Method for improving thermoelectric performance of bismuth telluride-based material

Abstract

The invention discloses a method for improving the thermoelectric performance of a bismuth telluride-based material, and the method comprises the steps: carrying out the vertical angular extrusion of a bismuth telluride-based ingot material, so as to improve the thermoelectric performance; the vertical corner extrusion is realized by adopting a vertical corner extrusion die, an inner cavity of the die is sequentially composed of a feeding area, a reducing area and an extrusion area which are communicated with one another, the feeding area is a longitudinally arranged cylinder, the extrusion area is a transversely arranged cuboid, and the feeding area is communicated with the extrusion area through the reducing area; the vertical corner extrusion precursor vertically enters the vertical corner extrusion die, and is extruded in the horizontal direction after being subjected to vertical corner extrusion. According to the method, directional arrangement and refinement of crystal grains in the bismuth telluride-based material are promoted through the vertical angular pressing process, the electrical transport performance of the material is remarkably improved through the process, meanwhile, the heat conductivity of the material is effectively reduced by enhancing the grain boundary scattering effect and introducing a defect structure, and finally the thermoelectric performance of the material is greatly optimized.

Description

Technical Field

[0001] The present invention belongs to the field of thermoelectric semiconductor materials, and in particular relates to a method for improving the thermoelectric performance of bismuth telluride-based materials. Background Art

[0002] Thermoelectric materials can achieve direct conversion of heat and electricity, and have broad prospects in the fields of thermoelectric power generation and thermoelectric refrigeration. Compared with traditional technologies, thermoelectric conversion has no moving parts, is noiseless, pollution-free, highly reliable, and has low maintenance costs, making it a highly promising green energy technology. Bismuth telluride (Bi2Te3)-based materials are the most widely studied and mature room-temperature high-performance thermoelectric materials. They have a high Seebeck coefficient, low thermal conductivity, and moderate electrical conductivity, resulting in excellent thermoelectric performance at room temperature. ZT The value is high, which can effectively realize heat and electricity conversion. It is an ideal material system for efficient thermoelectric energy conversion at room temperature.

[0003] Zone melting technology is the most widely used commercial preparation technology. It can obtain materials with coarse grains and nearly single crystal orientation characteristics. The samples show large carrier mobility and excellent electrical properties. However, the zone melting material ZT The value is low, and its coarse grains and nearly perfect texture make it easy to cleave along the basal plane, resulting in poor mechanical properties and low sample processing yield, which limits its application, especially in micro thermoelectric devices.

[0004] In order to obtain Bi2Te3-based materials with high mechanical properties, researchers have developed powder metallurgy preparation technology. Although the bismuth telluride-based thermoelectric materials prepared by powder metallurgy significantly enhance the grain boundary scattering of the material by reducing the grain size of the material and reducing the thermal conductivity of the material, it is difficult to obtain high texture of the material. ZT The value is lower.

[0005] In order to obtain highly textured materials, researchers have developed a hot extrusion deformation preparation technology. By effectively controlling the shear deformation process and dynamic recrystallization process during the extrusion process, although the material's grains can be significantly refined and a strong texture can be obtained, and a strong (110) orientation characteristic is exhibited perpendicular to the pressure direction, the orientation characteristics are uneven in the direction parallel to the pressure direction, and a feature similar to a silk texture is generated. However, this silk texture feature has a very limited ability to improve the sample's carrier mobility, and there is still much room for improvement in improving thermoelectric performance compared to zone-melting grown samples. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a method for improving the thermoelectric properties of bismuth telluride-based materials in response to the shortcomings of the above-mentioned existing technologies. Through the vertical angular extrusion process, the internal grains of bismuth telluride-based materials are oriented and refined. This process not only significantly improves the electrical transport performance of the material, but also effectively reduces the thermal conductivity of the material by enhancing the grain boundary scattering effect and introducing defect structures, ultimately significantly optimizing the thermoelectric performance of the material.

