03 Fakultät Chemie

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Start date: 2021Subject: search.filters.subject.500

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    Magnetic Moment Tensor Potentials for collinear spin-polarized materials reproduce different magnetic states of bcc Fe
    (2022) Novikov, Ivan; Grabowski, Blazej; Körmann, Fritz; Shapeev, Alexander
    We present the magnetic Moment Tensor Potentials (mMTPs), a class of machine-learning interatomic potentials, accurately reproducing both vibrational and magnetic degrees of freedom as provided, e.g., from first-principles calculations. The accuracy is achieved by a two-step minimization scheme that coarse-grains the atomic and the spin space. The performance of the mMTPs is demonstrated for the prototype magnetic system bcc iron, with applications to phonon calculations for different magnetic states, and molecular-dynamics simulations with fluctuating magnetic moments.
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    Untersuchungen zur Derivatisierung und Charakterisierung Carben-analoger N-heterozyklischer Halogenarsane
    (2021) Bender, Johannes; Gudat, Dietrich (Prof. Dr. Dr.)
    Die Chemie neutraler N-heterozyklischer Arsane konnte durch Synthese einer Reihe von Verbindungen mit funktionellen Substituenten am Arsen (Halogeno-, Pseudohalogeno-substituiert) erweitert sowie strukturelle und elektronische Verhältnisse aufgeklärt werden. Aus 2-Chloro-1,3,2-Diazaarsolidinen und -1,3,2-Diazaarsolenen konnten einige neue kationische Arsen-Analoga von N-heterozyklischen Carbenen hergestellt werden. Des Weiteren konnten noch unbekannte 2-Thiolato- und 2-Xanthogenato-1,3,2-Diazaarsolidine und -1,3,2-Diazaarsolene dargestellt und charakterisiert werden.
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    Miscibility, viscoelastic reinforcement, and transport properties of blend membranes based on sulfonated poly(phenylene sulfone)s
    (2021) Saatkamp, Torben; Maier, Joachim (Prof. Dr.)
    Chemical energy that hydrogen may generate during combustion and the corresponding electrical energy are interconvertible by means of a fuel cell (FC) and by the electrolysis of water (WE), which allows for the utilization of the complementary nature of these two key energy vectors towards energy sustainability. A proton exchange membrane (PEM) made from an ionomer is commonly employed as the electrolyte in mobile fuel cell applications and in water electrolyzers that require dynamic operability and pressurized product gases. New PEM materials are needed to increase performance, reduce environmental impact, and allow for a more targeted design of PEMFC and PEMWE systems, all of which is in some way limited by the use of the established perfluorosulfonic acid (PFSA) type ionomers. This work’s focus lies on sulfonated poly(phenylene sulfone)s (sPPS), a unique group of fluorine-free cation conducting ionomers. They are unique in terms of their chemical stability and transport properties, however, typical in terms of their salt-like brittleness in the dry state and extensive swelling at high humidity and in water. To make the unique properties of sPPS available in application, the goal of this work is to take a comprehensive approach to their viscoelastic reinforcement. Therefore, the structure of this thesis entails three related aspects along the process from pure materials to the optimization of robust PEMs for application. The first chapter focuses on the optimization of the intrinsic viscoelastic properties of a particularly suited sPPS (termed S360, with IEC 2.78 meq g-1, EW 360 g mol-1) which lays the groundwork for reliable and systematic further development. To achieve this, relevant properties of S360 are first characterized and viscoelastic shortcomings as seen in water uptake measurements and tensile tests under dry conditions (≤ 30% relative humidity, RH) discussed. The step-growth polymerization of S360 is optimized after finding significant inorganic contamination retained in the established purification process of the widely used monomer sulfonated difluorodiphenyl sulfone (sDFDPS), allowing for the preparation of the ionomer in reproducible high molecular weight. Relevant properties of high molecular weight S360 are characterized and an enhancement of mechanical properties at 30% RH as well as when submerged in water is found. Access to reproducible high quality of S360 enables its first-time use and study as a PEM in a completely fluorine-free WE cell. At 80 °C, record performance amongst fluorine free electrolytes in PEMWEs of 3.48 A cm-2 at 1.8 V is achieved, showcasing the potential of sPPS for application. The second chapter entails the identification and better understanding of a suitable and versatile reinforcement concept for creating robust membranes based on sPPS. To achieve this, the established homogeneously miscible acid-base polymer blends of sulfonated ionomers with poly(benzimidazole) (PBI, and its derivatives PBIO and PBIOO) are discussed in-depth and chosen for later systematic optimization in combination with sPPS. Since the origin of miscibility in PBI blends with sulfonated ionomers is insufficiently described in literature and could facilitate targeted design of new blend components, a model acid-base polymer blend system comprising pyridine-functionalized poly(sulfone) (PSU) is created. Pyridine groups of different basicity tethered to PSU in varying