Thesis advisor: Zhifeng Ren / Thermoelectrics, as one promising approach for solid-state energy conversion between heat and electricity, is becoming increasingly important within the last a couple of decades as the availability and negative environmental impact of fossil fuels draw increasing attention. Therefore, various thermoelectric materials in a wide working temperature range from room temperature to 1000 degrees Celsius for power generation or below zero for cooling applications have been intensively studied. In general, the efficiency of thermoelectric devices relies on the dimensionless figure-of-merit (ZT) of the material, defined as ZT=(S<super>2</sup>&sigma;)T/&kappa;, where S is the Seebeck coefficient, [sigma] the electrical conductivity, [kappa] the thermal conductivity (sum of the electronic part, the lattice part, and the bipolar contribution at high temperature region), and T the absolute temperature during operation. Techniques to measure those individual parameters will be discussed in the 2nd chapter while the 1st chapter mainly covers the fundamental theory of thermoelectrics. Recently, the idea of using various nanostructured materials to further improve the ZT of conventional thermoelectric materials has led to a renewed interest. Among these types of nanostructured materials, nanocomposites which mainly denote for the nano-grained bulk materials or materials with nano-sized inclusions are the major focus of our study. For nanocomposites, the enhancement in ZT mainly comes from the low lattice thermal conductivity due to the suppressed phonon transport by those interfaces or structure features in the nanometer scale without deteriorating the electron transport. In the last few years, we have successfully demonstrated in several materials systems (Bismuth Telluride, Skutterudites, Silicon Germanium) that ball milling followed by hot pressing is an effective way for preparing large quantities of those nanocomposite thermoelectric materials with high ZT values in the bulk form. Therefore, in the 3rd part of this thesis, I will talk about how I applied the same technique to the Thalllium (Tl) doped Lead Telluride (PbTe) which was reported for an improved Seebeck coefficient due to the creation of resonant states near the Fermi level, leading to a high ZT of about 1.5 at around 500 degrees Celsius. I showed that comparing with conventional tedious, energy consuming melting method, our fabrication process could produce such material with competing thermoelectric performance, but much simpler and more energy effective. Potential problems and perspectives for the future study are also discussed. The 4th chapter of my thesis deals with the challenge that in addition to those nanostructuring routes that mainly reduce the thermal conductivity to improve the performance, strategies to enhance the power factor (enhancing [sigma] or S or both) are also essential for the next generation of thermoelectric materials. In this part, modulation-doping which has been widely used in thin film semiconductor industry was studied in 3-D bulk thermoelectric nanocomposites to enhance the carrier mobility and therefore the electrical conductivity [sigma]. We proved in our study that by proper materials design, an improved power factor and a reduced thermal conductivity could be simultaneously obtained in the n-type SiGe nanocomposite material, which in turn gives an about 30% enhancement in the final ZT value. In order to further improve the materials performance or even apply this strategy to other materials systems, I also provided discussions at the end of chapter. In the last chapter, the structural and transport properties of a new thermoelectric compound Cu₂Se was studied which was originally regarded as a superionic conductor. The [beta]-phase of such material possesses a natural superlattice-like structure, therefore resulting in a low lattice thermal conductivity of 0.4-0.5 Wm^-1K^-1 and a high peak ZT value of ~1.6 at around 700 degrees Celsius. I also studied the phase transition behavior between the cubic [beta]-phase and the tetragonal [alpha]-phase of such material from the discontinuity of transport property curves and the change in crystal structure. In addition, I also talk about the abnormal decrease in specific heat with increasing temperature that I observed in the as-prepared Cu₂Se samples. I suggest this material is of general interest to a broad range of researchers in Physics, Chemistry, and Materials Science. / Thesis (PhD) — Boston College, 2012. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

This dissertation covers the study of the thermoelectric properties of GeTe and SnTe. The goal of this research is to develop high-performance lead-free thermoelectric materials that can replace PbTe-based systems so that thermoelectric technology could be bring into real application. During the study, extensive investigations on the electrical and thermal transport behaviors were conducted both experimentally and theoretically. In Chapter 1 ~ 3, the origin of thermoelectricity, modelling and characterization methods are discussed in detail. In Chapter 4, study on the thermoelectric properties of Bi, Zn and In co-doped GeTe was presented. Initial doping with Bi enhanced the performance by tuning the electronic properties and bringing down the thermal conductivity. Subsequent Zn doping permitted to maintain the high power factor by increasing carrier mobility and reducing carrier concentration. Subsequent In doping boosted the density of state effective mass. A peak zT value of 2.06 and an average zT value of 1.30 have been achieved in (Ge0.97Zn0.02In0.01Te)0.97(Bi2Te3)0.03. In Chapter 5, we thoroughly investigated the transport properties of SnTe-Sb2Te3 alloying system, provided useful insight of the mechanism of the enhanced Seebeck coefficient. To also overcome the poor carrier mobility, Pb compensation was performed which effectively optimized the carrier mobility. Meanwhile, Pb compensation broke the charge balance, allowing Sb to precipitate out of the structure. These second-phase particles provided additional source of phonon scattering, effectively suppressing the lattice thermal conductivity. As a result, a peak zT of 1.1 at 778K and an average zT of 0.56 from 300K to 778K was achieved in (Sn0.98Ge0.05Te)0.91 (Sb2Pb0.5Te)0.09, which is one of the best SnTe-based thermoelectric systems. / Thesis / Master of Science (MSc) / Thermoelectric materials can generate energy from temperature gradient, making them potential solutions for the escalating energy crisis. The state-of-the-art thermoelectric material is PbTe which shows outstanding performance and high stability. However, the toxicity of Pb element limits its practical application. It is the purpose of this work to develop high-performance GeTe and SnTe-based thermoelectrics to reduce the usage of PbTe. Combining theoretical calculations and experimental characterizations, detailed investigation on the transport properties, crystal structure and microstructure were performed on both GeTe and SnTe. Relations between their thermoelectric properties and their composition, synthesis method and microstructure were revealed. This work paves the path for the development of environmentally friendly and high-performance thermoelectric systems.

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.