Efficient Absorption Bandwidth Research Articles

Recycling waste chaff to prepare biochar with co-decorated CoCO3/CoNiO2 for anti-bacterial microwave dissipation

In order to address the recession of absorption properties and service life in electromagnetic absorbers caused by the bacterial propagation, the design of microwave absorbing composite with excellent antimicrobial character is an urgent requirement. Herein, the grain husk-derived carbon is co-decorated with CoNiO2 and CoCO3 nanoparticles through a facile hydrothermal-carbonization process. The defect structure and graphitization in biochar, complex interfaces between multi-components, and partial magnetic resonances can be regulated with the calcination temperature. With an appropriate impedance matching from balanceable electromagnetic parameters, the C4 composite treated by 650 °C realizes an optimal electromagnetic absorption of -41.87 dB at 2.5 mm thickness with its efficient absorption bandwidth of 4.45 GHz at 1.49 mm, which is mainly owing to the powerful dielectric dissipation caused by polarization and conduction losses together. Additionally, the composite exhibits a higher antimicrobial rate of 97.75 % against ubiquitous E. coli, indicating this work provides a reference for designing antimicrobial microwave absorption materials with better practicality.

Dimensional design of Fe0.9Co0.1 nano-alloys with enhanced low-frequency microwave absorption

Low-frequency microwave absorbing materials are critical for enabling radar stealth and interference resistance in gigahertz communication devices and electronic equipment. However, long-wavelength electromagnetic waves are not easily attenuated, due to the limitations of current absorbing materials, including low permeability and a lack of electromagnetic wave loss mechanisms at low frequencies. Herein, we design and prepare one- and two-dimensional Fe0.9Co0.1 nanoalloys with high permeability through an organic templating method combined with a constrained transformation strategy. We systematically investigate the impact of the anisotropy of monodisperse Fe0.9Co0.1 nanoalloys on low-frequency microwave absorption performance. The generous specific surface area of Fe0.9Co0.1 nanosheets facilitate multiple reflections and the scattering of electromagnetic waves within the absorber. This phenomenon, coupled with dielectric resonance, culminates in a maximum reflection loss value of − 48.7 dB at 4.2 GHz. More significantly, Fe0.9Co0.1 nanorods exhibit an absorption intensity of − 35.3 dB at a notably lower frequency of 2.5 GHz, outperforming existing materials, in addition to an efficient absorption bandwidth of 4.2 GHz at a thickness of only 0.7 mm. Through a combination of experimental investigations and finite element simulations, we identify that highly anisotropic magnetic nanorods demonstrate a robust magnetic coupling interaction, substantial interface polarization and quasi-antenna effect. These characteristics result in enhanced low-frequency electromagnetic losses. Our findings indicate that the high anisotropy of the Fe0.9Co0.1 nanoalloys significantly improves their microwave absorption performance, thus emphasizing their substantial potential in the development of efficient low-frequency microwave absorbers.

Facile and mass-producible Ni-added iron nanowires with excellent microwave absorbing performance

The application of magnetic nanocrystalline powders as radar absorption materials is increasingly attracting R&D interest. Severe agglomeration and mass production, however, are critical issues for practical application of magnetic nanoparticles. In the present study, iron nanowires with varying amounts of nickel addition (0, 1, 10, 30, and 50 wt%) were synthesized via direct reduction of iron salts with the aid of strong NdFeB magnets. The yield rate of these Ni-added iron nanowires (NiFe NWs) exceeded 1 g/min, making them suitable and was feasible for mass production. The characteristics of the so-obtained NiFe NWs were confirmed using field emission scanning electron microscopy, X-ray diffraction, and high-resolution transmission electron microscopy. Composite resins with NiFe NWs additions (3, 5, and 10 wt%) were prepared and examined using the coaxial line method to reveal their microwave absorption characteristics. Experimental results showed that composite resins with 10 wt% NiFe NWs additions possessed superior microwave absorbing properties, with the Ni1Fe99 NWs-added product exhibiting the best performance. When produced with a thickness of 1.7 mm, the reflection loss of the composites reached −39.28 dB at 12.53 GHz. Additionally, the efficient maximum absorption bandwidth was 3.33 GHz, ranging from 14.27 to 17.60 GHz.

