Thermoelectric conversion material and its manufacturing method

Abstract

To provide: an α-MgAgSb thermoelectric conversion material which involves little defect and has a high thermoelectric performance in the vicinity of 200°C; and a manufacturing method which enables the large-scale synthesis thereof.SOLUTION: A method for manufacturing a thermoelectric conversion material consisting of α-MgAgSb comprises the steps of: melting, by heating, a mixture having a given component composition including Mg, Ag and Sb or a mother alloy; heating and holding a coagulum thereof at a temperature in a temperature region that makes a β-MgAgSb single phase; and then, cooling it to make a molten coagulum of an α-MgAgSb single phase. The α-MgAgSb thermoelectric material consists of the molten coagulum of the α-MgAgSb single phase. The value of a thermoelectric performance index ZT at 470 K is 0.9 or larger.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a thermoelectric conversion material made of α-MgAgSb and a method for producing the same. [Background technology]

[0002] Thermoelectric conversion materials that have been put to practical use to date include bismuth-tellurium-based materials, lead-tellurium-based materials, and silicon-germanium-based materials. These materials exhibit high thermoelectric performance at temperatures between room temperature and 100°C, 400°C, and around 800°C, respectively. However, there are almost no known materials that exhibit high thermoelectric performance in the temperature range of 100 to 250°C, which accounts for the majority of industrial waste heat. Therefore, there is a demand for thermoelectric conversion materials that cover this temperature range.

[0003] In recent years, it has been reported that α-MgAgSb has high thermoelectric performance at around 200°C (Non-Patent Documents 1 and 2). Here, MgAgSb is known to have three different crystal structures: a tetragonal α-phase in the room temperature range (approximately up to 290°C), a Cu2Sb-type β-phase in the medium temperature range (approximately 290°C to 360°C), and a half-Heusler-type γ-phase in the high temperature range (approximately 360°C or higher) (Non-Patent Document 3). [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] H. Zhao et al., Nano Energy 7, 97 (2014) [Non-Patent Document 2] J. Lei et al., Phys.Chem.Chem.Phys. 20, 16729 (2018) [Non-Patent Document 3] Melanie J. Kirkham et al., Phys. Rev. B 85, ​​144120 (2012) Summary of the Invention [Problem to be solved by the invention]

[0005] By the way, the general manufacturing method of α-MgAgSb is to sinter powder, which is not suitable for mass synthesis. For example, there is a method of sintering powder made by alloying metal components using a ball mill, or a complicated process of melting and solidifying metal components, pulverizing them, and sintering the pulverized powder to reintegrate them. Therefore, it is difficult to commercialize it through mass synthesis, and a manufacturing method suitable for mass synthesis is required for practical use.

[0006] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a manufacturing method capable of mass-synthesizing a thermoelectric conversion material made of α-MgAgSb having high thermoelectric performance at around 200°C, and to provide the thermoelectric conversion material. [Means for solving the problem]

[0007] The production method according to the present invention is a method for producing a thermoelectric conversion material made of α-MgAgSb, and is characterized in that a mixture or master alloy of a predetermined component composition made of Mg, Ag and Sb is heated and melted, the solidified material is heated and held at a temperature range where the solidified material becomes a single phase β-MgAgSb, and then cooled to form a molten solidified material of the single phase α-MgAgSb.

[0008] The thermoelectric conversion material according to the present invention is characterized in that it is made of a molten solid of α-MgAgSb and has a thermoelectric figure of merit ZT value at 470K of 0.9 or more. Effect of the Invention

[0009] The thermoelectric conversion material made of α-MgAgSb according to the present invention can be produced by a simple method in which a mixture or a master alloy in which raw metals Mg, Ag, and Sb are mixed in a predetermined composition ratio is heated and melted, and then annealed at a predetermined temperature for a predetermined period of time. This enables the mass synthesis of α-MgAgSb, a thermoelectric material that exhibits high performance and can generate electricity using a temperature difference in the temperature range of 100°C to 250°C, which has not been put to practical use until now, and thus shows the way to practical use of thermoelectric power generation using industrial waste heat. [Brief description of the drawings]

