Novel thermoelectric conversion molten synthetic body and use thereof

Abstract

Provided are: a thermoelectric conversion molten synthetic body or a thermoelectric conversion sintered body which is composed of a strontium silicide polycrystalline body, wherein the content ratio of a cubic crystal SrSi2 phase is 99.0% by mass or more, and the content ratio of the tetragonal crystal SrSi2 phase and the content ratio of the trigonal crystal SrSi2 phase are both less than 0.1% by mass as quantitative values obtained by Rietveld analysis on the powder X-ray diffraction pattern; and a method for manufacturing the same. Also provided are: a thermoelectric conversion element which uses the above-described thermoelectric conversion molten synthetic body or the thermoelectric conversion sintered body; and a thermoelectric conversion module which is provided with the thermoelectric conversion element.

Description

Novel thermoelectrically convertible molten composite and its applications

[0001] The present invention relates to a thermoelectrically convertible molten composite and a method for producing the same, a thermoelectrically convertible sintered body and a method for producing the same, a thermoelectric conversion element, and a thermoelectric conversion module.

[0002] BACKGROUND ART Conventionally, attempts have been made to generate electricity using thermoelectric conversion materials in order to effectively utilize heat (exhaust heat) emitted from various facilities and the like as energy.

[0003] Until now, bismuth telluride (Bi) has been used as a thermoelectric conversion material that exhibits high thermoelectric conversion performance in the low temperature range of 0 to 300°C. ２ Te ３ However, the constituent elements bismuth and tellurium are toxic and have a large environmental impact, and there are also problems with the limited reserves of these elements in the earth's crust.

[0004] In light of this background, there is a need for the development of thermoelectric materials that have a low environmental impact, are composed of elements that are abundant in the earth's crust, and exhibit high thermoelectric conversion performance in low temperature ranges. In recent years, strontium silicide (SrSi ２ ) has been studied. ２ It is known that SrSi exists in cubic, tetragonal, trigonal and other crystal forms, among which cubic SrSi ２ (α-SrSi ２ Since only ZnO (also called ZnO) exhibits semiconductor properties, it is expected to be used as a thermoelectric conversion material.

[0005] Patent Document 1 describes a method in which a raw material containing strontium and silicon is arc-melted to obtain a strontium silicide alloy, and then the powder of this strontium silicide alloy is sintered to produce a strontium silicide alloy whose main crystal phase is cubic SrSi ２ However, the strontium silicide bulk polycrystalline body of Patent Document 1 is characterized by its bulk density and oxygen content, and no measurement data on thermoelectric conversion performance is given.

[0006] In addition, Patent Document 2 discloses a method for producing a strontium silicide bulk polycrystalline body (sintered body) in the same manner as Patent Document 1, and then sputtering the strontium silicide bulk polycrystalline body to form a cubic SrSi ２ The phase is mainly trigonal SrSi ２ It is described that a thin film containing a SiO 2 phase and / or Si was obtained. Patent Document 2 also shows the thermoelectric conversion performance of the obtained thin film at 300°C.

[0007] In addition, Non-Patent Document 1 discloses that a composition raw material containing strontium and silicon is converted into SrSi by the commonly known vertical Bridgman method. ２ The melt is heated to a temperature above the melting point of SrSi, and a crystal is grown from the melt, so that the main crystalline phase is cubic SrSi. ２ It is described that a strontium silicide polycrystalline body (melt synthesis body) having a phase structure was obtained.

[0008] In addition, Non-Patent Document 2 describes commercially available SrSi ２ After crushing the ingot, the resulting powder was sintered at a temperature of 1073K by spark plasma sintering, and the main crystalline phase was cubic SrSi ２ It is described that a strontium silicide sintered body having a strontium silicide phase was obtained.

[0009] JP 2019-147728 A JP 2019-149523 A

[0010] D. Shiojiri et al., J. Appl. Phys., 129, 115101 (2021)K. Hashimoto et al., J. Appl. Phys., 102, 063703 (2007)

[0011] When strontium silicide alloys are synthesized by the arc melting method as in Patent Documents 1 and 2, the raw materials are rapidly locally heated, resulting in the resulting strontium silicide alloys containing many cracks and voids. Furthermore, since temperature control is difficult with the arc melting method, it is difficult to obtain a strontium silicide alloy whose composition matches that of the raw materials. In fact, in Example 2 of Patent Document 1, although the atomic weight ratio of strontium to silicon in the raw materials is 1:2, the atomic weight ratio of strontium to silicon in the resulting strontium silicide alloy is 30:70. Furthermore, the thin film obtained in Patent Document 2 exhibited a maximum power factor at 300°C of 0.46 × 10 in Example 4. －３ W / mK ２ (film thickness: 1400 nm), which is extremely low. In the case of a thin-film thermoelectric conversion material such as that disclosed in Patent Document 2, the generated electric power is very small, and therefore its applications are thought to be limited.

[0012] On the other hand, when strontium silicide polycrystalline is melt-synthesized by the normal vertical Bridgman method as in Non-Patent Document 1, it is possible to obtain a strontium silicide polycrystalline (melt-synthesized product) whose composition matches that of the raw material. However, when the present inventors analyzed the strontium silicide polycrystalline obtained by the manufacturing method in Non-Patent Document 1, they found that the quantitative value obtained by Rietveld analysis of the powder X-ray diffraction pattern was about 3 mass % of tetragonal SrSi ２ phase (β-SrSi ２ It was confirmed that the tetragonal SrSi ２ is not a semiconductor but a conductor at a metal level. Non-Patent Document 1 describes that the band gap value of the obtained strontium silicide polycrystalline body was 13.1 meV, and this low band gap value is due to the fact that the strontium silicide polycrystalline body contains tetragonal SrSi ２ This is thought to be due to the presence of a phase. Strontium silicide polycrystalline bodies with such a low band gap value are unsuitable as semiconductor thermoelectric conversion materials.

[0013] In addition, Non-Patent Document 2 reports that the power factor of a strontium silicide sintered body manufactured from a commercially available ingot is 1.19 × 10 at 331 K in the low temperature range. －３ W / mK ２ However, it is still insufficient for practical use.

[0014] Incidentally, when manufacturing materials (referred to as thermoelectric conversion members) constituting the thermoelectric conversion units of thermoelectric conversion elements used in thermoelectric conversion modules, it is common to crush and sinter a thermoelectric conversion ingot obtained by arc melting or melt synthesis to form a sintered body, and then cut out a thermoelectric conversion member of the desired shape and size from the sintered body. In this regard, if the thermoelectric conversion member could be cut out directly from the thermoelectric conversion ingot itself, the crushing and sintering steps could be omitted, which would be convenient. However, the arc melting method described in Patent Documents 1 and 2 results in the strontium silicide alloy obtained containing many cracks and voids, making it unsuitable for cutting out thermoelectric conversion members. On the other hand, the melt synthesis method described in Non-Patent Document 1 makes it possible to obtain a strontium silicide polycrystalline body (melt synthesis body) without cracks or voids. However, the present inventors have confirmed that the strontium silicide polycrystalline body contains tetragonal SrSi ２ If a large amount of hydroxyapatite is contained, the cut portion collapses when cutting out the thermoelectric conversion member, making it difficult to obtain a thermoelectric conversion member of the desired shape and size.

[0015] The present invention has been made in view of the above, and has an object to provide a novel thermoelectrically convertible molten composite body that is a non-sintered body useful as a thermoelectric conversion material in the low temperature range (approximately 0 to 300°C) and has properties that allow thermoelectric conversion members of desired shapes and sizes to be cut out, and a method for producing the same; a thermoelectrically convertible sintered body that is useful as a thermoelectric conversion material in the low temperature range and from which thermoelectric conversion members of desired shapes and sizes can be cut out, and a method for producing the same; a thermoelectric conversion element using the above thermoelectrically convertible molten composite or the above thermoelectrically convertible sintered body; and a thermoelectric conversion module that includes the above thermoelectric conversion element.

[0016] Specific means for solving the above problems include the following embodiments: <1> A thermoelectrically convertible molten composite composed of a strontium silicide polycrystal, wherein the strontium silicide polycrystal is a cubic SrSi as determined by Rietveld analysis of a powder X-ray diffraction pattern. ２ The content of the tetragonal SrSi phase is 99.0 mass% or more. ２ phase and trigonal SrSi ２ A thermoelectrically convertible molten composite in which the content of each of the phases is less than 0.1 mass %.

[0017] <2> The thermoelectric convertible molten composite according to <1>, wherein the Si content is less than 0.1 mass % as determined by Rietveld analysis of a powder X-ray diffraction pattern.

[0018] <3> The thermoelectric convertible molten composite according to <1> or <2>, wherein the Sr content is less than 0.1 mass % as a quantitative value determined by Rietveld analysis of a powder X-ray diffraction pattern.

[0019] <4> The power factor at 310K is 2.50 × 10 －３ W / mK ２ The thermoelectric convertible molten composite material according to any one of <1> to <3> above.

[0020] <5> The thermoelectric convertible melt composite material according to any one of <1> to <4>, having a band gap of 45.0 meV or more.

[0021] <6> A thermoelectric convertible sintered body composed of a strontium silicide polycrystalline body, wherein the strontium silicide polycrystalline body is a cubic SrSi as a quantitative value obtained by Rietveld analysis of a powder X-ray diffraction pattern. ２ The content of the tetragonal SrSi phase is 99.0 mass% or more. ２ phase and trigonal SrSi ２ A thermoelectrically convertible sintered body, in which the content of each of the phases is less than 0.1 mass %.

[0022] <7> The thermoelectrically convertible sintered body according to <6>, obtained by sintering the thermoelectrically convertible molten composite body according to any one of <1> to <5>.

