Thermoelectric material and method for manufacturing the same

Abstract

To provide a Mg-Ge-based thermoelectric material that is low in toxicity, inexpensive, and has an excellent dimensionless figure of merit zT at high temperatures.SOLUTION: A thermoelectric material according to an embodiment contains Mg, Ge, and a doping element X that dopes carriers, and is a single crystal body consisting of a single crystal or a single crystal body consisting of a single crystal containing precipitates of different phases, in which α, a ratio of vacancy defects in Mg, is greater than 0% and 20.0% or less, and edge dislocations are present.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a thermoelectric material and a method for manufacturing the thermoelectric material. [Background technology]

[0002] Development of thermoelectric materials that can generate electrical energy from waste heat is progressing. Known thermoelectric materials include Bi2Sb3 and PbTe. However, conventional thermoelectric materials have problems in that the constituent elements are rare, and elements such as Te are highly toxic.

[0003] Mg-Sn thermoelectric materials, which are made up of Mg and Sn, are low-toxicity and inexpensive thermoelectric materials. Mg2Sn, a Mg-Sn thermoelectric material, has the problem of low electrical conductivity and high thermal conductivity, resulting in a low dimensionless figure of merit zT, which indicates the performance of thermoelectric materials. Here, zT is expressed as PF×T / κ, where PF is the power factor, T is the absolute temperature, and κ is the thermal conductivity.

[0004] As a technology for improving the zT of an Mg-Sn-based thermoelectric material, Patent Document 1 discloses a thermoelectric material that contains Mg, Sn, and a doping element X that dopes the carrier, is a single crystal made of a single crystal, and has α, which is the ratio of vacancy defects in the Mg, exceeding 0% and not exceeding 20.0%. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2022-125757 [Non-patent literature]

[0006] [Non-Patent Document 1] Kamila et al., Adv. Sci. 7 (2020) 2000070 Summary of the Invention [Problem to be solved by the invention]

[0007] Currently, there is a demand for thermoelectric materials that are less toxic, inexpensive, and have superior power generation performance at high temperatures compared to the Mg—Sn-based thermoelectric material of Patent Document 1.

[0008] The present invention has been made in view of the above problems, and aims to provide an Mg-Ge-based thermoelectric material that is low in toxicity, inexpensive, and has an excellent dimensionless figure of merit zT at high temperatures, and a method for producing the thermoelectric material. [Means for solving the problem]

[0009] In order to solve the above problems, the present invention proposes the following means. (1) The thermoelectric material of the first aspect of the present invention is Mg and Ge and a doping element X that dopes the carriers; Contains A single crystal body consisting of a single crystal or a single crystal body consisting of a single crystal containing precipitates of different phases, α, the ratio of Mg vacancy defects, is more than 0% and 20.0% or less, A thermoelectric material with edge dislocations. (2) A second aspect of the present invention is the thermoelectric material of the first aspect, The molar ratio (Mg:Ge:X) of the Mg, the Ge, and the doping element X may satisfy the following formula (1) or (2), and also satisfy the following formula (3). Mg:Ge:X=2×(100-α) / 100:1-a:a...(1) Mg:Ge:X=2×(100-α) / 100-a:1:a...(2) 0 <a≦0.12····(3) (3) A third aspect of the present invention is a thermoelectric material according to the first or second aspect, wherein The doping element X may be at least one element selected from the group consisting of Li, Cu, Ga, Ag, Sn, Sb, and Bi. (4) A fourth aspect of the present invention is a thermoelectric material according to any one of the first to third aspects, The pressure-applying element Z having an atomic radius smaller than the atomic radius of any one of the Mg, Ge, and doping element X may further be contained. (5) A fifth aspect of the present invention is the thermoelectric material of the fourth aspect, The pressure-applying element Z may be at least one selected from the group consisting of B and Si. (6) A sixth aspect of the present invention relates to the thermoelectric material of the fourth or fifth aspect, wherein the molar ratio (Mg:Ge:X:Z) of the Mg, the Ge, the doping element X, and the pressure-applying element Z satisfies any one of the following formulae (4) to (7), and also satisfies the following formulae (8) and (9): Mg:Ge:X:Z=2×(100-α) / 100:1-ab:a:b···(4)Mg:Ge:X:Z=2×(100-α) / 100-ab:1:a:b···(5) Mg:Ge:X:Z=2×(100-α) / 100-a:1-b:a:b...(6) Mg:Ge:X:Z=2×(100-α) / 100-b:1-a:a:b...(7) 0 <a≦0.12···(8) 0 <b≦0.05···(9) (7) A seventh aspect of the present invention is a thermoelectric material according to any one of the first to sixth aspects, The density of the edge dislocation is 1.0 × 10 16 ~1.0×10 18 (m ―2 ) may also be used. (8) A method for producing a thermoelectric material according to an eighth aspect of the present invention includes the steps of: Mg and Ge and a doping element X that dopes the carriers; After mixing at least the above to prepare a mixture, Melting the mixture by heating the mixture in an inert gas until the temperature of the mixture reaches a melting temperature; After the mixture is melted, it is cooled so that the pressure p [atm] of the inert gas satisfies the following formula (10) and the average cooling rate from the melting temperature to the slow-cooling stop temperature is 16.7 K / h or less. 0≦p≦48 (10) (9) A ninth aspect of the present invention is a method for producing a thermoelectric material according to the eighth aspect, comprising: The molar ratio (Mg:Ge:X) of the Mg, the Ge, and the doping element X may satisfy the following formula (11) or (12), and may also satisfy the following formulas (13) and (14). Mg:Ge:X = d:1 − a:a (11) Mg:Ge:X=da:1:a····(12) 1.8≦d≦2.2 (13) 0 <a≦0.12····(14) (10) A tenth aspect of the present invention is a method for producing a thermoelectric material according to the eighth or ninth aspect, further comprising: The doping element X may be at least one element selected from the group consisting of Li, Cu, Ga, Ag, Sn, Sb, and Bi. (11) In an eleventh aspect of the present invention, in the method for producing a thermoelectric material according to any one of the eighth to tenth aspects, the mixture may further contain a pressure-applying element Z having an atomic radius smaller than the atomic radius of any one of the Mg, the Ge, and the doping element X. (12) A twelfth aspect of the present invention may be the method for producing a thermoelectric material according to the ninth aspect, wherein the pressure-applying element Z is at least one selected from the group consisting of B and Si. (13) A thirteenth aspect of the present invention is a method for producing a thermoelectric material according to the eleventh or twelfth aspect, further comprising: The molar ratio (Mg:Ge:X:Z) of the Mg, the Ge, the doping element X, and the pressure-applying element Z may satisfy any one of the following formulas (15) to (18), and may also satisfy the following formulas (19), (20), and (21): Mg:Ge:X:Z=d:1-ab:a:b (15) Mg:Ge:X:Z=dab:1:a:b (16) Mg:Ge:X:Z=da:1-b:a:b (17) Mg:Ge:X:Z=db:1-a:a:b (18) 1.8≦d≦2.2 (19) 0 <a≦0.12···(20) 0 <b≦0.05···(21) (14) A fourteenth aspect of the present invention is a method for producing a thermoelectric material according to any one of the eighth to thirteenth aspects, further comprising: The melting temperature may be 1443K to 1343K. (15) A fifteenth aspect of the present invention is a method for producing a thermoelectric material according to any one of the eighth to fourteenth aspects, further comprising: The slow cooling stop temperature may be 1434K to 1173K. (16) A sixteenth aspect of the present invention relates to the method for producing a thermoelectric material according to any one of the eighth to fifteenth aspects, The pressure p may satisfy the following formula (22). 0.2≦p≦48 (22) [Effects of the Invention]

