Hierarchical Carbon Nanotube-Coated Carbon Fiber: Ultra Lightweight, Thin, and Highly Efficient Microwave Absorber

Enhancing Low-Frequency Microwave Absorption Through Structural Polarization Modulation of MXenes

Two-dimensional carbon-based materials have shown promising electromagnetic wave absorption capabilities in mid- and high-frequency ranges, but face challenges in low-frequency absorption due to limited control over polarization response mechanisms and ambiguous resonance behavior. In this study, we propose a novel approach to enhance absorption efficiency in aligned three-dimensional (3D) MXene/CNF (cellulose nanofibers) cavities by modifying polarization properties and manipulating resonance response in the 3D MXene architecture. This controlled polarization mechanism results in a significant shift of the main absorption region from the X-band to the S-band, leading to a remarkable reflection loss value of - 47.9 dB in the low-frequency range. Furthermore, our findings revealed the importance of the oriented electromagnetic coupling in influencing electromagnetic response and microwave absorption properties. The present study inspired us to develop a generic strategy for low-frequency tuned absorption in the absence of magnetic element participation, while orientation-induced polarization and the derived magnetic resonance coupling are the key controlling factors of the method.

Microwave-assisted ZnO-decorated carbon urchin resembling 'shish-kebab' morphology with self-healing and enhanced electromagnetic shielding properties

Herein, inspired by Acacia auriculiformis fruit, the shish-kebab-like growth of ZnO on carbon urchin (ZnO@CU) was designed using microwave radiation, thus leading to a hierarchal 3D structure that can promote multiple internal reflections through polarization centers. This hierarchal structure was then dispersed in a designer polyetherimide (PEI) matrix containing dynamic covalent bonds that can undergo metathesis, triggered by temperature, to harness self-healing properties in the composite. Such key attributes are required for their potential use in EMI shielding applications where frequent repairs are indispensable. Morphological investigation revealed that the ZnO flower was periodically nucleated like 'shish-kebab' structures on CU surfaces. CU was designed from short carbon fibers using a facile modified method. The EMI shielding performance of the resulting composites was investigated in the X-band, illustrating a shielding effectiveness of -40.6 dB for 2 wt% of ZnO@CU loading, and the composite can be preserved after the self-healing procedure. The ZnO 'kebabs' on 'CU shish' facilitated multiple scattering and numerous polarization centers to improve the EMI shielding performances at extremely low filler contents. In addition, the mechanical and thermal properties of the composite showed improved structural integrity and superior resistance to extreme temperatures, respectively. Overall, the proposed ZnO@CU/PEI composite has great potential to fulfill the increasing demands for lightweight EMI shielding materials in many fields.

Multifunctional Graphene Metasurface for Highly Flexible Control of Microwave Absorption

Reconfigurable multifunctional electromagnetic absorbers have shown broad application prospects in effectively dealing with a series of problems caused by complex electromagnetic environments due to their dynamic reflection wave control characteristics. In this work, we experimentally propose a multifunctional absorber based on a graphene metasurface. Its absorption mode can be flexibly switched among three modes of dual band, broadband, and single band. The reflection amplitude in each absorption mode can be controlled simultaneously. The measurement results of the prepared graphene metasurface indicate that the absorption modes and amplitudes can be dynamically controlled by changing two independent sets of bias voltages applied to the patterned graphene sandwich structures. The proposed graphene metasurface achieves peak absorption rates above 99.9% in both dual-band and single-band absorption modes. Specifically, in the broadband absorption mode, the bandwidth with an absorption rate greater than 90% reaches 17.8 GHz. In addition, it also integrates many advantages, such as optical transparency, polarization-insensitivity, stability of oblique incidence angles, and conformability to the application targets. Therefore, the proposed graphene metasurface is expected to be applied in platforms with optical windows that require resistance to electromagnetic interference and avoidance of electromagnetic radiation.

Enhanced Mechanical and Self-Healing Properties of Carbon Fiber-Reinforced Epoxy Laminates Using In Situ-Grown ZnO Nanorods and Thermo-Reversible Bonds

Advanced hierarchical carbon fiber epoxy laminates with an engineered interface using in situ-grown ZnO nanorods on carbon fiber resulted in strong mechanical interlocking with the matrix. To further strengthen the interface, "site-specific" modification was realized by modifying the ZnO nanorods with bismaleimide (BMI), which facilitates "thermo-reversible" bonds with graphene oxide (GO) present in the matrix. The resulting laminates exhibited an improvement in flexural strength by 20% and in interlaminar shear strength (ILSS) by 28%. In order to gain a mechanistic insight, few laminates were prepared by "nonselectively" modifying the ZnO-grown carbon fiber (CF) with BMI. The "nonselectively" modified laminates showed flexural strength and ILSS improvement by 43 and 39%, respectively. The "nonselective" modification resulted in a strong improvement in mechanical properties; however, the "site-specific" modification yielded a higher self-healing efficiency (81%). Raman spectroscopy, scanning electron microscopy (SEM) micrographs, atomic force microscope (AFM) analysis, and contact angle analysis indicated a strong interaction of the modified CFs with the resin. Enhanced surface area and energy, along with a decrease in segmental molecular mobility observed from dynamic mechanical analysis, confirmed the mechanism for a better performance. Microscopic images revealed an improved interfacial behavior of the fractured samples, indicating a higher interfacial adhesion in the modified laminates. Besides mechanical properties, these laminates also showed excellent electromagnetic interference (EMI) shielding performance. The laminates with only ZnO-modified CF showed a high shielding effectiveness of -47 dB.

