FeCo-Anchored Reduced Graphene Oxide Framework-Based Soft Composites Containing Carbon Nanotubes as Highly Efficient Microwave Absorbers with Excellent Heat Dissipation Ability

Elastomeric Sensor-Triboelectric Nanogenerator Coupled System for Multimodal Strain Sensing and Organic Vapor Detection

Stretchable, flexible sensors are one of the most critical components of smart wearable electronics and Internet of Things (IoT), thereby attracting multipronged research interest in the last decades. Following miniaturization and multicomponent development of several sensors in one could further propel the demand for wireless, multimodal platforms. Greener substitutes to conventional sensors that can operate in a self-powered configuration are highly desirable in terms of all-in-one sensor utilities. However, fabrication of composite-based ultrastretchable, self-powered sensors with multifunctionality, robustness, and conformability is still only partially achieved and, therefore, demands further investigation. In this work, we report a triboelectric nanogenerator (TENG)-based multifunctional strain and organic vapor sensor using cross-linked ethylene propylene diene monomer (EPDM) elastomer and conducting carbon black as active fillers in the presence of an ionic liquid. The resulting piezoresistive sensor demonstrates ultrahigh gauge factor (GF > 220k) and wide range strain sensitivity and is, therefore, suitable for subtle-to-high frequency motion detection devices. Supported by excellent triboelectric outputs (force sensitivity 0.5 V/N in the range of 50-300 N, maximum output voltage VOC ∼ 178 V, short circuit current ISC ∼ 18 μA, maximum power density 0.11 mW/cm2), the hybrid sensors offer remarkable mechanical toughness and seamless voltage generation under contact-separation, even after several thousand cycles of operations. Furthermore, the sensor substrates exhibited reproducible organic vapor-sensing behavior, with high responsivity of 1.92 and 1 for ethanol and acetone, respectively, under flowing vapor conditions. This work lays a strong foundation for developing a truly multimodal, TENG-based, self-powered organic vapor and strain sensors.

A review on recent progress in polymer composites for effective electromagnetic interference shielding properties - structures, process, and sustainability approaches

The rapid proliferation and extensive use of electronic devices have resulted in a meteoric increase in electromagnetic interference (EMI), which causes electronic devices to malfunction. The quest for the best shielding material to overcome EMI is boundless. This pursuit has taken different directions, right from materials to structures to process, up to the concept of sustainable materials. The emergence of polymer composites has substituted metal and metal alloy-based EMI shielding materials due to their unique features such as light weight, excellent corrosion resistance, and superior electrical, dielectric, thermal, mechanical, and magnetic properties that are beneficial for suppressing the EMI. Therefore, polymer nanocomposites are an extensively explored EMI shielding materials strategy. This review focuses on recent research developments with a major emphasis on structural aspects and processing for enhancing the EMI shielding effectiveness of polymer nanocomposites with their underlying mechanisms and some glimpses of the sustainability approaches taken in this field.

3D-Networks Based Polymer Composites for Multifunctional Thermal Management and Electromagnetic Protection: A Mini Review

The rapid development of miniaturized, high-frequency, and highly integrated microelectronic devices has brought about critical issues in electromagnetic compatibility and thermal management. In recent years, there has been significant interest in lightweight polymer-based composites that offer both electromagnetic interference (EMI) shielding and thermal conductivity. One promising approach involves constructing three-dimensional (3D) interconnection networks using functional fillers in the polymer matrix. These networks have been proven effective in enhancing the thermal and electrical conductivity of the composites. This mini-review focuses on the preparation and properties of 3D network-reinforced polymer composites, specifically those incorporating metal, carbon, ceramic, and hybrid networks. By comparing the effects of different filler types and distribution on the composite materials, the advantages of 3D interconnected conductive networks in polymer composites are highlighted. Additionally, this review addresses the challenges faced in the field of multifunctional thermal management and electromagnetic protection materials and provides insights into future development trends and application prospects of 3D structured composites.

Facile Construction of Chestnut-Like Structural Fireproof PDMS/Mxene@BN for Advanced Thermal Management and Electromagnetic Shielding Applications

Composite polymer materials featured superior thermal conductivity, flame retardancy, and electromagnetic shielding performance are increasingly in demand due to the rapid development of highly miniaturized, portable, and flexible electronic devices. Herein, a facile and green ball milling shear method is utilized for generating MXene@Boron nitride (MXene@BN). The multi-functional fillers (MXene@BN) are constructed and incorporated into polydimethylsiloxane (PDMS) to prepare a multifunctional composite (PDMS/MXene@BN) for achieving improved electromagnetic interference (EMI) shielding performance and thermal conductivity as well as flame retardancy simultaneously. When the PDMS/MXene@BN composite has a MXene@BN loading of 2.4 wt.%, it exhibits a high thermal conductivity of 0.59 W m-1K-1, which is 210% higher than that of the pure PDMS matrix. This is attributed to its unique chestnut-like double-layer structure. With a smoke production rate (SPR) of 0.04 m2 s-1 and total smoke production (TSP) of 3.51 m2, PDMS/MXene@BN 2.4 composite exhibits superb smoke suppression properties. These SPR and TSP values are 63.20% and 63.50% lower than the corresponding values of pure PDMS. Moreover, the EMI SE of the PDMS/MXene@BN 2.4 can reach 26.3 dB at 8.5 GHz. The work reported herein provides valuable insight into developing composites with multiple functions, which show strong potential for application in advanced packaging materials.

