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.     

A novel SiC-based microwave absorption ceramic with Sc2Si2O7 as transparent matrix

For enhancing the dielectric and electromagnetic wave (EMW) absorption ability, SiC-Sc₂Si₂O₇ ceramics were fabricated by introducing SiC into porous Sc₂Si₂O₇ ceramics through precursor infiltration and pyrolysis (PIP). The Sc₂Si₂O₇ powders were synthesized by sol-gel method and Sc₂Si₂O₇ ceramics were prepared by pressure-less sintering. The results indicate that SiC nano-crystalline and turbostratic carbon derived from polycarbosilane distributed uniformly in electrically insulating matrix (Sc₂Si₂O₇ matrix), resulting in tunable dielectric permittivity and EMW absorbing properties. Additionally, the content of C-rich SiC can be efficiently adjusted by PIP times. The SiC-Sc₂Si₂O₇ ceramic showed excellent microwave absorption performance when the content of C-rich SiC was 25.3?wt.%. A minimum reflection coefficient of ?51.3?dB was obtained at 9.56?GHz with the specimen thickness of 3.6?mm. The effective absorption bandwidth covered 3.6?GHz (from 8.7 to 12.3?GHz). The excellent microwave absorption abilities of SiC-Sc₂Si₂O₇ ceramic were mainly attributed to uniform distribution of C-rich SiC in Sc₂Si₂O₇ matrix. The special structure can improve the impedance matching and enhance microwave absorption performance. Moreover, the defects, interfaces and conductive network existed in the materials, which can synergistically improve the EMW absorption ability.     

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.     

Enhanced electromagnetic wave absorption properties of a novel SiC nanowires reinforced SiO2/3Al2O3·2SiO2 porous ceramic

To realize the broad-bandwidth and high-efficiency absorption characteristics, a novel SiC nanowires reinforced SiO₂/3Al₂O₃·2SiO₂ porous ceramic was successfully fabricated by method of precursor infiltration pyrolysis (PIP). Polycarbosilane (PCS) and ferrocene (Fe(C₅H₅)₂) were used as the precursor and catalyst to incorporate SiC nanowires into the SiO₂/3Al₂O₃·2SiO₂ porous ceramic. The curvy SiC nanowires formed three-dimensional (3D) networks with a proper nanometer heterostructure, thereby consuming the microwave energies. The influence of SiC nanowires contents on the microwave absorption properties was investigated. The results indicate that the SiC nanowires contents can be tuned by controlling the PIP cycles, thereby modifying the dielectric properties of as-prepared composite ceramics. The dielectric and electromagnetic wave absorption performances are gradually enhanced with an increasing of SiC nanowires contents. The SiC nanowires reinforced SiO₂/3Al₂O₃·2SiO₂ composite ceramic exhibits excellent electromagnetic wave absorption abilities when the SiC nanowires content is 23.9% (PIP5). The minimum reflection coefficient (RC_min) of the composite ceramic is −30 dB at 10.0 GHz, corresponding to more than 99.9% of the electromagnetic wave consumption. The effective absorption bandwidth (EAB) can cover the frequency ranges of 8.2–12.4 GHz (the entire X-band) at the thickness of 5.0 mm. In general, the novel SiC nanowires reinforced SiO₂/3Al₂O₃·2SiO₂ composite ceramic can be considered as a promising electromagnetic wave absorbing material.     

Polymer-derived Co2Si(Co)/SiCN composite ceramics with tunable microwave absorption properties

In this paper, Co₂Si(Co)/SiCN composite ceramics were synthesized by simple precursor-derived ceramics method. The phase composition, morphology, and microwave absorption properties of Co₂Si(Co)/SiCN composite ceramics at different pyrolysis temperatures (1000–1400°C) were studied. When pyrolysis temperature was 1300°C, carbon nanowires (CNWs), Co₂Si, Si₂N₂O, SiC and Si₃N₄ were in situ generated and the best electromagnetic wave (EMW) absorption performance was obtained. The minimum reflection loss reached−50.04 dB at 4.81 mm, and the effective absorption bandwidth broadened to 3.48 GHz (14.52–18 GHz) at 1.31 mm. The excellent EMW absorption performance mainly comes from the coexistence of multiple loss mechanisms, including the magnetic loss of Co₂Si, the conduction loss of CNWs, and the heterogeneous interfaces polarization between varieties of nanocrystals and amorphous ceramic matrix. By adjusting the sample thickness from 1 to 5 mm, the effective absorption of S1300 can cover the entire X and Ku bands, from 3.36 to 18 GHz. This study provides a simple way to synthesize high performance ceramic-based microwave absorbing materials.     