[0010] According to the above-described aspects of the present invention, it is possible to provide an Mg—Ge-based thermoelectric material that is low in toxicity, inexpensive, and has an excellent dimensionless figure of merit zT at high temperatures, and a method for producing the thermoelectric material. [Brief explanation of the drawings]

[0011] [Figure 1] Figure 1(a) shows a TEM image of the thermoelectric material, Figure 1(b) shows the Fourier transform of the area enclosed by the dotted line in Figure 1(a), and Figure 1(c) shows the inverse Fourier transform of the area enclosed by the circle in Figure 1(b). [Figure 2] 1 shows the measurement results of the temperature dependence of the Seebeck coefficient of the thermoelectric materials of Examples 1 to 4 and Comparative Examples 1 and 2. [Figure 3] 1 shows the results of measuring the temperature dependence of the electrical conductivity of the thermoelectric materials of Examples 1 to 4 and Comparative Examples 1 and 2. [Figure 4]1 shows the results of measuring the temperature dependence of thermal conductivity of the thermoelectric materials of Examples 1 to 4 and Comparative Examples 1 and 2. [Figure 5] 1 shows the measurement results of the mobility and carrier density of the thermoelectric materials of Examples 1 to 4 and Comparative Examples 1 and 2. [Figure 6] 1 shows the measurement results of the dimensionless figure of merit zT of the thermoelectric materials of Examples 1 to 4 and Comparative Examples 1 and 2. DETAILED DESCRIPTION OF THE INVENTION

[0012] <Thermoelectric materials> A thermoelectric material according to one embodiment of the present invention contains Mg, Ge, and a doping element X that dopes the carriers, and is a single crystal body consisting of a single crystal or a single crystal body consisting of a single crystal containing precipitates of a different phase, in which α, the ratio of Mg vacancy defects, is greater than 0% and 20.0% or less, and edge dislocations are present. Hereinafter, a thermoelectric material according to an embodiment of the present invention and a method for manufacturing the thermoelectric material will be described.

[0013] (Containing Mg, Ge, and doping element X that dopes carriers) The thermoelectric material according to this embodiment contains Mg, Ge, and a doping element X that dopes the carriers. Since the thermoelectric material according to this embodiment contains Mg and Ge, it is low-cost and has low toxicity. Furthermore, since the band gap of the Mg-Ge thermoelectric material is large, the Seebeck coefficient is unlikely to decrease in a high-temperature environment (200°C or higher). Therefore, it is possible to maintain power generation performance in a high-temperature environment. Furthermore, the thermoelectric material according to this embodiment has a large band gap, and the Seebeck coefficient is unlikely to decrease in a high-temperature environment (200°C or higher). Mg ) form clusters, which can increase phonon scattering. Furthermore, the doping element X and edge dislocations also increase phonon scattering, allowing for further reduction in thermal conductivity. In addition, the inclusion of the doping element X dopes the carriers, improving the power factor PF.

[0014] The doping element X in the thermoelectric material according to this embodiment is preferably at least one element selected from the group consisting of Li, Cu, Ga, and Ag. When the doping element X is at least one element selected from the group consisting of Li, Cu, Ga, and Ag, hole carriers can be doped. This allows the thermoelectric material according to this embodiment to function as a p-type thermoelectric material.

[0015] The doping element X in the thermoelectric material according to this embodiment is preferably at least one element selected from the group consisting of Sn, Sb, and Bi. When the doping element X is at least one element selected from the group consisting of Sn, Sb, and Bi, electron carriers can be doped. This allows the thermoelectric material according to this embodiment to function as an n-type thermoelectric material.

