Method for producing thermoelectric conversion material

Abstract

To obtain wadalite with a small environmental impact and simplicity.SOLUTION: A manufacturing method 1 for wadalite includes a mixing step S3 in which clay minerals are mixed with salt to obtain a mixture and a heat treatment step S4 in which the mixture is heat-treated. The clay minerals are weathered layered clay minerals. The salt is a metal chloride that becomes a molten salt in the heat treatment step S4.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention ,heat The present invention relates to a method for producing an electric conversion material. [Background technology]

[0002] Conventional thermoelectric conversion materials have been produced using inorganic compounds such as lead, tellurium, bismuth, and antimony.

[0003] Patent Document 1 states: A thermoelectric material that can be mass-synthesized and that has high thermoelectric performance at temperatures around 200°C, consisting of α-MgAgSb. A method for producing the same is disclosed. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent Publication No. 2021-036579 Summary of the Invention [Problem to be solved by the invention]

[0005] Conventional thermoelectric conversion materials have been difficult to use because they are rare, expensive, toxic, and have a large environmental impact. In addition, most conventional thermoelectric conversion materials are only suitable for use in the low-temperature range of room temperature to 600°C, making them unsuitable for use in high-temperature ranges.

[0006] The present invention has been made in view of the above, and has a small environmental impact and Thermoelectric conversion materials that can be used in high-temperature environments using inexpensive materials The purpose is to obtain [Means for solving the problem]

[0007] In order to solve the above problems, the present invention provides a method for producing a thermoelectric conversion material, comprising: a mixing step of mixing a clay mineral with a salt to obtain a mixture; a heat treatment step of heat treating the mixture; a pressure molding step of pressure molding the heat-treated mixture to obtain a molded product; and a firing step of firing the molded product, The clay mineral is a weathered layered clay mineral, The salt is a metal chloride that becomes a molten salt in the heat treatment step, and is one type of the metal chloride or a mixed salt of two or more types of the metal chloride. The heat treatment step involves heating the mixture at a heat treatment temperature of 700°C or higher and lower than 850°C, and the firing step involves raising the temperature to a predetermined firing temperature at a temperature increase rate of 200°C / hour or less. It is characterized by: [Effects of the Invention]

[0008] According to the present invention, the environmental load is small and Thermoelectric conversion materials that can be used in high-temperature environments using inexpensive materials can be obtained. [Brief explanation of the drawings]

[0009] [Figure 1] 1A to 1C are diagrams showing a method for manufacturing Wadalite according to the first embodiment. [Figure 2] X-ray diffraction spectrum of weathered biotite. [Figure 3] A diagram showing the X-ray diffraction spectrum of biotite. [Figure 4] FIG. 2 is a diagram illustrating the effect of the adsorption process shown in FIG. 1 on clay minerals. [Figure 5] A diagram showing the difference in X-ray diffraction spectra of weathered biotite with or without cesium ion adsorption. [Figure 6] A diagram showing the difference in the composition of weathered biotite depending on whether or not cesium ions are adsorbed. [Figure 7] FIG. 2 is a diagram showing the production conditions and results in Examples 1 to 5 and Comparative Examples 1 to 4 of the production method shown in FIG. [Figure 8] FIG. 8 is a diagram comparing the X-ray diffraction spectrum of the product of Example 1 shown in FIG. 7 with the X-ray diffraction spectrum of Wadalite. [Figure 9] FIG. 8 is a diagram comparing the X-ray diffraction spectrum of the product of Example 2 shown in FIG. 7 with the X-ray diffraction spectrum of Wadalite. [Figure 10] FIG. 8 is a diagram comparing the X-ray diffraction spectrum of the product of Comparative Example 1 shown in FIG. 7 with the X-ray diffraction spectrum of Wadalite. [Figure 11] 8 showing the X-ray diffraction spectrum of the product of Example 1 and the main peak of FIG. 9 showing the X-ray diffraction spectrum of the product of Example 2. FIG. [Figure 12] FIG. 8 is a diagram showing the results of analyzing the compositions of the products of Examples 1 to 5 and Comparative Examples 1 to 4 shown in FIG. 7 by fluorescent X-ray analysis. [Figure 13] FIG. 10 is a diagram comparing the X-ray diffraction spectrum of the product in Example 6 of the manufacturing method according to the second embodiment with the X-ray diffraction spectrum of wadalite. [Figure 14] 5A to 5C are diagrams showing a method for producing a thermoelectric conversion material according to a third embodiment. [Figure 15] FIG. 15 is a diagram showing the manufacturing conditions in Examples 7 to 11 and Comparative Example 5 of Manufacturing Method 1 shown in FIG. [Figure 16] FIG. 16 is a graph showing the measurement results of the electrical conductivity of each product of Examples 7 to 11 and Comparative Example 5 shown in FIG. [Figure 17] FIG. 16 is a diagram showing the measurement results of the Seebeck coefficient of each product of Examples 7 to 11 and Comparative Example 5 shown in FIG. [Figure 18] FIG. 16 is a diagram showing the results of measuring the thermal diffusivity of each product of Examples 7 to 11 and Comparative Example 5 shown in FIG. [Figure 19] 16 is a graph showing the evaluation results of the dimensionless figure of merit ZT of each product of Examples 7 to 11 and Comparative Example 5 shown in FIG. 15. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, embodiments of the present invention will be described with reference to the drawings. Unless otherwise specified, components or functions denoted by the same reference numerals in the respective embodiments have the same components or functions, and the description thereof will be omitted.

[0011] [First embodiment] A method 1 for manufacturing a wadalite according to the first embodiment will be described with reference to Figures 1 to 12. Figure 1 is a diagram showing a method 1 for manufacturing a wadalite according to the first embodiment.

[0012] The features of the method 1 for producing wadalite according to this embodiment are that it uses clay minerals that are abundant in nature as starting materials, and that wadalite can be produced at lower temperatures than conventional methods by heat treatment after mixing the clay minerals with salt.

[0013] 1, the production method 1 according to this embodiment includes a crushing and classification step S1 in which a clay mineral as a starting material is crushed and classified, and an adsorption step S2 in which a predetermined metal element is adsorbed onto the classified clay mineral. Furthermore, the production method 1 according to this embodiment includes a mixing step S3 in which a salt is mixed with the clay mineral to obtain a mixture, and a heat treatment step S4 in which the mixture is heat treated.

