Preparation method and extrusion die of bismuth telluride-based thermoelectric material

Abstract

The invention discloses a preparation method of a bismuth telluride-based thermoelectric material and an extrusion die. A die groove of the extrusion die comprises an inlet section, a connecting section and an outlet section which are connected in sequence; the connecting section sequentially comprises a plurality of reducing sections from the inlet section to the outlet section; the diameter of the outlet end of any one of the multiple reducing sections is larger than that of the outlet end of the next reducing section. By improving orientation in a traditional hot extrusion process, the bismuth telluride-based thermoelectric material with high mechanical performance and high thermoelectric performance is prepared, and the problems of low mechanical strength, non-uniform deformation and the like in the preparation process of the traditional bismuth telluride-based thermoelectric material are solved.

Description

Technical Field

[0001] The present invention belongs to the technical field of thermoelectric materials, and particularly relates to a preparation method of bismuth telluride-based thermoelectric materials and an extrusion die. Background Art

[0002] In recent years, countries around the world have advocated sustainable development and a low-carbon emission lifestyle. Refrigerants commonly used in traditional compression refrigeration technologies, such as Freon and other gases, have a strong greenhouse effect. Therefore, developing new environmentally friendly refrigeration technologies is an urgent and serious task. Thermoelectric materials can achieve the mutual conversion of thermal energy and electrical energy. Based on the Seebeck effect and Peltier effect of semiconductors, they have extensive applications in thermoelectric power generation and temperature-controlled refrigeration, and have the advantages of small size, no noise, high reliability, stable operation, and long service life.

[0003] Bismuth telluride-based thermoelectric materials are one of the thermoelectric conversion materials with the best performance near room temperature and are also the only commercial thermoelectric materials. However, due to the layered crystal structure of bismuth telluride-based thermoelectric materials, Bi atoms and Te atoms are alternately linked by covalent bonds to form a 5-layer Bi 2 Te 3 atomic structure, and adjacent Bi 2 Te 3 layers are linked by weak intermolecular forces, resulting in better thermoelectric performance in the in-plane direction, and the material is prone to interlayer cleavage, resulting in serious processing damage and being unable to be applied to micro-refrigeration devices. Emerging thermoelectric materials with good performance near room temperature, such as Bi 1-x Sb x 、CsBi 4 Te 6 、SnSe, etc. all have one-dimensional or two-dimensional crystal structures and strong thermoelectric anisotropic transport. Therefore, how to improve the grain orientation of bismuth telluride-based thermoelectric materials and other anisotropic thermoelectric materials and improve their mechanical strength to meet the requirements of high-performance thermoelectric refrigeration devices is a scientific and technological problem faced in the current thermoelectric field.

[0004] At present, there are three mainstream processes for the batch synthesis of bismuth telluride materials. One is the zone melting method to grow rod-shaped crystals. This method can achieve good grain orientation, thus ensuring the thermoelectric performance along the growth direction. Currently, the maximum ZT of the N-type material grown by the zone melting method reaches about 0.9. However, the mechanical strength of the material obtained by this process is very poor, it is easy to dissociate, and segregation of components occurs during the melt crystallization process, resulting in poor uniformity of the mass-produced materials (for example, patent CN202311741656.4 uses this method to prepare n-type bismuth telluride); the second is the powder metallurgy method, that is, the raw material powders are mixed and mechanically alloyed by ball milling or obtained by melt spinning to obtain ultrafine bismuth telluride powders, and then densified by sintering, or further subjected to secondary hot deformation treatment. Due to the high mixing uniformity of the powders in this process and the significant improvement of the mechanical strength of the material by the fine grain strengthening effect, but due to the simultaneous refinement of the grains, the orientation degree decreases greatly, and it is easy to introduce external impurities or cause oxidation during the powder making process, and the performance is lower than that of the crystal rod with preferred orientation. In addition, although hot deformation can effectively improve the orientation and performance of the material, it is not compatible with the existing material processing technology, and processing problems will be faced in industrial applications (for example, patent CN202211633482.5 uses this method to prepare n-type bismuth telluride); the third is the hot extrusion method. The high-purity raw materials are crushed by a crusher to obtain bismuth telluride nano-scale powders, cold pressed by a tablet press at room temperature, and then hot extruded through a hot extrusion die. While the grains are refined, the grains are promoted to turn to ensure the texture orientation degree, which can not only increase the mechanical strength of the material, but also obtain good thermoelectric performance; at the same time, due to the continuity of the hot extrusion method, it is suitable for commercial batch production, so it is expected to become the next priority development technology direction. Summary of the Invention

