JP2023163934A - Thermoelectric conversion material and method for producing the same, thermoelectric conversion element, and thermoelectric conversion module - Google Patents

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

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

In response to the trend toward securing a stable energy supply and preventing global warming, there is a strong desire to realize an energy-saving society. In this regard, currently about 60% of the primary energy supply is discarded as thermal energy, and waste heat recovery technology and its widespread use are desired. Under such circumstances, thermoelectric conversion modules such as thermoelectric generation modules and thermoelectric cooling modules are attracting attention.

Thermoelectric conversion modules are solid-state devices that allow direct conversion of thermal energy and electrical energy. Since there are no mechanical moving parts, it is highly reliable, maintenance-free, and can operate quietly. Also, no waste is produced during energy conversion. Therefore, thermoelectric conversion modules are evaluated as an environmentally symbiotic energy technology.

Thermoelectric generation modules and thermoelectric cooling modules are known as thermoelectric conversion modules. Thermoelectric power generation modules generate electricity from heat by utilizing the thermoelectromotive force generated by creating a temperature difference in materials, that is, the Seebeck effect of solids. On the contrary, a thermoelectric cooling module (Peltier cooling module) utilizes the phenomenon in which a temperature difference is created by providing a potential difference in materials, that is, the Peltier effect. Both modules utilize the property of thermoelectric conversion materials (thermoelectric materials), that is, the property of directly converting thermal energy and electrical energy.

An example of a schematic cross-sectional view of a general thermoelectric conversion module is shown in FIG. 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 are formed from molded bodies of thermoelectric conversion material. Further, electrodes (6) are provided above and below the P-type thermoelectric conversion element (2) and the N-type thermoelectric conversion element (4) to connect these elements in series. Furthermore, a set of ceramic plates (8) is provided that sandwich all of the P-type thermoelectric conversion element (2), the N-type thermoelectric conversion element (4), and the electrode (6) from above and below. By providing a temperature difference in the vertical direction of the module, it is possible to extract electricity commensurate with this temperature difference, thereby making it possible to function as a thermoelectric power generation module. Conversely, by passing current through the module, a temperature difference can be created between the top and bottom of the module, thereby allowing it to work as a thermoelectric cooling module.

The maximum energy conversion efficiency η _max obtained in the thermoelectric conversion module is determined according to the following equation (1) using the thermoelectric figure of merit (dimensionalless figure of merit) ZT of the thermoelectric conversion element. Note that the thermoelectric figure of merit ZT is the product of the figure of merit Z and the temperature T. In the following equation (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).

The first term on the right side of equation (1) above is the maximum energy conversion efficiency (Carnot efficiency) in 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, it is effective to increase the temperature difference (T _H −T _C ) across the module and to improve the thermoelectric figure of merit ZT of the thermoelectric conversion element.

The thermoelectric properties of a thermoelectric conversion material are evaluated by the thermoelectric figure of merit zT. Strictly speaking, this thermoelectric figure of merit zT is distinguished from the thermoelectric figure of merit ZT of the module, but it serves as an index for improving the thermoelectric performance ZT of the element. Therefore, in order to improve the performance of a thermoelectric conversion module (thermoelectric generation module, Peltier cooling module), it is required to improve the thermoelectric figure of merit zT of the material in the operating range in which the module operates.

The thermoelectric figure of merit zT of the thermoelectric conversion material is determined according to the following equation (2). In the following equation (2), S is the Seebeck coefficient of the material, σ is the electrical conductivity, T is the temperature, κ _total is the thermal conductivity, and PF is the power factor. The thermal conductivity κ _total is the sum of the lattice thermal conductivity κ _lat and the electronic thermal conductivity κ _el (κ _lat +κ _el ). Here, the lattice thermal conductivity κ _lat corresponds to a portion of the thermal conductivity κ _total in which quantized lattice vibrations, that is, phonons serve as heat carriers. Further, the electronic thermal conductivity κ _el corresponds to a portion where electrons or holes become heat carriers.

As can be seen from equation (2) above, in order to increase the figure of merit zT, it is necessary to increase the Seebeck coefficient S and the electrical conductivity σ and at the same time reduce 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 carrier concentration, it is difficult to control them independently. Specifically, the higher the carrier concentration, the higher the conductivity, but the smaller the Seebeck coefficient. Furthermore, high electrical conductivity and low electronic thermal conductivity are in an antinomic relationship. Therefore, in order to increase zT, it is effective to adjust the carrier concentration so that the power factor PF is maximized and to make the lattice thermal conductivity (κ _lat ) as small as possible. A thermoelectric conversion material whose carrier concentration is adjusted within an appropriate range is usually an N-type or P-type semiconductor with a finite bandgap.

Conventionally, compounds such as Bi ₂ Te ₃ , PbTb, CoSb ₃ , and La ₃ Te ₄ have been known as N-type thermoelectric conversion materials. Furthermore, compounds such as SnSe, BiSbTe, MgAgSb, NaPb _m SbTe _m+2 , (GeTe) _0.8 (AgSbTe ₂ ) _0.2 , and PbTe-4SrTe-2Na are known as P-type thermoelectric conversion materials. These compounds have a relatively high figure of merit zT. However, it contains harmful or rare elements such as cobalt (Co), lead (Pb), silver (Ag), and germanium (Ge). Therefore, in order to commercialize and industrialize thermoelectric power generation such as exhaust heat regeneration from automobiles and factories, there is a need for high-performance thermoelectric conversion materials that do not contain harmful or rare elements and exhibit a high performance index in the operating temperature range. There is. As such materials, Heusler-type compounds, particularly half-Heusler-type compounds, are considered to be promising.

Half-Heusler type compounds have a wide variety of combinations of component elements. Therefore, it is possible to design a composition that does not contain harmful or rare elements, and it is possible to obtain a non-toxic and inexpensive material. It also has the advantage of high mechanical strength and can be used stably even at high temperatures. Therefore, thermoelectric conversion materials mainly composed of half-Heusler type compounds are attracting attention as promising materials that can be applied mainly to exhaust heat regeneration devices for automobiles.

