JP7728660B2 - Co-Mn-Ga alloy powder, conductive compact, their manufacturing methods, and thermoelectric conversion element - Google Patents

Description

The present invention relates to a Co-Mn-Ga alloy powder useful as a material for thermoelectric conversion elements, and a method for producing the same. The present invention also relates to a conductive compact of the Co-Mn-Ga alloy powder, a method for producing the same, and a thermoelectric conversion element using the conductive compact.

In recent years, research has been progressing on thermoelectric conversion elements that utilize the anomalous Nernst effect. 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. Utilizing the anomalous Nernst effect allows current to be extracted perpendicular to the heat flow, which, unlike when using the Seebeck effect, offers the advantage of being able to construct thin, sheet-like thermoelectric conversion devices.

The ferromagnetic intermetallic compound Co ₂ MnGa is known as a material that exhibits a large anomalous Nernst effect at room temperature.

Patent Document 1 describes an example of an experiment in which a _Co2MnGa single crystal was produced by the Czochralski method and the anomalous Nernst coefficient was measured. The Nernst coefficient of _{the Co2MnGa} single crystal at room temperature (300 K) reached a high value of about 6 μV/K regardless of whether the magnetic field direction was parallel to the [100], [110], or [111] direction of the crystal (paragraph 0021, Figure 4).

Patent Document 2 describes the use of an anomalous Nernst material film in a composite sensor having a heat flow sensor and a temperature sensor. Several substances are listed as anomalous Nernst materials, one of which is _Co2MnGa (paragraph 0026). A sputtering method is presented as a method for forming the anomalous Nernst material film (paragraph 0030).

International Publication No. 2019/009308 Japanese Patent Application Laid-Open No. 2020-153668

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

The thermoelectric conversion elements used to realize thermoelectric power generation devices are preferably bulk bodies with a thickness of approximately several millimeters. It is possible to produce bulk bodies of the above size using single crystals produced by the Czochralski method. However, single crystal manufacturing techniques such as the Czochralski method are expensive and have low productivity, making them impractical for the 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 the industrial production of bulk materials for thermoelectric power generation devices.

The thermoelectric conversion elements used to create heat flow sensors are desirably small, considering that they will be incorporated into a small circuit pattern. Cutting a large number of small elements from a single crystal obtained using methods such as the Czochralski method is cost-prohibitive for industrial use. It is also difficult to directly form small elements of a specified shape using the Czochralski method. On the other hand, using film-forming techniques such as sputtering, it is possible to form small elements corresponding to a specified circuit pattern directly on an insulating substrate. However, such film-forming methods have low productivity, resulting in high manufacturing costs for heat flow sensors.

The present invention aims to provide technology suitable for the productive manufacture of thermoelectric conversion elements that utilize the anomalous Nernst effect and achieve a high Nernst coefficient, and in particular technology that can be widely adapted to the manufacture of elements of various shapes and sizes. It also aims to provide thermoelectric conversion elements with a high Nernst coefficient obtained using this technology.

To achieve the above objectives, this specification discloses the following invention.

[1] A Co-Mn-Ga alloy powder containing an intermetallic compound Co ₂ MnGa as a main component, in which, 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] an alloy ingot preparation step of solidifying a molten metal consisting of Co, Mn, and Ga to obtain a Co—Mn—Ga-based alloy ingot containing the intermetallic compound Co ₂ MnGa as a main component;
a pulverization step of pulverizing the Co—Mn—Ga-based alloy ingot to obtain a powder;
The method for producing the Co—Mn—Ga alloy powder according to the above [1].
[3] a classification step of classifying the powder obtained in the pulverization step using a sieve;
The method for producing a Co—Mn—Ga based alloy powder according to [2] above, further comprising:
[4] A conductive compact of powder containing the intermetallic compound Co ₂ MnGa as a main component.
[5] The powder mainly composed of the intermetallic compound Co ₂ MnGa 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, the conductive molded body according to [4].
[6] The conductive molded body according to the above [4] or [5], which is a sintered body of a powder containing an intermetallic compound Co ₂ MnGa as a main component.
[7] The conductive molded body according to any one of the above [4] to [6], which exhibits a Nernst coefficient of 6.0 μV/K or more at a temperature of 300K.
[8] A method for producing a conductive compact, comprising a sintering step of sintering a powder containing an intermetallic compound Co ₂ MnGa as a main component to obtain a conductive compact.
[9] The method for producing a conductive molded body according to the above-mentioned [8], wherein the powder mainly composed of the intermetallic compound Co ₂ MnGa 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.
[10] A thermoelectric conversion element using the conductive molded article according to any one of [4] to [7] above.

