Thermoelectric material, method for producing same, and thermoelectric conversion element

Abstract

A thermoelectric material according to the present invention contains an inorganic compound that contains Mg, Ag, and Sb, and a fatty acid and / or a fatty acid-based derivative. In addition, the thermoelectric material may be doped with Cu, Fe, Zn, Yb, Ca, or the like. A thermoelectric conversion element according to the present invention uses the thermoelectric material. A method for producing a thermoelectric material according to the present invention includes a pulverization step and a sintering step. In the pulverization step, a starting material constituting an inorganic compound that contains Mg, Ag, and Sb and a starting material containing specific amounts of a fatty acid and / or a fatty acid-based derivative are mixed with each other, and subsequently subjected to ball milling so as to prepare a powder. In the sintering step, the thus-produced powder is sintered. The thermoelectric material according to the present invention can have a reduced thermal conductivity by means of the addition of a fatty acid and / or a fatty acid-based derivative, and as a result, the thermoelectric material can have an improved dimensionless performance index ZT.

Description

Thermoelectric material, its manufacturing method, and thermoelectric conversion element

[0001] The present invention relates to a thermoelectric material, a manufacturing method thereof, and a thermoelectric conversion element, and more particularly to a thermoelectric material containing a magnesium (Mg), silver (Ag), and antimony (Sb)-based thermoelectric material, a manufacturing method thereof, and a thermoelectric conversion element.

[0002] As research into the efficient use of energy, including energy conservation and the use of renewable energy, progress is being made, and thermoelectric materials are attracting attention as materials that can be used in energy-harvesting devices that directly convert waste thermal energy into electrical energy for reuse, as well as in cooling devices that utilize the reverse conversion.

[0003] Patent Document 1 describes a method for doping magnesium silver antimony (MgAgSb) with copper (Cu) to achieve a power density of 25 μW cm －１ K －２ A thermoelectric material that achieves a power factor PF exceeding .

[0004] Non-Patent Document 1 describes silver tin selenium (Ag) as a thermoelectric material. ８ SnSe ６ ) has been reported to achieve a dimensionless figure of merit ZT of over 1.2 and an average of about 0.8 due to its low sound velocity and consequently low thermal conductivity.

[0005] International Publication No. 2022 / 059443

[0006] Wen Li, et al. , “Low Sound Velocity Contributing to the High Thermoelectric Performance of Ag8SnSe6”, Advanced Science, July 7, 2016.

[0007] An object of the present invention is to further improve the dimensionless figure of merit ZT in an MgAgSb-based thermoelectric material that can achieve a high power factor PF, that is, to provide an MgAgSb-based thermoelectric material with an improved dimensionless figure of merit ZT, a method for manufacturing the same, and a thermoelectric conversion element using the same.

[0008] The thermoelectric material according to the present invention solves the above-mentioned problems by including an inorganic compound consisting of magnesium (Mg), silver (Ag), and antimony (Sb), and a fatty acid and / or a fatty acid derivative. The fatty acid and / or a fatty acid derivative is stearic acid (C １８ H ３６ O ２ ), palmitic acid (C １６ H ３２ O ２ ), and icosanoic acid (C ２０ H ４０ O ２ The fatty acid may be at least one fatty acid selected from the group consisting of stearic acid (C １８ H ３６ O ２ ) and the fatty acid derivative is zinc stearate (C ３６ H ７０ ZnO ４ ), magnesium stearate (C ３６ H ７０ MgO ４ ), barium stearate (C ３６ H ７０ BaO ４ ), calcium stearate (C ３６ H ７０ CaO ４ ), sodium stearate (C １８ H ３５ NaO ２ ), and N,N'-ethylenebisstearamide (C ３８ H ７６ N ２ O ２ The content of the fatty acid and / or fatty acid derivative may be greater than 0% by weight and less than 1% by weight. ａ Sb ｂThe thermoelectric material may be expressed by the following formula, satisfying 0.95≦a≦1.05 and 0.95≦b≦1.05. The thermoelectric material may further contain at least one element selected from the group consisting of copper (Cu), iron (Fe), zinc (Zn), ytterbium (Yb), and calcium (Ca). The inorganic compound may be Mg １－ｃ X ｃ Ag ａ Sb ｂ X may be Cu, Fe, Zn, Yb, or Ca, and may satisfy the following conditions: 0.95≦a≦1.05, 0.95≦b≦1.05, and 0<c≦0.1. A thermoelectric conversion element according to the present invention includes a thermoelectric conversion layer containing the above-described thermoelectric material, thereby solving the above-described problem. A method for producing the above-described thermoelectric material according to the present invention includes a grinding step and a sintering step, in which raw materials constituting an inorganic compound containing Mg, Ag, and Sb and raw materials containing predetermined amounts of a fatty acid and / or a fatty acid-based derivative are mixed and ball milled to prepare a powder, and in the sintering step, the produced powder is sintered, thereby solving the above-described problem. The sintering step may be characterized in that the firing temperature does not exceed the decomposition temperature of the fatty acid and / or a fatty acid-based derivative. The inorganic compound may further contain at least one element selected from the group consisting of Cu, Fe, Zn, Yb, and Ca.

[0009] The effects obtained by the embodiment are briefly explained as follows: In an MgAgSb-based thermoelectric material capable of realizing a high power factor PF, the thermal conductivity k can be reduced by adding a fatty acid and / or a fatty acid derivative, and the dimensionless figure of merit ZT can be improved.

