Gd-Co-BASED METALLIC POWDER, METHOD FOR PRODUCING SAME, ELECTROCONDUCTIVE MOLDED BODY, AND THERMOELECTRIC CONVERSION ELEMENT

Abstract

[Problem] To provide a technique suitable for manufacturing, with high productivity, a thermoelectric conversion element that utilizes an anomalous Nernst effect to obtain a high Nernst coefficient in a zero magnetic field. [Solution] Provided is a metallic powder having the intermetallic compound GdCo5 as a main component, in which, according to a volume-based particle size distribution as determined by a laser diffraction and scattering method, the cumulative 50% particle diameter D50 is 1-150 μm and the cumulative 90% particle diameter D90 is 250 μm or less. This metallic powder can be obtained, for example, by pulverizing an alloy mass having the intermetallic compound GdCo5 as a main component.

Description

Gd-Co based metal powder, its manufacturing method, conductive compact, and thermoelectric conversion element

[0001] The present invention relates to a Gd-Co based metal powder useful as a material for thermoelectric conversion elements, a method for producing the same, a conductive molded body of the Gd-Co based metal powder, and a thermoelectric conversion element using the conductive molded body.

[0002] In recent years, research into thermoelectric conversion elements that utilize the anomalous Nernst effect has been progressing. The anomalous Nernst effect is a phenomenon in which, when a heat flow perpendicular to the magnetization is applied to a spontaneously magnetized magnetic material, an electromotive force is generated in a direction perpendicular to both the magnetization and the heat flow. The anomalous Nernst effect allows current to be extracted in a direction perpendicular to the heat flow, which has the advantage that, unlike when the Seebeck effect is used, a thin sheet-like thermoelectric conversion device can be constructed. For example, the ferromagnetic intermetallic compound Co ２ MnGa is known.

[0003] Patent Document 1 describes a method for producing Co by the Czochralski method. ２ The paper describes an experimental example in which MnGa single crystals were produced and the anomalous Nernst coefficient was measured. ２ The Nernst coefficient of MnGa single crystal at room temperature (300 K) reaches a high value of approximately 6 μV / K regardless of whether the magnetic field direction is parallel to the

[100] ,

[110] , or

[111] direction of the crystal (paragraph 0021, Figure 4).

[0004] Patent Document 2 describes a composite sensor having a heat flow sensor and a temperature sensor, in which an anomalous Nernst material film is used in the heat flow sensor. Several substances are listed as anomalous Nernst materials (paragraph 0026). A sputtering method is presented as a method for forming the anomalous Nernst material film (paragraph 0030).

[0005] The main applications of thermoelectric conversion elements include thermoelectric power generation devices and heat flow sensors.

[0006] A thermoelectric conversion element for realizing a thermoelectric power generation device is preferably a bulk body with a thickness of approximately several millimeters. It is possible to produce a bulk body of the above size using a single crystal grown by the Czochralski method. However, single crystal manufacturing techniques such as the Czochralski method are expensive and have low productivity, making them impractical for industrial production of materials for thermoelectric power generation devices. On the other hand, it is difficult to apply film formation techniques such as sputtering to industrial production of bulk materials for thermoelectric power generation devices.

[0007] Considering that the thermoelectric conversion element used to realize a heat flow sensor will be incorporated into a small circuit pattern, it is desirable for the element to be small. Cutting a large number of small elements from a single crystal obtained by the Czochralski method or other methods is difficult to commercialize industrially due to the cost. It is also difficult to directly form small elements of a predetermined shape using the Czochralski method. On the other hand, film formation techniques such as sputtering make it possible to directly form small elements corresponding to a predetermined circuit pattern on an insulating substrate. However, such film formation methods have low productivity, resulting in high manufacturing costs for heat flow sensors.

[0008] Patent Document 3 describes Co as a material that can be widely used to manufacture thermoelectric conversion elements of various shapes and sizes. ２ The powder of MnGa is disclosed. In the conductive compact using this powder, single crystal Co ２ A high Nernst coefficient is obtained, similar to that of MnGa.

