Effect of SiC nanowires on the high-temperature microwave absorption properties of SiCf/SiC composites

SiC-nanowire-reinforced SiC_f/SiC composites were successfully fabricated through an in situ growth of SiC nanowires on SiC fibres via chemical vapour infiltration. The dielectric and microwave absorption properties of the composites were investigated within the frequency range of 8.2–12.4 GHz at 25–600 °C. The electric conductivity and complex permittivity of the composites displayed evident temperature-dependent behaviour and were enhanced with increasing temperature. The composites exhibited superior microwave absorption abilities with a minimum reflection loss value of ?47.5 dB at 11.4 GHz and an effective bandwidth of 2.8 GHz at 600 °C. Apart from the contribution of the interconnected SiC nanowire network and multiple reflections, the excellent microwave absorption performance was attributed to dielectric loss that originated from SiC nanowires with abundant stacking faults and heterostructure interfaces. Results suggested that the composites are promising candidates for high-temperature microwave absorbing materials.     

Dielectric permittivity and microwave absorption properties of transition metal Ni and Mn doped SiC nanowires

In order to enhance the microwave absorption properties of SiC nanowires, two transition metals Ni and Mn were selected as doping elements to improve their electromagnetic parameters. The experimental results indicate that Ni and Mn as catalysts reduce the stacking defect density of SiC nanowires, which will weaken the interfacial polarization loss induced by stacking defects. However, they can increase the electrical conductivity of SiC nanowires and generate new impurity defects, thereby effectively improving the conductance loss and dipole polarization loss. Therefore, the dielectric loss of SiC nanowires is significantly enhanced, but they still do not have considerable magnetic loss capability. In addition, Ni and Mn doping also improves the impedance matching characteristics of SiC nanowires. Therefore, the microwave absorption ability of SiC nanowires is effectively enhanced. As the nanowire filling ratio is 20 wt%, the minimum reflection loss of the Ni_0.01Si_0.99C nanowire is −11.1 dB and the effective absorption bandwidth is 1.1 GHz (9.3–12.4 GHz) at a thickness of 2.8 mm; Mn_0.01Si_0.99C nanowires have a minimum reflection loss of −16.8 dB and an effective absorption bandwidth of 3.1 GHz (9.3–12.4 GHz) at a thickness of 2.8 mm.     

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.     

Core–shell spherical graphite@SiC attenuating agent for AlN-based microwave attenuating ceramics with high–efficiency thermal conduction and microwave absorption abilities

A core–shell structured spherical graphite (SG)@SiC attenuating agent with a tunable silicon carbide (SiC) shell thickness was synthesized via in-situ solid-liquid reaction of SG and Si. Then, fully dense 10 wt%SG@SiC/AlN microwave attenuating composite ceramics were prepared through hot-pressing sintering, and the morphology of SG@SiC particle was well maintained. By moderately modulating the thickness of the SiC shell with relatively low complex permittivity and thermal conductivity, an effectively inhibited solid solution of SiC into AlN, weakened dipole and electron polarization, enhanced conduction loss, and an improved impedance matching, thermal conductivity and microwave loss capacity were simultaneously achieved. Thus, the SG@SiC/AlN composite exhibit excellent and impressive thermal conductivity of 63.92 W m⁻¹·K⁻¹ and minimum reflection loss of −34.2 dB. The outstanding performance of SG@SiC/AlN composite indicates that the composite is promising microwave attenuating ceramic with excellent thermal conduction and microwave absorption ability. This work opens up a new core–shell structure strategy for designing and developing a high-efficiency attenuating agent and microwave attenuating ceramic.     

A low-cost lightweight microwave absorber: Silicon carbide synthesized from tissue

Recently, microwave absorbing materials have been widely used with the development of electromagnetic wave technology such as 5G communication. It is urgent to develop low-cost electromagnetic wave absorbing materials to meet the increasing civil market demand. Herein, we report a novel and simple strategy to synthesize lightweight silicon carbide microwave absorber by calcining tissue and glass microspheres at high temperature. The results show that higher synthesis temperature can improve the crystallinity of SiC, generate more whiskers and reduce the impurity content. This in turn increases the dielectric loss of the material. The products synthesized at 1600 °C have excellent microwave absorption properties. At the thickness of 1.3 mm, it achieves a reflection loss of ?32.5 dB and an effective absorption bandwidth of 4.1 GHz. This study broadens the way for the potential reuse of waste paper and glass, and provides a useful reference for the preparation of low-cost microwave absorbing materials.     

