Thermoelectric material, manufacturing method for the same and thermoelectric conversion element

Abstract

To achieve higher thermoelectric conversion efficiency in MgAgSb-based thermoelectric materials.SOLUTION: A thermoelectric material includes an inorganic compound containing magnesium (Mg), silver (Ag), and antimony (Sb). The inorganic compound further contains iron (Fe), and the content of Fe is more than 0% and equal to or less than 1.0% of the Mg composition. A method for manufacturing the thermoelectric material includes a material preparation step, a grinding step, a sintering step, and an annealing step. The material preparation step prepares a mixture containing a raw material containing Mg, a raw material containing Ag, and a raw material containing Fe. The grinding step includes a step of performing ball-milling on the mixture individually at a plurality of times, a step of adding Sb, and a stirring step.SELECTED DRAWING: Figure 1

Description

[Technical field]

[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 silver antimony (MgAgSb)-based thermoelectric material, a manufacturing method thereof, and a thermoelectric conversion element. [Background technology]

[0002] As research into the efficient use of energy, including energy conservation and the use of renewable energy, progresses, 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, and in cooling devices that utilize the reverse conversion.

[0003] In Patent Document 1, it is reported that by doping magnesium silver antimony (MgAgSb) with copper (Cu), it is possible to obtain a luminance of 25 μW cm -1 K -2 A thermoelectric material that achieves a power factor exceeding [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication WO2022 / 059443 Summary of the Invention [Problem to be solved by the invention]

[0005] An object of the present invention is to achieve higher thermoelectric conversion efficiency in an MgAgSb-based thermoelectric material. [Means for solving the problem]

[0006] According to one embodiment, the following is true. That is, it is a thermoelectric material including an inorganic compound containing magnesium (Mg), silver (Ag), and antimony (Sb), and the inorganic compound further contains iron (Fe), and the content of the Fe is greater than 0% and not more than 1.0% relative to the Mg composition.

[0007] According to another embodiment, a method for producing a thermoelectric material includes a material preparation step, a grinding step, a sintering step, and an annealing step. In the material preparation step, a mixture containing a raw material containing Mg, a raw material containing Ag, and a raw material containing Fe is prepared. In the grinding step, the mixture is subjected to ball milling multiple times, Sb is added, and a stirring step is performed. Effect of the Invention

[0008] The effects obtained by the embodiment are briefly described below. That is, in the MgAgSb-based thermoelectric material, higher thermoelectric conversion efficiency can be achieved. [Brief description of the drawings]

[0009] [Figure 1] FIG. 1 is a flow chart showing an example of a method for producing a thermoelectric material of the present invention. [Diagram 2] FIG. 2 is an explanatory diagram that illustrates a schematic configuration example of a thermoelectric conversion element using the thermoelectric material of the present invention. [Diagram 3] FIG. 3 is a graph showing the measurement results of the electrical conductivity of a thermoelectric material according to one embodiment of the present invention. [Figure 4] FIG. 4 is a graph showing the measurement results of the Seebeck coefficient of a thermoelectric material according to one embodiment of the present invention. [Diagram 5] FIG. 5 is a graph showing the measurement results of the power factor of a thermoelectric material according to one embodiment of the present invention. [Figure 6] FIG. 6 is a graph showing the results of measuring the thermal conductivity of a thermoelectric material according to one embodiment of the present invention. [Figure 7] FIG. 7 is a graph showing the results of measuring the figure of merit of a thermoelectric material according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] 1. Overview of the embodiment First, a summary of representative embodiments disclosed in the present application will be described. In the summary of the representative embodiments, reference numerals in parentheses in the drawings are used only to illustrate components included in the concept of the components to which they are attached.

[0011] [1] Fe-doped MgAgSb-based thermoelectric materials A representative embodiment disclosed in the present application is a thermoelectric material including an inorganic compound containing Mg, Ag, and Sb, the inorganic compound further containing Fe, and the content of the Fe is greater than 0% and less than or equal to 1.0% with respect to the Mg composition. This makes it possible to achieve higher thermoelectric conversion efficiency in MgAgSb-based thermoelectric materials.

[0012] [2] Optimal composition range In the thermoelectric material of [1], the inorganic compound is Mg 1-c Fe c Ag a Sb b and each parameter satisfies the following relationship: 0 <c≦0.01 0.95≦a≦1.05 0.95≦b≦1.05 This makes it possible to realize the highest thermoelectric conversion efficiency in Fe-doped MgAgSb-based thermoelectric materials.