[0016] (molar ratio of Mg, Ge and doping element) In the thermoelectric material according to this embodiment, the molar ratio of Mg, Ge, and doping element X (Mg:Ge:X) preferably satisfies the following formula (1) or (2) and also satisfies the following formula (3). In the following formulas (1) and (2), α represents the ratio α of Mg vacancies. When multiple doping elements X are included, a represents a value based on the molar ratio of the doping elements. When the molar ratio of Mg, Ge, and doping element X in the thermoelectric material according to this embodiment satisfies the following formula (1) or (2) and also satisfies formula (3) (a: greater than 0 and equal to or less than 0.12), a higher power factor PF can be obtained while suppressing the precipitation of a second phase, and the thermal conductivity can be further reduced. In the following formula (3), a is preferably equal to or greater than 0.08. In the following formula (3), a is preferably equal to or less than 0.10. Mg:Ge:X=2×(100-α) / 100:1-a:a...(1) Mg:Ge:X=2×(100-α) / 100-a:1:a...(2) 0 <a≦0.12····(3)

[0017] (Pressure applied element Z) The thermoelectric material according to this embodiment preferably further contains a pressure-applying element Z having an atomic radius smaller than the atomic radius of any one of Mg, Ge, and the doping element X. By including a pressure-applying element Z having an atomic radius smaller than the atomic radius of any one of Mg, Ge, and the doping element X in the thermoelectric material, the lattice constant can be reduced and pressure can be applied to the thermoelectric material. This reduces the energy required to form Mg vacancy defects, and the introduction of more Mg vacancy defects can further reduce the thermal conductivity of the thermoelectric material.

[0018] The pressure-applying element Z in the thermoelectric material according to this embodiment is preferably at least one selected from the group consisting of B and Si. When the pressure-applying element Z is at least one selected from the group consisting of B and Si, the thermal conductivity can be further reduced.

[0019] (molar ratio of Mg, Ge, doping element X, and pressure application element Z) In the thermoelectric material according to this embodiment, the molar ratio (Mg:Ge:X:Z) of Mg, Ge, doping element X, and pressure-applying element Z preferably satisfies any one of the following formulae (4) to (7), and also satisfies the following formulae (8) and (9). In the following formulae (4) to (7), α represents the ratio α of Mg vacancies. When the thermoelectric material contains multiple doping elements X, a is a value based on the molar ratio of the doping elements X. When the thermoelectric material contains multiple pressure-applying elements Z, b is a value based on the total molar ratio of the pressure-applying elements Z. When the molar ratio of Mg, Ge, doping element X, and pressure-applying element Z in the thermoelectric material according to this embodiment satisfies any one of the following formulae (4) to (7), and also satisfies the following formulae (8) (a: more than 0 and 0.12 or less) and (9) (b: more than 0 and 0.05 or less), the thermal conductivity can be further reduced. Mg:Ge:X:Z=2×(100-α) / 100:1-ab:a:b (4) Mg:Ge:X:Z=2×(100-α) / 100-ab:1:a:b (5) Mg:Ge:X:Z=2×(100-α) / 100-a:1-b:a:b...(6) Mg:Ge:X:Z=2×(100-α) / 100-b:1-a:a:b...(7) 0 <a≦0.12···(8) 0 <b≦0.05···(9)

[0020] The thermoelectric material according to this embodiment may be composed of Mg, Ge, a doping element X, and an impurity. Alternatively, the thermoelectric material according to this embodiment may be composed of Mg, Ge, a doping element X, a pressure-applying element Z, and an impurity. The impurities are not limited to those contained in the raw materials, but also include those mixed in during the manufacturing process. The thermoelectric material according to this embodiment may contain impurities to the extent that the effects of the present invention can be obtained.

[0021] The thermoelectric material according to this embodiment may contain heterogeneous precipitates, such as antisite defect precipitates (Ge-rich precipitates) in which Ge occupies the site of Mg.

[0022] The chemical composition of the thermoelectric material according to this embodiment can be measured by a combination of methods such as single crystal X-ray diffraction, inductively coupled plasma mass spectrometry, energy dispersive X-ray analysis, and neutron diffraction, and the molar ratios can be determined by complementary determinations based on the contents of each element obtained by these measurement methods. For example, if the thermoelectric material is composed of Mg, Ge, and Li, the contents can be determined by the following method. The Mg content is measured by single crystal X-ray diffraction and inductively coupled plasma mass spectrometry, the Ge content is measured by single crystal X-ray diffraction, inductively coupled plasma mass spectrometry, and energy dispersive X-ray analysis, and the Li content is measured by inductively coupled plasma mass spectrometry. The molar ratios of each element can be determined in a complementary manner based on the results obtained.

[0023] (single crystal body consisting of a single crystal) The thermoelectric material according to this embodiment is a single crystal body made of a single crystal or a single crystal body made of a single crystal containing precipitates of different phases. When the thermoelectric material is a single crystal body made of a single crystal, high carrier mobility can be obtained. In addition, the single crystal region and the V region having Mg vacancy defects can be obtained. Mg A semi-coherent interface is formed between the two regions. This semi-coherent interface does not affect carrier conduction, but can increase phonon scattering. Therefore, if the thermoelectric material is a single crystal consisting of a single crystal, the thermal conductivity can be reduced. Whether it is a single crystal or not can be determined by the back-reflection Laue method using X-rays. By rotating the thermoelectric material relative to the irradiated X-rays and obtaining Laue images with four-fold symmetry and three-fold symmetry, it can be concluded that it is a single crystal.

[0024] (Mg vacancy defect ratio α: over 0% and up to 20.0%) In the thermoelectric material according to this embodiment, the ratio α of Mg vacancy defects is greater than 0% and not greater than 20.0%. By having the ratio α of Mg vacancy defects greater than 0% and not greater than 20.0%, phonon scattering can be increased, thereby reducing the thermal conductivity of the thermoelectric material. A more preferable ratio α of Mg vacancy defects is greater than 0% to 15.0%. An even more preferable ratio α of Mg vacancy defects is greater than 0% to 10.0%.

[0025] The ratio of Mg vacancy defects can be measured, for example, by the following method. Microcrystals measuring tens of micrometers or less are collected from the thermoelectric material. Single crystal X-ray diffraction is performed on the obtained crystals. Single crystal structure analysis is performed using the diffraction intensity and measurement angle information obtained from the X-ray diffraction to determine the occupancy rates of Mg and Ge. If there are defects (vacancy defects), the Mg occupancy rate will be less than 100%. The amount of defects, α%, is taken as the amount of Mg vacancy defects (ratio of Mg vacancy defects).

