Process for producing spherical silver powder

Abstract

A method for manufacturing spherical silver powder with a smaller primary particle size deviation compared to conventional methods is provided, as well as the spherical silver powder obtained therefrom. The method for manufacturing spherical silver powder of the present invention includes a reduction precipitation step in which silver particles are reduced and precipitated by mixing a reducing agent composed of hydrazine carbonate in an aqueous reaction system containing silver ions.

Description

[0001] (This application was filed on July 17, 2019, with application number 201980045792.5 and invention title:

[0002] (A divisional application of the application for "Method for Manufacturing Spherical Silver Powder") Technical Field

[0003] This invention relates to a method for manufacturing spherical silver powder. In particular, this invention relates to a method for manufacturing spherical silver powder for use as a conductive paste in circuits for forming internal electrodes of multilayer capacitors, solar cells, plasma display panels, and touch panels. Background Technology

[0004] In the past, the following methods have been widely used as methods for forming internal electrodes of multilayer capacitors, conductor patterns of circuit boards, electrodes or circuits of substrates for solar cells or plasma display panels: for example, by mixing silver powder and glass powder together with an organic binder to produce a calcined conductive paste, forming the calcined conductive paste into a specified pattern on a substrate, and then heating it at a temperature of 500°C or higher to remove the organic components, thereby sintering the silver powder together to form a conductive film.

[0005] For conductive pastes used in this application, the miniaturization of electronic components necessitates high-density and fine-line conductor patterns. Therefore, the silver powder used must have a moderately small and concentrated particle size, dispersed within an organic binder.

[0006] As a method for manufacturing such conductive pastes, for example, in Patent Document 1, a wet reduction method is known to reduce and precipitate spherical silver powder by mixing a reducing agent in an aqueous reaction solution containing silver ions.

[0007] Furthermore, as a method for manufacturing spherical silver powder with a concentrated particle size, Patent Document 2 proposes a method of reducing and precipitating the powder by mixing seed particles before reduction, while Patent Document 3 proposes a method of reducing and precipitating the powder by mixing aqueous solutions containing silver ions and aqueous solutions of reducing agents that flow out from different flow paths. Additionally, Patent Document 4 proposes a method for reducing and precipitating copper powder using multiple reducing agents.

[0008] Existing technical documents

[0009] Patent documents

[0010] Patent Document 1: Japanese Patent Application Publication No. 2005-220380

[0011] Patent Document 2: Japanese Patent Application Publication No. 2009-235474

[0012] Patent Document 3: Japanese Patent Application Publication No. 2010-070793

[0013] Patent Document 4: International Publication No. 2014 / 104032 Summary of the Invention

[0014] The problem the invention aims to solve

[0015] As mentioned above, with the miniaturization of electronic components, there is a need for conductive pastes capable of depicting fine wiring. However, if coarse particles are mixed into the powder used in the conductive paste, it will cause flyback during printing, which may result in broken wiring. In addition, if spherical silver powder with uneven particle size is pasted, the viscosity characteristics will also deviate, making it difficult to exhibit stable printing properties.

[0016] Furthermore, in the fabrication of electrodes for solar cells, sintering is typically performed over extremely short periods, such as tens of seconds. During sintering, if the conductive paste contains a large number of coarse particles, it will cause insufficient sintering. On the other hand, if the conductive paste contains a large number of fine particles, it will promote sintering and cause over-sintering. Therefore, in order to achieve the appropriate sintering state in a short time, the technique of appropriately controlling the particle size of spherical silver powder is also very important.

[0017] For the reasons mentioned above, spherical silver powder with small particle size deviation is required.

[0018] In addition, the manufacturing methods for metal powders with concentrated particle size disclosed in the aforementioned patent documents 2 to 4 have the following problems.

[0019] First, in manufacturing methods using seed particles, as described in Patent Document 2, the manufacturing process becomes more complex due to the added step of producing seed particles. Furthermore, the dispersant used to produce seed particles may have side effects during the production of the target particles.

[0020] In addition, in the method described in Patent Document 3, in which an aqueous solution containing silver ions and an aqueous solution of a reducing agent flow out from different flow paths and come into contact and mix to reduce and precipitate the silver ion, blockage may occur in the pipe when the silver mirror reaction occurs in the contact pipe.

