Universität Stuttgart

Permanent URI for this communityhttps://elib.uni-stuttgart.de/handle/11682/1

Browse

Search Results

Now showing 1 - 10 of 22

Open Access
Novel X-ray lenses for direct and coherent imaging
(2019) Sanli, Umut Tunca; Schütz, Gisela (Prof. Dr.)
Open Access
Hydrogen transport in thin films : Mg-MgH2 and Ti-TiH2 systems
(2018) Hadjixenophontos, Efi
Hydrogen storage has become progressively important due to increasing energy demand. Magne-sium (Mg/MgH2) is one of the most promising elements of hydrogen uptake, however, the slow kinetics and need for high temperatures during dehydrogenation make this material challenging for mobile applications. Meanwhile, Titanium (Ti/TiH2/TiO2) draws attention due to its catalytic effect in hydrogenation of other metals with higher capacities. A comprehensive way to quantitatively char-acterize the kinetics of hydride formation in both systems (Mg and Ti) is shown here. A technique allowing a large range of pressures and temperatures (room temperature to 300 °C and from 0.05 bar up to 100 bar) is developed successfully. Thin films (50-1000 nm), deposited by ion beam sput-tering (PVD), are used because of their smooth surface and defined structure. In order to study hydrogen transport precisely, X-ray diffraction (XRD), electron microscopy (SEM/FIB/TEM) and electric resistance measurements are used. In the case of Mg, while a Pd coating is used as catalyst, the hydride is formed from the surface towards the substrate and transformation in the morpholo-gy is observed. Parabolic law is followed and the diffusion coefficient of hydrogen in MgH2 is ob-tained at room temperature (2.67 · 10-17 cm2/s). Additionally, a model is created to fit the experi-mental change in resistance during hydrogen loading and shows the changes in the behavior of thicker layers. The interface between Pd/Mg is discussed, since Mg5Pd2 and Mg6Pd are formed at high temperatures and are most dominant over dehydrogenation. However, at room temperature, this interface appears to be more stable. The activation energy of hydrogenation is calculated ex-perimentally from an Arrhenius plot to be equal to Ea = 22.6 ± 2.0 kJ/mol and the pre-factor D0 = 3904 cm2/s. Additional attention is given to magnesium hydride as an anode electrode in Li-ion bat-teries. TEM investigations of thin film electrodes demonstrate the complete lithiation of the mate-rial however, with drastic volume changes, leading to bad reversibility. In Ti the thin oxide layer naturally formed on the surface, appears to play a dominant role in the kinetics of hydrogen transport leading to a linear kinetics. A pressure dependency is observed, while an experimental evaluation of the permeation coefficient in the oxide is also discussed. Important information on the hydrogen transport is obtained in both systems, giving an input for further improvements of such hydrides.
Open Access
Gold nanoparticle-mediated DNA origami nanoarchitectures
(2024) Peil, Andreas; Na Liu, Laura (Prof. Dr.)
