An oxidation-resistant packaging material for hot rolling of thermoelectric materials and a method for preparing the same

Abstract

This invention discloses an antioxidant encapsulation material for hot rolling of thermoelectric materials and its preparation method, relating to the field of new materials technology. The encapsulation material is a three-layer composite tubular structure, comprising, from the inside out: an antioxidant barrier layer, a composite material with Fe-Al intermetallic compounds as the matrix and dispersed Y2O3 particles; a transition layer, a Ni-Cr-Fe-Mn-C solid solution alloy with Ni as the matrix; and a structural support layer, a micro-alloyed low-carbon bainitic steel. Each layer is metallurgically bonded by hot isostatic pressing, and then cold-drawn to form a hollow composite tube. This encapsulation material exhibits excellent high-temperature oxidation resistance, good plastic deformation capacity, and structural strength. It can effectively isolate oxygen, coordinate strain, and maintain overall integrity during high-temperature hot rolling. It is suitable for the protection and densification of Bi2Te3-based, PbTe-based, and SiGe-based thermoelectric materials, significantly improving the thermoelectric performance and production consistency of thermoelectric materials.

Description

Technical Field

[0001] This invention relates to the field of new materials technology, specifically to an antioxidant encapsulation material for hot rolling of thermoelectric materials and its preparation method. Background Technology

[0002] Thermoelectric materials, as functional materials capable of directly converting heat energy into electrical energy, have significant application prospects in fields such as thermoelectric power generation and solid-state refrigeration. Among them, bismuth telluride (Bi2Te3)-based, lead telluride (PbTe)-based, and silicon-germanium (SiGe)-based alloy systems have become key research and application areas due to their excellent thermoelectric properties. In recent years, with the continuous development of manufacturing processes, hot rolling technology, due to its high efficiency, continuous operation, and suitability for large-scale production, has been gradually introduced into the forming process of thermoelectric materials. During hot rolling, materials typically undergo high-temperature (200-1500℃) and large-deformation (reduction of 50-90%) processing environments, requiring thermoelectric materials to maintain good structural integrity and compositional stability during processing.

[0003] However, thermoelectric materials are mostly oxygen-sensitive functional materials. Especially during high-temperature hot rolling, direct exposure to air or oxygen-containing environments can easily lead to surface oxidation, component segregation, and even structural damage, resulting in a significant decrease in their thermoelectric performance. To prevent this problem, existing technologies often employ an encapsulation process, where a metal or alloy sheath is coated around the thermoelectric powder, and a sealed environment is formed through welding and vacuuming before hot rolling. While this method can isolate oxygen to some extent, it still faces several challenges in actual industrial production.

[0004] First, existing encapsulation materials primarily focus on their mechanical properties (such as strength and elongation) to accommodate hot rolling deformation, while neglecting the material's inherent oxidation resistance at high temperatures. Commonly used encapsulation materials such as copper, low-carbon steel (Q215, Q235, etc.), medium-carbon steel (45 steel, etc.), and some stainless steels can still undergo surface oxidation during sustained high temperatures and deformation, and may even lead to seal failure due to oxide layer peeling, resulting in contamination of internal thermoelectric materials.

[0005] Secondly, existing packaging processes mostly rely on single-material encapsulation sleeves, whose oxidation resistance is limited by the material's composition and structure. Even under vacuum conditions, if the encapsulation material itself is prone to oxidation at high temperatures, oxygen permeation may still occur at interfaces, welds, or material defects, affecting the final performance and consistency of the thermoelectric material.

[0006] Furthermore, as thermoelectric material systems expand into higher temperature ranges (e.g., SiGe-based alloys require processing above 1300℃), higher demands are placed on the high-temperature stability and oxidation resistance of encapsulation materials. Currently, there is a lack of a dedicated encapsulation material that combines excellent high-temperature oxidation resistance, good plastic deformation capacity, and compatibility with thermoelectric materials. This has become a technical bottleneck restricting the efficient, stable, and large-scale hot rolling of thermoelectric materials.

[0007] Therefore, developing an antioxidant encapsulation material specifically for the hot rolling process of thermoelectric materials and its preparation method is of great significance for improving the production efficiency of thermoelectric materials, ensuring product performance consistency, and promoting their industrial application. Summary of the Invention

[0008] The purpose of this invention is to overcome the shortcomings of the prior art and provide an antioxidant encapsulation material for hot rolling of thermoelectric materials and its preparation method. This encapsulation material can effectively protect and efficiently densify mainstream thermoelectric materials such as Bi2Te3-based, PbTe-based and SiGe-based materials.

[0009] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0010] An antioxidant encapsulation material for hot rolling of thermoelectric materials has a three-layer composite tubular structure, comprising, from the inside out:

[0011] Antioxidant barrier layer: It is a composite material with Fe-Al intermetallic compound as matrix and Y2O3 particles dispersed therein, wherein the Al content is 27-30 at.%, the average particle size of Y2O3 is 120 nm, the volume fraction is 0.1-1.0 vol.%, and the balance is Fe and unavoidable trace impurities. The thickness of this layer is 0.2-2.0 mm.

