Universität Stuttgart
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Item Open Access Bioinspired structure tailoring of tin oxide based materials for high capacity electrodes(2021) Jahnke, Timotheus; Bill, Joachim (Prof. Dr.)Item Open Access Extracting thermodynamic information from local composition fluctuations in solids : extended theory and its application to simulated and experimental atom probe data(2024) Zheng, Jianshu; Schmitz, Guido (Prof. Dr. Dr. h.c.)In case of liquids, thermodynamic fluctuation theory has been applied for decades to obtain direct thermodynamic information (e.g. miscibility gap, mixing/demixing tendencies, critical solution temperature) from local composition fluctuations. Recently, this theory has been extended to solids by introducing an additional elastic work term between the evaluated sub-system and the entire system, which does not arise in liquids. This extended theory has been verified via atomistic simulations in an exemplary Cu-Ni embedded-atom system using Monte Carlo simulations at a fixed temperature over the entire composition range. Composition fluctuations in the system that are represented by the relative variance of the composition histogram are tracked in various-sized subvolumes over time, revealing a systematic dependence on the size of the evaluation volume due to interface effects. Nonetheless, these surface effects can be excluded by extrapolation to an infinitely large subvolume, leading to perfect agreement with the prediction by the extended theory. Thus, the recovery of the Gibbs free energy of mixing from evaluation of the fluctuations is possible also in the case of solids. Atom Probe Tomography (APT) delivers combined high-resolution chemical and sub nanometric three-dimensional (3D) spatial information, and is therefore the perfect technique to determine local composition fluctuations by using spatial frequency distribution analysis in practical applications. In this work, the applicability of the extended theory is tested on the Cu-Ni alloy and ionic CuO systems via frequency distribution analysis on simulated and experimental atom probe data, and eventually compared to available phase diagram data, thereby proving the validity of extracting the Gibbs free energy from local composition fluctuations in solids. In the first part of this work, the spatial frequency distribution analysis is applied to simulated crystals of long-range ordered L12 and monoclinic structures numerically modeled disregarding thermodynamic interaction between atoms. The relative variance displays an evaluation size dependence, but goes to zero (i.e. no composition fluctuations) if extrapolated to sufficiently large evaluation size. This result meets the expectation as no composition fluctuations should be found in perfectly ordered materials. In the second part, this approach is applied to simulated alloys including thermodynamic interactions. Cu-Ni alloys of various compositions are firstly equilibrated using a Monte Carlo simulation with an embedded-atom potential. Afterwards, the alloys are numerically field-evaporated by the evaporation simulation package TAPSim and the 3D coordinates of the field-evaporated sample are recovered through the usual reconstruction algorithm. Throughout this process, two practical considerations related to the atom probe technique have been effectively addressed: i) The newly developed model tackles the challenges associated with the limited detection efficiency and allows the reconstruction of the relative variance for the bulk system from limited atom probe data scaled by detection efficiency; ii) An additional correction term which is proportional to the evaluation size and magnitude of composition inhomogeneity is introduced. It enables the separation of thermodynamic fluctuations from artificial composition variations inherent in the experimental method based on their different size dependence, so that the extrapolation still recovers the intrinsic thermodynamic composition fluctuations. In the third part, this approach is finally applied to experimental atom probe data. The Cu-Ni alloys are prepared by induction melting of pure Cu and Ni and CuO thin films are prepared via ion beam sputtering. After sufficient equilibration by heat treatment, Cu-Ni and CuO specimens for the APT measurement are fabricated via focused ion beam cutting. By experimentally conducting the same approach as developed theoretically, local composition fluctuations are obtained for both Cu-Ni and CuO systems. After