Method for manufacturing silver powder

Abstract

To provide a silver powder production method of, when silver powder is pasted to execute electrode formation, making the same into low resistance without changing the kind of a surface treatment agent.SOLUTION: In a silver powder production process, a slurry of silver powder is added with a micelle-containing O / W type emulsion of a surface treatment agent in which a cumulative 50% particle diameter D50 on a volume standard obtained by a laser diffraction type particle size distribution measurement method is 1.5 μm or less, and the surface of the silver powder is coated with the surface treatment agent is thereafter further coated with polyvalent carboxylic acid to obtain silver powder in which an electrode is made into low resistance when the silver powder is pasted to execute electrode formation.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for producing silver powder, and more particularly to a method for producing silver powder suitable for use in conductive pastes for forming electrical conduction paths in elements such as electrodes and circuits of various electronic components. [Background technology]

[0002] Traditionally, resin-molded and sintered silver pastes have been widely used to form electrodes and circuits in electronic components. In recent years, conductive pastes using silver powder have been required to achieve higher density of conductor patterns and thinner wiring due to the miniaturization of electronic components. Furthermore, thinner finger electrodes are required to increase the light-collecting area of ​​solar cells and improve power generation efficiency. If coarse particles are present in the silver powder contained in the silver paste, they can clog the printing plate when forming patterns, causing circuit breaks. Therefore, silver powder free of coarse particles has been sought for conductive pastes. Furthermore, in the formation of electrodes for solar cells, improved conductivity of the electrodes leads to improved conversion efficiency; therefore, there is a demand for improved conductivity of electrodes formed using resin-curing conductive paste.

[0003] As a method for producing silver powder for conductive paste, for example, Patent Document 1 discloses a method for producing silver powder with excellent low-temperature sinterability, in which formalin, a reducing agent, is added to an aqueous solution containing a silver ammine complex, and then a stearic acid emulsion, which acts as a dispersant for the reduced and precipitated silver powder, is added to obtain silver powder. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2006-002228 [Overview of the project] [Problems that the invention aims to solve]

[0005] In the method for producing silver powder disclosed in Patent Document 1, the silver powder is surface-treated with stearic acid, a dispersant, to improve the dispersibility of the reductively precipitated silver powder, but the stearic acid is added in emulsion form. In the manufacturing method disclosed in Patent Document 1, fatty acids such as stearic acid, which are surface treatment agents, are added in emulsion form because the melting point of these fatty acids is high and they are poorly soluble in water, making it difficult to uniformly disperse them in an aqueous solution containing dispersed silver powder. Therefore, the fatty acids are emulsified beforehand, that is, an emulsion in which fine fatty acid micelles are dispersed in water is created, and this emulsion is added to a slurry containing silver powder. However, it was found that the manufacturing method disclosed in Patent Document 1 cannot completely prevent the generation of coarse secondary silver particles.

[0006] The sinterability of silver paste is greatly influenced by the surface condition of the silver powder, particularly the surface treatment agents adhering to its surface. Furthermore, the combination of organic solvents, organic resin binders, and various additives that make up the silver paste must be changed when the surface treatment agents adhering to the silver powder are altered. Therefore, it is desirable to improve the dispersibility of the silver powder without changing the type of surface treatment agent, thereby suppressing the formation of coarse silver powder particles. In this regard, the inventors have found that reducing the particle size of the surface treatment agent micelles contained in the emulsion of the surface treatment agent improves the dispersibility of silver powder in a slurry containing reduced-precipitation silver powder, and have filed a Japanese Patent Application No. 2021-047972. However, the conductive film formed using silver particles obtained by the manufacturing method disclosed in Japanese Patent Application No. 2021-047972 has a lower volume resistivity than conductive films obtained using conventional silver powder, but it is not necessarily sufficient for the conductivity required in recent years. The technical problem to be solved in this invention is to provide a method for producing silver powder that results in low resistance when the obtained silver powder is made into a paste and used to form electrodes, without changing the type of surface treatment agent used for the silver powder from the conventional method. [Means for solving the problem]

[0007] As a result of intensive research to achieve the above problems, the present inventor has reduced the particle size of the micelles of the surface treatment agent contained in the emulsion of the surface treatment agent added in the silver powder production process, and by attaching the surface treatment agent using the emulsion, when the silver powder coated with the surface treatment agent on its surface is further coated with a polyvalent carboxylic acid, it has been found that the resistance becomes low when the silver powder coated with the polyvalent carboxylic acid is made into a paste to form an electrode. Based on the above findings, the present inventor has completed the present invention described below.

