Nickel powder, nickel slurry, and method for producing nickel slurry

Abstract

Provided are a nickel powder and a nickel slurry which are fine particles and contain few deformed nickel particles, and a method for producing the nickel slurry. [Solution] Nickel powder having a number average particle size of 0.02 μm or more and less than 0.20 μm, a proportion of coarse particles with a particle size of more than 0.8 μm of 3,200 mass ppm or less, a number proportion of particles with an aspect ratio of 0.5 or less of 7% or less, and a number proportion of flattened particles of 5% or less.

Description

[Technical Field]

[0001] The present invention relates to a nickel powder, a nickel slurry, and a method for producing the nickel slurry. [Background technology]

[0002] Nickel powder is used as a material for capacitors in electronic circuits, particularly as a thick-film conductor material that forms the internal electrodes of multilayer ceramic components such as multilayer ceramic capacitors (MLCCs) and multilayer ceramic substrates, i.e., as an electrode material.

[0003] In recent years, the capacity of multilayer ceramic capacitors has increased, and the amount of internal electrode paste used to form the internal electrodes of multilayer ceramic capacitors has also increased significantly. As a result, inexpensive base metals such as nickel are being used as the metal powder for the internal electrode paste that constitutes the thick film conductor, instead of expensive precious metals.

[0004] In the process of manufacturing multilayer ceramic capacitors, an internal electrode paste made by kneading nickel powder, binder resin such as ethyl cellulose, and organic solvent such as terpineol is screen-printed onto a dielectric green sheet. The dielectric green sheet on which the internal electrode paste has been printed and dried is then laminated so that the internal electrode paste printed layers and the dielectric green sheet are alternately stacked and pressed together to obtain a laminate.

[0005] This laminate is cut to a predetermined size, then subjected to a debindering process in which the binder resin is removed by heat treatment, and then fired at a high temperature of about 1300°C to obtain a ceramic molded body.

[0006] Then, external electrodes are attached to the resulting ceramic compact to obtain a multilayer ceramic capacitor. Because base metals such as nickel are used as the metal powder in the internal electrode paste that becomes the internal electrodes, the binder removal process for the laminate is carried out in an atmosphere with an extremely low oxygen concentration, such as an inert atmosphere, to prevent the base metals from oxidizing.

[0007] As multilayer ceramic capacitors become smaller and their capacitance increases, the internal electrodes and dielectric layers are becoming thinner. This has led to the particle size of the nickel powder used in the internal electrode paste becoming finer, with nickel powder with an average particle size of 0.5 μm or less being required, and nickel powder with an average particle size of 0.3 μm or less becoming the norm.

[0008] As internal electrodes become thinner, the particle size of the nickel powder used becomes finer, and there is also a need to reduce the amount of coarse particles contained in the nickel powder. Coarse particles can be primary particles with significantly large particle sizes, but they also include connected particles and agglomerates formed by bonding multiple particles together. Crushing can be used to loosen the bonds between these connected particles.

[0009] Methods for crushing nickel powder can be broadly divided into dry crushing and wet crushing. For example, Patent Document 1 describes a dry crushing mechanism in which the material to be crushed is introduced into the nozzle through a high-pressure gas injection nozzle and a supply passage connected to the middle of the nozzle, and then injected toward a collision plate to crush it.

[0010] Dry crushing is an effective method for crushing large nickel powder particles (over 0.2 μm), but as the particle size of the nickel powder becomes smaller (lighter in weight), the kinetic energy decreases, resulting in a weaker crushing force at the time of collision. Furthermore, nickel powders smaller than 0.2 μm pose an increased risk of dust explosions and fires, so in order to crush them safely, an inert gas must be used, which increases manufacturing costs.

[0011] On the other hand, as an example of wet crushing, Patent Document 2 discloses a production method in which nickel powder with few coarse particles (connected particles) can be obtained without the risk of fire by crushing a nickel powder slurry with a specific slurry concentration using a crushing device having a specific crushing mechanism. [Prior art documents] [Patent documents]

[0012] [Patent Document 1] Japanese Patent Application Laid-open No. 58-143853 [Patent Document 2] Japanese Patent Application Publication No. 2018-178218 Summary of the Invention [Problem to be solved by the invention]

[0013] The nickel powder manufacturing method described in Patent Document 2 as described above is effective in reducing the coarse particles contained in the nickel powder, but in recent years, the electrode films of the internal electrodes of MLCCs have become thinner and thinner, and there is a demand for further reduction in coarse particles.

[0014] However, while conventional nickel powder crushing methods can achieve a certain degree of effect in reducing coarse particles, crushing can deform the spherical nickel particles, resulting in the occurrence of scattered deformed nickel particles after crushing. Deformed nickel particles make it difficult to thin the internal electrodes of MLCCs, and the presence of a certain amount of deformed nickel particles can result in a decrease in the performance of the MLCC.

[0015] Therefore, an object of the present invention is to provide a nickel powder, a nickel slurry, and a method for producing a nickel slurry, which are fine particles and contain few deformed nickel particles. [Means for solving the problem]

[0016] The present inventors have found that by mixing nickel powder with an organic solvent and then crushing it in a bead mill, it is possible to obtain nickel powder that maintains a fine particle size while containing few coarse particles (connected particles and agglomerates) and few deformed nickel particles, and nickel slurry obtained by dispersing this nickel powder in an organic solvent. The present invention was completed based on this finding.

[0017] In order to solve the above problems, the nickel powder of the present invention has a number average particle diameter of 0.02 μm or more and less than 0.20 μm, a proportion of coarse particles with a particle diameter of more than 0.8 μm of 3,200 mass ppm or less, a number proportion of particles with an aspect ratio of 0.5 or less of 7% or less, and a number proportion of flattened particles of 5% or less.

[0018] In order to solve the above problems, the nickel slurry of the present invention contains the nickel powder of the present invention and an organic solvent.

[0019] The nickel slurry of the present invention may have a water content of 0.5 mass % or less.

[0020] In addition, in order to solve the above problems, the method for producing a nickel slurry of the present invention includes a mixed solution preparation step of mixing nickel powder having a number average particle diameter of 0.02 μm or more and less than 0.20 μm with an organic solvent to prepare a mixed solution having a nickel powder concentration of 5 mass% or more and 60 mass% or less, and a crushing step of crushing the nickel powder in the mixed solution using a bead mill.

[0021] In the crushing step, crushing may be performed under conditions that satisfy at least one of the following: a filling rate of the beads in the vessel is more than 70% by volume; and an agitator peripheral speed is more than 6 m / s.

[0022] The nickel powder mixed in the mixed solution preparation step may have a moisture content of 0.5 mass % or less.

[0023] The mixed solution preparation step may be a step of preparing a mixed solution having a water content of 0.5% by mass or less. [Effects of the Invention]

[0024] According to the present invention, it is possible to provide a nickel powder, a nickel slurry, and a method for producing a nickel slurry, which are fine particles and contain few deformed nickel particles. DETAILED DESCRIPTION OF THE INVENTION

[0025] An example of a method for producing nickel powder will be described below, followed by a description of a nickel powder, a nickel slurry, and a method for producing the nickel slurry according to one embodiment of the present invention.

[0026] [1. Nickel powder manufacturing method] <1-1. Mixing process> (1-1-1. Mixing Agents) <1-2. Crystallization process> (1-2-1. Reduction reaction) (1-2-2. Mixture temperature) 1-3. Recovery of nickel crystallized powder [2. Nickel Slurry Manufacturing Method] <2-1. Mixed liquid creation process> <2-2. Crushing process> (2-2-1. Bead Mill) (2-2-2. Crushing conditions) [3. Nickel Slurry] [4. Nickel powder]

[0027] [1. Nickel powder manufacturing method] As a representative example of a method for producing nickel powder, a procedure for producing raw material nickel powder (nickel crystallized powder) by a reduction reaction using a wet method will be described. Note that the method for producing nickel powder is not limited to the following example, and examples include dry reduction methods such as a solid-phase reduction method in which a solid nickel salt is reduced with a reducing agent and a gas-phase reduction method in which a nickel salt is reduced with hydrogen gas, and the present invention is applicable not only to the wet method but also to nickel powder produced by these methods for producing nickel powder.