[0007] The technical solution adopted by the present invention to solve the above-mentioned problems is: A preparation method for improving the thermoelectric performance of bismuth telluride-based materials comprises the following steps: (1) According to the stoichiometric ratio of each element in the bismuth telluride-based material, the single-substance raw material is weighed, and then stirred, melted, crushed, ground, cold-pressed and annealed to obtain a cold-pressed block of the bismuth telluride-based material; (2) hot forming the cold-pressed block of bismuth telluride-based material obtained in step (1) to obtain a vertical angular extrusion precursor; the vertical angular extrusion precursor is a cylinder, and the cross section is adapted to the size of the vertical angular extrusion die inlet in step (3); (3) The vertical angular extrusion precursor obtained in step (2) is transferred to a vertical angular extrusion die, and vertical angular extrusion is performed under vacuum conditions. The resulting vertical angular extrusion ingot is a bismuth telluride-based material with improved thermoelectric performance. The inner cavity of the vertical angular extrusion die is composed of a feed zone, a diameter-changing zone, and an extrusion zone, which are interconnected. The feed zone is a cylindrical shape arranged longitudinally, and the extrusion zone is a rectangular parallelepiped shape arranged transversely. The feed zone and the extrusion zone are connected through the diameter-changing zone. According to the above scheme, the vertical angle extrusion precursor can be replaced by a commercially available bismuth telluride-based ingot material obtained by an extrusion process. The commercially available bismuth telluride-based ingot material or the bismuth telluride-based ingot material prepared by an extrusion process using existing technology can achieve improved thermoelectric performance through the above step (3).

[0008] According to the above scheme, in step (3), the vertical angle extrusion is carried out by heating the temperature in a vacuum at a heating rate of 5-20°C / min to a vertical angle extrusion temperature range of 400-550°C, and holding the temperature for 60-90 minutes before vertical angle extrusion.

[0009] According to the above scheme, in step (3), the vertical angular extrusion speed in the longitudinal direction is 0.2-1.0 mm / min, and the uniaxial pressure is 100-200 MPa. The longitudinal direction refers to the vertical direction, and vertical angular extrusion means that the vertical angular extrusion precursor enters the vertical angular extrusion die vertically, and after vertical angular extrusion, it is extruded in the horizontal direction.

[0010] According to the above scheme, in step (3), the cross-sectional area ratio of the feed port and the extrusion port is (2-5):1, preferably (3-4):1.

[0011] According to the above scheme, the bismuth telluride-based material is p-type or n-type; the purity of the raw materials for preparing p-type or n-type bismuth telluride-based materials is not less than 99.99%. The chemical formula of p-type bismuth telluride-based materials is Bi 2-x Sb x Te3, using elemental bismuth, antimony, and tellurium as raw materials, x = 1.5~1.6; the chemical formula of n-type bismuth telluride-based materials is Bi2Te 2.81 Se 0.19 +ymol%I2, using elemental bismuth, tellurium, selenium and iodine as raw materials, y=0.03~0.04.

[0012] According to the above scheme, in step (1), the heating rate of the stirring and melting is 10~15℃ / min, the stirring and melting temperature is 550~650℃, the stirring frequency is 30~60r / min, the stirring time is 10~20min, and after stirring and melting, it is cooled to room temperature in the furnace.

[0013] According to the above scheme, in step (1), when the bismuth telluride-based material is p-type, the particle size of the powder after crushing and grinding needs to be less than 0.05 cm; when the bismuth telluride-based material is n-type, the particle size of the powder after crushing and grinding needs to be less than 0.1 cm.

[0014] According to the above scheme, in step (1), the uniaxial pressure of the cold pressing is 200~300Mpa, the holding time is 10~20min, and the deformation rate is 25~35%.

[0015] According to the above scheme, in step (1), the annealing is divided into two stages. The heating rate of the first stage is 10~20℃ / min, the annealing temperature is 350~400℃, and the annealing time is 0.5~1.0h; the heating rate of the second stage is 5~10℃ / min, the annealing temperature is 500~550℃, and the annealing time is 15~20h.

[0016] According to the above scheme, in step (2), the heating rate of thermoforming is 5~10℃ / min, the thermoforming temperature is 400~450℃, the holding time is 30~60min, the uniaxial pressure is 100~200Mpa, and the deformation rate is 15~25%.

[0017] The bismuth telluride-based thermoelectric material prepared by the above scheme is p-type, and its chemical formula is Bi 2-x Sb x Te3 (x = 1.5 ~ 1.6) has the characteristics of high texture and fine grains, and the orientation factor perpendicular to the extrusion direction is F (00l)The orientation factor parallel to the extrusion direction is 0.48~0.55 F (110) The conductivity is 8.5×10 4 ~10.5×10 4 S·m -1 , Seebeck coefficient is 180.0~230.0μV·K -1 .