Electromagnetic interference shielding of flexible carboxymethyl cellulose/MWCNT@Fe3O4 composite film with ultralow reflection loss

Nowadays, various high-performance electromagnetic interference (EMI) shielding materials have enormous application potential in electronic field. However, traditional EMI shielding materials often have high conductivity, resulting in the serious mismatch between the impedance of the material surface and the free space, which will cause a large amount of reflection of electromagnetic (EM) waves, leading to secondary reflection pollution. In this paper, we report a novel flexible EMI shielding composite film with extremely low reflection loss and efficient EM wave absorption, which was prepared by assisted self-assembly based on simple vacuum filtration using carboxymethyl cellulose as the matrix and MWCNT@Fe3O4 synthesized by chemical coprecipitation as the composite functional filler. By adjusting the Fe3O4 coating degree of MWCNTs in the filler, the composite film achieved the construction of a conductive network with high Fe3O4 content. Benefit by the good adaptability of conductivity and magnetic permeability, the composite film has good impedance matching ability and microwave absorption performance. The reflection loss of the composite film with the thickness of 28 μm in the X-band was only 0.23 dB, accounting for 1.7 % of the total loss. This work provides new insights for the development of EMI materials and effective mitigation secondary EM wave reflection pollution.

Tracking Regulatory Mechanism of Trace Fe on Graphene Electromagnetic Wave Absorption

Polarization and conductance losses are the fundamental dielectric attenuation mechanisms for graphene-based absorbers, but it is not fully understood in revealing the loss mechanism of affect graphene itself. For the first time, the reduced graphene oxide (RGO) based absorbers are developed with regulatory absorption properties and the absorption mechanism of RGO is mainly originated from the carrier injection behavior of trace metal Fe nanosheets on graphene. Accordingly, the minimum reflection loss (RLmin) of Fe/RGO-2 composite reaches - 53.38 dB (2.45 mm), and the effective absorption bandwidth achieves 7.52 GHz (2.62 mm) with lower filling loading of 2 wt%. Using off-axis electron hologram testing combined with simulation calculation and carrier transport property experiments, we demonstrate here the carrier injection behavior from Fe to graphene at the interface and the induced charge accumulation and rearrangement, resulting in the increased interfacial and dipole polarization and the conductance loss. This work has confirmed that regulating the dielectric property of graphene itself by adding trace metals can not only ensure good impedance matching, but also fully exploit the dielectric loss ability of graphene at low filler content, which opens up an efficient way for designing lightweight absorbers and may be extended to other types materials.

Multi-hierarchically Structural Polycaprolactone Composites with Tunable Electromagnetic Gradients for Absorption-Dominated Electromagnetic Interference Shielding

Achieving absorption-dominated electromagnetic interference (EMI) shielding composites with high shielding effectiveness (SE) remains a great challenge due to their minimization of secondary EM radiation pollution, which is highly desired for next-generation electronic devices. Herein, an ingenious approach is proposed to develop asymmetric hierarchical polycaprolactone (PCL) composites composed of an impedance matching layer and a conductive layer through the combination of alternate casting and electroless plating methods, while the polarization loss caused by the difference in conductivity between the two layers would further attenuate the EM waves. The gradient distribution of the shielding fillers creates a positive conductive gradient and a negative magnetic gradient; the higher the gradient, the more it induces magnetic and dielectric losses, which results in an enhanced absorption mechanism that could overcome the restrictions of the nonadjustable reflective properties. The obtained Fe₃O₄@rGO/Ni/Ag/PCL composite possesses a remarkable EMI SE of 47.6 dB, while the power coefficient of reflectivity (R) could be significantly reduced to 0.27. This research provides a feasible strategy for developing absorption-dominated shielding materials with tunable EM performance that are appropriate for the next generation of electronic devices.

