Size effects in thermoelectric cobaltate heterostructures
Prof. dr. ing. A.J.H.M. Rijnders
Thermoelectric energy conversion is a promising method to convert (waste) heat into useful electrical energy. To improve the efficiency of this process, which is currently limited, materials with improved thermoelectric performance are required. The performance indicator for thermoelectric materials is determined by a combination of the Seebeck coefficient, the electrical conductivity and the thermal conductivity. Because of the different dependencies of these three parameters on the carrier concentration, improving the thermoelectric performance is a complex challenge.
In this thesis heterostructures of thermoelectric oxides are studied. Thin films and superlattices of NaxCoO2 and Ca3Co4O9 are deposited by pulsed laser deposition. The aim is to understand the relation between structural and thermoelectric properties properties in these heterostructures and to use this knowledge to engineer materials with enhanced thermoelectric performance.
To ensure that the thin films remain stable in air, an AlOx capping layer is developed. With this capping layer, the intrinsic properties of NaxCoO2 thin films can be measured for the first time. As a next step, possibilities to engineer the structural properties are investigated, aiming to improve the thermoelectric performance. It is demonstrated that by controlling the crystallographic orientation and grain size in these epitaxial thin films, a significant improvement of the thermoelectric properties can be obtained. Size effects in NaxCoO2 thin films are also studied. The effect of reducing the layer thickness between 5 and 250nm on the structural as well as thermoelectric properties is studied and it is demonstrated that the thermoelectric performance can be preserved for ultra-thin films. The high-temperature properties and stability of NaxCoO2 and Ca3Co4O9 thin films are also investigated. Thin films of Ca3Co4O9 are shown to have superior thermal stability compared to NaxCoO2 thin films and the high temperature thermoelectric properties of Ca3Co4O9 thin films are enhanced compared to literature values. Finally, a new approach is introduced, which combines NaxCoO2 and Ca3Co4O9 into superlattices. It is demonstrated that if a high degree of crystalline ordering can be achieved, the electronic properties are preserved. These preliminary results demonstrate that such superlattices form a new and promising approach to improve the thermoelectric properties of these oxides.