Classification method and classification apparatus

Abstract

This invention provides a classification method that allows the use of an aqueous solvent as the solvent for the slurry used in the classification process, and enables the classification of fine particles from a target powder through a simple process. [Solution] The method comprises the steps of: preparing a slurry containing the target powder and a solvent; applying ultrasonic vibrations to the slurry to generate a mist; and recovering the fine particles of the target powder contained in the mist.

Description

[Technical Field]

[0001] This disclosure relates to a method and apparatus for classifying powders. [Background technology]

[0002] In recent years, powders with small average particle sizes (average particle size of several tens to several hundreds of nanometers) have been used in various fields such as electronics, pharmaceuticals, optics, and food. For example, in the field of electronics, conductive pastes containing conductive powders, resins, and organic solvents are used to form conductive layers, electrode layers, interlayer connectors, and electrode layers of electronic components that make up wiring boards for electrical circuits.

[0003] As these wiring boards and electronic components become smaller and denser, the width and thickness of conductive layers and electrode layers tend to decrease. Consequently, the conductive powders that make up the conductive pastes used in these components are also becoming smaller in diameter. Powders normally contain coarse particles exceeding a certain size (particle size), but as conductive powders become smaller in diameter, the adverse effects of the presence of coarse particles on the properties of conductive powders are becoming more serious. Therefore, conductive powders are required to contain no coarse particles, or only very few coarse particles.

[0004] Methods for removing coarse particles from conductive powders include granulation treatment and classification treatment. For example, Japanese Patent Publication No. 2005-342581 discloses a method for classifying metal ultrafine powders, in which at least one of the pH and electrolyte concentration of an aqueous slurry in which metal ultrafine powders are dispersed in water is adjusted, and then the slurry is passed through a glass or ceramic tube. The coarse particles are removed by adsorption to the tube wall using the attractive force between the particles and the tube wall, and the aqueous slurry from which the coarse particles have been removed is then recovered.

[0005] Japanese Patent Publication No. 2002-177743 discloses a method for removing coarse metal particles by circulating the metal particle slurry in the slurry chamber through a filter with a predetermined mesh size while circulating the metal particle slurry in the slurry chamber through the circulation line, which is provided in a circulation line that extracts metal particle slurry from a slurry chamber that functions as a filter pre-chamber and circulates it through a predetermined path back into the slurry chamber.

[0006] Japanese Patent Publication No. 2001-062332 discloses a method for classifying nickel powder into coarse and fine particles by dispersing nickel powder in water to form a slurry and supplying this slurry to a liquid cyclone.

[0007] Japanese Patent Publication No. 2011-156520 discloses a method for wet-grinding a metal particle slurry, which is prepared by dispersing metal fine powder in a solvent, using a wet-grinding apparatus, and then wet-classifying the ground metal particle slurry using a wet-classifier. In the method described in Japanese Patent Publication No. 2011-156520, the dispersion state of the metal fine powder is improved by using an aqueous solution of sodium hexametaphosphate as the solvent for dispersing the metal fine powder. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2005-342581 [Patent Document 2] Japanese Patent Publication No. 2002-177743 [Patent Document 3] Japanese Patent Publication No. 2001-062332 [Patent Document 4] Japanese Patent Publication No. 2011-156520 [Overview of the project] [Problems that the invention aims to solve]

[0009] In the method described in JP-A-2005-342581, when the tube diameter of glass or ceramics increases, the distance from the inner wall of the tube becomes longer, and it becomes difficult for coarse particles passing through the central part of the tube to adsorb to the inner wall of the tube. Even when the injection rate of the water slurry is increased, the adsorption efficiency when the coarse particles adsorb to the inner wall of the tube deteriorates. Therefore, it is difficult to efficiently classify a large amount of metal powder. If classification is performed on a small scale in order to prioritize classification performance, the working efficiency deteriorates and the manufacturing cost increases.

[0010] In the method described in JP-A-2002-177743, in order to remove metal particles having an average particle diameter of about several tens of nm to several hundreds of nm or more, it is necessary to use a filter having a mesh size of several tens of nm to several hundreds of nm or less. The filter is likely to be clogged, and when clogging occurs, the filtration rate rapidly decreases, resulting in a problem that productivity decreases.