Patent Document 1 discloses a method for producing a thermoelectric material in which a solid phase of a half-Heusler alloy consisting of intermetallic compounds [Ti _a Zr _b Hf _c ] [ _Nid Co _e ] [Sn _f Sb _g ] is generated. . Non-Patent Document 1 discloses investigating the characteristics of ScNiSb _1-x Te _x , which is a half-Heusler phase (Abstract of Non-Patent Document 1, etc.). Non-Patent Document 2 discloses investigating the thermoelectric properties of a compound having a composition such as MgNiSb (full text of Non-Patent Document 2). Non-Patent Document 3 discloses doping a Ti ₂ FeNiSb ₂ double-half Heusler compound with tin (Abstract of Non-Patent Document 3).

Japanese Patent Application Publication No. 2009-084689

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) A.V.Morozkin et al., Thermoelectric properties of ScCoSb, ScNi0.86Sb and MgNiSb compounds, Journal of Alloys and Compounds 400 (2005) 62-66 Rahidul Hasan et al., Enhanced Thermoelectric Properties of Ti2FeNiSb2 Double Half-Heusler Compound by Sn Doping, Adv. Energy Sustainability Res. 2022, 2100206

Although it has been proposed in the past to use half-Heusler compounds, which have various advantages, as thermoelectric conversion materials, conventional half-Heusler compounds have a problem in that they have high thermal conductivity. That is, as can be seen from equation (2) above, the thermoelectric figure of merit zT is inversely proportional to the thermal conductivity κ _total (=κ _lat + κ _el ), so the smaller the thermal conductivity, the higher the thermoelectric figure of merit zT. However, conventional half-Heusler type compounds have a high thermal conductivity, particularly a high lattice thermal conductivity κ _lat , and there is a limit to increasing the thermoelectric figure of merit zT.

The present inventors conducted extensive studies in view of such problems. As a result, they found that by incorporating magnesium (Mg) and vanadium (V), which have different valences, in different ratios into the same atomic site, it is possible to synthesize a new thermoelectric conversion material that has a half-Heusler type compound as its main component. Ta. It has also been found that this thermoelectric conversion material has low thermal conductivity and exhibits excellent thermoelectric effects in the temperature range from room temperature to 500°C. Furthermore, they found that by finely adjusting the composition, it is possible to control the properties of the thermoelectric conversion material to either P-type or N-type.

The present invention was completed based on such findings, and has low thermal conductivity, exhibits excellent thermoelectric effects in the temperature range from room temperature to 500°C, and has semiconductor characteristics that can be changed to either P-type or N-type. An object of the present invention is to provide a new thermoelectric conversion material whose main component is a half-Heusler type compound that can be adjusted, and a method for producing the same. Another object of the present invention is to provide a thermoelectric conversion element and a thermoelectric conversion module containing this thermoelectric conversion material.

The present invention includes the following aspects (1) to (14). Note that in this specification, the expression "~" includes numerical values at both ends thereof. That is, "X to Y" is synonymous with "more than or equal to X and less than or equal to Y."

(1) 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 (however, p and q satisfies -0.5≦p≦0.5 and 0≦q≦1.0, M has a composition represented by one or both of tin (Sn) and tellurium (Te),
A thermoelectric conversion material whose main component is a half-Heusler type compound.

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

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

(4) The thermoelectric conversion material according to (1) or (2) above, wherein the thermoelectric conversion material exhibits P-type semiconductor characteristics.

(5) The thermoelectric conversion material according to (1) or (2) above, wherein the thermoelectric conversion material exhibits N-type semiconductor characteristics.

(6) The thermoelectric conversion material according to (1) or (2) above, wherein the thermoelectric conversion material has a thermal conductivity (κ _total ) of 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 according to (1) or (2) above, wherein the power factor (PF) of the thermoelectric conversion material is 0.2 mW/(mK ² ) or more in a temperature range of 300 K or more and 800 K or less.

(8) The thermoelectric conversion material according to (1) or (2) above, wherein the power factor (PF) of the thermoelectric conversion material is 0.5 mW/(mK ² ) or more in a temperature range of 600 K or more and 800 K or less.

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

(10) 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 molten solidified product, and the The method according to (9) above, wherein a magnesium (Mg) source is further mixed into the molten solidified material, and the mixture of the molten solidified material and the magnesium (Mg) source is used as the mixed raw material.

(11) When preparing the mixed raw material, a vanadium (V) source, a nickel (Ni) source, and an antimony (Sb) source are melted and cooled to produce a molten solidified product, and magnesium is added to the molten solidified product. The method according to (9) above, wherein a (Mg) source is mixed and a mixture of the molten solidified product and the magnesium (Mg) source is used as the mixed raw material.

(12) 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 produce a molten solidified product, and the The method according to (9) above, wherein a molten solidified product is used as the mixed raw material.

(13) A thermoelectric conversion element comprising the thermoelectric conversion material of (1) or (2) above.

(14) A thermoelectric conversion module comprising the thermoelectric conversion element of (13) above.

According to the present invention, the main component is a half-Heusler type compound that has low thermal conductivity, exhibits an excellent thermoelectric effect in the temperature range from room temperature to 500°C, and whose semiconductor characteristics can be adjusted to either P-type or N-type. A novel thermoelectric conversion material and a method for manufacturing the same are provided. Further, according to the present invention, a thermoelectric conversion element and a thermoelectric conversion module including this thermoelectric conversion material are provided.

FIG. 2 is a schematic cross-sectional view showing the structure of a thermoelectric conversion module. FIG. 2 is a schematic diagram showing the crystal structure of triple half-Heusler (THH) type compound Mg ₂ VNi ₃ Sb ₃ . FIG. 2 is a diagram illustrating a triple half-Heusler (THH) type compound. The X-ray diffraction patterns are shown (Examples 1 to 3). An elemental mapping image is shown (Example 2). 2 shows temperature changes in Seebeck coefficient S (Examples 1 to 3). The temperature change of power factor PF is shown (Examples 1 to 3). 2 shows the temperature change in thermal conductivity κ _total (Examples 1 and 2). Figure 2 shows temperature changes in lattice thermal conductivity κ _lat (Examples 1 and 2). 2 shows temperature changes in thermoelectric figure of merit zT (Examples 1 and 2). The results of repeated tests on power factor PF are shown (Example 1). TG/DTA analysis results are shown (Example 2). 2 shows temperature changes in Seebeck coefficient S (Examples 4 to 7). 2 shows temperature changes in Seebeck coefficient S (Examples 8 to 14). Figure 2 shows the temperature change in thermal conductivity κ _total (Examples 4, 5, 8, 9, 13, and 14). Figure 3 shows temperature changes in lattice thermal conductivity κ _lat (Examples 4, 5, 8, 9, 13, and 14). The magnitude of the lattice thermal conductivity κ _lat of various half-Heusler type compounds is shown in comparison.

