Thermoelectric conversion material and manufacturing method thereof, thermoelectric conversion element, and thermoelectric conversion module

Abstract

To provide a novel thermoelectric conversion material which is low in thermal conductivity, exhibits excellent thermoelectric effects in the temperature range from room temperature to 500°C, and contains, as main constituents, a half-Heusler compound having semiconductor characteristics that can be adjusted to both P type and N type, and a method for producing the same, and also to provide a thermoelectric conversion element and a thermoelectric conversion module which include the thermoelectric conversion material.SOLUTION: Provided is a thermoelectric conversion material which contains at least magnesium (Mg), vanadium (Vd), nickel (Ni), and antimony (Sb), which has a composition represented by general formula: Mg2-pV1+pNi3Sb3-qMq (where, p and q satisfy -0.5≤p≤0.5 and 0≤q≤1.0, and M represents one or both of tin (Sn) and tellurium (Te)), and which contains, as main constituents, a half-Heusler compound.SELECTED DRAWING: Figure 17

Description

[Technical Field]

[0001] The present invention relates to a thermoelectric conversion material, a method for producing the same, a thermoelectric conversion element, and a thermoelectric conversion module. [Background technology]

[0002] In response to the trend toward stable energy security and the prevention of global warming, there is a strong desire to realize an energy-saving society. Currently, approximately 60% of primary energy supply is wasted as heat energy, and there is a need for waste heat recovery technology and its widespread use. Under these circumstances, thermoelectric conversion modules such as thermoelectric power generation modules and thermoelectric cooling modules are attracting attention.

[0003] Thermoelectric conversion modules are solid-state devices that enable direct conversion between thermal energy and electrical energy. Because they have no mechanical moving parts, they are highly reliable, maintenance-free, and operate quietly. Furthermore, they do not produce waste during energy conversion. For these reasons, thermoelectric conversion modules are highly regarded as an environmentally symbiotic energy technology.

[0004] Known thermoelectric conversion modules include thermoelectric power generation modules and thermoelectric cooling modules. Thermoelectric power generation modules generate electricity from heat by utilizing the thermoelectric power that occurs when a temperature difference is created in a material, i.e., the Seebeck effect of a solid. Conversely, thermoelectric cooling modules (Peltier cooling modules) utilize the phenomenon in which a temperature difference occurs when a potential difference is created in a material, i.e., the Peltier effect. Both modules utilize the properties of thermoelectric conversion materials (thermoelectric materials), namely, their ability to directly convert thermal energy into electrical energy.

[0005] Figure 1 shows an example of a cross-sectional schematic diagram of a typical thermoelectric conversion module. The thermoelectric conversion module (10) includes a P-type thermoelectric conversion element (2) and an N-type thermoelectric conversion element (4). Both the P-type thermoelectric conversion element (2) and the N-type thermoelectric conversion element (4) are made of molded thermoelectric material. Electrodes (6) are provided above and below the P-type thermoelectric conversion element (2) and the N-type thermoelectric conversion element (4), connecting these elements in series. Furthermore, a pair of ceramic plates (8) is provided, sandwiching the P-type thermoelectric conversion element (2), the N-type thermoelectric conversion element (4), and the electrodes (6) from above and below. By creating a temperature difference between the top and bottom of the module, electricity commensurate with this temperature difference can be extracted, allowing the module to function as a thermoelectric power generation module. Conversely, by passing an electric current through the module, a temperature difference can be created between the top and bottom of the module, allowing it to function as a thermoelectric cooling module.

[0006] Maximum energy conversion efficiency η obtained in a thermoelectric conversion module max is calculated according to the following formula (1) using the thermoelectric figure of merit (dimensionless figure of merit) ZT of the thermoelectric conversion element. The thermoelectric figure of merit ZT is the product of the figure of merit Z and the temperature T. In the following formula (1), T H is the high temperature side temperature (unit: K), T C is the low-temperature side temperature (unit: K), and T is the average temperature (unit: K).

[0007]

number

[0008] The first term on the right side of the above equation (1) is the maximum energy conversion efficiency (Carnot efficiency) of an ideal heat engine. On the other hand, the second term increases as the thermoelectric figure of merit ZT increases. Therefore, in order to increase the conversion efficiency, the temperature difference (T H -T C ) and improving the thermoelectric figure of merit ZT of the thermoelectric conversion element.

[0009] The thermoelectric properties of thermoelectric conversion materials are evaluated by the thermoelectric figure of merit zT. Strictly speaking, this thermoelectric figure of merit zT is distinct from the thermoelectric figure of merit ZT of the module, but it serves as an indicator for improving the thermoelectric performance ZT of the element. Therefore, in order to improve the performance of thermoelectric conversion modules (thermoelectric power generation modules, Peltier cooling modules), it is necessary to improve the thermoelectric figure of merit zT of the material in the operating range in which the module operates.

[0010] The thermoelectric figure of merit zT of a thermoelectric conversion material can be calculated according to the following formula (2): where S is the Seebeck coefficient of the material, σ is the electrical conductivity, T is the temperature, and κ is the total is the thermal conductivity, PF is the power factor, and κ total is the lattice thermal conductivity κ lat and electronic thermal conductivity κ el The sum of (κ lat +κ el ) where lattice thermal conductivity κ lat is the thermal conductivity κ total The quantized lattice vibrations, i.e., phonons, correspond to the heat carriers. el corresponds to the portion where electrons or holes act as heat carriers.

[0011]

number

[0012] As can be seen from the above formula (2), in order to increase the figure of merit zT, it is necessary to increase the Seebeck coefficient S and electrical conductivity σ while also increasing the thermal conductivity κ total (=κ lat +κ el) is important. However, since the Seebeck coefficient, electrical conductivity, and electronic conductivity are all functions of the carrier concentration, it is difficult to control them independently. Specifically, the higher the carrier concentration, the higher the electrical conductivity, but the smaller the Seebeck coefficient. In addition, high electrical conductivity and low electronic thermal conductivity are in a contradictory relationship. Therefore, in order to increase zT, it is necessary to adjust the carrier concentration so that the power factor PF is maximized, and also to control the lattice thermal conductivity (κ lat ) is effective to make as small as possible. Thermoelectric conversion materials with carrier concentrations adjusted within an appropriate range are usually N-type or P-type semiconductors with a finite band gap.

[0013] Compounds such as Bi2Te3, PbTb, CoSb3, and La3Te4 have been known as N-type thermoelectric conversion materials. SnSe, BiSbTe, MgAgSb, and NaPb are also known as P-type thermoelectric conversion materials. m SbTe m+2 , (GeTe) 0.8 (AgSbTe2) 0.2 Compounds such as PbTe-4SrTe-2Na are known. These compounds have a relatively high figure of merit (zT). However, they contain hazardous or rare elements such as cobalt (Co), lead (Pb), silver (Ag), and germanium (Ge). Therefore, to commercialize and commercialize thermoelectric power generation, such as exhaust heat recovery from automobiles and factories, high-performance thermoelectric conversion materials that do not contain hazardous or rare elements and exhibit high figures of merit within the operating temperature range are required. Heusler-type compounds, especially half-Heusler-type compounds, are considered promising as such materials.

