Thermoelectric conversion material preservation method, sintered body manufacturing method, and thermoelectric conversion material

Abstract

To provide a preservation method advantageous in terms of ease of storage and transportation of a thermoelectric conversion material containing Mg.SOLUTION: A thermoelectric conversion material 10 is surrounded by a low-polarity substance 20. The thermoelectric conversion material 10 contains Mg. The thermoelectric conversion material 10 surrounded by the low-polarity substance 20 is stored or transported.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a method for storing a thermoelectric conversion material, a method for producing a sintered body, and a thermoelectric conversion material. [Background technology]

[0002] Thermoelectric conversion technology is known. Thermoelectric conversion technology directly converts temperature differences into electrical energy and is based on three physical phenomena: the Seebeck effect, the Peltier effect, and the Thomson effect. For example, by using a circuit in which a P-type semiconductor and an N-type semiconductor are junctioned, it is possible to extract the electricity generated by creating a temperature difference at the junction between the P-type and N-type semiconductors.

[0003] The performance of a thermoelectric conversion material can be evaluated by the thermoelectric conversion figure of merit ZT. ZT is calculated by using the Seebeck coefficient S, electrical conductivity σ, and thermal conductivity κ of the thermoelectric conversion material, ZT=S 2 It is defined by the formula σT / κ. The higher the ZT, the higher the energy conversion efficiency, and the better the performance of the thermoelectric conversion module is expected. This type of thermoelectric conversion technology is being put to practical use in the fields of precision cooling and waste heat recovery.

[0004] For example, Mg3(Sb,Bi)2-based thermoelectric conversion materials are known as one of the thermoelectric conversion materials that exhibit a high ZT (see Non-Patent Documents 1 and 2). (Sb,Bi) means that at least one selected from the group consisting of Sb and Bi is contained. In addition, Patent Document 1 lists Mg3(Sb,Bi)2-based thermoelectric conversion materials as examples. 3+m A a B b D 2-e E e In this thermoelectric conversion material, the element D is at least one element selected from the group consisting of Sb and Bi.

[0005] Mg3(Sb,Bi)2-based thermoelectric conversion materials can react with moisture or oxygen in the atmosphere at room temperature to produce magnesium hydroxide, magnesium oxide, metallic antimony, and metallic bismuth (see Non-Patent Document 3). In this case, the electrical resistance of the thermoelectric conversion material increases, and the electrical conductivity σ of the thermoelectric conversion material decreases, which can result in a decrease in the thermoelectric figure of merit ZT.

[0006] Patent Document 2 describes a method for producing rare earth permanent magnets. In this method, a specified fine powder for rare earth permanent magnets is mixed with mineral oil, synthetic oil, or vegetable oil, and the resulting mixture is wet-pressed in a magnetic field to produce a compact. The resulting compact is then desolvated and sintered. The compact immersed in mineral oil, synthetic oil, or vegetable oil is stored in a vacuum or inert gas atmosphere. This significantly reduces the increase in oxygen content, suppressing the increase in oxygen content of the sintered body and enabling the stable production of high-performance rare earth magnets with excellent magnetic properties. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] International Publication No. 2017 / 072982 [Patent Document 2] Japanese Patent Application Publication No. 10-97937 [Non-patent literature]

[0008] [Non-Patent Document 1] H. Tamaki, HKSato, and T. Kanno, Isotropic Conduction Network and Defect Chemistry in Mg3+δSb2-Based Layered Zintl Compounds with High Thermoelectric Performance, Advanced Materials, (Germany), September 30, 2016, vol. 28 issue 46, p.10182-10187, https: / / doi.org / 10.1002 / adma.201603955 [Non-patent document 2] K. Imasato, M. Wood, S. Anand, JJ Kuo, and GJ Snyder, Understanding the High Thermoelectric Performance of Mg3Sb2-Mg3Bi2 Alloys, Advanced Energy and Sustainability Research, (Germany), February 9, 2022, vol. 3 issue 6, 2100208, https: / / doi.org / 10.1002 / aesr.202100208 [Non-patent document 3] X. Wu, X. Ma, H. Yao, K. Liang, P. Zhao, S. Hou, L. Yin, H. Yang, J. Sui, X. Lin, F. Cao, Q. Zhang, and J. Mao, Revealing the Chemical Instability of Mg3Sb2-xBix-Based Thermoelectric Materials, ACS Appl. Mater. Interfaces, (USA), October 20, 2023, vol. 15, issue 43, 50216-50224, https: / / doi.org / 10.1021 / acsami.3c12290 Summary of the Invention [Problem to be solved by the invention]

