Method for manufacturing metal powder, and metal powder

Abstract

A method for manufacturing a metal powder, having: a first step for dispersing, at a concentration of 1.0 g / L or below in a gas phase by means of a carrier gas, a raw material powder that is produced by chemically reacting metal ions in a liquid phase and in which the CV value, defined separately, is 0.40 or below; a second step for heat-treating the raw material powder in a state of being dispersed in the gas phase at a temperature that is at least (Tm - 100)°C and lower than Tb°C (where Tm°C is the melting point of a metal comprising metal atoms contained in the raw material powder, and Tb°C is the boiling point of the metal comprising metal atoms contained in the raw material powder), and thereby producing a metal powder precursor in a state of being dispersed in the gas phase; and a third step for cooling the metal powder precursor in a state of being dispersed in the gas phase, and thereby producing a metal powder.

Description

Method for producing metal powder and metal powder

[0001] This invention relates to a method for producing metal powder and to metal powder itself.

[0002] In recent years, there has been a growing demand for metal powders with excellent crystallinity and a narrow particle size distribution for use in electrode formation of electronic components. Methods for producing metal powders with excellent crystallinity include spray pyrolysis, atomization, and plasma vaporization, which are processes that vaporize the raw metal to produce the metal powder. These methods involve generating the metal powder under high-temperature conditions in the gas phase. While these methods yield metal powders with very high crystallinity, the narrowness of the particle size distribution depends on the uniformity / homogeny of the raw materials and the temperature distribution in the gas phase. Therefore, obtaining metal powders with an even narrower particle size distribution requires even more precise control, which can lead to increased costs if the manufacturing equipment is made larger and more complex to accommodate this.

[0003] Furthermore, wet reduction (a method of producing metal powder by reducing metal ions in a liquid phase) is known as a simple method for producing metal powder with a narrow particle size distribution. However, it is widely known that the crystallinity of metal powder obtained by wet reduction is completely insufficient to meet the levels required in recent years. Therefore, various studies have been conducted to improve the crystallinity of metal powder produced by wet reduction and other methods, as described below.

[0004] Patent Document 1 discloses a method for producing nickel powder, which involves mixing a nickel raw material consisting of nickel fine powder or nickel compound fine powder with a spacer consisting of fine powder of at least one salt selected from alkali metal salts or alkaline earth metal salts, grinding the mixture, heat-treating the pulverized mixture to thermally decompose and / or grow the nickel fine powder in the presence of the spacer, then acid-treating the heat-treated mixture to dissolve the spacer, washing off the dissolved material with water, and drying the residue to obtain nickel powder with grown grains.

[0005] Patent Document 2 discloses a method for producing spherical nickel powder, comprising: a first step of mixing alkaline earth metal compound powder and nickel compound powder; a second step of heating the mixture obtained in the first step in a hydrogen atmosphere while preventing the formation of complex oxides in order to reduce the nickel in the mixture obtained in the first step to metallic nickel; a third step of heating the heated material obtained in the second step in a non-oxidizing atmosphere while preventing the coarsening of the particles in order to improve the sphericity of the metallic nickel particles in the heated material obtained in the second step; and a fourth step of dissolving and removing the alkaline earth metal in the heated material obtained in the third step with acid.

[0006] Patent Document 3 discloses a method for producing metal powder with a crystallite size of 800 angstroms or more, characterized by mixing metal powder with at least one of alkali metal halides, alkaline earth metal halides, and rare earth halides, heating the mixture to above the melting point of the halide, cooling it, and then removing the halide from the resulting reaction product by wet processing to recover the metal powder.

[0007] Patent Document 4 describes a method for processing metal powder to modify its density, characterized by forming a coating of alkaline earth metal oxide, hydroxide, or carbonate on the particle surface, then heat-treating the metal powder below its melting point, leaching the resulting heat-treated product with acid, and then separating the solid and liquid to collect the metal powder.

[0008] Patent Document 5 discloses a method for producing spherical nickel powder, comprising 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 the first step in a hydrogen atmosphere, and / or further heating it in a non-oxidizing atmosphere after reduction.

[0009] Patent Document 6 describes a spherical nickel or spherical nickel compound powder containing SiO ２ nH ２ O(n≧0), Al ２ O ３ nH ２A method for producing spherical nickel powder is disclosed, comprising: a first step of adding and mixing at least one selected from O (n≧0) in an amount of 0.01 wt% to 30 wt% on an anhydrous basis; 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 it in a non-oxidizing atmosphere after reduction.

[0010] Patent Document 7 describes a dispersion solution of metal powder in which B is added per 100 parts by weight of metal powder. ２ O ３ A method for producing conductive powder is disclosed, characterized by comprising the steps of: (A) adding 40 parts by weight or more and 100 parts by weight or less of boron oxide or boric acid, mixing, and then drying the dispersion solution to obtain a mixture consisting of 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 with water or acid to obtain metal powder.

[0011] Patent Document 8 discloses a method for producing nickel powder, characterized by adding a sintering inhibitor and pure water to nickel powder obtained by a wet reduction method and mixing them, drying this mixed slurry by spray heat treatment, sintering the nickel powder by heat treatment in a reducing or inert atmosphere at a temperature below the melting point of the sintering inhibitor, and then separating and removing the sintering inhibitor from the sintered nickel powder.

[0012] Patent Document 9 discloses a method for producing highly dispersible platinum powder with a crystallite size in the range of 30 to 100 nm, comprising: a mixing step of mixing platinum powder with a mixture consisting of at least one powder of an oxide of any metal element from 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 predetermined temperature; a dissolution step of dissolving the mixture by treating the heat-treated mixed powder with an acid or alkali; and a removal step of removing the mixture by subjecting the mixed powder from which the mixture has been dissolved to a washing treatment.

[0013] Patent Document 10 discloses a method for producing nickel powder, characterized by mixing nickel powder with fine powder of an alkaline earth metal compound selected from the group consisting of oxides, hydroxides, carbonates, and bicarbonates of alkaline earth metals, or by coating the surface of each particle of nickel powder with the alkaline earth metal compound, and then performing heat treatment in an inert gas or slightly reducing gas atmosphere at a temperature below the melting point of the alkaline earth metal compound.

[0014] Japanese Patent Publication No. 10-102109, Japanese Patent Publication No. 11-140513, Japanese Patent Publication No. 2000-096110, Japanese Patent Publication No. 2001-040401, Japanese Patent Publication No. 2001-098337, Japanese Patent Publication No. 2001-107103, Japanese Patent Publication No. 2004-176120, Japanese Patent Publication No. 2004-339601, Japanese Patent Publication No. 2006-299385, Japanese Patent Publication No. 2002-146401

[0015] In the methods described in Patent Documents 1 to 10, various substances are added to and mixed with the metal powder before heat treatment to prevent sintering of the metal powders during heat treatment. As a result, these substances remain on the surface of the metal powder even after heat treatment. Therefore, in order to obtain a metal powder suitable for use as a conductive material, it is necessary to remove the aforementioned substances after heat treatment. However, since acids or alkalis are used in this removal operation, it is unavoidable that the surface of the metal powder will be affected, such as by dissolution of the metal powder surface or the formation of metal compounds on the surface of the metal powder. Furthermore, even when the aforementioned removal operation is performed, it is difficult to completely remove the added and mixed substances, and they remain on the surface of the metal powder. In other words, the methods described in Patent Documents 1 to 10 have various problems as methods for producing metal powder that can be suitably used as a conductive material. Moreover, even if the methods described in Patent Documents 1 to 10 are implemented, the level of excellent crystallinity required in recent years cannot be obtained, and there is a need for even better methods to improve the crystallinity of metal powder.

[0016] Therefore, the present invention aims to provide a method for producing metal powder that has excellent crystallinity and a narrow particle size distribution.

[0017] The present invention for solving the above problems is as described below. (1) A first step of dispersing a raw material powder, which is generated by chemically reacting metal ions in a liquid phase and has a CV value defined below of 0.40 or less, into a gas phase at a concentration of 1.0 g / L or less by a carrier gas; a step of heat-treating the raw material powder dispersed in the gas phase at a temperature of (Tm - 100) °C or higher and lower than Tb °C (where Tm °C is the melting point of the metal composed of metal atoms contained in the raw material powder, and Tb °C is the boiling point of the metal composed of metal atoms contained in the raw material powder.) to generate a metal powder precursor in a state of being dispersed in the gas phase; a third step of generating metal powder by cooling the metal powder precursor in a state of being dispersed in the gas phase; A method for producing metal powder having. CV value: When the cumulative 50% particle diameter based on the number standard calculated based on the particle diameters of 100 randomly selected particles measured by scanning electron microscope observation is defined as D50, the ratio of the standard deviation calculated based on the particle diameters of 100 randomly selected particles measured by scanning electron microscope observation to D50 Particle diameter: The diameter of a perfect circle having the same area as the projected area of the particles observed by scanning electron microscope observation (2) Before the first step, there is a raw material powder preparation step of generating the raw material powder having a CV value of 0.40 or less by chemically reacting metal ions in a liquid phase. The method for producing metal powder according to (1) above. (3) The solubility of the raw material powder in 100 g of the solvent used when forming the liquid phase at 20 °C is 1 × 10 －１ [g / 100 g of the solvent] or less. The method for producing metal powder according to (1) or (2) above. (4) The solubility of the raw material powder in 100 g of H ２ O at 20 °C is 1 × 10 －１ [g / 100 g of H ２ O] or less. The method for producing metal powder according to any one of (1) to (3) above. (5) The method for producing metal powder according to any one of (1) to (4) above, wherein the raw material powder is a metal compound powder. (6) The method for producing metal powder according to (5) above, wherein the metal compound powder is a powder of a metal compound represented by the following formula (1) or its hydrate. M ｗ C ｘ Hｙ O ｚ(1) (However, in formula (1), M is a metal element, w is a number between 1 and 10, x is a number between 0 and 10, y is a number between 0 and 20, and z is a number greater than 0 and less than or equal to 20.) (7) The method for producing metal powder according to (6) above, wherein the metal compound represented by formula (1) does not have a C-H bond (C-H bond). (8) The method for producing metal powder according to (6) or (7) above, wherein the mass fraction (%) of the metal element in the metal compound or its hydrate is 30% or more and less than 100%. (9) The method for producing metal powder according to any one of (5) to (8) above, wherein the metal compound powder includes at least one selected from the group consisting of metal oxide powder, metal hydroxide powder, inorganic weak acid metal salt powder, and organic weak acid metal salt powder. (10) The method for producing metal powder according to (5) above, wherein in the second step, the heat treatment is performed in a reducing atmosphere. (11) The method for producing a metal powder according to any one of (1) to (10), wherein in the second step, the raw material powder dispersed in the gas phase is heat-treated at a temperature of Tm°C or higher and less than Tb°C to produce a liquid metal powder precursor dispersed in the gas phase. (12) The method for producing a metal powder according to any one of (1) to (11), wherein in the second step, the raw material powder dispersed in the gas phase is heat-treated at a temperature of (Tm-100)°C or higher and (Tb-900)°C or lower. (13) The method for producing a metal powder according to any one of (1) to (12), wherein in the first step, the raw material powder is dispersed in the gas phase by the carrier gas while the raw material powder is suspended in a liquid. (14) A method for producing metal powder according to any one of (1) to (13) above, wherein in the third step, a metal powder is produced by cooling the metal powder precursor dispersed in the gas phase, and then the metal powder dispersed in the gas phase is cooled to a temperature at which the metal powder does not sinter. (15) A method for producing metal powder according to any one of (1) to (14) above, wherein the ratio of the D50 of the metal powder to the D50 of the raw material powder as defined below is 0.2 or more and 1.4 or less, and the ratio of the CV value of the metal powder to the CV value of the raw material powder is 1.0 or more and 1.4 or less.D50: The cumulative 50% particle diameter on a number basis, calculated based on the particle diameters measured by randomly selecting 100 particles by scanning electron microscopy observation. (16) A method for producing metal powder according to any one of (1) to (15) above, wherein the D50 defined below for the raw material powder is 10 nm or more and 15 μm or less, and the D50 defined below for the metal powder is 10 nm or more and 15 μm or less. D50: The cumulative 50% particle diameter on a number basis, calculated based on the particle diameters measured by randomly selecting 100 particles by scanning electron microscopy observation. Particle diameter: The diameter of a perfect circle having the same area as the projected area of ​​the particle observed by scanning electron microscopy observation. (17) A method for producing metal powder according to any one of (1) to (16) above, wherein the CV value of the metal powder produced in the third step is 0.50 or less. (18) The CV value defined below is 0.50 or less, the crystallite size defined below is 30.0 nm or more, and the crystallite size ratio defined below is 1.0 × 10. －２ The above describes the metal powder. CV value: The ratio of the standard deviation calculated based on the particle diameter of 100 randomly selected particles measured by scanning electron microscopy, with D50 being the cumulative 50% particle diameter based on the number of particles. Particle diameter: The diameter of a perfect circle with the same area as the projected area of ​​the particle observed by scanning electron microscopy. Crystallite diameter: The value calculated by Scherrer's formula using X-ray diffraction measurements. Crystallite diameter ratio: The ratio of the crystallite diameter to D50, with the value calculated by Scherrer's formula using X-ray diffraction measurements being the crystallite diameter.

