Universität Stuttgart
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Item Open Access Ambient pressure oxidation of Ag(111) surfaces : an in-situ X-ray study(2008) Reicho, Alexander; Dosch, Helmut (Prof. Dr.)The oxidation of metals plays an outstanding role in everyday life. Typical phenomena are the formation of rust on steel or oxide scales on copper, showing up as a green patina. The formation of metal oxides is not always an unwanted process. The functionality of many materials is directly related to their controlled oxidation. The most prominent examples are passivating oxide layers on stainless steel. Relevant for this thesis are industrially applied heterogeneous catalytic reactions for the synthesis of many chemical products, where gaseous reactants are in contact with the solid surface of the catalyst. Oxidation reactions are very important in this context, leading to a big need of understanding of these processes in research and development. Thereby, the active oxygen species on the surface and selectivity and poisoning of the catalyst have to be studied on an atomic scale. The high temperature and high pressure oxidation of the 4d transition metals Ru, Rh, Pd and Ag is a matter of particular interest, because these metals are widely used as oxidation catalysts. On Ruthenium one observes the formation of RuO2(110) bulk oxide islands at elevated temperatures and oxygen pressure. In the case of the Pd(100) and Rh(111) surface oxidation can lead to the formation of so-called surface oxides. These oxides are structurally related to the bulk oxide of the respective element. Furthermore, surface oxides are ultra thin oxides containing one metallic layer surrounded by two oxygen layers, giving rise to an oxygen-metal-oxygen sequence perpendicular to the surface plane. A future vision is to get a direct microscopic control of the emerging surface structures and ultimately of the real-time oxidation/reduction dynamics allowing one to tailor such catalytic reactions to better performance. A necessary prerequisite to the microscopic control is the full atomistic understanding of the surface structures which form at high temperature and at high oxygen pressures. Silver plays a unique role in heterogeneous catalysis. Supported Ag catalysts are used for the selective oxidation ('epoxidation') of ethylene and for the partial oxidation of methanol to formaldehyde. Ethylene oxide and its derivates are basic chemicals for industry, used in a many technologies with a world-wide production of more than 10 million tons as in medicine for disinfection, sterilization, or fumigation, or in transport and energy technologies for engine antifreeze and heat transfer. Because of its ability to kill most bacteria, formaldehyde is extensively used as disinfectant and as preservative in vaccinations. Therefore, the optimisation of these two Ag-supported catalytic reactions is of paramount importance. Current strategies employed in the industrial process to enhance selectivity include the empirical use of inhibitors (Cl) and promoters (Cs), however, on the way to a knowledge-based control of these reactions one has first to understand the surface structure of oxidized silver under relevant conditions in full detail. The formation of extended Ag(111) facets is observed on polycrystalline silver during the above industrial catalytic oxidation reactions, in turn fundamental research (experiment and theory) has been devoted to the detailed understanding of oxidation of this surface. The formation of an oxygen induced p(4x4) reconstruction on the Ag(111) surface is known since the early 70s. A surface oxide trilayer model, based on a three-layer slab of Ag2O(111), was proposed. Accordingly, the Ag(111) surface seemed to show a similar behaviour like Pd and Rh, being neighbours in the periodic table. Further theoretical calculations predicted the stability of this reconstruction under industrially relevant conditions. Nevertheless, several questions remained unsolved: the stability of the p(4x4) reconstruction under industrially relevant conditions was not checked experimentally, the structural model of the p(4x4) structure was not proven by a crystallographic method and previously unknown structures might play an important role for the catalytic activity of Ag(111) facets. Our experimental approach is based on the nowadays routinely available highly brilliant x-ray radiation produced by third generation synchrotron light sources. This radiation is used by us in three surface sensitive x-ray techniques. In-situ surface x-ray diffraction (SXRD) allows the identification and determination