A reverse extrusion device and an extrusion method for producing homogeneous high-performance bismuth telluride thermoelectric materials

Abstract

This invention relates to a reverse extrusion apparatus and extrusion method for producing homogeneous, high-performance bismuth telluride thermoelectric materials, and pertains to the field of thermoelectric material preparation. This invention solves the problems of easy cracking during material forming, poor microstructure uniformity, and poor mass production adaptability in existing bismuth telluride hot extrusion processes. In this invention, the main cylinder and reverse extrusion cylinder of the dual-power extrusion unit drive different crossbeams to move independently, achieving step-by-step extrusion. The mandrel of the triaxial compressive stress forming unit is fixed to the crossbeam driven by the main cylinder, and the hollow plunger is fixed to the crossbeam driven by the reverse extrusion cylinder and sleeved on the outside of the mandrel. The lower ends of both extend into the mold and cooperate with the inner wall of the mold, forming axial and radial synergistic constraints, placing the material in a triaxial compressive stress state. The extrusion method includes loading and vacuuming, heating and heat preservation, main cylinder pre-pressing, reverse extrusion forming, pressure holding and cooling, and demolding. This invention can achieve homogeneous and dense forming of bismuth telluride materials, suppress cracks, and improve thermoelectric and mechanical properties. This invention is used in the preparation of thermoelectric materials.

Description

Technical Field

[0001] This invention relates to the field of thermoelectric material preparation equipment technology, specifically to a reverse extrusion equipment and extrusion method for the production of homogeneous high-performance bismuth telluride thermoelectric materials. Background Technology

[0002] Bismuth telluride-based alloys are the best-performing and only commercially available thermoelectric conversion materials in the room temperature range. Leveraging the Seebeck and Peltier effects, they have become core components in fields such as 5G optical modules, micro-refrigeration, and waste heat recovery. Their performance directly determines the energy conversion efficiency and reliability of thermoelectric devices. Commercial applications place multiple core requirements on bismuth telluride thermoelectric materials: they must simultaneously possess high thermoelectric figure of merit (ZT) and high mechanical strength, along with excellent material homogeneity and ease of processing. Furthermore, the manufacturing process must meet the characteristics of mass production, low cost, and low complexity.

[0003] Currently, the mainstream preparation methods for bismuth telluride-based thermoelectric materials in the industry are zone melting, powder metallurgy, and hot extrusion. However, all existing technologies have significant shortcomings and cannot simultaneously meet the four core commercialization requirements: high thermoelectric performance, high mechanical strength, high uniformity, and mass production. Materials prepared by zone melting have excellent thermoelectric properties but poor mechanical properties, while those prepared by powder metallurgy are the opposite. As the requirements for miniaturization, high reliability, and low power consumption of thermoelectric devices continue to increase in fields such as 5G and aerospace, existing preparation technologies are no longer suitable for the needs of industrial development. The industry urgently needs new preparation technologies to achieve synergistic optimization of thermoelectric and mechanical properties, a two-way balance between process complexity and mass production efficiency, and dual guarantees of material uniformity and molding quality.

[0004] Hot extrusion is a process that involves heating zone-melting rods or powder blanks to above their recrystallization temperature, then plastically deforming and extruding them under pressure. This method simultaneously introduces dynamic recrystallization and texturing, refining grains to improve mechanical strength while promoting grain orientation, thus balancing thermoelectric and mechanical properties. It also features continuous processing capabilities and possesses potential for commercial mass production. However, this method is currently still in the technology optimization stage, with three core challenges—forming quality, performance uniformity, and process adaptability—remaining major obstacles to its industrialization.

[0005] 1. Uneven material flow and poor performance consistency: Traditional forward extrusion has severe external friction. The friction between the inner wall of the die and the edge of the material is much greater than that in the central area, resulting in uneven plastic deformation of the material. The microstructure and properties of the extruded rods vary significantly along the axial and radial directions, and the layered structure of the crystal rods is incomplete.

[0006] 2. Tensile stress at the diameter change point makes it prone to cracking and poor formability: Traditional hot extrusion often uses unidirectional channel diameter change, curved diameter change or single diameter change structure. The material is prone to tensile stress at the diameter change point. Tensile stress will induce the initiation and propagation of microcracks, causing surface cracking and internal through cracks, resulting in low yield.

[0007] 3. Difficult demolding and low automation, resulting in insufficient mass production capacity: Bismuth telluride is prone to sticking to the mold at high temperatures, making demolding difficult. In addition, the equipment has a low degree of automation, and demolding and other processes rely on manual operation, resulting in low production efficiency and making it impossible to achieve true mass production.

