Hierarchical Structures Advance Thermoelectric Properties of Porous n-type β-Ag₂Se

Owing to the intrinsically good near-room-temperature thermoelectric performance, β-Ag₂Se has been considered as a promising alternative to n-type Bi₂Te₃ thermoelectric materials. Herein, we develop an energy- and time-efficient wet mechanical alloying and spark plasma sintering method to prepare porous β-Ag₂Se with hierarchical structures including high-density pores, a metastable phase, nanosized grains, semi-coherent grain boundaries, high-density dislocations, and localized strains, leading to an ultralow lattice thermal conductivity of ∼0.35 W m^-1 K^-1 at 300 K. A relatively high carrier mobility is obtained by adjusting the sintering temperature to obtain pores with an average size of ∼260 nm, therefore resulting in a figure of merit, zT, of ∼0.7 at 300 K and ∼0.9 at 390 K. The single parabolic band model predicts that zT of such porous β-Ag₂Se can reach ∼1.1 at 300 K if the carrier concentration can be tuned to ∼1 × 10¹⁸ cm^-3, suggesting that β-Ag₂Se can be a competitive candidate for room-temperature thermoelectric applications.