JP7813740B2 - Nickel powder manufacturing method and its use - Google Patents

Description

The technology disclosed herein relates to a method for producing nickel powder and its use.

Electrodes for electronic components such as multilayer ceramic capacitors (MLCCs) are formed, for example, by firing an electrode paste containing nickel powder. In addition to elemental nickel (Ni), nickel oxide (NiO) and nickel hydroxide (Ni(OH) ₂ ) are present on the surfaces of Ni particles in this nickel powder. The surface condition of these Ni particles is a factor that affects the amount of shrinkage during firing of the electrode paste and the dispersibility within the electrode paste. For this reason, techniques for controlling the surface condition of Ni particles in nickel powder have been proposed in recent years.

For example, Patent Document 1 discloses a technique for treating nickel powder with a nitrogen-containing compound (primary alkylamine, aliphatic amide). This increases the ratio of elemental Ni to Ni-containing components (elemental Ni, Ni(OH) ₂ , NiO) on the particle surface to 50% or more. Nickel powder with a high ratio of elemental Ni exhibits a reduced amount of thermal shrinkage during firing. As a result, it is believed that structural defects (cracks, etc.) caused by the difference in the amount of thermal shrinkage between the substrate and the electrode can be suppressed. Furthermore, Patent Document 2 discloses Ni particles in which the particle surface composition is controlled to Ni: 3-22 mol%, Ni(OH) ₂ : 64-85 mol%, and NiO: 0-14 mol%. Patent Document 2 also discloses that Ni particles with the above configuration can improve the dispersion state of the Ni powder and the co-material.

Patent No. 6647458 Patent No. 6292014

In recent years, attempts have been made to miniaturize nickel powder used to form electrodes. For example, using an electrode paste containing Ni particles with an average particle size of 100 nm or less significantly reduces the thickness of the electrode after firing, which can greatly contribute to the miniaturization of electronic components. However, Ni particles with an average particle size of 100 nm or less have a large surface area, which tends to result in poor dispersibility within the electrode paste. Furthermore, if coarse secondary particles are generated by aggregation, the surface smoothness of the electrode after firing may be reduced. The technology disclosed herein was developed in light of these circumstances, and aims to improve the dispersibility of nickel powder containing Ni particles with an average particle size of 100 nm or less.

To solve the above problems, the technology disclosed herein provides a method for producing nickel powder (hereinafter simply referred to as the "production method") having the following configuration.

The manufacturing method disclosed herein includes a preparation step of preparing a Ni slurry in which Ni particles having an average particle size of 100 nm or less based on FE-SEM observation are dispersed in an organic solvent, and a Ni hydroxide reduction step of reducing Ni(OH) ₂ on the surfaces of the Ni particles by carrying out a bubbling treatment in which an oxidizing gas or a reducing gas is supplied to the Ni slurry while heating it.

The present inventors conducted extensive research to solve the above-mentioned problems and concluded that Ni particles with a high amount of Ni(OH) ₂ on their particle surfaces have many hydrophilic groups (OH groups) and are therefore difficult to disperse in electrode pastes containing large amounts of organic components such as dispersion media and binders. Based on this knowledge, the present inventors investigated means for reducing the amount of Ni(OH) ₂ on Ni particle surfaces and discovered a bubbling treatment in which heating is performed while supplying an oxidizing or reducing gas. Specifically, when a bubbling treatment is performed using an oxidizing gas, the Ni(OH) ₂ on the particle surfaces is oxidized to produce NiO. On the other hand, when a bubbling treatment is performed using a reducing gas, the Ni(OH) ₂ on the particle surfaces is reduced to produce Ni. Reducing the amount of Ni(OH) ₂ on Ni particle surfaces through these bubbling treatments can improve the dispersibility of Ni particles in electrode pastes.

In one aspect of the manufacturing method disclosed herein, Ni particles with a CV value of 0.2 or less are used in the preparation process. Using Ni particles with a small CV value of particle size contributes to the formation of an electrode with a thin film thickness and a smooth surface.

In one embodiment of the production method disclosed herein, the bubbling time in the Ni hydroxide reduction step is 5 minutes or longer, which allows the amount of Ni(OH) ₂ on the surfaces of the Ni particles to be more suitably reduced.

In one embodiment of the production method disclosed herein, the oxidizing gas contains one selected from oxygen and ozone. By using an oxidizing gas containing these, the amount of Ni(OH) ₂ on the particle surface can be more suitably reduced.

In one embodiment of the production method disclosed herein, the reducing gas contains one selected from hydrogen and carbon monoxide. By using a reducing gas containing these, the amount of Ni(OH) ₂ on the particle surface can be more suitably reduced.

In one aspect of the manufacturing method disclosed herein, the Ni particles are core-shell particles comprising a core particle and a shell that covers at least a portion of the surface of the core particle, and the shell contains elemental nickel. This allows for the production of a large amount of particles while maintaining a small particle diameter.

Another aspect of the technology disclosed herein provides a nickel powder primarily composed of Ni particles, wherein the Ni particles of the nickel powder have an average particle size of 100 nm or less based on FE-SEM observation, and in a photoelectron spectrum of the Ni particles measured by X-ray photoelectron spectroscopy, the ratio of the peak area of Ni(OH) ₂ to the total peak area of Ni, NiO, and Ni(OH) ₂ in the region showing the Ni 2p orbital is 60% or less.

As described above, the manufacturing method disclosed herein can produce Ni particles with a reduced amount of Ni(OH) ₂ on the particle surface. In XPS analysis of the particle surface of these Ni particles, the ratio of the Ni(OH) ₂ peak area to the total peak area of nickel-containing substances (Ni, NiO, Ni(OH) ₂ ) is reduced to 60% or less. These Ni particles have excellent dispersibility in electrode paste.

In one embodiment of the nickel powder disclosed herein, the Ni particles have a Ni peak area ratio of 10% to 40% relative to the total peak area, and a NiO peak area ratio of 10% to 50% relative to the total peak area. As described above, when Ni(OH) ₂ on the Ni particle surface is oxidized in the Ni hydroxide reduction step, Ni particles with an increased amount of NiO on the particle surface are produced.