[0010] FIG. 1 is a flowchart illustrating an example of a method for manufacturing a thermoelectric material according to the present invention. FIG. 2 is an X-ray diffraction pattern of a thermoelectric material according to Example 1 of the present invention. FIG. 3 is a graph showing measured values ​​of Vickers hardness of the thermoelectric material according to Example 1. FIG. 4 is a graph showing measured values ​​of the sound velocity of the thermoelectric material according to Example 1. FIG. 5 is a graph showing the temperature dependence of the thermal conductivity of the thermoelectric material according to Example 1. FIG. 6 is a graph showing the temperature dependence of the electrical conductivity of the thermoelectric material according to Example 1. FIG. 7 is a graph showing the temperature dependence of the Seebeck coefficient of the thermoelectric material according to Example 1. FIG. 8 is a graph showing the temperature dependence of the power factor PF of the thermoelectric material according to Example 1. FIG. 9 is a graph showing the temperature dependence of the dimensionless figure of merit ZT of the thermoelectric material according to Example 1. FIG. 10 is an explanatory diagram schematically illustrating an example of the configuration of a thermoelectric conversion element (two-pair module) using a thermoelectric material according to the present invention. FIG. 11 is a graph showing the measurement results of the thermoelectric conversion efficiency η of a thermoelectric conversion element (single leg of MgAgSb thermoelectric material) using a thermoelectric material according to the present invention. Fig. 12 is a graph showing the measurement results of the thermoelectric conversion efficiency η of a thermoelectric conversion element (two-pair module) using the thermoelectric material of the present invention. Fig. 13 is a graph showing the measured sound velocity of the thermoelectric material of Example 2. Fig. 14 is a graph showing the temperature dependence of the thermal conductivity of the thermoelectric material of Example 2. Fig. 15 is a graph showing the temperature dependence of the thermal conductivity of the thermoelectric material of Example 3. Fig. 16 is a graph showing the temperature dependence of the dimensionless figure of merit ZT of the thermoelectric material of Example 3.

[0011] 1. Solution Principle of the Present Invention The solution principle of the present invention will be briefly explained. An important performance characteristic factor of a thermoelectric material is the dimensionless figure of merit ZT, which is expressed by the following equation:

[0012]

[0013] where S is the Seebeck coefficient, σ is the electrical conductivity, T is the absolute temperature, and k is the thermal conductivity. ２ σ is the power factor PF (also called the electrical output factor), which corresponds to the power generated per unit temperature. In order to improve ZT, it is effective to increase the power factor PF and reduce the thermal conductivity k.

[0014] It is known that there is a positive correlation between the speed of sound and thermal conductivity (see Non-Patent Document 1, etc.), and lowering the speed of sound can be expected to have the effect of lowering the thermal conductivity k and improving the dimensionless figure of merit ZT. Since the speed of sound is the speed at which sound propagates through a material, the speed of sound decreases when the material becomes weaker, that is, when the bonds between the atoms that make up the material are weakened. Adding a soft material to a thermoelectric material weakens the thermoelectric material, which can correspondingly lower the speed of sound and reduce the thermal conductivity (see Non-Patent Document 1, etc.). As shown in the above formula, the dimensionless figure of merit ZT is inversely proportional to the thermal conductivity k, so ZT can be improved by reducing the thermal conductivity k.

[0015] The inventors have found that when the inorganic compound that is the thermoelectric material is an MgAgSb system, a fatty acid or a fatty acid derivative can be added, which weakens the MgAgSb system and reduces its hardness.

[0016] The fatty acid to be added may be a fatty acid having 10 to 24 carbon atoms, and may be branched or linear. For example, the fatty acid may be stearic acid (C １８ H ３６ O ２ ), palmitic acid with 16 carbon atoms (C １６ H ３２ O ２ ), icosanoic acid with 20 carbon atoms (C ２０ H ４０ O ２ ) can be adopted.

[0017] Along with or in addition to these fatty acids, fatty acid metal salts or fatty acid derivatives such as fatty acid amides may be added. The fatty acid metal salts include alkali metal salts of the above fatty acids and alkaline earth metal salts of the above fatty acids. For example, the metal salt of stearic acid is zinc stearate (C ３６ H ７０ ZnO ４ ), magnesium stearate (C ３６ H ７０ MgO ４ ), barium stearate (C ３６ H ７０ BaO ４ ), calcium stearate (C３６ H ７０ CaO ４ ), sodium stearate (C １８ H ３５ NaO ２ ) and the like. Examples of fatty acid amides include stearic acid amide, palmitic acid amide, eicosanoic acid amide, and N,N'-ethylenebisstearic acid amide (C ３８ H ７６ N ２ O ２ ) etc.

[0018] Although it is sufficient to add one type of fatty acid or fatty acid derivative, multiple types of fatty acids and / or fatty acid derivatives may also be added. Furthermore, MgAgSb-based thermoelectric materials may be doped with copper (Cu), iron (Fe), zinc (Zn), ytterbium (Yb), calcium (Ca), or the like. The effect of improving the power factor PF by doping (see Patent Document 1, etc.) and the effect of reducing the thermal conductivity k by the present invention act synergistically, further improving the dimensionless figure of merit ZT.

