Date of Award

8-2016

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Physics

Committee Member

Dr. Terry M. Tritt, Committee Chair

Committee Member

Dr. Jian He

Committee Member

Dr. Donald Liebenberg

Committee Member

Dr. Catalina Marinescu

Abstract

With the ever increasing consumption rate of energy, we will run out of fossil fuel resources sooner than we expect. Also the environmental concerns associated with the use of fossil fuel become a severe issue. As such, the need for alternative energy becomes extremely impending. Thermoelectricity is the simplest technology applica-ble to direct heat-electricity energy conversion with electricity being the best quality form of energy and heat being the lowest. However, comparing to the front row can-didates such as wind, photovoltaics, solar heat and biofuel that is possible to replace fossil fuel, thermoelectrics has received less heed due to its low conversion efficiency. Nonetheless, its ability to directly convert heat into electricity in an all solid-state manner still makes it pretty tempting for application as one of the energy sources. This can be well-justified by the fact that a huge amount of heat exhausted from a car, a power station or an industrial process is all amendable to thermoelectric (TE) conversion. What’s more, TE energy conversion is green and environmentally friendly, the TE devices have no movable parts and are susceptible to be miniaturized, so they can be shaped as needed and maintenance is minimized. Since modern TE study is efficiency driven and material oriented fundamental re-search, developing higher performance TE materials has thus become the ultimate goal so as to make thermoelectricity a crucial part in this big energy picture. Among the state of art TE materials, Co4Sb12 based skutterudites have become increasingly favorable for room temperature (300 K) to 800 K applications. The interest mainly lies on how such high performance is achieved via ”engineering” its unique crystal structure - the naturally formed nano-sized ”cage”. Properties of Co4Sb12 based skutterudites can be ”tuned” by filling guest atoms and/or substitutionally doping. The function of guest atoms is two-fold: One is to decrease the lattice ther-mal conductivity; the other is to improve the electrical properties. While guest atoms directly help optimize TE properties, they have certain solubility limit or Filling frac-tion limit (FFL) in the ”cage”, if surpassed, secondary phases would still contribute constructively to TE performance. From a solid state physics point of view, those fillers, dopants and secondary phases are all defects of different dimensionality, there-fore, Co4Sb12 skutterudites provide a material template to study the interplay of defects. Recently, the study of ”cagey” material mainly focuses on the optimization of perfor-mance of ”multiple-filled” skutterudites. In contrast, single filling is less sufficiently studied. To study the defect chemistry, a combined theoretical and experimental study of the single filled skutterudites is indispensable. Also, although the single-filled Co4Sb12 may not exhibit high performance, understanding a high performance material is equally important as understanding a low performance material. To this end, La was chosen to be the guest atom in our work for being the first element in rare earth group and not having f electrons, which make theoretical calculations more feasible. So the work herein presented: 1) Experimentally using only one control variable La to implement the filling-doping- nanocomposite approach that can significantly enhance the TE efficiency compared to an unfilled one. 2) Theoretically studying the interplay between defects, i.e., the filler atoms and sb vacancies, that cannot be unequivocally interpreted by experiments. More specifically, despite the elements normally filled into the ”cage” are rare earth elements (Ce,Pr,Nd and Eu) and are named ”rattler” - by definition, they are weakly bounded in the cages and ”rattle” about their equilibrium position substantially more than the other atoms in the structure - but the study on ”rattlers” always focus on their influence on thermal properties yet scarcely on electrical properties. Therefore, this dissertation seeks to answer questions about: i) the physical identity of the filler, whether it is a real filler or a ”rattler”. ii) The influence of f electrons on the rattling behavior. iii) how does the interplay between the filler atoms and Sb deficiency affect the TE properties in the system. We combine a series of experiment study with Density Functional Theory (DFT) cal-culations. The result not only show a obvious enhancement of TE efficiency compared to the prisine Co4Sb12 skutterudite, but also revealing an approach to help further improve the TE properties of other skutterudites and cagey materials.

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