In one embodiment of the nickel powder disclosed herein, the Ni particles have a Ni peak area ratio of 40% to 70% of the total peak area, and a NiO peak area ratio of 1% to 10% of the total peak area. As described above, when Ni(OH) ₂ on the Ni particle surface is reduced in the Ni hydroxide reduction step, Ni particles with an increased amount of Ni on the particle surface are produced.

Another aspect of the technology disclosed herein provides an electrode paste. The electrode paste disclosed herein includes the nickel powder having the above-described configuration and a dispersion medium for dispersing the nickel powder. This electrode paste contains Ni particles with a reduced amount of Ni(OH) ₂ on the Ni particle surface. These Ni particles are suitably dispersed in the electrode paste. This allows for the easy formation of an electrode with a thin film thickness and excellent surface smoothness.

FIG. 1 is a flow diagram showing a method for producing nickel particles according to one embodiment.

The following describes preferred embodiments of the technology disclosed herein. Matters other than those specifically mentioned in this specification that are necessary for implementing the technology disclosed herein can be understood as design matters for a person skilled in the art based on the prior art in the relevant field. The technology disclosed herein can be implemented based on the content disclosed in this specification and common technical knowledge in the relevant field. The following explanation is not intended to limit the technology disclosed herein to specific embodiments. In this specification and claims, when a specific numerical range is expressed as A to B (A and B are arbitrary numbers), this means "greater than A and less than B." Therefore, it also encompasses "greater than A but less than B."

1. Method for Producing Nickel Particles Hereinafter, a method for producing nickel particles according to this embodiment will be described. FIG. 1 is a flow diagram showing a method for producing nickel particles according to this embodiment. As shown in FIG. 1, the production method according to this embodiment includes a preparation step S10 and a Ni hydroxide reduction step S20. Furthermore, in the production method according to this embodiment, a separation step S30 is carried out after the Ni hydroxide reduction step S20 is carried out. Each step will be described below.

(1) Preparation step S10
In the preparation step S10, a Ni slurry in which Ni particles are dispersed in an organic solvent is prepared. For example, the Ni slurry can be prepared by adding Ni particles to an organic solvent and performing a dispersion process. Alternatively, the Ni particles can be generated by, for example, a thermal decomposition method in which a Ni salt is added to an organic solvent to generate a Ni complex, followed by a heat treatment to precipitate the Ni particles. This thermal decomposition method results in Ni particles dispersed in the organic solvent, which can then be used as a Ni slurry as is. Note that the preparation step S10 in the technology disclosed herein does not necessarily require the Ni slurry to be prepared by hand; a commercially available Ni slurry may be purchased, for example. In other words, the detailed procedure of the preparation step S10 does not limit the technology disclosed herein, and conventionally known means can be employed without particular limitation.

Next, we will explain the raw materials for the Ni slurry that can be used in the manufacturing method of this embodiment.

(a) Ni Particles Nickel particles (Ni particles) are a material that constitutes the main component of electrodes after firing. By firing these Ni particles, high-performance (low-resistance) conductive materials can be formed inexpensively. For this reason, Ni particles are suitable as an electrode material for electronic components such as MLCCs. In this specification, "Ni particles" refers to particles in which Ni is the main element in the particle surface layer. That is, the term "Ni particles" in this specification encompasses not only particles composed entirely of Ni (single Ni particles), but also Ni alloy particles, core-shell particles, and the like. The above-mentioned phrase "Ni is the main element in the particle surface layer" means that Ni is the metal element most commonly identified in the particle surface layer (a region 10 nm thick from the particle surface) in elemental analysis using XPS. Specifically, the term "Ni particles" as used herein refers to particles in which, when the total number of metal elements constituting the particle surface layer is taken as 100 mol%, 50 mol% or more (preferably 60 mol% or more, more preferably 70 mol% or more, even more preferably 80 mol% or more, and particularly preferably 90 mol% or more) is Ni element. Metal elements other than Ni that can be contained in Ni particles include gold (Au), platinum (Pt), silver (Ag), palladium (pd), copper (Cu), aluminum (Al), zinc (Zn), lithium (Li), sodium (Na), potassium (K), magnesium (Mg), calcium (Ca), and barium (Ba).

The "core-shell particles" mentioned above are multilayer particles comprising a core particle and a shell covering the surface of the core particle. When core-shell particles are used as Ni particles, a shell containing Ni element is formed. Examples of core particles include Cu particles, Au particles, Pt particles, Ag particles, and Pd particles. Forming a Ni shell on the surface of these core particles stabilizes particle size control during Ni particle production. Therefore, the production of this type of core-shell particle allows for the production of large quantities of particles while maintaining a small particle size, thereby improving productivity. Among the above core particles, Cu particles are preferred from the perspective of obtaining Ni particles with low material costs while suppressing a decrease in conductivity. The shell need not completely cover the entire core particle; it is sufficient that at least a portion of the core particle's surface is covered with the shell. For example, if the average coverage of the Ni shell based on SEM observation is 50% or more (more preferably 70% or more, even more preferably 80% or more, and particularly preferably 90% or more), core-shell particles (Ni particles) with sufficient conductivity can be obtained. The upper limit of the average coverage rate of the Ni shell may be 100% or less, 99% or less, or 95% or less.

In this embodiment, Ni particles with an average particle diameter of 100 nm or less based on FE-SEM observation are used. An electrode paste containing such fine Ni particles can reduce the thickness of the electrode after firing, contributing to the miniaturization of electronic components. On the other hand, the surface area of the fine Ni particles increases, making them more likely to aggregate in the electrode paste. As a result, if coarse secondary particles are generated, the coarse particles may protrude from the surface of the thin electrode, reducing surface smoothness. However, the manufacturing method according to this embodiment can improve the dispersibility of Ni particles in the electrode paste, thereby preventing aggregation even when the average particle diameter of the Ni particles is 100 nm or less. The average particle diameter of the Ni particles is preferably 80 nm or less, more preferably 70 nm or less, and particularly preferably 60 nm or less. This facilitates the formation of thinner electrodes. The lower limit of the average particle diameter of the Ni particles is not particularly limited and may be 1 nm or more, 5 nm or more, or 10 nm or more. In this specification, the term "average particle size based on FE-SEM observation" refers to the cumulative 50% particle size (D ₅₀ ) based on the number in the particle size distribution of 1,000 Ni particles extracted from an image of nickel powder taken using a field emission scanning electron microscope (FE-SEM).

