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The preparation technology of powder metallurgy is an important way to prepare Bi2Te3-based bulk materials with excellent mechanical properties and thermoelectric properties. However, the loss of sample orientation during the preparation of powder metallurgy results in low thermoelectric properties of the materials. The development of high-performance Bi2Te3-based thermoelectric materials with strong plate texture and fine grains is the focus of research on high-performance Bi2Te3-based thermoelectric materials. In this paper, a series of p-type Bi2Te3-based materials is prepared by vertical corner extrusion preparation technology. The influences of extrusion temperature on the microstructure and texture characteristics of the material and its influence on the thermoelectric properties of the material are systematically studied. In the vertical corner extrusion process, grains preferentially grow along the minimum resistance direction perpendicular to the pressure, that is, along the extrusion direction, thereby further enhancing the (00l) texture of the original hot-pressed sample; in the direction parallel to the pressure, due to friction with the inner wall of the die in the extrusion process, this frictional resistance will promote the inversion of the grains, so that the grains are arranged in a directional manner to reduce the frictional resistance, thus forming the (110) texture, which is not present in the original hot-pressed sample, in the extruded sample, and finally completing the transition from the hot-pressed sample to the plate texture of the extruded sample. When the extrusion temperature is low, the atomic diffusion rate is low, which limits the dynamic recrystallization of the grain, the grain growth process, and the grain deflection speed. With the increase of the extrusion temperature, these processes can be carried out rapidly, thus forming a more obvious plate texture characteristic. The 773 K extruded sample achieves high orientation factors of F(00l) = 0.51 and F(110) = 0.30 in the directions perpendicular to the pressure and parallel to the pressure, respectively, and the carrier mobility is as high as 345.4 cm2·V–1·s–1 at room temperature, which is comparable to the carrier mobility of the zone melt sample, showing excellent electrical transport performance. The power factor reaches 4.43 mW·m–1·K–2 at room temperature. At the same time, the sum of lattice thermal conductivity and bipolar thermal conductivity of the 773 K extruded sample decreases to a minimum value of 0.78 W·m–1·K–1 at 323 K. Finally, the 773 K extruded sample obtains a maximum ZT value of 1.13 at 323 K, which is nearly 70% higher than that of the hot-pressed sample. This research provides a new way for preparing high-performance strong plate textures and fine-grained Bi2Te3-based thermoelectric materials, and lays an important foundation for fabricating micro thermoelectric devices.
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Keywords:
- vertical angular extrusion preparation technology /
- extrusion temperature /
- microstructure /
- thermoelectric properties
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图 1 (a)高温热压装置及晶粒取向示意图; (b)高温垂直转角挤压装置及晶粒取向示意图. 块体Bi2Te3基样品的XRD谱图及取向因子 (c)热压样品和挤压样品垂直于压力方向; (d)热压样品和挤压样品平行于压力方向
Figure 1. (a) Schematic diagram of high-temperature hot pressing device and grain orientation; (b) high temperature vertical angle extrusion device and schematic diagram of grain orientation. XRD spectra and orientation factors of bulk Bi2Te3 based samples: (c) Hot pressed and extruded samples perpendicular to the pressure direction; (d) hot pressed samples and extruded samples are parallel to the direction of pressure.
图 2 垂直和平行于压力方向上热压样品和挤压样品的场发射扫描电镜(FESEM)图像, 垂直于压力方向上抛光表面的背散射电子像, 以及Sb, Te和元素Bi的面扫描EDS能谱图 (a1)—(a3)热压样品; (b1)—(b3) 673 K挤压样品; (c1)—(c3) 773 K挤压样品
Figure 2. Field emission scanning electron microscopy (FESEM) images of hot pressed and extruded samples in the vertical and parallel directions of pressure, as well as backscattered electron images of polished surfaces in the vertical direction of pressure, and surface scan EDS spectra of Sb, Te, and elemental Bi: (a1)–(a3) Hot pressed samples; (b1)–(b3) 673 K extruded sample; (c1)–(c3) 773 K extruded sample.
