METHOD FOR PRODUCING METAL POWDER, AND METAL POWDER

Abstract

A method of producing metal powder comprising a first process of dispersing raw metal powder, which is formed by reducing metal ions in the liquid phase, into the gas phase using a carrier gas, a second process of forming metal powder precursors dispersed in said gas phase by heat-treating the raw metal powder dispersed in the gas phase at a temperature of (Tm−100)w° C. or higher, wherein Tm° C. is a temperature at which the raw metal powder melts, and a third process of producing a metal powder by cooling said metal powder precursors dispersed in said gas phase. The present invention provides metal powders with excellent crystallinity and narrow particle size distribution.

Description

TECHNICAL FIELD

This invention relates to a method of manufacturing metal powders, and metal powders.

BACKGROUND ART

In recent years, there has been a growing demand for metal powders with excellent crystallinity and narrow particle size distribution as conductive powders used to form electrodes for electronic components.

As a method for producing metal powders with excellent crystallinity, metal powder production methods are known in which metal powders are formed through a high-temperature state in the gas phase, including the spray pyrolysis method, the atomization method, and the method of producing metal powders through a process of vaporizing the raw metal by plasma, etc. While these methods produce metal powders with extremely high crystallinity, the distribution width of the particle size distribution depends on the uniformity and homogeneity of the raw materials and the temperature distribution in the gas phase, etc. To obtain metal powders with a narrower particle size distribution requires even stricter control, which may lead to higher costs if the production facilities are made larger and more complex.

The wet reduction method and the like is known as a simple method to produce metal powders with narrow particle size distributions. However, it is widely known that the crystallinity of metal powders obtained by the wet reduction method is not sufficient for the level required in recent years. Therefore, various studies have been conducted to improve the crystallinity of metal powders produced by the wet reduction method, etc., as described below.

In Patent Literature 1, a nickel raw material consisting of fine nickel powder or fine nickel compound powder and a spacer consisting of fine powder of at least one salt selected from alkali metal salts or alkaline earth metal salts are mixed and milled, and then the milled material is heat-treated to pyrolyze the nickel fine powder and/or induce grain growth in the nickel fine powder under the presence of the spacer. Afterwards, the heat-treated material is acid-treated to dissolve the spacer, and the dissolved material is rinsed off and the residue is dried to obtain a nickel powder that has undergone grain growth.

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

Patent Literature 3 discloses a method for producing a metal powder with a crystallite diameter of 800 angstroms or greater. This method is characterized in that the metal powder is mixed with at least one of alkali metal halides, alkaline earth metal halides, and rare earth halides, the temperature is raised to above the melting point of the halides, and then cooled, and the resulting reaction product is treated in a wet process to remove the halides and recover the metal powder.

Patent Literature 4 discloses a method of treating metal powder. In the method for treating metal powder to modify the density of metal powder, an oxide, hydroxide, or carbonate coating of an alkaline earth metal is formed on the particle surface, the metal powder is heat-treated below its melting point, the resulting heat-treated product is leached with acid, and then solid-liquid separated to obtain metal powder.

In Patent Literature 5, a method for producing spherical nickel powder is disclosed, which consists of a first process of adding 0.01 wt % to 30 wt % of rare earth compounds to spherical nickel or spherical nickel compound powder, and a second process in which the nickel in the mixture obtained in the said process is reduced by heating in a hydrogen atmosphere, and/or, further heated in a non-oxidizing atmosphere after reduction.

Patent Literature 6 discloses a method for producing spherical nickel powder. The method for producing spherical nickel powder consists of a first process in which at least one or more selected from SiO₂·nH₂O (n≥0) and Al₂O₃·nH₂O (n≥0) by 0.01 wt % to 30 wt % in terms of anhydride are added to spherical nickel or spherical nickel compound powder and mixed, and a second process, in which nickel in the mixture obtained in the first process is reduced by heating in a hydrogen atmosphere, or further heated in a non-oxidizing atmosphere after reduction.

Patent Literature 7 discloses a method of producing conductive powder comprising (A) a step of boron oxide or boric acid is added to a dispersion solution of metal powder in a ratio of not less than 40 parts by weight and not more than 100 parts by weight (B₂O₃ equivalent) per 100 parts by weight of the metal powder, and after mixing, the dispersion solution is dried to obtain a mixture comprising the metal powder and boron oxide or boric acid, (B) a step where the mixture is heat-treated under a neutral atmosphere to obtain a heat-treated product, and (C) a step of dissolving the heat-treated product in water or acid to obtain a metallic powder.

Patent Literature 8 discloses a method of producing nickel powder, wherein nickel powder obtained by the wet reduction method is mixed with anti-sintering agent and pure water to obtain mixed slurry, the mixed slurry is dried by spray heat treatment, and the dried mixture is then heat-treated in a reducing or inert atmosphere at a temperature below the melting point of the anti-sintering agent. After sintering the nickel powder, the anti-sintering agent is separated and removed from the sintered nickel powder.

Patent Literature 9 discloses a method for producing platinum powder having high dispersibility with crystallite size in the range of 30 to 100 (nm), wherein the method comprises a mixing process for mixing platinum powder and a mixture consisting of at least one type of powder of an oxide of a metallic element of any of groups 3 to 15 of the long periodic table, a heat-treatment process in which the mixed powder obtained in the mixing process is heat-treated at a predetermined temperature, a dissolving process wherein the mixed agent is dissolved by treating the heat-treated mixed powder with an acid or alkali, and a removal process for removing the mixing agent by washing the mixed powder in which the mixing agent has been dissolved.

Patent Literature 10 discloses a method for producing nickel powder, wherein after 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 after coating each particle surface of nickel powder with the alkaline earth metal compound, heat treatment is carried out in an inert or slightly reducing gas atmosphere at a temperature below the melting temperature of the alkaline earth metal compound.

PRIOR ARTS

Patent Literatures

• • PTL 1: Japanese Patent Application Publication No. 1998-102109
  • PTL 2: Japanese Patent Application Publication No. 1999-40513
  • PTL 3: Japanese Patent Application Publication No. 2000-096110
  • PTL 4: Japanese Patent Application Publication No. 2001-040401
  • PTL 5: Japanese Patent Application Publication No. 2001-098337
  • PTL 6: Japanese Patent Application Publication No. 2001-107103
  • PTL 7: Japanese Patent Application Publication No. 2004-176120
  • PTL 8: Japanese Patent Application Publication No. 2004-339601
  • PTL 9: Japanese Patent Application Publication No. 2006-299385
  • PTL 10: Japanese Patent Application Publication No. 2002-146401

DISCLOSURE OF THE INVENTION

Problem to be Solved by the Invention

In the methods described in these Patent Literatures 1-10, various substances are added and mixed with the metal powder before heat treatment to prevent sintering of the metal powders during heat treatment. Therefore, these substances remain on the surface of the metal powder after the heat treatment. Therefore, it is necessary to remove the aforementioned substances after heat treatment in order to obtain metal powders suitable as conductive materials. However, since acids and alkalis and the like are used in the removal process, effects on metal powder surfaces, such as dissolution of metal powder surface or formation of metal compounds on metal powder surface, are inevitable. Even when the aforementioned removal operations are performed, complete removal of the aforementioned substances that have been added or mixed, and they remain on the metal powder surface. Thus, the methods described in Patent Literatures 1-10 have various problems as a method for producing metal powders that can be suitably used as conductive materials.

In addition, even if the methods described in Patent Literatures 1-10 are implemented, excellent crystallinity at the level required in recent years cannot be obtained, thereby an even better method for improving the crystallinity of metal powder is required.

Therefore, it is an object of the present invention to provide a method of producing a metal powder with excellent crystallinity and narrow particle size distribution.

The present invention, which solves the above problems, is as described below.

(1) A method of producing a metal powder comprising a first process in which a raw metal powder, which is formed by reducing metal ions in the liquid phase, is dispersed into the gas phase using a carrier gas,

• • a second process of forming metal powder precursors dispersed in the gas phase by heat-treating the raw metal powder dispersed in the gas phase at a temperature of (Tm−100)° C. or higher (where Tm° C. is the temperature at which the raw metal powder melts),
    and a third process of producing the metal powder by cooling the metal powder precursors dispersed in the gas phase.

(2) The method of producing the metal powder described in (1) above, wherein in the first process, the raw metal powder is dispersed in the gas phase at a concentration of 1.0 g/L or less using the carrier gas, in the second process, the raw metal powder dispersed in the gas phase at the above concentration is heat-treated at a temperature of (Tm−100)° C. or higher to form the metal powder precursors dispersed in the gas phase, and in the third process, the metal powder precursors dispersed in the gas phase at the aforementioned concentration are cooled to produce the metal powder.

(3) The method of producing the metal powder described in (1) or (2) above, wherein in the second process, the raw metal powder dispersed in the gas phase is heat-treated at a temperature of Tm° C. or higher to form the metal powder precursors (metal droplets) in the liquid state dispersed in the gas phase.

