Thermoelektrische Phänomene können in Nanomaterialien im Vergleich zum Volumenmaterial stark modifiziert werden. Die Bestimmung der elektrischen Leitfähigkeit, des absoluten Seebeck-Koeffizienten (S) und der Wärmeleitfähigkeit ist eine wesentliche Herausforderung für die Messtechnik in Hinblick auf Mikro- und Nanostrukturen aufgrund dessen, dass die Transporteigenschaften vom Volumenmaterial sich durch Oberflächen- und Einschränkungseffekte verändern können. Im Rahmen dieser Abschlussarbeit wird der Einfluss von Größeneffekten auf die thermoelektrischen Eigenschaften von dünnen Platinschichten untersucht und mit dem Volumenmaterial verglichen. Dafür wurde eine Messplattform als standardisierte Methode entwickelt, um S einer dünnen Schicht zu bestimmen. Strukturelle Eigenschaften wie Schichtdicke und Korngröße werden variiert. Grenz- und Oberflächenstreuung reduzieren S der dünnen Schichten im Vergleich zum Volumenmaterial. Außerdem wird eine Methode demonstriert um S von einzelnen metallischen Nanodrähten zu bestimmen. Für hochreine und einkristalline Silber-Nanodrähte wird der Einfluss von Nanostrukturierung auf die Temperaturabhängigkeit von S gezeigt. Ein Modell ermöglicht die eindeutige Zerlegung des temperaturabhängigen S von Platin und Silber in einen Thermodiffusions- und Phononen-Drag-Anteil. Des Weiteren werden die thermoelektrischen Transporteigenschaften von einzelnen auf Bismut-basierenden Kern/Hülle-Nanodrähten untersucht. Der Einfluss des Hüllenmaterials (Tellur oder Titandioxid) und der räumlichen Dimension des Nanodrahts auf die Transporteigenschaften wird diskutiert. Streuung an Oberflächen, Einkerbungen und Grenzflächen zwischen dem Kern und der Hülle reduzieren die elektrische und thermische Leitfähigkeit. Eine Druckverformung induziert durch die Hülle kann zu einer Bandöffnung bei Bismut führen, sodass S gesteigert werden kann. Das Kern/Hülle-System zeigt in eine Richtung, um die thermoelektrischen Eigenschaften von Bismut erfolgreich anzupassen. / Thermoelectric phenomena can be strongly modified in nanomaterials compared to the bulk. The determination of the electrical conductivity, the absolute Seebeck coefficient (S) and the thermal conductivity is a major challenge for metrology with respect to micro- and nanostructures because the transport properties of the bulk may change due to surface and confinement effects. Within the scope of this thesis, the influence of size effects on the thermoelectric properties of thin platinum films is investigated and compared to the bulk. For this reason, a measurement platform was developed as a standardized method to determine S of a thin film. Structural properties, like film thickness and grain size, are varied. Boundary and surface scattering reduce S of the thin films compared to the bulk. In addition, a method is demonstrated to determine S of individual metallic nanowires. For highly pure and single crystalline silver nanowires, the influence of nanopatterning on the temperature dependence of S is shown. A model allows the distinct decomposition of the temperature-dependent S of platinum and silver into a thermodiffusion and phonon drag contribution. Furthermore, the thermoelectric transport properties of individual bismuth-based core/shell nanowires are investigated. The influence of the shell material (tellurium or titanium dioxide) and spatial dimension of the nanowire on the transport properties are discussed. Scattering at surfaces, indentations and interfaces between the core and the shell reduces the electrical and the thermal conductivity. A compressive strain induced by the shell can lead to a band opening of bismuth increasing S. The core/shell system points towards a route to successfully tailor the thermoelectric properties of bismuth.

No description available.