[0012] Transition layer: a Ni-Cr-Fe-Mn-C solid solution alloy with Ni as the matrix, comprising by mass percentage: Cr not less than 14 wt.%, Fe 6-10 wt.%, Mn 1.5-2.5 wt.%, C 0.08-0.12 wt.%, with the balance being Ni and unavoidable trace impurities, and the thickness of this layer is 1.0-5.0 mm;

[0013] Structural support layer: It is a microalloyed low-carbon bainitic steel, which by weight percentage contains: C 0.03-0.07 wt.%, Si 0.2-0.5 wt.%, Mn 1.2-1.6 wt.%, Mo 0.3-0.5 wt.%, Nb 0.03-0.05 wt.%, Ti 0.01-0.02 wt.%, with the balance being Fe and unavoidable trace impurities. The grain size of this layer is 4.2-5 μm and the thickness is 3.0-15.0 mm.

[0014] Furthermore, the antioxidant barrier layer, transition layer, and structural support layer are integrally formed through a metallurgical bonding method.

[0015] Furthermore, the antioxidant barrier layer also contains 0.15-0.22 at.% Zr element.

[0016] Furthermore, the transition layer also contains 0.6-1.0 wt.% Co.

[0017] Furthermore, the structural support layer also contains 0.01-0.03 wt.% of B element.

[0018] Furthermore, the inner surface of the antioxidant barrier layer is coated with a boron nitride release coating with a thickness of 2.5-3 μm, wherein the boron nitride has a purity of 99.6% and a particle size of 0.8-1.0 μm; the inner surface roughness Ra of the three-layer composite tubular structure is 0.8-3.2 μm.

[0019] On the other hand, the present invention provides a method for preparing the antioxidant encapsulation material according to any one of the above claims, comprising the following steps:

[0020] S1: Preparation of antioxidant barrier layer material: The composite material with Fe-Al intermetallic compound as matrix and Y2O3 particles dispersedly distributed is homogenized and annealed at 1000-1200℃ for 10-12 hours, and then hot rolled at 900-1000℃ into a thin strip with a thickness of 0.4-2.2 mm.

[0021] S2: Prepare the transition layer material, cast it into a slab after vacuum induction melting, and hot roll it to a thickness of 1.2-5.5 mm at 1100-1200℃;

[0022] S3: Prepare structural support layer material, form slabs by continuous casting, and then perform controlled rolling and cooling to obtain bainitic structure. Specifically, hot rolling is carried out at 850-900℃ to form 3.5-16.5 mm thick plates, and after final rolling, the plates are accelerated to 450-550℃ at a cooling rate of 15-25℃ / s.

[0023] S4: Concentrically fit the above three layers of material in the inner-middle-outer order, controlling the gap within 0.08 mm, and evacuate to 5×10. -4 After Pa, hot isostatic pressing is performed at 900-1000℃ for 1.8-2 hours, with a pressure of 140-160 MPa, to form a metallurgical bonding interface;

[0024] S5: Hollow composite pipes with an inner diameter of 45-50 mm, a wall thickness of 10-12 mm, and a length of 1800-2200 mm, manufactured by cold drawing process.

[0025] Furthermore, in step S5, the cold drawing process includes multiple cold drawing passes, with each pass reducing the diameter by no more than 7%, and annealing at 620-650°C for 35-40 minutes between passes.

[0026] Furthermore, after step S5, the hollow composite pipe is subjected to quality inspection, which includes ultrasonic flaw detection, X-ray weld inspection, and helium mass spectrometry leak detection. The leakage rate of the helium mass spectrometry leak detection is ≤1×10⁻⁶. -9 Pa·m 3 / s.

[0027] Furthermore, the hot isostatic pressing treatment in step S4 is performed at a temperature of 950-1000℃, a pressure of 150-160MPa, and a time of 2 hours.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] (1) This invention employs a three-layer composite structure design. The inner anti-oxidation barrier layer directly blocks oxygen diffusion and inhibits element migration; the middle transition layer provides excellent high-temperature ductility and strain coordination capabilities within the hot rolling temperature range, matching the densification behavior of thermoelectric powder; and the outer structural support layer possesses high strength and creep resistance, ensuring the stability of the overall structure under large deformation during hot rolling. The three functionally graded materials are metallurgically bonded to form an integrated encapsulation system, maintaining complete sealing even under high temperature and large deformation conditions, effectively solving the problems of easy oxidation and failure of single-material encapsulation.

[0030] (2) In this invention, the Al content in the antioxidant barrier layer is controlled at 27-30 at.%, ensuring the formation of a continuous and dense Al2O3 protective film at high temperatures. The dispersed Y2O3 particles further enhance the high-temperature oxidation resistance and structural stability. In the transition layer, the content of elements such as Cr and Mn is adjusted to maintain good plasticity at high temperatures. The structural support layer uses microalloyed low-carbon bainitic steel, and a fine-grained bainitic structure (grain size 4.2-5 μm) is obtained through controlled rolling and cooling, which has both high strength and certain toughness. The material composition and structure of each layer are optimized and designed to have good compatibility with thermoelectric materials, avoiding interfacial reactions.

[0031] (3) This invention adopts a process route of first preparing the three layers separately, then hot isostatic pressing (HIP) composite, and finally cold drawing, which can achieve a tight metallurgical bond and precise dimensional control of the three-layer material. The HIP process ensures that the interface is free of pores and oxide inclusions; the cold drawing process can flexibly adjust the final thickness of each layer and the roughness of the inner surface of the tube to meet the encapsulation requirements of different thermoelectric materials. The preparation method has a clear process flow and controllable parameters, making it suitable for large-scale production.

