Hydrophobic SiC@C Nanowire Foam with Broad-Band and Mechanically Controlled Electromagnetic Wave Absorption

Compositional and Hollow Engineering of Silicon Carbide/Carbon Microspheres as High-Performance Microwave Absorbing Materials with Good Environmental Tolerance

Microwave absorbing materials (MAMs) characterized by high absorption efficiency and good environmental tolerance are highly desirable in practical applications. Both silicon carbide and carbon are considered as stable MAMs under some rigorous conditions, while their composites still fail to produce satisfactory microwave absorption performance regardless of the improvements as compared with the individuals. Herein, we have successfully implemented compositional and structural engineering to fabricate hollow SiC/C microspheres with controllable composition. The simultaneous modulation on dielectric properties and impedance matching can be easily achieved as the change in the composition of these composites. The formation of hollow structure not only favors lightweight feature, but also generates considerable contribution to microwave attenuation capacity. With the synergistic effect of composition and structure, the optimized SiC/C composite exhibits excellent performance, whose the strongest reflection loss intensity and broadest effective absorption reach - 60.8 dB and 5.1 GHz, respectively, and its microwave absorption properties are actually superior to those of most SiC/C composites in previous studies. In addition, the stability tests of microwave absorption capacity after exposure to harsh conditions and Radar Cross Section simulation data demonstrate that hollow SiC/C microspheres from compositional and structural optimization have a bright prospect in practical applications.

State-of-the-art in carbides/carbon composites for electromagnetic wave absorption

Electromagnetic wave absorbing materials (EWAMs) have made great progress in the past decades, and are playing an increasingly important role in radiation prevention and antiradar detection due to their essential attenuation toward incident EM wave. With the flourish of nanotechnology, the design of high-performance EWAMs is not just dependent on the intrinsic characteristics of single-component medium, but pays more attention to the synergistic effects from different components to generate rich loss mechanisms. Among various candidates, carbides and carbon materials are usually labeled with the features of chemical stability, low density, tunable dielectric property, and diversified morphology/microstructure, and thus the combination of carbides and carbon materials will be a promising way to acquire new EWAMs with good practical application prospects. In this review, we introduce EM loss mechanisms related to dielectric composites, and then highlight the state-of-the-art progress in carbides/carbon composites as high-performance EWAMs, including silicon carbide/carbon, MXene/carbon, molybdenum carbide/carbon, as well as some uncommon carbides/carbon composites and multicomponent composites. The critical information regarding composition optimization, structural engineering, performance reinforcement, and structure-function relationship are discussed in detail. In addition, some challenges and perspectives for the development of carbides/carbon composites are also proposed after comparing the performance of some representative composites.

Facile preparation and high microwave absorption of flower-like carbon nanosheet aggregations embedded with ultrafine Mo2C

Rational regulation of microstructure and components is vital for excellent microwave absorption performance. In this study, we report flower-like carbon nanosheet architectures embedded with ultrafine Mo₂C as a microwave absorber, prepared via simple carbothermal reduction using Mo-polydopamine as the precursor. We found that the particle size of the obtained Mo₂C/C composites could be simply tailored by the added ammonium molybdate content in the initial solution for the preparation of the Mo-polydopamine precursor. This helped tailor the BET surface area, which significantly impacts microwave absorption performance. The sample with a BET surface area of 173.31 m²g^-1 displayed high-efficiency microwave absorption, and the effective absorbing band reached up to 7.04 GHz (10.96-18 GHz) with the matching thickness of 2.9 mm at relatively low filler loading (only 10 wt%). Thus, the excellent microwave absorption performance and simple preparation process of the flower-like Mo₂C@C composites are promising for applications requiring lightweight and broadband microwave absorption.

Fabrication and Applications of Ceramic-Based Nanofiber Materials Service in High-Temperature Harsh Conditions—A Review

Ceramic-based nanofiber materials have attracted attention due to their high-temperature resistance, oxidation resistance, chemical stability, and excellent mechanical performance, such as flexibility, tensile, and compression, which endow them with promising application prospects for filtration, water treatment, sound insulation, thermal insulation, etc. According to the above advantages, we, therefore, reviewed the ceramic-based nanofiber materials from the perspectives of components, microstructure, and applications to provide a systematical introduction to ceramic-based nanofiber materials as so-called blankets or aerogels, as well as their applications for thermal insulation, catalysis, and water treatment. We hope that this review will provide some necessary suggestions for further research on ceramic-based nanomaterials.

