Die vorliegende Dissertation beschäftigt sich mit der Optimierung und der Untersuchung der Materialeigenschaften des thermoelektrischen Materials Ag1-xPb18Sb1+yTe20 (englisch Lead-Antimony-Silver-Telluride: LAST). Bei LAST handelt es sich um Bleitellurid mit geringen Anteilen von Silber und Antimon, welche teilweise gelöst den Gitterplatz von Blei substituieren (Einbau in PbTe-Matrix) bzw. Fremdphasen auf m- und nm-Skala bilden. Seine hohe thermoelektrische Güte wird dabei hauptsächlich der geringen thermischen Gitterleitfähigkeit zugeschrieben, die in ersten Veröffentlichungen mit dem Auftreten nanoskaliger, Silber- und Antimonreicher Einschlüsse und deren Funktion als Phononenstreuer erklärt wurde. Das durch Schmelzsynthese hergestellte Bulkmaterial wurde im Rahmen der Arbeit durch Gefügeabbildung und Elementanalytik untersucht. In Kooperation mit den Projektpartnern sollte daraus eine Korrelation von Struktur- und Funktionseigenschaften abgeleitet, sowie eine reproduzierbare Syntheseroute entwickelt werden. Die elektronenmikroskopische Abbildung der Mikrostruktur erfolgte dabei auf zwei Größenskalen. Auf der µm-Skala wurde die Oberfläche des Bulkmaterials auf Homogenität und Zusammensetzung sowie Anteil des Fremdphasenbestands untersucht. Trotz des sehr komplexen Phasenbestandes aufgrund des quaternären Phasendiagramms und der Vielzahl relevanter Syntheseparameter konnte ein Zusammenhang zwischen Zusammensetzung (Regulierung des Silber- und Antimonanteils bzw. dessen Verhältnis), Temperbedingungen und thermoelektrischen Eigenschaften hergestellt werden. Mithilfe des detektierten Phasenbestandes konnte die Existenz einer Mischungslücke im quasibinären Phasendiagramm 2PbTe-AgSbTe2 nachgewiesen werden. Dabei bilden die Zusammensetzungen zwei der ermittelten Fremdphasen die Phasengrenzen. Die beobachtete spinodale Entmischung erzeugte eine extrem hohe Grenzflächendichte und kann somit ebenfalls einen Beitrag zur Senkung der Wärmeleitfähigkeit liefern. Für die Analyse der Mikrostruktur auf nm-Skala wurden aus der LAST-Matrix mithilfe der fokussierten Ionenstrahltechnik elektronentransparente Schnitte gefertigt. Abhängig von Temperbedingungen und dem Verhältnis von Silber und Antimon wurden auch hier fremdphasige Einschlüsse entdeckt. Dabei konnte ein optimaler Temperbereich von 500 bis 550 °C (bezogen auf einen hohen Gütewert) mit dem Auftreten dieser Einschlüsse korreliert werden. Eine allgemeine, direkte Zuordnung des Vorhandenseins von Nanostrukturen zu guten oder schlechten thermoelektrischen Eigenschaften konnte im Allgemeinen jedoch nicht nachgewiesen werden. Vielmehr wurden deutliche Hinweise gefunden, dass auch die Anordnung von Punktdefekten (Blei-Substitution durch Silber und Antimon) und ggf. Agglomerate aus Punktdefekten in der LAST-Matrix eine Rolle bei der Senkung der Wärmeleitfähigkeit spielen. Im hochaktuellen Entwicklungsgebiet selbstorganisierender Nanostrukturen mit Auswirkungen auf thermoelektrische Eigenschaften wurden substantielle Fortschritte bei der Entwicklung geeigneter, LAST-basierter Thermoelektrika für mittlere Einsatztemperaturen erzielt. Die gewonnenen Erkenntnisse dieser Arbeit zeigen Optionen zur Erzeugung hocheffizienter thermoelektrischer Bauelemente auf, wie unter anderem die bestätigte Stabilität bis zu relativ hohen Einsatztemperaturen (> 500 °C) zeigt.

