阅读说明：本技术 热电转换材料、热电转换组件和热电转换材料的制造方法 (Thermoelectric conversion material, thermoelectric conversion module, and method for producing thermoelectric conversion material ) 是由 岛田武司 王楠 松田三智子 于 2019-01-15 设计创作，主要内容包括：提供无量纲品质因数ZT高的热电转换材料、热电转换组件和热电转换材料的制造方法。本发明涉及的热电转换材料(1)具有：具有包含Yb、Co和Sb而成的方钴矿型晶体结构的多个多晶晶粒(11)、和存在于相邻的上述多晶晶粒(11)之间、具有O相对于Yb的原子比超过0.4且小于1.5的晶体(13)的粒间层(12)。本发明涉及的热电转换材料的制造方法包括：称量工序(S1)、混合工序(S2)、将上述原材料的熔液利用液体急冷凝固法进行急冷凝固而制作带的带制作工序(S3)、在调节了氧浓度的非活性气氛中进行热处理的第一热处理工序(S4)、在还原气氛中进行热处理的第二热处理工序(S5)和在非活性气氛中进行加压烧结而制造上述热电转换材料(1)的加压烧结工序(S6)。(Provided are a thermoelectric conversion material having a high dimensionless figure of merit ZT, a thermoelectric conversion module, and a method for manufacturing the thermoelectric conversion material. A thermoelectric conversion material (1) according to the present invention comprises: a plurality of polycrystalline crystal grains (11) having a skutterudite-type crystal structure comprising Yb, Co and Sb, and an intergranular layer (12) which is present between adjacent ones of the polycrystalline crystal grains (11) and has crystals (13) in which the atomic ratio of O to Yb exceeds 0.4 and is less than 1.5. The method for producing a thermoelectric conversion material according to the present invention includes: a weighing step (S1), a mixing step (S2), a ribbon production step (S3) in which a melt of the raw material is rapidly solidified by a liquid rapid solidification method to produce a ribbon, a first heat treatment step (S4) in which heat treatment is performed in an inert atmosphere in which the oxygen concentration is adjusted, a second heat treatment step (S5) in which heat treatment is performed in a reducing atmosphere, and a pressure sintering step (S6) in which pressure sintering is performed in an inert atmosphere to produce the thermoelectric conversion material (1).)

1. A thermoelectric conversion material is characterized by comprising:

a plurality of polycrystalline crystal grains having a skutterudite-type crystal structure including Yb, Co, and Sb; and

and an intergranular layer which is present between adjacent polycrystalline grains and has crystals having an atomic ratio of O to Yb of more than 0.4 and less than 1.5.

2. The thermoelectric conversion material according to claim 1, wherein the skutterudite-type crystal structure comprising Yb, Co and Sb is a compositional formula of Yb_xCo₄Sb₁₂Wherein x is more than 0 and 0.3 or less.

3. The thermoelectric conversion material according to claim 1 or 2, wherein the oxygen concentration is 1200 ppm by volume or less.

4. A thermoelectric conversion material according to any one of claims 1 to 3, wherein the thickness of the inter-particle layer is 5nm or more and 1 μm or less.

5. The thermoelectric conversion material according to any one of claims 1 to 4, characterized in that at least one of thermal conductivity and electrical conductivity has anisotropy.

6. A thermoelectric conversion module comprising a plurality of elements in which the thermoelectric conversion material according to any one of claims 1 to 5 and a thermoelectric conversion material having a skutterudite crystal structure and having an opposite phase to the thermoelectric conversion material are connected via an electrically conductive material.

7. A method for producing a thermoelectric conversion material, comprising:

a weighing step of weighing a Yb-containing raw material, a Co-containing raw material and a Sb-containing raw material, respectively;

a mixing step of melting and mixing the raw materials;

a belt production step of subjecting the melt of the raw material to rapid solidification by a liquid rapid solidification method to produce a belt;

a first heat treatment step of subjecting the produced ribbon to a heat treatment in an inert atmosphere in which the oxygen concentration is adjusted and a step of crushing the ribbon to obtain polycrystalline grains;

a second heat treatment step of heat-treating the polycrystalline crystal grains heat-treated in the first heat treatment step in a reducing atmosphere; and

and a pressure sintering step of pressure sintering the polycrystalline crystal grains heat-treated in the second heat treatment step in an inert atmosphere to produce a thermoelectric conversion material.

8. The method for producing a thermoelectric conversion material according to claim 7, wherein the pressure sintering step is a step of heat-treating the polycrystalline grains in a reducing atmosphere and then pressure sintering the polycrystalline grains in an atmosphere substituted with argon gas.

9. The method for producing a thermoelectric conversion material according to claim 7 or 8, wherein a heat treatment temperature in the pressure sintering step is 600 ℃ or higher and 800 ℃ or lower.

10. The method for producing a thermoelectric conversion material according to any one of claims 7 to 9, wherein the oxygen concentration in the first heat treatment step is 10 ppm by volume or less.

Technical Field

The invention relates to a thermoelectric conversion material, a thermoelectric conversion module, and a method for manufacturing the thermoelectric conversion material.

Background

Currently, a large amount of heat energy is discharged from large-scale power generation systems such as thermal power stations, iron and steel-related furnaces, garbage incinerators, automobiles running on fossil fuel energy, and the like. A part of the discharged heat energy is used as a heat source for supplying hot water or heating, but most of the discharged heat energy is discarded without being used. The waste heat energy which is not used and discarded is referred to as unused waste heat energy or the like. If the unused waste heat energy can be effectively used and recovered, the energy consumption of the entire social system is reduced, and the solution can be greatly contributed to solving environmental problems such as energy problems and global warming.

