JP7596681B2 - Nickel particles, surface treatment method for nickel particles, and method for producing nickel powder - Google Patents

Description

The present invention relates to nickel particles, a method for surface treatment of nickel particles, and a method for producing nickel powder.

Nickel particles are used as a material for conductive pastes to make thick-film conductors, and are used to form electrical circuits and as electrodes for multilayer ceramic components such as multilayer ceramic capacitors (MLCCs) and multilayer ceramic substrates.

The MLCC in which this electrode is used is manufactured in the following manner, for example when nickel particles are used as the metal powder.

First, a conductive paste is obtained by kneading nickel particles, a resin such as ethyl cellulose, and an organic solvent such as terpineol, and the like. The conductive paste is then screen-printed onto a dielectric green sheet (ceramic green sheet) with a thickness of 10 μm or less, and then dried to produce a nickel coating for the internal electrodes.

Next, the printed nickel coating for the internal electrodes and the dielectric green sheets are layered alternately and pressed together to create a laminate.

The laminate thus produced is cut to a specified size, and a debindering process is carried out to burn off the resins such as ethyl cellulose used as the organic binder. The dielectric and the internal electrodes (nickel films) are then sintered by high-temperature firing at around 1300°C, resulting in a ceramic body in which the dielectric layers and the internal electrode layers are laminated together. External electrodes are then attached to this ceramic body to form a multilayer ceramic capacitor.

The binder removal process for the laminate is carried out in an atmosphere containing a very small amount of oxygen to prevent the nickel particles from oxidizing.

Nickel paste used in the internal electrodes of MLCCs is generally produced by kneading nickel powder in a vehicle, and contains many nickel powder agglomerates. The nickel powder production process usually includes a drying step at the final stage, regardless of the nickel powder production method (gas phase method or liquid phase method). Because the drying process promotes the agglomeration of nickel particles, the resulting nickel powder generally contains coarse particles of agglomerates that arise during drying.

In order to achieve a small size and large capacity in recent MLCCs, there has been a demand to increase the number of laminated ceramic green sheets with internal electrode layers from several hundred to around 1,000 layers. For this reason, there has been research into thinning the thickness of the internal electrode layers from the conventional level of several microns to the sub-micron level, and in conjunction with this, progress has been made in reducing the particle size of the nickel powder used as the electrode material for the internal electrodes.

However, the smaller the particle size, the larger the surface area of the nickel powder, and the greater the surface energy, making it easier to form agglomerates. In addition, metal powders such as nickel powder have poor dispersibility, and if agglomerates are present, the nickel powder may penetrate the ceramic sheet layer when sintered during the firing process in the manufacture of MLCCs, resulting in defective products with short-circuited electrodes. Even if the nickel powder does not penetrate the ceramic sheet layer, the distance between the electrodes in the MLCC may become shorter, causing partial current concentration, and this current concentration causes the deterioration of the life of the multilayer ceramic capacitor. Thus, in MLCCs, it is important to produce nickel paste with fewer coarse particles, including agglomerates, and to obtain internal electrodes with a smooth surface without unevenness. In addition, there is a concern that the presence of agglomerates of nickel particles may cause product defects, so it is desirable to improve the surface condition of the nickel particles to prevent agglomerates from occurring.

Patent Document 1 discloses a technique for oxidizing the surface of nickel particles, and discloses the technical details of adding nickel powder produced by a liquid phase method to pure water to form a slurry, and then oxidizing it with hydrogen peroxide. However, surface oxidation treatment using hydrogen peroxide must be carried out in an aqueous system, and is not suitable when using a non-aqueous organic compound as the surface treatment agent.

Japanese Patent Application Publication No. 11-343501

When nickel powder is stored, it may be oxidized by oxygen in the air over time, forming nickel hydroxide on the surface. Coarse nickel particles may occur when adjacent nickel particles are firmly bound together by the nickel hydroxide on the surface. For this reason, it is important to suppress the oxidation of nickel powder during storage, particularly for nickel powder used in MLCCs, to prevent problems caused by the generation of coarse particles due to oxidation during storage.

The present invention has been proposed in light of these circumstances, and aims to provide nickel particles that suppress the generation of coarse particles due to oxidation during storage, a surface treatment method for nickel particles, and a method for producing nickel powder.

The inventors of the present invention have conducted extensive research to solve the above-mentioned problems. As a result, they discovered that by coating nickel particles with a Lewis base compound, it is possible to suppress the aggregation of nickel particles and thereby suppress the generation of coarse particles, and thus completed the present invention.

To solve the above problems, the Lewis base-containing nickel particles of the present invention have a nitrogen-containing Lewis base present on the surface of the nickel particles, and the mass ratio of the Lewis base to the nickel particles is 0.16 to 3.0:100.

The Lewis base may be coordinated to the surface of the nickel particles.

The Lewis base may include a compound represented by the following formula (1):

In formula (1), R ¹ represents an alkyl group having 1 to 18 carbon atoms or an alkenyl group having 1 to 18 carbon atoms, R ² represents an alkylene group having 1 to 6 carbon atoms, and R ³ and R ⁴ represent H or CH ₃ .

The Lewis base may include one or more selected from N-oleoyl sarcosine, N-lauroyl sarcosine, and myristoylmethyl-β-alanine.

The Lewis base-containing nickel particles of the present invention may have a number average particle size of 0.03 μm to 0.4 μm, a content of particles with a particle size exceeding 0.8 μm of 400 mass ppm or less, and a content of particles with a particle size exceeding 1.2 μm of 200 mass ppm or less.

In addition, in order to solve the above problems, the surface treatment method for nickel particles of the present invention includes a mixing step of mixing nickel particles with a nitrogen-containing Lewis base.

In addition, to solve the above problem, the method for producing nickel powder of the present invention includes a drying step for drying the surface-treated nickel particles after the above mixing step.

The present invention provides nickel particles that suppress the generation of coarse particles due to oxidation during storage, a surface treatment method for nickel particles, and a method for producing nickel powder.

FIG. 1 is a flow chart showing a surface treatment method for nickel particles and a method for producing nickel powder according to the present invention as a series of steps. 1 is a SEM photograph of nickel particles of Example 1 and Comparative Example 2.

The following describes in detail the embodiments of the present invention with reference to the drawings. Figure 1 is a flow diagram showing the surface treatment method for nickel particles and the manufacturing method for nickel powder of the present invention as a series of steps. However, the present invention is not limited to the following embodiments.

[Surface treatment method for nickel particles]
Nickel particles to be surface treated
As nickel particles to be surface-treated, various nickel powders can be used regardless of the manufacturing method such as wet method or dry method. For example, nickel powders produced by so-called dry methods such as CVD method, evaporation quenching method, and hydrogen reduction method using nickel salt or nickel hydroxide can be used as nickel particles to be treated. Nickel powders produced by so-called wet methods such as wet reduction method using a reducing agent such as hydrazine for a nickel salt solution can be used as nickel particles to be treated. Among them, nickel powders produced by so-called wet methods such as wet reduction method are suitable as materials for MLCC because they are spherical and have small variation in particle size.

<Mixing process>
The method for treating the surface of nickel particles includes a mixing step of mixing nickel particles with a nitrogen-containing Lewis base. By the mixing step, the surfaces of the nickel particles can be surface-treated with the nitrogen-containing Lewis base.

(Nickel slurry)
The method of surface treating nickel particles using the Lewis base includes a method of contacting nickel particles with Lewis base by wet mixing using various solvents containing alcohol as a medium. That is, nickel slurry in which nickel particles are dispersed in various solvents containing alcohol and Lewis base containing nitrogen are added and mixed in various solvents containing alcohol to form a slurry, thereby uniformly treating the particle surface of nickel particles.

In the mixing step, the nickel particles to be surface-treated may be added to and mixed with a solution in which the Lewis base has been dissolved, or the Lewis base may be added to and mixed with a nickel slurry. However, from the viewpoint of effectively and uniformly treating the surfaces of fine nickel particles, it is preferable to dissolve the Lewis base in advance in various solvents, including alcohol.

Specifically, the nickel slurry is stirred, the Lewis base solution in which the Lewis base is dissolved is dropped into the nickel slurry and mixed, and then the mixed solution is stirred to perform the surface treatment.

For example, a Lewis base solution can be prepared so that the Lewis base content is 0.1 to 30% by mass, and the Lewis base solution can be dripped at a rate of 2 ml/min into nickel slurry being stirred at a peripheral speed of 5 to 10 m/sec. Agitator blades in the shape of a soft cross, paddle cross, four-blade inclined paddle, etc. can be used to stir the nickel slurry.

The temperature of the mixed solution of nickel particles and Lewis base during surface treatment is preferably 20 to 50°C. If the temperature of the mixed solution is lower than 20°C, the rate at which the Lewis base adheres to the nickel particles decreases, and the stirring time of the mixed solution during surface treatment may become longer. Furthermore, if the temperature of the mixed solution is higher than 50°C, the evaporation of the solvent is accelerated, which may make the surface treatment difficult.

Furthermore, the stirring time of the mixed solution in the surface treatment is preferably 0.5 to 24 hours. If the stirring time is less than 0.5 hours, the amount of Lewis base attached to the nickel particles will be small, and the effect of suppressing the generation of coarse particles due to the surface treatment may not be sufficiently obtained. Furthermore, even if the stirring time is longer than 24 hours, the amount of Lewis base attached will hardly increase, and if the treatment time is longer, the surface treatment time of the nickel particles will also be longer, which may increase the cost. The mixed solution may be stirred at a peripheral speed of 5 to 10 m/sec, as in the mixing process.

<Nitrogen-containing Lewis bases>
A nitrogen-containing Lewis base is used as the compound used for the surface treatment. It is believed that the surface treatment method of the present invention allows the lone electron pair of the nitrogen-containing Lewis base to donate electrons to the surface of the nickel particles to form a coordinate bond, forming a structure. The nitrogen-containing Lewis base compound is capable of efficiently adsorbing to the surface of the nickel particles and forming a coordinate bond, and can suppress the generation of coarse particles due to the aggregation of the nickel particles.

The amount of the nitrogen-containing Lewis base added is such that the mass ratio of the Lewis base to the nickel particles is 0.16 to 3.0:100 in the state of the Lewis base-containing nickel particles after surface treatment. Specifically, based on experience, it is preferable to add the Lewis base in the range of 0.2 mass% to 4.0 mass% relative to 100 mass% of the nickel particles. By adding an amount of the nitrogen-derived Lewis base of 0.2 mass% or more relative to 100 mass% of the nickel particles, the amount of adsorption to the nickel particles can be increased, and as a result, the dispersibility of the nickel microparticles can be further improved. On the other hand, by adding an amount of the nitrogen-derived Lewis base of 4.0 mass% or less, it is possible to prevent a change in the viscosity of the nickel paste due to an increase in the amount of the nitrogen-derived Lewis base.

Examples of nitrogen-containing Lewis bases include compounds represented by the following formula (1):

In formula (1), R ¹ represents an alkyl group having 1 to 18 carbon atoms or an alkenyl group having 1 to 18 carbon atoms, R ² represents an alkylene group having 1 to 6 carbon atoms, and R ³ and R ⁴ represent H or CH ₃ .

In particular, one or more types selected from N-oleoyl sarcosine, N-lauroyl sarcosine, and myristoylmethyl-β-alanine can be used as the Lewis base that is soluble in a solvent capable of forming a slurry of nickel particles. The structures of N-oleoyl sarcosine, N-lauroyl sarcosine, and myristoylmethyl-β-alanine are shown in the following general formulas (2) to (4).

(solvent)
The solvent for dispersing nickel particles to be surface-treated to form a slurry and the solvent for dissolving the nitrogen-containing Lewis base are not particularly limited as long as they can disperse nickel particles and dissolve the Lewis base.Specific examples include terpineol, dihydroterpineol, dihydroterpineol acetate, isobornyl propionate, isobornyl isobutyrate, mineral spirits, No. 0 solvent, butyl carbitol, isobutyl acetate, methyl ethyl ketone, cyclohexane, hexane, ethanol, nonane, nonanol, decanol, etc.Furthermore, aliphatic hydrocarbons, aromatic hydrocarbons, etc. can also be used, and specific examples include dimethyloctane, ethylmethylcyclohexane, methylpropylcycloheptane, trimethylhexane, butylcyclohexane, tridecane, tetradecane, methylnonane, ethylmethylheptane, trimethyldecane, pentylcyclohexane, decane, undecane, dodecane, toluene, etc.These organic solvents can be used alone or in combination of two or more.

[Method of manufacturing nickel powder]
Drying process
Next, a method for producing nickel powder will be described. This method includes a drying step of drying the surface-treated nickel particles after the mixing step described in the above-mentioned method for surface-treating nickel particles.

For example, nickel powder can be obtained by drying the surface-treated nickel particles at 50 to 300°C, preferably 80 to 150°C, using a general-purpose drying device such as an inert gas atmosphere dryer or vacuum dryer to prevent the particles from oxidizing.

