Method for manufacturing thermoelectric conversion element, thermoelectric conversion element, thermoelectric conversion module, thermoelectric conversion system, heat transport method, and power generation method

Abstract

This disclosure provides a feature for preventing degradation in performance of a thermoelectric conversion element including Mg and at least one selected from the group consisting of Sb and Bi even when a cutting liquid containing water is used. A method for manufacturing a thermoelectric conversion element according to the present disclosure includes: bringing an alkaline aqueous solution into contact with a wafer (20) that is a workpiece including Mg and at least one selected from the group consisting of Sb and Bi; and cutting the wafer (20) using the alkaline aqueous solution as a cutting liquid (21).

Description

Thermoelectric conversion element manufacturing method, thermoelectric conversion element, thermoelectric conversion module, thermoelectric conversion system, heat transport method, and power generation method

[0001] The present disclosure relates to a method for manufacturing a thermoelectric conversion element, a thermoelectric conversion element, a thermoelectric conversion module, a thermoelectric conversion system, a heat transport method, and a power generation method.

[0002] Thermoelectric power generation and thermoelectric cooling are well known. Thermoelectric power generation is a technology that directly converts thermal energy into electrical energy by utilizing the Seebeck effect, i.e., a thermoelectric power generated between two ends of a material in proportion to the temperature difference between the two ends of the material. Thermoelectric cooling is a technology that utilizes the Peltier effect, i.e., a phenomenon in which heat is transferred by electrons carried by an electric current. Thermoelectric power generation is used, for example, as a power source in remote areas or in space, and thermoelectric cooling is used, for example, to precisely regulate the temperature of electronic or optical devices.

[0003] The efficiency of energy conversion between heat and electricity in thermoelectric power generation and thermoelectric cooling is determined by the figure of merit ZT of the material used. The higher the figure of merit ZT, the higher the energy conversion efficiency of the material, and a semiconductor injected with electrically conductive carriers can have a high figure of merit ZT.

[0004] For example, as described in Patent Document 1, Non-Patent Document 1, and Non-Patent Document 2, a material containing Mg and at least one element selected from the group consisting of Sb and Bi is known as a semiconductor having a high figure of merit ZT.

[0005] Patent No. 6127281

[0006] H. Tamaki et al., Advanced Materials 28, 10182 (2016).A.Li et al. “Chemical stability and degradation mechanism of Mg3Sb2-xBix thermoelectrics towards room-temperature applications”, Acta Materialia, Vol. 239, pp.118301 (2022). [DOI: 10.1016 / j.actamat.2022.118301]

[0007] The present disclosure provides a method for manufacturing a thermoelectric conversion element, in which the performance of the thermoelectric conversion element containing Mg and at least one element selected from the group consisting of Sb and Bi is unlikely to be reduced even when a cutting fluid containing water is used.

[0008] The method for manufacturing a thermoelectric conversion element according to the present disclosure includes bringing an alkaline aqueous solution into contact with a workpiece containing Mg and at least one element selected from the group consisting of Sb and Bi, and cutting the workpiece using the alkaline aqueous solution as a cutting fluid.

[0009] According to the present disclosure, the performance of a thermoelectric conversion element containing Mg and at least one selected from the group consisting of Sb and Bi is unlikely to be reduced even when a cutting fluid containing water is used.

[0010] FIG. 1 is a perspective view showing an example of a thermoelectric conversion module. FIG. 2 is a flowchart showing an example of a manufacturing method for a thermoelectric conversion module. FIG. 3 is a diagram schematically showing a state in which an ingot is sliced ​​with a wire saw. FIG. 4 is a diagram schematically showing a state in which a wafer is diced with a wire saw. FIG. 5 is a diagram schematically showing a state in which a wafer is diced with a blade dicer. FIG. 6 is a cross-sectional view showing an example of a thermoelectric conversion element. FIG. 7 is a cross-sectional view showing another example of a thermoelectric conversion element. FIG. 8 is a side view showing an example of a thermoelectric conversion system. FIG. 9A is a scanning electron microscope (SEM) photograph showing a cross-section of a thermoelectric conversion element according to Example 1A. FIG. 9B is an SEM photograph showing a cross-section of a thermoelectric conversion element according to Comparative Example 1A. FIG. 10 is a graph showing the results of energy dispersive X-ray analysis (SEM-EDX) of the surface layer of the thermoelectric conversion element according to Example 1A.

[0011] (Findings underlying the present disclosure) As described in the above patent documents and non-patent documents, the use of a material containing Mg and at least one element selected from the group consisting of Sb and Bi is expected to improve the performance of thermoelectric conversion elements. When manufacturing a thermoelectric conversion element using such a material, it may be necessary to cut a workpiece containing Mg and at least one element selected from the group consisting of Sb and Bi into a desired shape. Because precise machining accuracy is required for cutting such workpieces, it is considered important to circulate and supply an oil-based or water-based cutting fluid to the cutting area. From the perspective of suppressing oxidation of the workpiece surface, the use of an oil-based cutting fluid is considered desirable. On the other hand, water-based cutting fluids have advantages over oil-based cutting fluids in terms of their resistance to ignition, ease of cleaning, and ease of cooling the workpiece during cutting.