Implementing an Air Suction Effect Induction Strategy to Create Super Thermally Insulating and Superelastic SiC Aerogels

Silicon carbide (SiC) aerogels are promising thermal insulators that are lightweight and possess high thermal stability. However, their application is hindered by their brittleness. Herein, an air suction effect induction (ASEI) strategy is proposed to fabricate a super thermally insulating SiC aerogel (STISA). The ASEI strategy exploits the air suction effect to subtly regulate the directional flow of the SiO gas, which can induce directional growth and assembly of SiC nanowires to form a directional lamellar structure. The sintering time is significantly reduced by >90%. Significant improvements in the compression and elasticity performance of the STISA are achieved upon the formation of a directional lamellar structure through the ASEI strategy. Moreover, the lamellar structure endows the STISA with an ultralow thermal conductivity of 0.019 W m^-1 K^-1 . The ASEI strategy paves the way for structural design of advanced ceramic aerogels for super thermal insulation.

Additive Manufacturing of Resilient SiC Nanowire Aerogels

Resilient ceramic aerogels are emerging as a fascinating material that features light weight, low thermal conductivity, and recoverable compressibility, promising widespread prospects in the fields of heat insulation, catalysis, filtration, and aerospace exploration. However, the construction of the resilient ceramic aerogels with rational designed multiscale architectures aiming for tunable physical and mechanical performances remains a major challenge. Here, 3D constructed resilient SiC nanowire aerogels possessing programmed geometries and engineered mechanical properties are created via additive manufacturing. The Young's modulus of the fabricated SiC nanowire aerogel lattices are tuned systematically from 0.012 MPa to 5.800 MPa spanning over 2 orders of magnitude. More importantly, the customized lightweight and resilient SiC nanowire aerogels show a low thermal conductivity (0.046 W m^-1 K^-1). The present work provides another approach to the design and rapid fabrication of resilient ceramic aerogels toward flexible thermal management devices, lightweight engineered structures, and other potential applications.

Controllable Synthesis of Mo3C2 Encapsulated by N-Doped Carbon Microspheres to Achieve Highly Efficient Microwave Absorption at Full Wavebands: From Lemon-like to Fig-like Morphologies

Mo₃C₂@N-doped carbon microspheres (Mo₃C₂@NC) have been discovered to be a family of superior microwave absorbing materials. Herein, Mo₃C₂@NC was synthesized through a simple high-temperature carbonization process by evaporating a graphite anode and Mo wire in Ar and N₂ atmospheres with an N-doping content of 6.4 at. %. Attributing to the self-assembly mechanism, the number of Mo wires inserted into the graphite anode determined the morphologies of Mo₃C₂@NC, which were the unique lemon-like (1- and 2-Mo₃C₂@NC) and fig-like (3-, 4-, and 5-Mo₃C₂@NC) microstructures. 1- and 2-Mo₃C₂@NC exhibited powerful reflection losses (RLs) of -45.60, -45.59, and -47.11 dB at the S, C and X bands, respectively, which corresponded to thinner thicknesses. 3-, 4-, and 5-Mo₃C₂@NC showed outstanding absorption performance at the C, X, and Ku bands, respectively, with each value of a minimum RL less than -43.00 dB. In particular, the strongest RL (-43.56 dB) for 5-Mo₃C₂@NC corresponded to an ultrathin thickness of 1.3 mm. In addition, the maximum effective absorption bandwidth was 6.3 GHz for 4-Mo₃C₂@NC. After analysis, all Mo₃C₂@NC samples showed well-matched impedance due to the enhanced dielectric loss caused by the unique carbon structure and moderate magnetic loss derived from the weak magnetic property of Mo₃C₂. More importantly, the unique lemon-like and fig-like microstructures created sufficient interfaces and differentiated multiple reflection paths, which greatly contributed to the strong microwave absorptions at full wavebands. In full consideration of the simple preparation method and tunable absorption properties, Mo₃C₂@NC composites can be regarded as excellent electromagnetic wave absorption materials.