A thermoelectric power generation system that converts thermal energy into electrical energy is receiving attention in the sense of recycling without using waste heat energy. A thermoelectric power generation system is a power generation system using a thermoelectric conversion module including a plurality of elements in which an n-type thermoelectric conversion material having electrons as carriers and a p-type thermoelectric conversion material having holes (holes) as carriers are connected via a conductive material. When a temperature gradient is generated between one of the thermoelectric conversion elements and the other of the thermoelectric conversion elements, electrons in a high-temperature region are activated (kinetic energy increases) in the n-type thermoelectric conversion material, the electrons are conducted to a low-temperature region to generate a thermoelectric force, and the high-temperature side becomes a high potential. On the other hand, in the p-type thermoelectric conversion material, holes in a high-temperature region are activated, the holes move to a low-temperature region to generate a thermoelectromotive force, and the low-temperature side becomes a high potential. When the n-type thermoelectric conversion material and the p-type thermoelectric conversion material are connected via the conductive material, current flows between them (also referred to as the seebeck effect), like a type of battery. The thermoelectric power generation system supplies the electric energy thus obtained.

That is, the thermoelectric conversion material used in the thermoelectric conversion module performs direct energy exchange based on solids without discharging carbon dioxide and without cooling with a refrigerant such as fluorocarbon gas. Therefore, in recent years, the value of energy technology has been newly recognized as an energy technology coexisting with the environment.

Techniques related to thermoelectric conversion materials are disclosed in non-patent documents 1 to 3 and patent document 1, for example.

In non-patent document 1, Ba is reported_xR_yCo₄Sb₁₂Low temperature transport properties of polycrystalline, two-component, filled skutterudites of (R ═ La, Ce, and Sr).

In non-patent document 2, it is reported that skutterudite CoSb having a plurality of co-filler materials of Ba, La and Yb is synthesized₃A very high thermoelectric figure of merit (dimensionless figure of merit) ZT of 1.7 is achieved at 850K.

Non-patent document 3 reports that high-performance n-type Yb is produced_xCo₄Sb₁₂(x is 0.2 to 0.6) in the case of filling skutterudite, Yb_0.3Co₄Sb₁₂The maximum ZT at 850K is 1.5. Non-patent document 3 describes the following: in order to obtain the above product, annealing was performed at 750 ℃ for 168 hours (7 days). In addition, in non-patent documentsIn document 3, the following description is given: powder X-ray diffraction (XRD) analysis after annealing resulted in the formation of Yb₂O₃、YbSb₂。

Patent document 1 describes a method for producing a filled skutterudite alloy, which includes melting an alloy material containing a rare earth metal R (where R is at least one of La, Ce, Pr, Nd, Sm, Eu, and Yb), a transition metal T (where T is at least one of Fe, Co, Ni, Os, Ru, Pd, Pt, and Ag), and metallic antimony (Sb), and rapidly solidifying the melt (molten solution) by a strip casting method.

Patent document 1 describes that a substantially uniform-filled skutterudite-based alloy can be produced in a large amount and easily by a casting method using a strip casting method. Further, patent document 1 describes that the produced filled skutterudite-based alloy can be used as it is for a thermoelectric conversion element without a step of pulverizing and sintering, and therefore the production cost of the thermoelectric conversion element can be significantly reduced.

Disclosure of Invention

Problems to be solved by the invention

The performance of the thermoelectric conversion material is evaluated in an amount called a dimensionless figure of merit (performance index) ZT, which is greater than 1, and is considered as a standard for practical use. Note that, in the thermoelectric conversion material whose ZT ≈ 1, the theoretical power generation efficiency is considered to be about 9%.

ZT＝S²σT/κ (1)

Here, in the above formula (1), S: seebeck coefficient, σ: conductivity, κ: thermal conductivity, T: absolute temperature.

As shown in the above formula (1), the thermoelectric conversion material having good performance, that is, high efficiency is a material having a large electrical conductivity σ and seebeck coefficient S and a small thermal conductivity κ. However, in general, the following correlation exists for thermoelectric conversion materials: the higher the electrical conductivity σ, the higher the thermal conductivity κ, and the lower the electrical conductivity σ, the lower the thermal conductivity κ; it is therefore difficult to improve the dimensionless figure of merit ZT.

The thermoelectric conversion material filled with only Ba described in non-patent document 1 has a problem of low dimensionless figure of merit ZT because of low electrical conductivity σ and high thermal conductivity κ. Further, even if a high ZT is obtained as in non-patent documents 1 and 2 and patent document 1, introduction of active materials such as Ba, La, and Sr causes a problem of significantly reducing reliability. In order to obtain the product described in non-patent document 3, annealing is required for a long time as described above, and thus the method is not suitable for mass production.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a thermoelectric conversion material, a thermoelectric conversion module, and a method for manufacturing a thermoelectric conversion material, which have a high dimensionless figure of merit ZT and high reliability.

Means for solving the problems

The object of the present invention can be achieved by the following means.

The thermoelectric conversion material according to the present invention comprises: a plurality of polycrystalline crystal grains having a skutterudite-type crystal structure including Yb, Co, and Sb; and an intergranular layer which is present between adjacent ones of the polycrystalline grains and has crystals having an atomic ratio of O to Yb of more than 0.4 and less than 1.5.

A thermoelectric conversion module according to the present invention includes a plurality of elements each having the thermoelectric conversion material according to the present invention and a thermoelectric conversion material having a skutterudite crystal structure and having an opposite phase to the thermoelectric conversion material, the thermoelectric conversion material being connected to each other through an electrically conductive material.

The method for producing a thermoelectric conversion material according to the present invention includes: a weighing step of weighing a Yb-containing raw material, a Co-containing raw material and a Sb-containing raw material, respectively; a mixing step of melting and mixing the raw materials; a belt production step of subjecting the melt of the raw material to rapid solidification by a liquid rapid solidification method to produce a belt; a first heat treatment step of performing heat treatment in an inert atmosphere in which the oxygen concentration is adjusted and a step of crushing the produced ribbon to obtain polycrystalline grains; a second heat treatment step of heat-treating the polycrystalline crystal grains heat-treated in the first heat treatment step in a reducing atmosphere; and a pressure sintering step of pressure sintering the polycrystalline crystal grains heat-treated in the second heat treatment step in an inert atmosphere to produce a thermoelectric conversion material.