Cette thèse a été consacrée à l’étude du Si poreux comme matériaux à faible conductivité thermique (k) pour application aux dispositifs thermoélectriques à base de Si. D’autres paramètres thermoélectriques, comme par exemple le coefficient Seebeck de ce matériau, ont été également étudiés.Si poreux est un matériau complexe composé de nanostructures de Si séparées de vide. Quand la porosité est élevée, sa conductivité thermique est bien inférieure à celle de Si cristallin. Nous avons étudié la conductivité thermique de Si poreux de différentes morphologies et porosités dans la gamme de températures 4.2-350K. Les mesures à T<20K sont les premières dans la bibliographie et ont montré une saturation de k en fonction de T pour ces températures. A des températures supérieures à 20K, k augmente régulièrement avec la température. La dépendance de température de k de Si poreux a été interprétée en considérant des modèles théoriques, basées sur la nature “fractal” de Si poreux. Nous avons calculé la dimension fractale de Si poreux par des images de microscopie électronique à balayage (SEM) et l’algorithme de “box counting”.Deux méthodes différentes ont été utilisées pour mesurer k: la méthode à courant direct (dc) combinée avec une analyse FEM et la méthode 3ω. Nous avons proposé deux approches améliorées pour extraire k du signal de potentiel 3ω en fonction de la fréquence. La première considère l’accord des résultats expérimentaux avec la solution asymptotique intégrale de la formule de Cahill, et la seconde fait une analyse des résultats expérimentaux en solvant l’équation temporelle de transfert de chaleur par des éléments finis. Plus précise est la méthode 3ω combinée avec des éléments finis. Les résultats correspondants sont en bon accord avec ceux obtenus par la méthode dc.Nous avons aussi étudié le Si poreux comme matériau thermoélectrique. Dans ce cas, le Si poreux peut être intéressant si il a une faible porosité, car le matériau à haute porosité est très résistive. Dans ce but, nous avons déterminé le coefficient Seebeck (S) des membranes de Si poreux de différentes porosités dans la gamme 40-84%, en utilisant un dispositif de mesure spécialement développé à cet effet. Pour des échantillons de porosité 51%, la valeur de coefficient S est de 1mV/K, bien supérieure à celle le Si cristallin. La dépendance de S de la porosité n’est pas monotone, et ceci est attribué à une combinaison des effets de filtrage d’énergie, des collisions des phonons et interactions phonon-porteurs électriques. Les résultats obtenus sont basées sur des mesures de photoluminescence (PL) et observations microscopiques à transmission (TEM). Nous avons enfin conclue que, malgré le coefficient S très élevé, le Si poreux n’est pas adéquat comme matériau thermoélectrique à cause de sa faible conductivité électrique, qui diminue en augmentant la porosité à cause de la résultante déplétion de porteurs.Nous avons aussi étudié des films minces polycristallins dopés avec du Bore. Ces films sont très intéressants comme matériaux thermoélectrique, car ils sont compatibles avec les procédés de fabrication des circuits intégrés de Si. Leur performance thermoélectrique est améliorée par diminution de la taille des grains. Des films minces polycristallins d’épaisseur entre 100 et 500nm ont été étudiés. Tous leurs paramètres thermoélectriques ont été mesurés et nous avons trouvé que le facteur de performance thermoélectrique zT augmente d’un facteur 3 en diminuant l’épaisseur de 500 à 100nm ceci étant attribué à la diminution de la taille des grains dans les films, conduisant à zT = 0.033, qui est la meilleure valeur reporté dans la littérature.Ce résultat compétitif augmente le potentiel d’utilisation des films polycristallins dans des dispositifs thermoélectriques efficaces, compatibles à la technologie de Si. / This thesis is devoted to the thermal conductivity and other thermoelectric properties of porous silicon (PSi) and thin polycrystalline Si films (thickness: 100-500 nm).PSi is a complex material composed of a Si skeleton of interconnected nanowires and dots, separated by voids. When it is highly porous, its thermal conductivity is very low, even below that of the amorphous Si. This makes it a good material for use as a thermal isolation platform on the Si wafer. In addition, its Seebeck coefficient is much higher than that of bulk c-Si.We studied k of PSi layers with different morphologies and porosities, in the temperature range 4.2-350K. The measurements below 20K are the first reported in the literature. A plateau-like dependence on temperature was observed for T below 20K, while above this temperature a monotonic increase with T is observed. The observed behaviour was interpreted using known theoretical models, based mainly on the fractal nature of PSi. PSi was characterized as a fractal material by calculating its fractal dimension using SEM images and the box counting algorithm.Two different methods were used to determine porous Si thermal conductivity: the DC method combined with FEM analysis and the 3ω method. Concerning the 3ω method, two improved approaches were proposed for extracting k from the 3ω voltage as a function of frequency: the first