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Crystal structure of the complex of 2,4,6-triethyl- 1,3,5-tris[(4-methyl-1H-indazol-1-yl)methyl]- benzene with NH4PF6
No full textThe complex of 2,4,6-triethyl-1,3,5-tris[(4-methyl-1H-indazol-1-yl)methyl]benzene with ammonium hexafluorophosphate, C39H42N6·NH4+·PF6−, crystallizes in the monoclinic space group P21 with two molecules of the receptor, two NH4+ and two PF6− ions in the asymmetric unit. In each of the complexes the ammonium ion resides in the cavity of the receptor molecule and is fixed in its position by three N—H⋯N bonds, while the remaining hydrogen atom of the cation acts as a bifurcated binding site for N—H⋯F bonding to the counter-anion. The crystal is composed of one-dimensional supramolecular aggregates extending along the a-axis direction
Üben ohne Druck: Förderung der motorischen Entwicklung von Kindergartenkindern mit Hemiparese
No full textDer Forschungsartikel widmet sich der Entwicklung architektonischer Lösungen zur Förderung der motorischen Entwicklung von Kindern mit Hemiparese in Folge eines Schlaganfalls. Besonders das Lernen mit möglichst wenig Druck steht im Vordergrund. Dabei wurde untersucht, welchen Einfluss ein Kindergarten auf die motorische Entwicklung nehmen kann und welche spezifischen architektonischen Aspekte Einfluss nehmen. Es wurden sowohl kleinere als auch größere Maßnahmen ausgearbeitet und Gestaltungsvorschläge erbracht. [... aus dem Text
Commodity production and African migration to Turkey, now and in the premodern past
No full textAfrican Migration in Turkey is an under-researched area despite the long history of migration between West Africa and the Ottoman Empire and the large number of African migrants in Turkey. The connection of this historical and contemporary migration movement with commodity production reveals not only the basic dynamics and patterns but also the global character of this mobility. While the flow of labor and commodities between Africa and Turkey continues, especially with Turkey's new Africa policy after 2002, African migrants are also implementing their own interests and agendas. Thus, an interdisciplinary approach encompassing history, economics, and anthropology reveals intertwined transformations and networks that provide a perspective for historical changes.A migração africana na Turquia é uma área pouco pesquisada, apesar da longa história de migração entre a África Ocidental e o Império Otomano e do grande número de migrantes africanos na Turquia. A conexão entre esse movimento migratório histórico e contemporâneo e a produção de commodities revela não apenas as dinâmicas e padrões básicos, mas também o caráter global dessa mobilidade. Enquanto o fluxo de trabalho e commodities entre a África e a Turquia continua, especialmente com o surgimento de uma nova política africana da Turquia após 2002, os migrantes africanos também estão implementando seus próprios interesses e agendas. Assim, uma abordagem interdisciplinar que abrange história, economia e antropologia revela transformações e redes entrelaçadas que nos fornecem uma perspectiva para as mudanças históricas
Effect of mechanical strain on the structure and properties of 2D materials and their heterostructures
No full textTwo-dimensional (2D) materials are a fascinating class of substances with diverse properties that make them suitable for various applications. They are particularly promising for optoelectronic and energy applications due to their thinness. They offer the potential to address the limitations of the silicon industry such as the increasing leakage currents (i.e., current which is non-functional and lost due to, e.g., spontaneous discharge of capacitors) with decreasing feature sizes and the heat management. However, using these materials in devices still requires overcoming several unresolved challenges. A major obstacle is to obtain precise control over the properties of these materials. Tuning these characteristics can be performed through various methods, such as adjusting the chemical composition of the materials or applying external fields. Strain is particularly attractive as it can be applied either during synthesis or dynamically during operation. Moreover, it can be applied in various forms, both uniform and non-uniform. Theoretical investigations have already provided detailed insight into the influence of uniform strain on optoelectronic properties of 2D materials. Yet, due to their low bending moduli, 2D materials are particularly susceptible to non-uniform strain fields, allowing them to bend easily in the out-of-plane direction. Although these strain fields are versatile for property modulation, non-uniform strain fields are rarely studied in theoretical works. This is mainly due to the computational complexity that arises after the breaking of symmetries by the strain field.