[0018] According to the present invention, it is possible to provide a method for producing metal powder that has excellent crystallinity and a narrow particle size distribution.

[0019] <Method for Manufacturing Metal Powder> The present invention provides a method for manufacturing metal powder, comprising: a first step of dispersing a raw material powder, which is produced by chemically reacting metal ions in a liquid phase and has a CV value of 0.40 or less, in a gas phase with a carrier gas at a concentration of 1.0 g / L or less; a second step of generating a metal powder precursor in the gas phase by heat-treating the raw material powder dispersed in the gas phase at a temperature of (Tm-100)°C or higher and less than Tb°C (where Tm°C is the melting point of the metal composed of metal atoms contained in the raw material powder, and Tb°C is the boiling point of the metal composed of metal atoms contained in the raw material powder); and a third step of generating metal powder by cooling the metal powder precursor dispersed in the gas phase.

[0020] Conventional methods had a problem in that, when heat-treated at high temperatures, the raw material powders or the metal powders produced from them would sinter or coalesce, forming a metal mass. Therefore, these methods were based on the premise of heat treatment at temperatures where the raw material powders and the metal powders produced from them would not sinter or coalesce, resulting in insufficient crystallinity of the metal powder obtained after heat treatment.

[0021] In contrast, the present invention involves dispersing a raw material powder, which is produced by chemically reacting metal ions in a liquid phase and has a CV value of 0.40 or less, in a gas phase, and then heat-treating the powder while it is dispersed in the gas phase. This allows for heat treatment at high temperatures, such as [the melting point of the metal composed of metal atoms contained in the raw material powder - 100°C], which would cause sintering or coalescence of the raw material powders or the metal powders produced from them in conventional methods. Furthermore, since the metal powder precursor produced by the heat treatment is cooled while the metal powder precursor remains dispersed in the gas phase, coalescence of the metal powders can be suppressed. With the above configuration, the present invention can produce metal powder that maintains a narrow particle size distribution while also possessing excellent crystallinity that could not be obtained by conventional methods.

[0022] As described above, the metal powder manufacturing method of the present invention provides a metal powder with superior crystallinity and a narrow particle size distribution compared to conventional methods. However, if an even narrower particle size distribution is required, the metal powder obtained by the present invention may be classified. As mentioned 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.

[0023] Furthermore, the present invention's method for producing metal powder allows for heat treatment at a higher temperature compared to conventional methods, thus shortening the heat treatment time. This results in the following effects, such as a dramatic increase in the amount of metal powder produced per unit time.

[0024] The following describes each step that constitutes the method for producing metal powder according to the present invention. In this invention, "D50", "CV value", "crystallite diameter", "crystallite diameter ratio", and "aspect ratio" are defined as follows. (D50) D50 is defined as the cumulative 50% particle diameter based on the number of particles, calculated based on the particle diameter measured by randomly selecting 100 particles by scanning electron microscopy observation. In this invention, the particle diameter is defined as the diameter of a perfect circle having the same area as the projected area of ​​the particle. (CV value) The CV value is defined as the ratio of the standard deviation (standard deviation / D50) to D50, calculated based on the particle diameter measured by randomly selecting 100 particles by scanning electron microscopy observation. (Crystallite diameter) The crystallite diameter is defined as the value d calculated by Scherrer's formula shown in the following formula (2) using measurements obtained by X-ray diffraction (in the following formula (2), K is Scherrer's constant, λ is the X-ray wavelength, β is the full width at half maximum of the diffraction peak, and θ is the diffraction angle). Scherrer's formula: d = Kλ / βcosθ (2) (Cryslite size ratio) The ratio of the crystallite size to D50 is defined as the crystallite size ratio. (Aspect ratio) 100 particles are randomly selected by scanning electron microscopy observation, and the ratio of the short side to the long side (short side / long side) of the rectangle circumscribing the particle to minimize its area is measured for each particle, and the average value of this ratio is defined as the aspect ratio.

[0025] The present invention's method for producing metal powder comprises the following first, second, and third steps. (First step) The first step in the present invention is to disperse a raw material powder, which is produced by chemically reacting metal ions in a liquid phase and has a CV value of 0.40 or less, in a gas phase at a concentration of 1.0 g / L or less using a carrier gas. Because the raw material powder is produced by chemically reacting metal ions in a liquid phase and has a CV value of 0.40 or less, a raw material powder with a narrow particle size distribution is produced. Furthermore, by dispersing the raw material powder in the gas phase at a concentration of 1.0 g / L or less using a carrier gas, heat treatment can be performed at a much higher temperature than in conventional methods in the second step described later. As a result, a metal powder with excellent crystallinity while maintaining a narrow particle size distribution can be produced. In addition, because heat treatment can be performed at a much higher temperature than in conventional methods in the second step described later, the heat treatment time in the second step can be shortened, and the amount of metal powder produced per unit time can be dramatically increased, as described later. Furthermore, in the second step described later, by heat treatment at a temperature of Tm°C or higher, spherical metal powder with a good aspect ratio can be produced. In this invention (specified specification), an aspect ratio closer to 1.0 is considered a good aspect ratio, and a particularly preferred aspect ratio is 1.0.

[0026] The raw material powder is produced by chemically reacting metal ions in a liquid phase, and has a CV value of 0.40 or less. Having the above composition in the raw material powder makes it easier to obtain a metal powder with excellent crystallinity and a narrow particle size distribution.

[0027] A CV value of 0.40 or less of the raw material powder makes it easier to obtain metal powder with a narrow particle size distribution. The CV value of the raw material powder is 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 material powder is not particularly limited and can be, for example, 0.01 or more. In this specification (the present invention), "CV value" is used as an indicator of the breadth or narrowness of the particle size distribution, and a smaller CV value indicates a narrower particle size distribution.

[0028] The solubility of the raw material powder in 100 g of the solvent used to form the liquid phase at 20°C is 1 × 10⁻⁶. －１ It is preferable that the solvent content is less than or equal to [g / 100g of solvent]. The solubility of the raw material powder in 100g of the solvent at 20°C is 1 × 10⁻⁶. －１ The solvent content is less than or equal to [g / 100g of solvent], which makes it easier to produce raw material powder with a narrow particle size distribution. The solvent may be a single solvent or a mixed solvent. The solubility of the raw material powder in 100g of the solvent at 20°C is 1 × 10⁻⁶. －２ It is more preferable that the amount is less than or equal to [g / 100g of solvent], and 1 × 10 －３ It is more preferable that the amount is less than or equal to [g / 100g of solvent], and 1 × 10 －４ It is more preferable that the amount is less than or equal to [g / 100g of solvent], and 1 × 10 －５ It is more preferably less than or equal to [g / 100g of solvent], and 1 × 10 －６ It is particularly preferable that the amount is less than or equal to [g / 100g of solvent]. The solubility of the raw material powder in 100g of the solvent at 20°C is 1 × 10⁻⁶. －２５ It is preferable that the amount is [g / 100g solvent] or more, and 1 × 10 －１５ It is more preferable that the amount is [g / 100g solvent] or more, 1 × 10 －７ It is particularly preferable that the amount is [g / 100g of solvent] or more.

[0029] If the solubility of the raw material powder in 100 g of the solvent at 20°C is within the above range, it becomes easier to produce raw material powder with a narrow particle size distribution. In this invention (specified specification), "solubility of the raw material powder in 100 g of the solvent used to form the liquid phase (the solvent) at 20°C" refers to "the solubility of a substance (for example, a metal oxide or metal hydroxide) that constitutes the raw material powder and becomes a metal powder after heat treatment, in 100 g of the solvent used to form the liquid phase, at 20°C." That is, for example, if the raw material powder is cuprous oxide powder, it refers to "the solubility of the cuprous oxide contained in the cuprous oxide powder in 100 g of the solvent used to form the liquid phase, at 20°C."

[0030] In this invention, the solubility of the raw material powder can be determined by preparing a solvent to be used to form the liquid phase, immersing the raw material powder in the solvent at 20°C, separating the solid and liquid by filtration or the like, and then performing elemental analysis of the separated liquid or the dried liquid by ICP or the like. In this invention, "solubility at 20°C per 100g of solvent used to form the liquid phase" refers to the solubility at 1 atmosphere. In this invention, "solubility at 20°C per 100g of solvent used to form the liquid phase" refers to the solubility at the pH of the solvent used to produce the raw material powder. In this invention, the unit of solubility at 20°C per 100g of solvent is [g / 100g of solvent].

[0031] 100g of raw material powder H ２ The solubility of O at 20°C is 1 × 10⁻⁶ －１ [g / 100g H ２ It is preferable that the H content of 100g of raw material powder is less than or equal to 0. ２ The solubility of O at 20°C is 1 × 10⁻⁶ －１ [g / 100g H ２ When the H content is below 0, it becomes easier to produce raw material powder with a narrow particle size distribution. ２ The solubility of O at 20°C is 1 × 10⁻⁶ －２ [g / 100g H ２ It is more preferable that it be less than or equal to 0, 1 × 10 －３ [g / 100g H ２ It is more preferable that it be less than or equal to 0, 1 × 10 －４ [g / 100g H ２ It is more preferable that it be less than or equal to 0, 1 × 10 －５ [g / 100g H ２ It is even more preferable that it be less than or equal to 0, 1 × 10 －６ [g / 100g H ２ It is particularly preferable that the H content of 100g of the raw material powder be less than or equal to 0. ２ The solubility of O at 20°C is 1 × 10⁻⁶ －２５ [g / 100g H ２ It is preferable that it be 0 or more, 1 x 10－１５ [g / 100g H ２ It is more preferable that it be 0 or greater, 1 × 10 －７ [g / 100g H ２ It is especially preferable that it be 0 or higher. 100g of H ２ The solubility of O at 20°C is within the above range, which makes it easier to produce raw material powder with a narrow particle size distribution. In this invention (specified herein), "100g of H of the raw material powder" ２ "Solubleness of O at 20°C" means "the solubility of 100g of H of a substance (for example, a metal oxide or metal hydroxide) that is a component of the raw material powder and becomes a metal powder after heat treatment, as described below." ２ This refers to the solubility of oxygen at 20°C. In other words, for example, if the raw material powder is cuprous oxide powder, it refers to the solubility of 100g of cuprous oxide contained in the cuprous oxide powder. ２ This refers to the solubility of O at 20°C. In this invention (specified specification), the solubility of the raw material powder is H ２ As O, ultrapure water (with a resistivity of 18 MΩ·cm or more at 25°C) is prepared, the raw material powder is immersed in the ultrapure water at 20°C, solid-liquid separation is performed by filtration, etc., and then the separated liquid or the dried liquid is subjected to elemental analysis by ICP, etc., to determine the O. In addition, "100g of H" in this invention (specified specification) ２ The solubility of O at 20°C refers to the solubility at 1 atmosphere. Furthermore, in this invention (specified herein), 100 g of H ２ The unit of solubility of raw material powder in O is [g / 100g of H] ２ Let's call it O.

[0032] The raw material powder is preferably a metal compound powder. Because the raw material powder is a metal compound powder, some of the constituent elements of the metal compound are removed from the powder by the heat treatment in the second step described later. This makes it easier to reduce the volume of individual particles constituting the powder, thus facilitating the production of powders with small particle sizes. Furthermore, even if the particles constituting the powder are aggregated, coalesced, or in contact with each other before the heat treatment, as mentioned above, some of the constituent elements of the metal compound are removed from the powder by the heat treatment in the second step described later. This, or the resulting reduction in the volume of individual particles, makes it easier to separate the metal powder precursors in the second step described later, thus facilitating the reduction in the CV value of the metal powder obtained by this invention. In particular, the smaller the size of the raw material powder, the more difficult it is to disperse it in the gas phase. Therefore, when producing small powders, deliberately using a metal compound makes it easier to produce powders with small particle sizes and a narrow particle size distribution. In this invention (specified specification), "metal compound powder" refers to "a powder of a metal compound or its hydrate."

[0033] The metal compound powder is preferably a powder of a metal compound represented by the following formula (1) or its hydrate. ｗ C ｘ H ｙ O ｚ (1) (However, in formula (1) above, M is a metallic element, w is a number between 1 and 10, x is a number between 0 and 10, y is a number between 0 and 20, and z is a number greater than 0 and less than or equal to 20.)

