A method of plastic forming of thermoelectric materials based on multi-directional hot compression

Abstract

The application provides a thermoelectric material plastic forming method based on multi-directional hot compression, through a multi-pass and multi-directional compression path, brittle thermoelectric material is successfully deformed by 60% of a large plastic deformation in a specific temperature range, and the forming condition is good. The method significantly improves the mechanical properties and plastic forming capacity of the material, and does not occur dynamic recrystallization in the material deformation process, and the microstructure is mainly optimized through dislocation slip and other mechanisms, effectively ensuring that the excellent thermoelectric performance does not attenuate, and breaking through the bottleneck of mutual restriction of mechanical properties and thermoelectric performance in the traditional process.

Description

Technical Field

[0001] This invention relates to the field of thermoelectric material preparation technology, and in particular to a method for plasticizing thermoelectric materials based on multi-directional thermal compression. Background Technology

[0002] Thermoelectric materials, as solid-state functional materials capable of converting heat energy into electrical energy, possess core advantages in high reliability, noiselessness, long lifespan, and precise and rapid temperature control response resulting from all-solid-state operation. Utilizing the Seebeck effect, they can directly convert waste energy sources such as automotive exhaust and industrial waste heat into electrical energy, providing power for IoT nodes, remote monitoring equipment, and deep space probes. Utilizing the Peltier effect, they can provide precise and efficient solid-state cooling and thermal management solutions for high-end chips, lidar, biological instruments, and consumer electronics.

[0003] However, most high-performance thermoelectric materials are inherently brittle. Their layered structure or covalent bond-dominated characteristics result in poor ductility and insufficient mechanical strength at room temperature. This inherent brittleness prevents the materials from being formed or customized using conventional plastic processing methods such as rolling, forging, and extrusion. More importantly, during subsequent machining processes such as cutting, drilling, and grinding, the materials are highly sensitive to stress concentration, easily leading to microcracks, chipping, or overall breakage. This results in low yield rates and severely limits their usability and reliability in practical engineering components. While patent application CN120265093A employs graphite lubrication and encapsulation technology, it is largely limited to unidirectional compression at room temperature. Due to the lack of thermal activation energy, dislocation slip within the material is insufficient, making it highly susceptible to cleavage fracture. Patent application CN116985324A utilizes unidirectional large deformation, which easily leads to severe anisotropic texture in the material and readily induces dynamic recrystallization at sustained high temperatures, resulting in grain coarsening and a significant increase in phonon thermal conductivity, thus fatally damaging the thermoelectric figure of merit. Therefore, balancing the material's plastic deformation capacity with the stability of its thermoelectric properties has become a challenging problem in this field. Summary of the Invention

[0004] In view of this, this invention proposes a plasticizing forming method for thermoelectric materials based on multi-directional thermal compression. Through multiple compression paths in multiple directions, brittle thermoelectric materials successfully achieve a large plastic deformation with a cumulative deformation of 60% within a specific temperature range (400℃-450℃), and the forming condition is excellent. This method significantly improves the material's mechanical properties (such as hardness) and plastic forming ability, while preventing dynamic recrystallization during the deformation process. Its microstructure is optimized mainly through mechanisms such as dislocation slip, effectively ensuring that its excellent thermoelectric properties do not diminish, thus overcoming the bottleneck of mutual constraint between mechanical and thermoelectric properties in traditional processes. This addresses the problem that traditional thermoelectric materials, due to their inherent brittleness, are extremely sensitive to stress concentration during machining processes such as cutting and drilling, easily generating microcracks, chipping, or overall breakage, making it difficult to meet the requirements of high-precision micro-machining with micro-cutting smaller than 1mm.

[0005] The technical solution of this invention is implemented as follows: In a first aspect, the present invention provides a method for plasticizing thermoelectric materials based on multi-directional hot compression, comprising the following steps: S1, heating a thermoelectric material sample to a set temperature and holding it at that temperature, and then performing a first pass of hot compression to obtain a first sample; S2. Take out the first sample, repeat the heating to the set temperature and keep it warm in step S1, change the compression direction, and perform a second hot compression to obtain the second sample. S3. Take out the second sample, repeat the heating to the set temperature and keep it warm in step S1, change the compression direction again, and perform a third hot compression to obtain the plasticized thermoelectric material.

[0006] Based on the above technical solution, further, the compression direction of the first hot compression includes: performing the first hot compression along the central axis of the thermoelectric material sample.

[0007] Based on the above technical solution, further, the change of compression direction in step S2 includes: rotating 90° around the axis perpendicular to the central axis of the first sample and then compressing along the central axis; The change of compression direction in step S3 includes: rotating 90° around the central axis of the second sample and then compressing along the central axis.

[0008] This invention utilizes a strict 90° rotation between passes to break the initial texture formed in the previous pass by leveraging the compressive stress of the subsequent pass. This rotational design, combined with the precise spatial constraint of the limiting groove, causes dislocations within the material to cyclically intersect in orthogonal directions, inducing a unique wavy fold structure. This structure exhibits a macroscopic plastic deformation capacity of 60% and effectively suppresses the penetration of brittle cleavage cracks at the microscopic level. Thus, while ensuring that the thermoelectric figure of merit does not decrease, the hardness is increased by 31% to 66% compared to the original billet.

[0009] Step S3 is a repetition and refinement of step S2. After the first two compression passes, the two-dimensional texture of the material has been broken. A third compression pass is then performed, rotating the material 90° around its central axis. This aims to apply the principal compressive stress in the only non-perpendicular direction. This completely eliminates residual local anisotropy, allowing the previously induced wavy structure to further evolve into a fully interwoven three-dimensional folded network. This enables the cumulative large plastic strain of up to 60% to be evenly distributed across the three spatial dimensions, greatly reducing the risk of internal shear cracking that is easily triggered by unidirectional or bidirectional overcompression. Ultimately, true isotropic optimization of the material's microstructure and macroscopic mechanical properties is achieved. Simultaneously, based on the actual deformation dimensions of the sample after the second compression pass, the shims are readjusted or replaced to ensure absolute uniformity and effective constraint of the stress transfer in the third compression pass.

