Nickel particles, surface treatment method for nickel particles, and method for producing nickel powder

Abstract

To provide nickel grains in which the generation of coarse grains caused by oxidation during storage is suppressed, a surface treatment method for nickel grains and method for producing nickel powder.SOLUTION: A surface treatment method for nickel grains comprises a mixing step in which a reduction reaction solution comprising nickel grains crystallized by reducing a water-soluble nickel salt with oleic acid and oleylamine, in which the mass ratio between the oleic acid and the oleylamine is 3:7 to 7:3.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to nickel particles, a method for surface treatment of nickel particles, and a method for producing nickel powder. [Background technology]

[0002] Nickel particles are used as an ingredient in conductive pastes for producing thick-film conductors, and are used in the formation of electrical circuits and in the electrodes of multilayer ceramic components such as multilayer ceramic capacitors (MLCCs) and multilayer ceramic substrates.

[0003] An MLCC using this electrode is manufactured by the following method, for example, when nickel particles are used as the metal powder.

[0004] First, a conductive paste obtained by kneading nickel particles, a resin such as ethyl cellulose, and an organic solvent such as terpineol is 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 film for the internal electrodes.

[0005] Next, the printed nickel coatings for the internal electrodes and the dielectric green sheets are layered alternately and pressed together to produce a laminate.

[0006] The laminate thus produced is cut to a specified size, and after a binder removal process is performed to burn off the ethyl cellulose and other resins used as organic binders, the dielectric and internal electrodes (nickel films) are sintered by high-temperature firing at about 1300°C to obtain a ceramic body in which the dielectric layers and internal electrode layers are laminated together. External electrodes are then attached to this ceramic body to form a multilayer ceramic capacitor.

[0007] The binder removal treatment of the laminate is carried out in an atmosphere containing an extremely small amount of oxygen so as to prevent the nickel particles from being oxidized.

[0008] Generally, nickel paste used in the internal electrodes of MLCCs is manufactured by kneading nickel powder in a vehicle, and contains many nickel powder aggregates. The manufacturing process of nickel powder usually has a drying process at the final stage, regardless of the manufacturing method of nickel powder (gas phase method or liquid phase method). Since the drying process promotes the aggregation of nickel particles, the resulting nickel powder generally contains aggregates generated during drying as coarse particles.

[0009] In order to achieve 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 1000. For this reason, there is a study being conducted to thin the thickness of the internal electrode layers from the conventional several microns level to the submicron level, and accordingly, progress is being made in reducing the particle size of nickel powder, the electrode material for the internal electrodes.

[0010] However, the smaller the particle size, the larger the surface area of ​​the nickel powder becomes, and the larger the surface energy becomes, making it easier to form aggregates. In addition, metal powders such as nickel powder have poor dispersibility, and if aggregates are present, the nickel powder may penetrate the ceramic sheet layer when sintered in the firing process during MLCC manufacturing, 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 has been the cause of the deterioration of the life of the multilayer ceramic capacitor. Thus, in MLCC, it is important to manufacture a nickel paste with few coarse particles, including aggregates, and to obtain internal electrodes with a smooth surface without unevenness. In addition, since there is a concern that the presence of aggregates of nickel particles may cause product defects, it is desired to improve the surface condition of the nickel particles so that aggregates do not occur.

[0011] 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, in the surface oxidation treatment using hydrogen peroxide, it may be difficult to uniformly control the oxidation treatment. [Prior art documents] [Patent documents]

[0012] [Patent Document 1] Japanese Patent Application Publication No. 11-343501 Summary of the Invention [Problem to be solved by the invention]

[0013] 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 nickel hydroxide on the surface. Therefore, it is important to suppress the oxidation of nickel powder during storage, especially for nickel powder used in MLCCs, to prevent defects caused by the generation of coarse particles due to oxidation during storage.