Carbon nanotubes are made into films or bulks, their surface or junction morphology in the networks can be modified to obtain desired electrical transport properties by various surface modification methods. The methods include incorporation of organic molecules or inorganic nanoparticles, debundling of nanotubes by dispersing agents, and microwave irradiation. Because carbon nanotubes have unique carrier transport characteristics along a sheet of graphite in a cylindrical shape, the properties can be dramatically changed by the modification. This is ideal for developing high-performance materials for thermoelectric and photovoltaic energy conversion applications. In this research, decoration of various organic/inorganic nanomaterials on carbon nanotubes was employed to enhance their electrical conductivity, to improve thermoelectric power factor by modulating their electrical conductance and thermopower, or to obtain n-type converted carbon nanotube. The electrical conductivity of double-wall nanotubes (DWNTs) decorated with tetrafluoro-tetracyanoquinodimethane (F4TCNQ) was increased up to 5.9 × 10^5 S/m. The sheet resistances were measured to be 42 Ω/sq at 75% of transmittance for HNO3/SOCl2-treated DWNT films, making their electrical conductivities 200~300% better than those of the pristine DWNT films. A series of experiments at different ion concentrations and reaction time periods were systematically performed in order to find optimum nanomaterial formation conditions and corresponding electronic transport changes for better thermoelectric power factor. For example, the thermoelectric power factors were improved by ~180% with F4TCNQ on DWNTs, ~200% with Cu on SWNTs, and ~140% with Fe on single-walled nanotubes (SWNTs). Also SWNTs was converted from p-type to n-type with a large thermopower (58 μV/K) by using polyethyleneimine (PEI) without vacuum or controlled environment. This transport behavior is believed to be from charge interactions resulted from the difference between the work functions/reduction potentials of nanotubes and nanomaterials. In addition, different dispersing agents were utilized with DWNT and SWNTs to see a debundling effect in a film network. The highest electrical conductivity of ~1.72×10^6 S/m was obtained from DWNT film which was fabricated with a nanotube solution dispersed by chlorosulfonic acid. Debundling of nanotubes in the film network has been demonstrated to be a critical parameter in order to get such high electrical property. In the last experiment, Au nanoparticle decoration on carbon nanotube bundle was performed and a measurement of themophysical properties has done before and after modifying carbon nanotube surface. Carbon nanotube bundle, herein, was bridged on microdevice to enable the measurement work. This study demonstrates a first step toward a breakthrough in order to extract the potential of carbon nanotubes regarding electron transport properties.

Intermetallic clathrates have long been of interest for materials science research. The promise these materials hold for useful applications ranges from thermoelectrics to photovoltaics and optoelectronics to potentially ultra-hard materials and magnetic cooling applications. Their unique physical properties are intimately related to their intriguing structural properties. Thus a fundamental understanding of the chemistry and physics of inorganic clathrates offers the possibility to assess their potential for use in the various applications mentioned above. The purpose of the current work is to expand the current knowledge of the synthetic routes for obtaining clathrate materials, their structural, chemical, and physical properties, particularly those that from in the type I, II and VIII crystal structures. New synthesis routes are presented and used for preparation of single crystals of Na 8Si46 and Na 24Si136. Single-crystal X-ray analysis, and resistivity, Seebeck coefficient and thermal conductivity measurements are presented. In addition, two "inverse" clathrates with compositions Sn 24P19.3Br8 and Sn17Zn7P22Br8 have been characterized in terms of their transport properties. Since the magnetic refrigeration based on the magnetocaloric effect is a topic of great interest, type VIII Eu 8Ga16Ge30 clathrates are also explored in terms of their application for magnetic cooling.

Clathrates are a class of new materials that have an open-framework structure that allows guest atoms or molecules to be enclosed inside of their polyhedral framework. Varying the number, weight, and size of the guest species in a particular framework allows one to alter the physical properties of the clathrate. This relationship enables one to further the fundamental understanding of the physics and chemistry of the clathrate structure and use this knowledge to "tune" certain properties. This "tunability" of inorganic clathrates is of great interest as it allows one to optimize their physical properties; making them promising candidates for a range of applications such as thermoelectric, optoelectronic, and superconductivity. In this study, new synthesis methods of type I and II clathrates of group 14 are introduced, along with two new compositions of type I clathrates. A new synthesis method used to produce single crystal and microcrystalline Na8Si46 and Na24Si136 clathrates by the spark plasma sintering technique is introduced. Microcrystalline type I Na8Si46 and type II Na24Si136 are also selectively synthesized with no phase impurity of the other type using the low temperature ionic liquid method. In addition, the synthesis of microcrystalline Na8Ge3Si38 and single crystal Ba2Cs6Ga8Sn38 type I clathrates are presented for the first time.