(4) The method of producing the metal powder described in any one of (1) to (3) above, wherein in the second process, the raw metal powder dispersed in the gas phase is heat-treated at a temperature of (Tm-100)° C. or higher and less than Tb° C. (where Tm° C. is the melting temperature of the raw metal powder and Tb° C. is the vaporization temperature of the raw metal powder).

(5) The method of producing the metal powder described in any one of (1) to (4) above, wherein in the first process, the raw metal powder is dispersed in the gas phase using the carrier gas while the raw metal powder is suspended in a liquid.

(6) The method of producing the metal powder described in any one of (1) to (5) above, wherein in the third process, the metal powder precursors dispersed in the gas phase are cooled to produce the metal powder, and then the metal powder dispersed in the gas phase is cooled to a temperature at which the metal powder does not sinter.

(7) The method of producing the metal powder described in any one of (1) to (6) above, wherein a CV value of the raw metal powder, as defined below, is 0.40 or less.

CV value: When D₅₀ is defined as the cumulative number-based 50% particle diameter, calculated from the measured particle diameters of not less than 100 randomly selected particles observed by scanning electron microscopy, CV value is defined as a ratio of the standard deviation calculated from the measured particle diameters of not less than 100 randomly selected particles observed by scanning electron microscopy to D₅₀.

(8) The method of producing the metal powder described in any one of (1) to (7) above, wherein a ratio of the crystallite diameter ratio, as defined below, of the metal powder to the crystallite diameter ratio, as defined below, of the raw metal powder is 4.0 or higher.

Crystallite diameter ratio: the ratio of a crystal diameter to D₅₀, wherein D₅₀ is defined as the cumulative number-based 50% particle diameter, calculated from the measured particle diameters of not less than 100 randomly selected particles observed by scanning electron, and crystallite diameter is defined as a value calculated from Scherrer's formula using a value measured by X-ray diffraction.

(9) The method of producing the metal powder described in any one of (1) to (8) above, wherein a ratio of the crystallite diameter, as defined below, of the metal powder to the crystallite diameter, as defined below, of the raw metal powder is 4.0 or higher.

Crystallite diameter: a value calculated from Scherrer's formula using a value measured by X-ray diffraction.

(10) The method of producing the metal powder described in any one of (1) to (9) above, wherein a ratio of a D₅₀, as defined below, of the metal powder to a D₅₀, as defined below, of the raw metal powder is not less than 1.0 and not more than 2.0, and the ratio of the CV value, as defined below, of the metal powder to the CV value, as defined below, of the raw metal powder is preferably not less than 1.0 and not more than 2.0.

D₅₀: D₅₀ is defined as the cumulative number-based 50% particle diameter, calculated from the measured particle diameters of not less than 100 randomly selected particles observed by scanning electron microscopy.

CV value: When D₅₀ is defined as the cumulative number-based 50% particle diameter, calculated from the measured particle diameters of not less than 100 randomly selected particles observed by scanning electron microscopy, CV value is defined as a ratio of the standard deviation calculated from the measured particle diameters of not less than 100 randomly selected particles observed by scanning electron microscopy to D₅₀.

(11) The method of producing the metal powder described in any one of (1) to (10) above, wherein a D₅₀, as defined below, of the raw metal powder is not less than 10 nm and not more than 15 μm, and a D₅₀, as defined below, of the metal powder is not less than 10 nm and not more than 15 μm.

D₅₀: D₅₀ is defined as the cumulative number-based 50% particle diameter, calculated from the measured particle diameters of not less than 100 randomly selected particles observed by scanning electron microscopy.

(12) A metal powder with a CV value, as defined below, of 0.50 or less, a crystallite diameter, as defined below, of 30.0 nm or more, and a crystallite diameter ratio, as defined below, of 1.0×10⁻² or more.

CV value: When D₅₀ is defined as the cumulative number-based 50% particle diameter, calculated from the measured particle diameters of not less than 100 randomly selected particles observed by scanning electron microscopy, CV value is defined as a ratio of the standard deviation calculated from the measured particle diameters of not less than 100 randomly selected particles observed by scanning electron microscopy to D₅₀.

Crystallite diameter: a value calculated from Scherrer's formula using a value measured by X-ray diffraction.

Crystallite diameter ratio: Crystallite diameter ratio is defined as a ratio of a crystal diameter to D₅₀, wherein the crystallite diameter is defined as a value calculated from Scherrer's formula using a value measured by X-ray diffraction.

(13) The metal powder described in (12) above, wherein the metal powder has the CV value, as defined below, of 0.40 or less, the D₅₀, as defined below, of not less than 10 nm and not more than 200 nm, the crystallite diameter ratio of 0.40 or more, and the aspect ratio, as defined below, of not less than 0.95 and not more than 1.0.

D₅₀: D₅₀ is defined as the cumulative number-based 50% particle diameter, calculated from the measured particle diameters of not less than 100 randomly selected particles observed by scanning electron microscopy.

Aspect ratio: Aspect ratio is defined as the average ratio of the short axis diameter to the long axis diameter (short axis diameter/long axis diameter) of a rectangle that circumscribes each particle so that the area is minimized, measured for not less than 100 randomly selected particles by scanning electron microscope observation.

Advantageous Effects of Invention

The present invention provides a method of producing metal powder with excellent crystallinity and narrow particle size distribution.

DESCRIPTION OF EMBODIMENTS

Method of Producing Metal Powder

The method of producing metal powder of the present invention comprises a first process of dispersing raw metal powder, which is formed by reducing metal ions in the liquid phase, into the gas phase using a carrier gas, a second process of forming metal powder precursors dispersed in said gas phase by heat-treating the raw metal powder dispersed in the gas phase at a temperature of (Tm−100)° C. or higher (where Tm° C. is the temperature at which the raw metal powder melts), and a third process of producing a metal powder by cooling said metal powder precursors dispersed in said gas phase.

The conventional method, having a problem of causing the raw metal powder particles to sinter together and form a metallic clump when heat-treated at high temperatures, assumes heat treatment at a temperature at which the raw metal powder does not sinter, thus the crystallinity of the metal powder obtained after heat treatment is insufficient.

In contrast, in the present invention, the raw metal powder formed by reducing metal ions in the liquid phase is dispersed in the gas phase, and the powder is heat-treated in a dispersed state in the gas phase, the heat-treatment can be carried out at high temperatures, such as [the temperature at which the raw metal powder melts−100° C.], which cause the raw metal powder particles to sinter with each other in the conventional method. In addition, the metal powder precursors formed by the heat treatment are cooled while the metal powder precursors are dispersed in the gas phase, which prevents the metal powder particles from coalescing each other. The above configuration of the present invention makes it possible to produce a metal powder that maintains a narrow particle size distribution and also has excellent crystallinity, which could not be obtained by conventional methods.

As described above, by using the method of producing metal powder in accordance with the present invention, a metal powder with excellent crystallinity and narrower particle size distribution than before can be obtained. If a metal powder with an even narrower particle size distribution is required, the metal powder obtained by the present invention may be classified. As mentioned above, the particle size distribution of the metal powder obtained by the present invention is narrow enough so that the proportion of powder to be removed by classification is small. In other words, the powder can be classified with a good yield.

In addition, the method of producing the metal powder in accordance with the present invention allows heat treatment at a higher temperature than conventional methods, and thus the heat treatment time can be shortened. In other words, the amount of metal powder produced per unit time can be dramatically improved.

The following describes each process in the method of producing the metal powder according to the present invention.

The “D₅₀,” “CV value,” “crystallite diameter,” “crystallite diameter ratio,” and “aspect ratio” specified in the present invention are defined as follows.

(D₅₀)

D₅₀ is defined as the cumulative number-based 50% particle diameter, calculated from the measured particle diameters of not less than 100 randomly selected particles observed by scanning electron microscopy. In the present invention, particle diameter is defined as the diameter of a perfect circle having the same area as the projected area of the particle.

(CV Value)

CV value is defined as the ratio of the standard deviation calculated from the measured particle diameters of not less than 100 randomly selected particles observed by scanning electron microscopy to D₅₀ (standard deviation/D₅₀).

(Crystallite Diameter)

Crystallite diameter is defined as the value d calculated from Scherrer's formula expressed in the following formula (1) using the value measured by X-ray diffraction (in the following formula (1), K is Scherrer's constant, λ is the X-ray wavelength, ß is the half width of the diffraction peak, and θ is the diffraction angle).

$\begin{array}{cc}\mathrm{Scherrer}\text{'}s\mathrm{formula}:d=K\lambda /\mathrm{\beta cos\theta }& \left(1\right)\end{array}$

(Crystallite Diameter Ratio)

Crystallite diameter ratio is defined as the ratio of crystallite diameter to D₅₀.

(Aspect Ratio)

Aspect ratio is defined as the average ratio of the short axis diameter to the long axis diameter (short axis diameter/long axis diameter) of a rectangle that circumscribes each particle so that the area is minimized, measured for not less than 100 randomly selected particles by scanning electron microscope observation.

The method of manufacturing the metallic powder according to the present invention includes the following first, second, and third processes.

