Flexible,hydrophobic SiC ceramic nanofibers used as high frequency electromagnetic wave absorbers

In this paper, flexible hydrophobic SiC ceramic nanofibers have been successfully fabricated via electrospinning and subsequent high temperature heat treatment. The synthesized SiC ceramic nanofibers show excellent flexibility without any breakage even under a bending angle of 142.6°, and high hydrophobicity with a water contact angle of 149.05°. The SiC nanofibers exhibit excellent electromagnetic (EM) wave absorption properties with an effective absorption bandwidth (reflection loss (RL) <−10 dB, 90% EM wave absorbed) of 4–18 GHz. The maximum reflection loss of SiC ceramic nanofibers reaches −19.4 dB at 5.84 GHz. In addition, the nanofibers are environmentally stable in 2 mol/L NaOH solution for 2 h and high temperature of 500 °C in air atmosphere. The excellent EM wave absorption performance, flexibility, hydrophobic properties, corrosion resistant properties in alkali environment and high temperature stability make SiC ceramic nanofibers to be a potential candidate for EM wave absorption used in harsh environment.     

MXene nanohybrids: Excellent electromagnetic properties for absorbing electromagnetic waves

MXenes have been long sought-after for their exceptional electrical conductivity, large family members, mechanical stability, tunable surface groups, and ease of processability. In particular, they have become the leading materials for electromagnetic (EM) wave absorption. Herein, MXenes’ crystal and electronic structures, as well as EM wave absorption related to those properties, are discussed. The structures and EM functions of MXene hybrids are systematically arranged. The crystal and electronic structure, EM characteristics, and EM response of MXene-based hybrids, as well as their relationship are revealed. Moreover, we propose the main challenges, and predict future directions in this rapidly broadening field.     

Nano-SiC-decorated Y2Si2O7 ceramic for enhancing electromagnetic waves absorption performance

To improve the electromagnetic (EM) wave absorption performance of rare earth silicate in harsh environments, this work synthesized dense SiC–Y₂Si₂O₇ composite ceramics with excellent EM wave absorption properties by using the polymer permeation pyrolysis (PIP) process, which introduced carbon and SiC into a porous Y₂Si₂O₇ matrix to form novel composite ceramics. SiC–Y₂Si₂O₇ composite ceramics with different numbers of PIP cycles were tested and analysed. The results show that the as-prepared composites exhibit different microstructures, porosities, dielectric properties and EM wave absorption properties. On the whole, the SiC–Y₂Si₂O₇ composite ceramics (with a SiC/C content of 29.88 wt%) show superior microwave absorption properties. The minimum reflection loss (RL_min) reaches ?16.1 dB when the thickness is 3.9 mm at 9.8 GHz. Moreover, the effective absorption bandwidth (EAB) included a broad frequency from 8.2 GHz to 12.4 GHz as the absorbent thickness varied from 3.15 mm to 4.6 mm. In addition, the EM wave absorption mechanism was analysed profoundly, which ascribed to the multiple mediums of nanocrystalline, amorphous phases and turbostratic carbon distributed in the Y₂Si₂O₇ matrix. Therefore, SiC–Y₂Si₂O₇ composite ceramics with high-efficiency EM wave absorption performance promise to be a novel wave absorbing material for applications in harsh environments.     

Lightweight and resilient SiOC/SiC foam with efficient electromagnetic wave absorption and high-temperature stability

Integrating multiple functions such as high electromagnetic (EM) wave absorption, thermal insulation, and resilience into one material is critical, especially for applications in harsh environment. SiC ceramic has received considerable attention as high-temperature wave absorber, but its applications are limited by common wave absorption performance and brittleness of ceramics. Here by incorporating SiO₂ with SiC in a unique three-dimensional network structure, SiOC/SiC foam consisting of abundant SiOC thin flakes interconnected by numerous long interweaving SiC nanowires have been prepared. The foam shows high EM wave absorption with minimum reflection loss of −30.23 dB, broad effective absorption bandwidth of 5.4 GHz, and a nearly complete compressive resilience from 10% strain. Besides, the foam displays high-temperature resistance up to 1400°C in air and good thermal insulation performance. Such multifunctional material is promising for applications in advanced aerospace industry under extreme conditions.     

