Spherical silver powder and method for producing spherical silver powder

Abstract

To provide a spherical silver powder that can impart excellent thin line printability to conductive pastes.SOLUTION: The present invention is a spherical silver powder in which a surface treatment agent is present; for which, in measurement of the thermal expansion coefficient, the maximum value of the thermal expansion coefficient with reference to the value at 50°C is 0.3% or less; for which the BET specific surface area is 0.1 m2 / g or more and 0.8 m2 / g or less; and for which the value of D90 is 2.0 μm or more and 4.0 μm or less.SELECTED DRAWING: None

Description

[Technical Field]

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

[0002] A method of forming a conductive film such as an electrode or electrical wiring by applying or printing a conductive paste containing a conductive metal powder onto a substrate such as a film, a board, or an electronic component, and then heating it to dry, harden, or bake it has been widely used. However, with the recent increase in the performance of electronic devices, conductive films formed using conductive pastes are required to have lower resistance, and this requirement is becoming stricter every year.

[0003] In response to the above demands, for example, Patent Document 1 proposes a silver powder containing a predetermined surface treatment agent and having a predetermined coefficient of thermal expansion, BET value (specific surface area), and ignition loss difference, in order to suppress swelling of a conductive coating film during firing of a sinterable conductive paste and to reduce the electrical resistance value of the cured film (conductive film). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 6174301 Summary of the Invention [Problem to be solved by the invention]

[0005] In recent years, conductive films have been made thinner, and there is a demand for conductive pastes that can be printed in fine lines.

[0006] Conventional silver powders have room for further improvement in terms of imparting excellent fine line printability to conductive pastes.

[0007] Therefore, an object of the present invention is to provide a spherical silver powder that can impart excellent fine line printability to a conductive paste. Another object of the present invention is to provide a method for producing spherical silver powder that can impart excellent fine line printability to a conductive paste. [Means for solving the problem]

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

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

[0010] [1] A surface treatment agent is present, In thermal expansion coefficient measurements, the maximum value of the thermal expansion coefficient is 0.3% or less, based on the value at 50°C. BET specific surface area is 0.1m 2 / g or more 0.8m 2 / g or less, D 90 The spherical silver powder has a value of 2.0 μm or more and 4.0 μm or less.

[0011] [2]D 50 The spherical silver powder according to [1], wherein the value of is 1.0 μm or more and 2.5 μm or less.

[0012] [3]D 10 The spherical silver powder according to [1] or [2], wherein the value is 0.5 μm or more and 1.2 μm or less.

[0013] [4] The spherical silver powder according to any one of [1] to [3], wherein the surface treatment agent is one or more surface treatment agents selected from the group consisting of fatty acids, compounds having an azole structure, and fatty acid salts.

[0014] [5] A method for producing spherical silver powder by adding a reducing agent to an aqueous reaction system containing silver ions and a chelating agent made of a polymer to reduce and precipitate silver particles, a carbonic acid concentration adjusting step of adjusting a ratio of the total molar concentration of carbonic acid to the total molar concentration of silver in the aqueous reaction system to be 0.004 or more and 0.051 or less before adding the reducing agent to the aqueous reaction system; a surface treatment agent addition step of adding a surface treatment agent to the aqueous reaction system after the silver particles have precipitated; A method for producing spherical silver powder, comprising:

[0015] [6] The method for producing spherical silver powder according to [5], wherein the surface treatment agent is one or more surface treatment agents selected from the group consisting of fatty acids, compounds having an azole structure, and fatty acid salts.

[0016] [7] The method for producing spherical silver powder according to [5] or [6], wherein the chelating agent is polyethyleneimine having a weight-average molecular weight of 600 or less.

[0017] [8] The method for producing spherical silver powder according to any one of [5] to [7], wherein the reducing agent is hydrazine. [Effects of the Invention]

[0018] According to the present invention, it is possible to provide a spherical silver powder that can impart excellent fine line printability to a conductive paste. The present invention also provides a method for producing spherical silver powder that can impart excellent fine line printability to conductive paste. [Brief explanation of the drawings]

[0019] [Figure 1] 1 is a graph showing the results of thermomechanical analysis of the spherical silver powder obtained in Example 1. [Figure 2] 2 is an enlarged graph of a portion of the graph in FIG. 1. [Figure 3] 1 is a graph showing the results of thermomechanical analysis of the spherical silver powder obtained in Example 2. [Figure 4] 4 is a graph showing an enlarged portion of the graph in FIG. 3. [Figure 5] 1 is a graph showing the results of thermomechanical analysis of the spherical silver powder obtained in Example 3. [Figure 6]6 is a graph showing an enlarged portion of the graph in FIG. 5. [Figure 7] 1 is a graph showing the results of thermomechanical analysis of the spherical silver powder obtained in Example 4. [Figure 8] 8 is a graph showing an enlarged portion of the graph in FIG. 7. [Figure 9] 1 is a graph showing the results of thermomechanical analysis of the spherical silver powder obtained in Example 5. [Figure 10] 10 is a graph showing an enlarged portion of the graph in FIG. 9. [Figure 11] 1 is a graph showing the results of thermomechanical analysis of the spherical silver powder obtained in Example 6. [Figure 12] 12 is a graph showing an enlarged portion of the graph in FIG. 11. [Figure 13] 1 is a graph showing the results of thermomechanical analysis of the spherical silver powder obtained in Example 7. [Figure 14] 14 is a graph showing an enlarged portion of the graph in FIG. 13. [Figure 15] 1 is a graph showing the results of thermomechanical analysis of the spherical silver powder obtained in Comparative Example 1. [Figure 16] FIG. 15 is an enlarged view of a portion of the graph. [Figure 17] 1 is a graph showing the results of thermomechanical analysis of the spherical silver powder obtained in Comparative Example 2. [Figure 18] 18 is a graph showing an enlarged portion of the graph in FIG. 17. [Figure 19] 1 is a graph showing the results of thermomechanical analysis of the spherical silver powder obtained in Comparative Example 3. [Figure 20] 20 is a graph showing an enlarged portion of the graph in FIG. 19. [Figure 21] 1 is a graph showing the results of thermomechanical analysis of the spherical silver powder obtained in Comparative Example 4. [Figure 22] 22 is a graph showing an enlarged portion of the graph in FIG. 21. [Figure 23] 1 is a 10,000x SEM image of the spherical silver powder obtained in Example 1. [Figure 24] 1 is a 10,000x SEM image of the spherical silver powder obtained in Example 2. [Figure 25]1 is a 10,000x SEM image of the spherical silver powder obtained in Example 3. [Figure 26] 1 is a 10,000x SEM image of the spherical silver powder obtained in Example 4. [Figure 27] 1 is a 10,000x SEM image of the spherical silver powder obtained in Example 5. [Figure 28] 1 is a 10,000x SEM image of the spherical silver powder obtained in Example 6. [Figure 29] 1 is a 10,000x SEM image of the spherical silver powder obtained in Example 7. [Figure 30] 1 is a 10,000x SEM image of the spherical silver powder obtained in Comparative Example 3. DETAILED DESCRIPTION OF THE INVENTION

