A Barrier Layer Suitable for Mg3Sb2-Mg3Bi2-Based Thermoelectric Materials and Its Usage Method

Abstract

The present invention relates to a barrier layer applicable to Mg3Sb2-Mg3Bi2-based thermoelectric materials and a method for using the same. By using the barrier layer material of the present invention, the Mg3Sb2-Mg3Bi2-based thermoelectric joint has a lower interfacial resistance and good high-temperature stability. The barrier layer material is almost chemically inert to the matrix material at high temperatures, enabling the medium-temperature power generation application of the Mg3Sb2-Mg3Bi2-based thermoelectric material and effectively improving the reliability and service life of its thermoelectric devices. In addition, the preparation method of the present invention has the advantages of simple process, low preparation cost, suitability for large-scale production, etc.

Description

Technical Field

[0001] The present invention belongs to the field of thermoelectric devices, and particularly relates to a barrier layer suitable for Mg3Sb2-Mg3Bi2-based thermoelectric materials and a method for using the same. Background Art

[0002] The maximum conversion efficiency of a thermoelectric device is related to the temperature at both ends of the device and the average ZT value of the thermoelectric material. The calculation formula is as follows:

[0003]

[0004] In the actual process of fabricating a device, in addition to considering the material properties, that is, the average ZT value, it is also necessary to consider the topological structure design of the device and the connection interface between the electrode and the material. The interface resistance will have a great impact on the conversion efficiency of the final device. Generally speaking, commonly used metal electrodes, such as Cu and Ni, will react with the thermoelectric material at high temperatures, resulting in a large interface resistance near the joint and deteriorating the thermoelectric properties of the material near the interface. Therefore, an additional barrier layer needs to be added between the electrode and the thermoelectric material to prevent the mutual diffusion between the material and the electrode, thereby ensuring the stability of the material chemical composition and reducing the interface loss.

[0005] The Mg3Sb2-based thermoelectric material has a hexagonal structure of anti-La2O3, which is composed of anion [Mg2Sb2] 2- layers stacked in the c-axis direction and cation Mg 2+ layers. The N-type Mg3Sb2-based thermoelectric material has good electrical properties due to having a valley number of 6. Its intrinsic thermal conductivity is also very low, ensuring its excellent thermoelectric properties. At the same time, the constituent elements of Mg3Sb2 are non-toxic, abundant in content, and low in cost, and are considered to be one of the thermoelectric materials most promising for large-scale applications.

[0006] In recent years, the research on Mg3Sb2-Mg3Bi2 solid solution-based thermoelectric materials has significantly improved the thermoelectric properties and stability of this system. Chen et al. through co-doping of Mn and Te, in Mg 3.15 Mn 0.05 Sb 1.5 Bi 0.49 Te 0.01An extremely high thermoelectric performance with a maximum ZT of 1.85 (723 K) and an average ZT value of 1.25 (300 - 723 K) was obtained (Chen, X. et al. Extraordinary thermoelectric performance in n-type manganese doped Mg3Sb2 Zintl: High band degeneracy, tuned carrier scattering mechanism and hierarchical microstructure. Nano Energy, 2018, 52, 246–255); Shi et al. doped Y into n-type Mg3SbBi materials and not only obtained a maximum ZT close to 1.8, but also achieved good high-temperature stability. After working at 750 K for 75 h, the performance of this material hardly decayed (Shi, X. et al. Extraordinary n-type Mg3SbBi thermoelectrics enabled by Yttrium doping. Adv. Mater. 2019, 31, 1903387).