uses a fitting of the experimental data to the asymptotic solution of the Cahill’s integral formula, and the second is based on the analysis of the experimental data by combining them with a solution of the transient heat transfer equation using FEM analysis. The results in this second case were more accurate and in very good agreement with the DC method.We also measured the Seebeck coefficient (S) of PSi membranes with porosities 40-84% using a home-built setup, which was fabricated, calibrated and tested within this thesis. A value as high as 1mV/K was obtained for the 51% porosity sample. An anomalous porosity dependence of S was obtained, which was attributed to the interplay between energy filtering, phonon scattering and phonon drag effects. The results were explained by combining them with PL and TEM measurements, used for the determination of nanocrystal sizes. We concluded that, despite of the extremely low k and the high S of PSi, the material with the studied high porosities is not adequate for use as a “good thermoelectric” material, because of its significantly low electrical conductivity, which decreases with increasing porosity, resulting from carrier depletion during formation.We also studied the thermoelectric properties of thin, boron-doped, polycrystalline silicon films, which are much more attractive for use as Si-based thermoelectrics than porous Si. Their thermoelectric performance is improved by decreasing film thickness, due to a decrease in polysilicon grain size. Thin films with thickness between 100-500nm were investigated. We measured their thermal conductivity, resistivity and Seebeck coefficient and extracted their thermoelectric figure of merit, which showed threefold increase by reducing film thickness down to 100nm. A value as high as 0.033 was achieved, which is the highest reported in the literature so far for boron-doped polysilicon films at room temperature. This increase is attributed to a decrease in the grain size of the material. The obtained value shows the interest of nanocrystalline Si films for integration in efficient Si-based thermoelectric generators, compatible with CMOS processing.

Thermoelectric (TE) ZrNiSn samples with a half-Heusler atomic structure were synthesized by mechanical alloying (MA) and consolidation by either Spark Plasma Sintering (SPS) or hot pressing (HP). X-Ray diffraction patterns of as milled powders and consolidated samples were compared and analyzed for phase purity. Thermal conductivity, electrical conductivity and Seebeck coefficient are measured as a function of temperature in the range 300 K to 800 K and compared with measurements reported for high temperature solid state reaction synthesis of this compound. HP samples, compared to SPS samples, demonstrate increased grain growth due to longer heating times. Reduced grain size achieved by MA and SPS causes increased phonon scattering due to the increased number of grain boundaries, which lowers the thermal conductivity without doping the base system with addition phonon scattering centers. Mechanical characterization of the samples by microindentation and depth sensing indentation for hardness and elastic modulus will be discussed.

This PhD thesis presents investigations of perovskite-related compounds in systems of interest for applications in components in solid oxide fuel cells. The compound compositions derive from substitutions in the parent compounds LaCoO3, LaCrO3 and SrFeO3. Novel phases La2Co1+z(MgxTi1-x)1-zO6 were synthesized and investigated with regard to structure, thermal expansion, electronic and magnetic properties. The study focused on the composition lines La2Co(MgxTi1-x)O6 (z=0), where the oxidation state of Co nominally changes from +2 (x=0.0) to +3 (x=0.5), and La2Co1+z(Mg0.5Ti0.5)1-zO6, with a varying fraction of Co3+ ions. XANES data show that the Co ions in the system have discrete oxidation states of +2 and +3. The TEC increases with increasing x due to an increasing contribution from spin state transitions of the Co3+ ions. Novel compounds La2Cr(M2/3Nb1/3)O6 with M=Mg, Ni, Cu were synthesized and characterized with respect to structure and magnetic properties. XRPD and NPD data indicate Pbnm symmetry; however, SAED patterns and HREM images indicate a P21/n symmetry for M=Mg, and Cu. The magnetic measurements results were rationalized using the Goodenough-Kanamori rules. Oxygen-deficient phases with x≥0.63 in SrxY1-xFeO3-δ and Sr0.75Y0.25Fe1-yMyO3-δ (M=Cr, Mn, Ni and y=0.2, 0.33, 0.5), were synthesized and characterized with respect to structure, oxygen content, thermogravimetry, TEC, conductivity and magnetic properties. Powder patterns of phases agree with cubic  perovskite structures. NPD data for x=0.75 reveal anisotropic displacement for the O atom, related to local effects from Fe3+/Fe4+ ions. SAED patterns for x=0.75 reveal the presence of an incommensurate modulation. The compounds start to lose oxygen in air at ~ 400°C. The TEC up to ~400°C for x=0.75 is ~10.5 ppm/K and increase to ~17.5 ppm/K at higher temperatures. The conductivity for x=0.91 is 164 S/cm at 400°C. Partial substitution of Fe by Cr, Mn or Ni does not increase the conductivity or decrease TEC.

