Thermoelectric material and thermoelectric material manufacturing method

Abstract

To provide a thermoelectric material with high design freedom by obtaining desired physical properties by compounding a ceramic material with metal particles and improving the dimensionless figure of merit ZT by adjusting the flatness of the metal particles.SOLUTION: A thermoelectric material is a composite material in which metal particles are blended with an insulating ceramic material, and the dimensionless figure of merit ZT has a value greater than the ZT of the metal itself depending on the blending ratio.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a thermoelectric material, and more particularly to a thermoelectric material that relatively improves the dimensionless figure of merit ZT. [Background technology]

[0002] Conventionally, PbTe (lead telluride) and Bi2Te3 (bismuth telluride) are known as materials with good thermoelectric properties. However, Pb and Te are highly toxic and have limited reserves, so the development of alternative materials is desired.

[0003] One of the useful indices of thermoelectric materials is the dimensionless figure of merit ZT, which is calculated using the Seebeck coefficient S, temperature T, electrical resistivity ρ, and thermal conductivity κ. ZT=S 2 T / (ρκ) (A large value is preferable for a thermoelectric material.)

[0004] As is clear from the above formula, if the electrical resistivity ρ and thermal conductivity κ are both small, ZT becomes large and it is preferable; however, metals generally conduct both electricity and heat well. In other words, if ρ is small, κ becomes large, so the two compensate each other, making it difficult to obtain the desired material. [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] S. Katsuyama, 'Possible enhancement of thermoelectric performance for Si-SiO2 sintered composite by percolation effect', J. Alloys Compd., 860 (2021) 157919 Summary of the Invention [Problem to be solved by the invention]

[0006] The present invention has been made in consideration of the above, and aims to provide a thermoelectric material with a high degree of design freedom by combining a ceramic material with metal particles to obtain desired physical properties, while improving ZT by adjusting the flatness of the metal particles. [Means for solving the problem]

[0007] The thermoelectric material described in claim 1 is a composite material in which metal particles are blended with an insulating ceramic material, and is characterized in that, depending on the blending ratio, it has a dimensionless figure of merit ZT that is greater than the ZT of the metal itself.

[0008] Here, ZT=S 2 T / (ρκ), S: Seebeck coefficient, T: temperature, ρ: electrical resistivity, κ=thermal conductivity. The term "ceramics" is used in a broad sense to refer to sintered bodies made by heating and sintering inorganic materials, and there are no particular limitations on the material. Considering versatility, availability, and low cost, silicon dioxide (glass) is an example of a material. The composite material may be formed by, for example, sintering or pressure sintering. In this case, the composite material in the claims may be rephrased as a "sintered body." Furthermore, the Seebeck coefficient S of this composite material is generally constant regardless of the mixing ratio (Figure 2(c)), but the electrical resistivity ρ and thermal conductivity κ of the composite change depending on the mixing ratio. Therefore, as the metal mixing ratio decreases (as the proportion of ceramics increases), ZT initially increases and then decreases (Figure 2(d)).

[0009] The thermoelectric material according to claim 2 is the thermoelectric material according to claim 1, characterized in that flat metal particles having an aspect ratio of 3 to 20 are used as the metal particles, and the thermoelectric material has a larger ZT value than when non-flat metal particles are used.

[0010] Flattening refers to a state where the particle is crushed in the thickness direction. The aspect ratio is defined as the long diameter / thickness of the particle. It is acceptable if the peak of the dispersion curve is between 3 and 20. If the aspect ratio is less than 3, the effect of the flattening process is unclear, and if the aspect ratio exceeds 20, the particle is relatively too thin and bends during pressure sintering, and the degree of flattening essentially reaches a plateau. The major axis is generally 1 μm to 200 μm, and preferably 3 μm to 100 μm. The thickness is preferably 1 μm to 20 μm within the above aspect ratio.

[0011] The thermoelectric material according to claim 3 is characterized in that in the thermoelectric material according to claim 1, the metal contains at least one selected from constantan, nichrome, stainless steel, permalloy, super heat-resistant alloys, or Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W, Fe, Ni, Co, Cu, Al, Mg, Zn, alloys thereof, or intermetallic compounds thereof.

[0012] The method for producing a thermoelectric material described in claim 4 is characterized in that an insulating ceramic material powder and a metal powder are mixed and sintered, and a thermoelectric material is obtained by adjusting the dimensionless figure of merit ZT based on the mixing ratio and the flatness of the metal powder.