[0010] Based on the above technical solution, furthermore, the set temperature in steps S1, S2, and S3 is 400℃~450℃, and the heat preservation time is 10~60min.

[0011] Because thermoelectric materials are inherently brittle, if the temperature is too low, it is difficult to provide sufficient thermal activation energy, resulting in poor material plasticity and difficulty in initiating effective dislocation slip. Under multi-directional alternating stress, brittle cleavage fracture is very likely to occur. If the temperature is too high, although the material softens and is easier to deform, it will cross the material's recrystallization critical point, leading to large-scale dynamic recrystallization and abnormal grain coarsening. This will significantly increase the phonon thermal conductivity, thus fatally reducing the thermoelectric performance of the thermoelectric material. The holding time is 10~60min. If the time is too short, the center temperature may not reach the set value, resulting in uneven deformation. If the time is too long, it may lead to excessive grain growth or reduced production efficiency.

[0012] Based on the above technical solutions, furthermore, the first hot compression, the second hot compression, and the third hot compression include compression cycles of 0.0005~1s. -1 The strain rate is used for thermal compression.

[0013] Excessively high strain rates result in significant work hardening, high deformation resistance, and a tendency to crack; excessively low strain rates lead to low production efficiency and may promote the recovery process, thus weakening the processing effect.

[0014] Based on the above technical solutions, the thermoelectric material sample further includes at least one of Bi2Te3, lead telluride, or silicon-germanium alloy.

[0015] Based on the above technical solution, further, in steps S2 and S3, the first sample or the second sample is taken out and cooled to room temperature before heating.

[0016] Cooling to room temperature ensures the material's microstructure is stable, facilitating safe handling.

[0017] Based on the above technical solution, step S1 further includes: coating the surface of the thermoelectric material sample with graphite and encapsulating it in a metal sleeve to obtain an encapsulated sample, then heating it to a set temperature and holding it at that temperature, and then performing a first-pass hot compression to obtain a first sample.

[0018] Based on the above technical solutions, the material of the metal sleeve further includes at least one of TA1, TA2, TA3 or TC4.

[0019] Based on the above technical solution, further, the thickness of the metal sleeve is 10% to 50% of the diameter of the cylindrical thermoelectric material sample; The height of the metal sleeve is equal to the height of the cylindrical thermoelectric material sample; The inner diameter of the metal sleeve is 0.25% to 0.75% larger than the diameter of the cylindrical thermoelectric material sample.

[0020] Based on the above technical solutions, the coating thickness of the graphite is further 0.1% to 1% of the diameter of the thermoelectric material.

[0021] Based on the above technical solutions, the shape of the thermoelectric material sample further includes any one of a cylinder, a cuboid, or a cube.

[0022] Based on the above technical solution, further, step S2 includes: taking out the first sample, placing it in the limiting groove, adding a gasket to fix it, placing it in the mold, repeating the heating to the set temperature and keeping it warm in step S1, changing the compression direction, and performing a second heat compression to obtain the second sample. Step S3 includes: taking out the second sample, adjusting the shim in the limiting groove, placing it into the mold, repeating the heating to the set temperature and holding it at the set temperature in step S1, changing the compression direction, performing a third hot compression, and obtaining the plasticized thermoelectric material.

[0023] The purpose of rotating 90° is to transform the lateral expansion direction of the previous pass into the compression direction of the next pass. This orthogonal path switching prevents the crystal cleavage planes within the sample from penetrating in a single direction. Three passes of hot compression, combined with the lateral support of the limiting groove, enable this rotational pressing to achieve large plastic deformation, a core technological guarantee for achieving 60% cumulative deformation without breakage. This path design aims to break up any initial texture, promote multi-directional deformation, and obtain a more isotropic microstructure. The limiting groove fixes the spatial orientation of the sample after rotation, ensuring that the second pass of compression proceeds along the preset direction. The shims fill the gaps caused by dimensional changes from the previous pass, ensuring uniform force application from the indenter.

[0024] Based on the above technical solutions, furthermore, the hardness of the limiting groove is less than the hardness of the mold; The hardness of the limiting groove is greater than the hardness of the metal sleeve.

[0025] Based on the above technical solutions, furthermore, the height of the limiting groove is 7% to 12% larger than the width of the first sample; The width of the limiting groove is 5% to 15% larger than the height of the first sample; The length of the limiting groove is 10% to 15% greater than the width of the first sample.

[0026] Based on the above technical solutions, furthermore, the hardness of the gasket is greater than the hardness of the metal sleeve.

[0027] Thermal compression is performed using a pressure head.

[0028] Based on the above technical solutions, the materials of the limiting groove, gasket, and pressure head are selected from either H13 or GCr15.

[0029] Secondly, the present invention also provides a plasticized thermoelectric material prepared by the thermoelectric material plasticizing method based on multi-directional thermal compression as described above.

[0030] Compared with the prior art, the present invention has the following beneficial effects: (1) This invention enables brittle thermoelectric materials to achieve a large plastic deformation with a cumulative deformation of 60% in a specific temperature range (400℃-450℃) through a multi-pass, multi-directional compression path, and the forming condition is good. While significantly improving the mechanical properties (such as hardness) and plastic forming ability of the material, this method does not cause dynamic recrystallization during the material deformation process. Its microstructure is mainly optimized through mechanisms such as dislocation slip, which effectively ensures that its excellent thermoelectric properties do not decay, and breaks through the bottleneck of mutual restriction between mechanical properties and thermoelectric properties in traditional processes.

