Method for producing silver powder

Abstract

To provide a method for producing silver powder by a wet reduction process, the method enabling production of silver powder which contains silver fine particles having internal voids and smooth surfaces and in which aggregation among the silver fine particles is inhibited, without the need for additional steps such as surface-smoothing treatment.SOLUTION: The present invention provides a method for producing silver powder, comprising a first liquid preparation step for preparing a first alkaline liquid containing silver ions and an amine compound, and a second liquid preparation step for preparing a second liquid by mixing the first liquid and formaldehyde, wherein the amine compound is a compound represented by formula (1): NR1R2R3 (1). [In formula (1), R1 represents a C1-6 hydrocarbon group having a carboxyl group, R2 represents a C1-12 hydrocarbon group having a hydroxyl group, and R3 represents a hydrogen atom, or a C1-12 hydrocarbon group optionally having a substituent.]SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a method for producing silver powder. [Background technology]

[0002] Silver powder is used as a material (filler) for conductive pastes used in electrical contacts such as wiring and electrodes of various electronic components such as solar cells, semiconductors, and capacitors. Patent Document 1 describes silver powder and a method for producing the same. The silver powder described in Patent Document 1 is produced by subjecting silver powder produced by a wet reduction method to a surface smoothing treatment in which particles are mechanically collided with each other, and then removing agglomerates by classification. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2005-240092 A Summary of the Invention [Problem to be solved by the invention]

[0004] The wiring and contacts of electronic components manufactured by applying a conductive paste are usually obtained by applying the conductive paste by printing or the like, and then heating it (typically by firing it).

[0005] In recent years, conductive pastes are required to have the property of being capable of low-temperature firing when obtaining electrodes, etc. In addition, in recent years, wiring, etc., has been made thinner, but thinner wiring makes it more susceptible to breakage, so there is a demand for conductive pastes that can produce thin wiring, etc. that are less likely to break.

[0006] Here, if the silver microparticles used in the conductive paste contain voids, the temperature at which they start to shrink when heated (fired) is faster than when they do not contain voids. Such thermal behavior of silver microparticles containing voids inside is advantageous in that it allows for low-temperature firing. The method disclosed in Patent Document 1 uses formalin as a reducing agent, and silver microparticles containing voids inside are obtained. Furthermore, in order to make the wiring less susceptible to breakage even when it is thinned, that is, to make it possible to thin the wiring, it is desirable that the surface of the silver fine particles is smooth and that aggregation of the silver fine particles is suppressed. In the method disclosed in Patent Document 1, a surface treatment agent such as oleic acid is used in the production of silver powder, and after the surface smoothing treatment, the aggregates are removed by classification. The additional treatments such as the surface smoothing treatment and classification leave room for improvement in terms of the production cost of the silver powder.

[0007] Therefore, an object of the present invention is to provide a method for producing silver powder using a wet reduction method, which contains silver microparticles that have internal voids and a smooth surface, and in which aggregation of the silver microparticles is suppressed, without requiring additional processes such as surface smoothing treatment. [Means for solving the problem]

[0008] As a result of extensive research by the inventors to solve the above-mentioned problems, the inventors have completed the present invention described below.

[0009] That is, the gist of the present invention for solving the above-mentioned problems is as follows.

[0010] [1] A method for producing an alkaline first liquid containing silver ions and an amine compound, comprising: a first liquid preparation step; and a second liquid preparation step of mixing the first liquid with formaldehyde to produce a second liquid, wherein the amine compound is represented by the formula (1): NR 1 R 2 R 3 (1) [In formula (1), R 1 is a hydrocarbon group having 1 to 6 carbon atoms and a carboxyl group; R2 is a hydrocarbon group having 1 to 12 carbon atoms and a hydroxyl group; R 3 is a hydrogen atom or a hydrocarbon group having 1 to 12 carbon atoms which may have a substituent. A method for producing silver powder, which is a compound represented by the formula:

[0011] [2] In the formula (1), R 2 and R 3 The method for producing silver powder according to [1], wherein the total number of hydroxyl groups contained in the silver powder is 2 or more.

[0012] [3] The method for producing silver powder described in [1] or [2], wherein the amine compound does not contain a sulfur atom.

[0013] [4] A method for producing silver powder described in any one of [1] to [3], wherein the amine compound is tricine or bicine.

[0014] [5] A method for producing silver powder described in any one of [1] to [4], wherein the pH of the first liquid is 9 or more and 13 or less.

[0015] [6] A method for producing silver powder as described in [1] to [5], in which the formaldehyde is added all at once to the first liquid while stirring the liquid, and the time required for the pH of the first liquid to decrease by 2 from the start of the addition of the formaldehyde is 15 seconds or more.