Carbon@SiC(SiCnws)-Sc2Si2O7 ceramics with multiple loss mediums for improving electromagnetic shielding performance

Multiple loss media can be applied, not only to improve the performance of microwave absorption materials, but also to promote the electromagnetic (EM) shielding efficacy of materials. In this study, multiple loss mediums (C, SiC, and SiCnws) were introduced into Sc₂Si₂O₇ ceramics to synthesise carbon@SiC(SiCnws)-Sc₂Si₂O₇ ceramics by H₂ reduction and CVI technology. After 1–3 CVI process cycles, the open porosity decreases from 34.7 %±0.5 % to 8.4 %±0.3 % filled with multiple loss media. The results show that CH₃SiCl₃ provides the raw material for synthesising SiC(SiCnws) and carbon in ceramics, resulting in adjustable EM shielding properties. Furthermore, the EM shielding mechanism consists mainly of the combined synergistic effect of C, SiC, and SiCnws, which contributes to the reflective shielding efficacy, and promotes the absorption shielding effectiveness, including dipole polarization loss, and interface polarization loss. Subsequently, carbon@SiC(SiCnws)-Sc₂Si₂O₇ ceramics with a multiple loss medium of 31.2 wt% ±0.22 wt% achieve a total EM shielding effectiveness value of 31.8 dB (99.92 % electromagnetic waves are attenuated) with an X-band thickness of 2 mm. Therefore, carbon@SiC(SiCnws)-Sc₂Si₂O₇ ceramics are a promising novel composite, applicable in future EM shielding in high temperature environments.     

Improved electromagnetic absorbing properties of Si3N4–SiC/SiO2 composite ceramics with multi-shell microstructure

Porous Si₃N₄–SiC composite ceramic was fabricated by infiltrating SiC coating with nano-scale crystals into porous β-Si₃N₄ ceramic via chemical vapor infiltration (CVI). Silica (SiO₂) film was formed on the surface of rod-like Si₃N₄–SiC grains during oxidation at 1100 °C in air. The as-received Si₃N₄–SiC/SiO₂ composite ceramic attains a multi-shell microstructure, and exhibits reduced impedance mismatch, leading to excellent electromagnetic (EM) absorbing properties. The Si₃N₄–SiC/SiO₂ fabricated by oxidation of Si₃N₄–SiC for 10 h in air can achieve a reflection loss of ?30 dB (>99.9% absorption) at 8.7 GHz when the sample thickness is 3.8 mm. When the sample thickness is 3.5 mm, reflection loss of Si₃N₄–SiC/SiO₂ is lower than ?10 dB (>90% absorption) in the frequency range 8.3–12.4 GHz, the effective absorption bandwidth is 4.1 GHz.     

In-situ synthesis of ternary layered Y3Si2C2 ceramic on silicon carbide fiber for enhanced electromagnetic wave absorption

A novel ternary layered ceramic of Y₃Si₂C₂ was successfully in-situ synthesized on the surface of home-made third-generation KD-SA SiC fiber for the first time by molten salt method aimed at improving the electromagnetic wave (EMW) absorption. After in-situ synthesis of Y₃Si₂C₂ ceramic layer on SiC fiber (SiC_f/Y₃Si₂C₂), significantly improved EMW absorption performance was obtained. The minimum reflection loss (RL_min) of ?16.97 dB was reached in SiC_f/Y₃Si₂C₂ composites at the thickness of only 2.19 mm, and the effective absorption bandwidth (EAB) was up to 5.44 GHz (12.56–18 GHz) at a thin thickness of 2.64 mm. The improvement in EMW absorption of SiC_f/Y₃Si₂C₂ is mainly attributed to enhanced dielectric loss and conduction loss resulting from increased heterogeneous interfaces and multiple reflections and scattering originating from net structure. The SiC_f/Y₃Si₂C₂ could be a promising EMW absorber for application in high-performance EMW absorbing materials.     

