WO2024204211A1 - Method for producing metal powder, and metal powder - Google Patents

Abstract

This method is for producing a metal powder and comprises: a first step for dispersing, in a gas phase by means of a carrier gas, a raw material metal powder produced by reducing metal ions in a liquid phase; a second step for subjecting the raw material metal powder dispersed in the gas phase to a thermal treatment at a temperature of at least (Tm-100)°C (where Tm°C is the melting point of the raw material metal powder) to produce a metal powder precursor dispersed in the gas phase; and a third step for cooling the metal powder precursor dispersed in the gas phase to produce a metal powder. With the present invention, it is possible to produce a metal powder having excellent crystallinity and a narrow particle size distribution.

Description

METAL POWDER METHOD AND METAL POWDER

The present invention relates to a method for producing metal powder and to metal powder.

In recent years, there has been an increasing demand for metal powders with excellent crystallinity and a narrow particle size distribution as conductive powders used to form electrodes for electronic components.
Known methods for producing metal powders with excellent crystallinity include spray pyrolysis, atomization, and methods for producing metal powders through a process of vaporizing raw metals using plasma, in which metal powders are produced through high temperature conditions in a gas phase. While these methods can produce metal powders with extremely high crystallinity, the breadth and narrowness of the particle size distribution depends on the uniformity/homogeneity of the raw materials and the temperature distribution in the gas phase, and therefore more strict control is required to obtain metal powders with an even narrower particle size distribution. In order to achieve this, the manufacturing equipment must be enlarged and complicated, which may lead to increased costs.

In addition, a wet reduction method and the like are known as a method for easily producing metal powders with a narrow particle size distribution. However, it is widely known that the crystallinity of metal powders obtained by the wet reduction method is completely insufficient to meet the level required in recent years.
In view of this, various studies have been carried out on methods for improving the crystallinity of metal powder produced by the wet reduction method or the like, as described below.

Patent Document 1 discloses a method for producing nickel powder in which a nickel raw material consisting of nickel fine powder or nickel compound fine powder is mixed and ground with a spacer consisting of a fine powder of at least one type of salt selected from alkali metal salts or alkaline earth metal salts, the ground product is heat-treated to cause the nickel fine powder to thermally decompose and/or grow in grains with the spacer in between, and then the heat-treated product is acid-treated to dissolve the spacer, the dissolved material is washed away with water, and the remainder is dried to obtain nickel powder with grain growth.

Patent Document 2 discloses a method for producing spherical nickel powder, which comprises a first step of mixing an alkaline earth metal compound powder with a nickel compound powder, a second step of heating the mixture obtained in the first step in a hydrogen atmosphere while preventing the formation of a complex oxide in order to reduce the nickel in the mixture to metallic nickel, a third step of heating the heated product obtained in the second step in a non-oxidizing atmosphere while preventing the particles from coarsening in order to improve the sphericity of the metallic nickel particles in the heated product obtained in the second step, and a fourth step of dissolving and removing the alkaline earth metal in the heated product obtained in the third step with an acid.

Patent Document 3 discloses a method for producing metal powder with a crystallite size of 800 angstroms or more, which comprises mixing metal powder with at least one of an alkali metal halide, an alkaline earth metal halide, and a rare earth halide, heating the mixture to a temperature equal to or higher than the melting point of the halide, and then cooling the mixture, and removing the halide by wet processing the resulting reaction product to recover the metal powder.

Patent Document 4 describes a method for treating metal powder to modify its density, which is characterized in that the particle surfaces are coated with an oxide, hydroxide or carbonate of an alkaline earth metal, the metal powder is then heat-treated at or below its melting point, the heat-treated product is leached with an acid, and the metal powder is then collected by solid-liquid separation.

Patent Document 5 discloses a method for producing spherical nickel powder, which comprises a first step of adding 0.01 wt% to 30 wt% of a rare earth compound to spherical nickel or spherical nickel compound powder, and a second step of heating and reducing the nickel in the mixture obtained in this step in a hydrogen atmosphere, and/or further heating the nickel in a non-oxidizing atmosphere after reduction.

Patent Document 6 discloses a method for producing spherical _nickel powder, which comprises a first step of adding and _mixing at least one selected from _SiO2.nH2O (n≧0) and _Al2O3.nH2O (n≧0) in an amount of 0.01 wt% to 30 wt%, calculated on an anhydrous basis, to spherical _nickel or spherical nickel compound powder, and a second step of heating and reducing the nickel in the mixture obtained in the first step in a hydrogen atmosphere and/or further heating the mixture in a non-oxidizing atmosphere after reduction.

Patent Document 7 discloses a method for producing a conductive powder _, which comprises the steps of: (A) adding 40 parts by weight or more and 100 parts by weight or less of boron oxide or boric acid, calculated as _B2O3 , per 100 parts by weight of metal powder to a dispersion solution of a metal powder, mixing the mixture, and then drying the dispersion solution to obtain a mixture of the metal powder and boron oxide or boric acid; (B) heat-treating the mixture in a neutral atmosphere to obtain a heat-treated product; and (C) dissolving the heat-treated product in water or acid to obtain a metal powder.

Patent Document 8 discloses a method for producing nickel powder, which comprises adding and mixing a sintering inhibitor and pure water to nickel powder obtained by a wet reduction method, drying this mixed slurry by spray heat treatment, and then heat treating the dried mixture in a reducing or inert atmosphere at a temperature below the melting point of the sintering inhibitor to sinter the nickel powder, and then separating and removing the sintering inhibitor from the sintered nickel powder.

Patent Document 9 discloses a method for producing platinum powder with high dispersibility and a crystallite size in the range of 30 to 100 (nm), which includes a mixing step of mixing platinum powder with an admixture consisting of a powder of at least one type of oxide of a metal element in any one of Groups 3 to 15 of the long periodic table, a heat treatment step of subjecting the mixed powder obtained in the mixing step to a heat treatment at a predetermined temperature, a dissolving step of treating the heat-treated mixed powder with an acid or alkali to dissolve the admixture, and a removal step of removing the admixture by cleaning the mixed powder in which the admixture has been dissolved.

Patent Document 10 discloses a method for producing nickel powder, which comprises mixing nickel powder with a fine powder of an alkaline earth metal compound selected from the group consisting of oxides, hydroxides, carbonates, and hydrogen carbonates of alkaline earth metals, or coating the surface of each particle of the nickel powder with the alkaline earth metal compound, and then carrying out a heat treatment in an inert gas or slightly reducing gas atmosphere at a temperature below the melting temperature of the alkaline earth metal compound.

Japanese Patent Application Publication No. 10-102109 Japanese Patent Application Publication No. 11-140513 JP 2000-096110 A JP 2001-040401 A JP 2001-098337 A JP 2001-107103 A JP 2004-176120 A JP 2004-339601 A JP 2006-299385 A JP 2002-146401 A

In the methods described in Patent Documents 1 to 10, various substances are added to and mixed with the metal powder before the heat treatment in order to prevent the metal powders from sintering during the heat treatment. Therefore, the substances remain on the surface of the metal powder even after the heat treatment. Therefore, in order to obtain a metal powder suitable as a conductive material, it is necessary to remove the above-mentioned substances after the heat treatment. However, since an acid or an alkali is used in the removal operation, it is inevitable that the metal powder surface will be affected, such as dissolving the metal powder surface or forming a metal compound on the metal powder surface. Furthermore, even if the above-mentioned removal operation is performed, it is difficult to completely remove the above-mentioned substances that have been added or mixed, and they will remain on the metal powder surface. In other words, the methods described in Patent Documents 1 to 10 have various problems as methods for producing metal powders that can be suitably used as conductive materials.
Furthermore, even if the methods described in Patent Documents 1 to 10 are carried out, the excellent crystallinity required in recent years cannot be obtained, and there is a demand for an even better method for improving the crystallinity of metal powders.

The present invention therefore aims to provide a method for producing metal powder that produces metal powder with excellent crystallinity and a narrow particle size distribution.

