Impedance Spectroscopy Analysis Of Thermoelectric Materials For Radioisotope Space Power System Applications

The European Space Agency (ESA) is currently supporting the research and development of a radioisotope thermoelectric generator (RTG) utilising americium-241 as a heat source and thermoelectric modules for thermal to electrical energy conversion. Initial design studies and a successful laboratory breadboard experimental campaign have demonstrated that bismuth telluride-based thermoelectric modules are a viable power conversion option with proven commercial manufacturing routes. However, although usable thermoelectric materials like bismuth telluride-based alloys have good heritage, their implementation into robust modules still to this day requires solving coupled challenges in both material development and systems engineering. Not to mention, for an RTG to continuously and reliably provide electrical power to spacecraft and or landers, their thermoelectric modules must be able to withstand mechanical trauma arising from pyroshock or random vibration associated with launch, stage separation, payload jettison and landing environments, as well as continuous thermal, thermomechanical and irradiation induced degradation. A proven understanding of their mechanical and thermoelectric properties in relation to the degradation mechanisms associated with operating in an RTG environment, is therefore highly necessary for mission acceptance. To be able to effectively address these challenges, advanced characterisation techniques are currently needed which rely on more factors than just traditional thermoelectric material performance metrics like the figure-of-merit.

The work presented in this thesis details an advance implementation of impedance spectroscopy for characterising not only fundamental material-level properties of the constituent materials which make up a thermoelectric module, but also system-level performance and characteristics which correlate with manufacturing defects and in-service degradation mechanisms. Subsequently, three practical case studies were undertaken in this study which for the first time succefully demonstrates the feasibility of using impedance spectroscopy to characterise; the degradation of thermoelectric modules under in-service conditions like neutron irradiation, the inherent performance variation between identically manufactured modules from batch production, and the system-level performance of a newly synthesised thermoelectric material. For each case, candidate Bi2Te3-based thermoelectric modules for the European RTG program was investigated.