Effects of the invention

According to the present invention, a thermoelectric conversion material, a thermoelectric conversion module, and a method for manufacturing a thermoelectric conversion material, which have a high dimensionless figure of merit ZT and high reliability, can be provided.

Drawings

Fig. 1A is a schematic view schematically showing the structure of a thermoelectric conversion material according to the present embodiment.

Fig. 1B is an enlarged view of a portion B of fig. 1A.

Fig. 2 is an observation view (observation range 20 μm × 20 μm) in which the electric resistance of the thermoelectric conversion material according to the present embodiment is mapped (mapping).

Fig. 3 is an observation view (observation range 20 μm × 20 μm) in which the electric resistance of an example of the thermoelectric conversion material according to the conventional example is mapped without any treatment on the atmosphere of the pressure sintering pretreatment.

Fig. 4 is a perspective view showing the structure of the thermoelectric conversion module according to the present embodiment. Fig. 4 shows a state before the upper substrate is mounted.

Fig. 5 is a perspective view showing the structure of the thermoelectric conversion module according to the present embodiment. Fig. 5 shows a state after the upper substrate is mounted.

Fig. 6 is a flowchart showing the contents of the method for producing a thermoelectric conversion material according to the present embodiment.

FIG. 7 is an EPMA image of the sample referred to in example No. 10. In the figure, (a) is an EPMA image, and (b) to (e) are diagrams showing the results of element mapping for Yb, Co, Sb, and O, respectively. The scale bars located below (a) to (e) each represent 50 μm.

FIG. 8 is an EPMA image of the sample referred to in example No. 16. In the figure, (a) is an EPMA image, and (b) to (e) are diagrams showing the results of element mapping for Yb, Co, Sb, and O, respectively. The scale bars located below (a) to (e) each represent 50 μm.

FIG. 9 is an EPMA image of the sample of comparative example No. 3. In the figure, (a) is an EPMA image, and (b) to (e) are diagrams showing the results of element mapping for Yb, Co, Sb, and O, respectively. The scale bars located below (a) to (e) each represent 50 μm.

FIG. 10 is a Transmission Electron Microscope (TEM) photograph and the results of element mapping of the sample of example No. 10. In the figure, (a) is a TEM photograph of the sample of No.10, and (b) to (e) are graphs showing the results of element mapping with respect to Co, Sb, Yb, and O, respectively. The respective scales located below (b) to (e) represent 20.00 nm.

Fig. 11A is a TEM photograph showing the sample of example No.29 and an electron beam diffraction pattern at a portion surrounded by a dotted circle in the drawing. The scale bar located at the lower right in the figure represents 20 nm.

FIG. 11B is a table showing EDX analysis values (at%) at positions 1 to 5 in FIG. 11A.

Fig. 12A is a TEM photograph showing a sample of comparative example No.7 and an electron beam diffraction pattern at a portion surrounded by a dotted circle in the drawing. The scale bar located at the lower right in the figure represents 20 nm.

Fig. 12B is a TEM photograph showing the sample of comparative example No.7 and an electron beam diffraction pattern at a portion surrounded by a dotted circle in the drawing. The scale bar located at the lower right in the figure represents 20 nm.

FIG. 12C is a table showing EDX analysis values (at%) at positions 1 to 4 in FIGS. 12A and 12B.

FIG. 13 is a TEM photograph and an electron beam diffraction pattern of the sample of example No. 10. In the figure, (a) is a TEM photograph. (b) Is an electron beam diffraction pattern relating to the portion (polycrystalline grains) at position 1 in (a). (c) Is the electron beam diffraction pattern of the portion (interparticle layer) at position 2 in (a). (d) Is an electron beam diffraction pattern relating to a portion (interface-a of the interparticle layer) at position 3 in (a). (e) Is an electron beam diffraction pattern relating to a portion (interface-b of the interparticle layer) at position 3 in (a). The scale bar at the lower right of (a) represents 100 nm.

Fig. 14(a) to (c) are diagrams showing the evaluation results from the electron beam diffraction pattern and the simulation.

FIG. 15A is an SEM photograph of the high-resistance portion of the sample of comparative example No. 7. The scale bar located at the lower right in the figure represents 10 μm.

Fig. 15B is a graph showing the result of high-sensitivity line analysis performed for the line analysis position shown in fig. 15A.

FIG. 16 is a SEM photograph showing a cross section of the sample of comparative example No.7 in the vicinity of the boundary line of the polycrystalline grains. The scale bar located at the lower right in the figure represents 10 μm.

Detailed Description

Next, an embodiment of a thermoelectric conversion material, a thermoelectric conversion module, and a method for manufacturing a thermoelectric conversion material according to the present invention will be described in detail with reference to the accompanying drawings.

[ thermoelectric conversion Material ]

Fig. 1A is a schematic view schematically showing the structure of a thermoelectric conversion material according to the present embodiment. Fig. 1B is an enlarged view of a portion B of fig. 1A. Note that fig. 1A and 1B are diagrams schematically illustrating an electron beam microanalyzer (EPMA) image.

