Thermoelectric conversion element, thermoelectric conversion module, thermoelectric conversion system, power generation method, thermoelectric conversion material, and method for manufacturing thermoelectric conversion element

Abstract

Provided is a thermoelectric conversion element which is advantageous from the perspective of reducing the resistance of the thermoelectric conversion element. This thermoelectric conversion element (10a) is provided with a thermoelectric conversion layer (11) and a metal layer (12). The thermoelectric conversion layer (11) contains a thermoelectric conversion material that contains Mg and at least one selected from the group consisting of Sb and Bi. The thermoelectric conversion layer (11) has a first surface (11a) in contact with the metal layer (12). The cross sections of the thermoelectric conversion layer 11 and the metal layer 12 are observed in the thickness direction of the metal layer 12 according to an energy dispersive X-ray analysis (EDX). In this case, the arithmetic average roughness Ra of the first surface 11a satisfies the condition represented by Ra < = 4.2 [mu] m.

Description

Technical Field

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

[0002] Conventionally known thermoelectric conversion elements are known. For example, a thermoelectric conversion module can be provided by electrically connecting an N-type thermoelectric conversion element containing an N-type thermoelectric conversion material and a P-type thermoelectric conversion element containing a P-type thermoelectric conversion material. In order to facilitate the electrical bonding of the thermoelectric conversion element, it is known to form a layer of a metal material on the end face portion of the thermoelectric conversion material. If the end face portion of the thermoelectric conversion material is formed of a layer of a metal material, the processing and assembly of the thermoelectric conversion element are facilitated.

[0003] For example, Patent Document 1 describes a magnesium-antimony-based thermoelectric element. The thermoelectric element includes: a magnesium-antimony-based thermoelectric material matrix layer located at the center of the thermoelectric element; intermediate layers attached to both surfaces of the matrix layer; and electrode layers attached to the surfaces of the two intermediate layers. The material of the intermediate layer is magnesium copper alloy and / or magnesium aluminum alloy. The intermediate layer is formed by magnetron sputtering or spraying method.

[0004] Patent Document 2 describes a magnesium-antimony-based thermoelectric element. The thermoelectric element includes: a magnesium-antimony-based thermoelectric material matrix layer located at the center of the thermoelectric element; intermediate layers attached to both surfaces of the matrix layer; and electrode layers attached to the surfaces of the two intermediate layers. The material of the intermediate layer is titanium copper alloy or magnesium copper alloy. The intermediate layer is formed by ion beam sputtering method or magnetron sputtering method.

[0005] Patent Document 3 describes a thermoelectric conversion element including an electrode, an intermediate layer, and a thermoelectric conversion layer. The intermediate layer is provided between the thermoelectric conversion layer and the electrode. The intermediate layer is in contact with the thermoelectric conversion layer. The thermoelectric conversion layer contains at least one element selected from Mg, Sb, and Bi and at least one element selected from Se and Te. The electrode is made of a CuZn alloy. The composition of the intermediate layer is different from the composition of the electrode and the composition of the thermoelectric conversion layer. The intermediate layer contains Cu, Zn, and Mg. The intermediate layer is formed by spark plasma sintering method (SPS).

[0006] Patent Document 4 describes reducing the interfacial resistance by roughening the surfaces of the thermoelectric element and the electrode. Examples of the material of the thermoelectric element are bismuth telluride, cesium bismuth telluride, and bismuth antimony.

[0007] Non-Patent Document 1 describes the contact resistivity at the contact interface between thermoelectric boundary materials such as Fe, Ni, Mg, Cu, Al, Zn, Ag, and Ti and Mg3Sb 1.5 Bi 0.5 and Bi.

[0008] Prior Art Documents

[0009] Patent document

[0010] Patent document 1: Specification of Chinese Patent No. 111613715

[0011] Patent document 2: Specification of Chinese Patent No. 110635020

[0012] Patent document 3: International Publication No. 2020 / 003554

[0013] Patent document 4: Japanese Patent Publication No. 2022-523127

[0014] Non-patent document

[0015] Non-patent document 1: X. Wu et.al., Acta Mater. 226 117616(2022). Summary of the invention

[0016] The present disclosure provides a thermoelectric conversion element that is advantageous from the viewpoint of reducing the resistance of the thermoelectric conversion element.

[0017] The thermoelectric conversion element of the present disclosure includes a thermoelectric conversion layer and a metal layer. The thermoelectric conversion layer contains a thermoelectric conversion material, the thermoelectric conversion material contains Mg and at least one selected from Sb and Bi, and the thermoelectric conversion layer has a first surface in contact with the metal layer.

[0018] When observing the cross section of the thermoelectric conversion layer and the metal layer along the thickness direction of the metal layer by energy dispersive X-ray analysis method, the arithmetic mean roughness Ra of the first surface satisfies a first condition expressed by Ra≤4.2μm.

[0019] According to the present disclosure, it is possible to provide a thermoelectric conversion element that is advantageous from the viewpoint of reducing the resistance of the thermoelectric conversion element. Brief description of the drawings

[0020] Figure 1 is a schematic cross-sectional view showing the thermoelectric conversion element of Embodiment 1.

[0021] Figure 2 is a schematic view showing the crystal structure of the La2O3 type.

[0022] Figure 3 is a flowchart showing an example of the process for calculating the arithmetic mean roughness Ra.

[0023] Figure 4 is an example of an image showing the distribution of Mg in the cross section of the thermoelectric conversion element.

[0024] Figure 5is obtained from the Figure 4 binary image after noise removal processing shown in the figure.

[0025] Figure 6 is a flowchart showing a method for manufacturing a thermoelectric conversion element according to Embodiment 1.

[0026] Figure 7 is a cross-sectional view showing an example of a thermoelectric conversion module according to Embodiment 2.

[0027] Figure 8 is a cross-sectional view showing another example of a thermoelectric conversion module according to Embodiment 2.

[0028] Figure 9 is a cross-sectional view showing still another example of a thermoelectric conversion module according to Embodiment 2.

[0029] Figure 10 is a side view showing a thermoelectric conversion system according to Embodiment 3.

[0030] Figure 11 is a diagram schematically showing a method for measuring the resistance of a thermoelectric conversion element.

[0031] Figure 12 is a diagram showing an example of the measurement result of the resistance of a thermoelectric conversion element according to Example 1.