The present invention, which solves the above problems, is as described below.
(1) a first step of dispersing a raw metal powder produced by reducing metal ions in a liquid phase into a gas phase by a carrier gas;
a second step of heat-treating the raw metal powder dispersed in the gas phase at a temperature of (Tm-100)°C or higher (where Tm°C is the temperature at which the raw metal powder melts) to produce a metal powder precursor dispersed in the gas phase;
a third step of cooling the metal powder precursor dispersed in the gas phase to produce a metal powder;
A method for producing a metal powder having the above structure.
(2) The method for producing a metal powder according to (1) above, wherein in the first step, the raw metal powder is dispersed in a gas phase by the carrier gas at a concentration of 1.0 g/L or less, in the second step, the raw metal powder dispersed in the gas phase at the concentration is heat-treated at a temperature of (Tm-100)°C or more to produce a metal powder precursor dispersed in the gas phase, and in the third step, the metal powder precursor dispersed in the gas phase at the concentration is cooled to produce the metal powder.
(3) The method for producing a metal powder according to (1) or (2) above, wherein in the second step, the raw metal powder dispersed in the gas phase is heat-treated at a temperature of Tm°C or higher to produce a metal powder precursor in a liquid state dispersed in the gas phase.
(4) A method for producing a metal powder according to any one of (1) to (3) above, wherein in the second step, the raw metal powder dispersed in the gas phase is heat-treated at a temperature of (Tm-100)°C or higher and lower than Tb°C (where Tm°C is the temperature at which the raw metal powder melts, and Tb°C is the temperature at which the raw metal powder vaporizes).
(5) A method for producing a metal powder described in any one of (1) to (4) above, wherein in the first step, the raw metal powder is dispersed in the gas phase by the carrier gas while the raw metal powder is suspended in a liquid.
(6) The method for producing a metal powder according to any one of (1) to (5) above, wherein in the third step, the metal powder precursor dispersed in the gas phase is cooled to produce a metal powder, and then the metal powder dispersed in the gas phase is cooled to a temperature at which the metal powder does not sinter.
(7) The method for producing a metal powder according to any one of (1) to (6) above, wherein the raw metal powder has a CV value, as defined below, of 0.40 or less.
CV value: When the cumulative 50% particle diameter based on the number calculated based on the particle diameters of 100 or more particles randomly selected and measured by scanning electron microscope observation is defined as _D50 , the ratio of the standard deviation calculated based on the particle diameters of 100 or more particles randomly selected and measured by scanning electron microscope observation to _D50 . (8) The method for producing a metal powder according to any one of (1) to (7) above, wherein the ratio of the crystallite diameter ratio of the metal powder defined below to the crystallite diameter ratio of the raw metal powder defined below is 4.0 or more.
Crystallite size ratio: The ratio of the crystallite size to _D50, where D50 is the cumulative 50% particle size based on the number calculated based on the particle sizes of 100 or more randomly selected particles measured by scanning electron microscope observation, and _D50 is the crystallite size calculated by Scherrer's formula using the measured values by X-ray diffraction. (9) A method for producing a metal powder according to any one of (1) to (8), wherein the ratio of the crystallite size of the metal powder defined below to the crystallite size of the raw metal powder defined below is 4.0 or more.
Crystallite size: A value calculated by the Scherrer formula using measured values by X-ray diffraction. (10) The ratio of the _D50 defined below of the metal powder to the _D50 defined below of the raw metal powder is 1.0 to 2.0, and the ratio of the CV value defined below of the metal powder to the CV value defined below of the raw metal powder is 1.0 to 2.0. The method for producing a metal powder according to any one of (1) to (9) above.
_D50 : cumulative 50% particle diameter on a number basis calculated based on particle diameters measured by observing 100 or more particles randomly selected by scanning electron microscope. CV value: cumulative 50% particle diameter on a number basis calculated based on particle diameters measured by observing 100 or more particles randomly selected by scanning electron microscope is defined as _D50 . Ratio of standard deviation calculated based on particle diameters measured by observing 100 or more particles randomly selected by scanning electron microscope to _D50 . (11) The method for producing a metal powder according to any one of (1) to (10) above, wherein the raw metal powder has a _D50 defined below of 10 nm or more and a _D50 defined below of 10 nm or more and a D50 defined below of 10 nm or more and a D50 of the metal powder.
_D50 : Cumulative 50% particle size based on the number calculated based on particle sizes measured by randomly selecting 100 or more particles by scanning electron microscope observation (12) A metal powder having a CV value defined below of 0.50 or less, a crystallite size defined below of 30.0 nm or more, and a crystallite size ratio defined below of 1.0 x ^10-2 or more.
CV value: The ratio of the standard deviation calculated based on the particle diameters of 100 or more randomly selected particles measured by scanning electron microscope observation to _D50 , where _D50 is the cumulative 50% particle diameter based on the number of particles measured by randomly selecting 100 or more particles by scanning electron microscope observation. Crystallite size: A value calculated by Scherrer's formula using the measured value by X-ray diffraction. Crystallite size ratio: A ratio of the crystallite size to _D50 , where D50 is the value calculated by Scherrer's formula using the measured value by X-ray diffraction. (13) The metal powder according to (12) above, in which the CV value is 0.40 or less, the _D50 defined below is 10 nm or more and 200 nm or less, the crystallite size ratio is 0.40 or more, and the aspect ratio defined below is 0.95 or more and 1.0 or less.
_D50 : Cumulative 50% particle size based on the number calculated based on the particle diameters measured by randomly selecting 100 or more particles by scanning electron microscope observation. Aspect ratio: A value calculated by measuring the ratio of the minor axis diameter to the major axis diameter of a rectangle circumscribing the particle so as to have the smallest area (minor axis diameter/major axis diameter) for each particle by randomly selecting 100 or more particles by scanning electron microscope observation, and averaging the measured values.

The present invention provides a method for producing metal powder that has excellent crystallinity and a narrow particle size distribution.

<Metal Powder Manufacturing Method>
The method for producing metal powder of the present invention includes a first step of dispersing a raw metal powder, produced by reducing metal ions in a liquid phase, in a gas phase using a carrier gas; a second step of heat-treating the raw metal powder dispersed in the gas phase at a temperature of (Tm-100)°C or higher (wherein Tm°C is the temperature at which the raw metal powder melts) to produce a metal powder precursor dispersed in the gas phase; and a third step of cooling the metal powder precursor dispersed in the gas phase to produce a metal powder.

　Previously known methods had the problem that when heat-treated at high temperatures, the raw metal powder would sinter together and become a metal lump. For this reason, the method was premised on heat-treating at a temperature at which the raw metal powder would not sinter, and the crystallinity of the metal powder obtained after heat treatment was insufficient.

In contrast, the present invention disperses raw metal powder produced by reducing metal ions in a liquid phase into a gas phase, and then heat-treats the powder while it is dispersed in the gas phase. This allows heat treatment at a high temperature (the temperature at which the raw metal powder melts -100°C) at which the raw metal powder would sinter together in conventional methods. In addition, the metal powder precursor produced by heat treatment is cooled while still dispersed in the gas phase, so coalescence of the metal powder particles can be suppressed. The above configuration of the present invention makes it possible to produce metal powder that maintains a narrow particle size distribution while also having excellent crystallinity that was not possible with conventional methods.

As described above, by using the method for producing metal powder of the present invention, it is possible to obtain metal powder that has better crystallinity than conventional methods and also has a narrow particle size distribution. However, if a metal powder with an even narrower particle size distribution is required, the metal powder obtained by the present invention may be classified. As described above, the particle size distribution of the metal powder obtained by the present invention is sufficiently narrow, so the proportion of powder removed by classification is small. In other words, classification can be performed with a good yield.

In addition, the metal powder manufacturing method of the present invention allows heat treatment to be performed at a higher temperature than conventional methods, shortening the heat treatment time. In other words, the amount of metal powder produced per unit time can be dramatically improved.

Hereinafter, each step constituting the method for producing metal powder according to the present invention will be described.
In the present invention, "D ₅₀ ", "CV value", "crystallite size", "crystallite size ratio" and "aspect ratio" are defined as follows.
( _D50 )
The cumulative 50% particle diameter based on the number calculated based on the particle diameters measured by randomly selecting 100 or more particles by scanning electron microscope observation is defined as _D50 . In the present invention, the particle diameter is defined as the diameter of a perfect circle having the same area as the projected area of a particle.
(CV value)
The ratio of the standard deviation calculated based on the particle diameters of 100 or more particles randomly selected by scanning electron microscope observation to _D50 (standard deviation/ _D50 ) is defined as the CV value.
(Crystallite size)
The value d calculated by the Scherrer formula represented by the following formula (1) using the measured values by X-ray diffraction is defined as the crystallite size (in the following formula (1), K is the Scherrer constant, λ is the X-ray wavelength, β is the half-width of the diffraction peak, and θ is the diffraction angle).
Scherrer's formula: d = Kλ / β cos θ (1)
(Crystallite size ratio)
The ratio of the crystallite size to the _D50 is defined as the crystallite size ratio.
(Aspect Ratio)
At least 100 particles are randomly selected by observation under a scanning electron microscope, and the ratio of the minor axis to the major axis of a rectangle circumscribing the particle so as to have the smallest area (minor axis diameter/major axis diameter) is measured for each particle, and the average value of these ratios is defined as the aspect ratio.

