Papers by Keyword: Alkali Metals

Abstract: This work is a study of the segregation of alloying elements and salts to the surface of Direct Chilled-cast (DC-cast) ingots of alloy: (i) AA5083; and (ii) AA5182. The ingots only have slightly different chemical compositions. Alloy AA5182 contains Cu, while alloy AA5083 is grain refined, contains Cr, and is lower in Si and Fe compared to AA5182. The ingot microstructures were investigated using Optical Microscopy, Scanning Electron Microscope (SEM) and Secondary Ion Mass Spectrometry (SIMS). The results showed that the surfaces contained aluminum dendrites, large intermetallic crystals (Al₆(MnFe) and Al₃(Fe,Mn)) and impurities, and small crystals between dendrite arms. Between the dendrites small salt crystals containing Na, K, Ca, Mg, F and/or Cl were found. The conclusion is that salt and minority impurities are mainly located to grain boundaries in the segregation zone.

Abstract: We investigate the passivation of interface traps by method of oxidizing Si-face 4H-SiC in the presence of potassium as well as examining the thermal stability of this passivation process. It is observed that this type of dry oxidation leads to a strong passivation of interface traps at the SiO2/4H-SiC interface with energy levels near the SiC conduction band edge. Furthermore, it is observed that if potassium ions residing at the SiO2/SiC interface are moved towards the sample surface by exposing them to ultraviolet light (UV) under an applied depletion bias stress at high temperatures the interface traps become electrically active again and are evidently depassivated. These findings are in line with recently a published model of the effect of sodium on such interface states

Abstract: This paper has built a mathematical model for the evaporating characteristics of the grooved micro heat pipe’s thin film region and computed them in a specific working condition. The evaporating model of Wayner was employed in this mathematical model. The results from computation showed, for the H2O and NH3 as working fluid, at the beginning of the thin film region, the heat flux raised rapidly to a peak value and then declined to almost 0 also rapidly in a very short distance. Differently, for the Na and K as working fluid, the heat flux raised quickly but declined slower. Therefore, the alkali metals working fluids had larger area of high heat flux covered. The results indicated that the alkali metals working fluid has better evaporating characteristics for the high-temperature heat pipe than normal working fluids.

Abstract: The modified embedded atom method (MEAM) can describe the physical properties of bulk systems for a wide range of advanced engineering materials. However, the MEAM is found to return negative surface energy for Li(100), Li(110) and Li(111), if the relaxation of atomic positions on the surface is taken into account. In order to solve this problem, a new scheme of MEAM for lithium has been developed, by modifying the expression of embedding function. In this work, the new scheme is also applied to the other alkali metals, and the parameter sets of MEAM have been determined by fitting to not only bulk properties but also some non-bulk properties. The new MEAM potentials for alkali metals have been applied to calculate the elastic stiffness of crystal, the vacancy formation energy, the surface energies for low index crystal faces and the bond length and the binding energy for dimer. The results have been compared with experimental values.