[0007] However, the current research on devices based on Mg3Sb2-Mg3Bi2 thermoelectric materials is relatively scarce. In particular, there are few reports on devices for medium-temperature (500 - 800 K) power generation applications and the corresponding barrier layer materials. Mao et al. first proposed that Fe and Ni could be used as the diffusion barrier layer for Mg 3.2 Bi 1.498 Sb 0.5 Te 0.002 and fabricated a thermoelectric couple, and characterized its thermoelectric cooling performance near room temperature (Mao, J. et al. High thermoelectric cooling performance of n-type Mg3Bi2-based materials. Science. 2019, 365, 495–498). After that, most Mg3Sb2-Mg3Bi2-based thermoelectric devices chose Fe as the barrier layer material. The team led by Professor Pei Yanzhong of Tongji University used Fe as the barrier layer for Mg 3.05 Y 0.015 SbBi and found that after aging at 600 K for 240 h, its interface resistance increased from 14 μΩcm 2 to 30 μΩcm 2(Bu, Z. et al. An over 10% module efficiency obtained using non-Bi2Te3 thermoelectric materials for recovering heat of <600K. Energy Environ. Sci. 2021, 14, 6506–6513). In addition, since the interface thermal aging temperature in this study is lower than the operating temperature of thermoelectric devices in the medium temperature range (∼800K), it cannot yet reflect the situation of using the Fe barrier layer in the medium temperature power generation of Mg3Sb2-Mg3Bi2-based materials. The research team of Professor Zhang Qian from Harbin Institute of Technology conducted in-depth research and optimization on the Fe barrier layer and finally found that 304 stainless steel can achieve a lower interface resistance (5.36 μΩ cm 2 ), better mechanical properties (shear strength 35.74 MPa), and better service stability (interface resistance 11.64 μΩ cm after 400h aging 2 ). However, the thermal aging experimental temperature of its barrier layer is even lower, only 523K, which also cannot meet the application requirements of Mg3Sb2-Mg3Bi2-based thermoelectric materials in the medium temperature range (Yin, L. et al. Reliable N-type Mg 3.2 Sb 1.5 Bi 0.49 Te 0.01 / 304 stainless steel junction for thermoelectric applications. Acta Mater. 2020, 198, 25–34). Therefore, there is still a lack of a suitable barrier layer material for medium temperature power generation devices based on Mg3Sb2-Mg3Bi2-based thermoelectric materials. Summary of the Invention

[0008] The technical problem to be solved by the present invention is to provide a barrier layer suitable for Mg3Sb2-Mg3Bi2-based thermoelectric materials and its use method. By using the barrier layer material, the Mg3Sb2-Mg3Bi2-based thermoelectric joint has a lower interface resistance and good high-temperature stability. The barrier layer material is almost chemically inert to the matrix material at high temperatures, enabling the medium temperature power generation application of Mg3Sb2-Mg3Bi2-based thermoelectric materials and effectively improving the reliability and service life of its thermoelectric devices.

[0009] The present invention provides a barrier layer suitable for Mg3Sb2-Mg3Bi2-based thermoelectric materials, and the barrier layer is metal niobium or its alloy.

[0010] The barrier layer is niobium powder, niobium foil, and a niobium-based metal layer deposited by evaporation or electroplating.

[0011] The thickness of the barrier layer is 0.1 μm - 2 mm. Preferably, it is 30 μm - 0.1 mm. The thickness of the barrier layer is related to the difference in the coefficient of thermal expansion between it and the matrix of the thermoelectric material. The greater the difference, the greater the stress generated at the interface. Accordingly, the stress problem needs to be improved by reducing the thickness of the barrier layer. The Mg3Sb2-Mg3Bi2 solid solution has a relatively high coefficient of thermal expansion, while niobium metal has a relatively low coefficient of thermal expansion. Therefore, its thickness should be less than 0.1 mm. Alloying with other elements to increase its coefficient of thermal expansion can increase the thickness, but it should not be too thick (greater than 2 mm) to prevent cracking at the interface.

[0012] The thermoelectric material is a compound based on the Mg3Sb2-Mg3Bi2 solid solution.

[0013] The Mg3Sb2-Mg3Bi2 solid solution is synthesized into powder by vacuum melting or ball milling, and combined with sintering technology to prepare a bulk, or a bulk is obtained by one-step sintering using the powder raw material.