The method for producing metal powder of the present invention includes the following first, second and third steps.
(First step)
The first step in the present invention is a step of dispersing a raw metal powder produced by reducing metal ions in a liquid phase into a gas phase by a carrier gas. Since the raw metal powder is produced by reducing metal ions in a liquid phase, a raw metal powder with a narrow particle size distribution is produced, and the raw metal powder is further dispersed in a gas phase by a carrier gas, so that in the second step described later, heat treatment can be performed at a much higher temperature than in the conventional method, and therefore a metal powder having excellent crystallinity while maintaining a narrow particle size distribution can be produced. In addition, since heat treatment can be performed at a much higher temperature than in the conventional method in the second step described later, the heat treatment time in the second step can be shortened, and the production amount of metal powder per unit time can be dramatically improved. In addition, by performing heat treatment at a temperature of Tm ° C. or higher in the second step described later, a spherical metal powder with a good aspect ratio can be produced. In the present invention (this specification), the closer the shape of the metal powder is to a perfect sphere, that is, the closer the aspect ratio is to 1.0, the better the aspect ratio is, and a particularly preferred aspect ratio is 1.0.

　A known method can be used to produce raw metal powder by reducing metal ions in the liquid phase. For example, a metal salt can be dissolved in water in advance to form an aqueous metal salt solution, and a reducing agent can be added to the aqueous solution to reduce the metal ions, producing raw metal powder. Raw metal powder of the desired particle size and particle size distribution can be obtained by known methods such as appropriately adjusting the metal ion concentration in the liquid phase, the concentration and addition timing of the reducing agent, and the concentration and addition timing of other additives.

There are no particular limitations on the metal salt, and examples that can be used include nickel nitrate, nickel sulfate, nickel acetate, silver nitrate, copper nitrate, and copper sulfate.

The reducing agent is not particularly limited, and for example, hydrazine or formaldehyde can be used.

The type of metal constituting the raw metal powder is not particularly limited. That is, the raw metal powder in the present invention may be, for example, a metal powder selected from nickel, copper, silver, palladium, gold, platinum, and iron, or may be an alloy powder of two or more of the above metals, or may be an alloy powder and/or composite powder to which an appropriate amount of other metal elements, such as magnesium, calcium, ruthenium, rhodium, rhenium, iridium, osmium, titanium, tungsten, and tin, which are widely known to be added to the above metals, or nonmetal elements, such as silicon, bismuth, phosphorus, and sulfur, have been added.

The D ₅₀ of the raw metal powder is not particularly limited, and can be, for example, 10 nm or more and 15 μm or less. The D ₅₀ of the raw metal powder when generating a metal powder with a small particle size can be, for example, 10 nm or more and 200 nm or less, 60 nm or more and 200 nm or less, 70 nm or more and 200 nm or less, 80 nm or more and 200 nm or less, 80 nm or more and 180 nm or less, 80 nm or more and 150 nm or less, or 80 nm or more and 120 nm or less. The D ₅₀ of the raw metal powder when generating a metal powder with a medium particle size can be, for example, 200 nm or more and 1.0 μm or less. The D ₅₀ of the raw metal powder when generating a metal powder with a large particle size can be, for example, 1.0 μm or more and 15 μm or less, or 1.0 μm or more and 5.0 μm or less.

The CV value of the raw metal powder is preferably 0.40 or less, more preferably 0.39 or less, more preferably 0.38 or less, more preferably 0.37 or less, more preferably 0.36 or less, and particularly preferably 0.35 or less. The lower limit of the CV value of the raw metal powder is not particularly limited, and can be, for example, 0.01 or more. In this specification (the present invention), the "CV value" is used as an index of the width or narrowness of the particle size distribution, and the smaller the CV value, the narrower the particle size distribution.

There are no particular limitations on the type of gas that constitutes the carrier gas; for example, nitrogen gas can be used.

The concentration of the raw metal powder in the gas phase dispersed in the gas phase by the carrier gas is preferably 1.0 g/L or less, more preferably 0.5 g/L or less, even more preferably 0.1 g/L or less, and particularly preferably 0.05 g/L or less. The lower limit of the raw metal powder concentration in the gas phase is not particularly limited, but from the viewpoint of production efficiency, it is preferably 0.001 g/L or more. When the concentration of the raw metal powder in the gas phase is in the above range, the dispersibility in the gas phase is excellent, so that the raw metal powder particles are less likely to come into contact with each other, and as a result, it is easier to obtain metal powder with a narrow particle size distribution. In particular, by setting the heat treatment temperature to Tm°C or more and setting the concentration of the raw metal powder in the gas phase to the above range, it is particularly easy to obtain metal powder with excellent crystallinity and narrow particle size distribution, for the same reasons as those described in the paragraph explaining the suitable concentration of the metal powder precursor in the second step described later.

The raw metal powder dispersed in the gas phase by the carrier gas may be dispersed in the gas phase in a dry state or in a state suspended in a liquid. From the viewpoint of energy efficiency during the heat treatment in the second step described later, it is preferable to disperse the raw metal powder in a dry state in the gas phase. From the viewpoint of dispersibility of the raw metal powder in the gas phase, it is preferable to disperse the raw metal powder in a suspended state in a liquid in the gas phase. By improving dispersibility in the gas phase, the raw metal powder particles are less likely to come into contact with each other, and as a result, it is easier to obtain metal powder with a narrow particle size distribution. Note that the raw metal powder is suspended in a liquid when it is produced by reducing metal ions in the liquid phase, and the liquid can be removed by an appropriate method to make the raw metal powder dry. Even when the raw metal powder is dispersed in the gas phase in a suspended state, the liquid in which the raw metal powder is suspended can be replaced with another liquid before dispersion in the gas phase.

The present invention may be a batch type or a continuous type, but from the viewpoint of production volume per unit time and energy efficiency, a continuous type is preferable. In the case of a continuous type, for example, the raw metal powder dispersed in the gas phase can be heat-treated while being transported by a carrier gas. The direction of the above-mentioned transport is not particularly limited, and the heat treatment can be performed while being transported in various directions, such as the direction of gravity, the horizontal direction, or the direction opposite to the direction of gravity. From the viewpoint of maintaining a narrow particle size distribution, it is preferable to heat-treat the raw metal powder dispersed in the gas phase while being transported in the direction of gravity by a carrier gas.

(Second step)
The second step in the present invention is a step of producing a metal powder precursor dispersed in a gas phase by heat treating the raw metal powder dispersed in a gas phase at a temperature of (Tm-100) ° C. or higher (where Tm ° C. is the temperature at which the raw metal powder melts). This allows heat treatment at a much higher temperature than before, so that metal powder that maintains a narrow particle size distribution and also has excellent crystallinity can be produced. In addition, since heat treatment can be performed at a much higher temperature than before, the heat treatment time can be shortened, and the amount of metal powder produced per unit time can be dramatically improved. In addition, since heat treatment can be performed at a temperature of Tm ° C. or higher, which is much higher than before, spherical metal powder with a good aspect ratio can be produced.

The heat treatment temperature in the second step may be (Tm-100)°C or higher, where Tm°C is the temperature at which the raw metal powder melts and Tb°C is the temperature at which the raw metal powder vaporizes, but is preferably (Tm-50)°C or higher, more preferably Tm°C or higher, even more preferably (Tm+50)°C or higher, and particularly preferably (Tm+100)°C or higher. This makes it easier to obtain metal powder with excellent crystallinity. In particular, by setting the heat treatment temperature at Tm°C or higher, the raw metal powder can be melted, making it easier to obtain metal powder with excellent crystallinity. In other words, the second step is preferably a step of generating a metal powder precursor (metal droplets) in a liquid state dispersed in a gas phase by heat treating the raw metal powder dispersed in a gas phase at a temperature of Tm°C or higher. Note that Tm is the temperature at which the raw metal powder melts, but does not necessarily mean the temperature at which the raw metal powder melts completely, and may be any temperature at which 90% by mass or more of the raw metal powder melts. That is, even if a part of the raw metal powder is not melted, the metal powder produced by the present invention is heat-treated at a temperature sufficiently higher than that of the conventional example, and therefore a metal powder having excellent crystallinity can be obtained. The upper limit of the heat treatment temperature in the second step is preferably less than Tb°C, more preferably (Tb-500)°C or less, more preferably (Tb-800)°C or less, more preferably (Tb-850)°C or less, more preferably (Tb-900)°C or less, even more preferably (Tb-950)°C or less, and particularly preferably (Tb-1000)°C or less. In addition, the upper limit of the heat treatment temperature in the second step is more preferably (Tm + 500) ° C or less, more preferably (Tm + 450) ° C or less, more preferably (Tm + 400) ° C or less, more preferably (Tm + 350) ° C or less, more preferably (Tm + 300) ° C or less, more preferably (Tm + 250) ° C or less, even more preferably (Tm + 200) ° C or less, and particularly preferably (Tm + 150) ° C or less. By setting the heat treatment temperature in the second step within the above range, the raw metal powder is less likely to vaporize, making it easier to obtain a metal powder that maintains a narrow particle size distribution while also having excellent crystallinity. In this invention (this specification), the "temperature at which the raw metal powder vaporizes" refers to the boiling point of the bulk of the metal.