[0005] Based on the above background technology, the present invention provides a preparation method and an extrusion die for bismuth telluride-based thermoelectric materials, and by improving the orientation in the traditional hot extrusion process, a bismuth telluride-based thermoelectric material with high mechanical properties and high thermoelectric properties is prepared.

[0006] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0007] On the one hand, the present invention provides an extrusion die for bismuth telluride-based thermoelectric materials. The die groove of the extrusion die includes an inlet section, a connection section, and an outlet section connected in sequence; the connection section includes a plurality of reduced-diameter sections in sequence from the inlet section to the outlet section; the outlet end diameter of any one reduced-diameter section in the connection section is greater than the outlet end diameter of the next reduced-diameter section.

[0008] In the technical solution of the present invention, the diameter of any one reduced-diameter section is convergent from one end connecting the previous section to one end connecting the next section, and the outlet end diameter is greater than the outlet end diameter of the next reduced-diameter section.

[0009] As a preferred embodiment, the connecting section includes a primary diameter reduction section and a secondary diameter reduction section from the inlet section to the outlet section; one end of the primary diameter reduction section is connected to the inlet section, and the other end is connected to the secondary diameter reduction section; the diameter of the primary diameter reduction section converges from the end connected to the inlet section to the end connected to the secondary diameter reduction section; one end of the secondary diameter reduction section is connected to the primary diameter reduction section, and the other end is connected to the outlet section; the diameter of the secondary diameter reduction section converges from the end connected to the primary diameter reduction section to the end connected to the outlet section; the diameter of the outlet end of the primary diameter reduction section is larger than the diameter of the outlet end of the secondary diameter reduction section.

[0010] As a preferred embodiment, the cross-sectional angle of the plurality of diameter reduction sections is 20-85°, preferably 40-80°; in the technical solution of the present invention, there is no particular limitation on the size of the cross-sectional angle of any one diameter reduction section and the cross-sectional angle of the next diameter reduction section, and it can be greater than or less than.

[0011] As a preferred embodiment, the inlet section and the outlet section are equal-diameter sections, and the area ratio of their cross-sections is 4-10:1.

[0012] As a preferred embodiment, the extrusion die further includes a heating mechanism for heating the die cavity, and the heating mechanism is arranged outside the die cavity;

[0013] Preferably, the heating mechanism includes multiple groups of heating rods; the heating rods are resistance wire type heating rods; the heating power of the multiple groups of heating rods is not less than 200W for each group;

[0014] Preferably, the multiple groups of heating rods are uniformly arranged outside the die cavity; in some specific embodiments, the multiple groups of heating rods are arranged outside the inlet section, the connecting section and the outlet section; wherein, the heating rods arranged outside the inlet section are parallel to the radial direction of the inlet section and are axially equally spaced with the inlet section as the center; the heating rods arranged outside the outlet section are parallel to the radial direction of the outlet section and are axially equally spaced with the outlet section as the center.

[0015] Preferably, the extrusion die further includes a temperature detection mechanism and a heating control mechanism; the temperature detection mechanism includes multiple groups of thermocouples, and the multiple groups of thermocouples are arranged between the heating rods and the wall of the die cavity; the heating control mechanism is a PID controller; the PID controller is electrically connected to the multiple groups of thermocouples; in the technical solution of the present invention, setting multiple groups of thermocouples can simultaneously detect the temperature information of the extrusion die. After being fed back to the PID controller, local precise heating temperature control can be realized, and the temperature control accuracy is preferably within ±1°C.

[0016] As a preferred embodiment, the extrusion die is made of high-strength stainless steel, tungsten carbide or molybdenum alloy.

[0017] As a preferred embodiment, the extrusion die further includes a pressure bar.

[0018] In the technical solution of the present invention, the shape of the cross-section of the die groove of the extrusion die is not particularly limited and may be circular or square.