The present invention makes it possible to obtain thermoelectric conversion elements with a high Nernst coefficient. Because the magnetic material used in these thermoelectric conversion elements is a powder, the technology of the present invention can be used to produce elements of a wide variety of shapes and sizes. Furthermore, the technology of the present invention is superior in terms of cost and productivity compared to conventional technologies that use single crystals or thin films produced by sputtering.

X-ray diffraction pattern of the powder after the first classification step. 10 is an SEM photograph of the powder after the first classification process. 10 is an SEM photograph of the powder after the second classification process. FIG. 2 is a diagram schematically showing the attachment positions of terminals for power measurement and probes for temperature measurement, as well as the directions of heat flow and magnetic field application, for a sample for measuring the Nernst effect used in the examples. 10 is a graph showing the results of measuring the Nernst coefficient of the conductive compact powder after the second classification step.

[Co-Mn-Ga alloy powder]
In this invention, a Co- _Mn -Ga alloy powder containing the intermetallic compound Co2MnGa as its main component is used as a material suitable for thermoelectric conversion elements. _Co2MnGa is a type of Heusler alloy with an _L21 crystal structure. This intermetallic compound is a Weyl ferromagnetic material, and is known to exhibit a large anomalous Nernst effect, with a Nernst coefficient of approximately 6 μV/K at room temperature.

In a certain composition range where the composition ratio of Co, Mn, and Ga is close to the stoichiometric composition of Co ₂ MnGa, an intermetallic compound having a Co ₂ MnGa-type crystal structure can exist stably as a single phase. It is believed that a Co-Mn-Ga-based alloy can be obtained around this composition range, in which an intermetallic compound phase having a Co ₂ MnGa-type crystal structure and a different phase are mixed. In this specification, an intermetallic compound phase having a Co ₂ MnGa-type (i.e., L2 ₁ ) crystal structure in a Co-Mn-Ga-based ternary alloy is referred to as the "intermetallic compound Co ₂ MnGa" or simply the "Co ₂ MnGa phase." Therefore, even if an intermetallic compound has a composition ratio slightly different from the stoichiometric composition of Co ₂ MnGa, one having a Co ₂ MnGa-type crystal structure is included in the "intermetallic compound Co ₂ MnGa" referred to in this specification.

"Mainly composed of the intermetallic compound Co ₂ MnGa" means that the metal phase with the largest mass fraction among the metal phases contained in the powder is the Co ₂ MnGa phase. In a thermoelectric conversion element using a Co-Mn-Ga alloy powder, even if a heterophase other than the Co ₂ MnGa phase is contained, the thermoelectric conversion action occurs due to the anomalous Nernst effect of the Co ₂ MnGa phase. However, to achieve efficient thermoelectric conversion characteristics, it is desirable that the amount of heterophases that do not exhibit the anomalous Nernst effect be small. For example, the proportion of the Co ₂ MnGa phase in the powder is preferably 50 mass% or more, and more preferably 90 mass% or more. Furthermore, powders consisting of the intermetallic compound Co ₂ MnGa, i.e., powders in which the intermetallic compound Co ₂ MnGa and other components are unavoidable impurities, are particularly preferred.

The particle size distribution of the particles that make up the Co-Mn-Ga alloy powder, measured by volumetric particle size distribution using a laser diffraction/scattering method, should be such that the cumulative 50% particle diameter D50 is 1 to 150 μm and the cumulative 90% particle diameter D90 is 250 μm or less. If the cumulative 50% particle diameter D50 is too large, the average particle diameter will be large, leading to a decrease in coercivity. If the cumulative 90% particle diameter D90 is too large, the proportion of coarse particles will increase, also leading to a decrease in coercivity. From the perspective of ensuring coercivity, the cumulative 50% particle diameter D50 is preferably 100 μm or less, and more preferably 60 μm or less.

Examples of methods for producing Co—Mn—Ga-based alloy powder include:
an alloy ingot preparation step of solidifying a molten metal consisting of Co, Mn, and Ga to obtain a Co—Mn—Ga-based alloy ingot containing an intermetallic compound Co ₂ MnGa as a main component;
a pulverization step of pulverizing the Co—Mn—Ga-based alloy ingot to obtain a powder;
If necessary, a classification step of classifying the powder obtained in the pulverization step using a sieve;
A process having the following can be applied.

