Metal powder and metal powder-containing slurry

Abstract

This invention provides metal powders and metal powder-containing slurries that have excellent dispersibility, crushability, and sedimentation suppression properties, and can be applied to aqueous slurries or pastes with minimal use of organic components, resulting in a low environmental impact. [Solution] The metal powder consists of composite metal particles in which boundaries are observed within the particles by scanning electron microscopy, and single metal particles in which boundaries are not observed within the particles by scanning electron microscopy, with the composite metal particles accounting for 20% to 90% of the total number of particles constituting the metal powder.

Description

[Technical Field]

[0001] This disclosure relates to materials for forming electrodes and wiring in electronic devices, particularly metal powders used as materials for forming internal electrodes in multilayer ceramic capacitors, and slurries containing such metal powders. [Background technology]

[0002] Slurries and pastes containing metal powders or ceramic powders have traditionally been used as materials for forming electrodes and wiring in many electronic devices such as semiconductor devices, solar cells, multilayer ceramic capacitors (MLCCs), and displays, particularly for forming the internal electrodes of multilayer ceramic capacitors. Electrodes and wiring in electronic devices are formed by pattern printing these slurries and pastes using methods such as screen printing, followed by firing.

[0003] Methods using slurries and pastes have advantages over vacuum process methods such as PVD (sputtering, vacuum deposition, etc.) and CVD, which require expensive vacuum equipment, including lower costs, the ability to handle non-planar shapes, and the ability to process large areas. However, because these slurries and pastes contain many organic components, methods using slurries and pastes inherently have environmental impact issues.

[0004] Specifically, for example, multilayer ceramic capacitors have a structure in which internal electrode layers and dielectric layers are alternately stacked, but currently they are obtained by the following manufacturing process. That is, a green sheet which will become the dielectric layer is formed from a ceramic dielectric slurry containing barium titanate powder as the ceramic powder, an internal electrode paste containing nickel powder as the metal powder is printed onto the obtained green sheet by a method such as screen printing, the green sheets printed with the internal electrode paste are alternately stacked and laminated, and the resulting laminate is heat-treated to form a laminated structure consisting of a sintered film of ceramic powder and metal powder.

[0005] Ceramic dielectric slurries and pastes for internal electrodes contain not only particulate components such as barium titanate powder and nickel powder, which are constituent materials of the dielectric layer and internal electrodes, but also large amounts of organic components such as resin binders, organic solvents, and dispersants, which are necessary to maintain a smooth and dense film structure from the printing process to the heat treatment process.

[0006] These organic components must ultimately be removed from the film structure, and current electronic device manufacturing processes incorporate steps to thermally decompose these organic components. Therefore, current electronic device manufacturing processes, including those for multilayer ceramic capacitors, require not only the cost of organic components contained in the slurry or paste, but also a significant amount of thermal energy for removing these organic components in addition to the thermal energy required for powder sintering. This presents a major challenge in reducing environmental impact in a society where carbon neutrality is increasingly demanded.

[0007] From the perspective of reducing environmental impact, the application of aqueous slurries and pastes containing metal powders and ceramic powders, with the use of organic components minimized, is being considered. In particular, there is a desire to apply aqueous slurries directly to substrates without adding binders to form pastes.

[0008] However, the application of aqueous slurries and pastes presents challenges, particularly in the case of non-precious metal powders such as nickel powder. These challenges include degradation of particle components due to oxidation and leaching, and coarsening of particle components due to increased aggregation. Therefore, their realization is not easy. In particular, particles that have coarsened due to degradation of particle components become firmly bound together, making them extremely difficult to break down. Consequently, the application of aqueous slurries and pastes requires practical characteristics such as the ability to easily break down aggregated particles during the process and the ability to suppress the sedimentation of particles dispersed by the crushing. In particular, when aqueous slurries are directly applied to substrates, high sedimentation suppression is required.

[0009] While it is possible to suppress the degradation and aggregation of particle components by applying a protective film to the particle surface with a large amount of organic components beforehand, this not only reduces the particle properties due to the replacement of constituent elements on the particle surface, but also necessitates a process to remove the large amount of organic components.

[0010] Investigations are also underway regarding aqueous slurries and pastes that can easily break down aggregated particles during the process. For example, Japanese Patent Publication No. 2006-028320 discloses an aqueous electrode ink in which metal powder such as nickel powder is contained in an amount of 25% to 40% by weight relative to the total weight of the aqueous electrode ink, at least two water-soluble resins with different average molecular weights selected from water-soluble resins are contained in an amount of 3 to 6 parts by weight per 100 parts by weight of metal powder, an aqueous plasticizer is contained in an amount of 1.5 to 1.9 times the weight of the water-soluble resins, and the remainder is water, wherein the average molecular weight of one water-soluble resin is 5,000 to 10,000 and the average molecular weight of the other water-soluble resin is 150,000 to 360,000.

[0011] Japanese Patent Publication No. 2006-210301 discloses a conductive ink comprising nickel powder having an average primary particle size of 100 nm or less dispersed in a dispersion medium mainly composed of water, alcohols, glycols, or saturated hydrocarbons having a boiling point of 300°C or less at atmospheric pressure, wherein the dispersion medium contains one or more metal salts or metal oxides selected from those containing Ti, V, Ni, Cu, Zn, Y, Nb, Mo, Ag, In, Sn, Ta, and W as film density improving agents for improving the film density of a conductor formed using the conductive ink.

[0012] Japanese Patent Publication No. 2002-317201 discloses an aqueous nickel slurry comprising water, nickel fine powder in which an insoluble inorganic oxide, which is an oxide or complex oxide containing silicon, aluminum, or zirconium, is fixed to the surface of individual nickel fine particles, polyacrylic acid, its ester or a salt thereof, and organic group-substituted ammonium hydroxide.

[0013] Japanese Patent Publication No. 2003-147414 discloses a method for obtaining a metal powder slurry that does not re-aggregate even after being left for a long time, by adding dried metal powder or metal powder before drying to water to a certain metal powder concentration, and grinding the surface of the metal powder with a wet grinder.

[0014] On the other hand, with the thinning of the internal electrode layers of multilayer ceramic capacitors, it has also been proposed to construct metal powders in the form of flattened flake-shaped particles by physical or chemical means. Japanese Patent Publication No. 2004-084055 discloses nickel flakes obtained by flaking a mixture consisting of a grinding medium, nickel powder, and an organic solvent using a grinding apparatus, having an average particle size of 0.5 to 10 μm, an average thickness of 0.03 to 0.5 μm, and an aspect ratio of 10 to 100, as well as an electrode-forming ink using these nickel flakes. [Prior art documents] [Patent Documents]

[0015] [Patent Document 1] Japanese Patent Publication No. 2006-028320 [Patent Document 2] Japanese Patent Publication No. 2006-210301 [Patent Document 3] Japanese Patent Publication No. 2002-317201 [Patent Document 4] Japanese Patent Publication No. 2003-147414 [Patent Document 5] Japanese Patent Publication No. 2004-084055 [Overview of the project] [Problems that the invention aims to solve]

[0016] In JP-A-2006-028320 and JP-A-2006-210301, although particles such as nickel powder can be sufficiently dispersed in an aqueous slurry, a large amount of resin, organic salts, inorganic salts, etc. must be used as a dispersion aid, which makes it difficult to reduce the energy for separating the particles and the dispersion aid.