[0019] 2. Overview of the Embodiments First, an overview of the representative embodiments disclosed in the present application will be described. Reference numerals in parentheses in the drawings used in the overview of the representative embodiments merely illustrate components included in the concept of the components to which the reference numerals are attached.

[0020] [1] Addition of a fatty acid and / or a fatty acid derivative to an MgAgSb-based thermoelectric material A representative embodiment disclosed in the present application is a thermoelectric material containing an inorganic compound containing Mg, Ag, and Sb, and a fatty acid and / or a fatty acid derivative. By adding the fatty acid and / or a fatty acid derivative to the MgAgSb-based thermoelectric material, the thermal conductivity k can be reduced, and as a result, the dimensionless figure of merit ZT can be improved.

[0021] [2] Fatty acid and / or fatty acid derivative In the thermoelectric material of [1], the fatty acid and / or fatty acid derivative is stearic acid (C １８ H ３６ O ２ ), palmitic acid (C１６ H ３２ O ２ ), and icosanoic acid (C ２０ H ４０ O ２ ) and / or a derivative thereof. The fatty acid is preferably stearic acid (C １８ H ３６ O ２ The fatty acid derivative is preferably zinc stearate (C ３６ H ７０ ZnO ４ ), magnesium stearate (C ３６ H ７０ MgO ４ ), barium stearate (C ３６ H ７０ BaO ４ ), calcium stearate (C ３６ H ７０ CaO ４ ), sodium stearate (C １８ H ３５ NaO ２ ), N,N'-ethylenebisstearic acid amide (C ３８ H ７６ N ２ O ２ In [1], the fatty acid and / or fatty acid derivative means any combination of fatty acid and / or one or more fatty acid derivatives, and may be any combination of fatty acid only, one type of fatty acid derivative, fatty acid and one type of fatty acid derivative, fatty acid and two types of fatty acid derivative, two or more types of fatty acid derivative, etc. This makes it possible to determine that a suitable additive is fatty acid C. １８ H ３６ O ２ and suitable derivatives are also identified.

[0022] [3] Suitable content of fatty acid and / or fatty acid derivative In the thermoelectric material of [1] or [2], the content of the fatty acid and / or fatty acid derivative is greater than 0 wt % and less than 1 wt %. This specifies a suitable content of the fatty acid and / or fatty acid derivative to be added. Here, the content of fatty acid and / or fatty acid derivative means the mass content when one type of fatty acid and / or fatty acid derivative is added, and means the total mass content when multiple types of fatty acids and / or fatty acid derivatives are added.

[0023] [4] Composition of inorganic compound In the thermoelectric material according to any one of [1] to [3], the inorganic compound is MgAg ａ Sb ｂ and satisfies 0.95≦a≦1.05 and 0.95≦b≦1.05. This identifies a suitable stoichiometric ratio for the MgAgSb-based thermoelectric material.

[0024] [5] Doping of MgAgSb-based thermoelectric material In the thermoelectric material according to any one of [1] to [4], the thermoelectric material further contains at least one element selected from the group consisting of Cu, Fe, Zn, Yb, and Ca. This allows the effect of improving the power factor PF by doping the MgAgSb-based thermoelectric material to be synergistically achieved. In addition, a suitable dopant is identified.

[0025] [6] Composition of doped inorganic compound In the electric heating material of [5], the inorganic compound is Mg １－ｃ X ｃ Ag ａ Sb ｂ Here, X is Cu, Fe, Zn, Yb, or Ca, and the parameters a, b, and c satisfy the following conditions: 0.95≦a≦1.05 0.95≦b≦1.05 0<c≦0.1 This identifies a suitable doping amount for an MgAgSb-based thermoelectric material.

[0026] [7] Thermoelectric conversion element Another representative embodiment disclosed in the present application is a thermoelectric conversion element having a thermoelectric conversion layer made of any one of the thermoelectric materials [1] to [6]. This allows the thermoelectric conversion element to enjoy the same effects as those of [1] to [6].

[0027] [8] Manufacturing Method of Thermoelectric Material (FIG. 1) Yet another representative embodiment disclosed in the present application is a manufacturing method of a thermoelectric material including a milling step and a sintering step, in which in the milling step, raw materials constituting an inorganic compound containing Mg, Ag, and Sb and raw materials each containing a predetermined amount of a fatty acid and / or a fatty acid-based derivative are mixed (S1), and ball milled (S2) to prepare a powder, and in the sintering step, the produced powder is sintered (S3). As in [1], this allows the addition of a fatty acid and / or a fatty acid-based derivative to reduce the thermal conductivity of an MgAgSb-based thermoelectric material, and as a result, the dimensionless figure of merit ZT can be improved.

[0028] [9] Sintering Temperature: The method for producing a thermoelectric material according to [8] is characterized in that the firing temperature in the sintering step does not exceed the decomposition temperature of the fatty acid and / or fatty acid derivative. When multiple types of fatty acids and / or fatty acid derivatives are added, the firing temperature is adjusted so as not to exceed the lowest decomposition temperature among them. This allows the firing temperature to be kept within an appropriate range.

[0029]

[10] Doping of MgAgSb-based thermoelectric material In the method for producing a thermoelectric material according to [8] or [9], the inorganic compound further contains at least one element selected from the group consisting of Cu, Fe, Zn, Yb, and Ca. This allows the doping of the MgAgSb-based thermoelectric material to synergistically improve the power factor (PF). A suitable dopant is also specified.