(First Process)

The first process in the present invention is to disperse the raw metal powder, which is produced by reducing metal ions in the liquid phase, into the gas phase using a carrier gas. This process enables the production of a metal powder with excellent crystallinity while maintaining a narrow particle size distribution, because, in this process, a raw metal powder is formed by reducing metal ions in the liquid phase, resulting in a raw metal powder with a narrow particle size distribution, and because by dispersing the raw metal powder in the gas phase using a carrier gas, allowing for performing the heat treatment at a much higher temperature than conventional methods in the second process described below. In addition, because the heat treatment can be performed at a much higher temperature in the second process described below, the heat treatment time in the second process can be shortened, and the amount of the metal powder produced per unit time can be dramatically improved. Furthermore, by heat-treating at a temperature of Tm° C. or higher in the second process described below, a spherical metal powder with a good aspect ratio can be produced. In the present invention (herein), the closer the shape of the metal powder is to a true sphere, i.e., the closer the aspect ratio is to 1.0, the better the aspect ratio is, and a particularly preferred aspect ratio is 1.0.

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

There are no particular restrictions on metal salts. For example, nickel nitrate, nickel sulfate, nickel acetate, silver nitrate, copper nitrate, and copper sulfate can be used.

There are no particular restrictions on reducing agents. For example, hydrazine and formaldehyde can be used as reducing agents.

The type of metal contained in the raw metal powder is not particularly restricted. That is, the raw metal powder in the present invention may be, for example, a metal powder selected from nickel, copper, silver, palladium, gold, platinum, and iron, or an alloy powder selected from two or more of the above metals, and furthermore, alloy powders and/or composite powders containing appropriate amounts of other metallic elements such as magnesium, calcium, ruthenium, rhodium, rhenium, iridium, osmium, titanium, tungsten, and tin, which are widely known to be added to the aforementioned metals, as well as non-metallic elements such as silicon, bismuth, phosphorus, and sulfur.

The D₅₀ of the raw metal powder is not restricted and may be, for example, not less than 10 nm and not more than 15 μm. The D₅₀ of the raw metal powder in the case of producing metal powder with small particle size can be, for example, not less than 10 nm and not more than 200 nm, not less than 60 nm and not more than 200 nm, not less than 70 nm and not more than 200 nm, not less than 80 nm and not more than 200 nm, not less than 80 nm and not more than 180 nm, not less than 80 nm and not more than 150 nm, or not less than 80 nm and not more than 120 nm. The D₅₀ of the raw metal powder in the case of producing metal powder with medium particle size can be, for example, not less than 200 nm and not more than 1.0 μm. The D₅₀ of the raw metal powder in the case of producing a metal powder with a large particle diameter can be, for example, not less than 1.0 μm and not more than 15 μm, or not less than 1.0 μm and not more than 5.0 μm.

The CV value of the raw metal powder should preferably be 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 especially preferably 0.35 or less. The lower limit of the CV value of the raw metal powder is not particularly restricted and can be, for example, 0.01 or higher. In this specification (the present invention), “CV value” is used as an indicator of the width and narrowness of the particle size distribution, and a smaller CV value indicates a narrower particle size distribution.

The type of gas contained in the carrier gas is not particularly restricted, for example, nitrogen gas can be used.

The concentration in the gas phase of the raw metal powder dispersed in the gas phase using the carrier gas should preferably be 1.0 g/L or less, more preferably 0.5 g/L or less, even more preferably 0.1 g/L or less, and especially preferably 0.05 g/L or less. The lower limit of the concentration of raw metal powder in the gas phase is not particularly restricted, but from the perspective of manufacturing efficiency, it is desirable that it be 0.001 g/L or higher. When the concentration of the raw metal powder in the gas phase is in the above range, dispersibility in the gas phase becomes excellent, which makes it difficult for the raw metal powder particles to come into contact with each other, and as a result, a metal powder with narrow particle size distribution can be easily obtained. In particular, by setting the heat treatment temperature to Tm° C. or higher and maintaining the concentration of the raw metal powder in the gas phase within the above-mentioned 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 appropriate concentration of the metal powder precursors in the second process described below.

The raw metal powder dispersed in the gas phase using the carrier gas may be dispersed in the gas phase in a dry state or suspended in a liquid. From the viewpoint of energy efficiency during the heat treatment in the second process described below, it is preferable to disperse the raw metal powder in a dry state in the gas phase. From the viewpoint of dispersibility of the raw metal powder in the gas phase, the raw metal powder in a suspended state in the liquid should preferably be dispersed in the gas phase. Excellent dispersibility in the gas phase makes it difficult for the raw metal powder particles to come into contact with each other, and as a result, a metal powder with narrow particle size distribution can be easily obtained. The raw metal powder is suspended in liquid at the stage where it is formed by reducing metal ions in the liquid phase, and the liquid can be removed by an appropriate method to make the raw metal powder dry. Even in the case of dispersion in the gas phase while suspended in liquid, the liquid in which the raw metal powder is suspended can be replaced with another liquid in advance before dispersion in the gas phase.

The present invention can be a batch process or a continuous process, but the continuous process is preferred from the viewpoints of production volume per unit time and energy efficiency. In the case of the continuous process, for example, the raw metal powder dispersed in the gas phase can be heat-treated while being transferred by a carrier gas. The direction of the aforementioned transfer is not particularly limited, and the heat treatment can be performed while transferring in various directions, for example, in the direction of gravity, horizontally, or in the reverse direction of gravity. From the viewpoint of maintaining a narrow particle size distribution, it is preferable to heat treat the raw metal powder dispersed in the gas phase while transferring it in the direction of gravity by a carrier gas.

(Second Process)

The second process of the present invention is a process of forming metal powder precursors dispersed in the gas phase by heat treating the raw metal powder dispersed in the gas phase at a temperature of (Tm−100)° C. or higher (where Tm° C. is the temperature at which the raw metal powder melts). This allows the heat treatment at a much higher temperature than in the conventional methods, making it possible to produce a metal powder that maintains a narrow particle size distribution and has excellent crystallinity. In addition, because the heat treatment can be performed at a much higher temperature than before, the heat treatment time can be shortened and the amount of metal powder produced per unit of time can be dramatically increased. In addition, because the heat treatment can be performed at a temperature of Tm° C. or higher, which is a much higher temperature than before, spherical metal powder with a good aspect ratio can be produced.

If the temperature at which the raw metal powder melts is Tm° C. and the temperature at which the raw metal powder vaporizes is Tb° C., the heat treatment temperature in the second process should be (Tm−100)° C. or higher, preferably (Tm−50)° C. or higher, more preferably Tm° C. or higher, even more preferably (Tm+50)° C. or higher, and especially preferably (Tm+100)° C. or higher. This makes it easier to obtain the metal powder with excellent crystallinity. In particular, the heat treatment temperature of Tm° C. or higher allows melting of the raw metal powder, which facilitates obtaining metal the powder with excellent crystallinity. In other words, the second process is preferably a process to form metal powder precursors (metal droplets) in the liquid state dispersed in the gas phase by the heat treatment of the raw metal powder dispersed in the gas phase at a temperature of Tm° C. or higher. Tm is the temperature at which the raw metal powder melts should be high enough to melt at least 90% of the raw metal powder by mass, and it does not necessarily mean the temperature at which the raw metal powder completely melts. In other words, even if a part of the raw metal powder is not melted, the metal powder produced by the present invention is heat-treated at a sufficiently high temperature compared to prior arts, enabling the production of a metal powder with excellent crystallinity. The upper limit of the heat treatment temperature in the second process should preferably be less than Tb° C., (Tb−500)° C. or less, more preferably (Tb−800)° C. or less, more preferably (Tb−850)° C. or less, still more preferably (Tb−900)° C. or less, even more preferably (Tb−950)° C. or less, and especially preferably (Tb−1000)° C. or less. The upper limit of the heat treatment temperature in the second process should more preferably be (Tm+500)° C. or less, more preferably (Tm+450)° C. or less, more preferably (Tm+400)° C. or less, more preferably (Tm+350)° C. or less, more preferably (Tm+250)° C. or less, even more preferably (Tm+200)° C. or less, and especially preferably (Tm+150)° C. or less. The heat treatment temperature in the second process is within the above range, which makes it difficult for the raw metal powder to vaporize, thus facilitating the production of a metal powder that maintains a narrow particle size distribution and has excellent crystallinity. In the present invention (this specification), the “temperature at which the raw metal powder vaporizes” refers to the boiling point of the bulk of the metal.

As the heat treatment method in the second process, heat treatment by radiation heat is preferred, but heat treatment can also be performed by bringing the raw metal powder directly into contact with a heat source. For heat treatment by radiant heat, an electric furnace (electric heater) can be used, or radiant heat generated by a flame can be used. From the viewpoint of uniform heating, heat treatment by radiant heat is preferred. As a method of heat treating by bringing the raw metal powder directly into contact with a heat source, for example, a heat treatment method in which the raw metal powder is brought into direct contact with a flame can be used. From the viewpoint of energy efficiency, the method of heat treating by bringing the raw metal powder directly into contact with a heat source is preferred. It is also possible to heat treat the raw metal powder by bringing it into contact with heated, high-temperature gas. In addition, each of the above heat treatment methods can be combined as appropriate. For example, heat treatment can be performed by direct heating with a flame and then by radiant heat from an electric furnace.