图 3 样品在垂直于压力方向上的电子反向散射衍射(EBSD)分析, 包括插入{000l}极图的反极图(IPF)图、晶界分布图、晶粒取向扩展(GOS)图和核平均取向偏差(KAM)图 (a1)—(a4)热压样品; (b1)—(b4) 673 K挤压样品; (c1)—(c4) 773 K挤压样品; (d)平均晶粒尺寸; (e)小角度晶界(Lagd)比例; (f)不同组织百分比. CDRX, PDRX和Def分别代表完全再结晶组织、部分再结晶组织和形变组织
Figure 3. Electron backscatter diffraction (EBSD) analysis of the sample in the direction perpendicular to the pressure, including the inverse pole map (IPF) of the inserted {000l} pole plot, grain boundary distribution, grain orientation spread (GOS) plot, and nucleus mean orientation deviation (KAM) plot: (a1)–(a4) Hot-pressed sample; (b1)–(b4) 673 K extruded sample; (c1)–(c4) 773 K extruded sample; (d) mean grain size; (e) small angle grain boundary (Lagd) ratio; (f) different tissue percentages. CDRX, PDRX, and Def represent fully recrystallized, partially recrystallized, and deformed tissues, respectively.
图 4 样品在平行于压力方向上的电子反向散射衍射(EBSD)分析, 包括插入{000l}极图的反极图(IPF)图、晶界分布图、晶粒取向扩展(GOS)图和核平均取向偏差(KAM)图 (a1)—(a4)热压样品; (b1)—(b4) 673 K样品; (c1)—(c4) 773 K挤压样品; (d)平均晶粒尺寸; (e)小角度晶界(Lagd)比例; (f)不同组织百分比. CDRX, PDRX和Def分别代表完全再结晶组织、部分再结晶组织和形变组织
Figure 4. Electron Backscatter Diffraction (EBSD) analysis of the sample in the direction parallel to the pressure, including the inverse pole pattern (IPF) diagram of the inserted {000l} pole pattern, grain boundary distribution, grain orientation spread (GOS) diagram, and nucleus mean orientation deviation (KAM) diagram: (a1)–(a4) Hot-pressed sample; (b1)–(b4) 673 K sample; (c1)–(c4) 773 K extruded sample; (d) mean grain size; (e) small angle grain boundary (Lagd) ratio; (f) different tissue percentages. CDRX, PDRX, and Def represent fully recrystallized, partially recrystallized, and deformed tissues, respectively.
图 5 样品的电输运性能 (a)电导率; (b)Seebeck系数; (c)室温下载流子迁移率与载流子浓度的函数关系; (d)功率因子
Figure 5. The electrical transport properties of the sample: (a) Conductivity; (b) Seebeck coefficient; (c) carrier mobility as a function of carrier concentration at room temperature; (d) power factor.
图 6 样品的热输运性能 (a)总热导率κ; (b)晶格热导率与双极热导率之和κL + κb; (c)不同样品的ZT值, 以及与文献报道三元p型Bi2Te3的ZT值对比
Figure 6. Thermal transport properties of samples: (a) Total thermal conductivity κ; (b) sum of lattice thermal conductivity and bipolar thermal conductivity κL + κb; (c) the ZT values of different samples, and the comparison with the ZT values of ternary p-type Bi2Te3 reported in the literature.
表 1 热压样品和挤压样品的室温物理性能参数
Table 1. Room temperature physical performance parameters of hot pressed sintered samples and vertical angular pressing samples.
Sample σ/(104 S·m–1) S/(μV·K–1) n/(1019 cm–3) μ/(cm2·V–1·s–1) PF/(mW·m–1·K–2) m*/m0 HP 10.9 157.6 4.02 144.1 2.72 1.13 673 K-HE 9.6 179.5 2.99 224.6 3.11 1.11 698 K-HE 10.6 189.6 2.60 252.2 3.83 1.10 723 K-HE 10.5 204.2 2.24 274.3 4.40 1.12 748 K-HE 10.1 205.4 2.15 308.5 4.26 1.10 773 K-HE 8.6 226.9 1.71 345.4 4.43 1.12 
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