[0014] Half-Heusler compounds have a wide variety of combinations of component elements. This makes it possible to design compositions that do not contain harmful or rare elements, resulting in non-toxic and inexpensive materials. They also have the advantage of high mechanical strength and can be used stably even at high temperatures. Therefore, thermoelectric conversion materials primarily composed of half-Heusler compounds are attracting attention as promising materials that can be applied primarily to exhaust heat recovery devices in automobiles.

[0015] Patent Document 1 describes an intermetallic compound [Ti a Zr b Hf c ][Ni d Co e ][Sn f Sb g ] is disclosed as a method for producing a thermoelectric material that generates a solid phase of a half-Heusler alloy consisting of ScNiSb 1-x Te x (Abstract of Non-Patent Document 1, etc.). Non-Patent Document 2 discloses investigating the thermoelectric properties of compounds having compositions such as MgNiSb (full text of Non-Patent Document 2). Non-Patent Document 3 discloses doping tin into a Ti2FeNiSb2 double half-Heusler compound (Abstract of Non-Patent Document 3). [Prior art documents] [Patent documents]

[0016] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-084689 [Non-patent literature]

[0017] [Non-Patent Document 1] Kamil Ciesielski et al., Mobility Ratio as a Probe for Guiding Discovery of Thermoelectric Materials: The Case of Half-Heusler Phase ScNiSb1-xTex, PHYSICAL REVIEW APPLIED 15,044047 (2021) [Non-patent document 2] AVMorozkin et al., Thermoelectric properties of ScCoSb, ScNi0.86Sb and MgNiSb compounds, Journal of Alloys and Compounds 400 (2005) 62-66 [Non-patent document 3] Rahidul Hasan et al., Enhanced Thermoelectric Properties of Ti2FeNiSb2 Double Half-Heusler Compound by Sn Doping, Adv. Energy Sustainability Res. 2022, 2100206 Summary of the Invention [Problem to be solved by the invention]

[0018] Although the use of half-Heusler compounds, which have various advantages, as thermoelectric conversion materials has been proposed, conventional half-Heusler compounds have the problem of high thermal conductivity. That is, as can be seen from the above formula (2), the thermoelectric figure of merit zT is proportional to the thermal conductivity κ total (=κ lat +κ el ), the thermoelectric figure of merit zT increases as the thermal conductivity decreases. However, conventional half-Heusler compounds have low thermal conductivity, especially the lattice thermal conductivity κ lat was high, which limited the ability to increase the thermoelectric figure of merit zT.

[0019] The present inventors have conducted extensive research in light of these problems. As a result, they have discovered that a novel thermoelectric conversion material primarily composed of a half-Heusler compound can be synthesized by incorporating magnesium (Mg) and vanadium (V), which have different valences, into the same atomic site in different proportions. They have also discovered that this thermoelectric conversion material has low thermal conductivity and exhibits excellent thermoelectric effects in the temperature range from room temperature to 500°C. They have also discovered that by fine-tuning the composition, the properties of the thermoelectric conversion material can be controlled to either P-type or N-type.

[0020] The present invention was completed based on these findings, and aims to provide a novel thermoelectric conversion material containing as its main component a half-Heusler compound that has low thermal conductivity, exhibits excellent thermoelectric effect in the temperature range from room temperature to 500°C, and whose semiconductor properties can be adjusted to either P-type or N-type, as well as a method for producing the same. Another aim of the present invention is to provide a thermoelectric conversion element or a thermoelectric conversion module that includes this thermoelectric conversion material. [Means for solving the problem]

[0021] The present invention encompasses the following aspects (1) to (14). In this specification, the expression "to" includes the numerical values ​​on both ends. In other words, "X to Y" is synonymous with "at least X and at most Y."

[0022] (1) A composite material containing at least magnesium (Mg), vanadium (V), nickel (Ni), and antimony (Sb), and having the general formula: Mg 2―p V 1+p Ni3Sb 3-q M q (wherein p and q satisfy -0.5≦p≦0.5 and 0≦q≦1.0, and M is one or both of tin (Sn) and tellurium (Te), A thermoelectric conversion material whose main component is a half-Heusler compound.

[0023] (2) The thermoelectric conversion material according to (1) above, wherein the thermoelectric conversion material is composed of a single phase of a half-Heusler compound.

[0024] (3) The thermoelectric conversion material according to (1) or (2) above, wherein the thermoelectric conversion material is a polycrystalline material.

[0025] (4) The thermoelectric conversion material according to (1) or (2) above, which exhibits P-type semiconductor properties.

[0026] (5) The thermoelectric conversion material according to (1) or (2) above, which exhibits N-type semiconductor properties.

[0027] (6) The thermal conductivity (κ total ) is 6.0 W / (mK) or less in a temperature range of 300 K or more and 800 K or less.

[0028] (7) The power factor (PF) of the thermoelectric conversion material is 0.2 mW / (mK 2 ) or more.

[0029] (8) The power factor (PF) of the thermoelectric conversion material is 0.5 mW / (mK 2 ) or more.

[0030] (9) A method for producing the thermoelectric conversion material according to (1) or (2) above, comprising the following steps: A step of preparing a mixed raw material containing at least magnesium (Mg), vanadium (V), nickel (Ni), and antimony (Sb): A step of subjecting the mixed raw material to a mechanical alloying treatment to produce a mechanically alloyed product; and a step of pressure sintering the mechanically alloyed product A method comprising:

[0031] (10) The method according to (9), wherein, in preparing the mixed raw material, a magnesium (Mg) source, a vanadium (V) source, a nickel (Ni) source, and an antimony (Sb) source are melted and solidified to produce a molten solid, a magnesium (Mg) source is further mixed with the molten solid, and a mixture of the molten solid and the magnesium (Mg) source is used as the mixed raw material.

[0032] (11) The method according to (9), wherein, in preparing the mixed raw material, a vanadium (V) source, a nickel (Ni) source, and an antimony (Sb) source are melted and cooled to prepare a molten solidify, a magnesium (Mg) source is further mixed with the molten solidify, and a mixture of the molten solidify and the magnesium (Mg) source is used as the mixed raw material.

[0033] (12) The method according to (9), wherein, when preparing the mixed raw material, a magnesium (Mg) source, a vanadium (V) source, a nickel (Ni) source, and an antimony (Sb) source are melted and cooled to prepare a molten solidification product, and the molten solidification product is used as the mixed raw material.

[0034] (13) A thermoelectric conversion element containing the thermoelectric conversion material of (1) or (2) above.