The shape of the Ni particles is not particularly limited and may be spherical or non-spherical. Examples of non-spherical Ni particles include plate-shaped, scale-shaped, flake-shaped, and irregularly shaped particles. When spherical Ni particles are used, the aspect ratio of the Ni particles is preferably 1.2 or less, more preferably 1.15 or less, and particularly preferably 1.1 or less. This facilitates improving the packing density of the Ni particles in the electrode paste. The lower limit of the aspect ratio of such spherical Ni particles is 1 or more. On the other hand, when non-spherical Ni particles are used, the aspect ratio of the Ni particles is preferably 1.3 or more, more preferably 1.5 or more, even more preferably 1.7 or more, and particularly preferably 2 or more. When Ni particles with such a high aspect ratio are used, the Ni particles are oriented along a predetermined direction in the electrode paste, making it easier to form suitable conductive paths in the fired electrode. On the other hand, taking into consideration the ease of particle production, the upper limit of the aspect ratio of non-spherical Ni particles is preferably 5 or less, more preferably 4 or less, and particularly preferably 3 or less. Furthermore, the nickel powder in this embodiment may be a mixed powder of spherical particles and non-spherical particles.

Furthermore, when generating Ni particles by thermal decomposition in this process, a nickel salt is used as the Ni source. Examples of nickel salts include nickel formate, nickel nitrate, nickel sulfate, nickel carbonate, nickel carboxylate, nickel hydroxide, and nickel chloride. Adding these nickel salts to an organic solvent generates a nickel complex. Heating this nickel complex produces Ni particles. When generating core-shell particles with a Ni shell by thermal decomposition, a salt of the metal element that will form the core particles can be added to the organic solvent along with the nickel salt. If the standard potential of the added metal element is higher than that of Ni, the metal element will precipitate preferentially. This allows for the formation of a Ni shell on the surface of the core particles after the core particles containing the desired metal element are generated. Examples of salts of metal elements that can generate such core particles include copper formate, copper acetate, copper chloride, copper sulfate, copper nitrate, silver acetate, silver nitrate, silver chloride, silver oxalate, palladium chloride, chloroplatinic acid, and chloroauric acid.

(b) Organic Solvent In the production method according to this embodiment, an organic solvent is used as a dispersion medium for dispersing Ni particles. This prevents moisture from coming into contact with the surface of the Ni particles, which would otherwise increase the amount of Ni(OH) ₂ . The organic solvent is not particularly limited as long as it can disperse Ni particles. Examples of such organic solvents include low-polarity solvents such as ethylene glycol, alcohols, amine compounds, N,N-dimethylformamide, dimethyl sulfoxide, and acetone. When producing Ni particles by a thermal decomposition method, it is preferable to use an organic solvent that forms a nickel complex when a nickel salt is dissolved in it. Examples of such organic solvents include amine compounds such as n-octylamine, 2-ethylhexylamine, n-decylamine, n-dodecylamine, n-tetradecylamine, n-hexadecylamine, stearylamine, and oleylamine.

In addition, the Ni slurry in this embodiment preferably has a Ni particle content relative to the organic solvent adjusted to a predetermined range. Specifically, by dispersing a certain amount or more of Ni particles in the organic solvent, the Ni hydroxide reduction step S20, described below, can be carried out efficiently. From this perspective, the Ni particle content, when the weight (g) of the organic solvent is taken as 100%, is preferably 0.5% or more, more preferably 1% or more, and particularly preferably 5% or more. On the other hand, if the Ni particle content is too high, the Ni particles may not be properly dispersed in the organic solvent, which may result in non-uniform Ni(OH) ₂ reduction in the Ni hydroxide reduction step S20. From this perspective, the upper limit of the Ni particle content is preferably 30% or less, more preferably 25% or less, and particularly preferably 20% or less.

(2) Ni hydroxide reduction step S20
In this process, a bubbling process is performed in which an oxidizing gas or a reducing gas is supplied to the Ni slurry while heating it. This reduces the amount of Ni(OH) ₂ on the surface of the Ni particles. This reduces the hydrophilic groups (OH groups) on the Ni particle surface, improving dispersibility in electrode pastes containing organic solvents and binders. As a result, nickel powder containing Ni particles that are less likely to agglomerate in electrode pastes can be produced.

For example, when an oxidizing gas is bubbled in this process, Ni(OH) ₂ on the Ni particle surface is oxidized to generate NiO. This reduces the number of hydrophilic groups (OH groups) on the Ni particle surface. Furthermore, the generated NiO coats the Ni particle surface. This also prevents the amount of Ni(OH) ₂ on the particle surface from increasing due to moisture in the air after the Ni particles are separated from the Ni slurry. The oxidizing gas can be any conventional gas that has an oxidizing effect on Ni(OH) ₂ and is not particularly limited. For example, the oxidizing gas may contain an oxidizing component such as oxygen or ozone. The oxidizing gas may also contain an inert component such as nitrogen, helium, or argon as a secondary component. When using a mixed gas containing such an inert component, it is preferable that the mixed gas contains 20% or more of the above-mentioned oxidizing component (more preferably 50% or more, particularly preferably 70% or more). That is, the oxidizing gas in this embodiment may be air, which is a mixed gas of oxygen and nitrogen. Even in this case, the Ni particle surface can be suitably oxidized. On the other hand, the upper limit of the content of the oxidizing component in the mixed gas is not particularly limited, and may be 100% or 90% or less.

