Method of Efficient Ag Doping for Fermi Level Tuning of Thermoelectric Bi0.5Sb1.5Te3 Alloys Using a Chemical Displacement Reaction

Silver is a well-known element for use as a p-type dopant in Bi2Te3-related compounds. In this paper, an efficient method for incorporating ultralow Ag dopant concentrations (<1300 ppm) into Bi0.5Sb1.5Te3 via a simple chemical displacement reaction is described. Powders of Bi0.5Sb1.5Te3 synthesized by mechanical alloying were reacted with Ag ions in dilute HNO3 solutions (pH 0.2), resulting in the deposition of Ag on the surface of the powders due to the difference in reduction potential between Ag and Bi0.5Sb1.5Te3. The Ag/Bi0.5Sb1.5Te3 powders were then sintered by spark plasma sintering (SPS), and the thermoelectric properties of the dense Ag-doped samples were measured. Low Ag-doped samples showed behavior characteristic of partially degenerate semiconductors, while highly doped specimens exhibited properties associated with fully degenerate semiconductors. From the measured transport properties and theoretical estimations, successful tuning of the Fermi level with Ag was confirmed. Consequently, the temperature at which the peak dimensionless figure of merit (ZT) value was obtained increased from SO to 250 degrees C. Such findings may be beneficial in the utilization of waste heat over a wide temperature range, as the Ag-doped samples could be employed as functionally graded materials for thermoelectric modules.