[0017] On the other hand, in JP-A-2002-317201, an insoluble inorganic oxide is fixed to the particle surface of nickel powder instead of an organic component. However, since the particle surface of nickel powder is covered with an insoluble inorganic oxide, there is a problem that the particle characteristics and uses are greatly limited unless it is decomposed and removed by heating or the like.

[0018] In JP-A-2003-147414, an aqueous slurry that does not form hard aggregates that are difficult to redisperse during long-term storage can be obtained without using a dispersion aid, but it is said that metal powder settles in the aqueous slurry, and there is a problem that sufficient sedimentation suppression performance cannot be obtained.

[0019] In JP-A-2004-084055, there is a problem that the thickness of nickel flakes varies and it is extremely difficult to obtain a very thin internal electrode in a reproducible state. Further, when nickel flakes are fine powder, they are said to aggregate significantly, and there are problems in application to aqueous slurries and pastes.

[0020] Even when the present disclosure is put into an aqueous solvent to form an aqueous slurry, without applying a protective film to the particle surface with a large amount of organic components and without causing the problem that the particle characteristics and uses are limited, without using an insoluble non-electrolyte fixed to the particle surface, particles aggregated during the process can be easily crushed, and sedimentation of the particles dispersed by the crushing can be suppressed, and an object is to provide a metal powder with a low environmental load and an aqueous slurry containing the metal powder.

Means for Solving the Problems

[0021] A metal powder according to one aspect of this disclosure is It consists of composite metal particles in which boundaries are observed within the particles by scanning electron microscopy, and single metal particles in which boundaries are not observed within the particles by scanning electron microscopy. The composite metal particles make up 20% to 90% of the total number of particles constituting the metal powder. It is characterized by the following:

[0022] In one embodiment of the present disclosure, the number-average particle diameter of the particles constituting the metal powder in terms of the equivalent circular diameter can be 10 nm or more and 550 nm or less.

[0023] A metal powder according to one aspect of the present disclosure may include at least one powder selected from nickel, copper, gold, silver, platinum, palladium, or alloys thereof.

[0024] A metal powder-containing slurry according to one aspect of the present disclosure comprises a metal powder and a solvent containing a quaternary ammonium hydroxide and water, wherein the absolute value of the zeta potential is 2 mV or more, and the metal powder used is the metal powder according to one aspect of the present disclosure.

[0025] In a metal powder-containing slurry according to one aspect of the present disclosure, it is preferable that the content of the metal powder relative to the total amount of the metal powder-containing slurry is 1% by mass or more and 25% by mass or less.

[0026] In a metal powder-containing slurry according to one aspect of this disclosure, it is preferable that the quaternary ammonium hydroxide has 16 or fewer carbon atoms.

[0027] In this case, the quaternary ammonium hydroxide is preferably at least one selected from tetramethylammonium hydroxide, tetraethylammonium hydroxide, tetrapropylammonium hydroxide, and tetrabutylammonium hydroxide.

[0028] In a metal powder-containing slurry according to one aspect of this disclosure, it is preferable that the pH is 8 or higher based on a liquid temperature of 25°C.

[0029] In a metal powder-containing slurry according to one aspect of the present disclosure, the solvent may include an organic solvent with a relative permittivity of 20 or more.

[0030] In this case, the organic solvent is preferably at least one selected from methanol, ethanol, and 1-propanol.

[0031] A metal powder-containing slurry according to one aspect of the present disclosure may contain a polymer electrolyte, and the absolute value of the zeta potential may be 15 mV.

[0032] In this case, the polymer electrolyte is preferably at least one selected from polydiallyldimethylammonium chloride, polyallylamine quaternary ammonium, sodium polystyrene sulfonate, polyethyleneimine, sodium polyacrylate, and ammonium polyacrylate. [Effects of the Invention]

[0033] One embodiment of the present disclosure, the metal powder and metal powder-containing slurry, possesses excellent dispersibility, crushability, and sedimentation suppression, can be applied to aqueous slurries or pastes that do not contain organic components, and has a low environmental impact. Therefore, it can be said that it makes a very high contribution to the manufacturing field of electronic devices such as multilayer ceramic capacitors, in line with a carbon-neutral society. [Brief explanation of the drawing]

[0034] [Figure 1] Figure 1 shows scanning electron microscope (SEM) images of the metal powder (nickel powder) obtained in Example 1 (magnification: 50,000x). Figure 1(a) shows the entire metal powder (nickel powder), Figure 1(b) shows the composite metal particles, and Figure 1(c) shows the single metal particles. [Figure 2]Figure 2 shows scanning electron microscope (SEM) images of the metal powder (nickel powder) obtained in Example 6 (magnification: 50,000x). Figure 2(a) shows the entire metal powder (nickel powder), Figure 2(b) shows the composite metal particles, and Figure 2(c) shows the single metal particles. [Figure 3] Figure 3 shows the classification of composite metal particles among the metal powder (nickel powder) obtained in Example 6. [Figure 4] Figure 4 shows a scanning electron microscope (SEM) image of the metal powder (nickel powder) obtained in Comparative Example 1 (magnification: 40,000x). [Modes for carrying out the invention]

[0035] The inventors of this disclosure have diligently studied means to realize a metal powder-containing slurry that has excellent dispersibility, crushability, and sedimentation suppression, even when the metal powder is added to an aqueous solvent to form an aqueous slurry, without using insoluble non-electrolytes that adhere to the particle surface. They have found that by incorporating composite metal particles having specific particle properties into the metal powder, it is possible to suppress the sedimentation of the metal powder in an aqueous metal powder-containing slurry containing quaternary ammonium hydroxide and with water as the main solvent component, thereby obtaining a metal powder-containing slurry that is excellent in all aspects of dispersibility, crushability, and sedimentation suppression, and have completed this disclosure.

[0036] The following describes in detail a metal powder and a metal powder-containing slurry, which are examples of one aspect of this disclosure.

[0037] (1) Metal powder The metal powder in this example consists of composite metal particles in which boundaries are observed within the particles by scanning electron microscopy and single metal particles in which boundaries are not observed within the particles by scanning electron microscopy, and is characterized in that the composite metal particles make up 20% to 90% of the total number of particles constituting the metal powder.

[0038] Composite metal particles are particles formed when two or more ordinary, non-aggregated metal particles (primary particles), i.e., single metal particles that do not show boundaries within the particle when observed with a scanning electron microscope, are bonded together to form a single particle. Therefore, in composite metal particles, boundaries are observed within the particle as traces of the particles before bonding when observed with a scanning electron microscope. Composite metal particles are single particles and are different from aggregated particles (secondary particles) that are formed by the aggregation of multiple metal particles.

[0039] Composite metal particles are thought to be formed when some of the single metal particles combine due to collisions caused by the movement of individual metal particles, resulting from stirring during a crushing process using a mixing device, under conditions that cause some of the raw material particles to deform. Therefore, the boundaries within the composite metal particles are thought to represent traces of the individual metal particles (raw material particles) that remain after two or more single metal particles combine through collision to form a single particle.

