JP7600782B2 - 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 debinding 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 a technique for adding nickel powder produced by a liquid phase method to pure water to form a slurry, and then oxidizing it with hydrogen peroxide. However, with surface oxidation treatment using hydrogen peroxide, it can be difficult to uniformly control the oxidation treatment.

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 nickel particle aggregation can be suppressed by coating the nickel particles with oleic acid and oleylamine, and have completed the present invention.

In order to solve the above problems, the surface treatment method for nickel particles of the present invention includes a mixing step of mixing a reduction reaction solution containing nickel particles crystallized by reducing a water-soluble nickel salt with oleic acid and oleylamine, and the mass ratio of the oleic acid to the oleylamine is 3:7 to 7:3.

The mixing process may cause the oleic acid and oleylamine to be coordinately bonded to the surface of the nickel particles.

The mass ratio of the sum of the oleic acid and the oleylamine to the nickel particles may be 0.16 to 3.0:100.

In order to solve the above problems, the method for producing nickel powder of the present invention includes a crystallization step of reducing a water-soluble nickel salt in a reduction reaction solution to crystallize nickel particles, and a mixing step of mixing the reduction reaction solution containing the nickel particles after the crystallization step with oleic acid and oleylamine, and the mass ratio of the oleic acid to the oleylamine is 3:7 to 7:3.

The mixing process may cause the oleic acid and oleylamine to be coordinately bonded to the surface of the nickel particles.

The mass ratio of the sum of the oleic acid and the oleylamine to the nickel particles may be 0.16 to 3.0:100.

After the mixing step, a drying step may be included to dry the surface-treated nickel particles.

In order to solve the above problems, the nickel particles of the present invention are nickel particles containing oleic acid and oleylamine, in which oleic acid and oleylamine are present on the surface of the nickel particles, the mass ratio of the sum of the oleic acid and the oleylamine to the nickel particles is 0.005 to 0.1:100, and the mass ratio of the oleic acid to the oleylamine is 3:7 to 7:3.

In the nickel particles of the present invention, the oleic acid and the oleylamine may be coordinately bonded to the surface of the nickel particles.

The 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.

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, Example 2, and Comparative Example 1.

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.

[Method of manufacturing nickel powder]
According to the method for producing nickel powder of the present invention, nickel powder can be obtained by a so-called wet method, such as a wet reduction method using a reducing agent such as hydrazine for a nickel salt solution. The method for producing nickel powder of the present invention includes a crystallization step and a mixing step, which will be described below as an example. It may also include a nickel salt aqueous solution preparation step, a washing/filtration step, and a drying step.

<1: Nickel salt aqueous solution preparation step>
The nickel salt aqueous solution may be appropriately selected from nickel salt compounds such as nickel chloride, nickel sulfate, and nickel nitrate. Since elements resulting from the anions of the selected nickel salt remain in the synthesized nickel powder to a certain extent, the nickel salt may be selected from elements that do not affect the performance even if they remain. For example, nickel chloride is preferred as the nickel salt because the chloride ions of nickel chloride are not easily incorporated into the nickel powder and are easily removed by washing. Nickel chloride is also preferred from the viewpoint of ease of treatment of waste liquid after wet reduction.

When nickel chloride is selected, the concentration of the nickel chloride aqueous solution is preferably adjusted so that the nickel concentration is in the range of 50 to 200 g/liter. If the nickel concentration is too low, it is not efficient because it requires relatively large equipment and increases the amount of waste liquid. On the other hand, if the concentration is too high, it becomes difficult to control the reduction reaction, and it may be difficult to obtain nickel particles with the desired particle size. Also, when using other nickel salts, it is preferable to adjust the nickel concentration to the range of 50 to 200 g/liter for the same reason.

The nickel salt aqueous solution can be prepared, for example, by dissolving a predetermined amount of nickel salt in pure water. The nickel salt aqueous solution may be prepared by the user, or a prepared nickel salt aqueous solution may be purchased.

<2: Crystallization process>
In the crystallization step, the water-soluble nickel salt is reduced in a reduction reaction solution to crystallize nickel particles. For example, a particle size control agent is added to an aqueous nickel salt solution, and then a reducing agent is added to perform a reduction treatment in which the water-soluble nickel salt is reduced in the reduction reaction solution.

