Excellent Microwave Absorption Research Articles

Microwave‐Assisted Fabrication of Highly Crystalline, Robust COF Membrane for Organic Solvent Nanofiltration

AbstractFabrication of crystalline, robust covalent organic framework (COF) membranes based on disorder‐to‐order strategy is promising yet highly challenging. Herein, a microwave‐assisted method for fabricating COF membranes is proposed. Initially, monomers polymerize rapidly on the surface of porous Al2O3 substrate at room temperature to form an amorphous pristine membrane. Subsequently, a microwave field is exerted to trigger fast crystallization, acquiring a crystalline COF membrane within 60 min. The amorphous pristine membrane exhibits a high dissipation factor, indicating excellent microwave absorption capability, which accelerates the dynamic reversible reactions during the microwave treatment and thus ensures a rapid transition from the amorphous to the crystalline state. Owing to the high‐crystallinity and robust structure, the COF membranes exhibit high rejection rates for solute molecules with molecular weights exceeding 700 Da (e.g., Evans blue: 98.7%) and high solvent permeance for organic solvents (e.g., ethanol: 87.8 Lm−2h−1 bar−1, n‐hexane: 222.3 Lm−2h−1 bar−1). Surprisingly, the COF membranes exhibit superior mechanical properties, with Young's modulus of 33.91 ± 3.94 GPa, outperforming previously reported polycrystalline COF membranes and are close to those for inorganic zeolite membranes. The microwave‐assisted COF crystallization method opens a new avenue to fabricating a variety of crystalline membranes for advanced separations.

Effects of La3+ doping on the structural, vibrational, elemental, and dielectric characteristics of Sr-Ba-Zr2 R-type hexaferrite

Hexagonal ferrites possess physicochemical and dielectric features that make them highly favorable for absorption purposes. Due to its excellent characteristics at high frequencies, R-type hexagonal ferrite is specifically designed for efficient microwave absorption. Samples of R-type hexaferrite were prepared using the sol-gel auto-combustion process. The samples had the chemical composition Sr0.7Ba0.3Zr2Fe4-xLaxO11, where x ranged from 0.00 to 0.09 in increments of 0.03. This replacement seeks to enhance the understanding of how the La3+ ions affect the physicochemical characteristics of these ferrites, such as their structure, dielectric properties, XPS, and FTIR properties. The single phase of R-type hexaferrite is confirmed by the FTIR and XRD methods. The crystallite size of the samples of R-type hexaferrite doped with La was determined using three different methods. The crystallite size, determined using Sherrer's formula, ranged from 25.865 to 41.393 nm. Meanwhile, the size calculated using Williamson's method was between 36.084 nm and 62.32 nm. The Size-strain analysis revealed a size range of 24.26 nm to 27.31 nm. The FTIR spectra exhibit two intrinsic vibrational bands, labelled as υ1 and υ2, in the wavenumber range of 400 to 600 cm-1. The XPS examination verifies the elemental composition of a representative specimen that has been swapped with La. As the La3+ content increases, additional semicircles are formed, suggesting the existence of more mechanisms for polarization relaxation and a greater capacity for polarization loss. Out of all the compositions that were created, the material with a composition of x = 0.09 exhibits the highest absorption capabilities. This is because of its minimal reflection loss. By replacing the La3+ element in R-type hexaferrites, the peak of the sample moves towards the higher frequency range, indicating excellent microwave absorption properties. R-type hexaferrites are utilized in transformers to reduce eddy current losses and in microwave absorption devices.