A universal approach to 'host' carbon nanotubes on a charge triggered 'guest' interpenetrating polymer network for excellent 'green' electromagnetic interference shielding

The widespread use of miniaturized electronic gadgets today faces stiff reliability obstacles from factors like stray electromagnetic signals. The challenge is to design lightweight shielding materials that combine small volume and high-frequency operations to reliably reduce/eliminate electromagnetic interference. Herein, in the first of its kind, a sequential interpenetrating polymeric network (IPN) membrane was used to host a CNT construct through a stimuli-responsive trigger. The proposed construct besides being robust, sustainable, and scalable is a universal approach to fabricate a CNT construct where conventional strategies are not amenable. This approach of self-assembling counter-charged CNTs also maximizes the number of CNTs in the final construct, thereby greatly enhancing the shielding performance dominated by 90% absorption in a wide frequency band of 8.2-26.5 GHz. The IPN-CNT construct achieves specific shielding effectiveness in the range of ca. 1607-5715 dB cm² g^-1 by tuning the thickness of the CNT construct with an endearing green index (g_s ≈ 1.8). The performance of such an ultra-thin, light-weight IPN-CNT construct remained unchanged when subjected to 10 000 bending cycles and on exposure to different chemical environments, indicating outstanding mechanical/chemical stability.

Flexible Polydimethylsiloxane Composite with Multi-Scale Conductive Network for Ultra-Strong Electromagnetic Interference Protection

Highlights

A multi-scale conductive network was constructed in flexible PDMS/Ag@PLASF/CNT composite with micro-size Ag@PLASF and nano-size CNT. The PDMS/Ag@PLASF/CNT composite showed outstanding electrical conductivity of 440 S m-1 and superior electromagnetic interference shielding effectiveness of up to 113 dB. The PDMS/Ag@PLASF/CNT composites owned good retention (> 90%) of electromagnetic interference shielding performance even after subjected to a simulated aging strategy or 10,000 bending-releasing cycles.

Abstract

Highly conductive polymer composites (CPCs) with excellent mechanical flexibility are ideal materials for designing excellent electromagnetic interference (EMI) shielding materials, which can be used for the electromagnetic interference protection of flexible electronic devices. It is extremely urgent to fabricate ultra-strong EMI shielding CPCs with efficient conductive networks. In this paper, a novel silver-plated polylactide short fiber (Ag@PLASF, AAF) was fabricated and was integrated with carbon nanotubes (CNT) to construct a multi-scale conductive network in polydimethylsiloxane (PDMS) matrix. The multi-scale conductive network endowed the flexible PDMS/AAF/CNT composite with excellent electrical conductivity of 440 S m−1 and ultra-strong EMI shielding effectiveness (EMI SE) of up to 113 dB, containing only 5.0 vol% of AAF and 3.0 vol% of CNT (11.1wt% conductive filler content). Due to its excellent flexibility, the composite still showed 94% and 90% retention rates of EMI SE even after subjected to a simulated aging strategy (60 °C for 7 days) and 10,000 bending-releasing cycles. This strategy provides an important guidance for designing excellent EMI shielding materials to protect the workspace, environment and sensitive circuits against radiation for flexible electronic devices.

Supplementary Information

The online version contains supplementary material available at 10.1007/s40820-022-00990-7.

Corrosion-Resistant Graphene-Based Magnetic Composite Foams for Efficient Electromagnetic Absorption

Designing and fabricating high-performance microwave absorption materials with efficient electromagnetic absorption and corrosion resistance becomes a serious and urgent concern. Herein, novel corrosion-resistant graphene-based carbon-coated iron (Fe@C) magnetic composite foam is fabricated via self-assembly of iron phthalocyanine/Fe₃O₄ (FePc hybrid) on the graphene skeletons under solvothermal conditions and then annealing at high temperature. As a result, the rational construction of a hierarchical impedance gradient between graphene skeletons and Fe@C particles can facilitate the optimization in impedance matching and attenuation characteristic of the foam, realizing the efficient dissipation for incident electromagnetic waves. Additionally, the performance of electromagnetic absorption can be controllably regulated by optimizing annealing temperature and/or time. More importantly, the formation of a carbon-coated iron structure substantially improves the corrosion resistance of magnetic particles, endowing the composite foam with excellent stability and durability in microwave absorption performance.

Experiment and simulation of flexible CNT/SA/PDMS electromagnetic shielding composite

Flexible electromagnetic shielding composites have a great potential for wide range applications. In this study, two flexible composites were produced by plating Ni nanoparticles on carbon nanotubes (CNTs) or infiltrating carbon nanofibers/polydimethylsiloxane (CNF/PDMS) polymer into CNT/sodium alginate (CNT/SA) sponge skeleton (CNT/SA/CNF/PDMS composites). The composites are tested under the X band in the frequency range of 8.2 - 12.4 GHz, the electromagnetic interference shielding effectiveness (EMI-SE) values of the above two composites are almost as twice as that of CNT/SA/PDMS composite at a same CNT loading. Introducing nano-sized Ni particles on CNT improved the microwave absorption capacity of the composite, while adding CNF on the PDMS matrix enhanced the conductivity of these composites. Under 10% strain, both flexible composites show stable conductivity. Simulation and calculation results shown that increasing the cladding rate of Ni nanoparticles on the surface of CNT, reducing the average size of Ni particles, and increasing the loading of CNF in PDMS matrix can significantly improve conductivity and then EMI performance of the materials. All of these could benefit for the design of flexible electromagnetic shielding composites.