If necessary, a washing and filtering process described below can be carried out to produce a nickel powder hydrous cake, and the adhering water in the cake can be replaced with a low-temperature volatile organic solvent such as ethanol, and then the cake can be dried in the inert gas atmosphere dryer or vacuum dryer described above to weaken the drying aggregation between nickel particles that occurs during drying due to the large surface tension of water.

(Washing and filtering process)
In the method for producing nickel powder of the present invention, a washing and filtering step may be performed before the drying step. Specifically, after carrying out the above-mentioned surface treatment method for nickel particles, a slurry in which nickel particles surface-treated with a Lewis base are dispersed is obtained, and this slurry is first filtered under reduced pressure using a suction filter. Then, the nickel particles remaining on the filter paper after filtration are mixed with the above-mentioned solvent and washed by stirring at a peripheral speed of 2 to 5 m/sec for 0.5 to 3 hours. By repeating this washing and filtering process, unreacted Lewis bases and impurities can be removed from the nickel particles. Furthermore, infrared spectroscopy of the obtained filtrate is performed to confirm that no unreacted Lewis bases are detected, and the washing process can be terminated.

A general-purpose solid-liquid separation device can be used to filter the nickel particles, and specific examples include, but are not limited to, a Denver filter, a filter press, a centrifuge, a decanter, etc. After the washing operation, the solvent may be removed by blowing it off from the nickel particles using a spray dryer or the like.

(Crushing and classification process)
In addition, in the manufacturing method of nickel powder of the present invention, a crushing/classification step may be performed after the drying step. For example, when nickel particles are weakly aggregated due to dry aggregation in the drying step, a crushing step can be provided in the middle or at the end of the manufacturing process to crush the nickel particles. For example, dry crushing methods such as spiral jet crushing and counter jet mill crushing, wet crushing methods such as high-pressure fluid collision crushing, and other general-purpose crushing methods can be applied.

The nickel powder may further be classified by any method, such as dry classification, wet classification, or sieving, to remove extremely coarse particles.

[Lewis base-containing nickel particles]
Next, the Lewis base-containing nickel particles of the present invention will be described. The nickel particles can be obtained, for example, by the above-mentioned surface treatment method of nickel particles of the present invention or the manufacturing method of nickel powder. However, the manufacturing method is not limited to these.

In the Lewis base-containing nickel particles of the present invention, a nitrogen-containing Lewis base is present on the surface of the nickel particles, and the mass ratio of the Lewis base to the nickel particles is 0.16 to 3.0:100. If the mass ratio is less than 0.16:100, there is a risk that oxidizable regions will remain on the surface of the nickel particles, which will generate nickel hydroxide over time and result in the generation of coarse particles. In addition, it is difficult to have a Lewis base present at a mass ratio exceeding 3.0:100, and a mass ratio of 3.0:100 is considered to be the upper limit for the amount of Lewis base present.

It is believed that the lone electron pair of the nitrogen-containing Lewis base donates an electron to the surface of the nickel particle, forming a coordinate bond, which causes the Lewis base to coordinate to the surface of the nickel particle. Nitrogen-containing Lewis base compounds can efficiently adsorb to the surface of the nickel particle and form coordinate bonds, preventing the generation of coarse particles due to the aggregation of nickel particles.

Examples of nitrogen-containing Lewis bases include compounds represented by the following formula (1):

In formula (1), R ¹ represents an alkyl group having 1 to 18 carbon atoms or an alkenyl group having 1 to 18 carbon atoms, R ² represents an alkylene group having 1 to 6 carbon atoms, and R ³ and R ⁴ represent H or CH ³ .

In particular, one or more types selected from N-oleoyl sarcosine, N-lauroyl sarcosine, and myristoylmethyl-β-alanine can be used as the Lewis base that is soluble in a solvent capable of forming a slurry of nickel particles. The structures of N-oleoyl sarcosine, N-lauroyl sarcosine, and myristoylmethyl-β-alanine are shown in the following general formulas (2) to (4).

The Lewis base-containing nickel particles of the present invention preferably have a number average particle size of 0.03 μm to 0.4 μm in order to accommodate the recent trend of thinner internal electrodes in multilayer ceramic capacitors. The number average particle size is the number average particle size determined from a scanning electron microscope (SEM) image of the nickel particles.

Similarly, from the viewpoint of being compatible with thin layers, it is preferable that the particle shape is approximately spherical, and an approximately spherical shape includes not only a true sphere, but also an ellipsoid having a ratio of the minor axis to the major axis (minor axis/major axis) of 0.8 to 1.0 in a specified cross section (e.g., a cross section passing through the center of the particle).

