Master Theses

Construction of an interatomic potential for the LiTiO battery anodes. Transition metal oxides are known to exhibit variable oxidation states. Therefore, they are preferred as storage materials in lithium ion batteries. We aim to construct a state-of-the-art interatomic potential for the LiTiO system, which will be used in atomistic Monte Carlo/molecular dynamics simulations. The atomistic framework incorporates Coulomb interactions, metallic bonding character via an embedded-atom formalism, as well as charge transfer due to the different oxidation states of the species. Varying oxidation states may induce different crystal structures which we could observe in recent experiments. We want to further explain this from an atomistic point of view. (supervisors: Sebastian Eich, Yug Joshi)

Machine learning concepts in atomic resolution tomography. In several impressive cases, concepts of machine learning (ML) have proven to be effective tools for materials science. The project tries to develop a computational algorithm for the measurement of particle / precipitates sizes in atom probe tomography data. The preferred machine learning strategy will be the “support vector regression”, which should be trained with data sets that are generated by simulated field evaporation. For this simulation step, a simulation tool (TAPSim) is already available. So, practically the project has to develop suitable Phython scripts that control the available basic software and simulation packages. After training, the ML code should be applied to experimental data sets that were presented in one of our recent publications, however there, they were evaluated with traditional fitting and calibration concepts. (supervisors: Patrick Stender, Sebastian Eich)

High resolution analysis of the grain boundaries in LiFePO₄ cathodes. In previous studies, we collected several indicators that the grain boundaries (GB) of LiFePO₄ cathodes must have very special properties. Probably, they represent purely ion-conductive pathways in the mixed electronic-ionic conducting matrix. Therefore, we would like to elucidate this issue by the analysis of the GBs in highest possible resolution. The cutting-edge, aberration-corrected electron microscope of the University will be applied. This allows imaging in world-leading resolution and chemical analysis of local oxidation states via electron energy loss spectroscopy. The cathode material will be produced by thin film deposition. Electron transparent slices are prepared by focused-ion-beams in the lift-out technique. The project is especially suited for those who like to work with top instrumentation tools. (supervisors: Guido Schmitz, Yug Joshi)

SEI formation at the anodes of Na ion batteries. Sodium ion batteries have the potential to replace Li-ion batteries, should the Li resource become too limited. However, the present knowledge on suitable materials (anodes, cathodes electrolytes) with sodium is rather limited. In this project, suitability and stability of Ge or Si anodes for cycling with Na+ ions should be explored. The anodes will be deposited on oscillating quartz microbalances to measure the lithiation and the corrosion of the anodes in contact to Diglyme electrolyte. This gravimetry measures the mass change of the electrodes in sub-monolayer resolution and so can identify reversibly and irreversibly adsorbed species to the electrode. We have already applied this technique to several thin film electrodes of Li-based systems and now would like to transfer the technique to sodium batteries. The experiments will be complemented with electron microscopy and X ray photoelectron spectroscopy. (supervisor: Jug Yoshi).

V₂O₅ nanowires as high power battery electrodes. In order to enable a fast charging, battery electrodes should offer high amount of surface for Li penetration. In this sense nanowires can be expected to offer particularly high rates. They also have advantages in relaxing mechanical stress. With this work, we will try producing V₂O₅ Nanowires by tricky oxidation treatments of thin vanadium films. The electrochemical performance of the wire ensemble in cyclo-voltametry will be directly compared to that of conventional thin film electrodes, which we had already investigated in the past. In a second stage of the project, we will also try depositing the wires on quartz crystals, which allows a mass spectroscopy of reactions to appear at the wire surfaces. (supervisors: Yug Joshi, Guido Schmitz)

Measurement of hydrogen permeation by high resolution gravimetry. Hydrogen storage, in the form of metallic hydrides, is a promising concept, since it enables particularly high volume density in H storage. With this aim, we would like to study the hydrogenation of FeTi alloys, since these alloys are available for quite the economic prices. We would like to understand the hydrogen uptake and the nucleation of the hydrides. An innovative method of measurement shall be applied for the first time. We deposit FeTi layers as thin films to oscillating quartz microbalances and measure the gain in weight when solving hydrogen in the metallic films. Furthermore, formation of hydride phase leads to expansion due to reaction excess volume. Thus, nuclei of the hydride will become visible as surface protrusions, which we will image by white light interferometry to measure their number and size in depending on the hydrogenation treatment. (supervisor: Guido Schmitz) 

Detection of hydrogen by atom probe tomography. Future hydrogen technology needs new construction materials for the 700 to 1000 bar hydrogen pressure range. Hydrogen embrittlement is a phenomenon feared when using steel constructions in hydrogen atmosphere. In order to progress the understanding of this phenomenon, one needs to measure tiny amounts of hydrogen in high spatial resolution. We would like to explore atom probe tomography for this. Cryo-preparation and direct transfer into the atom probe should prevent hydrogen from escaping before the measurement. We will first test and benchmark the method at NiTi multilayers, where hydrogen should preferentially segregate to the Ti layers. Afterwards, new high-entropy alloys should be analyzed. Are they single- or multiphase? In which phase or structural defects does hydrogen prefer to segregate? (supervisors: Patrick Stender, Guido Schmitz)

Hysteresis in reactive wetting on rough surfaces. In a previous PhD thesis (Samuel Griffith) we developed a new thermodynamic model to understand the spreading of liquid solder alloys on rough substrates when downsizing the solder connection. Wetting hysteresis, meaning that an advancing and a receding wetting angle must be distinguished depending on the samples history, is a fundamental phenomenon which explains e.g. why liquid droplets can stick to vertical windows. However, wetting hysteresis is only weakly understood and, especially in the case of a reactive soldering system, practically no quantitative prediction is possible. However, our new model makes indeed a clear prediction, to which the wetting hysteresis should depend on the size ratio between the liquid droplet and the roughness scale of the substrate. We plan to critically check this theory. Nano-structured rough surfaces will be produced by electron beam lithography, for which we can exactly control the amplitude (RMS) and periodicity of the roughness. On these, the spreading of solder droplets with a size downwards of 10 micrometers will be studied by in-situ optical and SEM observation. (supervisor: Guido Schmitz)

Pump-probe experiments in atom probe tomography. In Atom Probe Tomography (APT), very fine samples are evaporated as tiny molecular fragments by superposition of high electric field and laser pulses. Especially with organic matter, the size and the charge state of the evaporated fragments have not been understood, yet. Likewise, oxygen and nitrogen can be lost due to insufficient ionization. The aim of the project is to investigate whether a second, subsequent laser pulse, that hits the desorbed species right after leaving the sample surface, can positively influence the level of ionization and the fragmentation through multi-photon ionization. This requires laser powers > 10²⁰ W/cm², which we plan to obtain with ultra-short (40 fs), highly focused laser pulses. (supervisors: Patrick Stender, Guido Schmitz)

Development of cryo-APT preparation techniques. Samples for Atom Probe Tomography must be in a needle-like shape with a radius of curvature of 50 nm at the apex. Preparation of conventional materials is available by using Focused Ion Beam techniques. Currently, liquids are of great interest, with the goal of studying processes at biological interfaces like cell membranes, viruses or bacteria in atomic resolution. Aqueous solutions are cryo-frozen and transferred into the FIB for tip shaping and finally to the atom probe. The sample must be permanently cooled to preserve them in solid state. To avoid artifacts, a crystallization has to be prevented. Through innovative ink-jet printing we try to avoid a crystallization of ice crystals. The goal of the study is the measurement of a virus by atom probe tomography. (supervisor: Patrick Stender)