abstract: The challenging search for clean, reliable and environmentally friendly energy sources has fueled increased research in thermoelectric materials, which are capable of recovering waste heat. Among the state-of-the-art thermoelectric materials β-Zn4Sb3 is outstanding because of its ultra-low glass-like thermal conductivity. Attempts to explore ternary phases in the Zn-Sb-In system resulted in the discovery of the new intermetallic compounds, stable Zn5Sb4In2-δ (δ=0.15) and metastable Zn9Sb6In2. Millimeter-sized crystals were grown from molten metal fluxes, where indium metal was employed as a reactive flux medium.Zn5Sb4In2-δ and Zn9Sb6In2 crystallize in new structure types featuring complex framework and the presence of structural disorder (defects and split atomic positions). The structure and phase relations between ternary Zn5Sb4In2-δ, Zn9Sb6In2 and binary Zn4Sb3 are discussed. To establish and understand structure-property relationships, thermoelectric properties measurements were carried out. The measurements suggested that Zn5Sb4In2-δ and Zn9Sb6In2 are narrow band gap semiconductors, similar to β-Zn4Sb3. Also, the peculiar low thermal conductivity of Zn4Sb3 (1 W/mK) is preserved. In the investigated temperature range 10 to 350 K Zn5Sb4In2-δ displays higher thermoelectric figure of merits than Zn4Sb3, indicating a potential significance in thermoelectric applications. Finally, the glass-like thermal conductivities of binary and ternary antimonides with complex structures are compared and the mechanism behind their low thermal conductivities is briefly discussed. / Dissertation/Thesis / Ph.D. Chemistry 2011

L'intérêt porté au développement de matériaux thermoélectriques est grandissant car ils permettent de créer des sources d'énergie renouvelable, dites « vertes », ce qui s'inscrit pleinement dans la stratégie de lutte contre le réchauffement climatique. A ce jour le rendement de tels systèmes reste faible, le coût de développement élevé, et les plages de températures d'utilisation sont limitées. Dans ces travaux de thèse différentes pistes sont explorées pour développer des matériaux innovants à base de chalcogènes, principalement le tellure. Les principaux résultats portent sur les points suivants. (i) Une étude par spectroscopies couplée à des calculs théoriques a permis de mieux comprendre les phénomènes de conduction dans les verres du système Cu-As-Te. (ii) La recristallisation complète de verres de formulation Ge20Te77Se3 dopés a été réalisée pour pousser à son terme la logique dite du Phonon Glass Electron Crystal (PGEC).(iii) Différents modes de synthèses ont été mis en œuvre pour suivre les propriétés thermoélectriques de matériaux de formulation CuPb18SbTe20 (frittage, SPS, flash-SPS, hybrid flash-SPS). (iv) Accroissement de 170% des performances d'alliage du système Pb-Sb-Te en générant des vacances de sites (composés non-stœchiométriques). (v) Le suivi des conséquences du dopage de GeTe par un seul élément a montré la nécessité d'un co-dopage pour simultanément accroître la conductivité électronique et le Seebeck. (vi) Le co-dopage In-Bi de GeTe a permis de créer des niveaux résonants (In) et d'accroitre la diffusion thermique (Bi). (vii) Enfin, le résultat le plus remarquable porte sur le co-dopage Ga-Sb de GeTe qui permet d'effectuer de l'ingénierie de structure de bandes. Couplé à une synthèse par hybrid flash SPS ces matériaux prometteurs permettent d'obtenir un zT 2 sur une large gamme de température (600–773 K). / With the performance of direct conversion between thermal and electrical energy, thermoelectric materials, which are crucial in the renewable energy conversion roadmap, provide an alternative for power generation and refrigeration to solve the global energy crisis. But the low efficiency of the current materials, their usual costs, availability, and limited working temperatures, drastically constrain their application. Hence, the search for new and more efficient