JP2021050384A - Manufacturing method of nickel fine particles - Google Patents

Abstract

To provide a manufacturing method of nickel fine particles less in coarse particles, adequate for the downsizing and increased capacity of multilayer ceramic capacitors, excellent in dispersibility when manufacturing a paste, and excellent in sinterability when formed into the paste.SOLUTION: A manufacturing method of nickel fine particles characterized in that nickel or a nickel compound is heated in a reducing flame to generate nickel fine particles, powder containing the nickel fine particles is dispersed in a liquid medium in the presence of a dispersant to prepare a dispersion, the dispersion is filtered to remove coarse particles in the powder from the dispersion, then, the liquid medium and the dispersant are removed from the dispersion.SELECTED DRAWING: None

Description

The present invention relates to a method for producing nickel fine particles.

Multilayer ceramic capacitors (MLCCs) are known. Metal fine particles are used for the internal electrodes of the MLCC. As metal fine particles for MLCC, metal powder having an average particle diameter of 1.0 μm or less and a substantially spherical shape is known.
In recent years, MLCCs are required to be further miniaturized and have a large capacity. As one means for realizing miniaturization and large capacity of MLCC, finer metal fine particles having an average particle size of, for example, 0.2 μm or less are required.

As a method for producing fine metal fine particles, for example, the methods described in Patent Documents 1 and 2 are known.
Patent Document 1 describes a high-temperature reduction atmosphere in a furnace by partially burning a fuel using oxygen or oxygen-enriched air in a method of forming a high-temperature reduction atmosphere in a furnace using a burner to obtain a metal from a metal compound. , And the oxygen ratio of the burner is adjusted, and the powdery metal compound is ejected into the high temperature reducing airflow generated by the burner to heat and reduce the metal compound, thereby controlling the particle size of the spherical shape. A method for producing an ultrafine metal powder is described, which comprises producing an ultrafine metal powder.
Patent Document 2 is characterized in that a metal powder as a raw material is blown into a reducing flame formed in a furnace by a burner, and the metal powder is melted in the flame to be in an evaporated state to obtain a spherical metal ultrafine powder. A method for producing ultrafine metal powder is described.

Japanese Patent No. 4304212 Japanese Patent No. 4304221

However, when nickel fine particles are produced by the production methods described in Patent Documents 1 and 2, the powder may contain coarse particles, although the amount is very small. In MLCCs that are required to be miniaturized and have a large capacity, coarse particles mixed in the internal electrodes may penetrate the ceramic layer. Therefore, there is a problem that the coarse particles cause a short circuit between the electrodes and a defective product may be generated.

On the other hand, when manufacturing electrodes and the like, a paste in which nickel fine particles are dispersed is used. In the production of the nickel fine particle paste, the nickel fine particles are required to have sufficient dispersibility in order to disperse the nickel fine particles in the medium. This is because if the dispersibility of the nickel fine particles is insufficient, it becomes difficult to produce the paste.
In addition, the paste in which nickel fine particles are dispersed is required to have few sintering defects such as cracks during sintering, that is, excellent sintering property.

The present invention provides a method for producing nickel fine particles, which has a small number of coarse particles, is suitable for miniaturization and large capacity of a multilayer ceramic capacitor, has excellent dispersibility in producing a paste, and has excellent sinterability when formed into a paste. The task is to do.

The present invention relates to the following [1] to [12].
[1] Nickel fine particles are generated by heating nickel or a nickel compound in a reducing flame, and the powder containing the nickel fine particles is dispersed in a liquid medium in the presence of a dispersant to prepare a dispersion liquid. A method for producing nickel fine particles, in which coarse particles in the powder are removed from the dispersion by filtering the dispersion, and then the liquid medium and the dispersant are removed from the dispersion.
[2] The method for producing nickel fine particles according to [1], wherein the dispersant has a polyoxyalkylene chain, a carboxyl group, and an amino group.
[3] The method for producing nickel fine particles according to [1] or [2], wherein the dispersant is removed by subjecting an acid treatment with an organic acid.
[4] The method for producing nickel fine particles according to [3], wherein the organic acid is at least one selected from the group consisting of citric acid, acetic acid, and tartaric acid.
[5] The method for producing nickel fine particles according to [3] or [4], wherein the treatment time of the acid treatment is 1 to 6 hours.
[6] The method for producing nickel fine particles according to any one of [3] to [5], wherein the nickel fine particles are subjected to an alkali treatment after the acid treatment.
[7] The method for producing nickel fine particles according to [6], wherein the treatment time for the alkali treatment is 1 to 6 hours.
[8] The method for producing nickel fine particles according to any one of [1] to [7], wherein the dispersant is removed by performing heat treatment in an oxygen-containing atmosphere of 200 ° C. or lower.
[9] The method for producing nickel fine particles according to [8], wherein the oxygen content of the oxygen-containing atmosphere is 0.1 to 50%.
[10] The method for producing nickel fine particles according to [8] or [9], wherein the heat treatment treatment temperature is 170 to 200 ° C.
[11] The method for producing nickel fine particles according to any one of [8] to [10], wherein the heat treatment treatment time is 10 to 60 minutes.
[12] The carbon concentration of the obtained nickel fine particles is 0.1% by mass or less, the oxygen concentration is 2% by mass or less, and there are no coarse particles in the powder of the nickel fine particles. Either manufacturing method.

According to the present invention, a method for producing nickel fine particles, which has a small number of coarse particles, is suitable for miniaturization and large capacity of a multilayer ceramic capacitor, has excellent dispersibility in producing a paste, and has excellent sinterability when formed into a paste. Is provided.