As a heat treatment method in the second step, heat treatment by radiant heat is preferable, but heat treatment can also be performed by directly contacting the raw metal powder with a heat source. For heat treatment by radiant heat, an electric furnace (electric heater) can be used, and radiant heat generated by a flame can also be used. From the viewpoint of uniform heating, heat treatment by radiant heat is preferable. As a method of heat treatment by directly contacting the raw metal powder with a heat source, for example, a method of heat treatment by directly contacting the raw metal powder with a flame can be used. From the viewpoint of energy efficiency, a method of heat treatment by directly contacting the raw metal powder with a heat source is preferable. Also, heat treatment can be performed by contacting the raw metal powder with a heated high-temperature gas. Also, heat treatment can be performed by appropriately combining each of the heat treatment methods shown above, for example, heat treatment can be performed by directly heating with a flame and then by radiant heat from an electric furnace.

The concentration of the raw metal powder in the gas phase in the second step is preferably 1.0 g/L or less, more preferably 0.5 g/L or less, even more preferably 0.1 g/L or less, and particularly preferably 0.05 g/L or less. The lower limit of the concentration of the raw metal powder in the gas phase is not particularly limited, but from the viewpoint of production efficiency, it is preferably 0.001 g/L or more. When the concentration of the raw metal powder in the gas phase is in the above range, the dispersibility in the gas phase is excellent, and the raw metal powder and the metal powder precursor are less likely to come into contact with each other, making it easier to obtain metal powder with a narrow particle size distribution. In particular, by setting the heat treatment temperature to Tm°C or more and setting the concentration of the raw metal powder in the gas phase to the above range, it is particularly easy to obtain metal powder with excellent crystallinity and narrow particle size distribution, for the same reasons as those described in the paragraph explaining the suitable concentration of the metal powder precursor in the second step described later.

The heat treatment time in the second step is not particularly limited, but is preferably 1 minute or less, more preferably 30 seconds or less, more preferably 15 seconds or less, and particularly preferably 10 seconds or less. This allows the production amount of metal powder per unit time to be increased.

The metal powder precursor is a precursor of the metal powder generated in the third step described below, and is generated in a state dispersed in the gas phase by heat-treating the raw metal powder dispersed in the gas phase at a temperature of (Tm-100) ° C or higher. The metal powder precursor may be in a solid state, a mixed state of solid and liquid, or a liquid state. From the viewpoint of the crystallinity and aspect ratio of the metal powder generated in the third step described below, a mixed state of solid and liquid metal powder precursor or a liquid state metal powder precursor (metal droplets) is preferable, and a liquid state metal powder precursor (metal droplets) is particularly preferable. By heat-treating the raw metal powder at a temperature of (Tm-100) ° C or higher and lower than Tm ° C, lattice defects are reduced and a solid state metal powder precursor is generated. By generating a solid state metal powder precursor in this step, a metal powder with excellent crystallinity can be manufactured in the third step described below. In addition, by heat treating the raw metal powder at a temperature of Tm ° C. or higher, the raw metal powder melts and a liquid metal powder precursor (metal droplets) is generated. By generating a liquid metal powder precursor (metal droplets) in this step, a metal powder with excellent crystallinity can be produced in the third step described below. Since the raw metal powder in the present invention is generated by reducing metal ions in a liquid phase, organic substances are used in most cases when generating the raw metal powder, and a certain amount of organic substances are often present inside the particles of the raw metal powder. Therefore, by melting the metal powder precursor in this step and moving the organic substances to the surface of the metal powder precursor, the organic substances that inhibit the improvement of crystallinity can be removed from the inside of the particles, making it easier to generate a metal powder with excellent crystallinity. That is, when the metal powder precursor is a metal powder precursor in a mixed state of solid and liquid or a metal powder precursor (metal droplets) in a liquid state, it becomes easier to generate a metal powder with excellent crystallinity, and when the metal powder precursor is a metal powder precursor (metal droplets) in a liquid state, it becomes easier to generate a metal powder with excellent crystallinity. In the present invention, the above-mentioned solid state, mixed solid and liquid state, and liquid state produced by heat treatment at a temperature of (Tm-100)°C or higher are referred to as "metal powder precursors."

The concentration of the metal powder precursor in the gas phase in the second step is preferably 1.0 g/L or less, more preferably 0.5 g/L or less, even more preferably 0.1 g/L or less, and particularly preferably 0.05 g/L or less. The lower limit of the concentration of the metal powder precursor in the gas phase is not particularly limited, but from the viewpoint of production efficiency, it is preferable that it is 0.001 g/L or more. Since the concentration of the metal powder precursor in the gas phase is in the above range, the dispersibility in the gas phase is excellent, and therefore, in the third step described below, it becomes easier to cool the metal powder precursor and metal powder while avoiding contact, and as a result, it becomes easier to obtain metal powder with a narrow particle size distribution. In addition, when the heat treatment temperature is set to Tm ° C or more to generate a metal powder precursor in a mixed state of solid and liquid or a metal powder precursor in a liquid state, the precursors are more likely to coalesce when they come into contact with each other than when the heat treatment temperature is set to (Tm-100) ° C or more and less than Tm ° C to generate a metal powder precursor in a solid state. That is, the particle size distribution of the metal powder produced in the third step described below tends to be broad. However, when the concentration of the metal powder precursor in the gas phase is within the above-mentioned range, the dispersibility in the gas phase is excellent and the metal powder precursors are less likely to come into contact with each other, making it easier to obtain metal powder with a narrow particle size distribution. That is, by setting the heat treatment temperature to Tm°C or higher and setting the concentration of the metal powder precursor in a mixed state of solid and liquid in the gas phase or the metal powder precursor in a liquid state within the above-mentioned range, it is particularly easy to obtain metal powder with excellent crystallinity and a narrow particle size distribution.

(Third step)
The third step in the present invention is to produce a metal powder by cooling the metal powder precursor dispersed in the gas phase. By cooling the metal powder precursor dispersed in the gas phase, the metal powder precursor and the metal powder are less likely to come into contact with each other, making it easier to obtain a metal powder with a narrow particle size distribution.

In the third step of the present invention, it is preferable to generate a metal powder dispersed in a gas phase by cooling the metal powder precursor dispersed in the gas phase, and then cool the metal powder dispersed in the gas phase to a temperature at which the metal powder does not sinter. The method for cooling the metal powder dispersed in the gas phase to a temperature at which the metal powder does not sinter is not particularly limited, but for example, it can be cooled to less than 250°C, less than 220°C, or less than 200°C. In addition, for example, by using an organic compound such as acetic acid or oleic acid as a surface treatment agent, it becomes easier to suppress sintering of the metal powder even at a temperature higher than the above-mentioned temperature. That is, it is possible to generate a metal powder dispersed in a gas phase by cooling the metal powder precursor dispersed in the gas phase, then attach an organic compound to the surface of the metal powder, and then cool the metal powder dispersed in the gas phase to a temperature at which the metal powder does not sinter. By the above-mentioned aspect of the third step, the metal powder precursor and the metal powder are less likely to come into contact at a temperature at which they coalesce or sinter, so that it becomes easier to obtain a metal powder with a narrow particle size distribution. In this specification (the present invention), the "temperature at which the metal powder does not sinter" refers to the temperature at which, when the metal powder is cooled in the third step of the present invention, the metal powder is then recovered, and 100 or more particles of the recovered metal powder are randomly selected and observed under a scanning electron microscope, the number of particles that are sintered (necked) is 5% or less by number.

The metal powder precursor to be cooled in the third step may be in a solid state, a mixed state of solid and liquid, or a liquid state, but from the viewpoint of the crystallinity and aspect ratio of the metal powder produced by this step, a mixed state of solid and liquid metal powder precursor or a liquid state metal powder precursor (metal droplets) is preferable, and a liquid state metal powder precursor (metal droplets) is particularly preferable.

When cooling the solid-state metal powder precursor, it is preferable to produce the metal powder by cooling the solid-state metal powder precursor dispersed in the gas phase to a temperature below (Tm-100)°C, and it is particularly preferable to produce the metal powder dispersed in the gas phase by cooling the solid-state metal powder precursor dispersed in the gas phase to a temperature below (Tm-100)°C, and then cool the metal powder dispersed in the gas phase to a temperature at which the metal powder does not sinter. This allows cooling while avoiding contact between the solid-state metal powder precursor and the metal powder, making it easier to obtain a metal powder that maintains a narrow particle size distribution while also having excellent crystallinity.

