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Among emerging materials systems for thermoelectric applications, the early transition-metal nitrides based on ScN and CrN show unexpectedly promising properties. These properties are determined by high Seebeck coefficient, low thermal conductivity, and high electrical conductivity. There is, however, still a need to improve the thermoelectric properties. One idea is to introduce dopants or alloying elements to alter the concentration, mobility, and/or type of charge carriers, such as in (Cr1-xVx)N. Another is to focus on the different scattering mechanisms, such as to reduce the scattering of electrons and increase the scattering of phonons, thus increasing the electrical conductivity while lowering the thermal conductivity. Electrical conductivity can be altered by grain boundary modifications, such as larger grains as well as metallic inclusions, for a smoother interface for the electrons. Such nanoinclusions could potentially also act as phonon scattering centra. Phonon scattering can also occur by site substitution of isoelectronic but heavier atoms, which could reduce the phonon mean free path while retaining the electrical conductivity. For CrN, the obvious substitutions are Mo or W.This thesis investigates these effects for CrN-based materials, and how to control the growth of them in a DC-magnetron sputtering system.First, an optimization study for CrN was made, here the power was fixed while the temperature and nitrogen content were varied. Second, the effect of in-situ annealing at the deposition temperature was investigated. Here, both films with pure CrN and films with a mix of CrN and Cr2N were annealed. It was found that temperature, pressure, and ambient gas have a large effect on the decomposition of CrN to Cr2N. Third, alloying with V and Mo was implemented. It was found that Mo quickly breaks the rock-salt structure, and for further investigation of CrMoN and Cr(Mo,V)N systems, other deposition methods which allows for lower deposition rates need to be investigated.The sample deemed best was the CrVN-sample, showing a Seebeck coefficient of -141 μV/K, resistivity of 1520 μΩ∙cm and a power factor of 1.3 mW/mK2.

Local thermal management has important implications regarding comfort, energy consumption, and electronic device performance/lifetime. While organic thermoelectrics have emerged as promising materials for flexible thermoelectric energy harvesting devices, their potential as Peltier cooling element has been largely overlooked. Here, micro-thermoelectric coolers based on doped small molecule thin-films with a fast response time (around 25 µs) which is among the fastest micro-thermoelectric coolers reported are presented. This experimental cooling performance is supported by simulation using the finite-element method for thermal transport. The results show that organic thermoelectrics offer great potential for flexible and wearable micro-thermoelectric cooling applications.

