Valence band study of thermoelectric Zintl-phase SrZn_2Sb_2 and YbZn_2Sb_2: X-ray photoelectron spectroscopy and density functional theory

The electronic structure of SrZn_2Sb_2 and YbZn_2Sb_2 is investigated using density functional theory and high-resolution x-ray photoemission spectroscopy. Both traditional Perdew-Burke-Ernzerhof and state-of-the-art hybrid Heyd-Scuseria-Ernzerhof functionals have been employed to highlight the importance of proper treatment of exchange-dependent Zn  3d states, Yb 4f states, and band gaps. The role of spin-orbit corrections in light of first-principles transport calculations are discussed and previous claims of Yb^(3+) valence are investigated with the assistance of photoelectron as well as scanning and transmission electron microscopy

Effects of spatial variability on the estimation of erosion rates for cohesive riverbanks

River morphodynamics and sediment transportBank erosion and protectio

Turbulent Flow through Idealized Emergent Vegetation

This paper presents results of several large-eddy simulations (LES) of turbulent flow in an open channel through staggered arrays of rigid, emergent cylinders, which can be regarded as idealized vegetation. In this study, two cylinder Reynolds numbers, RD=1,340 and RD=500, and three vegetation densities are considered. The LES of the lowest density and at RD=1,340 corresponds to a recently completed laboratory experiment, the data of which is used to validate the simulations. Fairly good agreement between calculated and measured first- and second-order statistics along measurement profiles is found, confirming the accuracy of the simulations. The high resolution of the simulations enables an explicit calculation of drag forces, decomposed into pressure and friction drag, that are exerted on the cylinders. The effect of the cylinder Reynolds number and the cylinder density on the drag and hence on the flow resistance is quantified and in agreement with previous experimental studies. Turbulence structures are visualized through instantaneous pressure fluctuations, isosurfaces of the Q-criterion and contours of vertical vorticity in horizontal planes. Analysis of velocity time signals and distributions of drag and lift forces over time reveals that flow and turbulence are more influenced by the vegetation density than by the cylinder Reynolds number

The End of the Lines for OX 169: No Binary Broad-Line Region

We show that unusual Balmer emission line profiles of the quasar OX 169, frequently described as either self-absorbed or double peaked, are actually neither. The effect is an illusion resulting from two coincidences. First, the forbidden lines are quite strong and broad. Consequently, the [N II]6583 line and the associated narrow-line component of H-alpha present the appearance of twin H-alpha peaks. Second, the redshift of 0.2110 brings H-beta into coincidence with Na I D at zero redshift, and ISM absorption in Na I D divides the H-beta emission line. In spectra obtained over the past decade, we see no substantial change in the character of the line profiles, and no indication of intrinsic double-peaked structure. The H-gamma, Mg II, and Ly-alpha emission lines are single peaked, and all of the emission-line redshifts are consistent once they are correctly attributed to their permitted and forbidden-line identifications. A systematic shift of up to 700 km/s between broad and narrow lines is seen, but such differences are common, and could be due to gravitational and transverse redshift in a low-inclination disk. Stockton & Farnham (1991) had called attention to an apparent tidal tail in the host galaxy of OX 169, and speculated that a recent merger had supplied the nucleus with a coalescing pair of black holes which was now revealing its existence in the form of two physically distinct broad-line regions. Although there is no longer any evidence for two broad emission-line regions in OX 169, binary black holes should form frequently in galaxy mergers, and it is still worthwhile to monitor the radial velocities of emission lines which could supply evidence of their existence in certain objects.Comment: 19 pages, 5 figures, accepted for publication in Ap.

Accelerated material development for laser powder-bed fusion using the arc melting process

Metal additive manufacturing has in recent years gained an increasing amount of attention, especially the subgroup of laser powder-bed fusion and aluminium alloys. However, established alloys are designed for casting and forging and often require alterations to make them eligible for the challenging processing conditions. The material selection is limited and calls for new alloys tailored specifically for additive manufacturing. In this work, an analysis suite is proposed as a tool to investigate material systems quickly and in-expensively for use in additive manufacturing. The selected material system is the Al7075 aluminium alloy, which is susceptible to cracking caused by hot tearing. To resolve this issue, it is mixed with varying quantities of silicon. The effect of silicon on solidification, grain refinement, and the resulting crack susceptibility is investigated with thermodynamical calculations considering the columnar to equiaxed transition, optical microscopy, and scanning electron microscopy after being processed by arc melting. The thermodynamical calculations of the compositions indicated a trend between the decreased columnar to equiaxed transition point at elevated temperature gradients to the silicon concentration. The experimental results reflected a similar trend by observing the reduction of the average grain size in the material system from 2470 μm2 to 323 μm2 for a composition with 0 wt.% and 10.5 wt.% silicon respectively. A composition of interest from the result was further mixed with zirconium hydride to investigate its grain refining properties on the alloy. The average grain size was reduced from 1055 μm2 to 453 μm2 by the inclusion of 0.24 wt.% zirconium. As such, this work provides a new approach to investigating a material system for use in additive manufacturing.acceptedVersio