Thermodynamic Study On Some Subsystems Of The Skutterudite Thermoelectric Materials ?Va,Ce,Yb??Co,Fe,Ni?₄?Sb,Ge?₁₂

The family of skutterudites based on CoSb₃ with the cage-like open structure has been extensively studied and regarded as a potential candidate of novel thermoelectric material due to its high Seebeck coefficient and high electrical conductivity. However, its high thermal conductivity makes it difficult to be an efficient thermoelectric material. In attempts to lower the thermal conductivity, nowadays, the study of the skutterudite thermoelectric materials is focused on doping foreign atoms to improve their thermoelectric properties. Small and heavy rare earth atoms such as Ce and Yb have been used to fill the crystal structure cages and the "rattling" of the filled atoms scatters phonons strongly and drastically reduces the thermal conductivity. At the same time, substituting Fe or Ni for Co and substituting Ge for Sb can maximize the influence of point-defect scattering by creating large mass difference at the lattice sites to lower the lattice thermal conductivity. The rare earth-filled skutterudites with the substitutions of constituent atoms of the framework structure mainly involve the elements Ce, Yb, Co, Fe, Ni, Sb, Ge and so on. Utilizing the CALPHAD approach, the present work mainly focuses on the experimental measurements and the thermodynamic optimizations of the Ce-Sb, the Fe-Sb, the Ce-Ge, the Yb-Ge and the Yb-Ni binary systems and the Ce-Fe-Sb and the Co-Fe-Sb ternary systems, in order to get a set of the self-consistent, reasonable and reliable thermodynamic parameters for each of the systems. The basic thermodynamic data and the phase equilibrium analyses are provided for the material design and the process control of the CoSb₃-based thermoelectric materials.The first aspect of the present work is the phase equilibrium measurements. With the comprehensive consideration of the microstructure observation, the phase identification and the composition measurement of the as-cast Ce-Ge binary and the Ce-Fe-Sb ternary alloys, their solidification processes are analyzed. The phase equilibrium relations of the Ce-Ge binary system in the range of 40-60 at.% Ge is modified on the basis of the previous literature reported phase diagram and the liquidus projection of the Ce-Fe-Sb ternary system over the entire composition range is firstly constructed. In the meanwhile, through the study on the equilibrium microstructure, the phase identification and the composition measurement of the Ce-Fe-Sb ternary alloys after the isothermal heat treatment at 823 K for two weeks, the phase equilibrium relations and the isothermal section over the entire composition range were determined.The second aspect of the present work is the thermodynamic assessments and the developments of the thermodynamic databases by the CALPHAD approach. According to the features of the crystalline structures of the phases, the reasonable thermodynamic models for all the phases are selected. In consideration of the newly reported phase equilibrium data and thermochemical data, the Ce-Sb and the Fe-Sb binary systems are re-assessed, and the Ge-Yb and the Ni-Yb binary system and the Co-Fe-Sb ternary system are firstly assessed. Considering the literature reported and the present investigated data of phase equilibria in the composition range of 40-60 at.% Ge as well as the literature reported thermochemical data, the Ce-Ge binary system is firstly assessed. Based on the experimental liquidus projection and isothermal sections constructed by combining the literature results and the present work, the Ce-Fe-Sb ternary system are firstly assessed. A set of self-consistent thermodynamic parameters is obtained for each of the above mentioned sub-systems of the skutterudite thermoelectric materials ?Va,Ce,Yb??Co,Fe,Ni?4?Sb,Ge?12, the related thermodynamic database is developed, and the calculated results can reproduce the experimental data well.The third aspect of the present work is the application of CALPHAD approach in the analysis of solidification microstructures and phase equilibrium relations. Especially, the constituent phases and the phase compositions of each of the typical Ce-Fe-Sb as-cast alloys are calculated by using the Scheil-Gulliver model of the Padat software and are compared with the experimental observation of the same alloy. The solidification simulation can be helpful to explain the experimental measurement, analyze the microstructure evolution and guidethe composition design. The thermodynamic analysis of the present work on the ?Ce,Yb?-?Co,Fe,Sb?-?Sb,Ge? systems is of important reference to the composition design and the process control of the CoSb₃-based skutterudite thermoelectric materials.