A Bi-Sb thermoelectric material for cryogenic refrigeration and its preparation method

Abstract

The present invention relates to the field of thermoelectric materials. To solve the problem that due to the large segregation coefficient of the Bi-Sb material system, the material utilization rate of the produced Bi-Sb alloy is often less than 10%, the present application proposes a Bi-Sb thermoelectric material for low-temperature refrigeration and its preparation method. The chemical formula of the Bi-Sb thermoelectric material is Bi 0.88‑ x Sb 0.12 Hf x , where 0.001 ≤ x ≤ 0.01. The prepared Bi-Sb thermoelectric material has several times higher material utilization rate than that of general Bi-Sb alloys.

Description

Technical Field

[0001] The present invention relates to the field of thermoelectric materials, and particularly to a high-uniformity Bi-Sb thermoelectric material for low-temperature refrigeration and a preparation method thereof. Background Art

[0002] The performance of thermoelectric cooler (TEC) products mainly depends on thermoelectric materials. Improving the performance of thermoelectric materials is a key way to enhance market competitiveness. Thermoelectric materials are a new type of energy material that can directly convert thermal energy and electrical energy, and can be used for solid-state refrigeration and waste heat power generation. The former is based on the Peltier effect, and the latter is based on the Seebeck effect. The two are inverse effects and are related by the Thomson effect, which are collectively called the three major effects of thermoelectricity. The main performance index of thermoelectric materials is the dimensionless figure of merit zT = ( a 2 s / k )T, where a is the Seebeck coefficient, s is the electrical conductivity, k is the thermal conductivity of the material, and T is the service temperature of the material. a 2 s is an index to measure the electrical properties of the material and is called the power factor. The thermal conductivity of the material is divided into two parts, the electronic thermal conductivity k e and the lattice thermal conductivity k l. The former is coupled with the electrical conductivity by the Lorenz equation, and the latter can be regulated relatively independently.

[0003] Bismuth telluride alloy is currently the only thermoelectric material that has been commercially applied on a large scale, and is mainly used for refrigeration and power generation near room temperature. However, due to its relatively large band gap (~0.13 eV), the Fermi level deviates far from the valence band top or the conduction band bottom at low temperatures, and the Seebeck coefficient decreases significantly; at the same time, the scattering effect of the U process is not obvious at low temperatures, and the thermal conductivity increases rapidly with the decrease of temperature. Eventually, the thermoelectric performance of bismuth telluride decreases continuously with the decrease of temperature below room temperature.

[0004] Currently, the market demand for deep refrigeration is increasing day by day. Relevant enterprises mainly produce multi-stage TEC products through bismuth telluride alloy to achieve deep refrigeration. When an electric current is applied to the TEC, the heat at the cold end of the first-stage TEC is transported to the hot end of the TEC, and the temperature decreases; at this time, the cold end of the first-stage TEC becomes the hot end of the second-stage TEC, and the temperature has decreased significantly, resulting in a decrease in the thermoelectric performance of bismuth telluride. Therefore, the refrigeration performance of the second-stage TEC is not as good as that of the first-stage TEC; by analogy, the higher the number of stages of the TEC, the worse the performance, mainly because the thermoelectric performance of the bismuth telluride alloy decreases with the decrease of temperature. Therefore, developing excellent thermoelectric materials at low temperatures has important application value.

[0005] Bi-Sb alloy is a thermoelectric material with relatively high performance at low temperatures discovered so far. However, due to its obvious anisotropy, its thermoelectric performance is only the most excellent along the c-axis direction. The zone melting method, as a typical directional solidification process, can be used to produce highly oriented Bi-Sb alloy. However, due to the large segregation coefficient (k~3) of the Bi-Sb material system, the material utilization rate of the produced Bi-Sb alloy is often less than 10%, resulting in a great waste of materials and making it difficult to be used in commercial production. Summary of the Invention

[0006] To solve the problem that the material utilization rate of the produced Bi-Sb alloy is often less than 10% due to the large segregation coefficient of the Bi-Sb material system. This application proposes a Bi-Sb thermoelectric material for low-temperature refrigeration and its preparation method, which can increase the material utilization rate by several times compared with general Bi-Sb alloys.

[0007] The present invention is realized through the following technical solutions: A Bi-Sb thermoelectric material for low-temperature refrigeration, the chemical formula of the Bi-Sb thermoelectric material is Bi 0.88-x Sb 0.12 Hf x , where 0.001 ≤ x ≤ 0.01.