Flexible Ceramic Fibers: Recent Development in Preparation and Application

Flexible ceramic fibers (FCFs) have been developed for various advanced applications due to their superior mechanical flexibility, high temperature resistance, and excellent chemical stability. In this article, we present an overview on the recent progress of FCFs in terms of materials, fabrication methods, and applications. We begin with a brief introduction to FCFs and the materials for preparation of FCFs. After that, various methods for preparation of FCFs are discussed, including centrifugal spinning, electrospinning, solution blow spinning, self-assembly, chemical vapor deposition, atomic layer deposition, and polymer conversion. Recent applications of FCFs in various fields are further illustrated in detail, including thermal insulation, air filtration, water treatment, sound absorption, electromagnetic wave absorption, battery separator, catalytic application, among others. Finally, some perspectives on the future directions and opportunities for the preparation and application of FCFs are highlighted. We envision that this review will provide readers with some meaningful guidance on the preparation of FCFs and inspire them to explore more potential applications.

Construction of one-dimensional MoO2/NC heteronanowires for microwave absorption

A combination of a special micro–nanostructure and multiple components has been proven as an effective strategy to strengthen the microwave attenuation capacity. In this work, one-dimensional MoO₂/N-doped carbon (NC) nanowires with a heterostructure have been successfully prepared by utilizing mild in situ chemical oxidative polymerization and pyrolysis treatment. After compounding them with a thermoplastic polyurethane (TPU) matrix, the flexible composites exhibit tunable wave absorbing performance by modulating the filler loading of MoO₂/NC heteronanowires. Experimental results demonstrate that the minimum reflection loss value of the MoO₂/NC–TPU hybrid is up to −35.0 dB at 8.37 GHz under a thickness of only 2.3 mm with 40 wt% filler amounts. Moreover, the effective absorption bandwidth enables 3.26 GHz to be achieved (8.49–11.75 GHz) when the thickness changes to 2.0 mm, covering almost the whole X-band. Meanwhile, when the filler loading becomes 30 wt%, dual-absorption peaks appear. The relevant absorption mechanism is mainly attributed to the dielectric loss including strong dipolar/interfacial polarizations, Debye relaxation loss and multiple reflection and scattering.

A combination of a special micro–nanostructure and multiple components has been proven as an effective strategy to strengthen the microwave attenuation capacity.

Fabrication of Hollow Cube Dual-Semiconductor Ln2O3/MnO/C Nanocomposites with Excellent Microwave Absorption Performance

Metal-organic frameworks (MOFs) have been verified as ideal precursors for preparing highly effective microwave absorbers. However, it is still challenging to fabricate a thin, lightweight, and well-organized nanostructure with strong microwave absorption (MA) capability and wide absorption bandwidth. In this study, hollow cube dual-semiconductor Ln₂O₃/MnO/C (Ln = Nd, Gd, Er) nanocomposites, which are effective microwave absorbers, have been fabricated via one-step high-temperature carbonization of Ln-Mn-MOFs. The effect of band gap on the MA performance of various nanocomposites synthesized at the same carbonization temperature is investigated. Gd₂O₃/MnO/C-800 shows superior MA capacity with maximum reflection loss (RL_max) of -64.4 dB at 12.8 GHz and 1.86 mm-thickness. When the thickness is 1.44 mm, the RL value is obtained as -52.7 dB at 16.8 GHz, and at a low frequency of 4.36 GHz and thickness of 4.59 mm, the RL value reaches -56.4 dB. Further, the effect of temperature on the MA properties of Gd₂O₃/MnO/C is examined. The results reveal that Gd₂O₃/MnO/C-700 has an ultrahigh MA bandwidth of 6.6 GHz, covering the entire Ku bands at 2.09 mm-thickness. Overall, this work demonstrates a facile strategy to construct hollow, homogeneous ternary composites with outstanding MA performance.

Enhanced interfacial reaction of silicon carbide fillers onto the metal substrate in carbon nanotube paste for reliable field electron emitters

Adhesion of carbon nanotube (CNT) onto a cathode substrate is very crucial for field electron emitters that are operating under high electric fields. As a supporting precursor of CNT field emitters, we adopted silicon carbide (SiC) nano-particle fillers with Ni particles and then enhanced interfacial reactions onto Kovar-alloy substrates through the optimized wet pulverization process of SiC aggregates for reliable field electron emitters. As-purchased SiC aggregates were efficiently pulverized from 20 to less than 1 micro-meter in a median value (D50). CNT pastes for field emitters were distinctively formulated by a mixing process of the pulverized SiC aggregates and pre-dispersed CNTs. X-ray photoelectron spectroscopy studies showed that the optimally pulverized SiC-CNT paste-emitter had a stronger Si 2p^3/2 signal in the Ni₂Si phase than the as-purchased one. The Si 2p^3/2 signal would represent interfacial reaction of the SiC nano-particle onto Ni from the CNT paste and the Kovar substrate, forming the supporting layer for CNT emitters. The optimal paste-emitter even in a vacuum-sealed tube exhibited a highly reliable field emission current with a high current density of 100 mA cm^-2 for over 50 h along with good reproducibility. The enhanced interfacial reaction of SiC filler onto the metal substrates could lead to highly reliable field electron emitters for vacuum electronic devices.

