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McCloy J.S.,Pacific Northwest National Laboratory | Wolf W.,Materials Design Inc. | Wimmer E.,Materials Design Inc. | Zelinski B.J.,Raytheon Co.
Journal of Applied Physics | Year: 2013

The lattice parameter of cubic chemical vapor deposited (CVD) ZnS with measured oxygen concentrations <0.6 at. % and hydrogen impurities of <0.015 at. % has been measured and found to vary between -0.10% and +0.09% relative to the reference lattice parameter (5.4093 Å) of oxygen-free cubic ZnS as reported in the literature. Defects other than substitutional O must be invoked to explain these observed volume changes. The structure and thermodynamic stability of a wide range of native and impurity induced defects in ZnS have been determined by ab initio calculations. Lattice contraction is caused by S-vacancies, substitutional O on S sites, Zn vacancies, H in S vacancies, peroxy defects, and dissociated water in S-vacancies. The lattice is expanded by interstitial H, H in Zn vacancies, dihydroxy defects, interstitial oxygen, Zn and [ZnHn] complexes (n = 1,⋯,4), interstitial Zn, and S2 dumbbells. Oxygen, though present, likely forms substitutional defects for sulfur resulting in lattice contraction rather than as interstitial oxygen resulting in lattice expansion. It is concluded based on measurement and calculations that excess zinc atoms either at anti-sites (i.e., Zn atoms on S-sites) or possibly as interstitial Zn are responsible for the relative increase of the lattice parameter of commercially produced CVD ZnS. © 2013 American Institute of Physics.


Christensen M.,Materials Design Inc. | Angeliu T.M.,Knolls Atomic Power Laboratory | Ballard J.D.,Knolls Atomic Power Laboratory | Vollmer J.,Knolls Atomic Power Laboratory | And 2 more authors.
Journal of Nuclear Materials | Year: 2010

Effects of twenty impurity and alloy elements on the strength of a Zr(0 0 0 1)/Zr(0 0 0 1) ∑7 twist grain boundary were studied using a first-principles density functional approach. A ranking in the order of most weakening to most strengthening was: Cs, I, He, Te, Sb, Li, O, Sn, Cd, H, Si, C, N, B, U, Ni, Hf, Nb, Cr, and Fe. Segregation energies for these elements to the grain boundary and the Zr(0 0 0 1) surface were also calculated. Calculations showed that the weakening grain boundary elements He, I, and Cs have a strong driving force for segregation to the grain boundary from bulk Zr. Zircaloy cladding failures (pellet-clad interactions) in commercial fuel systems and separate effects test results provide context for these computational results. © 2010 Elsevier B.V. All rights reserved.


Gao M.C.,U.S. National Energy Technology Laboratory | Gao M.C.,URS Corporation | Suzuki Y.,Los Alamos National Laboratory | Schweiger H.,Materials Design Inc. | And 3 more authors.
Journal of Physics Condensed Matter | Year: 2013

V is the only element in the periodic table that forms a complete solid solution with Cr and thus is particularly important in alloying strategy to ductilize Cr. This study combines first-principles density functional theory calculations and experiments to investigate the phase stability and elastic properties of Cr-V binary alloys. The cluster expansion study reveals the formation of various ordered compounds at low temperatures that were not previously known. These compounds become unstable due to the configurational entropy of bcc solid solution as the temperature is increased. The elastic constants of ordered and disordered compounds are calculated at both T = 0 K and finite temperatures. The overall trends in elastic properties are in agreement with measurements using the resonant ultrasound spectroscopy method. The calculations predict that addition of V to Cr decreases both the bulk modulus and the shear modulus, and enhances the Poisson's ratio, in agreement with experiments. Decrease in the bulk modulus is correlated to decrease in the valence electron density and increase in the lattice constant. An enhanced Poisson's ratio for bcc Cr-V alloys (compared to pure Cr) is associated with an increased density of states at the Fermi level. Furthermore, the difference charge density in the bonding region in the (110) slip plane is highest for pure Cr and decreases gradually as V is added. The present calculation also predicts a negative Cauchy pressure for pure Cr, and it becomes positive upon alloying with V. The intrinsic ductilizing effect from V may contribute, at least partially, to the experimentally observed ductilizing phenomenon in the literature. © 2013 IOP Publishing Ltd.


