(199h) Phase Diagrams, Defect Models and Thermoelectric Properties: ß-Ag2se and CoSb3

Thermoelectric materials have attracted tremendous research efforts owing to their potential contributions in enhancing energy usage efficiency and as renewable energy. ß-Ag₂Se and CoSb₃ are promising thermoelectric materials for room- temperature and mid-temperature applications. It has been reported that their thermoelectric propertied can be further enhanced with Ga-doping and In-doping, respectively. Phases and microstructures are critical to the thermoelectric properties. Phase diagrams provide fundamental phase equilibria information, and are important for the understanding and illustration of phases and microstructural evolution of materials. Ternary Co-Ga-Sb and Ag-In-Se alloys are prepared, and their equilibrium phases are determined. The Co-Ga-Sb and Ag-In-Se phase diagrams are determined based on the phase diagrams of their constitute binary systems and the experimental phase equilibria results of ternary alloys. The crystal structure of the ß-Ag₂Se phase is orthorhombic and that of the CoSb₃ phase is skutterudite. The solubilities of indium in ß-Ag₂Se and gallium in CoSb₃ are small, and are less than 0.5at.% and about 1at.%, respectively. Since the third element doping is important for the thermoelectric properties of both compounds, efforts are carried out to describe the small solubilities of indium in b-Ag₂Se and gallium in CoSb₃. Defect-type models are used for thermodynamic modeling. The formation energies of perfect ß-Ag₂Se and CoSb₃ phases are determined by the Calphad-type modeling. The defect types of the compounds are proposed, and the Gibbs energies of the compounds related with temperatures and concentrations are formulated. Phase diagrams are then calculated using the Calphad-type approach. Besides thermodynamic properties, thermoelectric properties can also be calculated based on the defect models. The calculated thermoelectric properties are compared with elemental measurements. Based on the results of both thermodynamic and thermoelectric calculations, it is confirmed that Ga atoms occupy both the void and Sb sites in CoSb_3. The defect type of the binary ß-Ag₂Se is a cationic Frenkel type disorder, and indium atoms are assumed to be on both Ag and Se sites.