Thermoelectric material, lateral 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 composed of inexpensive raw materials; a lateral thermoelectric module using the thermoelectric material; and a method for manufacturing the thermoelectric material.SOLUTION: A thermoelectric material represented by the general formula (1), Mg3-x+dYxSb2-yBiy, where 0≤x≤1, y=0 or 2, and -0.1≤d≤1, provided that if y=0 then 0<x≤0.1, and is a single crystal in which the difference (absolute value) between the in-plane Seebeck coefficient (SIP) and the cross-plane Seebeck coefficient (SCP) at the measurement temperature of 300 [K] is 20 [μV / K] or greater.SELECTED DRAWING: Figure 1

Description

[Technical field]

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

[0002] Thermoelectric materials are materials that can extract electricity by creating a temperature difference. Because they have no moving parts and are unlikely to break down, 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 by the following formula (A), and materials with a high value of this are considered to have excellent 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 contacting the interface between the thermoelectric material and the electrode with a high-temperature heat source. However, reactions such as element diffusion occur at the interface, which makes the thermoelectric module susceptible to degradation.

[0004] On the other hand, materials in which the carrier polarity (p-type and 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 "horizontal structure" in which the temperature difference direction and the power generation direction 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 (B). 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 shows high performance at high temperatures (700°C), and rhenium is an expensive metal raw material. Therefore, there is a demand for the development of a thermoelectric material that shows performance at lower temperatures and is made from inexpensive raw materials. Materials using Mg3Sb2 single crystals, which are cheaper than Re, have also been reported, but goniopolarity was not observed (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 invention has been made in view of the above-mentioned situation in the conventional technology, and has an object to provide a new thermoelectric material that exhibits goniopolarity in a low temperature range and is made from inexpensive raw materials, and further 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. As a result, they have discovered a thermoelectric material represented by a specific general formula (1), which is a single crystal and has an in-plane Seebeck coefficient (S IP ) and the interfacial Seebeck coefficient (S CP The inventors have found that the above problems can be solved by using a thermoelectric material in which the difference (absolute value) between the potential and the resistance is 20 [μV / K] or more, and have completed the present invention.

[0010] That is, the present disclosure relates to the following [1] to [6]. [1]: A thermoelectric material represented by the following general formula (1): Mg 3-x+d Y x Sb 2-y Bi y (1) (In the general formula (1), 0≦x≦1, y=0 or 2, and −0.1≦d≦1. When y=0, 0 <x≦0.1である) It is a single crystal, In-plane Seebeck coefficient (S IP ) and the interfacial Seebeck coefficient (SCP ) is 20μV / K or more (absolute value). [2]: In the general formula (1), when y = 0, the carrier density (n) at a measurement temperature of 300 [K] is -1.0 × 10 20 [cm -3 ] and more When y=2, the carrier density (n) at the measurement temperature of 300[K] is -5.0×10 19 [cm -3 ] or more 10.0 x 10 19 [cm -3 The thermoelectric material according to [1], wherein [3]: Interfacial electrical resistivity (ρ CP ) is 8.0 [Ωm] or less. [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], In the general formula (1), when y=0, a simple metal of an element included in the chemical composition of the general formula (1) is heated to 700° C. or more and 900° C. or less, and then cooled; A method for producing a thermoelectric material, comprising heating a simple metal of an element contained in the chemical composition of general formula (1) at y=2 to 620°C or higher and 830°C or lower, and then cooling the simple metal. Effect of the Invention

[0011] The present disclosure makes it possible to provide a novel thermoelectric material that exhibits goniopolarity in a low temperature range and is made from inexpensive raw materials, as well as a lateral thermoelectric module that uses this thermoelectric material and a method for producing this thermoelectric material. [Brief description of the drawings]

[0012] [Figure 1]FIG. 1A is a graph showing the relationship between the Seebeck coefficient and the carrier density for the thermoelectric materials obtained in Examples 1-1 to 1-5 and Comparative Example 1-1 of the present application, and FIG. 1B is a graph showing the relationship between the Seebeck coefficient and the electron carrier density for the thermoelectric materials obtained in Examples 2-1 to 2-2 and Comparative Example 2-1 of the present application. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Each embodiment of the present invention will be described. Note that each embodiment described below is based on claims. The present invention is not limited to the scope of the present invention, and all of the elements and combinations thereof described in each embodiment are not necessarily essential to the solution of the present invention.