thermoelectric materials is one of the most dynamic objectives of this thesis. The key milestones achieved from this thesis work includes: (i) elucidating the mechanism for hole conductivity in Cu-As-Te glasses by X-ray absorption spectroscopy and quantum simulations; (ii) formulating a novel approach to achieve phonon-glass electron-crystal mechanism by crystallizing the Ge20Te77Se3 glasses by excess doping with metals or semi-metals (glass-ceramics); (iii) demonstrating the effect of processing route on the thermoelectric performance of CuPb18SbTe20 and highlighting the advantage of hybrid-flash spark plasma sintering technique, i.e., better optimization of electrical and thermal transport properties and achieving multi-scale hierarchical architectures; (iv) improving the thermoelectric performance of Pb-Sb-Te alloys (enhancement by 170%) by tuning their cation vacancies (Pb deficiencies); (v) understating the impact of doping just a group-11 coinage metal, or group-13 element on GeTe solid-state solution and recapitulating the need for pair substitution; (vi) substantially enhancing the average zT of In-Bi codoped GeTe; (vii) achieving a remarkably high and stable zT of close to 2 over a wide temperature range (600 – 773 K) by manipulating the electronic bands in Ga-Sb codoped GeTe, which has been processed by hybrid flash-spark plasma sintering, thus making it a serious candidate for energy harvesting systems.

The development of a complete thermoelectric generator and its application on a display polarizer film was successfully accomplished in this thesis. A systematic study of the prospective thermoelectric materials, PEDOT:PSS-based and ${ZnON}$, used for the present application is presented. To the best of our knowledge, this is the first exploration of the thermoelectric parameters of ${ZnON}$ reported here. Thin-film deposition of these materials was performed via both solution- and vacuum-based techniques. In addition, certain doping mechanisms were tested in an attempt to further understand the correlation between electrical conductivity and Seebeck coefficient. A maximum power factor of $42{\mu}Wm^{-1}K^{-2}$ was achieved for the PEDOT:PSS-based thin film at room temperature. It was initially doped via 5vol% of DMSO and sequentially treated with ethylene glycol. Specifically, its electrical conductivity displayed a 2-fold increase after EG treatment, reaching a value of about 1632 Scm$^{-1}$. Systematic studies performed on the association between thin-film thickness and its Seebeck coefficient shows a decrease in the latter as the number of multilayers printed increases. Among the different $O_{2}/N_{2}$ ratios that were tested for ${ZnON}$ thin films, a maximum power factor value of 163${\mu}Wm^{-1}K{-2}$ was achieved with the lowest $O_{2}$ flow rate configuration. In contrast to PEDOT:PSS-based thin films, the ${ZnON}$ displayed the opposite effect on the relation of the Seebeck coefficient with respect to thin-film thickness. Furthermore, a heterostructure was also developed by implementing ${ZnO}$ nanowires into the ${ZnON}$ thin film. ${ZnO}$ nanowires have been fabricated through the hydrothermal method on inkjet-printed patterns of zinc acetate dihydrate. It has been demonstrated that with the right inkjet-printing parameters and substrate temperature, ${ZnO}$ nanowires can be effortlessly fabricated in accordance with the desired pattern variations under low temperature and mild conditions. Finally, a complete device of the thermoelectric generator was fabricated using the above materials and a special set-up developed in order to test the device on the polarizer. The power output achieved from a 1-thermoelectric couple under normal backlight illumination and ambient conditions was 23pW. Overall, it is thought that the particular design and proof of concept presented here can be the basis of a prospective energy harvesting scheme via thermoelectrics in future display-based handheld devices.