Perovskite oxides show wide range of applications in the area of magnetism, ferroelectricity, piezoelectricity, thermoelectricity, gas sensing, catalyst development, solid oxide fuel cell, etc. This is due to flexibility in the structure and compositions that can be tuned by specific element doping. In the perovskite oxide (ABO3), large cation (A) is 12 -coordinated and smaller B-cation is 6 coordinated with oxide ions. Oxide materials are considered as better candidates for thermoelectric applications (interconversion of thermal into electrical energy) due to its non-toxicity and thermal stability at elevated temperature. These are insulating in nature and the conductivity can be increased by doping A and / or B –sites. Perovskite oxides are also used for oxygen monitoring in different applications including control and optimization of combustion of fossil fuels in industries and automobiles, biological and defines places, etc. In the present study, we focused on thermoelectric properties in single perovskite oxides of lanthanum cobaltite and calcium manganite and a double perovskite oxide of dysprosium barium cobaltite. Also, the oxygen sensing behaviour of dysprosium barium cobaltite at elevated temperatures is studied. The thesis contains seven chapters and a summary of respective chapters are given below. The first chapter outlines the basics of thermoelectric and gas sensing applications of both perovskite and double perovskite oxides. In the initial part, thermoelectric phenomena are explained. Thermoelectric effect is the conversion of thermal energy to electrical energy and vice-versa. Higher thermoelectric efficiency (η) can be achieved by maintaining a large temperature difference across the material. The efficiency depends on the thermoelectric figure of merit (zT) of material, which depends on thermopower (S), electrical resistivity (ρ) and thermal conductivity (κ) of the material and hence needs to be optimized. The latter part discusses the oxygen sensing property of distorted double perovskite 112 structure type as it shows advantages over other materials due to oxygen nonstoichiometric. Further, an overview of the relevant literature, objective and scope of the thesis are mentioned. The second chapter elucidates the materials and methods used for the present work. The materials viz. LaCoO3, CaMnO3-δ and DyBaCo2O5+δ, were selected for thermoelectric and oxygen sensing studies. Both the conventional solid state and soft chemistry methods were adopted for the synthesis of these materials. Powders were densified into pellets by hot uniaxial pressing / cold isostatic pressing and various heat treatments were carried out. Samples thus prepared were phase pure as confirmed using powder x-ray diffraction and Rietveld refinement performed for structural analysis. Morphological studies were carried out using scanning electron microscopy and transmission electron microscopy. Further Raman and x-ray photoelectron spectroscopic characterization of these materials were discussed. The transport properties viz. electrical resistivity, thermopower and thermal conductivity of compact pellets were measured at elevated temperatures. Further, the home-built apparatus for room temperature See beck measurements and chemo resistive oxygen sensing were explained in detail as a part of this work. The third chapter describes the effect of monovalent ion doping (Na+ and K+) at A-site of lanthanum cobaltite on thermoelectric properties. Lanthanum cobaltite system exhibit exotic behaviour due to commensuration phenomena of spin, lattice, charge and metal insulator transition. The synthesis, followed by structural refinements by Rietveld method using Fullprof suit program are explained. The results of the transport properties indicate that there is no appreciable change in the See beck Coefficient of K-doped samples throughout the studied temperature range. The Na-doped samples exhibit a decrease in the Seebeck value with increasing Na content at room temperature. At higher temperatures Seebeck value matches with that of the parent sample. This may be due to a change in the ratio of the concentration of Co4+/Co3+ ions which increases the configurational entropy of the system. In conclusion, the highest figure of merit (0.01) found for the Na / K- doped lanthanum cobaltite is for 15 atomic wt. % of doping amongst the studied samples. The fourth chapter explains about Tb/Nb co-doped calcium manganite for thermoelectric applications. The CaMnO3-δ shows enhanced thermoelectric properties, exhibits n-type behavior and the absolute thermopower is found to be 129 µV/K. Here, we investigated the Terbium and Niobium codoped at Ca and Mn-sites respectively. The presence of oxygen non-stoichiometry was confirmed using Raman spectroscopy (Mn3+ peak at 614 cm-1) and δ value was evaluated by iodometric titration. The thermoelectric properties of cold isostatic pressed (CIP) pellets prepared by the solid state and soft chemistry routes are compared. The non-monotonous behavior of absolute thermopower may be due to the increase of Mn3+ in the Mn4+ matrix and also the presence of oxygen defects in compounds. The thermoelectric figure of merit of solid state sample CaMnO3-δ estimated of 0.036 at 825K. The fifth chapter describes the thermoelectric properties of double Perovskite AA’B2O6 (112 type): (RE)BaCo2O5+δ. It is a disordered double perovskite with non-stoichiometry in oxygen and exhibits mixed valences of Cobalt. Resistivity of DyBaCo2O5+δ was found to be 0.09 Ω cm and Seebeck coefficient is found to be 42 µV/K. In order to improve the thermopower value, the Fe is substituted at Co-site. This varies the valences of Cobalt that in turn leads to a higher thermopower. Also, the morphology of thermally etched CIP pellets recorded and correlated with the transport properties. It shows the highest thermoelectric figure of merit of 0.25 at 773 K for 20 at wt % of Fe substituted sample. The sixth chapter explains about oxygen sensing studies of DyBaCo2O5+δ (112 type). The detailed structural and morphological characterization studies were carried out. Thermogravimetric analysis at isothermal temperature 873 K shows fast intake/release of oxygen of this disordered double perovskite structure. The higher chemo resistive oxygen sensitivity at the elevated temperature was measured. Further, the systematic study on the effect of oxygen sensing on the substitution of Fe and Cu at Co-site in DyBaCo2-xM xO5+δ was investigated. The possible bulk diffusion mechanism at higher temperature due to movement of oxygen defects were explained. The highest sensitivity was obtained for x = 0.4 at % of Fe and 0.2 at % of Cu at 973 K and 823 K respectively. The key findings and future aspects are summarized in the chapter-7.