[0009] The present disclosure provides a method for preserving a thermoelectric conversion material that is advantageous from the viewpoint of ease of storage and transportation of a Mg-containing thermoelectric conversion material. [Means for solving the problem]

[0010] The method for storing a thermoelectric conversion material according to the present disclosure includes: Surrounding a thermoelectric conversion material containing Mg with a low-polarity substance; and storing or transporting the thermoelectric conversion material surrounded by the low-polarity substance. [Effects of the Invention]

[0011] According to the present disclosure, it is possible to provide a storage method that is advantageous from the viewpoint of ease of storage and transportation of a thermoelectric conversion material containing Mg. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a diagram schematically illustrating an example of a method for storing a thermoelectric conversion material. [Figure 2] FIG. 2 is a diagram schematically illustrating another example of a method for storing a thermoelectric conversion material. [Figure 3] FIG. 3 is a diagram schematically illustrating yet another example of a method for storing a thermoelectric conversion material. [Figure 4] FIG. 4 is a flowchart showing an example of a method for producing a sintered body. [Figure 5] FIG. 5 is a diagram schematically illustrating an example of the structure of a thermoelectric conversion material that is a sintered body. [Figure 6A] FIG. 6A is a graph showing an X-ray diffraction (XRD) pattern of the sintered body according to Example 1. [Figure 6B] FIG. 6B is a graph showing an XRD pattern of the sintered body according to Comparative Example 1. [Figure 7] FIG. 7 is a diagram schematically showing a method for measuring the surface resistivity of a sintered body. [Figure 8A] FIG. 8A is a graph showing the distribution of resistivity on the surface of the sintered body according to Example 1. [Figure 8B] FIG. 8B is a graph showing the distribution of resistivity on the surface of the sintered body according to Comparative Example 1. [Figure 8C] FIG. 8C is a graph showing the distribution of resistivity on the surface of the sintered body according to Comparative Example 2. [Figure 9A] FIG. 9A is a scanning electron microscope (SEM) observation image of the surface of the sintered body according to Example 1. [Figure 9B] FIG. 9B is an SEM image of the surface of the sintered body according to Comparative Example 3. [Figure 10] FIG. 10 is a Mg distribution image of the surface of the sintered body according to Example 1, obtained by energy dispersive X-ray spectroscopy (SEM-EDX). DETAILED DESCRIPTION OF THE INVENTION

[0013] (Findings that formed the basis of this disclosure) As can be seen from the above formula for the thermoelectric conversion figure of merit ZT, the thermoelectric conversion figure of merit ZT of a thermoelectric conversion material decreases as the electrical resistance of the thermoelectric conversion material increases. In a thermoelectric conversion material containing Mg, moisture can cause the generation of products exhibiting high electrical resistance. A certain amount of moisture is present in the atmosphere. Therefore, to prevent an increase in the electrical resistance of a thermoelectric conversion material during storage or transportation of a Mg-containing thermoelectric conversion material, it is possible to handle the thermoelectric conversion material in an environment where the presence of moisture is minimized. For example, it is possible to store or transport a Mg-containing thermoelectric conversion material in an inert gas atmosphere, a vacuum atmosphere, a dry atmosphere, or other such environment. However, constantly creating such an environment during storage or transportation of a thermoelectric conversion material is cumbersome and may increase the cost of storing or transporting the thermoelectric conversion material.