[0016] [7] A method for producing silver powder described in any one of [1] to [6], wherein the content of the silver ions in the first liquid is greater than the content of the amine compound. Effect of the Invention

[0017] According to the present invention, a method can be provided that can produce silver powder using a wet reduction method, which contains silver microparticles that have internal voids and smooth surfaces, and in which aggregation of the silver microparticles is suppressed, without requiring additional processes such as surface smoothing treatment. [Brief description of the drawings]

[0018] [Figure 1] 1 is a 5,000x SEM image of the silver powder obtained in Example 1. [Diagram 2] 1 is a 10,000x SEM image of the silver powder obtained in Example 1. [Diagram 3] 1 is a 20,000x SEM image of the silver powder obtained in Example 1. [Figure 4] 1 is a 5,000x cross-sectional SEM image of the silver powder obtained in Example 1. [Diagram 5] 1 is a 10,000x cross-sectional SEM image of the silver powder obtained in Example 1. [Figure 6] 1 is a 20,000x magnification SEM image of a cross section of the silver powder obtained in Example 1. [Figure 7] 1 is a graph showing the particle size distribution of the silver powder obtained in Example 2. [Figure 8] 1 is a graph showing the particle size distribution of the silver powder obtained in Example 3. [Figure 9] 1 is a 5,000x SEM image of the silver powder obtained in Example 3. [Figure 10] 1 is a 10,000x SEM image of the silver powder obtained in Example 3. [Figure 11] 1 is a 20,000x SEM image of the silver powder obtained in Example 3. [Figure 12] 1 is a 5,000x cross-sectional SEM image of the silver powder obtained in Example 3. [Figure 13] 1 is a 10,000x cross-sectional SEM image of the silver powder obtained in Example 3. [Figure 14] 1 is a 20,000x cross-sectional SEM image of the silver powder obtained in Example 3. [Figure 15] 1 is a graph showing the pH change of the first liquid in the second liquid preparation step of Example 4. [Figure 16] 2 is a 20,000x SEM image of the silver powder obtained in Comparative Example 1. [Figure 17] 1 is a graph showing the pH change of the first liquid in the second liquid preparation step of Comparative Example 1. [Figure 18] 2 is a 20,000x SEM image of the silver powder obtained in Comparative Example 2. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] (Terminology and Measurement Methods) Prior to describing the embodiments, the terms and measurement methods used in this specification will be described.

[0020] <Reaction delay time> In this specification, the "reaction delay time" refers to how much longer the time it takes for the first liquid to turn black after adding formaldehyde as a reducing agent to the first liquid (the time until particles are generated) is compared to the time until particles are generated in Comparative Example 1 described below. The time until particle generation was measured by videotaping the addition of the reducing agent to the first liquid with a video camera, adding formaldehyde as a reducing agent, and visually checking the video for the time from the moment the reducing agent came into contact with the first liquid until the entire first liquid turned black (time until particle generation).

[0021] <Particle size distribution> 0.1 g of silver powder was added to 40 mL of isopropyl alcohol (IPA) and dispersed for 2 minutes using an ultrasonic homogenizer (US-150T, 19.5 kHz, manufactured by Nippon Seiki Co., Ltd.) with a tip diameter of 18 mm. The volumetric particle size distribution of the silver powder was determined in total reflection mode (by wet laser diffraction / scattering particle size distribution measurement) using the obtained sample using a laser diffraction / scattering particle size distribution analyzer (MICROTRAC MT3300EXII, manufactured by Microtrac-Bell Corporation).

[0022] <Specific surface area> A BET specific surface area measuring device (Macsorb HM-model 1210, Mountec Co., Ltd.) was used, and He-N was placed in the measuring device at 60°C for 10 minutes. 2 After degassing by flowing a mixed gas (nitrogen 30% by volume), the material was measured by the BET one-point method.

[0023] <Ignition loss (Ig-loss)> The ignition loss value of the silver powder was measured based on the mass loss of the sample after heating the silver powder sample. Specifically, the silver powder sample was precisely weighed (weight: w1), placed in a magnetic crucible, and heated to 800°C. Then, it was heated at 800°C for 30 minutes, which was a sufficient time to reach a constant weight. It was then cooled and reweighed (weight: w2). The weights w1 and w2 were then substituted into the following formula a to determine the ignition loss value. In the examples of this specification described later, the weight w1 was set to 2 g. Ignition loss (mass%) = (w1-w2) / w1 × 100 (a)

[0024] <pH of first solution> The pH of the first liquid was measured as follows. First, three types of standard solutions with pH values ​​of 10.23, 10.51, and 10.83 were prepared by appropriately adding a 20% by mass aqueous sodium hydroxide solution or a 61% by mass aqueous nitric acid solution to the first standard solution. The pH measurements were performed at 25°C using a pH meter and electrode (HORIBA, D-73). These three types of standard solutions were colored by adding 0.1 ml of Alizarin Yellow R (Tokyo Chemical Industry, colored). Next, light (covering the wavelength band of 250 to 800 nm) from a halogen light source (HL-2000-LL, Ocean Photonics) was passed through the colored solutions, and the transmitted light was measured by spectrometry using a spectrometer (FLAME-S, Ocean Photonics), and the ratio of absorbance at two specific wavelengths (381 nm and 460 nm) was calculated for each standard pH solution. From these results, a calibration curve of pH-absorbance ratio was created. Then, 0.1 ml of Alizarin Yellow R (Tokyo Chemical Industry Co., Ltd., colored) was added to the first liquid to be measured to color the solution, and the absorbance ratio was determined in the same manner as above. The pH of the first liquid to be measured was calculated by applying this to the created calibration curve.