One-step solvothermal synthesis of hollow Fe3O4/single walled carbon nanohorns composites with excellent microwave absorption properties

One-step solvothermal synthesis of hollow Fe3O4/single walled carbon nanohorns composites with excellent microwave absorption properties

Poly(Vinylidene Fluoride‐Hexafluoropropylene) Composites With Carbon Based Nanofillers for Electromagnetic Interference Shielding

ABSTRACTIn the current work, a series of flexible polymer matrix composites (PMCs) based on poly(vinylidene fluoride‐hexafluoropropylene) (PVDF‐HFP) as the polymer matrix and carbon nanofillers, namely single‐walled nanotubes (SWCNT) or carbon blacks (CB), were fabricated via a facile preparation method of blending and solution casting. The electrical conductivity, morphology, dielectric constant, mechanics, and electromagnetic interference (EMI) shielding properties of the composite films were adjusted by varying the carbon type (CB = Ketjenblack, Vulcan, or SWCNT = Tuball) and amount (0.1–15 wt%). The influence of the conductive nanofillers on the EMI shielding effectiveness (SE) and microwave absorbing properties in the X‐band frequency range (8–12 GHz) were investigated. By increasing the carbon content, the EMI shielding properties of the composite films were improved due to an increase in the magnitude of the electromagnetic properties. As the loading of these carbon fillers approach the percolation threshold, the dielectric constant of the composite can dramatically increase because of the construction of a microcapacitor network. However, these nanofillers/polymer composites generally have high dielectric loss near the percolation threshold due to the direct connection between carbon nanofillers. The PVDF‐HFP/Tuball composite particularly contributed low dielectric loss to the electromagnetic (EM) wave, resulting in excellent microwave absorption properties with a SE per unit thickness of 337.5 dB/mm and 2.5× less weight compared with conventional aluminum with the same SE, demonstrating the promise of using these materials as practical EMI shields.

Multifunctional Ti3C2Tx/Fe3O4/glass fiber paper composites for flexible microwave absorption materials with ultralow filling ratio

The preparation of multifunctional microwave absorbing materials with the characteristics of stronger absorption, wider bandwidth, lighter weight, higher strength and thinner thickness is an urgent research direction. Due to the difficulty of controllable distribution of absorbers, excellent microwave absorption performance is usually accompanied by large absorber amount and thickness, which can result in microwave absorption composite materials lack flexibility, mechanical properties, and adaptability to the environment. In this study, Ti3C2Tx/Fe3O4/glass fiber paper composite was prepared by vacuum-assisted filtration. The controllable pore structure constructed by glass fiber enables uniform distribution of MXene/Fe3O4 and effectively spreading of MXene monolayer film, enhancing conductivity loss and interface loss. By controlling the diameter of glass fiber, the pore size and the distribution of absorber can be regulated, thereby achieving a minimum reflection loss of up to −57.2 dB at an ultra-low filling ratio of 1.66 wt%. With the addition amount of 1.90 wt%, the effective absorption bandwidth can cover the whole X-band. The ultra-low specific reflection loss exceeds most of fabric materials and MXene-based absorbers. Meanwhile, the composite material exhibited excellent mechanical properties with a tensile strength of 3.054 kN/m and a burst resistance of 290 kPa. In addition, the adhesion between the filler and the glass fiber was increased by adding acrylic resin-based aqueous adhesive, and the surface grafting functional group (-F) made the composite material superhydrophobic. This work provides a new strategy to synthesize multifunctional absorbing material with low filling ratio and strong environmental practicality.

Metal organic frameworks derived core-shell structured C@TiC nanocomposites with excellent microwave absorption performance

With aim to prepare nano-microwave absorption material with excellent microwave absorption performance, core-shell structured C@TiC nanocomposites with tunable nanostructures and morphologies were successfully synthesized through one-step pyrolysis of the Ti-based MOFs precursors at a low temperature. Effects of various metal/linker ratio, solvent types and Hacac addition on the microstructures and properties of the C@TiC nanocomposites were thoroughly investigated, demonstrating that the TiC core-C shell structure could be effectively tailored. Compared to pure TiC nanoparticles, the C@TiC nanocomposites exhibited significantly improved microwave absorption performance, including the stronger RL peak of -35.64 dB (10.72 GHz) at 2.4 mm thicknesses and the enhanced effective microwave wave absorption width (EAB, RL≤-10 dB) spanning the entire C-band and X-band, which is ascribed to the better impedance matching and richer microwave loss mechanisms. As a result, C@TiC nanocomposites show great potential to be applied as absorbers with strong microwave absorption and wide absorption bandwidth.