[0032] Figure 13 is a photograph of a thermoelectric conversion element according to Example 1.

[0033] Figure 14 is a photograph showing the peeling of a coating film in Comparative Example 3. Detailed implementation mode

[0034] (Insight on which the present disclosure is based)

[0035] It is possible to consider forming a metal layer by methods such as electrolytic plating and electroless plating. Generally, in the bonding of dissimilar materials, by roughening the interface of the dissimilar materials, an anchoring effect can be exerted, and an improvement in the adhesion of the dissimilar materials can be expected.

[0036] For example, it is possible to consider forming a metal layer containing Cu or the like on the end face of a thermoelectric conversion layer containing a thermoelectric conversion material by a method such as plating, and the thermoelectric conversion material contains Mg and at least one selected from Sb and Bi. In this case, according to the research of the present inventors, if the arithmetic mean roughness Ra of the end face of the thermoelectric conversion layer is large, the interface resistance at the interface between the thermoelectric conversion layer and the metal layer tends to increase. In addition, it was found that if the arithmetic mean roughness Ra of the end face of the thermoelectric conversion layer is large, the adhesion of the metal layer to the thermoelectric conversion layer decreases, and peeling of the metal layer occurs.

[0037] According to Patent Documents 1, 2, and 3, several materials are described as the metal layer in contact with the end face of the thermoelectric conversion layer containing a thermoelectric conversion material containing Mg and at least one selected from Sb and Bi. However, there is no research on the surface state of the end face of the thermoelectric conversion layer in contact with the metal layer. Patent Document 4 describes that the interface resistance can be reduced by roughening the surface of a thermoelectric element containing a material such as bismuth telluride. Although Non-Patent Document 1 describes that thermoelectric boundary materials such as Fe, Ni, Mg, Cu, Al, Zn, Ag, and Ti are in contact with Mg3Sb 1.5 Bi 0.5 The contact resistivity at the contact interface is 1.5 Bi 0.5 The surface state was not studied.

[0038] Therefore, the inventors have repeatedly conducted a large number of exploratory tests to solve the above-mentioned problem in the case where a metal layer including Cu or the like is formed on the end surface of a thermoelectric conversion layer including a thermoelectric conversion material containing Mg and at least one selected from Sb and Bi. As a result, it has been newly discovered that by adjusting the roughness of the first surface of the thermoelectric conversion layer in contact with the metal layer, the interface resistance at the interface between the thermoelectric conversion layer and the metal layer becomes lower, which is advantageous from the perspective of reducing the resistance of the thermoelectric element. Based on this new insight, the inventors have completed the thermoelectric conversion element disclosed in the present invention.

[0039] (Embodiments of the present disclosure)

[0040] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings.

[0041] (Implementation Method 1)

[0042] Figure 1 Schematically showing a cross-sectional view of a thermoelectric conversion element according to Embodiment 1. Figure 1 As shown, the thermoelectric conversion element 10a includes a thermoelectric conversion layer 11 and a metal layer 12. The thermoelectric conversion layer 11 contains a thermoelectric conversion material, and the thermoelectric conversion material contains Mg and at least one selected from Sb and Bi. The thermoelectric conversion layer 11 has a first surface 11a in contact with the metal layer 12. The cross-section of the thermoelectric conversion layer 11 and the metal layer 12 is observed along the thickness direction of the metal layer 12 according to the energy dispersive X-ray analysis method (EDX). In this case, the arithmetic mean roughness Ra of the first surface 11a satisfies the condition represented by Ra≤4.2μm. As a result, for example, the interface resistivity at the interface between the thermoelectric conversion layer 11 and the metal layer 12 is likely to be lowered. In addition, when the metal layer 12 is formed by a method such as plating, the adhesion of the metal layer 12 to the thermoelectric conversion layer 11 is likely to be high, and it is difficult for the metal layer 12 to peel off.

[0043] The arithmetic mean roughness Ra of the first surface 11a is not limited to a specific value as long as the above conditions are satisfied. The arithmetic mean roughness Ra of the first surface 11a is, for example, 0.05 μm or more. In this case, the time required to adjust the arithmetic mean roughness Ra of the first surface 11a can be shortened. The arithmetic mean roughness Ra can be 0.1 μm or more, can be 0.15 μm or more, and can be 0.2 μm or more.

[0044] The arithmetic mean roughness Ra of the first surface 11a can also be within the range defined by the following upper limit value and the following lower limit value. The upper limit value is, for example, a value selected from 4.2 μm, 4.0 μm, 3.5 μm, 3.0 μm, 2.5 μm, and 2.0 μm. The lower limit value is a value selected from 0.05 μm, 0.1 μm, 0.15 μm, and 0.2 μm.

[0045] The shape of the thermoelectric conversion element 10a is not limited to a specific shape. The thermoelectric conversion element 10a is, for example, substantially rectangular parallelepiped-shaped. The shape of the thermoelectric conversion element 10a only needs to be a shape capable of forming the thermoelectric conversion layer 11 containing the thermoelectric conversion material, and can be, for example, rectangular parallelepiped-shaped, cubic-shaped, substantially cubic-shaped, other prismatic-shaped, substantially prismatic-shaped, cylindrical-shaped, or tubular-shaped.

[0046] The thickness of the thermoelectric conversion layer 11 is not limited to a specific value. Its thickness is, for example, 0.1 mm or more and 5.0 mm or less.

[0047] The thermoelectric conversion material contained in the thermoelectric conversion layer 11 is not limited to a specific material as long as it contains Mg and at least one of Sb and Bi. The thermoelectric conversion material contained in the thermoelectric conversion layer 11 is, for example, an N-type thermoelectric conversion material.

[0048] The thermoelectric conversion material contained in the thermoelectric conversion layer 11, for example, has a La2O3-type crystal structure. In this case, the thermoelectric conversion element 10a is likely to exhibit the desired thermoelectric conversion characteristics. Whether the thermoelectric conversion material has a La2O3-type crystal structure can be determined, for example, based on the X-ray diffraction measurement results of a sample of the thermoelectric conversion material. Figure 2 is a diagram schematically showing the La2O3-type crystal structure. When the thermoelectric conversion material has a La2O3-type crystal structure, for example, Mg is located at Figure 2 the C1 site shown, and at least one of Sb and Bi is located at the C2 site. As Figure 2 shown, a bond as indicated by a dashed line is formed between the C1 site and the C2 site.

