Processing and forming method of bismuth telluride thermoelectric material

Abstract

The present invention provides a processing and forming method of bismuth telluride thermoelectric material, including configuring P-type and N-type materials according to a predetermined molar ratio, and pre-treating the materials respectively, and then placing the pre-treated P-type and N-type materials respectively into a hot extrusion die containing a multi-stage variable diameter structure, and placing the pre-treated P-type and N-type materials together with the die into a heater at a temperature of 300°C to 500°C, applying a certain vertical pressure to the material, and under the combined action of temperature and pressure, the material passes through the inner cavity of the variable diameter hot extrusion die, and is discharged along the direction of the extrusion force to form a rod-shaped material with a certain cross-section, and then the rod-shaped material is heat-treated. Through experimental verification, the thermoelectric material prepared by the method of the present invention has an orientation degree of at least 16% higher than that of the material obtained by the existing processing method, a compressive strength of at least 6%, and a thermoelectric performance of at least 3%, and the thermoelectric performance and mechanical strength of the material are both improved.

Description

Technical Field

[0001] The invention belongs to the technical field of thermoelectric material preparation, and relates to a processing and molding method of bismuth telluride thermoelectric material. Background Art

[0002] Thermoelectric materials are key materials for thermoelectric cooling or power generation devices. Bi2Te3-based thermoelectric materials are still the best commercial materials near room temperature so far. The industrial preparation methods include zone melting and hot pressing sintering. The zone melting method can make the material obtain a certain degree of orientation and good thermoelectric performance, but it is prone to cleavage and has weak mechanical strength. Although the mechanical strength of the material prepared by the hot pressing sintering method is significantly improved compared with the zone melting method, the thermoelectric performance of the material is relatively low. Excellent thermoelectric performance and high mechanical strength have become the goals pursued by the thermoelectric industry.

[0003] The processing of bismuth telluride thermoelectric materials has been studied relatively early and extensively. For example, Xu Desheng's "Semiconductor Refrigeration and Application Technology" (Shanghai Jiaotong University Press, 1991) introduced the Bridgman and zone melting methods for growing Bi2Te3 crystals; patent numbers ZL03150425.6) and 200810038766.3 introduced methods for preparing thermoelectric materials by sintering; the Chinese invention patent with patent number CN114210978A provides a hot extrusion molding method for bismuth telluride thermoelectric materials, combining powder metallurgy with a one-time variable diameter hot extrusion process, achieving precision molding and performance improvement of highly brittle bismuth telluride thermoelectric materials, and solving the problems of low reliability and performance degradation of thermoelectric coolers.

[0004] Variable diameter extrusion is widely used in the processing of bismuth telluride thermoelectric materials, but most of them are operated by one variable diameter extrusion. In theory, continuous multiple variable diameter extrusion helps the lattice to rotate multiple times in a predetermined direction, thereby obtaining a larger number of grains with ideal orientations, and through this multi-stage variable diameter continuous hot extrusion method, the grains can be further refined, making it easier to process rods with a large compression ratio, and the thermoelectric properties and mechanical strength of the corresponding materials will also change accordingly, but there is little research on continuous multiple variable diameter extrusion in the prior art. Summary of the invention

[0005] The present invention aims at the deficiencies in the research of the prior art, adopts a multi-stage variable diameter continuous hot extrusion processing and forming process, and under the joint action of heat and mechanical pressure, the bismuth telluride blank to be processed is continuously passed through a multi-stage variable diameter hot extrusion die cavity with a certain compression ratio. Under the shearing action of multiple variable diameter hot extrusions, the lattice rotates multiple times in a predetermined direction, thereby obtaining a larger number of grains with an ideal orientation, and optimizing the orientation of the material; after multiple variable diameter hot extrusions, the grains are further refined, so that the thermoelectric performance and mechanical strength of the material are further improved.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is as follows:

[0007] In a first aspect, the present invention provides a method for processing and forming a bismuth telluride thermoelectric material, comprising the following steps: configuring a P-type and an N-type material according to a predetermined molar ratio, and pre-treating the materials respectively, and then placing the pre-treated P-type and N-type materials respectively into a variable diameter hot extrusion die, and placing the pre-treated P-type and N-type materials together with the die into a heater at a temperature of 300°C to 500°C, applying a certain vertical pressure to the material, and under the combined action of temperature and pressure, the material passes through the inner cavity of the variable diameter hot extrusion die and is discharged in the direction of the extrusion force to form a rod-shaped material with a certain cross-section, and then heat-treating the rod-shaped material.