[0040] The composite metal particles may be flake-shaped (flat) composite metal particles formed by the two-dimensional bonding of multiple single metal particles, or amorphous granular composite metal particles formed by the three-dimensional bonding of multiple single metal particles.

[0041] The composite metal particles make up 20% to 90% of the total particles constituting the metal powder, based on their number. The content of these composite metal particles relative to the total particles is preferably 30% to 85%, more preferably 40% to 80%, and even more preferably 50% to 80%.

[0042] If the proportion of composite metal particles relative to the total number of particles is less than 20%, when a metal powder-containing slurry is formed using the metal powder, the settling rate of the particles constituting the metal powder in the slurry may increase, impairing the shelf life of the metal slurry. On the other hand, if the proportion of composite metal particles in the metal powder exceeds 90% by number, it may become difficult to achieve a uniform film thickness in the metal powder-containing slurry or in the metal powder film coated with a metal paste processed from the metal powder-containing slurry.

[0043] The percentage of composite metal particles relative to the total number of particles is calculated by distinguishing between composite metal powder and single metal powder based on the presence or absence of boundaries within the particles, as captured using a scanning electron microscope, and then counting the number of each type of metal particle.

[0044] When a metal powder-containing slurry is formed using the metal powder in this example, the composite metal particles are present in the solvent in an appropriate number ratio, and the floating of the composite metal particles in the solvent suppresses the settling of the metal powder in the slurry. Therefore, in the metal powder-containing slurry using the metal powder in this example, the metal powder as a whole exhibits excellent dispersibility and settling suppression, and thus its disintegration properties are also excellent.

[0045] On the other hand, single metal particles can take on any shape, and may have the shape of the raw material particles applied to the crushing process using a mixing device, or the shape of the raw material particles after being deformed during the crushing process. For example, single metal particles may include particles that have been deformed into a flake shape during the crushing process using a mixing device, even if they were originally spherical raw material particles. In other words, single metal particles can include both particles that are the original raw material particles and particles that have been deformed into a flake shape.

[0046] In this example, the number-average particle diameter at the equivalent circular diameter of the particles constituting the metal powder is preferably 10 nm or more and 550 nm or less. More preferably, the number-average particle diameter at the equivalent circular diameter of the particles is 20 nm or more and 250 nm or less, and even more preferably 20 nm or more and 150 nm or less.

[0047] If the number-average particle diameter of the particles constituting the metal powder exceeds 550 nm in equivalent circular diameter, problems may arise such as difficulty in achieving a uniform film thickness in a metal powder film formed by coating with a slurry using the metal powder. Furthermore, if the number-average particle diameter of the particles exceeds 550 nm, the mass of the metal particles increases, which increases the settling velocity of the metal particles in the metal slurry, potentially impairing the long-term stability of the metal slurry.

[0048] The lower limit of the number-average particle diameter at the equivalent circular diameter of the particles constituting the metal powder depends on the availability of the raw material powder. Furthermore, if the number-average particle diameter at the equivalent circular diameter of the particles falls below 10 nm, problems may arise in handling the metal powder.

[0049] The number-average particle diameter of the composite metal particles constituting the metal powder, in terms of the equivalent circular diameter, is preferably 30 nm to 550 nm, and more preferably 30 nm to 300 nm.

[0050] Furthermore, the number-average particle diameter of a single metal particle in the equivalent circular diameter is preferably 10 nm to 250 nm, and more preferably 10 nm to 100 nm.

[0051] If the particle size of the composite metal particles and the particle size of the single metal particles are within the above range, it is possible to achieve the number-average particle size of the metal powder in this example.

[0052] The number-average particle diameter in terms of the equivalent circle diameter is obtained by using a scanning electron microscope (SEM) to analyze images taken at a magnification that allows a total of 100 or more particles to be observed within the same field of view. This analysis is performed to select single particles (particles corresponding to primary particles) that constitute the observable metal powder. The number-average value of the equivalent circle diameter (the diameter of a circle with equal area) of these particles is then calculated from their contours. Note that single particles include not only single metal particles but also composite metal particles.

[0053] In this example, the number-average particle diameter at the equivalent circular diameter of the particles constituting the metal powder can also be obtained from the number-average particle diameters calculated for both the composite metal particles and the single metal particles. In this case, the distinction between composite metal powder and single metal powder is determined by the presence or absence of boundaries within the particles, as observed using a scanning electron microscope.

[0054] In this example, the metal powder has a content of composite metal particles, based on the number of particles, of 20% to 90%. Therefore, when the slurry is prepared by adding the metal powder to a solution containing quaternary ammonium hydroxide and water, with an absolute zeta potential of 2 mV or more, so that the concentration of the metal powder relative to the entire slurry is 1.0% to 20% by mass, and the slurry is left to stand at room temperature (for example, 25°C) for 1 hour, the particles constituting the metal powder settle, and the slurry can be maintained without forming a clear separation interface. A separation interface refers to the interface between the particle settling layer and the supernatant layer that appears when the flow of the slurry is stopped and it is left to stand. On the other hand, in conventional metal slurries using water-containing solvents other than the metal slurry in this example, the metal powder particles settle in less than 1 hour when left to stand at room temperature, and a separation interface appears.

[0055] The type of metal powder is not particularly limited; for example, at least one selected from nickel, copper, gold, silver, platinum, palladium, or alloys thereof can be used as the metal powder. In particular, when manufacturing multilayer ceramic capacitors using the metal powder in this example, it is preferable to use nickel or copper, which are relatively inexpensive.

[0056] Furthermore, with the increasing miniaturization of electronic devices such as multilayer ceramic capacitors, there is a demand for thinner ceramic dielectric layers and internal electrode layers. The metal powder-containing slurry in this example is particularly suitable as a material for forming internal electrodes because the thickness of the metal powder film formed by coating it is uniform.

[0057] Furthermore, beyond multilayer ceramic capacitors, the metal powder-containing slurry in this example, or the metal paste obtained by processing the slurry, can also be used as a wiring material by applying it to ceramic substrates such as alumina or resin substrates, or by forming a pattern in the shape of a circuit during application and then applying or printing it, followed by heat treatment that causes the particles constituting the metal powder to neck together.

[0058] The metal powder in this example is not particularly limited and can be manufactured by any method.

[0059] For example, the metal powder in this example is produced by adjusting the crushing conditions in a crushing process using a mixing device such as a paint shaker, a rotational and orbital pulverizer, a bead mill, or a ball mill, which causes some of the raw material particles constituting the raw material powder to deform into flakes, or by causing the raw material particles and the deformed flakes to combine to form composite metal particles. The raw material powder may be in the form of spherical particles or flakes.

[0060] For example, although not limited to this, in crushing processes using a mixing device, by crushing the raw material powder under conditions that result in over-crushing, which is usually avoided because some of the particles in the raw material powder become too fine, it becomes possible to obtain a metal powder in which composite metal particles make up 20% to 90% of the total particle count.

[0061] When obtaining metal powder by crushing using a mixing device, more specifically, a raw material powder having a number average particle diameter of 10 nm or more and 550 nm or less in the equivalent circle diameter is first suspended in water or an organic solvent so that the raw material powder content is 1.0% by mass or more and 20% by mass or less to obtain a suspension.

