Synthesis of SiC nanowires with in-situ deposition of carbon coating

SiC nanowires are effective reinforcement materials in ceramic matrix composites. A compliant coating such as carbon on nanowires is necessary in order to moderate the nanowire/matrix interfacial bounding for taking the most advantages of SiC nanowires. SiC nanowires with an in-situ deposition of carbon shell coating were fabricated by a novel chemical vapor growth process. Highresolution transmission electron microscopy examinations showed that the nanowires consisted of a single crystal beta-SiC core with an amorphous carbon shell 2-5 nm in thickness. The nanowires were straight with a length generally over 10 microm and a diameter 15-150 nm. The growth direction of the core SiC nanowires is (111). A simple three-step growth model for SiC nanowires was proposed based on a vapor-solid growth mechanism. Because the carbon-coated nanowires were grown directly on continuous Tyranno-SA SiC fibers, in-situ application of the present technique on the fabrication of SiC nanowire-reinforced SiC/SiC composites is expected.     

界面涂层与基体对先进CVI-SiC/SiC复合材料性能的影响

对含有几种典型界面结构和SiC纳米线的CVI-SiC/SiC复合材料的弯曲性能和断裂韧性进行了比较研究. 研究表明: 界面涂层对SiC/SiC的力学性能至关重要, 120nm厚的碳界面涂层使材料的强度与韧性都增加一倍; 在用140nm厚的SiC层将该碳层分为更薄的两层, 形成C/SiC/C多层界面涂层时, 材料的强度没有明显的变化, 而断裂韧性则略有提高. 对基体中弥散分布有SiC纳米线的SiC/SiC的力学性能研究表明, SiC纳米线具有非常高的强化效率, 使SiC/SiC复合材料具有更高的强度和韧性.     

平纹编织SiC/SiC复合材料多尺度建模及强度预测

连续SiC纤维增强SiC基体复合材料（SiC/SiC）具有优异的高温力学性能、辐照稳定性及较低的氚渗透率,在核工程结构领域具有良好的应用前景,掌握其承载状态下的损伤演化和强度性能,对SiC/SiC复合材料的应用具有重要指导意义。本文基于平纹编织SiC/SiC复合材料的制备过程和组分材料分布的多尺度特性,考虑复合材料微观结构的局部近似周期性,建立了纤维丝尺度和纤维束尺度单胞模型。使用有限元分析软件对纤维丝尺度模型的弹性性能和强度性能进行预测,将这些性能参数代入纤维束尺度模型,引入Tsai-Wu失效准则,根据材料的不同失效模式并对失效单元进行方向性刚度折减,模拟了平纹编织SiC/SiC复合材料在单轴拉伸载荷下的渐进损伤过程。数值模拟曲线与试验曲线吻合较好,实现了对平纹编织SiC/SiC复合材料强度的有效预测。      

Synthesis of the tube-brush-shaped SiC nanowire array on carbon fiber and its photoluminescence properties

Tube-brush-shaped nanostructure of SiC nanowires was synthesized on polyacrylonitrile-based carbon fibers. The morphology and microstructure of the nanowires were characterized by X-ray powder diffraction, field emission scanning electron microscopy and high-resolution transmission electron microscopy. A quasi-periodically twin structure with (111) plane as the boundary along the SiC nanowires was observed. The vapor-solid growth mechanism of the SiC nanowire brush is also discussed. Moreover, some separated blue-shifted photoluminescence peaks around 469 nm were measured. The separated blue-shifted emission peaks are attributed to the quantum confinement of nanoscaled twin segments along each nanowire rather than the apparent diameters of the nanowires. The SiC nanowire brushes hopefully can find potential applications in nanotechnology.     

碳化硅纳米线增强多孔碳化硅陶瓷基复合材料的制备

构建多孔碳化硅纳米线(SiCNWs)网络并控制化学气相渗透(CVI)过程,可设计并获得轻质、高强度和低导热率SiC复合材料。首先将SiCNWs和聚乙烯醇(PVA)混合,制备具有最佳体积分数(15.6%)和均匀孔隙结构的SiCNWs网络;通过控制CVI参数获得具有小而均匀孔隙结构的SiCNWs增强多孔SiC(SiCNWs/SiC)陶瓷基复合材料。SiC基体形貌受沉积参数(如温度和反应气体浓度)的影响,从球状颗粒向六棱锥颗粒形状转变。SiCNWs/SiC陶瓷基复合材料的孔隙率为38.9%时,强度达到(194.3±21.3) MPa,导热系数为(1.9 ± 0.1) W/(m∙K),显示出增韧效果,并具有低导热系数。     