[0013] [3] Optimal crystal structure In the thermoelectric material of [1] or [2], the inorganic compound is an α-phase with a half-Heusler structure and has the symmetry of the space group I-4c2, where "-4" represents 4 with an overscore throughout this specification. This makes it possible to realize the highest thermoelectric conversion efficiency in Fe-doped MgAgSb-based thermoelectric materials.

[0014] [4] p-type The thermoelectric material according to any one of items [1] to [3] is a p-type. This makes it easier to combine with other n-type thermoelectric materials.

[0015] [5] Powder, sintered body or thin film The thermoelectric material according to any one of items [1] to [4] may be in the form of a powder, a sintered body, or a thin film. This makes it possible to configure various thermoelectric elements that exhibit high thermoelectric performance at room temperature.

[0016] [6] Contains organic materials The thermoelectric material in [5] is in the form of a thin film and further contains an organic material. This makes it possible to realize a flexible film-like thermoelectric material.

[0017] [7] Thermoelectric conversion elements A representative embodiment disclosed in the present application is a thermoelectric conversion element having a thermoelectric conversion layer made of the thermoelectric material according to any one of claims 1 to 6. This makes it possible to realize a thermoelectric conversion element that achieves higher thermoelectric conversion efficiency in the MgAgSb-based thermoelectric material.

[0018] [8] Manufacturing method of thermoelectric materials The method for producing a thermoelectric material according to any one of items [1] to [6] includes a material preparation step, a mechanical alloying step, a sintering step, and an annealing step. This makes it possible to produce a MgAgSb-based thermoelectric material that achieves higher thermoelectric conversion efficiency.

[0019] [9] Manufacturing method of thermoelectric materials (Fig. 1) A representative embodiment disclosed in the present application is a method for producing a thermoelectric material, which includes a material preparation step (S1), a grinding step (S2), a sintering step (S3), and an annealing step (S4), and is configured as follows:

[0020] Material preparation step (S1): A mixture containing a raw material containing Mg, a raw material containing Ag, and a raw material containing Fe is prepared.

[0021] Grinding step (S2): This step includes steps of ball milling the mixture several times (S21, S23, S25, S27), a step of adding Sb (S22), and a stirring step (S24, S26).

[0022] Sintering step (S3): The pulverized mixture is sintered by applying high temperature and high pressure. Annealing step (S4): Heat treatment is performed.

[0023] This makes it possible to produce a MgAgSb-based thermoelectric material that achieves higher thermoelectric conversion efficiency.

[0024] 2. Details of the embodiment The embodiment will now be described in further detail.

[0025] [Composition of Thermoelectric Materials] The thermoelectric material of the present invention is composed of an inorganic compound containing Mg, Ag, and Sb doped with Fe. The Fe content is greater than 0% and less than or equal to 1.0% of the Mg composition. This allows the MgAgSb-based thermoelectric material to achieve higher thermoelectric conversion efficiency.

[0026] The optimal composition is the above inorganic compound with Mg 1-c Fe c Ag a Sb b The ranges in which each parameter satisfies the following relationships are expressed as:

[0027] 0 <c≦0.01 0.95≦a≦1.05 0.95≦b≦1.05

[0028] As a result, the highest thermoelectric conversion efficiency can be achieved in the Fe-doped MgAgSb-based thermoelectric material. In such a composition range, it is considered that in the inorganic compound composed of Mg, Ag, and Sb, a part of Mg constituting the matrix phase is substituted with doped Fe. More specifically, this matrix phase is a MgAgSb-based crystal, which becomes the α-phase (tetragonal phase) of the half-Heusler structure at low temperatures and belongs to the I-4c2 space group (the 120th in the International Tables for Crystallography). It is considered that a part of this Mg site is substituted with Fe. Examples of the matrix phase include Mg 1.0 Ag 1.0 Sb 1.0 、Mg 0.98 Ag 1.02 Sb 1.00 、Mg 1.00 Ag 1.01 Sb 1.02 、Mg 0.98 Ag 0.97 Sb 0.99 、Mg 1.0 Ag 0.98 Sb 0.98 etc. are adopted.

[0029] As for the content of Fe, the above range is appropriate, but 0 < c ≤ 0.005 is more preferable, and 0 < c ≤ 0.0025 is even more preferable.

[0030] The thermoelectric material may be in the form of a powder, a sintered body, or a thin film, and can be applied to a thermoelectric conversion element that exhibits high thermoelectric performance at room temperature. For example, in this embodiment, a sintered body of the thermoelectric material can be manufactured by a manufacturing method described later. The sintered body is crushed to obtain a powder, and a thin film is formed by a physical vapor deposition method using the sintered body as a target. Here, generally, the powder may include a powder that has been crushed and finely processed. A green compact can be formed by pressing the powder with a powder pressing machine or the like. Generally, a green compact refers to a powder compressed into a predetermined shape. When heated at a temperature below the melting point of the powder components, the contact surfaces of the powder particles are bonded together, and the green compact shrinks and densifies with the increase in heating time. This phenomenon is called sintering, and the one 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 condensing a gas phase on a solid surface (substrate).