Enhanced electromagnetic absorption properties of Fe-doped Sc2Si2O7 ceramics

Doping transition metal elements in a crystal causes distortion and defects in the lattice structure, which change the electronic structure and magnetic moment, thereby adjusting the electrical conductivity and electromagnetic properties of the material. Fe-doped Sc₂Si₂O₇ ceramics were synthesized using the sol-gel method for application to microwave absorption. The effect of Fe-doped content on the electromagnetic (EM) and microwave absorption properties was investigated in the Ku-band (12.4–18 GHz). As expected, the dielectric and magnetic properties improve substantially with increasing Fe content. Fe doping causes defects and impurity levels, which enhance polarization loss and conductance loss, respectively. Fe replaces Sc atoms in the ScO₆ octahedral structure, creating a difference in spin magnetic moments, which increases the magnetic moment. Moreover, the magnetic coupling of Fe and O atoms occurs at the Fermi level, which benefits magnetic loss. In particular, when the Fe content is 6%, the fabricated Fe-doped Sc₂Si₂O₇ ceramics show an absorption property with absorption peaks located at 14.5 GHz and a minimum reflection loss (RL_min) of ?12.8 dB. Therefore, Fe-doped Sc₂Si₂O₇ ceramics with anti-oxidation and good microwave absorption performance have a greater potential for application in high-temperature and water-vapor environments.     

Porous SiC–Si3N4 composite ceramics with excellent EMW absorption property prepared by gelcasting using DMAA

The porous SiC–Si₃N₄ composite ceramics with good EMW absorption properties were prepared by combination of gelcasting and carbothermal reduction. The pre-oxidation of Si₃N₄ powders significantly improved the rheological properties of slurries (0.06 Pa s at 103.92 s⁻¹) and also suppressed the generation of NH₃ and N₂ from Si₃N₄ hydrolysis and reaction between Si₃N₄ and initiator APS, thereby reducing the pore defects in green bodies and enhancing mechanical properties with a maximum value of 42.88 MPa. With the extension of oxidation time from 0 h to 10 h, the porosity and pore size of porous SiC–Si₃N₄ composite ceramics increased from approximately 41.86% and 1.0–1.5 μm to 46.33% and ~200 μm due to the production of CO, N₂ and gaseous SiO, while the sintering shrinkage decreased from 16.24% to 10.50%. With oxidation time of 2 h, the Si₂N₂O fibers formed in situ by the reaction of Si₃N₄ and amorphous SiO₂ effectively enhanced the mechanical properties, achieving the highest flexural strength of 129.37 MPa and fracture toughness of 4.25 MPa m^1/2. Compared with monolithic Si₃N₄ ceramics, the electrical conductivity, relative permittivity and dielectric loss were significantly improved by the in-situ introduced PyC from the pyrolysis of three-dimensional network DMAA-MBAM gel in green bodies and the SiC from the carbothermal reduction reaction between PyC and SiO₂ and Si₃N₄. The porous SiC–Si₃N₄ composite ceramics prepared by the unoxidized Si₃N₄ powders demonstrated the optimal EMW absorption properties with reflection loss of −22.35 dB at 8.37 GHz and 2 mm thickness, corresponding to the effective bandwidth of 8.20–9.29 GHz, displaying great application potential in EMW absorption fields.     

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.     

Enhanced wet-oxidation resistance of Y2O3 modified SiC ceramics by the formation of Y2Si2O7 protective layer

The poor wet-oxidation resistance limits the long-life service of SiC_f/SiC composites as the hot end components of aero-engines. The stability of SiC_f/SiC composites under high-temperature wet oxygen environment can be promoted by more robust SiC matrix. In this work, the effect of Y₂O₃ on the corrosion behaviors of SiC ceramics in flowing O₂/H₂O atmosphere at 1400 ℃ was studied. Duo to the continuous Y₂Si₂O₇ layer formed on the surface, SiC-Y₂O₃ ceramics exhibit much better wet-oxidation resistance than original SiC ceramics. During the oxidation process, Y₂O₃ dispersed in the ceramics migrates to the surface and reacts with SiO₂ to form β-Y₂Si₂O₇. Subsequently, the β-Y₂Si₂O₇ aggregates and grows to form a continuous Y₂Si₂O₇ layer, inhibiting the corrosion from oxidizing medium to the inner SiC matrix. This study is expected to provide important ideas for the design and structure regulation of wet-oxidation resistant SiC_f/SiC composites.     

Adjustable iron-containing SiBCN ceramics with high-temperature microwave absorption and anti-oxidation properties

Quaternary siliconboron carbonitride (SiBCN) ceramics show excellent high-temperature stability and oxidation resistance, indicating great potential as high-temperature electromagnetic (EM) wave absorbing materials. In this contribution, an efficient and facile method was developed to prepare bulk iron-containing SiBCN (Fe–SiBCN) ceramics with remarkable EM wave absorption at high temperature by pyrolyzing boron and iron containing precursors (PSZV-B–Fe). The introduction of boron and iron not only improves the high-temperature stability but also influences the complex permittivity and EM wave absorption. The minimum reflection coefficient (RC_min) is −61.05 dB, and the effective bandwidth absorption (EAB) is 3.35 GHz (9.05–12.4 GHz). The RC_min will be decreased to −52.3 dB at 600°C as well as the EAB covers more than 67% of the X band (2.8 GHz). The high-temperature stable Fe–SiBCN ceramics with adjustable dielectric properties can be utilized as high-performance EM wave-absorbing materials in high-temperature and harsh environments.     