[0014] <Starting materials> Figure 2 shows the X-ray diffraction spectrum of weathered biotite. Figure 3 shows the X-ray diffraction spectrum of biotite.

[0015] The clay mineral used as the starting material is a layered clay mineral. Examples of layered clay minerals include 1:1 type clay minerals and 2:1 type clay minerals. Specific examples of 1:1 type clay minerals include serpentine group minerals (e.g., kaolinite). Specific examples of 2:1 type clay minerals include talc-pyrophyllite group minerals (e.g., talc), smectite group minerals (e.g., montmorillonite), vermiculite group minerals (e.g., weathered biotite), mica group minerals (e.g., biotite, illite, biotite), brittle mica group minerals, and chlorite group minerals.

[0016] The clay mineral serving as the starting material for Production Method 1 according to this embodiment is preferably a weathered layered clay mineral, with weathered biotite being particularly preferred. Weathered biotite is naturally abundant and easily available. Biotite itself has a layered crystalline structure consisting of thin, sheet-like clay layers stacked one on top of the other. Weathering of weathered biotite often causes cleavage, in which the sheet-like clay layers peel off in specific directions, resulting in swelling, particularly at the outer edges. This increases the spacing between the clay layers in weathered biotite, facilitating the entry of cations between the clay layers and promoting reaction with salt in the heat treatment step S4. Furthermore, weathered biotite is prone to adsorb metal elements in the adsorption step S2, making it easy to produce high-purity wadalite, as described below. For these reasons, weathered biotite is preferred as the starting material for Production Method 1 according to this embodiment.

[0017] As shown in Figure 2, the X-ray diffraction spectrum of weathered biotite has an asymmetric and broad peak at diffraction angles of around 6° to 10°. This is thought to be due to the disruption of the layered crystal structure of weathered biotite due to weathering. Because biotite has high crystallinity, the X-ray diffraction spectrum of biotite has sharp peaks, as shown in Figure 3. In other words, weathered biotite and biotite can be distinguished by comparing their X-ray diffraction spectra. The X-ray diffraction spectrum is a spectrum that represents the diffraction intensity measured by X-ray diffraction at each diffraction angle.

[0018] <Crushing / classification process S1> The clay mineral as the starting material may be in bulk form, but in order to produce uniform wadalite with high efficiency, it is preferable to pulverize it to a predetermined particle size. The particle size of the clay mineral pulverized in the pulverization and classification step S1 is not particularly limited, and may be, for example, an average particle size of 5 μm or less, or an average particle size of more than 5 μm and 20 μm or less. The method for pulverizing the clay mineral in the pulverization and classification step S1 is not particularly limited, and may be, for example, a pulverization method using an automatic mortar.

[0019] The method for classifying the clay mineral in the pulverization and classification step S1 is not particularly limited, and may be, for example, a classification method using elutriation. A classification method using elutriation is a method in which pulverized clay mineral is placed in water, stirred to remove impurities, and then dried to classify the clay mineral. A classification method using elutriation is preferable in that it can obtain clay minerals of any particle size and remove impurities, making it possible to produce more uniform wadalite.

[0020] <Adsorption process S2> Fig. 4 is a diagram illustrating the effect of the adsorption step S2 shown in Fig. 1 on clay minerals. Fig. 5 is a diagram showing the difference in X-ray diffraction spectrum of weathered biotite depending on whether or not cesium ions are adsorbed. Fig. 6 is a diagram showing the difference in composition of weathered biotite depending on whether or not cesium ions are adsorbed.

[0021] In the adsorption step S2, a predetermined metal element ion is adsorbed onto the clay mineral obtained in the pulverization and classification step S1. The metal element to be adsorbed is not particularly limited, as long as it has an ionic radius equal to or greater than that of the metal (e.g., calcium) contained in the salt (e.g., calcium chloride) used in the subsequent mixing step S3 and heat treatment step S4. In other words, the ionic radius of the adsorbed metal element is equal to or greater than that of the metal contained in the salt mixed in the mixing step S3. Examples of the adsorbed metal element include sodium, potassium, rubidium, and cesium. When the clay mineral is weathered biotite, cesium is preferably the adsorbed metal element.

[0022] In the adsorption step S2, first, as shown in FIG. 4(a), the metal element ions (cesium ions) to be adsorbed penetrate from the outer edge of the clay mineral into the spaces between the clay layers that make up the clay mineral and are adsorbed. Then, as shown in FIG. 4(b), new metal element ions are adsorbed next to the adsorbed metal element ions, and as shown in FIG. 4(c), the metal element ions are adsorbed continuously (or collectively) between the clay layers. Then, as shown in FIG. 4(d), the adsorption of the mass of metal element ions causes the interlayer distance of the clay layers to which the metal element ions are adsorbed to shrink. Then, as shown in FIG. 4(e), adjacent clay layers peel off, and surface adsorption sites for the metal element ions appear.

[0023] As shown in Figures 4(a) to 4(e), in the adsorption step S2, metal element ions are adsorbed between the clay layers, shrinking the interlayer distance between the clay layers and uniforming the interlayer distance along the clay layer. In fact, the interlayer distance of weathered biotite without cesium ion adsorption is approximately 10.54 Å, while the interlayer distance of weathered biotite with cesium ion adsorption is approximately 10.20 Å. Furthermore, as shown in Figure 5, the X-ray diffraction spectrum of weathered biotite with cesium ion adsorption has a sharper peak around 6° to 10° than the X-ray diffraction spectrum of weathered biotite without cesium ion adsorption. This is thought to be due to the fact that the adsorbed cesium ions increase the regularity of the clay layers in the weathered biotite with cesium ion adsorption, resulting in a more uniform layered structure. Weathered biotite absorbs cesium ions, which have a large ionic radius, and exchanges them with potassium ions that were originally present between the clay layers. In fact, as shown in Figure 6, the amount of cesium ions in weathered biotite with cesium ions adsorbed is higher than that in weathered biotite without cesium ion adsorption, but the amount of potassium ions is reduced.