[0011] <​​​​​​​​​​​​​

[0014] A classification method according to one aspect of the present disclosure includes: preparing a slurry containing a target powder and a solvent; applying ultrasonic vibration to the slurry to generate mist; recovering fine particles of the target powder encapsulated in the mist. The classification method according to one aspect of the present disclosure includes the above steps.

[0015] In the classification method according to one aspect of the present disclosure, the number average particle diameter in terms of the equivalent diameter of the projected area circle of the fine particles can be 1 nm or more and 500 nm or less, preferably 5 nm or more and 250 nm or less, and more preferably 10 nm or more and 200 nm or less.

[0016] In the classification method according to one aspect of the present disclosure, the concentration of the target powder with respect to the whole slurry is preferably 2% by mass or more and 12% by mass or less.

[0017] In the classification method according to one aspect of the present disclosure, the target powder preferably contains at least one powder selected from metal powder, ceramic powder, or a mixed powder thereof.

[0018] In this case, the solvent may contain quaternary ammonium hydroxide and water.

[0019] The quaternary ammonium hydroxide preferably has 16 or less carbon atoms.

[0020] The quaternary ammonium hydroxide is preferably at least one selected from tetramethylammonium hydroxide, tetraethylammonium hydroxide, tetrapropylammonium hydroxide, or tetrabutylammonium hydroxide.

[0021] The solvent may contain an organic solvent having a relative permittivity of 20 or more.

[0022] The organic solvent is preferably at least one selected from methanol, ethanol, or 1-propanol.

[0023] A classification apparatus in one aspect of this disclosure is: A mist generating device that generates mist by applying ultrasonic vibrations to a slurry containing the target powder and solvent, A mist recovery device for recovering the mist generated by the mist generator, It is equipped with. [Effects of the Invention]

[0024] A classification method and classification apparatus according to one aspect of this disclosure can use a common solvent such as water as the solvent for the slurry used in the classification process, and can classify fine particles from a target powder through a simple process. [Brief explanation of the drawing]

[0025] [Figure 1] Figure 1 is a schematic diagram showing an example of an embodiment of the classification apparatus of this disclosure. [Figure 2] Figure 2(a) is an SEM image (observation magnification 30,000x) of nickel powder obtained in Example 1 by heating the slurry before classification to remove the solvent, Figure 2(b) shows the result of extracting particle contours using image analysis software, and Figure 2(c) is a graph showing the particle size distribution. [Figure 3] Figure 3(a) is an SEM image (observation magnification 30,000x) of nickel powder obtained in Example 1 by heating the recovered slurry after classification to remove the solvent, Figure 3(b) shows the result of extracting particle contours using image analysis software, and Figure 3(c) is a graph showing the particle size distribution. [Modes for carrying out the invention]

[0026] This disclosure relates to a classification method and apparatus for classifying powders. The powders covered by this disclosure are those used in various fields such as electronics, pharmaceuticals, optics, and food.

[0027] In particular, the classification method and apparatus of this disclosure are suitably applicable to the classification of metal powders used as materials for forming electrodes and wiring in many electronic devices used in the field of electronics, such as semiconductor devices, solar cells, multilayer ceramic capacitors (MLCCs), and displays.

[0028] The present disclosure will be described in detail below, as an example of an embodiment, based on a classification method and classification apparatus for classifying metal powders. However, the present disclosure is not limited thereto and can be broadly applied to classification methods and classification apparatus for classifying ceramic powders or mixed powders of metal powders and ceramic powders for forming the ceramic dielectric layer of multilayer ceramic capacitors, or powders that constitute pharmaceuticals, optics, food, etc.

[0029] The classification method of this disclosure is characterized by comprising the steps of: preparing a slurry containing a target powder and a solvent; applying ultrasonic vibration to the slurry to generate a mist; and recovering the fine particles of the target powder contained in the mist. It is possible to crush the slurry in a crushing device before the step of misting the slurry. The classification method of this disclosure can be carried out with any device as long as it has the above characteristics. For example, the classification method of this disclosure can be carried out using a classification device 1 conceptually shown in Figure 1.

[0030] (1) Classification device The classification apparatus 1 is configured to obtain fine particles classified from the target powder by applying ultrasonic vibrations to a slurry S containing the target powder and solvent to generate mist M, and then recovering the generated mist M. The classification apparatus 1 comprises a mist generator 2 that generates mist M by applying ultrasonic vibrations to the slurry S, and a mist recovery device 3 that recovers the mist M generated by the mist generator 2.

[0031] The mist generator 2 comprises a container 4 and an ultrasonic transducer 5.

