(Bulk) metallic glasses ((B)MGs) are known to exhibit the highest yield strength of any metallic material (up to 5GPa), and show an elastic strain at ambient conditions, which is about ten times larger than that of crystalline materials. Despite these intriguing mechanical properties, BMGs are not used as structural materials in service, so far. The major obstacle is their inherent brittleness, which results from severe strain localization in so-called shear bands. MGs fail due to formation and propagation of shear bands. A very effective way to attenuate the brittle behaviour is to incorporate crystals into the glass. The resulting BMG composites exhibit high strength as well as plasticity. Cu-Zr-Al-based BMG composites are special to that effect, since they combine high strength, plasticity and work-hardening. They are comprised of the glass and shape-memory B2 CuZr crystals, which can undergo a deformation-induced martensitic transformation. The work-hardening originates from the martensitic transformation and overcompensates the work-softening of the glass. The extent of the plasticity of BMG composites depends on the volume fraction, size and particularly on the distribution of the B2 CuZr crystals. Nowadays, it is very difficult, if not impossible to prepare BMG composites with uniformly distributed crystals in a reproducible manner by melt-quenching, which is the standard preparation method. Flash-annealing of BMGs represents a new approach to overcome this deficiency in the preparation of BMG composites and is the topic of the current thesis. Cu46Zr46Al8 and Cu44Zr44Al8Hf2Co2 BMGs were flash-annealed and afterwards investigated in terms of phase formation, crystallization kinetics and mechanical properties. Flash-annealing is a process, which is characterized by the rapid heating of BMGs to predefined temperatures followed by instantaneous quenching. A temperature-controlled device was succesfully developed and built. The Cu-Zr-Al-based BMGs can be heated at rates ranging between 16 K/s and about 200 K/s to temperatues above their melting point. Rapid heating is followed by immediate quenching where cooling rates of the order of 1000 K/s are achieved. As a BMG is flash-annealed, it passes the glass-transition temperature, Tg, and transforms to a supercooled liquid. Further heating leads to its crystallization and the respective temperature, the crystallization temperature, Tx, divides the flash-annealing of BMGs into two regimes: (1) sub-Tx-annealing and (2) crystallization. The structure of the glass exhibits free volume enhanced regions (FERs) and quenched-in nuclei. Flash-annealing affects both heterogeneities and hence the structural state of the glass. FERs appear to be small nanoscale regions and they can serve as initiation sites for shear bands. Flash-annealing of Cu-Zr-Al-based BMGs to temperatures below Tg leads to structural relaxation, the annihilation of FERs and the BMG embrittles. In contrast, the BMG rejuvenates, when flash-annealed to temperatures of the supercooled liquid region (SLR). Rejuvenation is associated with the creation of FERs. Compared to the as-cast state, rejuvenated BMGs show an improved plasticity, due to a proliferation of shear bands, which are the carrier of plasticity in MGs. Flash-annealing enables to probe the influence of the free volume in bulk samples on their mechanical properties, which could not be studied, yet. In addition, B2 CuZr nanocrystals precipitate during the deformation of flash-annealed Cu44Zr44Al8Hf2Co2 BMGs. Deformation-induced nanocrystallization does not occur for the present as-cast BMGs. Flash-annealing appears to stimulate the growth of quenched-in nuclei, which are subcritical in size and can also dissolve, once the BMG is heated to temperatures in the SLR. Rejuvenation represents a disordering process, whereas the growth of quenched-in nuclei is associated with ordering. There is a competition between both processes during flash-annealing. The ordering seems to lead to a “B2-like” clustering of the medium range of Cu44Zr44Al8Hf2Co2 BMGs with increasing heating duration. So far, there does not exist another method to manipulate the MRO of BMGs. If Cu44Zr44Al8Hf2Co2 BMGs are flash-annealed to temperatures near Tx, most likely compressive resiudal stresses develop near the surface, which is cooled faster than the interior of the BMG specimen. They hinder the propagation of shear bands and increase the plasticity of flash-annealed BMGs in addition to rejuvenation and deformation-induced nanocrystallization. If BMGs are heated to temperatures above Tx, they start to crystallize. Depending on the exact temperature to which the BMG is flash-annealed and subsequently quenched, one can induce