Universität Stuttgart
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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 Misfit-layered cobalt oxides for thermoelectric energy conversion(2017) Büttner, Gesine; Weidenkaff, Anke (Prof. Dr.)The conversion of waste heat into electrical current by a thermoelectric converter can significantly contribute to a more sustainable usage of our resources. The p-type misfit-layered [Ca2CoO3-δ][CoO2]1.62 is known for its promising conversion efficiency, which yet needs to be improved significantly for commercial applications. The efficiency of a material increases with the Figure of Merit ZT=σα^2/κ, with Seebeck coefficient α, electrical conductivity σ, and thermal conductivity κ. The aim of this thesis is to provide a better understanding of the electrical and the thermal properties of the complex [Ca2CoO3 δ][CoO2]1.62 and to use this understanding to improve the efficiency of converters. Accordingly, (i) the increase of ZT via cation substitution is shown; (ii) a better understanding of the electrical transport above room temperature is developed; (iii) the effect of stoichiometric defects and secondary phases on the thermoelectric properties is investigated. Finally, (iv) [Ca2CoO3 δ][CoO2]1.62 - CaMn0.97W0.03O3 δ - converters are fabricated and the efficiency is increased by a suitable converter design. More specifically, the unexplored influence of Ru and In substitution on the thermoelectric properties of the polycrystalline [Ca2CoO3 δ][CoO2]1.62 is investigated. While In does not have a positive effect, Ru for Co substitution increases ZT up to 20 %. This increase stems from a strong reduction of the thermal conductivity - which is probably induced by resonance scattering - while the decrease of the power factor α^2 σ is minor. The electrical transport mechanism of pure and Ru-substituted [Ca2CoO3 δ][CoO2]1.62 between room temperature and 800 K so far lacks a coherent theoretical model. Surprisingly, the framework of Anderson localization, which was developed to describe conduction in an impurity band of semiconductors, can be applied to the oxide. The Anderson model assumes that transport happens via charge-carrier hopping in a random Coulomb potential. For [Ca2CoO3 δ][CoO2]1.62, charges are considered to hop between Co sites in the CoO2 layer, while the random potential originates from interactions with the mismatched Ca2CoO3 δ layer. The presence of the ionized Ru atoms further alters the Coulomb potential, which increases the activation energy of the transport behavior. This understanding might contribute to the development of better theoretical models for the prediction of the thermoelectric properties of substituted [Ca2CoO3 δ][CoO2]1.62 compounds. A further improvement of the materials efficiency can be achieved by systematic introduction of stoichiometric defects and impurity phases. Here, the unexplored influence of the Co/Ca ratio on the thermoelectric properties of [Ca2 wCoO3 δ][CoO2]1.62, and the effect of Co3O4 impurity phase are investigated. It is shown that an increasing Co/Ca ratio in the [Ca2 wCoO3 δ][CoO2]1.62 phase leads to a larger figure of merit ZT induced by a strong resistivity drop. The decrease of resistivity stems from additional p-type charge carriers created by the formation of Ca vacancies. The Co3O4 impurity phase increases the thermal conductivity of the composite samples and leads to a reduction of ZT when the volume fraction of the Co3O4 phase is increased from 1% to 3%. Hence, the best figure of merit is expected close to the upper phase boundary of the [Ca2 wCoO3 δ][CoO2]1.62 phase. Not only the figures of merit of the materials, but also the design of a thermoelectric converter determines the device efficiency. In a converter, a p-type and a suitable n-type thermoelectric material are connected electrically in series and thermally in parallel. Here, [Ca2 wCoO3 δ][CoO2]1.62 is combined with the n-type CaMn0.97W0.03O3-δ and the device efficiency is improved by a variation of the ratio A_p/A_n of the cross section areas of the legs. The good agreement between the experimental values and the predictions of the compatibility model show the high quality of the fabricated devices and the value of the model for the optimization of the converter design. The adjustment of A_p/A_n improves the power output and the efficiency of the converters, where the best volume and area power densities exceed published high temperature values. The achieved efficiency of 1.08 % at a temperature of 1085 K at the hot side is close to the theoretical expected efficiency and can be further improved via ZT.Item Open Access Atom probe reconstruction with a locally varying tip shape(2019) Beinke, Daniel; Schmitz, Guido (Prof. Dr. Dr. h.c.)In this thesis, a new approach for the reconstruction of data taken from an atom probe tomography experiment is presented. The goal of the study is to develop an algorithm, which is able to overcome well-known drawbacks of the conventional reconstruction technique, mainly caused by local magnification