[0030] 3. Details of the embodiment The embodiment will be described in further detail.

[0031] FIG. 1 is a flowchart showing an example of a method for manufacturing a thermoelectric material according to the present invention. First, in the preparation step S1, raw materials containing Mg, Ag, and Sb for an MgAgSb-based thermoelectric material and fatty acid and / or fatty acid derivatives are prepared. Next, in the milling step S2, these materials are mixed and milled by ball milling to produce powder. The addition of fatty acid and / or fatty acid derivative in the milling step S2 provides a lubricating effect, which can shorten the ball milling time and / or reduce the milling power. In the sintering step S3, the produced powder is sintered. For example, spark plasma sintering can be used. The sintering temperature is, for example, 300°C, and must be controlled so as not to exceed the decomposition temperature of the fatty acid or fatty acid derivative contained in the raw material powder, which is approximately 360°C.

[0032] The thermoelectric material to be fabricated is MgAg ａ Sb ｂ It is preferable that the relationship 0.95≦a≦1.05 and the relationship 0.95≦b≦1.05 are satisfied.

[0033] The added fatty acids are the above-mentioned fatty acids and / or fatty acid derivatives. The fatty acids are preferably stearic acid (C １８ H ３６ O ２ ), and the fatty acid derivative is preferably zinc stearate (C ３６ H ７０ ZnO ４ ), magnesium stearate (C ３６ H ７０ MgO ４ ), barium stearate (C ３６ H ７０ BaO ４ ), calcium stearate (C ３６ H ７０ CaO ４ ), sodium stearate (C １８ H ３５ NaO ２ ), and N,N'-ethylenebisstearamide (C ３８ H ７６ N ２ O２ At least one selected from the group consisting of

[0034] The content of the fatty acid and / or fatty acid derivative is adjusted to a range of 0 wt% to 1 wt%. As described above, the effect is fully exerted by adding only one type of fatty acid or fatty acid derivative, but multiple types of fatty acids and / or fatty acid derivatives may also be added. Furthermore, the material may be doped with Cu, Fe, Zn, Yb, Ca, etc.

[0035] The thermoelectric material may be in the form of a powder, sintered body, or thin film, which allows it to be used in thermoelectric conversion elements that exhibit high thermoelectric performance at room temperature. For example, a sintered body of this thermoelectric material can be produced using the above-mentioned manufacturing method. The sintered body can be pulverized to obtain a powder, and a thin film can be formed by physical vapor deposition or other methods using the sintered body as a target. Here, the term "powder" generally refers to powder that has been crushed and finely processed. A compact can be formed by pressing the powder using a powder compactor or other device. Generally, a compact refers to a powder compressed into a predetermined shape. When a compact is heated at a temperature below the melting point of the powder components, the contact surfaces of the powder particles bond together, and the compact shrinks and densifies with increasing heating time. This phenomenon is called sintering, and the product obtained by sintering can also be called a sintered body. A thin film refers to a thin film, and may include a layer formed by condensation of a gas phase on a solid surface (substrate).

[0036] The thermoelectric material of the present invention can also be processed into a film by mixing it with other organic materials in addition to the inorganic compound powder and fatty acid and / or fatty acid derivative described above. In this case, the organic material can be at least one selected from the group consisting of poly(3,4-ethylenedioxythiophene)polystyrenesulfonate (PEDOT:PSS), poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT), polyaniline (PANI), tetrathiafulvalene (TTF), and benzodifurandione paraphenylenevinylidene (BDPPV). The use of these organic materials allows the thermoelectric material of the present invention to be processed into a flexible film.

[0037] In this case, the content of the powder is not particularly limited, but the powder is preferably contained in the range of 4 wt % to 80 wt %, more preferably 4 wt % to 50 wt %, even more preferably 4 wt % to 10 wt %, and even more preferably 4 wt % to 7 wt %, relative to the organic material. This makes it possible to realize a film that has flexibility and thermoelectric conversion performance.

[0038] The thermoelectric material of the present invention functions as a p-type thermoelectric material with holes as carriers. By combining it with other n-type thermoelectric materials, a thermoelectric power generation device, which is a type of thermoelectric conversion element, can be constructed. The thermoelectric conversion element is constructed by alternately connecting p-type thermoelectric conversion legs and n-type thermoelectric conversion legs, each with electrodes connected to both ends, in series.

[0039] The p-type thermoelectric conversion leg includes a thermoelectric conversion layer made of the above-mentioned thermoelectric material. On the other hand, the n-type thermoelectric conversion leg includes a thermoelectric conversion layer made of, for example, magnesium antimony (Mg ２ Sb ３ ) system, bismuth telluride selenium (BiTeSe) system, cobalt antimony (CoSb ３ The thermoelectric conversion layer can be arbitrarily selected from thermoelectric materials such as ZnO (Ni) and Cu (CuSb). The electrode material can be any material, for example, Fe, aluminum (Al), Cu, Ag, nickel (Ni), or MgCuSb. The low-temperature electrode requires wiring to an external circuit, so Cu, Ni, or the like, which have a high affinity with solder, is suitable. The high-temperature electrode is preferably a stable metal material that can be attached to a high-temperature object with good thermal conductivity, and can be, for example, Fe, Al, or Cu.