Molten metal consisting of Co, Mn, and Ga can be obtained by placing the raw Co, Mn, and Ga metals, weighed to the specified composition, into a crucible and heating and melting them in a non-oxidizing atmosphere. The molten metal is then solidified to obtain a Co-Mn-Ga alloy ingot. The molten metal can be solidified by cooling it in the crucible or by casting it into a mold.

The Co-Mn-Ga alloy ingot is then pulverized to obtain a powder. Known mechanical pulverizers, such as stamp mills, hammer mills, and sample mills, can be used as the pulverizing means. It is more efficient to combine multiple pulverizing means to gradually reduce the particle size. When the pulverizing process is carried out in stages, classification using a sieve or other method may be performed between each pulverizing stage. Furthermore, to adjust the final particle size distribution, it is preferable to classify the powder obtained after the final pulverizing step using a sieve.

[Conductive molded body]
In this specification, an object that is formed into a predetermined shape using powder as a material and has the definite shape to maintain that shape in the usage environment is called a "powder compact." In particular, a powder compact that has conductivity is called a "conductive powder compact." A conductive powder compact that uses the above Co-Mn-Ga alloy powder as a material and contains the intermetallic compound Co ₂ MnGa as its main component is useful as a thermoelectric conversion element.

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.

An example of a form of conductive powder compact other than a compact or sintered compact is a compact formed 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.

When a sintered body is used as a conductive compact of a powder containing the intermetallic compound Co ₂ MnGa as the main component, a sintered body of a powder containing the intermetallic compound Co ₂ MnGa as the main component can be produced by using a known sintering method.When a part or all of a circuit pattern formed on an insulating substrate is used as a conductive compact of a powder containing the intermetallic compound Co ₂ MnGa as the main component, a method can be applied in which a coating film of the circuit pattern is formed on the insulating substrate using a paint containing the powder as a filler, and then the coating film is heated and sintered.

By using Co-Mn-Ga alloy powder adjusted to the particle size distribution described above, it is possible to create a conductive powder compact that exhibits a Nernst coefficient of 6.0 μV/K or more at a temperature of 300 K.

[Example 1]
(Production of Co—Mn—Ga-based alloy ingot)
The raw materials, metallic Co (manufactured by Rare Metallic Co., Ltd., purity 3N), metallic Mn (manufactured by Rare Metallic Co., Ltd., purity 3N), and metallic Ga (manufactured by Rare Metallic Co., Ltd., purity 6N), were weighed out to a molar ratio of Co:Mn:Ga = 2:1:1 and placed in an alumina crucible. This crucible was placed in a vertical electric furnace and, under an Ar gas atmosphere, the temperature was raised from room temperature to 1000°C over 4 hours, raised from 1000°C to 1250°C over 2 hours, held at 1250°C for 12 hours, and then allowed to cool in the furnace for 5 hours. This heating pattern allowed the molten metal of the Co—Mn—Ga alloy to solidify in the crucible, yielding a Co—Mn—Ga alloy ingot.

(Powder preparation)
553.47 g of the obtained Co—Mn—Ga alloy ingot was subjected to the following three-stage pulverization process and two-stage classification process to produce powder.

(First pulverization step)
The above Co-Mn-Ga alloy ingot was pulverized into particles of about 5 mm or less in an air atmosphere using a stamp mill (ANS-143PS, manufactured by Nitto Kagaku Co., Ltd.).

(Second pulverization step)
The pulverized material obtained in the first pulverization step was pulverized in a glove box under a nitrogen gas atmosphere using a hammer mill (Hammer Crusher NH-34S, manufactured by Sansho Industry Co., Ltd., screen mesh: 0.3 mm). The oxygen concentration in the atmosphere during pulverization was less than 0.0% by volume. After pulverization was completed, the container was gradually opened to the atmosphere, and the pulverized material was collected.

(Third pulverization step)
The pulverized material obtained in the second pulverization step was pulverized in a glove box under a nitrogen gas atmosphere using a sample mill (SK-M10 model, manufactured by Kyoritsu Riko Co., Ltd.). The pulverization procedure consisted of repeating four cycles of "30-second pulverization treatment → cooling." The oxygen concentration in the atmosphere during the pulverization and cooling cycles was less than 0.0% by volume. After the final cooling period, the container was gradually opened to the atmosphere, and the pulverized powder was collected.