[0032] (4) The embodiments of the present invention show that the Bi2Te3-based, PbTe-based, and SiGe-based thermoelectric materials hot-rolled using the packaging material of the present invention all have a density higher than 95% and an oxygen content lower than 100 ppm. Compared with the control group using 304 stainless steel or pure copper packaging, the present invention has significant improvements in oxidation resistance, packaging integrity, and thermoelectric performance protection, and has important engineering application value. Detailed Implementation

[0033] The present invention will be further described below with reference to the embodiments. The embodiments of the present invention include, but are not limited to, the following embodiments.

[0034] This invention provides an antioxidant encapsulation material for hot rolling of thermoelectric materials. The antioxidant encapsulation material is a three-layer composite tubular structure that can effectively protect and efficiently densify mainstream thermoelectric materials such as Bi2Te3-based, PbTe-based, and SiGe-based materials over a wide temperature range.

[0035] The aforementioned three-layer composite tubular structure comprises, from the inside out, an antioxidant barrier layer, a transition layer, and a structural support layer. Each layer is integrally formed using a metallurgical bonding method, maintaining structural integrity during hot rolling and large deformation processes, without interlayer delamination or interface cracking.

[0036] The antioxidant barrier layer directly contacts the thermoelectric powder, and its function is to block the diffusion of oxygen, water vapor, and other reactive gases into the material, and to inhibit the migration of elements from the encapsulation material itself into the thermoelectric material. This layer uses a composite material with Fe-Al intermetallic compound as the matrix and dispersed Y2O3 particles. The Al content is 27-30 at.%, ensuring the formation of a continuous and dense Al2O3 surface film at high temperatures; Y2O3 serves as the dispersed reinforcing phase, with an average particle size of 120 nm and a volume fraction of 0.1-1.0 vol.%; the balance being iron (Fe) and unavoidable trace impurities. The thickness of the antioxidant barrier layer is 0.2-2.0 mm. It is prepared by mechanical alloying, specifically by mixing Fe powder, Al powder and Y2O3 nanoparticles in a certain proportion, and ball milling them in a high-energy ball mill at a ball-to-material ratio of 10-15:1, a rotation speed of 250-350 rpm, and under argon protection for 40-60 hours to obtain composite powder. Then, the powder is packed into a sleeve, vacuum sealed, and hot isostatic pressing is performed at 1050-1150℃ and 150-180 MPa for 2-4 hours to obtain a fully dense billet.

[0037] The transition layer, located between the inner and outer layers, provides sufficient high-temperature ductility and strain compatibility within the hot-rolling temperature range to match the volume shrinkage and flow behavior of the thermoelectric powder during densification. This layer employs a Ni-Cr-Fe-Mn-C solid solution alloy with a Ni matrix. The composition, by mass percentage, is as follows: Ni is the balance; Cr content is not less than 14 wt.% to maintain high-temperature strength; Fe content is 6-10 wt.%; Mn content is 1.5-2.5 wt.% to reduce stacking fault energy and promote dynamic recrystallization, thereby improving high-temperature plasticity; C content is controlled within the range of 0.08-0.12 wt.% to prevent grain boundary embrittlement. The transition layer thickness is 1.0-5.0 mm.

[0038] The primary function of the structural support layer is to maintain mechanical load and overall structural integrity. It must possess high yield strength, high creep resistance, and good process compatibility with hot rolling equipment. This layer is made of microalloyed low-carbon bainitic steel, with the following composition by weight percentage: C 0.03-0.07 wt.%, Si 0.2-0.5 wt.%, Mn 1.2-1.6 wt.%, Mo 0.3-0.5 wt.%, Nb 0.03-0.05 wt.%, Ti 0.01-0.02 wt.%, with the balance being Fe and unavoidable trace impurities. The thickness of this structural support layer is 3.0-15.0 mm, and the grain size is 4.2-5 μm.

[0039] To achieve a tight metallurgical bond between the three layers of materials and ultimately form a composite pipe that meets dimensional requirements, this invention employs the following preparation method, the core steps of which include pre-preparation of each layer of blanks, hot isostatic pressing, and cold drawing:

[0040] S1, the composite material with Fe-Al intermetallic compound as matrix and Y2O3 particles dispersedly distributed is homogenized and annealed at 1000-1200℃ for 10-12 hours, and then hot rolled at 900-1000℃ into a thin strip with a thickness of 0.4-2.2 mm.

[0041] S2, the transition layer Ni-Cr-Fe-Mn-C alloy is vacuum induction melted and then cast into a slab, which is then hot rolled at 1100-1200℃ to a thickness of 1.2-5.5 mm;

[0042] S3 involves continuously casting microalloyed low-carbon bainitic steel for the structural support layer into slabs, followed by controlled rolling and cooling: hot rolling at 850-900℃ into plates of 3.5-16.5 mm thickness, followed by accelerated cooling at a rate of 15-25℃ / s to 450-550℃, and then air cooling or coiling to obtain a low-carbon bainitic structure with a grain size of 4.2-5 μm.

[0043] S4. Concentrically fit the above three layers of material in the inner-middle-outer order, controlling the gap within 0.08 mm, and evacuate to 5×10⁻⁶ mm. -4 After Pa, hot isostatic pressing (HIP) is performed at 900-1000℃ for 1.8-2 hours, with a pressure of 140-160 MPa, to induce atomic-level diffusion bonding at the interface, forming a metallurgical interface free of pores and oxide inclusions. Testing shows that the shear strength at the interface is not less than 90% of the strength of the base material, and metallographic observation reveals a continuous interface without delamination.