Dans son ensemble, cette thèse porte sur la synthèse, l’étude structurale et l’étude des propriétés magnétiques et de transport de différentes familles d’oxydes de ruthénium, présentant des comportements électroniques et magnétiques variés, de structure rutile, hexaferrite de type R et hollandite. Le but de ce travail était l’étude des particularités des oxydes de ruthénium sondées par l’effet Seebeck dans les matériaux suivants : RuO2 de structure rutile (chaînes d’octaèdres de RuO6 liés par leurs arêtes, interconnectées par leurs sommets) à transport de type Boltzmann dominé par les interactions électron-phonon, les hexaferrites de type R BaCo2Ru4O11 et BaMn2Ru4O11 (octaèdres de RuO6 liés par les arêtes, formant des plans kagomé, et octaèdres de RuO6 liés par une face) ferromagnétiques doux et mauvais métaux, et deux nouvelles hollandites Sr1.5Ru6.1Cr1.9O16 et Ba1.5Ru6.1Cr1.9O16 (doubles chaînes de RuO6 liés par les arêtes, interconnectées par les sommets) avec agglomérats de spins localisés. La synthèse de ces deux nouvelles hollandites par réaction à l’état solide a permis de mettre en évidence l’existence de magnétorésistance négative dans cette famille de composés. Cette thèse montre que le comportement du coefficient Seebeck S d'oxydes de ruthénium à structures constituées d'octaèdres RuO6 majoritairement liés par leurs arêtes présente deux comportements différents. À basse T, S dépend fortement de la structure cristallographique et de la structure électronique associée. Par contre, dans la limite haute T, S tend vers une valeur commune indépendamment de la structure comme rapporté ici pour les hexaferrites de type R et les hollandites, et comme observé précédemment dans la pérovskite SrRuO3 (octaèdres RuO6 liés par les sommets) ferromagnétique métallique et dans la quadruple pérovskite LaCu3Ru4O12 (octaèdres RuO6 liés par les sommets) métallique présentant un magnétisme de type Pauli. Dans ces hexaferrites de type R BaCo2Ru4O11 et BaMn2Ru4O11 et dans ces deux nouvelles hollandites Sr1.5Ru6.1Cr1.9O16 et Ba1.5Ru6.1Cr1.9O16, le coefficient Seebeck à haute température atteint une valeur dominée par l’entropie de spin du ruthénium. / This thesis deals with the synthesis, the structural study and the magnetic properties and electronic transport studies of different ruthenium oxide families, presenting various magnetic and electronic behaviors, with rutile, R-type hexaferrite and hollandite structures. The goal of this thesis was the study of the ruthenium oxide peculiarities probed by the Seebeck effect in the following materials: RuO2 rutile (edge-shared RuO6 chain interconnected by their vertices) with Boltzmann type transport dominated by electron-phonon interactions, BaCo2Ru4O11 et BaMn2Ru4O11 R-type hexaferrites (edge-shared RuO6 octahedra, forming kagome planes, and face-shared RuO6 octahedra) soft ferromagnetic bad metals, and two new hollandites Sr1.5Ru6.1Cr1.9O16 et Ba1.5Ru6.1Cr1.9O16 (double chains of edge-shared RuO6 octahedra, interconnected by their vertices) with localized transport and cluster-glass behavior. The synthesis of both new hollandites by solid state reaction allowed us to show the existence of negative magnetoresistance in this compound family. This thesis shows that the behavior of the Seebeck coefficient of ruthenium oxides with structures mainly consisting of edge-shared RuO6 octahedra presents two different behaviors. At low T, S strongly depends on the crystallographic structure and on the associated electronic structure. On the other hand, in the high T limit, S tends a common value independently of the structure as reported here for the R-type hexaferrites and the hollandites and as previously observed in the ferromagnetic metal SrRuO3 perovskite (apex-shared RuO6 octahedra) and in the metallic with Pauli-type magnetism quadruple perovskite LaCu3Ru4O12 (apex-shared RuO6 octahedra). In these R-type hexaferrites BaCo2Ru4O11 and BaMn2Ru4O11 and these two new hollandites Sr1.5Ru6.1Cr1.9O16 and Ba1.5Ru6.1Cr1.9O16, the high temperature Seebeck coefficient reaches a value dominated by the Ru spin entropy term.