[0008] That is, in the present invention to achieve the above problems, (1) A method for producing silver powder in which silver ions are made into a silver complex with a complexing agent and the silver complex is reduced to obtain silver powder, comprising: a silver complexing step of using ammonium ions as a complexing agent for complexing the silver ions to form an aqueous solution of a silver-ammine complex; a reduction step of adding a reducing agent to the aqueous solution containing the silver complex and reducing the silver complex with the reducing agent to obtain a slurry of silver powder; an emulsion addition step of adding an O / W type emulsion containing micelles of a surface treatment agent having a volume-based cumulative 50% particle diameter D 50 of 1.5 μm or less obtained by a laser diffraction type particle size distribution measurement method to the silver powder slurry to surface-treat the silver powder; a carboxylic acid coating step of coating the silver powder coated with the surface treatment agent in the emulsion addition step with a polyvalent carboxylic acid; A method for producing silver powder is provided. (2) In the production method of (1) above, it is preferable that the polyvalent carboxylic acid is one or more selected from the group consisting of adipic acid, succinic acid, diglycolic acid, glutaric acid, and maleic acid. (3) In the production method of (1) or (2) above, the surface treatment agent contained in the emulsion is preferably a fatty acid having a linear carbon number of 8 or more. (4) In the manufacturing methods described in (1) to (3) above, the surface treatment agent contained in the emulsion is preferably a long-chain fatty acid having 12 or more carbon atoms. (5) In the manufacturing methods described in (1) to (4) above, the surface treatment agent contained in the emulsion may be one or two selected from the group consisting of palmitic acid and stearic acid. (6) In the manufacturing methods described in (1) to (4) above, the surface treatment agent contained in the emulsion may be linoleic acid or linolenic acid. [Effects of the Invention]

[0009] By using the manufacturing method of the present invention, it is possible to obtain silver powder that exhibits low resistance when the silver powder is paste-formed into an electrode, without changing the type of surface treatment agent for the silver powder. [Brief explanation of the drawing]

[0010] [Figure 1] This figure compares the measurement results of the particle size distribution of micelles contained in the emulsions obtained in Example 1 and Comparative Example 1. [Figure 2] This figure compares the measurement results of the particle size distribution of micelles contained in the emulsions obtained in Example 2 and Comparative Example 2. [Figure 3] This figure compares the measurement results of the particle size distribution of micelles contained in the emulsions obtained in Example 3 and Comparative Example 3. [Modes for carrying out the invention]

[0011] [Starting materials] In the method for producing silver powder of the present invention, an aqueous solution containing silver(1) ions is used as a starting material, to which a complexing agent is added to form a silver complex. As a source of silver ions, known inorganic silver salts used industrially, such as silver(1) nitrate, silver(1) sulfate, silver(1) carbonate, silver(1) chloride, and silver(1) oxide, can be used. Although not specifically defined in this invention, the silver ion concentration in the aqueous solution is preferably 0.1% by mass or more and 10% by mass or less before the addition of the reducing agent described later. A silver ion concentration of less than 0.1% by mass is undesirable because it reduces the amount of silver powder that can be produced in one reaction. Furthermore, a silver ion concentration exceeding 10% by mass is undesirable because it increases the viscosity of the reaction solution after the deposition of silver particles, making it difficult to uniformly stir the reaction solution. [Complexing agent] As complexing agents for silver ions, ammonium ions such as aqueous ammonia and ammonium salts, and chelate compounds such as salts of ethylenediaminetetraacetic acid (EDTA) can be used. However, it is preferable to use ammonium ions because they readily form complexes with silver ions, are easy to wash, and do not leave behind many impurities. When ammonium ions are used as complexing agents, a silver-ammine complex is formed in aqueous solution. In this case, since the coordination number of the ammine complex is 2, at least 2 moles of ammonium ions are added per mole of silver ions. [Complex formation aid] Furthermore, azoles such as benzotriazole or its salts, or oxycarboxylic acids such as citric acid, may be added before the addition of the reducing agent described later, as additives to assist in the formation of the silver complex.

[0012] [Reducing agent] In the method for producing silver powder of the present invention, known reducing agents can be used to reduce the silver complex and precipitate metallic silver. Examples of reducing agents include formalin, ascorbic acid, hydrazine, alkanolamines, hydroquinone, oxalic acid, formic acid, aldehydes, alcohols, lower oxides of metals such as organic substances and sugars, and sodium borohydride. However, it is preferable to use one or more of ascorbic acid, glucose, formaldehyde, hydrazine, and hydrazine carbonate, as they are relatively stable in reactivity and can rapidly reduce silver. In particular, it is preferable to use formaldehyde, hydrazine, or hydrazine carbonate. The amount of reducing agent added is preferably 1 equivalent or more relative to the silver to increase the silver yield. If a reducing agent with weak reducing power is used, it may be 2 equivalents or more relative to the silver, for example, 10 to 20 equivalents. Regarding the method of adding the reducing agent, it is preferable to add it at a rate of 1 equivalent / min or more relative to the amount of silver ions in order to prevent aggregation of the reduced and precipitated silver powder. Furthermore, during reduction, it is preferable to stir the silver-ammine complex aqueous solution and the reaction solution after the deposition of silver particles from before the addition of the reducing agent until the end of the reduction deposition process. In addition, the temperature when adding the reducing agent and reducing and precipitating the silver particles is preferably between 5°C and 80°C, and more preferably between 5°C and 40°C.