Monte Carlo based statistical approach to solve Boltzmann Transport Equation (BTE) has become a norm to investigate heat transport in semiconductors at sub-micron regime, owing mainly to its ability to characterize realistically sized device geometries qualitatively. One of the primary issues with this technique is that the approach predominantly uses empirically fitted phonon dispersion relations as input to determine the properties of phonons so as to predict the thermal conductivity of specified material geometry. The empirically fitted dispersion relations assume harmonic approximation thereby failing to account for thermal expansion, interaction of lattice waves, effect of strain on spring stiffness, and accurate phonon-phonon interaction. To circumvent this problem, in this work, a coupled molecular mechanics-Monte Carlo (MM-MC) platform has been developed and used to solve the phonon Boltzmann Transport Equation (BTE) for the calculation of thermal conductivity of several novel and emerging nanostructures. The use of the quasi-anharmonic MM approach (as implemented in the open source NEMO 3-D software toolkit) not only allows one to capture the true atomicity of the underlying lattice but also enables the simulation of realistically-sized structures containing millions of atoms. As compared to the approach using an empirically fitted phonon dispersion relation, here, a 17% increase in the thermal conductivity for a silicon nanowire due to the incorporation of atomistic corrections in the LA (longitudinal acoustic) branch alone has been reported. The atomistically derived thermal conductivity as calculated from the MM-MC framework is then used in the modular design and analysis of (i) a silicon nanowire based thermoelectric cooler (TEC) unit, and (ii) a GaN/InN based nanostructured light emitting device (LED). It is demonstrated that the use of empirically fitted phonon bandstructure parameters overestimates the temperature difference between the hot and the cold sides and the overall cooling efficiency of the system, thereby, demanding the use of the BTE derived thermal conductivity in the calculation of thermal conductivity. In case of the light-emitting device, the microscopically derived material parameters, as compared to their bulk and fitted counterparts, yielded ~3% correction (increase) in optical efficiency. A non-deterministic approach adopted in this work, therefore, provides satisfactory results in what concerns phonons transport in both ballistic and diffusive regimes to understand and/predict the heat transport phenomena in nanostructures.

xvii, 189 p. ： ill. (some col.) / Low-dimensional electronic systems, systems that are restricted to single energy levels in at least one of the three spatial dimensions, have attracted considerable interest in the field of thermoelectric materials. At these scales, the ability to manipulate electronic energy levels offers a great deal of control over a device's thermopower, that is, its ability to generate a voltage due to a thermal gradient. In addition, low-dimensional devices offer increased control over phononic heat flow. Mesoscale geometry can also have a large impact on both electron and phonon dynamics. Effects such as ballistic transport in a two-dimensional electron gas structure can lead to the enhancement or attenuation of electron transmission probabilities in multi-terminal junctions. The first half of this dissertation investigates the transverse thermoelectric properties of a four-terminal ballistic junction containing a central symmetry-breaking scatterer. It is believed that the combined symmetry of the scatterer and junction is the key component to understanding non-linear and thermoelectric transport in these junctions. To this end, experimental investigations on this type of junction were carried out to demonstrate its ability to generate a transverse thermovoltage. To aid in interpreting the results, a multi-terminal scattering-matrix theory was developed that relates the junction's non-linear electronic properties to its thermoelectric properties. The possibility of a transverse thermoelectric device also motivated the first derivation of the transverse thermoelectric efficiency. This second half of this dissertation focuses on heat flow phenomena in InAs/InP heterostructure nanowires. In thermoelectric research, a phononic heat flow between thermal reservoirs is considered parasitic due to its minimal contribution to the electrical output. Recent experiments involving heterostructure nanowires have shown an unexpectedly large heat flow, which is attributed in this dissertation to an interplay between electron-phonon interaction and phononic heat flow. Using finite element modeling, the recent experimental findings have provided a means to probe the electron-phonon interaction in InAs nanowires. In the end, it is found that electron-phonon interaction is an important component in understanding heat flow at the nanoscale. This dissertation includes previously unpublished co-authored material. / Committee in charge: Dr. Richard Taylor， Chair； Dr. Heiner Linke， Advisor； Dr. David Cohen， Member； Dr. John Toner， Member； Dr. David Johnson， Outside Member

