A method for preparing p-type bismuth telluride-based ultrafine crystal thermoelectric material based on copper-assisted extrusion forming

Abstract

The application belongs to the technical field of bismuth telluride-based thermoelectric materials, and particularly relates to a preparation method of p-type bismuth telluride-based ultrafine crystal thermoelectric materials based on copper-assisted extrusion forming. The application provides a method for preparing p-type bismuth telluride-based ultrafine crystal thermoelectric materials by using a copper mask-assisted extrusion forming technology. High-strength and high-ductility copper masks are used to apply axial constraints on the materials, so that the brittle cracking of bismuth telluride in the process of high-speed and large deformation hot extrusion is effectively avoided, the non-uniform deformation in the process of hot extrusion is relieved, and the deformation energy is fully accumulated to induce recrystallization and refine the grains. The grains are selectively oriented in the direction of directional deformation, so that the carrier mobility is improved. The bismuth telluride material is deformed under constraints, so that twinning is induced and coherent twin boundaries are generated, so that the mechanical and thermoelectric properties of the p-type bismuth telluride-based thermoelectric materials are simultaneously greatly strengthened.

Description

Technical Field

[0001] This invention belongs to the field of bismuth telluride-based thermoelectric materials technology, and in particular, a method for preparing p-type bismuth telluride-based ultrafine-grained thermoelectric materials based on copper-assisted extrusion molding. Background Technology

[0002] Thermoelectric conversion materials enable the mutual conversion of thermal and electrical energy, and have broad application prospects in waste heat power generation, solid-state refrigeration, and other fields, thus occupying an important position in new energy materials. Bismuth telluride alloy is currently the only commercially available thermoelectric material and a key component of thermoelectric devices. It is generally produced through directional solidification, but the mechanical strength and processing performance of bismuth telluride materials produced by this process are low, and thermoelectric elements can only be cut to a size of 0.8 mm or larger. Therefore, many researchers have adopted powder metallurgy technology to prepare polycrystalline Bi2Te3-based thermoelectric materials. Powder metallurgy technology can control the grain size of materials to a minimum of nanometer or submicron (<1 μm) through high-energy ball milling, solvothermal methods, etc., comprehensively improving the thermoelectric and mechanical properties of bismuth telluride-based alloys. However, this process, on the one hand, loses the anisotropy of the material, forming isotropy, which limits further improvement of the material's thermoelectric performance; on the other hand, ball milling, solvothermal methods, and other techniques have low powder production efficiency and high cost, which is not conducive to large-scale mass production.

[0003] In recent years, theoretical and experimental studies have shown that plastic deformation can induce bismuth telluride grains to rotate and rearrange, resulting in preferred orientation and improved carrier mobility. Furthermore, it can refine grains through recovery and recrystallization, reducing thermal conductivity and increasing strength. Therefore, plastic deformation technology has shown great potential in simultaneously improving the thermoelectric and mechanical properties of p-type bismuth telluride-based alloys. However, bismuth telluride-based alloys are brittle and have poor hot workability. During plastic deformation, their thermoplasticity is severely insufficient, leading to easy cracking, stress release, and hindering the accumulation of deformation energy. This results in insufficient recrystallization driving force, weakened grain preferred orientation, and severely impacts the surface quality and the improvement of thermoelectric and mechanical properties. Therefore, seeking a simple, efficient, reliable, and high-performance preparation process is of great significance for the preparation of p-type bismuth telluride-based thermoelectric materials. Summary of the Invention

[0004] This invention provides a method for preparing p-type bismuth telluride-based ultrafine-grained thermoelectric materials using a copper-mask-assisted extrusion molding technique. The high-strength, high-ductility copper mask applies axial constraint to the material, effectively preventing brittle cracking of bismuth telluride during high-speed, large-deformation hot extrusion and mitigating uneven deformation. Simultaneously, the accumulated deformation energy induces recrystallization to refine the grains. The grains develop selective orientation in the direction of directional deformation, improving carrier mobility. Deformation of the bismuth telluride material under constraint induces twinning and generates coherent twin boundaries, thereby significantly enhancing the mechanical and thermoelectric properties of the p-type bismuth telluride-based thermoelectric material. Furthermore, at high temperatures, Cu in the copper mask readily diffuses into the Te-Te interstices in the bismuth telluride lattice, increasing the formation energy of Te vacancies to suppress donor-like effects and improve the repeatability of its thermoelectric performance.

[0005] To achieve the above objectives, the technical solution adopted in this invention is a method for preparing p-type bismuth telluride-based ultrafine-grained thermoelectric materials based on copper-assisted extrusion molding, the specific steps of which are as follows:

[0006] Step 1: Using Bi, Te, and Se as raw materials, according to the stoichiometric ratio of Bi... x Sb 2-x Te3 is formulated into a dense mass, wherein 0.3 ≤ x ≤ 0.5;

[0007] Step 2: Encapsulate the dense bulk material obtained in Step 1 into a copper shield to obtain the precursor.