(1st classification process)
The powder obtained in the third pulverization step was classified using a sieve with 100 μm openings, and the powder that passed through the sieve was obtained.

(Second classification process)
The powder obtained in the first classification step was further classified using a sieve with 45 μm openings, and the powder that passed through the sieve was obtained.

(Measurement of particle size distribution)
The pulverized material after the second pulverization step, the powder after the third pulverization step, the powder after the first classification step, and the powder after the second classification step were measured for volumetric particle size distribution by laser diffraction/scattering using a dry laser diffraction particle size distribution analyzer (HELOS & RODOS, manufactured by Japan Laser Co., Ltd.) with a lens having a focal length of 1000 mm. The cumulative 10% particle diameter D10, cumulative 50% particle diameter D50, and cumulative 90% particle diameter D90 calculated based on the obtained particle size distribution are shown in Table 1.

(Measurement of X-ray diffraction pattern)
The powder after the first classification step was subjected to measurement of an X-ray diffraction pattern using Cu-Kα radiation using an X-ray diffractometer (Shimadzu Corporation, XRD-6100 LabX). An example of the X-ray diffraction pattern is shown in Figure 1. Figure 1 also shows the calculated diffraction peak positions that appear in a Co ₂ MnGa crystal with a lattice constant a = 0.578 nm. The X-ray diffraction pattern of the powder sample is in good agreement with the calculated diffraction pattern of a Co ₂ MnGa crystal.

(Measurement of powder magnetic properties)
The magnetic properties of the powders after the third pulverization step, the powders after the first classification step, and the powders after the second classification step were measured at 300 K using a SQUID (Super Quantum Interference Device) magnetometer in the following manner. That is, the temperature of the powder samples was set to 300 K, and a maximum magnetic field of 3 T (30,000 Oe) was applied using a superconducting magnet attached to the SQUID magnetometer. Then, the magnetic field was swept from 3 T to -3 T and from -3 T to 3 T, with the magnetic field fixed at each measurement point, and the magnetization (μ _B /fu) was measured. The saturation magnetization and coercivity were calculated based on the magnetization curves. The results are shown in Table 1.

(SEM photo of powder)
SEM (scanning electron microscope) photographs of the powder after the first classification step and the powder after the second classification step are shown in Figures 2 and 3, respectively. The length of the white scale bar displayed at the bottom of the photograph corresponds to 10 μm. The SEM used was an FE-SEM JSM-7200F manufactured by JEOL Ltd.

(Powder Composition Analysis by EDX)
The powder after the first classification step was subjected to composition analysis using an EDX (energy dispersive X-ray analysis) device (X-Max20, manufactured by Oxford Instruments) attached to an SEM, and the atomic ratio was found to be Co:Mn:Mg=51.0:29.1:19.9.

(Preparation of conductive molded body)
Here, a sintered body was produced as a conductive powder compact using the powder after the second classification process described above as follows: Approximately 5.9 g of the powder after the second classification process 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 produced a cylindrical sintered body with a diameter of 10 mm and a height of approximately 8 mm. The heating pattern was as follows: the temperature was raised to 650°C, held at 650°C for 10 minutes, raised to 750°C, held at 750°C for 10 minutes, and then allowed to cool.

(Measurement of the anomalous Nernst effect)
A rectangular parallelepiped sample with a length ( _L1 ) of 8.269 mm, a width (W) of 1.460 mm, and a thickness (H) of 0.624 mm was cut out from the above-mentioned conductive molded body (sintered body). Figure 4 shows the attachment positions of the terminals for measuring electromotive force and the probes for measuring temperature, as well as the directions of heat flow and magnetic field application. Terminals 2a and 2b for measuring electromotive force were attached to the center of opposite 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, and is therefore designated as V _ANE . Two temperature measurement probes were attached to the top surface of sample 1 at two locations (positions indicated by reference numerals 31 and 32) with a 5.0 mm gap (L ₂ ) between them using conductive epoxy adhesive, allowing the temperature difference ΔT between them to 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 Quantum Design Physical Property Measurement System (PPMS) device. The electromotive force generated between both ends of the sample in the width direction was measured at room temperature (300 K). The solid arrow (reference numeral 4) in Figure 4 indicates the direction of heat flow in the sample. After temperatures _T1 and _T2 stabilized, a magnetic field was applied to the sample, and the voltage V _ANE (V) was measured. The open arrow (reference numeral 5) in Figure 4 indicates the direction of the magnetic field. The magnetic field was swept between 3 T and -3 T and between -3 T and 3 T.
The Nernst coefficient S _ANE (μV/K) was calculated using the following formula (1).
S _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: width of sample (mm),
ΔT: temperature difference between the two temperature probe mounting positions (K),
_L2 : Distance in the longitudinal direction of the sample between the two temperature probe attachment positions (mm),
is.