controlled partial crystallization. Consequently, BMG composites can be prepared. Both Cu-Zr-Al-based BMGs are flash-annealed at various heating rates to study the phase formation as a function of the heating rate. In addition, Tg and Tx are identified for each heating rate, so that a continuous heating transformation diagram is constructed for both glass-forming compositions. An increasing heating rate kinetically constrains the crystallization process, which changes from eutectic (Cu10Zr7 and CuZr2) to polymorphic (B2 CuZr). If the Cu-Zr-Al-based BMGs are heated above a critical heating rate, exclusively B2CuZr crystals precipitate, which are metastable at these temperatures. Thus, flash-annealing of Cu46Zr46Al8 and Cu44Zr44Al8Hf2Co2 BMGs followed by quenching enables the preparation of B2 CuZr BMG composites. The B2 precipitates are small, high in number and uniformly distributed when compared to conventional BMG composites prepared by melt-quenching. Such composite microstructures allow the direct observation of crystal sizes and numbers, so that crystallization kinetics of deeply supercooled liquids can be studied as they are flash-annealed. The nucleation kinetics of devitrified metallic glass significantly diverge from the steady-state and at high heating rates above 90 K/s transient nucleation effects become evident. This transient nucleation phenomenon is studied experimentally for the first time in the current thesis. Once supercritical nuclei are present, they begin to grow. The crystallization temperature, which depends on the heating rate, determines the crystal growth rate. At a later stage of crystallization a thermal front traverses the BMG specimen. In levitation experiments, this thermal front is taken as the solid-liquid interface and its velocity as the steady-state crystal growth rate. However, the thermal front observed during flash-annealing, propagates through the specimen about a magnitude faster than is known from solidification experiments of levitated supercooled liquids. As microstructural investigations show, crystals are present in the whole specimen, that means far ahead of the thermal front. Therefore, it does not represent the solid-liquid interface and results from the collective growth of crystals in confined volumes. This phenomenon originates from the high density of crystals and becomes evident during the heating of metallic glass. It could be only observed for the first time in the current thesis due to the high temporal resolution of the high-speed camera used. The heating rate and temperature to which the BMG is flash-annealed determine the nucleation rate and the time for growth, respectively. The size and number of B2 CuZr crystals can be deliberately varied. Thus mechanical properties of B2 CuZr BMG composites can be studied as a function of the volume fraction and average distance of B2 particles. Cu44Zr44Al8Hf2Co2 BMG specimens were flash-annealed at a lower and higher heating rate (35 K/s and 180 K/s) to different temperatures above Tx and subsequently subjected to uniaxial compression. BMG composites prepared at higher temperatures show a lower yield strength and larger plastic strain due to the higher crystalline volume fraction. They not only exhibit plasticity in uniaxial compression, but also ductility in tension as a preliminary experiment demonstrates. Furthermore, nanocrystals precipitate in the amorphous matrix of BMG composites during deformation. They grow deformation-induced from quenched-in nuclei, which are stimulated during flash-annealing. In essence, flash-annealing of BMGs is capable of giving insight into most fundamental scientific questions. It provides a deeper understanding of how annealing affects the structural state of metallic glasses. The number and size of structural heterogeneities can be adjusted to prepare BMGs with improved plasticity. Furthermore, crystallization kinetics of liquids can be studied as they are rapidly heated. Transient nucleation effects arise during rapid heating of BMGs and they cannot be described using the steady-state nucleation rate. Therefore, an effective nucleation rate was introduced. Besides, the flash-annealing process rises the application potential of BMGs. The microstructure of BMG composites comprised of uniformly distributed crystals and the glass, can be reliably tailored. Thus, flash-annealing constitutes a novel method to design the mechanical properties of BMG composites in a reproducible manner for the first time. BMG composites, which exhibit high strength, large plasticitiy and as in the case of B2 CuZr BMG composites as well work-hardening behaviour, can be prepared, so that the intrinsic brittleness of monolithic BMGs is effectively overcome.

