Papers by Keyword: Mechanism of Formation

Abstract: Composite core-shell SiC-SiO_x nanowires can be produced by heating quartz and SiC powders, with addition of Ar(g) or He(g). The two powders are mixed to create pellets, which will react to SiO(g) and CO(g) at elevated temperatures. The two gases will react on a colder surface, producing a web of SiC-SiO_x nanowires. The product serves as a precursor for SiC nanowires production. During the process, silicon and oxygen accumulate at high energy points, forming SiO_x nodules. Nodules can either generate in proximity of stacking faults, or where two or more nanowires are close to each other. The present work investigates the role of crystal defects in the wettability between silica and silicon carbide. Samples were collected and analyzed under Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). The results show that β-SiC grows mainly in the [111] direction. Crystal defects are located in the SiC core-phase. SiO_x initially develops a uniform layer as thick as the core-phase itself. SiO_x nodules would first form where the defects are present, by accumulating at high energy sites. Droplets on a flat surface imply poor wettability. The mechanism of formation of the nodules is compared to two earlier proposed theories. In conclusion, the wettability of SiO_x and SiC at nanoscale is controlled by the presence of crystallographic defects. Continuous SiO_x layers and bead-like structures can be found in the same temperature interval. The microstructural changes depend on the local energy balance.

Abstract: Buried mudstones general have undercompacted phenomenon. Undercompacted mudstones have the characteristics that the porosity and pore fluid pressure are abnormal bigger. In order to disclosure the seepage mechanics mechanism of undercompacted mudstones formation, this paper has summed up the seepage mechanics relationship when fluid flows through the mudstone, and has verified the relationships between the key physical quantities with the minimal pressure (pressure that can let the fluid flow in the mudstone) by the experiments in physics. This paper has also analysis the formations process of undercompacted mudstone. The result shows that, the flow regime of fluid in the mudstone is the low speed seepage, and it is not applicable by Darcy equation; the fluid what flow through the thick and heavy compacted mudstone has the big minimal pressure. At the beginning or during the deposit, the rule of fluid flow in the mudstone decides that the fluid inside of the mudstone is more difficult to flow out than the fluid surface of the mudstone, and the inside mudstone becomes undercompacted. Because of the undercompacted mudstone is more important for the exploration of oil and gas, it has theoretic and practical sense to analysis the formations mechanism of the undercompacted mudstone.

Abstract: Sand-cast compacted graphite (CG) cast iron typically presents a surface layer (the casting skin) whose microstructure is significantly different than that of the bulk material. It is generally believed that the casting skin is the result on interaction between the metal and the mold moisture and atmosphere. This reaction results in a decarburized or graphite-free layer. The thickness of this layer is a function of process variables (e.g. cooling rate, pouring temperature, binder, coating, sand fineness, and Mg and inoculation level). The paper presents a summary of recent findings on the negative effect of casting skin on the static properties of CG iron through the use of the skin quality factor for tensile strength, defined as the ratio between the as-cast and machined tensile strength. A mechanism of casting skin formation is proposed and supported with experimental and computational data.