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

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.