S. Cottenier

Precipitation in simultaneously nitrided and aged Mo-containing maraging steel

A. Verdiere, C. Hofer, S. De Waele, V. Bliznuk, S. Primig, S. Cottenier, M. D. Tran, B. Pennings, L. A. I. Kestens, R. H. Petrov
Materials Characterization
131, 21-30
2017
A1

Abstract 

The excellent mechanical properties of maraging steels are ascribed to nanometer-sized intermetallics which precipitate during aging in the ductile very low carbon Ni-martensite. Their wear and fatigue properties can be improved by nitriding. The non-equilibrium precipitation reactions in Fe-Ni-Co-Mo maraging steels are studied during an aging heat treatment executed in a nitriding atmosphere. The precipitates formed during the initial stages of precipitation are characterized with transmission electron microscopy and atom probe tomography. Spherical intermetallic precipitates having a diameter of around 3 nm were detected in the aged, bulk material. These ω-type precipitates formed during the early stages of aging, have a trigonal crystal lattice and their chemical composition is close to (Fe,Ni)7Mo2. In the nitrided layer, Mo-N disc-shaped nitrides on the {100} martensitic lattice having a diameter of 3 to 4 nm were found but their exact crystal structure could not be determined with microstructural characterization techniques. Density functional theory calculations confirmed that a single layer of Mo atoms, substituting Fe on the {100} plane of the Fe-matrix, is stable and showed that the N atoms prefer to be in the Mo-layer, on the octahedral sites with Fe as nearest neighbors.

Adsorption of volatile polonium and bismuth species on metals in various gas atmospheres: Part I - Adsorption of volatile polonium and bismuth on gold

E. A. Maugeri, J. Neuhausen, R. Eichler, R. Dressler, K. Rijpstra, S. Cottenier, D. Piguet, A. Vogele, D. Schumann, A. Bronselaer
RADIOCHIMICA ACTA
104 (11), 757-767
2017
A1

Abstract 

Polonium isotopes are considered the most hazardous radionuclides produced during the operation of accelerator driven systems (ADS) when lead-bismuth eutectic (LBE) is used as the reactor coolant and as the spallation target material. In this work the use of gold surfaces for capturing polonium from the cover gas of the ADS reactor was studied by thermochromatography. The results show that gaseous monoatomic polonium, formed in dry hydrogen, is adsorbed on gold at 1058 K. Its adsorption enthalpy was calculated as -250 +/- 7 kJ mol(-1), using a Monte Carlo simulation code. Highly volatile polonium species that were observed in similar experiments in fused silica columns in the presence of moisture in both inert and reducing gas were not detected in the experiments studying adsorption on gold surfaces. PoO2 is formed in both dry and moist oxygen, and its interaction with gold is characterized by transport reactions. The interaction of bismuth, present in large amounts in the atmosphere of the ADS, with gold was also evaluated. It was found that bismuth has a higher affinity for gold, compared to polonium, in an inert, reducing, and oxidizing atmosphere. This fact must be considered when using gold as a material for filtering polonium in the cover gas of ADS.

High-Throughput Screening of Extrinsic Point Defect Properties in Si and Ge: Database and Applications

M. Sluydts, M. Pieters, J. Vanhellemont, V. Van Speybroeck, S. Cottenier
Chemistry of Materials
29 (3), pp 975–984
2016
A1

Abstract 

Increased computational resources now make it possible to generate large data sets solely from first principles. Such “high-throughput” screening is employed to create a database of embedding enthalpies for extrinsic point defects and their vacancy complexes in Si and Ge for 73 impurities from H to Rn. Calculations are performed both at the PBE and HSE06 levels of theory. The data set is verified by comparison of the predicted lowest-enthalpy positions with experimental observations. The effect of temperature on the relative occupation of defect sites is estimated through configurational entropy. Potential applications are demonstrated by selecting optimal vacancy traps, directly relevant to industrial processes such as Czochralski growth as a means to suppress void formation.

