JP7813741B2 - Nickel powder - Google Patents

Description

The technology disclosed herein relates to nickel powder.

Electrodes for electronic components such as multilayer ceramic capacitors (MLCCs) are formed, for example, by firing an electrode paste containing nickel powder. In recent years, various techniques have been proposed to attach surface modifiers to the Ni particles in the nickel powder in order to improve the performance of electrode pastes and MLCCs.

For example, the surfaces of Ni particles described in Patent Document 1 are coated with one or more surface modifiers selected from organosilicon compounds, coupling agents, and surfactants. The Ni particles described in Patent Document 1 are attached to a dielectric substance (such as _BaTiO3 ) via these surface modifiers. Furthermore, the surfaces of Ni particles described in Patent Document 2 are modified with a phosphate-based, phosphorous-based, or hypophosphorous-based compound. The surface modifiers described in these patent documents can approximate the thermal shrinkage characteristics of the nickel powder during firing to those of other components (such as substrates and dielectric layers), thereby suppressing structural defects such as delamination and cracks.

Patent No. 4492785 Patent No. 3155948

In recent years, attempts have been made to miniaturize nickel powder used to form electrodes. For example, using nickel powder containing Ni particles with an average particle size of 150 nm or less significantly reduces the thickness of the electrode film after firing, which can greatly contribute to the miniaturization of electronic components. However, because Ni particles with an average particle size of 150 nm or less have a large surface area, they tend to be less dispersible when preparing 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 here was developed in light of these circumstances, and aims to create a nickel powder that exhibits excellent dispersibility when preparing electrode paste, even though the average particle size of the Ni particles is 150 nm or less.

To solve the above problems, the technology disclosed herein provides nickel powder with the following composition.

The nickel powder disclosed herein is primarily composed of Ni particles. The Ni particles of this nickel powder have an average particle size of 150 nm or less based on FE-SEM observation, and a surface modifier is attached to the particle surface. The surface modifier includes an aliphatic monoamine having 8 to 18 carbon atoms and a polyamine compound having an acid value of 5 mg KOH/g to 30 mg KOH/g and an amine value of 10 mg KOH/g to 100 mg KOH/g.

The nickel powder surface modifier disclosed herein contains a polyamine compound having a specified range of acid value and amine value. Because such polyamine compounds contain sufficient functional groups with amine values, they adhere firmly to acid sites on the surface of Ni particles through coordinate bonds. Furthermore, because such polyamine compounds contain sufficient functional groups with acid values, they also adhere to basic sites on the surface of Ni particles, making them less likely to detach. Therefore, the polyamine compound described above adheres favorably to the surface of Ni particles, effectively improving dispersibility during electrode paste preparation. However, excessive adhesion of this type of polyamine compound to Ni particles can increase viscosity during electrode paste preparation. In this case, it becomes difficult to apply a thin layer of electrode paste to a substrate or the like. In contrast, the technology disclosed herein adheres an aliphatic monoamine having 8 to 18 carbon atoms to the Ni particles in addition to the polyamine compound. By occupying a portion of the Ni particle surface with this aliphatic monoamine, the increase in viscosity due to excessive adhesion of the polyamine compound can be suppressed. In addition, in the technology disclosed herein, the polyamine compound and aliphatic monoamine having the above-described configuration are not added to the electrode paste as dispersants, but are instead pre-attached to the surface of the Ni particles as surface modifiers. This allows each surface modifier to efficiently adhere to the Ni particles and exert a steric hindrance effect, significantly improving the dispersibility of the nickel powder. As described above, the nickel powder disclosed herein can exhibit excellent dispersibility during electrode paste preparation, even when the average particle size of the Ni particles is 150 nm or less.

In one embodiment of the nickel powder disclosed herein, the aliphatic monoamine includes at least one selected from the group consisting of oleylamine, tetradecylamine, dodecylamine, and octylamine. This more effectively suppresses an increase in paste viscosity due to the adhesion of polyamine compounds.