[0020] The spherical silver powder of the present invention is suitable for use as a conductive filler for conductive pastes. A conductive paste using the spherical silver powder of the present invention can be used to form conductive patterns or electrodes on a substrate. A conductive paste using the spherical silver powder of the present invention can be printed on a substrate by, for example, screen printing, offset printing, photolithography, or the like, to form a conductive film such as a conductive pattern or an electrode.

[0021] (Terminology and Measurement Methods) First, prior to describing the embodiments, the terms and measurement methods used in this specification will be explained.

[0022] <Confirmation of spherical silver powder (particle shape)> As used herein, spherical silver powder refers to silver powder in which the average shape factor of 400 or more particles observed by image analysis based on scanning electron microscope (SEM) images is in the range of 1.0 or more but less than 1.7. The scanning electron microscope is not particularly limited, but examples that can be used include the JSM-6100 and JSM-IT300L manufactured by JEOL Ltd. In the present invention, the silver powders of Examples 1 and 5 were observed using the JSM-IT300L, and the silver powders of Examples 2, 3, 4, 6, 7, and Comparative Example 3 were observed using the JSM-6100. The shape factor in this specification refers to the ratio of the area of ​​a virtual circle, the diameter of which is the average maximum length of 400 or more particles observed by the image analysis, to the average particle area of ​​the silver particles obtained by tracing the outlines of the particles, and is the value obtained by dividing the area of ​​the virtual circle by the average particle area. The shape factor is calculated using the formula π(average maximum length / 2) 2 / average particle area.

[0023] <Measurement of the thermal expansion coefficient of spherical silver powder> The thermal expansion coefficient of the spherical silver powder was measured as follows. First, 0.3 g of spherical silver powder was weighed out. The spherical silver powder was then placed in a specified mold with a diameter of 5 mm and pressed for 1 minute under a 50 kg load using a press to create a cylindrical measurement sample. This measurement sample was placed in the sample holder of a thermomechanical analysis (TMA) device (Thermo plus EVO 2 series TMA8311). A measurement load of 98 mN was applied using the measurement probe, and the sample was heated from room temperature to 900°C at a heating rate of 10°C / min. Thermomechanical analysis (TMA) of the measurement sample was then performed. The thermal expansion coefficient at each temperature was calculated using the following formula (1), with the value at 50°C as the reference temperature. Thermal expansion coefficient (%) when heated from 50°C to T°C = (L T -L 50 ) / L 50 ×100 (1) where L 50 is the axial length (mm) of the cylindrical measurement sample at a sample temperature of 50°C, L T is the axial length (mm) of the cylindrical measurement sample at sample temperature T°C.

[0024] <BET specific surface area> In this specification, the "BET specific surface area" was measured by the BET one-point method using nitrogen adsorption with Macsorb HM-model 1210 (manufactured by MOUNTECH). In addition, for the measurement of the BET specific surface area, the sample weight was set to 3.0 g, a N2 / He (30 / 70) mixed gas was used, the gas flow rate was set to 25 mL / min, and the degassing conditions before measurement were 60°C for 10 minutes.

[0025] <Quantification of surface treatment agent> In this specification, for example, when the surface treatment agent of spherical silver powder is a fatty acid, the content of the fatty acid was measured according to the quantitative analysis method of fatty acids described in Japanese Patent No. 5622543. Specifically, first, after dissolving the spherical silver powder in an acid, an organic solvent was mixed, and after extracting the entire amount of the surface treatment agent into the organic solvent phase, a predetermined amount of the organic solvent phase was aliquoted, evaporated to dryness, and the remaining solid was determined by measuring the carbon amount with a carbon-sulfur analyzer and calculated.

[0026] For example, when the surface treatment agent is specified as stearic acid and no carbon source other than stearic acid is contained in the spherical silver powder, the measurement method of stearic acid is as follows.

[0027] When a calibration curve was obtained by measuring the respective carbon amounts (intensities) with a carbon-sulfur analyzer in standard solutions with different contents (mg) of stearic acid, the slope was designated as A (intensity / mg). And for the stearic acid mass X (mg) and concentration Y (%) in the spherical silver powder, from the extraction of the treatment agent into the total amount of organic solvent a (mL) by the treatment of the above spherical silver powder, a predetermined amount b (mL) was aliquoted, and when the carbon amount C (intensity) obtained by measuring the remaining solid and the amount of spherical silver powder dissolved in the acid was M (g), the stearic acid mass X and concentration Y were calculated by the following formulas (A) and (B), respectively. X (mg) = (C / A × a / b) ··· (A) Y (%) = X / (M × 1000) × 100 ··· (B)

[0028] When oleic acid was used as the surface treatment agent, the carbon content was measured and determined in the same manner as above. Calculations were also made for oleic acid using the calibration curve for stearic acid. Since the molecular weight of stearic acid is 284.48, of which the carbon content is 216.19, and the molecular weight of oleic acid is 282.46, of which the carbon content is 216.19, the oleic acid concentration Y' was calculated using the following formula (C). Oleic acid concentration Y' (%) = Y × (216.19 / 284.48) × (282.46 / 216.19) (C)

[0029] In addition, in this specification, for example, when the surface treatment agent for the spherical silver powder is benzotriazole, the content of the benzotriazole was measured according to the quantitative analysis method for benzotriazole and benzotriazole salts described in Japanese Patent No. 5523153. Specifically, the spherical silver powder was first washed with an aqueous hydrochloric acid solution, and the washings were subjected to quantitative analysis by absorptiometry.