[0021] Furthermore, in methods using multiple reducing agents, such as in Patent Document 4, it is argued that the drainage process becomes more complex compared to using a reducing agent alone, resulting in increased manufacturing costs.

[0022] Therefore, the object of the present invention is to provide a method for manufacturing spherical silver powder that can solve the above-mentioned problems and can easily manufacture spherical silver powder with smaller primary particle size deviation compared to the past.

[0023] Solution for solving the problem

[0024] To address the aforementioned issues, the inventors conducted in-depth research and discovered that by mixing hydrazine carbonate as a reducing agent in an aqueous reaction system containing silver ions to reduce and precipitate silver particles, spherical silver powder with a concentrated primary particle size can be produced. The reason for the concentrated primary particle size when using hydrazine carbonate as a reducing agent, compared to using an aqueous hydrazine solution (hydrazine hydrate), is not yet clear, but it is speculated that it is because the reduction begins after the carbonic acid attached to the hydrazine carbonate molecules (NH₂NH₂)₂·CO₂ is removed. Therefore, the time from the addition of the reducing agent to the start of reduction allows sufficient time for the hydrazine (N₂H₄) before the reaction to be fully dispersed in the aqueous reaction system containing silver ions, achieving uniform nucleation and growth within the mixed solution of the aqueous reaction system containing silver ions and hydrazine carbonate.

[0025] This invention is based on the above insights, as detailed below.

[0026] (1) A method for manufacturing spherical silver powder, comprising a reduction precipitation step in which a reducing agent composed of hydrazine carbonate is mixed in an aqueous reaction system containing silver ions to reduce and precipitate silver particles.

[0027] (2) The method for manufacturing spherical silver powder according to (1) above, wherein the amount of the hydrazine carbonate mixed in the reduction precipitation step is 1 to 6 molar equivalents relative to silver.

[0028] (3) The method for manufacturing spherical silver powder according to (1) or (2) above, wherein the aqueous reaction system containing the above-mentioned silver ions is a silver ammonia complex.

[0029] The silver ammonia complex is prepared by adding ammonia or an ammonium salt to an aqueous solution containing at least one of silver nitrate, a silver complex, and a silver intermediate.

[0030] (4) The method for manufacturing spherical silver powder according to any one of (1) to (3) above, wherein, in the reduction precipitation step, the temperature of the aqueous reaction system containing the silver ions when the reducing agent is mixed is 10 to 50°C.

[0031] (5) The method for manufacturing spherical silver powder according to any one of (1) to (4) above, wherein the cumulative 50% particle size D50 of the primary particle size of the spherical silver powder by SEM is 0.1 to 1.5 μm, and the coefficient of variation in the particle size distribution is 0.2 or less.

[0032] The effects of the invention

[0033] According to the present invention, a method for manufacturing spherical silver powder can be provided, which can easily produce spherical silver powder with smaller primary particle size deviation compared to the past. Attached Figure Description

[0034] Figure 1 The image shows a SEM photograph of the spherical silver powder obtained in Example 1.

[0035] Figure 2 The results of particle size distribution analysis of the SEM images obtained in Example 1 are shown.

[0036] Figure 3 The image shows a SEM photograph of the spherical silver powder obtained in Example 2.

[0037] Figure 4 The results of particle size distribution analysis of the SEM images obtained in Example 2 are shown.

[0038] Figure 5 The image shows a SEM photograph of the spherical silver powder obtained in Comparative Example 1.

[0039] Figure 6 To compare the particle size distribution analysis results of the SEM images obtained in Example 1. Detailed Implementation

[0040] In this specification, "SEM primary particle size" refers to the primary particle size obtained by scanning electron microscopy (SEM). The cumulative 50% particle size D50 of this SEM primary particle size is determined as follows: Silver particles are observed using a scanning electron microscope (SEM) at a magnification of 10,000. From the silver particles (primary particles) observed in a randomly selected field of view that do not overlap or combine with other particles and have clear outlines, 100 particles are randomly selected. Using image analysis particle size distribution measurement software (Mac-View manufactured by Mountech Co., Ltd.), the equivalent circular diameter (Heywood diameter) of each silver particle is converted. The particle size of each silver particle is then determined, and the cumulative value of the particle size distribution, representing the number of particles, is the 50% particle size.

