The abundance of low-temperature waste heat produced by industry and automobile exhaust necessitates the development of power generation with thermoelectric (TE) materials. Commercially available bismuth telluride-based alloys are generally used near room temperature. Materials that are composed of p-type bismuth telluride, which are suitable for low-temperature power generation (near 380 K), were successfully obtained through Sb-alloying, which suppresses detrimental intrinsic conduction at elevated temperatures by increasing hole concentrations and material band gaps. Furthermore, hot deformation (HD)-induced multi-scale microstructures were successfully realized in the high-performance p-type TE materials. Enhanced textures and donor-like effects all contributed to improved electrical transport properties. Multiple phonon scattering centers, including local nanostructures induced by dynamic recrystallization and high-density lattice defects, significantly reduced the lattice thermal conductivity. These combined effects resulted in observable improvement of ZT over the entire temperature range, with all TE parameters measured along the in-plane direction. The maximum ZT of 1.3 for the hot-deformed Bi 0.3 Sb 1.7 Te 3 alloy was reached at 380 K, whereas the average ZT av of 1.18 was found in the range of 300-480 K, indicating potential for application in low-temperature TE power generation. Keywords: bismuth telluride; donor-like effect; hot deformation; low-temperature power generation; texture INTRODUCTION Thermoelectric (TE) devices have attracted extensive interest over the past few decades because of their potential use in direct thermal-toelectrical energy conversion and solid-state refrigeration. The TE conversion efficiency of a material can be gauged by the dimensionless figure of merit ZT ¼ a 2 sT/k, where a, s, k and T are the Seebeck coefficient, the electrical conductivity, the thermal conductivity and the operating temperature, respectively. 1 Continuous effort has been invested toward improving the ZT values of TE materials, resulting in significant advances through phonon engineering 2-9 and band engineering. [10][11][12][13][14] For example, remarkable increases in ZT have been achieved in bulk nanomaterials via the enhancement of phonon scattering at boundaries to reduce lattice thermal conductivities. 2,4,6,7 Currently, the best commercial TE materials near room temperature are still rhombohedral bismuth tellurides and related solid solutions fabricated by unidirectional crystal growth. [15][16][17] Nanostructuring strategies have been devised to prepare highperformance bismuth telluride-based alloys, including bottom-up

Nanostructuring has proved effective in improving the figure of merit in the widely used Bi 2 Te 3 based thermoelectric materials. In this work, a hot deformation induced in situ nanostructuring process is directly applied to the commercial unidirectionally grown p-type Bi 0.5 Sb 1.5 Te 3 ingots to explore the possibility of commercial application of the "top down" nanostructuring approach, and the thermoelectric properties are investigated over a wide temperature range of 15 K to 520 K. In comparison to the commercial zone melted ingot and the hot pressed sample, it is found that the hot deformed samples exhibit much less texture and significantly reduced lattice thermal conductivity due to in situ formed nanostructures and defects. A high ZT of $1.3 is achieved near room temperature, $50% improvement compared to that of the zone melted ingot. The hot deformation process thus provides a promising top down approach to prepare high performance Bi 2 Te 3 based thermoelectric materials in a way that is more readily incorporated into the existing procedure of device manufacturing.

For decades, zone-melted Bi 2 Te 3 -based alloys have been the most widely used thermoelectric materials with an optimal operation regime near room temperature. However, the abundant waste heat in the mid-temperature range poses a challenge; namely, how and to what extent the service temperature of Bi 2 Te 3 -based alloys can be upshifted to the mid-temperature regime. We report herein a synergistic optimization procedure for Indium doping and hot deformation that combines intrinsic point defect engineering, band structure engineering and multiscale microstructuring. Indium doping modulated the intrinsic point defects, broadened the band gap and thus suppressed the detrimental bipolar effect in the mid-temperature regime; in addition, hot deformation treatment rendered a multiscale microstructure favorable for phonon scattering and the donor-like effect helped optimize the carrier concentration. As a result, a peak value of zT of~1.4 was attained at 500 K, with a state-of-the-art average zT av of~1.3 between 400 and 600 K in Bi 0.3 Sb 1.625 In 0.075 Te 3 . These results demonstrate the efficacy of the multiple synergies that can also be applied to optimize other thermoelectric materials. INTRODUCTIONThermoelectricity is the simplest technology for direct heat-toelectricity power generation. Thermoelectric (TE) devices are all solid state, without rotation parts or working fluids, and are thus easy to miniaturize. These modular characteristics make TE devices reliable, durable and easy to use in tandem with other energy conversion technologies. 1,2 The energy conversion efficiency of a TE device is primarily determined by the TE material's dimensionless figure of merit, defined as zT = α 2 σT/κ, where α is the Seebeck coefficient, σ is the electrical conductivity, κ is the total thermal conductivity (including the lattice contribution κ L and the carrier contribution κ e ) and T is the absolute temperature. 2 zT is generally a function of temperature whereas waste heat is the energy source for TE power generation. It is useful to compare the optimal operation temperature ('service temperature') of state-of-the-art TE materials and the temperature range in which most of the waste heat is produced. State-of-the-art TE materials have their best zT values (those between 1 and 2) in different temperature regimes; for example, Bi 2 Te 3 works best near room temperature, 3,4 whereas PbTe, Mg 2 Si 1-x Sn x , Half-Heusler compounds and filled skutterudites generally reach their best performance above 600 K. [5][6][7][8][9][10][11][12][13][14] There is a conspicuous lack of high-performance TE materials between 400 and 600 K, the

No abstract

No abstract