JP2021158355A - Manufacturing method of thermoelectric conversion element and manufacturing method of thermoelectric conversion module using the same - Google Patents

Abstract

To solve a problem in which a crack and the like occur in a diffusion prevention layer at a joint, and it is difficult to make the diffusion prevention layer dense at the same time as the powder of a thermoelectric conversion material.SOLUTION: A manufacturing method of a thermoelectric conversion element includes a step of sintering a thermoelectric conversion material powder containing Sb in contact with an alloy foil containing Ti and Al to obtain a thermoelectric conversion material having a diffusion prevention layer, and in the alloy foil containing Ti and Al, in a cross-section observation method that can determine an orientation difference of a crystal, when an area of 5000 μm2 or more is observed, the number of crystal grains defined by the grain boundaries with an orientation difference of 10 degrees or more is 1.0 or less/5000 μm2.SELECTED DRAWING: Figure 1

Description

The present invention relates to a method for manufacturing a thermoelectric conversion element that converts heat into electricity, and a method for manufacturing a thermoelectric conversion module using the same.

For example, in order to effectively utilize the heat of a high temperature part such as the exhaust heat of a factory, a thermoelectric conversion module that converts heat into electricity is being studied. The thermoelectric conversion module is composed of a large number of thermoelectric conversion elements. The thermoelectric conversion element generates an electromotive force according to the temperature difference between the high temperature side and the low temperature side, and N-type and P-type thermoelectric conversion elements are connected in series, one surface is the high temperature side and the other surface is. By arranging it on the low temperature side, heat can be converted into electricity.

In recent years, as a thermoelectric conversion material having a high electrical characteristic (figure of merit ZT), an Sb-based thermoelectric conversion material having a scutterdite structure and containing Sb has been attracting attention. By using this scutterdite-based thermoelectric conversion element, heat can be efficiently converted into electricity, for example, when the high temperature portion is about 300 ° C. to 500 ° C.

When such a thermoelectric conversion element is used as a thermoelectric conversion module, electrodes are bonded to a high temperature side and a low temperature side, respectively. However, especially on the high temperature side, solid phase diffusion may proceed between the electrode and the thermoelectric conversion element, and a part of the thermoelectric conversion element may be deteriorated. Further, especially on the high temperature side, a thermal cycle occurs, so that there is a possibility that cracks or the like may occur in the diffusion layer at the joint. Such a state causes an increase in electrical resistance and causes a decrease in electrical performance.

On the other hand, in Patent Document 1, in order to suppress the diffusion between the Sb-based sinterdite thermoelectric element and the electrode material, Ti powder or a mixed powder of Ti powder and Al powder is filled at the end of the thermoelectric element alloy powder at the same time. A method of sintering has been proposed. Further, in Patent Document 2, it is composed of a sintered body containing a thermoelectric conversion material made of an alloy containing Sb and a diffusion prevention layer made of an alloy containing Ti and Al laminated on the thermoelectric conversion material, and the diffusion prevention. A thermoelectric conversion element is disclosed in which the neck portion of the sintered body constituting the layer is Ti-Al alloyed and the Al concentration in each portion of the sintered body is 50 at% or less. As a result, the Al-concentrated portion (Al-Sb), which is mainly composed of Al and Sb and is fragile compared to the surroundings, is suppressed, and cracks originating from the portion are reduced, so that the electrical characteristics can be maintained. It is said that it can provide a thermoelectric conversion element with high performance. Here, the notation such as Ti—Al alloy means that the alloy contains an element of Ti and an element of Al as a main element (upper element in the content).

Japanese Unexamined Patent Publication No. 2011-249442 JP-A-2019-169534

Both the diffusion prevention layer described in Patent Document 1 and Patent Document 2 were sintered using powder at the same time as the powder of the thermoelectric conversion material. In this case, the problem is that it is difficult to control sintering, which is densified at the same time as the powder of the thermoelectric conversion material.

An object of the present invention is to solve the problem of cracks or the like occurring in the diffusion layer at the joint portion described in Patent Documents 1 and 2, and the problem of densification by simultaneous sintering of the thermoelectric conversion material with powder. The purpose is to solve the problem.