[0035] (14) A thermoelectric conversion module including the thermoelectric conversion element of (13) above. [Effects of the Invention]

[0036] The present invention provides a novel thermoelectric conversion material containing as its main component a half-Heusler compound that has low thermal conductivity, exhibits excellent thermoelectric effects in the temperature range from room temperature to 500°C, and whose semiconductor properties can be adjusted to either P-type or N-type, as well as a method for producing the same. The present invention also provides a thermoelectric conversion element and a thermoelectric conversion module that include this thermoelectric conversion material. [Brief explanation of the drawings]

[0037] [Figure 1] FIG. 2 is a cross-sectional view showing a structure of a thermoelectric conversion module. [Figure 2] 1 is a schematic diagram showing the crystal structure of the triple half-Heusler (THH) compound Mg2VNi3Sb3. [Figure 3] 1 is a diagram illustrating a triple half-Heusler (THH) compound. [Figure 4] 1 shows X-ray diffraction patterns (Examples 1 to 3). [Figure 5] Elemental mapping images are shown (Example 2). [Figure 6] 1 shows the temperature change of the Seebeck coefficient S (Examples 1 to 3). [Figure 7] 1 shows the temperature change of the power factor PF (Examples 1 to 3). [Figure 8] 1 shows the temperature change of thermal conductivity κtotal (Examples 1 and 2). [Figure 9] 1 shows the temperature change of lattice thermal conductivity κlat (Examples 1 and 2). [Figure 10] 1 shows the temperature change of the thermoelectric figure of merit zT (Examples 1 and 2). [Figure 11] 1 shows the results of repeated tests of the power factor PF (Example 1). [Figure 12] The results of TG / DTA analysis are shown below (Example 2). [Figure 13] 1 shows the temperature change of the Seebeck coefficient S (Examples 4 to 7). [Figure 14] 1 shows the temperature change of the Seebeck coefficient S (Examples 8 to 14). [Figure 15] The temperature change of thermal conductivity κtotal is shown (Examples 4, 5, 8, 9, 13, and 14). [Figure 16] 1 shows the temperature change of lattice thermal conductivity κlat (Examples 4, 5, 8, 9, 13, and 14). [Figure 17] The magnitude of the lattice thermal conductivity κlat of various half-Heusler compounds is compared. DETAILED DESCRIPTION OF THE INVENTION

[0038] A specific embodiment of the present invention (hereinafter referred to as "the present embodiment") will be described. Note that the present invention is not limited to the following embodiment, and various modifications are possible within the scope of the present invention.

[0039] <<1. Thermoelectric conversion materials>> The thermoelectric conversion material of this embodiment contains at least magnesium (Mg), vanadium (V), nickel (Ni), and antimony (Sb), and is represented by the general formula: Mg 2―p V 1+p Ni3Sb 3-q M q (where p and q satisfy -0.5≦p≦0.5 and 0≦q≦1.0, and M is one or both of tin (Sn) and tellurium (Te)), and the main component is a half-Heusler compound.

[0040] The thermoelectric conversion material of this embodiment contains a half-Heusler compound as a main component. Here, "main component" means that the content of the half-Heusler compound in the thermoelectric conversion material is 50% by mass or more. From the viewpoint of making the most of the excellent properties of the half-Heusler compound, the higher the content of the main component, the better. The content is preferably 60% by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, and particularly preferably 90% by mass or more. When defined based on the X-ray diffraction pattern, the peak intensity (I HH ) to the peak intensity of the main peak from the heterogeneous phase (I HP ) ratio (I HP / I HH ) is preferably 0.4 or less, more preferably 0.3 or less, even more preferably 0.2 or less, and particularly preferably 0.1 or less. Most preferably, the thermoelectric conversion material is composed of a single phase of a half-Heusler compound. Here, being composed of a single phase means that the peak intensity ratio (I HP / I HH ) is zero (0).

[0041] A half-Heusler compound is a compound represented by the chemical formula: XYZ, and has a cubic crystal structure. The crystal structure of a half-Heusler compound is shown in Figure 2. In a crystal of a half-Heusler compound, X atoms and Z atoms form a rock-salt structure, and Y atoms occupy four of the eight sublattices in the structure. The composition formula of the main component compound of this embodiment is: Mg 2―p V 1+p Ni3Sb 3-q M q Rewriting the formula (XYZ) gives (Mg 2 / 3―p / 3 V 1 / 3+p / 3 )Ni(Sb 1-q / 3 M q / 3 As can be seen, magnesium (Mg) and vanadium (V) occupy the X atomic site, nickel (Ni) occupies the Y atomic site, and antimony (Sb) and the M element (Sn and / or Te) occupy the Z atomic site.

[0042] Half-Heusler compounds exhibit a band gap and semiconducting properties when the total number of valence electrons is 18. As a result, they are known to have a large power factor PF. This is called the 18-electron rule. In the thermoelectric conversion material of this embodiment, when p = q = 0, the ratio of divalent magnesium (Mg) atoms to pentavalent vanadium (V) atoms is 2:1, and the average valence of the X atoms (Mg, V) at this time is trivalent. Therefore, the sum of the average valence (trivalent) of the X atoms (Mg, V), the valence (decavalent) of the Y atoms (Ni), and the valence (pentavalent) of the Z atoms (Sb) is 18, which follows the 18-electron rule. Therefore, it is expected that the thermoelectric properties will be improved around this composition.

[0043] As described above, in the half-Heusler compound of this embodiment, magnesium (Mg) occupying the X atomic site is a divalent element, while vanadium (V) is a pentavalent element. Furthermore, the ratio of Mg to V is approximately 2:1, which is not equivalent. The half-Heusler compound Mg2VNi3Sb3 can be considered an intermetallic compound in which MgNiSb and VNiSb are dissolved in a solid solution in a ratio of approximately 2:1, as shown in FIG. 3. For reference, FIG. 3 also shows the conventional ternary half-Heusler compound ScNiSb. Compounds containing multiple elements with different valences (heterovalent elements) in different ratios at the same atomic site are called triple half-Heusler (THH) compounds. Triple half-Heusler (THH) compounds have low thermal conductivity and therefore have the potential to exhibit excellent thermoelectric properties. The "triple" refers to the tripling of formula units, and well-known examples include double perovskites (usually ABO3 versus A2B'B"O6).

[0044] In this respect, the compounds proposed so far are not triple half-Heusler (THH) type compounds. For example, the alloy [Ti a Zr b Hf c ][Ni d Co e ][Sn f Sb g] contains tetravalent elements titanium (Ti), zirconium (Zr), and hafnium (Hf) at the X atomic site, and does not contain any heterovalent element at the same atomic site. The double half-Heusler (DHH) compound Ti2FeNiSb2 disclosed in Non-Patent Document 3 contains heterovalent elements Fe and Ni at the Y atomic site, but in the same ratio (1:1). This double half-Heusler (DHH) compound Ti2FeNiSb2 can be considered an intermetallic compound in which TiFeSb and TiNiSb are solid-solved in a ratio of 1:1, and is clearly different from triple half-Heusler compounds.

[0045] The thermoelectric conversion material of this embodiment, which is mainly composed of a triple half-Heusler (THH) compound, has a thermal conductivity κ total , especially the lattice thermal conductivity κ lat The detailed mechanism is unknown and should not be interpreted in a restrictive manner. However, the following mechanism is thought to be the case.

[0046] Lattice thermal conductivity κ lat is proportional to the product of the lattice specific heat C, the sound velocity v, and the phonon mean free path l, as shown in the following equation (3), where phonons are quantized lattice vibrations in a crystal.