[0031] The thermoelectric material of the present invention can also be processed into a film by mixing the powder of the inorganic compound described above with an organic material. In this case, the organic material can be at least one selected from the group consisting of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT), polyaniline (PANI), tetrathiafulvalene (TTFF), and benzodifurandione paraphenylene vinylidene (BDPPV). By using these organic materials, the thermoelectric material of the present invention can be processed into a flexible film.

[0032] In this case, the powder content is not particularly limited, but the powder content is preferably in the range of 4% by mass to 80% by mass, more preferably 4% by mass to 50% by mass, even more preferably 4% by mass to 10% by mass, and even more preferably 4% by mass to 7% by mass, relative to the organic material. This makes it possible to realize a film that has flexibility and thermoelectric conversion performance.

[0033] [Thermoelectric conversion element] The thermoelectric material of the present invention, which satisfies the above-mentioned composition, functions as a p-type thermoelectric material in which holes are carriers. By combining it with other n-type thermoelectric materials, it is possible to configure a thermoelectric power generation device, which is a type of thermoelectric conversion element.

[0034] 2 is an explanatory diagram showing a schematic configuration example of a thermoelectric conversion element using the thermoelectric material of the present invention. The thermoelectric conversion element is configured by alternately connecting in series p-type thermoelectric conversion layers 1 and n-type thermoelectric conversion layers 2, each of which has an electrode connected to both ends. The electrode 3-1 connected to the first n-type thermoelectric conversion layer 2-1, the electrode 3-2 connecting the first p-type thermoelectric conversion layer 1-1 and the second n-type thermoelectric conversion layer 2-2, and the electrode 3-3 of the second p-type thermoelectric conversion layer 1-2 are arranged on the low temperature side, while the electrode 4-1 connecting the first n-type thermoelectric conversion layer 2-1 and the first p-type thermoelectric conversion layer 1-1 and the electrode 4-2 connecting the second n-type thermoelectric conversion layer 2-2 and the second p-type thermoelectric conversion layer 1-2 are arranged on the high temperature side. The potential difference generated across both ends of the p-type thermoelectric conversion layer 1 and the n-type thermoelectric conversion layer 2 due to the Seebeck effect is integrated by the series connection and output between the electrodes 3-1 and 3-3. Due to space limitations, Fig. 2 illustrates an example of two p-type and two n-type thermoelectric conversion elements connected in series, but the number of elements is arbitrary.

[0035] Here, the p-type thermoelectric conversion layer 1 is a thermoelectric conversion layer made of the above-mentioned thermoelectric material. On the other hand, the n-type thermoelectric conversion layer 2 can be arbitrarily selected from thermoelectric materials such as Mg2Sb3-based, BiTeSe-based, and CoSb3-based thermoelectric materials. As described later, the thermoelectric material of the present invention exhibits particularly high thermoelectric conversion performance at absolute temperatures of 450K to 550K, so that it is preferable to select a thermoelectric material that exhibits high thermoelectric conversion performance in the same high temperature range for the n-type thermoelectric conversion layer 2.

[0036] The electrode material is arbitrary, and may be, for example, Fe, aluminum (Al), Cu, Ag, or nickel (Ni). Since electrodes 3-1 to 3-3 are on the low-temperature side and require wiring to an external circuit, Cu, Ni, or the like, which have a high affinity with solder, are suitable, and electrodes 4-1 to 4-2 are attached to a high-temperature object with good thermal conductivity and are suitable for use as a stable metal material, and for example, Fe, Al, or Cu may be used.

[0037] 2, a Π-type thermoelectric power generation element is illustrated, but any other type of thermoelectric conversion element, for example, a U-shape, etc., can be configured. This makes it possible to realize a thermoelectric conversion element that achieves higher thermoelectric conversion efficiency in an MgAgSb-based thermoelectric material.

[0038] [Manufacturing method] 1 is a flow chart showing an example of a method for producing a thermoelectric material of the present invention. The method for producing a thermoelectric material of the present invention includes a material preparation step (S1), a crushing step (S2), a sintering step (S3), and an annealing step (S4), and is configured as follows:

[0039] Material preparation step (S1): A mixture containing a raw material mainly containing Mg, a raw material mainly containing Ag, and a raw material mainly containing Fe, each in a predetermined stoichiometric composition ratio, is prepared.