The method for producing a thermoelectric conversion element of the present invention is to obtain a thermoelectric conversion material provided with a diffusion prevention layer by sintering the powder of the thermoelectric conversion material containing Sb in contact with an alloy foil containing Ti and Al. The alloy foil containing Ti and Al has an orientation difference of 10 degrees or more when observing a region having an area of ^{5000 μm 2} or more in a cross-sectional observation method capable of discriminating the orientation difference of crystals. The number of crystal grains defined by the grain boundaries is 1.0 or less / 5000 μm ² .

Further, the alloy foil containing Ti and Al preferably has an Al content of 8 mass% or more and 36 mass% or less, and has a Ti ₃ Al compound.

Further, the method for manufacturing a thermoelectric conversion module of the present invention preferably includes a step of joining the thermoelectric conversion element obtained by the above-mentioned method for manufacturing a thermoelectric conversion element and an electrode via the diffusion prevention layer.

According to the present invention, it is possible to provide a method for manufacturing a thermoelectric conversion element that suppresses the occurrence of cracks or the like in the diffusion layer at the joint portion and can be densified at the same time as the powder of the thermoelectric conversion material.

The perspective view which shows the whole structure of a thermoelectric conversion module. An enlarged cross-sectional view of the thermoelectric conversion module. The figure which shows the X-ray diffraction result of the Ti-Al foil. A cross-sectional SEM photograph of the bonding interface of a thermoelectric conversion element containing Ti-Al foil and Sb. EDX line analysis result near the junction interface in FIG. The manufacturing process flow diagram of the thermoelectric conversion material with Ti-Al foil. SEM-EDX analysis result of Example 1. Results of SEM-EDX analysis of Example 1 in which a high-temperature standing test was performed. SEM-EDX analysis result of Example 2. SEM-EDX analysis result of Example 3. SEM-EDX analysis result of Example 4. Results of SEM-EDX analysis of Example 5 in which a high-temperature standing test was performed. SEM observation result of Comparative Example 1.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a perspective view showing the overall structure of the thermoelectric conversion module 10 of the present embodiment. The thermoelectric conversion module 10 is composed of a P-type thermoelectric conversion element 21, an N-type thermoelectric conversion element 22, an electrode 30, and a ceramic wiring board 40. Adjacent P-type thermoelectric conversion elements 21 and N-type thermoelectric conversion elements 22 are arranged in a houndstooth pattern, and are electrically connected in series via an electrode 30 and a ceramic wiring board 40. The N-type thermoelectric conversion element 22 and the P-type thermoelectric conversion element are thermoelectric conversion materials containing Sb, and those having a scutterdite structure are particularly preferable. E.g., P-type thermoelectric conversion element 21 is _{Ce x Fe y Mn 4-y} Sb 12, N -type thermoelectric conversion element 22 is suitably _Yb x _Co 4 _{Sb 12.} In the case of a scutterdite structure containing Sb, other trace elements may be contained in the above composition. The thermoelectric conversion material having a scutterdite structure has high power generation performance in a temperature range of 300 to 500 ° C., and it is possible to obtain a high output by obtaining a large temperature difference. Therefore, it is preferable that the thermoelectric conversion material containing Sb is identified by XRD and has a single phase having a scutterdite structure. The thermoelectric conversion material containing Sb means that Sb is contained in a state in which Sb easily reacts, such as a composition having more Sb than other elements and a composition with other elements that are more stable than Sb.

The electrode 30 may be a member that can be energized even in a temperature range of 300 to 500 ° C. Further, the member may have a thermal expansion coefficient close to that of the P-type thermoelectric conversion element 21 and the N-type thermoelectric conversion element 22, and may have a structure composed of a single layer of pure metal or alloy or a plurality of layers of pure metal or alloy. .. In particular, in the case of a thermoelectric conversion material containing Sb, it is preferable that it is a composite material in which Cu or Cu and Mo are mixed (composite in granular form, composite in layers, etc.) to adjust the thermal expansion.

The ceramic wiring board 40 may have insulating properties, and it is preferable to use a ceramic material such as alumina, aluminum nitride, or silicon nitride because it can be used in a wide temperature range from low temperature to high temperature.