of structural models of surface reconstructions under industrially relevant conditions. This technique is combined with high resolution core level spectroscopy (HRCLS) and normal incidence x-ray standing wave absorption (NIXSW), giving insight into the local binding geometry of the oxygen and silver atoms.Item Open Access Prozessbausteine(2014) Eberle, Hanna; Leymann, Frank (Prof. Dr.)Gegenwärtig existierenden Modellierungssprachen und Werkzeugen zur Umsetzung prozessbasierter Anwendungen liegt im Allgemeinen die Annahme eines zur Entwicklungszeit bekannten und in seiner Struktur vollständig ausmodellierten Prozessmodells zugrunde. Für Szenarien, in welchen eine prozessbasierte Anwendung neben stabilen, d.h. zur Modellierungszeit des Prozesses bekannten, auch durch dynamische, d.h. erst zur Anwendungslaufzeit geltende, Rahmenbedingungen beeinflusst wird, ist eine derartige statische Prozessmodellierung nur bedingt geeignet. In diesen Szenarien ist es vielmehr wünschenswert, (i) zur Entwicklungszeit bereits bekannte Prozessteile der Anwendung detailliert ausmodellieren zu können, und diese (ii) zur Laufzeit der Anwendung unter Berücksichtigung der zum Ausführungszeitpunkt geltenden dynamischen Rahmenbedingungen zum vollständigen Prozess der Anwendung zu integrieren. Das im Verlauf dieser Arbeit vorgestellte Konzept der Prozessbausteine setzt an diesem Punkt an und schafft ein Rahmenwerk für die Modellierung und Ausführung prozessbasierter Anwendungen unter Berücksichtigung sowohl stabiler als auch dynamischer Rahmenbedingungen. Kerngedanke des Konzepts ist die Abbildung stabiler Rahmenbedingungen zur Entwicklungszeit in Form teilweise unvollständiger Prozessmodelle, sogenannter Prozessbausteine. Zu einem späteren Zeitpunkt im Lebenszyklus der Anwendung werden diese Prozessbausteine dann, motiviert durch die jeweils geltenden dynamischen Rahmenbedingungen, mit weiteren Prozessbausteinen zum vollständigen Prozessmodell der Anwendung integriert. Zur vollständigen Unterstützung der Entwicklung von Anwendungen auf Grundlage dieses Konzepts umfasst die vorliegende Arbeit die Definition eines Metamodells für sowohl die Modellierung einzelner als auch die Repräsentation integrierter Prozessbausteine, die Beschreibung der Ausführung integrierter Prozessbausteine, sowie die Vorstellung einer Architektur für die Ausführung integrierter Prozessbausteine.Item Open Access Colloidal monolayers on quasiperiodic laser fields(2010) Mikhael, Jules; Bechinger, Clemens (Prof. Dr.)Quasicrystals are somewhat paradoxical structures which exhibit many amazing properties distinguishing them from ordinary crystals. Although the atoms are not localized at periodic positions, quasicrystals posses perfect long-range order. Until the early 1980s it was unanimously established that ordered matter is always periodic. Accordingly, the rotational symmetry in real space was thought to be limited to n=2,3,4 and 6. However more than a hundred complex metal alloys, for instance the discretely diffracting icosahedral AlPdMn or decagonal AlNiCo, have defied these crystallographic rules and self-organized into quasicrystals. Although the majority of the identified quasicrystals are complex metal alloys synthesized in the laboratory, recent experimental results proved that quasiperiodic order is not limited to metals. Matter also organizes itself aperiodically at larger length scales where thermal fluctuations play an important role. Recent experiments have shown that quasiperiodic order is also oberved in soft matter systems, such as micellars, polymers, and binary nanoparticles. Quasicrystals show many interesting properties which are quite different from that of periodic crystals. Accordingly, they are considered as materials with high technological potential e.g. as surface coatings, thermal barriers, catalysts or photonic materials. Quasicrystalline structures have been theoretically predicted also in systems with a single type of particles. Nevertheless, experimentally their spontaneous formation has been only observed in binary, ternary or even more complex alloys. Accordingly, their surfaces exhibit a high degree of structural and chemical complexity and show intriguing properties. In order to understand the origin of those characteristics it would be helpful to disentangle structural and chemical aspects which can be achieved by growing single-element monolayers to quasicrystalline surfaces. Apart from understanding how quasicrystalline properties can be transferred to such monolayers, this approach might allow fabrication of materials with novel properties. First heteroepitatic growth