On the other hand, when a reducing gas is bubbled in this process, Ni(OH) ₂ on the Ni particle surface is reduced to produce Ni. In this case, the hydrophilic groups (OH groups) on the Ni particle surface are reduced, improving dispersibility in the electrode paste. The reducing gas can be any conventional gas that has a reducing effect on Ni(OH) ₂ and is not particularly limited. For example, the reducing gas may contain a reducing component such as hydrogen or carbon monoxide. Similarly to the oxidizing gas, the reducing gas may also contain an inert component such as nitrogen, helium, or argon as a secondary component. When using a mixed gas containing such an inert component, it is preferable that the mixed gas contains 1% or more of the above-mentioned reducing component (more preferably 3% or more, particularly preferably 5% or more). This allows for favorable reduction of the Ni particle surface. On the other hand, the upper limit of the content of the reducing component in the mixed gas is not particularly limited and may be 100% or less.

If the gas (oxidizing gas or reducing gas) used in the bubbling treatment contains moisture, the amount of Ni(OH) ₂ on the Ni particle surface may actually increase. Therefore, the gas used for the bubbling treatment is preferably a dry gas. Specifically, the moisture content of the gas used for the bubbling treatment is preferably 500 ppm or less, more preferably 100 ppm or less, and particularly preferably 10 ppm or less, and is controlled to, for example, 1 ppm. This allows the amount of Ni(OH) ₂ on the Ni particle surface to be suitably reduced.

The gas supply rate in the bubbling treatment is preferably 100 cm ³ /min or more, more preferably 200 cm ³ /min or more, and particularly preferably 300 cm ³ /min or more, per 1 L of Ni slurry. This allows the amount of Ni(OH) ₂ on the Ni particle surface to be efficiently reduced. On the other hand, the upper limit of the gas supply rate is not particularly limited, and may be 7000 cm ³ /min or less, 5000 cm ³ /min or less, or 2000 cm ³ /min or less.

The heating temperature in this step is preferably 80°C or higher, more preferably 100°C or higher, and particularly preferably 150°C or higher. This allows the Ni(OH) ₂ reduction reaction to be favorably promoted. On the other hand, the heating temperature in this step is preferably 350°C or lower, more preferably 300°C or lower, and particularly preferably 200°C or lower. This prevents the organic solvent from thermally decomposing and generating nickel carbide.

The bubbling time is preferably 1 minute or more, more preferably 5 minutes or more, even more preferably 15 minutes or more, and particularly preferably 30 minutes or more. This allows the amount of Ni(OH) ₂ on the Ni particle surface to be sufficiently reduced. On the other hand, the bubbling time is preferably 150 minutes or less, more preferably 120 minutes or less, more preferably 90 minutes or less, and particularly preferably 60 minutes or less. This prevents the bubbling process, which involves heating, from being prolonged, and prevents the generation of nickel carbide due to decomposition of the organic solvent.

Furthermore, in this process, a water-insoluble thiourea compound may be added to the Ni slurry before the bubbling treatment. This thiourea compound easily decomposes by heat treatment and forms a Ni sulfide layer, mainly composed of nickel sulfide, on the surface of the Ni particles. Ni particles having this Ni sulfide layer have an excellent sintering suppression effect, and therefore can effectively suppress the occurrence of cracks during the firing treatment. An example of the thiourea compound is a compound represented by the following formula (1). The thiourea compound represented by formula (1) can effectively promote the formation of the Ni sulfide layer. In formula (1), _R1 and _R2 are linear or cyclic alkyl groups having 6 or more carbon atoms. The linear alkyl group here may have a linear or branched chain structure.
R ₁ -HNCSNH-R ₂ (1)

Note that examples of thiourea compounds represented by the above formula (1) include N,N'-diphenylthiourea, N,N'-dihexylthiourea, and 1,3-dicyclohexylthiourea. These thiourea compounds are readily soluble in organic solvents and readily decompose upon heat treatment. Therefore, using the above-mentioned thiourea compounds can further promote the formation of a Ni sulfide layer. Furthermore, these thiourea compounds have lower reactivity with oxygen than other sulfur compounds (such as alkylthiols), and can therefore suppress gelation of the Ni slurry. Therefore, the above-mentioned thiourea compounds can suppress non-uniform reactions caused by gelation of the Ni slurry.

Furthermore, it is preferable to set the amount of thiourea compound added so that the Ni weight and S weight in the Ni slurry satisfy a predetermined ratio. For example, it is preferable to set the amount of thiourea compound added so that the ratio of S weight to Ni weight (100%) in the Ni slurry is 0.01% or more (more preferably 0.05% or more, even more preferably 0.1% or more, and particularly preferably 0.5% or more). This allows sufficient S element to be supplied to the surface of the Ni particles, allowing for the favorable formation of a Ni sulfide layer. On the other hand, if the amount of thiourea compound added exceeds a certain level, not only will the sintering suppression effect saturate, but sulfur contamination of the heating furnace may occur, resulting in increased equipment costs. From this perspective, it is preferable to set the amount of thiourea compound added so that the ratio of S element content to Ni element content is 3.0% or less (more preferably 2.5% or less, even more preferably 2.0% or less, and particularly preferably 1.5% or less).

(3) Separation step S30
As shown in FIG. 1 , in the manufacturing method according to this embodiment, the separation step S30 is carried out after the Ni hydroxide reduction step S20. In this separation step S30, Ni particles are separated from the Ni slurry. This step can be carried out using any conventionally known separation means used to separate powder from a slurry, without any particular restrictions. Examples of such separation means include static separation, centrifugation, and filter filtration. The Ni particles separated from the Ni slurry are then preferably washed with a predetermined organic solvent and then dried. This allows for the production of dried nickel powder from which impurities have been removed.

Note that the separation step S30 is not an essential step in the manufacturing method disclosed herein. For example, depending on the type of organic solvent used in preparing the Ni slurry, the Ni slurry after the Ni hydroxide reduction step S20 has been performed can be used as an electrode paste as is.

