Method for producing nickel powder

Abstract

To provide a method for producing nickel powder, the method enabling production of nickel powder having a large crystallite diameter even if the nickel powder is synthesized by a wet method.SOLUTION: A method for producing nickel powder comprises a crystallization step of heating reaction solution by irradiating reaction solution including a water-soluble nickel salt, a complexing agent, water, and a reducing agent with microwaves, thereby precipitating nickel particles. The crystallization step includes a heating stage of raising the temperature of the reaction solution to 150°C or more and 300°C or less under a condition of a heating rate of 50°C / min or more by irradiation with microwaves, and a temperature holding stage of holding the temperature of the reaction solution at 150°C or more and 300°C or less thereafter. In the crystallization step, a nickel powder in which a crystallite diameter of the nickel powder composed of the nickel particles is 25% or more of a number average particle diameter of the nickel powder is precipitated.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing nickel powder used in conductive pastes for forming electrodes, including electrode materials for multilayer ceramic capacitors. [Background technology]

[0002] Nickel powder is used as a material for capacitors in electronic circuits, particularly as a material for thick-film conductors that form internal electrodes of multilayer ceramic components such as multilayer ceramic capacitors (MLCCs) and multilayer ceramic substrates.

[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 such as palladium.

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

[0005] This laminate is cut to a predetermined size, the binder resin is then removed by heat treatment (binder removal treatment), and the laminate is 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 both 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 a number average particle size of 0.5 μm or less being required, and the use of nickel powder with a number average particle size of 0.3 μm or less becoming the norm.

[0008] Methods for producing nickel powder can be broadly divided into gas phase methods and wet methods. Gas phase methods include, for example, a method described in Patent Document 1 in which nickel chloride vapor is reduced with hydrogen to produce nickel powder, and a method described in Patent Document 2 in which nickel metal is vaporized in plasma to produce nickel powder. Wet methods include, for example, a method described in Patent Document 3 in which a reducing agent is added to a nickel salt solution to produce nickel powder.

[0009] The vapor-phase method is advantageous because it uses a high-temperature process at temperatures above 1000°C. Due to the large crystallite size, shrinkage during firing begins at a temperature close to that of the ceramic particles that make up the dielectric green sheet. However, powders synthesized using the vapor-phase method have the problem of a broad particle size distribution. As mentioned above, thinning the internal electrodes requires nickel powder with a number-average particle size of 0.5 μm or less, without containing coarse particles, and with a relatively narrow particle size distribution. Therefore, obtaining such nickel powder using the vapor-phase method requires an additional post-process step of classifying the nickel powder using expensive classification equipment.

[0010] Furthermore, when using nickel powder with a number average particle size of 0.2 μm or less in the gas phase method, it becomes difficult to remove coarse particles through classification, making it impossible to respond to future trends toward thinner internal electrodes.

[0011] On the other hand, the wet method has the advantage that the particle size distribution of the obtained nickel powder is narrower than that of the gas phase method. In particular, in the method described in Patent Document 3, in which a solution containing hydrazine as a reducing agent is added to a solution containing nickel salt and palladium, nickel salt (nickel ions (Ni 2+ ), or nickel complex ions) is reduced with hydrazine, which allows the number of nuclei generated to be controlled, thereby controlling the particle size, and it is known that nuclei generation and particle growth are uniform, resulting in the production of fine nickel powder. [Prior art documents] [Patent documents]

[0012] [Patent Document 1] Japanese Patent Application Publication No. 4-365806 [Patent Document 2] Special Publication No. 2002-530521 [Patent Document 3] Japanese Patent Application Laid-Open No. 2004-332055 Summary of the Invention [Problem to be solved by the invention]

[0013] However, nickel powder obtained by the existing wet method tends to have a smaller crystallite size than nickel powder obtained by the dry method.

