Construction of Hollow Carbon Nanofibers with Graphene Nanorods as Nano-Antennas for Lower-Frequency Microwave Absorption

Corn silk-derived biomass carbon materials for low-frequency microwave absorption and energy storage

Biomass carbon (BC) materials derived from agricultural waste have shown great potential in microwave absorption (MA). However, current research mainly focuses on high-frequency (8-18 GHz) MA, and much less effort has been spent on low-frequency (2-8 GHz) MA and other important functionalities such as energy storage. Herein, corn silk rich in carbon is utilized to prepare BC materials with uniform pores and large specific surface area through a straightforward chemical activation and carbonization process. Attributed to its optimized impedance matching, interfacial polarization and (N and O) heteroatom-induced dipole polarization, the optimal sample exhibits superior low-frequency MA capability, including a strong reflection loss (RL) of -75 dB at 6.88 GHz, an effective absorption bandwidth (EAB, RL ≤ -10 dB) down to 2.8 GHz, and excellent radar cross-section reduction. Furthermore, it achieves a high initial discharge specific capacity of 1015.54 mA·h g-1 and stable cycling performance at 0.5 A g-1 in lithium-ion batteries owing to its heteroatom-rich porous structure with a large specific surface area. Our study offers a simple and low-cost way to fabricate high-performance multifunctional BC materials for low-frequency MA and lithium-ion energy storage.

Design and fabrication of 1D nanomaterials for electromagnetic wave absorption

The design and fabrication of high-performance electromagnetic wave (EMW) absorbing materials are essential in developing electronic communication technology for defense and civilian applications. These materials function by interacting with EMWs, creating various effects such as polarization relaxation, magnetic resonance, and magnetic hysteresis in order to absorb EMWs. Significant progress has been made to improve the dimensional performance of such materials, emphasizing the 'thin, light, broad, and strong' functional specifications. One-dimensional (1D) nanostructures are characterized by high surface area, low density, and unique electromagnetic properties, providing promising solutions to address some of the challenges in facilitating multiple reflections and wideband resonances, which are crucial for effective EMW attenuation. This paper provides an overview of recent advances in exploring 1D structures for enhancing EMW absorption and their controllability. The design and fabrication of nanofibers, nanowires, and other 1D nanostructures are highlighted. The advantages of 1D nanomaterials in EMW absorption are also described. Challenges and future directions are discussed, focusing on developing new design concepts and fabrication methods for achieving high-performance and lightweight EMW absorbers and enhancing fundamental understanding of EMW absorption mechanisms.

Construction of Chiral-Magnetic-Dielectric Trinity Structures with Different Magnetic Systems for Efficient Low-Frequency Microwave Absorption

The fabrication of carbon nanocoil (CNC)-based chiral-dielectric-magnetic trinity composites holds great significance in low-frequency microwave absorption fields. However, it is not clear that how the different magnetic systems affect the magnetic and frequency responses of the composites. Herein, four types of magnetic metals, FeCo, CoNi, FeNi, and FeCoNi, are selected to be combined with the chiral templates respectively, resulting in four types of chiral-dielectric-magnetic composites with similar morphology. The CNC templates endow all the composites with excellent dielectric loss. Further permeability analysis and the micro-magnetic simulation confirm that the frequency response region can be well adjusted by changing the magnetic systems with specific magnetic resonance modes and magnetic domain motion. Due to the synergistic effect between magnetism, chirality, and dielectricity, the FeNi-based composites exhibit the best low-frequency microwave absorption performance. The minimum RL of -60.7 dB is achieved at 6.7 GHz with an ultra-low filling ratio of 10%, and the EAB value in low-frequency region is extended to 3.7 GHz. This study provides further guidelines for the design of the chiral-dielectric-magnetic trinity composites in low-frequency microwave absorption.

A Nanoconfinement Strategy to Construct Co@CNTs for Lightweight and Ultra-Broadband Microwave Absorption

The construction of stable and efficient nanocomposites with low addition and light weight has always been the goal pursued in the field of electromagnetic wave (EMW) absorption. In this study, the Co@CNTs nanocomposites with Co nanoparticles (13 nm) nanoconfined in the carbon nanotube (CNT) are successfully synthesized by a simple hydrothermal method and phenolic assisted pyrolysis method. The degree of graphitization of CNTs and the microstructure of Co nanoparticles can be effectively regulated by controlling the calcination temperature. The sample calcined at 700 °C can obtain excellent absorption performance at a low filling capacity of 10 wt.%: the minimum reflection loss (RL) is -41.2 dB and the effective absorption bandwidth (EAB) reaches a maximum width of 14.2 GHz. When the sample thickness is only 2.2 mm, the EAB of <-20 dB reaches 8.3 GHz, which is the maximum EAB of most current Co-based absorbers. In particular, the polarization and ferromagnetic coupling behaviors are elucidated in depth with the aid of electromagnetic field simulations using the High-Frequency Structure Simulator (HFSS). This work provides a new nanoconfinement strategy for constructing the Co@CNTs nanocomposites as lightweight and ultra-broadband absorbing materials for EMW protection and EMW pollution control.