[0024] When cesium ions are adsorbed between the clay layers of weathered biotite in the adsorption step S2, not only do cesium ions and ions with similar chemical properties become more easily adsorbed between the layers, but the interlayer space also becomes more uniform. This allows calcium ions, which have a large ionic radius, to penetrate deep between the clay layers and easily exchange for the cesium ions adsorbed between the layers during the subsequent heat treatment step S4, when a salt (e.g., calcium chloride) reacts with the weathered biotite. Therefore, adsorbing cesium ions in the adsorption step S2 effectively promotes the reaction between the weathered biotite and salt in the subsequent heat treatment step S4. However, the adsorption step S2 is not essential to the production method 1 according to this embodiment.

[0025] One method for implementing the adsorption step S2 is to mix a solution containing the metal element ions to be adsorbed (e.g., a cesium chloride solution) with a clay mineral (e.g., weathered biotite) to adsorb the metal element ions onto the clay mineral. In this case, it is preferable to mix the metal element with the clay mineral so that the metal element is added in an amount of 2 wt% to 6 wt% based on the weight of the clay mineral. This allows the metal element ions to be adsorbed onto the clay mineral in the correct amount in the adsorption step S2. Specifically, for example, an elutriation material is dispersed in a cesium chloride solution, which is then mixed with weathered biotite and gently stirred overnight to obtain weathered biotite with adsorbed cesium ions. The cesium chloride solution and weathered biotite are mixed so that the cesium is added in an amount of 2 wt% to 6 wt% based on the weight of the weathered biotite.

[0026] <Mixing process S3> In the mixing step S3, salt is mixed with the clay mineral obtained in the crushing and classification step S1 or the adsorption step S2 to obtain a mixture. The salt to be mixed is a metal chloride that will become a molten salt in the subsequent heat treatment step S4. This is because if the mixed salt becomes a molten salt when heated to a high temperature in the subsequent heat treatment step S4, it will act as an ion and be able to produce wadalite. However, if the mixed salt sublimes when heated to a high temperature, wadalite cannot be produced. For example, calcium chloride is a preferred salt to be mixed. For example, calcium chloride powder having an average particle size equivalent to that of calcium chloride powder manufactured by Showa Chemical Co., Ltd. and having product codes 0305-2250, 0305-2260, or 0305-2280 is preferred.

[0027] In the mixing step S3, the clay mineral (e.g., weathered biotite) obtained in the crushing and classification step S1 or the adsorption step S2 may be mixed with salt (e.g., calcium chloride) in equal amounts. Specifically, for example, weathered biotite and calcium chloride may be mixed in equal amounts. This means that it is sufficient to prepare a large amount of calcium chloride for reaction. This is thought to be due to the structural characteristics of weathered biotite, namely, the clay layers swell and the interlayer spaces widen due to weathering. In the conventional Patent Document 1 (JP 2013-23400 A), as described in paragraph 0067 thereof, a large amount of calcium chloride—four times the amount of amorphous material—waddalite was required. In contrast, the manufacturing method 1 according to the present embodiment allows for the production of wadalite even when a smaller amount of salt is mixed than in the past.

[0028] <Heat treatment step S4> In the heat treatment step S4, the mixture obtained in the mixing step S3 is heat-treated. Specifically, for example, a mixture of weathered biotite and calcium chloride is heated at a heat treatment temperature of 600°C or higher and lower than 850°C using a general electric furnace such as a circular electric furnace. The heat treatment time is not particularly limited, but may be, for example, 2 hours. After the heat treatment step S4, in the manufacturing method 1 according to this embodiment, the heat-treated product obtained in the heat treatment step S4 is washed with water and subjected to solid-liquid separation by centrifugation, after which the liquid phase is discarded and the remaining solid phase is dried at 80°C to obtain wadalite.

[0029] When the mixture is a mixture of weathered biotite with cesium adsorption and calcium chloride, the heat treatment temperature in the heat treatment step S4 may be 600°C or higher and lower than 850°C. A heat treatment temperature of 700°C or higher and lower than 850°C in the heat treatment step S4 is preferable because high-purity wadalite can be produced. Furthermore, when the mixture is a mixture of weathered biotite without cesium adsorption and calcium chloride, the heat treatment temperature in the heat treatment step S4 may be 700°C or higher and lower than 850°C. Wadalite can be produced even at a low heat treatment temperature of less than 850°C in the heat treatment step S4 because the clay layers of the weathered biotite have wide gaps between them, making it easy for calcium ions to penetrate between the layers and react with the weathered biotite.

[0030] In addition, when producing a film-like wadalite on a substrate, if wadalite can be produced at a low temperature as in the heat treatment step S4 of this embodiment, the heat resistance of the substrate is less restricted. Therefore, the manufacturing method 1 according to this embodiment is less restricted in the type of substrate used during production, which improves the flexibility of the manufacturing environment and makes it easier to produce wadalite.

[0031] <Example> Fig. 7 is a diagram showing the production conditions and results for Examples 1 to 5 and Comparative Examples 1 to 4 of Production Method 1 shown in Fig. 1. In the results shown in Fig. 7, a circle indicates that wadalite was produced, a double circle indicates that high-purity wadalite was produced, and an x ​​indicates that wadalite could not be produced.

[0032] In Example 1, weathered biotite from Ono Town, Fukushima Prefecture was prepared as the starting clay mineral. The weathered biotite was then crushed and classified using a crusher. The particle size of the crushed and classified weathered biotite was measured using a laser diffraction scattering method (using a Zetasizer Nano-ZS, manufactured by Malvern Panalytical). The average particle size was found to be 5 μm. The X-ray diffraction spectrum of the weathered biotite is shown in Figure 2, confirming its crystalline nature. This is clearly different from the conventional method described in Patent Document 1, which uses amorphous materials to produce wadalite.

[0033] Next, in Example 1, 0.5 g of crushed and classified weathered biotite was mixed with 0.5 g of calcium chloride and stirred to obtain a uniform mixture. That is, in Example 1, the same mass of calcium chloride was mixed with weathered biotite without cesium ion adsorption to obtain a mixture. The obtained mixture was heat-treated at 700°C for 2 hours. The heat-treated product obtained by the heat treatment was washed with water and centrifuged for solid-liquid separation. The liquid phase was discarded, and the remaining solid phase was dried at 80°C to obtain the product of Example 1.

[0034] Next, the X-ray diffraction spectrum of the product of Example 1 was measured by powder X-ray diffraction method and compared with the X-ray diffraction spectrum of Wadalite.