[0014] In view of the above circumstances, an object of the present invention is to provide a method for producing nickel powder that can produce nickel powder having a large crystallite size even when synthesized by a wet method. [Means for solving the problem]

[0015] As a result of intensive research, the inventors have found that it is possible to produce highly crystalline nickel powder, i.e., nickel powder with a large crystallite diameter, by combining the use of a reducing agent and the heating of an aqueous solution in which a precursor containing nickel ions, a complexing agent, and a reducing agent is dispersed with microwave irradiation, which can rapidly increase the temperature, to promote the reduction reaction of nickel.

[0016] In order to solve the above problems, the method for producing nickel powder of the present invention includes a crystallization step in which a reaction solution containing a water-soluble nickel salt, a complexing agent, water, and a reducing agent is irradiated with microwaves to heat the reaction solution, thereby precipitating nickel particles. The crystallization step includes a temperature increase step in which the reaction solution is heated to 150°C or higher and 300°C or lower by irradiating microwaves at a temperature increase rate of 50°C / min or higher, and a temperature holding step in which the temperature of the reaction solution is then held at 150°C or higher and 300°C or lower. The crystallization step precipitates nickel powder in which the crystallite diameter of the nickel powder comprising the nickel particles is 25% or more of the number average particle diameter of the nickel powder.

[0017] The water-soluble nickel salt may be one or more selected from chlorides, sulfates, and acetates.

[0018] The complexing agent may be ammonia.

[0019] The reducing agent may be hydrazine.

[0020] The method for producing nickel powder of the present invention may include a mixing step of preparing a reaction liquid containing the water-soluble nickel salt, the complexing agent, the water, and the reducing agent.

[0021] The temperature holding step may hold the temperature of the reaction solution for 45 seconds to 10 minutes. [Effects of the Invention]

[0022] According to the present invention, it is possible to provide a method for producing nickel powder that can produce nickel powder having a large crystallite size even when synthesized by a wet method. DETAILED DESCRIPTION OF THE INVENTION

[0023] The nickel powder and the method for producing the nickel powder according to the present invention will be described below in the following order. Note that the present invention is not limited to the following examples and can be modified as desired without departing from the gist of the present invention. 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. Recovery of nickel crystallized powder 2. Nickel powder

[0024] <1. Nickel powder manufacturing method> First, a method for producing nickel powder according to one embodiment of the present invention will be described. The method for producing nickel powder according to one embodiment of the present invention mainly comprises a crystallization step in which a reaction solution containing a water-soluble nickel salt, a complexing agent, a reducing agent, and water is irradiated with microwaves to heat the reaction solution to a temperature of 150°C to 300°C at a heating rate of 50°C / min or more, and then the temperature of the reaction solution is maintained to reduce the water-soluble nickel salt by a reduction reaction with hydrazine, thereby obtaining nickel crystallized powder.

[0025] In the present invention, "powder" and "powder" refer to a state in which a large number of particles gather together to form an aggregate, and a slurry of crystallized nickel particles or a solid aggregate formed by drying corresponds to nickel powder or nickel powder. For example, "nickel crystallized powder" is nickel powder in a reduced state in the crystallization process.

[0026] [1-1. Mixing process] In the present invention, it is important to add a specific complexing agent in advance. To achieve this, a mixing step is performed before the crystallization step. The mixing step is a step in which a water-soluble nickel salt, a complexing agent, water, and a reducing agent are mixed to obtain a mixed solution.

[0027] Here, the water used as the solvent is preferably high-purity water such as ultrapure water (electrical conductivity: ≦0.06 μS / cm) or pure water (electrical conductivity: ≦1 μS / cm) from the viewpoint of reducing the amount of impurities in the resulting nickel powder, and among these, it is preferable to use pure water, which is inexpensive and easily available. Each of the above-mentioned various chemicals will be described in detail below.

[0028] (1-1-1. Mixing Agents) (a) Water-soluble nickel salts The water-soluble nickel salt used in the present invention 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 acetate can be used. Among these nickel salts, nickel chloride, nickel sulfate, or a mixed solution thereof is more preferably used. Furthermore, the water-soluble nickel salt does not need to be dissolved when the reaction solution is mixed.

