Thermoelectric properties of Ag-doped bismuth sulfide polycrystals prepared by mechanical alloying and spark plasma sintering

Single orthorhombic phase Bi2−xAgxS3 (x = 0–0.07) bulk materials were prepared by combining mechanical alloying (MA) with spark plasma sintering (SPS) technique. The microstructure and electrical transport properties were investigated with a special emphasis on the influence of Ag content. All the samples have a high density (91.1–95.9%) and homogeneous grains of about 100–500 nm. Optimizing the Ag content greatly improves the electrical conductivity and leads to a peak value of 2.8 × 103 S m−1 at 523 K for the Bi1.99Ag0.01S3 composition. The absolute value of Seebeck coefficient shows an inversely varying trend to the electrical conductivity with Ag content, whereby a power factor at 523 K by adjusting Ag content is enhanced from 91 μW mK−2 for Bi2S3 to 235 μW mK−2 for Bi1.99Ag0.01S3. The thermal conductivity of Bi2−xAgxS3 ranges from 0.48 to 0.73 W mK−1, which is lower than the reported values of Bi2S3 compounds. An enhanced maximum ZT value 0.25 is achieved at 573 K for the Bi1.99Ag0.01S3 sample, which is the highest value in Bi2S3 system in literature. The present work reveals that the non-toxic and cheap Bi2S3 based compound is a promising candidate for the thermoelectric applications.