[0026] (edge dislocation) The thermoelectric material according to this embodiment has edge dislocations. An edge dislocation is a crystal defect in which the dislocation line intersects perpendicularly with the Burgers vector, which indicates the direction of crystal displacement. The Burgers vector is a vector that indicates the magnitude and direction of lattice strain caused by dislocations within a crystal lattice. The presence of edge dislocations in the thermoelectric material according to this embodiment can further reduce the thermal conductivity of the thermoelectric material.

[0027] The density of edge dislocations in the thermoelectric material according to this embodiment is 1.0×10 16 ~1.0×10 18 (m -2 ) is preferable. The density of edge dislocations in the thermoelectric material is 1.0 × 10 16 ~1.0×10 18 (m -2 ) can further reduce the thermal conductivity of the thermoelectric material.

[0028] The density of edge dislocations in the thermoelectric material according to this embodiment can be measured, for example, by the following method. Microcrystals measuring several tens of micrometers or less are extracted from the thermoelectric material. The dislocation measurement method will be described below with reference to FIG. 1. FIG. 1(a) shows a TEM image of the thermoelectric material, FIG. 1(b) shows a Fourier transform of the area enclosed by the dotted line in FIG. 1(a), and FIG. 1(c) shows an inverse Fourier transform of the area enclosed by the circle in FIG. 1(b). The extracted sample is observed with a transmission electron microscope (TEM) at a magnification of 500,000 times to obtain a TEM image (FIG. 1(a)). A fast Fourier transform (FFT) is performed on the TEM image obtained by the observation to obtain an FFT image (FIG. 1(b)). An inverse FFT image of the same size (16 nm × 16 nm) as the TEM image is created from the moiré spots observed in the FFT pattern (FIG. 1(c)). Edge dislocations are identified as points in the image where the number of lines increases or decreases. The dislocation density can be obtained by dividing the number of edge dislocations (points indicated by ⊥) confirmed in the image by the area of the image. The density of each edge dislocation is found in the four inverse FFT images, and the average of these is taken as the edge dislocation density. If there is one or more edge dislocations in the image, it is considered that an edge dislocation is present.

[0029] (Seebeck coefficient S) When the thermoelectric material according to the embodiment is p-type, the Seebeck coefficient S of the thermoelectric material according to the embodiment at 800 K is preferably 150 (μV / K) to 300 (μV / K). When the thermoelectric material is n-type, the Seebeck coefficient S of the thermoelectric material according to the embodiment at 800 K is preferably -250 (μV / K) to -100 (μV / K). The Seebeck coefficient S can be measured by a steady-state direct current method using a thermoelectric performance measuring device (for example, the RZ2001 manufactured by OZAWA Scientific).

[0030] (Conductivity σ) When the thermoelectric material is p-type, the conductivity σ of the thermoelectric material according to this embodiment at 800 K is preferably 300 (S / cm) to 900 (S / cm). When the thermoelectric material is n-type, the conductivity σ of the thermoelectric material according to this embodiment at 800 K is preferably 200 (S / cm) to 800 (S / cm). The conductivity σ can be measured by a DC four-terminal method using a thermoelectric performance measuring device (for example, the RZ200li manufactured by OZAWA Scientific).

[0031] (thermal conductivity κ) When the thermoelectric material is p-type, the thermal conductivity κ of the thermoelectric material according to this embodiment at 800 K is preferably 6.0 (W / Km) or less. More preferably, the thermal conductivity κ of the thermoelectric material according to this embodiment at 800 K is 3.0 (W / Km) or less. When the thermoelectric material is n-type, the thermal conductivity κ of the thermoelectric material according to this embodiment at 80 K is preferably 6.0 (W / Km) or less. The thermal conductivity κ can be measured by the laser flash method using a thermal constant measurement device (for example, LFA 467 HyperFlash manufactured by Netzsch).

[0032] (carrier density n) When the thermoelectric material is p-type, the carrier density n of the thermoelectric material according to this embodiment at room temperature is 5.0 (10 18 ×cm -3 )~3.0(10 20 ×cm -3When the thermoelectric material is an n-type, the carrier density n of the thermoelectric material according to this embodiment at room temperature is preferably 5.0 (10 18 ×cm -3 )~3.0(10 20 ×cm -3 The carrier density n can be calculated from the Hall coefficient obtained by the Hall effect measurement method using a physical property measurement device (for example, PPMS manufactured by Quantum Design), the elementary charge, and the electrical conductivity.

[0033] (carrier mobility μ) When the thermoelectric material is p-type, the carrier mobility μ of the thermoelectric material according to this embodiment at room temperature is 40 (cm 2 / Vs)~200(cm 2 When the thermoelectric material is n-type, the carrier mobility μ of the thermoelectric material according to this embodiment at room temperature is preferably 40 (cm 2 / Vs)~200(cm 2 The carrier mobility μ can be calculated from the Hall coefficient obtained by a Hall effect measurement method using a physical property measurement device (for example, PPMS manufactured by Quantum Design), the elementary charge, and the electrical conductivity.

[0034] (Power factor PF) When the thermoelectric material is p-type, the power factor PF of the thermoelectric material according to this embodiment at 800 K is 1 (10 -3 ×W / K 2 m)~5.5(10 -3 ×W / K 2 When the thermoelectric material is n-type, the power factor PF of the thermoelectric material according to this embodiment at 800 K is preferably 1 (10 -3 ×W / K 2 m)~5.5(10 -3 ×W / K 2 The power factor PF can be calculated from the Seebeck coefficient S and the electrical conductivity σ.

[0035] (Dimensionless figure of merit zT) When the thermoelectric material is p-type, the dimensionless figure of merit zT at 800 K of the thermoelectric material according to this embodiment is preferably 0.3 or more, and more preferably 0.8 or more. When the thermoelectric material is n-type, the dimensionless figure of merit zT at 800 K of the thermoelectric material according to this embodiment is preferably 0.3 or more. The dimensionless figure of merit zT at 800 K is 0.5 or more. The dimensionless figure of merit zT can be calculated from the Seebeck coefficient S, electrical conductivity σ, thermal conductivity κ, and absolute temperature T.