[0008] The present invention utilizes the balance of carrier concentration caused by the low segregation coefficient of Hf in Bi and the large segregation coefficient of Sb in Bi to prepare a Bi-Sb zone-melted alloy with uniform thermoelectric performance. The segregation phenomenon refers to the difference in the solid solubility of solute atoms in the solid solvent and the liquid solvent. When the liquid phase begins to partially crystallize into the solid phase, some solute atoms cannot dissolve into the crystallized solid phase (the segregation coefficient is less than 1), so the doping concentration of this part will be lower than the equilibrium concentration. On the contrary, when the segregation coefficient is greater than 1, the doping concentration will be higher than the equilibrium concentration.

[0009] The preparation method of the Bi-Sb thermoelectric material for low-temperature refrigeration is as follows:

[0010] (1) Crush the matrix raw materials;

[0011] (2) Weigh the raw materials in step (1) according to the stoichiometric ratio of each element in the matrix composition, and load the raw materials into the mold;

[0012] The mold is a high-melting-point ceramic, including a quartz tube, graphite, and magnesium oxide;

[0013] Preferably, the top of the mold is flat-bottomed instead of the conventional conical shape. The conical top is difficult to withstand the volume expansion generated during the solidification process of the material, resulting in the mold cracking and the material flowing out of the mold and being oxidized.

[0014] Preferably, a graphite paper is fixed on the inner wall of the mold to absorb the stress generated during the solidification of the material.

[0015] Preferably, the mold should be cleaned before use. The cleaning steps are as follows: Pour dilute nitric acid into the mold, ultrasonically vibrate for 15 - 20 min, pour out the nitric acid and wash it twice with clean water, wash it once with absolute ethanol, and then put the mold into an oven and dry it at 120 °C for 12 h for standby;

[0016] (3)Vacuum the mold in step (2) to ≤10 -3 Pa, and seal the mold with a high heat source;

[0017] The high heat source includes a hydrogen - oxygen flame and an acetylene flame.

[0018] (4)Place the sealed mold in a rocking melting furnace and melt it for 10 - 12 h to obtain a polycrystalline ingot;

[0019] Preferably, melt it in a rocking melting furnace at 700 - 1000 °C for 10 - 12 h.

[0020] (5)Place the polycrystalline ingot obtained in step (4) in a vertical zone - melting furnace for zone - melting growth to prepare a Bi - Sb thermoelectric material.

[0021] Preferably, the zone - melting heating methods include resistance - wire heating, optical heating, and induction heating. The zone - melting temperature is 280 - 400 °C, preferably 350 °C.

[0022] Preferably, the zone - melting zone is 1 - 2 cm, the growth rate is 8 - 50 mm / h. After the polycrystalline ingot is zone - melted from start to end, it is cooled to room temperature.

[0023] Compared with the prior art, the beneficial effects of the present invention are: The Bi - Sb - Hf (0.001 ≤ x ≤ 0.01) thermoelectric alloy prepared by the present invention has high uniformity and high orientation; at the same time, the material has high thermoelectric performance at low temperatures. i0.88-x Sb 0.12 Hf x (0.001≤ x≤ 0.01) Brief Description of the Drawings

[0024] Figure 1 It is a flow chart of the preparation method of the Bi - Sb zone - melting ingot of the present invention;

[0025] Figure 2 It is the Seebeck coefficient diagram of different parts of the Bi - Sb - Hf ( 0.88-x Sb 0.12 Hf x ( x =0.001, 0.002, 0.005, 0.01)ingots prepared in Examples 1 - 4. DETAILED DESCRIPTION

[0026] In order to better understand the present invention, the content of the present invention is further explained in conjunction with the following examples, but the content of the present invention is not limited to the following examples. The raw materials used in the examples can be purchased commercially or prepared by conventional methods.

[0027] Example 1

[0028] (1) Crush the raw materials Bi block, Sb block and Hf block;

[0029] (2) Seal one end of a quartz tube with an inner diameter of about 28 mm with an oxyhydrogen flame, pour dilute nitric acid into the quartz tube, and ultrasonically vibrate for 15 to 20 minutes. Pour out the nitric acid and wash it twice with clean water and once with anhydrous ethanol. Then put the quartz tube into an oven and dry it at 120°C for 12 hours for later use.

[0030] According to the chemical formula Bi 0.879 Sb 0.12 Hf 0.001 The stoichiometric ratio of each element in step (1) is 1600 g and placed in a dried quartz tube with graphite paper on the inner wall.

[0031] (3) Pump the vacuum degree of the quartz tube in step (2) to 10 -3 Pa, and seal the other end of the quartz tube with an oxyhydrogen flame;

[0032] (4) placing the quartz tube in step (3) in a rotary melting furnace at 800° C. and melting for 10 h, shaking the tube during the melting process to ensure that the raw materials are fully mixed, and cooling the tube to room temperature to obtain a polycrystalline ingot;

[0033] (5) Placing the polycrystalline ingot obtained in step (4) in a vertical zone melting furnace for zone melting growth, the zone melting temperature is 350°C, the growth rate is 25 mm / h, and after the polycrystalline ingot is zone melted from beginning to end, it is cooled to room temperature to obtain Bi 0.879 Sb 0.12 Hf 0.001 Zone melting ingot.