In situ growth of one-dimensional carbon-rich SiC nanowires in porous Sc2Si2O7 ceramics with excellent microwave absorption properties

For enhancing the absorption ability of dielectric and electromagnetic wave (EMW), C-rich SiC NWs /Sc₂Si₂O₇ ceramics are successfully fabricated through in-situ growth of SiC nanowires (NWs) into porous Sc₂Si₂O₇ ceramics by precursor infiltration and pyrolysis (PIP) at 1400?°C in Ar. SiC NWs are in-situ formed in the pore channels via a vapor-liquid-solid (VLS) mechanism, the relative complex permittivity increases notably with the content of absorber (C-rich SiC NWs), which tune the microstructure and dielectric property of C-rich SiC NWs/Sc₂Si₂O₇ ceramics. Meanwhile, the minimum reflection coefficient (RC) of C-rich SiC NWs/Sc₂Si₂O₇ ceramic decreases from ?9.5?dB to ??35.5?dB at 11?GHz with a thickness of 2.75?mm, and the effective absorption bandwidth (EAB) covers the whole X band (8.2–12.4?GHz) when the content of absorber is 24.5?wt%. The results indicate that Sc₂Si₂O₇ ceramics decorated with SiC NWs and nanosized carbon have a superior microwave-absorbing ability, which can be contributed to the Debye relaxation, interfacial polarization and conductivity loss enhanced by in-situ formed SiC NWs and nanosized carbon phases. The C-rich SiC NWs /Sc₂Si₂O₇ ceramics can be a promising microwave absorbing materials within a broad bandwidth.     

Flexible SiC-CNTs hybrid fiber mats for tunable and broadband microwave absorption

Flexible microwave absorbers with high stability are in increasing demand for the applications under harsh conditions. SiC as a functional ceramic material has the feature of high environmental tolerance and adjustable electromagnetic (EM) absorbing properties, making them suitable to be applied for harsh environments. However, the electrical property of SiC requires to be further enhanced to obtain qualified EM absorbing performance. In this work, multiwall carbon nanotubes (CNTs) were introduced to SiC to enhance the electrical properties. Flexible two-dimensional (2D) CNTs loaded SiC fiber mats were prepared as EM absorbers via electrospinning and polymer-derived-ceramic (PDC) methods. The CNTs inside the fibers can form conductive networks and act as reinforcement to ensure high flexibility and enhance the microwave absorption properties of SiC mats. Thus, a reflection loss of ?61 dB and an effective absorption band (EAB) of 2.9 GHz were obtained. More importantly, the EM absorption can be adjusted by tuning the content of CNTs and the EAB can cover the entire X-band by adjusting the material thickness. The work provided a facile strategy to fabricated flexible 2D ceramic mats with high environmental stability and tunable electrical properties, which may shed light on the production of reliable EM absorber for broadband EM absorption applications.     

Strong effect of atmosphere on the microstructure and microwave absorption properties of porous SiC ceramics

Excellent microwave absorption properties of porous SiC ceramics were successfully synthesized using SiC/camphene slurries with various polycarbosilane (PCS) contents related to the SiC powder. The compositions of the nanowires (NWs) growth in the pore channels of porous SiC ceramics strongly depended on the pyrolysis atmosphere, with N₂-generating Si₃N₄ NWs and Ar SiC NWs. With the increase of PCS content, the minimum reflection coefficient (RC) of porous SiC ceramics decreased from ?7.6 dB to ?67.4 dB in Ar and from ?10.9 dB to ?24.7 dB in N₂, respectively. The effective absorption bandwidth (EAB) of porous SiC ceramics could be up to 8.1 GHz in Ar and 4.5 GHz in N₂. The enhanced microwave absorption properties of porous SiC ceramics could be attributed to the formation of SiC nano-crystalline, nanosized carbon and the NWs, which would increase the amount of boundaries and defects, leading to the electronic dipole polarization and interfacial scattering.     