[0036] <Method of manufacturing thermoelectric materials> An example of a method for manufacturing a thermoelectric material according to this embodiment will be described. The method for manufacturing a thermoelectric material according to this embodiment involves mixing at least Mg, Ge, and a doping element X for doping the carriers to prepare a mixture (mixing step), then heating the mixture in an inert gas until the temperature of the mixture reaches the melting temperature to melt the mixture (melting step), and after the mixture has melted, cooling the mixture so that the pressure p [atm] of the inert gas satisfies the following formula (10) and the average cooling rate from the melting temperature to the slow-cooling stop temperature is 16.7 K / h or less (cooling step). Each step will be described below. 0≦p≦48 (10)

[0037] (Mixing process) In the mixing step according to this embodiment, a mixture is prepared by mixing at least Mg, Ge, and a doping element X for doping the carriers.

[0038] "mixture" The mixture contains at least Mg, Ge, and a doping element X that dopes the carriers.

[0039] The doping element X in the mixture is preferably at least one element selected from the group consisting of Sn, Sb, and Bi. When the doping element X is at least one element selected from the group consisting of Sn, Sb, and Bi, electron carriers can be doped. This allows the thermoelectric material according to this embodiment to function as an n-type thermoelectric material.

[0040] The doping element X in the mixture is preferably at least one element selected from the group consisting of Li, Cu, Ga, and Ag. When the doping element X is at least one element selected from the group consisting of Li, Cu, Ga, and Ag, hole carriers can be doped.

[0041] It is preferable that the molar ratio of Mg, Ge, and doping element X (Mg:Ge:X) in the mixture satisfies the following formula (11) or (12), and also satisfies the following formulas (13) and (14). Here, when the mixture contains multiple doping elements X, a is a value based on the molar ratio of the doping elements. When the molar ratio of Mg, Ge, and doping element X in the mixture satisfies the following formula (11) or (12), and also satisfies the following formulas (13) and (14), the ratio α of Mg vacancy defects can be controlled more precisely. Mg:Ge:X = d:1 − a:a (11) Mg:Ge:X=da:1:a····(12) 1.8≦d≦2.2 (13) 0 <a≦0.12····(14)

[0042] The mixture preferably further contains a pressure-applying element Z having an atomic radius smaller than the atomic radius of any one of Mg, Ge, and the doping element X. By containing the pressure-applying element Z in the mixture, the ratio α of Mg vacancy defects can be more precisely controlled.

[0043] The pressure-applying element Z in the mixture is preferably at least one selected from the group consisting of B and Si. When the pressure-applying element Z is at least one selected from the group consisting of B and Si, it becomes easier to control the ratio α of Mg vacancy defects.

[0044] When the mixture contains a pressure-applying element Z, it is preferable that the molar ratio (Mg:Ge:X:Z) of Mg, Ge, doping element X, and pressure-applying element Z in the mixture satisfy one of the following formulas (15) to (18), and also satisfy the following formulas (19), (20), and (21). Here, a is a value based on the molar ratio of the doping element X. When the mixture contains multiple pressure-applying elements Z, b is a value based on the total molar ratio of the pressure-applying elements Z. When the molar ratio of Mg, Ge, doping element X, and pressure-applying element Z in the mixture satisfies one of the following formulas (15) to (18), and also satisfies the following formulas (19), (20), and (21), the ratio α of Mg vacancy defects can be more precisely controlled. Mg:Ge:X:Z=d:1-ab:a:b (15) Mg:Ge:X:Z=dab:1:a:b (16) Mg:Ge:X:Z=da:1-b:a:b (17) Mg:Ge:X:Z=db:1-a:a:b (18) 1.8≦d≦2.2 (19) 0 <a≦0.12···(20) 0 <b≦0.05···(21)

[0045] The mixture is obtained by weighing and mixing grains or powders of each element. The mixing method is not particularly limited, and for example, mixing may be performed in a mortar. The purity of each element is preferably 4N or higher.

[0046] (Melting process) In the melting step according to the present embodiment, the mixture is melted by heating it in an inert gas until the temperature of the mixture reaches the melting temperature. The melting temperature is preferably 1443K to 1343K.

[0047] The melting step may be carried out, for example, by filling a Tammann tube with the mixture, placing the Tammann tube filled with the mixture in a quartz tube, and then heating the quartz tube. During this process, the quartz tube and the Tammann tube are filled with an inert gas (e.g., Ar gas). The rate of temperature rise to the melting temperature is not particularly limited, but it is preferable to control it so that overshoot does not occur. The inside of the Tammann tube may be coated with BN as a release agent. To prevent Mg from volatilizing from the Tammann tube during the melting step, the space between the Tammann tube and the lid sealing the Tammann tube may be filled with BN to seal the tube.

[0048] (cooling process) In the cooling step according to this embodiment, after the mixture is melted, the mixture is cooled so that the pressure p [atm] of the inert gas satisfies the above formula (10) and the average cooling rate from the melting temperature to the slow-cooling stop temperature is 16.7°C / h or less.

[0049] In the cooling step, the pressure p [atm] of the inert gas satisfies the above formula (10). When the inert gas pressure p satisfies the above formula (10), the ratio of vacancy defects in Mg can be improved, and the thermal conductivity of the thermoelectric material can be reduced. It is more preferable that the inert gas pressure p satisfies the following formula (22). When the inert gas pressure p satisfies the following formula (22), the ratio α of vacancy defects in Mg can be improved, and the thermal conductivity can be further reduced. The inert gas pressure p may be controlled from the melting step onwards. 0.2≦p≦48 (22)

[0050] The slow cooling stop temperature in the cooling step is preferably 1443 K to 1343 K. If the slow cooling stop temperature is in this range, the ratio α of vacancy defects in Mg can be improved and the thermal conductivity can be further reduced. However, the slow cooling stop temperature may be lower than the slow cooling stop temperature of 1343 K.