[0031] (2) The graphite lubrication and metal sheath used in this invention effectively reduce frictional resistance and interfacial reaction during deformation, preventing adhesion between the sample and the mold and the generation of surface cracks. The titanium alloy sheath not only serves as a conventional stress-enhancing layer, but more importantly, its excellent high-temperature strength demonstrates strong load-bearing and conformal capabilities during multi-stage cyclic heating and orthogonal deformation at 400℃~450℃. Even after the sample undergoes multiple 90° rotations and large-amplitude geometric deformations, the titanium alloy sheath can still closely adhere to and constrain the deformed blank, uniformly transmitting the triaxial compressive stress that is constantly changing direction, thereby effectively suppressing surface or internal shear cracking caused by abrupt changes in the deformation path and violent kneading, thus ensuring the uniformity of the multi-directional large plastic deformation process and the integrity of the blank. Attached Figure Description

[0032] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0033] Figure 1 A schematic flowchart of a thermoelectric material plasticizing forming method based on multi-directional thermal compression provided by the present invention; Figure 2 This is a schematic diagram of the multi-directional thermal compression path in this invention (showing the rotational relationship of the X, Y, and Z axes). Figure 3 This is a schematic diagram and a physical image of a bismuth telluride sample after triaxial compression according to the present invention. Figure 4 Image a is a SEM image of the sintered bismuth telluride sample from Example 1 at 450℃ with a deformation of 10% in each of the three dimensions; image b is a SEM image of the sintered bismuth telluride sample from Example 2 at 450℃ with a deformation of 15% in each of the three dimensions; image c is a SEM image of the zone-melted bismuth telluride sample from Example 3 at 400℃ with a deformation of 20% in each of the three dimensions; image d is a SEM image of the zone-melted bismuth telluride sample from Example 4 at 450℃ with a deformation of 20% in each of the three dimensions. Figure 5 In the figure, a is a comparison of the hardness of the undeformed sintered bismuth telluride sample with the hardness of the sintered bismuth telluride samples in Examples 1 and 2 at 450℃ with a deformation amount of 10% and 15% in each of the three dimensions, respectively; b is a comparison of the hardness of the undeformed zone-melted bismuth telluride sample with the hardness of the zone-melted bismuth telluride samples in Examples 3 and 4 at 400℃ and 450℃ with a deformation amount of 20% in each of the three dimensions, respectively. Detailed Implementation

[0034] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0035] Reference Figure 1 , Figure 2 The present invention provides a method for multi-directional hot compression plasticizing and a schematic diagram of a multi-directional hot compression path, comprising the following steps: S1. Heat the thermoelectric material sample to a set temperature and hold it at that temperature, then perform the first hot compression to obtain the first sample. Preferably, the surface of the thermoelectric material sample is coated with graphite and encapsulated in a metal sleeve to obtain an encapsulated sample. The encapsulated sample is heated to a set temperature and kept at that temperature, and then subjected to a first heat compression to obtain a first sample. Preferably, the thermoelectric material sample is cylindrical in shape.

[0036] The coating process ensures that the graphite layer is continuous, intact, and of uniform thickness, thereby forming a stable and effective solid lubrication interface between the thermoelectric material blank and the inner wall of the metal sleeve during subsequent hot compression. The encapsulation process must ensure that the cylindrical thermoelectric material sample is centered inside the metal sleeve and has no significant axial displacement, laying the foundation for uniform transmission of axial pressure in the future.

[0037] Preferably, the ratio of the diameter to the height of the cylindrical thermoelectric material sample is 1:1 to ensure the symmetry and uniformity of the stress state during deformation. The inner diameter of the metal sleeve needs to be slightly larger than the outer diameter of the sample after the graphite layer is coated. The difference (single-sided gap) should be controlled between 0.25% and 0.75% of the sample diameter. If the gap is too small, it will lead to difficulties in sealing and may generate pre-tightening stress due to uneven expansion when heated. If the gap is too large, it may cause the cylindrical thermoelectric material sample to tilt or become unstable in the metal sleeve during the initial compression stage.

[0038] S2. Take out the first sample, heat it to the set temperature and keep it warm, change the compression direction, and perform a second hot compression to obtain the second sample. Preferably, the encapsulated sample obtained in step S1 is taken out, heated to a set temperature and kept at that temperature, and then hot-compressed at a certain strain rate to obtain a second sample.

[0039] Preferably, the heating process must ensure that the entire encapsulated sample is heated uniformly to avoid cracking of the metal sleeve due to uneven thermal stress; the heat preservation process ensures that the internal temperature field of the sample is uniform and stable, providing the necessary thermal activation conditions for subsequent plastic deformation.

[0040] Preferably, the set temperature is 400℃~450℃. This temperature range is strictly based on the intrinsic physical and thermodynamic properties of the thermoelectric material. The coated graphite layer has excellent chemical and thermal stability within this temperature range, serving only as a solid lubricant and an isolation layer to prevent adhesion. Because thermoelectric materials are inherently brittle, excessively low temperatures make it difficult to provide sufficient thermal activation energy, resulting in poor material plasticity and difficulty in initiating effective dislocation slip. Under multi-directional alternating stress, brittle cleavage fracture is highly likely to occur. While excessively high temperatures soften the material and make it more easily deformable, they can cross the material's recrystallization critical point, leading to large-scale dynamic recrystallization and abnormal grain coarsening. This significantly increases phonon thermal conductivity, thus fatally reducing the thermoelectric performance of the thermoelectric material.

[0041] The heat preservation time is 10~60min. If the time is too short, the center temperature may not reach the set value and the deformation may be uneven; if the time is too long, it may lead to excessive grain growth or reduced production efficiency. Preferably, the first, second, and third hot compression passes include compression cycles of 0.0005 to 1 s. -1 The strain rate is used for thermal compression.

[0042] Excessively high strain rates result in significant work hardening, high deformation resistance, and a tendency to crack; excessively low strain rates lead to low production efficiency and may promote the recovery process, thus weakening the processing effect.

[0043] S3. Take out the second sample, heat it to the set temperature and keep it warm, change the compression direction again, and perform a third hot compression to obtain the plasticized thermoelectric material.