Impedance matching optimization of SiCf/Si3N4–SiOC composites for excellent microwave absorption properties

SiC fiber reinforced ceramic matrix composites (SiC_f-CMCs) are considered to be one of the most promising materials in the electromagnetic (EM) stealth of aero-engines, which is expected to achieve strong absorption and broad-band performance. Multiscale structural design was applied to SiC_f/Si₃N₄–SiOC composites by construction of micro/nanoscale heterogeneous interfaces and macro double-layer impedance matching structure. SiC_f/Si₃N₄–SiOC composites were fabricated by using SiC fibers with different conductivities and SiOC–Si₃N₄ matrices with gradient impedance structures to improve impedance matching effectively. Owing to its unique structure, SiC_f/Si₃N₄–SiOC composites (A3-composites) achieved excellent EM wave absorption performance with a minimum reflection coefficient (RC_min) of ?25.1 dB at 2.45 mm and an effective absorption bandwidth (EAB) of 4.0 GHz at 2.85 mm in X-band. Moreover, double-layer SiC_f/Si₃N₄–SiOC with an improved impedance matching structure obtained an RC_min of ?56.9 dB and an EAB of 4.2 GHz at 3.00 mm, which means it can absorb more than 90% of the EM waves in the whole X-band. The RC is less than ?8 dB at 2.6–2.8 mm from RT to 600 °C in the whole X-band, displaying excellent high-temperature absorption performance. The results provide a new design opinion for broad-band EM absorbing SiC_f-CMCs at high temperatures.     

Dielectric properties of Si3N4–SiCN composite ceramics in X-band

Si₃N₄–SiCN composite ceramics were successfully fabricated through precursor infiltration pyrolysis (PIP) method using polysilazane as precursor and porous Si₃N₄ as preform. After annealed at temperatures varying from 900 °C to 1400 °C, the phase composition of SiCN ceramics, electrical conductivity and dielectric properties of Si₃N₄–SiCN composite ceramics over the frequency range of 8.2–12.4 GHz (X-band) were investigated. With the increase of annealing temperature, the content of amorphous SiCN decreases and that of N-doped SiC nano-crystals increases, which leads to the increase of electrical conductivity. After annealed at 1400 °C, the average real and imaginary permittivities of Si₃N₄–SiCN composite ceramics are increased from 3.7 and 4.68 × 10^?3 to 8.9 and 1.8, respectively. The permittivities of Si₃N₄–SiCN composite ceramics show a typical ternary polarization relaxation, which are ascribed to the electric dipole and grain boundary relaxation of N-doped SiC nano-crystals, and dielectric polarization relaxation of the in situ formed graphite. The Si₃N₄–SiCN composite ceramics exhibit a promising prospect as microwave absorbing materials.     

MXene-derived TiC/SiBCN ceramics with excellent electromagnetic absorption and high-temperature resistance

Demand for high-performance electromagnetic (EM) wave absorbing materials with high-temperature resistance is always urgent for application in a harsh environment. In this contribution, two-dimensional material, Ti₃C₂T_x MXene, was introduced into a hyperbranched polyborosilazane. After pyrolyzation, the as-prepared TiC/SiBCN ceramics present excellent EM wave absorption in X-band. The TiC nanograins appearing after annealing provide multilevel reflection and interface polarization. Dipole polarization formed at interface defects, in company with interfacial polarization, also makes a great contribution to enhanced EM wave absorption. The TiC/SiBCN nanocomplex prepared with 5 wt% Ti₃C₂T_x MXene possesses a minimum reflection coefficient of −45.44 dB at 10.93 GHz and abroad bandwidth 8.4 and 12.4 GHz, almost covering the entire X-band. Tuning the thickness in the range of 2.35-2.54 mm, the effective absorption band can achieve the entire X-band. And the EM wave absorbing performance has been maintained to a large extent at 600°C with the minimum reflection coefficient of −26.12 dB at 12.13 GHz and the effective absorption bandwidth of 2 GHz. Last but not the least, TiC/SiBCN ceramics offer a good thermal stability in argon as well as in air atmosphere, making it possible to serve in high-temperature detrimental environments. This study is expected to provide a new perspective for the design of high-performance absorbing materials that are able to be used in harsh environments, especially in high temperatures.     

