Universität Stuttgart
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Item Open Access Precipitation of nitrides in iron-based binary and ternary alloys; influence of defects and transformation-misfit stresses(2015) Akhlaghi, Maryam; Mittemeijer, Eric Jan (Prof. Dr. Ir.)The initial microstructure of the unnitrided specimen has a significant influence on the nitriding response of binary Fe-Me (Me: Mo or Al) alloys specimens. This effect was not investigated until now for the case of nitrided ternary Fe-Me1-Me2 alloys, the role of the initial microstructure was studied upon nitriding Fe-4.1 at.% Cr-7.9 at.% Al specimens. To this end, the recrystallized and cold-rolled specimens were nitrided at low nitriding temperature of 400 °C. Upon precipitation of misfitting coherent nitrides during nitriding of thin-foils of binary Fe-Me (Me: Cr and V) alloys, a hydrostatic tensile lattice-stain component results from the elastic accommodation of volume misfit of nitrides and ferrite matrix. The change of the ferrite-matrix lattice parameter can be traced upon precipitation of the nitrides by X-ray diffraction measurements. The theory originally developed for the case of imperfections (by Eshelby) in solids can be applied for quantitatively describing the lattice-parameter changes of the matrix, the nitrides and the aggregate (matrix+ nitrides) as function of volume fraction and type of nitrides.Item Open Access Formation of lath martensite(2015) Löwy, Sarah; Mittemeijer, Eric Jan (Prof. Dr. Ir.)In this thesis the formation of different lath martensites was investigated upon cooling, particularly with regard to the mechanisms contributing to the transformation process. Upon very slow cooling of different Fe-Ni alloys and a maraging steel, all forming lath martensite, a discontinuous transformation behaviour was observed. This modulation of the transformation rate is ascribed to the interplay of chemical driving force, developing strain energy and its relaxation upon slow cooling. It is proposed that the modulation is caused by simultaneous formation of blocks in different martensite packages. Additionally, the influence of the Ni content on the transformation behaviour is presented as well as the influence of an externally applied force.Item Open Access Manganese-based cathode materials for Li-ion batteries(2015) Surace, Yuri; Weidenkaff, Anke (Prof. Dr.)Li-ion batteries are one of the most commercialized solutions to store electrochemical energy, but until now their broad use is limited to small electronic devices. Higher specific energy and longer cycle life are needed to open the way to a broader range of applications (i.e. electric vehicles or stationary batteries). The specific energy of Li-ion batteries is a function of the anode and cathode capacity for lithium intercalation and the cell voltage. However, capacity and voltage of current state-of-the-art cathode materials are the main specific energy-limiting factors of Li-ion batteries. For this reason, much of the attention during the past few years focused on cathode materials with either high voltage or high capacity or in the best of all cases both, coupled with high stability. Manganese is one of the most common transition metals used in battery materials due to its multiple (and at least partially accessible) oxidation states, its low toxicity and its high availability. Mn-based cathode materials benefit from the Mn3+/Mn2+ or Mn4+/Mn3+ redox couples which allow obtaining a potential range between 3.0 V and 4.2 V vs Li+/Li depending on the crystal structure and the chemical composition. The aim of this work was to study unexplored and scarcely explored Mn-based cathode materials and to improve their electrochemical performances through structural, morphological and chemical modifications. In the initial part of the thesis, a study of calcium manganite Ruddlesden-Popper phases Ca2MnO4 was carried out. Although the pristine material was not electrochemically active, Ca2MnO4 was activated for Li intercalation by Ca extraction using a novel and simple treatment with sulphuric acid. The influence of the amount of Ca extracted, and of the particle size were studied and correlated with the electrochemical properties. It was proposed that the acid treated materials had a bi-functional crystalline-amorphous structure, composed by a Ca2MnO4 crystalline bulk phase for the stability and an amorphous MnO2•xH2O surface for the electrochemical response. For each 25at% of calcium extracted, capacities of 40 Ah/kg and 55Ah/kg were obtained for micron-sized particles and for nano-sized particles, respectively. A stability improvement of a factor of 10 was reached in comparison to bare amorphous hydrated manganese oxide. The work focused then on Li3MnO4, a lithium rich phase containing manganese (V). Developing a novel freeze drying (FD) synthesis-route, the micro- and