[0047]

number

[0048] In the triple half-Heusler (THH) compound of this embodiment, the magnesium (Mg) atom and the vanadium (V) atom occupying the same atomic site have different valences and significantly different masses. Furthermore, the inventors' investigations revealed that the Mg atom and the V atom are irregularly arranged at the X atomic site. It is speculated that the irregular arrangement of atoms (Mg, V) with different valences and masses causes a disturbance in the lattice periodicity. Furthermore, the disturbance in the lattice periodicity makes phonons, particularly long-period phonons, more susceptible to scattering, resulting in a smaller mean free path l and a lower lattice thermal conductivity κ. latI think it has become smaller.

[0049] The thermoelectric conversion material of this embodiment has semiconductor properties, and therefore has a lower carrier concentration than metals, and the electron thermal conductivity κ is a value in which electrons and holes are responsible for thermal conduction. el is small. Lattice thermal conductivity κ lat and electronic thermal conductivity κ el The thermal conductivity κ total I think it has become smaller.

[0050] p is limited to the range of -0.5 to 0.5 (-0.5≦p≦0.5). p represents the vanadium (V) excess. That is, when p = 0, the composition follows the 18-electron rule. In contrast, when p > 0, the composition is V-excessive, and when p < 0, the composition is Mg-excessive. By adjusting p within the above range, the semiconducting properties of the thermoelectric conversion material can be controlled to either P-type or N-type. However, when p is less than -0.5 or more than 0.5, deviation from the 18-electron rule becomes significant. Furthermore, heterophases are more likely to form, resulting in a significant decrease in thermoelectric properties, particularly the Seebeck coefficient S. p may be -0.3 to 0.3 (-0.3≦p≦0.3) or -0.2 to 0 (-0.2≦p≦0).

[0051] The value of q is limited to a range of 0 to 1.0 (0≦q≦1.0). q is the doping amount of tin (Sn) and / or tellurium (Te), which acts as a dopant. By adjusting q within the above range, the carrier type (P-type, N-type) or carrier concentration of the thermoelectric conversion material can be controlled. However, if q exceeds 1.0, the amount of dopant becomes excessive. As a result, the thermoelectric properties, particularly the Seebeck coefficient, decrease significantly with increasing carrier concentration, and heterogeneous phases are likely to be formed. q may be 0 to 0.5 or 0 to 0.3. q may be zero (0). The dopant is at least one of tin (Sn) and tellurium (Te). The dopant may be only Sn, only Te, or both Sn and Te.

[0052] The thermoelectric conversion material of this embodiment is preferably dense. Dense materials have a high Seebeck coefficient S and electrical conductivity σ. This makes it possible to further increase the power factor PF and thermoelectric figure of merit zT. The relative density of the thermoelectric conversion material is preferably 80% or more, more preferably 90% or more, and even more preferably 95% or more.

[0053] The thermoelectric conversion material of this embodiment may be a polycrystalline or single-crystalline body. However, a polycrystalline body, particularly a sintered body, is preferred. As described below, the thermoelectric conversion material of this embodiment can easily produce a polycrystalline body with a homogeneous composition and crystal structure. Furthermore, in a polycrystalline body, phonons are scattered at the grain boundaries, which can be expected to further reduce the lattice thermal conductivity. In contrast, single-crystalline bodies are not easy to produce. In particular, the thermoelectric conversion material of this embodiment contains multiple components with different melting points and vapor pressures, making it difficult to produce a homogeneous single-crystalline body. When the thermoelectric conversion material is a polycrystalline body, its particle size is not limited. However, the average particle size is typically 100 nm to 100 μm, 500 nm to 100 μm, or 10 μm to 50 μm.

[0054] The thermoelectric conversion material of this embodiment may exhibit P-type semiconductor characteristics, or may exhibit N-type semiconductor characteristics. The semiconductor characteristics (P-type or N-type) of the thermoelectric conversion material can be adjusted by controlling the vanadium (V) excess amount p and the dopant amount q. In particular, by using a combination of both the P-type thermoelectric conversion material of this embodiment and the N-type thermoelectric conversion material, it is possible to fabricate a thermoelectric conversion module using the same material system. A thermoelectric conversion module using the same material system can prevent stress caused by differences in the thermal expansion coefficients of the materials and the resulting deterioration of characteristics and reliability. Therefore, constructing a thermoelectric conversion module using the same material system is an important factor for practical application.

[0055] The thermoelectric conversion material of this embodiment is characterized by low thermal conductivity. Although not limited thereto, the thermal conductivity κ of the thermoelectric conversion material totalis preferably 10.0 W / (mK) or less, more preferably 8.0 W / (mK) or less, and even more preferably 6.0 W / (mK) or less in the temperature range of 300 K or more and 800 K or less. lat is preferably 5.0 W / (mK) or less, more preferably 4.0 W / (mK) or less, and even more preferably 3.0 W / (mK) or less in the temperature range of 300 K or more and 800 K or less. total and lattice thermal conductivity κ lat However, the lower limit of κ is typically total is 0.5W / (mK) or more, κ lat is 0.4W / (mK) or more.

[0056] The thermoelectric conversion material of this embodiment is characterized by exhibiting an excellent thermoelectric effect in the temperature range from room temperature to 500° C. Although not limited thereto, the power factor PF of the thermoelectric conversion material is preferably 0.1 mW / (mK 2 ) or more, more preferably 0.2mW / (mK 2 ) or more, more preferably 0.25mW / (mK 2 ) or more. The power factor PF is preferably 0.25 mW / (mK 2 ) or more, more preferably 0.5mW / (mK 2 ) or more, more preferably 0.75mW / (mK 2 ) or more, particularly preferably 1.0 mW / (mK 2 ) or more. There is no upper limit for the power factor PF. However, it is typically 10.0 mW / (mK 2 ) or less, or 5.0mW / (mK 2 ) is as follows.

[0057] <<2. Thermoelectric conversion element>> The thermoelectric conversion element of this embodiment includes the thermoelectric conversion material described above. Specifically, it includes a molded body of the thermoelectric conversion material. The molded body may be a polycrystalline body or a single crystal body. However, a polycrystalline body, particularly a sintered body, is preferable. The thermoelectric conversion element may also include components other than the molded body of the thermoelectric conversion material. For example, a protective layer may be provided on the surface of the molded body to prevent oxidation or damage to the material. The thermoelectric conversion element may be either P-type or N-type. A P-type thermoelectric conversion element is made of a molded body of a P-type thermoelectric conversion material. An N-type thermoelectric conversion element is made of a molded body of an N-type thermoelectric conversion material.

[0058] <<3. Thermoelectric conversion module>> The thermoelectric conversion module of this embodiment includes the above-described thermoelectric conversion element. That is, at least one of, and preferably both of, the P-type and N-type thermoelectric conversion elements included in the thermoelectric conversion module are the above-described thermoelectric conversion elements. For example, as shown in Fig. 1, the thermoelectric conversion module may be composed of a P-type thermoelectric conversion element, an N-type thermoelectric conversion element, electrodes connecting the P-type and N-type thermoelectric conversion elements in series, and a pair of ceramic plates sandwiching all of the P-type thermoelectric conversion element, the N-type thermoelectric conversion element, and the electrodes from above and below.

