JP6129909B2 - Spherical silver powder and method for producing the same - Google Patents

Description

  The present invention relates to spherical silver powder and a method for producing the same, and particularly for conductive pastes used for electronic components such as internal electrodes of multilayer capacitors and conductor patterns of circuit boards, plasma display panels and solar cell boards and circuits. The present invention relates to spherical silver powder and a method for producing the same.

  Conventionally, as a method for forming internal electrodes of multilayer capacitors, conductor patterns of circuit boards, electrodes and circuits of substrates for solar cells and plasma display panels (PDP), silver powder is added to an organic vehicle together with glass frit and kneaded. After forming the baking type conductive paste manufactured by the above in a predetermined pattern on the substrate, the organic component is removed by heating at a temperature of 500 ° C. or higher, and the silver particles are sintered together to form the conductive film. The forming method is widely used.

  Silver powder for conductive paste used in such a method is suitable for reducing the size of electronic components, increasing the density of conductive patterns, making fine lines, etc., so that the particle size is moderately small and the particle size is uniform. It is requested.

  As a method for producing such silver powder for conductive paste, there is known a wet reduction method in which spherical silver powder is reduced and precipitated by adding a reducing agent to an aqueous reaction system containing silver ions (for example, Patent Document 1). reference).

JP-A-8-176620 (paragraph numbers 0008-0013)

  However, when spherical silver powder having the same particle size as that of spherical silver powder produced by a conventional wet reduction method is used as a baking type conductive paste, the silver particles are sufficiently separated even when heated at a temperature of about 600 ° C. In some cases, a good conductive film could not be formed.

  Therefore, in view of such conventional problems, the present invention provides a spherical silver powder having a particle size comparable to that of spherical silver powder produced by a conventional wet reduction method and capable of being fired at a lower temperature, and a method for producing the same. The purpose is to provide.

  As a result of earnest research to solve the above problems, the present inventors mixed a reducing agent-containing solution containing an aldehyde as a reducing agent while generating cavitation in an aqueous reaction system containing silver ions, By reducing and precipitating silver particles, it was found that spherical silver powder having a particle size comparable to that of spherical silver powder produced by a conventional wet reduction method and calcinable at a lower temperature can be produced. It came to be completed.

  That is, in the method for producing spherical silver powder according to the present invention, an aqueous reaction system containing silver ions is mixed with a reducing agent-containing solution containing an aldehyde as a reducing agent while generating cavitation to reduce and precipitate silver particles. It is characterized by that. In this method for producing spherical silver powder, it is preferable to generate cavitation by irradiating an aqueous reaction system containing silver ions with ultrasonic waves. The aqueous reaction system containing silver ions is preferably an aqueous solution containing a silver ammonia complex, and the reducing agent-containing solution is preferably a solution containing formaldehyde or acetaldehyde. Furthermore, it is preferable that the silver particles that have been reduced and precipitated are separated into solid and liquid, washed, and then dried at 100 ° C. or lower. Moreover, it is preferable that spherical silver powder has the space | gap closed inside the particle | grain.

In addition, the spherical silver powder according to the present invention is characterized by having voids closed inside the particles. In the spherical silver powder, the average particle diameter _{D 50} of the spherical silver powder by a laser diffraction method is preferably in the range of 0.1 to 10 [mu] m. The true specific gravity of the spherical silver powder is preferably 9.8 g / cm ³ or less. Furthermore, it is preferable that the content of impurity elements other than carbon, nitrogen, oxygen and hydrogen contained in the spherical silver powder is less than 100 ppm.

  Furthermore, the conductive paste according to the present invention is characterized by using the spherical silver powder as a conductor.

  According to the present invention, an aqueous reaction system containing silver ions is mixed with a reducing agent-containing solution containing an aldehyde as a reducing agent while generating cavitation, thereby reducing and precipitating silver particles. A spherical silver powder having the same particle size as that of the spherical silver powder produced by the reduction method and capable of being fired at a lower temperature can be produced.

It is a figure which shows the field emission type | mold scanning electron microscope (FE-SEM) photograph of the cross section of the spherical silver powder obtained in Example 1. FIG. It is a figure which shows the FE-SEM photograph of the cross section of the spherical silver powder obtained in Example 2. FIG. It is a figure which shows the FE-SEM photograph of the cross section of the spherical silver powder obtained in Example 3. FIG. It is a figure which shows the FE-SEM photograph of the cross section of the spherical silver powder obtained by the comparative example 1.

  In the embodiment of the method for producing spherical silver powder according to the present invention, silver particles are reduced by mixing an aqueous reaction system containing silver ions with a reducing agent-containing solution containing aldehyde as a reducing agent while generating cavitation. Precipitate.

