Thermoelectric material, horizontal thermoelectric module, and method for manufacturing thermoelectric material

Abstract

To provide a novel thermoelectric material that exhibits goniopolarity in a low-temperature region and is made from inexpensive raw materials, and to provide a horizontal thermoelectric module using this thermoelectric material and a method for manufacturing this thermoelectric material. The thermoelectric material is represented by the following formula (1), and the in-plane Seebeck coefficient (S IP ) and the interfacial Seebeck coefficient (S CP ) is 20 μV / K or more. Mg 3-x+d Y x Bi 2-y Sb y (1) (In formula (1), 0.00≦x≦1.00, 0.05≦y≦1.00, and −0.10≦d≦1.00.)

Description

[Technical Field]

[0001] The present disclosure relates to novel thermoelectric materials, lateral thermoelectric modules, and methods for manufacturing thermoelectric materials. [Background technology]

[0002] Thermoelectric materials are materials that can extract electricity by creating a temperature difference. Because they have no moving parts and are less prone to breakdowns, they are attracting attention as a maintenance-free waste heat recovery technology. The performance of thermoelectric materials can be expressed by the dimensionless figure of merit ZT, calculated using the following formula (A), and a higher value of this is considered to indicate superior characteristics. Thermoelectric modules can be constructed using such thermoelectric materials. ZT=S 2 T / ρκ (A) [S: Seebeck coefficient (V / K), T: temperature (K), ρ: electrical resistivity (Ωm), κ: thermal conductivity (w / m K)]

[0003] Conventional thermoelectric modules have a "vertical structure" in which the direction of the temperature difference and the direction of power generation are the same, and generate power by bringing the interface between the thermoelectric material and the electrode into contact with a high-temperature heat source. However, reactions such as element diffusion occur at the interface, making the thermoelectric module susceptible to degradation.

[0004] On the other hand, materials in which the carrier polarity (p-type or n-type) changes depending on the direction are called "goniopolar materials." When goniopolar materials are used as thermoelectric materials, it is possible to construct a thermoelectric module with a "lateral structure" in which the direction of the temperature difference and the direction of power generation are perpendicular to each other.

[0005] As a goniopolar material, Re4Si7 has a lateral figure of merit Z xy It is known that the transverse Seebeck coefficient S xy can be calculated using the following formula (B), and the lateral performance index Z xy T can be calculated using the following formula (C). S xy =(S CP -S IP )sin(θ)cos(θ) (B) Z xy T=(S xy ) 2 T / ρ xx κ yy (C) [S CP : Interfacial Seebeck coefficient (V / K), S IP : in-plane Seebeck coefficient (V / K), θ: angle between the in-plane direction (CP) and the y direction, T: temperature (K), ρ xx : Transverse electrical resistivity (Ωm), κ yy : longitudinal thermal conductivity (w / m K)]

[0006] However, Re4Si7 exhibits high performance in the high-temperature region (700°C), and rhenium is an expensive metallic raw material. Therefore, there is a need to develop a thermoelectric material that exhibits performance in lower temperature regions and is made from inexpensive raw materials. Materials using Mg3Sb2 single crystals, which are cheaper than Re, have also been reported, but they do not exhibit goniopolarity (Non-Patent Documents 2-3). [Prior art documents] [Non-patent literature]

[0007] [Non-Patent Document 1] Scudder, M. et al., “Highly efficient transverse thermoelectric devices with Re4Si7 crystals”, Energy & Environmental Science, 14, 4009 (2021) [Non-patent document 2] Li, A. et al., “Demonstration of valley anisotropy utilized to enhance the thermoelectric power factor.”, Nature Communications, 12, 5408 (2021) [Non-patent document 3] Jin, M. et al., “Nearly isotropic transport properties in anisotropically structured n-type single-crystalline Mg3Sb2.”, Materials Today Physics, 21, 100508 (2021) Summary of the Invention [Problem to be solved by the invention]

[0008] The present disclosure has been made in consideration of the above-described situation in the conventional technology, and aims to provide a novel thermoelectric material that exhibits goniopolarity in a low temperature range and is made from inexpensive raw materials, as well as to provide a horizontal thermoelectric module using this thermoelectric material and a method for manufacturing this thermoelectric material. [Means for solving the problem]

[0009] The present inventors have conducted extensive research to solve the above problems, and as a result have found that the following thermoelectric material can solve the above problems, thereby completing the present invention.