[0013] [Surface treatment agent] In the method for producing silver powder of the present invention, the silver powder precipitated by reduction is treated with a surface treatment agent to improve its dispersibility. A hydrophobic dispersant is preferred as the surface treatment agent, and fatty acids or salts thereof can be used. By using fatty acids or salts thereof, it is possible to achieve both the adsorption of the surface treatment agent to silver and the dispersibility of silver particles. Examples of fatty acids (number of carbon atoms in parentheses) include propionic acid (3), caprylic acid (8), lauric acid (12), myristic acid (14), palmitic acid (16), stearic acid (18), behenic acid (22), acrylic acid (3), oleic acid (18), linoleic acid (18), linolenic acid (18), and arachidonic acid (20). However, in the present invention, it is preferable to use fatty acids with 8 or more carbon atoms in a straight chain, more preferably long-chain fatty acids with 12 or more carbon atoms, even more preferably long-chain fatty acids with 16 or more carbon atoms, and most preferably with 20 or fewer carbon atoms. As a surface treatment agent, it is particularly preferable to use one or both of palmitic acid (melting point: 62.9°C) and stearic acid (melting point: 69.6°C). Furthermore, linoleic acid and linolenic acid are particularly preferable because they reduce the viscosity when the mixture is pasteurized. Fatty acids with fewer than 8 carbon atoms are water-soluble, thus reducing the need for emulsification, and also exhibit weak adsorption of surface treatment agents to silver. Fatty acids with 12 or more carbon atoms are chosen because they easily provide the dispersibility required for silver powder. Palmitic acid and stearic acid are readily available. Fatty acids with more than 20 carbon atoms are difficult to adjust in terms of viscosity during paste formation. Note that commercially available fatty acids as exemplified above may contain other fatty acids. For example, stearic acid reagents typically contain other fatty acids that are difficult to separate during the manufacturing process, rather than being 100% stearic acid by mass. Therefore, the fatty acids used for emulsification only need to contain 50% or more of the fatty acid as the main component (also known as a purity of 50% or more) as determined by GC-MS analysis; other fatty acids may be present in addition to the main component.

[0014] [Emulsion] Many of the fatty acids used as surface treatment agents are solid at room temperature and poorly soluble in water. Therefore, they are first emulsified into a liquid state and added to the slurry of silver powder precipitated by reduction, thereby adhering the surface treatment agent to the surface of the silver particles. It is preferable to use a surfactant during emulsification. Examples of surfactants include Kao Corporation's Leodol TW-P120, Emulgen 350, and Emulgen 120. During emulsification, the fatty acids form micelles and disperse in the aqueous solution as fine droplets. The concentration of fatty acids in the emulsion is preferably 0.1% by mass or more and less than 50% by mass. Furthermore, the concentration of fatty acids in the emulsion after dilution, similar to when they are added to the aqueous solution during the production of silver powder, is preferably 0.1% by mass or more and 5% by mass, and more preferably 1% by mass or more and 5% by mass or less. The present invention provides a method for producing silver powder, which involves the volume-based cumulative 50% particle size D of micelles containing fatty acids and surfactants in a pre-prepared emulsion. 50 It is characterized by having a particle size of 1.5 μm or less. Cumulative 50% particle size D 50 The particle size is preferably 1.0 μm or less, and the cumulative 50% particle size D 50is more preferably 0.7 μm or less, and even more preferably 0.4 μm or less in order to enhance the effect of reducing coarse particles. In the present invention, the cumulative 50% particle size D 50 of the micelles has no particular lower limit, but is, for example, 1 nm or more. The method for measuring the cumulative 50% particle size D 50 of the micelles will be described later. When measuring the volume-based cumulative 50% particle size D 50 of the micelles contained in a commercially available stearic acid emulsion described in Patent Document 1, it was 4.0 μm. In the method for producing silver powder of the present invention, by making the particle size of the micelles smaller than before, when an emulsion containing a surface treatment agent is added to a slurry containing silver powder, the speed at which the emulsion disperses in the slurry becomes faster, and it becomes possible to uniformly adhere the surface treatment agent to the surface of the silver particles in the slurry, and the ability to suppress aggregation of the silver particles is improved. Further, even when the silver powder is made into a conductive paste, compared with the case where a surface treatment agent having a conventionally large size adheres to the surface of the silver particles, when a surface treatment agent having a small size adheres, it is more compatible with the solvent, resin, additive, etc. used in the conductive paste, and as a result, it is considered that this leads to a reduction in the resistance of the electrode using the conductive paste.

