Preparation method of bismuth telluride block with uniform performance

Abstract

The invention discloses a preparation method of a bismuth telluride block with uniform performance, and relates to the technical field of thermoelectric materials. According to the preparation method provided by the invention, the rectifying ring and the heat preservation layer are introduced into the mold for SPS sintering, and meanwhile, a staged sintering procedure is adopted for sintering; the bismuth telluride block with excellent consistency in temperature fields (the maximum temperature difference of all positions on the surface of a sample is not higher than 6.1 DEG C) and thermoelectric performance (the difference of thermoelectric figure of merit of different positions is not higher than 4%) at different positions can be obtained, the production efficiency is improved, and the production cost is reduced.

Description

Technical Field

[0001] This invention relates to the field of thermoelectric materials technology, and more specifically, to a method for preparing a bismuth telluride bulk with uniform properties. Background Technology

[0002] Thermoelectric materials are a class of functional materials capable of directly converting thermal energy into electrical energy, possessing significant advantages such as no mechanical structure, high reliability, and small size. Among them, bismuth telluride-based thermoelectric materials are among the most mature and high-performing room-temperature thermoelectric materials currently available for commercial application, showing promising industrial prospects in thermoelectric power generation and refrigeration.

[0003] Bismuth telluride thermoelectric materials are typically in bulk form, and their preparation usually employs powder sintering technology. This involves densifying bismuth telluride powder using methods such as hot pressing or spark plasma sintering (SPS) to achieve excellent thermoelectric properties. SPS technology, utilizing pulsed current heating, can directly heat the powder in a short time, offering significant advantages such as rapid densification, low-temperature sintering, and inhibited grain growth, making it one of the key processes for preparing high-performance bismuth telluride thermoelectric materials. However, during SPS sintering, the heating path is from the inside of the powder outwards. Furthermore, there are significant differences in thermal conductivity between the sintering mold, bismuth telluride powder, and graphite paper, which often leads to uneven temperature fields within the sample. This results in lower temperatures at the sample edges, varying degrees of sintering in different areas, and consequently, uneven density and poor internal performance consistency in the sintered sample. The non-uniform properties of bismuth telluride bulk materials can lead to uneven heat flow, current, and stress conduction during device operation. This not only reduces overall thermoelectric conversion efficiency but may also cause localized stress accumulation, interface failure, decreased temperature control accuracy, and shortened device lifespan. However, directly removing the insufficiently sintered areas at the edges of the sample bulk material reduces material utilization, resulting in significant waste and increased manufacturing costs. Therefore, there is an urgent need for a preparation method that can improve the uniformity of the properties of sintered bismuth telluride bulk materials. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a method for preparing bismuth telluride bulk materials with uniform performance.

[0005] The above-mentioned objective of this invention is achieved through the following technical solution: A method for preparing a bismuth telluride bulk with uniform properties includes the following steps: Bismuth telluride powder is loaded into a mold and sent to an SPS sintering furnace for sintering. After sintering, a bismuth telluride bulk with uniform properties is obtained. The sintering procedure is as follows: Sinter at T1 and p1 for 10-20 min, then sinter at T2 and p2 for 10-20 min; T1 = 190-230℃, T2-T1 = 170-260℃; p1 = 15-23 MPa, p2-p1 = 22-40 MPa; The mold, from top to bottom, includes an upper pressure head (1), a sleeve (5), and a lower pressure head (8). The sleeve (5) is a hollow structure that runs vertically through the mold, and the space enclosed by its inner wall forms a mold cavity (3). The upper pressure head (1) includes a first base (11) and a first protruding rod (12) embedded in the first base. An upper gasket (2) is provided at the end of the first protruding rod (12). The lower pressure head (8) includes a second base (81) and a second protruding rod (82) embedded in the second base. A lower gasket (4) is provided at the end of the second protruding rod (82). The first base (11) is located at the top of the mold, and the first protruding rod (12) is fitted and connected to the inner wall of the mold cavity (3). The second base (81) is located at the bottom of the mold cavity, and the second protruding rod (82) is fitted and connected to the inner wall of the mold cavity (3). The outer wall of the sleeve (5) is fitted with a heat insulation layer (6); the heat insulation layer includes an inner layer material (61) and an outer layer material (62), the thermal conductivity of the inner layer material is 0.02-0.1 W / (m·K), and the outer layer material includes aluminum foil; An annular rectifier ring (7) is provided between the sleeve and the second base of the lower pressure head. The rectifier ring is a silicon nitride ring, a zirconium oxide ring, or an aluminum oxide ring.