Metals that act as particle size control agents for nickel powder can be selected from copper, palladium, platinum, rhodium, etc., which have a smaller tendency to ionize than nickel, and palladium and copper, which are solid-soluble with nickel, are preferable. Metals that act as particle size control agents act as nuclei when nickel powder is synthesized from nickel salt, and the particle size of the nickel powder can be controlled by controlling the number of nuclei. Even if an element does not form a solid solution with nickel, it is desirable to add it if it can form a solid solution with a metal that acts as a nuclei, as this can make the nuclei finer and reduce the content of metals other than nickel contained in each nickel particle, making it possible to highly purify the nickel powder.

The amount of elements that dissolve with nickel should be 500 ppm by mass or less, since adding more than 500 ppm by mass to the total amount of nickel will not refine the nickel powder. Elements that dissolve with nucleus elements such as copper and palladium should be 50 ppm by mass or less to the total amount of nickel. Adding more than this amount will not have the effect of refining the nucleus and refining the nickel powder.

If the metal particles that serve as nuclei are agglomerated, the number of nuclei that actually act as nuclei will be reduced, and the amount contained in each nickel powder particle will increase, reducing the purity of the nickel powder and even increasing the average particle size of the nickel powder, so it is desirable for them to be monodisperse.

To suppress the aggregation of the core metal particles and maintain monodispersity, it is desirable to add molecules with protective colloidal properties to the reduction reaction solution. Polymeric materials such as gelatin and polyvinylpyrrolidone, and surfactants are suitable as materials with protective colloidal properties. The amount of this material added is desirably 0.0025 to 0.5% by mass or less relative to the nickel. If too much is added, it may inhibit the reduction of nickel ions, and if too little is added, the desired protective colloid effect may not be obtained.

Furthermore, it is beneficial to add a complexing agent that forms a complex with nickel ions to the nickel salt aqueous solution in the crystallization process. The complexing agent can be any agent capable of forming a nickel complex, and compounds having amino groups, carboxyl groups, sulfo groups, hydroxyl groups, alkyl groups, etc. are suitable. When nickel forms a complex ion by adding a complexing agent, the shape of the nickel powder obtained through the crystallization process tends to be spherical.

<3: Mixing step>
In the mixing step (surface treatment step in FIG. 1), the reduction reaction solution containing nickel particles after the crystallization step is mixed with oleic acid and oleylamine. As a method for surface treating nickel powder with oleic acid and oleylamine, for example, a method of wet mixing the reduction reaction solution containing precipitated nickel particles with a mixed solution of oleic acid and oleylamine using water or alcohol containing oleic acid and oleylamine as a medium can be mentioned. That is, the reduction reaction solution containing nickel particles obtained by the crystallization step is mixed with a water or alcohol solution of oleic acid and oleylamine to form a slurry, so that oleic acid and oleylamine are uniformly adsorbed on the particle surface of the nickel powder.

Specifically, the reduction reaction solution containing nickel particles is stirred, and oleic acid and oleylamine dissolved in a water-soluble organic solvent are added thereto, followed by stirring. In this case, an organic solvent such as alcohol may be used as the organic solvent, and the surface treatment temperature during stirring is preferably 20 to 50°C. If the treatment temperature is lower than 20°C, the rate at which oleic acid and oleylamine adhere to the nickel particles decreases, and the stirring time may be long. Furthermore, if the treatment temperature is higher than 50°C, the evaporation of the water-soluble organic solvent is promoted, which may make the surface treatment difficult.

Furthermore, it is preferable that the stirring time is 0.5 to 24 hours. If the stirring time is less than 0.5 hours, the amount of oleic acid and oleylamine adhering to the nickel particles may be small. Furthermore, even if the stirring time is longer than 24 hours, the amount of oleic acid and oleylamine adhering hardly increases, and if the processing time is longer, the nickel powder production time will also be longer, which may increase costs.