nanostructure of the material were modified with relevant consequences on the electrochemical properties. Smaller particles size in conjunction with smaller grains size allowed obtaining a first discharge capacity of 290 Ah/kg with an improvement of up to 31%, in comparison to Li3MnO4 synthesized by the solid state route. Moreover, measurements carried out at different cycling rates showed improvements in rate capability. In addition, this new route allowed reducing the re-action temperature and time. However, considerable modifications in the Li3MnO4 structure occurred during the first cycle and the capacity improvement vanished after a few cycles due to structural instability of this material under cycling. To gain deeper insight into the reason of the capacity fading of this material, a post mortem analysis was carried out which allowed to create a model for the degradation mechanism. Briefly, the lithium extraction or insertion in the structure caused the amorphization of the material with conversion to the more stable amorphous manganese oxide. In the last part of this thesis, preliminary studies on lithium manganese borate LiMnBO3 were carried out. It was shown in a proof of concept study that the FD synthesis was applicable for this material as well. Nanocrystalline material was obtained with electrochemical performance comparable to the state of the art by gaining in synthesis simplicity.Item Open Access Kinetics of phase transformations(2015) Rheingans, Bastian; Mittemeijer, Eric Jan (Prof. Dr. Ir.)In this thesis, the kinetics of heterogeneous solid-state phase transformations in different prototype experimental systems are investigated with focus on the development of new strategies for kinetic modelling using mean-field kinetic models. Topics cover the interrelation between the kinetic model description and the amount of available experimental information, the interpretation of kinetic model parameters determined upon model fitting and the coupling of kinetic models to external (thermodynamic) input data. Experimental studies include the crystallisation kinetics of metallic glasses and precipitation kinetics in supersaturated alloys.Item Open Access Template controlled mineralization of functional ZnO thin films(2017) Blumenstein, Nina; Bill, Joachim (Prof. Dr.)In this thesis, the influence of different organic templates on the bioinspired deposition of ZnO thin films is investigated. Depending on the polarity of the templates, the growth and the properties of the films can be influenced. On a non-polar template, film growth is inhibited whereas homogeneous films grow on polar templates. Additionally, it was shown that on a template with high polarity a crystallographic texture is observed. This leads to a macroscopically measurable piezoelectric response of these samples. In the last part of this work, the incorporation of Al, Ga and In into the ZnO films was investigated. Measurements showed a blue shift of the UV photoluminescence emission and an improved electrical conductivity with increasing doping content.Item Open Access Thermodynamics and kinetics of the oxidation of amorphous Al-Zr alloys(Stuttgart : Max-Planck-Institut für Intelligente Systeme (ehemals Max-Planck-Institut für Metallforschung), 2015) Weller, Katharina; Mittemeijer, Eric J. (Prof. Dr. Ir.)The present thesis presents a comprehensive investigation of the thermal oxidation of amorphous Al-Zr alloys. The oxide composition and microstructure, as well as the oxidation kinetics and oxidation mechanism of amorphous Al-Zr alloys upon thermal oxidation at relatively low oxidation temperatures of 350 - 400 °C and at high oxidation temperatures of 500 - 560 °C have been investigated. The phase and microstructural development upon oxidation of amorphous Al-Zr alloys, as well as the oxidation-induced changes in the alloy substrate, were investigated by a combinatorial experimental approach using X-ray diffraction (XRD), cross-sectional (analytical) transmission electron microscopy (TEM), Auger electron spectroscopy (AES) sputter-depth profiling and spectroscopic ellipsometry (SE). Furthermore, the corrosion behavior of as-deposited and oxidation-treated amorphous Al-Zr alloys have been investigated as function of the alloy composition and oxidation conditions by employing an electrochemical microcapillary technique.Item Open Access Synthesis and thermoelectric properties of chalcogenide and half-Heusler phases(2018) Zou, Tianhua; Weidenkaff, Anke (Prof. Dr.)Item Open Access Light emission from lanthanide-doped AlN and AlxIn1-xN layers(2015) Yang, Miao; Schmitz, Guido (Prof. Dr. Dr. h.c.)Optical properties of lanthanide doped phosphors and semiconductors are of great interest especially in view of solid-state light emitting devices of the next generation. Radiative intra-4f electron transitions of trivalent lanthanide ions present