[0023] <8> The thermoelectric convertible sintered body according to <6> or <7>, wherein the Si content is less than 0.8 mass % as a quantitative value obtained by Rietveld analysis of a powder X-ray diffraction pattern.

[0024] <9> The thermoelectric convertible sintered body according to any one of <6> to <8>, wherein the Sr content is less than 0.1 mass % as a quantitative value obtained by Rietveld analysis of a powder X-ray diffraction pattern.

[0025] <10> The power factor at 310K is 2.35 × 10 －３ W / mK ２ The thermoelectric convertible sintered body according to any one of <6> to <9> above.

[0026] <11> The thermoelectric convertible sintered body according to any one of <6> to <10>, having a band gap of 50.0 meV or more.

[0027] <12> A thermoelectric conversion element comprising: a thermoelectric conversion part made of the thermoelectric convertible molten composite according to any one of <1> to <5> or the thermoelectric convertible sintered body according to any one of <6> to <11>; and a first electrode and a second electrode provided on the thermoelectric conversion part.

[0028] <13> A thermoelectric conversion module including the thermoelectric conversion element according to <12>.

[0029] <14> A method for producing a thermoelectrically convertible molten composite according to any one of <1> to <5>, comprising: ２ and heating at a temperature below the melting point of SrSi ２ a solid-liquid phase reaction step for synthesizing SrSi synthesized in the solid-liquid phase reaction step; ２ a melting step of heating the above to obtain a molten liquid; and a crystal growing step of growing crystals from the molten liquid to obtain a strontium silicide polycrystalline body.

[0030] <15> The method for producing a thermoelectric convertible molten composite according to <14>, wherein the atomic ratio of strontium to silicon in the raw material composition is 33.0:67.0 to 34.5:65.5.

[0031] <16> The method for producing a thermoelectric convertible molten composite according to <14> or <15>, wherein in the crystal growth step, the temperature of the molten liquid is decreased at a temperature decreasing rate of 0.223 to 4.72°C / h until the temperature of the molten liquid reaches 1100°C, and after the temperature of the molten liquid reaches 1100°C, at a temperature decreasing rate of 0.494 to 10.38°C / h until the temperature of the molten liquid reaches 1000°C.

[0032] <17> The method for producing a thermoelectric convertible molten composite according to any one of <14> to <16>, wherein the solid-liquid phase reaction step, the melting step, and the crystal growth step are carried out using a vertical Bridgman furnace.

[0033] <18> A method for producing a thermoelectrically convertible sintered body, comprising a sintering step of sintering a thermoelectrically convertible molten composite body obtained by the production method according to any one of <14> to <17>.

[0034] According to the present invention, it is possible to provide a novel thermoelectrically convertible molten composite and a method for producing the same, a novel thermoelectrically convertible sintered body and a method for producing the same, a thermoelectric conversion element using the above-mentioned thermoelectrically convertible molten composite or the above-mentioned thermoelectrically convertible sintered body, and a thermoelectric conversion module including the above-mentioned thermoelectric conversion element.

[0035] 1 is a diagram showing an example of a vertical Bridgman furnace; FIG. 2 is a diagram showing the movable range (0 mm - 200 mm) of the heating part of the vertical Bridgman furnace; FIG. 3 is a diagram showing an example of a sintering apparatus; FIG. 4 is a diagram showing an example of a thermoelectric conversion module; FIG. 5 is a diagram showing another example of a thermoelectric conversion module; FIG. 6 is a diagram showing the relationship between the position of the heating part of the vertical Bridgman furnace (0 mm - 200 mm) and the temperature directly below the crucible; FIG. 7 is a diagram showing powder X-ray diffraction patterns of each sample obtained from the ingot of Example 1; FIG. 8 is a diagram showing powder X-ray diffraction patterns of each sample obtained from the ingot of Example 1; FIG. 9 is a diagram showing powder X-ray diffraction patterns of each sample obtained from the ingot of Example 2; FIG. 10 is a graph showing the relationship between temperature and Seebeck coefficient for test pieces obtained from the ingots of Examples 1 and 2 and for test pieces obtained from the sintered body of Example 3; FIG. 11 is a graph showing the relationship between temperature and electrical conductivity for test pieces obtained from the ingots of Examples 1 and 2 and for test pieces obtained from the sintered body of Example 3. 1 is a graph showing the relationship between temperature and power factor for test pieces obtained from the ingots of Examples 1 and 2 and the test piece obtained from the sintered body of Example 3. FIG. 2 is a diagram showing a backscattered electron image and a secondary electron image of the ingot of Example 4 observed with a scanning electron microscope. FIG. 3 is a diagram showing powder X-ray diffraction patterns of each sample obtained from the ingot of Example 4. FIG. 4 is a diagram showing powder X-ray diffraction patterns of each sample obtained from the ingot of Example 4. FIG. 5 is a diagram showing backscattered electron images and secondary electron images of the ingot of Example 5 observed with a scanning electron microscope. FIG. 6 is a diagram showing powder X-ray diffraction patterns of each sample obtained from the ingot of Example 5. FIG. 7 is a diagram showing powder X-ray diffraction patterns of each sample obtained from the ingot of Example 5. FIG. 8 is a diagram showing powder X-ray diffraction patterns of the sample obtained from the ingot of Comparative Example 1. FIG. 9 is a diagram showing powder X-ray diffraction patterns of the sample obtained from the ingot of Comparative Example 1. FIG. 10 is a diagram showing powder X-ray diffraction patterns of the sample obtained from the ingot of Comparative Example 2. FIG. 11 is a graph showing the relationship between temperature and Seebeck coefficient for test pieces obtained from the ingots of Examples 4 and 5 and Comparative Example 1. 1 is a graph showing the relationship between temperature and electrical conductivity in test pieces obtained from the ingots of Examples 4 and 5 and Comparative Example 1.1 is a graph showing the relationship between temperature and power factor for test pieces obtained from each ingot of Examples 4 and 5 and Comparative Example 1. It is a diagram showing optical microscope images of each sintered body of Examples 6 to 8 observed with an optical microscope and backscattered electron images of each sintered body of Examples 6 to 8 observed with a scanning electron microscope. It is a diagram showing powder X-ray diffraction patterns of samples obtained from each sintered body of Examples 6 to 8. It is a diagram showing powder X-ray diffraction patterns of samples obtained from each sintered body of Examples 6 to 8. It is a graph showing the relationship between temperature and Seebeck coefficient for test pieces obtained from each sintered body of Examples 6 to 8 and Comparative Example 3. It is a graph showing the relationship between temperature and electrical conductivity for test pieces obtained from each sintered body of Examples 6 to 8 and Comparative Example 3. It is a graph showing the relationship between temperature and power factor for test pieces obtained from each sintered body of Examples 6 to 8 and Comparative Example 3.

[0036] Specific embodiments to which the present invention is applied will be described in detail below. In this specification, the expression "x to y" using numerical values ​​x and y means "greater than or equal to x and less than or equal to y" unless otherwise specified. In such an expression, when a unit is assigned only to the numerical value y, the unit also applies to the numerical value x.

[0037] <Thermoelectrically convertible melt composite> The thermoelectrically convertible melt composite according to this embodiment (hereinafter also simply referred to as "melt composite") has a crystalline phase of substantially cubic SrSi ２ The term "melt synthesized body" refers to a solidified body obtained by heating raw material components to prepare a melt, and then growing crystals while lowering the temperature of the melt.

[0038] The melt synthesized body according to this embodiment has a cubic SrSi as a quantitative value obtained by Rietveld analysis of the powder X-ray diffraction pattern. ２ The melt synthesized body according to this embodiment has a tetragonal SrSi phase content of 99.0% by mass or more, preferably 99.5% by mass or more, as a quantitative value obtained by Rietveld analysis of the powder X-ray diffraction pattern. ２ phase and trigonal SrSi ２The contents of the phases are preferably less than 0.1% by mass, and are preferably below the detection limit. Furthermore, in the melt composite according to this embodiment, the Sr and Si contents are preferably less than 0.1% by mass, and more preferably below the detection limit, as quantitative values ​​determined by Rietveld analysis of the powder X-ray diffraction pattern. The conditions for the powder X-ray diffraction analysis and Rietveld analysis are the same as those described in the Examples below.

[0039] Generally, when a thermoelectric conversion element is manufactured using a molten composite, if an attempt is made to cut a thermoelectric conversion member directly from the molten composite, the cut portion collapses, making it difficult to obtain a thermoelectric conversion member of the desired shape. For this reason, the molten composite is powdered and then sintered to form a sintered body, from which the thermoelectric conversion member is cut.

[0040] In contrast, the melt composite according to this embodiment has a crystalline phase of substantially cubic SrSi ２ Since the melt composite according to this embodiment is composed of only a strontium silicide polycrystalline body, the crystalline phase is homogeneous and has excellent density, as well as excellent mechanical properties such as Young's modulus and fracture toughness. Therefore, with the melt composite according to this embodiment, it is possible to cut out a thermoelectric conversion member of a desired shape using a wire saw or the like without the cut-out portion crumbling, and as a result, it is possible to omit the conventional steps of crushing and sintering.

[0041] The thermoelectrically convertible sintered body according to the present embodiment, which will be described later, is obtained by sintering the powder of the melt composite according to the present embodiment, and therefore has minute voids and interfaces between powder particles that do not affect the thermoelectric conversion performance. In contrast, the melt composite according to the present embodiment is obtained by growing crystals from a molten liquid obtained by melting the raw material components, and therefore has no voids and no interfaces between powder particles. This difference can be confirmed by an optical microscope (Nomarski contrast) and a reflected electron microscope.

