Microwave Attenuation Research Articles

Synergistic Dielectric–Magnetic Enhancement via Phase‐Evolution Engineering and Dynamic Magnetic Resonance

AbstractDielectric polarization and magnetic resonance associated with intrinsic constituent and extrinsic structure are two kinds of fundamental attenuation mechanisms for microwave absorbers, but remain extremely challenging in revealing the composition‐morphology‐performance correlation. Herein, hierarchical MXene/metal‐organic framework derivatives with coherent boundaries and magnetic units below critical grain size are constructed to realize synergistic dielectric–magnetic enhancement by phase‐evolution engineering and dynamic magnetic resonance. Specifically, phase‐evolution induced inseparable interfaces, diverse incompatible phases, and defects/vacancies contribute to dielectric polarization, while closely distributed magnetic units simultaneously realize nanoscale multi‐domain coupling and long‐range magnetic interaction. As results, the hierarchical derivatives promise an exceptional reflection loss of −59.5 dB and an effective absorption bandwidth of 6.1 GHz. Both experimental results and theoretical calculations indicate that phase‐evolution engineering and dynamic magnetic resonance maximize the absorption capability and demonstrate a versatile methodology for manipulating microwave attenuation. More importantly, the proposed multi‐domain coupling and long‐range magnetic interaction theories innovatively offer dynamic magnetic resonance mechanism for magnetic loss within critical grain size.

Optimized electromagnetic wave absorption of α-Fe2O3@MoS2 nanocomposites with core–shell structure

Core–shell structures and interfacial polarization are of great significance to meet the diversified requirements of microwave attenuation. Herein, α-Fe2O3@MoS2 nanocomposites are fabricated via a simple two-step hydrothermal process, in which MoS2 nanosheets as the shell are self-assembled and α-Fe2O3 microdrums are used as the core to constitute a special flower-like morphology with core–shell structure. This structure can provide more interface contact to achieve strong interfacial polarization and possibly offer more multiple reflection and scattering of electromagnetic waves. Furthermore, the microwave dissipation performances of α-Fe2O3@MoS2 nanocomposites can be significantly improved through construction of core–shell structure and flower-like morphology, controlling the content of α-Fe2O3 microdrums and adjusting the filler loading ratios. This work proves that the as-synthesized nanocomposites achieve excellent effective absorption bandwidth and outstanding electromagnetic wave absorption capabilities due to their special interfaces, core–shell structures and good impedance matching conditions. Therefore, α-Fe2O3@MoS2 nanocomposites are expected to be a novel and desirable candidate for high-performance electromagnetic wave absorbers.

Experimental Study on the Damage Characteristics and Acoustic Properties of Red Sandstone with Different Water Contents under Microwave Radiation.

Rock breaking is one of the most basic issues in deep underground engineering. Water plays an important role in the rock response under microwave radiation. Consequently, microwave radiation experiments using red sandstone with different water contents were conducted. The damage characteristics and ultrasonic properties of red sandstone after microwave radiation were primarily investigated, and the representative conclusions were drawn as follows: With the increase in water content, the time of complete formation of the rupture surface of the rock sample gradually decreased, and the decreasing range gradually increased. When the fracture surface is completely formed, the samples with a higher water content have more powdery rock cuttings and less surface roughness. The damage degree of the samples does not increase significantly with the increase in the water content when the sample is radiated at the same time. As the microwave radiation time is increased, the damage degree of the sample will increase significantly. Through the ultrasonic velocity test, it can be suggested that the sample exhibits obvious zonal damage characteristics under the action of a microwave. Generally speaking, it is a very effective means of improving the degree of microwave attenuation of the rock by increasing the water content of the rock mass.