When cooling a metal powder precursor in a mixed state of solid and liquid or a metal powder precursor in a liquid state (metal droplets), it is preferable to produce a metal powder by cooling the liquid metal powder precursor (metal droplets) dispersed in a gas phase to a temperature below Tm°C, and it is particularly preferable to produce a metal powder dispersed in a gas phase by cooling the liquid metal powder precursor (metal droplets) dispersed in a gas phase to a temperature below Tm°C, and then cool the metal powder dispersed in the gas phase to a temperature at which the metal powder does not sinter. This allows cooling while avoiding contact between the liquid metal powder precursor (metal droplets) and the metal powder, making it easier to obtain a metal powder that maintains a narrow particle size distribution while also having excellent crystallinity.

The concentration of the metal powder precursor in the gas phase in the third step is preferably 1.0 g/L or less, more preferably 0.5 g/L or less, even more preferably 0.1 g/L or less, and particularly preferably 0.05 g/L or less. The lower limit of the concentration of the metal powder precursor in the gas phase is not particularly limited, but from the viewpoint of production efficiency, it is preferable that it is 0.001 g/L or more. When the concentration of the metal powder precursor in the gas phase is in the above range, the dispersibility in the gas phase is excellent, so that it is easy to cool the metal powder precursor and metal powder while avoiding contact with each other, and as a result, it is easy to obtain a metal powder with a narrow particle size distribution. In particular, by setting the heat treatment temperature to Tm°C or more and setting the concentration of the metal powder precursor in the gas phase to the above range, it is particularly easy to obtain a metal powder with excellent crystallinity and a narrow particle size distribution, for the same reasons as those described in the paragraph explaining the suitable concentration of the metal powder precursor described above.

The concentration of the metal powder in the gas phase produced by cooling in the third step is preferably 1.0 g/L or less, more preferably 0.5 g/L or less, even more preferably 0.1 g/L or less, and particularly preferably 0.05 g/L or less. There is no particular lower limit to the concentration of the metal powder in the gas phase, but from the viewpoint of production efficiency, it is preferably 0.001 g/L or more. When the concentration of the metal powder in the gas phase is within the above range, the dispersibility in the gas phase is excellent, making it easier to cool the metal powder precursor and the metal powder while avoiding contact with each other, and as a result, it becomes easier to obtain metal powder with a narrow particle size distribution.

The metal powder produced in the third step may be, for example, the metal powder described below.

The method for producing metal powder of the present invention may include the above-mentioned first, second, and third steps, but may also include a further step before the first step, a further step between the above steps, or a further step after the third step.

The method for producing metal powder of the present invention can have a step of recovering the generated metal powder after the third step. That is, it can have a fourth step of recovering the generated metal powder. In particular, it is preferable to cool the metal powder dispersed in the gas phase in the third step to a temperature at which the metal powder does not sinter, and then recover the metal powder. This makes it easier to obtain metal powder with a narrow particle size distribution, since the metal powder precursor and the metal powder are less likely to come into contact at temperatures at which they coalesce or sinter.

The ratio of the crystallite diameter ratio of the metal powder to the crystallite diameter ratio of the raw metal powder is preferably 4.0 or more, more preferably 4.5 or more, and particularly preferably 5.0 or more. The upper limit of the ratio of the crystallite diameter ratio of the metal powder to the crystallite diameter ratio of the raw metal powder is not particularly limited, but can be, for example, 1000 or less. If the raw metal powder is amorphous and therefore no crystal peak is detected by the method for measuring the crystallite diameter described below, the crystallite diameter ratio of the raw metal powder is set to 0, and if the crystallite diameter ratio of the generated metal powder is greater than 0, the ratio of the crystallite diameter ratio of the metal powder to the crystallite diameter ratio of the raw metal powder is set to 4.0 or more.

The ratio of the crystallite diameter of the metal powder to the crystallite diameter of the raw metal powder is preferably 4.0 or more, more preferably 4.5 or more, and particularly preferably 5.0 or more. The upper limit of the ratio of the crystallite diameter of the metal powder to the crystallite diameter of the raw metal powder is not particularly limited, but can be, for example, 1000 or less. If the raw metal powder is amorphous and therefore no crystal peak is detected by the method for measuring the crystallite diameter described below, the crystallite diameter of the raw metal powder is set to 0 nm, and if the crystallite diameter of the generated metal powder is greater than 0 nm, the ratio of the crystallite diameter of the metal powder to the crystallite diameter of the raw metal powder is set to 4.0 or more.

The ratio of the _D50 of the metal powder to the _D50 of the raw metal powder is preferably 1.0 to 2.0, more preferably 1.0 to 1.8, even more preferably 1.0 to 1.6, even more preferably 1.0 to 1.4, and particularly preferably 1.0 to 1.2.

The ratio of the CV value of the metal powder to the CV value of the raw metal powder is preferably 1.0 or more and 2.0 or less, more preferably 1.0 or more and 1.8 or less, more preferably 1.0 or more and 1.6 or less, more preferably 1.0 or more and 1.4 or less, and particularly preferably 1.0 or more and 1.2 or less.

<Metal powder>
According to the manufacturing method of the present invention, it is possible to manufacture a metal powder having a CV value of 0.50 or less, a crystallite size of 30.0 nm or more, and a crystallite size ratio of 1.0 × 10 ^-2 or more. In other words, it is possible to manufacture a metal powder having a narrow particle size distribution and excellent crystallinity.

The CV value of the metal powder of the present invention may be 0.50 or less, preferably 0.45 or less, more preferably 0.40 or less, more preferably 0.39 or less, more preferably 0.38 or less, more preferably 0.37 or less, more preferably 0.36 or less, and particularly preferably 0.35 or less. There is no particular limit to the lower limit of the CV value of the metal powder, and it can be, for example, 0.01 or more.

The crystallite size of the metal powder of the present invention may be 30.0 nm or more, preferably 35.0 nm or more, preferably 40.0 nm or more, preferably 45.0 nm or more, preferably 50.0 nm or more, preferably 60.0 nm or more, preferably 70.0 nm or more, and particularly preferably 80.0 nm or more. The upper limit of the crystallite size is not particularly limited, but can be, for example, D ₅₀ or less of the metal powder. When the crystallite size of the metal powder is in the above range, the crystallinity of the metal powder is excellent. In this specification (the present invention), the "crystallite size" is used as one of the indicators of crystallinity, and the larger the crystallite size, the better the crystallinity (higher the crystallinity).

The type of metal constituting the metal powder of the present invention is not particularly limited. That is, the metal powder of the present invention may be, for example, a metal powder selected from nickel, copper, silver, palladium, gold, platinum and iron, or an alloy powder of two or more of the above metals, or an alloy powder and/or composite powder to which an appropriate amount of the above-mentioned other metal elements or nonmetal elements is added. The higher the ratio of the metal components selected from nickel, copper, silver, palladium, gold, platinum and iron to the entire metal powder, the more preferable, and it is preferably 99% by mass or more, more preferably 99.9% by mass or more, and particularly preferably 99.99% by mass or more. The higher the ratio of the metal components selected from nickel, copper, silver, palladium, gold, platinum and iron to the entire metal components constituting the metal powder, the more preferable, and it is preferably 99% by mass or more, more preferably 99.9% by mass or more, and particularly preferably 99.99% by mass or more. The less impurities contained in the metal powder, the better. For example, the ratio of halogen elements to the entire metal powder is preferably 100 ppm by mass or less, more preferably 50 ppm by mass or less, even more preferably 10 ppm by mass or less, and it is particularly preferable that no halogen elements are contained. Also, for example, the ratio of alkaline earth metals to the entire metal powder is preferably 100 ppm by mass or less, more preferably 50 ppm by mass or less, even more preferably 10 ppm by mass or less, and it is particularly preferable that no alkaline earth metals are contained.