As shown in fig. 1A, a thermoelectric conversion material 1 (not shown in fig. 1, see fig. 4.) according to the present embodiment has the following structure: a plurality of polycrystalline crystal grains 11 having a skutterudite-type crystal structure including Yb, Co, and Sb, and an intergranular layer 12 which is present between adjacent polycrystalline crystal grains 11 and has crystals 13 (see fig. 1B) in which the atomic ratio of O to Yb exceeds 0.4 and is less than 1.5. As will be described later, the polycrystalline grains 11 are pulverized particles produced in the production process, and the thermoelectric conversion material 1 according to the present embodiment is a sintered body obtained by pressure sintering the polycrystalline grains 11. In the above-described grain boundary layer 12, a reaction intermediate product (not shown) of the polycrystalline grains 11 and the precipitated grains (not shown) may coexist. The thermoelectric conversion material 1 can be suitably obtained by a method for producing a thermoelectric conversion material described later.

The skutterudite crystal structure containing Yb, Co and Sb has a crystal structure (cubic crystal) in which Co is surrounded in an octahedral structure of Sb having a common apex, and Yb is filled in the gaps of the crystal structure (referred to as a filled skutterudite structure or the like). As such a skutterudite-type crystal structure comprising Yb, Co and Sb, Yb having a compositional formula is preferably used_xCo₄Sb₁₂(wherein x is more than 0 and 0.3 or less). X is preferably 0.1 or more, and more preferably 0.2 or more. In addition, x is preferably less than 0.3. The polycrystalline crystal grains 11 including the crystal grains having such a crystal structure, preferably the polycrystalline crystal grains 11 including the crystal grains having a crystal structure represented by the above composition formula, suitably function as the thermoelectric conversion material 1.

The thermoelectric conversion material 1 preferably has anisotropy in at least one of thermal conductivity and electric conductivity. As shown in fig. 1A, such anisotropy can be obtained by aligning the polycrystalline grains 11 having different aspect ratios constituting the thermoelectric conversion material 1 in a substantially constant direction. That is, it is considered that the anisotropy of the electrical conductivity and the thermal conductivity is caused by the anisotropy of the shape of the polycrystalline grains 11 having different aspect ratios, and is generated by the difference in the number of the above-described grain layers 12 per unit path.

The polycrystalline grains 11 having different aspect ratios can be obtained, for example, as described in the production method described later, by: the ribbon is produced by a liquid rapid solidification method, and then the ribbon is crushed to obtain polycrystalline grains 11 in the form of crushed particles, and the polycrystalline grains 11 are tapped (tapping) and then pressure-sintered to produce the polycrystalline element. That is, such anisotropy can be achieved by performing pressure sintering after tapping to make the shape of the polycrystalline grains 11a shape having a different aspect ratio and align the shape in a substantially constant direction. Since the thermal conductivity and the electrical conductivity are given by the shape of the polycrystalline grains 11 formed by pressure sintering after tapping and the intergranular layer 12 formed therewith, the directions of anisotropy of the thermal conductivity and the electrical conductivity are in agreement in many cases. In order to facilitate the anisotropy, it is preferable to perform pressure sintering after applying vibration to the polycrystalline grains 11 to improve the stability by tapping or the like as described above.

The size of the polycrystalline grains 11 is preferably 10 to 500 μm in the maximum length, for example. The maximum length is, for example, the length between two points at which the phase is farthest from each other in one polycrystalline crystal grain 11 in an image taken by an electron microscope photograph or the like. When the size of the polycrystalline grains 11 is within this range, the oxygen concentration in the sintered body after pressure sintering can be easily controlled. The size of the polycrystalline grains 11 can be controlled by appropriately changing the pulverization time when the ribbon is pulverized, for example.

The polycrystalline crystal grains 11 are formed to have a metallographic structure including a plurality of crystal grains having a skutterudite-type crystal structure including Yb, Co, and Sb. Since the directions of atomic arrangement differ from one grain to another, a region in which the atomic arrangement of the crystal structure is disordered, that is, a grain boundary (crystal grain boundary) is formed between adjacent grains. The size of the crystal grains constituting the polycrystalline grains 11 is preferably, for example, 1 to 50 μm, but is not limited to this range. When the size of the crystal grains is in this range, high denseness can be obtained. The size of the crystal grains can be controlled by appropriately changing, for example, the quenching speed in the tape production step, the heat treatment temperature in the first heat treatment step, the second heat treatment step, and the pressure sintering step.

The thermoelectric conversion material 1 has: a plurality of polycrystalline crystal grains 11 having the above-described structure; and an intergranular layer 12 present between adjacent polycrystalline grains 11 and having crystals 13 with an atomic ratio of O to Yb exceeding 0.4 and less than 1.5. The crystal 13 precipitated in the inter-particle layer 12 has an atomic ratio of O to Yb of preferably 1 or less, more preferably 0.7 or less, and still more preferably 0.67 or less.

In the present embodiment, the grain layer 12 having the crystal 13 in which the atomic ratio of O to Yb exceeds 0.4 and is less than 1.5 may be provided. That is, the atomic ratio of O to Yb of the crystals 13 contained in the intergranular layer 12 is not necessarily all within the above range. The thermoelectric conversion material 1 may contain crystals having an atomic ratio of O to Yb of 0.4 or less in a mixed manner, or may contain crystals having an atomic ratio of O to Yb of 1.5 or more in a mixed manner, the crystals being precipitated in the inter-granular layer 12.

When the atomic ratio of O to Yb of the crystals 13 precipitated in the intergranular layer 12 is within the above range, the crystals 13 contain not only the oxide of Yb but also the metal Yb at a high ratio. The reason for this is considered to be that, for example, oxygen is derived from Yb₂O₃The Yb oxide, which is very stable and has low electrical conductivity σ and thermal conductivity κ, is dissociated in the linkage, thereby generating a stable oxide having oxygen defects. Further, it is considered that the Yb oxide having such defects must generate electrons in order to ensure electrical neutrality, and therefore the electrical conductivity inevitably increases. Therefore, it is considered that the thermal conductivity κ and the electrical conductivity σ of the thermoelectric conversion material 1 can be reduced and the dimensionless figure of merit ZT can be improved. The anisotropy of the electrical conductivity and the thermal conductivity is considered to be caused by the anisotropy of the shapes of the polycrystalline grains 11 having different aspect ratios, and is generated by the difference in the number of the inter-granular layers 12 (that is, the inter-granular layers 12 including the crystals 13 having the atomic ratio of O to Yb within the above range) per unit path.