[0029] (b) Complexing agent The use of a complexing agent can accelerate the reduction reaction by forming a complex with the water-soluble nickel salt in the reaction solution. If a complexing agent is not added, the reduction reaction will not proceed sufficiently.

[0030] As the complexing agent, an organic compound having a coordinating atomic group such as a carbonyl group or an amino group, or ammonia can be used. Of these, ammonia is preferred as the complexing agent. Here, the reason why ammonia is preferably used as the complexing agent is that the ligands of a nickel ammine complex coordinated with ammonia are easily released, which promotes decomposition of the complex in the crystallization step described below and facilitates the growth of crystals of nickel particles.

[0031] Furthermore, since the nickel complex is six-coordinated, the molar ratio of nickel atoms to coordinating atomic groups is 1:6 to 18, and preferably 1:6 to 12. Furthermore, when ammonia is used as the complexing agent, the presence of an excess of ammonia molecules relative to the number of nickel atoms allows the pH of the reaction solution to be set to 9.5 or higher, thereby accelerating the reduction of nickel ions by hydrazine.

[0032] (c) Hydrazine In one embodiment of the present invention, a method for producing nickel powder uses hydrazine (N2H4, molecular weight: 32.05) as a reducing agent. Hydrazine can be used in the form of anhydrous hydrazine or its hydrate, hydrazine hydrate (N2H4·H2O, molecular weight: 50.06). Either can be used. The reduction reaction of hydrazine is shown in Equation (2), which will be described later. Hydrazine is particularly suitable as a reducing agent because it has high reducing power in alkaline solutions, produces nitrogen gas and water as by-products of the reduction reaction, so no impurities are generated in the reaction solution, contains few impurities, and is readily available. For example, commercially available industrial-grade 60% by weight hydrazine hydrate can be used.

[0033] (d) Nucleating agent Furthermore, a metal salt of a metal more noble than nickel can be added as a nucleating agent, as it has catalytic activity against the reducing agent. Metal salts of metals more noble than nickel have a lower ionization tendency than nickel, and are therefore reduced before nickel during reduction and precipitation. Therefore, they can act as a nucleating agent, forming the initial nuclei for the crystallization of nickel particles. By growing particles from these initial nuclei, even finer nickel crystallized powder (nickel powder) can be produced. However, a nucleating agent is not an essential agent.

[0034] The salt of a metal more noble than nickel may be any metal salt that has catalytic activity toward the reducing agent used, is water-soluble, and has a lower ionization tendency than nickel. For example, when hydrazine is used as the reducing agent, examples of water-soluble noble metal salts include water-soluble platinum salts, palladium salts, rhodium salts, iridium salts, etc. Examples of water-soluble palladium salts that can be used include, but are not limited to, sodium palladium(II) chloride, ammonium palladium(II) chloride, palladium(II) nitrate, and palladium(II) sulfate.

[0035] 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 within the range of 0.2 mol ppm to 300 mol ppm, preferably 0.5 mol ppm to 250 mol ppm, and more preferably 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 300 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.

[0036] (e) Amine compounds An amine compound can also be added to inhibit the self-decomposition of hydrazine, promote the reduction reaction, and inhibit the bonding between nickel particles. Examples of the amine compound include alkyleneamines and / or alkyleneamine derivatives. More specifically, the alkyleneamine can be one or more selected from ethylenediamine (HNCHNH), diethylenetriamine (HNCHNHCHNH), triethylenetetramine (HN(CHNH)CHNH), tetraethylenepentamine (HN(CHNH)CHNH), and pentaethylenehexamine (HN(CHNH)CHNH). In addition, the alkyleneamine derivative may be one or more selected from tris(2-aminoethyl)amine (N(C2H4NH2)3) and (2-aminoethyl)-2-aminoethanol (H2NC2H4NHC2H4OH). These alkyleneamines and alkyleneamine derivatives are water-soluble, and examples thereof include ethylenediamine and diethylenetriamine. However, the amine compound is not an essential drug.