[0049] The thermoelectric conversion material, for example, also contains Te. In this case, the thermoelectric conversion element 10a is more likely to exhibit the desired thermoelectric conversion characteristics.

[0050] Thermoelectric conversion materials, for example, have a composition represented by Mg 3+m R a T b Sb 2-e-c Bi c Z e In this composition, element R is at least one element selected from Ca, Sr, Ba, and Yb. Element T is at least one element selected from Mn and Zn. Element Z is at least one element selected from Te, Se, Sc, Y, and La. The value of m satisfies -0.39 ≤ m ≤ 0.42. The value of a satisfies 0 ≤ a ≤ 0.12. The value of b satisfies 0 ≤ b ≤ 0.48. The value of c satisfies 0 ≤ c ≤ 1.6. The value of e satisfies 0.001 ≤ e ≤ 0.06. In this case, the thermoelectric conversion element 10a is more likely to exhibit the desired thermoelectric conversion characteristics.

[0051] As Figure 1 shown, the metal layer 12 forms, for example, the end face of the thermoelectric conversion element 10a. The thermoelectric conversion element 10a includes, for example, a first metal layer 12a and a second metal layer 12b as the metal layer 12, and the first metal layer 12a and the second metal layer 12b are arranged so as to form both end faces of the thermoelectric conversion element 10a in the thickness direction of the metal layer 12. The thickness of the metal layer 12 is not limited to a specific value. Its thickness is, for example, 0.5 μm or more and 100 μm or less, preferably 0.5 μm or more and 10 μm or less. The thicknesses of the respective metal layers 12 may be the same or different.

[0052] The metal contained in the metal layer 12 is not limited to a specific metal. The metal layer 12 contains, for example, Cu. In the metal layer 12, Cu may exist in the form of a single substance or in the form of an alloy containing Cu. In this case, the thermoelectric conversion element 10a is more likely to exhibit the desired thermoelectric conversion characteristics.

[0053] Metal diffusion may also occur between the metal layer 12 and the thermoelectric conversion layer 11. For example, when the metal layer 12 contains an alloy containing Cu, the alloy may contain Cu and at least one of Mg, Sb, Bi, Te, and Zn. Mg, Sb, Bi, Te, and Zn are derived from the thermoelectric conversion layer 11. The portion generated by the metal diffusion between the metal layer 12 and the thermoelectric conversion layer 11 may be formed only in one of the first metal layer 12a and the second metal layer 12b, or may be formed in both the first metal layer 12a and the second metal layer 12b.

[0054] The thermoelectric conversion element 10a may also include another layer such as an electrode layer on a surface that does not contact the first surface 11a in the thickness direction of the metal layer 12 for the purpose of bonding to a circuit and preventing oxidation of the electrode, etc. The other layer may be a single layer or multiple layers. An example of the other layer is a layer having a structure in which a Ni layer and an Au layer are sequentially formed on the metal layer 12 in the thickness direction of the metal layer 12. In the electrode layer formed on the first metal layer 12a or the second metal layer 12b, the composition of the material, the layer structure, and the thickness of each layer may be the same or different. A diffusion layer may or may not exist between the layers.

[0055] Figure 3 FIG. is a flowchart showing an example of a process for calculating the arithmetic mean roughness Ra of the first surface 11a of the thermoelectric conversion layer 11. As Figure 3 shown, in step S1, the cross-section of the thermoelectric conversion layer 11 and the metal layer 12 is observed in the thickness direction of the metal layer 12 according to energy dispersive X-ray analysis (EDX) to obtain an EDX image of a single element. The EDX image can be obtained, for example, using a field emission scanning electron microscope (FE-SEM) (model: SU8220) manufactured by Hitachi High-Tech Corporation and QUANTAX FlatQUAD manufactured by Bruker AXS. A process of smoothing the cross-section of the thermoelectric conversion layer 11 and the metal layer 12 using a method such as ion milling can also be performed before EDX.

[0056] In step S1, the single element selected to obtain the EDX image is not limited to a specific element. For example, the single element may be Mg, Sb, or Bi that can be contained in the thermoelectric conversion layer 11, or Cu that can be contained in the metal layer 12. The size of one pixel of the EDX image is, for example, half or less of the arithmetic mean roughness Ra of the first surface 11a. In addition, the size of the field of view of the EDX image is, for example, sufficiently large compared to the arithmetic mean roughness Ra of the first surface 11a. The representative size of the field of view of the EDX image is preferably 10 times or more, more preferably 100 times or more, the value of the arithmetic mean roughness Ra of the first surface 11a. Figure 4 FIG. is an example of an image showing the distribution of Mg in the cross-section of the thermoelectric conversion element obtained by EDX.

[0057] The processing after step S2 can be performed using, for example, OpenCV of the Python library. In step S2, the EDX image obtained in step S1 is subjected to a grayscale conversion process to obtain a grayscale image. Next, in step S3, the grayscale image is binarized based on a specified threshold value regarding brightness to obtain a binary image. For example, in the case where the EDX image is an image representing the distribution of Mg as a single element, a threshold value regarding brightness is selected so that the boundary between the part containing Mg and the part not containing Mg becomes clear. Next, in step S4, a noise removal process is performed on the obtained binary image. The noise removal process can be performed, for example, in such a way that small white dots in the black part or small black dots in the white part are eliminated. As the noise removal process, a pair of processes consisting of a dilation process and a contraction process, called a closing / opening process, performed once or twice is preferably used. Figure 5 is the binary image after the noise removal process obtained from the Figure 4 image shown.

[0058] Next, in step S5, the interface coordinate point sequence is obtained using the binary image after the noise removal process. For example, in the Figure 5 binary image shown, the coordinate in actual size units of each pixel in the left - right direction (horizontal) is defined as x, and the coordinate in actual size units of each pixel in the up - down direction is defined as y. Among each x n value (n = 1, 2,..., N; N is the number of horizontal pixels), the boundary position y n is determined to obtain the interface coordinate point sequence. In the case where multiple boundary positions y n are determined for the same x n value, in accordance with the idea of the stylus - type evaluation method in Japanese Industrial Standards JIS B 0601:2013 and JIS B 0633:2001, one boundary position y n is selected in the determination of the interface coordinate point sequence. For example, in the case where the EDX image is an image regarding Mg, Sb, or Bi, the y n value closest to the metal layer 12 among the multiple boundary positions y n is selected in the determination of the interface coordinate point sequence. On the other hand, in the case where the EDX image is an image regarding Cu, the y n value closest to the thermoelectric conversion layer 11 among the multiple boundary positions y n is selected in the determination of the interface coordinate point sequence.