[0030] For example, the benzotriazole content was determined by weighing 0.2 g of spherical silver powder, washing it with a hydrochloric acid solution, and quantitatively analyzing it by absorptiometry according to the following procedure. First, concentrated hydrochloric acid (Kanto Scientific Co., Ltd., special grade) was diluted with pure water to prepare an 18% by mass hydrochloric acid solution. Next, 0.2 g of silver powder and 20 mL of the hydrochloric acid solution were placed in a 100 mL glass beaker and heated to a boil. After boiling began, heating was continued for 15 minutes to maintain the boiling state. During heating, 18% by mass hydrochloric acid solution was added to the solution so that the volume did not exceed the volume before heating, so as not to evaporate the solution to dryness. After heating, the solution was cooled to 25°C and filtered. An 18% by mass hydrochloric acid solution was added to the filtrate to adjust the volume to 20 mL, and a measurement sample solution for absorptiometry was prepared. Furthermore, the absorbance of the measurement sample solution was measured using a spectrophotometer (Hitachi, U-3210) to measure the peak absorbance at 272.8 nm ± 0.5 nm. A calibration curve was previously prepared to determine the relationship between benzotriazole concentration and absorbance. The concentration of benzotriazole in the filtrate was calculated from this calibration curve and the absorbance value of the measurement sample solution. The amount (mass%) of spherical silver powder contained in the filtrate was then calculated based on this concentration, the volume of the filtrate, and the weight of the silver powder.

[0031] The type of surface treatment agent can be identified by qualitative analysis by gas chromatography of the surface treatment agent vaporized by heating the spherical silver powder.

[0032] <Particle size distribution> In this specification, the volume-based cumulative 10% particle diameter (D 10 ), cumulative 50% particle diameter (D 50 ), cumulative 90% particle diameter (D 90 ) was measured using a laser diffraction / scattering particle size distribution analyzer (Microtrac MT-3300 EXII, manufactured by Microtrac Bell Corporation). For the measurement, 0.1 g of sample (silver powder) was added to 40 mL of isopropyl alcohol (IPA) and dispersed. An ultrasonic homogenizer (manufactured by Nippon Seiki Seisakusho, device name: US-150T; 19.5 kHz, tip diameter 18 mm) was used for dispersion. The dispersion time was 2 minutes. The dispersed sample was loaded into the above-mentioned device, and the particle size distribution was determined using the attached analysis software.

[0033] (spherical silver powder) The spherical silver powder of the present invention contains a surface treatment agent, and in thermal expansion coefficient measurements, the maximum value of the thermal expansion coefficient is 0.3% or less based on the value at 50°C, and the BET specific surface area is 0.1 m 2 / g or more 0.8m 2 / g or less, and D 90The value of is 2.0 μm or more and 4.0 μm or less. Hereinafter, "thermal expansion coefficient based on the value at 50°C in thermal expansion coefficient measurement" may be simply referred to as "thermal expansion coefficient." As described above, the thermal expansion coefficient measurement can be performed by increasing the temperature from room temperature to 900°C at a rate of 10°C / min and measuring the expansion coefficient in the axial direction of a cylindrical measurement sample. The spherical silver powder described above can impart excellent fine-line printability to the conductive paste and can reduce the electrical resistance of the conductive film. This is presumably because, compared with flake powder, it is less likely to produce coarse particles that clog printing nozzles when made into a paste, and the specific surface area can be made smaller relative to the particle diameter, which prevents an increase in viscosity and suppresses swelling of the conductive coating film during firing.

[0034] Examples of surface treatment agents include fatty acids, compounds having an azole structure, fatty acid salts, surfactants, organometallic chelating agents, and protective colloids. Here, the surface treatment agent is preferably one or more surface treatment agents selected from the group consisting of fatty acids, compounds having an azole structure, and fatty acids, from the viewpoint of being able to adhere uniformly to the surface of the silver powder and achieving high dispersibility.

[0035] Examples of fatty acids include behenic acid, stearic acid, palmitic acid, myristic acid, lauric acid, ricinoleic acid, oleic acid, linoleic acid, and linolenic acid. These may be used alone or in combination of two or more. Among these, stearic acid and oleic acid are preferred. Examples of fatty acid salts include salts of the fatty acids listed above, such as sodium salts and potassium salts.

[0036] Examples of compounds having an azole structure include benzotriazole, sodium salt of benzotriazole, and potassium salt of benzotriazole. These may be used alone or in combination of two or more. Among these, benzotriazole and sodium benzotriazole are preferred.

[0037] The content of the surface treatment agent in the spherical silver powder is preferably 0.01% by mass or more, more preferably 0.03% by mass or more, and is preferably 1.00% by mass or less, more preferably 0.60% by mass or less, and even more preferably 0.40% by mass or less. If the content of the surface treatment agent in the spherical silver powder is 0.01% by mass or more, the dispersibility of the silver powder can be improved. On the other hand, if the content of the surface treatment agent in the spherical silver powder is 1.00% by mass or less, the dispersibility of the silver powder is maintained and sintering is not inhibited, thereby improving the electrical conductivity.

[0038] The maximum value of the thermal expansion coefficient of the spherical silver powder is 0.30% or less, preferably 0.25% or less, and more preferably 0.20% or less. If the maximum value of the thermal expansion coefficient of the spherical silver powder is 0.30% or less, swelling does not occur during firing, short circuits do not occur in the resulting cured film, and the electrical resistance value is maintained low. On the other hand, the thermal expansion coefficient of spherical silver powder Maximum may be 0% (no expansion) or may be 0.01% or more.