[0025] <Add all at once> In this specification, the term "add all at once" means adding the first liquid to the second liquid all at once without taking time, rather than gradually or intermittently adding the first liquid to the second liquid over time, for example by tilting a container containing the first liquid significantly and pouring it into the second liquid.

[0026] (Silver powder manufacturing method) The method for producing silver powder of the present invention (hereinafter sometimes simply referred to as the "production method") comprises a first liquid preparation step of obtaining an alkaline first liquid containing silver ions and an amine compound, and a second liquid preparation step of mixing the first liquid with a formaldehyde-containing liquid (e.g., an aqueous formaldehyde solution) to obtain a second liquid. In the production method of the present invention, the amine compound is represented by the formula (1): NR 1 R 2 R 3 (1) [In formula (1), R 1 is a hydrocarbon group having 1 to 6 carbon atoms and a carboxyl group; R 2 is a hydrocarbon group having 1 to 12 carbon atoms and a hydroxyl group; R 3 is a hydrogen atom or a hydrocarbon group having 1 to 12 carbon atoms which may have a substituent. It is a compound represented by the formula: The above-described manufacturing method makes it possible to obtain silver powder that contains silver fine particles having internal voids and smooth surfaces, and in which aggregation of the silver fine particles is suppressed, without carrying out a surface smoothing treatment (and subsequent classification treatment) as in the conventional technology. The reason for this is presumably as follows.

[0027] First, formaldehyde functions as a reducing agent, and by using formaldehyde, it is possible to obtain silver fine particles having internal voids. It is presumed that the amine nitrogen atoms and carboxyl oxygen atoms of the amine compound represented by formula (1) (hereinafter sometimes simply referred to as "amine compound") coordinate to the silver ions, thereby covering the silver nuclei as they are being generated and effectively suppressing the rate at which silver microparticles are generated (the rate at which silver ions are reduced) from becoming excessively fast, thereby smoothing the surface of the silver microparticles and suppressing the aggregation of the silver microparticles themselves. The portion where the amine nitrogen atoms, etc. are coordinated to the silver ions is considered to be hydrophobic. In addition, due to the hydroxyl group (hydrophilic part) of the amine compound, a micelle-like structure is formed in the aqueous solution around the silver nucleus as the particle grows, allowing the silver microparticles to grow while keeping the silver ions separated from each other, and as a result, it is believed that the aggregation of the silver microparticles can be suppressed. Furthermore, it is presumed that the carboxyl group of the amine compound exerts a buffering ability mainly in the alkaline region, suppressing the sudden change in pH to the acidic side caused by formic acid produced as a result of the reaction of formaldehyde as a reducing agent, thereby suppressing the sudden change in reaction conditions, and as a result, making it possible to smooth the surface of the silver microparticles.

[0028] In addition, the production method of the present invention may further include, in addition to the first liquid preparation step and the second liquid preparation step, a third liquid preparation step, a separation step, etc., which will be described later.

[0029] <First liquid preparation process> In the first liquid preparation step, an alkaline first liquid containing silver ions and an amine compound is obtained. The solvent is usually water. A pH adjuster may be added to the first liquid. As the pH adjuster, a common acid or base may be used, for example, nitric acid, sodium hydroxide, etc.

[0030] [Silver ions] As the silver ion source, known inorganic silver salts such as silver nitrate, silver sulfate, silver carbonate, silver chloride, and silver oxide that are industrially used can be used.

[0031] The inventors have found that the amine compound can function sufficiently even when added in a small amount, and therefore the content of silver ions in the first liquid is preferably greater than the content of the amine compound.

[0032] [Amine compounds] The amine compound has the formula (1): NR 1 R 2 R 3 (1) [In formula (1), R 1 is a hydrocarbon group having 1 to 6 carbon atoms and a carboxyl group; R 2 is a hydrocarbon group having 1 to 12 carbon atoms and a hydroxyl group; R 3 is a hydrogen atom or a hydrocarbon group having 1 to 12 carbon atoms which may have a substituent. It is a compound represented by the formula:

[0033] As described above, this amine compound has a buffering ability in the alkaline region, and also has N atoms and R atoms. 1 It is believed that the carboxyl group (oxygen atom) of R coordinates with the silver ion. The part that forms a complex with the silver ion is considered to be hydrophobic, while R 2 , or R 2 and R 3 is a hydrophilic structure. It is preferable that the amine compound does not contain a sulfur atom. If the amine compound does not contain a sulfur atom, the obtained silver powder can be suitably used for various electronic component applications such as semiconductors.

[0034] In formula (1), R 1 is a hydrocarbon group having 1 to 6 carbon atoms and a carboxyl group. 1 The hydrocarbon group must have at least one carboxyl group, and may have two or more carboxyl groups, but preferably has one carboxyl group. In addition, R 1 The number of carbon atoms in the hydrocarbon group does not include the carbon atoms in the carboxyl group. For example, R1 "-CH 2 C(=O)OH”, R 1 The number of carbon atoms in the hydrocarbon group is "1".