Precision of Electric-Field Gradient Predictions by Density Functional Theory and Implications for the Nuclear Quadrupole Moment and Its Error Bar of the 111Cd 245 keV 5/2+ Level

L.A. Errico, K. Lejaeghere, J. Runco, S.N. Mishra, M. Renteria, S. Cottenier
Journal of Physical Chemistry C
120 (40), 23111-23120
2016
A1

Abstract 

We present ab initio calculated electric-field gradient tensors at Cd sites in a set of simple. yet diverse noncubic metals. By combining these predictions with carefully selected published experimental data, the nuclear quadrupole moment of the 245 keV 5/2(+) level of Cd-111 is determined to be 0.76(2) b. Knowing this quadrupole moment is important for time-differential perturbed angular correlation spectroscopy: decades of experimentally obtained nuclear quadrupole coupling constants for solids can now be more reliably converted into electronic structure information. For nuclear physics systematics, this is a rare opportunity to have reliable quadrupole moment information for a short-lived, level, that is not accessible to regular experimental methods. Much effort is spent on the determination of a meaningful error bar, which is an-aspect that gained only recently more attention in the context of density-functional theory predictions. This required assessing the numerical uncertainty in density functional theory predictions for electric-field gradient-tensors in solids. In contrast to quantum chemistry methods, these density functional theory predictions cannot detect systematic errors. By comparing our quadrupole moment value with an independent value Obtained from quantum chemistry calculations and experiment, we show that systematic errors are small for the systems studied here. Yet; there are indications that-density functional theory underestimates by a few percent the electric-field gradient, and therefore overestimates the quadrupole Moment by the saute amount. We point out which future work needs' to be done to characterize the possible deviations inherent to density functional theory.

Solving the Christoffel equation: phase and group velocities

J. Jaeken, S. Cottenier
Computer Physics Communications
207, 445–451
2016
A1

Abstract 

We provide christoffel, a Python tool for calculating direction-dependent phase velocities, polarization vectors, group velocities, power flow angles and enhancement factors based on the stiffness tensor of a solid. It is built in a modular way to allow for efficient and flexible calculations, and the freedom to select and combine results as desired. All derivatives are calculated analytically, which circumvents possible numerical sampling problems. GNUPlot scripts are provided for convenient visualization.

Open Access version available at UGent repository

Error estimates for density-functional theory predictions of surface energy and work function

S. De Waele, K. Lejaeghere, M. Sluydts, S. Cottenier
Physical Review B
Vol. 94, Iss. 23 — 15 December 2016
2016
A1

Abstract 

Density-functional theory (DFT) predictions of materials properties are becoming ever more widespread. With increased use comes the demand for estimates of the accuracy of DFT results. In view of the importance of reliable surface properties, this work calculates surface energies and work functions for a large and diverse test set of crystalline solids. They are compared to experimental values by performing a linear regression, which results in a measure of the predictable and material-specific error of the theoretical result. Two of the most prevalent functionals, the local density approximation (LDA) and the Perdew-Burke-Ernzerhof parametrization of the generalized gradient approximation (PBE-GGA), are evaluated and compared. Both LDA and GGA-PBE are found to yield accurate work functions with error bars below 0.3 eV, rivaling the experimental precision. LDA also provides satisfactory estimates for the surface energy with error bars smaller than 10%, but GGA-PBE significantly underestimates the surface energy for materials with a large correlation energy.

Open Access version available at UGent repository

Electronic structure and magnetic properties of dilute U impurities in metals

S.K. Mohanta, S. Cottenier, S.N. Mishra
Journal of Magnetism and Magnetic Materials
405, 107-116
2016
A1

Abstract 

The electronic structure and magnetic moment of dilute U impurity in metallic hosts have been calculated from first principles. The calculations have been performed within local density approximation of the density functional theory using Augmented plane wave+local orbital (APW + lo) technique, taking account of spin-orbit coupling and Coulomb correlation through LDA+U approach. We present here our results for the local density of states, magnetic moment and hyperfine field calculated for an isolated U impurity embedded in hosts with sp-, d- and f-type conduction electrons. The results of our systematic study provide a comprehensive insight on the pressure dependence of 5f local magnetism in metallic systems. The unpolarized local density of states (LDOS), analyzed within the frame work of Stoner model suggest the occurrence of local moment for U in sp-elements, noble metals and f-block hosts like La, Ce, Lu and Th. In contrast, U is predicted to be nonmagnetic in most transition metal hosts except in Sc, Ti, Y, Zr, and Hf consistent with the results obtained from spin polarized calculation. The spin and orbital magnetic moments of U computed within the frame of LDA+U formalism show a scaling behavior with lattice compression. We have also computed the spin and orbital hyperfine fields and a detail analysis has been carried out. The host dependent trends for the magnetic moment, hyperfine field and 5f occupation reflect pressure induced change of electronic structure with U valency changing from 3(+) to 4(+) under lattice compression. In addition, we have made a detailed analysis of the impurity induced host spin polarization suggesting qualitatively different roles off -band electrons on moment stability. The results presented in this work would be helpful towards understanding magnetism and spin fluctuation in U based alloys. (C) 2015 Elsevier B.V. All rights reserved.