(4) Summary As described above, the manufacturing method according to this embodiment includes a Ni hydroxide reduction step S20 in which Ni(OH) ₂ on the surface of Ni particles is reduced by bubbling. This reduces the hydrophilic groups on the surface of the Ni particles, improving their dispersibility in the electrode paste. As a result, a nickel powder containing Ni particles that are less likely to aggregate in the electrode paste can be manufactured.

Furthermore, the manufacturing method according to this embodiment can also suppress the occurrence of structural defects such as cracks in the electrode after the paste is fired. Specifically, Ni particles containing a large amount of Ni(OH) ₂ tend to have a large amount of thermal shrinkage due to the release of a large amount of water ( _H2O ) during firing. This results in a large difference in the amount of thermal shrinkage between the electrode paste and the base material, which can cause structural defects such as cracks in the electrode after firing. In contrast, the manufacturing method according to this embodiment can reduce the amount of Ni(OH) ₂ in the Ni particles, thereby contributing to the suppression of structural defects in the electrode after firing.

2. Nickel Powder Next, the nickel powder produced by the manufacturing method described above will be described. The Ni particles contained in the nickel powder according to this embodiment have a reduced amount of Ni(OH) ₂ on the particle surface. The nickel powder having such a configuration will be specifically described below.

First, the term "nickel powder" in this specification refers to a powder material (a group of fine particles) primarily composed of Ni particles. Here, "primarily composed of Ni particles" means that, among the inorganic fine particles contained in the powder material, the inorganic fine particles contained in the largest amount by weight are Ni particles with a reduced amount of Ni(OH) _{2 .} More specifically, the term "nickel powder" in this specification refers to a powder material containing 50% by weight or more (preferably 60% by weight or more, more preferably 70% by weight or more, even more preferably 80% by weight or more, and particularly preferably 90% by weight or more) of Ni particles satisfying the Ni(OH)2 amount described below. In other words, the nickel powder disclosed herein may contain inorganic fine particles other than the Ni particles having the above-described configuration, as long as the sintering-inhibiting effect of the technology disclosed herein is not significantly impaired. Examples of such subcomponents include Ni particles that do not satisfy the Ni(OH) ₂ content described below, and inorganic fine particles containing other metal elements as the main component (Cu particles, Au particles, Ag particles, Pd particles, Pt particles, etc.).

As described above, the Ni particles produced by the manufacturing method according to this embodiment have a reduced amount of Ni(OH) ₂ on the particle surface. Here, "the amount of Ni(OH) ₂ on the particle surface is reduced" means that when the surface of the Ni particles is analyzed using X-ray photoelectron spectroscopy (XPS), the proportion of Ni(OH) ₂ relative to the total amount of Ni-containing components (simple Ni, NiO, and Ni(OH) ₂ ) is 60% or less. Specifically, when a photoelectron spectrum of the Ni particles is obtained using XPS, multiple peaks derived from the Ni-containing components are observed in the 845-870 eV region, which indicates the Ni 2p orbital. Among these multiple peaks, a peak derived from simple Ni is observed near 852.2 eV. Furthermore, a peak derived from NiO is observed near 854.3 eV. The peak derived from Ni(OH) ₂ is observed near 855.6 eV. Therefore, by calculating the ratio (PA Ni(OH) ₂ /PA _all ) of the peak area of Ni(OH) ₂ to the total peak area PA _all of Ni, _{NiO, and Ni(} OH)2, the proportion of Ni(OH) ₂ relative to the total amount of Ni-containing components can be obtained. In general, Ni particles have a large amount of Ni(OH) ₂ generated on the particle surface by moisture in the air, so the peak area ratio (PA _Ni(OH)2 /PA _all ) exceeds 60%. In contrast, Ni particles produced through the production method according to this embodiment have a reduced amount of Ni(OH) ₂ on the particle surface, so the peak area ratio (PA _Ni(OH)2 /PA _all ) is reduced to 60% or less (preferably 57% or less, more preferably 56% or less, and particularly preferably 55% or less). Such Ni particles have few hydrophilic groups on the particle surface, and therefore can exhibit excellent dispersibility in electrode paste. The lower limit of the peak area ratio (PA _Ni(OH) /PA _all ) is not particularly limited, and may be 25% or more, or 30% or more.

Furthermore, as described above, when an oxidizing gas is used in the Ni hydroxide reduction step S20, Ni(OH) ₂ on the particle surface is oxidized to NiO. In this case, Ni particles with a large amount of NiO present on the particle surface are generated. Specifically, when bubbling treatment with an oxidizing gas is performed, the ratio of the NiO peak area _PANiO to the total peak area _PAall in the XPS analysis ( _PANiO / _PAall ) increases to 12% or more (preferably 13% or more, particularly preferably 20% or more). Since the particle surfaces of such Ni particles are coated with NiO, the generation of Ni(OH) ₂ due to moisture absorption can be more effectively prevented. On the other hand, the upper limit of the peak area ratio ( _PANiO / _PAall ), which indicates the proportion of NiO present, is not particularly limited and may be 60% or less, 55% or less, or 50% or less. Furthermore, when bubbling treatment with an oxidizing gas is performed, Ni on the particle surface is also oxidized to NiO. Therefore, in Ni particles that have been bubbling treatment with an oxidizing gas, the ratio of the Ni peak area PA _Ni to the total peak area PA _all (PA _Ni /PA _all ) is reduced to 45% or less (preferably 40% or less, particularly preferably 35% or less). The lower limit of the peak area ratio (PA _Ni /PA _all ) indicating the abundance ratio of Ni is not particularly limited and may be 10% or more, or may be 15% or more.