Fabrication of hollow Fe3O4 microspheres encapsulated by single-walled carbon nanohorns for high-performance microwave absorption

Due to the increasingly serious electromagnetic radiation and pollution problems, it is urgent to develop high-performance electromagnetic wave absorbing materials. Herein, the hollow Fe3O4 microspheres encapsulated by single-walled carbon nanohorns (H-Fe3O4@SWCNHs) have been synthesized via facile solvothermal approach. The obtained H-Fe3O4@SWCNHs composite exhibits a unique hollow core-shell structure and superior microwave absorption performance. When the matching thickness is 2.0 mm, the composite shows a minimum reflection loss value of −42.2 dB at 13.6 GHz and a maximum effective absorption broadband value of 6.2 GHz covering from 11.8 to 18 GHz. The excellent microwave absorption performance of the H-Fe3O4@SWCNHs composite is mainly attributed to the unique hollow core-shell structure, optimal impedance matching, and synergistic effects of dielectric loss and magnetic loss. Therefore, the present work provides a new kind of ideal microwave absorbing material with light weight, thin thickness, wide bandwidth and strong absorption capability for high-performance microwave absorption.

Achieving excellent microwave absorption performance in ultralight Ti3C2T MXene with M−O bonds (M = Fe, Co, Ni) as surface terminating groups

Achieving excellent microwave absorption performance in ultralight Ti3C2T MXene with M−O bonds (M = Fe, Co, Ni) as surface terminating groups

Fe2O3-decorated multiwall carbon nanotube composites for boosted microwave absorption

Developing microwave absorption (MA) materials with a strong absorption ability over a wide bandwidth through a simple and environmentally friendly approach remains a tremendous challenge. Herein, we propose to use an Fe3+–tannic acid framework to assist the dispersion of multi-walled carbon nanotubes (MWCNTs) and successfully prepare MWCNTs/porous carbon/α-Fe2O3 composites through freeze-drying and subsequent heat treatment. The dielectric properties and MA performance can be regulated by the heat treatment temperature, which leads to tunable crystalline structure, composition, and graphitization degree of MWCNTs. Consequently, the MWCNTs/porous carbon/α-Fe2O3 composite heat-treated at 300 °C exhibits a high reflection loss (RL) of −58.9 dB and an effective absorption bandwidth (5.28 GHz) with a matched thickness of 2.26 mm at a filler proportion of only 5 wt%, and the related frequency bandwidth with RL below −10 dB reaches 14.3 GHz at a thickness of 2–5 mm. In conclusion, the balance between conduction and polarization loss endows the composite with excellent impedance matching and boosting MA performance. This study offers a guideline for fabricating excellent MA materials through a simple, environmentally friendly method.

Efficient Nanofibrous Electromagnetic Wave Absorber Inspired by Spider Silk Hunting.

With the rapid advancements in wireless communication and radar detection technologies, there has been a significant uptick in the utilization frequency of electromagnetic waves across both civilian and military sectors, consequently generating substantial electromagnetic radiation and interference. These electromagnetic pollutants present a considerable threat to public health and information security. Consequently, materials capable of absorbing and mitigating electromagnetic pollution have garnered significant attention.The nanomaterials with multiple components and various heterogeneous interfaces have been considered as preferred efficient electromagnetic wave (EMW) absorbers. In this work, carbon nanofibers doped with magnetic metal oxide particles (Co/Ni-CNFs) are prepared by electrospinning and heat treatment. In these samples, the minimum reflection loss can reach -40.13dB at 4.6mm, and the widest effective absorption bandwidth is 4.4GHz. The excellent microwave absorption is attributed to the appropriate impedance matching. The electromagnetic parameters of Co/Ni-CNFs are balanced by adjusting the concentration of magnetic metal-organic framework material (MOFs) in the precursor solution. It is believed that this work can provide a reference for developing lightweight, flexible, and robust electromagnetic wave-absorbing materials.