[0009] On the other hand, Patent Document 4 discloses an antiferromagnetic Mn ３ It is described that Sn powder exhibits a coercive force of about 0.25 to 0.5 T, and exhibits thermoelectric conversion action due to the anomalous Nernst effect even in a state where no magnetic field is applied (hereinafter, this may be referred to as "zero magnetic field").

[0010] International Publication No. 2019 / 009308 JP 2020-153668 A JP 2023-2425 A JP 2021-145116 A

[0011] Currently, the above-mentioned Co ２Materials that exhibit high Nernst coefficients, such as MnGa, are known. ２ Because MnGa has poor coercive force, it is virtually impossible to obtain thermoelectric conversion action due to the anomalous Nernst effect when no magnetic field is applied (zero magnetic field). Thermoelectric conversion elements used in sensors are expected to be installed in small spaces inside electronic devices, and it is practically difficult to incorporate thermoelectric conversion elements into devices together with powerful magnets. Therefore, it is desirable to use materials that exhibit thermoelectric conversion action even in zero magnetic field for thermoelectric conversion elements.

[0012] On the other hand, Mn ３ If Sn powder is used, thermoelectric conversion can be achieved even in zero magnetic field. However, the Nernst coefficient is Co ２ It is much smaller than MnGa and is not sufficient to construct a practical thermoelectric conversion element. ３ The coercive force of Sn powder is said to be approximately 0.25 to 0.5 T (Patent Document 4), but in order to provide reliability that allows stable and high thermoelectric conversion even when exposed to disturbances such as magnetic fields generated by equipment, it is desirable to develop an anomalous Nernst effect-expressing material with an even higher coercive force.

[0013] The present invention aims to provide a new material that has a high coercive force, a high Nernst coefficient in zero magnetic field, and is easily applicable to the manufacture of thermoelectric conversion elements of various shapes and sizes, and also to provide a thermoelectric conversion element with a high Nernst coefficient obtained using the material.

[0014] In order to achieve the above object, the present specification discloses the following invention.

[0015] [1] Intermetallic compound GdCo ５ [2] A metal powder containing the intermetallic compound GdCo by solidifying molten metal of a Gd-Co alloy. ５[3] The method for producing a metal powder according to the above [1], comprising: an alloy ingot preparation step for obtaining an alloy ingot containing the intermetallic compound GdCo as a main component; and a pulverization step for obtaining a powder by pulverizing the alloy ingot. ５ [4] The intermetallic compound GdCo ５ The powder mainly composed of the intermetallic compound GdCo has a cumulative 50% particle diameter D50 of 1 to 150 μm and a cumulative 90% particle diameter D90 of 250 μm or less in a volume-based particle size distribution determined by a laser diffraction / scattering method. ５ [6] The conductive molded body according to any one of [3] to [5] above, which is a sintered body of a powder mainly composed of [3]. [7] A thermoelectric conversion element using the conductive molded body according to any one of [3] to [6] above.

[0016] According to the present invention, it is possible to obtain a thermoelectric conversion element that exhibits a high Nernst coefficient in zero magnetic field and has excellent resistance to disturbances caused by external magnetic fields. Because the material used for the thermoelectric conversion element is a powder, the technology of the present invention can be widely adapted to elements of various shapes and sizes. Furthermore, the technology of the present invention is superior in terms of cost and productivity compared to technologies using single crystals or thin films prepared by sputtering.

[0017] 1 is an SEM photograph of a Gd-Co-based metal powder that has been ground using a hammer mill. 2 is an SEM photograph of a Gd-Co-based metal powder that has been ground using a sample mill. 3 is an SEM photograph of a Gd-Co-based metal powder that has been ground using a planetary ball mill. 4 is a graph illustrating particle size distribution curves of the hammer mill-ground product, sample mill-ground product, and planetary ball mill-ground product of the Gd-Co-based metal powder obtained in Example 1. 5 is a diagram illustrating X-ray diffraction patterns of the hammer mill-ground product, sample mill-ground product, and planetary ball mill-ground product of the Gd-Co-based metal powder obtained in Example 1. 6 is a graph illustrating magnetization curves of the hammer mill-ground product, sample mill-ground product, and planetary ball mill-ground product of the Gd-Co-based metal powder obtained in Example 1. 7 is a diagram schematically illustrating the attachment positions of a terminal for power measurement and a probe for temperature measurement, as well as the direction of heat flow and magnetic field application, for a sample for measuring the Nernst effect. 8 is a graph illustrating the attachment positions of a terminal for power measurement and a probe for temperature measurement, as well as the direction of heat flow and magnetic field application, for a sample mill-ground product of GdCo ５ 1 is a graph showing the results of measuring the Nernst coefficient for a powder conductive compact.