<p>In the United States, buildings are responsible for almost 40% of the country’s total energy consumption and 38% of the total greenhouse gas emissions. Researchers are constantly seeking sustainable and efficient energy generation solutions for buildings as society continues to cope with the intensifying energy crisis and environmental deterioration. Thermoelectric technology is one such solution that potentially can lead to significant energy recovery and conversion between waste or excess thermal energy and electrical energy. One promising application is integrating thermoelectric materials into the building envelope (TBE) for power generation and building heating and cooling without transporting energy among subsystems and refrigerant use. TBE can combine structural support and thermal storage with power generation and thermal-activated cooling and heating, thereby contributing to sustainable living and energy. </p> <p>TBE technology is still in its early development stages. This dissertation aimed to develop a fundamental understanding of the characteristics, behaviors, operation, and control of TBE systems as energy-efficient measures for thermal energy harvesting and thermal comfort regulation and to address the significant research gaps concerning high-conversion efficiency materials and optimal module configuration as well as system deployment related to real-world applications. Accordingly, this dissertation focused on the following three key objectives: (1) development and characterization of new thermoelectric composite materials; (2) identification of optimal designs and controls of TBE and established mathematical models for performance simulation; and (3) quantification of the energy-saving benefits of TBE. </p> <p>The following five aspects specifically were investigated:</p> <p>(1)<em> Material development and characterization</em>. New thermoelectric cement composites were developed with cement and various additives, material concentrations, and fabrication methods in the laboratory. Their thermoelectric properties (e.g., Seebeck coefficient, thermal conductivity, electrical conductivity, power factor, and the figure of merit) were measured simultaneously and characterized at 300–350 K.</p> <p>(2)<em> Module evaluation.</em> Commercially available thermoelectric modules (TEMs) were assessed using well-designed test apparatus in both the heat pumping and power generation modes. The test results validated the numerical model, which assisted with performance comparison and material selection between cement-based and commercial TEMs for the TBE prototype.</p> <p>(3)<em> Prototype assessment. </em>A convective TBE prototype and a radiant TBE prototype were designed, assembled, and evaluated in a pair of controlled testing chambers. The TBE’s surface temperature, thermal capacity, and COP were assessed under summer and winter conditions. </p> <p>(4)<em> Prototype modeling. </em>The first-principle-based numerical models of both the convective and radiant TBE prototypes were developed in Modelica. The modeling results indicated good agreement with the experimental data. The verified models were used to study the impacts of the design parameters and operating conditions on the heat pumping performance of TBE.</p> <p>(5)<em> System simulation. </em>A TBE building system model was established by integrating the TBE prototype model within a building’s heat balance model, considering the building construction, climate condition, power control, etc. Its seasonal performance under various climate conditions was studied to identify the potential optimal operation and energy savings. </p> <p>This dissertation confirmed several key findings in the areas of material development, system design and operation, and energy savings. The TBE achieved higher efficiency with a heat pump for heating than for cooling generally. The TBE heating system performed better than a conventional electric heater (efficiency assumed at 0.9). The measures that improved TBE heating efficiency were enhancing the material’s thermoelectric properties, optimizing the geometry and number of TEMs, and improving the boundary heat transfer of TEMs. </p> <p>This dissertation concluded that the TBE system is a promising alternative to conventional heating systems in buildings. Furthermore, the knowledge gained will strengthen the understanding of thermoelectrics in the building domain and guide further development in TBE, as well as facilitate the operation of net-zero energy and carbon-neutral buildings. </p>

