03 Fakultät Chemie
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Item Open Access Experimental and computational phase studies of the ZrO2-based systems for thermal barrier coatings(2006) Wang,Chong; Aldinger, Fritz (Prof.)The ZrO2-based materials are practically important as the thermal barrier coatings (TBC) for high temperature gas turbines, due to their low thermal conductivity, high temperature thermal stability and excellent interfacial compatibility. Studies of the phase equilibira, phase transformation, and thermodynamics of the ZrO2-based systems can provide the necessary basic knowledge to develop the next generation TBC materials. In the thesis, the systems ZrO2 - HfO2, ZrO2 - LaO1.5, ZrO2 - NdO1.5, ZrO2 - SmO1.5, ZrO2 - GdO1.5, ZrO2 - DyO1.5, ZrO2 - YbO1.5 and ZrO2 - GdO1.5 - YO1.5 were experimentally studied. The samples were prepared by the chemical co-precipitation method, with aqueous solutions Zr(CH3COO)4, HfO(NO3)2, and RE(NO3)3×xH2O (RE=La, Nd, Sm, Gd, Dy, Yb) as starting materials. Various experimental techniques, X-ray diffraction (XRD), scanning electron microscopy (SEM), electron probe microanalysis (EPMA), transmission electron microscopy (TEM), differential thermal analysis (DTA), and high temperature calorimetry were employed to study the phase transformation, phase equilibria between 1400 and 1700°C, heat content and heat capacity of the materials. A lot of contradictions in the literature were resolved and the phase diagrams were reconstructed.Item Open Access High rate electrochemical dissolution of iron-based alloys in NaCl and NaNO3 electrolytes(2002) Wagner, Thomas; Mittemeijer, Eric Jan (Prof. Dr. Ir.)With the investigations presented in this work, the reaction mechanisms and principles of steels upon the high rate electrochemical dissolution in activating NaCl electrolytes and passivating NaNO3 electrolytes are revealed and the role of anodic surface films developing at the substrate surface is included in schematic dissolution models. For the development of accurate dissolution models, mask-less Electrochemical Machining (ECM) experiments with the flow channel cell at high electrolyte flow rates (up to 7 m/s) and current densities up to 70 A/cm2 were carried out in combination with following ex situ surface analysis. On the basis of these experiments a satisfactory dissolution model for heterogeneous steel substrates is presented, with special respect to the influence of local turbulences in the flowing electrolyte. To specify and characterize the electrochemical behaviour of the examined electrolyte / substrate combination, polarization measurements with the rotating cylinder electrode(RCE) are presented.Item Open Access Mechanische Spektroskopie an dünnen Kupferschichten(2001) Hagen, Joachim von der; Arzt, Eduard (Prof. Dr. phil.)In dieser Arbeit wurden erstmalig dünne Kupferschichten, die für die Mikroelektronik von zunehmendem technologischen Interesse sind, systematisch mit Hilfe der mechanischen Spektroskopie untersucht. Dabei handelt es sich um eine empfindliche und zerstörungsfreie Messmethode, mit der man Informationen über Defektstrukturen in der Schicht und in der Substrat/Schicht-Grenzfläche erhalten kann. Darüber hinaus wurden die spektroskopischen Ergebnisse ebenfalls erstmalig vor dem Hintergrund der thermomechanischen Eigenschaften dünner Schichten diskutiert. Die Voraussetzung hierfür wurde durch eine apparative Neuentwicklung geschaffen. Bei den untersuchten Systemen handelte es sich um Kupferschichten auf den Trägermaterialien Silizium und Saphir. Die Messungen beruhen auf der Dämpfung von Eigenschwingungen zwischen 20 bis 530°C. Daneben wird die Eigenfrequenz gemessen, aus der man prinzipiell Rückschlüsse auf den E-Modul von Schicht und Substrat, bzw. auf die Haftung ziehen kann. Es wurden vor allem passivierte und unpassivierte Kupferschichten zwischen 1 und 4 µm auf Siliziumsubstraten untersucht. Kupferschichten auf Silizium-Substraten zeigen ein breites, bei Temperaturzyklen stabiles, Dämpfungsmaximum zwischen 280 und 380°C. Mit zunehmender Schichtdicke wächst dessen Intensität, während sich seine Position zu höheren Temperaturen verschiebt. Auf Grund seiner Aktivierungsenthalpie kann dieses Maximum auf Versetzungsbewegungen zurückgeführt werden. Man nimmt an, dass die Versetzungen thermisch aktivierte, lokale Bewegungen um