[0051] In the cooling step, the average cooling rate from the melting temperature to the cooling stop temperature after melting is 16.7 K / h or less. By setting the average cooling rate to 16.7 K / h or less, the crystallinity of the thermoelectric material can be made into a single crystal. Since the proportion of vacancy defects increases as the cooling time becomes shorter, the average cooling rate is preferably 2 K / h or more.

[0052] The technical scope of the present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the spirit of the present invention. [Example]

[0053] Next, examples of the present invention will be described, but the conditions in the examples are merely examples adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to these examples. Various conditions can be adopted in the present invention as long as they do not deviate from the gist of the present invention and the object of the present invention is achieved.

[0054] Example 1 Mg (particle size 4 mm, purity 4N), Ge (particle size 150 μm, purity 4N), and Li (rod-shaped, purity 3N) were weighed to a molar ratio (Mg:Ge:Li) of 1.99:1.0:0.01 and mixed in a mortar. Li was added as a dopant. The resulting mixture was filled into a BN-coated Tammann tube (made of alumina), which was then placed inside a quartz tube. Ar was then filled into the tube, and the Ar pressure (Ar partial pressure) was controlled to 1.3 atm. The mixture was melted at 1443 K and slowly cooled to 1343 K over 48 hours. The mixture was then cooled to room temperature, yielding a single crystal of the thermoelectric material of Example 1.

[0055] Example 2 Mg (particle size 4 mm, purity 4N), Ge (particle size 150 μm, purity 4N), and Li (rod-shaped, purity 3N) were weighed to a molar ratio (Mg:Ge:Li) of 1.97:1.0:0.03 and mixed in a mortar. Li was added as a dopant. The resulting mixture was filled into a BN-coated Tammann tube (made of alumina), which was then placed inside a quartz tube. Ar was added, and the Ar pressure (Ar partial pressure) was controlled at 1.3 atm. The mixture was melted at 1443 K and slowly cooled to 1343 K over 48 hours. It was then cooled to room temperature, yielding a single crystal of the thermoelectric material of Example 2.

[0056] Example 3 Mg (particle size 4 mm, purity 4N), Ge (particle size 150 μm, purity 4N), and Li (rod-shaped, purity 3N) were weighed out to a molar ratio (Mg:Ge:Li) of 1.95:1.0:0.05 and mixed in a mortar. Li was added as a doping element. The resulting mixture was filled into a BN-coated Tammann tube (made of alumina), which was then placed inside a quartz tube. Ar was filled into the tube, and the Ar pressure (Ar partial pressure) was controlled to 1.3 atm. The mixture was melted at 1443 K and slowly cooled to 1343 K over 48 hours. After that, it was cooled to room temperature and used as the thermoelectric material of Example 3 (Mg 1.85 Ge 1.00 Li 0.05 ) single crystal was obtained.

[0057] Example 4 Mg (particle size 4 mm, purity 4N), Ge (particle size 150 μm, purity 4N), and Li (rod-shaped, purity 3N) were weighed out so that the molar ratio (Mg:Ge:Li) was 1.90:1.0:0.10, and mixed in a mortar. Here, Li was added as a doping element. The resulting mixture was filled into a BN-coated Tammann tube (made of alumina), and the Tammann tube was placed inside a quartz tube. At this time, Ar was filled, and the Ar pressure (Ar partial pressure) was controlled to 1.3 atm. The mixture was melted at 1443 K and slowly cooled to 1343 K over 48 hours. Thereafter, it was cooled to room temperature, and the thermoelectric material of Example 4 (Mg 1.83 Ge 1.00 Li 0.10 ) single crystal was obtained.

[0058] (Comparative Example 1) Mg (particle size 4 mm, purity 4N) and Ge powder (particle size 150 μm, purity 4N) were weighed and mixed so that the molar ratio (Mg:Ge) was 2:1. The resulting mixture was filled into a Tammann tube (made of alumina), and the Tammann tube was then filled into a quartz tube. At this time, Ar was filled and the Ar pressure (Ar partial pressure) was controlled to 1.3 atm. The mixture was melted at 1443 K and slowly cooled to 1343 K over 48 hours. Thereafter, it was cooled to room temperature and the thermoelectric material of Comparative Example 1 (Mg 1.90 Single crystals of Ge were obtained.

[0059] (Comparative Example 2) As a thermoelectric material of Comparative Example 2, the results of Non-Patent Document 1 are shown. According to Non-Patent Document 2, Mg (turnings), Ge (purity 2N), and Li (purity >99.5%) were weighed out to a molar ratio of 1.95:1:0.05, and the mixture was made into fine powder. The mixture was compressed (66 MPa) with a graphite punch and sintered at 650°C to form a thermoelectric material (Mg 1.95 Ge 1.00 Li 0.05 ) polycrystalline bodies have been reported to have been obtained.

[0060] (Seebeck coefficient S) The Seebeck coefficient S (μV / K) of the thermoelectric materials of Examples 1 to 4 and Comparative Example 1 was measured by the following method. -3 Measurements were carried out using a thermoelectric performance measuring device (OZAWA Scientific RZ2001i) at a constant temperature of 300K to 850K, using the steady-state direct current method. The results are shown in Tables 1 and 2 and Figure 2. The results in Table 1 are values at room temperature (300K), and the results in Table 2 are values at 800K.

[0061] (Conductivity σ) The conductivity σ (S / cm) of the thermoelectric materials of Examples 1 to 4 and Comparative Example 1 was measured by the following method. -3Measurements were carried out using a thermoelectric performance measuring device (OZAWA Scientific RZ2001i) at temperatures of 300K to 850K using the DC four-terminal method. The results are shown in Tables 1 and 2 and Figure 3. The results in Table 1 are values at room temperature (300K), and the results in Table 2 are values at 800K.