Fabrication of ultra-high strength MWCNTs/CI /PI rigid composite foam with excellent microwave absorption performance by pressure foaming method

Polyimide (PI) foam exhibits excellent high-temperature resistance and outstanding physical and chemical stability, making it an ideal matrix for absorbing materials. However, the growing demand for absorbing materials that can serve as load-bearing components has revealed that existing polyimide composite foams often fall short of practical application requirements due to their low mechanical properties. The study employed a simple foaming method using isocyanate-based polyimide as the foam base under a 0.2 MPa atm. Multi-walled carbon nanotubes (MWCNTs) and carbonyl iron (CI) powder were added sequentially to the precursor solution. Under the filling parameters of 2.0 % MWCNTs and 10 % CI powder, the compressive strength of polyimide composite foam reaches 16.2 MPa and exhibits excellent microwave absorption properties, with a minimum reflection loss (RLmin) of −62.51 dB and an effective absorption bandwidth (EAB) of 7.86 GHz. Additionally, the prepared composite foam has potential infrared stealth capability. This work offers new insights for rapid and low-cost manufacturing of high-strength absorbing foam.

Synthesis of in situ grown CNTs on MOF-derived Ni@CNT with tailorable microstructures toward regulation of electromagnetic wave absorption performance

Metal-Organic framework (MOF) derivatives have been applied as electromagnetic wave absorption (EMA) materials in recent years. Carbon nanotubes (CNTs) can achieve in situ growth on the surface of MOF derivatives. However, there is limited research on the EMA performance by regulating the morphology of the in situ grown CNTs. In this work, we prepared MOF-derived Ni@carbon nanotubes (Ni@CNT) with controllable length of CNTs by solvothermal and simple one-step pyrolysis method and revealed the CNT growth mechanism. We further investigated the effect of in situ grown CNT morphology on electromagnetic parameters and EMA performance. In situ grown CNTs of similar length on the surface of MOF derivatives lead to similar electromagnetic parameters. When the length of CNTs is short, the relaxation strength (Δε=εs−ε∞) is high, leading to excellent microwave absorption performance. For Ni@CNT-600, the reflection loss of −44.4 dB can be achieved at merely 1.72 mm. Finite element (FE) simulation was used to evaluate the EMA mechanism of different samples by calculating electric field and polarization strength. This work has explored the electromagnetic wave absorption performance from the perspective of microstructural tailoring, helpful for understanding the unique role of the in situ grown CNTs on the surface of MOF derivatives and investigating the ultra-thin electromagnetic wave absorption materials.

Synthesis of nano-diamond modified Ti3C2Tx MXene heterostructure for enhanced electromagnetic wave absorption

The highly efficient electromagnetic wave (EMW) absorption materials are considered a promising approach to combatting electromagnetic radiation pollution. We synthesized a novel heterostructure 2D Ti3C2Tx MXene composite film modified by 0D nano-diamond (ND) particles using simple mechanical stirring and a vacuum filtration method. Subsequently, we investigated its microstructure and EMW absorption properties. The ND modification not only regulated the impedance matching of the composite films but also enhanced the diversity of EMW losses, demonstrating excellent microwave absorption performance. With a 40 wt% ND content, the minimum reflection loss (RLmin) of the films reached −43.0 dB, corresponding to a film thickness of 2.5 mm. This study explored a novel and promising microwave absorption film, expanding the application of diamond in microwave absorption, and addressing the theoretical gap in diamond-enhanced MXene microwave absorption performance.

CNTs/lamellar VSe2/Fe3O4 self-assembled a variety of unique three-dimensional multilayer interface structures and microwave absorption properties

Fabrication of microwave absorption (MA) materials of matched small thickness, low density, broad effective absorption bandwidth (EAB), as well as high absorption performance remains a technical challenge to be solved today. In this paper, three-dimensional (3D) VSe2/CNTs/Fe3O4 ternary composites with a variety of self-assembled structures were fabricated by a two-step hydrothermal and one-step carbothermal reduction method. Via varying the content of Fe3O4 in the composites, different 3D multilayer interfacial structures, such as star fruit-, bitter melon-, goldfish-, starfish-, peony- and chrysanthemum-shaped structures were self-assembled, and excellent MA absorption characteristics were obtained. The best reflection loss (RLmin) was −50.96 dB at 1.8 mm thickness. The maximum EAB was 4.93 GHz at 2.0 mm thickness. Carbon nanotubes mainly contributed to conduction and polarization losses. In situ generated VSe2 nanosheets and Fe3O4 nanoparticles promoted the assembly of multilevel interfacial structures, thus contributing to interfacial polarization loss and dipole polarization loss. Fe3O4 also created magnetic losses. This study lays the foundation for fabricating highly efficient MA materials in special self-assembled heterostructures with ternary synergistic effects at the nanoscale.