[0044] Preferably, in steps S2 and S3, the first or second sample is removed and cooled to room temperature before heating. This ensures the stability of the material microstructure and facilitates safe operation.

[0045] The change of compression direction in step S2 includes: rotating 90° around the axis perpendicular to the central axis of the first sample, and then compressing along the axial direction; The change of compression direction in step S3 includes: rotating 90° around the central axis of the second sample and then compressing along the axial direction.

[0046] The purpose of rotating 90° is to transform the lateral expansion direction of the previous pass into the compression direction of the next pass. This switching of orthogonal paths prevents the crystal cleavage planes inside the sample from penetrating in a single direction. Combined with the lateral support of the limiting groove, this rotational pressing can achieve the effect of large plastic deformation, which is the core process guarantee for achieving 60% cumulative deformation without breakage.

[0047] This path design aims to break the initial possible texture, promote multi-directional deformation, and obtain a more isotropic structure.

[0048] Preferably, step S2 includes: taking out the first sample, placing it in the limiting groove, adding a gasket to fix it, placing it in the mold, heating it to the set temperature and keeping it warm, changing the compression direction, and performing a second round of hot compression to obtain the second sample; Step S3 includes: taking out the second sample, adjusting the shim in the limiting groove, placing it into the mold, heating it to the set temperature and keeping it warm, changing the compression direction, and performing a second round of hot compression to obtain the second sample.

[0049] The limiting groove is used to fix the spatial posture of the sample after rotation, ensuring that the second compression is performed in the preset direction. The shim is used to fill the gap caused by the dimensional changes in the previous compression, ensuring that the pressure head applies force evenly.

[0050] Preferably, the width of the limiting groove is 5% to 15% larger than the height of the encapsulated sample. If the gap is too small, the sample will be difficult to insert or easily scratched during insertion; if the gap is too large, it will not be effectively limited, resulting in deviation of the compression path. The gasket needs to have sufficient hardness to ensure that it does not deform under pressure.

[0051] Preferably, step S3 is a repetition and refinement of step S2. After the first two compression passes, the two-dimensional texture of the material has been broken. A third compression pass is then performed, rotating the material 90° around its central axis. This aims to apply the principal compressive stress in the only non-perpendicular direction. This completely eliminates residual local anisotropy, allowing the previously induced wavy structure to further evolve into a fully interwoven three-dimensional folded network. This enables the cumulative large plastic strain of up to 60% to be evenly distributed across the three spatial dimensions, significantly reducing the risk of internal shear cracking easily caused by unidirectional or bidirectional overcompression. Ultimately, true isotropic optimization of the material's microstructure and macroscopic mechanical properties is achieved. Simultaneously, based on the actual deformation dimensions of the sample after the second compression pass, the shims are readjusted or replaced to ensure absolute uniformity and effective constraint of the stress transfer in the third compression pass.

[0052] Preferably, the heating temperature and holding time are consistent with the parameters set in step S2 to ensure the stability and repeatability of the process. After these three compressions, the cumulative true strain can reach over 60%, thereby ensuring the thermoelectric properties of the thermoelectric material while enhancing its plastic deformation capacity. Microstructural observation shows that no significant dynamic recrystallization occurred during the multi-directional hot compression process. The original layered structure mainly coordinates plastic deformation through dislocation slip, lamellar bending, and the formation of wavy folds. Due to the absence of recrystallization and significant grain coarsening, the original orientation structure and carrier transport channels of the material are not significantly damaged; at the same time, the increase in folded layered structure, dislocations, and substructures can enhance phonon scattering, which is beneficial to reducing lattice thermal conductivity. Therefore, the present invention can improve the plastic deformation capacity of thermoelectric materials while maintaining their thermoelectric properties and has the potential for further improvement.

[0053] Experiments have shown that the graphite lubrication and metal sheath used effectively reduced frictional resistance and interfacial reaction during deformation, preventing the sample from sticking to the mold and the generation of surface cracks. The titanium alloy sheath serves as an ideal stress-reinforcing and insulating layer. Its excellent high-temperature strength can effectively transfer triaxial compressive stress to the billet, suppressing the initiation and propagation of surface cracks. At the same time, its thermal expansion coefficient, which is similar to that of thermoelectric materials, reduces thermal stress, thus ensuring the uniformity of the deformation process and the integrity of the billet. The combination design of the limiting groove and the gasket enables precise control of the deformation path of multiple passes, so that the compressive load of each pass can be applied evenly in the preset direction, effectively avoiding internal shear bands or cracking caused by stress concentration.

[0054] By employing multi-pass, multi-directional compression paths, brittle thermoelectric materials successfully achieved large plastic deformation with a cumulative deformation of 60% within a specific temperature range (400℃-450℃), exhibiting excellent forming properties. This method significantly improves the material's mechanical properties (such as hardness) and plastic forming ability, while preventing dynamic recrystallization during deformation. The microstructure is optimized primarily through mechanisms such as dislocation slip, effectively ensuring that its excellent thermoelectric properties do not diminish, thus overcoming the bottleneck of mutual constraint between mechanical and thermoelectric properties in traditional processes.

[0055] The present invention will be further described in detail below with reference to specific embodiments.

[0056] Example 1 This embodiment provides a method for plasticizing thermoelectric materials based on multi-directional thermal compression, using sintered P-type Bi2Te3 preform as the forming material, including the following steps: Step S1: Pre-treatment of thermoelectric material Bi2Te3: Using thermoelectric material Bi2Te3 with a diameter of 19.8 mm and a height of 20 mm, place the thermoelectric material Bi2Te3 coated with 0.2 mm of graphite into a titanium alloy sleeve made of TA2 with an inner diameter of 20 mm, an outer diameter of 25 mm, and a height of 20 mm.

[0057] Step S2: Transfer the encapsulated sample to a heating furnace, heat to 450°C and hold for 15 minutes. Then, reduce the temperature by 0.0005 s. -1 The strain rate is adjusted along the axial direction of the specimen for the first compression pass, achieving a deformation of 10%.

