The current high demand for sustainable and renewable energy sources to solve world energy crisis has enormously increased interest in looking at alternative sources of energy. All the machines used in manufacturing process, electricity generation, residential applications, transportation etc., rejects energy in the form of heat into environment. Thermoelectric materials can convert thermal-to-electrical and electrical-to-thermal energy and can be utilized in waste-heat harvesting, more efficient cooling to reduce energy usage and CO2 emissions. Significant research efforts have been devoted over the past decade to thermoelectric materials, with particular emphasis being placed on combining materials selection with nanostructuring. The overarching goal was to reduce thermal conductivity through selective phonon scattering and thus boost the thermoelectric figure-of-merit (ZT). SiGe alloys, as well as superlattices and nanocomposites made from them, showed significant improvements upon nanostructuring and ZT exceeding one at high temperatures. Other group IV alloys were not studied in the context of thermoelectrics. However, SiSn alloys are widely studied for their optoelectronic properties because they were predicted to become direct-gap materials when Sn composition increased beyond about 50%. To address this gap, we study the thermoelectric properties of SiSn alloys. Furthermore, we develop an iterative full-band solver for the electron Boltzmann transport equation and use it to compute the electron and hole mobility and Seebeck coeffcient in SiSn alloys. The electronic structure of SiSn alloys was computed in the virtual crystal approximation from non-local empirical pseudopotentials, while the application of strain allowed us to extract the electron-phonon coupling deformation potentials for each alloy composition. We benchmark our code against available mobility data for Si and SiGe alloys and find that it accurately reproduces the measured values. Full phonon dispersion was computed from the adiabatic bond charge model, which was shown to accurately reproduce measured dispersion, and used in our phonon BTE solver to compute lattice thermal conductivities. Scattering rates include anharmonic phonon-phonon, impurity, isotope, alloy, and boundary mechanisms. The lowest thermal conductivity was obtained in SiSn alloys, which have been experimentally demonstrated with up to 18% Sn composition. This carries through when combined with calculations of electronic power factor, where mobilities and Seebeck coeffcients of SiSn alloys are comparable to those of SiGe. Furthermore, ZT is optimized through doping for every composition. The ZT improves dramatically at higher temperatures, reaching ZT of 1.9, 2.36 is obtained for Sn composition of 10% and 50% in a n-doped bulk SiSn alloys at a temperature of 1480 K. However, such high Sn composition of 50% is unlikely to be synthesized due to low solid solubility of Sn in Si. Lastly, we study the impact of nanostructuring in thin films on the ZT. We also establish the limits on how much the ZT can be improved through nanostructuring by studying thin films of SiSn alloys across temperature from room temperature up to 1500 K. We conclude that in bulk SiSn alloys, even at modest Sn concentration of 10%, ZT can reach 1.9, while in 20 nm thin films of n-type SiSn alloys, it can reach the long-sought target of ZT>3 and ZT of 2.16 is obtained in p-type nanostructured SiSn alloys.

