Liquid-Phase Hot Deformation to Enhance Thermoelectric Performance of n-type Bismuth-Telluride-Based Solid Solutions.

Yehao Wu,Xinbing Zhao,Tiejun Zhu,Yuan Yu,Qi Zhang,Renshuang Zhai

doi:10.1002/advs.201901702

Yehao Wu, Xinbing Zhao + Show 4 more

Open Access

https://doi.org/10.1002/advs.201901702

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Abstract

Bismuth‐telluride‐based solid solutions are the best commercial thermoelectric materials near room temperature. For their n‐type polycrystalline compounds, the maximum figures of merit (zTs) are often less than 1.0 due to the degraded carrier mobility resulting from the loss of texture. Herein, a liquid‐phase hot deformation procedure, during which the Bi2(Te,Se)3 ingots are directly hot deformed with the extrusion of liquid eutectic phase, is performed to enhance the thermoelectric performance of n‐type Bi2(Te,Se)3 alloys. The deformation‐induced dynamic recrystallization is remarkably suppressed due to the reduction of nucleation sites and the release of deformation stress by liquid phase, contributing to a weakened carrier scattering and enhanced carrier mobility. The liquid eutectic phase also facilitates the rotation of grains and enhanced (000l) texture, further improving carrier mobility. In addition, the dense dislocations and lattice distortion introduced into the matrix reduce the lattice thermal conductivity. As a result, a high zT value of 1.1 at 400 K is obtained, about 75% increment over the normal one‐step hot deformed alloys. This work not only demonstrates a simple and efficient technique for achieving superior n‐type Bi2Te3‐based materials, but also elucidates the important role of liquid eutectic phase in hot deformation.

Highlights

Bismuth-telluride-based solid solutions are the best commercial thermoelectric materials near room temperature
The X-ray diffraction (XRD) patterns in Figure 1a indicate that the Bi2Te2.7Se0.3 melted ingot has a pure rhombohedral R3m phase, while the Bi2Te2.7Se0.3 + 16 wt% Te-melted ingot has a little Te second phase remaining in the Bi2Te3 matrix
The Te-rich eutectic phase is introduced into the Bi2(Te,Se)3 ingot at the melting stage and plays important roles in subsequent hot deformation stage

Summary

Results and Discussion

The X-ray diffraction (XRD) patterns in Figure 1a indicate that the Bi2Te2.7Se0.3 melted ingot (named as M-0Te) has a pure rhombohedral R3m phase, while the Bi2Te2.7Se0.3 + 16 wt% Te-melted ingot (named as M-16Te) has a little Te second phase remaining in the Bi2Te3 matrix. Note that, compared to the HD-0Te sample, the number of recrystallized grains is significantly decreased in the LPHD sample (see Figure 6d and Figure S6 in the Supporting Information) combined with a reduction of total boundary length from 2.15 m (Figure 6a) to 1.58 m (Figure 6c) in an area of 4 mm by 6 mm, which is beneficial for the reduction of carrier scattering and enhancement of μH. As mentioned above, both the diminution of recrystallized grains and enhancement of texture during the LPHD process are beneficial for the increase in μH. ZTs in some multiple HD alloys are slightly higher than zT in the LPHD sample, the process flow of one-step LPHD technique is much shorter, which makes it a more efficient and energy-saving route for large-scale production of superior n-type Bi2(Te,Se) materials

Conclusion

Experimental Section

Conflict of Interest