[0034] The raw material powder is a metal compound represented by formula (1) (M ｗ C ｘ H ｙ O ｚIt is preferable that the powder is of ) or its hydrate, as this makes it easier for the particles to separate when the metal powder M is produced by the heat treatment in the second step described later. It is also preferable that carbon (C) elements are less likely to remain in the metal powder produced through the steps described later. Furthermore, since the formula (1) does not contain elements other than M, C, H, and O, such as sulfur (S) and nitrogen (N), it is preferable that impurities such as S and N elements are less likely to remain in the metal powder produced through the steps described later. Note that S and N elements may be included as impurities in the metal compound powder as long as the effects of the present invention are not impaired. That is, for example, the metal compound (M) represented by the formula (1) described above ｗ C ｘ H ｙ O ｚ ) or trace amounts of S elements, N elements, etc. may be present on the surface of the hydrate powder. Furthermore, the aforementioned w may be 1 to 8, 1 to 5, or 1 to 3. Furthermore, the aforementioned x is more preferably 0 to 8, more preferably 0 to 6, more preferably 0 to 4, more preferably 0 to 2, more preferably 0 to 1, and particularly preferably 0. This is preferable because it makes it less likely for carbon (C) elements to remain in the metal powder produced through the process described later, and furthermore, because the crystallinity of the metal powder tends to be good due to the less likely for C elements to remain. Furthermore, the aforementioned y is more preferably 0 to 15, more preferably 0 to 10, more preferably 0 to 5, more preferably 0 to 2, and particularly preferably 0. Furthermore, the aforementioned z is more preferably greater than 0 to 15, more preferably greater than 0 to 10, more preferably greater than 0 to 5, more preferably greater than 0 to 3, and particularly preferably greater than 0 to 1. The values ​​of y and / or z being within the above range is preferable because it makes it less likely for oxides to remain when the metal powder is produced by reduction through the process described later, and furthermore, because the metal powder tends to have good crystallinity due to the less residual oxides, it is preferable. Also, the metal compound represented by formula (1) (M ｗ C ｘ Hｙ O ｚ Preferably, the metal compound (M) represented by formula (1) does not have a C-H bond. This makes it easier to narrow the particle size distribution of the raw material powder, and thus easier to narrow the particle size distribution of the metal powder produced through the process described later. Furthermore, it is preferable because carbon (C) elements are less likely to remain in the metal powder produced through the process described later, and furthermore, because the metal powder tends to have good crystallinity due to the less residual C elements, it is preferable. Also, the metal compound (M) represented by formula (1) is preferable. ｗ C ｘ H ｙ O ｚ The number of waters of hydration in the hydrate of () can be any number, for example, greater than 0 and less than or equal to 10, greater than 0 and less than or equal to 8, greater than 0 and less than or equal to 6, greater than 0 and less than or equal to 4, or greater than 0 and less than or equal to 2. Note that w, x, y, z and the number of waters of hydration do not necessarily have to be integers, but may be decimals or fractions.

[0035] From the viewpoint of productivity and cost (transportation costs, storage costs, and the cost of thermal energy required during the heat treatment described later), the mass fraction (%) of the metal element in the metal compound or its hydrate is preferably 30% or more and less than 100%, more preferably 32% or more and less than 100%, more preferably 40% or more and less than 100%, more preferably 45% or more and less than 100%, more preferably 50% or more and less than 100%, more preferably 60% or more and less than 100%, more preferably 70% or more and less than 100%, and more preferably 80% or more and less than 100%. From the viewpoint of balancing productivity and cost with the ease of obtaining a powder with a narrow particle size distribution, the mass fraction (%) of the metal element in the metal compound or its hydrate is preferably 45% or more and 65% or less.

[0036] The metal compound powder is preferably a powder of a metal compound or its hydrate that, under atmospheric pressure, does not have a boiling point or sublimation point within the range below the boiling point of the metal element contained in the metal compound. This makes it difficult for gaseous metal to be generated in the second step described later, making it easier to produce a metal powder with a narrow particle size distribution.

[0037] The metal compound powder preferably contains at least one selected from the group consisting of metal oxide powder, metal hydroxide powder, inorganic weak acid metal salt powder, and organic weak acid metal salt powder, and more preferably contains at least one selected from the group consisting of metal oxide powder, metal hydroxide powder, and inorganic weak acid metal salt powder. Furthermore, the metal compound powder preferably contains at least one selected from the group consisting of cuprous oxide powder, basic copper carbonate powder, copper oxalate powder, nickel hydroxide powder, basic nickel carbonate powder, nickel oxalate powder, and silver carbonate powder. The inorganic weak acid metal salt powder is a salt of an inorganic acid classified as a weak acid and a metal, and is not particularly limited; for example, known ones can be used, but it is preferably a metal carbonate powder. The metal carbonate powder may also be a basic metal carbonate powder. The organic weak acid metal salt powder is a salt of an organic acid classified as a weak acid and a metal, and is not particularly limited; for example, known ones can be used, but it is preferably a metal carboxylate powder, and particularly preferably a metal dicarboxylate powder. Including at least one selected from the group consisting of the aforementioned metal compound powders in the raw material powder makes it easier to produce a raw material powder with a narrow particle size distribution.

[0038] From the viewpoint of producing a raw material powder with a narrow particle size distribution in the liquid phase, metal oxides are preferred. From the viewpoint of improving the dispersibility of the raw material powder in the gas phase in the second step described later, metal hydroxides are preferred. The metal oxide is not particularly limited and includes base metal oxides and noble metal oxides. Examples of base metal oxides include nickel oxide, copper oxide (copper(II) oxide), cuprous oxide (copper(I) oxide), iron(II) oxide, and iron(III) oxide. An example of a noble metal oxide is silver oxide. Note that 100g of these base metal oxides and noble metal oxides contains H ２ The solubility of O at 20°C is 1 × 10⁻⁶ －１ [g / 100g H ２is as follows. From the viewpoint of producing raw material powder with a narrow particle size distribution in the liquid phase, base metal oxides are preferred, and in particular, cuprous oxide is preferred. Also, from the perspective of cost, base metal oxides are preferred. The metal hydroxides are not particularly limited, and examples include base metal hydroxides and noble metal hydroxides. Examples of base metal hydroxides include nickel hydroxide, copper(I) hydroxide, copper(II) hydroxide, iron(II) hydroxide, and iron(III) hydroxide. An example of a noble metal hydroxide is silver hydroxide. Incidentally, the solubility in 100 g of H ２ O at 20 °C for these base metal hydroxides and noble metal hydroxides is 1 × 10 －１ [g / 100 g of H ２ O] or less. From the viewpoint of producing raw material powder with a narrow particle size distribution in the liquid phase, base metal hydroxides are preferred, and in particular, nickel hydroxide is preferred. Also, from the perspective of cost, base metal hydroxides are preferred. The metal carbonates are not particularly limited, and examples include base metal carbonates and noble metal carbonates.

[0039] Examples of base metal carbonates include nickel carbonate, copper carbonate, and iron carbonate. An example of a noble metal carbonate is silver carbonate. Incidentally, the solubility in 100 g of H ２ O at 20 °C for these base metal carbonates and noble metal carbonates is 1 × 10 －１ [g / 100 g of H ２ O] or less. From the viewpoint of producing raw material powder with a narrow particle size distribution in the liquid phase, base metal carbonates are preferred, and in particular, nickel carbonate and copper carbonate are preferred. Also, from the perspective of cost, base metal carbonates are preferred. The basic metal carbonates are not particularly limited, and examples include basic base metal carbonates and basic noble metal carbonates. Examples of basic base metal carbonates include basic nickel carbonate and basic copper carbonate. Incidentally, the solubility in 100 g of H ２ O at 20 °C for these basic carbonates is 1 × 10 －１ [g / 100 g of H ２O] or less. From the viewpoint of producing a raw material powder with a narrow particle size distribution in the liquid phase, basic base metal carbonates are preferred, and basic nickel carbonate and basic copper carbonate are particularly preferred. Also, from the viewpoint of cost, basic base metal carbonates are preferred. Note that the metal elements in the basic metal carbonate and CO ３ It may exist in any molar ratio, for example, CO ３ While the ratio is 1, the metal element may be greater than 1 and less than or equal to 10. Also, CO in a basic metal carbonate ３ The OH groups may be present in any molar ratio, for example, 1:1 to 1:8.

[0040] The metal carboxylate powder is not particularly limited and includes base metal carboxylates and noble metal carboxylates. From the viewpoint of producing a raw material powder with a narrow particle size distribution in the liquid phase, base metal dicarboxylates and noble metal dicarboxylates are preferred. Examples of base metal dicarboxylates include nickel oxalate, copper oxalate, and iron oxalate, and an example of a noble metal dicarboxylate is silver oxalate. From the viewpoint of cost, base metal carboxylates are preferred. Note that 100g of these dicarboxylates contains H ２ The solubility of O at 20°C is 1 × 10⁻⁶ －１ [g / 100g H ２ It is less than or equal to O.

[0041] The method for producing raw material powder by chemically reacting metal ions in a liquid phase is not particularly limited, and known methods can be used, such as dissolving a metal salt in a solvent and then chemically reacting the metal ions in the liquid phase to produce the raw material powder. For example, raw material powder can be produced by first dissolving a metal salt in water to form an aqueous solution of the metal salt, and then adding an alkaline aqueous solution to this aqueous solution to chemically react the metal ions. More specifically, for example, nickel nitrate can be dissolved in water to form an aqueous solution of nickel nitrate, and then an aqueous potassium hydroxide solution can be added to this aqueous solution of nickel nitrate to chemically react nickel ions and hydroxide ions in the liquid phase to produce nickel hydroxide powder as a metal hydroxide powder. Alternatively, for example, copper sulfate can be dissolved in water to form an aqueous copper sulfate solution, and then an aqueous potassium hydroxide solution and an aqueous glucose solution can be added to this aqueous copper sulfate solution to chemically react (reduce) copper(II) ions in the liquid phase to produce cuprous oxide powder. Furthermore, for example, raw material powder can also be produced by mixing an insoluble metal compound with a solvent and then adding an acid to chemically react metal ions in the liquid phase. More specifically, for example, nickel hydroxide can be mixed with water, and then oxalic acid can be added to chemically react nickel ions in the liquid phase and produce nickel oxalate powder. Alternatively, for example, nickel hydroxide produced by the method described above can be used to produce nickel oxalate powder using the method described above. By appropriately adjusting the metal ion concentration in the liquid phase, the type, concentration, and timing of addition of the alkaline aqueous solution, and the concentration and timing of addition of other additives, etc., a raw material powder with a desired particle size and particle size distribution can be obtained by known methods.

[0042] The solvent is not particularly limited; for example, water, polyols, trioctylphosphine, trioctylphosphine oxide, oleylamine, etc., can be used, but water is particularly preferred among these.

[0043] The metal salt is not particularly limited; for example, nitrates, sulfates, carbonates, oxalates, and acetates can be used. More specifically, nickel nitrate, nickel sulfate, nickel carbonate, silver nitrate, silver sulfate, silver carbonate, copper nitrate, copper sulfate, copper carbonate, iron nitrate, iron sulfate, and iron carbonate can be used.

[0044] The alkaline aqueous solution is not particularly limited; for example, potassium hydroxide aqueous solution, sodium hydroxide aqueous solution, or ammonia water can be used.

[0045] The types of metal elements that make up the raw material powder are not particularly limited. That is, the raw material powder in the present invention may contain, for example, metal elements selected from nickel, copper, silver, palladium, gold, platinum, and iron, or it may contain two or more metal elements selected from the above metals. Furthermore, it may also contain appropriate amounts of other metal elements such as magnesium, calcium, ruthenium, rhodium, rhenium, iridium, osmium, titanium, tungsten, and tin, as well as nonmetal elements such as silicon, bismuth, phosphorus, and sulfur. As described above, the types of metal elements are not particularly limited, but it is preferable to include transition elements, and more preferable to include base metal elements such as nickel, copper, and iron. The inclusion of base metal elements in the raw material powder is preferable because it increases the stability of the raw material powder, making it easier to produce raw material powder with a narrow particle size distribution. Furthermore, from the viewpoint of making it easier to obtain the said effect, the content of base metal elements relative to metal elements in the raw material powder is preferably 5% by mass or more, more preferably 10% by mass or more, more preferably 20% by mass or more, more preferably 30% by mass or more, more preferably 40% by mass or more, more preferably 50% by mass or more, more preferably 60% by mass or more, more preferably 70% by mass or more, more preferably 80% by mass or more, more preferably 90% by mass or more, even more preferably 95% by mass or more, and particularly preferably 100% by mass.

[0046] The D50 of the raw material powder is not particularly limited and can be, for example, 10 nm to 15 μm or 10 nm to 5.0 μm. When producing metal powder with small particle size, the D50 of the raw material powder can be, for example, 10 nm to 200 nm, 60 nm to 200 nm, 70 nm to 200 nm, 80 nm to 200 nm, 80 nm to 180 nm, 80 nm to 150 nm, or 80 nm to 120 nm. When producing metal powder with medium particle size, the D50 of the raw material powder can be, for example, 200 nm to 1.0 μm. When producing metal powder with large particle size, the D50 of the raw material powder can be, for example, 1.0 μm to 15 μm or 1.0 μm to 5.0 μm.

[0047] In the first step, it is preferable to disperse a raw material powder, which is produced by chemically reacting metal ions in a liquid phase and has a CV value of 0.40 or less, in the gas phase with a carrier gas at a concentration of 1.0 g / L or less, while maintaining substantially the same D50 as at the time of production. This makes it easier to produce metal powder with a narrow particle size distribution. By not performing a process to reduce D50, such as the grinding process described later, it is possible to suppress the widening of the particle size distribution caused by the generation of fine powder and aggregation due to the grinding process. In this invention (specified specification), a D50 of 0.9 times or more and 1.1 times or less of the D50 at the time of production is defined as "substantially the same D50".

[0048] In the first step, it is preferable to disperse a raw material powder, which is produced by chemically reacting metal ions in a liquid phase and has a CV value of 0.40 or less, in the gas phase with a carrier gas at a concentration of 1.0 g / L or less, while maintaining substantially the same CV value as at the time of production. This makes it easier to produce metal powder with a narrow particle size distribution. In this invention (specified specification), a CV value of 0.9 to 1.1 times the CV value at the time of production is defined as "substantially the same CV value".

