Evaluation of Thermoelectric Properties of Cu3.21Bi4.79S9 Bismuth Chalcogenide

Cu3.21Bi4.79S9 was synthesized from Cu, Bi and S element powders using mechanical alloying method. The formation of Cu3.21Bi4.79S9 was identified using XRD and the changes of morphologies of the mixtures of Cu, Bi, and S powders during milling were observed using table top SEM. The milled powders were sintered using Hot-isostatic pressing at 230°C with a pressure of 50 MPa. Electrical resistivity and Seebeck coefficient of sintered samples were measured using ZEM-3 (Electrical resistivity and Seebeck Coefficient measuring System). Cu3.21Bi4.79S9 and some secondary phases were found in the 5h milled powder but single phase Cu3.21Bi4.79S9 was only obtained after milling for 15 h. A minimum electrical resistivity of sintered Cu3.21Bi4.79S9 sample was found to be 0.66 Ω.m at 170°C. We observed that a n- to p-type conversion at temperature of around 75 °C. However, a maximum n-type Seebeck coefficient of Cu3.21Bi4.79S9 was of -214 μV/K at 45 °C. The Seebeck coefficient decreases with increasing temperature and it reaches zero value at around 75 °C and then p-type Seebeck coefficient increases with increasing the temperature. The maximum p-type Seebeck coefficient was observed of 202 μV/K at 170°C.