High-temperature oxidation of SiC in a glow-discharge oxygen plasma

The oxidation kinetics and structure of the oxide scales formed on high-density SiC were studied in molecular oxygen at 740 Torr and in a glow-discharge oxygen plasma at 0.1 Torr at temperatures of 1000, 1100, and 1200°C. The monatomic oxygen formed by the glow discharge markedly increased the reaction rate and the vaporization of some of the oxidation products. The marked differences in kinetics suggest that the rate-controlling step during oxidation in molecular oxygen is the dissociation of adsorbed diatomic oxygen to the monatomic species. Films formed in molecular oxygen were mostly amorphous SiO₂ with small inclusions of SiC and graphite, whereas films formed in dissociated oxygen were primarily amorphous SiO₂ containing SiO, S₂O₃, and the coesite form of SiO₂.     

Numerical modeling of high-temperature corrosion processes

Numerical modeling of the diffusional transport associated with high-temperature corrosion processes is reviewed. These corrosion processes include external scale formation and internal subscale formation during oxidation, coating degradation by oxidation and substrate interdiffusion, carburization, sulfidation and nitridation. The studies that are reviewed cover such complexities as concentration-dependent diffusivities, cross-term effects in ternary alloys, and internal precipitation where several compounds of the same element may form (e.g., carbides of Cr) or several compounds exist simultaneously (e.g., carbides containing varying amounts of Ni, Cr, Fe or Mo). In addition, the studies involve a variety of boundary conditions that vary with time and temperature. Finite-difference (F-D) techniques have been applied almost exclusively to model either the solute or corrodant transport in each of these studies. Hence, the paper first reviews the use of F-D techniques to develop solutions to the diffusion equations with various boundary conditions appropriate to high-temperature corrosion processes. The bulk of the paper then reviews various F-D modeling studies of diffusional transport associated with high-temperature corrosion.     

Influence of Reinforcement Content and Matrix Composition on the Oxidation Resistance of Aluminum-Matrix Composites (A3xx.x/SiCp)

A study was made on the influence of the SiCp proportion and the matrix concentration of four composites (A360/SiC/10p, A360/SiC/20p, A380/SiC/10p, A380/SiC/20p) on their oxidation behavior. Gravimetric tests were used in a kinetics study of the corrosion process at different temperatures (350, 400, 450, and 500°C). The influence of corrosion on mechanical properties was evaluated by hardness measurements. The nature of corrosion products and the influence of the microstructure on the morphology and growth of the oxidation layer were analyzed by SEM and low-angle XRD. The extent of the damage due to oxidation for Al/SiCp composites increases with the SiCp concentration due to the increase of nucleation sites. One of these nucleation sites is in the interface region between the matrix and the particles. Oxidation was influenced more by the percentage of alloy elements in the matrix than by the proportion of SiCp reinforcement. The presence of Cu and Ni in the matrix favors the oxidation process through the formation of different intermetallic compounds.     

Effects of electrical discharge surface modification of superalloy Haynes 230 with aluminum and molybdenum on oxidation behavior

The effects of the electrical discharge alloying (EDA) process on improving the high temperature oxidation resistance of the Ni-based superalloy Haynes 230 have been investigated. The 85 at.% Al and 15 at.% Mo composite electrode provided the surface alloying materials. An Al-rich layer is produced on the surface of the EDA specimen alloyed with positive electrode polarity, whereas, many discontinuous piled layers are attached to the surface of the EDA superalloy when negative electrode polarity is selected. The oxidation resistance of the specimen alloyed with positive electrode polarity is better than that of the unalloyed superalloy, and the effective temperature of oxidation resistance of the alloyed layer can be achieved to 1100 °C. Conversely, the oxidation resistance of the other EDA specimen alloyed with negative electrode polarity is even worse than that of the unalloyed superalloy.     

Chip geometries during high-speed machining for orthogonal cutting conditions

The originality of this work consists in taking photographs of chips during the cutting process for a large range of speeds. Contrary to methods usually used such as the quick stop in which root chips are analyzed after an abrupt interruption of the cutting, the proposed process photographs the chip geometry during its elaboration. An original device reproducing perfectly orthogonal cutting conditions is used because it allows a good accessibility to the zone of machining and reduces considerably the vibrations found in conventional machining tests. A large range of cutting velocities is investigated (from 17 to 60 m/s) for a middle hard steel (French Standards XC18). The experimental measures of the root chip geometry, more specifically the tool-chip contact length and the shear angle, are obtained from an analysis of the pictures obtained with a numerical high-speed camera. These geometrical characteristics of chips are studied for various cutting speeds, at the three rake angles −5, 0, +5° and for different depths of cut reaching 0.65 mm.     