[0013] Alternatively, the ceramic material powder and the metal powder may be mixed by ball milling, and the metal powder may be flattened to a desired aspect ratio. Effect of the Invention

[0014] In the present invention, the small electrical resistance κ is provided by the metal particles, while the small thermal conductivity ρ is provided by the ceramics, thereby relatively improving ZT, and by using flat metal particles, ZT is further improved, thereby making it possible to obtain a thermoelectric material with a high degree of freedom in designing according to required specifications. [Brief description of the drawings]

[0015] [Figure 1] 1 is an optical microscope photograph of a cross section of a test specimen. [Diagram 2] 1 is a graph showing the measurement results of the electrical resistivity ρ, the absolute value S of the Seebeck coefficient, the thermal conductivity κ, and the figure of merit ZT of the sintered body. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] The present inventors focused on the percolation phenomenon in the composite of metal particles and ceramics, and based on this, came up with the idea of ​​reducing the denominator of ZT in the composite material, i.e., ρ·κ. As a result of extensive investigations, they arrived at the present invention. Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings.

[0017] Here, we decided to use soda-lime glass (SLG) as the insulating ceramic and constantan (Cu-45Ni) as the metal for the evaluation.

[0018] First, SLG powder (Unitika Ltd.: particle size ≦75 μm, density 2.5 g / cm) was used as the matrix. 3 ) was used, and constantan particles (manufactured by Sanyo Special Steel Co., Ltd.: particle size ≦45 μm, density 8.9 g / cm 3 The mixture was mixed in a wet ball mill using ethanol as a solvent and zirconia balls.

[0019] This mixing causes the metal particles to become flattened, and their shape becomes, for example, φ100 μm or more and thickness 10 μm or less (aspect ratio 10 or more) depending on the processing time. In addition, the glass powder is finely crushed, and the glass powder adheres evenly around the metal particles.

[0020] This mixed powder was placed in a cylindrical graphite mold and heated to 700°C under a vacuum atmosphere at a pressure of 40 MPa and held for 5 minutes by plasma discharge sintering to obtain a test specimen. In addition, we also prepared test specimens that were plasma sintered under similar conditions using constantan that was not ball milled, i.e., constantan particles with an aspect ratio of 1 or less, at 50 vol%, 60 vol%, and 70 vol%, for comparison.

[0021] Figure 1 is an optical microscope photograph of the cross section of a test specimen. The black parts in the image are the matrix, and the white parts are constantan. As is clear from the image, the test specimen that was not ball milled naturally has constantan in granular form, while the test specimen that was ball milled has constantan that has been stretched thinly in the thickness direction (aspect ratio is approximately 20). During pressure sintering, the flat constantan becomes oriented so that it aligns in the direction perpendicular to the pressure direction (the stretching direction).

[0022] Next, the electrical resistivity ρ, the absolute value of the Seebeck coefficient S, the thermal conductivity κ, and the figure of merit ZT of the test specimen (sintered body) were measured. All measurements were performed at a temperature of 300K. Figure 2 is a graph showing the measurement results of the electrical resistivity ρ, absolute value of the Seebeck coefficient S, thermal conductivity κ, and figure of merit ZT of the sintered body. In the figure, test bodies that were not ball milled are marked with a circle, and test bodies in which the metal particles have been flattened by ball milling are marked with a triangle. The results of test bodies measured in the pressure direction of pressure sintering are shown with a lower triangle ▽, and the results of test bodies measured in the direction perpendicular to the pressure direction are shown with an upper triangle △.

[0023] As is clear from Figure 2(a), when the constantan content exceeds 50 vol%, the electrical resistivity ρ rapidly decreases, becoming approximately the same order of magnitude as the electrical resistivity of 100% constantan (pure constantan). At 50 vol% or more, the percolation phenomenon becomes prominent, and it was confirmed that even when a composite is formed and an insulator (even up to 50 vol%) is included, the electrical resistivity ρ can remain as low as a metal. In addition, when the sample was flattened, the reduction in electrical resistivity ρ was remarkable, especially in the direction perpendicular to the pressure direction, compared to when the sample was not flattened. It was found that even when the sample contained 30 vol% constantan, the reduction was on the same order of magnitude as that of pure constantan.

[0024] As is clear from FIG. 2(b), it can be confirmed that the Seebeck coefficient S remains approximately the same as that of pure constantan, so long as the constantan content is at least 30 vol%. It is generally believed that the electrical resistivity ρ and the Seebeck coefficient S cannot be controlled independently. However, in the present invention, it has been surprisingly discovered that it is possible to lower the electrical resistivity ρ while keeping the Seebeck coefficient S constant by combining the two.