Clathrate-like porous graphene/TiO2 composite with strong dielectric polarization for electromagnetic microwave absorption

Porous carbon frameworks functionalized with multi-scale particles are an important approach in highly efficient microwave absorbent design since impedance matching and polarization/conductivity loss are well balanced. The absorbent with a versatile structure that is not easily deformed within application is still in need. In this work, open-cell porous graphene (PG) functionalized with TiO2 nanorods is synthesized by the solvothermal method, creating a high specific surface area structure. Complex electromagnetic parameters indicate that the introduction of TiO2 nanorods improves the microwave absorption (MA) performance by optimizing the impedance matching and enhancing the polarization relaxation due to the large numbers of PG@TiO2 interfaces. Polarization loss plays a dominant role in microwave attenuation. In addition, the designed clathrate-like structure is anticipated to further attenuate the MA wave by multiple reflections and scatterings. Interface polarization and dipole polarization can be identified with frequencies in Cole–Cole plotting. The optimized PG@TiO2 composite with a thickness of 1.92 mm exhibits a minimum reflection loss of −55.10 dB at 14.29 GHz with a low filler ratio of 10 wt. %. This work is heuristic in the absorbent structure design and is helpful in enhancing and identification of polarization relaxation loss.

Generation of Defective Few-Layered Graphene Mesostructures by High-Energy Ball Milling and Their Combination with FeSiCuNbB Microwires for Reinforcing Microwave Absorbing Properties

Defective few-layered graphene mesostructures (DFLGMs) are produced from graphite flakes by high-energy milling processes. We obtain an accurate control of the generated mesostructures, as well as of the amount and classification of the structural defects formed, providing a functional material for microwave absorption purposes. Working under far-field conditions, competitive values of minimum reflection loss coefficient (RLmin) = -21.76 dB and EAB = 4.77 dB are achieved when DFLGMs are immersed in paints at a low volume fraction (1.95%). One step forward is developed by combining them with the excellent absorption behavior that offers amorphous Fe73.5Si13.5B9Cu1Nb microwires (MWs), varying their filling contents, which are below 3%. We obtain a RLmin improvement of 47% (-53.08 dB) and an EAB enhancement of 137% (4 dB) compared to those obtained by MW-based paints. Furthermore, a fmin tunability is demonstrated, maintaining similar RLmin and EAB values, irrespective of an ideal matching thickness. In this scenario, the Maxwell-Garnet standard model is valid, and dielectric losses mainly come from multiple reflections, interfacial and dielectric polarizations, which greatly boost the microwave attenuation of MWs. The present concept can remarkably enhance not only the MW attenuation but can also apply to other microwave absorption architectures of technological interest by adding low quantities of DFLGMs.

Enhanced Magnetic and Microwave Absorbing Properties of Nd3+ Ion Doped CoFe2O4 by Solid‐State Reaction Method

CoNdxFe(2−x)O4 (x = 0.0–0.05) spinel ferrites are successfully synthesized by mechanical milling using solid‐state reaction. The effects of Nd3+ ions on structural, microstructure, and magnetic and microwave‐absorbing properties of samples are analyzed. The X‐ray diffraction pattern shows a single‐phase cobalt ferrite for all compositions. Refinement of the structure shows that an increase in the concentration of Nd3+ ions leads to the structure parameter and volume of unit cell to decrease. Lattice strain arises in the crystal, and limiting the crystallite growth is confirmed from the Williamson–Hall analysis. The particle size of the doping samples decreases compared to that without doping. The magnetic parameters, namely coercivity field and saturation magnetization, are strongly affected by the insertion of Nd3+ ions. Another important role of Nd3+ ions is in tuning the performance of microwave absorption. The maximum reflection loss values are gradually increased from −12.65 dB (≈94.35% microwave attenuation) up to −28.57 dB (≈99.86% microwave attenuation) in the concentration of x = 0.0–0.05. Their effective bandwidth also shows an increase from 0.05 up to 1 GHz. The results exhibit that Nd3+ ion‐doped cobalt ferrite has potential properties in designing novel microwave‐absorbing materials.

