Impedance spectroscopy characterization of neutron irradiated thermoelectric modules for space nuclear power

Abstract

The European Space Agency is currently supporting the research and development of advanced radioisotope power systems utilising thermoelectric modules. The performance of thermoelectric modules following exposure to neutron radiation is of significant interest due to the likely application of radioisotope thermoelectric generators in deep space exploration or planetary landers requiring prolonged periods of operation. This study utilises impedance spectroscopy to characterise the effects of neutron irradiation on the performance of complete thermoelectric modules, as opposed to standalone material. For a 50 We americium-241 radioisotope thermoelectric generator design, it is estimated that the TE modules could be exposed to a total integrated flux of approximately 5 × 1013 neutrons cm-2 (>1 MeV). In this study, an equivalent neutron dose was simulated experimentally via an acute 2-hour exposure in a research pool reactor. Bi2Te3-based thermoelectric modules with different leg aspect ratios and microstructures were investigated. Gamma-ray spectroscopy was initially used to identify activated radionuclides and hence quantify irradiation induced transmutation doping. To evaluate the thermoelectric properties pre- and post-irradiation, impedance spectroscopy characterization was employed. Isochronal thermal annealing of defects imparted by the irradiation process, revealed that polycrystalline based modules required significantly higher temperature than those with a monolithic microstructure. Whilst this may indicate a greater susceptibility to neutron irradiation, all tested modules demonstrated sufficient radiation hardness for use within an americium-241 radioisotope thermoelectric generator. Furthermore, the work reported demonstrates that impedance spectroscopy is a highly capably diagnostic tool for characterising the in-service degradation of complete thermoelectric devices.

Thermoelectric materials for space explorations

“…Mesalam et al have demonstrated, through 10 000 hours of vacuum testing, that RTG systems incorporating these Bi 2 Te 3 -based modules are relatively unaffected by changes over time. 22,23 The test results indicate an annual efficiency degradation of less than 1% for these TE converters, capable of delivering a 10 W electrical power output. Future development efforts are focused on achieving a 50 W power output using the same design principles.…”

“…7 Recent advancements by the ESA have led to the development of Bi 2 Te 3 -based TE modules, specifically designed for deep-space probes using 241 Am as the fuel source. These modules operate at lower hot side temperatures than those powered by 238 Pu, 8,9,22 enabling the use of Bi 2 Te 3 -based alloys as TE materials. Traditionally, Bi 2 Te 3 alloys have seen limited use in RTGs due to their low operating temperature range of 300-600 K. However, their integration with 241 Am heat sources is highly promising.…”

“…Various trends in zT after irradiation in the literature are attributed to the effect of irradiation on the TE material's atomic structure, leading to the formation of vacancies and interstitials, thereby impacting carrier concentration, s and S values. 22,122 3.5. Skutterudites (CoSb 3 ) Skutterudite, a CoSb 3 -based alloy, crystallizes into a cubic crystal structure characterized by octahedrons within the Im% 3 space group, as illustrated in Fig.…”

“…Various trends in zT after irradiation in the literature are attributed to the effect of irradiation on the TE material's atomic structure, leading to the formation of vacancies and interstitials, thereby impacting carrier concentration, σ and S values. 22,122…”