[0025] The measurement results of thermal conductivity κ are shown in Figure 2 (c). When the constantan content is 80 vol% or more, the thermal conductivity κ is one order of magnitude larger than that of pure SLG, making it difficult to realize the benefits of combining the two. Furthermore, containing constantan to this extent also leads to an increase in the cost of the composite material. Regarding the flattening effect, a tendency for thermal conductivity to increase in the direction perpendicular to the pressure direction can be seen, but the increase is kept to a level that cannot be described as significant.

[0026] Figure 2(d) is a graph of ZT calculated based on the above measurement results. When flattening is not performed, ZT decreases as the constantan mixing ratio is reduced, peaking at a constantan mixing ratio of about 70 vol% to 80 vol%, but when flattening is performed, ZT can be improved (recovered) to the peak order without flattening, even with a mixing ratio of about 50 vol%. In particular, the degree of recovery of ZT in the pressure direction is large (although this depends on the mixing ratio). In other words, it can be confirmed that ZT can be increased according to the degree of flattening. Furthermore, when looking at the 30 vol% and 50 vol% flat constantan composite materials, it can be confirmed that the performance index, particularly in the direction perpendicular to the pressure application direction, is stable.

[0027] The evaluation was made as an absolute value. ZT (pure Constantan) = 0.026, ZT (70 vol% Constantan without flattening treatment) = 0.038, and ZT (50 vol% Constantan with flattening treatment) = 0.038. That is, it is possible to increase ZT by compounding, and it is also possible to increase ZT by flattening if the constantan compounding ratio is the same. As a guideline, the compounding ratio of constantan metal particles can be said to be 30 vol% to 80 vol%, preferably 50 vol% to 80 vol%.

[0028] As described above, the ZT of a sintered body of an insulating ceramic powder and a metal powder can be adjusted (and in some cases increased) by adjusting the mixing ratio. Furthermore, if the metal powder is flattened by ball mill processing or the like during mixing, the ZT can be increased accordingly. When flattened, the bending strength is also improved. Therefore, the thermoelectric material of the present invention not only offers freedom in designing the thermoelectric performance, but also allows the mechanical strength and other physical properties to be maintained or controlled by selecting and combining materials, thereby enabling a wide range of applications.

[0029] The above description is an example, and the present invention is not limited to the above. As the matrix material, for example, oxide-based glass or non-oxide-based glass such as crystallized glass or general-purpose glass can be used. A combination of multiple types of glass can also be used. As the matrix material, ceramics such as Al2O3, ZrO2, MgO, mullite, MgO / Al2O3, and Al2O3 / Y2O3 can be used. A combination of multiple types of these can also be used. When using glass powder, the particle size is preferably 100 μm or less. When using ceramic powder, there is no particular restriction on the particle size, but from the viewpoint of sinterability, the average particle size is preferably 1 μm or less.

[0030] Examples of metal materials that can be used include constantan, as well as nichrome, stainless steel, permalloy, super heat-resistant alloys, or Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W, Fe, Ni, Co, Cu, Al, Mg, Zn, or alloys or intermetallic compounds thereof. When the metal powder is flattened while being mixed with the matrix material using a ball mill or the like, the initial unflattened particle size is preferably 1 μm to 200 μm, particularly 3 μm to 100 μm. If the particle size is smaller than 1 μm, it is difficult to flatten the powder due to its fineness, and if it is larger than 200 μm, separation from the powder of the matrix material occurs, and if metal powder of this size remains, it is difficult to sinter it due to its coarseness. It is more preferable to narrow the particle size range of the metal particles to 32 to 20 μm. [Industrial Applicability]

[0031] According to the present invention, it is possible to obtain a thermoelectric material that is inexpensive and has good thermoelectric properties while having bending strength and other physical properties.

Claims

1. A composite material in which metal particles are blended with an insulating ceramic material, A thermoelectric material characterized in that, depending on the compounding ratio, the dimensionless figure of merit ZT has a value larger than the value of the dimensionless figure of merit ZT of the metal itself.

2. The thermoelectric material according to claim 1, characterized in that the metal particles used are flat metal particles having an aspect ratio of 3 to 20, and the ZT value is greater than that of the case where non-flat metal particles are used.

3. Metals are Constantan, Nichrome, Stainless Steel, Permalloy or Superalloy, or Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W, Fe, Ni, Co, Cu, Al, Mg, Zn, or alloys or intermetallic compounds thereof, The thermoelectric material according to claim 1 , comprising at least one selected from the following:

4. A method for producing a thermoelectric material, comprising the steps of: mixing and sintering an insulating ceramic material powder and a metal powder; and adjusting the dimensionless figure of merit ZT based on the mixing ratio and the flatness of the metal powder to obtain a thermoelectric material.

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