[0008] Step 3: Place the precursor in the extrusion mold, heat it, and extrude it directly from the mold to form the shape. Cool it in air or an inert atmosphere.

[0009] Step 4: Remove the copper shield to obtain the n-type bismuth telluride-based ultrafine-grained thermoelectric material.

[0010] Moreover, the purity of Bi, Sb, and Te in step 1 is above 99.99%.

[0011] Moreover, the density of the dense bulk material in step 1 is above 95%, and the dense bulk material is a smelted ingot, a zone-melted single crystal rod, or a hot-pressed sintered bulk material.

[0012] Moreover, the copper shield in step 2 is a shield made of copper or copper alloy with a thickness greater than 0.1 mm. The copper shield structure is a closed cylindrical surface, a closed free elongation end face, and an open or closed end face under compression. The gap between the copper shield and the dense block is <0.1 mm, and the copper shield and the dense block are flush on the top and bottom surfaces. When compressed, the copper shield constrains the dense block.

[0013] Furthermore, in step 3, the extrusion angle of the extrusion die is 90° to 180°, the extrusion ratio is greater than 9:1, the extrusion conditions are heating to 390°C to 530°C, holding for 2 minutes to 2 hours, and the extrusion pressure is 40 to 1000 MPa.

[0014] Furthermore, if the dense bulk body is a zone-melting single crystal rod, then the extrusion direction in step 3 is parallel to the axis of the zone-melting single crystal rod; if the dense bulk body is a hot-pressed sintered bulk body, then the extrusion direction in step 3 is perpendicular to the hot-pressing direction of the sintered bulk body.

[0015] Compared with existing technologies, the beneficial effects of this invention are as follows: 1. The p-type bismuth telluride-based ultrafine-grained thermoelectric material products prepared by this invention have uniform microstructures, significant selective grain orientation, and are all nanocrystalline and submicron-sized (<1μm). Their mechanical and thermoelectric properties are far superior to single-crystal products prepared by directional solidification technology, hot-pressed sintered products prepared by powder metallurgy, and extruded products prepared by conventional hot deformation processes, thus possessing high practical value; 2. The technical method provided by this invention is reliable. Samples are directly extruded through a mold, rapidly cooled in air or an inert atmosphere (avoiding grain growth), and sampled. There is no need to wait for the mold to cool slowly and demolding is unnecessary. The operation is simple, production efficiency is high, and cost is low; 3. This invention, in principle, utilizes a high-strength, high-ductility copper shield to apply axial constraint to the material, effectively avoiding… The high-speed, large-deformation hot extrusion process induces brittle cracking in bismuth telluride, mitigating uneven deformation during hot extrusion. High-speed, large deformation allows for the accumulation of deformation energy, inducing recrystallization and refining the grains. Furthermore, due to directional deformation, the grains develop selective orientation in that direction, significantly improving carrier mobility. Moreover, the constrained deformation of bismuth telluride can induce twinning, generating coherent twin boundaries that strongly scatter low-frequency phonons and low-energy electrons, further increasing the Seebeck coefficient and electrical conductivity while reducing thermal conductivity. This simultaneously and significantly enhances the mechanical and thermoelectric properties of p-type bismuth telluride-based thermoelectric materials. 4. The copper shield used in this invention can undergo a diffusion reaction with bismuth telluride at high temperatures. Cu readily diffuses into the Te-Te basal planes of the bismuth telluride lattice, increasing the formation energy of Te vacancies to suppress the formation of Te vacancies and Bi. Te The formation of antisite defects reduces the donor-like effect, thereby improving the repeatability of thermoelectric properties of p-type bismuth telluride alloys. Attached Figure Description

[0016] Figure 1 The polar diagram of the (0 0 0 1) crystal plane of the sample surface perpendicular to the axis in Example 1;

[0017] Figure 2 The cross-sectional morphology of the sample perpendicular to the axis in Example 1;

[0018] Figure 3 The resistivity is shown in the embodiment.

[0019] Figure 4 The Seebeck coefficient in the embodiment;

[0020] Figure 5 The thermal conductivity is shown in the example.

[0021] Figure 6 This refers to the zT value in the example. Detailed Implementation

[0022] The present invention will now be described in detail with reference to the accompanying drawings, comparative examples, and embodiments. However, the scope of the present invention is not limited to the following embodiments.

[0023] Comparative Example 1:

[0024] The commercially available p-type bismuth telluride-based thermoelectric material prepared by zone melting has a maximum ZT value of ~0.95 (300K), a compressive strength of 22MPa at room temperature, and a flexural strength of 15MPa.