[0059] At least one of the P-type thermoelectric conversion element and the N-type thermoelectric conversion element may be the thermoelectric conversion element of this embodiment. For example, the P-type thermoelectric conversion element may be the thermoelectric conversion element of this embodiment, and the N-type thermoelectric conversion element may be made of another material, or vice versa. However, preferably, both the P-type thermoelectric conversion element and the N-type thermoelectric conversion element are made of the thermoelectric conversion element of this embodiment. By making both thermoelectric conversion elements out of the same material, the thermal expansion coefficients of both can be made the same, and as a result, stress generation when a temperature difference occurs and the resulting deterioration of characteristics and reliability can be prevented.

[0060] The thermoelectric conversion module is preferably a thermoelectric power generation module or a thermoelectric cooling module. By creating a temperature difference between the top and bottom of the module, electricity corresponding to this temperature difference can be extracted, allowing the module to function as a thermoelectric power generation module. Conversely, by passing a current through the module, a temperature difference can be created between the top and bottom of the module, which can be used to allow the module to function as a thermoelectric cooling module.

[0061] The thermoelectric conversion module of this embodiment has no mechanical moving parts, making it highly reliable, maintenance-free, and quiet. It also has the advantage of not producing waste materials during energy conversion. Furthermore, it has a wide variety of component element combinations and does not contain any hazardous or rare elements. It also has high mechanical strength and can be used stably even at high temperatures. Therefore, this thermoelectric conversion module is expected to be used in power generation devices that use exhaust heat from factories and automobiles as a heat source, power generation devices for wearable devices that use body heat as a heat source, power generation devices for artificial satellites and space probes, and various cooling devices.

[0062] <<4. Methods for manufacturing thermoelectric conversion materials>> The thermoelectric conversion material of this embodiment may be produced by any method as long as it satisfies the above-described requirements. However, a preferred production method includes the following steps: a step of preparing a mixed raw material containing at least magnesium (Mg), vanadium (V), nickel (Ni), and antimony (Sb) (preparation step); a step of subjecting this mixed raw material to mechanical alloying treatment to produce a mechanically alloyed product (mechanical alloying step); and a step of pressure-sintering this mechanically alloyed product (sintering step). If necessary, a step of subjecting the resulting sintered body to post-treatment (post-treatment step) may also be included. Each step will be described in detail below.

[0063] <Preparation process> In the preparation step, a mixed raw material containing at least magnesium (Mg), vanadium (V), nickel (Ni), and antimony (Sb) is prepared. The mixed raw material contains the constituent elements of the thermoelectric conversion material (Mg, V, Ni, Sb, and optionally Te and / or Sn) and is produced from raw materials (Mg source, V source, Ni source, Sb source, Te source, and Sn source). The form of the raw material mixture is not limited as long as it contains the constituent elements. However, it is preferable that the mixed raw material contains a molten solid containing the constituent elements. In the molten solid, the constituent elements are distributed relatively uniformly. By using the mixed raw material containing the molten solid, it is possible to ensure a uniform composition distribution of the thermoelectric conversion material obtained as a result. Furthermore, the method for producing the mixed raw material is not limited. However, a method including a melting step of the raw materials is preferred. For example, the following first to third embodiments can be mentioned, and among these, the first or second embodiment, which is particularly preferable, is preferable, as it is easy to obtain a single phase half-Heusler compound.

[0064] In a first embodiment, when preparing the mixed raw material, a magnesium (Mg) source, a vanadium (V) source, a nickel (Ni) source, and an antimony (Sb) source are melted and solidified to produce a melt-solidified product, and a magnesium (Mg) source is further mixed with the obtained melt-solidified product, and the mixture of the melt-solidified product and the magnesium (Mg) source is used as the mixed raw material. If necessary, a tellurium (Te) source and / or a tin (Sn) source may be added to the raw materials before the melting step and / or the melt-solidified product after the melting step.

[0065] Among the components of thermoelectric conversion materials, magnesium (Mg) is a highly volatile component. Therefore, it volatilizes during high-temperature melting, and the composition of the final material is likely to be Mg-deficient. By adding a Mg source to the raw materials before the melting process and then adding an additional Mg source to the molten solid obtained after the melting process, a material with the desired composition can be easily obtained. This also makes it possible to achieve uniform composition and structure in the final thermoelectric conversion material.

[0066] Known molten raw materials can be used as the magnesium (Mg) source, vanadium (V) source, nickel (Ni) source, antimony (Sb) source, tellurium (Te) source, and tin (Sn) source. Examples include powders or pellets containing each of the constituent elements (Mg, V, Ni, Sb, Te, Sn) alone, or powders or pellets containing a combination of multiple constituent elements.

[0067] The raw materials (e.g., Mg source) may be melted by a known method. For example, the raw materials placed in a crucible may be subjected to heat treatment using resistance heating, high-frequency induction heating, arc discharge, plasma heating, pulse current, or electron beam heating. To prevent oxidation of the raw materials, they are preferably melted in an inert gas atmosphere or under reduced pressure. The melting temperature is not limited as long as the raw materials are melted. However, an excessively high melting temperature may cause the raw materials to volatilize, resulting in a deviation in the material composition. The melting temperature is preferably 1000°C or higher and 3000°C or lower. Mechanical or electromagnetic stirring may also be applied to the molten metal during melting. After melting, the molten metal is cooled to obtain a molten solid. A magnesium (Mg) source is then added to the resulting molten solid. The amount of Mg source added may be adjusted to obtain a thermoelectric conversion material with the desired composition.

[0068] In a second embodiment, when preparing the mixed raw material, a vanadium (V) source, a nickel (Ni) source, and an antimony (Sb) source are melted and cooled to form a melt-solidified product, and a magnesium (Mg) source is further mixed with the obtained melt-solidified product, and the mixture of the melt-solidified product and the magnesium (Mg) source is used as the mixed raw material. If necessary, a tellurium (Te) source and / or a tin (Sn) source may be added to the raw materials before the melting step and / or the melt-solidified product after the melting step.

[0069] In the second embodiment, a magnesium (Mg) source is not added to the raw materials before the melting step, but is added to the molten solid obtained after the melting step. Since the volatilization of Mg can be effectively suppressed, a material with the desired composition can be easily obtained. Furthermore, it becomes possible to achieve uniform composition and structure of the final thermoelectric conversion material. The types of raw materials and melting conditions are the same as those in the first embodiment.

[0070] In a third embodiment, a magnesium (Mg) source, a vanadium (V) source, a nickel (Ni) source, and an antimony (Sb) source are melted and cooled to prepare a molten solid, and the resulting molten solid is used as the mixed raw material. If necessary, a tellurium (Te) source and / or a tin (Sn) source may be added to the raw materials before the melting step and / or to the molten solid after the melting step.

[0071] In the third embodiment, all of the magnesium (Mg) source is added to the raw materials before the melting step, but not to the molten solid obtained after the melting step. Because Mg volatilizes during the melting step, it is desirable to incorporate a larger amount of Mg source in anticipation of the volatilization. The required amount varies depending on the melting conditions, making it difficult to determine a specific amount. However, it is conceivable to incorporate an amount of 0.5% to 10% of the Mg content in the final composition, for example. The types of raw materials and melting conditions are the same as those in the first embodiment.

