J. J. Shen

Recrystallization induced in situ nanostructures in bulk bismuth antimony tellurides: a simple top down route and improved thermoelectric properties

Recrystallization induced in situ nanostructure formation was used as a new means to obtain high performance polycrystalline thermoelectric materials, which was realized by a simple hot forging process to the coarse-grained Bi0.5Sb1.5Te3 alloys. The pole figure measurement showed that the oriented textures were weakened or eliminated after hot forging of the alloys with a quasi-layered crystal structure, implying the presence of deformation recrystallization. Transmission electron microscopy observation revealed the recrystallization induced in situ nanostructures and high density of defects in the hot forged samples. Transport property measurements indicated that the hot forged samples had both increased electrical power factor and reduced thermal conductivity, compare to the initial alloys without hot forging. The maximum ZT values of >1.3 at room temperature were reproducibly obtained for the hot forged samples, suggesting that the simple new method can be applied for large scale production of high performance polycrystalline thermoelectric materials with in situ nanostructures.

Enhancement in thermoelectric performance of bismuth telluride based alloys by multi-scale microstructural effects

Decoupling of interdependent thermoelectric parameters was considered as a crucial strategy to enhance the thermoelectric performance of bulk materials. Here multi-scale microstructural effects have been introduced by a simple hot deformation process to obtain high-performance n-type bismuth telluride based alloys. Highly preferred orientation enables a significant improvement in in-plane electrical conductivity. The donor-like effect (an interaction of antisite defects and vacancies), which can be adjusted by varying hot deformation temperature, was also considered responsible for the remarkable enhancement in power factor. Besides, the in-plane lattice thermal conductivity was greatly reduced by in situ nanostructures and high-density lattice defects generated during the hot deformation process. The present study experimentally demonstrates a successful combination of microscale texture enhancement, atomic scale lattice defects and donor-like effect and recrystallization induced nanostructures as a new approach to improve thermoelectric properties. These effects led to a maximum ZT of 0.95 for the Bi2Te2Se1 sample hot deformed at 823 K, about 80% improvement over that without hot deformation.

The texture related anisotropy of thermoelectric properties in bismuth telluride based polycrystalline alloys

Large size bismuth telluride based polycrystalline alloys have been prepared to investigate the texture-related anisotropy of thermoelectric properties. X-ray diffraction analysis indicated that the n-type alloy was more easily (001) oriented than the p-type one. Both the electrical and thermal conductivities showed strong anisotropy in the textured samples, while the Seebeck coefficient and the ZT were almost isotropic. However, as the electrical and thermal conductivities were measured along different directions, the ZT could be overestimated up to ∼60%. A relationship between the texture degree and the anisotropy of the thermal conductivity has been proposed.

Hot deformation induced bulk nanostructuring of unidirectionally grown p-type (Bi,Sb)2Te3 thermoelectric materials

Nanostructuring has proved effective in improving the figure of merit in the widely used Bi2Te3 based thermoelectric materials. In this work, a hot deformation induced in situ nanostructuring process is directly applied to the commercial unidirectionally grown p-type Bi0.5Sb1.5Te3 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 Bi2Te3 based thermoelectric materials in a way that is more readily incorporated into the existing procedure of device manufacturing.

Effects of yttrium doping on the thermoelectric properties of Hf0.6Zr0.4NiSn0.98Sb0.02 half-Heusler alloys

The (Y,Sb) codoped ( Hf 0.6 Zr 0.4 ) 1 − x Y x NiSn 0.98 Sb 0.02 ( x = 0 , 0.01, 0.015, 0.02, and 0.025) half-Heusler alloys were prepared by levitation melting and spark plasma sintering. The effects of Y doping on the electrical conductivity, the Seebeck coefficient, and the thermal conductivity have been investigated in the temperature range of 300–900 K. It was found that the Y doping decreased the carrier concentration and electrical conductivity due to the introduction of hole carriers. The thermal conductivity was also reduced upon Y doping, mainly due to the reduced carrier thermal conductivity. The Y-doping substantially increased the Seebeck coefficient because of the decrease in carrier concentration. Pisarenko plot showed that the measured room temperature Seebeck coefficients agrees well with the predicted single parabolic band behavior as a function of the carrier concentration, suggesting that no additional mechanisms cause the extra enhancement of Seebeck coefficient in the Y–Sb codoped half-Heusler alloys. The figure of merit Z T of 1% Y-doped sample was increased by a factor of about 25% than that of the undoped sample.

