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

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 Cu-29 decreased (Ni-28, Co-27, Fe-26, and Mn-25), the lattice constant increased; however, the lattice constant also increased when Cu-29 was substituted with Zn-30 (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.