A specific embodiment of the present invention (hereinafter referred to as "this embodiment") will be described. Note that the present invention is not limited to the following embodiments, and various changes can be made without departing from the gist of the present invention.

<<1. Thermoelectric conversion materials >>
The thermoelectric conversion material of this embodiment includes 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 (However, p and q satisfy -0.5≦p≦0.5 and 0≦q≦1.0, and M is represented by one or both of tin (Sn) and tellurium (Te)) The main component is a half-Heusler type compound.

The thermoelectric conversion material of this embodiment has a half-Heusler type compound as a main component. Here, the main component means that the content of the half-Heusler type compound in the thermoelectric conversion material is 50% by mass or more. From the viewpoint of taking advantage of the excellent properties of the half-Heusler type compound, the content of the main component is preferably as high as possible. 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. If defined based on the X-ray diffraction pattern, the ratio ( _{I HP /} I HH ) of the peak intensity of the main peak derived from the half-Heusler type compound (I _HH ) to the peak intensity of the main peak derived from the different phase (I _HP ) is 0. .4 or less is preferable, 0.3 or less is more preferable, 0.2 or less is even more preferable, and 0.1 or less is particularly preferable. Most preferably, the thermoelectric conversion material is composed of a single phase of a half-Heusler type compound. Here, being composed of a single phase means that the peak intensity ratio (I _HP /I _HH ) in the X-ray diffraction pattern is zero (0).

A half-Heusler type compound is a compound represented by the chemical formula: XYZ and has a cubic crystal structure. The crystal structure of the half-Heusler type compound is shown in FIG. In the crystal of a half-Heusler type compound, X and Z atoms constitute a rock salt type structure, and Y atoms occupy four of the eight sublattices in the structure. Compositional formula of the main component compound of this embodiment: Mg _2-p V _1+p Ni ₃ Sb _3-q If M _q is rewritten according to the chemical formula (XYZ), (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 atom site. Further, nickel (Ni) occupies the Y atomic site, and antimony (Sb) and M elements (Sn and/or Te) occupy the Z atomic site.

A half-Heusler type compound has a band gap when the total number of valence electrons is 18, and exhibits semiconductor properties. As a result, it is known to have a large power factor PF. This is called the 18 electronic rule. In the thermoelectric conversion material of this embodiment, when p=q=0, the number ratio of divalent magnesium (Mg) atoms and pentavalent vanadium (V) atoms is 2:1, and the X atoms at that time The average valence of (Mg, V) is trivalent. Therefore, the sum of the average valence of the X atoms (Mg, V) (trivalent), the valence of the Y atom (Ni) (10 valence), and the valence of the Z atom (Sb) (5 valence) is 18, which is 18 Follow electronic rules. Therefore, it is expected that the thermoelectric properties will be high near this composition.

As described above, in the half-Heusler type compound of this embodiment, magnesium (Mg) occupying the X atom site is a divalent element, whereas vanadium (V) is a pentavalent element. Further, the ratio of Mg and V is approximately 2:1, which is not equivalent. The half-Heusler type compound Mg ₂ VNi ₃ Sb ₃ can be regarded as an intermetallic compound in which MgNiSb and VNiSb are dissolved in solid solution at a ratio of approximately 2:1, as shown in FIG. Note that in FIG. 3, a conventional ternary half-Heusler type compound ScNiSb is also shown for reference. A compound containing a plurality of elements with different valences (different valence elements) in different proportions at the same atomic site is called a triple half-Heusler (THH) type compound. Triple half-Heusler (THH) type compounds have low thermal conductivity and therefore have the potential to exhibit excellent thermoelectric properties. "Triple" means triplicate of formula units, and well-known examples include double perovskites (usually A ₂ B'B"O ₆ for ABO ₃ ).

In this regard, the conventionally proposed compounds are not triple half-Heusler (THH) type compounds. For example, the alloy [Ti _a Zr _b Hf _c ] [ _Nid Co _e ] [Sn _f Sb _g ] disclosed in Patent Document 1 contains titanium (Ti), zirconium (Zr), and hafnium (all of which are tetravalent). Hf) is included at the X atomic site, and no different element is included at the same atomic site. Further, the double-half-Heusler (DHH) compound Ti ₂ FeNiSb ₂ disclosed in Non-Patent Document 3 contains Fe and Ni, which are heterovalent elements, at the Y atom site, but the ratio thereof is the same (1:1). This double-half-Heusler (DHH) type compound Ti ₂ FeNiSb ₂ can be regarded as an intermetallic compound in which TiFeSb and TiNiSb are dissolved in a solid solution at a ratio of 1:1, and is clearly different from the triple-half-Heusler type compound.

The thermoelectric conversion material of this embodiment, which has a triple half-Heusler (THH) type compound as its main component, is characterized by a low thermal conductivity κ _total , particularly a low lattice thermal conductivity κ _lat . The detailed mechanism is unknown and should not be interpreted in a restrictive manner. However, the following mechanism can be considered.

The 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 equation (3) below. Here, phonons are quantized lattice vibrations in the crystal.

In the triple half-Heusler (THH) type compound of this embodiment, the magnesium (Mg) atom and the vanadium (V) atom occupying the same atomic site have different valences and greatly differ in mass. Further, the present inventors investigated and found that Mg atoms and V atoms are irregularly arranged at the X atom site. It is speculated that the irregular arrangement of atoms (Mg, V) with different valences and masses may have caused disturbances in the periodicity of the lattice. It is also believed that due to the disturbance in the periodicity of the lattice, phonons, especially long-period phonons, are more likely to be scattered, and as a result, the mean free path l becomes smaller and the lattice thermal conductivity κ _lat becomes smaller.

