Abstract
Cu–Sn-based sulfides are earth-abundant and nontoxic compounds of special interest for low-cost energy harvesting applications. In the present work, we have investigated the effect of grain size on the thermoelectric properties of Cu2SnS3 (CTS). Three dense CTS samples with nanometric grains were produced by mechanical alloying combined with spark plasma sintering, preserving the small size of crystalline domains to 12, 25, and 37 nm, respectively. The experimental results show that the Seebeck coefficient (S) and electrical resistivity (ρ) decrease with decreasing domain sizes, while the thermal conductivity (κ) increases. A smaller domain size correlates with a lower resistivity and a degenerate semiconductor-like behavior due to higher carrier concentration. At the same time, our synthesis method leads to materials with very low lattice thermal conductivity, thanks to the nanometric size of grains and structural disorder. As a result, the sample with the smallest grain size exhibits the highest zT of ∼0.4 at 650 K. First-principles density functional theory (DFT) simulations on various CTS crystallite surfaces revealed localized states near the Fermi level and the absence of band gap, indicating the metallic nature of the surfaces. Various CTS systems were tested by DFT, showing the following order of increasing formation energy: stoichiometric CTS, Cu vacancy, Cu-rich, Sn vacancy, and Sn-rich.
Highlights
Most commercially available thermoelectric (TE) devices use toxic and scarce materials, making them expensive and potentially hazardous, for example, Sb2Te3, Bi2Te3, and so forth
We have studied the effects of crystalline domain size on the TE properties using experimental analyses and first-principles simulations
Rietveld refinement of the XRD patterns revealed average domain sizes below 50 nm for samples sintered under different conditions
Summary
Most commercially available thermoelectric (TE) devices use toxic and scarce materials, making them expensive and potentially hazardous, for example, Sb2Te3, Bi2Te3, and so forth. The search for high-performance, nontoxic, ecofriendly, and earth-abundant TE materials has led to the exploration of multinary sulfides.[1,2] Chalcogenides,[3] colusites,[4,5] and other metal-based sulfides[6,7] could be viable alternatives to existing materials.[8] Cu-based sulfides have low formation energy so that it is possible to produce them by short-period reactive milling using a planetary or vibrating mill. As shown in the present work, high-energy reactive ball milling, called mechanical alloying, can be employed with success to synthesize new and disordered phases. Milling offers the advantage of facile and scalable production for industrial use.[9]
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