氮化硅结合碳化硅树料的抗渣侵蚀性

借助光学显微镜、扫描电子显微镜和Ｘ射线衍射分析等手段，研究了矿渣侵蚀后的氮化硅结合碳化硅材料的显微结构和物相组成．探讨了氮化硅结合碳化硅材料的损坏机理。     

氮化物结合碳化硅耐火材料的研究现状

分别概述了以氮化硅、赛隆和氧氮化硅作为结合相 的SiC材料的结构特点、理化性能、生产工艺和应用情况,详细 介绍了国内这3种材料的研究现状,并对今后氮化物结合SiC 材料的研究内容提出了自己的观点。     

Silicon carbide nano-fibers in-situ grown on carbon fibers for enhanced microwave absorption properties

Silicon carbide nano-fibers (SiCNFs) were in-situ grown on the surface of carbon fibers by catalysis chemical vapor deposition (CCVD) with Ni nano-particles as catalyst at 1000 °C. The phase composition, microstructures, oxidation resistance and microwave absorption properties of the SiCNFs coated carbon fibers were investigated by X-ray diffraction (XRD), Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), Thermal gravity analysis (TGA) and Vector network analyzer, respectively. The results show that the as-grown nano-fibers which are mainly composed of β-SiC, present a withe-like morphology with diameter of 20–50 nm and aspect ratio of 100–150. Additionally, the TGA curves indicate that the oxidation resistance of the SiCNFs coated carbon fibers is significantly improved in comparison to the pure carbon fibers. Moreover, the investigation reveals that the microwave absorption properties of the SiCNFs coated carbon fibers are effectively enhanced. The reflectivity of the SiCNFs coated carbon fibers is less than −10 dB within the frequency ranging from 9.2 to 11.7 GHz and the lowest value of reflectivity can approach −19.9 dB when the thickness of specimen is 2 mm. While the reflection loss of the pure carbon fibers is higher than −2.1 dB within the whole band ranging from 2 and 18 GHz. The superior microwave absorbing performance of the SiCNFs coated carbon fibers is mainly attributed to the improved impedance matching as well as dissipation resulted from hopping migration. In conclusion, this study provides an effective modification approach to improve the microwave absorption properties of carbon fibers. Finally, the SiCNFs coated carbon fibers could be considered as a promising candidate in light-weight microwave absorbing materials.     

Open-cell reaction bonded silicon nitride foams: Fabrication and characterization

In this study, a newly designed fabrication procedure was utilized to produce silicon nitride foams. The main goal of the present study was to obtain Si₃N₄ foams with high levels of porosity and pore interconnectivity via an economical fabrication procedure including sacrificial template technique, gel-casting and reaction bonding processes. The fabrication procedure was studied and optimized in terms of suspension preparation and rheology, gel-casting parameters, and reaction bonding conditions. The produced foams have a precisely controlled level of porosity which can be varied up to 87 vol%. BET analysis showed that the surface area of the foam is of the order of 2.01 m²/g. The pore interconnectivity of the foam was investigated via polyester resin infiltration. Based on XRD and SEM analysis, the dominant nitriding reactions are the gas-phase reactions which lead to α-Si₃N₄ in the form of whiskers.     

Recent advances in carbon nanotubes-based microwave absorbing composites

With the blossom of information industry, electromagnetic wave technology shows increasingly potential in many fields. Nevertheless, the trouble caused by electromagnetic waves has also drawn extensive attention. For instance, electromagnetic pollution can threaten information safety in vital fields and the normal function of delicate electronic devices. Consequently, electromagnetic pollution and interference become an urgent issue that needs to be addressed. Carbon nanotubes (CNTs) have become a potential candidate to deal with these problems due to many advantages, such as high dielectric loss, remarkable thermodynamic stability, and low density. With the appearance of climbing demands, however, the carbon nanotubes combining various composites have shown greater prospects than the single CNTs in microwave absorbing materials. In this short review, recent advances in CNTs-based microwave absorbing materials were comprehensively discussed. Typically, we introduced the electromagnetic wave absorption mechanism of CNTs-based microwave absorbing materials and generalized the development of CNTs-based microwave absorbers, including CNTs-based magnetic metal composites, CNTs-based ferrite composites, and CNTs-based polymer composites. Ultimately, the growing trend and bottleneck of CNTs-based composites for microwave absorption were analyzed to provide some available ideas to more scientific workers.     