FIG. 2 shows an enlarged cross-sectional view of a part of the thermoelectric conversion module. A first conductive member 41 and a second conductive member 42 are formed on both surfaces of the ceramic wiring board 40. The first conductive member 41 and the second conductive member 42 may be members such as Cu as well as the electrode 30. The first conductive member 41 may form a metal film (not shown) such as Ni for the purpose of improving the bondability with respect to the bonding layer 60.

As the bonding layer 60, solder, a brazing material, a metal nanoparticle sintered layer, or the like may be used. When a sintered layer using a metal nanoparticle paste is formed as the bonding layer 60, melting does not occur even when the thermoelectric conversion module operates at a temperature higher than the bonding temperature, which is desirable because of high reliability. When the first conductive member 41 is Cu, the bonding layer 60 is more preferably a sintered layer of Cu. That is, a Cu sintered layer is formed by using a metal nanoparticle paste containing Cu.

The metal layer 51 has a role of facilitating bonding between the ceramic wiring board and the P-type thermoelectric conversion element 21 and the N-type thermoelectric conversion element 22 via the bonding layer 60. The metal layer 51 can be selected in various ways in consideration of the bondability with the bonding layer 60, and when the bonding layer 60 is a Cu sintered layer, Ni is particularly preferable. As is clear from the equilibrium phase diagram, Cu and Ni are completely solid-solved. Therefore, the neck is easily formed and can be firmly joined.

The method for forming the metal layer 51 may be appropriately selected depending on the presence or absence of the diffusion prevention layer 50. For example, the diffusion prevention layer 50 may be formed on at least one of the electrodes 30 side. That is, the diffusion prevention layer 50 on the ceramic wiring board side does not have to be formed. In that case, the metal layer 51 may be formed on one surface of the P-type thermoelectric conversion element 21 and the N-type thermoelectric conversion element 22. When the metal layer 51 is formed on the surfaces of the P-type thermoelectric conversion element 21 and the N-type thermoelectric conversion element 22, a plating method or the like can be applied. When forming the diffusion prevention layer 50, Ni may be bonded as the metal layer 51 at the same time as the thermoelectric conversion material and the diffusion prevention layer 50 are bonded.

The diffusion prevention layer 50 is formed of a Ti—Al foil manufactured from a cast alloy, and is preferably a crystal grain composed of grain boundaries having an orientation difference of less than 10 degrees in a cross-sectional observation range of 100 μm × 50 μm. .. More preferably, it is preferably composed of crystal grains having a crystal grain boundary orientation difference of 2 degrees or less. Here, when one or more crystal grains are present in the metal structure, they are collectively referred to as a crystal grain group. Each crystal grain in the crystal grain group can be discriminated by a color map or the like using EBSD (Electron Backscatter Diffraction). In the case of a crystal grain group having a crystal grain orientation difference of 10 degrees or more, it can be discriminated by a BSE image (Back Scattered Electron) or the like. In the evaluation using EBSD, by designating the reference orientation difference, if crystal grains on both sides of the crystal grain boundary are present at an angle equal to or greater than the specified orientation difference, it is determined to be individual crystal grains. In other words, when there are crystal grains on both sides of the crystal grain boundary less than the reference orientation difference, it is determined as one crystal grain without considering it as an individual crystal grain. The foil may be referred to as a plate, a thin piece, a thin plate, a film, or the like as long as the function as a diffusion prevention layer can be obtained during use as a thermoelectric conversion element, and the thickness and form are not limited.

It is desirable that the Ti-Al foil is mechanically cut from the Ti-Al ingot using a wire saw or the like, and then adjusted to a predetermined thickness by wrapping or the like. By using a Ti—Al foil composed of crystal grains having a crystal grain boundary orientation difference of less than 10 degrees, the thickness of the reaction layer 501 shown in FIG. 2 can be suppressed to 30 μm or less, and reliability is achieved. It is possible to prevent the formation of an Al—Sb compound phase that causes a decrease. The thickness of the reaction layer may be an average thickness calculated from a cross-sectional observation of a SEM (Scanning Electron Microscope) or the like.