Carbon nanowires reinforced porous SiO2/3Al2O3·2SiO2 ceramics with tunable electromagnetic absorption properties

Porous SiO₂/3Al₂O₃·2SiO₂ ceramics decorated with carbon nanowires (CNWs) were manufactured by catalytic chemical vapor deposition (CCVD) for wide absorption band and high absorption application. The results show that the as-prepared CNWs grow uniformly in porous SiO₂/3Al₂O₃·2SiO₂ ceramics to form the three dimensional(3D) structure. The content of CNWs can be effectively controlled by changing the weight of catalyst, and the dielectric properties of CNWs-SiO₂/3Al₂O₃·2SiO₂ composite ceramics can be tuned. As expected, the CNWs-SiO₂/3Al₂O₃·2SiO₂ composite ceramics possess wonderful electromagnetic (EM) absorption properties due to the effects of conductive loss, impedance match and dipoles polarization. It can be concluded that the minimum reflection coefficient (RC_min) of CNWs-SiO₂/3Al₂O₃·2SiO₂ composite ceramics reaches ?31?dB?at 9.1?GHz (which means 99.9% of EM wave powers are absorbed) and the effective absorption bandwidth (EAB) covers the whole X-band (8.2–12.4?GHz) at the thickness of 5.0?mm.     

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.     

Hollow porous magnetic/magnetic heterostructured CoFe/defective spinel microspheres template-free synthesis and effective modulation of microwave absorption performance

To create a spinel ferrite with excellent performance for electromagnetic (EM) wave absorption in the low frequency range of 4–6 GHz, compositions based on Co_0.75Zn_0.125Fe_0.125Fe₂O₄ (CZF–1) and Co_0.5Zn_0.25Fe_0.25Fe₂O₄ (CZF–2) with multiple elements substituted for A sites were synthesized by using solvothermal method. Hollow porous magnetic/magnetic heterostructure microspheres (HHMs) of CZF–A1 and CZF–A2 with multiple interfaces were prepared by hydrogen–thermal reduction of CZF–1 and CZF–2, and their unique structure and EM absorption properties were investigated in detail. The widest effective absorption bandwidth (EAB) of CZF–A1 and CZF–A2 was 4.1 GHz (13.6–17.7 GHz) and 3.7 GHz (8.0–11.7 GHz) for a corresponding thickness of 1.4 mm and 2.0 mm, respectively. In addition, the minimum reflection loss (R.L_min) of CZF–A1 and CZF–A2 reached –49.1 dB (at f_m = 13.4 GHz) and –45.0 dB (at f_m = 4.2 GHz) at a thicknesses of 1.6 mm and 3.7 mm, respectively. More specifically, in the low frequency region of 4–6 GHz, CZF–A1 and CZF–A2 exhibited excellent EM wave absorption due to the effective regulation of their natural resonance frequency. The EM wave absorption frequency band of CZF–A1 and CZF–A2 samples was able to completely cover the 4–6 GHz frequency region for at coating thickness of CZF–A1 and CZF–A2 was only 3.5 mm and 3.3 mm respectively, and their R.Lmin reached –36.5 dB and –22.6 dB. Moreover, the absorption mechanisms of CZF–A1 and CZF–A2 including magnetic resonance, eddy current loss, interfacial polarization and dipole polarization were also investigated in detail. This research provides new insights and guidance for the development of spinel ferrite-based EM absorbers for high efficiency EM wave absorption in the low frequency (4–6 GHz) region.     

Multifunction properties of SiOC reinforced with carbon fiber and in-situ SiC nanowires

Herein, the SiC nanowires were successfully fabricated via chemical vapor infiltration (CVI) into carbon fiber felts (CFs) and then the SiOC/SiCnws/CFs composites were synthesized by precursor infiltration and pyrolysis (PIP) processes. Results indicated that the lightweight composites possessed enhanced mechanical performance, low thermal conductivity, and excellent electromagnetic wave absorption properties. Detailedly, the compressive strength reached to 22.0 MPa and 9.6 MPa after two PIP processes cycles in z and x/y directions, respectively. Meanwhile, the composites exhibited tailored electromagnetic wave absorption performance with the effective absorption bandwidth of 3.06 GHz, and the minimum reflection loss (RL_min) was -48.2 dB with a thickness of 3.6 mm. The present work has a guidance to prepare and design multifunction properties for application in harsh environment.