A novel sponge-like 2D Ni/derivative heterostructure to strengthen microwave absorption performance.

One of the major hurdles of Ni-based microwave absorbing materials is the preparation of two-dimensional (2D) Ni flakes that can improve magnetic anisotropy to tune complex permeability. In this study, we successfully synthesized porous 2D sponge-like Ni/derivative heterostructures composed of Ni, NiO and Ni(OH)2 through a controllable hydrogen reduction method. Thanks to the larger grain size of the Ni/derivative heterostructure prepared at 600 °C (Ni-600) under hydrogen flow, good magnetic properties and high magnetic loss could be obtained, which is beneficial for the enhancement of microwave absorption properties. For the Ni-600 samples, the minimal reflection loss (RL) is -37.3 dB at 7.1 GHz and the effective bandwidth (RL < -10 dB, 90% microwave dissipation) could be tuned in the range of 4.5-18.0 GHz with the thickness of 1.5-4.5 mm. High attenuation ability, including dielectric loss and magnetic loss, and good impedance matching are the requirements for excellent microwave absorption properties. In addition, the porous 2D heterostructure flake structure also significantly contributes to microwave absorption. Multiple reflections and scattering caused by the porous flakes, interfacial polarizations in the heterostructures, tunable impedance matching in the porous structure, strong natural resonance induced by the 2D flakes and plentiful micro-capacitors in the separate flakes account for the enhanced microwave absorption performance. This study demonstrates a fresh exploration of designing novel electromagnetic wave absorbing materials.

Hierarchical porous Ni@boehmite/nickel aluminum oxide flakes with enhanced microwave absorption ability.

In this article, composites consisting of porous Ni cores coated with boehmite/nickel aluminum oxide nanoflakes were successfully prepared by a versatile method. The crystal constituents and shapes of the boehmite/nickel aluminum oxide nanoflakes were strongly influenced by reaction temperature, and their microwave absorption properties were investigated in terms of complex permittivity and permeability. The results reveal that the composites comprising porous Ni cores coated with boehmite/nickel aluminum oxide synthesized at 180 °C present superior absorption properties. The optimal reflection loss is -44.3 dB (>99.99% attenuation) at 14.4 GHz, and the effective absorption (below -10 dB) bandwidth can be monitored in the frequency range of 5.8-18.0 GHz for an absorber with thickness in the range of 1.5-3.5 mm. The high dissipation capability, good impedance match and multiple reflection of the porous flaky structure are responsible for the improvement in microwave absorption. Moreover, a new absorption mechanism was proposed for the porous structure. In this mechanism, the porous structure serves as a spreading container, which attenuates electromagnetic energy by prolonging the travel path and constrains waves in the void space to gradually consume energy. This method paves a new avenue to design porous magnetic-dielectric absorbing materials.

Multifunctional Magnetic Ti3C2Tx MXene/Graphene Aerogel with Superior Electromagnetic Wave Absorption Performance.

Ingenious microstructure design and a suitable multicomponent strategy are still challenging for advanced electromagnetic wave absorbing (EMA) materials with strong absorption and a broad effective absorption bandwidth (EAB) at thin sample thickness and low filling level. Herein, a three-dimensional (3D) dielectric Ti3C2Tx MXene/reduced graphene oxide (RGO) aerogel anchored with magnetic Ni nanochains was constructed via a directional-freezing method followed by the hydrazine vapor reduction process. The oriented cell structure and heterogeneous dielectric/magnetic interfaces benefit the superior absorption performance by forming perfect impedance matching, multiple polarizations, and electric/magnetic-coupling effects. Interestingly, the prepared ultralight Ni/MXene/RGO (NiMR-H) aerogel (6.45 mg cm-3) delivers the best EMA performance in reported MXene-based absorbing materials up to now, with a minimal reflection loss (RLmin) of -75.2 dB (99.999 996% wave absorption) and a broadest EAB of 7.3 GHz. Furthermore, the excellent structural robustness and mechanical properties, as well as the high hydrophobicity and heat insulation performance (close to air), guarantee the stable and durable EMA application of the NiMR-H aerogel to resist deformation, water or humid environments, and high-temperature attacks.

Enhanced Electromagnetic Wave-Absorbing Performance of Magnetic Nanoparticles-Anchored 2D Ti3C2Tx MXene.