[0014] The connection between the niobium metal or its alloy barrier layer and the Mg3Sb2-Mg3Bi2-based thermoelectric material can be achieved by methods such as sintering, electrochemical deposition, and magnetron sputtering. Specifically, when niobium metal and its alloy are selected as the barrier layer material, the powder or foil of niobium metal and its alloy can be directly added during the sintering process of the Mg3Sb2-based thermoelectric material for one-step sintering connection, or the niobium metal and its alloy can be prepared into a target, and after the Mg3Sb2-based thermoelectric material is sintered into a bulk, it is deposited on the surface of the thermoelectric material by magnetron sputtering.

[0015] Specifically, the present invention also provides a method for using a barrier layer suitable for the Mg3Sb2-Mg3Bi2-based thermoelectric material, including:

[0016] (1) Weigh the corresponding raw materials according to the stoichiometric ratio of the Mg3Sb2-Mg3Bi2 solid solution and put them into a ball mill for ball milling to obtain the powder of the Mg3Sb2-Mg3Bi2-based thermoelectric material;

[0017] (2) Load the powder of the Mg3Sb2-Mg3Bi2-based thermoelectric material and niobium metal or its alloy with a thickness of 0.1 μm - 2 mm into a graphite mold according to the three-layer structure of thermoelectric material powder, niobium metal or its alloy, and thermoelectric material powder, transfer it to a spark plasma sintering furnace, and sinter at 700 - 800 °C for 3 - 5 minutes.

[0018] The ball milling speed in step (1) is 500 - 600 rpm, and the ball milling time is 12 - 20 h.

[0019] The present invention also provides an application of a barrier layer suitable for Mg3Sb2-Mg3Bi2 based thermoelectric materials in connecting Mg3Sb2-Mg3Bi2 based thermoelectric materials and metal electrode materials.

[0020] The present invention also provides an application of a barrier layer suitable for Mg3Sb2-Mg3Bi2 based thermoelectric materials in the interconnection of Mg3Sb2-Mg3Bi2 based thermoelectric materials.

[0021] Beneficial effects

[0022] The present invention uses niobium metal or its alloy as the barrier layer, which itself has good electrical conductivity and thermal conductivity. Compared with iron and 304 stainless steel alloy used in most current low-temperature Mg3Sb2 based thermoelectric devices, it has a better barrier effect, shows good chemical inertness with the thermoelectric material matrix at high temperatures and lower interfacial resistance. After undergoing accelerated aging for a long time at a temperature 200 °C higher than the currently reported temperature, it can still maintain chemical inertness and low interfacial resistance. Using niobium metal as the barrier layer of Mg3Sb2 based thermoelectric devices enables the realization of its medium-temperature power generation application and can effectively improve the reliability and service life of its thermoelectric devices. Description of the Drawings

[0023] Figure 1 is the microscopic morphology and element distribution of the interface between 0.1 mm niobium foil and Mg3Sb2-Mg3Bi2 based thermoelectric materials before aging in Example 1 of the present invention.

[0024] Figure 2 is the interfacial resistivity of the joint between 0.1 mm niobium foil and Mg3Sb2-Mg3Bi2 based thermoelectric materials before and after aging in Example 1 of the present invention.

[0025] Figure 3 is the comparison of the interfacial resistivity of the joint between 0.1 mm niobium foil and Mg3Sb2-Mg3Bi2 based thermoelectric materials before aging in Example 1 of the present invention with the literature values.

[0026] Figure 4 is the microscopic morphology and element distribution of the interface between 0.1 mm niobium foil and Mg3Sb2-Mg3Bi2 based thermoelectric materials after aging in Example 1 of the present invention.

[0027] Figure 5 is the elemental line scan results of the interface between 0.1 mm niobium foil and Mg3Sb2-Mg3Bi2 based thermoelectric materials before and after aging in Example 1 of the present invention.