[0032] Container 4 contains a slurry S containing the target powder and solvent. Container 4 has a lid to prevent the mist M from diffusing into the atmosphere.

[0033] The volume of container 4 is not limited to this, but is 100 cm³. 3 More than 5000cm 3 It can be less than or equal to 500 cm, preferably 500 cm. 3 More than 3000cm 3 It can be less than or equal to 1000 cm², and more preferably 1000 cm². 3 More than 2000cm 3 It can be the following:

[0034] The volume of slurry S contained in container 4 is not limited to this, but can be 1% to 30% of the volume of container 4, preferably 3% to 20%, and more preferably 5% to 10%. The liquid depth of slurry S contained in container 4 is not limited to this, but can be 0.1 cm to 10 cm, preferably 0.3 cm to 5 cm, and more preferably 0.5 cm to 3 cm.

[0035] In this example, container 4 is placed inside a water tank 6 filled with water. Specifically, container 4 is placed on a raised platform 7 that is placed on the bottom of the water tank 6.

[0036] The ultrasonic vibrator 5 atomizes the slurry S. The ultrasonic vibrator 5 is configured to be able to atomize the slurry S accommodated in the container 4 based on the ultrasonic vibration applied to the slurry S. In this example, the ultrasonic vibrator 5 is placed on the bottom of the water tank 6 so as to face the lower surface of the container 4. The oscillation part of the ultrasonic vibrator 5 faces the lower surface of the container 4 through water. Therefore, when the ultrasonic vibrator 5 is vibrated, the vibration is propagated (applied) to the slurry S accommodated in the container 4 through water, and the slurry S is atomized.

[0037] The frequency and output of the ultrasonic waves generated by the ultrasonic vibrator 5 are appropriately adjusted according to the amount of the slurry S, the type of the solvent, etc., so that the size (particle diameter) of one particle of the mist and the generation amount of the mist can be adjusted to an appropriate range. For example, the frequency of the ultrasonic waves can be 0.5 MHz or more and 5 MHz or less, preferably 0.8 MHz or more and 3 MHz or less, and more preferably 1 MHz or more and 2 MHz or less. Also, the output (intensity) of the ultrasonic waves can be 20 W / m 2 or more and 600 W / m 2 or less, preferably 50 W / m 2 or more and 400 W / m 2 or less, and more preferably 100 W / m 2 or more and 300 W / m 2 or less.

[0038] The mist recovery device 3 includes a recovery container 8, a recovery pipe 9 that conveys the mist M in the container 4 to the recovery container 8, and a gas supply source 10 that supplies a carrier gas into the container 4 and the recovery pipe 9.

[0039] The mist M generated in the container 4 along with the vibration of the ultrasonic vibrator 5 is collected from one end of the recovery pipe 9 that opens into the container 4 by supplying a carrier gas from the gas supply source 10 into the container 4. The mist M collected in the recovery pipe 9 is conveyed through the recovery pipe 9 by the carrier gas supplied from the gas supply source 10 into the recovery pipe 9, and is supplied to and recovered by the recovery container 8 from the other end of the recovery pipe 9.

[0040] The inner diameter of the recovery pipe 9 is not limited to this, but can be 1 mm or more and 30 mm or less, preferably 3 mm or more and 20 mm or less, and more preferably 5 mm or more and 10 mm or less.

[0041] The mist M can be recovered into the solvent stored in the recovery container 8, or it can be recovered by simply dropping it into the recovery container 8 without using a solvent. When recovering the mist M into the solvent stored in the recovery container 8, it is preferable that the solvent is the same as the solvent that makes up the slurry S. In this example, the mist recovery device 3 recovers the mist M into the solvent stored in the recovery container 8.

[0042] In the mist recovery device 3 of this example, the flow rate of carrier gas supplied to the container 4 and the recovery pipe 9 can be adjusted by adjustment valves 12a and 12b provided in the gas supply pipes 11a and 11b, which supply carrier gas from the gas supply source 10 into the container 4 and the recovery pipe 9.

[0043] In this example, the carrier gas is supplied to the container 4 and the recovery pipe 9 from the same gas supply source 10. However, in the classification apparatus of this disclosure, the carrier gas may also be supplied to the container and the recovery pipe from separate gas supply sources.

[0044] As the carrier gas, air or inert gases such as nitrogen or argon can be used, but considering the cost, it is preferable to use air.