In one embodiment of the nickel powder disclosed herein, the polyamine compound is a chain amine compound having an amino group and a carboxy group in the main chain. This allows for better dispersibility when preparing the electrode paste. One example of such a chain amine compound is at least one selected from the group consisting of polyamide, polyaminoamide, and polyesteramide.

In one embodiment of the nickel powder disclosed herein, the weight ratio of the surface modifier to the Ni particles is 0.5% by mass or more and 7.0% by mass or less. This allows for a higher level of both improved dispersibility and viscosity suppression effects.

In one embodiment of the nickel powder disclosed herein, the weight ratio of polyamine compound to aliphatic monoamine is 10% by mass or more and 120% by mass or less. This allows for a higher level of both improved dispersibility and viscosity suppression effects.

FIG. 1 is a flow diagram illustrating an example of a method for producing nickel particles according to an 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. Nickel Powder The nickel powder according to this embodiment is a powder material (a group of fine particles) primarily composed of Ni particles, as described below. Here, "primarily composed of Ni particles" means that the inorganic particles contained in the powder material in the greatest amount by weight are the "Ni particles having a surface modifier attached thereto," as described below. More specifically, the "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 having a surface modifier attached thereto. In other words, the nickel powder disclosed herein may contain other inorganic particles as long as the effects of the technology disclosed herein are not significantly impaired. Examples of such minor components include Ni particles not having a surface modifier attached thereto and inorganic particles primarily composed of other metal elements (such as Cu particles, Au particles, Ag particles, Pd particles, and Pt particles).

As mentioned above, the Ni particles, which are the main component of the nickel powder according to this embodiment, have a surface modifier attached to their particle surfaces. Below, we will explain each of the Ni particles and the surface modifier.

(1) Ni Particles Nickel particles (Ni particles) are the main component of electrodes after paste 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, "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 using core-shell particles as Ni particles, a shell containing Ni is formed on the surface of a core particle containing a metal element other than Ni. This allows Ni to be the primary element of the particle surface layer. 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 the production of Ni particles (core-shell particles). Therefore, this type of core-shell particle can be produced in large quantities while maintaining a small particle size. This significantly improves Ni particle 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 Ni shell need only cover at least a portion of the surface of the core particle; it is not necessary to completely cover the entire surface of the core particle. For example, if the average coverage rate 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.

Here, the nickel powder according to this embodiment is primarily composed of Ni particles having an average particle diameter of 150 nm or less. Using nickel powder containing such fine Ni particles allows for a thinner electrode film thickness after firing, contributing to the miniaturization of electronic components. On the other hand, because the surface area of the fine Ni particles is increased, they are more likely to aggregate when added to an electrode paste. This can result in the generation of coarse secondary particles, which may protrude from the surface of the thin electrode, reducing surface smoothness. However, the technology disclosed herein can improve dispersibility during electrode paste preparation, thereby preventing aggregation even when using Ni particles with an average particle diameter of 150 nm or less. The average particle diameter of the Ni particles is preferably 120 nm or less, more preferably 100 nm or less, and particularly preferably 80 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.

(2) Surface Modifier Next, in the nickel powder according to this embodiment, a surface modifier is attached to the surface of the Ni particles. Here, the surface modifier in this embodiment contains an aliphatic monoamine and a polyamine compound. This makes it possible to realize a nickel powder that exhibits good dispersibility while suppressing an increase in viscosity during preparation of an electrode paste.

The weight ratio of the surface modifier to the Ni particles is preferably 0.5% by mass or more (more preferably 1.0% by mass or more, even more preferably 1.5% by mass or more, and particularly preferably 2.0% by mass or more). This allows the dispersibility-improving effect of the surface modifier, described below, to be optimally exerted. On the other hand, the upper limit of the weight ratio of the surface modifier to the Ni particles is preferably 7.0% by mass or less (more preferably 6.5% by mass or less, even more preferably 6.0% by mass or less, and particularly preferably 5.5% by mass or less). This prevents an increase in paste viscosity due to excessive adhesion of the surface modifier.