[0034] Example 2

[0035] (1) Crush the raw materials Bi block, Sb block and Hf block;

[0036] (2) Seal one end of a quartz tube with an inner diameter of about 28 mm with an oxyhydrogen flame, pour dilute nitric acid into the quartz tube, and ultrasonically vibrate for 15 to 20 minutes. Pour out the nitric acid and wash it twice with clean water and once with anhydrous ethanol. Then put the quartz tube into an oven and dry it at 120°C for 12 hours for later use.

[0037] According to the chemical formula Bi 0.878 Sb0.12 Hf 0.002 Weigh a total of 1600 g of the raw materials in step (1) and place them into a quartz tube with a dried inner wall lined with graphite paper.

[0038] (3) Pump the quartz tube in step (2) to a vacuum of 10 -3 Pa, and seal the other end of the quartz tube with a hydrogen-oxygen flame;

[0039] (4) Place the quartz tube in step (3) in a rotary melting furnace at 800 °C and melt for 10 h. Keep swinging during the melting process to ensure sufficient mixing of the raw materials. Cool to room temperature to obtain a polycrystalline ingot;

[0040] (5) Place the polycrystalline ingot obtained in step (4) on a vertical zone melting furnace for zone melting growth. The zone melting temperature is 350 °C and the growth rate is 25 mm / h. After zone melting the polycrystalline ingot from start to end, cool to room temperature to obtain a Bi 0.878 Sb 0.12 Hf 0.002 zone-melted ingot.

[0041] Example 3

[0042] (1) Crush the raw material Bi blocks, Sb blocks and Hf blocks;

[0043] (2) Seal one end of a quartz tube with an inner diameter of about 28 mm with a hydrogen-oxygen flame. Pour dilute nitric acid into the quartz tube, ultrasonically vibrate for 15 - 20 min, pour out the nitric acid, wash twice with clear water and once with anhydrous ethanol, and then place the quartz tube in an oven and dry at 120 °C for 12 h for standby;

[0044] Weigh a total of 1600 g of the raw materials in step (1) according to the stoichiometric ratio of each element in the chemical formula Bi 0.875 Sb 0.12 Hf 0.005 and place them into a quartz tube with a dried inner wall lined with graphite paper.

[0045] (3) Pump the quartz tube in step (2) to a vacuum of 10 -3 Pa, and seal the other end of the quartz tube with a hydrogen-oxygen flame;

[0046] (4) Place the quartz tube in step (3) in a rotary melting furnace at 800 °C and melt for 10 h. Keep swinging during the melting process to ensure sufficient mixing of the raw materials. Cool to room temperature to obtain a polycrystalline ingot;

[0047] (5) Place the polycrystalline ingot obtained in step (4) on a vertical zone melting furnace for zone melting growth. The zone melting temperature is 350 °C and the growth rate is 25 mm / h. After zone melting the polycrystalline ingot from start to end, cool to room temperature to obtain a Bi 0.875 Sb0.12 Hf 0.005 Zone melting ingot.

[0048] Example 4

[0049] (1) Crush the raw materials Bi block, Sb block and Hf block;

[0050] (2) Seal one end of a quartz tube with an inner diameter of about 28 mm with an oxyhydrogen flame, pour dilute nitric acid into the quartz tube, and ultrasonically vibrate for 15 to 20 minutes. Pour out the nitric acid and wash it twice with clean water and once with anhydrous ethanol. Then put the quartz tube into an oven and dry it at 120°C for 12 hours for later use.

[0051] According to the chemical formula Bi 0.87 Sb 0.12 Hf 0.01 The stoichiometric ratio of each element in step (1) is 1600 g and placed in a dried quartz tube with graphite paper on the inner wall.

[0052] (3) Pump the vacuum degree of the quartz tube in step (2) to 10 -3 Pa, and seal the other end of the quartz tube with an oxyhydrogen flame;

[0053] (4) placing the quartz tube in step (3) in a rotary melting furnace at 800° C. and melting for 10 h, shaking the tube during the melting process to ensure that the raw materials are fully mixed, and cooling the tube to room temperature to obtain a polycrystalline ingot;

[0054] (5) Placing the polycrystalline ingot obtained in step (4) in a vertical zone melting furnace for zone melting growth, the zone melting temperature is 350°C, the growth rate is 25 mm / h, and after the polycrystalline ingot is zone melted from beginning to end, it is cooled to room temperature to obtain Bi 0.87 Sb 0.12 Hf 0.01 Zone melting ingot.