[0059] Next, in step S6, using the interface coordinate point sequence obtained in step S5, the arithmetic mean roughness Ra of the first surface 11a is calculated according to the following calculation formula (1). The function f(x) is expressed as f(x)=ax + b, and by using a linear function for the interface coordinate point sequence (xn , y n ) is approximated. The function f(x) is a function for correcting the deviation from the horizontal state in the case of an EDX image that is not completely horizontal due to the influence of the method of selecting the observation field of view. For example, the arithmetic mean roughness Ra of the first surface 11a is determined in this way. It is also possible to determine the arithmetic mean roughness Ra of the first surface 11a as the average value of the calculated values obtained by calculating the arithmetic mean roughness Ra for each of a plurality of portions of a cross section extending in the same direction of one thermoelectric conversion element 10a.

[0060]

[0061] An example of the correspondence relationship between the arithmetic mean roughness Ra determined based on the EDX image and the arithmetic mean roughness Ra measured according to the above JIS standard is shown in Table 1. Furthermore, the arithmetic mean roughness Ra based on the JIS standard is the measured value for the end face of the thermoelectric conversion layer 11 corresponding to the first surface 11a before the formation of the metal layer 12. According to Table 1, there is a correlation function of 0.9 or more between the arithmetic mean roughness Ra determined based on the EDX image and the arithmetic mean roughness Ra measured according to the JIS standard, and there is a strong positive correlation between their arithmetic mean roughness Ra.

[0062] Table 1

[0063]

[0064] The relationship of the arithmetic mean roughness Ra in each case where Mg, Sb, Bi, or Cu is selected as a single element in the acquisition of the EDX image for the same thermoelectric conversion element 10a is shown in Table 2. As shown in Table 2, regardless of the type of single element selected in the acquisition of the EDX image, the arithmetic mean roughness Ra is determined to be a similar value. According to the method of determining the arithmetic mean roughness Ra using the EDX image of the present disclosure, the state of the roughness of the first surface 11a can be quantitatively evaluated.

[0065] Generally, the surface roughness is evaluated according to the methods specified in JIS B 0601:2013, JIS B 0633:2001, etc. On the other hand, it is considered that forming the metal layer 12 in contact with the first surface 11a during the manufacture of the thermoelectric conversion element 10a may affect the arithmetic mean roughness Ra of the first surface 11a. For example, in the case of forming the metal layer 12 by plating, denaturation of the surface corresponding to the first surface 11a of the thermoelectric conversion layer 11 occurs, which can affect the arithmetic mean roughness Ra of the first surface 11a. In addition, in the case of forming the metal layer 12 by DC sputtering or the like, the surface of the thermoelectric conversion layer 11 serving as the substrate is easily damaged, which can affect the arithmetic mean roughness Ra of the first surface 11a. According to the method of determining the arithmetic mean roughness Ra using an EDX image, it is possible to quantitatively evaluate the roughness state of the first surface 11a on the basis of also considering the influence on the arithmetic mean roughness Ra of the first surface 11a associated with the formation of the metal layer 12.

[0066] Table 2

[0067]

[0068] The thermoelectric conversion element 10a can be manufactured, for example, by a method including the following (I) and (II).

[0069] (I) Adjust the arithmetic mean roughness Ra of the first surface 11a of the thermoelectric conversion layer 11 containing a thermoelectric conversion material, the thermoelectric conversion material containing Mg and at least one selected from Sb and Bi.

[0070] (II) Form a metal layer in contact with the first surface 11a.

[0071] Figure 6 is a flowchart showing a manufacturing method of the thermoelectric conversion element 10a. As Figure 6 shown, in step S11, a thermoelectric conversion material is produced. An example of the method of producing the thermoelectric conversion material is shown below. The method of producing the thermoelectric conversion material is not limited to the following method.

[0072] A solid-phase reaction is caused in a state where Mg particles and at least one kind of particles selected from Sb particles and Bi particles as raw materials are mixed to obtain a powdery alloy containing Mg and at least one selected from Sb and Bi. In the solid-phase reaction, powder of an element as a dopant can also be mixed as needed. The raw material can be particles or powder. An example of the method of causing the solid-phase reaction is mechanical alloying. The alloy can also be obtained by a method other than the solid-phase reaction, such as a melting method or other methods.

[0073] Next, the powdered alloy is sintered to obtain a single-crystalline or polycrystalline alloy. Sintering of the powdered alloy can be carried out, for example, by spark plasma sintering or hot pressing. The obtained sintered body can be used as a thermoelectric conversion material as it is. It is also possible to perform heat treatment on the obtained sintered body, and the heat-treated sintered body can also be used as a thermoelectric conversion material.

[0074] Next, as Figure 6 shown, in step S12, adjustment of the arithmetic mean roughness Ra of the first surface 11a of the thermoelectric conversion layer 11 including the obtained thermoelectric conversion material is performed. Step S12 corresponds to the above (I).

[0075] As a method for adjusting the arithmetic mean roughness Ra of the first surface 11a, for example, a prescribed machining can be applied. An example of machining is dry grinding. It is also possible to adjust the arithmetic mean roughness Ra of the first surface 11a as a cut surface by cutting out the thermoelectric conversion material using a wire saw or a blade saw. In the adjustment of the arithmetic mean roughness Ra of the first surface 11a, cleaning of the thermoelectric conversion layer 11 can also be performed. Thereby, the amount of foreign substances such as abrasive materials attached to the first surface 11a can be reduced, and the adhesiveness between the thermoelectric conversion layer 11 and the metal layer 12 is likely to become high. The thus obtained first surface 11a has an arithmetic mean roughness Ra that satisfies the condition expressed by Ra ≤ 5.2 μm. In this case, the arithmetic mean roughness Ra of the first surface 11a is determined in accordance with JIS B 0601:2013 and JIS B 0633:2001. The first surface 11a is at least one surface that comes into contact with the outside of the thermoelectric conversion material. The adjustment of the arithmetic mean roughness Ra of the first surface 11a can be carried out in such a way as to satisfy the condition of Ra ≤ 4.7 μm, can be carried out in such a way as to satisfy the condition of Ra ≤ 4.2 μm, and can be carried out in such a way as to satisfy the condition of Ra ≤ 3.7 μm.

