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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.

Der thermische Widerstand ist eine wichtige Kenngröße von Silicium-Germanium-Hetero-Bipolartransistoren (SiGe-HBTs). Bisher kam es bei der quantitativen Bestimmung der thermischen Widerstände von SiGe-HBTs zu deutlichen Abweichungen zwischen Simulation und Messung. Der Unterschied zwischen Simulation und Messung betrug bei den untersuchten HBTs mehr als 30 Prozent. Diese Arbeit widmet sich der Aufklärung und Beseitigung der möglichen Ursachen hierfür. Zu diesem Zweck werden als erstes die Messmethoden analysiert. Es zeigt sich, dass die bisher verwendete Extraktionsmethode sensitiv auf den Early-Effekt (Basisweitenmodulation) reagiert. Im Rahmen der Untersuchungen wurde ein neues Extraktionsverfahren entwickelt. Die neue Extraktions­methode ist unempfindlich gegenüber dem Early-Effekt. Mit Bauelemente­simulationen wird erstmalig die Wirkung des Seebeck-Effektes (Thermospannungen) auf die elektrisch extrahierten thermischen Widerstände demonstriert. Der Seebeck-Effekt bewirkt, dass die elektrisch extrahierten thermischen Widerstände der untersuchten HBTs nahezu 10 Prozent kleiner als die erwarteten Werte sind. Dieser Effekt wurde bisher nicht beachtet und wird hier erstmals nachgewiesen. Weiterhin wird die Abhängigkeit des thermischen Widerstandes vom Arbeitspunkt untersucht. Dabei hat sich gezeigt, dass bis zu einer Basis-Emitter-Spannung von 0,91 Volt die geometrische Form des Wärme abgebenden Gebietes unabhängig vom Arbeitspunkt ist. Anhand von Messungen wird gezeigt, dass die Dotierung die spezifische Wärmeleitfähigkeit von Silicium reduziert. Die Abnahme wird für Dotierungen größer als 1*1019 cm‑3 deutlich sichtbar. Ist die Dotierung größer als 1*1020 cm‑3, beträgt die Abnahme der spezifischen Wärmeleitfähigkeit mehr als 75 Prozent. Mithilfe einer Simulatorkalibrierung wird die spezifische Wärmeleitfähigkeit als Funktion der Dotierung bestimmt. Die erhaltene Funktion kann künftig beim thermischen Entwurf von HBTs verwendet werden. Somit können zukünftig genauere Vorhersagen zum thermischen Widerstand der HBTs gemacht werden. Dies ermöglicht zuverlässigere Aussagen darüber, wie Änderungen des Transistordesigns zur Minimierung des thermischen Widerstandes beitragen. / The thermal resistance is an important parameter of silicon-germanium heterojunction bipolar transistors (SiGe HBTs). Until now, the quantitative determination of the thermal resistance showed significant differences between measurements and simulations. The difference between simulation and measurement of the investigated HBTs was more than 30 percent. This thesis devotes the clarification and elimination of potential sources for it. For this purpose, the measurement methods are analyzed at first. It is shown, that the currently used extraction method is sensitive to the Early effect (basewidth modulation). A now extraction method was developed, which is not sensitive to the Early effect. For the first time, the influence of the Seebeck effect (thermoelectric voltages) on the electrically extracted thermal resistance is shown by device simulations. The Seebeck effect leads to a 10 percent lower extracted thermal resistances compared to the expected values of the investigated HBTs. This effect was not taken into account up to now and is demonstrated here for the first time. Furthermore, the dependence of the thermal resistance on the operating point was investigated. The results show that the shape of the heat source is independent of the operating point if the base emitter voltage is smaller than 0.91 volt. The thermal conductivity of silicon is decreased by increasing doping concentrations. This is shown by measurements. The reduction of the thermal conductivity is well observable for doping concentrations higher than 1*1019 cm‑3. For doping concentration higher than 1*1020 cm‑3 the reduction amounts to more than 75 percent. The thermal conductivity was determined as a function of the doping concentration with the aid of a simulator calibration. This function can be used in the future thermal design of HBTs. It facilitates the optimization of the HBTs with respect to a minimal thermal resistance.