[0019] On the other hand, the present invention provides a method for preparing a bismuth telluride-based thermoelectric material by the above extrusion die, including the step of loading a bismuth telluride-based thermoelectric material ingot into the extrusion die and performing hot extrusion.

[0020] In the technical solution of the present invention, the bismuth telluride-based thermoelectric material ingot is prepared by a conventional method in the art. In some specific embodiments, it can be prepared by the following steps: crushing the bismuth telluride-based thermoelectric material into nanoscale powder and cold pressing it into an ingot.

[0021] In the technical solution of the present invention, the bismuth telluride-based thermoelectric material is a conventional material in the art, which includes N-type materials and P-type materials. Among them, the chemical formula of the N-type material is Bi 2 Te 3-x-y Se x / M y , M is selected from at least one of I, Br, S, SbI 3 and TeI 4 where 0 ≤ x ≤ 1 and 0 ≤ y ≤ 0.5; the chemical formula of the P-type material is Bi 2-p-q Sb p Te 3 / N q , N is selected from at least one of Cu, Ag, In, Ga, Sn and Te, 1.2 ≤ p ≤ 1.8, and 0 ≤ q ≤ 0.1.

[0022] In the technical solution of the present invention, the bismuth telluride-based thermoelectric material ingot is loaded into the extrusion die from the inlet section and extruded from the outlet section.

[0023] As a preferred embodiment, the hot extrusion is vacuum hot extrusion or hot extrusion in an inert gas protection atmosphere;

[0024] Preferably, the temperature of the inlet section is 350 - 450 °C;

[0025] Preferably, the temperature of the connection section is 350 - 450 °C;

[0026] Preferably, the temperature of the outlet section is 300 - 450 °C;

[0027] Preferably, the extrusion ratio of the hot extrusion is 4 - 10:1;

[0028] Preferably, the extrusion pressure of the hot extrusion is 80 - 500 MPa;

[0029] Preferably, the extrusion rate of the hot extrusion is 0.1 - 5 mm / min.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] The die groove of the extrusion die provided by the present invention includes an inlet section, a connection section, and an extrusion section. The connection section sequentially includes a plurality of reduced-diameter sections. This extrusion die forms a unique extrusion flow channel during the preparation of the thermoelectric material, greatly enhancing the plastic deformation and deformation uniformity of the ingot during extrusion, enhancing the plastic deformation of all parts of the ingot, and improving the overall orientation of the hot-extruded ingot. Among them, the setting of the connection section increases the stress distribution and strain uniformity of the initial ingot during hot extrusion, and improves the overall uniformity and orientation degree. In addition, the present invention further embeds a heating mechanism, a temperature detection mechanism, and a heating control mechanism in different parts of the hot extrusion die, which can perform local independent temperature control, realize precise control of the temperature during the hot extrusion process, greatly improve the stability and consistency of the hot extrusion process, and finally obtain a bismuth telluride material with controllable grain composition and high texture degree, realizing the dual improvement of thermoelectric performance and mechanical performance.

[0032] The hot extrusion die provided by the present invention not only realizes the batch production of high-orientation fine-grained hot-extruded bismuth telluride ingot materials, but also can be widely applied to the preparation of other strongly anisotropic materials. Description of the Drawings

[0033] By reading the detailed description of the non-limiting embodiments with reference to the following drawings, the present invention and its features, shape, and advantages will become more obvious. The same reference numerals indicate the same parts in all the drawings. The drawings are not deliberately drawn to scale, and the focus is on showing the gist of the present invention.

[0034] Figure 1 and Figure 2 is a cross-sectional view of the extrusion die in the present invention;

[0035] Figure 3 is a graph of the ZT value of the bismuth telluride-based thermoelectric material prepared in the embodiment and comparative example of the present invention;

[0036] Figure 4 is a graph of the flexural strength performance of the bismuth telluride-based thermoelectric material prepared in the embodiment and comparative example of the present invention;

[0037] Figure 5 is a cross-sectional view of the extrusion die in the comparative example of the present invention. Detailed Embodiments

[0038] The following embodiments are only a part of the embodiments of the present invention, rather than all embodiments. Therefore, the detailed description of the embodiments of the present invention provided below is not intended to limit the scope of the claimed invention, but merely represents selected embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative efforts fall within the protection scope of the present invention.