[0018] The bismuth telluride-based material prepared by the above method is n-type, and its general chemical formula is Bi2Te 2.81 Se 0.19 +ymol%I2 (y=0.03~0.04), with high texture, fine grain characteristics, and orientation factor perpendicular to the extrusion direction F (00l) The orientation factor parallel to the extrusion direction is 0.30~0.56 F (110) The conductivity is 8.5~10.5×10 at 300K. 4 S·m -1 , Seebeck coefficient is -200.0~-180.0μV·K -1 .

[0019] Compared with the prior art, the present invention has the following beneficial effects: The present invention adopts vertical angular extrusion technology to prepare bismuth telluride-based materials, making full use of the shear deformation process and dynamic recrystallization process in the vertical angular extrusion process to refine the grains. During the vertical angular extrusion process, in the direction perpendicular to the pressure, the grains preferentially grow along the direction of minimum resistance, that is, along the extrusion direction, so that the (00l) texture of the extruded sample is enhanced; while in the direction parallel to the pressure, due to the friction with the inner wall of the die during the extrusion process, this frictional resistance promotes the flipping of the grains, so that the grains are arranged in a directional manner, achieving the effect of reducing frictional resistance, thereby forming a (110) texture in the extruded sample and forming a plate texture feature. This texture feature greatly improves the electrical transport performance of the material; at the same time, inside the material, the grains undergo a dynamic recrystallization process, which triggers grain refinement. This process not only enhances the grain boundary scattering effect, but also introduces a large number of defect structures, effectively enhancing phonon scattering, thereby reducing the thermal conductivity of the material. Ultimately, the material obtains excellent thermoelectric properties. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 2. The vertical corner extrusion die in the embodiment is a cross-sectional view, which includes a front cross-sectional view and a side cross-sectional view, with the extrusion port side as the front; Figure 2It is a top view of the vertical corner extrusion die in the embodiment; wherein, 1-the outer wall forming the inner cavity (i.e., the die body), 2-the feeding area, 3-the diameter-changing area, and 4-the extrusion area.

[0021] Figure 3 XRD patterns of the final product of Example 1 in the directions perpendicular to and parallel to the pressure; Figure 4 Free cross-sectional SEM images of the final products of Example 1 and Comparative Example 1; Figure 5 Thermoelectric performance data of the final products of Example 1 and Comparative Example 1; Figure 6 XRD patterns of the final product of Example 2 in directions perpendicular to the extrusion direction and parallel to the pressure direction; Figure 7 Free cross-sectional SEM images of the final products of Example 2 and Comparative Example 2; Figure 8 Thermoelectric performance data of the final products of Example 2 and Comparative Example 2; Figure 9 XRD patterns of the final product of Example 3 in directions perpendicular to the extrusion direction and parallel to the pressure direction; Figure 10 Free cross-sectional SEM images of the final products of Example 3 and Comparative Example 3; Figure 11 Thermoelectric performance data of the final products of Example 3 and Comparative Example 3. DETAILED DESCRIPTION

[0022] In order to better understand the present invention, the content of the present invention is further illustrated below in conjunction with examples, but the present invention is not limited to the following examples.

[0023] In the present invention, unless otherwise specified, the raw materials and equipment used can be purchased from the market or are commonly used in the art.

[0024] In the following embodiment, the hot pressing mold 1 is made of Dievar hot working die steel, and the inner cavity is composed of a feed area 2, a diameter-changing area 3, and an extrusion area 4, which are interconnected. The feed area 2 is a cylindrical shape arranged longitudinally (the longitudinal direction is the vertical direction), and the extrusion area 4 is a rectangular parallelepiped shape arranged transversely (the transverse direction is the horizontal direction). The feed area 2 and the extrusion area 4 are connected through the diameter-changing area 3. The diameter of the feed area is 32 mm (the cross section of the feed opening is about 804 mm). 2 The height of the feeding area is 160 mm, and the vertical distance from the feeding port to the discharging port is 200 mm; the extrusion area is a rectangular parallelepiped with a width of 30 mm × a height of 8 mm × a length of 50 mm, and the cross section of the discharging port is a rectangle with a cross section of 30 mm × 8 mm (the cross section of the discharging port is 240 mm). 2, the cross-sectional area ratio of the feed port and the extrusion port is 3.3:1). Figure 1-2 shown.