[0008] In summary, the existing hot extrusion process for bismuth telluride has problems such as easy cracking during material forming, poor microstructure uniformity, and poor adaptability to mass production. Summary of the Invention

[0009] The purpose of this invention is to solve the problems of easy cracking during material forming, poor microstructure uniformity, and poor mass production adaptability in the existing hot extrusion process of bismuth telluride, and to provide a reverse extrusion equipment and extrusion method for the production of homogeneous high-performance bismuth telluride thermoelectric materials.

[0010] The technical solution of this invention is:

[0011] This invention provides a reverse extrusion apparatus for the production of homogeneous high-performance bismuth telluride thermoelectric materials. It includes a frame assembly, a dual-power extrusion unit, a triaxial compressive stress forming unit, and an auxiliary unit. The frame assembly includes a guide column and multiple crossbeams slidably mounted on the guide column. The dual-power extrusion unit includes a main cylinder and a reverse extrusion cylinder, which respectively drive different crossbeams to move independently along the guide column, achieving step-by-step extrusion. The triaxial compressive stress forming unit includes a mold, a mandrel, and a hollow plunger. The mold is fixedly configured such that the mandrel is fixed to a crossbeam driven by the main cylinder, and the hollow plunger is fixed to the crossbeam driven by the reverse extrusion cylinder and sleeved on the outside of the mandrel. The hollow plunger can slide axially relative to the mandrel. The lower ends of the mandrel and the hollow plunger extend into the mold and cooperate with the inner wall of the mold, forming a coordinated axial and radial constraint on the material, so that the material is in a triaxial compressive stress state during the extrusion molding process. The auxiliary unit includes a vacuum pump and a hydraulic station to provide process environment protection for the equipment.

[0012] Furthermore, the frame assembly includes two guide columns, a first crossbeam, a second crossbeam, a third crossbeam, and a base frame. The two guide columns are arranged vertically, and the lower ends of the two guide columns are fixedly installed on the base frame. The first crossbeam, the second crossbeam, and the third crossbeam are arranged sequentially from top to bottom and slide in cooperation with the two guide columns.

[0013] Preferably, both the main cylinder and the reverse extrusion cylinder are fixed on the upper surface of the first crossbeam, the piston rod of the main cylinder is connected to the upper surface of the second crossbeam, and the piston rod of the reverse extrusion cylinder is connected to the upper surface of the third crossbeam.

[0014] Preferably, the main cylinder and the reverse extrusion cylinder are controlled by independent hydraulic circuits to form a step-by-step extrusion mode; wherein, the main cylinder is used to pre-press the billet, and the reverse extrusion cylinder is used to perform reverse hot extrusion molding on the pre-pressed billet.

[0015] Furthermore, the spindle is a solid rigid structure, with its upper end fixed to a second crossbeam driven by the main cylinder. The hollow plunger is a hollow cylinder sleeved on the outside of the spindle, with its upper end fixed to a third crossbeam driven by the reverse extrusion cylinder. The hollow plunger moves linearly along the spindle with the third crossbeam.

[0016] Preferably, the mold is a modular, detachable hollow cylindrical structure, fixed to the working platform of the frame assembly. A heating cylinder is sleeved on the outside of the mold, and the mold and the heating cylinder are located inside a vacuum chamber, which is connected to the vacuum pump.

[0017] Preferably, the hollow plunger has a water-cooling groove inside for introducing cooling water to achieve cooling after the material is reverse-extruded.

[0018] Furthermore, the vacuum pump is used to evacuate and maintain pressure in the vacuum chamber; the hydraulic station is connected to the main cylinder and the reverse extrusion cylinder through hydraulic oil pipes, providing hydraulic power to the main cylinder and the reverse extrusion cylinder; the auxiliary unit also includes an air compressor and a water chiller, the air compressor is connected to the vacuum chamber and is used to break the vacuum after extrusion; the water chiller is connected to the water cooling tank in the hollow plunger and the cooling pipes of each unit for circulating cooling.

[0019] The present invention also provides an extrusion method using a reverse extrusion apparatus for the production of homogeneous high-performance bismuth telluride thermoelectric materials, comprising the following steps: Step 1: Load the bismuth telluride raw material into the mold cavity, close the vacuum chamber door, start the vacuum pump to evacuate and maintain pressure; Step 2: Start the heating cylinder to heat up to the set temperature and then maintain the temperature; Step 3: Start the main cylinder to drive the second crossbeam with the fixed column shaft downward to pre-press the bismuth telluride raw material in Step 1 and improve the initial density of the billet; Step 4: Keep the main cylinder stationary, start the reverse extrusion cylinder, drive the third crossbeam with the fixed hollow plunger downward, and drive the hollow plunger to extrude downward along the central axis. The bismuth telluride raw material flows upward and reverses along the annular gap between the lower end of the central axis and the inner wall of the hollow plunger to form the material. Step 5: After reaching the extrusion stroke, maintain pressure, turn on the water cooler to cool the formed bar and the mold, the main cylinder moves up to the preset stroke node one, the reverse extrusion cylinder moves up to the preset stroke node two, the formed bar begins to separate from the hollow plunger under the push force of the mandrel, then the air compressor is started to break the vacuum, the door is opened, the clamps are prepared to hold the formed bar, the reverse extrusion cylinder moves up again to the preset stroke node three, the bar completely separates from the hollow plunger under the push force of the mandrel, and the bar is taken out.