[0039] In the present invention, unless otherwise specified, all devices and raw materials can be purchased from the market or are commonly used in this industry. The methods in the following embodiments are conventional methods in this field unless otherwise specified.

[0040] Referring to Figure 1-2 , the present invention provides an extrusion die for bismuth telluride-based thermoelectric materials. In the technical solution of the present invention, the shape of the cross-section of the die groove of the extrusion die is not particularly limited and can be circular or square. When the cross-section of the die groove is square, the following diameter is the side length of the cross-section. The die groove of the extrusion die provided by the present invention includes an inlet section 2, a connection section 3, and an outlet section 4 that are connected in sequence; the connection section 3 includes a plurality of diameter-reducing sections in sequence from the inlet section 2 to the outlet section 4; the outlet end diameter of any one diameter-reducing section is greater than the outlet end diameter of the next diameter-reducing section.

[0041] In the technical solution of the present invention, the diameter of any one diameter-reducing section is convergent from one end connecting the previous section (including the inlet section and the previous diameter-reducing section) to one end connecting the next section (including the outlet section and the next diameter-reducing section), and the outlet end diameter is greater than the outlet end diameter of the next diameter-reducing section.

[0042] Further, the connection section 3 includes a primary diameter-reducing section 3-1 and a secondary diameter-reducing section 3-2 from the inlet section 2 to the outlet section 4; one end of the primary diameter-reducing section 3-1 is connected to the inlet section 2, and the other end is connected to the secondary diameter-reducing section 3-2; the diameter of the primary diameter-reducing section 3-1 is convergent from one end connecting the inlet section 2 to one end connecting the secondary diameter-reducing section 3-2; one end of the secondary diameter-reducing section 3-2 is connected to the primary diameter-reducing section 3-1, and the other end is connected to the outlet section 4; the diameter of the secondary diameter-reducing section 3-2 is convergent from one end connecting the primary diameter-reducing section 3-1 to one end connecting the outlet section 4; the outlet end diameter of the primary diameter-reducing section 3-1 is greater than the outlet end diameter of the secondary diameter-reducing section 3-2.

[0043] Further, the profile angle of the plurality of diameter-reducing sections is 20-85°, preferably 40-80°; in the technical solution of the present invention, the size of the profile angle of any one diameter-reducing section and the profile angle of the next diameter-reducing section is not particularly limited and can be greater (such as Figure 1 ) or less (such as Figure 2 ).

[0044] Further, the inlet section 2 and the outlet section 4 are of equal diameter, and the area ratio of their cross-sections is 4-10:1, that is, the extrusion ratio is 4-10:1.

[0045] Further, the extrusion die further includes a heating mechanism 8 for heating the die groove, and the heating mechanism 8 is arranged outside the die groove.

[0046] Further, the heating mechanism 8 includes multiple groups of heating rods; the heating rod 5 is arranged outside the inlet section 2, the heating rod 6 is arranged outside the connecting section 3, and the heating rod 7 is arranged outside the outlet section 4; the heating rod is a resistance wire type heating rod; the heating power of multiple groups of heating rods is not less than 200W for a single group.

[0047] Further, multiple groups of heating rods are evenly arranged outside the die groove; among them, the heating rod 5 arranged outside the inlet section 2 is parallel to the radial direction of the inlet section 2 and is axially equally spaced with the inlet section 2 as the center; the heating rod 7 arranged outside the outlet section 4 is parallel to the radial direction of the outlet section 4 and is axially equally spaced with the outlet section 4 as the center.

[0048] Further, the extrusion die further includes a temperature detection mechanism and a heating control mechanism; the temperature detection mechanism includes multiple groups of thermocouples, and multiple groups of thermocouples are arranged between the heating rod and the groove wall of the die groove; the heating control mechanism is a PID controller; the PID controller is electrically connected to multiple groups of thermocouples; in the technical solution of the present invention, setting multiple groups of thermocouples can simultaneously detect the temperature information of the extrusion die. After being fed back to the PID controller, local precise heating temperature control can be realized, and the temperature control accuracy is preferably within ±1°C.