[0041] The method for manufacturing spherical silver powder of the present invention includes a reduction precipitation step that reduces and precipitates silver particles. Other steps may be included as needed. Examples of other steps include a silver ion dispersion preparation step, a dispersant adsorption step, a recovery and washing step, a drying step, and a dry treatment step. That is, in addition to the reduction precipitation step that reduces and precipitates silver particles, the method for manufacturing spherical silver powder of the present invention may optionally include a silver ion dispersion preparation step, a dispersant adsorption step, a recovery and washing step, a drying step, and a dry treatment step.

[0042] The embodiments of the present invention, including specific solutions, will be described below in the following order.

[0043] 1-A) Preparation process of silver ion dispersion

[0044] 1-B) Reduction and precipitation process

[0045] 1-C) Adsorption process of dispersant

[0046] 1-D) Recycling and Cleaning Process

[0047] 1-E) Drying process

[0048] 1-F) Dry processing steps

[0049] 1-A) Preparation process of silver ion dispersion

[0050] This process involves preparing a silver ion dispersion for generating silver particles, which serve as the basis for spherical silver powder. The silver ion dispersion obtained through this process can be used as an aqueous reaction system containing silver ions.

[0051] As an aqueous reaction system containing silver ions, an aqueous solution or slurry containing at least one of silver nitrate, a silver complex, and a silver intermediate can be used. Seed particles, which serve as nuclei for the growth of silver particles, can also be used, but since the reaction system becomes more complex, it is preferable not to use seed particles.

[0052] Aqueous solutions containing silver complexes can be generated by adding ammonia or ammonium salts to an aqueous solution of silver nitrate or a suspension of silver oxide. To ensure the spherical silver powder has a suitable particle size and shape, an aqueous solution of silver ammonia complex obtained by adding ammonia to an aqueous solution of silver nitrate is preferred.

[0053] Since the coordination number of ammonia in the silver-ammonia complex is 2, 2 moles of ammonia react with 1 mole of silver. It is preferable to add 1 molar equivalent or more of ammonia relative to silver, and more preferably 2 molar equivalents or more. In this case, 1 molar equivalent of ammonia is equivalent to 2 moles of ammonia relative to 1 mole of silver. Furthermore, to facilitate the reaction of the complex to some extent, the amount of ammonia added can be 8 molar equivalents or less relative to silver, preferably 6 molar equivalents or less. Additionally, a pH adjuster can be added to the aqueous reaction system containing silver ions. Conventional acids or bases can be used as pH adjusters, such as nitric acid and sodium hydroxide.

[0054] It should be noted that the aforementioned silver intermediates refer to substances produced during the reaction process to manufacture the target substance. Examples of silver intermediates include silver oxide (Ag₂O) and silver carbonate (Ag₂CO₃). These substances dissolve by adding ammonia during the manufacture of the ammonia complex, and most of the silver ions exist in the form of silver ammonia complexes.

[0055] 1-B) Reduction and precipitation process

[0056] In this process, silver is reduced and precipitated from an aqueous reaction system (silver ion dispersion) containing silver ions using a reducing agent.

[0057] Hydrazine carbonate is used as a reducing agent. It should be noted that "hydrazine carbonate" here also includes aqueous solutions of hydrazine carbonate obtained by diluting hydrazine carbonate. In this invention, it is preferable not to use reducing agents other than hydrazine carbonate; therefore, it is preferable that the "hydrazine carbonate" does not contain reducing agents of a type different from those having aldehyde groups. Furthermore, to simplify the drainage treatment, it is preferable to use the reducing agent only in this step; in this case, the reducing agent is used only in the reduction step 1-B) after the preparation step of the silver ion dispersion in step 1-A). For example, commercially available hydrazine carbonate manufactured by Otsuka Chemical Co., Ltd. or JAPAN FINECHEM COMPANY, INC. can be used. Alternatively, hydrazine carbonate prepared by blowing carbon dioxide into a common hydrazine aqueous solution can also be used (as long as it is mostly hydrazine carbonate, it may also contain some hydrazine that has not been converted to carbonate).

