Method for improving thermoelectric performance of p-type bismuth telluride crystal bar

Abstract

The invention discloses a method for improving thermoelectric performance of a p-type bismuth telluride crystal bar, and relates to the technical field of thermoelectric materials. According to the method provided by the invention, spark plasma sintering is firstly carried out on a blank formed by pressing a bismuth telluride raw material at 180-230 DEG C, then the temperature is increased by 50-320 DEG C, heat preservation heat treatment is carried out on a sintered block, a large number of grain boundaries can be reserved through the step of SPS sintering, and grains are preliminarily connected to form a crystal bar framework; the step of heat treatment can reduce dislocation density and distribution between bismuth telluride crystal boundaries, meanwhile, part of extremely small crystal grains can be recrystallized and grow, and a small amount of precipitated phases can be formed in the system. Therefore, according to the method provided by the invention, the thermal conductivity can be reduced and the Seebeck coefficient can be improved under the condition that the conductivity is prevented from being greatly reduced, so that the thermoelectric figure of merit of the crystal bar is improved.

Description

Technical Field

[0001] This invention relates to the field of thermoelectric materials technology, and more specifically, to a method for improving the thermoelectric properties of p-type bismuth telluride crystal rods. Background Technology

[0002] Spark plasma sintering (SPS) technology, through direct heating with pulsed current and surface activation mechanisms, enables rapid and densified sintering of bismuth telluride-based alloy powders at low temperatures, simultaneously optimizing the mechanical strength and thermoelectric properties of the alloys. However, adjusting the crystal rod formulation and SPS sintering process parameters offers limited improvement to the thermoelectric properties of bismuth telluride crystal rods, and such process optimization often requires significant time and raw material costs. Therefore, providing a simple method to further enhance the thermoelectric properties of bismuth telluride is of great importance. Summary of the Invention

[0003] To address the shortcomings of existing technologies, this invention provides a method for improving the thermoelectric properties of p-type bismuth telluride crystal rods.

[0004] The above-mentioned objective of this invention is achieved through the following technical solution: A method for improving the thermoelectric properties of p-type bismuth telluride crystal rods includes the following steps: S1. Mix and melt the raw materials for preparing p-type bismuth telluride crystal rods, and press them into billets; S2. The billet obtained in step S1 is subjected to SPS sintering at T1 for 5-60 min, and a sintered block is obtained after sintering; T1 = 180-230℃. S3. Place the sintered block obtained in step S2 at T2 for 1-24 h to obtain the desired temperature; T2-T1=50-320℃.

[0005] The method for improving the thermoelectric properties of p-type bismuth telluride crystal rods provided by this invention first involves SPS (spark plasma sintering) to locally melt and react the raw material particles in the crystal rod blank, forming bismuth telluride crystal nuclei. In conventional SPS sintering processes for bismuth telluride crystal rods, the sintering temperature is typically no lower than 400℃ to obtain crystal rods with internal densification and superior performance. However, the inventors of this application discovered that by lowering the temperature conditions for SPS sintering, sintering at a temperature T1 of 180-230°C, and then performing heat treatment at a temperature T2 that is 50-320°C higher than T1, the SPS sintering step can first nucleate bismuth telluride and form a preliminary framework for the crystal rod, but avoids the difficulty of orientation caused by excessive crystal nucleus growth, while retaining a large number of grain boundaries; the subsequent heat treatment step can reduce the dislocation density and distribution between bismuth telluride grain boundaries, while some extremely small grains will recrystallize and grow, and a small amount of precipitates will also form in the system. Thus, the bismuth telluride crystal rod prepared by this invention can not only hinder phonon thermal conduction through grain boundaries, but also promote carrier transport as much as possible with fewer dislocations and smaller specific surface area grain boundaries. At the same time, when carriers cross, grain boundaries can also act as a barrier to filter carriers with higher energy. Therefore, this invention can reduce the thermal conductivity of the material and increase the Seebeck coefficient without losing too much electrical conductivity, thereby improving the thermoelectric figure of merit of the material. It should be noted that if the heat treatment temperature is too high, the crystal nuclei are prone to melting and growing, leading to a reduction in the number of grain boundaries and an increase in thermal conductivity. While the reduced number of grain boundaries improves electrical conductivity, it severely impacts the filtering effect of grain boundaries on low-energy carriers, thus affecting the thermoelectric figure of merit. If the heat treatment temperature is too low, it is impossible to achieve a dense arrangement of crystals within the crystal rod. This not only simultaneously reduces both thermal and electrical conductivity, but the presence of defects such as dislocations and porosity can also negatively affect the band degeneracy within the crystal, leading to a decrease in the Seebeck coefficient. Therefore, a low heat treatment temperature will also result in a decrease in the thermoelectric figure of merit. Therefore, the heat treatment temperature T2 in step S3 should be 50-320°C higher than the SPS sintering temperature T1. To ensure that the SPS sintering in step S2 and the heat treatment in step S3 can work synergistically to improve the thermoelectric performance of the bismuth telluride crystal rod, the duration of each step also needs to be limited.