The main focus of this work is the preparation, chemical and structural characterization along with the investigation of physical properties of intermetallic clathrates. Starting from the history of clathrate research, classification of clathrate types, their structural properties and possible application areas are evaluated in chapter 2. The methodologies of sample preparation and materials characterization as well as quantum chemical calculations are discussed in chapter 3. The complete characterization of Ba8Ge433 ( is a Schottky-symbol standing for vacancies),12-14 which is a parent compound for the variety of ternary variants, is the subject of chapter 4. Ba8Ge433 is a high temperature phase,12 which was prepared for the first time as single phase bulk material in this work.15, 16 In this way, the intrinsic transport properties could be investigated without influence of grain boundary and impurity effects. The transport behavior is analyzed at low and high temperatures and referred to the former results. In addition, crystal structure and vacancy ordering in terms of the reaction conditions are discussed. Chemical bonding in Ba8Ge433 is investigated by topological analysis of the electron localizability indicator and the electron density. Chapter 5 deals with the preparation, phase analysis, crystal structure and physical properties of BaGe5, which constitutes a new clathrate type oP60.17, 18 So far, two clathrate types were known in the binary system Ba – Ge, namely the clathrate cP124 Ba6Ge25,19-21 and the clathrate-I Ba8Ge433. Originally, BaGe5 was detected by optical and scanning electron microscopy within the grains of Ba8Ge433.12 Once the preparation of phase-pure Ba8Ge433 was achieved, it became possible to make detailed investigations of its decomposition along with the formation of BaGe5. A detailed theoretical and experimental analysis on the relation between crystal structure and physical properties of BaGe5 is presented. In chapter 6, a thorough structural characterization and the physical properties of clathrates in the system Ba – Ni – Ge is presented based on the subtle relation between the crystal structure containing vacancies and the thermoelectric properties. During the investigations in this system, a large single crystal was grown by Nguyen et al. 22, 23 from the melt with the composition Ba8Ni3.5Ge42.10.4. A systematic reinvestigation of the phase relations in this system was performed and the influence of different Ni content to the crystal structure and physical properties is evaluated. The Si-based ternary clathrate with composition Ba8–δNixySi46–x–y is the subject of chapter 7. The phase relations and the homogeneity range are established. The crystal structure taking into account vacancies in the framework is discussed. Physical properties of bulk pieces are analyzed and the results are related to the sample composition. In addition, first-principles electronic structure calculations are carried out to assess variations in the electronic band structure, phase stability and chemical bonding.24 Chapter 8 reports on the intermetallic compound Ba3Si4,25, 26 which was encountered during the investigations on the Ba – Ni – Si phase diagram. The discussion covers issues related to preparation, crystal structure, phase diagram analysis, electrical and magnetic properties, NMR measurements, quantum mechanical calculations and oxidation to nanoporous silicon with gaseous HCl. Besides my contributions to the NoE CMA, I studied under the Priority Program 1178 of Deutsche Forschungsgemeinschaft “Experimental electron density as the key for understanding chemical interactions” with the project of “Charge distribution changes by external electric fields: investigations of bond selective redistributions of valence electron densities”. Chapter 9 deals with the preparation of chalcopyrites ZnSiP2 and CuAlS2 for experimental charge density analysis. Both phases show semiconducting properties and have non-centrosymmetric structures with high space group symmetry as needed to investigate the structural changes induced by external electric field. In this chapter, I describe the preparation and the crystal structure analyses of ZnSiP2 and CuAlS2 including issues related to the data collection as well as the results of NMR investigation.

This dissertation presents the development of two unique measurement platforms. <br><br>The first system is a high-temperature Z-Meter. This system is designed to simultaneously measure the electrical resistivity, Seebeck coefficient, and thermal conductivity of a thermoelectric sample to accurately determine the figure of merit, ZT, for that material. It is designed to operated at sample temperatures of up to 1000C, and with temperature gradients on the order of 500C across the sample. This system also provides <i>in situ</i> load monitoring for contact pressure and allows the user to adjust loading during the experiment. <br><br>The second part of this dissertation focuses on the development of enhanced composite phase change material (PCM) heat sinks to improve passive thermal management in mobile electronics. We present a new design for a composite PCM heat sink and utilize off-the-shelf PCMs to show characterize the performance. In order to accurately investigate the performance enhancement of these designs, we develop a turn-key thermal management evaluation platform to allow the user complete control over the power profiles and cycling applied to the test chip, as well as providing <i>in situ</i> temperature monitoring within the chip. The proposed package designs show significant improvement in the length of time extended before reaching the cut-off temperature within the heatfluxes tested, 6 - 14 W/cm^2, and accomplish this while weighing less than the equivalent sensible heat storage design.<br><br><br><br>

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.

Enormous advances in computing power in recent decades have made it possible to perform accurate numerical simulations of a wide range of systems in condensed matter physics. At the forefront of this progress has been density functional theory (DFT), a very popular approach to tackling the complexity of quantum-mechanical systems that very often strikes a good balance between accuracy and tractability in light of the finite computational resources available to researchers. This thesis describes work utilising DFT methods to tackle two distinct problems. Firstly, the theoretical prediction of stable and metastable periodic structures under specified conditions using the ab initio random structure searching (AIRSS) method, which involves a large scale exploration of the Born-Oppenheimer energy surface, and secondly the use of a vibrational self-consistent field (VSCF) approach to investigate the effects of nuclear motion and anharmonicity in crystal systems, which involves a local exploration of the Born-Oppenheimer energy surface. The AIRSS crystal structure prediction method is here applied to a study of defect structures in graphene. It is also applied to a study of the xenon-oxygen binary system under a range of geological pressures (83–200 GPa). Novel xenon oxide structures are predicted and characterised theoretically. This work was carried out in collaboration with an experimental study of the system at the lower end of the pressure range. The VSCF approach to investigating anharmonicity is here applied to the study of tin selenide (SnSe), a material that has recently been shown to demonstrate consider- able promise as a thermoelectric material. In this thesis, the effects of the anharmonic nuclear motion on the vibrational and electronic properties of SnSe are investigated quantitatively.