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.     

Mechanical and dielectric properties of functionalized boron nitride nanosheets/silicon nitride composites

Uniformly dispersed boron nitride nanosheets (BNNSs) reinforced silicon nitride (Si₃N₄) composites were prepared by surface modification assisted flocculation combined with SPS sintering. In order to improve the dispersibility of the BNNSs in the composites, the liquid phase stripped BNNSs are surface functionalized by a two-step covalently modification. The amino-modified BNNSs (NH₂-BNNSs) and Si₃N₄ powders have opposite surface potential, mixed evenly by electrostatic interaction during flocculation. The results showed that mechanical properties of Si₃N₄ composites were obviously enhanced by adding NH₂-BNNSs. The fracture toughness and bending strength of Si₃N₄ composites added 0.75 wt% NH₂-BNNSs were increased by 34% and 28%, respectively, compared with monolithic Si₃N₄. Toughening mechanisms are synergistic action of the torn, pull-out or bridging of BNNSs and crack deflection mechanisms with microstructural analyzes. The dielectric properties of the Si₃N₄ ceramics are also improved after the addition of NH₂-BNNSs.     

硅粉对刚玉-氮化硅复合材料表面氧化膜的影响

将T-60氧化铝粉、氮化硅粉和硅粉分别按50%、37.5%和12.5%的质量分数配料混合,经压制成型后,在空气中于1600℃保温2h烧成制得刚玉-氮化硅复合材料试样,借助于XRD、SEM、EDS等研究了加入硅粉对刚玉-氮化硅复合材料表面氧化膜组成和结构的影响。结果表明:在空气中烧成的刚玉-氮化硅复合材料表面氧化膜主要由富硅玻璃相、刚玉和莫来石组成;在刚玉-氮化硅复合材料中引入硅粉能减小氧化膜厚度,并能提高氧化膜的致密程度。     

硅复合刚玉-氮化硅材料的性能、组成及显微结构

将由50% (质量分数,下同)氧化铝粉、37. 5%氮化硅粉和12. 5%硅粉组成的混合料和由62. 5%氧化铝粉和37. 5%氮化硅粉组成的混合料分别压制成试样,分别在空气和埋炭气氛中于1600℃保温2h烧成,然后检测试样的体积密度、显气孔率和常温耐压强度,并采用XRD、SEM、EDS等手段分析试样的物相组成和显微结构。结果表明:与未加硅粉的试样相比,加入硅粉的试样在两种气氛中烧成后,显气孔率较低,强度较大,说明硅对刚玉-氮化硅材料具有助烧结作用;在空气中烧成后,试样中残留有较多的单质硅,这些单质硅均匀分布于刚玉和氮化硅颗粒的空隙间;在埋炭气氛中烧成后,单质硅原位反应生成了O’SiAlON和SiC。     

Carbon fiber reinforced composites from industrial waste for microwave absorption and electromagnetic interference shielding applications

Lightweight materials with hybrid microstructures are getting great attention in the area of electromagnetic wave absorption. In the present study, carbon fiber and fly ash reinforced composites are prepared by mixing them with ground granulated blast furnace slag, followed by compaction and sintering at 1000 °C under an argon atmosphere. Akermanite-gehlenite was observed to be the primary crystalline phase present in the prepared samples. Porous composites are obtained with the addition of fly ash and carbon fiber as they inhibit densification. The resultant microstructure has homogeneous carbon fiber dispersion and uniform fly ash anchoring on the matrix phase. This enhanced interface polarization, defect polarization, electron transportation, and impedance matching characteristics of the composites. Hence, the developed composites' microwave absorption and electromagnetic interference shielding properties exhibited an outstanding performance at low thickness with a reflection loss value of ?41.24 dB and total shielding effectiveness of 42.29 dB at the X-band.     

