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.</div

Towards a Comprehensive Model for Characterising and Assessing Thermoelectric Modules by Impedance Spectroscopy

Thermoelectric devices have potential energy conversion applications ranging from space exploration through to mass-market products. Standardised, accurate and repeatable high-throughput measurement of their properties is a key enabling technology. Impedance spectroscopy has shown promise as a tool to parametrically characterise thermoelectric modules with one simple measurement. However, previously published models which attempt to characterise fundamental properties of a thermoelectric module have been found to rely on heavily simplified assumptions, leaving its validity in question. In this paper a new comprehensive impedance model is mathematically developed. The new model integrates all relevant transport phenomena: thermal convection, radiation, and spreading-constriction at junction interfaces. Additionally, non-adiabatic internal surface boundary conditions are introduced for the first time. These phenomena were found to significantly alter the low and high frequency response of Nyquist spectra, showing their necessity for accurate characterisation. To validate the model, impedance spectra of a commercial thermoelectric module was experimentally measured using a new and parametrically fitted. Technique precision is investigated using a Monte-Carlo residual resampling approach. A complete characterisation of all key thermoelectric properties as a function of temperature is validated with material property data provided by the module manufacturer. Additionally, by firstly characterising the module in vacuum, the ability to characterise a heat transfer coefficient for free and forced convection is demonstrated. The model developed in this study is therefore a critical enabler to potentially allow impedance spectroscopy to characterise and monitor manufacturing and operational defects in practical thermoelectric modules across multiple sectors, as well as promote new sensor technologies

GAKTpore: Stereological Characterisation Methods for Porous Foams in Biomedical Applications

In tissue engineering, scaffolds are a key component that possess a highly elaborate pore structure. Careful characterisation of such porous structures enables the prediction of a variety of large-scale biological responses. In this work, a rapid, efficient, and accurate methodology for 2D bulk porous structure analysis is proposed. The algorithm, “GAKTpore”, creates a morphology map allowing quantification and visualisation of spatial feature variation. The software achieves 99.6% and 99.1% mean accuracy for pore diameter and shape factor identification, respectively. There are two main algorithm novelties within this work: (1) feature-dependant homogeneity map; (2) a new waviness function providing insights into the convexity/concavity of pores, important for understanding the influence on cell adhesion and proliferation. The algorithm is applied to foam structures, providing a full characterisation of a 10 mm diameter SEM micrograph (14,784 × 14,915 px) with 190,249 pores in ~9 min and has elucidated new insights into collagen scaffold formation by relating microstructural formation to the bulk formation environment. This novel porosity characterisation algorithm demonstrates its versatility, where accuracy, repeatability, and time are paramount. Thus, GAKTpore offers enormous potential to optimise and enhance scaffolds within tissue engineering

GAKTpore: Stereological Characterisation Methods for Porous Metal Foams in Biomedical Applications- Data

In this work, a rapid, efficient, and accurate methodology for 2D bulk porous structure analysis and pore morphology classification is proposed. The algorithm, “GAKTpore”, creates a morphology map allowing visualisation of spatial feature variation, which enables optimisation of pore sizes/shapes/ranges/dispersion within any porous structure. The micrographs used in this study with their data outputted from the GAKTpore algorithm and below. Micrographs are stored as .tiff files and data is outputted and saved as a .CSV file

Am-241 Powered Dynamic Radioisotope Power System (DRPS) for Long Duration Lunar Rovers

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Thermal Properties and Behaviour of Am-Bearing Fuel in European Space Radioisotope Power Systems

The European Space Agency is funding the research and development of 241Am-bearing oxide-fuelled radioisotope power systems (RPSs) including radioisotope thermoelectric generators (RTGs) and European Large Heat Sources (ELHSs). The RPSs’ requirements include that the fuel’s maximum temperature, Tmax, must remain below its melting temperature. The current prospected fuel is (Am0.80U0.12Np0.06Pu0.02)O1.8. The fuel’s experimental heat capacity, Cp, is determined between 20 K and 1786 K based on direct low temperature heat capacity measurements and high temperature drop calorimetry measurements. The recommended high temperature equation is Cp(T/K) = 55.1189 + 3.46216 × 102 T − 4.58312 × 105 T−2 (valid up to 1786 K). The RTG/ELHS Tmax is estimated as a function of the fuel thermal conductivity, k, and the clad’s inner surface temperature, Ti cl, using a new analytical thermal model. Estimated bounds, based on conduction-only and radiation-only conditions between the fuel and clad, are established. Estimates for k (80–100% T.D.) are made using Cp, and estimates of thermal diffusivity and thermal expansion estimates of americium/uranium oxides. The lowest melting temperature of americium/uranium oxides is assumed. The lowest k estimates are assumed (80% T.D.). The highest estimated Tmax for a ‘standard operating’ RTG is 1120 K. A hypothetical scenario is investigated: an ELHS Ti cl = 1973K-the RPSs’ requirements’ maximum permitted temperature. Fuel melting will not occur