Ab initio modeling study of boron diffusion in silicon

We present investigations of boron diffusion in crystalline silicon using ab initio calculations (W. Windl et al., Phys. Rev. Lett. 83 (1999) 4345). Based on these results, a new mechanism for B diffusion mediated by Si self-interstitials was proposed. Rather than kick-out of B into a mobile channel-interstitial, one- or two-step diffusion mechanisms have been found for the different charge states. The predicted activation energy of 3.5–3.8 eV, migration barrier of 0.4–0.7 eV, and diffusion-length exponent of −0.6 to −0.2 eV are in excellent agreement with experiment. We also present results of ab initio calculations for the structure and energetics of boron-interstitial clusters in Si. We show how these first-principles results can be used to create a physical B diffusion model within a continuum simulator which has strongly enhanced predictive power in comparison to traditional diffusion models.     

The spall strength of silicon carbide and boron carbide ceramics processed by spark plasma sintering

The spall strength of silicon carbide (SiC) and boron carbide (B₄C) ceramics processed by Spark Plasma Sintering (SPS) has been studied as a function of the loading stress. In the course of the planar impact experiments, the velocity of either the sample free surface or of the sample–window interface was continuously monitored by a Velocity Interferometer System for Any Reflector (VISAR). With the increase of impact stress the spall strength of both ceramics, increases initially and then declines monotonously until it vanishes almost completely, as the impact stress approaches the respective Hugoniot Elasic Limit (HEL). The mechanisms that may account for that behavior and, in particular, the role of the compressive wing cracks in the onset of the spall strength decline are discussed.     

Oxidation kinetics of reaction-sintered silicon carbide

The oxidation kinetics of reaction-sintered silicon carbide has been studied over the temperature range 1200° to 1350°C. The material has a bulk density of 3·00 g/cm³ and the unreacted Si content is 22·5% (v/v). The activation energy for oxidation is 28·75 ± 2·61 kcal/mol. It is proposed that the diffusion of oxygen through the growing oxide film is the rate-controlling process.     

Effect of microstructure on the high temperature strength of nitride bonded silicon carbide composite

Four compositions of nitride bonded SiC were fabricated with varying particle size of SiC of ∼ 9.67, ∼ 13.79, ∼ 60 μ and their mixture with Si of ∼ 4.83 μ particle size. The green density and hence the open porosity of the shapes were varied between 1.83 to 2.09 g/cc and 33.3 to 26.8 vol.%, respectively. The effect of these parameters on room temperature and high temperature strength of the composite up to 1300°C in ambient condition were studied. The high temperature flexural strength of the composite of all compositions increased at 1200 and 1300°C because of oxidation of Si₃N₄ phase and blunting crack front. Formation of Si₃N₄ whisker was also observed. The strength of the mixture composition was maximum.     

Sintering of nano crystalline α silicon carbide by doping with boron carbide

Sinterable nano silicon carbide powders of mean particle size (37 nm) were prepared by attrition milling and chemical processing of an acheson type alpha silicon carbide having mean particle size of 0.39 μm (390 nm). Pressureless sintering of these powders was achieved by addition of boron carbide of 0.5 wt% together with carbon of 1 wt% at 2050° C at vacuum (3 mbar) for 15 min. Nearly 99% sintered density was obtained. The mechanism of sintering was studied by scanning electron microscopy and transmission electron microscopy. This study shows that the mechanism is a solid-state sintering process. Polytype transformation from 6H to 4H was observed.     

Mechanical characteristics of microwave sintered silicon carbide

The present work deals with the sintering of SiC with a low melting additive by microwave technique. The mechanical characteristics of the products were compared with that of conventionally sintered products. The failure stress of the microwave sintered products, in biaxial flexure, was superior to that of the products made by conventional sintering route in ambient condition. In firing of products by conventionally sintered process, SiC grain gets oxidized producing SiO₂ (∼ 32 wt%) and deteriorates the quality of the product substantially. Partially sintered silicon carbide by such a method is a useful material for a varieties of applications ranging from kiln furniture to membrane material.     

新型双孔碳化硅的合成与表征

以正硅酸乙酯(TEOS)与蔗糖为原料,采用溶胶-凝胶法制备碳硅干凝胶,利用过量的碳源作为造孔剂,在氩气气氛1500℃下进行碳热还原反应合成出新型双孔的SiC.用XRD、SEM、TEM和低温N2吸附-脱附等手段对合成的样品进行表征,结果表明:合成的样品为"刺球"状双孔的新型碳化硅,这种两种孔分别是"刺"碳化硅晶体上的孔(孔半径约为2～6nm)和"刺"与"刺"之间的隙间孔(孔半径约为6～40nm).其比表面积为113m2/g,孔体积为0.38m3/g.     