[0014] The present invention has been proposed in consideration of the above-mentioned circumstances, and aims to provide nickel particles that suppress the generation of coarse particles due to oxidation during storage, a method for surface treatment of nickel particles, and a method for producing nickel powder. [Means for solving the problem]

[0015] The present inventors have conducted extensive research to solve the above-mentioned problems, and as a result, have found that the aggregation of nickel particles can be suppressed by coating the nickel particles with oleic acid and oleylamine, thereby completing the present invention.

[0016] 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, wherein the mass ratio of the oleic acid to the oleylamine is 3:7 to 7:3.

[0017] The mixing step may cause the oleic acid and oleylamine to be coordinated to the surfaces of the nickel particles.

[0018] A mass ratio of the total of the oleic acid and the oleylamine to the nickel particles may be 0.16 to 3.0:100.

[0019] In addition, in order to solve the above problems, the method for producing nickel powder of the present invention includes a crystallization process in which a water-soluble nickel salt is reduced in a reduction reaction solution to crystallize nickel particles, and a mixing process in which the reduction reaction solution containing the nickel particles after the crystallization process is mixed with oleic acid and oleylamine, and the mass ratio of the oleic acid to the oleylamine is 3:7 to 7:3.

[0020] The mixing step may cause the oleic acid and oleylamine to be coordinated to the surfaces of the nickel particles.

[0021] A mass ratio of the total of the oleic acid and the oleylamine to the nickel particles may be 0.16 to 3.0:100.

[0022] After the mixing step, a drying step of drying the surface-treated nickel particles may be included.

[0023] In addition, in order to solve the above problems, the nickel particles of the present invention are oleic acid- and oleylamine-containing nickel particles, 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.

[0024] 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.

[0025] 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 having a particle size exceeding 0.8 μm of 400 mass ppm or less, and a content of particles having a particle size exceeding 1.2 μm of 200 mass ppm or less. Effect of the Invention

[0026] According to the present invention, it is possible to provide nickel particles that suppress the generation of coarse particles due to oxidation during storage, a method for treating the surface of nickel particles, and a method for producing nickel powder. [Brief description of the drawings]

[0027] [Figure 1] 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. [Diagram 2] 1 is a SEM photograph of nickel particles of Example 1, Example 2, and Comparative Example 1. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028] Hereinafter, the embodiments of the present invention will be described in detail with reference to the drawings. Fig. 1 is a flow diagram showing the surface treatment method of nickel particles and the manufacturing method of nickel powder of the present invention as a series of steps. However, the present invention is not limited to the following embodiments.

[0029] [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.

[0030] <1: Nickel salt aqueous solution preparation process> 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.

[0031] When nickel chloride is selected, the concentration of the nickel chloride aqueous solution is preferably adjusted to a range of 50 to 200 g / L of nickel. If the nickel concentration is too low, a relatively large facility is required and the amount of waste liquid increases, which is not efficient. On the other hand, if the concentration is too high, it may be difficult to control the reduction reaction, making it difficult to obtain nickel particles with the desired particle size. Also, when other nickel salts are used, it is preferable to adjust the nickel concentration to a range of 50 to 200 g / L for the same reason.

[0032] The nickel salt aqueous solution can be prepared, for example, by dissolving a predetermined amount of nickel salt in pure water, etc. The nickel salt aqueous solution may be prepared by the user, or a prepared nickel salt aqueous solution may be purchased, etc.

[0033] <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 reducing agent is added, and a reduction treatment is performed in which the water-soluble nickel salt is reduced in the reduction reaction solution.

[0034] The metal serving as the particle size control agent for nickel powder may be selected from copper, palladium, platinum, rhodium, etc., which have a smaller ionization tendency than nickel, and palladium or copper, which is solid-soluble with nickel, is preferable. The metal serving as the particle size control agent acts as a nucleus 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 dissolve in nickel, it is desirable to add it if it can dissolve in a metal acting as a nucleus, because it can make the nucleus fine and reduce the content of metals other than nickel contained in one nickel particle, thereby enabling high purification of the nickel powder.

[0035] The amount of elements that dissolve with nickel should be 500 ppm by mass or less, since the nickel powder cannot be refined even if more than 500 ppm by mass is added to the total amount of nickel. Elements that dissolve with the core 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 produce the effect of refining the core and refining the nickel powder.