[0058] To improve the reaction yield of silver, the amount of reducing agent relative to silver can be 1 molar equivalent or more, or 1.1 molar equivalents or more. On the other hand, excessive use of reducing agent may increase the cost of raw materials and wastewater treatment. Therefore, the amount of mixed reducing agent is preferably 6 molar equivalents or less relative to silver, more preferably 5 molar equivalents or less. Furthermore, when hydrazine carbonate is mixed in an aqueous reaction system containing silver ions, the concentration of hydrazine carbonate is preferably in the range of 1 to 70% by mass. It should be noted that the molecular formula of hydrazine carbonate is (N₂H₄)₂·CO₂, therefore one molecule of hydrazine carbonate has the same effect as two ordinary hydrazine molecules. Since hydrazine releases four electrons during reduction, hydrazine carbonate reacts at a rate of 1 / 8 mole relative to 1 mole of silver. That is, 1 molar equivalent of hydrazine carbonate relative to silver is equivalent to 1 / 8 mole of hydrazine carbonate relative to 1 mole of silver.

[0059] Furthermore, in this process, the temperature of the aqueous reaction system containing the aforementioned silver ions during the mixing of the reducing agent is preferably 10–50°C, more preferably 20–40°C. The preferred temperature range for the aqueous reaction system is 20–40°C. This is because: at higher temperatures, carbonic acid is released quickly, potentially leaving insufficient time for the hydrazine (N₂H₄) before the reaction to diffuse; at lower temperatures, the reactivity of the hydrazine after the carbonic acid is released decreases, potentially preventing sufficient reduction and precipitation.

[0060] Furthermore, this process can be carried out using either continuous mixing or intermittent mixing. However, in the case of intermittent mixing, it is preferable to minimize the time required to mix all the reducing agent with the aqueous reaction system containing silver ions (i.e., the silver ion dispersion). In intermittent mixing, the larger the volume (the amount of silver powder collected in the intermittent process), the more difficult it is to mix the reducing agent in a short time. Moreover, even if the silver ion dispersion being mixed is thoroughly stirred, if a reduction precipitation reaction occurs during the period when a concentration difference of the reducing agent exists between the region close to and far from the reducing agent introduction region, this can also be one of the reasons for the increased deviation in the primary particle size of the resulting spherical silver powder.

[0061] Therefore, unlike conventional techniques, this invention uses hydrazine carbonate as a reducing agent. Because of the use of hydrazine carbonate, a grace period is created from the moment the reducing agent comes into contact with the aqueous reaction system containing silver ions until the reduction reaction begins. This allows for a longer period of time to stir the reducing agent to reduce the concentration gradient. When comparing the yield of silver powder obtained using batch processing at the same volume, this invention is advantageous in that it can easily produce spherical silver powder with smaller particle size deviations compared to conventional methods. On the other hand, when comparing by changing the volume, this invention is also advantageous in that it can improve mass production (i.e., the ability to simultaneously and uniformly produce large quantities of silver powder).

[0062] It should be noted that while hydrazine hydrochloride and hydrazine sulfate, other hydrazine derivatives besides hydrazine carbonate, can be considered as reducing agents, the chlorine or sulfur components within these molecules will react with silver to form silver chloride or silver sulfide. Furthermore, since chlorine or sulfur components remain in the powder after sintering, they can promote corrosion. Additionally, in other hydrazine compounds, some hydrazine undergoes substitution reactions, resulting in fewer electrons released per molecule, potentially increasing the required amount of reducing agent to be mixed. Moreover, the increased cost due to hydrazine substitution reactions is expected to increase the manufacturing cost of spherical silver powder, and the cost of wastewater treatment is also higher than with ordinary hydrazine. For these reasons, hydrazine carbonate is suitable as the reducing agent in this invention.

[0063] 1-C) Adsorption process of dispersant

[0064] In this process, the dispersant is adsorbed onto the surface of each silver particle.

[0065] Before and after the reduction precipitation of spherical silver powder, a dispersant can be added to the solution to allow the dispersant to adsorb onto the surface of the silver powder. The dispersant can be added only before reduction, only after reduction, or both before and after reduction.

[0066] The amount of dispersant such as organic matter added in the adsorption process is preferably 0.05% by mass or more and 3.0% by mass or less relative to the mass of silver powder, more preferably 0.1% by mass or more and 1.0% by mass or less.