Cette thèse concerne la réalisation des films minces d’oxydes et l’étude de leurs propriétés physiques pour les cellules photovoltaïques (PV) et les modules thermoélectriques. Dans une première partie, les propriétés de l’oxyde de titane TiOx (1,45<x<2) sont mises en évidence pour une utilisation en tant qu’oxyde transparent conducteur optiquement actif à disposer en face avant des cellules PV ou, comme couche de couplage optique à intercaler entre le métal réflecteur et la couche absorbante d’une cellule PV. Les couches sont déposées par ablation laser pulse (PLD). Cette méthode permet d’obtenir des couches stoechiométriques ou déficitaires en oxygène grâce au contrôle de la pression d’oxygène pendant le dépôt. Les couches sont dopées par Nb pour un gain en conductivité électrique et/ou par Nd pour la conversion des photons UV en photons du Proche IR. Les films d’une part, isolants, transparents et luminescents ou d’autre part, conducteurs et absorbants ont été obtenus. La présence de polarons et/ou de bipolarons dans les couches TiO₁,₄₅₋₁,₆₀ explique la discontinuité observée sur leurs courbes de thermoconductivité. Une seconde partie du manuscrit concerne la thermoelectricité ou il est question de modifier les propriétés des cobaltites de calcium pour la conversion en énergie électrique des gradients de température faibles, centres autour de 300-365 K. Le contrôle de la concentration en oxygène des films a permis d’obtenir les phases polymorphes CaxCoO₂, Ca₃Co₄O₉, et Ca₃Co₄O₆,₄₋₆,₈ présentant des comportements semiconducteurs ou métalliques en fonction de la température de dépôt. Les films Ca₃Co₄O₆,₄₋₆,₈ montrent de faibles résistivités (3,8-6 mΩ.cm) et des coefficients de Seebeck élevés (S) ≥ 1000 μV/K qui doivent être confirmes pour que de tels films soient utilisés dans les thermogénérateurs. / This thesis concerns the realization of oxide thin films and the study of their properties for photovoltaic or thermoelectric devices. In the first part, the TiOx properties are studied for use as an optically active transparent conductive oxide to put in front of the PV cells or, as optical coupling layer to interpose between the metal reflector and the absorbent layer of a PV cell. The layers are deposited by pulsed laser deposition (PLD). This method allows to get stoichiometric or oxygen deficient layers by controlling the oxygen partial pressure during the growth. The layers are doped with Nb to enhance electrical conductivity and/or with Nd for the conversion of Ultra-Violet photons to Near Infra-Red photons. Insulating and transparent layers, luminescent layers or conducting and absorbent layers are obtained. The TiO₁,₄₅₋₁,₆₀ films show polaronic or bipolaronic conductivity and exhibited the jump of electrical conductivity with jump height and temperature depending on the nature of the dopants. A second part of the manuscript concerns thermoelectricity in which the properties of cobalt calcium oxide are modulated for an efficient conversion of low temperature gradients centered at 300-365K. The control of the oxygen concentration of films allows to obtain the polymorphic phases CaxCoO₂,Ca₃Co₄O₉ and Ca₃Co₄O₆,₄₋₆,₈ having metallic or semiconducting behavior depending on the deposition temperature. The Ca₃Co₄O₆,₄₋₆,₈ films show high Seebeck coefficients (S) ≥ 1 000 μV/K and low electrical resistivity (3.8 to 6 mΩ.cm). Such interesting values have to be confirmed by additional experiments in order to be used as thermoelectric films.

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.

Die thermoelektrische Energiekonversion auf der Basis des Seebeck-Effekts ist eine Methode zur direkten Erzeugung elektrischer Energie aus thermischer Energie. Für die wesentlichen anwendungsrelevanten Parameter Temperaturbereich, elektrische Leistung und Herstellungskosten sind Materialauswahl und Aufbau der TEG entscheidend. In der vorliegenden Arbeit wurden erstmalig thermoelektrische Oxidkeramiken in monolithischen TEG verwendet, die auf der Grundlage des transversalen thermoelektrischen Effekts arbeiten. Die TEG wurden mit industriell skalierbaren Keramiktechnologien hergestellt, untersucht und hinsichtlich ihrer Parameter detailliert theoretisch und experimentell bewertet. Als Modellsystem für die Materialien wurde La1-xSrxCuO4 in Kombination mit Ag bzw. Ag6Pd1 verwendet. Es konnte belegt werden, dass diese monolithischen TEG im Bereich kleiner elektrischer Leistungen eine vorteilhafte Alternative zu herkömmlichen longitudinalen thermoelektrischen Generatoren sein können.