[0062] If the amount of raw material powder in the suspension falls outside the above range, the contact between the particles constituting the raw material powder and the crushing media may be insufficient during the crushing process, or the convection of the suspension in the processing container may be insufficient. As a result, the crushing of the raw material powder and the formation of flake-like particles or composite metal particles may not occur sufficiently.

[0063] Next, the suspension and the crushed media are filled into a processing container, and the raw material powder in the suspension is crushed for a predetermined time.

[0064] As the crushing media, balls or beads with a particle size (average particle size) of 0.05 mm or more and 0.5 mm or less, preferably 0.05 mm or more and 0.3 mm or less, can be used. If the particle size of the crushing media falls outside the above range, during the crushing process, contact between the particles constituting the raw material powder and the crushing media may be insufficient, or the convection of the suspension in the processing container may be insufficient, which may result in insufficient crushing of the raw material powder and insufficient formation of flake-like particles or composite metal particles.

[0065] Through this crushing process of the raw material powder in the suspension, the metal powder in the suspension will contain composite metal particles at a concentration of 20% to 90% based on the number of particles.

[0066] When the metal powder in this example is added to an aqueous solvent to form a slurry, the suspension obtained by suspending the raw material powder in water using a bead mill or ball mill can be subjected to a crushing treatment and then used as is as a slurry. In this case, the timing of adding quaternary ammonium hydroxide, organic solvent, polymer electrolyte, etc., can be either before or after the crushing treatment. Furthermore, if the content of metal powder relative to the total amount of metal powder-containing slurry after the crushing treatment falls outside the desired value, the aforementioned solvent can be added additionally.

[0067] To obtain metal powder from a suspension after crushing, the suspension is heated to remove the solvent, thereby obtaining the metal powder.

[0068] The raw material powder is not particularly limited, and any raw material powder can be used. For example, it may be either an experimentally synthesized product or a commercially available product.

[0069] For example, when nickel powder is used as the raw material, it can be produced by known granulation methods using reactions in the liquid phase or gas phase. Examples of granulation methods using reactions in the gas phase include the CVD method, the evaporative quenching method, and the hydrogen reduction method using nickel salts or nickel hydroxide. Examples of granulation methods using reactions in the liquid phase include the electroless reduction method, in which nickel powder is deposited in a nickel salt solution using a reducing agent such as hydrazine, and the electrolytic reduction method, in which nickel powder is deposited by placing electrodes in an aqueous nickel salt solution and passing an electric current between the electrodes.

[0070] In this case, the particle size of the nickel powder used as the raw material can be appropriately controlled by known methods depending on the granulation method.

[0071] (2) Slurry containing metal powder A metal powder-containing slurry according to an example of one aspect of the present disclosure comprises a metal powder, a quaternary ammonium hydroxide, and a solvent containing water, wherein the absolute value of the zeta potential is 2 mV or more, and the metal powder used is the metal powder according to an example of one aspect of the present disclosure.

[0072] The solvent that makes up the metal powder-containing slurry in this example is mainly water. By using water as the main component of the solvent, it is possible to achieve a low environmental impact.

[0073] (2-1) Metal powder content The amount of metal powder in the metal powder-containing slurry can be any amount depending on the application of the metal powder-containing slurry. For example, the amount of metal powder can be within a concentration range that can be crushed by a crusher, although this is not limited to the above. More specifically, the amount of metal powder is preferably 1.0% by mass or more and 25% by mass or less, more preferably 1.0% by mass or more and 15% by mass or less, and even more preferably 1.0% by mass or more and 12% by mass or less, based on the total amount of the metal powder-containing slurry.

[0074] (2-2) Quaternary ammonium hydroxide The metal powder-containing slurry in this example contains a quaternary ammonium hydroxide. Quaternary ammonium hydroxide has the general formula: [R 1 , R 2 , R 3 , R 4 N + ][OH - It has the composition represented by [ ]. In the general formula, R represents an alkyl group.

[0075] Quaternary ammonium hydroxides almost completely dissociate in aqueous solutions, simultaneously generating hydroxyl ions and ammonium ions. The hydroxyl ions raise the pH of the aqueous solution to the alkaline side, i.e., pH 7 or higher, which suppresses the progression of aggregation due to metal leaching in an acidic atmosphere. Furthermore, the presence of charged quaternary ammonium ions between the particles constituting the metal powder reduces the aggregation tendency of the metal powder, making its disintegration process easier.

[0076] To effectively achieve these effects, it is preferable that the alkyl group has a high degree of dissociation in aqueous solution, and that its length is such that the action of the positive charge of the nitrogen atom is not inhibited by steric hindrance.

[0077] From this perspective, the number of carbon atoms in the quaternary ammonium hydroxide is preferably 16 or less. When the number of carbon atoms is in the range of 4 or more and 16 or less, due to the synergistic effect between the formation of an alkaline environment in the aqueous solution and the interparticle mediation of the charged quaternary ammonium ions, the cohesiveness of the particles in the metal powder-containing slurry is significantly reduced. Therefore, even if the addition amount of the quaternary ammonium hydroxide to the total amount of the solvent is extremely small, it is possible to provide an easily disintegratable and aqueous metal powder-containing slurry.

[0078] General formula representing quaternary ammonium hydroxide: [R 1 、R 2 、R 3 、R 4 N + [OH - In this formula, R 1 、R 2 、R 3 、R 4 are each preferably an alkyl group having 1 to 4 carbon atoms, and are preferably composed of the same or different alkyl groups from each other.

[0079] Examples of the quaternary ammonium hydroxide having such a structure include tetramethylammonium hydroxide, tetraethylammonium hydroxide, tetrapropylammonium hydroxide, tetrabutylammonium hydroxide, trimethylethylammonium hydroxide, trimethylpropylammonium hydroxide, dimethyldipropylammonium hydroxide, monomethyldiethylammonium hydroxide, monomethyltripropylammonium hydroxide, and the like. These quaternary ammonium hydroxides may be used alone or in combination of two or more.

[0080] For reasons unknown, among these, those in which the charge is easily distributed isotropically when ionized in a solvent are preferred. More specifically, the quaternary ammonium hydroxide is preferably at least one selected from tetramethylammonium hydroxide, tetraethylammonium hydroxide, tetrapropylammonium hydroxide, and tetrabutylammonium hydroxide.

[0081] In this example, the amount of quaternary ammonium hydroxide in the metal powder-containing slurry is restricted to an amount that can raise the pH to 8 or higher at a liquid temperature of 25°C.

[0082] In other words, when producing the metal powder-containing slurry in this example, a quaternary ammonium hydroxide is added to a solvent containing water until the pH reaches 8 or higher.

[0083] Specifically, although it varies depending on factors such as the amount of carbon dioxide dissolved in the solvent from the atmosphere, the concentration of quaternary ammonium hydroxide in metal powder-containing slurries is approximately several tens of millimol / L.

[0084] (2-3) pH In this example, the metal powder-containing slurry has a pH of 7 or higher at a liquid temperature of 25°C. It is preferable to add a quaternary ammonium hydroxide to the solvent to raise the pH of the metal powder-containing slurry to 8 or higher at a liquid temperature of 25°C.

