Thermal transport in van der Waals Solids and Inorganic Clathrates from first-principles calculations

Doctoral thesis, 2018

Energy management is arguably one of the defining challenges for our modern societies. An ever-increasing demand for energy has to be balanced with the requirement for a sustainable energy economy that minimizes the human impact on the environment. Materials and their ability to transport both electrical and thermal currents play a key role in this area as they are essential components in energy extraction, transport, storage, and consumption technologies. On the macroscopic level, electrical and thermal transport in materials can be described by a set of coupled phenomenological relations that contain material specific transport coefficients. On the microscopic level, these transport coefficients are governed by chemical composition and the specific arrangement of the constituent atoms, the so-called microstructure. Since relatively small differences in this regard can have a dramatic impact on the macroscopic behavior of a material, a detailed understanding of the underlying processes and couplings is essential for materials development and optimization.

In this thesis, the thermal conductivity in two classes of materials of current and future technological importance has been investigated using electronic structure calculations (density functional theory) in combination with methods from statistical physics (Boltzmann transport theory). The first two papers included in this thesis deal with van-der-Waals solids, layered materials that are currently attracting tremendous attention in the scientific community due to their exciting combination of electrical, optical, and thermal properties. In this context, the present thesis provides predictions and a detailed analysis of the lattice thermal conductivity in Mo and W-based transition metal dichalcogenides. Furthermore, a model is developed to explain the extreme structure sensitivity of the conductivity and calculations are presented that elucidate chemical trends and establish bounds.

The third and fourth paper deal with clathrates, inclusion compounds that have been found to exhibit a combination of transport properties that are very well suited for thermoelectric applications. These materials exhibit extremely small thermal conductivities. The present thesis provides an in-depth analysis of the lattice dynamics of these materials, with a special focus on the thermal conductivity and the so-called phonon-glass behavior.

The present thesis provides a stepping stone for future investigations of transport processes in van-der-Waals solids and clathrates, which eventually should lead to the development of devices with higher energy efficiency and better materials for energy extraction technologies.