[0076] Next, in step S13, a metal layer 12 that contacts the first surface 11a is formed. Step S13 corresponds to the above (II). The method for forming the metal layer 12 is not limited to a specific method. Examples of the method for forming the metal layer 12 are electroplating, electroless plating, sputtering, and spraying. In the case where the metal layer 12 is formed by electroplating, a commercially available electroplating solution can also be used. After the plating is finished, a cleaning process for removing the plating solution can be performed. The thermoelectric conversion element 10a is obtained in this way. It is also possible to form another film by a method such as electroplating after forming the metal layer 12 in order to prevent oxidation of the metal layer 12.

[0077] (Embodiment 2)

[0078] Figure 7 is a cross-sectional view showing an example of the thermoelectric conversion module of Embodiment 2. AsFigure 7 As shown in Figure 7 , the thermoelectric conversion module 100 includes a P-type thermoelectric element 20a, an N-type thermoelectric element as the thermoelectric conversion element 10a, and an electrode 31. The electrode 31 electrically connects one end portion of the P-type thermoelectric conversion element 20a and one end portion of the N-type thermoelectric conversion element 10a. The metal layer 12 can be directly connected to the electrode 31, or another layer can be disposed between the electrode 31 and the metal layer 12.

[0079] As Figure 7 shown in Figure 7 , the P-type thermoelectric element 20a includes, for example, a thermoelectric conversion layer 21 and a pair of electrode layers 22. The electrode layer 22 is in contact with the second surface 21a of the thermoelectric conversion layer 21. The second surface 21a is an end surface of the thermoelectric conversion layer 21 in the thickness direction of the electrode layer 22. The pair of electrode layers 22 are in contact with both end surfaces of the thermoelectric conversion layer 21 in the thickness direction of the electrode layer 22. The electrode layer 22 can be directly connected to the electrode 31, or another layer can be disposed between the electrode 31 and the electrode layer 22.

[0080] The thermoelectric conversion layer 21 contains a P-type thermoelectric conversion material. Examples of the P-type thermoelectric conversion material are bismuth telluride, cesium bismuth telluride, germanium telluride, bismuth antimony, Mg3(Sb, Bi)2, and MgAgSb. (Sb, Bi) means containing at least one selected from Sb and Bi. The N-type thermoelectric conversion material contained in the thermoelectric conversion layer 11 and the P-type thermoelectric conversion material contained in the thermoelectric conversion layer 21 can be materials of the same alloy system or materials of different alloy systems. The same alloy system means that the elements constituting the alloy are the same. From the viewpoint of reducing the thermal stress in the thermoelectric conversion module 100, it is important that the difference in the coefficient of thermal expansion between the N-type thermoelectric conversion material contained in the thermoelectric conversion layer 11 and the P-type thermoelectric conversion material contained in the thermoelectric conversion layer 21 is small. For example, when the P-type thermoelectric conversion material is Mg3(Sb, Bi)2 or bismuth telluride, the difference in the coefficient of thermal expansion between the N-type thermoelectric conversion material and the P-type thermoelectric conversion material is likely to be small.

[0081] When the P-type thermoelectric conversion material is bismuth telluride, the electrode layer 22 is, for example, a layer such as a Ni plating layer, a Ni spray plating layer, a Ni sputtering layer, or a Mo spray plating layer. When the P-type thermoelectric conversion material is bismuth telluride and is represented by the composition of Bi2Te x , the x in this composition satisfies, for example, the condition of 2 < x < 4. The composition of bismuth telluride can be Bi2Te3. It is also possible to contain at least one selected from antimony and selenium in bismuth telluride. Bismuth telluride containing antimony has, for example, a composition of (Bi 1-y Sb y )2Te x . When the composition of the P-type thermoelectric conversion material has (Bi 1-y Sb y )2Te xIn the case of the composition, for example, the condition 0 < y < 1 is satisfied, and preferably the condition 0.6 < y < 0.9 is satisfied.

[0082] Figure 8 It is a cross-sectional view showing another example of the thermoelectric conversion module. As Figure 8 shown, the thermoelectric conversion module 200 is configured in the same manner as the thermoelectric conversion module 100, except that the thermoelectric conversion element 10b is provided instead of the thermoelectric conversion element 10a as the N-type thermoelectric conversion element. The thermoelectric conversion element 10b is configured in the same manner as the thermoelectric conversion element 10a, except for the parts specifically described. The components of the thermoelectric conversion element 10b that are the same as or corresponding to the components of the thermoelectric conversion element 10a are labeled with the same reference numerals, and detailed descriptions are omitted. The description of the thermoelectric conversion element 10a is also applicable to the thermoelectric conversion element 10b as long as there is no technical contradiction.

[0083] As Figure 8 shown, the thermoelectric conversion element 10b includes a metal layer 12, a first electrode layer 13, and a second electrode layer 14. In the thermoelectric conversion element 10b, the metal layer 12, the first electrode layer 13, and the second electrode layer 14 are sequentially arranged from the first surface 11a of the thermoelectric conversion layer 11 in the thickness direction of the metal layer 12. The thermoelectric conversion element 10b is electrically connected to the electrode 31 through the metal layer 12, the first electrode layer 13, and the second electrode layer 14.

[0084] Figure 9 It is a cross-sectional view showing still another example of the thermoelectric conversion module. Figure 9 The thermoelectric conversion module 300 shown is configured in the same manner as the thermoelectric conversion module 100, except for the parts specifically described. As Figure 9 shown, the thermoelectric conversion module 300 further includes an electrode 32 and an electrode 33. The other end of the N-type thermoelectric conversion element 10a is electrically connected to the electrode 32 through the metal layer 12. The other end of the P-type thermoelectric conversion element 20a is electrically connected to the electrode 33 through the electrode layer 22. The thermoelectric conversion module 300 further includes a first wiring 41 and a second wiring 42. The first wiring 41 is connected to the electrode 32. The second wiring 42 is connected to the electrode 33. The first wiring 41 and the second wiring 42, for example, are responsible for applying a voltage to the N-type thermoelectric conversion element 10a and the P-type thermoelectric conversion element 20a. The first wiring 41 and the second wiring 42 may also be responsible for supplying the electricity generated in the thermoelectric conversion module 300 to the outside of the thermoelectric conversion module 100.