[0072] <Mechanical alloying process> In the mechanical alloying process, the resulting mixed raw materials are subjected to mechanical alloying to produce a mechanically alloyed product. Mechanical alloying (MA) is a technique in which metal powders are pulverized and mixed using a high-energy mill, resulting in mechanical alloying. Ductile metal powders undergo repeated plastic deformation, fracture, and agglomeration due to the compression of newly formed surfaces during processing. This results in the formation of a lamellar structure consisting of thin layers of components, which becomes ultrafine as the processing progresses. Furthermore, the application of mechanical energy increases the temperature of the metal powder. The combined effects of the refinement of the lamellar structure and the temperature increase result in alloying at the atomic level, thereby enabling the production of a uniform alloy (intermetallic compound) phase. In particular, the thermoelectric conversion material (half-Heusler compound) of this embodiment contains constituents with different melting points and specific gravities, making it difficult to obtain a single phase through melting alone. However, a uniform single-phase intermetallic compound phase can be obtained by performing a mechanical alloying process after melting.

[0073] The mill used for the mechanical alloying treatment is not limited as long as it is a high-energy mill. Examples include an attritor, a planetary ball mill, and a vibrating ball mill. The treatment may be carried out under conditions that allow sufficient alloying. For example, when using a planetary ball mill, the rotational and revolution speeds may be set to 300 rpm or more and 1200 rpm or less, and the treatment may be carried out for 10 minutes or more and 50 hours or less.

[0074] <Sintering process> In the sintering process, the mechanically alloyed material is pressure-sintered. Through this sintering process, a thermoelectric conversion material consisting of a dense sintered body can be obtained. Furthermore, even if a heterogeneous phase remains after the mechanical processing process, alloying progresses during sintering, making it possible to obtain an intermetallic compound phase with a small amount of heterogeneous phase.

[0075] Pressure sintering may be performed by a known method, such as a hot press (HP), hot isostatic press (HIP), or pressure sintering apparatus such as a pulsed electric current sintering (PECS) apparatus. While not limited to this, for example, sintering may be performed under conditions of applying a pressure of 20 MPa to 100 MPa at a temperature of 800 K to 1300 K for 2 minutes to 120 minutes.

[0076] <Post-processing process> If necessary, the obtained sintered body may be subjected to post-treatment. For example, the sintered body may be subjected to processing such as grinding or polishing to adjust the dimensions. Furthermore, a protective layer may be provided on the surface of the sintered body to prevent oxidation.

[0077] In this manner, the thermoelectric conversion material of this embodiment can be produced. [Example]

[0078] The present invention will be described in more detail using the following examples, but the present invention is not limited to the following examples.

[0079] [Experimental Example A] In Experimental Example A, a thermoelectric conversion material with a Mg2VNi3Sb3 composition (p=q=0) was prepared by a different method and evaluated. Specifically, the sample was prepared according to the following procedure.

[0080] (1) Preparation of thermoelectric conversion materials [Example 1] (p=q=0, first embodiment) <Preparation process> The raw materials, magnesium (Mg): 0.393 g, vanadium (V): 0.398 g, nickel (Ni): 1.374 g, and antimony (Sb): 2.851 g, were mixed and melted in an arc furnace. To improve the sample uniformity, the melted and solidified sample was turned over and melted again, a process repeated three times. The removed molten solid was then placed in a glove box, and 0.085 g of magnesium (Mg) was added to form a mixed raw material.

[0081] <Mechanical alloying process> The resulting mixed raw material was then placed in a tungsten carbide ball mill container, which was then set in a planetary ball mill and mechanically alloyed by pulverizing for 10 hours at a rotation and revolution speed of 600 rpm.

[0082] <Sintering process> The powder sample was then removed from the ball mill and pressure-sintered using a pulsed electric current sintering (PECS) device. Pressure sintering was performed in a vacuum atmosphere at a temperature of 873 K, applying a pressure of 30 MPa to the sample for 20 minutes. In this way, a polycrystalline sample of thermoelectric conversion material consisting of a sintered body was produced.

[0083] [Example 2] (p=q=0, second embodiment) In the preparation process, the raw materials, vanadium (V): 0.398 g, nickel (Ni): 1.374 g, and antimony (Sb): 2.851 g, were mixed and melted in an arc furnace. To improve the uniformity of the sample, the melted and solidified sample was turned over and melted again, a procedure repeated three times. The removed molten solid was then placed in a glove box, and 0.384 g of magnesium (Mg) was added to form a mixed raw material. A polycrystalline sample of thermoelectric conversion material was prepared in the same manner as in Example 1.

[0084] [Example 3] (p=q=0, third embodiment) In the preparation step, the raw materials, magnesium (Mg): 0.379 g, vanadium (V): 0.397 g, nickel (Ni): 1.374 g, and antimony (Sb): 2.850 g, were mixed and melted in an arc furnace. To improve the uniformity of the sample, the melted and solidified sample was turned over and melted again, a procedure repeated three times. This resulted in a mixed raw material consisting of a molten solid. A polycrystalline sample of thermoelectric conversion material was prepared in the same manner as in Example 1.

[0085] (2) Evaluation of thermoelectric conversion materials The samples (thermoelectric conversion materials) obtained in Examples 1 to 3 were evaluated for various properties as follows.

[0086] <density> The density was measured by the Archimedes method.

[0087] <xrd> The samples were analyzed by powder X-ray diffraction to evaluate the produced phase. The analysis was performed by polishing the surface of the sintered sample and irradiating the polished surface with X-rays. The X-ray diffraction measurement was performed under the following conditions.

[0088] - X-ray diffraction equipment: Rigaku Corporation, MiniFlex600 - Source: CuKα -Tube voltage: 40kV -Tube current: 15mA - Scan speed: 2° / min - Scan range: 20°~80°

[0089] <sem-eds> The microstructure of the sample was examined using a scanning electron microscope (SEM; Thermo Fisher Scientific, Helios 5 Hydra Dual Beam). Elemental mapping images were also obtained using an attached energy dispersive X-ray analyzer (EDS; Oxford Instruments, Ultim Max 170) for elemental analysis. Specifically, observation and analysis were performed under the following conditions: magnification: 500–20,000x, voltage: 10–30 kV, and current: 1.6–3.2 nA.

[0090] <Thermoelectric properties> Thermoelectric properties (Seebeck coefficient S, electrical resistivity ρ, thermal conductivity κ) in a predetermined temperature range (from approximately 300 K (room temperature) to approximately 900 K) total ) was measured. The Seebeck coefficient S was determined using a thermoelectric property evaluation device (Advance Riko Co., Ltd., ZEM-3 series) according to the following procedure. That is, a temperature gradient ΔT was applied to the thermoelectric conversion material, and the thermoelectromotive force VE generated by this was measured. The Seebeck coefficient S was then calculated according to the following formula (4). In the following formula (4), S wire is the absolute Seebeck coefficient of the metal probe used in the thermoelectric power measurement.

[0091]

number

[0092] The electrical resistivity ρ was measured by the four-terminal method using a thermoelectric property evaluation device (Advance Riko Co., Ltd., ZEM-3 series). Then, the electrical conductivity σ was calculated using the electrical resistivity ρ according to the following equation (5).

