Abstract
Bismuth telluride based compounds have been widely employed in commercial thermoelectric cooling/generation devices owing to their decent thermoelectric properties near room temperature regime. Some trace elements, such as iodine, silver and copper, are commonly added to modulate transport properties of bismuth telluride based compounds. In addition, the figure of merit of thermoelectric materials can be effectively promoted by electrical stressing. In this study, Ag elements were driven into a piece of Bi0.5Sb1.5Te3 sample in contact to a silver foil when applying a high density of electric current at elevated temperature. We demonstrate that Ag concentration profile can be tailored unidirectionally by electrical stressing. The evolution of Ag concentration profile was measured by energy dispersive spectroscopy and modeled based on an electromigration-enhanced diffusion mechanism. It is found that the supersaturated Ag may react with excess Te by forming Ag2Te secondary phase near the grain boundaries. With continuous electrical stressing, Ag2Te will be decomposed by hole wind force. These Ag elements migrate from anode side to cathode side of the Bi0.5Sb1.5Te3 sample under an electric field and a hole wind driving force simultaneously. Due to multiple Ag defect sites, such as interstitials, Sb lattice and grain boundaries, Ag atoms may migrate through different paths and lead to a superposition of different diffusion pockets. A physical mechanism is proposed to explain the evolution of Ag concentration profiles in the electrically stressed Bi0.5Sb1.5Te3 compounds. The effects of electrical stressing and Ag doping on the defect mechanism and thermoelectric transport properties of Bi0.5Sb1.5Te3 compounds are investigated.