JP7647221B2 - Surface treatment method for nickel particles and manufacturing method for nickel powder - Google Patents

Description

The present invention relates to 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.

A multilayer ceramic capacitor using this electrode is manufactured in the following way, 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 is 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 is a study being conducted to thin 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 is being 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 in the firing process during the manufacture of MLCC, 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 MLCC, it is important to produce a nickel paste with few 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 was proposed in light of these circumstances, and aims to provide a surface treatment method for nickel particles and a method for producing nickel powder that suppress the generation of coarse particles due to oxidation during storage.

The present inventors have conducted extensive research to solve the above-mentioned problems. As a result, they have discovered that by coating the surfaces of nickel particles crystallized by reducing a water-soluble nickel salt with a Lewis base compound without drying the nickel particles even once so that the surfaces do not react with oxygen in the air and become oxidized, it is possible to suppress the aggregation of nickel particles in a powder state after drying, and as a result, the generation of coarse particles can be suppressed, which led to the completion of the present invention.

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 a phosphorus-containing Lewis base and a nitrogen-containing Lewis base.

In addition, in the surface treatment method for nickel particles of the present invention, the phosphorus-containing Lewis base and the nitrogen-containing Lewis base may be coordinately bonded to the surface of the nickel particles.

In addition, in the surface treatment method for nickel particles of the present invention, the mass ratio of the total amount of the phosphorus-containing Lewis base and the nitrogen-containing Lewis base to the nickel particles may be 0.16 to 3.0:100.

In addition, in the surface treatment method for nickel particles of the present invention, the mass ratio of the phosphorus-containing Lewis base to the nitrogen-containing Lewis base may be 3:7 to 7:3.

In addition, in the surface treatment method for nickel particles of the present invention, the phosphorus-containing Lewis base may include one or more selected from the phosphate ester and polyphosphate ester shown in the following formula (1), the phosphate ester and phosphate polyester shown in (2), and the phosphate ether and phosphate polyether shown in (9), and the nitrogen-containing Lewis base may include a compound shown in the following formula (3).

In formula (1), n is a natural number of 1 to 5, R ₁ represents H or an alkyl group having 7 to 15 carbon atoms, and R ₂ represents an alkyl group having 7 to 15 carbon atoms or a tert-butyl group represented by formula (A).
In formula (2), m is a natural number of 1 to 5, _R3 represents H or an alkyl group having 7 to 15 carbon atoms, and _R4 represents an alkyl group having 7 to 15 carbon atoms or the tert-butyl group represented by formula (A).
In formula (9), r is a natural number of 1 to 5, R ₉ represents H or an alkyl group having 7 to 15 carbon atoms, and R ₁₀ represents an alkyl group having 1 to 15 carbon atoms or the tert-butyl group represented by formula (A).
In formula (3), ^R5 represents an alkyl group having 1 to 18 carbon atoms or an alkenyl group having 1 to 18 carbon atoms, ^R6 represents an alkylene group having 1 to 6 carbon atoms, and ^R7 and ^R8 represent H or _CH3 .

The method for producing nickel powder of the present invention includes a crystallization step in which a water-soluble nickel salt is reduced in a reduction reaction solution to crystallize nickel particles, and a mixing step in which the reduction reaction solution containing the nickel particles after the crystallization step is mixed with a phosphorus-containing Lewis base and a nitrogen-containing Lewis base.

In addition, in the method for producing nickel powder of the present invention, the mass ratio of the total amount of the phosphorus-containing Lewis base and the nitrogen-containing Lewis base to the nickel particles may be 0.16 to 3.0:100.

In addition, in the method for producing nickel powder of the present invention, the mass ratio of the phosphorus-containing Lewis base to the nitrogen-containing Lewis base may be 3:7 to 7:3.

The method for producing nickel powder of the present invention may also include a drying step for drying the surface-treated nickel particles after the mixing step.