Two-dimensional Ti3C2Tx MXene-based hybrids-anchored magnetic metal nanoparticles show a huge potential application as effective wave absorbers due to the synergistic electromagnetic (EM) loss effect. In this work, uniform and size-controllable nickel, cobalt, or nickel-cobalt alloy nanoparticles were in situ grown on the surface of MXene via a facile and moderate co-solvothermal method for the first time. As an example, a nickel nanoparticles-anchored MXene (Ni@MXene) hybrid was homodispersed into dielectric polyvinylidene fluoride to develop its EM wave-absorbing capacity to a great extent. As expected, the results showed strong reflection loss (RLmin = -52.6 dB at 8.4 GHz), broad effective absorption bandwidth (EAB = 3.7 GHz including 71% of X-band), low loading (10 wt % Ni@MXene), and thin thickness (3.0 mm). By adjusting the sample thickness, EAB can cover completely the whole X-band with a maximum of 6.1 GHz, showing a huge potential of Ni@MXene hybrid applying as aircraft stealth coating. The mechanism analyses revealed that the excellent impedance matching, magnetocoupling effect, conductance, magnetic loss, and multiple scatterings contribute to the splendid EM wave-absorbing performance of the Ni@MXene hybrid. Considering the excellent overall performance, the Ni@MXene hybrid was identified as a promising candidate for EM wave absorption.

Promising Ti3C2Tx MXene/Ni Chain Hybrid with Excellent Electromagnetic Wave Absorption and Shielding Capacity.

Electromagnetic (EM) pollution affecting people's normal lives and health has attracted considerable attention in the current society. In this work, a promising EM wave absorption and shielding material, MXene/Ni hybrid, composed of one-dimensional Ni nanochains and two-dimensional Ti3C2Tx nanosheets (MXene), is successfully designed and developed. As expected, excellent EM wave absorption and shielding properties are obtained and controlled by only adjusting the MXene content in the hybrid. A minimum reflection loss of -49.9 dB is obtained only with a thickness of 1.75 mm at 11.9 GHz when the MXene content is 10 wt %. Upon further increasing the MXene content to 50 wt %, the optimal EM shielding effectiveness (SE) reaches 66.4 dB with an absorption effectiveness (SEA) of 59.9 dB. Mechanism analysis reveals that the excellent EM wave absorption and shielding performances of the hybrid are contributed to the synergistic effect of conductive MXene and magnetic Ni chains, by which, the dielectric properties and electromagnetic loss can be easily controlled to obtain appropriate impedance matching conditions and good EM wave dissipation ability. This work provides a simple but effective route to develop MXene-based EM wave absorption and shielding materials. A universal guideline for designing the absorbing and shielding materials for the future is also proposed.

Quick Heat Dissipation in Absorption-Dominated Microwave Shielding Properties of Flexible Poly(vinylidene fluoride)/Carbon Nanotube/Co Composite Films with Anisotropy-Shaped Co (Flowers or Chains).

The facile fabrication of thin flexible electromagnetic interference (EMI) shielding materials with fast heat dissipation for adaptable tuning in both civil and military applications is in urgent demand. In our work, the flexible poly(vinylidene fluoride) (PVDF)/carbon nanotube (CNT) composite films decorated with anisotropy-shaped Co in flowers or chains were prepared and studied. The results showed that by increasing the Co filler contents, the EC (electrical conductivity), TC (thermal conductivity), and EMI shielding properties of such PVDF/CNT/Co (flowers or chains) flexible films were significantly improved. In contrast, the PVDF/CNT/Co-chain flexible films exhibit higher performance with respect to the EC, TC, and EMI shielding properties. Total shielding of 35.3 and 32.2 dB were, respectively, obtained by the PVDF/CNT/6 wt % Co-chain with an EC of 2.28 S/cm and the PVDF/CNT/6 wt % Co-flower with an EC of 1.94 S/cm at a film thickness of 0.3 mm. Possibly owing to the conductive dissipation, interfacial polarization, magnetic loss, multiple reflections, and scattering of EM waves, such flexible composite films possessed a remarkable absorption-dominated EMI shielding behavior. These new composite films with enhanced TC are easily able to transform microwave energy into Joule heating systems, making themselves greatly potential for effective EMI shielding as well as rapid heat dissipation.

Enhancing the microwave absorption properties of amorphous CoO nanosheet-coated Co (hexagonal and cubic phases) through interfacial polarizations.

Core-shell flower-like composites were successfully prepared by a simple polyol method. These composites were formed by coating dual-phased (face-centered cubic [fcc] and hexagonal close-packed [hcp]) Co with amorphous CoO nanosheets. The microwave absorption properties of the flower-like Co@CoO paraffin composites with various Co@CoO amounts were then investigated. Results showed that the paraffin-based composite containing 70wt% flower-like Co@CoO displayed excellent microwave absorption properties (RE=24.74dB·GHz/mm). The minimum reflection loss of -30.4dB was obtained at 16.1GHz with a small thickness of 1.5mm, and 1.5mm bandwidth reached 4.6GHz (13.4-18GHz) below -10dB (90% microwave absorption). The excellent microwave absorption properties of flower-like Co@CoO are attributed to the synergetic effect between magnetic loss and dielectric loss, and the magnetic loss makes a main contribution to absorption. The core-shell flower-like structures with dual Co phases also contributed to microwave absorption. The amorphous CoO nanosheets were able to generate multiple reflections and exhibit scattering. In addition, the novel absorption mechanism that enhanced interfacial polarization was proposed. This enhancement resulted from the presence of interfaces between the hcp and fcc phases and between the core-shell Co@CoO composites.