[0067] There are no particular restrictions on the use of dispersants; they can be selected appropriately according to the purpose. Examples include fatty acids and their salts, surfactants, organometallic compounds, chelating agents, and polymeric dispersants. Dispersants can be used alone or in combination with two or more.

[0068] 1-D) Recycling and Cleaning Process

[0069] Following the steps outlined above, this step involves recovering and cleaning the resulting silver particles. Recovery and cleaning can be performed as separate steps, repeated multiple times, or simultaneously.

[0070] The silver powder obtained through the above reduction process mostly contains impurities, so it is preferable to clean it. Here, pure water is preferred as the cleaning solvent.

[0071] There are no particular restrictions on the methods of recycling and cleaning; appropriate methods can be selected based on the purpose. Examples include decantation and filtration. The endpoint of cleaning can be determined using the conductivity of the cleaned water; preferably, cleaning should continue until the conductivity drops below 0.5 mS / m.

[0072] 1-E) Drying process

[0073] In this process, the aggregates of silver particles after the recycling and cleaning process are dried.

[0074] The aggregates of silver particles after the above-mentioned recycling and cleaning process are filter cakes or slurries containing a large amount of moisture. Therefore, in order for the silver particles to be ultimately usable as silver powder, it is necessary to remove the moisture from the filter cakes or slurries.

[0075] Methods for removing moisture include blowing dry air, reducing pressure, immersing in a drying solvent, drying with compressed air, and centrifugal drying, but heating under reduced pressure is relatively simple. The preferred drying temperature is below 100°C, which prevents sintering between silver powder particles.

[0076] 1-F) Dry processing steps

[0077] The silver powder obtained through the above drying process can be subjected to dry processing steps such as dry crushing and grading. Alternatively, surface smoothing can be performed, which involves feeding the silver powder into a device that allows it to flow mechanically, causing mechanical collisions between the silver particles and smoothing the sharp edges of the particles. In addition, grading can be performed after crushing and smoothing. It should be noted that an integrated device capable of drying, crushing, and grading can also be used. The cumulative 50% particle size (D50) of the SEM primary particle size of the spherical silver powder obtained through the above processes can be set to 0.1–1.5 μm, and the coefficient of variation in the particle size distribution can be set to below 0.2. Furthermore, the D50 can be set to 0.2–1.0 μm.

[0078] Compared to conventional manufacturing methods, the above-described method for manufacturing spherical silver powder eliminates the need for expensive reagents or complex processes, and the drainage treatment can be performed in the same manner as in conventional methods using hydrazine aqueous solutions. Therefore, it does not significantly increase production costs. Furthermore, this manufacturing method allows for the easy production of spherical silver powder with a smaller primary particle size deviation compared to conventional methods. The spherical silver powder obtained by this manufacturing method exhibits a small primary particle size deviation.

[0079] Example

[0080] The following describes in detail the embodiments of the spherical silver powder of the present invention, but the present invention is not limited to the following embodiments in any way.

[0081] (Example 1)

[0082] Prepare 3.2 L of a silver nitrate aqueous solution containing 0.12 mol / L silver ions. Add 137.7 g of a 28% by mass ammonia aqueous solution (equivalent to 2.7 molar equivalents relative to silver) to the above silver nitrate aqueous solution to obtain a silver ammonia complex aqueous solution. Separately, dilute 14.6 g of a 70% by mass hydrazine carbonate aqueous solution (manufactured by Otsuka Chemical Co., Ltd., equivalent to 1.8 molar equivalents relative to silver) with 131.4 g of pure water to obtain a reducing agent. Maintain the temperature of the silver ammonia complex aqueous solution at 30.0 °C, and mix the reducing agent with the thoroughly stirred silver ammonia complex solution to obtain a slurry containing silver powder. The solution begins to change color 1.0 second after the start of mixing, indicating that a reduction precipitation reaction has occurred.

[0083] Furthermore, 3.6 g of a solution obtained by dissolving oleic acid in ethanol at a mass of 5.0% by mass was added as a dispersant to the resulting slurry containing silver powder. After thorough stirring, the solution was allowed to mature. The amount of oleic acid added was 0.4% by mass relative to the mass of the silver powder. The matured slurry was filtered, washed with water, and then dried in a vacuum dryer at 73°C for 10 hours. The resulting dried powder was then fed into a crusher (manufactured by Kyoritsu Rikko Co., Ltd., SK-M10 model) and crushed twice for 30 seconds each time. This yielded the spherical silver powder of Example 1.

