Transient Inclusion Evolution During Modification of Alumina Inclusions by Calcium in Liquid Steel: Part I. Background,Experimental Techniques and Analysis Methods

In aluminum-killed steels, modification of solid alumina inclusions is often carried out by calcium treatment, converting the alumina to liquid calcium aluminates. When calcium treatment is performed, calcium can either react with sulfur in the melt or with solid alumina. Calcium sulfide inclusions are solid at steel casting temperatures and thus would be detrimental to castability if they remained in the steel after calcium treatment. The aim was to study the transient evolution of inclusions after calcium treatment, testing the hypothesis that calcium sulfide may form as an intermediate reaction product, which can subsequently react with alumina to form modified calcium aluminates. The first part gives the project background and describes the experimental and quantification techniques adopted, including the effect of sampler size in laboratory melts. Results of the formation of intermediate calcium reaction products in laboratory and industrial heats are presented in the second part.     

Effects of substrate orientation on opto-electronic properties in self-assembled InAs/GaAs quantum dots

Electronic structure and optical transition characteristics in (100), (110), and (111) oriented InAs/GaAs quantum dots (containing \({\sim }2\) million atoms) were studied using a combination of valence force-field molecular mechanics and 20-band \(sp^{3}d^{5}s^{*}\) atomistic tight-binding framework. These quantum dots are promising candidates for non-traditional applications such as spintronics, quantum cryptography and quantum computation, but suffer from the deleterious effects of various internal fields. Here, the dependence of strain and polarization fields on the substrate orientation is reported and discussed. It is found that, compared to the (100) and (110) oriented counterparts, quantum dots grown on the (111) oriented substrate exhibit a smaller splitting (non-degeneracy) in the excited \(P\) states and enhanced isotropy in the interband optical emission characteristics.     

Electronic structure and transmission characteristics of SiGe nanowires

Atomistic disorder such as alloy disorder, surface roughness and inhomogeneous strain are known to influence electronic structure and charge transport. Scaling of device dimensions to the nanometer regime enhances the effects of disorder on device characteristics and the need for atomistic modeling arises. In this work SiGe alloy nanowires are studied from two different points of view: (1) Electronic structure where the bandstructure of a nanowire is obtained by projecting out small cell bands from a supercell eigenspectrum and (2) Transport where the transmission coefficient through the nanowire is computed using an atomistic wave function approach. The nearest neighbor sp³d⁵s^* semi-empirical tight-binding model is employed for both electronic structure and transport. The connection between dispersions and transmission coefficients of SiGe random alloy nanowires of different sizes is highlighted. Localization is observed in thin disordered wires and a transition to bulk-like behavior is observed with increasing wire diameter.     

Performance analysis of a Ge/Si core/shell nanowire field-effect transistor

We ana/lyze the performance of a recently reported Ge/Si core/shell nanowire transistor using a semiclassical, ballistic transport model and an sp3d5s* tight-binding treatment of the electronic structure. Comparison of the measured performance of the device with the effects of series resistance removed to the simulated result assuming ballistic transport shows that the experimental device operates between 60 and 85% of the ballistic limit. For this approximately 15 nm diameter Ge nanowire, we also find that 14-18 modes are occupied at room temperature under ON-current conditions with ION/IOFF = 100. To observe true one-dimensional transport in a 110 Ge nanowire transistor, the nanowire diameter would have to be less than about 5 nm. The methodology described here should prove useful for analyzing and comparing on a common basis nanowire transistors of various materials and structures.     

Quasiparticle band gap engineering of graphene and graphone on hexagonal boron nitride substrate

Graphene holds great promise for post-silicon electronics; however, it faces two main challenges: opening up a band gap and finding a suitable substrate material. In principle, graphene on hexagonal boron nitride (hBN) substrate provides a potential system to overcome these challenges. Recent theoretical and experimental studies have provided conflicting results: while theoretical studies suggested a possibility of a finite band gap of graphene on hBN, recent experimental studies find no band gap. Using the first-principles density functional method and the many-body perturbation theory, we have studied graphene on hBN substrate. A Bernal stacked graphene on hBN has a band gap on the order of 0.1 eV, which disappears when graphene is misaligned with respect to hBN. The latter is the likely scenario in realistic devices. In contrast, if graphene supported on hBN is hydrogenated, the resulting system (graphone) exhibits band gaps larger than 2.5 eV. While the band gap opening in graphene/hBN is due to symmetry breaking and is vulnerable to slight perturbation such as misalignment, the graphone band gap is due to chemical functionalization and is robust in the presence of misalignment. The band gap of graphone reduces by about 1 eV when it is supported on hBN due to the polarization effects at the graphone/hBN interface. The band offsets at graphone/hBN interface indicate that hBN can be used not only as a substrate but also as a dielectric in the field effect devices employing graphone as a channel material. Our study could open up new way of band gap engineering in graphene based nanostructures.     