[0035] Figure 8 is a diagram comparing the X-ray diffraction spectrum of the product of Example 1 shown in Figure 7 with the X-ray diffraction spectrum of Wadalite. In Figure 8, the gray line shows the X-ray diffraction spectrum of the product of Example 1, and the dashed line shows the X-ray diffraction spectrum of Wadalite.

[0036] As shown in FIG. 8, the X-ray diffraction spectrum of the product of Example 1 had a main peak with the highest diffraction intensity at a diffraction angle of approximately 33 degrees, which matched the X-ray diffraction spectrum of wadalite. The X-ray diffraction spectrum of the product of Example 1 also matched the X-ray diffraction spectrum of wadalite for peaks other than the main peak. Therefore, the product of Example 1 was confirmed to be wadalite. However, the X-ray diffraction spectrum of the product of Example 1 has a peak near 6 degrees to 10 degrees. This peak is a peak derived from the layered structure of weathered biotite. In other words, it was confirmed that the product of Example 1 was mostly wadalite, but that the layered structure of weathered biotite remained in a portion of it.

[0037] In Example 2, a cesium chloride solution was mixed with weathered biotite that had been crushed and classified in the same manner as in Example 1, allowing cesium ions to be adsorbed onto the weathered biotite. The cesium chloride solution was mixed with the weathered biotite so that 4.0 wt% of cesium was added relative to the weight of the weathered biotite. Other production conditions in Example 2 were the same as those in Example 1.

[0038] Figure 9 is a diagram comparing the X-ray diffraction spectrum of the product of Example 2 shown in Figure 7 with the X-ray diffraction spectrum of Wadalite. In Figure 9, the gray line shows the X-ray diffraction spectrum of the product of Example 2, and the dashed line shows the X-ray diffraction spectrum of Wadalite.

[0039] As shown in Figure 9, the X-ray diffraction spectrum of the product of Example 2 had a main peak with the highest diffraction intensity at a diffraction angle of approximately 33 degrees, which matched the X-ray diffraction spectrum of wadalite. The X-ray diffraction spectrum of the product of Example 2 also matched the X-ray diffraction spectrum of wadalite for peaks other than the main peak. Furthermore, the X-ray diffraction spectrum of the product of Example 2 did not have a peak derived from the layered structure of weathered biotite that appears around 6 degrees to 10 degrees. Therefore, the layered structure of weathered biotite completely disappeared from the product of Example 2, confirming that the product is wadalite of higher purity than Example 1.

[0040] In Example 3, a mixture of weathered biotite with cesium ions adsorbed thereon and calcium chloride was heat-treated at a temperature of 600°C for 2 hours. Other production conditions in Example 3 were the same as those in Example 2.

[0041] The X-ray diffraction spectrum of the product of Example 3, like the X-ray diffraction spectrum of the product of Example 1, has peaks derived from the layered structure of weathered biotite, but most of the peaks, including the main peak, match the X-ray diffraction spectrum of wadalite. It was confirmed that the product of Example 3 is mostly wadalite, but the layered structure of weathered biotite remains in some parts.

[0042] In Example 4, the weathered biotite was mixed with a cesium chloride solution so that 2.2 wt% of cesium was added to the weight of the weathered biotite, and cesium ions were adsorbed onto the weathered biotite. The other production conditions in Example 4 were the same as those in Example 2.

[0043] The X-ray diffraction spectrum of the product of Example 4 did not have peaks derived from the layered structure of weathered biotite, similar to the X-ray diffraction spectrum of the product of Example 2, and matched the X-ray diffraction spectrum of wadalite. The product of Example 4 was confirmed to be high-purity wadalite.

[0044] In Example 5, a mixture of weathered biotite with cesium ions adsorbed thereon and calcium chloride was heat-treated at a temperature of 600°C for 2 hours. Other production conditions in Example 5 were the same as those in Example 4.

[0045] The X-ray diffraction spectrum of the product of Example 5 has peaks derived from the layered structure of weathered biotite, similar to the X-ray diffraction spectra of the products of Examples 1 and 3. However, most of the peaks, including the main peak, matched the X-ray diffraction spectrum of wadalite. It was confirmed that the product of Example 5 was mostly wadalite, but that the layered structure of weathered biotite remained in some parts.

[0046] In Comparative Example 1, biotite from India was prepared as the clay mineral starting material, and the biotite was crushed and classified in a crusher in the same manner as in Example 1. Other production conditions in Comparative Example 1 were the same as in Example 1.

[0047] Fig. 10 is a diagram comparing the X-ray diffraction spectrum of the product of Comparative Example 1 shown in Fig. 7 with the X-ray diffraction spectrum of Wadalite. In Fig. 10, the gray line shows the X-ray diffraction spectrum of the product of Comparative Example 1, and the dashed line shows the X-ray diffraction spectrum of Wadalite.

[0048] As shown in Figure 10, the X-ray diffraction spectrum of the product of Comparative Example 1 did not have a main peak at a diffraction angle of around 33 degrees, and did not match the X-ray diffraction spectrum of wadalite. Therefore, it was confirmed that the product of Comparative Example 1 was not wadalite. The inventors confirmed that pyroxene (pyroxene) or hematite (hematite) was produced under the production conditions of Comparative Example 1.

[0049] In Comparative Example 2, a mixture of weathered biotite without cesium ion adsorption and calcium chloride was heat-treated at 600°C for 2 hours. Other production conditions for Comparative Example 2 were the same as those for Example 1.

[0050] The X-ray diffraction spectrum of the product of Comparative Example 2 did not have a main peak at a diffraction angle of around 33 degrees, and did not match the X-ray diffraction spectrum of wadalite. Most of the product of Comparative Example 2 maintained the layered structure of weathered biotite, and it was confirmed that it was not wadalite.

[0051] In Comparative Example 3, a mixture of weathered biotite with cesium ions adsorbed thereon and calcium chloride was heat-treated at 500°C for 2 hours. Other production conditions for Comparative Example 3 were the same as those for Example 2.

[0052] The X-ray diffraction spectrum of the product of Comparative Example 3 did not have a main peak at a diffraction angle of around 33 degrees, and did not match the X-ray diffraction spectrum of wadalite. Most of the product of Comparative Example 3 maintained the layered structure of weathered biotite, and it was confirmed that it was not wadalite.

[0053] In Comparative Example 4, a mixture was obtained by mixing 10 times the mass of calcium chloride with the weathered biotite having cesium ions adsorbed thereon. Other production conditions for Comparative Example 4 were the same as those for Comparative Example 3.