[0028] The nickel powder manufacturing method mainly involves a crystallization step in which a reaction solution containing a water-soluble nickel salt, a metal salt of a metal nobler than nickel, hydrazine as a reducing agent, an alkali hydroxide as a pH adjuster, water, and optionally an amine compound is reduced by hydrazine to obtain nickel crystallized powder. The method also includes a mixing step in which the water-soluble nickel salt, the metal salt of a metal nobler than nickel, hydrazine, and the alkali hydroxide are mixed within 10 seconds to obtain a mixture, thereby crystallizing fine nickel powder. An optional crushing step is also added as a post-treatment step. In this specification, "powder" and "powder" refer to a state in which many particles are aggregated together. Crystallized nickel particles dispersed in a slurry or dried into a solid aggregate are considered nickel powder. For example, "nickel crystallized powder" refers to nickel powder reduced in the crystallization step.

[0029] <1-1. Mixing process> The mixing step is a step of mixing a water-soluble nickel salt, a metal salt of a metal nobler than nickel, hydrazine, and an alkali hydroxide to obtain a mixture. Here, the water used as the solvent is preferably high-purity water such as ultrapure water (conductivity: ≦0.06 μS / cm) or pure water (conductivity: ≦1 μS / cm) in order to reduce the amount of impurities in the resulting nickel powder. Among these, pure water is preferred because it is inexpensive and easily available. The reduction reaction begins when the reaction solution is prepared. Each of the above-mentioned chemicals will be described in detail below.

[0030] (1-1-1. Mixing Agents) (a) Water-soluble nickel salts The water-soluble nickel salt to be used is not particularly limited as long as it is a water-soluble nickel salt that is easily soluble in water, and for example, one or more selected from nickel chloride, nickel sulfate, and nickel nitrate can be used. Of these nickel salts, nickel chloride, nickel sulfate, or a mixture thereof is more preferred.

[0031] (b) Salts of metals more noble than nickel Metal salts of metals more noble than nickel have a lower ionization tendency than nickel, and are therefore reduced before nickel when nickel is reduced and precipitated. This allows them to act as nucleating agents, which act as initial nuclei for the crystallization of nickel particles. Particle growth from these initial nuclei allows the production of even finer nickel crystallized powder (nickel powder).

[0032] The salt of a metal more noble than nickel may be any water-soluble metal salt of a metal that has a lower ionization tendency than nickel, such as water-soluble copper salts, gold salts, silver salts, platinum salts, palladium salts, rhodium salts, iridium salts, etc. For example, copper sulfate can be used as a water-soluble copper salt, silver nitrate can be used as a water-soluble silver salt, and sodium palladium(II) chloride, ammonium palladium(II) chloride, palladium(II) nitrate, palladium(II) sulfate, etc. can be used as a water-soluble palladium salt, but the examples are not limited to these.

[0033] As the salt of a metal more noble than nickel, it is preferable to use the above-mentioned palladium salt, because although the particle size distribution becomes somewhat broader, it is possible to more finely control the particle size of the resulting nickel powder. When a palladium salt is used, the ratio of palladium salt to nickel [mol ppm] (moles of palladium salt / moles of nickel × 10 6 ) can be appropriately selected depending on the desired number-average particle size of the nickel powder. For example, if the number-average particle size of the nickel powder is to be set to 0.1 μm or less, the ratio of palladium salt to nickel is preferably set within the range of 0.2 mol ppm to 500 mol ppm, and more preferably within the range of 0.5 mol ppm to 200 mol ppm. If this ratio is less than 0.2 mol ppm, it may be difficult to sufficiently refine the nickel crystallized powder (nickel powder). On the other hand, if this ratio exceeds 500 mol ppm, a large amount of expensive palladium salt will be used, which may lead to an increase in the cost of producing the nickel powder.

[0034] (c) Complexing agent The complexing agent may be any complexing agent containing a carboxyl group in the molecule, such as citric acid, nitrilotriacetic acid, 1,3,5-benzenetricarboxylic acid (trimesic acid), ethylenetetracarboxylic acid, 1,2,3,4-butanetetracarboxylic acid, and salts and derivatives thereof. It is more preferable to use one or more compounds selected from sodium nitrilotriacetate and its salts. The amount of complexing agent added is preferably 0.01 to 5 moles per mole of nickel. If the amount added is less than 0.01 mole per mole of nickel, the effect of homogenizing the particle size distribution will be insufficient. On the other hand, if the amount added exceeds 5 moles per mole of nickel, the cost of producing the nickel powder may increase.

[0035] (d) Hydrazine In the nickel powder manufacturing method, hydrazine (N2H4, molecular weight: 32.05) is used as a reducing agent. Hydrazine can be used in the form of anhydrous hydrazine or its hydrated form, hydrazine hydrate (N2H4·H2O, molecular weight: 50.06). Either can be used. The reduction reaction of hydrazine is shown in equation (2), which is described below. Hydrazine is particularly suitable as a reducing agent because it has high reducing power, is particularly alkaline, produces nitrogen gas and water as by-products, and therefore does not produce impurities in the reaction solution. Hydrazine contains few impurities, and is easily available. For example, commercially available industrial-grade 60% by weight hydrazine hydrate can be used.

[0036] (e) Alkali hydroxide The reducing power of hydrazine increases as the reaction solution becomes more alkaline (see formula (2) below). Therefore, in a method for producing nickel powder according to one embodiment of the present invention, an alkali hydroxide is used as a pH adjuster to increase the alkalinity. The alkali hydroxide is not particularly limited, but it is preferable to use an alkali metal hydroxide in terms of availability and cost. Specifically, it is more preferable to use one or more selected from sodium hydroxide and potassium hydroxide.

[0037] The amount of alkali hydroxide to be added should be determined so that the pH of the reaction solution is 9.5 or higher, preferably 10 or higher, and more preferably 10.5 or higher at the reaction temperature, in order to sufficiently increase the reducing power of hydrazine as a reducing agent.

[0038] In the nickel powder manufacturing method, the mixing step involves mixing solution B containing hydrazine and an alkali hydroxide with solution A containing a water-soluble nickel salt, a metal salt of a metal nobler than nickel, and a complexing agent (referred to as the "former step"), or mixing solution D containing an alkali hydroxide with solution C containing a water-soluble nickel salt, a metal salt of a metal nobler than nickel, a complexing agent, and hydrazine (referred to as the "latter step"). The presence of water-soluble nickel salt, hydrazine, and an alkali hydroxide can cause nickel crystallization due to reduction, although this is affected by temperature and pH. Therefore, by separating solutions A and B, or solutions C and D, and then mixing them all at once under conditions favorable for crystallization, fine nickel powder with a narrow particle size distribution can be obtained.

[0039] The amount of hydrazine used in the mixing step is preferably 1.0 mol or less, more preferably 0.5 mol or less, per mol of nickel. If the amount of hydrazine used is more than 1.0 mol per mol of nickel, the reducing power is high, and nuclei tend to form in localized areas where solutions A and B, or solutions C and D, intermix when they are mixed, making it difficult to obtain fine nickel crystallized powder or a narrow particle size distribution.

[0040] (The former stage) In the former stage, solution B containing hydrazine, which has been made highly alkaline by the addition of alkali hydroxide to increase its reducing power, is added to and mixed with solution A containing the substance to be reduced, such as a water-soluble nickel salt and a metal salt of a metal nobler than nickel, and a complexing agent.