Research progress and future perspectives on electromagnetic wave absorption of fibrous materials

Electromagnetic waves have caused great harm to military safety, high-frequency electronic components, and precision instruments, and so forth, which urgently requires the development of lightweight, high-efficiency, broadband electromagnetic waves (EMW) absorbing materials for protection. As the basic fibrous materials, carbon fibers (CFs) and SiC fibers (SiCf) have been widely applied in EMW absorption due to their intrinsic characteristics of low density, high mechanical properties, high conductivity, and dielectric loss mechanism. Nevertheless, it has remained a great challenge to develop lightweight EMW-absorbing fibrous materials with strong absorption capability and broad frequency range. In this review, the fundamental electromagnetic attenuation mechanisms are firstly introduced. Furthermore, the preparation, structure, morphology, and absorbing performance of CFs and SiCf-based EMW absorbing composites are summarized. In addition, prospective research opportunities are highlighted toward the development of fibrous absorbing materials with the excellent absorption performance.

Review of Polymer-Based Composites for Electromagnetic Shielding Application

The rapid advancement of electronic communication technology has greatly aided human productivity and quality of life, but it has also resulted in significant electromagnetic pollution issues. Traditional metals and alloys are often used for electromagnetic interference (EMI) shielding due to their excellent electrical conductivity. However, they have drawbacks such as being heavy, expensive, and having low corrosion resistance, which limits their application in electromagnetic shielding. Therefore, it is crucial to develop novel EMI shielding materials. Polymers, being highly flexible, corrosion-resistant, and possessing high specific strength, are frequently employed in electromagnetic shielding materials. In this review, we firstly introduce the basic theory of electromagnetic shielding. Then, we outline the processing methods and recent developments of polymer-based electromagnetic shielding composites, including uniform-, foam-, layered-, and segregated structures. Lastly, we present the challenges and prospects for the field, aiming to provide direction and inspiration for the study of polymer-based electromagnetic shielding composite materials.

3D lamellar skeletal network of porous carbon derived from hull of water chestnut with excellent microwave absorption properties

Biomass derived carbon has attracted extensive attention in the field of microwave absorption because of its sustainability and porous structure beneficial to microwave attenuation. In this study, 3D lamellar skeletal network porous carbon was successfully obtained from hull of water chestnut using biomass waste as raw material by controlling the ratio of KOH and precursors in a one-step carbonization process. The optimization of biomass carbon morphology was achieved and its microwave absorption properties were investigated. At the temperature of 600 °C, when the ratio of hull of water chestnut to KOH is 1:1, the porous carbon material with filling ratio of 35% can reach the effective absorption bandwidth (RL < -10 dB) of 6.0 GHz (12-18 GHz) at the matching thickness of 1.90 mm, covering the whole Ku band. When the thickness is 2.97 mm, the optimal reflection loss reaches -60.76 dB. The surface defects, interface polarization and dipole polarization of 3D porous skeleton network structure derived from hull of water chestnut contribute to the excellent reflection loss and bandwidth of porous carbon materials. The porous carbon with low density, low cost and simple preparation method has broad application prospects in the preparation of biomass-derived microwave absorbers.

Ultralight Hierarchically Structured RGO Composite Aerogels Embedded with MnO2/Ti3C2Tx for Efficient Microwave Absorption

Although intensive efforts have been devoted to fabricating Ti₃C₂T_x MXene composites for microwave absorption, it remains a great challenge to achieve excellent MA performance at low loading and thin thickness. Herein, a three-dimensional (3D) lightweight hierarchically structured MnO₂/Ti₃C₂T_x/RGO composite aerogel with abundant heterointerfaces was fabricated via a hydrothermal and chemical reduction self-assembly method. The RGO aerogel embedded with laminated MnO₂/Ti₃C₂T_x provides a lot of heterogeneous interfaces, 3D porous interconnected conductive networks, and reasonable combination of various loss materials for rich interfacial polarization, conductivity loss, multiple reflections and scattering, and good impedance matching. Benefiting from the synergy of different loss mechanisms, the maximum reflection loss (RL) is up to -66.5 dB (>99.9999% energy absorption) at only 10 wt % loading and 2.0 mm thickness, and even at only 1.5 mm thickness, the maximum RL value remains at -36 dB (>99.9% energy absorption). The work provides a promising route to construct 3D hierarchically heterogeneous composite aerogels for efficient MA at thin thickness and low loading.

