  Thermoelectric (TE) materials research plays a vital role in
heat-to-electrical energy conversion and refrigeration applications.
Bismuth-antimony (Bi-Sb) alloy is a promising material for thermoelectric
cooling. Herein, a high figure of merit, ZT, near 0.6 at cryogenic temperatures
(100-150K) has been achieved in melt-spun n-type Bi85Sb15 bulk samples
consisting of micron-size grains. The achieved ZT is nearly 50 percents higher
than polycrystalline averaged single crystal ZT of ~0.4, and it is also
significantly higher than ZT of less than ~0.3 measured below 150K in Bi-Te
alloys commonly used for cryogenic cooling applications. The improved
thermoelectric properties can be attributed to the fine-grained microstructure
achieved from rapid solidification, which not only significantly reduced the
thermal conductivity but also mitigated a segregation effect. A record low
thermal conductivity of ~1.5 W m-1 K-1 near 100 K was measured using the hot
disk method. The thermoelectric properties for this intriguing
semimetal-semiconductor alloy system were analyzed within a two-band effective
mass model. The study revealed a gradual narrowing of the band gap at
increasing temperature in Bi-Sb alloy for the first time.
Magneto-thermoelectric effects of this Bi-Sb alloy further improved the TE
properties, leading to ZT of about 0.7. The magneto-TE effect was further
demonstrated in a combined NdFeB/BiSb/NdFeB system. The compactness of the
BiSb-magnet system with high ZT enables the utilization of magneto-TE effect in
thermoelectric cooling applications.