MOFs-Derived Strategy and Ternary Alloys Regulation in Flower-Like Magnetic-Carbon Microspheres with Broadband Electromagnetic Wave Absorption

Broadband electromagnetic (EM) wave absorption materials play an important role in military stealth and health protection. Herein, metal-organic frameworks (MOFs)-derived magnetic-carbon CoNiM@C (M = Cu, Zn, Fe, Mn) microspheres are fabricated, which exhibit flower-like nano-microstructure with tunable EM response capacity. Based on the MOFs-derived CoNi@C microsphere, the adjacent third element is introduced into magnetic CoNi alloy to enhance EM wave absorption performance. In term of broadband absorption, the order of efficient absorption bandwidth (EAB) value is Mn > Fe = Zn > Cu in the CoNiM@C microspheres. Therefore, MOFs-derived flower-like CoNiMn@C microspheres hold outstanding broadband absorption and the EAB can reach up to 5.8 GHz (covering 12.2-18 GHz at 2.0 mm thickness). Besides, off-axis electron holography and computational simulations are applied to elucidate the inherent dielectric dissipation and magnetic loss. Rich heterointerfaces in CoNiMn@C promote the aggregation of the negative/positive charges at the contacting region, forming interfacial polarization. The graphitized carbon layer catalyzed by the magnetic CoNiMn core offered the electron mobility path, boosting the conductive loss. Equally importantly, magnetic coupling is observed in the CoNiMn@C to strengthen the magnetic responding behaviors. This study provides a new guide to build broadband EM absorption by regulating the ternary magnetic alloy.

A Novel Nano-Laminated GdB2C2 with Excellent Electromagnetic Wave Absorption Performance and Ultra-High-Temperature Thermostability

A novel nano-laminated GdB2C2 material was successfully synthesized using GdH2, B4C, and C via an in situ solid-state reaction approach for the first time. The formation process of GdB2C2 was revealed based on the microstructure and phase evolution investigation. Purity of 96.4 wt.% GdB2C2 was obtained at a low temperature of 1500 °C, while a nearly fully pure GdB2C2 could be obtained at a temperature over 1700 °C. The as-obtained GdB2C2 presented excellent thermal stability at a high temperature of 2100 °C in Ar atmosphere due to the stable framework formed by the high-covalence four-member and eight-member B-C rings in GdB2C2. The GdB2C2 material synthesized at 1500 °C demonstrated a remarkably low minimum reflection loss (RLmin) of -47.01 dB (3.44 mm) and a broad effective absorption bandwidth (EAB) of 1.76 GHz. The possible electromagnetic wave absorption (EMWA) mechanism could be ascribed to the nano-laminated structure and appropriate electrical conductivity, which facilitated good impedance matching, remarkable conduction loss, and interfacial polarization, along with the reflection and scattering of electromagnetic waves at multiple interfaces. The GdB2C2, with excellent EMWA performance as well as remarkable ultra-high-temperature thermal stability, could be a promising candidate for the application of EMWA materials in extreme ultra-high temperatures.

Three-dimensional biotemplate-loaded nickel sulfide vacancies engineered to promote the absorption of electromagnetic waves

Vacancy engineering offers an appealing strategy for modifying the electronic structure of transition metals. Transition metals with abundant sulfur vacancies can significantly contribute to the microwave absorption capabilities of absorbers. In this study, an NixSy@De composite material was synthesized through a straightforward hydrothermal synthesis technique. The effective absorption bandwidth (EAB) of this composite material reached 9.86 GHz at 1.44 mm. A minimum reflection loss (RLmin) of -33.61 dB at 1 mm was achieved, and after mild etching, the RLmin further improved to -93.53 dB at 1.16 mm to achieve a high-attenuation microwave absorption. The exceptional performance of NixSy@De for the absorption of electromagnetic waves (EMWs) is based on its high dielectric loss, substantial magnetic loss, and excellent impedance matching. This work combines transition metal sulfides with three-dimensional biotemplated diatomite, providing valuable insights into the design of advanced EMW absorbing materials.

GO-decorated chain-like Fe2O3/FeMn2O4 NPs (GO-Fe2O3/FeMn2O4 nanocomposites) with ultrabroad band microwave absorption

In this study, GO-Fe2O3/FeMn2O4 nanocomposites were synthesized in which the chain-like Fe2O3/FeMn2O4 NPs were decorated on GO sheets. The crystalline phases of Fe2O3 and FeMn2O4 were recognized from the XRD pattern. TEM images showed that the oval-shaped Fe2O3/FeMn2O4 NPs with an almost narrow size distribution (60-80 nm) were connected to form chains. The Fe2O3/FeMn2O4 NP chains were decorated on the GO sheets in different weight ratios and GO-Fe2O3/FeMn2O4 (1 : 3), GO-Fe2O3/FeMn2O4 (1 : 4), and GO-Fe2O3/FeMn2O4 (1 : 5) nanocomposites, which were respectively named S1, S2, and S3, were finally produced. The microwave attenuation performance of all samples was investigated based on their EM parameters. The results demonstrated the superior attenuation ability of S1 and S2 in terms of reflection loss and absorption bandwidth. The minimum reflection losses (RLmin) for S1 and S2 reached over -85 dB and -88 dB and at the rest of the frequency band, the RL varied from -10 dB to -40 dB for samples thicker than 2.4 mm. The effective bandwidth (RL ≤ -10 dB) was 10 GHz, which covered the entire Ku and X bands for S1, and was -8.3 GHz, which eliminated the entire Ku band and half of the X band, for S2 at 2.4-3.6 mm with matching thicknesses. S3 exhibited a relatively weaker absorption performance. The results confirmed that achieving the maximum EM absorption performance of GO-based composites is guaranteed by optimizing the weight ratio of the decorative material (Fe2O3/FeMn2O4 NPs).