[0037] (f) Alkali hydroxide An alkali hydroxide can also be added to adjust the pH of the reaction solution. As the alkali hydroxide, it is more preferable to use one or more selected from sodium hydroxide and potassium hydroxide. However, the alkali hydroxide is not an essential agent.

[0038] The mixing step is not an essential step, and may be carried out by the manufacturer as one step in the method for producing nickel powder, or the reaction liquid obtained in the mixing step may be obtained by purchase or the like and used to carry out the crystallization step.

[0039] [1-2. Crystallization process] The crystallization step is a step in which the water-soluble nickel salt in the mixed solution is reduced with a reducing agent to obtain nickel crystallized powder.

[0040] (1-2-1. Reduction reaction) In the mixed solution (i.e., reaction solution), the water-soluble nickel salt is reduced with hydrazine in the presence of a complexing agent to obtain a crystallized nickel powder.

[0041] First, the reduction reaction in the crystallization process 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). Stoichiometrically (theoretical value), 0.5 moles of hydrazine (N2H4) are required for 1 mole of nickel (Ni).

[0042] Here, from the reduction reaction of hydrazine in formula (2), it can be seen that the stronger the alkalinity of hydrazine, the greater its reducing power.

[0043] [C1] Ni 2+ +2e - →Ni↓ (two-electron reaction) (1) N2H4+4OH - →N2↑+4H2O+4e - (four-electron reaction) (2)

[0044] In the method for producing nickel powder of this embodiment, the molar ratio of nickel atoms to hydrazine is preferably in the range of 1:0.6 to 5, and more preferably 1:0.6 to 4. If the total amount of hydrazine is less than the lower limit, that is, if the molar ratio of nickel atoms to hydrazine is less than 1:0.6, there is a possibility that not all of the nickel in the reaction solution will be reduced.

[0045] In the crystallization reaction, the reaction solution is irradiated with microwaves at a rate of 50°C / min or more to raise the temperature from the working temperature (5°C to 40°C) to a holding temperature of 150°C to 300°C. Irradiating the reaction solution with microwaves (frequency 100 MHz to 5 GHz) at such a rate to raise the temperature of the reaction solution to 150°C to 300°C promotes the oxidative decomposition of the reducing agent and the decomposition of the nickel complex, thereby rapidly generating the particle nuclei necessary for the crystal growth reaction. The holding temperature is then maintained at a constant temperature set in the range of 150°C to 300°C. Here, the constant temperature is a range of -5°C to +5°C around the set temperature.

[0046] If the microwave output irradiated to the reaction solution is at a heating rate of less than 50°C / min, the oxidative decomposition of the reducing agent and the decomposition of the nickel complex will proceed during the heating process up to the holding temperature, which will not promote crystal growth of nickel particles and will result in nickel powder with a crystallite size less than 25% of the number average particle size.

[0047] Similarly, if the reaction solution is maintained at a temperature below 150°C, the crystal growth of nickel particles will not proceed. On the other hand, if the reaction solution is maintained at a temperature above 300°C, the effect will be the same as if the temperature were 300°C or lower, and this is not economical.

[0048] Hydrothermal synthesis is a known wet process for producing highly crystalline metal particles, which can promote crystal growth at temperatures above the boiling point of water. However, if the heating rate in hydrothermal synthesis is not fast enough, the reduction reaction may terminate midway through heating, i.e., before the holding temperature is reached. Furthermore, if the reaction raw materials form a complex, heating alone may not be enough to completely decompose them. This makes it difficult to achieve a crystallite size of metal particles that is sufficiently large relative to the primary particle size. Therefore, the nickel powder manufacturing method of this embodiment uses microwaves, which can increase the heating rate.

[0049] In the nickel powder manufacturing method of this embodiment, the holding time at the holding temperature can be 45 seconds to 10 minutes, and preferably 45 seconds to 5 minutes. If the holding time is less than 45 seconds, the reduction of nickel ions may not be completed, and if the holding time exceeds 10 minutes, the reduction reaction has already ended and will not proceed.