ihre Gleichgewichtslage ausführen, während sie an ihren Enden fest verankert sind. Als Verankerungspunkte sind vor allem die Grenz-, bzw. die Oberfläche, sowie weitere Versetzungen anzusehen. Die Relaxationsparameter der Dämpfungsmaxima zeigen, dass Einengungseffekte die Mobilität der beweglichen Versetzungssegmente maßgeblich bestimmen, wie es im Zusammenhang mit den hohen inneren Spannungen in dünnen Schichten diskutiert wird.Item Open Access Field evaporation and atom probe tomography of pure water tips(2020) Schwarz, T. M.; Weikum, E. M.; Meng, K.; Hadjixenophontos, E.; Dietrich, C. A.; Kästner, J.; Stender, P.; Schmitz, G.Measuring biological samples by atom probe tomography (APT) in their natural environment, i.e. aqueous solution, would take this analytical method, which is currently well established for metals, semi-conductive materials and non-metals, to a new level. It would give information about the 3D chemical structure of biological systems, which could enable unprecedented insights into biological systems and processes, such as virus protein interactions. For this future aim, we present as a first essential step the APT analysis of pure water (Milli-Q) which is the main component of biological systems. After Cryo-preparation, nanometric water tips are field evaporated with assistance by short laser pulses. The obtained data sets of several tens of millions of atoms reveal a complex evaporation behavior. Understanding the field evaporation process of water is fundamental for the measurement of more complex biological systems. For the identification of the individual signals in the mass spectrum, DFT calculations were performed to prove the stability of the detected molecules.Item Open Access Thermal cycling creep of a fiber reinforced aluminum alloy(2000) Flaig, Alexander; Arzt, Eduard (Prof. Dr.)In der vorliegenden Arbeit wurde das Kriechverhalten eines Metall-Matrix-Verbundwerkstoffes unter thermozyklischen Bedingungen untersucht. Bei dem Werkstoff handelt es sich um eine eutektische Al-Si Legierung, die mit 15 Vol.% diskontinuierlicher Aluminiumoxid-Fasern verstärkt wurde. Fortgeschrittene Methoden der mechanischen Hochtemperaturprüfung wurden verwendet, um die spannungsabhängige Verformungsrate während der thermischen Zyklen zu messen. Speziell untersucht wurden die Einflüsse der Maximaltemperatur, der thermischen Amplitude, von Haltezeiten, des Vorzeichens der Belastung und der Orientierung der Verstärkung untersucht. Bei geringen Spannungen wurde im Vergleich zu isothermen Bedingungen eine Beschleunigung des Kriechens gefunden und der beobachtete Spannungsexponent sank auf kleine Werte. Eine genaue Analyse der Dehnungen in einzelnen Zyklen lieferte Belege für das Vorhandensein beträchtlicher inelastischer Dehnungen, die sich von Halbzyklus zu Halbzyklus gegenseitig größtenteils kompensierten. Ein kontinuumsmechanisches Modell wurde aufgestellt, das den Verbundwerkstoff durch zwei aufeinander gestapelte und sich homogen verformende Platten annäherte. Das Modell wurde verwendet, um auf Basis von berechneten inneren Spannungen die Kriechraten und die innerzyklische Dehnungsentwicklung unter thermozyklischen Bedingungen zu erklären und vorherzusagen. Die experimentell gemachten Beobachtungen wurden im Vergleich zu den Simulationsergebnissen interpretiert und diskutiert. Die beobachteten Phänomene konnten auf Basis des Modells erklärt werden. Das Material zeigte ausgeprägtes Übergangsverhalten beim Be- und Entlasten. Dieses wurde mit Hilfe von spannungszyklischen Kriechexperimenten näher untersucht. Das Phänomen des Rückwärts-Kriechens nach äußerer Entlastung wurde ausgenutzt, um Effekte der Last-Übertragung von der Matrix auf die Fasern zu untersuchen und um auf die Entwicklung der Dehnrate eines Verbundwerkstoffes zurückzuschließen.Item Open Access Adjustable polystyrene nanoparticle templates for the production of mesoporous foams and ZnO inverse opals(2020) Abitaev, Karina; Qawasmi, Yaseen; Atanasova, Petia; Dargel, Carina; Bill, Joachim; Hellweg, Thomas; Sottmann, ThomasThe manifold applications of porous materials, such as in storage, separation, and catalysis, have led to an enormous interest in their cost-efficient preparation. A promising strategy to obtain porous materials with adjustable pore size and morphology is to use templates exhibiting the appropriate