| Summary: | Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2018. === This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. === Cataloged student-submitted from PDF version of thesis. === Includes bibliographical references. === High temperature (>900°C) industrial waste heat recovery remains a key challenge for thermoelectric (TE) materials. The unique combination of high temperature, low heat-flux, and large surface area of waste heat generation in industrial processes shows that active material cost is the main metric inhibiting application. Low cost molten compounds with semiconducting properties are therefore proposed as a cost-effective addition to solid-state materials for these conditions. The performance of a laboratory-scale TE test cell based on molten SnS is demonstrated which reports Seebeck coefficient, electrical conductivity, thermal conductivity and, for the first time, the Figure of Merit and TE conversion efficiency of a molten semiconductor at the device level. The heat transfer modes of molten SnS in the TE test cell is investigated. The results suggest a domination of natural convection over intrinsic thermal conduction and radiative thermal conduction as primary heat transfer mechanism. In addition, a change of structure and thermophysical properties is found to occur at around 1000'C for molten SnS. The structure and property change is further connected to a semiconductor-to-metal (SC-M) transition, or metallization, which is known to take place in all molten semiconductors at high temperatures. The relationship between SC-M transition and structure/property changes connected by a proposed thermodynamic framework is verified with molten SnS. The outcome of this thesis confirms the opportunity offered by molten thermoelectric compounds and discusses the remaining materials and engineering challenges that need to be tackled in order to envision future deployment of thermoelectric devices based on molten semiconductors. === by Youyang Zhao. === Ph. D.
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