[0035] R 1 From the viewpoint of availability of the amine compound, the number of carbon atoms of R is preferably 5 or less, and more preferably 4 or less. 1 When the carbon number is 1 or 2, both the carboxyl group and the N atom can be coordinated to the silver ion, and in this case, it is believed that a stable structure of a 5- to 6-membered ring is formed. This is considered to be particularly preferable for suppressing the generation rate of silver fine particles.

[0036] In formula (1), R 2 R is a hydrocarbon group having 1 to 12 carbon atoms and a hydroxyl group. 2 R constitutes the hydrophilic portion of the amine compound. 2 has at least one hydroxyl group, and may have two or more hydroxyl groups.

[0037] R 2 If R has two or more hydroxyl groups, 2 From the viewpoint of producing silver powder having excellent smoothness and suppressed aggregation, the number of hydroxyl groups is preferably 2 or more, while from the viewpoint of easy availability of the amine compound, the number is preferably 4 or less.

[0038] R 2 From the viewpoint of hydrophilicity of R, the ratio of the number of hydroxyl groups to the number of carbon atoms (number of hydroxyl groups: number of carbon atoms) is preferably 1:1 to 3. 2 From the viewpoints of hydrophilicity and ease of availability of the amine compound, the number of carbon atoms of is preferably 2 or more and 6 or less, taking into consideration the above ratio.

[0039] In formula (1), R 3 is a hydrogen atom or a hydrocarbon group having 1 to 12 carbon atoms which may have a substituent. When the substituent is a hydrophilic functional group, R 3 is R 2Together, they constitute the hydrophilic part of the amine compound. Examples of such functional groups include a hydroxyl group, a phosphate group, a sulfonic acid group, an amino group, and a mercapto group. Among these, from the viewpoint of the availability of the amine compound and the suitability of the silver powder produced using the compound for electronic components, a hydroxyl group is preferred. When the substituent contains carbon, the number of such carbon atoms is not included in the number of carbon atoms of the hydrocarbon group constituting R. 3 The number of substituents is not particularly limited. For example, it is 1 to 5. Also, similar to the case of R, 2 the ratio of the number of substituents to the number of carbon atoms of R (number of substituents: number of carbon atoms) is preferably 1:1 to 3. 3

[0040] R 3 is a hydrocarbon group having 1 or more and 12 or less carbon atoms which may have a substituent. The number of carbon atoms of the hydrocarbon group of R 3 is preferably 2 or more and 6 or less in consideration of the above ratio from the viewpoints of its hydrophilicity and the availability of the amine compound.

[0041] Here, in formula (1), the total number of hydroxyl groups possessed by R 2 and R 3 is preferably 2 or more. If the total number of hydroxyl groups possessed by R 2 and R 3 is 2 or more, the smoothness of the silver fine particle surface can be effectively improved, and the aggregation of silver fine particles can be effectively suppressed.

[0042] In one embodiment, when R 2 has two or more hydroxyl groups, R 3 is preferably a hydrogen atom.

[0043] In one embodiment, when R 2 has one hydroxyl group, R 3 is preferably a hydrocarbon group having 1 or more and 12 or less carbon atoms which may have a substituent, more preferably a hydrocarbon group having 1 or more and 12 or less carbon atoms having a hydroxyl group. R 2It is more preferable that the group is the same as the group represented by the formula:

[0044] Specifically, the amine compound is preferably tricine or bicine, since it can effectively improve the smoothness of the surface of the silver fine particles and effectively suppress the aggregation of the silver fine particles.

[0045] The content of the amine compound in the first liquid is, for example, 0.008 mol or more, or may be 0.01 mol or more, and for example, 0.2 mol or less, or may be 0.1 mol or less, per mol of silver in the first liquid.

[0046] [ammonia] It is preferable to add ammonia water or an ammonium salt to the first liquid, since this forms a silver ammine complex and effectively narrows the particle size distribution of the resulting silver powder. When ammonia water or an ammonium salt is added to the first liquid, the concentration of ammonia in the first liquid is, for example, 1 mol or more and 3 mol or less per mol of silver in the first liquid.

[0047] [Properties of the first liquid] The first liquid is alkaline. By bringing the silver ion-containing liquid into an alkaline state and contacting it with a reducing agent, silver powder that meets the general requirement for sharp particle size distribution can be obtained. The pH of the first liquid is preferably 9 or more, more preferably 10 or more, from the viewpoint of obtaining silver powder with excellent smoothness and sharp particle size distribution, while from the viewpoint of chemical costs, it is preferably 13 or less, more preferably 12.5 or less. By setting the pH of the first liquid within the above range, the particle size distribution of the resulting silver powder can be effectively adjusted. The pH of the first liquid can be adjusted using the above-mentioned pH adjuster.