[0014] In this specification, the description of "X or more and Y or less" or "X to Y" representing a numerical range means a numerical range including the lower and upper limits which are the endpoints, unless otherwise specified. When a numerical range is described in stages, the upper and lower limits of each numerical range can be arbitrarily combined.

[0015] In this specification, "carrier density" means "electron carrier density" or "hole carrier density". When the carrier density is a negative value, it means that the absolute value of the carrier density is the electron carrier density. When the carrier density is a positive value, it means that the carrier density is the hole carrier density.

[0016] [Thermoelectric materials] A thermoelectric material according to one embodiment of the present invention (hereinafter, simply referred to as a "thermoelectric material") is a thermoelectric material represented by the following general formula (1). Mg 3-x+d Y x Sb 2-y Bi y (1) (In the general formula (1), 0≦x≦1, 0≦y≦2, and −0.1≦d≦1, and when 0≦y<2, 0 <x≦0.1である)

[0017] In general formula (1), preferably, -0.5 ≦ d ≦ 0.5, when 0 ≦ y < 2, 0 < x ≦ 0.5, and when y = 2, 0 ≦ x ≦ 0.5. More preferably, y = 0 or 2, and x = d = 0.

[0018] The thermoelectric material represented by general formula (1) can be manufactured, for example, as described in [Manufacturing method of thermoelectric material] below, by heating and then cooling simple metals of the elements included in the chemical composition of general formula (1) as starting materials. The chemical composition of the thermoelectric material can be measured by a fluorescent X-ray analyzer. Magnesium, yttrium, antimony, and bismuth used as constituent elements are all inexpensive metal raw materials.

[0019] The ratio of the constituent elements can be controlled by adjusting the charged amount of the simple metal used as the starting material. Specifically, magnesium, yttrium, and bismuth are charged so that the chemical composition and the charged composition of general formula (1) are equal, and antimony is charged so that the charged composition is 3.0 to 4.0 times the chemical composition of general formula (1), whereby a thermoelectric material having a desired chemical composition can be obtained.

[0020] The thermoelectric material is a single crystal. Since the thermoelectric material is a single crystal, goniopolarity in which the carrier polarity (p-type and n-type) changes depending on the direction can be expressed in the material.

[0021] In order to make the thermoelectric material a single crystal, it can be manufactured, for example, as described in [Manufacturing method of thermoelectric material] below, by heating simple metals of each constituent element at an appropriate temperature. For example, in general formula (1), when y < 1, it is preferably heated at 700 °C or higher and 900 °C or lower, and when y ≧ 1, it is preferably heated at 620 °C or higher and 830 °C or lower. Also, in general formula (1), when 0 ≦ y < 2, by making the charged amount of antimony an excessive amount, the melting point of the mixture can be lowered and single crystal growth can be promoted.

[0022] The in-plane Seebeck coefficient (S IP ) and the inter-plane Seebeck coefficient (S CP) (absolute value) (hereinafter also referred to as the "Seebeck coefficient difference") 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, that is, the higher the goniopolarity.

[0023] The lower limit of the Seebeck coefficient difference of the thermoelectric material at a measurement temperature of 300 [K] is preferably 30 [μV / K] or more, more preferably 60 [μV / K] or more, and even more preferably 90 [μV / K] or more. The upper limit of the Seebeck coefficient difference is not particularly limited, but it is preferable that the electrical resistivity is appropriately controlled and a high transverse figure of merit Z xy From the viewpoint of obtaining T, the resistance is preferably 1500 [μV / K] or less, and more preferably 600 [μV / K] or less.

[0024] The Seebeck coefficient difference may be 20 to 1500 [μV / K], 30 to 1500 [μV / K], 60 to 300 [μV / K], or 90 to 300 [μV / K].

[0025] The difference in Seebeck coefficient can be adjusted, for example, by appropriately controlling the carrier density (n) of the thermoelectric material.

[0026] In general formula (1), when y<1, the carrier density (n) at a measurement temperature of 300[K] is -1.0×10 20 [cm -3

[0046] By controlling the Seebeck coefficient difference within the above range, the Seebeck coefficient difference can be appropriately adjusted. Specifically, by increasing the carrier density (n), the Seebeck coefficient difference can be increased.