sharp and well-defined emission lines at the wavelengths from UV to IR part of the spectrum. However, their intensity is comparatively weak hitherto. In the scope of this dissertation, wide band gap III-nitride semiconductors, AlN and AlxIn1-xN, are chosen as host materials and doped with Pr, Sm, Tb and Tm ions respectively. According to our elaborate optoelectronic, structural and compositional characterisations, we attempt to find an innovative guideline how to obtain the lanthanide luminescence and increase their intensity. In the light of crystal field (CF) theory, electrostatic perturbation with a non-central symmetry on the lanthanide ions is a fundamental requirement for the selection of host material to achieve their intra-4f transitions. Splitting of low-temperature photoluminescence (PL) peaks from AlN:Sm and AlN:Tb layers reveals that most of the radiative lanthanide ions are substitutionally located in a C3v local symmetry. Physical and structural information of this architecture in AlN:Sm system (namely the effective point charge of four surrounding N ions felt by the 4f-electrons of Sm ion and their spatial positions related to 4f-electron orbital of Sm ion) is determined through our preliminary efforts by using computer-assisted fitting procedure. Once the host material is defined, intensity of the lanthanide luminescence can be “extrinsically” enhanced by three ways. (1) Appropriate thermal treatment is a conventional technique for this purpose. The atomic rearrangements activated thereby can be considered as a joint reaction of first order. By introducing a concept of “extended lanthanide luminescence centres” we are able to simplify the description of this intensity enhancement in AlN:Ln system with only two thermodynamic and kinetic parameters. From the PL spectra of AlN with and without lanthanide doping, we confirm that the luminescence generated by carrier recombination within O-associated defect states can be strengthened particularly after annealing at intermediate temperatures (300 - 600 °C). The peak of this defect luminescence covers the required energy for the excitation of lanthanide luminescence. Both of them exhibit therefore a correlated development. This result enables us to assist the excitation processes of lanthanide luminescence centre through (2) utilising available defect states within the band gap. Inspired of this, lanthanide luminescence can be further intensified by (3) establishing additional excitation path via engineering the band structure of host material. This method is proven to be effective in Al0.87In0.13N:Tm and Al0.84In0.16N:Pr systems. Due to decomposition at proper temperature, we observe an almost instantaneous formation of nano-sized In-rich AlxIn1-xN quantum dots (QDs) with subsequent comparatively slow coarsening. This coarsening permits us to modify the band gap energy of QDs by altering their size, which is a function of the annealing temperature and duration. If this band gap energy is in resonance with the 4f-levels to be excited in the lanthanide ions, luminescence intensity increases. An elaborate model, relating thermal formation of “extended lanthanide luminescence centres”, time-dependent variation of band gap energy and resonant energy transfer, can sufficiently describe the development of lanthanide luminescence intensity during the annealing.Item Open Access Hydrogen transport in thin films : Mg-MgH2 and Ti-TiH2 systems(2018) Hadjixenophontos, EfiHydrogen 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.Item Open Access Synthese und Hochtemperaturverhalten von nahezu stöchiometrischem SiC aus molekularen Vorstufen(2016) Lehmann, Tobias; Bill, Joachim (Prof. Dr.)Die Herstellung von Siliciumcarbid aus polymeren Vorstufen (Precursoren) ist eine vielversprechende Herstellungsmethode zur Erzeugung von Bauteilen, die hohen Temperaturen ausgesetzt sind. Allerdings ist für deren Herstellung zunächst ein grundlegendes Verständnis über den Zusammenhang zwischen der Molekularstruktur der Vorstufen mit deren Reaktionsausbeute, Konsistenz, Pyrolyseverhalten sowie mit dem Aufbau und Kristallisationsverhalten der korrespondierenden amorphen Materialien unerlässlich. Bisher existieren dazu keine systematischen Untersuchungen. Weiterhin weist das nach der Pyrolyse der Precursoren erhaltene Material nach dem derzeitigen Stand der Forschung einen Kohlenstoffüberschuss auf, der dessen Oxidationsbeständigkeit verschlechtert. Die Voraussetzung zur Verbesserung der Oxidationsbeständigkeit ist daher die Reduktion des Kohlenstoffgehalts in den nach der Pyrolyse erhaltenen Materialien. Ziel dieser Arbeit war es deshalb, eingehend den Einfluss der Molekularstruktur der Precursoren auf die Verarbeitbarkeit, das Kristallisationsverhalten der Pyrolysate, den Aufbau des amorphen und kristallinen Zustandes sowie dem Oxidationsverhalten der Keramik zu untersuchen. Die