[0041] In the former stage, depending on the temperature at the time when solutions A and B are mixed, i.e., at the time when the reduction reaction begins (hereinafter sometimes referred to as the "reaction initiation temperature"), if the time required to mix an aqueous solution containing a nickel salt (solution A) with an aqueous reducing agent solution (solution B) whose alkalinity has been increased with an alkali hydroxide increases, the alkalinity increases locally in the mixing region of solutions A and B during mixing, increasing the reducing power of hydrazine. This tends to cause a time lag in nucleation, making it difficult to obtain fine nickel crystallized powder or a narrow particle size distribution. This tendency is more pronounced when weakly acidic solution A is mixed with alkaline solution B. This tendency can be suppressed by shortening the mixing time of solutions A and B, thereby obtaining fine nickel crystallized powder with a narrow particle size distribution. Therefore, a short mixing time is desirable. Here, the mixing time is defined as the moment when solution B comes into contact with solution A and the moment when solution B has completely entered solution A.

[0042] When mixing solution A with solution B, it is preferable to perform stirring and mixing while stirring solution A. Good stirring and mixing properties reduce the non-uniformity between solutions A and B, depending on the location of nucleation, and also reduce the dependency of nucleation on the mixing time as described above, making it easier to obtain a finely divided nickel crystallized powder with a narrow particle size distribution. Any known method may be used for stirring and mixing, and from the standpoint of controlling stirring and mixing properties and equipment costs, it is preferable to use, for example, a stirring blade.

[0043] (The latter stage) The latter stage involves mixing the reducing agent hydrazine with the substance to be reduced, such as a water-soluble nickel salt or a metal salt of a metal nobler than nickel, and a complexing agent to form solution C, and then mixing solution D, which contains an alkali hydroxide, into solution C. This stage differs from the former stage in that the pH in the environment where the reducing agent and substance to be reduced coexist is adjusted with an alkali hydroxide to increase the reducing power, thereby causing crystallization.

[0044] In the latter stage, the hydrazine, water-soluble nickel salt, metal salt of a metal nobler than nickel, and complexing agent are already in uniform concentrations, so the time lag in nucleation that occurs when mixing the alkali hydroxide is not as large as in the former stage, making it easier to obtain finer nickel crystallized powder and a narrow particle size distribution. However, since there may be a slight time lag in nucleation, it is desirable to mix solutions C and D for a short time. Here, the mixing time begins when solution D comes into contact with solution C and ends when solution D has completely mixed into solution C.

[0045] When mixing solution C with solution D, it is preferable to perform stirring and mixing while stirring solution C. Good stirring and mixing properties reduce the non-uniformity between solutions C and D, depending on the location of nucleation, and also reduce the dependency of nucleation on the mixing time as described above, making it easier to obtain a finely divided nickel crystallized powder with a narrow particle size distribution. Any known method may be used for stirring and mixing, and from the standpoint of controlling stirring and mixing properties and equipment costs, it is preferable to use, for example, a stirring blade.

[0046] (f) Amine compounds The amine compound acts as an inhibitor of hydrazine self-decomposition, a reduction reaction accelerator, and an inhibitor of nickel particle bonding. It is a compound containing two or more primary amino groups (-NH2) in the molecule, or one primary amino group (-NH2) and one or more secondary amino groups (-NH-) in the molecule. Although it is not an essential agent in the present invention, it is preferable to use it in order to obtain these effects.

[0047] The amine compound may be at least one of alkyleneamines or alkyleneamine derivatives. More specifically, the alkyleneamine may be at least one selected from ethylenediamine (HNCHNH), diethylenetriamine (HNCHNHCHNH), triethylenetetramine (HN(CHNH)CHNH), tetraethylenepentamine (HN(CHNH)CHNH), and pentaethylenehexamine (HN(CHNH)CHNH). The alkyleneamine derivative may be at least one selected from tris(2-aminoethyl)amine (N(CHNH)), and (2-aminoethyl)-2-aminoethanol (HNCHNHCHOH). These alkyleneamines and alkyleneamine derivatives are water-soluble, and among them, ethylenediamine and diethylenetriamine are preferred because they are easily available and inexpensive.

[0048] The action of the above amine compound as a reduction reaction accelerator is due to the ionization of nickel ions (Ni 2+ It is thought that this is due to its function as a complexing agent, complexing hydrazine with hydrazine to form nickel complex ions. In addition, it is speculated that the actions of the amine compound as an inhibitor of the self-decomposition of hydrazine and as an inhibitor of the bonding between nickel particles are due to the interaction between the primary amino group (-NH2) and secondary amino group (-NH-) in the amine compound molecule and the surface of the hydrazine and nickel crystallized powder.

[0049] Here, the ratio of the amine compound to nickel in the reaction solution [mol %] ((moles of amine compound / moles of nickel) × 100) is preferably in the range of 0.01 mol % to 5 mol %, and more preferably in the range of 0.03 mol % to 2 mol %. If the ratio is less than 0.01 mol %, the amount of the amine compound is too small, and it may not be possible to obtain the functions of an inhibitor of hydrazine self-decomposition, an accelerator of the reduction reaction, or an inhibitor of linking of nickel particles together. On the other hand, if the ratio exceeds 5 mol %, the amine compound's function as a complexing agent for forming nickel complex ions may become too strong, which may cause abnormal particle growth in the nickel crystallized powder. This may result in the nickel powder losing its granularity and sphericity, resulting in an irregular shape, or the formation of many coarse particles in which nickel particles are linked together, which may result in deterioration of the nickel powder's properties.

[0050] In a method for producing nickel powder according to one embodiment of the present invention, either solution A or B may contain an amine compound, and either solution C or D may contain an amine compound. That is, the amine compound may be mixed in advance with solution A or B before the mixing step, which is before the start of the reduction reaction. The timing of mixing the amine compound can be appropriately selected based on a comprehensive judgment depending on the purpose.

[0051] In particular, the action of the amine compound as a reduction reaction accelerator can be such that by blending the amine compound before the start of the reduction reaction, a synergistic effect with the salt of a metal more noble than nickel can be achieved, allowing for the stable crystallization of fine nickel crystallization powder (nickel powder).

[0052] In other words, by adding an amine compound in advance, the amine compound has the advantage of acting as an inhibitor of hydrazine self-decomposition and a reduction reaction accelerator (complexing agent) from the start of the reduction reaction. On the other hand, the interaction of the amine compound with the nickel particle surface, for example, through adsorption, may contribute to nucleation and affect the particle size and particle size distribution of the resulting nickel crystallized powder. Therefore, it is important to appropriately consider various conditions, such as the type and amount of amine compound.

[0053] Furthermore, the method for producing nickel powder may include, after the mixing step, an amine compound mixing step of mixing the mixture with an amine compound. That is, the amine compound may be mixed after the mixing step and after the start of the reduction reaction.

[0054] In this case, the amine compound is added to and mixed with the reaction solution after the very early stage of the crystallization process, when nucleation occurs. Therefore, although the action of the amine compound as an inhibitor of hydrazine self-decomposition and as a reduction reaction accelerator (complexing agent) is somewhat delayed, the amine compound is no longer involved in nucleation, and therefore the particle size and particle size distribution of the resulting nickel crystallized powder are less likely to be affected by the amine compound, which has the advantage of making them easier to control.

[0055] Here, the mixing time for mixing the amine compound into the mixture in the amine compound mixing step may be a single mixing within a few seconds, or may be a divided mixing or dropwise mixing over a period of several to 30 minutes. Because the amine compound also acts as a reduction reaction accelerator (complexing agent), adding it slowly will cause crystal growth to proceed more slowly, resulting in a highly crystalline nickel crystallized powder. However, the hydrazine self-decomposition inhibitory effect also gradually takes effect, and the effect of reducing hydrazine consumption decreases. Therefore, the mixing time can be appropriately determined while balancing these two factors. Here, the mixing time for mixing the amine compound into the mixture begins the moment the amine compound comes into contact with the mixture and ends the moment the amine compound has completely entered the mixture.