Tuning Dielectric Loss of SiO2@CNTs for Electromagnetic Wave Absorption

We developed a simple method to fabricate SiO₂-sphere-supported N-doped CNTs (NCNTs) for electromagnetic wave (EMW) absorption. EMW absorption was tuned by adsorption of the organic agent on the precursor of the catalysts. The experimental results show that the conductivity loss and polarization loss of the sample are improved. Meanwhile, the impedance matching characteristics can also be adjusted. When the matching thickness was only 1.5 mm, the optimal 3D structure shows excellent EMW absorption performance, which is better than most magnetic carbon matrix composites. Our current approach opens up an effective way to develop low-cost, high-performance EMW absorbers.

Graphene and Iron Reinforced Polymer Composite Electromagnetic Shielding Applications: A Review

With advancements in the automated industry, electromagnetic inferences (EMI) have been increasing over time, causing major distress among the end-users and affecting electronic appliances. The issue is not new and major work has been done, but unfortunately, the issue has not been fully eliminated. Therefore, this review intends to evaluate the previous carried-out studies on electromagnetic shielding materials with the combination of Graphene@Iron, Graphene@Polymer, Iron@Polymer and Graphene@Iron@Polymer composites in X-band frequency range and above to deal with EMI. VOSviewer was also used to perform the keyword analysis which shows how the studies are interconnected. Based on the carried-out review it was observed that the most preferable materials to deal with EMI are polymer-based composites which showed remarkable results. It is because the polymers are flexible and provide better bonding with other materials. Polydimethylsiloxane (PDMS), polyaniline (PANI), polymethyl methacrylate (PMMA) and polyvinylidene fluoride (PVDF) are effective in the X-band frequency range, and PDMS, epoxy, PVDF and PANI provide good shielding effectiveness above the X-band frequency range. However, still, many new combinations need to be examined as mostly the shielding effectiveness was achieved within the X-band frequency range where much work is required in the higher frequency range.

A generalizable strategy for constructing ultralight three-dimensional hierarchical network heterostructure as high-efficient microwave absorber

Using previous models and theories to construct and develop high-efficient microwave absorbers (MAs) should be a strategic and effective ways to optimize the electromagnetic wave attenuation. Herein, the ultralow density and flexible graphene oxide foam (GOF) and reduced graphene oxide foam (RGOF)/MoS₂ nanosheets were designed and fabricated by the method of chemical vapor deposition and hydrothermal reaction. The obtained GOF and RGOF/MoS₂ samples exhibited very excellent microwave absorption properties while their densities were merely 0.0082 and 0.0084 g•cm^-3, respectively. More importantly, benefiting from the excellent synergistic effect between RGOF and MoS₂, the designed RGOF/MoS₂ well inherited the combined advantages of GOF and MoS₂ in terms of strong absorption abilities, broad absorption bandwidth and thin matching thicknesses. The values of minimum reflection loss and effective frequency bandwidth for RGOF/MoS₂ sample could reach up to -62.92 dB with the matching thickness of 2.27 mm and 4.48 GHz with the matching thickness of 2.12 mm, which were very desirable for high-performance MAs. Moreover, the obtained results indicated that the microwave absorption properties of RGOF/MoS₂ sample could be further optimized by regulating the MoS₂ content. Therefore, a new and effective strategy was proposed to develop high efficiency MAs with ultra-lightweight, wide-band, thin thickness and strong absorption capabilities.

Electromagnetic Interference Shield of Highly Thermal-Conducting, Light-Weight, and Flexible Electrospun Nylon 66 Nanofiber-Silver Multi-Layer Film

A light-weight, flexible electromagnetic interference (EMI) shield was prepared by creating a layer-structured metal-polymer composite film consisting of electrospun nylon 66 nanofibers with silver films. The EMI shielding effectiveness (SE), specific SE, and absolute SE of the composite were as high as 60.6 dB, 67.9 dB cm³/g, and 6792 dB cm²/g in the X- and K_u-bands, respectively. Numerical and analytical calculations suggest that the energy of EM waves is predominantly absorbed by inter-layer multiple reflections. Because the absorbed EM energy is dissipated as heat, the thermal conductivity of absorption-dominant EMI shields is highly significant. Measured thermal conductivity of the composite was found to be 4.17 Wm⁻¹K⁻¹ at room temperature, which is higher than that of bulk nylon 66 by a factor of 16.7. The morphology and crystallinity of the composite were examined using scanning electron microscopy and differential scanning calorimetry, respectively. The enhancement of thermal conductivity was attributed to an increase in crystallinity of the nanofibers, which occurred during the electrospinning and subsequent hot pressing, and to the high thermal conductivity of the deposited silver films. The contribution of each fabrication process to the increase in thermal conductivity was investigated by measuring the thermal conductivity values after each fabrication process.