[0045] The flow rate of the carrier gas introduced into container 4 is not limited to this, but can be 0.1 L / min or more and 10 L / min or less, preferably 0.5 L / min or more and 5 L / min or less, and more preferably 1 L / min or more and 3 L / min or less. Similarly, the flow rate of the carrier gas introduced into recovery pipe 9 is not limited to this, but can be 0.1 L / min or more and 10 L / min or less, preferably 0.5 L / min or more and 5 L / min or less, and more preferably 1 L / min or more and 3 L / min or less.

[0046] (2) Classification method This section describes a method for classifying the target powder using classification apparatus 1 in this example.

[0047] First, prepare slurry S by adding a solvent to the target powder.

[0048] The concentration of the target powder relative to the entire slurry S is not limited as long as a mist containing fine particles can be generated from the slurry S, and can be appropriately determined depending on the type of target powder and the type of solvent constituting the slurry S. Generally, the concentration of the target powder relative to the entire slurry S is not limited to this, but can be 2% by mass or more and 12% by mass or less, and preferably 3% by mass or more and 8% by mass or less. In particular, when the target powder is a metal powder, it is preferable to have a concentration of 3% by mass or more and 10% by mass or less, and more preferably 3% by mass or more and 5% by mass or less.

[0049] If the concentration of the target powder relative to the entire slurry S is less than 2% by mass, when the slurry S is atomized, the proportion of mist M that does not contain fine particles will increase, potentially reducing the efficiency of recovering the fine particles. Conversely, if the concentration of the target powder relative to the entire slurry S is greater than 12% by mass, it may become difficult to form mist M from the slurry S.

[0050] The number-average particle diameter at the equivalent circular diameter of the projected area of ​​the target powder (hereinafter also referred to as "number-average particle diameter at the equivalent circular diameter") is not particularly limited as long as the target powder contains fine particles having the desired particle size. That is, in addition to fine particles, the target powder may contain coarse particles larger than the fine particles or aggregated particles formed by the aggregation of fine particles. From the viewpoint of yield, it is preferable that the number-average particle diameter at the equivalent circular diameter of the target powder is as close as possible to the number-average particle diameter at the equivalent circular diameter of the fine particles obtained by classification, even if it contains coarse particles and aggregated particles. For example, the number-average particle diameter at the equivalent circular diameter of the target powder is larger than the number-average particle diameter at the equivalent circular diameter of the fine particles, preferably about 10 times or less the number-average particle diameter at the equivalent circular diameter of the fine particles, more preferably 5 times or less the number-average particle diameter at the equivalent circular diameter of the fine particles, and even more preferably 3 times or less the number-average particle diameter at the equivalent circular diameter of the fine particles. In this disclosure, particle size (particle diameter) means the equivalent circular diameter of the projected area.

[0051] The volume-based median diameter D50 (hereinafter also referred to as "median diameter D50") of the target powder based on the projected area equivalent diameter is not particularly limited, but it is preferable that it be as close as possible to the number-average particle diameter of the fine particles obtained by classification, similar to the number-average particle diameter of the target powder at the equivalent diameter. For example, the median diameter D50 of the target powder is preferably 0.1 to 10 times the number-average particle diameter of the fine particles at the equivalent diameter, more preferably 0.2 to 5 times, and even more preferably 0.3 to 3 times.

[0052] The median diameter D50 refers to the particle diameter at which, when the number of particles at each particle diameter is accumulated from the smallest particle diameter, the accumulated volume is 50% of the total volume of all particles.

[0053] The maximum particle diameter in the projected area equivalent diameter of the target powder (hereinafter also referred to as "maximum particle diameter in the equivalent diameter") is not particularly limited, but from the viewpoint of yield, it is preferably about 20 times or less the number average particle diameter in the equivalent diameter of the fine particles, more preferably 10 times or less the number average particle diameter in the equivalent diameter of the fine particles, and even more preferably 5 times or less the number average particle diameter in the equivalent diameter of the fine particles.

[0054] The number-average particle diameter, median diameter D50, and maximum particle diameter in the projected area equivalent diameter can be determined as follows. Specifically, a scanning electron microscope (SEM) is used to photograph the metal powder obtained by heating slurry S- to remove the solvent, and an SEM image is obtained. From the SEM image, the contours of 100 or more particles of the target powder are extracted using image analysis software, and the diameter of a circle with the same area as the projected area of ​​each particle is determined as the equivalent diameter. Based on the obtained equivalent diameters, the number-average value, median value, and maximum value are calculated to determine the number-average particle diameter, median diameter D50, and maximum particle diameter in the projected area equivalent diameter.