[0055] Comparative Example

[0056] (1) Crush the raw materials Bi and Sb blocks;

[0057] (2) Seal one end of a quartz tube with an inner diameter of about 28 mm with an oxyhydrogen flame, pour dilute nitric acid into the quartz tube, and ultrasonically vibrate for 15 to 20 minutes. Pour out the nitric acid and wash it twice with clean water and once with anhydrous ethanol. Then put the quartz tube into an oven and dry it at 120°C for 12 hours for later use.

[0058] According to the chemical formula Bi 0.88 Sb 0.12 The stoichiometric ratio of each element in step (1) is 1600 g and placed in a dried quartz tube with graphite paper on the inner wall.

[0059] (3) Pump the vacuum degree of the quartz tube in step (2) to 10 -3 Pa, and seal the other end of the quartz tube with an oxyhydrogen flame;

[0060] (4) placing the quartz tube in step (3) in a rotary melting furnace at 800° C. and melting for 10 h, shaking the tube during the melting process to ensure that the raw materials are fully mixed, and cooling the tube to room temperature to obtain a polycrystalline ingot;

[0061] (5) Placing the polycrystalline ingot obtained in step (4) in a vertical zone melting furnace for zone melting growth, the zone melting temperature is 350°C, the growth rate is 25 mm / h, and after the polycrystalline ingot is zone melted from beginning to end, it is cooled to room temperature to obtain Bi 0.88 Sb 0.12 Zone melting ingot.

[0062] Test Case

[0063] According to Figure 1 As shown in the process flow diagram, Examples 1-4 and Comparative Examples were prepared using the process parameters of a single melting zone, a melting zone length of 1-2 cm, and a growth rate of 25 mm / h. The Seebeck coefficients of different parts of the Bi0.88-xSb0.12Hfx (x=0.001, 0.002, 0.005, 0.01) ingots prepared in Examples 1-4 of the prepared Bi-Sb thermoelectric materials. Figure 2 As shown, the Seebeck coefficient of different parts in the comparative example (without Hf doping) fluctuates greatly, reaching 150%; in the embodiment, due to the segregation effect of Hf, the Seebeck coefficient fluctuates less. The Seebeck coefficient is the most sensitive parameter to the carrier concentration among the thermoelectric parameters. The homogenization of the Seebeck coefficient indicates the uniformity of the thermoelectric properties of the material. At the same time, the maximum temperature difference of the 4-layer TEC prepared by Bi-Sb alloy is 3 degrees higher than that of the 4-layer TEC prepared by conventional bismuth telluride. The problem that Bi-Sb alloy is difficult to commercialize due to material utilization has been solved.

Claims

1. A Bi-Sb thermoelectric material for cryogenic refrigeration, characterized in that, The chemical formula of the Bi-Sb thermoelectric material is Bi 0.88-x Sb 0.12 Hf x , where 0.001 ≤ x ≤ 0.

01.

2. A preparation method of the Bi-Sb thermoelectric material for low-temperature refrigeration according to claim 1, characterized in that, The preparation method comprises the following steps: (1) Crushing the matrix raw materials; (2) Weighing the raw materials in step (1) according to the stoichiometric ratios of the elements in the matrix composition, and loading the raw materials into a mold; (3) Pump the vacuum degree of the mold in step (2) to ≤ 10 -3 Pa, and seal the mold with a high heat source; (4) Placing the sealed mold in a rocking melting furnace and melting for 10 - 12 h to obtain a polycrystalline ingot; (5) Placing the polycrystalline ingot obtained in step (4) in a vertical zone melting furnace for zone melting growth to produce a Bi - Sb thermoelectric material.

3. The preparation method of the Bi-Sb thermoelectric material for low-temperature refrigeration according to claim 2, characterized in that, The mold is a high - melting - point ceramic, and the top of the mold is flat - bottomed.

4. The preparation method of the Bi-Sb thermoelectric material for low-temperature refrigeration according to claim 2 or 3, characterized in that, Graphite paper is fixed on the inner wall of the mold.

5. The preparation method of the Bi-Sb thermoelectric material for low-temperature refrigeration according to claim 2, characterized in that, In step (3), the high - heat source includes an oxy - hydrogen flame and an acetylene flame.

6. The preparation method of the Bi-Sb thermoelectric material for low-temperature refrigeration according to claim 2, characterized in that, In step (5), the zone melting heating method includes resistance wire heating, optical heating, and induction heating.

7. The preparation method of the Bi-Sb thermoelectric material for low-temperature refrigeration according to claim 2, wherein In step (5), the zone melting temperature is 280 - 400 degrees Celsius.

8. The preparation method of the Bi-Sb thermoelectric material for low-temperature refrigeration according to claim 2, wherein, In step (5), the zone melting area is 1 - 2 cm.

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