[0027] In general formula (1), when y ≧ 1, the carrier density (n) at a measurement temperature of 300 [K] is -5.0 × 10 19 [cm -3 ] or more 10.0 x 10 19 [cm -3 ], the Seebeck coefficient difference can be appropriately adjusted. Specifically, the carrier density (n) can be controlled within a range of -5.0×1019 [cm -3 ] or more 3.1×10 19 [cm -3 ] or less, or 4.7 x 10 19 [cm -3 ] or more, the Seebeck coefficient difference can be made larger. 19 [cm -3 ] or more 4.7×10 19 [cm -3 ] The Seebeck coefficient difference can be made larger by making it smaller within the range below.

[0028] In the general formula (1), when y<1, the lower limit of the carrier density (n) of the thermoelectric material at a measurement temperature of 300 [K] is preferably −1.0×10 20 [cm -3 ] or more, and more preferably −1.0×10 19 [cm -3 ] or more, and more preferably −1.0×10 18 [cm -3 When the carrier density is within the above range, the Seebeck coefficient difference can be easily increased. The upper limit of the carrier density (n) when y<1 is not particularly limited, but it is necessary to appropriately control the electrical resistivity and obtain a high lateral figure of merit Z. xy From the viewpoint of obtaining T, it is preferable to obtain −1.0×10 13 [cm -3 ] or less, and more preferably -1.0 × 10 14 [cm -3 ]below.

[0029] In general formula (1), when y<1, the carrier density (n) of the thermoelectric material is −1.0×10 20 ~-1.0×10 13 [cm -3 ], or -1.0 x 10 19 ~-1.0×10 13 [cm -3 ], or -1.0 x 10 18 ~-1.0×10 14 [cm -3] may also be used.

[0030] In general formula (1), when y<1, the carrier density (n) can be increased, i.e., the electron carrier density can be decreased, by increasing the amount of yttrium in the starting material, increasing the temperature of the magnesium atmosphere annealing, increasing the time of the magnesium atmosphere annealing, etc. The carrier density (n) can be decreased, i.e., the electron carrier density can be increased, by decreasing the amount of yttrium in the starting material, decreasing the temperature of the magnesium atmosphere annealing, decreasing the time of the magnesium atmosphere annealing, etc.

[0031] In the general formula (1), when y≧1, the carrier density (n) of the thermoelectric material at a measurement temperature of 300 [K] is preferably −5.0×10 19 ~10.0×10 19 [cm -3 ], and more preferably −3.0×10 19 ~4.5×10 19 [cm -3 ], or 4.7 × 10 19 ~8.0×10 19 [cm -3 ], and more preferably −1.0×10 19 ~4.0×10 19 [cm -3 ], or 4.7 × 10 19 ~6.0×10 19 [cm -3 When the carrier density is within the above range, the Seebeck coefficient difference can be easily increased.

[0032] In the general formula (1), when y≧1, the carrier density (n) can be increased, i.e., the electron carrier density can be decreased or the hole carrier density can be increased by increasing the amount of magnesium and yttrium in the starting materials, using a crucible made of graphite, decreasing the heating temperature within the range of 620° C. to 830° C., shortening the heating time, etc. In order to decrease the carrier density (n), i.e., to increase the electron carrier density or to decrease the hole carrier density, the amount of magnesium and yttrium in the starting materials can be decreased, using a crucible made of alumina, increasing the heating temperature within the range of 620° C. to 830° C., lengthening the heating time, etc.

[0033] The inter-plane electrical resistivity (ρ CP The upper limit of the interplanar electrical resistivity (ρ CP ) within the above range, a higher lateral performance index Z xy T can be obtained. Interfacial electrical resistivity (ρ CP The lower limit of ) is not particularly limited, but is usually 1.0 × 10 -8 [Ωm] or more. Interfacial electrical resistivity (ρ CP ) is 1.0 × 10 -8 ~8.0 [Ωm], and may be 1.0×10 -8 7.0 [Ωm] or 1.0×10 -8 It may be up to 4.0 [Ωm].

