Recently a significant figure-of-merit (ZT) improvement in the most-studied existing thermoelectric materials has been achieved by creating nanograins and nanostructures in the grains using the combination of high-energy ball milling and a direct-current-induced hot-press process. Thermoelectric transport measurements, coupled with microstructure studies and theoretical modeling, show that the ZT improvement is the result of low lattice thermal conductivity due to the increased phonon scattering by grain boundaries and structural defects. In this article, the synthesis process and the relationship between the microstructures and the thermoelectric properties of the nanostructured thermoelectric bulk materials with an enhanced ZT value are reviewed. It is expected that the nanostructured materials described here will be useful for a variety of applications such as waste heat recovery, solar energy conversion, and environmentally friendly refrigeration.

Enhancement of Thermoelectric Figure-of-Merit by a Bulk Nanostructuring Approach

Combining high energy ball-milling and hot-pressing, significant enhancements of the thermoelectric figure-of-merit (ZT) have been reported for p-type Bi0.4Sb1.6Te3 nanocomposites. However, applying the same technique to n-type Bi2Te2.7Se0.3 showed no improvement on ZT values, due to the anisotropic nature of the thermoelectric properties of n-type Bi2Te2.7Se0.3. Even though texturing was effective in improving peak ZT of Bi2Te2.7Se0.3 from 0.85 to 1.04, reproducibility from batch to batch remains unsatisfactory. Here, we show that good reproducibility can be achieved by introducing an optimal concentration of 0.01 copper (Cu) per Bi2Te2.7Se0.3 to make Cu0.01Bi2Te2.7Se0.3 samples. A peak ZT value of 0.99 was achieved in Cu0.01Bi2Te2.7Se0.3 samples without texturing. With texturing by re-pressing, the peak ZT was increased to 1.06. Aging in air for over 5 months did not deteriorate but further improved the peak ZT to 1.10. The mechanism by which copper improves the reproducibility, enhances the carrier mobility, and reduces the lattice thermal conductivity is also discussed.

Thermoelectric Property Studies on Cu-Doped n-type CuxBi2Te2.7Se0.3 Nanocomposites

By ball milling alloyed bulk crystalline ingots into nanopowders and hot pressing them, we had demonstrated high figure-of-merit in nanostructured bulk bismuth antimony telluride. In this study, we use the same ball milling and hot press technique, but start with elemental chunks of bismuth, antimony, and tellurium to avoid the ingot formation step. We show that a peak ZT of about 1.3 in the temperature range of 75 and 100 °C has been achieved. This process is more economical and environmentally friendly than starting from alloyed bulk crystalline ingots. The ZT improvement is caused mostly by the lower thermal conductivity, similar as the case using ingot. Transmission electron microscopy observations of the microstructures suggest that the lower thermal conductivity is mainly due to the increased phonon scattering from the increased grain boundaries of the nanograins, precipitates, nanodots, and defects. Our material also exhibits a ZT of 0.7 at 250 °C, similar to the value obtained when ingot was used. This study demonstrates that high ZT values can be achieved in nanostructured bulk materials with ball milling elemental chunks, suggesting that the approach can be applied to other materials that are hard to be made into ingot, in addition to its advantage of lower manufacturing cost.

https://www.bc.edu/content/dam/files/schools/cas_sites/physics/pdf/Ren/Nanoletters%208%202580-2584%202008.pdf

Enhanced thermoelectric figure-of-merit in p-type nanostructured bismuth antimony tellurium alloys made from elemental chunks.

Defects and defect engineering are at the core of many regimes of material research, including the field of thermoelectric study. The 60-year history of V2VI3 thermoelectric materials is a prime example of how a class of semiconductor material, considered mature several times, can be rejuvenated by better understanding and manipulation of defects. This review aims to provide a systematic account of the underexplored intrinsic point defects in V2VI3 compounds, with regard to (i) their formation and control, and (ii) their interplay with other types of defects towards higher thermoelectric performance. We herein present a convincing case that intrinsic point defects can be actively controlled by extrinsic doping and also via compositional, mechanical, and thermal control at various stages of material synthesis. An up-to-date understanding of intrinsic point defects in V2VI3 compounds is summarized in a (χ, r)-model and applied to elucidating the donor-like effect. These new insights not only enable more innovative defect engineering in other thermoelectric materials but also, in a broad context, contribute to rational defect design in advanced functional materials at large.