[0028] Figure 6 is the comparison of the interfacial resistivity of the joint between 0.1 mm niobium foil and Mg3Sb2-Mg3Bi2 based thermoelectric materials after aging in Example 1 of the present invention with the literature values.

[0029] Figure 7 It is the element distribution after aging at the interface between the 50-μm niobium foil and the Mg3Sb2-Mg3Bi2-based thermoelectric material in Example 2 of the present invention.

[0030] Figure 8 It is the element distribution after aging at the interface between the 30-μm niobium foil and the Mg3Sb2-Mg3Bi2-based thermoelectric material in Example 3 of the present invention. Detailed implementation manners

[0031] 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 not 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 also fall within the scope defined by the appended claims of this application.

[0032] Example 1

[0033] (1) Weigh a total of 7 g of Mg, Sb, Bi, and Se raw materials in a glove box, put them into a ball mill, and ball mill at 500-600 rpm for 12-20 h to obtain the powder of the Mg3Sb2-Mg3Bi2-based thermoelectric material.

[0034] (2) In the glove box, load the material powder and 0.1-mm niobium metal foil into a graphite mold according to the three-layer structure of mixed powder, niobium metal foil, and material powder, and quickly transfer it to a spark plasma sintering furnace. Sinter at 750 °C for 3 minutes to make the niobium barrier layer and the Mg3Sb2-Mg3Bi2 thermoelectric material matrix have good connection.

[0035] (3) Use a diamond wire saw to cut the obtained block containing the niobium barrier layer into the required shape, observe the microscopic morphology, element distribution, and diffusion at the interface, and measure its interface resistance.

[0036] (4) Vacuum seal the particles obtained after cutting, age at 500 °C for 15 days, and perform the same interface morphology and element observation and interface resistance measurement on the aged samples.

[0037] According to Figure 1 It can be observed that the niobium metal foil does not show obvious diffusion during the high-temperature sintering process at 750 °C, showing good chemical inertness. And there is no cracking at the connection with the matrix, indicating good interface connection. The interface resistance is 9.7 μΩ·cm 2 ( Figure 2 ), which is at a relatively good level in the Mg3Sb2-Mg3Bi2-based thermoelectric material ( Figure 3 ). After 15 days of high-temperature accelerated aging, the interface still maintains good connection and does not crack (Figure 4 ) and the thickness of the diffusion layer only increased by less than 1 μm Figure 5 ). The interfacial resistance increased to 26 μΩcm 2 ( Figure 2 ) and still remained at a low level Figure 6 ), indicating the feasibility and reliability of the niobium barrier layer for medium-temperature power generation devices of Mg3Sb2-based thermoelectric materials.

[0038] Example 2

[0039] (1) Weigh a total of 7 g of Mg, Sb, Bi, and Se raw materials in a glove box, put them into a ball milling jar, and ball mill at 500 - 600 rpm for 12 - 20 h to obtain the powder of Mg3Sb2-Mg3Bi2-based thermoelectric materials.

[0040] (2) In the glove box, load the material powder and a 50-μm niobium metal foil into a graphite mold according to the three-layer structure of mixed powder, niobium metal foil, and material powder, and quickly transfer it to a spark plasma sintering furnace. Sinter at 750 °C for 3 minutes to make the niobium barrier layer have a good connection with the Mg3Sb2-Mg3Bi2-based thermoelectric material matrix.

[0041] (3) Use a diamond wire saw to cut the obtained block containing the niobium barrier layer into the required shape, observe the microscopic morphology, element distribution, and diffusion at the interface, and measure its interfacial resistance.

[0042] (4) Vacuum seal the cut particles and age them at 500 °C for 15 days. For the obtained aged samples, also observe the interface morphology and elements and measure the interfacial resistance. Using a 50-μm niobium metal foil still maintains almost the same chemical inertness as a 0.1-mm-thick metal foil, and no obvious diffusion occurred during the sintering at 750 °C and the accelerated aging at 500 °C Figure 7 ).