Die vorliegende Arbeit ist eine umfassende Studie über das Wachstum mittels Molekularstrahlepitaxie (MBE) und die gemessenen Seebeck Koeffizienten und Lochtransport Eigenschaften von p‑Typ Oxiden, eine Materialklasse welche die optische Transparenz und die einstellbare Leitfähigkeit verbindet. Insbesondere, Nickeloxid (NiO) und Zinnmonoxid (SnO) wurden mittels plasmaunterstützter MBE unter Einsatz von einer Metall‑Effusionszelle und einem Sauerstoffplasma gewachsen. Für das NiO Wachstum wurden vor allem die Wachstumsgrenzen bei hohen Temperaturen festgelegt, welche von der Substratstabilität im Falle von Magnesiumoxid und Galliumnitrid abhängen. Es wird die Möglichkeit der Qualitätsbewertung mittels Ramanspektroskopie für Natriumchlorid-Strukturen gezeigt. Untersuchung der NiO Dotierung durch Oberflächen-Akzeptoren und der damit verbundenen Oberflächen‑Loch‑Anreicherungsschicht offenbart eine neue Dotierungsmöglichkeit für p‑leitende Oxide im Allgemeinen. Die metastabile Phase des SnO wird mittels PAMBE unter Verwendung bekannter Wachstumskinetik von Zinndioxid und verschiedener in‑situ Methoden stabilisiert, die anwendungsrelevante thermische Stabilität wird untersucht. Anschließende ex‑situ Charakterisierungen durch XRD und Ramanspektroskopie identifizieren das kleine Wachstumsfenster für das epitaktische Wachstum von SnO. Elektrische Messungen bestätigen die p‑Typ Ladungsträger mit vielversprechenden Löcherbeweglichkeiten welche auch für Hall Messungen zugänglich sind. Temperaturabhängige Hall Messungen zeigen einen bandähnlichen Transport welcher auf eine hohe Qualität der gewachsenen Schichten hindeutet. Die Funktionalität der gewachsenen Schichten wird durch verschiedene Anwendungen nachgewiesen. Zum Beispiel werden pn‑Heteroübergänge wurden durch das heteroepitaktische Wachstum der SnO Schichten auf einem Galliumoxid-Substrat erlangt. Die ersten bisher berichteten SnO-basierten pn‑Übergänge mit einem Idealitätsfaktor unter zwei wurden erreicht. / This thesis presents a comprehensive study on the growth by molecular beam epitaxy (MBE) and the measured Seebeck coefficients and hole transport properties of p‑type oxides, a material class which combines transparency and tunable conductivity. Specifically, Nickel oxide (NiO) and tin monoxide (SnO) were grown by plasma‑assisted MBE using a metal effusion cell and an oxygen plasma. For NiO growth, the focus lies on high temperature growth limits which were determined by the substrate stability of magnesium oxide and gallium nitride. Quality evaluation by Raman spectroscopy for rock‑salt crystal structures is demonstrated. Investigations of NiO doping by surface acceptors and the related surface hole accumulation layer reveal a new doping possibility for p‑type oxides in general. The meta‑stable SnO is stabilized by PAMBE utilizing known growth kinetics of tin dioxide and various in‑situ methods, its application-relevant thermal stability is investigated. Following ex‑situ characterizations by XRD and Raman spectroscopy identify secondary phases and a small growth window for the epitaxial growth of SnO. Electrical measurements confirm the p‑type carriers with promising hole mobilities accessible to Hall measurements. Temperature dependent Hall measurements show band‑like transport indicating a high quality of the grown layers. The functionality of the grown layers is proven by various applications. For example, pn‑heterojunctions were achieved by heteroepitaxial growth of the SnO layers on gallium oxide substrates. The first reported SnO based pn‑junction with an ideality factor below two is accomplished.