(a) Aliphatic Monoamine The surface modifier in this embodiment contains an aliphatic monoamine. This aliphatic monoamine has a functional group (such as an amino group) with an amine value, and therefore adheres firmly to the Ni particles by coordinate bonding. The aliphatic monoamine in this embodiment is a low-molecular-weight compound having 8 to 18 carbon atoms. When this aliphatic monoamine occupies a certain amount or more of the surface of the Ni particles, an increase in viscosity due to excessive adhesion of the polyamine compound can be suppressed.

The aliphatic monoamine in this embodiment is sufficient as long as it has an amine value (amino group) for forming a coordinate bond with the Ni particles and has a carbon number small enough to suppress viscosity increase. In other words, the aliphatic monoamine in this embodiment can be appropriately selected from conventionally known aliphatic monoamines that meet the above requirements. For example, the aliphatic monoamine may contain either a primary amine or a tertiary amine. In either case, it can be suitably attached to the Ni particle surface. Furthermore, the alkyl group in the aliphatic monoamine may be a chain alkyl group, a saturated hydrocarbon, or an unsaturated hydrocarbon. The chain alkyl group here may have a linear or branched structure. As long as the carbon number is within the range of 8 to 18, a sufficient viscosity suppression effect can be achieved regardless of the alkyl group structure. Specific examples of aliphatic monoamines include primary amines such as n-octylamine, 2-ethylhexylamine, n-decylamine, n-dodecylamine, n-tetradecylamine, n-hexadecylamine, stearylamine, and oleylamine, and tertiary amines such as octanenitrile, 2-ethylhexanenitrile, decanenitrile, dodecanenitrile, tetradecanenitrile, hexadecanenitrile, stearonitrile, and oleylnitrile.

The amine value of the aliphatic monoamine is preferably 100 mgKOH/g or more, more preferably 150 mgKOH/g or more, and particularly preferably 200 mgKOH/g or more. For example, by using an aliphatic monoamine with a higher amine value than the polyamine compound described below, a more favorable viscosity suppression effect can be achieved. On the other hand, the upper limit of the amine value of the aliphatic monoamine is not particularly limited, and may be 600 mgKOH/g or less, 500 mgKOH/g or less, or 450 mgKOH/g or less. It is also preferable that the aliphatic monoamine is a compound without an acid value. This allows for a more favorable viscosity suppression effect to be achieved.

When the weight of the Ni particles is taken as 100%, the amount of aliphatic monoamine attached is preferably 0.5% or more, more preferably 1.0% or more, even more preferably 1.5% or more, and particularly preferably 2.0% or more. The viscosity suppression effect tends to improve as the amount of aliphatic monoamine attached to the Ni particles increases. On the other hand, the amount of aliphatic monoamine attached to the weight of the Ni particles is preferably 7.0% or less, more preferably 6.5% or less, even more preferably 6.0% or less, and particularly preferably 5.5% or less. As the amount of aliphatic monoamine attached to the Ni particles decreases, the amount of amine-based compound (described below) attached increases, which tends to enhance the dispersibility-improving effect.

(b) Polyamine Compound The surface modifier in this embodiment includes a polyamine compound having an acid value of 5 mg KOH/g or more and 30 mg KOH/g or less and an amine value of 10 mg KOH/g or more and 100 mg KOH/g or less. Because such a polyamine compound has a sufficient number of functional groups (such as amino groups) with an amine value, it adheres firmly to the acid sites of the Ni particles through coordinate bonds. Furthermore, because this polyamine compound also has a sufficient number of functional groups (such as carboxyl groups and phosphate groups) with an acid value, it also adheres to the basic sites on the surface of the Ni particles and is less likely to detach. Therefore, the polyamine compound having the above-described configuration adheres well to the surface of the Ni particles, thereby improving dispersibility during electrode paste preparation.