[0008] The inner cavity of the hot extrusion die contains at least two sections of variable diameter structures, and the larger diameter side transitions to the smaller diameter side through a first conical surface with a certain angle to the central axis, and the first conical surface is connected to the first cylindrical surface below to form a first variable diameter section; the first cylindrical surface is connected to a second conical surface with a certain angle to the central axis below, and the two are transitioned through a smooth arc surface, and the second conical surface is connected to the second cylindrical surface below to form a second variable diameter section, and then the subsequent variable diameter sections are formed according to the same method.

[0009] In each variable diameter section, the angle between the conical surface and the central axis is 45° to 70°, the area ratio of the upper and lower circles of the conical surface is 2 to 15; the length and diameter ratio of the cylindrical surface is 0.3 to 1.0, and the total compression ratio is the product of the compression ratios of each variable diameter section.

[0010] Preferably, in the processing and forming method of the bismuth telluride thermoelectric material provided by the present invention, the configuration method of the P-type and N-type materials is as follows: Bi, Te and Sb are calculated and weighed according to the molar ratio of Bi2Te3 and Sb2Te3 of 15% to 25%: 85% to 75%, and Te is added as a dopant to configure the P-type material; Bi, Te and Se materials are calculated and weighed according to the molar ratio of Bi2Te3 and Bi2Se3 of 88% to 96%: 12% to 4%, and a dopant is added to configure the N-type material, and then the P-type and N-type raw materials weighed according to the above ratios are respectively placed in a quartz container, which is vacuumed and sealed;

[0011] Among them, the added amount of Te, the dopant of P-type material, is 0.1% to 0.5% of the total weight of Bi2Te3 and Sb2Te3 materials; the dopant of N-type material is selected from one or more of halogen elements F, Cl, Br, I and their compounds, preferably SbI3, and the added amount is 200ppm to 800ppm of the total weight of Bi2Te3 and Bi2Se3 materials.

[0012] The quartz container is a quartz ampoule or a quartz test tube, and the air pressure in the quartz container is less than 2Pa after vacuuming.

[0013] Preferably, in the processing and forming method of bismuth telluride thermoelectric material provided by the present invention, the pretreatment method of P-type and N-type materials is as follows: place a quartz container in a tubular furnace at 700°C to 800°C, heat for 1 hour, shake for 1 hour, take it out of the furnace and cool it in the air, mechanically crush the high-temperature smelted material, press it at room temperature, place it in a heating furnace at a temperature of 500°C to 600°C, and perform heat treatment for 5 to 15 hours in a low oxygen partial pressure atmosphere.

[0014] Among them, the mechanical crushing time is preferably 1 min to 3 min; when pressing and molding at room temperature, the pressure is preferably 100 MPa to 180 MPa; and the oxygen partial pressure in the heating furnace is preferably controlled below 20 ppm.

[0015] Furthermore, there is a certain difference in the heating temperature of the P-type material and the N-type material. The temperature of the heater for the P-type material is set to 300°C to 390°C, and the temperature of the heater for the N-type material is set to 430°C to 500°C.

[0016] Preferably, in the processing and forming method of bismuth telluride thermoelectric material provided by the present invention, when the rod-shaped material is heat-treated, the rod-shaped material is heat-treated again in an atmosphere with an oxygen partial pressure of less than 20 ppm, and the heat treatment temperature is 350°C to 400°C, and the time is 5 hours to 15 hours.

[0017] In a second aspect, the present invention provides a bismuth telluride thermoelectric rod, which is processed by the method described above.