Influence of different matrices on the mechanical and microwave absorption properties of SiC fiber-reinforced oxide matrix composites

Fiber-reinforced ceramic matrix composites have excellent mechanical and microwave absorption properties, but still present considerable challenges. We prepared a SiC_f/mullite-SiO₂ composite (composite A) and a SiC_f/Al₂O₃-SiO₂ composite (composite B) by a precursor infiltration and sintering (PIS) process. Compared with the composite B, the composite A was easily densified. The flexural strength of the composite A reached 216 MPa, whereas that of the composite B was 159 MPa. The imaginary part of permittivity for composites A and B, which was determined by the contents of matrix and porosity, varied in the range of 2.5–3.5 and 3.6–5, respectively. The microwave absorption properties of the composite A were significantly enhanced in the range of 8.2–12.4 GHz. The results indicate that an optimal reflection loss of ?44 dB was reached at 12 GHz with a thickness of 2.9 mm for the composite A. These SiC fiber-reinforced oxide matrix materials have promising applications in microwave absorption, especially at high temperatures.     

Tunable microwave absorption band via rational design of C@TiC nanospheres

To effectively tune material's microwave absorption band, it is necessary to build a special structure and composition. Titanium carbide nanoparticles decorated carbon nanospheres (C@TiC) were prepared by using carbon nanospheres as a initial structure-directing agent, and their absorption band was tuned by rational design of TiC content. With increasing dosage of tetrabutyl titanate (TBT), absorption band of C@TiC dispersed in paraffin (30 wt%) gradually shifted from Ku to S-band, realizing the adjustment of absorption band. For example, when the amount of TBT was 1.0, 1.5 and 2.0 mL, the minimum reflection loss (RL) of C@TiC was about ?49.8, ?52.4 and ?50.3 dB at 17.4, 13.1 and 6.6 GHz, respectively. Excellent performance was attributed to good impedance match and synergistic effect between carbon spheres and TiC nanoparticles. This in-situ phase transition induced nanoheterostructure would provide a way for microwave absorbers with tunable band.     

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.     

烧结助剂对反应烧结氮化硅陶瓷的影响

以Si粉和C粉为主要原料 ,在氮气流量为1.2L·min- 1,氮化温度为 1380℃ ,保温时间为 2 0h的条件下 ,研究了分别以 10wt%的MgO、Al、Al2 O3和Al2 O3+Y2 O3粉为烧结助剂对反应烧结氮化硅陶瓷的影响。结果表明 :以MgO粉作烧结助剂时 ,试样的主要成分是MgSiO3,另外还有Si2 N2 O ,但没有Si3N4 生成 ;以Al粉作烧结助剂时 ,试样的主要成分是SiO2 ,仅有少量Si3N4 存在 ;以Al2 O3作烧结助剂时 ,试样的主要成分是β Si3N4 和α Si3N4 ;以 2wt%Al2 O3+8wt%Y2 O3作烧结助剂时 ,试样的主要成分为 β Si3N4 ,同时含有少量α Si3N4 。     

Tunable microwave absorption properties of nickel-carbon nanofibers prepared by electrospinning

The nickel-carbon nanofibers (Ni-C NFs) were fabricated by the electrospinning of poly(vinyl alcohol) (PVA) and nickel acetate tetrahydrate (NiAc) solution precursor with succedent PVA pyrolyzation and calcination process. The microwave absorption performance and electromagnetic (EM) parameters of the NFs were researched over the frequency range of 2.0–18.0?GHz. Both the impedance matching and EM wave absorption properties of the Ni-C NFs were improved by changing the carbonization temperature. The effect of graphitization degree on reflection loss (RL) and the possible loss mechanisms were directly displayed in the comparative study of each sample. The optimal RL value of ??44.9?dB and an effective frequency bandwidth of 3.0?GHz under a thickness of 3.0?mm can be reached by a sample calcined at 650?°C. These lightweight Ni-C NFs composites can be promising candidates for EM wave absorbers due to the combination of multiple loss mechanisms, nano-size effect and good impedance matching between Ni nanoparticles and CNFs.     

Dielectric regulation of high-graphitized fine ash wrapped cube-like ZnSnO3 composites with boosted microwave absorption performance

Intrinsic dielectric properties and tuning conductivity play important roles in microwave absorption. Novel multi-interfaced ZnSnO₃@ fine ash (ZSFA) composite was successfully synthesized by coating cube-like ZnSnO₃ particles with highly graphitized gasification fine ash. After hydrothermal reaction and Ostwald ripening process, fine ash was tightly wrapped around the assembly of ZnSnO₃ particles. Related electromagnetic parameters and dielectric dissipation ability were discussed with different mass additions. Owing to the strong polarization relaxation, special conductive network, and multi-interface structural design, the as-synthesized ZSFA exhibited adjustable dielectric loss behaviors and efficient microwave absorption ability. When 50% mass added, the maximum reflection loss value of the obtained ZSFA-2 is ?47.8 dB at 2.5 mm thickness, showing the enhanced dielectric loss ability. Meanwhile, the widest effective absorption bandwidth (RL ≤ ?10 dB) can cover 7.0 GHz (11.0–18.0 GHz) at a thickness of only 2.2 mm, which included the entire Ku band. This unique pure dielectric composite exhibited high-performance electromagnetic wave attenuation property and broadband frequency response, thereby providing a new approach to the production of a superior microwave absorber.     