concentration are used to investigate the effect that interpolymer acid-base interaction strength and concentration have on miscibility in blends with 80 wt% S360, as derived from the blend membranes’ cross-sectional SEMs. High mutual compatibility is achieved at high concentration of weak interpolymer interaction, which is interpreted with regards to the observed miscibility in PBI blends. Based on the derived role that hydrogen bonds may play in PBI blends, the difference of interpolymer interaction in solution (during membrane formation) and in the dry membrane is described. This could enable the development of new blend concepts in the future. An exemplary miscible blend that comprises interpolymer hydrogen bonds only in solution but not in the final membrane is shown. The third chapter describes the optimization and balance of properties in the previously described polymer blends with PBIO, following the goal to prepare membranes which can be evaluated in fuel cells and fabricated on a wider scale in order to bring the attractive properties of sPPS into application. To achieve this, S360-blend membranes of varying PBIO content are characterized with regard to conductivity and mechanical properties in various conditions. High mechanical robustness is achieved in S360 blends with 30 wt% PBIO but is accompanied by dramatic reduction of conductivity, due to the charge-consuming acid-base interaction. The findings are translated into blends with fully sulfonated sPPS (termed S220, with IEC 4.54 meq g-1, EW 220 g mol-1) which allows for the creation of membranes that combine mechanical toughness with high conductivity at a ratio of 25 wt% PBIO in S220, making the material suited for production on a commercial casting line and fuel cell testing. Membranes based on S360 that comprise 15 wt% PBIO are designated for further studies in PEMWEs, where membrane requirements differ significantly from that in PEMFCs, highlighting the versatility of the reinforcement approach chosen in this work. Finally, first fuel cell tests of thin spray coated PBIO blend membranes are conducted, and initial durability testing of sPPS-based membranes in fuel cells is possible. Overall, the results presented in this work are strongly interrelated which underlines the importance of comprehensiveness in the successful viscoelastic reinforcement of sulfonated poly(phenylene sulfone)s. Ultimately, the blend membranes resulting from this work can be used as a platform for further development of sPPS-based PEMs in the future.
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    Probing molecular quantum bits
    (2021) Lenz, Samuel; Slageren, Joris van (Prof. Dr.)
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    Characterization and optimization of sulfurized poly(acrylonitrile) cathodes and silicon anodes
    (2023) Niesen, Stefan; Buchmeiser, Michael R. (Prof. Dr.)
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    Extracting thermodynamic information from local composition fluctuations in solids : extended theory and its application to simulated and experimental atom probe data
    (2024) Zheng, Jianshu; Schmitz, Guido (Prof. Dr. Dr. h.c.)
    In case of liquids, thermodynamic fluctuation theory has been applied for decades to obtain direct thermodynamic information (e.g. miscibility gap, mixing/demixing tendencies, critical solution temperature) from local composition fluctuations. Recently, this theory has been extended to solids by introducing an additional elastic work term between the evaluated sub-system and the entire system, which does not arise in liquids. This extended theory has been verified via atomistic simulations in an exemplary Cu-Ni embedded-atom system using Monte Carlo simulations at a fixed temperature over the entire composition range. Composition fluctuations in the system that are represented by the relative variance of the composition histogram are tracked in various-sized subvolumes over time, revealing a systematic dependence on the size of the evaluation volume due to interface effects. Nonetheless, these surface effects can be excluded by extrapolation to an infinitely large subvolume, leading to perfect agreement with the prediction by the extended theory. Thus, the recovery of the Gibbs free energy of mixing from evaluation of the fluctuations is possible also in the case of solids. Atom Probe Tomography (APT) delivers combined high-resolution chemical and sub nanometric three-dimensional (3D) spatial information, and is therefore the perfect technique to determine local composition fluctuations by using spatial frequency distribution analysis in practical applications. In this work, the applicability of the extended theory is tested on the Cu-Ni alloy and ionic CuO systems via frequency distribution analysis on simulated and experimental atom probe data, and eventually compared to available phase diagram data, thereby proving the validity of extracting the Gibbs free energy from local composition fluctuations in solids. In the first part of this work, the spatial frequency distribution analysis is applied to simulated crystals of long-range ordered L12 and monoclinic structures numerically modeled disregarding thermodynamic interaction between atoms. The relative variance displays an evaluation size dependence, but goes to zero (i.e. no composition fluctuations) if extrapolated to sufficiently large evaluation size. This result meets the