[0006] The preparation method provided by this invention involves adding a heat insulation layer to the outside of the sintering mold sleeve used in conventional SPS sintering. This restricts the heat conduction path from the inside of the sample to the outside during SPS sintering, reduces heat dissipation inside the sintering mold, and alleviates heat loss at the edges of the bismuth telluride sample during sintering. Ultimately, a stable and uniform temperature field is formed inside the bismuth telluride sample located within the mold. Simultaneously, the introduction of a rectifier ring optimizes the current conduction path, isolates the electrical contact between the lower base (second base) and the sleeve, and guides most of the current through the bismuth telluride powder within the mold cavity. This enhances micro-arc discharge and interface effects, causing the current-heat effect to be concentrated inside the sample. This improves sintering efficiency while avoiding excess waste heat generated by current dispersion through the mold sleeve, which could affect the external temperature field of the bismuth telluride sample and lead to abnormal grain enlargement. Based on this, the inventors of this application discovered that a specific sintering procedure is required to fully utilize the heat insulation and heat homogenization effects of the insulation layer and the rectifier ring during the sintering process. The reason for this is speculated to be that while the introduction of the insulation layer and the rectifier ring improves the radial temperature uniformity of the sample, temperature gradients and thermal stresses still exist along the axial direction and at the particle scale of the powder. By employing a stepped heating sintering procedure, the entire sintering system (including powder, mold, and insulation layer) can smoothly and controllably transition to a high-temperature state as a cohesive whole, allowing the sample bulk to simultaneously reach a state of near thermal equilibrium at both the microscopic and macroscopic scales, thereby obtaining a bismuth telluride bulk with more uniform overall performance.

[0007] Preferably, the sleeve has a coin-shaped cross-section, with a circular outer surface and a rectangular inner surface.

[0008] Preferably, the sleeve is a graphite sleeve with an outer surface diameter of 200-400mm and an axial height of 100-200mm.

[0009] Preferably, the dimensions of the inner surface of the sleeve are (20-40mm) * (140-160mm).

[0010] Preferably, both the first base and the second base are solid cylinders.

[0011] Preferably, the height of the first base and the second base is 50-100 mm.

[0012] Preferably, both the first protruding rod and the second protruding rod are solid rectangles.

[0013] Preferably, the direction in which the first protruding rod connects to the first base is a first direction, and the dimension of the first protruding rod in a second direction matches the dimension of the inner surface of the sleeve, with a dimension difference ≤1%, and the second direction is perpendicular to the first direction; the direction in which the second protruding rod connects to the second base is a third direction, and the dimension of the second protruding rod in a fourth direction matches the dimension of the inner surface of the sleeve, with a dimension difference ≤1%, and the third direction is perpendicular to the fourth direction. Through this dimensional matching, the first and second protruding rods can be fitted and connected to the inner wall of the sleeve during the sintering process. By changing the connection relationship between the protruding rods and the sleeve, the sealing and opening of the mold cavity inside the sleeve can be achieved (sealed when fitted, open when not fitted).

[0014] Preferably, the height of both the first protruding rod and the second protruding rod is 50-100 mm.

[0015] Preferably, the thickness of both the upper and lower gaskets is 1-8 mm.

[0016] Preferably, T2 = 400-450℃.

[0017] Preferably, p2 = 45-55 MPa.

[0018] Preferably, before sintering at T1 and p1, the temperature and pressure are increased from room temperature and atmospheric pressure to T1 and p1 at a heating rate of 20-30℃ / min and a pressure increase rate of 1.5-3 MPa / min.