[0006] Preferably, T2-T1 = 110-220℃.

[0007] Preferably, the temperature T2 in step S3 is 300-500℃.

[0008] Preferably, the raw materials for preparing the p-type bismuth telluride crystal rod in step S1 include Bi, Sb, Te and doping elements in a molar ratio of (12-40):(75-180):(100-350):(1-2), wherein the doping elements include at least one of Se and Cu.

[0009] Preferably, the melting temperature in step S1 is 580-750℃ and the melting time is 30-150 min.

[0010] More preferably, the melting method described in step S1 includes oscillating melting.

[0011] Preferably, step S1 further includes a crushing step after smelting, wherein the crushing includes crushing the alloy ingot obtained from smelting to a particle size ≤100μm.

[0012] More preferably, the crushing method in step S1 includes at least one of mechanical crushing and ball milling.

[0013] Preferably, the pressing in step S1 is hot isostatic pressing and / or cold isostatic pressing, wherein the pressure of hot isostatic pressing is 30-80 MPa and the pressure of cold isostatic pressing is 130-160 MPa.

[0014] Preferably, the pressure of SPS sintering in step S2 is 20-60 MPa.

[0015] More preferably, in step S2, the pressure is increased to 20-60 MPa at a rate of 0.3-12 MPa / min.

[0016] More preferably, in step S2, the temperature is increased to T1 at a heating rate of 3-25°C / min.

[0017] In a specific embodiment of the present invention, the SPS sintering time in step S2 is the time for holding the pressure and temperature after increasing the pressure to a specific pressure and heating to T2.

[0018] Preferably, the SPS sintering in step S2 is carried out under a vacuum of <10 Pa.

[0019] Preferably, in step S3, the temperature is increased to T2 at a heating rate of 1-5℃ / min.

[0020] Preferably, step S3 involves heat preservation under normal pressure.

[0021] Preferably, the heat preservation in step S3 is carried out in an air atmosphere.

[0022] Compared with the prior art, the present invention has the following beneficial effects: The method provided by this invention involves SPS sintering of bismuth telluride crystal rod blanks at a lower temperature, followed by a heat treatment step. This can reduce thermal conductivity and increase Seebeck coefficient without causing excessive damage to electrical conductivity, thereby improving the thermoelectric figure of merit of the obtained bismuth telluride crystal rods. Attached Figure Description

[0023] Figure 1The graph shows the trends of thermal conductivity (κ), electrical conductivity (σ), Seebeck coefficient (S), and thermoelectric figure of merit (ZT) of the bismuth telluride crystal rods obtained in Example 1 (after heat treatment) and Comparative Example 9 (before heat treatment) as a function of temperature.

[0024] Figure 2 The graphs show the trends of thermal conductivity (κ), electrical conductivity (σ), Seebeck coefficient (S), and thermoelectric figure of merit (ZT) of the bismuth telluride crystal rods obtained in Example 7 (after heat treatment) and Comparative Example 10 (before heat treatment) as a function of temperature.

