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 Genetisch modifizierte Biotemplate zur Erzeugung von Zr-basierten Nanomaterialien(2019) Eisele, Rahel; Bill, Joachim (Prof. Dr.)In Biomineralisationsprozessen aus der belebten Natur scheiden sich anorganische Materialien auf organischen Templaten (Biomakromoleküle) ab. Funktionelle Gruppen der Makromoleküle steuern dabei die Abscheidung aus einer wässrigen Lösung sowie die Strukturierung des anorganischen Materials. Dabei sind spezifische Wechselwirkungen zwischen dem organischen Templat und dem anorganischen Material von Bedeutung. Die Materialbildung findet unter Umgebungsbedingungen in wässrigen Systemen statt. Für technisch interessante Materialien wie Zirkoniumdioxid (ZrO2) stellt die energieeffiziente Herstellung präziser Nanostrukturen eine technische Herausforderung dar. Daher wurden im Rahmen dieser Arbeit die Prinzipien der Biomineralisation auf die Herstellung von Zirkonium-basiertem Material (ZrbM) übertragen. Hierzu gehörte die Materialbildung durch Mineralisation aus einer ZrOCl2-Lösung sowie eine gezielte Mineralisation auf bioorganischen M13-Bakteriophagentemplaten. Um die „biologische Spezifität“ in Biomineralisationsprozessen auf die Bildung von ZrbM zu übertragen, wurden Peptide mittels Phagen-Display identifiziert, die spezifisch an ZrO2 binden. Mittels genetischer Modifikation wurden diese ZrO2 Bindepeptide auf der Phagenoberfläche präsentiert. Hierdurch wurde eine hohe Bindepeptiddichte und damit viele Interaktionspunkte zum anorganischen Material erzielt. Bevor der Einfluss dieser Bindepeptide auf die Mineralisation von ZrbM untersucht werden konnte, wurde zunächst der Partikelbildungs- und Partikelwachstumsprozess von ZrbM in einer ZrOCl2-Lösung und einem Ethanol-Wasser Lösungsmittelgemisch bei verschiedenen System- und Prozessparametern beschrieben. Auf Grundlage dieser Ergebnisse wurde eine Mineralisationslösung etabliert mit der der Einfluss der Bindepeptide - präsentiert auf der Phagenoberfläche - auf die Mineralisation von ZrbM untersucht werden konnte. Die Bindepeptide zeigten einen deutlichen Einfluss auf die Mineralisation von ZrbM. Im Vergleich zu Bakteriophagen ohne Bindepeptid wurde mit den genetisch modifizierten Bakteriophagen eine deutlich höhere Abscheiderate erzielt. Dieser Einfluss der Bindepeptide wurde auf Hydroxygruppen in Serineinheiten zurückgeführt. Diese führen zum einen zu einer starken Anziehung von molekularen Zr-Spezies an das Biotemplat. Zum anderen induzieren die Hydroxygruppen die heterogene Keimbildung von ZrbM durch Kondensationsreaktionen zwischen dem Biotemplat und molekularen Zr-Spezies. Somit ist es nun möglich genetisch kontrolliert Zr-basierte Nanomaterialien zu mineralisieren. Im Rahmen dieser Arbeit gelang es nicht nur einzelne Phagen zu mineralisieren, sondern auch dünne homogene Schichten aus ZrbM. Diese ZrbM-Schichten wurden im letzten Teil dieser Arbeit vergleichend zu Phagenschichten und SiO2-Schichten auf die Adhäsion von Staphylococcus aureus (S. aureus) getestet. S. aureus ist ein pathogenes Bakterium, welches zur Bildung von Biofilmen, zum Beispiel auf Implantaten, und dadurch zu einem Implantatverlust bis hin zu lebensbedrohlichen Komplikationen führen kann. Die Biofilmbildung kann effektiv unterbunden werden, indem die Bakterienadhäsion auf Oberflächen verhindert wird. Daher wurde im Rahmen dieser Arbeit untersucht, ob bestimmte chemische Oberflächen, das heißt bestimmte Materialien oder auch bestimmte funktionelle Gruppen, die Bakterienadhäsion unterdrücken können. Die Untersuchung der Bakterienadhäsion auf den verschiedenen Oberflächen ergab, dass auf der Phagenschicht im Vergleich zur SiO2-Schicht und einer Schicht aus ZrbM eine sehr geringe Bakterienadhäsion vorlag. Untersuchungen verschiedener Einflussfaktoren auf die Bakterienadhäsion zeigten, dass die Bakterienadhäsion an der SiO2-Schicht und der ZrbM-Schicht durch die Oberflächenrauigkeit, die Hydrophobizität und die Oberflächenladung beeinflusst werden kann. Bei der Phagenschicht korrelierten weder die Oberflächenladung, noch die Oberflächenrauigkeit und die Hydrophobizität im Vergleich zu den anorganischen Materialoberflächen mit der Bakterienadhäsion. Dies ließ darauf schließen, dass die geringe Bakterienadhäsion auf der Phagenschicht auf die biochemische Zusammensetzung der Hüllproteine, vor allem auf die Abwesenheit spezifischer Bindedomänen (Ligand-Rezeptor-Wechselwirkungen), zurückzuführen ist.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 Interaction of carbon and nitrogen in iron(Stuttgart : Max-Planck-Institut für Intelligente Systeme (ehemals Max-Planck-Institut für Metallforschung), 2016) Göhring, Holger; Mittemeijer, Eric Jan (Prof. Dr. Ir.)Item Open Access Grain growth and texture evolution in copper thin films(2010) Sonnweber-Ribic, Petra; Arzt, Eduard (Prof. Dr. phil)An improved basic understanding of mechanisms causing grain growth and texture evolution in Cu thin films contains the potential to improve performance and reliability of components and devices. In this work, the influence of film thickness, strain and temperature on grain growth and texture evolution in Cu thin films was investigated. By varying the parameters, information about the underlying mechanisms were revealed. The 0.5 to 10 micrometer thick Cu films were deposited on 125 micrometer thick polyimide substrates (Kapton®, DuPont) using a UHV magnetron sputtering system. For detailed observation of grain growth and texture evolution an EBSD-based in situ testing appliance was constructed. This system allowed the simultaneous observation of grain growth and texture evolution, giving new insight into growth kinetics and details of grain growth. In a first step, Cu thin films of thicknesses in between 0.5 and 10 micrometer were deposited on polymer substrates and annealed at 330°C for 30 min. Their resulting texture and microstructure were investigated by EBSD. A texture transition from (111) to (100) was observed at film thicknesses between 3 and 5 micrometer. The experimental findings were explained by the texture evolution model of Thompson and Carel. A significant observation which cannot be explained by a purely energetic argument is the broad texture transition. In order to get more information about the critical role of strain energy, uniaxial tensile tests were carried out on 3 micrometer thick films. In contrast to theoretical predictions, various tensile tests revealed no influence of strain on grain growth behaviour. Neither at room temperature nor at elevated temperatures, further (100) grain growth was observed. In a next step, the abnormal growth of individual (100) oriented grains was recorded for more than 24 hours at temperatures between 90 and 118°C. Annealing was carried out inside a Leo 1530-VP SEM equipped with a heating facility. Detailed analysis of grain growth and estimates of the possibly acting driving forces indicated that the reduction of dislocation density played an important role for abnormal grain growth. A further hint for the critical importance of defect density was given by the HWHM of the (100) texture fraction. Nevertheless, it was not clear why this driving force favours the growth of (100) oriented grains. A possible answer could be given by the strain energy release maximization (SERM) model developed by Lee. In addition, when analysing the activation energy for grain growth, they were found to possess a higher grain boundary mobility, supporting the preferred growth of (100) oriented grains. A new texture map, considering dislocation density as driving force, was constructed. Assuming dislocation density to play a significant role for grain growth and texture evolution in Cu thin films, the influence of deposition parameters is pointed out.Item Open Access Mechanisms of intrinsic stress formation in thin film systems(2013) Flötotto, David; Mittemeijer, Eric Jan (Prof. Dr.)Functionalization of thin-film systems on the basis of their functional properties requires precise control of the intrinsic film stresses that develop during the growth process. Although the intrinsic stress evolution during thin film growth has been extensively studied for a huge diversity of different materials, deposition techniques and deposition conditions, the technological potential to optimize and control the properties of thin film systems is still limited due to the lack of fundamental and comprehensive understanding of the stress-inducing mechanisms and their complex correlation with atomic scale processes during thin film growth. The effect of the adatom surface diffusivity on the evolution of the microstructure and the intrinsic stress of thin metal films was investigated for the case of growth of polycrystalline Ag films on amorphous SiO2 and amorphous Ge substrates, with high and low Ag adatom surface diffusivity, respectively. As evidenced by AR-XPS, Ge continuously segregates at the surface of the growing film and thus suppresses the surface diffusivity of the deposited Ag adatoms on the a-Ge substrate also after coalescence of Ag islands and subsequent thickening of the laterally closed Ag film. The abatement of the Ag adatoms surface diffusivity by (segregated) Ge leads to the development of a fine, equiaxed, texture less microstructure for Ag film growth on a-Ge substrates, which is in striking contrast to the development of a columnar, surface energy minimizing {111} fiber textured microstructure for Ag film growth on a-SiO2 substrates. Nevertheless, the real-time in-situ stress measurements revealed a compressive-tensile-compressive stress evolution for the developing Ag films on both types of substrates, however on different time scales and with stress-component values of largely different magnitudes. On the basis of experimental results an assessment could be made of the role of adatom surface diffusivity on the microstructural development and the intrinsic stress evolution during film growth: The microstructural development of polycrystalline metallic thin films is predominated by the surface diffusivity of the adatoms, and the intrinsic stress evolution is largely controlled by the developing microstructure and the grain-boundary diffusivity of atoms. It is revealed that the in-plane film stress oscillates with increasing film thickness at the initial stage of epitaxial