Ligand Addition Energies and the Stoichiometry of Colloidal Nanocrystals

M. Sluydts, K. De Nolf, V. Van Speybroeck, S. Cottenier, Z. Hens
ACS Nano
10 (1), 1462-1474
2016
A1

Abstract 

Experimental non-stoichiometries of colloidal nanocrystals such as CdSe and PbS are accounted for by attributing to each constituent atom and capping ligand a formal charge equal to its most common oxidation state to obtain an overall neutral nanocrystal. In spite of its apparent simplicity, little theoretical support of this approach - called here the oxidation-number sum rule - is present in the current literature. Here, we introduce the ligand addition energy, which we define as the energy gained or expended upon the transfer of one ligand from a reference state to a (metal-rich) solid surface. For the combination of CdSe, ZnSe and InP with either chalcogen, halogen or hydrochalcogen ligands, we compute successive ligand addition energies using ab initio methods and determine the thermodynamically stable surface composition as that composition where ligand addition turns endothermic. We find that the oxidation-number sum rule is valid in many situations, although exceptions occur for each material studied most notably when exposed to small oxidative ligands. In the case of InP violations are more severe, extending towards the entire chalcogen ligand family. In addition, we find that electronegativity rather than chemical hardness is a reasonable predictor for ligand addition energies, with the most electronegative ligands yielding the most exothermic addition energies. Finally, we argue that the ligand addition energy will be a most useful quantity for future computational studies on the structure, stability and reactivity of nanocrystal surfaces.

Is the error on first-principles volume predictions absolute or relative?

K. Lejaeghere, L. Vanduyfhuys, T. Verstraelen, V. Van Speybroeck, S. Cottenier
Computational Materials Science
117, 390-396
2016
A1

Abstract 

Many benchmarks of density-functional theory with respect to experiment suggest the error on predicted equilibrium volumes to scale with the volume. Relative volume errors are therefore often used as a decisive argument to select one exchange-correlation functional over another. We show that the error on the volume (after correcting for systematic deviations) is only approximately relative. A simple analytic model, validated by rigorous Monte Carlo simulations, reveals that a more accurate error estimate can be derived from the inverse of the bulk modulus. This insight is not only instrumental for the selection or design of suitable functionals. It also calls for a new attitude towards computational errors: to report computational errors on electronic-structure calculations, identify systematic deviations and distinguish between relative and absolute effects. (C) 2016 Elsevier B.V. All rights reserved.

Open Access version available at UGent repository

Ab initio study of the trapping of polonium on noble metals

K. Rijpstra, A. Van Yperen-De Deyne, E. A. Maugeri, J. Neuhausen, M. Waroquier, V. Van Speybroeck, S. Cottenier
Journal of Nuclear Materials
472, 35-42
2016
A1

Abstract 

In the future MYRRHA reactor, lead bismuth eutectic (LBE) will be used both as coolant and as spallation target. Due to the high neutron flux a small fraction of the bismuth will transmute to radiotoxic 210Po. Part of this radiotoxic element will evaporate into the gas above the coolant. Extracting it from the gas phase is necessary to ensure a safe handling of the reactor. An issue in the development of suitable filters is the lack of accurate knowledge on the chemical interaction between a candidate filter material and either elemental polonium or polonium containing molecules. Experimental work on this topic is complicated by the high radiotoxicity of polonium. Therefore, we present in this paper a first-principles study on the adsorption of polonium on noble metals as filter materials. The adsorption of monoatomic Po is considered on the candidate filter materials palladium, platinum, silver and gold. The case of the gold filter is looked upon in more detail by examining how bismuth pollution affects its capability to capture polonium and by studying the adsorption of the heavy diatomic molecules Po2, PoBi and PoPb on this gold filter.

Open Access version available at UGent repository

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