<Oleic acid and oleylamine>
The compounds used for the surface treatment are oleic acid and oleylamine. It is believed that the method for producing nickel powder according to the present invention allows the lone electron pairs of oleic acid and oleylamine to donate electrons to the surface of the nickel particles to form a coordinate bond, forming a structure. The oleic acid and oleylamine compounds can be efficiently adsorbed to the surface of the nickel particles and form coordinate bonds, thereby suppressing the generation of coarse particles due to the aggregation of nickel particles.

Oleic acid and oleylamine are effective when used in combination, with the mass ratio of oleic acid to oleylamine being 3:7 to 7:3. If the proportion of oleic acid or oleylamine is higher or lower than this mass ratio range, it may not be possible to suppress the generation of coarse particles due to the aggregation of nickel particles.

The amount of oleic acid and oleylamine compound added is preferably in the range of 0.16% by mass or more and 3.0% by mass or less for 100% by mass of nickel particles before surface treatment. In other words, the mass ratio of the total of oleic acid and oleylamine to the nickel particles is preferably 0.16 to 3.0:100. By having the total content of oleic acid and oleylamine be 0.16% by mass or more for 100% by mass of 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 having the total content of oleic acid and oleylamine be 3.0% by mass or less, a change in the viscosity of the nickel paste due to an increase in the amount of oleic acid and oleylamine can be prevented.

(solvent)
The organic solvent is not particularly limited as long as it can disperse nickel particles and dissolve oleic acid and oleylamine.Specific examples of the organic solvent 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, and decanol. Furthermore, aliphatic hydrocarbons represented by C _n H _2n+2 , C _n H _2n , C _n H _2n-2 , aromatic hydrocarbons represented by C _n H _2n-6, etc. can also be used, and specific examples thereof 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.

<4: Washing and filtration 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 oleic acid and oleylamine 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 oleic acid, oleylamine, and impurities can be removed from the nickel particles. Furthermore, infrared spectroscopy of the obtained filtrate is performed to confirm that unreacted oleic acid and oleylamine are not detected, and the washing process can be completed.

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.

<5: Drying process>
In the method for producing nickel powder of the present invention, a drying step of drying the surface-treated nickel particles may be included after the mixing step. For example, the drying step can be performed after the washing and filtering steps.

For example, nickel powder can be obtained by drying the nickel 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 nickel from oxidizing. The nickel powder may also be passed through a mesh if necessary.

If necessary, a washing and filtering process 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.

[Surface treatment method for nickel particles]
Next, an example of the method for surface treating nickel particles of the present invention will be described. This method includes a mixing step of mixing a reduction reaction solution containing nickel particles crystallized by reducing a water-soluble nickel salt with oleic acid and oleylamine.

For example, the crystallization step in the nickel powder manufacturing method described above may be carried out to reduce the water-soluble nickel salt to obtain a reduction reaction solution containing crystallized nickel particles, or the reduction reaction solution may be obtained by purchasing it, etc.

The mixing process is the same as the mixing process in the nickel powder manufacturing method described above, and it is believed that this process causes oleic acid and oleylamine to form coordinate bonds on the surface of the nickel particles.

The nickel particles to be surface-treated are not limited to nickel particles crystallized by reducing water-soluble nickel salt, and various nickel powders can be used regardless of the manufacturing method, such as wet method or dry method. In other words, nickel particles may be surface-treated in the state of a reduction reaction solution without drying, or dried powdery nickel particles may be surface-treated by making them into a slurry. For example, nickel powder produced by a so-called dry method, such as a CVD method, an evaporation quenching method, or a hydrogen reduction method using nickel salt or nickel hydroxide, can be used as the nickel particles to be treated. Nickel powder produced by a so-called wet method, such as a wet reduction method using a reducing agent such as hydrazine for a nickel salt solution, can be used as the nickel particles to be treated. Among them, nickel powder produced by a so-called wet method, such as a wet reduction method, is suitable as a material for MLCC because it is spherical and has a small variation in particle size.

Below, we will explain the surface treatment method for dried powdered nickel particles.

<Mixing process>
The method for treating the surface of nickel particles includes a mixing step of mixing nickel particles with oleic acid and oleylamine. By the mixing step, the surfaces of the nickel particles can be surface-treated with oleic acid and oleylamine.

