Enhanced Cryogenic Thermoelectric Performance of Textured Bi_1–<i>x</i>Sb_<i>x</i> Ribbons with Electronic Phase Transition

Bi_1-<i>x</i>Sb_<i>x</i> alloys are promising cryogenic thermoelectric materials for generator and refrigeration devices at temperatures below 200 K. Herein, we prepared highly (00<i>l</i>) textured Bi_1-<i>x</i>Sb_<i>x</i> (<i>x</i> = 0-0.05) ribbons by a melt-spinning technique and tuned its band structure with a Dirac electronic phase transition via Sb doping for improving the thermoelectric performance. The results indicate that the lamellar grains with (00<i>l</i>) orientation facilitate the alignment of the Fermi pocket of ribbon samples and cause a higher Seebeck coefficient compared with the nonoriented Bi_1-<i>x</i>Sb_<i>x</i> bulk. Meanwhile, the Fermi level of Bi_1-<i>x</i>Sb_<i>x</i> ribbons moves down by Sb doping, inducing the decrease of the carrier concentration and the increment of the Seebeck coefficient. Particularly, the Dirac electron phase is modulated when <i>x</i> reaches 0.04, which enlarges the carrier mobility and results in a well-maintained conductivity. Therefore, the optimized transport properties yield a large power factor of 61.1 μW·cm^-1·K^-2 at 140 K for the <i>x</i> = 0.04 sample, a significant 65% increase compared to the <i>x</i> = 0 ribbon. Besides, a planar thermoelectric device composed of 8 legs was assembled with the optimized ribbon, which produces a high open-circuit voltage of 39.8 mV. The output power maximum and the corresponding power density reach up to 402 nW and 125.7 μW·cm^-2 under a temperature gradient of 80 K, respectively. Our work suggests that modulating the Dirac phase transition can effectively enhance the thermoelectric performance of Bi_1-<i>x</i>Sb_<i>x</i> alloys.