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Thermoelectric semiconductors can convert temperature differences into electricity or electricity into temperature differences. This offers great potential for the use of wasted heat or cooling. These materials can be used in a variety of fields, from healthcare to space exploration. The effectiveness of the materials is evaluated by their thermoelectric properties such as the Seebeck coefficient, electrical conductivity, and thermal conductivity. The aim of this PhD thesis is to investigate the electronic structure using first-principle methods for potential thermoelectric applications. Materials of interest include Copper and Tin based ternary /quaternary compounds, and monolayers of SnS2, SnSe2 and Janus SnSSe. Density functional theory, ab initio molecular dynamics and Boltzmann transport theory are used to study the electronic and phonon transport properties. In the first part of the thesis, electronic structure calculations were performed on both monoclinic and disordered cubic forms of Cu2SnS3(CTS). The impact of structural disorder on thermoelectric properties was examined through these simulations. The results, obtained through first-principle calculations, revealed the existence of band tails in the electronic density of states for the disordered structure, and low-lying optical modes in the disordered cubic structure. This was found to be caused by a significant variation in Sn bonding, leading to strong anharmonicity as measured by the Grüneisen parameter. The findings from the first principle calculations were supported by Nuclear inelastic scattering experiments. Furthermore, the effect of grain size on Cu2SnS3 was studied using first-principles calculations on various ordered and disordered surfaces. The density of states (DOS) revealed that the surface of CTS is conductive due to the presence of dangling bonds. Furthermore, calculations of the formation energy showed that the stoichiometric CTS, Cu-vacant and Cu-rich systems are energetically more favourable, while the formation of Sn-vacant and Sn-rich systems is less likely. In the subsequent study, the impact of Ag substitution at the Sn site at various concentrations was investigated. The Fermi level for Ag-substituted systems was found to lie deep within the valence band, with the shift of the Fermi level inside the valence band increasing with substitution increasing the carrier concentration. The incorporation of Ag into the system decreases the root mean squared displacement of the other cations and anions, which reduces the scattering of phonons and thereby increases the lattice thermal conductivity. A comparative study of various polymorphs of CTS, Cu2ZnSnS4 and Cu2ZnSnSe4 was done. Ab-initio molecular dynamics was performed on CTS, CZTS and CZTSe. The root mean squared displacement value for the disordered polymorph was higher than for the ordered phase, indicating increased static disorder. This corresponds to the static (temperature-independent) distortion of the crystalline lattice due to the disorder of the cations and is associated with higher anharmonicity and bond inhomogeneity in the disordered phase, which is then directly responsible for the ultra-low thermal conductivity. In the final part of the thesis, thermoelectric properties of dichalcogenide monolayer of SnS2, SnSe2 and Janus SnSSe was performed. Density functional theoretical calculations points out the hexagonal Janus SnSSe monolayer as a potential high-performing thermoelectric material. Results for the Janus SnSSe monolayer show an ultra-low thermal conductivity originating from the low group velocity of the low-lying optical modes, leading to superior zT values of 0.5 and 3 at 300 K and 700 K for the p-type doping, respectively. The successful calculation of properties for materials shows that the computational work done in this thesis can be used for further research into thermoelectricity.