Since the thermoelectric conversion material of this embodiment has semiconducting properties, it has a lower carrier concentration than metals, and has a lower electronic thermal conductivity κ _el in which electrons and holes are responsible for heat conduction. It is believed that because both the lattice thermal conductivity κ _lat and the electron thermal conductivity κ _el become smaller, the thermal conductivity κ _total , which is the sum of these, becomes smaller.

p is limited to a range of −0.5 or more and 0.5 or less (−0.5≦p≦0.5). p represents an excess amount of vanadium (V). That is, when p=0, the composition follows the 18-electron rule. On the other hand, when p>0, the composition becomes excessive V, and when p<0, the composition becomes excessive Mg. By adjusting p within the above range, the semiconductor characteristics of the thermoelectric conversion material can be controlled to P-type or N-type. However, when p becomes less than -0.5 or more than 0.5, the deviation from the 18-electron rule becomes large. Furthermore, since different phases are likely to be generated, the thermoelectric properties, particularly the Seebeck coefficient S, are significantly reduced. p may be -0.3 or more and 0.3 or less (-0.3≦p≦0.3), and may be -0.2 or more and 0 or less (-0.2≦p≦0).

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 act as dopants. 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, when q is more than 1.0, the amount of dopant becomes excessive, and as a result, the thermoelectric properties, especially the Seebeck coefficient, decrease significantly as the carrier concentration increases, and at the same time, foreign phases are also likely to be generated. q may be 0 or more and 0.5 or less, and may be 0 or more and 0.3 or less. q may be zero (0). Further, the dopant is at least one of tin (Sn) and tellurium (Te). The dopant may be Sn only, Te only, or both Sn and Te.

The thermoelectric conversion material of this embodiment is preferably dense. Dense materials have a high Seebeck coefficient S and conductivity σ. Therefore, it is possible to further increase the power factor PF and the 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.

The thermoelectric conversion material of this embodiment may be polycrystalline or single crystalline. However, polycrystalline bodies, especially sintered bodies, are preferred. As described later, in the thermoelectric conversion material of this embodiment, a polycrystalline body having a homogeneous composition and crystal structure can be easily obtained. Furthermore, in polycrystalline materials, phonons are scattered at grain boundaries, which can be expected to further reduce lattice thermal conductivity. On the other hand, single crystals are not easy to manufacture. In particular, since the thermoelectric conversion material of this embodiment includes a plurality of components having different melting points and vapor pressures, it is difficult to produce a homogeneous single crystal. When the thermoelectric conversion material is polycrystalline, its particle size is not limited. However, typically the average particle size is 100 nm or more and 100 μm or less, 500 nm or more and 100 μm or less, or 10 μm or more and 50 μm or less.

The thermoelectric conversion material of this embodiment may exhibit P-type semiconductor characteristics. Alternatively, it may exhibit N-type semiconductor characteristics. The semiconductor properties (P type, N type) of the thermoelectric conversion material can be adjusted by controlling the excess vanadium (V) amount p and the dopant amount q. In particular, by using both the P-type thermoelectric conversion material and the N-type thermoelectric conversion material of this embodiment in combination, it is possible to produce a thermoelectric conversion module using the same material system. Thermoelectric conversion modules made of the same material can prevent the generation of stress due to differences in the thermal expansion coefficients of the materials and the resulting deterioration of characteristics and reliability. Therefore, constructing thermoelectric conversion modules using the same material system is an important element toward practical use.

The thermoelectric conversion material of this embodiment is characterized by low thermal conductivity. Although not limited, the thermal conductivity κ _total of the thermoelectric conversion material is preferably 10.0 W/(mK) or less, more preferably 8.0 W/(mK) or less in a temperature range of 300 K or higher and 800 K or lower. , more preferably 6.0 W/(mK) or less. In addition, the lattice thermoelectric coefficient κ _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) in the temperature range of 300 K or more and 800 K or less. ) is below. The lower limits of the thermal conductivity κ _total and the lattice thermal conductivity κ _lat are not limited. However, typically κ _total is 0.5 W/(mK) or more, and κ _lat is typically 0.4 W/(mK) or more.

The thermoelectric conversion material of this embodiment is characterized in that it exhibits an excellent thermoelectric effect in the temperature range from room temperature to 500°C. Although not limited, the power factor PF of the thermoelectric conversion material is preferably 0.1 mW/(mK ² ) or more, more preferably 0.2 mW/(mK ² ) or more in a temperature range of 300 K or more and 800 K or less. , more preferably 0.25 mW/(mK ² ) or more. In addition, the power factor PF is preferably 0.25 mW/(mK ² ) or more, more preferably 0.5 mW/(mK ² ) or more, and even more preferably 0.75 mW/(mK ² ) in the temperature range of 600 K or more and 800 K or less. ) or more, particularly preferably 1.0 mW/(mK ² ) or more. The upper limit of the power factor PF is not limited. However, it is typically 10.0 mW/(mK ² ) or less, or 5.0 mW/(mK ² ) or less.

<<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 thermoelectric conversion material. The molded body may be a polycrystalline body or a single crystalline body. However, polycrystalline bodies, especially sintered bodies, are preferred. Further, the thermoelectric conversion element may include members 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. Further, the thermoelectric conversion element may be of P type or N type. The P-type thermoelectric conversion element is made of a molded body of P-type thermoelectric conversion material. The N-type thermoelectric conversion element is made of a molded body of N-type thermoelectric conversion material.

<<3. Thermoelectric conversion module >>
The thermoelectric conversion module of this embodiment includes the above-mentioned thermoelectric conversion element. That is, at least one, preferably both, of the P-type thermoelectric conversion element and the N-type thermoelectric conversion element included in the thermoelectric conversion module are the above-mentioned thermoelectric conversion elements. For example, as shown in FIG. 1, a P-type thermoelectric conversion element, an N-type thermoelectric conversion element, an electrode connecting the P-type thermoelectric conversion element and the N-type conversion element in series, It may be configured from a pair of ceramic plates that sandwich all of the conversion element and electrodes from above and below.