[0025] Example 1 A method for improving the thermoelectric performance of bismuth telluride-based materials, comprising the following steps: (1) Preparation of cold-pressed blocks of bismuth telluride-based materials According to the chemical composition of n-type bismuth telluride-based thermoelectric materials Bi 0.4 Sb 1.6 A total of 1000g of single-element raw materials, including high-purity bismuth blocks, antimony blocks, and tellurium blocks, are weighed according to the stoichiometric ratio of each element in Te3. The purity of the raw materials is all 99.99%. The raw materials are vacuum-sealed in a stirring melting furnace, and then the furnace body is heated to 600℃ at a heating rate of 10℃ / min, the stirring frequency is 40r / min, and the stirring time is 20min. After stirring and melting, the materials are cooled to room temperature with the furnace to obtain a molten ingot.

[0026] The molten ingot was placed in a jaw crusher for crushing, and the crushed blocks were placed in a rotary grinder with a grinding frequency of 5000 r / min. The particle size of the obtained p-type powder was less than 0.05 cm.

[0027] The p-type powder obtained by crushing and grinding was transferred to a cold pressing mold with a diameter of 32 mm, and then cold pressed using a uniaxial pressure of 200 MPa. The holding time was about 15 minutes, the deformation rate was about 30%, and a cylinder with a cross-sectional diameter of 32 mm was prepared.

[0028] The cylinder obtained by the cold pressing was placed in an annealing furnace. The heating rate of the first stage was 20°C / min, the annealing temperature was 350°C, and the annealing time was 0.5h. The heating rate of the second stage was 5°C / min, the annealing temperature was 500°C, and the annealing time was 15h to obtain a cold-pressed block of bismuth telluride-based material.

[0029] (2) Preparation of vertical angular extrusion precursor The annealed cold-pressed block was moved to a hot pressing mold with a diameter of 32 mm, heated to 400°C at a rate of 10°C / min, kept at 400°C for 20 minutes, and then hot-pressed using a uniaxial pressure of 100 MPa with a deformation rate of about 20%, resulting in a cylinder with a cross-sectional diameter of 32 mm, which is the vertical angle extrusion precursor.

[0030] (3) Preparation of vertical angular extrusion ingots The vertical angular extrusion precursor is placed in a vertical angular extrusion die, heated to 500°C at a rate of 10°C / min, kept at this temperature for 60 minutes, and then extrusion is started at an extrusion speed of 0.2mm / min. The upper limit of the uniaxial pressure is 500MPa. The vertical angular extrusion ingot is extruded from the discharge port of the vertical angular extrusion die to obtain a bismuth telluride-based thermoelectric material with improved thermoelectric performance.

[0031] Comparative Example 1 The main difference from Example 1 is that step (3) is omitted, and the cylinder formed by hot pressing in step (2) is the final product, which is named hot extrusion ingot.

[0032] Figure 3 The bulk XRD patterns of Example 1 perpendicular to and parallel to the extrusion direction are shown. Calculation of the orientation factor shows that the orientation factors of Example 1 perpendicular to and parallel to the extrusion direction are as high as 0.55 and 0.30, respectively, indicating that the present invention enables the bismuth telluride-based thermoelectric material to exhibit obvious orientation characteristics in both the vertical and pressure directions, that is, a plate texture is obtained. This texture feature can significantly improve the electrical transport performance of the material.

[0033] Figure 4 The grain structures of the final products of Comparative Example 1 and Example 1 are compared. Compared with the grain size greater than 20 μm in Comparative Example 1, the grain size of Example 1 is significantly reduced during the vertical angular extrusion process due to the occurrence of dynamic recrystallization behavior, and its grain size can be refined to 2~10 μm. This significant refinement of the grains can effectively reduce the lattice thermal conductivity.

[0034] Figure 5 The electrothermal transport performance and thermoelectric figure of merit of the final product of comparative example 1 and example 1 are ZT Comparison. The hot pressed ingot of Comparative Example 1 has a very high carrier concentration, so the material has a high electrical conductivity and a high thermal conductivity. The electrical conductivity at room temperature is greater than 1.0×10 5 S·m -1 , thermal conductivity exceeds 1.40W·m -1 ·K -1 , the Seebeck coefficient is only 157.6μV·K -1 , the material at room temperature ZT The value is only 0.56. In contrast, the thermal conductivity of the vertical angular extrusion ingot of Example 1 is significantly reduced to ~1.2W·m at room temperature due to the synergistic effect of the highly oriented structure and grain refinement effect, and the introduction of rich defect structure. -1 ·K -1 , the conductivity dropped to 8.6×10 4 S·m -1 , the Seebeck coefficient increases to 227μV·K -1, at room temperature ZT The value is increased to 1.08, the highest ZT The value is close to 1.13.