Rozanska X.,Materials Design SARL | Ungerer P.,Materials Design SARL | Leblanc B.,Materials Design SARL | Saxe P.,Materials Design Inc. | Wimmer E.,Materials Design SARL
Oil and Gas Science and Technology | Year: 2015

This work demonstrates the systematic prediction of thermodynamic properties for batches of thousands of molecules using automated procedures. This is accomplished with newly developed tools and functions within the Material Exploration and Design Analysis (MedeAβ) software environment, which handles the automatic execution of sequences of tasks for large numbers of molecules including the creation of 3D molecular models from 1D representations, systematic exploration of possible conformers for each molecule, the creation and submission of computational tasks for property calculations on parallel computers, and the post-processing for comparison with available experimental properties. After the description of the different MedeAβ functionalities and methods that make it easy to perform such large number of computations, we illustrate the strength and power of the approach with selected examples from molecular mechanics and quantum chemical simulations. Specifically, comparisons of thermochemical data with quantum-based heat capacities and standard energies of formation have been obtained for more than 2 000 compounds, yielding average deviations with experiments of less than 4% with the Design Institute for Physical PRoperties (DIPPR) database. The automatic calculation of the density of molecular fluids is demonstrated for 192 systems. The relaxation to minimum-energy structures and the calculation of vibrational frequencies of 5 869 molecules are evaluated automatically using a semi-empirical quantum mechanical approach with a success rate of 99.9%. The present approach is scalable to large number of molecules, thus opening exciting possibilities with the advent of exascale computing. © X. Rozanska et al.


Christensen M.,Materials Design Inc. | Wolf W.,Materials Design Inc. | Freeman C.M.,Materials Design Inc. | Wimmer E.,Materials Design Inc. | And 4 more authors.
Journal of Nuclear Materials | Year: 2014

The effect of the alloying elements Sn, Fe, Cr, Ni, Nb, and O on hydrogen-containing alpha-zirconium and zirconium hydrides is investigated using ab initio quantum mechanical calculations and classical simulations. Cr, Fe, and Ni atoms attract interstitially dissolved H atoms whereas interstitial oxygen atoms show no pronounced interaction with H atoms. The alloying elements destabilize the hydride phases in the order Sn > Fe > Cr > Ni > Nb. Hence, substitutional Sn (if atomically dispersed), Cr and Fe atoms are likely to delay hydride precipitation, effectively increasing the hydrogen solubility. Nb and Sn influence the mobility of Zr self-interstitial atoms (SIA's), which diffuse rapidly and preferentially parallel to the basal planes forming interstitial dislocations loops perpendicular to the basal planes (a-loops). Nb suppresses this diffusion of SIA's, thereby reducing the rate of formation of interstitial a-loops. Sn atoms, if present on substitutional sites, have a similar, but smaller effect. If SIA's approach substitutional Fe, Cr, and Ni atoms, the simulations indicate a spontaneous swap promoting the smaller transition metal atoms into interstitial atoms, which diffuse very rapidly with a preference in the c-direction, thereby facilitating their segregation to energetically more favorable sites such as vacancies, vacancy c-loops, grain boundaries, surfaces, and intermetallic precipitates. © 2013 Elsevier B.V. All rights reserved.


Yiannourakou M.,Materials Design Sarl | Ungerer P.,Materials Design Sarl | Leblanc B.,Materials Design Sarl | Rozanska X.,Materials Design Sarl | And 4 more authors.
Oil and Gas Science and Technology | Year: 2013

The development of industrial software, the decreasing cost of computing time, and the availability of well-tested force fields make molecular simulation increasingly attractive for chemical engineers. We present here several applications of Monte-Carlo simulation techniques, applied to the adsorption of fluids in microporous solids such as zeolites and model carbons (pores < 2 nm). Adsorption was computed in the Grand Canonical ensemble with the MedeA® -GIBBS software, using energy grids to decrease computing time. MedeA® -GIBBS has been used for simulations in the NVT or NPT ensembles to obtain the density and fugacities of fluid phases. Simulation results are compared with experimental pure component isotherms in zeolites (hydrocarbon gases, water, alkanes, aromatics, ethanethiol, etc.), and mixtures (methane-ethane, n-hexane-benzene), over a large range of temperatures. Hexane/benzene selectivity inversions between silicalite and Na-faujasites are well predicted with published forcefields, providing an insight on the underlying mechanisms. Also, the adsorption isotherms in Na-faujasites for light gases or ethane-thiol are well described. Regarding organic adsorbents, models of mature kerogen or coal were built in agreement with known chemistry of these systems. Obtaining realistic kerogen densities with the simple relaxation approach considered here is encouraging for the investigation of other organic systems. Computing excess sorption curves in qualitative agreement with those recently measured on dry samples of gas shale is also favorable. Although still preliminary, such applications illustrate the strength of molecular modeling in understanding complex systems in conditions where experiments are difficult. © 2013, IFP Energies nouvelles.