It is a schematic diagram which shows the structure of the manufacturing apparatus used in the manufacturing method of the nickel fine particle of one Embodiment. FIG. 2 is a sectional view taken along line II-II of a combustion burner included in the manufacturing apparatus of FIG. FIG. 3 is a sectional view taken along line III-III of the combustion burner shown in FIG. 6 is an SEM image of the nickel fine particles obtained in Example 1. It is an SEM image of the nickel fine particles obtained in Comparative Example 1.

The meanings of the following terms in the present specification are as described below.
"Nickel fine particles" refer to nickel particles having an average particle size of less than 300 nm.
"Coarse particles" refer to nickel particles having a length in the major axis direction of 400 nm or more.
The "oxygen ratio" refers to a value when the amount of oxygen required for complete combustion of fuel is defined as 1.
"~" Indicating a numerical range means that the numerical values described before and after the numerical range are included as the lower limit value and the upper limit value.

Hereinafter, the method for producing nickel fine particles according to the present embodiment will be described in detail with reference to the drawings. In addition, in the drawings used in the following description, in order to make the features easy to understand, the featured parts may be enlarged for convenience, and the dimensional ratios of each component may not be the same as the actual ones. Absent.

In the method for producing nickel fine particles according to the present embodiment, nickel fine particles are produced by heating nickel or a nickel compound in a reducing flame.
Next, the powder containing the nickel fine particles is dispersed in a liquid medium in the presence of a dispersant to prepare a dispersion, and the dispersion is filtered to remove coarse particles in the powder from the dispersion. ..
Then, the liquid medium and the dispersant are removed from the dispersion.

It can be said that the method for producing nickel fine particles according to the present embodiment includes the following first step, second step, and third step.
First step: A step of producing nickel fine particles by heating nickel or a nickel compound in a reducing flame.
Second step: The powder containing the nickel fine particles is dispersed in a liquid medium in the presence of a dispersant to prepare a dispersion liquid, and the dispersion liquid is filtered to obtain coarse particles in the powder from the dispersion liquid. The process of removing.
Third step: Next, a step of removing the liquid medium and the dispersant from the dispersion liquid.

(First step)
In the first step, nickel or a nickel compound is heated in a reducing flame to generate nickel fine particles, and the powder containing the nickel fine particles is recovered. In the production of nickel fine particles, nickel or a nickel compound can be heated to evaporate and reduced to obtain nickel fine particles having a desired average particle size.

FIG. 1 is a schematic view showing a configuration of a manufacturing apparatus used in the method for manufacturing nickel fine particles according to the present embodiment. The manufacturing apparatus 10 shown in FIG. 1 includes a flammable gas supply unit 11, a raw material feeder 12, a nickel compound supply unit 13, a flammable gas supply unit 14, a combustion burner 15, a water-cooled furnace 16, an inert gas supply source 17, and a plurality of them. The inert gas supply unit 18, the cooling gas supply unit 19, the bag filter 20, and the blower 21 are provided.

The flammable gas supply unit 11 is connected to the raw material feeder 12. The flammable gas supplied from the flammable gas supply unit 11 is supplied to the combustion burner 15 together with the raw material powder supplied from the raw material feeder 12. The flammable gas also functions as a carrier gas for transporting the raw material powder.
As the flammable gas, for example, natural gas, propane gas and the like can be used. The amount of carbon in the flammable gas may be appropriately adjusted in consideration of the carbon concentration on the surface of the desired nickel fine particles.

The raw material feeder 12 is connected to the flammable gas supply unit 11 and the combustion burner 15. The raw material feeder 12 supplies the raw material powder to the combustion burner 15.
The raw material powder is not particularly limited as long as it contains particles of nickel or a nickel compound. For example, metal particles of elemental nickel; metal particles of a nickel compound such as nickel oxide and nickel hydroxide can be used. Further, it is preferable that the nickel purity of the raw material powder is sufficiently high enough to obtain the desired nickel fine particles.

The nickel compound supply unit 13 is connected to the combustion burner 15. The nickel compound supply unit 13 supplies the nickel compound to the combustion burner 15. The nickel compound may be a nickel salt or a nickel oxide. Even if it is a nickel compound other than nickel oxide, the nickel compound is not particularly limited as long as it is a nickel compound such as nickel nitrate or nickel hydroxide that can produce nickel oxide by heating.
As the raw material powder, either nickel particles or nickel compound particles may be used alone, or these particles may be used in combination.

The combustion-supporting gas supply unit 14 is connected to the combustion burner 15. The combustion-supporting gas supply unit 14 supplies a combustion-supporting gas such as oxygen to the combustion burner 15.

The combustion burner 15 forms a reducing flame. FIG. 2 is a cross-sectional view taken along the line II-II of the combustion burner 15 included in the manufacturing apparatus 10 shown in FIG. FIG. 3 is a sectional view taken along line III-III of the combustion burner 15 shown in FIG.
As shown in FIG. 2, the combustion burner 15 comprises a raw material supply pipe 31 for supplying a raw material powder and a flammable gas, a primary flammable gas supply pipe 32 for supplying a flammable gas, and a flammable gas. It has a secondary combustion-supporting gas supply pipe 33 for supplying and a cooling pipe 34 for cooling the combustion burner 15. A spout 31a is formed at the tip of the raw material supply pipe 31, a spout 32a is formed at the tip of the primary flammable gas supply pipe 32, and a spout 33a is formed at the tip of the secondary flammable gas supply pipe 33. Is formed.