Thesis advisor: Cyril P. Opeil / Thermoelectric (TE) materials are of great interest to contemporary scientists because of their ability to directly convert temperature differences into electricity, and are regarded as a promising mode of alternative energy. The TE conversion efficiency is determined by the Carnot efficiency, η_C and is relevant to a commonly used figure of merit ZT of a material. Improving the value of ZT is presently a core mission within the TE field. In order to advance our understanding of thermoelectric materials and improve their efficiency, this dissertation investigates the low-temperature behavior of the p-type thermoelectric Cu2Se through chemical doping and nanostructuring. It demonstrates a method to separate the electronic and lattice thermal conductivities in single crystal Bi2Te3, Cu, Al, Zn, and probes the electrical transport of quasi 2D bismuth textured thin films. Cu2Se is a good high temperature TE material due to its phonon-liquid electron-crystal (PLEC) properties. It shows a discontinuity in transport coefficients and ZT around a structural transition. The present work on Cu2Se at low temperatures shows that it is a promising p-type TE material in the low temperature regime and investigates the Peierls transition and charge-density wave (CDW) response to doping [1]. After entering the CDW ground state, an oscillation (wave-like fluctuation) was observed in the dc I-V curve near 50 K; this exhibits a periodic negative differential resistivity in an applied electric field due to the current. An investigation into the doping effect of Zn, Ni, and Te on the CDW ground state shows that Zn and Ni-doped Cu2Se produces an increased semiconducting energy gap and electron-phonon coupling constant, while the Te doping suppresses the Peierls transition. A similar fluctuating wave-like dc I-V curve was observed in Cu1.98Zn0.02Se near 40 K. This oscillatory behavior in the dc I-V curve was found to be insensitive to magnetic field but temperature dependent [2]. Understanding reducing thermal conductivity in TE materials is an important facet of increasing TE efficiency and potential applications. In this dissertation, a magnetothermal (MTR) resistance method is used to measure the lattice thermal conductivity, κ_ph of single crystal Bi2Te3 from 5 to 60 K. A large transverse magnetic field is applied to suppress the electronic thermal conduction while measuring thermal conductivity and electrical resistivity. The lattice thermal conductivity is then calculated by extrapolating the thermal conductivity versus electrical conductivity curve to a zero electrical conductivity value. The results show that the measured phonon thermal conductivity follows the e^(Δ_min⁄T) temperature dependence and the Lorenz ratio corresponds to the modified Sommerfeld value in the intermediate temperature range. These low-temperature experimental data and analysis on Bi2Te3 are important compliments to previous measurements and theoretical calculations at higher temperatures, 100 – 300 K. The MTR method on Bi2Te3 provides data necessary for first-principles calculations [4]. A parallel study on single crystal Cu, Al and Zn shows the applicability of the MTR method for separating κ_e and κ_ph in metals and indicates a significant deviation of the Lorenz ratio between 5 K and 60 K [3]. Elemental bismuth is a component of many TE compounds and in this dissertation magnetoresistance measurements are used investigate the effect of texturing in polycrystalline bismuth thin films. Electrical current in bismuth films with texturing such that all grains are oriented with the trigonal axis normal to the film plane is found to flow in an isotropic manner. By contrast, bismuth films with no texture such that not all grains have the same crystallographic orientation exhibit anisotropic current flow, giving rise to preferential current flow pathways in each grain depending on its orientation. Textured and non-textured bismuth thin films are examined by measuring their angle-dependent magnetoresistance at different temperatures (3 – 300 K) and applied magnetic fields (0 – 90 kOe). Experimental evidence shows that the anisotropic conduction is due to the large mass anisotropy of bismuth and is confirmed by a parallel study on an antimony thin film [5]. [1] Mengliang Yao, Weishu Liu, Xiang Chen, Zhensong Ren, Stephen Wilson, Zhifeng Ren, and Cyril Opeil, J. Alloys Compd. 699, 718 (2017). [2] Mengliang Yao, Weishu Liu, Xiang Chen, Zhensong Ren, Stephen Wilson, Zhifeng Ren, and Cyril P. Opeil, J. Materiomics 3, 150 (2017). [3] Experimental determination of phonon thermal conductivity and Lorenz ratio of single crystal metals: Al, Cu and Zn, Mengliang Yao, Mona Zebarjadi, and Cyril P. Opeil, under review. [4] Experimental determination of phonon thermal conductivity and Lorenz ratio of single crystal bismuth telluride, Mengliang Yao, Stephen Wilson, Mona Zebarjadi, and Cyril Opeil, under review. [5] Albert D. Liao, Mengliang Yao, Ferhat Katmis, Mingda Li, Shuang Tang, Jagadeesh S. Moodera, Cyril Opeil, Mildred S. Dresselhaus, Appl. Phys. Lett. 105, 063114 (2014). / Thesis (PhD) — Boston College, 2017. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