The acid value of the polyamine compound is preferably 7 mgKOH/g or more, more preferably 10 mgKOH/g or more, and particularly preferably 12 mgKOH/g or more. As the acid value of the polyamine compound increases, dispersibility during electrode paste preparation tends to improve. On the other hand, the acid value of the polyamine compound is preferably 28 mgKOH/g or less, more preferably 25 mgKOH/g or less, and particularly preferably 20 mgKOH/g or less. If the acid value of the polyamine compound becomes too high, the polyamine compound will adhere excessively to the basic sites on the surface of the Ni particles, making it difficult for the anionic dispersant (e.g., a carboxylic acid dispersant) added to the electrode paste to adhere to the Ni particles. In this case, the dispersibility-enhancing effect of the anionic dispersant may not be properly exerted.

On the other hand, the amine value of the polyamine compound is preferably 12 mgKOH/g or more, more preferably 14 mgKOH/g or more, and particularly preferably 15 mgKOH/g or more. As the amine value increases, the particle adhesion of the polyamine compound tends to improve. On the other hand, the amine value of the polyamine compound is preferably 90 mgKOH/g or less, more preferably 80 mgKOH/g or less, and particularly preferably 70 mgKOH/g or less. If the amine value of the polyamine compound is too high, the polyamine compound may adhere across multiple Ni particles, making them more likely to aggregate.

The specific structure of the polyamine compound in this embodiment is not particularly limited. This is because particle adhesion and dispersibility are optimally achieved as long as the amine value and acid value are within the above-mentioned ranges. In other words, the polyamine compound in this embodiment can be appropriately selected from conventionally known polyamine compounds that satisfy the above-mentioned requirements. One example of this type of polyamine compound is a chain amine compound having an amino group and a carboxy group in the main chain. Examples of such chain amine compounds include polyamide, polyaminoamide, polyesteramide, and derivatives thereof. The chain amine compound may also have a branched chain structure in which a side chain is bonded to the main chain. This side chain may be a chain alkyl group, a saturated hydrocarbon, or an unsaturated hydrocarbon. The side chain may also be an alkyl group containing an ester bond (e.g., a polyester obtained by dehydration condensation of 12-hydroxystearic acid). An example of a polyamide having such a branched chain structure is polyalkyleneimine. The polyamine compound may also be a block copolymer in which multiple monomers having an amine value and an acid value are copolymerized.

When the weight of the Ni particles is taken as 100%, the amount of polyamine compound attached is preferably 0.5% or more, more preferably 1.0% or more, even more preferably 1.5% or more, and particularly preferably 2.0% or more. As the amount of polyamine compound attached to the Ni particles increases, the dispersibility improvement effect tends to be further improved. On the other hand, polyamine compounds having an acid value and an amine value tend to easily adhere to Ni particles, and if the amount attached to the Ni particles becomes too high, the paste viscosity may increase. For this reason, in this embodiment, a portion of the surface of the Ni particles is occupied by an aliphatic monoamine, thereby suppressing an increase in viscosity due to excessive attachment of the polyamine compound. From this perspective, in the nickel powder according to this embodiment, the amount of polyamine compound attached to the Ni particles can be 7.0% or less (preferably 6.5% or less, more preferably 6.0% or less, and particularly preferably 5.5% or less) relative to the weight of the Ni particles.

The weight ratio of polyamine compound to aliphatic monoamine is preferably 10% or more, more preferably 15% or more, even more preferably 20% or more, and particularly preferably 25% or more. This allows the dispersibility-enhancing effect of the polyamine compound to be more suitably exerted. On the other hand, the weight ratio of polyamine compound to aliphatic monoamine is preferably 120% or less, more preferably 115% or less, even more preferably 110% or less, and particularly preferably 105% or less. This prevents viscosity increases due to excessive adhesion of the polyamine compound.

(4) Summary As described above, in this embodiment, a polyamine compound having a predetermined acid value and amine value is adhered to Ni particles. This polyamine compound has excellent adhesion to the Ni particle surface and can form suitable steric hindrance during electrode paste preparation. Therefore, Ni particles with this polyamine compound attached can exhibit excellent dispersibility during electrode paste preparation. On the other hand, if an excessive amount of the polyamine compound described above adheres to the Ni particles, the paste viscosity may increase during electrode paste preparation. Therefore, in this embodiment, an aliphatic monoamine with a small carbon number is attached to the Ni particle surface. By occupying a portion of the Ni particle surface with this aliphatic monoamine, the viscosity increase due to excessive adhesion of the polyamine compound can be suppressed. As described above, the nickel powder according to this embodiment can exhibit suitable dispersibility while effectively suppressing viscosity increase during electrode paste preparation.