The primary flammable gas supply pipe 32 and the secondary flammable gas supply pipe 33 extend in a direction parallel to the central axis of the raw material supply pipe 31. The primary flammable gas supply pipe 32 is arranged outside the raw material supply pipe 31, and the secondary flammable gas supply pipe 33 is arranged outside the primary flammable gas supply pipe 32.
The secondary flammable gas supply pipe 33 is configured to eject the flammable gas toward a point on the extension line of the central axis of the raw material supply pipe 31.
The cooling pipe 34 is arranged outside the secondary flammable gas supply pipe 33. A cooling medium can flow through the cooling pipe 34. The cooling pipe 34 has a structure in which the cooling medium flows toward the tip of the combustion burner 15 and the cooling medium can be folded back at the tip of the combustion burner 15. This makes it possible to prevent the tip of the combustion burner 15 from becoming excessively hot.
The configuration of the combustion burner 15 is not limited to the example shown in FIG. The position, angle, number, etc. of the ejection holes of the combustion burner can be appropriately changed according to the properties of the desired nickel fine particles.

As shown in FIG. 1, the combustion burner 15 is arranged at the top (upper end) of the water cooling furnace 16. The tip of the combustion burner 15 is housed in the upper end of the water cooling furnace 16. As a result, the combustion burner 15 forms a reducing flame in the upper part of the water-cooled furnace 16.
Then, a reducing flame is formed inside the water-cooled furnace 16 by the combustion burner 15, and the nickel compound is heated to evaporate and reduced. This makes it possible to generate nickel fine particles.

The water cooling furnace 16 has a cylindrical shape and extends in the vertical direction. The inside of the water cooling furnace 16 is isolated from the outside air. A combustion burner 15 is attached to the top (upper end) of the water-cooled furnace 16 so that the tip of the combustion burner 15 faces downward.
The heating in the water-cooled furnace 16 may be performed only by the reducing flame of the combustion burner 15 into which the raw material powder is charged, and the water-cooled furnace 16 may further include other heating mechanisms in addition to the reducing flame of the combustion burner 15. Good. Further, the water-cooled furnace 16 may have a function of supplying a fluid that is not directly involved in heating for the purpose of adjusting the temperature inside the furnace, preventing adhesion to the furnace wall, and the like.
The water-cooled furnace 16 is a water-cooled heating furnace, but in other embodiments, the water-cooled furnace 16 may be replaced with a refractory-structured furnace.

The inert gas supply source 17 is connected to the plurality of inert gas supply units 18. The plurality of inert gas supply units 18 supply the inert gas into the water-cooled furnace 16 from the inert gas supply source 17 and eject the inert gas into the water-cooled furnace 16. By ejecting the inert gas supplied from the plurality of inert gas supply units 18, a swirling flow of the inert gas can be generated in the water cooling furnace 16.

The inert gas supply unit 18 is, for example, a port. The plurality of inert gas supply units 18 are provided on the side wall of the water cooling furnace 16. Further, the plurality of inert gas supply units 18 are arranged in the circumferential direction of the side wall of the water cooling furnace 16 and in the extending direction (vertical direction) of the water cooling furnace 16.

The cooling gas supply unit 19 cools the powder conveyed from the water cooling furnace 16. For cooling, an inert gas such as nitrogen gas or argon; air can be used.
The bag filter 20 separates powder and combustion exhaust gas. The bag filter 20 has a collection unit 20a at the lower end. Powder containing nickel fine particles can be recovered from the recovery unit 20a.
The blower 21 sucks the gas in the bag filter 20 and discharges the gas as exhaust gas.

When the manufacturing apparatus 10 is used in the first step, nickel or a nickel compound as a raw material is quantitatively supplied from the feeder and transported into the water-cooled furnace 16 by a flammable gas. Nickel and the like put into the reducing flame evaporate by heating and are reduced to nickel fine particles having a particle size smaller than that of the raw material metal compound. A swirling flow is generated in the water cooling furnace 16 due to the inert gas (nitrogen). The nickel particles have a desired particle size due to this swirling flow, and powder containing nickel fine particles is transported from the water cooling furnace 16.

In the method for producing nickel fine particles according to the present embodiment, the carbon concentration on the surface of the nickel fine particles can be controlled by adjusting the amount of carbon in the flammable gas supplied to the burner. By controlling the carbon concentration on the surface of the nickel fine particles, the sinterability such as the sintering temperature of the nickel fine particles can be adjusted.
Here, the "carbon amount" when adjusting the carbon amount in the combustible gas supplied to the combustion burner 15 is the ratio of the carbon element concentration contained in the fuel. This carbon content is, for example, a mixed gas of _{methane (CH 4} ): 1.175 m ³ / h and hydrogen (H ₂ ): 3.9 m ³ / h when the fuel is methane + 50% hydrogen. The amount of carbon at this time is given by the following equation {(1.175 × 1) / (1.175 × (1 + 4) + 3.9 × 2) × 100 = 8.6%}.

The oxygen ratio when heating the raw material powder can be, for example, in the range of 0.6 to 1.2. This may form a reducing flame. The amount of oxygen does not necessarily have to be less than the amount of complete combustion of the flammable gas, and the amount of oxygen may be excessive.