Simultaneously enhancing mechanical and microwave absorption properties of Cf/SiC composites via SiC nanowires additions

C_f/SiC composite is an excellent structural and functional material, silicon carbide nanowires (SiCnws) are not only a toughening material but also a great application in the field of microwave absorption. In this study, SiCnws are grown on the surface of carbon fiber (C_f) by polymer impregnation and pyrolysis, and the SiC matrix was prepared by chemical vapor osmosis method. The SiCnws are introduced to enhance the mechanical and microwave absorption properties simultaneously. After 3 impregnations, the flexural strength of the composite was 107.35 ± 10 MPa. When the thickness is 1.86 mm, the minimum reflection loss value is ?41.08 dB, and the effective absorption bandwidth (RL ≤ ?10 dB) is 3.86 GHz. Furthermore, the microwave absorption mechanism of the material is discussed. This work provides a new method to prepare lightweight, stable and high-performance microwave absorption materials, and these materials are expected to be used in high temperature environments.     

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.     

Microwave‐absorbing performance and mechanical properties of poly(vinyl chloride)/acrylonitrile–butadiene rubber thermoplastic elastomers filled with multiwalled carbon nanotubes and silicon carbide

Poly(vinyl chloride) (PVC)/acrylonitrile–butadiene rubber (NBR) were mixed with multiwalled carbon nanotubes (MWCNTs) and silicon carbide (SiC) to prepare microwave‐absorbing composites. The complex permittivity, direct‐current (dc) conductivity, microwave‐absorbing performance, morphology, and mechanical properties of the composites were studied. The real and imaginary parts of the permittivity of the composites increased with increasing MWCNT content. The premixing of the MWCNTs with PVC was more beneficial to the dispersion of MWCNTs; this led to a higher dc conductivity and permittivity and better microwave‐absorbing performance than the premixing of MWCNTs with NBR for the PVC/NBR/MWCNT composites. The PVC/NBR/MWCNT composites had a minimum reflection loss (RL_min) of ?49.5 dB at the optimum thickness of 1.96 mm. The efficient microwave absorption of the PVC/NBR/MWCNT composites was due to a high dielectric loss and moderate permittivity. The incorporation of SiC into the PVC/NBR/MWCNT composites increased the real and imaginary parts of permittivity of the composites. When the SiC content was 70 phr, RL_min decreased to ?34.9 dB at a thickness of 3 mm. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

SiC Matrix Structure–Function Composites Prepared by Pressureless Sintering and Their Microwave Attenuation Property in the X Band

For developing excellent microwave attenuation materials in a wide band, two series of SiC‐C (graphite) composites with different C additions were fabricated by pressureless sintering, using α‐SiC and β‐SiC green powder, respectively. β‐SBC composites were more suitable for microwave attenuation materials than α‐SBC composites. β‐SiC composites with 3 wt% C additions exhibited the best microwave absorption: The most significant microwave attenuation was ?40.5 dB, and most other attenuations were above ?30 dB in the whole X band. The composites were prepared with cost‐effective and easily controllable manufacturing process and can be considered as structure–function materials for particular applications.     

Effect of boron content on the microstructure and electromagnetic properties of SiBCN ceramics

Electromagnetic wave (EMW) absorbing materials have excellent potential for various applications in civil engineering and the military. In this study, siliconboron carbonitride (SiBCN) ceramics with excellent EMW absorption capability and oxidation resistance were obtained by adjusting the boron content. The results revealed that the graphite crystallite size in the SiBCN ceramics increased from 3.42 to 3.78 nm, whereas the thickness of the oxide layer decreased from 16.6 to 8.2 μm. The highest electrical conductivity and permittivity for the SiBCN ceramics were obtained when the boron content was 5%. The minimum reflection loss was ?35.25 dB at 10.57 GHz and a ceramic thickness of 2.0 mm. At a temperature of 600 °C, the SiBCN ceramic exhibited excellent EMW attenuation ability; particularly, the minimum reflection loss reached ?29.18 dB at 9.65 GHz and a ceramic thickness of 2.5 mm. The superior EMW absorption properties of the SiBCN ceramics at high temperatures can be ascribed to the synergistic effect of relaxation and conductivity. The results suggest that boron could enhance the transformation of amorphous carbon into crystalline graphite and increase the number of heterointerfaces and conductive paths. This work provides a method for obtaining SiBCN ceramics with excellent EMW absorption properties.     