[0050] (1-2-2. 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 dried 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.

[0051] <2. Nickel powder> Nickel powder can be produced inexpensively by the production method of the present invention, has high performance, and is suitable as a material for internal electrodes of multilayer ceramic capacitors, etc. The production method of the present invention can produce nickel powder having the following properties. The average particle size (number average particle size) and crystallite size have the following properties.

[0052] (number average particle size) In order to accommodate the recent trend toward thinner internal electrodes in multilayer ceramic capacitors and the like, the nickel powder of this embodiment preferably has a number average particle size of 0.02 μm to 0.3 μm, and more preferably 0.02 μm to 0.25 μm. However, considering that there are many types of multilayer ceramic capacitors and that nickel powder with a number average particle size of more than 0.15 μm to less than 0.4 μm is still widely used, the number average particle size of the nickel powder can be set to 0.02 μm to 0.4 μm. The number average particle size of the nickel powder in the present invention can be calculated from particles photographed using a scanning electron microscope (SEM image).

[0053] (crystallite diameter) The crystallite size of the nickel powder of this embodiment is 25% or more of the number average particle size of the nickel powder, and preferably 28% to 50%. If the crystallite size is 25% or more of the number average particle size, the shrinkage of the nickel powder during firing begins at a temperature close to the shrinkage of the ceramic particles that make up the dielectric green sheet during firing. Therefore, the nickel powder obtained by the manufacturing method of the present invention is more suitable as a material for internal electrodes of multilayer ceramic capacitors and the like. The crystallite size of the nickel powder can be calculated, for example, from the half-width of the peak obtained by powder X-ray diffraction. [Example]

[0054] EXAMPLES The present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples. Evaluation methods used in the examples and comparative examples will be described below.

[0055] (1) Measurement method of number average particle size In Examples 1 to 6 and Comparative Examples 1 and 2, the "number average particle size" was determined by image analysis of images observed using a scanning electron microscope (SEM), where 100 observable primary particles of nickel powder were randomly selected and the number average value of the measured circle-equivalent diameters (diameters of circles having the same area) was used.

[0056] (2) Method for measuring crystallite size In Examples 1 to 6 and Comparative Examples 1 and 2, the "crystallite size" was calculated from the half-width at 2θ of the main peak, which is the maximum intensity of nickel, by powder X-ray diffraction (XRD (X-ray diffraction)) analysis using CuKα as the X-ray source, using the Scherrer equation. In the case of nickel, the peak of the Ni(111) plane, which exists near 2θ = 45 degrees, was the subject of analysis.

[0057] [Example 1] (1) Preparation of nickel powder 1.4 g of nickel sulfate hexahydrate (NiSO4·6H2O) as a water-soluble nickel salt, 10.3 g of 10% by weight aqueous ammonia (NH3) as a complexing agent, and 1.7 g of commercially available industrial-grade 60% by weight hydrazine hydrate (manufactured by Otsuka-MGC Chemical Co., Ltd.) as a reducing agent were added to 478 g of distilled water and stirred to prepare 489 mL of an aqueous solution containing a dispersed nickel powder precursor. The nickel powder precursor here is a mixture of nickel ions dissolved in the water-soluble nickel salt, undissolved water-soluble nickel salt, a complexing agent, and a reducing agent. The pH of the prepared reaction solution was approximately 10. Next, 5 mL of the aqueous solution containing the dispersed precursor at 20 °C was placed in a 35 mL reaction vessel and heated at a heating rate of 80 °C / min using a microwave synthesis system (Discover 2.0, manufactured by CEM Japan Co., Ltd.) capable of rapid heating, with microwave irradiation at a maximum power density of 60 W / mL. After reaching 200°C, the liquid temperature was held at 200°C for 1 minute, resulting in the precipitation of 3 mg of nickel powder in the aqueous solution. The nickel powder precipitated in the aqueous solution was captured with a magnet, washed with ethanol by decantation, and then dried under reduced pressure. Nickel powder was precipitated from 5 mL of aqueous solution in which the precursor was dispersed, and washing and drying under reduced pressure were repeated 50 times to obtain approximately 150 mg of nickel powder.