experiments on decagonal and icosahedral surfaces indeed demonstrate the formation of Pb, Bi and Sb monolayers with a high degree of quasicrystalline order as determined by low-energy electron diffraction and elastic helium atom scattering experiments. Compared to reciprocal space studies, only recently atomically resolved scanning tunneling microscopy investigations of the adsorbate morphology became possible. Even then, however, it is difficult to relate the structure of the adsorbate to that of the underlying substrate. In that respect, the study of the phase behaviour of colloidal particles interacting with quasiperiodic laser fields can throw new light on fundamental problems of broad interest in the physics of quasicrystals and in condensed matter physics. In fact colloidal systems are meanwhile established as excellent model for atomic systems and colloidal physics have demonstrated that such systems can give answers to many basic physics questions. Depending on the pair-interaction and the concentration, colloidal systems show analogues of all the states of atomic systems: gas, liquid and solid states. The mesoscopic size (nm-µm), the time scales (ms-s) and the tunability of the pair interaction in colloidal systems make them a convenient model system for experimental and theoretical studies. As a consequence, real space analysis by means of video microscopy allows tracking the trajectories of the individual particles and makes the time evolution of the system accessible in detail. Such information is inaccessible in systems investigated by diffraction experiments, as the scattering information is available only averaged over the scattering area. Because in a colloidal system there is direct access to real space information, the strength and nature of the different interactions, the origins of the complex phase behavior could be in different examples identified. In conclusion, the study of the rich phase behavior of colloidal suspensions provides ideal conditions for experimental and theoretical studies. In this Thesis, we report on a real-space investigation of the phase behaviour of charged colloidal monolayers interacting with quasicrystalline decagonal or tetradecagonal substrates created by interfering five or seven laser beams. Different starting configurations, such as dense fluid and triangular crystals with different densities, are prepared. At low intensities and high particle densities, the electrostatic colloidal repulsion dominates over the colloid-substrate interaction and the crystalline structure remains mainly intact. As expected, at very high intensities the colloid-substrate interaction dominates and a quasiperiodic ordering is observed. Interestingly, at intermediate intensities we observe the alignment of crystalline domains along the 5 directions of the quasicrystalline substrate. This is in agreement with observations of Xenon atoms adsorbed on the ten-fold decagonal Al-Ni-Co surface and numerical simulations of weakly adsorbed atomic systems. Intermediate phases are observed for colloid-substrate interactions strong enough to produce defects in the crystal. These defects adapt the form of rows of quadratic tiles. Surprisingly, for specific particle densities (at which the colloid-substrate interaction is minimized) we identify a novel pseudomorphic ordering. This intermediate phase which exhibits likewise crystalline and quasicrystalline structural properties can be described by an Archimedean-like tiling consisting of alternating rows of quadratic and triangular tiles. The calculated diffraction pattern of this phase is in agreement with recent observations of copper adsorbed on icosahedral AlPdMn surfaces. Interestingly, we also observe the formation of the same phase on tetradecagonal substrates also at densities for which the potential energy of the colloidal system is minimized. Although the structure can also be described by rows of triangles and rows of squares, a closer analysis reveals substantial differences. Here, large domains with almost periodic ordering are found. We show that this behavior is closely related to the low density of highly symmetric local motifs in the substrate potential. In the second part of this Thesis the conditions under which quasicrystals form are investigated. Currently, it is not clear why most quasicrystals hold 5- or 10-fold symmetry but no single example with 7 or 9-fold symmetry has ever been observed. Since the properties of quasicrystals are strongly connected to their atomic structure, a better understanding of their growth mechanisms is of great importance. In contrast to crystals which are periodic in all three dimensions, quasiperiodicity is always (except for icosahedral quasicrystals) restricted to two dimensions. Accordingly, three-dimensional quasicrystals are comprised of a periodic stacking of quasiperiodic layers and any hurdle in the formation of quasiperiodic order within a single layer will eventually prohibit their growth along the periodic direction. In this Thesis, we also report on geometrical constraints which impede the formation of quasicrystals with certain symmetries in a colloidal model system. This is achieved by subjecting a colloidal monolayer to N=5- and 7-beam quasiperiodic potential landscapes. Our results clearly demonstrate that quasicrystalline order is much easier established for N = 5 compared to N = 7. With increasing laser intensity we observe that the colloids first adopt quasiperiodic order at local areas which then laterally grow until an extended quasicrystalline layer forms. As nucleation sites where quasiperiodicity originates, we identify highly symmetric motifs in the laser pattern. We find that their density strongly varies with n and surprisingly is smallest exactly for those quasicrystalline symmetries which have never been observed in atomic systems. Since such high symmetry motifs also exist in atomic quasicrystals where they act as preferential adsorption sites, this suggests that it is indeed the deficiency of such motifs which accounts for the absence of e.g. materials with 7-fold symmetry. In addition to the fundamental aspects, we report in this Thesis on the fabrication of large colloidal quasiperiodic layers incorporated in a polymer hydrogel matrix. Because quasicrystals have higher point group symmetry than ordinary crystals, micrometer-scale quasicrystalline materials are expected to exhibit large and isotropic photonic bandgaps in the visible range. In our case, the quasiperiodic symmetries are induced using extended light fields. The reported gelled colloidal quasicrystals are unique in that they have large sizes as well as good optical uniformity. With laser diffraction the in situ variable length scale of such materials is demonstrated. In conclusion, we have studied the phase behavior of charged colloidal particles interacting with quasiperiodic laser fields. We showed that novel pseudomorphic growth can lead to the formation of a phase which exhibits likewise crystalline and quasicrystalline structural properties. We also performed unconventional measurements in order to understand why the formation of quasicrystals is limited to specific rotational symmetries. We have found that geometrical hurdles play a crucial role in the proliferation of quasiperiodicity and that such hurdles can hindered or even prohibited the formation of e.g. 7- or 9-fold symmetry. And finally, we have shown that the combination of extended light fields and hydrogel matrices leads to the formation of large quasiperiodically ordered colloidal materials.Item Open Access Towards spin injection into silicon(2007) Dash, Saroj Prasad; Carstanjen, Heinz Dieter (Prof. Dr.)The efficient spin injection into semiconductors could pave the way to a new generation of electronics devices such as spin memories, spin transistors, and spin quantum computers. The most important semiconductor for industrial application, Si has been studied for the purpose of spin injection extensively in this thesis. Three different concepts for spin injection into Si have been addressed: (1) spin injection through a ferromagnet-Si Schottky contact, (2) spin injection using MgO tunnel barriers in between the ferromagnet and Si, and (3) spin injection from Mn-doped Si (DMS) as spin aligner. (1) FM-Si Schottky contact for spin injection: In a heterostructure of a ferromagnetic thin film on a Si substrate, any structural disorder at the interface would drastically reduce the spin polarization at the interface and, hence, the spin injection efficiency. To be able to improve the interface qualities one needs to understand the atomic processes involved in the formation of such silicide phases. In order to obtain more detailed insight into the formation of such silicide phases the initial stages of growth of Co and Fe were studied in situ by HRBS with monolayer depth resolution. As understood, it was important to prohibit the in-diffusion of Co into interstitial sites at the initial stages of growth and the out-diffusion of Si atoms in the latter stages. So in order to control and improve the interface, equilibrium growth conditions were followed (i) by lowering the growth temperature and (ii) by surfactant-mediated growth. Low temperature growth