[0010] That is, the present disclosure relates to the following: [1] A thermoelectric material comprising: The thermoelectric material is represented by the following formula (1): Mg 3-x+d Y x Bi 2-y Sb y (1) (In formula (1), 0.00≦x≦1.00, 0.05≦y≦1.00, and −0.10≦d≦1.00.) The in-plane Seebeck coefficient (S IP ) and the interfacial Seebeck coefficient (S CP ) is 20 μV / K or more. [2] Interfacial electrical resistivity (ρ CP) is 300 [μΩm] or less. [3] The thermoelectric material according to [1] or [2], which is polycrystalline. [4] The thermoelectric material according to any one of [1] to [3], which is for use in a horizontal thermoelectric module. [5] A horizontal thermoelectric module using the thermoelectric material according to any one of [1] to [4]. [6] A method for producing a thermoelectric material according to any one of [1] to [4], The manufacturing method comprises: a melting process in which a mixture of elemental metals is heated to 830°C or higher to obtain a molten material; a crystallization step of cooling the melt at a rate of 0.5 to 5.5°C / hour to crystallize it; and The method for producing a thermoelectric material, wherein the mixture of elemental metals contains magnesium, antimony, and bismuth. [7] The method for producing a thermoelectric material according to [6], wherein the content of the magnesium in the mixture of elemental metals is 1.005 to 1.100 times, on a molar basis, the chemical composition represented by formula (1). [8] The method for producing a thermoelectric material according to [6] or [7], wherein the mixture of elemental metals further contains yttrium. [9] The method for producing a thermoelectric material according to any one of [6] to [8], wherein one or more steps selected from the group consisting of the melting step and the crystallization step are carried out in a state where the mixture of elemental metals or the melt is placed in a graphite crucible.

[10] The method for producing a thermoelectric material according to any one of [6] to [9], wherein one or more steps selected from the group consisting of the melting step and the crystallization step are carried out in an inert gas atmosphere. [Effects of the Invention]

[0011] The present disclosure provides a novel thermoelectric material that exhibits goniopolarity in a low temperature range and is made from inexpensive raw materials, as well as a horizontal thermoelectric module using this thermoelectric material and a method for manufacturing this thermoelectric material. DETAILED DESCRIPTION OF THE INVENTION

[0012] Each embodiment of the present disclosure will be described. Note that each embodiment described below does not limit the invention according to the claims, and not all of the elements and combinations thereof described in each embodiment are necessarily essential to the solution of the present disclosure.

[0013] In this specification, unless otherwise specified, the expressions "X or more and Y or less" or "X to Y" representing a numerical range mean a numerical range including the lower and upper limits, which are the endpoints. When a numerical range is described in stages, the upper and lower limits of each numerical range can be combined in any way.

[0014] [Thermoelectric materials] A thermoelectric material according to an embodiment of the present disclosure (hereinafter also simply referred to as a "thermoelectric material") is a thermoelectric material represented by the following formula (1). Mg 3-x+d Y x Bi 2-y Sb y (1) (In formula (1), 0.00≦x≦1.00, 0.05≦y≦1.00, and −0.10≦d≦1.00.)