[0012] For example, Non-Patent Document 2 describes that a material containing Mg and at least one selected from the group consisting of Sb and Bi is very sensitive to moisture. Therefore, if a water-based cutting fluid is used to cut a workpiece containing Mg and at least one selected from the group consisting of Sb and Bi, the electrical resistance of the thermoelectric conversion element may increase, possibly resulting in a decrease in the performance of the thermoelectric conversion element.

[0013] In view of these circumstances, the present inventors have conducted extensive research into whether it is possible to suppress a decrease in the performance of a thermoelectric conversion element when a cutting fluid containing water is used to cut a workpiece containing Mg and at least one element selected from the group consisting of Sb and Bi. As a result, it has been newly discovered that the performance of a thermoelectric conversion element is less likely to decrease when a specific aqueous solution is used to cut a workpiece containing Mg and at least one element selected from the group consisting of Sb and Bi. Based on this new finding, the present inventors have completed a thermoelectric conversion element according to the present disclosure.

[0014] (Embodiments of the Present Disclosure) Hereinafter, embodiments of the present disclosure will be described with reference to the drawings.

[0015] 1 is a perspective view showing an example of a thermoelectric conversion module. As shown in Fig. 1, a thermoelectric conversion module 100 includes a thermoelectric conversion element 1a and an electrode 3. The electrode 3 is electrically connected to the thermoelectric conversion element 1a.

[0016] The thermoelectric conversion module 100 is, for example, a thermoelectric conversion module having a π-type structure. The thermoelectric conversion module 100 further includes a thermoelectric conversion element 1b. In the thermoelectric conversion module 100, a plurality of thermoelectric conversion elements 1a and a plurality of thermoelectric conversion elements 1b are alternately arranged, and adjacent thermoelectric conversion elements 1a and thermoelectric conversion elements 1b are electrically connected by electrodes 3. As a result, the plurality of thermoelectric conversion elements 1a and the plurality of thermoelectric conversion elements 1b are electrically connected in series. One of the thermoelectric conversion elements 1a and the thermoelectric conversion elements 1b is an N-type thermoelectric conversion element, and the other of the thermoelectric conversion elements 1a and the thermoelectric conversion elements 1b is a P-type thermoelectric conversion element.

[0017] 1 , the thermoelectric conversion module 100 further includes, for example, a pair of substrates 4. The substrates 4 are, for example, ceramic plates, and the thermoelectric conversion elements 1 a, 1 b, and electrodes 3 are disposed between the pair of substrates 4. For example, the electrodes 3 are patterned on one surface of each substrate 4.

[0018] 2 is a flowchart showing an example of a method for manufacturing a thermoelectric conversion module. In FIG. 2, thermoelectric conversion elements 1a and 1b are obtained through steps S11 to S14. Steps S11 to S14 will be described using thermoelectric conversion element 1a as an example.

[0019] As shown in FIG. 2 , in step S11, an ingot 10 of thermoelectric conversion material is produced. The ingot 10 contains, for example, Mg and at least one selected from the group consisting of Sb and Bi. The method for producing the ingot 10 is not limited to a specific method, and for example, the methods described in Patent Document 1 and Non-Patent Document 1 may be adopted. For example, the ingot 10, which is a sintered body, may be obtained by pressure sintering a powder of a compound containing Mg and at least one selected from the group consisting of Sb and Bi. For example, the ingot 10 may be obtained as a dense sintered body by spark plasma sintering, hot pressing, hot forging, or extrusion. The ingot 10 is, for example, a sintered body having a density of 90% or more of the theoretical density.

[0020] The powder of the compound used to produce the ingot 10 can be prepared, for example, by dissolving Mg and at least one element selected from the group consisting of Sb and Bi, together with other additives as necessary, and pulverizing the resulting compound. The powder of the compound may also be prepared by mechanical alloying.

[0021] Next, in step S12, the ingot 10 is sliced. The ingot 10 is sliced ​​using, for example, a wire saw. FIG. 3 is a schematic diagram illustrating the ingot being sliced ​​using a wire saw. As shown in FIG. 3, the ingot 10 is fixed to a jig 12, for example, with wax 14. In this state, a wire 13 reciprocates at high speed, approaching the jig 12 and slicing the ingot 10. Abrasive grains, such as diamond, alumina, and SiC, are embedded in the wire 13. As shown in FIG. 3, when slicing the ingot 10, a cutting fluid 11 is supplied toward the ingot 10. For example, a nozzle 11a is disposed around the ingot 10, and the cutting fluid 11 is supplied toward the ingot 10 from the nozzle 11a. This removes chips generated by cutting the ingot 10, enabling precise machining of the ingot 10. The cutting fluid 11 may contain abrasive grains. In this case, abrasive grains may or may not be embedded in the wire 13. The cutting fluid 11 does not need to contain abrasive grains.

[0022] For example, ultrasonic cleaning is performed on plate-shaped pieces obtained by slicing the ingot 10 while immersed in a hydrocarbon solution or an aqueous solution containing alcohol, an aqueous solution containing an amine-based organic compound, or ion-exchanged water. This removes wax and cuttings adhering to the pieces. An organic solvent such as acetone may also be used to clean the plate-shaped pieces.

