(40f) Phase Diagram, Microstructure and Thermoelectric Properties

Thermoelectric materials and devices have attracted intensive research and development interests due to their ability to convert waste heat directly into electricity. Energy usage efficiency could then be improved which is a very critical concern nowadays. Good thermoelectric properties require low thermal conductance but good electrical conductance. Phonon scattering caused by nanoprecipitates has been reported to be an efficient mechanism to enhance the thermal resistance but causing no significant hindrance on electrical conductance. Microstructures of thermoelectric materials are essential for their thermoelectric properties. Phase diagrams provide information of phase equilibria, and are fundamentally important for processing routes selection and understanding of materials microstructures.

Ternary thermoelectric materials systems, Bi-Te-In, Ag-Pb-Te and Co-Sb-Ga, are used as examples to illustrate the relationships between phase diagrams, microstructures and thermoelectric properties. Two kinds of ternary alloys were prepared. Ternary alloys of one kind were equilibrated at pre-determined temperatures, and the equilibrium phases were determined. Alloys of the other kind were quenched, and their primary solidification alloys were determined. Isothermal sections of ternary phase equilibria were proposed based on the results of determined ternary equilibrium phases together with the information obtained from their constituent binary phase diagrams, while the liquidus projections were determined based on results of the primary solidification phases.

According to the determined liquidus projections, ternary alloys with compositions at the ternary eutectics in the Bi-Te-In and Ag-Sb-Te systems, L (Bi- 13.0at%In-77.0at%Te)=Bi₂Te₃+Te+In₂Te₅, and L (Ag-40.at%Sb-36.at%Te)=AgSbTe₂ +Ag₂Te+δ-Sb₂Te, were prepared. Their microstructures were determined, and their thermoelectric properties were measured. Nanoprecipitates of Ag₂Te phase were observed. This ternary eutectic Ag-Pb-Te alloy has very low thermal conductivity (0.3w/mK at 415K), presumably due to the nano-sized lamellar structure and the nanoprecipitates enhancing phonon scattering. Very fine microstructures with nano- whisker-type precipitates were observed in the as-solidified ternary Bi-Te-In eutectic. These nanoprecipitates contribute to the alloys’ thermoelectric properties.  

Ternary alloys in the Bi-Te-In and Co-Sb-Ga systems with compositions close to the ternary solubility limits of Bi₂Te₃ and CoSb₃ phases were also prepared. The thermoelectric properties of the Ga-doped CoSb₃ alloys were improved with the concentration of Ga doping and the figure of merit increased to 0.7 with a 0.1at% Ga doping. Similar results were observed in the In-doped Bi₂Te₃ alloys. A small amount alloying would enhance the phonon scattering and reduce lattice thermal conductivity.       However, secondary phases would form if the doping concentrations are higher than the solubility limits. Since the secondary phase might not have good thermoelectric properties, its existence would deteriorate the thermoelectric properties of the alloys. The phase diagrams are thus fundamentally important.