On the other hand, when a reducing gas is used in the Ni hydroxide reduction step S20, Ni(OH) ₂ is reduced to Ni. In this case, Ni particles are generated in which Ni is abundantly present on the particle surface. That is, Ni particles are generated in which the peak area ratio (PA _Ni /PA _all ) is increased to 40% or more (preferably 42% or more, particularly preferably 45% or more). These Ni particles also have reduced hydrophilic groups on the particle surface, improving their dispersibility in the electrode paste. The upper limit of the peak area ratio (PA _Ni /PA _all ), which indicates the proportion of Ni present, is not particularly limited and may be 70% or less, 65% or less, or 60% or less. When bubbling with a reducing gas is performed, NiO on the particle surface is also reduced to Ni. Therefore, in Ni particles subjected to a bubbling treatment with a reducing gas, the peak area ratio (PA _NiO /PA _all ) indicating the abundance ratio of the NiO decreases to 10% or less (typically 9% or less, for example 8% or less). The lower limit of the peak area ratio (PA _NiO /PA _all ) indicating the abundance ratio of the NiO is preferably 1% or more, more preferably 2.5% or more, and particularly preferably 5% or more. By having a certain amount of NiO present on the surface of the Ni particles in this way, the generation of Ni(OH) ₂ due to moisture absorption can be suppressed.

When a thiourea compound is added in the Ni hydroxide reduction step S20, a Ni sulfide layer containing nickel sulfide is formed on the surface of the Ni particles. This Ni sulfide layer has an excellent sintering suppression effect, thereby preventing cracks from occurring in the electrode after firing. In this case, nickel sulfide is confirmed in the particle surface analysis using the above-mentioned XPS. Specifically, when a photoelectron spectrum of Ni particles is obtained using XPS, peaks derived from components constituting the Ni particle surface are confirmed in the 158-170 eV region, which indicates the S2p orbital. Among these, the peak derived from Ni sulfide is confirmed in the 162±0.5 eV region. Therefore, the proportion of Ni sulfide present on the surface of the _Ni particles can be analyzed by calculating the ratio (PA _NiS /PA _158-170 ) of the area of the peak derived from Ni sulfide to the total area PA _158-170 of the peaks confirmed in the region indicating the S2p orbital. In this case, if a Ni sulfide layer is formed by adding the thiourea compound, the peak area ratio (PA _NiS /PA _158-170 ) will be 80% or more. This peak area ratio (PA _NiS /PA _158-170 ) is preferably 85% or more, more preferably 90% or more, and particularly preferably 95% or more. This makes it possible to obtain nickel powder with better sintering resistance. The upper limit of the peak area ratio (PA _NiS /PA _158-170 ) is not particularly limited, and may be 100% or less or 99% or less.

3. Electrode Paste The nickel powder according to this embodiment can be dispersed in an appropriate dispersion medium to prepare a paste for forming an electrode (electrode paste). Because this electrode paste contains nickel powder with excellent dispersibility, it is possible to easily form an electrode with a thin film thickness.

The dispersion medium may be any suitable one capable of dispersing nickel powder well, and any conventionally known dispersion medium for use in electrode pastes can be used without particular limitation. As described above, the Ni particles contained in the nickel powder according to this embodiment have reduced hydrophilic groups on their surfaces, which allows them to exhibit favorable dispersibility in organic (non-aqueous) components. For this reason, the dispersion medium for the electrode paste is preferably an organic (non-aqueous) dispersion medium. Examples of such dispersion mediums include petroleum hydrocarbons (especially aliphatic hydrocarbons) such as mineral spirits, ethylene glycol and diethylene glycol derivatives, and high-boiling organic solvents such as toluene, xylene, butyl carbitol (BC), isobornyl acetate, terpineol, and dihydroterpineol. Such organic (non-aqueous) dispersion mediums can also suppress the generation of Ni(OH) ₂ due to moisture absorption.

While not intending to limit the technology disclosed herein, the content of nickel particles relative to the total weight of the electrode paste is preferably approximately 30% to 70% (e.g., 40% to 60%). Furthermore, the viscosity of the electrode paste is preferably approximately 10 mPa·s to 100 mPa·s (e.g., approximately 20 mPa·s to 50 mPa·s). This makes it easy to obtain an electrode paste that facilitates the formation of precise electrode patterns. The viscosity of the electrode paste can be measured using an E-type viscometer.

The electrode paste may contain additives other than nickel particles. Such additives include binders, conductive materials, dispersants, viscosity adjusters, etc. As long as these additives do not significantly impair the effects of the technology disclosed herein (dispersibility of Ni particles), any conventional additives that can be added to electrode pastes can be used without particular restrictions.

The above describes one embodiment of the technology disclosed herein. However, the above-described embodiment is not intended to limit the technology disclosed herein. In other words, the technology disclosed herein may include various modifications of the above-described embodiment.

[Test Example]
Test examples relating to the technology disclosed herein will be described below, but the technology disclosed herein is not limited to the following test examples.

1. Sample Preparation In this test example, six types of nickel powder (samples 1 to 6) were prepared using different manufacturing methods. The manufacturing procedures for each sample are described below.

(1) Sample 1
For Sample 1, a Ni slurry in which Ni particles (Cu/Ni core-shell particles) were dispersed in an organic solvent (oleylamine) was prepared according to the following procedure. Specifically, 5.1 g of nickel formate dihydrate (Ni amount: 1.62 g) was added to 957.7 g of oleylamine. Heat treatment at 120°C was then performed for 120 minutes. This resulted in the formation of a nickel formate-oleylamine complex in the solution. Next, 0.69 g of copper formate tetrahydrate (Cu amount: 0.19 g) was added to this solution. Heat treatment at 60°C was then performed for 30 minutes. This resulted in the formation of a copper formate-oleylamine complex in the solution. The solution containing these two complexes was heated at 190°C in a nitrogen atmosphere for 10 minutes. When the solution containing these two complexes was heated in this manner, Cu, which has a higher standard potential, preferentially precipitated, resulting in the formation of Cu core particles. Further heating resulted in the formation of Ni shells on the surfaces of the Cu core particles. This produces Cu/Ni core-shell particles. Furthermore, in this test, the produced Cu/Ni core-shell particles were used as seed particles, and the Ni shells on the surfaces of the seed particles were further grown. Specifically, 341.9 g of nickel acetate tetrahydrate (Ni amount: 74.27 g) was added to a seed particle slurry containing the above-mentioned seed particles. Next, a heat treatment at 135°C was carried out for 120 minutes. This produced a nickel acetate-oleylamine complex in the slurry. The seed particle slurry containing this complex was then heated at 200°C in a nitrogen atmosphere for 30 minutes. This caused Ni to precipitate so as to adhere to the surfaces of the seed particles. As a result, a Ni slurry was prepared in which Cu/Ni core-shell particles having Ni shells of sufficient thickness were dispersed in oleylamine.

