Phase stability and thermoelectric properties of Cu10.5Zn1.5Sb4S13 tetrahedrite
Graphical abstract
Introduction
Thermoelectric (TE) materials required for power generation applications need to possess the high figure of merit (ZT), readily available in large-scale, stable at the operating temperatures [1], [2]. Most of the currently investigated TE materials use costly rare earth elements, whose availability are limited and have toxicity issues. Recently, environmentally friendly and earth-abundant materials are being explored for thermoelectric applications [3], [4]. Tetrahedrite, a naturally occurring mineral is one such material of interest as it shows promising ZT, a parameter used to evaluate the efficiency of TE materials. ZT is a dimensionless parameter given by ZT = S2T/ρκ where S, ρ and T represent Seebeck coefficient, electrical resistivity and absolute temperature T. κ is thermal conductivity which is the sum of carrier thermal conductivity (κe) and lattice thermal conductivity (κL).
The tetrahedrite compound is a p-type ternary I-V-VI semiconductor of copper antimony sulfide (Cu12Sb4S13) with an estimated band gap of 1.7 eV [5]. Naturally occurring tetrahedrite has the composition of Cu12-xMx(Sb, As)4S13 which is a solid solution of As rich tennantite (Cu12As4S13) and Sb rich tetrahedrite (Cu12Sb4S13). Famatinite (Cu3SbS4) and skinnerite (Cu3SbS3) are some of the other compounds in Cu-Sb-S system [6]. The pure form of the tetrahedrite phase does not occur naturally, and normally transition metals such as Zn, Fe, Hg, Ni, and Mn substitutes the Cu site in Cu12-xTrxSb4S13 . Tetrahedrite has a complex crystal structure with 58 atoms arranged in a high symmetry cubic unit cell (I3m) constituted by CuS4 tetrahedra, CuS3 triangles and SbS3 pyramids [7].
The thermoelectric properties of synthetic tetrahedrite Cu12-xTrxSb4S13 (Tr = Mn, Ni, Fe, Co, Zn) have been studied and reported in the recent past [8], [9], [10]. Mn substituted tetrahedrite of nominal composition Cu11MnSb4S13 showed ZT of as high as 1.13 at 575 K [11]. Ni and Te substitution also yielded an impressive ZT of 0.7 at 665 K [10] and 0.92 at 723 K [12] respectively. Natural tetrahedrite was also found to give power factor and ZT similar to synthetic compound [13] with modification in their chemical compositions. The thermal conductivity of both synthetic and purified natural mineral tetrahedrites are <1 W/(m.K), which mainly arises due to inherent nature of the complex crystal structure having a large number of atoms in a unit cell of the tetrahedrite phase. Grain refinement of this phase can further enhance the ZT through the reduction in the lattice thermal conductivity (κL) by enhanced phonon scattering. Until now, only a few studies have been reported on the synthesis of nanocrystalline tetrahedrite and their electronic properties [14], [15].
In the present investigation, we report synthesis of single phase nanocrystalline Zn-doped tetrahedrite (Cu10.5Zn1.5Sb4S13) compound. The phase stability was studied using in-situ high-temperature X-ray diffraction data obtained at different temperatures (298–773 K). The thermoelectric properties of the materials after processing the compound by spark plasma sintering into bulk samples having relative density >98% are reported.
Section snippets
Experimental details
The Cu10.5Zn1.5Sb4S13 tetrahedrite compound was synthesized by mixing Copper (I) sulfide (Cu2S, 99.5%, Alfa Aesar), Zinc Sulfide (ZnS, 99.99%, Aldrich) and Antimony (III) Sulfide (Sb2S3, 99.995%, Aldrich) in stoichiometric ratio (the exact stoichiometry was Cu10.5Zn1.5Sb4S12.75) followed by ball milling and annealing process. Ball milling was carried out in a hardened steel vial with 10 mm stainless steel balls in a planetary ball mill (Fritsch Pulverisette-5) with the ball to powder ratio of
XRD investigation
Fig. 1a shows the XRD patterns of the powders milled for various time. From the XRD peaks, it can be seen that, after 10 h milling, the Cu12Sb4S13 tetrahedrite phase (diffraction peaks corresponding to d (222) of ICDD: 042–0560) starts appearing. The powder is converted into nearly single phase tetrahedrite after 30 h milling with traces of unreacted Sb and ZnS phases.
The formation of the tetrahedrite phase during ball milling of Cu2S, ZnS and Sb2S3 can be understood by the following chemical
Conclusions
In summary, we have synthesized nanocrystalline tetrahedrite powder of nominal composition Cu10.5Zn1.5Sb4S13 successfully by a simple ball milling and annealing of Cu2S, Sb2S3 and ZnS powders. The high temperature in situ x-ray diffraction of the nanocrystalline powders showed stable tetrahedrite phase up to 773 K with marginal growth in the crystallite size. Samples densified to >98% by spark plasma sintering at 723 K for 10 min, showed p-type behavior with highest Seebeck coefficient of
Acknowledgment
The authors are thankful to Prof. G. Sundararajan, Director, ARCI for his keen interest and support in carrying out this work. The DSC/TG thermal analysis support by Dr. N.Rajalakshmi, ARCI, Chennai, INDIA and thermal diffusivity measurement by Dr. Anilkumar Rai, IGCAR, Kalpakkam, INDIA are gratefully acknowledged.
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