Ce travail a été réalisé dans le cadre d’une étude sur les matériaux massifs thermoélectriques nanostructurés, composés dont les propriétés peuvent potentiellement être améliorées grâce aux nombreuses interfaces. Il concerne la réalisation et la caractérisation de composites à matrices skutterudites (CoSb3 ou In0,4Co4Sb12) contenant différents taux d’inclusions nanométriques de PbTe ou de ZnO. Des techniques de métallurgie des poudres et de fracturation laser en milieu liquide ont été conjuguées à divers moyens de caractérisation (RX, MEB, MET) pour mettre au point le protocole d’élaboration des matériaux. L’observation des microstructures des composites a révélé que l’étape de dispersion des particules dans le matériau est la plus délicate de la préparation. Celles-ci forment des amas localisés aux joints de grains. La porosité est également relativement importante lorsque le taux d’inclusions est élevé en particulier avec ZnO. Les propriétés électriques (pouvoir thermoélectrique, résistivité électrique, effet Hall) et thermiques (conductivité thermique) ont été mesurées sur une large gamme de température (2-800 K) puis corrélées aux microstructures. L’analyse des résultats a permis de montrer que le PbTe tend à dégrader les propriétés électriques des deux matrices du fait notamment d’une réaction des nanoparticules avec les skutterudites. Par contre, l’ajout de ZnO semble plus prometteur même s’il est difficile de conclure définitivement quant à son rôle réel compte tenu de la complexité des microstructures (défauts, pores, joints de grains). Un modèle théorique développé afin de mieux comprendre l’impact des tailles de particules sur les propriétés thermiques a mis en évidence qu’il est plus intéressant de travailler avec des skutterudites partiellement remplies plutôt qu’avec CoSb3 puisque les inclusions affectent majoritairement les phonons les moins énergétiques. Les tendances qui se dégagent de cette étude vont plutôt dans le sens d’une détérioration des propriétés avec ce type de nanostructuration lorsque les taux de nanoparticules excèdent quelques pourcents mais l’utilisation de quantités plus faibles et une porosité mieux maîtrisée pourrait avoir un effet positif / This work was carried out as part of a study on nanostructured bulk thermoelectric materials, compounds whose properties can potentially be improved with many interfaces. It is related to the synthesis and characterization of skutterudites (CoSb3 or In0.4Co4Sb12) containing nanoinclusions of PbTe or ZnO. Techniques of powder metallurgy and laser fragmentation in liquid medium were combined to X-rays diffraction analyses, SEM and TEM observations to develop the experimental procedure for the preparation of materials. The microstructures of the composites show that the dispersion step is the most difficult part of the preparation as it leads to agglomerates located at the grain boundaries. The porosity is also relatively large when the quantity of inclusions is high especially with ZnO. The electrical and thermal properties (thermoelectric power, electrical resistivity, Hall effect, thermal conductivity) have been measured over a wide temperature range (2-800 K) and correlated with the microstructures. The results analysis showed that PbTe tends to degrade the electrical properties of the two matrixes because of a reaction between the nanoparticles and the skutterudites. Contrarily, the addition of ZnO seems more promising although it is difficult to conclude definitively on its effectiveness given the complexity of the microstructures (defects, pores, grain boundaries). A theoretical model developed to better understand the impact of the particle sizes on the thermal properties showed that it is more interesting to work with partially filled skutterudites instead of pure CoSb3 since the inclusions mostly affect the scattering of long wavelength phonons. The trends that emerge from this study are rather in the sense of a deterioration of the thermoelectric properties with this type of nanostructure when the quantities of nanoparticles exceed a few percentage. The use of smaller quantities and a better controlled porosity should be considered