Broadband electromagnetic wave absorption performance of CuS-biomass carbon composites derived from moss

The absorption performance of broadband electromagnetic waves is of utmost importance in safeguarding diverse electronic equipment against electromagnetic wave pollution, necessitating materials with low density, thin thickness, and cost-effectiveness to be employed. This study presents the successful synthesis of biomass-derived carbon (BC) composites via the hydrothermal method, utilizing moss coated with flower-like CuS. The CuS was uniformly dispersed on BC matrix, contribute to dipole polarization relaxation. The formation of heterogeneous interfaces between CuS and the BC phase resulted in significant polarization loss. Additionally, the wrinkled BC and flower like CuS constructed by nanosheets boosted scattering of electromagnetic wave due to its large specific surface area, which can provide additional channels for charge transfer and avail dissipation of electromagnetic wave energy. With an efficient absorption bandwidth (EAB) that encompasses the whole X band, the electromagnetic waves absorption (EMA) performance of CuS/BC composites was able to reach a minimum reflection loss of − 28.8 dB.

P-doped in the shell of Ni@WO3-δ nanocomposites induced electronic structure change for improving microwave absorption performance

In this work, we reported P-doped WO3-δ coated Ni (Ni@P-WO3-δ) nanocomposites prepared by the arc discharge method with a subsequent phosphorylation treatment, where the Ni@P-WO3-δ nanocomposites exhibited a tunable electromagnetic wave absorption (EMA) performances by the doping P into the surface shell WO3-δ compared with the Ni@WO3-δ. The experimental evidence combined with the theory calculation results based on First-principles revealed that the doping P into the WO3-δ endowed improvable electrical conductivity, resulting in higher conduction loss for the Ni@P-WO3-δ nanocomposites. Meanwhile, P doping causes more defects and distorted lattice for the WO3-δ shell, which enhances defect-induced polarization loss. As a result, enhanced conduction loss and polarization relaxation loss of the Ni@P-WO3-δ nanocomposites are dominant for improving EMA performance. Further, the study found that the EMA properties of the Ni@P-WO3-δ nanocomposites are related to P doping content and can be tailored efficiently by controlling the amount of P dopant source. Thus, the Ni@P-WO3-δ nanocomposites with appropriate P doping can optimize their balance between impedance matching and dielectric loss, achieving excellent microwave absorption performance. The minimum reflection loss (RL) value of the Ni@P-WO3-δ nanocomposites was − 55.62 dB, and the efficient absorption bandwidth achieved in 12.58 GHz, with a thickness rang of 1.61–5.00 mm. Therefore, this work confirms that doping engineering has a significant application for the design and synthesis of dielectric-typed microwave absorbers.

Urchin-like Fe3O4@C with core-shell structure modified 2D MXene for high-performance microwave absorption

As a typical two-dimensional material, Ti3C2Tx MXene has been considered a novel potential microwave-absorbing material because of its outstanding metallic-like conductivity and rich surface functional groups. Unfortunately, the microwave absorption performance is constrained by its intrinsic high conductivity and single loss mechanism. Herein, an urchin-like Fe3O4@C hollow sphere with core-shell structure was built by hydrothermal and annealing processes, which was subsequently used to modify MXene via electrostatic self-assembly. The Fe3O4@C/Ti3C2Tx hybrid with 30 wt% Fe3O4@C loading displayed a minimum reflection loss (RLmin) value of –64.0 dB at a thickness of 3.32 mm and a maximum efficient absorption bandwidth (EABmax) of 3.02 GHz at 3.24 mm. The exceptional microwave absorption performance is mainly attributed to the impedance matching brought about by the special urchin-like core-shell structure of Fe3O4@C, as well as the rich interfacial polarization and multiple reflections/scatterings. This work offers a novel idea for the design of efficient MXene-based microwave-absorbing materials.

Construction of one-dimensional hierarchical MoS2/Ni3S2 composites with enhanced interfacial polarization and improved wideband microwave absorption

The construction of one-dimensional (1D) sulfides has attracted extensive attention for improving microwave absorption (MA) performance owing to the anisotropic conductive networks. However, the synthesis of conductive 1D hierarchical materials with unique interfacial polarization and excellent MA properties remains challenging. In this study, cable-like MoS2/Ni3S2 was synthesized by a one-step hydrothermal strategy. The complex permittivity of the binary composites could be improved by tuning the thickness of the MoS2 coating. Importantly, the construction of heterogeneous contacts by MoS2 and Ni3S2 contributed to enhanced polarization loss, and the charge distribution was validated by electron holography. The wide efficient absorption bandwidth can reach above 4.8 GHz at a thin thickness. These new discoveries shed light on novel structures for 1D sulfide materials and the design of functional core-shell composites for microwave absorption.