[0018] [Gd-Co based metal powder] In the present invention, the intermetallic compound GdCo is used as a material suitable for thermoelectric conversion elements. ５ The main component of the powder is Gd-Co based intermetallic compound GdCo. ５ has a hexagonal crystal structure and exhibits antiferromagnetism at room temperature.

[0019] The composition ratio of Gd (gadolinium) and Co (cobalt) is GdCo ５ In the composition range close to the stoichiometric composition of GdCo ５ An intermetallic compound with a GdCo type crystal structure can exist stably as a single phase. ５ It is believed that a Gd-Co alloy containing a mixture of an intermetallic compound phase with a crystalline structure of the GdCo type and a different phase is obtained. ５ Even if the composition is slightly different from the stoichiometric composition of GdCo ５ type crystal structure (i.e., stoichiometric GdCo in the X-ray diffraction pattern) ５ The crystalline phase having a crystal structure in which diffraction peaks corresponding to the diffraction peaks from each crystal plane of the crystal are observed is referred to as the "intermetallic compound GdCo" in this specification. ５ " is included in.

[0020] “Intermetallic compound GdCo ５ "Powder mainly composed of GdCo" means that the X-ray diffraction pattern of the powder shows that GdCo ５ The integrated intensity of the diffraction peak with the highest peak height among the diffraction peaks of the type crystal structure is I ０ , heterogeneous phase (GdCo ５ The integrated intensity of the diffraction peak with the highest peak height among the diffraction peaks of the crystalline phase (phase other than the crystalline phase of the crystalline structure) is I １ When I １ <I ０ This means that the powder satisfies the relationship of I. １ = 0, and the above I １ <I ０ The intermetallic compound GdCo satisfies the relationship ５ In a thermoelectric conversion element using powder mainly composed of GdCo ５ Even if other phases than GdCo are included, ５ However, in order to achieve efficient thermoelectric conversion characteristics, it is desirable to minimize the amount of heterogeneous phases that do not exhibit the anomalous Nernst effect. １ <0.5 I ０ More preferably, I １ <0.3 I ０ A particularly preferred powder is GdCo, in which no other phase is detected. ５ Examples include single-phase powders.

[0021] The particle size distribution of the particles constituting the Gd—Co-based metal powder is determined by a volume-based particle size distribution measured by a laser diffraction / scattering method, with a cumulative 50% particle diameter D50 ranging from 1 μm to 150 μm, and a cumulative 90% particle diameter D90 ranging from 250 μm to 250 μm. If the cumulative 50% particle diameter D50 is too large, the average particle diameter increases, leading to a decrease in coercivity. If the cumulative 90% particle diameter D90 is too large, the proportion of coarse particles increases, also leading to a decrease in coercivity. The optimal particle size distribution conditions vary depending on the application and the element manufacturing process, but optimal conditions can usually be set within the above particle size distribution range. From the perspective of prioritizing the maintenance of coercivity, the cumulative 50% particle diameter D50 is preferably 70 μm or less, more preferably 50 μm or less, and even more preferably 20 μm or less. The cumulative 90% particle diameter D90 is preferably 150 μm or less, more preferably 120 μm or less, and even more preferably 60 μm or less.

[0022] The Gd-Co based metal powder can be produced, for example, by solidifying a molten metal containing Gd and Co to form an intermetallic compound GdCo. ５ and a crushing step of crushing the alloy ingot to obtain a powder. If necessary, a classification step of classifying the powder obtained in the crushing step using a sieve or the like may be further carried out.