[0034] The in-plane electrical resistivity (ρ IP ) is preferably 5.5×10 -2 [Ωm] or less, and more preferably 4.5×10 -2 [Ωm] or less, and more preferably 1.0×10 -2 [Ωm] or less. IP ) within the above range, a higher lateral performance index Z xyT can be obtained. The in-plane electrical resistivity (ρ IP The lower limit of ) is not particularly limited, but is usually 1.0 × 10 -6 [Ωm] or more. In-plane electrical resistivity (ρ IP ) is 1.0 × 10 -6 ~5.5×10 -2 [Ωm], 1.0×10 -6 ~4.5×10 -2 [Ωm], 1.0×10 -6 ~1.0×10 -2 [Ωm] may also be used.

[0035] In order to reduce the electrical resistivity, when 0≦y<2 in the general formula (1), the amount of yttrium added can be increased.

[0036] [Thermoelectric module] The thermoelectric material according to the present embodiment can be used for a thermoelectric module. In addition, since the thermoelectric material according to the present embodiment is a goniopolar material, it can be used for a thermoelectric module having a "horizontal structure" in which the temperature difference direction and the power generation direction are perpendicular to each other, that is, for a horizontal thermoelectric module.

[0037] [Method of manufacturing thermoelectric materials] The method for producing the thermoelectric material according to this embodiment is not particularly limited, but an example of the production method will be described below.

[0038] First, the elemental metals contained in the chemical composition of the general formula (1) mentioned above are treated as graphite or is placed in an alumina crucible, sealed in a stainless steel tube under an argon atmosphere, heated, and then cooled.

[0039] It is preferable to adjust the amount of magnesium, yttrium, and bismuth to be equal to the chemical composition of the general formula (1), and adjust the amount of antimony to be 3.0 to 4.0 times the chemical composition of the general formula (1). By adjusting in this way, a thermoelectric material having a desired chemical composition can be obtained. By adding an excess amount of antimony, the melting point of the mixture can be lowered and single crystal growth can be promoted. Excess antimony can be removed as described below.

[0040] In general formula (1), when y<1, it is preferable to use an alumina crucible from the viewpoint of cost, and when y≧1, it is preferable to use a graphite crucible from the viewpoint of low reactivity with magnesium. Also, from the viewpoint of preventing evaporation of magnesium, it is preferable to heat in an argon atmosphere rather than in a vacuum.

[0041] When y<1 in general formula (1), the heating temperature is preferably 700 to 900° C., more preferably 750 to 890° C., and even more preferably 800 to 880° C. When y≧1 in general formula (1), the heating temperature is preferably 620 to 830° C., more preferably 640 to 810° C., and even more preferably 650 to 800° C. When the heating temperature is within the above range, the thermoelectric material is easily converted into a single crystal, the carrier density (n) of the thermoelectric material is easily adjusted, and evaporation of magnesium is easily prevented.

[0042] The heating time is preferably adjusted according to the heating temperature. Specifically, when the heating temperature is 700° C. or less, the heating time is preferably 5 to 200 hours, and when the heating temperature is over 700° C., the heating time is preferably 5 to 50 hours. When the heating time is within the above range, the thermoelectric material is easily converted into a single crystal, the carrier density (n) of the thermoelectric material is easily adjusted, and evaporation of magnesium is easily prevented.

[0043] After heating, when y<1 in general formula (1), the cooling rate can be slowed down to facilitate single crystal growth. When y<1 in general formula (1), the cooling rate may be, for example, 0.1 to 10°C per hour, or 0.2 to 5°C per hour. It is also preferable to remove excess antimony, which can be removed by, for example, centrifugation at 700°C. Thereafter, it is preferable to cool to room temperature.

[0044] When y<1, it is preferable to seal the collected sample together with magnesium in an argon atmosphere and heat it (magnesium atmosphere annealing). By performing magnesium atmosphere annealing, it becomes easier to adjust the carrier density (n).

[0045] The temperature of the magnesium atmosphere annealing is preferably 500 to 900° C., more preferably 550 to 850° C., and further preferably 580 to 820° C. The time of the magnesium atmosphere annealing is preferably 2 to 30 hours, and more preferably 5 to 20 hours.

[0046] The measurement methods for 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.

[0047] <Crystallization confirmation> This is confirmed by X-ray diffraction. For example, the X-ray diffraction of the plate surface of a single crystal grown in a plate shape is measured, If only the diffraction peak with 00l index is observed, it can be determined that the single crystal has grown on the ab plane.