[0040] In the examples described below, a Π-type two-pair thermoelectric conversion module will be described as an example, but any other type of thermoelectric conversion element can be configured, such as a U-shaped one, etc. This makes it possible to realize a thermoelectric conversion element that achieves higher thermoelectric conversion efficiency using an MgAgSb-based thermoelectric material.

[0041] Example 1 The inventors discovered that adding a fatty acid and / or a fatty acid derivative to an MgAgSb-based thermoelectric material can reduce the thermal conductivity, and as a result, improve the dimensionless figure of merit ZT. They fabricated samples with the following compositions and measured their properties. The fabricated samples were prepared by adding stearic acid (C ) as a fatty acid to an MgAgSb thermoelectric material. １８ H ３６ O ２ The four samples contained 0.25 wt%, 0.50 wt%, 0.75 wt%, and 1.0 wt% of fatty acids, respectively, and a control sample containing no fatty acids (0.0 wt%).

[0042] [Manufacturing of Thermoelectric Material] Preparation step S1: As raw materials, Mg (turned material 5-25 mm, 99.95%), Ag (shot 2-5 mm, 99.99%), and Sb (lump 2 mm, 99.999%) were weighed to prepare a predetermined stoichiometric composition ratio. In addition, fatty acids were weighed to have respective contents of 0.25 wt%, 0.50 wt%, 0.75 wt%, and 1.0 wt%, respectively.

[0043] Pulverization step S2: A ball mill (8000D Mixer / Mill manufactured by SPEA SamplePrep) was used. The prepared raw materials were loaded into a stainless steel ball milling jar and then ball milled for 5 hours in an argon (Ar) atmosphere. According to Patent Document 1, the milling process without adding fatty acid required a total of 10 hours or more, with mixing and ball milling treatments repeated several times. However, the milling process in this example was able to be shortened to 5 hours. This is thought to be due to the lubricating effect of the added fatty acid.

[0044] Sintering step S3: The powder was sintered at 300°C under a uniaxial pressure of 60 MPa for 5 minutes using a spark plasma sintering (SPS) device (Fuji Electric Power Co., Ltd., Doctor Labo Series 322La). The sintering temperature was set at 300°C so as not to exceed the decomposition temperature of the added fatty acid, which was approximately 360°C.

[0045] [Characteristics of the produced thermoelectric material] Four MgAgSb thermoelectric material samples (MgAg ａ Sbｂ + x wt% C １８ H ３６ O ２ The measurements were performed on samples with x=0.25, 0.50, 0.75, 1.0, a=0.97, b=0.99) and a comparative sample (x=0.0).

[0046] Figure 2 shows the X-ray diffraction pattern of the fabricated thermoelectric material. The horizontal axis shows the detection angle 2θ, and the vertical axis shows the diffraction intensity in arbitrary units. The bottom row shows the diffraction pattern of MgAgSb obtained by simulation, and the diffraction patterns of each sample are arranged vertically above it. The diffraction patterns of each sample match the diffraction pattern of MgAgSb obtained by simulation, indicating that MgAgSb crystals are formed as the parent phase. No significant diffraction peaks were detected for the added fatty acid because the amount added was small.

[0047] Figure 3 shows the measured Vickers hardness of the fabricated thermoelectric materials (each sample). The change in Vickers hardness was observed relative to the fatty acid content. The horizontal axis represents the fatty acid content of each sample, and the vertical axis represents the Vickers hardness. Error bars are shown above and below the average value. While the Vickers hardness of the comparison sample (x = 0) was 250, the Vickers hardness of the samples with x = 0.25, 0.5, and 0.75 decreased to 245, 239, and 234, respectively.

[0048] Figure 4 shows the measured values ​​of the sound velocity of the fabricated thermoelectric materials (each sample). The sound propagation velocity (sound velocity) in each sample, which is an MgAgSb sintered body by SPS, was measured using an ultrasonic measuring device (UVM-2 model, manufactured by Ultrasonic Industrial Co., Ltd.). The sound velocity in the longitudinal direction (v ｌ ) and the transverse speed of sound (v ｔ The horizontal axis represents the content of fatty acids, and the vertical axis on the left represents the speed of sound in the horizontal direction (v ｔ ) is the vertical axis on the right, and the vertical speed of sound (v ｌ ) is shown. ｔ ) is 1.62 × 10 for the comparative sample (x = 0). ３ m / s, whereas for the samples with x = 0.25, 0.5, and 0.75, the values ​​were 1.59, 1.55, and 1.54 (× 10３ The speed of sound in the longitudinal direction (v ｌ ) is 3.57 × 10 for the comparative sample (x = 0). ３ m / s, whereas for the samples with x = 0.25, 0.5, and 0.75, the values ​​were 3.42, 3.39, and 3.31 (×10 ３ m / s).

[0049] The thermal conductivity k was determined by the following procedure: The thermal conductivity k is calculated by the following formula 2.

[0050] where D is the thermal diffusivity, ρ is the density, and C ｐ is the heat capacity. The thermal diffusivity D was measured on a disk-shaped sample using a tabletop xenon flash analyzer LFA467 (uncertainty ±3%, manufactured by NETZSCH). The density ρ was calculated by the Archimedes method, and the heat capacity C ｐ was calculated according to the Dulong-Petit law.