nanostructure. In this study, close-packed polystyrene (PS) nanoparticles, synthesized by emulsion polymerization, were used to produce porous PS and ZnO inverse opals. The size and distribution of the polystyrene nanoparticles, characterized by dynamic light scattering (DLS), small-angle neutron scattering (SANS), and scanning electron microscopy (SEM), were controlled via the concentration of sodium dodecyl sulfate (SDS). Systematic measurements of the water/styrene-interfacial tension show that the critical micelle concentration (CMC) of the ternary water–styrene–SDS system, which determines whether monodisperse or polydisperse PS particles are obtained, is considerably lower than that of the binary water–SDS system. The assemblies of close-packed PS nanoparticles obtained via drying were then studied by small-angle X-ray scattering (SAXS) and SEM. Both techniques prove that PS nanoparticles synthesized above the CMC result in a significantly unordered but denser packing of the particles. The polystyrene particles were subsequently used to produce porous polystyrene and ZnO inverse opals. While the former consists of micrometer-sized spherical pores surrounded by extended open-cellular regions of mesopores (Rpore ≈ 25 nm), the latter are made of ZnO-nanoparticles forming a structure of well-aligned interconnected pores.Item Open Access Peptide controlled shaping of biomineralized tin(II) oxide into flower-like particles(2019) Kilper, Stefan; Jahnke, Timotheus; Wiegers, Katharina; Grohe, Vera; Burghard, Zaklina; Bill, Joachim; Rothenstein, DirkThe size and morphology of metal oxide particles have a large impact on the physicochemical properties of these materials, e.g., the aspect ratio of particles affects their catalytic activity. Bioinspired synthesis routes give the opportunity to control precisely the structure and aspect ratio of the metal oxide particles by bioorganic molecules, such as peptides. This study focusses on the identification of tin(II) oxide (tin monoxide, SnO) binding peptides, and their effect on the synthesis of crystalline SnO microstructures. The phage display technique was used to identify the 7-mer peptide SnBP01 (LPPWKLK), which shows a high binding affinity towards crystalline SnO. It was found that the derivatives of the SnBP01 peptide, varying in peptide length and thus in their interaction, significantly affect the aspect ratio and the size dimension of mineralized SnO particles, resulting in flower-like morphology. Furthermore, the important role of the N-terminal leucine residue in the peptide for the strong organic-inorganic interaction was revealed by FTIR investigations. This bioinspired approach shows a facile procedure for the detailed investigation of peptide-to-metal oxide interactions, as well as an easy method for the controlled synthesis of tin(II) oxide particles with different morphologies.Item Open Access The effect of substrate orientation on the kinetics and thermodynamics of initial oxide-film growth on metals(2007) Reichel, Friederike; Mittemeijer, Eric J. (Prof. Dr. Ir.)This thesis addresses the effect of the parent metal-substrate orientation on the thermodynamics and kinetics of ultra-thin oxide-film growth on bare metals upon their exposure to oxygen gas at low temperatures (up to 650 K). For such thin oxide overgrowths on their metals, the resulting oxide-film microstructures often differ from those predicted by bulk thermodynamics, because of the relatively large contributions of interface and surface energies to the total energetics of the various metal-substrate/oxide-film systems. To this end, a model description has been developed to predict the thermodynamically stable microstructure of a thin oxide film grown on its bare metal substrate as function of the oxidation conditions and the substrate orientation. An amorphous state for ultra-thin oxide films grown on e.g. Al, Ti, Zr or Si can be thermodynamically, instead of kinetically, preferred up to a certain critical oxide-film thickness, because of the lower sum of surface and interface energies as compared to the corresponding crystalline modification. Beyond this critical oxide-film thickness, bulk thermodynamics will strive to stabilize the competing crystalline oxide phase. For Mg and Ni, the critical oxide-film thickness is less than 1 oxide monolayer and therefore the