Surface Modification of Carbon Fiber for Enhancing the Mechanical Strength of Composites

The surface of carbon fibers (CFs) is often modified by multi-walled carbon nanotubes (MWCNTs), and the effect of the interface on the mechanical properties has been reported mostly for epoxy matrices. We achieved effective surface modification of CFs by a simple two-step process to graft a large amount of MWCNTs using a highly reactive polymer to enhance the bonding between CFs and MWCNTs. The first step was the reactive mono-molecular coating of a reactive polymer (poly-2-isopropenyl-2-oxazoline; Pipozo) that has high reactivity with COOH from CFs and MWCNTs. The high reactivity between the oxazoline group and COOH or phenol OH was confirmed for low-molecular-weight reactions. The second step was the coating of MWCNTs from a dispersion in a solvent. This simple process resulted in a substantial amount of MWCNTs strongly bonded to CF, even after washing. The MWCNTs grafted onto CFs remained even after melt-mixing. The effect on the interface, i.e., physical anchoring, led to an improvement of the mechanical properties. The novelty of the present study is that Pipozo acted as a molecular bonding layer between CFs and MWCNTs as a physical anchoring structure formed by a simple process, and the interface caused a 20% improvement in the tensile strength and modulus. This concept of a composite having a physical anchoring structure of MWCNTs on CFs has potential applications for lightweight thermoplastics, such as in the automotive industry.

Fabrication of one-dimensional M (Co, Ni)@polyaniline nanochains with adjustable thickness for excellent microwave absorption properties

The development of microwave absorbing materials with strong absorption capacity, wide bandwidth and light weight has always been a topic of concern. Herein, one-dimensional (1D) M (Co, Ni)@polyaniline (PANI) nanochains (NCs) with adjustable thickness have been successfully synthesized by reducing the mental ions under a parallel magnetic field, pretreating metal nanochains with KH550 and pre-oxidization of aniline monomer. It is found that Co has a more favorable absorption width for electromagnetic waves (EMW) and Ni aims at the absorption intensity. Furthermore, the effect of metal elements on adjusting impedance matching is more significant than their magnetic loss for composites. The minimum reflection loss (RLmin) of CoP2 can be up to -73.16 dB at 4.63 mm and the effective absorption bandwidth (EAB) is 4.98 GHz at 2.17 mm, while those of NiP2 are -65.06 dB at 3.88 mm and 5.02 GHz at 2.05 mm. The increase of PANI content can significantly reduce the matching thickness. And the RLmin of CoP3 and NiP3 can reach -58.72 dB at 2.32 mm and -65.96 dB at 1.59 mm, respectively. The absorption mechanism reveals that the matching thickness of the quarter-wavelength determines frequency location. And high absorption intensity is attributed to the synergistic effects of impedance matching, conduction loss, polarization loss, and magnetic loss. This work provides a theoretical basis for designing PANI or other conducting polymers coating magnetic nanochains for electromagnetic absorbing materials with strong absorption capacity, wide bandwidth and light weight.

Metal-Organic Framework-Derived Core-Shell Nanospheres Anchored on Fe-Filled Carbon Nanotube Sponge for Strong Wideband Microwave Absorption

Metal-organic frameworks (MOFs) are booming as a promising precursor for constructing lightweight, high-efficiency microwave absorbing (MA) material. However, it is still a challenge to rationally design three-dimensional (3D), porous MOF-derived MA materials with a stable structure and strong and wideband MA performance. Herein, a 3D hybrid nanostructure (CNT/FeCoNi@C) comprising MOF-derived magnetic nanospheres and Fe-filled carbon nanotube (CNT) sponge has been controllably fabricated to enhance the absorption ability and broaden the effective absorption bandwidth (EAB). The magnetic nanospheres are uniformly anchored on the CNT skeleton, forming hybrid network structures, which enhance interface polarization, electron transportation, and impedance matching. The minimum reflection loss (RL) and EAB of the as-prepared CNT/FeCoNi@C sponges reach -51.7 dB and 6.0 GHz, respectively, outperforming most reported MOF-based wave absorbers. This work provides not only a novel design of MOF-derived 3D nanostructures but also an effective guide for the optimization of electromagnetic properties and absorbing performance in MA material.