The concentration of the raw metal powder in the gas phase in the second process should preferably be 1.0 g/L or less, more preferably 0.5 g/L or less, even more preferably 0.1 g/L or less, and especially preferably 0.05 g/L or less. The lower limit of the concentration of raw metal powder in the gas phase is not particularly restricted, but from the perspective of manufacturing efficiency, it is desirable that it be 0.001 g/L or higher. When the concentration of the raw metal powder in the gas phase is in the above range, dispersibility in the gas phase becomes excellent, which makes it difficult for the raw metal powder and the metal powder precursors to come into contact, and as a result, a metal powder with narrow particle size distribution can be easily obtained. In particular, by setting the heat treatment temperature to Tm° C. or higher and maintaining the concentration of the raw metal powder in the gas phase within the above-mentioned range, it becomes particularly easy to obtain metal powder with excellent crystallinity and a narrow particle size distribution for the same reasons as those described in the paragraph explaining the appropriate concentration of the metal powder precursors in the second process described below.

The heat treatment time in the second process is not particularly limited, but 1 minute or less is preferred, 30 seconds or less is more preferred, 15 seconds or less is more preferred, and 10 seconds or less is particularly preferred. This allows a larger amount of metal powder to be produced per unit time.

The metal powder precursors are the precursors of the metal powder produced in the third process described below, and are formed by heat treating the raw metal powder dispersed in the gas phase at a temperature of (Tm−100)° C. or higher. The metal powder precursors may be in the solid state, in a mixture of solid and liquid, or the liquid state. From the perspective of the crystallinity and aspect ratio of the metal powder produced in the third process described below, it is preferable that the metal powder precursors be in a mixture of solid and liquid or in the liquid state (metal droplets), and it is particularly preferable that the metal powder precursors be in the liquid state (metal droplets). By heat treating the raw metal powder at a temperature of not less than (Tm−100)° C. and less than Tm° C., lattice defects are reduced and solid-state metal powder precursors are formed. By forming the solid-state metal powder precursors in this process, a metal powder with excellent crystallinity can be produced in the third process described below. Furthermore, the heat treatment of the raw metal powder at a temperature of Tm° C. or higher melts the raw metal powder and forms metal powder precursors (metal droplets) in the liquid state. By forming metal powder precursors (metal droplets) in the liquid state in this process, the metal powder with excellent crystallinity can be produced in the third process described below. Since the raw metal powder in the present invention is formed by reducing metal ions in the liquid phase, in most cases, organic materials are used in the formation of the raw metal powder, and organic materials are often present to a certain degree inside the particles of the raw metal powder. Therefore, by melting the metal powder precursors and moving the organic matter to the surface of the metal powder precursors in this process, the organic matter that inhibits 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, the metal powder precursors are in a mixture of solid and liquid or in the liquid state (metal droplets), making it easier to produce a metal powder with excellent crystallinity. Moreover, the metal powder precursors are in the liquid state (metal droplets), making it particularly easy to produce a metal powder with excellent crystallinity. In the present invention, those in the solid state, those in a mixture of solid and liquid, and those in the liquid state, which are formed by the heat treatment at a temperature of (Tm−100)° C. or higher, are referred to as “metal powder precursors”.

The concentration of the metal powder precursors in the gas phase in the second process should preferably be 1.0 g/L or less, more preferably 0.5 g/L or less, even more preferably 0.1 g/L or less, and especially preferably 0.05 g/L or less. The lower limit of the concentration of the metal powder precursors in the gas phase is not particularly restricted, but from the perspective of manufacturing efficiency, it is desirable that it be 0.001 g/L or higher. The concentration of the metal powder precursors in the gas phase in the above range provides excellent dispersibility in the gas phase, which facilitates cooling while avoiding contact of the metal powder precursors and metal powder particles in the third process described below, and as a result, a metal powder with narrow particle size distribution can be easily obtained. Furthermore, when forming the metal powder precursors in a mixture of solid and liquid or in the liquid state by setting the heat treatment temperature to Tm° C. or higher, the metal powder precursors more easily coalesce when they come into contact with each other, compared to forming solid-state metal powder precursors by setting the heat treatment temperature to not less than (Tm−100)° C. and less than Tm° C. In other words, the particle size distribution of the metal powder produced in the third process described below becomes wider. However, when the concentration of the metal powder precursors in the gas phase is in the above range, dispersibility in the gas phase becomes excellent, which makes it difficult for the metal powder precursors to come into contact with each other, and as a result, a metal powder with narrow particle size distribution can be easily obtained. In other words, by setting the heat treatment temperature to Tm° C. or higher and the concentration of the metal powder precursors in a mixture of solid and liquid or in the liquid state in the gas phase to the above range, it becomes particularly easy to obtain a metal powder with excellent crystallinity and a narrow particle size distribution

(Third Process)

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

The third process in the present invention is preferably to cool the metal powder precursors dispersed in the gas phase to produce a metal powder dispersed in the gas phase, and then cool the metal powder dispersed in the gas phase to a temperature at which said metal powder does not sinter. The method of 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 can be cooled to less than 250° C., 220° C., or 200° C., for example. For example, the use of an organic compound such as acetic acid or oleic acid as a surface treatment agent can easily inhibit sintering of the metal powder even at temperatures higher than those mentioned above. In other words, the metal powder precursors dispersed in the gas phase can be cooled to produce a metal powder dispersed in the gas phase, followed by adsorbing an organic compound onto the surface of said metal powder, and then the metal powder dispersed in the gas phase can be cooled to a temperature where said metal powder does not sinter. The third process in the embodiment described above makes it easier to obtain a metal powder with a narrow particle size distribution because the metal powder precursors and the metal powder particles are less likely to come into contact with each other at the temperature at which they coalesce or sinter. In this specification (the present invention), “temperature at which the metal powder does not sinter” means the temperature at which the metal powder is cooled in the third process of the present invention, and then the metal powder is collected, and 100 or more particles are randomly selected and observed by scanning electron microscopy of the collected metal powder, and of the observed particles, the number of sintered (necking) particles is less than 5% of the number of the observed particles.

The metal powder precursors to be cooled in the third process may be in the solid state, in a mixture of solid and liquid, or the liquid state. From the perspective of the crystallinity and aspect ratio of the metal powder produced in this process, it is preferable that the metal powder precursors be in a mixture of solid and liquid or in the liquid state (metal droplets), and it is particularly preferable that the metal powder precursors be in the liquid state (metal droplets).

When cooling the solid-state metal powder precursors, it is preferable to cool the solid-state metal powder precursors dispersed in the gas phase to a temperature less than (Tm−100)° C. to produce metal powder, and is especially preferable to cool the solid-state metal powder precursors dispersed in the gas phase to a temperature less than (Tm−100)° C. to produce a metal powder dispersed in the gas phase, and then cool the metal powder dispersed in the gas phase to a temperature at which said metal powder does not sinter. This allows cooling while avoiding contact of the solid-state metal powder precursors and the metal powder particles, making it easier to obtain a metal powder that maintains a narrow particle size distribution and has excellent crystallinity.

When cooling the metal powder precursors in a mixture of solid and liquid or in the liquid state (metal droplets), it is preferable to cool the metal powder precursors in the liquid state (metal droplets) dispersed in the gas phase to a temperature less than Tm° C. to produce metal powder, and is especially preferable to cool the metal powder precursors in the liquid state (metal droplets) dispersed in the gas phase to a temperature less than Tm° C. to produce metal powder dispersed in the gas phase, and then cool the metal powder dispersed in the gas phase to a temperature at which said metal powder does not sinter. This allows cooling while avoiding contact of the metal powder precursors in the liquid state (metal droplets) and the metal powder, making it easier to obtain a metal powder that maintains a narrow particle size distribution and has excellent crystallinity.

The concentration of the metal powder precursors in the gas phase in the third process should preferably be 1.0 g/L or less, more preferably 0.5 g/L or less, even more preferably 0.1 g/L or less, and especially preferably 0.05 g/L or less. The lower limit of the concentration of the metal powder precursors in the gas phase is not particularly restricted, but from the perspective of manufacturing efficiency, it is desirable that it be 0.001 g/L or higher. The concentration of the metal powder precursors in the gas phase in the above range provides excellent dispersibility in the gas phase, which facilitates cooling while avoiding contact of the metal powder precursors and metal powder particles, and as a result, a metal powder with narrow particle size distribution can be easily obtained. In particular, by setting the heat treatment temperature to Tm° C. or higher and maintaining the concentration of the metal powder precursors in the gas phase within the above-mentioned 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 appropriate concentration of the metal powder precursors as described above.

The concentration of the metal powder produced by cooling in the third process in the gas phase should preferably be 1.0 g/L or less, more preferably 0.5 g/L or less, even more preferably 0.1 g/L or less, and especially preferably 0.05 g/L or less. The lower limit of the concentration of the metal powder in the gas phase is not particularly restricted, but from the perspective of manufacturing efficiency, it is desirable that it be 0.001 g/L or higher. The concentration of the metal powder in the gas phase in the above range provides excellent dispersibility in the gas phase, which facilitates cooling while avoiding contact of the metal powder precursors and the metal powder particles, and as a result, a metal powder with narrow particle size distribution can be easily obtained.