[0020] Furthermore, in step three, the pre-compression pressure is 0–15T; in step four, the reverse extrusion pressure is 0–100T, and the extrusion speed is 0.05–0.2 mm / s.

[0021] Compared with the prior art, the present invention has the following advantages:

[0022] 1. The step-by-step dual-power extrusion design integrates pre-compression and reverse extrusion, eliminating the need for separate cold-pressing pretreatment:

[0023] This invention employs an independent, step-by-step extrusion mode with a main cylinder for pre-extrusion and a reverse-extrusion cylinder for reverse forming. The main cylinder uniformly pre-presses the bismuth telluride powder within the mold, expelling interstitial gas and ensuring the initial density and flatness of the billet. Subsequently, the main cylinder remains stationary, while the reverse-extrusion cylinder applies force independently to drive the hollow plunger downwards, with the plunger shaft remaining fixed, causing the material to flow upwards under pressure within the mold cavity to complete the forming process. This design continuously completes pre-pressing and hot extrusion within the same mold and heating cavity, eliminating the necessary pre-processes such as cold pressing, transfer, and reloading required in traditional processes. This avoids damage, oxidation, or delamination of the billet during transfer, resulting in a simpler process flow, higher production stability, and better material density and uniformity.

[0024] 2. Triaxial compressive stress constraint to inhibit crack initiation:

[0025] This invention provides a stable triaxial compressive stress field by applying a counter-pressure force from the mandrel at the front end of the material, an axial extrusion force from the counter-extrusion cylinder, and a radial constraint force from the inner wall of the mold. This ensures that the bismuth telluride material remains in a triaxial compressive stress state throughout the entire process of extrusion, flow, diameter change, and molding. This completely eliminates the tensile stress that easily occurs in the diameter change zone and extrusion end in traditional forward extrusion, diameter change extrusion, or corner extrusion, effectively suppressing the initiation and propagation of microcracks, and ensuring that the finished product has no surface cracks and no through-hole defects.

[0026] 3. Precise connection of all components, high extrusion molding accuracy:

[0027] The main cylinder and the reverse extrusion cylinder are rigidly connected to the corresponding crossbeams. The crossbeams slide precisely along the guide column. The hollow plunger and the column shaft are coaxially matched to ensure the coaxiality and gap consistency during the extrusion process. The material is subjected to uniform force, flows stably, and has a consistent wall thickness during molding. The formed bar does not bend or become eccentric, and the uniformity of the structure and properties along the length direction is significantly improved.

[0028] 4. Integrated auxiliary units, high degree of automation, and strong batch production capability:

[0029] The equipment integrates auxiliary units such as vacuum pumps, hydraulic stations, air compressors, and water chillers to achieve full automation of the process, including vacuuming, heating and heat preservation, step extrusion, pressure holding, pressure release, and demolding. Only material handling and loading require manual operation. It can achieve kilogram-level batch production and solves the problem of difficult demolding of bismuth telluride due to high-temperature adhesion.

[0030] 5. Fully adjustable process parameters, modular and replaceable molds, and strong versatility:

[0031] The equipment's core process parameters, such as main cylinder pressure, reverse extrusion cylinder pressure, heating temperature, and extrusion speed, are all continuously adjustable, adaptable to the preparation of bismuth telluride materials with different formulations and density requirements. The mold adopts a modular and detachable structure, which can be quickly replaced according to different extrusion ratio requirements. It is also compatible with the preparation of N-type and P-type bismuth telluride-based thermoelectric materials. The equipment has strong versatility, low conversion costs, and a wide range of applications. Attached Figure Description

[0032] Figure 1 This is a schematic diagram of the main structure of the reverse extrusion device of the present invention;

[0033] Figure 2 This is a schematic diagram of the layout of each unit of the reverse extrusion device of the present invention;

[0034] Figure 3 The conductivity diagrams are for the products of Examples 1 and 2.