Ungerer P.,Materials Design S.A.R.L. | Rigby D.,Materials Design Inc. | Leblanc B.,Materials Design S.A.R.L. | Yiannourakou M.,Materials Design S.A.R.L.
Molecular Simulation | Year: 2014

Asphaltenes are heavy crude oil compounds, defined as soluble in toluene and precipitating in alkanes. To understand the relation between asphaltene structure and aggregation, we perform equilibrium molecular dynamics with Large-scale Atomic and Molecular Massively Parallel Software (LAMMPS), using the atomistic force field PCFF+ in the MedeA® environment. The following three molecular models are considered: the continental model (1350 g/mol) that has a large polyaromatic core and long alkyl chains, the island model (780 g/mol) that has a smaller polyaromatic unit and shorter chains and the archipelago model (1350 g/mol) that has three polyaromatic nuclei bridged with alkyl chains. The aggregation in a given solvent is monitored by visualising solvent-free configurations over 15 ns trajectories at 350 K. Nanoaggregates are characterised by stacked polyaromatic units separated by 0.33-0.4 nm. Irreversible aggregation is found with the continental model in both solvents. Aggregation of the island model is significant in n-heptane and low in toluene. The archipelago model does not aggregate significantly. Our results confirm that the island model is a reasonable average model of asphaltenes [Headen TF, Boek ES, Skipper NT. Energy Fuels 2009;23:1220-1229]. The open structure of nanoaggregates and the limited number of stacked molecules are also in agreement with previous interpretations of experimental data [Fenistein D. et al. Langmuir 1998;14:1013-1020]. © 2014 © 2013 Taylor & Francis.


Kwon J.,University of Texas at Dallas | Saly M.,SAFC Hitech | Halls M.D.,Materials Design Inc. | Kanjolia R.K.,SAFC Hitech | Chabal Y.J.,University of Texas at Dallas
Chemistry of Materials | Year: 2012

Tertbutylallylcobalttricarbonyl ( tBu-AllylCo(CO) 3) is shown to have strong substrate selectivity during atomic layer deposition of metallic cobalt. The interaction of tBu-AllylCo(CO) 3 with SiO 2 surfaces, where hydroxyl groups would normally provide more active reaction sites for nucleation during typical ALD processes, is thermodynamically disfavored, resulting in no chemical reaction on the surface at a deposition temperature of 140 °C. On the other hand, the precursor reacts strongly with H-terminated Si surfaces (H/Si), depositing ∼1 ML of cobalt after the first pulse by forming Si-Co metallic bonds. This remarkable substrate selectivity of tBu-AllylCo(CO) 3 is due to an ALD nucleation reaction process paralleling a catalytic hydrogenation, which requires a coreactant that acts as a hydrogen donor rather than a source of bare protons. The chemical specificity demonstrated in this work suggests a new paradigm for developing selective ALD precursors. Namely, selectivity can be achieved by tailoring the ligands in the coordination sphere to obtain structural analogues to reaction intermediates for catalytic transformations that exhibit the desired chemical discrimination. © 2012 American Chemical Society.


Kwon J.,University of Texas at Dallas | Dai M.,University of Texas at Dallas | Halls M.D.,Materials Design Inc. | Chabal Y.J.,University of Texas at Dallas
Applied Physics Letters | Year: 2010

We demonstrate that interfacial SiO2, usually formed during high- κ oxide growth on silicon using ozone (O3), is suppressed during Al2 O3 atomic layer deposition (ALD) by decreasing the O3 flow rate. First-principles calculations indicate that oxygen introduced by the first low-dose O3 exposure is inserted into the surface nucleation layer rather than the Si lattice. Subsequent Al2 O3 deposition further passivates the surface against substrate oxidation. Aluminum methoxy [-Al (OCH3)2] and surface Al-O-Al linkages formed after O3 pulses are suggested as the reaction sites for trimethylaluminum during ALD of Al2 O3. © 2010 American Institute of Physics.


PubMed | Materials Design Incorporated
Type: Journal Article | Journal: Journal of physics. Condensed matter : an Institute of Physics journal | Year: 2011

A workshop, Theory Meets Industry, was held on 12-14 June 2007 in Vienna, Austria, attended by a well balanced number of academic and industrial scientists from America, Europe, and Japan. The focus was on advances in abinitio solid state calculations and their practical use in industry. The theoretical papers addressed three dominant themes, namely (i)more accurate total energies and electronic excitations, (ii)more complex systems, and (iii)more diverse and accurate materials properties. Hybrid functionals give some improvements in energies, but encounter difficulties for metallic systems. Quantum Monte Carlo methods are progressing, but no clear breakthrough is on the horizon. Progress in order-N methods is steady, as is the case for efficient methods for exploring complex energy hypersurfaces and large numbers of structural configurations. The industrial applications were dominated by materials issues in energy conversion systems, the quest for hydrogen storage materials, improvements of electronic and optical properties of microelectronic and display materials, and the simulation of reactions on heterogeneous catalysts. The workshop is a clear testimony that abinitio computations have become an industrial practice with increasingly recognized impact.

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