Highly aligned flax/polypropylene nonwoven preforms for thermoplastic composites

A major challenge for natural fibre composites is to achieve high mechanical performance at a competitive price. Composites constructed from unidirectional yarns and woven fabrics are known to perform significantly better than composites made from random nonwoven mats, but unidirectional yarns and fabrics are much more expensive to manufacture than random nonwoven mats. Here, we report on highly aligned natural fibre nonwoven mats that can be used as a replacement for unidirectional woven fabrics. A drawing operation is added to the conventional nonwoven process to improve fibre alignment in the nonwoven preforms and the final composites. The modified nonwoven manufacturing process is much simpler and cheaper than the unidirectional woven fabric process because of the elimination of expensive spinning and weaving operations. The composites fabricated from the highly aligned nonwoven mats showed similar mechanical strength as the composites made from unidirectional woven fabrics.     

Field emission performance of SiC nanowires directly grown on graphite substrate

Lawn-like SiC nanowire arrays were successfully synthesized on graphite substrates by thermal evaporation of silicon powders at high temperature. The morphology, microstructure and composition of the nanowires were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM) measurements. The product grown on graphite substrates was hexagonal prism-shaped single-crystal 3C-SiC nanowires with high aspect ratio. Planar defects, such as microtwins and stacking faults were observed in SiC nanowires. Field emission measurements of the SiC nanowires grown on graphite substrate showed a very low threshold field of 2.1 V μm⁻¹, high brightness and stable field emission performance.     

不同界面SiC/SiC复合材料的断裂行为研究

碳化硅纤维增强碳化硅复合材料(SiC/SiC)是极具前景的高温结构材料。通过先驱体浸渍裂解(PIP)工艺分别制备了PyC界面和CNTs界面SiC/SiC复合材料, 对两种SiC/SiC复合材料的整体力学性能以及界面剪切强度等进行了测试表征, 并对材料中裂纹的产生与扩展进行了原位观测。结果表明, 两种界面SiC/SiC复合材料弯曲强度相近, 但PyC界面SiC/SiC复合材料的断裂韧性约为CNTs界面SiC/SiC复合材料的两倍。在PyC界面SiC/SiC复合材料中, 裂纹沿纤维-基体界面扩展, PyC涂层能够偏转或阻止裂纹, 材料呈现伪塑性断裂特征; 而在CNTs界面SiC/SiC复合材料中, 裂纹在扩展路径上遇到界面并不偏转, 初始裂纹最终发展为主裂纹, 材料呈现脆性断裂模式。     

生物态碳-陶瓷基复合材料制备方法的研究现状EI北大核心CSCD

近年来,碳-陶瓷基复合材料因其耐高温、低密度、抗腐蚀性能好、热膨胀系数低、性能可设计性强等特点成为研究热点之一,将生物态材料的多孔结构引入陶瓷基体中制备具有生物形态的碳-陶瓷复合材料的研究已引起关注。本文综述了生物态碳-陶瓷基复合材料的多孔结构、制备工艺、性能以及应用前景。强调设计材料微观结构的重要性,并详细介绍了碳-陶瓷基复合材料制备过程中的关键技术——渗透技术,包括:化学气相渗透、熔融渗透、溶胶凝胶渗透、料浆渗透、聚合物前驱体渗透、熔盐渗透六种渗透技术,并对其存在的问题提出解决方案。综述了生物态碳-陶瓷基复合材料压缩强度和断裂强度等性能,对未来的性能研究方向提出建议,指出应测试高温、强酸强碱、冷热交替环境下材料的力学性能。探讨生物态碳-陶瓷基复合材料在航空发动机叶片、汽车尾气净化器、催化剂载体三个方面的潜在应用,概述在复杂成型、较强的力学性能和热稳定性等方面的挑战和实际局限性。最后,对生物形态的碳-陶瓷基复合材料制备工艺的改进、力学性能的研究进行展望,为生物态碳-陶瓷基复合材料的研制和应用提供理论依据和参考。     

Effects of trace amount of nanometric SiC additives with wire or particle shapes on the mechanical and thermal properties of alumina matrix composites

SiC nanowires (SiCNWs) are suitable candidates used as additives to improve the thermal conductivity of alumina, since they exhibit superior properties such as high chemical and thermal stability. In this study, alumina matrix composites reinforced with very small amount of β-SiC/SiO₂ core–shell nanowires were fabricated by hot-pressing. They were first characterized and compared with alumina matrix specimens containing SiC nanopowder. It was found out that with 0.2 wt% SiC additives, the grain sizes of the alumina specimens were reduced by 20 % of that of the monolithic one, regardless of the shape of the SiC additives. Vickers hardness of specimen containing both SiCNWs and SiC nanopowders slightly increased, while fracture toughness decreased more than that of the monolithic alumina. Thermal conductivity of the specimens increased with increased amount of SiCNWs and was better than those of the specimens containing SiC nanopowders. The alumina composite containing 0.2 wt% SiCNWs had higher thermal conductivity than that of the monolithic alumina by as much as 45 %. From these results, it is clear that only small amount of nanosized SiC as an additive material, particularly SiCNWs, has a significant effect on the properties of alumina matrix composites.     