As mentioned above, oleic acid and oleylamine are effective when used together, and the mass ratio of oleic acid to oleylamine is 3:7 to 7:3.

As described above, the amount of oleic acid and oleylamine compound added is preferably in the range of 0.16% by mass or more and 3.0% by mass or less in total for 100% by mass of nickel particles before surface treatment.

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

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

Specifically, the nickel slurry is stirred, and the oleic acid and oleylamine solution in which the oleic acid and oleylamine are dissolved is dropped into and mixed, and then the mixed solution is stirred to perform the surface treatment.

For example, an oleic acid and oleylamine solution can be prepared so that the total content of oleic acid and oleylamine is 0.1 to 30 mass%, and the oleic acid and oleylamine 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.

During surface treatment, the temperature of the mixed solution of nickel particles with oleic acid and oleylamine is preferably 20 to 50°C. If the temperature of the mixed solution is lower than 20°C, the rate at which oleic acid and oleylamine adhere 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 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 oleic acid and oleylamine 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 oleic acid and oleylamine 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 production cost. The mixed solution may be stirred at a peripheral speed of 5 to 10 m/sec, the same as in the mixing process.

(Oleic acid and oleylamine-containing nickel particles)
Nickel particles containing oleic acid and oleylamine can be obtained by the above-mentioned method for producing nickel powder or the method for surface treating nickel particles of the present invention.

In the oleic acid and oleylamine-containing nickel particles of the present invention, oleic acid and oleylamine are present on the surface of the nickel particles, and the mass ratio of oleic acid and oleylamine to the nickel particles is preferably 0.005 to 0.1:100. If the mass ratio is less than 0.005:100, 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 oleic acid and oleylamine present at a mass ratio exceeding 0.1:100, and a mass ratio of 0.1:100 is considered to be the upper limit of the amount of oleic acid and oleylamine present.

It is believed that oleic acid and oleylamine form coordinate bonds to the surface of nickel particles by donating electrons to the surface of the nickel particles through the lone electron pairs of oleic acid and oleylamine. Oleic acid and oleylamine can be efficiently adsorbed to the surface of nickel particles and form coordinate bonds, which can suppress the generation of coarse particles due to the aggregation of nickel particles.

The nickel particles containing oleic acid and oleylamine 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 scanning electron microscope (SEM) images 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 oleic acid and oleylamine-containing nickel particles of the present invention preferably have a particle content of 400 mass ppm or less having a particle size exceeding 0.8 μm, and a particle content of 200 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]
Specifically, the alkaline colloidal solution for reducing nickel was prepared as follows. First, 0.12 g of gelatin was dissolved in 6 L of pure water, and then hydrazine was mixed so that the hydrazine concentration was 0.02 g/L. Next, a mixed solution was prepared of 10 mg of tetraamminepalladium (II) dichloride solution with a palladium content of 100 g/L and 0.5 mg of diamminesilver (I) chloride solution with a silver content of 50 g/L. Then, 2 ml of this mixed solution was dropped into the solution containing gelatin and hydrazine to obtain a colloidal solution.

Sodium hydroxide was added to this colloidal solution to adjust the pH to 10 or higher, and then hydrazine was added until the hydrazine concentration reached 26 g/L to produce an alkaline hydrazine solution mixed with composite colloidal particles consisting of palladium and a trace amount of silver, which was used as an alkaline colloidal solution for reducing nickel.

0.5 L of a nickel chloride aqueous solution with a nickel concentration of 100 g/L was then dropped into this alkaline colloidal solution to reduce nickel, obtaining nickel powder.

<Surface treatment of nickel particles>
A solution in which 0.025 g of oleic acid and 0.025 g of oleylamine were dissolved as surface treatment agents in 10 g of dihydroterpineol was added to about 100 g of the reduction reaction solution containing 10 g of precipitated nickel particles, and the mixture was stirred at 300 rpm by a three-one motor for 1 hour and then at room temperature of 25° C. for 3 hours to obtain a nickel particle slurry. The slurry was subjected to solid-liquid separation by filtration and dried at 120° C. under a nitrogen atmosphere to obtain the desired nickel particles.