Confined magnetic vortex motion from metal-organic frameworks derived Ni@C microspheres boosts electromagnetic wave energy dissipation

Magnetic domain structure plays an important role in regulating the electromagnetic properties, which dominates the magnetic response behaviors. Herein, unique magnetic vortex domain is firstly obtained in the Ni nanoparticles (NPs) reduced from the Ni-based metal-organic frameworks (MOFs) precursor. Due to both the high symmetry spheres and boundary restriction of graphited carbon shell, confined magnetic vortex structure is generated in the nanoscale Ni core during the annealing process. Meanwhile, MOFs-derived [email protected] assembly powders construct special magnetic flux distribution and electron migration routes. MOFs-derived [email protected] microspheres exhibit outstanding electromagnetic (EM) wave absorption performance. The minimum reflection loss value of [email protected]–V microspheres with vortex domain can reach −54.6 ​dB at only 2.5 ​mm thickness, and the efficient absorption bandwidth up to 5.0 ​GHz at only 2.0 ​mm. Significantly, configuration evolution of magnetic vortex driven by the orientation and reversion of polarity core boosts EM wave energy dissipation. Magnetic coupling effect among neighboring [email protected] microspheres significantly enhances the magnetic reaction intensity. Graphitized carbon matrix and heterojunction Ni–C interfaces further offer the conduction loss and interfacial polarization. As result, MOFs-derived [email protected]–V powders display unique magnetic vortex, electronic migration network, and high-performance EM wave energy dissipation.

Fabrication of Zn0.25Co0.75(MnZr)xFe2–2xO4 spinel ferrites as broadband microwave absorbers in 26.5–40 GHz frequency range

With the advent of 5 G technologies, developing microwave-absorbing materials is an urgent need to address the issues caused by harmful electromagnetic pollution. For this purpose, a novel ferrite series with composition Zn0.25Co0.75(MnZr)xFe2–2xO4 (0.05 ≤ x ≤ 0.25, ∆x = 0.05) was successfully synthesized using the sol-gel citrate route. Also, the effects of the co-substitution of Mn and Zr on the structural, morphological, magnetic, electromagnetic, and microwave absorption characteristics of zinc-cobalt spinel ferrites were studied. The X-ray diffraction patterns indicated the formation of nanoparticles having a cubic spinel lattice structure. Coercivity was found to decrease whereas saturation magnetization increased with the increasing level of substitution from MZ 0.05 to MZ 0.25. In Ka-band, the experimental results showed that MZ 0.15 outperformed all the prepared compositions by exhibiting a minimum reflection loss of −27.99 dB and an efficient absorption bandwidth of 6.5 GHz at a narrow sample thickness of 2.2 mm. The simulation performed by varying the sample thickness revealed that the high values of gross loss tangent and excellent impedance matching lead the composition MZ 0.20 to achieve the optimum value of reflection loss of − 42.71 dB at a matching frequency of 29.20 GHz and a matching thickness of 1.1 mm. These findings suggested that the fabricated materials prove to be efficient absorbers in the 26.5–40 GHz frequency range.

Electromagnetic absorption behavior regulation in bimetallic polyphthalocyanine derived CoFe-alloy/C 0D/2D nanocomposites

Efficient tuning of electrical and magnetic property of a material is of great importance in its functional material design, mechanism understanding, and application expanding. In this work, we propose an efficient strategy for the electromagnetic absorption (EMA) behavior regulation of bimetallic polyphthalocyanine (PPc) derived CoFe-alloy/C nanocomposites. Through the adjusting of calcination temperature and Fe/Co atomic ratio, the microstructure (0D/2D nanocomposite), electrical conductivity, magnetic property, and EMA behavior can be efficiently tuned. After optimization, the efficient absorption bandwidth of CoFe-alloy/C achieves 7.64 GHz, while the maximal absorption gets close to −60 dB, exhibiting superiority to most reported carbon related materials. It is thought that the absorbing mechanism of CoFe-alloy/C is mainly ascribed to the combination of conductance loss, multiple polarization, resonance phenomenon, and hysteresis loss. This research greatly expands the application scope of PPc derivatives, and provides an efficient way to understand the interaction between material and electromagnetic waves.