Ce mémoire de thèse s’inscrit dans le développement d’un microscope thermique à sonde locale.Ce système d’imagerie présente deux modes de fonctionnement permettant de déterminer soit unetempérature de surface soit des propriétés thermophysiques de matériaux. Un micro-thermocouplebifilaire a été utilisé comme capteur thermique. Il est peu invasif et permet d’accéder à destempératures de surface sur une large gamme de température. De plus, le microscope offrel’avantage d’être moins sensible à la nature optique des échantillons que les méthodes en champlointain. Dans le but de maitriser le contact entre la sonde et la surface, un résonateur à quartz(diapason) a été utilisé comme capteur de force. Un système d’excitation original basé sur l’effetphoto-thermo-élastique a été mis au point. Le microscope fonctionne donc comme un SThM puisqu’ilpermet d’extraire simultanément des images topographiques et thermiques (régime périodique 2 et3 oméga). En revanche, les résultats obtenus ont permis de mettre en évidence les avantages dumicro-thermocouple en termes de résolutions spatiales topographiques vis-à-vis des techniques àsondes résistives fonctionnant en mode 3 oméga. / This PhD thesis deals with the development of a thermalmicroscope using a local probe. This imagingsystem presents two functioning modes that allow determining either surface temperature or thermalproperties of materials. A micro-wire thermocouple is used as a thermal sensor. It is less invasiveand allows measuring the surface temperature with a large temperature range. Furthermore, themicroscope offers an advantage to be less sensitive to the optical nature of a sample surface thanoptical methods. To control the contact between the probe and the surface, a quartz tuning fork hasbeen used as a force sensor. An original excitation system has been developed based on the photothermaleffect. The microscope works also as a SThM since it permits to extract simultaneouslytopographical and thermal pictures (2 and 3 omega periodical modes). Results underlining themicro-thermocouple advantages, in terms of topographical compared to resistive probe techniquesfunctioning with the 3 omega method, have been obtained.

When a temperature differential is applied across a semiconductor, a thermal voltage develops across it in response. The ratio of this thermal voltage to the applied temperature differential is the Seebeck coefficient, a transport coefficient that complements measurements of electrical and thermal conductivity. The physical interpretation of the Seebeck coefficient is the entropy per charge carrier divided by its charge and is hence a direct measurement of the carrier entropy in the solid state. This PhD thesis has three major outcomes. The first major outcome is a demonstration of how the Seebeck coefficient can be used as a tool to quantify the role of energetic disorder in organic semiconductors. To this end, a microfabricated chip was designed to perform accurate measurements of the Seebeck coefficient within the channel of the active layer in a field-effect transistor (FET). When measured within an FET, the Seebeck coefficient can be modulated using the gate electrode. The extent to which the Seebeck coefficient is modulated gives a clear idea of charge carrier trapping and the distribution of the density of states within the organic semiconductor. The second major outcome of this work is the observation that organic semiconducting polymers show Seebeck coefficients that are temperature independent and strongly gate voltage modulated. The extent to which the Seebeck coefficient is modulated in the polymer PBTTT is found to be larger than that in the polymer IDTBT. Taken together with conventional charge transport measurements on IDTBT, the voltage modulated Seebeck coefficient confirms the existence of a vanishingly small energetic disorder in this material. In the third and final outcome of this thesis, the magnitude of the Seebeck coefficient is shown to be larger for organic small molecules as compared to organic polymers. The basis for this is not yet clear. There are reports that such an observation is substantiated through a larger contribution from vibrational entropy that adds to the so called entropy-of-mixing contribution so as to boost the magnitude of the Seebeck coefficient in organic small molecules. As of now, this remains an open question and is a potential starting point for future work. The practical implications of this PhD thesis lie in building cost-effective and environmentally friendly waste-heat to useful energy converters based on organic polymers. The efficiency of heat to energy conversion by organic polymers tends to be higher than that for conventional semiconductors owing to the presence of narrow bands in organic polymer semiconductors.