[0023] The molten metal of the Gd-Co alloy can be obtained by, for example, melting raw material metals of Gd and Co weighed to a predetermined composition using a plasma arc or by heating and melting them in a crucible. The molten metal is solidified to form the intermetallic compound GdCo. ５ The alloy block containing the intermetallic compound GdCo as the main component is obtained. ５Since Gd—Co alloys are relatively brittle, their alloy blocks can be pulverized by mechanical pulverization. For example, the desired particle size can be achieved by using known pulverization techniques such as a hammer mill, a sample mill, or a planetary ball mill. Furthermore, to adjust the final particle size distribution, the powder obtained by the pulverization technique can be classified using a sieve. The Gd—Co alloy powder of the present invention can be used as a material for thermoelectric conversion elements.

[0024] [Conductive compact] In this specification, an object formed into a predetermined shape using powder as a material and having a definite shape that allows it to maintain its shape in the usage environment is called a "powder compact." In particular, a powder compact having conductivity is called a "powder conductive compact." The intermetallic compound GdCo, which uses the above-mentioned Gd-Co-based metal powder as a material, ５ A conductive molded body of powder containing the above as a main component is useful as a thermoelectric conversion element.

[0025] Typical forms of conductive powder compacts include green compacts and sintered compacts. Green compacts can also be used as thermoelectric conversion elements by incorporating them into devices in a way that allows them to maintain their shape in the usage environment. Sintered compacts are preferred to ensure stable shapeability.

[0026] Examples of forms of conductive powder compacts other than compacts and sintered bodies include compacts obtained by solidifying powder into a predetermined shape using a binder component such as a resin. When a non-conductive binder component is used, the powder particles must be solidified in a state where they are in contact with each other. When a conductive binder component is used, contact between the powder particles is not essential.

[0027] Intermetallic compound GdCo ５ When a sintered body is used as a conductive compact of a powder containing GdCo as the main component, a known sintering method is used to sinter the GdCo intermetallic compound. ５ A sintered body can be produced from powder containing GdCo as the main component. ５When the powder containing the powder as a main component is used as a conductive molded body, a method can be applied in which a coating film of a circuit pattern is formed on an insulating substrate using a paint containing the powder as a filler, and then the coating film is heated and sintered.

[0028] By using a Gd-Co-based metal powder adjusted to the above-mentioned particle size distribution as a material, it is possible to construct a conductive molded body that has a large coercive force and exhibits excellent thermoelectric conversion properties in a zero magnetic field environment at room temperature.

[0029] [Example 1] (Preparation of alloy ingot) Metallic Gd (manufactured by Nippon Yttrium Co., Ltd., purity 3N) and metallic Co (manufactured by Rare Metallic Co., Ltd., purity 3N) as raw materials were weighed out so that the atomic ratio of Ga:Co was 1.0:5.0, and a molten metal of Ga-Co alloy was produced in an argon atmosphere using an arc melting furnace (manufactured by Nisshin Giken Co., Ltd.), which was then solidified on a water-cooled copper plate to obtain an alloy ingot of approximately 50 g.

[0030] (Preparation of Powder) The obtained alloy ingot was coarsely pulverized in a mortar in a glove box under a nitrogen atmosphere. The pulverized product at this stage is called a "coarsely pulverized product." Using this coarsely pulverized product, the following three types of pulverized products were prepared.

[0031] (i) Hammer Mill Crushed Product The coarsely crushed product was crushed in the glove box using a hammer mill (Hammer Crusher NH-34S, manufactured by Sansho Industries Co., Ltd., screen mesh: 0.3 mm) to obtain a hammer mill crushed product. Figure 1 shows an example of an SEM (scanning electron microscope) photograph of the hammer mill crushed product. The SEM used was an FE-SEM JSM-7200F manufactured by JEOL Ltd. (the same applies to each of the following crushed products). Elemental analysis of the hammer mill crushed product was performed using an EDX (energy dispersive X-ray analysis) device (X-Max20 manufactured by Oxford Instruments) attached to the SEM (the same applies to each of the following crushed products). As a result, the composition of the hammer mill crushed product was Ga:Co = 1.2:5.0 in atomic ratio, and the oxygen content was 1.0 mass%. The particle size distribution of the hammer-milled product was also measured using a dry laser diffraction particle size distribution analyzer (HELOS & RODOS, manufactured by Nippon Laser Co., Ltd.) with a lens having a focal length of 200 mm (the same was true for each of the following pulverized products). As a result, the volume-based cumulative 10% particle diameter D10 of the hammer-milled product measured by the laser diffraction / scattering method was 5 μm, the cumulative 50% particle diameter D50 was 44 μm, and the cumulative 90% particle diameter D90 was 111 μm.

