Structure of and ion segregation to an alumina grain boundary: implications for growth and creep

Using periodic density-functional theory (DFT), we investigated the structure and cohesive properties of the �-alumina �11 tilt grain boundary, with and without segregated elements, as a model for the thermally grown oxide in jet engine thermal barrier coatings. We identified a new low-energy structure different from what was proposed previously based on electron microscopy and classical potential simulations. We explored the structure and energy landscape at the grain boundary for segregated Al, O, and early transition metals (TMs) Y and Hf. We predict that the TMs preferentially adsorb at the same sites as Al, while some adsites favored by O remain unblocked by TMs. All segregated atoms have a limited effect on grain boundary adhesion, suggesting that adhesion energies alone cannot be used for predictions of creep inhibition. These findings provide some new insights into how TM dopants affect alumina growth and creep kinetics. I

Investigations of the stress and deformation behaviour of large backup and work rolls under operation-near loads

Stütz- und Arbeitswalzen in Grobblech- und Warmbandwalzwerken unterliegen im Walzprozess starken äußeren Belastungen, welche in einem komplexen Spannungs- und Deformationszustand resultieren. Wirkende Kräfte haben eine elastische Deformation der Walzen zur Folge, die die Qualität und Maßhaltigkeit des Walzgutes maßgeblich bestimmen. Die Dimensionierung der Walzen zum Einsatz in einem Walzgerüst wird in der Regel durch Erfahrungswerte oder Festigkeitsberechnungen bestimmt, stützen sich jedoch nicht auf Messwerte von Untersuchungen realer Größenordnungen. Experimentelle Ergebnisse sind in der Literatur höchstens von Walzen im Labormaßstab zu finden. Eine Übertragung auf reale Abmessungen wird häufig vollzogen, ist jedoch aufgrund der Komplexität der Walzen mit allen Randbedingungen (Inhomogenitäten, Struktur) nicht sinnvoll. Um auch Erkenntnisse über das Spannungs- und Deformationsverhalten von Arbeits- und Stützwalzen mit realitätsgetreuen Abmessungen zu erhalten, wurde als Ziel dieser Arbeit ein Großprüfstand konzipiert und gebaut, um eine Kombination aus einer Arbeits- und Stützwalze unter betriebsnahen Lasten zu untersuchen. So konnte neben dem Spannungszustand auch das Durchbiegeverhalten der Walzen gemessen werden. Zur Beeinflussung der Biegelinie wurde zusätzlich der Einfluss einer Walzengegenbiegung und einer Walzenschränkung untersucht. Ein weiteres Ziel dieser Arbeit war das Aufstellen von Rechenmodellen, welche durch die experimentellen Untersuchungen validiert werden sollten. Neben einem Balkenmodell auf Basis der finiten Elemente wurde der gesamte Prüfstand mit der FE-Software COMSOL Multiphysics simuliert. Diese Arbeit verschafft zusätzlich einen Überblick über den Stand der Technik hinsichtlich der zur Warmband- und Grobblecherzeugung eingesetzten Walzenwerkstoffe, den Herstellungsverfahren von Verbundgusswalzen, welche in diesen Werken überwiegend eingesetzt werden und den Möglichkeiten zur Beeinflussung der Planheit eines Bandes oder Bleches durch gezielte Modifikation der Walzen oder des Walzprozesses.Such rolls as for instance backup and work rolls used in plate and hot-strip rolling mills are exposed to severe external loads which can be derived from stress and deformation conditions of complex nature. Furthermore, active forces result in roll deformations which have a decisive and substantial impact on quality and dimensional stability of the rolled stock. Dimensioning of the rolls that are going to be used in a roll stand will usually be determined by empirical values or strength calculations; however, neither empirical values nor strength calculations are based on measured values of actual magnitudes as examined. Experimental results can at best be found in literature on rolls in lab-scale. Although transmission to real dimensions is rather customary, it is not reasonable to do so because of the complexity of the rolls, including all marginal conditions (as for instance non-homogeneity and structure). It was the aim of this study to draft and build a large-scale test bench in order to also gain knowledge of the stress and deformation behaviour of backup and work rolls with realistic dimensions on the one hand, and to be able to examine the combination of backup and work roll with operation-near loads on the other. This way, both the stress condition as well as the roll bending behaviour could be measured. For influencing the bending line, investigations have additionally been made on the influence of a roll counter bending system and a roll crossing system. Another goal of this study was to establish calculation models which were to be validated by means of experimental investigations. In addition to a beam model based on finite elements, the entire test bench was simulated by means of the COMSOL Multiphysics FEsoftware. This study provides an additional survey on the state-of-the-art technology in regard to the roll materials used for production of plates and hot strips, the production processes of compound- cast rolls which will predominantly be employed in these mills as well as the possibilities of influencing the strip or plate flatness owing to a tailor-made modification of the rolls or the rolling process