Electromagnetic wave absorbing performance of multiphase (SiC/HfC/C)/SiO2 nanocomposites with an unique microstructure

Dielectric properties and electromagnetic (EM) wave absorbing performance of monolithic (SiC/HfC/C)/SiO₂ nanocomposites (denoted as SHCOs) have been investigated in the X-band (8.2–12.4 GHz). The multiphase SHCOs are composed of insulating SiO₂ and SiC/HfC/C nanocomposite fillers (SHC), which fillers composed of semiconducting β-SiC, conductive HfC-Carbon core-shell nanoparticles, and interconnected carbon nanoribbons. Dielectric response indicates that the increased SHC content results in an enhanced imaginary part of the permittivity and dielectric loss, leading to an improved EM absorbing performance. The unique microstructure with an EM wave-transparent SiO₂ matrix is favorable for impedance matching and effective EM wave propagation. The enhanced interface polarization and conduction loss are considered as the key mechanisms for EM wave attenuation. The minimum reflection loss of the SHCOs achieves – 60.7 dB containing 20 vol% of SHC (at 9.98 GHz) with the sample thickness of 3.33 mm, and the effective absorbing bandwidth (EAB) covers ca. 72 % of the X-band. The monolithic (SiC/HfC/C)/SiO₂ nanocomposites with outstanding EM wave absorbing performance are promising candidates for EM application at high temperatures.     

High temperature anti-oxidative and tunable wave absorbing SiC/Fe3Si/CNTs composite ceramic derived from a novel polysilyacetylene

With the rapid development of high power electromagnetic (EM) equipment and high-speed aircraft, the powerful and high oxidation-resistance absorbers are fundamentally desirable for the EM field. Herein, a novel high temperature anti-oxidative SiC/Fe₃Si/CNTs composite is synthesized by a facile polymer derived ceramic (PDC) route from a Fe-containing polysilyacetylene (PSA). The microstructure of as-prepared SiC/Fe₃Si/CNTs composite absorber is featured by micro-sized SiC ceramic grains with spherical Fe₃Si nanoparticles and carbon nanotubes (CNTs) attached to. The vector network analyzer tests show a tunable wave-absorbing performance by adjusting the thickness of layer, and the effective bandwidth (the reflection loss < −10 dB) is 3.3–16.8 GHz for the sample S-1400 (heat treatment at 1400 °C in nitrogen flow). The minimal RL value is −41.2 dB at 10.5 GHz at a thickness of 2 mm and an effective bandwidth is nearly 4 GHz (12.9–16.9 GHz) at the thickness of only 1.5 mm. Moreover, after the oxidation treatment at 800 °C in the air, this absorber maintains the main structure and shows a good high temperature oxidation resistance. This absorber still remains excellent wave absorption property, in view of a minimal RL value of −40 dB at the thickness of 3 mm and a bandwidth of 4.8 GHz (10.4–15.2 GHz) at the thickness of 2.5 mm. The mechanism of high EM wave absorption performance is studied and attributed to the impendence matching, polarization, and the magnetic properties. Thus, the SiC/Fe₃Si/CNTs composite is a promising EM absorber for high-temperature EM wave-absorbing applications.     

The microstructure and electromagnetic wave absorption properties of near-stoichiometric SiC fibre

The microstructure and electromagnetic (EM) properties of near-stoichiometric SiC fibres (with C/Si ratio of 1.125) were analyzed and evaluated in detail. The SiC fibres consisted of β-SiC nanocrystallines and free carbon, and exhibited a uniquely specific skin-core structure with thin carbon layer of 5 nm on their surfaces. The relative complex permittivity increased with the increasing fibre volume fraction from 13 vol% to 27.5 vol%. The imaginary part of permittivity increased from 1.36 to 2.13 at 10 GHz, due to more SiC nanocrystallines and interfaces generating. The EM wave absorption properties were enhanced by the increasing fibre volume fraction and the effective absorption bandwidth was approximately 2.6 GHz when the fibre volume fraction was 27.5 vol%.     