Polyindole-Derived Nitrogen-Doped Carbon Nanosphere/Al2O3 Composites with High-Performance Microwave Absorption

In this study, polyindole-derived nitrogen-doped carbon nanosphere/Al2O3 (PINNC/Al2O3) composites are fabricated by hydrothermal, in situ aggregation and high-temperature pyrolysis using potassium aluminum sulfate dodecahydrate, urea, and indole as raw materials. The results show that the introduction of alumina modulates the impedance matching and results in the formation of a great deal of interfaces with polyindole-derived nitrogen-doped carbon nanospheres, which promotes the reinforcement of interfacial polarization. Moreover, the unique semicircular spherical structure of alumina and the pleated structure of carbon nanospheres facilitate multiple microwave attenuation. The abundance of in situ nitrogen atom doping in the carbon material and the defects that formed after pyrolysis can supply a great deal of polarization sites for dipole polarization, thus enhancing dipole polarization loss. As the matching thickness reached 2.5 mm, the minimum reflection loss (RL) of the PINNC/Al2O3 composites is −46.08 dB at 16.72 GHz (fill ratio of 0.5). And as the matching thickness is merely 3.0 mm, the effective absorption bandwidth (EAB) is 7.04 GHz. This study can serve as a good guide for the devising of polyindole-derived nitrogen-doped carbon-based electromagnetic microwave absorbents.

Estimating leaf moisture content at global scale from passive microwave satellite observations of vegetation optical depth

Abstract. The moisture content of vegetation canopies controls various ecosystem processes such as plant productivity, transpiration, mortality, and flammability. Leaf moisture content (here defined as the ratio of leaf water mass to leaf dry biomass, or live-fuel moisture content, LFMC) is a vegetation property that is frequently used to estimate flammability and the danger of fire occurrence and spread, and is widely measured at field sites around the globe. LFMC can be retrieved from satellite observations in the visible and infrared domain of the electromagnetic spectrum, which is however hampered by frequent cloud cover or low sun elevation angles. As an alternative, vegetation water content can be estimated from satellite observations in the microwave domain. For example, studies at local and regional scales have demonstrated the link between LFMC and vegetation optical depth (VOD) from passive microwave satellite observations. VOD describes the attenuation of microwaves in the vegetation layer. However, neither were the relations between VOD and LFMC investigated at large or global scales nor has VOD been used to estimate LFMC. Here we aim to estimate LFMC from VOD at large scales, i.e. at coarse spatial resolution, globally, and at daily time steps over past decadal timescales. Therefore, our objectives are: (1) to investigate the relation between VOD from different frequencies and LFMC derived from optical sensors and a global database of LFMC site measurements; (2) to test different model structures to estimate LFMC from VOD; and (3) to apply the best-performing model to estimate LFMC at global scales. Our results show that VOD is medium to highly correlated with LFMC in areas with medium to high coverage of short vegetation (grasslands, croplands, shrublands). Forested areas show on average weak correlations, but the variability in correlations is high. A logistic regression model that uses VOD and additionally leaf area index as predictor to account for canopy biomass reaches the highest performance in estimating LFMC. Applying this model to global VOD and LAI observations allows estimating LFMC globally over decadal time series at daily temporal sampling. The derived estimates of LFMC can be used to assess large-scale patterns and temporal changes in vegetation water status, drought conditions, and fire dynamics.

Polarization genes dominated heteroatom-doped graphene aerogels toward super-efficiency microwave absorption

The cellular structure of heteroatom-doped graphene aerogel improves the impedance matching, and the polarization gene dominated cell walls generate a great quantity of polarization sites, synergistically promoting its microwave attenuation.

Metal-organic framework‑derived carbon‑based composites for electromagnetic wave absorption: Dimension design and morphology regulation

• The recent progress of dimension design and morphology regulation of MOF-derived carbon-based MAMs has been summarized. • The effectiveness of dimension and morphology design has been demonstrated in tuning the EMW absorption performance of MAMs. • The fundamental dimension-morphology-function relationship is discussed based on the selected representative examples. • The facing challenges and future perspective of dimension and morphology engineering for MAMs are rationally proposed. Developing highly efficient microwave absorbing materials (MAMs) to ameliorate the electromagnetic (EM) response and facilitate energy absorption is crucial in both the civil and military industries. Metal-organic framework (MOF) derived nanoporous carbon composites have emerged as advanced MAMs owing to their rich porosity, tunable compositions, facile functionalization, and morphology diversity. Together with the flourishing development of composition-tuning strategy, the rational dimension design and elaborate control over the architectures have also evolved into an effective approach to regulating their EM properties. Herein, we provide a comprehensive review of the recent advances in using dimension and morphology modulation to adjust the microwave attenuation capacities for MOF-derived carbon composites. The underlying design rules and unique advantages for the MAMs of various dimensions were discussed with the selection of representative work, providing general concepts and insight on how to efficiently tune the morphologies. Accordingly, the fundamental dimension-morphology-function relationship was also elucidated. Finally, the challenges and perspectives of dimension design and morphology control over MOF-derived MAMs were also presented.