In addition, in this embodiment, the polyamine compound and aliphatic monoamine having the above-described configuration are not added to the electrode paste as dispersants, but are instead attached to the surface of the Ni particles as surface modifiers. This allows the surface modifier to efficiently adhere to the Ni particles, creating a steric hindrance effect, thereby more suitably improving dispersibility during electrode paste preparation.

2. Method for Producing Nickel Particles Next, an example of a method for producing the nickel powder having the above-described configuration will be described. Figure 1 is a flow diagram showing an example of a method for producing nickel particles according to this embodiment. The production method shown in Figure 1 includes a preparation step S10, a surface modification step S20, and a separation step S30. 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 the above-mentioned 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.

When generating Ni particles by thermal decomposition, nickel salts are 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 nickel complexes. Heating these nickel complexes produces Ni particles. When generating core-shell particles with Ni shells 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 Ni shells on the surfaces of core particles containing the desired metal element. 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.

The organic solvent used to prepare the Ni slurry is not particularly limited, as long as it can disperse Ni particles and is compatible with the surface modifier. Examples of such organic solvents include low-polarity solvents such as alcohols, amine compounds, N,N-dimethylformamide, dimethyl sulfoxide, and acetone. When generating Ni particles using a thermal decomposition method, it is preferable to use an organic solvent that forms a nickel complex when the nickel salt mentioned above 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.

Furthermore, it is preferable that the Ni slurry prepared in this process 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, Ni particles with a surface modifier attached can be efficiently produced. From this perspective, when the weight (g) of the organic solvent is taken as 100%, the Ni particle content is preferably 50% or more, more preferably 60% or more, and particularly preferably 70% 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, and the surface modifier may adhere unevenly. From this perspective, the upper limit of the Ni particle content is preferably 95% or less, more preferably 90% or less.

(2) Surface modification step S20
In this process, a surface modifier is added to the Ni slurry and then heated. This allows the surface modifier containing an aliphatic monoamine and a polyamine compound to adhere to the surface of the Ni particles. The surface modifier used in this process has already been described, so a redundant description will be omitted. In this process, it is preferable to add the surface modifier at an appropriate weight ratio relative to the weight of Ni in the Ni slurry.

For example, the amount of aliphatic monoamine added relative to the weight of Ni in the Ni slurry (100%) is preferably 5% or more (more preferably 10% or more, even more preferably 15% or more, and particularly preferably 20% or more). This allows sufficient aliphatic monoamine to adhere to the surface of the Ni particles, thereby suppressing an increase in viscosity due to excessive adhesion of polyamine compound. On the other hand, the amount of aliphatic monoamine added relative to the weight of Ni is preferably 180% or less (more preferably 170% or less, even more preferably 150% or less, and particularly preferably 100% or less). This increases the amount of polyamine compound adhered, which tends to enhance the dispersibility-improving effect.

On the other hand, the amount of polyamine compound added relative to the weight of Ni in the Ni slurry (100%) is preferably 5% or more (more preferably 10% or more, even more preferably 15% or more, and particularly preferably 20% or more). This allows sufficient polyamine compound to adhere to the surface of the Ni particles, thereby favorably improving dispersibility during electrode paste preparation. On the other hand, the amount of polyamine compound added relative to the weight of Ni is preferably 65% or less (more preferably 60% or less, even more preferably 55% or less, and particularly preferably 50% or less). This makes it easier to suppress an increase in viscosity during electrode paste preparation.