[0023] Next, in step S13, a metal layer 1k is formed on both sides of the plate-shaped piece obtained by slicing the ingot 10, thereby obtaining a wafer 20. The thickness of the wafer 20 is, for example, 0.1 mm or more and 10 mm or less. The metal layer 1k is formed, for example, by sputtering, thermal spraying, or plating. The metal contained in the metal layer 1k is not limited to a specific metal. The metal layer 1k contains, for example, at least one selected from the group consisting of copper, copper alloy, nickel, nickel alloy, iron, and iron alloy. The metal layer 1k may have a single-layer structure or a multi-layer structure. In order to improve wettability with the solder paste or silver paste used for electrical connection with the electrode 3, the outermost surface of the metal layer 1k may contain at least one selected from the group consisting of gold, silver, and tin.

[0024] The thickness of the metal layer 1k is not limited to a particular value and may be, for example, 0.1 μm to 10 μm.

[0025] Next, in step S14, the wafer 20 is diced. Dicing the wafer 20 yields, for example, rectangular parallelepiped pieces. The wafer 20 is diced using, for example, a wire saw or a blade dicer. FIG. 4 is a schematic diagram illustrating the process of dicing the wafer with a wire saw. As shown in FIG. 4, the wafer 20 is fixed to a jig 22, for example, with wax or adhesive tape (not shown). In this state, a wire 23 moves back and forth at high speed toward the jig 22 to diced the wafer 20. Abrasive grains, such as diamond, alumina, and SiC, are embedded in the wire 23. As shown in FIG. 4, during dicing of the wafer 20, a cutting fluid 21 is supplied toward the wafer 20. For example, a nozzle 21a is disposed around the wafer 20, and the cutting fluid 21 is supplied toward the wafer 20 from the nozzle 21a. This removes chips generated by cutting the wafer 20, enabling precise processing of the wafer 20. Abrasive grains may be mixed in the cutting fluid 21. In this case, abrasive grains may or may not be embedded in the wire 23. The cutting fluid 21 does not necessarily have to contain abrasive grains.

[0026] FIG. 5 is a schematic diagram showing a state in which a wafer is diced with a blade dicer. As shown in FIG. 5, a wafer 20 is fixed to the center of a ring frame 25 by a dicing tape 26. In this state, a dicing blade 24 rotates at high speed and moves while cutting the wafer 20, thereby dicing the wafer 20. As shown in FIG. 5, during dicing of the wafer 20, cutting fluid 21 is supplied toward the wafer 20. For example, nozzles 21a are arranged around the wafer 20, and cutting fluid 21 is supplied from the nozzles 21a toward the wafer 20. This removes cutting debris generated by cutting the wafer 20, enabling precise processing of the wafer 20.

[0027] The rectangular parallelepiped pieces obtained by dicing the wafer 20 are washed with, for example, a cleaning solution containing water. This removes wax or cutting chips adhering to the pieces. The pieces are washed, for example, by pouring the cleaning solution over the pieces or by immersing the pieces in the cleaning solution. The washed pieces are dried to obtain the thermoelectric conversion elements 1a.

[0028] Thus, the method for manufacturing the thermoelectric conversion element 1a includes cutting a workpiece containing Mg and at least one element selected from the group consisting of Sb and Bi, such as an ingot 10 or a wafer 20, using a cutting fluid. An alkaline aqueous solution is preferably used as the cutting fluid, and the alkaline aqueous solution is brought into contact with the workpiece. Cutting the workpiece using the cutting fluid may include, for example, forming a coating containing Mg on the surface of the workpiece. Specifically, the coating is the surface portion 1h shown in FIG. 6 (described later). In this case, even if the cutting fluid contains water, the electrical resistance of the thermoelectric conversion element 1a is unlikely to increase, and the performance of the thermoelectric conversion element 1a is likely to be improved. The cutting fluid is, for example, supplied toward the workpiece.

[0029] The recovered cutting fluid from which cutting chips have been removed by filtration or the like may be supplied again toward the workpiece.

[0030] For example, in slicing the ingot 10, the ingot 10 corresponds to the workpiece. The cutting fluid 11 used in slicing the ingot 10 is, for example, an alkaline aqueous solution. In some cases, the cutting fluid 11 may be an oil-based cutting fluid instead of an alkaline aqueous solution.

[0031] For example, in dicing the wafer 20, the wafer 20 corresponds to the workpiece. The cutting fluid 21 used in dicing the wafer 20 is, for example, an alkaline aqueous solution. As described above, the wafer 20 has, for example, a metal layer 1k on its surface.

[0032] When the thermoelectric conversion element 1 a is manufactured, a cutting fluid that is an alkaline aqueous solution may be supplied to the workpiece in cutting the workpiece other than slicing the ingot 10 and dicing the wafer 20 .

[0033] The pH of the alkaline aqueous solution is not limited to a specific value. For example, the alkaline aqueous solution satisfies the condition of 8≦pH≦14. In this case, the electrical resistance of the thermoelectric conversion element 1 a is less likely to increase, and the performance of the thermoelectric conversion element 1 a is more likely to be improved.