Microwave absorption property of GO-Fe/FeO-NiO HNFs: GO decorated Fe/FeO-NiO hexagonal flakes with a 2D/0D/2D structure.

In this study, Fe/FeO-NiO HNFs in which 2D NiO hexagonal nanoflakes (NiO HNFs) were decorated by 0D Fe/FeO NPs were prepared by a facile hydrothermal method. Then, 0D/2D Fe/FeO-NiO HNFs were loaded on 2D GO sheets in three different weight ratios of GO (1 : 3), (1 : 4), and (1 : 5) to Fe/FeO-NiO HNFs and a novel GO-Fe/FeO-NiO HNF composite with a 2D/0D/2D structure was successfully produced. TEM images revealed the interesting morphology of the GO-Fe/FeO-NiO HNF composite in which individual FeO NPs with a narrow size distribution (∼15 nm) were arranged in hexagonal NiO nanoflakes, decorated on the GO substrate. Since the morphology of nanomaterials has an important effect on their microwave absorption properties, designing a composite with an asymmetric morphology, which is the combination of zero, one, and two-dimensional nanostructures can be very efficient for adjusting the microwave absorption property. The microwave absorption ability of GO-Fe/FeO-NiO HNF composites was surveyed. All samples of Fe/FeO-NiO HNF composites exhibited superior microwave attenuation performance in terms of reflection loss with a suitable bandwidth. The minimum reflection losses for GO-Fe/FeO-NiO HNFs (1 : 3), (1 : 4), and (1 : 5) reached -75.22, -53, and -18 dB, respectively, and the effective absorption bandwidths (RL ≤ -10 dB) for GO-Fe/FeO-NiO HNFs (1 : 3), (1 : 4), and (1 : 5) were 2, 3 and 3.2 GHz, respectively.

Rational design of hierarchical yolk-double shell Fe@NCNs/MnO2 via thermal-induced phase separation toward wideband microwave absorption

Rational design of yolk-shell architectures with synergistic effect are vital for improving microwave attenuation ability, but still face challenges. In this work, we employ the thermal-induced phase separation engineering and redox strategy to fabricate hierarchical yolk-double shell Fe@NCNs/MnO2, in which magnetic Fe units are individually confined in N-doped carbon nanocubes (NCNs) cavity and hierarchical MnO2 nanosheets are intimately anchored on NCNs surface. It is found that the phase separation process between Prussian blue and poly-dopamine is significantly determined by the thermal-induced inward contraction owing to their different thermal stabilities and independent crystal structures. As results, the hierarchical composites exhibit matched impedance and wideband microwave absorption owing to the internal cavity architectures, multiple hetero-interfaces, dielectric-magnetic synergistic effect and hierarchical conformations. The minimum reflection loss reaches −46.7 dB with 3.3 mm thickness, and more importantly, the effective bandwidth (<-10 dB) achieves as strong as 10.8 GHz at 3.1 mm. This work opens up a novel and meaningful thinking for the construction of yolk-double shell architectures and the synthesized hierarchical composites can be used as promising candidates in addressing electromagnetic pollution.