It is desirable to prepare the Ti-Al ingot by the cold crucible dissolution method. A Ti-Al ingot can be obtained by melting and holding Al and Ti in a water-cooled copper crucible at a predetermined ratio, discharging hot water to a graphite mold installed in a melting furnace, and cooling the mixture. The Ti-Al ingot has a difference in crystal grain size and crystal quality due to the influence of cooling. Therefore, it is desirable to dissolve Al and Ti by the cold crucible dissolution method and cool them with a graphite mold for 1 hour. When cooling the ingot in this melting method, the temperature gradient of the molten metal in the crucible is taken into consideration, and the molten metal is drawn out from the low temperature side such as below the crucible and poured into the mold to suppress the occurrence of sink marks. May be replaced with. In addition, HIP (Hot Isostatic Pressing), which is left for a long time in an inert atmosphere while pressurizing and heating the Ti-Al ingot after coagulation, can be added to the process to eliminate sink marks. preferable.

In the case of a Ti-Al compound on the Al-rich side (for example, TiAl ₃ ), there is a concern that Sb in the thermoelectric conversion material reacts with Al to promote the formation of Al-Sb. _{Therefore, it is desirable to use Ti 3} Al on the Ti-rich side as the composition of the Ti-Al foil. Therefore, it is preferable to prepare a Ti-Al ingot at a ratio of Ti-19.5 mass% Al as a raw material ratio for cold crucible dissolution. Further, TiAl is Al content increases than _Ti 3 Al, because the melting point is higher intermetallic compound, may be produced TiAl ingot in a ratio of Ti-34.2mass% Al. These are those both aimed at the stoichiometric ratio of the compound, such as TiAl and Ti 3 _Al, it may be a composition ratio obtained these compounds. Therefore, the raw material ratio of Al to the whole is preferably 8 mass% or more and 36 mass% or less, and more preferably 19.5 mass% to 34.2 mass%.

FIG. 3 shows the results of X-ray diffraction evaluation of the surface of the Ti—Al foil. The Ti-Al ingot is produced by a cold crucible dissolution method with a dissolution raw material ratio of Ti-19.5 mass% Al. This is an X-ray diffraction result obtained by cutting a Ti-Al ingot to prepare a Ti-Al foil and analyzing the surface thereof. The diffraction peak of Ti _{3 Al is obtained.} As shown in FIG. 3, it is desirable to use a Ti—Al foil having the maximum peak intensity on the (201) plane of _{Ti 3 Al.}

FIG. 4 shows an example of an electron micrograph of a joining cross section when the N-type thermoelectric conversion element 22 is joined using Ti—Al foil as the diffusion prevention layer 50. The electron micrograph is shown as a BSE image. In the case of a BSE image, since it is affected by the channeling contrast, the contrast of the image changes depending on the angle of incidence of electrons with respect to the crystal orientation. Therefore, when the orientation difference of the crystal grain boundaries is 10 degrees or more, the grain boundaries are clearly observed. Since the Ti-Al foil is composed of the orientation difference of the crystal grain boundaries of less than 10 degrees by EBSD analysis, the difference in contrast is not clearly seen in the BSE image.

Grain boundaries with a large orientation difference include many defects such as dislocations. Therefore, due to the disorder of the atomic arrangement, the diffusion rate of atoms at grain boundaries is much faster than that of body diffusion within grains. That is, when the grain boundaries having a large orientation difference are contained in the Ti—Al alloy, the diffusion of Sb and the like in the thermoelectric conversion material is likely to occur, so that a phase change is likely to occur. When Ti—Al undergoes a phase change, a reaction layer is formed at the bonding interface with the thermoelectric conversion material. As the phase change of Ti—Al increases, the thickness of the reaction layer increases, and a compound phase such as Al—Sb that causes a decrease in reliability is generated.