[0049] Preferably, the raw material powder is produced by chemically reacting metal ions in a liquid phase, has not undergone pulverization, and has a CV value of 0.40 or less. Because it is not pulverized, it is easier to produce a raw material powder with a narrow particle size distribution, thus making it easier to produce a metal powder with a narrow particle size distribution. The raw material powder, produced by chemically reacting metal ions in a liquid phase, has not undergone pulverization, and has a CV value of 0.40 or less, may also be subjected to crushing. In this specification, "pulverization" refers to the operation of reducing the size of a substance to a smaller size than its original size by applying external force. In the case of powder, this refers to the operation of reducing the size of individual particles constituting the powder. Furthermore, "crushing" refers to the operation of applying external force to a material that has aggregated with relatively weak force, such as a particle aggregate or granules, to disperse or break it up.

[0050] The type of gas constituting the carrier gas is not particularly limited. For example, inert gases such as nitrogen gas or reducing carrier gases that can reduce the raw material powder or products derived from the raw material powder during heat treatment can be used. When the raw material powder is a metal compound powder, using a reducing carrier gas is preferable because it facilitates the reduction of the metal compound to metal. Even without using a reducing carrier gas, if the metal compound is thermally decomposed and a reducing gas is produced, this reducing gas can reduce the metal compound to metal. However, using a reducing carrier gas further facilitates the reduction of the metal compound in the metal compound powder. This increased reduction of the metal compound makes it easier to obtain metal powder and improves the crystallinity of the resulting metal powder. When the raw material powder contains base metal elements, it is particularly preferable to use a reducing carrier gas. The reducing substance does not necessarily have to exist as a gas as the reducing carrier gas; for example, it may be a gas-liquid mixed fluid of nitrogen and an aqueous methanol solution.

[0051] The concentration of the raw material powder dispersed in the gas phase by the carrier gas should be 1.0 g / L or less, preferably 0.5 g / L or less, 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 raw material powder in the gas phase, but from the viewpoint of manufacturing efficiency, it is preferably 0.001 g / L or more. When the concentration of the raw material powder in the gas phase is within the above range, the dispersibility in the gas phase is excellent, so the raw material powders are less likely to come into contact with each other, and as a result, it becomes easier to obtain metal powder with a narrow particle size distribution. In particular, by setting the heat treatment temperature to Tm°C or higher and the concentration of the raw material powder in the gas phase to the above range, it becomes particularly easy to obtain metal powder with excellent crystallinity and a narrow particle size distribution for the same reasons as described in the paragraph explaining the preferred concentration of the metal powder precursor in the second step described later.

[0052] The raw material powder dispersed in the gas phase by the carrier gas may be dispersed in a dry state or in a suspended state in a liquid state. From the viewpoint of energy efficiency during the heat treatment in the second step described later, it is preferable to disperse the raw material powder in a dry state in the gas phase. From the viewpoint of the dispersibility of the raw material powder in the gas phase, it is preferable to disperse the raw material powder in a suspended state in a liquid state in the gas phase. Excellent dispersibility in the gas phase makes it less likely for the raw material powder particles to come into contact with each other, and as a result, it becomes easier to obtain metal powder with a narrow particle size distribution. The raw material powder is suspended in a liquid at the stage when it is produced by chemically reacting metal ions in the liquid phase, and the liquid can be removed by an appropriate method to dry the raw material powder. Even when dispersing the raw material powder in a suspended state in a liquid state in the gas phase, the liquid in which the raw material powder is suspended can be replaced with another liquid beforehand before dispersing it in the gas phase.

[0053] The present invention may be a batch system or a continuous system, but a continuous system is preferred from the viewpoint of production volume per unit time and energy efficiency. In the case of a continuous system, for example, the raw material powder dispersed in the gas phase can be heat-treated while being transported by a carrier gas. The direction of transport is not particularly limited, and the heat treatment can be performed while transporting in various directions, such as in the direction of gravity, horizontally, or in the opposite direction of gravity. From the viewpoint of maintaining a narrow particle size distribution, it is preferable to heat-treat the raw material powder dispersed in the gas phase while transporting it in the direction of gravity by a carrier gas.

[0054] (Second Step) The second step in the present invention is a step to produce a metal powder precursor dispersed in the gas phase by heat-treating a raw material powder dispersed in the gas phase at a temperature of (Tm-100)°C or higher and less than Tb°C (where Tm°C is the melting point of the metal composed of metal atoms contained in the raw material powder, and Tb°C is the boiling point of the metal composed of metal atoms contained in the raw material powder). As a result, heat treatment can be performed at a much higher temperature than in the conventional method, making it possible to produce a metal powder that maintains a narrow particle size distribution while also possessing excellent crystallinity. Furthermore, because heat treatment can be performed at a much higher temperature than in the conventional method, the heat treatment time can be shortened, and the amount of metal powder produced per unit time can be dramatically increased. When manufactured in a continuous process, shortening the heat treatment time makes the equipment smaller (i.e., for example, in the case of a tubular container, the heat treatment section can be made narrower and / or shorter), thus saving space and reducing installation and maintenance costs.

[0055] The heat treatment temperature in the second step may be (Tm-100)°C or higher and less than Tb°C, where Tm°C is the melting point of the metal composed of metal atoms contained in the raw material powder and Tb°C is the boiling point of the metal composed of metal atoms contained in the raw material powder. Preferably, the heat treatment temperature in the second step is (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 a metal powder with excellent crystallinity. In particular, a heat treatment temperature of Tm°C or higher makes it easier to generate a liquid metal powder precursor (metal droplet), making it easier to obtain a metal powder with particularly good crystallinity and a good aspect ratio. That is, the second step is preferably a step in which a liquid metal powder precursor (metal droplet) dispersed in the gas phase is generated by heat-treating the raw material powder dispersed in the gas phase at a temperature of Tm°C or higher. In this invention (specified specification), "melting point" refers to the melting point at 1 atmosphere. Furthermore, it is not necessary for the metal powder precursor described later to be completely melted; it is sufficient if 90% or more of the metal powder precursor is melted. In other words, even if a portion of the metal powder precursor is not melted, the metal powder produced by the present invention is heat-treated at a sufficiently higher temperature than conventional examples, thus enabling the production of a metal powder with excellent crystallinity.

[0056] Furthermore, the heat treatment temperature in the second step is more preferably (Tb-500)°C or lower, more preferably (Tb-800)°C or lower, more preferably (Tb-850)°C or lower, more preferably (Tb-900)°C or lower, even more preferably (Tb-950)°C or lower, and particularly preferably (Tb-1000)°C or lower. Furthermore, the heat treatment temperature in the second step is more preferably (Tm+500)°C or lower, more preferably (Tm+450)°C or lower, more preferably (Tm+400)°C or lower, more preferably (Tm+350)°C or lower, more preferably (Tm+300)°C or lower, more preferably (Tm+250)°C or lower, even more preferably (Tm+200)°C or lower, and particularly preferably (Tm+150)°C or lower. When the heat treatment temperature in the second step is within the above range, the metal powder precursor described later is less likely to vaporize, making it easier to obtain a metal powder that maintains a narrow particle size distribution while also possessing excellent crystallinity. In particular, when the heat treatment temperature in the second step is between Tm°C and (Tb-900)°C, it is preferable to obtain a metal powder with excellent crystallinity and a narrow particle size distribution. Among these, it is particularly preferable to use a metal compound powder, which is a metal compound represented by the above formula (1) and does not have a C-H bond (C-H bond), or a powder of the hydrate thereof, as the raw material powder, and to heat treat it in the second step at a temperature between Tm°C and (Tb-900)°C, as it is extremely easy to obtain a metal powder with excellent crystallinity and a narrow particle size distribution. In this invention (specified specification), "boiling point" refers to the boiling point at 1 atmosphere.

[0057] As for the heat treatment method in the second step, it is preferable to heat treat by radiant heat, but it is also possible to heat treat by directly contacting the raw material 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 treating by directly contacting the raw material powder with a heat source, for example, a method of heat treating by directly contacting the raw material powder with a flame can be used. From the viewpoint of energy efficiency, a method of heat treating by directly contacting the raw material powder with a heat source is preferable. It is also possible to heat treat by contacting the raw material powder with a heated high-temperature gas. Furthermore, the heat treatment methods described above can be combined as appropriate, for example, by directly heating with a flame and then heat treating with radiant heat from an electric furnace.

[0058] In the second step, the concentration of the raw material powder in the gas phase 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 raw material powder in the gas phase, but from the viewpoint of manufacturing efficiency, it is preferably 0.001 g / L or more. When the concentration of the raw material powder in the gas phase is within the above range, the dispersibility in the gas phase is excellent, and the raw material powder and metal powder precursor are less likely to come into contact, 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 higher and less than Tb°C, and setting the concentration of the raw material powder in the gas phase within the above range, it becomes particularly easy to obtain metal powder with excellent crystallinity and a narrow particle size distribution for the same reasons as described in the paragraph explaining the preferred concentration of the metal powder precursor in the second step described later.

[0059] 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 for a higher production volume of metal powder per unit time. Furthermore, when manufacturing continuously, shortening the heat treatment time allows for a smaller apparatus (i.e., for example, in the case of a tubular container, the heat treatment section can be made narrower and / or shorter), resulting in space savings and reduced installation and maintenance costs.

[0060] The metal powder precursor is a precursor to the metal powder produced in the third step described later. It is produced in a dispersed state in the gas phase by heat-treating the raw material powder, which is dispersed in the gas phase, at a temperature of (Tm-100)°C or higher and less than Tb°C. The metal powder precursor may be in a solid state, a mixture of solid and liquid, or in a liquid state. However, from the viewpoint of the crystallinity and aspect ratio of the metal powder produced in the third step described later, it is preferable that the metal powder precursor be in a mixed state of solid and liquid or in a liquid state (metal droplet), and it is particularly preferable that it be in a liquid state (metal droplet). By heat-treating the raw material powder at a temperature of (Tm-100)°C or higher and less than Tm°C, a solid state metal powder precursor is produced. By producing a solid state metal powder precursor in this step, a metal powder with excellent crystallinity can be produced in the third step described later. Furthermore, by heat-treating the raw material powder at a temperature of Tm°C or higher and less than Tb°C, the raw material powder melts, and a liquid metal powder precursor (metal droplet) is generated. By generating a liquid metal powder precursor (metal droplet) in this step, a metal powder with excellent crystallinity can be produced in the third step described later. In the present invention, the raw material powder is produced by chemically reacting metal ions in a liquid phase, so organic matter is almost always used in the production of the raw material powder, and a certain amount of organic matter is often present inside the particles of the raw material powder. Therefore, by melting the metal powder precursor in this step and moving the organic matter to the surface of the metal powder precursor, the organic matter that hinders the improvement of crystallinity can be removed from inside the particles, making it easier to produce a metal powder with excellent crystallinity. In other words, if the metal powder precursor is a metal powder precursor in a state in which solid and liquid are mixed, or a liquid metal powder precursor (metal droplet), it becomes easier to produce a metal powder with excellent crystallinity, and if the metal powder precursor is a liquid metal powder precursor (metal droplet), it becomes particularly easier to produce a metal powder with excellent crystallinity. In this invention, the aforementioned solid, solid-liquid mixture, and liquid substances produced by heat treatment at a temperature of (Tm-100)°C or higher and less than Tb°C are referred to as "metal powder precursors."

[0061] In the second step, the concentration of the metal powder precursor in the gas phase 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 precursor in the gas phase, but from the viewpoint of manufacturing efficiency, it is preferably 0.001 g / L or more. When the concentration of the metal powder precursor in the gas phase is within the above range, the dispersibility in the gas phase is excellent, making it easier to cool while avoiding contact between the metal powder precursor and metal powder in the third step described later, and as a result, it is easier to obtain metal powder with a narrow particle size distribution. When the heat treatment temperature is Tm°C or higher to produce a metal powder precursor in a mixed state of solid and liquid or in a liquid state, the metal powder precursors are more likely to coalesce when they come into contact with each other compared to when the heat treatment temperature is (Tm-100)°C or higher and less than Tm°C to produce a solid state of metal powder precursor. In other words, the particle size distribution of the metal powder produced in the third step described later tends to be broad. However, because the concentration of the metal powder precursor in the gas phase is within the range described above, 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. Specifically, by setting the heat treatment temperature to Tm°C or higher and less than Tb°C, and by keeping the concentration of the metal powder precursor in a mixed state of solid and liquid in the gas phase, or in a liquid state, within the range described above, it becomes particularly easy to obtain metal powder with excellent crystallinity and a narrow particle size distribution.

[0062] When the raw material powder is a metal compound powder, it is preferable in the second step to generate a metal powder precursor dispersed in the gas phase by thermally decomposing the metal compound through heat treatment. Thermal decomposition of the metal compound makes it easier to separate the generated metal powder precursors, thus making it easier to reduce the CV value of the metal powder obtained by the present invention.

[0063] If the raw material powder is a metal compound powder, in the second step, the raw material powder is heat-treated and reduced to produce a metal powder precursor dispersed in the gas phase. If the raw material powder is a metal compound powder, it is preferable to perform the heat treatment in a reducing atmosphere in the second step. This makes it easier to reduce the metal compound, making it easier to obtain metal powder and also makes it easier to improve the crystallinity of the obtained metal powder.