[0034] The alkaline aqueous solution preferably satisfies the condition of 9≦pH≦12. In this case, the electrical resistance of the thermoelectric conversion element 1a is less likely to increase, and the performance of the thermoelectric conversion element 1a is more likely to be improved. In addition, the cutting fluid, which is an alkaline aqueous solution, is easy to handle. When the alkaline aqueous solution satisfies the condition of pH≦12, components used in cutting the workpiece, such as the housing of the dicing device, the piping of the dicing device, and the dicing tape 26, are less likely to be deteriorated by the cutting fluid.

[0035] The alkaline aqueous solution contains, for example, water as its main component, and the pH of the alkaline aqueous solution is adjusted to a predetermined range by adding a predetermined additive. In this specification, the term "main component" refers to the component that is contained in the largest amount by mass. The alkaline aqueous solution contains, for example, at least one selected from the group consisting of triethanolamine, diethanolamine, magnesium hydroxide, sodium hydroxide, and potassium hydroxide. In this case, the electrical resistance of the thermoelectric conversion element 1a is less likely to increase, and the performance of the thermoelectric conversion element 1a is likely to be improved.

[0036] The alkaline aqueous solution may optionally contain an oil component. The oil component is, for example, a component that can be used as a base oil in a lubricant. The oil component may be a mineral oil or a synthetic oil.

[0037] As described above, a processed product obtained by cutting a workpiece, such as a rectangular parallelepiped piece obtained by dicing the wafer 20, can be cleaned with a cleaning liquid containing water. In such a case, if a cutting liquid that is an alkaline aqueous solution is used in cutting the workpiece to obtain the processed product, cleaning the processed product with a cleaning liquid containing water is unlikely to increase the electrical resistance of the thermoelectric conversion elements 1a, and the performance of the thermoelectric conversion elements 1a is likely to be improved. The cleaning liquid containing water may be ion-exchanged water or an alkaline aqueous solution.

[0038] As shown in FIG. 2 , in step S15, the thermoelectric conversion module 100 is assembled. The thermoelectric conversion element 1b, which is fabricated separately from the thermoelectric conversion element 1a, is used to assemble the thermoelectric conversion module 100. The thermoelectric conversion element 1b can be fabricated, for example, by the same method as the thermoelectric conversion element 1a. If the workpiece to be machined to obtain the thermoelectric conversion element 1b does not contain Mg and at least one element selected from the group consisting of Sb and Bi, a cutting fluid other than an alkaline aqueous solution may be used as the cutting fluid. For example, a neutral aqueous solution or an oil-based cutting fluid may be used.

[0039] For example, in the thermoelectric conversion module 100, the thermoelectric conversion element 1a is an N-type thermoelectric conversion element and contains, for example, an Mg(Sb,Bi)-based thermoelectric conversion material. (Sb,Bi) means that at least one selected from the group consisting of Sb and Bi is included. The thermoelectric conversion element 1b is, for example, a P-type thermoelectric conversion element. The thermoelectric conversion material included in the P-type thermoelectric conversion element is not limited to a specific material. The P-type thermoelectric conversion element can use, for example, a (Bi,Sb)(Te,Se)-based thermoelectric conversion material, a GeTe-based thermoelectric conversion material, an MgAgSb-based thermoelectric conversion material, or an Mg(Sb,Bi)-based thermoelectric conversion material. (Bi,Sb) means that at least one selected from the group consisting of Bi and Sb is included. (Te,Se) means that at least one selected from the group consisting of Te and Se is included.

[0040] For example, on one substrate 4 on which electrodes 3 are patterned, the metal layer 1k of the thermoelectric conversion element 1a and the metal layer (not shown) of the thermoelectric conversion element 1b are alternately arranged so as to be in contact with the electrodes 3. One substrate 4 is coated with either solder paste, conductive paste, or brazing paste depending on the required module heat resistance temperature and bonding temperature. Next, the other substrate 4 on which electrodes 3 are patterned is placed on the thermoelectric conversion elements 1a and 1b. The other substrate 4 is coated with either solder paste, conductive paste, or brazing paste. In this state, a heat treatment is performed. As a result, the thermoelectric conversion elements 1a and 1b are bonded to the electrodes 3, and the thermoelectric conversion module 100 is obtained.

[0041] 6 is a cross-sectional view showing an example of a thermoelectric conversion element. As shown in FIG. 6, the thermoelectric conversion element 1a includes a thermoelectric conversion body 1g and a surface portion 1h. The thermoelectric conversion body 1g contains a thermoelectric conversion material containing, for example, Mg and at least one selected from the group consisting of Sb and Bi. The surface portion 1h is in contact with a first surface 1m of the thermoelectric conversion body 1g. The atomic ratio r of the content of Mg to the total content of Sb and Bi in the surface portion 1h is h is the atomic ratio r of the Mg content to the total content of Sb and Bi in 1 g of the thermoelectric converter.g In this case, the electrical resistance of the thermoelectric conversion element 1a is less likely to be high, and the thermoelectric conversion element 1a is more likely to have high performance. h and atomic ratio r g is obtained, for example, based on the results of SEM-EDX on a sample including a cross section of the thermoelectric conversion element 1a.