[0055] In this disclosure, the slurry S can be crushed as a prior step. In particular, when the target powder is a metal powder, it is preferable to dissolve the aggregated particles by crushing and disperse the metal powder in the slurry S. As a crushing device that can be used for the crushing process, a wet crushing device that can be applied to the slurry can be used. Examples of wet crushing devices include ball mills, bead mills, planetary mills, Henschel mixers, feather mills, ultrasonic homogenizers, automatic mortars, and thin-film swirling high-speed mixers.

[0056] The target powder is not particularly limited and can be any powder used in various fields such as electronics, pharmaceuticals, optics, and food. If the target powder is a metal powder, the metal powder is also not particularly limited and can be at least one selected from, for example, nickel, copper, gold, silver, platinum, palladium, or alloys thereof. 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 low-cost.

[0057] The metal powder used in this example is not particularly limited, and any metal powder can be used. For example, it may be either an experimentally synthesized product or a commercially available product.

[0058] Any solvent can be selected as long as sufficient dispersibility of the metal powder in the slurry S is ensured. For example, either an organic solvent such as alcohol or an aqueous solvent such as water may be used.

[0059] However, if the slurry after classification contains an organic solvent, the manufacturing process of electronic devices including multilayer ceramic capacitors requires a significant amount of thermal energy to remove the organic components, in addition to the thermal energy required for powder sintering. Therefore, from the viewpoint of reducing environmental impact, it is preferable to use an aqueous solvent, preferably one in which water is the main component, as the solvent.

[0060] Furthermore, when using an aqueous solvent, it is preferable to include a quaternary ammonium hydroxide to ensure the dispersibility of the metal powder in the slurry S. In this example, an aqueous solvent containing water and a quaternary ammonium hydroxide is used as the solvent.

[0061] Quaternary ammonium hydroxides have 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.

[0062] 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.

[0063] 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.

[0064] From this perspective, it is preferable that the carbon number of quaternary ammonium hydroxide is 16 or less. By having a carbon number in the range of 4 to 16, the synergistic effect of the formation of an alkaline environment in the aqueous solution and the interparticle interposition of charged quaternary ammonium ions significantly reduces the aggregation of particles in the metal slurry. Therefore, even if the amount of quaternary ammonium hydroxide added relative to the total amount of solvent is very small, it is possible to provide an easily disintegrable, aqueous metal slurry.

[0065] The general formula for quaternary ammonium hydroxide is: [R 1 , R 2 , R 3 , R 4 N + ][OH - ]Middle, R 1 , R 2 , R 3 , R 4 Each of these is preferably an alkyl group having 1 to 4 carbon atoms, and it is preferable that they are composed of the same or different alkyl groups.

[0066] Examples of quaternary ammonium hydroxides with such a structure include tetramethylammonium hydroxide, tetraethylammonium hydroxide, tetrapropylammonium hydroxide, tetrabutylammonium hydroxide, trimethylethylammonium hydroxide, trimethylpropylammonium hydroxide, dimethyldipropylammonium hydroxide, monomethyltriethylammonium hydroxide, and monomethyltripropylammonium hydroxide. These quaternary ammonium hydroxides may be used individually or in combination of two or more.

[0067] 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.

[0068] The amount of quaternary ammonium hydroxide in slurry S is restricted to an amount that can raise the pH to 8 or higher at a liquid temperature of 25°C.

[0069] In other words, when preparing the metal slurry in this example, a quaternary ammonium hydroxide is added to a solvent containing water until the pH reaches 8 or higher.

[0070] 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 slurries is typically around several tens of millimoles / L.

[0071] In this example, slurry S 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 slurry to 8 or higher at a liquid temperature of 25°C.

[0072] 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 coarse particles, accompanied by degradation of the particulate components, will bind together firmly, making it difficult to break them down. In addition, the settling of the metal powder in slurry S cannot be sufficiently suppressed. However, depending on the application and storage period of the metal slurry, the pH of slurry S at a liquid temperature of 25°C can be set to 8 or below.

[0073] While there is no particular upper limit to the pH of slurry S 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.