[0015] In formula (1), x preferably satisfies 0.00≦x≦0.50, more preferably 0.00≦x≦0.10. Furthermore, x further preferably satisfies x=0.00. As the value of x increases, it becomes possible to control the Fermi level in formula (1) toward the electron doping side, and within the above range, it becomes easier to achieve both low electrical resistivity and a high Seebeck coefficient. In formula (1), y preferably satisfies 0.10≦y≦0.80, more preferably 0.30≦y≦0.70, and even more preferably 0.40≦y≦0.60. As the value of y decreases, the band gap widens, and the electrical resistivity and Seebeck coefficient tend to increase. Within the above range, it becomes easier to achieve both low electrical resistivity and a high Seebeck coefficient. In formula (1), d preferably satisfies 0.00≦d≦1.00, more preferably 0.00≦d≦0.50, and even more preferably 0.00≦d≦0.30. Furthermore, d particularly preferably satisfies d=0.03. As the value of d increases, it becomes possible to control the Fermi level toward the electron doping side. Within the above range, it becomes easier to achieve both low electrical resistivity and a high Seebeck coefficient.

[0016] The thermoelectric material represented by formula (1) can be produced by heating and then cooling elemental metals of the elements contained in the chemical composition of formula (1) as starting materials, as described in the "Method for producing a thermoelectric material" below. The chemical composition of the thermoelectric material can be measured using an X-ray fluorescence analyzer. Magnesium, yttrium, antimony, and bismuth, which can be used as constituent elements, are all inexpensive metal raw materials.

[0017] The ratio of the constituent elements can be controlled by adjusting the amount of elemental metals used as starting materials, as will be described in detail later.

[0018] The in-plane Seebeck coefficient (S IP ) and the interfacial Seebeck coefficient (S CP The absolute value of the difference between the Seebeck coefficient (hereinafter referred to as the "Seebeck coefficient difference") and the goniopolarity is 20 μV / K or more. The larger the Seebeck coefficient difference, the larger the difference in carrier polarity depending on the direction in the material, i.e., the higher the goniopolarity. The fact that the Seebeck coefficient difference at a measurement temperature of 100°C is within the above range indicates that the thermoelectric material has high performance at 100°C. For example, when using a thermoelectric material to extract electricity from waste heat in a factory, the waste heat is typically in the temperature range of 100 to 300°C. Therefore, the thermoelectric material disclosed herein can suitably extract electricity from such waste heat.

[0019] The absolute value of the Seebeck coefficient difference is preferably 40 [μV / K] or more, more preferably 100 [μV / K] or more, even more preferably 150 [μV / K] or more, even more preferably 200 [μV / K] or more, and particularly preferably 250 [μV / K] or more. The upper limit of the absolute value of the Seebeck coefficient difference is not particularly limited, but it is preferable to appropriately control the electrical resistivity and obtain a high transverse figure of merit Z. xy From the viewpoint of obtaining T, the value is preferably 1500 [μV / K] or less, and more preferably 600 [μV / K] or less.

[0020] The absolute value of the Seebeck coefficient difference may be, for example, 20 to 1500 [μV / K], 40 to 1500 [μV / K], 100 to 1500 [μV / K], 150 to 1500 [μV / K], 200 to 1500 [μV / K], or 200 to 600 [μV / K].

[0021] The absolute value of the Seebeck coefficient difference can be adjusted, for example, by changing the starting composition of the thermoelectric material or by changing the cooling rate in the crystallization process described below. Specifically, by increasing the cooling rate, it is possible to control the Fermi level toward the electron doping side. On the other hand, by decreasing the cooling rate, it is possible to control the Fermi level toward the hole doping side.

[0022] In-plane Seebeck coefficient (S IP The value of is not particularly limited, but may be, for example, −250 to 100 μV / K, −200 to 50 μV / K, or −150 to −30 μV / K. Interfacial Seebeck coefficient (S CPThe value of is not particularly limited, but may be, for example, −200 to 150 μV / K, −150 to 150 μV / K, or 50 to 150 μV / K.

[0023] S IP and S CP However, it is preferable that the following formula (X) is satisfied. S IP ×S CP <0 (X) S IP and S CP However, satisfying the above formula (X) means that S IP is positive and S CP is negative, or S IP is negative and S CP A positive Seebeck coefficient indicates that the thermoelectric material behaves as a p-type semiconductor, and a negative Seebeck coefficient indicates that the thermoelectric material behaves as an n-type semiconductor. IP and S CP However, satisfying the above formula (X) means that the extremely unique property of the material is realized, for example, that it behaves as an n-type semiconductor in the in-plane (IP) direction and as a p-type semiconductor in the inter-plane (CP) direction. In this case, it becomes possible to obtain a high transverse Seebeck coefficient according to formula (B).

