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Universitat de Barcelona. Facultat de Física

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[eng] In this thesis, it is detailed the bottom-up production and characterization of thermoelectric (TE) nanomaterials with significant enhanced performance by using colloidal nanocrystals (NCs) as building blocks. The production of TE nanomaterials with significant improved figure of merit (ZT), has to do, not only with the precise control of the NCs properties, but also with the further fine control over the crystallographic alignment of nanograins of highly anisotropic materials. The first part of the thesis correspond to the study of synthetic routes to produce high quality chalcogenide NCs that are doped during the NC synthesis, in order to control the charge carrier concentration. The system studied was I−V−VI chalcogenide semiconductor, specifically it was produced the materials: AgSbSe2 and Cu3SbSe4. A low-cost, high-yield and scalable synthesis route to produce monodisperse of AgSbSe2 and Cu3SbSe4 NCs was obtained. After ligand displacement, the NCs were used as building blocks to produce TE nanomaterials. Additionally, by means of substitutional doping, a large increment in the power factor and relatively lower thermal conductivities were observed. The optimization of the doping concentration resulted in ZT values of 1.10 at 640 K for AgSb0.98Bi0.02Se2, and of 1.26 at 673 K for Cu3Sb0.88Sn0.10Bi0.02Se4, which represents a significant increase beyond the state of the art in Te-free multinary Ag/Cu-based chalcogenide materials. In the second part of the thesis, the work about PbS-metal (Cu and Sn) nanocomposites produced by blending procedure is presented. The low work function metal is able to inject electrons to the intrinsic PbS matrix, which is another strategy to control the charge carrier concentration. The power factor is dramatically enhanced due to the increase of the electrical conductivity in the nanocomposites. Consequently, the ZTmax was remarkably enhanced by two times as compared with the pristine PbS. Furthermore, we also compared the TE performance of microcrystalline composites with the same composition as in nanocrystalline composites; commercial PbS host with Cu particles. The results revealed that with the same metal addition, higher electrical conductivities were obtained in the nanocomposite, but higher Seebeck coefficients were maintained in the microcomposite. Moreover, higher thermal conductivities were also obtained in the microcomposite. Finally, the figure of merit ZT were higher for the microcomposite system in the low temperature range, but much lower in the higher temperature range compared with the nanocomposites system. In the last block, the process of production of crystallographically textured materials is presented. We face here the challenge of bottom-up approaches to control the crystallographic alignment of nanograins. The production of nanostructured Bi2Te3-based alloys is presented. This can be done with controlled stoichiometry by solution-processing, and crystallographic texture by liquid-phase sintering using multiple pressure and release steps at 480 °C, above the tellurium melting point. Additionally, we explain the possible mechanism to produce the highly textured nanomaterials. This strategy results in record TE figures of merit: ZT=1.83 at 420 K for Bi0.5Sb2.5Te3 and ZT=1.31 for Bi2Te2.7Se0.3 at 440 K when averaged over 5 materials in the c direction, respectively. These high figures of merit extended over a wide temperature range, which results in energy conversion efficiencies a 50% higher than commercial ingots in the similar temperature range. In summary, different strategies to improve the TE performance of bulk nanostructured materials produced by bottom-up engineering of NCs, have been studied and confirmed in this thesis. Additionally, it has been proven that the solution-processed synthesis approach is low-cost, compatible with the scale-up engineering, and also versatile in tuning the size, shape, composition, and microstructure, among others parameters of different nanomaterials to optimize their TE properties.

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[spa] Los nanocristales (NCs) coloidales tienen excelentes propiedades para diferentes aplicaciones, como la conversión de energía, la catálisis, los dispositivos electrónicos y optoelectrónicos, entre otros. Así mismo, la síntesis coloidal de NCs tiene ventajas en el control del tamaño, forma y composición a nivel de la nanoescala; las bajas temperaturas de reacción; y la no necesidad de equipos especializados. Este proyecto se concentra en el diseño racional y la ingeniería de materiales termoeléctricos (TE) nanoestructurados de alta eficiencia, usando la estrategia del ensamblado ascendente (bottom-up) de NCs coloidales. Primero, se diseñó una ruta de síntesis de bajo costo, alto rendimiento, con la cual, se obtuvieron NCs de AgSbSe2 y Cu3SbSe4. La optimización de la concentración de dopaje resultó en valores para la figura de mérito TE, ZT, de 1.10 a 640 K para AgSb0.98Bi0.02Se2, y de 1.26 at 673 K para Cu3Sb0.88Sn0.10Bi0.02Se4. El material con mejores propiedades se usó para la producción de un generador TE en forma de anillo, para acoplarlo a los tubos de escape de gases, obteniendo una potencia eléctrica de 1mW por elemento TE con una diferencia de temperatura de 160 °C. En la segunda parte, se presenta el trabajo de la producción de nanocopuestos de PbS-metal (Cu y Sn) usando un procedimiento versátil de mezcla de NCs. La función de trabajo del metal es capaz de inyectar electrones a la matriz intrínseca de PbS. El factor de potencia TE, se ve dramáticamente incrementado debido al aumento en la conductividad eléctrica en los nanocompuestos TE. Consecuentemente, el valor máximo de ZT se vio excepcionalmente incrementado por el doble del valor comparado con el material original PbS. Finalmente, se presenta el proceso de producción de materiales texturizados cristalográficamente, produciendo materiales tipo p BixSb2-xTe3 y tipo n Bi2Te3-xSex. Se controló la estequiometria durante el procesamiento en solución y la textura cristalográfica, por medio de la sinterización en fase líquida con un procedimiento de múltiples pasos de presión y relajación a una temperatura de 480°C. Los valores de la figura de mérito TE presentan el record de: ZT=1.83 a 420 K para Bi0.5Sb2.5Te3 y ZT=1.31 para Bi2Te2.7Se0.3 a 440 K.