expectation as no composition fluctuations should be found in perfectly ordered materials. In the second part, this approach is applied to simulated alloys including thermodynamic interactions. Cu-Ni alloys of various compositions are firstly equilibrated using a Monte Carlo simulation with an embedded-atom potential. Afterwards, the alloys are numerically field-evaporated by the evaporation simulation package TAPSim and the 3D coordinates of the field-evaporated sample are recovered through the usual reconstruction algorithm. Throughout this process, two practical considerations related to the atom probe technique have been effectively addressed: i) The newly developed model tackles the challenges associated with the limited detection efficiency and allows the reconstruction of the relative variance for the bulk system from limited atom probe data scaled by detection efficiency; ii) An additional correction term which is proportional to the evaluation size and magnitude of composition inhomogeneity is introduced. It enables the separation of thermodynamic fluctuations from artificial composition variations inherent in the experimental method based on their different size dependence, so that the extrapolation still recovers the intrinsic thermodynamic composition fluctuations. In the third part, this approach is finally applied to experimental atom probe data. The Cu-Ni alloys are prepared by induction melting of pure Cu and Ni and CuO thin films are prepared via ion beam sputtering. After sufficient equilibration by heat treatment, Cu-Ni and CuO specimens for the APT measurement are fabricated via focused ion beam cutting. By experimentally conducting the same approach as developed theoretically, local composition fluctuations are obtained for both Cu-Ni and CuO systems. After the elastic work term correction, the CALPHAD-style parametrization of the Gibbs free energy is obtained by linking it to the measured local composition fluctuations. In this way, the Cu-Ni miscibility gap is successfully reconstructed from data measured at elevated temperature (800 K), and the resulting phase diagram is in agreement with the CALPHAD results in literature. The frequency distribution analysis of the reconstructed CuO tends to approach the binomial distribution (i.e. behavior of random alloys), since field evaporation of molecules (e.g. CuO, Cu2O) but not only single ions destroys the long-range order structure and deteriorates the resolution in the reconstruction. This effect indicates the partial limitation of this method on ionic compounds. In summary, the present work has systematically extended and proven the application of the composition fluctuation theory to metallic alloys, and makes it possible to directly access thermodynamic information from local composition fluctuations. APT is demonstrated as a new technique to extract direct thermodynamic information, and a general route from the APT measurement to the Gibbs free energy is presented. Given that the composition fluctuation is a local property and only a substantially short diffusion length for equilibration is required, this represents an efficient methodology especially for systems where slow diffusion hinders the establishment of large scale thermodynamic equilibrium. APT, as a sub-nanometric resolution technique, promises to extract more accurate thermodynamic information in a wider temperature composition range. Besides, this study advances our understanding of the size dependence in the traditional frequency distribution analysis. It is pointed out that potential misinterpretation could happen and is presented in literature, if a sample evaluation size in the frequency distribution analysis is arbitrarily chosen. Only the bulk relative variance obtained via extrapolation to infinitely large sub-system is thermodynamically meaningful.
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    Deposition and characterization of multi-functional, complex thin films using atomic layer deposition for copper corrosion protection
    (2022) Dogan, Gül; Schütz, Gisela (Prof. Dr.)
    This thesis focuses on ALD thin film protection properties against corrosion of copper to develop an understanding of material interface properties and to develop novel thin films processes. This understanding is then applied to enhance materials with potential use in semiconductor devices. The main research objectives are listed below: Understanding corrosion protection properties of ALD thin films: - Development of protective thin films by combining different oxide layers - To characterize the protection properties at high temperatures and in aggressive environments, - To understand the interaction of copper and ALD protection layers when exposed to high temperatures, - Finding the optimum deposition parameters to achieve defect-free thin layers for best corrosion protection Application of ALD oxide thin films for copper corrosion protection in semiconductor devices: - Structuring the ALD thin films to make reliable interface for copper-copper interconnects with micromachining methods such as laser drilling and plasma etching - To remove ALD layers in a localized, selective way without degradation of the underlying copper layer
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    Atom probe study on CuNi thin films : miscibility gap and grain boundary segregation
    (2023) Duran, Rüya; Schmitz, Guido (Prof. Dr. Dr. h. c.)
