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Die vorliegende Arbeit verbindet die Thematik der magnetoaktiven Hybridmaterialien mit Hydrogelen. Hydrogele stellen eine breite Klasse an Materialien dar, die sich unter anderem durch ein Stimuli-sensitives Quellverhalten auszeichnen. In dieser Arbeit wurde ein magnetoaktives Hydrogel hergestellt und der auftretende magnetorheologische Effekt (MRE) und die Auswirkung von Quellgradänderungen auf diesen untersucht. Hierzu wurde zunächst eine geeignete Kombination von Matrix und Partikeln ausgewählt und das Material synthetisiert. Hierauf folgten eine rheologische und eine tomographische Untersuchung. Die verwendeten Methoden mussten für eine Anwendung auf Hydrogele angepasst werden. Die rheologischen Daten erlauben eine Charakterisierung der mechanischen Eigenschaften des Materials und eine phänomenologische Beschreibung des magnetorheologischen Effektes durch eine empirisch ermittelte Formel. Durch Hinweise auf die mikroskopisch stattfindenden Prozesse aus den tomographischen Daten konnte eine äquivalente Formel aus theoretischen Überlegungen hergeleitet werden.

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Manipulating grain boundaries to pursue favorable physical or chemical properties is essential in materials design. As a prominent candidate for direct heat-to-electricity conversion applications, the performance of thermoelectric (TE) materials is strongly affected by the chemical compositions and physical properties of grain boundaries. As a layer-by-layer deposition technique, atomic layer deposition (ALD) is recognized as a unique method for depositing highly uniform films in a controlled manner. This gives us fresh insight into applying ALD approaches on a powder surface to realize a uniform coating on each particle with a specific thickness and composition of ALD layers. Powder ALD also provides a new way to construct complex layer structures, such as multiple layers, with precision layer composition control. In this thesis, a strategy of interface modification based on powder ALD is introduced in various TE materials (Bi, CuNi, and Zn4Sb3) to accurately control and modify the phase boundaries by oxide layer coating. For elemental bismuth, as the first discovered TE material, ultrathin layers of Al2O3, TiO2, and ZnO are typically deposited on powders via 1–20 cycles. All of the oxide layers significantly alter the microstructure and suppress grain growth. The hierarchical interface modifications aid the formation of an energy barrier by the oxide layer, resulting in a substantial increase in the Seebeck coefficient that is superior to that of most pure polycrystalline metals. Conversely, taking advantage of strong electron and phonon scattering, an exceptionally large decrease in thermal conductivity is obtained. A maximum figure of merit (zT) of 0.15 at 393 K and an average zT of 0.14 at 300–453 K were achieved in 5 cycles of Al2O3-coated Bi. Additionally, newly developed Sb2O5 thin films produced from SbCl5 and H2O2 were formed on the surfaces of Bi powders. Because of the high Kapitza resistance generated by Sb2O5 layers on Bi particles, a substantial decrease in total thermal conductivity from 7.8 to 5.7 W/m·K was obtained with just 5 cycles of Sb2O5 layer deposition and a 16% reduction in lattice thermal conductivity. Because of strong phonon scattering, the maximum zT values increased by approximately 12% and were relocated to 423 K. For CuNi, first, single-type ZnO and Al2O3 layers were deposited on the surface of CuNi powder, and their effect on the TE performance of the bulk was thoroughly investigated. The enhancement of the Seebeck coefficient, caused by the energy filtering effect, compensates for the electrical conductivity deterioration due to the low electrical conductivity of the oxide layers. Furthermore, the oxide layers may significantly increase the phonon scattering. Therefore, to reduce the resistance of phase boundaries, a multiple-layer structure was constructed by inserting Al2O3 into ZnO. Atom probe tomography shows that the Al atoms diffused into ZnO and realized the doping effect after pressing. Al diffusion has great potential to increase the electrical conductivity of coating layers. In comparison to pure CuNi, zT increased by 128% as a result of the decrease in resistance and stronger phonon scattering at the phase boundaries. The oxide layer coating not only yields a significant enhancement in the TE performance but also behaves as an energy barrier to suppress the migration of Zn ions in Zn4Sb3. With increasing ZnO layer cycle numbers, the layer thickness can be precisely tuned, and Zn migration can be effectively blocked with an oxide barrier. In the 100 cycle ZnO-coated sample, there was little deterioration of the power factor due to increasing resistivity. However, the decrease in total thermal conductivity results in similar zT values compared with pure Zn4Sb3, indicating that the TE performance of the 100 cycle ZnO layer-coated sample did not degrade. Additionally, 100 cycles of ZnO layers result in significantly enhanced thermal stability and effectively block Zn atom movement after 10 thermal cycling tests. The study demonstrates that ALD-based interface modification is a versatile method for decoupling TE parameters and precisely modifying phase boundaries, which is practical for other TE materials.:Abstract Kurzfassung Contents Chapter 1 Introduction Chapter 2 Background and motivation 2.1 Fundamental knowledge of thermoelectricity 2.1.1 Thermoelectric effects 2.1.2 Thermoelectric parameters 2.2 Interface/surface modification of thermoelectric materials 2.2.1 Principles 2.2.2 Discontinuous interface modification 2.2.3 Continuous interface modification 2.3 Background of atomic layer deposition 2.3.1 An ALD process example 2.3.2 Growth characteristics 2.3.3 Powder ALD 2.4 Powder ALD in thermoelectricity 2. 5 State-of-art in Bi, CuNi alloys, and Zn4Sb3 2.5.1 Bi and CuNi 2.5.2 β-Zn4Sb3 Chapter 3 Experimental techniques 3.1 Material syntheses and preparations 3.2 Material characterizations 3.2.1 ICP-OES 3.2.2 APT 3.2.3 LSR and LFA 3.2.4 GIXRD 3.2.5 XPS Chapter 4 Effect of powder ALD interface modification on the thermoelectric properties of Bismuth 4.1 Introduction 4.2 The influence of Al2O3, TiO2, and ZnO layers on TE properties 4.2.1 Characterizations of Al2O3, ZnO, and TiO2 ALD thin films 4.2.2 Microstructural characterizations of bulks 4.2.3 Effect of ALD surface modification on the TE properties of Bi 4.3 The influence of newly developed Sb2O5 layers on TE properties 4.3.3 New developed Sb2O5 ALD films 4.3.2 Microstructural characterizations of bulks 4.3.3 Effect of ALD surface modification on the TE properties of Bi Chapter 5 Precision interface engineering of CuNi alloys by multilayers of powder ALD 5.1 Introduction 5.2 Analysis of CuNi powders coated with ZnO and Al2O3 5.3 The effect of single-kind oxides on TE performance 5.4 The effect of multilayers on TE performance 5.5 Summary Chapter 6 Blocking ion migration in Zn4Sb3 by powder ALD 6.1 Introduction 6.2 Zn ion migration analysis in pure Zn4Sb3 6.3 The powder ALD effect on microstructure and thermoelectric properties 6.3 Stability testing on ZnO-coated samples 6.4 Summary Chapter 7 Summary and outlook 7.1 Summary 7.2 Outlook Appendix Appendix A: XRD patterns of Al2O3, TiO2 and ZnO-coated Bi Appendix B: TE properties of Al2O3 and ZnO-coated CuNi alloys References Abbreviations and symbols Acknowledgments List of publications List of awards List of attending conferences