Alternating Multilayered Si3N4/SiC Aerogels for Broadband and High-Temperature Electromagnetic Wave Absorption up to 1000 °C

Lightweight electromagnetic (EM) wave absorbers made of ceramics have sparked tremendous interest for applications in EM wave interference protection at high temperatures. However, EM wave absorption by pure ceramics still faces huge challenges due to the lack of efficient EM wave attenuation modes. Inspired by the energy dissipation mechanism during fracture of lobster shells with a soft and stiff multilayered structure, we fabricate a high-performance EM wave absorption ceramic aerogel composed of an alternating multilayered wave transparent Si₃N₄ (N) layer and wave absorption SiC (C) layer by a simple restack method. The obtained N/C aerogel shows ultralow density (∼8 mg/cm³), broad effective absorption bandwidth (8.4 GHz), strong reflection loss (-45 dB) at room temperature, and excellent EM wave absorption performance at high temperatures up to 1000 °C. The attenuation of EM wave mainly results from a "reflection-absorption-zigzag reflection" process caused by the alternating multilayered structure. The superior absorption performance, especially at high temperatures, makes the N/C aerogel promising for next-generation wave absorption devices served in high-temperature environments.

Structural and Electronic Properties of Heterostructures Composed of Antimonene and Monolayer MoS2

Antimonene is found to be a promising material for two-dimensional optoelectronic equipment due to its broad band gap and high carrier mobility. The van der Waals heterostructure, as a unique structural unit for the study of photoelectric properties, has attracted great attention. By using ab initio density functional theory with van der Waals corrections, we theoretically investigated the structural and electronic properties of the heterostructures composed of antimonene and monolayer MoS₂. Our results revealed that the Sb/MoS₂ hetero-bilayer is an indirect semiconductor with type-II band alignment, which implies the spatial separation of photogenerated electron–hole pairs. Due to the weak van der Waals interlayer interactions between the adjacent sheets of the hetero-bilayer systems, the band structures of isolated antimonene and monolayer MoS₂ are preserved. In addition, a tunable band gap in Sb/MoS₂ hetero-bilayer can be realized by applying in-plane biaxial compressing/stretching. When antimonene and monolayer MoS₂ are stacked into superlattices, the indirect semiconductors turn into direct semiconductors with the decreased band gaps. Our results show that the antimonene-based hybrid structures are good candidate structures for photovoltaic devices.

Implications from Broadband Microwave Absorption of Metal-Modified SiC Fiber Mats

Understanding the physical requirements for a broad bandwidth is vital for the design of high-efficiency microwave absorber. Our recent works on silicon carbide (SiC) fiber mats-based absorbers imply that metal modification (e.g., Fe or Hf) could benefit their bandwidth effectively. For verification, we fabricated a Co/SiC fiber mat via a similar electrospinning process and subsequent pyrolysis at 1400 °C in Ar atmosphere. The results indicate that after Co modification, the SiC fiber mats show elevated permittivity and tangent loss. With a proper amount of Co adding, the mats could exhibit a wide bandwidth of around 8 GHz (ranging from 10 to 18 GHz) for effective absorption (reflection loss (RL) less than -10 dB) at 2.8 mm thickness. This is similar to our previous findings, confirming that metal modification could be an effective approach to extend the bandwidth of SiC mat absorbers. Explanations can be found through theoretical analysis with the quarter wavelength (λ/4) cancellation theory. It suggests that the declining permittivity (with the increase of frequency) is the key to keep the wavelength in material (λ_m) nearly unchanged within a frequency range. As a result, in this range, λ/4 cancellation could still be satisfied without changing thickness, which could explain the reasons for the broad bandwidth of metal-modified SiC fiber mats. With this model, it is further predicted that the effective absorption bandwidth could be even extended to be around 12 GHz with appropriate tangent loss. It should be emphasized that the implications obtained in this study could also be applicable to other dielectric absorbers. The requirement of permittivity and the proposed approach could serve as guidelines to achieve a wide bandwidth on a dielectric absorber relying on the λ/4 cancellation principle.