[0085] If the pH is below 8, the metal powder will oxidize and dissolve due to acidification by dissolved carbon dioxide in the water and reaction with dissolved oxygen, and its aggregation will progress more easily. The aggregated and coarser particles, accompanied by the degradation of the particulate components, will bind together firmly, making it difficult to break them up. In addition, the settling of the metal powder in the metal powder-containing slurry cannot be sufficiently suppressed. However, depending on the application and storage period of the metal powder-containing slurry, it is possible to set the pH of the metal powder-containing slurry to 8 or below at a liquid temperature of 25°C.

[0086] While there is no particular upper limit to the pH of a metal powder-containing slurry at a liquid temperature of 25°C, a pH above 13 is undesirable because it may increase the measurement error (alkalinity error) when using a pH meter with a typical glass electrode.

[0087] (2-4) Solvent The solvent used in the metal powder-containing slurry in this example is an aqueous solution of water mixed with a quaternary ammonium hydroxide. From the viewpoint of ensuring sufficient dissociation of the quaternary ammonium hydroxide, the solvent must be a polar solvent with a remarkably high dielectric constant, such as water.

[0088] In the metal powder-containing slurry of this example, the solvent may include an organic solvent with a relative permittivity of 20 or higher. That is, from the viewpoint of not excessively inhibiting the ionization of electrolytes in aqueous solution, a mixed solution containing an organic solvent with lower polarity than water can be used as the solvent. However, in order for the solvent to retain the properties of water, it is preferable to use an organic solvent whose volume ratio to the total solvent is less than 50% and which has a relative permittivity of 20 or higher.

[0089] Examples of organic solvents with a relative permittivity of 20 or higher include methanol, ethanol, and 1-propanol. These organic solvents may be used individually or in combination of two or more.

[0090] (2-5) Polymer electrolyte The metal powder-containing slurry in this example may contain a polymer electrolyte. By adding a polymer electrolyte that is highly dissociable in aqueous solution and carries a positive or negative charge, the agglomeration relaxation effect due to interparticle mediation can be enhanced together with the charged quaternary ammonium ions, thereby further improving the ease of disintegration of the metal powder-containing slurry.

[0091] Examples of polymer electrolytes include polydiallyldimethylammonium chloride, polyallylamine quaternary ammonium, sodium polystyrene sulfonate, polyethyleneimine, sodium polyacrylate, and ammonium polyacrylate. These polymer electrolytes may be used individually or in combination of two or more types.

[0092] The amount of polymer electrolyte added is preferably in the range of 0.01% by mass or more and 10% by mass or less relative to the metal powder, and more preferably 0.1% by mass or more and 5% by mass or less.

[0093] Furthermore, inorganic bases such as sodium hydroxide may promote aggregation by increasing the ionic strength of the aqueous solution simply by making the pH alkaline; therefore, it is preferable that the metal powder-containing slurry in this example does not contain inorganic bases.

[0094] (2-6) Absolute value of the zeta potential In this example, the zeta potential of the metal powder-containing slurry is 2mV or higher. The zeta potential is the potential of the slip surface where liquid flow begins to occur in the electric double layer formed around the particles in an aqueous solution. When the zeta potential approaches zero, the repulsive force between particles weakens, causing them to aggregate. In other words, if the interparticle mediation effect of quaternary ammonium hydroxide is normal, the absolute value of the zeta potential of the metal powder-containing slurry will be 2mV or higher. If it is less than 2mV, there is a possibility that the interparticle mediation effect of quaternary ammonium hydroxide is not being performed normally.

[0095] In this example, if the metal powder-containing slurry also contains a polymer electrolyte, the interparticle interaction of the polymer electrolyte contributes to making it possible to achieve an absolute value of 15 mV or higher for the zeta potential.

[0096] The zeta potential can be measured using any known method, such as electrophoresis, electroacoustics, or colloidal vibrational current spectroscopy.

[0097] (2-7) Decomposition Before using the metal powder-containing slurry in this example, known methods can be used to re-crush the metal powder. Examples of crushing methods include ultrasonic homogenizers, ball mills, bead mills, automatic mortars, and thin-film swirling high-speed mixers.

[0098] (2-8) Applications of metal powder-containing slurry The metal powder-containing slurry in this example can be used in the same way as slurry and paste materials for electronic materials.

[0099] In the production of a conductive paste using the metal powder-containing slurry in this example, the metal powder in the metal powder-containing slurry is re-crushed by any method, and then a binder such as cellulose resin (ethylcellulose, nitrocellulose, etc.), acrylic resin, or phenolic resin is added to form a paste.

[0100] The metal powder-containing slurry of this example and the metal powder-containing paste manufactured using this slurry can be used, for example, as internal electrode materials for multilayer ceramic capacitors, electrode materials for electronic devices including multilayer ceramic capacitors, wiring formation materials, printed circuit boards, internal wiring for semiconductors, and bonding between printed circuit boards and electronic components.

[0101] In these applications, known methods can be used to apply the metal powder-containing slurry of this example and the metal powder-containing paste produced using this slurry to various substrates. Such application methods include, for example, gravure printing, screen printing, spray coating, spin coating, dip coating, bar coating, knife coating, offset printing, flexographic printing, inkjet printing, and dispenser printing. Of these, dip coating or spin coating is preferred from the viewpoint of efficiently forming a coating film. [Examples]

[0102] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0103] First, the evaluation methods used in this embodiment and comparative example will be described.

[0104] (1) Measurement of the average particle size of composite metal particles and single metal particles in metal powder, and the content of composite metal particles. The average particle sizes of composite metal particles and single metal particles were determined by image analysis using a scanning electron microscope (SEM (JEOL Ltd., model: JSM-7200F)) at a magnification that allowed a total of 100 or more particles to be observed within the same field of view. Composite metal particles and single metal particles were selected, their equivalent circle diameters (diameters of circles with equal area) were measured, and their number-average values ​​were calculated. The percentage of composite metal particles was calculated as: number of composite metal particles in the observed image / (number of single metal particles + number of composite metal particles) × 100.

[0105] (3) Method for measuring zeta potential The "zeta potential" was measured using an electroacoustic zeta potential meter (Colloidal Dynamics LLC, model: ZetaProbe) while stirring the slurry at 250 rpm.

[0106] (4) Evaluation of the effect of suppressing sedimentation A metal powder-containing slurry was dispersed using a thin-film swirling high-speed mixer (Primix Corporation, model: Filmix 30-L) at a rotation speed of 22,000 rpm for 5 minutes. Then, the median value of the sedimentation velocity (μm / sec) was measured using a centrifugal sedimentation type dispersion-stabilizing particle size distribution analyzer (LUM Corporation, model: LUMiSizer 611) at a rotation speed of 300 rpm. The "median value" is the sedimentation velocity value that gives 50% of the integrated distribution.

[0107] In all of the examples, water and quaternary ammonium hydroxide were used as solvents for the slurry used in the preparation of the metal powder (primary crushing treatment to obtain the metal powder) and the preparation of the metal powder-containing slurry (secondary crushing treatment to obtain the metal powder-containing slurry).