[0042] In addition, the melt synthesized body according to this embodiment has a crystal phase of substantially cubic SrSi ２Since the melt composite according to this embodiment is composed of only the strontium silicide polycrystalline phase, it exhibits semiconductor properties. ｇ ) is preferably 45.0 meV or more, more preferably 46.0 meV or more, and even more preferably 47.0 meV or more. ｇ ) is not particularly limited, and is, for example, about 60.0 meV.

[0043] In addition, the melt synthesized body according to this embodiment has a crystal phase of substantially cubic SrSi ２ Since the melt composite according to this embodiment is composed of only the strontium silicide polycrystalline phase, it exhibits high thermoelectric conversion performance in the low temperature range (approximately 0 to 300°C). The power factor at 310K of the melt composite according to this embodiment is 2.50 × 10 －３ W / mK ２ It is preferable that the value is equal to or greater than 2.60×10 －３ W / mK ２ More preferably, it is 2.70×10 or more. －３ W / mK ２ The upper limit of the power factor at 310K is not particularly limited, and is, for example, 3.20 W / mK. ２ That's about it.

[0044] Furthermore, the melt composite according to this embodiment exhibits excellent mechanical properties. The Young's modulus of the melt composite according to this embodiment is preferably 85.0 GPa or more, more preferably 88.0 GPa or more, and even more preferably 90.0 GPa or more. The upper limit of the Young's modulus is not particularly limited, and is, for example, about 100.0 GPa. Furthermore, the fracture toughness value of the melt composite according to this embodiment is 1.0 MPa m １／２ It is preferable that the viscosity is 1.2 MPa m or more. １／２ More preferably, it is 1.3 MPa m or more. １／２ The upper limit of the fracture toughness value is not particularly limited, and is, for example, 2.0 MPa m １／２ The Young's modulus and fracture toughness of the molten composite are measured by the method described in the Examples below.

[0045] The inventors have found that the tetragonal SrSi ２ It has been confirmed that the higher the content of the tetragonal SrSi phase, the more susceptible the molten composite is to oxidation degradation, and the more difficult it is to cut out the thermoelectric conversion element. ２ In this regard, the melt synthesized body according to this embodiment has a crystal phase that is substantially cubic SrSi ２ Since it is made of strontium silicide polycrystalline material, which is only a phase, it has excellent durability and can be easily cut out into thermoelectric conversion components.

[0046] The molten composite according to this embodiment may contain a dopant, such as a p-type dopant such as Ba, Ag, or Ga; or an n-type dopant such as N, P, or Sb.

[0047] <Method of Manufacturing Melt Composite> The melt composite according to this embodiment is a SrSi ２ (1121°C) to form SrSi ２ and a solid-liquid phase reaction process for synthesizing SrSi synthesized in the solid-liquid phase reaction process. ２ SrSi ２ SrSi above the melting point ２ The strontium silicide can be produced by a production method (melt synthesis method) including a melting step of heating the strontium silicide at a temperature at which it does not decompose to obtain a molten liquid, and a crystal growing step of growing crystals from the molten liquid to obtain a strontium silicide polycrystalline body.

[0048] Generally, when melt synthesizing an alloy, the raw materials are heated to a temperature equal to or higher than the melting point of the alloy, and a molten liquid of the alloy is prepared in one step while synthesizing the alloy, and then a crystal is grown while the temperature of the molten liquid is lowered. In other words, it is common to prepare a molten liquid of the alloy while synthesizing the alloy, and then grow a crystal from the molten liquid. In Non-Patent Document 1, the raw materials are also SrSi ２ By heating at a temperature higher than the melting point (1121°C) of SrSi ２ While synthesizing SrSi ２However, according to the investigations of the present inventors, a melt of SrSi ２ While synthesizing SrSi ２ When preparing a melt of SrSi ２ The high temperature during the synthesis of tetragonal SrSi ２ and other impurities precipitate.

[0049] In contrast, in the method for producing a molten composite according to this embodiment, SrSi ２ While synthesizing SrSi ２ The melt is not prepared in one step, but is divided into two steps, a solid-liquid phase reaction step and a melting step, to form SrSi ２ At temperatures below the melting point of SrSi ２ After completing the synthesis of SrSi ２ According to this manufacturing method, the crystal phase is substantially cubic SrSi ２ It is possible to obtain a homogeneous strontium silicide polycrystalline body (melt synthesis body) that is only a single phase.

[0050] The molten composite produced by the method for producing a molten composite according to this embodiment exhibits a high band gap value, high thermoelectric conversion performance, and excellent mechanical properties because the crystalline phase is substantially cubic SrSi ２ This is because the strontium silicide is composed of a homogeneous polycrystalline strontium silicide consisting of only the strontium silicide phase. According to the method for producing a molten composite according to this embodiment, it is possible to obtain a large molten composite that is useful in industry.

[0051] The raw materials used in the method for producing a molten composite according to this embodiment and each step of the method will be described in detail below.

[0052] [Raw Material Composition] The raw material composition used in producing the molten composite according to this embodiment contains strontium and silicon.

[0053] As the strontium in the composition raw material, for example, high-purity strontium having a purity of 2N or more (99% by mass or more) is preferably used. Since strontium becomes liquid in the solid-liquid phase reaction step described below, the size of strontium in the composition raw material is not particularly limited. However, from the viewpoint of reaction efficiency in the solid-liquid phase reaction step, it is preferable that the size of strontium in the composition raw material is 3 mm square or less.

[0054] As the silicon in the composition raw material, for example, high-purity silicon with a purity of 5N or more (99.999% by mass or more) is preferably used. Since silicon becomes a solid phase in the solid-liquid phase reaction process described below, it is preferable to powder it and then classify it from the viewpoint of reaction efficiency in the solid-liquid phase reaction process. The particle size of the silicon powder is preferably 1 to 100 μm, and more preferably 25 to 75 μm.

[0055] The atomic weight ratio of strontium to silicon in the raw material is about 1:2, and preferably 33.0:67.0 to 34.5:65.5.

[0056] [Melt Synthesis Method] The solid-liquid phase reaction step, melting step, and crystal growth step described below are usually carried out by loading the raw material components into a crucible, sealing it with a lid, and placing it in a heating furnace.

[0057] The pressure conditions during melt synthesis may be atmospheric pressure, but a positive pressure of about +0.5 to 3.5 kPa is preferable. Furthermore, the atmospheric conditions during melt synthesis are preferably an inert gas atmosphere such as Ar gas, and more preferably a reducing atmosphere in which an inert gas is mixed with about 3 to 7 volume % of hydrogen gas. By performing melt synthesis in such a reducing atmosphere, oxide generation is suppressed and the ingot after melt synthesis tends to be easier to remove from the crucible.

[0058] (Solid-liquid phase reaction process) In the solid-liquid phase reaction process, the raw material is heated to 1000°C or more to form SrSi ２ and reacting strontium with silicon to form SrSi ２ Synthesize the following.

[0059] The heating temperature in the solid-liquid phase reaction step (solid-liquid phase reaction temperature) must be lower than the melting temperature in the subsequent step. However, as the heating temperature decreases, the tetragonal SrSi ２ Therefore, the heating temperature (solid-liquid phase reaction temperature) is preferably, for example, 1035 to 1055° C. The rate of temperature increase until the solid-liquid phase reaction temperature is reached is not particularly limited.

[0060] The heating time in the solid-liquid phase reaction step may be any time that allows the solid-liquid phase reaction between strontium and silicon to be completed, and is preferably, for example, 1 to 20 hours, and more preferably 3 to 10 hours.

[0061] (Melting step) In the melting step, the SrSi synthesized in the solid-liquid phase reaction step is melted. ２ SrSi ２ SrSi above the melting point ２ The mixture is heated at a temperature at which the melt does not decompose, to obtain a homogeneous melt without unevenness.

[0062] In order to obtain a homogeneous molten liquid, the heating temperature (melting temperature) in the melting step is, for example, preferably 1121 to 1150° C., and more preferably 1130 to 1135° C. The rate of temperature increase until the melting temperature is reached is not particularly limited.

[0063] The heating time in the melting step is determined based on the synthesized SrSi ２ The time may be any time long enough for the above to be completely melted, and for example, it is preferably 1 to 10 hours, and more preferably 3 to 5 hours.

[0064] (Crystal Growth Step) In the crystal growth step, the temperature of the melt is lowered to grow crystals from the melt, thereby obtaining a strontium silicide polycrystalline body (melt synthesized body).

[0065] The crystalline phase is substantially cubic SrSi ２To obtain a strontium silicide polycrystalline body consisting only of the tetragonal phase, it is preferable to reduce the cooling rate, especially in the initial stage of crystal growth. Specifically, it is preferable to reduce the cooling rate by 0.223 to 4.72°C / h until the temperature of the molten liquid reaches 1100°C, and then by 0.494 to 10.38°C / h until the temperature of the molten liquid reaches 1100°C. After the temperature of the molten liquid reaches 1000°C, it is slowly cooled to room temperature. In this way, by growing a crystal while adjusting the cooling rate, it is possible to obtain tetragonal SrSi ２ and trigonal SrSi ２ Therefore, a strontium silicide polycrystalline body can be obtained that is substantially free of voids and cracks.

[0066] In the crystal growth process, it is preferable to provide a temperature gradient such that the temperature at the bottom of the crucible is relatively low and the temperature at the mouth is relatively high. By providing such a temperature gradient, unreacted silicon and Sr are deposited on the upper side (the mouth side of the crucible) which is the terminal side of the crystal growth. ２ (SiO ４ By removing these impurities, a molten composite body made of highly pure strontium silicide polycrystalline material can be obtained.