Cette thèse porte sur l’étude de la synthèse et des données thermodynamiques des clathrates intermétalliques du système Na-Si, qui sont des matériaux pertinents pour les applications thermoélectriques et qui sont les précurseurs pour de nouvelles formes de Si prometteuses pour des applications photovoltaïques et photoniques. Dans ce travail la formation de clathrates Na-Si a été étudiée in situ et ex situ sous haute pression et haute température. Un nombre de nouvelles formes de silicium a été observé en chauffant des clathrates sous vide. Sous pression inférieure à 3 GPa les clathrates ne se forment pas par l’interaction des éléments. A la place, le siliciure Na4Si4 se forme et reste stable jusqu’à la fusion. Au-dessus de 3 GPa, Na4Si4 formé au début du chauffage, réagit avec l’excès de Si et donne Na30Si136 (structure II) et ensuite, si Na30Si136 réagit avec l’excès de Si pour former Na8Si46 (structure I) au dessous de 7 GPa et le clathrate NaSi6 au-dessus de 7 GPa. Des expériences ont été réalisées en utilisant des cellules standard Paris-Edimbourg (enclumes opposées) jusqu'à 6 GPa et la presse multi-enclumes pour les pressions au-dessus de 6 GPa. La forte sensibilité des produits de cristallisation à la concentration en sodium a été observée. Un schéma de transformations de clathrate pour les pressions entre 0 à 10 GPa et températures entre 300 et 1500 K a été proposé. Le diagramme de phase (concentration – température sous 4 GPa) qui représente les transformations observées et les domaines de stabilité a également été proposé et suggère l’existence de trois équilibres eutectiques : entre Si et structure I, entre structures I et II, et entre structure II et Na4Si4. Pour comprendre la thermodynamique et les interactions dans le système Na-Si, on a également effectué la modélisation du diagramme de phase à pression ambiante, ce que nous a permis de trouver les caractéristiques thermodynamiques de fusion et les équations des liquidus observés. Pour prendre en compte l’impact de la pression sur la thermodynamique, les équations d’état p-V-T de Si à structure diamant, structure I et la forme haute pression de structure II ont été utilisés. / The objective of this thesis is to study the synthesis and thermodynamics of intermetallic clathrates of the Na-Si system, which are the important materials for thermoelectric applications and are the promising precursors for new forms of Si for the photovoltaic and photonic applications. In this work the formation of the Na-Si clathrates have been studied in situ and ex situ at high pressure and high temperature. A number of new forms of silicon have been observed while heating the clathrates under vacuum. Under pressures below 3 GPa the clathrates does not form by the interaction of elements. Instead, the silicide Na4Si4 forms and remains stable up to melting. Above 3 GPa, Na4Si4 formed in the beginning of the heating, react with the excess of Si and gives Na30Si136 (structure II) and after that, if Na30Si136 react with the excess of Si, to form Na8Si46 (structure I) below 7 GPa and the clathrate NaSi6 above 7 GPa. The experiments have been performed by using the standard Paris-Edinburg cell (opposite anvils) up to 6 GPa and multianvil press for pressures above 6 GPa. The strong sensibility of the crystallization product to the concentration of sodium has been observed. The diagram of transformations in clathrates for pressures between 0 and 10 GPa and temperatures between 300 and 1500 K has been proposed. The phase diagram (concentration – temparature at 4 GPa) that represents the observed transformations and stability domains has been also proposed and suggests the existence of three eutetic equilibria: between Si and structure I, between structures I and II, and between structure II and Na4Si4. To understand the thermodynamics and interactions in the system Na-Si, the modelisation of phase diagram at ambient pressure has also been performed that allowed us to establish the thermodynamic characteristics of melting and the equations of observed liquidi. To take into account the impact of pressure on the thermodynamics, the p-V-T equations of state of diamond Si, structure I and high-pressure form of structure II have been used.

Soft matter, here, liquids and polymers, have adaptability to a surrounding geometry. They intrinsically have advantageous characteristics from a mechanical perspective, such as flowing and wetting on surrounding surfaces, giving compliant, conformal and deformable behavior. From the behavior of soft matter for heterogeneous surfaces, compliant structures can be engineered as embedded liquid microstructures or patterned liquid microsystems for emerging compliant microsystems. Recently, skin electronics and soft robotics have been initiated as potential applications that can provide soft interfaces and interactions for a human-machine interface. To meet the design parameters, developing soft material engineering aimed at tuning material properties and smart processing techniques proper to them are to be highly encouraged. As promising candidates, Ga-based liquid alloys and silicone-based elastomers have been widely applied to proof-of-concept compliant structures. In this thesis, the liquid alloy was employed as a soft and stretchable electrical and thermal conductor (resistor), interconnect and filler in an elastomer structure. Printing-based liquid alloy patterning techniques have been developed with a batch-type, parallel processing scheme. As a simple solution, tape transfer masking was combined with a liquid alloy spraying technique, which provides robust processability. Silicone elastomers could be tunable for multi-functional building blocks by liquid or liquid-like soft solid inclusions. The liquid alloy and a polymer additive were introduced to the silicone elastomer by a simple mixing process. Heterogeneous material microstructures in elastomer networks successfully changed mechanical, thermal and surface properties. To realize a compliant microsystem, these ideas have in practice been useful in designing and fabricating soft and stretchable systems. Many different designs of the microsystems have been fabricated with the developed techniques and materials, and successfully evaluated under dynamic conditions. The compliant microsystems work as basic components to build up a whole system with soft materials and a processing technology for our emerging society.

