Synthesis and Characterization of Composite Materials for Thermoelectric Power Generation
In a thermoelectric material, a temperature difference will give rise to an electric potential that is proportional to the temperature gradient. In a closed circuit, this potential can keep a current flowing as long as the temperature gradient is maintained. This is a type of power conversion that has received increasingly more attention in science over the past two decades because of the world-wide growing concern for the environment and the depletion of fossil fuel resources. With thermoelectric materials it is possible to build electric power generators that run on heat. The generators can be adapted to the temperature the heat is supplied at by engineering of the thermoelectric materials used in the generator. Thermoelectric generators are therefore interesting for applications in waste heat recovery.
A challenge with thermoelectric materials for power conversion is the relatively low conversion efficiency compared to their cost of fabrication. Since the mid 1990’s a number of new concepts have been introduced for the development of more efficient materials. Many of these are based on nanostructuring.
Thermoelectric materials are usually semiconductors with relatively high charge carrier concentrations. The main focus of this thesis has been to explore different synthesis methods for making nanostructured composite thermoelectric materials and to analyze the effect of the obtained structures on the thermoelectric properties.
Most of this thesis has been devoted to improving the clathrate Ba8Ga16Ge30 system by employing several approaches to engineering its nano- and microstructure. Different types of nanoparticles have been introduced to the clathrate matrix but also phase separation has been evaluated. It was seen that even very small amounts of nanoparticles can have dramatic effects on the thermoelectric properties. By adding TiO2 nanoparticles to Ba8Ga16Ge30 it was possible to enhance the material figure-of-merit, zT, through a reduction of the thermal conductivity as well as through a complex doping effect. For a mixed guest clathrate system with part substitution of Sr for Ba on the guest position in Ba8Ga16Ge30 it was possible to achieve a system that phase separates into Sr8Ga16Ge30 and Ba8Ga16Ge30 during cooling after heat treatment, resulting in a composite material. However, sintering of this material proved challenging and the final material had low thermoelectric performance due to high electrical resistivity and structural changes. The Mg2Si system was modified by employing different quenching procedures after heat treatment of Mg2Si0.94Sn0.06 resulting either in a solid solution or a material with a novel compositional microstructure. The microstructured material had lower thermal conductivity as well as enhanced Seebeck coefficient, resulting in improved zT.
Finally, a thermoelectric module has been constructed and evaluated. The module uses Ba8Ga16Ge30 as n-type material and La-doped Yb14MnSb11 as p-type material. The tested module performed well over the whole temperature range for which it was designed, from room temperature to 800°C. However, challenges were identified in achieving good electrical contacts as well as in protecting the sensitive Yb14MnSb11 compound from oxidization and sublimation.