[0025] Figure 3 The graphs show the trends of thermal conductivity (κ), electrical conductivity (σ), Seebeck coefficient (S), and thermoelectric figure of merit (ZT) of the bismuth telluride crystal rods obtained in Example 8 (after heat treatment) and Comparative Example 11 (before heat treatment) as a function of temperature.

[0026] Figure 4 This is an EDS mapping diagram of the bismuth telluride crystal rod obtained in Example 1 of the present invention. Detailed Implementation

[0027] 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.

[0028] Examples 1-5 and Comparative Examples 1-6 This embodiment and comparative example provide a series of methods for improving the thermoelectric properties of p-type bismuth telluride crystal rods, including the following steps: S1. Mix the raw materials according to the molar ratio of Bi:Sb:Te:Se=15:85:150:1, and melt them by shaking at 620℃ for 120 min. Then, ball mill the powder until the particle size is ≤75μm (passing through a 200-mesh sieve), and press it into a billet by hot isostatic pressing at 30 MPa. S2. The temperature and pressure are increased to the set values ​​according to the heating time of 30 min and the pressure increase time of 10 min respectively, so that the billet obtained in step S1 is subjected to SPS sintering at T1, pressure of 35 MPa and vacuum degree of 7 Pa, and a sintered block is obtained after sintering. S3. Under normal pressure and in an air atmosphere, heat the material to T2 at a heating rate of 1℃ / min, and place the sintered block obtained in step S2 at T2 for a set time to obtain the final product.

[0029] The differences between Examples 1-13 and Comparative Examples 1-8 are shown in Table 1 below: Table 1. Example 6 A method for improving the thermoelectric properties of p-type bismuth telluride crystal rods includes the following steps: S1. Mix the raw materials according to the molar ratio of Bi:Sb:Te:Se=33:167:300:2, and melt them by shaking at 630℃ for 60 min. Then, ball mill them until the powder particle size is ≤75μm (passes through a 200-mesh sieve), and press them into billets by hot isostatic pressing at 40 MPa. S2. Increase the temperature and pressure to the set values ​​according to the heating time of 35 min and the pressure increase time of 10 min respectively, so that the billet obtained in step S1 is sintered under T1=200℃, pressure 30 MPa, vacuum degree 6 Pa for 10 min, and a sintered block is obtained after sintering. S3. Heat the material to T2 = 350℃ at a heating rate of 2℃ / min and keep the sintered block obtained in step S2 at T2 for 4 h to obtain the final product.

[0030] Example 7 A method for improving the thermoelectric properties of p-type bismuth telluride crystal rods includes the following steps: S1. Mix the raw materials according to the molar ratio of Bi:Sb:Te:Se=20:80:150:1, and melt them by shaking at 660℃ for 30 min. Then, ball mill them until the powder particle size is ≤75μm (passes through a 200-mesh sieve), and press them into billets by hot isostatic pressing at 150 MPa. S2. The temperature and pressure are increased to the set values ​​according to the heating time of 30 min and the pressure increase time of 10 min respectively, so that the billet obtained in step S1 is sintered under T1=195℃, pressure of 25 MPa and vacuum degree of 5 Pa for 10 min, and a sintered block is obtained after sintering. S3. Heat the material to T2 = 320℃ at a heating rate of 1℃ / min and keep the sintered block obtained in step S2 at T2 for 6 h to obtain the final product.

[0031] Example 8 A method for improving the thermoelectric properties of p-type bismuth telluride crystal rods includes the following steps: S1. Mix the raw materials according to the molar ratio of Bi:Sb:Te:Se=18:82:150:1, and melt them by shaking at 650℃ for 50 min. Then, ball mill them until the powder particle size is ≤75μm (passes through a 200-mesh sieve), and press them into billets by hot isostatic pressing at 160 MPa. S2. The temperature and pressure are increased to the set values ​​according to the heating time of 30 min and the pressure increase time of 10 min respectively, so that the billet obtained in step S1 is sintered under T1=195℃, pressure of 25 MPa and vacuum degree of 5 Pa for 10 min, and a sintered block is obtained after sintering. S3. Heat the material to T2 = 370℃ at a heating rate of 5℃ / min and keep the sintered block obtained in step S2 at T2 for 4 hours to obtain the final product.