[0018] Experimental verification shows that the thermoelectric material prepared by the method of the present invention has an orientation degree increased by at least 16% over that of the material obtained by the existing processing method, a compressive strength increased by at least 6%, and a thermoelectric performance increased by about 3%, thereby effectively improving both the mechanical properties and the thermoelectric properties. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is a schematic diagram of the cross-sectional structure of the inner cavity of the variable diameter hot extrusion die according to an embodiment of the present invention. DETAILED DESCRIPTION

[0020] The present invention will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and are not intended to limit the scope of the present invention. In addition, it should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms fall within the scope limited by the appended claims of the application equally.

[0021] Example

[0022] Take raw materials Te, Bi, Sb, and Se with a purity greater than 99.99%. Calculate the stoichiometric ratio of each element according to the ratio of 18% molBi2Te3:82% molSb2Te3, weigh the corresponding element materials, with a total weight of 5000g, add 0.5% by weight of Te as a dopant, and configure P-type material; calculate the stoichiometric ratio of each element according to the ratio of 93% molBi2Te3:7% molBi2Se3, weigh the corresponding element materials, with a total weight of 6000g, add 500ppm by weight of SbI3 as a dopant, and configure N-type material.

[0023] Put the P-type material and N-type material into clean quartz glass tubes respectively, evacuate until the pressure inside the tube is less than 2Pa, and seal it. Then put the quartz glass tube into a tubular heating furnace, set the temperature to 800℃, heat it up for one hour, shake it at a constant temperature for one hour, and cool it outside the furnace.

[0024] The high-temperature synthesized material was mechanically crushed for 3 minutes and then pressed into a primary blank, which was kept at 540°C with an oxygen partial pressure of less than 20 PPm in the furnace. After 15 hours, it was cooled with the furnace.

[0025] Place the blank into a hot extrusion die with a three-stage variable diameter structure. See the schematic diagram of the die cavity structure for details. Figure 1 The diameter of the first reducing section is becomes The diameter of the second reducing section is becomes The diameter of the third reducing section is becomes The side with larger diameter transitions to the side with smaller diameter through a first conical surface which has a certain angle with the central axis, and the first conical surface is connected to the first cylindrical surface below to form a first diameter-changing section; the first cylindrical surface is connected to a second conical surface which has a certain angle with the central axis below, and the two are transitioned through a smooth arc surface, and the second conical surface is connected to the second cylindrical surface below to form a second diameter-changing section, and then the third diameter-changing section is formed according to the same method.

[0026] In this embodiment, the angle θ between the conical surface and the central axis in the three diameter-changing sections is 50°, and the maximum circle diameter is 100 mm; the area ratios ε of the upper and lower circles of the conical surface are ε1=4.7, ε2=4.7, and ε3=4.55 from large to small; the ratios of the length and diameter of the cylindrical surface are λ1=0.6, λ2=0.6, and λ3=1.0, respectively, and the total compression ratio is the product of the compression ratios of the diameter-changing sections, which is approximately 100.

[0027] The blank and the die are placed in the heater, the temperature of the P-type material is set to 360°C, and the temperature of the N-type material is set to 450°C. A pressure of 200 MPa is applied in the extrusion direction, and a rod with a diameter of 10 mm is extruded in the pressure direction.

[0028] The extruded rods were kept at 360°C for 12 hours and then cooled in the furnace, with the oxygen partial pressure in the furnace maintained below 20 ppm.

[0029] Comparative Example

[0030] The experimental materials and processing methods are the same as those in the embodiment, the only difference is that the variable diameter extrusion die used is a single variable diameter section, and the diameter is becomes

[0031] Effect comparison

[0032] The rods prepared in the embodiment and the comparative example were processed into samples of the same specifications, and the thermoelectric properties, orientation, compressive strength and maximum temperature difference of the materials were compared. The results are shown in Table 1:

[0033] Table 1 Comparison of performance test results of embodiments and comparative examples

[0034]

[0035] According to the above results, the thermoelectric material prepared by the method of the present invention has an orientation degree increased by at least 16% compared with the material obtained by the existing processing method, the compressive strength is increased by at least 6%, the thermoelectric performance is increased by about 3%, and both the mechanical properties and thermoelectric properties are effectively improved.