Here, fig. 2 is an explanatory diagram (observation range 20 μm × 20 μm) in which the electric resistance of the thermoelectric conversion material according to the present embodiment is mapped. Fig. 2 shows a thermoelectric conversion material obtained by subjecting the atmosphere of the pressure sintering pretreatment to 3% hydrogen, then subjecting the resultant to pressure of argon 0.1MPa so that the oxygen concentration of the sintered body becomes 470 ppm by volume, and observing the electric resistance thereof with a scanning probe microscope. Fig. 3 is an explanatory view (observation range 20 μm × 20 μm) in which the electric resistance of the thermoelectric conversion material according to the conventional example is mapped. Fig. 3 is a view of the resistance of the thermoelectric conversion material without any treatment in the atmosphere of the pressure sintering pretreatment, which is observed with a scanning probe microscope.

It is understood that the thermoelectric conversion material 1 according to the present embodiment shown in fig. 2 (see fig. 4) has lower electrical resistance (higher electrical conductivity σ) and no black portion having higher electrical resistance than the thermoelectric conversion material according to the conventional example shown in fig. 3. The resistance per unit area shown in FIG. 2 was 6.3. mu. OMEGA.m, and the resistance per unit area shown in FIG. 3 was 32.2. mu. OMEGA.m.

The above atomic ratio of O to Yb of the crystals 13 precipitated in the intergranular layer 12 is preferably 0.5 (i.e., Yb₂O), 0.67 (i.e., Yb)₃O₂). When set in this way, a stable oxide having oxygen defects and being stable can be maintained in the inter-granular layer 12.

The above atomic ratio of O to Yb of the crystal 13 precipitated in the inter-granular layer 12 can be obtained by performing the first heat treatment step, the second heat treatment step, and the pressure sintering step in the production method described later.

The thermoelectric conversion material 1 according to the present embodiment has an oxygen concentration of preferably 1200 ppm by volume or less, more preferably 700 ppm by volume or less, and still more preferably 510 ppm by volume or less. With this arrangement, it is possible to easily obtain the intergranular layer 12 having the crystal 13 in which the atomic ratio of O to Yb exceeds 0.4 and is less than 1.5 (i.e., without performing the second heat treatment step S5 described later at a hydrogen concentration of 100%). As a result, the electric resistance of the thermoelectric conversion material 1 can be reduced. On the other hand, the lower limit of the oxygen concentration is desirably as low as possible, and preferably 0ppm by volume, but in reality, for example, 50 ppm by volume or more is preferable in view of measurement accuracy and the like.

The thickness of the inter-particle layer 12 is preferably 5nm or more and 1 μm or less. When the thickness of the inter-granular layer 12 is within this range, the number of crystals 13 present in the inter-granular layer 12 increases, and the high-resistance oxide can be reduced. Therefore, the thermal conductivity κ of the thermoelectric conversion material 1 becomes small and the resistivity σ also becomes high, so the dimensionless figure of merit ZT can be further improved. From the viewpoint of more reliably obtaining this effect, the thickness of the inter-particle layer 12 is more preferably 5nm or more and 100nm or less.

The thickness of the inter-particle layer 12 can be controlled by appropriately changing the quenching speed in the tape production step and the heat treatment temperature in the pressure sintering step, for example.

The thickness of the interlayer 12 can be observed and measured by, for example, a Scanning Electron Microscope (SEM), a Transmission Electron Microscope (TEM), or the like. The thickness of the granular layer 12 can be measured, for example, by: when observing the range including the grain layer 12, the distance between any two adjacent polycrystalline grains 11 is measured.

Note that the thermoelectric conversion material 1 according to the present embodiment may contain impurities as long as the impurities do not adversely affect thermoelectric conversion.

The thermoelectric conversion material 1 according to the present embodiment described above includes: a plurality of polycrystalline crystal grains 11 having a skutterudite-type crystal structure including Yb, Co, and Sb; and an intergranular layer 12 present between adjacent polycrystalline grains 11 and having crystals 13 with an atomic ratio of O to Yb exceeding 0.4 and less than 1.5. The thermoelectric conversion material 1 according to the present embodiment may not contain an active material such as Ba, La, or Sr. Therefore, the thermoelectric conversion material 1 according to the present embodiment has a high dimensionless figure of merit ZT and high reliability.

[ thermoelectric conversion Module ]

Next, a thermoelectric conversion module according to the present embodiment will be described.

Fig. 4 and 5 are perspective views showing the structure of the thermoelectric conversion module according to the present embodiment. Fig. 4 shows a state before the upper substrate 6 is mounted, and fig. 5 shows a state after the upper substrate 6 is mounted.

As shown in fig. 4 and 5, the thermoelectric conversion material 1 according to the present embodiment described above can be mounted on a thermoelectric conversion module 10. The thermoelectric conversion module 10 includes a plurality of elements 4 each having a columnar thermoelectric conversion material 1 and a columnar thermoelectric conversion material 2 opposite to the thermoelectric conversion material 1, and having ends connected to each other via an electrically conductive material 3. Although not particularly limited, the element 4 can connect the thermoelectric conversion material 1 and the reverse-phase thermoelectric conversion material 2 alternately and electrically in series via the electrically conductive material 3 as shown in fig. 4 and 5. The element 4 in which the thermoelectric conversion material 1, the reverse-phase thermoelectric conversion material 2, and the conductive material 3 are connected is disposed between the lower substrate 5 and the upper substrate 6 (see fig. 5). In the conductive material 3, the first conductive material 3a is connected to the thermoelectric conversion material 1, and the second conductive material 3b is connected to the thermoelectric conversion material 2. The first conductive material 3a is connected to a first wiring 7 for voltage extraction, and the second conductive material 3b is connected to a second wiring 8 for voltage extraction.

