Particle dynamics modeling of boundary effects in granular couette flow

This study provides insights in understanding boundary effects on the flow of dry granular materials composed of identical, smooth, inelastic spheres between parallel, bumpy walls in the absence of gravity. The results of this study are useful in providing a basis upon which developing theories can be modified as well as substantiating previous and current experiments. This in turn has a major importance in many industries which are concerned with handling of particulates, such as coal, mineral processing, powder metallurgy, and agriculture. The particle dynamics or discrete element method is used to model this flow, thereby providing a means of obtaining macroscopic information from the detailed particle-level multi-body dynamics. A shearing flow is induced by allowing the upper and lower walls to move with the same constant velocity in opposite directions. The wall geometry is characterized by several parameters - the spacing between the wall half-spheres and their geometric arrangement, diameter ratio of the wall to flow spheres, shear gap height and particle inelasticity measured by a constant normal restitution coefficient. Boundary roughness is either increased or decreased by appropriately adjusting the spacing between wall half-spheres or by changing the diameter ratio. For small systems, a large stress drop occurs for dence [sic] flows when the wall particles are tightly packed. In this case, a layered shearing flow is present. However, as the shear gap height is increased while maintaining the same wall conditions, the stress drop does not occur. Computed wall stress components and slip velocities are very sensitive to boundary parameters. The ratio of shear to normal wall stresses is found to be independent of shear gap heights for fixed wall conditions. As wall roughness increases or flow particles become more elastic, slip velocity decreases thereby reducing the effectiveness of the walls in supplying momentum to the flow. A pronounced reduction in slip velocity is observed with increasing wall roughness for relatively elastic particles

Growth of superconducting MgB2 thin films via postannealing techniques

We report the effect of annealing on the superconductivity of MgB2 thin films as functions of the postannealing temperature in the range from 700 C to 950 C and of the postannealing time in the range from 30 min to 120 min. On annealing at 900 C for 30 min, we obtained the best-quality MgB2 films with a transition temperature of 39 K and a critical current density of ~ 10^7 A/cm^2. Using the scanning electron microscopy, we also investigated the film growth mechanism. The samples annealed at higher temperatures showed the larger grain sizes, well-aligned crystal structures with preferential orientations along the c-axis, and smooth surface morphologies. However, a longer annealing time prevented the alignment of grains and reduced the superconductivity, indicating a strong interfacial reaction between the substrate and the MgB2 film.Comment: 7 pages, 4 figures include