[0035] Figure 4 This is a Seebeck coefficient plot of the products of Examples 1 and 2;

[0036] Figure 5 These are the thermal conductivity diagrams of the products from Examples 1 and 2;

[0037] Figure 6 These are the thermoelectric figure of merit diagrams for the products of Examples 1 and 2;

[0038] Figure 7 These are the compressive strength diagrams of the products from Examples 1 and 2;

[0039] in:

[0040] 1. Main cylinder, 2. Reverse extrusion cylinder, 3. Mold, 4. Core shaft, 5. Hollow plunger, 6. Guide column, 7. First crossbeam, 8. Second crossbeam, 9. Third crossbeam, 10. Base frame, 11. Vacuum pump, 12. Hydraulic station, 13. Vacuum chamber, 14. Heating cylinder, 15. Vacuum chamber, 16. Air compressor, 17. Water chiller. Detailed Implementation

[0041] Specific implementation method one: Combining Figures 1 to 2 This embodiment describes a reverse extrusion apparatus for producing homogeneous high-performance bismuth telluride thermoelectric materials, comprising a frame assembly, a dual-power extrusion unit, a triaxial compressive stress forming unit, and an auxiliary unit. The frame assembly includes a guide column and multiple crossbeams slidably mounted on the guide column. The dual-power extrusion unit includes a main cylinder 1 and a reverse extrusion cylinder 2, which drive different crossbeams to move independently along the guide column, achieving step-by-step extrusion. The triaxial compressive stress forming unit includes a mold 3, a mandrel 4, and a hollow plunger 5. The mold 3 is fixedly installed, the spindle 4 is fixed to the crossbeam driven by the main cylinder 1, the hollow plunger 5 is fixed to the crossbeam driven by the reverse extrusion cylinder 2 and sleeved on the outside of the spindle 4, the hollow plunger 5 can slide axially relative to the spindle 4, the lower ends of the spindle 4 and the hollow plunger 5 extend into the mold 3 and cooperate with the inner wall of the mold 3 to form axial and radial co-constraints on the material, so that the material is in a triaxial compressive stress state during the extrusion molding process; the auxiliary unit includes a vacuum pump 11 and a hydraulic station 12 to provide process environment protection for the equipment.

[0042] The dual-power extrusion unit in this embodiment is the core power output component of the reverse extrusion equipment, and is mounted on the frame assembly. The main cylinder and the reverse extrusion cylinder adopt an independent control design to realize step-by-step extrusion action, providing graded extrusion force for bismuth telluride material. The specific connection relationship, power parameters and working mode are as follows:

[0043] The connection is as follows: both the main cylinder and the reverse extrusion cylinder are fixed to the upper surface of the first crossbeam 7. The piston rod of the main cylinder is bolted to the upper surface of the second crossbeam 8, and the extension and retraction of the main cylinder can directly drive the second crossbeam 8 to move vertically up and down along the guide column. The piston rod of the reverse extrusion cylinder is bolted to the upper surface of the third crossbeam 9, and the extension and retraction of the reverse extrusion cylinder can directly drive the third crossbeam 9 to move vertically up and down along the guide column.

[0044] The power parameters are as follows: the main cylinder outputs an extrusion pressure that is continuously adjustable from 0 to 50T, providing the expected extrusion pressure for bismuth telluride materials; the reverse extrusion cylinder outputs an extrusion pressure that is continuously adjustable from 0 to 100T, providing the core power for reverse extrusion molding of bismuth telluride materials.

[0045] The working mode is as follows: the main cylinder and the reverse extrusion cylinder are controlled by independent hydraulic circuits, realizing a step-by-step extrusion mode in which only the main cylinder and the reverse extrusion cylinder move independently. During the first extrusion, the main cylinder applies force, and during the second extrusion, the reverse extrusion cylinder applies force, so as to achieve uniform force distribution on the material.

[0046] The triaxial compressive stress forming unit in this embodiment is set on the working platform of the frame assembly. It is the core unit for realizing the reverse flow forming of bismuth telluride and triaxial compressive stress constraint, ensuring uniform material structure and no crack initiation. The specific structure and parameters are as follows:

[0047] The mold adopts a modular and detachable structure. The main body of the mold is preferably a hollow cylinder, which is fixed to the working platform of the frame assembly. The mold cavity volume is 1400 cm³.

[0048] The core shaft is a solid rigid structure with a preferred diameter of 100mm. When the equipment is not in operation, it is flush with the bottom of the hollow plunger. The upper part is fixed to the lower surface of the second crossbeam 8 with bolts to provide continuous axial pressure for the bismuth telluride billet. The hollow plunger is preferably a hollow cylinder with an outer diameter of 300mm and an inner diameter of 100mm. It is sleeved on the outside of the core shaft, and its upper end is bolted to the lower surface of the third crossbeam 9. It can move precisely in a straight line along the core shaft with the third crossbeam 9.