Next, in Sample 1 of this test, a process was carried out to reduce Ni(OH) ₂ on the surfaces of the Ni particles (Cu/Ni core-shell particles) (Ni hydroxide reduction process). Specifically, a heat treatment was carried out at 200°C for 5 minutes while bubbling oxygen gas into the Ni slurry. This oxidized the surfaces of the Ni particles, converting the Ni(OH) ₂ on the particle surfaces to NiO.

Next, for Sample 1, Ni particles were separated from the Ni slurry. Specifically, the Ni particles were allowed to settle in the Ni slurry by static settling. The supernatant liquid was then removed to separate the Ni particles. These Ni particles were washed with isobornyl acetate and then dried to obtain a dry nickel powder (Sample 1) composed primarily of Cu/Ni core-shell particles.

(2) Sample 2
In this sample, nickel powder was produced following the same procedure as in Sample 1, except that the bubbling treatment time was changed to 10 minutes.

(3) Sample 3
In this sample, nickel powder was produced following the same procedure as in Sample 1, except that the bubbling treatment was carried out in the presence of a thiourea compound (1.17 g of diphenylthiourea).

(4) Sample 4
In this sample, nickel powder was produced following the same procedure as in Sample 1, except that the bubbling treatment time was changed to 30 minutes.

(5) Sample 5
In this sample, nickel powder was produced following the same procedure as in Sample 2, except that the gas used in the bubbling treatment was changed to a reducing gas (nitrogen gas containing 1% hydrogen).

(6) Sample 6
For Sample 6, nickel powder was produced according to the same procedure as for Sample 1, except that the bubbling treatment was not performed.

2. Evaluation Test (1) Analysis of Particle Size Distribution Each sample of nickel powder was observed using an FE-SEM (Hitachi High-Tech Corporation, Model: SU8230). Then, 1,000 particles whose entire peripheries could be seen from the SEM images were randomly selected, and the Heywood diameter was measured to determine the particle size distribution. Then, based on the particle size distribution, the average particle diameter ( _D50 particle diameter) and coefficient of variation (CV value) of Samples 1 to 6 were measured. The measurement results are shown in Table 1.

(2) Surface analysis of Ni particles First, an indium wire was pressed onto aluminum foil, and nickel powder from each sample was sprinkled onto the indium and pressed again to prepare a sample for XPS analysis. Then, a photoelectron spectrum was obtained for each sample in a narrow scan range of 845 to 870 eV using a photoelectron spectrometer (manufactured by ULVAC-PHI, Model: XPS PHI5000 VersaProbe). The measurement conditions are as follows:

X-ray source: Monochromated Al-Kα
Tube voltage: 15 kV
Output: 200W
Neutralization gun: Not used Pass energy: 11.75 eV
Step: 0.1 eV
Acquisition time: 200 ms
Accumulation count: 30 times

Next, the region of photoelectron energy (845-870 eV) derived from the Ni 2p orbital was separated from this photoelectron spectrum. Then, the peak areas of the Ni-derived peak (85.2 eV), NiO-derived peak (854.3 eV), and Ni(OH) _2- derived peak (855.6 eV) in this region were measured. The total peak area PA _all of these Ni-containing components was then calculated. Next, the ratio of the Ni peak area PA _Ni to this total peak area _PA all (PA _Ni /PA _all ) was calculated. Similarly, the ratio of the _NiO peak area PA _NiO to the total peak area PA all (PA _NiO /PA _all ) was calculated. Then, the ratio of the Ni(OH) ₂ peak area PA _Ni(OH)2 to the total peak area PA _all (PA _Ni(OH)2 /PA _all ) was calculated. The calculation results are shown in Table 1.

(3) Evaluation of Sintering Characteristics In this test, simultaneous differential thermal and thermogravimetry (TG-DTA) was performed to evaluate the sintering characteristics of each sample. Specifically, 20 mg of nickel powder was placed in a differential thermobalance (manufactured by Rigaku Corporation, model: TG-DTA8122) and heated from room temperature to 400°C at a heating rate of 10°C/min while supplying nitrogen gas at a flow rate of 0.5 L/min. The weight loss rate (%) during heating was then measured.

(4) Evaluation of Dispersibility Electrode pastes were prepared using the nickel powders of Samples 1 to 6, and the dispersibility of Ni particles in the electrode pastes was evaluated. Specifically, nickel powder (45 wt%), barium titanate powder (4.5 wt%), ethyl cellulose resin (2 wt%), carboxylic acid dispersant (2.0 wt%), and isobornyl acetate (the remainder) were mixed and dispersed using a three-roll mill. The prepared electrode paste was then applied to a glass slide to a thickness of 2 μm. The coating was then dried at 120°C for 5 minutes, and the surface of the coating was observed using an optical microscope (magnification: 200x). Secondary particles with a particle diameter of 1 μm or greater were considered "agglomerated particles." Samples with 15 or fewer agglomerated particles per field of view were evaluated as "good," samples with 16 to 25 agglomerated particles per field of view were evaluated as "fair," and samples with more than 25 agglomerated particles per field of view were evaluated as "poor." The evaluation results are shown in Table 1.

As shown in Table 1 above, in Samples 1 to 5, the proportion of Ni(OH) ₂ present on the particle surface was lower than in Sample 6. This indicates that the amount of Ni(OH) ₂ present can be reduced by heating the Ni slurry while bubbling oxygen gas or reducing gas. For example, in Samples 1 to 4, which were bubbled with oxygen gas, a tendency was observed in which Ni(OH) ₂ decreased and NiO increased. This is believed to be due to the Ni(OH) ₂ being oxidized by the oxygen gas to form NiO. On the other hand, in Sample 5, which was bubbled with reducing gas, a tendency was observed in which Ni(OH) ₂ decreased and Ni increased. This is believed to be due to the Ni(OH) ₂ being reduced by the reducing gas to form Ni. Furthermore, as a result of the dispersibility evaluation, Samples 1 to 5 were confirmed to have superior dispersibility compared to Sample 6. This indicates that by reducing Ni(OH) ₂ on the particle surface by bubbling treatment, Ni particles can be suitably dispersed in the electrode paste even when fine particles with an average particle diameter of 100 nm or less are used.