[0058] (2) Evaluation (a) Number average particle size of nickel powder Image analysis of the images observed with a scanning electron microscope (SEM) revealed that the number average particle size in equivalent circle diameter of the nickel powder obtained in (1) above was 145 nm.

[0059] (b) Ratio of crystallite size to number average particle size The nickel powder obtained in (1) above was analyzed by powder X-ray diffraction and found to have a crystallite size of 61 nm, which means that the ratio of the crystallite size to the number average particle size was 42%.

[0060] [Example 2] (1) Preparation of nickel powder About 150 mg of nickel powder was obtained in the same manner as in Example 1, except that microwaves were irradiated and the liquid temperature was maintained at 300°C.

[0061] (2) Evaluation (a) Average primary particle size of nickel powder As a result of image analysis of the images observed by a scanning electron microscope (SEM), the number average particle size in equivalent circle diameter of the nickel powder obtained in (1) above was 134 nm.

[0062] (b) Ratio of crystallite size to number average particle size The nickel powder obtained in (1) above was analyzed by powder X-ray diffraction and found to have a crystallite size of 72 nm, which means that the ratio of the crystallite size to the number average particle size was 54%.

[0063] [Example 3] (1) Preparation of nickel powder Approximately 150 mg of nickel powder was obtained in the same manner as in Example 1, except that the liquid temperature was maintained at 150° C. for 5 minutes after the liquid temperature reached 150° C. by microwave irradiation.

[0064] (2) Evaluation (a) Number average particle size of nickel powder Image analysis of the images observed with a scanning electron microscope (SEM) revealed that the number average particle size of the nickel powder obtained in (1) above was 212 nm in terms of the circle-equivalent diameter.

[0065] (b) Ratio of crystallite size to number average particle size The nickel powder obtained in (1) above was analyzed by powder X-ray diffraction and found to have a crystallite size of 53 nm, which means that the ratio of the crystallite size to the number average particle size was 25%.

[0066] [Example 4] (1) Preparation of nickel powder About 150 mg of nickel powder was obtained in the same manner as in Example 1, except that the heating rate was 50° C. / min.

[0067] (2) Evaluation (a) Number average particle size of nickel powder As a result of image analysis of the images observed by a scanning electron microscope (SEM), the number average particle size of the nickel powder obtained in (1) above was 185 nm in terms of the circle-equivalent diameter.

[0068] (b) Ratio of crystallite size to number average particle size The nickel powder obtained in (1) above was analyzed by powder X-ray diffraction and found to have a crystallite size of 56 nm, which means that the ratio of the crystallite size to the number average particle size was 30%.

[0069] [Example 5] (1) Preparation of nickel powder Approximately 150 mg of nickel powder was obtained in the same manner as in Example 1, except that 1.3 g of nickel chloride hexahydrate (NiCl2·6H2O) was used as the water-soluble nickel salt.

[0070] (2) Evaluation (a) Number average particle size of nickel powder As a result of image analysis of the images observed by a scanning electron microscope (SEM), the number average particle size in equivalent circle diameter of the nickel powder obtained in (1) above was 170 nm.

[0071] (b) Ratio of crystallite size to number average particle size The nickel powder obtained in (1) above was analyzed by powder X-ray diffraction and found to have a crystallite size of 48 nm, which means that the ratio of the crystallite size to the number average particle size was 28%.

[0072] [Example 6] (1) Preparation of nickel powder Approximately 150 mg of nickel powder was obtained in the same manner as in Example 1, except that 1.3 g of nickel acetate tetrahydrate (Ni(CH3COO)2·4H2O) was used as the water-soluble nickel salt.

[0073] (2) Evaluation (a) Number average particle size of nickel powder Image analysis of the images observed with a scanning electron microscope (SEM) revealed that the number average particle size of the nickel powder obtained in (1) above was 150 nm in terms of the circle-equivalent diameter.