[0040] Grinding step (S2): includes a step (S21, S23, S25, S27) of ball milling the prepared mixture in a plurality of times, a step (S22) of adding Sb, and a stirring step (S24, S26). More specifically, after the first ball milling (S21), a predetermined amount of Sb is added (S22), a second ball milling (S23) is performed, and then the stirring (S24, S26) and ball milling (S25, S27) are repeated twice. Note that this is only an example, and it is important that each raw material is finely and uniformly ground and dispersed. For example, the grinding step (S2) can be appropriately changed depending on the state of the prepared electrode material. When the electrode material is a finer powder (smaller particle size), the number of ball millings and / or the milling time can be shortened or the milling power can be reduced.

[0041] Sintering step (S3): The pulverized mixture is sintered by applying high temperature and pressure. Annealing step (S4): Heat treatment is performed. This makes it possible to produce a MgAgSb-based thermoelectric material that achieves higher thermoelectric conversion efficiency. EXAMPLES

[0042] The inventors discovered that extremely high thermoelectric conversion efficiency can be achieved by doping Fe into MgAgSb-based thermoelectric materials, and prepared samples with the following composition and measured their characteristics. 1-c Fe c Ag 0.97 Sb 0.99 There are four possibilities for c=0, 0.0025, 0.005, and 0.01.

[0043] [Manufacturing method] Material preparation process (S1): As raw materials, Mg (turned material 5-25 mm, 99.95%), Ag (shot 2-5 mm, 99.99%), and Fe (powder <45 μm, 99.9%) were weighed to have a predetermined stoichiometric composition ratio and loaded into a stainless steel ball milling jar in an argon (Ar) atmosphere.

[0044] Grinding process (S2): The first ball milling process was carried out for 10 hours using a ball mill (SPEX SamplePrep 8000D Mixer / Mill). Next, Sb (2 mm chunks, 99.999%) was added, and the second ball milling process was carried out for 4 hours in an Ar atmosphere. To further improve uniformity, manual mixing (S24, S26) and 2-hour ball milling (S25, S27) were repeated twice while still in an Ar atmosphere. Here, manual mixing refers to peeling off the raw materials stuck to the side walls of the ball milling jar from the side walls and mixing them in the ball milling jar.

[0045] Sintering step (S3): Using a spark plasma sintering device (Fuji Electric Industrial Co., Ltd., Dr. Labo Series 322Lx), the pulverized mixture was subjected to spark plasma sintering at a temperature of 573 K and a uniaxial pressure of 60 MPa for 5 minutes.

[0046] Annealing step (S4): The sintered sample was placed in a quartz tube under vacuum (<5 × 10 -3 The tube was sealed in a sealed container (Pa) and then heat-treated at 573K for 6 hours.

[0047] [Measurement results] The measurement results of the samples are shown in Table 1 and Figures 3 to 7. Figure 3 shows the electrical conductivity σ, Figure 4 shows the Seebeck coefficient S, Figure 5 shows the power factor PF, Figure 6 shows the thermal conductivity κ, and Figure 7 shows the dimensionless figure of merit ZT.

[0048] Figure 3 shows the electrical conductivity σ (Sm -1 As shown in Fig. 3, an increase in the electrical conductivity of the MgAgSb compound was observed by doping it with a small amount of Fe. The electrical conductivity (σ) is a function of the carrier mobility (μ H ) and carrier concentration (n H ) and is proportional to the equation σ = eμ H n H(where e is the elementary charge). Carrier mobility is an index that indicates how fast carriers can move within a base material, so an increase in electrical conductivity (σ) means that the carrier mobility has improved.

[0049] The carrier concentration (n H ) and carrier mobility (μ H The measurement results are shown in Table 1.

[0050] [Table 1]

[0051] As the amount of Fe added increases, the carrier mobility increases significantly, but the carrier concentration remains almost unchanged. This indicates that the improvement in carrier mobility is the main cause of the improvement in electrical conductivity due to Fe doping.

[0052] Figures 4 and 5 show the Seebeck coefficient S (μVK -1 ) and power factor PF(mWm -1 K -2 ) on the vertical axis. While the electrical conductivity σ increases in the Fe-doped samples (Fig. 3), the Seebeck coefficient S of all samples, except for the sample with c = 0.01, changes within a similar range of values ​​(Fig. 4). In other words, the Seebeck coefficient S is maintained at a high value even in the Fe-doped samples, and a high power factor PF is achieved because the electrical resistance (the reciprocal of the electrical conductivity σ) is reduced by Fe doping (Fig. 5). However, when the amount of Fe added, c, reaches 0.01, the power factor PF decreases.