the elastic work term correction, the CALPHAD-style parametrization of the Gibbs free energy is obtained by linking it to the measured local composition fluctuations. In this way, the Cu-Ni miscibility gap is successfully reconstructed from data measured at elevated temperature (800 K), and the resulting phase diagram is in agreement with the CALPHAD results in literature. The frequency distribution analysis of the reconstructed CuO tends to approach the binomial distribution (i.e. behavior of random alloys), since field evaporation of molecules (e.g. CuO, Cu2O) but not only single ions destroys the long-range order structure and deteriorates the resolution in the reconstruction. This effect indicates the partial limitation of this method on ionic compounds. In summary, the present work has systematically extended and proven the application of the composition fluctuation theory to metallic alloys, and makes it possible to directly access thermodynamic information from local composition fluctuations. APT is demonstrated as a new technique to extract direct thermodynamic information, and a general route from the APT measurement to the Gibbs free energy is presented. Given that the composition fluctuation is a local property and only a substantially short diffusion length for equilibration is required, this represents an efficient methodology especially for systems where slow diffusion hinders the establishment of large scale thermodynamic equilibrium. APT, as a sub-nanometric resolution technique, promises to extract more accurate thermodynamic information in a wider temperature composition range. Besides, this study advances our understanding of the size dependence in the traditional frequency distribution analysis. It is pointed out that potential misinterpretation could happen and is presented in literature, if a sample evaluation size in the frequency distribution analysis is arbitrarily chosen. Only the bulk relative variance obtained via extrapolation to infinitely large sub-system is thermodynamically meaningful.Item Open Access Structural design of advanced electrodes employing nanostructured V2O5 nanofibers for metal-ion battery application(2021) Diem, Achim M.; Bill, Joachim (Prof. Dr.)Item Open Access Atom probe study on CuNi thin films : miscibility gap and grain boundary segregation(2023) Duran, Rüya; Schmitz, Guido (Prof. Dr. Dr. h. c.)In dieser Arbeit wurde die Lage der Mischungslücke, und die Korngrenzsegregation im Legierungssystem, Kupfer-Nickel, per Atomsondentomographie (APT) analysiert. Zur Untersuchung der Mischungslücke eines binären Systems mit langsamer Diffusion wurde ein neues Verfahren verwendet. Multilagen aus Cu- und Ni- Dünnschichten wurden mittels Ionenstrahlbeschichtung (IBS) auf Wolframpfosten beschichtet und durch fokussierte Ionenstrahlung (FIB) geformt. Bei drei unterschiedlichen Temperaturen, zwischen 573 und 673 K, wurden isotherme Auslagerungssequenzen an einem Ultrahochvakuumofen (UHV) durchgeführt und der Mischungsprozess analysiert. Ein Modell des Diffusionsprozesses wurde mittels mathematischer Überlegungen erstellt. Durch das Fitten der experimentellen Kompositionsprofile mittels dieses Modells konnten die Gleichgewichtskonzentrationen der Schichten auch mit relativ kurzen Auslagerungszeiten ermittelt werden. Darüber hinaus konnten aus den diffusionskontrollierten Zeit- und Temperaturdaten physikalische Eigenschaften wie der effektive Diffusionskoeffizient (Gitterdiffusion einschließlich Defektdiffusion) bestimmt werden. Dieser betrug Deff = 1.86 ∙ 10-10 m2/s ∙ exp(-164 kJ mol-1/RT). Während dem Vermischen wurde die Änderung der multilagigen Mikrostruktur bis zur vollständigen Mischung bei 623 und 673 K beobachtet, wobei Korngrenzen als schneller Diffusionsweg eine wichtige Rolle spielen. Bei 573 K wurde Nichtmischbarkeit experimentell deutlich nachgewiesen, wobei die Phasengrenzen bei cNi=26 at.% und cNi=66 at.% liegen. Mit diesen Phasengrenzen wurde die Mischungslücke über eine Redlich-Kister-Parametrisierung der Gibbs‘schen freien Energie über den gesamten Konzentrationsbereich rekonstruiert. Hierin wurde für die kritische Temperatur, TC, 608 K bei einer Konzentration von 45 at% Ni gefunden. Im zweiten Teil wurde die Korngrenzsegregation durch die FIB/tEBSD- (Transmissions-Elektronen-Rückstreubeugung) Technik, in Korrelation zu APT-Messung charakterisiert. Vier Legierungen mit einem Ni-Anteil zwischen 25 und 85 at.