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.

Waste heat recovery is a multi-billion-dollar industry with a compound annual growth rate of 8.8% assessed between 2016 to 2024 and low-grade waste heat (< 230^oC ± 20^oC) makes up 66% of this ubiquitous resource. Thermoelectric generators are preferred for the recovery process because they are cheap and are well suited for this temperature range. They generate power by converting thermal potential to electric potential, known as the Seebeck effect. Since they have no moving parts, they are inherently immune to mechanical failure or an intermittent need for maintenance. However, the challenge has been to effectively harvest waste heat with these modules to generate power, using passive processes. This work is focused on designing a device for optimized harvesting of waste energy from the ambient with a custom, evaporatively-cooled heat sink. This heat sink is designed to passively handle the cooling of the other side of the thermoelectric module so as to enable the attainment of a minimum of 5V, which is the minimum voltage required to power small mobile devices. The heat sink model is similar to a loop heat pipe but engineered for compactness. To ensure this level of efficacy is attained, several studies are made to optimize the wick. Non-metal wicks were considered as they do not contribute to an increase in temperature of the compensation chamber in loop heat pipes. A non-metal wick integrated with nanoparticles is tested and results show a clear thermal management enhancement over similar but virgin non-metal wicks, at over 16%. The heat source section of the device is optimized for energy-harvesting in low grade temperature regimes by incorporating a near-black body coating on the metal heat source section. Experimental results show that both the heat source and sink sections were able to induce sufficient thermal potential for the thermoelectric modules to passively generate up to 5V using eight 40mm by 40mm Bismuth Telluride modules in 3.5 minutes. The prototype is relatively cheap, inherently reliable and presents the possibility of passively harvesting low-grade waste heat for later use, including powering small electronic devices.

This study centered on a novel thermoelectric generator (TEG) integrated into the built environment. Designed by Watts Thermoelectric LLC, the TEG is essentially a novel assembly of thermoelectric modules whose required temperature differential is supplied by hot and cold streams of water flowing through the TEG. Per its recommended operating conditions, the TEG nominally generates 83 Watts of electrical power. In its default configuration in the built environment, solar-thermal energy serves as the TEG’s hot stream source and geothermal energy serves as its cold stream source. Two systems-level, thermodynamic analyses were performed, which were based on the TEG’s upcoming characterization testing, scheduled to occur later in 2011 in Detroit, Michigan. The first analysis considered the TEG coupled with a solar collector system. A numerical model of the coupled system was constructed in order to estimate the system’s annual energetic performance. It was determined numerically that over the course of a sample year, the solar collector system could deliver 39.73 megawatt-hours (MWh) of thermal energy to the TEG. The TEG converted that thermal energy into a net of 266.5 kilowatt-hours of electricity in that year. The second analysis focused on the TEG itself during operation with the purpose of providing a preliminary thermodynamic characterization of the TEG. Using experimental data, this analysis found the TEG’s operating efficiency to be 1.72%. Next, the annual emissions that would be avoided by implementing the zero-emission TEG were considered. The emission factor of Michigan’s electric grid, RFCM, was calculated to be 0.830 tons of carbon dioxide-equivalent (CO2e) per MWh, and with the TEG’s annual energy output, it was concluded that 0.221 tons CO2e would be avoided each year with the TEG. It is important to note that the TEG can be linearly scaled up by including additional modules. Thus, these benefits can be multiplied through the incorporation of more TEG units. Finally, the levelized cost of electricity (LCOE) of the TEG integrated into the built environment with the solar-thermal hot source and passive ground-based cold source was considered. The LCOE of the system was estimated to be approximately $8,404/MWh, which is substantially greater than current generation technologies. Note that this calculation was based on one particular configuration with a particular and narrow set of assumptions, and is not intended to be a general conclusion about TEG systems overall. It was concluded that while solar-thermal energy systems can sustain the TEG, they are capital-intensive and therefore not economically suitable for the TEG given the assumptions of this analysis. In the end, because of the large costs associated with the solar-thermal system, waste heat recovery is proposed as a potentially more cost-effective provider of the TEG’s hot stream source. / text