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

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 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 can be appropriately selected from aqueous solutions in which nickel salt compounds such as nickel chloride, nickel sulfate, and nickel nitrate are dissolved. Since elements resulting from the anions of the selected nickel salt may remain in the synthesized nickel powder to some extent, the nickel salt may be selected in consideration of 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, after adding a particle size control agent to an aqueous nickel salt solution, a reduction treatment is performed to reduce the water-soluble nickel salt in a reduction reaction solution to which a reducing agent has been added.

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 relative 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 relative 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, the reduction reaction solution containing nickel particles after the crystallization step is mixed with a phosphorus-containing Lewis base and a nitrogen-containing Lewis base. By using a phosphorus-containing Lewis base and a nitrogen-containing Lewis base as compounds used for the surface treatment of nickel particles in the mixing step, it is considered that the lone electron pairs of the phosphorus-containing Lewis base and the nitrogen-containing Lewis base provide electrons to the surface of the nickel particles to form a coordinate bond structure. The phosphorus-containing Lewis base and the nitrogen-containing Lewis base can be efficiently adsorbed and coordinate bonded to the surface of the nickel particles, and can suppress the generation of coarse particles due to the aggregation of nickel particles.

One method for surface treating nickel powder with a phosphorus-containing Lewis base and a nitrogen-containing Lewis base is, for example, wet mixing a reduction reaction solution containing precipitated nickel particles with a solution of a phosphorus-containing Lewis base and a nitrogen-containing Lewis base in water or alcohol as a medium, which contains a phosphorus-containing Lewis base and a nitrogen-containing Lewis base. That is, by mixing the reduction reaction solution containing nickel particles obtained by the crystallization process with a water or alcohol solution of a phosphorus-containing Lewis base and a nitrogen-containing Lewis base to form a slurry, the phosphorus-containing Lewis base and the nitrogen-containing Lewis base are uniformly adsorbed onto the particle surfaces of the nickel powder.

Specifically, the reduction reaction solution containing nickel particles is stirred, and then a phosphorus-containing Lewis base and a nitrogen-containing Lewis base dissolved in a water-soluble organic solvent are added to the solution, 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 the Lewis base adheres 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 Lewis base adhering to the nickel particles may be small. Furthermore, even if the stirring time is longer than 24 hours, the amount of Lewis base adhering hardly increases, and if the processing time is longer, the manufacturing time of the nickel powder will also be longer, which may increase the manufacturing cost.

Phosphorus-containing Lewis bases
The compounds used for the surface treatment are a phosphorus-containing Lewis base compound together with a nitrogen-containing Lewis base compound described below. It is believed that the surface treatment method according to the present invention results in a structure in which the lone electron pair of the phosphorus-containing Lewis base provides electrons to the surface of the nickel particles to form a coordinate bond. The phosphorus-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.

Examples of Lewis bases containing phosphorus include phosphate esters and polyphosphate esters shown in the following formula (1), phosphate esters and polyester phosphates shown in (2), and phosphate ethers and polyether phosphates shown in (9). As Lewis bases containing phosphorus, phosphate esters and polyphosphate esters shown in the following formula (1), phosphate esters and polyester phosphates shown in (2), and phosphate ethers and polyether phosphates shown in (9) may be used alone or in combination of two or more.

In formula (1), n is a natural number of 1 to 5, R ₁ represents H or an alkyl group having 7 to 15 carbon atoms, and R ₂ represents an alkyl group having 7 to 15 carbon atoms or a tert-butyl group represented by formula (A).

In formula (2), m is a natural number of 1 to 5, _R3 represents H or an alkyl group having 7 to 15 carbon atoms, and _R4 represents an alkyl group having 7 to 15 carbon atoms or the tert-butyl group represented by formula (A).

In formula (9), r is a natural number of 1 to 5, R ₉ represents H or an alkyl group having 7 to 15 carbon atoms, and R ₁₀ represents an alkyl group having 1 to 15 carbon atoms or the tert-butyl group represented by formula (A).

