A cold-pressing-hot-rolling synergistic packaging method of high-zt thermoelectric materials

Abstract

The application discloses a cold-pressing-hot-rolling collaborative packaging method of high-ZT thermoelectric material, and relates to the technical field of thermoelectric materials.The packaging method comprises the following steps: cold-pressing thermoelectric material powder, applying a pressure of 100-500 tons at room temperature to form a block-shaped precursor with an initial density not lower than 70% of the theoretical density; loading the block-shaped precursor into a metal steel sleeve; vacuumizing the interior of the steel sleeve to an air pressure lower than 0.06 Pa and then welding and sealing; keeping the sealed steel sleeve at a temperature of 500-1500 DEG C for 10-120 min; hot-rolling the steel sleeve at the keeping temperature, with a cumulative reduction rate not lower than 60%; and removing the outer steel sleeve to obtain a densified thermoelectric block material.Under the premise of ensuring or even improving the thermoelectric performance (high ZT value) of the material, the application simultaneously realizes high material utilization, low oxidation loss, low equipment dependence and good process adaptability, effectively breaks through the bottleneck of high cost and low efficiency in the current industrialization process of thermoelectric materials, and has important practical value and popularization prospect.

Description

Technical Field

[0001] This invention relates to the field of thermoelectric materials technology, specifically to a cold-pressing-hot-rolling co-packaging method for high-ZT thermoelectric materials. Background Technology

[0002] Thermoelectric materials, as functional materials capable of directly converting heat energy into electrical energy, have demonstrated significant application value in recent years in fields such as thermoelectric power generation, solid-state refrigeration, and industrial waste heat recovery. Their performance is primarily determined by the dimensionless thermoelectric figure of merit (ZT), with a high ZT value depending on the synergistic optimization of high electrical conductivity, low thermal conductivity, and a high Seebeck coefficient. With the global push for energy efficiency and carbon neutrality, developing thermoelectric materials that combine high performance, high stability, and scalable fabrication has become an important research direction in this field. Among these, the densification and microstructure control of bulk thermoelectric materials are key aspects for improving the ZT value.

[0003] Among these methods, powder metallurgy combined with hot deformation is considered an effective pathway for preparing high-performance thermoelectric bulk materials. Traditionally, spark plasma sintering (SPS) and hot pressing sintering have been widely used to obtain high-density thermoelectric samples, but they heavily rely on high vacuum environments to suppress high-temperature oxidation, resulting in high equipment costs and limited single-batch processing capacity. Furthermore, additional machining is often required after sintering, leading to significant material loss and lengthy processes. Although some studies have attempted to introduce hot working methods such as upsetting or forging for subsequent densification, problems such as uneven deformation, abnormal grain growth, and deterioration of edge properties still necessitate the removal of large amounts of ineffective areas, resulting in low material utilization and difficulty in achieving precise dimensional control and continuous production.

[0004] While existing technologies have explored combining encapsulation and hot rolling, they mostly employ a two-step method: sintering for densification followed by encapsulation and hot rolling. This approach fails to eliminate reliance on high vacuum and complex sintering equipment, instead increasing process complexity. Furthermore, conventional hot rolling processes, if conducted without a protective atmosphere, are highly susceptible to oxidation of thermoelectric powders, damaging carrier concentration and grain boundary structure, and significantly reducing ZT values. Conversely, maintaining a vacuum or inert atmosphere throughout the process leads back to the old path of high cost and low efficiency. Therefore, there is an urgent need for a large-scale preparation method that can effectively isolate oxidation, achieve high densification and grain size control, be compatible with continuous hot rolling under atmospheric conditions, and be applicable to thermoelectric materials in various temperature ranges (such as BiTe, PbTe, and SiGe), in order to overcome the efficiency and cost bottlenecks in the current industrialization of thermoelectric materials. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and provide a new method based on "cold pressing-hot rolling co-packaging", which aims to achieve efficient, low-cost and controllable preparation of high-performance thermoelectric materials through process integration and optimization.