[0074] (b) Ratio of crystallite size to number average particle size The nickel powder obtained in (1) above was analyzed by powder X-ray diffraction and found to have a crystallite size of 60 nm, which means that the ratio of the crystallite size to the number average particle size was 40%.

[0075] [Comparative Example 1] (1) Preparation of nickel powder Approximately 150 mg of nickel powder was obtained in the same manner as in Example 1, except that after the temperature reached 140°C by irradiating with microwaves, the temperature was maintained for 5 minutes and then heating was terminated, and the liquid temperature was not allowed to exceed 150°C.

[0076] (2) Evaluation (a) Number average particle size of nickel powder As a result of image analysis of the images observed by a scanning electron microscope (SEM), the number average particle size of the nickel powder obtained in (1) above was 231 nm in terms of the circle-equivalent diameter.

[0077] (b) Ratio of crystallite size to number average particle size The nickel powder obtained in (1) above was analyzed by powder X-ray diffraction and found to have a crystallite size of 40 nm, which means that the ratio of the crystallite size to the number average particle size was 17%.

[0078] Comparative Example 2 (1) Preparation of nickel powder About 150 mg of nickel powder was obtained in the same manner as in Example 1, except that the reaction solution was heated at a temperature increase rate of 40°C / min.

[0079] (2) Evaluation (a) Number average particle size of nickel powder Image analysis of the images observed with a scanning electron microscope (SEM) revealed that the number average particle size in equivalent circle diameter of the nickel powder obtained in (1) above was 195 nm.

[0080] (b) Ratio of crystallite size to number average particle size The nickel powder obtained in (1) above was analyzed by powder X-ray diffraction and found to have a crystallite size of 43 nm, which means that the ratio of the crystallite size to the number average particle size was 22%.

[0081] Table 1 shows the evaluation results of Examples 1 to 6 and Comparative Examples 1 and 2.

[0082] [Table 1]

[0083] [Considerations on evaluation results] From the ratio of the crystallite size to the average primary particle size, it was clear that nickel powders obtained in Examples 1 to 6, which satisfied the manufacturing method of the present invention, had higher crystallinity than the manufacturing methods of Comparative Examples 1 and 2. The reason why highly crystalline nickel powders could be produced in the Examples is thought to be that the combined use of a reducing agent and the heating of the reaction solution by microwave irradiation, which can rapidly increase the temperature of the reaction solution, effectively promotes the crystal growth of the nickel particles precipitated by reduction at high temperatures.

[0084] As described above, it was confirmed that highly crystalline nickel powder can be produced by using the production method of the present invention.

Claims

1. a crystallization step of irradiating a reaction solution containing a water-soluble nickel salt, a complexing agent, water, and a reducing agent with microwaves to heat the reaction solution, thereby precipitating nickel particles; the crystallization step includes a temperature-raising step of raising the temperature of the reaction solution to 150° C. or higher and 300° C. or lower by irradiating the microwaves at a temperature-raising rate of 50° C. / min or higher, and a temperature-holding step of subsequently holding the temperature of the reaction solution at 150° C. or higher and 300° C. or lower, A method for producing nickel powder, wherein the crystallization step precipitates nickel powder in which the crystallite diameter of the nickel powder comprising the nickel particles is 25% or more of the number average particle diameter of the nickel powder.

2. 2. The method for producing nickel powder according to claim 1, wherein the water-soluble nickel salt is one or more selected from chlorides, sulfates, and acetates.

3. The method for producing nickel powder according to claim 1 or claim 2, wherein the complexing agent is ammonia.

4. The method for producing nickel powder according to claim 1 or claim 3, wherein the reducing agent is hydrazine.

5. The method for producing nickel powder according to claim 1 , further comprising a mixing step of preparing a reaction solution containing the water-soluble nickel salt, the complexing agent, the water, and the reducing agent.

6. The method for producing nickel powder according to claim 1 , wherein the temperature maintaining step maintains the temperature of the reaction solution for 45 seconds to 10 minutes.

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