[0053] In Fig. 6 and Fig. 7, the horizontal axis represents absolute temperature T (K) and the thermal conductivity κ (Wm -1 K -1) and figure of merit ZT (dimensionless) are shown on the vertical axis, respectively. Thermal conductivity κ increases with increasing Fe doping. This is due to improved carrier mobility. The dimensionless figure of merit ZT of the sample with doping c=0.0025 was the largest in all measured temperature ranges, with a maximum ZT=1.52 observed at 473K. The measurement results also showed that the Seebeck coefficient was positive, indicating that the Fe-doped sample behaved as a p-type semiconductor.

[0054] [Comparison with results reported in the literature] The maximum ZT measured in this example, 1.52 (absolute temperature 473K), was a higher value than the ZT of various thermoelectric materials doped with impurities other than Fe, as reported below. Therefore, it was found that doping with Fe is most effective in increasing the thermoelectric performance of MgAgSb-based thermoelectric materials.

[0055] In paper 1 (Zihang Liu, et.al., “Lithium Doping to Enhance Thermoelectric Performance of MgAgSb with Weak Electron-Phonon Coupling”, Advanced Energy Materials, vol. 6, Issue 7, 2016), it was reported that for a MgAgSb-based thermoelectric material doped with 1% lithium (Li), ZT = 1.1 at 300 to 548K.

[0056] In paper 2 (Zihang Liu, et.al., “The microscopic origin of low thermal conductivity for enhanced thermoelectric performance of Yb doped MgAgSb”, Acta Materialia, vol.128, pp.227-234, April 15, 2017), a ZT of 1.4 at 550K was reported for a MgAgSb-based thermoelectric material doped with 0.5% ytterbium (Yb).

[0057] In paper 3 (Pingjun Ying, et.al., “High Performance α-MgAgSb Thermoelectric Materials for Low Temperature Power Generation”, Chemistry of Materials, 2015, 27, 3, 909-913, January 16, 2015), it was reported that ZT=1.1 at 525K for a MgAgSb-based thermoelectric material doped with 1% indium (In).

[0058] In paper 4 (Zihang Liu, et.al., “The influence of doping sites on achieving higher thermoelectric performance for nanostructured α-MgAgSb”, Nano Energy, vol.31, pp.194-200, January 2017), a ZT of ~1.3 was reported at approximately 550K for a MgAgSb-based thermoelectric material doped with 1% calcium (Ca).

[0059] 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. [Explanation of symbols]

[0060] 1. p-type thermoelectric conversion layer 2. n-type thermoelectric conversion layer 3-1~3-3 Low temperature electrode 4-1~4-2 High temperature side electrode S1 Material preparation process S2 Grinding process S21, S23, S25, S27 Ball milling process S22 Sb addition process S24, S26 Mixing process S3 Sintering process S4 Annealing process

Claims

1. It contains an inorganic compound containing magnesium (Mg), silver (Ag), and antimony (Sb). The inorganic compound further contains iron (Fe), The Fe content is greater than 0% and 1.0% or less relative to the Mg composition. Thermoelectric materials.

2. In claim 1, The inorganic compound is Mg １－ｃ Fe ｃ Ag ａ Sb ｂ It is represented as, 0 < c ≤ 0.01, 0.95 ≤ a ≤ 1.05, and Satisfying 0.95 ≤ b ≤ 1.05, Thermoelectric materials.

3. In claim 1 or claim 2, The inorganic compound is an α-phase with a half-Heusler structure and has space group I-4c2 symmetry. Thermoelectric materials.

4. In claim 1 or claim 2, It is p-type Thermoelectric materials.

5. In claim 1 or claim 2, The form of the thermoelectric material is a powder, a sintered body, or a thin film. Thermoelectric materials.

6. In claim 5, The aforementioned thermoelectric material is in the form of a thin film and further contains an organic material. Thermoelectric materials.

7. The thermoelectric material according to claim 1 or claim 2 is used as the thermoelectric conversion layer. Thermoelectric conversion element.

8. A method for manufacturing a thermoelectric material according to claim 1 or claim 2, This process includes a material preparation step, a grinding step, a sintering step, and an annealing step. A method for manufacturing thermoelectric materials.

9. A method for manufacturing thermoelectric materials, This process includes a material preparation step, a grinding step, a sintering step, and an annealing step. In the material preparation step, a mixture containing a raw material containing Mg, a raw material containing Ag, and a raw material containing Fe is prepared. The grinding step includes a step of ball milling the mixture in multiple stages, a step of adding Sb, and a stirring step. A method for manufacturing thermoelectric materials.

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