The D ₅₀ of the metal powder of the present invention is not particularly limited, and can be, for example, 10 nm or more and 15 μm or less. When a metal powder having a small particle size is produced, the D ₅₀ of the metal powder can be, for example, 10 nm or more and 200 nm or less, preferably 60 nm or more and 200 nm or less, more preferably 70 nm or more and 200 nm or less, more preferably 80 nm or more and 200 nm or less, more preferably 80 nm or more and 180 nm or less, even more preferably 80 nm or more and 150 nm or less, and particularly preferably 80 nm or more and 120 nm or less. When the D ₅₀ of the metal powder is in the above range, it can be suitably used particularly for the internal electrodes of multilayer ceramic capacitors. That is, when used for the internal electrodes of multilayer ceramic capacitors, in addition to excellent crystallinity and narrow particle size distribution, the D ₅₀ of the metal powder in the above range can form thin and highly continuous internal electrodes while suppressing short circuit defects, and it is easy to suppress delamination and cracks of the multilayer ceramic capacitor. When a raw metal powder having a medium particle size is produced, the D ₅₀ of the metal powder can be, for example, 200 nm or more and 1.0 μm or less. When the D ₅₀ of the metal powder is in the above range, it can be used particularly suitably for terminal electrodes of multilayer ceramic electronic components. That is, when used for terminal electrodes of multilayer ceramic electronic components, in addition to excellent crystallinity and narrow particle size distribution, the D ₅₀ of the metal powder in the above range makes it easy to form thin, dense, and highly continuous terminal electrodes. When a metal powder having a large particle size is produced, the D _{50 of the metal powder can be, for example, 1.0 μm or more and 15 μm or less, or can be 1.0 μm or more and 5.0 μm or less. When the D 50 of} the metal powder is in the above range, it can be used particularly suitably for internal electrodes of multilayer inductors. That is, when used for internal electrodes of a multilayer inductor, the metal powder has excellent crystallinity and a narrow particle size distribution, and since the _D50 of the metal powder is within the above range, it becomes easy to form internal electrodes with low resistivity and it becomes easy to form a multilayer inductor in which short circuit defects are suppressed.

The crystallite size ratio of the metal powder of the present invention may be 1.0 x 10 ^-2 or more, preferably 2.0 x 10 ^-2 or more, more preferably 3.0 x 10 ^-2 or more, more preferably 5.0 x 10 ^-2 or more, more preferably 0.1 or more, more preferably 0.2 or more, more preferably 0.3 or more, more preferably 0.4 or more, more preferably 0.5 or more, and particularly preferably 0.6 or more. The upper limit of the crystallite size ratio is not particularly limited, but can be, for example, 1.0 or less. When D ₅₀ is 10 nm or more and 200 nm or less, the crystallite size ratio is preferably 0.40 or more, more preferably 0.50 or more, and particularly preferably 0.60 or more. When D ₅₀ is 10 nm or more and 200 nm or less, the upper limit of the crystallite size ratio is not particularly limited, but can be, for example, 1.0 or less. When D ₅₀ is 200 nm or more and 1.0 μm or less, the crystallite diameter ratio is more preferably 5.0 × 10 ^-2 or more, and particularly preferably 1.0 × 10 ^-1 or more. When D ₅₀ is 200 nm or more and 1.0 μm or less, the upper limit of the crystallite diameter ratio is not particularly limited, but can be, for example, 1.0 or less. When _{D 50} is 1.0 μm or more and 15 μm or less, the crystallite diameter ratio is more preferably 2.0 × 10 ^-2 or more, more preferably 3.0 × 10 ^-2 or more, even more preferably 4.0 × 10 ^-2 or more, and particularly preferably 5.0 × 10 ^-2 or more. When _{D 50} is 1.0 μm or more and 15 μm or less, the upper limit of the crystallite diameter ratio is not particularly limited, but can be, for example, 1.0 or less. When the crystallite diameter ratio of the metal powder is in the above range, the crystallinity of the metal powder is excellent. In this specification (the present invention), the "crystallite size ratio" is used as one index of crystallinity, and a larger crystallite size ratio indicates better crystallinity (higher crystallinity).

The aspect ratio of the metal powder of the present invention is preferably 0.70 or more and 1.0 or less, more preferably 0.80 or more and 1.0 or less, even more preferably 0.90 or more and 1.0 or less, even more preferably 0.95 or more and 1.0 or less, and particularly preferably 1.0. When the aspect ratio of the metal powder is in the above range, it becomes easier to narrow the particle size distribution of the metal powder.

When 100 or more particles are randomly selected and observed under a scanning electron microscope, it is preferable that the number of particles that are sintered (necked) among the particles observed in the metal powder of the present invention is 5% or less by number. This makes it easier to obtain a metal powder with a narrow particle size distribution.

It is particularly preferred that the metal powder of the present invention has a _D50 of 10 nm or more and 200 nm or less, a CV value of 0.40 or less, a crystallite diameter of 30.0 nm or more, and a crystallite diameter ratio of 0.40 or more. The _D50 , CV value, crystallite diameter, and crystallite diameter ratio of the metal powder are within the above ranges, so that the metal powder can be particularly suitably used for internal electrodes of multilayer ceramic capacitors. That is, the metal powder has a small particle size, excellent crystallinity, and a narrow particle size distribution, so that when used for internal electrodes of multilayer ceramic capacitors, a thin and highly continuous internal electrode can be formed while suppressing short circuit defects, and delamination and cracks of the multilayer ceramic capacitor can be easily suppressed.

<Measurement method>
( _D50 )
_D50 as defined in the present invention can be measured, for example, by the following method: A powder is observed using a scanning electron microscope, 100 particles constituting the powder are randomly selected from the observation, their particle diameters are measured, and the number-based cumulative 50% particle diameter _D50 can be calculated based on the particle diameters.

(CV value)
The CV value defined in the present invention can be measured, for example, by the following method: A powder is observed using a scanning electron microscope, 100 particles constituting the powder are randomly selected from the observation, their particle sizes are measured, a standard deviation is calculated based on the particle sizes, and the CV value is calculated as the ratio of the standard deviation to _D50 calculated by the above-mentioned method.

(Crystallite size)
The crystallite size defined in the present invention can be measured, for example, by the following method. That is, using an XRD measurement device, CuKα radiation (wavelength λ: 1.5418 Å), tube voltage 40 kV, tube current 30 mA, step angle 0.01°, scanning speed 10.0°/min, XRD measurement of metal powder is performed for diffraction angle 2θ: 20.0 to 100.0°, the main peak with the maximum peak intensity is detected, the half-width of the peak is measured, and the crystallite size can be calculated using Scherrer's formula. In addition, the main peak can be, for example, a peak corresponding to the (111) plane, around 44° in the case of nickel powder, around 38° in the case of silver powder, and around 43° in the case of copper powder.

(Aspect Ratio)
By observing 100 or more particles at random, the ratio of the minor axis diameter to the major axis diameter of a rectangle circumscribing the particle so as to have the smallest area (minor axis diameter/major axis diameter) is measured for each particle, and the aspect ratio can be calculated as the average value of these ratios.

The present invention will be explained in more detail below using examples, but the present invention is not limited to the following examples. Note that the melting point and boiling point of nickel used in the following examples are about 1455°C and about 2913°C, the melting point and boiling point of silver are about 962°C and about 2162°C, and the melting point and boiling point of copper are about 1085°C and about 2562°C, respectively.

<Production of Metal Powder>
(Examples 1 to 4)
First, hydrazine was added to an aqueous solution of nickel nitrate to reduce the nickel ions in the solution to metallic nickel, thereby obtaining a suspension of nickel powder, and the nickel powder was then separated and dried from the suspension to prepare a raw nickel powder. The properties of the raw nickel powder prepared are shown in Table 1.
Next, using a vertical tubular container equipped with a nozzle at the top for spraying powder, the raw material nickel powder was dispersed in the gas phase from the nozzle with a cross-sectional area of an opening of ² cm2 using a carrier gas (nitrogen gas) with a flow rate of 2200 L/min at a concentration of 0.05 g per unit volume (1 L) of the gas phase.
Then, while the raw material nickel powder was dispersed in the gas phase at the concentration, it was passed through the above-mentioned vertical tubular container and heat-treated at 1600°C for 3 seconds to generate nickel droplets of molten metallic nickel. An electric furnace was installed on the outside of the vertical tubular container used for the heat treatment, and the inside of the tubular container was set to the above-mentioned temperature.
Next, the nickel droplets were cooled while dispersed in the gas phase at the above concentration to produce nickel powder, and the nickel powder thus produced was further cooled to 180° C. while dispersed in the gas phase, and the nickel powder was recovered.
The properties of the raw nickel powder and the nickel powder were evaluated by the evaluation methods described below.
The evaluation results are shown in Table 1.

(Comparative Example 1)
The raw nickel powder prepared in Example 1 was subjected to a heat treatment at the temperature and time shown in Table 1, and then cooled to 25° C. to produce nickel powder, which was then recovered. The properties of the nickel powder were then evaluated using the same evaluation method as in Example 1. The evaluation results are shown in Table 1.

(Comparative Examples 2 and 3)
The raw nickel powder prepared in Example 1 was heat-treated at the temperature and time shown in Table 1, and then cooled to 25°C, whereupon the particles constituting the raw nickel powder were sintered together to form a metal lump.