[0058] Step S3: After cooling the sample to room temperature following the first compression, remove it and rotate it 90° about the axis perpendicular to its central axis. Place it in the center of a rectangular limiting groove made of H13 with dimensions of 50mm long, 18mm wide, and 38mm high, and groove dimensions of 32mm long, 18mm wide, and 38mm high, open at one end. Select a gasket made of H13 and place it into the gap between the sealed sample and the two ends of the limiting groove to fix it. Then, place the whole thing into the mold, heat it to 450℃ and hold it for 15 minutes, then... (The sentence is incomplete and ends abruptly). -1 The strain rate is used for a second compression to achieve a cumulative deformation of 20%.

[0059] Step S4: After cooling the packaged sample to room temperature again, remove it and rotate it 90° around its central axis. Replace the adjusting shim in the limiting groove, then place the entire sample into the mold. Heat to 450℃ and hold for 15 minutes, then... (The sentence is incomplete and requires more context to translate accurately.) -1 The strain rate is used for the third compression to achieve a cumulative total deformation of 30%.

[0060] Step S5: After the encapsulated sample cools to room temperature, remove the bismuth telluride sample and peel off the TA2 titanium alloy sleeve to obtain the plasticized thermoelectric material.

[0061] Figure 3 This is a schematic diagram and a physical image of the bismuth telluride sample after triaxial compression in this embodiment.

[0062] Upon testing, the thermoelectric material formed under these conditions, as observed by SEM microstructure, exhibits a wavy morphology formed by the bending of layered structures. Its hardness is 45.31 HV, which is 31% higher than that of the undeformed sample. Furthermore, its thermoelectric figure of merit is 28% higher than that of the original material, indicating improvements in both mechanical and thermoelectric properties.

[0063] Example 2 This embodiment provides a method for plasticizing thermoelectric materials based on multi-directional thermal compression, using sintered P-type Bi2Te3 preform as the forming material, including the following steps: Step S1: Pre-treatment of thermoelectric material Bi2Te3: Using thermoelectric material Bi2Te3 with a diameter of 19.92 mm and a height of 20 mm, place the thermoelectric material Bi2Te3 coated with 0.08 mm of graphite into a titanium alloy sleeve made of TA1 with an inner diameter of 20 mm, an outer diameter of 25 mm, and a height of 20 mm.

[0064] Step S2: Transfer the encapsulated sample to a heating furnace, heat to 450°C and hold for 15 minutes. Then, reduce the temperature by 0.0005 s. -1 The strain rate is adjusted along the axial direction of the specimen for the first compression pass, achieving a deformation of 15%.

[0065] Step S3: After cooling the sample to room temperature following the first compression, remove it and rotate it 90° about the axis perpendicular to its central axis. Place it in the center of a rectangular limiting groove made of H13 with dimensions of 50mm long, 18mm wide, and 38mm high, and groove dimensions of 32mm long, 18mm wide, and 38mm high, open at one end. Select a gasket made of H13 and place it into the gap between the sealed sample and the two ends of the limiting groove to fix it. Then, place the whole thing into the mold, heat it to 450℃ and hold it for 15 minutes, then... (The sentence is incomplete and ends abruptly). -1 The strain rate is adjusted to perform a second compression, achieving a cumulative deformation of 30%.

[0066] Step S4: After cooling the packaged sample to room temperature again, remove it and rotate it 90° around its central axis. Replace the adjusting shim in the limiting groove, then place the entire sample into the mold. Heat to 450℃ and hold for 15 minutes, then... (The sentence is incomplete and requires more context to translate accurately.) -1 The strain rate is used for the third compression, achieving a cumulative total deformation of 45%.

[0067] Step S5: After the encapsulated sample cools to room temperature, remove the bismuth telluride sample and peel off the TA1 titanium alloy sleeve to obtain the plasticized thermoelectric material.

[0068] Upon testing, the thermoelectric material formed under these conditions showed no cracks on its surface, and its microstructure exhibited a wavy morphology formed by the bending of a layered structure, which was more pronounced than the wavy morphology in Example 1. Its hardness was 51.96 HV, which was 50% higher than that of the undeformed sample, and its thermoelectric figure of merit was 14% higher than that of the original material. While ensuring its thermoelectric properties, it significantly improved its plastic deformation capacity and mechanical properties.

[0069] Example 3 This embodiment provides a plasticizing forming method for thermoelectric materials based on multi-directional thermal compression, using zone-melting P-type Bi2Te3 billet as the forming material, including the following steps: Step S1: Pre-treatment of thermoelectric material Bi2Te3: Using thermoelectric material Bi2Te3 with a diameter of 19.94 mm and a height of 20 mm, place the thermoelectric material Bi2Te3 coated with 0.06 mm of graphite into a titanium alloy sleeve made of TA3 with an inner diameter of 20 mm, an outer diameter of 25 mm, and a height of 20 mm.

[0070] Step S2: Transfer the encapsulated sample to a heating furnace, heat to 400°C and hold for 15 minutes. Then, at 0.001s... -1 The strain rate is adjusted along the axial direction of the specimen for the first compression pass, achieving a deformation of 20%.

[0071] Step S3: After cooling the sample to room temperature following the first compression, remove it and rotate it 90° about the axis perpendicular to its central axis. Place it in the center of a rectangular limiting groove made of H13 with dimensions of 50mm long, 18mm wide, and 38mm high, and groove dimensions of 32mm long, 18mm wide, and 38mm high, open at one end. Select a gasket made of H13 and place it into the gap between the sealed sample and the two ends of the limiting groove to fix it. Then, place the whole thing into the mold, heat it to 400℃ and hold it for 15 minutes, then... (The sentence is incomplete and ends abruptly). -1 The strain rate is used for a second compression to achieve a cumulative deformation of 40%.