Since its origin in the 1980s, DNA (deoxyribonucleic acid) nanotechnology has established itself as a captivating nanofabrication technique with ever increasing impact that combines aspects from physics, chemistry, and biology to construct artificial nanosystems by means of molecular self assembly. Within the field of DNA nanotechnology, the DNA origami technique represents one of the most versatile fabrication tools to craft functional two-dimensional (2D) and three-dimensional (3D) nanostructures from the bottom up. These structures offer precisely tailored geometries along with programmable functions, featuring positional addressability with sub-5 nm resolution and exceptional spatiotemporal accuracy. This thesis discusses strategies to employ the DNA origami technique to assemble intricate hybrid nanosystems with synergistically integrated gold nanoparticles (AuNPs). The AuNPs take over different roles; they grant (i) structural and (ii) functional features and enable the (iii) optical monitoring of the systems. This approach allows the fabrication of nanostructures piece by piece to explore their structural and functional properties at the nanoscale in detail. The first publication covers different strategies for the hierarchical assembly of topological DNA origami structures using a AuNP-templated self-assembly approach. The assembly of [2], [3], and [4]catenanes with interconnecting AuNPs is elucidated. The AuNPs can be controllably released to disconnect the individual rings, leaving only the mechanical bond of the catenane chain. In the second publication, a dynamic AuNP-DNA origami gear system is presented that is designed to emulate a planetary gearset with precise spatiotemporal control over its rotation dynamics. The AuNPs serve three crucial tasks. They (i) structurally link the origami ring modules, (ii) mediate the rotation and (iii) enable the real time optical tracking of the rotation via fluorescence spectroscopy. The system enables tightly orchestrated and programmable bidirectional rotations. In the third publication, reconfigurable chiral metastructures comprising multiple plasmonic particles that are accurately positioned in a helical manner around a DNA origami template are discussed. The implementation of a DNA ‘swingarm strategy’ enables the simultaneous and efficient relocation of multiple closely spaced AuNPs over large distances to precisely tune the chiroptical response of the system. The presented publications illustrate the beneficial synergies between DNA origami systems and rationally integrated AuNPs with the aim to advance and expand the application spectrum of these hybrid nanosystems within their scientific disciplines.
Open Access
Modeling and simulation of closed low-pressure adsorbers for thermal energy storage
(2019) Schäfer, Micha; Thess, André (Prof. Dr. rer. nat.)
Closed low-pressure adsorption systems can be applied for thermal energy storage. Their performance is determined by the mass and heat transport processes in the adsorber. Therefore, thorough knowledge of these transport processes is required for further storage development. The present thesis contributes to this by providing detailed models of closed low-pressure adsorbers and by conducting simulations over a broad range of parameters and configurations. The focus is on adsorbers of larger scale (length L = 0.1 . . . 1 m) and on the discharging process. As the adsorption pair, binderless zeolite 13X with water is examined. The models are developed in a stepwise manner from pore to storage scale. The Finite-Difference-Method is implemented to numerically solve the models. Simulations are conducted for defined reference cases as well as over a broad range of geometric and process parameters. The reference cases are analyzed in detail to gain a better understanding of the transport processes. Furthermore, the results are analyzed with respect to two particular modeling aspects: equilibrium assumptions and rarefaction effects (e. g. slip effect). With respect to the application, the discharging performance is analyzed in terms of thermal power and a defined discharging degree. Both the adsorber and the adsorbent configurations are varied. In addition, the effect of the discharging conditions is evaluated. Finally, one exemplary charging process is examined. The detailed analysis of the reference cases reveals that the mass and heat transport and the adsorption processes are strongly coupled and can only be understood in their interaction. For onedimensional adsorber configurations, that is the mass and heat transport are in the same direction, the discharging process is generally limited by the heat transport. This leads to insufficient thermal power and unsuitable discharging durations of up to one year. In contrast, for two-dimensional adsorber configurations, that is the mass and heat transport are in perpendicular directions, the discharging process can be limited either by the mass or heat transport or by the adsorption. The limitation depends on the configuration of the adsorber and adsorbent. Moreover, the twodimensional adsorber configurations can provide sufficient thermal power. With respect to the modeling, it is found that the assumption of a uniform pressure distribution is applicable for one-dimensional adsorber configurations. In contrast, for two-dimensional configurations, no equilibrium assumptions can be applied in general. However, for powder adsorbent it is always valid to assume local adsorption equilibrium. Regarding the rarefaction effects in twodimensional adsorber configurations with honeycombs and granules, the slip effect is relevant for small channel and particle diameters (d = 1 mm). For adsorbers with powder adsorbent, the reduction of the effective heat conductivity due to the rarefaction effect becomes relevant. With respect to the application, the variation of the adsorber configuration shows that the volumetric thermal power generally decreases with increasing adsorber length. Furthermore, the power decreases with increasing width between the parallel heat exchanger plates in the adsorber. Regarding the adsorbent configuration in two-dimensional adsorber configurations, it is found that the volumetric thermal power can be optimized by variation of the channel or particle diameter. Interestingly, the optima for peak and mean power do not coincide. In addition, the discharging degree is found to strongly depend on the discharging conditions in terms of discharging temperature and volume flow of the heat transfer fluid extracting the heat from the adsorber. In general, the discharging degree decreases with increasing discharging temperature. Similarly, the discharging degree decreases with increasing volume flow of the heat transfer fluid. Finally, the analysis of an exemplary charging process revealed that the pressure in the adsorber can increase significantly (> 50%) due to the desorption.