(Comparative Examples 4 and 5)
The raw material nickel powder prepared in Example 1 was mixed with magnesium carbonate and stirred, then heat-treated at the temperature and time shown in Table 1, and then cooled to 25° C. to produce nickel powder, which was then recovered. The properties of the nickel powder were then evaluated using the same evaluation method as in Example 1. The evaluation results are shown in Table 1.

(Comparative Example 6)
The raw nickel powder prepared in Example 1 was mixed with magnesium carbonate and stirred, then heat-treated at the temperature and time shown in Table 1, and then cooled to 25°C. As a result, the particles constituting the raw nickel powder were sintered together to form a metal lump.

(Comparative Example 7)
Nickel powder was produced, cooled, and recovered in the same manner as in Example 1, except that the heat treatment temperature was 1100°C, the heat treatment time was 4 seconds, the heat treatment did not go into a molten state during or after the heat treatment, and the nickel powder was cooled to 180°C in a state where it was dispersed in a gas phase after the heat treatment. The raw nickel powder used was that prepared in Example 1. The properties of the nickel powder were evaluated using the same evaluation method as in Example 1. The evaluation results are shown in Table 1.

Example 5
The nickel powder was produced, cooled, and recovered in the same manner as in Example 1, except that the heat treatment temperature was 1400°C, the heat treatment did not go through a molten state during or after the heat treatment, the heat treatment was cooled to 1350°C while maintaining the above-mentioned dispersion concentration, and the nickel powder was further cooled to 180°C in a state where the nickel powder was dispersed in the gas phase. The raw nickel powder used was that prepared in Example 1. The properties of the nickel powder were evaluated using the same evaluation method as in Example 1. The evaluation results are shown in Table 1.

(Example 6)
Nickel powder was produced, cooled, and collected in the same manner as in Example 1, except that the produced nickel droplets were cooled to 250° C. in a state where they were dispersed in the gas phase and collected. The raw nickel powder used was that prepared in Example 1. The properties of the nickel powder were evaluated using the same evaluation method as in Example 1. The evaluation results are shown in Table 1.

(Example 7)
Nickel powder was produced, cooled and collected in the same manner as in Example 1, except that the produced nickel droplets were cooled to 365° C. while dispersed in the gas phase, and then a diluted liquid of oleic acid was sprayed onto the nickel powder to adhere to it, and the nickel powder was further cooled to 250° C. and collected. The raw nickel powder used was that prepared in Example 1. The properties of the nickel powder were evaluated using the same evaluation method as in Example 1. The evaluation results are shown in Table 1.

(Examples 8 and 9)
Nickel powder was produced, cooled, and recovered in the same manner as in Example 1, except that the raw nickel powder produced in Example 1 was dispersed in a gas phase at the concentration shown in Table 1. The raw nickel powder used was that prepared in Example 1. The properties of the nickel powder were evaluated using the same evaluation method as in Example 1. The evaluation results are shown in Table 1.

(Examples 10 to 15)
First, hydrazine was added to an aqueous silver nitrate solution to reduce the silver ions in the solution to metallic silver, thereby obtaining a suspension of silver powder, and the silver powder was then separated and dried from the suspension to prepare a raw silver powder. The properties of the raw silver powder prepared are shown in Table 2.
Next, using a vertical tubular container equipped with a nozzle at the top for spraying powder, the raw silver powder was dispersed in the gas phase from the nozzle with a cross-sectional area of ² cm2 at an opening using a carrier gas (nitrogen gas) with a flow rate of 2,200 L/min at a concentration of 0.05 g per unit volume (1 L) of the gas phase.
Then, while the raw silver powder was dispersed in the gas phase at that concentration, it was passed through the above-mentioned vertical tubular container and subjected to heat treatment for 4 seconds at the temperature shown in Table 2 to generate silver droplets of molten metallic silver. An electric furnace was installed on the outside of the vertical tubular container used for the heat treatment, and was set so that the inside of the tubular container was at the above-mentioned temperature.
Next, the silver droplets were dispersed in the gas phase at the above concentration, and the mixture was cooled to 120° C. to produce silver powder, which was then recovered.
The properties of the raw silver powder and the silver powder were evaluated using the evaluation methods described below. The evaluation results are shown in Table 2.

(Example 16)
Without separating and drying the silver powder from the suspension of silver powder obtained in Example 15, the liquid in which the silver powder was suspended was replaced with water, and the content ratio of silver powder in the suspension was adjusted to 20% by mass to obtain raw silver powder suspended in water. The raw silver powder was then dispersed in the gas phase while the raw silver powder was suspended in water. The silver powder was produced, cooled, and recovered in the same manner as in Example 15. The properties of the silver powder were evaluated by the same evaluation method as in Example 15. The evaluation results are shown in Table 2.

(Comparative Example 8)
The raw silver powder produced in Example 15 was heat-treated at the temperature and time shown in Table 2, and then cooled to 25°C, whereupon the particles constituting the raw silver powder sintered together to form a metal lump.

(Comparative Example 9)
The raw silver powder produced in Example 15 was mixed with magnesium carbonate and stirred, then heat-treated at the temperature and time shown in Table 2, and then cooled to 25°C. As a result, the particles constituting the raw silver powder sintered together to form a metal lump.

(Examples 17 and 18)
First, hydrazine was added to an aqueous solution of copper nitrate to reduce the copper ions in the solution to metallic copper, thereby obtaining a suspension of copper powder, and the raw copper powder was separated from the suspension and dried to prepare a raw copper powder. The properties of the prepared raw copper powder are shown in Table 2.
Next, using a vertical tubular container equipped with a nozzle at the top for spraying powder, the raw copper powder was dispersed in the gas phase from the nozzle with a cross-sectional area of ² cm2 at an opening using a carrier gas (nitrogen gas) at a flow rate of 2200 L/min at a concentration of 0.05 g per unit volume (1 L) of the gas phase.
The raw copper powder was dispersed in the gas phase at the concentration, and was passed through the vertical tubular vessel described above while being heat-treated at 1300°C for 4 seconds to generate copper droplets of molten metallic copper. An electric furnace was installed on the outside of the vertical tubular vessel used for the heat treatment, and was set so that the temperature inside the tubular vessel was set to the above-mentioned temperature.
Next, the copper droplets were dispersed in the gas phase at the above concentration, and the mixture was cooled to 60° C. to produce copper powder, which was then recovered.
The properties of the raw copper powder and the copper powder were evaluated by the methods described below. The evaluation results are shown in Table 2.

(Comparative Example 10)
The raw copper powder produced in Example 18 was heat-treated at the temperature and time shown in Table 2, and then cooled to 25°C, whereupon the particles constituting the raw copper powder sintered together to form metal lumps.

(Comparative Example 11)
The raw copper powder produced in Example 18 was mixed with magnesium carbonate and stirred, then heat-treated at the temperature and time shown in Table 2, and then cooled to 25°C. As a result, the particles constituting the raw copper powder sintered together to form metal lumps.

In the above-mentioned examples and comparative examples, except for "Comparative Examples 2, 3, 6, 8, 9, 10, 11 and Examples 6 and 9," 100 or more particles were randomly selected from the recovered metal powder and observed using a scanning electron microscope, and it was confirmed that the number of particles that were sintered (necked) was 5% or less by number. In Examples 6 and 9, the number of particles that were sintered (necked) was more than 5% by number.

(Reference example)
As metal powders produced by a known dry method, nickel powder produced by a known method of heating raw metal nickel as a raw metal with plasma to melt and vaporize it, and cooling the generated metal vapor to obtain a metal powder, nickel powder produced by a known method of heating raw nickel powder with a flame to vaporize it, and cooling the generated metal vapor to obtain a metal powder, nickel powder produced by a known method of spraying nickel acetate tetrahydrate powder as a thermally decomposable compound powder into a gas phase, and then heat-treating it at a temperature equal to or higher than the thermal decomposition temperature to obtain a metal powder, and nickel powder produced by spraying molten nickel and cooling it were prepared, and the properties of the nickel powders were evaluated by the same evaluation method as in Example 1. In each of the nickel powders, the crystallite size was 30.0 nm or more, the crystallite size ratio was 1.0 × 10 ^-2 or more, but the CV value was more than 0.50.

<Evaluation method>
( _D50 )
The metal powder was observed using a scanning electron microscope (SU-1510, manufactured by Hitachi High-Technologies Corporation), and 100 particles constituting the powder were randomly selected from the observation, their particle diameters were measured, and the cumulative 50% particle diameter _D50 based on the number was calculated based on the particle diameters. The particle diameter was measured as the diameter of a perfect circle having the same area as the projected area of the particle.

(CV value)
The metal powder was observed using a scanning electron microscope (SU-1510, manufactured by Hitachi High-Technologies Corporation), and 100 particles constituting the powder were randomly selected from the observation, their particle diameters were measured, and the standard deviation was calculated based on the particle diameters. The CV value was then calculated as the ratio of the standard deviation to _D50 calculated by the above-mentioned method.