FABRICATION OF SiCp-AI COMPOSITE MATERIALS

Liquid phase fabrication methods for aluminum matrix composites reinforced with SiC whiskers, or SiC particles have been investigated and the mechanical properties of fabricated composites have been evaluated. Three kinds of liquid phase fabrication methods; hot extrusion, hot pressing and pressure infiltration, were studied. Commercial SiC whiskers and SiC powders of alpha type and beta type were used as the reinforcements for an aluminum matrix. Among the fabrication methods investigated, the best results were achieved by the pressure infiltration. The mechanical properties and the wear resistance of the fabricated composites were measured. The SiC whisker reinforced aluminum matrix composites have high strength, so that they can be used as high specific strength materials. The SiC particulate reinforced aluminum matrix composites are not strong as the SiC whisker reinforced composites. However, the SiC particulate reinforced aluminum matrix composites have a good potential for use as wear resistant material. The hardening effect of beta type particles on the aluminum matrix was larger than that of alpha type particles.     

Thermomechanical durability of CVI-processed two-dimensionally woven and three-dimensionally braided SIC fibre-reinforced SiC composites

The thermomechanical durability of a two-dimensionally woven and a three-dimensionally braided SiC/SiC composite fabricated by chemical vapour infiltration technique were studied. The effects of thermal shock, thermal ageing, and thermal cycling at high temperatures on mechanical properties were determined. The thermal degradation mechanisms were also investigated. The results indicated that the fibre geometry, matrix porosity, interlaminar shear strength and testing conditions affect the thermomechanical durability of the SiC/SiC composites.     

Investigations on NiAl composites fabricated by matrix coated single crystalline AlO-fibers with and without hBN interlayer

The intermetallic compound NiAl has excellent potential for high temperature structural applications but suffers from low temperature brittleness and insufficient high temperature strength. One way to remove these deficiencies is the reinforcement by high strength ceramic fibers. Such intermetallic matrix composites can be conveniently fabricated by the hot pressing of matrix coated fibers. Al₂O₃ single crystal fibers show excellent chemical stability with the NiAl matrix, but the residual thermal compressive stresses during cool down dramatically degrades the fiber strength and thus, renders the composite useless for structural applications. We report on an experimental and computational study to mitigate this problem and to fabricate Al₂O₃/NiAl composites with sufficient high temperature strength. Analytical TEM, mechanical testing and push-out tests were employed to characterize chemistry, microstructure and mechanical properties of the composites. It will be shown that a processing window exists that allows producing intermetallic matrix composites with promising mechanical properties.     

Porous SiC nanowire arrays as stable photocatalyst for water splitting under UV irradiation

In this study, we report the fabrication and photocatalytic properties of the oriented arrays of SiC nanowires on the Si substrate. The SiC nanowire arrays were prepared by carbonizing the Si nanowire arrays with the graphite powder at 1250 °C. The as-prepared SiC nanowires are highly porous, which endows them with a high surface-to-volume ratio. Considering the large surface areas and the high stability, the porous SiC nanowire arrays were used as photocatalyst for water splitting under UV irradiation. It was found that such porous SiC structure exhibited an enhanced and extremely stable photocatalytic performance.     

Determination of mechanical behavior of natural fibre based hybrid composites experimentally

This paper deals with the production and the mechanical testing of natural fibre‐based hybrid composites. Hybrid composite is produced by vacuum assisted resin infusion method of woven jute fabric and nonwoven wool felt along with glass fabric. Tensile, flexure and impact properties have been evaluated and compared. The tensile strength and the Young's modulus were found to be 70.66 MPa and is 5.63 GPa, respectively. Flexure and impact tests were performed on both woven jute fabric surface and on nonwoven wool felt surface of the composite specimens. The highest flexural strength was obtained in the specimen loaded at the woven jute fabric surface. Impact test results show that the specimens impacted at the nonwoven wool felt surface were carrying higher loads than the specimens impacted at the woven jute fabric surface.     