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

The method of producing the metal powder of the present invention need only have the first, second, and third processes described above, but may have one or more additional processes before the first process, one or more additional processes between each of the above processes, or one or more additional processes after the third process.

The method of producing the metal powder in accordance with the present invention may have a process for recovering the produced metal powder after the third process. That is, it can have a fourth process to recover the produced metal powder. In particular, it is preferable to cool the metal powder dispersed in the gas phase in the third process to a temperature at which said metal powder does not sinter, and then recover said metal powder. This makes it easier to obtain a metal powder with a narrow particle size distribution because the metal powder precursors and the metal powder particles are less likely to come into contact with each other at the temperature at which they coalesce or sinter.

The ratio of the crystallite diameter ratio of the metal powder to the crystallite diameter ratio of the raw metal powder should preferably be 4.0 or higher, more preferably 4.5 or higher, and especially preferably 5.0 or higher. The upper limit of the ratio of the crystallite diameter ratio of the metal powder to the crystallite diameter ratio of the raw metal powder is not particularly limited, but can be 1000 or less, for example. If the crystallite peak is not detected by the method of measuring crystallite diameter described below because the raw metal powder is amorphous, the crystallite diameter ratio of the raw metal powder shall be 0. If the crystallite diameter ratio of the produced metal powder exceeds 0, the ratio of the crystallite diameter ratio of the metal powder to that of the raw metal powder is 4.0 or more.

The ratio of the crystallite diameter of the metal powder to the crystallite diameter of the raw metal powder should preferably be 4.0 or higher, more preferably 4.5 or higher, and especially preferably 5.0 or higher. The upper limit of the ratio of the crystallite diameter of the metal powder to the crystallite diameter of the raw metal powder is not particularly limited, but can be 1000 or less, for example. If the crystallite peak is not detected by the method of measuring crystallite diameter described below because the raw metal powder is amorphous, the crystallite diameter of the raw metal powder shall be 0 nm. If the crystallite diameter of the produced metal powder exceeds 0 nm, the ratio of the crystallite diameter of the metal powder to that of the raw metal powder is 4.0 or more.

The ratio of the D₅₀ of the metal powder to the D₅₀ of the raw metal powder is preferably not less than 1.0 and not more than 2.0, preferably not less than 1.0 and not more than 1.8, preferably not less than 1.0 and not more than 1.6, preferably not less than 1.0 and not more than 1.4, and particularly preferably not less than 1.0 and not more than 1.2.

The ratio of the CV value of the metal powder to the CV value of the raw metal powder is preferably not less than 1.0 and not more than 2.0, preferably not less than 1.0 and not more than 1.8, preferably not less than 1.0 and not more than 1.6, preferably not less than 1.0 and not more than 1.4, and particularly preferably not less than 1.0 and not more than 1.2.

<Metallic Powder>

By the manufacturing method of the present invention, it is possible to produce metal powder with a CV value of 0.50 or less, a crystallite diameter of 30.0 nm or more, and a crystallite diameter ratio of 1.0×10⁻² or more. In other words, a metal powder with a narrow particle size distribution and excellent crystallinity can be produced.

The CV value of the metal powder of the present invention may be 0.50 or less, but it should preferably be 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 especially preferably 0.35 or less. The lower limit of the CV value of the metal powder is not particularly restricted and can be, for example, 0.01 or higher.

The crystallite diameter of the metal powder of the present invention should be 30.0 nm or larger, but should preferably be 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 especially preferably 80.0 nm or larger. The upper limit of the crystallite diameter is not particularly limited, but can be, for example, less than D₅₀ of the metal powder. When the crystallite diameter of the metal powder is in the above range, the crystallinity of said metal powder is excellent. In this specification (the present invention), “crystallite diameter” is used as one of the indices of crystallinity, and a larger crystallite diameter indicates superior crystallinity (higher crystallinity).

The type of metal contained in the metal powder is not restricted. That is, the metal powder 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 a composite powder to which metal elements other than the metal elements mentioned above or non-metal elements are added in appropriate amounts. The higher the ratio of the metal component selected from nickel, copper, silver, palladium, gold, platinum, and iron to the total metal powder, the higher the ratio, and 99.9% or more by mass is preferred, 99.9% or more by mass is more preferred, and 99.99% or more by mass is especially preferred. The higher the ratio of the metal component selected from nickel, copper, silver, palladium, gold, platinum, and iron to the total metal component contained in the metal powder, the more desirable, and 99.9 mass % or more is preferred, 99.9 mass % or more is more preferred, and 99.99 mass % or more is especially preferred. Furthermore, it is preferable that the metal powder contain fewer impurities. For example, the ratio of halogen elements to the entire metal powder should preferably be 100 mass ppm or less, more preferably 50 mass ppm ore less, even more preferably 10 mass ppm or less, it is especially preferred that halogen elements are not contained. For example, the ratio of alkaline earth metals to the entire metal powder should preferably be 100 mass ppm or less, more preferably 50 mass ppm or less, even more preferably 10 mass ppm or less, and it is especially preferred that alkaline earth metals are not contained.

The D₅₀ of the metal powder is not restricted and may be, for example, not less than 10 nm and not more than 15 μm. The D₅₀ of the raw metal powder in the case of producing metal powder with small particle size can be, for example, not less than 10 nm and not more than 200 nm, preferably not less than 60 nm and not more than 200 nm, more preferably not less than 70 nm and not more than 200 nm, more preferably not less than 80 nm and not more than 200 nm, preferably not less than 80 nm and not more than 180 nm, not less than 80 nm and not more than 150 nm, or not less than 80 nm and not more than 120 nm. The D₅₀ of the metal powder in the above range makes it particularly suitable for use as the internal electrode of multilayer ceramic capacitors. In other words, when used for the internal electrodes of multilayer ceramic capacitors, in addition to having excellent crystallinity and narrow particle size distribution, the D₅₀ of the metallic powder in the above range enables the formation of thin and highly continuous internal electrodes while suppressing short defects, and also makes it easier to suppress delamination and cracking of the multilayer ceramic capacitor. The D₅₀ of the metal powder in the case of producing metal powder with medium particle size can be, for example, not less than 200 nm and not more than 1.0 μm. The D₅₀ of the metal powder in the above range makes it particularly suitable for use in terminal electrodes of multilayer ceramic electronic components. That is, when used for terminal electrodes of multilayer ceramic electronic components, in addition to excellent crystallinity and narrow particle size distribution, the D₅₀ of the metal powder in the above range facilitates the formation of thin, dense, and highly continuous terminal electrodes. In the case of producing a metal powder with a large particle diameter, the D₅₀ of the metal powder can be, for example, not less than 1.0 μm and not more than 15 μm, or not less than 1.0 μm and not more than 5.0 μm. The D₅₀ of the metal powder in the above range makes it particularly suitable for use as the internal electrode of multilayer inductors. In other words, when used for the internal electrodes of multilayer inductors, in addition to having excellent crystallinity and narrow particle size distribution, the D₅₀ of the metallic powder in the above range enables the formation of internal electrodes with low specific resistance and multilayer inductors with suppressed short defects.

The crystallite diameter ratio of the metal powder of the present invention should be 1.0×10⁻² or higher, but should preferably be 2.0×10⁻² or higher, more preferably 3.0×10⁻² or higher, more preferably 5.0×10⁻² or higher, more preferably 0.1 or higher, more preferably 0.2 or higher, more preferably 0.3 or higher, more preferably 0.4 or higher, more preferably 0.5 or higher, and especially preferably 0.6 or higher. The upper limit of the crystallite diameter ratio is not particularly limited, but can be 1.0 or less, for example. When D₅₀ is not less than 10 nm and not more than 200 nm, a crystallite diameter ratio should preferably be 0.40 or higher, more preferably 0.50 or higher, and especially preferably 0.60 or more. The upper limit of the crystallite diameter ratio when D₅₀ is not less than 10 nm and not more than 200 nm is not particularly limited, but can be 1.0 or less, for example. When D₅₀ is not less than 200 nm and not more than 1.0 μm, a crystallite diameter ratio should more preferably be 5.0×10⁻² or higher, and especially preferably 1.0×10⁻¹ or higher. The upper limit of the crystallite diameter ratio when D₅₀ is not less than 200 nm and not more than 1.0 μm is not particularly limited, but can be 1.0 or less, for example. When D₅₀ is not less than 1.0 μm and not more than 15 μm, a crystallite diameter ratio should preferably be 2.0×10⁻² or higher, more preferably 3.0×10⁻² or higher, even more preferably 4.0×10⁻² or higher, and especially preferably 5.0×10⁻² or higher. The upper limit of the crystallite diameter ratio when D₅₀ is not less than 1.0 μm and not more than 15 μm is not particularly limited, but can be 1.0 or less, for example. When the crystallite diameter ratio of the metal powder is in the above range, the crystallinity of said metal powder is excellent. In this specification (the present invention), “crystallite diameter ratio” is used as one of the indices of crystallinity, and a higher crystallite diameter ratio indicates superior crystallinity (higher crystallinity).