  Cavitation (cavity phenomenon) is a physical phenomenon in which bubbles are generated and disappeared in a short time due to a local pressure difference in the liquid, and can be generated by ultrasonic irradiation or a homogenizer for emulsification. However, in order to generate | occur | produce in the whole aqueous reaction system containing silver ion, it is preferable to generate | occur | produce by irradiating an ultrasonic wave to the aqueous reaction system containing silver ion. The oscillation frequency of the ultrasonic wave to be irradiated may be any oscillation frequency that can generate cavitation, but is preferably 28 to 40 kHz. Moreover, what is necessary is just to set the output of an ultrasonic wave according to the liquid quantity with which it uses for the reductive precipitation reaction of silver particle. The ultrasonic irradiation may be performed when the reducing agent-containing solution is added to reduce and precipitate silver particles, and may be performed before and after that.

  As an aqueous reaction system containing silver ions, an aqueous solution or slurry containing silver nitrate, a silver complex or a silver intermediate can be used. An aqueous solution containing a silver complex can be produced by adding aqueous ammonia or ammonium salt to an aqueous silver nitrate solution or a silver oxide suspension. Among these, it is preferable to use a silver ammine complex aqueous solution obtained by adding ammonia water to a silver nitrate aqueous solution so that the silver powder has an appropriate particle size and a spherical shape. Since the coordination number of ammonia in the silver ammine complex is 2, 2 mol or more of ammonia is added per 1 mol of silver. Further, if the amount of ammonia added is too large, the complex becomes too stable and the reduction is difficult to proceed. Therefore, the amount of ammonia added is preferably 8 moles or less per mole of silver. If adjustments such as increasing the amount of reducing agent added are made, it is possible to obtain spherical silver powder having an appropriate particle size even if the amount of ammonia added exceeds 8 mol.

  As the reducing agent, a compound having an aldehyde group can be used, but formaldehyde or acetaldehyde is preferably used. Moreover, an aldehyde single-piece | unit may be used, an aldehyde compound may be used in combination, and a mixture with water or alcohol like formalin may be used. When a reducing agent-containing solution containing aldehyde is added as such a reducing agent to reduce and precipitate silver particles, spherical silver powder having voids closed inside the particles can be obtained by generating cavitation. . The addition amount of the reducing agent may be excessive with respect to the aqueous reaction system containing silver ions, but the silver ions remaining in the aqueous reaction system containing silver ions are reduced to reduce the loss of silver as a noble metal. In order to suppress it, the amount of the reducing agent required for making the unreduced portion of silver ions 10 ppm or less is sufficient.

  Moreover, you may add a pH adjuster to the aqueous reaction system containing a silver ion. As the pH adjuster, general acids and bases can be used. For example, nitric acid, sodium hydroxide, and the like can be used.

  Further, a surface treating agent may be added to an aqueous reaction system containing silver ions. As the surface treatment agent, fatty acids, fatty acid salts, surfactants, organometallic compounds, chelating agents, polymer dispersing agents and the like can be used. Examples of fatty acids include propionic acid, caprylic acid, lauric acid, myristic acid, palmitic acid, stearic acid, behenic acid, acrylic acid, oleic acid, linoleic acid, arachidonic acid and the like. Examples of fatty acid salts include lithium, sodium, potassium, barium, magnesium, calcium, aluminum, iron, cobalt, manganese, lead, zinc, tin, strontium, zirconium, silver, copper, and other fatty acids and salts formed. Things can be mentioned. Examples of surfactants include anionic surfactants such as alkylbenzene sulfonates and polyoxyethylene alkyl ether phosphates, cationic surfactants such as aliphatic quaternary ammonium salts, and amphoteric compounds such as imidazolinium betaine. Nonionic surfactants such as surfactants and polyoxyethylene alkyl ethers and polyoxyethylene fatty acid esters can be mentioned. Examples of organometallic compounds include acetylacetone tributoxyzirconium, magnesium citrate, diethylzinc, dibutyltin oxide, dimethylzinc, tetra-n-butoxyzirconium, triethylindium, triethylgallium, trimethylindium, trimethylgallium, monobutyltin oxide, tetra Examples include isocyanate silane, tetramethyl silane, tetramethoxy silane, polymethoxy siloxane, monomethyl triisocyanate silane, silane coupling agent, titanate coupling agent, aluminum coupling agent and the like. Examples of chelating agents include imidazole, oxazole, thiazole, selenazole, pyrazole, isoxazole, isothiazole, 1H-1,2,3-triazole, 2H-1,2,3-triazole, 1H-1,2,4. -Triazole, 4H-1,2,4-triazole, 1,2,3-oxadiazole, 1,2,4-oxadiazole, 1,2,5-oxadiazole, 1,3,4-oxa Diazole, 1,2,3-thiadiazole, 1,2,4-thiadiazole, 1,2,5-thiadiazole, 1,3,4-thiadiazole, 1H-1,2,3,4-tetrazole, 1,2 , 3,4-oxatriazole, 1,2,3,4-thiatriazole, 2H-1,2,3,4-tetrazole, 1,2,3,5-oxatriazole, , 3,5-thiatriazole, indazole, benzimidazole, benzotriazole or salts thereof, or oxalic acid, succinic acid, malonic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, dodecane Diacid, maleic acid, fumaric acid, phthalic acid, isophthalic acid, terephthalic acid, glycolic acid, lactic acid, oxybutyric acid, glyceric acid, tartaric acid, malic acid, tartronic acid, hydroacrylic acid, mandelic acid, citric acid, ascorbic acid, etc. Can be mentioned. Examples of the polymer dispersant include peptides, gelatin, collagen peptides, albumin, gum arabic, protalbic acid, lysalbic acid and the like. These surface treatment agents may be used individually by 1 type, and may use 2 or more types together.