Structural,thermal and dielectric properties of AlN–SiC composites fabricated by plasma activated sintering

Highly dense AlN–SiC composites with various SiC additions (0–50?wt-%) were fabricated at 1800°C by plasma activated sintering. The effect of SiC addition on structural, thermal and dielectric properties as well as microwave absorbing performance of the composites was investigated. The thermal conductivity decreases with increasing SiC addition, from 68.7 W (m?K)^?1 for 0?wt-% SiC to 19.38?W (m?K)^?1 for 50?wt-% SiC. On the contrary, the permittivity and dielectric loss increase gradually, from 7.6–8.5 to 22–26.7 and from 0.02–0.1 to 0.2–0.53, respectively. AlN–SiC composite with better thermal and dielectric properties in 30?wt-% SiC, whose thermal conductivity and dielectric loss are found to be 24.88?W (m?K)^?1 and 0.15–0.74, respectively. Furthermore, the composite exhibits microwave absorbing performance with the minimum reflection loss (RL) of ?16.5 dB at 15.5 GHz and the frequency range of 2.6 GHz for RL below ?10 dB (90% absorption).     

Enhancement of the microwave absorption properties of PyC-SiCf/SiC composites by electrophoretic deposition of SiC nanowires on SiC fibers

The employment of coating technique on the silicon carbide fibers plays a pivotal role in preparing SiC fiber-reinforced SiC composites (SiC_f/SiC) toward electromagnetic wave absorption applications. In this work, SiC nanowires (SiCNWs) are successfully deposited onto the pyrolytic carbon (PyC) coated SiC fibers by an electrophoretic deposition method, and subsequently densified by chemical vapor infiltration to obtain SiCNWs/PyC-SiC_f/SiC composites. The results reveal that the introduction of SiCNWs could markedly enhance the microwave absorption properties of PyC-SiC_f/SiC composites. Owing to the increasing of SiCNWs loading, the minimum reflection loss of composites raises up to −58.5 dB in the SiCNWs/PyC-SiC_f/SiC composites with an effective absorption bandwidth (reflection loss ≤ −10 dB) of 6.13 GHz. The remarkable enhancement of electromagnetic wave absorption performances is mainly attributed to the improved dielectric loss ability, impedance matching and multiple reflections. This work provides a novel strategy in preparing SiC_f/SiC composites with excellent electromagnetic wave absorption properties.     

Decoration of worm-like Cu2S particles on CuCo2S4 micro-spheres: An effective strategy to improve the electromagnetic dissipation feature

Civilization can be shielded from the dangerous electromagnetic spectrum by using microwave absorption materials, however, absorbing electromagnetic radiation with thin thickness and high bandwidth remains a challenge, especially at scales that are significant. Herein, we propose a novel architecture where worm-like Cu₂S particles are decorating CuCo₂S₄ micro-spheres were decorated, and this method is thought to be a successful one for enhancing the created nanocomposite's ability to dissipate electromagnetic radiation. Changing the filler loading percentage allows the nanohybrids' electromagnetic characteristics and microwave dissipation effectiveness to be efficiently changed. This leads to the creation of ultra-bandwidth absorbers with thin thickness, which are then tested using waveguide and free-space techniques. The sample with a thickness of 1.4 mm has a maximum reflection loss of ?18 dB and a maximum bandwidth of 3.6 GHz. The hetero-structures, multi-interfaces, and multiple relaxations phenomena, as well as the combined effects of the two components, are credited with the superior microwave absorption performance compared with the state-of-the-art. This finding demonstrates that CuCo₂S₄/Cu₂S nanohybrids pave the way for the development of future high-performance microwave absorption materials.     

Electromagnetic and microwave absorption properties of FeSiAl and flaky graphite filled Al2O3 composites with different FeSiAl particle size

The increasing electromagnetic interference problems have drawn much attention to microwave absorbing materials. To satisfy the needs of practical application, FeSiAl and flaky graphite filled Al₂O₃ composites were sintered by hot-pressing for microwave absorption application. The effect of FeSiAl particle size on the electromagnetic and microwave absorption properties was investigated in the X-band (8.2–12.4 GHz). The results show that the dielectric properties enhance significantly with increasing FeSiAl particle size, which is attributed to the increased interfacial polarization and conductance loss. As a result of the favorable impedance matching and appropriate electromagnetic attenuation, the reflection loss (RL) of the composites filled with 25–48 μm flaky FeSiAl achieves -15.2 dB at 10.6 GHz and the effective absorption bandwidth (RL < -10 dB) is 1.2 GHz in 10.0–11.2 GHz with a matching thickness of 1.0 mm. It indicates that FeSiAl and flaky graphite filled Al₂O₃ composites are potential candidates for thin-thickness microwave absorbing materials, and the microwave absorption properties can be enhanced by adjusting absorbent particle size.     