Highly dispersed Co/Co9S8 nanoparticles encapsulated in S, N co-doped longan shell-derived hierarchical porous carbon for corrosion-resistant, waterproof high-performance microwave absorption

It is highly desirable, but challenging to develop multifunctional electromagnetic wave (EMW) absorbing material for practical applications in some harsh environments. Herein, we successfully embedded highly dispersed Co/Co9S8 nanoparticles into a three-dimensional (3D) honeycomb porous carbon skeleton (the carbon skeleton is derived from longan shell-derived S, N co-doped porous carbon) as a multifunctional material with outstanding EMW absorption properties, hydrophobicity and corrosion resistance. Its superior versatility is attributed to synergistic effects of the S and N dopants, large specific surface area, abundant carbon defects, and 3D porous characteristics. Minimal reflection loss (RLmin) and efficient absorption bandwidth (EAB) of the optimized material as EMW absorbers can achieve −59.9 dB and 6.8 GHz at a thickness of 2.7 mm, respectively, which are superior to most of the reported carbon-based absorbents. Meanwhile, theoretical simulations of the radar scattering cross section (RCS) further confirm that this multifunctional material has outstanding EMW attenuation performance and actual application potential. In addition, the material possesses strong hydrophobicity (124°) and anti-corrosion properties, expanding the scope of potential applications of microwave absorbers. Therefore, this work provides an effective development strategy for the design of anti-corrosion, super-hydrophobic, and high-performance EMW absorbing materials.

Facile fabrication of exfoliated g-C3N4/MWCNTs/Fe3O4 ternary composites with multi-component functional synergy for high-performance microwave absorption

A facile synthesis strategy of multi-component functional synergy absorbers with a minimum reflection loss value (RLmin) of −51.74 dB at only 1.9 mm (11.12 GHz) and the efficient absorption bandwidth (EAB) of 4.28 GHz at 1.4 mm.

NiO/Ni Heterojunction on N-Doped Hollow Carbon Sphere with Balanced Dielectric Loss for Efficient Microwave Absorption.

Hollow carbon spheres are potential candidates for lightweight microwave absorbers. However, the skin effect of pure carbon-based materials frequently induces a terrible impedance mismatching issue. Herein, small-sized NiO/Ni particles with heterojunctions on the N-doped hollow carbon spheres (NHCS@NiO/Ni) are constructed using SiO2 as a sacrificing template. The fabricated NHCS@NiO/Ni displayed excellent microwave absorbability with a minimum reflection loss of -44.04dB with the matching thickness of 2mm and a wider efficient absorption bandwidth of 4.38GHz with the thickness of 1.7mm, superior to most previously reported hollow absorbers. Experimental results demonstrated that the excellent microwave absorption property of the NHCS@NiO/Ni are attributed to balanced dielectric loss and optimized impedance matching characteristic due to the presence of NiO/Ni heterojunctions. Theoretical calculations suggested that the redistribution of charge at the interfaces and formation of dipoles induced by N dopants and defectsareresponsible for the enhanced conduction and polarization losses of NHCS@NiO/Ni. The simulations for the surface current and power loss densities reveal that the NHCS@NiO/Nihas-applicable attenuation ability toward microwave under the practical application scenario. This work paves an efficient way for the reasonable design of small-sized particles with well-defined heterojunctions on hollow nanostructures for high-efficiency microwave absorption.

Facile synthesis of yolk-shell pompon-like Fe@void@CeO2@Ni nanospheres with enhanced microwave absorption properties

Due to the multiple scattering of microwaves and the existence of electron aggregation polarization at heterogeneous interfaces, the design of hollow or anisotropic microstructures has always been one of the research focuses of microwave absorption materials (MAMs). In this work, an effective strategy has been proposed reasonably to manufacture yolk-shell pompon-like Fe@void@CeO2@Ni nanospheres via hydrogen reduction and multiple solvothermal procedures. The results show that the metallic iron core with a diameter of 300 nm and the outer CeO2 layer with a thickness of 70 nm compound to form a yolk-shell structure, and the metallic nickel nanosheets with a length of about 100 nm are evenly embedded in the CeO2 layer to form a pompon-like structure. These unique microstructures provide heterogeneous interfacial polarization, three-dimensional conductive networks, and multiple electromagnetic scattering which bring excellent dielectric loss to the composites. Combined with the inherent strong magnetic loss of magnetic metals, as-prepared composites exhibit excellent microwave absorption properties. The minimal RL value reaches −48.9 dB and the efficient absorption bandwidth reaches 14 GHz with the thickness ranges from 2.5 to 7 mm. Hence, this unique microstructural design scheme has great prospects in the field of MAMs.