When Ti—Al foil is used as the diffusion prevention layer 50, it is possible to prevent the formation of Al—Sb and join them. FIG. 5 shows an example in which the junction interface between the diffusion prevention layer 50 using Ti-Al foil and the N-type thermoelectric conversion element 22 is line-analyzed by EDX (Energy Dispersive x-ray Spectroscopy). It can be seen that there is no portion where Al and Sb are relatively high, and a clear reaction layer such as the reaction layer 501 in FIG. 2 does not appear. That is, Al-Sb, which causes a decrease in reliability, is not confirmed. Depending on the site, the reaction layer 501 is formed, but the thickness of the reaction layer 501 is 30 μm or less, and Al-Sb, which causes a decrease in reliability, is not confirmed. The reaction layer 501 only needs to be able to prevent the formation of Al—Sb, and there is no problem as long as it is a reaction layer formed of _{TiSb 2, Ti—Co—Sb, and Ti—Al—Sb.} By setting the orientation difference of the crystal grain boundaries constituting the Ti—Al foil to less than 10 degrees, it is possible to suppress the thickness of the reaction layer. As described above, it is assumed that the Ti-Al foil may react with the adjacent thermoelectric conversion material powder, metal layer, or the like. The diffusion prevention layer may have at least a function of preventing the diffusion of the reaction layer that causes cracks during use as a thermoelectric conversion element, for example, between adjacent thermoelectric conversion material powders and metal layers. At least a Ti-Al layer is sufficient. Here, the length of the crack does not have to interfere with the electrical properties of the thermoelectric conversion material. For example, the presence or absence of cracks may be determined by confirming cracks having a length exceeding 5 μm existing in 20 μm square of the observation surface of the electron micrograph. In this 20 μm square observation field of view, any part including the interface may be observed, and it is preferable to confirm a plurality of parts.

The phase of the Ti—Al foil in the region that does not contribute to the formation of the reaction layer 501 does not change before and after sintering with the thermoelectric conversion material. For example, when a Ti-Al foil is produced with a dissolution raw material ratio of Ti-19.5 mass% Al, the main phase is Ti ₃ Al, but the diffusion prevention layer 50 after sintering also _{maintains Ti 3} Al. Therefore, the diffusion prevention layer 50 having a high diffusion prevention effect can be obtained.

FIG. 6 shows a manufacturing process flow of a thermoelectric conversion material with a diffusion prevention layer of Ti—Al foil. As for the method for producing the Ti-Al foil, as described above, a Ti-Al ingot is obtained by pouring hot water into a graphite mold by a cold crucible method or the like, and then the Ti-Al ingot is cut into a foil having a thickness of 50 μm to 300 μm by a wire saw or the like. .. The cut out Ti-Al foil is polished and washed to obtain a Ti-Al foil as the diffusion prevention layer 50.

Next, a thermoelectric conversion material containing Ni foil, Ti—Al foil, and Sb is filled in the mold. When forming Ti-Al foil on both sides of the thermoelectric conversion material, the Ni foil, the Ti-Al foil, the thermoelectric conversion material containing Sb, the Ti-Al foil, and the Ni foil may be laminated in this order from the bottom surface of the mold. When the Ti-Al foil is formed on only one side of the thermoelectric conversion material, for example, the Ni foil, the Ti-Al foil, and the thermoelectric conversion material containing Sb may be laminated in this order from the bottom surface of the mold. The material and Ni foil may be exchanged and laminated. The thermoelectric conversion material containing Sb may be directly filled in a predetermined amount mold, or may be molded in advance by a simple press or the like and laminated in the mold in the state of a green compact.

Next, the thermoelectric conversion material, the Ti-Al foil, and the Ni foil are pressure-sintered using a hot press or the like. For example, in the case of an N-type thermoelectric conversion material containing Sb, sintering is carried out by holding at 700 ° C. for 60 minutes in an inert gas such as Ar and pressurizing at 15 MPa to 68 MPa to obtain N-type thermoelectric including Ti-Al foil and Sb. The conversion material and the Ti-Al foil and Ni foil can be bonded.

_{For example, in the case of Yb x} Co ₄ Sb ₁₂ , which is an N-type thermoelectric conversion material, the Ti—Al foil of the present embodiment is used even if a sintering aid alloy for the purpose of improving sinterability is contained. By doing so, the same effect can be exhibited. That is, the thickness of the reaction layer formed at the interface between the thermoelectric conversion material and the Ti—Al foil can be suppressed to 30 μm or less, and the formation of Al—Sb can be prevented.

The Ti—Al foil and the Ni foil are solid-phase bonded, and a reaction layer such as Ti—Ni, Ni—Al, or Ti—Ni—Al may be partially formed. Further, an Al layer (660 ° C.) having a low melting point may be sandwiched in order to improve the bondability between the Ti—Al foil and the Ni foil. In this case, a reaction layer of Ti—Ni, Ni—Al, or Ti—Ni—Al is formed at the bonding interface as in the case of solid phase bonding.