[0074] Aqueous solvents can include organic solvents with a relative permittivity of 20 or higher. That is, from the viewpoint of not excessively inhibiting the ionization of electrolytes in aqueous solutions, 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.

[0075] 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.

[0076] Slurry S 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 charged quaternary ammonium ions, thereby further improving the ease of disintegration of slurry S.

[0077] 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.

[0078] 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.

[0079] 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 slurry S in this example does not contain inorganic bases.

[0080] The slurry S is thoroughly stirred before being placed in the container 4, and / or after being placed in the container 4, it is thoroughly stirred. The method of stirring the slurry S is not particularly limited and can be done, for example, by using a mixing rod or by shaking the container 4. The slurry S is poured into the container 4 until the liquid depth is between 0.1 cm and 10 cm.

[0081] The container 4 containing the slurry S is placed inside a water tank 6 filled with water. Specifically, the container 4 is placed on a raised platform 7 that is placed on the bottom of the water tank 6.

[0082] In this state, the ultrasonic transducer 5 is vibrated, with a frequency between 0.5 MHz and 5 MHz, and an output of 20 W / m². 2 More than 600W / m 2 By emitting the following ultrasonic waves, ultrasonic vibrations are applied to the slurry S in the container 4 to atomize it, and carrier gas is supplied from the gas supply source 10 into the container 4 and the recovery pipe 9.

[0083] The average particle size of a single mist particle M varies depending on the type of solvent, the ultrasonic frequency, etc. In this example, when an aqueous solvent is used and the ultrasonic frequency is 1 MHz or more and 2 MHz or less, the average particle size of a single mist particle M is several μm to several tens of μm, preferably 2 μm to 5 μm. The method for determining the average particle size of a single mist particle M is not particularly limited, but for example, it can be determined from the integrated volume measured by a laser diffraction scattering particle size analyzer.

[0084] The mist M generated based on ultrasonic vibrations contains particles of the target powder (metal powder) dispersed in the slurry S. The target powder contained in the mist M are fine particles with a particle size smaller than the particle size of a single mist M. A single mist M does not necessarily contain only one fine particle; a single mist M can contain multiple fine particles. Furthermore, not all mist Ms need to contain fine particles of the target powder; some mist Ms may not contain any fine particles.

[0085] The mist M, which contains the fine particles of the target powder, is transported by a carrier gas through the recovery pipe 9 to the recovery container 8 for collection. This allows the target powder to be classified and fine particles to be obtained. In this example, the mist M is recovered by dissolving it in the solvent stored in the recovery container 8, and the solvent is removed as needed. However, when manufacturing a slurry or paste using the fine particles obtained by classification, it is also possible to add the necessary additives without removing the solvent.

[0086] The number-average particle diameter in the equivalent circular diameter of the projected area of ​​the fine particles obtained by classification is not limited to this, but can be between 1 nm and 500 nm, preferably between 5 nm and 250 nm, and more preferably between 10 nm and 200 nm. In particular, when the fine particles are metal powder, the number-average particle diameter in the equivalent circular diameter of the fine particles can preferably be between 10 nm and 200 nm.

[0087] The volume-based median diameter D50 of the fine particles obtained by classification, based on the projected area equivalent diameter, is not limited to this, but can be between 1 nm and 500 nm, preferably between 5 nm and 250 nm, and more preferably between 10 nm and 200 nm. In particular, when the fine particles are metal powder, the median diameter D50 can preferably be between 5 nm and 250 nm, and more preferably between 10 nm and 200 nm. Furthermore, the difference between the number-average particle diameter at the equivalent diameter of the fine particles obtained by classification and the median diameter D50 tends to be smaller than the difference between the number-average particle diameter at the equivalent diameter of the target powder and the median diameter D50. This clearly indicates that coarse particles have been excluded from the target powder.

[0088] The maximum particle diameter of the fine particles obtained by classification, based on the projected area circle equivalent diameter, is not limited to this, but can be 1000 nm or less, preferably 500 nm or less, and more preferably 300 nm or less. In particular, when the fine particles are metal powder, the maximum particle diameter can be 500 nm or less, and more preferably 200 nm or less.

[0089] When the fine particles are metal powder, problems such as powder re-aggregation may occur if the number-average particle diameter in the equivalent circle diameter is less than 10 nm. On the other hand, if the number-average particle diameter exceeds 200 nm, it may not be possible to adequately thin the conductive layer or electrode layer.

