The best thermoelectric materials are believed to be heavily doped semiconductors. The presence of a band gap is assumed to be essential to achieve large thermoelectric power factor and figure of merit. In this work, we propose semi-metals with large asymmetry between conduction and valence bands as an alternative class of thermoelectric materials. To illustrate the idea, we study semi-metallic HgTe in details experimentally and theoretically. We employ ab initio calculations with hybrid exchange-correlation functional to accurately describe the electronic band structure in conjunction with the Boltzmann Transport theory to investigate the electronic transport properties. We calculate the lattice thermal conductivity using first principles calculations and evaluate the overall figure of merit. To validate our theoretical approach, we prepare semi-metallic HgTe samples and characterize their transport properties. Our first-principles calculations agree well with the experimental data. We show that intrinsic HgTe, a semimetal with large disparity in its electron and hole masses, has a high thermoelectric power factor that is comparable to the best known thermoelectric materials. Finally, we propose other possible materials with similar band structures as potential candidates for thermoelectric applications.

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 (100–150 K) has been achieved in melt-spun n-type Bi85Sb15 bulk samples consisting of micron-size grains. The achieved ZT is nearly 50% 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 150 K 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.

We have investigated the doping effects of elements (Sn, Pb, Ge) with valence 4 in Bi-Sb alloys. The solubilities of these elements in Bi-Sb alloy are known to range from ∼3 to ∼0.5 to zero atomic percent, respectively. We employed a rapid solidification method to prepare samples that allowed us to achieve a high doping level of Ge and a high doping efficiency of Pb in Bi85Sb15. Using this method, unprecedented high Ge content (up to 13 atomic percent) could be solutionized in Bi-Sb. Microstructure analyses and thermoelectric transport property measurements have been performed on the doped samples which all showed p-type properties at cryogenic temperatures. The doping efficiency of the elements (Sn, Pb, Ge) was calculated based on the Hall measurements and discussed in light of donor and acceptor sites in Bi-Sb. A two-band effective mass model was used to simulate the thermoelectric transport properties within the Boltzmann transport theory. The present study revealed a gap-decreasing phenomenon in the undoped and doped samples. This gap-decreasing behavior is a heretofore unrealized obstacle to achieving a high thermoelectric figure of merit in p-type Bi-Sb alloys.

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