Quantum Dimensional Zeeman Effect in the Magneto-optical Absorption Spectrum for Quantum Dot - Impurity Center Systems

Magneto-optical properties of the quantum dot - impurity center (QD-IC) systems synthesized in a transparent dielectric matrix are considered. For the QD one-electron state description the parabolic model of the confinement potential is used. Within the framework of zero-range potential model and the effective mass approach, the light impurity absorption coefficient for the case of transversal polarization with respect to the applied magnetic field direction, with consideration of the QD size dispersion, has been analytically calculated. It is shown that for the case of transversal polarization the light impurity absorption spectrum is characterized by the quantum dimensional Zeeman effect.Comment: 18 pages, 1 figure, PDF fil

On the feasibility of N2 fixation via a single-site FeI/FeIV cycle: Spectroscopic studies of FeI(N2)FeI, FeIV=N, and related species

The electronic properties of an unusually redox-rich iron system, [PhBPR 3]FeNx (where [PhBPR 3] is [PhB(CH2PR2)3]−), are explored by Mössbauer, EPR, magnetization, and density-functional methods to gain a detailed picture regarding their oxidation states and electronic structures. The complexes of primary interest in this article are the two terminal iron(IV) nitride species, [PhBPiPr 3]FeN (3a) and [PhBPCH2Cy 3]FeN (3b), and the formally diiron(I) bridged-Fe(μ-N2)Fe species, {[PhBPiPr 3]Fe}2(μ-N2) (4). Complex 4 is chemically related to 3a via a spontaneous nitride coupling reaction. The diamagnetic iron(IV) nitrides 3a and 3b exhibit unique electronic environments that are reflected in their unusual Mössbauer parameters, including quadrupole-splitting values of 6.01(1) mm/s and isomer shift values of −0.34(1) mm/s. The data for 4 suggest that this complex can be described by a weak ferromagnetic interaction (J/D < 1) between two iron(I) centers. For comparison, four other relevant complexes also are characterized: a diamagnetic iron(IV) trihydride [PhBPiPr 3]Fe(H)3(PMe3) (5), an S = 3/2 iron(I) phosphine adduct [PhBPiPr 3]FePMe3 (6), and the S = 2 iron(II) precursors to 3a, [PhBPiPr 3]FeCl and [PhBPiPr 3]Fe-2,3:5,6-dibenzo-7-aza bicyclo[2.2.1]hepta-2,5-diene (dbabh). The electronic properties of these respective complexes also have been explored by density-functional methods to help corroborate our spectral assignments and to probe their electronic structures further

Local surface structure and composition control the hydrogen evolution reaction on iron nickel sulfides

In order to design more powerful electrocatalysts, developing our understanding of the role of the surface structure and composition of widely abundant bulk materials is crucial. This is particularly true in the search for alternative hydrogen evolution reaction (HER) catalysts to replace platinum. We report scanning electrochemical cell microscopy (SECCM) measurements of the (111)‐crystal planes of Fe4.5Ni4.5S8, a highly active HER catalyst. In combination with structural characterization methods, we show that this technique can reveal differences in activity arising from even the slightest compositional changes. By probing electrochemical properties at the nanoscale, in conjunction with complementary structural information, novel design principles are revealed for application to rational material synthesis

Rational Design Rules for Molecular Water Oxidation Catalysts based on Scaling Relationships

Lowering the overpotential required for water oxidation is of paramount importance for the efficient production of carbon-neutral fuels. This article highlights the intrinsic influence of the water oxidation mechanism used by molecular catalysts on the theoretically achievable minimal overpotential, based on scaling relationships typically used for heterogeneous catalysts. Due to such scaling relationships, catalysts that operate through the water nucleophilic attack mechanism have a fundamental minimal overpotential of about 0.3V, whereas those that follow the dinuclear radical oxo coupling mechanism should in principle be able to operate with a lower overpotential. Therefore, it is recommended to design catalysts operating through the latter mechanism to achieve very efficient water oxidation systems