[0015] [Method for Preparing Emulsion] Specifically, the following three methods can be mentioned as the method for preparing the above emulsion. (1) Using a commercially available emulsion (starting emulsion: Cellosolve 920 manufactured by Chukyo Yushi Co., Ltd. satisfies this condition) in which the volume-based cumulative 50% particle size D 50 of the micelles of the surface treatment agent is 2 μm or more as a starting material, heating it to a temperature not lower than the melting point of the surface treatment agent contained in the emulsion and not lower than the temperature at which the surfactant separates from the surface treatment agent, stirring it by a known stirring means, and cooling it after the volume-based cumulative 50% particle size D 50 of the micelles of the surface treatment agent becomes 1 μm or less. In that case, it is preferable to use a homogenizer as the stirring means. (2) The volume-based cumulative 50% particle size D 50Starting with a commercially available emulsion (original emulsion) with a particle size of 2 μm or larger, the mixture is held for 1 minute or more at a temperature below the melting point of the surface treatment agent contained in the emulsion, and at a temperature at which the surfactant separates from the surface treatment agent. After confirming that a solid of the surface treatment agent has formed in the liquid, it is heated above the melting point of the surface treatment agent and stirred using a known stirring method to obtain the cumulative 50% particle size D of the micelles of the surface treatment agent based on volume. 50 Cooling is performed after the particle size becomes 1 μm or less. In this case, it is preferable to use a homogenizer as the stirring means. Furthermore, the preparation method described in (2) can yield an emulsion with smaller micelle particle sizes. (3) Add a surfactant to the fatty acid which is the surface treatment agent described above, melt it at a temperature above the melting point of each, then add boiling water to prevent solidification, and stir using a known stirring method to obtain the cumulative 50% particle size D of the micelles of the surface treatment agent by volume. 50 Cooling is performed after the particle size becomes 1 μm or less. In this case, it is preferable to use a homogenizer as the stirring means. When emulsifying surface treatment agents such as fatty acids, micelles are formed by bonding the fatty acids with a surfactant. The temperature at which the nonionic surfactant, which has bonded to the surface treatment agent to form micelles, separates from the surface treatment agent is called the cloud point. Here, the temperature at which the nonionic surfactant separates from the surface treatment agent is the temperature at which, as the temperature of the emulsion is increased, the surface treatment agent whose bond with the nonionic surfactant has broken begins to be observed (in Example 2 below, this is the temperature at which the solid fatty acid begins to be visible). In the case of the aforementioned surfactant, the cloud point is approximately 30-80°C, and the cloud point of Cellosol in the example below is 60°C. A commercially available homogenizer can be used. Preferably, the homogenizer has a structure in which the tip of the shaft consists of a fixed outer blade and a rotating inner blade, and can finely crush and homogenize the material through the effects of ultrasound, high frequency, etc., that occur between the window between the inner and outer blades. For example, a Biomixer (model: BM-4) manufactured by Nippon Seiki Seisakusho can be used. The stirring conditions using the homogenizer described above depend on the amount of emulsion being processed, but for example, for a volume of emulsion of 50 mL, the rotation speed of the homogenizer's inner blade is preferably 7000 rpm or higher, and more preferably 10000 rpm or higher. Furthermore, the stirring time using the homogenizer is preferably 10 seconds or higher, and more preferably 1 minute or higher. This is because if the amount of stirring (rotation speed × time) relative to the volume of liquid is small, it becomes more difficult to reduce the micelle particle size. Furthermore, the above-mentioned cooling method can be a standard cooling method; for example, it is sufficient to cool it to room temperature or the liquid temperature of the slurry containing silver particles in the silver powder manufacturing method described later, by letting it cool at room temperature, or by using the temperature control function of a hot bath to slow cooling or water cooling.

[0016] [Method for measuring the particle size distribution of micelles contained in emulsions] The particle size distribution of micelles of the surface treatment agent contained in the emulsion is measured using a laser diffraction particle size distribution analyzer (e.g., Microtrac MT3300EXII manufactured by Nikkiso Co., Ltd.). The dispersion medium is pure water, and volume-based particle size distribution measurement is performed. The volume-based cumulative 10% particle size (D) is automatically calculated by the aforementioned analyzer. 10 ), cumulative 50% particle size (D 50 ), cumulative 90% particle size (D 90 ), and cumulative 95% particle size (D 95 ), maximum particle size (D max Use ). The steepness of the particle size distribution is (D 90 -D 10 ) / D 50 Evaluate using ratios. When measuring the particle size distribution, it is preferable to measure the emulsion diluted with pure water to the same dilution ratio (e.g., 10 times) as when it is added to the aqueous solution during the production of silver powder, as described later.

[0017] [Method for manufacturing silver powder] In an embodiment of the present invention's method for producing silver powder, ammonium ions are added to an aqueous solution containing silver ions to form a silver-ammine complex (silver complex formation step), and a reducing agent is added to the resulting aqueous solution of the silver-ammine complex to reduce and precipitate silver particles (reduction step). After reducing and precipitating the silver particles with the reducing agent, the cumulative 50% particle size D of micelles of the surface treatment agent is added to the slurry containing the silver particles. 50 An emulsion with a particle size of 1.5 μm or less is added to adhere the surface treatment agent to the surface of the silver particles (emulsion addition step). The amount of fatty acids in the added emulsion is preferably 0.1% by mass or more and 1.2% by mass or less relative to the amount of silver in the silver-ammine complex aqueous solution. If the amount of fatty acids is less than 0.1% by mass relative to the amount of silver, the frequency of coarse silver particles may increase. Also, if the amount of fatty acids exceeds 1.2% by mass relative to the amount of silver, the frequency of coarse silver particles may increase. More preferably, the amount of fatty acids in the added emulsion is 0.1% by mass or more and 1.0% by mass or less relative to the amount of silver in the silver-ammine complex aqueous solution, and even more preferably 0.8% by mass or less. Any reducing agent that reduces and precipitates silver particles is acceptable, and as mentioned above, one or more of ascorbic acid, glucose, formaldehyde, hydrazine, and hydrazine carbonate can be used, with formaldehyde, hydrazine, or hydrazine carbonate being preferred.