[0090] According to the classification method of this disclosure, a common solvent such as water can be used as the solvent for the slurry used in the classification process, and by a simple process of misting the slurry, it is possible to classify fine particles with a volume-average particle size (MV) of 500 nm or less, preferably 100 nm or less, from the target powder. [Examples]

[0091] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0092] [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.

[0093] 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.

[0094] 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.

[0095] When the obtained nickel powder was observed using a scanning electron microscope (SEM), the particle shape was found to be approximately spherical.

[0096] (2) Preparation of metal slurry (nickel slurry) 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.

[0097] 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.

[0098] 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.

[0099] 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.

[0100] 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.

[0101] (3) Classification of metal slurries (nickel slurries) The prepared slurry is placed in a glass container (container thickness 1.7 mm, container volume 1500 cm³) that constitutes the mist generator of the classification apparatus. 3The slurry was introduced into the glass container to a depth of 1 cm, and an ultrasonic oscillator (manufactured by Kaijo Co., Ltd., model: QUAVA mini 30110), which constitutes the mist generator, was operated at a frequency of 1.6 MHz and an output of 100 W to irradiate the slurry with ultrasound. After sufficient mist had been generated in the glass container, air was supplied to the glass container as a carrier gas from the gas supply source that constitutes the mist recovery device at a flow rate of 3 L / min, thereby transporting the mist through the recovery tube (inner diameter: 8 mm) that constitutes the mist recovery device to the recovery container that constitutes the mist recovery device. More than 10 mL of slurry, which is the amount necessary for evaluation, was repeatedly prepared using the classification operation described above. During the classification operation, no clogging that would make it difficult to recover the slurry was observed in the classification device, especially in the recovery tube.

[0102] (4) Particle size evaluation of metal powder (nickel powder) in metal slurry before and after classification treatment Nickel powder obtained by heating the nickel slurry before classification and the nickel slurry recovered in the recovery container to remove the solvent was photographed using a scanning electron microscope (SEM (JEOL Ltd., model: JSM-7200F)) to obtain SEM images. From these images, the contours of 700 particles of each nickel powder were extracted using image analysis software (Lightstone Co., Ltd., MIPAR), and the equivalent circle diameter of the projected area of ​​each particle was determined. The number average value, median value, and maximum value of these values ​​were calculated to determine the number average particle diameter at the equivalent circle diameter, the median diameter D50, and the maximum particle diameter at the equivalent circle diameter. As a result, in the nickel slurry before classification, the number average particle diameter at the equivalent circle diameter of the nickel particles was 86 nm, the median diameter was 78 nm, and the maximum particle diameter was 318 nm. In the nickel slurry recovered using the recovery container, the number-average particle diameter in the equivalent circular diameter of nickel particles was 76 nm, the median diameter was 72 nm, and the maximum particle diameter was 169 nm.

[0103] Table 1 shows the characteristics of the slurry before classification, and Table 2 shows the classification method, presence or absence of clogging, and characteristics of the slurry after classification. Similarly, Tables 1 and 2 also show the characteristics of Examples 2-3 and Comparative Examples 1-2.

[0104] [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.

[0105] In the preparation of the metal powder (nickel powder)-containing slurry, nickel powder and nickel powder-containing slurry were prepared in the same manner as in Example 1, except that in the preparation step of the metal powder (nickel powder)-containing slurry, tetrabutylammonium hydroxide, which has 16 carbon atoms, was added as a quaternary ammonium hydroxide until the pH became 8, and the solvent obtained using this solvent was adjusted to a nickel concentration of 12% by mass. The amount of tetrabutylammonium hydroxide added to water was 0.1 mmol / L.

[0106] The prepared slurry was subjected to classification under the same conditions as in Example 1. When evaluated in the same manner as in Example 1, the nickel slurry before classification had a number-average particle diameter of 250 nm at the equivalent circular diameter of nickel particles, a median diameter D50 of 188 nm, and a maximum particle diameter of 500 nm. On the other hand, the nickel slurry recovered in the recovery container had a number-average particle diameter of 121 nm at the equivalent circular diameter of nickel particles, a median diameter D50 of 104 nm, and a maximum particle diameter of 170 nm.

[0107] [Example 3] In the process of preparing the raw material nickel powder, nickel powder was prepared in the same manner as in Example 1.