The heating temperature in this process is preferably 50°C or higher, more preferably 70°C or higher, and particularly preferably 90°C or higher. This facilitates the adhesion reaction between the surface modifier and the Ni particles. On the other hand, the heating temperature in this process is preferably 200°C or lower, more preferably 180°C or lower, and particularly preferably 150°C or lower. This prevents the organic solvent from thermally decomposing and producing nickel carbide. Furthermore, the heating time is preferably 5 minutes or longer, more preferably 10 minutes or longer, and particularly preferably 15 minutes or longer. This allows the surface modifier to adhere sufficiently to the Ni particles. On the other hand, the heating time is preferably 180 minutes or shorter, more preferably 150 minutes or shorter, and particularly preferably 120 minutes or shorter. This prevents the organic solvent from decomposing.

(3) Separation step S30
In this step, the Ni particles to which the surface modifier is attached are separated from the Ni slurry. This allows the nickel powder according to this embodiment to be obtained. In this step, any conventionally known separation means used to separate powder from a slurry can be used without any particular restrictions. Examples of such separation means include static separation, centrifugation, and filtration. This separation step S30 is not an essential step in producing the nickel powder according to this embodiment. For example, depending on the type of organic solvent used in the Ni slurry, the Ni slurry after the surface modification step S20 can be used as an electrode paste as is.

(4) Other Steps It is also preferable to wash the Ni particles separated from the Ni slurry with a predetermined cleaning medium. For example, when washing Ni particles, a cleaning process can be performed in which the separated Ni particles are dispersed in a liquid cleaning medium and then the Ni particles are separated again from the cleaning medium. This allows the surface modifier that has not adhered to the Ni particles to be removed. As a result, it is possible to prevent an increase in viscosity due to the inclusion of excess surface modifier during electrode paste preparation. It is also preferable to repeat this cleaning process multiple times (e.g., 2 to 5 times). This ensures that excess surface modifier is removed.

The cleaning medium is not particularly limited, as long as it does not significantly impair the dispersibility-enhancing effect of the surface modifier on the Ni particle surface. It can be appropriately selected from conventionally known organic solvents capable of dispersing Ni particles. Examples of such cleaning media include acetate solvents such as isobornyl acetate, isobornyl propionate, isobornyl butyrate, isobornyl isobutyrate, dihydroterpinyl acetate, ethylene glycol monobutyl ether acetate, dipropylene glycol methyl ether acetate, 3-methoxy-3-methylbutyl acetate, and 1-methoxypropyl-2-acetate. Depending on the type of cleaning medium, a dispersion of Ni particles dispersed in the cleaning medium can also be used as an electrode paste.

Furthermore, it is preferable to perform a drying process on the Ni particles after the separation step S30 (or after the cleaning process). This allows for the production of dry nickel powder. The heating temperature during the drying process is preferably set in the range of 50°C to 120°C (preferably 90°C to 110°C). This allows the organic solvent (or cleaning medium) to be efficiently removed without denaturing (or burning off) the surface modifier attached to the Ni particles.

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). In this case, the nickel powder according to this embodiment has a polyamine compound having a predetermined acid value and amine value attached to the surface of the Ni particles, allowing the Ni particles to be easily dispersed in the dispersion medium. Furthermore, in the prepared electrode paste, the generation of coarse particles due to aggregation of Ni particles is suppressed, so that even when a thin electrode is formed, the surface smoothness of the electrode can be maintained at a high level. Furthermore, in the nickel powder according to this embodiment, the aliphatic monoamine suppresses excessive adhesion of the polyamine compound to the surface of the Ni particles. Therefore, when the electrode paste is prepared, a sudden increase in viscosity of the paste can be prevented.

Any dispersant can be used as long as it can disperse nickel powder well, and any conventional dispersant that can be used in electrode pastes can be used without particular restrictions. Examples of such dispersants 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, and terpineol.

In the manufacture of MLCCs, an electrode paste may be applied to a sheet material (BT sheet) made of a mixture of barium titanate (BaTiO ₃ ) and resin. Using a low-polarity solvent as the dispersion medium for the electrode paste can suppress dissolution of the resin in the BT sheet (sheet attack). The nickel powder according to this embodiment exhibits particularly favorable dispersibility in low-polarity solvents because a polyamine compound having an acid value adheres to the Ni particles. In other words, the nickel powder according to this embodiment is particularly suitable for use in electrode pastes that require the use of low-polarity solvents. Here, a low-polarity solvent refers to a solvent with an HLB of 5 or less. Examples of such low-polarity solvents include hexane, benzene, toluene, terpineol, dihydroterpineol, and dihydroterpinyl acetate.