Arylene Diimide Derivatives as Anolyte Materials with Two-Electron Storage for Ultrastable Neutral Aqueous Organic Redox Flow Batteries

Arylene Diimide Derivatives as Anolyte Materials with Two-Electron Storage for Ultrastable Neutral Aqueous Organic Redox Flow Batteries

Microwave Attenuation in Wireless Link Caused by Sand and Dust Storms

In this letter, the attenuation by spheroidal sand particles under different relative humidity ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RH</i> ) is investigated with consideration of contributions of water vapor and excess charges carried by particles, which is much closer to real conditions. The microwave (MW) attenuation will be enhanced when the sand particles are considered as the spheroids. The contributions of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RH</i> and excess charges to the MW attenuation are much greater by the spheroidal sand particles than that by the equal-volume spherical ones. The increasing effects of excess charges and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RH</i> on the MW attenuation are mutually reinforced. The contribution of water vapor to the MW attenuation by sand–dust storms is significant when the visibility is high, which makes up the gap between the experimental results and calculated results. An improved attenuation model by sand and dust storms is given with an incident wave frequency of 4–45 GHz.

MOF-derived core-shell Co9S8 @MoS2 nanocubes anchored on RGO to construct heterostructure for high-efficiency microwave attenuation

Elegant material construction to realize high-efficiency microwave absorption with low filler loading is in strong demand yet remains challenging. Here we combine a self-assembly method followed by pyrolysis to fabricate core-shell Co9S8 @MoS2 nanocubes, which were uniformly anchored on reduced graphene oxide nanosheets (A-G/Co9S8 @MoS2) to form hierarchical heterostructures. The heterostructure was deliberately designated via chemical etching of in situ formed Co9S8 derived from ZIF-67 and decoration of flower-like MoS2 nanosheets. Owing to its synergic effect in compositions and well-designed heterostructure, the product A-G/Co9S8 @MoS2 exhibits excellent microwave absorption performance with low loading content. A minimum reflection loss of − 55.13 dB and effective absorption bandwidth of 6.61 GHz with filler loading of only 8 wt% was achieved, contributed by the good impedance matching and dual loss mechanism. This work provides a new pathway for designing low-filler loading high-performance microwave absorption materials.

Willow catkins–derived Co/C nanoparticles anchored on porous carbon fibers with lightweight and enhanced microwave absorption performances

In this work, a self-polymerization route was adopted to construct willow catkins/Co-based metal–organic frameworks, which was followed by subsequent sintering to fabricate composites with Co/C nanoparticles anchored on porous carbon fibers. The annealing temperature for the composites had a significant influence on the tuning of the chemical composition, micromorphology, and absorption performance. The sample annealing at 700℃ exhibited excellent microwave absorption performance, in which the effective absorption bandwidth (reflection loss ≤−10 dB) was up to 5.44 GHz with a thickness of 1.7 mm and filling content of 25 wt%. The key factor for realizing excellent absorption was ascribed to its moderate microwave attenuation ability and better impedance matching. This work could contribute to the design of novel biomass-derived microwave absorption materials with low thickness, large bandwidth, and lightweight.

Effect of heat treatment on the structure and electromagnetic properties of FeSiAl films

FeSiAl films were prepared by magnetron sputtering and heat-treated in different atmospheres. The films were characterized by SEM, XRD, XPS, VSM and VNA. Relationship between the composition, microstructure and static magnetic and electromagnetic properties of FeSiAl films was investigated. The results showed that an insulating oxide layer was formed on the surface of heat-treated FeSiAl films. The oxide layer significantly affects the resistivity, saturation magnetization strength and electromagnetic properties of films. The films heat-treated in vacuum show relatively high resistivity and excellent electromagnetic properties with saturation magnetization strength of 2190 emu/cm3 and resistivity of 250.5 μΩ/cm. The ferromagnetic resonance frequency of the film is 1.8 GHz. The extrema of the imaginary part of the complex permeability reaches 1699, which is nearly 7.4 times higher than that of the as-deposited FeSiAl film, showing excellent microwave attenuation performance.

Induced Crystallization-Controllable Nanoarchitectonics of 3D-Ordered Hierarchical Macroporous Co@N-Doped Carbon Frameworks for Enhanced Microwave Absorption.

