Boosting thermoelectric efficiency of Ag₂Se through cold sintering process with Ag nano-precipitate formation

Abstract

Silver selenide (Ag₂Se) stands out as a promising thermoelectric (TE) material, particularly for applications near room temperatures. This research presents a novel approach for the fabrication of bulk Ag₂Se samples at a relatively low temperature (170°C) using the cold sintering process (CSP) with AgNO₃ solution as a transient liquid agent. The effect of AgNO₃ addition during CSP on the microstructure and TE properties was investigated. The results from phase, composition and microstructure analyses showed that the introduction of AgNO₃ solution induced the formation of Ag nano-precipitates within the Ag₂Se matrix. Although the nano-precipitates do not affect the phase and crystal structure of orthorhombic β-Ag₂Se, they suppressed crystal growth, leading to reduced crystallite sizes. The samples containing Ag nano-precipitates also exhibited high porosity and low bulk density. Consequently, these effects contributed to significantly enhanced electrical conductivity and a slight decrease in the Seebeck coefficient when small Ag concentrations were incorporated. This resulted in an improved average power factor from ∼1540 µW·m⁻¹·K⁻² for pure Ag₂Se to ∼1670 µW·m⁻¹·K⁻² for Ag₂Se with additional Ag precipitates. However, excessive Ag addition had a detrimental effect on the power factor. Furthermore, thermal conductivity was effectively suppressed in Ag₂Se fabricated using AgNO₃-assisted CSP, attributed to enhanced phonon scattering at crystal interfaces, pores, and Ag nano-precipitates. The highest figure-of-merit (zT) of 0.92 at 300 K was achieved for the Ag₂Se with 0.5wt% Ag during CSP fabrication, equivalent to >20% improvement compared to the controlled Ag₂Se without extra Ag solution. Thus, the process outlined in this study presents an effective strategy to tailor the microstructure of bulk Ag₂Se and enhance its TE performance at room temperature.

Keywords

thermoelectric / silver selenide / chalcogenide / cold sintering process / nano-precipitate

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