EFFECTS OF BI DOPING ON MICROSTRUCTURE EVOLUTION OF AG-ALLOYED PBTE FOR THERMOELECTRIC POWER GENERATION

One of the directions applied in the search for new alternative energy sources is capturing waste heat and directly converting it into electrical power using thermoelectric (TE) generators, which are based on the Seebeck effect. Among the various TE compounds, PbTe has been considered to be one of the best materials applied for the mid-temperature range due to its high figure of merit, ZT.

A common way to improve the TE properties is additions of alloying elements with concentrations above the solubility limit. This enables formation of second phase precipitates, which reduce lattice thermal conductivity due to phonon scattering. Our approach is to enhance the TE properties of PbTe by alloying with 3.3 at. % Ag, with or without 0.2 or 0.4at. % Bi; herein, Ag-atoms serve as Ag₂Te-precipitate forming elements, whereas Bi atoms act as electron donors. We observe the microstructure evolution of both alloys for different aging times at 380 °C.

The thermal conductivity values of this system exhibit interesting behavior: whereas the specimens containing 0.4 at. % Bi show thermal conductivities that increase with aging time, their 0.2 at. % Bi counterparts exhibit reduction of thermal conductivity upon aging for 6 h, followed by increase of thermal conductivity upon over-aging for 48 h. We elucidate these observations in terms of interplay between strained, super-saturated PbTe-matrix attained for the as-quenched state and precipitation of Ag₂Te-phase for the aged states. We suggest that Bi atoms suppress the evolution of the Ag₂Te-precipitates, which stabilize TE performance against deterioration for long durations at service temperatures. We take advantage of these effects to tailor TE properties of PbTe.