Enhancing thermoelectric properties of silver selenide through cold sintering process using aqua regia as a liquid medium

Silver selenide (Ag2Se) is a promising thermoelectric material for near-room temperature energy conversion. This research developed an innovative approach to enhance the thermoelectric properties of Ag2Se through low-temperature cold sintering process (CSP). Aqua regia, a mixture of hydrochloric and nitric acids, was employed as a liquid medium in CSP, facilitating the localized atomic rearrangement and bonding at intergranular interfaces between Ag2Se particles. This yielded a densely packed sample with merged sub-micron grains and coherent interfaces. Aqua regia-assisted CSP notably improved thermoelectric properties compared to a reference sample without aqua regia. Carrier concentration decreased and carrier mobility increased for the CSP sample with 0.1 M aqua regia, leading to the enhanced electrical conductivity, Seebeck coefficient, and the maximum power factor of 2.13 mWm−1K−2. The addition of aqua regia in CSP consolidation also suppressed phonon thermal conductivity due to sub-micron pores and nanoprecipitates, resulting in low thermal conductivity of 0.5–0.7 Wm−1K−1, and significantly enhanced figure-of-merit (zT) reaching 1.13. Remarkably, the zT of the CSP Ag2Se was very stable against temperatures (±6 % fluctuations), with an average value of 1.06. These findings highlight CSP with aqua regia as an effective route to fabricate high-performance thermoelectric Ag2Se for near-room temperature applications.