Solvent-Free Synthesis of Ultrafine Tungsten Carbide Nanoparticles-Decorated Carbon Nanosheets for Microwave Absorption

Carbides/carbon composites are emerging as a new kind of binary dielectric systems with good microwave absorption performance. Herein, we obtain a series of tungsten carbide/carbon composites through a simple solvent-free strategy, where the solid mixture of dicyandiamide (DCA) and ammonium metatungstate (AM) is employed as the precursor. Ultrafine cubic WC1-x nanoparticles (3-4 nm) are in situ generated and uniformly dispersed on carbon nanosheets. This configuration overcomes some disadvantages of conventional carbides/carbon composites and is greatly helpful for electromagnetic dissipation. It is found that the weight ratio of DCA to AM can regulate chemical composition of these composites, while less impact on the average size of WC1-x nanoparticles. With the increase in carbon nanosheets, the relative complex permittivity and dielectric loss ability are constantly enhanced through conductive loss and polarization relaxation. The different dielectric properties endow these composites with distinguishable attenuation ability and impedance matching. When DCA/AM weight ratio is 6.0, the optimized composite can produce good microwave absorption performance, whose strongest reflection loss intensity reaches up to - 55.6 dB at 17.5 GHz and qualified absorption bandwidth covers 3.6-18.0 GHz by manipulating the thickness from 1.0 to 5.0 mm. Such a performance is superior to many conventional carbides/carbon composites.

Hybrid Reduced Graphene Oxide with Special Magnetoresistance for Wireless Magnetic Field Sensor

Very few materials show large magnetoresistance (MR) under a low magnetic field at room temperature, which causes the barrier to the development of magnetic field sensors for detecting low-level electromagnetic radiation in real- time. Here, a hybrid reduced graphene oxide (rGO)-based magnetic field sensor is produced by in situ deposition of FeCo nanoparticles (NPs) on reduced graphene oxide (rGO). Special quantum magnetoresistance (MR) of the hybrid rGO is observed, which unveils that Abrikosov's quantum model for layered materials can occur in hybrid rGO; meanwhile, the MR value can be tunable by adjusting the particle density of FeCo NPs on rGO nanosheets. Very high MR value up to 21.02 ± 5.74% at 10 kOe at room temperature is achieved, and the average increasing rate of resistance per kOe is up to 0.9282 Ω kOe-1. In this paper, we demonstrate that the hybrid rGO-based magnetic field sensor can be embedded in a wireless system for real-time detection of low-level electromagnetic radiation caused by a working mobile phone. We believe that the two-dimensional nanomaterials with controllable MR can be integrated with a wireless system for the future connected society.

Highly Efficient Wideband Microwave Absorbers Based on Zero-Valent Fe@γ-Fe2O3 and Fe/Co/Ni Carbon-Protected Alloy Nanoparticles Supported on Reduced Graphene Oxide

Electronic systems and telecommunication devices based on low-power microwaves, ranging from 2 to 40 GHz, have massively developed in the last decades. Their extensive use has contributed to the emergence of diverse electromagnetic interference (EMI) phenomena. Consequently, EMI shielding has become a ubiquitous necessity and, in certain countries, a legal requirement. Broadband absorption is considered the only convincing EMI shielding solution when the complete disappearance of the unwanted microwave is required. In this study, a new type of microwave absorber materials (MAMs) based on reduced graphene oxide (rGO) decorated with zero-valent Fe@γ-Fe2O3 and Fe/Co/Ni carbon-protected alloy nanoparticles (NPs) were synthesized using the Pechini sol-gel method. Synthetic parameters were varied to determine their influence on the deposited NPs size and spatial distribution. The deposited superparamagnetic nanoparticles were found to induce a ferromagnetic resonance (FMR) absorption process in all cases. Furthermore, a direct relationship between the nanocomposites' natural FMR frequency and their composition-dependent saturation magnetization (Ms) was established. Finally, the microwave absorption efficiency (0.4 MHz to 20 GHz) of these new materials was found to range from 60% to 100%, depending on the nature of the metallic particles grafted onto rGO.

Dopamine-derived cavities/Fe3O4 nanoparticles-encapsulated carbonaceous composites with self-generated three-dimensional network structure as an excellent microwave absorber

Dopamine-derived cavities/Fe₃O₄ nanoparticles-encapsulated carbonaceous composites with self-generating three-dimensional (3D) network structure were successfully fabricated by a facile synthetic method, in which sodium alginate provided carbon matrix pores and excellent microwave absorption performance was established. The hollow cavities derived from the core–shell-like CaCO₃@polydopamine were creatively introduced into the 3D absorber to significantly improve the absorption performance. The sample calcined at 700 °C exhibited the most outstanding microwave absorption performance, with minimal reflection loss up to −50.80 dB at 17.52 GHz with a rare thickness of only 1.5 mm when filler loading was 35% in paraffin matrix. The effective absorption bandwidth of reflection loss < −10 dB reached 3.52 GHz from 14.48 GHz to 18 GHz, corresponding to the same thickness of 1.5 mm. In contrast, the sample without hollow dopamine-derived cavities showed poor performance due to poor impedance matching, and this highlights the role of hollow cavities brought into the 3D structure, which led to a difference in interfacial polarization, multiple reflections and scattering. The novel dopamine-derived cavities/Fe₃O₄ nanoparticles-encapsulated carbonaceous composites with 3D network structure can be regarded as a promising candidate for application as a microwave absorber with strong absorption.