Eine der Schlüsselkomponenten der Brennstoffzelle ist die Bipolarplatte (BPP) welche den Kern eines Brennstoffzellensystems bildet und hierbei die mechanische Hauptstruktur des Brennstoffzellenstapels bildet. Die Bipolarplatte sorgt aber nicht nur für die Stabilität des Stacks, sondern übernimmt weitere Aufgaben wie die elektrische Verbindung der Zellen, die Gasverteilung über die Fläche der Platten, Gastrennung zwischen angrenzenden Zellen, Dichtung nach außen und Kühlung. Zentrale Zielstellung ist somit mittels alternativer Anbindungs- bzw. Fügeverfahren, wie der Einsatz leitfähiger Kleber, isolierender Kleber, sowie Laser- und Widerstandsschweißung eine anforderungsgemäße Verbindung der metallischen Bipolarplatten zu schaffen bei möglichst geringer Einschränkung der Korrosionsbeständigkeit. Gleichzeitig im jeweiligen Verfahren soll dabei eine gleichmäßigere Strom- u. Wärmeverteilung auf der Platte erzielt werden. Darüber hinaus beeinflusst die Anzahl der Schweiß- und/oder Klebepunkte, welche auf einer metallischen Bipolarplatte aufgebracht werden, die Alterung der Zellen, bedingt durch Wechselwirkungen zwischen dem Material der Bipolarplatten sowie der eingesetzten Klebematerialien, oder die Beschädigung der Korrosionsschutzbeschichtung der metallischen Bipolarplatten durch verschiedenste Schweißverfahren. Somit müssen die Anbindungspunkte der metallischen Bipolarplatte so ausgeführt werden, dass die Entstehung von Hot Spots und Korrosionsschwerpunkten vermieden wird, wobei eine ausreichende Strom- und Wärmeleitfähigkeit erzielt wird. / One of the key components of the fuel cell is the bipolar plate (BPP), which forms the core of a fuel cell system and is the main mechanical structure of the fuel cell stack. However, the bipolar plate not only ensures the stability of the stack, but also performs other tasks such as the electrical connection of the cells, gas distribution over the surface of the plates, gas separation between adjacent cells, sealing to the outside and cooling. The central objective is thus to create a connection between the metallic bipolar plates that meets the requirements using alternative connection and joining processes, such as the use of conductive adhesives, insulating adhesives, as well as laser and resistance welding, with the least possible restriction of corrosion resistance. At the same time, a more even current and heat distribution on the plate is to be achieved in the respective process. Furthermore, the number of welding and/or bonding points applied to a metallic bipolar plate influences the ageing of the cells due to interactions between the material of the bipolar plates and the bonding materials used, or the damage to the anti-corrosion coating of the metallic bipolar plates caused by various welding processes. Thus, the connection points of the metallic bipolar plate must be designed in such a way that the formation of hot spots and corrosion foci is avoided, while sufficient current and heat conductivity is achieved.

Two major challenges that impede fuel cell technology breakthrough are the insufficient activity of the electrocatalysts for the oxygen reduction reaction and their degradation during operation, caused by the potential-induced corrosion of their carbon-support upon fuel cell operation. Unsupported electrocatalysts derived from tailored noble-metal nanostructures are superior to the conventional carbon-supported Pt nanoparticle catalysts and address these barriers by fine-tuning the surface composition and eliminating the support. Herein, recent efforts and achievements in the design, synthesis and characterization of unsupported electrocatalysts are reviewed, paying special attention to noble-metal aerogels, nano/meso-structured thin films and template-derived metal nanoarchitectures. Their electrocatalytic performances for oxygen reduction are compared and discussed, and examples of successful catalyst transfer to polymer electrolyte fuel cells are highlighted. This report aims to demonstrate the potential and challenges of implementing unsupported catalysts in fuel cells, thereby providing a perspective on the further development of these materials.