[0036] The preferred embodiments of the present invention have been specifically described above, but the present invention is not limited to the embodiments. Those skilled in the art may make various equivalent modifications or substitutions without violating the spirit of the present invention. These equivalent modifications or substitutions are all included in the scope defined by the claims of this application.

Claims

1. A method for processing and forming bismuth telluride thermoelectric material, characterized in that: P-type and N-type materials are configured according to a predetermined molar ratio, and the materials are pretreated respectively. Then, the pretreated P-type and N-type materials are placed in a variable diameter hot extrusion die respectively, and the die is placed in a heater at a temperature of 300°C to 500°C. A certain vertical pressure is applied to the materials. Under the combined action of temperature and pressure, the materials pass through the inner cavity of the variable diameter hot extrusion die and are discharged along the direction of the extrusion force to form a rod-shaped material with a certain cross-section. The rod-shaped material is then heat treated. Among them, Bi, Te and Sb are calculated and weighed according to the molar ratio of Bi2Te3 and Sb2Te3 of 15% to 25%: 85% to 75%, and Te is added as a dopant to configure a P-type material; Bi, Te and Se materials are calculated and weighed according to the molar ratio of Bi2Te3 and Bi2Se3 of 88% to 96%: 12% to 4%, and a dopant is added to configure an N-type material, and then the P-type and N-type raw materials weighed according to the above molar ratio are respectively placed in a quartz container, which is vacuumed and sealed. The dopant Te of the P-type material is added in an amount of 0.1% to 0.5% of the total weight of the Bi2Te3 and Sb2Te3 materials; the dopant of the N-type material is selected from one or more of the halogen elements F, Cl, Br, I and their compounds, and the addition amount is 200ppm to 800ppm of the total weight of the Bi2Te3 and Bi2Se3 materials. The pretreatment method of P-type and N-type materials is as follows: put the quartz container into a tube furnace at 700℃~800℃, heat for 1 hour, shake for 1 hour, take it out of the furnace and cool it in the air, mechanically crush the high-temperature smelted material, press it at room temperature, put it into a heating furnace at 500℃~600℃, and heat treat it for 5~15 hours in a low oxygen partial pressure atmosphere. The inner cavity of the hot extrusion die contains at least two sections of variable diameter structures, and the larger diameter side transitions to the smaller diameter side through a first conical surface with a certain angle to the central axis, and the first conical surface is connected to the first cylindrical surface below to form a first variable diameter section; the first cylindrical surface is connected to a second conical surface with a certain angle to the central axis below, and the two are transitioned through a smooth arc surface, and the second conical surface is connected to the second cylindrical surface below to form a second variable diameter section, and then the subsequent variable diameter sections are formed according to the same method. In each variable diameter section, the angle between the conical surface and the central axis is 45° to 70°, the area ratio of the upper and lower circles of the conical surface is 2 to 15; the length and diameter ratio of the cylindrical surface is 0.3 to 1.0, and the total compression ratio is the product of the compression ratios of each variable diameter section.

2. The method for processing and forming bismuth telluride thermoelectric material according to claim 1, characterized in that: in, The quartz container is a quartz ampoule or a quartz test tube. After vacuuming, the pressure inside the quartz container is less than 2Pa. The dopant of the N-type material is SbI3.

3. The method for processing and forming bismuth telluride thermoelectric material according to claim 1, characterized in that: in, The mechanical crushing time is 1 min to 3 min; when pressing and molding at room temperature, the pressure is 100 MPa to 180 MPa; the oxygen partial pressure in the heating furnace is controlled below 20 ppm.

4. The method for processing and forming bismuth telluride thermoelectric material according to claim 1, characterized in that: in, The temperature for placing the P-type material heater is set to 300°C to 390°C, and the temperature for placing the N-type material heater is set to 430°C to 500°C.

5. The method for processing and forming bismuth telluride thermoelectric material according to claim 1, characterized in that: in, When the rod-shaped material is heat-treated, the rod-shaped material is heat-treated again in an atmosphere with an oxygen partial pressure of less than 20 ppm, with a heat treatment temperature of 350° C. to 400° C. and a time of 5 hours to 15 hours.

6. A bismuth telluride thermoelectric rod, characterized in that: The method is used to obtain the product.

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