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 configuring both thermoelectric conversion elements with the same material, the thermal expansion coefficients of both can be made the same, and as a result, it is possible to prevent the generation of stress when a temperature difference occurs and the resulting deterioration of characteristics and reliability. can.

The thermoelectric conversion module is preferably a thermoelectric generation module or a thermoelectric cooling module. By providing a temperature difference in the upper and lower directions of the module, electricity commensurate with this temperature difference can be extracted, and the module can function as a thermoelectric power generation module. Conversely, by passing current through the module, a temperature difference can be created between the top and bottom of the module, and this can be used to cause the module to function as a thermoelectric cooling module.

Since the thermoelectric conversion module of this embodiment has no mechanical moving parts, it is highly reliable and maintenance-free, and its operation is quiet. It also has the advantage of not producing waste during energy conversion. Moreover, it has a rich combination of constituent elements and does not contain harmful or rare elements. Furthermore, it has high mechanical strength and can be used stably even at high temperatures. Therefore, this thermoelectric conversion module can 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 types of cooling devices. There are high expectations for this device.

<<4. Manufacturing method of thermoelectric conversion material >>
The method of manufacturing the thermoelectric conversion material of this embodiment is not limited as long as it satisfies the above-mentioned requirements. However, a preferred manufacturing 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); The method includes a step of performing alloying treatment to produce a mechanically alloyed product (mechanical alloying step) and a step of pressurizing and sintering the mechanically alloyed product (sintering step). Further, if necessary, a step of post-processing the obtained sintered body (post-processing step) may be provided. Each step will be explained in detail below.

<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 constituent elements of the thermoelectric conversion material (Mg, V, Ni, Sb, and if necessary Te and/or Sn), and contains raw materials (Mg source, V source, Ni source, Sb source, Te source, Sn source). The form of the raw material mixture is not limited as long as it contains the constituent elements. However, it is preferable to include a molten solidified material containing the constituent elements. In the molten solidified material, the constituent elements are relatively uniformly distributed. By using a mixed raw material containing a molten solidified material, uniformity in the composition distribution of the thermoelectric conversion material finally obtained can be ensured. Further, the method for producing the mixed raw material is not limited. However, methods involving a step of melting the raw materials are preferred. For example, the following first to third embodiments may be mentioned, and among these, the first or second embodiment is particularly preferred since it is easy to obtain a single phase of a half-Heusler type compound.

In the first aspect, when preparing a 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 solidified product. A magnesium (Mg) source is further mixed into the obtained molten solidified material, and the mixture of the molten solidified material and the magnesium (Mg) source is used as a mixed raw material. If necessary, a tellurium (Te) source and/or a tin (Sn) source may be added to the raw material before the melting process and/or the molten solidified product after the melting process.

Among the constituent components of the thermoelectric conversion material, magnesium (Mg) is an easily volatile component. Therefore, the composition of the material that is finally obtained by volatilization during high-temperature melting tends to be an Mg-deficient composition. By adding an Mg source to the raw material before the melting process and additionally adding the Mg source to the molten solidified material obtained after the melting process, a material with a desired composition can be easily obtained. Furthermore, it is possible to make the composition and structure of the finally obtained thermoelectric conversion material uniform.

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 and pellets containing each constituent element (Mg, V, Ni, Sb, Te, and Sn) alone, or powders and pellets containing a combination of multiple constituent elements.

The raw material (Mg source, etc.) may be melted by a known method. For example, the raw material charged into the crucible may be subjected to heat treatment such as resistance heating, high frequency induction heating, arc discharge, plasma heating, pulse energization, or electron beam heating. In order to prevent oxidation of the raw material, it is preferable to melt the material under an inert gas atmosphere or under reduced pressure. The melting temperature is not limited as long as the raw material melts. However, if the melting temperature is too high, volatilization of the raw material will proceed, and there is a risk that the composition of the material will shift. The melting temperature is preferably 1000°C or more and 3000°C or less. Further, mechanical or electromagnetic stirring may be applied to the molten metal during melting. After melting, the molten metal is cooled to obtain a molten solid. Then, a magnesium (Mg) source is added to the obtained molten solidified product. The amount of the Mg source added may be adjusted so as to obtain a thermoelectric conversion material with a desired composition.

In the second aspect, when preparing a mixed raw material, a vanadium (V) source, a nickel (Ni) source, and an antimony (Sb) source are melted and cooled to produce a molten solidified product, and the obtained molten solidified product is A magnesium (Mg) source is further mixed into the product, and the mixture of the molten solidified product and the magnesium (Mg) source is used as a mixed raw material. If necessary, a tellurium (Te) source and/or a tin (Sn) source may be added to the raw material before the melting process and/or the molten solidified product after the melting process.

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

In the third aspect, when preparing a 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 produce a molten solidified product. The obtained molten solidified product is used as a mixed raw material. If necessary, a tellurium (Te) source and/or a tin (Sn) source may be added to the raw material before the melting process and/or the molten solidified product after the melting process.

In a third embodiment, all of the magnesium (Mg) source is added to the raw material before the melting step, but not to the molten solidified product obtained after the melting step. Since Mg volatilizes during the melting process, it is desirable to incorporate a large amount of Mg source in consideration of volatile content. Since the required amount varies depending on the melting conditions, it is difficult to determine it unambiguously. However, for example, it is conceivable to mix it in an amount of 0.5% or more and 10% or less with respect to the amount of Mg in the final composition. The types of raw materials and melting conditions are the same as in the first embodiment.

<Mechanical alloying process>
In the mechanical alloying step, the obtained mixed raw material is subjected to mechanical alloying treatment to produce a mechanically alloyed product. Mechanical alloying (MA) is a method of grinding and mixing metal powders using a high-energy mill, thereby mechanically alloying them. Ductile metal powder repeatedly undergoes plastic deformation and fracture during processing, and agglomeration due to pressure bonding of newly formed surfaces. Therefore, a layered structure consisting of thin layers of components is formed, and as the treatment progresses, the layered structure becomes ultra-fine. Furthermore, the temperature of the metal powder increases due to the application of mechanical energy. The refinement of the layered structure and the rise in temperature act in combination to cause alloying at the atomic level, thereby making it possible to obtain a uniform alloy (intermetallic compound) phase. In particular, since the thermoelectric conversion material (half-Heusler type compound) of this embodiment includes components having different melting points and specific gravity, it is difficult to obtain a single phase only by a melting process. By performing mechanical alloying treatment after melting, a homogeneous single-phase intermetallic compound phase can be obtained.