Excellent Low-Frequency Microwave Absorption and High Thermal Conductivity in Polydimethylsiloxane Composites Endowed by Hydrangea-Like CoNi@BN Heterostructure Fillers.

The advancement of thin, lightweight, and high-power electronic devices has increasingly exacerbated issues related to electromagnetic interference and heat accumulation. To address these challenges, a spray-drying-sintering process is employed to assemble chain-like CoNi and flake boron nitride (BN) into hydrangea-like CoNi@BN heterostructure fillers. These fillers are then composited with polydimethylsiloxane (PDMS) to develop CoNi@BN/PDMS composites, which integrate low-frequency microwave absorption and thermal conductivity. When the volume fraction of CoNi@BN is 44 vol% and the mass ratio of CoNi to BN is 3:1, the CoNi@BN/PDMS composites exhibit optimal performance in both low-frequency microwave absorption and thermal conductivity. These composites achieve a minimum reflection loss of -49.9dB and a low-frequency effective absorption bandwidth of 2.40GHz (3.92-6.32GHz) at a thickness of 4.4mm, fully covering the n79 band (4.4-5.0GHz) for 5G communications. Meanwhile, the in-plane thermal conductivity (λ∥) of the CoNi@BN/PDMS composites is 7.31W m-1 K-1, which is ≈11.4 times of the λ∥ (0.64W m-1 K-1) for pure PDMS, and 32% higher than that of the (CoNi/BN)/PDMS composites (5.52W m-1 K-1) with the same volume fraction of CoNi and BN obtained through direct mixing.

Excellent C-band microwave absorption properties of Ni@nitrogen-doped porous carbon nanocomposite via polymer bubbling technique

Nitrogen-doped porous carbon nanocomposites are highly sought-after materials for electromagnetic wave absorption due to their unique combination of electrical and magnetic properties. However, achieving optimal absorption performance within the critical C-band (4–8 GHz) frequency range remains a significant challenge. Ni@NPC nanocomposites, synthesized through the polymer bubbling technique, demonstrated exceptional microwave absorption (MWA) performance in the C-band. These materials exhibited a reflection loss (RL) of −65.20 dB at a thickness of 4.8 mm. When carbonized at 600 °C, a thickness of 2.0 mm achieved an effective absorption bandwidth (EAB) of 5.74 GHz. The remarkable absorption performance is ascribed to the combined effect of their porous composition and the addition of nitrogen dopants, which efficiently foster various microwave attenuation processes. Furthermore, the material’s effectiveness in real-world applications was evaluated using computer simulation technology (CST). This work highlights the potential of the one-pot fabrication method for developing high-performance MWA materials with practical applications.

The low-dimensional units modulation of 3D floral Fe/Ni@C towards efficient microwave absorption

Metal-organic frameworks (MOF) derivatives have great potential in microwave absorption (MA) due to their customizable components, topology, and high porosity. However, the correlation between the structural parameters of the low-dimensional units and the MA performance of the 3D MOF derivatives remains unclear. Herein, we prepared a series of floral Fe/Ni@C with different petal sizes and aspect ratios to investigate the effect of low-dimensional structural parameters on the MA properties of 3D MOF derivatives. Specifically, we proposed a competitive coordination strategy. By adjusting the ratio of coordination metals, we successfully obtained floral FeNi-MOF-74 with different petal sizes and aspect ratios. Subsequently, the floral Fe/Ni@C was obtained by thermal-field modulation. It shows that, on the one hand, as the petal size increases, the conductive path is extended and the electron hopping is enhanced, thus facilitating the conductivity loss. And, with the rise of the petal aspect ratio, the tip effect is enhanced, which strengthens the polarization loss. On the other hand, the graphitization degree of the carbon components as well as the size and magnetic properties of the alloy particles can be effectively modulated by optimizing the thermal field modulation temperature. Noteworthily, under the conditions of a coordination metal ratio of 1:1 and an annealing temperature of 700 °C, the obtained MOF derivative exhibited excellent microwave absorption (−65.47 dB at 1.56 mm) and broadband (6.09 GHz at 1.76 mm). This strategy provides new ideas for modulating the microstructure and electromagnetic properties of MOF derivatives.