In particular, examples of Lewis bases that can be dissolved in a solvent capable of forming a slurry of nickel particles include phosphate esters, polyphosphate esters, phosphate polyesters, phosphate ethers, phosphate polyethers, and polyether phosphate esters, including the compounds shown in the above formulas (1), (2), and (9). For example, DISPERBYK (registered trademark) (hereinafter the same) 110, 111, 142, and 145 (manufactured by BYK Japan), Solsperse (registered trademark) (hereinafter the same) 41000 and 3600 (manufactured by Lubrizol), Disparlon (registered trademark) DA-375 (manufactured by Kusumoto Chemical Industries, Ltd.), Hiplat (registered trademark) ED152, ED153, ED154, ED174, and ED251 (manufactured by Kusumoto Chemical Industries, Ltd.), and Phosphanol (registered trademark) (hereinafter the same) RS-610 and RS-710 (manufactured by Toho Chemical Industries, Ltd.) can be used.

<Nitrogen-containing Lewis bases>
The compounds used for the surface treatment are the above-mentioned phosphorus-containing Lewis base compound and a nitrogen-containing Lewis base compound. It is believed that the surface treatment method of the present invention results in a structure in which the lone electron pair of the nitrogen-containing Lewis base provides electrons to the surface of the nickel particles to form a coordinate bond. 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.

Examples of nitrogen-containing Lewis bases include compounds represented by the following formula (3):

In formula (3), ^R5 represents an alkyl group having 1 to 18 carbon atoms or an alkenyl group having 1 to 18 carbon atoms, ^R6 represents an alkylene group having 1 to 6 carbon atoms, and ^R7 and ^R8 represent H or _CH3 .

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 formulas (4) to (6).

The phosphorus-containing Lewis base and the nitrogen-containing Lewis base are effective when used in combination, and the mass ratio of the phosphorus-containing Lewis base to the nitrogen-containing Lewis base can be set to 3:7 to 7:3. By setting the ratio of the phosphorus-containing Lewis base to the nitrogen-containing Lewis base in this range, the generation of coarse particles due to the aggregation of nickel particles can be more effectively suppressed.

(Amount added)
The amount of the phosphorus-containing Lewis base and the nitrogen-containing Lewis base added is preferably such that the mass ratio of the total amount of the phosphorus-containing Lewis base and the nitrogen-containing Lewis base to the nickel particles is 0.16 to 3.0:100 in the state of the Lewis base-containing nickel particles after the surface treatment. By having the Lewis base content of 0.16% by mass or more relative to 100% by 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 fine particles can be further improved. On the other hand, by having the Lewis base content of 3.0% by 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 Lewis base.

(solvent)
The solvent for dispersing nickel particles to be surface-treated to form a slurry, and the solvent for dissolving the phosphorus-containing Lewis base and 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, 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 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.

<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 surface treatment method of 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 a solution containing a Lewis base containing phosphorus and a Lewis base containing nitrogen.

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 the Lewis base to form a coordinate bond on the surface of the nickel particles.