[0014] In view of these circumstances, the present inventors have conducted extensive research into methods for suppressing an increase in the electrical resistance of a thermoelectric conversion material containing Mg even when the thermoelectric conversion material is stored or transported in a moisture-containing environment. As a result, they have newly discovered that by surrounding the thermoelectric conversion material with a specific substance, it is possible to suppress the generation of products that exhibit high electrical resistance even in a moisture-containing environment, and thus to suppress an increase in the electrical resistance of the thermoelectric conversion material. Based on this new finding, the present inventors have completed the method for storing a thermoelectric conversion material of the present disclosure.

[0015] (Embodiments of the present disclosure) Hereinafter, embodiments of the present disclosure will be described with reference to the drawings.

[0016] (Embodiment 1) FIG. 1 is a schematic diagram illustrating an example of a method for storing a thermoelectric conversion material. As shown in FIG. 1, a thermoelectric conversion material 10 is surrounded by a low-polarity substance 20. Thus, the thermoelectric conversion material 10 is surrounded by the low-polarity substance 20. The thermoelectric conversion material 10 surrounded by the low-polarity substance 20 is stored or transported in this manner. In this specification, "storage" refers to maintaining an object in a specific state. "Storage" refers to managing an object without moving it from a specific location. The thermoelectric conversion material 10 contains Mg. According to this storage method, since the thermoelectric conversion material 10 is surrounded by the low-polarity substance 20, even if the thermoelectric conversion material 10 is stored or transported in a moisture-containing environment, the thermoelectric conversion material 10 is less susceptible to moisture. Therefore, even if the thermoelectric conversion material 10 is not stored or transported in an environment where moisture is minimized, such as an inert gas atmosphere, a vacuum atmosphere, or a dry atmosphere, the electrical resistance of the thermoelectric conversion material 10 is less likely to increase. According to this storage method, the thermoelectric conversion material 10 can be easily stored and transported. For example, thermoelectric conversion materials such as BiTe are generally stored in a sealed container with a dry interior or in a cool, dark place with a dry interior.

[0017] The thermoelectric conversion material 10 may be stored or transported in an environment where moisture is present. As described above, even in such an environment, the thermoelectric conversion material 10 is surrounded by the low-polarity substance 20, so that the thermoelectric conversion material 10 is less susceptible to the effects of moisture, and the electrical resistance of the thermoelectric conversion material 10 is less likely to increase. The thermoelectric conversion material 10 may be stored or transported in the atmosphere. Because the thermoelectric conversion material 10 is surrounded by the low-polarity substance 20, the low-polarity substance 20 exists between the thermoelectric conversion material 10 and the atmosphere, and the thermoelectric conversion material 10 is isolated from the atmosphere. In other words, because the thermoelectric conversion material 10 is surrounded by the low-polarity substance 20, the thermoelectric conversion material 10 is less susceptible to the effects of moisture contained in the atmosphere, and the electrical resistance of the thermoelectric conversion material 10 is less likely to increase.

[0018] The low-polarity substance 20 is, for example, a substance that is insoluble or immiscible in water. The low-polarity substance 20 is, for example, a substance that has a relative dielectric constant of 8.0 or less at 20°C. The low-polarity substance 20 may be a liquid or a solid. For example, whether the low-polarity substance 20 is a liquid or a solid can be determined according to ASTM D4359. The relative dielectric constant of the low-polarity substance 20 at 20°C may be 7.0 or less, 6.0 or less, 5.0 or less, 4.0 or less, or 3.0 or less.

[0019] 1, the low-polarity substance 20 is, for example, a liquid. In this case, for example, the liquid low-polarity substance 20 is contained inside a predetermined container 30. In addition, the thermoelectric conversion material 10 is surrounded by the low-polarity substance 20 in a state where the thermoelectric conversion material 10 is immersed in the low-polarity substance 20.

[0020] The low-polarity substance 20 contains, for example, an oil agent. In this case, the thermoelectric conversion material 10 is less susceptible to the effects of moisture, and the electrical resistance value of the thermoelectric conversion material 10 is less likely to increase. Furthermore, when the low-polarity substance 20 contains an oil agent, the low-polarity substance 20 is easier to handle. Examples of oil agents are synthetic oil, mineral oil, and vegetable oil.

[0021] The oil agent contains, for example, a hydrocarbon compound. In this case, the thermoelectric conversion material 10 is less susceptible to the effects of moisture, and the electrical resistance value of the thermoelectric conversion material 10 is less likely to increase. Examples of hydrocarbon compounds are aromatic hydrocarbons, paraffinic hydrocarbons, and naphthenic hydrocarbons. The low-polarity substance 20 may contain such a hydrocarbon compound as a main component. In this specification, a main component refers to the component that is contained in the largest amount by mass.

[0022] The container 30 has an inner surface formed of, for example, a material that does not react with the low-polarity substance 20. The container 30 may be a bottle, a tank, or a drum.

[0023] During storage or transportation of the thermoelectric conversion material 10, for example, the inside of the container 30 is sealed. The inside of the container 30 is sealed, for example, by attaching a lid 40 to the container 30. The lid 40 liquid-tightly seals the inside of the container 30 by applying pressure or heat. The lid 40 and the container 30 may be chemically bonded together. The inside of the container 30 may be sealed by bonding the lid 40 and the container 30 together with a predetermined bonding material.

[0024] Fig. 2 is a diagram schematically illustrating another example of a method for storing a thermoelectric conversion material. As shown in Fig. 2, the low-polarity substance 20 may be solid. The thermoelectric conversion material 10 may be coated with the solid low-polarity substance 20, for example. The thermoelectric conversion material 10 may be embedded in the solid low-polarity substance 20. An example of the solid low-polarity substance 20 is paraffin.

[0025] The thermoelectric conversion material 10 is not limited to a specific thermoelectric conversion material as long as it contains Mg. The thermoelectric conversion material 10 may further contain, for example, at least one element selected from the group consisting of Sb and Bi. In this case, the thermoelectric conversion material 10 is likely to exhibit high thermoelectric conversion performance.

[0026] 1, the thermoelectric conversion material 10 is, for example, a powder. In this case, the thermoelectric conversion material 10 can be surrounded by, for example, immersing the powdered thermoelectric conversion material 10 in a liquid low-polarity substance 20, or coating the powdered thermoelectric conversion material 10 with the low-polarity substance 20. In this case, the powdered thermoelectric conversion material 10 can be easily stored and transported.

[0027] FIG. 3 is a diagram schematically illustrating yet another example of a method for storing a thermoelectric conversion material. As shown in FIG. 3, the thermoelectric conversion material 10 may be in a bulk form. In this specification, "bulk" refers to a bulky form other than a thin film, powder, or granules. For example, the powdered thermoelectric conversion material 10 can be produced by pulverizing the bulk thermoelectric conversion material 10. Alternatively, the bulk thermoelectric conversion material 10 may be a sintered body produced from the powdered thermoelectric conversion material 10. The bulk thermoelectric conversion material 10 may be a sintered body produced from a powder obtained by pulverizing and mixing the single raw materials of the constituent elements of the thermoelectric conversion material 10.

[0028] For example, a sintered body can be produced from powdered thermoelectric conversion material 10. The sintered body is produced, for example, by a method including the following (I) and (II). In (I), thermoelectric conversion material 10 is surrounded by a low-polarity substance 20. According to such a method, even if powdered thermoelectric conversion material 10 is stored or transported in an environment where moisture is present, the electrical resistance value of the powdered thermoelectric conversion material 10 is unlikely to increase, and the electrical resistance value of the sintered thermoelectric conversion material 10 is also unlikely to increase. Therefore, a thermoelectric conversion element obtained from the sintered body is likely to exhibit the desired thermoelectric conversion performance. (I) A powdered thermoelectric conversion material 10 containing Mg is supplied. (II) Sintering the thermoelectric conversion material 10.

[0029] 4 is a flowchart showing an example of a method for producing a sintered body. As shown in FIG. 4, in step S11, powdered thermoelectric conversion material 10 is supplied as described above in (I). For example, in step S11, thermoelectric conversion material 10 surrounded by low-polarity substance 20 is taken out from container 30. In this case, thermoelectric conversion material 10 is taken out from container 30 in a mixed state with low-polarity substance 20. Thermoelectric conversion material 10 is filled, for example, together with low-polarity substance 20 into a die for sintering.

