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Les matériaux thermoélectriques (TE) offrent la possibilité de convertir directement un flux de chaleur en courant électrique pour recycler la chaleur perdue, par exemple par nos automobiles. Les skutterudites AyFe4-xCoxSb12, (A = Ce, Yb, …, 0 ≤ y < 1; x < 4) sont déjà de bons matériaux thermoélectriques dans le domaine de température 400–800K. Pour améliorer le coefficient Seebeck, des nano-inclusions de InSb ou GaSb (~50 nm) ont été générées à l’étape de frittage flash dans Ce0,3Fe1,5Co2,5Sb12 de type p. Elles n’ont pas eu l’effet escompté de filtrage en énergie des trous mais ont conduit à l’insertion de ~ 0,1 mol d’indium ou de gallium dans Ce0,3Fe1,5Co2,5Sb12 et à un facteur de mérite TE amélioré ZTmax = 0,7 (+ 20%) dans les deux cas . Pour réduire la conductivité thermique et améliorer leur performances TE, nous avons entrepris d’élaborer pour Co0,91Ni0,09Sb3 et Yb0,25Co4Sb12 de type n des microstructures à grains ultrafins (~ 100 nm) par broyage à haute énergie et frittage flash (SPS). Pour inhiber la croissance des grains lors du frittage, nous avons utilisé des additifs nanométriques (10 – 20nm), soit ajoutés ex-situ (CeO2, SiO2), soit générés in-situ (Yb, Yb2O3). Des facteurs de mérite TE ZTmax = 0,8 (+ 30%) et ZTmax = 1,4 ( + 10%) ont été obtenus respectivement pour Co0,91Ni0,09Sb3 et Yb0,25Co4Sb12 / The thermoelectric materials (TE) offer the possibility to convert a heat flow into an electric current for recycling heat wasted for example, by our automobiles. AyFe4-xCoxSb12 skutterudites, (A = Ce, Yb, …, 0 ≤ y < 1; x < 4) are already good thermoelectric materials in the 400 – 800 K temperature range. To improve the Seebeck coefficient, nano-inclusions of InSb or GaSb (~ 50 nm) were introduced during the spark plasma sintering step in p type Ce0.3Fe1.5Co2.5Sb12. They did not led to expected charge carriers energy filtering and but led to the insertion of ~ 0.1 mol of indium or gallium in Ce0.3Fe1.5Co2.5Sb12 and to figure of merit improved by 20 % (ZTmax = 0.7) in both cases. To reduce the thermal conductivity and improve their TE performance, we have developed for n type Co0.91Ni0.09Sb3 et Yb0.25Co4Sb12 an ultrafine grained microstructure (~ 100 nm) by high energy milling and spark plasma sintering (SPS). To inhibit grain growth during sintering, we used nanoscale additives (10 – 20nm) either added ex-situ (CeO2, SiO2) or precipitated in-situ (Yb, Yb2O3). The figure of merit ZTmax = 0,8 (+ 30%) et ZTmax = 1,4 ( + 10%) were thus obtained respectively in Co0,91Ni0,09Sb3 and Yb0,25Co4Sb12

An increasing need for effective thermal sensors, together with dwindling energy resources, have created renewed interests in thermoelectric (TE), or solid-state, energy conversion and refrigeration using semiconductor-based nanostructures. Effective control of electron and phonon transport due to confinement, interface, and quantum effects has made nanostructures a good way to achieve more efficient thermoelectric energy conversion. This thesis studies the two well-known approaches: confinement and energy filtering, and implements improvements to achieve higher thermoelectric performance. The effect of confinement is evaluated using a 2D material with a gate and utilizing the features in the density of states. In addition to that, a novel controlled scattering approach is taken to enhance the device thermoelectric properties. The shift in the onset of scattering due to controlled scattering with respect to sharp features in the density of states creates a window shape for transport integral. Along with the controlled scattering, an effective utilization of Fermi window can provide a considerable enhancement in thermoelectric performance. The conclusion from the results helps in selection of materials to achieve such enhanced thermoelectric performance. In addition to that, the electron filtering approach is studied using the Wigner approach for treating the carrier-potential interactions, coupled with Boltzmann transport equation which is solved using Rode's iterative method, especially in periodic potential structures. This study shows the effect of rapid potential variations in materials as seen in superlattices and the parameters that have significant contribution towards the thermoelectric performance. Parameters such as period length, height and smoothness of such periodic potentials are studied and their effect on thermoelectric performance is discussed. A combination of the above two methods can help in understanding the effect of confinement and key requirements in designing a nanostructured thermoelectric device that has a enhanced performance.

