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Universitat de Barcelona. Facultat de Física

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[eng] TE devices have countless potential application, but their high manufacturing and material costs hamper their cost-effectiveness and limit their widespread implementation. To overcome these drawbacks, TE devices must be manufactured employing high throughput technologies and using materials based on low cost and abundant elements. This thesis focuses on the development of scalable and low consumption methods for the production of TE nanomaterials with optimized performance. In these directions, the general goals of this thesis is to develop cost-effective TE devices with improved TE conversion efficiency and low production cost. The main research objectives are: a) Exploring earth abundant, low-cost and less toxic materials with high TE performance; b) Developing a simple, scalable and cost-effective methodologies to produce TE materials and modules; c) Optimize the power factor (PF) and figure of merit (ZT) through optimized either via doping or defect engineering. d) Reduce the lattice thermal conductivity (κL) through the introduction of phonon scattering sources such as point defects, dislocations and interfaces. The thesis is divided into 5 chapters. Chapter 1 introduces the research status and development prospects of TE materials. Chapter 2 reports p-type polycrystalline SnSe materials with marked crystallographic texture were produced from blends of SnSe NCs and Te NRs through hot-pressing them at a temperature above the Te melting point. The presence of Te promoted the material crystallization in the form of a laminar structure with the a-axis along the pressure direction. SnSe nanomaterials consolidated in the presence of Te showed higher electrical conductivities and lower Seebeck coefficients and thermal conductivities than pure SnSe. The TE figures of merit up to ZT ̴̴ 1.4 at 790 K were measured from the SnSe nanomaterial consolidated in the presence of Te and measured along the pressing direction. Chapter 3 presents a synthetic protocol for large scale production of covellite CuS nanoparticles at ambient temperature and atmosphere, and using water as a solvent. The crystal phase and stoichiometry of the particles are afterward tuned through an annealing process at a moderate temperature under inert or reducing atmosphere. By optimizing the charge carrier concentration through the annealing time, Cu2-xS with record figures of merit in the middle temperature range, up to 1.41 at 710 K, is obtained. We finally demonstrate that this strategy, based on a low-cost and scalable solution synthesis process, is also suitable for the production of high performance Cu2-xS layers using high throughput and cost-effective printing technologies The Chapter 4 report a large scale method to synthesize N-type Bi2Se3 nanosheets. And further doped with Sn which present notable TE performance. Incorporating Sn in Bi2Se3 increases the Seebeck coefficient and thus improves the power factor to 0.65 mWm-1K-2. The low thermal conductivity, down to 0.91 Wm-1K-1, of Bi1.93Sn0.07Se3 results in ZT values well above higher than those of pure Bi2Se3. The Chapter 5 report that semiconductor-metal composites are produced by just blending nanoparticles of a sulfide semiconductor obtained in aqueous solution and at room temperature, with a metallic Cu powder. During the annealing process, the presence of metallic copper activates a partial reduction of the PbS, resulting in the formation of PbS-Pb-CuxS composites, the introduction of metallic copper in the initial blend results in a significant improvement of the TE performance of PbS, reaching a dimensionless TE figure of merit ZT = 1.1 at 750 K, which represents about a 400% increase over bare PbS. Besides, an average ZTave = 0.72 in the temperature range 320-773 K is demonstrated. Finally, the main conclusions of this thesis and some perspectives for future work are presented.

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[spa] Esta tesis consta de cinco capítulos. El primer capítulo presenta principalmente los conceptos básicos de termoelectricidad (TE) y el estado actual de desarrollo y los desafíos en el campo de la TE. Los capítulos 2 a 5 presentan principalmente la parte experimental. En el marco general de la optimización de materiales TE a través de la ingeniería bottom-up, el trabajo presentado en esta tesis tiene como objetivo resolver los principales desafíos en este campo: 1) Producción de nanomateriales de textura cristalográfica. 2) Ajuste la fase cristalina controlando la atmósfera del tratamiento térmico para optimizar la conductividad eléctrica y la conductividad térmica. 3) Aumentar la concentración de portadores de nanomateriales TE mediante el dopado de partículas metálicas. 4) Incrementar el coeficiente de Seebeck de los nanomateriales aumentando la banda prohibida con el dopaje. En el capítulo dos, se produjeron materiales de SnSe policristalinos de tipo p con una textura cristalográfica marcada a partir de mezclas de nanocristales de SnSe (NCs) y nanorods de Te (NRs) mediante prensado en caliente a una temperatura por encima del punto de fusión del Te. La presencia de Te promovió la cristalización del material en forma de estructura laminar con el eje a a lo largo de la dirección de la presión. Los nanomateriales de SnSe consolidados en presencia de Te mostraron conductividades eléctricas más altas y coeficientes de Seebeck y conductividades térmicas más bajas que el SnSe puro. En el capítulo tres, presento un protocolo sintético para la producción potencial a gran escala de nanopartículas de covellite CuS a temperatura ambiente y atmósfera. La atmósfera, la temperatura y el tiempo del tratamiento térmico permiten ajustar la densidad de las vacantes de cobre y, por lo tanto, ajustar la concentración del portador de carga y las propiedades de transporte del material. En el capítulo cuatro, presento los nanomateriales Bi2Se3 con propiedades TE mejoradas al doparse con Sn. En el capítulo cinco, presento una ruta versátil, escalable, a temperatura ambiente y libre de tensioactivos para la síntesis de nanopartículas de PbS en solución acuosa, y además caracterizo las propiedades termoeléctricas del PbS tipo n obtenido y demuestro que la incorporación de Cu puede mejorar el rendimiento del material.