of Co on Si (100): Already at very low coverage Co diffusion into the bulk Si has been observed. The amount of in-diffused Co is, however, less than at room temperature. In contradiction to room temperature growth, Co atoms form layers of pure Co on top of the Si surface already at very low coverage. Every second Si layer, starting with the first Si layer, is Co depleted. This leads to an oscillatory Co distribution in the Si lattice which is preserved up to higher coverages (1.3 ML). Surfactant-mediated growth of Co on Si (100) : The lower surface free energy of Sb in comparison to Co and Si, makes it a potential candidate for surfactant mediated growth. By the use of one monolayer of Sb adsorbed on a Si (100) surface, Co-Si intermixing at the interface is strongly reduced in comparison to the interface without Sb as surfactant. The improved interface quality with Sb-mediated growth is also reflected in magnetic measurements. Co with Sb-mediated growth shows a higher magnetic moment. It was shown that simple solutions can reduce the FM-Si inter diffusion at the interface and improve the interface quality. However these non-equilibrium growth conditions could not stop the silicide formation completely. (2) MgO tunnel barrier for spin injection into Si: On the other hand, using an ultra-thin tunnel barrier between FM and Si will have three advantages: (i) form a chemical barrier between the FM and Si, (ii) circumvent the conductivity mismatch problem, and (iii) in addition act as a spin filter. The fabrication and characterization of ultra-thin crystalline MgO tunnel barriers on Si (100) was presented. Some of the important properties required for tunnel barriers on Si have been addressed. Ultra-thin stoichiometric MgO tunnel barriers with sharp interface with Si (100), very homogeneous, without pin-holes, and crystalline in structure could be fabricated by reactive molecular beam epitaxy. Co and Fe on an ultra thin MgO tunnel barrier were found to have island-like growth with a rough surface. Ultra-thin Co and Fe films are found to be thermally quite stable up to 450 °C. (3) Mn doped Si for spin injection: For spin injection purpose, instead of contacting the Si with a ferromagnetic metal, the contact could be made with another semiconductor, one with ferromagnetic properties. This solves the conductivity mismatch problem by ensuring that the resistivities of the materials on both side of the interface are comparable in magnitude. Si-based diluted magnetic semiconductor samples were prepared by doping Si with Mn by two different methods i) by Mn ion implantation and ii) by in-diffusion of Mn atoms (solid state growth). In the case of implanted samples, Mn atoms do not substitute Si sites. The implanted samples show room temperature ferromagnetism as measured by a SQUID magnetometer. The magnetic moment per Mn atom is found to decrease with increasing implantation dose. It has been observed that the implanted samples show carrier mediated ferromagnetism and, more importantly, mediated by both holes and electrons in contrast to statements in the literature. Solid state growth of Mn doped Si : For evaporation of Mn on Si (100), Mn atoms diffuse deep into the Si bulk already at room temperature, even for very low coverage (0.25 ML) with an oscillatory concentration depth profile as observed by HRBS with monolayer depth resolution. This results in natural MnxSi1-x/Si digital layers on the surface. Surprisingly, the samples prepared by this solid state diffusion process show room-temperature ferromagnetism having a magnetic moment of 1.8 µB per Mn atom, which is much higher than that of the ion-implanted samples. In contrast to ion-implanted samples the ferromagnetism in these samples does not show any carrier mediation.Item Open Access Application-specific UML profiles for multidisciplinary product data integration(2011) Reichwein, Axel; Rudolph, Stephan (Priv.Doz. Dr.-Ing.)This thesis examines the suitability of the UnifiedModeling Language (UML) to establish a central product model for multidisciplinary product data integration. Computer-aided product design involves the use of specialized discipline-specific software applications in order to model and simulate various product aspects. Dependencies between models are thereby frequent as the same product information often appears redundantly in various engineering models. In addition, dependencies exist due to relationships between distinct features of various models. As a result, model modifications frequently require the update of dependent models. Data consistency between models is achieved automatically through