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 50% (e.g., 35% to 45%). The viscosity of the electrode paste is preferably approximately 0.1 Pa·s to 10 Pa·s (e.g., approximately 0.2 Pa·s to 5 Pa·s). The nickel powder according to this embodiment easily achieves such low-viscosity electrode paste because the aliphatic monoamine prevents excessive adhesion of polyamine compounds. This makes it possible to apply the electrode paste at a thickness of 3 μm or less, contributing to thinner electrodes. 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. These additives may be any conventional additives that can be added to electrode pastes, and are not particularly limited, as long as they do not significantly impair the effects of the technology disclosed herein.

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. Preparation of Nickel Powder In this test example, 10 types of nickel powder (Examples 1 to 10) with different surface modifiers attached were prepared. The manufacturing procedures for each example are described below.

In this test, a Ni slurry containing Ni particles (Cu/Ni core-shell particles) dispersed in an organic solvent (oleylamine) was prepared according to the following procedure. Specifically, 5.1 g of nickel formate dihydrate (Ni content: 1.62 g) was added to 957.7 g of oleylamine. The mixture was then heated at 120°C 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 content: 0.19 g) was added to the solution. This mixture was then heated at 60°C 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, Cu, which has a higher standard potential, preferentially precipitated, resulting in the formation of Cu core particles. As the heat treatment continues, a Ni shell is formed on the surface of the Cu core particle. This results in the formation of Cu/Ni core-shell particles. Furthermore, in this test, the formed Cu/Ni core-shell particles were used as seed particles, and the Ni shells on the surface 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 was performed at 135°C for 120 minutes. This resulted in the formation of 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 resulted in the precipitation of Ni so as to adhere to the surface of the seed particles. As a result, a Ni slurry was prepared in which Cu/Ni core-shell particles having a sufficiently thick Ni shell were dispersed in oleylamine. Measurements based on FE-SEM observations revealed that the average particle size of the Ni particles (Cu/Ni core-shell particles) in the Ni slurry was 50 nm. The CV value was 0.15.

Next, in this test, a surface modification process was carried out to attach a surface modifier to the Ni particles in the Ni slurry. Specifically, the Ni particles were first allowed to settle in the Ni slurry, and a portion of the supernatant was removed. This adjusted the Ni particle content in the Ni slurry to 80%. Next, a predetermined amount of surface modifier was added to the Ni slurry (80 g) after re-dispersion. A heat treatment was then performed at 100°C for 15 minutes to attach the surface modifier to the Ni particles. Six types of surface modifiers were prepared for this test, as shown in Table 1 below. As shown in Table 2, the amount and type of surface modifier added varied in each of Examples 1 to 9. Note that the amount of surface modifier added in Table 2 is a weight ratio when the weight of the Ni particles in the Ni slurry (64 g = 80 g x 80%) is considered to be 100 wt%.

Next, in this test, Ni particles were allowed to settle in the Ni slurry by static settling, and then all of the supernatant liquid was removed. Next, a cleaning medium (isobornyl acetate) was added to the settled Ni particles and ultrasonically stirred. This cleaning process of separating the Ni particles and dispersing them in the cleaning medium was repeated three times. A drying process was then performed at 120°C to obtain a nickel dry powder primarily composed of Ni particles (Cu/Ni core-shell particles) with a surface modifier attached.

2. Evaluation Test In this test, electrode pastes were prepared using the nickel dry powders of Examples 1 to 9. The dispersibility of Ni particles in each paste was then evaluated. Specifically, an electrode paste was prepared by mixing 45 wt% nickel powder, 4.5 wt% barium titanate powder, 2 wt% ethyl cellulose resin, 2.0 wt% carboxylic acid dispersant, and 46.5 wt% isobornyl acetate. This electrode paste was prepared using a three-roll dispersion process. The prepared electrode paste was then applied to a glass slide. In this test, a thin film coating was attempted, with the paste thickness reaching 2 μm.