Effect of Microstructures of Carbon Nanoproducts Grown on Carbon Fibers on the Interfacial Properties of Epoxy Composites

Two kinds of carbon nanoproducts with different microstructures, namely, carbon nanotubes (CNTs) and carbon nanofibers (CNFs), were grown on the surface of carbon fibers (CFs) by chemical vapor deposition (CVD) at low temperatures to improve the interface bonding between fibers and resins. The short-beam method and the micro-debonding method were used to test the interlaminar shear strength (ILSS) and interfacial shear strength (IFSS) of the composites. The results showed that the contribution of CNTs to the improvement of interfacial properties was better than that of CNFs. Specifically, the ILSS and IFSS of the CF-CNFs/epoxy composites increased by 18.59 and 24.39%, respectively, while the ILSS and IFSS of the CF-CNTs/epoxy composites increased by 26.97 and 47.79%, respectively. Compared with CNFs, the high degree of graphitization of CNTs and the π-interactions with the resin can better induce the formation of an interphase between the fiber and the resin, which suppressed the initiation of cracks and extended the propagation path of the cracks in the composites.

The excellent electromagnetic wave absorbing properties of carbon fiber composites: effect of metal content

In this work, the effect of metal loading on electromagnetic (EM) absorbing performance was investigated to obtain an excellent EM absorber through a simple process. Specifically, iron/cobalt/carbon nanocomposite fibers were...

Fabrication of Electrospun Ni0.5Zn0.5Fe2O4 Nanofibers Using Polyvinyl Pyrrolidone Precursors and Electromagnetic Wave Absorption Performance Improvement

Ni0.5Zn0.5Fe2O4 nanofibers with an average diameter of 133.56 ± 12.73 nm were fabricated by electrospinning and calcination. According to our thermogravimetric-differential thermal analysis and X-ray diffraction results, the calcination temperature was 650 °C. The microstructure, crystal structure, and chemical composition of the nanofibers were observed using field-emission scanning electron, X-ray diffraction, and energy-dispersive X-ray spectroscopy. Commercial particle samples and samples containing 10 wt% and 20 wt% nanofibers were fabricated, and the electromagnetic properties were analyzed with a vector network analyzer and a 7.00 mm coaxial waveguide. Regardless of the nanofiber content, Ni0.5Zn0.5Fe2O4 was dominantly affected by the magnetic loss mechanism. Calculation of the return loss based on the transmission line theory confirmed that the electromagnetic wave return loss was improved up to -59.66 dB at 2.75 GHz as the nanofiber content increased. The absorber of mixed compositions with Ni0.5Zn0.5Fe2O4 nanofibers showed better microwave absorption performance. It will be able to enhance the performance of commercial electromagnetic wave absorbers of various types such as paints and panels.

Yolk-shelled Co@SiO2@Mesoporous carbon microspheres: Construction of multiple heterogeneous interfaces for wide-bandwidth microwave absorption

Nowadays, in the practical application of microwave absorption, it is still urgent and challenging to develop the microwave absorber with broadened bandwidth at a single thickness. Constructing composites with multi-component and multi-structure has been an effective strategy to obtain enhanced microwave absorption performance. Herein, yolk-shelled Co@SiO₂@Mesoporous carbon (Co@SiO₂@MC) microspheres were prepared by in-situ one-pot synthesis, carbonization reduction, and subsequent etching. The mesoporous carbon shell and hollow cavity structure were obtained simultaneously by controlling the etching of SiO₂. The large carbon-air interface in the mesoporous shell and interior voids extend the propagation path of electromagnetic wave and enhance scattering. Owing to strong dielectric/magnetic loss, synergistic effect between different components and microstructures, as well as excellent impedance matching, Co@SiO₂@MC microspheres exhibit desirable microwave absorption performance. Notably, for the sample with mesoporous carbon shell thickness of 25 nm, the effective absorption bandwidth (reflection loss below -10 dB) is as wide as 9.6 GHz (8.4-18 GHz), completely covering the whole X and Ku bands at 3.7 mm. The ultra-wide absorption bandwidth of the yolk-shelled Co@SiO₂@MC microspheres highlight their potential application in the field of microwave absorption. Furthermore, this work provides new insights for the preparation of multi-component/multi-structure microwave absorbers.

Research Progress on Durability of Cellulose Fiber-Reinforced Cement-Based Composites

The performance of cellulose fiber-reinforced cement-based composites (CFCCs) depends not only on the characteristics of the cement matrix and fibers but also on the bonding property of the matrix and fibers. The durability of cement-based composites including various properties such as impermeability, frost resistance, and carbonization resistance has an important impact on the long-term service life of the matrix structure. The presence of a large number of hydroxyl groups on the molecular chain of cellulose can promote the formation of intra- and intermolecular hydrogen bonds of cellulose. This special structure imparts the cellulose high hydrophilicity, which leads the cement hydration C-S-H gel to adhere to the surface of cellulosic fibers (CFs) and induce its growth. The cavity of CFs has good water absorption and can be used as an internal curing fiber for the continuous hydration of cement-based composites. But CFs in the Portland cement matrix tend to deteriorate under strong alkali conditions. This paper presents a review of the research on the durability of CFCCs. The methods and paths to improve the durability of CFCCs are summarized and analyzed from the perspectives of the internal curing of CFs, the deterioration of the performance of CFs in the matrix, and the use of many types of supplementary cementitious materials. Finally, the development and engineering application of CFCCs have been prospected.