% wurden auf Wolframpfosten per IBS beschichtet, und bei 700 K für 24 h wärmebehandelt. Die Segregation von Cu in die Korngrenzen wurde beobachtet. Durch die Verwendung eines theoretischen Models wurde die Exzess-Kurve über den gesamten Konzentrationsbereich, und die Korngrenz-Formationsenergie auf Basis der experimentellen Daten berechnet. Die tEBSD-Analyse während der FIB-Präparation erlaubt die Identifikation der Körner und deren Orientierung. Ein neues Verfahren wurde entwickelt, um mithilfe der Orientierung benachbarter Körner, Berechnungen zur Ermittlung der Korngrenzorientierung durchzuführen und somit die Orientierung natürlicher Korngrenzen zu bestimmen. Mit diesem Verfahren konnte der zeitliche Aufwand dieser anspruchsvollen Auswertung (verglichen zur herkömmlichen Methode mittels TEM-Untersuchung) stark reduziert werden, so dass eine quantitative Analyse vieler Korngrenzen möglich wurde. Aus den einzelnen Korngrenzorientierungen wurde die Korngrenzrotation, und die jeweiligen Anteile an Kippung und Drehung berechnet. Eine Abhängigkeit der Feststoffsegregation vom Kipp- und Drehanteil der Korngrenze wurde beobachtet, die am kleinsten für die reine Kipp- und Drehrotation war. Die ermittelten Segregationsweiten sind signifikant größer als die strukturellen Korngrenzweiten und bewegen sich zwischen 12 und 85 Å. Dieses Verhalten wurde durch eine künstliche Verbreiterung der Korngrenze erklärt, die durch eine Flugbahnabweichung der Korngrenzatome während der Verdampfung verursacht wurde. Eine Korngrenzweite von w0 = (10.1 ± 1.5) Å wurde für eine unverfälschte Korngrenze gefunden.Item Open Access Three dimensional analytical study of thin film battery electrodes(2021) Abdelkarim, Ahmed; Schmitz, Guido (Prof. Dr. Dr. h. c.)Li ions play a major role in batteries for energy storage. On the other hand, Li is notoriously challenging to be reliably detected with most microscpic techniques.Owing to its weak scattering form factor, low X-ray emission or peak overlaps in EELS spectroscopy, precise microscopic analysis of Li in battery materials is delicate. The aim of this study was to investigate a very sensitive analysis of the ionic transport on a microscopic, even on an atomic level scale. Well controlled amorphous lithium iron phosphate (LFP) thin films were prepared by ion-beam sputter deposition. In a subsequent annealing step, amorphous films were then crystallized. The electrochemical performance of both LFP phases is checked in cyclo-voltammetry, while structure and microstructure are confirmed by XRD and TEM, respectively. Cycling reversibility over 7000 cycles with a retention more than 92 % is accomplished for the crystalline LFP, whereas the amorphous phase is electrochemically nonfunctional. Intercalation of LFP thin films was studied as a function of film thickness (25 - 250 nm). The intercalation kinetics is systematically quantified over a wide range of scanning rates (0.004 to 400 mV s-1 in cyclic voltammetry experiments. Two different diffusion regimes for the material undergoing two phase reaction were explained with the help of the modified Randles-Sevcik equation. Slow Li diffusion in the thickest films was recorded. Dependence of the peak current on the layer thickness is explained in terms of increasing the grain boundary (GB) area. Opposite to the peak fluxes, the overpotential was interestingly found to be independent of the layer thickness. Less electrical driving force is required to force the same current in thick film. The grain boundaries represent an electroactive interface at which the overpotential appears. And hence, the grain boundaries work as fast conduction paths for faster Li ions diffusion. Thus, the total current is controlled by the total grain boundary area rather than the thin film surface. LiFePO4 (LFP) is then 3-dimensionally studied by laser-assisted Atom Probe Tomography (APT). The effects of laser power on the quantitative analysis of the amorphous phase by atom probe tomography were considered. The systematic investigation of amorphous samples presented herein demonstrates quantification of constituent elements, particularly lithium. Stoichiometric ratios relative to all elements (Li+Fe+P+O) and to the stable element (Fe) were calculated; P and O reveal reverse behavior against laser power. Li, on the other hand, after considering its migration, increases with rising laser power. Even though APT measurements at cryogenic temperatures (60 K) were performed, migration of Li ions in some LFP states was observed. In response to the applied measurement fields, Li ions are undoubtedly redistributed. In the amorphous LFP