[0018] Preferably, after reducing and precipitating the silver particles, the silver-containing slurry containing the surface-treated silver particles is subjected to solid-liquid separation by adding an emulsion containing micelles of a surface treatment agent, and the resulting solid is washed with pure water to remove impurities from the solid. The end point of this washing can be determined by the electrical conductivity of the water after washing. Preferably, washing is continued until the electrical conductivity of the water after washing is 0.5 mS / m or less. The lump-like cake obtained after washing contains a lot of moisture, so it is preferable to obtain dried silver powder using a dryer such as a vacuum dryer. In this case, the drying temperature is preferably 100°C or lower in order to prevent the silver particles from sintering together during drying. The obtained silver powder may also be subjected to dry crushing treatment or classification treatment. Dry crushing treatment is a crushing treatment carried out for the purpose of crushing aggregates of silver powder that have formed when the particles have aggregated together during drying. There are no particular restrictions on the crushing method, and it can be appropriately selected according to the purpose, but it is preferable to use a crusher that crushes and flows the powder by rotating a stirring blade, for example, a Henschel mixer, sample mill, blender, coffee mill, etc. The rotation speed, time, and number of treatments should be set appropriately so as not to heat the powder as much as possible. When dry crushing treatment is performed, it may also be combined with the carboxylic acid coating process described later.

[0019] [Carboxylic acid coating process] In the manufacturing method of the present invention, the silver powder coated with the surface treatment agent obtained in the emulsion addition step is further coated with a polycarboxylic acid. The reason why the electrode obtained when the silver powder coated with polycarboxylic acid is made into a paste and used to form an electrode is not currently precisely known, but the inventors of the present invention surmise that by attaching a polycarboxylic acid to the surface of the silver powder, volume shrinkage between the silver powder particles during thermal curing is promoted, thereby improving the contact between the silver powder particles. The following are examples of specific embodiments of polycarboxylic acid coating, but the embodiments of polycarboxylic acid coating in the present invention are not limited to these. (1) When performing the dry crushing treatment of silver powder, a polycarboxylic acid dissolved in a solvent that can be air-dried is added and mixed. (2) After washing the silver powder, a polycarboxylic acid is added to the silver powder before drying and mixed, and then dried. (3) After drying, polycarboxylic acid and pure water or alcohol are added to the silver powder to form a slurry, which is then wet-crushed and dried. Of the three embodiments described above, it is preferable to use dry crushing in (1) from the viewpoint of preventing deterioration of polycarboxylic acids due to heating temperature when drying moisture, etc.

[0020] [Polyhydric carboxylic acids] The most significant technical feature of the manufacturing method of the present invention is coating silver powder with a polycarboxylic acid containing two or more carboxyl groups. Examples of polycarboxylic acids include adipic acid, succinic acid, diglycolic acid, glutaric acid, and maleic acid. In particular, it is preferable to use adipic acid, which has the effect of reducing volume resistivity with the addition of a small amount. The amount of polycarboxylic acid to coat the silver powder is preferably 0.01% to 0.5% by mass relative to the mass of the material silver powder, and more preferably 0.02% to 0.4% by mass. This is because if the amount of polycarboxylic acid added is outside the range of 0.01% to 0.5% by mass relative to the mass of the material silver powder, the conductivity-improving effect of the resulting conductive film cannot be sufficiently obtained. In one embodiment of this invention, when adding polycarboxylic acid to silver powder material for coating, the polycarboxylic acid is added to the silver powder material in a dissolved state in a solvent. The concentration of the dissolved polycarboxylic acid at this time is preferably 1% to 20% by mass. This is because if the dissolution concentration of the polycarboxylic acid is less than 1% by mass, the amount of solution will be large, and the solution may become unevenly distributed during drying to remove the solvent, potentially preventing the polycarboxylic acid from uniformly coating the silver powder material. Conversely, if the dissolution concentration exceeds 20% by mass, the amount of solution will be insufficient, potentially preventing the polycarboxylic acid from uniformly coating the silver powder material. Furthermore, any solvent capable of dissolving the polycarboxylic acid is acceptable. In the embodiment described in (1) above, a solvent that can be evaporated at room temperature (natural drying is possible), has a boiling point of 83°C or lower, and is volatile is preferable because it facilitates the removal of the solvent after coating. Examples include alcohols, acetone, and ethers such as methyl ethyl ether and ethyl ether. In the embodiment described in (1) above, it is more preferable to select the type and amount of solvent such that the silver powder coated with the polycarboxylic acid is dry when the dry crushing treatment is completed.

[0021] In the case of silver powder material to which polycarboxylic acid has been added, it is preferable to perform dry crushing so that the polycarboxylic acid uniformly coats the material. Dry crushing is performed by placing the silver powder material to which polycarboxylic acid has been added into, for example, a Henschel mixer, sample mill, blender, coffee mill, etc. Then, if necessary, the solvent used to add the polycarboxylic acid is evaporated by frictional heat generated by crushing or by a drying process. This results in silver powder material coated with polycarboxylic acid. When silver powder coated with a surface treatment agent by the emulsion addition process is further coated with polycarboxylic acid, although the state of adhesion between the silver and the surface treatment agent on the surface of the silver powder is not certain, it is thought that the surface treatment agent and the polycarboxylic acid coexist on the silver surface.