[0064] If the raw material powder contains base metal elements, it is preferable to perform heat treatment in an inert atmosphere or a reducing atmosphere in the second step, and more preferably in a reducing atmosphere. This makes it easier to obtain metal powder and improves the crystallinity of the resulting metal powder. Furthermore, if the raw material powder contains base metal compound powder, the heat treatment in a reducing atmosphere is performed in the second step. This reduces the base metal compound to produce base metal, and thus base metal powder is obtained.

[0065] (Third step) The third step in the present invention is a step of producing metal powder by cooling a metal powder precursor dispersed in the gas phase. By cooling the metal powder precursor dispersed in the gas phase, contact between the metal powder precursor and the metal powder becomes less likely, making it easier to obtain metal powder with a narrow particle size distribution.

[0066] The third step in the present invention preferably involves generating metal powder dispersed in a gas phase by cooling a metal powder precursor dispersed in a gas phase, and then cooling 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 below 250°C, below 220°C, or below 200°C.

[0067] Furthermore, by using organic compounds such as acetic acid or oleic acid as surface treatment agents, for example, it becomes easier to suppress the sintering of the metal powder even at temperatures higher than those mentioned above. That is, by cooling a metal powder precursor dispersed in the gas phase, metal powder dispersed in the gas phase is generated, then an organic compound is attached to the surface of the metal powder, and then the metal powder dispersed in the gas phase can be cooled to a temperature at which the metal powder does not sinter. Because the third step is in the manner described above, contact between the metal powder precursor and the metal powder becomes less likely at temperatures at which they coalesce or sinter, making it easier to obtain metal powder with a narrow particle size distribution. In this specification (the present invention), "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 recovered, and 100 particles are randomly selected and observed using a scanning electron microscope, the number of particles that are sintered (necked) among the observed particles is 5 percent or less.

[0068] The metal powder precursor cooled in the third step may be in a solid state, a mixture of solid and liquid, or in a liquid state. However, from the viewpoint of the crystallinity and aspect ratio of the metal powder produced in this step, it is preferable that the metal powder precursor be in a mixture of solid and liquid or in a liquid state (metal droplet), and it is particularly preferable that it be in a liquid state (metal droplet).

[0069] When cooling a solid metal powder precursor, it is preferable to generate metal powder by cooling the solid metal powder precursor dispersed in the gas phase to a temperature below (Tm-100)°C. It is particularly preferable to generate metal powder dispersed in the gas phase by cooling the solid metal powder precursor dispersed in the gas phase to a temperature below (Tm-100)°C, and then to cool the metal powder dispersed in the gas phase to a temperature at which the metal powder does not sinter. This allows for cooling while avoiding contact between the solid metal powder precursor and the metal powder, making it easier to obtain metal powder that maintains a narrow particle size distribution while also possessing excellent crystallinity.

[0070] When cooling a metal powder precursor in a mixed solid-liquid state or a liquid metal powder precursor (metal droplet), it is preferable to generate metal powder by cooling the liquid metal powder precursor (metal droplet) dispersed in the gas phase to a temperature below Tm°C. It is particularly preferable to generate metal powder dispersed in the gas phase by cooling the liquid metal powder precursor (metal droplet) dispersed in the gas phase to a temperature below Tm°C, and then to cool the metal powder dispersed in the gas phase to a temperature at which the metal powder does not sinter. This allows for cooling while avoiding contact between the liquid metal powder precursor (metal droplet) and the metal powder, making it easier to obtain metal powder that maintains a narrow particle size distribution while also possessing excellent crystallinity.

[0071] In the third step, the concentration of the metal powder precursor in the gas phase 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 precursor in the gas phase, but from the viewpoint of manufacturing efficiency, it is preferably 0.001 g / L or more. When the concentration of the metal powder precursor in the gas phase is within the above range, the dispersibility in the gas phase is excellent, making it easier to cool while avoiding contact between the metal powder precursor and the metal powder, and as a result, it becomes easier to obtain a metal powder with a narrow particle size distribution. In particular, by setting the heat treatment temperature to Tm°C or higher and the concentration of the metal powder precursor in the gas phase within the above range, it becomes 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 preferred concentration of the metal powder precursor mentioned above.

[0072] In the third step, the concentration of the metal powder in the gas phase generated by cooling 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 manufacturing 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 while avoiding contact between the metal powder precursor and the metal powder, and as a result, it becomes easier to obtain metal powder with a narrow particle size distribution.

[0073] The metal powder produced in the third step may be, for example, one of the metal powders described later.

[0074] The present invention provides a method for producing metal powder, which may include the first, second, and third steps described above. However, it may also include another step before the first step, another step between each of the above steps, and another step after the third step.

[0075] The present invention's method for producing metal powder may include a raw material powder preparation step before the first step, in which a raw material powder having a CV value of 0.40 or less is produced by chemically reacting metal ions in a liquid phase, or it may include a raw material powder preparation step in which a metal salt is dissolved in a solvent, and then a raw material powder having a CV value of 0.40 or less is produced by chemically reacting metal ions in a liquid phase. It is preferable that the raw material powder has not been pulverized. Not being pulverized makes it easier to produce a raw material powder with a narrow particle size distribution. The raw material powder that has not been pulverized may be crushed. The solvent and metal salt can be those mentioned above. Furthermore, the raw material powder produced in the raw material powder preparation step may be used as a suspension that has been dried, or it may be used as a suspension without drying, or the liquid in which the raw material powder is suspended may be replaced with another liquid. The other liquid is not limited, and for example, water can be used.

[0076] The method for producing the metal powder of the present invention can include a step of recovering the produced metal powder after the third step. That is, it can include a fourth step of recovering the produced metal powder. In particular, it is preferable to cool the metal powder in a state of being 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. Thereby, since it becomes difficult to contact at the temperature at which the metal powder precursor or the metal powder coalesces or sinters, it becomes easier to obtain a metal powder with a narrow particle size distribution.

[0077] The ratio of D50 of the metal powder to D50 of the raw material powder is preferably 0.2 or more and 2.0 or less, more preferably 0.5 or more and 2.0 or less, more preferably 0.8 or more and 2.0 or less, more preferably 1.0 or more and 2.0 or less, preferably 1.0 or more and 1.8 or less, preferably 1.0 or more and 1.6 or less, preferably 1.0 or more and 1.4 or less, and particularly preferably 1.0 or more and 1.2 or less. Also, the ratio of D50 of the metal powder to D50 of the raw material powder is more preferably 0.2 or more and 1.4 or less, more preferably 0.5 or more and 1.4 or less, more preferably 0.8 or more and 1.4 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.

[0078] The ratio of the CV value of the metal powder to the CV value of the raw material powder is preferably 1.0 or more and 2.0 or less, preferably 1.0 or more and 1.8 or less, preferably 1.0 or more and 1.6 or less, preferably 1.0 or more and 1.4 or less, and particularly preferably 1.0 or more and 1.2 or less.

[0079] When the raw material powder is a metal compound powder and is a powder of the metal compound represented by the above formula (1), the ratio of the atomic weight of the metal element M to the above molecular weight is preferably 0.8 or more and less than 1.0.

[0080] <Metal powder> By the production method of the present invention, the CV value is 0.50 or less, the crystallite diameter is 30.0 nm or more, and the crystallite diameter ratio is 1.0×10 －２The above-mentioned metal powder can be manufactured. In other words, it is possible to manufacture a metal powder with a narrow particle size distribution and excellent crystallinity.

[0081] The CV value of the metal powder of the present invention may be 0.50 or less, but is 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. The lower limit of the CV value of the metal powder is not particularly limited and can be, for example, 0.01 or more.

[0082] The crystallite size of the metal powder of the present invention may be 30.0 nm or larger, but is preferably 35.0 nm or larger, preferably 40.0 nm or larger, preferably 45.0 nm or larger, preferably 50.0 nm or larger, preferably 60.0 nm or larger, preferably 70.0 nm or larger, and particularly preferably 80.0 nm or larger. The upper limit of the crystallite size is not particularly limited, but for example, it can be D50 or less of the metal powder. When the crystallite size of the metal powder is within the above range, the crystallinity of the metal powder is excellent. In this specification (the present invention), "crystallite size" is used as one of the indicators of crystallinity, and a larger crystallite size indicates better crystallinity (higher crystallinity).

[0083] 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 selected from two or more of the above metals, or an alloy powder and / or composite powder to which other metal elements or nonmetal elements mentioned above are added in appropriate amounts. The ratio of the metal component selected from nickel, copper, silver, palladium, gold, platinum, and iron to the total metal powder is preferably as high as possible, preferably 99% by mass or more, more preferably 99.9% by mass or more, and particularly preferably 99.99% by mass or more. Furthermore, it is preferable that the amount of impurities contained in the metal powder be small. For example, the ratio of halogen elements to the total 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 particularly preferably no halogen elements at all. Also, for example, the ratio of alkaline earth metals to the total 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 particularly preferably no alkaline earth metals at all.

[0084] The D50 of the metal powder of the present invention is not particularly limited and can be, for example, 10 nm to 15 μm or 10 nm to 5.0 μm. When producing metal powder with small particle size, the D50 of the metal powder can be, for example, 10 nm to 200 nm, preferably 60 nm to 200 nm, more preferably 70 nm to 200 nm, more preferably 80 nm to 200 nm, more preferably 80 nm to 180 nm, even more preferably 80 nm to 150 nm, and particularly preferably 80 nm to 120 nm. When the D50 of the metal powder is within the above range, it can be suitably used for internal electrodes of multilayer ceramic capacitors. That is, when used for internal electrodes of multilayer ceramic capacitors, in addition to excellent crystallinity and a narrow particle size distribution, the D50 of the metal powder being within the above range allows for the formation of thin, highly continuous internal electrodes while suppressing short-circuit defects, and also makes it easier to suppress delamination and cracking of the multilayer ceramic capacitor. When producing metal powder with a medium particle size, the D50 of the metal powder can be, for example, 200 nm to 1.0 μm. Having the D50 of the metal powder within this range makes it particularly suitable for use as internal electrodes and terminal electrodes in multilayer ceramic electronic components. Specifically, when used as internal electrodes and terminal electrodes in multilayer ceramic electronic components, in addition to excellent crystallinity and a narrow particle size distribution, having the D50 within this range allows for the formation of thin, highly continuous internal electrodes while suppressing short-circuit defects when used as internal electrodes, and also facilitates the suppression of delamination and cracking in multilayer ceramic capacitors. When used as terminal electrodes, it facilitates the formation of thin, dense, and highly continuous terminal electrodes. When producing metal powder with a large particle size, the D50 of the metal powder can be, for example, 1.0 μm to 15 μm, or 1.0 μm to 5.0 μm. Having the D50 of the metal powder within this range makes it particularly suitable for use as internal electrodes in multilayer inductors.In other words, when used for the internal electrodes of a multilayer inductor, its excellent crystallinity and narrow particle size distribution, combined with the fact that the D50 of the metal powder is within the above range, make it easier to form internal electrodes with low resistivity and to form a multilayer inductor with suppressed short-circuit defects.

[0085] The crystallite size ratio of the metal powder of the present invention is 1.0 × 10⁻⁶. －２ The above is acceptable, but 2.0 × 10 －２ Preferably, it is 3.0 × 10 －２ It is more preferable that the above be the case, 5.0 × 10 －２ It is more preferable that the crystallite size ratio be greater than or equal to 0.1, more preferably 0.2, more preferably 0.3, more preferably 0.4, more preferably 0.5, and particularly preferably 0.6. There is no particular upper limit to the crystallite size ratio, but for example it can be 1.0 or less. When D50 is 10 nm or more and 1.0 μm or less, the crystallite size ratio is 5.0 × 10 －２ It is more preferable that the above be the case, 1.0 × 10 －１ The above is particularly preferable. When D50 is 10 nm or more and 1.0 μm or less, the upper limit of the crystallite size ratio is not particularly limited, but for example, it can be 1.0 or less. When D50 is 1.0 μm or more and 15 μm or less, the crystallite size ratio is 2.0 × 10 －２ It is more preferable that the above be the case, 3.0 × 10 －２ It is more preferable that the above be the case, 4.0 × 10 －２ It is even more preferable that the above be the case, 5.0 × 10 －２ The above is particularly preferable. The upper limit of the crystallite size ratio when D50 is 1.0 μm or more and 15 μm or less is not particularly limited, but for example it can be 1.0 or less. When the crystallite size ratio of the metal powder is within the above range, the crystallinity of the metal powder is excellent. In this specification (the present invention), "crystallite size ratio" is used as one of the indicators of crystallinity, and a larger crystallite size ratio indicates better crystallinity (higher crystallinity).

[0086] 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, 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. Having the aspect ratio of the metal powder within the above range makes it easier to narrow the particle size distribution of the metal powder.

[0087] In the present invention, when 100 particles are randomly selected and observed using a scanning electron microscope, it is preferable that the number of sintered (necked) particles among the observed particles is 5 percent or less. This makes it easier to obtain a metal powder with a narrow particle size distribution.