single-crystalline Al(111) film growth on a Si(111) substrate, with a periodicity of two times the Fermi wavelength of bulk Al and a stress amplitude as large as 100 MPa. Such macroscopic stress oscillations are shown to be caused by a hitherto unrecognised stress generating mechanism resulting from the quantum confinement of free electrons in the ultrathin metal film: A freestanding film would energetically prefer specific thicknesses and lateral dimensions as a result of the optimal, net effect of quantum confinement of the electrons and the associated elastic deformation (straining). Because the film is attached to the (rigid) substrate, the (oscillating) preferred lateral dimensions cannot be realized and consequently an oscillating component of stress is induced in the plane of the film. The amplitude, period and phase of the observed stress oscillations are consistent with predictions based on the free electron model and continuum elasticity. Furthermore, it is revealed that oxygen exposure of clean Si(111)-7x7, Si(100)-2x1 and a-Si surfaces results in compressive adsorption-induced surface stress changes for all three surfaces due to the incorporation of O atoms into Si backbonds. The measured surface-stress change decreases with decreasing atomic packing density of the clean Si surfaces, in correspondence with the less-densily packed Si surface regions containing more free volume for the accomodation of adsorbed O atoms. It is demonstrated for the first time that pronounced intrinsic stresses can be generated in ultrathin amorphous Al2O3 films formed by thermal oxidation of bare Al surfaces at low temperatures. The magnitude of the growth stress strongly depends on the Al surface orientation: Oxide films formed on Al(100) are stress free, whereas oxide films formed on Al(111) exhibit a thickness averaged in-plane tensile film stress as large as 1.9 GPa. The striking dependence of the stress evolution on the Al surface orientation at the very beginning of oxygen exposure is a direct consequence of the different initial oxygen-adsorbate structures at the Al surfaces inducing distinctly different adsorption induced changes of surface stress. In contrast, the observed striking dependence of the stress evolution on the Al surface orientation during continued oxide-film growth is the result of competing processes of free volume generation and structural relaxation originating from the different microstructural developments for amorphous oxide film growth on Al(111) and Al(100).Item Open Access Aluminum-induced crystallization of semiconductor thin films(2015) Qu, Fei; Schmitz, Guido (Prof. Dr.)Thin film materials of the semiconductors, such as silicon (Si), germanium (Ge) or their alloys, are turning into the most promising functional materials in the energy technology. However, the morphologies of these semiconductor thin films must be varied to be suitable for the different applications, e.g. a large-grained layer as the seed layer of thin film solar cells, a porous structure for anode materials of high energy rechargeable lithium (Li) ion batteries. Due to the collective interdiffusion process during the aluminum (Al)-induced crystallization, in this thesis, the suitable morphologies are achieved for the corresponding applications under the different fabrication conditions. A large-grained Si layer can be formed by the crystallization of Si in a porous Al layer, which is obtained by applying a bias voltage. Since the Al grain boundaries are contaminated by e.g. oxygen (O), the diffusion of Si in the Al grain boundaries is retarded. It can lead to a reduction of the nucleation density of Si. At a certain high temperature, a collective diffusion process of Si in Al is activated. Consequently, a large-grained Si layer with (100) texture can be formed. By purposely interrupting the annealing of nanocrystalline Al/amorphous Si (a-Si) bilayers, a porous structure of the crystallized Si can be developed due to the incomplete intermixing of Si and Al. Due to the different dominant diffusion processes of Si in Al at the different annealing temperatures, the most Si diffuses along the different paths in the Al layer, such as triple junction, grain boundary and Al bulk. Therefore, it can develop the different morphologies of the porous Si layers after the selectively etching of Al. By introducing an amorphous Ge interlayer between the crystalline Al and amorphous Si layer, the Al grain boundaries are not essential for the crystallization of the amorphous Si in contrast to the case in Al/Si bilayer system. Si crystallizes continuously on the pre-crystallized Ge seeds which form initially at the original interface of crystalline Al and amorphous Ge. The thermodynamic models to interpret the fundamentals of these different crystallization behaviors of Si are established based on the change of the interface energy between the different phases of the whole system during the crystallization. Using the effective diffusivity, the dominant diffusion process of Si in Al can be investigated to explore the morphological dependence of the crystallized Si layer on the annealing conditions.Item Open Access Initial oxidation