[0022] [Particle size distribution measurement] The particle size distribution of silver powder was determined by first crushing the silver powder using a sample mill SK-M10 manufactured by Kyōritsu Riko Co., Ltd., then dispersing 0.1 g of silver powder in isopropyl alcohol (IPA), stirring for 2 minutes using an ultrasonic homogenizer (model: US-150T) manufactured by Nippon Seiki Seisakusho, and finally measuring the particle size distribution using a laser diffraction particle size analyzer (Microtrac MT3300EXII manufactured by Nikkiso Co., Ltd.). [Specific surface area measurement] The specific surface area of ​​the silver powder is determined by the BET method. The BET method can be used to measure the specific surface area using a specific surface area measuring device. In this embodiment, the value measured using the Macsorb HM-model 1210 manufactured by Mountec Co., Ltd. is used as the specific surface area measuring device for the BET method. In this embodiment, the specific surface area is measured by the BET single-point method after degassing by passing a He-N2 mixed gas (30% nitrogen) through the measuring device at 60°C for 10 minutes.

[0023] [Method for manufacturing silver paste] The silver powder obtained in each of the examples and comparative examples described later was mixed with flake silver powder (FA-S-20 manufactured by DOWA High-Tech Co., Ltd.) in a ratio of 4:6, resulting in a total of 92.6% by mass of silver powder, 4.88% by mass of epoxy resin, 0.24% by mass of curing agent (boron trifluoride monoethylamine complex manufactured by Wako Pure Chemical Industries), and 2.28% by mass of solvent (BCA: butyl carbitol acetate). This mixture was stirred at 1200 rpm for 30 seconds using a propellerless self-rotating stirring and defoaming device (VMX-N360 manufactured by EME Corporation), and then kneaded using a three-roll machine (80S manufactured by EXAKT Corporation), passing the mixture through a roll gap from 100 μm to 20 μm to obtain a conductive paste (silver paste). After preparation, the viscosity was measured and adjusted using a BCA (Brookfield DV-3, CP-52 cone) viscometer at a cone rotation speed of 1 rpm to achieve a viscosity of 300 Pa·s (viscosity measured after 5 minutes at 25°C).

[0024] [Screen Printing] Using the conductive paste obtained by the above procedure, a line pattern with a width of 500 μm and a length of 37.5 mm was printed on the conductive paste using a screen printing machine (Microtec MT-320T) at a squeegee pressure of 0.18 MPa to form a conductive paste film. The obtained film was heat-cured in an air-circulating dryer at 200°C for 30 minutes to form a conductive film. For each of the obtained conductive films, the average thickness of the conductive film was determined by measuring the step difference between the unprinted area and the conductive film area on the alumina substrate using a surface roughness meter (Tokyo Seimitsu Co., Ltd. Surfcom 480B-12). Meanwhile, the resistance value of each conductive film was measured using a digital multimeter (ADVANTEST R6551). The volume of the conductive film was determined from the size (film thickness, width, length), and the volume resistivity was calculated from this volume and the measured resistance value. [Examples]

[0025] [Comparative Example 1] 5g of a commercially available stearic acid emulsion (Cellosol 920, manufactured by Chukyo Oil & Fat Co., Ltd., containing 82% water), an example of a commercially available emulsion, was mixed with pure water to make a total volume of 50mL (10-fold dilution) in accordance with the dilution ratio used in silver powder manufacturing described later. The particle size distribution of micelles contained in the emulsion was then measured using a Microtrac MT3300EXII manufactured by Nikkiso Co., Ltd. The measurement results of the particle size distribution are shown in Figure 1. Cumulative 10% particle size by volume of micelles contained in a 10-fold diluted emulsion D 10 This is 0.8 μm, cumulative 50% particle size D 50 This is 4.0 μm, cumulative 90% particle size D 90 The size was 9.8 μm. The measurement results are shown in Table 1. To 3375 g of an aqueous silver nitrate solution containing 45.3 g of silver, 3.3 g of a 60% aqueous nitric acid solution was added, and 76.5 g of 28% by mass industrial ammonia water (corresponding to 1.5 molar equivalents of ammonia per mole of silver) was added to obtain an aqueous silver-ammine complex solution. After adjusting the temperature of this aqueous silver-ammine complex solution to 35°C, while stirring, an aqueous solution of sodium benzotriazole was added to the aqueous silver-ammine complex solution so that the amount of sodium benzotriazole relative to silver was 0.5% by mass. After stirring, an aqueous solution of 12.5 g of 80% by mass hydrated hydrazine diluted with 130.2 g of pure water was added to reduce the silver-ammine complex and obtain a slurry containing silver particles. Furthermore, to the obtained slurry containing silver particles, 17.54 g of the above-mentioned stearic acid emulsion diluted 10 times was added, and stirring was stopped to allow the surface-treated silver particles to settle. The liquid containing the precipitated silver particles was filtered, washed with water until the electrical conductivity of the liquid after passing water through was 0.2 mS / m or less, and then vacuum-dried at 73°C. After crushing using a sample mill SK-M10 manufactured by Kyōritsu Riko Co., Ltd., a 10% by mass adipic acid ethanol solution was added so that the amount of adipic acid was 0.07% by mass relative to the silver powder. Using a sample mill SK-M10 manufactured by Kyōritsu Riko Co., Ltd., the mixture was mixed twice for 45 seconds each time to ensure that the adipic acid ethanol solution added to 120 g of silver powder was evenly mixed. The silver powder, which had been coated with a surface treatment agent using stearic acid emulsion, was further coated with adipic acid to obtain the silver powder relating to Comparative Example 1. The particle size distribution and specific surface area of ​​the obtained silver powder were measured. Then, a silver paste was prepared as described above and its volume resistivity was measured. The measurement results are shown in Table 2.