[0019] Preferably, before sintering at T2 and p2, the temperature is increased from T1 and p1 to T2 and p2 at a heating rate of 15-27℃ / min and a pressure increase rate of 1-2.5 MPa / min.

[0020] Preferably, the inner layer material of the insulation layer includes at least one of graphite felt, alumina fiber, and zirconium oxide fiber.

[0021] Preferably, the thickness of the insulation layer is 20-75 mm.

[0022] More preferably, the thickness of the inner layer material is 20-73 mm, and the thickness of the outer layer material is 0.01-2 mm.

[0023] The insulation layer is fitted onto the outer wall of the sleeve and is tightly attached to the outer wall of the sleeve.

[0024] Preferably, the difference between the outer and inner surface radii of the annular rectifier ring is 20-60 mm.

[0025] The rectifier ring described in this invention is a circular ring with a certain thickness in the vertical direction (axial direction), and the cross-section along the axial direction is rectangular.

[0026] More preferably, the thickness of the annular rectifier ring is 5-15 mm.

[0027] Preferably, carbon paper is provided between each pair of the upper pressure head, upper gasket, sleeve, lower gasket, and lower pressure head.

[0028] Carbon paper transition can reduce contact resistance, thereby avoiding the generation of local hot spots that affect the uniformity of the thermal field.

[0029] Preferably, the average particle size of the bismuth telluride powder is 5-13 μm.

[0030] Preferably, the method for preparing the bismuth telluride powder includes the following steps: The raw materials are mixed and smelted to obtain polycrystalline bismuth telluride bulk material, which is then mechanically crushed and ball-milled to obtain bismuth telluride powder.

[0031] More preferably, the raw material elements include Bi powder, Sb powder, Te powder and Se powder in a molar ratio of 2:2:(2.8-3):(0.01-0.05).

[0032] More preferably, the melting is any one of vacuum melting, swing melting, or zone melting.

[0033] More preferably, the mechanical crushing is any one of jaw crusher, roller crusher, or shear crusher.

[0034] More preferably, the ball mill rotates at a speed of 300-600 rpm.

[0035] Compared with the prior art, the present invention has the following beneficial effects: The preparation method provided by this invention can obtain bismuth telluride bulk materials with excellent consistency in temperature field (maximum temperature difference at each location on the sample surface is no higher than 6.1℃) and thermoelectric properties (thermoelectric figure of merit difference at different locations is no higher than 4%) in different parts through SPS sintering, thereby improving production efficiency and reducing production costs. Attached Figure Description

[0036] Figure 1 This is a schematic diagram of the mold provided by the present invention. In the figure: 1 is the upper pressure head, 11 is the first base, 12 is the first protruding rod, 2 is the upper gasket, 3 is the mold cavity, 4 is the lower gasket, 5 is the sleeve, 6 is the insulation layer, 61 is the inner layer material, 62 is the outer layer material, 7 is the annular rectifier ring, 8 is the lower pressure head, 81 is the second base, and 82 is the second protruding rod.

[0037] Figure 2 This is a ZT value-temperature distribution diagram of the homogenized bismuth telluride-based thermoelectric material prepared in Example 1 of the present invention. In the diagram, #1, #2, and #3 represent three different locations.

[0038] Figure 3 The temperature distribution diagram in the sintering process of bismuth telluride bulk material prepared by the bismuth telluride preparation method provided in Embodiment 1 of the present invention is shown in the figure; in the figure: 9-bismuth telluride sample; 10-graphite sleeve; 11-inner layer of heat preservation layer; 12-aluminum foil.

[0039] Figure 4 This is a temperature distribution diagram of the bismuth telluride preparation method provided in Comparative Example 8 of the present invention during the sintering process of bismuth telluride bulk. Detailed Implementation

[0040] The present invention will be further described below with reference to specific embodiments, but the embodiments do not limit the present invention in any way. Unless otherwise stated, the raw materials and reagents used in the embodiments of the present invention are conventionally purchased raw materials and reagents.