The conveyed powder containing nickel fine particles is cooled by the cooling gas supplied from the cooling gas supply unit 19. For example, air may be cooled with an inert gas such as nitrogen or argon. The exhaust gas temperature when the cooling gas is mixed is, for example, 200 to 700 ° C. Therefore, after cooling with the cooling gas, it is advisable to mix the cooling gas so that the temperature becomes 100 ° C. or lower.
In this way, the powder containing the nickel fine particles is conveyed together with the combustion exhaust gas and collected by the bag filter 20.

(Second step)
Next, the nickel fine particles obtained in the first step are dispersed in a liquid medium in the presence of a dispersant to prepare a dispersion liquid.
Since the nickel fine particles obtained in the first step have a very fine particle size, they have a strong cohesive force, and agglomerated particles are likely to be generated during the preparation of the dispersion liquid. Therefore, by dispersing the nickel fine particles in the liquid medium in the presence of the dispersant, the formation of agglomerated particles is reduced. By sufficiently reducing the formation of agglomerated particles, the loss due to the filtration of the nickel fine particles due to the removal of the agglomerated particles is reduced, and the yield of the finally obtained nickel fine particles is improved.

In preparing the dispersion, for example, the nickel fine particles obtained in the first step and the liquid medium are mixed in the presence of a dispersant. The specific mode of mixing is not particularly limited. For example, ultrasonic stirring, self-revolving mixer, mill stirring, stirrer stirring and the like can be used.

The liquid medium is not particularly limited as long as it is a compound in which nickel fine particles can be dispersed in combination with a dispersant. For example, an aqueous medium such as water can be mentioned. However, specific examples of the liquid medium are not limited to these examples.

The dispersant is not particularly limited as long as it is a compound capable of dispersing nickel fine particles in the liquid medium in combination with the liquid medium.
For example, a graft polymer having an ionic group in the main chain and a polyoxyalkylene chain in the graft chain can be mentioned.
Examples of the ionic group include a carboxyl group and an amino group. The dispersant is preferably a graft polymer having both a carboxyl group and an amino group as an ionic group in the main chain and a polyoxyalkylene chain in the graft chain.
The graft polymer having an ionic group in the main chain and a polyoxyalkylene chain in the graft chain may be a synthetic one or a commercially available product. Specific examples of commercially available graft polymers include the Mariarim series manufactured by NOF CORPORATION.

In the second step, after preparing the dispersion liquid, the coarse particles in the powder are removed from the dispersion liquid by filtering the dispersion liquid. The powder containing nickel fine particles obtained in the first step may contain coarse particles. Since the coarse particles may cause a short circuit between the electrodes of the MLCC, the coarse particles are removed in the second step to obtain a nickel fine particle dispersion liquid from which the coarse particles have been removed. The nickel fine particle dispersion mainly contains nickel fine particles, a liquid medium, and a dispersant. In the nickel fine particle dispersion liquid, nickel fine particles are dispersed in a liquid medium.

The mode of filtering the dispersion is not particularly limited. For example, suction filtration, filter press, wet classification and the like can be used. However, the specific embodiment of the filtration of the dispersion liquid is not limited to these examples.
When preparing the dispersion, agglomerated particles of nickel fine particles may be generated. At the same time as the removal of the coarse particles, the aggregated particles of the nickel fine particles may be removed from the dispersion liquid. According to the wet classification, the powder in the dispersion can be separated according to the particle size of the coarse particles, the agglomerated particles, and the nickel fine particles.

(Third step)
Next, the liquid medium and the dispersant are removed from the nickel fine particle dispersion obtained in the second step. By removing the dispersant, the dispersibility of the nickel fine particles in producing the paste is improved, and the paste can be produced. For example, if the dispersant used in the preparation of the dispersion liquid in the second step remains on the surface of the nickel fine particles, the dispersant for paste production used in the production of the paste adheres to the surface of the nickel fine particles. It's hard to do. Therefore, the dispersibility of the nickel fine particles in the production of the paste deteriorates, and the production of the paste may become difficult.
Therefore, in the third step, the dispersant used in the preparation of the dispersion liquid in the second step is removed, and the dispersibility of the nickel fine particles in producing the paste is improved.

The liquid medium can be removed, for example, by solid-liquid separation of the nickel fine particle dispersion. As a result, coarse particles, a liquid medium, and nickel fine particles from which aggregated particles have been removed, if necessary, can be obtained.
The mode of solid-liquid separation is not particularly limited. For example, a centrifuge, a filter press, or the like can be used. However, the specific mode of solid-liquid separation is not limited to these examples.

The dispersant can be removed, for example, by applying an acid treatment with an organic acid to the nickel fine particles. Generally, nickel fine particles cannot be subjected to excessive heat treatment at a high temperature because sintering proceeds. Therefore, even if heat treatment is performed, the treatment temperature is relatively low, and the dispersant cannot be sufficiently removed.
Therefore, the method for producing nickel fine particles according to the present embodiment has an advantage that removal of the dispersant can be promoted by applying an acid treatment with an organic acid to the nickel fine particles.

As the acid treatment, for example, the nickel fine particles obtained by solid-liquid separation and the organic acid are mixed and stirred, so that the nickel fine particles can be subjected to the acid treatment with the organic acid.
As the organic acid, at least one or more selected from the group consisting of citric acid, acetic acid and tartaric acid can be used. However, specific examples of organic acids are not limited to these examples. One type of organic acid may be used alone, or two or more types may be used in combination.
As the stirring method, for example, ultrasonic stirring, stirring with a stirring blade, or the like can be used.