Next, the paste on the glass slide was dried (120°C, 5 minutes). The surface of the dried film was then observed under an optical microscope (200x magnification) to check for the presence or absence of agglomerated particles. In this evaluation, if secondary particles with a particle diameter exceeding 0.4 μm were confirmed during microscopic observation, it was determined that "agglomerated particles were generated." Dispersibility was rated as "Good" when the number of agglomerated particles generated per field of view was 10 or less, and "Poor" when the number was more than 10. The evaluation results are shown in Table 2.

First, the nickel powders in Examples 1 to 9 all successfully suppressed an increase in paste viscosity to the extent that they could be applied to a thickness of 2 μm or less. Next, as shown in Table 1, it was confirmed that the Ni particles were well dispersed in the electrode paste in Examples 1 to 7. On the other hand, in Examples 8 and 9, as in Examples 1 to 7, no improvement in dispersibility was confirmed despite the addition of two types of surface modifiers. The reason for this is that the surface modifier D used in Example 8 was a polyamine compound with no acid value. This suggests that improving dispersibility during electrode paste preparation requires that the surface modifier (polyamine compound) have a certain acid value or higher. Furthermore, in Example 9, the acid value of surface modifier E was 50 mg KOH/g. This suggests that polyamine compounds with too high an acid value have a strong repulsive force against the surface of Ni particles, making them less likely to adhere to Ni particles even if they have an amine value.

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 6 below.

<Item 1>
A nickel powder mainly composed of Ni particles,
the Ni particles have an average particle size of 150 nm or less based on FE-SEM observation, and a surface modifier is attached to the particle surface;
The surface modifier is
an aliphatic monoamine having 8 to 18 carbon atoms;
and a polyamine compound having an acid value of 5 mgKOH/g or more and 30 mgKOH/g or less and an amine value of 10 mgKOH/g or more and 100 mgKOH/g or less.

<Item 2>
2. The nickel powder according to item 1, wherein the aliphatic monoamine includes at least one selected from the group consisting of oleylamine and octylamine.

<Item 3>
3. The nickel powder according to item 1 or 2, wherein the polyamine compound is a chain amine compound having an amino group and a carboxy group in the main chain.

<Item 4>
4. The nickel powder according to item 3, wherein the chain amine compound includes at least one selected from the group consisting of polyamides, polyaminoamides, and polyesteramides.

<Item 5>
5. The nickel powder according to any one of items 1 to 4, wherein the weight ratio of the surface modifier to the Ni particles is 0.5% by mass or more and 7.0% by mass or less.

<Item 6>
6. The nickel powder according to any one of items 1 to 5, wherein a weight ratio of the polyamine compound to the aliphatic monoamine is 10% by mass or more and 120% by mass or less.

Claims (6)

A nickel powder mainly composed of Ni particles,
the Ni particles have an average particle size of 150 nm or less based on FE-SEM observation, and a surface modifier is attached to the particle surface;
The surface modifier is
an aliphatic monoamine having 8 to 18 carbon atoms;
a polyamine compound having an acid value of 5 mgKOH/g or more and 30 mgKOH/g or less and an amine value of 10 mgKOH/g or more and 100 mgKOH/g or less ,
The nickel powder , 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 . The nickel powder according to claim 1, wherein the aliphatic monoamine includes at least one selected from the group consisting of oleylamine, octylamine, and hexylamine. The nickel powder according to claim 1, wherein the polyamine compound is a chain amine compound having an amino group and a carboxy group in the main chain. The nickel powder according to claim 3, wherein the chain amine compound includes at least one selected from the group consisting of polyamides, polyaminoamides, and polyesteramides. The nickel powder according to claim 1, wherein the weight ratio of the surface modifier to the Ni particles is 0.5% by mass or more and 7.0% by mass or less. The nickel powder according to claim 1, wherein the weight ratio of the polyamine compound to the aliphatic monoamine is 10% by mass or more and 120% by mass or less.