[0085] (Embodiment 3)

[0086] Figure 10 It is a side view showing the thermoelectric conversion system of Embodiment 3. As Figure 10As shown, the thermoelectric conversion system 500 includes a thermoelectric conversion module 400 and a heat source 70. The thermoelectric conversion module 400 is configured in the same manner as the thermoelectric conversion module 100 except for the specifically described parts. The thermoelectric conversion module 400 includes an electrode 31, an electrode 32, and an electrode 33. The heat source 70 is disposed on the side of the electrode 31. The electrode 32 and the electrode 33 electrically connect the other end portions of the P-type thermoelectric conversion element 20a and the N-type thermoelectric conversion element 10a.

[0087] The thermoelectric conversion system 500 further includes, for example, a pair of substrates 60. One of the pair of substrates 60 is disposed in contact with the electrode 31, and the other of the pair of substrates 60 is disposed in contact with the electrode 32 and the third electrode 33. With such a configuration, in the thermoelectric conversion system 500, it is difficult to generate a temperature deviation in the direction parallel to the main surface of the substrate 60. The material of the substrate 60 is not limited to a specific material. The substrate 60 includes, for example, alumina or aluminum nitride.

[0088] According to the thermoelectric conversion system 500, the following power generation method can be provided, which includes generating an electric current by generating a temperature difference using heat from the heat source 70 in the thermoelectric conversion module 400.

[0089] The thermoelectric conversion element of the above-described embodiment can be used, for example, in various applications including those of conventional thermoelectric conversion elements.

[0090] (Supplementary Note)

[0091] Based on the above description, the following technology is disclosed.

[0092] (Technology 1)

[0093] A thermoelectric conversion element includes a thermoelectric conversion layer and a metal layer. The thermoelectric conversion layer includes a thermoelectric conversion material containing Mg and at least one selected from Sb and Bi.

[0094] The thermoelectric conversion layer has a first surface in contact with the metal layer.

[0095] When observing the cross section of the thermoelectric conversion layer and the metal layer along the thickness direction of the metal layer by energy dispersive X-ray analysis method, the arithmetic mean roughness Ra of the first surface satisfies a first condition expressed by Ra ≤ 4.2 μm.

[0096] (Technology 2)

[0097] The thermoelectric conversion element according to Technology 1, wherein the metal layer contains Cu.

[0098] (Technology 3)

[0099] According to the thermoelectric conversion element of technology 1 or 2, the arithmetic mean roughness Ra of the first surface satisfies a second condition represented by 0.05 μm ≤ Ra.

[0100] (Technique 4)

[0101] In the thermoelectric conversion element according to any one of techniques 1 to 3, the thermoelectric conversion material has a La 2 O 3 type crystal structure.

[0102] (Technique 5)

[0103] In the thermoelectric conversion element according to any one of techniques 1 to 3, the thermoelectric conversion material further contains Te.

[0104] (Technique 6)

[0105] The thermoelectric conversion element according to any one of techniques 1 to 5,

[0106] The thermoelectric conversion material has Mg 3+m R a T b Sb 2-e-c Bi c Z e The composition of the representation,

[0107] In the composition,

[0108] The element R is at least one element selected from the group consisting of Ca, Sr, Ba and Yb,

[0109] The element T is at least one element selected from the group consisting of Mn and Zn,

[0110] The element Z is at least one element selected from Te, Se, Sc, Y and La,

[0111] The value of m satisfies -0.39≤m≤0.42,

[0112] The value of a satisfies 0≤a≤0.12,

[0113] The value of b satisfies 0≤b≤0.48,

[0114] The value of c satisfies 0≤c≤1.6,

[0115] The value of e satisfies 0.001≤e≤0.06.

[0116] (Technique 7)

[0117] A thermoelectric conversion module, comprising:

[0118] P-type thermoelectric conversion element;

[0119] N-type thermoelectric conversion element; and

[0120] An electrode that electrically connects one end of the P-type thermoelectric conversion element and one end of the N-type thermoelectric conversion element

[0121] The N-type thermoelectric conversion element is the thermoelectric conversion element described in any one of Technologies 1 to 6.

[0122] (Technology 8)

[0123] A thermoelectric conversion system includes the thermoelectric conversion module described in Technology 7 and a heat source disposed on the electrode side.

[0124] (Technology 9)

[0125] A power generation method includes: generating an electric current by generating a temperature difference using heat from a heat source in the thermoelectric conversion module described in Technology 7.

[0126] (Technology 10)

[0127] A thermoelectric conversion material

[0128] contains Mg and at least one selected from Sb and Bi

[0129] includes at least one surface that has an arithmetic mean roughness Ra satisfying a third condition expressed by Ra ≤ 5.2 μm and is in external contact with the thermoelectric conversion material

[0130] The arithmetic mean roughness Ra is determined in accordance with JIS B 0601:2013 and JIS B 0633:2001.

[0131] (Technology 11)

[0132] A method for manufacturing a thermoelectric conversion element includes:

[0133] adjusting the arithmetic mean roughness Ra of the first surface of a thermoelectric conversion layer that includes a thermoelectric conversion material containing Mg and at least one selected from Sb and Bi; and

[0134] forming a metal layer in contact with the first surface;

[0135] When observing a cross-section of the thermoelectric conversion layer and the metal layer along the thickness direction of the metal layer by energy dispersive X-ray analysis method, the arithmetic mean roughness Ra of the first surface satisfies a fourth condition expressed by Ra ≤ 4.2 μm.

[0136] Examples

[0137] The present disclosure will be described in detail below with reference to examples. However, the thermoelectric conversion element of the present disclosure is not limited to the examples shown below.

