Yasmin Ahmed Salem, M.A.
Yasmin Ahmed Salem
Press and Public Relations Officer

Phone: +49 211 6792 722
Room: 222

Scientific Events

Scientific Events


18630 1557316852

Deformation mechanisms in metals under a tribological load

In 1950, Bowden and Tabor pointed out that in metallic tribological contacts the majority of the dissipated energy is spend to change the contacting materials’ microstructures. This – in part – explains why most metals show a highly dynamic subsurface microstructure under the shear load imposed by a sliding contact. In order to understand these processes, the elementary mechanisms accommodating the shear strain and acting in the material need to be revealed and understood. In this presentation, three examples of research avenues following this hypothesis will be given. During the very early stages of sliding, dislocations show an interesting self-organization phenomenon. How these structures interfere with twin boundaries and what might be learned about the dislocation motion under the slider will be the first part of the talk. Second, we will address how the high entropy alloy (HEA) CoCrFeMnNi reacts to a tribological load and whether there is evidence for mechanisms specific to HEAs. Third, we will focus our attention at tribo-chemically activated oxidation process studied for high-purity copper. [more]

18319 1555331518

Making quantum transport visible in thermoelectric Bi2Te3 nanoparticles

Bi2Te3, Sb2Te3, and Bi2Se3, well established thermoelectric materials, are also three-dimensional (3D) topological insulators (TI) exhibiting a bulk bandgap and highly conductive, robust, gapless surface states. While the transport properties of 3D TIs are of utmost importance for potential applications, they are difficult to characterize. The reason is that transport in those materials is always dominated by bulk carriers. Still, the signature of the nontrivial electronic band structure on the thermoelectric transport properties can be evidenced in transport experiments using nanostructures with a high surface-to-volume ratio. Using a nanoparticle-based materials’ design, the highly porous macroscopic sample features a carrier density of the surface states in a comparable order of magnitude as the bulk carrier density. Further, the sintered nanoparticles impose energetic barriers for the transport of bulk carriers (hopping transport), while the connected surfaces of the nanoparticles provide a 3D percolation path for surface carriers. Within this work, I will discuss the nanoparticle processing as well as the transport properties of these combined thermoelectric and 3D TI samples. [more]

MPIE Seminar

17321 1549460332

Computational Modeling of Moving Boundary Problems

The focus of this presentation is on computational methods for moving boundary/interface problems and its applications including fracture, fluid structure interaction, inverse analysis and topology optimization. First, two computational methods for dynamic fracture will be presented, i.e. the cracking particles method (CPM) and dual-horizon peridynamics (DH-PD). These methods do neither require a representation of the crack surface and associated complex crack tracking algorithms nor criteria for crack branching and crack interactions. They also do not need to distinguish between crack nucleation and crack propagation. Complex discrete fracture patterns are the natural outcome of the simulation. The performance of these methods will be demonstrated by several benchmark problems for non-linear quasi-brittle dynamic fracture and adiabatic shear bands. Subsequently, a local partition of unity-enriched meshfree method for non-linear fracture in thin shells -- based on Kirchhoff-Love theory -- exploiting the higher order continuity of the meshfree approximation will be presented. The method does not require rotational degrees of freedom and the discretization of the director field. This also drastically simplifies the enrichment strategy accounting for the crack kinematics. Based on the meshfree thin shell formulation, an immersed particle method (IPM) for modeling fracturing thin-structures due to fluid-structure interaction is proposed. The key feature of this method is that it does not require any modifications when the structure fails and allows fluid to flow through the openings between crack surfaces naturally.The last part of the presentation focuses on inverse analysis and topology optimization with focus on computational materials design of piezoelectric/flexoelectric nanostructures and topological insulators. In the first application of piezo/flexoelectricity, we use isogeometric basis functions (NURBS or RHT-splines) in combination with level sets since C1 continuity is required for the numerical solution of the flexoelectric problem. Hence, only the electric potential and the displacement field is discretized avoiding the need of a complex mixed formulation. The level set method will be used to implicitly describe the topology of the structure. In order to update the level set function, a stabilized Hamilton-Jacobi equation is solved and an adjoint method is employed in order to determine the velocity normal to the interface of the voids/inclusions, which is related to the sensitivity of the objective function to variations in the material properties over the domain. The formulation will be presented for continua though results will also be shown for thin plates. The method will be extended to composites consisting of flexible inclusions with poor flexoelectric constants. Nonetheless, it will be shown that adding these flexible inclusions will result in a drastic increase in the energy conversion factor of the optimized flexoelectric nanostructures. In the second application, we propose a computational methodology to perform inverse design of quantum spin hall effect (QSHE)-based phononic topological insulators. We first obtain two-fold degeneracy, or a Dirac cone, in the bandstructure using a level set- based topology optimization approach. Subsequently, four-fold degeneracy, or a double Dirac cone, is obtained by using zone folding, after breaking of translational symmetry, which mimics the effect of strong spin-orbit coupling and which breaks the four-fold degeneracy resulting in a bandgap, is applied. We use the approach to perform inverse design of hexagonal unit cells of C6 and C3 symmetry. The numerical examples show that a topological domain wall with two variations of the designed metamaterials exhibit topologically protected interfacial wave propagation, and also demonstrate that larger topologically- protected bandgaps may be obtained with unit cells based on C3 symmetry. [more]