effects. At the same time, the algorithm should be easy to use and also fast enough, so that it might be routinely used as an improved alternative to the established reconstruction technique. The idea is based on the already existing possibility to simulate an entire atom probe experiment on a realistic length. Since the successive calculation of ion trajectories starting at the emitter surface and hitting the detector after a flight of a few centimeters can be realized, the concept is designed to invert the field evaporation process by making use of this trajectory calculation. To this end, the detected emitter volume needs to be rebuilt from the bottom to the top, which is an important difference compared to the conventional technique. In a first test, this inversion of the simulated experiment is demonstrated for a few prominent example cases. The decisive criterion for the positioning of an atom at a specific lattice site on the current emitter surface is the accordance of the impact position of the corresponding calculated trajectory with the measured coordinates on the detector. For every possible surface position, first an ion trajectory is calculated and its detector impact position is compared to the measured impact position. Finally, the best-matching trajectory defines the reconstruction coordinates. The approach is performed for some prominent example emitter structures with strongly varying evaporation fields of the involved material, which is known for causing tremendous artifacts in the reconstruction derived by the standard technique. In this first attempt, the algorithm is restricted to a rigid lattice, which means that detected atoms can only be positioned at sites belonging to the former lattice of the emitter. In a second step, the restriction to a rigid lattice is dropped. In this way, the reconstruction algorithm describes a more realistic scenario, since the exact lattice structure and its orientation might be unknown in the majority of experiments. The possibilities and limitations of the approach are discussed. It is found that an additional criterion for the determination of the reconstruction coordinates is needed in this case, since the algorithm is very sensitive to the misplacement of atoms. The stability can be significantly improved by the consideration of an inter-atomic potential, which acts as a filter that exclusively allows surface sites with a sufficiently high amount of neighbor atoms. For a perfect detector efficiency the algorithm yields promising results, but a decrease of the efficiency towards realistic values gives rise to artifacts. As a consequence of these numerical experiments, a new concept has been developed, which neglects the consideration of exact ion trajectories in order to make the algorithm more stable and fast. This third approach assumes rotational symmetry for the investigated emitter volume. An absolutely new characteristic of the technique is the capability to extract the shape of a field emitter directly from the observed pattern of ion impacts on the detector. This feature is a very important difference to the conventional technique, which assumes a constant spherical emitter shape. To the best of the authors knowledge, such a technique with this capability did not exist before. The promising features are demonstrated for several simulated but nevertheless realistic emitter structures. The improved quality of the reconstruction that can be achieved by the application of the here developed technique is shown by direct comparison to the result of the established reconstruction approach. The impressive benefits are illustrated for relevant emitter structures containing either precipitates or layers of different materials with strongly varying evaporation fields (44% or 56% relative variation). In addition, a simple modification of the technique is described, which yields homogenized atomic densities in the reconstructed volumes. Without this modification, the emitter surface is treated like a rigid curved plane, which is shifted upwards with every reconstructed atom during reconstruction. Once the surface is no longer considered to be rigid, individual parts can be lifted separately, yielding a significantly homogenized atomic density. Finally, the new concept of shape extraction is extended for the application to arbitrary emitter structures. The main idea of extracting the information about the emitter shape from the local density of measured events on the detector is maintained. In order to extend the approach to the application to structures without rotational symmetry, a relation between the local density of events on the detector and the Gaussian curvature on the emitter surface is derived. With the help of an iterative finite difference method, the Gaussian curvature at several positions on the tip surface is set. Consequently, a reasonable description of the emitter surface can be obtained and the reconstruction of an arbitrary data set can be performed. The concept is tested and discussed for a simulated example emitter structure.