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Synthesis and Thermoelectric Studies of a Narrow Gap Semiconducting Alloy for Cooling: BiSb

Gao, Sheng
Thesis/Dissertation; Online
Gao, Sheng
Poon, Joseph
Thermoelectric (TE) technology plays a vital role in heat-to-electrical energy conversion and refrigeration. Bismuth antimony (Bi-Sb) is one of the most promising materials for thermoelectric cooling. Therefore my research target was focused on designing, preparing and characterizing Bi-Sb alloys to achieve a high figure of merit ZT. Herein a high figure of merit ZT near 0.6 at cryogenic temperatures (100-150K) in zero magnetic field has been measured in nanocrystalline bulk n-type Bi85Sb15, nearly 50% higher than the polycrystalline-averaged ZT ∼ 0.4 of single crystal Bi-Sb from Yim and Amith. Magneto-thermoelectric effects of this nanobulk Bi-Sb further improved the TE properties. ZT was enhanced to ∼0.7 in a portable and compact BiSb/NdFeB material system. The improved thermoelectric properties can be attributed to the nanoscale microstructure achieved from rapid solidification, which not only significantly reduced the thermal conductivity but also avoided a segregation effect. A record low thermal conductivity of 1.5 W/m·K was measured using the “hot disk” method. The thermoelectric properties were analyzed within a two-band effective mass model, providing a deeper understanding of the transport mechanisms for this intriguing semimetal-semiconductor alloy system. The study revealed a gradual narrowing of the band gap at increasing temperature for the first time. The easily accessible low magnetic field for achieving high ZT enables the utilization of n-type BiSb in thermoelectric cooling applications. On the other hand, because of the demand of similar n-p TE legs in TE devices, great efforts were put into the investigation of doping effects to develop TE properties of p-type Bi-Sb system to build a similar counterpart to the intrinsic n-type Bi-Sb. Transition metals were used to suppress the electron mobility for a stronger p-type performance. One of the best ZT so far ∼ 0.13 around 200K was achieved with Ni and Pb co-doped p-type Bi-Sb alloy. The gap decreasing behavior mentioned before could be a heretofore unrealized obstacle to achieving a high thermoelectric figure of merit in p-type Bi-Sb alloys comparing with the n-type counterpart. After all, the present nano-structured Bi-Sb alloys can already be readily utilized in thermoelectric cooling applications. This thesis started with the introduction of basic concepts of TE physics followed by a detailed description of experiments procedure and analysis of measurements data.
University of Virginia, Department of Physics, PHD (Doctor of Philosophy), 2019
Published Date
PHD (Doctor of Philosophy)
Sponsoring Agency
Defense Advanced Research Projects Agency MATRIX Program
Libra ETD Repository
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