It is possible to manufacture a thermoelectric conversion material with a diffusion prevention layer by the above method. The thermoelectric conversion material with a diffusion prevention layer is cut into the shapes of the P-type thermoelectric conversion element 21 and the N-type thermoelectric conversion element 22 shown in FIGS. 1 and 2 using a wire saw or the like. After cutting out, the thermoelectric conversion module 10 shown in FIG. 1 can be obtained by arranging the ceramic wiring board 40 on the ceramic wiring board 40 via a bonding material using a jig or the like, mounting electrodes, and heating under pressure.

(Example 1)
A thermoelectric conversion element was produced by joining a thermoelectric conversion material containing Ni foil, Ti-Al foil and Sb. The Ti-Al ingot, which is a raw material for the Ti-Al foil, was produced by the cold crucible method. The pellets of Ti and Al were mixed in a ratio of Ti-19.5mass% Al, after charged into a water-cooled copper crucible, the melting furnace was evacuated at 6.6 × ¹⁰ -2 Pa, Ar gas was introduced .. After introducing Ar gas, the molten metal is melted by high-frequency melting, then the hot water is discharged to a columnar graphite mold installed in the melting furnace, cooled, and after 1 hour, the Ti-Al alloy is taken out from the graphite mold to remove Ti-Al. I got an ingot.

The obtained round bar-shaped Ti-Al ingot was machined into a Ti-Al round bar having a diameter of 30 mm by wire electric discharge machining, and further, a Ti-Al foil having a thickness of about 0.3 mm was produced by a multi-wire saw. The Ti-Al foil was processed to about 0.2 mm by wrapping and then washed with acetone to obtain a Ti-Al foil. The obtained Ti-Al foil was produced in the same lot as the Ti-Al foil evaluated in FIG. The Ti-Al foil used was one in which no crystal grain group having an orientation difference of 10 degrees or more was present from the cross-sectional EBSD analysis in the range of 100 μm × 50 μm.

As the thermoelectric conversion material, _{a powder of Yb 0.3} Co ₄ Sb ₁₂ was compactly molded at a pressure of 21 MPa. Ni foil, Ti-Al foil, Yb _0.3 Co ₄ Sb ₁₂ green compact, Ti-Al foil, and Ni foil were laminated in this order in a graphite mold, and pressure sintering was performed. Sintering was carried out in an Ar atmosphere at 700 ° C. for 60 minutes under a pressure of 68 MPa. By embedding the obtained bonded body in resin, polishing it, and observing the cross section, it is possible to confirm the thickness of the reaction layer formed at the interface between the thermoelectric conversion material and the Ti-Al foil and the presence or absence of Al-Sb formation. went.

FIG. 7 shows the SEM-EDX mapping analysis result of Example 1. Since the mapping analysis results are displayed in grayscale, the abundance ratio of the elements is not necessarily displayed absolutely. It was confirmed that no reaction layer exceeding 1 μm was formed in the field of view of about 30 μm along the interface between the Ti—Al foil and Yb _0.3 Co ₄ Sb _{12, and Al—Sb was not formed.} Furthermore, it was confirmed that there were no cracks having a length exceeding 5 μm in the observation field of view of 20 μm square shown by the broken line.

The thermoelectric conversion element of Example 1 was subjected to a high temperature standing test of holding at 600 ° C. for 100 hours in an argon atmosphere. The result of SEM-EDX mapping analysis after the high temperature standing test is shown in FIG. It was confirmed that no reaction layer exceeding 1 μm was formed in the field of view in the range of about 10 μm along the interface between the Ti—Al foil and Yb _0.3 Co ₄ Sb _{12, and Al—Sb was not formed.} bottom. Furthermore, it was confirmed that there were no cracks having a length exceeding 5 μm in the observation field of view of 20 μm square shown by the broken line.