[0042] In the thermoelectric conversion element 1a, the atomic ratio r h is the atomic ratio r g As long as the atomic ratio r h and atomic ratio r g is not limited to a specific value. h and atomic ratio r g For example, 1.01≦r h / r g ≦2.0, 1.02≦r h / r g ≦1.5, or 1.05≦r h / r g The condition of ≦1.3 may be satisfied.

[0043] Atomic ratio r h is, for example, 1.6 or more and 1.9 or less, 1.7 or more and 1.9 or less, or 1.75 or more and 1.85 or less. g is, for example, 1.4 or more and 1.7 or less, 1.45 or more and 1.65 or less, or 1.45 or more and 1.6 or less.

[0044] The surface portion 1h forms a layer along the first surface 1m, for example. The thickness of the surface portion 1h is not limited to a specific value. The thickness is, for example, 10 nm or more and 1000 nm or less. It is considered that the presence of the surface portion 1h makes it difficult for the electrical resistivity of the thermoelectric converter 1g to become high, and the thermoelectric conversion element 1a is likely to have high performance.

[0045] It is believed that the surface portion 1h is formed when the cutting fluid, for example, an alkaline aqueous solution, comes into contact with the workpiece, and therefore it is believed that the surface portion 1h includes a mixed phase mainly composed of an alloy containing Mg and at least one element selected from the group consisting of Sb and Bi, Mg(OH)2, and MgO.

[0046] The atomic ratio r of the Bi content to the Sb content in the surface portion 1h H is, for example, the atomic ratio r of the Bi content to the Sb content in the thermoelectric converter 1 g G In this case, the electrical resistance of the thermoelectric conversion element 1a is less likely to increase, and the thermoelectric conversion element 1a is more likely to have high performance. H and atomic ratio r G is obtained, for example, based on the results of SEM-EDX on a sample including a cross section of the thermoelectric conversion element 1a.

[0047] In the thermoelectric conversion element 1a, the atomic ratio r H and atomic ratio r G is not limited to a specific value. H and atomic ratio r G For example, 1.2≦r H / r G ≦2.2, 1.4≦r H / r G ≦1.9, or 1.5≦r H / r G The condition of ≦1.7 may be satisfied.

[0048] Atomic ratio r H is, for example, 0.3 or more and 2.0 or less, 0.35 or more and 1.2 or less, or 0.4 or more and 0.6 or less. g is, for example, 0.1 or more and 1.1 or less, 0.15 or more and 0.7 or less, or 0.2 or more and 0.4 or less.

[0049] 6, the thermoelectric conversion element 1a includes, for example, a metal layer 1k. The metal layer 1k is in contact with the second surface 1n of the thermoelectric conversion body 1g. The first surface 1m extends along a direction intersecting with the second surface 1n. For example, when the thermoelectric conversion element 1a has a rectangular parallelepiped shape, a pair of metal layers 1k is formed in contact with a pair of second surfaces 1n that are parallel to each other. The first surfaces 1m extend along a direction that perpendicularly intersects with the second surface 1n, for example.

[0050] As described above, the metal layer 1k is formed on a plate-like piece obtained by slicing the ingot 10. The surface portion 1h can be formed, for example, during dicing of the wafer 20, by bringing the cutting fluid 21, which is an alkaline aqueous solution, into contact with the side surface of the wafer 20 exposed by cutting on which the metal layer 1k is not formed.

[0051] FIG. 7 is a cross-sectional view showing another example of a thermoelectric conversion element. As shown in FIG. 7, in the thermoelectric conversion element 1a, the metal layer 1k may have a multi-layer structure. The metal layer 1k includes, for example, a first metal layer 1p, a second metal layer 1q, and a third metal layer 1r, with the first metal layer 1p, the second metal layer 1q, and the third metal layer 1r arranged outward in this order. The first metal layer 1p includes, for example, at least one selected from the group consisting of copper, copper alloy, nickel, nickel alloy, iron, and iron alloy. The second metal layer 1q includes, for example, at least one selected from the group consisting of copper, copper alloy, nickel, nickel alloy, iron, and iron alloy. The third metal layer 1r includes, for example, at least one selected from the group consisting of gold, silver, and tin.

[0052] 7, the thermoelectric conversion element 1a may include, for example, an intermediate layer 1c between the metal layer 1k and the thermoelectric conversion body 1g. The intermediate layer 1c may contain Mg and a component contained in the first metal layer 1p. The intermediate layer 1c may be formed, for example, by diffusion of Mg contained in the thermoelectric conversion body 1g and a component contained in the first metal layer 1p. The thermoelectric conversion element 1a may include, for example, a boundary portion 1d at the boundary between the intermediate layer 1c and the surface portion 1h. The boundary portion 1d is thought to mainly contain, for example, MgO, Mg(OH)2, and an Mg-containing alloy.

[0053] According to the thermoelectric conversion module 100, heat can be transported by passing a current through the thermoelectric conversion module 100. When a current is generated in the thermoelectric conversion module 100, the Peltier effect generated in the thermoelectric conversion elements 1a and 1b causes heat to be transported between the pair of substrates 4, creating a temperature difference between these substrates. This allows, for example, cooling of an object.

[0054] Electric power can also be generated by generating a temperature difference using heat from a heat source in the thermoelectric conversion module 100. In this case, the temperature difference causes the Seebeck effect in the thermoelectric conversion elements 1a and 1b, generating an electromotive force.