Diese Arbeit demonstriert die vollständige thermoelektrische Charakterisierung einzelner einkristalliner Bismuttellurid- und Silbernanodrähte und deren anschließende lokale strukturelle und chemische Charakterisierung mittels analytischer Transmissionselektronenmikroskopie. Die lokale strukturelle, chemische und morphologische Charakterisierung entlang der Nanodrähte trägt essentiell zum Verständnis des thermoelektrischen Transportes bei und bestätigt die Korrelation zwischen Oberflächen- und den thermoelektrischen Eigenschaften. Für durchmesservariierte Bismuttelluridnanodrähte wird der Einfluss der Morphologie auf die Wärmeleitfähigkeit bei Raumtemperatur quantifiziert. Im Vergleich zu einem glatten Referenznanodraht zeigt der durchmesservariierte Nanodraht gleicher Zusammensetzung und Kristallorientierung eine Reduktion der Wärmeleitfähigkeit um 55 %. Diese Reduktion kann durch Phononenrückstreuung an der eingekerbten Oberfläche erklärt werden. Die elektrische Leitfähigkeit und der Seebeckkoeffizient der Bismuttelluridnanodrähte deuten auf einen topologischen Oberflächenzustand hin. Für Silbernanodrähte werden die elektrische Leitfähigkeit und die Wärmeleitfähigkeit im Temperaturbereich von 1,4 K bis 300 K gemessen. Mit fallender Temperatur steigt die relative Reduktion der Wärmeleitfähigkeit im Verhältnis zur elektrischen Leitfähigkeit stärker, sodass die Lorenzzahl die klassische Wiedemann-Franz-Relation nicht erfüllt und eine Funktion der Temperatur darstellt. Der Temperaturverlauf der Lorenzzahl der Silbernanodrähte entspricht der 1938 von Makinson aufgestellten Theorie für hochreine Metalle und ist im Tieftemperaturbereich um bis zu zwei Größenordnungen zum Sommerfeldwert reduziert. / This work demonstrates the full thermoelectric characterisation of individual single crystalline bismuth telluride and silver nanowires and their subsequent local structural and chemical characterisation via analytical transmission electron microscopy along the whole nanowires. Therefore, the correlation between the structure, in particular the surface morphology, and the thermoelectric transport properties is unambiguously shown. For diameter varied bismuth telluride nanowires the influence of the morphology on the thermal conductivity is quantified at room temperature. The diameter varied nanowire shows a reduction of 55 % with respect to the smooth nanowire of the same chemical composition and structural orientation. This reduction can be explained by phonon backscattering at the indents. The electrical conductivity and the Seebeck coefficient indicate the presence of a topological surface state. For silver nanowires the electrical and thermal conductivity are determined in the temperature range between 1.4 K and 300 K. With decreasing temperature the relative reduction of the thermal conductivity is higher than the reduction of the electrical conductivity resulting in a temperature-dependent Lorenz number, so that the classical Wiedemann-Franz relation is not fulfilled. The temperature characteristic of the silver nanowires'' Lorenz number is in agreement with the theory Makinson established for highly pure metals in 1938 and is reduced by two orders of magnitude with respect to the Sommerfeld value in the low temperature regime.

Increasing interest in an effciency enhancement of existing energy sources led to an extended research in the field of thermoelectrics. Especially skutterudites with their high power factor (electric conductivity times Seebeck coefficient squared) are suitable thermoelectric materials. However, a further improvement of their thermoelectric properties is necessary. The relatively high thermal conductivity can be decreased by introducing loosely bound guest ions, whereas atom substitution or nanostructuring (as thin films) could yield an increased power factor. The present work proves the feasibility to deposit single phase skutterudite thin films by MBE technique. In this regard CoSby and FeSby film series were deposited with three different methods: i) codeposition at elevated temperatures, ii) codeposition at room temperature followed by post-annealing, and iii) modulated elemental reactant method. The structural and thermoelectric properties of these films were investigated by taking the thermal stability of the film and the substrate properties into account. Compared to the stoichiometric Sb content of skutterudites of 75 at.%, a small excess of Sb is necessary for achieving single phase skutterudite films. It was found, that the deposited single phase CoSb3 films reveal bipolar conduction (and therefore a low Seebeck coefficient), whereas FeSb3 films show p-type conduction and very promising power factors at room temperature. The need of substrates with a low thermal conductivity and a suitable thermal expansion coefficient is also demonstrated. A high thermal conductivity influences the measurements of the Seebeck coefficient and the obtained values will be underestimated by thermal shortening of the film by the substrate. If the thermal expansion coefficient of film and substrate differ strongly from each other, crack formation at the film surface was observed. Furthermore, the realization of controlled doping by substitution as well as the incorporation of guest ions was successfully shown. Hence, this work is a good starting point for designing skutterudite based thin film structures. Two successful examples for such structures are given: i) a thickness series, where a strong decrease of the resistivity was observed for films with a thickness lower than 10nm, and ii) a FexCo1-xSb3 gradient film, for which the gradient was maintained even at an annealing temperature of 400°C.