[0039] The BET specific surface area of ​​spherical silver powder is 0.10m 2 / g or more, and 0.25m 2 / g or more, and 0.30m 2 / g or more is more preferable, and 0.80m 2 / g or less, and 0.65m 2 / g or less, and 0.60m 2 It is more preferable that the saturation coefficient is 1 / g or less. The BET specific surface area of ​​the spherical silver powder is 0.10m 2 If the density is less than 1 / g, the particle size becomes too large and is therefore unsuitable for fine line printing. On the other hand, the BET specific surface area of ​​spherical silver powder is 0.80m 2 If the particle size is greater than / g, the particle size becomes too small, and the viscosity of the paste becomes high, making it unsuitable for fine line printing.

[0040] Spherical silver powder D 90 The value of is 2.0 μm or more, preferably 2.1 μm or more, and is 4.0 μm or less, preferably 3.7 μm or less. D 90 If the value is greater than 4.0 μm, the particles tend to clog the printing plate, making printing difficult, and this is unsuitable.

[0041] Spherical silver powder D 50 The value of is preferably 1.0 μm or more, more preferably 1.3 μm or more, and is preferably 2.5 μm or less, more preferably 2.2 μm or less.

[0042] Spherical silver powder D 10 The value of is preferably 0.5 μm or more, more preferably 0.7 μm or more, and is preferably 1.2 μm or less. Spherical silver powder D 90 , D 50 , D 10 If the value is above the upper limit, the particle size is too large and the particle is unsuitable for fine line printing, and if it is below the lower limit, the viscosity of the paste becomes too high and the paste is unsuitable for fine line printing.

[0043] (Method of manufacturing spherical silver powder) The method for producing spherical silver powder of the present invention (hereinafter sometimes simply referred to as the "production method") involves adding a reducing agent to an aqueous reaction system containing silver ions and a chelating agent made of a polymer, thereby reducing and precipitating silver particles. The production method of the present invention includes a carbon dioxide concentration adjustment step in which, before adding the reducing agent to the aqueous reaction system, the ratio of the total molar concentration of carbon dioxide (CO3) to the total molar concentration of silver (Ag) in the aqueous reaction system (hereinafter sometimes referred to as "CO3 / Ag") is adjusted to between 0.004 and 0.051, and a surface treatment agent addition step in which, after the silver particles have precipitated, a surface treatment agent is added to the aqueous reaction system.

[0044] The above-described manufacturing method can produce spherical silver powder that can impart excellent fine-line printability to conductive pastes. This is presumably because, by maintaining the CO3 / Ag ratio in the aqueous reaction system at or above a predetermined level, the rapid reduction of silver ions by the reducing agent—in other words, the reducing action of the reducing agent—is moderated, effectively suppressing the formation of irregularly shaped particles such as confetti-like particles. Furthermore, by maintaining the CO3 / Ag ratio in the aqueous reaction system at or below a predetermined level, it is presumably possible to effectively prevent the particle size of the resulting silver particles from becoming too small, thereby effectively preventing the viscosity of the conductive paste from becoming too high when used in the paste. In this specification, confetti-like particles refer to particles having numerous protrusions radiating from the center of the particle.

[0045] The aqueous reaction system contains silver ions and a polymeric chelating agent (hereinafter, simply referred to as "chelating agent"). The aqueous reaction system can be obtained, for example, by adding the chelating agent to an aqueous solution containing silver ions.

[0046] Here, the aqueous solution containing silver ions is not particularly limited, but an aqueous silver nitrate solution or the like can be used. In one embodiment, it is preferable to obtain a silver ammine complex by adding ammonia water or an ammonium salt to an aqueous solution containing silver ions, since this allows for effective control of the shape and particle size distribution of the resulting spherical silver powder. Note that the ammonia water or ammonium salt is preferably added in an amount equal to or greater than the molar amount of ammonia per mol of silver.

[0047] Specific examples of preferred chelating agents according to the present invention include amino compounds and imine compounds. Among these, polyethyleneimine (PEI) is preferred. In particular, the imine compound PEI has a network structure containing both primary amines (-NH) and secondary amines (=NH) in the molecule, which provides preferred results in the present invention.

[0048] The chelating agent according to the present invention preferably has a weight-average molecular weight of less than 600, and more preferably 145 or more and 600 or less. This is because a chelating agent with a weight-average molecular weight of 145 or more has the effect of producing highly dispersible silver particles. On the other hand, a polymeric amine with a weight-average molecular weight of 600 or less is believed to ensure the water solubility of the polymeric amine, and to prevent the polymeric amine from remaining on the surface or inside the produced silver particles. The weight average molecular weight of the chelating agent can be measured by the GPC-MALS method.

[0049] In the aqueous reaction system, the proportion of the chelating agent relative to the total mass of silver is preferably 0.001% by mass or more, more preferably 0.040% by mass or more, and is preferably 1.000% by mass or less.

[0050] The reducing agent to be added to the aqueous reaction system (after the carbon dioxide concentration adjustment step described below) is not particularly limited, but is preferably a reducing agent that does not contain carbon dioxide. Specific reducing agents that can be used include, for example, hydrazine, formalin, sodium borohydride, glucose, and hypophosphorous acid. Among these, hydrazine is particularly preferred from the viewpoints of stable reactivity and ability to rapidly reduce silver ions.

[0051] The carbonation concentration adjusting step and the surface treatment agent adding step included in the production method of the present invention will be described below, but the production method of the present invention is not limited to a method including only these steps. For example, the production method of the present invention may optionally include steps other than the carbonation concentration adjusting step and the surface treatment agent adding step (hereinafter, sometimes referred to as "other steps"). Examples of other steps include a separation step in which the reduced and precipitated silver particles are separated from the aqueous reaction system and dried.