The measurement results of the Nernst coefficient are shown in Figure 5. The Nernst coefficient at a temperature of 300 K reached 6.68 μV/K. It was confirmed that the conductive compact of powder containing the intermetallic compound _Co2MnGa as the main component according to the present invention exhibits a Nernst coefficient equal to or greater than that of single crystal _Co2MnGa .

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 (20)

A Co-Mn-Ga-based alloy powder containing an intermetallic compound Co ₂ MnGa as a main component , in which the proportion of the Co ₂ MnGa phase in the powder is 90 mass % or more, and 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. an alloy ingot preparation step of solidifying a molten metal composed of Co, Mn, and Ga to obtain a Co—Mn—Ga-based alloy ingot containing an intermetallic compound Co ₂ MnGa as a main component and having a proportion of the Co 2 MnGa phase of 90 mass % or more _;
a pulverization step of pulverizing the Co—Mn—Ga-based alloy ingot to obtain a powder;
2. The method for producing Co—Mn—Ga based alloy powder according to claim 1, wherein the a classification step of classifying the powder obtained in the pulverization step using a sieve;
3. The method for producing Co-Mn-Ga based alloy powder according to claim 2, further comprising: A conductive compact of powder containing an intermetallic compound Co ₂ MnGa as a main component , with the proportion of the Co ₂ MnGa phase being 90 mass % or more . The powder, which is mainly composed of the intermetallic compound Co ₂ MnGa and has a ratio of 90 mass% or more of the Co 2 MnGa phase _, 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. The conductive molded body according to claim 4. 6. The conductive molded body according to claim 4, which is a sintered body of powder containing the intermetallic compound _Co2MnGa as a main component , with the proportion of the _Co2MnGa phase being 90 mass % or more . The conductive molded article according to any one of claims 4 to 6, exhibiting a Nernst coefficient of 6.0 μV/K or greater at a temperature of 300 K. A method for producing a conductive molded body, comprising: a sintering step of sintering a powder containing an intermetallic compound Co ₂ MnGa as a main component , with the proportion of the Co ₂ MnGa phase being 90 mass % or more, to obtain a conductive molded body. The powder, which is mainly composed of the intermetallic compound Co ₂ MnGa and has a ratio of 90 mass% or more of the Co 2 MnGa phase _, 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. The method for producing a conductive molded body according to claim 8. A thermoelectric conversion element using the conductive molded article described in any one of claims 4 to 7. A Co-Mn-Ga alloy powder is a powder made of an intermetallic compound Co ₂ MnGa, and 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. an alloy ingot preparation step of solidifying a molten metal consisting of Co, Mn, and Ga to obtain a Co—Mn—Ga-based alloy ingot consisting of an intermetallic compound Co ₂ MnGa;
a pulverization step of pulverizing the Co—Mn—Ga-based alloy ingot to obtain a powder;
The method for producing Co-Mn-Ga based alloy powder according to claim 11 , which comprises: a classification step of classifying the powder obtained in the pulverization step using a sieve;
The method for producing Co-Mn-Ga based alloy powder according to claim 12 , further comprising: A conductive compact of powder made of the intermetallic compound Co ₂ MnGa. The powder made of the intermetallic compound Co ₂ MnGa 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. The conductive molded body according to claim 14 or 15 , which is a sintered body of powder made of the intermetallic compound Co ₂ MnGa. The conductive molded article according to any one of claims 14 to 16 , which exhibits a Nernst coefficient of 6.0 µV/K or more at a temperature of 300K. A method for producing a conductive compact, comprising: a sintering step of sintering powder made of an intermetallic compound Co ₂ MnGa to obtain a conductive compact. The powder made of the intermetallic compound Co ₂ MnGa 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. A thermoelectric conversion element using the conductive molded article according to any one of claims 14 to 17 .