Improvement of thermoelectric properties in nanostructures with constrictions

Abstract

The results of a study into the development of novel Thermoelectric (TE) materials by engineering nanoscale constrictions are presented in this thesis. The aim of this investigation lies in the development of an enhanced TE material. To achieve this, the dependence of TE properties, specifically the TE figure of merit (𝑍𝑇) on the material properties has been considered. The enhancement of the 𝑍𝑇 was achieved by reducing the thermal conductivity (𝑘) of the material. During this phase, the effects of different nanoscale modifications to the material structure on its electrical properties are contemplated to ensure that the TE 𝑍𝑇 does not get vitiated. Here, a novel nanostructure formed by the sintering of individual Silicon nanoparticles in a linear fashion has been used and is referred to as a Nanoparticle Chain (NPC) structure. The nanoparticle arrangement in an NPC structure causes nanoscale constrictions to be formed along the transport direction of the structure. This is seen to cause extremely low lattice 𝑘 (reaching 0.614 W/mK) while preserving a considerable amount of crystallinity. The fabrication procedure of the NPC structure has also been considered through this study thereby ensuring that results can be translated to real-world applications using existing technologies. During the investigation, an interesting competing effect between two, phonon transport aspects has been observed to cause a nonmonotonic trend in the 𝑘 of the structure, while a variation in the phonon density of states along the transport direction was identified to cause a 𝑘 reduction to values lower than those attained with comparably sized nanowires. Further variations of the structure are obtained by expanding the zero-dimensional constriction of NPC structures to a one-dimensional form referred to as Nanowire Chain (NWC) structures. Subsequently, the electrical properties of the structures in consideration are evaluated, and a three-order of magnitude enhancement in the TE ZT is observed in comparison to the bulk material. Thus, it is shown that nanoscale constrictions can be engineered to enhance the TE performance of materials. Keywords: Nanoparticles, Thermoelectric, Phonon transport, Electron transport, Ab-initio modelling, Constriction engineering