[0055] Fig. 8 is a side view showing an example of a thermoelectric conversion system. As shown in Fig. 8, the thermoelectric conversion system includes a thermoelectric conversion module 100 and a heat source 6. The heat source 50 is disposed in contact with one of the substrates 4, for example.

[0056] According to the thermoelectric conversion system 300, for example, heat from the heat source 6 can be used to generate a temperature difference in the thermoelectric conversion module 100, thereby generating electric power.

[0057] (Additional Note) From the above description, the following techniques are disclosed.

[0058] (Technology 1) A method for manufacturing a thermoelectric conversion element, comprising: bringing an alkaline aqueous solution into contact with a workpiece containing Mg and at least one element selected from the group consisting of Sb and Bi; and cutting the workpiece while using the alkaline aqueous solution as a cutting fluid.

[0059] (Technology 2) The method for producing a thermoelectric conversion element according to Technology 1, wherein the alkaline aqueous solution satisfies the condition of 8≦pH≦14.

[0060] (Technology 3) The method for manufacturing a thermoelectric conversion element according to Technology 1 or 2, wherein cutting the workpiece further includes forming a coating containing Mg on a surface of the workpiece.

[0061] (Technology 4) The method for producing a thermoelectric conversion element according to any one of Technologies 1 to 3, wherein the alkaline aqueous solution contains at least one selected from the group consisting of triethanolamine, diethanolamine, magnesium hydroxide, sodium hydroxide, and potassium hydroxide.

[0062] (Technology 5) The method for manufacturing a thermoelectric conversion element according to any one of Technologies 1 to 4, further comprising washing a processed product obtained by cutting the workpiece with a cleaning solution containing water, and drying the processed product.

[0063] (Technology 6) The method for manufacturing a thermoelectric conversion element according to any one of Technologies 1 to 5, wherein the workpiece is a wafer.

[0064] (Technology 7) The method for manufacturing a thermoelectric conversion element according to Technology 6, wherein the wafer has a metal layer on a surface thereof.

[0065] (Technology 8) The method for manufacturing a thermoelectric conversion element according to any one of Technologies 1 to 7, wherein the workpiece is an ingot of a sintered body containing Mg and at least one selected from the group consisting of Sb and Bi.

[0066] (Technology 9) A thermoelectric conversion element comprising: a thermoelectric conversion body including a thermoelectric conversion material containing Mg and at least one selected from the group consisting of Sb and Bi; and a surface portion in contact with a first surface of the thermoelectric conversion body, wherein an atomic ratio of the Mg content to the total content of Sb and Bi in the surface portion is higher than an atomic ratio of the Mg content to the total content of Sb and Bi in the thermoelectric conversion body.

[0067] (Technology 10) The thermoelectric conversion element according to Technology 9, further comprising a metal layer in contact with a second surface of the thermoelectric conversion body, wherein the first surface extends along a direction intersecting the second surface.

[0068] (Technology 11) The thermoelectric conversion element according to Technology 9 or 10, wherein an atomic ratio of the Bi content to the Sb content in the surface portion is higher than an atomic ratio of the Bi content to the Sb content in the thermoelectric conversion body.

[0069] (Technology 12) A thermoelectric conversion module comprising: the thermoelectric conversion element according to any one of Technologies 9 to 11; and an electrode electrically connected to the thermoelectric conversion element.

[0070] (Technology 13) A thermoelectric conversion system comprising: the thermoelectric conversion module according to Technology 12; and a heat source disposed in contact with the electrodes.

[0071] (Technology 14) A heat transport method, comprising: transporting heat from a heat source by passing an electric current through the thermoelectric conversion module according to Technology 12.

[0072] (Technology 15) A power generation method, comprising generating electric power by generating a temperature difference using heat from a heat source in the thermoelectric conversion module according to Technology 12.

[0073] The present disclosure will be described in detail below with reference to examples, but the thermoelectric conversion element of the present disclosure is not limited to the examples shown below.

[0074] (Ingot Production) Two types of MgSb-based ingots having compositions A and B were produced by the following procedure. Composition A was Mg 3.2 Sb 1.5 Bi 0.49 Te 0.01 and composition B is Mg 3.1 Mn 0.01 Sb 1.0 Bi 0.997 Te 0.003 The formula was expressed as follows. Predetermined amounts of granular antimony, bismuth, magnesium, tellurium, and manganese were weighed and placed in a carbon crucible, and heat-treated for 10 seconds at a temperature of 800 to 1200°C by high-frequency induction heating in an argon atmosphere. This resulted in a melt. The melt was pulverized in a mortar to obtain a compound powder having the desired composition. The compound powder was sintered in an argon atmosphere by hot pressing to obtain an MgSb-based ingot. The maximum temperature reached by hot pressing and the duration at that temperature were 850°C and 10 minutes, respectively, to obtain an MgSb-based ingot having composition A. The maximum temperature reached by hot pressing and the duration at that temperature were 800°C and 10 minutes, respectively, to obtain an MgSb-based ingot having composition B. These MgSb-based ingots were cylindrical with a diameter of 30 mm and a thickness of 20 mm.