[0067] From the viewpoint of easily realizing the temperature gradient, it is preferable that the solid-liquid reaction step, the melting step, and the crystal growth step are all performed using a vertical Bridgman furnace. A vertical Bridgman furnace allows the temperatures of multiple heaters arranged vertically to be set independently, making it possible to easily realize a temperature gradient in which the temperature is relatively low at the bottom and relatively high at the top. The vertical Bridgman furnace may be configured such that the crucible is fixed and the heating unit surrounding the crucible is movable in the vertical direction, or may be configured such that the heating unit surrounding the crucible is fixed and the crucible is movable in the vertical direction.

[0068] Although this may vary depending on the temperature gradient formed in the vertical Bridgman furnace, when a strontium silicide polycrystalline body (a molten composite) is obtained using a vertical Bridgman furnace in which the heating part is configured to be movable in the vertical direction, it is generally preferable to set the moving speed of the heating part to 0.5 to 7.2 mm / h. ２ and trigonal SrSi ２ In order to obtain a strontium silicide polycrystalline body that is substantially free of , the moving speed of the heating part is more preferably 0.5 to 5.5 mm / h, and even more preferably 1.0 to 4.5 mm / h.

[0069] An example of a vertical Bridgman furnace that can be used in the method for producing a molten composite according to this embodiment is shown in Fig. 1. The vertical Bridgman furnace 1 shown in Fig. 1 includes a furnace body 10 and a heating section 20 that surrounds the furnace body 10.

[0070] The furnace body 10 includes a cylindrical quartz glass tube 11 arranged with its axis oriented vertically, a graphite sample stage 12 arranged inside the quartz glass tube 11 and on which the crucible 30 and quartz cover 31 are placed, a heat shield 13 arranged above the interior of the quartz glass tube 11, and a thermocouple 14 for measuring the temperature directly below the crucible. A pressure reduction mechanism for reducing the pressure inside the quartz glass tube 11 and an Ar gas supply mechanism for supplying Ar gas into the quartz glass tube 11 are connected to the quartz glass tube 11.

[0071] The heating unit 20 has three heaters 21A, 21B, and 21C arranged vertically around the quartz glass tube 11, a heat insulating material 22 covering the heaters 21A, 21B, and 21C, and thermocouples 23A, 23B, and 23C for measuring the temperatures of the heaters 21A, 21B, and 21C, respectively. The temperatures of the heaters 21A, 21B, and 21C can be set independently, and when producing a molten composite, a temperature gradient is formed in which the temperature on the lower side (the heater 21C side) is relatively low and the temperature on the upper side (the heater 21A side) is relatively high.

[0072] The vertical Bridgman furnace 1 shown in Fig. 1 is configured so that the heating unit 20 can be moved vertically. Specifically, the heating unit 20 can be moved vertically from the bottom position at 0 mm (left side of Fig. 2) to the top position at 200 mm (right side of Fig. 2). The temperature of the crucible can be increased by moving the heating unit 20 downward, and the temperature of the crucible can be decreased by moving the heating unit 20 upward.

[0073] <Thermoelectrically convertible sintered body> The thermoelectrically convertible sintered body according to this embodiment (hereinafter also simply referred to as "sintered body") has a crystal phase of substantially cubic SrSi ２ The melt synthesized body according to the present embodiment is excellent in sinterability, and therefore a dense sintered body without cracks can be obtained.

[0074] The sintered body according to this embodiment, like the melt synthesized body according to this embodiment, has a cubic SrSi ２ The sintered body according to this embodiment has a tetragonal SrSi phase content of 99.0% by mass or more, preferably 99.5% by mass or more, as a quantitative value obtained by Rietveld analysis of the powder X-ray diffraction pattern. ２ phase and trigonal SrSi ２ The contents of all of the phases are preferably less than 0.1% by mass, and are preferably below the detection limit. Furthermore, the sintered body according to this embodiment preferably has a Sr content of less than 0.1% by mass, and more preferably below the detection limit, as a quantitative value determined by Rietveld analysis of a powder X-ray diffraction pattern. Furthermore, the sintered body according to this embodiment preferably has a Si content of less than 0.8% by mass, and more preferably less than 0.1% by mass, and even more preferably below the detection limit, as a quantitative value determined by Rietveld analysis of a powder X-ray diffraction pattern.

[0075] The sintered body according to this embodiment exhibits semiconductor properties, similar to the melt synthesized body according to this embodiment. The band gap value (E ｇ) is preferably 50.0 meV or more, more preferably 51.0 meV or more, and even more preferably 52.0 meV or more. ｇ ) is not particularly limited, and is, for example, about 70.0 meV.

[0076] Furthermore, the sintered body according to this embodiment exhibits high thermoelectric conversion performance in the low temperature range (approximately 0 to 300°C), similar to the melt-synthesized body according to this embodiment. The power factor of the sintered body according to this embodiment at 310K is 2.35 × 10 －３ W / mK ２ It is preferable that the value is equal to or greater than 2.40×10 －３ W / mK ２ More preferably, it is 2.45×10 or more. －３ W / mK ２ The upper limit of the power factor at 310 K is not particularly limited, and is, for example, 3.00×10 －３ W / mK ２ That's about it.

[0077] Furthermore, the sintered body according to this embodiment exhibits excellent mechanical properties, similar to the melt-synthesized body according to this embodiment. The Young's modulus of the sintered body according to this embodiment is preferably 80.0 GPa or more, more preferably 85.0 GPa or more, and even more preferably 86.0 GPa or more. The upper limit of the Young's modulus is not particularly limited, and is, for example, about 100.0 GPa. Furthermore, the fracture toughness value of the sintered body according to this embodiment is 1.3 MPa m １／２ It is preferable that the viscosity is 1.5 MPa m or more. １／２ More preferably, it is 1.6 MPa m or more. １／２ The upper limit of the fracture toughness value is not particularly limited, and is, for example, 2.0 MPa m １／２ That's about it.

[0078] <Method for Manufacturing Sintered Body> The sintered body according to this embodiment can be manufactured by a manufacturing method including a sintering step of sintering the melt-synthesized body according to this embodiment described above.

[0079] Before sintering, it is preferable to powderize the molten composite according to this embodiment and then classify the powder. This tends to produce a dense sintered body without voids. The particle size of the powdered molten composite is preferably 10 to 100 μm, and more preferably 25 to 45 μm.

[0080] For sintering, pressure compression sintering methods such as hot press sintering (HP), hot isostatic pressing (HIP), and spark plasma sintering can be used, with spark plasma sintering being preferred.

[0081] For example, a jig as shown in Fig. 3 is used for pressure compression sintering. During sintering, first, the powder of the molten composite is filled into the space surrounded by the graphite die 50 and graphite punches 51a and 51b shown in Fig. 3. At this time, to prevent adhesion, it is preferable to sandwich carbon paper between the contact points of the powder of the molten composite and the graphite die 50 and the graphite punches 51a and 51b. Thereafter, sintering is performed using a sintering apparatus such as a spark plasma sintering apparatus. Sintering is preferably performed under reduced pressure and in an inert gas atmosphere.

[0082] The sintering pressure for the pressure compression sintering is preferably 60 to 150 MPa. By setting the sintering pressure to 60 MPa or more, it tends to be easier to obtain a sintered body having a sufficient density (for example, a relative density of 97% or more with respect to the theoretical density).

[0083] The sintering temperature for the pressure compression sintering is preferably 850 to 950°C. By setting the sintering temperature to 850°C or higher, a sintered body with sufficient density tends to be obtained. On the other hand, by setting the sintering temperature to 950°C or lower, damage to the strontium silicide polycrystalline body and the generation of voids tend to be suppressed, and scattering of strontium tends to be prevented.

[0084] <Thermoelectric Conversion Element> The thermoelectric conversion element according to this embodiment includes a thermoelectric conversion part made of the molten composite or sintered body according to this embodiment described above, and a first electrode and a second electrode provided on the thermoelectric conversion part.

[0085] The thermoelectric conversion section can be a thermoelectric conversion element obtained by cutting the molten composite or sintered compact according to this embodiment to a desired size using a wire saw or the like. As described above, the molten composite according to this embodiment is homogeneous and has excellent density and mechanical properties (Young's modulus, fracture toughness, etc.), so the thermoelectric conversion element can be cut directly from the molten composite without the cut-out portion collapsing. Furthermore, the molten composite according to this embodiment also has excellent sinterability, so a sintered body can be obtained that is free of cracks, has excellent density, and is also excellent in mechanical properties (Young's modulus, fracture toughness, etc.). This tends to improve the yield when cutting out thermoelectric conversion elements from a sintered body.

[0086] The method for forming the first electrode and the second electrode is not particularly limited. Examples include plating the molten composite or sintered compact with electroless nickel plating or the like, or applying a conductive paste to the molten composite or sintered compact followed by firing. Another example is a method in which electrodes are formed simultaneously during the production of the sintered compact by layering a metal powder for forming the electrodes, a powder of the molten composite, and a metal powder for forming the electrodes in this order, and then subjecting the sintered compact to pressure compression sintering.

[0087] <Thermoelectric Conversion Module> The thermoelectric conversion module according to this embodiment includes the thermoelectric conversion element according to the embodiment described above. The configuration of the thermoelectric conversion module according to this embodiment is not particularly limited, and any configuration such as a π type or a uni-leg type can be adopted.