  When the silver-containing slurry obtained by reducing and precipitating the silver particles is solid-liquid separated and washed with water, a lump cake containing 1 to 200% by mass of water with respect to silver and almost no fluidity is obtained. In order to accelerate the drying of the cake, the moisture in the cake may be replaced with a lower alcohol or the like. By drying this cake with a dryer such as a forced circulation air dryer, vacuum dryer, airflow dryer, etc., dry silver powder is obtained, but in order to maintain closed voids inside the silver powder particles, The drying temperature is preferably 100 ° C. or lower.

  Moreover, you may give a dry-type crushing process and a classification process to the obtained silver powder. Instead of this crushing, silver powder is put into an apparatus that can fluidize the particles mechanically, and the silver powder particles are mechanically collided with each other, so that irregularities and angular portions on the surface of the silver powder particles are removed. You may perform the surface smoothing process which makes it smooth. Moreover, you may perform a classification process after crushing and a smoothing process. In addition, you may dry, grind | pulverize, and classify | categorize using the integrated apparatus (Drymeister made from Hosokawa Micron Corporation, a micron dryer etc.) which can perform drying, grind | pulverize, and a classification.

  By the above method for producing spherical silver powder, spherical silver powder having a plurality of voids (a plurality of sealed holes) closed inside the particles can be produced. In general, the steps of baking a conductive paste using silver powder to form a conductive film are as follows: (1) evaporation of diluted solvent, (2) combustion of organic components (surface treatment agent and resin), (3) It consists of softening glass frit as a sintering aid and (4) liquid phase sintering of silver particles. In order to form the conductive film wiring at a lower temperature, it is necessary to lower the temperature for the above (2) to (4). When ordinary spherical silver powder is used, the burning of the organic component (2) above occurs only on the surface of the silver particles, so the effect of the burning of the organic component on the sintering of the silver particles is limited, When spherical silver powder having voids inside the particles is used, in addition to the combustion of the organic components on the surface of the silver particles, it is considered that the expansion of the substance in the voids and the energy of the combustion advantageously contribute to the sintering of the silver particles. . On the other hand, in the spherical silver powder in which the void inside the particle is open to the outside of the particle, the substance in the particle disappears during washing and drying during production, so that the expansion and combustion of the substance in the void during firing Does not occur and is almost the same as ordinary spherical silver powder. In addition, as a method for producing silver powder having voids inside the particles, a method in which metal particles lower than silver are used as base particles, silver is deposited on the surface of the metal particles by a substitution reaction, and the base particles are dissolved and removed can be considered. However, in this method, if the void inside the particle is a closed void, the metal component of the base particle remains, which may lead to a decrease in conductivity and a decrease in reliability due to oxidation. It is necessary to make these voids open to the outside of the particles.

  Spherical silver powder with voids closed inside the particles prevents transition metal and alkaline earth as impurities (contained by entrainment of the reaction mother liquor during reduction) in order to prevent an increase in electrical resistance and a decrease in reliability due to oxidation. It is preferable that the content of impurity elements such as metal species, alkali metal elements, aluminum and magnesium is less than 100 ppm.

The true specific gravity of the spherical silver powder having voids closed inside the particles is preferably 9.8 g / cm ³ or less. For a true specific gravity of the bulk silver is 10.5 g / cm ^3, if 9.8 g / cm ³ or less, so that the density of 7% or more with respect to the bulk silver is reduced. When the true specific gravity is larger than 9.8 g / cm ³ , the voids are too small or too small, or the voids are opened to the outside, and the effect of promoting the sintering due to the expansion of the material in the voids and the combustion thereof is ineffective. It may be enough.