[0035] Example 2 The hot-extruded ingot purchased on the market was placed in a vertical angular extrusion die, heated to 500°C at a rate of 10°C / min, kept at this temperature for 60 minutes, and then extruded at an extrusion speed of 0.2mm / min. The vertical angular extrusion time was 60~90 minutes, and the upper limit of the uniaxial pressure was 500MPa. The vertical angular extruded ingot was obtained, which is the bismuth telluride-based thermoelectric material with improved thermoelectric performance.

[0036] The specific specifications of the hot-extruded ingot are as follows: the composition is Bi2Te3-Sb2Te3 alloy, the ingot length is 240±1mm, the weight is 5.6kg, the conductivity at 25°C is 0.850-1.150milliohm*cm, the production method is extrusion process, and it was purchased from Xiamen Xi'an Technology Co., Ltd.

[0037] Comparative Example 2 The hot extruded ingot purchased in Example 2 was directly used as Comparative Example 2.

[0038] Figure 6 This is the bulk XRD pattern of Example 2 perpendicular to the extrusion direction. The calculation of the orientation factor shows that the orientation factors of Example 1 perpendicular to and parallel to the extrusion direction are as high as 0.45 and 0.30, respectively, indicating that the present invention can further enhance the texture characteristics in the material, thereby further improving the electrical transport performance of the material.

[0039] Figure 7 The grain structures of Comparative Example 2 and Example 2 are compared. Due to the occurrence of dynamic recrystallization during vertical angular extrusion, the grain size of Example 2 is significantly smaller than that of Comparative Example 2. This significant grain refinement effect can reduce lattice thermal conductivity by enhancing grain boundary scattering.

[0040] like Figure 8 The electrothermal transport performance and thermoelectric figure of merit of comparative example 2 and example 2 are shown. ZT Comparative Example 2 showed 9.3×10 4 S·m -1 conductivity, 1.37W·m -1 ·K -1 The thermal conductivity and 227.1μV·K -1 The Seebeck coefficient is the largest ZTThe value of 0.90 was obtained at 348 K. In contrast, Example 2 maintained a 9.3×10 4 S·m -1 The electrical conductivity is reduced to 1.15 W·m -1 ·K -1 , the Seebeck coefficient is 221.1μV·K -1 , its maximum ZT The value increases to 1.21 at 348K.

[0041] Example 3 A method for improving the thermoelectric performance of bismuth telluride-based materials, comprising the following steps: (1) Preparation of cold-pressed blocks of bismuth telluride-based materials According to the chemical composition of n-type bismuth telluride-based thermoelectric materials Bi2Te 2.81 Se 0.19 I 0.0004， A total of 1000 g of single-element raw materials, including high-purity bismuth blocks, antimony blocks, tellurium blocks, and iodine powder, were weighed according to the stoichiometric ratio of each element. The purity of the raw materials was all 99.99%. The raw materials were vacuum-sealed in a stirring melting furnace, and then the furnace was heated to 650°C at a heating rate of 8°C / min, the stirring frequency was 50r / min, the stirring time was 20min, and after stirring and melting, the materials were cooled to room temperature with the furnace to obtain a molten ingot.

[0042] The molten ingot was placed in a jaw crusher for crushing, and the crushed blocks were placed in a rotary grinder with a grinding frequency of 5000 r / min. The particle size of the obtained n-type powder was less than 0.1 cm.

[0043] The crushed and ground n-type powder was transferred to a cold pressing mold with a diameter of 32 mm, and then cold pressed using a uniaxial pressure of 200 MPa. The holding time was about 15 minutes, the deformation rate was about 30%, and a cylinder with a cross-sectional diameter of 32 mm was prepared.

[0044] The cylinder obtained by the cold pressing was placed in an annealing furnace. The heating rate of the first stage was 20°C / min, the annealing temperature was 400°C, and the annealing time was 0.5h. The heating rate of the second stage was 5°C / min, the annealing temperature was 550°C, and the annealing time was 20h to obtain a cold-pressed block of bismuth telluride-based material.