[0054] The X-ray diffraction spectrum of the product of Comparative Example 4 did not have a main peak at a diffraction angle of around 33 degrees, and did not match the X-ray diffraction spectrum of wadalite. Most of the product of Comparative Example 4 maintained the layered structure of weathered biotite, and it was confirmed that it was not wadalite.

[0055] <Considerations on the Examples> From the results of Example 1, Comparative Example 1, and Comparative Example 2, it can be seen that when the clay mineral is biotite, wadalite cannot be produced at a heat treatment temperature of 700°C or less, but when the clay mineral is weathered biotite, wadalite can be produced at a heat treatment temperature of 700°C or more. As mentioned above, this is thought to be because, in weathered biotite, the clay layers swell due to weathering and the spaces between the layers widen, making it easier for cations to enter between the clay layers and facilitating the reaction with salt in the heat treatment step S4.

[0056] The results of Examples 1 and 2 show that high-purity wadalite can be produced by adsorbing cesium ions to weathered biotite. As mentioned above, this is thought to be because the adsorption of cesium ions not only makes it easier for ions with similar chemical properties to cesium ions to be adsorbed between the layers, but also because the spaces between the layers are uniformly spread, making it easier for the ions to penetrate deep between the clay layers and facilitating their reaction with salt in the heat treatment step S4.

[0057] The results of Examples 1 and 3 show that wadalite can be produced even when the heat treatment temperature is lowered to 600°C by adsorbing cesium ions to weathered biotite. This is thought to be because, as mentioned above, the adsorption of cesium ions easily promotes the reaction with salt in the heat treatment step S4. In other words, the adsorption of cesium ions allows the heat treatment temperature to be lowered.

[0058] The results of Comparative Examples 3 and 4 show that when the heat treatment temperature is 500°C, wadalite cannot be produced even if the amount of calcium chloride is increased by 10 times.

[0059] From these findings, it was found that it is important to use weathered layered clay minerals (e.g., weathered biotite) in order to produce wadalite at a low heat treatment temperature of 700°C. Furthermore, it was found that it is important to adsorb metal element ions (e.g., cesium ions) onto the weathered layered clay minerals in order to lower the heat treatment temperature.

[0060] FIG. 11 is a diagram comparing the main peaks in FIG. 8, which shows the X-ray diffraction spectrum of the product of Example 1, with the main peaks in FIG. 9, which shows the X-ray diffraction spectrum of the product of Example 2.

[0061] 11, the main peak of the product of Example 1 had a full width at half maximum (FWHM) of 0.258 and a diffraction intensity of about 7000 cps (counts per second), while the main peak of the product of Example 2 had a full width at half maximum (FWHM) of 0.197 and a diffraction intensity of about 8000 cps.

[0062] In other words, the main peak of Example 2, in which high-purity wadalite was produced without a peak derived from the layered structure of weathered biotite, has a smaller half-width and higher diffraction intensity than the main peak of Example 1, in which wadalite was produced with a peak derived from the layered structure of weathered biotite.

[0063] Here, if impurities are present inside the crystal, when measured by X-ray diffraction, the X-rays are scattered inside the crystal, causing the diffraction intensity peak to broaden and the diffraction intensity of the peak to decrease. Therefore, if impurities are present inside the crystal, the half-width of the main peak tends to increase. Furthermore, if the impurities inside the crystal vary, dispersion of the diffraction angle also occurs, so the half-width of the main peak tends to increase further. Therefore, the smaller the half-width of the main peak and the higher the diffraction intensity, the higher the purity of the wadalite can be considered.

[0064] Therefore, by evaluating not only the peaks derived from the layered structure of weathered biotite but also the half-width and diffraction intensity of the main peak, it is possible to accurately determine whether high-purity wadalite has been produced.

[0065] Furthermore, from the results of Examples 1 and 2 and Figure 11, etc., it can be seen that by subjecting a mixture of weathered biotite and calcium chloride in equal masses to a heat treatment at a temperature of 700°C or higher and lower than 850°C, it is possible to produce wadalite having a main peak full width at half maximum (FWHM) of 0.197 or higher and 0.258 or lower. In other words, the wadalite produced by production method 1 according to this embodiment includes wadalite having a main peak full width at half maximum (FWHM) of 0.197 or higher and 0.258 or lower.

[0066] FIG. 12 is a diagram showing the results of analyzing the compositions of the products of Examples 1 to 5 and Comparative Examples 1 to 4 shown in FIG. 7 by fluorescent X-ray analysis.

[0067] In Examples 1 to 5, naturally occurring weathered biotite was used as the starting material, and therefore it was found that the wadalite produced in Examples 1 to 5 contained at least one element of iron, magnesium, titanium, potassium, and manganese. That is, the wadalite produced by Production Method 1 according to this embodiment includes wadalite containing at least one element of iron, magnesium, titanium, potassium, and manganese.

[0068] Figure 12 also shows the results of X-ray fluorescence analysis of the composition of weathered biotite without cesium ion adsorption, weathered biotite with cesium ion adsorption, and biotite. It can be seen that the proportion of elements such as iron is smaller in weathered biotite than in biotite. This is thought to be due to the leaching of elements such as iron caused by the weathering of biotite.

[0069] [Second embodiment] A method 1 for manufacturing a Wadalite according to the second embodiment will be described with reference to Fig. 13. In the manufacturing method 1 according to the second embodiment, the description of the same configuration as in the first embodiment will be omitted.

[0070] The method 1 for manufacturing Wadalite according to the second embodiment differs from the first embodiment in the adsorption step S2, but the steps other than the adsorption step S2 are the same as those in the first embodiment.

[0071] That is, in the adsorption step S2 according to the first embodiment, a solution containing the metal element ions to be adsorbed (e.g., a cesium chloride solution) is mixed with a clay mineral (e.g., weathered biotite), thereby adsorbing the metal element ions onto the clay mineral.

[0072] In contrast, in the adsorption step S2 according to the second embodiment, a metal element is mixed with a clay mineral and heat-treated, thereby adsorbing the metal element ions onto the clay mineral. Specifically, in the adsorption step S2 according to the second embodiment, for example, cesium chloride powder is mixed with weathered biotite and heat-treated, thereby adsorbing cesium ions onto the weathered biotite.