The mass ratio of the phosphorus-containing Lewis base to the nitrogen-containing Lewis base, and the mass ratio of the total amount of the phosphorus-containing Lewis base and the nitrogen-containing Lewis base to the nickel particles can be the same as in the mixing step in the above-mentioned nickel powder manufacturing method. Similarly to the above-mentioned nickel powder manufacturing method, the phosphorus-containing Lewis base may contain one or more phosphorus-containing Lewis base compounds selected from the compounds represented by the above formulas (1), (2), and (9), and the nitrogen-containing Lewis base may contain a nitrogen-containing Lewis base compound represented by the formula (3). In particular, examples of phosphorus-containing Lewis bases include phosphate esters, polyphosphate esters, phosphate polyesters, phosphate ethers, phosphate polyethers, polyether phosphate esters, and the like, which contain the compounds represented by the above formulas (1), (2), and (9). For example, DISPERBYK (registered trademark) (hereinafter the same) 110, 111, 142, 145 (manufactured by BYK Japan), Solsperse (registered trademark) (hereinafter the same) 41000, 3600 (manufactured by Lubrizol), Disparlon (registered trademark) DA-375 (manufactured by Kusumoto Chemical Industries, Ltd.), Hyplad (registered trademark) ED152, ED153, ED154, ED174, ED251 (manufactured by Kusumoto Chemical Industries, Ltd.), Phosphanol (registered trademark) (hereinafter the same) RS-610, RS-710 (manufactured by Toho Chemical Industries, Ltd.), etc. can be used. In addition, as the nitrogen-containing Lewis base, one or more types selected from N-oleoyl sarcosine, N-lauroyl sarcosine, and myristoyl methyl-β-alanine can be used as the Lewis base.

[Lewis base-containing nickel powder]
By the above-mentioned method for producing nickel powder or the method for surface treating nickel particles of the present invention, a Lewis base-containing nickel powder can be obtained.

In the Lewis base-containing nickel particles of the present invention, a phosphorus-containing Lewis base and a nitrogen-containing Lewis base are present on the surface of the nickel particles, and the mass ratio of the total amount of the phosphorus-containing Lewis base and the nitrogen-containing 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 of the amount of Lewis base present.

It is believed that the lone electron pair of the phosphorus-containing Lewis base and the lone electron pair of the nitrogen-containing Lewis base donate electrons to the surface of the nickel particles to form a coordinate bond, resulting in the Lewis base being coordinated to the surface of the nickel particles. Lewis base compounds are capable of efficiently adsorbing to the surface of nickel particles and forming coordinate bonds, which can suppress the generation of coarse particles due to the aggregation of nickel particles.

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]
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 and obtain nickel powder.

<Surface treatment of nickel particles>
A Lewis base solution prepared by dissolving 0.05 g of N-oleoyl sarcosine represented by chemical formula (4) and 0.05 g of phosphate polyester represented by chemical formula (2) as Lewis bases 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 for 1 hour using a three-one motor, and then stirred at room temperature of 25° C. for 3 hours to obtain nickel particle slurry. This 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 in which 0.05 g of N-lauroyl sarcosine represented by chemical formula (5) and 0.05 g of phosphate polyester represented by chemical formula (2) were 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. This 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 0.05 g of myristoylmethyl-β-alanine represented by chemical formula (6) and 0.05 g of phosphate polyester represented by chemical formula (2) 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 in which 0.05 g of N-oleoyl sarcosine represented by chemical formula (4) and 0.05 g of phosphoric ether represented by chemical formula (9) were 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. This 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 the Lewis base compound, the conditions were the same as in Example 1. That is, nickel particles were produced without carrying out surface treatment with a Lewis base.

[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 (7) and (8).

[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) (7)

[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 (8)

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 Example 1 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 a SEM image 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 a diameter 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 4 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.

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 1 at initial stage and after standing for 180 days.

The results of Examples 1 to 4 and Comparative Example 1 show that the surface treatment did not significantly change the average particle size of the nickel particles, and no increase in 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 in Comparative Example 1 showed that when the surface was not treated with a phosphorus-containing Lewis base or a nitrogen-containing Lewis base, an increase in coarse particles was observed after 180 days in air.

In addition, in the SEM image of Comparative Example 1 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 surface treatment method for nickel particles and the manufacturing method for nickel powder of the present invention can suppress the generation of coarse particles due to oxidation during storage by surface treatment with a phosphorus-containing Lewis base and a nitrogen-containing Lewis base.