The aspect ratio of the metal powder of the present invention should preferably be not less than 0.70 and not more than 1.0, more preferably not less than 0.80 and not more than 1.0, more preferably not less than 0.90 and not more than 1.0, more preferably not less than 0.95 and not more than 1.0, and especially preferably 1.0. The aspect ratio of the metal powder in the above range facilitates narrowing the particle size distribution of the metal powder.

For the metal powder of the present invention, 100 or more particles are randomly selected and observed by scanning electron microscopy of the collected metal powder, and of the observed particles, the number of sintered (necking) particles should preferably be less than 5% of the number of the observed particles. This makes it easier to obtain metal powder with narrow particle size distribution.

It is particularly preferred that the metal powder has D₅₀ of not less than 10 nm and not more than 200 nm, a CV value of 0.40 or less, a crystallite diameter of 30.0 nm or more, and a crystallite diameter ratio of 0.40 or more. The D₅₀, CV value, crystallite diameter, and crystallite diameter ratio of the metal powder in the above range make it particularly suitable for use as the internal electrode of multilayer ceramic capacitors. In other words, when used for the internal electrodes of multilayer ceramic capacitors, having excellent crystallinity and narrow particle size distribution of the metallic powder enables the formation of thin and highly continuous internal electrodes while suppressing short defects, and also makes it easier to suppress delamination and cracking of the multilayer ceramic capacitor.

Method of Measurement

(D₅₀)

D₅₀ as defined in this invention can be measured, for example, by the following method. The powder is observed using a scanning electron microscope, 100 particles contained in the powder are randomly selected and the particle diameters are measured for the particles from the observation, and D₅₀ of the cumulative number-based 50% particle diameter can be calculated based on the particle diameter.

(CV Value)

The CV value as defined in this invention can be measured, for example, by the following method. The powder is observed using a scanning electron microscope, 100 particles contained in the powder are randomly selected and the particle diameters are measured for the particles from the observation, 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 the D₅₀ calculated by the above method.

(Crystallite Diameter)

Crystallite diameter as defined in this invention can be measured, for example, by the following method. Using an XRD measurement device, XRD measurement of the metal powder at a diffraction angle of 2θ: 20.0-100.0° is conducted using a CuKα ray (wavelength A: 1.5418 Å), under the conditions of a tube voltage of 40 kV, tube current of 30 mA, step angle of 0.01° and scanning speed of 10.0°/min. The main peak is detected, the width at half maximum of the peak is measured, and the crystallite diameter can be calculated using Scherrer's formula. For example, the main peak corresponding to the (111) plane can be used as the peak near 44° for nickel powder, near 38° for silver powder, and near 43° for copper powder.

(Aspect Ratio)

Aspect ratio can be calculated as the average ratio of the short axis diameter to the long axis diameter (short axis diameter/long axis diameter) of a rectangle that circumscribes each particle so that the area is minimized, measured for not less than 100 randomly selected particles by scanning electron microscope observation.

EXAMPLES

The present invention will be described more specifically with the use of examples below, but the invention is not limited to the following examples. The melting point and boiling point of nickel used in the following examples are about 1455° C. and 2913° C., respectively; the melting point and boiling point of silver are about 962° C. and 2162° C., respectively; and the melting point and boiling point of copper are about 1085° C. and 2562° C., respectively.

Producing Metal Powder

Examples 1-4

First, nickel powder suspension was obtained by adding hydrazine to an aqueous nickel nitrate solution to reduce nickel ions in the solution to metallic nickel, and then nickel powder was separated from the suspension and dried to prepare raw nickel powder. The properties of the raw nickel powder are shown in Table 1.

The raw nickel powder was dispersed into the gas phase at a concentration of 0.05 g per unit volume (1 L) of gas phase by a carrier gas (nitrogen gas) at a flow rate of 2200 L/minute through a nozzle with a cross-sectional area of 2 cm² at the opening, using a vertical tubular vessel with a nozzle for ejecting powder at the top. With the raw nickel powder dispersed in the gas phase at said concentration, the heat treatment was conducted at 1600° C. for 3 seconds while passing through the aforementioned vertical tubular vessel to form molten nickel droplets of metallic nickel. An electric furnace was installed on the outside of the vertical tubular vessel used for the heat treatment, and the temperature inside the tubular vessel is set to the aforementioned temperature.

Next, the aforementioned nickel droplets were cooled while dispersed in the gas phase at the aforementioned concentration to produce nickel powder. Furthermore, the produced nickel powder was cooled to 180° C. while dispersed in the gas phase, then the nickel powder was recovered.

The properties of the raw nickel powder and nickel powder were evaluated by the evaluation method described below.

The evaluation results are shown in Table 1.

Comparative Example 1

The raw nickel powder prepared in Example 1 was heat-treated at the temperature and time shown in Table 1, then cooled to 25° C. to produce nickel powder, and the nickel powder was collected. Then, the properties of the nickel powder were evaluated using the same evaluation method as in Example 1. The evaluation results are shown in Table 1.

Comparative Examples 2 and 3

The raw nickel powder prepared in Example 1 was heat-treated at the temperature and time shown in Table 1, and then cooled to 25° C. The particles in the raw nickel powder sintered together to form a metallic mass.

Comparative Examples 4 and 5

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

Comparative Example 6

The raw nickel powder prepared in Example 1 was mixed with magnesium carbonate, stirred, and heat-treated at the temperature and time shown in Table 1, and then cooled to 25° C. The particles in the raw nickel powder sintered together to form a metallic mass.

Comparative Example 7

Nickel powder was formed, cooled, and recovered in the same manner as in Example 1, except that the heat treatment temperature was set to 1100° C., the heat treatment time was set to 4 seconds, and the nickel powder was cooled to 180° C. while dispersed in the gas phase after the heat treatment, without undergoing a molten state during and after the heat treatment. Nickel powder prepared in Example 1 was used as the raw material. Then, the properties of the nickel powder were evaluated using the same evaluation method as in Example 1. The evaluation results are shown in Table 1.

Example 5

Nickel powder was formed, cooled, and recovered in the same manner as in Example 1, except that the heat treatment temperature was set to 1400° C., the nickel powder was cooled to 1350° C. while maintaining the aforementioned dispersion concentration, and furthermore the nickel powder was cooled to 180° C. while dispersed in the gas phase after the heat treatment, without undergoing a molten state during and after the heat treatment. Nickel powder prepared in Example 1 was used as the raw material. Then, the properties of the nickel powder were evaluated using the same evaluation method as in Example 1. The evaluation results are shown in Table 1.

Example 6

Nickel powder was formed, cooled, and recovered in the same manner as in Example 1, except that the nickel powder was cooled to 250° C. and recovered with the formed nickel droplets dispersed in the gas phase. Nickel powder prepared in Example 1 was used as the raw material. Then, the properties of the nickel powder were evaluated using the same evaluation method as in Example 1. The evaluation results are shown in Table 1.

Example 7

Nickel powder was formed, cooled, and recovered in the same manner as in Example 1, except that the formed nickel droplets were cooled to 365° C. while dispersed in the gas phase, then a dilute solution of oleic acid in liquid form was sprayed and adsorbed onto the nickel powder, and said nickel powder was further cooled to 250° C. and recovered. Nickel powder prepared in Example 1 was used as the raw material. Then, the properties of the nickel powder were evaluated using the same evaluation method as in Example 1. The evaluation results are shown in Table 1.

Examples 8 and 9

Nickel powder was formed, cooled, and recovered in the same manner as in Example 1, except that the raw nickel powder formed in Example 1 was dispersed in the gas phase at the concentrations shown in Table 1. Nickel powder prepared in Example 1 was used as the raw material. Then, the properties of the nickel powder were evaluated using the same evaluation method as in Example 1. The evaluation results are shown in Table 1.

Examples 10-15

First, silver powder suspension was obtained by adding hydrazine to an aqueous silver nitrate solution to reduce silver ions in the solution to metallic silver, and then silver powder was separated from the suspension and dried to prepare raw silver powder. The properties of the raw silver powder are shown in Table 2.

The raw silver powder was dispersed into the gas phase at a concentration of 0.05 g per unit volume (1 L) of gas phase by a carrier gas (nitrogen gas) at a flow rate of 2200 L/minute through a nozzle with a cross-sectional area of 2 cm² at the opening, using a vertical tubular vessel with a nozzle for ejecting powder at the top. With the raw silver powder dispersed in the gas phase at said concentration, the heat treatment was conducted at the temperatures shown in Table 2 for 4 seconds while passing through the aforementioned vertical tubular vessel to form molten silver droplets of metallic silver. An electric furnace was installed on the outside of the vertical tubular vessel used for the heat treatment, and the temperature inside the tubular vessel is set to the aforementioned temperature.

Next, the aforementioned silver droplets were cooled to 120° C. while dispersed in the gas phase at the aforementioned concentration to produce silver powder, then the silver powder was recovered.