The treatment time of the acid treatment varies depending on the concentration of the acid component, but is preferably 1 to 6 hours. When the stirring time is 1 hour or more, the dispersant tends to be sufficiently removed. When the stirring time is 6 hours or less, the dispersant is less likely to be concentrated due to the complexation of the nickel fine particles in the mixed solution containing the organic acid and the nickel fine particles, and the dispersant tends to be removed more sufficiently.

When the nickel fine particles are subjected to acid treatment with an organic acid, for example, a mixed solution containing the organic acid and the nickel fine particles is solid-liquid separated to obtain nickel fine particles. The mode of solid-liquid separation after the acid treatment is also not particularly limited. For example, a centrifuge, a filter press, or the like can be used. However, the specific embodiment of the solid-liquid separation after the acid treatment is not limited to these examples.

When acid treatment with an organic acid is applied to nickel fine particles, the nickel fine particles obtained by solid-liquid separation after the acid treatment are washed. For example, nickel fine particles and pure water are mixed and stirred to wash the nickel fine particles.
As a method for cleaning the nickel fine particles after the acid treatment, for example, ultrasonic stirring, stirring with a stirring blade, or the like can be used. The washing time can be, for example, 5 minutes. It is sufficient that the number of washings is about 1 to 3 times.
After cleaning is completed, for example, nickel fine particles can be obtained by solid-liquid separation of the mixed solution used for cleaning. The mode of solid-liquid separation after washing is also not particularly limited. For example, a centrifuge, a filter press, or the like can be used. However, the specific mode of solid-liquid separation after washing is not limited to these examples.

When the acid treatment with an organic acid is applied to the nickel fine particles, it is preferable to apply an alkali treatment to the nickel fine particles after the acid treatment for the purpose of removing the dispersant, neutralizing the fine particles and the like. As a result, the dispersant can be further removed from the nickel fine particles, and high quality nickel fine particles can be produced.
For example, the alkali treatment can be applied to the nickel fine particles by mixing the nickel fine particles after the acid treatment with a basic compound such as an aqueous ammonia solution and stirring the mixture.
As the basic compound, ammonia, methylamine, dimethylamine, trimethylamine and the like can be used. However, specific examples of basic compounds are not limited to these examples.
As the stirring method, for example, ultrasonic stirring, stirring with a stirring blade, or the like can be used.

The treatment time of the alkaline treatment varies depending on the concentration of the basic component, but is preferably 1 to 6 hours. When the stirring time is 1 hour or more, the dispersant tends to be sufficiently removed. When the stirring time is 6 hours or less, the dispersant is less likely to be concentrated due to the complexation of the nickel fine particles, and the dispersant tends to be removed more sufficiently.

When the nickel fine particles are subjected to alkali treatment with a basic compound, for example, a mixed solution containing the basic compound and the nickel fine particles is solid-liquid separated to obtain nickel fine particles. The mode of solid-liquid separation after the alkali treatment is also not particularly limited. For example, a centrifuge, a filter press, or the like can be used. However, the specific embodiment of the solid-liquid separation after the alkali treatment is not limited to these examples.

When the nickel fine particles after the acid treatment are subjected to the alkali treatment, the nickel fine particles obtained by the solid-liquid separation after the alkali treatment are washed with pure water.
As a method for cleaning the nickel fine particles after the alkali treatment, for example, ultrasonic stirring, stirring with a stirring blade, or the like can be used. For example, the washing time can be 5 minutes. It is sufficient that the number of washings is about 1 to 3 times.
After cleaning is completed, for example, nickel fine particles can be obtained by solid-liquid separation of the mixed solution used for cleaning. The mode of solid-liquid separation after washing is also not particularly limited. For example, a centrifuge, a filter press, or the like can be used. However, the specific mode of solid-liquid separation after washing is not limited to these examples.

In the method for producing nickel fine particles according to the present embodiment, it is preferable to remove the dispersant by performing heat treatment in an oxygen-containing atmosphere of 200 ° C. or lower. When the heat treatment is performed in an oxygen-containing atmosphere of 200 ° C. or lower, it tends to be easy to obtain high-quality nickel fine particles as a powder while suppressing oxidation of the nickel fine particles.
For example, the nickel fine particles after the acid treatment or the nickel fine particles after both the acid treatment and the alkali treatment can be heat-treated.

The oxygen content of the oxygen-containing atmosphere is preferably 0.01 to 50%, more preferably 0.1 to 20%. When the oxygen content of the oxygen-containing atmosphere is within the above numerical range, the removal of the dispersant tends to be more sufficient.

For the heat treatment, for example, a batch type heating furnace equipped with a heater can be used. Gas can be flowed into the heating furnace to create an oxygen-containing atmosphere in the heating furnace. The heating furnace may be provided with a mechanism for stirring the atmosphere in the furnace, or may be a continuous type provided with a transfer mechanism such as a conveyor.
When heating in the heating furnace, a flame such as a burner may be used, or the heated gas may flow into the heating furnace. When a burner or the like is used, the indirect heating method is preferable from the viewpoint of controlling the atmosphere of the heating furnace.

The heat treatment treatment temperature is preferably 170 to 200 ° C. When the treatment temperature of the heat treatment is 170 ° C. or higher, there is a tendency that a sufficient effect of removing the dispersant by the heat treatment can be obtained. When the heat treatment treatment temperature is 200 ° C. or lower, the dispersant can be removed while preventing the progress of sintering of the nickel fine particles.
The heat treatment treatment time is preferably 10 to 60 minutes. When the heat treatment treatment time is 10 minutes or more, the dispersant is sufficiently thermally decomposed, and there is a tendency that a sufficient effect of removing the dispersant by the heat treatment can be obtained. Even if the heat treatment treatment time exceeds 60 minutes, it is difficult to expect the effect of removing the dispersant any more.