[0056] When the amine compound is mixed into the mixture in the amine compound mixing step, stirring and mixing is preferred, in which the mixture is stirred while being mixed. Good stirring and mixing properties reduce the non-uniformity between the mixture and the amine compound, and also reduce the dependency on the mixing time of the alkali hydroxide as described above, making it easier to obtain a finely divided nickel crystallized powder with a narrow particle size distribution. Any known stirring and mixing method may be used, and from the standpoint of controlling the stirring and mixing properties and equipment costs, it is preferable to use, for example, a stirring blade.

[0057] (g) Other ingredients In addition to nickel salt, metal salt of a metal nobler than nickel, hydrazine, and alkali hydroxide, the reaction solution used in the crystallization step may contain various additives such as dispersants, complexing agents other than those mentioned above, and antifoaming agents. For example, the use of an appropriate dispersant or complexing agent in an appropriate amount can improve the granularity (sphericity) and surface smoothness of the nickel crystallized powder, and can reduce the number of coarse particles. Furthermore, the use of an appropriate antifoaming agent in an appropriate amount can suppress foaming during the crystallization step due to nitrogen gas (see Equations (2) to (4) below) generated during the crystallization reaction, thereby preventing, for example, the aqueous solution from overflowing from the container.

[0058] Known substances can be used as dispersants, such as alanine (CH3CH(COOH)NH2), glycine (H2NCH2COOH), triethanolamine (N(C2H4OH)3), and diethanolamine (also known as iminodiethanol) (NH(C2H4OH)2). Known substances can be used as complexing agents, such as hydroxycarboxylic acids, carboxylic acids (organic acids containing at least one carboxyl group), hydroxycarboxylic acid salts and derivatives, carboxylic acid salts and derivatives, specifically tartaric acid, malic acid, ascorbic acid, formic acid, acetic acid, pyruvic acid, and their salts and derivatives. Furthermore, the defoaming agent is not particularly limited as long as it has excellent foam-breaking properties under alkaline conditions, and oil-based or solvent-based silicone or non-silicone defoaming agents can be used. Methionine can function as an auxiliary agent for suppressing the self-decomposition of hydrazine and as an inhibitor of the formation of coarse particles. It also contributes to the spheroidization (surface smoothing) of nickel particles.

[0059] <1-2. Crystallization process> The crystallization step is a step of reducing the water-soluble nickel salt in the mixture to obtain nickel crystallized powder.

[0060] (1-2-1. Reduction reaction) In the mixture (i.e., reaction solution), the water-soluble nickel salt is reduced with hydrazine in the presence of an alkali hydroxide to obtain a crystallized nickel powder. Simultaneously with this reduction reaction, the self-decomposition of hydrazine can be significantly suppressed by the action of a trace amount of a specific amine compound.

[0061] First, the reduction reaction in the crystallization step will be explained. The reaction in which nickel ions crystallize to form nickel (Ni) is a two-electron reaction shown in the following formula (1). The reaction with hydrazine (N2H4) is a four-electron reaction shown in the following formula (2). For example, as described above, when nickel chloride (NiCl2) is used as the nickel salt and sodium hydroxide (NaOH) is used as the alkali hydroxide, the overall reduction reaction is expressed as shown in the following formula (3), in which nickel hydroxide (Ni(OH)2), produced by the neutralization reaction of nickel chloride and sodium hydroxide, is reduced with hydrazine. Stoichiometrically (theoretical value), 0.5 moles of hydrazine (N2H4) are required for 1 mole of nickel (Ni).

[0062] Here, from the reduction reaction of hydrazine in equation (2), it can be seen that the stronger the alkalinity of hydrazine, the greater its reducing power. The alkali hydroxide is used as a pH adjuster to increase alkalinity, and it plays a role in accelerating the reduction reaction of hydrazine.

[0063] [C1] Ni 2+ +2e - →Ni↓ (two-electron reaction) (1) N2H4→N2↑+4H + +4e - (four-electron reaction) (2) 2NiCl2+N2H4+4NaOH→2Ni(OH)2+N2H4+4NaCl →2Ni↓+N2↑+4NaCl+4H2O ···(3)

[0064] As mentioned above, in conventional crystallization processes, the active surface of nickel crystallization powder acts as a catalyst to promote the self-decomposition reaction of hydrazine, as shown in Equation (4) below, resulting in the consumption of large amounts of hydrazine as a reducing agent for purposes other than reduction. Therefore, depending on crystallization conditions such as the reaction initiation temperature, for example, approximately 2 moles of hydrazine per mole of nickel, approximately four times the theoretical amount required for the reduction, was typically used. Furthermore, as shown in Equation (4), the self-decomposition of hydrazine produces a large amount of ammonia as a by-product, resulting in a high concentration of ammonia in the reaction solution, resulting in the generation of nitrogen-containing wastewater. Thus, the use of excessive amounts of hydrazine, an expensive chemical, and the cost of treating nitrogen-containing wastewater contribute to the increased production costs of nickel powder (wet-process nickel powder).

[0065] [C2] 3N2H4→N2↑+4NH3 (4)

[0066] Therefore, in the method for producing nickel powder, it is preferable to use a specific amine compound to significantly suppress the self-decomposition reaction of hydrazine and significantly reduce the amount of hydrazine used, which is an expensive chemical. The reason why the amine compound can suppress the self-decomposition of hydrazine is thought to be (I) because the molecules of the specific amine compound are adsorbed on the surface of the nickel crystallized powder in the reaction solution, preventing contact between the active surface of the nickel crystallized powder and the hydrazine molecules, or (II) because the molecules of the specific amine compound act on the surface of the nickel crystallized powder, deactivating the catalytic activity of the surface.

[0067] In the conventional wet crystallization process, nickel ions (Ni ) such as tartaric acid and citric acid are used to shorten the reduction reaction time (crystallization reaction time) to a practical range. 2+Complexing agents that form complex ions with hydrazine and increase the concentration of ionic nickel are commonly used as reduction reaction accelerators. However, these complexing agents, such as tartaric acid and citric acid, do not function as inhibitors of the self-decomposition of hydrazine, as the specific amine compounds do, nor as inhibitors of linkage that inhibit the formation of coarse particles caused by linkage of nickel particles together during crystallization.

[0068] On the other hand, the specific amine compounds also function as complexing agents, similar to tartaric acid and citric acid, and have the advantage of simultaneously acting as an inhibitor of hydrazine self-decomposition, an inhibitor of linkage, and an accelerator of the reduction reaction.

[0069] (1-2-2. Mixture temperature) The temperature of the mixture obtained in the mixing step, i.e., the temperature of the mixture at the time the mixture is prepared in the mixing step, is 10°C to 30°C. If this temperature is below 10°C, the cost and time required for cooling each solution may be high. Furthermore, if this temperature is higher than 30°C, the crystallization reaction may begin in earnest during mixing, making it difficult to obtain finely divided nickel crystallized powder with a narrow particle size distribution. A temperature of 10°C to 25°C is more preferable because it makes it easier to obtain finely divided nickel crystallized powder with a narrow particle size distribution. Note that when the temperature of the mixture is 10°C to 30°C, the temperatures of the individual solutions A to D can be freely set without any particular restrictions, as long as the temperature of the mixture obtained by mixing them is within the above temperature range.