The properties of the raw silver powder and silver powder were evaluated by the evaluation method described below. The evaluation results are shown in Table 2.

Example 16

Silver powder was formed, cooled, and recovered in the same manner as in Example 15, except that raw silver powder suspended in water is obtained by, without separating and drying the silver powder obtained from the silver powder suspension in Example 15, replacing the liquid in which the silver powder is suspended with water and adjusting the silver powder content ratio in the suspension to 20 mass %, and then the raw silver powder is dispersed into the gas phase while suspended in water. Then, the properties of the silver powder were evaluated using the same evaluation method as in Example 15. The evaluation results are shown in Table 2.

Comparative Example 8

The raw silver powder produced in Example 15 was heat-treated at the temperature and time shown in Table 2, and then cooled to 25° C. The particles in the raw silver powder sintered together to form a metallic mass.

Comparative Example 9

The raw silver powder produced in Example 15 was mixed with magnesium carbonate, stirred, and heat-treated at the temperature and time shown in Table 2, and then cooled to 25° C. The particles in the raw silver powder sintered together to form a metallic mass.

Examples 17 and 18

First, copper powder suspension was obtained by adding hydrazine to an aqueous copper nitrate solution to reduce copper ions in the solution to metallic copper, and then copper powder was separated from the suspension and dried to produce raw copper powder. The properties of the raw copper powder are shown in Table 2.

The raw copper powder was dispersed into the gas phase at a concentration of 0.05 g per unit volume (1 L) of gas phase by a carrier gas (nitrogen gas) at a flow rate of 2200 L/minute through a nozzle with a cross-sectional area of 2 cm² at the opening, using a vertical tubular vessel with a nozzle for ejecting powder at the top. With the raw copper powder dispersed in the gas phase at said concentration, the heat treatment was conducted at 1300° C. for 4 seconds while passing through the aforementioned vertical tubular vessel to form molten copper droplets of metallic copper. An electric furnace was installed on the outside of the vertical tubular vessel used for the heat treatment, and the temperature inside the tubular vessel is set to the aforementioned temperature.

Next, the aforementioned copper droplets were cooled to 60° C. while dispersed in the gas phase at the aforementioned concentration to produce copper powder, then the copper powder was recovered.

The properties of the raw copper powder and copper powder were evaluated by the evaluation method described below. The evaluation results are shown in Table 2.

Comparative Example 10

The raw copper powder produced in Example 18 was heat-treated at the temperature and time shown in Table 2, and then cooled to 25° C. The particles in the raw copper powder sintered together to form a metallic mass.

Comparative Example 11

The raw copper powder produced in Example 18 was mixed with magnesium carbonate, stirred, and heat-treated at the temperature and time shown in Table 2, and then cooled to 25° C. The particles in the raw copper powder sintered together to form a metallic mass.

For the metal powder recovered in the above-described Examples and Comparative Examples other than “Comparative Examples 2, 3, 6, 8, 9, 10, 11, and Examples 6 and 9”, 100 or more particles are randomly selected and observed by scanning electron microscopy of the collected metal powder, and of the observed particles, the number of sintered (necking) particles was confirmed to be less than 5% of the number of the observed particles. In Examples 6 and 9, the number of sintered (necking) particles was more than 5 number %.

Reference Example

Nickel powder produced by a known method in which raw nickel metal is heated with plasma to melt and vaporize it, and the resulting metal vapor is cooled to obtain metal powder; nickel powder produced by a known method in which raw nickel powder is heated with a flame to vaporize it, and the resulting metal vapor is cooled to obtain metal powder; nickel powder produced by a known method in which nickel acetate tetrahydrate powder, a thermally decomposable compound powder, is sprayed into the gas phase, and then heat-treated at or above the thermal decomposition temperature to obtain metal powder; and nickel powder produced by spraying and cooling nickel melt. These powders were prepared, and the nickel powder properties were evaluated using the same evaluation method as in Example 1. In all cases, the crystal grain size was 30.0 nm or more, the crystal grain size ratio was 1.0×10⁻² or more, but the CV value was over 0.50.

Evaluation Method

(D₅₀)

The metal powder is observed using a scanning electron microscope (Hitachi High-Tech, SU-1510), 100 particles contained in the powder are randomly selected and the particle diameters are measured for the particles from the observation, and D₅₀ of the cumulative number-based 50% particle diameter was calculated based on the particle diameter. The diameter of a perfect circle having the same area as the projected area of the particle was measured as the particle diameter.

(CV Value)

The metal powder is observed using a scanning electron microscope (Hitachi High-Tech, SU-1510), 100 particles contained in the powder are randomly selected and the particle diameters are measured for the particles from the observation, and the standard deviation was calculated based on the particle diameter. The CV value was then calculated as the ratio of the aforementioned standard deviation to the D₅₀ calculated by the aforementioned method.

(Crystallite Diameter and Crystallite Diameter Ratio)

Using an XRD measurement device (SmartLab, manufactured by Rigaku), XRD measurement of the metal powder at a diffraction angle of 2θ: 20.0-100.0° is conducted using a CuKα ray (wavelength λ: 1.5418 Å), under the conditions of a tube voltage of 40 kV, tube current of 30 mA, step angle of 0.01° and scanning speed of 10.0°/min. Peaks corresponding to the (111) plane (nickel powder: around 44°, silver powder: around 38°, copper powder: around 43°) were detected and the width at half maximum was measured.

$\mathrm{Scherrer}\text{'}s\mathrm{formula}:d=K\lambda /\mathrm{\beta cos\theta }$

(In the above formula, K is Scherrer's constant, λ is the X-ray wavelength, β is the width at half maximum of the diffraction peak, and θ is the diffraction angle.)

(Aspect Ratio)

Aspect ratio was calculated as the average ratio of the short axis diameter to the long axis diameter (short axis diameter/long axis diameter) of a rectangle that circumscribes each particle so that the area is minimized, measured for not less than 100 randomly selected particles by scanning electron microscope observation. The aspect ratio was evaluated as “X” for particles with an aspect ratio of less than 0.70, “Δ” for particles with an aspect ratio of not less than 0.70 and not more than 0.95, and “0” for particles with an aspect ratio of not less than 0.95 and not more than 1.0.

	TABLE 1
	CV					CV
	D	( /		( /				D	( /		( /
		[nm]	D )	[nm]	D )	[g/L]	[° C.]			[nm]	D )	[nm]	D )		D	CV
1	Ni	80	0.35	8.1	0.10	0.05	1600		Ni	84	0.37			◯	1.1	1.1		5.3
1	Ni	80	0.35	8.1	0.10	—	200		Ni	80	0.35	13.9	0.17	◯	1.0	1.0	1.7	1.7
2	Ni	80	0.35	8.1	0.10	—	400		Ni
3	Ni	80	0.35	8.1	0.10	—	1100		Ni
4	Ni		0.35	8.1	0.10	—	200		Ni	80	0.35	13.6	0.17	◯	1.0	1.0	1.7	1.7
5	Ni	80	0.35	8.1	0.10	—	400		Ni		0.36		0.25	◯	1.0	1.0	2.5	2.5
6	Ni		0.35	8.1	0.10	—	1100		Ni
7	Ni	80	0.35	8.1	0.10	0.05	1100		Ni	81	0.35		0.27	◯	1.0	1.0	2.7	2.7
2	Ni		0.31	7.9	0.13	0.05	1600		Ni		0.33			◯	1.1	1.1	5.1	4.8
3	Ni		0.38	8.3	0.05	0.05			Ni		0.40			◯	1.1	1.1	11.3	10.7
4	Ni	80	0.20	8.1	0.10	0.05	1600		Ni	84	0.23	38.0		◯	1.1	1.2	4.7	4.5
5	Ni	80	0.35	8.1	0.10	0.05	1400		Ni	82		30.1	0.37	◯	1.0	1.0	3.7	3.6
6	Ni	80	0.35	8.1	0.10	0.05	1600		Ni				0.43	Δ	1.3	1.3		4.2
7	Ni		0.35	8.1	0.10	0.05	1600		Ni	84	0.37	43.6		◯	1.1	1.1	5.4	5.1
8	Ni	80	0.35	8.1	0.10	0.50	1600		Ni	92				◯	1.2	1.2	5.7
9	Ni		0.35	8.1	0.10	1.50			Ni		0.48	46.4		Δ	1.5	1.4	5.7	3.9
indicates data missing or illegible when filed