Hollow dopamine-derived cavities/Fe₃O₄ nanoparticles-encapsulated carbonaceous composites with self-generating 3D network structure were fabricated for potential application as excellent microwave absorbers.

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

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

Direct Growth of a Polypyrrole Aerogel on Hollow CuS Hierarchical Microspheres Yields Particles with Excellent Electromagnetic Wave Properties

A current hot topic in polymer science is the development of electromagnetic wave-absorbing materials with desired properties (i.e., proper impedance matching and strong attenuation capability), but it presents a considerable challenge. In this work, solvothermal, and self-assembled polymerization were employed for the controlled fabrication of a uniform polypyrrole (PPy) aerogel coated on hollow CuS hierarchical microspheres (CuS@PPy). The PPy coating thickness of the heterostructure could be tuned by varying the feeding weight ratios of CuS/pyrrole monomer. The electromagnetic wave absorption properties of the CuS@PPy composites were estimated to be in the frequency range 2–18 GHz. The as-prepared Sample B (fabricated by the addition of 35 mg CuS) showed a maximum reflection loss (RL) of −52.85 dB at a thickness of 2.5 mm. Moreover, an ultra-wide effective bandwidth (RL ≤ −10 dB) from 9.78 to 17.80 GHz (8.02 GHz) was achieved. Analysis of the electromagnetic properties demonstrated that the CuS@PPy had a remarkable enhancement compared to pure CuS platelet-based spheres and pure PPy, which can be attributed to the increased relatively complex permittivity and the promoted dielectric loss by the intense interfacial dielectric polarizations. We believe that the as-fabricated CuS@PPy can be a good reference for the fabrication of lightweight and optimal broadband absorbers.

Core-Multishell Heterostructure with Excellent Heat Dissipation for Electromagnetic Interference Shielding

Herein, we report high electromagnetic interference (EMI) shielding effectiveness of -40 dB in the K_u-band (for a 600 μm thick film) through a unique core-shell heterostructure consisting of a ferritic core (Fe₃O₄) and a conducting shell (multiwalled carbon nanotubes, MWCNTs) supported onto a dielectric spacer (here SiO₂). In recent times, materials with good flexibility, heat dissipation ability, and sustainability together with efficient EMI shielding at minimal thickness are highly desirable, especially if they can be easily processed into thin films. The resulting composites here shielded EM radiation mostly through absorption driven by multiple interfaces provided by the heterostructure. The shielding value obtained here is fairly superior among the different polymer nanocomposite-based EMI shielding materials. In addition to EMI shielding capability, this composite material exhibits outstanding heat dissipation ability (72 °C to room temperature in less than 90 s) as well as high heat sustainability. The composite material retained its EMI shielding property even after repeated heat cycles, thereby opening new avenues in the design of lightweight, flexible, and sustainable EMI shielding materials.

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

Strong EM wave absorption and lightweight are the foremost important factors that drive the real-world applications of the modern microwave absorbers. This work mainly deals with the design of highly efficient microwave absorbers, where a hierarchical carbon nanotube (CNT) forest is first grown on the carbon fiber (CF) through the catalytic chemical vapor deposition method. The hierarchical carbon nanotube grown on the carbon fiber (CNTCF) is then embedded in the epoxy matrix to synthesize lightweight nanocomposites for their use as efficient microwave absorbers. The morphological study shows that carbon nanotubes (CNTs) self-assemble to form a trapping center on the carbon fiber. The electromagnetic characteristics of resultant nanocomposites are investigated exclusively in the X-band (8.2-12.4 GHz) using the network analyzer. The synthesized nanocomposites, containing 0.35 and 0.50 wt % CNTCFs, exhibit excellent microwave absorption properties, which could be attributed to the better impedance matching conditions and high dielectric losses. The reflection loss (RL) of -42.0 dB (99.99% absorption) with -10 dB (90% absorption) and -20 dB (99% absorption) bandwidths of 2.7 and 1.16 GHz, respectively, is achieved for 0.35 wt % CNTCF loading at 2.5 mm thickness. The composite with 0.50 wt % CNTCF loading illustrates substantial absorption efficiency with the RL reaching -24.5 dB (99.65% absorption) at 9.8 GHz and -10 dB bandwidth comprising 84.5% of the entire X band. The excellent microwave properties obtained here are primarily due to the electric dipole polarization, interfacial polarization, and unique trapping center. These trapping centers basically induce multiple reflections and scatterings, which attenuate more microwave energy. This investigation opens a new approach for the development of extremely lightweight, small-thickness, and highly efficient microwave absorbers for X-band applications.