Relationship between microstructure and electromagnetic properties of SiC fibers

The microstructure and electromagnetic (EM) properties of four kinds of SiC fibers have been studied. These fibers are composed of amorphous SiC_xO_y, SiC nanocrystallines, free carbon, and nanopores, whose volume fractions are analyzed quantitatively. The content of free carbon notably affects the fiber's conductivity: the logarithm of conductivity increases linearly with the increase in the free carbon content when the volume fraction sum of free carbon and SiC exceed the percolation threshold. The EM loss mechanism is mainly composed of the conduction loss caused by free carbon and the polarization loss caused by SiC nanocrystallines. The content of free carbon is the decisive factor for the type of EM loss mechanism: the proportion of conduction loss increases linearly with the increase in free carbon content. Conduction loss is necessary for good EM absorption property, and polarization loss favors broadband absorption. For SiC fibers dominated by polarization loss, excellent absorption properties can be obtained in composites with higher fiber volume fraction (>20 vol%), which is crucial for structural absorbing materials.     

Oxidation behavior of SiC nanowires: A nanowire radius-dependent growth of oxide scale thickness

SiC nanowires (SiC NWs) possess both high thermal stability of SiC ceramic and one-dimensional nanoscale features, which makes them highly attractive as reinforcements in ceramics or building units in resilient ceramic nanowires aerogels (NWAs) as well as blocks for electronic nanodevices. Understanding the oxidation behavior of SiC NWs at high temperatures is essential for their practical applications. Herein, we investigated the oxidation behavior of SiC NWs at 900–1200°C in air. Two oxidation stages were found, including an initial stage controlled by the reaction between oxygen and SiC at the SiO₂/SiC interface and a subsequent oxygen diffusion–dependent stage. The oxide scale thickness was strongly influenced by the radius of the SiC NWs. With the increase of the NW radius from 40 to 120 nm, the oxidation activation energy of the oxidation process increases from 84.05 to 98.32 kJ/mol. The thermal insulation performances of SiC NWA, which is composed of SiC NWs, have been improved after oxidation. The evolution of the thermal insulation performance of SiC NWA during oxidation is consistent with the trends of the growth of the amorphous oxide layer, which indicates that exploring the oxidation kinetics is of great significance in understanding the high-temperature behavior of SiC NW-based materials. The present work provides insight into exploring the size effects on oxidation of SiC NWs, which may be helpful to further understanding the high-temperature applications of SiC NWA.     

Construction of Si3N4/SiO2/SiC–Y2Si2O7 composite ceramics with gradual impedance matching structure for high-temperature electromagnetic wave absorption

Good impedance matching is vital in upgrading the performance of electromagnetic (EM) wave-absorbing materials. In this study, Si₃N₄/SiO₂/SiC–Y₂Si₂O₇ composite ceramics were synthesized by sintering and chemical vapor infiltration (CVI) technology with gradual impedance matching. The relationship between the microstructure of the as-prepared composite ceramics and EM wave absorption characteristics was thoroughly explored. It was found that the amorphous Si₃N₄, SiO₂, and SiC layers were constructed with a gradual impedance matching structure, which not only improved impedance matching but also increased the number of nano interfaces. More importantly, SiC nanocrystals effectively increased the conduction loss, and the presence of defects and nanoscale heterogeneous interfaces further increased the polarization loss. Consequently, the as-prepared composite ceramics displayed enhanced EM wave absorption properties, with a minimum reflection coefficient (RC_min) value of less than ?20 dB over a temperature range of 25 °C (RT)-300 °C, and an effective absorption bandwidth (EAB) maintained at 4.2 GHz with the thickness range of 3.75–4.75 mm. These results demonstrated the practical significance of high-performance EM wave absorption materials that can be applied in high-temperature and water vapor environments.     