Magnetic carbon composites derived from biomass with excellent microwave absorption capacity

It is well known that magnetic iron oxide (Fe3O4) and carbon-based materials are good microwave absorbing materials. However, the frequency bands in which Fe3O4 absorbs electromagnetic waves are usually concentrated in the low-frequency region, whereas carbon materials are in the high-frequency region. Therefore, this study aims to deal with the limitation of the absorption bands of these two substances. In this work, this Fe3O4/C composite with high performance microwave absorption was successfully adoption of hydrothermal-calcination process using mangosteen shells as a biomass carbon material, and with a thickness of 1.97 mm, the optimal reflection loss of the sample can reach −67.49 dB, and the frequency band that can be absorbed is 4.32 GHz. The interface polarization, dipolar polarization and eddy current loss in Fe3O4 and carbon materials produce synergistic effects, and the porous conductive network increases the electromagnetic wave transmission path and generates conductive losses, which results in composites with excellent electromagnetic wave absorption properties. The findings indicated that a composite compound of Fe3O4 and carbon-based material was successfully synthesized by hydrothermal method and obtained a materials with outstanding electromagnetic wave absorption properties and high absorption bandwidth.

Regulation of PPy Growth States by Employing Porous Organic Polymers to Obtain Excellent Microwave Absorption Performance.

Regulating the different growth states of polypyrrole (PPy) is a key strategy for obtaining PPy composites with high electromagnetic wave (EMW) absorption properties. This work finds that the growth states of PPy is regulated by controlling the amount of pyrrole added during the preparation of composites, so as to regulate the development of conductive networks to obtain excellent EMW absorption performance. The POP/PPy-200 composite achieves an effective absorption bandwidth (EAB) of 6.24GHz (11.76-18.00GHz) at a thickness of only 2.34mm, covering 100% of the Ku band. The minimum reflection loss of -73.05dB can be demonstrated at a thickness of only 2.29mm, while at the same time showing an EAB of 5.96GHz to meet the requirements of "thin", "light", "wide", and "strong". Such excellent EMW absorption performance is attributed to the conductive loss caused by the regulation of the growth states of PPy and the polarization loss caused by the heterostructure. This work also addresses the key challenge that porous organic polymers (POPs) cannot be applied to EMW absorption due to poor conductivity and providing new insights into the candidates for EMW absorbing materials.

C/Co3O4/Diatomite Composite for Microwave Absorption.

Transition metal oxides have been widely used in microwave-absorbing materials, but how to improve impedance matching is still an urgent problem. Therefore, we introduced urea as a polymer carbon source into a three-dimensional porous structure modified by Co3O4 nanoparticles and explored the influence of different heat treatment temperatures on the wave absorption properties of the composite. The nanomaterials, when calcined at a temperature of 450 °C, exhibited excellent microwave absorption capabilities. Specifically, at an optimized thickness of 9 mm, they achieved a minimum reflection loss (RLmin) of -97.3 dB, accompanied by an effective absorption bandwidth (EAB) of 9.83 GHz that comprehensively covered both the S and Ku frequency bands. On the other hand, with a thickness of 3 mm, the RLmin was recorded as -17.9 dB, with an EAB of 5.53 GHz. This excellent performance is attributed to the multi-facial polarization and multiple reflections induced by the magnetic loss capability of Co3O4 nanoparticles, the electrical conductivity of C, and the unique three-dimensional structure of diatomite. For the future development of bio-based microwave absorption, this work provides a methodology and strategy.