Control of anisotropic electrical transport property of Bi2S3 thermoelectric polycrystals

After fabricating Bi2S3 nanorods with preferential (001) orientation by hydrothermal synthesis (HS) using Bi(NO3)(3) and Na2S2O3 as raw materials and ethylenediamine (EDA) as a pH modifier, Bi2S3 bulk materials were consolidated by spark plasma sintering (SPS). A single-phase, rod-like Bi2S3 crystal with a length in range of 2-10 mu m and a diameter in range of 100-400 nm was formed by optimizing the hydrothermal process parameters. The rod-like structures were maintained in the Bi2S3 bulk, and as a result the orientation degree of the bulk sample prepared from Bi2S3 nanorods powders reached 0.69, being higher than 0.18 found for mechanical alloying (MA)-derived powder. The measurement of electric transport properties also confirmed that the Bi2S3 bulk has obvious anisotropy. The electric resistivity normal to the pressing direction is one order higher than that in parallel to the pressing direction due to their anisotropic orientation. The power factor of the bulk sample along the direction perpendicular to the pressing direction is twice higher than that in parallel to the pressing direction, and also higher than the sample using the MA-processed powders.