Climate change is a subject of deep distress in today’s world. Over dependence on hydrocarbon has resulted in serious environmental problems. Rising sea level, global warming and ozone layer depletion are the mainstream of any discuss world over. The collective goal of cutting carbon emission by the year 2020has prompted the search for clean, alternative energy sources. This effort are already yielding good reward as other forms of energy such as solar, wind, nuclear and hydro have received huge investment and renew interest over the past decade. Thermoelectric materials over the past decades have been tipped to replace conventional means of power generations as these materials have the ability to convert heat to electrical energy and vice versa. They are simple, have no moving parts and use no greenhouse gases. But the major drawback of these materials is their low conversion efficiency. Hence there is a need to enhance the efficiency of thermoelectric material to fulfill their undeniable potentials. A parameter called the thermoelectric figure of merit, ZT defines the efficiency of a thermoelectric material. ZT relates three non-mutually exclusive transport properties namely Seebeck coefficient, electrical conductivity and thermal conductivity. Efficient thermoelectric material should possess high Seebeck coefficient, high electrical conductivity and low thermal conductivity. Hence, one of the interesting ideas in the area of thermoelectric research is the concept of designing a bulk material with high density of phonon scattering centers so has to reduce the lattice contribution to thermal conductivity but at the same time have minimum impact oncharge carriers. This is usually achieved by utilizing interphase and grain boundaries which are localized defects to scatter phonons. The volume fraction of the grain/interphase boundaries can be control through phase modification and microstructure design. This thesis is centered on Bi-Sb-Te systems which are the present room temperature state of the earth thermoelectric material. The investigation revolves around developing a new kind of microstructure in the well-studied Bi-Sb-Te system that shows tremendous potential as a means to reduce lattice contribution to thermal conductivity. The idea of having both p and n-type thermoelectric material preferably from the same material was also a motivation in our investigation. The thesis isdivided into six chapters. The first chapter introduces the concept of thermoelectricity i.e. the direct conversion of thermal energy into electricity. The physics involved and contribution of individual to the science of thermoelectricity were enumerated. Efficiency, optimization and material selection for better thermoelectric performance were briefly enumerated. Prospective materials that are currently been investigated for better thermoelectric properties were also mentioned. The structure of the Bi-Sb-Te system which is the focus of this thesis is present in this chapter including doping effect on the thermoelectric performance of the system as well as the various methods present been employed to improve the thermoelectric properties of the system. Finally the chapter enumerates the scope and object of the present thesis. The different experimental procedures adopted in the present thesis arediscussed in chapter 2. The details of different processing routes followed to synthesize flame-melted ingots, flame-melted + low temperature milled (cryo milling) + spark plasma sintering (SPS) alloy and flame-melted + melt spinning + spark plasma sintering (SPS) alloy, are discussed followed by the various structural and functional characterization techniques. The unique advantage of the spark plasma sintering techniques over the conventional sintering method was talked out in detail. The structural characterizations performed on the synthesized alloys include XRD, SEM and whilethe functional characterizations comprised of Hall measurement, Seebeck coefficient, electrical resistivity and thermal conductivity measurements. Thermoelectric properties of selected composition of Bi-Sb-Te synthesized via flame-melting are presented in chapter 3.Detail study of four analyzed compositions namelyBi24Sb20Te56, Bi20Sb12Te69, Bi16Sb5Te79 and Bi29Sb11Te60resulted in four unique microstructure and different volume fraction of primary and secondary phases. The resultant morphologies of the microstructure were observed to have influence the thermoelectric behavior corresponding to each composition. The sole influence of anti-structural defects on the conductivity type and the role of microstructure morphologies and length scale were understood in this chapter. Samples with segregated Te and a solid solution BiSbTe3(eutectic morphology) form an n-type thermoelectric material while samples with only solid solution BiSbTe3 forms a p-type thermoelectric material. Pair of n-type and p-type material was obtained without the introduction of external dopant.The pair shows good compatibility factorsuitable for thermoelectric device. In chapter 4, the thermoelectric properties of four selected composition of Bi-Sb-Te synthesized via low temperature milling plus spark plasma sintering is addressed. The analyzed compositions are as follows Bi24Sb20Te56, Bi18Sb11Te71, Bi17Sb6Te77, and Bi28Sb15Te57 respectively. The effect of low temperature milling combine with the prospect of minimum grain growth of spark plasma sintering on the thermoelectric properties of the selected compositions were determined. Samples with eutectic morphology which would otherwise scatter charge carriers were observed to have the highest carrier mobility as a result of high volume fraction of Te phase which serves as a donor injecting excess electrons into the system. The impact of small grain size was observed on the transport properties of the sample Bi28Sb15Te57 with the highest electrical resistivity, the best Seebeck coefficient and the lowest thermal conductivity. Pair of n-type and p-type material was obtained without the introduction of external doping elements. The pairshows good compatibility factor suitable for segmented thermoelectric device. Chapter 5 narrates the thermoelectric properties of four compositions namely Bi30Sb13Te58, Bi23Sb13Te65, Bi18Sb5Te77 and Bi23Sb20Te58subjected to melt spinning plus spark plasma sintering.High cooling rate obtained during melt spinning process was observed in this chapter to cause a shift of composition which resulted in a microstructure morphology with eutectic colonies that is predominantly Te rich. These Te rich colonies in the sample Bi30Sb13Te58 was observed to change the conductivity type of the sample from an otherwise p-type to n-type while also aiding bipolar conduction which was detrimental to the overall thermoelectric performance of the alloy. Segregated Te in the form of eutectic morphology helps to inject excess electron into the bulk of the sample Bi23Sb13Te65 and Bi18Sb5Te77hereby increases the observed electrical conductivity which by virtue of the microstructure morphology is expected to be low. As a result of the processing routes, all four compositions in this chapter shown-type conductivity. Chapter 6 presents the summary of the important conclusions drawn from this work.