[0070] Furthermore, in the presence of water-soluble nickel salt, hydrazine, and alkali hydroxide, crystallization of nickel powder begins even at low temperatures. However, in one embodiment of the present invention, the temperature of the mixture may be adjusted to 40°C to 90°C using a water bath or the like to activate the reduction reaction and crystallize the nickel powder. The higher the mixture temperature, the more accelerated the reduction reaction and the more highly crystallized the nickel crystallized powder tends to be (higher crystallinity and larger crystallite size of nickel particles). However, the self-decomposition reaction of hydrazine tends to be accelerated even more. This increases hydrazine consumption and causes intense foaming of the reaction solution, which may cause the crystallization reaction to become unsustainable. On the other hand, if the mixture temperature is too low, the crystallinity of the nickel crystallized powder decreases significantly and the reduction reaction slows, significantly extending the crystallization process time and reducing productivity. For these reasons, by maintaining the mixture temperature in the range of 40°C to 90°C during the crystallization process, high-performance nickel crystallized powder can be produced inexpensively while suppressing hydrazine consumption and maintaining high productivity.

[0071] 1-3. Recovery of nickel crystallized powder The nickel crystallized powder produced by the reduction reaction in the crystallization step can be separated from the reaction solution using known procedures, for example, by washing, solid-liquid separation, and drying to obtain nickel powder. If desired, a sulfur compound such as a mercapto compound or a disulfide compound may be added to the reaction solution or washing solution containing the nickel crystallized powder to obtain nickel powder (nickel crystallized powder) that has been subjected to a surface treatment (sulfur coating treatment) to modify the surface of the nickel crystallized powder with a sulfur component.

[0072] Alternatively, nickel powder can be obtained by subjecting the obtained nickel powder to heat treatment, for example, at about 200°C to 300°C in an inert or reducing atmosphere. These sulfur coating treatments and heat treatments can control the binder removal behavior in internal electrodes and the sintering behavior of the nickel powder during the manufacture of the aforementioned multilayer ceramic capacitor, etc., and are therefore very effective when used within appropriate ranges. Furthermore, if necessary, it is more preferable to add a crushing process (post-processing process) described below in which the nickel powder obtained in the crystallization process is crushed, thereby obtaining nickel powder in which the number of coarse particles due to the connection of nickel particles generated during the nickel particle generation process in the crystallization process is reduced.

[0073] Specifically, nickel crystallized powder is subjected to solid-liquid separation from the reaction solution using a Denver filter, filter press, centrifuge, decanter, or the like, and is then thoroughly washed with high-purity water such as pure water (electrical conductivity: ≦1 μS / cm), and dried at 50°C to 300°C, preferably 80°C to 150°C, using a general-purpose drying device such as an atmospheric dryer, hot air dryer, inert gas atmosphere dryer, or vacuum dryer, to obtain nickel crystallized powder (nickel powder).

[0074] In addition, when drying is performed at approximately 200°C to 300°C in an inert, reducing, or vacuum atmosphere using drying equipment such as an inert gas atmosphere dryer or vacuum dryer, it is possible to obtain nickel crystallized powder (nickel powder) that has not only been dried but also been heat-treated. Heat treatment can change the surface condition of the nickel powder (ratio of nickel metal, nickel oxide, and nickel hydroxide). Specifically, the ratio of nickel oxide increases and the ratio of nickel hydroxide decreases. In addition, because heat treatment promotes crystal growth, the higher the drying temperature, the larger the crystallite diameter of the resulting nickel powder.

[0075] [2. Nickel Slurry Manufacturing Method] Next, an example of a method for producing a nickel slurry of the present invention will be described. As mentioned above, the raw nickel powder (nickel crystallized powder) obtained in the crystallization process by a wet method contains only a small proportion of coarse particles formed by nickel particles linking together during the reduction and precipitation process because the amine compound acts as a nickel particle linkage inhibitor during crystallization. However, as the particle size of the nickel powder decreases, the cohesive force between nickel particles increases, resulting in an insufficient dispersion. Therefore, a crushing process may be performed following the crystallization process to break down the coarse particles formed by linking nickel particles at their linkages and reduce the coarse particles. Reducing the coarse particles is particularly important for raw nickel powder with a number-average particle size of 0.02 μm or more but less than 0.20 μm, such as that used as a material for the internal electrodes of MLCCs.

[0076] In addition, even in nickel powders produced by other methods, such as solid-phase reduction methods in which solid nickel salts are reduced with a reducing agent or dry reduction methods such as gas-phase reduction methods in which nickel salts are reduced with hydrogen gas, the smaller the particle size, the stronger the cohesive force between nickel particles, making it more likely that coarse particles will be formed by bonding together. Therefore, it is necessary to reduce the coarse particles by crushing. In this case, too, it is important to reduce the coarse particles, especially for raw nickel powders with a number-average particle size of 0.02 μm or more but less than 0.20 μm.

[0077] However, while conventional nickel powder crushing methods can achieve a certain degree of effect in reducing coarse particles, crushing can deform the spherical nickel particles, resulting in the occurrence of scattered deformed nickel particles after crushing. Deformed nickel particles make it difficult to thin the internal electrodes of MLCCs, and the presence of a certain amount of deformed nickel particles can result in a decrease in MLCC performance.

[0078] Therefore, in the method for producing nickel slurry of the present invention, by carrying out the mixed liquid preparation step and the crushing step described below, not only can the amount of coarse particles be reduced, but deformation of spherical nickel particles due to crushing can also be suppressed.

[0079] <2-1. Mixed liquid creation process> In this process, nickel powder with a number-average particle size of 0.02 μm or more and less than 0.20 μm is mixed with an organic solvent to prepare a mixed solution with a nickel powder concentration of 5% by mass or more and 60% by mass or less. To prepare the mixed solution, the nickel powder can be dispersed in the organic solvent by a known method such as stirring. By using an organic solvent used in pastes for multilayer ceramic capacitor internal electrodes as the organic solvent, it is possible to prepare a nickel paste without replacing the organic solvent in the nickel slurry with an organic solvent for the nickel paste. Furthermore, a dispersant may be added as needed to improve the dispersibility of the nickel powder in the nickel slurry.

[0080] Examples of organic solvents used in pastes for multilayer ceramic capacitor internal electrodes include terpene alcohol-based and aliphatic hydrocarbon-based organic solvents. More specifically, examples of terpene alcohol-based organic solvents include terpineol (terpineol), dihydroterpineol, terpineol acetate, borneol, geraniol, and linalool. Examples of aliphatic hydrocarbon-based organic solvents include n-decane, n-dodecane, and mineral spirits. These organic solvents may be used alone or in combination of two or more.

[0081] In this case, the concentration of nickel powder in the mixed solution is preferably 5% by mass or more and 60% by mass or less, and more preferably 10% by mass or more and 50% by mass or less, taking into consideration productivity and ease of crushing treatment. If the concentration is less than 5% by mass, the nickel powder concentration is too low, which may result in low productivity and may be impractical. On the other hand, if the concentration exceeds 60% by mass, the viscosity of the mixed solution is high, which may make it difficult to crush the nickel powder using a bead mill.

[0082] (moisture content) Ideally, the moisture content of the nickel powder mixed in the mixed solution preparation step is 0% by mass, but because moisture of 0.5% by mass or less may remain when the nickel particles are dried, the moisture content of the nickel powder mixed in the mixed solution preparation step is preferably 0.5% by mass or less.Furthermore, the mixed solution preparation step is preferably a step of preparing a mixed solution with a moisture content of 0.5% by mass or less.

[0083] The reason for this is that the presence of water facilitates the oxidation of nickel particles. When oxidation of nickel powder occurs, coarse particles, primarily composed of plate-like nickel hydroxide, in which nickel particles are firmly bound together by nickel hydroxide, are likely to be generated. Therefore, when a crushing process is performed to reduce the coarse particles, the presence of water during the crushing process and before and after the process may also result in the generation of coarse particles.