The construction of ordered hierarchical porous structures in metal-organic frameworks (MOFs) and their derivatives is highly promising to meet the low-density and high-performance demands of microwave absorption materials. However, traditional methods based on sacrificial templates or corrosive agents inevitably suffer from the collapse of the microporous framework and the accumulation of nanoparticles during the carbonization transformation, resulting in the deteriorating impedance match, which greatly limits the incident and attenuation of microwaves. Herein, an induced crystallization and controllable nanoarchitectonics strategy is employed to replace traditional growing-etching methods and successfully synthesize carbonized 3D-ordered macroporous Co@N-doped carbon (3DOM Co@NDC) based on the 3D-ordered template. The obtained 3D-ordered macroporous structure ensures the stable growth of hybrid carbon frameworks and CoC nanoparticles without collapse, preserves abundant interfaces for both the incident and attenuation performance, so as to significantly improve the impedance matching and absorption properties compared to conventional MOFs derivatives. The minimum reflection loss of 3DOM Co@NDC is -57.36dB at the thickness of 1.9mm, and the effective bandwidth is 7.36GHz at 1.6mm. Moreover, the innovative strategy to prepare 3D-ordered hierarchical macroporous structures opens up a new avenue for advanced MOFs-derived absorbers with excellent performance.

Highly effective and tunable microwave absorber integrating multiscale attenuation behaviours derived from prussian blue analogue/graphene oxide aerogel

Tunable and efficient absorption of graphene-based microwave absorbers are essential on the realms of electromagnetic compatibility and protection in various application scenarios. However, challenges arise owing to their limited microwave attenuation behaviors. Herein, CoNiFe Prussian blue analogue (PBA)-derived magnetic alloy@carbon nanocubes anchored on N-doped reduced graphene oxide (rGO) aerogels were achieved via CoNiFe-PBA nanocubes assisting assembly of GO and subsequent thermal annealing approach. Such three-dimensional (3D) graphene-based macroscopic architecture integrates multiple attenuation behaviours occurred across multiple length scales. Attributed to the synergy of multiple scattering, conduction loss, multiple heterogeneous interface and dipolar polarizations, and magnetic loss, the optimized CoNiFe-PBA/GO aerogel derivative simultaneously exhibits strong reflection loss and wide effective bandwidth with an ultralow filling content (1.1 wt%) at both X band (−66.01 dB and 5.2 GHz at 3.2 mm) and Ku band (−66.23 dB and 6.6 GHz at 2.6 mm). Multiscale assembly strategy of graphene-based electromagnetic functional materials from molecular level to macroscale proposed and demonstrated by this work shows promise for exploring tunable and efficient microwave absorbers.

Hierarchical core-shell FeS2/Fe7S8@C microspheres embedded into interconnected graphene framework for high-efficiency microwave attenuation

With the imminent era dominated by high-tech electronic products, the development of high-efficiency electromagnetic wave absorption materials has become a critical task. Rational component regulation and structural design are considered to be effective methods to optimize the electromagnetic wave absorption performance of materials. Herein, we first prepared a Fe-PBA@ PDA@GO precursor via solvothermal method; then, vulcanization was used to develop a FeS2/Fe7S8@C@rGO composite with a core-shell heterostructure, in which core-shell FeS2/Fe7S8@C microspheres were embedded on the wrinkled reduced graphene oxide layer. FeS2/Fe7S8@C@rGO, as a multi-component composite, demonstrates superior impedance matching and can thus capture more electromagnetic waves, while the multiple losses facilitate the dissipation of incident electromagnetic waves. Concretely, FeS2/Fe7S8@C@rGO can display significant attenuation of incident electromagnetic waves through multi-interface polarization, dipole polarization, resonance loss and eddy current loss. Additionally, the three-dimensional hierarchical structure not only improves the conductive loss, but also promotes multiple reflections and scattering. The FeS2/Fe7S8@C@rGO composite exhibits a minimum reflection loss of −62.7 dB and an effective absorption broadband (6.1 GHz) with a filler loading of 20 wt% at 2.2 mm, and the effective absorption bandwidth reaches 7.6 GHz at a layer thickness of 2.5 mm. These results provide a valuable reference for the preparation of high-performance microwave absorbers through composition adjustment and controlled structural design.