[0051] Figure 5 is a graph showing the temperature dependence of the thermal conductivity k of the fabricated thermoelectric materials (each sample). The horizontal axis represents absolute temperature T (K), and the vertical axis represents thermal conductivity k (W / mK). For example, at 523 K (250 °C), the thermal conductivity k of the comparative sample (x = 0) was 1.05 W / mK, whereas the samples with x = 0.25, 0.5, and 0.75 were able to reduce it to 0.93, 0.89, and 0.84 (W / mK), respectively. The same is true for other temperature ranges.

[0052] The dimensionless figure of merit ZT of the fabricated thermoelectric material was determined by the following procedure: The dimensionless figure of merit ZT can be calculated by Equation 1 as described above.

[0053] First, the electrical conductivity σ and Seebeck coefficient S were measured in a helium gas atmosphere for each sample processed into a rectangular parallelepiped shape using a thermoelectric property evaluation device ZEM-3 (uncertainty ±5%, manufactured by Advance Riko Co., Ltd.). Figure 6 shows the temperature dependence of the electrical conductivity σ of the produced thermoelectric materials (each sample) (the horizontal axis is absolute temperature T (K), and the vertical axis is electrical conductivity σ (10 ４FIG. 7 shows the temperature dependence of the Seebeck coefficient S of the thermoelectric material (each sample) (abscissa is absolute temperature T (K), ordinate is Seebeck coefficient S (μV / K)).

[0054] The power factor PF (= S) is calculated from the measured electrical conductivity σ and Seebeck coefficient S. ２ 8 is a graph showing the temperature dependence of the power factor PF of the thermoelectric material (each sample) of this example. The horizontal axis is the absolute temperature T (K), and the vertical axis is the power factor PF (μW / cmK ２ At a temperature of 300K, the power factor PF is about 25 μW / cmK for the comparative sample without added fatty acid. ２ From the sample with x = 0.75, it is about 20 μW / cmK ２ The same decrease occurs in other temperature ranges.

[0055] The dimensionless figure of merit ZT is calculated by dividing the determined power factor PF by the thermal conductivity k (Figure 5) according to Equation 1 as described above. Figure 9 shows the temperature dependence of the dimensionless figure of merit ZT of the produced thermoelectric materials (each sample) (the horizontal axis is absolute temperature T (K), and the vertical axis is the dimensionless figure of merit ZT). The average value of the dimensionless figure of merit ZT (273K to 573K) was 1.09 for no fatty acid added, but improved to 1.17 for the sample with 0.75 wt% fatty acid added.

[0056] As shown in FIG. 8, by adding fatty acids to MgAgSb-based electrothermal materials, the power factor PF decreases, but the thermal conductivity k decreases even more, resulting in an improvement in the dimensionless figure of merit ZT.

[0057] [Fabrication of Thermoelectric Conversion Element] In order to measure the thermoelectric conversion characteristics, a thermoelectric conversion element was fabricated using the thermoelectric material of the present invention.

[0058] 10 is an explanatory diagram showing a schematic diagram of an example of the configuration of a thermoelectric conversion element (two-pair module) using the thermoelectric material of the present invention. The MgAgSb thermoelectric material of the present invention (containing 0.75 wt% fatty acid) was sandwiched between two layers of magnesium copper antimony (MgCuSb) as an interface material, and sintered by SPS to produce an MgCuSb / MgAgSb / MgCuSb laminate. The resulting MgCuSb / MgAgSb / MgCuSb laminate had dimensions of approximately 3.8 × 3.8 × 6 mm. ３ The MgAgSb thermoelectric material was cut into a rectangular parallelepiped shape to obtain single legs (1-1, 1-2). ３ Sb １．５ Bi ０．５ SUS304 / Mg sandwiched between Sb-rich 304 stainless steel SUS304 ３ Sb １．５ Bi ０．５ Single legs (2-1, 2-2) of p-type MgCuSb / MgAgSb / MgCuSb stacks were fabricated. Single legs (1-1, 1-2) of p-type MgCuSb / MgAgSb / MgCuSb stacks were fabricated. ３ Sb １．５ Bi ０．５ The single legs (2-1, 2-2) of the SUS304 laminate were alternately electrically connected in series to form a Π configuration, thereby producing a thermoelectric conversion module.

[0059] [Measurement of Thermoelectric Conversion Characteristics] Using a thermoelectric conversion efficiency evaluation device for small modules, Mini-PEM (manufactured by Advance Riko Co., Ltd.), the temperature on the low-temperature side was kept at 293 K in a vacuum, and the temperature on the high-temperature side was changed between 373 K and 573 K. The output power of the single leg and two-pair module of the MgAgSb thermoelectric material was measured, and the conversion efficiency was calculated.

[0060] The measurement results of the thermoelectric conversion efficiency η of the fabricated single leg and two-pair module of the MgAgSb thermoelectric material are shown in Figures 11 and 12. The maximum measured thermoelectric conversion efficiency η of the single leg of the MgAgSb thermoelectric material was approximately 8.6%, and the maximum measured thermoelectric conversion efficiency η of the two-pair module was approximately 7.0%.

[0061] The thermoelectric conversion efficiency η depends on the dimensionless figure of merit ZT as expressed by the following equation 3. Therefore, it is considered that the thermoelectric conversion efficiency η improves as the dimensionless figure of merit ZT improves.

[0062]

[0063] Here, T Ｈ and T Ｌ are the temperatures on the high and low sides, respectively, and T ａｖｅ is the average temperature.

