Silicon Carbide Monofilament-Reinforced Silicon Nitride or Silicon Carbide Matrix Composites

SiC-monofilament-reinforced SiC or Si₃N₄ matrix composites were fabricated by hot-pressing, and their mechanical properties and effects of filaments and filament coating layers were studied. Relationships between frictional stress of filament/matrix interface and fracture toughness of SiC monofilament/Si₃N₄ matrix composites were also investigated. As a result, it was confirmed experimentally that in the case of composites fractured with filament pullout, the fracture toughness increased as the frictional stress increased. On the other hand, when frictional stress was too large (>about 80 MPa) for the filament to be pulled out, fracture toughnesses of the composites were almost the same and not so much improved over that of Si₃N₄ monolithic ceramics. The filament coating layers were found to have a significant effect on the frictional stress of the SiC monofilament/Si₃N₄ matrix interface and consequently the fracture toughness of the composites. Also the crack propagation behavior in the SiC monofilament/Si₃N₄ matrix composites was observed during flexural loading and cyclic loading tests by an in situ observation apparatus consisting of an SEM and a bending machine. The filament effect which obstructed crack propagation was clearly observed. Fatigue crack growth was not detected after 300 cyclic load applications.     

Silicon Carbide/Silicon and Silicon Carbide/Silicon Carbide Composites Produced by Chemical Vapor Infiltration

Composites of SiC/Si and SiC/SiC were prepared from single yarns of SiC. The use of carbon coatings on SiC yarn prevented the degradation normally observed when chemically vapor deposited Si is applied to SiC yarn. The strength, however, was not retained when the composite was heated at elevated temperatures in air. In contrast, the strength of a SiC/C/SiC composite was not reduced after this composite was heated at elevated temperatures, even when the fiber ends were exposed.     

以二氧化硅为硅源制备纯硅的方法

介绍了纯硅材料的类型及其主要用途,叙述了国内外以二氧化硅为硅源,热还原法(包括金属、非金属和耦合热还原法)、熔盐电解法(包括氟化物、氯化物和耦合熔盐电解法)等制备纯硅的方法,评叙了各种方法的优缺点。指出氯化钙熔盐电解法工艺简单、产品纯度高、能耗小、成本低、经济效益好、环境友好、市场潜力与前景较好。     

Silicon Carbide Platelet/Silicon Carbide Composites

α-silicon carbide platelet/β-silicon carbide composites have been produced in which the individual platelets were coated with an aluminum oxide layer. Hot-pressed composites showed a fracture toughness as high as 7.2 MPa·m^1/2. The experiments indicated that the significant increase in fracture toughness is mainly the result of crack deflection and accompanying platelet pullout. The coating on the platelets also served to prevent the platelets from acting as nucleation sites for the α- to β-phase transformation, so that the advantageous microstructure remains preserved during high-temperature processing.     

氮化钒铁中硅含量的硅钼蓝分光光度法研究

采用硅钼蓝分光光度法测定氮化钒铁中的硅含量,研究了氮化钒铁溶样方法,通过溶样效果,选择确定溶样酸。结果表明,该方法准确度高、重复性好,操作简便,可广泛运用于氮化钒铁中硅的检测。     

Silicon/Silicon Carbide Composites Fabricated by Infiltration of a Silicon Melt into Charcoal

Dense Si/SiC composites were fabricated via a conventional reaction-bonding process, using oak charcoal that exhibited a honeycomb structure. The silicon melt was infiltrated into the porous oak charcoal (density of ~0.6 g/cm³) while the sample was heated to 1700°C under vacuum (10^-3 torr (~0.133 Pa)), which resulted in in situ silicon-fiber/SiC composites. The reaction product had an average density of 2.8 g/cm³ and showed three-point flexural strengths of 330 MPa at room temperature and 280 MPa at 1300°C. Good oxidation resistance also was observed at temperatures up to 1300°C in flowing air. This process provided excellent shape-making capability, because the charcoal that was used as a preform was readily machinable.     