polymeren Precursoren wurden aus chlorhaltigen Monosilanen synthetisiert. Die statistische Auswertung der Ergebnisse zeigte in Kombination mit spektroskopischen Methoden, dass niedrige Molekularmassen zu hohen Reaktionsausbeuten führten. Höhere Molekularmassen führten hingegen zu höheren keramischen Ausbeuten und viskoelastischem Fließverhalten. Kleinere Moleküle konnten mit einer höheren Dichlorodimethylsilan-Konzentration erhalten werden, während eine höhere Phenyltrichlorosilan-Konzentration eine Erhöhung der Molekularmasse bewirkte. Eine noch stärkere Erhöhung der Molekularmasse konnte mit Dichloromethylvinylsilan erreicht werden, da über die Vinylgruppen sowohl während der Synthese als auch während der Pyrolyse Vernetzungsreaktionen stattgefunden haben. Die Zusammensetzung der erhaltenen Pyrolysate lag im Bereich von SiC(1,04) bis SiC(2,63), wobei der Kohlenstoffgehalt direkt mit dem der eingesetzten Monosilane korrelierte. Ein größerer Anteil an Dichlorodimethylsilan sowie Dichloromethylvinylsilanin den Polysilanen führte zu niedrigen Kohlenstoffgehalten, während höhere Phenyltrichlorosilan-Konzentrationen hohe Kohlenstoffgehalte bewirkten. Die statistische Auswertung ermöglichte es, den quantitativen Einfluss der einzelnen Monosilankonzentrationen auf die Reaktionsausbeute und die keramische Ausbeute der Precursoren sowie auf den Kohlenstoffgehalt der aus den Precursoren erhaltenen amorphen Materialien zu ermitteln. Dadurch konnten gezielt unterschiedliche Kohlenstoffgehalte realisiert und deren Einfluss auf das Oxidations- und Kristallisationsverhalten der amorphen Pyrolysate untersucht werden. Für die Untersuchung des Kristallisationsverhaltens wurden die Proben isotherm bei unterschiedlichen Temperaturen ausgelagert und mittels Röntgendiffraktometrie untersucht. Ein höherer Kohlenstoffgehalt hatte eine höhere Kristallisationstemperatur zur Folge. Oberhalb ihrer jeweiligen Kristallisationstemperatur kristallisierten die Proben so schnell, dass keine Keimbildung beobachtet werden konnte. Der Kohlenstoffgehalt hatte einen starken Einfluss auf das Kristallitwachstum. Ein hoher Kohlenstoffgehalt führte zu einem matrixkontrollierten Wachstum mit einer Aktivierungsenergie von 195 4 kJ/mol . Das Kristallitwachstum bei der nahezu stöchiometrischen Zusammensetzung war dagegen grenzflächenkontrolliert. Die Aktivierungsenergie betrug 420 38 kJ/mol . Unterstützt wurden diese Aussagen mit Aufnahmen der hochauflösenden Transmissionselektronenmikroskopie. Bei hohen Kohlenstoffgehalten waren die SiC-Kristallite in eine kohlenstoffreiche Matrix eingebettet, während bei niedrigen Kohlenstoffgehalten die SiC-Kristallite von dünnen Kohlenstoffschichten umgeben waren. Diese Schichten sind für das grenzflächenkontrollierte Wachstum verantwortlich. Es wurden die drei Pyrolysate SiC(1) (33,0 Masse% Kohlenstoff), SiC(2) (37,0 Masse% Kohlenstoff) und SiC(3) (46,6 Masse% Kohlenstoff) oxidiert. Es zeigte sich, dass die Oxidationsbeständigkeit der Proben abhängig vom Kohlenstoffgehalt ist. Je höher der Kohlenstoffgehalt war, desto stärker nahm bei einer Temperatur von 860 ° C und einem Sauerstoffpartialdruck von 0,2 bar der Sauerstoffgehalt der Proben zu und desto stärker nahm die Masse der Proben ab. Bei SiC(1) findet passive Oxidation statt. Sowohl der Sauerstoffgehalt als auch die Masse sind nach der Oxidation im Vergleich zu den Pyrolysaten erhöht. SiC(3) oxidiert aktiv. Während der Oxidation wandeln sich die Pyrolysate vollständig in SiO2 um. Die Bildung einer geschlossenen SiO2-Schutzschicht wird nicht beobachtet. Der bei SiC(3) beobachtete Masseverlust kann nur mit der Abnahme des Siliziumgehaltes um 40% im Vergleich zu dem Pyrolysat erklärt werden. Durch den hohen Kohlenstoffgehalt reagiert der Großteil des eindiffundierenden Sauerstoffs zu CO/CO2. Das führt zu einer Reduktion des Sauerstoffpartialdrucks. Deshalb erfolgt aktive Oxidation unter Bildung von gasförmigem SiO. Es kann sich keine SiO2-Schutzschicht bilden. Dadurch kann der Sauerstoff weiter in das Material diffundieren und gasförmiges SiO aus dem Materialentweichen. Die Ergebnisse der Oxidation von SiC(2) liegen zwischen denen von SiC(1) und SiC(3). Der Anstieg des Sauerstoffgehaltes der oxidierten Proben ist in SiC(2) höher als in SiC(1). Zusätzlich wird eine Massenabnahme der Proben nach der Oxidation im Vergleich zu den Pyrolysaten bei SiC(2) beobachtet. Wegen des höheren Kohlenstoffanteils in SiC(2) kann sich in diesem Material mehr CO/CO2 bilden als in SiC(1). Dadurch ist der Sauerstoffpartialdruck zu Beginn der Oxidation in SiC(2) niedriger als in SiC(1). Demgemäß erfolgt in SiC(2) zunächst aktive Oxidation. Durch das entweichende CO/CO2 wird der Kohlenstoffgehalt an der Oberfläche des Materials niedriger. Dadurch steigt der Sauerstoffpartialdruck. Anders als in SiC(3) erfolgt nun passive Oxidation und es bildet sich eine SiO2-Schutzschicht.