[0024] The thermoelectric material may be single crystal or polycrystalline. The thermoelectric material is preferably polycrystalline. Polycrystalline is more suitable for producing a thermoelectric module. The thermoelectric material of the present disclosure tends to have a large absolute value of the difference in Seebeck coefficients even when it is polycrystalline. This is because the thermoelectric material of the present disclosure has a layered crystal structure. This is because crystal growth reflecting the layered structure progresses during the crystallization process, and it is possible to increase the absolute value of the difference in Seebeck coefficients even when it is polycrystalline. To make a thermoelectric material polycrystalline, for example, as described in [Method of manufacturing thermoelectric materials] below, In another method, a mixture of elemental metals is heated at an appropriate temperature to obtain a melt. In another method, the cooling rate in the crystallization step is set within the range described below.

[0025] The inter-plane electrical resistivity (ρ CP ) is preferably 300 [μΩm] or less, more preferably 200 [μΩm] or less, and even more preferably 150 [μΩm] or less. CP When is in the above range, a higher lateral performance index Z xy You can get T. ρ CP The lower limit of ρ is not particularly limited, CP The range is, for example, 1×10 -2 ~300[μΩm], 1×10 -1 It may be up to 200 [μΩm], or 1 to 150 [μΩm]. ρ CP can be adjusted, for example, by adjusting the magnesium content in the mixture of elemental metals or by adjusting the yttrium content.

[0026] The in-plane electrical resistivity (ρ IP ) is preferably 300 [μΩm] or less, more preferably 200 [μΩm] or less, and even more preferably 150 [μΩm] or less. IP The lower limit of ρ is not particularly limited, IP The range is, for example, 1×10 -2 ~300[μΩm], 1×10 -1 It may be up to 200 [μΩm], or 1 to 150 [μΩm].

[0027] [Thermoelectric module] The thermoelectric material according to the present embodiment can be used for a thermoelectric module. Furthermore, since the thermoelectric material according to the present embodiment is a goniopolar material, it can be used to construct a thermoelectric module with a "horizontal structure" in which the temperature difference direction and the power generation direction are perpendicular to each other, i.e., it can be used for a horizontal thermoelectric module. The horizontal thermoelectric module according to the present disclosure can use the thermoelectric material according to the present disclosure.

[0028] [Method of manufacturing thermoelectric materials] The method for producing a thermoelectric material is not particularly limited, but preferably includes a melting step in which a mixture of elemental metals is heated to 830°C or higher to obtain a melt, and a crystallization step in which the melt is cooled at a rate of 0.5 to 5.5°C / hour to crystallize it. The mixture of elemental metals preferably contains magnesium, antimony, and bismuth, where magnesium, antimony, and bismuth are elemental metals.

[0029] The method for obtaining the mixture of elemental metals is not particularly limited, and for example, the mixture of elemental metals can be obtained by mixing elemental metals of the elements included in the chemical composition of the above formula (1) by placing them in a crucible, etc. In other words, the method for producing a thermoelectric material preferably includes a step of obtaining a mixture of elemental metals.

[0030] In the melting step, the mixture of elemental metals is heated to 830°C or higher to obtain a molten material. By heating the mixture of elemental metals to 830°C or higher, the elemental metals are easily melted. The temperature in the melting step may be, for example, 830 to 1000° C., or 830 to 900° C. The time for the melting step is not particularly limited, and may be, for example, 1 to 72 hours, 12 to 72 hours, or 24 to 72 hours.