[0044] S5 is ultimately manufactured into hollow composite tubes with an inner diameter of 45-50 mm, a wall thickness of 10-12 mm, and a length of 1800-2200 mm through a cold drawing process. The cross-sectional shape is circular to meet the feeding requirements of standard hot rolling dies. During the cold drawing process, each layer of material deforms synchronously. By controlling the total deformation, the final thickness of each layer is ensured to meet the aforementioned range. Furthermore, the surface roughness Ra of the inner surface of the composite tube can be controlled within the range of 0.8-3.2 μm by adjusting the surface condition of the cold drawing die.

[0045] In this invention, by selecting and combining single-layer materials of different thicknesses prepared in steps S1-S3, and then performing hot isostatic pressing composite in step S4, the deformation amount is precisely controlled in the cold drawing process in step S5, resulting in a composite pipe product with each layer thickness conforming to the above-mentioned range and an overall wall thickness of 10-12 mm. Examples 1-3 provide several feasible examples of specific thickness combinations.

[0046] In a preferred embodiment of the present invention, 0.15-0.22 at.% Zr is further added to the inner alloy to refine the grains and improve high-temperature strength; 0.6-1.0 wt.% Co is introduced into the intermediate Ni-based alloy to improve hot corrosion resistance; and 0.01-0.03 wt.% B is added to the outer bainitic steel to inhibit austenite grain growth.

[0047] In another preferred embodiment of the present invention, before filling with thermoelectric powder, a boron nitride (BN) release coating with a thickness of 2.5-3 μm, a purity of 99.6%, and a particle size of 0.8-1.0 μm is coated on the inner layer surface. The coating is dispersed in ethanol, sprayed, and dried at 140-160°C for 1-1.2 hours. This coating remains chemically inert at hot rolling temperatures, preventing the thermoelectric material from adhering to the inner layer.

[0048] In another preferred embodiment of the present invention, the inner surface roughness Ra of the three-layer composite pipe is controlled at 0.8-3.2 μm to enhance the frictional anchoring effect during powder filling.

[0049] In addition, the preparation method of the encapsulation material in this invention also includes ultrasonic flaw detection of the hollow composite tube to ensure that there are no delamination, porosity or inclusion defects; real-time X-ray imaging detection of the weld to confirm that there are no incomplete fusion or cracks; and helium mass spectrometry leak detection of the encapsulated body after vacuum sealing, with a leakage rate of 8×10⁻⁶. -10 Pa·m 3 / s, satisfying ≤1×10 -9 Pa·m 3 / s requirement.

[0050] Example 1

[0051] The antioxidant encapsulation material provided in this embodiment is used for Bi2Te3-based thermoelectric materials, and it comprises, from the inside out:

[0052] Antioxidant barrier layer: A composite material with Fe-Al intermetallic compound as the matrix and dispersed Y2O3 particles (Zr was not added in this example). The Al content is 28 at.%; the average particle size of Y2O3 is 120 nm, and the volume fraction is 0.5 vol.%; the balance is Fe and unavoidable trace impurities. The oxidation weight gain rate of this alloy at 1300℃ is 0.08 mg / cm³. 2 It does not undergo significant interfacial reactions with thermoelectric elements such as Bi, Te, Pb, Se, Si, and Ge below 1400℃. The thickness of this antioxidant barrier layer is 0.5 mm.

[0053] Transition layer: A Ni-Cr-Fe-Mn-C solid solution alloy with Ni as the matrix is ​​used. The composition, by mass percentage, is as follows: Ni balance; Cr content 15 wt.%; Fe content 8 wt.%; Mn content 2 wt.%; C content controlled at 0.1 wt.%. This transition layer alloy exhibits an elongation of 48% at 1000℃ and maintains 27% elongation at 1300℃. Its Young's modulus at 800℃ is 95 GPa, significantly lower than that of the outer structural steel, thus effectively buffering the difference in thermal expansion between the inner and outer layers. The transition layer thickness is 2.0 mm.

[0054] Structural support layer: Made of microalloyed low-carbon bainitic steel, its composition by weight percentage is: C 0.05 wt.%, Si 0.3 wt.%, Mn 1.4 wt.%, Mo 0.4 wt.%, Nb 0.04 wt.%, Ti 0.015 wt.%, with the balance being Fe. The grain size is 4.5 μm. At room temperature, its yield strength is 465 MPa with an elongation of 30%; after holding at 1000℃ for 1 hour, its compressive yield strength is 85 MPa, sufficient to withstand the external pressure during hot rolling with 90% reduction without buckling instability. The thickness of this structural support layer is 8.0 mm.

[0055] The above three-layer structure achieves metallurgical bonding through hot isostatic pressing. The specific method is as follows:

[0056] S1, a composite material with Fe-Al intermetallic compound as matrix and Y2O3 particles dispersedly distributed is homogenized and annealed at 1100℃ for 12 hours, and then hot rolled at 950℃ into a thin strip with a thickness of 0.6 mm.

[0057] S2, the transition layer Ni-Cr-Fe-Mn-C alloy is vacuum induction melted and then cast into a slab, which is then hot rolled to a thickness of 2.2 mm at 1150℃;

[0058] S3, the microalloyed low-carbon bainitic steel of the structural support layer is continuously cast into slabs, and then subjected to controlled rolling and cooling: hot rolling at 880℃ into 8.5 mm thick plates, and after final rolling, accelerated cooling to 500℃ at a rate of 20℃ / s, followed by air cooling or coiling, to obtain a low-carbon bainitic structure with a grain size of 4.5 μm.