[0043] Example 3

[0044] (1) Weigh a total of 7 g of Mg, Sb, Bi, and Se raw materials in a glove box, put them into a ball milling jar, and ball mill at 500 - 600 rpm for 12 - 20 h to obtain the powder of Mg3Sb2-Mg3Bi2-based thermoelectric materials.

[0045] (2) In the glove box, load the material powder and a 30-μm niobium metal foil into a graphite mold according to the three-layer structure of mixed powder, niobium metal foil, and material powder, and quickly transfer it to a spark plasma sintering furnace. Sinter at 750 °C for 3 minutes to make the niobium barrier layer have a good connection with the Mg3Sb2-Mg3Bi2-based thermoelectric material matrix.

[0046] (3) Cut the obtained bulk containing the niobium barrier layer into the desired shape using a diamond wire saw, observe the microscopic morphology, element distribution, and diffusion at the interface, and measure its interface resistance.

[0047] (4) Vacuum seal the particles obtained after cutting, age them at 500 °C for 15 days, and perform the same interface morphology and element observation and interface resistance measurement on the aged samples. The 30-μm-thick niobium metal foil still maintains almost the same chemical inertness as the 0.1-mm-thick metal foil, and no obvious diffusion occurred during the sintering at 750 °C and the accelerated aging at 500 °C ( Figure 8 ).

[0048] All the above embodiments of the present invention exhibit good contact and high-temperature stability with the Mg3Sb2-Mg3Bi2-based thermoelectric material, and at the same time have a low interface resistance, providing a solution for the application of Mg3Sb2-Mg3Bi2-based thermoelectric materials in medium-temperature power generation devices, and can effectively improve the reliability and service life of their thermoelectric devices.

Claims

1. A barrier layer applicable to Mg3Sb2-Mg3Bi2 based thermoelectric materials, characterized in that: The barrier layer is niobium metal or its alloy.

2. The barrier layer according to claim 1, wherein: The barrier layer is niobium powder, niobium foil, and a niobium-based metal layer deposited by evaporation or electroplating.

3. The barrier layer according to claim 1, wherein: The thickness of the barrier layer is 0.1 micrometer to 2 millimeters.

4. The barrier layer according to claim 1, wherein: The thermoelectric material is a compound based on the Mg3Sb2-Mg3Bi2 solid solution.

5. The barrier layer according to claim 4, wherein: The Mg3Sb2-Mg3Bi2 solid solution is synthesized into powder by vacuum melting or ball milling, and combined with sintering technology to prepare a bulk, or a bulk is obtained by one-step sintering using the powder raw material.

6. Application of the barrier layer for Mg3Sb2-Mg3Bi2-based thermoelectric materials according to any one of claims 1-5 in connecting Mg3Sb2-Mg3Bi2-based thermoelectric materials and metal electrode materials.

7. Application of the barrier layer for Mg3Sb2-Mg3Bi2-based thermoelectric materials according to any one of claims 1-5 in the interconnection of Mg3Sb2-Mg3Bi2-based thermoelectric materials.

8. A preparation method of a barrier layer for Mg3Sb2-Mg3Bi2-based thermoelectric materials, comprising the following steps: (1) Weigh the corresponding raw materials according to the stoichiometric ratio of the Mg3Sb2-Mg3Bi2 solid solution, and put them into a ball mill for ball milling to obtain the powder of the Mg3Sb2-Mg3Bi2-based thermoelectric material; (2) Load the powder of the Mg3Sb2-Mg3Bi2-based thermoelectric material and 0.1 micrometer to 2 millimeters of niobium metal or its alloy into a graphite mold according to a three-layer structure of thermoelectric material powder, niobium metal or its alloy, and thermoelectric material powder, transfer it to a spark plasma sintering furnace, and sinter at 700-800 °C for 3-5 minutes.

9. The preparation method according to claim 8, wherein: The ball milling speed in step (1) is 500-600 rpm, and the ball milling time is 12-20 h.

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