[0088] An example of a thermoelectric conversion module according to this embodiment is shown in Fig. 4. In the thermoelectric conversion module shown in Fig. 4, electrodes 101 and 111 are provided at the upper ends of an n-type thermoelectric conversion unit 100 and a p-type thermoelectric conversion unit 110 that are arranged side by side, and electrodes 102 and 112 are provided at the lower ends. The electrodes 101 and 111 are connected to form an integrated electrode, while the electrodes 102 and 112 are configured separately. The molten composite or sintered body according to this embodiment is used, for example, for the p-type thermoelectric conversion unit 110.

[0089] 4, by heating the electrodes 101, 111 and dissipating heat from the electrodes 102, 112, a positive temperature difference (Th-Tc) occurs between the electrodes 101, 111 and the electrodes 102, 112, and thermally excited carriers cause the p-type thermoelectric conversion unit 110 to have a higher potential than the n-type thermoelectric conversion unit 100. At this time, by connecting a load 130 between the electrodes 102 and 112, a current flows from the p-type thermoelectric conversion unit 110 to the n-type thermoelectric conversion unit 100.

[0090] Another example of the thermoelectric conversion module according to this embodiment is shown in Fig. 5. In the thermoelectric conversion module shown in Fig. 5, an electrode 121 is provided at the upper end of a thermoelectric conversion unit 120, and an electrode 122 is provided at the lower end thereof.

[0091] 5, by heating the electrode 121 side and dissipating heat from the electrode 122 side, a positive temperature difference (Th-Tc) occurs between the electrodes 121 and 122, and the electrode 121 side has a higher potential than the electrode 122 side. At this time, by connecting a load 130 between the electrodes 121 and 122, a current flows from the electrode 121 side to the electrode 122 side.

[0092] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples.

[0093] Example 1 Preparation of Raw Materials Irregularly shaped strontium (purity 2N, manufactured by Kojundo Chemical Laboratory Co., Ltd.) that had been stored in oil was washed three times with acetone in a glove box under an Ar gas atmosphere, and then cut into 1-3 mm squares.

[0094] Furthermore, 2 to 5 mm granular silicon (purity 5N, manufactured by Kojundo Chemical Laboratory Co., Ltd.) and 20 mm diameter zirconia balls were placed in a zirconia container and pulverized using a planetary ball mill (PULVERISETTE 7, manufactured by FRITSCH) at 400 rpm for 2 minutes. After pulverization, the mixture was classified using a stainless steel sieve to obtain a powder with a particle size of 75 to 212 μm.

[0095] [Preparation of Crucible] A crucible made of boron nitride (BN) (manufactured by Denka Co., Ltd.) was prepared as the crucible. The internal height of the crucible was 70 mm, and the upper side from the mouth to a position 56.5 mm in the depth direction was approximately cylindrical, while the lower side had an approximately conical internal shape to facilitate the formation of a temperature gradient. The inner diameter of the mouth was 38.75 mm, the inner diameter at a position 56.5 mm in the depth direction from the mouth was 31.2 mm, and the inner diameter of the bottom was 3 mm. To prevent a reaction between the strontium melt and the crucible, a mold release agent (Whitey Seven, manufactured by Odec Co., Ltd.) diluted with ethanol was poured into the crucible to coat the inner wall, and the crucible was dried in a drying furnace (80°C, 1 hour or more) and then baked in an electric furnace (1140°C, 10 hours). Thereafter, BN spray (manufactured by Kaken Tech Co., Ltd.) was applied to the entire inner wall.

[0096] An alumina lid (custom-made by Nikkato Corp.) was prepared as the lid for the crucible. The outer surface of the lid was coated with the above-mentioned mold release agent.

[0097] [Melt Synthesis of Strontium Silicide Polycrystalline Body] A vertical Bridgman furnace (gas replacement type vertical vacuum heating furnace, custom-made by Heattech Co., Ltd.) having the configuration shown in FIG. 1 was used as the heating furnace.

[0098] Strontium (20.000 g) and silicon (12.822 g) were weighed out and loaded into a crucible as raw materials so that the atomic weight ratio of strontium to silicon was 33.33:66.67. The crucible was then covered with a lid and placed on a graphite sample holder (manufactured by Ueki Corporation), and the entire crucible was covered with a quartz cover. The interior of the heating furnace was then evacuated to a negative pressure of -98 kPa or less, and then filled with Ar gas three times until the pressure reached -50 kPa. Finally, Ar gas was filled until the pressure reached +2.4 kPa.

[0099] During the melt synthesis of strontium silicide polycrystalline, the heating unit was first fixed at a position 123 mm from the bottom end, and the temperatures of the upper heater (heater 21A) and middle heater (heater 21B) were set to 1130°C, while the temperature of the lower heater (heater 21C) was set to 890°C. Figure 6 shows the relationship between the heating unit position (0 mm-200 mm) and the temperature directly below the crucible, with the heating unit located at the bottom end at 0 mm and the heating unit located at the top end at 200 mm. The temperature directly below the crucible reached 1040°C after 90 minutes. This state was maintained for 10 hours to allow the solid-liquid phase reaction of the raw material composition to proceed, resulting in the formation of SrSi ２ was synthesized (solid-liquid phase reaction process).

[0100] Next, the heating part was moved to the bottom end (0 mm position) over 15 minutes. At that time, the temperature directly below the crucible was 1131°C. This state was maintained for 5 hours, and SrSi ２ A melt was obtained (melting step).

[0101] Next, the heating section was moved from the bottom (0 mm position) to the top (200 mm position) at a rate of 3.6 mm / h to lower the temperature of the melt and grow a crystal from the melt (crystal growth step). The rate of temperature drop of the melt was an average of 1.84°C / h (minimum: 1.23°C / h, maximum: 2.36°C / h) until it reached 1100°C, and then an average of 4.32°C / h (minimum: 3.01°C / h, maximum: 5.19°C / h) until it reached 1000°C.

[0102] Thereafter, the temperatures of the upper, middle, and lower heaters were all set to 600°C, and the temperature directly below the crucible was allowed to drop to 600°C over 5 hours. Furthermore, the temperatures of the upper, middle, and lower heaters were all set to 25°C, and the temperature directly below the crucible was allowed to drop to 25°C over 5 hours, after which the ingot was removed from the crucible. No voids were visually observed in the obtained ingot.

[0103] [Powder X-ray Diffraction Analysis and Rietveld Analysis] Approximately ⅕ of the obtained ingot, which was the upper side (the crucible mouth side) where the crystal growth terminated, was cut with a wire saw perpendicular to the height of the crucible to obtain an upper sample. Furthermore, the remaining portion was cut in half with a wire saw perpendicular to the height of the crucible to obtain a middle sample and a lower sample. A portion of each sample was then powdered and subjected to powder X-ray diffraction analysis using an X-ray diffractometer (MiniFlex300, manufactured by Rigaku Corporation) under the following conditions: characteristic X-ray: CuKα radiation, tube voltage: 40 kV, tube current: 15 mA, scan rate: 3 deg / min, scan step: 0.02 deg.

[0104] The powder X-ray diffraction patterns of each sample are shown in Figures 7A and 7B. As shown in Figures 7A and 7B, the upper sample contained Si and Sr as impurities. ２ (SiO ４ ) was confirmed in the middle and bottom samples, but no impurities were confirmed in the middle and bottom samples. This result indicates that impurities segregate in the upper part of the ingot (the end of the crystal growth).

[0105] Furthermore, Rietveld analysis of the powder X-ray diffraction pattern was performed. The analysis conditions for Rietveld analysis are shown in Table 1 below, and the quantitative values ​​obtained by Rietveld analysis are shown in Table 2 below.

[0106]

[0107]

[0108] As shown in Table 2, the crystalline phase of the upper sample contained Si phase and Sr ２ (SiO ４ ) phase, while the middle and bottom samples contained cubic SrSi ２ The middle and lower samples were composed of only the molten phase.

[0109] [Measurement of Band Gap Value] Using a Hall measurement device (ResiTest 8300, manufactured by Toyo Corporation), the carrier concentration of the lower sample was measured at measurement temperatures of 80 to 400 K, and an Arrhenius plot was made. The fitting range was set to 280 to 400 K, and the band gap value was calculated from the slope of the plot line within that range. As a result, the band gap value (E ｇ ) was 48.0 meV.

[0110] Example 2 Melt synthesis of strontium silicide polycrystalline body In Example 2, an ingot was obtained in the same manner as in Example 1, except that the melt synthesis conditions were changed as follows.

[0111] In the melt synthesis of strontium silicide polycrystalline material, first, the heating unit was fixed at a position 106 mm from the bottom end, and the temperatures of the upper heater (heater 21A) and the middle heater (heater 21B) were set to 1130°C, and the temperature of the lower heater (heater 21C) was set to 890°C. The temperature directly below the crucible reached 1050°C after 90 minutes. This state was maintained for 10 hours to allow the solid-liquid phase reaction of the raw material components to proceed, resulting in the formation of SrSi ２ was synthesized (solid-liquid phase reaction process).

[0112] Next, the heating part was moved to the bottom end (0 mm position) over 15 minutes. At that time, the temperature directly below the crucible was 1133°C. This state was maintained for 5 hours, and SrSi ２ A melt was obtained (melting step).

[0113] Next, the heating section was moved from the bottom (0 mm position) to the top (200 mm position) at a rate of 1.08 mm / h to gradually lower the temperature of the melt and grow a crystal from the melt (crystal growth step). The rate of temperature drop of the melt was an average of 0.617°C / h (minimum: 0.445°C / h, maximum: 0.745°C / h) until it reached 1100°C, and then an average of 1.152°C / h (minimum: 0.988°C / h, maximum: 1.339°C / h) until it reached 1015°C.