[0059] S4. Concentrically fit the above three layers of material in the inner-middle-outer order, controlling the gap within 0.08 mm, and evacuate to 5×10⁻⁶ mm. -4 After Pa, hot isostatic pressing is performed at 950℃ for 2 hours with a pressure of 150 MPa to cause atomic-level diffusion bonding at the interface, forming a metallurgical bonding interface without pores or oxide inclusions.

[0060] S5 is ultimately manufactured into a hollow composite pipe with an inner diameter of 50 mm, a wall thickness of 10.5 mm, and a length of 2000 mm through a cold drawing process. Its cross-sectional shape is circular. After cold drawing deformation, the thickness of each layer is uniformly reduced, and finally a composite structure with an anti-oxidation barrier layer of 0.5 mm, a transition layer of 2.0 mm, and a structural support layer of 8.0 mm is obtained.

[0061] The encapsulation material prepared above is used for thermoelectric materials (Bi). 0.5 Sb 1.5 The main process steps for packaging Te3 are as follows:

[0062] First, p-type Bi 0.5 Sb 1.5 Te3 thermoelectric powder was packed into a glove box protected by high-purity argon gas, with both oxygen and water content controlled at 0.08 ppm. The powder was then subjected to high-energy ball milling, resulting in an average particle size of 15 μm, a loose packing density of 73% of the theoretical density, and an oxygen content of 42 ppm.

[0063] Subsequently, the hollow composite pipe was placed in a glove box, with a 304 stainless steel flange pre-welded to one end, while the other end remained open. Thermoelectric powder was uniformly filled into the pipe cavity using a vibration filling method, achieving a filling density of 80% of the theoretical density. After filling, the open end was sealed using TIG pulse welding with the following parameters: current 100 A, pulse frequency 5 Hz, and shielding gas 99.999% pure argon. The weld depth penetrated the three layers of the structure, ensuring a full-section seal.

[0064] Next, a KF16 standard vacuum nozzle was installed on the welded package, and a molecular pump-turbo pump combined vacuum system was connected to evacuate the system to an absolute pressure of 0.1 Pa. After reaching the target vacuum level, high-frequency induction heating was used to partially seal the base of the nozzle, cutting off the vacuum path and forming a completely sealed vacuum chamber. The entire evacuation and sealing process was completed in an inert atmosphere.

[0065] Subsequently, the package was placed in a box-type resistance furnace for preheating and heat preservation. For Bi2Te3-based materials, the heat preservation temperature was set at 350℃ for 20 minutes. During the heat preservation process, the inner layer of the package material formed a dense Al2O3 film with a thickness of 60 nm on the surface; the middle layer of Ni-based alloy underwent dynamic recovery, eliminating work hardening; and the outer layer of bainitic steel maintained its high strength.

[0066] After the heat treatment period, the material was immediately transferred to a two-roll hot rolling mill for hot rolling deformation. The rolling temperature was 350℃, the total reduction was 85%, and five passes were used, with a heat treatment time between passes not exceeding 4 minutes. During the rolling process, the three layers of encapsulating material deformed synergistically: the outer layer provided rigid constraint, the middle layer coordinated the internal powder flow through high plasticity, and the inner layer remained chemically inert and extended synchronously with deformation. Its Al2O3 film maintained continuity due to the pinning of the Y2O3 dispersed phase, without cracks or peeling. The thermoelectric material density of the hot-rolled bar was 96.2% of the theoretical density, the grain orientation factor was 0.73, and there were no macroscopic pores or compositional segregation.

[0067] After hot rolling, the package was longitudinally slit using wire cutting to remove the outer and middle layers, retaining the inner thin shell as a temporary protective layer. It was then immersed in 5 wt.% dilute hydrochloric acid for 10 minutes to dissolve the residual Fe-Al phase. After rinsing with deionized water, it was vacuum dried to obtain a pure thermoelectric material sample. The final sample had an oxygen content of 85 ppm and a ZT value of 0.98 at 350 K, exhibiting good thermoelectric performance.

[0068] Example 2

[0069] The antioxidant encapsulation material provided in this embodiment is used for PbTe-based thermoelectric materials, and it comprises, from the inside out:

[0070] Antioxidant barrier layer: A Fe-Al-Zr-Y₂O₃ quaternary composite material is used, with Fe-Al intermetallic compounds as the matrix. Al 29 at.%, Zr 0.18 at.%, Y₂O₃ (average particle size 120 nm) 0.8 vol.%, with the balance being Fe and unavoidable trace impurities. The addition of Zr significantly refines the grain size to 3.8 μm, reducing the oxidation weight gain rate at 1300℃ to 0.06 mg / cm². 2 •h, and does not react with elements such as Pb, Te, and Na at the interface. The layer is 0.6 mm thick and is prepared by mechanical alloying (ball-to-material ratio 12:1, rotation speed 320 rpm, ball milling time 45 hours) and hot isostatic pressing (1200℃, 155 MPa, holding time 3.5 hours), with a relative density ≥99.6%.