Faser-Kunststoff-Verbunde (FKV) mit duroplastischer Matrix zeichnen sich dadurch aus, dass ihre guten physikalischen und thermomechanischen Eigenschaften wesentlich durch den Verarbeitungsprozess und die sich dabei bildende polymere Netzwerkstruktur bestimmt werden. Die Einstellung und Kontrolle des Vernetzungsgrades stellt eine fertigungstechnische Herausforderung dar, weshalb zumeist auf vollständige Aushärtung abgezielt wird. Die Ausnutzung der Eigenschaften im teilvernetzten gelierten Zustand birgt hingegen immenses Potential für eine flexiblere Gestaltung von Fertigungsprozessen und ist zudem von außerordentlicher Relevanz für ein verbessertes Verständnis konventioneller Verarbeitungsverfahren auf Basis von FKV mit duroplastischer Matrix. Die vorliegende Arbeit zeigt die experimentelle Analyse der werkstoffmechanischen Phänomene bei der Deformation teilvernetzter Matrices auf und erarbeitet basierend auf physikalisch begründeten Wirkmechanismen geeignete Materialmodelle zu deren praxisgerechter Beschreibung. Im Ergebnis stehen Lösungswege bereit, die das besondere mit der Netzwerkbildung einhergehende Potential duroplastischer Matrixpolymere nutzbar machen. Die Anwendung der entwickelten Modelle in Prozess- und Struktursimulationen ermöglicht eine erhöhte Prognosegüte bei der Berechnung von Eigenspannungen und ein deutlich verbessertes Verständnis der matrixdominierten Schädigungs- und Ermüdungsphänomene in FKV.