[0032] (ii) Sample Mill Crushed Product A portion of the hammer mill crushed product was crushed in the glove box using a sample mill (SK-M10 model, manufactured by Kyoritsu Riko Co., Ltd.) to obtain a sample mill crushed product. Figure 2 shows an example of an SEM photograph of the sample mill crushed product. The composition of the sample mill crushed product measured by EDX was Ga:Co = 1.1:5.0 in atomic ratio, and the oxygen content was 1.0 mass%. Furthermore, the volume-based cumulative 10% particle diameter D10 of the sample mill crushed product measured by laser diffraction / scattering method was 2 μm, the cumulative 50% particle diameter D50 was 13 μm, and the cumulative 90% particle diameter D90 was 44 μm.

[0033] (iii) Planetary ball milled product A portion of the hammer milled product was pulverized in a planetary ball mill (Fritsch, P-7) placed in a sealed container under a nitrogen atmosphere to obtain a planetary ball milled product. Figure 3 shows an example of an SEM photograph of the planetary ball milled product. The composition of the planetary ball milled product measured by EDX was Ga:Co = 1.1:5.0 in atomic ratio, and the oxygen content was 1.3% by mass. Furthermore, the volume-based cumulative 10% particle diameter D10 of the planetary ball milled product measured by laser diffraction / scattering method was 3 μm, the cumulative 50% particle diameter D50 was 10 μm, and the cumulative 90% particle diameter D90 was 40 μm.

[0034] FIG. 4 shows particle size distribution curves of the hammer mill-ground product, the sample mill-ground product, and the planetary ball mill-ground product.

[0035] (Measurement of X-ray diffraction pattern) The X-ray diffraction patterns of the hammer mill crushed product, sample mill crushed product, and planetary ball mill crushed product were measured using an X-ray diffractometer (Rigaku, Ultima IV) under the conditions of Cu-Kα radiation, tube voltage 40 kV, tube current 40 mA, measurement step 0.02 degrees, and scan speed 2 degrees / min. Figure 5 shows an example of the X-ray diffraction pattern. All crushed products were composed of hexagonal GaCo ５ The diffraction pattern of the crystal is almost single-phase GaCo. ５ It was confirmed that the powder consisted of a crystalline phase.

[0036] (Measurement of powder magnetic properties) The magnetic properties of each powder sample of the hammer mill crushed product, sample mill crushed product, and planetary ball mill crushed product were measured at room temperature using a VSM (Model-5, manufactured by Toei Kogyo Co., Ltd.). The measurement conditions were a maximum applied magnetic field of 6 T and a sweep rate of 1 min / F.S. As a result of the measurement, it was found that the magnetization of each crushed product was not saturated in a magnetic field of 6 T, and the magnetization of the hammer mill crushed product in a magnetic field of 6 T was 20.0 A m ２ / kg, and the sample mill crushed product was 21.3 A m ２ / kg, and the planetary ball mill crushed product was 21.3 A m ２ / kg. The coercive force was 0.74 T (589 kA / m) for the hammer mill-ground product, 1.48 T (1178 kA / m) for the sample mill-ground product, and 1.86 T (1480 kA / m) for the planetary ball mill-ground product. Figure 6 shows the magnetization curves of the hammer mill-ground product, sample mill-ground product, and planetary ball mill-ground product.

[0037] (Preparation of Conductive Compacts) The hammer mill-ground, sample mill-ground, and planetary ball mill-ground powders were used to prepare sintered compacts as follows: Approximately 5 g of each powder was placed in a cylindrical graphite cell cylinder with an inner diameter of 10 mm, and heated in a spark plasma sintering apparatus under a vacuum atmosphere of approximately 1 Pa while applying a pressure of 90 MPa (7.065 kN) using upper and lower pistons. This resulted in a cylindrical sintered compact with a diameter of 10 mm and a height of approximately 8 mm. The heating pattern was to heat the sample to 500°C, hold it at 500°C for 10 minutes, and then allow it to cool.

