Atomic Ordering at the Liquid-Al/MgAl2O4 Interfaces from Ab Initio Molecular Dynamics Simulations

EPSRC (UK) grant number EP/N007638/

Prenucleation at the Interface Between MgO and Liquid Magnesium: An Ab Initio Molecular Dynamics Study

Magnesia (MgO) particles inevitably exist in liquid Mg and may be used as potential sites for heterogeneous nucleation to achieve effective grain refinement. Understanding of the atomic configurations on MgO surfaces and in the liquid Mg adjacent to the liquid Mg/MgO interfaces is therefore of both scientific and practical interests. We investigate the surface structures of MgO in liquid Mg and the atomic arrangements of liquid Mg adjacent to liquid/substrate interfaces, using an ab initio molecular dynamics (MD) simulation technique. We find that an atomically rough terminating Mg layer forms on the {1 1 1} terminated MgO substrate (octahedral MgO) in liquid Mg. The simulations also reveal that on the structurally flat {0 0 1} terminated MgO substrate (cubic MgO) a rough Mg layer forms due to the unique chemical interactions between the ions on the substrate and the liquid metals. The surface roughness together with the large lattice misfits with solid Mg makes both octahedral and cubic MgO substrates impotent for heterogeneous nucleation of α-Mg. The present results may shed new light on grain refinement of Mg-alloys.EPSRC (UK

Defects in half-metals and finite temperature

The influence of intrinsic defects in half-metals is calculated in the case of NiMnSb. Of the 14 cases of intrinsic defects, five affect the half-metallic properties. They are energetically very unlikely to occur. Circumstances are discussed under which defects may even have a beneficial effect on the spin polarization of the conduction electrons. Non-intrinsic defects, like deliberate doping by rare-earth atoms, as well as the effect of nano-structured contacts may influence the magnon spectrum, improving the behaviour at finite temperature.</p

Effect of Substrate Chemistry on Prenucleation

© 2018, The Authors. Understanding atomic ordering in a metallic liquid adjacent to a solid substrate is of both scientific and technological importance. In this study, we used ab initio molecular dynamics (MD) simulations to investigate systematically the effect of substrate chemistry on atomic ordering in the liquid adjacent to artificially created substrates that have the same crystal structure as the solid phase upon solidification but different chemical characteristics. We found that for a given liquid, an attractive chemical interaction (negative heat of mixing) between the liquid and the substrate strengthens atomic ordering in the liquid at the interface, while a repulsive interaction (positive heat of mixing) weakens atomic ordering. It is realized that although both structure and chemistry of the substrate affect atomic ordering in the liquid adjacent to the liquid/substrate interface, the structural effect dominates the atomic ordering process, while the chemical effect is but a secondary factor and affects atomic ordering through either strengthening or weakening the structural templating. Such knowledge of atomic ordering may help us to understand both prenucleation and heterogeneous nucleation, and to develop more effective approaches to grain refinement during solidification through effective manipulation of the interplay between structural and chemical effects of the substrate on atomic ordering in the liquid adjacent to the interface

Formation, structure and magnetism of the γ-(Fe,M)23C6 (M = Cr, Ni) phases: A first-principles study

The γ-(Fe,M)23C6 phases constitute an important class of iron carbides. They occur both as precipitates in steels and iron alloys, thereby increasing their strength, and as common minerals in meteorites and in iron-rich parts of the Earth's mantle. Here we investigate the composition-dependent relative stability of these phases and the role of magnetism therein. The γ-(Fe,M)23C6 phases have mineral names isovite (M = Cr) and haxonite (M = Ni), and have a complex crystal structure (116 atoms in the cubic unit cell) in which the metal atoms have a rich variety of atomic coordination numbers, ranging from 12 to 16. First-principles calculations show a narrow formation range for γ-(Fe1−xNix)23C6 (x = 0–0.043), while the formation range for γ-(Fe1−xCrx)23C6 is very broad (x = 0–0.85), in good agreement with available experimental data. The present study also shows the importance of magnetism on the formation and stability of these compounds. The conditions of formation and several factors enhancing or hampering the formation of γ-(Fe,M)23C6 in man-made steels and in meteorites are discussed.MvH acknowledges the Dutch Science Foundation NWO for a VIDI (Grant No. 723.012.006)

Evaluation of Xpert® MTB/RIF and ustar easyNAT™ TB IAD for diagnosis of tuberculous lymphadenitis of children in Tanzania : a prospective descriptive study