[0072] Step S4: After cooling the packaged sample to room temperature again, remove it and rotate it 90° around its central axis. Place the adjusting shim back into the limiting groove and then insert the entire sample into the mold. Heat to 400℃ and hold for 15 minutes, then... (The sentence is incomplete and requires more context to translate accurately.) -1 The strain rate is used for the third compression, achieving a cumulative total deformation of 60%.

[0073] Step S5: After the encapsulated sample cools to room temperature, remove the bismuth telluride sample and peel off the TA3 titanium alloy sleeve to obtain the plasticized thermoelectric material.

[0074] Tests showed that the thermoelectric material formed under these conditions had no cracks on its surface, exhibited a wrinkled microstructure, and had a hardness of 34.58 HV, which was 41% higher than that of the undeformed sample, significantly improving its mechanical properties and machinability.

[0075] Example 4 This embodiment provides a plasticizing forming method for thermoelectric materials based on multi-directional thermal compression, using zone-melting P-type Bi2Te3 billet as the forming material, including the following steps: Step S1: Pre-treatment of thermoelectric material Bi2Te3: Using thermoelectric material Bi2Te3 with a diameter of 19.91 mm and a height of 20 mm, place the thermoelectric material Bi2Te3 coated with 0.09 mm of graphite into a titanium alloy sleeve made of TC4 with an inner diameter of 20 mm, an outer diameter of 25 mm, and a height of 20 mm.

[0076] Step S2: Transfer the encapsulated sample to a heating furnace, heat to 450°C and hold for 15 minutes. Then, at 0.001s... -1 The strain rate is adjusted along the axial direction of the specimen for the first compression pass, achieving a deformation of 20%.

[0077] Step S3: After cooling the sample to room temperature following the first compression, remove it and rotate it 90° about the axis perpendicular to its central axis. Place it in the center of a rectangular limiting groove made of H13 with dimensions of 50mm long, 18mm wide, and 38mm high, and groove dimensions of 32mm long, 18mm wide, and 38mm high, open at one end. Select a gasket made of H13 and place it into the gap between the sealed sample and the two ends of the limiting groove to fix it. Then, place the whole thing into the mold, heat it to 450℃ and hold it for 15 minutes, then... (The sentence is incomplete and ends abruptly). -1 The strain rate is used for a second compression to achieve a cumulative deformation of 40%.

[0078] Step S4: After cooling the packaged sample to room temperature again, remove it and rotate it 90° around its central axis. Place the adjusting shim back into the limiting groove and then insert the entire sample into the mold. Heat to 450℃ and hold for 15 minutes, then... (The sentence is incomplete and requires more context to translate accurately.) -1 The strain rate is used for the third compression, achieving a cumulative total deformation of 60%.

[0079] Step S5: After the encapsulated sample cools to room temperature, remove the bismuth telluride sample and peel off the TC4 titanium alloy sleeve to obtain the plasticized thermoelectric material.

[0080] Testing revealed numerous and clearly defined wrinkled morphological features in the cross-section of the thermoelectric material sample formed under these conditions, both vertically and horizontally. Its hardness was 40.7 HV, a 66% increase compared to the undeformed sample, and its thermoelectric figure of merit (ZT value) remained stable compared to the original material. This demonstrates that this plasticizing method based on multi-directional hot compression can significantly improve the mechanical properties while maintaining the thermoelectric performance.

[0081] Example 5 The difference between this embodiment and Embodiment 1 is that lead telluride is used to replace the thermoelectric material Bi2Te3, and the heating temperature in steps S2, S3, and S4 is adjusted to 430℃ and the holding time is adjusted to 60min.

[0082] Example 6 The difference between this embodiment and Embodiment 1 is that: the thermoelectric material Bi2Te3 is replaced with a silicon-germanium alloy, and the holding time in steps S2, S3, and S4 is adjusted to 10 minutes, with a strain rate of 1 second. -1 .

[0083] Comparative Example 1 The difference between this comparative example and Example 1 is that this comparative example does not include steps S3, S4, and S5.

[0084] Furthermore, step two involves transferring the encapsulated sample to a heating furnace, heating it to 450°C, and holding it at that temperature for 15 minutes. Then, the temperature is increased by 0.001 s. -1 The strain rate is adjusted along the axial direction of the specimen for the first compression pass, achieving a deformation of 20%.

[0085] After single-pass isothermal forming, a complete sample with a deformation of 20% can be obtained. However, the internal structure of this sample mainly exhibits unidirectional bending and local wrinkling characteristics, and the uniformity of the structure and the synergistic improvement effect on mechanical and thermoelectric properties are lower than those of Example 1. Testing showed that after single-pass isothermal hot compression treatment, the sample remained intact when the deformation was 20%, indicating that the Bi2Te3 material has a certain plastic deformation capacity under these conditions. However, because this comparative example only underwent compression in one direction, the deformation path inside the sample was singular, and the layered structure mainly exhibited bending and wrinkling along one direction, with obvious orientation characteristics remaining in some areas. The uniformity of the structure and the degree of multi-directional coordinated deformation were insufficient. Although the hardness was improved compared to the undeformed sample, the improvement was lower than that of Example 1, and the thermoelectric properties did not show a significant synergistic improvement. In contrast, Example 1, through three-pass hot compression and two 90° rotations, allowed the compressive load to act sequentially in different directions, effectively breaking the unidirectional deformation structure and promoting the formation of a uniform multi-directional wavy wrinkled structure in the layered structure. This achieved a higher cumulative deformation while improving the material's mechanical properties and maintaining stable thermoelectric properties.

[0086] Comparative Example 2 The difference between this comparative example and Example 1 is that in step three of this comparative example, "rotate 90° with the axis perpendicular to its central axis as the axis of rotation" is modified to "rotate 90° with its central axis as the axis of rotation".