The mill used for the mechanical alloying process is not limited as long as it is a high energy mill. Examples include attritors, planetary ball mills, and vibratory ball mills. The treatment may be performed under conditions that allow alloying to proceed sufficiently. For example, when using a planetary ball mill, the rotation speed and the revolution speed may be set to 300 rpm or more and 1200 rpm or less, and the treatment may be performed for 10 minutes or more and 50 hours or less.

<Sintering process>
In the sintering process, the mechanically alloyed material is sintered under pressure. Through the sintering process, a thermoelectric conversion material made of a dense sintered body can be obtained. Furthermore, even if foreign phases remain after the mechanical treatment step, alloying progresses during sintering, and it is possible to obtain an intermetallic compound phase with a small amount of foreign phases.

Pressure sintering may be performed using a known method. For example, a pressure sintering device such as a hot press (HP), a hot isostatic press (HIP), or a pulsed electric current sintering (PECS) device may be used. Although not limited, for example, sintering may be performed under conditions of applying a pressure of 20 MPa or more and 100 MPa or less, and at a temperature of 800 K or more and 1300 K or less for 2 minutes or more and 120 minutes or less.

<Post-processing process>
If necessary, the obtained sintered body may be subjected to post-treatment. For example, the dimensions may be adjusted by subjecting the sintered body to processing such as grinding or polishing. Further, in order to prevent oxidation of the sintered body, a protective layer may be provided on the surface of the sintered body.

In this way, the thermoelectric conversion material of this embodiment can be manufactured.

The present invention will be explained in more detail using the following examples. However, the present invention is not limited to the following examples.

[Experiment example A]
In Experimental Example A, thermoelectric conversion materials having a composition of Mg ₂ VNi ₃ Sb ₃ (p=q=0) were produced using different methods and evaluated. Specifically, samples were prepared according to the following procedure.

(1) Production of thermoelectric conversion material [Example 1] (p=q=0, first embodiment)
<Preparation process>
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 uniformity of the sample, the procedure of turning the solidified sample after melting and melting it again was repeated three times. Next, the taken out molten solidified product was placed in a glove box, and 0.085 g of magnesium (Mg) was further added to prepare a mixed raw material.

<Mechanical alloying process>
Next, the obtained mixed raw material was sealed in a tungsten carbide ball mill container. The ball mill container after sealing was set in a planetary ball mill device, and pulverized for 10 hours at an autorotation and revolution speed of 600 rpm for mechanical alloying.

<Sintering process>
Next, the powder sample taken out from the ball mill container was pressure sintered using a pulsed electric current sintering (PECS) device. Pressure sintering was performed at a temperature of 873 K in a vacuum atmosphere by applying a pressure of 30 MPa to the sample for 20 minutes. In this way, a polycrystalline sample of a thermoelectric conversion material made of a sintered body was prepared.

[Example 2] (p=q=0, second aspect)
In the preparation process, 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 procedure of turning the solidified sample after melting and melting it again was repeated three times. Next, the taken out molten solidified product was placed in a glove box, and 0.384 g of magnesium (Mg) was added thereto to prepare a mixed raw material. A polycrystalline sample of the thermoelectric conversion material was produced in the same manner as in Example 1 except for the above.

[Example 3] (p=q=0, third aspect)
In the preparation process, 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 heated in an arc furnace. Melted. To improve the uniformity of the sample, the procedure of turning the solidified sample after melting and melting it again was repeated three times. As a result, a mixed raw material consisting of a molten solidified material was obtained. A polycrystalline sample of the thermoelectric conversion material was produced in the same manner as in Example 1 except for the above.

(2) Evaluation of thermoelectric conversion materials Various properties of the samples (thermoelectric conversion materials) obtained in Examples 1 to 3 were evaluated as shown below.

<Density>
Density was measured by Archimedes method.

<XRD>
The sample was analyzed by powder X-ray diffraction to evaluate the formed phase. The analysis was performed by polishing the sintered sample surface and irradiating the polished surface with X-rays. Moreover, the X-ray diffraction measurement was performed under the following conditions.

-X-ray diffraction device: Rigaku Co., Ltd., MiniFlex600
-Radiation source: CuKα
-Tube voltage: 40kV
-Tube current: 15mA
-Scan speed: 2°/min -Scan range: 20°~80°

<SEM-EDS>
The microstructure of the samples was investigated using a scanning electron microscope (SEM; Thermo Fisher Scientific, Helios 5 Hydra DualBeam). Further, an elemental mapping image was obtained using an energy dispersive X-ray analyzer (EDS; Oxford Instruments, Ultim Max 170) attached to a microscope, and elemental analysis was performed. Specifically, observation and analysis were performed under conditions of magnification: 500 to 20,000 times, voltage: 10 to 30 kV, and current: 1.6 to 3.2 nA.

<Thermoelectric properties>
Thermoelectric properties (Seebeck coefficient S, electrical resistivity ρ, thermal conductivity κ _total ) in a predetermined temperature range (from about 300 K (room temperature) to about 900 K) were measured. The Seebeck coefficient S was determined by the following procedure using a thermoelectric property evaluation device (Advance Riko Co., Ltd., ZEM-3 series). That is, a temperature gradient ΔT was provided in the thermoelectric conversion material, and the thermoelectromotive force VE generated thereby was measured. Then, the Seebeck coefficient S was calculated according to the following equation (4). Note that in the following equation (4), S _wire is the absolute Seebeck coefficient of the metal probe used for thermoelectromotive force measurement.

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

Thermal diffusivity D was measured by the flash method using a flash analyzer (NETZSCH, LFA467 HyperFlash). Using the obtained thermal diffusivity D, constant pressure specific heat Cp, and density d measured by the Archimedes method, the thermal conductivity κ _total was determined according to the following equation (6).