Here, the thermoelectric conversion material 2 in the opposite phase is, for example, n-type when the thermoelectric conversion material 1 is p-type, and p-type when the thermoelectric conversion material 1 is n-type.

Examples of the conductive material 3 include a thin plate material, a film, and a thin film using copper, silver, gold, platinum, aluminum, or an alloy of any one metal selected from these metals. The conductive material 3 may have conductivity, and is not limited to the above-described metal, form, and the like. For example, if the conductive material 3 is disposed on the low temperature side, a film made of a conductive polymer or the like can be used.

For example, aluminum nitride (AlN) or silicon nitride (Si) can be used as the lower substrate 5 and the upper substrate 6₃N₄) Alumina (Al)₂O₃) And the like.

In the thermoelectric conversion module 10 according to the above-described embodiment, for example, the upper substrate 6 is heated or brought into contact with a high heat portion, whereby a temperature gradient can be generated in the same direction with respect to the thermoelectric conversion material 1 and the opposite-phase thermoelectric conversion material 2. Thereby, the thermoelectric conversion material 1 and the reverse-phase thermoelectric conversion material 2 generate a thermoelectromotive force due to the seebeck effect. At this time, the thermoelectromotive forces are generated in the thermoelectric conversion material 1 and the inverted thermoelectric conversion material 2 in opposite directions with respect to the temperature gradient, and the thermoelectromotive forces are added without being cancelled out. Therefore, the thermoelectric conversion module 10 can generate a large thermoelectromotive force. In addition to the above-described embodiment, the lower substrate 5 may be cooled or brought into contact with the low-temperature portion. Further, the upper substrate 6 may be heated or brought into contact with a high-temperature portion, and the lower substrate 5 may be cooled or brought into contact with a low-temperature portion.

The thermoelectric conversion module according to the present embodiment described above uses the thermoelectric conversion material 1 according to the present embodiment described above. Therefore, the thermoelectric conversion module according to the present embodiment has a high dimensionless figure of merit ZT and high reliability for the same reason as described for the thermoelectric conversion material 1 according to the present embodiment.

[ method for producing thermoelectric conversion Material ]

Next, a method for producing the thermoelectric conversion material of the present embodiment will be described.

Fig. 6 is a flowchart showing the contents of a method for producing a thermoelectric conversion material according to the present embodiment (hereinafter referred to as the present production method). The present manufacturing method is, for example, a manufacturing method of the thermoelectric conversion material 1 according to the present embodiment described above.

As shown in fig. 6, the manufacturing method includes a weighing step S1, a mixing step S2, a tape producing step S3, a first heat treatment step S4, a second heat treatment step S5, and a pressure sintering step S6. In the present manufacturing method, it is preferable that the oxidation amount is suppressed and the reduction amount is reduced between the first heat treatment step S4 and the second heat treatment step S5 performed before the pressure sintering step S6 and between the second heat treatment step S5 and the pressure sintering step S6 so as not to contact an oxidizing atmosphere (for example, the atmosphere). After the second heat treatment step S5, it is also desirable to perform the pressure sintering step S6 without contacting an oxidizing atmosphere (e.g., the atmosphere). Preferably, the second heat treatment process S5 and the pressure sintering process S6 are performed using a furnace that is kept airtight.

These steps will be described in order below.

(weighing step S1)

The weighing step S1 is a step of weighing each of the Yb-containing material, the Co-containing material, and the Sb-containing material. The raw materials can be weighed by using a general weighing machine. The form of the raw material may be any form. That is, the raw material may be, for example, ore, waste material, or the like, or may be a purified product having high purity which is preliminarily purified. Here, Yb, Co and Sb are preferably analyzed in advance for the content ratio contained in each raw material, and based on this analysis, the composition formula of the weighed raw materials is expressed asYb_xCo₄Sb₁₂(wherein x is more than 0 and 0.3 or less).

The weighing is preferably performed in a sealed operation device such as a glove box that can be operated in a state of being isolated from outdoor air. The sealing operation apparatus is preferably an apparatus capable of supplying an inert gas such as nitrogen or argon into the apparatus. The weighing is preferably carried out in a closed operation apparatus having an oxygen concentration of 0.1 to 100ppm by volume. The raw material weighed in the closed processing apparatus is preferably placed in a heat-resistant container such as a graphite crucible. The raw material weighed in the closed processing apparatus is preferably placed in a quartz container or the like and vacuum-sealed. Although only one of these schemes can prevent excessive (more than necessary) oxidation of the raw material, it is preferable to perform all of the schemes. As will be described later, excessive oxidation is not preferable in the present embodiment, but after oxidation to such an extent that the surface of the belt is slightly oxidized, the oxide of part of Yb of the crystals 13 precipitated in the inter-granular layer 12 is reduced by performing heat treatment in the second heat treatment step S5, and the electric resistance can be reduced.

(mixing step S2)

The mixing step S2 is a step of melting and mixing the above-described raw materials. The melting of the raw material is carried out, for example, at 1020 ℃ or higher, preferably 1050 ℃ or higher. From the viewpoint of cost and maintenance of the heating apparatus, the melting of the raw material is preferably performed at 1300 ℃ or lower, more preferably at 1100 ℃ or lower.

The melting of the raw material is preferably performed while being maintained at the above temperature for several hours to several tens of hours, and thereafter, is rapidly cooled. For example, it is preferable that the melting of the raw material is maintained at the above temperature for 20 hours, and then the raw material is put into water cooled to 20 ℃ or lower and quenched. In the case where the amount of the raw material is large, the raw material may be slowly cooled. After the rapid cooling, the ingot is preferably taken out from a quartz container in an inert atmosphere and loaded in a heat-resistant container such as a graphite crucible. In the case of a short time of 1 hour or less, the ingot can be taken out in the air and loaded in a heat-resistant container. Thereafter, the temperature is raised to 1100 ℃ by, for example, a high-frequency heating furnace, and the resultant is reheated to form a melt.