[0038] (Measurement of the anomalous Nernst effect) From the above conductive molded body (sintered body), the length (L １ A rectangular parallelepiped sample with a thickness (H) of approximately 0.5 mm, a width (W) of approximately 1.5 mm, and a thickness (H) of approximately 8.0 mm was cut out. Figure 7 shows the attachment positions of the terminals for measuring electromotive force and the probes for measuring temperature, as well as the directions of the heat flow and magnetic field. Terminals 2a and 2b for measuring electromotive force were attached to the center of the opposing sides of the Nernst effect measurement sample 1 with conductive epoxy adhesive, so that the voltage (V) generated between the two terminals could be measured with a voltmeter. This voltage is generated by the anomalous Nernst effect, so V ＡＮＥ The sample 1 was placed at two locations (positions indicated by symbols 31 and 32) on the top surface of the sample 1 with a distance of 5.0 mm (L ２ ) and attached a temperature measurement probe with conductive epoxy adhesive so that the temperature difference ΔT between them could be monitored. A magnetic field was applied in the thickness direction of the sample while a heat flow was generated in the longitudinal direction of the sample in a physical property measurement system PPMS device manufactured by Quantum Design, and the electromotive force generated between both ends of the sample in the width direction was measured at room temperature (300 K). The black arrow (symbol 4) in Figure 7 indicates the direction of heat flow in the sample. Temperature T １ and T２ After stabilizing, a magnetic field is applied to the sample, and a voltage V ＡＮＥ The magnetic field was swept between 3 T and -3 T and between -3 T and 3 T. The Nernst coefficient S was calculated using the following formula (1): ＡＮＥ (μV / K) was calculated. ＡＮＥ (μV / K) = V ＡＮＥ (V) / W / ΔT(K) / L ２ ...(1) where V ＡＮＥ : electromotive force (V) generated at both ends of the sample in the width direction, W: length of the sample in the width direction (mm), ΔT: temperature difference (K) between the two temperature probe attachment positions, L ２ : the distance (mm) in the longitudinal direction of the sample between the two temperature probe attachment positions.

[0039] The results are shown in Table 1. Figure 8 illustrates the measurement results of the Nernst coefficient for the conductive compacts made using the sample mill pulverized product. In this example, the Nernst coefficient at a temperature of 300K in the absence of a magnetic field (zero magnetic field) was 0.4 μV / K. ５ It has been confirmed that a conductive compact of powder containing this as the main component has a coercive force exceeding 0.5 T in a measurement of the Nernst coefficient, and exhibits the anomalous Nernst effect in zero magnetic field.

[0040]

[0041] 1 Nernst effect measurement sample 2a, 2b electromotive force measurement terminals 31, 32 temperature measurement positions 4 heat flow direction in sample 5 magnetic field direction

Claims

1. Intermetallic compound GdCo ５ A powder mainly composed of the above, wherein in a volume-based particle size distribution measured by a laser diffraction / scattering method, the cumulative 50% particle diameter D50 is 1 to 150 μm and the cumulative 90% particle diameter D90 is 250 μm or less.

2. The molten metal of the Gd-Co alloy is solidified to produce the intermetallic compound GdCo. ５ 2. The method for producing metal powder according to claim 1, comprising: an alloy ingot preparation step for obtaining an alloy ingot mainly composed of the above; and a crushing step for obtaining powder by crushing the alloy ingot.

3. Intermetallic compound GdCo ５ A conductive compact made of powder with this as the main component.

4. The intermetallic compound GdCo ５ The powder mainly composed of the conductive molded body according to claim 3 has a cumulative 50% particle diameter D50 of 1 to 150 μm and a cumulative 90% particle diameter D90 of 250 μm or less in a volume-based particle size distribution measured by a laser diffraction / scattering method.

5. Intermetallic compound GdCo ５ 4. The conductive molded body according to claim 3, which is a sintered body of a powder containing the above as a main component.

6. The conductive molded body according to claim 3, which exhibits a Nernst coefficient of 0.2 μV / K or more at a temperature of 300 K in the absence of an applied magnetic field.

7. A thermoelectric conversion element using the conductive molded article according to any one of claims 3 to 6.

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