[0084] Therefore, it is ideal for the water content of the nickel powder and the mixed solution to be 0% by mass, but considering the effort and cost involved in strictly eliminating water contamination, a small amount of water, for example, 0.5% by mass or less, is acceptable. For example, it is not necessary to strictly eliminate water when the nickel powder contains a small amount of moisture before being converted into the mixed solution from the dried nickel powder, or when a small amount of water is mixed into the organic solvent used to prepare the mixed solution.

[0085] That is, it is not possible to strictly exclude water that may be unavoidably mixed in, and although it is acceptable, it is preferable to intentionally avoid environments, treatments, operations, etc. that result in water being mixed in. Specifically, it is preferable not to store the nickel powder to be crushed in a high-humidity environment, not to add water to an organic solvent, not to perform a mixing step or crushing step in a high-humidity environment, not to mix water into a vessel, and not to store the produced nickel slurry in a high-humidity environment or add water to it.

[0086] For example, in Patent Document 2, nickel powder is crushed in water. However, to subsequently produce an internal electrode paste using this nickel powder, it is necessary to remove the water by drying the nickel powder once or by replacing the water with the solvent of the internal electrode paste. Therefore, the presence of water increases the number of steps required to produce the internal electrode paste, thereby increasing the production cost of the internal electrode paste. Furthermore, as the production volume of nickel powder is scaled up, the nickel slurry remains in water for a longer time during crushing, which increases the amount of coarse particles generated due to the oxidation of the nickel particles.

[0087] In Patent Document 2, nickel powder is disintegrated by causing a nickel powder slurry, in which nickel powder is dispersed in water, to collide with a collision plate. Therefore, a possible disintegration method would be to disintegrate the nickel powder by causing a nickel powder slurry, in which nickel powder is dispersed in an organic solvent instead of water, to collide with a collision plate. However, organic solvents are more viscous than water, and when the nickel powder slurry collides with the collision plate, the organic solvent acts as a shock absorber, reducing the efficiency of disintegrating the nickel powder compared to when water is used. Therefore, when an organic solvent is used, it is difficult to further reduce agglomerates using the method of Patent Document 2.

[0088] Therefore, in the present invention, nickel powder is crushed using an organic solvent by a bead mill as follows.

[0089] <2-2. Crushing process> In this step, the nickel powder in the mixed solution is crushed using a bead mill.

[0090] (2-2-1. Bead Mill) As the bead mill, a media agitation grinder capable of crushing nickel powder using beads can be used. For example, a bead mill using a separation method such as a slit type, a screen type, or a centrifugal separation type can be used. In the present invention, considering that the particle size of the nickel powder is in the nano to micron range, it is preferable to use a bead mill using a screen type suitable for submicron to nano-pulverization or a centrifugal separation type suitable for nano-pulverization, rather than a slit type suitable for submicron pulverization.

[0091] In addition, bead mills have vertical and horizontal vessels (crushing and dispersion chambers) for crushing the material to be processed, but because there is no fundamental difference in performance between them, the appropriate type can be selected taking into consideration installation space, workability, etc. In addition, the appropriate size of the bead mill can be selected taking into consideration processing capacity, installation space, etc.

[0092] The shape of the agitator of the bead mill can be disc type, pin type, single rotor type, etc. In the present invention, it is preferable to use a single rotor type agitator in consideration of the crushability of the nickel powder.

[0093] The beads to be used may be made of glass, alumina, zircon, zirconia, steel, etc. Beads made of an appropriate material should be selected taking into consideration the efficiency of crushing the nickel powder and problems caused by impurities generated by the beads. For example, zirconia beads or alumina beads can be used.

[0094] The size of the beads can be selected appropriately by taking into consideration the T pores, R pores, mass of the beads, and empirical rules relative to the particle size of the nickel powder. For example, beads with a diameter of 0.03 mm to 0.5 mm can be used.

[0095] (2-2-2. Crushing conditions) In the crushing step, crushing is preferably performed under conditions that satisfy at least one of the following: a bead filling rate relative to the vessel is greater than 70% by volume, or an agitator peripheral speed is greater than 6 m / s. By satisfying these conditions, the number of coarse particles in the nickel powder can be reduced by crushing, and deformation of the nickel powder can be suppressed.

[0096] For example, the bead filling rate can be set to more than 70% by volume and 90% by volume, and the peripheral speed can be set to more than 6 m / s and 15 m / s. Note that both or either of these conditions may be satisfied.

[0097] The time for crushing the nickel powder may be set in consideration of the amount of nickel powder to be processed, and may be, for example, 10 minutes to 5 hours.

[0098] After the crushing step, the crushed nickel powder can be taken out in the form of a nickel slurry from the vessel of the bead mill.

[0099] [3. Nickel Slurry] The nickel slurry obtained by the above-mentioned method for producing a nickel slurry contains an organic solvent and the nickel powder of the present invention. The nickel powder will be described later. The content of the nickel powder in the nickel slurry and the type of organic solvent vary depending on the conditions of the above-mentioned method for producing a nickel slurry.

[0100] The water content in the nickel slurry also varies depending on the conditions of the nickel slurry manufacturing method. As described in the section (Water content), the water content in the nickel slurry is preferably 0 mass %, but is not limited to this.

[0101] The nickel slurry obtained through the nickel slurry manufacturing method may be used as a raw material for nickel paste or the like as a finished product, or the nickel slurry may be subjected to solid-liquid separation and then dried to form nickel powder.

[0102] The specific methods for carrying out the solid-liquid separation and drying treatment are as described above. For example, nickel powder and organic solvent are separated from the nickel slurry by solid-liquid separation using a Denver filter, filter press, centrifuge, decanter, or the like, and then the nickel powder is dried at 40 to 300°C, preferably 60 to 150°C, using a drying device such as an atmospheric dryer, hot air dryer, inert gas atmosphere dryer, or vacuum dryer, to obtain dried nickel powder.

[0103] Furthermore, the obtained nickel powder may be subjected to heat treatment, for example, in an inert atmosphere or a reducing atmosphere at about 200°C to 300°C. Heat treatment is very effective if used within an appropriate range, because it can control the binder removal behavior and sintering behavior of the nickel powder in the internal electrodes during the production of the multilayer ceramic capacitor described above.

[0104] [4. Nickel powder] The nickel powder of the present invention is obtained by the above solid-liquid separation and drying treatment. The obtained nickel powder is inexpensive, has high performance, and is suitable for use in internal electrodes of multilayer ceramic capacitors. The nickel powder has the following properties, such as number-average particle size, proportion of coarse particles, aspect ratio, and proportion of flattened particles.

[0105] (number average particle diameter) In order to accommodate the recent trend toward thinner internal electrodes in multilayer ceramic capacitors, the number average particle size of the nickel powder is 0.02 μm or more but less than 0.2 μm, and more preferably 0.18 μm or less. The number average particle size is the number average particle size determined from a scanning electron microscope (SEM) image of the nickel powder.

[0106] (Proportion of coarse particles) If nickel powder contains coarse particles, when it is used in the internal electrodes of a multilayer ceramic capacitor, the continuity of the internal electrode layer may be reduced or the adjacent dielectric layers may be compressed, causing short-circuiting. Taking into consideration the frequency with which these defects caused by coarse particles occur, the nickel powder of the present invention contains coarse particles with a particle diameter of more than 0.8 μm at a content of 3,200 ppm by mass or less.

[0107] The amount of coarse particles can be determined, for example, by ultrasonically dispersing 0.05 g of nickel powder or a nickel slurry with a nickel content of 0.05 g in 50 mL to 100 mL of a slurry solvent to obtain a nickel powder dispersion, filtering this dispersion through a porous track-etched membrane filter with uniformly sized linear pores of 0.8 μm in diameter, capturing coarse particles larger than the pore size of the membrane filter (greater than 0.8 μm) on the filter, and calculating the amount of coarse particles by ICP emission spectroscopy of a solution in which the total amount of coarse particles is dissolved in acid. The ratio of the amount of coarse particles to the nickel content of the nickel powder or nickel slurry is the proportion of coarse particles present. In the following examples, the proportion of coarse particles present was calculated using this method.