[0010] [Figure 1] Powder X-ray diffraction patterns of a heat-melted sample of MgAgxSb (x=0.97) before and after annealing. [Diagram 2] This figure shows the dependence of impurity phases on the starting composition and annealing temperature (T = 270°C, 290°C, 300°C, 310°C, 320°C, 330°C, 340°C) in samples annealed after heating and melting the starting composition MgAgxSb (x = 1, 0.99, 0.97, or 0.95). Iα and Iγ on the vertical axis Iγ / Iα represent the intensities of the strongest lines in the X-ray diffraction of α-MgAgSb and γ-MgAgSb, respectively. [Diagram 3] This figure shows the temperature dependence of the dimensionless figure of merit ZT for samples annealed after heating and melting the starting composition MgAgxSb (x = 0.99, 0.97, 0.95). [Figure 4] SEM images comparing the surface of α-MgAgSb obtained by heating and melting followed by annealing, and the surface of a pressure-sintered body of polycrystalline α-MgAgSb powder. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] In summary, one method for producing α-MgAgSb according to the present invention is to: 1) mix Mg, Ag, and Sb in a solution of MgAg x Sb yThe process comprises the steps of: weighing and mixing the components so as to satisfy the composition ratio of (0.9≦x≦1.0, 0.9≦y≦1.0), 2) heating the mixture to 900-1100° C. and then cooling to obtain a melt, and 3) annealing the melt at 270° C.≦T≦340° C. The melt obtained by this production method is composed of α-MgAgSb with a relative density of 97% or more on the surface or cross section observed by SEM.

[0012] That is, the inventors have found that by combining the melting and solidification method with heat treatment, it is possible to obtain an α-MgAgSb bulk material with high purity and few defects. That is, Mg, Ag, and Sb, which are metals constituting the low-temperature phase α-MgAgSb, are weighed out in a composition ratio corresponding to the desired α-MgAgSb, for example, a composition ratio in which the ratio of Ag and / or Sb is slightly reduced. This mixture or mother alloy is heated and melted to melt or remove impurities such as compounds. After solidification into a predetermined bulk, the bulk, which retains the low-temperature phase α-MgAgSb and the high-temperature phase γ-MgAgSb, is annealed (heat treated) for a predetermined time at a predetermined temperature to obtain a single phase β-MgAgSb, which is a medium-temperature phase. By cooling this, a melt consisting of almost a single phase of α-MgAgSb can be produced, which does not contain the high-temperature phase γ-MgAgSb or impurity phases consisting of compounds such as MgAg and Ag3Sb.

[0013] It was confirmed that this α-MgAgSb molten solid has fewer defects than conventional sintered bodies, and has a high ZT value in the temperature range of about 200° C. Annealing is performed in the temperature range of 270° C. to 340° C., which is the temperature at which the β-MgAgSb single phase, which is a medium temperature phase, is formed. Although it depends on the temperature, annealing is preferably performed for a time period that can form the β-MgAgSb single phase, for example, for 2 days or more, preferably about one week.

[0014] Previously, α-MgAgSb was produced by a method that creates an alloy powder of metal components using a ball mill, which is not suitable for mass synthesis, and then sintering the powder, or by melting the metal components, crushing and powdering them, and then sintering them together again, which were complicated processes that made it difficult to commercialize the material through mass synthesis. In contrast, the present invention has established a method for producing a thermoelectric conversion material using α-MgAgSb by a melting and solidification method that is suitable for mass synthesis, and as a result, has succeeded in easily producing high-performance α-MgAgSb. This method also makes it easy to control the crystal grain size, which can affect the thermoelectric properties.

[0015] In addition, in the case of α-MgAgSb produced by conventional methods such as sintering, a considerable number of pores are observed between the sintered crystals under SEM, but in the α-MgAgSb molten solid produced by the production method of the present invention, no such pores are observed and it has the characteristic of being almost homogeneous.

[0016] Examples will be described below. However, the examples are merely illustrative of the present invention and the present invention is not limited to the examples.