(tape production Process S3)

The ribbon producing step S3 is a step of rapidly solidifying the melt of the raw material by a liquid rapid solidification method to produce a ribbon. The liquid rapid solidification method is a method of dropping molten metal (melt) onto a rolling metal roll and rapidly cooling the metal roll at the nucleation rate of crystals to produce an amorphous metal ribbon.

The thickness of the tape obtained in the tape producing step S3 is preferably 10 to 200 μm. When the thickness of the tape is within this range, the uniformity of the structure becomes high, and the degree of oxidation can be easily controlled. The tape forming step S3 is also performed in an inert atmosphere as described above.

(first Heat treatment step S4)

The first heat treatment step S4 is a step of obtaining the polycrystalline grains 11 in the form of ground particles by performing a heat treatment (not shown in fig. 6) and a grinding step (not shown in fig. 6) on the produced ribbon in an inert atmosphere in which the oxygen concentration is adjusted. That is, in the first heat treatment step S4, the ribbon may be heat-treated in an inert atmosphere in which the oxygen concentration is adjusted, and then the ribbon may be pulverized to obtain the polycrystalline grains 11 in the form of a pulverized powder. In the first heat treatment step S4, after the ribbon is crushed to obtain the polycrystalline crystal grains 11 in the form of crushed powder, the polycrystalline crystal grains 11 may be heat-treated in an inert atmosphere in which the oxygen concentration is adjusted.

Before the heat treatment, the heat-resistant container such as a graphite crucible is preferably mounted in a closed operation apparatus (oxygen concentration of 0.1 to 100ppm by volume) and covered with a lid. Since the oxygen concentration in the heat-resistant container can be reduced, a lid having a deep depth is preferably used as the lid of the heat-resistant container. The cover of the heat-resistant container is preferably 5 to 10cm in depth, for example. In order to reduce the oxygen concentration in the heat-resistant container, a getter material such as metal Ti is preferably mounted.

The oxygen concentration in the inert gas in the first heat treatment step S4 is preferably 10 ppm by volume or less, and more preferably 5 ppm by volume or less, for example. When this setting is made, only the surface of the belt can be slightly oxidized. In the present embodiment, excessive oxidation is not preferable, but only the surface of the band or the surface of the polycrystalline grains 11 can be oxidized by performing the treatment in such an atmosphere. Then, by performing heat treatment in the second heat treatment step S5 performed later to reduce a part of the Yb oxide of the crystal grains 13 precipitated in the intergranular layer 12, the electric resistance can be reduced. In addition, when set in this manner, the atomic ratio of O to Yb of the crystal 13 precipitated in the intergranular layer 12 can be made to exceed 0.4 and be less than 1.5. Therefore, the thermal conductivity κ and the electrical conductivity σ of the thermoelectric conversion material 1 can be reduced, and the dimensionless figure of merit ZT can be improved.

The inert atmosphere includes, but is not limited to, at least one of nitrogen and argon. The oxygen concentration of the inert atmosphere can be adjusted by using a mixed gas of hydrogen and argon, a mixed gas of hydrogen and nitrogen, or hydrogen alone, for example. Note that the inert atmosphere may contain water vapor. When water vapor is included, oxidation may be slightly promoted.

The heat treatment conditions in the first heat treatment step S4 may be, for example, a heat treatment temperature of 500 to 800 ℃. The heat treatment conditions in the first heat treatment step S4 may be, for example, 3 hours or more and less than 168 hours when the heat treatment temperature is 700 ℃. From the viewpoint of preventing extreme oxidation of the band or the polycrystalline grains 11, the heat treatment time is preferably 48 hours or less.

The belt is preferably pulverized in a closed operation apparatus (oxygen concentration of 0.1 to 100 ppm). The median diameter (d50) of the polycrystalline grains 11 obtained by the pulverization is preferably 10 to 100 μm. When set in this manner, oxygen concentration control is facilitated by contact with oxygen, and a dense sintered body is easily obtained by pressure sintering described later.

The pulverization of the belt can be performed, for example, by using a mortar and a pestle, or by using a ball mill, a rod mill, a high-pressure pulverizing roller, a vertical impact mill, a jet mill, or the like.

The polycrystalline crystal grains 11 obtained in the first heat treatment step S4 are weighed in a closed processing device, loaded in a carbon mold (carbon die) in a ventilation chamber, and introduced into a pressure sintering device such as a hot press.

(second Heat treatment step S5)

The second heat treatment step S5 is a step of heat-treating the polycrystalline grains 11 heat-treated in the first heat treatment step S4 in a reducing atmosphere. By performing this second heat treatment step S5, a part of the Yb oxide of the crystal 13 precipitated in the intergranular layer 12 is reduced. By performing the pressure sintering step S6 described later after this step, the atomic ratio of O to Yb of the crystal 13 precipitated in the intergranular layer 12 can be finally made to be more than 0.4 and less than 1.5. In the present embodiment, since the atomic ratio of O to Yb of the crystals 13 precipitated in the intergranular layer 12 can be made to be in the above-described range by performing the second heat treatment step S5, not only the oxide of Yb but also the ratio of the metal Yb can be increased. The reason for this is not clear, but it is considered that the reason is that oxygen is derived from Yb, for example, as described above₂O₃The Yb oxide, which is very stable and has low electrical conductivity σ and thermal conductivity κ, is released from the linkage, thereby producing a stable oxide having oxygen defects. Further, it is considered that the Yb oxide having such defects must generate electrons in order to ensure electrical neutrality, and therefore the electrical conductivity inevitably increases. Therefore, it is considered that the thermal conductivity κ of the thermoelectric conversion material 1 heat-treated in the second heat treatment step S5 can be reduced and the electrical conductivity σ can be improved, and as a result, the dimensionless figure of merit ZT becomes high. In other words, the second heat treatment process S5 may be referred to as a process of performing heat treatment to such an extent that conduction electrons are generated to maintain some or all of the crystalline Yb suboxide generated on the surface of the polycrystalline crystal grains 11 in the first heat treatment process S4 immediately before sintering.