The flaky porous Fe3O4 with tunable dimensions for enhanced microwave absorption performance in X and C bands

Special electric and magnetic characteristics make Fe₃O₄ widely applied in the electromagnetic (EM) wave absorption region. However, for pure Fe₃O₄, it is still a challenge to simultaneously obtain high absorption intensity and broadband absorption at a low thickness, owing to its low dielectric property. As we realized, flake configuration and the porous structure have obviously promote the EM wave absorption property. Because the former can lead to multi-reflection between flakes and the latter is conductive to interface polarization, flaky Fe₃O₄ with a porous and coarse surface was designed to overcome the deficiency of traditional Fe₃O₄ particles. The experimental results demonstrate that the flaky configuration is conductive to enhancing the dielectric coefficient and optimizing impedance matching. Moreover, the complex permittivity rises with the aspect ratio of the sheet. Under a suitable dimension, the flaky Fe₃O₄ could acquire targeted EM wave absorption capacity in the X band (8-12 GHz). In detail, the maximum reflection loss (RL) could reach a strong intensity of -49 dB at 2.05 mm. The effective absorption bandwidth (EAB) with RL below -10 dB is 4.32 (7.52-11.84) GHz, which is almost equivalent to the whole X band (8-12 GHz). Even more exciting, when regulating the thickness between 2.05 and 3.05 mm, the EAB could cover the entire C and X bands (4-12 GHz). This study provides a good reference for the future development of other ferromagnetic materials toward specific microwave bands.

Enhanced shielding effectiveness in nanohybrids of graphene derivatives with Fe3O4 and ε-Fe3N in the X-band microwave region

Novel nanocomposites of reduced graphene oxide (rGO)-Fe3O4, denoted as 'rGO:IO, and nitrogen doped rGO-ε-Fe3N, denoted as 'NrGO:IN', were prepared by a modified polyol method, wherein both the reduction of graphene oxide and oxidation of Fe2+/Fe3+ ions occurred simultaneously, followed by ammonia nitridation. The electron microscopy analysis of the rGO:IO and NrGO:IN nanocomposites revealed unique morphologies. In rGO:IO, the Fe3O4 nanoparticles having a mean diameter of 38 nm were found to be uniformly anchored to the rGO sheet surface, whereas in NrGO:IN, the ε-Fe3N nanoparticles (∼150 nm) were shielded by the NrGO sheets. Superparamagnetic and weak ferromagnetic characteristics with saturation magnetization values of 39.5 and 46 emu g-1 were observed in the rGO:IO and NrGO:IN nanocomposites respectively, which can be attributed to the nature of the constituent magnetic nanoparticles, Fe3O4 and ε-Fe3N. In addition, the graphene derivatives such as rGO and NrGO contributed to the enhanced electrical properties of the nanocomposite. The electrochemical impedance spectroscopy analysis showed that, compared to pure Fe3O4 and ε-Fe3N nanoparticles, the total electrical resistance of rGO:IO and NrGO:IN was reduced by 33 344.8 and 1569.87 Ω cm-2, respectively, when combined with the rGO and NrGO sheets. Further, the electromagnetic shielding performance of the NrGO:IN nanocomposite was investigated for the first time and was compared with the other samples. Of the two prepared nanocomposites, NrGO:IN exhibited electromagnetic shielding effectiveness of 35.33 dB at 11.4 GHz, which is considerably larger than that of rGO:IO (14.4 dB at 8 GHz). This enhanced shielding effectiveness is not only due to the high inherent magnetic and electrical properties of ε-Fe3N nanoparticles, but also due to the 'particle shielded by sheet' morphology of the NrGO:IN, which enhances the charge accumulation at the heterogeneous interfaces of NrGO sheets/ε-Fe3N nanoparticles.

Gradient Structure Design of Flexible Waterborne Polyurethane Conductive Films for Ultraefficient Electromagnetic Shielding with Low Reflection Characteristic

