The Co-Sb-Ga System: Isoplethal Section and Thermodynamic Modeling
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INTRODUCTION
THERMOELECTRIC materials and devices have attracted substantial research interest, primarily in waste heat recovery and efficiency enhancement of energy usage. Various studies have been carried out to develop materials with better thermoelectric properties.[1–5] Skutterudite compounds have a body-centered cubic structure, and there are two major voids per unit cell. When a third atom is incorporated into the voids, the compound is described as RM4X12, and is referred to as filled skutterudite. CoSb3 is a skutterudite compound;[6–10] it has been reported that filled CoSb3 skutterudites have promising thermoelectric properties, and Ga is one of the suitable filling atoms.[6–10] Phase equilibria information is fundamentally important for materials’ development, processing-route selection, and related products’ reliability assessment. Although Co-Sb-Ga is of great interest in thermoelectric applications, there are only limited phase equilibria results.[10–12] Markovski et al.[11] determined the Co-SbGa isothermal section at 773 K (500 °C). Qiu et al.[10] WOJCIECH GIERLOTKA, Assistant Professor, is with the Department of Materials Science and Engineering, National Dong Hwa University, Hualien, Taiwan R.O.C. SINN-WEN CHEN, Professor, WEI-AN CHEN, Master Student, and JUI-SHEN CHANG, Doctoral Student, are with the Department of Chemical Engineering, National Tsing Hua University, Hsinchu, Taiwan R.O.C. Contact e-mail: [email protected] G. JEFFREY SNYDER, Professor, is with the Materials Science, California Institute of Technology, Pasadena, CA, and also with the Department of Materials Science and Engineering, Northwestern University, Evanston, IL. YINGLU TANG, Doctoral Student, is with the Materials Science, California Institute of Technology. Manuscript submitted October 22, 2014. Article published online 10 February 2015 1488—VOLUME 46A, APRIL 2015
reported the solubility of Ga in the CoSb3 phase and the equilibrium phase relationships around the CoSb3 phase at 923 K (650 °C). Chen et al.[12] determined the liquidus projection including the invariant reaction temperatures and the isothermal section at 923 K (650 °C) of the Co-Sb-Ga ternary system. The phase diagrams of the three binary constituent systems have been determined,[13–15] but the CoSb3 phase has been described as a line compound in all previous studies.[15–17] Since ternary solubility in the CoSb3 phase is critical, a thermodynamic model of the CoSb3 phase which can properly describe its ternary solubility needs to be developed. This study remodels the Co-Sb binary system using a defect model, and also experimentally determines the CoSb3-GaSb isoplethal section due to the insufficiency of ternary phase equilibria data.[10–12] The thermodynamic models of the Co-Sb-Ga system are then reassessed with the new constituent binary models and the ternary Co-Sb-Ga phase equilibria results.
II.
EXPERIMENTAL PROCEDURES
Ternary Co-Sb-Ga alloys were prepared using pure Co foil (99.95 wt pct purity, Alfa Aesar, USA), Sb shots (99.9999 wt pct purity, Alfa Ae
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