[0049] Specific Implementation Method Two: Combining Figure 1 This embodiment describes a frame assembly comprising two guide columns 6, a first crossbeam 7, a second crossbeam 8, a third crossbeam 9, and a base frame 10. The two guide columns 6 are arranged vertically, and their lower ends are fixedly mounted on the base frame 10. The first crossbeam 7, the second crossbeam 8, and the third crossbeam 9 are arranged sequentially from top to bottom and slide in cooperation with the two guide columns 6.

[0050] In this embodiment, the frame assembly serves as the basic support structure for the equipment, used for the installation, positioning, and guidance of each functional unit. It includes two guide columns, three crossbeams, and a base frame. The three crossbeams are all slidably engaged with the two guide columns, enabling each crossbeam to move precisely and linearly along the guide columns without jamming, thus providing guidance for the step-by-step extrusion of the dual-power extrusion unit.

[0051] Specific implementation method three: Combining Figure 1 In this embodiment, the main cylinder 1 and the reverse extrusion cylinder 2 are both fixed on the upper surface of the first crossbeam 7. The piston rod of the main cylinder 1 is connected to the upper surface of the second crossbeam 8, and the piston rod of the reverse extrusion cylinder 2 is connected to the upper surface of the third crossbeam 9.

[0052] Specific implementation method four: Combination Figure 1 In this embodiment, the main cylinder 1 and the reverse extrusion cylinder 2 are controlled by independent hydraulic circuits to form a step-by-step extrusion mode; wherein, the main cylinder 1 is used to pre-press the billet, and the reverse extrusion cylinder 2 is used to perform reverse hot extrusion molding on the pre-pressed billet.

[0053] Specific Implementation Method Five: Combining Figure 1In this embodiment, the central shaft 4 is a solid rigid structure, with its upper end fixed to the second crossbeam 8 driven by the main cylinder 1. The hollow plunger 5 is a hollow cylinder sleeved on the outside of the central shaft 4, with its upper end fixed to the third crossbeam 9 driven by the reverse extrusion cylinder 2. The hollow plunger 5 moves linearly along the central shaft 4 with the third crossbeam 9.

[0054] Specific Implementation Method Six: Combination Figure 1 In this embodiment, the mold 3 is a modular, detachable hollow cylindrical structure, fixed to the working platform of the frame assembly. A heating cylinder 14 is sleeved on the outside of the mold 3. The mold 3 and the heating cylinder 14 are located inside the vacuum chamber 15, which is connected to the vacuum pump 11.

[0055] The heating cylinder is sleeved on the outside of the mold, and the heating power is 1-4KW;

[0056] Specific implementation method seven: Combination Figure 1 This embodiment describes a hollow plunger 5 with a water-cooling groove inside, which is used to introduce cooling water to achieve cooling after the material is reverse-extruded.

[0057] The hollow plunger has a water-cooling tank for circulating cooling water to cool the material after the reverse extrusion is completed.

[0058] Specific implementation method eight: Combination Figure 2 In this embodiment, the vacuum pump 11 is used to evacuate and maintain pressure in the vacuum chamber 15; the hydraulic station 12 is connected to the main cylinder 1 and the reverse extrusion cylinder 2 through hydraulic oil pipes, and provides hydraulic power to the main cylinder 1 and the reverse extrusion cylinder 2.

[0059] The auxiliary unit also includes an air compressor 16 and a water chiller 17. The air compressor 16 is connected to the vacuum chamber 15 and is used to break the vacuum after extrusion. The water chiller 17 is connected to the water cooling tank in the hollow plunger 5 and the cooling pipes of each unit for circulating cooling.

[0060] The auxiliary units in this embodiment include a vacuum pump, an air compressor, a hydraulic station, and a water chiller, providing process guarantees such as vacuum environment, pneumatic pressure, hydraulic power, and circulating cooling for the normal operation of the equipment. The vacuum pump is preferably 0.25-0.75KW, which can pump the vacuum level in the vacuum chamber to ≤10Pa and maintain the pressure, providing a low-oxygen vacuum environment for the reverse extrusion of bismuth telluride material and avoiding performance degradation caused by high-temperature oxidation. After extrusion, the air compressor provides compressed air to quickly fill the vacuum chamber and break the vacuum, achieving rapid balance of internal and external pressure. The hydraulic station is preferably 3-5.5KW, connected to the main cylinder and reverse extrusion cylinder through high-pressure hydraulic oil pipes, providing high-pressure hydraulic oil to the dual-power extrusion unit. It is the core source of power output for the main cylinder and reverse extrusion cylinder, and works with pressure sensors to achieve precise control of extrusion force and pressure. The water chiller prevents each unit from operating under overload conditions at high temperatures, ensuring the stability and service life of each unit.

