Fast synthesis and sintering of Zn and Ni substituted tetrahedrite for thermoelectric applications

A current challenge in the thermoelectric field is the discovery of new materials which are inexpensive, easy to synthesize, and comprised of earth-abundant elements. Recently natural mineral tetrahedrites received great interest from thermoelectric community because of their non-toxicity, environmental friendliness and relatively high abundance. Natural "tetrahedrite" exists as a solid solution of tetrahedrite (Cu12Sb4S13) and tennantite (Cu12As4S13), where the Cu atoms are partially replaced by transition metal elements such as Zn, Fe, Mn, Hg or Ni [1-3]. The tetrahedrite/tennantite mineral family is the most widespread sulfosalt on Earth. The ZT values obtained for pure synthetic tetrahedrite Cu12Sb4S13 is 0.56 at 673 K [4]. Synthetic tetrahedrites were variously doped, with interesting results for Ni doped samples with a maximum ZT of 0.7 at 665 K for Cu10.5Ni1.5Sb4S13 [5]. All the synthesis reported in thermoelectric literature are based on a direct melting method starting from pure constituent elements and are characterized by high pressures, high temperatures and long time (up to 2-3 week) [4-6]. In this work, we developed a new fast synthesis and sintering method to obtain Ni-Zn substituted tetrahedrite pellets by means of the combination of ball milling and open die pressing sintering (ODP) methods [7]. X-Ray diffraction (with a profile Rietveld refinements) and Scanning Electron Microscope (equipped with energy dispersive X-Ray spectroscopy) were employed for a structural, morphological and compositional characterization of the obtained samples. Furthermore, the samples were thermoelectric characterized by electrical conductivity and Seebeck coefficient measurements, and Laser Flash Analysis for the thermal conductivity