Architecting functionalized carbon microtube/carrollite nanocomposite demonstrating significant microwave characteristics

Biomass-derived materials have recently received considerable attention as lightweight, low-cost, and green microwave absorbers. On the other hand, sulfide nanostructures due to their narrow band gaps have demonstrated significant microwave characteristics. In this research, carbon microtubes were fabricated using a biowaste and then functionalized by a novel complementary solvothermal and sonochemistry method. The functionalized carbon microtubes (FCMT) were ornamented by CuCo2S4 nanoparticles as a novel spinel sulfide microwave absorber. The prepared structures illustrated narrow energy band gap and deposition of the sulfide structures augmented the polarizability, desirable for dielectric loss and microwave attenuation. Eventually, the architected structures were blended by polyacrylonitrile (PAN) to estimate their microwave absorbing and antibacterial characteristics. The antibacterial properties against Gram-negative Escherichia coli (E. coli) and Gram-positive Staphylococcus aureus (S. aureus) were scrupulously assessed. Noteworthy, the maximum reflection loss (RL) of the CuCo2S4/PAN with a thickness of 1.75 mm was 61.88 dB at 11.60 GHz, while the architected FCMT/PAN composite gained a broadband efficient bandwidth as wide as 7.91 GHz (RL > 10 dB) and 3.25 GHz (RL > 20 dB) with a thickness of 2.00 mm. More significantly, FCMT/CuCo2S4/PAN demonstrated an efficient bandwidth of 2.04 GHz (RL > 20 dB) with only 1.75 mm in thickness. Interestingly, FCMT/CuCo2S4/PAN and CuCo2S4/PAN composites demonstrated an electromagnetic interference shielding efficiency of more than 90 and 97% at the entire x and ku-band frequencies, respectively.

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 Ti₃C₂T_x 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 (RL_min) 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.

Recent progress of microwave absorption microspheres by magnetic-dielectric synergy

Designing and developing high-performance microwave absorption (MA) materials for electromagnetic protection and radar detection have received widespread attention. Recently, magnetic-dielectric MA materials have become a research hotspot due to their unique complementary functions and synergy loss mechanism. Herein, we review important research progress of excellent MA systems combining strong magnetic components and dielectric substrates. The functional materials involve magnetic materials, carbon components, semiconductors, polymer and so on. For a comprehensive analysis, current development and challenges are firstly introduced in the background. Modern requirements for microwave energy conversion are elaborated in the following part. To highlight the key points, more attention has been paid to the magnetic-dielectric synergy microsphere: (i) core/yolk-shell structure, (ii) multi-component assembly and (iii) MOF-derived synergy composites. Meanwhile, classical and typical high-performance MA composites with a multi-loss mechanism are also mentioned in this review paper. Finally, the design principles, electromagnetic synergy, future mechanism exploration and device application are presented, which provides guidance for understanding MA materials.

A Review on Metal-Organic Framework-Derived Porous Carbon-Based Novel Microwave Absorption Materials

The development of microwave absorption materials (MAMs) is a considerable important topic because our living space is crowed with electromagnetic wave which threatens human's health. And MAMs are also used in radar stealth for protecting the weapons from being detected. Many nanomaterials were studied as MAMs, but not all of them have the satisfactory performance. Recently, metal-organic frameworks (MOFs) have attracted tremendous attention owing to their tunable chemical structures, diverse properties, large specific surface area and uniform pore distribution. MOF can transform to porous carbon (PC) which is decorated with metal species at appropriate pyrolysis temperature. However, the loss mechanism of pure MOF-derived PC is often relatively simple. In order to further improve the MA performance, the MOFs coupled with other loss materials are a widely studied method. In this review, we summarize the theories of MA, the progress of different MOF-derived PC‑based MAMs, tunable chemical structures incorporated with dielectric loss or magnetic loss materials. The different MA performance and mechanisms are discussed in detail. Finally, the shortcomings, challenges and perspectives of MOF-derived PC‑based MAMs are also presented. We hope this review could provide a new insight to design and fabricate MOF-derived PC-based MAMs with better fundamental understanding and practical application.

Modulating dielectric loss of mesoporous carbon fibers with radar cross section reduction performance via computer simulation technology

Mesoporous carbon fibers, a kind of light weight microwave absorbing material, are prepared by electrospinning, through which the pore structure and microwave absorbing properties are influenced by the addition of tetraethyl orthosilicate.