model-to-model data exchange software. The use of a central product model enables to reduce the required number of data exchange connections. Central product models store product information which is spread across several models and achieve data consistency through data exchange connections between themselves and specific models as in a hub-and-spoke network. Central product models are especially useful for automatic data consistency in design scenarios which include a high number of inter-model dependencies and model modifications. The integration of geometry and therefrom derived models such as structural analysis or computational fluid dynamics models has already been successfully addressed in numerous central product models. However, the multidisciplinary integration of more diverse models, such as geometric, software, controller and multibody system models, currently presents a challenge. Although several central product models have been developed for multidisciplinary design, none has yet gained, in contrast to geometry-focused central product models, wide acceptance nor reached the status of an international standard. The unmanageable high number of diverse discipline- and application-specific modeling concepts hinders the development of a standardized holistic central product representation. This thesis investigates the possibility of establishing an interdisciplinary central product model based on the common modular structure of models from various disciplines. Most models which are edited with current state-of-the-art software applications are composed of modular components in order to support the exchange and reuse of model information. Models from different disciplines therefore share common modeling concepts for the specification of modular model components. However, there is yet no overarching modeling standard to describe the common characteristics of modular model components from various disciplines. Object-oriented modeling concepts currently mainly describe software modules called objects. Object-oriented modeling concepts are generic and can be used to represent modular components in general. The Unified Modeling Language (UML) has been since its emergence in 1997 the de facto standard for object-oriented modeling. This thesis examines the use of the object-oriented modeling concepts of the UML to uniformly describe widely used application-specific geometric, dynamic and multibody system models in a central product model. Application-specific model information was represented in UML through generic UML modeling concepts in combination with lightweight UML extensions in the form of stereotypes. UML profiles regrouped stereotypes which corresponded to a specific modeling application. The automatic translation of UML model information into the specific models and vice versa was implemented in order to test and validate the application-specific UML profiles. The UML-based central product model was used in several test cases to automatically generate consistent models for the simulation and evaluation of various product configurations. The test cases included models for the simulation of slider-crank mechanisms, the evaluation of cabin pressure control systems, the design of conveyor system configurations, the evaluation of satellite configurations and the generation of customized aircraft geometry. The workflows within the test cases included the automatic creation and modification of UML models as well as the invocation of data exchange connections. The workflows were described in executable UML activity diagrams or Java programs. The thesis demonstrates that the UML can be used beyond conventional software modeling to establish a central holistic product representation. The modeling concepts of geometric, dynamic and multibody system models were translated mostly according to one-to-one mappings into corresponding UML modeling concepts with their respective stereotype. As a result, the specific model information is easily recognizable in the UML-based central product model. Furthermore, the use of a UML-based central product model is facilitated for the many modelers who are already familiar with the widespread and standardized UML modeling language.Item Open Access Mechanistic studies on the DNA methyltransferases DNMT3A and DNMT3B(2021) Dukatz, Michael; Jeltsch, Albert (Prof. Dr.)In this work, both regulatory and catalytic mechanisms of de novo methyltransferases were investigated, which include interactions with other proteins and the specific recognition of the substrate sequence. Another part of this work strived to elucidate how enzymatic