[0088] The metal powder of the present invention has a D50 of 10 nm or more and 1.0 μm or less, a CV value of 0.40 or less, a crystallite size of 40.0 nm or more, and a crystallite size ratio of 1.0 × 10⁻⁶. －１ The above conditions are particularly preferable. When the D50, CV value, crystallite diameter, and crystallite diameter ratio of the metal powder are within the above range, it can be suitably used for internal electrodes and / or terminal electrodes of multilayer ceramic capacitors. Specifically, when the metal powder has a small particle size, excellent crystallinity, and a narrow particle size distribution, it is possible to form thin, highly continuous internal electrodes while suppressing short-circuit defects when used for internal electrodes of multilayer ceramic capacitors, and it is also easier to suppress delamination and cracking of the multilayer ceramic capacitor. Furthermore, when the metal powder has a small particle size, excellent crystallinity, and a narrow particle size distribution, it is easier to form thin, dense, and highly continuous terminal electrodes when used for terminal electrodes of multilayer ceramic capacitors.

[0089] <Measurement Method> (D50) D50 as defined in the present invention can be measured, for example, by the following method. That is, the powder is observed using a scanning electron microscope, 100 particles constituting the powder are randomly selected based on the observation and their particle diameters are measured, and the cumulative 50% particle diameter D50 based on the number of particles can be calculated based on these particle diameters.

[0090] (CV value) The CV value as defined in the present invention can be measured, for example, by the following method. That is, the powder is observed using a scanning electron microscope, 100 particles constituting the powder are randomly selected based on the observation and their particle diameters are measured, the standard deviation is calculated based on the particle diameters, and the CV value can be calculated as the ratio of the standard deviation to D50 calculated by the method described above.

[0091] (Crystallite Size) The crystallite size as defined in this invention can be measured, for example, by the following method. That is, using an XRD measuring device, an XRD measurement of the metal powder is performed using CuKα line (wavelength λ: 1.5418 Å), under the conditions of tube voltage 40 kV, tube current 30 mA, step angle 0.01°, and scanning speed 10.0° / min, for diffraction angles 2θ: 20.0 to 100.0°, the main peak with the maximum peak intensity is detected, the full width at half maximum of the peak is measured, and the crystallite size can be calculated using Scherrer's formula. The main peak can be, for example, a peak corresponding to the (111) plane, which can be around 44° for nickel powder, around 43° for copper powder, and around 38° for silver powder.

[0092] (Aspect Ratio) By randomly selecting 100 particles using scanning electron microscopy, the ratio of the shorter side to the longer side (short side / long side) of a rectangle circumscribing the particle to minimize its area can be measured for each particle, and the aspect ratio can be calculated as the average of these ratios.

[0093] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples. The melting and boiling points of nickel used in the following examples are approximately 1455°C and 2913°C, respectively; the melting and boiling points of copper are approximately 1085°C and 2562°C, respectively; and the melting and boiling points of silver are approximately 962°C and 2162°C, respectively. Furthermore, Table 1 shows the physicochemical properties of copper, copper(II) acetate, cuprous oxide, basic copper(II) carbonate, copper(II) oxalate, nickel, nickel acetate, nickel hydroxide, basic nickel carbonate, nickel oxalate, silver, silver carbonate, or hydrates thereof related to the present invention. The following atomic weights were used to calculate the mass fractions: Cu: 63.546, Ni: 58.693, Ag: 107.868, C: 12.011, H: 1.008, O: 15.999.

[0094]

[0095] <Production of Metal Powder> (Experimental Example A1) First, potassium hydroxide aqueous solution and glucose aqueous solution were added to copper sulfate aqueous solution and stirred to chemically react (reduce) the copper(II) ions in the solution, thereby obtaining a suspension of cuprous oxide powder. The raw material cuprous oxide powder was then prepared by separating and drying the cuprous oxide powder from this suspension. The properties of the prepared raw material cuprous oxide powder are shown in Table 2-1. Next, using a vertical tubular container equipped with a nozzle at the top for ejecting the powder, the raw material cuprous oxide powder was dispensed into an opening with a cross-sectional area of ​​2 cm². ２From the nozzle, the raw material cuprous oxide powder was dispersed in the gas phase at a concentration of 0.05 g per unit volume (1 L) of the gas phase by a carrier gas (a mixture of nitrogen and methanol) at a flow rate of 2200 L / min. Then, with the raw material cuprous oxide powder dispersed in the gas phase at a concentration of 0.05 g / L or less, it was heat-treated at the temperature and time shown in Table 2-1 while passing through the aforementioned vertical tubular container, generating copper droplets of molten metallic copper. An electric furnace was installed on the outside of the vertical tubular container used for the heat treatment, and it was set to maintain the temperature inside the tubular container as described above. Next, the copper droplets, with the concentration of 0.05 g / L or less dispersed in the gas phase, were cooled to generate copper powder. Furthermore, the generated copper powder, with the concentration of 0.05 g / L or less dispersed in the gas phase, was cooled to 60°C and the copper powder was recovered. The properties of the raw material cuprous oxide powder and copper powder were evaluated using the evaluation method described later. The evaluation results are shown in Table 2-1.

[0096] (Experimental Examples A2, A3) In Experimental Example A1, instead of dispersing the raw material cuprous oxide powder in the gas phase, the raw material cuprous oxide powder was heat-treated in a mixed gas atmosphere of nitrogen and methanol at the temperatures and times shown in Table 2-1, and then cooled to 25°C. As a result, the particles constituting the copper powder sintered or melted together to form a metal mass.

[0097] (Experimental Example A4) In Experimental Example A1, instead of dispersing the raw material cuprous oxide powder in the gas phase, magnesium carbonate was mixed with the raw material cuprous oxide powder and stirred. Then, heat treatment was performed in a mixture of nitrogen and methanol at the temperature and time shown in Table 2-1, and then it was cooled to 25°C to produce copper powder, which was then recovered. The properties of the copper powder were then evaluated using the same evaluation method as in Experimental Example A1. The evaluation results are shown in Table 2-1.

[0098] (Experimental Example A5) In Experimental Example A1, instead of dispersing the raw material cuprous oxide powder in the gas phase, magnesium carbonate was mixed with the raw material cuprous oxide powder and stirred. Then, heat treatment was performed in a mixture of nitrogen and methanol at the temperature and time shown in Table 2-1, and then cooled to 25°C. As a result, the particles constituting the copper powder sintered or melted together to form a metal mass.

[0099] (Experimental Examples A6, A7) Copper powder was obtained using the same method as in Experimental Example A1, except that the heat treatment temperature and time were as shown in Table 2-1, and the copper powder did not go through a molten state during or after the heat treatment. The properties of the copper powder were then evaluated using the same evaluation method as in Experimental Example A1. The evaluation results are shown in Table 2-1.

[0100] (Experimental Examples A8, A9) Copper powder was obtained in the same manner as in Experimental Example A1, except that the D50 and CV values ​​of the raw material cuprous oxide powder were as shown in Table 2-1. The properties of the copper powder were then evaluated using the same evaluation method as in Experimental Example A1. The evaluation results are shown in Table 2-1.

[0101] (Experimental Examples A10-A13) Copper powder was obtained in the same manner as in Experimental Example A1, except that the raw material cuprous oxide powder was dispersed in the gas phase at the concentrations shown in Table 2-1, heat treatment was performed at concentrations below those shown in Table 2-1, and copper powder was produced by cooling at concentrations below those shown in Table 2-1. The properties of the copper powder were then evaluated using the same evaluation method as in Experimental Example A1. The evaluation results are shown in Table 2-1.

[0102] (Experimental Example A14) In Experimental Example A1, copper powder was obtained in the same manner as in Experimental Example A1, except that the cuprous oxide powder was not separated and dried from the suspension of cuprous oxide powder obtained in Experimental Example A1, but rather the liquid in which the cuprous oxide powder was suspended was replaced with water, and the content ratio of cuprous oxide powder in the suspension was adjusted to 30% by mass to obtain raw material cuprous oxide powder suspended in water, and then the raw material cuprous oxide powder suspended in water was dispersed in the gas phase. The properties of the copper powder were then evaluated using the same evaluation method as in Experimental Example A1. The evaluation results are shown in Table 2-1.

[0103] (Experimental Example A15) First, an aqueous solution containing copper nitrate and urea was heated and stirred to chemically react with copper(II) ions in the solution, thereby obtaining a suspension of basic copper carbonate powder. The basic copper carbonate powder was then separated and dried from this suspension to prepare the raw material basic copper carbonate powder. The properties of the prepared raw material basic copper carbonate powder are shown in Table 2-1. Next, using a vertical tubular container equipped with a nozzle at the top for ejecting the powder, the raw material basic copper carbonate powder was dispensed into an opening with a cross-sectional area of ​​2 cm². ２From the nozzle, the raw material basic copper carbonate powder was dispersed in the gas phase at a concentration of 0.05 g per unit volume (1 L) of the gas phase by a carrier gas (a mixture of nitrogen and methanol) at a flow rate of 2200 L / min. Then, with the raw material basic copper carbonate powder dispersed in the gas phase at a concentration of 0.05 g / L or less, it was heat-treated at the temperature and time shown in Table 2-1 while passing through the aforementioned vertical tubular container, generating copper droplets of molten metallic copper. An electric furnace was installed on the outside of the vertical tubular container used for the heat treatment, and it was set to maintain the temperature inside the tubular container as described above. Next, the copper droplets, with the concentration of 0.05 g / L or less dispersed in the gas phase, were cooled to generate copper powder. Furthermore, the generated copper powder, with the concentration of 0.05 g / L or less dispersed in the gas phase, was cooled to 60°C and the copper powder was recovered. The properties of the raw material basic copper carbonate powder and the copper powder were evaluated using the evaluation method described later. The evaluation results are shown in Table 2-1.

[0104] (Experimental Example A16) First, copper sulfate aqueous solution and sodium oxalate aqueous solution were added to EDTA aqueous solution while stirring, and a suspension of copper oxalate powder was obtained by chemically reacting the copper(II) ions in the solution. Then, the raw material copper oxalate powder was prepared by separating and drying the copper oxalate powder from this suspension. The properties of the prepared raw material copper oxalate powder are shown in Table 2-1. Next, using a vertical tubular container equipped with a nozzle for ejecting the powder at the top, the raw material copper oxalate powder was dispensed into an opening with a cross-sectional area of ​​2 cm². ２From the nozzle, the raw material copper oxalate powder was dispersed in the gas phase at a concentration of 0.05 g per unit volume (1 L) of the gas phase by a carrier gas (a mixture of nitrogen and methanol) at a flow rate of 2200 L / min. Then, with the raw material copper oxalate powder dispersed in the gas phase at a concentration of 0.05 g / L or less, it was heat-treated at the temperature and time shown in Table 2-1 while passing through the aforementioned vertical tubular container, generating copper droplets of molten metallic copper. An electric furnace was installed on the outside of the vertical tubular container used for the heat treatment, and it was set to maintain the temperature inside the tubular container as described above. Next, the copper droplets, which were dispersed in the gas phase at a concentration of 0.05 g / L or less, were cooled to generate copper powder, and then the generated copper powder, which was dispersed in the gas phase, was cooled to 60°C and recovered. The properties of the raw material copper oxalate powder and the copper powder were evaluated using the evaluation method described later. The evaluation results are shown in Table 2-1.

[0105] (Experimental Example A17) First, copper acetate was produced by crystallization (i.e., copper acetate was precipitated by cooling a high-temperature aqueous solution of copper acetate to below room temperature to a supersaturated state) to obtain a suspension of copper acetate. Then, the copper acetate was separated and dried from this suspension, and further pulverized using a dry pulverizer to prepare the raw material copper acetate powder. The properties of the prepared raw material copper acetate powder are shown in Table 2-1. Next, using a vertical tubular container equipped with a nozzle at the top for ejecting the powder, the raw material copper acetate powder was pulverized through an opening with a cross-sectional area of ​​2 cm². ２From the nozzle, the raw material copper acetate powder was dispersed in the gas phase at a concentration of 0.05 g per unit volume (1 L) of the gas phase by a carrier gas (a mixture of nitrogen and methanol) at a flow rate of 2200 L / min. Then, with the raw material copper acetate powder dispersed in the gas phase at a concentration of 0.05 g / L or less, it was heat-treated at the temperature and time shown in Table 2-1 while passing through the aforementioned vertical tubular container, generating copper droplets of molten metallic copper. An electric furnace was installed on the outside of the vertical tubular container used for the heat treatment, and it was set to maintain the temperature inside the tubular container as described above. Next, the copper droplets, with the concentration of 0.05 g / L or less dispersed in the gas phase, were cooled to generate copper powder. Furthermore, the generated copper powder, with the concentration of 0.05 g / L or less dispersed in the gas phase, was cooled to 60°C and the copper powder was recovered. The properties of the raw material copper acetate powder and copper powder were evaluated using the evaluation method described later. The evaluation results are shown in Table 2-1.

[0106] (Experimental Example A18) Copper powder was obtained in the same manner as in Experimental Example A11, except that the D50 and CV values ​​of the raw material cuprous oxide powder were as shown in Table 2-1. The properties of the copper powder were then evaluated using the same evaluation method as in Experimental Example A11. The evaluation results are shown in Table 2-1.