[0036] 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, which will reduce the purity of the nickel powder and even increase the average particle size of the nickel powder, so it is desirable for them to be monodisperse.

[0037] In order to suppress the aggregation of the core metal particles and maintain monodispersity, it is desirable to add molecules having a protective colloid effect to the reduction reaction solution. Polymer materials such as gelatin and polyvinylpyrrolidone and surfactants are suitable as materials having a protective colloid effect. The amount of this material added is desirably 0.0025 to 0.5 mass % or less relative to nickel. If the amount is too large, the reduction of nickel ions may be inhibited, and if the amount is too small, the desired protective colloid effect may not be obtained.

[0038] 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 may be any agent capable of forming a nickel complex, and compounds having an amino group, a carboxyl group, a sulfo group, a hydroxyl group, an alkyl group, or the like 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.

[0039] <3:Mixing process> In the mixing step (surface treatment step in FIG. 1), the reduction reaction solution containing the nickel particles after the crystallization step is mixed with oleic acid and oleylamine. As a method for surface treating the nickel powder with oleic acid and oleylamine, for example, a method of wet mixing the reduction reaction solution containing the precipitated nickel particles with a mixed solution of oleic acid and oleylamine in water or alcohol containing oleic acid and oleylamine is given. That is, the reduction reaction solution containing the 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 the oleic acid and oleylamine are uniformly adsorbed on the particle surfaces of the nickel powder.

[0040] 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. At this time, 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 adhesion rate of oleic acid and oleylamine to the nickel particles may decrease, and the stirring time may be long. Also, if the treatment temperature is higher than 50°C, the volatilization of the water-soluble organic solvent is promoted, which may make the surface treatment difficult.

[0041] Furthermore, the stirring time 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 may be small. Even if the stirring time is longer than 24 hours, the amount of oleic acid and oleylamine attached hardly increases, and if the treatment time is longer, the nickel powder production time will also be longer, which may increase the cost.

[0042] <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.

[0043] Oleic acid and oleylamine are effective when used in combination, and the mass ratio of oleic acid to oleylamine is 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.

[0044] 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 of oleic acid and oleylamine compound relative to 100% by mass of nickel particles before surface treatment. That is, the mass ratio of the total of oleic acid and oleylamine to nickel particles is preferably 0.16-3.0:100. By making the total content of oleic acid and oleylamine 0.16% by mass or more relative to 100% by mass of nickel particles, the amount of adsorption to nickel particles can be increased, and as a result, the dispersibility of nickel microparticles can be further improved. On the other hand, by making the total content of oleic acid and oleylamine 3.0% by mass or less, the change in viscosity of the nickel paste due to the increase in the amount of oleic acid and oleylamine can be prevented.

[0045] (solvent) The organic solvent is not particularly limited as long as it can disperse nickel particles and dissolve oleic acid and oleylamine. 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, C n H 2n+2 , C n H 2n , C n H 2n-2 Aliphatic hydrocarbons represented by C n H 2n-6 Specific examples of the aromatic hydrocarbons 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.

[0046] <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 terminated.

[0047] For filtering the nickel particles, a general-purpose solid-liquid separation device can be used, and specific examples thereof 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.

[0048] <5: Drying process> The method for producing nickel powder of the present invention may include a drying step of drying the surface-treated nickel particles after the mixing step. For example, the drying step can be performed after the washing and filtering steps.

[0049] For example, nickel powder can be obtained by drying the nickel powder at 50 to 300° C., preferably 80 to 150° C., using a general-purpose drying device such as an inert gas atmosphere dryer or a vacuum dryer so as not to oxidize the nickel. The nickel powder may be passed through a mesh as necessary.

[0050] If necessary, a washing and filtration process may be carried out to obtain a nickel powder hydrous cake, and the adhering water in this cake may be replaced with a low-temperature volatile organic solvent such as ethanol, and then the cake may be dried in the above-mentioned inert gas atmosphere dryer or vacuum dryer to weaken the drying aggregation between nickel particles that occurs during drying due to the large surface tension of water.