Thermoelectricity can directly convert a temperature difference into a voltage or charge current. Recently, the development of spin caloritronics has introduced spin as another degree of freedom in traditional thermoelectrics. This discovery bodes a new generation of magnetic random access memories (MRAMs), where thermal spin-transfer torque (TSTT) rather than voltage driven spin-transfer torque (STT) is used to switch the magnetization in magnetic tunnel junctions (MTJs). To advance the rising trend of spin caloritronics, the coupling of charge, spin, and heat flow during electron transport in MTJs was systematically studied in this thesis. To begin with, the static transport properties of MTJs were studied by observing current dependent tunnel magnetic resistance (TMR). The observed decrease of TMR with a biased current is attributed to the change in spin polarization of the free ferromagnetic layer. A phenomenological model has been built based on the current dependent polarization, which agrees with our experimental results. Next, the Seebeck rectification effect in MTJs was studied. By applying microwave currents to MTJs, an intrinsic thermoelectric coupling effect in the linear response regime of MTJs was discovered. This intrinsic thermoelectric coupling contributes a nonlinear correction to Ohm's law. In addition, this effect can be controlled magnetically since the Seebeck coefficient is related to magnetization configuration. Finally, TSTT in MTJs was systematically studied. A laser heating technique was employed to apply a temperature difference across the tunnel barrier and ferromagnetic resonance (FMR) spectra were measured electrically through spin rectification. By analyzing the FMR spectra, TSTT in MTJs was observed and the angular dependence of TSTT was found to be different from dc-biased STT. By solving the Landau-Lifshitz-Gilbert equation including STT, the experimental observations were well explained. The discovery of Seebeck rectification refines the previous understanding of magneto-transport and microwave rectification in MTJs and provides a new possibility for utilizing spin caloritronics in high-frequency applications. The study of TSTT in MTJs shows clear experimental evidence of TSTT in MTJs. Further optimization of the design of MTJs may succeed in decreasing the necessary switching fields strength or even achieve a switching by only TSTT in MTJs. / February 2017

Environmental challenges, such as global warming, growing demand on energy, and diminishing oil sources have accelerated research on alternative energy conversion methods. Thermoelectric power generation is a promising method to convert wasted heat energy into useful electrical energy form. A temperature gradient imposed on a thermoelectric device produces a Seebeck potential. However, this temperature gradient causes thermal stresses due to differential thermal expansions and mismatching of the bonded components of the device. Thermal stresses are critical for thermoelectric devices since they can generate failures, including dislocations, cracks, fatigue fractures, and even breakdown of the entire device. Decreases in power-generation performance and operation lifetime are major consequences of these failures. In order to minimize thermal stresses in the legs without affecting power-generation capabilities, this study concentrates on structural solutions. Thermoelectric devices with non-segmented and segmented legs were modeled. Specifically, the possible effect of various leg geometries, configurations, and dimensions were evaluated using finite-element and statistical methods. Significant changes in the magnitudes and distributions of thermal stresses occurred. Specifically, the maximum equivalent stresses in the rectangular-prism and cylindrical legs were 49.9 MPa and 43.3 MPa, respectively for the temperature gradient of 100ºC. By using cylindrical legs with modified dimensions, decreases in the maximum stresses in legs reached 21.2% without affecting power-generation performance. Moreover, the effect of leg dimensions and coaxial-leg configurations on power generation was significant; in contrast, various leg geometries and rotated-leg configurations had very limited affect. In particular, it was possible to increase power output from 20 mW to 65 mW by simply modifying leg widths and heights within the defined range. It should be noted, however, this modification also increased stress levels. It is concluded that leg geometries, configurations, and dimensions can be redesigned for improved durability and overall performance of thermoelectric devices.