The low-temperature oxidation of calcium by water vapor

The oxidation kinetics of calcium in water vapor have been studied over the temperature range 25–300°C. There is a change in the form of the oxidation kinetics with temperature, from essentially linear at temperatures below 150°C to logarithmic at 300°C. This is coupled with a change in the manner of growth of the oxide layer as well as the chemical composition of the reaction product. In addition, the oxidation rate decreases with temperature, reaching a minimum at about 150°C. At temperatures below 150°C oxidation appears to be a result of the formation of cracks or fissures in the oxide film. Above 150°C no single oxidation mechanism can be deduced.     

计算机编程语言与输入设备的发展问题初探

现代化建设事业的发展离不开计算机水平的提高。计算机编程语言作为人与机器沟通的桥梁,在计算机系统里有着非常重要的地位。本文从探讨计算机编程语言的概念出发,详细阐述了计算机编程语言所包含的内容和基本分类。接着笔者又深入分析了计算机输入设备的发展问题。最后,针对计算机编程语言和输入之间的联系问题,笔者做了观点性和理论性的论述分析。     

基于DSOF的网格资源发现模型研究

为了解决传统资源发现机制不能很好地适应网格资源环境的问题,在有效组建网格资源虚拟组织的基础上,提出了资源虚拟组织中联系结点的选取原则和选取算法,通过该算法从具有相同资源类型的联系结点中选出管理结点.联系结点和管理结点之间采用类似二部图的方式进行连接,构成基于动态自组织覆盖架构的网格资源发现模型.从资源发现能力、资源发现效率和系统可扩展性等方面对该模型进行了综合评价,结果显示该模型适合网格资源的特征,在大规模网格系统中能够有效提高资源发现性能,仿真实验验证了该模型的有效性.     

Effect of pressure on three catalytic partial oxidation reactions at millisecond contact times

The effects of pressure on reactant conversion and product selectivities in three catalytic oxidation systems have been examined at pressures between 1 and > 5 atm. Reaction was sustained autothermally near adiabatic operating conditions at temperatures of 1000°C with residence times over the noble metal catalysts between 10^–4 and 10^–2 s. The three systems investigated were (1) HCN synthesis over Pt-10% Rh gauze catalysts, (2) methane oxidation to synthesis gas (CO and H₂) over rhodium-coated monoliths, and (3) ethane conversion to ethylene over platinum-coated monoliths. We find that selectivities in all three reactions do not change dramatically with approximately a five-fold increase in pressure. This strongly suggests that free radical homogeneous chain reactions are not significant in these processes and that they can be operated reliably above atmospheric pressure. For the synthesis of HCN over Pt-10% Rh gauzes, the selectivity to HCN can be maintained above 0.75 at pressures up to 5.5 atm. Selectivities to synthesis gas (CO and H₂) from a methane-air mixture over a Rh-coated foam monolith at pressures up to 5.5 atm were maintained above 0.90. Over a Pt-coated foam monolith, the selectivity to ethylene from ethane-air and ethane-O₂ mixtures was independent of pressure up to 6.5 atm and conversion rose slightly although it was necessary to maintain constant velocity and residence time over the catalyst to avoid carbon formation.This research was supported by DOE under Grant No. DE-FG02-88ER13878.     

In-situ synthesis of TiO2 nanostructures on Ti foil for enhanced and stable photocatalytic performance

TiO_2 nanostructures with strong interfacial adhesion and diverse morphologies have been in-situ grown on Ti foil substrate through a multiple-step method based on conventional plasma electrolytic oxidation(PEO) technology, hydrothermal reaction and ion exchange process. The PEO process is critical to the formation of TiO_2 seeding layer for the nucleation of Na_2Ti_3O_7 and H_2Ti_3O_7 mediates that are strongly attached to the Ti foil. An ion exchange reaction can finally lead to the formation of H_2Ti_3O_7 nanostructures with diverse morphologies and the calcination process can turn the H_2Ti_3O_7 nanostructures into TiO_2 nanostructures with enhanced crystallinity. The morphology of the TiO_2 nanostructures including nanoparticles(NP), nanowhiskers(NWK), nanowires(NW) and nanosheets(NS) can be easily tailored by controlling the NaOH concentration and reaction time during hydrothermal process. The morphology, composition and optical properties of TiO_2 photocatalysts were analyzed using scanning electron microscope(SEM), X-ray diffraction(XRD), photoluminescence(PL) spectroscopy and UV–vis absorption spectrum. Photocatalytic tests indicate that the TiO_2 nanosheets calcined at 500?C show good crystallization and the best capability of decomposing organic pollutants. The decoration of Ag cocatalyst can further improve the photocatalytic performance of the TiO_2 nanosheets as a result of the enhanced charger separation efficiency. Cyclic photocatalytic test using TiO_2 nanostructures grown on Ti foil substrate demonstrates the superior stability in the photodegradation of organic pollutant, suggesting the promising potential of in-situ growth technology for industrial application.