Highly efficient electromagnetic shielding materials entailing strong electromagnetic wave absorption and low reflection have become an increasing requirement for next-generation communication technologies and high-power electronic instruments. In this study, a new strategy is employed to provide flexible waterborne polyurethane composite films with an ultra-efficient electromagnetic shielding effectiveness (EMI SE) and low reflection by constructing gradient shielding layers with a magnetic ferro/ferric oxide deposited on reduced graphene oxide (rGO@Fe₃O₄) and silver-coated tetraneedle-like ZnO whisker (T-ZnO/Ag) functional nanoparticles. Because of the differences in density between rGO@Fe₃O₄ and T-ZnO/Ag, a gradient structure is automatically formed during the film formation process. The gradient distribution of rGO@Fe₃O₄ over the whole thickness range forms an efficient electromagnetic wave absorption network that endows the film with a strong absorption ability on the top side, while a thin layer of high-density T-ZnO/Ag at the bottom constructs a highly conductive network that provides an excellent electromagnetic reflection ability for the film. This specific structure results in an "absorb-reflect-reabsorb" process when electromagnetic waves penetrate into the composite film, leading to an excellent EMI shielding performance with an extremely low reflection characteristic at a very low nanofiller content (0.8 vol % Fe₃O₄@rGO and 5.7 vol % T-ZnO/Ag): the EMI SE reaches 87.2 dB against the X band with a thickness of only 0.5 mm, while the shielding effectiveness of reflection (SE_R) is only 2.4 dB and the power coefficient of reflectivity ( R) is as low as 0.39. This result means that only 39% of the microwaves are reflected in the propagation process when 99.9999998% are attenuated, which is the lowest value among the reported references. This composite film with remarkable performance is suitable for application in portable and wearable smart electronics, and this method offers an effective strategy for absorption-dominated EMI shielding.

Decorating MOF-Derived Nanoporous Co/C in Chain-Like Polypyrrole (PPy) Aerogel: A Lightweight Material with Excellent Electromagnetic Absorption

To clear away the harmful effects of the increment of electromagnetic pollution, high performance absorbers with appropriate impedance matching and strong attenuation capacity are strongly desired. In this study, a chain-like PPy aerogel decorated with MOF-derived nanoporous Co/C (Co/C@PPy) has been successfully prepared by a self-assembled polymerization method. With a filler loading ratio of 10 wt %, the composite of Co/C@PPy could achieve a promising electromagnetic absorption performance both in intensity and bandwidth. An optimal reflection loss value of −44.76 dB is achieved, and the effective bandwidth (reflection loss lower than −10 dB) is as large as 6.56 GHz. Furthermore, a composite only loaded with 5 wt % Co/C@PPy also achieves an effective bandwidth of 5.20 GHz, which is even better than numerous reported electromagnetic absorption (EA) materials. The result reveals that the as-fabricated Co/C@PPy—with high absorption intensity, broad bandwidth, and light weight properties—can be utilized as a competitive absorber.

Microwave absorption properties of reduced graphene oxide strontium hexaferrite/poly(methyl methacrylate) composites

The key factors to consider when designing microwave absorber materials for eradication of electromagnetic (EM) pollution are absorption of incident EM waves and good impedance matching. By keeping these things in mind, flexible microwave absorber composite films can be fabricated by simple gel casting techniques using reduced graphene oxide (RGO) and strontium ferrite (SF) in a poly(methyl methacrylate) (PMMA) matrix. SF nanoparticles are synthesized by the well known sol-gel method. Subsequently, reduced graphene oxide (RGO) and SF nanocomposite (RGOSF) are prepared through a chemical reduction method using hydrazine. The structure, morphology, chemical composition, thermal stability and magnetic properties of the nanocomposite are characterized in detail by various techniques. The SF particles are found to be nearly 500 nm and decorated on RGO sheets as revealed by field emission scanning electron microscopy and transmission electron microscopy analysis. Fourier transform infrared and and Raman spectroscopy clearly show the presence of SF in the graphene sheet by the lower peak positions. Finally, ternary polymer composites of RGO/SF/PMMA are prepared by an in situ polymerization method. Magnetic and dielectric studies of the composite reveal that the presence of RGO/SF/PMMA lead to polarization effects contributing to dielectric loss. Also, RGO surrounding SF provides a conductive network in the polymer matrix which is in turn responsible for the magnetic loss in the composite. Thus, the permittivity as well as the permeability of the composite can be controlled by an appropriate combination of RGO and SF in PMMA. More than 99% absorption efficiency is achieved by a suitable combination of magneto-dielectric coupling in the X-band frequency range by incorporating 9 wt% of RGO and 1 wt% of SF in the polymer matrix.

Two-dimensional materials and one-dimensional carbon nanotube composites for microwave absorption

In this work, hierarchical architecture MoS₂/CNT nanohybrids synthesized by the hydrothermal method, with different CNT proportions are systematically investigated for their microwave absorption. MoS₂ nanoflowers are anchored uniformly on the surface of a CNT when the proportion of the MoS₂/CNT nanohybrids was 10:2, and the reflection loss can attain -20 dB in the range of 3.4-13.9 GHz with multiple thicknesses from 1.5-5.0 mm, while an optimal consequence of -46 dB can be reached at 6.6 GHz at 2.9 mm. The excellent performance indicates that the MoS₂/CNT = 10:2 nanohybrids have the potential for use as microwave absorbing materials.

Fe3O4 nanoparticles decorated on a CuS platelet-based sphere: a popcorn chicken-like heterostructure as an ideal material against electromagnetic pollution

A popcorn chicken-like CuS/Fe₃O₄ heterostructure was fabricated via the solvothermal deposition method to investigate its potential as a high electromagnetic wave absorber.