One of the biggest obstacles to broadening the implementation of thermoelectric generators are the currently available thermoelectric materials, which tend to feature rare/expensive and toxic elements such as Bi, Pb or Te. In the pursuit of novel materials, researchers have been studying a p-type semiconductor mineral, named tetrahedrite (Cu
12Sb
4S
13). This mineral has low toxicity and is abundant in the surface of the Earth, displaying good electrical and thermal properties for thermoelectric applications, which led to a figure of merit, zT, of 0.6 at 700 K. Although current materials have higher performances (zT > 1.0), tetrahedrite has great potential for improvement via isovalent doping, as reported by various studies [
1].
In this work, the effect in tetrahedrite thermoelectric properties of simultaneous isovalent doping of two elements, selenium (Se) and nickel (Ni), are presented. By employing Wien2K software [
2] for calculating electrical band structures and density of states, coupled with BoltzTrap software [
3], for calculating thermoelectric properties, simulations were conducted, which suggest that the optimum stoichiometric content for the highest thermoelectric performance (maximum zT), would be achieved with stoichiometric content of 0.5 Ni and 0.5 Se.
Thus, samples following the formula Cu12−xNixSb4S13−ySey were produced and characterized by powder X-ray diffraction, Raman spectroscopy and scanning electron microscopy, with energy dispersive spectrometry. After annealing at 723 K for 7 days (the current stage in the production process), the samples display a major tetrahedrite phase, with the lowest estimated molar content being ≈84%, and a minor presence of secondary phases, mostly covellite and famatinite.
Measurements of the Seebeck coefficient and electrical resistivity not only indicate that co-doping improves the power factor, but they also are congruent with simulations, with the Cu11.5Ni0.5Sb4S12.5Se0.5 sample achieving the highest power factor (1277.73 µW/m·K2 at 300 K). After estimating the thermal conductivity using the Wiedemann-Franz law and previously reported values for the lattice contribution, a figure of merit of zT = 0.32 at 300 K is obtained, which is one of the highest for this type of thermoelectric materials.
Author Contributions
Conceptualization: D.M., E.L., L.F.S., J.F.M. and A.P.G.; Methodology: D.M., E.L., L.F.S. and A.P.G.; Software: J.F.M.; Validation: D.M., E.L., L.F.S. and A.P.G.; Formal analysis: D.M., E.L., L.F.S. and A.P.G.; Investigation: D.M., E.L., L.F.S., J.F.M. and A.P.G.; Data curation: D.M., E.L., L.F.S., J.F.M. and A.P.G.; Writing—original draft preparation: D.M.; Writing—review and editing: E.L., L.F.S. and A.P.G.; Supervision: E.L., L.F.S. and A.P.G.; Project administration: A.P.G. All authors have read and agreed to the published version of the manuscript.
Funding
Work was supported by the portuguese Foundation for Science and Technology (FCT) through the contract number: PD/BD/135926/2018.
Institutional Review Board Statement
Not applicable.
Informed Consent Statement
Not applicable.
Data Availability Statement
Not applicable.
Conflicts of Interest
The authors declare no conflict of interest.
References
- Chetty, R.; Balia, A.; Mallik, R.C. Tetrahedrites as thermoelectric materials: An overview. J. Mater. Chem. C 2015, 48, 12364–12378. [Google Scholar] [CrossRef]
- Blaha, P.; Schwarz, K.; Madsen, G.K.H.; Kvasnicka, D.; Luitz, J. WIEN2k: An Augmented Plane Wave Local Orbitals Program for Calculating Crystal Properties; Karlheinz Schwarz; Technische Universität Wien: Vienna, Austria, 2001; ISBN 3-9501031-1-2. [Google Scholar]
- Madsen, G.K.H.; Singh, D.J.; BoltzTraP. A code for calculating band-structure dependent quantities. Comput. Phys. Commun. 2006, 175, 67–71. [Google Scholar] [CrossRef] [Green Version]
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