The carbon concentration on the surface of the nickel fine particles obtained by the method for producing nickel fine particles according to the present embodiment is, for example, 0.1% by mass or less, and the oxygen concentration is 2% by mass or less. When the carbon concentration is not more than the upper limit value, it can be said that the removal of the dispersant is more sufficient. When the oxygen concentration is not more than the upper limit value, the nickel fine particles are not oxidized and the quality of the powder tends to be further improved. Therefore, when the carbon concentration on the surface of the nickel fine particles is 0.1% by mass or less and the oxygen concentration is 2% by mass or less, it can be determined that very high quality nickel fine particles are obtained.
There are no coarse particles in the nickel fine particle powder obtained by the method for producing nickel fine particles according to the present embodiment. The presence or absence of coarse particles can be determined by, for example, observation with an electron microscope such as SEM.

The average particle size of the nickel fine particles obtained by the method for producing nickel fine particles according to the present embodiment is, for example, 30 to 300 nm. The average particle size of the nickel fine particles can be measured by the method described in Examples.

(Action effect)
In the method for producing metal fine particles according to the present embodiment described above, the powder containing nickel fine particles is dispersed in a liquid medium in the presence of a dispersant. Therefore, even if the nickel fine particles have an average particle size of about 100 nm and have a strong cohesive force, a uniform dispersion liquid having few agglomerated particles can be obtained. As described above, since the dispersion liquid in which the formation of agglomerated particles is reduced is used, the loss due to the filtration of the nickel fine particles due to the removal of the agglomerated particles is reduced, and the yield of the nickel fine particles finally obtained is improved.
In addition, in the method for producing metal fine particles according to the present embodiment, in order to remove coarse particles in the powder from the dispersion liquid by filtering a uniform dispersion liquid, in MLCC where miniaturization and large capacity are required. , Coarse particles are less likely to penetrate the ceramic layer. As a result, short circuits between the electrodes due to coarse particles are unlikely to occur, and the occurrence of defective MLCCs is reduced.
Further, in the method for producing metal fine particles according to the present embodiment, since the dispersant is removed from the dispersion liquid, the dispersant is unlikely to adhere to the surface of the nickel fine particles and remain. Therefore, the dispersant for paste production used in the production of the paste tends to sufficiently adhere to the surface of the nickel fine particles. As a result, the dispersibility of the nickel fine particles in producing the paste is improved, and the paste can be produced. In addition, gas generation due to volatilization of the dispersant during sintering and sintering defects such as cracks are reduced, and the sintering property when nickel fine particles are used as a paste is improved.
From the above, according to the method for producing nickel fine particles according to the present embodiment, there are few defective products when used as a raw material for the internal electrode of MLCC, the dispersibility of the nickel fine particles in producing a paste is excellent, and High-quality nickel fine particles suitable for use as internal electrodes of MLCC can be produced because they are excellent in sinterability when nickel fine particles are used as a paste.

Although some embodiments of the present invention have been described above, the present invention is not limited to such specific embodiments. In addition, the present invention may be added, omitted, replaced, or otherwise modified within the scope of the gist of the present invention described in the claims.
Further, as the manufacturing apparatus used in the method for producing metal fine particles of the above-described embodiment, the manufacturing apparatus 10 having the configurations shown in FIGS. 1 to 3 has been described as an example, but the configurations of the burner and the furnace are not limited to this example.

Hereinafter, the present invention will be specifically described with reference to Examples. However, the present invention is not limited to the following description.

<Measurement method>
(Carbon concentration)
The carbon concentration on the surface of the nickel fine particles was measured using a carbon / sulfur analyzer (manufactured by HORIBA, Ltd .: EMIA-920V).

(Oxygen concentration)
The oxygen concentration on the surface of the nickel fine particles was measured using an oxygen / nitrogen analyzer (manufactured by LECO: TC-600 type).

(Specific surface area: BET)
The BET of the nickel fine particles was measured using a specific surface area meter (manufactured by Mountech: Macsorb HM model-1201).

(Average particle size)
The average particle size (nm) of the nickel fine particles was calculated from the following formula (1) based on ^{the above-mentioned BET measurement value (m 2 / g).}
Average particle size (nm) = 6 / (BET × ρ) × 10 ⁹ ... (1)
However, in the formula (1), "BET" is the BET (m ² / g) of the nickel fine particles measured using a specific surface area meter, and "ρ" is the density of nickel (g / m ³ ). ..

<Manufacturing example 1>
As the raw material powder, nickel oxide powder having an average particle size of 10 μm was used, and the manufacturing apparatus 10 was used.
Methane gas was used as the flammable gas, oxygen gas was used as the flammable gas, and a reducing flame was formed by the burner flame 15. The nickel oxide powder transported together with the flammable gas was heated with a reducing flame to evaporate it in the reducing flame, and nickel fine particles having a size of submicron or less were produced. Next, the powder containing the nickel fine particles was recovered from the recovery unit 20a of the bag filter 20.
Table 1 shows the combustion conditions for the formation of nickel fine particles in the first step.

Next, a powder containing nickel fine particles: 4 g, a dispersant A: 40 g, and pure water: 360 ml were mixed for 30 minutes using an ultrasonic homogenizer to prepare a dispersion. Dispersant A is "Marialim HKM-50A" manufactured by NOF CORPORATION. Further, the dispersant A is a graft polymer having a carboxyl group and an amino group as ionic groups in the main chain and a polyoxyalkylene chain in the graft chain.
Next, the dispersion was suction-filtered to pass through a 0.45 μm cartridge filter to obtain a nickel fine particle dispersion from which aggregated particles and coarse particles were removed. The nickel fine particle dispersion was solid-liquid separated by a centrifuge to obtain nickel fine particles before removing the dispersant.