[0032] Example 9 A method for improving the thermoelectric properties of p-type bismuth telluride crystal rods includes the following steps: S1. Mix the raw materials according to the molar ratio of Bi:Sb:Te:Se=35:165:300:2, and melt them by shaking at 640℃ for 80 min. Then, ball mill them until the powder particle size is ≤75μm (passes through a 200-mesh sieve), and press them into billets by hot isostatic pressing at 60 MPa. S2. The temperature and pressure are increased to the set values ​​according to the heating time of 35 min and the pressure increase time of 10 min respectively, so that the billet obtained in step S1 is sintered under T1=200℃, pressure of 30 MPa and vacuum degree of 4 Pa ​​for 10 min, and a sintered block is obtained after sintering. S3. Heat the material to T2 = 380℃ at a heating rate of 5℃ / min and keep the sintered block obtained in step S2 at T2 for 4 hours to obtain the final product.

[0033] Example 10 A method for improving the thermoelectric properties of p-type bismuth telluride crystal rods includes the following steps: S1. Mix the raw materials according to the molar ratio of Bi:Sb:Te:Se=20:80:150:1, and melt them by shaking at 660℃ for 30 min. Then, ball mill them until the powder particle size is ≤75μm (passes through a 200-mesh sieve), and press them into blanks by hot isostatic pressing at 160 MPa. S2. The temperature and pressure are increased to the set values ​​according to the heating time of 30 min and the pressure increase time of 10 min respectively, so that the billet obtained in step S1 is sintered under T1=195℃, pressure of 25 MPa and vacuum degree of 3 Pa for 10 min, and a sintered block is obtained after sintering. S3. Heat the material to T2 = 390℃ at a heating rate of 1℃ / min and keep the sintered block obtained in step S2 at T2 for 6 h to obtain the final product.

[0034] Comparative Example 7 A method for improving the thermoelectric properties of p-type bismuth telluride crystal rods, wherein the only difference from Example 1 is: Replace SPS sintering in step S2 with hot pressing sintering, while keeping the sintering temperature and pressure unchanged.

[0035] Comparative Example 8 A method for improving the thermoelectric properties of p-type bismuth telluride crystal rods, wherein the only difference from Example 1 is: Reverse the order of steps S2 and S3, perform heat treatment first, and then perform SPS sintering.

[0036] Comparative Example 9 A method for improving the thermoelectric properties of p-type bismuth telluride crystal rods, wherein the only difference from Example 1 is: Step S3 is skipped.

[0037] Comparative Example 10 A method for improving the thermoelectric properties of p-type bismuth telluride crystal rods, wherein the only difference from Example 6 is: Step S3 is skipped.

[0038] Comparative Example 11 A method for improving the thermoelectric properties of p-type bismuth telluride crystal rods, wherein the only difference from Example 7 is: Step S3 is skipped.

[0039] Comparative Example 12 A method for improving the thermoelectric properties of p-type bismuth telluride crystal rods, wherein the only difference from Example 8 is: Step S3 is skipped.

[0040] Comparative Example 13 A method for improving the thermoelectric properties of p-type bismuth telluride crystal rods, wherein the only difference from Example 9 is: Step S3 is skipped.

[0041] Comparative Example 14 A method for improving the thermoelectric properties of p-type bismuth telluride crystal rods, wherein the only difference from Example 10 is: Step S3 is skipped.

[0042] Performance testing Thermal conductivity test: The thermal diffusivity, specific heat and density of the sample in the thickness direction were tested and calculated according to the formula: thermal conductivity = thermal diffusivity * specific heat * density. The specific heat was measured using a Netzsch DSC300 Caliris Classic differential scanning calorimeter, the density was measured using the Archimedes displacement method, and the thermal diffusivity was measured using a laser thermal conductivity / thermal diffusivity measurement system (CLA) provided by Beijing KRIO. The sample was 10 mm in diameter and 2 mm thick.

[0043] Conductivity test: The conductivity of the material was measured using a Seebeck coefficient and resistivity testing system. The sample size was 3*3*12.5mm.