[0031] In the crystallization step, the melt obtained in the melting step is cooled at a rate of 0.5 to 5.5°C / hour to crystallize it. A cooling rate of 0.5 to 5.5°C / hour facilitates the absolute value of the Seebeck coefficient difference to fall within the above range. A cooling rate of 5.5°C / hour or less facilitates crystal growth, making it easier to increase the absolute value of the Seebeck coefficient difference even in polycrystals. A slower cooling rate facilitates crystal growth and makes it possible to control the Fermi level toward the hole doping side. A cooling rate of 0.5°C / hour or less significantly shifts the Fermi level toward the hole doping side, resulting in a small absolute value of the Seebeck coefficient difference. The cooling rate is preferably 1.0 to 5.0°C / hour, more preferably 1.5 to 4.0°C / hour, and even more preferably 1.8 to 2.5°C / hour. The cooling temperature of the melt in the crystallization step is not particularly limited as long as the melt crystallizes, but it is preferable to cool the melt to 660°C. Alternatively, the melt may be cooled to 630°C or 600°C. By cooling to the above temperatures, the melt is more likely to crystallize.

[0032] The melting step is preferably carried out with the mixture of elemental metals placed in a graphite crucible. The crystallization step is preferably carried out with the molten material placed in a graphite crucible. That is, it is preferable that one or more steps selected from the group consisting of the melting step and the crystallization step be carried out with the mixture or molten material of elemental metals placed in a graphite crucible. Graphite crucibles have a low reactivity with magnesium, which allows for a more precise stoichiometry of the resulting thermoelectric material.

[0033] The atmosphere in which the melting step and the crystallization step are performed is not particularly limited, but it is preferable that one or more steps selected from the group consisting of the melting step and the crystallization step are performed in an inert gas atmosphere, and it is more preferable that the melting step and the crystallization step are performed in an inert gas atmosphere. By performing the steps in an inert gas atmosphere, it becomes easier to control the evaporation of magnesium. The inert gas is not particularly limited, and known inert gases such as nitrogen gas and argon gas can be used, with argon gas being preferred. The pressure of the inert gas in the inert gas atmosphere is not particularly limited, and is 1×10 0 ~2×10 5 It can be Pa.

[0034] The magnesium content in the mixture of elemental metals is not particularly limited, but is preferably 1.005 to 1.100 times, and more preferably 1.010 to 1.050 times, on a molar basis relative to the chemical composition represented by the above formula (1). Because magnesium has a low boiling point, it may evaporate during the melting and crystallization processes. Therefore, by setting the magnesium content in the elemental metals within the above range, it becomes easier to obtain the thermoelectric material represented by the above formula (1). Furthermore, by performing the crystallization process in an inert gas atmosphere, the amount of magnesium evaporation can be suppressed, making it easier to set the absolute value of the Seebeck coefficient difference within the above range.

[0035] The mixture of elemental metals can further contain yttrium. When the mixture of elemental metals contains yttrium, divalent magnesium is partially substituted with trivalent yttrium, which can shift the Fermi level to the electron-doped side. As a result, the absolute value of the Seebeck coefficient difference can be easily set within the above range. The content of yttrium in the mixture of elemental metals is not particularly limited, but may be 0.1 to 1.0 part by mol, or 0.1 to 0.5 part by mol, when the content of magnesium in the mixture of elemental metals is taken as 100 parts by mol.

[0036] The content of bismuth in the mixture of elemental metals is preferably equal to the chemical composition represented by the above formula (1). The content of antimony in the mixture of elemental metals is preferably equal to the chemical composition represented by the above formula (1).

[0037] The methods for measuring each physical property are described below. <Chemical composition of thermoelectric materials> It is determined by measuring using an X-ray fluorescence analyzer, for example, an energy dispersive X-ray analyzer.

[0038] <Confirmation of crystallinity> This can be confirmed by X-ray diffraction. For example, if X-ray diffraction is measured on the plate surface of a single crystal grown in a plate shape and only a diffraction peak with an 00l index is observed, it can be determined to be a single crystal grown on the ab plane. If diffraction peaks other than the 00l index are observed, it can be determined to be polycrystalline.

[0039] <In-plane Seebeck coefficient (S IP ) and interfacial Seebeck coefficient (S CP )> A temperature difference is applied to the sample by connecting a heater or other device, and the thermoelectric power generated in the direction of the temperature difference is measured. The Seebeck coefficient is calculated from the proportionality coefficient between the thermoelectric power and the temperature difference. A temperature difference is applied in the in-plane direction for in-plane measurements, and in the inter-plane direction for inter-plane measurements.

