Abstract
Thermoelectric (TE) materials are of great interest to many researchers because they directly convert electric and thermal energy in a solid state. Various materials such as chalcogenides, clathrates, skutterudites, eutectic alloys, and intermetallic alloys have been explored for TE applications. The Ga-Sn-Te system exhibits promising potential as an alternative to the lead telluride (PbTe) based alloys, which are harmful to environments because of Pb toxicity. Therefore, in this study, thermodynamic optimization and critical evaluation of binary Ga-Sn, binary Sn-Te, and ternary Ga-Sn-Te systems have been carried out over the whole composition range from room temperature to above liquidus temperature using the CALPHAD method. It is observed that Sn-Te and Ga-Te liquids show the strong negative deviation from the ideal solution behavior. In contrast, the Ga-Sn liquid solution has a positive mixing enthalpy. These different thermodynamic properties of liquid solution were explicitly described using Modified Quasichemical Model (MQM) in the pair approximation. The asymmetry of ternary liquid solution in the Ga-Sn-Te system was considered by adopting the toop-like interpolation method based on the intrinsic property of each binary. The solid phase of SnTe was optimized using Compound Energy Formalism (CEF) to explain the high temperature homogeneity range, whereas solid solution, Body-Centered Tetragonal (BCT) was optimized using a regular solution model. Thermodynamic properties and phase diagram in the Ga-Sn-Te and its sub-systems were reproduced successfully by the optimized model parameters. Using the developed database, we also suggested several ternary eutectic compositions for designing TE alloy with improved properties.
Highlights
Thermoelectric (TE) materials are being continuously developed for application in waste heat recovery and electronic cooling because of their ability to interchange between thermal energy and electrical energy in a solid-state [1,2]
The Ga-Sn system has been thermodynamically assessed by Anderson and Ansara [35]. They proposed the Ga-Sn phase diagram based on the extensive review of the thermodynamic and phase diagram data
The liquidus line was determined by Delcroix et al [37], Predel [38], and Puschin et al [39] using thermal analysis and by Shurai et al [40], Trebukhov et al [41], and Zivkovic et al [42] using differential thermal analysis (DTA)
Summary
Thermoelectric (TE) materials are being continuously developed for application in waste heat recovery and electronic cooling because of their ability to interchange between thermal energy and electrical energy in a solid-state [1,2]. The low conversion efficiency of the TE devices is one of the major limiting factors for commercial applications This conversion efficiency is described by a dimensionless thermoelectric figure of merit (ZT) represented as S2 σT/(k), where S, σ, k, and T are Seebeck coefficient, electrical conductivity, thermal conductivity, and temperature, respectively. It is challenging to maximize the collective term, S2 σ Another approach is to reduce thermal conductivity by introducing phonon scattering sites such as solid solutions, multi-phase microstructure, precipitations, or nanostructuring. They have demonstrated that Ga doping in SnTe improves the electronic properties by activating several hole packets, thereby enhancing the Seebeck Coefficient [26] Eutectic alloys in this ternary system have not been explored for TE application. The Gibbs free energy of ternary liquid solution was estimated by combining the optimized model parameters of the three sub-binary systems with a reasonable interpolation method
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