Open Access
PFG-NMR studies of ATP diffusion in PEG-DA hydrogels and aqueous solutions of PEG-DA polymers
(2018) Majer, Günter; Southan, Alexander
Adenosine triphosphate (ATP) is the major carrier of chemical energy in cells. The diffusion of ATP in hydrogels, which have a structural resemblance to the natural extracellular matrix, is therefore of great importance to understand many biological processes. In continuation of our recent studies of ATP diffusion in poly(ethylene glycol) diacrylate (PEG-DA) hydrogels by pulsed field gradient nuclear magnetic resonance (PFG-NMR), we present precise diffusion measurements of ATP in aqueous solutions of PEG-DA polymers, which are not cross-linked to a three-dimensional network. The dependence of the ATP diffusion on the polymer volume fraction in the hydrogels, φ, was found to be consistent with the predictions of a modified obstruction model or the free volume theory in combination with the sieving behavior of the polymer chains. The present measurements of ATP diffusion in aqueous solutions of the polymers revealed that the diffusion coefficient is determined by φ only, regardless of whether the polymers are cross-linked or not. These results seem to be inconsistent with the free volume model, according to which voids are formed by a statistical redistribution of surrounding molecules, which is expected to occur more frequently in the case of not cross-linked polymers. The present results indicate that ATP diffusion takes place only in the aqueous regions of the systems, with the volume fraction of the polymers, including a solvating water layer, being blocked for the ATP molecules. The solvating water layer increases the effective volume of the polymers by 66%. This modified obstruction model is most appropriate to correctly describe the ATP diffusion in PEG-DA hydrogels.
Open Access
Spectroscopic investigations of the magnetic anisotropy of lanthanide- and cobalt-based molecular nanomagnets
(2016) Rechkemmer, Yvonne; Slageren, Joris van (Prof. Dr.)
Single-molecule magnets are metal complexes exhibiting an energy barrier for spin reversal, leading to magnetic bistability and slow relaxation of the magnetization. Their potential for practical applications such as high-density magnetic data storage was recognized early on and with the goal of achieving high energy barriers, different kinds of single-molecule magnets have been synthesized. The quadratic dependence of the barrier height on the spin motivated chemists to synthesize metal complexes with very high total spins; however, with limited success. It was shown that high spins come along with low anisotropies and increased interest thus focused on the synthesis and investigation of (mononuclear) complexes of highly anisotropic metal centers, e.g. lanthanide or cobalt complexes. Although rather high energy barriers can be achieved in such systems, practical application remains problematic and has not been realized yet. Reasons are for example the lack of rational design criteria and the complex interplay of different magnetic relaxation pathways. The aim of this work was therefore the comprehensive magnetic and spectroscopic investigation of selected molecular lanthanide and cobalt compounds in order to obtain a deeper insight into the correlation of molecular and electronic structures as well as the corresponding magnetic properties. The applied spectroscopic methods included electron paramagnetic resonance spectroscopy, far-infrared spectroscopy and optical methods. Special emphasis was placed on magnetic circular dichroism (MCD) spectroscopy, which served as a main tool for electronic structure determination. However, since the MCD-spectrometer was not part of the available experimental equipment at the University of Stuttgart, its design, setup and characterization were the first part of this work. In the further course of this work MCD-spectroscopy was employed for the electronic structure determination of selected lanthanide and cobalt compounds. The studied