[0073] <Example> In Example 6, an example of Manufacturing Method 1 according to the second embodiment, weathered biotite was prepared by pulverizing and classifying it in the same manner as in Example 2 shown in FIG. 7 . Then, in Example 6, the weathered biotite was mixed with cesium chloride powder in the same mass, and the mixture was heat-treated at 700°C for 2 hours. The mixture was then washed with water to remove excess salt. This resulted in a powder material of weathered biotite with adsorbed cesium ions. The adsorption ratio of cesium ions to the weathered biotite in the obtained powder material was 13.0 wt% (weight of cesium relative to the weight of weathered biotite). Other manufacturing conditions for Example 6 were the same as those for Example 2.

[0074] The inventors have confirmed that the adsorption process S2 according to the second embodiment allows cesium ions to be adsorbed onto the weathered biotite so that cesium is added in an amount of 13.0 wt% or more and 15 wt% or less of the weight of the weathered biotite.

[0075] 13 is a diagram comparing the X-ray diffraction spectrum of the product in Example 6 of Manufacturing Method 1 according to the second embodiment with the X-ray diffraction spectrum of wadalite. In Fig. 13, the gray line shows the X-ray diffraction spectrum of the product in Example 6, and the dashed line shows the X-ray diffraction spectrum of wadalite.

[0076] The X-ray diffraction spectrum of the product of Example 6 did not have peaks derived from the layered structure of weathered biotite, similar to the X-ray diffraction spectrum of the product of Example 2, and matched the X-ray diffraction spectrum of wadalite. The product of Example 6 was confirmed to be high-purity wadalite.

[0077] The adsorption step S2 of the second embodiment involves heat treatment, which eliminates the need to stir the cesium chloride solution mixed with weathered biotite, and therefore allows cesium ions to be adsorbed onto the weathered biotite in a shorter time than in the first embodiment.

[0078] [Third embodiment] 14 to 19, a method 1 for producing a thermoelectric conversion material according to the third embodiment will be described. In the method 1 according to the third embodiment, the same configuration as in the first embodiment will not be described.

[0079] FIG. 14 is a diagram showing a method 1 for producing a thermoelectric conversion material according to the third embodiment.

[0080] The manufacturing method 1 according to the third embodiment is a method for manufacturing a thermoelectric conversion material by adding a pressure-molding step S5 and a firing step S6 to the manufacturing method 1 according to the first embodiment.

[0081] Specifically, in the production method 1 according to the third embodiment, the pulverization and classification step S1 and the heat treatment step S4 may be the same steps as in the first embodiment. The production method 1 according to the third embodiment can produce a thermoelectric conversion material regardless of whether or not the adsorption step S2 is included. That is, the production method 1 according to the third embodiment may include the adsorption step S2 similar to the first or second embodiment, but may not include the adsorption step S2 from the perspective of process simplification. The production method 1 according to the third embodiment differs from the first embodiment in the mixing step S3. The production method 1 according to the third embodiment has a pressure-molding step S5 and a firing step S6 added to the first embodiment. Below, the steps of the production method 1 according to the third embodiment that differ from the first embodiment will be described.

[0082] <Mixing process S3> In the mixing step S3, one or more alkali metal chlorides are mixed with the clay mineral obtained in the crushing and classification step S1 to obtain a mixture. The alkali metal salt to be mixed is not particularly limited, but calcium chloride, sodium chloride, potassium chloride, or magnesium chloride is preferred. For example, the melting points of calcium chloride, potassium chloride, and magnesium chloride are approximately 700°C to 800°C, but a mixed salt obtained by mixing two or more alkali metal chlorides has a lower eutectic temperature at a specific mixing ratio. Therefore, the mixed salt can be expected to lower the reaction temperature with the clay mineral. Furthermore, by adjusting the mixing ratio, the mixed salt can produce a thermoelectric conversion material exhibiting desired performance. For these reasons, the mixing step S3 is a characteristic step of the production method 1 according to this embodiment.

[0083] <Pressure molding process S5> In the manufacturing method 1 according to the third embodiment, similar to the first embodiment, the heat-treated product obtained in the heat treatment step S4 is washed with water, subjected to solid-liquid separation by centrifugation, and the remaining solid phase is dried to obtain a powder material (Wadalite). In the pressure molding step S5, the powder material is subjected to a pressure molding process to obtain a molded product. Specifically, in the pressure molding step S5, the powder material is placed in a tablet molder with a diameter of 16 mm, and pressed into a pellet shape using a hydraulic press under a load of 10 T.

[0084] The pressure molding step S5 can reduce the amount of voids remaining inside the molded product. This can prevent the voids remaining inside the molded product from expanding due to the influence of heat during the subsequent firing step S6, causing cracks or distortion of the molded product. The pressure molding step S5 can be performed to an extent that the voids remaining inside the molded product can be reduced as much as possible, and the method for performing the pressure molding is not particularly limited. The pressure molding step S5 may be performed together with the firing step S6.

[0085] <Baking process S6> In the firing step S6, the molded product obtained in the pressure molding step S5 is again subjected to a heat treatment to fire the molded product. Specifically, in the firing step S6, a general electric furnace such as a circular electric furnace is used to heat the molded product to a predetermined firing temperature at a rate of 100°C / h to 200°C / h, and the firing temperature is maintained for a predetermined firing time to fire the molded product. The firing temperature is not particularly limited, but may be, for example, 900°C to 950°C. The firing time is also not particularly limited, but may be, for example, 5 hours.

[0086] Regarding the heating rate, a rapid temperature increase is undesirable because it causes voids in the molded product to rapidly escape to the outside, resulting in cracks or distortion. Because molded products have low thermal conductivity, the heating rate conditions must be strictly controlled. Through repeated experiments, the inventors found that a heating rate exceeding 200°C / h causes cracks and other defects in the molded product. Therefore, a heating rate of 200°C / h or less is preferable. From the perspective of shortening the manufacturing time, a heating rate of 100°C / h or more and 200°C / h or less is more preferable. As mentioned above, since cracks or distortions in the molded product are thought to be due to voids within the molded product, it is important to thoroughly dry the moisture in the molded product. Manufacturing method 1 according to the third embodiment may include a drying step of thoroughly drying the molded product prior to firing step S6.