[0108] [Example 1] (1) Preparation of raw material powder (raw material nickel powder) 405 g of nickel chloride hexahydrate (NiCl2·6H2O) as a nickel source, 1.271 g of L-methionine (CH3SC2H4CH(NH2)COOH) and 1.024 g of ethylenediamine (H2NC2H4NH2) as stabilizers, 40.0 mg of palladium(II) ammonium chloride (also known as tetrachloropalladium(II)ate) ((NH4)2PdCl4) as a nucleating agent, and 230 g of sodium hydroxide (NaOH) as a pH adjuster were added to 2570 g of distilled water and stirred to prepare a 3 L reaction aqueous solution. The pH of the prepared reaction aqueous solution was measured at a liquid temperature of 25°C using a pH meter (Horiba, Ltd., model: LAQUAact D-73), and the pH was found to be approximately 13.

[0109] Next, 207 g of commercially available industrial-grade 60% by mass hydrated hydrazine (manufactured by MGC Otsuka Chemical Co., Ltd.) was gradually added to the above reaction aqueous solution as a reducing agent. The reaction aqueous solution was then heated to a bath temperature of 85°C while stirring, and the electroless reduction reaction was continued for 60 minutes. As a result, nickel powder, a metal powder, precipitated in the solution.

[0110] The nickel powder precipitated in the reaction aqueous solution was washed by decantation with distilled water, and then dried under reduced pressure to obtain 100 g of nickel powder.

[0111] When the obtained nickel powder was observed using a scanning electron microscope (SEM), the particle shape was found to be approximately spherical.

[0112] (2) Preparation of metal powder (nickel powder) To form the slurry, tetraethylammonium hydroxide (TEAH) was added to water, which was used as a solvent for slurry formation. The pH was measured using a pH meter at a liquid temperature of 25°C until the pH reached 12. This aqueous solution was then mixed with the roughly spherical raw material powder obtained in the raw material nickel powder preparation step to obtain a 40% by mass slurry.

[0113] Next, zirconia beads with a particle size of 0.1 mm were packed into the vessel (capacity 0.2 L) of a bead mill (manufactured by Ashizawa Finetech Co., Ltd., model: HFM02) to 80% of its internal volume. Then, the slurry was introduced into the vessel, and the total amount of zirconia beads and metal powder-containing slurry was packed to 100% of the vessel's internal volume. Next, the bead mill was operated at a peripheral speed of 10 m / s for 240 minutes to continuously process the slurry at high speed, thereby performing a primary crushing treatment.

[0114] (3) Preparation of a slurry containing metal powder (nickel powder) After the initial crushing treatment, the beads were separated from the slurry by filtering the slurry in the tank. The slurry after bead removal was then diluted by adding an aqueous solution of water and tetraethylammonium hydroxide (TEAH) with a pH of 12 at a liquid temperature of 25°C, so that the nickel concentration was 5% by mass.

[0115] The diluted slurry was placed into a thin-film swirling high-speed mixer (Primix Corporation, model: Filmix 30-L) and subjected to a secondary crushing treatment at a rotation speed of 22,000 rpm for 5 minutes to prepare a slurry containing 5% by mass of nickel powder.

[0116] The amount of tetraethylammonium hydroxide (TEAH) added to the water was 17 mmol / L. The amount of slurry required for evaluation was repeatedly prepared using the procedure described above.

[0117] (3) Evaluation (a) Composition of metal powder (nickel powder) When nickel powder obtained by heating a nickel powder-containing slurry to remove the solvent was observed using a scanning electron microscope (SEM), the average particle size of the composite nickel particles was 91 nm, and the average particle size of the single nickel particles was 47 nm. The composite nickel particle content was 66%.

[0118] (b) Zeta potential measurement of a slurry containing metal powder (nickel powder) The zeta potential of the obtained nickel powder-containing slurry was analyzed using an electroacoustic zeta potential meter and found to be -2.0 mV.

[0119] (c) Evaluation of the effect of suppressing sedimentation When the obtained nickel powder-containing slurry was subjected to an evaluation test for its settling suppression effect, the settling velocity was found to be 340 μm / second.

[0120] Figures 1(a), 1(b), and 1(c) show scanning electron microscope images of the metal powder according to Example 1 (a), and images obtained by separating the image into composite metal particles (b) and single metal particles (c).

[0121] Table 1 shows the metal species, average particle size of composite metal particles, average particle size of single metal particles, and content of composite metal particles; Table 2 shows the concentration of metal powder, solvent, type of quaternary ammonium compound, and type of polymer electrolyte; and Table 3 shows the evaluation test results for the settling inhibition effect. The same applies to Examples 2-11 and Comparative Examples 1-5.

[0122] [Example 2] Nickel powder was prepared in the same manner as in Example 1, except that the amount of palladium(II) ammonium chloride added as a nucleating agent in the raw material nickel powder preparation process was 0.134 mg.

[0123] Nickel powder and nickel powder-containing slurry were prepared in the same manner as in Example 1, except that tetrabutylammonium hydroxide, which has 16 carbon atoms, was added as a quaternary ammonium hydroxide in the preparation process of the metal powder (nickel powder) and the metal powder (nickel powder)-containing slurry. The amount of tetrabutylammonium hydroxide added to water was 17 mmol / L.

[0124] Observation of the nickel powder in the obtained nickel powder-containing slurry revealed that the average particle size of composite nickel particles was 550 nm, and the average particle size of single nickel particles was 250 nm. Furthermore, the composite nickel particle content was 70%.

[0125] The zeta potential of the nickel powder-containing slurry was -2.7 mV.

[0126] When the obtained nickel powder-containing slurry was subjected to an evaluation test for its settling suppression effect, the settling velocity was found to be 395 μm / second.

[0127] [Example 3] Nickel powder was prepared in the same manner as in Example 1, except that the amount of palladium(II) ammonium chloride added as a nucleating agent in the preparation process of the raw material nickel powder was 5.3 mg.

[0128] In the preparation of the metal powder (nickel powder) and the metal powder (nickel powder)-containing slurry, the nickel powder and nickel powder-containing slurry were prepared in the same manner as in Example 1, except that the solvent used was obtained by mixing water and 1-propanol in a volume ratio of 9:1, and then adding tetrapropylammonium hydroxide, which has 12 carbon atoms, as a quaternary ammonium compound until the pH reached 8. The amount of tetrapropylammonium hydroxide added to the water was 0.1 mmol / L.

[0129] Observation of the nickel powder in the obtained nickel powder-containing slurry revealed that the average particle size of composite nickel particles was 143 nm, and the average particle size of single nickel particles was 95 nm. The composite nickel particle content was 60%.

[0130] The zeta potential of the nickel powder-containing slurry was -4.4 mV.

[0131] When the obtained nickel powder-containing slurry was subjected to an evaluation test for its settling suppression effect, the settling velocity was found to be 363 μm / second.

[0132] [Example 4] The raw nickel powder was prepared in the same manner as in Example 1, except that in the preparation process for the raw nickel powder, 20.2 g of nickel chloride hexahydrate was used as the nickel source, 5.084 g of L-methionine and 1.024 g of ethylenediamine were used as stabilizers, and 53.4 mg of palladium(II) ammonium chloride was used as a nucleating agent.