[0048] The temperature of the first liquid is, for example, 5°C or higher, and may be 20°C or higher, and is, for example, 50°C or lower, and may be 40°C or lower.

[0049] <Second liquid preparation process> In the second liquid preparation step, the first liquid and formaldehyde are mixed to obtain the second liquid. As described above, formaldehyde functions as a reducing agent, and by using formaldehyde, silver particles having internal voids are obtained. Therefore, the second liquid contains precipitated silver particles. The second liquid is usually a suspension (so-called slurry) or a dispersion in which silver particles are dispersed.

[0050] As a source of formaldehyde, for example, an aqueous formaldehyde solution (so-called formalin) can be used. From the viewpoints of completion of the reduction reaction and cost, the amount of formaldehyde used is preferably 3 to 8 mol, more preferably 4 to 6 mol, per mol of silver in the first liquid to be mixed.

[0051] The method of mixing the first liquid and formaldehyde is not particularly limited, but it is preferable to add an aqueous formaldehyde solution to the first liquid and stir and mix. This allows a milder reduction reaction to be carried out than when the first liquid is added to an aqueous formaldehyde solution, effectively improving the smoothness of the silver microparticle surface and effectively suppressing aggregation between the silver microparticles. Formaldehyde may be added to the first liquid all at once or continuously. From the viewpoint of effectively suppressing aggregation between the silver microparticles, it is preferable to add the aqueous formaldehyde solution to the first liquid while stirring the liquid. Note that, from the viewpoint of effectively generating voids, ultrasound or the like may be used during reduction precipitation.

[0052] Here, the first liquid contains an amine compound which exhibits buffering ability. When formaldehyde is mixed with the first liquid, formaldehyde functions as a reducing agent and itself becomes formic acid. As a result, the pH changes to acidic. However, due to the buffering action of the amine compound, the pH fluctuation is small and gradual. Specifically, when formaldehyde is added all at once to the first liquid in the second liquid preparation step, the time required for the pH of the first liquid to decrease by 2 from the start point of adding formaldehyde is, for example, 15 seconds or more (usually 100 seconds or less). It is considered that this can suppress a sudden change in the environment of the reduction reaction, more effectively improve the smoothness of the silver fine particle surface, and more effectively suppress the aggregation of silver fine particles. In the present invention, when the first liquid and formaldehyde are brought into contact, the generation of silver fine particles starts immediately from that moment and the pH also fluctuates. However, it is considered that the second liquid is obtained when the addition of formaldehyde is completed and the pH fluctuation has settled. This does not strictly define the end point of the generation of silver fine particles.

[0053] In the second liquid preparation step, the liquid temperature until the first liquid and formaldehyde are mixed to obtain the second liquid is, for example, 20°C or higher, may be 30°C or higher, for example, 50°C or lower, and may be 40°C or lower.

[0054] <Third liquid preparation step> In an arbitrary third liquid preparation step, a surface treatment agent is added to the second liquid obtained in the second liquid preparation step to obtain a third liquid containing surface-treated silver fine particles (hereinafter, may be referred to as "surface-treated silver fine particles"). Note that the third liquid containing surface-treated silver fine particles is usually a suspension (so-called slurry) or dispersion in which the surface-treated silver fine particles are dispersed.

[0055] Examples of the surface treatment agent include behenic acid, stearic acid, palmitic acid, myristic acid, lauric acid, ricinoleic acid, oleic acid, linoleic acid, linolenic acid, benzotriazole, and metal salts thereof. These may be used alone or in combination of two or more. Examples of the metal include sodium, potassium, magnesium, and calcium.

[0056] The amount of the surface treatment agent added in the third liquid preparation step is usually 0.05% by mass or more and 0.5% by mass or less, based on the mass of silver contained in the second liquid obtained in the second liquid preparation step.

[0057] <Separation process> In the optional separation step, the silver particles obtained in the second liquid preparation step or the surface-treated silver particles (hereinafter, these may be collectively referred to as "silver particles, etc.") are separated from the solution by a solid-liquid separation technique such as filtration and dried to obtain dried silver powder. In addition, the separation step may optionally include a washing and recovery step of recovering and washing the separated silver particles and the like.

[0058] In the washing and recovery step, for example, an aggregate of the separated silver particles and the like is formed into a cake, and this cake is washed. This washing may be performed, for example, using pure water. Dehydration in the washing and recovery step may be performed, for example, by decantation or a filter press. The end point of the washing may be determined using the electrical conductivity of the washing water. Specifically, the end of the washing may be determined when the electrical conductivity of the washing water becomes equal to or lower than a predetermined value. The silver particles and the like after washing may be subjected to a drying step.

[0059] In the drying step, the aggregate of the silver particles containing moisture is dried. The drying step may be performed by vacuum drying or using an airflow dryer. In the drying step, a high-pressure air flow may be blown onto the aggregate of the silver particles, or the cake or silver powder in the drying process may be put into a stirrer having a stirring rotor and stirred to apply a dispersing force to the cake or silver powder in the drying process, thereby promoting dispersion or drying.