[0049] Further, the extrusion die is made of high-strength stainless steel, tungsten carbide or molybdenum alloy.

[0050] Further, the extrusion die further includes a pressure rod 1.

[0051] Example 1

[0052] This example uses the Figure 2 extrusion die in to prepare a thermoelectric material. In this die, the diameter of the inlet section is 90 mm, the diameter of the outlet section is 30 mm; the diameter at the connection of the primary reduction section and the secondary reduction section is 60 mm; β1 = 30°, β2 = 45°; the specific preparation process is as follows:

[0053] According to the stoichiometric ratio, using the chemical formula Bi 2 Te 2.7 Se 0.3 / TeI 4 as the basis, accurately weigh the elemental raw materials Bi, Te, Se and TeI with a purity ≥ 99.99% 4;Load the prepared raw materials into a quartz tube with a diameter of 50 mm. After vacuum packaging, place it in a rocking furnace for melting to obtain an ingot. Grind the surface of the ingot and use a crusher to break it into nano-sized powder. Load it into a clean and pollution-free container and anneal it in a vacuum atmosphere furnace at 380 °C for 12 h under a vacuum of 1 Pa or in an argon atmosphere. Cold press the annealed powder into a square ingot at room temperature with a pressure of 50 MPa using a tablet press. Rotate the ingot 90 degrees and load it into the inlet section channel of the extrusion die. Set the temperature control during hot extrusion as follows: 400 °C for the inlet section, 420 °C for the connecting section, and 400 °C for the outlet section. Set the hot extrusion pressure as 200 MPa and the extrusion rate as 0.1 mm / min. Finally, obtain an N-type bismuth telluride hot-extruded bar with a smooth surface and metallic luster.

[0054] Comparative Example 1

[0055] This comparative example uses Figure 5 the single-stage diameter-reducing die shown in

[0056] Example 2

[0057] This example uses Figure 2 the extrusion die in

[0058] According to the stoichiometric ratio, based on the chemical formula Bi 0.905 Sb 0.095 accurately weigh the elemental raw materials Bi and Sb with a purity of ≥99.99%. Load the prepared raw materials into a quartz tube with a diameter of 50 mm. After vacuum packaging, place it in a rocking furnace for melting to obtain an ingot. Grind the surface of the ingot and use a crusher to break it into nano-sized powder. Load it into a clean and pollution-free container and anneal it in a vacuum atmosphere furnace at 200 °C for 12 h under a vacuum of 1 Pa or in an argon atmosphere. Cold press the annealed powder into a square ingot at room temperature with a pressure of 50 MPa using a tablet press. Rotate the ingot 90 degrees and load it into the inlet section channel of the extrusion die. Set the temperature control during hot extrusion as follows: 200 °C for the die inlet section, 210 °C for the connecting section, and 200 °C for the outlet section. Set the hot extrusion pressure as 200 MPa and the extrusion rate as 0.1 mm / min. Finally, obtain Bi 0.905 Sb 0.095Bar

[0059] Thermoelectric properties and mechanical strength tests were respectively carried out on the central part and the area near the surface of the obtained bar, and the test results are as Figure 3 and Figure 4 shown

[0060] Comparative Example 2

[0061] In this comparative example, a single-stage diameter-reducing die shown in Figure 5 was used to thermally extrude the same material in Example 2 to prepare a thermoelectric material. The die includes an upper die 9, a diameter-changing section die 10 and a lower die 11. The diameter of the upper die 9 is 90 mm, the diameter of the lower die 11 is 30 mm, and the profile angle β of the diameter-changing section die 10, which is the connecting part between the upper die and the lower die, is 50°. Other conditions are the same as those in Example 2