Att minska energianvändningen är något det pratats mer och mer om de senaste åren. Det finns olika sätt att minska energianvändningen på och ett av dessa är att återvinna värmeenergi. Det kan gälla både spillvärme och nyttig värme. Detta går att tillämpa i industrin, transportsektorn, hushåll och till vardags. Gemensamt för dessa processer är att det används stora mängder energi vilket till stor grad består av förluster till omgivningen eller att processerna inte optimeras. På senare tid har det forskats kring teknologi som kan ta vara på denna värmeenergi och på så vis minska förlusterna. En teknologi för detta är termoelektriska generatorer (TEG) som bygger på Seebeckeffekten för att generera elektricitet från temperaturskillnader. När ett TEG-element utsätts för värme på en sida och kyla på den andra sidan så genereras en elektrisk spänning. En elektrisk ström och effekt kan tas ur kretsen om elementet kopplas till en elektrisk last. Materialet i elementet består av halvledarmaterial med låg värmeledningsförmåga och en hög elektrisk ledningsförmåga. Teknologin har funnits länge men aldrig tillämpats i någon större grad. Nu på senare år har intresset ökat och kommersiella produkter med TEG-element har tagits fram. I detta arbete har en sådan produkt testats för att se hur lämpligt det skulle vara att använda dessa vid hushåll som inte är anslutna till elnätet och har en vedkamin för uppvärmning. TEG-enheten testas på en värmeplatta där ställbara temperaturer är möjliga för att testa prestandan vid temperaturerna 150° C, 200° C och 230° C. En krets sätts ihop för att kunna mäta av värden på spänning och ström vid olika laster som sätts med resistorer. Mätningarna görs med en ökning på 0,1 A vid varje mätning. Resultatet från dessa tester visar att maximal effekt på 14 W uppnås hos produkten vid 230° C. När modifiering av produkten görs för att öka temperaturskillnaden uppnås 17,8 W vilket tyder på att effekten ökar när delta T ökar. Den spänning som uppnås vid öppen krets var som högst 31 V och vid maximal effekt var den 17,8 V. Strömmen var då 1 A. De resultat som testerna gav levde inte upp till de 25 W som produkten sägs kunna ge. Produkten saknar även viktiga komponenter så som spänningsreglerare.Det går av både teori och tester avgöra att det är ett lämpligt sätt att använda sig av TEG-enheter för att generera små mängder elektricitet vid hushåll utan koppling till elnätet. / In recent years the topic of reducing the energy usage has been on the agenda. There are several ways of reducing the energy usage and one of these is to recycle heat energy. It could be both waste heat and useful heat. This can be implied to the industry, transport sector, households and on daily activities. The common factor between these is that large quantities of energy is used and to a large extent consists of losses to the surrounding or from processes that are not optimized. In recent time there has been done research around technology that can recycle and use this heat energy and in return reduce the energy usage. One technology to do this is thermoelectric generators (TEG) that are implementing the Seebeck effect to generate electricity from temperature differences. When a TEG-element have one side that is exposed to a heat source and one side being cooled down an electric voltage is being generated. An electric current and power can then be used from the circuit if the element is connected to an electric load. The material in the element exists of semiconductive materials with low heat conductivity and high electric conductivity. The technology has existed for a long time but has never been implemented to a larger extent. It is only in recent years that the interest has grown and some commercial products with TEG-elements has been developed. In this thesis one of these products has been tested to see how viable it would be to use these within a household that is not connected to the electrical grid and where the house is heated with a wood-burning stove. The TEG-product is tested on a heat plate where it is possible to set a desired temperature. The temperatures of 150° C, 200° C and 230° C are chosen for testing the performance of the product. A circuit is put together to be able to read the values of the voltage and current at different loads that are set with resistors. The measurements are done with an increase of 0,1 A for every measurement. The result from these tests shows that the maximum power of 14 W is achieved at 230° C on the hot side. But when modification of the product is made to increase the temperature difference a value of 17,8 W is attained. This indicate that the power is increasing when the temperature difference is increasing. The attained voltage at open circuit was as highest 31 V and at maximum power it was 17,8 V. The current was then 1 A. The results that the testing gave did not match the value of 25 W that the datasheet says the product can deliver. Also, the product is missing important components such as voltage regulator.It is possible from both the theory and the testing to see that it is suitable to use a TEG-product to generate small amount of electricity to households that are not connected to the electrical grid.