shell structure FeSe2@MoSe2 nanocomposites with the filler loadings of 30 wt% and 40 wt% presented the optimal reflection loss (RLopt) value of −59.87 dB at 11 GHz with a matching thickness of 3.10 mm and −60.53 dB at 13.52 GHz with a matching thickness of 2.47 mm. And their corresponding effective frequency bandwidth (fb) values were up to 10.0 GHz with a thickness of 3.66 mm and 6.00 GHz with a thickness of 2.12 mm, respectively. It was worth pointing out that the as-prepared flower-like FeSe2@MoSe2 nanocomposite with filler loading of 30 wt% could simultaneously present very extraordinary electromagnetic wave absorption capabilities and broad absorption bandwidth with the very thin matching thicknesses, which was desirable for high-efficient microwave absorbers. Therefore, a simple and effective strategy was proposed to produce flower-like core@shell structure MoSe2-based nanocomposites, which could be applied as the very desirable candidates for high-performance microwave absorption materials.

Constructing flower-like core@shell MoSe2-based nanocomposites as a novel and high-efficient microwave absorber

Magnetic-dielectric synergy and interfacial engineering to design yolk–shell structured CoNi@void@C and CoNi@void@C@MoS2 nanocomposites with tunable and strong wideband microwave absorption

Ren Xie

Papers

Constructing and optimizing core@shell structure CNTs@MoS2 nanocomposites as outstanding microwave absorbers

Optimization, selective and efficient production of CNTs/CoxFe3−xO4 core/shell nanocomposites as outstanding microwave absorbers

Positive and Reverse Core/Shell Structure CoxFe3–xO4/MoS2 and MoS2/CoxFe3–xO4 Nanocomposites: Selective Production and Outstanding Electromagnetic Absorption Comprehensive Performance

Constructing flower-like core@shell MoSe2-based nanocomposites as a novel and high-efficient microwave absorber

Magnetic-dielectric synergy and interfacial engineering to design yolk–shell structured CoNi@void@C and CoNi@void@C@MoS2 nanocomposites with tunable and strong wideband microwave absorption