Nanosecond laser-induced thermal evaporation of silicon carbide

Excimer (XeCl) laser pulses, 15 ns in duration and with fluences up to 10 J · cm^–2, have been employed to induce melting and evaporation of 6H-SiC thin layers in vacuum. Sample surface modification in the nanosecond time scale have been tnonitorizedin situ by optical probing. Eventually, the ablation product was collected on silicon single-crystal substrates placed in front of the SiC target. Modeling of the heating and the thermal evaporation processes resulted in estimation of surface temperatures as high as 10,000 K, evaporation rates of the order of 10²⁵ molecules · cm^–2 · s^–1 and recoil pressures of the order of 1 GPa. Comparison with experiments showed that the simple mechanism of purely thermal evaporation is able to describe the process of particle removal from a surface by short laser pulses only in the low-energy density range. Above a certain threshold the model breaks down and other mechanisms have to be considered.Paper presented at the Fourth International Workshop on Subsecond Thermophysics, June 27–29, 1995, Köln, Germany.     

The effect of silicon carbide additives on the stab resistance of shear thickening fluid treated fabrics

Applications of shear thickening fluids (STFs) with ballistic fabrics improve the protection performance of body protective systems. This article presents an innovative view of STF-impregnated ballistic fabrics by integrating silicon carbide (SiC) particles into targets. In this study, SiC particles were added into silica-based STFs, and Twaron fabrics were impregnated with this novel suspension. The effect of the SiC particles in the STFs was investigated with rheological testing. The results show that SiC particles are able to increase the viscosity profile of the suspension. In the stab testing, two types of impactors, such as spikes and knives, were dropped on the composite targets. According to the results, SiC particles enhance the protection performance of the STF-treated ballistic fabrics while keeping the flexibility.     

碳化硅陶瓷膜在油水分离中的应用研究

以孔径为0.1μm的碳化硅陶瓷膜,在跨膜压差0.2MPa、温度20℃、伴有固定时间间隔的反冲洗浓排条件下,通过死端过滤处理含油废水.结果表明,膜通量大,出水质量满足<碎屑岩油藏注水水质推荐指标及分析方法>标准(SY/T-532994)的要求,可以作为回注水;且膜简易清洗后,通量可100%恢复.     

磁性碳化硅功能陶瓷的制备

采用低分子量的聚硅烷(LPS)与二茂铁合成聚铁碳硅烷(PFCS),后者经高温烧成可制得磁性碳化硅陶瓷,XRD分析表明碳化硅陶瓷具有磁性的原因是由于生成了Fe3Si。铁能够促进β-SiC的形成和生长。     

大尺寸拼接式碳化硅反射镜的研制及环境模拟试验

随着空间技术的快速发展,对大尺寸空间反射镜部件的需求愈发强烈。本文从材料和制备技术角度分析了大尺寸碳化硅反射镜的发展趋势,并以Φ1.0 m口径常压烧结碳化硅拼接式技术验证镜研制过程为例,对拼接式反射镜的分块镜设计制备、连接、光学加工等过程进行了探讨。此外,结合实际的工程化应用需求,对1.0 m口径碳化硅拼接式技术验证镜进行了包括热真空、振动和抗辐照等在内的环境模拟试验。试验结果表明：通过合理的制备技术,Φ1.0 m口径的碳化硅拼接式技术验证镜光学加工后面形RMS达到了0.038λ (λ=632.8 nm),经历热真空和振动的环境模拟考核后,其面形RMS分别为0.037λ和0.036λ;此外,拼接式反射镜经过⁶⁰Co γ射线辐照测试后,反射率指标基本保持不变,显示了良好的工程应用前景。

     

高比表面碳化硅的制备与应用

利用碳化硅陶瓷材料耐磨、耐蚀、高热导等优良特性 ,同时攻克其比表面积低的缺陷 ,可使其成为一种催化剂载体的理想候选材料 ,应用于高温、强腐蚀等苛刻反应条件中。归纳了近年来高比表面碳化硅材料的研究进展 ,并阐述了其制备原理、方法及应用。     