In der vorliegenden Dissertation werden verschiedene plasmonisch aktive Kern/Schale- Nanopartikel synthetisiert, experimentell und theoretisch charakterisiert und in analytischen Anwendungen der oberflächenverstärkten Raman-Spektroskopie (engl. surface-enhanced Raman scattering, SERS) eingesetzt. Es werden die optischen Eigenschaften von Gold/Silber-Nanoschalen mit durchstimmbaren Plasmonbanden behandelt. Motivation dafür ist die Frage nach optimalen SERS-Markern für die rote Laseranregung (λ = 632.8 nm). In SERS-Anwendungen gibt es die Möglichkeit mehrere Marker-Moleküle auf die Oberfläche der Nanopartikel aufzubringen, um so eine erhöhte Multiplexing-Kapazität zu generieren. Diese Option der gemischten Monolagen wird in der vorliegenden Arbeit untersucht. Es werden SERS-Marker-Konzepte für die rote Laseranregung basierend auf einzelnen Nanopartikeln gezeigt. Außerdem wird dargestellt, inwieweit sich durch die Anordnung von Nanopartikeln in allen drei Raumdimensionen neue SERS-Marker- Konzepte mit sehr guten plasmonischen Eigenschaften realisieren lassen. In den oben beschriebenen Kapiteln übernehmen Nanopartikel die Rolle des SERS-Substrats für den selektiven Nachweis eines bestimmten Zielmoleküls (z.B. Antigens). Neben diesen Anwendungen können Nanopartikel jedoch auch noch als SERS-Substrat für die markierungsfreie Detektion von Analytmolekülen eingesetzt werden. In dieser Dissertation wird die Herstellung, Charakterisierung und der Einsatz eines integrierten SERS-Substrats für die kombinierte Festphasensynthese und Analytik mittels plamonisch aktiver Gold/Glas-Kern/Schale-Nanopartikel auf Harz-Mikrokugeln behandelt.

There are various approaches available for the durability design of non-metallic reinforcement in concrete, such are ACI 440.1R-15, BISE-99, CHBDC-07, NS3473-98, and JSCE-97.These available design approaches are very general and consolidate all effects into a single factor for each type of Fiber Reinforced Polymer (FRP) material depending on the type of fiber, which does not actually resemble the real concrete service life. Thus, more reliable design approaches are in need and have been developed in recent years which tried to simulate real-life conditions. They provide safety factors by not only considering the type of fiber, but also the moisture conditions, temperature conditions, diameter of the bar, and service life of the structure. Such design approaches need to be considered in the standards that can be applied in the concrete design context. This study was a part of my master’s thesis.

Die Anwendung des Verfahrens der inversen Parameteridentifikation auf die Nanoindentation mit einer neuen Materialklasse (amorphe Legierungen) ist Hauptgegenstand der vorliegenden Arbeit. Um die Methode auf ihre Zuverlässigkeit hin zu überprüfen, werden darüber hinaus die drei Härtevergleichsplatten HV240, HV400 und HV720 sowie das oxidische Glas BK7, deren Nanoindentationsmessungen von Dipl.-Ing. André Clausner schon zu einem früheren Zeitpunkt vorgenommen wurden, zur Berechnung herangezogen. Die Auswahl der Materialien erfolgte so, dass diese einen möglichst großen Bereich von Y abdecken, von BK7 bis hin zu HV240. Damit soll gezeigt werden, dass das Verfahren der inversen Parameteridentifikation für einen großen Bereich von natürlich vorkommenden Materialien genutzt werden kann. Der Schwerpunkt liegt dabei auf der Bestimmung des Fließverhaltens, das durch die Parameter Fließgrenze1 Y und Verfestigungsexponent n erfolgt. Ziel ist es, in Zukunft auf weitere Experimente, die bisher zur Bestimmung der mechanischen Materialeigenschaften genutzt wurden und häufig zur Zerstörung der Proben führten, verzichten zu können. Für viele Gläser, z.B. BK7, sind derartige zerstörende Versuche nicht anwendbar, weil spröde Materialien splittern statt plastisch zu fließen. Dieser Arbeit liegt die Methode der Finiten-Elemente zugrunde, um eine inverse Parameteridentifikation zu realisieren. Sie wird hier eingesetzt, weil es sich bei plastischer Verformung um einen nichtlinearen Prozess2 handelt, der analytisch nicht mehr geschlossen gelöst werden kann. Die Simulationssoftware ANSYS R und ein Optimierungsmodul (SPC-OPT) der Fakultät für Maschinenbau dienen zur Berechnung. Bei der Simulation werden dabei ein zweidimensionales Modell und ein realitätsnahes dreidimensionales Modell eingesetzt.