[0087] <Example> FIG. 15 is a diagram showing the manufacturing conditions in Examples 7 to 11 and Comparative Example 5 of Manufacturing Method 1 shown in FIG.

[0088] In Example 7, weathered biotite was prepared as the starting clay mineral, and the weathered biotite was crushed and classified using a crusher. The particle size of the crushed and classified weathered biotite was measured using a laser diffraction scattering method (using a Zetasizer Nano-ZS, manufactured by Malvern Panalytical) and found to be 5 μm in average particle size.

[0089] Next, in Example 7, 0.5 g of crushed and classified weathered biotite was mixed with 0.5 g of calcium chloride and stirred to obtain a uniform mixture. That is, in Example 7, the same mass of weathered biotite and calcium chloride were mixed to obtain a mixture. The obtained mixture was heat-treated at 700°C for 2 hours. The heat-treated product obtained by the heat treatment was washed with water and centrifuged for solid-liquid separation. The liquid phase was discarded, and the remaining solid phase was dried at 80°C to obtain a powder material.

[0090] Next, in Example 7, the obtained powder material was placed in a tablet molding machine with a diameter of 16 mm and pressed into a pellet shape using a hydraulic press under a load of 10 T to obtain a molded product. Next, in Example 7, the obtained molded product was heat-treated again at a heating rate of 100 ° C. / h, a firing temperature of 950 ° C., and a firing time of 5 hours, and the molded product was fired. The obtained fired product was then naturally cooled to obtain the product of Example 7.

[0091] In Example 8, 0.5 g of weathered biotite, which had been crushed and classified in the same manner as in Example 7, was uniformly mixed with 0.5 g of a mixed salt obtained by mixing calcium chloride and sodium chloride in a mixing ratio of calcium chloride:sodium chloride = 1:1 to obtain a mixture. Other production conditions in Example 8 were the same as in Example 7.

[0092] In Example 9, calcium chloride and potassium chloride were selected as the mixed salts, and the mixing ratio of calcium chloride:potassium chloride was 1:1. Other production conditions in Example 9 were the same as those in Example 8.

[0093] In Example 10, the mixing ratio of the mixed salts was calcium chloride:potassium chloride=1:3. Other production conditions in Example 10 were the same as those in Example 9.

[0094] In Example 11, the mixing ratio of the mixed salts was calcium chloride:potassium chloride = 3: 1. Other production conditions in Example 11 were the same as those in Example 9.

[0095] In Comparative Example 5, the heat treatment in the heat treatment step S4 was carried out without mixing salt with weathered biotite, which is a clay mineral. Other production conditions in Comparative Example 5 were the same as those in Example 7.

[0096] 16 to 19, the performance evaluation of each product of Examples 7 to 11 and Comparative Example 5 shown in FIG. 15 will be described.

[0097] The performance of thermoelectric conversion materials can be evaluated using the dimensionless figure of merit ZT of the thermoelectric conversion material. The dimensionless figure of merit ZT is ZT=S 2 It is expressed by the formula σT / κ, where S is the Seebeck coefficient of the thermoelectric conversion material, T is the absolute temperature of the thermoelectric conversion material, σ is the electrical conductivity of the thermoelectric conversion material, and κ is the thermal conductivity of the thermoelectric conversion material. The thermal conductivity κ can be calculated by multiplying the specific heat and density by the thermal diffusivity. Therefore, in order to evaluate the performance of each product of Examples 7 to 11 and Comparative Example 5, the electrical conductivity [S / cm], Seebeck coefficient [μV / K], thermal diffusivity [mm 2 / s] was measured, and ZT in the temperature range was calculated.

[0098] Fig. 16 is a diagram showing the measurement results of the electrical conductivity of each product of Examples 7 to 11 and Comparative Example 5 shown in Fig. 15. Fig. 17 is a diagram showing the measurement results of the Seebeck coefficient of each product of Examples 7 to 11 and Comparative Example 5 shown in Fig. 15. Fig. 18 is a diagram showing the measurement results of the thermal diffusivity of each product of Examples 7 to 11 and Comparative Example 5 shown in Fig. 15. Fig. 19 is a diagram showing the evaluation results of the dimensionless figure of merit ZT of each product of Examples 7 to 11 and Comparative Example 5 shown in Fig. 15.

[0099] Generally, the conductivity of clay minerals at room temperature is 10 -13 , but the conductivity of each product of Examples 7 to 11 is on the order of 10 -5The improvement is on the order of . It is clear from the equation for the dimensionless figure of merit ZT that an improvement in electrical conductivity contributes to an improvement in the dimensionless figure of merit ZT. Therefore, it can be said that the products of Examples 7 to 11 have improved thermoelectric conversion performance compared to simple clay minerals. Furthermore, as shown in FIG. 19, the dimensionless figure of merit ZT of each of the products of Examples 7 to 11 shows fluctuations in the high temperature range (650°C to 850°C). Therefore, it was confirmed that each of the products of Examples 7 to 11 is useful as a thermoelectric conversion material.

[0100] There are few examples of conventional thermoelectric conversion materials being evaluated for their thermoelectric performance in high-temperature regions. This is because many conventional thermoelectric conversion materials are structurally unstable in high-temperature regions, and thermoelectric performance cannot be expected. In addition, there was concern that conventional thermoelectric conversion materials would crack or deform due to the effects of heat when placed in a high-temperature environment.

[0101] In contrast, the products of Examples 7 to 11 exhibit performance as thermoelectric conversion materials even in high-temperature regions, as shown in Figure 19. Furthermore, because clay minerals are used as starting materials, which are also used in pottery, they are less susceptible to heat and have heat resistance, making them suitable for use in high-temperature regions. Therefore, production method 1 according to the third embodiment can produce thermoelectric conversion materials that can be used in high-temperature regions.

[0102] <Considerations on the Examples> The evaluation results of Examples 7 to 11 and Comparative Example 5 revealed that the dimensionless figure of merit ZT was improved when one or more alkali metal chlorides were mixed compared to when no alkali metal salts were mixed. Therefore, it can be seen that the inclusion of the mixing step S3 in which one or more alkali metal salts are mixed has a favorable effect on thermoelectric conversion performance. This is thought to be due to the additive contribution of electronic conduction carried out by conduction electrons and holes and ionic conduction carried out by point defects. Therefore, any alkali metal salt that can be expected to have this effect is thought to achieve the effects of this embodiment.