material, we observe a strong Li gradient towards the tip front, which hinders reliable analysis. Obviously, during measurement, Li is drawn towards the tip front and this effect increases with increasing laser power. The remaining host elements, Fe, P, and O, remain homogeneously distributed. New unique insights into the mechanisms of Li movements are provided. Li is pulled and Li enrichment/depletion regions are observed. A new term "Li shooting" is addressed to describe these Li movements. It is demonstrated that the ions indeed experience a field-dependent drift. By mathematically modelling the resulting composition profiles, the Li diffusivity is quantitatively evaluated. In a direct comparison between the amorphous and the crystalline LFP films of identical chemical composition, it is shown that the diffusivity of the amorphous structure is orders of magnitude faster than that of the crystalline state at a temperature of 60 K. Most notably, this is the first study to investigate the capabilities of APT in LFP at different de-/lithiation states. Li compositions show a wave-like distribution as a result of existence the Li rich/poor phases. 3D iso-surfaces and 2D orthoslices were provided to differentiate between the two phases. Li, in the fully lithiated phase, reaches its ideal stoichiometric ratio, while it is overestimated in the fully delithiated phase. Obviously, the thin films include inactive LFP regions. They were highlighted in this thesis for the first time by atom probe analysis. To quantify the two-phase nature of LFP, statistical analyses of the dis/charged LFP at different lithiation states were performed. Observed frequency distributions of the concentrations of small clusters were compared to the binomial distributions and discussed in detail. Deviations between observed and binomial distributions were represented in the Pearson coefficient to demonstrate the phase separation in the atom probe analysis. Our results provide an evidence to statistically understand the local microstructure evolution in battery materials, which is a pivotal characteristic of battery performance. On the other hand, APT has shown some constraints to microscopy differentiate between the two phases. Although APT measurements were performed at cryo-temperature, Li showed a displacement during measurements. Contrastingly, most of materials are fractured early at ultra low temperatures.Item Open Access Li-ion transport and optical modulation in thin-film battery electrodes(2021) Joshi, Yug; Schmitz, Guido (Prof. Dr.)The optical modulation of lithium manganese oxide (LiMn2O4, LMO) and lithium titanate (Li4Ti5O12, LTO) due to Li-ion insertion is quantitatively characterized. Ion beam sputtering is used to deposit the layers of respective materials on top of already sputtered platinum which acts as the current collector/reflector. The structure and morphology of the layers were probed using X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Well-defined intercalation states were prepared electrochemically and investigated by optical spectrometry in reflectance geometry. The obtained dispersion curves were then modeled using the Clausius-Mossotti dispersion equation to obtain the complex refractive index as a function of wavelength at various intercalation states. A continuous change in the effective resonant wavelength with lithium intercalation was observed. This was found to be consistent with the evolution of the band structure upon ion insertion. In LMO, two significant resonances were identified in the visible region of the spectrum, which shifts with the degree of intercalation. By associating this shift with the evolving band structure, the resonances were attributed to electronic transitions between the O-2p band and the split Mn-d band. In the case of LTO, the mechanism and effect of the phase transformation (from spinel structured Li4Ti5O12 to rock-salt type Li7Ti5O12 upon lithium insertion) on the optical response is studied. The same model (using Clausius-Mossotti dispersion) unveils the presence of one and two major resonant wavelengths/frequencies in the case of Li4Ti5O12 and Li7Ti5O12, respectively, in the UV/visible/NIR region of light. The single resonance in the case of Li4Ti5O12 is allocated to a transition from O-2p to Ti-t2g i.e., across the band-gap. Whereas for the Li7Ti5O12 phase, the two resonances were characterized for the electronic transitions from O-2p to empty Ti-t2g and from filled Ti t2g to empty Ti-eg. The concentration dependence of the