[0052] <Carbonation concentration adjustment process> In the carbon dioxide concentration adjusting step, the ratio of the total molar concentration of carbon dioxide to the total molar concentration of silver in the aqueous reaction system is adjusted to 0.004 or more and 0.051 or less before adding a reducing agent to the aqueous reaction system. The CO3 / Ag ratio in the aqueous reaction system is 0.004 or more and 0.051 or less, preferably 0.031 or less.

[0053] Here, the adjustment of CO3 / Ag is not particularly limited, and can be carried out by, for example, a method of adding a predetermined amount of carbonated water and / or carbonate to the aqueous reaction system, a method of blowing carbon dioxide gas into the aqueous reaction system (so-called bubbling), etc. From the viewpoint of operability, the method of adding a predetermined amount of carbonate to the aqueous reaction system is preferred.

[0054] When a carbonate is used to prepare CO3 / Ag, the carbonate may be used in the form of an aqueous solution. The concentration of the aqueous solution of carbonate is, for example, 1% by mass or more and 30% by mass or less. The carbonate is not particularly limited, and examples thereof include sodium carbonate and potassium carbonate.

[0055] <Surface treatment agent addition process> In the surface treatment agent addition step, after the silver particles have precipitated, a surface treatment agent is added to the aqueous reaction system, thereby obtaining surface-treated silver particles. The aqueous reaction system containing silver particles is usually a suspension (so-called slurry) or dispersion in which silver particles are dispersed.

[0056] Examples of surface treatment agents include fatty acids, compounds having an azole structure, fatty acid salts, surfactants, organometallic chelating agents, and protective colloids. Here, the surface treatment agent is preferably one or more surface treatment agents selected from the group consisting of fatty acids, compounds having an azole structure, and fatty acid salts, from the viewpoint of being easily and uniformly attached to the surface of the silver powder.

[0057] Examples of fatty acids include behenic acid, stearic acid, palmitic acid, myristic acid, lauric acid, ricinoleic acid, oleic acid, linoleic acid, and linolenic acid. These may be used alone or in combination of two or more. Among these, stearic acid and oleic acid are preferred. Examples of fatty acid salts include salts of the fatty acids listed above, such as sodium salts and potassium salts.

[0058] Examples of compounds having an azole structure include benzotriazole, sodium salt of benzotriazole, and potassium salt of benzotriazole. These may be used alone or in combination of two or more. Among these, benzotriazole and sodium benzotriazole are preferred.

[0059] The amount of the surface treatment agent added in the surface treatment agent addition step is usually 0.01% by mass or more and 1.00% by mass or less relative to the mass of silver contained in the aqueous reaction system.

[0060] <Separation process> In an optional separation step, the reduced and precipitated silver particles are separated from the aqueous reaction system and dried. In addition, the separation step may optionally include a washing and recovery step in which the separated silver particles and the like are recovered and washed.

[0061] In the washing and recovery step, for example, the aggregates of the separated silver particles, etc. are formed into a cake-like form, and the cake of aggregates of the silver particles, etc. is washed. Washing in the washing and recovery step may be performed using, for example, pure water. Dehydration in the washing and recovery step may be performed by, for example, decantation or a filter press. The end point of washing may be determined using the electrical conductivity of the washing water. Specifically, the end of washing may be determined when the electrical conductivity of the washing water becomes a predetermined value or less. The silver particles, etc. after washing may be subjected to a drying step in an aggregated state, such as a cake-like form.

[0062] In the drying step, aggregates of silver particles and the like that contain moisture and are in an agglomerated state are 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 aggregates of silver particles and the like, or the cake or spherical silver powder in the drying process may be placed in a mixer having a stirring rotor and stirred, thereby applying a dispersing force to the cake or spherical silver powder in the drying process and promoting dispersion and drying.

[0063] In the drying step, the temperature of the spherical silver powder is usually 100° C. or less. If the temperature of the spherical silver powder is 100° C. or less, it is possible to effectively prevent the silver particles in the spherical silver powder from sintering with each other.

[0064] Since the spherical 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 spherical silver powder. Here, improving the handleability of the spherical silver powder means, for example, ensuring fluidity to the extent that it does not interfere with the supply operation into an apparatus, or loosening the spherical silver powder to an appropriate extent so that processing in the apparatus proceeds efficiently.

[0065] The method for 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 a stirring blade to crush the particles and fluidize the spherical silver powder, and for example, a sample mill, a blender, a coffee mill, or the like can be used. [Example]

[0066] 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 confirmation of the spherical silver powder (particle shape), thermomechanical analysis of the spherical silver powder (calculation of the maximum thermal expansion coefficient), measurement of the BET specific surface area, quantification of the surface treatment agent, and measurement of the particle size distribution were carried out by the methods described above.

[0067] <Evaluation of fine line printability> First, the silver powder and aluminum powder (SEM average diameter 2.0 μm) obtained in the examples and comparative examples were mixed with glass powder (containing PbO as the main component, and also containing B2O3, SiO2, and other oxides) as glass frit, ethyl cellulose, Texanol, butyl carbitol acetate, tributyl citrate, 1-octanol, oleic acid, triacetin, methylphenylpolysiloxane, hydrogenated castor oil, and fatty acid amide to obtain the composition shown in Table 1, thereby obtaining a mixture. Next, the obtained mixture was premixed under the conditions of a planetary stirrer (revolution 1000 rpm), and then kneaded with a three-roll mill (manufactured by EXAKT) to obtain a conductive paste. The conductive paste obtained above was used to print a line shape using screen printing. The line had a design line width of 12 μm and a length of 150 mm. A Microtec printer was used for printing, with a squeegee speed of 350 mm / s. A silicon substrate (for solar cell applications, textured and coated with SiNx) approximately 170 μm thick was used for printing. After printing, the substrate was dried for 5 minutes in a dryer set at 200°C and then baked in a solar cell baking furnace (NGK) at a peak temperature of 750°C on the wafer's top surface to produce a sample. The resulting sample was observed with a digital microscope (Keyence VHX-5000) to check for any breaks, and the fine line printability was evaluated using the following criteria. A: No disconnection was detected. B: A break was confirmed. C: Printing was not possible because the paste viscosity was too high.