[0138] (Example 1)

[0139] Weigh approximately 1.3 g of the powder of the alloy with the composition of Mg 3.2 Sb 1.5 Bi 0.49 Te 0.01 inside the glove box. The inside of the glove box is maintained in an argon atmosphere until the thermoelectric conversion material is obtained. Next, the weighed powder is filled into the sintering space of a carbon mold, and powder pressing is performed using a carbon punch. The inner diameter of the mold is 10 mm. Next, the mold is placed in the chamber of a spark plasma sintering apparatus. The chamber is maintained in an argon atmosphere. Next, while applying a pressure of 50 MPa to the filler in the mold, an electric current is applied to the mold by the sintering apparatus. By applying the electric current, after the temperature of the mold reaches 840 °C as the sintering temperature, this temperature is maintained for 10 minutes. Then, the electric current flowing through the mold is reduced, and the heating of the mold is stopped. After confirming that the temperature of the mold has dropped to room temperature, the sintered body is taken out from the inside of the mold, and the thermoelectric conversion material of Example 1 is obtained. Grind the surface of the taken-out sintered body, i.e., the thermoelectric conversion material, that is in contact with the inner surface of the mold, and then clean it with acetone. The thickness of the sintered body of Example 1 is approximately 3 mm.

[0140] Grind both end faces of the sintered body in the thickness direction using sandpaper. The grinding is dry grinding, and a sample grinding machine IS-POLISHER manufactured by Ikemura Seiki Co., Ltd. is used. The thickness of the sintered body after dry grinding is approximately 2.9 mm. Measure the arithmetic mean roughness Ra of the ground surface of the sintered body of Example 1 using the method based on JIS B 0601:2013 and JIS B 0633:2001. As a result, the arithmetic mean roughness Ra of the ground surface is 3.41 μm. A SURFCOM TOUCH50 manufactured by Tokyo Seimitsu Co., Ltd. is used in the measurement of the arithmetic mean roughness Ra.

[0141] Electroplate Cu on the ground surface of the sintered body of Example 1 to form a metal layer. After electroplating Cu, the sintered body is cleaned.

[0142] Use a wire saw to process the sintered body with the metal layer formed thereon to obtain the thermoelectric conversion element of Example 1. This thermoelectric conversion element is in the shape of a rectangular parallelepiped, and the thermoelectric conversion element has a square-shaped contour with a side length of approximately 3 mm in the plane parallel to the metal layer. In the thermoelectric conversion element of Example 1, the distance between the metal layers is approximately 2.9 mm. Figure 13 is a photograph showing the thermoelectric conversion element of Example 1.

[0143] Install external electrodes on the metal layer of the thermoelectric conversion element of Example 1, and perform an aging treatment under the use conditions of the thermoelectric conversion element of Example 1.

[0144] The interfacial resistivity at the interface between the thermoelectric conversion layer, which is a sintered body, and the metal layer in the thermoelectric conversion element of Example 1 was determined as follows. Figure 11 FIG. is a diagram schematically showing a method for measuring the resistance of a thermoelectric conversion element. As Figure 11 shown, a pair of external electrodes 71 are mounted on the thermoelectric conversion element 10a. A galvanometer 84 is electrically connected to both of the pair of electrodes 71 through a wire 85. A voltmeter 81 is connected to one external electrode 71 through a wire 82, and a probe 83 is connected to the end of the wire 82 on the side opposite to the one external electrode 71. While moving the probe 83 from one external electrode 71 to the other external electrode 71 at intervals of 50 μm in the direction indicated by the arrow J, the resistance was measured using the four-terminal measurement method. A source meter (model: 2400) manufactured by KEITHLEY was used for the measurement of the resistance. Figure 12 FIG. is an example showing the measurement results of the resistance of the thermoelectric conversion element of Example 1. Figure 12 In, “A” is the part corresponding to one metal layer 12 and one external electrode 71. “B” is the part corresponding to the thermoelectric material layer 11. “C” is the part corresponding to the other metal layer 12 and the other external electrode 71. The change in resistance at the boundary between A and B and the change in resistance at the boundary between B and C correspond to the interfacial resistance at the interface between the thermoelectric conversion layer and the metal layer. The interfacial resistivity at the interface between the thermoelectric conversion layer and the metal layer in the thermoelectric conversion element of Example 1 was obtained from the cross-sectional area parallel to the metal layer of the thermoelectric conversion element of Example 1 and the above-described interfacial resistance. As a result, the interfacial resistivity was 2.53 mΩ·mm 2 .

[0145] Next, argon ion milling was performed in a direction perpendicular to the interface between the thermoelectric conversion layer and the metal layer of the thermoelectric conversion element of Example 1 to form a smooth first cross-section with the interface exposed. In this smooth first cross-section, compositional analysis was performed using a field emission scanning electron microscope (FE-SEM) (model: SU8220) manufactured by Hitachi High-Tech Corporation and an energy dispersive X-ray analysis (EDX) device FlatQUAD manufactured by Bruker AXS Inc., to obtain an EDX image of a single element Mg including the boundary between the thermoelectric conversion layer and the metal layer. Using the obtained EDX image, the arithmetic mean roughness Ra of the surface of the thermoelectric conversion layer in contact with the metal layer was obtained in the same manner as the calculation method of the arithmetic mean roughness Ra based on the EDX image described in Embodiment 1. As a result, the arithmetic mean roughness Ra was 4.16 μm. In addition, argon ion milling was performed in a direction perpendicular to the interface between the thermoelectric conversion layer and the metal layer to form a smooth second cross-section perpendicular to the first cross-section with the interface exposed. Similarly to the first cross-section, compositional analysis was also performed on the second cross-section to obtain an EDX image of a single element Mg including the boundary between the thermoelectric conversion layer and the metal layer. The arithmetic mean roughness Ra of the surface of the thermoelectric conversion layer in contact with the metal layer calculated using this EDX image was 3.85 μm. In Table 1, the larger value among the arithmetic mean roughness Ra calculated using the EDX image of the first cross-section and the arithmetic mean roughness Ra calculated using the EDX image of the second cross-section was recorded.