(Example 2)
MnSb powder, which is a sintering aid, was mixed with the thermoelectric conversion material powder of Yb _0.3 Co ₄ Sb ₁₂ at a ratio of 1 mass%, and the green compact was molded at a pressure of 68 MPa. The same procedure as in Example 1 was carried out except that the Al foil was bonded to the Al foil under a pressure of 15 MPa. FIG. 9 shows the SEM-EDX mapping analysis result of Example 2. At the interface between the Ti-Al foil and Yb _0.3 Co ₄ Sb ₁₂ , a reaction layer composed of Ti-Sb, Ti-Co-Sb, Ti-Al-Sb, and Co-Al having an average thickness of 20 μm or less is formed. It was confirmed that Al-Sb was not generated. Furthermore, it was confirmed that there were no cracks having a length exceeding 5 μm in the observation field of view of 20 μm square shown by the broken line. From the results of semi-quantitative analysis of EDX, it is considered that _{Ti-Sb contains a known compound such as TiSb 2.}

(Example 3)
The same procedure as in Example 2 was carried out except that MnSb powder, which is a sintering aid, was mixed with the thermoelectric conversion material powder of _{Yb 0.3} Co ₄ Sb _{12 at a ratio of 1 mass% and bonded under a pressure of 68 MPa.} FIG. 10 shows the SEM-EDX mapping analysis result of Example 3.
At the interface between the Ti-Al foil and Yb _0.3 Co ₄ Sb ₁₂ , a reaction layer composed of Ti-Sb, Ti-Co-Sb, Ti-Al-Sb, and Co-Al having a layer thickness of 30 μm or less is formed, and Al is formed. It was confirmed that -Sb was not generated. Furthermore, it was confirmed that there were no cracks having a length exceeding 5 μm in the observation field of view of 20 μm square shown by the broken line.

(Example 4)
_{The same procedure as in Example 3 was carried out except that CeFe 3.925} Mn _0.075 Sb ₁₂ powder was used as the thermoelectric conversion material powder, held at 580 ° C. for 80 minutes in an Ar atmosphere, and pressure-sintered at 68 MPa. FIG. 11 shows the SEM-EDX mapping analysis result of Example 4.
It was confirmed that a reaction layer composed of Ti-Sb having a layer thickness of 1 μm or less was formed at the interface between the Ti-Al foil and CeFe _3.925 Mn _0.075 Sb _{12, and Al-Sb was not formed.} Furthermore, it was confirmed that there were no cracks having a length exceeding 5 μm in the observation field of view of 20 μm square shown by the broken line.

(Example 5)
When the ingot produced, from the lower side of the crucible to pull solidifying the melt, and a dense Ti-Al foil which underwent HIP process on the ingot after solidification, Yb _0.3 was green compact molded at a pressure of 68MPa The same procedure as in Example 1 was carried out except that the thermoelectric conversion material powder of _{Co 4} Sb _{12 was used.}
At the interface between the Ti-Al foil and Yb _0.3 Co ₄ Sb ₁₂ , a reaction layer composed of Ti-Sb, Ti-Co-Sb, Ti-Al-Sb, and Co-Al having a layer thickness of 30 μm or less is formed, and Al is formed. It was confirmed that -Sb was not generated. Furthermore, it was confirmed that there were no cracks having a length exceeding 5 μm in the observation field of view of 20 μm square shown by the broken line.

The thermoelectric conversion element of Example 5 was subjected to a high temperature standing test of holding at 600 ° C. for 100 hours. The result of SEM-EDX mapping analysis after the high temperature standing test is shown in FIG. A reaction layer composed of 30 μm Ti-Sb, Ti-Co-Sb, Ti-Al-Sb, and Co-Al is formed at the interface between the Ti-Al foil and Yb _0.3 Co ₄ Sb _{12, and the Al-Sb} It was confirmed that there was no formation or cracking.

After the high temperature standing test, the power generation characteristics were evaluated using a thermoelectric characteristic evaluation device (ZEM-3 manufactured by ULVAC Riko). ^{The output factor PF (PF = S 2} / ρ) calculated from the Seebeck coefficient S of the N-type thermoelectric material conversion element and the electrical resistivity ρ was a value that matched within the error range before and after the high temperature standing test.

(Example 6)
The composition of the Ti-Al foil was the same as that of Example 3 except that the composition was Ti-34.2 mass% Al.
A reaction layer composed of Ti-Sb, Ti-Co-Sb, Ti-Al-Sb, and Co-Al having a layer thickness of 30 μm or less was formed at the interface between the Ti-Al foil and Yb _0.3 Co ₄ Sb _12. Although a small amount of Al-Sb was generated in the reaction layer, it was confirmed that cracks exceeding 5 μm did not occur in the observation field of view of 20 μm square.