[0017] Example 1: Preparation of α-MgAgSb sample Material: MgAg x Sb

[0018] (1) Manufacturing method: As starting materials, Mg (purity 99.9%), Sb (purity 99.9999%), and Ag (purity 99.99%) were used. These were weighed so that Mg:Ag:Sb = 1:x:1 (x = 1, 0.99, 0.97, or 0.95), and placed in an alumina Tamman tube, which was then sealed in a SUS316L pipe. This was heated to 1000 ° C in an electric furnace, held at that temperature for 10 hours, and then cooled in the furnace. The resulting melt was cut, wrapped in a carbon sheet, sealed in a SUS316 pipe, and annealed at T ° C (T = 270 ° C, 290 ° C, 300 ° C, 310 ° C, 320 ° C, 330 ° C, or 340 ° C) for one week.

[0019] (2) Sample evaluation: Figure 1 shows the results of powder X-ray diffraction of a sample with x=0.97 before and after annealing at 310°C. It can be seen that the impurity phases of γ-MgAgSb, MgAg, and Ag3Sb that were present before annealing are dramatically reduced by annealing. In particular, by reducing the γ-MgAgSb phase, the power factor (S 2 / ρ; S is the thermoelectromotive force and ρ is the electrical resistance) was found to be significantly improved.

[0020] As shown in Figure 2, we investigated the dependence of the impurity phase on the annealing temperature and the starting composition. As a result, we found that the α-MgAgSb sample with the least impurity phase was obtained when the starting samples with x = 0.99, 0.97, or 0.95, which were weighed with a slight Ag deficiency, were heated to melt and annealed at 310°C.

[0021] Example 2: Thermoelectric properties of the α-MgAgSb sample obtained in Example 1

[0022] FIG. 3 shows the thermoelectric figure of merit ZT (=S 2 The thermoelectric figure of merit ZT of the obtained single-phase α-MgAgSb was 0.9 at x = 0.95 and 470 K, which exceeds the thermoelectric performance of bismuth-tellurium-based materials in this temperature range.

[0023] Example 3: SEM surface observation of the α-MgAgSb sample obtained in Example 1

[0024] Figure 4 shows SEM images of the surfaces of α-MgAgSb obtained by annealing the melt in Example 1, and pellets obtained by hot pressing polycrystals of α-MgAgSb as in Non-Patent Documents 1 and 2. It is clear from Figure 4 that the melt produced by the manufacturing method of the present invention is almost homogeneous, whereas the pellets obtained by hot pressing have a considerable number of pores between the grains of the sintered polycrystals. The relative density of the pellets obtained by hot pressing was 95%. [Industrial Applicability]

[0025] The thermoelectric conversion material comprising α-MgAgSb obtained by the present invention has high thermoelectric conversion performance in the temperature range of 100°C to 250°C and is useful for applications such as converting waste heat in this temperature range, such as waste heat from various industries, into electric power and recovering energy.

Claims

1. A method for producing a thermoelectric conversion material made of α-MgAgSb, comprising the steps of: A mixture or mother alloy of a predetermined composition consisting of Mg, Ag, and Sb is heated and melted in an electric furnace, and is held there to melt or remove impurity compounds, and then the furnace is cooled and solidified. The solidified body is heated and held at a temperature in the range of 270 to 340°C for at least two days or more to reduce the amount of γ-MgAgSb (γ phase), which is a high-temperature phase, and then cooled to obtain MgAgSb. ｘ A method for producing a thermoelectric conversion material, comprising: a molten solid of α-MgAgSb (α phase) having a composition ratio of 0.95≦x≦0.99; and a thermoelectric figure of merit ZT value at 470 K of 0.8 or more, the molten solid being made of α-MgAgSb (α phase).

2. 2. The method for producing a thermoelectric conversion material according to claim 1, wherein the thermoelectric figure of merit ZT value at 470K is 0.9 or more.

3. A thermoelectric conversion material made of α-MgAgSb, MgAg ｘ A thermoelectric conversion material comprising a molten solid of α-MgAgSb (α phase) having a composition ratio of Sb (0.95≦x≦0.99), the thermoelectric conversion material having an intensity ratio of the strongest ray of X-ray diffraction of γ-MgAgSb (γ phase), which is a high-temperature phase relative to the α phase, of 0.1 or less and a thermoelectric figure of merit ZT value at 470 K of 0.8 or more.

4. 4. The thermoelectric conversion material according to claim 3, wherein the thermoelectric figure of merit ZT value at 470K is 0.9 or more.

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