1,681 Works

On the direct kinematics problem of parallel mechanisms

Arthur Seibel, Stefan Schulz & Josef Schlattmann
The direct kinematics problem of parallel mechanisms, that is, determining the pose of the manipulator platform from the linear actuators’ lengths, is, in general, uniquely not solvable. For this reason, instead of measuring the lengths of the linear actuators, we propose measuring their orientations and, in most cases, also the orientation of the manipulator platform. This allows the design of a low-cost sensor system for parallel mechanisms that completely renounces length measurements and provides a...

Surface excess elasticity of gold: ab initio coefficients and impact on the effective elastic response of nanowires

Beatrix A. M. Elsner, Stefan Müller, Swantje Bargmann & Jörg Weißmüller
Predicting the influence of the surface on the effective elastic properties of nanoscale structures and nanomaterials remains a challenge, which we here address on both levels, continuum and atomic. Density Functional Theory (DFT) computation at the atomic level yields the first reliable surface excess elastic parameters for the (111) and (001) surfaces of gold. At the continuum level, we derive closed-form expressions for the effective elastic behavior that can be combined with the DFT-derived excess...

Modeling chemistry and flow in catalytic fixed-bed reactors with detailed geometry

Ying Dong
In this work, three-dimensional CFD models were developed and applied to simulate catalytic fixed bed reactors with different types of packing configuration and reactions. A spatially resolved profile measurement technique was applied in this work to critically validate the simulation results, and to calibrate parameters required in the model. This work demonstrates that detailed reactor modeling and profile measurements are useful tools for achieving an in-depth understanding of the interactions between flow, heat and mass...

Suppression of material transfer at contacting surfaces: the effect of adsorbates on Al/TiN and Cu/diamond interfaces from first-principles calculations

Gregor Feldbauer, Michael Wolloch, Pedro O Bedolla, Josef Redinger, András Vernes & Peter Mohn
The effect of monolayers of oxygen (O) and hydrogen (H) on the possibility of material transfer at aluminium/titanium nitride (Al/TiN) and copper/diamond (Cu/Cdia) interfaces, respectively, were investigated within the framework of density functional theory (DFT). To this end the approach, contact, and subsequent separation of two atomically flat surfaces consisting of the aforementioned pairs of materials were simulated. These calculations were performed for the clean as well as oxygenated and hydrogenated Al and Cdia surfaces,...

Lösender Angriff auf zementgebundene Baustoffe – Veränderungen der Porenstruktur und Folgen für Transport- und Korrosionsprozesse

Holle Kerstin Goedeke
Es wurden Korrosionsversuche an Zementmörteln mit stationären und instationären Randbedingungen durchgeführt. In weiteren Versuchen wurden die Veränderung der Porosität und Porenstruktur sowie die damit verbundenen Auswirkungen auf die Stofftransportprozesse betrachtet. Weiterhin wurden Simulationsberechnungen durchgeführt. Mit dem Ziel einer Verbesserung der Berechnung von Transportprozessen erfolgte eine Anwendung verschiedener Porenstrukturmodelle auf experimentell ermittelte Daten. Den Abschluss bildet die Vorstellung eines neuen Modellansatzes zur Beschreibung der Porenstrukturänderung infolge von Korrosionsprozessen.

Robust and linear parameter-varying control of aeroservoelastic systems

Julian Theis
The present dissertation considers the control of aeroservoelastic systems within the framework of robust control and linear parameter-varying (LPV) systems. Aeroservoelastic systems are characterized by a strong interdependence of rigid-body dynamics, structural dynamics, aerodynamics, and feedback control systems. As models for these systems are of high order, the first contribution of this thesis is a set of tools for LPV model order reduction. Specifically, an interpolation method based on local modal decomposition and a parameter-varying...

A Comparison of German and Indian innovation pathways in the auto component industry

Katharina Kalogerakis, Luise Fischer & Rajnish Tiwari
This paper is a part of a series of analyses conducted within a BMBF-funded research project to investigate potentials of frugal innovations for Germany. One of the objectives of this project has been to identify innovation pathways that foster frugal innovations. Since auto component suppliers based in India are known to contribute significantly to the development of “affordable and good quality” vehicles, it was considered useful to conduct comparative studies of the prevalent innovation pathways...

To live in the city centre : housing and tenants in central neighbourhoods of Latin American cities

Hans Harms
After reviewing structural changes in metropolitan areas of Latin America and the implications for housing, this paper describes five central neighbourhoods with a predominance of rental housing: three in Lima and one each in Havana and Rio de Janeiro. This includes the types of buildings, how, when and why they were built, who occupies them and who owns them. It also describes what influence government policy has had on them and what measures (if any)...