[0041] Example 1 A method for preparing a bismuth telluride bulk with uniform properties includes the following steps: Bismuth telluride powder is loaded into a mold and sent to an SPS sintering furnace for sintering. After sintering, a bismuth telluride bulk with uniform properties is obtained. The sintering procedure is as follows: The sample was sintered at 210℃ and 20 MPa for 15 min at room temperature and pressure (heating and pressure increase time was 8 min), and then sintered at 450℃ and 50 MPa for 15 min (heating and pressure increase time was 10 min). The mold, from top to bottom, includes an upper pressure head (1), a sleeve (5), and a lower pressure head (8). The sleeve (5) is a hollow structure that runs vertically through the mold, with an outer surface diameter of 300 mm. The space enclosed by its inner wall forms a mold cavity (3) with a cross-section of 30 mm * 150 mm. The axial height of the sleeve is 170 mm. The upper pressure head (1) includes a first base (11) with a height of 80 mm and a first protruding rod (12) embedded in the first base. The first protruding rod has a height of 70 mm and a cross-section of 30 mm * 150 mm. The end of the first protruding rod (12) is provided with an upper gasket (2) with a thickness of 5 mm. The lower pressure head (8) includes a second base (81) with a height of 80 mm and a second protruding rod (82) embedded in the second base. The second protruding rod has a height of 70 mm and a cross-section of 30 mm * 150 mm. The end of the second protruding rod (82) is provided with an upper gasket (2) with a thickness of 5 mm. mm lower pad (4); the first base (11) is located at the top of the mold, and the first protruding rod (12) is fitted and connected to the inner wall of the mold cavity (3); the second base (81) is located at the bottom of the mold cavity, and the second protruding rod (82) is fitted and connected to the inner wall of the mold cavity (3); The outer wall of the sleeve is fitted with a 70.01 mm thick insulation layer (6); the insulation layer includes an inner layer material (61) and an outer layer material (62), wherein the inner layer material is 70 mm thick zirconium oxide and the outer layer material is 0.01 mm thick single-layer aluminum foil; A circular rectifier ring (7) is provided between the sleeve and the second base of the lower pressure head. The rectifier ring is a silicon nitride ring with a thickness of 10 mm. The difference between the outer surface radius and the inner surface radius of the ring is 50 mm, and the inner surface radius is 100 mm.

[0042] The bismuth telluride powder has an average particle size of 10 μm, and its preparation method includes the following steps: Bi, Sb, Te, and Se raw materials are mixed in the following order: (BiSb) 2Te 3Se 0.04 The mixture was mixed in proportion and smelted at 420°C to obtain polycrystalline bismuth telluride bulk material. After being crushed into large bulk materials, it was ball-milled at 400 rpm for 8 hours to obtain bismuth telluride powder.

[0043] Examples 2-5 and Comparative Examples 1-6 This embodiment and comparative example provide a series of methods for preparing bismuth telluride bulk materials with uniform performance. The only difference from Example 1 is the difference in the sintering process T1, p1, T2, and p2, as shown in Table 1 below: Table 1. Note: In Comparative Examples 3-4, room temperature and pressure were raised to specific temperatures and pressures over a period of 8 minutes, after which no further heating or pressurization was performed.

[0044] Example 6 A method for preparing a bismuth telluride bulk with uniform properties, wherein the only difference from Example 1 is: The inner layer of the insulation layer is made of 20mm thick zirconium oxide.

[0045] Example 7 A method for preparing a bismuth telluride bulk with uniform properties, wherein the only difference from Example 1 is: The inner layer of the insulation layer is made of zirconia with a thickness of 80mm.

[0046] Example 8 A method for preparing a bismuth telluride bulk with uniform properties, wherein the only difference from Example 1 is: The average particle size of the bismuth telluride powder is 5 μm.

[0047] The method for preparing bismuth telluride powder in this embodiment differs from that in Example 1 only in that: The ball milling time is 12 hours.

[0048] Example 9 A method for preparing a bismuth telluride bulk with uniform properties, wherein the only difference from Example 1 is: The average particle size of the bismuth telluride powder is 20 μm.