[0133] In the preparation of nickel powder and nickel powder-containing slurry, the nickel powder and nickel powder-containing slurry were prepared in the same manner as in Example 1, except that water and methanol were mixed as the solvent in a volume ratio of 6:4, and then tetramethylammonium hydroxide, a quaternary ammonium compound with 4 carbon atoms, was added until the pH reached 13, and the primary crushing treatment time in a beer mill was set to 360 minutes. The amount of tetramethylammonium hydroxide added to the water was 90 mmol / L.

[0134] Observation of the nickel powder in the obtained nickel powder-containing slurry revealed that the average particle size of composite nickel particles was 30 nm, and the average particle size of single nickel particles was 10 nm. Furthermore, the composite nickel particle content was 50%.

[0135] The zeta potential of the nickel powder-containing slurry was -5.5 mV.

[0136] When the obtained nickel slurry was subjected to an evaluation test for its settling suppression effect, the settling velocity was found to be 235 μm / second.

[0137] [Example 5] In the preparation of nickel powder and nickel powder-containing slurry, a nickel slurry was prepared in the same manner as in Example 1, except that 0.05 g of polydiallyldimethylammonium chloride (molecular weight 8500) was added to the aqueous solution as a polymer electrolyte, and the primary crushing treatment time using a beer mill was set to 360 minutes.

[0138] Observation of the nickel powder in the obtained nickel powder-containing slurry revealed that the average particle size of composite nickel particles was 192 nm, and the average particle size of single nickel particles was 55 nm. Furthermore, the composite nickel particle content was 90%.

[0139] The zeta potential of the nickel powder-containing slurry was +21.0 mV.

[0140] When the obtained nickel slurry was subjected to an evaluation test for its settling suppression effect, the settling velocity was found to be 150 μm / second.

[0141] [Example 6] Nickel powder and nickel powder-containing slurry were prepared in the same manner as in Example 1, except that 0.05 g of sodium polystyrene sulfonate (molecular weight 10,000) was added to the aqueous solution as a polymer electrolyte in the preparation step of the nickel powder and nickel powder-containing slurry.

[0142] Observation of the nickel powder in the obtained nickel powder-containing slurry revealed that the average particle size of composite nickel particles was 142 nm, and the average particle size of single nickel particles was 61 nm. Furthermore, the composite nickel particle content was 72%.

[0143] The zeta potential of the nickel powder-containing slurry was -30.0 mV.

[0144] When the obtained nickel slurry was subjected to an evaluation test for its settling suppression effect, the settling velocity was found to be 74 μm / second.

[0145] Figures 2(a), 2(b), and 2(c) show scanning electron microscope images (a) of the metal powder according to Example 6, and images obtained by separating the image into composite metal particles (b) and single metal particles (c). Figure 3 also illustrates the separation of composite metal particles in the metal powder according to Example 6.

[0146] [Example 7] In the preparation of nickel powder and nickel powder-containing slurry, nickel powder and nickel powder-containing slurry were prepared in the same manner as in Example 1, except that the solvent used was a mixture of water and ethanol in a volume ratio of 8:2, followed by the addition of tetraethylammonium hydroxide, and an aqueous solution to which 0.05 g of polyethyleneimine (molecular weight 1200) was added as a polymer electrolyte.

[0147] Observation of the nickel powder in the obtained nickel powder-containing slurry revealed that the average particle size of composite nickel particles was 122 nm, and the average particle size of single nickel particles was 61 nm. The composite nickel particle content was 58%.

[0148] The zeta potential of the nickel powder-containing slurry was +15.0 mV.

[0149] When the obtained nickel powder-containing slurry was subjected to an evaluation test for its settling suppression effect, the settling velocity was found to be 95 μm / second.

[0150] [Example 8] In the preparation of nickel powder and nickel powder-containing slurry, nickel powder and a slurry containing 5% by mass of nickel powder were prepared in the same manner as in Example 1, except that 0.05 g of sodium polyacrylate (molecular weight 3500) was added to the aqueous solution as a polymer electrolyte and the primary crushing treatment time in a beer mill was set to 360 minutes. This slurry was mixed with the nickel powder-containing slurry of Example 1 in a volume ratio of 1:3.5 to obtain the nickel powder-containing slurry of this example.

[0151] Observation of the nickel powder in the obtained nickel powder-containing slurry revealed that the average particle size of composite nickel particles was 301 nm, and the average particle size of single nickel particles was 60 nm. The composite nickel particle content was 20%.

[0152] The zeta potential of the nickel powder-containing slurry was -45.0 mV.

[0153] When the obtained nickel powder-containing slurry was subjected to an evaluation test for its settling suppression effect, the settling velocity was found to be 380 μm / second.

[0154] [Example 9] In the preparation process for nickel powder and nickel powder-containing slurry, nickel powder and a slurry containing 5% by mass of nickel powder were prepared in the same manner as in Example 1, except that 0.05 g of ammonium polyacrylate (molecular weight 6000) was added to the aqueous solution as a polymer electrolyte and the primary crushing treatment time in a beer mill was set to 360 minutes. This slurry was mixed with the nickel powder-containing slurry of Example 1 in a volume ratio of 1:2 to obtain the nickel powder-containing slurry of this example.

[0155] Observation of the nickel powder in the obtained nickel powder-containing slurry revealed that the average particle size of composite nickel particles was 241 nm, and the average particle size of single nickel particles was 60 nm. The composite nickel particle content was 30%.

[0156] The zeta potential of the nickel powder-containing slurry was -46.0 mV.

[0157] When the obtained nickel powder-containing slurry was subjected to an evaluation test for its settling suppression effect, the settling velocity was found to be 355 μm / second.

[0158] [Example 10] In the preparation of nickel powder and nickel powder-containing slurry, the nickel slurry was prepared in the same manner as in Example 1, except that an aqueous solution of water to which tetraethylammonium hydroxide was added until the pH reached 12 was used as the solvent, and after primary crushing treatment using a beer mill, a nickel powder-containing slurry containing 1% by mass of nickel powder was prepared. The amount of tetraethylammonium hydroxide added to the water was 17 mmol / L.

[0159] Observation of the nickel powder in the obtained nickel powder-containing slurry revealed that the average particle size of composite nickel particles was 93 nm, and the average particle size of single nickel particles was 50 nm. Furthermore, the composite nickel particle content was 60%.

[0160] The zeta potential of the nickel powder-containing slurry was -2.0 mV.

[0161] When the obtained nickel powder-containing slurry was subjected to an evaluation test for its settling suppression effect, the settling velocity was found to be 275 μm / second.

[0162] [Example 11] In the preparation of nickel powder and nickel powder-containing slurry, nickel powder and nickel powder-containing slurry were prepared in the same manner as in Example 1, except that an aqueous solution of water to which tetraethylammonium hydroxide was added until the pH reached 12 was used as the solvent, and after primary crushing treatment using a beer mill, a 20% by mass nickel powder-containing slurry was prepared. The amount of tetraethylammonium hydroxide added to the water was 17 mmol / L.

[0163] Observation of the nickel powder in the obtained nickel powder-containing slurry revealed that the average particle size of composite nickel particles was 101 nm, and the average particle size of single nickel particles was 52 nm. Furthermore, the composite nickel particle content was 65%.