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

[0007] A cold-pressing-hot-rolling co-packaging method for high-ZT thermoelectric materials includes the following steps:

[0008] S1: Cold-press the thermoelectric material powder and apply a pressure of 100 to 500 tons at room temperature to form a block precursor with an initial density of not less than 70% of the theoretical density;

[0009] S2: The block-shaped precursor is inserted into a metal sleeve;

[0010] S3: Evacuate the inside of the steel sleeve until the air pressure is below 0.06Pa, then weld and seal it;

[0011] S4: Keep the sealed steel sleeve at 500℃-1500℃ for 10min-120min;

[0012] S5: The steel sleeve is hot-rolled at the insulation temperature, and the cumulative reduction rate is not less than 60%;

[0013] S6: Remove the outer steel sleeve to obtain a denser thermoelectric bulk material.

[0014] Furthermore, in step S1, the particle size of the thermoelectric material powder is 0.1μm-100μm, and it is dried at 100℃-150℃ for 2h-4h before cold pressing, and the cold pressing holding time is 30s-120s.

[0015] Furthermore, in step S2, the metal sleeve is made of low-carbon steel or stainless steel with a wall thickness of 2mm-8mm. The inner wall of the sleeve is coated with molybdenum disulfide or boron nitride release agent with a coating thickness of 5μm-20μm.

[0016] Furthermore, in step S4, the heat preservation temperature is selected according to the thermoelectric material system: 500℃-600℃ for BiTe-based materials, 550℃-800℃ for PbTe-based materials, and 900℃-1200℃ for SiGe-based materials.

[0017] Furthermore, in step S5, hot rolling is carried out in air at a rolling speed of 0.1 m / min to 2 m / min and a hot rolling pressure of 500 T to 3000 T.

[0018] Furthermore, in step S5, the hot rolling is a single-pass or multi-pass rolling process; when the hot rolling is a multi-pass rolling process, the reduction amount of each pass is designed to be incremental or gradient distributed, and the total cumulative reduction rate is not less than 80%.

[0019] Furthermore, steps S4 and S5 are carried out consecutively. After heat preservation, the steel sleeve is directly fed into the rolling mill, and the temperature fluctuation is maintained within ±10℃ through a heat-insulated transition channel.

[0020] Furthermore, in step S6, the steel sleeve is removed by wire cutting or laser cutting, and the surface of the thermoelectric material is ground after cutting.

[0021] Furthermore, the thermoelectric material is one of BiTe-based, PbTe-based, or SiGe-based thermoelectric materials.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] (1) This invention, through the synergy of "cold pressing pretreatment" and "hot rolling under vacuum steel sleeve protection", successfully increases the density of various thermoelectric materials (BiTe-based, PbTe-based, SiGe-based) to over 95%, approaching or reaching the level of traditional vacuum sintering, without relying on high-cost, high-vacuum sintering equipment (such as SPS). At the same time, due to the effective suppression of oxidation and regulation of grain orientation, the ZT value of the prepared bulk materials is significantly improved (e.g., the ZT value of the SiGe-based material in the example is 69.1% higher than that of the unprotected process), thus optimizing the overall performance of the materials.

[0024] (2) This invention integrates encapsulation, pre-densification (cold pressing), and final densification deformation (hot rolling) into a continuous process, avoiding the lengthy process caused by traditional two-step or multi-step methods such as "sintering and densification first, then machining" or "sintering first, then encapsulation and hot rolling". Since the hot rolling deformation is uniform and carried out under protection, the finished product does not need to remove a large number of cracked or degraded ineffective areas, and the material utilization rate is as high as 91.8% or more, which is much higher than the traditional SPS process (about 72.3%), significantly reducing raw material loss and production costs.

[0025] (3) This invention provides a stable micro-vacuum protection environment for the internal thermoelectric powder by using a weldable and sealable metal sleeve and evacuating it to a high vacuum (<0.06Pa) after filling. This allows subsequent heat preservation and hot rolling processes to be carried out directly in air, completely eliminating the high cost and low efficiency constraints of relying on vacuum or inert gas protection throughout the process. Example data show that the material oxidation weight gain rate is extremely low (≤0.1%), comparable to vacuum sintering, fundamentally solving the problem of material oxidation and deterioration caused by atmospheric hot rolling.

[0026] (4) The encapsulation method provided by this invention is adaptable to different thermoelectric materials. Its core parameters (such as cold pressing pressure, heat preservation temperature, hot rolling reduction rate, etc.) can be flexibly adjusted according to the characteristics of thermoelectric materials in different temperature ranges (from low temperature BiTe to high temperature SiGe), and the process has strong universality. At the same time, the continuous operation mode of "heat preservation-hot rolling" reduces the intermediate cooling and reheating links, has high thermal energy utilization efficiency, and is easy to connect with continuous rolling production lines, providing a practical and feasible technical path for the large-scale and batch preparation of high-performance thermoelectric materials. Attached Figure Description

[0027] Figure 1 The packaging process flow diagram provided for this invention.