In this thesis, I theoretically investigate the influence of strain on the properties of 2D materials within the framework of density functional theory (DFT). Initially, I discuss the role of wrinkling in 2D transition metal dichalcogenides (TMDCs, \ch{MX2}, where M = W, Mo,... and X = S, Se,...). Wrinkles convey a non-uniform, periodic strain field to the 2D structure and experimental observations have suggested the emergence of new optoelectronic phenomena, yet the impact of these fields have not been fully investigated theoretically. In my work, I investigate the role of wrinkles in nanoscale monolayers and bilayers of TMDCs, and I extend the analysis to heterobilayers. Furthermore, I highlight the critical role of spin-orbit coupling (SOC) in determining the electronic structure of these systems.
Next, I address the effect of strain at the edges of 2D h--BN. When they are exfoliated from bulk and thinned under ion radiation, edges with different number of layers are formed. At these edges, localized (non-uniform) strain accumulates. This local strain can influence the charge density of the sample. Here, I particularly focus on connecting the DFT simulation results with experimental data obtained from aberration-corrected scanning transmission electron microscopy (TEM). I present how the correlation can be deciphered and linked to the experimental observations.
Finally, I concentrate on atomic layer deposition (ALD) synthesized \ch{Sb2Te3}-\ch{Sb2Se3} multilayered material. These materials experience another strain field, namely the interfacial strain, during synthesis. Moreover, defects are present in this synthesis. These defects introduce a local strain field that can influence the thermoelectricity in these materials. It is noteworthy that these layered samples are at the crossover to the 2D materials, since further reduction of the stack size will result in 2D structures. Here, I explain how strain affects the thermoelectric coefficients of \ch{Sb2Te3}-\ch{Sb2Se3} heterostructures and how this effect can be correlated with experimental data
Ultrasonic Guided Waves for Liquid Water Localization in Fuel Cells: An Ex Situ Proof of Principle
No full textWater management is a key issue in the design and operation of proton exchange membrane fuel cells (PEMFCs). For an efficient and stable operation, the accumulation of liquid water inside the flow channels has to be prevented. Existing measurement methods for localizing water are limited in terms of the integration and application of measurements in operating PEMFC stacks. In this study, we present a measurement method for the localization of liquid water based on ultrasonic guided waves. Using a sparse sensing array of four piezoelectric wafer active sensors (PWAS), the measurement requires only minor changes in the PEMFC cell design. The measurement method is demonstrated with ex situ measurements for water drop localization on a single bipolar plate. The wave propagation of the guided waves and their interaction with water drops on different positions of the bipolar plate are investigated. The complex geometry of the bipolar plate leads to complex guided wave responses. Thus, physical modeling of the wave propagation and tomographic methods are not suitable for the localization of the water drops. Using machine learning methods, it is demonstrated that the position of a water drop can be obtained from the guided wave responses despite the complex geometry of the bipolar plate. Our results show standard deviations of 4.2 mm and 3.3 mm in the x and y coordinates, respectively. The measurement method shows high potential for in situ measurements in PEMFC stacks as well as for other applications that require deposit localization on geometrically complex waveguides
Schädigungsentwicklung und Spannungsumlagerungen in ermüdungsbeanspruchten Betondruckzonen
No full textDas Ermüdungsverhalten von Beton wurde in der Literatur bislang sehr ausführlich anhand von zentrisch beanspruchten Zylinderprobekörpern untersucht. Untersuchungen, die sich mit zyklisch biegebeanspruchten Betonbauteilen im Druckschwellbereich beschäftigen, existieren nur in geringer Zahl. Solche Bauteile sind beispielsweise in Turmstrukturen von Windenergieanlagen vorhanden, die aktuell in großem Umfang errichtet werden. Für hochfesten Beton, wie er in diesen Bauwerken meist verwendet wird, liegen noch weniger Untersuchungsergebnisse vor. Aufgrund der hohen auftretenden Lastwechselzahlen und der großen Querschnittsabmessungen ist die versuchstechnische Untersuchung des Ermüdungsverhaltens solcher Strukturen mit großen Herausforderungen verbunden. Mit Hilfe von numerischen Simulationen kann hingegen eine Vielzahl an Parametervariationen innerhalb kürzester Zeit untersucht werden, wodurch eine beschleunigte Untersuchung des Ermüdungsverhaltens von Bauteilen möglich wird. Die dafür verwendeten Materialmodelle werden schnell sehr komplex und benötigen eine aufwendige Kalibrierung der Eingangsparameter an Bauteilversuchen. Ein Modell zur Abbildung der makroskopischen Schädigungsprozesse im Beton unter Ermüdungsbeanspruchung in Querschnitten unter Druckschwellbeanspruchung, das anhand von üblichen Zylinderversuchen kalibriert werden kann, existiert bisher nicht.
