Impedance Spectroscopy: A Tool for Assessing Thermoelectric Modules for Radioisotope Power Systems

Thermoelectric energy convertors in the form of solid state modules are utilised in space nuclear power systems such as a radioisotope thermoelectric generator (RTG). However, to ensure that implemented thermoelectric modules are reliable, efficient, and capable of delivering power and energy over a required lifespan, Standardised, accurate and repeatable high-throughput measurement systems are needed. Recently, Impedance spectroscopy has shown promise as a tool to parametrically characterise thermoelectric modules with one simple measurement, showcasing itself as a potentially key enabling technology. This paper investigates the use of impedance spectroscopy as a measurement system for assessing the health state of practical thermoelectric modules from batch production runs. Particularly for the case of candidate thermoelectric modules which are required to operate for long durations within an RTG system. The complex impedance response of non-ideal modules, in the form of Nyquist spectra, is reported. The characteristic high frequency response is here linked with a new parametric model which considered the influences of the copper contacts as well as the thermal contact resistance between the semiconductor-copper region. From these results it was found that the thermally capacitive nature of the copper contacts, as well as an unavoidable thermal contact resistance, was highly necessary for an accurate fitting of the mid-high frequency response of Nyquist spectra.