Improvements in the thermoelectric performance of n-type Bi2Te3 materials to more closely match their p-type counterparts are critical to promote the continued development of bismuth telluride thermoelectric devices. Here the unconventional heteroatom dopant, niobium, has been employed as a donor in Bi2Te3. Nb substitutes for Bi in the rhombohedral Bi2Te3 structure and exhibits multiple roles in its modulation of electrical transport and defect-induced phonon scattering. The carrier concentration is significantly increased as electrons are afforded by aliovalent doping and formation of vacancies on the Te sites. In addition, incorporation of Nb in the pseudoternary Bi2–x Nb x Te3−δ system increases the effective mass, m*, which is consistent with cases of “conventional” elemental doping in Bi2Te3. Lastly, inclusion of Nb induces both point and extended defects (tellurium vacancies and dislocations, respectively), enhancing phonon scattering and reducing the thermal conductivity. As a result, an optimum zT of 0.94 was achieved in n-type Bi0.92Nb0.08Te3 at 505 K, which is dramatically higher than an equivalent undoped Bi2Te3 sample. This study suggests not only that is Nb an exciting and novel electron dopant for the Bi2Te3 system but also that unconventional dopants might be utilized with similar effects in other chalcogenide thermoelectrics.
Polycrystalline BiCuSeO is considered as a promising thermoelectric material due to its intrinsically low thermal conductivity and moderate Seebeck coefficient. However, its low electrical conductivity and coupled electron−phonon transport properties restrict the further improvement of the thermoelectric performance. In this work, Pb and Yb dopants are incorporated into BiCuSeO to substitute for Bi sites via ball milling and high-pressure and high-temperature sintering, leading to a synergistic optimization of the electron and phonon transport and improved thermoelectric performance. The carrier concentration exhibits an enhancement with increasing Pb&Yb co-doping contents. Meanwhile, the decreased carrier mobility is suppressed appropriately by coordinating with the interplay of Pb and Yb dopants on the electronic structure. Besides, Pb&Yb co-doping combined with high-pressure and high-temperature sintering introduces abundant grain boundaries, dislocations, and point defects to effectively decrease the lattice thermal conductivity by scattering phonons in a broad frequency range. Coupled with the synergistic optimization of the electrical and thermal properties, a maximum zT of 1.2 is achieved in Bi 0.88 Pb 0.06 Yb 0.06 CuSeO at 850 K, which significantly outperforms the majority of oxygen-containing thermoelectric materials. Our study suggests that dual doping of bivalent ions and rare-earth elements at Bi sites is an effective strategy for improving the thermoelectric performance of BiCuSeO.
Pressure is a fundamental thermodynamic variable that can create exotic materials and modulate transport properties, motivating prosperous progress in multiple fields. As for inorganic thermoelectric materials, pressure is an indispensable condition during a preparation process, which is employed to compress raw powders into the specific shape of solid-state materials for performing properties characterization. In addition to this function, the extra influence of pressure on thermoelectric performance is frequently underestimated and even overlooked. In this review, we summarize recent progress and achievements of pressure-induced structure and performance in thermoelectrics, emphatically involving the modulation of pressure on crystal structure, electrical transport properties, microstructure, and thermal conductivity. According to various studies, the modulated mechanism of pressure on these items above has been discussed in detail, and the perspectives and strategies have been proposed with respect to applying pressure to improve thermoelectric performance. Overall, the purpose of the review is supposed to enrich the understanding of the mechanisms in pressure-induced transport properties and provide a guidance to rationally design a structural pattern to improve thermoelectric performance.
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