[0030] Next, in step S12, the powdered thermoelectric conversion material 10 is sintered. The sintering method of the thermoelectric conversion material 10 is not limited to a specific method as long as it can solidify the powdered thermoelectric conversion material 10 into a predetermined shape. The sintering of the thermoelectric conversion material 10 can be performed, for example, according to a spark plasma method or a hot pressing method. If necessary, the obtained sintered body may be subjected to a predetermined heat treatment for homogenization or the like. A sintered body is obtained in this manner, and the sintered thermoelectric conversion material 10 can be provided.

[0031] FIG. 5 is a schematic diagram illustrating an example of the structure of a sintered thermoelectric conversion material. As shown in FIG. 5, the sintered thermoelectric conversion material 10 includes, for example, grains 10g. The grains 10g contain, for example, Mg and at least one element selected from the group consisting of Sb and Bi. The grains 10g have an average grain size of, for example, 40 μm or more and 200 μm or less. This average grain size can be determined, for example, by a sectioning method. For example, the average grain size of the grains 10g can be determined from an SEM image of the surface of the thermoelectric conversion material 10 according to the method described in the Examples. In FIG. 5, the three concentric circles indicated by dashed double-dashed lines represent the circular test lines used in the sectioning method. The average grain size of the grains 10g calculated in this manner differs from the circle-equivalent average diameter or average Feret diameter calculated by methods other than the sectioning method. The circle-equivalent diameter is the diameter of a circle having an area equal to the projected area of ​​the grains 10g. The average Feret diameter is the distance between two parallel lines when 10 g of grain is sandwiched between two lines.

[0032] In the thermoelectric conversion material 10, for example, there is a region at the boundary 10b of the grain 10g where Mg is present at a higher concentration than the Mg concentration inside the grain 10g. For example, the region extends along the boundary 10b. Whether or not such a region exists in the thermoelectric conversion material 10 can be determined based on, for example, a Mg distribution image obtained by energy dispersive X-ray spectroscopy (SEM-EDX) of the surface of the thermoelectric conversion material 10. For example, International Publication No. 2020 / 194776 discloses a region where Mg is present at a higher concentration than the Mg concentration inside the grain 10g. 3+m Sb a Bi 2-a-c A c In this thermoelectric conversion material, A is at least one element selected from the group consisting of Se and Te, and this thermoelectric conversion material has an Mg-rich portion. Regarding the portion where Mg is present at a concentration higher than the Mg concentration inside grain 10g, the description of the Mg-rich portion in WO 2020 / 194776 can be referred to.

[0033] The following non-patent document 4 describes Mg 3.05 Sb 2-x-y Bi y-x Te x (x≦0.04, y≦1.5) is described. In this thermoelectric material, Mg is uniformly distributed in the sample made from coarse grains as the raw material, and the average crystal grain size is 60 μm. Non-patent document 4 X. Shi, C. Sun, Z. Bu, X. Zhang, Y. Wu, S. Lin, W. Li, A. Faghaninia, A. Jain, and Y. Pei, Revelation of Inherently High Mobility Enables Mg3Sb2as a Sustainable Alternative to n-Bi2Te3Thermoelectrics, Advanced Science, (Germany), July 13, 2019, Vol. 6 Issue 16, 1802286, https: / / doi.org / 10.1002 / advs.201802286

[0034] In the thermoelectric conversion material 10 that is a sintered body, for example, the grains 10g have an average grain size of 40 μm or more and 200 μm or less, and a region where Mg is present at a high concentration is present at the boundary 10b of the grains 10g. With this configuration, the thermoelectric conversion material 10 that is a sintered body is less likely to become hot. In the thermoelectric conversion material 10 that is a sintered body, for example, even when the crystal grains are about 60 μm, a region where Mg is present at a high concentration is present at the boundary 10b.

[0035] The average particle size of 10 g of grains may be 50 μm or more, 60 μm or more, or 70 μm or more, and may be 180 μm or less, 150 μm or less, 120 μm or less, or 100 μm or less.

