Enhanced thermoelectric properties of bismuth sulfide polycrystals prepared by mechanical alloying and spark plasma sintering

Bismuth sulfide powders were synthesized by mechanical alloying (MA) and then consolidated by spark plasma sintering (SPS) technique. In order to improve the electrical transport properties of bismuth sulfides, the carrier concentration was optimized by modifying chemical composition of sulfur through producing sulfur vacancies, and the carrier mobility was enhanced by a two-step SPS as a hot-forging process through increasing grain orientation. The electrical resistivity of bismuth sulfides was reduced to 10^-4 from 10^-2 Ω m by optimizing sulfur content, and further lowered by hot-forging, whereby the power factor was significantly increased from 91 to 254 μW/mK². The hot-forged Bi₂S_2.90 sample showed the highest ZT=0.11 (at 523 K), which is higher than the reported value. The present work revealed that bismuth sulfide compounds as a promising candidate of thermoelectric materials can be synthesized by a simple process.