[0028] Figure 2 This is a photograph of the appearance of the PbTe thermoelectric material prepared in Example 1 of the present invention. Detailed Implementation

[0029] The present invention will be further described below with reference to the accompanying drawings and embodiments. The embodiments of the present invention include, but are not limited to, the following embodiments.

[0030] like Figure 1 As shown, this invention provides a cold-pressing-hot-rolling co-packaging method for high ZT thermoelectric materials, comprising the following steps:

[0031] Step S1: The thermoelectric material powder is cold-pressed, and a pressure of 100 to 500 tons is applied at room temperature to form a block precursor with a certain initial strength and shape. The cold pressing pressure is applied to the powder system to make it reach more than 70% of the theoretical density, so as to provide structural stability for subsequent encapsulation and thermal deformation.

[0032] Step S2: The cold-pressed thermoelectric powder is loaded into the metal sleeve, which is a weldable and sealable hollow container. Its inner cavity size matches the cold-pressed block to ensure that the powder is fully contained without any loose gaps. After the filling is completed, the two ends of the steel sleeve are welded and sealed to form a closed cavity.

[0033] Step S3: Before or during the sealing process of the steel sleeve, a vacuum treatment is performed inside it. A mechanical pump or molecular pump system is used to reduce the air pressure inside the steel sleeve to below 0.06 Pa, effectively removing oxygen and other active gases and inhibiting the oxidation and deterioration of the thermoelectric material during the high temperature process.

[0034] Step S4: Perform heat preservation treatment on the encapsulated steel sleeve assembly. Heat it in the temperature range of 500℃-1500℃ and maintain the temperature for 10min-120min. This temperature range covers the applicable temperature range of low temperature BiTe, medium temperature PbTe and high temperature SiGe thermoelectric materials, so that the material is in a thermoplastic state and the diffusion mechanism is activated.

[0035] Step S5: Under the condition of keeping the heat preservation temperature constant, the steel sleeve is sent into the hot rolling mill for rolling deformation, and the cumulative reduction rate of a single or multiple passes is controlled to be no less than 60%. The ductility of the steel sleeve shell is used to drive the internal thermoelectric material to undergo plastic flow synchronously, so as to achieve densification and grain orientation control.

[0036] Step S6: After hot rolling, a composite strip with high density and uniform microstructure is obtained. Then, the outer steel sleeve is removed by wire cutting, laser cutting or mechanical sawing to separate the internal pure thermoelectric bulk material.

[0037] In step S1 above, the cold pressing pressure is set to 100 to 500 tons, which is sufficient to induce significant plastic deformation and cold welding effects between the original thermoelectric powder particles, thereby improving the interparticle bonding force. Simultaneously, it avoids mold damage or energy waste caused by excessive pressure. The density of the powder after cold pressing is not less than 70% of its theoretical density. This density level ensures the structural integrity of the powder during handling and packaging, and provides sufficient space for densification in the subsequent hot rolling stage. When the initial density is lower than this threshold, phenomena such as pore aggregation and fracture instability are prone to occur during hot rolling, affecting the consistency of the final material properties.

[0038] The raw material thermoelectric powder has a particle size distribution ranging from 0.1μm to 100μm and can be obtained through zone melting crushing, high-energy ball milling, or commercial procurement. Before filling, it is dried at 100℃-150℃ for 2-4 hours to remove adsorbed moisture and prevent steam pressure buildup at high temperatures after encapsulation. The cross-sectional shape of the cold-pressing mold cavity can be designed as rectangular, circular, or polygonal according to the final product requirements. The cold-pressing operation is carried out at room temperature, with a pressing and holding time of 30s to 120s to fully release elastic recovery stress and reduce dimensional springback after demolding.