    In dieser Arbeit wurde die Lage der Mischungslücke, und die Korngrenzsegregation im Legierungssystem, Kupfer-Nickel, per Atomsondentomographie (APT) analysiert. Zur Untersuchung der Mischungslücke eines binären Systems mit langsamer Diffusion wurde ein neues Verfahren verwendet. Multilagen aus Cu- und Ni- Dünnschichten wurden mittels Ionenstrahlbeschichtung (IBS) auf Wolframpfosten beschichtet und durch fokussierte Ionenstrahlung (FIB) geformt. Bei drei unterschiedlichen Temperaturen, zwischen 573 und 673 K, wurden isotherme Auslagerungssequenzen an einem Ultrahochvakuumofen (UHV) durchgeführt und der Mischungsprozess analysiert. Ein Modell des Diffusionsprozesses wurde mittels mathematischer Überlegungen erstellt. Durch das Fitten der experimentellen Kompositionsprofile mittels dieses Modells konnten die Gleichgewichtskonzentrationen der Schichten auch mit relativ kurzen Auslagerungszeiten ermittelt werden. Darüber hinaus konnten aus den diffusionskontrollierten Zeit- und Temperaturdaten physikalische Eigenschaften wie der effektive Diffusionskoeffizient (Gitterdiffusion einschließlich Defektdiffusion) bestimmt werden. Dieser betrug Deff = 1.86 ∙ 10-10 m2/s ∙ exp(-164 kJ mol-1/RT). Während dem Vermischen wurde die Änderung der multilagigen Mikrostruktur bis zur vollständigen Mischung bei 623 und 673 K beobachtet, wobei Korngrenzen als schneller Diffusionsweg eine wichtige Rolle spielen. Bei 573 K wurde Nichtmischbarkeit experimentell deutlich nachgewiesen, wobei die Phasengrenzen bei cNi=26 at.% und cNi=66 at.% liegen. Mit diesen Phasengrenzen wurde die Mischungslücke über eine Redlich-Kister-Parametrisierung der Gibbs‘schen freien Energie über den gesamten Konzentrationsbereich rekonstruiert. Hierin wurde für die kritische Temperatur, TC, 608 K bei einer Konzentration von 45 at% Ni gefunden. Im zweiten Teil wurde die Korngrenzsegregation durch die FIB/tEBSD- (Transmissions-Elektronen-Rückstreubeugung) Technik, in Korrelation zu APT-Messung charakterisiert. Vier Legierungen mit einem Ni-Anteil zwischen 25 und 85 at.% wurden auf Wolframpfosten per IBS beschichtet, und bei 700 K für 24 h wärmebehandelt. Die Segregation von Cu in die Korngrenzen wurde beobachtet. Durch die Verwendung eines theoretischen Models wurde die Exzess-Kurve über den gesamten Konzentrationsbereich, und die Korngrenz-Formationsenergie auf Basis der experimentellen Daten berechnet. Die tEBSD-Analyse während der FIB-Präparation erlaubt die Identifikation der Körner und deren Orientierung. Ein neues Verfahren wurde entwickelt, um mithilfe der Orientierung benachbarter Körner, Berechnungen zur Ermittlung der Korngrenzorientierung durchzuführen und somit die Orientierung natürlicher Korngrenzen zu bestimmen. Mit diesem Verfahren konnte der zeitliche Aufwand dieser anspruchsvollen Auswertung (verglichen zur herkömmlichen Methode mittels TEM-Untersuchung) stark reduziert werden, so dass eine quantitative Analyse vieler Korngrenzen möglich wurde. Aus den einzelnen Korngrenzorientierungen wurde die Korngrenzrotation, und die jeweiligen Anteile an Kippung und Drehung berechnet. Eine Abhängigkeit der Feststoffsegregation vom Kipp- und Drehanteil der Korngrenze wurde beobachtet, die am kleinsten für die reine Kipp- und Drehrotation war. Die ermittelten Segregationsweiten sind signifikant größer als die strukturellen Korngrenzweiten und bewegen sich zwischen 12 und 85 Å. Dieses Verhalten wurde durch eine künstliche Verbreiterung der Korngrenze erklärt, die durch eine Flugbahnabweichung der Korngrenzatome während der Verdampfung verursacht wurde. Eine Korngrenzweite von w0 = (10.1 ± 1.5) Å wurde für eine unverfälschte Korngrenze gefunden.