[0049] The method for preparing bismuth telluride powder in this embodiment differs from that in Example 1 only in that: The ball milling time is 4 hours.

[0050] Example 10 A method for preparing a bismuth telluride bulk with uniform properties, wherein the only difference from Example 1 is: The average particle size of the bismuth telluride powder is 2 μm.

[0051] The method for preparing bismuth telluride powder in this embodiment differs from that in Example 1 only in that: The ball milling time is 10 hours.

[0052] Comparative Example 7 A method for preparing bismuth telluride bulk, wherein the only difference from Example 1 is: No rectifier ring is provided between the sleeve and the second base of the lower pressure head.

[0053] Comparative Example 8 A method for preparing bismuth telluride bulk, wherein the only difference from Example 1 is: The inner layer of the insulation layer is made of 10mm thick zirconium oxide.

[0054] Performance testing The bismuth telluride bulk material prepared by this invention has uniform properties, with a length of 150 mm, a width of 30 mm, and a height of 30 mm.

[0055] Sample temperature range test: The uniformity of the temperature field inside the sample is characterized by calculating the temperature range of the entire sample during the sintering process using a simulation method.

[0056] ZT value difference test at different locations: The ZT values ​​at three locations of the sample were tested to characterize the uniformity of the thermoelectric properties of the bismuth telluride sample. The results are shown in Table 2.

[0057] Table 2. As shown in Table 2 above, the preparation method provided by this invention can obtain bismuth telluride bulk materials with excellent consistency in temperature field (maximum temperature difference at different locations on the sample surface is no higher than 6.1℃) and thermoelectric properties (thermoelectric figure of merit difference at different locations is no higher than 4%) in different parts through SPS sintering, thereby improving production efficiency and reducing production costs. Furthermore, comparing Examples 1-3 and Examples 4-5 shows that sintering using the preferred sintering procedure of this invention results in better sample uniformity. Comparing Examples 1 and Examples 6-7 shows that an inappropriate insulation layer thickness can also lead to a decrease in sintering effect, mainly because an excessively thick insulation layer can hinder the necessary radial heat conduction within the sample to some extent, forming tangible heat accumulation. Comparing Examples 1 and Examples 8-10 shows that within the preferred powder particle size range of this invention, bismuth telluride exhibits better sintering effect and higher temperature and performance consistency.

[0058] According to Comparative Examples 1-2, the temperature and pressure gradients in the two sintering steps of the sintering process deviate from the scope protected by this invention, which will lead to insufficient conduction of pressure and heat, affecting the sintering uniformity. Comparative Examples 3-4 lack a segmented heating and pressurizing process and do not perform pre-sintering at low temperatures. The contact between powder particles is relatively loose, and the particle bonding rate is inconsistent in different regions, resulting in over-sintering in some areas and under-sintering in others, ultimately affecting the uniformity of the material structure and properties.

[0059] Comparative Examples 5 and 6 show the first pre-sintering stage temperatures as too low and too high, respectively. At too low a temperature, the pre-sintering is insufficient, while at too high a temperature, the grain growth rate in different regions will be different, and local regions will be densified first. Therefore, it can be seen that the ZT of the bismuth telluride bulk obtained in Comparative Examples 5 and 6 varies greatly at different locations.

[0060] In Comparative Example 7, no rectifier ring was set up, and the current was uniformly dispersed and flowed through the graphite sleeve. This not only weakened the promoting effect of micro-arc discharge and interface effect on powder sintering, but also led to the generation of additional waste heat. This waste heat failed to participate in the sintering process effectively, which in turn aggravated the temperature gradient in the sintering area. The difference in thermoelectric performance at different locations reached 30.8%.

[0061] The data from Comparative Example 8 show that an excessively thin insulation layer significantly reduces the uniformity of sample performance, due to substantial heat loss from the sample edges via conduction and radiation; (Refer to...) Figure 4 During the sintering process, the thermal field of the sample was uneven, the edge temperature was lower, and the internal temperature difference reached 60℃.