(Comparative Example 1)
The sieved Ti powder having a mesh size of 45 μm or less and the sieved Al powder having a mesh size of 30 μm or less were mixed at a ratio of 66 mass% of Ti powder and 34 mass% of Al powder, and held at 900 ° C. for 2 hours. The obtained powder was crushed to obtain a Ti—Al alloy powder. The obtained Ti—Al alloy powder and Yb _0.3 Co ₄ Sb ₁₂ powder were mixed at a ratio of 35 mass%, and a green compact was molded by a simple press. The same procedure as in Example 1 was carried out except that a green compact composed of Ti—Al alloy powder and Yb _0.3 Co ₄ Sb _{12 powder was used as the diffusion prevention layer.}

13 (a) and 13 (a)'are cross-sectional SEM observation results of the Ti—Al alloy powder 70 used. The range surrounded by the dotted line shows the crystal grain boundaries, and it can be seen that the inside of each Ti-Al powder is composed of crystal grain groups having crystal grain boundaries having different orientation differences. As a result of the observation, the total number of individual crystal grains of the Ti-Al powder was 67, and the total area of the Ti-Al powder from the cross-sectional observation was 0.0027 mm ² . When this is converted into a rectangular area, 67 crystal grains are included in the range of 50 μm in length × 54 μm in width.

FIG. 13B shows a junction formed between the N-type thermoelectric conversion element 22 and the diffusion prevention layer 50 by sintering using a thermoelectric conversion element _{made of Yb 0.3} Co ₄ Sb _{12 and Ti—Al powder.} 6 is a cross-sectional SEM image showing an interfacial reaction layer. The reaction layer 501 is formed with a reaction layer having a thickness of more than 40 μm, and when it is inferred including the result of the composition analysis indicated by EDX, a structure in which the Al—Sb reaction layer 503 is dispersed in the _{TiSb 2 reaction layer 502 is formed.} Had had. Further, FIG. 13 (c) shows a cross section showing a bonded interfacial reaction layer sintered using a thermoelectric conversion element and Ti—Al powder when heat-treated at a temperature of 500 ° C. for 125 hours in a reduced pressure atmosphere of 10 Pa. It is an SEM image. In the 20 μm square field of view shown by the broken line observed in FIG. 13 (c), it seems that cracks are generated starting from the formed Al—Sb reaction layer 503, and the formation of the Al—Sb reaction layer 503 is generated. It was found to cause a decrease in reliability.

The present invention has been described with reference to examples. The results are summarized in Table 1. The present invention is not limited to these, and the effect can be sufficiently exhibited even when various combinations are used.

10 ... Thermoelectric conversion module 21 ... P-type thermoelectric conversion element 22 ... N-type thermoelectric conversion element 30 ... Electrode 40 ... Ceramics wiring substrate 41 ... First conductive member 42 ... Second conductive member 50 ... Diffusion prevention layer 501 ... Reaction layer 502 ... TiSb ₂ reaction layer 503 ... Al-Sb reaction layer 51 ... metal layer 60 ... bonding layer 70 ... Ti-Al alloy powder 80 ... interface

Claims (3)

It has a step of sintering a powder of a thermoelectric conversion material containing Sb in a state where an alloy foil containing Ti and Al are in contact with each other to obtain a thermoelectric conversion material having a diffusion prevention layer.
The alloy foil containing Ti and Al is used in a cross-section observation method capable of discriminating the orientation difference of crystals.
When observing a region having an area of 5000 μm ² or more, the number of crystal grains defined by the grain boundaries having an orientation difference of 10 degrees or more is 1.0 or less / 5000 μm ² . Production method. The alloy foil containing Ti and Al is
The method for producing a thermoelectric conversion element according to claim 1, wherein the Al content is 8 mass% or more and 36 mass% or less, and the _{mixture has a Ti 3 Al compound.} A thermoelectric conversion module comprising a step of joining a thermoelectric conversion element obtained by the method for manufacturing a thermoelectric conversion element according to claim 1 or 2 and an electrode via the diffusion prevention layer. Production method.

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