[0048] <Carrier density (n)> The carrier density is determined by measuring the Hall coefficient. The Hall coefficient can be determined, for example, by the van der Pauw method. From the obtained Hall coefficient, the carrier density is determined by first-principles calculation.

[0049] <In-plane Seebeck coefficient (S IP ) and interfacial Seebeck coefficient (SCP )> A temperature difference is applied to the sample by connecting a heater or the like, and the thermoelectromotive force generated in the direction of the temperature difference is measured. The Seebeck coefficient is calculated from the proportionality coefficient between the thermoelectromotive force 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.

[0050] <In-plane electrical resistivity (ρ IP ) and interfacial electrical resistivity (ρ CP )> The four-terminal method is used. A 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. EXAMPLES

[0051] The present invention will be described in detail below using examples, but the scope of the present invention is not limited to the descriptions of the examples.

[0052] <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).

[0053] These were weighed out to obtain the respective charging compositions shown in Table 1. The weighed elemental metals were placed in crucibles shown in Table 1 and sealed in stainless steel tubes under an argon atmosphere. After sealing, the materials were heated under the heating conditions shown in Table 1 and cooled to room temperature to obtain thermoelectric materials. In Comparative Example 2-1 and Examples 2-1 and 2-2, the cooling rate was 1° C. per hour.

[0054] In Comparative Example 2-1 and Examples 2-1 and 2-2, after cooling the thermoelectric material, excess antimony was removed by centrifugation at 700° C. The thermoelectric material after centrifugation was sealed together with magnesium in an argon atmosphere and heated under the conditions shown in Table 1 (magnesium atmosphere annealing).

[0055] The obtained thermoelectric materials were all single crystals. The physical properties of the obtained thermoelectric materials are shown in Table 2. The relationship between the Seebeck coefficient and the carrier density of the thermoelectric materials obtained in Examples 1-1 to 1-5 and Comparative Example 1-1 is shown in Figure 1(A), and the relationship between the Seebeck coefficient and the electron carrier density of the thermoelectric materials obtained in Examples 2-1 to 2-2 and Comparative Example 2-1 is shown in Figure 1(B).

[0056] [Table 1]

[0057] [Table 2]

[0058] As shown in Table 2, the Seebeck coefficient differences were large, at 20 μV / K or more, in Examples 1-1 to 1-5 and Examples 2-1 to 2-2. In other words, it was found that a new thermoelectric material was obtained that exhibited goniopolarity in the low temperature region (300 K) and was made from inexpensive raw materials.

[0059] The transverse Seebeck coefficient S xy and lateral performance index Z xy T was calculated and is shown in Table 3.

[0060] [Table 3]

Claims

1. A thermoelectric material represented by the following general formula (1): *| ３－ｘ＋ｄ ﹹ ｘ 3) ２－ｙ 3) ｙ () (In the general formula (1), 0≦x≦1, y=0 or 2, and −0.1≦d≦1, and when y=0, 0<x≦0.1.) It is a single crystal, In-plane Seebeck coefficient (S ＩＰ ) and the interfacial Seebeck coefficient (S ＣＰ ) is 20 [μV / K] or more (absolute value).

2. In the general formula (1), when y = 0, the carrier density (n) at a measurement temperature of 300 [K] is -1.0 × 10 ２０ [cm －３ ] and above, When y = 2, the carrier density (n) at the measurement temperature of 300 [K] is -5.0 × 10 １９ [cm －３ ] or more 10.0 x 10 １９ [cm －３ 2. The thermoelectric material according to claim 1 , wherein

3. Interfacial electrical resistivity (ρ ＣＰ 2. The thermoelectric material according to claim 1, wherein the thermal conductivity is 8.0 [Ωm] or less.

4. The thermoelectric material according to any one of claims 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 claims 1 to 3.

6. A method for producing the thermoelectric material according to any one of claims 1 to 3, comprising the steps of: In the general formula (1), when y = 0, a simple metal of an element included in the chemical composition of the general formula (1) is heated to 700 ° C. or more and 900 ° C. or less, and then cooled; A method for producing a thermoelectric material, comprising heating a simple metal of an element contained in the chemical composition of general formula (1) at y=2 to 620° C. or more and 830° C. or less, and then cooling the simple metal.

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