The growing need of energy worldwide has lead to an increasing demand for alternative sources of power generation. Thermoelectric materials are one of the ‘green energy sources’ which convert directly heat into electricity, and vice–versa. The efficiency of this conversion is dependent on ‘figure of merit’ (z T), which depends on the material’s Seebeck coefficient (S), electrical resistivity (ρ) and thermal conductivity (κ) through the relation z T=S2T/ρκ, where T is the temperature. High values of z T lead to high efficiency, and therefore, z T must be maximized. Lead telluride is well–established thermoelectric material in the temperature range 350 K and 850 K. The aim of this thesis is to improve the z T of the material by adopting two different approaches – (i) doping/alloying and (ii) introducing additional interfaces in bulk i.e. having two phase PbTe. In this thesis, first an introduction about the thermoelectric phenomenon is given, along with the material parameters on which z T depends. A survey of literature associated with PbTe is done and the current status of thermoelectric devices is summarized briefly. This is followed by a description of the synthesis procedure and the measurement techniques adopted in this work. The first approach is the conventional alloying and doping of the material by which carrier concentration of the material is controlled so that maximum power factor Sρ2 is achieved and a simultaneous reduction of thermal conductivity takes place by mass fluctuation scattering. Under this, two systems have been studied. The first system is PbTe1−ySey alloys doped with In (nominal composition: Pb0.999In0.001Te1−ySey, y=0.01, 0.05, 0.10, 0.20, 0.25, 0.30). The compounds were single phase and polycrystalline. Lattice constants obtained from Rietveld refinement of X–ray diffraction (XRD) data showed that Vegard’s law was followed, indicating solid solution formation between PbTe and PbSe. Compositional analysis showed lower indium content than the nominal composition. Temperature dependent Seebeck coefficient showed all the samples to be n–type while Pisarenko plots showed that indium did not act as a resonant dopant. Electrical resistivity increased with temperature, while mobility vs T fitting showed a mixed scattering mechanism of acoustic phonon and ionized impurity scattering. Thermal conductivity followed a T1 dependence, which indicated acoustic phonon scattering. At high temperature, slight bipolar effect was observed, which showed the importance of control-ling carrier concentration for good thermoelectric properties. A z T of 0.66 was achieved at 800 K. The second alloy studied under this approach was Mn doped Pb1−ySnyTe alloy (nominal composition Pb0.96−yMn0.04SnyTe (y=0.56, 0.64, 0.72, 0.80)). All the samples followed Vegard’s law, showing formation of complete solid solution between PbTe and SnTe. Microstructure analysis showed grain size distribution of <1 µm to more than 10 µm. Seebeck coefficient showed all samples were p-type and the role of two valence band conduction in p–type PbTe based materials. Electrical resistivity showed a de-crease possibly due to (i) large carrier concentration or (ii) increased mobility due to Mn2+ ions. Thermal conductivity decreased systematically with decreasing Sn content. Bipolar effect was observed at high temperatures. Accordingly, the highest z T of 0.82 at 720 K was obtained for the sample with Sn (y=0.56) content due to optimum carrier concentration and maximum disorder. The second approach of having additional interfaces in bulk focuses on reducing thermal conductivity by scattering phonons. Under this approach, three systems were studied. The first system is PbTe with bismuth (Bi) secondary phase. The XRD and Ra-man studies showed that bismuth was not a dopant in PbTe, while micrographs showed micrometer–sized Bi secondary phase dispersed in bulk of PbTe. Reduction in Seebeck coefficient showed possible hole donation across PbTe–Bi interfaces, while electrical re-sistivity and thermal conductivity showed that the role of electrons at the interfaces was more important than phonons for the present bismuth concentrations. For the parent PbTe, z T of 0.8 at 725 K was reached, which, however decreased for bismuth added samples. The second system studied under the two phase approach was indium (In) added PbTe. Indium was not found to act as dopant in PbTe, while micrometer sized indium phase was found in PbTe bulk. A decrease in the electronic thermal conductivity ac-companied by a simultaneous increase of the electrical resistivity and Seebeck coefficient throughout the measurement range indicated increased scattering of electrons at PbTe-In interfaces. Higher values of the lattice thermal conductivity showed that the PbTe–In interfaces were ineffective at scattering phonons, which was initially expected due to the lattice mismatch between PbTe and In. For PbTe with 3 at. % In phase, z T value of 0.78 at 723 K was achieved. Under the two phase approach, as a comparative study, PbTe with both micrometer sized Bi and In phases together was prepared, in which no improvement in z T was found. A comparison of both the approaches showed that the alloying approach is better than the two–phase approach. This is because micrometer sized secondary phase scatter the electrons more than the phonons, leading to adverse effect on the transport coef-ficients, and hence, on z T. Alloying, on the other hand, is more beneficial in reducing thermal conductivity by mass fluctuation scattering, along with a simultaneous reduction of electrical resistivity.