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The Cu–S compounds have been reported as promising thermoelectric materials with abundant element composition, low price, and low toxicity. In this work, SnxCu1.8
xS samples with different Sn contents (x 5 0.005, 0.01, 0.03, and 0.05) were fabricated by mechanical alloying combined with spark plasma sintering. The phase structure and microstructure of all the bulk samples were checked by X-ray diffraction (XRD) and field emission scanning electron microscopy respectively. The thermoelectric transport properties, such as electrical conductivity, Seebeck coefficient, carrier concentration, carrier mobility, and thermal conductivity, were measured. The effect of second phase introduced by Sn addition on the thermoelectric properties of Cu–S system was investigated. The thermoelectric properties of samples were improved by the precipitations of two different second phases (Cu2SnS3 and Cu4SnS4). The second phase species depend on the Sn contents. Finally, the Sn0.01Cu1.79S bulk sample obtained the highest ZT value of 0.81 at 773 K, which is 1.6-fold higher than that of the pristine Cu1.8S sample due to the significantly reduced thermal conductivity by second phase and nanopores scattering.

I. INTRODUCTION

Fossil fuels are becoming increasingly depleted, human demand for energy has increased drastically, and energy has become a worldwide problem facing the 21st century, making improving the comprehensive utilization of existing energy sources and finding new renewable energy urgent. Thermoelectric (TE) devices with n-type and p-type TE material with a lightweight structure; that are simple, environmentally friendly, safe, and stable, etc.; and have no moving parts, no maintenance, and no noise have attracted widespread attention in recent years.1–4 Dimensionless thermoelectric figure of merit (ZT) is used to measure the most important indicators of the performance of thermoelectric materials, where ZT 5 S2rT/j (S, r, j, and T represent the Seebeck coefficient, electrical conductivity, thermal conductivity, and absolute temperature, respectively). The electrical properties of the material (power factor S2r) are closely related; ideal thermoelectric material should have a high S absolute value, high r, and low j.5 Over the years, Bi2Te3-based alloys were used as room-temperature TE materials,6,7 PbQ (Q 5 Te, Se, or S) alloys,8,9 Skutterudite compounds were used as middle-temperature TE materials10,11; and SiGe alloy, oxides were used as high temperature TE materials.12,13 Unfortunately, the Contributing Editor: Terry M. Tritt a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2017.288

high-performance thermoelectric materials used commercially are limited by expensive and toxic raw materials, especially Pb and Te.6–9 Therefore, TE materials made of cheap, and earth-abundant elements such as SnSe,14,15 Cu2Se,16,17 Cu–S compounds,18–20 and AgCuSe21–23 have received increasing attention. Copper sulfides [Cu2
xS (0 # x # 1)] are superionic conductors with good conductivity, and according to varyi