Authors:	Heinrich, Christophe Paul
Title:	Structure-property relationships in pnictide and chalcogenide thermoelectric materials
Online publication date:	17-Jul-2016
Year of first publication:	2016
Language:	english
Abstract:	The goal of this work was to gain a deeper understanding of the structural chemistry of selected pnictide and chalcogenide thermoelectric materials. In the ongoing search for new materials for thermoelectric energy conversion, materials composed of inexpensive and earth abundant elements need to be explored and their thermoelectric efficiencies have to be improved. Different material classes provide diverse chemical optimization opportunities to enhance the thermoelectric properties. However, a profound knowledge of the underlying chemistry and the structure-property relationships is needed to seize these opportunities. This work focuses on three different material classes that have been investigated in regard to their structural chemistry and their thermoelectric properties. First, inspired by the thermoelectric properties of adamantine-like chalcogenides, the substitution series Cu2ZnGeSe4−xSx was investigated as a model system for isovalent anion substitutions. Throughout the substitution series, the induced changes of bonding character and structure had a significant effect on several material properties. A phase transition was discovered throughout the complete solid solution series. Using variable temperature X-Ray diffraction and 63Cu MAS-NMR we revealed that this phase transition is correlated to a superionic transition of Cu+ ions. A detailed analysis of the thermal conductivity exhibited a 42% reduction of the room temperature lattice thermal conductivity due to disorder scattering. Further, the Callaway model was used to calculate the lattice thermal conductivity and revealed mass contrast to have a greater influence than strain contrast on these materials. Secondly, filled Skutterudites were investigated, which are a very promising and deeply investigated class of thermoelectric materials. To overcome the hitherto existing problems of long reaction times and material inhomogeneities, an easy two-step synthesis for indium-filled skutterudites was developed. By separating the kieftite host formation from the topotactic filler insertion, the reaction time could be dramatically reduced and the formation of side phases was effectively circumvented. Thereby, an unadulterated investigation of the influence of the indium-filling degree on the structure and the thermoelectric properties was possible. To this end, a series of indium-filled skutterudites InxCo4Sb12 with nominal composition x = 0.12, 0.15, 0.18 and 0.20 were synthesized in bulk quantities. The structural and microstructural aspects of the obtained powders and short-time sintered pellets were investigated in addition to their thermal stability. The excellent sample homogeneity of the obtained materials was demonstrated via SEM/EDX, PSM measurements, 121Sb Mosbauer spectroscopy, and powder X-Ray diffraction of standard laboratory and synchrotron data. Rietveld refinements revealed a marginal volume fraction of InSb in few samples, and allowed to determine the actual indium-filling degree. This enabled to correlate the obtained transport data to the actual indium-filling degree and showed that the indium-filling has a positive effect on both, the electronic and thermal properties. The thermoelectric figure of merit was consequently enhanced up to a value close to unity at 420 °C. Although a marginal presence of an InSb side phase (≈ 0.1 vol%) was reveald in some samples, this had no significant influence on the transport properties. Nevertheless, the resulting lower actual indium-filling degree had a dramatic effect on the thermoelectric properties, and resulted in a 18% reduction of the thermoelectric figure of merit. The detailed investigation of the thermoelectric properties revealed that a reduction of the lattice thermal conductivity by enhanced phonon scattering provides a very large optimization potential. Finally, the search for new, cheap and non-toxic, n-type thermoelectric materials has led us to the investigation of tetragonal tungsten bronzes. In this work, a first assessment of the potential of tetragonal tungsten bronzes for thermoelectric energy conversion was conducted. These compounds of general composition Nb8−xW9+xO47−δ can be characterized by their infinitely adaptive structure. We chose this class of materials, due to their structural complexity, and high potential for electronic optimization by cation substitution (x) and controlled oxygen deficiency (δ). To this end, a series of compounds with low substitution degree x = 0, 0.075, 0.1, and 1, was synthesized and structurally characterized by X-ray diffraction and high-resolution transmission electron microscopy. Further, the thermal stability and oxygen deficiency was analyzed using elaborated thermogravimetric analyses. Thermoelectric transport measurements revealed that both the cation substitution and the controlled oxygen defect concentration leads to enhanced electronic properties.
DDC:	540 Chemie 540 Chemistry and allied sciences
Institution:	Johannes Gutenberg-Universität Mainz
Department:	FB 09 Chemie, Pharmazie u. Geowissensch.
Place:	Mainz
ROR:	https://ror.org/023b0x485
DOI:	http://doi.org/10.25358/openscience-4087
URN:	urn:nbn:de:hebis:77-diss-1000005775
Version:	Original work
Publication type:	Dissertation
License:	In Copyright
Information on rights of use:	https://rightsstatements.org/vocab/InC/1.0/
Extent:	XXIX, 140 Seiten
Appears in collections:	JGU-Publikationen