[0061] The basic workflow of the extrusion equipment of this invention is as follows: Open the vacuum chamber door—align the mold—load the material—close the vacuum chamber door—evacuate the vacuum—heat and start—keep the temperature—the main cylinder descends to the set stroke to achieve pre-compression of the material—the reverse extrusion cylinder descends—the material flows upward into the mandrel to achieve the reverse extrusion process—after reaching the set stroke node, maintain the pressure—the main cylinder moves upward—after the reverse extrusion cylinder moves upward, the material begins to separate from the hollow plunger under the top force of the mandrel—the suction valve opens—open the chamber door—prepare the clamp to hold the material—the reverse extrusion cylinder moves upward again—after reaching the stroke node, remove the material—complete one operation.

[0062] Specific Implementation Method Nine: Combining Figures 1 to 2 This embodiment describes an extrusion method that includes the following steps: Step 1: Load the bismuth telluride raw material into the mold cavity of mold 3, close the vacuum chamber door, start the vacuum pump 11 to evacuate and maintain pressure; Step 2: Start the heating cylinder 14 to heat up to the set temperature and then maintain the temperature; Step 3: Start the main cylinder 1 to drive the second crossbeam 8 of the fixed column shaft 4 downward to pre-press the bismuth telluride raw material in Step 1 and improve the initial density of the billet; Step 4: Main cylinder 1 remains stationary, start reverse extrusion cylinder 2, drive the third crossbeam 9 of the fixed hollow plunger 5 downward, drive the hollow plunger 5 to extrude downward along the central shaft 4, and the bismuth telluride raw material flows upward in the opposite direction along the annular gap between the lower end of the central shaft 4 and the inner wall of the hollow plunger 5 to form the material. Step 5: After reaching the extrusion stroke, maintain pressure, turn on the water cooler 17 to cool the formed bar and the mold 3, the main cylinder 1 moves up to the preset stroke node one, the reverse extrusion cylinder 2 moves up to the preset stroke node two, the formed bar begins to separate from the hollow plunger 5 under the push force of the mandrel 4, then the air compressor 16 is started to break the vacuum, the door is opened, and after the clamp is prepared to hold the formed bar, the reverse extrusion cylinder 2 moves up again to the preset stroke node three, the bar completely separates from the hollow plunger 5 under the push force of the mandrel 4, and the bar is taken out.

[0063] This embodiment improves the density of the billet by pre-pressurizing and venting, then uses reverse extrusion to make the material flow and form under triaxial compressive stress, suppressing crack generation, and finally uses water cooling and ejection structure to achieve rapid demolding, ensuring that the bar has a uniform structure and a complete surface.

[0064] Specific Implementation Method Ten: Combining Figures 1 to 2 In this embodiment, the pre-compression pressure in step three is 0-15T; the back-extrusion pressure in step four is 0-100T, and the extrusion speed is 0.05-0.2mm / s.

[0065] In this embodiment, the pre-compression pressure of 0-15T ensures that the billet is initially compacted without breaking, and the reverse extrusion pressure of 0-100T combined with low-speed extrusion of 0.05-0.2mm / s allows the material to flow plastically under high pressure and refine the grains, while avoiding the rapid generation of cracks.

[0066] Combination Figures 1 to 7 Description of embodiments of the present invention:

[0067] Example 1

[0068] This example demonstrates the production of N-type bismuth telluride using a reverse extrusion apparatus for producing homogeneous, high-performance bismuth telluride materials. The specific implementation steps are as follows:

[0069] 1. Material Synthesis: Bi, Se, and Te are synthesized according to the stoichiometric ratio Bi₂Te. 3-x Se x (x=0.3-0.7, purity≥99.99%), place them into the quartz tubes in order of increasing melting point, seal the tubes so that the pressure inside the tubes is less than 10Pa, place them in a swing melting furnace, melt for 1-2 hours, and then take out the quartz tubes and cool them in the air.

[0070] 2. Powder crushing: The material synthesized in step 1 is crushed using a crusher.

[0071] 3. Loading and vacuuming: Load the material fragments obtained in step two into the mold cavity of the equipment, close the mold cavity, start the vacuum pump, and evacuate to 10Pa.

[0072] 4. Heating and heat preservation: Start the heating cylinder, raise the temperature to 470℃, and keep it at the temperature for 25 minutes after reaching the temperature.

[0073] 5. Pre-extrusion: Start the main cylinder, set the pressure to 20T, and hold the pressure for 5 minutes to improve the initial density of the billet.

