Synthesis and Thermoelectric Properties of Cu-Sb-S Compounds
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The Cu-Sb-S compounds (Cu12Sb4S13, CuSbS2, Cu3SbS3 and Cu3SbS4) have the advantages of earth-abundance, low-toxicity and low-cost, compared with conventional thermoelectric materials. This work provides a comprehensive study on the synthesis methods, crystal structures and thermoelectric properties of Cu-Sb-S compounds. All of the samples were prepared by mechanical alloying combined with SPS, which had high density, high purity and very fine microstructure. The lone-pair electrons of Sb and the [CuS3] plane play important roles in realizing very low lattice thermal conductivity of these compounds. Except for Cu12Sb4S13, which is known as a good thermoelectric material, the other three compounds showed very poor thermoelectric performance due to their high electrical resistivities. A phase transition at 398 K was found in Cu3SbS3, which makes it unsuitable for applications and attempts to optimize electrical properties of CuSbS2 failed. Different p-type dopants were studied to improve the electrical properties of Cu3SbS4. Both Ge-doping and Sn-doping on Sb sites increased the carrier concentration of Cu3SbS4 significantly. The electrical transport properties were analyzed using SPB model, and a large effective mass of 3.0 me was found for all of the samples. A maximum zT value of 0.69 was obtained at 623 K in 5 mol. % Sn-doped sample which was about 6 times higher than that of undoped sample. The solid-solutions of Cu3SbS4(1-y)Se4y were studied to further improve the thermoelectric properties. The lattice thermal conductivity was reduced in solid-solution due to the local mass contrast and alloying scattering, but there was no further improvement in zT value due to the decrease in Seebeck coefficient. Another solid solution of Cu3Sb1-xBixS4 was studied, but Bi had very low solubility and a second phase was formed instead of forming the solid solution. Future work should focus on reducing the lattice thermal conductivity of Cu3SbS4 without impacting its electrical properties.
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