[0026] [Example 1] When 5g of the above-mentioned commercially available stearic acid emulsion was placed in a 100mL beaker and heated to 62.8°C in a water bath (constant temperature water bath), the gradual formation of a white solid was observed. This is because the cloud point of the emulsifier contained in Cellosol is 60°C. The surface treatment agents contained in the above-mentioned commercially available stearic acid emulsion are the fatty acids stearic acid (melting point: 69.6°C) and palmitic acid (melting point: 62.9°C). Since the mixture was heated at a temperature below the melting point of these fatty acids, the bonds between these fatty acids and the surfactant were broken, and the white solid solidified. The white suspension is thought to be a suspension containing the surfactant that covered the surface of the fatty acids during micelle formation, the fatty acid solid, and the solvent water. After 2 minutes from heating, no further formation of the white solid occurred in the white suspension, so it was held in that state for 5 minutes. Subsequently, the mixture was heated to 95°C on a heater, melting the white solid stearic acid and palmitic acid to form oil droplets. With the oil droplets of the fatty acids floating in the white suspension, 10 mL of boiling water was added using a pipette, followed by the addition of boiling water until the total volume reached 50 mL. The beaker was then transferred to an 80°C water bath (constant temperature water bath), and the mixture was stirred at 10,000 rpm for 1.5 minutes using the homogenizer. After removing it from the water bath, it was allowed to cool for 6 hours to room temperature to obtain the emulsion of Example 1. The particle size distribution of the micelles contained in the obtained emulsion was measured, and the results are shown in Figure 1. For comparison, the measurement results of Comparative Example 1 are also shown in Figure 1. Cumulative 10% particle size D based on volume of micelles in Example 1 10 This is 0.1 μm, volume-based cumulative 50% particle size D 50 This is 0.2 μm, cumulative 90% particle size D 90 The particle size was 0.5 μm, and a finely milled emulsion was obtained. The measurement results are shown in Table 1. The particle size distribution of this emulsion was measured two weeks after preparation, but the particle size distribution had hardly changed. Silver powder according to Example 1 was obtained using the same procedure as in Comparative Example 1, except that 17.54 g was taken from the emulsion obtained by the procedure described above. The particle size distribution and specific surface area of ​​the obtained silver powder were measured. Then, a silver paste was prepared as described above, and its volume resistivity was measured. The measurement results are shown in Table 2.

[0027] [Comparative Example 2] Distribute 1.75g ​​of oleic acid (manufactured by NOF Corporation, NAA-34), a surface treatment agent, and 0.525g of surfactant (manufactured by Kao Corporation, Emulgen 350) into a 100mL beaker and heat to approximately 80°C to dissolve the surfactant. Add boiling water until the total volume reaches 50mL, and stir at 3000rpm for 1.5 minutes using a biomixer (model: BM-4) manufactured by Nippon Seiki Seisakusho, then cool to room temperature and process the micelles. 50 This yielded an oleic acid emulsion with a particle size of 4.00 μm. The particle size distribution of micelles contained in the obtained emulsion was measured and the results are shown in Figure 2. Cumulative 10% particle size D based on volume of micelles in Comparative Example 2 10 This is 1.26 μm, cumulative 50% particle size D 50 This is 4.00 μm, cumulative 90% particle size D 90 The size was 10.1 μm. The measurement results are shown in Table 1. Silver powder for Comparative Example 2 was obtained using the same procedure as in Comparative Example 1, except that 7.77 g of the oleic acid emulsion was used instead of a 10-fold diluted stearic acid emulsion, and a 60% nitric acid aqueous solution was not added. The particle size distribution and specific surface area of ​​the obtained silver powder were measured. Then, a silver paste was prepared as described above, and its volume resistivity was measured. The measurement results are shown in Table 2.

[0028] [Example 2] Except for setting the biomixer stirring conditions to 10,000 rpm for 1.5 minutes, the procedure was the same as in Comparative Example 2, and the micelles were D 50A 0.15 μm oleic acid emulsion was obtained. The particle size distribution of micelles contained in the obtained emulsion was measured, and the results are shown in Figure 2. For comparison, the measurement results for Comparative Example 2 are also shown in Figure 2. Cumulative 10% particle size D based on volume of micelles in Example 2 10 This is 0.09 μm, volume-based cumulative 50% particle size D 50 This is 0.15 μm, cumulative 90% particle size D 90 The particle size was 0.27 μm, and a finely milled emulsion was obtained. The measurement results are shown in Table 1. The particle size distribution of this emulsion was measured two weeks after preparation, but the particle size distribution had hardly changed. Silver powder according to Example 2 was obtained using the same procedure as in Comparative Example 2, except that the emulsion obtained by the above procedure was used. The particle size distribution and specific surface area of ​​the obtained silver powder were measured. Then, a silver paste was prepared as described above, and its volume resistivity was measured. The measurement results are shown in Table 2.