L’étude de matériaux thermoélectriques nanocomposites et nanostructurés est en plein essor suite à l’intérêt de multiplier le nombre d’interfaces et de diminuer la taille des objets pour améliorer les performances. Nous avons mis au point une nouvelle méthode de préparation de nanopoudres thermoélectriques de type n (Bi0.95Sb0.05)2(Te0.95Se0.05)3 et de type p (Bi0.2Sb0.8)2Te3, à partir de la fracturation laser en milieu aqueux de poudres de taille micrométrique. La cellule de préparation développée permet d’obtenir par jour environ 200 mg de poudres nanométriques cristallisés présentant la structure cristallographique des poudres initiales et dont la taille moyenne est comprise entre 7 et 12 nm. Les mécanismes mis en jeu dans l’obtention des nanoparticules ont été abordés. Ils dépendent fortement de la densité d’énergie du faisceau laser. Les nanopoudres ont ensuite été mélangées mécaniquement aux poudres micrométriques de même nature et ont été compactées à froid. Les propriétés thermoélectriques (résistivité électrique, pouvoir thermoélectrique, conductivité thermique) des nanocomposites ont été évaluées à température ambiante. Les premiers résultats montrent que même si le pouvoir thermoélectrique est maintenu dans les matériaux nanostructurés et nanocomposites et que la conductivité thermique totale peut, de manière tout à fait exceptionnelle, être diminuée d’un facteur deux, la résistivité électrique obtenue est jusqu’alors trop élevée pour conduire à de bonnes performances en terme de facteur de mérite adimensionnel, par rapport à un matériau massif conventionnel de même composition / The study of thermoelectric nanostructured and nanocomposite materials is expanding because of the interest to multiply the number of interfaces and to decrease the size of the objects in order to improve the thermoelectric performance. We developed a new method to prepare thermoelectric n type (Bi0.95Sb0.05)2(Te0.95Se0.05)3 and p type (Bi0.2Sb0.8)2Te3 nanopowders, from the laser fracture in a liquid medium of powders of micrometric size. The developed cell preparation makes it possible to obtain per day approximately 200 mg of crystallized nanometric powders having the crystallographic structure of the initial powders and whose mean size lies between 7 and 12 nm. The mechanisms concerned in obtaining the nanoparticules were approached. They strongly depend on the density of energy of the laser beam. The nanopowders then were mechanically mixed with the micrometric powders of comparable nature and were cold pressed. The thermoelectric properties (electrical resistivity, thermoelectric power, thermal conductivity) of the nanocomposites were evaluated at room temperature. The first results show that even if the thermoelectric power is maintained in nanostructured and nanocomposite materials and that the total thermal conductivity can, in a completely exceptional way, being decreased by a factor two, the electrical resistivity obtained is hitherto too high to lead to high values of the dimensionless thermoelectric figure of merit, with regard to conventional bulk materials of same composition

Les préoccupations environnementales actuelles ont conduit à un regain d’intérêt pour la conversion d'énergie par effets thermoélectriques au cours de ces 20 dernières années. Le challenge lié à cette technologie consiste à découvrir des matériaux qui possèdent à la fois une faible conductivité thermique, une forte conductivité électrique et un fort pouvoir thermoélectrique. Les travaux présentés dans ce mémoire se sont orientés vers l'étude de phases cristallines complexes à base de Mo3Sb7. Contrôler finement les propriétés électriques et thermiques de ces matériaux par le biais de substitutions appropriées et relier les propriétés physiques aux propriétés structurales et électroniques ont été au coeur de ces travaux de recherche. Des résultats significatifs ont ainsi pu être obtenus tant au niveau de la synthèse et de la caractérisation physico-chimique qu'au niveau des propriétés magnétiques et de transport. En particulier, nous avons pu mettre en évidence les propriétés exotiques du composé Mo3Sb7 dont la compréhension s'est révélée indispensable pour l'étude des propriétés de transport des matériaux substitués ternaires et quaternaires. Les différentes possibilités de substitution ont alors permis d'améliorer de façon substantielle les performances thermoélectriques du composé Mo3Sb7 et ont, de ce fait, conduit à la découverte de nouveaux matériaux surpassant les meilleurs matériaux connus à ce jour (Si-Ge) et utilisés sur la gamme 900 - 1200 K dans des applications en génération d'électricité / Due to current environmental concerns, a resurgence of interest in thermoelectricity have been