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Die vorgelegte Dissertation untersucht die Anwendbarkeit des Ultraschalllaufzeit-Verfahrens (UTTT) zur Analyse des Aufstiegs von Gasblasen in Flüssigmetallen. Aufbauend auf ersten Messungen zum Einzelblasenaufstieg in Wasser, welche der Validierung von UTTT dienten, erfolgten die Untersuchungen zum Aufstieg einzelner Blasen im Flüssigmetall GaInSn. Vergleichsmessungen mittels Röntgenradiografie führten zu einer zusätzlichen Visualisierung der Blasen, sodass dadurch die Interpretation der Messwerte von UTTT in GaInSn validiert werden konnte. Der Einfluss eines homogenen horizontalen statischen Magnetfeldes auf die Blasenbewegung wurde untersucht. Abschließend wurden ebenfalls Blasenketten untersucht, dabei wurde sowohl der Blasenaufstieg in Wasser als auch in Flüssigmetall betrachtet. Dabei zeigte sich, dass UTTT die Bewegung von größeren Blasenvolumina sowohl in Wasser als auch in Flüssigmetall verlässlich messen kann.

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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