derived dielectric constants indicates a fast lithium-ion transport through the grain boundaries. This helps in nucleating the grain boundaries with a conductive lithium-rich phase. This increases the electronic conductivity of the thin films in the initial stages of intercalation and explains the debated understanding of the fast dis-/charge capability of Li4Ti5O12 electrodes on a nanoscale. On a micrometer scale, the diffusion is controlled by the bulk diffusion. To investigate the kinetics of lithium migration at this length scale, an innovative technique is developed that employs optical microscopy in a constrained region of the sputtered thin-film sample. At this constrained region, lithium is blocked from entering the LTO structure directly from the electrolyte. Therefore, the technique enables the observation of the lateral transport of lithium through the electrode due to the optical contrast generated in this material during the ion insertion and subsequent phase transformation. The poor diffusivity of lithium in its end phases (or Li4Ti5O12 and Li7Ti5O12) is confirmed but, this poor diffusivity challenges the notion of high dis-/charging performance reported in this material. Surprisingly, the movement of the phase boundary is hindered which has been refuted in prior reports. However, this hindrance is confirmed here by the slow, linear growth kinetics of the Li-rich phase in the initial stages of the lithium transport. Interestingly, the partial solubility of lithium in the spinel structured Li4+δTi5O12 phase increases the diffusivity of lithium in this spinel phase drastically. This drastic increase in diffusivity along with the reduction in the size of the electrode seems to be compensating for the kinetic hindrance experienced by the phase boundary.Item Open Access Ultrathin organic thin-film transistors : investigating hybrid gate dielectrics and stable semiconductor monolayers(2021) Acharya, Rachana; Schmitz, Guido (Prof. Dr. Dr. h.c.)Organic electronics is an emerging field of research which includes the investigation of novel materials such as organic semiconductors and the development of devices such as organic thin-film transistors (TFTs). The unique properties of organic semiconductors, such as the ability to process them at relatively low temperatures, enable the development of potential applications of organic TFTs in flexible and wearable electronics such as rollable and foldable displays, conformable sensors and electronic skin. In order to facilitate the portable and lightweight nature of flexible electronics by powering them with small batteries or solar cells, a low operating voltage and an overall low power consumption are some of the main requirements of organic TFTs. High-capacitance gate dielectrics, such as hybrid gate dielectrics with an ultrathin metal oxide and an organic self-assembled monolayer (SAM), are an essential choice towards fulfilling these operation requirements. Organic TFTs are fabricated by depositing different materials as thin films by a variety of processes, and the individual film properties of the different components influence the overall electrical characteristics of organic TFTs. The main contribution of this thesis is to establish a correlation between the material properties of the individual components and the electrical properties of the organic TFTs, and moreover, suitably modify the fabrication process to achieve better electrical characteristics in organic TFTs. In this thesis, hybrid gate dielectrics consisting of an ultrathin aluminum oxide (AlOx) film and a phosphonic acid SAM are investigated. The AlOx films are fabricated by exposing the surface of the underlying aluminum gate electrode to an oxygen plasma, and the SAMs are processed from solution. Phosphonic acid molecules with an alkyl or a fluoroalkyl chain with different chain lengths have been chosen to form the SAMs. Two small-molecule organic semiconductors are selected as the active material in the organic TFTs: dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) and 2,7-diphenyl[1]benzothieno[3,2-b][1]benzothiophene (DPh-BTBT). The significance of both components of the hybrid gate dielectric in simultaneously achieving a leakage-current density of 10-7A/cm2 as well as an operating voltage of 3 V has been established. The thickness of the AlOx films was measured by Transmission Electron Microscopy and the films were characterized by Electron Energy Loss Spectroscopy and Atom Probe Tomography to determine the thin-film composition. Depending