https://onlinelibrary.wiley.com/doi/epdf/10.1002/advs.201600004

New Insights into Intrinsic Point Defects in V2VI3 Thermoelectric Materials

The thermoelectric properties of the p-type (Bi0.25Sb0.75)2Te3 doped with 8 wt. % excess Te and the n-type Bi2(Te0.94Se0.06)3 doped substantially with 0.07 wt. % I, 0.02 wt. % Te, and 0.03 wt. % CuBr which were grown by the Bridgman method at a rate of 6 cm/h were measured before and after annealing, where annealing was done in the temperature range from 473 up to 673 K for 2–5 h in a vacuum and a hydrogen stream. No annealing effect on the power factor was observed for the p-type specimen, but the as-grown p-type specimen exhibited a large power factor of 5.53×10−3 W/mK2 at 308 K and a low thermal conductivity of 1.21 W/mK. When the n-type specimen was annealed at 473 K for 2 h in a hydrogen stream, however, the power factor at 308 K increased significantly from 3.26×10−3 to 4.73×10−3 K−1 but its thermal conductivity then increased by about 3% from 1.26 to 1.30 W/mK. As a result, the maximum thermoelectric figure of merits Z at 308 K for the as-grown p- and annealed n-type specimens reached surprisingly ...

Bismuth telluride compounds with high thermoelectric figures of merit

Thermoelectric properties of the hot-pressed p-type (Bi0.25Sb0.75)2Te3 alloy were characterized with variation of the hot-pressing temperature and the starting powder size. The roles of the factors which affect the Seebeck coefficient of the hot-pressed (Bi0.25Sb0.75)2Te3 alloy has been elucidated in this study. The donor-like behavior of oxygen could be one of the possible explanations for the higher Seebeck coefficient of the hot-pressed (Bi0.25Sb0.75)2Te3 alloy. Te vacancies formed by mechanical deformation during the powdering process significantly promote the diffusion of second phase Te atoms into their lattice sites so that the matrix Te solubility approaches its equilibrium value at a given temperature in a relatively short length of time. Using the Seebeck coefficient at various hot-pressing temperatures, the micro-phase diagram near the stoichiometric composition of (Bi0.25Sb0.75)2Te3 was evaluated.

Thermoelectric properties of (Bi0.25Sb0.75)2Te3 alloys fabricated by hot-pressing method

Effect of excess te addition on the thermoelectric properties of the 20% Bi2Te3-80% Sb2Te3 single crystal and hot-pressed alloy

The temperature dependence of the Hall mobility, Seebeck coefficient, electrical resistivity, thermal conductivity, and figure-of-merit of the SbI3 and CuBr-doped 85% Bi2Te3-15% Bi2Se3 single crystals have been characterized at temperatures ranging from 77 K to 600 K. The scattering parameter in 85% Bi2Te3-15% Bi2Se3 single crystal was determined as 0.1 from the temperature dependence of the carrier mobility. With increasing the amount of Sbl3 or CuBr doping, the Seebeck coefficient of 85% Bi2Te3-15% Bi2Se3 decreased and the temperature at which the Seebeck coefficient shows a maximum shifted to higher temperature. Compared to the Sbl3-doped specimens, the CuBr-doped single crystals exhibited higher (m* / m0)3/2 μc, implying that CuBr is a more effective dopant to improve the material factor and thus the figure-of-merit of 85% Bi2Te3-15% Bi2Se3. The maximum figure-of-merit of 2.0 × 10−3/K and 2.2 × 10−3/K was obtained for 0.1 wt % Sbl3-doped specimen and 0.03 wt % CuBr-doped specimen, respectively.

Thermoelectric properties of the n-type 85% Bi2 Te3-15% Bi2 Se3 alloys doped with Sbl3 and CuBr

Electrical and thermoelectric properties of 90% Bi2Te3-5% Sb2Te3-5% Sb2Se3 single crystals doped with SbI3

Bi/sub 2/Te/sub 3/-Sb/sub 2/Te/sub 3/ alloys were prepared by the mechanical alloying method and sintered at the temperatures ranging from 50 to 550/spl deg/C to ensure the phase equilibrium at low temperatures. A micro-phase diagram near the stoichiometric composition was evaluated with the carrier concentration which was estimated from the values of the Seebeck coefficient. Singular properties of the Seebeck coefficient and the electrical resistivity observed at the composition of 20% Bi/sub 2/Te/sub 3/-80% Sb/sub 2/Te/sub 3/ were also discussed.

Jong-Seung Hwang

Papers

Thermoelectric properties of (Bi0.25Sb0.75)2Te3 alloys fabricated by hot-pressing method

Effect of excess te addition on the thermoelectric properties of the 20% Bi2Te3-80% Sb2Te3 single crystal and hot-pressed alloy

Thermoelectric properties of the n-type 85% Bi2 Te3-15% Bi2 Se3 alloys doped with Sbl3 and CuBr

Electrical and thermoelectric properties of 90% Bi2Te3-5% Sb2Te3-5% Sb2Se3 single crystals doped with SbI3

Evaluation of the micro-phase diagram near the stoichiometric composition of the Bi/sub 2/Te/sub 3/-Sb/sub 2/Te/sub 3/ alloys