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

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

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

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

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

Recent Joint Studies Related to the Development of Space Radioisotope Power Systems

Over the last several years there has been a mutually beneficial ongoing technical interchange between the U.K and the U.S. related to various aspects of space radioisotope power systems (RPS). While this interchange has been primarily focused on materials based activities, it has also included some aspects related to safety, environmental, and lessons learned during the application of RPSs by the U.S. during the last fifty years. Recent joint technical RPS endeavors have centered on the development of a possible “cold” ceramic surrogate for 238PuO2 and 241AmOx and the irradiation of thermoelectrics and other materials at expected RPS related neutron fluences. As the U.S. continues to deploy and Europe develops RPS capability, on-going joint RPS technical interfaces will continue to enhance each entities’ endeavors in this nuclear based power technology critical for deep space exploration

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

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