lanthanide compounds were literature-known molecular tetra-carbonates of erbium (1-Er) and dysprosium (1-Dy). Detailed magnetometric studies showed that both 1-Er and 1-Dy are field-induced single-molecule magnets; however, 1-Er and 1-Dy show significant differences in their magnetic relaxation behavior. The magnetic studies were complemented by detailed spectroscopic investigations.The combination of far-infrared-, luminescence- and MCD-spectroscopy allowed for the experimental determination of 48 energy levels for 1-Er and 55 levels for 1-Dy, which built the foundation for the subsequent crystal field analysis and electronic structure determination. In addition, the results of EPR-spectroscopic studies were used for fine-tuning and verifying the respectively determined crystal field parameters. Calculating the magnetic dipole strengths for transitions between the relevant states led to a quantitative understanding of the magnetic relaxation pathways. Besides the investigation of lanthanide compounds, this thesis deals with two classes of cobalt complexes. The first class comprises mononuclear complexes in which one Co(II) ion is ligated by the nitrogen donors of two doubly deprotonated 1,2-bis(methanesulfonamido)-benzene-ligands. Rather acute N-Co-N bite angles indicate strong deviations from ideal tetrahedral symmetry. The static magnetic properties hint at very high energy barriers for spin reversal and with the help of far-infrared spectroscopy, largely negative axial zero-field splitting parameters were determined. The corresponding energy barriers belong to the highest ever reported for 3d-transition metal complexes and investigating the dynamic magnetic properties confirmed single-molecule magnet behavior. The unique magnetic properties were fully explained by analyzing spectroscopic results. The MCD-spectra showed intense signals that were assigned to spin-allowed d-d-transitions. Subsequent crystal field analysis revealed that the strong axial crystal field generated by the ligands leads to a large splitting of the electronic terms and thus in turn to a relatively small energy gap between the electronic ground state and the first excited state. The resulting increase in second-order spin-orbit coupling explains the high energy barriers observed in the studied complexes. The second class of cobalt compounds studied in this work included dimers of distorted octahedrally coordinated Co(II) ions bridged by symmetrical or asymmetrical quinone based bridging ligands. The main focus of investigation lay on the impact of the bridging ligand on the magnetic coupling between the cobalt centers. Thus, the magnetic properties of the complexes were studied with the help of static susceptibility and magnetization measurements and analyzed by means of different models. Depending on the bridging ligand, different signs for the exchange coupling constants were found. The varying signs can be explained by different relative contributions of possible exchange paths, influenced by the different substituents at the bridging ligands or slight geometry differences. The observations indicate that electron withdrawing substituents favor ferromagnetic couplings, which are preferred in the context of molecular magnetism. All in all, it can be concluded that this work provides a contribution to the deeper understanding of the features relevant for single-molecule magnets. The electronic structure determination for selected lanthanide and cobalt complexes applying advanced magnetometric and spectroscopic techniques not only led to an understanding of the static and dynamic magnetic properties but also allowed for the development of design criteria and new approaches for improved single-molecule magnets in the future.
Open Access
Entwicklung laserspektroskopischer Methoden zur Analyse der Verdunstungseigenschaften von Brennstofftropfen
(Stuttgart : Deutsches Zentrum für Luft- und Raumfahrt, Institut für Verbrennungstechnik, 2021) Werner, Stefanie; Riedel, Uwe (Prof. Dr. rer. nat.)