异形截面SiC陶瓷纤维的制备工艺与性能研究进展

异形纤维是指截面为非圆形的纤维,由于异形截面SiC纤维具有特殊的电磁性能,已受到研究者的广泛重视。从熔融纺丝成型和化学成型两方面综述了近年来C形、中空形、三叶形、条形、十字形等异形截面SiC陶瓷纤维的制备方法与性能,概述了部分异形纤维的应用进展,针对目前异形截面SiC纤维的制备方法存在的不足,对后续发展方向作了展望。     

Ab initio study on the lattice instability of silicon and aluminum under [0 0 1] tension

Two different single crystals, Si with the diamond structure and Al with face-centered-cubic, are subjected to [0 0 1] tension in ab initio molecular dynamics (static) simulations based on Bachelet–Hammann–Schlüter (BHS) pseudopotential. Not only the ideal tensile strength under isotropic Poisson contraction, but also the crystal stability and bifurcation to anisotropic contraction are discussed in terms of the elastic stiffness matrix and the change in the charge density. The ideal tensile strengths are overestimated to as high as =0.18, σ=18.7 GPa for Si and =0.25, σ=23.5 GPa for Al, respectively. These values are inconsistent with the experimentally observed characteristics such as the hardness of Al being lower than that of Si. The elastic stiffness matrix reveals that the crystals become unstable at far lower strain and stress, =0.094, σ=10.7 GPa for Si and =0.055, σ=5.65 GPa for Al, and bifurcate to the lower energy pass of the anisotropic contraction. The change in the electronic structure suggests that nucleation/passage of a partial dislocation would take place in the bifurcated anisotropic contraction. Thus the instability point indicates the onset of the nonelastic deformation and is much more important than the ideal tensile strength.     

Effect of infiltrants on the electrical resistivity of reaction-sintered silicon carbide

The effect of infiltrants on the electrical resistivity of reaction-sintered silicon carbide, at temperatures ranging from RT to 1000°C, has been studied. Electrical resistivity decreases with increase in temperature up to 1000°C in VC and MoSi₂, whereas minimum electrical resistivity is observed at ∼600°C in B₄C infiltrant.     

温度对化学气相沉积碳化硅涂层的影响

以三氯甲基硅烷和氢气为气源,研究了化学气相沉积碳化硅过程中,温度(850-1350℃)对沉积速率、反应物消耗效应、涂层形貌和相结构的影响.用磁悬浮天平在线实时称量基体质量变化进行动力学研究;采用扫描电镜和X射线衍射对样品做了表征.结果表明,沉积过程存在四个控制机理:a区(<1000℃)为表面反应动力学控制;b区(1000-1050℃)主要是HCl对沉积的抑制作用;c区(1050-1300℃)是表面化学反应和传质共同控制;d(>1300℃)为传质为限速步骤.由于不同的控制机制,导致所得涂层的形貌存在差异.含碳含硅中间物质浓度的减小、HCl增多和MTS的分解共同导致反应物消耗效应.涂层由热解碳和碳化硅两相组成,温度的升高使热解碳相减少,碳硅比接近1.     

Low temperature synthesis of mesoporous silicon carbide via magnesiothermic reduction

Mesoporous silicon carbides (SiC) with high surface areas (above 300 m²/g) have been prepared successfully at a relative low temperature of 650 °C via magnesiothermic reduction of mesoporous silica/carbon (SiO₂/C) composites. The physicochemical properties and the structure of the products were characterized by various techniques such as XRD, FT-IR, SEM, TEM and N₂ adsorption-desorption isotherm. The experimental results indicate that the obtained SiC materials by this new method have similar structure to corresponding silica matrix templates. It was found that the magnesium (Mg) plays an important role in determining the structure and properties of the final products, which is used as a dual role agent both reducer and catalyst. The formation mechanism of mesoporous SiC has been also discussed.     

聚碳硅烷先驱体转化法制备SiC涂层研究

以自合成的聚二甲基硅烷(PDMS)为原料,常压裂解合成聚碳硅烷(PCS),通过FT-IR分析PCS的结构,用GPC测定其分子量及分布,用熔点分析仪测定其熔点.在此基础上,采用聚碳硅烷先驱体转化法在石墨基体上制备SiC涂层,通过X射线衍射对涂层进行晶相分析,用扫描电镜分析涂层表面和横断面的形貌.结果表明,在石墨基体上形成了明显的β-SiC晶相,可以获得均匀、致密的SiC涂层,其厚度可通过涂层次数的改变进行调节,单次涂层最大厚度大约为2μm.