[0071] Transition layer: A Ni-Cr-Fe-Mn-Co-C solid solution alloy is used, with the following composition by mass percentage (wt.%): Ni (balance), Cr 16 wt.%, Fe 9 wt.%, Mn 2.2 wt.%, Co 0.8 wt.%, and C 0.11 wt.%. The introduction of Co improves hot corrosion resistance, achieving an elongation of 32% at 1200℃ and a Young's modulus of 92 GPa at 800℃, effectively coordinating the thermal expansion difference between the inner and outer layers (the CTE of PbTe is approximately 10 × 10⁻⁶). -6 / K, the CTE of this transition layer is 9.5×10 -6 / K). The transition layer thickness is 2.5 mm, and it is produced by vacuum induction melting (1580℃, vacuum degree ≤5×10). -3 Pa) is cast into slabs.

[0072] Structural support layer: Constructed of boron-containing microalloyed low-carbon bainitic steel, with the following composition by weight percentage (wt.%): C 0.06 wt.%, Si 0.4 wt.%, Mn 1.5 wt.%, Mo 0.45 wt.%, Nb 0.045 wt.%, Ti 0.018 wt.%, B 0.02 wt.%, with the balance being Fe. Boron inhibits austenite grain growth, resulting in a final grain size of 4.3 μm. The yield strength at room temperature is 480 MPa, with an elongation of 28%. After holding at 600℃ for 1 hour, the compressive yield strength reaches 92 MPa, allowing it to withstand 80% rolling load reduction without instability. This layer is 7.0 mm thick.

[0073] The above-mentioned packaging material preparation process is as follows:

[0074] S1, the Fe-Al-Zr-Y2O3 composite material was homogenized and annealed at 1150℃ for 11 hours, and then hot-rolled at 980℃ into a thin strip with a thickness of 0.7 mm, with the inner surface roughness Ra controlled at 1.2 μm;

[0075] S2, Ni-Cr-Fe-Mn-Co-C alloy slab is hot rolled to a thickness of 2.7 mm at 1180℃, and the surface is mechanically polished to remove oxide scale;

[0076] S3, low-carbon bainitic steel containing boron microalloying is continuously cast into slabs, and then subjected to controlled rolling and cooling: hot-rolled at 900℃ into 7.3 mm thick plates, and after final rolling, accelerated cooling to 550℃ at a rate of 25℃ / s, followed by air cooling or coiling, to obtain a low-carbon bainitic structure with a grain size of 4.3 μm.

[0077] S4, three layers of material are concentrically assembled in the inner-middle-outer order, with the gap controlled within 0.06 mm, and vacuumed to 5×10. -4 After Pa, hot isostatic pressing was performed at 980℃ for 2 hours with a pressure of 155 MPa. After the interface atoms diffused and bonded, there were no pores or oxide inclusions.

[0078] S5 is a circular hollow composite tube with an inner diameter of 48 mm, a wall thickness of 10.1 mm, and a length of 2100 mm, produced by a multi-pass cold drawing process (diameter reduction ≤7% per pass, annealing at 620℃ for 35 minutes between passes). The thicknesses of each layer after cold drawing are as follows: 0.6 mm for the anti-oxidation barrier layer, 2.5 mm for the transition layer, and 7.0 mm for the structural support layer. Subsequently, a 3.0 μm thick BN release coating (99.6% purity, 0.9 μm particle size) is coated on the inner surface of the composite tube, and then dried at 150℃ for 1.1 hours.

[0079] Packaging and hot rolling process: The above-mentioned composite tube is used for PbTe-based (Pb 0.98 Na 0.02 Thermoelectric powder (Te) was used for encapsulation, and the filling, welding, and vacuuming steps were the same as in Example 1. The preheating temperature was set to 600°C and held for 40 minutes, forming an 80 nm thick dense Al2O3 film in the inner layer. The hot rolling temperature was 600°C, the total reduction was 85%, and five-pass rolling was used, with the holding time between passes not exceeding 4 minutes.

[0080] After testing, Pb 0.98 Na 0.02 The Te thermoelectric material has a density of 95.8% (theoretical density) and an oxygen content of 92 ppm. Its ZT value is 1.42 at 700 K. The encapsulated body exhibited no bulging or cracking during hot rolling, maintaining intact interfacial bonding and preventing oxygen permeation.

[0081] Example 3

[0082] The antioxidant encapsulation material provided in this embodiment is used for SiGe-based thermoelectric materials, and it comprises, from the inside out:

[0083] Antioxidant Barrier Layer: Employs a high-temperature resistant Fe-Al-Zr-Y₂O₃ composite material. The composition, by atomic percentage (at.%), is: Al 30 at.%, Zr 0.22 at.%, Y₂O₃ (average particle size 120 nm) 1.0 vol.%, with the balance being Fe and unavoidable trace impurities. The high Al content ensures the formation of a continuous and dense Al₂O₃ film (120 nm thick) at 1300℃. Zr and Y₂O₃ synergistically refine the grain size to 3.5 μm, resulting in an oxidation weight gain rate of only 0.09 mg / cm² at 1500℃. 2 •h, does not diffuse at the interface with Si or Ge. The thickness of this layer is 1.5 mm.

[0084] Transition layer: A Ni-Cr-Fe-Mn-Co-C high-temperature adaptable alloy is used, with the following composition by mass percentage (wt.%): Ni (balance), Cr 18 wt.%, Fe 10 wt.%, Mn 2.5 wt.%, Co 1.0 wt.%, and C 0.12 wt.%. The high Cr / Co ratio improves high-temperature strength and creep resistance, achieving an elongation of 25% at 1300℃ and a Young's modulus of 88 GPa at 1000℃. Adjusting the Fe / Ni ratio reduces the CTE to 4.8 × 10⁻⁶. -6 / K, with CTE (4×10) of SiGe -6 The K / K pair exhibits excellent compatibility. The transition layer thickness is 2.0 mm.