In addition, the electronic thermal conductivity κ _el is calculated according to the following equation (7) using the electrical conductivity σ, and the lattice is calculated based on the relationship of the following equation (8) using the thermal conductivity κ _total and the electronic thermal conductivity κ _el . The thermal conductivity κ _lat was determined. In the following equation (7), L is the Lorentz number and T is the absolute temperature.

Next, the power factor PF and thermoelectric figure of merit zT were determined according to the following equation (9) using the Seebeck coefficient S, the electrical conductivity σ, and the thermal conductivity κ _total .

<Heat resistance>
The thermoelectric properties were repeatedly measured in a determined temperature range (approximately 300 K (room temperature) to approximately 900 K) to evaluate the heat resistance of the sample. In addition, thermal analysis was performed in each of the heating process and cooling process using a thermogravimetric differential thermal analysis measuring device (TG/DTA; Shimadzu Corporation, DTG-60).

(3) Evaluation results <density>
The density of the sample obtained in Example 1 was 6.605 g/cm ³ . When the relative density was calculated using the theoretical density (6.61 g/cm ³ ) determined from the lattice constant, the value was 95% or more. In addition, the samples obtained in Example 2 (density: 6.78 g/cm ³ ) and Example 3 (density: 6.67 g/cm ³ ) had a density of 6.5 g/cm ³ or more and a relative density of 95 % or more.

<XRD>
FIG. 5 shows the X-ray diffraction (XRD) patterns of the samples (thermoelectric conversion materials) obtained in Examples 1 to 3. It was confirmed that the samples of Examples 1 and 2 were composed of a single phase of a half-Heusler type compound Mg ₂ VNi ₃ Sb ₃ . That is, all the diffraction peaks were identified as originating from the half-Heusler phase (HH phase), and the peaks originating from the second phase (different phase) were below the detection limit. The ratio (I HP /I _HH ) of the peak intensity of the main peak derived from the different phase (I _HP ) to the peak intensity of the main peak derived from the _HH phase (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 from the second phase (different phase) was also observed. Specifically, I _HP /I _HH was 0.28.

<SEM-EDS>
An elemental mapping image of the sample obtained in Example 2 is shown in FIG. In the observation range, all elements (Mg, V, Ni, Sb) were uniformly dispersed, and no precipitates or second phases (different phases) were present.

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

The Seebeck coefficient S of each sample showed a positive value (FIG. 6), which revealed that the sample exhibited P-type semiconductor characteristics. Although not shown in the figure, the conductivity σ of all samples increased as the temperature increased, and this revealed that the samples exhibited semiconductor characteristics. Further, all samples had a thermal conductivity κ _total of 6 W/(Km) or less and a lattice thermal conductivity κ _lat of 4 W/(Km) or less within the measurement temperature range (FIGS. 8 and 9). The thermoelectric figures of merit PF and zT were high for the samples of Examples 1 and 2 (FIG. 7, FIG. 10). In particular, the thermoelectric figure of merit PF and zT of Example 1 were high, reaching their maximum values at about 700 K, and the respective values at that time exceeded 0.9 mW/(mK ² ) and 0.15. Although not shown, the thermoelectric figure of merit zT and PF of the sample of Example 3 were low. The reason for this was thought to be the presence of different phases or a difference in the parent composition.

<Heat resistance>
FIG. 11 shows the results of a repeated power factor PF test performed on the sample of Example 1 in a temperature range of about 300K to about 900K. Further, the thermal analysis results of the sample of Example 2 in the temperature range of about 300K to about 1000K are shown in FIG.

In the repeated test results of the power factor PF, there was almost no difference between the first measurement result and the second measurement result (FIG. 11). Furthermore, as a result of thermal analysis, no peaks associated with reactions or phase changes were observed, and the value of heat flow after cooling was almost the same as the value before the start of temperature rise (FIG. 12). From this, it was found that the thermoelectric conversion material of this embodiment does not change in quality even when heated to about 1000K, and has excellent heat resistance.

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

(1) Production of thermoelectric conversion material [Examples 4, 5, 7] (p=0.05 to 0.3, q=0, second embodiment)
In the preparation process, the amounts of raw materials (Mg, V, Ni, and Sb) were changed as shown in Table 1 below. A polycrystalline sample of the thermoelectric conversion material was produced in the same manner as in Example 2 except for the above.

[Example 6] (p=0.2, q=0, second aspect)
In the preparation process, the amounts of raw materials (Mg, V, Ni, and Sb) were changed as shown in Table 1 below. The sintering conditions were 873K for 1 hour. A polycrystalline sample of the thermoelectric conversion material was produced in the same manner as in Example 2 except for the above.

[Examples 8 to 10] (p=0, q=0.01 to 0.3, M=Te, second embodiment)
In the preparatory step, tellurium (Te) was added as a dopant together with magnesium (Mg) to the molten solidified material. In addition, the blending amounts of raw materials (Mg, V, Ni, Sb, and Te) were changed as shown in Table 1 below. A polycrystalline sample of the thermoelectric conversion material was produced in the same manner as in Example 2 except for the above.

[Example 11] (p=0.1, q=0.3, M=Te, second aspect)
In the preparatory step, tellurium (Te) was added as a dopant together with magnesium (Mg) to the molten solidified material. In addition, the blending amounts of raw materials (Mg, V, Ni, Sb, and Te) were changed as shown in Table 1 below. A polycrystalline sample of the thermoelectric conversion material was produced in the same manner as in Example 2 except for the above.

[Example 12] (p=0, q=0.3, M=Sn, second aspect)
In the preparation process, tin (Sn) was added as a dopant when mixing vanadium (V), nickel (Ni), and antimony (Sb). In addition, the blending amounts of 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 produced in the same manner as in Example 2 except for the above.

[Example 13] (p=0.1, q=0.3, M=Te, second aspect)
In the preparatory step, tellurium (Te) was added as a dopant together with magnesium (Mg) to the molten solidified material. In addition, the blending amounts of raw materials (Mg, V, Ni, Sb, and Te) were changed as shown in Table 1 below. The sintering conditions were 1073K for 1 hour. A polycrystalline sample of the thermoelectric conversion material was produced in the same manner as in Example 2 except for the above.