[0074] 6. Reverse hot extrusion: The main cylinder remains stationary, and the reverse extrusion cylinder is started. The pressure is 75T and the extrusion speed is 0.1mm / s. The material flows upward and reverses between the mandrel and the hollow plunger to form the shape. After reaching the extrusion stroke, the pressure is held for 10 minutes to fix the grain structure. Then the main cylinder moves upward to the stroke node, and the reverse extrusion cylinder moves upward again. The formed bar begins to separate from the hollow plunger under the action of the mandrel.

[0075] 7. Cooling: Turn on the water chiller to cool the sample and mold to below 50°C.

[0076] 8. Automatic demolding: Start the suction valve, open the door, prepare the clamp, and the reverse extrusion cylinder 2 moves upward again to remove the bar material, completing one operation process.

[0077] Example 2

[0078] This example demonstrates the production of P-type bismuth telluride using a reverse extrusion apparatus for producing homogeneous, high-performance bismuth telluride materials. The specific implementation steps are as follows:

[0079] 1. Material Synthesis: Bi, Sb, and Te are synthesized according to the stoichiometric ratio of Bi... 2-y Sb y Te3 (y=1.2-1.5, purity ≥99.99%) is placed into quartz tubes in order of increasing melting point. After sealing the tubes, the pressure inside the tubes is made less than 10 Pa. The tubes are then placed in a swing melting furnace and melted for 1-2 hours. After melting, the quartz tubes are removed and cooled in the air.

[0080] 2. Powder crushing: The material synthesized in step 1 is crushed using a crusher.

[0081] 3. Loading and vacuuming: Load the material fragments obtained in step two into the mold cavity of the equipment, close the mold cavity, start the vacuum pump, and evacuate to 10Pa.

[0082] 4. Heating and heat preservation: Start the heating cylinder, raise the temperature to 490℃, and keep it at the temperature for 35 minutes after reaching the temperature.

[0083] 5. Pre-extrusion: Start the main cylinder, set the pressure to 20T, and hold the pressure for 5 minutes to improve the initial density of the billet.

[0084] 6. Reverse hot extrusion: The main cylinder remains stationary, and the reverse extrusion cylinder is started. The pressure is 85T and the extrusion speed is 0.15mm / s. The material flows upward and reverses between the mandrel and the hollow plunger to form the shape. After reaching the extrusion stroke, the pressure is held for 10 minutes to fix the grain structure. Then the main cylinder moves upward to the stroke node, and the reverse extrusion cylinder moves upward again. The formed bar begins to separate from the hollow plunger under the action of the mandrel.

[0085] 7. Cooling: Turn on the water chiller to cool the sample and mold to below 50°C.

[0086] 8. Automatic demolding: Start the suction valve, open the door, prepare the clamp, and the reverse extrusion cylinder 2 moves upward again to remove the bar material, completing one operation process.

[0087] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A reverse extrusion apparatus for the production of homogeneous high-performance bismuth telluride thermoelectric materials, characterized in that: It includes a frame assembly, a dual-power extrusion unit, a triaxial compressive stress forming unit, and auxiliary units; The frame assembly includes a guide column and multiple crossbeams that are slidably mounted on the guide column; The dual-power extrusion unit includes a main cylinder (1) and a counter-extrusion cylinder (2). The main cylinder (1) and the counter-extrusion cylinder (2) drive different crossbeams to move independently along the guide column to achieve step-by-step extrusion. The triaxial compressive stress forming unit includes a mold (3), a mandrel (4), and a hollow plunger (5). The mold (3) is fixedly set. The mandrel (4) is fixed on a crossbeam driven by the main cylinder (1). The hollow plunger (5) is fixed on a crossbeam driven by the anti-extrusion cylinder (2) and sleeved on the outside of the mandrel (4). The hollow plunger (5) can slide axially relative to the mandrel (4). The lower ends of the mandrel (4) and the hollow plunger (5) extend into the mold (3) and cooperate with the inner wall of the mold (3) to form a axial and radial co-constraint on the material, so that the material is in a triaxial compressive stress state during the extrusion forming process. The auxiliary unit includes a vacuum pump (11) and a hydraulic station (12) that provide process environment protection for the equipment.

2. The reverse extrusion equipment for producing homogeneous high-performance bismuth telluride thermoelectric materials according to claim 1, characterized in that: The frame assembly includes two guide columns (6), a first crossbeam (7), a second crossbeam (8), a third crossbeam (9), and a base frame (10). The two guide columns (6) are arranged vertically, and the lower ends of the two guide columns (6) are fixedly installed on the base frame (10). The first crossbeam (7), the second crossbeam (8), and the third crossbeam (9) are arranged sequentially from top to bottom and slide in cooperation with the two guide columns (6).