Nonaqueous Processing of Silicon for Reaction-Bonded Silicon Nitride

Ethanolic silicon suspensions, with and without a polyethoxylated amine of low molecular weight, were studied by rheological, adsorption, electrophoretic, and sedimentation methods. Pellets were pressure-cast and nitrided to form reaction-bonded silicon nitride. Density and binding strength in the green state relate well to rheological behavior and other collodial aspects of the suspensions used, particularly the additive's role and distribution. Density and degree of nitridation in the final state are not importantly affected by the additive's use. Its greatest benefit is to modify the binding strength in the green state. The mode by which this small molecule affects the processing of silicon consists of adsorption, combined with an increased electrostatic interparticle repulsion which increases the suspension viscosity and that of undried pellets. Although the improved binding strength is accompanied by decreased green and nitrided densities, high degrees of conversion to silicon nitride are still achieved.     

Oxidation of Chemically-Vapor-Deposited Silicon Nitride and Slnglem-Crystal Silicon

Oxidation of {111} single-crystal silicon and dense, chemically-vapor-deposited silicon nitride was done in clean silica tubes at temperatures of 1000° to woo°C. The oxidation rates of silicon nitride under various atmospheres (dry O₂, wet O₂, wet inert gas, and steam) were several orders of magnitude slower than those of silicon under the identical conditions. The activation energy for the oxidation of silicon nitride decreased from 330 to 259 kJ/mol in going from dry O₂ to steam while that for Si decreased from 120 to 94 kJ/mol. The parabolic rate constant for Si increased linearly as the water vapor pressure increased. However, the parabolic rate constant for silicon nitride showed nonlinear dependency on the water vapor pressure in the presence of oxygen. The oxidation kinetics of silicon nitride is explained by the formation of nitrogen compounds (NO and NH₃) at the reaction interface and the counterpermeation of these reaction products.     

Silicon Carbide Powder Synthesis from Silicon Monoxide and Methane

SiC was synthesized via the gas-phase reaction between SiO and CH₄ at 1500° and 1560°C in a tubular flow reactor. SiO vapor was generated from equimolar powder mixtures of Si and SiO₂ in the reactor while CH₄ was externally supplied. Products of different morphologies were collected at different longitudinal locations: whiskers, crystal aggregates, scale, and powder. The total yield of SiC, based on the amount of SiO generated, reached as high as 99%, of which 25–46% by mass was fine powder with sizes ranging from 60 to 300 nm.     

ICP—AES法测定高硅钢中的硅

研究了用ICP-AES法快速测定高硅钢中的硅,确定了最佳测定条件.在此条件下测定,获得满意结果.实验结果表明,该方法简便、快速、灵敏,检出限为0.000 9 μg/mL;RSD＜0.5 %;回收率为99 %～101 %,结果令人满意.     

The Interaction of Molten Silicon with Silicon Aluminum Oxynitrides

Silicon aluminum oxynitride solid solutions (sialons) based on β-Si₃N₄ and Si₂N₂O behave differently in contact with molten sih'con. The Si₂N₂O-based sialons convert to almost pure Si₃N₄, apparently through a two-step decomposition and solution-precipitation reaction, whereas the β-Si₃N₄ sialons are preferentially attacked at the grain boundaries. The composition of the grain-boundary phase appears to control the rate of reaction.     

四氯化硅为原料制备二氧化硅粉体的研究

以多晶硅生产中的副产物四氯化硅为原料,在碱性溶液中制备了二氧化硅.研究了碱性溶液种类、浓度以及后处理、表面活性剂对于二氧化硅性能的影响.结果表明,采用氢氧化钠质量分数为5%的氢氧化钠水溶液时,二氧化硅邻苯二甲酸二丁酯(DBP)的吸附值为2.92 mL/g,一次粒子直径在20 nm左右,BET比表面积为260.382 m2/g;不论采用何种碱液,随着碱液浓度的增大,二氧化硅的DBP吸附值都减小,相应的粒子直径也都增大;正丁醇恒沸蒸馏处理可较大幅度提高二氧化硅的DBP吸附值;制备过程中表面活性剂的加入只能一定程度上提高产品二氧化硅的DBP吸附值.     

Studies on Abrasive Wear of Monolithic Silicon Nitride and a Silicon Carbide Whisker-Reinforced Silicon Nitride Composite

Results presented in this paper demonstrate the roles of the most important parameters that govern abrasive wear of experimental ceramics. SEM studies of the abraded surfaces evidenced two main kinds of failures: microdropping of grain(s) and powder-type wear track(s) resulting from the brittle microfracturing of parts of grain(s). There were whiskers in the surface, and whisker pullouts occurring during wear processes are believed to be the reasons for the lower wear rates of SiCw/Si₃N₄ composite ceramics under experimental conditions.     