[0062] (thermal conductivity κ) The thermal conductivity κ (W / Km) of the thermoelectric materials of Examples 1 to 4 and Comparative Example 1 was measured by the following method. -3 Measurements were carried out by the laser flash method using a thermal constant measurement device (Netzsch TC-7000H) at temperatures of 300K to 850K (Pa). The results are shown in Tables 1 and 2 and Figure 4. The results in Table 1 are for room temperature (300K), and the results in Table 2 are for 800K.

[0063] (carrier density n and carrier mobility μ) The carrier density n(10 20 ×cm -3 ) and carrier mobility (cm 2 The sample size was 2.5 mm x 6 mm x 0.8 mm, and measurements were carried out in a vacuum (up to 1 Pa), at a measurement temperature of 300 K, in a magnetic field range of -5 T to 5 T, using a physical property measurement device (Quantum Design PPMS) and the Hall effect measurement method. The carrier density n and carrier mobility μ were measured using the Hall coefficient R measured by the Hall effect measurement. H and the elementary charge e, the values were calculated using the following equations (25) and (26). The results are shown in Figure 5 and Table 1. The results in Table 1 are values at 300K. n=|1 / (eR H )|···(25) μ=σR H ···(26)

[0064] (Power factor PF and dimensionless figure of merit zT) The power factor PF and dimensionless figure of merit zT of the thermoelectric materials of Examples 1 to 4 and Comparative Example 1 were calculated using the following equations (27) and (28). In the following equations (27) and (28), S represents the Seebeck coefficient, σ represents electrical conductivity, T represents absolute temperature, and κ represents thermal conductivity. The results are shown in Tables 1 and 2. The temperature dependence of zT is shown in Figure 6. The results in Table 1 are values at room temperature (300 K), and the results in Table 2 are values at 800 K. zT=S 2 σT / κ (27) PF=S 2 σ (28)

[0065] (Determination of single crystallinity and lattice constants) The crystallinity and lattice constant of the thermoelectric materials of Examples 1 to 4 and Comparative Example 1 were determined by the back reflection Laue method using X-rays using a D8 QUEST manufactured by Bruker, and the results are shown in Table 1. The numbers in parentheses indicate the standard deviation.

[0066] (Mg vacancy defect ratio) The ratio of Mg vacancy defects in the thermoelectric materials of Examples 1 to 4 and Comparative Example 1 was measured as follows. Small crystals measuring several tens of micrometers or less were collected from the thermoelectric materials of Examples 1 to 4 and Comparative Example 1. Single-crystal X-ray diffraction was performed on the obtained crystals. Single-crystal structure analysis was performed using the diffraction intensity and measurement angle information obtained by X-ray diffraction to determine the occupancy rates of Mg and Ge. If there are vacancies (vacancy defects), the occupancy rate of Mg or Ge will be less than 100%. The average vacancy amount α% was defined as the amount of Mg vacancy defects. The results are shown in Table 1. The numbers in parentheses indicate standard deviations. In the case of Mg2Ge in Comparative Example 1, the molar ratio (Mg:Ge) of the thermoelectric material was 1.90:1. The molar ratio (Mg:Ge:Li) of the thermoelectric material of Example 4 was 1.83:1.00:0.10.

[0067] Tiny crystals measuring less than several tens of micrometers were extracted from the thermoelectric material. The extracted sample was observed using a transmission electron microscope (TEM) at 500,000x magnification to obtain a TEM image. A fast Fourier transform (FFT) was performed on the TEM image obtained from the observation to obtain an FFT image. An inverse FFT image of the same size (16nm x 16nm) as the TEM image was created from the moiré spots seen in the FFT pattern. Edge dislocations were identified as areas in the image where the number of lines observed increased or decreased. The density of each edge dislocation was calculated for four inverse FFT images, and the average value was taken as the edge dislocation density. If there was one or more edge dislocations in the image, it was considered to be present. The results are shown in Table 1. The numbers in parentheses indicate the standard deviation.

[0068] [Table 1]

[0069] [Table 2]

[0070] As shown in Table 1, the thermoelectric materials of Examples 1 to 4 satisfied the requirements of the present invention, and therefore had high power factors PF and low thermal conductivities κ, resulting in high dimensionless figures of merit zT. On the other hand, the thermoelectric materials of Comparative Examples 1 and 2 did not satisfy the requirements of the present invention, and therefore had low PFs and inferior dimensionless figures of merit zT.

[0071] Figure 2 shows the measurement results of the temperature dependence of the Seebeck coefficient of the thermoelectric materials of Examples 1 to 4 and Comparative Examples 1 and 2. The vertical axis of Figure 2 represents the Seebeck coefficient (µV / K), and the horizontal axis represents the temperature T (K). 2, the thermoelectric material of Comparative Example 1 had a negative Seebeck coefficient, while the thermoelectric materials of Examples 1 to 4 had positive Seebeck coefficients. The Seebeck coefficient tended to decrease as the Li content increased.

[0072] Figure 3 shows the measurement results of the temperature dependence of the electrical conductivity of the thermoelectric materials of Examples 1 to 4 and Comparative Examples 1 and 2. The vertical axis of Figure 3 represents electrical conductivity (S / cm), and the horizontal axis represents temperature T (K). 3, the thermoelectric materials of Examples 1 to 4 had higher electrical conductivities than the thermoelectric material of Comparative Example 1. The thermoelectric materials of Examples 3 and 4 had higher electrical conductivities than the thermoelectric material of Comparative Example 2. The thermoelectric material of Example 1 had a decrease in electrical conductivity at higher temperatures, but the electrical conductivities of the thermoelectric materials of Examples 2 to 4 increased above 650 K.