[0108] [Flattened particle count ratio] The proportion of flattened nickel particles in the nickel powder (flattening rate) can be calculated by the following formula (5): For example, the nickel powder is imaged with a scanning electron microscope, or a small amount of nickel slurry is placed on a sample stage, dried, and imaged with a scanning electron microscope, and the flattening rate can be calculated from the ratio of the number of flattened particles to the total number of detected particles in the nickel powder.

[0109] [Formula 5] Flattening rate (%) = (number of flattened particles (pieces) / total number of detected particles (pieces)) × 100 (5)

[0110] Here, the flattened particles are defined as particles whose maximum particle size is 5 / 3 or more of the number average particle size of nickel particles.

[0111] Flattened nickel particles make it difficult to thin the internal electrodes of MLCCs, and flattening them may result in a decrease in the performance of the MLCC. Therefore, in the nickel powder of the present invention, the proportion of flattened particles is set to 5% or less, taking into consideration the proportion that does not decrease the performance of the MLCC. More preferably, it is set to 2.5% or less.

[0112] Aspect Ratio The aspect ratio of nickel particles can be determined by scanning electron microscope (SEM) observation using the same method as described in [Number ratio of flattened particles]. The aspect ratio is the length of the minimum Feret diameter to the maximum Feret diameter of the nickel particle (minimum Feret diameter / maximum Feret diameter). If the aspect ratio is greater than 0.5, the shape of the nickel particles is close to spherical, and the film density will be high when the nickel paste used is used to form a coating film.

[0113] If there are many nickel particles with an aspect ratio of 0.5 or less, the film density will be low, which may result in a decrease in the performance of the MLCC. Therefore, in the nickel powder of the present invention, the number proportion of nickel particles with an aspect ratio of 0.5 or less is set to 7% or less, taking into consideration the proportion at which the performance of the MLCC will not be decreased. More preferably, it is set to 5% or less. The number proportion of nickel particles with an aspect ratio of 0.5 or less can be calculated using the following formula (6).

[0114] [Formula 6] Quantity ratio (%) = (Number of nickel particles with an aspect ratio of 0.5 or less (pieces) / Total number of detected particles (pieces)) x 100 (6) [Example]

[0115] Hereinafter, one embodiment of the present invention will be described in more detail using examples, but the present invention is not limited to the following examples in any way.

[0116] In the examples, nickel powder was prepared by a wet method, and a nickel slurry was prepared using the prepared nickel powder, and the physical properties of the nickel powder in the nickel slurry were evaluated.

[0117] [Nickel powder manufacturing] (Preparation of Solution C) The water-soluble nickel salt was 405 g of nickel chloride hexahydrate (NiCl2·6H2O, molecular weight: 237.69), and the metal salt of a metal more noble than nickel was 53.5 mg of ammonium palladium(II) chloride (also known as ammonium tetrachloropalladate(II)) ((NH4)2PdCl4, molecular weight: 284.31). The complexing agent was sodium nitrilotriacetate monohydrate (molecular weight: 275.10). Solution C was prepared by dissolving 46.9 g of methionine (CH3SC2H4CH(NH2)COOH, molecular weight: 149.21) in 1880 mL of pure water. The reducing agent was hydrazine hydrate (NH4·HO, molecular weight: 50.06). This was diluted 1.67 times with pure water to prepare 35 g of commercially available industrial-grade 60% hydrazine hydrate (Otsuka-MGC Chemical Co., Ltd.). The palladium (Pd) content in Solution C was 200 ppm by mass (110.3 mol ppm) relative to nickel (Ni), the molar ratio of hydrazine to nickel was 0.25, and the molar ratio of sodium nitrilotriacetate to nickel was 0.1.

[0118] (Preparation of Solution D) 290 g of sodium hydroxide (NaOH, molecular weight: 40.0) was dissolved in 560 mL of pure water as an alkali hydroxide to prepare solution D, an aqueous solution containing sodium hydroxide as the main component. The molar ratio of sodium hydroxide to nickel contained in solution D was 4.3.

[0119] (Amine compound aqueous solution) An amine compound aqueous solution containing ethylenediamine as the main component was prepared by dissolving 2.05 g of ethylenediamine (abbreviated as EDA) (H2NC2H4NH2, molecular weight: 60.1), an alkyleneamine containing two primary amino groups (-NH2) in the molecule, in 18 mL of pure water. The amount of ethylenediamine contained in the amine compound aqueous solution was a trace amount, at a molar ratio of 0.02 (2.0 mol%) relative to nickel.

[0120] The materials used in solutions C, D, and the aqueous amine compound solution were all reagents manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., except for 60% hydrazine hydrate and sodium nitrilotriacetate.

[0121] (Mixing process and crystallization process) Solution C at a liquid temperature of 25°C was placed in a Teflon (registered trademark) coated stainless steel container equipped with a stirring blade and stirred, and then, while continuing to stir, Solution D at a liquid temperature of 25°C was added and mixed to obtain a reaction liquid (mixture) (mixing step), and the reduction reaction (crystallization reaction) was initiated.The stainless steel container was then placed in an 80°C water bath, and the reaction liquid was heated while continuing to stir. The amine compound aqueous solution was added dropwise to the reaction solution over a 10-minute period (from 8 to 18 minutes after the start of the reaction) (amine compound aqueous solution mixing step). Then, over a 35-minute period (from 45 to 80 minutes after the start of the reaction), 172 g of commercially available industrial-grade 60% hydrazine hydrate (manufactured by Otsuka-MGC Chemical Co., Ltd.), prepared by diluting hydrazine hydrate (N2H4·HO, molecular weight: 50.06) 1.67 times with purified water, was added dropwise. This reduced reaction proceeded while suppressing hydrazine self-decomposition, resulting in the precipitation of nickel crystal powder in the reaction solution (crystallization step). The reduction reaction was complete within 90 minutes of the start of the reaction, and the clear supernatant of the reaction solution confirmed that all nickel components in the reaction solution had been reduced to metallic nickel. The molar ratio of hydrazine to nickel added dropwise during the crystallization step was 1.21.

[0122] (Recovery of nickel crystallized powder) The reaction solution containing the nickel crystallized powder was in the form of a slurry in which crystallized nickel particles were dispersed. This slurry containing the nickel crystallized powder was repeatedly filtered and washed using pure water with a conductivity of 1 μS / cm. When the conductivity of the filtrate filtered from the nickel crystallized powder-containing slurry reached 10 μS / cm or less, the nickel particles and water were separated by solid-liquid separation, and the nickel particles were dried in a vacuum dryer set at a temperature of 150°C to obtain nickel crystallized powder to be used in the production of nickel slurry. The moisture content of the obtained nickel crystallized powder was measured multiple times using a Karl Fischer moisture meter; the moisture content at 110°C was always 0.1 to 0.5% by mass and never exceeded 0.5% by mass.

[0123] (Physical properties of nickel crystallized powder) The nickel crystallized powder thus obtained was observed with a scanning electron microscope (SEM). Image analysis software (Mac-View, manufactured by Mountec Co., Ltd.) was used to measure the particle size distribution of the nickel crystallized powder in multiple fields of view in the SEM image, and the average particle size for each field was calculated. The number-average particle size (mn) was 65 nm to 68 nm, and the volume-average particle size (mv) was 68 to 71 nm. The coefficient of variation (CV) was calculated, and the CV value (number) was 12.23% and the CV value (volume) was 11.69%. Furthermore, the proportion of coarse particles with a particle size of over 0.8 μm in the nickel crystallized powder before crushing was 90% by mass or more. These results indicate that although the amount of coarse particles was high, the average particle size and coefficient of variation were both small, resulting in a fine nickel powder with a narrow particle size distribution.