[0060] In the drying step, the temperature of the silver powder is usually not higher than 100° C. If the temperature of the silver powder is not higher than 100° C., sintering of the silver particles in the silver powder can be effectively prevented.

[0061] Since the silver powder after drying may be in the form of lumps, a dry crushing treatment or classification operation may be carried out simultaneously with or after the drying step in order to improve the handleability of the silver powder, etc. Here, improving the handleability of the silver powder means, for example, ensuring a degree of fluidity that does not interfere with the supply operation into an apparatus, or moderately loosening the silver powder so that the processing in the apparatus proceeds efficiently.

[0062] The method of the dry crushing treatment is not particularly limited and can be appropriately selected depending on the purpose. However, it is preferable to use a crusher that rotates an agitator blade to crush the silver powder and fluidize it, and for example, a sample mill, a blender, a coffee mill, etc. can be used.

[0063] <Silver powder properties> The volume-based cumulative 10% particle size (D 10 ) is, for example, 0.10 μm or more, may be 0.30 μm or more, or may be 0.50 μm or more, and is, for example, 2.00 μm or less, may be 1.50 μm or less, or may be 1.20 μm or less.

[0064] The cumulative 50% particle size (D 50 ) is, for example, 0.30 μm or more, may be 0.50 μm or more, or may be 0.80 μm or more, and is, for example, 5.00 μm or less, may be 3.50 μm or less, or may be 2.50 μm or less.

[0065] The volume-based cumulative 90% particle size (D 90 ) is, for example, 1.00 μm or more, may be 1.20 μm or more, or may be 1.50 μm or more, and is, for example, 6.00 μm or less, may be 4.00 μm or less, or may be 3.50 μm or less.

[0066] The BET specific surface area of ​​the silver powder obtained by the production method of the present invention is, for example, 0.100 m 2 / g or more, and 0.200m 2 / g or more, 0.300m 2 / g or more, for example 1.000m 2 / g or less, and 0.800m 2 / g or less, 0.750m 2 / g or less.

[0067] The ignition loss (Ig-Loss) value of the silver powder obtained by the manufacturing method of the present invention is, for example, 0.50 mass% or more, or may be 1.00 mass% or more, or may be 1.20 mass% or more, and may be, for example, 5.00 mass% or less, or may be 3.00 mass% or less, or may be 2.50 mass% or less. EXAMPLES

[0068] The present invention will be described in more detail below using examples, but the present invention is not limited to the following examples in any way. The reaction delay time, particle size distribution, specific surface area, and ignition loss (Ig-loss) were measured by the methods described above. The pH measurement of the first liquid, the scanning electron microscope (SEM) image capture, and the cross-sectional SEM image capture were performed according to the following procedure.

[0069] <pH of first solution> The first liquid in Example 1 described below was selected as the standard first liquid. Using this, the pH of the first liquid in each of the examples and comparative examples was calculated by the method described above.

[0070] <Scanning Electron Microscope (SEM) Image> The silver powder was observed at 5,000x, 10,000x, and 20,000x magnifications using a scanning electron microscope (JEOL JSM-IT300LV, manufactured by JEOL Ltd.) at an accelerating voltage of 15 kV to confirm the surface condition of the silver particles and the state of aggregation between the silver particles.

[0071] <Cross-section SEM image capture> The cross-sections of the silver particles in the silver powder were examined by solidifying each sample in resin and polishing it with a cross-section polisher to expose the cross-sections of the silver particles. The cross-sections of each particle were then observed at 5,000x, 10,000x, and 20,000x magnifications with a scanning electron microscope (JEOL JSM-IT300LV, manufactured by JEOL Ltd.) at an accelerating voltage of 15 kV to confirm the presence or absence of voids inside the silver particles.

[0072] Example 1 <First liquid preparation process> 239 g of a 0.1 mol / L aqueous tricine solution (equivalent to 0.05 mol of tricine per mol of silver) was added to 3283 g of an aqueous silver nitrate solution containing 49 g of silver, and the temperature of the aqueous solution was adjusted to 22° C. Furthermore, 113.3 g of an industrial ammonia solution with a concentration of 28% by mass (equivalent to 2.05 mol of ammonia per mol of silver) was added. While stirring the obtained silver solution, 3.11 g of a 20 mass % aqueous sodium hydroxide solution was added as a pH adjuster (corresponding to 0.26 mol of sodium hydroxide per mol of silver) to obtain a first liquid.

[0073] <Second liquid preparation process> While stirring the obtained first liquid, an aqueous solution of 175.1 g of 37% by mass formalin diluted with 350 g of pure water was added all at once as a reducing agent to the liquid (equivalent to 4.8 moles of formaldehyde per mole of silver), and the first liquid to which the aqueous solution was added was stirred at 30°C for 3 minutes to obtain a slurry containing silver particles as the second liquid.

[0074] <Third liquid preparation process> To the obtained second liquid, a stearic acid emulsion (stearic acid content: 15.5% by mass) was added in an amount of 1.3% by mass relative to the Ag amount, and the mixture was stirred at 160 rpm for 6 minutes to obtain a third liquid.