[0085] Structural support layer: Made of microalloyed low-carbon bainitic steel, with the following composition by weight percentage (wt.%): C 0.07 wt.%, Si 0.5 wt.%, Mn 1.6 wt.%, Mo 0.5 wt.%, Nb 0.05 wt.%, Ti 0.02 wt.%, B 0.03 wt.%, with the balance being Fe. This steel, after controlled rolling and cooling, forms a fine-grained bainitic structure (grain size 4.2 μm), with a room temperature yield strength of 510 MPa, an elongation of 27%, and a compressive yield strength of 78 MPa after holding at 1300℃ for 1 hour. It can withstand extreme rolling loads of 75% reduction. The thickness of the structural support layer is 10.0 mm.

[0086] The above-mentioned packaging material preparation process is as follows:

[0087] S1, the Fe-Al-Zr-Y2O3 composite material was homogenized and annealed at 1200℃ for 12 hours, and then hot-rolled at 1000℃ into a thin strip with a thickness of 1.6 mm, with the inner surface roughness Ra controlled at 2.0 μm;

[0088] S2, Ni-Cr-Fe-Mn-Co-C alloy slab is hot rolled to a thickness of 2.2 mm at 1200℃, and the oxide film on the surface is removed by plasma cleaning;

[0089] S3, the microalloyed bainitic steel thick plate is prepared into a billet with a thickness of 10.5 mm according to the aforementioned controlled rolling and controlled cooling process; the microalloyed low carbon bainitic steel is continuously cast into a slab, and then subjected to controlled rolling and controlled cooling: hot rolling at 850℃ into a 10.0 mm thick plate, and after final rolling, it is accelerated to 450℃ at a rate of 16℃ / s, and then air-cooled or coiled to obtain a low carbon bainitic structure with a grain size of 4.2 μm;

[0090] S4, three layers of material are concentrically assembled in the inner-middle-outer order, with the gap controlled within 0.08 mm, and vacuumed to 5×10. -4 After Pa, it is subjected to hot isostatic pressing at 1000℃ for 2.0 hours with a pressure of 160 MPa, and the interface forms an atomic-level metallurgical bond without oxide inclusions and pores.

[0091] S5 is produced by multi-pass cold drawing (diameter reduction ≤6% per pass, annealing at 650℃ for 40 minutes between passes) to form a circular hollow composite tube with an inner diameter of 45mm, a wall thickness of 12mm, and a length of 2200mm. The thicknesses of each layer after cold drawing are as follows: 1.5mm for the anti-oxidation barrier layer, 2.0mm for the transition layer, and 10.0mm for the structural support layer. Subsequently, a 3.0 μm thick BN release coating (99.6% purity, 1.0 μm particle size) is applied to the inner surface of the composite tube, and it is dried at 160℃ for 1.2 hours.

[0092] Packaging and hot rolling process: The above-mentioned composite tube is used for SiGe-based (Si 0.85 Ge 0.15 Thermoelectric powder was encapsulated, and the filling, welding, and vacuuming steps were the same as in Example 1. The preheating temperature was set at 1300℃ and held for 60 minutes, forming a dense Al2O3 film in the inner layer. The hot rolling temperature was 1300℃, the total reduction was 75%, and five-pass rolling was used, with the holding time between passes not exceeding 4 minutes.

[0093] After testing, Si 0.85 Ge 0.15The thermoelectric material has a density of 95.1% (theoretical density) and an oxygen content of 98 ppm. Its ZT value is 0.95 at 1200 K. The encapsulation remains completely sealed at 1300℃, with no interlayer delamination or oxygen permeation, meeting the stringent requirements for large-scale hot rolling of SiGe-based materials.

[0094] Comparative Example 1

[0095] The Bi in Example 1 is encapsulated using a single material, 304 stainless steel tube (10 mm wall thickness, with no special coating on the inner surface). 0.5 Sb 1.5 Te3 thermoelectric powder was hot-rolled under the same process conditions (350℃, 85% reduction). The oxygen content of the hot-rolled sample was 320 ppm, the density was 91.5%, and the ZT value was 0.72.

[0096] Comparative Example 2: A single-material pure copper tube (10 mm wall thickness, inner surface without special coating) was used to encapsulate Bi from Example 1. 0.5 Sb 1.5 Te3 thermoelectric powder was hot-rolled under the same conditions. Due to the insufficient strength of copper at 350℃, local bulging occurred during the rolling process, leading to seal failure. The oxygen content of the sample was as high as 510 ppm, the density was only 88.3%, and the ZT value was 0.65.

[0097] Table 1. Performance comparison of encapsulation materials prepared in Example 1 and Comparative Examples 1 and 2

[0098]

[0099] As can be seen from the data in the table, the three-layer composite packaging material provided by this invention exhibits excellent comprehensive performance during the hot rolling process. It not only ensures the structural integrity of the package but also significantly reduces oxygen contamination of the thermoelectric material and improves density and thermoelectric performance.