Die steigenden Emissionen des klimaschädlichen Treibhausgases CO2 durch die Verbrennung von fossilen, endlichen Energieträgern müssen möglichst schnell und nachhaltig reduziert werden. Ein vielversprechender Lösungsansatz zur Reduzierung der Schadstoffemissionen bei der Verbrennung liegt in dem Einsatz von alternativen und erneuerbaren Brennstoffen. Als Energieträger bieten sich auf Grund ihrer hohen Energiedichte vor allem flüssige Brennstoffe an. Diese werden typischerweise durch Druckzerstäubung in die Brennkammer eingebracht, verdunstet und dann mit dem Oxidationsmittel vermischt und verbrannt. Die Verdunstung der kleinen Brennstofftropfen des sogenannten Sprays ist von entscheidender Bedeutung für den Gesamtverbrennungsprozess in Verbrennungsmotoren und Gasturbinen. Im Allgemeinen bestimmt die Verdunstungsrate die Verbrennungsrate. Daher sind Modelle notwendig, die eine genaue Vorhersage der Brennstoffverdunstung ermöglichen. Zur Validierung dieser Modelle werden quantitative Messungen unter genau definierten Randbedingungen benötigt. Da die Prozesse in technischen Brennkammern sehr komplex sind, werden Experimente zur Tropfenverdunstung häufig mit linearen, monodispersen Tropfenketten durchgeführt, um die Kopplung zwischen den verschiedenen Effekten zu minimieren. Durch die geringe Größe der Tropfen (typischerweise wenige hundert Mikrometer oder weniger), erfordert die experimentelle Untersuchung eine hohe räumliche Auflösung. In dieser Arbeit wurden quantitative, laseroptische Messtechniken mit hoher räumlicher Auflösung zur experimentellen Untersuchung der Tropfenverdunstung an monodispersen Tropfenketten entwickelt. Mit den Messtechniken wurden Validierungsdaten für die Verdunstungseigenschaften von verschiedenen Brennstoffen bestimmt. Konzentrationsmessungen von verdunsteten Kohlenwasserstoffen wurden unter Verwendung von Infrarot-Laserabsorptionsspektroskopie und laserinduzierter Fluoreszenzspektroskopie (LIF) durchgeführt. Tropfenketten wurden mit einem Tropfenkettengenerator erzeugt, welcher vertikal in einem Strömungskanal installiert wurde. Die untersuchten Brennstoffe waren Cyclohexan, iso-Octan, n-Heptan, n-Pentan, 1-Butanol und Anisol. Der Strömungskanal wurde mit einer laminaren Luftströmung bei verschiedenen Temperaturen (313 K - 430 K) durchströmt. Da die untersuchten Tropfen einen Durchmesser in der Größenordnung von 120 bis 160 µm hatten und die Konzentrationsgradienten nahe der Tropfenoberfläche groß waren, war eine hohe räumliche Auflösung der Messtechniken erforderlich. Die Absorptionsmessungen wurden mit der Infrarotstrahlung eines HeNe-Lasers bei λ = 3,39 µm durchgeführt, um die CH-Streckschwingung der Kohlenwasserstoffe anzuregen. Die für die Quantifizierung der Brennstoffkonzentrationen benötigten Absorptionsquerschnitte wurden in einer beheizten Gaszelle für Temperaturen von 300 K - 773 K bestimmt. Die räumliche Auflösung im Strömungskanal betrug < 50 µm über eine Länge von 2 mm (Halbwertsbreite). Durch die Zylindersymmetrie und gute Stabilität der Tropfenketten konnten zeitliche Mittelungs- und Tomografieverfahren angewandt werden. Hierdurch konnten radiale Konzentrationsprofile an mehreren Positionen im Strömungskanal erhalten werden. Aus dem Anstieg der Dampfkonzentration an verschiedenen Messpositionen konnte die Verdunstungsrate bestimmt werden. Die Verdunstungsraten wurden in Abhängigkeit von der Mantelstromtemperatur (313 K - 430 K), der Tropfengeschwindigkeit (8 m/s - 23 m/s), der Tropfenerzeugungsfrequenz (12 kHz - 75 kHz) und dem Tropfenabstand (300 µm - 685 µm) gemessen. Im untersuchten Temperaturbereich steigt die Verdunstungsrate des Brennstoffs linear mit der Temperatur an. Die Reihenfolge der Brennstoffe in Bezug auf die Verdunstungsrate entspricht den Siedepunkten der einzelnen Brennstoffe. Da technische Brennstoffe häufig eine Mischung mehrerer Komponenten sind, ist die Untersuchung von Brennstoffgemischen von großem Interesse. Daher wurde ein Messverfahren entwickelt, um binäre Gemische zu untersuchen. Das Verfahren wurde verwendet, um eine Mischung aus Cyclohexan und Anisol zu untersuchen. Zwei Messtechniken - laserinduzierte Fluoreszenz (LIF) und Infrarot Absorptionsspektroskopie - wurden verwendet, um beide Spezies zu messen. Um λ = 3,39 µm ist der Absorptionsquerschnitt von Cyclohexan um etwa den Faktor 8 größer als von Anisol. Im untersuchten Fall war die Konzentration aufgrund des höheren Dampfdrucks ebenfalls deutlich größer. Daher konnte das Infrarot-Absorptionssignal praktisch ausschließlich Cyclohexan zugeordnet werden. Anisol hat bei Anregung bei λ = 266 nm eine sehr gute Fluoreszenzquantenausbeute, während Cyclohexan keine Fluoreszenz zeigt. LIF ermöglicht daher die Quantifizierung von Anisol (oder anderen Aromaten) ohne Interferenz durch Kohlenwasserstoffe. Es wurde ein Messverfahren entwickelt, welches Halationseffekte vermeidet, die typischerweise in planaren LIF-Experimenten an Tropfenketten auftreten. Kalibrationsmessungen, die im gleichen Strömungskanal durchgeführt wurden, ermöglichten die Quantifizierung der verdunsteten Anisolkonzentrationen. Die räumliche Auflösung betrug 80 µm. Ähnlich wie bei den Einzelkomponentenmessungen wurden Verdunstungsraten bestimmt. Wie aufgrund des niedrigeren Dampfdrucks zu erwarten, ist die Verdunstungsrate von Anisol niedriger als die von Cyclohexan. Die Verdunstungsrate von Cyclohexan in der binären Mischung stimmt gut mit den Einzelkomponentenmessungen überein. Das entwickelte Messverfahren ist sehr vielversprechend für weitere Untersuchungen an Mehrkomponentenmischungen. In dieser Arbeit konnte damit erstmals mit hoher räumlicher Auflösung die Verdunstung von Brennstoffkomponenten mittels Absorptionsspektroskopie in der Nähe von Brennstofftropfen untersucht werden. Zusätzlich wurden in Kombination mit laserinduzierter Fluoreszenzspektroskopie Messungen an binären Mischungen durchgeführt. Damit steht ein wertvoller Datensatz zur Validierung von numerischen Simulationen zur Verfügung.
Open Access
Monodisperse highly ordered and polydisperse biobased solid foams
(2018) Andrieux, Sébastien; Stubenrauch, Cosima (Prof. Dr.)
The aim of this work was the synthesis of monodisperse highly ordered biobased polymer foams and a comparison with their polydisperse counterparts. We used the biobased and biodegradable polymer chitosan, which we cross-linked with genipin. The polymer foams were synthesised via foam templating, i.e. via a liquid foam whose continuous phase contains a polymer and can be solidified. In order to obtain monodisperse highly ordered polymer foams, one first has to generate monodisperse highly ordered liquid foam templates. We did so by using microfluidics, which allows to produce monodisperse liquid foams with bubble sizes from 200 µm to 800 µm and polydispersities below 5%. The monodisperse foams were collected outside of the microfluidic channels and left to self-order under the influence of gravity and confinement. We studied the kinetics of the cross-linking reaction to find the optimal storage conditions during cross-linking. Once cross-linked we freeze-dried the gelled foams to obtain solid chitosan foams. We compared the morphological properties of the solid foams with those of the liquid templates in order to test the efficiency of the developed templating route. We observed how modifying the cross-linking and drying conditions can strongly affect the morphology of the solid foams. The main issue was to maintain the key properties of the liquid foam template throughout the solidification process, namely the bubble size distribution, the structural order and the density. We then compared the synthesised monodisperse polymer foams with their polydisperse counterparts. Although easy foaming methods exist for the generation of polydisperse foams, they do not allow the control over the polydispersity. We thus used microfluidics to generate liquid chitosan foams with tunable polydispersities from below 5% up to 26%. Microfluidics allows to match the average bubble size and density of the polydisperse liquid chitosan foam with those of the monodisperse counterpart. After solidifying the liquid templates we obtained solid foams with controlled polydispersities and studied the in uence of the polydispersity on the mechanical properties. However, we observed that not the polydispersity but the foam density was the main parameter at play. Moreover, the solid chitosan foams had weak mechanical properties with elastic moduli below 100 kPa. To overcome this issue, we incorporated cellulose nanofibres to the original chitosan solution and followed the developed route for foam templating. We had to adapt the microfluidic parameters to account for the viscosity changes brought about by the nanofibres. However, we managed to produce monodisperse liquid foams having the same bubble size, i.e. ~300 µm, but different amounts of cellulose nanofibres. The cellulose content had a strong influence on the solid foam morphology in general and on the pore connectivity in particular.