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

(2) Evaluation of thermoelectric conversion materials The samples (thermoelectric conversion materials) obtained in Examples 4 to 14 were subjected to X-ray diffraction analysis (XRD) and evaluation of thermoelectric properties in the same manner as in Experimental Example A.

(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), and 6.961 g/cm ³ (Example 6), respectively. , 6.512g/cm ³ (Example 7), 6.825g/cm ³ (Example 8), and 6.812g/cm ³ (Example 9).

<XRD>
Although not shown, in the X-ray diffraction (XRD) patterns of the samples obtained in Examples 4 to 14, the strongest peak derived from the half-Heusler (HH) phase was observed in all samples. Specifically, the ratio (I _HP /I _HH ) of the peak intensity of the main peak derived from the HH phase (I _HH ) to the peak intensity (I _HP ) of the main peak derived from the different phase is 0.04 to 0.24. there were.

<Thermoelectric properties>
The Seebeck coefficient S, lattice thermal conductivity κ _lat , and thermoelectric figure of merit zT of the samples obtained in Examples 4 to 14 are shown in FIGS. 13 to 16.

When the excess vanadium amount p and the dopant amount q were changed, the value of the Seebeck coefficient S changed from positive to negative (FIGS. 13 and 14). In particular, the Seebeck coefficient showed a negative value in the samples with excessive V composition (q>0.1) (Examples 5 to 6) or the samples with Te added composition (q>0.1) (Examples 11 and 13). , was found to exhibit N-type semiconductor properties. From this, it was found that by adjusting the excess vanadium amount p and the dopant amount q, the semiconductor characteristics of the thermoelectric conversion material can be controlled from P-type to N-type.

All of the samples measured (Examples 4, 8, 9, 13, and 14) had low thermal conductivity κ _total and low lattice thermal conductivity κ _lat (FIGS. 15 and 16). Specifically, in the region of 700K or less, the thermal conductivity κ _total was 6 W/(Km) or less, and the lattice thermal conductivity κ _lat was 4 W/(Km) or less.

(4) Summary From the above results, the thermoelectric conversion material of this embodiment, which mainly consists of a half-Heusler type compound with a specific composition, has low thermal conductivity and exhibits an excellent thermoelectric effect in the temperature range from room temperature to 500°C. In addition, it was found that the semiconductor characteristics can be adjusted to either P-type or N-type.

In particular, since the thermoelectric conversion material of this embodiment contains a triple half-Heusler (THH) type compound as a main component, its thermal conductivity, particularly lattice thermal conductivity, is significantly low. This will be explained using FIG. 17. FIG. 17 shows the lattice thermal conductivity κ _{l at} at 300K or 700K of the triple half-Heusler (THH) type compound of this embodiment (Example 1), and that of the conventional half-Heusler (HH) type compound and double-half Heusler (DHH). It is shown in comparison with the lattice thermal conductivity κ _lat of the type compound.

The triple half-Heusler type compound of this embodiment has a lattice thermal conductivity κ _lat that is significantly smaller than that of conventional half-Heusler type compounds and double-half Heusler type compounds. For example, looking at the lattice thermal conductivity at 300K, the minimum of the conventional half-Heusler type compound is about 7W/(mK), whereas the triple half-Heusler type compound of this embodiment is about 2W/(mK). It is.

As described above, it can be said that the thermoelectric conversion material of this embodiment, which has a triple half-Heusler (THH) type compound with a significantly low thermal conductivity as a main component, has high potential as a material exhibiting excellent thermoelectric properties.

2 P-type thermoelectric conversion element 4 N-type thermoelectric conversion element 6 Electrode 8 Ceramic plate 10 Thermoelectric conversion module

Claims (14)

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 (however, p and q are - 0.5≦p≦0.5 and 0≦q≦1.0, M has a composition represented by one or both of tin (Sn) and tellurium (Te),
A thermoelectric conversion material whose main component is a half-Heusler type compound. The thermoelectric conversion material according to claim 1, wherein the thermoelectric conversion material is composed of a single phase of a half-Heusler type compound. The thermoelectric conversion material according to claim 1 or 2, wherein the thermoelectric conversion material is polycrystalline. The thermoelectric conversion material according to claim 1 or 2, wherein the thermoelectric conversion material exhibits P-type semiconductor characteristics. The thermoelectric conversion material according to claim 1 or 2, wherein the thermoelectric conversion material exhibits N-type semiconductor characteristics. The thermoelectric conversion material according to claim 1 or 2, wherein the thermoelectric conversion material has a thermal conductivity (κ _total ) of 6.0 W/(mK) or less in a temperature range of 300 K or more and 800 K or less. The thermoelectric conversion material according to claim 1 or 2, wherein the power factor (PF) of the thermoelectric conversion material is 0.2 mW/(mK ² ) or more in a temperature range of 300 K or more and 800 K or less. The thermoelectric conversion material according to claim 1 or 2, wherein the power factor (PF) of the thermoelectric conversion material is 0.5 mW/(mK ² ) or more in a temperature range of 600 K or more and 800 K or less. A method for producing a thermoelectric conversion material according to claim 1 or 2, comprising the following steps;
Step of preparing a mixed raw material containing at least magnesium (Mg), vanadium (V), nickel (Ni), and antimony (Sb):
A method comprising: performing mechanical alloying on the mixed raw material to produce a mechanically alloyed product; and pressurizing and sintering the mechanically alloyed product. 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 molten solidified product, and the molten solidified product is The method according to claim 9, further comprising mixing a magnesium (Mg) source into the molten solidified material and using a mixture of the molten solidified material and the magnesium (Mg) source as the mixed raw material. When preparing the mixed raw material, a vanadium (V) source, a nickel (Ni) source, and an antimony (Sb) source are melted and cooled to produce a molten solidified product, and magnesium (Mg) is further added to the molten solidified product. 10. The method according to claim 9, wherein a mixture of the molten solidified material and the magnesium (Mg) source is used as the mixed raw material. 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 produce a molten solidified product, and the molten solidified product is The method according to claim 9, wherein: is used as the mixed raw material. A thermoelectric conversion element comprising the thermoelectric conversion material according to claim 1 or 2. A thermoelectric conversion module comprising the thermoelectric conversion element according to claim 13.

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