[0164] The zeta potential of the nickel powder-containing slurry was -2.1 mV.

[0165] When the obtained nickel powder-containing slurry was subjected to an evaluation test for its settling suppression effect, the settling velocity was found to be 381 μm / second.

[0166] [Comparative Example 1] Nickel powder and nickel powder-containing slurry were prepared in the same manner as in Example 1, except that, in the preparation process of nickel powder and nickel powder-containing slurry, only water was used as the solvent without adding quaternary ammonium hydroxide, and primary crushing treatment using a beer mill was not performed.

[0167] Observation of the nickel powder in the obtained nickel powder-containing slurry revealed no composite nickel particles; only single nickel particles were observed. The average particle size of the single nickel particles was 60 nm.

[0168] The zeta potential of the nickel powder-containing slurry was +1.0 mV.

[0169] When the obtained nickel slurry was subjected to an evaluation test for its settling suppression effect, the settling velocity was found to be 562 μm / second.

[0170] Figure 4 shows an example of the metal powder (nickel powder) related to Comparative Example 1.

[0171] [Comparative Example 2] In the preparation process for nickel powder and nickel powder-containing slurry, a nickel slurry was prepared in the same manner as in Comparative Example 1, except that an aqueous solution of water with sodium hydroxide added to adjust the pH to 12 was used as the solvent.

[0172] Observation of the nickel powder in the obtained nickel powder-containing slurry revealed no composite nickel particles; only single nickel particles were observed. The average particle size of the single nickel particles was 60 nm.

[0173] The zeta potential of the nickel powder-containing slurry was -1.5 mV.

[0174] When the obtained nickel powder-containing slurry was subjected to an evaluation test for its settling suppression effect, the settling velocity was found to be 519 μm / second.

[0175] [Comparative Example 3] In the preparation process for nickel powder and nickel powder-containing slurry, an aqueous solution of trioctylmethylammonium chloride, a quaternary ammonium compound but not a quaternary ammonium hydroxide with 25 carbon atoms, was used as the solvent. This solution was prepared at a concentration of 17 mmol / L relative to water without pH adjustment, and primary crushing treatment using a beer mill was not performed. In addition, nickel powder and nickel powder-containing slurry were prepared in the same manner as in Example 2.

[0176] Observation of the nickel powder in the obtained nickel powder-containing slurry revealed no composite nickel particles; only single nickel particles were observed. The average particle size of the single nickel particles was 250 nm.

[0177] The zeta potential of the nickel powder-containing slurry was +1.8 mV.

[0178] When the obtained nickel powder-containing slurry was subjected to an evaluation test for its settling suppression effect, the settling velocity was found to be 630 μm / second.

[0179] [Comparative Example 4] In the preparation process for nickel powder and nickel powder-containing slurry, nickel powder and nickel powder-containing slurry were prepared in the same manner as in Example 3, except that an aqueous solution without quaternary ammonium hydroxide was used as the solvent and primary crushing treatment using a beer mill was not performed.

[0180] Observation of the nickel powder in the obtained nickel powder-containing slurry revealed no composite nickel particles; only single nickel particles were observed. The average particle size of the single nickel particles was 100 nm.

[0181] The zeta potential of the nickel powder-containing slurry was +1.2 mV.

[0182] When the obtained nickel powder-containing slurry was subjected to an evaluation test for its settling suppression effect, the settling velocity was found to be 502 μm / second.

[0183] [Comparative Example 5] In the preparation process for nickel powder and nickel powder-containing slurry, nickel powder and nickel powder-containing slurry were prepared in the same manner as in Example 4, except that an aqueous solution without quaternary ammonium hydroxide was used as the solvent and primary crushing treatment using a beer mill was not performed.

[0184] Observation of the nickel powder in the obtained nickel powder-containing slurry revealed no composite nickel particles; only single nickel particles were observed. The average particle size of the single nickel particles was 10 nm.

[0185] The zeta potential of the nickel powder-containing slurry was +0.7 mV.

[0186] When the obtained nickel slurry was subjected to an evaluation test for its settling suppression effect, the settling velocity was found to be 450 μm / second.

[0187] [Table 1]

[0188] [Table 2]

[0189] [Table 3]

[0190] [Discussion on the evaluation results] Tables 1 to 3 show that the metal powder and metal powder-containing slurry according to an example of one embodiment of this disclosure exhibit superior settling inhibition compared to the comparative example. This superior settling inhibition effect in the example, despite being an aqueous metal powder-containing slurry, is thought to be due to the relatively large buoyancy acting on the composite metal particles (bound particles) and the flake-like particles among the single metal particles. Furthermore, it is thought that the addition of quaternary ammonium hydroxide significantly reduces the aggregation of metal particles in the metal powder-containing slurry due to the synergistic effect of forming an alkaline environment in the aqueous solution and the interparticle interposition of charged quaternary ammonium ions, resulting in an easily crushable aqueous metal powder-containing slurry.

Claims

1. It consists of composite metal particles in which boundaries are observed within the particles by scanning electron microscopy, and single metal particles in which boundaries are not observed within the particles by scanning electron microscopy. A metal powder in which the composite metal particles constitute 20% to 90% of the total number of particles comprising the metal powder.

2. The metal powder according to claim 1, wherein the number-average particle diameter of the metal powder at the equivalent circular diameter is 10 nm or more and 550 nm or less.

3. The metal powder according to claim 1, comprising at least one powder selected from nickel, copper, gold, silver, platinum, palladium, or alloys thereof.

4. The solution comprises a metal powder, a quaternary ammonium hydroxide, and a solvent containing water. The absolute value of the zeta potential is 2 mV or more. The metal powder used is the metal powder described in any one of claims 1 to 3. Slurry containing metal powder.

5. The metal powder-containing slurry according to claim 4, wherein the content of the metal powder relative to the total amount of the metal powder-containing slurry is 1.0% by mass or more and 25% by mass or less.

6. The metal powder-containing slurry according to claim 4, wherein the quaternary ammonium hydroxide has 16 or fewer carbon atoms.

7. The metal powder-containing slurry according to claim 6, wherein the quaternary ammonium hydroxide is at least one selected from tetramethylammonium hydroxide, tetraethylammonium hydroxide, tetrapropylammonium hydroxide, or tetrabutylammonium hydroxide.

8. The metal powder-containing slurry according to claim 4, wherein the pH is 8 or higher based on a liquid temperature of 25°C.

9. The metal powder-containing slurry according to claim 4, wherein the solvent comprises an organic solvent with a relative dielectric constant of 20 or more.

10. The metal powder-containing slurry according to claim 9, wherein the organic solvent is at least one selected from methanol, ethanol, or 1-propanol.

11. The metal powder-containing slurry according to claim 4, comprising a polymer electrolyte, wherein the absolute value of the zeta potential is 15 mV or more.

12. The metal powder-containing slurry according to claim 11, wherein the polymer electrolyte is at least one selected from polydiallyldimethylammonium chloride, polyallylamine quaternary ammonium, sodium polystyrene sulfonate, polyethyleneimine, sodium polyacrylate, or ammonium polyacrylate.

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