Multistage reclamation of Co2+-containing alginate hydrogels as excellent reduction catalyst and subsequent microwave absorber by facile transformation

The multistage reclamation of materials has made contributions to sustainable development, but further progress is still sought after. In this work, functionalized seaweed-based composites were successfully prepared and utilized in multiple stages. Specifically, Co²⁺-containing alginate hydrogels (CHB-Co²⁺) prepared by sol-gel self-assembly and adsorption method using interior/exterior co-functionalized calcium alginate as raw materials were utilized for efficient reduction of p-nitrophenol. After coupling with freeze-drying and carbonization procedures, a high-performance Co/N co-doped carbonaceous microwave absorber was obtained and investigated in detail. By virtue of unique 3D interconnected network, heterogeneous interfaces and doped heteroatom N species, by which endowing the absorber with optimal impedance matching and attenuation ability, as-fabricated NC-Co-700 exhibited prominent microwave absorption performance with -54.2 dB of RL_min at 6.4 GHz and 5.3 GHz of maximum absorption bandwidth (from 12.7 to 18.0 GHz). Additionally, in view of the dielectric loss and magnetic loss caused by the synergy effect among the functional components, the underlying absorption mechanism was proposed. This work provided a novel idea for designing biomass-based functional materials and simultaneously achieved economic benefits through the rational utilization of other products in the preparation process.

A Facile Synthesis of Novel Amorphous TiO2 Nanorods Decorated rGO Hybrid Composites with Wide Band Microwave Absorption

Amorphous structures may play important roles in achieving highly efficient microwave absorption performance due to the polarization losses induced by the disorders, vacancies and other functional groups existed in them. Herein, a kind of amorphous TiO2/rGO composite (a-TiO2/rGO) was successfully fabricated via a facile one-step solvothermal method. The complex permittivity of the composites can be regulated by adjusting the addition of precursor solution. The minimum reflection loss of a-TiO2/rGO composites reached -42.8 dB at 8.72 GHz with a thickness of 3.25 mm, and the widest efficient absorption bandwidth (EAB) was up to 6.2 GHz (11.8 to 18 GHz) with a thickness of only 2.15 mm, which achieved the full absorption in Ku band (12 to 18 GHz). Furthermore, the EAB was achieved ranging from 3.97 to 18 GHz by adjusting the thickness of the absorber, covering 87.7% of the whole radar frequency band. It is considered that the well-matched impedance, various polarization processes, capacitor-like structure and conductive networks all contributed to the excellent microwave absorption of a-TiO2/rGO. This study provides reference on constructing amorphous structures for future microwave absorber researches and the as-prepared a-TiO2/rGO composites also have great potential owing to its facile synthesis and highly efficient microwave absorption.

Heterogeneous Interface Induced the Formation of Hierarchically Hollow Carbon Microcubes against Electromagnetic Pollution

Carbon materials with multilevel structural features are showing great potentials in electromagnetic (EM) pollution precaution. With ZIF-67 microcubes as a self-sacrificing precursor, hierarchical carbon microcubes with micro/mesoporous shells and hollow cavities have been successfully fabricated with the assistance of rigid SiO₂ coating layers. It is found that the SiO₂ layer can effectively counteract the inward shrinkage of organic frameworks during high-temperature pyrolysis due to intensive interfacial interaction. The obtained hollow porous carbon microcubes (HPCMCs) exhibit larger Brunauer-Emmett-Teller surface area and pore volume than porous carbon microcubes (PCMCs) directly derived from ZIF-67 microcubes. The unique microstructure is confirmed to be favorable for conductive loss and interfacial polarization, thus boosting the overall dielectric loss capability of carbon materials. Besides, hollow cavity will also promote multiple reflection of incident EM waves and intensify the dissipation of EM energy. As expected, HPCMCs harvest better microwave absorption performance, including strong reflection loss intensity and broad response bandwidth, than many traditional microporous/mesoporous carbon materials. This study provides a new strategy for the construction of hierarchical carbon materials and may inspire the design of carbon-based composites with excellent EM functions.

Anisotropic Electromagnetic Absorption of Aligned Ti3C2Tx MXene/Gelatin Nanocomposite Aerogels

Assembling Ti₃C₂T_x MXene nanosheets into three-dimensional (3D) architecture with controllable alignment is of great importance for electromagnetic wave absorption (EMA) application. However, it is a great challenge to realize it due to the weak van der Waals interconnection between MXene nanosheets. Herein, we propose to introduce gelatin molecules as a "chemical glue" to fabricate the 3D Mxene@gelatin (M@G) nanocomposite aerogel using a unidirectional freeze casting method. The Ti₃C₂T_x MXene nanosheets are well aligned in the M@G nanocomposite aerogel, yielding much enhanced yet anisotropic mechanical properties. Due to the unidirectional aligned microstructure, the M@G nanocomposite aerogel shows significantly anisotropic EMA properties. M@G-45 shows a -59.5 dB minimum reflection loss (RL_min) at 14.04 GHz together with a 6.24 GHz effective absorption bandwidth in the parallel direction (relative to the direction of unidirectional freeze casting). However, in the vertical direction of the same M@G aerogel, RL_min is shifted to a much lower frequency (4.08 GHz) and the effective absorption bandwidth decreases to 0.86 GHz. The anisotropic electromagnetic energy dissipation mechanism was deeply investigated, and the impendence match plays a critical role for electromagnetic wave penetration. Our lightweight M@G nanocomposite aerogel with controllable MXene alignment is very promising in EMA application.

