Improving Thermoelectric Properties of Inorganic Clathrates by Atomic and Microscale Structure Engineering

Abstract: As more countries are aiming for a carbon-neutral economy, technologies that utilize renewable energies are increasingly being considered. Thermoelectric materials enable the direct conversion between a thermal gradient and an electrical potential gradient, and are thus exploited for applications such as waste heat recovery. One of the prominent thermoelectric materials is the inorganic clathrate. Extensive research has been conducted over the past few decades to utilize its properties. However, several problems still remain ambiguous. In this thesis, we have studied the atomic and microscale structure of the clathrates and investigated its impact on the thermoelectric properties. Especially, with a combination of experiment and theoretical calculations, the existence of an order-disorder phase transition is confirmed. It further influences the electrical transport properties, since the band structure changes after the phase transition. The degree of chemical ordering can be controlled by the synthesis method, because materials reach different equilibrium states in either route. In addition, it is found that atomic vacancies can induce a peculiar transition effect in the electrical resistivity. In order to investigate its influence on the thermoelectric performance, a novel method is employed, where Ba8(AlxGa1–x)16Ge30 clathrates are synthesized by alloying Ba8Al16Ge30 with Ba8Ga16Ge30. This way a heterostructure is created, which contains the quaternary clathrate main phase and aggregates of Al particles. Consequently, the charge carrier mobility is largely improved to a value higher than that of the single crystal, while the lattice thermal conductivity is reduced due to the enhanced phonon scattering at different length scales. A greatly improved understanding of the process-structure-property relationship of clathrates is achieved in this thesis. The methodologies used, as well as the key findings, can be applicable for other material systems, and hence facilitate the future research in the thermoelectrics field.