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Hermanson J.C.,U.S. Department of Agriculture | Iliopoulos A.,Center for Computational Material Science
Proceedings of the ASME Design Engineering Technical Conference | Year: 2010

Automated inverse methods for material constitutive characterization under multidimensional loading conditions has motivated the custom design, manufacturing and utilization of mechatronic loading machines. This present paper reports on the architecture of a mechatronic system capable of enforcing 6-DoF kinematic boundary conditions on deformable material specimens under testing, while at the same time measuring both the imposed kinematics and the corresponding reaction forces in a fully automated manner. This system has a recursive nature as it consists of a hexapod configuration that repeats itself six times. In addition to the architecture, we also present the historical evolution, and current status of its manufacturing implementation and the initial fielding of our system for composite material testing and characterization. Copyright © 2010 by ASME.

Pillay D.,Center for Computational Material Science | Johannes M.D.,Center for Computational Material Science | Garsany Y.,Alternative Energy Section | Swider-Lyons K.E.,Alternative Energy Section
Journal of Physical Chemistry C | Year: 2010

Density functional theory calculations and rotating ring disk electrode experiments were performed to investigate the poisoning effects of sulfur species on the catalytic properties of elemental Pt and Pt3Co alloy surfaces. Experimental data indicates that there is a positive shift in the oxidation overpotential of Pt3Co accompanied by less oxidation/reduction cycles necessary in rotating ring disk electrode experiments (RRDE) in order to remove most of the sulfur species. Our theoretical calculations suggest that OH clustering is substantially reduced on the Pt 3C(111) surface irrespective of the presence of Co atoms versus Pt(111). While the presence of Co does enhance adsorption of electronegative atoms/molecules on neighboring Pt sites, once Co atoms are oxidized or a Co-S bond is formed, they serve as a pin for the poison and subsequently reduce bonding of additional electronegative atoms/molecules at nearby sites. Additionally, our calculations indicate that a combination of effects due to less Pt3Co surface oxidation, more weakly adsorbed S species, and lower reaction barriers for SO2 oxidation on Pt3Co versus Pt subsequently leads to easier cleaning of the surface.

Mones L.,University of Cambridge | Jones A.,University of Edinburgh | Gotz A.W.,University of California at San Diego | Laino T.,IBM | And 4 more authors.
Journal of Computational Chemistry | Year: 2015

The implementation and validation of the adaptive buffered force (AdBF) quantum-mechanics/molecular-mechanics (QM/MM) method in two popular packages, CP2K and AMBER are presented. The implementations build on the existing QM/MM functionality in each code, extending it to allow for redefinition of the QM and MM regions during the simulation and reducing QM-MM interface errors by discarding forces near the boundary according to the buffered force-mixing approach. New adaptive thermostats, needed by force-mixing methods, are also implemented. Different variants of the method are benchmarked by simulating the structure of bulk water, water autoprotolysis in the presence of zinc and dimethyl-phosphate hydrolysis using various semiempirical Hamiltonians and density functional theory as the QM model. It is shown that with suitable parameters, based on force convergence tests, the AdBF QM/MM scheme can provide an accurate approximation of the structure in the dynamical QM region matching the corresponding fully QM simulations, as well as reproducing the correct energetics in all cases. Adaptive unbuffered force-mixing and adaptive conventional QM/MM methods also provide reasonable results for some systems, but are more likely to suffer from instabilities and inaccuracies. © 2015 Wiley Periodicals, Inc.

Iliopoulos A.P.,Research Applications Corporation | Michopoulos J.G.,Center for Computational Material Science | Lambrakos S.G.,Center for Computational Material Science | Bernstein N.,Center for Computational Material Science
Journal of Computational Science | Year: 2011

The present work has been motivated by the continuous growth of General Purpose Graphic Processor Unit (GPGPU) technologies as well as the necessity of linking usability with multiscale materials processing and design. The inverse problem of determining the phenomenological interparticle Lenard-Jones potential governing the fracture dynamics of a two dimensional structure under tension, is used to examine the feasibility and efficiency of utilizing GPGPU architectures. The implementation of this inverse problem under a molecular dynamics framework provides verification of this methodology. The main contribution of this paper is a performance evaluation driven sensitivity analysis that is contacted on GPGPU-enabled hardware in order to examine efficiency relative to various combinations of GPGPU and Central Processing Unit (CPU) cores as a function of problem size. In particular, speedup factors are determined relative to various number of core combinations of a quad core processor. © 2011.

Iliopoulos A.P.,Research Applications Corporation | Michopoulos J.G.,Center for Computational Material Science
Proceedings of the ASME Design Engineering Technical Conference | Year: 2010

To pursue characterization of composite materials, contemporary automated material testing machines are programmed to follow loading paths in multidimensional spaces. A computational methodology for selecting the best loading subspace among all those possible is formulated and presented in this paper. The criterion for subspace selection employed is based on the assessment of which among the possible subspaces generates the richest set of strain-states as compared to those of the union of all possible 4D loading spaces. A systematic program of simulation sequences of virtual experiments is presented and the concept of strain state cloud (SSC) is introduced as a high dimensional volumetric histogram describing the frequency of appearance of each strain state within the corresponding strain space. Comparison of the SSCs for each of the fifteen 4D subspaces relative to the full 6D space allows a ranking classification of each subspace. Based on this ranking we select the three top cases as being those considered for actual testing. Copyright © 2010 by ASME.

Lambrakos S.G.,Center for Computational Material Science | Michopoulos J.G.,Center for Computational Material Science
Journal of Materials Engineering and Performance | Year: 2010

This paper examines multiscale inverse analysis of rapid and localized energy deposition, where there exists extremely strong filtering of spatial and temporal structure within the associated diffusion pattern. This strong filtering tends to establish conditions where system identification, or in particular reconstruction of detailed features of the energy source, based on data-driven inverse analysis of the diffusion pattern alone is not well posed. An accurate and well-posed characterization of rapid energy deposition processes should be in terms of two distinctly separate scales for both spatial and temporal structures. Accordingly, the inverse rapid and localized energy deposition problem requires a formulation with respect to system identification and parameterization that should be cast in terms of two separate sets of parameters. One should represent energy source characteristics on spatial and temporal scales commensurate with that of thermal diffusivity within the material. The other parameter set should represent energy source characteristics on spatial and temporal scales commensurate with those of surface phenomena. The general procedure presented here for inverse analysis of rapid and localized energy deposition is formulated in terms of these two separate sets of parameters. © ASM International.

Wang T.,James Franck Institute | Vaxenburg R.,Technion - Israel Institute of Technology | Liu W.,James Franck Institute | Rupich S.M.,James Franck Institute | And 4 more authors.
ACS Nano | Year: 2015

The electronic structure of single InSb quantum dots (QDs) with diameters between 3 and 7 nm was investigated using atomic force microscopy (AFM) and scanning tunneling spectroscopy (STS). In this size regime, InSb QDs show strong quantum confinement effects which lead to discrete energy levels on both valence and conduction band states. Decrease of the QD size increases the measured band gap and the spacing between energy levels. Multiplets of equally spaced resonance peaks are observed in the tunneling spectra. There, multiplets originate from degeneracy lifting induced by QD charging. The tunneling spectra of InSb QDs are qualitatively different from those observed in the STS of other III-V materials, for example, InAs QDs, with similar band gap energy. Theoretical calculations suggest the electron tunneling occurs through the states connected with L-valley of InSb QDs rather than through states of the -valley. This observation calls for better understanding of the role of indirect valleys in strongly quantum-confined III-V nanomaterials. © 2014 American Chemical Society.

Michopoulos J.G.,Center for Computational Material Science | Iliopoulos A.P.,SAIC | Furukawa T.,Virginia Polytechnic Institute and State University
Proceedings of the ASME International Design Engineering Technical Conferences and Computers and Information in Engineering Conference 2009, DETC2009 | Year: 2010

The present paper reports on the progress towards the evaluation of the Mesh Free Random Grid Method (MFRGM) for the inverse constitutive characterization of composite materials. The method provides the capability for the remote (non contact) measurement of displacement and strain fields of structures bounded by flat surfaces that deform under various mechanical and generalized loading conditions. The known forward solution of an anisotropic plate with an open hole, loaded at infinity, is used to generate synthetic images MFRG. The inverse problem for determining the constitutive parameters formulated directly on the generalized constitutive law. Performance of the technique is evaluated by the usage of just one frame corresponding to one set of strain state for various amounts of noise. The evaluation is repeated by utilizing frames corresponding to different rotations of the laminate relative to the loading direction. Finally the exceedingly accurate behavior of the methodology is discussed. Copyright © 2009 by ASME.

PubMed | Center for Computational Material Science
Type: Journal Article | Journal: Journal of physics. Condensed matter : an Institute of Physics journal | Year: 2011

Simulation of a cluster representing a finite portion of a larger covalently bonded system requires the passivation of the cluster surface. We compute the effects of an explicit hybrid orbital passivation (EHOP) on the atomic structure in a model bulk, three-dimensional, narrow gap semiconductor, which is very different from the wide gap, quasi-one-dimensional organic molecules where most passivation schemes have been studied in detail. The EHOP approach is directly applicable to minimal atomic orbital basis methods such as tight-binding. Each broken bond is passivated by a hybrid created from an explicitly expressed linear combination of basis orbitals, chosen to represent the contribution of the missing neighbour, e.g.a sp(3) hybrid for a single bond. The method is tested by computing the forces on atoms near a point defect as a function of cluster geometry. We show that, compared to alternatives such as pseudo-hydrogen passivation, the force on an atom converges to the correct bulk limit more quickly as a function of cluster radius, and that the force is more stable with respect to perturbations in the position of the cluster centre. The EHOP method also obviates the need for parameterizing the interactions between the system atoms and the passivating atoms. The method is useful for cluster calculations of non-periodic defects in large systems and for hybrid schemes that simulate large systems by treating finite regions with a quantum-mechanical model, coupled to an interatomic potential description of the rest of the system.

News Article | April 11, 2016
Site: www.nrl.navy.mil

U.S. Naval Research Laboratory (NRL) research physicist, Dr. Carl Stephen Hellberg, is elected Fellow by the American Physical Society (APS) for creative and influential contributions in the fields of strongly correlated materials, quantum dots, defects, and heterostructures. Arriving at NRL in 1996 as a National Research Council (NRC) research associate, Hellberg has concentrated on researching the physics of surfaces and interfaces using density functional theory, concentrating on low-dimensional systems and the surfaces and interfaces of bulk crystals. "Dr. Hellberg is recognized for his work demonstrating the limits of strontium titanate to coherently grow beyond a few layers on silicon and how chemical substitutions at the interface can produce a better interface and more uniform thin films," said Dr. Michael Mehl, head, Center for Computational Material Science. "I am very delighted he has been elected a Fellow of the American Physical Society for his groundbreaking work in the field of computational physics." Hellberg's current research includes first principles calculations of surfaces, interfaces, and thin films. He is focusing on oxides and chalcogenides, including polarity mismatched interfaces, topological insulators, and monolayer heterostructures, with a particular interests in electrical properties and metal-insulator transitions. He also works on strongly correlated electron systems, including quantum dots, nanocrystals, and transition metal oxides. Hellberg received his undergraduate degree in physics from Princeton University in 1987, and then studied for a year at the Ludwig Maximilian University of Munich on a Fulbright Fellowship. He enrolled in graduate school at the University of Pennsylvania, where he worked with professor Eugene J. Mele, receiving his Ph.D. in 1993. He spent three years working with professor Efstratios Manousakis at Florida State University in a postdoctoral appointment developing computational techniques to study strongly correlated electrons. The APS is a non-profit membership organization working to advance and diffuse the knowledge of physics. APS Fellows are elected on the criterion of exceptional contributions to the physics enterprise that are comprised of outstanding physics research, important applications of physics, leadership in or service to physics, or significant contributions to physics education. About the U.S. Naval Research Laboratory The U.S. Naval Research Laboratory provides the advanced scientific capabilities required to bolster our country's position of global naval leadership. The Laboratory, with a total complement of approximately 2,500 personnel, is located in southwest Washington, D.C., with other major sites at the Stennis Space Center, Miss., and Monterey, Calif. NRL has served the Navy and the nation for over 90 years and continues to advance research further than you can imagine. For more information, visit the NRL website or join the conversation on Twitter, Facebook, and YouTube.

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