[0087] Testing revealed the presence of localized microcracks on the sample surface after three compression passes. Scanning electron microscopy (SEM) observation showed uneven distribution of the wavy structure within the sample, with some areas still exhibiting significant unidirectional texture. While the hardness increase was relatively small (approximately 20%), the thermoelectric figure of merit showed a certain degree of decline. This is because, in this comparative example, the second pass only rotated 90° around the sample's central axis, failing to truly transform the lateral expansion direction formed in the first pass into the main compression direction for the next pass. The actual compression path still exhibited strong coaxial or near-coaxial characteristics. Consequently, the texture and localized strain concentration formed in the previous pass were not sufficiently dispersed, making it prone to forming localized shear bands and microcracks during subsequent compression. In contrast, in this invention, the second pass rotated 90° around an axis perpendicular to the sample's central axis, and the third pass rotated another 90° around the sample's central axis. This allows the compressive load to act sequentially in different spatial directions, forming a more complete orthogonal deformation path, promoting the formation of a three-dimensional wrinkle network, thereby improving molding uniformity and reducing thermoelectric performance degradation.

[0088] Comparative Example 3 The preparation method described in patent CN 116985324 A is adopted.

[0089] Step 1: Pre-treatment of the sealing structure of the thermoelectric material blank: Using a Bi2Te3 blank with a diameter of 9.7 mm and a height of 11.9 mm, the Bi2Te3 blank is inserted into a TC4 hollow metal sleeve. The inner diameter of the hollow metal sleeve is 10.9 mm, the height of the hollow metal sleeve is 12.6 mm, the thickness of the hollow metal sleeve is 4 mm, and the gap between the hollow metal sleeve and the thermoelectric material blank is 0.6 mm. Bentonite is filled into the gap and the upper and lower end faces to form a sealing structure. Step 2: Heating the sealing structure: The entire sealing structure containing the Bi2Te3 blank is heated to 500℃ and held for 15 minutes; Step 3, Hot Compression of the Billet: The thermoelectric material billet after heat preservation is hot compressed at a speed of 0.01s. -1 The strain rate was used for compression forming, with a deformation of 40%. Step 4: Remove the metal sleeve: Take out the sealing structure and cool it, remove the outer hollow metal sleeve, take out the inner Bi2Te3 material, and obtain the formed N-type Bi2Te3 thermoelectric material.

[0090] Testing revealed that the Bi2Te3 thermoelectric material prepared using this method could obtain a shaped sample after 40% hot compression deformation, but localized cracking or peeling occurred at the sample edges, resulting in lower overall integrity compared to the embodiments of this invention. SEM observation showed uneven internal microstructure distribution, with localized recrystallization and grain coarsening, and the absence of a continuous, uniform wavy wrinkled structure. Hardness testing indicated only a slight increase in hardness compared to the undeformed sample, a smaller increase than that of the embodiments of this invention. Thermoelectric performance testing showed a significant decrease in thermoelectric figure of merit compared to the original sample. This is because the method involves high-temperature hot compression at around 500℃, and the high temperature and holding process easily induce dynamic recrystallization and grain growth. Furthermore, unidirectional compression can lead to concentrated microstructure orientation and uneven local strain, thereby damaging the microstructure conducive to thermoelectric transport and resulting in deterioration of thermoelectric performance. This demonstrates that while high-temperature hot compression can achieve a certain degree of plastic deformation, it is difficult to simultaneously achieve significant improvements in mechanical properties, sample integrity, and thermoelectric performance stability.

[0091] Comparative Example 4 The preparation method described in patent CN 120265093A is adopted. Step one involves pretreatment of the thermoelectric material Bi2Te3: A 19.8mm diameter, 20mm high Bi2Te3 thermoelectric material coated with 0.0198mm of graphite is placed in an isolation sleeve made of 45 steel with openings at both ends. This sleeve has an inner diameter of 20mm, an outer diameter of 25mm, and a height of 20mm. Then, it is placed into a mold made of 42CrMo with an inner diameter of 25.2mm, an outer diameter of 40mm, a height of 25mm, and a wall thickness of 7.4mm. The selected pressure head is made of H13 mold steel with a diameter of 25.1mm and a height of 27.5mm. The selected gasket is also made of H13 mold steel with a diameter of 25mm. 0.1mm thick; H13 mold steel can achieve the required hardness level and has good fatigue and creep resistance. H13 mold steel is harder than the stress-plasticized layer made of 40CrMo, thus preventing damage to the indenter during compression.

[0092] Step 2: Incubate the thermoelectric material Bi2Te3 at room temperature for 1 second. -1 The thermoelectric material Bi2Te3 sample was compressed under a strain rate of 372 MPa triaxial compressive stress, which enabled the sample to complete 50% of the deformation and the forming condition was good.

[0093] Step 3: Remove the thermoelectric material: After compression, peel off the mold made of 42CrMo and the isolation sleeve made of 45 steel, and remove the pressure head and gasket made of H13 mold steel to obtain the thermoelectric material after plastic forming.

[0094] Testing revealed that the Bi2Te3 sample achieved 50% deformation using the room-temperature triaxial compressive stress compression method, exhibiting good macroscopic shaping with no obvious surface breakage. The hardness was also improved compared to the undeformed sample, indicating that this method can improve the material's resistance to deformation to some extent. However, thermoelectric performance tests showed that the conductivity and power factor of this sample were lower than in Example 1, the thermoelectric figure of merit was not significantly improved, and the overall thermoelectric performance was lower than in Example 1. This is because the comparative example primarily relied on a large triaxial compressive stress at room temperature to achieve plastic deformation. Although external constraints could suppress macroscopic cracking, the lack of thermal activation conditions resulted in insufficient dislocation slip and lamellar coordinated deformation within the material, leading to strain concentration, microcracks, or residual defects in localized areas. These defects enhance carrier scattering, reduce carrier mobility and conductivity, and consequently decrease the power factor. In contrast, Example 1, which involves multi-pass, multi-directional hot compression at 400°C to 450°C, can promote the formation of layered, interlaced, and wavy folded structures without significant dynamic recrystallization. This improves plastic deformation capacity while maintaining good carrier transport channels, resulting in superior thermoelectric performance compared to the comparative example.