[0068] Example 1 87,410 g of a silver nitrate aqueous solution containing 1,260 g (11.7 mol) of Ag was prepared, and 2,560 g of ammonia water with a concentration of 28 mass % was added thereto to prepare an aqueous solution containing silver ions, and the liquid temperature was set to 35°C. To the aqueous solution containing silver ions, 63 g of a 10% by weight sodium carbonate solution was added to adjust the ratio of the total molar concentration of carbon dioxide to the total molar concentration of silver in the aqueous reaction system (CO3 / Ag) to 0.005. Next, 13 g of an aqueous solution containing 5% by weight of polyethyleneimine (PEI) with a weight-average molecular weight of 600 as a chelating agent (0.052% by weight relative to the Ag mass) was added to the aqueous reaction system after the CO3 / Ag adjustment to prepare an aqueous reaction system containing silver ions and a chelating agent. 2684 g of a 6.2% by weight hydrazine solution as a reducing agent was then added to the aqueous reaction system and thoroughly stirred to obtain a slurry containing silver powder. 65 g of a 3.5% by weight oleic acid (surface treatment agent) solution in neoethanol was then added to the resulting slurry, thoroughly stirred, and then aged. The aged slurry was filtered, and the residue was washed with water. The washed residue was dried, and 150 g of silver powder was added to a sample mill (SK-M10, manufactured by Kyoritsu Riko Co., Ltd.) and crushed twice for 90 seconds to obtain the silver powder according to Example 1. The obtained silver powder was used to confirm the spherical silver powder (particle shape), to perform thermomechanical analysis of the spherical silver powder (calculation of the maximum thermal expansion coefficient), to measure the BET specific surface area, to measure the content of the surface treatment agent, to measure the particle size distribution, and to evaluate the fine-line printability. The results are shown in Table 3. Graphs of the thermomechanical analysis of the silver powder are shown in Figures 1 and 2, and a 10,000x SEM image of the silver powder is shown in Figure 23.

[0069] Example 2 96,690 g of an aqueous silver nitrate solution containing 1,620 g (15.0 mol) of Ag was prepared, and 3,280 g of ammonia water with a concentration of 28 mass % was added thereto to prepare an aqueous solution containing silver ions, and the liquid temperature was set to 35°C. To the aqueous solution containing silver ions, 65 g of a 10% by weight sodium carbonate solution was added to adjust the ratio of the total molar concentration of carbon dioxide to the total molar concentration of silver in the aqueous reaction system (CO3 / Ag) to 0.004. Next, 16 g of an aqueous solution containing 5% by weight of polyethyleneimine (PEI) with a weight-average molecular weight of 600 as a chelating agent (0.049% by weight relative to the Ag mass) was added to the aqueous reaction system after the CO3 / Ag adjustment to prepare an aqueous reaction system containing silver ions and a chelating agent. 3441 g of a 6.2% by weight hydrazine aqueous solution as a reducing agent was then added to the aqueous reaction system and thoroughly stirred to obtain a slurry containing silver powder. Furthermore, 138.8 g of a 4.0% by weight benzotriazole (surface treatment agent) Solmix solution (133.3 g of Solmix (Solmix AP-7, manufactured by Japan Alcohol Sales Co., Ltd.) was added to 5.5 g of benzotriazole) and thoroughly stirred, followed by aging. The aged slurry was filtered, and the residue was washed with water. The washed residue was then dried and crushed twice for 90 seconds using a sample mill (SK-M10, manufactured by Kyoritsu Riko Co., Ltd.) containing 150 g of silver powder, to obtain the silver powder of Example 2. The obtained silver powder was used to confirm the spherical silver powder (particle shape), to perform thermomechanical analysis of the spherical silver powder (calculation of the maximum thermal expansion coefficient), to measure the BET specific surface area, to measure the content of the surface treatment agent, to measure the particle size distribution, and to evaluate the fine-line printability. The results are shown in Table 3. Graphs of the thermomechanical analysis of the silver powder are shown in Figures 3 and 4, and a 10,000x SEM image of the silver powder is shown in Figure 24.

[0070] Example 3 96,190 g of an aqueous silver nitrate solution containing 1,620 g (15.0 mol) of Ag was prepared, and 3,280 g of ammonia water with a concentration of 28 mass % was added thereto to prepare an aqueous solution containing silver ions, and the liquid temperature was set to 35°C. To the aqueous solution containing silver ions, 324 g of a 5% by weight sodium carbonate solution was added to adjust the ratio of the total molar concentration of carbon dioxide to the total molar concentration of silver in the aqueous reaction system (CO3 / Ag) to 0.010. Next, 202 g of an aqueous solution containing 0.4% by weight of polyethyleneimine (PEI) with a weight-average molecular weight of 600 as a chelating agent (0.050% by weight relative to the Ag mass) was added to the aqueous reaction system after the CO3 / Ag adjustment to prepare an aqueous reaction system containing silver ions and a chelating agent. Then, 3512 g of a 6.2% by weight hydrazine aqueous solution as a reducing agent was added to the aqueous reaction system and thoroughly stirred to obtain a slurry containing silver powder. Furthermore, 340 g of a 0.86% by weight stearic acid (surface treatment agent) emulsion was added to the resulting slurry, thoroughly stirred, and then aged. The aged slurry was filtered, and the residue was washed with water. The washed residue was dried, and 150 g of silver powder was added to a sample mill (SK-M10, manufactured by Kyoritsu Riko Co., Ltd.) and crushed twice for 90 seconds to obtain the silver powder according to Example 3. The obtained silver powder was used to confirm the spherical silver powder (particle shape), to perform thermomechanical analysis of the spherical silver powder (calculation of the maximum thermal expansion coefficient), to measure the BET specific surface area, to measure the content of the surface treatment agent, to measure the particle size distribution, and to evaluate the fine-line printability. The results are shown in Table 3. Graphs of the thermomechanical analysis of the silver powder are shown in Figures 5 and 6, and a 10,000x SEM image of the silver powder is shown in Figure 25.