Enhanced broadband microwave absorption of Fe/C core-shell nanofibers in X and Ku bands

Fe/C core-shell nanofibers were prepared via a coaxial electrospinning technique integrated with high-temperature carbonization, in which the Fe(NO3)3-containing PAN solution and Fe(NO3)3 constituted the shell and core structures, respectively. The core-shell structure was perfectly maintained after carbonization, and Fe nanoparticles were uniformly distributed within the carbon nanofibers. The effects of the microscopic morphology and crystallinity of the carbon components on the electromagnetic properties, matching characteristics, and electromagnetic wave absorption properties of the Fe/C nanofibers were investigated by adjusting the carbonization temperature. As-prepared Fe/C-800 °C core-shell nanofibers mixed with 10 wt% of paraffin showed a minimal reflection loss (RLmin) of −20.6 dB under 11.52 GHz at 2.5 mm of thickness and lower than −20 dB under 2.3–5.0 mm of thickness. The efficient absorption bandwidth (EAB) was 6.24 GHz in the range of 11.76–18 GHz at 2.1 mm, covering the whole Ku band. The absorption bandwidth can cover the whole X and Ku bands by varying the sample thickness. The efficient absorption bandwidth was 3.84 GHz in the range of 8.08–11.92 GHz at 2.9 mm, covering the X band. The effective complementarities between the magnetic loss of Fe nanoparticles, the dielectric loss of PAN-based carbon, and the core-shell structure promoted impedance matching and multiple interface polarization, endowing the produced composites with prominent absorption characteristics of microwaves. Therefore, the prepared Fe/C core-shell nanofibers are an ideal lightweight and broadband electromagnetic wave (EMW)-absorbing material that exhibits high-performance microwave absorption.

N-doped carbon nanofiber embedded with TiN nanoparticles: A type of efficient microwave absorbers with lightweight and wide-bandwidth

In this work, TiN-embedded in N-doped carbon nanofiber (TiN/NCNF) composites were synthesized by the electrospinning method and heat treatment process subsequently. The TiN/NCNF composites possess different TiN nanoparticles (TiNNPs) content, which can acquire by tuning the percentage of TiNNPs in the polyacrylonitrile precursor solutions. The study shows that TiN/NCNF composites hold more intense and tunable electromagnetic wave (EMW) absorption capabilities, as the embedded TiNNPs with more defects lead to enhanced electrical conductivity and cause room-temperature ferromagnetic. As a result, TiN/NCNF composites can achieve the well-matched impedance and contribute to significant electromagnetic wave attenuation capabilities due to their more interfacial polarization, defect dipole relaxation polarization, enhanced conductance loss, and magnetic loss caused by the defect, resulting in excellent microwave absorbing performance. When the mass fraction of the TiN/NCNF composites (S2 sample with nominal 3 wt% TiNNPs content) was only 30% in the absorbing coating, the optimal reflection loss (RL) value was achieved in − 45.19 dB at 15.6 GHz with a thickness of 1.91 mm, corresponding efficient absorption bandwidth (RL≤−10 dB) of 4.93 GHz. This work provides a novel method of realizing lightweight, wide-bandwidth, and efficient microwave absorbing characteristics of carbon fiber absorbents, using obtained particles from defect engineering strategies as loaded ones. As may be expected, the TiN/NCNF composites would have a significant potential application prospect as microwave absorbing materials.