[0114] Thereafter, the temperatures of the upper, middle, and lower heaters were all set to 600°C, and the temperature directly below the crucible was allowed to drop to 600°C over 5 hours. Furthermore, the temperatures of the upper, middle, and lower heaters were all set to 25°C, and the temperature directly below the crucible was allowed to drop to 25°C over 5 hours, after which the ingot was removed from the crucible. No voids were visually observed in the obtained ingot.

[0115] [Powder X-ray diffraction analysis and Rietveld analysis] Approximately one-fifth of the obtained ingot, which was the upper side (the side toward the mouth of the crucible) where the crystal growth terminated, was cut with a wire saw perpendicular to the height of the crucible to obtain an upper sample. Furthermore, the remaining portion was cut in half with a wire saw perpendicular to the height of the crucible to obtain a middle sample and a lower sample. A portion of each sample was then powdered and subjected to powder X-ray diffraction analysis in the same manner as in Example 1.

[0116] The powder X-ray diffraction patterns of each sample are shown in Figures 8A and 8B. As shown in Figures 8A and 8B, the upper sample contained Si and Sr as impurities. ２ (SiO ４ ) was confirmed in the middle and bottom samples, but no impurities were confirmed in the middle and bottom samples. This result indicates that impurities segregate in the upper part of the ingot (the end of the crystal growth).

[0117] Furthermore, Rietveld analysis of the powder X-ray diffraction pattern was carried out in the same manner as in Example 1. The quantitative values ​​obtained by Rietveld analysis are shown in Table 3 below.

[0118]

[0119] As shown in Table 3, the crystalline phase of the upper sample contained Si phase and Sr ２ (SiO ４ ) phase, while the middle and bottom samples contained cubic SrSi ２ The middle and lower samples were composed of only the molten phase.

[0120] [Measurement of Band Gap Value] The band gap value of the lower sample of Example 2 was measured in the same manner as in Example 1. As a result, the band gap value (Eｇ ) was 49.4 meV.

[0121] <Example 3> [Production of sintered body] In a glove box with an Ar gas atmosphere, a part of the lower sample of Example 1 was pulverized using a mortar. After pulverization, the pulverized material was classified using a stainless steel sieve to obtain a sample powder with a particle size of 25 to 45 μm.

[0122] 3.75 g of sample powder was placed in a space surrounded by a graphite die (manufactured by Ueki Corporation) with an inner diameter of 10.4 mm and two graphite punches (manufactured by Ueki Corporation) (see Figure 3). Carbon paper was sandwiched between the top and bottom of the powder to prevent it from adhering to the punches. In this state, the powder was pressed at a pressure of 60 MPa using a foot-operated hydraulic press (manufactured by Riken Seiki Co., Ltd.) to produce a powder compact.

[0123] Next, the graphite die was placed in a spark plasma sintering apparatus (Ed-PAS III, manufactured by Elenix Co., Ltd.), the press pressure was increased to 100 MPa, and spark plasma sintering was performed in an Ar atmosphere. The temperature conditions during sintering were as follows. First, the temperature was raised from room temperature to 500°C over 1 minute, and then to 800°C over 1 minute. Next, the temperature was raised at a heating rate of 30°C / min until it reached the sintering temperature of 920°C and held at 920°C for 10 minutes. Next, the temperature was lowered at a heating rate of 50°C / min until it reached 600°C, during which time the press pressure was gradually reduced to 42 MPa. Then, the sample was allowed to cool to room temperature. Temperature measurements were performed by measuring the temperature of the graphite die. A K-type thermocouple was used from room temperature to 580°C, and an infrared radiation thermometer was used at temperatures above that.

[0124] After sintering, the adhering carbon paper was removed with sandpaper to obtain a sintered body. The shape of the obtained sintered body was cylindrical (the top and bottom surfaces were circles with a diameter of 10 mm, and the height was 5 mm).

[0125] <Evaluation of Thermoelectric Conversion Performance> The lower samples of Examples 1 and 2 were cut perpendicular to the height direction of the crucible using a wire saw to obtain approximately disk-shaped samples with a thickness of 2 mm. Furthermore, test pieces measuring 2 mm x 2 mm x 7.5 mm were cut from the approximately disk-shaped samples using a wire saw. The sintered body of Example 3 was cut perpendicular to the axial direction of the cylinder using a wire saw to obtain a disk-shaped sample with a thickness of 2 mm. Furthermore, test pieces measuring 2 mm x 2 mm x 7.5 mm were cut from the disk-shaped samples using a wire saw.

[0126] For each of the test pieces of Examples 1 to 3, the Seebeck coefficient and electrical conductivity were measured in the temperature range of 300 to 600 K using a thermoelectric property evaluation device (ZEM-3, manufactured by Advance Riko Co., Ltd.). Then, the power factor was calculated based on the obtained Seebeck coefficient and electrical conductivity. The Seebeck coefficient, electrical conductivity, and power factor of each of the test pieces of Examples 1 to 3 are shown in Figs. 9 to 11. As can be seen from Fig. 11, each of the test pieces of Examples 1 to 3 had high thermoelectric conversion performance in the low temperature range. The power factor at 310 K was 2.736 × 10 for Example 1, －３ W / mK ２ , and Example 2 is 2.919 × 10 －３ W / mK ２ , and Example 3 is 2.845 × 10 －３ W / mK ２ It was.

[0127] Example 4 Melt synthesis of strontium silicide polycrystalline body In Example 4, an ingot was obtained in the same manner as in Example 1, except that the crucible into which the raw material composition was charged was changed to an alumina crucible (manufactured by Nikkato Corporation) with boron nitride pressed onto the inner surface, the gas filled in the heating furnace was changed to Ar gas containing 5% by volume of hydrogen gas, and the melt synthesis conditions were changed as follows:

[0128] In melt synthesis of strontium silicide polycrystalline, first, the heating unit was fixed at a position 123 mm from the bottom end, and the temperatures of the upper heater (heater 21A) and the middle heater (heater 21B) were set to 1130°C, while the temperature of the lower heater (heater 21C) was set to 890°C. The temperature directly below the crucible reached 1035°C after 90 minutes. This state was maintained for 10 hours to allow the solid-liquid phase reaction of the raw material components to proceed, resulting in SrSi ２ was synthesized (solid-liquid phase reaction process).

[0129] Next, the heating part was moved to the bottom end (0 mm position) over 15 minutes. At that time, the temperature directly below the crucible was 1130°C. This state was maintained for 5 hours, and SrSi ２ A melt was obtained (melting step).

[0130] Next, the heating section was moved from the bottom (0 mm position) to the top (200 mm position) at a speed of 3.6 mm / h to gradually lower the temperature of the melt and grow a crystal from the melt (crystal growth step). The cooling rate of the melt was an average of 1.84°C / h (minimum: 1.23°C / h, maximum: 2.36°C / h) until it reached 1100°C, and then an average of 4.32°C / h (minimum: 3.01°C / h, maximum: 5.19°C / h) until it reached 1000°C.

[0131] Thereafter, the temperatures of the upper, middle, and lower heaters were all set to 600°C, and the temperature directly below the crucible was allowed to drop to 600°C over 5 hours. Furthermore, the temperatures of the upper, middle, and lower heaters were all set to 25°C, and the temperature directly below the crucible was allowed to drop to 25°C over 5 hours, after which the ingot was removed from the crucible. No voids were visually observed in the resulting ingot. A backscattered electron image and a secondary electron image of the resulting ingot observed with a scanning electron microscope (SEM) are shown in FIG. 12 .

[0132] [Powder X-ray diffraction analysis and Rietveld analysis] Approximately one-fifth of the obtained ingot, which was the upper side (the side toward the mouth of the crucible) where the crystal growth terminated, was cut with a wire saw perpendicular to the height of the crucible to obtain an upper sample. Furthermore, the remaining portion was cut in half with a wire saw perpendicular to the height of the crucible to obtain a middle sample and a lower sample. A portion of each sample was then powdered and subjected to powder X-ray diffraction analysis in the same manner as in Example 1.

[0133] The powder X-ray diffraction patterns of each sample are shown in Figures 13A and 13B. As shown in Figures 13A and 13B, Si was confirmed as an impurity in the top sample, but no impurities were confirmed in the middle and bottom samples. This result indicates that impurities segregate at the top side of the ingot (the end side of the crystal growth).

[0134] Furthermore, Rietveld analysis of the powder X-ray diffraction pattern was carried out in the same manner as in Example 1. The quantitative values ​​obtained by Rietveld analysis are shown in Table 4 below.

[0135]

[0136] As shown in Table 4, the crystalline phase of the upper sample contained a Si phase, while the crystalline phase of the middle and lower samples was cubic SrSi ２ The middle and lower samples were composed of only the molten phase.

[0137] [Measurement of Band Gap Value] The band gap value of the lower sample of Example 4 was measured in the same manner as in Example 1. As a result, the band gap value (E ｇ ) was 49.4 meV.

[0138] [Measurement of Young's modulus and fracture toughness] For the lower sample of Example 4, Young's modulus E was calculated according to the following formula: E = 9BG / (3B + G) based on density functional theory (DFT) using Hill's approximation equation. In the formula, G represents the shear modulus (Gpa), and B represents the isotropic bulk modulus (Gpa). Furthermore, for the lower sample of Example 4, the fracture toughness was measured in accordance with the IF method of JIS R 1607:2015. As a result, the Young's modulus of the lower sample of Example 4 was 90.17 GPa, and the fracture toughness was 1.452 MPa m １／２ It was.

[0139] Example 5 Melt synthesis of strontium silicide polycrystalline body In Example 5, an ingot was obtained in the same manner as in Example 4, except that the melt synthesis conditions were changed as follows.