This work contributes to the structure investigation on the basis of small-angle neutron scattering (SANS). A new analytical scattering function for polydispers precipitates with diffusion zones is presented and used in SANS experiments. For diluted and dense packed systems structure describing parameter values were obtained. These results lead to a deeper understanding of the process of nanocristallization of amorphous alloys. The investigation of SANS on Fe73.5Si15.5B7Cu1Nb3 shows that the Fe3Si type nanocrystals created in the amorphous matrix during annealing are covered by Nb-atoms. The accumulation of Nb-atoms or Nb-B-aggregates acting as inhibitors at the surface of the nanocrystals is assumed to be the basic mechanism controlling the evolution of the precipitates. For the first time this inhibitor-model is shown to be correct without doubts. In the Zr32Ti7.5Al10Cu20Ni8 amorphous alloy the formation of ultrafine nanocystals of about 2-3 nm in diameter was observed. The nanocrystallization starts after ordered clusters achieved particular sizes and a certain packing fraction. This leads to a new model for the microscopic formation procedure of ultrafine nanocrystals in this amorphous alloy. Theoretical models of fractal systems are applied to complicated polydisperse materials. Both the theory for an exact surface fractal of Hermann (1994)and the model for coupled volume and surface fractals in the formulation of Wong (1992) are shown to be applicable. The latter approach is applied to experimental data here for the first time. With computer simulations conditions for scattering experiments were optained therewith predictions about the quality and grade of fractality in real specimens become possible. / Die vorliegende Arbeit ist ein Beitrag zur Strukturaufklärung mittels Neutronen-Kleinwinkel-Streuung (SANS). Es wird eine neu entwickelte analytische Streufunktion für polydisperse Ausscheidungen mit Diffusionszonen genutzt, um SANS Experimente auszuwerten. Sowohl für verdünnte, als auch für dicht gepackte Systeme werden auf diese Weise quantitative Strukturparameter gewonnen. Diese liefern einen Beitrag zum Verständnis des Nanokristallisationsverhaltens amorpher metallischer Gläser. Die Auswertung der Experimente an on Fe73.5Si15.5B7Cu1Nb3 zeigt, dass Fe3Si-artige Nanokristalle, die während der Temperaturbehandlung in der amorphen Matrix entstehen, von Nb-Atomen bedeckt werden. Diese Ansammlung von Nb-Atomen oder von entsprechenden Nb-B-Aggregaten auf der Oberfläche dieser Ausscheidungen hemmt das Größenwachstum der entstehenden Nanokristalle. Dieses Inhibitor-Modell wurde hier erstmals zweifelsfrei bestätigt. In Proben des amorphen metallischen Glases Zr32Ti7.5Al10Cu20Ni8 werden ultrafeine Ausscheidungen mit Durchmessern von 2-3 nm beobachtet. Diese entstehen verzögert nach der Ausprägung dicht gepackter Gebiete mit erhöhter Nahordnungsstruktur. Es wird ein Modell vorgeschlagen, das diesen Prozess erklären kann. Theoretisch diskutierte Modelle für fraktale Systeme werden auf komplizierte polydisperse Materialien angewendet. Sowohl die Formulierung von Hermann (1994) für ein exaktes Oberflächenfraktal, als auch der erstmals auf experimentelle Daten angewendete Ansatz von Wong (1992) für ein gekoppeltes Volumen- und Oberflächenfraktal erweisen sich als praktisch nutzbar. Mittels Computersimulationen wurden Bedingungen abgeleitet, die an Streuexperimente zu stellen sind, damit Aussagen über Qualität und Grad von Fraktalität in realen Proben getroffen werden können.

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