In addition, the Lewis base-containing nickel particles of the present invention preferably have a particle content of 200 mass ppm or less having a particle size exceeding 0.8 μm, and a particle content of 100 mass ppm or less having a particle size exceeding 1.2 μm.

The effect of coarse particles in nickel particles depends on the film thickness of the internal electrode layer of the multilayer ceramic capacitor in which the nickel particles are used. In recent thin internal electrode layers, if the content of coarse particles with a particle size exceeding 0.8 μm exceeds 400 mass ppm, or if the content of coarse particles with a particle size exceeding 1.2 μm exceeds 200 mass ppm, the occurrence of short circuits between electrodes may become significant. It goes without saying that the lower the content of coarse particles in nickel particles, the better. If the content of coarse particles with a particle size exceeding 0.8 μm is 200 mass ppm or less and the content of coarse particles with a particle size exceeding 1.2 μm is 100 mass ppm or less, the occurrence rate of short circuits between electrodes can be sufficiently reduced. The particle size of the coarse particles may be the minor axis diameter determined from the SEM image.

The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples in any way.

[Example 1]
10 g of nickel particles was added to 100 g of dihydroterpineol, a soft cross-shaped stirring blade was attached to a three-one motor, and the mixture was stirred at 300 rpm for 1 hour so that the peripheral speed was 8 m / sec to obtain a nickel slurry. While maintaining the stirring conditions, a Lewis base solution in which 0.2 g of N-oleoyl sarcosine was dissolved as a Lewis base in 10 g of dihydroterpineol was dropped at a rate of 2 ml / min into the nickel slurry. Thereafter, the mixture was stirred at room temperature of 25 ° C. for 3 hours without changing the stirring conditions to obtain a slurry of surface-treated nickel particles. The slurry was separated into solid and liquid by suction filtration, and the separated nickel particles were added with 100 g of dihydroterpineol to form a slurry, and this operation was repeated three times to wash the nickel particles. After washing, the nickel particles were separated into solid and liquid by suction filtration, and the separated nickel particles were dried at 120 ° C. under a nitrogen atmosphere to obtain a powder of nickel particles after the intended surface treatment.

[Example 2]
Surface treatment was carried out in the same manner as in Example 1, except that N-lauroyl sarcosine was used as the Lewis base, to obtain a powder of surface-treated nickel particles.

[Example 3]
Surface treatment was carried out in the same manner as in the Example, except that myristoylmethyl-β-alanine was used as the Lewis base, to obtain a powder of surface-treated nickel particles.

(Comparative Example 1)
Except for the step of dropping the Lewis base solution, the conditions were the same as in Example 1. That is, the nickel particles were slurried, solid-liquid separated, washed, and dried in the same manner as in Example 1, without using a Lewis base.

(Comparative Example 2)
The nickel particles themselves used as the raw material in Examples 1 to 3 and Comparative Example 1 were used as Comparative Example 2. That is, unlike Comparative Example 1, the nickel particles were not subjected to any of the following treatments: slurrying, solid-liquid separation, washing, or drying.

[Evaluation of Nickel Particles]
Evaluation of the amount of carbon on the surface of nickel particles
The carbon content was measured using a carbon/sulfur analyzer (CS844 manufactured by LECO Corporation).

<Analysis of nickel particle surface treatment state>
The dihydroterpineol after solid-liquid separation by suction filtration and the dihydroterpineol after washing the nickel particles were recovered, and were diluted with methanol to adjust the concentration for analysis, and the content of the Lewis base not consumed in the surface treatment was quantified using a liquid chromatography-mass spectrometer (Agilent 1290 infinity2-Agilent 6530). Using this value, the Lewis base content contained in the nickel particles after the surface treatment was calculated from the following formulas (5) and (6).

[Equation 1]
Amount of Lewis base contained in Ni particles after surface treatment=(Total amount of Lewis base used−Amount of Lewis base not consumed in surface treatment) (5)

[Equation 2]
Lewis base content (mass%) in Ni particles after surface treatment
= (amount of Lewis base contained in Ni particles after surface treatment/amount of Ni particles after surface treatment) × 100 (6)

Measurement of average particle size
The average particle size of the nickel particles was calculated as the number average particle size by measuring the particle size obtained from the results of image analysis of an observation image (SEM image) of the nickel powder using a scanning electron microscope (SEM, JSM-6360, manufactured by JEOL Ltd.).

