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.
Highlights
Space nuclear power systems are presently under development as part of a European Space Agency (ESA) funded programme.1 These systems could potentially supply electrical and thermal energy derived from the radiogenic decay of sintered radioisotope oxide pellets
This study utilises impedance spectroscopy to characterise the effects of neutron irradiation on the performance of complete thermoelectric modules, as opposed to standalone material
The work reported in this paper demonstrates the use of the impedance spectroscopy technique to diagnose in-service degradation
Summary
Space nuclear power systems are presently under development as part of a European Space Agency (ESA) funded programme. These systems could potentially supply electrical and thermal energy derived from the radiogenic decay of sintered radioisotope oxide pellets. An increase in internal electrical resistance would have detrimental effects on modulelevel zT and desired match loading conditions, while a polarity reversal of individual thermoelements within a module would generate high resistance bridges which could render the device inoperative When evaluating both properties as a TE power factor (S2/ρ), an overall decrease is exhibited due to the relatively higher increase in electrical resistivity. Candidate Bi2Te3-based modules of different aspect ratios and microstructures were irradiated with a neutron fluence of 5 × 1013 neutrons cm-2 (>1 MeV), equivalent to the neutron yield expected from a 50 We Americium-241 powered RTG containing 10 kg of oxide fuel with a nominal mission time of 10-years.
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