[0075] (Ingot Slicing and Plating) Each of the MgSb-based ingots described above was fixed to a jig with wax, and the MgSb-based ingot was sliced ​​using a wire saw as shown in FIG. 3 to obtain disk-shaped pieces with a thickness of 1.2 mm. When slicing the MgSb-based ingot, an oil-based cutting fluid containing a lubricating base oil as a main component was supplied toward the ingot as a cutting fluid, and the cutting fluid passed over or near the cut surface of the ingot. The disk-shaped pieces were washed with acetone and then dried. Then, a 3 μm Cu layer, a 1 μm Ni layer, and a 0.01 μm Au layer were formed on both sides of the disk-shaped pieces by plating. The Cu layer, Ni layer, and Au layer were arranged in this order toward the outside of the disk-shaped pieces. In this manner, a wafer having a metal layer on its surface was obtained.

[0076] (Wafer Dicing) The wafer was fixed to a jig with dicing tape and diced using a blade dicer into rectangular parallelepiped pieces with square sides measuring 1 mm in plan view. During wafer dicing, cutting fluid was supplied toward the wafer. The cutting fluid passed through the cut surface of the wafer or its vicinity. A circulating cutting fluid stored in a bath was used. Four types of liquids shown in Table 1 were compared and evaluated as cutting fluids. Thermoelectric conversion elements according to each example and comparative example were fabricated in this manner. Furthermore, a thermoelectric conversion element according to a reference example was fabricated using only an oil-based cutting fluid containing no water instead of the liquid shown in Table 1.

[0077]

[0078] (Electrical Resistance of Thermoelectric Conversion Element) The electrical resistance of the thermoelectric conversion elements according to each Example, Comparative Example, and Reference Example was measured at room temperature between 15°C and 25°C using a four-terminal measurement method. This measurement was performed on 10 samples for each Example, Comparative Example, and Reference Example, and the electrical resistance value of the thermoelectric conversion element according to each Example, Comparative Example, and Reference Example was determined as the average of the measured values ​​for the 10 samples. The results are shown in Table 2. In Table 2, the ratio of the electrical resistance of the thermoelectric conversion element according to the Reference Example indicates the ratio of the electrical resistance of the thermoelectric conversion element according to the Example or Comparative Example to the electrical resistance of the thermoelectric conversion element according to the Reference Example obtained from an ingot having the same composition. It can be seen from Comparative Examples 1A and 1B that when ion-exchanged water with a pH of 7 was used as the cutting fluid for dicing, the electrical resistance of the resulting thermoelectric conversion element increased by 10% or more compared to the electrical resistance of the thermoelectric conversion element according to the Reference Example. On the other hand, according to each example, it can be seen that when an alkaline aqueous solution is used as the cutting fluid in dicing, the electrical resistance of the resulting thermoelectric conversion element remains at 3% or less of the electrical resistance of the thermoelectric conversion element according to the reference example.

[0079]

[0080] (Cross-sectional structure of thermoelectric conversion element) The cross sections of the thermoelectric conversion elements according to Example 1A and Comparative Example 1A, perpendicular to the metal layer, were exposed, and the cross sections were smoothed using an ion milling device. Then, cross-sectional observation and composition analysis by energy dispersive X-ray spectroscopy (EDX) were performed using a Hitachi High-Technologies Corporation SU8220 field emission scanning electron microscope (FE-SEM). Figure 9A is an SEM photograph showing a cross section of the thermoelectric conversion element according to Example 1A. Figure 9B is an SEM photograph showing a cross section of the thermoelectric conversion element according to Comparative Example 1A. Figure 9A confirms the presence of a surface layer with a thickness of approximately 300 nm on the surface of the thermoelectric conversion element, which has a composition different from the composition of the central portion of the thermoelectric conversion element. The surface layer extended in a direction perpendicular to the metal layer.

[0081] In the cross section of the thermoelectric conversion element shown in FIG. 9A, the composition ratio in atomic percent was analyzed by SEM-EDX at measurement points P1, P2, P3, P4, P5, and P6. The atomic ratios Mg / (Sb+Bi) and Bi / Sb at these measurement points are shown in Table 3. FIG. 10 is a graph showing the results of SEM-EDX on the surface layer of the thermoelectric conversion element according to Example 1A. In FIG. 10, the vertical axis represents the count number, and the horizontal axis represents energy [eV]. As shown in FIG. 10, a relatively high concentration of oxygen was detected in the surface layer. Therefore, it is believed that a mixture of Mg3(Sb, Bi)2, MgO, and Mg(OH)2 was formed in the surface layer.

[0082] As shown in Table 3, the atomic ratio Mg / (Sb+Bi) at measurement points P1, P2, and P3, which belong to the surface layer, is greater than the atomic ratio Mg / (Sb+Bi) at measurement points P4, P5, and P6, which belong to the base of the thermoelectric conversion element. In addition, the atomic ratio Bi / Sb at measurement points P1, P2, and P3 is greater than the atomic ratio Bi / Sb at measurement points P4, P5, and P6. Therefore, it is believed that the use of an alkaline aqueous solution as a cutting fluid during dicing suppressed the reaction of excessive Mg dissolution into the cutting fluid, resulting in a small amount of Sb dissolution. This results in a higher Mg content in the surface layer than in the base, and it is believed that the surface layer functions as a protective layer that suppresses a decrease in the electrical resistance of the base.