[0075] <Separation process> The stirring of the third liquid was stopped to allow the silver particles to settle, and the third liquid in which the silver particles had settled was filtered and washed with water. The washing with water was continued until the electrical conductivity of the washing liquid was 0.5 mS / m or less. The electrical conductivity was measured using a CM-31P electrical conductivity meter manufactured by Toa DKK Co., Ltd. The cake after filtration was vacuum dried at 73°C for 10 hours, and further crushed using a sample mill SK-M10 manufactured by Kyoritsu Riko Co., Ltd. at a rotation speed of 15,000 rpm for 240 seconds. As a result, a dried silver powder was obtained. The particle size distribution and BET specific surface area of ​​the obtained silver powder were evaluated. The evaluation results of the particle size distribution and BET specific surface area of ​​the silver powder are shown in Table 1 below. In addition, SEM images and cross-sectional SEM images of the obtained silver powder were taken to confirm the presence or absence of voids inside the silver particles, the surface condition, and the state of aggregation between the silver particles. The confirmation results are shown in Table 1 below, and the SEM images and cross-sectional SEM images of the silver powder are shown in Figures 1 to 6.

[0076] Example 2 Silver powder was obtained in the same manner as in Example 1, except that in the first liquid preparation step, the amount of tricine aqueous solution added was changed from 239 g to 47 g (equivalent to 0.01 mol of tricine per 1 mol of silver). The particle size distribution, BET specific surface area, and Ig-loss of the obtained silver powder were evaluated. The evaluation results of the particle size distribution, BET specific surface area, and Ig-loss of the silver powder are shown in Table 1 below, and the particle size distribution of the silver powder is shown in Figure 7. In addition, SEM images and cross-sectional SEM images of the obtained silver powder were taken to confirm the presence or absence of voids inside the silver particles, the surface condition, and the state of aggregation between the silver particles. The results are shown in Table 1 below. Furthermore, in the second liquid preparation step, the time until particle generation was measured, and the reaction delay time was calculated as the amount of delay compared to the corresponding time in Comparative Example 1 described below (hereinafter, this may also be simply expressed as "the reaction delay time was measured"). The measurement results of the reaction delay time are shown in Table 1.

[0077] Example 3 Silver powder was obtained in the same manner as in Example 2, except that 0.84 g of a 61% by mass aqueous nitric acid solution (equivalent to 0.018 moles of nitric acid per mole of silver) was added as a pH adjuster to the first liquid before the addition of the reducing agent. The particle size distribution, BET specific surface area, and Ig-loss of the obtained silver powder were evaluated. The evaluation results of the particle size distribution, BET specific surface area, and Ig-loss of the silver powder are shown in Table 1 below, and the particle size distribution of the silver powder is shown in Figure 8. In addition, SEM images and cross-sectional SEM images of the obtained silver powder were taken to confirm the presence or absence of voids inside the silver particles, the surface condition, and the state of aggregation between the silver particles. The confirmation results are shown in Table 1 below, and the SEM images and cross-sectional SEM images of the silver powder are shown in Figures 9 to 14. Furthermore, the reaction delay time was measured in the second liquid preparation step. The measurement results of the reaction delay time are shown in Table 1 below.

[0078] Example 4 Silver powder was obtained in the same manner as in Example 1, except that the first liquid preparation step and the second liquid preparation step were carried out as follows.

[0079] <First liquid preparation process> 50 g of a 0.1 mol / L aqueous tricine solution (equivalent to 0.048 mol of tricine per mol of silver) was added to 760.7 g of an aqueous silver nitrate solution containing 11.3 g of silver, and the temperature of the aqueous solution was adjusted to 22° C. Furthermore, 23.8 g of an industrial ammonia solution with a concentration of 28% by mass (equivalent to 1.87 mol of ammonia per mol of silver) was added. While stirring the obtained silver solution, 0.54 g of a 20% by mass aqueous sodium hydroxide solution was added as a pH adjuster (corresponding to 0.11 mol of sodium hydroxide per 1 mol of silver) to obtain a first liquid.

[0080] <Second liquid preparation process> While stirring the obtained first liquid, an aqueous solution of 36.9 g of 37% by mass formalin diluted with 15.8 g of pure water was added all at once as a reducing agent to the liquid (equivalent to 4.4 moles of formaldehyde per mole of silver). The first liquid to which the aqueous solution was added was stirred at 30°C for 3 minutes to obtain a slurry containing silver particles as the second liquid.

[0081] <Evaluation etc.> SEM images and cross-sectional SEM images of the obtained silver powder were taken to confirm the presence or absence of voids inside the silver particles, the surface condition, and the state of aggregation between the silver particles. The results are shown in Table 1 below. In addition, the reaction delay time was measured in the second liquid preparation step. The reaction delay time measurement results are shown in Table 1 below. Furthermore, the pH change of the first liquid in the second liquid preparation step is shown in Figure 15. The time required for the pH of the first liquid to decrease by 2 from the start of addition of formalin to the first liquid was 42 seconds.