Enhanced microwave absorption properties of Fe-doped SiOC ceramics by the magnetic-dielectric loss properties

The combination of multiple loss characteristics is an effective approach to achieve broadband microwave wave absorption performance. The Fe-doped SiOC ceramics were synthesized by polymer derived ceramics (PDCs) method at 1500 °C, and their dielectric and magnetic properties were investigated at 2–18 GHz. The results showed that adding Fe content effectively controlled the composition and content of multiphase products (such as Fe₃Si, SiC, SiO₂ and turbostratic carbon). Meanwhile, the Fe promoted the change of the grain size. The Fe₃Si enhanced the magnetic loss, and the SiC and turbostratic carbon generated by PDCs process significantly increased the polarization and conductance loss. Besides, the magnetic particles Fe₃Si and dielectric particles SiO₂ improved the impedance matching, which was beneficial to EM wave absorption properties. Impressively, the Fe-doped SiOC ceramics (with Fe addition of 3 wt %) presented the minimum reflection coefficient (RC_min) of ?20.5 dB at 10.8 GHz with 2.8 mm. The effective absorption bandwidth (EAB, RC < ?10 dB) covered a wide frequency range from 5 GHz to 18 GHz (covered the C, X and Ku-band) when the absorbent thickness increased from 2 mm to 5 mm. Therefore, this research opens up another strategy for exploring novel SiOC ceramics to design the good EM wave-absorbing materials with broad absorption bandwidth and thin thickness.     

Photoluminescence property of inexpensive flexible SiC nanowires membrane by electrospinning and carbothermal reduction

Silicon carbide nanowire (SiC NW), as a typical wide band gap semiconductor was used as light-emitting materials and devices in high-temperature and harsh environments due to its excellent properties. In this paper, flexible ultra-long SiC NWs membrane was successfully synthesized by electrospinning and subsequently high-temperature sintering using phenolic resin and silica sol as precursors. Results of system characterization reveal that SiC NWs possess a smooth and uniform surface with diameter distribution mainly between 50-300 nm and a length of more than tens of micrometers, forming a network structure. The growth mechanism of synthesized nanowires was mainly carbothermal reduction in situ and was accompanied by vapor-solid (V-S) reaction. The present work provides a simple and cost-effective way for controllable fabrication of SiC NWs membrane. The photoluminescence spectrum of SiC NWs membrane emerged a clear blue shift, indicating a potential application in optoelectronic devices and discussed the potential applications of SiC NWs membrane in other fields.     

Ultra-light 3D bamboo-like SiC nanowires felt for efficient microwave absorption in the low-frequency region

Nonmagnetic ceramics are ideal microwave absorbing materials used in high-temperature and oxidizing environments. However, low-frequency absorbing properties of this material are rarely reported because low-frequency absorbing requires nonmagnetic materials to have much higher permittivity. In this research, a series of three-dimensional architectures formed by SiC nanowires with different microstructures felt were fabricated to address this issue. The morphology of the SiC_nw (linear, bamboo-shaped, and worm-like) dominated by the VLS growth mechanism can be manipulated by the silicon vapor concentration, which is governed by the vaporization temperature of the mixed silicon source (Si and SiO₂) in different sintering processes. The spontaneously overlapped bamboo-shaped SiC nanowires in these felt enhance the permittivity and conductivity loss and produce multiple scattering effects on the incident EM waves, thus increasing the low-frequency wave absorption ability. The RL_min of the bamboo-shaped SiC_nw felt reaches ?44.3 dB at 3.85 GHz with the corresponding EAB of 0.64 GHz (3.6–4.24 GHz) at a thickness of 3.5 mm. The density of the SiC_nw felt is as low as 0.022 g/cm³ due to the high porosity (99.3%) of 3D networks, which fulfills lightweight requirements and highly efficient electromagnetic wave absorption.     

Enhanced electromagnetic microwave absorption of SiC nanowire-reinforced PDC-SiC ceramics catalysed by rare earth

Rare earth elements can modulate the dielectric constant of materials and significantly improve their dielectric properties. Herein, SiCnws/SiC ceramics were prepared through polymer derived ceramics (PDCs) technology with rare earth Sc particles as the catalyst. The Sc particles promote the precipitation of SiC and C from the matrix. Furthermore, the SiCnws, grown via the vapour-liquid-solid (VLS) mechanism, construct the three dimensional (3D) network structure to improve impedance matching and loss characteristics. Remarkably, the SiCnws/SiC ceramics minimum reflection coefficient (RC_min) achieved a value of ?33.2 dB at 9.4 GHz with a thickness of 2.75 mm, and the effective absorption bandwidth (EAB) was 4.2 GHz covering the whole X band. When microwaves permeated into the SiCnws/SiC ceramics, those trapped in the 3D network structure underwent a variety of microwave energy dissipation processes, including multiple reflections, scattering, and interface and dipole polarisation. Consequently, SiCnws-reinforced PDC-SiC ceramics catalysed by rare earth emerge as a promising new approach to enhance electromagnetic (EM) wave absorption performance.     