Open Access
Phase-space resolved decay rates of driven systems near the transition state
(2020) Feldmaier, Matthias; Main, Jörg (Prof. Dr.)
Die Bewegung einzelner Atome oder Moleküle bei chemischen Reaktionen lässt sich in vielen Fällen durch klassische Mechanik auf einer Born-Oppenheimer Potentialfläche beschreiben. Hierbei sind die Reaktanten oft durch eine Rang-1 Barriere von den Produkten getrennt. Eine solche Barriere ist durch einen instabilen Freiheitsgrad, die Reaktionskoordinate und eine gegebene Anzahl an stabilen Freiheitsgraden, die orthogonalen Moden, charakterisiert. Eine reagierende Trajektorie wird die Barriere meist in der Sattelregion, d. h. in einer direkten Umgebung des Sattels, überqueren. Diese Region fungiert als Flaschenhals für die Reaktion. Im Rahmen der Theorie der Übergangszustände (engl. transition state theory, TST) können Reaktionsraten über den Fluss reaktiver Trajektorien durch eine nur einmal durchstoßene Trennfläche (engl. dividing surface, DS) berechnet werden. Eine solche Trennfläche ist an der normal hyperbolischen invarianten Mannigfaltigkeit (NHIM) des Sattels verankert und trennt das System in Reaktanten und Produkte. Die NHIM ist dabei ein spezieller Unterraum des vollen Phasenraums und enthält Trajektorien, welche für alle Zeiten an die Sattelregion gebunden sind. Da diese Trajektorien somit weder zur Reaktanten- noch zur Produktseite gehören, bildet die NHIM einen Übergangszustand (engl. transition state, TS) der Reaktion. Für getriebene Systeme ist dieser zeitabhängig. In dieser Arbeit werden anhand eines zweidimensionalen, getriebenen Modellsystems mehrere Methoden zur Berechnung von NHIM und DS im Phasenraum vorgestellt. Basierend auf der Dynamik in einer direkten Umgebung der NHIM werden außerdem verschiedene Ansätze zur Berechnung des zugehörigen Zerfalls der Reaktantenpopulation nahe des TS diskutiert. Anschließend werden die vorgestellten Methoden auf ein realistischeres chemisches Modell angewandt, der getriebenen LiCN <-> LiNC Isomerisationsreaktion. Ein wichtiges Resultat hierbei ist, dass das externe Treiben dieses Systems einen großen Einfluss hat, sowohl auf die Dynamik von Trajektorien in der NHIM, als auch auf den zugehörigen Zerfall der Reaktantenpopulation nahe des TS.
Open Access
Calculation of pure substance and mixture viscosities using PCP-SAFT and entropy scaling
(Stuttgart : Universität Stuttgart, Institut für Technische Thermodynamik und Thermische Verfahrenstechnik, 2020) Lötgering-Lin, Oliver; Gross, Joachim (Prof. Dr.-Ing.)

Universität Stuttgart

Browse

Filters

Settings

Sort By

Results per page

Search Results