Bulk Bi-Sb polycrystals underpinned by high electron/phonon mean free path ratio enabling thermoelectric cooling under 77 K
Xiaowei Wu, Zhen Fan, Hangtian Zhu, Tianyu Wang, Meng Liu, Jun Li, Nan Chen, Qiulin Liu, Zhen Lu, Guodong Li, Xin Qian, Te‐Huan Liu, Ronggui Yang, Xiaoyan Niu, Qi Zhao, Zhiliang Li, Shufang Wang, Huaizhou Zhao
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 (λele ~ 103 nm) is found of two orders of magnitude larger than that of phonons (λph ~ 101 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 Bi0.88Sb0.12 polycrystal, with a ZTmax of 0.48 at 150 K, is fabricated and demonstrates a ΔTmax of 4 K at a Th of 75 K. This marks the first report of n-p paired thermoelectric cooling modules operating below liquid nitrogen temperature. The authors reveal nearly two-order magnitude of decoupling of the mean free path of the dominant electrons to that of phonons in Bi-Sb alloy, achieving an atomically homogeneous, coarse-grained sample through ultrafast quenching, enabling thermoelectric cooling down to the liquid nitrogen regime.