High-Performance Electromagnetic Wave Absorbing CNT/SiCf Composites: Synthesis, Tuning, and Mechanism

High-performance electromagnetic (EM) wave absorbing materials are strongly desired in many fields like portable devices and aircraft. Introducing carbon nanotubes (CNTs) to certain materials has been proved to be an effective method leading to good EM wave absorption capability. In this work, CNTs are successfully synthesized on SiC fibers with high speed by using a newly developed method which is far more efficient than the commonly used one. The obtained CNT/SiC_f composites exhibit high-performance EM wave absorption capability. With 0.72 wt % CNTs, the reflection loss of the 4 mm composite with only 20 wt % filler loading reaches -62.5 dB with the broad effective absorption bandwidth of 8.8 GHz, covering almost the entire Ku band and three-quarters X band. Moreover, the composites can be added to varying matrices so as to modify their EM wave absorption and other properties. The EM wave absorption performance can be easily tuned in a wide range by varying the CNT content, thickness, and filler loading. This work offers a new route for efficiently synthesizing CNTs but, more importantly, for designing high-performance and multifunctional EM wave absorbing materials.

Fe3O4 Nanoparticle/N-Doped Carbon Hierarchically Hollow Microspheres for Broadband and High-Performance Microwave Absorption at an Ultralow Filler Loading

Although the existing Fe₃O₄-based microwave absorbing materials (MAMs) have shown promising microwave absorbing (MA) capacity, it is highly desired but still remains a great challenge to achieve strong minimum reflection loss (RL_min) and broad effective frequency bandwidth (f_e) at an ultralow filler loading. Herein, for the first time, by carbonizing hierarchical poly(urea-formaldehyde) microcapsules with Fe₃O₄ nanoparticle cores in a nitrogen atmosphere, Fe₃O₄ hybrid and N-doped hollow carbon microspheres (Fe₃O₄/CMs) with a hierarchical micro/nanostructure are prepared on a large scale and at a low cost to achieve extremely superior MA performances. Benefitting from their unique structure and diverse composition, which synergetically contribute to good impedance matching, strong dielectric/magnetic loss, and abundant multiscattering/reflection, Fe₃O₄/CM composites possessed a RL_min value reaching -60.3 dB and an f_e of as broad as 6.4 GHz (7.2-13.6 GHz, covering the full X-band) at an ultralow filler loading of 10 wt % in paraffin wax, which are significantly superior to those of the previously reported state-of-the-art Fe₃O₄-based or hollow MAMs. Furthermore, the f_e can be adjusted in the range of 4.5-18 GHz, covering 85% of the whole measured frequency range, via changing the thickness between 2.5 and 5.5 mm. This work offers new insights for developing advanced lightweight MAMs with strong absorption and a broad absorbing frequency range at a low filler loading.

NiFe2O4 Nanoparticles Synthesized by Dextrin from Corn-Mediated Sol-Gel Combustion Method and Its Polypropylene Nanocomposites Engineered with Reduced Graphene Oxide for the Reduction of Electromagnetic Pollution

In this work, nickel ferrite (NiFe2O4) nanoparticles were synthesized by dextrin from corn-mediated sol-gel combustion method and were annealed at 600, 800, and 1000 °C. The structural and physical characteristics of prepared nanoparticles were studied in detail. The average crystallite size was 20.6, 34.5, and 68.6 nm for NiFe2O4 nanoparticles annealed at 600 °C (NFD@600), 800 °C (NFD@800), and 1000 °C (NFD@1000), respectively. The electromagnetic interference shielding performance of prepared nanocomposites of NiFe2O4 nanoparticles (NFD@600 or NFD@800 or NFD@1000) in polypropylene (PP) matrix engineered with reduced graphene oxide (rGO) have been investigated; the results indicated that the prepared nanocomposites consisted of smaller-sized nickel ferrite nanoparticles exhibited excellent electromagnetic interference (EMI) shielding characteristics. The total EMI shielding effectiveness (SET) for the prepared nanocomposites have been noticed to be 45.56, 36.43, and 35.71 dB for NFD@600-rGO-PP, NFD@800-rGO-PP, and NFD@1000-rGO-PP nanocomposites, respectively, at the thickness of 2 mm in microwave X-band range (8.2-12.4 GHz). The evaluated values of specific EMI shielding effectiveness (SSE) were 38.81, 32.79, and 31.73 dB·cm3/g, and the absolute EMI shielding effectiveness (SSE/t) values were 388.1, 327.9, and 317.3 dB·cm2/g for NFD@600-rGO-PP, NFD@800-rGO-PP, and NFD@1000-rGO-PP, respectively. The prepared lightweight and flexible sheets can be considered useful nanocomposites against electromagnetic radiation pollution.