[0064] As described above, it has been experimentally demonstrated that adding a fatty acid to an MgAgSb-based thermoelectric material can reduce the thermal conductivity, and as a result, can improve the dimensionless figure of merit ZT.

[0065] Example 2 Adding a fatty acid derivative to an MgAgSb-based thermoelectric material instead of a fatty acid can produce the same effect, reducing the thermal conductivity and, as a result, improving the dimensionless figure of merit ZT. In this Example 2, samples with the following compositions were fabricated and their properties were measured. The fabricated samples were MgAg ａ Sb ｂ The thermoelectric material (a = 0.97, b = 0.99) was mixed with a sample containing 0.75 wt% of fatty acid (fatty acid), magnesium stearate (C ３６ H ７０ MgO ４ ) 0.75 wt% (magnesium stearate), zinc stearate (C ３６ H ７０ ZnO ４ ), and a comparative sample (No Addition) containing neither fatty acids nor their derivatives.

[0066] The manufacturing method of the thermoelectric material was the same as in Example 1. The sound velocity of each manufactured sample was measured, and the thermal conductivity k was calculated. The method of measuring the sound velocity, including the measuring instrument, was the same as in Example 1, and the method of calculating the thermal conductivity k was also the same as in Example 1.

[0067] FIG. 13 is a graph showing the measured sound velocity for each sample. The longitudinal sound velocity (v ｌ ) and the transverse speed of sound (v ｔ The vertical axis on the left shows the transverse sound velocity (v ｔ ) is the vertical axis on the right, and the vertical speed of sound (v ｌ ) is shown. ｔ ) and the longitudinal speed of sound (v ｌ ) and the comparative sample (No. addition) were 1.62 × 10 ３ m / s and 3.57 x 10 ３ m / s, whereas the sample containing 0.75 wt% fatty acid (fatty acid) had a 1.54 × 10 ３ m / s and 3.31 x 10 ３ m / s, and magnesium stearate (C ３６ H ７０ MgO ４ ) in the sample containing 0.75 wt%, respectively, ３ m / s and 3.31 x 10 ３ m / s, and zinc stearate (C ３６ H ７０ ZnO ４ ) in the sample containing 0.75 wt% (zinc stearate), the ３ m / s and 3.40 x 10 ３ Although not as much as in the case of the sample with added fatty acid, it was found that the addition of fatty acid derivatives also contributed to a similar level of reduction in the sound velocity.

[0068] FIG. 14 is a graph showing the temperature dependence of the thermal conductivity k of the thermoelectric material (each sample) of Example 2. The horizontal axis is absolute temperature T (K), and the vertical axis is thermal conductivity k (W / mK). For example, the thermal conductivity k at 523 K (250° C.) was 1.05 W / mK for the comparative sample (No Addition), whereas it was reduced to 0.84 W / mK for the sample containing 0.75 wt % fatty acid (see Example 1, FIG. 5). ３６ H ７０ ZnO ４) and a sample containing 0.75 wt% magnesium stearate (C ３６ H ７０ MgO ４ It was found that the sample containing 0.75 wt% of each of these compounds also reduced the thermal conductivity k to a similar extent. Although not as much as the sample with added fatty acid, the addition of fatty acid derivatives also contributed to a similar level of reduction in thermal conductivity k. The same was true in other temperature ranges.

[0069] As described above, in Example 2, it was experimentally demonstrated that the thermal conductivity can be reduced by adding a fatty acid derivative to an MgAgSb-based thermoelectric material. Therefore, it was found that adding a fatty acid derivative to an MgAgSb-based thermoelectric material instead of a fatty acid can also improve the dimensionless figure of merit ZT, similar to the case of adding a fatty acid.

[0070] Example 3 The phenomenon of improving the dimensionless figure of merit ZT by adding a fatty acid and / or a fatty acid derivative to an MgAgSb-based thermoelectric material of the present invention is different in mechanism from the phenomenon of improving the power factor PF by doping with Cu, Fe, Zn, Yb, Ca, etc. (see Patent Document 1), and is therefore expected to act synergistically.

[0071] In this Example 3, a sample was prepared in which Cu was doped into an MgAgSb-based thermoelectric material and stearic acid was further added as a fatty acid (MgAgSb + Cu doping + fatty acid addition, Mg １－ｃ Cu ｃ Ag ａ Sb ｂ In this example, a = 0.97, b = 0.99, and c = 0.01) were fabricated and their properties were measured. The manufacturing method of the thermoelectric material was the same as in Example 1. The method of calculating the thermal conductivity k of each fabricated sample was also the same as in Examples 1 and 2. The dimensionless figure of merit ZT was calculated by dividing the determined power factor PF by the thermal conductivity k according to Equation 1 as described above.

