Thermal Stability, Mechanical Properties and Thermoelectric Performance of Cu₁₁TrSb₄S₁₃ (Tr = Mn, Fe, Co, Ni, Cu, and Zn)

Abstract

Tetrahedrites substituted with transition elements, Cu₁₁TrSb₄S₁₃ (Tr = Mn, Fe, Co, Ni, Cu, and Zn), were synthesized by mechanically alloying and hot pressing, and their thermal stability, mechanical properties and thermoelectric performance, including phase transition (decomposition), elemental redistributions, microstructures, thermoelectric parameters, hardness, and bending strength, were examined. Hot-pressed compacts showed relative densities of 97.4–99.8%. As the atomic number of the transition element substituted for ²⁹Cu decreased (²⁸Ni, ²⁷Co, ²⁶Fe, and ²⁵Mn), the lattice constant increased; however, the lattice constant also increased when ²⁹Cu was substituted with ³⁰Zn (higher atomic number). The electrical conductivity of tetrahedrites doped with transition elements decreased compared with that of intrinsic tetrahedrite Cu₁₂Sb₄S₁₃. This was because transition elements were successfully substituted at Cu⁺ sites, and the carrier (hole) concentration decreased owing to electron donation. The Seebeck coefficient of Cu₁₁TrSb₄S₁₃ was greater than that of Cu₁₂Sb₄S₁₃, except for Cu₁₁FeSb₄S₁₃. However, the thermal conductivity of the tetrahedrite decreased upon the substitution of transition elements owing to enhanced impurity phonon scattering. Endothermic reactions were observed at temperatures between 882 K and 984 K, which corresponded to each melting point, and the tetrahedrite melting point increased upon doping with transition elements. The Vickers hardness and three-point bending strength of Cu₁₂Sb₄S₁₃ were 2.2 GPa and 23 MPa, respectively. However, the hardness (2.5–2.7 GPa) and bending strength (23–44 MPa) increased for Cu₁₁TrSb₄S₁₃ as the result of the solid-solution hardening effect.