[0140] In the melt synthesis of strontium silicide polycrystalline material, first, the heating unit was fixed at a position 106 mm from the bottom end, and the temperatures of the upper heater (heater 21A) and the middle heater (heater 21B) were set to 1130°C, and the temperature of the lower heater (heater 21C) was set to 890°C. The temperature directly below the crucible reached 1035°C after 90 minutes. This state was maintained for 10 hours to allow the solid-liquid phase reaction of the raw material components to proceed, resulting in the formation of SrSi ２ was synthesized (solid-liquid phase reaction process).

[0141] Next, the heating part was moved to the bottom end (0 mm position) over 15 minutes. At that time, the temperature directly below the crucible was 1130°C. This state was maintained for 5 hours, and SrSi ２ A melt was obtained (melting step).

[0142] Next, the heating section was moved from the bottom (0 mm position) to the top (200 mm position) at a rate of 1.08 mm / h to gradually lower the temperature of the melt and grow a crystal from the melt (crystal growth step). The rate of temperature drop of the melt was an average of 0.617°C / h (minimum: 0.445°C / h, maximum: 0.745°C / h) until it reached 1100°C, and then an average of 1.152°C / h (minimum: 0.988°C / h, maximum: 1.339°C / h) until it reached 1015°C.

[0143] Thereafter, the temperatures of the upper, middle, and lower heaters were all set to 600°C, and the temperature directly below the crucible was allowed to drop to 600°C over 5 hours. Furthermore, the temperatures of the upper, middle, and lower heaters were all set to 25°C, and the temperature directly below the crucible was allowed to drop to 25°C over 5 hours, after which the ingot was removed from the crucible. No voids were visually observed in the resulting ingot. A backscattered electron image and a secondary electron image of the resulting ingot observed with an SEM are shown in FIG. 14 .

[0144] [Powder X-ray diffraction analysis and Rietveld analysis] Approximately one-fifth of the obtained ingot, which was the upper side (the side toward the mouth of the crucible) where the crystal growth terminated, was cut with a wire saw perpendicular to the height of the crucible to obtain an upper sample. Furthermore, the remaining portion was cut in half with a wire saw perpendicular to the height of the crucible to obtain a middle sample and a lower sample. A portion of each sample was then powdered and subjected to powder X-ray diffraction analysis in the same manner as in Example 1.

[0145] The powder X-ray diffraction patterns of each sample are shown in Figures 15A and 15B. As shown in Figures 15A and 15B, Si was confirmed as an impurity in the top sample, but no impurities were confirmed in the middle and bottom samples. This result indicates that the impurities segregate at the top side of the ingot (the end side of the crystal growth).

[0146] Furthermore, Rietveld analysis of the powder X-ray diffraction pattern was carried out in the same manner as in Example 1. The quantitative values ​​obtained by Rietveld analysis are shown in Table 5 below.

[0147]

[0148] As shown in Table 5, the crystalline phase of the upper sample contained a Si phase, while the crystalline phase of the middle and lower samples was cubic SrSi ２ The middle and lower samples were composed of only the molten phase.

[0149] [Measurement of Band Gap Value] The band gap value of the lower sample of Example 5 was measured in the same manner as in Example 1. As a result, the band gap value (E ｇ) was 46.1 meV.

[0150] [Measurement of Young's modulus and fracture toughness] The Young's modulus and fracture toughness of the lower sample of Example 5 were measured in the same manner as in Example 4. As a result, the Young's modulus of the lower sample of Example 5 was 91.14 GPa, and the fracture toughness was 1.370 MPa m １／２ It was.

[0151] Comparative Example 1 Melt synthesis of strontium silicide polycrystalline material In Comparative Example 1, an ingot was obtained in the same manner as in Example 4, except that the atomic weight ratio of strontium to silicon was changed to 33.8:66.2, the fill gas in the heating furnace was changed to Ar gas, and the melt synthesis conditions were changed as follows:

[0152] In the melt synthesis of strontium silicide polycrystalline, the heating unit was first fixed at a position 197 mm from the bottom end, and the temperature of the upper heater (heater 21A) was set to 1100°C, the temperature of the middle heater (heater 21B) to 1130°C, and the temperature of the lower heater (heater 21C) to 890°C. The temperature directly below the crucible reached 900°C after 90 minutes. This state was maintained for 3 hours to allow the solid-liquid phase reaction of the raw material components to proceed, resulting in the formation of SrSi ２ was synthesized.

[0153] Next, the heating part was moved to the bottom end (0 mm position) over 15 minutes. At that time, the temperature directly below the crucible was 1130°C. This state was maintained for 3 hours, and SrSi ２ A melt of 1000 ppm was obtained.

[0154] Next, the heating section was moved from the bottom (0 mm position) to the top (200 mm position) at a speed of 7.2 mm / h to gradually lower the temperature of the melt and grow a crystal from the melt (crystal growth step). The cooling rate of the melt was an average of 2.13°C / h (minimum: 0.88°C / h, maximum: 3.35°C / h) until it reached 1100°C, and then an average of 7.02°C / h (minimum: 3.56°C / h, maximum: 9.73°C / h) until it reached 1000°C.

[0155] Thereafter, the temperatures of the upper, middle, and lower heaters were all set to 600° C., and the temperature directly below the crucible was allowed to drop to 600° C. over 5 hours. Furthermore, the temperatures of the upper, middle, and lower heaters were all set to 25° C., and the temperature directly below the crucible was allowed to drop to 25° C. over 5 hours, after which the ingot was removed from the crucible.

[0156] [Powder X-ray Diffraction Analysis and Rietveld Analysis] A powder sample was collected from the lower side (bottom side of the crucible) of the obtained ingot, which is the starting side of the crystal growth, and subjected to powder X-ray diffraction analysis in the same manner as in Example 1.

[0157] 16A and 16B show the powder X-ray diffraction patterns of the sample of Comparative Example 1. As shown in FIGS. 16A and 16B, the sample of Comparative Example 1 contains tetragonal SrSi ２ was confirmed.

[0158] Furthermore, Rietveld analysis of the powder X-ray diffraction pattern was carried out in the same manner as in Example 1. The quantitative values ​​obtained by Rietveld analysis are shown in Table 6 below.

[0159]

[0160] As shown in Table 6, the crystalline phase of the sample of Comparative Example 1 contained 1.88 mass % of tetragonal SrSi ２ phase was included.

[0161] [Measurement of Band Gap Value] The band gap value of the sample of Comparative Example 1 was measured in the same manner as in Example 1. As a result, the band gap value (E ｇ ) was 30.9 meV.

[0162] Comparative Example 2 Melt Synthesis of Strontium Silicide Polycrystalline Body In Comparative Example 2, an ingot was obtained in the same manner as in Example 4, except that the melt synthesis conditions were changed as follows.

[0163] In the melt synthesis of strontium silicide polycrystalline material, the heating unit was first fixed at the bottom end (0 mm position), and the temperatures of the upper heater (heater 21A) and the middle heater (heater 21B) were set to 1130°C, while the temperature of the lower heater (heater 21C) was set to 890°C. The temperature directly below the crucible reached 1130°C after 90 minutes. This state was maintained for 5 hours, and SrSi ２ While synthesizing SrSi ２ A melt of 1000 ppm was obtained.

[0164] The heating element was then moved from the bottom (0 mm) to the top (200 mm) at a rate of 3.6 mm / h to gradually lower the temperature of the melt and grow a crystal from the melt. The cooling rate of the melt was an average of 1.84°C / h (minimum: 1.23°C / h, maximum: 2.36°C / h) until it reached 1100°C, and then an average of 4.32°C / h (minimum: 3.01°C / h, maximum: 5.19°C / h) until it reached 1000°C.

[0165] Thereafter, the temperatures of the upper, middle, and lower heaters were all set to 600° C., and the temperature directly below the crucible was allowed to drop to 600° C. over 5 hours. Furthermore, the temperatures of the upper, middle, and lower heaters were all set to 25° C., and the temperature directly below the crucible was allowed to drop to 25° C. over 5 hours, after which the ingot was removed from the crucible.

[0166] Powder X-ray diffraction analysis: Powder samples 1 and 2 were collected from two different locations on the lower side (bottom side of the crucible) of the obtained ingot, which was the starting point of crystal growth. Powder X-ray diffraction analysis was then performed in the same manner as in Example 1.

[0167] The powder X-ray diffraction patterns of Samples 1 and 2 of Comparative Example 2 are shown in Figure 17. As shown in Figure 17, both Samples 1 and 2 are tetragonal SrSi ２ and many other impurities were found.

[0168] <Evaluation of Thermoelectric Conversion Performance> The lower samples of Examples 4 and 5 were cut perpendicular to the height direction of the crucible using a wire saw to obtain approximately disk-shaped samples with a thickness of 2 mm. Furthermore, test pieces measuring 2 mm x 2 mm x 7.5 mm were cut from the approximately disk-shaped samples using a wire saw. Test pieces were also cut from the ingot of Comparative Example 1 in the same manner.

[0169] For each of the test pieces of Examples 4 and 5 and Comparative Example 1, the Seebeck coefficient and electrical conductivity were measured in the temperature range of 300 to 600 K using a thermoelectric property evaluation device (ZEM-3, manufactured by Advance Riko Co., Ltd.). Then, the power factor was calculated based on the obtained Seebeck coefficient and electrical conductivity. The Seebeck coefficient, electrical conductivity, and power factor of each of the test pieces of Examples 4 and 5 and Comparative Example 1 are shown in Figs. 18 to 20. As can be seen from Fig. 20, each of the test pieces of Examples 4 and 5 had high thermoelectric conversion performance in the low temperature region. The power factor at 310 K was 2.707 × 10 for Example 4, 2.707 × 10 for －３ W / mK ２ , and Example 5 is 2.893 × 10 －３ W / mK ２ , and Comparative Example 1 is 2.275 × 10 －３ W / mK ２ It was.