Fluid dynamic optimization of HVAC-components with adjoint methods

Anne Gerdes
This thesis is concerned with the theory and application of continuous adjoint shape and topology optimization for heating, ventilation and air-conditioning components of aircrafts. The focal point is the applicability of adjoint theory to industrial practice, referring to step size computation for the gradient descent, design constraints and a correct computation of the gradient from the sensitivity. The implemented approaches are validated and verified via simulation of proper fluid dynamic test-cases. The arising framework is...

Media judgment of entrepreneurial failure - implications for founders

Matthias Jacobi
The thesis with the topic „Media judgment of entrepreneurial failure – implications for founders“ compares media reporting about failure of startups in Germany and the US. It can be shown, that the US media reports more positively about failed startups, than the German media does.

Synthesis and thermal stability of zirconia and yttria-stabilized zirconia microspheres

Elisabeth Leib, Ulla Vainio, Robert M. Pasquarelli, Jonas Kus, Christian Czaschke, Nils Walter, Rolf Janßen, Martin Müller, Andreas Schreyer, Horst Weller & Tobias Vossmeyer
Zirconia microparticles produced by sol-gel synthesis have great potential for photonic applications. To this end, identifying synthetic methods that yield reproducible control over size uniformity is mportant. Phase transformations during thermal cycling can disintegrate the particles. Therefore, understanding the parameters driving these transformations is essential for enabling high-temperature applications. Particle morphology is expected to influence particle processability and stability. Yttria-doping should improve the thermal stability of the particles, as it does in bulk zirconia.

Ethics and technology : some issues ; a booklet

Mirko Daniel Garasic & Steffen Steinert
Bioethical questions have a crucial role both in people’s personal choices and in the political domain. The aim of this booklet is to introduce you to the main controversies in contemporary bioethics, especially to those concerning the intersection between ethics and technology. At the end of reading the booklet, you will be equipped with the conceptual tools required to understand the main debates in ethics and technology and the most relevant positions in the debate....

Elastic and plastic poisson’s ratios of nanoporous gold

Lukas Lührs, Celal Soyarslan, Jürgen Markmann, Swantje Bargmann & Jörg Weißmüller
We explore the elastic and plastic Poisson’s ratios, mE and mP, of nanoporous gold, using digital image correlation during compression experiments including load/unload segments. The two coefficients differ significantly, with mE independent of the ligament size, L, and with a trend for mP / L at not too large L. Disorder in the network of ligaments may explain why mE is smaller than predicted by lattice-based models. Finite element simulations, based on the Deshpande–Fleck constitutive...

Elastic behaviour at the nanoscale of innovative composites of nanoporous gold and polymer

Emma Griffiths, Swantje Bargmann & B. Dayanand Reddy
Nanoporous gold is a material with unique properties that make it attractive for use in multiple areas. It has an extremely high compressive strain capability but is very brittle in tension which limits its usage. One means of addressing this limitation is to fill the voids of the nanoporous material with a polymer, resulting in a more ductile and stronger material. In this work, the mechanical properties of a nanocomposite representative sample are studied numerically....

Exceptionally strong, stiff and hard hybrid material based on an elastomer and isotropically shaped ceramic nanoparticles

Prokopios Georgopanos, Gerold A. Schneider, Axel Dreyer, Ulrich A. Handge, Volkan Filiz, Artur Feld, Ezgi D. Yilmaz, Tobias Krekeler, Martin Ritter, Horst Weller & Volker Abetz
In this work the fabrication of hard, stiff and strong nanocomposites based on polybutadiene and iron oxide nanoparticles is presented. The nanocomposites are fabricated via a general concept for mechanically superior nanocomposites not based on the brick and mortar structure, thus on globular nanoparticles with nanosized organic shells. For the fabrication of the composites oleic acid functionalized iron oxide nanoparticles are decorated via ligand exchange with an α,ω-polybutadiene dicarboxylic acid. The functionalized particles were processed...

On the impact of capillarity for strength at the nanoscale

Nadiia Mameka, Jürgen Markmann & Jörg Weißmüller
The interior of nanoscale crystals experiences stress that compensates for the capillary forces and that can be large, in the order of 1 GPa. Various studies have speculated on whether and how this surface-induced stress affects the stability and plasticity of small crystals. Yet, experiments have so far failed to discriminate between the surface contribution and other, bulk-related size effects. To clarify the issue, here we study the variation of the flow stress of a nanomaterial...