[0039] In step S2 above, the steel sleeve is made of low-carbon steel or stainless steel, with a wall thickness of 2mm-8mm to balance encapsulation strength and ease of subsequent cutting. The cross-sectional shape of the steel sleeve is rectangular, circular, or polygonal, and can be flexibly customized according to the final product requirements. Before vacuuming, molybdenum disulfide or boron nitride is coated on the inner wall of the steel sleeve as a release agent. The release agent is uniformly sprayed or brushed onto the inner surface in the form of a suspension, and the coating thickness is controlled between 5μm and 20μm. This layer remains chemically inert at high temperatures, preventing the steel sleeve from undergoing interfacial reactions and metallurgical bonding with the thermoelectric material, facilitating subsequent separation without damaging the thermoelectric body.

[0040] The filling operation is carried out in a dry and clean environment. The cold-pressed block is slowly pushed into the steel sleeve axially to avoid structural damage due to impact. One end of the steel sleeve is pre-sealed, while the other end is left as a loading port and a vacuum port; after filling, this open end is not welded temporarily to facilitate connection to the vacuum system. A 2mm-5mm axial allowance is reserved in the inner length of the steel sleeve to accommodate thermal expansion and deformation flow, preventing the steel sleeve from cracking due to its incompressibility during hot rolling.

[0041] In step S3 above, the ultimate vacuum level of the vacuum process is controlled below 0.06 Pa. This vacuum level can reduce the residual oxygen partial pressure inside the steel sleeve to a level insufficient to cause significant oxidation of the thermoelectric materials. The vacuuming operation is started after a pre-reserved evacuation port at one end of the steel sleeve. Welding and sealing are performed after the target vacuum level is reached, avoiding air backflow contamination throughout the process. The vacuuming process lasts 10-30 minutes, during which the vacuum gauge reading is monitored. Once the pressure is confirmed to be stable below 0.06 Pa, the evacuation port is immediately spot-welded and sealed using an automatic argon arc welding machine.

[0042] In step S4 above, the holding temperature is set to 500℃-1500℃. This holding temperature can be adjusted according to the recrystallization temperature and thermal stability of different types of thermoelectric materials. For BiTe-based materials, 500℃-600℃ is selected; for PbTe-based materials, 550℃-800℃ is selected; and for SiGe-based materials, 900℃-1200℃ is used to ensure that the material has suitable rheological properties during hot rolling without excessive grain growth. The holding time is controlled between 10min and 120min. If the time is too short, the powder particles will not be sufficiently preheated, resulting in increased deformation resistance during hot rolling; if the time is too long, abnormal grain growth will begin, destroying the nanoscale phonon scattering structure and reducing the thermoelectric figure of merit.

[0043] In one embodiment of the invention, heating is performed using a box-type resistance furnace or an induction heating furnace. The steel sleeve assembly is placed on a high-temperature resistant ceramic support to avoid direct contact with the furnace bottom, which could lead to localized overheating. The heating rate is 5°C / min-20°C / min to prevent thermal shock from causing deformation of the steel sleeve or internal stress concentration.

[0044] In step S5 above, the reduction rate is defined as the ratio of the original thickness of the steel sleeve minus the rolled thickness to the original thickness. A high reduction rate induces large plastic strain in the internal thermoelectric material, promoting pore closure and grain boundary purification, while simultaneously inducing preferred grain orientation and enhancing carrier transport capacity. During multi-pass rolling, the reduction amount for each pass is designed in an increasing or gradient distribution to optimize the stress transmission path. In one embodiment of the invention, the hot rolling pressure is set to 500T-3000T, the roll surface roughness is controlled within Ra 0.8μm, and the rolling speed is controlled between 0.1m / min and 2m / min. Hot rolling is performed directly in air without the need for additional inert atmosphere protection. Thanks to the physical isolation effect of the steel sleeve, the internal thermoelectric material is always in a vacuum or near-vacuum environment, completely avoiding the risk of oxidation.

[0045] Specifically, the hot rolling process is divided into three stages: the first stage is the bite stage, where the rolls contact the steel sleeve surface at a low speed (0.1m / min-0.5m / min); the second stage is the main deformation stage, where the rolling speed is increased to 0.5m / min-2m / min, and the main rolling force is applied to achieve the target reduction rate; the third stage is the exit stabilization stage, where the rolled piece enters the cooling conveyor belt after leaving the rolls. Hot rolling adopts a single-pass or multi-pass rolling mode. Single-pass rolling is suitable for small deformations with a reduction rate of 60%-75%, while multi-pass rolling is used to achieve deep deformations with a total reduction rate of over 80%. Brief heat preservation is allowed between each pass. The heat preservation treatment is carried out continuously with the hot rolling process, that is, the steel sleeve is immediately transferred to the rolling mill for deformation after the heating and heat preservation is completed, without a cooling and dwell process in between, ensuring efficient use of heat energy and a compact process flow; preferably, a heat-insulated transition channel can be provided between the heating furnace outlet and the rolling mill inlet to reduce heat loss and maintain temperature fluctuations within ±10℃.