[0044] Seebeck coefficient test: The Seebeck coefficient of the material is measured by a Seebeck coefficient and resistivity testing system. The sample size is 3*3*12.5mm.

[0045] Thermoelectric figure of merit test: The thermoelectric figure of merit is given by the formula zT=(S 2 The value is obtained by calculating σ / κ)·T, where S, σ, κ and T are Seebeck coefficient, electrical conductivity, total thermal conductivity and absolute temperature, respectively.

[0046] The results of the above performance tests are shown in Table 2-3 below: Table 2. Table 3. As can be seen from the data in Table 2-3 above, introducing a heat treatment step after SPS sintering can reduce thermal conductivity and increase Seebeck coefficient without causing excessive damage to electrical conductivity, thereby improving the thermoelectric figure of merit of the obtained bismuth telluride crystal rod.

[0047] According to Comparative Examples 1-2, an unsuitable temperature difference T2-T1 will prevent the two steps from forming a good fit, thus affecting the performance of the resulting crystal rod.

[0048] According to Comparative Examples 3-4, an unsuitable sintering temperature T1 in the first step can lead to excessively large crystal nuclei, making rotation difficult, or an inability to form enough crystal nuclei, all of which affect the thermoelectric properties of the crystal rod. In Comparative Example 3, the SPS sintering temperature was the conventional 400℃.

[0049] According to Comparative Examples 5-6, inappropriate sintering time can prevent each step from achieving its full potential.

[0050] 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 improving the thermoelectric properties of p-type bismuth telluride crystal rods, characterized in that, Includes the following steps: S1. Mix and melt the raw materials for preparing p-type bismuth telluride crystal rods, and press them into billets; S2. The billet obtained in step S1 is subjected to SPS sintering at T1 for 5-60 min, and a sintered block is obtained after sintering; T1 = 180-230℃. S3. Place the sintered block obtained in step S2 at T2 for 1-24 h to obtain the desired temperature; T2-T1=50-320℃.

2. The method for improving the thermoelectric properties of p-type bismuth telluride crystal rods as described in claim 1, characterized in that, T2-T1=110-220℃.

3. The method for improving the thermoelectric properties of p-type bismuth telluride crystal rods as described in claim 1 or 2, characterized in that, In step S3, T2 = 300-500℃.

4. The method for improving the thermoelectric properties of p-type bismuth telluride crystal rods as described in claim 1, characterized in that, The raw materials for preparing the p-type bismuth telluride crystal rod in step S1 include Bi, Sb, Te and doping elements in a molar ratio of (12-40):(75-180):(100-350):(1-2), wherein the doping elements include at least one of Se, Cu, Sn and Yb.

5. The method for improving the thermoelectric properties of p-type bismuth telluride crystal rods as described in claim 1 or 4, characterized in that, The melting temperature in step S1 is 580-750℃, and the melting time is 30-150 min.

6. The method for improving the thermoelectric properties of p-type bismuth telluride crystal rods as described in claim 1, characterized in that, Step S1, after smelting, also includes a crushing step, which includes crushing the alloy ingot obtained from smelting to a particle size ≤100μm.

7. The method for improving the thermoelectric properties of p-type bismuth telluride crystal rods as described in claim 1 or 6, characterized in that, The pressing in step S1 is hot isostatic pressing and / or cold isostatic pressing, wherein the pressure of hot isostatic pressing is 30-80 MPa and the pressure of cold isostatic pressing is 130-160 MPa.

8. The method for improving the thermoelectric properties of p-type bismuth telluride crystal rods as described in claim 1, characterized in that, The pressure for SPS sintering in step S2 is 20-60 MPa.

9. The method for improving the thermoelectric properties of p-type bismuth telluride crystal rods as described in claim 1 or 8, characterized in that, The SPS sintering in step S2 is carried out under a vacuum of <10 Pa.

10. The method for improving the thermoelectric properties of p-type bismuth telluride crystal rods as described in claim 1, characterized in that, Step S3 raises the temperature to T2 at a heating rate of 1-5℃ / min.

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