[0093]

number

[0094] The thermal diffusivity D was measured by the flash method using a flash analyzer (NETZSCH, LFA467 HyperFlash). The thermal conductivity κ was calculated according to the following equation (6) using the obtained thermal diffusivity D, the specific heat at constant pressure Cp, and the density d measured by the Archimedes method. total asked for.

[0095]

number

[0096] In addition, the electrical conductivity σ is used to calculate the electronic thermal conductivity κ according to the following equation (7): el and further calculate the thermal conductivity κ total and electronic thermal conductivity κ el Using the following equation (8), the lattice thermal conductivity κ lat In the following equation (7), L is the Lorentz number and T is the absolute temperature.

[0097]

number

[0098] Then the Seebeck coefficient S, electrical conductivity σ, and thermal conductivity κ total The power factor PF and thermoelectric figure of merit zT were calculated according to the following equation (9).

[0099]

number

[0100] <Heat resistance> The thermoelectric properties were repeatedly measured in a fixed temperature range (from approximately 300 K (room temperature) to approximately 900 K) to evaluate the heat resistance of the sample. Thermal analyses were also performed during the heating and cooling processes using a thermogravimetric / differential thermal analyzer (TG / DTA; Shimadzu Corporation, DTG-60).

[0101] (3) Evaluation results <density> The density of the sample obtained in Example 1 was 6.605 g / cm 3 The theoretical density (6.61 g / cm) calculated from the lattice constant 3 ) and the relative density was calculated using the same method, and the value was 95% or more. 3 ) and Example 3 (density: 6.67 g / cm 3 The sample obtained in this experiment had a density of 6.5 g / cm 3 In this way, the relative density was 95% or more.

[0102] <xrd> The X-ray diffraction (XRD) patterns of the samples (thermoelectric conversion materials) obtained in Examples 1 to 3 are shown in Figure 5. It was confirmed that the samples of Examples 1 and 2 consisted of a single phase of the half-Heusler compound Mg2VNi3Sb3. That is, all diffraction peaks were identified as originating from the half-Heusler phase (HH phase), and peaks originating from second phases (heterogeneous phases) were below the detection limit. The peak intensity (I HH ) to the peak intensity of the main peak from the heterogeneous phase (I HP ) ratio (I HP / I HH ) was 0.00. On the other hand, in Example 3, although a peak derived from the half-Heusler phase was strongly observed, a peak derived from a second phase (heterogeneous phase) was also observed. Specifically, I HP / I HH was 0.28.

[0103] <sem-eds> An elemental mapping image of the sample obtained in Example 2 is shown in Figure 6. Within the observation range, all elements (Mg, V, Ni, Sb) were uniformly dispersed, and no precipitates or second phases (heterogeneous phases) were present.

[0104] <Thermoelectric properties> Seebeck coefficient S, power factor PF, and thermal conductivity κ of the samples in Examples 1 to 3 total , lattice thermal conductivity κ lat , and the thermoelectric figure of merit zT are shown in FIGS. 6 to 10, respectively.

[0105] All samples had a positive Seebeck coefficient S (Fig. 6), which indicated that the samples exhibited P-type semiconductor properties. Although not shown, the electrical conductivity σ of all samples increased with increasing temperature, which indicated that the samples exhibited semiconductor properties. Furthermore, all samples had a thermal conductivity κ within the measurement temperature range. total is 6W / (Km) or less, lattice thermal conductivity κ lat The thermoelectric figures of merit PF and zT were high in the samples of Examples 1 and 2 (Figs. 7 and 10). The thermoelectric figures of merit PF and zT of Example 1 were particularly high, reaching a maximum value at approximately 700 K, with the respective values ​​at that time being 0.9 mW / (mK 2 ) and exceeded 0.15. Although not shown, the sample of Example 3 had low thermoelectric figures of merit zT and PF. This was thought to be due to the presence of a different phase or deviation in the matrix composition.

[0106] <Heat resistance> The results of repeated power factor PF tests on the sample of Example 1 in the temperature range of about 300 K to about 900 K are shown in Figure 11. The results of thermal analysis on the sample of Example 2 in the temperature range of about 300 K to about 1000 K are shown in Figure 12.

[0107] In the results of repeated power factor (PF) tests, almost no difference was observed between the first and second measurement results (Fig. 11). Furthermore, in the results of thermal analysis, no peaks associated with reactions or phase changes were observed, and the heat flow value after cooling was almost the same as the value before the temperature increase began (Fig. 12). This demonstrates that the thermoelectric conversion material of this embodiment does not change in quality even when heated to approximately 1000 K, and has excellent heat resistance.

[0108] [Experimental Example B] In Experimental Example B, thermoelectric conversion materials with modified compositions were prepared and evaluated. Specifically, samples were prepared according to the following procedure.

[0109] (1) Preparation of thermoelectric conversion materials [Examples 4, 5, and 7] (p=0.05 to 0.3, q=0, second embodiment) In the preparation step, the blending amounts of raw materials (Mg, V, Ni, and Sb) were changed as shown in the following Table 1. Otherwise, polycrystalline samples of thermoelectric conversion materials were prepared in the same manner as in Example 2.

[0110] [Example 6] (p=0.2, q=0, second embodiment) In the preparation step, the blending amounts of raw materials (Mg, V, Ni, and Sb) were changed as shown in Table 1 below. The sintering conditions were 873 K and 1 hour. Otherwise, polycrystalline samples of thermoelectric conversion materials were prepared in the same manner as in Example 2.

[0111] [Examples 8 to 10] (p=0, q=0.01 to 0.3, M=Te, Second Embodiment) In the preparation step, tellurium (Te) was added as a dopant to the molten solid together with magnesium (Mg). The blending amounts of the raw materials (Mg, V, Ni, Sb, and Te) were changed as shown in Table 1 below. Otherwise, polycrystalline samples of thermoelectric conversion materials were prepared in the same manner as in Example 2.

[0112] [Example 11] (p=0.1, q=0.3, M=Te, Second Embodiment) In the preparation step, tellurium (Te) was added as a dopant to the molten solid together with magnesium (Mg). The blending amounts of the raw materials (Mg, V, Ni, Sb, and Te) were changed as shown in Table 1 below. Otherwise, polycrystalline samples of thermoelectric conversion materials were prepared in the same manner as in Example 2.

[0113] [Example 12] (p=0, q=0.3, M=Sn, Second Embodiment) In the preparation step, tin (Sn) was added as a dopant when vanadium (V), nickel (Ni), and antimony (Sb) were mixed. The amounts of the raw materials (Mg, V, Ni, Sb, and Sn) were changed as shown in Table 1 below. A polycrystalline sample of the thermoelectric conversion material was otherwise prepared in the same manner as in Example 2.

[0114] [Example 13] (p=0.1, q=0.3, M=Te, Second Embodiment) In the preparation step, tellurium (Te) was added to the molten solid as a dopant together with magnesium (Mg). The amounts of the raw materials (Mg, V, Ni, Sb, and Te) were changed as shown in Table 1 below. The sintering conditions were 1073 K and 1 hour. A polycrystalline sample of the thermoelectric conversion material was prepared in the same manner as in Example 2.