[0170] <Examples 6 to 8> [Production of sintered body] In Examples 6 to 8, a part of the lower sample of Example 5 was used, and a sintered body was obtained in the same manner as in Example 3, except that the temperature conditions during sintering were changed as follows.

[0171] During sintering, the temperature was first raised from room temperature to 500°C over 1 minute, and then raised to 800°C over 1 minute. Next, the temperature was raised at a heating rate of 30°C / min until it reached the sintering temperature of 890°C (Example 6), 920°C (Example 7), or 950°C (Example 8), and held at each sintering temperature for 10 minutes. Next, the temperature was lowered at a heating rate of 50°C / min until it reached 600°C, during which time the pressing pressure was gradually reduced to 42 MPa. After that, the sample was allowed to cool to room temperature to obtain a sintered body.

[0172] Optical microscope images of the sintered bodies of Examples 6 to 8 observed with an optical microscope and backscattered electron images observed with an SEM are shown in Figure 21. As can be seen from Figure 21, the higher the sintering temperature, the fewer voids there were and the stronger the adhesion of the grain boundaries.

[0173] [Measurement of relative density] The apparent densities of the sintered bodies of Examples 6 to 8 were measured by Archimedes' method. In addition, the true densities of the sintered bodies of Examples 6 to 8 were measured by a gas displacement method using a gas pycnometer. The relative densities were then calculated from the ratios of the apparent densities to the true densities. As a result, the relative densities of the sintered bodies were 97.6% for Example 6, 97.7% for Example 7, and 99.7% for Example 8.

[0174] [Powder X-ray Diffraction Analysis and Rietveld Analysis] The ends of the sintered bodies of Examples 6 to 8 were powdered and subjected to powder X-ray diffraction analysis in the same manner as in Example 1.

[0175] 22A and 22B show the powder X-ray diffraction patterns of the sintered bodies of Examples 6 to 8. As shown in Fig. 22A and 22B, Si was confirmed as an impurity in the sintered body of Example 8, but no impurities were confirmed in the sintered bodies of Examples 6 and 7.

[0176] Furthermore, Rietveld analysis of the powder X-ray diffraction pattern was carried out in the same manner as in Example 1. The quantitative values ​​obtained by Rietveld analysis are shown in Table 7 below.

[0177]

[0178] As shown in Table 7, the crystalline phase of the sintered body of Example 8 contained a small amount of Si phase, while the crystalline phase of the sintered bodies of Examples 6 and 7 was cubic SrSi ２ It consisted of only the phase.

[0179] [Measurement of Band Gap Value] The band gap values ​​of the sintered bodies of Examples 6 to 8 were measured in the same manner as in Example 1. As a result, the band gap values ​​(E ｇ ) was 55.8 meV in Example 6, 53.8 meV in Example 7, and 52.6 meV in Example 8.

[0180] [Measurement of Young's modulus and fracture toughness value] The Young's modulus and fracture toughness value of the sintered bodies of Examples 6 to 8 were measured in the same manner as in Example 4. As a result, the Young's modulus was 86.92 GPa for Example 6, 88.56 GPa for Example 7, and 87.32 GPa for Example 8. The fracture toughness value was 1.445 MPa m １／２ , and Example 7 is 1.650 MPa m １／２ , and Example 8 is 1.445 MPa m １／２ It was.

[0181] Comparative Example 3 [Production of Sintered Body] In Comparative Example 3, a sintered body was obtained in the same manner as in Example 3, except that Sample 1 of Comparative Example 2 was used.

[0182] <Evaluation of Thermoelectric Conversion Performance> The sintered bodies of Examples 6 to 8 and Comparative Example 3 were cut perpendicular to the axial direction of the cylinder with a wire saw to obtain disk-shaped samples with a thickness of 2 mm. Furthermore, test pieces measuring 2 mm × 2 mm × 7.5 mm were cut out from the disk-shaped samples using the wire saw.

[0183] For each of the test specimens of Examples 6 to 8 and Comparative Example 3, the Seebeck coefficient and electrical conductivity were measured in the temperature range of 300 to 600 K using a thermoelectric property evaluation device (ZEM-3, manufactured by Advance Riko Co., Ltd.). Then, the power factor was calculated based on the obtained Seebeck coefficient and electrical conductivity. The Seebeck coefficient, electrical conductivity, and power factor of each of the test specimens of Examples 6 to 8 and Comparative Example 3 are shown in Figs. 23 to 25. As can be seen from Fig. 25, each of the test specimens of Examples 6 to 8 had high thermoelectric conversion performance in the low temperature region. The power factor at 310 K was 2.401 × 10 for Example 6, 2.401 × 10 for Example 6, and 2.401 × 10 for Comparative Example 3. －３ W / mK ２ , and Example 7 is 2.571 × 10 －３ W / mK ２ , and Example 8 is 2.442 × 10 －３ W / mK ２ , and Comparative Example 3 is 1.814 × 10 －３ W / mK ２ It was.

[0184] REFERENCE SIGNS LIST 1 Vertical Bridgman furnace 10 Furnace body 11 Quartz glass tube 12 Graphite sample stage 13 Heat shield 14 Thermocouple 20 Heating part 21A, 21B, 21C Heater 22 Heat insulator 23A, 23B, 23C Thermocouple 30 Crucible 31 Quartz cover 50 Graphite die 51a, 51b Graphite punch 100 n-type thermoelectric conversion part 101, 102 Electrode 110 p-type thermoelectric conversion part 111, 112 Electrode 120 Thermoelectric conversion part 121, 122 Electrode 130 Load

Claims

1. A thermoelectric conversion melt composite composed of strontium silicide polycrystals, which is quantitatively determined by Rietveld analysis of a powder X-ray diffraction pattern to be cubic SrSi ２ The content of the tetragonal SrSi phase is 99.0 mass% or more. ２ Phase and trigonal SrSi ２ A thermoelectrically convertible molten composite, each of which has a content of less than 0.1 mass % of each phase.

2. The thermoelectric convertible molten composite body according to claim 1, having a Si content of less than 0.1 mass % as a quantitative value by Rietveld analysis of a powder X-ray diffraction pattern.

3. The thermoelectric convertible molten synthetic body according to claim 1, wherein the Sr content is less than 0.1 mass % as a quantitative value by Rietveld analysis of a powder X-ray diffraction pattern.

4. The power factor at 310K is 2.50 x 10 －３ W / mK ２ The thermoelectric convertible molten composite material according to claim 1 .

5. The thermoelectrically convertible molten composite material according to claim 1, having a band gap of 45.0 meV or more.

6. A thermoelectric conversion sintered body composed of a strontium silicide polycrystal, which is quantitatively determined by Rietveld analysis of a powder X-ray diffraction pattern to be cubic SrSi ２ The content of the tetragonal SrSi phase is 99.0 mass% or more. ２ Phase and trigonal SrSi ２ A thermoelectrically convertible sintered body, in which the content of each of the phases is less than 0.1 mass %.

7. A thermoelectrically convertible sintered body according to claim 6, which is obtained by sintering the thermoelectrically convertible molten composite body according to claim 1.

8. A thermoelectrically convertible sintered body according to claim 6, having a Si content of less than 0.8 mass % as a quantitative value obtained by Rietveld analysis of a powder X-ray diffraction pattern.

9. A thermoelectrically convertible sintered body according to claim 6, in which the Sr content is less than 0.1 mass % as a quantitative value obtained by Rietveld analysis of a powder X-ray diffraction pattern.

10. The power factor at 310K is 2.35 x 10 －３ W / mK ２The thermoelectric convertible sintered body according to claim 6 .

11. The thermoelectric convertible sintered body according to claim 6, having a band gap of 50.0 meV or more.

12. A thermoelectric conversion element comprising: a thermoelectric conversion part constituted by the thermoelectric convertible molten composite body according to any one of claims 1 to 5, or the thermoelectric convertible sintered body according to any one of claims 6 to 11; and a first electrode and a second electrode provided on the thermoelectric conversion part.

13. A thermoelectric conversion module comprising the thermoelectric conversion element according to claim 12.

14. A method for producing a thermoelectric convertible molten composite according to any one of claims 1 to 5, comprising the steps of: heating a raw material containing strontium and silicon at 1000°C or higher to SrSi ２ Heating at a temperature below the melting point of SrSi ２ a solid-liquid phase reaction step for synthesizing SrSi ２ a melting step of heating the mixture to obtain a molten liquid; and a crystal growing step of growing crystals from the molten liquid to obtain a strontium silicide polycrystalline body.

15. The method for producing a thermoelectric convertible molten composite according to claim 14, wherein the atomic ratio of strontium to silicon in the raw material composition is 33.0:67.0 to 34.5:65.

5.

16. A method for producing a thermoelectrically convertible molten composite body as described in claim 14, wherein in the crystal growth step, the temperature of the molten liquid is decreased at a rate of 0.223 to 4.72°C / h until the temperature of the molten liquid reaches 1100°C, and after the temperature of the molten liquid reaches 1100°C, the temperature is decreased at a rate of 0.494 to 10.38°C / h until the temperature of the molten liquid reaches 1000°C.

17. The method for producing a thermoelectrically convertible molten composite body according to claim 14, wherein the solid-liquid phase reaction step, the melting step, and the crystal growing step are carried out using a vertical Bridgman furnace.

18. A method for producing a thermoelectrically convertible sintered body, comprising a sintering step of sintering a thermoelectrically convertible molten composite body obtained by the method according to claim 14.

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