In der vorliegenden Arbeit wurden diese Punkte aufgegriffen und ein Modell zur Simulation der Schädigungsentwicklung und von Spannungsumlagerungen in ermüdungsbeanspruchten Betondruckzonen numerisch implementiert. Hierfür wurde ein dreistufiges Vorgehen gewählt. Im ersten Schritt wurden experimentelle Untersuchungen konzipiert und durchgeführt. Die Versuche waren eine wichtige Grundlage für das numerische Modell und wurden an zylindrischen und balkenförmigen Probekörpern durchgeführt. Aus statischen und zyklischen Versuchen an den Betonzylindern wurden die Eingangsparameter für das Materialmodell ermittelt und die Balkenversuche dienten zur Validierung des Modells. Diese wurden in einem Resonanzprüfstand durchgeführt, mit dem sich die erforderlichen großen Kräfte mit Belastungsfrequenzen von 19 Hz realisieren ließen. Damit konnten die hohen auftretenden Lastwechselzahlen in kurzer Zeit erreicht werden. Im zweiten Schritt wurde ein additives Dehnungsmodell in den Berechnungsablauf der Finite-Elemente-Software ANSYS Mechanical implementiert. Die zugrundeliegenden Funktionen für die Steifigkeits- und Dehnungsverläufe wurden aus Zylinderversuchen abgeleitet. Im dritten Schritt erfolgte die Validierung der numerischen Implementierung anhand der Balkenversuche. Es wurden unterschiedliche Phänomene betrachtet, wie z. B. die Temperatur- und Dehnungsentwicklung sowie die Veränderungen in der Materialsteifigkeit und die daraus resultierenden Spannungsumlagerungen im Querschnitt.
In den experimentellen Untersuchungen der Balkenprobekörper wurde mit der Resonanzprüfmethode ein Ermüdungsversagen durch Abplatzen der Betondruckzone oder durch eine vollständige Zerstörung der Balken erzielt. Die ertragenen Lastwechselzahlen aller Balken lagen deutlich über den Bruchlastwechselzahlen nach fib Model Code 2010. Die Ursache dafür ist eine frühe Schädigung und Steifigkeitsabnahme der am stärksten beanspruchten Bereiche, die dort zu einer nachfolgenden Verringerung der Spannungen führte. In den numerischen Nachrechnungen konnten die in den Balkenversuchen beobachteten Effekte bestätigt werden. Vor allem die am stärksten geschädigten Bereiche konnten mit dem Modell sehr gut abgebildet werden. Es wurde zudem gezeigt, dass sich infolge der Entlastung der geschädigten Bereiche Spannungen in weniger stark beanspruchte Bereiche umlagerten. Dies bestätigte die positive Wirkung der Spannungsumlagerungen auf die Ermüdungslebensdauer der Bauteile. Mit dem Modell können Parameterstudien mit z. B. variierenden Belastungsniveaus oder Vorspannungsgraden durchgeführt werden, um wertvolle Erkenntnisse zur Vordimensionierung ermüdungsbeanspruchter Bauteile und zur Identifikation markanter Stellen in der Geometrie zu gewinnen. Für weiterführende Aussagen ist die Durchführung zusätzlicher Versuche notwendig, um das numerische Modell auf ein breiteres Parameterspektrum zu erweitern.The fatigue behavior of concrete has so far been studied intensively in the literature using centrically loaded cylindrical specimens. Only a few studies have been carried out on concrete structures subjected to cyclic bending loads. Such structures can be found in wind turbine towers, for example. Research results for high-strength concrete, which is usually used in such structures, are even scarcer. Due to the high number of occurring load cycles and the large cross-sectional dimensions, the testing of the fatigue behavior of such structures is associated with major challenges. Numerical simulations, in contrast, enable the investigation of a large number of parameter variations within a very short period of time, which accelerates the investigation of the fatigue behavior of components. The used material models quickly become very complex and require time-consuming calibration of the input parameters in structural tests. A model for simulating the macroscopic damage processes in concrete subjected to fatigue loading in compressed cross-sections, which can be calibrated using standard cylinder tests, does not yet exist.
