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Yasmin Ahmed Salem, M.A.
Yasmin Ahmed Salem
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Scientific Events

Scientific Events

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Molecular dynamics on the diffusive time scale

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Molecular dynamics on the diffusive time scale

We formulate a theory of non-equilibrium statistical thermodynamics for ensembles of atoms or molecules. The theory is an application of Jayne's maximum entropy principle, which allows the statistical treatment of systems away from equilibrium. In particular, neither temperature nor atomic fractions are required to be uniform but instead are allowed to take different values from particle to particle. In addition, following the Coleman-Noll method of continuum thermodynamics we derive a dissipation inequality expressed in terms of discrete thermodynamic fluxes and forces. This discrete dissipation inequality effectively sets the structure for discrete kinetic potentials that couple the microscopic field rates to the corresponding driving forces, thus resulting in a closed set of equations governing the evolution of the system. We complement the general theory with a variational meanfield theory that provides a basis for the formulation of computationally tractable approximations. We present several validation cases, concerned with equilibrium properties of alloys, heat conduction in silicon nanowires, hydrogen desorption from palladium thin films and segregation/precipitation in alloys, that demonstrate the range and scope of the method and assess its fidelity and predictiveness. These validation cases are characterized by the need or desirability to account for atomic-level properties while simultaneously entailing time scales much longer than those accessible to direct molecular dynamics. The ability of simple meanfield models and discrete kinetic laws to reproduce equilibrium properties and long-term behavior of complex systems is remarkable. [more]

Topological Optimization and Textile Manufacturing of 3D Lattice Materials

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Topological Optimization and Textile Manufacturing of 3D Lattice Materials

Recent advances in topological optimization methodologies for design of internal material architecture, coupled with the emergence of micro- and nanoscale fabrication processes, 3D imaging, and micron scale testing methodologies, now make it possible to design, fabricate, and characterize lattice materials with unprecedented control. This talk will describe a collaborative effort that employs scalable 3D textile manufacturing, location specific joining, and vapor phase alloying to produce metallic lattices with a wide range of internal architectures, alloy compositions, and mechanical and functional properties. The project involves three length scales. The highest level (component scale) spans centimeters to meters and encompasses gradients in unit cell architecture, porosity, and the creation of sandwich structures. The second level (architectural unit cells) spans tens of microns to millimeters and employs architectural optimization to design the geometry of the braided/woven structure. The smallest level (microstructure) spans nanometers to tens of microns focuses on vapor phase alloying of the wires after textile manufacturing. Topology optimization allows properties to be decoupled and tailored for specific applications. Dramatic enhancements in permeability have been balanced with modest reductions in stiffness and are being used to develop heat exchanger materials with high thermal transport, low impedance, low thermal gradients and high temperature strength. In a parallel effort, architectural designs to maximize both structural resonance and inter-wire friction are also being employed to develop metallic lattices capable of mechanical damping at elevated temperatures. These examples will be used to highlight the benefits to be gained by development of metallic lattice materials with a wide range of tailorable properties. [more]

Additive Manufacturing, 3D Printing, Porosity and Synchrotron Experiments

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Additive Manufacturing, 3D Printing, Porosity and Synchrotron Experiments

3D printing of metals has advanced rapidly in the past decade and is used across a wide range of industry. Many aspects of the technology are considered to be well understood in the sense that validated computer simulations are available. At the microscopic scale, however, more work is required to quantify and understand defect structures, which affect fatigue resistance, for example. Synchrotron-based 3D X-ray computed microtomography (µXCT) was performed at the Advanced Photon Source on a variety of AM samples using both laser (SLM) and electron beam (EBM) powder bed; this showed systematic trends in porosity. Optical and SEM characterization of powders used in additively manufacturing (AM) reveals a variety of morphologies and size distributions. Computer vision (CV), as a subset of machine learning, has been successfully applied to classify different microstructures, including powders. The power of CV is further demonstrated by application to detecting and classifying defects in the spreading in powder bed machines, where the defects often correspond to deficiencies in the printed part. In addition to the printed material, a wide range of powders were measured and invariably exhibited porosity to varying degrees. Outside of incomplete melting and keyholing, porosity in printed parts is inherited from pores or bubbles in the powder. This explanation is reinforced by evidence from simulation and from dynamic x-ray radiography (DXR), also conducted at the APS. DXR has revealed a wide range of phenomena, including void entrapment (from powder particles), keyholes (i.e., vapor holes) and hot cracking. Keyhole depth is linearly related to the excess power over a vaporization threshold. Concurrent diffraction provides information on solidification and phase transformation in, e.g., Ti-6Al-4V and stainless steel. High Energy (x-ray) Diffraction Microscopy (HEDM) experiments are also described that provide data on 3D microstructure and local elastic strain in 3D printed materials, including Ti-6Al-4V and Ti-7Al. The reconstruction of 3D microstructure in Ti-6Al-4V is challenging because of the fine, two-phase lamellar microstructure and the residual stress in the as-built condition. Both the majority hexagonal phase and the minority bcc phase were reconstructed. [more]

Early stages of high temperature oxidation and sulphidation studied by synchrotron X-ray diffraction and spectroscopy

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Early stages of high temperature oxidation and sulphidation studied by synchrotron X-ray diffraction and spectroscopy

Ferritic high temperature alloys are widely used as boiler tube and heat exchanger materials in thermal power plants. All technologies have in common that the applied materials are exposed to different temperatures, process pressures and reactive atmospheres which lead to a change of the material properties and a further degradation of the material. Material changes caused by ageing in highly corrosive and toxic gases such as SO2 are mainly studied ex situ after the reaction is finished.The presentation will focus on a novel approach to study high temperature oxidation and sulphidation of alloys aged in a strongly corrosive environment in real time by energy dispersive X-ray diffraction (EDXRD). A special designed corrosion reactor was used to combine high temperature gas corrosion experiments with the collection of diffraction pattern. For this technique high energetic white X-ray radiation (10-100 keV) was used instead of conventional monochromatic radiation. It enables us to study crystallization procedures on short and medium time scales (1 min < t < 24 h) as a function of process time.X-ray diffraction is not phase sensitive for structural very similar oxide phases such as Fe2O3 and Cr2O3. To enlighten the formation mechanism of protective Cr2O3 at high temperature in corrosive atmosphere for different ferritic alloys an experimental setup for X-ray absorption near edge structure spectroscopy (XANES) in corrosive environment was developed and put into operation. The presentation will provide an overview of the possibilities of high temperature corrosion analysis using synchrotron-based X-ray diffraction and spectroscopy techniques. [more]

New concepts in electrochemistry – from magnetic structuring of macroscopic layers to single nanoparticle analysis

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New concepts in electrochemistry – from magnetic structuring of macroscopic layers to single nanoparticle analysis

Electrochemistry is a well-established technique for the electrodeposition of thin films for corrosion protection or of 3D structures for integrated circuits. It is also key to most approaches for sustainable energy conversion and storage and it is widely utilized in sensors for the detection and quantification of ions and biomolecules. In this presentation novel concepts adopting classical electrochemical methods for the fabrication and characterization of magnetic materials at the micro- and nanoscale will be presented.First the influence of magnetic fields on electrochemical deposition will be discussed using the magnetic-field assisted fabrication of structured electrodeposits in the milli- and micrometer range as an example. The relevant magnetic forces and their effect on local mass transport control will be discussed.[1,2]Electrochemistry will then be highlighted as a powerful tool for the characterization of magnetic nanoparticles beyond conventional imaging methods. For superparamagnetic Fe3O4 core Au shell nanoparticles electrochemical analysis of the particle coating quality will be shown.[3] Advancing from this, single nanoparticle electrochemistry will be presented as a new method that provides hitherto inaccessible insights into magnetic field effects on single nanoparticles in suspensions. Thus, magnetic field enhanced particle agglomeration and altered particle corrosion dynamics can be detected on a single particle level.[4]Fig. 1: Magnetic field assisted structuring of electrodeposits (left) and electrochemical characterization of magnetic core shell nanoparticles (right).References:[1] K. Tschulik, C. Cierpka, A. Gebert, L. Schultz, C.J. Kähler, M. Uhlemann, , Anal. Chem. 2011, 83, 3275–3281.[2] K. Ngamchuea, K. Tschulik, R. G. Compton, Nano Res. 2015, 8, 3293–3306.[3] K. Tschulik, K. Ngamchuea, C. Ziegler, M. G. Beier, C. Damm, A. Eychmueller, R. G. Compton, Adv. Funct. Mater. 2015, 25, 5149–5158.[4] K. Tschulik, R. G. Compton, Phys. Chem. Chem. Phys. 2014, 16, 13909–13913. [more]

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MULTICOMPONENT AND HIGH-ENTROPY ALLOYS

Conventional strategy for developing metallurgical alloys is to select the main component based on a primary property requirement, and to use alloying additions to confer secondary properties. This strategy has led to the development of many successful alloys based on a single main component with a mix of different alloying additions to provide a balance of required in-service properties. Typical examples include high temperature Ni superalloys, wrought Al alloys and corrosion resistant stainless steels. However, conventional alloy development strategy leads to an enormous amount of knowledge about alloys based on one component, but little or no knowledge about alloys containing several main components in approximately equal proportions. Theories for the occurrence, structure and properties of crystalline phases are similarly restricted to alloys based on one or two main components. Information and understanding is highly developed about alloys close to the corners and edges of a multicomponent phase diagram, with much less known about alloys in the centre of the diagram. This talk describes a range of other multicomponent alloying strategies and gives a number of examples of high-entropy and other multicomponent alloys. [more]

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Hydrogen Storage Technology at the Helmholtz Zentrum Geesthacht

The use of fossil fuels as energy supply is growing increasingly problematic both from the point of view of environmental emissions and energy sustainability. As an alternative to fossil fuels, hydrogen is widely regarded as a key element for a potential energy solution. In this respect, hydrogen storage technologies are considered a key roadblock towards the use of H2 as energy carrier. Among the methods available to store hydrogen, solid-state storage appears to be a very interesting alternative, showing for example the highest volumetric storage densities and high safety. Within the Helmholtz “Advanced Engineering Materials” Programme, the Department of Nanotechnology focusses on the development of both nanostructured hydrogen storage materials and hydrogen storage systems. A detailed account of the actual and future research activities in the field of hydrogen technology at the Helmholtz-Zentrum Geesthacht will be presented. [more]

 
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