Towards high performance nanostructured thermoelectric materials. A bottom-up approach

Abstract

[spa] En esta tesis se usa la estrategia de “Bottom-up” para la producción de materiales termoeléctricos nano-estructurados con alta eficiencia termoeléctrica. Esta técnica proporciona un amplio rango de posibilidades para aumentar la eficiencia de los materiales termoeléctricos. Permite aumentar los valores de la figura de mérito ZT (parámetro que mide la eficiencia del material termoeléctrico), gracias al fino control del tamaño, la forma y la composición de los nanocristales (NCs) que se realiza en la síntesis en solución, donde los NCs se convierten en los componentes básicos del material nanoestructurado. Además la síntesis en solución permite combinar y/o sintetizar los NCs para obtener diferente tipo de nano-heteroestructuras con mejores propiedades electrónicas. En esta tesis se optimiza la síntesis coloidal para producir NCs con el tamaño, forma y composición deseada y a escala del gramo. Los materiales seleccionados fueron calcogenuros de Plata, Plomo, Bismuto y Cobre, debido a sus propiedades intrínsecas útiles para conseguir materiales termoeléctricos eficientes. EstÁ dividida en tres partes, la primera se relaciona con el desarrollo de nanocompositos o heteroestructuras, usando dos tipos de NCs coloidales. En esta, se demuestra la eficacia de este tipo de meta-materiales para incrementar el bloqueo fonónico, debido a la alta densidad de interfaces en los nano-granos y la diferencia de estructura cristalográfica que se obtiene al usar dos tipos diferentes de nanocristales. En la segunda parte se investiga el efecto de los ligandos orgánicos que quedan atados a los nanocristales, en la formación de materiales nanoestructurados en bloque o nanocompositos. Se propone un proceso simple, general y escalable, para realizar el intercambio de ligandos, con el cual se obtiene un gran aumento de la figura de Mérito (ZT) del material y además se abre la posibilidad de cambiar la concentración de portadores en el material. En la parte final se aborda el estudia la consolidación de los materiales nanoestructurados, mediante la utilización de la técnica de “spark plasma sintering” (SPS), para obtener nanomateriales altamente densos. Se demuestra la importancia de obtener un material altamente denso para producir materiales con altas eficiencias termoeléctricas. Estos resultados muestran que la producción de materiales nanoestructurados usando como unidades fundamentales los NCs obtenidos en solución, (estrategia del “Bottom-up”) es un método muy eficaz que permite obtener materiales altamente eficientes para usarlos en dispositivos termoeléctricos.

[eng] This thesis aims to develop high efficiency thermoelectric nanostructured materials from the bottom-up assembly of NCs. To achieve this, I have developed different approaches with the purpose to enhance the transport properties of the materials and to overcome the limitations of solution processed NCs. First of all, I optimized colloidal synthesis routes to produce NCs with the desired size, shape and composition at the gram scale. The materials I used were silver, lead, bismuth and copper chalcogenides, due to their intrinsic useful properties to obtain efficient thermoelectric materials. This thesis is divided in three parts. Firstly, I explore the production of binary nano-heterostructured materials; the modular design of multicomponent solids allows enhancing the thermoelectric efficiency of current materials. The combination of different stoichiometry and crystallographic structure provides effective phonon blocking. PbTe and Ag2Te colloidal NCs were assembled into Ag2Te-PbTe nanocomposites with homogeneous phase distributions and adjustable composition. The evolution of their electrical conductivity and Seebeck coefficient is discussed in terms of the blend composition and the characteristics of the constituent materials. Undoped (Ag2Te)0.75(PbTe)0.25 nanocomposites displayed best power factor (PF=S2σ) among the different nanocomposites tested and reached ZT values up to 0.38 at 670 K. Since the presence of organic ligands (OL) on the surface of colloidal NCs strongly limits their performance in technological applications, where charge carrier transfer/transport plays an important role, I developed a strategy to replace the OL from the NCs surface. The first strategy was to use metal salts, matched with the NCs composition to eliminate the surface OL without introducing extrinsic impurities in the final nanomaterial. The potential of this simple, general and scalable process were demonstrated by characterizing the thermoelectric properties of nanostructured bulk Ag2Te produced by the bottom up assembly of Ag2Te NCs. A 6-fold increase of the Ag2Te thermoelectric figure of merit was obtained when displacing organic ligands by AgNO3. In a second approach, I used sodium salts to carry out the OL replacement, with PbSe NCs. I tested salts including sodium azide, sodium nitrate and sodium amide with the aim of tuning the carrier concentration of the NCs. The electrical conductivity of the bulk nanocrystalline material, treated with sodium amide, increased more than one order of magnitude, and the resultant figure of merit at 600 K was 0.6. Additionally I discussed the effect that the scattering at the grain interfaces has in electronic transport using a model that takes into account the energy barrier at the NCs boundaries. The results of this work have been submitted for publication. Another challenge in the preparation of bulk nanostructured materials is to obtain a dense solid. The process of consolidation of the NCs into a dense solid is crucial to obtain a bulk nanomaterial with high thermoelectric properties, because the porosity strongly affects the transport properties. In order to obtain the dense solids, I started using cold press (CP) technique. Later, I was able to use hot press (HP) technique, and carried out the adjustment of the parameters like temperature and time, in order to avoid a highly increasing of the grain size. Another technique that has shown outstanding results in the thermoelectric fields is spark plasma sintering (SPS). Thus I analyzed the transport properties of the nanomaterials obtained by the consolidation of a colloids NCs-based powder into dense pellets by SPS. Using Cu2SnSe3 NCs, obtained by a novel and high yield colloidal synthesis route. These results show the bottom-up production of nanocrystalline materials from solution-processed NCs to be a potentially advantageous alternative to conventional methods in the production of efficient thermoelectric materials. At the same time the progress achieved here allows to overcome some of the main difficulties in the production of bulk nanomaterials with high thermoelectric efficiency from the bottom-up assembly of colloidal NCs.