Conversion of dissipated heat into electricity is the basic principle of thermoelectricity. It has wide variety of applications in the areas such as automobile engineering, refrigerating coolants, satellite etc. In search of such materials thermoelectrics has given wide scope to complex materials like Tellurides, Clatharates, Zintl compounds, Half Heusler alloys, Si-Ge, Skutterudite etc. The defining factor for thermoelectric materials is ZT, thermoelectric figure of merit. This attributes to the power factor (P=σS2, σ-electrical conductivity S-Seebeck coefficient) enhancement and thermal conductivity reduction. Band gap tunning, carrier concentration are a few ways to improve power factor where as in parallel, grain size reduction, point defects, dislocations are key ways to thermal conductivity reduction. These can be achieved experimentally through microstructural engineering and processing . The issues existing with available thermoelectric materials are associated with stability of performance for a long range of temperatures. Fine tuning of microstructure is the key factor in overcoming the thermal stability issues.
Currently we are exploring a program of microstructure- transport properties correlation of newly processed thermoelectric alloys by fine tuning the eutectic microstructure. In this current work, we have synthesized a set of thermoelectric alloys eutectic and off eutectic compositions in Sn-Te alloy system (Sn14Te86 (at%)). These as melted alloys show eutectic microstructure consisting of Sn50Te50 and Te phase with varied volume fraction. Variations of transport properties with respect to secondary phase fraction have also been studied. Antimony additions with various at% to Eutectic microstructure has been studied. To these alloys the effect of Directional solidification has been studied and has improved TE properties significantly. A power factor of 19μW/cm-K2 and seebeck coefficient of 150μV/K has been shown.
The elemental distribution mapping and the compositional analysis using electron probe microanalysis (WDS), demonstrates that the matrix is rich in tellurium and the continuous phase has the composition of Sn50Te50 (at%). Transport properties of the current alloy are attractive in terms of standard thermoelectric material. Further elemental additions have enhanced properties. They were directionally solidified at various speeds and TE properties were studied with microstructural modification. The detailed microstructure-transport properties correlation will be presented.