[0100] In all implementation cases, the three-layer composite structure is integrated into a functionally graded system through metallurgical bonding. The inner Fe-Al-Y2O3 alloy provides an oxidation barrier, the middle Ni-Cr-Fe-Mn alloy achieves high-temperature plasticity matching, and the outer microalloyed bainitic steel ensures structural strength. The synergistic effect of these three layers ensures that the encapsulation system remains completely sealed after hot rolling at 1300℃ with a 90% reduction, and the oxygen increment of the internal thermoelectric material is controlled to within 50 ppm.

[0101] The above embodiments are merely one of the preferred embodiments of the present invention and should not be used to limit the scope of protection of the present invention. Any modifications or refinements made to the main design concept and spirit of the present invention that are not of substantial significance, but solve the same technical problem as the present invention, should be included within the scope of protection of the present invention.

Claims

1. An antioxidant encapsulation material for hot rolling of thermoelectric materials, characterized in that, It is a three-layer composite tubular structure, consisting of the following layers from the inside out: Antioxidant barrier layer: It is a composite material with Fe-Al intermetallic compound as matrix and Y2O3 particles dispersed therein, wherein the Al content is 27-30 at.%, the average particle size of Y2O3 is 120 nm, the volume fraction is 0.1-1.0 vol.%, and the balance is Fe and unavoidable trace impurities. The thickness of this layer is 0.2-2.0 mm. Transition layer: a Ni-Cr-Fe-Mn-C solid solution alloy with Ni as the matrix, comprising by mass percentage: Cr not less than 14 wt.%, Fe 6-10 wt.%, Mn 1.5-2.5 wt.%, C 0.08-0.12 wt.%, with the balance being Ni and unavoidable trace impurities, and the thickness of this layer is 1.0-5.0 mm; Structural support layer: It is a microalloyed low-carbon bainitic steel, containing by weight percentage: C 0.03-0.07 wt.%, Si 0.2-0.5 wt.%, Mn 1.2-1.6 wt.%, Mo 0.3-0.5 wt.%, Nb 0.03-0.05 wt.%, Ti 0.01-0.02 wt.%, with the balance being Fe and unavoidable trace impurities. The grain size of this layer is 4.2-5 μm and the thickness is 3.0-15.0 mm.

2. The antioxidant encapsulation material for hot rolling of thermoelectric materials according to claim 1, characterized in that, The antioxidant barrier layer, transition layer, and structural support layer are integrally formed through a metallurgical bonding method.

3. The antioxidant encapsulation material for hot rolling of thermoelectric materials according to claim 2, characterized in that, The antioxidant barrier layer also contains 0.15-0.22 at.% Zr element.

4. The antioxidant encapsulation material for hot rolling of thermoelectric materials according to claim 2, characterized in that, The transition layer also contains 0.6-1.0 wt.% Co.

5. The antioxidant encapsulation material for hot rolling of thermoelectric materials according to claim 3, characterized in that, The structural support layer also contains 0.01-0.03 wt.% of B element.

6. The antioxidant encapsulation material for hot rolling of thermoelectric materials according to claim 5, characterized in that, The inner surface of the antioxidant barrier layer is coated with a boron nitride release coating with a thickness of 2.5-3 μm, wherein the boron nitride has a purity of 99.6% and a particle size of 0.8-1.0 μm; the inner surface roughness Ra of the three-layer composite tubular structure is 0.8-3.2 μm.

7. A method for preparing an antioxidant encapsulation material according to any one of claims 1-6, characterized in that, Includes the following steps: S1, Preparation of antioxidant barrier layer material: The composite material with Fe-Al intermetallic compound as matrix and Y2O3 particles dispersedly distributed is homogenized and annealed at 1000-1200℃ for 10-12 hours, and then hot rolled at 900-1000℃ into a thin strip with a thickness of 0.4-2.2 mm. S2, Preparation of transition layer material: After vacuum induction melting, it is cast into a slab and hot rolled at 1100-1200℃ to a thickness of 1.2-5.5mm; S3, Preparation of structural support layer material: The slab is continuously cast into a billet, and then controlled rolling and cooling are performed to obtain bainitic structure. Specifically, the slab is hot rolled at 850-900℃ to form a 3.5-16.5 mm thick plate, and after final rolling, it is accelerated to 450-550℃ at a cooling rate of 15-25℃ / s. S4. Concentrically fit the above three layers of material in the inner-middle-outer order, controlling the gap within 0.08 mm, and evacuate to 5×10⁻⁶ mm. -4 After Pa, hot isostatic pressing is performed at 900-1000℃ for 1.8-2 hours, with a pressure of 140-160 MPa, to form a metallurgical bonding interface; S5 is a hollow composite pipe with an inner diameter of 45-50 mm, a wall thickness of 10-12 mm, and a length of 1800-2200 mm, manufactured by cold drawing process.

8. The preparation method according to claim 7, characterized in that, In step S5, the cold drawing process includes multiple cold drawing passes, with each pass reducing the diameter by no more than 7%, and annealing at 620-650℃ for 35-40 minutes between passes.

9. The preparation method according to claim 8, characterized in that, Following step S5, the hollow composite pipe undergoes quality inspection, which includes ultrasonic flaw detection, X-ray weld inspection, and helium mass spectrometry leak detection. The leakage rate detected by helium mass spectrometry must satisfy ≤1×10⁻⁶. -9 Pa·m 3 / s.

10. The preparation method according to claim 8, characterized in that, The hot isostatic pressing treatment in step S4 is performed at a temperature of 950-1000℃, a pressure of 150-160 MPa, and a time of 2 hours.

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