Enhanced microwave absorption performance from abundant polarization sites of ZnO nanocrystals embedded in CNTs via confined space synthesis

Dielectric composites constructed using carbon and metal oxides have become a hot research topic; however, the strategy to strengthen the coupling of components still needs to be optimized to enhance dielectric loss. Herein, ultra-fine ZnO derived from ZIF-8 was uniformly distributed and tightly embedded in multi-wall carbon nanotubes (C-ZnO@CNTs) via a novel confined space synthesis. Due to the presence of a polypyrrole coating, ZnO nanocrystals could be formed in the space of the original polyhedron and inserted into the CNTs, promoting the generation of polarized CNTs and providing abundant polarization centers on the CNTs. The composites exhibited superior microwave absorption capacity with a reflection loss value of up to -48.2 dB at 6.0 GHz, and the effective bandwidth reached 14.9 GHz by adjusting their thickness. According to the geometric phase analysis, the strain driven by the tight-coupling between ZnO-CNTs was confirmed to exist in the interfaces, boosting their inherent electromagnetic properties. The improved dielectric loss was caused by the strong interfacial polarization among ZnO-ZnO or ZnO-CNTs and the conductive loss among intertwined CNTs network, as revealed by electron holography. Therefore, the overall electrical properties could be improved by the polarized C-ZnO@CNTs with high electron conductivity. The confined space strategy may have promising potential for the synthesis of new composites of polarized carbon materials tightly coupled with metal oxides nanocrystals.

Graphene oxide-ferrite hybrid framework as enhanced broadband absorption in gigahertz frequencies

The present investigation is focused on the in-situ synthesis of Graphene oxide (GO)-ferrite nanoparticle hybrid framework by gel-combustion method followed by fabrication of homogeneous, structurally stable thin (~100-120 μm) hybrid-polyurethane coating on a metallic aluminum substrate and its application on the properties of broadband absorption over the microwave frequency region. Microstructure studies of hybrid materials illustrated that small sized ferrite nanoparticles (~17 nm) are grafted on and through the graphene layers, which forms a homogeneous coating thereby. The hybrid-nanocomposite coating demonstrated superior broadband absorption properties with absorptivity higher than 90% throughout a bandwidth of ~6 GHz, and moreover, it was found that with increased loading of GO in the nanocomposite, the bandwidth range of absorption frequency increases with enhanced absorptivity. The real part and imaginary part of the surface impedance values of the coating was obtained as 377 Ω and 0 Ω, respectively, which imply that the free-space impedance of the hybrid-nanocomposite coating is matching correctly. The nanocomposite coating showed ultra-high absorptivity over the frequency band of 8-12 GHz, which has numerous practical applications as radar absorbing materials (RAM), stealth technology, electromagnetic shielding, and radiated electromagnetic interference (EMI) management in onboard spacecraft and many more.

Cobalt nanoparticles encapsulated in a nitrogen and oxygen dual-doped carbon matrix as high-performance microwave absorbers

Co@NOC composites showed impressive microwave absorption properties owing to multiple dielectric and magnetic loss mechanisms.

Retracted Article: A versatile strategy for alternately arranging the foam ratio layers of multilayer graphene/thermoplastic polyurethane composite foams towards lightweight and broadband electromagnetic wave absorption

A broadband electromagnetic wave (EW) absorbing material should possess both wider effective absorption bandwidth and lower minimum reflection loss, depending on good impedance matching between the absorber and air and strong attenuation of EW.

Performance enhanced electromagnetic wave absorber from controllable modification of natural plant fiber

Short, surface rough carbon rods, which were derived from natural sisal fiber and went through two different modifications, with excellent electromagnetic wave absorption performance, were studied in this work for the first time. The structure–property relationship was clearly established here. It was shown that these green, cheap and easily obtained carbon rods with mass preparation possibility presented eye-catching absorbing behaviors towards electromagnetic wave. Based on the natural structure of sisal fiber, the minimum reflection loss of KOH activated product reached −51.1 dB and the maximum effective absorbing bandwidth achieved 7.88 GHz. The magnetically modified sample presented −48.6 dB of minimum reflection loss and 4.32 GHz of optimal absorbing bandwidth. Its pioneering application in this field not only opens a new road for this traditional textile sisal fiber but also would possibly make a referable contribution to the design and synthesis of superior carbonaceous electromagnetic wave absorption materials based on bioresource.

Novel carbon rods derived from plant fiber with excellent electromagnetic wave absorption performance have been facilely accomplished via two different modifications.