[0095] Performance testing Figure 4 In the images, a is a SEM image of the sintered bismuth telluride sample from Example 1 at 450℃ with a deformation of 10% in each of the three dimensions; b is a SEM image of the sintered bismuth telluride sample from Example 2 at 450℃ with a deformation of 15% in each of the three dimensions; c is a SEM image of the zone-melted bismuth telluride sample from Example 3 at 400℃ with a deformation of 20% in each of the three dimensions; and d is a SEM image of the zone-melted bismuth telluride sample from Example 4 at 450℃ with a deformation of 20% in each of the three dimensions. Figure 5 In the figure, a is a comparison of the hardness of the undeformed sintered bismuth telluride sample with the hardness of the sintered bismuth telluride samples in Examples 1 and 2 at 450℃ with a deformation amount of 10% and 15% in each of the three dimensions, respectively; b is a comparison of the hardness of the undeformed zone-melted bismuth telluride sample with the hardness of the zone-melted bismuth telluride samples in Examples 3 and 4 at 400℃ and 450℃ with a deformation amount of 20% in each of the three dimensions, respectively.

[0096] Depend on Figure 4 , 5It can be seen that after multi-directional hot compression, the layered structure of the Bi2Te3 sample gradually transforms from a relatively flat lamellar structure to a wavy, wrinkled structure, and the wrinkling becomes more pronounced with increasing deformation. Simultaneously, the hardness of the deformed sample is significantly higher than that of the undeformed sample, indicating that multi-directional hot compression can effectively improve the mechanical properties of the material. The mechanism lies in the fact that after changing the compression direction between passes, the compressive stress can act sequentially in different spatial directions, causing multi-directional bending and interlacing deformation of the layered structure, and improving the material's resistance to deformation through dislocation slip, dislocation multiplication, and wrinkle strengthening. Since no significant dynamic recrystallization occurs during this process, the original thermoelectric transport structure of the material is not severely damaged, thus maintaining stable thermoelectric properties while achieving plasticization and strengthening.

[0097] In summary, this invention proposes a plasticizing forming method for thermoelectric materials based on multi-directional thermal compression. Through multiple compression paths in multiple directions, brittle thermoelectric materials successfully achieve a cumulative plastic deformation of 60% within a specific temperature range (400℃-450℃), with excellent forming results. This method significantly improves the material's mechanical properties (such as hardness) and plastic forming ability, while preventing dynamic recrystallization during deformation. Its microstructure is optimized primarily through mechanisms such as dislocation slip, effectively ensuring that its excellent thermoelectric properties do not diminish. This overcomes the bottleneck of mutual constraints between mechanical and thermoelectric properties in traditional processes. It addresses the problem that traditional thermoelectric materials, due to their inherent brittleness, are extremely sensitive to stress concentration during machining processes such as cutting and drilling, easily generating microcracks, chipping, or overall breakage, thus failing to meet the requirements of high-precision micro-machining with micro-cutting smaller than 1mm.

[0098] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for plasticizing thermoelectric materials based on multi-directional thermal compression, characterized in that, The process includes the following steps: S1, heating the thermoelectric material sample to a set temperature and holding it at that temperature, followed by the first pass of hot compression to obtain the first sample; S2. Take out the first sample, repeat the heating to the set temperature and keep it warm in step S1, change the compression direction, and perform a second hot compression to obtain the second sample. S3. Take out the second sample, repeat the heating to the set temperature and keep it warm in step S1, change the compression direction again, and perform a third hot compression to obtain the plasticized thermoelectric material.

2. The thermoelectric material plasticizing and forming method based on multi-directional thermal compression as described in claim 1, characterized in that, The compression direction of the first hot compression includes: performing the first hot compression along the central axis of the thermoelectric material sample.

3. The thermoelectric material plasticizing and forming method based on multi-directional thermal compression as described in claim 1, characterized in that, The change of compression direction in step S2 includes: rotating 90° around the axis perpendicular to the central axis of the first sample, and then compressing along the central axis. The change of compression direction in step S3 includes: rotating 90° about the central axis of the second sample and then compressing along the central axis.

4. The thermoelectric material plasticizing and forming method based on multi-directional thermal compression as described in claim 1, characterized in that, The set temperature in steps S1, S2, and S3 is 400℃~450℃, and the holding time is 10~60min.

5. The thermoelectric material plasticizing and molding method based on multi-directional thermal compression as described in claim 1, characterized in that, The first, second, and third hot compression passes include compression cycles of 0.0005 to 1 second. -1 The strain rate is used for thermal compression.

6. The thermoelectric material plasticizing forming method based on multi-directional thermal compression as described in claim 1, characterized in that, The thermoelectric material sample includes at least one of Bi2Te3, lead telluride, or silicon-germanium alloy.

7. The thermoelectric material plasticizing method based on multi-directional thermal compression as described in claim 1, characterized in that, Step S1 further includes: coating the surface of the thermoelectric material sample with graphite and encapsulating it in a metal sleeve to obtain an encapsulated sample, then heating it to a set temperature and holding it at that temperature, followed by a first pass of hot compression to obtain a first sample.

8. The thermoelectric material plasticizing method based on multi-directional thermal compression as described in claim 7, characterized in that, The metal sleeve is made of at least one of TA1, TA2, TA3 or TC4.

9. The thermoelectric material plasticizing forming method based on multi-directional thermal compression as described in claim 1, characterized in that, Step S2 includes: taking out the first sample, placing it in the limiting groove, adding a gasket to fix it, placing it in the mold, repeating the heating to the set temperature and keeping it warm in step S1, changing the compression direction, and performing a second round of hot compression to obtain the second sample. The height of the limiting groove is 7% to 12% greater than the width of the first sample; The width of the limiting groove is 5% to 15% larger than the height of the first sample; The length of the limiting groove is 10% to 15% greater than the width of the first sample.

10. The thermoelectric material after plasticizing prepared by the thermoelectric material plasticizing method based on multi-directional thermal compression as described in any one of claims 1 to 9.

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