[0107] (Experimental Example A19) First, hydrazine was added to an aqueous copper nitrate solution and stirred to reduce the copper ions in the solution to metallic copper, thereby obtaining a suspension of copper powder. The raw copper powder was then separated from this suspension and dried to prepare the raw copper powder. The properties of the prepared raw copper powder are shown in Table 2-1. Next, using a vertical tubular container equipped with a nozzle at the top for ejecting the powder, the raw copper powder was dispensed into an opening with a cross-sectional area of ​​2 cm². ２From the nozzle, the raw copper powder was dispersed in the gas phase at a concentration of 0.50 g per unit volume (1 L) of the gas phase by a carrier gas (nitrogen gas) at a flow rate of 2200 L / min. Then, with the raw copper powder dispersed in the gas phase at a concentration of 0.50 g / L or less, it was heat-treated at the temperature and time shown in Table 2-1 while passing through the aforementioned vertical tubular container, generating copper droplets of molten metallic copper. An electric furnace was installed on the outside of the vertical tubular container used for the heat treatment, and it was set to maintain the temperature inside the tubular container as described above. Next, the copper droplets, with their dispersion in the gas phase at a concentration of 0.50 g / L or less, were cooled to generate copper powder. Furthermore, the generated copper powder, with its dispersion in the gas phase, was cooled to 60°C and the copper powder was recovered. The properties of the raw copper powder and the copper powder were evaluated using the evaluation method described later. The evaluation results are shown in Table 2-1.

[0108] (Experimental Example B1) First, a potassium hydroxide aqueous solution was added to an aqueous nickel nitrate solution and stirred to chemically react the nickel ions in the solution, thereby obtaining a suspension of nickel hydroxide powder. The raw nickel hydroxide powder was then separated and dried from this suspension to prepare the raw nickel hydroxide powder. The properties of the prepared raw nickel hydroxide powder are shown in Table 2-2. Next, using a vertical tubular container equipped with a nozzle at the top for ejecting the powder, the raw nickel hydroxide powder was dispensed into an opening with a cross-sectional area of ​​2 cm². ２From the nozzle, the nickel hydroxide powder was dispersed in the gas phase at a concentration of 0.03 g per unit volume (1 L) of the gas phase by a carrier gas (a mixture of nitrogen and methanol) at a flow rate of 2200 L / min. Then, with the raw material nickel hydroxide powder dispersed in the gas phase at a concentration of 0.03 g / L or less, it was heat-treated at the temperature and time shown in Table 2-2 while passing through the aforementioned vertical tubular container, generating nickel droplets in which metallic nickel was molten. An electric furnace was installed on the outside of the vertical tubular container used for the heat treatment, and it was set to maintain the temperature inside the tubular container as described above. Next, the nickel droplets, with the aforementioned nickel droplets dispersed in the gas phase at a concentration of 0.03 g / L or less, were cooled to generate nickel powder, and then the generated nickel powder, with the dispersed nickel powder in the gas phase, was cooled to 180°C and recovered. The properties of the raw material nickel hydroxide powder and the nickel powder were evaluated using the evaluation method described later. The evaluation results are shown in Table 2-2.

[0109] (Experimental Examples B2, B3) In Experimental Example B1, instead of dispersing the raw nickel hydroxide powder in the gas phase, the raw nickel hydroxide powder was heat-treated in a mixed gas atmosphere of nitrogen and methanol at the temperatures and times shown in Table 2-2, and then cooled to 25°C. As a result, the particles constituting the nickel powder sintered or melted together to form a metal mass.

[0110] (Experimental Example B4) In Experimental Example B1, instead of dispersing the raw nickel hydroxide powder in the gas phase, magnesium carbonate was mixed with the raw nickel hydroxide powder and stirred. Then, heat treatment was performed in a mixed gas of nitrogen and methanol at the temperature and time shown in Table 2-2, and then it was 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 Experimental Example B1. The evaluation results are shown in Table 2-2.

[0111] (Experimental Example B5) In Experimental Example B1, instead of dispersing the raw nickel hydroxide powder in the gas phase, magnesium carbonate was mixed with the raw nickel hydroxide powder and stirred. Then, heat treatment was performed in a mixture of nitrogen and methanol at the temperature and time shown in Table 2-2, and then cooled to 25°C. As a result, the particles constituting the nickel powder sintered or melted together to form a metal mass.

[0112] (Experimental Examples B6, B7) Nickel powder was obtained using the same method as in Experimental Example B1, except that the heat treatment temperature and time were as shown in Table 2-2, and the molten state was not passed through during or after the heat treatment. The properties of the nickel powder were then evaluated using the same evaluation method as in Experimental Example B1. The evaluation results are shown in Table 2-2.

[0113] (Experimental Examples B8, B9) Nickel powder was obtained in the same manner as in Experimental Example B1, except that the D50 and CV values ​​of the raw material nickel hydroxide powder were as shown in Table 2-2. The properties of the nickel powder were then evaluated using the same evaluation method as in Experimental Example B1. The evaluation results are shown in Table 2-2.

[0114] (Experimental Example B10) Nickel powder was obtained in the same manner as in Experimental Example B1, except that the generated nickel droplets were cooled to 250°C while dispersed in the gas phase and then collected. The properties of the nickel powder were then evaluated using the same evaluation method as in Experimental Example B1. The evaluation results are shown in Table 2-2.

[0115] (Experimental Example B11) Nickel powder was obtained in the same manner as in Experimental Example B1, except that the generated nickel droplets were cooled to 365°C while dispersed in the gas phase, then a diluted solution of liquid oleic acid was sprayed onto the nickel powder, and the nickel powder was further cooled to 250°C and collected. The properties of the nickel powder were then evaluated using the same evaluation method as in Experimental Example B1. The evaluation results are shown in Table 2-2.

[0116] (Experimental Examples B12-B15) Nickel powder was obtained in the same manner as in Experimental Example B1, except that the raw material nickel hydroxide powder was dispersed in the gas phase at the concentrations shown in Table 2-2, heat treatment was performed at concentrations below those shown in Table 2-2, and nickel powder was produced by cooling at concentrations below those shown in Table 2-2. The properties of the nickel powder were then evaluated using the same evaluation method as in Experimental Example B1. The evaluation results are shown in Table 2-2.

[0117] (Experimental Example B16) Nickel powder was obtained in the same manner as in Experimental Example B1, except that instead of separating and drying the nickel hydroxide powder from the suspension of nickel hydroxide powder obtained in Experimental Example B1, the liquid in which the nickel hydroxide powder was suspended was replaced with water, and the content ratio of nickel hydroxide powder in the suspension was adjusted to 30% by mass to obtain raw material nickel hydroxide powder suspended in water, and then the raw material nickel hydroxide powder suspended in water was dispersed in the gas phase. The properties of the nickel powder were then evaluated using the same evaluation method as in Experimental Example B1. The evaluation results are shown in Table 2-3.

[0118] (Experimental Example B17) First, a nickel nitrate aqueous solution was added to a potassium carbonate aqueous solution and stirred to chemically react the nickel ions in the solution, thereby obtaining a suspension of basic nickel carbonate powder. The basic nickel carbonate powder was then separated from this suspension and dried to prepare the raw material basic nickel carbonate powder. The properties of the prepared raw material basic nickel carbonate powder are shown in Table 2-3. Next, using a vertical tubular container equipped with a nozzle at the top for ejecting the powder, the raw material basic nickel carbonate powder was dispensed into an opening with a cross-sectional area of ​​2 cm². ２From the nozzle, the raw material basic nickel carbonate powder was dispersed in the gas phase at a concentration of 0.03 g per unit volume (1 L) of the gas phase by a carrier gas (a mixture of nitrogen and methanol) at a flow rate of 2200 L / min. Then, with the raw material basic nickel carbonate powder dispersed in the gas phase at a concentration of 0.03 g / L or less, it was heat-treated at the temperature and time shown in Table 2-3 while passing through the aforementioned vertical tubular container, generating nickel droplets in which metallic nickel was molten. An electric furnace was installed on the outside of the vertical tubular container used for the heat treatment, and it was set to maintain the temperature inside the tubular container as described above. Next, the nickel droplets, with the aforementioned concentration of 0.03 g / L or less dispersed in the gas phase, were cooled to generate nickel powder. Furthermore, the generated nickel powder, with the dispersed state in the gas phase, was cooled to 180°C and the nickel powder was recovered. The properties of the raw material basic nickel carbonate powder and the nickel powder were evaluated using the evaluation method described later. The evaluation results are shown in Table 2-3.

[0119] (Experimental Example B18) First, a potassium hydroxide aqueous solution was added to an aqueous solution of nickel nitrate and stirred to chemically react the nickel ions in the solution, thereby obtaining a suspension of nickel hydroxide powder. Then, oxalic acid was added to obtain a suspension of nickel oxalate. The raw material nickel oxalate powder was then prepared by separating and drying the nickel oxalate powder from this suspension. The properties of the prepared raw material nickel oxalate powder are shown in Table 2-3. Next, using a vertical tubular container equipped with a nozzle at the top for ejecting the powder, the raw material nickel oxalate powder was dispensed into an opening with a cross-sectional area of ​​2 cm². ２From the nozzle, the nickel oxalate powder was dispersed in the gas phase at a concentration of 0.03 g per unit volume (1 L) of the gas phase by a carrier gas (a mixture of nitrogen and methanol) at a flow rate of 2200 L / min. Then, with the raw material nickel oxalate powder dispersed in the gas phase at a concentration of 0.03 g / L or less, it was heat-treated at the temperature and time shown in Table 2-3 while passing through the aforementioned vertical tubular container, generating nickel droplets in which metallic nickel was molten. An electric furnace was installed on the outside of the vertical tubular container used for the heat treatment, and it was set to maintain the temperature inside the tubular container as described above. Next, the nickel droplets, which were dispersed in the gas phase at a concentration of 0.03 g / L or less, were cooled to generate nickel powder, and the generated nickel powder was further cooled to 180°C while dispersed in the gas phase, and the nickel powder was recovered. The properties of the raw material nickel oxalate powder and the nickel powder were evaluated using the evaluation method described later. The evaluation results are shown in Table 2-3.

[0120] (Experimental Example B19) First, nickel acetate was produced by crystallization (i.e., nickel acetate was precipitated by cooling a high-temperature aqueous solution of nickel acetate to below room temperature to a supersaturated state) to obtain a suspension of nickel acetate. Then, the nickel acetate was separated and dried from this suspension, and further pulverized using a dry pulverizer to prepare raw nickel acetate powder. The properties of the prepared raw nickel acetate powder are shown in Table 2-3. Next, using a vertical tubular container equipped with a nozzle at the top for ejecting the powder, the raw nickel acetate powder was pulverized through an opening with a cross-sectional area of ​​2 cm². ２From the nozzle, the nickel acetate powder was dispersed in the gas phase at a concentration of 0.03 g per unit volume (1 L) of the gas phase by a carrier gas (a mixture of nitrogen and methanol) at a flow rate of 2200 L / min. Then, with the raw material nickel acetate powder dispersed in the gas phase at a concentration of 0.03 g / L or less, it was heat-treated at the temperature and time shown in Table 2-3 while passing through the aforementioned vertical tubular container, generating nickel droplets in which metallic nickel was molten. An electric furnace was installed on the outside of the vertical tubular container used for the heat treatment, and it was set to maintain the temperature inside the tubular container as described above. Next, the nickel droplets, which were dispersed in the gas phase at a concentration of 0.03 g / L or less, were cooled to generate nickel powder. Furthermore, the generated nickel powder was cooled to 180°C while still dispersed in the gas phase, and the nickel powder was recovered. The properties of the raw material nickel acetate powder and the nickel powder were evaluated using the evaluation method described later. The evaluation results are shown in Table 2-3.

[0121] (Experimental Example B20) Nickel powder was obtained in the same manner as in Experimental Example B13, except that the D50 and CV values ​​of the raw material nickel hydroxide powder were as shown in Table 2-3. The properties of the nickel powder were then evaluated using the same evaluation method as in Experimental Example B13. The evaluation results are shown in Table 2-3.

[0122] (Experimental Example B21) First, hydrazine was added to an aqueous solution of nickel nitrate and stirred to reduce the nickel ions in the solution to metallic nickel, thereby obtaining a suspension of nickel powder. The raw nickel powder was then separated from this suspension and dried to prepare the raw nickel powder. The properties of the prepared raw nickel powder are shown in Table 2-3. Next, using a vertical tubular container equipped with a nozzle at the top for ejecting the powder, the raw nickel powder was dispensed into an opening with a cross-sectional area of ​​2 cm². ２From the nozzle, the nickel powder was dispersed in the gas phase at a concentration of 0.50 g per unit volume (1 L) of the gas phase by a carrier gas (nitrogen gas) at a flow rate of 2200 L / min. Then, with the raw nickel powder dispersed in the gas phase at a concentration of 0.50 g / L or less, it was heat-treated at the temperature and time shown in Table 2-3 while passing through the aforementioned vertical tubular container, generating nickel droplets in which metallic nickel was molten. An electric furnace was installed on the outside of the vertical tubular container used for the heat treatment, and it was set to maintain the temperature inside the tubular container as described above. Next, the nickel droplets were cooled while dispersed in the gas phase at a concentration of 0.50 g / L or less to generate nickel powder, and then the generated nickel powder was 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 using the evaluation method described later. The evaluation results are shown in Table 2-3.