[0084] The spherical silver powder obtained in Example 1 was photographed using a scanning electron microscope (SEM) at a magnification of 10,000. The images are shown below. Figure 1 .

[0085] Additionally, for the captured SEM images ( Figure 1 The particle size distribution of the primary particle size of the obtained spherical silver powder was analyzed using image analysis software (Mac-View manufactured by Mountech Co., Ltd.).

[0086] This analysis software can calculate the particle area by depicting the outline of any particle, and then calculate the particle size by converting it to the equivalent circle diameter (Heywood diameter). This operation was performed on 100 particles within the image, and the results were plotted as a particle size distribution. This particle size distribution is shown below. Figure 2 The particles selected for measurement are those that do not overlap or combine with other particles in the SEM image and have clear outlines. In the particle size distribution, a narrow distribution width indicates a smaller standard deviation, suggesting silver powder with small and concentrated particle size deviations. Furthermore, for different particle sizes, the coefficient of variation is calculated to perform a relative evaluation of particle size deviation. The coefficient of variation is obtained by dividing the standard deviation of the particle size distribution obtained from the first SEM particle size determination by D50, and serves as an indicator of the magnitude of the deviation in the first SEM particle size determination.

[0087] The analysis results showed that the cumulative 50% particle size (D50) of the spherical silver powder was 0.34 μm, the standard deviation of the particle size distribution was 0.063 μm, and the coefficient of variation was 0.185.

[0088] (Example 2)

[0089] Prepare 3.2 L of a silver nitrate aqueous solution containing 0.12 mol / L silver ions. Add 137.7 g of a 28% by mass ammonia aqueous solution (equivalent to 2.7 molar equivalents relative to silver) to the above silver nitrate aqueous solution to obtain a silver ammonia complex aqueous solution. Separately, dilute 14.6 g of a 70% by mass hydrazine carbonate aqueous solution (manufactured by Otsuka Chemical Co., Ltd., equivalent to 1.8 molar equivalents relative to silver) with 131.4 g of pure water to obtain a reducing agent. Maintain the temperature of the silver ammonia complex aqueous solution at 30.0 °C, and add 0.59 g of a stearic acid-based emulsion solution (equivalent to 0.2% by mass of silver based on stearic acid) as a dispersant. Mix the above reducing agent into the thoroughly stirred silver ammonia complex solution to obtain a slurry containing silver powder. The solution begins to change color 1.2 seconds after the start of mixing, indicating that a reduction precipitation reaction has occurred.

[0090] Furthermore, 3.6 g of a solution obtained by dissolving oleic acid in ethanol at a mass of 5.0% by mass was added as a dispersant to the resulting slurry containing silver powder. After thorough stirring, the solution was allowed to mature. The amount of oleic acid added was 0.4% by mass relative to the mass of the silver powder. The matured slurry was filtered, washed with water, and then dried in a vacuum dryer at 73°C for 10 hours. It was then crushed to obtain the spherical silver powder of Example 2.

[0091] The spherical silver powder obtained in Example 2 was photographed using a scanning electron microscope (SEM) at a magnification of 10,000. The images are shown below. Figure 3 Furthermore, the SEM images obtained in Example 2 were analyzed using Mac-View in the same manner as in Example 1. The measurement results were then plotted as a particle size distribution. Figure 4 The results showed that the cumulative 50% particle size (D50) was 0.50 μm, the standard deviation of the particle size distribution was 0.069 μm, and the coefficient of variation was 0.138.

[0092] (Comparative Example 1)

[0093] Prepare 3.2 L of a silver nitrate aqueous solution containing 0.12 mol / L silver ions. Add 137.7 g of a 28% by mass ammonia aqueous solution (equivalent to 2.7 molar equivalents relative to silver) to the silver nitrate aqueous solution to obtain a silver ammonia complex aqueous solution. Separately, dilute 11.8 g of an 80% by mass hydrazine aqueous solution (equivalent to 1.8 molar equivalents relative to silver) with 123.3 g of pure water to obtain a reducing agent. Maintain the temperature of the silver ammonia complex aqueous solution at 30.0 °C, and mix the above reducing agent into the thoroughly stirred silver ammonia complex solution to obtain a slurry containing silver powder. The solution changes color immediately after mixing begins (after 0.3 seconds), indicating that a reduction precipitation reaction has occurred.