Improvement of SiC–Si/MAS Interface in the Joints via In Situ Synthesizing SiC Nanowires

Improving the SiC–Si/MAS interface is essential for enhancing the SiC–Si/MAS joint strength. In this work, well‐dispersed SiC nanowires (NWs) were in situ synthesized at the SiC–Si/MAS interface in the joints as the anchors, which made the fracture feature of the joints change from SiC–Si/MAS interface fracture to MAS fracture, resulting in an obvious improvement of the shear strength. The results showed that the average shear strength of the joints was increased by 49% by in situ synthesizing SiC NWs, which was primarily attributed to the excellent interface anchoring effects of SiC NWs involving the interfacial bonding anchoring between SiC NWs and SiC–Si layer and the mechanical interlocking anchoring between SiC NWs with the junctions and the surrounding grains. This study may provide new insight into the anchoring mechanisms and suggest a new design methodology for nanowire joints, leading to improved joint strength.     

Synthesis of SiC nanowires by a simple chemical vapour deposition route in the presence of ZrB2

The SiC nanowires (NWs) were fabricated by a simple chemical vapour deposition (CVD) method at high temperature using Si, phenolic resin, and ZrB₂ powder. The morphologies of the fabricated SiC NWs included SiC/SiO₂ chain-beads and straight wires with core-shell structures. The fabricated SiC NWs were micrometre-to-millimetre in length, with chains 100–300 nm in diameter and beads with diameters of less than 1 μm. The core-shell-structured SiC NWs consisted of crystalline SiC cores and thin amorphous SiO₂ shells. SiC crystals grew in the [111] direction governed by a vapour-solid (VS) mechanism. The added ZrB₂ promotes the generation of gaseous species at higher gas pressures, which contributes to the formation of SiC NWs by CVD. The fabricated SiC NWs exhibited good photoluminescence properties due to many stacking faults and the presence of amorphous SiO₂.     

Synthesis of CoNi/SiO2 core-shell nanoparticles decorated reduced graphene oxide nanosheets for enhanced electromagnetic wave absorption properties

In this research, the nanocomposites, CoNi/SiO₂ core-shell nanoparticles decorated reduced graphene oxide (RGO) nanosheets, are successfully synthesized via liquid-phase reduction reactions combined with a sol-gel route. The structures, morphologies, chemical composition and magnetic properties of CoNi nanoparticles, CoNi/SiO₂ core-shell nanoparticles and RGO/CoNi/SiO₂ nanocomposites have been investigated in exhaustive detail. The electromagnetic (EM) parameters of RGO/CoNi/SiO₂ nanocomposites are measured using a vector network analyzer. The results reveal that the RGO/CoNi/SiO₂ nanocomposites display enhanced EM wave absorption properties with the maximum reflection loss (R_L) of ??46.3?dB at 6.2?GHz with a matching thickness of 4.2?mm. Additionally, the absorption bandwidth corresponding to the R_L less than ??10?dB is up to 14.3?GHz (3.7–18.0?GHz) with a matching thickness range of 2.0–5.0?mm. To comprehensively consider the absorption bandwidth and the maximum R_L, the integrational method which defines ΔS as the integration area of R_L (R_L < ??10?dB) and R_E as EM wave absorption efficiency is adopted to reveal that the RGO/CoNi/SiO₂ nanocomposites exhibit the excellent absorption properties with the matching thickness of only 2.0?mm. Accordingly, the as-prepared RGO/CoNi/SiO₂ nanocomposites could be applied as promising EM wave absorption materials.