initial development of an amorphous oxide phase on these metal substrates is unlikely. Finally, for Cu and densely packed Cr and Fe metal surfaces, oxide overgrowth is predicted to proceed by the direct formation and growth of a crystalline oxide phase. Further, polished Al single-crystals with {111}, {100} and {110} surface orientations were introduced in an ultra-high vacuum system for specimen processing and analysis. After surface cleaning and annealing, the bare Al substrates have been oxidized by exposure to pure oxygen gas. During the oxidation, the oxide-film growth kinetics has been established by real-time in-situ spectroscopic ellipsometry. After the oxidation, the oxide-film microstructures were investigated by angle-resolved X-ray photoelectron spectroscopy and low energy electron diffraction. Finally, high-resolution transmission electron microscopic analysis was applied to study the microstructure and morphology of the grown oxide films on an atomic scale. The stoichiometric (i.e. Al2O3) oxide films grown on Al{111} are amorphous up to 450 K, whereas at higher temperatures epitaxial crystalline oxide films with a coherent metal/oxide interface develop. The oxide films grown on Al{100} are also overall stoichiometric, have uniform thicknesses and atomically flat metal/oxide interfaces. They are amorphous up to 400 K, but are transformed into crystalline gamma-Al2O3 upon annealing beyond a critical thickness. At more elevated temperatures (> 400 K), a crystalline Al2O3 film with a semi-coherent metal/oxide interface develops. For the crystalline gamma-Al2O3 overgrowth on Al{100}, an unexpected high lattice mismatch (> 15%) between the Al{100} substrate and the gamma-Al2O3 overgrowth is found with a semi-coherent metal/oxide interface. The oxide films grown on Al{110} for temperatures smaller than 550 K are also overall stoichiometric and amorphous. At more elevated temperatures (> 550 K), the original bare Al{110} surface becomes reconstructed at the onset of oxidation and {111}-facets develop. The kinetics of the oxide-film growth on the bare Al{100} and Al{110} substrates can be subdivided into a initial, very fast and a subsequent, very slow oxidation stage. For the oxidation of the bare Al{111} substrate up to 450 K, a distinction between an initial, very fast and a subsequent, very slow oxidation stage cannot be made. Instead, the initial oxide-film growth rate on Al{111} decreases only gradually with increasing oxidation time. The experimental growth curves for the thermal oxidation of Al single-crystals in the temperature regime of 350 – 600 K can be accurately described by considering the coupled currents of Al3+ cations and electrons in an uniform surface-charge field. As such, a gradual transformation of the initial amorphous oxide film on Al{100} into gamma-Al2O3, was observed with increasing oxidation temperature in the range of 350 – 600 K for Al{100}, as well as up to 450 K for Al{110}. Whereas, on Al{111}, the corresponding amorphous-to-crystalline transition was found to be more abrupt.Item Open Access The initial oxidation of Al-Mg alloys(2009) Panda, Emila; Mittemeijer Eric J. (Prof. Dr.)The oxide film present on an alloy surface influences many of its physical and chemical properties, such as corrosion resistance, adhesion, electrical and thermal conductivity, friction and wear resistance. The technological demand to control and optimize these properties by tailoring both the alloy-substrate and oxide-film microstructure has led to a large interest for the thermal oxidation behavior of metallic alloys in the last decades. However, up to date, investigations on the oxidation of metallic alloys have been performed mainly at relatively high temperatures (i.e. T > 800 K) and high pressures (i.e. 0.1 < p < 105 Pa). At these high temperatures, relatively thick (i.e. in the μm-range) oxide scales, composed of multiple, crystalline oxide phases, develop on the alloy surface by sequential, preferential oxidation of the alloy constituents. The oxidation of metallic alloys at low temperatures (i.e. T < 600 K), on the other hand, has been investigated only very scarcely up to date. At these low temperatures, thermally activated diffusion of reactants through the developing oxide-film is negligibly small and, consequently, ultra-thin (< 3 nm) oxide films of near-limiting