17498 1548412926

Nanoscale thermodynamics at complex oxide surfaces and interfaces for application in electronics, sensing, and energy conversion

Perovskite oxides exhibit a plethora of fascinating electronic material properties covering an exceptionally wide range of phenomena in solid state and surface physics. This has led to tremendous efforts to functionalize these materials in applications for energy technology, gas sensing, and electronics. Layered in an atomically defined epitaxial heterostructures and superlattices, diverse properties of perovskites can be combined on the nanoscale level. In such structures, even new functionality can arise at interfaces of layered materials, exhibiting properties that are absent in the bare bulk materials. In our approach, we utilize atomically-defined layer growth to obtain desired material properties. However, on top of that, we employ thermodynamic engineering of crystal defects as a unique approach to functionalize material properties at surfaces and interfaces: Even at material synthesis conditions close to perfection, device properties are often determined by imperfection, hence, by lattice disorder and crystal defects. As we discuss, we can intentionally control defect structure in nanoscale devices, by developing and utilizing thermodynamic routes to trigger surface and interface reactions in confined systems. While historically defects were seen as something to be avoided, a change of paradigm is required in the field of complex oxides today: In these materials, we can promote functionality, such as metallicity in nominally insulating compounds, by atomic defect-management. Therefore, rather than avoiding defect formation, it is an essential necessity to control and to utilize defect formation in oxides on the nanoscale. Here, we discuss fundamental aspects of lattice disorder effects in bulk oxides, and elaborate the special character of defect formation in thin films, surfaces and interfaces. Focusing on SrTiO3 as a perovskite model system, we will crosslink fundamental perspectives on lattice disorder to actual applications, addressing different examples, such as resistive switching memories, high-mobility electron gases and induced magnetism, oxygen sensors, and electro-catalysts. [more]

Seminar Talk

17274 1547029112

Phase Transitions in Non-Equilibrium Metallic Systems

Nearly all classes of materials show non-equilibrium phase transitions and the first technological use of quenching metals for designing properties is documented as ~800 BC. However, the decomposition towards equilibrium is still difficult to understand due to the strong non-equilibrium kinetics. Two examples are discussed: First the decomposition of a quenched super saturated solid solution and second the decomposition of a quenched metallic melt. In the first example the technological important AlMgSi alloys are addressed. Low temperature solute clustering, its implications on aging and the effect of trace elements are discussed. Moreover, it is shown which physical pre-requisites need to be fulfilled to modify diffusion by orders of magnitude and to examine a “diffusion on demand” concept. In the second example the first solid–solid transition via melting in a metal, detected upon the decomposition of a metallic glass, is demonstrated. The transformation path is discussed under its thermodynamic and kinetic prerequisites. Moreover, the capabilities of the applied novel technique of fast scanning calorimetry is addressed. Finally, it is outlined how this technique links the two examples via its potential for in-situ measuring the non-equilibrium vacancy evolution. [more]

MPIE Colloquium

17035 1545141786

3D Nano-Architected Metamaterials

Advances in 3D additive manufacturing techniques have enabled the fabrication of nanostructures with remarkable mechanical properties. Using the latest 3D printing techniques, novel material structures with specific architectures, often referred to as metamaterials, can be produced. They can exhibit superior mechanical and physical properties at extremely low mass densities and, thus, expand the current limits of the yet stiff and strong architectures, architectures with high mechanical resilience or with negative Poisson’s ratio. Mechanical size effects were shown to result in extraordinary strength values of different specific architectures. Understanding the underlying characteristics of these complex new materials, such as the deformation and failure mechanisms, and how they impact behavior of the structure, is critical and increasingly challenging. In this presentation, the principles underlying ultra-strong yet light 3D nano-architected metamaterials as well as strategies to tailor their properties will be discussed. [more]

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]

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