Abstract
Bi-Sb alloy, as a promising thermoelectric material at cryogenic temperatures, has seen stagnant progress due to challenges in understanding the transport behaviors of energy carriers, and difficulties in synthesizing high-homogeneity, large-grain samples. In this study, an inherent electron-phonon decoupling in Bi-Sb is revealed using the first-principles calculations based on the virtual crystal approximation. The mean free path of the dominant electrons (λ(ele) ~ 10(3) nm) is found of two orders of magnitude larger than that of phonons (λ(ph) ~ 10(1 )nm), suggesting that a grain size greater than 10 μm would be favorable for thermoelectric transport. Bulk Bi-Sb polycrystals with highly elemental homogeneity and large grain size (~80 μm) are successfully synthesized through an ultra-fast quenching method combined with annealing, delivering superior thermoelectric performance. A prototype module based on the Bi(0.88)Sb(0.12) polycrystal, with a ZT(max) of 0.48 at 150 K, is fabricated and demonstrates a ΔT(max) of 4 K at a T(h) of 75 K. This marks the first report of n-p paired thermoelectric cooling modules operating below liquid nitrogen temperature.