Codeposition of Free Silicon during CVD of Silicon Carbide

Factors influencing the concentration and distribution of elemental silicon codeposited during chemical vapor deposition (CVD) of SiC from MTS (CH₃SiCl₃) and hydrogen diluted by argon are reported. The experiments were carried out in both hot- and cold-wall reactors at 1383–1473 K at atmospheric pressure. Codeposition of free silicon was detected even at very low excess hydrogen, contrary to the prediction of thermochemical calculations. In the hot-wall reactor, under conditions of high exchange rate of the feed gases, deposits of uniform composition were obtained, containing 0%–90% free silicon, depending upon feed gas composition. The deposits of pure silicon carbide consisted of β-SiC with a microhardness of 2400 kg/mm² at a typical formation rate of 30 μm/h. Microhardness decreased to 800 kg/mm² with increasing silicon concentration. In the cold-wall reactor, under impinging gas flow conditions, nonuni-form deposition occurred: a local gradient of Si/SiC was obtained with free silicon concentrations varying gradually between 0% and 35%. Si/SiC ratios in the deposits were determined by a combination of XRD, scanning AES, and SMP.     

Anisotropy of Silicon Nitride with Aligned Silicon Nitride Whiskers

A model based on anisotropic sintering shrinkage of silicon nitride with aligned silicon nitride whisker seeds was built in order to provide an easy way to obtain information on how the large elongated grains were aligned. The method requires a simple measuring device for the information. XRD analysis showed a good correlation with predictions of the model. Both predictions of the model and experimental results indicated that the fraction of aligned large elongated grains increased as the whisker content increased.     

氮化硅加入量对碳化硅-氮化硅-莫来石复相材料氧化层的影响

以莫来石、红柱石、氮化硅、碳化硅为主要原料,在空气气氛下烧成制备碳化硅-氮化硅-莫来石复相材料,并采用XRD、SEM样品进行了表征。结果表明:碳化硅化硅-氮化硅-莫来石复合材料在烧结过程中会在试样表面形成氧化层,分为氧化膜和致密层。氧化膜的主要成分为Si O2,其主要是碳化硅和氮化硅的氧化产物,随着试样中氮化硅含量的增加,试样表面形成的Si O2逐渐增多;试样截面出现致密层,随着试样中氮化硅含量的增加,试样的致密层厚度逐渐减小。     

Oxidation of Silicon, Silicon Carbide, and Silicon Nitride in Gases Containing Oxygen and Chlorine

Chlorine contamination accelerates the oxidation of silicon-based ceramics through the formation of volatile silicon chloride or silicon oxychloride species which degrade the protective character of the SiO₂ film. Accelerated attack may occur by active corrosion or formation of bubbles in the oxide layer. Si₃N₄ is much more resistant to this attack than either silicon or SiC. This resistance may be related to the presence of a thin silicon oxynitride layer below the SiO₂ scale which forms on Si₃N₄.     

Stress-Corrosion Cracking of Silicon Carbide Fiber/Silicon Carbide Composites

Ceramic-matrix composites are being developed to operate at elevated temperatures and in oxidizing environments. Considerable improvements have been made in the creep resistance of SiC fibers and, hence, in the high-temperature properties of SiC fiber/SiC (SiC_f/SiC) composites; however, more must be known about the stability of these materials in oxidizing environments before they are widely accepted. Experimental weight change and crack growth data support the conclusion that the oxygen-enhanced crack growth of SiC_f/SiC occurs by more than one mechanism, depending on the experimental conditions. These data suggest an oxidation embrittlement mechanism (OEM) at temperatures <1373 K and high oxygen pressures and an interphase removal mechanism (IRM) at temperatures of ≳700 K and low oxygen pressures. The OEM results from the reaction of oxygen with SiC to form a glass layer on the fiber or within the fiber–matrix interphase region. The fracture stress of the fiber is decreased if this layer is thicker than a critical value ( d > d _c) and the temperature below a critical value ( T < T _g), such that a sharp crack can be sustained in the layer. The IRM results from the oxidation of the interfacial layer and the resulting decrease of stress that is carried by the bridging fibers. Interphase removal contributes to subcritical crack growth by decreasing the fiber-bridging stresses and, hence, increasing the crack-tip stress. The IRM occurs over a wide range of temperatures for d < d _c and may occur at T > T _g for d > d _c. This paper summarizes the evidence for the existence of these two mechanisms and attempts to define the conditions for their operation.