[0046] In step S6 above, the process of cutting to obtain the thermoelectric material adopts a non-contact processing method, such as wire cutting or laser cutting, to ensure that the complete thermoelectric core is accurately peeled out from the composite strip. After cutting, the material surface is slightly ground to remove trace amounts of residual metal contamination and restore a pure thermoelectric interface. After hot rolling, the density of the thermoelectric material reaches more than 95% of its theoretical density. This density level indicates that the internal porosity has been fully eliminated, approaching an ideal crystal structure.

[0047] Example 1

[0048] In this embodiment, a BiTe-based thermoelectric material is encapsulated:

[0049] Bi2Te3-Sb2Te3 composite powder with a particle size of 1-5μm was selected as the raw material and dried at 120℃ for 3 hours to remove adsorbed moisture. Subsequently, cold pressing was performed, applying 300 tons of pressure at room temperature and holding for 60 seconds to achieve 75% of the theoretical density of the cold-pressed powder, forming a structurally stable block precursor. In the encapsulation stage, a 3mm thick low-carbon steel sleeve with a rectangular cross-section was used. The inner wall was uniformly coated with a 10μm thick boron nitride release agent. The cold-pressed block was slowly pushed into the inner cavity of the steel sleeve (leaving a 3mm axial allowance). Then, the air pressure inside the steel sleeve was evacuated to 0.04Pa using a vacuum system, and the evacuation port was spot-welded to complete the seal. For the heat treatment, a box-type resistance furnace was used to heat the material to 550℃ at a heating rate of 10℃ / min and hold it at that temperature for 30 minutes to bring the material to a thermoplastic state. The hot rolling process is carried out in an atmospheric environment at a rolling speed of 0.8 m / min. A 70% reduction rate is achieved through single-pass rolling. The heat preservation and hot rolling processes are performed continuously, with an insulated transition channel maintaining temperature fluctuations within ±10℃. Finally, wire cutting is used to remove the steel sleeve, and the material surface is lightly ground to obtain a pure BiTe-based thermoelectric bulk material, as shown in the image. Figure 2 As shown.

[0050] Example 2

[0051] In this embodiment, a PbTe-based thermoelectric material is encapsulated:

[0052] Using PbTe-AgSbTe2 powder with a particle size of 5-20 μm as raw material, the material was dried at 150℃ for 2 hours and then cold-pressed. A pressure of 400 tons was applied at room temperature and held for 90 seconds, resulting in a density of 78% of the theoretical density after cold pressing. Encapsulation was performed using a 4 mm thick stainless steel sleeve with a circular cross-section. The inner wall was coated with a 15 μm thick molybdenum disulfide release agent. After filling the cold-pressed block, a vacuum was applied to 0.03 Pa, and the block was welded and sealed. Induction heating was used for heat treatment in an induction furnace at a heating rate of 15℃ / min, reaching 700℃ and holding at that temperature for 60 minutes. Hot rolling was carried out in an atmospheric environment at a rolling speed of 1.2 m / min using a two-pass rolling process. The first pass had a reduction rate of 40%, the second pass had a reduction rate of 40%, and the cumulative reduction rate was 80%, with the holding temperature maintained throughout. After hot rolling, the steel sleeve was removed by laser cutting, and the material surface was ground to obtain the PbTe-based thermoelectric block.