[0062] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A method for preparing a bismuth telluride bulk with uniform properties, characterized in that, Includes the following steps: Bismuth telluride powder is loaded into a mold and sent to an SPS sintering furnace for sintering. After sintering, a bismuth telluride bulk with uniform properties is obtained. The sintering procedure is as follows: Sinter at T1 and p1 for 10-20 min, then sinter at T2 and p2 for 10-20 min; T1 = 190-230℃, T2-T1 = 170-260℃; p1 = 15-23 MPa, p2-p1 = 22-40 MPa; The mold, from top to bottom, includes an upper pressure head (1), a sleeve (5), and a lower pressure head (8). The sleeve (5) is a hollow structure that runs vertically through the mold, and the space enclosed by its inner wall forms a mold cavity (3). The upper pressure head (1) includes a first base (11) and a first protruding rod (12) embedded in the first base. An upper gasket (2) is provided at the end of the first protruding rod (12). The lower pressure head (8) includes a second base (81) and a second protruding rod (82) embedded in the second base. A lower gasket (4) is provided at the end of the second protruding rod (82). The first base (11) is located at the top of the mold, and the first protruding rod (12) is fitted and connected to the inner wall of the mold cavity (3). The second base (81) is located at the bottom of the mold cavity, and the second protruding rod (82) is fitted and connected to the inner wall of the mold cavity (3). The outer wall of the sleeve (5) is fitted with a heat insulation layer (6); the heat insulation layer includes an inner layer material (61) and an outer layer material (62), the thermal conductivity of the inner layer material is 0.02-0.1 W / (m·K), and the outer layer material includes aluminum foil; An annular rectifier ring (7) is provided between the sleeve and the second base of the lower pressure head. The rectifier ring is a silicon nitride ring, a zirconium oxide ring, or an aluminum oxide ring.

2. The method for preparing a uniform bismuth telluride bulk as described in claim 1, characterized in that, The T2 is 400-450℃; And / or, the p2 = 45-55 MPa.

3. The method for preparing a bismuth telluride bulk with uniform properties as described in claim 1 or 2, characterized in that, Before sintering at T1 and p1, the temperature and pressure are increased from room temperature and atmospheric pressure to T1 and p1 at a heating rate of 20-30℃ / min and a pressure increase rate of 1.5-3 MPa / min. And / or, before sintering at T2 and p2, the temperature is increased from T1 and p1 to T2 and p2 at a heating rate of 15-27℃ / min and a pressure increase rate of 1-2.5 MPa / min.

4. The method for preparing a uniform bismuth telluride bulk material as described in claim 1, characterized in that, The inner layer material includes at least one of graphite felt, alumina fiber, and zirconium oxide fiber.

5. The method for preparing a bismuth telluride bulk with uniform properties as described in claim 1 or 4, characterized in that, The thickness of the insulation layer is 20-75 mm.

6. The method for preparing a bismuth telluride bulk with uniform properties as described in claim 5, characterized in that, The thickness of the inner layer material is 20-73 mm, and the thickness of the outer layer material is 0.01-2 mm.

7. The method for preparing a bismuth telluride bulk with uniform properties as described in claim 1, characterized in that, The difference between the outer and inner surface radii of the annular rectifier ring is 20-60 mm.

8. The method for preparing a bismuth telluride bulk material with uniform properties as described in claim 1, characterized in that, The average particle size of the bismuth telluride powder is 5-13 μm.

9. The method for preparing a bismuth telluride bulk with uniform properties as described in claim 1 or 8, characterized in that, The method for preparing the bismuth telluride powder includes the following steps: The raw materials are mixed and smelted to obtain polycrystalline bismuth telluride bulk material, which is then mechanically crushed and ball-milled to obtain bismuth telluride powder.

10. The method for preparing a bismuth telluride bulk with uniform properties as described in claim 9, characterized in that, The raw material elements include Bi powder, Sb powder, Te powder and Se powder in a molar ratio of 2:2:(2.8-3):(0.01-0.05).

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