Investigations on NiAl composites fabricated by matrix coated single crystalline Al2O3-fibers with and without hBN interlayer

The intermetallic compound NiAl has excellent potential for high temperature structural applications but suffers from low temperature brittleness and insufficient high temperature strength. One way to remove these deficiencies is the reinforcement by high strength ceramic fibers. Such intermetallic matrix composites can be conveniently fabricated by the hot pressing of matrix coated fibers. Al₂O₃ single crystal fibers show excellent chemical stability with the NiAl matrix, but the residual thermal compressive stresses during cool down dramatically degrades the fiber strength and thus, renders the composite useless for structural applications. We report on an experimental and computational study to mitigate this problem and to fabricate Al₂O₃/NiAl composites with sufficient high temperature strength. Analytical TEM, mechanical testing and push-out tests were employed to characterize chemistry, microstructure and mechanical properties of the composites. It will be shown that a processing window exists that allows producing intermetallic matrix composites with promising mechanical properties.     

三向正交碳纤维增强纳米碳化硅生物复合材料的制备和力学性能

碳纤维增强的纳米碳化硅陶瓷基复合材料力学性能优良,且具有一定的生物相容性,因此可作为一种新型的可取代钛合金的全尺寸整体人工骨骼。研究了具有三向正交结构的T300和M30碳纤维预制体对C/SiC复合材料制备过程和抗弯强度的影响规律。以聚碳硅烷为先驱体,以二乙烯基苯为溶剂和交连剂,采用聚合物浸渍热解法制备了C/SiC复合材料,采用阿基米德排水法测量其密度和气孔率,采用三点抗弯法测量其抗弯强度。M30 C/SiC抗弯强度比T300 C/SiC高6.7％,表明碳纤维弹性模量对复合材料基体开裂强度有显著影响,并通过增加纤维径向强度以及承担载荷的比例提高了复合材料的断裂强度。      

Microstructural characterization and fracture properties of SiC-based fibers annealed at elevated temperatures

Ceramic matrix composites (CMCs) have been proposed as potential structural materials for application of high temperature technologies. Excellent high temperature performance of CMCs requires that fibers must have high enough thermal stability and sufficient mechanical properties throughout the service life. In order to clarify the correlation between the mechanical properties and the microstructure of SiC-based fibers, SiC-based fibers were annealed at elevated temperatures in Ar for 1 h. After annealing, the fracture strengths on these fibers were evaluated at room temperature by tensile test; the microstructural features were characterized by X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM). Furthermore, the fracture mechanics was applied to estimate the fracture toughness and the critical fracture energy of these fibers. As a result, excellent microstructure and mechanical stabilities were observed for SiC fibers with near-stoichiometric composition and high-crystallite structure. Combining the microstructure examination with tensile test indicates that the thermal and mechanical stabilities of SiC fibers at high temperatures were mainly controlled by their crystallization and composition as well as other factors.     

Interfacial design of Cu/SiC composites prepared by powder metallurgy for heat sink applications

In order to dissipate the heat generated in electronic packages, suitable materials must be developed as heat spreaders or heat sinks. Metal matrix composites (MMCs) offer the possibility to tailor the properties of a metal (Cu) by adding an appropriate reinforcement phase (SiC) to meet the demands for high thermal conductivities in thermal management applications. Copper/SiC composites have been produced by powder metallurgy. Silicon carbide is not stable in copper at the temperature needed for the fabrication of Cu/SiC. The major challenge in development of Cu/SiC is the suppression of this reaction between copper and SiC. Improvements in bonding strength and thermo-physical properties of the composites have been achieved by a vapour deposited molybdenum coating on SiC powders to control the detrimental interfacial reactions.     

Flexible Transparent and Free‐Standing SiC Nanowires Fabric: Stretchable UV Absorber and Fast‐Response UV‐A Detector

Transparent and flexible materials are desired for the construction of photoelectric multifunctional integrated devices and portable electronics. Herein, 2H‐SiC nanowires are assembled into a flexible, transparent, self‐standing nanowire fabric (FTS‐NWsF). The as‐synthesized ultralong nanowires form high‐quality crystals with a few stacking faults. The optical transmission spectra reveal that FTS‐NWsF absorbs most incident 200–400 nm light, but remains transparent to visible light. A polydimethylsiloxane (PDMS)–SiC fabric–PDMS sandwich film device exhibits stable electrical output even when repeatedly stretched by up to 50%. Unlike previous SiC nanowires in which stacking faults are prevalent, the transparent, stretchable SiC fabric shows considerable photoelectric activity and exhibits a rapid photoresponse (rise and decay time < 30 ms) to 340–400 nm light, covering most of the UV‐A spectral region. These advances represent significant progress in the design of functional optoelectronic SiC nanowires and transparent and stretchable optoelectronic systems.