Furthermore, when the weight loss amounts of Samples 1 to 6 were compared, Samples 1 to 5 lost less weight during heating than Sample 6. This is thought to be because the amount of water desorbed from the Ni particles during firing was reduced as a result of reducing the amount of Ni(OH) ₂ on the particle surface. In other words, reducing the amount of Ni(OH) ₂ by bubbling treatment reduces the amount of shrinkage of the Ni particles during firing, which is expected to prevent structural defects such as cracks from occurring in the electrode after firing.

The technology disclosed herein has been described in detail above, but these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and variations of the specific examples given above. In other words, the technology disclosed herein encompasses the aspects described in items 1 to 12 below.

<Item 1>
a preparation step of preparing a Ni slurry in which Ni particles having an average particle size of 100 nm or less based on FE-SEM observation are dispersed in an organic solvent;
and a nickel hydroxide reduction step of reducing Ni(OH) ₂ on the surface of the nickel particles by carrying out a bubbling treatment in which an oxidizing gas or a reducing gas is supplied to the nickel slurry while heating it.

<Item 2>
Item 2. The method for producing nickel powder according to item 1, wherein Ni particles having a CV value of 0.2 or less are used in the preparation step.

<Item 3>
3. The method for producing nickel powder according to item 1 or 2, wherein the bubbling treatment is carried out for 5 minutes or more.

<Item 4>
4. The method for producing nickel powder according to any one of items 1 to 3, wherein the oxidizing gas contains one selected from oxygen and ozone.

<Item 5>
4. The method for producing nickel powder according to any one of items 1 to 3, wherein the reducing gas contains one selected from hydrogen and carbon monoxide.

<Item 6>
6. The method for producing nickel powder according to any one of items 1 to 5, wherein the Ni particles are core-shell particles having a core particle and a shell covering at least a part of the surface of the core particle, and the shell contains nickel element.

<Item 7>
A nickel powder mainly composed of Ni particles,
The Ni particles are
The average particle size based on FE-SEM observation is 100 nm or less,
In a photoelectron spectrum of the Ni particles measured by X-ray photoelectron spectroscopy, the ratio of the peak area of the Ni(OH) ₂ to the total peak area of Ni, NiO, and Ni(OH) ₂ in a region showing the Ni 2p orbital is 60% or less. Nickel powder.

<Item 8>
The Ni particles are
The ratio of the Ni peak area to the total peak area is 12% or more and 40% or less, and
8. The nickel powder according to item 7, wherein a ratio of the NiO peak area to the total peak area is 10% or more and 50% or less.

<Item 9>
The Ni particles are
The ratio of the Ni peak area to the total peak area is 40% or more and 70% or less, and
8. The nickel powder according to item 7, wherein a ratio of the NiO peak area to the total peak area is 1% or more and 10% or less.

<Item 10>
The nickel powder according to any one of items 7 to 9,
a dispersion medium for dispersing the nickel powder;
Electrode paste comprising:

Claims (10)

A method for producing nickel powder containing 50 mass% or more of surface-treated Ni particles, in which a ratio of a peak area of a peak derived from nickel sulfide to a total peak area in a region showing an S2p orbital in a photoelectron spectrum of the particles measured by X-ray photoelectron spectroscopy is 80% or more,
a preparation step of preparing a Ni slurry in which Ni particles having an average particle size of 100 nm or less based on FE-SEM observation are dispersed in an organic solvent;
A nickel hydroxide reduction step of reducing Ni(OH) ₂ on the surface of the nickel particles by carrying out a bubbling treatment in which an oxidation gas or a reduction gas is supplied to the nickel slurry while heating it ;
A method for producing nickel powder , wherein the Ni particles are core-shell particles having a core particle and a shell covering at least a portion of the surface of the core particle, and the shell contains nickel element . The method for producing nickel powder described in claim 1, wherein Ni particles having a CV value of 0.2 or less are used in the preparation process. The method for producing nickel powder according to claim 1 or 2, wherein the bubbling treatment is carried out for 5 minutes or more. The method for producing nickel powder described in claim 1 or 2, wherein the oxidizing gas includes one selected from oxygen and ozone. The method for producing nickel powder described in claim 1 or 2, wherein the reducing gas includes one selected from hydrogen and carbon monoxide. The method for producing nickel powder described in claim 1 or 2, wherein the Ni particles are core-shell particles having a core particle and a shell covering at least a portion of the surface of the core particle, and the shell contains nickel element. A nickel powder containing 50 mass% or more of surface-treated Ni particles,
The surface-treated Ni particles are
The average particle size based on FE-SEM observation is 100 nm or less,
In a photoelectron spectrum of the surface-treated Ni particles measured by X-ray photoelectron spectroscopy, the ratio of the peak area of Ni(OH) ₂ to the total peak area of Ni, NiO, and Ni(OH) ₂ in a region showing Ni 2p orbitals is 60% or less;
The surface-treated Ni particles are core-shell particles having a core particle and a shell covering at least a portion of the surface of the core particle, and the shell contains nickel element . The surface-treated Ni particles are
The ratio of the Ni peak area to the total peak area is 12% or more and 40% or less, and
The nickel powder according to claim 7, wherein a ratio of the peak area of the NiO to the total peak area is 10% or more and 50% or less. The surface-treated Ni particles are
The ratio of the Ni peak area to the total peak area is 40% or more and 70% or less, and
The nickel powder according to claim 7, wherein a ratio of the NiO peak area to the total peak area is 1% or more and 10% or less. The nickel powder according to claim 7,
a dispersion medium for dispersing the nickel powder;
Electrode paste comprising: