Universität Stuttgart

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

Browse

Filters

2015 - 2019142020 - 202510

Settings

Start date: 2015Subject: search.filters.subject.540Publication Type: search.filters.UBSTyp.DissertationInstitute: search.filters.UBSInstitut.Institut für Materialwissenschaft

Search Results

Now showing 1 - 10 of 24
  • Thumbnail Image
    ItemOpen 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.
  • Thumbnail Image
    ItemOpen 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.
  • Thumbnail Image
    ItemOpen Access
    Growth of oxide materials with Ruddlesden-Popper- and garnet-type structures via the optical float zone method: investigations of scintillation, optical, and magnetic properties, and post-growth modifications of single crystals via topochemical routes
    (2025) Yilmaz, Hasan; Clemens, Oliver (Prof. Dr.)
    With the continuous progress in materials science, the global market demand for high-performance functional materials, especially those with customized optical and magnetic properties, has increased significantly due to their critical role in next-generation technologies such as photonic devices, data storage systems, quantum computing, and biomedical applications. With this motivation, this dissertation is focused on the growth of high-quality oxide single crystals with Ruddlesden-Popper (RP) and garnet-type structures using the optical floating zone (OFZ) method and the detailed investigation of their scintillation, optical and magnetic properties. Furthermore, the use of chemical doping and post-growth topochemical modifications as strategies to modify material properties and stabilise metastable structures is also covered. With these approaches, we can classify this study into two parts: optical and magnetic properties. In the first part of this work, RP-type n = 1 LaSrGaO4 (LSGO) and Garnet-type Gd3In2Ga3O12 (GIGG) single crystals doped with different Rare Earths (RE) including Eu, Sm, Ho, Nd have been grown via OFZ method to investigate their luminescence, decay time kinetics and scintillation properties. To compare the scintillation behaviour, a well-known commercial scintillator, Gd2.98Ce0.02Al2Ga3O12 (0.02Ce:GAGG) single crystal, was synthesised and characterised using the same methods. This limits deviations in scintillation properties caused by different synthesis methods. Powder X-ray diffraction (XRD) and Single Crystal X-ray diffraction (SC-XRD) measurements together with Rietveld analysis confirmed phase purity and crystallinity, while Scanning Electron Microscopy (SEM-EDX) and backscatter electron imaging (BSE) confirmed the homogeneous distribution of the dopants. In addition to these analytical methods, Photoluminescence Spectroscopy (PL) and luminescence decay measurements have demonstrated that these RE-doped single crystals have strong emission, which is related to the low phonon energy of their host lattices. However, scintillation measurements under 137Cs gamma-ray point source excitation resulted in a limited gamma ray response. This response was significantly weaker than observed for the well-known commercial scintillator 0.02Ce:GAGG. Inefficient absorption of high-energy excitation or inadequate energy transfer to the activator ions in these materials are responsible for this relatively low scintillation performance. The second part of the thesis is focused on bilayer Ruddlesden-Popper nickelates. Single crystals of Sr3Ni2-xAlxO7-δ (SNAO) and its Y-substituted modification YᵧSr3-yNi2-xAlxO7-δ (YSNAO) have been grown by using strategically chemical substitutions and high oxygen pressure conditions to stabilize the RP phase. XRD and SC-XRD analysis confirmed the existence of the n = 2 RP-type structure by doping with Al, while thermogravimetric analysis (TGA) and X-ray photoelectron spectroscopy (XPS) indicated successful oxygen incorporation and corresponding changes in the Ni oxidation state. Magnetic susceptibility measurements (SQUID magnetometry) have demonstrated the antiferromagnetic ordering transitions in these doped nickelate single crystals, and transport measurements have shown enhanced electrical conductivity compared to the undoped parent compound. In addition, a topochemical fluorination process has been investigated as a post-growth modification for n = 1 RP-type La2NiO4+δ single crystals. Fluorine has been introduced into the crystal lattice at moderate temperatures using polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), and CuF2 as fluorination reagent. PTFE has been found to be the most effective fluorinating agent, resulting in partially fluorinated La2NiO4+δ with a diffusion-limited penetration pattern. EDX and EDX-mapping analyses have demonstrated an inhomogeneous F distribution, indicating that fluorine incorporation is limited by diffusion kinetics. This inhomogeneity should be carefully considered in future structural and functional analyses as it may affect the accurate characterisation of intrinsic properties in fluorinated crystals. In summary, the results of this thesis demonstrate the flexibility of the optical floating zone technique for the growth of high-quality functional oxides and show how structural stability, and functional properties can be modified using specific chemical doping approaches. To optimise scintillation performance in novel garnet-type hosts and to improve the magnetic and electronic behaviour of RP-type nickelates, the combined approach of crystal growth and post-growth modification could provide an additional parameter for material modification.
  • Thumbnail Image
    ItemOpen Access
    Synthesis and thermoelectric properties of chalcogenide and half-Heusler phases
    (2018) Zou, Tianhua; Weidenkaff, Anke (Prof. Dr.)
  • Thumbnail Image
    ItemOpen Access
  • Thumbnail Image
    ItemOpen 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.
  • Thumbnail Image
    ItemOpen 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.
  • Thumbnail Image
    ItemOpen 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.
  • Thumbnail Image
    ItemOpen 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.
  • Thumbnail Image
    ItemOpen Access
    Bioinspired structure tailoring of tin oxide based materials for high capacity electrodes
    (2021) Jahnke, Timotheus; Bill, Joachim (Prof. Dr.)