[0103] In Examples 9 to 11, it was found that by preparing a mixed salt of two types of alkali metal chlorides and varying the mixing ratio, it was possible to shift the peak position of the dimensionless figure of merit ZT in the high temperature range. From these results, it can be seen that by adjusting the mixing ratio of two or more types of alkali metal chlorides, it is possible to produce a thermoelectric conversion material that exhibits better thermoelectric conversion performance in the desired high temperature range. This is thought to be because the conductivity characteristics of alkali metal chlorides depend on temperature. By utilizing this characteristic, it is possible to produce a thermoelectric conversion material that has the desired thermoelectric conversion performance to suit the environment in which the thermoelectric conversion material is installed.

[0104] From the evaluation results of Examples 8 and 9, the dimensionless figure of merit ZT when calcium chloride and potassium chloride were used as a mixed salt was higher than the dimensionless figure of merit ZT when calcium chloride and sodium chloride were used as a mixed salt. In other words, it was found that calcium chloride and potassium chloride are preferable as mixed salts of alkali metal chlorides. This is thought to be due to the large adsorption amount of alkali metal chlorides and the uniform adsorption structure. Therefore, it is thought that a combination of alkali metal chlorides that has a large adsorption amount and a uniform adsorption structure can improve thermoelectric conversion performance.

[0105] Conventional thermoelectric conversion materials have been produced using inorganic compounds such as lead, tellurium, bismuth, and antimony. These materials have the drawbacks of being rare, expensive, and toxic. In contrast, production method 1 according to the third embodiment can produce thermoelectric conversion materials using materials that are abundant on Earth and have a low environmental impact, making it possible to produce thermoelectric conversion materials more safely and inexpensively than conventional methods.

[0106] Furthermore, since the materials used to manufacture conventional thermoelectric conversion materials contain heavy elements, there is a problem that the weight of conventional thermoelectric conversion materials is large. In contrast, the manufacturing method 1 according to the third embodiment can manufacture thermoelectric conversion materials using materials composed of lighter elements than conventional materials, making it possible to manufacture thermoelectric conversion materials that are lighter than conventional materials.

[0107] Furthermore, most conventional thermoelectric conversion materials have a low operating temperature range of room temperature to 600°C, making them unsuitable for use in high-temperature ranges. In contrast, production method 1 according to the third embodiment can produce a thermoelectric conversion material that exhibits stable thermoelectric conversion performance even in high-temperature ranges of 600°C or higher. Therefore, the thermoelectric conversion material produced by production method 1 according to the third embodiment can be used in high-temperature environments such as nuclear power plants or boilers, thereby expanding its industrial applicability.

[0108] As described above, the manufacturing method 1 according to the third embodiment includes a mixing step S3 in which weathered biotite is mixed with an alkali metal chloride to obtain a mixture, a heat treatment step S4 in which the obtained mixture is heat-treated, a pressure molding step S5 in which the heat-treated mixture is pressure-molded to obtain a molded product, and a firing step S6 in which the obtained molded product is fired.

[0109] As a result, the manufacturing method 1 according to the third embodiment can produce silicate minerals containing wadalite in a simple manner with a small environmental impact, and can ultimately use the silicate minerals as intermediates to safely and inexpensively produce lightweight thermoelectric conversion materials that can be used in high-temperature ranges.

[0110] Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above embodiments and various modifications can be made without departing from the spirit of the present invention. In the present invention, the configuration of one embodiment can be added to the configuration of another embodiment, the configuration of one embodiment can be replaced with the configuration of another embodiment, or part of the configuration of one embodiment can be deleted. [Explanation of symbols]

[0111] 1…Manufacturing method, S1…Powder and grading process, S2…Adhesion process, S3…Mixing process, S4…Heat treatment process, S5…Pressure forming process, S6…Cooking process

Claims

1. a mixing step of mixing a clay mineral with salt to obtain a mixture; a heat treatment step of subjecting the mixture to heat treatment; a pressure molding step of pressure molding the heat-treated mixture to obtain a molded product; A firing step of firing the molded product, The clay mineral is a weathered layered clay mineral, The salt is a metal chloride that becomes a molten salt in the heat treatment step, a salt mixture of one kind of the metal chloride or two or more kinds of the metal chloride; The heat treatment step includes heating the mixture at a heat treatment temperature of 700°C or higher and lower than 850°C, In the firing step, the temperature is raised to a predetermined firing temperature at a rate of 200° C. / hour or less. A method for producing a thermoelectric conversion material comprising:

2. a mixing step of mixing a clay mineral with salt to obtain a mixture; a heat treatment step of subjecting the mixture to heat treatment; a pressure molding step of pressure molding the heat-treated mixture to obtain a molded product; A firing step of firing the molded product, The clay mineral is a weathered layered clay mineral, the salt is a mixed salt of two or more types of alkali metal chlorides that becomes a molten salt in the heat treatment step, The heat treatment step includes heating the mixture at a heat treatment temperature of 700°C or higher and lower than 850°C, By adjusting the mixing ratio of the mixed salts, the thermoelectric conversion performance of the thermoelectric conversion material can be adjusted. A method for producing a thermoelectric conversion material comprising:

3. The clay mineral is weathered biotite 3. A method for producing a thermoelectric conversion material according to claim 1 or 2.

4. The salt is a mixed salt of calcium chloride and potassium chloride. The method for producing a thermoelectric conversion material according to claim 3 .

5. The method further includes an adsorption step, which is carried out before the mixing step, in which ions of a metal element are adsorbed onto the clay mineral, The ionic radius of the metal element is equal to or greater than the ionic radius of the metal contained in the salt. The method for producing a thermoelectric conversion material according to claim 4 .

6. The metal element is cesium The method for producing a thermoelectric conversion material according to claim 5 .

7. The adsorption step involves mixing the clay mineral with the metal element and then performing a heat treatment to adsorb ions of the metal element onto the clay mineral. The method for producing a thermoelectric conversion material according to claim 5 .

8. The adsorption step adsorbs ions of the metal element onto the clay mineral by mixing the metal element with the clay mineral so that the metal element is added in an amount of 2 wt % to 6 wt % based on the weight of the clay mineral. The method for producing a thermoelectric conversion material according to claim 6 .

9. In the mixing step, the clay mineral and the salt are mixed in equal amounts by mass. The method for producing a thermoelectric conversion material according to claim 5 .

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