[0051] [Surface treatment method for nickel particles] Next, an example of the surface treatment method for 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.

[0052] For example, the crystallization step in the aforementioned method for producing nickel powder may be carried out to reduce a 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.

[0053] The mixing step is the same as the mixing step in the above-mentioned method for producing nickel powder, and it is believed that this step causes oleic acid and oleylamine to form coordinate bonds with the surfaces of the nickel particles.

[0054] The nickel particles to be surface-treated are not limited to nickel particles crystallized by reducing a water-soluble nickel salt, and various nickel powders can be used regardless of the manufacturing method, such as a wet method or a dry method. That is, nickel particles may be surface-treated in the state of a reduction reaction solution without drying, or dried powdery nickel particles may be slurried and surface-treated. 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.

[0055] The surface treatment method for dried powdered nickel particles will be described below.

[0056] <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.

[0057] As described above, the effect is exhibited by using oleic acid and oleylamine in combination, and the mass ratio of oleic acid to oleylamine is 3:7 to 7:3.

[0058] 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 of oleic acid and oleylamine compound relative to 100% by mass of nickel particles before surface treatment.

[0059] (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.

[0060] In the mixing step, nickel particles to be surface-treated may be added to a solution in which oleic acid and oleylamine are 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.

[0061] Specifically, the nickel slurry is stirred, 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.

[0062] For example, an oleic acid and oleylamine solution is 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 dropped at a dropping rate of 2 ml / min into nickel slurry being stirred at a peripheral speed of 5 to 10 m / sec. Note that a stirring blade with a shape such as a soft cross, a paddle cross, or a four-blade inclined paddle can be used to stir the nickel slurry.

[0063] The temperature of the mixed solution of nickel particles, oleic acid, and oleylamine during surface treatment is preferably 20 to 50° C. If the temperature of the mixed solution is lower than 20° C., the adhesion rate of oleic acid and oleylamine to the nickel particles may decrease, and the stirring time of the mixed solution during surface treatment may become longer. Also, if the temperature of the mixed solution is higher than 50° C., the evaporation of the solvent is promoted, which may make the surface treatment difficult.

[0064] 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. 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 stirring of the mixed solution may be performed under the condition of a peripheral speed of 5 to 10 m / sec, similar to the mixing process.

[0065] (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.

[0066] 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 an oxidizable region remains on the surface of the nickel particles, which may generate nickel hydroxide over time, resulting in the generation of coarse particles. In addition, it is difficult to make oleic acid and oleylamine exist when the mass ratio exceeds 0.1:100, and the mass ratio of 0.1:100 is considered to be the upper limit of the amount of oleic acid and oleylamine present.

[0067] It is believed that the lone electron pairs of oleic acid and oleylamine provide electrons to the surface of the nickel particles to form a coordinate bond, which causes the oleic acid and oleylamine to coordinate to the surface of the nickel particles. Oleic acid and oleylamine can be efficiently adsorbed to the surface of the nickel particles and coordinate to suppress the generation of coarse particles due to the aggregation of nickel particles.

[0068] In order to meet the recent trend of thinner internal electrodes in multilayer ceramic capacitors, the nickel particles containing oleic acid and oleylamine preferably have a number average particle size of 0.03 μm to 0.4 μm. The number average particle size is the number average particle size determined from a scanning electron microscope (SEM) image of the nickel particles.

[0069] Similarly, from the viewpoint of accommodating thin layers, it is preferable that the particle shape is approximately spherical, and the 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) and the like.

[0070] Furthermore, 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.

[0071] 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-layered 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 obtained from the SEM image. EXAMPLES

[0072] 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.

[0073] [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.

[0074] 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.

[0075] Then, 0.5 L of an aqueous nickel chloride solution with a nickel concentration of 100 g / L was dropped into this alkaline colloidal solution as an aqueous nickel salt solution to reduce nickel, thereby obtaining nickel powder.