[0029] [Comparative Example 3] Distribute 1.75g ​​of linoleic acid (manufactured by Fujifilm Wako Pure Chemical Industries, 88% purity by mass) and 0.525g of surfactant (manufactured by Kao, Emulgen 350) into a 100mL beaker and heat to approximately 80°C to dissolve the surfactant. Add boiling water until the total volume reaches 50mL, and stir at 3000rpm for 1.5 minutes using a biomixer (model: BM-4) manufactured by Nippon Seiki Seisakusho, then cool to room temperature and process the micelles. 50 A linoleic acid emulsion with a particle size of 3.1 μm was obtained. Figure 3 shows the results of particle size analysis of the micelles contained in the obtained emulsion. Cumulative 10% particle size D of emulsion micelles in Comparative Example 3 10 This is 0.97 μm, cumulative 50% particle size D 50 The particle size is 3.1 μm, with a cumulative 90% particle size of D. 90 The size was 10.1 μm. The measurement results are shown in Table 1. Silver powder for Comparative Example 3 was obtained using the same procedure as for Comparative Example 1, except that 8.83 g of the linoleic acid emulsion was used instead of a 10-fold diluted stearic acid emulsion, and a 60% nitric acid aqueous solution was not added. The particle size distribution and specific surface area of ​​the obtained silver powder were measured. Then, a silver paste was prepared as described above, and its volume resistivity was measured. The measurement results are shown in Table 2.

[0030] [Example 3] Except for setting the biomixer stirring conditions to 10,000 rpm for 1.5 minutes, the procedure was the same as in Comparative Example 3, and the micelles were D 50 A linoleic acid emulsion with a particle size of 0.15 μm was obtained. The particle size distribution of micelles contained in the obtained emulsion was measured, and the results are shown in Figure 3. For comparison, the measurement results for Comparative Example 3 are also shown in Figure 3. Cumulative 10% particle size D based on volume of micelles in Example 3 10 This is 0.09 μm, volume-based cumulative 50% particle size D 50 This is 0.15 μm, cumulative 90% particle size D 90 The particle size was 0.24 μm, and a finely milled emulsion was obtained. The measurement results are shown in Table 1. The particle size distribution of this emulsion was measured two weeks after preparation, but the particle size distribution had hardly changed. Silver powder according to Example 3 was obtained using the same procedure as in Comparative Example 3, except that the emulsion obtained in the above procedure was used. The particle size distribution and specific surface area of ​​the obtained silver powder were measured. Then, a silver paste was prepared as described above, and its volume resistivity was measured. The measurement results are shown in Table 2.

[0031] [Table 1]

[0032] [Table 2]

[0033] Based on the above results, the cumulative 50% particle size D of the present invention in Examples 1-3 50 It was found that silver powder obtained by first producing silver powder using an O / W type emulsion containing micelles of a surface treatment agent with a size of 1.5 μm or less, and then coating it with a polycarboxylic acid, exhibits low resistance when the resulting silver powder is formed into a paste and used to form electrodes.

[0034] In Comparative Example 1 and Example 1, silver powder was obtained using the same procedure as in Comparative Example 1 and Example 1, except that the adipic acid ethanol solution was not added and mixing of the adipic acid ethanol solution using the SK-M10 sample mill was not performed. The volume resistivity of the conductive film was then confirmed. As a result, the volume resistivity in Comparative Example 1 without adipic acid was 28.5 μΩ·cm, while in Comparative Example 1 with adipic acid added it decreased to 22.7 μΩ·cm. Similarly, in Example 1 without adipic acid added it was 21.4 μΩ·cm, while in Example 1 with adipic acid added it decreased to 18.9 μΩ·cm. In other words, it was found that adding adipic acid resulted in a lower volume resistivity than not adding adipic acid.

[0035] [Table 3]

Claims

1. A method for producing silver powder, which involves forming a silver complex with a complexing agent using silver ions and then reducing the silver complex to obtain silver powder, A silver complexation step is performed in which ammonium ions are used as a complexing agent to complex the silver ions, and an aqueous solution of the silver-ammine complex is formed. A reduction step is to add a reducing agent to an aqueous solution containing the silver complex, and reduce the silver complex with the reducing agent to obtain a silver powder slurry. The aforementioned silver powder slurry is subjected to a volume-based cumulative 50% particle size D obtained by laser diffraction particle size distribution analysis. ５０ The process involves adding an emulsion containing micelles of a surface treatment agent with a size of 1.5 μm or less to surface-treat the silver powder, and performing an emulsion addition step. The aforementioned emulsion addition step involves further coating the silver powder coated with a surface treatment agent with a polycarboxylic acid in a carboxylic acid coating step, A method for producing silver powder, including the method described above.

2. The method for producing silver powder according to claim 1, wherein the polycarboxylic acid is one or more selected from adipic acid, succinic acid, diglycolic acid, glutaric acid, and maleic acid.

3. The method for producing silver powder according to claim 1 or 2, wherein the surface treatment agent contained in the emulsion is a fatty acid with eight or more carbon atoms in a straight chain.

4. The method for producing silver powder according to claim 3, wherein the surface treatment agent contained in the emulsion is a long-chain fatty acid having 12 or more carbon atoms.

5. The method for producing silver powder according to claim 4, wherein the surface treatment agent contained in the emulsion is one or two selected from palmitic acid and stearic acid.

6. The method for producing silver powder according to claim 4, wherein the surface treatment agent contained in the emulsion is one or two selected from linoleic acid and linolenic acid.

Images