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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.
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.
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.
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.
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).
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.
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.
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.
Open Access
Phase-transformation kinetics of TiCr2 laves phases
(2010) Baumann, Wolfgang; Mittemeijer, Eric Jan (Prof. Dr. Ir.)
The thesis is divided into three parts: The first part deals with experimental progress in the field of differential thermal analysis DTA (see Chapter 2). This progress was necessary to enable the quantitative investigation of the phase transformation in TiCr2 Laves phases. For quantitative analysis of reaction/transformation kinetics using DTA a calibration of the DTA apparatus in terms of temperature and heat capacity is inevitable. Moreover the measurement signal has to be desmeared, since the measurement signal is smeared due to thermal lag [49], which affects the peak position and the peak shape. In the second part layer-stacking irregularities in C36-type Nb-Cr and Ti-Cr Laves were systematically studied by XRPD and HRTEM. The layer-stacking irregu-larities in both substances can be associated with a preceding C14 (2H)C36 (4H) phase transformation driven by synchro-Shockley dislocations dipoles, i.e. the syn-chro-Shockley dislocations dipoles leave behind a fingerprint in terms of certain layer stacking irregularities (see Chapter 3). The third part of this thesis is about phase transformation kinetics (Chapter 4, 5 and 6). During the investigation of the equilibrium phase transformation in TiCr2 occurring in the vicinity of the equilibrium phase-transformation temperature , it was found that application of the so-called Kissinger method [50] in the case of such an equilibrium transformation will lead to erroneous activation energies, since the original assumptions made for derivation of the Kissinger method are not valid. This was shown by theory and experiment in Chapter 4. Chapter 5 deals with the phase-transformation kinetics of the equilibrium phase transformations in TiCr2 Laves phases, i.e. the modular phase-transformation model has been successfully applied to DTA data of both transformations (i.e. for the phase transformation occurring upon heating and upon cooling). The model reveals the mechanism and the kinetics of the phase transformations and furthermore explains the differences of the phase transformations as a function of Laves-phase composition. The modular model presented in Chapter 5 was used in Chapter 6 to describe the influence of the plastic deformation on the phase-transformation kinetics in TiCr2 Laves phases. It is shown that plastic deformation considerably affects the phase-transformation kinetics. This is ascribed to the presence of dislocations formed upon plastic deformation during pressing.
Open Access
Synthesis and thermoelectric properties of chalcogenide and half-Heusler phases
(2018) Zou, Tianhua; Weidenkaff, Anke (Prof. Dr.)

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