No abstract

The microstructures and Seebeck coefficients of thermoelectric alloys in the pseudo-ternary PbTeAg 2 Te-Sb 2 Te 3 system were examined using samples that were compositionally graded by unidirectional solidification by the Bridgman method and diffusion couples. At compositions near the middle of the pseudo-binary PbTe-AgSbTe 2 line, a compositionally modulated microstructure has been found. From diffusion couple experiments, it is found that the PbTe-AgSbTe 2 system exhibits a miscibility gap at low temperatures while it forms a complete solid solution at high temperatures; the critical temperature is between 400 C and 450 C. The modulated microstructure originates from the decomposition of the high-temperature solid solution during cooling. Scanning Seebeck coefficient measurement on these samples covers a wide compositional space of the pseudo-ternary system. The Seebeck coefficient transitions from positive values at AgSbTe 2 -rich compositions to negative values at PbTe-rich compositions on the pseudo-binary PbTe-AgSbTe 2 line. Composition-graded samples prepared by the Bridgman method are thus useful to investigate thermoelectric materials in multicomponent systems.

Unidirectional solidification of PbTe-rich alloys in the pseudobinary PbTe-Sb2Te3 system using the zone leveling technique enables the production of large regions of homogeneous solid solutions for the formation of precipitate nanocomposites as compared with Bridgman solidification. (PbTe)0.940(Sb2Te3)0.060 and (PbTe)0.952(Sb2Te3)0.048 alloys were successfully grown using (PbTe)0.4(Sb2Te3)0.6 and (PbTe)0.461(Sb2Te3)0.539 as seed alloys, respectively, with 1 mm h–1 withdrawal velocity. In the unidirectionally solidified regions of both alloys, Widmanstätten precipitates are formed due to the decrease in solubility of Sb2Te3 in PbTe. To determine the compositions of the seed alloys for the zone leveling experiments, the solute distribution in solidification in the PbTe-richer part of the pseudobinary PbTe-Sb2Te3 system has been examined from the concentration profiles in the samples unidirectionally solidified by the Bridgman method.