generation of 3-methylcytosine by DNMT3A can occur.Item Open Access Fatigue of Al thin films at ultra high frequencies(2005) Eberl, Christoph; Arzt, Eduard (Prof. Dr. phil.)Ultra high-cycle fatigue at frequencies in the GHz regime leads to a characteristic void and extrusion formation in patterned metal thin films. Resulting from the microstructural damage formation a significant degradation in form of a shift of the resonance frequency and failures by short circuits in Surface Acoustic Wave (SAW) test devices take place. To study fatigue at ultra high cycles, SAW test devices were used to test continuous and patterned Al thin films at ultra high frequencies. For stress amplitudes as low as 14 MPa lifetime measurements showed no fatigue limit for 400 nm Al thin films. The resulting damage sites appeared in regions of cyclic stress concentration as identified by Finite Element Analysis. In situ measurements revealed that the characteristic extrusion/void formation mechanism operates on a short time scale. The post-test analysis of microstructural changes reveals extrusion and void formation concentrated at grain boundaries. This finding and the observed grain growth indicates a high material flux at the grain boundaries induced by the cyclic load. Quantitative analysis also shows a correlation between extrusion density and electrical devices performance. This direct correlation shows a functional agreement with a common theory on the influence of crack density on intrinsic stresses in thin metal films. Advanced Finite Element (FEM) calculations simulate very well the sensitivity of the resonance frequency to damage structure in interconnects such as cracks, voids and extrusions. The experimentally observed linear correlation between damage density and frequency shift is reproduced by the FEM model. The estimation of the short circuit probability from the extrusion length distribution revealed an exponential dependency on the electrode distance. The observed damage formation is explained by the combined action of dislocation motion and stress-induced diffusion processes.Item Open Access Eignung von metallorganischen Gerüstverbindungen als stationäre Phase in der Hochleistungsflüssigchromatographie (HPLC)(2017) Lieder, Christian; Klemm, Elias (Prof. Dr.-Ing.)Anwendung von metallorganischen Gerüstverbindungen als stationäre Phase in der HPLC, Vergleich mit klassischen Silika-Materialien. Synthese der metallorganischen Gerüstverbindungen, Modifizierung. Befüllung chromatographischer Säulen und Gegenüberstellung der Füllmethoden. Methodenentwicklung, Einflüsse auf chromatographische Ergebnisse. Chirale Erkennung, Untersuchung der Wechselwirkungen. Theoretisch chemische Berechnungen der Wechselwirkungen.Item Open Access Synthetic and mechanistic prospects of homogeneous gold catalysis(2009) Pankajakshan, Sreekumar; Hashmi, A. Stephen K. (Prof. Dr.)a) The first chapter of this thesis manifests the exploration of homogeneous gold-catalyzed conversions of furan containing aryl-ynamides and ynol ethers. Enynes boast to be the most explored substrate structures in the realm of homogeneous gold catalysis, whereas the reactivity of ene-ynamides and ene-ynol ethers are much less explored till date.34, 45 Hashmi and co-workers recently reported the homogeneous gold-catalyzed synthesis of phenols from furan containing ynamides/ynol ethers.45e These substrates showed excellent reactivity and selectivity apparently owing to the heteroatom directly attached to the alkyne unit. Impressed with the reactivity and high selectivity of these systems, we decided to explore the catalytic activity of the aryl substituted variants of these compounds. Gold catalysis of aryl-ynamides The aryl-ynamide substrates turned out to be interesting candidates for gold catalysis. The mode of reactivity depended on the tether length. The substrates with two carbon tether underwent a Friedel-Crafts type reaction upon subjected to gold catalysts and furnished benzannulated arenes in moderate to excellent yields (Scheme C). A combination of Ph3PAuCl /AgBF4 (5 mol%, 1:1) in CH2Cl2 or CHCl3 was found to be the best choice of catalyst. The reaction conditions were mild and a broad spectrum of polyarenes and polyheteroarenes were synthesized. The substrates carrying an unactivated arene failed to react even under higher temperature and prolonged reaction time. The gold catalysis of substrates 58r-58t with a longer tether took a completely different pathway and gave rise to cyclopentadiene structures (Scheme D). Similar to shorter tethered substrates, electron withdrawing groups on the arene were not