[0124] [Production of nickel slurry]

[0125] Dihydroterpineol was added to the obtained nickel crystallized powder to prepare a mixed solution of nickel crystallized powder and an organic solvent with a nickel crystallized powder concentration of 30 mass% (mixture preparation step), and the nickel crystallized powder in the mixed solution was subjected to a crushing treatment using a bead mill (crushing step) to obtain a nickel slurry.

[0126] The bead mill used was an HFM02 manufactured by Ashizawa Finetech (single rotor type agitator, 0.2 L vessel capacity), and zirconia beads with a diameter of 0.05 mm were used as media.

[0127] Table 1 shows the conditions of the agitator peripheral speed, bead filling rate relative to the vessel, and crushing treatment time in the crushing treatment using a bead mill in Examples 1 and 2 and Comparative Example 1. The bead filling rate is a value indicating how much of the vessel is filled with beads. In Table 1, the case where the vessel is filled to 60% of the capacity is defined as 60% by volume (Example 1), the case where it is filled to 80% by volume (Example 2), and the case where it is filled to 70% by volume (Comparative Example 1).

[0128] [Table 1]

[0129] [Evaluation of nickel powder properties] The nickel slurries obtained in Examples 1 and 2 and Comparative Example 1 were measured for the number average particle size of the nickel powder, the proportion of coarse particles with a particle size of more than 0.8 μm, the number proportion of particles with an aspect ratio of 0.5 or less, and the number proportion of flattened particles, as described below. The measurement results are shown in Table 2.

[0130] <Number average particle diameter> Similar to the nickel crystallized powder before crushing, a small amount of nickel slurry was sampled and dried at 120°C for 30 minutes to obtain nickel powder. The nickel powder was observed under a scanning electron microscope (SEM) at a magnification of 40,000 times, and the particle size distribution of the nickel crystallized powder in multiple fields of view of the SEM image was measured using image analysis software (Mac-View manufactured by Mountec Co., Ltd.) to determine the number average particle diameter mn of each field, which was 65 nm to 68 nm. In other words, there was no change in the number average particle diameter before and after the crushing treatment.

[0131] <Proportion of coarse particles with a particle size of over 0.8 μm> Approximately 1 g of nickel slurry was collected and dried at 120°C for 30 minutes. The solids concentration was calculated from the weight difference before and after drying. Based on the calculated solids concentration, nickel slurry was collected in a beaker so that the nickel content was 0.05 g. Approximately 50 mL of dihydroterpineol, the organic solvent used in the nickel slurry, was added to dilute the nickel powder and dispersed ultrasonically for 30 minutes. This was used as the evaluation sample. A porous track-etched membrane filter with uniformly sized, linear pores of 0.8 μm diameter was placed in the filter filtration device and pressurized to 0.1 MPa to filter the evaluation sample. 50 mL of dihydroterpineol was prepared, and the filtered filter was placed therein. The attached nickel was ultrasonically dispersed in the dihydroterpineol, and the dispersion was filtered again in the same manner. These filtration and dispersion procedures were repeated a total of four times, and after the fifth pressure filtration, the nickel powder remaining on the filter was dissolved in acid and subjected to ICP emission spectroscopy to determine the content (mass) of coarse particles contained in the nickel slurry, i.e., coarse particles with a particle diameter exceeding 0.8 μm.

[0132] <Number ratio of particles with aspect ratio of 0.5 or less> A small amount of the obtained nickel slurry was placed on a sample stage and dried at 120°C for 30 minutes to obtain nickel powder, which was observed under a scanning electron microscope (SEM) at a magnification of 40,000 times, and the aspect ratio (minimum Feret diameter / maximum Feret diameter) of the nickel particles in the field of view was calculated. The proportion of nickel particles with an aspect ratio of 0.5 or less was calculated using the following formula (6).

[0133] [Formula 6] Quantity ratio (%) = (Number of nickel particles with an aspect ratio of 0.5 or less (pieces) / Total number of detected particles (pieces)) x 100 (6)

[0134] <Proportion of flattened particles> A small amount of the resulting nickel slurry was placed on a sample stage and dried at 120°C for 30 minutes. SEM images of the resulting nickel powder were taken at 40,000x magnification. Using AI image analysis software (manufactured by Dai Nippon Printing Co., Ltd.), all nickel particles in the SEM images were detected and classified by particle size. The number ratio of flattened particles (flattening rate) was calculated using equation (5).

[0135] [Formula 5] Flattening rate (%) = (number of flattened particles (pieces) / total number of detected particles (pieces)) × 100 (5)

[0136] [Table 2]

[0137] In comparison with Comparative Example 1, in which bead mill processing was performed under low peripheral speed conditions, Example 1, in which processing was performed under conditions of high peripheral speed and low bead packing ratio, resulted in a reduced content of coarse particles while preventing a deterioration in the flattening ratio. Furthermore, Example 2, in which processing was performed under conditions of low peripheral speed and high bead packing ratio, resulted in an increased flattening ratio compared to Example 1, but within an acceptable range, and also resulted in a reduced content of coarse particles.

[0138] Looking at the flattening rate and aspect ratio, although the conditions of Comparative Example 1 suppressed deformation of nickel particles, the proportion of coarse particles was also high, and it is thought that the conditions were insufficient for crushing the nickel particles.

[0139] The results of Examples 1 and 2 show that the nickel slurry manufacturing method according to one embodiment of the present invention produced nickel powder and nickel slurry with fewer deformed nickel particles and fewer coarse particles.

[0140] From the above, it is clear that according to the present invention, even if the fine nickel powder has a strong cohesive force, it is possible to obtain a fine nickel powder with few deformed particles and coarse particles (connected particles and agglomerates) by performing a crushing process using a bead mill.

[0141] Although the embodiments and examples of the present invention have been described in detail above, it will be readily apparent to those skilled in the art that many modifications are possible without substantially departing from the novel features and effects of the present invention. Therefore, all such modifications are intended to be included within the scope of the present invention.

[0142] For example, a term described at least once in the specification together with a different term having a broader or equivalent meaning can be replaced with that different term anywhere in the specification. In addition, the configuration and operation of the nickel powder manufacturing method are not limited to those described in each embodiment and each example of the present invention, and various modifications are possible.

Claims

1. The number average particle size is 0.02 μm or more and less than 0.20 μm, The proportion of coarse particles having a particle diameter of more than 0.8 μm is 3200 mass ppm or less, The proportion of particles having an aspect ratio of 0.5 or less is 7% or less, The number ratio of flattened particles is 5% or less. Nickel powder.

2. The nickel powder according to claim 1, an organic solvent; 1. A nickel slurry comprising:

3. The nickel slurry according to claim 2, wherein the water content is 0.5 mass% or less.

4. a mixed solution preparation step of mixing nickel powder having a number average particle size of 0.02 μm or more and less than 0.20 μm with an organic solvent to prepare a mixed solution having a nickel powder concentration of 5% by mass or more and 60% by mass or less; a crushing step of crushing the nickel powder in the mixed solution using a bead mill; A method for producing a nickel slurry, comprising:

5. 5. The method for producing a nickel slurry according to claim 4, wherein in the crushing step, crushing is performed under conditions that satisfy at least one of a bead filling rate with respect to a vessel exceeding 70% by volume or an agitator peripheral speed exceeding 6 m / s.

6. The method for producing a nickel slurry according to claim 4, wherein the water content of the nickel powder mixed in the mixed solution preparation step is 0.5 mass% or less.

7. The method for producing a nickel slurry according to claim 4, wherein the mixed solution preparation step is a step of preparing a mixed solution having a water content of 0.5 mass% or less.

Images