[0073] FIG. 4 shows the measurement results of the temperature dependence of the thermal conductivity of the thermoelectric materials of Examples 1 to 4 and Comparative Examples 1 and 2. The vertical axis of FIG. 4 represents thermal conductivity (W / mK), and the horizontal axis represents temperature T (K). The thermal conductivity decreased as the Li content increased. Furthermore, the thermal conductivity decreased as the temperature increased. The thermal conductivity of the thermoelectric material of Example 4 was lower than that of Comparative Examples 1 and 2.

[0074] FIG. 5 shows the measurement results of the mobility and carrier density of the thermoelectric materials of Examples 1 to 4 and Comparative Example 1. The vertical axis of FIG. 5 represents the mobility (cm 2 / Vs, and the horizontal axis is the carrier density (cm -3 ) As the Li content increased, the mobility decreased and the carrier density increased. The thermoelectric materials of Examples 1 to 4 had higher mobilities than the thermoelectric material of Comparative Example 2.

[0075] FIG. 6 shows the measurement results of the dimensionless figure of merit zT of the thermoelectric materials of Examples 1 to 4 and Comparative Example 1. The vertical axis of FIG. 6 represents the dimensionless figure of merit zT, and the horizontal axis represents the temperature T (K). As shown in FIG. 6, the dimensionless figure of merit zT increased as the Li concentration increased. In particular, the dimensionless figure of merit zT of the thermoelectric material of Example 4 exhibited a high value of 1.0 at 850 K. This is thought to be due to increased phonon scattering caused by vacancy defects, doping elements, and edge dislocations. [Industrial Applicability]

[0076] The thermoelectric material according to this embodiment has low toxicity, is inexpensive, and has an excellent dimensionless figure of merit zT at high temperatures, and therefore has high industrial applicability.

Claims

1. Mg and Ge and a doping element X for doping the carrier; Contains A single crystal body consisting of a single crystal or a single crystal body consisting of a single crystal containing precipitates of different phases, α, which is the ratio of Mg vacancy defects, is more than 0% and 20.0% or less, A thermoelectric material with edge dislocations.

2. 2. The thermoelectric material according to claim 1, wherein a molar ratio (Mg:Ge:X) of the Mg, the Ge, and the doping element X satisfies the following formula (1) or (2), and also satisfies the following formula (3): Mg:Ge:X=2×(100-α) / 100:1-a:a...(1) Mg:Ge:X=2×(100-α) / 100-a:1:a...(2) 0<a≦0.12 (3)

3. 3. The thermoelectric material according to claim 1, wherein the doping element X is at least one element selected from the group consisting of Li, Cu, Ga, Ag, Sn, Sb, and Bi.

4. 3. The thermoelectric material according to claim 1, further comprising a pressure-applying element Z having an atomic radius smaller than the atomic radius of any one of the Mg, the Ge, and the doping element X.

5. The thermoelectric material according to claim 4 , wherein the pressure-applying element Z is at least one selected from the group consisting of B and Si.

6. 6. The thermoelectric material according to claim 5, wherein a molar ratio (Mg:Ge:X:Z) of the Mg, the Ge, the doping element X, and the pressure-applying element Z satisfies any one of the following formulas (4) to (7), and also satisfies the following formulas (8) and (9): Mg: Ge: Mg:Ge:X:Z=2×(100-α) / 100-a:1-b:a:b...(6) Mg:Ge:X:Z=2×(100-α) / 100-b:1-a:a:b...(7) 0 < a ≦ 0.12 (8) 0<b≦0.05 (9)

7. The density of the edge dislocations is 1.0 × 10 １６ ~1.0 x 10 １８ m ―２ The thermoelectric material according to claim 1 or 2,

8. Mg and Ge and a doping element X for doping the carrier; After mixing at least the above to prepare a mixture, Melting the mixture by heating the mixture in an inert gas until the temperature of the mixture reaches a melting temperature; After the mixture is melted, the mixture is cooled so that the pressure p [atm] of the inert gas satisfies the following formula (10), and the temperature of the mixture is cooled so that the average cooling rate from the melting temperature to a slow-cooling stop temperature is 16.7 K / h or less. 0≦p≦48 (10)

9. 9. The method for producing a thermoelectric material according to claim 8, wherein a molar ratio (Mg:Ge:X) of the Mg to the Ge to the doping element X satisfies the following formula (11) or (12), and also satisfies the following formulas (13) and (14): Mg:Ge:X=d:1-a:a...(11) Mg:Ge:X=da:1:a...(12) 1.8≦d≦2.2 (13) 0 < a ≦ 0.12 (14)

10. 10. The method for producing a thermoelectric material according to claim 9, wherein the doping element X is at least one element selected from the group consisting of Li, Cu, Ga, Ag, Sn, Sb, and Bi.

11. 10. The method for producing a thermoelectric material according to claim 8, wherein the mixture further contains a pressure-applying element Z having an atomic radius smaller than that of any one of the Mg, the Ge, and the doping element X.

12. The method for producing a thermoelectric material according to claim 11 , wherein the pressure-applying element Z is at least one selected from the group consisting of B and Si.

13. 13. The method for producing a thermoelectric material according to claim 12, wherein a molar ratio (Mg:Ge:X:Z) of the Mg, the Ge, the doping element X, and the pressure-applying element Z satisfies any one of the following formulas (15) to (18), and also satisfies the following formulas (19), (20), and (21): Mg:Ge:X:Z=d:1-ab:a:b...(15) Mg:Ge:X:Z=d-a-b:1:a:b...(16) Mg:Ge:X:Z=da:1-b:a:b...(17) Mg:Ge:X:Z=d-b:1-a:a:b...(18) 1.8≦d≦2.2 (19) 0 < a ≦ 0.12 (20) 0 < b ≦ 0.05 (21)

14. The method for producing a thermoelectric material according to claim 8 or 9, wherein the melting temperature is 1443K to 1343K.

15. The method for producing a thermoelectric material according to claim 8 or 9, wherein the slow cooling stop temperature is 1434K to 1173K.

16. The method for producing a thermoelectric material according to claim 8 or 9, wherein the pressure p satisfies the following formula (22): 0.2≦p≦48・・・(22)

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