[0036] (Addendum) From the above description, the following techniques are disclosed. (Technology 1) Surrounding a thermoelectric conversion material containing Mg with a low-polarity substance; and storing or transporting the thermoelectric conversion material surrounded by the low-polarity substance. A method for storing thermoelectric conversion materials. (Technology 2) The thermoelectric conversion material is stored or transported in an environment where moisture is present. A method for storing a thermoelectric conversion material according to claim 1. (Technology 3) The thermoelectric conversion material is in a powder or bulk form. A method for storing a thermoelectric conversion material according to any one of claims 1 to 2. (Technology 4) Surrounding the thermoelectric conversion material with the low-polarity substance includes immersing the powdered thermoelectric conversion material in the liquid phase of the low-polarity substance, or coating the powdered thermoelectric conversion material with the low-polarity substance. 4. A method for storing a thermoelectric conversion material according to any one of claims 1 to 3. (Technology 5) The thermoelectric conversion material further contains at least one selected from the group consisting of Sb and Bi. 5. A method for storing a thermoelectric conversion material according to any one of claims 1 to 4. (Technology 6) The low-polarity substance is a liquid. 6. A method for storing a thermoelectric conversion material according to any one of claims 1 to 5. (Technology 7) The low-polarity substance includes an oil agent. 7. A method for storing a thermoelectric conversion material according to any one of claims 1 to 6. (Technology 8) The oil agent includes a hydrocarbon compound. A method for storing a thermoelectric conversion material according to claim 7. (Technology 9) supplying a powdered thermoelectric conversion material containing Mg; sintering the thermoelectric conversion material; The thermoelectric conversion material is surrounded by a low-polarity substance. A method for manufacturing a sintered body. (Technology 10) grains containing Mg and at least one selected from the group consisting of Sb and Bi; The grains have an average particle size of 40 μm or more and 200 μm or less, a region where Mg is present at a higher concentration than the Mg concentration inside the grain exists at the grain boundary; Thermoelectric conversion materials. [Example]

[0037] The present disclosure will be described in detail below with reference to examples, but the present disclosure is not limited to the examples shown below.

[0038] Example 1 Mg 3.14 Mn 0.01 Sb 1.0 Bi 0.997 Te 0.003Powder of an Mg3(Sb,Bi)2-based thermoelectric conversion material having the composition shown above was immersed in liquid paraffin, a low-polarity liquid, in an argon-filled glove box. This thermoelectric conversion material powder was prepared by crushing an ingot obtained by melting the constituent elements or compounds, as in Example 1 of Patent Document 1, for example. A mixture containing the Mg3(Sb,Bi)2-based thermoelectric conversion material immersed in liquid paraffin was stored for 14 hours in a high-temperature, high-humidity environment at a temperature of 85°C and a relative humidity of 85%. The mixture was placed in a sealed glass container. It is believed that storing this mixture in a high-temperature, high-humidity environment rather than in a humid environment at room temperature allows for accurate evaluation of the effects of moisture in the atmosphere during long-term storage in the atmosphere. Next, the mixture was placed inside a carbon die-punch in the atmosphere and compacted. The powder of the Mg3(Sb,Bi)2-based thermoelectric conversion material was then sintered in an argon atmosphere using a hot press method. As a result, a sintered compact with a diameter of 25 mm and a thickness of 3 mm was obtained. The maximum temperature inside the die-punch during the hot press and the time for holding the maximum temperature were 840°C and 10 minutes, respectively. This is thought to have caused the liquid paraffin to almost completely disappear due to thermal decomposition or other reasons. Next, in order to homogenize the sintered body, the metallic magnesium and the sintered body were placed in a carbon crucible and subjected to a heat treatment at 600°C for 8 hours in an argon atmosphere. In this way, the sintered body according to Example 1 was obtained.

[0039] (Comparative Example 1) Except for not immersing the powder of the Mg(Sb,Bi)-based thermoelectric conversion material in liquid paraffin, it was stored in a high-temperature, high-humidity environment in the same manner as in Example 1. Thereafter, a sintered body according to Comparative Example 1 was obtained in the same manner as in Example 1, except for not performing the heat treatment at 600°C for 8 hours in an argon atmosphere.