<Manufacturing Examples 2 and 3>
As shown in Table 2, nickel fine particles before removing the dispersant were obtained in the same manner as in Production Example 1 except that the dispersant A was changed to the following dispersant B and dispersant C, respectively.
Dispersant B: A graft polymer having a carboxyl group as an ionic group in the main chain and a polyoxyalkylene chain in the graft chain.
Dispersant C: A graft polymer having an amino group as an ionic group in the main chain and a polyoxyalkylene chain in the graft chain.

Table 2 shows the results of the yields of the nickel fine particles before removing the dispersant in Production Examples 1 to 3.

As shown in Table 2, when dispersant A, which is a graft polymer having a carboxyl group and an amino group as ionic groups in the main chain and a polyoxyalkylene chain in the graft chain, was used, the nickel fine particles were uniformly dispersed. It was confirmed that the dispersion was obtained and that the dispersion could be filtered by a 0.45 μm cartridge filter.

<Manufacturing example 4>
A dispersion was not prepared from the nickel fine particles recovered from the manufacturing apparatus 10, and nickel fine particles were manufactured without removing the coarse particles in the powder.

<Example 1>
A 2 mass% citric acid aqueous solution: 50 ml was added to 4 g of nickel fine particles before removing the dispersant obtained in Production Example 1, and acid treatment was performed while stirring with an ultrasonic bath. The treatment time of the acid treatment was 6 hours. Solid-liquid separation was performed with a centrifuge, water was added to the obtained nickel fine particles, washed, and then solid-liquid separation was performed again with a centrifuge to obtain nickel fine particles after acid treatment.
Next, 50 ml of a 2% by mass aqueous ammonia solution was added to the nickel fine particles after the acid treatment, and the nickel particles were treated with an alkali while stirring with an ultrasonic bath. The treatment time for the alkali treatment was 6 hours. Then, solid-liquid separation was performed with a centrifuge, water was added to the obtained nickel fine particles, and the mixture was washed, and then solid-liquid separation was performed again with a centrifuge to obtain nickel fine particles.
Next, the nickel fine particles after the alkali treatment were heat-treated. As the oxygen-containing atmosphere, a gas obtained by mixing 20% of oxygen gas with nitrogen gas was used. The heat treatment temperature was 170 ° C. and the treatment time was 30 minutes.
The carbon concentration and oxygen concentration of the nickel fine particles after the heat treatment were measured.

<Examples 2 to 17>
Nickel fine particles were produced in the same manner as in Example 1 except that the conditions of the acid treatment treatment time, the alkali treatment treatment time, the heat treatment temperature and time, and the oxygen concentration of the oxygen-containing atmosphere were changed as shown in Table 3. did.
In Example 10, nickel fine particles were produced in the same manner as in Example 1 except that the alkali treatment with ammonia was not carried out.
Table 3 shows the measurement results of the carbon concentration and the oxygen concentration of the nickel fine particles obtained in Examples 2 to 17.

<Comparative example 1>
The nickel fine particles themselves obtained in Production Example 4 were used as the nickel fine particles of Comparative Example 1.

FIG. 4 is an SEM image of the nickel fine particles obtained in Example 1. FIG. 5 is an SEM image of the nickel fine particles obtained in Comparative Example 1. The magnification of the SEM images in FIGS. 4 and 5 is 50,000 times.
The length was measured by observing 20 fields of view (about 100,000 particles) in the SEM image at a magnification of 10,000, counting particles having a size of 400 nm or more, and measuring the frequency of coarse particles.
Table 4 shows the measurement results of the BET, average particle size, carbon concentration, oxygen concentration and the number of coarse particles of the nickel fine particles of Example 1 and Comparative Example 1 by image analysis.

As shown in Table 4, the BET value and carbon concentration of the nickel fine particles of Example 1 after removing the dispersant are those of the nickel fine particles (nickel fine particles of Comparative Example 1) before being mixed with the dispersant to prepare a dispersion liquid. It was at the same level as the BET value and carbon concentration. From this result, it was judged that the dispersant was sufficiently removed from the nickel fine particles of Example 1. Since the oxygen concentration was also at the same level in Example 1 and Comparative Example 1, the nickel fine particles of Example 1 did not undergo oxidation of the nickel fine particles even after the heat treatment. It was judged that the quality of the powder was very high.
Therefore, the nickel fine particles of Example 1 are expected to be excellent in dispersibility when producing a paste and excellent in sinterability when made into a paste.
Further, according to the nickel fine particles of Example 1, the number of coarse particles was 0. Therefore, according to the nickel fine particles of Example 1, it is expected that the multilayer ceramic capacitor is suitable for miniaturization and large capacity.

As shown in Table 3, although the alkali treatment was not performed in Example 10, the dispersant could be largely removed by the acid treatment and the heat treatment. However, in Example 10, the carbon concentration on the surface of the nickel fine particles was relatively high as compared with the results of other Examples in which the alkali treatment was performed (for example, Example 1 and the like; the same applies hereinafter). From this result, it was considered that when the alkali treatment was applied, the carbon concentration on the surface of the nickel fine particles became relatively low, and the dispersant could be removed more sufficiently.