[0062] Testing and characterization

[0063] Samples were respectively cut from the central part and the edge part close to the surface of the bars prepared in Examples 1-2 and Comparative Examples 1-3 to detect thermoelectric properties and mechanical strength properties. The size of the electrical property sample strip is 2 mm×2 mm×10 mm, and the size of the thermal conductivity square piece is 10 mm×10 mm×2 mm. The relationship curve between the ZT value and temperature and the flexural strength property were mainly tested. Among them, the ZT value was calculated by measuring the electrical conductivity σ, the Seebeck coefficient α and the thermal conductivity κ, and then according to the formula ZT = σα 2 / κ; the flexural strength property was tested by the three-point bending method, and the size of the sample strip is 3 mm×3 mm×20 mm; the cutting direction of the above samples is along the extrusion direction of the crystal bar. The test results are shown in Figure 3-4 . In Example 1, the properties of the material are basically the same in different parts. The maximum ZT value is 1.13, and the flexural strength reaches 124 MPa, indicating that the N-type hot-extruded bar prepared by this method has excellent thermoelectric properties and mechanical properties. The results show that the material prepared by the traditional die has a poor grain orientation degree, low mechanical strength, poor material uniformity, and is prone to local cracks. The present invention solves the problems of low mechanical strength and uneven deformation existing in the preparation process of traditional bismuth telluride-based thermoelectric materials through the design of the extrusion die combined with the hot extrusion process, and provides a new technical path for the development of high-performance thermoelectric refrigeration devices. The extrusion die and method in the present invention can improve the plastic deformation ability of the material during hot extrusion, and improve the mechanical properties and thermoelectric properties of the material

[0064] The above are only the preferred embodiments of the present invention. It should be noted that: for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention

Claims

1. An extrusion die for bismuth telluride-based thermoelectric material, characterized in that: The die groove of the extrusion die includes an inlet section, a connecting section and an outlet section connected in sequence; the connecting section includes multiple reduced diameter sections in sequence from the inlet section to the outlet section; the outlet end diameter of any reduced diameter section among the multiple reduced diameter sections is larger than the outlet end diameter of the next reduced diameter section.

2. The extrusion die according to claim 1, characterized in that: The connecting section includes a primary diameter reduction section and a secondary diameter reduction section from the inlet section to the outlet section; one end of the primary diameter reduction section is connected to the inlet section, and the other end is connected to the secondary diameter reduction section; the diameter of the primary diameter reduction section converges from the end connected to the inlet section to the end connected to the secondary diameter reduction section; one end of the secondary diameter reduction section is connected to the primary diameter reduction section, and the other end is connected to the outlet section; the diameter of the secondary diameter reduction section converges from the end connected to the primary diameter reduction section to the end connected to the outlet section; the outlet end diameter of the primary diameter reduction section is larger than the outlet end diameter of the secondary diameter reduction section.

3. The extrusion die according to claim 1, characterized in that: The cross-sectional angle of the plurality of reduced diameter sections is 20-85°, preferably 40-80°.

4. The extrusion die according to claim 1, characterized in that: The inlet section and the outlet section are equal-diameter sections, and the area ratio of their cross sections is 4 to 10:

1.

5. The extrusion die according to claim 1, characterized in that: The extrusion die further comprises a heating mechanism for heating the die groove, wherein the heating mechanism is arranged outside the die groove; Preferably, the heating mechanism comprises a plurality of groups of heating rods; Preferably, the multiple groups of heating rods are evenly arranged outside the mold groove.

6. The extrusion die according to claim 5, characterized in that: The extrusion die also includes a temperature detection mechanism and a heating control mechanism; the temperature detection mechanism includes multiple sets of thermocouples, and the multiple sets of thermocouples are arranged between the heating rod and the groove wall of the die groove; the heating control mechanism is a PID controller; the PID controller is electrically connected to the multiple sets of thermocouples.

7. The extrusion die according to claim 1, characterized in that: The extrusion die also includes a pressure rod.

8. A method for preparing a bismuth telluride-based thermoelectric material by using the extrusion die according to any one of claims 1 to 7, characterized in that: The method comprises the steps of placing a bismuth telluride-based thermoelectric material ingot into the extrusion die and performing hot extrusion.

9. The method according to claim 8, characterized in that The hot extrusion is vacuum hot extrusion or hot extrusion in an inert gas protective atmosphere; Preferably, the temperature of the inlet section is 350-450°C; Preferably, the temperature of the connecting section is 350-450°C; Preferably, the temperature of the outlet section is 300-450°C.

10. The method according to claim 8, characterized in that The extrusion ratio of the hot extrusion is 4 to 10:1; Preferably, the extrusion pressure of the hot extrusion is 80-500 MPa; Preferably, the extrusion rate of the hot extrusion is 0.1-5 mm / min.

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