[0094] Furthermore, 3.6 g of a solution obtained by dissolving oleic acid in ethanol at a mass of 5.0% by mass was added as a dispersant to the resulting silver powder slurry. After thorough stirring, the slurry was allowed to mature. The amount of oleic acid added was 0.4% by mass relative to the mass of the silver powder. The matured slurry was filtered, washed with water, and then dried in a vacuum dryer at 73°C for 10 hours. It was then crushed to obtain the spherical silver powder of Comparative Example 1.

[0095] The spherical silver powder of Comparative Example 1 was photographed using a scanning electron microscope (SEM) at a magnification of 10,000. The images are shown below. Figure 5 Furthermore, the SEM images of Comparative Example 1 were analyzed using Mac-View in the same manner as in Example 1. The measurement results were presented as a plot of particle size distribution. Figure 6 The results showed that the cumulative 50% particle size (D50) was 0.40 μm, the standard deviation of the particle size distribution was 0.096 μm, and the coefficient of variation was 0.238.

[0096] The particle size characteristics of the silver powder obtained in these examples and comparative examples are shown in Table 1. It can be seen that the primary particle size of Example 1 is smaller than that of Comparative Example 1, while that of Example 2 is larger. However, the coefficient of variation in all examples is smaller than that of Comparative Example 1. Therefore, it can be considered that when hydrazine carbonate is used as a reducing agent, the primary particle size is concentrated regardless of its particle size.

[0097] [Table 1]

[0098]

[0099] Furthermore, it is known that according to the present invention, compared with conventional manufacturing methods, the deviation in primary particle size can be reduced without the use of expensive reagents or complex processes. Additionally, it has been confirmed that drainage treatment can be performed in the same manner as in the conventional use of hydrazine aqueous solution.

[0100] Furthermore, as demonstrated by these examples and comparative examples, the spherical silver powder produced using hydrazine carbonate exhibits a narrower primary particle size distribution width compared to conventional spherical silver powder without significantly increasing production costs. In other words, it is possible to produce particles with small primary particle size deviations.

[0101] Industrial availability

[0102] As can be seen from the above, the spherical silver powder produced by this invention exhibits a high primary particle size distribution. Therefore, it is expected that the target powder characteristics can be easily achieved, and pastes capable of handling high-density and fine-line patterns in conductors can also be produced.

Claims

1. A method for manufacturing spherical silver powder, comprising a reduction precipitation step of mixing a reducing agent in an aqueous reaction system containing silver ions to reduce and precipitate silver particles. The reducing agent is either an aqueous solution of hydrazine carbonate obtained by diluting hydrazine carbonate, or an aqueous solution of hydrazine carbonate obtained by blowing carbon dioxide into an aqueous hydrazine solution. The molecular formula of the hydrazine carbonate is (NH₂NH₂)₂·CO₂. In the reduction precipitation process, the temperature of the aqueous reaction system containing the silver ions when the reducing agent is mixed is 10–50°C.

2. The method for manufacturing spherical silver powder according to claim 1, wherein, The amount of hydrazine carbonate in the reducing agent mixed in the reduction precipitation process is 1 to 6 molar equivalents relative to silver.

3. The method for manufacturing spherical silver powder according to claim 1, wherein, The aqueous reaction system containing the silver ions is a silver ammonia complex. The silver ammonia complex is prepared by adding ammonia or an ammonium salt to an aqueous solution containing at least one of silver nitrate, a silver complex, and a silver intermediate.

4. The method for manufacturing spherical silver powder according to claim 2, wherein, The aqueous reaction system containing the silver ions is a silver ammonia complex. The silver ammonia complex is prepared by adding ammonia or an ammonium salt to an aqueous solution containing at least one of silver nitrate, a silver complex, and a silver intermediate.

5. The method for manufacturing spherical silver powder according to any one of claims 1 to 4, wherein, The cumulative 50% particle size (D50) of the SEM primary particle size of the obtained spherical silver powder was 0.1–1.5 μm, and the coefficient of variation in the particle size distribution was below 0.2.

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