Um den CO2-Ausstoß von Kraftfahrzeugen zu reduzieren, ist die Beimischung von biogenen Kraftstoffen zum fossilen Ottokraftstoff interessant. Der Einsatz dieser führt zum veränderten Rußpartikelausstoß, welcher aufgrund der zunehmend verschärften gesetzlichen Regulierungen im Automobilsektor hinsichtlich Partikelmasse und -anzahl begrenzt ist. In der Arbeit werden Flammenstruktur und Rußbildungsprozess bei der Anwendung flüssiger Brennstoffe mit biogenem Anteil in laminaren, vorgemischten Modellflammen charakterisiert. Durch die Verwendung verschiedenster Messtechniken werden experimentelle Referenzdaten ohne den Einfluss von komplexen Wechselwirkungen, hervorgerufen durch den motorischen Betrieb, erhoben. Anhand der gemessenen Partikelgrößenverteilungen wird der chemisch hemmende Einfluss der biogenen Komponenten Ethanol und Butanol auf die Rußbildung gezeigt. Diese wird verlangsamt, sodass die Partikeldurchmesser abnehmen und die Rußmenge deutlich minimiert wird, wobei bzgl. der Gesamtpartikelanzahl kein eindeutiger Einfluss erkennbar ist.

Enormous demands for fast and low-power computing and memory building blocks for consumer electronics, such as smartphones or tablets, have led to the emergence of silicon nanowire transistors a decade ago. Along with the Si-based nanotechnology, the silicon compatible optical and chemical sensing applications have boosted the research on hybrid devices that combine the organic and inorganic materials. Apart from the revolution in the device dimensions, the rapid growth of artificial intelligence in the software industry brunch requires the next generation’s computers with the revolutionized hybrid device architecture. Implementing such new devices can effectively perform machine learning tasks without the massive consumption of energy. The hybrid Si nanowire devices have an excellent capability to replace the conventional computing element by providing new functionalities of combined materials to the traditional transistor devices preserving the advantage of CMOS technology. A goal of this thesis is to develop functional hybrid Si nanowire-based transistors modulated by the stimuli-dependent gate to go beyond the current digital building blocks. The hybrid devices converge semiconductor channel and various materials from organic molecules to silicate composite as a gate of the transistor. External stimuli change the electronic state of the gate materials which is transformed to the gate potential of the transistors. First, this thesis studies the electronic characteristics of the Si nanowire FETs under the optical stimulus. Optical stimulus induces the strong conductance change on bare Si nanowire FETs. Under the light with low power intensity, the transistor shows an unconventional negative photoconductance (NPC) which is dependent on the doping concentration of the nanowire and the wavelength of the incident light. The dopants ions and surface states cause photo-generated hot electrons trapping which restricts conventional photoconductance in the semiconductor. In the hybrid device, however, the gate material on the Si dioxide layer plays a significant role in the optoelectronic modulation of the FET device. This thesis demonstrates that an organic photochromic material, porphyrin, wrapping around the nanowire channel acts as an optical gate of the Si nanowire transistor. The diffusive property of electrons in the molecular film decides the optical switching dynamics and efficiency. Further, this thesis introduces new functional gate material, sol-gel derived ion-doped silicate film, based on the availability of stimulus-dependent gate modulation. This amorphous and transparent silicate film shows memristive property due to the ionic redistribution in the film under bias condition. Interestingly, the sol-gel film-coated Si nanowire FETs the devices show a double gate effect cooperating with a back gate under light illumination which is due to the channel separation in the fin structure of the nanowire. In addition, the sol-gel silicate film-coated Si nanowire transistor emulates the neuronal plasticity with pulsed gate stimulation, namely “neurotransistor.” Because of the mobile ions in the silicate film, the transistor has a short-term memory and mimics membrane potential change of the neuron cell. The neurotransistor could be used as a computing node in the physical neural network for hardware machine learning. This work demonstrates that the physical properties of the gate material decide the transfer characteristics and time-dependent dynamics of the hybrid Si nanowire transistors. The optical and neuromorphic gate features of the hybrid transistors would accelerate the advancement of an optical or brain-like computing machine.

Over the last two decades, electron tomography, the combination of tomographic methods and transmission electron microscopy (TEM), has considerably contributed to provide new insights into the three-dimensional structure of nanoscale materials. In particular, emerging advances in nanoscience are inevitably linked to developments in quantitative two-dimensional (2D) and three-dimensional (3D) TEM characterization techniques. In many cases, ET is employed to reconstruct the 3D shape (faceting of crystals) and the distribution or the arrangement (assembly) of nanoparticles down to the nanometer and atomic scale. Moreover, it is used to reconstruct the full 3D morphology of complex nanomaterials and composites, which can be evaluated further as a basis for quantitative modelling of physical properties. Beyond these capabilities, ET reveals the 3D chemical composition of nanostructures by combining it with spectroscopic methods, such as, electron energy-loss spectroscopy (EELS) and energy-dispersive X-ray spectroscopy (EDS). In specific cases, ET applied together with electron holography enables reconstructing electrostatic potentials in 3D, for example space-charge related diffusion potentials at pn-junctions in semiconductors. In ferromagnetic materials, this approach also allows for the 3D reconstruction of the internal remanent magnetic induction (B-field).