tolerated. b) The second chapter of this thesis deals with the development of gold-catalyzed transformations of furan containing alkynes that prove the potential intermediacy of a cationic intermediate in these types of reactions. The actual electronic state of the intermediate in gold-catalyzed enyne cycloisomerizations is a hot spot in recent scientific reports.20a-c A cyclopropyl "carbene" form of the intermediate has been used by most of the authors. But it is important to note that such a species is yet to be isolated or characterized spectroscopically. Recently Fürstner et al., proposed remarkable experimental evidence for the involvement of the cationic form of this intermediate in gold-catalyzed cyclizations.20a,20c The scenario demands the consideration of the actual electronic state of the intermediate as a mesomeric/tautomeric package of different canonical forms (Scheme H). In the context of this thesis it was found that furyl-alkynes carrying electron donating substituents in the α-position of the tether led to the formation of unexpected five-membered structures (instead of the expected phenols) when subjected to gold catalysts (Scheme I). The observed reactivity pattern for the substrates 116/118 was highly informative and served as an experimental evidence for the significant contribution of the cationic form of the intermediate AO in the mesomeric/tautomeric equilibria shown in the scheme H. A mechanistic proposal based on this intermediate was suggested for this hitherto unknown transformation in the gold catalysis of furyl-alkynes (Scheme J). c) The third chapter of this thesis elaborates the unexpected formation of N,O-acetals from oxanorbornadienes-the potential precursors for phenols- upon exposure to gold and other Lewis acids (Scheme K). AuCl was found to be the best Lewis acid catalyst under the given conditions, and Yb(CF3SO3)3 also showed remarkable reactivity. The generality and the absence of any other products/side products confirmed that the reactions followed general acid catalysis. The observation that there is a net reversal in the connectivity of the tether in the product hinted at a complex mechanism operating in the process. d) The fourth chapter of this thesis describes the investigations carried out on the aerobic oxidation reactions catalyzed by gold. A homogeneous catalytic system comprising of gold(I) chloride, n-butyl lithium and sodium carbonate was developed and was found to oxidize primary aromatic alcohols to aldehydes. The system was poor in activity towards aliphatic and secondary alcohols (Scheme L).Item Open Access Bemannte Missionen zum Mars mit kontinuierlichen Antrieben(2005) Schmidt, Tanja D.; Auweter-Kurtz, Monika (Prof. Dr.-Ing habil.)Im Rahmen dieser Arbeit werden bemannte Missionen zum Mars für unterschiedliche Antriebskonzepte im Hinblick auf Flexibilität, kurze Gesamtmissionsdauern, kurze Transferzeiten und moderate Startmassen untersucht und verglichen. Die untersuchten Antriebskonzepte sind kontinuierliche elektrische Antriebe sowie chemische und nuklear-thermische Antriebe, die zur Kategorie der impulsiven Antriebe gehören. Die im Rahmen dieser Arbeit hierzu erstellten Programme bzw. Routinen zur Bahnsimulation und -optimierung werden vorgestellt. Es werden die Ergebnisse einer detaillierten, systematischen Bahnanalyse für helio- und planetozentrische Bahnen und für Rundreise-Missionen aufgezeigt. Speziell für die kontinuierlichen elektrischen Antriebe werden Variationen der Antriebsparameter (Schub, spezifischer Impuls und Triebwerkswirkungsgrad) durchgeführt und deren Einfluß auf Flugzeit und Startmasse bzw. Treibstoffmasse untersucht. Es werden die hierfür notwendigen minimalen Antriebsparameter für bemannte Marsmissionen ausgearbeitet und verschiedene elektrische Antriebskonzepte hinsichtlich Anwendbarkeit untersucht. Die atmosphärischen Flugsegmente einer solchen Mission werden im Rahmen dieser Arbeit mittels Literaturstudien untersucht. Die in der Literatur diskutierten Konzepte für Schwerlaststartraketen an der Erde, Marsaufstiegsfahrzeuge sowie Aermanöver-Vehikel für Mars und Erde werden hinsichtlich Anwendbarkeit für bemannte Marsmissionen evaluiert und Konzeptvorschläge ausgearbeitet. Neben dem Antriebssystem werden im Rahmen dieser Arbeit die Lebenserhaltung, Habitate und die Energieversorgung untersucht. Es werden Modelle für diese Subsysteme erstellt und verschiedene Konzepte miteinander verglichen. Basierend auf den Ergebnissen dieser Arbeit wird ein Konzeptvorschlag einer bemannten Marsmission mit kurzer Gesamtmissionsdauer unter Verwendung kontinuierlicher elektrischer Antriebe vorgestellt.