[0072] FIG. 15 is a graph showing the temperature dependence of the thermal conductivity k of the thermoelectric material sample (MgAgSb + Cu doping + fatty acid addition) of Example 3. The horizontal axis represents absolute temperature T (K), and the vertical axis represents thermal conductivity k (W / mK). For comparison, FIG. 15 also shows the measured values ​​of the thermal conductivity k of a sample in which MgAgSb is doped with Cu (MgAgSb + Cu doping) described in Patent Document 1 and a comparative sample (MgAgSb) without Cu doping. The thermal conductivity k of the sample in which MgAgSb-based thermoelectric material is doped with Cu (MgAgSb + Cu doping) is approximately 10% to 20% higher than that of the comparative sample (MgAgSb) (Patent Document 1). However, the thermal conductivity k of the sample with added fatty acid (MgAgSb + Cu doping + fatty acid addition) is even lower than that of the comparative sample. In particular, in the relatively low temperature range of 300 K to 500 K, the decrease in thermal conductivity k due to the addition of fatty acid is more significant than the increase in thermal conductivity k due to Cu doping. For example, at 373 K, the thermal conductivity k of the comparative sample is 0.93 W / mK, while the thermal conductivity k of the Cu-doped sample (MgAgSb + Cu doping) increases to 1.0 W / mK, but decreases to 0.71 W / mK by further adding fatty acid.

[0073] FIG. 16 is a graph showing the temperature dependence of the dimensionless figure of merit ZT of the thermoelectric material of Example 3. The horizontal axis represents absolute temperature T (K), and the vertical axis represents the dimensionless figure of merit ZT. The dimensionless figure of merit ZT of the Cu-doped sample (MgAgSb + Cu doping) is improved over the entire temperature range measured (273 K to 573 K) compared to the comparative sample. This is the result of the improvement in the power factor PF, which more than compensates for the increase in thermal conductivity k due to Cu doping (see Patent Document 1). The dimensionless figure of merit ZT of the sample with added fatty acid (MgAgSb + Cu doping + fatty acid addition) is further improved over the entire temperature range measured (273 K to 573 K). This is due to the synergistic effect of the improvement in power factor PF due to Cu doping and the decrease in thermal conductivity k due to the addition of fatty acid. It is clear from Example 2 that the same action and effect can be achieved even when a fatty acid-based derivative is added instead of a fatty acid. Therefore, it has been experimentally clarified that the addition of a fatty acid and / or a fatty acid-based derivative to the MgAgSb-based thermoelectric material of the present invention acts synergistically with the effect of improving the power factor PF by doping with Cu, Fe, Zn, Yb, Ca, etc.

[0074] The invention made by the inventor has been specifically described above based on an embodiment, but it goes without saying that the invention is not limited thereto and can be modified in various ways without departing from the spirit of the invention.

[0075] 1-1, 1-2 Single leg of MgCuSb / MgAgSb / MgCuSb laminate 2-1, 2-2 SUS304 / Mg ３ Sb １．５ Bi ０．５ / Single leg of SUS304 laminate

Claims

1. A thermoelectric material comprising an inorganic compound consisting of magnesium (Mg), silver (Ag), and antimony (Sb), and a fatty acid and / or a fatty acid-based derivative.

2. The fatty acid and / or fatty acid derivative is stearic acid (C １８ H ３６ O ２ ), palmitic acid (C １６ H ３２ O ２ ), and icosanoic acid (C ２０ H ４０ O ２ 2. The thermoelectric material according to claim 1 , wherein the fatty acid is at least one fatty acid selected from the group consisting of:

3. The fatty acid is stearic acid (C １８ H ３６ O ２ ) and the fatty acid derivative is zinc stearate (C ３６ H ７０ ZnO ４ ), magnesium stearate (C ３６ H ７０ MgO ４ ), barium stearate (C ３６ H ７０ BaO ４ ), calcium stearate (C ３６ H ７０ CaO ４ ), sodium stearate (C １８ H ３５ NaO ２ ), and N,N'-ethylenebisstearamide (C ３８ H ７６ N ２ O ２ The thermoelectric material according to claim 1 , wherein the thermoelectric material is at least one selected from the group consisting of:

4. The thermoelectric material according to any one of claims 1 to 3, wherein the content of the fatty acid and / or fatty acid derivative is greater than 0 wt % and less than 1 wt %.

5. The inorganic compound is MgAg ａ Sb ｂ The thermoelectric material according to claim 1 , wherein the following conditions are satisfied: 0.95≦a≦1.05, and 0.95≦b≦1.

05.

6. The thermoelectric material according to any one of claims 1 to 5, further comprising at least one element selected from the group consisting of copper (Cu), iron (Fe), zinc (Zn), ytterbium (Yb), and calcium (Ca).

7. The inorganic compound is Mg １－ｃ X ｃ Ag ａ Sb ｂ The thermoelectric material according to claim 6 , wherein X is Cu, Fe, Zn, Yb, or Ca, and satisfies 0.95≦a≦1.05, 0.95≦b≦1.05, and 0<c≦0.

1.

8. A thermoelectric conversion element comprising a thermoelectric conversion layer, wherein the thermoelectric conversion layer contains the thermoelectric material according to any one of claims 1 to 7.

9. A method for producing a thermoelectric material, comprising a milling step and a sintering step, wherein in the milling step, raw materials constituting an inorganic compound containing Mg, Ag, and Sb and raw materials containing predetermined amounts of fatty acid and / or fatty acid derivative are mixed and ball milled to prepare a powder, and in the sintering step, the produced powder is sintered.

10. The method for producing a thermoelectric material according to claim 9, wherein the firing temperature in the sintering step does not exceed the decomposition temperature of the fatty acid and / or fatty acid derivative.

11. The method for producing a thermoelectric material according to claim 9 or 10, wherein the inorganic compound further contains at least one element selected from the group consisting of Cu, Fe, Zn, Yb, and Ca.

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