Organic semiconductors have wide applications in organic-based light-emitting diodes, field-effect transistors, and thermoelectrics due to the easily modified electrical and optical properties, excellent mechanical flexibility, and solution processability. To fabricate high performance devices, it is important to understand charge transport mechanisms, which are mainly affected by material energetics and material morphology. Currently it is difficult to control the charge transport properties of new organic semiconductors and organic-inorganic nanocomposites due to our incomplete understanding of the large number of influential variables. Molecular doping of π-conjugated polymers and surface modification of nanowires are two means through which charge transport can be manipulated. In molecular doping, both the energetics and microstructures of polymer films can be changed by controlling the degree of oxidation of the conjugated polymer backbone. For surface modification of inorganic nanowires, the energetics and morphology can be influenced by the properties of the surface modifiers. Meanwhile, the energy band alignment, which can be controlled by surface modification and molecular doping, may also alter the charge transport due to the variation in energetic barriers between the transport states in the organic and inorganic components. To reveal the effects of morphology and energetics on charge transport in conjugated polymers and organic-inorganic nanocomposites, the influence of surface modifier on the electrical and morphological properties of nanocomposites was first probed. Silver nanowires modified with different thiols were blended with poly (3,4-ethylenedioxythiophene)-poly(styrenesulfonate)(PEDOT:PSS) to fabricate thin films. The modified nanowires provided a means of controllably altering the nanowire dispersability and compatibility with solvents and polymers. The results also demonstrated that charge transport between the nanowires was facilitated due to low wire-to-wire junction resistance. To further figure out the charge transport mechanism in organic-inorganic nanocomposites and the potential applications, tellurium nanowires and ferric chloride doped poly (3-hexylthiophene-2,5-diyl)(P3HT) were used to characterize energy band alignment effects on charge transport, electrical conductivity, and thermoelectric properties. The results showed that charge transfer between nanowires can be mediated by the polymer and may potentially increase the electrical conductivity as compared to the pure polymer or pure nanowires; while the observed enhancement of power factor (equal to electrical conductivity times the square of Seebeck coefficient) may not be affected by the energy band alignment. It is important to investigate the change of polymer morphology caused by molecular doping and processing method to determine how the morphology will influence the electrical and thermoelectric properties. Various p-type dopants, including ferric chloride and molybdenum tris(1,2-bis(trifluoromethyl)ethane-1,2-dithiolene) (Motfd3), were examined for us in P3HT and other polymers. The results showed that: i) At light doping levels, the electrical conductivity and power factor of polymers doped with the large electron affinity (EA) dopants were larger than small EA dopants; ii) At heavy doping levels, the large size dopants cannot effectively dope polymers even for the dopants with large EAs; iii) For the same dopant, as the IE of the polymer increased, the doping efficiency gradually decreased.

L'objectif de cette thèse est l'étude des échanges et de la dissipation d'énergie aux échelles mésoscopiques, à travers l'étude de nanotubes, de nanofils ou de pointes taillées par exemple. Notre intérêt pour la dissipation d'énergie nous portera vers les NEMS (Nano Electro Mechanical Systems) et leur facteur de qualité. Pour étudier les échanges d'énergie nous nous intéresserons à la thermodynamique aux petites échelles et notamment aux machines thermiques qui exploitent ces échanges d'énergie pour extraire un travail utile (mécanique, électrique...). Ce travail se concentre dans un premier temps sur la dissipation d'énergie et plus particulièrement sur le facteur de qualité de nanotubes de carbone mono-paroi à température ambiante et sur la façon de l'augmenter par application d'une tension électrique. Cette tension électrique induit un fort tirage sur le nanotube et la modification concomitante de la forme du mode résonant modifie la dissipation d’énergie. Ce phénomène, couplé à une modification des propriétés de l’ancrage (effet d’ancrage mou ajustable en tension) résultant également de la tension, diminue drastiquement la dissipation d’énergie et on atteint alors des facteurs de qualité record. Dans un second temps, nous nous intéressons aux machines thermiques : une machine stochastique cyclique et une machine électrique continue. La machine thermique stochastique est réalisée avec un nanofil vibrant sous ultra haut vide. La thermodynamique stochastique permet de redéfinir le travail et la chaleur pour un objet qui stocke des quantités d’énergies similaires aux fluctuations du bain thermique avec lequel il est en contact. Le premier objectif est de réaliser un cycle de Carnot permettant d'atteindre le rendement du même nom, inaccessible pour les machines macroscopiques. Pour la machine thermique continue nous étudions numériquement un prototype de machine thermique électrique basé sur des effets de résonance d'effet tunnel qui pourrait être une amélioration du principe des machines thermoïoniques. L’utilisation de l’effet tunnel permet à priori de réduire la température de la source chaude de la machine puisque l’on a plus besoin de vaincre le travail de sortie des deux électrodes. Les résonances dans l’effet tunnel, obtenues par confinement dans une dimension, permettent un filtrage en énergie des électrons passant d’un réservoir thermique à l’autre, ce qui a pour effet d’améliorer le rendement de la machine / The purpose of this work is the study of energy transfer and dissipation at the mesoscopic scale, through the study of nanotubes, nanowires, or sharp tips for example. Our interest for energy dissipation will lead us to dive into Nano Electro Mechanical Systems (NEMS) and their quality factor. Energy transfers will be studied with small scale thermodynamics and stochastic heat engines which use those energy transfers to produce useful work (mechanical, electrical…). This work is focused in a first time on the energy dissipation and particularly on the quality factor of single wall carbon nanotubes at room temperature and the ways to improve it by applying an electrical voltage. This voltage induces a strong pulling on the nanotube and the resulting vibrating shape modification changes the dissipation. This phenomenon, coupled with a clamping modification (tunable soft clamping) also stemming from the voltage, drastically reduces the dissipation. We can then achieve record high quality factors. In a second time we take interest in heat engines: a stochastic cyclic heat engine and a continuous electrical heat engine. The stochastic heat engine is realized with a vibrating nanowire under high vacuum. The stochastic thermodynamics allow us to redefine work and heat for an object that stores energies of the order of magnitude of thermal fluctuations in the thermal bath it interacts with. The aim is to build a Carnot cycle and achieve the corresponding yield, out of reach for macroscopic engines. Concerning the continuous heat engine we study numerically a prototype for an electrical heat engine based on resonant tunneling which could be an improvement of the thermionic heat engines. Allowing the thermal reservoirs to exchange electrons through tunneling allows in principle to reduce the temperature of the hot source because overcoming the work function of both electrodes is not necessary anymore. The resonances in the tunnel effect, obtained through confinement of one dimension, is useful for filtering the energy of the electrons tunneling from one reservoir to another, thus increasing the yield of the heat engine