[0108] In the preparation of the metal powder (nickel powder)-containing slurry, nickel powder and nickel powder-containing slurry were prepared in the same manner as in Example 1, except that, as a solvent, water and ethanol were mixed in a volume ratio of 6:4, and then tetramethylammonium hydroxide, which has 4 carbon atoms, was added until the pH reached 13, so that the nickel concentration was 2% by mass. The amount of tetramethylammonium hydroxide added to the water was 90 mmol / L.

[0109] The prepared slurry was subjected to classification under the same conditions as in Example 1. When evaluated in the same manner as in Example 1, the nickel slurry before classification had a number-average particle diameter of 80 nm at the equivalent circular diameter of nickel particles, a median diameter D50 of 73 nm, and a maximum particle diameter of 305 nm. On the other hand, the nickel slurry recovered in the recovery container had a number-average particle diameter of 78 nm at the equivalent circular diameter of nickel particles, a median diameter D50 of 70 nm, and a maximum particle diameter of 166 nm.

[0110] [Comparative Example 1] Nickel powder and nickel powder-containing slurry were prepared in the same manner as in Example 1, except that the classification of the metal slurry (nickel slurry) was carried out using a filter filtration method. The filter filtration operation was performed by supplying 10 mL or more of the prepared nickel slurry from a syringe connected to a disc-type filter (manufactured by Advantec Toyo Co., Ltd., disc diameter 25 mm, pore size 0.2 μm). When checking for clogging during the classification operation, clogging occurred on the filter, but a portion of the slurry was recovered. When this partially recovered slurry was evaluated in the same manner as in Example 1, the number-average particle size in the equivalent circle diameter of nickel particles in the recovered slurry was 88 nm, the median diameter D50 was 79 nm, and the maximum particle size was 199 nm.

[0111] [Comparative Example 2] Nickel powder and nickel powder-containing slurry were prepared in the same manner as in Example 2, except that the classification of the metal slurry (nickel slurry) was carried out using a filter filtration method. The filter filtration operation was performed by supplying 10 mL or more of the prepared nickel slurry from a syringe connected to a disc-type filter (manufactured by Advantec Toyo Co., Ltd., disc diameter 25 mm, pore size 0.2 μm). When checking for clogging during the classification operation, clogging occurred on the filter, making it difficult to recover the slurry, so the test was terminated as classification was deemed impossible.

[0112] [Table 1]

[0113] [Table 2]

[0114] [Discussion on the evaluation results] Table 1 shows that in the example of the classification method according to one embodiment of the present disclosure, the requirements were met, and it was confirmed that fine particles could be recovered more efficiently than in the comparative example. The reason why the example had an excellent classification effect without clogging during the classification operation is thought to be that the mist itself, which contains coarse particles that tend to hinder the propagation of ultrasonic waves, is difficult to form, and that the larger the particle size in the mist, the more the mist falls and is not transported before reaching the collection container. [Explanation of Symbols]

[0115] 1 Classifier 2. Mist Generator 3. Mist collection device 4 containers 5. Ultrasonic transducer 6 Aquariums 7. Raising platform 8. Collection containers 9. Recovery pipe 10 Gas supply sources 11a, 11b Gas supply pipes 12a, 12b Adjustment valve

Claims

1. A step of preparing a slurry containing the target powder and solvent, A step of applying ultrasonic vibrations to the slurry to generate mist, A step of recovering the fine particles of the target powder contained in the mist, A classification method comprising the following features.

2. The classification method according to claim 1, wherein the number-average particle diameter of the fine particles in the projected area circle equivalent diameter is 1 nm or more and 500 nm or less.

3. The classification method according to claim 1, wherein the concentration of the target powder relative to the entire slurry is 2% by mass or more and 12% by mass or less.

4. The classification method according to claim 1, wherein the target powder includes at least one powder selected from metal powder, ceramic powder, or a mixture thereof.

5. The classification method according to claim 1, wherein the solvent comprises a quaternary ammonium hydroxide and water.

6. The classification method according to claim 5, wherein the quaternary ammonium hydroxide has 16 or fewer carbon atoms.

7. The classification method 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 classification method according to claim 1, wherein the solvent includes an organic solvent with a relative dielectric constant of 20 or more.

9. The classification method according to claim 8, wherein the organic solvent is at least one selected from methanol, ethanol, or 1-propanol.

10. A mist generator that generates mist by applying ultrasonic vibrations to a slurry containing the target powder and solvent, A mist recovery device for recovering the mist generated by the mist generator, A classification device equipped with the following features.

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