陶瓷纤维吸波材料的吸波机理及其结构设计

陶瓷纤维复合材料在结构及隐身技术中得到了广泛的应用。通过分析陶瓷纤维材料的吸波机理以及结构特性对纤维材料的电磁参数和吸波性能的影响,提出了综合不同方法来对吸波纤维进行结构设计的构思。     

Enhanced high-temperature dielectric properties and microwave absorption of SiC nanofibers modified Si3N4 ceramics within the gigahertz range

Si₃N₄ ceramics modified with SiC nanofibers were prepared by gel casting aiming to enhance the dielectric and microwave absorption properties at temperatures ranging from 25?°C to 800?°C within X-band (8.2–12.4?GHz). The results indicate that the complex permittivity and dielectric loss are significantly increased with increased weight fraction of SiC nanofibers in the Si₃N₄ ceramics. Meanwhile, both complex permittivity and dielectric loss of SiC nanofibers modified Si₃N₄ ceramics are obviously temperature-dependent, and increase with the higher test temperatures. Increased charges mobility along conducting paths made of self-interconnected SiC nanofibers together with multi-scale net-shaped structure composed of SiC nanofibers, Si₃N₄ grains and micro-pores are the main reason for these enhancements in dielectric properties. Moreover, the calculated microwave absorption demonstrates that much enhanced microwave attenuation abilities can be achieved in the SiC nanofibers modified Si₃N₄ ceramics, and temperature has positive effects on the microwave absorption performance. The SiC nanofibers modified Si₃N₄ ceramics will be promising candidates as microwave absorbing materials for high-temperature applications.     

Rapid one-pot synthesis of SiO2@SiO2/Carbon@Carbon core-shell composites with efficient microwave absorption in a short time

A carbon-silica nanomaterial for electromagnetic wave (EW) absorption was synthesized using a modified one-pot method. The unique hydrolysis-polymerization process forms a SiO₂@SiO₂/Carbon@Carbon core-shell structure. The growth process of the material was studied by transmission electron microscope (TEM) and thermogravimetric analysis (TGA). Nanoparticles were successfully synthesized with a core-shell structure after 6 h of reaction, and the composite material showed excellent EW absorption performance with a thickness of 3.8 mm. The minimum reflection loss (RL_min) was ?56.3 dB, and the broadest effective absorption bandwidth (EAB) (RL ≤ 10 dB, 90% absorption) covered 5.15 GHz (12.48–17.63 GHz) with 1.8 mm of thickness. There is no significant difference in the EW absorption performance with increasing reaction time. Thus, this study provides a method for synthesizing EW-absorbing materials with shorter reaction time, simpler process, and excellent absorption properties, possibly a candidate for further application.     

Fabrication and mechanical properties of boron nitride nanotube reinforced silicon nitride ceramics

In this study, silicon nitride (Si₃N₄) ceramics added with and without boron nitride nanotubes (BNNTs) were fabricated by hot-pressing method. The influence of sintering temperature and BNNTs content on the microstructures and mechanical properties of Si₃N₄ ceramics were investigated. It was found that both flexural strength and fracture toughness of Si₃N₄ were improved when sintering temperature increases. Moreover, α-Si₃N₄ phase could transform into β-Si₃N₄ phase completely when sintering temperature rises to 1800 °C and above. BNNTs can enhance the fracture toughness of Si₃N₄ dramatically, which increases from 7.2 MPa m^1/2 (no BNNTs) to 10.4 MPa m^1/2 (0.8 wt% BNNTs). However, excessive addition of BNNTs would reduce the fracture toughness of Si₃N₄. Meanwhile, the flexural strength and relative density of Si₃N₄ decreased slightly when BNNTs were added. The related toughening mechanism was also discussed.