witnessed by the last 20 years. The challenge raised by this technology lies in identifying materials that display low thermal conductivity as well as both high electrical conductivity and thermopower. The work presented in this manuscript deals with a thorough study on molybdenum-antimony based complex crystalline structure. To finely control the thermal and electrical properties of these compounds through judicious substitutions and to link up physical and structural properties were at the heart of this in-depth study. Not only did we obtain outstanding results regarding the synthesis and both the chemical and structural characterizations but we also discovered intriguing magnetic and transport properties. Particularly, we emphasized the exotic properties exhibited by the binary Mo3Sb7 compound whose a deep understanding were essential to study the transport properties of the ternary and quaternary alloys. The different substitutions we have considered were found to substantially improve the thermoelectric properties of the Mo3Sb7 compound and thus, led to the synthesis of new prospective thermoelectric materials that surpass the best compounds discovered up-to-now (Si-Ge) and used in power generation applications in the 900 – 1200 K temperature range

O presente trabalho investiga o fenômeno termodifusivo em dispersões coloidais de nanopartículas magnéticas de óxidos de ferro em água (ferrofluidos), com a formação de dupla camada elétrica em torno das partículas. A estabilidade da partícula em solução é controlada pela concentração de íons. Ao estabelecer uma diferença de temperatura através da amostra líquida, ocorre o efeito de termodifusão (efeito Soret) das partículas e de íons em solução. Este efeito é o movimento das partículas para o lado frio ou quente do gradiente de temperatura. O acúmulo para um dos lados do gradiente de temperatura depende das características da solução. O efeito Soret de ferrofluidos em soluções ácidas e básicas é descrito a partir da determinação experimental das grandezas físicas envolvidas na difusão das partículas. O coeficiente Soret ST e o coeficiente de difusão são determinados em experimentos ópticos de lente de matéria, utilizando o aparato experimental de Varredura-Z, e de espalhamento Rayleigh forçado para termodifusão. Para investigar a resposta dos íons ao gradiente de temperatura, são realizadas medidas do potencial termoelétrico em uma célula termoelétrica, gerado a partir da difusão das cargas dispersas no líquido. O potencial superficial das partículas também é investigado experimentalmente, para descrever a interação das partículas com o campo termoelétrico. Os experimentos são realizados em função da temperatura da amostra e usados para descrever os resultados ST(T) das partículas, a partir de equações dos principais modelos teóricos. Os resultados mostram as diferenças e semelhanças do efeito Soret das nanopartículas em soluções ácidas e básicas, e que em ambos os casos a termodifusão de nanopartículas reflete o comportamento termodifusivo dos íons dispersos em solução. / This work investigates the thermal diffusion phenomena in colloidal dispersions of iron oxide magnetic nanoparticles dispersed in water (ferrofluid). The particles are stable in water due to electrical double layer around the particles, controlled by the ionic concentration. A temperature gradient throughout the ferrofluid sample causes the thermodiffusion (Soret effect) of dispersed particles and ions. This effect is the movement of particles to the cold or hot side of the temperature gradient. The particles migration for a given side depends on the characteristics of the sample. The Soret effect of ferrofluids in acidic and basic solutions is described by the experimental measurements of the physical parameters associated to particles diffusion. The Soret coefficient ST and the mass diffusion coefficient are measured in the matter lens experiment in the Z-scan experimental setup, and by the use of Thermal Diffusion Forced Rayleigh Scattering experiments. Concerning the ionic response to the temperature gradient the thermoelectric field generated by charges diffusion is measured in a thermoelectric cell. The surface potential of the particles is also measured to describe its interactions with the thermoelectric field. These experiments are made as function of the temperature of the sample and the results are applied to describe the ST(T) of particles by the use of equations from the main theoretical models. The results show differences and resemblances of the Soret effect in acidic and basic nanoparticles solutions. In both kind of solutions the thermodiffusion of nanoparticles is mainly ruled by the thermodiffusion of ions dispersed in solution.