[0053] Example 3

[0054] In this embodiment, a SiGe-based thermoelectric material is encapsulated:

[0055] Si with a particle size of 20-50 μm was selected. 0.8 Ge 0.2 The powder was dried at 140℃ for 2.5 hours, then subjected to a cold-pressing pressure of 500 tons at room temperature for 120 seconds, achieving a density of 80% of the theoretical density after cold pressing. The encapsulation used a 6mm thick low-carbon steel sleeve with a rectangular cross-section design. The inner wall was coated with a 20μm thick boron nitride release agent. After filling, a vacuum was applied to 0.02Pa, and the sleeve was welded and sealed. Heat preservation was performed in a box-type resistance furnace at a heating rate of 8℃ / min, reaching 1100℃ and holding at that temperature for 90 minutes. Hot rolling was carried out in an atmospheric environment at a rolling speed of 0.5m / min, using a three-pass rolling process. The first pass had a reduction rate of 30%, the second pass 30%, and the final pass 25%, resulting in a cumulative reduction rate of 85%. Heat preservation and hot rolling were performed continuously to minimize heat loss. Finally, the steel sleeve was removed by wire cutting, and the surface was lightly ground to obtain the SiGe-based thermoelectric bulk material.

[0056] Comparative Example 1

[0057] This comparative example uses traditional SPS sintering (BiTe-based, the current mainstream process):

[0058] The same Bi₂Te₃-Sb₂Te₃ composite powder as in Example 1 was used as the raw material. Cold pressing was not performed; instead, spark plasma sintering (SPS) was directly employed. The sintering process was carried out in a high vacuum environment (0.01 Pa), with a sintering temperature of 580°C, a pressure of 50 MPa, and a holding time of 10 min. After sintering, due to cracked areas at the edges, approximately 15% of the material needed to be removed by machining, ultimately obtaining a BiTe-based thermoelectric bulk material.

[0059] Comparative Example 2

[0060] The process in this comparative example involves sintering followed by hot rolling (PbTe-based, currently a two-step method):

[0061] Using the same PbTe-AgSbTe2 powder as in Example 2, a pre-sintering treatment was first performed: in a vacuum hot-pressing sintering equipment, the temperature was 650°C and the pressure was 40MPa for 20 minutes at a vacuum of 0.05Pa, resulting in a density of 88%. Subsequently, it was encapsulated using a stainless steel sleeve of the same specifications as in Example 2, with a holding temperature of 700°C and a hot rolling reduction rate of 70%. Due to the uneven deformation of the material after pre-sintering, the deformed areas needed to be corrected by machining after hot rolling, resulting in a material loss of approximately 10%, ultimately yielding a PbTe-based thermoelectric bulk material.

[0062] Comparative Example 3

[0063] This comparative example uses direct atmospheric hot rolling without encapsulation (SiGe-based, without protective process):

[0064] The same Si as in Example 3 was selected. 0.8 Ge 0.2 The powder was cold-pressed according to the parameters of Example 3 (500 tons of pressure, holding for 120 seconds), but without steel sleeve encapsulation. It was then hot-rolled directly in the atmosphere at a holding temperature of 1100°C and a hot-rolling reduction rate of 80%. Due to the lack of encapsulation protection, severe oxidation occurred on the material surface, requiring subsequent removal of the surface oxide layer. Material loss was approximately 20%, ultimately yielding a SiGe-based thermoelectric bulk material.

[0065] The thermoelectric blocks prepared in the examples and comparative examples were tested for the following key performance indicators, and the test results are shown in Table 1:

[0066] (1) Density: Archimedes' displacement method, combined with X-ray diffraction to calculate theoretical density;

[0067] (2) ZT value: Thermal conductivity (κ) is measured by laser scintillation method, electrical conductivity (σ) is measured by four-probe method, and Seebeck coefficient (α) is measured by Seebeck coefficient tester, according to ZT=α 2 σT / κ calculation (T is the operating temperature);

[0068] (3) Degree of oxidation: the rate of change in material weight after hot rolling (Δm / m0×100%).

[0069] (4) Material utilization rate: effective thermoelectric core weight of finished product / total weight of raw materials × 100%.

[0070] Table 1. Performance test results of the products prepared in the Examples and Comparative Examples.