[0040] (Comparative Example 2) A sintered body according to Comparative Example 2 was obtained in the same manner as in Example 1, except that the powder of the Mg(Sb,Bi)-based thermoelectric conversion material was not immersed in liquid paraffin, and the powder was stored and sintered in an argon atmosphere.

[0041] (Comparative Example 3) A sintered body according to Comparative Example 3 was obtained in the same manner as in Comparative Example 2, except that the diameter of the sintered body was changed to 10 mm.

[0042] (Evaluation of X-ray diffraction patterns) X-ray diffraction (XRD) measurements were performed on samples prepared from the sintered bodies of Example 1 and Comparative Example 1, and XRD patterns were obtained. For this XRD measurement, a Malvern Panalytical Aeris X-ray diffractometer was used, using Cu-Kα radiation as the X-ray source. Figure 6A is a graph showing the XRD pattern of the sintered body of Example 1. Figure 6B is a graph showing the XRD pattern of the sintered body of Comparative Example 1. In Figures 6A and 6B, the vertical axis represents the diffraction intensity, and the horizontal axis represents the diffraction angle 2θ. As shown in Figure 6A, the XRD pattern of the sintered body of Example 1 mainly contained diffraction peaks derived from the Mg3(Sb,Bi)2-based thermoelectric conversion material. On the other hand, as shown in Figure 6B, the XRD pattern of the sintered body of Comparative Example 1 also contained diffraction peaks derived from the Mg3(Sb,Bi)2-based thermoelectric conversion material, as well as diffraction peaks believed to be derived from impurities. It was found that these diffraction peaks derived from impurities coincided with diffraction peaks derived from bismuth or magnesium oxide.

[0043] (Measurement of electrical resistance of sintered body) The change in resistivity at measurement positions on the surfaces of the sintered bodies according to Example 1, Comparative Example 1, and Comparative Example 2 was measured using the four-probe method. FIG. 7 is a diagram schematically illustrating a method for measuring the resistivity of the surface of a sintered body. As shown in FIG. 7, four probes T1, T2, T3, and T4 were moved at 1 mm intervals in the diameter direction (X-axis direction) from an origin O located on the contour of the sintered body sample S in a planar view, and these probes were brought into contact with the sintered body sample S. The four probes T1, T2, T3, and T4 were aligned at predetermined intervals on a line perpendicular to the X-axis in a planar view of the sintered body. The values ​​of current I at probes T1 and T4 when a voltage V was swept at probes T2 and T3 were read, and the resistivity at each measurement position was calculated. FIG. 8A is a graph showing the distribution of resistivity on the surface of the sintered body according to Example 1. FIG. 8B is a graph showing the distribution of resistivity on the surface of the sintered body according to Comparative Example 1. FIG. 8C is a graph showing the distribution of resistivity on the surface of the sintered body according to Comparative Example 2. In these graphs, the vertical axis represents resistivity, and the horizontal axis represents the measurement position as a distance from the origin O in the X-axis direction. As shown in FIGS. 8A and 8C, the resistivity of the surface of the sintered body according to Example 1 and the resistivity of the surface of the sintered body according to Comparative Example 2 were approximately 1.0 to 2.0 mΩ·cm. On the other hand, as shown in FIG. 8B, the resistivity of the surface of the sintered body according to Comparative Example 1 was 10 4 From 10 5 The electrical resistance was in the range of or near that range of mΩ cm, and was higher than those of the sintered body of Example 1 and the sintered body of Comparative Example 2. Therefore, it can be understood that by surrounding a Mg-containing thermoelectric conversion material such as an Mg3(Sb,Bi)2-based thermoelectric conversion material with a low-polarity substance, the electrical resistance value of the sintered body is less likely to increase even if the thermoelectric conversion material is stored in an environment where moisture is present.