[Example 2]
A solution of 0.015 g of oleic acid and 0.035 g of oleylamine dissolved in 10 g of dihydroterpineol was added to about 100 g of reduction reaction solution containing 10 g of precipitated nickel particles, and the mixture was stirred at 300 rpm by a three-one motor for 1 hour, and then stirred at room temperature of 25° C. for 3 hours to obtain nickel particle slurry. The slurry was separated into solid and liquid by filtration, and dried at 120° C. under a nitrogen atmosphere to obtain the desired nickel particles. Other conditions and procedures were the same as those in Example 1.

[Example 3]
A solution of 0.035 g of oleic acid and 0.015 g of oleylamine dissolved in 10 g of dihydroterpineol was added to about 100 g of reduction reaction solution containing 10 g of precipitated nickel particles, and the mixture was stirred at 300 rpm by a three-one motor for 1 hour, and then stirred at room temperature of 25° C. for 3 hours to obtain nickel particle slurry. The slurry was separated into solid and liquid by filtration, and dried at 120° C. under a nitrogen atmosphere to obtain the desired nickel particles. Other conditions and procedures were the same as those in Example 1.

[Example 4]
A solution of 0.015 g of oleic acid and 0.015 g of oleylamine dissolved in 10 g of dihydroterpineol was added to about 100 g of reduction reaction solution containing 10 g of precipitated nickel particles, and the mixture was stirred at 300 rpm by a three-one motor for 1 hour, and then stirred at room temperature of 25° C. for 3 hours to obtain nickel particle slurry. The slurry was separated into solid and liquid by filtration, and dried at 120° C. under a nitrogen atmosphere to obtain the desired nickel particles. Other conditions and procedures were the same as those in Example 1.

[Example 5]
A solution of 0.05 g of oleic acid and 0.05 g of oleylamine dissolved in 10 g of dihydroterpineol was added to about 100 g of reduction reaction solution containing 10 g of precipitated nickel particles, and the mixture was stirred at 300 rpm by a three-one motor for 1 hour, and then stirred at room temperature of 25° C. for 3 hours to obtain nickel particle slurry. The slurry was separated into solid and liquid by filtration, and dried at 120° C. under a nitrogen atmosphere to obtain the desired nickel particles. Other conditions and procedures were the same as those in Example 1.

(Comparative Example 1)
Except for the step of mixing the dihydroterpineol solution of oleic acid and oleylamine, the conditions were the same as in Example 1. That is, nickel particles were produced without carrying out surface treatment with oleic acid and oleylamine.

(Comparative Example 2)
Instead of a solution in which 0.025 g of oleic acid and 0.025 g of oleylamine were dissolved in 10 g of dihydroterpineol, a solution in which only 0.025 g of oleic acid was dissolved in 10 g of dihydroterpineol was mixed. The other conditions were the same as those in Example 1. In other words, nickel particles were produced without performing surface treatment with oleylamine.

(Comparative Example 3)
Instead of a solution in which 0.025 g of oleic acid and 0.025 g of oleylamine were dissolved in 10 g of dihydroterpineol, a solution in which only 0.025 g of oleylamine was dissolved in 10 g of dihydroterpineol was mixed. The other conditions were the same as those in Example 1. In other words, nickel particles were produced without performing a surface treatment with oleic acid.

Table 1 shows the mass ratio of nickel particles to surface treatment agent in Examples 1 to 5 and Comparative Examples 1 to 3. Note that the amount of surface treatment agent in Table 1 is the total amount of oleic acid and oleylamine.

[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 prepare concentrations for analysis, and the contents of oleic acid and oleylamine not consumed in the surface treatment were quantified using a liquid chromatography mass spectrometer (Agilent 1290 infinity2-Agilent 6530). Using these values, the contents of oleic acid and oleylamine contained in the nickel particles after the surface treatment were calculated from the following formulas (1) and (2).