thicknesses are formed, which are generally constituted of a metastable (often amorphous), multi-metal oxide phase. However, the detailed microstructures (i.e. thickness, morphology, crystallographic structure, chemical composition and constitution) of these initial oxide films as a function of the alloy microstructure (e.g. alloying content, surface orientation), the surface pretreatment (e.g. with or without a native oxide) and the growth conditions (e.g. temperature, time and partial oxygen pressure) are often unknown. This PhD thesis addresses the initial stages of dry, thermal oxidation of bare (i.e. without a native oxide) Al-based Al-Mg alloy surfaces as a function of the oxidation conditions (here: oxidation temperature, time and partial oxygen pressure) and for different pre-treatments of the bare alloy surface prior to oxidation. To this end, first, a thermodynamic model, which accounts for the crucial role of surface and interface energetics in such ultra-thin oxide-film systems, was developed to predict the initial, amorphous oxide overgrowth (i.e. am-Al2O3, am-MgO and/or am-MgAl2O4) developing on a bare AlMg alloy substrate as a function of the growth temperature, the Mg alloying content at the alloy/oxide interface and the oxide-film thickness (≤ 5 nm). To this end, experimental or empirically-estimated values for the surface energies of the competing amorphous oxide phases of am-Al2O3, am-MgO or am-MgAl2O4 (further denoted as , and ) as a function of the growth temperature were employed. Required values for the interface energy between the alloy substrate and the competing amorphous oxide phases as a function of the temperature and the Mg alloying content at the alloy/oxide interface were estimated from corresponding expressions, as derived on the basis of the macroscopic atom approach. Further, comprehensive experimental investigation of the interrelationships between the oxide growth kinetics, the microstructural evolutions in the oxide overgrowth and the alloy subsurface and the oxidation conditions was conducted. To this end, polycrystalline AlMg alloy specimens with nominal Mg alloying contents of 0.8 and 1.1 at. % were thermally oxidized in a dedicated UHV system (base pressure < 3×10-8 Pa) for specimen processing (i.e. cleaning, annealing and oxidation) and in-situ analysis. After introduction of the polished alloy sample surface in the UHV system, first the native oxide on the alloy surface was removed by sputter cleaning with a focussed 1 keV Ar+ ion beam rastering the entire alloy surface (of 7×7 mm2). The thus obtained sputter-cleaned (as verified by in-situ AR-XPS) alloy surfaces will be further designated as SC-substrate. These SC-substrates were subsequently in-situ exposed to pure O2 gas in the partial oxygen pressure range of pO2 = 10-4 - 10-2 Pa for durations varying from 15 s up to 6 hrs and for various temperatures in the range of T = 300 – 610 K. As an additional surface pretreatment, some SC-substrates were in-vacuo annealed for 1200 s at T = 460 K prior to the oxidation. The thus obtained sputter-cleaned, annealed alloy surfaces will be further designated as SC/Ann-substrate. Real-time in-situ spectroscopic ellipsometry (RISE) was applied to establish the oxide-film growth kinetics. The thicknesses, compositions and chemical constitutions of the grown films were determined by in-situ angle-resolved X-ray Photoelectron Spectroscopy (AR-XPS). Furthermore, the microstructures of some of the grown oxide films were analysed on an atomic scale by cross-sectional high resolution-Transmission Electron Microscopy (HR-TEM).Item Open Access Identification of the first sulfobetaine hydrogel‐binding peptides via phage display assay(2023) Ihlenburg, Ramona B. J.; Petracek, David; Schrank, Paul; Davari, Mehdi D.; Taubert, Andreas; Rothenstein, DirkUsing the M13 phage display, a series of 7- and 12-mer peptides which interact with new sulfobetaine hydrogels are identified. Two peptides each from the 7- and 12-mer peptide libraries bind to the new sulfobetaine hydrogels with high affinity compared to the wild-type phage lacking a dedicated hydrogel binding peptide. This is the first report of peptides binding to zwitterionic sulfobetaine hydrogels and the study therefore opens up the pathway toward new phage or peptide/hydrogel hybrids with high application potential.