MICROSTRUCTURE AND THERMOELECTRIC PROPERTIES OF (Zr,Hf)NiSn-BASED HALF-HEUSLER ALLOYS BY MELT SPINNING AND SPARK PLASMA SINTERING

(Zr,Hf)NiSn-based half-Heusler thermoelectric materials have been prepared by melt spinning and spark plasma sintering to refine the grain size. The grain sizes of the melt-spun thin ribbons varied from ~500 nm to ~3 μm and no significant grain growth were found for the bulk samples compacted by spark plasma sintering. Nanoscale precipitates dispersed in the matrix were observed, which should be more metallic due to the increase of the electrical conductivity. The reduction of lattice thermal conductivity was observed due to the refined grain sizes.

SELF-ASSEMBLY OF BISMUTH SELENIDE TWO-DIMENSIONAL SUPERSTRUCTURE FROM HEXAGONAL NANOSHEETS

A kind of novel superstructure array composed of hexagonal Bi2Se3 nanosheets were fabricated via a facile hydrothermal method. The structure was characterized by X-ray diffraction and field emission scanning electron micrioscopy. It is suggested that the two dimensional superstructure was self-assembled by sheet-like Bi2Se3 building blocks on naturally-formed Bi2SeO2 precusor template. The present result indicates one promising way in design and growth of self-assembled superstructures by controllable crystal growth from deoxided substrates with good lattice match.

Influence of Ag 2 Te Doping on the Thermoelectric Properties of p-type Bi 0.5 Sb 1.5 Te 3 Bulk Alloys: Influence of Ag 2 Te Doping on the Thermoelectric Properties of p-type Bi 0.5 Sb 1.5 Te 3 Bulk Alloys

采用真空熔炼、机械球磨及放电等离子烧结技术 (SPS) 制备得到了(Ag 2 Te)x(Bi 0.5 Sb 1.5 Te 3 ) 1- x ( x = 0, 0.025, 0.05, 0.1) 系列样品, 性能测试表明, Ag 2 Te的掺入可以显著改变材料的热电性能变化趋势, 掺杂样品在温度为450~550 K范围内具有较未掺杂样品更优的热电性能. 适当量的Ag2Te掺入能够有效地提高材料的声子散射, 降低材料的热导率. 在测试温度范围内, (Ag 2 Te) 0.05 (Bi 0.5 Sb 1.5 Te 3 ) 0.95 具有最低的晶格热导, 室温至575 K范围内保持在0.2 ~ 0.3 W/(m·K)之间, 在575 K时, (Ag 2 Te) 0.05 (Bi 0.5 Sb 1.5 Te 3 ) 0.95 试样具有最大热电优值 ZT = 0.84, 相较于未掺杂样品提高了约20%.采用真空熔炼、机械球磨及放电等离子烧结技术 (SPS) 制备得到了(Ag 2 Te)x(Bi 0.5 Sb 1.5 Te 3 ) 1- x ( x = 0, 0.025, 0.05, 0.1) 系列样品, 性能测试表明, Ag 2 Te的掺入可以显著改变材料的热电性能变化趋势, 掺杂样品在温度为450~550 K范围内具有较未掺杂样品更优的热电性能. 适当量的Ag2Te掺入能够有效地提高材料的声子散射, 降低材料的热导率. 在测试温度范围内, (Ag 2 Te) 0.05 (Bi 0.5 Sb 1.5 Te 3 ) 0.95 具有最低的晶格热导, 室温至575 K范围内保持在0.2 ~ 0.3 W/(m·K)之间, 在575 K时, (Ag 2 Te) 0.05 (Bi 0.5 Sb 1.5 Te 3 ) 0.95 试样具有最大热电优值 ZT = 0.84, 相较于未掺杂样品提高了约20%.