[0071] Example 4 The silver powder of Example 4 was obtained in the same manner as Example 2, except that the amount of 10 mass% sodium carbonate aqueous solution was changed from 65 g to 275 g to adjust the CO3 / Ag to 0.017, and the amount of 3.7 mass% benzotriazole sodium (surface treatment agent) aqueous solution was changed to 147 g. The obtained silver powder was used to confirm the spherical silver powder (particle shape), to perform thermomechanical analysis of the spherical silver powder (calculation of the maximum thermal expansion coefficient), to measure the BET specific surface area, to measure the content of the surface treatment agent, to measure the particle size distribution, and to evaluate the fine-line printability. The results are shown in Table 3. Graphs of the thermomechanical analysis of the silver powder are shown in Figures 7 and 8, and a 10,000x SEM image of the silver powder is shown in Figure 26.

[0072] Example 5 The silver powder of Example 5 was obtained in the same manner as Example 1, except that the amount of 10 mass % aqueous sodium carbonate solution was changed from 63 g to 189 g and the CO3 / Ag ratio was adjusted to 0.015. The obtained silver powder was used to confirm the spherical silver powder (particle shape), to perform thermomechanical analysis of the spherical silver powder (calculation of the maximum thermal expansion coefficient), to measure the BET specific surface area, to measure the content of the surface treatment agent, to measure the particle size distribution, and to evaluate the fine-line printability. The results are shown in Table 3. Graphs of the thermomechanical analysis of the silver powder are shown in Figures 9 and 10, and an SEM image of the silver powder at 10,000x magnification is shown in Figure 27.

[0073] Example 6 The silver powder of Example 6 was obtained in the same manner as Example 3, except that the amount of 5% by mass sodium carbonate aqueous solution was changed from 324 g to 971 g to adjust the CO3 / Ag to 0.031, and 340 g of 0.86% by mass stearic acid (surface treatment agent) emulsion was changed to 327 g of 1.3% by mass stearic acid emulsion aqueous solution. The obtained silver powder was used to confirm the spherical silver powder (particle shape), to perform thermomechanical analysis of the spherical silver powder (calculation of the maximum thermal expansion coefficient), to measure the BET specific surface area, to measure the content of the surface treatment agent, to measure the particle size distribution, and to evaluate the fine-line printability. The results are shown in Table 3. Graphs of the thermomechanical analysis of the silver powder are shown in Figures 11 and 12, and a 10,000x SEM image of the silver powder is shown in Figure 28.

[0074] Example 7 The silver powder of Example 7 was obtained in the same manner as Example 1, except that 421 g of a 15 mass% sodium carbonate aqueous solution was added to adjust the CO3 / Ag to 0.051, and the amount of a 3.5 mass% oleic acid (surface treatment agent) neoethanol solution was changed from 65 g to 126 g. The obtained silver powder was used to confirm the spherical silver powder (particle shape), to perform thermomechanical analysis of the spherical silver powder (calculation of the maximum thermal expansion coefficient), to measure the BET specific surface area, to measure the content of the surface treatment agent, to measure the particle size distribution, and to evaluate the fine-line printability. The results are shown in Table 3. Graphs of the thermomechanical analysis of the silver powder are shown in Figures 13 and 14, and an SEM image of the silver powder at 10,000x magnification is shown in Figure 29.

[0075] (Comparative Example 1) 3887 g of an aqueous silver nitrate solution containing 43.16 g (0.40 mol) of Ag was prepared, and 97.1 g of ammonia water with a concentration of 28 mass % was added thereto to prepare an aqueous solution containing silver ions, and the liquid temperature was set to 34.5°C. To the aqueous solution containing silver ions, 0.043 g (0.100 mass% relative to the mass of Ag) of polyethyleneimine (PEI) with a weight-average molecular weight of 300 was added as a chelating agent to prepare an aqueous reaction system containing silver ions and the chelating agent. Next, 7.5 g of a hydrazine aqueous solution as a reducing agent was added to the aqueous reaction system and thoroughly stirred to obtain a slurry containing silver powder. 0.12 mass% of stearic acid relative to the mass of silver was added to the obtained slurry, which was then thoroughly stirred and aged. The aged slurry was filtered, and the residue was washed with water. The washed residue was then pulverized and dried to obtain the silver powder of Comparative Example 1. The obtained silver powder was used to confirm the spherical silver powder (particle shape), to perform thermomechanical analysis of the spherical silver powder (calculation of the maximum thermal expansion coefficient), to measure the BET specific surface area, to measure the content of the surface treatment agent, to measure the particle size distribution, and to evaluate the fine line printability. The results are shown in Table 3. Graphs of the thermomechanical analysis of the silver powder are shown in Figures 15 and 16.

[0076] (Comparative Example 2) The silver powder of Comparative Example 2 was obtained in the same manner as Comparative Example 1, except that the chelating agent was changed from polyethyleneimine (PEI) with a weight-average molecular weight of 300 to polyethyleneimine (PEI) with a weight-average molecular weight of 600, and the surface treatment agent was changed from stearic acid to benzotriazole. The obtained silver powder was used to confirm the spherical silver powder (particle shape), to perform thermomechanical analysis of the spherical silver powder (calculation of the maximum thermal expansion coefficient), to measure the BET specific surface area, to measure the content of the surface treatment agent, to measure the particle size distribution, and to evaluate the fine line printability. The results are shown in Table 3. Graphs of the thermomechanical analysis of the silver powder are shown in Figures 17 and 18.

