Engineering of Thermoelectric Composites Based on Silver Selenide in Aqueous Solution and Ambient Temperature

Abstract

The direct, solid state, and reversible conversion between heat and electricity using thermoelectric devices finds numerous potential uses, especially around room temperature. However, the relatively high material processing cost limits their real applications. Silver selenide (Ag 2 Se) is one of the very few n-type thermoelectric (TE) materials for room-temperature applications. Herein, we report a room temperature, fast, and aqueous-phase synthesis approach to produce Ag 2 Se, which can be extended to other metal chalcogenides. These materials reach TE figures of merit (zT) of up to 0.76 at 380 K. To improve these values, bismuth sulfide (Bi 2 S 3 ) particles also prepared in an aqueous solution are incorporated into the Ag 2 Se matrix. In this way, a series of Ag 2 Se/Bi 2 S 3 composites with Bi 2 S 3 wt % of 0.5, 1.0, and 1.5 are prepared by solution blending and hot-press sintering. The presence of Bi 2 S 3 significantly improves the Seebeck coefficient and power factor while at the same time decreasing the thermal conductivity with no apparent drop in electrical conductivity. Thus, a maximum zT value of 0.96 is achieved in the composites with 1.0 wt % Bi 2 S 3 at 370 K. Furthermore, a high average zT value (zT ave ) of 0.93 in the 300−390 K range is demonstrated.

A Recipe for a Great Meal: A Benchtop Route from Elemental Se to Superior Thermoelectric β-Ag2Se

“…15 Benefiting from its narrow band gap and complex crystal structure, β-Ag 2 Se exhibits high electrical conductivity, moderate Seebeck coefficient, high Hall mobility, and intrinsically low lattice thermal conductivity resulting in an overall high TE figure-of-merit (zT). 16 Despite these interesting properties, the material's sensitivity to stoichiometry and synthesis makes it challenging to tune TE performance. There have been a number of synthetic approaches developed to boost the TE performance of this material.…”

“…There have been a number of synthetic approaches developed to boost the TE performance of this material. zT values ranging from 0.3 to 1.2 at room temperature have been reported for bulk samples; however, achieving consistent zT values within the working temperature has been challenging. − The inconsistencies in zT both at room temperature and across measured temperatures have been attributed to the poor reproducibility of utilized synthetic methods and the coexistence of metastable phases with the orthorhombic structure, which mainly arise due to the migration of Ag ions at various temperatures …”

“…The orthorhombic chiral β-Ag 2 Se crystal structure is stable up to 134 °C; above this temperature, it transforms to the high-temperature cubic modification, α-Ag 2 Se, which is a superionic conductor with a high level of disorder in the Ag sublattice . Benefiting from its narrow band gap and complex crystal structure, β-Ag 2 Se exhibits high electrical conductivity, moderate Seebeck coefficient, high Hall mobility, and intrinsically low lattice thermal conductivity resulting in an overall high TE figure-of-merit ( zT ) . Despite these interesting properties, the material’s sensitivity to stoichiometry and synthesis makes it challenging to tune TE performance.…”

“…Moreover, substituted/doped phases often have a metastable nature and decompose when heated to high temperatures, resulting in the formation of a secondary phase of the dopant metal selenide as inclusions with adverse TE performance. Solution-based methods have recently been adopted as alternatives for synthesizing single-phase β-Ag 2 Se-based materials with decent TE performances. ,− However, ubiquitous use of these methods is hindered by toxicity of the starting materials, the bulky nature of the solvents/surfactants employed (which can also adversely impact the electronic transport of charge carriers), inconsistency in the product yield and quality, and difficulty in controlling the reaction rate. Samples made via these solution-based routes usually require further high-temperature annealing to improve material performance.…”