[0045] (2) Preparation of vertical angular extrusion precursor The annealed cold-pressed block was moved to a hot pressing mold with a diameter of 32 mm, heated to 450°C at a rate of 10°C / min, kept at 450°C for 20 minutes, and then hot-pressed using a uniaxial pressure of 150 MPa with a deformation rate of about 20%, resulting in a cylinder with a cross-sectional diameter of 32 mm, which is the vertical angle extrusion precursor.

[0046] (3) Preparation of vertical angular extrusion ingots The vertical angle extrusion precursor is placed in a vertical angle extrusion die, heated to 550°C at a rate of 10°C / min, kept at this temperature for 60 minutes, and then extrusion is started at an extrusion speed of 0.2mm / min. The upper limit of the uniaxial pressure is 500MPa, and a vertical angle extrusion ingot is obtained, which is a bismuth telluride-based thermoelectric material with improved thermoelectric performance.

[0047] Comparative Example 3 The main difference from Example 3 is that step (3) is omitted, and the cylinder formed by hot pressing in step (2) is the final product, which is named hot extrusion ingot.

[0048] Figure 9 The bulk XRD patterns of Example 3 perpendicular to and parallel to the extrusion direction are shown. Calculation of the orientation factor shows that the orientation factors of Example 3 perpendicular to and parallel to the extrusion direction are as high as 0.56 and 0.32, respectively, indicating that the present invention enables the bismuth telluride-based thermoelectric material to exhibit obvious orientation characteristics in both the vertical and pressure directions, that is, a plate texture is obtained. This texture feature can significantly improve the electrical transport performance of the material.

[0049] Figure 10 The grain structures of the final products of Comparative Example 3 and Example 3 are compared. Compared with the grain size of Comparative Example 3 which is larger than 20 μm, during the vertical angular extrusion process, due to the occurrence of dynamic recrystallization behavior, the grain size of Example 1 is more uniform, which is conducive to the formation of a strong texture.

[0050] Figure 11 The electrothermal transport performance and thermoelectric figure of merit of the final product of comparative example 3 and example 3 are ZT Comparison. The conductivity of the hot pressed ingot in Comparative Example 3 at room temperature is less than 7.0×10 4 S·m -1 , thermal conductivity is 1.20W·m -1 ·K -1 , the Seebeck coefficient is -208.6μV·K -1 , the material at room temperature ZT The value is only 0.75. In contrast, the vertical angular extrusion ingot of Example 3 has a highly oriented structure, and its conductivity increases to 10.5×10 4 S·m -1, the Seebeck coefficient is reduced to -195.1μV·K -1 , at room temperature ZT The value increased to 0.81, the highest ZT The value is close to 0.88.

[0051] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this field, several improvements and changes can be made without departing from the creative concept of the present invention, which all fall within the scope of protection of the present invention.

Claims

1. A method for improving the thermoelectric performance of bismuth telluride-based materials, characterized in that: Bismuth telluride-based ingot materials are subjected to vertical angular extrusion to improve thermoelectric performance; the vertical angular extrusion is achieved using a vertical angular extrusion die; the inner cavity of the vertical angular extrusion die is sequentially composed of a feed area, a diameter-changing area, and an extrusion area that are interconnected, the feed area being a longitudinally arranged cylindrical shape, the extrusion area being a transversely arranged rectangular parallelepiped shape, and the feed area and the extrusion area being connected through the diameter-changing area.

2. The method for improving the thermoelectric performance of bismuth telluride-based materials according to claim 1, characterized in that: The vertical angular extrusion is performed by heating the material to a temperature within the vertical angular extrusion range of 400-550° C. in a vacuum or protective atmosphere and then maintaining the temperature for a certain period of time. The vertical angular extrusion has a longitudinal extrusion speed of 0.2-1.0 mm / min and a uniaxial pressure of 100-200 MPa. The area ratio between the feed port of the feed zone and the extrusion port of the extrusion zone is (2-5):

1.

3. The method for improving the thermoelectric performance of bismuth telluride-based materials according to claim 1, characterized in that: The bismuth telluride-based ingot material is a commercially available bismuth telluride-based ingot material obtained by an extrusion process.