[0123] (Experimental Examples C1-C3) First, a potassium carbonate aqueous solution was added to a silver nitrate aqueous solution and stirred to chemically react the silver ions in the solution, thereby obtaining a suspension of silver carbonate powder. The raw material silver carbonate powder was then prepared by separating and drying the silver carbonate powder from this suspension. The properties of the prepared raw material silver carbonate powder are shown in Table 2-3. Next, using a vertical tubular container equipped with a nozzle at the top for ejecting the powder, the raw material silver carbonate powder was dispensed into an opening with a cross-sectional area of ​​2 cm². ２ From the nozzle, the raw material silver carbonate powder was dispersed in the gas phase at a concentration of 0.05 g per unit volume (1 L) of the gas phase by a carrier gas (air) at a flow rate of 2200 L / min. Then, with the raw material silver carbonate powder dispersed in the gas phase at a concentration of 0.05 g / L or less, it was heat-treated at the temperature and time shown in Table 2-3 while passing through the aforementioned vertical tubular container, generating 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 it was set to maintain the temperature inside the tubular container as described above. Next, the aforementioned silver droplets were cooled while dispersed in the gas phase at a concentration of 0.05 g / L or less to generate silver powder, and then the generated silver powder was cooled to 120°C while dispersed in the gas phase, and the silver powder was recovered. The properties of the raw material silver carbonate powder and silver powder were evaluated using the evaluation method described later. The evaluation results are shown in Table 2-3.

[0124] (Experimental Example C4) Silver powder was obtained in the same manner as in Experimental Example C1, except that the raw material silver carbonate powder was dispersed in the gas phase at the concentrations shown in Table 2-3, heat treatment was performed at concentrations below those shown in Table 2-3, and silver powder was produced by cooling at concentrations below those shown in Table 2-3. The properties of the silver powder were then evaluated using the same evaluation method as in Experimental Example C1. The evaluation results are shown in Table 2-3.

[0125] (Experimental Example C5) First, hydrazine was added to an aqueous silver nitrate solution and stirred to reduce the silver ions in the solution to metallic silver, thereby obtaining a suspension of silver powder. The raw material silver powder was then prepared by separating and drying the silver powder from this suspension. The properties of the prepared raw material silver powder are shown in Table 2-3. Next, using a vertical tubular container equipped with a nozzle at the top for ejecting the powder, the raw material silver powder was dispensed into an opening with a cross-sectional area of ​​2 cm². ２ The raw material silver powder was dispersed in the gas phase at a concentration of 0.50 g per unit volume (1 L) of the gas phase by a carrier gas (air) at a flow rate of 2200 L / min from the nozzle. Then, with the raw material silver powder dispersed in the gas phase at a concentration of 0.50 g / L or less, it was subjected to heat treatment for 4 seconds at the temperature shown in Table 2-3 while passing through the aforementioned vertical tubular container, generating 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 it was set to maintain the aforementioned temperature inside the tubular container. Next, with the aforementioned silver droplets dispersed in the gas phase at a concentration of 0.50 g / L or less, it was cooled to 120°C to generate silver powder, which was then recovered. The properties of the raw material silver powder and the silver powder were evaluated using the evaluation method described later. The evaluation results are shown in Table 2-3.

[0126] <Evaluation Method> (D50) Metal powder was observed using a scanning electron microscope (Hitachi High-Tech Corporation, SU-1510). Based on this observation, 100 particles constituting the powder were randomly selected and their particle diameters were measured. The cumulative 50% particle diameter D50, based on the number of particles, was calculated based on these particle diameters. The diameter of a perfect circle with the same area as the projected area of ​​the particle was measured as the particle diameter.

[0127] (CV value) Metal powder was observed using a scanning electron microscope (Hitachi High-Tech Corporation, SU-1510). 100 particles constituting the powder were randomly selected based on this observation, and their particle sizes were measured. The standard deviation was then calculated based on these particle sizes. Next, the CV value was calculated as the ratio of the standard deviation to D50 calculated using the method described above.

[0128] (Cryslite size and crystallite size ratio) Using an XRD measuring device (SmartLab, manufactured by Rigaku Corporation), CuKα rays (wavelength λ: 1.5418 Å) were used, and XRD measurements of metal powders were performed for diffraction angles 2θ: 20.0 to 100.0° under the conditions of tube voltage 40 kV, tube current 30 mA, step angle 0.01°, and scanning speed 10.0° / min. Peaks corresponding to the (111) plane (nickel powder: around 44°, copper powder: around 43°, silver powder: around 38°) were detected and the full width at half maximum was measured, and the crystallite size was calculated using Scherrer's formula below. Scherrer's formula: d = Kλ / βcosθ (In the above formula, K is Scherrer's constant, λ is the X-ray wavelength, β is the full width at half maximum of the diffraction peak, and θ is the diffraction angle). Next, the crystallite size ratio was calculated as the ratio of the crystallite size to D50 measured by the method described above.

[0129] (Aspect Ratio) 100 particles were randomly selected using scanning electron microscopy, and the ratio of the shorter side to the longer side of a rectangle circumscribing the particle (short side / long side) was measured for each particle to minimize its area. The aspect ratio was calculated as the average of these ratios. Examples with an aspect ratio of less than 0.70 were evaluated as "C", examples between 0.70 and 0.95 as "B", and examples between 0.95 and 1.0 as "A".

[0130]

[0131]

[0132]

[0133] As is clear from the results in Tables 2-1 to 2-3, in experimental examples within the scope of the present invention, it was possible to produce metal powder with excellent crystallinity and a narrow particle size distribution. On the other hand, in experimental examples outside the scope of the present invention, it was not possible to produce metal powder that satisfied both "excellent crystallinity" and "narrow particle size distribution".

[0134] Furthermore, as is clear from the results of experimental examples A1 to A7 and B1 to B7, by dispersing raw material powder with a narrow particle size distribution in the gas phase and heat-treating it, and by heat-treating it at a temperature of (Tm-100)°C or higher, it was possible to produce metal powder with excellent crystallinity and a narrow particle size distribution, and by heat-treating it at a temperature of Tm°C or higher, it was possible to produce metal powder with particularly excellent crystallinity. Furthermore, as is clear from the results of experimental examples B1, B10, and B11, by cooling the metal powder generated and dispersed in the gas phase to a temperature at which the metal powder does not sinter and recovering it, it was possible to produce metal powder with a particularly narrow particle size distribution. Furthermore, as is clear from the results of experimental examples A1, A10 to 13, B1, B12 to B15, the lower the concentration of raw material powder dispersed in the gas phase, the lower the concentration of raw material powder during heat treatment, and the lower the concentration of metal powder precursor during cooling, the more possible it was to produce metal powder with a particularly narrow particle size distribution.

[0135] Furthermore, as is clear from the results of experimental examples C1 to C3, by setting the heat treatment temperature of the raw material powder to [Tb-900]°C or lower, it was possible to produce metal powder with a narrower particle size distribution. Even when the heat treatment temperature does not reach Tb°C, as it approaches Tb°C, the amount of metal vaporized by the heat treatment increases, generating fine powder. However, by setting the temperature to [Tb-900]°C or lower, the generation of this fine powder can be greatly suppressed, and as a result, it is thought that it was possible to produce metal powder with a narrower particle size distribution.

[0136] Furthermore, as is clear from the results of experimental examples A1, A14, B1, and B16, dispersing raw material powder suspended in liquid in the gas phase resulted in the production of metal powder with a narrower particle size distribution compared to dispersing dry raw material powder in the gas phase. Also, as is clear from the results of experimental examples A1, A15-A17, B1, B17-B19, etc., using raw material powder with a narrow particle size distribution generated in the liquid phase resulted in the production of metal powder with excellent crystallinity and a narrow particle size distribution, while using raw material powder with a wide particle size distribution generated by grinding did not result in the production of metal powder with a narrow particle size distribution. Furthermore, as is clear from the results of experimental examples A18, A19, B20, B21, C4, and C5, using metal compound powder as the raw material powder resulted in the production of metal powder with a narrower particle size distribution compared to using metal powder as the raw material powder.

Claims

1. A method for producing metal powder, comprising: a first step of dispersing a raw material powder, which is produced by chemically reacting metal ions in a liquid phase and has a CV value of 0.40 or less as defined below, in a gas phase with a carrier gas at a concentration of 1.0 g / L or less; a second step of generating a metal powder precursor in the gas phase by heat-treating the raw material powder dispersed in the gas phase at a temperature of (Tm-100)°C or higher and less than Tb°C (where Tm°C is the melting point of the metal composed of metal atoms contained in the raw material powder, and Tb°C is the boiling point of the metal composed of metal atoms contained in the raw material powder); and a third step of generating metal powder by cooling the metal powder precursor in the gas phase. CV value: The ratio of the standard deviation calculated based on the particle diameter of 100 randomly selected particles measured by scanning electron microscopy, with D50 being the cumulative 50% particle diameter based on the number of particles. Particle diameter: The diameter of a perfect circle having the same area as the projected area of ​​a particle observed by scanning electron microscopy.

2. The method for producing metal powder according to claim 1, further comprising a raw material powder preparation step, which involves chemically reacting metal ions in a liquid phase to produce the raw material powder having a CV value of 0.40 or less, prior to the first step.

3. The solubility of the raw material powder in 100 g of the solvent used to form the liquid phase at 20°C is 1 × 10⁻⁶. －１ A method for producing metal powder according to claim 1, wherein the amount of the solvent is less than or equal to [g / 100g].

4. 100g of the above raw material powder, H ２ The solubility of O at 20°C is 1 × 10⁻⁶ －１ [g / 100g H ２ A method for producing metal powder according to claim 1, wherein the result is less than or equal to O.

5. The method for producing a metal powder according to claim 1 or 2, wherein the raw material powder is a metal compound powder.

6. The method for producing a metal powder according to claim 5, wherein the metal compound powder is a powder of a metal compound represented by the following formula (1) or a hydrate thereof. ｗ C ｘ H ｙ O ｚ (1) (However, in formula (1) above, M is a metallic element, w is a number between 1 and 10, x is a number between 0 and 10, y is a number between 0 and 20, and z is a number greater than 0 and less than or equal to 20.) 7. The method for producing metal powder according to claim 6, wherein the metal compound represented by formula (1) does not have a C-H bond (C-H bond).

8. The method for producing metal powder according to claim 6, wherein the mass fraction (%) of the metal element in the metal compound or its hydrate is 30% or more and less than 100%.

9. The method for producing a metal powder according to claim 5, wherein the metal compound powder comprises at least one selected from the group consisting of metal oxide powder, metal hydroxide powder, inorganic weak acid metal salt powder, and organic weak acid metal salt powder.

10. The method for producing metal powder according to claim 5, wherein the heat treatment is performed in a reducing atmosphere in the second step.

11. The method for producing metal powder according to claim 1 or 2, wherein in the second step, the raw material powder dispersed in the gas phase is heat-treated at a temperature of Tm°C or higher and less than Tb°C to produce a liquid metal powder precursor dispersed in the gas phase.

12. The method for producing metal powder according to claim 1 or 2, wherein in the second step, the raw material powder dispersed in the gas phase is heat-treated at a temperature of (Tm-100)°C or higher and (Tb-900)°C or lower.

13. The method for producing metal powder according to claim 1 or 2, wherein in the first step, the raw material powder is dispersed in the gas phase by the carrier gas while the raw material powder is suspended in a liquid.

14. The method for producing metal powder according to claim 1 or 2, wherein in the third step, metal powder is produced by cooling the metal powder precursor dispersed in the gas phase, and then the metal powder dispersed in the gas phase is cooled to a temperature at which the metal powder does not sinter.

15. A method for producing metal powder according to claim 1 or 2, wherein the ratio of the D50 of the metal powder to the D50 of the raw material powder as defined below is 0.2 or more and 1.4 or less, and the ratio of the CV value of the metal powder to the CV value of the raw material powder is 1.0 or more and 1.4 or less. D50: The cumulative 50% particle diameter on a particle count basis, calculated based on the particle diameter measured by randomly selecting 100 particles by scanning electron microscopy observation.

16. A method for producing a metal powder according to claim 1 or 2, wherein the D50 of the raw material powder, as defined below, is 10 nm or more and 15 μm or less, and the D50 of the metal powder, as defined below, is 10 nm or more and 15 μm or less. D50: The cumulative 50% particle diameter based on the number of particles, calculated based on the particle diameter measured by randomly selecting 100 particles by scanning electron microscopy. Particle diameter: The diameter of a perfect circle having the same area as the projected area of ​​the particle observed by scanning electron microscopy.

17. The method for producing metal powder according to claim 1 or 2, wherein the CV value of the metal powder produced in the third step is 0.50 or less.

18. The metal powder has a CV value defined below of 0.50 or less, a crystallite diameter defined below of 30.0 nm or more, and a crystallite diameter ratio defined below of 1.0×10 －２ or more. CV value: When the cumulative 50% particle diameter based on the number standard, which is calculated based on the particle diameters of 100 randomly selected particles by scanning electron microscope observation, is defined as D50, the ratio of the standard deviation calculated based on the particle diameters of 100 randomly selected particles by scanning electron microscope observation to D50 Particle diameter: The diameter of a perfect circle having the same area as the projected area of the particle observed by scanning electron microscope observation Crystallite diameter: The value calculated by the Scherrer's formula using the measurement value by X-ray diffraction Crystallite diameter ratio: When the value calculated by the Scherrer's formula using the measurement value by X-ray diffraction is defined as the crystallite diameter, the ratio of the crystallite diameter to D50

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