Small domestic hot water boilers burning solid fuels represent a significant source of air pollu-tion. It is therefore an effort to increase their combustion efficiency and to reduce the produc-tion of harmful emissions. For this reason, the operation of older and currently unsatisfactory types of household boilers has been legally restricted. Preferred types of boilers are low-emission boilers, especially automatic or gasification boilers. Most of them, however, in compar-ison with previous types of boilers, also require connection to the electricity grid. If there is a long-term failure in electricity grid, the operation of newer boiler types is limited. Wood and coal gasification boilers are currently available on the market and can be operated even in the event of a power failure, but only in heating systems with natural water circulation. In heating systems with forced water circulation, these boilers, fireplaces or fireplace inserts with hot-water heat exchangers cannot be operated without external battery supply in the event of a power failure. The dissertation thesis therefore deals with the question of whether it would be possible by thermoelectric conversion of waste heat of flue gases of small-scale low-emission combustion hot water domestic boilers to obtain sufficient electricity, to power supply their circulation pumps and to ensure operation in systems with forced water circulation independently of elec-tricity supply from the grid. In order to answer this question, a simulation tool predicting the power parameters of ther-moelectric generators was created. Compared to previously published works, the calculations and simulations include the influence of the generator on the boiler flue gas functionality. To verify the simulation tool, an experimental thermoelectric generator was built using the waste heat of the flue gas of an automatic hot water boiler for wood pellets. In addition to this genera-tor, there was also created an experimental thermoelectric fireplace insert and other equipment related to these experiments.

Les excellentes propriétés électroniques, thermiques et mécaniques du graphène confèrent à ce matériau planaire (bi-dimensionnel) un énorme potentiel applicatif, notamment en électronique. Néanmoins, ce matériau présente de sérieux inconvénients qui pourraient limiter son champ d'applications. Par exemple, sa structure de bandes électronique sans bande interdite rend difficile le blocage du courant dans un dispositif. De plus, pour les applications thermoélectriques, sa forte conductance thermique est aussi une forte limitation. Il y a donc beaucoup de défis à relever pour rendre ce matériau vraiment utile pour des applications. Cette thèse porte sur l'étude des propriétés électroniques et thermoélectriques dans les hétérostructures planaires constituées de graphène et de nitrure de bore hexagonal (BN). Différentes configuration de ce nouveau matériau hybride permettent de moduler la bande interdite, la conductance thermique et le coefficient Seebeck. Cette étude a été menée au moyen de calculs atomistiques basés sur les approches des liaisons fortes (TB) et du modèle à constantes de force (FC). Le transport d'électrons et de phonons a été simulé dans le formalisme des fonctions de Green hors équilibre. Les résultats montrent que, grâce à la modulation de la bande interdite, des transistors à base d'hétérostructures de BN et de graphène peuvent présenter un très bon rapport courant passant / bloqué d'environ 10⁴ à 10⁵. En outre, nous montrons l'existence d'états quantiques hybrides à l'interface zigzag entre le graphène et le BN donnant lieu à des propriétés de transport électronique très intéressantes. Enfin, ce travail montre qu'en agençant correctement des nano-flocons de BN sur les côtés d'un nanoruban de graphène, la conductance des phonons peut être fortement réduite alors que l'ouverture de bande interdite conduit à un accroissement important du coefficient Seebeck. Il en résulte qu'un facteur de mérite thermoélectrique ZT plus grand que l'unité peut être réalisé à température ambiante. / Graphene is a fascinating 2-dimensional material exhibiting outstanding electronic, thermal and mechanical properties. Is this expected to have a huge potential for a wide range of applications, in particular in electronics. However, this material also suffers from a strong drawback for most electronic devices due to the gapless character of its band structure, which makes it difficult to switch off the current. For thermoelectric applications, the high thermal conductance of this material is also a strong limitation. Hence, many challenges have to be taken up to make it useful for actual applications. This thesis work focuses on the theoretical investigation of a new strategy to modulate and control the properties of graphene that consists in assembling in-plane heterostructures of graphene and Boron Nitride (BN). It allows us to tune on a wide range the bandgap, the thermal conductance and the Seebeck coefficient of the resulting hybrid nanomaterial. The work is performed using atomistic simulations based on tight binding (TB), force constant (FC) models for electrons and phonons, respectively, coupled with the Green's function formalism for transport calculation. The results show that thanks to the tunable bandgap, it is possible to design graphene/BN based transistors exhibiting high on/off current ratio in the range 10⁴-10⁵. We also predict the existence hybrid quantum states at the zigzag interface between graphene and BN with appealing electron transport. Finally this work shows that by designing properly a graphene ribbon decorated with BN nanoflakes, the phonon conductance is strongly reduced while the bandgap opening leads to significant enhancement of Seebeck coefficient. It results in a thermoelectric figure of merit ZT larger than one at room temperature.

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