<Measurement of the content of coarse particles>
The nickel powder after the surface treatment operation was placed in a 100 ml high vessel container (BHB-100 manufactured by Kinki Container Co., Ltd.), and the nickel particles of Examples 1 to 3 and Comparative Example 1 were left at room temperature for 1 day in an air atmosphere, and a scanning electron microscope (SEM, JSM-6360, manufactured by JEOL Ltd.) was used to obtain SEM images at a magnification of 5000 times. Then, using image analysis software Mac-View (manufactured by Mountec Co., Ltd.), the area and number of particles in which the entire particle shape was visible in the obtained SEM image were measured, and the diameter of each particle was obtained from these, and particles with diameters of 0.8 μm or more and 1.2 μm or more were counted as coarse particles. Then, the content of coarse particles contained in the nickel powder (when the particle size exceeded 0.8 μm and when the particle size exceeded 1.2 μm) was obtained as an initial value.

After taking the SEM images, the nickel particles of Examples 1 to 3 and Comparative Example 1 were left at room temperature in an air atmosphere for 180 days, and the content of coarse particles was then measured in the same manner. The results of the content of coarse particles after leaving them for 180 days are shown in Table 1.

For the nickel particles of Comparative Example 2, SEM images were similarly taken of the nickel particles immediately before use as a raw material in Examples 1 to 3 and Comparative Example 1, and the initial content of coarse particles was similarly determined. After obtaining the initial SEM images, the nickel particles of Comparative Example 2 were stored in the same place and environment as Examples 1 to 3 and Comparative Example 1, and the nickel particles of Comparative Example 2 were left at room temperature in an air atmosphere for 180 days, after which the content of coarse particles was similarly measured.

The results for the amount of carbon on the nickel particle surface, the average particle size, and the content of coarse particles are shown in Table 1. Figure 2 shows SEM photographs of nickel particles in Example 1 and Comparative Example 2 at initial and 180-day intervals.

The results of Examples 1 to 3 and Comparative Example 2 show that the average particle size of nickel particles did not change significantly due to the surface treatment, and no generation of coarse particles was observed (Table 1, Figure 2). In addition, the surface treatment was able to suppress the increase in coarse particles after 180 days in an air atmosphere.

However, the results for the nickel particles of Comparative Example 1 showed that when the nickel particles were slurried, solid-liquid separated, washed, and dried without using a Lewis base, the content of coarse particles increased compared to the nickel particles of Comparative Example 2. Furthermore, the amount of coarse particles further increased after 180 days in an air atmosphere (Table 1).

In addition, in the SEM image of Comparative Example 2 after 180 days in Figure 2, an area was observed in the center where multiple particles appeared to have been crushed and coalesced into a dark mass. This area can be counted as one coarse particle caused by oxidation.

[summary]
As described above, it is clear that the nickel particles of the present invention are surface-treated with a nitrogen-containing Lewis base, thereby suppressing the generation of coarse particles due to oxidation during storage.

Claims (6)

A nitrogen-containing Lewis base is present on the surface of the nickel particles,
a mass ratio of the Lewis base to the nickel particles is 0.16 to 3.0:100;
A dry nickel powder comprising Lewis base-containing nickel particles , wherein the Lewis base is a compound represented by the following formula (1) :

(In formula (1), R1 ^represents an alkyl group having 1 to 18 carbon atoms or an alkenyl group having 1 to 18 carbon atoms, R2 ^represents an alkylene group having 1 to 6 carbon atoms, and R3 ^and R4 ^represent H or CH3 _. ) The nickel powder according to claim 1, wherein the Lewis base is coordinated to the surface of the nickel particles. The nickel powder according to claim 1 or 2 , wherein the Lewis base comprises one or more selected from N-oleoyl sarcosine, N-lauroyl sarcosine, and myristoyl methyl-β-alanine. The number average particle size of the Lewis base-containing nickel particles is 0.03 μm to 0.4 μm;
The content of particles having a particle size exceeding 0.8 μm is 400 ppm by mass or less,
The nickel powder according to any one of claims 1 to 3 , wherein the content of particles having a particle size exceeding 1.2 µm is 200 mass ppm or less. 5. The method for surface treatment of nickel particles containing a Lewis base according to claim 1, further comprising a mixing step of mixing nickel particles with a nitrogen-containing Lewis base. A method for producing nickel powder, comprising the step of drying the surface-treated nickel particles after the mixing step according to claim 5 .

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