The reducing atmosphere in the second heat treatment step S5 may be, for example, a hydrogen concentration of 3 vol% or 100 vol%. The second heat treatment step S5 is also preferably carried out in a closed operation apparatus (oxygen concentration of 0.1 to 100 ppm).

The second heat treatment step S5 is preferably performed in a pressure sintering apparatus for performing pressure sintering. In this setting, the second heat treatment step S5 and the pressure sintering step S6 may be performed continuously in the same furnace. Therefore, the oxidation reaction of the polycrystalline grains 11 heat-treated in the second heat treatment step S5 can be suppressed. In the present embodiment, it is preferable to evacuate the inside of the pressure sintering apparatus, for example, to about 10Pa or so, and replace it with an inert atmosphere. The inert atmosphere includes, but is not limited to, at least one atmosphere of nitrogen and argon. This operation is repeated 2 or more times, and after the final evacuation is finished, the hydrogen concentration is preferably set to 3 vol% or 100 vol% as described above. Then, once the gas pressure in the pressure sintering apparatus becomes 0.1 to 0.6MPa (1 to 6 atm) in terms of gauge pressure, heat treatment is performed at about 400 to 600 ℃ for 1 to 6 hours, and then cooling is performed.

(pressure sintering step S6)

The pressure sintering step S6 is a step of pressure sintering the polycrystalline grains 11 heat-treated in the second heat treatment S5 in an inert atmosphere to produce the thermoelectric conversion material 1 according to the present embodiment described above. That is, the pressure sintering step S6 is a step of performing pressure sintering without contact with an oxidizing atmosphere (for example, the atmosphere) after the second heat treatment step S5 and before the pressure sintering step S6 is completed, to produce the thermoelectric conversion material 1 according to the present embodiment described above. The term "inactive" means not oxidizing. Such an inert atmosphere can be suitably realized, for example, by: the furnace was evacuated to 10Pa or less, replaced with Ar gas, and the process was repeated 3 times to discharge oxygen in the atmosphere from the furnace.

In the pressure sintering step S6, it is preferable to replace the oxidizing atmosphere with an inert atmosphere before the pressure sintering. For example, as described above, the reducing atmosphere in the pressure sintering apparatus is evacuated to about 10Pa and then replaced with an inert atmosphere. In this case, the pressure sintering apparatus is preferably operated without opening to the atmosphere. The inert atmosphere includes, but is not limited to, at least one atmosphere of nitrogen and argon. The inert atmosphere is preferably an argon atmosphere. When the reducing atmosphere is replaced with the inert atmosphere, the replacement operation is preferably performed 2 or more times, more preferably 3 or more times. As the pressure sintering device, for example, a hot press can be used. In this step, if the time is short, e.g., about 2 hours before evacuation, the atmosphere may be opened.

After the substitution to the inert atmosphere, the inside of the pressure sintering apparatus is heated at a temperature rise rate of, for example, 300 to 600 ℃/h, and is held at 600 to 800 ℃ for 1 hour. At this time, the pressurization is performed at a rate equivalent to the temperature rise rate so that the pressurization pressure is, for example, 50 to 70MPa, specifically, 68MPa, at the holding temperature. After the completion of the holding, the mixture is cooled at a cooling rate of 500 ℃/h or less and then depressurized. The cooling is preferably performed by natural air cooling in the pressure sintering apparatus. The thermoelectric conversion material 1 according to the present embodiment described above can be suitably manufactured by the operation up to now.

(post-treatment)

The produced thermoelectric conversion material 1 was taken out from the pressure sintering apparatus together with the carbon mold, and the thermoelectric conversion material 1 (sintered body) was taken out from the heat-resistant container. After removing chips (debris) from the surface of the thermoelectric conversion material 1, an arbitrary size and shape are cut out by a precision machining machine. These operations may be performed in air. The precision machining machine preferably uses a machining machine having a diamond blade. In addition, the seebeck coefficient, the electric conductivity, the specific heat, the thermal conductivity, and the like can be measured by a conventional method as needed.

As described above, the method for producing a thermoelectric conversion material according to the present embodiment proceeds from the weighing step S1 to the pressure sintering step S6, and thus a thermoelectric conversion material 1 can be produced, the thermoelectric conversion material 1 including: a plurality of polycrystalline crystal grains 11 having a skutterudite-type crystal structure including Yb, Co, and Sb; and an intergranular layer 12 present between adjacent polycrystalline grains 11 and having crystals 13 with an atomic ratio of O to Yb exceeding 0.4 and less than 1.5. The method for producing a thermoelectric conversion material according to the present embodiment does not introduce active materials such as Ba, La, and Sr in the production process. Therefore, the method for manufacturing a thermoelectric conversion material according to the present embodiment can manufacture the thermoelectric conversion material 1 having a high dimensionless figure of merit ZT and high reliability. In addition, in the method for producing a thermoelectric conversion material according to the present embodiment, since the second heat treatment step S5 is performed so that the atomic ratio of O to Yb of the crystal 13 included in the inter-granular layer 12 exceeds 0.4 and is less than 1.5, the heat treatment time (annealing time) in the first heat treatment step S4 can be shortened. Therefore, the method for producing a thermoelectric conversion material according to the present embodiment is superior in mass productivity compared to conventional methods.