[Equation 1]
Amount of oleic acid and oleylamine contained in Ni particles after surface treatment=(total amount of oleic acid and oleylamine used−amount of oleic acid and oleylamine not consumed in surface treatment) (1)

[Equation 2]
Contents of oleic acid and oleylamine in Ni particles after surface treatment (mass%)
= (amount of oleic acid and oleylamine contained in Ni particles after surface treatment/amount of Ni particles after surface treatment) × 100 (2)

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 5 and Comparative Examples 1 to 3 were left at room temperature in an air atmosphere for 1 day. 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 5 and Comparative Examples 1 to 3 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 oleic acid and oleylamine contained in the nickel particles, the amount of carbon on the surface of the nickel particles, the average particle size, and the content of coarse particles initially and after 180 days of storage are shown in Table 2. Figure 2 also shows SEM photographs of the nickel particles initially and after 180 days of storage in Examples 1 and 2 and Comparative Example 1.

The results of Examples 1 to 5 and Comparative Examples 1 to 3 show that the average particle size of nickel particles did not change significantly due to surface treatment, and no increase in coarse particles was observed. In particular, the surface treatment using a combination of oleic acid and oleylamine was able to suppress the increase in coarse particles after 180 days in an air atmosphere (Examples 1 to 5, Comparative Examples 2 and 3).

However, the results for the nickel particles in Comparative Example 1 showed that when the surface was not treated with oleic acid and oleylamine, an increase in coarse particles was observed after 180 days in an air atmosphere. Furthermore, the results for the nickel particles in Comparative Examples 2 and 3 showed that treating the nickel particles with either oleic acid or oleylamine was effective in suppressing the increase in coarse particles, but this effect was more pronounced in Examples 1 to 5, in which oleic acid and oleylamine were used in combination.

In addition, in the SEM image of Comparative Example 1 taken 180 days later, a dark mass was observed in the center where multiple particles appeared to have been crushed and adhered together (Figure 2). This area can be counted as one large particle caused by oxidation.

[summary]
As described above, it is clear that the surface treatment method of nickel particles and the manufacturing method of nickel powder of the present invention can suppress the generation of coarse particles due to oxidation of nickel particles during storage by surface treating nickel particles with oleic acid and oleylamine.

Claims (10)

The method includes a mixing step of mixing a reduction reaction solution containing nickel particles crystallized by reducing a water-soluble nickel salt with oleic acid and oleylamine,
The mass ratio of the oleic acid to the oleylamine is 3:7 to 7:3.
A method for surface treatment of nickel particles. The surface treatment method for nickel particles according to claim 1, wherein the oleic acid and oleylamine are coordinately bonded to the surface of the nickel particles by the mixing process. The surface treatment method for nickel particles according to claim 1 or 2, wherein a mass ratio of the total of the oleic acid and the oleylamine to the nickel particles is 0.16 :100 to 3.0:100. a crystallization step of reducing a water-soluble nickel salt in a reduction reaction solution to crystallize nickel particles;
a mixing step of mixing the reduction reaction solution containing the nickel particles after the crystallization step with oleic acid and oleylamine;
Including,
The method for producing nickel powder, wherein the mass ratio of the oleic acid to the oleylamine is 3:7 to 7:3. The method for producing nickel powder according to claim 4, wherein the oleic acid and oleylamine are coordinately bonded to the surfaces of the nickel particles by the mixing process. The method for producing nickel powder according to claim 4 or 5, wherein a mass ratio of the total of the oleic acid and the oleylamine to the nickel particles is 0.16 :100 to 3.0:100. The method for producing nickel powder according to any one of claims 4 to 6, further comprising a drying step of drying the surface-treated nickel particles after the mixing step. Oleic acid and oleylamine are present on the surface of the nickel particles,
a mass ratio of the total of the oleic acid and the oleylamine to the nickel particles is 0.005 :100 to 0.1:100;
The mass ratio of the oleic acid to the oleylamine is 3:7 to 7:3.
Nickel particles containing oleic acid and oleylamine. The oleic acid and oleylamine-containing nickel particles of claim 8, wherein the oleic acid and the oleylamine are coordinated to the surface of the nickel particles. The number average particle size 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,
10. The oleic acid and oleylamine-containing nickel particles according to claim 8 or 9, wherein the content of particles having a particle size exceeding 1.2 μm is 200 mass ppm or less.

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