(Crystallite size and crystallite size ratio)
Using an XRD measurement device (Rigaku Corporation, SmartLab), the metal powder was subjected to XRD measurement at a diffraction angle 2θ of 20.0 to 100.0° using CuKα radiation (wavelength λ: 1.5418 Å) under the conditions of a tube voltage of 40 kV, a tube current of 30 mA, a step angle of 0.01°, and a scanning speed of 10.0°/min. Peaks corresponding to the (111) plane (nickel powder: near 44°, silver powder: near 38°, copper powder: near 43°) were detected and the half-width was measured, and the crystallite size was calculated using the following Scherrer formula.
Scherrer's formula: d = Kλ/β cos θ
(In the above formula, K is the Scherrer constant, λ is the X-ray wavelength, β is the half-width of the diffraction peak, and θ is the diffraction angle).
Next, the crystallite size ratio was calculated as the ratio of the above crystallite size to _D50 measured by the above method.

(Aspect Ratio)
By observing with a scanning electron microscope, 100 or more particles were randomly selected, and the ratio of the minor axis to the major axis of the rectangle circumscribing the particle so as to minimize the area (minor axis diameter/major axis diameter) was measured for each particle, and the aspect ratio was calculated as the average of the ratios. Examples with an aspect ratio of less than 0.70 were evaluated as "x", examples with an aspect ratio of 0.70 or more and less than 0.95 were evaluated as "△", and examples with an aspect ratio of 0.95 or more and 1.0 or less were evaluated as "◯".

As is clear from the results of Tables 1 and 2, in the examples, metal powders having excellent crystallinity and a narrow particle size distribution could be produced. On the other hand, in the comparative examples, metal powders satisfying both "excellent crystallinity" and "narrow particle size distribution" could not be produced. In addition, as is clear from the results of Example 1, Comparative Example 7, and Example 5, metal powders having excellent crystallinity could be produced by heat treating the raw metal powder at a temperature of (Tm-100) ° C. or higher, and metal powders having particularly excellent crystallinity could be produced by heat treating the raw metal powder at a temperature of Tm ° C. or higher. In addition, as is clear from the results of Examples 1, 6, and 7, metal powders having a particularly narrow particle size distribution could be produced by cooling and recovering the metal powder dispersed in the gas phase to a temperature at which the metal powder does not sinter. In addition, as is clear from the results of Examples 1, 8, and 9, the lower the concentration of the raw metal powder dispersed in the gas phase, the concentration of the raw metal powder during heat treatment, and the concentration of the metal powder precursor during cooling, the narrower the particle size distribution of the metal powder could be produced. Moreover, as is clear from the results of Examples 10, 11 and 12, the particle size distribution tended to become wider as the heat treatment temperature of the raw metal powder exceeded Tm° C. and became higher. Moreover, as is clear from the results of Examples 15 and 16, compared with Example 15 in which a raw metal powder in a dry state was dispersed in a gas phase, Example 16 in which a raw metal powder in a suspended state in a liquid was dispersed in a gas phase was able to produce a metal powder with a narrower particle size distribution.

Claims (13)

A first step of dispersing a raw metal powder produced by reducing metal ions in a liquid phase into a gas phase by a carrier gas;
a second step of heat-treating the raw metal powder dispersed in the gas phase at a temperature of (Tm-100)°C or higher (where Tm°C is the temperature at which the raw metal powder melts) to produce a metal powder precursor dispersed in the gas phase;
a third step of cooling the metal powder precursor dispersed in the gas phase to produce a metal powder;
A method for producing a metal powder having the above structure. The method for producing metal powder according to claim 1, wherein in the first step, the raw metal powder is dispersed in the gas phase by the carrier gas at a concentration of 1.0 g/L or less, in the second step, the raw metal powder dispersed in the gas phase at the concentration is heat-treated at a temperature of (Tm-100)°C or more to produce a metal powder precursor dispersed in the gas phase, and in the third step, the metal powder precursor dispersed in the gas phase at the concentration is cooled to produce the metal powder. The method for producing metal powder according to claim 1, wherein in the second step, the raw metal powder dispersed in the gas phase is heat-treated at a temperature equal to or higher than Tm°C to produce a liquid metal powder precursor dispersed in the gas phase. The method for producing metal powder according to claim 1, wherein in the second step, the raw metal powder dispersed in the gas phase is heat-treated at a temperature equal to or higher than (Tm-100)°C and lower than Tb°C (where Tm°C is the temperature at which the raw metal powder melts, and Tb°C is the temperature at which the raw metal powder vaporizes). The method for producing metal powder according to claim 1, wherein in the first step, the raw metal powder is dispersed in the gas phase by the carrier gas while the raw metal powder is suspended in the liquid. The method for producing metal powder according to claim 1, wherein in the third step, the metal powder precursor dispersed in the gas phase is cooled to produce a metal powder, and then the metal powder dispersed in the gas phase is cooled to a temperature at which the metal powder does not sinter. The method for producing a metal powder according to claim 1, wherein the raw metal powder has a CV value, as defined below, of 0.40 or less.
CV value: When the cumulative 50% particle diameter based on the number calculated based on the particle diameters of 100 or more particles randomly selected and measured by scanning electron microscope observation is defined as _D50 , the ratio of the standard deviation calculated based on the particle diameters of 100 or more particles randomly selected and measured by scanning electron microscope observation to _D50 2. The method for producing a metal powder according to claim 1, wherein the ratio of the crystallite size ratio of the metal powder, defined below, to the crystallite size ratio of the raw metal powder, defined below, is 4.0 or more.
Crystallite size ratio: The cumulative 50% particle size based on the number calculated based on the particle sizes of 100 or more randomly selected particles measured by scanning electron microscope observation is defined as _D50 , and the crystallite size is defined as the value calculated by Scherrer's formula using the measured values by X-ray diffraction. The ratio of the crystallite size to _D50 2. The method for producing a metal powder according to claim 1, wherein the ratio of the crystallite size, defined below, of the metal powder to the crystallite size, defined below, of the raw metal powder is 4.0 or more.
Crystallite size: Value calculated by Scherrer's formula using measured values by X-ray diffraction The method for producing a metal powder according to claim 1, wherein the ratio of the _D50 , defined below, of the metal powder to the _D50, defined below, of the raw metal powder is 1.0 to 2.0, and the ratio of the CV value, defined below, of the metal powder to the CV value, defined below, of the raw metal powder is 1.0 to 2.0.
_D50 : cumulative 50% particle diameter based on the number calculated based on particle diameters of 100 or more particles randomly selected by scanning electron microscope observation. CV value: When the cumulative 50% particle diameter based on the number calculated based on particle diameters of 100 or more particles randomly selected by scanning electron microscope observation is defined as _D50 , the ratio of the standard deviation calculated based on particle diameters of 100 or more particles randomly selected by scanning electron microscope observation to _D50 The method for producing a metal powder according to claim 1, wherein the raw metal powder has a _D50 defined below of 10 nm or more and 15 μm or less, and the metal powder has a _D50 defined below of 10 nm or more and 15 μm or less.
_D50 : cumulative 50% particle diameter based on the number calculated based on particle diameters measured by randomly selecting 100 or more particles by scanning electron microscope observation A metal powder having a CV value, as defined below, of 0.50 or less, a crystallite diameter, as defined below, of 30.0 nm or more, and a crystallite diameter ratio, as defined below, of 1.0×10 ⁻² or more.
CV value: When the cumulative 50% particle diameter based on the number calculated based on the particle diameters of 100 or more particles randomly selected by scanning electron microscope observation is taken as _D50 , the ratio of the standard deviation calculated based on the particle diameters of 100 or more particles randomly selected by scanning electron microscope observation to _D50. Crystallite diameter: A value calculated by Scherrer's formula using the measured value by X-ray diffraction. Crystallite diameter ratio: A ratio of the crystallite diameter to _D50 when the value calculated by Scherrer's formula using the measured value by X-ray diffraction is taken as the crystallite diameter. 13. The metal powder according to claim 12, wherein the CV value is 0.40 or less, the _D50 defined below is 10 nm or more and 200 nm or less, the crystallite size ratio is 0.40 or more, and the aspect ratio defined below is 0.95 or more and 1.0 or less.
_D50 : Cumulative 50% particle size based on the number calculated based on the particle diameters measured by randomly selecting 100 or more particles by scanning electron microscope observation. Aspect ratio: A value calculated by measuring the ratio of the minor axis diameter to the major axis diameter of a rectangle circumscribing the particle so as to have the smallest area (minor axis diameter/major axis diameter) for each particle by randomly selecting 100 or more particles by scanning electron microscope observation, and averaging the measured values.