[0076] <Surface treatment of nickel particles> A solution of 10 g of dihydroterpineol, 0.025 g of oleic acid and 0.025 g of oleylamine as surface treatment agents, 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 separated into solid and liquid by filtration, and dried at 120° C. under a nitrogen atmosphere to obtain the desired nickel particles.

[0077] [Example 2] A solution of 0.015g of oleic acid and 0.035g of oleylamine dissolved in 10g of dihydroterpineol was added to about 100g of reduction reaction solution containing 10g 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. 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.

[0078] [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 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.

[0079] [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 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. 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.

[0080] [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 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. 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.

[0081] 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 the surface treatment with oleic acid and oleylamine.

[0082] 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.

[0083] 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.

[0084] Table 1 shows the mass ratio of nickel particles to surface treatment agent in Examples 1 to 5 and Comparative Examples 1 to 3. In Table 1, the amount of surface treatment agent is the total amount of oleic acid and oleylamine.

[0085] [Table 1]

[0086] [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 (LECO CS844).

[0087] <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 these were diluted with methanol to adjust the concentration 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 this value, the contents of oleic acid and oleylamine contained in the nickel particles after surface treatment were calculated from the following formulas (1) and (2).

[0088] [Number 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)

[0089] [Number 2] Oleic acid and oleylamine contents (mass%) in Ni particles after surface treatment = (Amount of oleic acid and oleylamine contained in Ni particles after surface treatment / Amount of Ni particles after surface treatment) × 100 (2)

[0090] 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 the observation image (SEM image) of the nickel powder using a scanning electron microscope (SEM, JSM-6360, manufactured by JEOL Ltd.).

[0091] <Measurement of the content of coarse particles> The nickel powder after the surface treatment was placed in a 100 ml high vessel container (BHB-100 manufactured by Kinki Yoki 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 can be seen in the obtained SEM images were measured, and the diameter of each particle was calculated from these, and particles with diameters of 0.8 μm or more and 1.2 μm or more were counted as coarse particles. The content of coarse particles contained in the nickel powder (when the particle size exceeds 0.8 μm and when the particle size exceeds 1.2 μm) was calculated as an initial value.

[0092] In addition, after taking the SEM images, the nickel particles of Examples 1 to 5 and Comparative Examples 1 to 3 were left to stand at room temperature in an air atmosphere for 180 days, and the content of coarse particles was then measured in the same manner.

[0093] The contents of oleic acid and oleylamine contained in the nickel particles, the amount of carbon on the nickel particle surface, the average particle size, and the content of coarse particles initially and after being left for 180 days are shown in Table 2. Also, Fig. 2 shows SEM photographs of the nickel particles initially and after being left for 180 days in Examples 1 and 2 and Comparative Example 1.

[0094] [Table 2]

[0095] From the results of Examples 1 to 5 and Comparative Examples 1 to 3, the average particle size of nickel particles did not change significantly due to the surface treatment, and no increase in coarse particles was observed. In particular, the surface treatment using oleic acid and oleylamine in combination 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).

[0096] However, from the results of the nickel particles of Comparative Example 1, when the surface treatment with oleic acid and oleylamine was not performed, an increase in coarse particles was observed after 180 days in an air atmosphere. Also, from the results of the nickel particles of Comparative Examples 2 and 3, the effect of suppressing the increase in coarse particles by treating the nickel particles with either oleic acid or oleylamine was observed, but this effect was more remarkable in the cases of Examples 1 to 5 in which oleic acid and oleylamine were used in combination.

[0097] 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.

[0098] [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

1. 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.

2. The method for treating the surface of 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 step.

3. The surface treatment method for nickel particles according to claim 1 or 2, wherein the mass ratio of the total of the oleic acid and the oleylamine to the nickel particles is 0.16:100 to 3.0:

100.

4. 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.

5. 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 step.

6. 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.

7. 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.

8. Oleic acid and oleylamine are present on the surface of the nickel particles, a mass ratio of the sum 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.

9. 9. The oleic acid and oleylamine containing nickel particles of claim 8, wherein said oleic acid and said oleylamine are coordinated to the surface of the nickel particles.

10. 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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