A novel core/shell cuprous oxide-based structure with improved microwave absorbing and antibacterial performance

The cuprous oxide-based (Cu2O@NiCo2O4/RGO) composites were fabricated using a simple approach to achieve superior microwave absorbing (MA) performance and long-term antibacterial activity. The minimum reflection loss (RL) value reached −51.56 dB with an efficient absorption bandwidth (EAB) of 6.64 GHz. The enhanced MA performance was mainly due to increased interfacial polarization, efficient multiple reflection and scattering, and dipole polarization. Finite element method simulations were adopted to validate the improved MA performance of NiCo2O4 achieved with Cu2O and RGO. Its outstanding long-term antibacterial activity against S. aureus and P. aeruginosa is based on the synergistic effect between the continuous release of copper ions and ROS generated by photocatalysis. Density functional theory simulations were used to analyze the regulation of the work function of Cu2O@NiCo2O4/RGO, demonstrating the putative mechanism of ROS production. The Cu2O@NiCo2O4/RGO material is a promising MA antifouling agent for marine.

Hierarchical Porosities Strategy to Enhance the Absorbing Performance of Graphene Aerogels

Graphene aerogels ha shown fabulous capability in microwave absorption (MA) because of their high surface area, low density, and great chemical stability. To fully exploit its MA capability, the microstructural regulation of graphene aerogels should be carefully studied. Herein, 3D holey‐reduced graphene aerogels (3D‐HGAs) with hierarchical porosity structures composed of nanopores in the plane of graphene oxide (GO) sheets and macropores in aerogel for high‐performance absorbing materials are prepared. The efficient absorption bandwidth (EAB) of 3D‐HGA can reach 7.1 GHz and the minimum reflection loss is −36.4 dB, showing a significant increase contrasted with 3D‐reduced GO aerogels (3D‐RGA). Hierarchical porosity, interfacial polarization, multiple reflections, and matched impedance are all benefits of promoting MA performance. Consequently, the low filling content, low density, and special hierarchical porosity inspire the design of high‐performance radar absorption materials.

Rational Design of Yolk Core-Shell Structure MnO-Co@C Nanospheres for High-Performance Microwave Absorption

MnO-Co@C nanospheres were fabricated by in situ polymerizing and high-temperature carbonizing processes. This unique synthesis method does not require any template or reducing gas. The synthesized multicore-shell structure has a shell of about 500 nm and multiple nuclei of several tens of nanometers. Subsequently, extensive experiments were conducted to adjust the material composition of the nanospheres by adjusting the amount of resorcinol and formaldehyde. The results showed that the obtained material performed best when resorcinol and formaldehyde were added to 0.2 g MnCo2O4 at 0.3 g and 0.42 mL, respectively. The efficient absorption bandwidth (EAB) value reaches 3.3 GHz when the absorber thickness is 3 mm. The reflection loss (RL) is up to −23.8 dB when the frequency is at 8.6 GHz. The unique yolk core-shell structure gives the material a heterogeneous interface, and the enhanced interfacial polarization loss causes the enhanced dielectric loss. The carbon layer with microporosity also causes conduction loss and multiple reflections. The composite structure formed by metallic Co, MnO, and carbon has better impedance matching and improved microwave absorption capability.

Multifunctional graphene/carbon fiber aerogels toward compatible electromagnetic wave absorption and shielding in gigahertz and terahertz bands with optimized radar cross section

Lightweight absorption-dominated electromagnetic interference (EMI) shielding materials with multifunctional integration of multiband absorption, mechanical durability and thermal insulation are urgently required in the practical application of next-generation communication technology combined with flexible electronics. In this work, ultralight and mechanically durable graphene/carbon fiber composite aerogels (CGAs) are synthesized via solvothermal reaction and freeze-drying. The as-prepared CGAs demonstrate excellent compatible electromagnetic wave absorption and shielding performance in both gigahertz (GHz) and terahertz (THz) bands without addition of magnetic component. Specifically, a broad efficient absorption bandwidth of 8.72 GHz at 2–18 GHz and a high mean absorptivity of 97.4% at 0.3–1.5 THz are achieved, respectively. Furthermore, absorbing coatings with CGAs have been proved to contribute to remarkable radar cross section reduction of 39.06 dBm2 by computer-assisted simulation. Additionally, the compressible CGA demonstrates superior and stable EMI shielding performance under in-situ compression. The EMI shielding effectiveness exceeds 35 dB in X-band when a compression ratio of 78.9% is applied. Moreover, CGAs are thermally insulating and self-extinct, making them reliable in practical application. This work provides a series of promising multifunctional lightweight EMI shielding materials for future high-frequency wireless communication.