on the parameters of the oxygen-plasma process, AlOx films with a thickness ranging between 4 nm and 7 nm were fabricated, and consequently, organic TFTs with a gate-dielectric capacitance between 1 μF/cm2 and 1.6 μF/cm2. In particular, charge carrier mobilities ranging from 1.8 to 2.3 cm2/Vs were obtained for a number of favorable combinations of the plasma power and plasma duration that produce AlOx films with a small surface roughness and thus promote the formation of high-quality SAMs and well-ordered DNTT films on these gate dielectrics. The influence of the thickness of the SAM by employing different chain-length phosphonic acid molecules on different TFT characteristics such as the threshold voltage, gate-leakage current, charge-carrier mobility and the subthreshold swing has been examined in DNTT and DPh-BTBT TFTs. By employing the medium-chain-length phosphonic acid molecules, an optimum charge carrier mobility of 2 cm2/Vs for the DNTT TFTs, and a turn-on voltage of 0 V for the DPh-BTBT TFTs was achieved. The growth and morphology of the organic semiconductor DNTT on different gate-dielectric surfaces was observed by Scanning Electron Microscopy and Atomic Force Microscopy and was correlated with surface properties of the SAMs and the electrical characteristics of TFTs based on those films. The stability of ultrathin films with a thickness of one-two molecular monolayers of the organic semiconductor DNTT was investigated, and spontaneous morphological changes occurring in the films were observed and correlated with the stability of organic TFTs based on these films. The structural reconfiguration of the ultrathin DNTT films and the degradation of the charge-carrier mobility of organic TFTs based on these films were prevented to a certain extent by cryogenic cooling and in-situ encapsulation. The hybrid gate dielectric with its two components, the organic semiconductor and the gate dielectric-semiconductor interface are the focal points in this thesis. The material and surface properties of the individual components of the gate dielectric have been correlated with the film properties of the organic semiconductor and further with the electrical characteristics of organic TFTs.Item Open Access Basics of process physics and joint formation in resistance projection welding processes(2020) Wehle, Michael; Schmitz, Guido (Prof. Dr. Dr. h.c.)Diese Arbeit leistet einen Beitrag zur Verbesserung des Prozessverständnisses von Widerstands-Buckelschweißverfahren. Es werden die bekannten grundlegenden physikalischen Prozessgrößen von Widerstandsschweißprozessen auf diese Verfahren übertragen und deren Relevanz für den Prozess mittels analytischer und numerischer Methoden an Hand eines Prozessbeispiels abgeschätzt. Die Arbeit liefert somit ein physikalisches Prozessmodell, welches zum einen das Prozessverständnis vertieft zum anderen auch eine Grundlage für die Verbesserung der numerischen Finite Elemente Simulation bildet. Einen weiteren Fortschritt bringt die vorgelegte Arbeit durch die Untersuchungen zur Verbindungsbildung von Metallen (in diesem Fall rostfreie Stähle) im semi- festen Zustand mit sich. Durch die systematische Untersuchung der Einflussgrößen Temperatur, Druck und Scherung in der Grenzfläche zwischen zwei Probekörpern wird ein Kriterium erarbeitet, welches eine Abschätzung des Schweißergebnisses ermöglicht. Somit wird einerseits ein vertieftes allgemeines Verständnis des Festkörperschweißens sowie ein spezifisch für das Buckelschweißen gültige Verständnis aufgebaut. Eine Kombination des Verbindungsbildungskriteriums mit einem numerischen Modell zur Simulation des Gesamtprozesses ermöglicht eine Vorbewertung von Fügeaufgaben. Hierzu werden erste Ansätze präsentiert, um die Anwendbarkeit der Vorarbeiten in der numerischen Simulation aufzuzeigen. Durch die Ergebnisse dieser Arbeit wird somit eine Reduktion des experimentellen Aufwandes bei der Prozessentwicklung des untersuchten Verfahrens erreicht und die Vorbewertung künftiger Fügeaufgaben erleichtert. Des Weiteren lassen sich, durch die Gestaltung der durchgeführten Versuche, allgemeingültige Schlüsse ziehen, welche auch für artverwandte Widerstandsschweißverfahren einen Mehrwert darstellen.Item Open Access Reactive wetting of tin-based miniaturised solder joints(2022) Griffiths, Samuel J.; Schmitz, Guido (Prof. Dr.)Item Open Access Quantitative investigation of the cycling behavior and SEI formation via the time-resolved mass spectrometry(2023) Wang, Ke; Schmitz, Guido (Prof. Dr. Dr. h. c.)