[0083]

[0084] 9B, the composition ratio in atomic % was analyzed by SEM-EDX at measurement points P11, P12, P13, P14, P15, and P16. The atomic ratios Mg / (Sb+Bi) and Bi / Sb at these measurement points are shown in Table 4.

[0085] According to FIG. 9B and Table 4, in the thermoelectric conversion element of Comparative Example 1A, a surface layer with a large atomic ratio Mg / (Sb+Bi) as seen in Example 1A was not observed. Instead, voids were observed inside the thermoelectric conversion element of Comparative Example 1A. As shown in Table 4, the atomic ratio Mg / (Sb+Bi) at measurement points P13 and P14 around the voids was small, suggesting that the amount of Mg was reduced in the area around the voids. The area where the amount of Mg was reduced, including measurement points P13 and P14, was confirmed at a position corresponding to a depth of approximately 1 to 50 μm from the surface of the thermoelectric conversion element. It is believed that moisture that penetrated into specific parts of the thermoelectric conversion element of Comparative Example 1A caused a corrosion reaction of the solid components of the element, resulting in the high electrical resistance of the thermoelectric conversion element of Comparative Example 1A.

[0086] 9B, in the thermoelectric conversion element of Comparative Example 1A, measurement points P15 and P16 were located inside the thermoelectric conversion element, and measurement points P11, P12, P13, and P14 were located in a region closer to the surface of the thermoelectric conversion element than the region including measurement points P15 and P16. As shown in Table 4, the atomic ratio Mg / (Sb+Bi) at measurement points P11, P12, P13, and P14 of the thermoelectric conversion element of Comparative Example 1A was lower than the atomic ratio Mg / (Sb+Bi) at internal measurement points P15 and P16.

[0087]

[0088] The thermoelectric conversion elements of the present disclosure can be used in a variety of applications, including applications for conventional thermoelectric conversion elements.

[0089] REFERENCE SIGNS LIST 1a thermoelectric conversion element 1g thermoelectric conversion body 1h surface portion 1k metal layer 1m first surface 1n second surface 3 electrode 10 ingot 11 cutting fluid 20 wafer 21 cutting fluid 100 thermoelectric conversion module 300 thermoelectric conversion system

Claims

1. A method for manufacturing a thermoelectric conversion element, comprising bringing an alkaline aqueous solution into contact with a workpiece containing Mg and at least one selected from the group consisting of Sb and Bi, and cutting the workpiece while using the alkaline aqueous solution as a cutting fluid.

2. The method for manufacturing a thermoelectric conversion element according to claim 1, wherein the alkaline aqueous solution satisfies the condition of 8 ≤ pH ≤ 14.

3. The method for manufacturing a thermoelectric conversion element according to claim 1, wherein cutting the workpiece further includes forming a film containing Mg on the surface of the workpiece.

4. The method for manufacturing a thermoelectric conversion element according to claim 1, wherein the alkaline aqueous solution contains at least one selected from the group consisting of triethanolamine, diethanolamine, magnesium hydroxide, sodium hydroxide, and potassium hydroxide.

5. The method for manufacturing a thermoelectric conversion element according to claim 1, including washing the processed product obtained by cutting the workpiece with a cleaning liquid containing water and drying the processed product.

6. The method for manufacturing a thermoelectric conversion element according to claim 1, wherein the workpiece is a wafer.

7. The method for manufacturing a thermoelectric conversion element according to claim 6, wherein the wafer has a metal layer on its surface.

8. The method for manufacturing a thermoelectric conversion element according to claim 1, wherein the workpiece is an ingot of a sintered body containing Mg and at least one selected from the group consisting of Sb and Bi.

9. A thermoelectric conversion element, comprising a thermoelectric conversion body including a thermoelectric conversion material containing Mg and at least one selected from the group consisting of Sb and Bi, and a surface portion in contact with a first surface of the thermoelectric conversion body, wherein an atomic ratio of the content of Mg to the total content of Sb and Bi in the surface portion is higher than an atomic ratio of the content of Mg to the total content of Sb and Bi in the thermoelectric conversion body.

10. The thermoelectric conversion element according to claim 9, further comprising a metal layer in contact with a second surface of the thermoelectric conversion body, wherein the first surface extends along a direction intersecting the second surface.

11. The thermoelectric conversion element according to claim 9, wherein an atomic ratio of the content of Bi to the content of Sb in the surface portion is higher than an atomic ratio of the content of Bi to the content of Sb in the thermoelectric conversion body.

12. A thermoelectric conversion module comprising the thermoelectric conversion element according to any one of claims 9 to 11 and an electrode electrically connected to the thermoelectric conversion element.

13. A thermoelectric conversion system comprising the thermoelectric conversion module according to claim 12 and a heat source disposed in contact with the electrode.

14. A heat transport method including transporting heat from a heat source by passing a current through the thermoelectric conversion module according to claim 12.

15. A power generation method including generating a temperature difference by heat from a heat source in the thermoelectric conversion module according to claim 12 to generate electric power.

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