The present work addresses these aspects and implements an additive strain model to simulate the damage development and stress redistributions in fatigue-loaded concrete compression zones. A three-step approach was chosen for this. In the first step, experimental investigations were designed and carried out. The tests are essential for the numerical model and were carried out on cylindrical and beam-shaped specimens. The input parameters for the material model were determined from static and cyclic tests on the concrete cylinders and the beam tests were used to validate the model. The beam tests were carried out in a resonance-based testing facility, which allowed the realization of the required high forces with load frequencies of 19 Hz. This made it possible to apply the high number of load cycles in a short time. In the second step, the strain model was implemented in the simulation procedure of the finite element software ANSYS Mechanical. The functions for the stiffness and strain development were derived from cylinder tests. In the third step, the numerical implementation was validated using the beam tests. Different phenomena were considered, such as the temperature and strain development as well as the changes in material stiffness and the resulting stress redistributions in the cross-section.
In the experimental investigations of the beam specimens, fatigue failure due to spalling of the concrete compression zone or complete destruction of the beams was achieved using the resonance-based testing facility. All beams endured significantly more load cycles than the number of cycles to failure according to fib Model Code 2010. This was due to early damage and a reduction in stiffness in the most heavily loaded regions, which subsequently led to a reduction in stresses. The numerical simulations confirmed the effects observed in the beam tests. In particular, the model was able to simulate the most damaged regions very well. Moreover, it was shown that stresses redistributed to less heavily loaded regions as a result of the relief of the damaged regions. This confirmed the positive effect of stress redistributions on the fatigue life of the structures. The model can be used to carry out parameter studies with varying load levels or degrees of prestressing, for example, in order to gain valuable insights into the predimensioning of fatigue-loaded structures and to identify critical locations in the geometry. For more detailed conclusions, additional tests have to be carried out in order to expand the numerical model to a wider range of parameters
Market penetration of fuel cell vehicles: Analysis based on agent behaviour
No full textThis paper discusses the market penetration of fuel cell vehicles (FCVs) in Germany from the perspectives of different stakeholders. There are several economic studies and models describing the introduction of hydrogen-powered vehicles, but most of them focus on only one segment of the car market. Most studies analyse the impacts of FCVs on the automotive industry or the demand for FCVs separately, while others look at the required hydrogen infrastructure, but none of these analyses examines the car market as a whole. The analysis takes into account the actions of the whole market (consumers, automotive manufacturers, filling station owners and policymakers) and their interactions.
According to the results of the System Dynamics model the combination of tax-free hydrogen fuel, subsidies on FCVs and sufficient hydrogen infrastructure supply will lead to quick market penetration of FCVs. The analysis clearly shows that the government is recommended to support the installation of a sufficient number of hydrogen filling pumps at the beginning of the market introduction (e.g. 500 filling stations). Due to the model results, approximately 4.8 billion Euros are needed to reach a successful market penetration of FCVs. Thereby about one-third of all passenger cars will be driven with hydrogen in 2040 and even two-third in 2050. Primarily subsidies and tax allowances for the vehicles are necessary to reduce the price of FCVs to the average level of modern diesel cars; otherwise consumers' acceptance of FCVs will be very low. A rapid introduction of FCVs based on these policies is necessary in order to limit the cumulative subsidies for the vehicles and the fuel-tax deficit