[0040] <In-plane electrical resistivity (ρ IP ) and interfacial electrical resistivity (ρ CP )> The four-terminal method is used. Direct or alternating current is applied to two terminals connected to both ends of the sample, and the voltage effect on the sample is measured using two other terminals to measure the electrical resistivity. [Example]

[0041] The present disclosure will be specifically described below using examples, but the scope of the present disclosure is not limited to the description of the examples.

[0042] <Examples and Comparative Examples> The starting materials used were 99.9% pure Mg (particle size 3-7 mm), 99.9999% pure Sb (shot), 99.9999% pure Bi (shot), and 99.9% pure Y (-20 mesh).

[0043] These were weighed out to obtain the respective charging compositions shown in Table 1. The weighed elemental metals were placed in a graphite crucible and sealed in a stainless steel tube under an argon atmosphere. After sealing, the mixture was heated at 890°C for 48 hours, cooled to 660°C at the cooling rate shown in Table 1, and then cooled to room temperature at a cooling rate of 200°C / hour to obtain thermoelectric materials. [Table 1]

[0044] The thermoelectric materials obtained were all polycrystalline. The physical properties of the obtained thermoelectric materials are shown in Table 2. [Table 2]

[0045] As shown in Table 2, the difference in the Seebeck coefficient in the examples was large, at 20 μV / K or more. In other words, it was found that a new thermoelectric material was obtained that exhibited goniopolarity in the low temperature region (100°C) and was made from inexpensive raw materials.

[0046] The transverse Seebeck coefficient S xy and lateral performance index Z xy T was calculated and is shown in Table 3. [Table 3] In the table, the results for Examples 1 and 3 were not measured. For example, 6.7E-03 means 6.7×10 -3 Shows.

Claims

1. A thermoelectric material comprising: The thermoelectric material is represented by the following formula (1): *| ３－ｘ＋ｄ ﹹ ｘ 3) ２－ｙ 3) ｙ () (In formula (1), 0.00≦x≦1.00, 0.05≦y≦1.00, and −0.10≦d≦1.00.) The in-plane Seebeck coefficient (S ＩＰ ) and the interfacial Seebeck coefficient (S ＣＰ ) the absolute value of the difference between the voltage Vcc and the resistance Vcc is 20 [μV / K] or more.

2. The inter-plane electrical resistivity (ρ ＣＰ 2. The thermoelectric material according to claim 1, wherein the resistivity is 300 [μΩm] or less.

3. The thermoelectric material of claim 1 which is polycrystalline.

4. The thermoelectric material according to claim 1 , which is for use in a horizontal thermoelectric module.

5. A horizontal thermoelectric module using the thermoelectric material according to any one of claims 1 to 4.

6. A method for producing the thermoelectric material according to any one of claims 1 to 4, comprising the steps of: The manufacturing method comprises: a melting step of heating a mixture of elemental metals to 830°C or higher to obtain a melt; a crystallization step of cooling the melt at a rate of 0.5 to 5.5°C / hour to crystallize the melt; and The method for producing a thermoelectric material, wherein the mixture of elemental metals contains magnesium, antimony, and bismuth.

7. 7. The method for producing a thermoelectric material according to claim 6, wherein the content of the magnesium in the mixture of elemental metals is 1.005 to 1.100 times, on a molar basis, the chemical composition represented by formula (1).

8. The method for producing a thermoelectric material according to claim 6 , wherein the mixture of elemental metals further contains yttrium.

9. 7. The method for producing a thermoelectric material according to claim 6, wherein the one or more steps selected from the group consisting of the melting step and the crystallization step are performed in a state where the mixture of elemental metals or the melt is placed in a graphite crucible.

10. The method for producing a thermoelectric material according to claim 6 , wherein one or more steps selected from the group consisting of the melting step and the crystallization step are carried out in an inert gas atmosphere.

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