[0025] Comparative Example 2:

[0026] The p-type bismuth telluride-based polycrystalline thermoelectric material prepared by hot pressing sintering has a maximum ZT value of ~1.01 (423 K), a compressive strength of 62.1 MPa and a flexural strength of 40.3 MPa at room temperature.

[0027] Example 1:

[0028] The component is Bi 0.32 Sb 1.68 A hot-pressed sintered bulk of Te3 (97% density) was encapsulated in a 0.3 mm thick copper shield along a predetermined direction (ensuring a gap of less than 0.1 mm) to obtain a precursor. The precursor was placed in a mold (extrusion angle 90°, extrusion ratio 25:1), heated to 530°C, held for 40 min, and then directly extruded from the mold under a pressure of 200 MPa, followed by air cooling. The copper shield was then removed to obtain the final p-type bismuth telluride-based ultrafine-grained thermoelectric material. The grain size of the samples was all ~1 μm (see...). Figure 1 Preferred orientation along the (0 0 0 1) crystal plane (see...) Figure 2 The highest ZT value of 1.39 (383K) was obtained, and the compressive strength at room temperature was 66.1MPa and the flexural strength was 44.8MPa, both of which were higher than those of comparative examples 1 and 2.

[0029] Example 2:

[0030] The component is Bi 0.41 Sb 1.59A hot-pressed sintered Te3 mass (98% density) was encapsulated in a 0.5mm thick copper shield along a predetermined direction (ensuring a gap of less than 0.1mm) to obtain a precursor. The precursor was placed in a mold (mold extrusion angle of 150°, extrusion ratio of 20:1), heated to 510℃, held for 1 hour, and then directly extruded from the mold at a pressure of 330MPa, cooled by N2 gas. The copper shield was then peeled off to obtain the final p-type bismuth telluride-based ultrafine-grained thermoelectric material. The grain size of the sample was ~0.7μm, with a preferred orientation along the (0 0 0 1) crystal plane, achieving a maximum ZT value of 1.52 (383K). The compressive strength at room temperature was 67.5MPa, and the flexural strength was 46MPa, both higher than those of comparative examples 1 and 2.

[0031] Example 3:

[0032] The component is Bi 0.5 Sb 1.5 A hot-pressed sintered Te3 mass (99% density) was encapsulated in a 1mm thick copper shield along a predetermined direction (ensuring a gap of less than 0.1mm) to obtain a precursor. The precursor was placed in a mold (180° extrusion angle, 9:1 extrusion ratio), heated to 410℃, held for 1.5h, and then directly extruded from the mold at a pressure of 950MPa, followed by air cooling. The copper shield was then peeled off to obtain the final p-type bismuth telluride-based ultrafine-grained thermoelectric material. The grain size of the sample was approximately 0.58μm, with a preferred orientation along the (0 0 0 1) crystal plane. The maximum ZT value of 1.48 (343K) was obtained, and the compressive strength at room temperature was 69MPa and the flexural strength was 53MPa, both higher than those of comparative examples 1 and 2.

Claims

1. A method for preparing p-type bismuth telluride-based ultrafine-grained thermoelectric material based on copper-assisted extrusion molding, characterized in that... The specific steps are as follows: Step 1: Using Bi, Sb, and Te as raw materials, according to the stoichiometric ratio of Bi... x Sb 2-x Te3 is prepared into a dense bulk material, wherein 0.3≤x≤0.5, the density of the dense bulk material is above 95%, and the dense bulk material is a zone melting single crystal rod; Step 2: The dense block obtained in Step 1 is encapsulated in a copper shield to obtain a precursor. The copper shield is made of copper or copper alloy and has a thickness greater than 0.1 mm. The structure of the copper shield is a closed cylindrical surface, a closed free elongation end face, and an open or closed compression end face. The gap between the copper shield and the dense block is <0.1 mm, and the copper shield and the dense block are flush on the top and bottom surfaces. When compressed, the copper shield constrains the dense block. Step 3: Place the precursor in the extrusion die, heat it and extrude it directly from the die. There is no need to wait for the die to cool down slowly and no need to demold. It can be cooled in air or an inert atmosphere. The extrusion angle of the extrusion die is 90° to 180°, the extrusion ratio is greater than 9:1, the extrusion conditions are heating to 390° to 530°, holding for 2 min to 2 h, and the extrusion pressure is 40 to 1000 MPa. Step 4: Remove the copper shield to obtain the p-type bismuth telluride-based ultrafine-grained thermoelectric material.

2. The method for preparing p-type bismuth telluride-based ultrafine-grained thermoelectric material based on copper-assisted extrusion molding according to claim 1, characterized in that: In step 1, the purity of Bi, Sb, and Te is above 99.99%.

3. The method for preparing p-type bismuth telluride-based ultrafine-grained thermoelectric material based on copper-assisted extrusion molding according to claim 1, characterized in that: In step 3, the extrusion direction is parallel to the axis of the zone melting single crystal rod.

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