[0077] (Comparative Example 3) 3637 g of an aqueous silver nitrate solution containing 43.16 g (0.40 mol) of Ag was prepared, and 87.6 g of ammonia water with a concentration of 28 mass % was added thereto to prepare an aqueous solution containing silver ions, and the liquid temperature was set to 35°C. To the aqueous solution containing silver ions, 80 g of a 20% by mass sodium hydroxide solution was added. Next, 0.86 g (0.100% by mass relative to the Ag mass) of an aqueous solution containing 0.043 g of polyethyleneimine (PEI) with a weight-average molecular weight of 600 as a chelating agent was added to the aqueous reaction system after the addition of the sodium hydroxide solution, thereby preparing an aqueous reaction system containing silver ions and a chelating agent. 243 g of a 2.5% by mass hydrazine aqueous solution as a reducing agent was then added to the aqueous reaction system and thoroughly stirred to obtain a slurry containing silver powder. 3.34 g of a 1.5% stearic acid (surface treatment agent) emulsion was added to the obtained slurry, thoroughly stirred, and then aged. The aged slurry was filtered, and the filter cake was washed with water. The washed filter cake was then dried and milled twice for 90 seconds using a sample mill (SK-M10, manufactured by Kyoritsu Riko Co., Ltd.) with 150 g of silver powder. The mill mill was then milled twice for 90 seconds to obtain the silver powder according to Comparative Example 3. The obtained silver powder was used to confirm the spherical silver powder (particle shape), to perform thermomechanical analysis of the spherical silver powder (calculation of the maximum thermal expansion coefficient), to measure the BET specific surface area, to measure the content of the surface treatment agent, to measure the particle size distribution, and to evaluate the fine line printability. The results are shown in Table 3. Graphs of the thermomechanical analysis of the silver powder are shown in Figures 19 and 20, and a 10,000x SEM image of the silver powder is shown in Figure 30. Note that confetti-shaped particles were observed near the center of the SEM image in Figure 30.

[0078] Comparative Example 4 3572 g of an aqueous solution of silver nitrate containing 56.11 g (0.52 mol) of Ag was prepared, and 113.9 g of ammonia water with a concentration of 28 mass % was added thereto to prepare an aqueous solution containing silver ions, and the liquid temperature was set to 35°C. To the aqueous solution containing silver ions, 78.55 g of a 5% by weight sodium carbonate solution was added to adjust the ratio of the total molar concentration of carbon dioxide to the total molar concentration of silver in the aqueous reaction system (CO3 / Ag) to 0.071. Next, 0.561 g (0.050% by weight relative to Ag) of an aqueous solution containing 0.028 g of polyethyleneimine (PEI) with a weight-average molecular weight of 600 as a chelating agent was added to the aqueous reaction system after the addition of the sodium hydroxide solution after adjusting the CO3 / Ag ratio, to prepare an aqueous reaction system containing silver ions and a chelating agent. 300 g of a 2.5% by weight hydrazine aqueous solution as a reducing agent was then added to the aqueous reaction system and thoroughly stirred to obtain a slurry containing silver powder. 1.964 g of a 10% by weight oleic acid (surface treatment agent) solution in neoethanol was then added to the resulting slurry, thoroughly stirred, and then aged. The aged slurry was filtered, and the residue was washed with water. The washed residue was dried, and 150 g of silver powder was added to a sample mill (SK-M10, manufactured by Kyoritsu Riko Co., Ltd.) and crushed twice for 90 seconds to obtain the silver powder of Comparative Example 4. The obtained silver powder was used to confirm the spherical silver powder (particle shape), to perform thermomechanical analysis of the spherical silver powder (calculation of the maximum thermal expansion coefficient), to measure the BET specific surface area, to measure the content of the surface treatment agent, to measure the particle size distribution, and to evaluate the fine line printability. The results are shown in Table 3. Graphs of the thermomechanical analysis of the silver powder are shown in Figures 21 and 22.

[0079] [Table 1]

[0080] [Table 2]

[0081] [Table 3]

[0082] As is clear from the results in Tables 2 and 3, the spherical silver powders of Examples 1 to 7 can impart excellent fine-line printability to conductive pastes. When the CO3 / Ag ratio exceeds 0.051, the particle size becomes too small to print, as in Comparative Example 4. Furthermore, as is clear from the results in Tables 2 and 3, the spherical silver powders obtained by the manufacturing methods of Examples 1 to 7 can impart excellent fine line printability to conductive pastes. [Industrial Applicability]

[0083] According to the present invention, it is possible to provide a spherical silver powder that can impart excellent fine line printability to a conductive paste. The present invention also provides a method for producing spherical silver powder that can impart excellent fine line printability to conductive paste.

Claims

1. one or more surface treatment agents selected from the group consisting of fatty acids, compounds having an azole structure, and fatty acid salts are present; In thermal expansion coefficient measurement, the maximum value of the thermal expansion coefficient is 0.01% or more and 0.3% or less based on the value at 50°C, BET specific surface area is 0.1m ２ / g or more 0.8m ２ / g or less, D ９０ The spherical silver powder has a value of 2.0 μm or more and 4.0 μm or less.

2. D ５０ 2. The spherical silver powder according to claim 1, wherein the value of is 1.0 μm or more and 2.5 μm or less.

3. D １０ 2. The spherical silver powder according to claim 1, wherein the value of is 0.5 μm or more and 1.2 μm or less.

4. A method for producing spherical silver powder by adding a reducing agent to an aqueous reaction system containing silver ions and a chelating agent made of a polymer to reduce and precipitate silver particles, comprising: a carbonic acid concentration adjusting step of adjusting a ratio of a total molar concentration of carbonic acid to a total molar concentration of silver in the aqueous reaction system to be 0.004 or more and 0.051 or less before adding the reducing agent to the aqueous reaction system; a surface treatment agent addition step of adding a surface treatment agent to the aqueous reaction system after the silver particles have precipitated; Including, the chelating agent is polyethyleneimine having a weight average molecular weight of 600 or less, the reducing agent is hydrazine, The spherical silver powder thus obtained has a BET specific surface area of ​​0.10 m 2 / g or more and 0.80 m 2 / g or less, and a D 90 value of 2.0 μm or more and 4.0 μm or less.

5. 5. The method for producing spherical silver powder according to claim 4, wherein the surface treatment agent is one or more surface treatment agents selected from the group consisting of fatty acids, compounds having an azole structure, and fatty acid salts.

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