Fine needle aspiration biopsy has become a standard approach for diagnosis of peripheral tuberculous lymphadenitis. The aim of this study was to compare the performance of Xpert MTB/RIF and Ustar EasyNAT TB IAD nucleic acid amplification assays, against acid-fast bacilli microscopy, cytology and mycobacterial culture for the diagnosis of TB lymphadenitis in children from a TB-endemic setting in Tanzania.; Children of 8 weeks to 16 years of age, suspected of having TB lymphadenitis, were recruited at a district hospital in Tanzania. Fine needle aspirates of lymph nodes were analysed using acid-fast bacilli microscopy, liquid TB culture, cytology, Xpert MTB/RIF and EasyNAT. Latent class analysis and comparison against a composite reference standard comprising "culture and/or cytology" was done, to assess the performance of Xpert MTB/RIF and EasyNAT for the diagnosis of TB lymphadenitis.; Seventy-nine children were recruited; 4 were excluded from analysis. Against a composite reference standard of culture and/or cytology, Xpert MTB/RIF and EasyNAT had a sensitivity and specificity of 58 % and 93 %; and 19 % and 100 % respectively. Relative to latent class definitions, cytology had a sensitivity of 100 % and specificity of 94.7 %.; Combining clinical assessment, cytology and Xpert MTB/RIF may allow for a rapid and accurate diagnosis of childhood TB lymphadenitis. Larger diagnostic evaluation studies are recommended to validate these findings and on Xpert MTB/RIF to assess its use as a solitary initial test for TB lymphadenitis in children

Water Oxidation with Cobalt‐Loaded Linear Conjugated Polymer Photocatalysts

We report here the first examples of linear conjugated organic polymer photocatalysts that produce oxygen from water after loading with cobalt and in the presence of an electron scavenger. The oxygen evolution rates, which are higher than for related organic materials, can be rationalized by a combination of the thermodynamic driving force for water oxidation, the light absorption of the polymer, and the aqueous dispersibility of the relatively hydrophilic polymer particles. We also used transient absorption spectroscopy to study the best performing system and we found that fast oxidative quenching of the exciton occurs (picoseconds) in the presence of an electron scavenger, minimizing recombination

Intrinsic defects in and electronic properties of θ-Al13Fe4: an ab initio DFT study

© 2019 The Author(s). θ-Al13Fe4 exhibits a rich variety of crystal physics. It contains twenty crystallographically different atomic species with a diversity of chemical coordination. An understanding of its structural and physical properties is a prerequisite for controlling its formation and its use. Here we investigate systematically the intrinsic defects in θ-Al13Fe4 using a first-principles density-functional theory method. The calculations reveal that among the various intrinsic defects it is energetically favourable for Fe substitution of Al but on just three of the fifteen Al sites. This results in a new structural model, Al68Fe24(Al, Fe)4IX(Al, Fe)2VII(Al, Fe)4V (the Roman numerals represent the Al sites) which updates the thermodynamic model, currently in use, which is associated with the formation of vacancies on some of the Al sites. The calculations demonstrate that the addition of Fe induces magnetism which gives rise to clustering. The calculations provide the dependence of the lattice parameters on Fe concentration and explain the experimental data in the literature. The information obtained here provides insight into the formation and properties of θ-Al13Fe4 and its role in the solidification of Al alloys, in determination of the microstructure and related mechanical properties of the products, and in catalysis for organic reactions.EPSRC (Grant EP/N007638/1)

Measuring portfolio performance using a modified measure of risk

This paper reports the results of an investigation into the properties of a theoretical modification of beta proposed by Leland (1999) and based on earlier work of Rubinstein (1976). It is shown that when returns are elliptically symmetric, beta is the appropriate measure of risk and that there are other situations in which the modified beta will be similar to the traditional measure based on the capital asset pricing model. For the case where returns have a normal distribution, it is shown that the criterion either does not exist or reduces exactly to the conventional beta. It is therefore conjectured that the modified measure will only be useful for portfolios that have nonstandard return distributions which incorporate skewness. For such situations, it is shown how to estimate the measure using regression and how to compare the resulting statistic with a traditional estimated beta using Hotelling's test. An empirical study based on stocks from the FTSE350 does not find evidence to support the use of the new measure even in the presence of skewness.Journal of Asset Management (2007) 7, 388-403. doi:10.1057/palgrave.jam.225005