Examples of How Fundamental Knowledge can Improve Steelmaking: Desulphurisation Kinetics Calcium and Modification

Sulphur control is an essential part of steel production. This paper summarises two aspects of sulphur in secondary metallurgy. First, it is shown that silicon can contribute to ladle desulphurisation if the ladle slag is low in silica; the effect of silicon is primarily on the equilibrium sulphur level, rather than a specific kinetic effect. Second, sulphur is shown to capture calcium (as calcium sulphide) upon calcium injection to modify inclusions. In steels with less than approximately 100 ppm sulphur, the calcium sulphide subsequently back-reacts with alumina inclusions, to modify the oxide inclusions to calcium aluminates.     

Multiscale modeling of screening effects on conductivity of graphene in weakly bonded graphene-dielectric heterostructures

Graphene is often surrounded by different dielectric materials when integrated into realistic devices. The absence of dangling bonds allows graphene to bond weakly via the van der Waals interaction with the adjacent material surfaces and to retain its peculiar linear band structure. In such weakly bonded systems, however, the electronic properties of graphene are affected by the dielectric screening due to the long-range Coulomb interaction with the surrounding materials. Including the surrounding materials in the first principles density functional theory (DFT) calculations is computationally very demanding due to the large supercell size required to model heterogeneous interfaces. Here, we employ a multiscale approach combining DFT and the classical image-potential model to investigate the effects of screening from the surrounding materials (hBN, SiC, SiO₂, Al₂O₃, and HfO₂) on the dielectric function and charged impurity scattering limited conductivity of graphene. In this approach, the graphene layer is modeled using DFT and the screening from the surrounding materials is incorporated by introducing an effective dielectric function. The dielectric function and conductivity of graphene calculated using the simplified two-band Dirac model are compared with DFT calculations. The two-band Dirac model is found to significantly overestimate the dielectric screening and charged impurity scattering limited conductivity of graphene. The multiscale approach presented here can also be used to study screening effects in weakly bonded heterostructures of other emerging two-dimensional materials such as metal dichalcogenides.     

Calcium Modification of Spinel Inclusions in Aluminum-Killed Steel: Reaction Steps

Calcium treatment is a well-established way to modify solid alumina inclusions to liquid or partially liquid calcium aluminates. Spinels (Al₂O₃·xMgO) can also form in liquid steel after aluminum deoxidation. Like alumina, the spinels can be modified readily to liquid inclusions by a calcium treatment. The modification of spinels was studied by observing the transient evolution of inclusions, in laboratory and industrial heats. Spinel modification involves the preferential reduction of MgO from the spinel, with Mg dissolving in the steel, and it proceeds through transient calcium sulfide formation, just like in the case of alumina inclusions. Because magnesium dissolves in steel after the calcium treatment of spinels, the reoxidation of the melt will produce new spinels.     

Simulating low temperature diesel combustion with improved spray models

Current spray models based on the Lagrangian-droplet and Eulerian-fluid (LDEF) method in the KIVA-3V code are strongly mesh dependent due to errors in predicting the droplet–gas relative velocity and errors in describing droplet–droplet collision and coalescence processes. To reduce the mesh dependence, gas-jet theory is introduced to predict the droplet–gas relative velocity, and a radius of influence (ROI) of collision methodology is established for each gas phase cell to estimate the collision probability for each parcel in the cell. Spray and combustion processes in a low temperature combustion diesel engine under early and late injection strategies with a fine mesh were predicted using the conventional LDEF model and compared with the measurements of soot, OH, fuel liquid and vapor distributions obtained by laser based diagnostics including, PLIF, LII, and Mie scattering. Then, the KIVA-3V code implemented with the improved spray model based on the gas-jet model and modifications of the spray models was utilized to simulate the processes on a relatively coarse numerical mesh. Comparison of the simulations between the fine and coarse meshes shows that the improved spray model can greatly reduce the mesh dependence for low temperature combustion diesel engine CFD simulations.