of zirconium: oxide-film growth kinetics and mechanisms(2011) Bakradze, Georgijs; Mittemeijer, Eric Jan (Prof. Dr. Ir.)The present thesis addresses the growth kinetics, chemical constitution, morphology and atomic transport mechanisms of zirconium-oxide films, as grown by the dry, thermal oxidation of single-crystalline Zr surfaces at low oxidation temperatures. To this end, bare (i.e. without a native oxide) well-defined single-crystalline Zr(001) and Zr(100) surfaces were exposed to oxygen gas in the temperature range of T = 300-450 K. The current study, for the first time, presents a direct comparison of the initial oxidation of single-crystalline Zr surfaces with basal and prism orientations. The relationships between the oxidation kinetics, the developing microstructure and the crystallographic orientation of the parent metal substrate have been established by application of various (surface-)analytical techniques. On the basis of the thus-obtained results, a Zr oxidation mechanism in the low-temperature regime (< 500 K) has been proposed. As evidenced by ellipsometry, after a short initial stage of fast oxide-film growth, a near-limiting thickness of the oxide film is attained at T < 375 K on both surfaces. Distinct differences in the oxidation kinetics for the two Zr substrate orientations become apparent at T > 375 K: the Zr(100) plane oxidizes more readily than the more densely-packed Zr(001) plane under the same experimental conditions. At T > 375 K, the oxidation rate of Zr becomes governed by thermally-activated dissolution and diffusion of oxygen into the Zr substrate. The changes in the local chemical states of Zr and O in the thin zirconium-oxide films have been investigated with increasing oxidation temperature. To this end, the oxide-film valence-bands (VB) and the Auger-parameters of Zr and O in the grown oxide films were resolved from measured XPS spectra of the oxidized Zr surfaces in the oxidation-temperature range of T = 300-450 K. The changes in the shape of the oxide-film VB spectra and the local chemical states of Zr and O in the oxide films evidence that the oxide films grown at T ≤ 400 K are predominantly amorphous, whereas a tetragonal ZrO2-like phase develops at T > 400 K. The formation of the tetragonal zirconia modification in the oxide films developing at 450 K is supported by HR-TEM analysis. The exposure of the bare Zr surfaces to pure oxygen gas at low leads to the initial, very fast formation of a dense arrangement of small oxide nuclei clusters, which completely cover the bare Zr surface after 300 s of exposure. With increasing temperature the mobility of adsorbed O species and/or Zr species on the oxidizing surface and in the developing oxide becomes promoted, thereby promoting the restructuring/reorientation of the oxide clusters into bigger agglomerates, e.g. with increasing oxidation time at constant temperature, as driven by the Gibbs-Thomson effect. In this thesis for the first time, two-stage tracer oxidation experiments have been applied to study the atomic transport mechanisms in thin (< 10 nm) oxide films, as formed during the initial stages of the low-temperature oxidation of the bare, single-crystalline Zr(001) and Zr(100) surfaces at 450 K. The observed differences in shape of the measured tracer profiles for different stages of oxidation indicate a change in the oxide-film growth mechanism during oxidation: i.e. a change of the predominant transport mechanism from inward oxygen transport by a combination of lattice and short-circuit mechanisms during the initial oxidation stage to inward oxygen transport by only the lattice mechanism during later oxidation stages. A low-temperature oxidation mechanism for zirconium at T = 450 K was proposed. Oxygen transport through the developing oxide film requires coupled fluxes of inwardly migrating O anions and outwardly migrating O vacancies, as supplied by the slow continuous O dissolution into Zr. The resulting Zr(O) solid-solution phases formed at the metal/oxide interface are evidenced by an interfacial suboxide layer in the in-situ AR-XPS and ellipsometry analysis. O vacancies, as generated in the interfacial suboixde layer by the slow, but continuous, dissolution of O into the Zr substrate, diffuse outwardly through the O sublattice of the crystalline zirconia overlayer towards the oxide/gas interface to be filled by chemisorbed O surface species at the gas/oxide interface. The thus-established outward flux of O vacancies is accompanied by a net inward lattice flux of oxygen anions. At the early oxidation stage, oxygen is transported inwardly via both the lattice and short-circuit transport mechanism. At later oxidation stages, the contribution of O short-circuit transport becomes negligible, as attributed to a reduction of grain-boundary (GB) density in the oxide-film in association with an equilibration of the GB structure, in possible combination with the accumulation of space charge in the vicinity of the GB in the oxide-film. The overall oxide-film growth rate at 450 K is limited by the O dissolution rate (i.e. supply of oxygen vacancies into the growing oxide film) at the suboxide/metal interface.