Effect of nodal mass on macroscopic mechanical properties of nanoporous metals

Jingsi Jiao & Norbert Huber
The current work investigates the effect of the nodal mass on the macroscopic mechanical behavior of nanoporous metals using the Finite Element Method. A nodal corrected beam modeling concept is introduced that allows local incorporation of the effective elastoplastic mechanical behavior of the nodal mass in the nodal area of a representative volume element (RVE). The calibration to the corresponding Finite Element solid model is achieved by integrating additional geometry and material parameters to the...

Hydrogen bonding and thermoplastic elastomers - a nice couple with temperature-adjustable mechanical properties

Elisabeth Wittenberg, Andreas Meyer, Steffen Eggers & Volker Abetz
Styrene-butadiene copolymers are modified with varying fractions of benzoic acid moieties being able to perform hydrogen bonding. This is done by using a simple synthetic approach which utilizes click hemistry. Temperature-dependent dynamic mechanical properties are studied, and it turns out that even the apparently rather simple ydrogen bonding motif has a marked impact on the material properties due to the fact that it facilitates the formation of a supramolecular polymer network. Besides a glass transition,...

A ferroelectric liquid crystal confined in cylindrical nanopores : reversible smectic layer buckling, enhanced light rotation and extremely fast electro-optically active Goldstone excitations

Mark Busch, Andriy V. Kityk, Wiktor Piecek, Tommy Hofmann, Dirk Wallacher, Sylwia Calus, Przemyslaw Kula, Martin Steinhart, Manfred Eich & Patrick Huber
The orientational and translational order of a thermotropic ferroelectric liquid crystal (2MBOCBC) imbibed in self-organized, parallel, cylindrical pores with radii of 10, 15, or 20 nm in anodic aluminium oxide monoliths (AAO) are explored by high-resolution linear and circular optical birefringence as well as neutron diffraction texture analysis. The results are compared to experiments on the bulk system. The native oxidic pore walls do not provide a stable smectogen wall anchoring. By contrast, a polymeric...

Controlling thermal emission with refractory epsilon-near-zero metamaterials via topological transitions

Pavel N. Dyachenko, Sean Molesky, Alexander Petrov, Michael Störmer, Tobias Krekeler, Slawa Lang, Martin Ritter, Zubin Jacob & Manfred Eich
Control of thermal radiation at high temperatures is vital for waste heat recovery and for high-efficiency thermophotovoltaic (TPV) conversion. Previously, structural resonances utilizing gratings, thin film resonances, metasurfaces and photonic crystals were used to spectrally control thermal emission, often requiring lithographic structuring of the surface and causing significant angle dependence. In contrast, here, we demonstrate a refractory W-HfO2 metamaterial, which controls thermal emission through an engineered dielectric response function. The epsilon-near-zero frequency of a metamaterial...

The perfect match: the role of categorical fit between venture capitalists and their startup investments

Jan-Frederik Arnold
This dissertation sheds light on how the categorical fit between venture capitalists and startups affect the matching process before a funding round. The study uses market categories to measure new venture’s distinctiveness and the cultural distance between venture capitalists and new ventures. The empirical analysis of 11 years of venture capital investment data in the USA confirms that experienced and high status investors prefer more distinctive new ventures and that the negative effect of cultural...

3D stochastic bicontinuous microstructures: generation, topology and elasticity

Celal Soyarslan, Swantje Bargmann, Marc Pradas & Jörg Weißmüller
Motivated by recent experimental investigations of the mechanical behavior of nanoporous metal we explore an efficient and robust method for generating 3D representative volume elements (RVEs) with strikingly similar behavior. Our approach adopts Cahn's method of generating a Gaussian random field by taking a superposition of standing sinusoidal waves of fixed wavelength but random in direction and phase. In its theory part, our study describes closed-form xpressions for how the solid volume fraction affects the...

Noise impact in powered respirators

Klaus-Dieter Fröhner & Ze Li
In respirators the users are closely linked with the apparatus. The new types of respirators use powered ventilators so that the technical layout and the used elements have strong effects on the users. The most unwanted effect is noise that influences not only the hearing bility of the user but also impairs the communication between users. This study aims at a systematic investigation of influencing factors based on a study evaluating methods for measurements. It...

Wie kommt das Repositorium in die Suchmaschine?

Beate Rajski
Publication repositories use a special interface for the provision of their metadata: OAI-PMH ("Protocol for Metadata Harvesting" of the Open Archive Initiative). Since this is not used by search engines such as Google Scholar, metadata is also provided in the machine-readable HTML header for a publication.

Stress and strain of aging innovators in the product development process

Kerstin Nawroth
Workforces in industrialized countries in general are becoming older because the babyboom generation is reaching an advanced age and the percentage of young employees is constantly decreasing.

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