[0115] [Example 14] (p=0, q=0.3, M=Sn, Second Embodiment) In the preparation process, tin (Sn) was added as a dopant when vanadium (V), nickel (Ni), and antimony (Sb) were mixed. The amounts of the raw materials (Mg, V, Ni, Sb, and Sn) were changed as shown in Table 1 below. The sintering conditions were 1073 K and 1 hour. A polycrystalline sample of the thermoelectric conversion material was prepared in the same manner as in Example 2.

[0116] (2) Evaluation of thermoelectric conversion materials For the samples (thermoelectric conversion materials) obtained in Examples 4 to 14, X-ray diffraction analysis (XRD) and evaluation of the thermoelectric properties were carried out in the same manner as in Experimental Example A.

[0117] (3) Evaluation results <density> The densities of the samples obtained in Examples 4 to 9 were 6.78 g / cm 3 (Example 4), 6.735 g / cm 3 (Example 5), 6.961 g / cm 3 (Example 6), 6.512 g / cm 3 (Example 7), 6.825 g / cm 3 (Example 8), and 6.812 g / cm 3 (Example 9).

[0118] <xrd> Although not shown, in the X-ray diffraction (XRD) patterns of the samples obtained in Examples 4 to 14, the peak derived from the half-Heusler (HH) phase was observed to be the most intense in all samples. Specifically, the peak intensity (I HH ) to the peak intensity of the main peak from the heterogeneous phase (I HP ) ratio (I HP / I HH ) was 0.04 to 0.24.

[0119] <Thermoelectric properties> Seebeck coefficient S and lattice thermal conductivity κ of the samples obtained in Examples 4 to 14 lat , and the thermoelectric figure of merit zT are shown in FIGS.

[0120] When the vanadium excess amount p or the dopant amount q was changed, the Seebeck coefficient S changed from positive to negative (FIGS. 13 and 14). In particular, the samples with a V-excess composition (q>0.1) (Examples 5 and 6) or the Te-added composition (q>0.1) (Examples 11 and 13) showed negative Seebeck coefficients, indicating N-type semiconducting properties. This indicates that the semiconducting properties of the thermoelectric conversion material can be controlled from P-type to N-type by adjusting the vanadium excess amount p or the dopant amount q.

[0121] All of the samples measured (Examples 4, 8, 9, 13, and 14) had a thermal conductivity of κ total and lattice thermal conductivity κ lat Specifically, the thermal conductivity k in the region below 700 K was low (Fig. 15 and Fig. 16). total is 6W / (Km) or less, lattice thermal conductivity κ lat was less than 4W / (Km).

[0122] [Table 1]

[0123] (4) Summary From the above results, it was found that the thermoelectric conversion material of this embodiment, which contains a half-Heusler compound of a specific composition as its main component, has low thermal conductivity, exhibits excellent thermoelectric effect in the temperature range from room temperature to 500°C, and can adjust its semiconductor properties to either P-type or N-type.

[0124] In particular, since the thermoelectric conversion material of this embodiment contains a triple half-Heusler (THH) compound as a main component, the thermal conductivity, especially the lattice thermal conductivity, is significantly small. This will be explained using FIG. 17. FIG. 17 shows the lattice thermal conductivity κ of the triple half-Heusler (THH) compound of this embodiment (Example 1) at 300 K or 700 K. l at is the lattice thermal conductivity κ of the conventional half-Heusler (HH) compound and the double half-Heusler (DHH) compound. lat This is shown in contrast to

[0125] The triple half-Heusler compound of this embodiment has a lattice thermal conductivity κ lat is significantly smaller than those of conventional half-Heusler compounds and double half-Heusler compounds. For example, the lattice thermal conductivity at 300 K is at least about 7 W / (mK) for conventional half-Heusler compounds, while it is about 2 W / (mK) for the triple half-Heusler compound of this embodiment.

[0126] Thus, the thermoelectric conversion material of this embodiment, which is mainly composed of a triple half-Heusler (THH) compound with significantly low thermal conductivity, can be said to have great potential as a material that exhibits excellent thermoelectric properties. [Explanation of symbols]

[0127] 2 P-type thermoelectric conversion element 4 N-type thermoelectric conversion element 6 electrodes 8 ceramic plates 10 Thermoelectric conversion module< / xrd> < / xrd> < / xrd>

Claims

1. It contains at least magnesium (Mg), vanadium (V), nickel (Ni), and antimony (Sb), and has the general formula: Mg ２―ｐ V １＋ｐ Ni ３ Sb ３－ｑ M ｑ (wherein p and q satisfy −0.5≦p≦0.5 and 0≦q≦1.0, and M is one or both of tin (Sn) and tellurium (Te), A thermoelectric conversion material whose main component is a half-Heusler compound.

2. The thermoelectric conversion material according to claim 1 , wherein the thermoelectric conversion material is composed of a single phase of a half-Heusler compound.

3. The thermoelectric conversion material according to claim 1 or 2, wherein the thermoelectric conversion material is a polycrystalline material.

4. The thermoelectric conversion material according to claim 1 or 2, which exhibits P-type semiconductor properties.

5. The thermoelectric conversion material according to claim 1 or 2, which exhibits N-type semiconductor properties.

6. The thermal conductivity (κ ｔｏｔａｌ 3. The thermoelectric conversion material according to claim 1, wherein the thermal conductivity is 6.0 W / (mK) or less in a temperature range of 300 K or more and 800 K or less.

7. The thermoelectric conversion material has a power factor (PF) of 0.2 mW / (mK ２ 3. The thermoelectric conversion material according to claim 1, wherein the temperature is 0.15 to 1.5° C. or more.

8. The thermoelectric conversion material has a power factor (PF) of 0.5 mW / (mK ２ 3. The thermoelectric conversion material according to claim 1, wherein the temperature is 0.15 to 1.5° C. or more.

9. A method for producing the thermoelectric conversion material according to claim 1 or 2, comprising the following steps: A step of preparing a mixed raw material containing at least magnesium (Mg), vanadium (V), nickel (Ni), and antimony (Sb): A step of subjecting the mixed raw material to a mechanical alloying treatment to produce a mechanically alloyed product; and a step of pressure sintering the mechanically alloyed product A method comprising:

10. 10. The method according to claim 9, wherein, in preparing the mixed raw material, a magnesium (Mg) source, a vanadium (V) source, a nickel (Ni) source, and an antimony (Sb) source are melted and solidified to prepare a molten solidify, a magnesium (Mg) source is further mixed with the molten solidify, and a mixture of the molten solidify and the magnesium (Mg) source is used as the mixed raw material.

11. 10. The method according to claim 9, wherein, in preparing the mixed raw material, a vanadium (V) source, a nickel (Ni) source, and an antimony (Sb) source are melted and cooled to prepare a molten solidification product, a magnesium (Mg) source is further mixed with the molten solidification product, and a mixture of the molten solidification product and the magnesium (Mg) source is used as the mixed raw material.

12. 10. The method according to claim 9, wherein, when preparing the mixed raw material, a magnesium (Mg) source, a vanadium (V) source, a nickel (Ni) source, and an antimony (Sb) source are melted and cooled to form a molten solidified product, and the molten solidified product is used as the mixed raw material.

13. A thermoelectric conversion element comprising the thermoelectric conversion material according to claim 1 or 2.

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

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