	TABLE 2
	CV					CV
	D	( /		( /				D	( /		( /
		[nm]	D )	[nm]	D )	[g/L]	[° C.]			[nm]	D )	[nm]	D )		D	CV
10	Ag	1.0 × 103	0.37	11.4	1.1 × 10−2	0.05	1100		Ag	1.1 × 103	0.39	64.5	5.9 × 10	◯	1.1	1.1	5.7	5.1
11	Ag	1.0 × 10	0.37	11.4	1.1 × 10−2	0.05	1200		Ag	1.1 × 103	0.44	50.2	5.2 × 10	◯	1.0	1.2	4.4	4.4
12	Ag	1.0 × 103	0.37	11.4	1.1 × 10	0.05	1300		Ag	9.9 × 102	0.49	43.9	4.4 × 10−2	◯	1.0	1.3	3.9
13	Ag	2.6 × 103	0.37	13.2	5.1 × 10−3	0.05	1100		Ag	2.8 × 10	0.39	88.9	3.2 × 10	◯	1.1	1.1	6.7	6.3
14	Ag	4.5 × 103	0.3	10.7	2.4 × 10	0.05	1100		Ag	4.8 × 103	0.40	51.3	1.1 × 10	◯	1.1	1.1	4.8	4.5
15	Ag	109	0.34	11.2	0.10	0.05	1150		Ag	124	0.38	51.1	0.41	◯	1.1	1.1	4.6	4.0
16	Ag	109	0.34	11.2	0.10	0.05	1150		Ag	110	0.34	50.7	0.46	◯	1.0	1.0	4.5	4.5
8	Ag	109	0.34	11.2	0.10	—	400		Ag
9	Ag	109	0.34	11.2	0.10	—	400		Ag
17	Cu	2.4 × 103	0.35	9.9	4.1 × 10	0.05	1300		Cu	2.6 × 10	0.37	42.6	1.6 × 10−2	◯	1.1	1.1	4.3	4.0
18	Cu	5.0 × 102	0.34	12.6	2.5 × 10−2	0.05	1300		Cu	5.3 × 102	0.37		1.0 × 10	◯	1.1	1.1	4.3	4.0
10	Cu	5.0 × 102	0.34	12.6	2.5 × 10−2	—	400		Cu
11	Cu	5.0 × 10	0.34	12.6	2.5 × 10	—	400		Cu
indicates data missing or illegible when filed

As shown in Tables 1 and 2, in the Examples, metal powders with excellent crystallinity and narrow particle size distribution could be produced. On the other hand, in the Comparative Examples, a metal powder satisfying both “excellent crystallinity” and “narrow particle size distribution” could not be produced. As is clear from the results of Example 1, Comparative Example 7, and Example 5, metal powders with excellent crystallinity could be produced when the heat treatment temperature of the raw metal powder was (Tm−100)° C. or higher, and metal powders with particularly excellent crystallinity could be produced when the heat treatment temperature was Tm or higher. As is clear from the results of Example 1, Example 6, and Example 7, a metallic powder with a particularly narrow particle size distribution could be produced by cooling the metal powder dispersed in the gas phase to a temperature at which said metal powder does not sinter, and then recovering said metal powder. As is clear from the results of Example 1, Example 8, and Example 9, the lower the concentration of the raw metal powder dispersed in the gas phase, the concentration of the raw metal powder during heat treatment, and the concentration of the metal powder precursor during cooling, the narrower the particle size distribution of the resulting metal powder. As is clear from the results of Example 10, Example 11, and Example 12, the particle size distribution tended to become wider as the heat treatment temperature of the raw metal powder was further increased beyond Tm° C. As is clear from the results of Examples 15 and 16, metal powders with narrower particle size distributions could be produced in Example 16, where the raw metal powder suspended in liquid was dispersed into the gas phase, compared to Example 15, where the raw metal powder was dispersed in the dry state in the gas phase.

Claims

1. A method of producing a metal powder comprising:

a first process in which a raw metal powder, which is formed by reducing metal ions in a liquid phase, is dispersed into agas phase using a carrier gas,

a second process of forming metal powder precursors dispersed in the gas phase by heat-treating the raw metal powder dispersed in the gas phase at a temperature of (Tm−100)° C. or higher, wherein Tm° C. is a temperature at which the raw metal powder melts, and

a third process of producing the metal powder by cooling the metal powder precursors dispersed in the gas phase.

2. The method of producing the metal powder according to claim 1, wherein in the first process, the raw metal powder is dispersed in the gas phase at a concentration of 1.0 g/L or less using the carrier gas, in the second process, the raw metal powder dispersed in the gas phase at the above concentration is heat-treated at the temperature of (Tm−100)° C. or higher to form the metal powder precursors dispersed in the gas phase, and in the third process, the metal powder precursors dispersed in the gas phase at the aforementioned concentration are cooled to produce the metal powder.

3. The method of producing the metal powder according to claim 1, wherein in the second process, the raw metal powder dispersed in the gas phase is heat-treated at a temperature of Tm° C. or higher to form the metal powder precursors in a liquid state dispersed in the gas phase.

4. The method of producing the metal powder according to claim 1, wherein in the second process, the raw metal powder dispersed in the gas phase is heat-treated at a temperature of (Tm−100)° C. or higher and less than Tb° C., wherein Tm° C. is a melting temperature of the raw metal powder, and wherein Tb° C. is a vaporization temperature of the raw metal powder.

5. The method of producing the metal powder according to claim 1, wherein in the first process, the raw metal powder is dispersed in the gas phase using the carrier gas while the raw metal powder is suspended in a liquid.

6. The method of producing the metal powder according to claim 1, wherein in the third process, the metal powder precursors dispersed in the gas phase are cooled to produce the metal powder, and then the metal powder dispersed in the gas phase is cooled to a temperature at which the metal powder does not sinter.

7. The method of producing the metal powder according to claim 1, wherein a CV value of the raw metal powder is 0.40 or less, wherein the CV value is a ratio of a standard deviation to D50,

wherein the standard deviation is calculated from measured particle diameters of not less than 100 randomly selected particles observed by scanning electron microscopy, and wherein D50 is a cumulative number-based 50% particle diameter calculated from the measured particle diameters of the not less than 100 randomly selected particles observed by scanning electron microscopy.

8. The method of producing the metal powder according to claim 1, wherein a ratio of a crystallite diameter ratio of the metal powder to a crystallite diameter ratio of the raw metal powder is 4.0 or higher, wherein a crystallite diameter ratio is a ratio of a crystal diameter to D50,

wherein D50 is a cumulative number-based 50% particle diameter calculated from measured particle diameters of not less than 100 randomly selected particles observed by scanning electron, and wherein the crystallite diameter is a value calculated from Scherrer's formula using a value measured by X-ray diffraction.

9. The method of producing the metal powder according to claim 1, wherein a ratio of a crystallite diameter of the metal powder to a crystallite diameter of the raw metal powder is 4.0 or higher, wherein a crystallite diameter is a value calculated from Scherrer's formula using a value measured by X-ray diffraction.

10. The method of producing the metal powder according to claim 1, wherein a ratio of a D50 of the metal powder to a D50 of the raw metal powder is not less than 1.0 and not more than 2.0, and a ratio of a CV value of the metal powder to a CV value of the raw metal powder is preferably not less than 1.0 and not more than 2.0, wherein D50 is a cumulative number-based 50% particle diameter calculated from measured particle diameters of not less than 100 randomly selected particles observed by scanning electron microscopy, and wherein a CV value is a ratio of a standard deviation calculated from the measured particle diameters of the not less than 100 randomly selected particles observed by scanning electron microscopy to the D50.

11. The method of producing the metal powder according to claim 1, wherein a D50 of the raw metal powder is not less than 10 nm and not more than 15 μm, and a D50 of the metal powder is not less than 10 nm and not more than 15 μm, wherein D50 is a cumulative number-based 50% particle diameter calculated from measured particle diameters of not less than 100 randomly selected particles observed by scanning electron microscopy.

12. A metal powder with a CV value of 0.50 or less, a crystallite diameter of 30.0 nm or more, and a crystallite diameter ratio of 1.0×10-2 or more, wherein a CV value is a ratio of a standard deviation to D50, wherein the standard deviation is calculated from measured particle diameters of not less than 100 randomly selected particles observed by scanning electron microscopy, wherein D50 is a cumulative number-based 50% particle diameter, calculated from the measured particle diameters of the not less than 100 randomly selected particles observed by scanning electron microscopy, wherein crystallite diameter is a value calculated from Scherrer's formula using a value measured by X-ray diffraction, and wherein a crystallite diameter ratio is a ratio of a crystal diameter to the D50.

13. The metal powder according to claim 12, wherein the metal powder has a CV value of 0.40 or less, a D50 of not less than 10 nm and not more than 200 nm, a crystallite diameter ratio of 0.40 or more, and an aspect ratio of not less than 0.95 and not more than 1.0, wherein the aspect ratio is an average ratio of a short axis diameter to a long axis diameter of a rectangle that circumscribes each particle so that an area is minimized, measured for not less than 100 randomly selected particles by scanning electron microscope observation.

14. The method of producing the metal powder according to claim 1, wherein in the second process, the raw metal powder dispersed in the gas phase is heat-treated at a temperature of (Tm−100)° C. or higher and less than (Tb−900)° C. or less, wherein Tm° C. is a melting temperature of the raw metal powder, and wherein Tb° C. is a vaporization temperature of the raw metal powder.

15. The method of producing the metal powder according to claim 1, wherein in the first process, the raw metal powder having a CV value of 0.40 or less is produced by reducing metal ions in the liquid phase, and then the raw metal powder is dispersed in the gas phase at a concentration of 1.0 g/L or less using the carrier gas.

16. The metal powder according to claim 12, wherein the metal powder has a crystallite diameter ratio of 0.50 or more.