[0082] Example 5 Silver powder was obtained in the same manner as in Example 4, except that in the first liquid preparation step, 50 g of a 0.1 mol / L bicine aqueous solution (equivalent to 0.05 mol of bicine per 1 mol of silver) was added to the silver nitrate aqueous solution instead of a 0.1 mol / L tricine aqueous solution. SEM images and cross-sectional SEM images of the obtained silver powder were taken to confirm the presence or absence of voids inside the silver particles, the surface condition, and the state of aggregation between the silver particles. The results are shown in Table 1 below. In addition, the reaction delay time was measured in the second liquid preparation step. The reaction delay time measurement results are shown in Table 1 below.

[0083] Comparative Example 1 Silver powder was obtained in the same manner as in Example 1, except that the 0.1 mol / L aqueous tricine solution was not added in the first liquid preparation step. The particle size distribution, BET specific surface area, and Ig-loss of the obtained silver powder were evaluated. The evaluation results of the particle size distribution, BET specific surface area, and Ig-loss of the silver powder are shown in Table 1 below. In addition, in the second liquid preparation step, the time until the formation of fine silver particles was measured, and the fine silver particles precipitated immediately after the addition of formalin to the first liquid. In addition, SEM images of the obtained silver powder were taken to confirm the surface condition of the silver particles and the state of aggregation between the silver particles. The confirmation results are shown in Table 1 below, and an SEM image of the silver powder is shown in Figure 16. Note that the surfaces of the silver particles in the obtained silver powder were not smooth. Furthermore, the pH change of the first liquid in the second liquid preparation step is shown in Figure 17. The time required for the pH of the first liquid to decrease by 2 from the start of addition of formalin to the first liquid was 6 seconds.

[0084] Comparative Example 2 Silver powder was obtained in the same manner as in Example 1, except that in the first liquid preparation step, 239 g of a 1:1 molar mixture of a 0.1 mol / L aqueous trishydroxymethylaminomethane (Tris) solution and a 0.1 mol / L aqueous glycine solution (equivalent to 0.045 moles of Tris and 0.045 moles of glycine per mole of silver) was added to the aqueous silver nitrate solution instead of a 0.1 mol / L aqueous tricine solution. The particle size distribution of the obtained silver powder was evaluated. The evaluation results of the particle size distribution of the silver powder are shown in Table 1 below. Furthermore, the reaction delay time was measured in the second liquid preparation step, and it was found that fine silver particles precipitated immediately after the formalin was added to the first liquid. Furthermore, SEM images of the obtained silver powder were taken to confirm the surface condition of the silver particles and the state of aggregation between the silver particles. The confirmation results are shown in Table 1 below, and an SEM image of the silver powder is shown in Figure 18. Note that the silver particles in the obtained silver powder were still in an aggregated state even after crushing.

[0085] [Table 1]

[0086] As is clear from the evaluation results of the examples and comparative examples, the manufacturing method of silver powder of the examples makes it possible to obtain silver powder that contains silver microparticles with internal voids and smooth surfaces, and in which aggregation of the silver microparticles is suppressed, without the need for additional processes such as surface smoothing treatment.

[0087] As is clear from the particle size distribution results of Examples 2 and 3, it is possible to adjust the particle size distribution of the silver powder by adjusting the pH of the first liquid. [Industrial Applicability]

[0088] According to the present invention, a method can be provided that can produce silver powder using a wet reduction method, which contains silver microparticles that have internal voids and smooth surfaces, and in which aggregation of the silver microparticles is suppressed, without requiring additional processes such as surface smoothing treatment.

Claims

1. a first liquid preparation step of obtaining an alkaline first liquid containing silver ions and an amine compound; a second liquid preparation step of mixing the first liquid and formaldehyde to obtain a second liquid; Including, The amine compound is represented by the formula (1): NR １ R ２ R ３ ・・・(1) [In formula (1), R １ is a hydrocarbon group having 1 to 6 carbon atoms and having a carboxyl group, R ２ is a hydrocarbon group having 1 to 12 carbon atoms and having a hydroxyl group, R ３ is a hydrogen atom or a hydrocarbon group having 1 to 12 carbon atoms which may have a substituent. A method for producing silver powder, which is a compound represented by the formula:

2. In the formula (1), R ２ and R ３ The method for producing silver powder according to claim 1 , wherein the total number of hydroxyl groups contained in said silver powder is 2 or more.

3. The method for producing silver powder according to claim 1 , wherein the amine compound does not have a sulfur atom.

4. The method for producing silver powder according to claim 1 , wherein the amine compound is tricine or bicine.

5. The method for producing silver powder according to claim 1 , wherein the pH of the first liquid is 9 or more and 13 or less.

6. A method for producing silver powder as described in claim 1, wherein the formaldehyde is added to the first liquid all at once while stirring the liquid, and the time required for the pH of the first liquid to decrease by 2 from the start of the addition of the formaldehyde is 15 seconds or more.

7. A method for producing silver powder according to any one of claims 1 to 6, wherein the content of the silver ions in the first liquid is greater than the content of the amine compound.

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