[0071]

[0072] The densities of Examples 1-3 all reached over 95%, approaching the 96.5% of traditional SPS sintering (Comparative Example 1), and significantly higher than unencapsulated hot rolling (Comparative Example 3, 82.6%). This result fully demonstrates that the synergistic effect of cold pressing pretreatment and hot rolling can effectively achieve high densification of thermoelectric materials without relying on high-cost high-vacuum sintering equipment. Meanwhile, the ZT values ​​of the examples all showed significant improvements compared to the comparative examples, with BiTe-based materials showing an increase of 21.9%, PbTe-based materials an increase of 18.8%, and SiGe-based materials an increase of 69.1%. The core reason is the steel sleeve + vacuum encapsulation design adopted in this invention, which completely avoids the oxidation risk under atmospheric conditions, effectively protecting the carrier concentration and grain boundary structure of the thermoelectric material. Simultaneously, the grain orientation control during hot rolling further enhances the carrier transport capacity, ultimately achieving optimization of the thermoelectric figure of merit. Regarding oxidation resistance, the oxidation weight change rate of the examples was ≤0.1%, which is basically equivalent to the oxidation level of vacuum sintering (Comparative Example 1) and far superior to unencapsulated hot rolling (Comparative Example 3, 5.23%), strongly verifying that the combination of steel sleeve encapsulation and vacuum treatment has excellent oxidation resistance. In terms of material utilization, the material utilization rate of the examples was ≥91.8%, which is significantly improved compared to traditional SPS sintering (72.3%) and hot rolling after sintering and encapsulation (80.5%). This is because the process of the present invention does not require the removal of ineffective areas after sintering or deformation, greatly reducing material loss.

[0073] In summary, the "cold pressing-hot rolling co-packaging method" of the present invention achieves a synergistic unity of high density, high ZT value, high material utilization and low cost in the preparation of multi-temperature zone thermoelectric materials through process integration and optimization, effectively breaking through the efficiency and cost bottlenecks of the prior art.

[0074] 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. A cold-pressing-hot-rolling co-packaging method for high ZT thermoelectric materials, characterized in that, Includes the following steps: S1: The thermoelectric material powder is cold-pressed and subjected to a pressure of 100 to 500 tons at room temperature to form a block precursor with an initial density of not less than 70% of the theoretical density; the thermoelectric material is one of BiTe-based, PbTe-based, or SiGe-based thermoelectric materials; S2: The block precursor is loaded into a metal sleeve; the metal sleeve is made of low carbon steel or stainless steel, with a wall thickness of 2mm-8mm, and the inner wall of the sleeve is coated with molybdenum disulfide or boron nitride release agent with a coating thickness of 5μm-20μm. S3: Evacuate the inside of the steel sleeve until the air pressure is below 0.06Pa, then weld and seal it; S4: Keep the sealed steel sleeve at 500℃-1500℃ for 10min-120min; S5: The steel sleeve is hot-rolled at the insulation temperature, and the cumulative reduction rate is not less than 60%; S6: Remove the outer steel sleeve to obtain a denser thermoelectric bulk material.

2. The cold-pressing-hot-rolling co-packaging method for high ZT thermoelectric materials according to claim 1, characterized in that, In step S1, the particle size of the thermoelectric material powder is 0.1μm-100μm, and it is dried at 100℃-150℃ for 2h-4h before cold pressing, and the cold pressing holding time is 30s-120s.

3. The cold-pressing-hot-rolling co-packaging method for high ZT thermoelectric materials according to claim 2, characterized in that, In step S4, the insulation temperature is selected according to the thermoelectric material system: 500℃-600℃ for BiTe-based materials, 550℃-800℃ for PbTe-based materials, and 900℃-1200℃ for SiGe-based materials.

4. The cold-pressing-hot-rolling co-packaging method for high ZT thermoelectric materials according to claim 3, characterized in that, In step S5, hot rolling is carried out in air at a rolling speed of 0.1 m / min to 2 m / min and a hot rolling pressure of 500 T to 3000 T.

5. The cold-pressing-hot-rolling co-packaging method for high ZT thermoelectric materials according to claim 4, characterized in that, In step S5, the hot rolling is a single-pass or multi-pass rolling process; when the hot rolling is a multi-pass rolling process, the reduction amount of each pass is designed to be incremental or gradient distributed, and the total cumulative reduction rate is not less than 80%.

6. The cold-pressing-hot-rolling co-packaging method for high ZT thermoelectric materials according to claim 5, characterized in that, Steps S4 and S5 are performed consecutively. After heat preservation, the steel sleeve is directly fed into the rolling mill, and the temperature fluctuation is maintained within ±10℃ through a heat-insulated transition channel.

7. The cold-pressing-hot-rolling co-packaging method for a high-ZT thermoelectric material according to claim 6, characterized in that, In step S6, the steel sleeve is removed by wire cutting or laser cutting, and the surface of the thermoelectric material is ground after cutting.

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