[0044] (Observation of the structure of sintered bodies) Samples prepared from the sintered bodies of Example 1 and Comparative Example 3 were observed by SEM. A Hitachi High-Tech SEM SU8220 was used. Figure 9A shows an SEM image of the surface of the sintered body of Example 1. Figure 9B shows an SEM image of the surface of the sintered body of Comparative Example 3. From these SEM images, the average grain size of each sintered body was determined using a cross-section method. White dots P1 and P2 in Figures 9A and 9B represent the centers of the triple-circle circular test lines used in the cross-section method. The intersections of the triple-circle circumferences and grain boundaries were determined using image processing. In Figures 9A and 9B, the diameters of the triple-circle circular test lines were 0.05 mm, 0.11 mm, and 0.16 mm. In Figures 9A and 9B, areas with brightness levels of 128 or higher were defined as grain boundaries. The distance between adjacent grain boundaries on the circumference of the triple circle, which is the circular test line, was defined as the crystal grain size, and the average value of the crystal grain sizes was defined as the average grain size. The minimum grain size, maximum grain size, and average grain size calculated by the cutting method for the sintered body of Example 1 were 0.81 μm, 364 μm, and 83 μm, respectively. On the other hand, the minimum grain size, maximum grain size, and average grain size calculated by the cutting method for the sintered body of Comparative Example 3 were 0.4 μm, 94 μm, and 9.9 μm, respectively. It was confirmed that a sintered body with a large grain size can be obtained by surrounding a Mg-containing thermoelectric conversion material, such as an Mg(Sb,Bi)-based thermoelectric conversion material, with a low-polarity substance.

[0045] SEM-EDX was performed on a sample prepared from the sintered body of Example 1. Figure 10 shows an Mg distribution image obtained by SEM-EDX on the surface of the sintered body of Example 1. In Figure 10, the white areas are areas where magnesium element is present. As shown in Figure 10, areas where magnesium is present at high concentrations exist at the grain boundaries in the sintered body of Example 1. In this way, it was confirmed that by surrounding a Mg-containing thermoelectric conversion material such as an Mg3(Sb,Bi)2-based thermoelectric conversion material with a low-polarity substance, Mg-rich areas exist at the grain boundaries and the average grain size is relatively large. It is thought that the electrical resistance of the sintered body was not high because the sintered body had such a structure. [Industrial Applicability]

[0046] According to the method for storing a thermoelectric conversion material of the present disclosure, storage and transportation of a thermoelectric conversion material containing Mg can be simplified. [Explanation of symbols]

[0047] 10 Thermoelectric conversion materials 10g grain 10b boundary 20 Low polarity substances

Claims

1. Surrounding a thermoelectric conversion material containing Mg with a low-polarity substance; and storing or transporting the thermoelectric conversion material surrounded by the low-polarity substance. A method for storing thermoelectric conversion materials.

2. The thermoelectric conversion material is stored or transported in an environment where moisture is present. A method for storing the thermoelectric conversion material according to claim 1.

3. The thermoelectric conversion material is in a powder or bulk form. A method for storing the thermoelectric conversion material according to claim 1.

4. Surrounding the thermoelectric conversion material with the low-polarity substance includes immersing the powdered thermoelectric conversion material in the liquid phase of the low-polarity substance, or coating the powdered thermoelectric conversion material with the low-polarity substance. A method for storing the thermoelectric conversion material according to claim 1.

5. The thermoelectric conversion material further contains at least one selected from the group consisting of Sb and Bi. A method for storing the thermoelectric conversion material according to claim 1.

6. The low-polarity substance is a liquid. A method for storing the thermoelectric conversion material according to claim 1.

7. The low-polarity substance includes an oil agent. A method for storing the thermoelectric conversion material according to claim 1.

8. The oil agent includes a hydrocarbon compound. A method for storing the thermoelectric conversion material according to claim 7.

9. supplying a powdered thermoelectric conversion material containing Mg; sintering the thermoelectric conversion material; The thermoelectric conversion material is surrounded by a low-polarity substance. A method for manufacturing a sintered body.

10. The alloy comprises grains containing Mg and at least one selected from the group consisting of Sb and Bi, The grains have an average particle size of 40 μm or more and 200 μm or less, a region where Mg is present at a higher concentration than the Mg concentration inside the grain exists at the boundary of the grain; Thermoelectric conversion materials.

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