4. A preparation method for improving the thermoelectric performance of bismuth telluride-based materials, characterized in that: The following steps are involved: (1) According to the stoichiometric ratio of each element in the bismuth telluride-based material, the single-substance raw material is weighed, and then stirred, melted, crushed, ground, cold-pressed and annealed to obtain a cold-pressed block of the bismuth telluride-based material; (2) hot forming the cold-pressed block of bismuth telluride-based material obtained in step (1) to obtain a vertical angular extrusion precursor; the vertical angular extrusion precursor is a cylinder, and the cross section is adapted to the size of the vertical angular extrusion die inlet in step (3); (3) The vertical angular extrusion precursor obtained in step (2) is transferred to a vertical angular extrusion die, and vertical angular extrusion is performed under vacuum conditions. The obtained vertical angular extrusion ingot is a bismuth telluride-based material with improved thermoelectric performance; wherein the inner cavity of the vertical angular extrusion die is composed of a feed area, a diameter-changing area and an extrusion area which are interconnected in sequence, the feed area is a longitudinally arranged cylindrical shape, and the extrusion area is a transversely arranged rectangular parallelepiped shape, and the feed area and the extrusion area are connected through the diameter-changing area.

5. The method for improving the thermoelectric performance of bismuth telluride-based materials according to claim 4, characterized in that: In step (3), the vertical angular extrusion is carried out by heating the temperature in a vacuum at a heating rate of 5-20°C / min to a vertical angular extrusion temperature range of 400-550°C, and holding the temperature for 60-90 minutes; the cross-sectional area ratio of the feed port and the extrusion port is (2-5):

1.

6. The method for improving the thermoelectric performance of bismuth telluride-based materials according to claim 4, characterized in that: In step (3), the vertical angular extrusion has an extrusion speed in the longitudinal direction of 0.2-1.0 mm / min and a uniaxial pressure of 100-200 MPa; the vertical angular extrusion precursor vertically enters the vertical angular extrusion die, and after vertical angular extrusion, is extruded from the horizontal direction.

7. The method for improving the thermoelectric performance of bismuth telluride-based materials according to claim 4, characterized in that: The bismuth telluride-based material is p-type or n-type; the purity of the raw materials for preparing the p-type or n-type bismuth telluride-based material is not less than 99.99%; the chemical formula of the p-type bismuth telluride-based material is Bi 2-x Sb x Te3, using elemental bismuth, antimony, and tellurium as raw materials, x = 1.5~1.6; the chemical formula of n-type bismuth telluride-based materials is Bi2Te 2.81 Se 0.19 +ymol%I2, using elemental bismuth, tellurium, selenium and iodine as raw materials, y=0.03~0.

04.

8. The method for improving the thermoelectric performance of bismuth telluride-based materials according to claim 4, characterized in that: In step (1), the heating rate of the stirring and melting is 10-15°C / min, the stirring and melting temperature is 550-650°C, and the stirring and melting is followed by cooling to room temperature in the furnace; The deformation rate of the cold pressing is 25-35%; The annealing is divided into two stages. The heating rate of the first stage is 10-20°C / min, the annealing temperature is 350-400°C, and the annealing time is 0.5-1.0h; the heating rate of the second stage is 5-10°C / min, the annealing temperature is 500-550°C, and the annealing time is 15-20h.

9. The method for improving the thermoelectric performance of bismuth telluride-based materials according to claim 3, characterized in that: In step (2), the heating rate of the thermoforming is 5-10°C / min, the thermoforming temperature is 400-450°C, the holding time is 30-60min, the uniaxial pressure is 100-200 MPa, and the deformation rate is 15-25%.

10. The bismuth telluride-based thermoelectric material obtained by the method according to any one of claims 3 to 9, characterized in that: The bismuth telluride-based thermoelectric material is p-type, and its general chemical formula is Bi 2-x Sb x Te3, x = 1.5 ~ 1.6, orientation factor perpendicular to the extrusion direction F (00l) The orientation factor parallel to the extrusion direction is 0.48~0.55 F (110) The conductivity is 8.5×10 4 ~10.5×10 4 S·m -1 , Seebeck coefficient is 180.0~230.0μV·K -1 . The bismuth telluride-based material is n-type and has the general chemical formula Bi2Te 2.81 Se 0.19 +ymol%I2, y=0.03~0.04, orientation factor perpendicular to the extrusion direction F (00l) The orientation factor parallel to the extrusion direction is 0.30~0.56 F (110) The conductivity is 8.5×10 4 ~10.5×10 4 S·m -1 , Seebeck coefficient is -200.0~-180.0μV·K -1 .

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