Claims (6)

The method includes a mixing step of mixing a reduction reaction solution containing nickel particles crystallized by reducing a water-soluble nickel salt with a phosphorus-containing Lewis base and a nitrogen-containing Lewis base,
the phosphorus-containing Lewis base comprises at least one selected from the group consisting of a phosphoric acid ester and a polyphosphoric acid ester represented by the following formula (1), a phosphoric acid ester and a phosphoric acid polyester represented by the following formula (2), and a phosphoric acid ether and a phosphoric acid polyether represented by the following formula (9),
The nitrogen-containing Lewis base comprises a compound represented by the following formula (3):
a mass ratio of the total amount of the phosphorus-containing Lewis base and the nitrogen-containing Lewis base to the nickel particles is 0.16 to 3.0:100 .

In formula (1), n is a natural number of 1 to 5, R ₁ represents H or an alkyl group having 7 to 15 carbon atoms, and R ₂ represents an alkyl group having 7 to 15 carbon atoms or a tert-butyl group represented by formula (A).
In formula (2), m is a natural number of 1 to 5, R3 _represents H or an alkyl group having 7 to 15 carbon atoms, and R4 _represents an alkyl group having 7 to 15 carbon atoms or the tert-butyl group represented by formula (A).
In formula (9), r is a natural number of 1 to 5, R ₉ represents H or an alkyl group having 7 to 15 carbon atoms, and R ₁₀ represents an alkyl group having 1 to 15 carbon atoms or the tert-butyl group represented by formula (A).
In formula (3), R5 ^represents an alkyl group having 1 to 18 carbon atoms or an alkenyl group having 1 to 18 carbon atoms, R6 ^represents an alkylene group having 1 to 6 carbon atoms, and R7 ^and R8 ^represent H or _CH3 . The surface treatment method for nickel particles according to claim 1, wherein the phosphorus-containing Lewis base and the nitrogen-containing Lewis base are coordinated to the surface of the nickel particles. The surface treatment method for nickel particles according to claim 1 or 2 , wherein the mass ratio of the phosphorus-containing Lewis base to the nitrogen-containing Lewis base is 3:7 to 7:3. 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 a phosphorus-containing Lewis base and a nitrogen-containing Lewis base;
Including ,
the phosphorus-containing Lewis base comprises at least one selected from the group consisting of a phosphoric acid ester and a polyphosphoric acid ester represented by the following formula (1), a phosphoric acid ester and a phosphoric acid polyester represented by the following formula (2), and a phosphoric acid ether and a phosphoric acid polyether represented by the following formula (9),
The nitrogen-containing Lewis base comprises a compound represented by the following formula (3):
a mass ratio of the total amount of the phosphorus-containing Lewis base and the nitrogen-containing Lewis base to the nickel particles is 0.16 to 3.0:100 .

In formula (1), n is a natural number of 1 to 5, R ₁ represents H or an alkyl group having 7 to 15 carbon atoms, and R ₂ represents an alkyl group having 7 to 15 carbon atoms or a tert-butyl group represented by formula (A).
In formula (2), m is a natural number of 1 to 5, R3 _represents H or an alkyl group having 7 to 15 carbon atoms, and R4 _represents an alkyl group having 7 to 15 carbon atoms or the tert-butyl group represented by formula (A).
In formula (9), r is a natural number of 1 to 5, R ₉ represents H or an alkyl group having 7 to 15 carbon atoms, and R ₁₀ represents an alkyl group having 1 to 15 carbon atoms or the tert-butyl group represented by formula (A).
In formula (3), R5 ^represents an alkyl group having 1 to 18 carbon atoms or an alkenyl group having 1 to 18 carbon atoms, R6 ^represents an alkylene group having 1 to 6 carbon atoms, and R7 ^and R8 ^represent H or _CH3 . The method for producing nickel powder according to claim 4 , wherein the mass ratio of the phosphorus-containing Lewis base to the nitrogen-containing Lewis base is 3:7 to 7:3. The method for producing nickel powder according to claim 4 or 5 , further comprising a drying step of drying the surface-treated nickel particles after the mixing step.

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