In Example 11, the treatment time of the acid treatment is shorter than that of the other examples. In Example 11, the carbon concentration on the surface of the nickel fine particles was relatively high as compared with the results of other Examples in which the treatment time of the acid treatment was relatively long. From this result, it was considered that when the treatment time of the acid treatment was relatively long, the carbon concentration on the surface of the nickel fine particles was relatively low, and the removal of the dispersant tended to be more sufficient.

In Example 12, the treatment time of the acid treatment is longer than that of the other examples, but the carbon concentration on the surface of the nickel fine particles is compared with the result of the other example in which the treatment time of the acid treatment is relatively short. But it was relatively expensive. From this result, it was considered that even if the treatment time of the acid treatment was excessively lengthened, the effect of removing the dispersant could not be further improved.

In Example 13, the treatment time of the alkali treatment is longer than that of the other examples, but the carbon concentration on the surface of the nickel fine particles is compared with the result of the other example in which the treatment time of the alkali treatment is relatively short. But it was relatively expensive. From this result, it was considered that the effect of removing the dispersant could not be expected any more even if the treatment time of the alkali treatment was excessively lengthened.

In Example 14, the heat treatment treatment time is shorter than in other examples. In Example 14, the carbon concentration on the surface of the nickel fine particles was relatively high as compared with the results of other Examples in which the heat treatment treatment time was relatively long. From this result, it was considered that when the heat treatment treatment time was relatively long, the carbon concentration on the surface of the nickel fine particles was relatively low, and the removal of the dispersant tended to be more sufficient.

In Example 15, the treatment temperature of the heat treatment is lower than that of other Examples. In Example 15, the carbon concentration on the surface of the nickel fine particles was relatively high as compared with the results of other Examples in which the heat treatment treatment temperature was relatively high. From this result, it was considered that when the treatment temperature of the heat treatment was relatively high, the carbon concentration on the surface of the nickel fine particles was relatively low, and the removal of the dispersant tended to be more sufficient.

In Example 16, the heat treatment treatment temperature is higher than in other examples. In Example 16, the oxygen concentration on the surface of the nickel fine particles was relatively high as compared with the results of other Examples in which the heat treatment treatment temperature was relatively low. From this result, it was considered that when the heat treatment temperature was relatively low, the oxygen concentration of the nickel fine particles was low, and the oxidation of the nickel fine particles could be suppressed.

In Example 17, the oxygen concentration in the atmosphere during the heat treatment is lower than that in the other examples. In Example 17, the carbon concentration on the surface of the nickel fine particles was relatively high as compared with the results of other Examples in which the heat treatment treatment temperature was relatively high. From this result, when the oxygen concentration in the atmosphere during the heat treatment is relatively high, the amount of oxygen required to remove the dispersant is sufficient, the carbon concentration on the surface of the nickel fine particles is relatively low, and the dispersant is used. Was thought to be able to be removed even more sufficiently.

According to the method for producing nickel fine particles of the present invention, nickel fine particles having few coarse particles, suitable for miniaturization and large capacity of a multilayer ceramic capacitor, and excellent dispersibility and sinterability when made into a paste can be obtained.

10 ... Manufacturing equipment, 11 ... Combustible gas supply unit, 12 ... Raw material feeder, 13 ... Nickel compound supply unit, 14 ... Combustible gas supply unit, 15 ... Combustion burner, 16 ... Water-cooled furnace, 17 ... Inert gas supply Source, 18 ... Multiple inert gas supply units, 19 ... Cooling gas supply unit, 20 ... Bug filter, 21 ... Blower

Claims (12)

By heating nickel or a nickel compound in a reducing flame, nickel fine particles are produced.
The powder containing the nickel fine particles is dispersed in a liquid medium in the presence of a dispersant to prepare a dispersion, and the dispersion is filtered to remove coarse particles in the powder from the dispersion.
Next, a method for producing nickel fine particles, which removes the liquid medium and the dispersant from the dispersion. The method for producing nickel fine particles according to claim 1, wherein the dispersant has a polyoxyalkylene chain, a carboxyl group, and an amino group. The method for producing nickel fine particles according to claim 1 or 2, wherein the dispersant is removed by subjecting an acid treatment with an organic acid. The method for producing nickel fine particles according to claim 3, wherein the organic acid is at least one selected from the group consisting of citric acid, acetic acid, and tartaric acid. The method for producing nickel fine particles according to claim 3 or 4, wherein the treatment time of the acid treatment is 1 to 6 hours. The method for producing nickel fine particles according to any one of claims 3 to 5, wherein the nickel fine particles are subjected to an alkali treatment after the acid treatment. The method for producing nickel fine particles according to claim 6, wherein the treatment time of the alkali treatment is 1 to 6 hours. The method for producing nickel fine particles according to any one of claims 1 to 7, wherein the dispersant is removed by performing heat treatment in an oxygen-containing atmosphere of 200 ° C. or lower. The method for producing nickel fine particles according to claim 8, wherein the oxygen content of the oxygen-containing atmosphere is 0.1 to 50%. The method for producing nickel fine particles according to claim 8 or 9, wherein the heat treatment treatment temperature is 170 to 200 ° C. The method for producing nickel fine particles according to any one of claims 8 to 10, wherein the heat treatment treatment time is 10 to 60 minutes. The carbon concentration of the obtained nickel fine particles is 0.1% by mass or less, the oxygen concentration is 2% by mass or less, and there are no coarse particles in the powder of the nickel fine particles, according to any one of claims 1 to 11. The manufacturing method described.

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