[0146] (Examples 2 to 34 and Comparative Examples 1 and 2)

[0147] The composition of the alloy powder was adjusted as shown in Table 3 or Table 4, and otherwise, the same procedure as in Example 1 was carried out to obtain the thermoelectric conversion materials of Examples 2 to 34 and Comparative Examples 1 and 2. For each thermoelectric conversion material, the grinding conditions were adjusted so that the arithmetic mean roughness Ra of the ground surface became the value shown in Table 3 or Table 4, and otherwise, dry grinding was carried out in the same manner as in Example 1. The arithmetic mean roughness Ra of the ground surface of the sintered body was measured in the same manner as in Example 1. The results are shown in Table 3 or Table 4. As shown in Table 3 or Table 4, the interfacial resistivity at the interface between the thermoelectric conversion layer and the metal layer of the thermoelectric conversion element of the example was lower than the interfacial resistivity at the interface between the thermoelectric conversion layer and the metal layer of the thermoelectric conversion elements of Comparative Examples 1 and 2.

[0148] Except for using each thermoelectric conversion material after dry grinding, Cu was electroplated and processing was carried out using a wire saw in the same manner as in Example 1 to obtain the thermoelectric conversion elements of Examples 2 to 34 and Comparative Examples 1 and 2. For each thermoelectric conversion element, the same evaluation as the thermoelectric conversion element of Example 1 was carried out. The results are shown in Table 3 or Table 4.

[0149] (Comparative Example 3)

[0150] The same method as in Example 1 was used to obtain a thermoelectric conversion material. For the obtained thermoelectric conversion material, the grinding conditions were adjusted so that the arithmetic mean roughness Ra of the grinding surface became the value shown in Table 4. Except for this, dry grinding was performed in the same manner as in Example 1. Except for using the thermoelectric conversion material after this dry grinding, electroplating of Cu was attempted in the same manner as in Example 1. Figure 14 It is a photograph showing the peeling of the coating film in Comparative Example 3. As Figure 14 shown, in Comparative Example 3, peeling of the coating film occurred in the cleaning process after electroplating of Cu, and a metal layer could not be formed.

[0151] (Comparative Example 4)

[0152] Except for using the powder of the alloy having the composition shown in Table 4, the same procedure as in Example 1 was carried out to obtain the thermoelectric conversion material of Comparative Example 4. For the obtained thermoelectric conversion material, the grinding conditions were adjusted so that the arithmetic mean roughness Ra of the grinding surface became the value shown in Table 4. Except for this, dry grinding was performed in the same manner as in Example 1. Except for using the thermoelectric conversion material after this dry grinding, electroplating of Cu was attempted in the same manner as in Example 1. However, in Comparative Example 4, peeling of the coating film occurred in the cleaning process after electroplating of Cu, and a metal layer could not be formed.

[0153] Table 3

[0154]

[0155] Table 4

[0156]

[0157] Industrial Applicability

[0158] The thermoelectric conversion element of the present disclosure can be used in various applications including the applications of conventional thermoelectric conversion elements.

[0159] Explanation of Reference Numerals

[0160] 10a, 10b Thermoelectric conversion element

[0161] 11 Thermoelectric conversion layer

[0162] 11a First surface

[0163] 12 Metal layer

[0164] 20a P-type thermoelectric conversion element

[0165] 31 Electrode

[0166] 70 Heat source

[0167] 100, 200, 300, 400 thermoelectric conversion modules

[0168] 500 thermoelectric conversion system

Claims

1. A thermoelectric conversion element, comprising a thermoelectric conversion layer and a metal layer, wherein the thermoelectric conversion layer contains a thermoelectric conversion material, and the thermoelectric conversion material contains Mg and at least one selected from Sb and Bi, the thermoelectric conversion layer has a first surface in contact with the metal layer, when observing a cross-section of the thermoelectric conversion layer and the metal layer along the thickness direction of the metal layer by energy dispersive X-ray analysis method, the arithmetic mean roughness Ra of the first surface satisfies a first condition represented by Ra ≤ 4.2 μm.

2. The thermoelectric conversion element according to claim 1, wherein the metal layer contains Cu.

3. The thermoelectric conversion element according to claim 1, wherein the arithmetic mean roughness Ra of the first surface satisfies a second condition represented by 0.05 μm ≤ Ra.

4. The thermoelectric conversion element according to claim 1, wherein the thermoelectric conversion material has a crystal structure of La2O3 type.

5. The thermoelectric conversion element according to claim 1, wherein the thermoelectric conversion material further contains Te.

6. The thermoelectric conversion element according to claim 1, The thermoelectric conversion material has a composition represented by Mg 3+m R a T b Sb 2-e-c Bi c Z e and is represented by in the composition, element R is at least one element selected from Ca, Sr, Ba, and Yb, element T is at least one element selected from Mn and Zn, element Z is at least one element selected from Te, Se, Sc, Y, and La, the value of m satisfies -0.39 ≤ m ≤ 0.42, the value of a satisfies 0 ≤ a ≤ 0.12, the value of b satisfies 0 ≤ b ≤ 0.48, the value of c satisfies 0 ≤ c ≤ 1.6, the value of e satisfies 0.001 ≤ e ≤ 0.

06.

7. A thermoelectric conversion module, comprising: a P-type thermoelectric conversion element; an N-type thermoelectric conversion element; and an electrode for electrically connecting one end portion of the P-type thermoelectric conversion element and one end portion of the N-type thermoelectric conversion element, wherein the N-type thermoelectric conversion element is the thermoelectric conversion element according to claim 1.

8. A thermoelectric conversion system, comprising the thermoelectric conversion module according to claim 7 and a heat source disposed on the electrode side.

9. A power generation method, comprising: generating an electric current by generating a temperature difference using heat from a heat source in the thermoelectric conversion module according to claim 7.

10. A thermoelectric conversion material, containing Mg and at least one selected from Sb and Bi, comprising at least one surface having an arithmetic mean roughness Ra that satisfies a third condition represented by Ra ≤ 5.2 μm and is in contact with the outside of the thermoelectric conversion material, wherein the arithmetic mean roughness Ra is determined in accordance with JIS B 0601:2013 and JIS B 0633:2001.

11. A method for manufacturing a thermoelectric conversion element, comprising: adjusting the arithmetic mean roughness Ra of the first surface of the thermoelectric conversion layer, the thermoelectric conversion layer containing a thermoelectric conversion material, and the thermoelectric conversion material containing Mg and at least one selected from Sb and Bi; and forming a metal layer in contact with the first surface; When observing a cross section of the thermoelectric conversion layer and the metal layer in the thickness direction of the metal layer by energy dispersive X-ray analysis, the arithmetic mean roughness Ra of the first surface satisfies a fourth condition expressed as Ra ≤ 4.2 μm.

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