壳核结构SiC/C纤维的制备与吸波性能的研究

采用活性碳纤维转换法制备了壳核结构SiC/C纤维,采用拉曼光谱、SEM、XRD以及热重分析等测试方法对比研究了生成SiC的厚度对壳核结构SiC/C纤维样品的热重及吸波性能的影响。结果表明:包裹SiC壳层后样品吸波性能得到提高,样品厚度为3.0 mm时,保温4 h样品的最小反射损耗在8.24 GHz处达到-17.22 dB,低于-10 dB(90%的电磁波被吸收)的频宽在2.0 mm处达到4.8 GHz(11.12~15.92 GHz);保温3 h样品的最小反射损耗在8.23 GHz处达到-14.45 dB,低于-10 dB(90%的电磁波被吸收)的频宽在2.0 mm处达到4.56 GHz(10.88~15.44 GHz);且随着SiC含量的升高,试样微波吸收性能有所增强;制备的壳核结构SiC/C纤维样品起始氧化温度提高了150 ℃以上,并且最终残余质量在50%左右,即包裹SiC纤维后样品的抗氧化能力大大提高。     

Hierarchical carbon nanowires network modified PDCs-SiCN with improved microwave absorption performance

In order to optimize the dielectric performance of polymer derived SiCN ceramics (PDCs-SiCN), carbon nanowires (CNW) were deposited in SiCN by catalytic chemical vapor deposition (CCVD). Microstructure evolutions, dielectric property and electromagnetic (EM) wave absorption capacity of CNW/SiCN were investigated. Results show that carbon nanowires had plentiful pits/defects on their roughened surface and formed hierarchical network in SiCN which benefited the impedance match and generated strong conductivity and polarization loss, enhancing the absorption ability of CNW/SiCN. When CNW accounted for 5.61 wt%, RC_min reached −51 dB with EAB of 3.0 GHz at 2.7 mm in thick, showing excellent microwave absorbing performance. The favorable microwave absorption ability could be ascribed to three aspects including enhanced conductivity loss derived from the excellent conductivity of CNW, polarization loss generated by defects, and multiple reflection loss enhanced by hierarchical network. By comparing the variation tendency between defect concentration and electrical conductivity in CNW/SiCN, it is rational to conclude that the conductivity loss dominated the dielectric loss while the polarization loss and repeated multi-reflection simultaneously worked. This work can be further extended to study regarding the effect of heat-treatment temperature since CNW have the potential to promote the crystallization process of amorphous PDCs thereby improving their dielectric properties.     

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.     

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.     

Effect of preparation conditions on mechanical,dielectric and microwave absorption properties of SiC fiber/mullite matrix composite

Silicon carbide fiber-reinforced mullite matrix (SiC_f/Mu) composites were fabricated via an infiltration and sintering method. Effects of sintering parameters on microstructure, mechanical, dielectric and microwave absorption properties of SiC_f/Mu composites have been investigated. The flexural strength is significantly improved with increasing sintering temperature, and the highest flexural strength of 213?MPa is obtained in vacuum at 1000?°C for 2?h. The performances of composites with different holding time are further studied at 1000?°C. The flexural strengths of composites sintered at 1000?°C for 2 and 4?h reach 213 and 219?MPa, respectively. The failure displacement of the composite sintered at 1000?°C for 4?h reaches 0.39?mm. The excellent microwave absorption properties are achieved for the composite sintered at 1000?°C for 2?h. The minimum reflection loss (RL) reaches ?38?dB with a thickness of 2.9?mm?at 12?GHz and the effective absorbing bandwidth (RL?≤??10?dB) with a thickness of 3.4?mm covers the whole X?band, which indicate that SiC_f/Mu composite is a good candidate for microwave absorbing materials. These results provide valuable solutions to obtaining structural-functional materials for microwave absorption applications in civil and military areas.     

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.