3. The reverse extrusion equipment for producing homogeneous high-performance bismuth telluride thermoelectric materials according to claim 2, characterized in that: The main cylinder (1) and the reverse extrusion cylinder (2) are both fixed on the upper surface of the first crossbeam (7). The piston rod of the main cylinder (1) is connected to the upper surface of the second crossbeam (8), and the piston rod of the reverse extrusion cylinder (2) is connected to the upper surface of the third crossbeam (9).

4. The reverse extrusion equipment for producing homogeneous high-performance bismuth telluride thermoelectric materials according to claim 1, characterized in that: The main cylinder (1) and the reverse extrusion cylinder (2) are controlled by independent hydraulic circuits to form a step-by-step extrusion mode; wherein, the main cylinder (1) is used to pre-press the billet, and the reverse extrusion cylinder (2) is used to reverse hot extrusion molding of the pre-pressed billet.

5. The reverse extrusion equipment for producing homogeneous high-performance bismuth telluride thermoelectric materials according to claim 1, characterized in that: The spindle (4) is a solid rigid structure, and its upper end is fixed to the second crossbeam (8) driven by the main cylinder (1). The hollow plunger (5) is a hollow cylinder sleeved on the outside of the spindle (4), and its upper end is fixed to the third crossbeam (9) driven by the reverse extrusion cylinder (2). The hollow plunger (5) moves linearly along the spindle (4) with the third crossbeam (9).

6. The reverse extrusion equipment for producing homogeneous high-performance bismuth telluride thermoelectric materials according to claim 1, characterized in that, The mold (3) is a modular, detachable hollow cylindrical structure, fixed to the working platform of the frame assembly. A heating cylinder (14) is sleeved on the outside of the mold (3). The mold (3) and the heating cylinder (14) are located inside the vacuum chamber (15). The vacuum chamber (15) is connected to the vacuum pump (11).

7. The reverse extrusion equipment for producing homogeneous high-performance bismuth telluride thermoelectric materials according to claim 1, characterized in that, The hollow plunger (5) is equipped with a water-cooling groove inside, which is used to introduce cooling water to achieve cooling after the material is reverse-extruded.

8. The reverse extrusion equipment for producing homogeneous high-performance bismuth telluride thermoelectric materials according to claim 6, characterized in that: The vacuum pump (11) is used to evacuate and maintain pressure in the vacuum chamber (15); the hydraulic station (12) is connected to the main cylinder (1) and the reverse extrusion cylinder (2) through hydraulic oil pipes, and provides hydraulic power to the main cylinder (1) and the reverse extrusion cylinder (2); The auxiliary unit also includes an air compressor (16) and a water chiller (17). The air compressor (16) is connected to the vacuum chamber (15) and is used to break the vacuum after extrusion. The water chiller (17) is connected to the water cooling tank in the hollow plunger (5) and the cooling pipes of each unit for circulating cooling.

9. An extrusion method using a reverse extrusion apparatus for the production of homogeneous high-performance bismuth telluride thermoelectric materials as described in any one of claims 1 to 8, characterized in that: Includes the following steps: Step 1: Load the bismuth telluride raw material into the mold cavity of the mold (3), close the vacuum chamber door, start the vacuum pump (11) to evacuate and maintain pressure; Step 2: Start the heating cylinder (14) to heat up to the set temperature and then keep it warm; Step 3: Start the main cylinder (1) to drive the second crossbeam (8) of the fixed column spindle (4) to descend, pre-press the bismuth telluride raw material in Step 1, and improve the initial density of the billet; Step 4: Keep the main cylinder (1) stationary, start the reverse extrusion cylinder (2), drive the third crossbeam (9) of the fixed hollow plunger (5) to move downward, drive the hollow plunger (5) to extrude downward along the column shaft (4), and the bismuth telluride raw material flows upward in the opposite direction along the annular gap between the lower end of the column shaft (4) and the inner wall of the hollow plunger (5) to form the shape. Step 5: After reaching the extrusion stroke, maintain pressure and turn on the water cooler (17) to cool the forming rod and the mold (3). Then, the main cylinder (1) moves up to the preset stroke node one, and the reverse extrusion cylinder (2) moves up to the preset stroke node two. Under the action of the push-out force of the mandrel (4), the forming rod begins to separate from the hollow plunger (5). Then, the air compressor (16) is started to break the vacuum, the hatch is opened, and the forming rod is prepared with the clamp. Then, the reverse extrusion cylinder (2) moves up again to the preset stroke node three. Under the action of the push-out force of the mandrel (4), the rod completely separates from the hollow plunger (5) and is taken out.

10. The extrusion method according to claim 9, characterized in that: In step three, the pre-compression pressure is 0-15T; in step four, the reverse extrusion pressure is 0-100T, and the extrusion speed is 0.05-0.2mm / s.

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