The average particle size D ₅₀ by laser diffraction method of spherical silver powder having voids which are closed inside the particles is preferably from 0.1 to 10 [mu] m. Moreover, in order to use it for formation of the electrically conductive film which thinning progresses, it is more preferable that it is 5 micrometers or less. On the other hand, if the particle size is too small, the specific surface area increases, so that when used as a conductive paste, the viscosity increases, or when used as a photosensitive paste, the transmission of ultraviolet rays tends to be insufficient. It is preferably 1 μm or more.

  A conductive paste can be produced using the above spherical silver powder as a conductor. For example, a conductive paste can be produced by mixing the above spherical silver powder with a resin. Examples of the resin include an epoxy resin, an acrylic resin, a polyester resin, a polyimide resin, a polyurethane resin, a phenoxy resin, a silicone resin, and ethyl cellulose. These resins may be used alone or in combination of two or more. This conductive paste can be printed on the substrate by screen printing, offset printing, photolithography, or the like. In the case of screen printing, the viscosity of the conductive paste is preferably 30 to 100 Pa · s at 25 ° C. When the viscosity of the conductive paste is less than 30 Pa · s, “bleeding” may occur during printing, and when it exceeds 100 Pa · s, uneven printing may occur.

  Examples of the spherical silver powder and the production method thereof according to the present invention will be described in detail below.

[Example 1]
A 1 L beaker obtained by separating 753 g of an aqueous silver nitrate solution containing 8.63 g of silver is placed in an ultrasonic cleaning machine (US Cleaner USD-4R, output 160 W, manufactured by As One Co., Ltd.) containing water at a water temperature of 35 ° C., and an oscillation frequency of 40 kHz. Then, the ultrasonic irradiation was started and stirring was started.

  Next, 29.1 g of 28% by mass of ammonia water (equivalent to 3.0 equivalents with respect to silver) is added to the silver nitrate aqueous solution in the above beaker to form a silver ammine complex salt, and 30 seconds after the addition of the ammonia water. 0.48 g of a 20% by mass aqueous sodium hydroxide solution was added, and 20 minutes after the addition of aqueous ammonia, 48.7 g of a 27.4% by mass formaldehyde solution in which formalin was diluted with pure water (11. 30 equivalents later, 0.86 g of a 1.2 mass% stearic acid ethanol solution was added to obtain a slurry containing silver particles.

  Next, after the ultrasonic irradiation is finished, the slurry obtained by filtering the slurry containing silver particles and washing with water is dried for 10 hours with a vacuum dryer at 75 ° C., and the dried silver powder is dried with a coffee mill for 30 seconds. Crushing to obtain spherical silver powder.

  The spherical silver powder thus obtained was measured for particle size distribution, BET specific surface area, true specific gravity by laser diffraction method, observed the particle cross section, and the content of impurity elements and silver, and organic components (carbon, nitrogen, The content of oxygen and hydrogen was determined.

The particle size distribution by the laser diffraction method is as follows. 0.3 g of spherical silver powder is placed in 30 mL of isopropyl alcohol, and dispersed for 5 minutes by an ultrasonic cleaner with an output of 50 W, and a microtrack particle size distribution measuring device (Honeywell-Nikkiso 9320HRA (X -100)). As a result, D ₁₀ = 1.6 μm, D ₅₀ = 3.0 μm, and D ₉₀ = 5.3 μm.

The BET specific surface area was degassed at 60 ° C. for 10 minutes, and then measured by a BET one-point method by nitrogen adsorption using a specific surface area measuring device (Monosorb manufactured by Quanta Chrome). As a result, the BET specific surface area was 0.35 m ² / g.

The true specific gravity was measured using 10 g of spherical silver powder, isopropyl alcohol as an immersion liquid, and a 50 mL volume pycnometer. As a result, the true specific gravity was 9.6 g / cm ³ , and it was confirmed that the density was 9% lower than the true specific gravity of bulk silver of 10.5 g / cm ³ .

The observation of the particle cross-section was performed using a field emission scanning electron microscope (FE-SEM) (JSM-6700F manufactured by JEOL) on the cross section of the spherical silver powder cut by a focused ion beam (FIB) apparatus (JEM-9310FIB manufactured by JEOL). This was done by observing. As a result, as shown in the FE-SEM photograph of FIG. 1, it was confirmed that a closed void exists inside the spherical silver powder particles. Further, the area of all particles of the spherical silver powder in the FE-SEM photograph is 13.7 μm ² , the area of the particles having voids is 0.56 μm ² , and the ratio of the area of the particles having voids to the area of all particles is 4.1%. In view of the fact that the cross section of the observed spherical silver powder does not necessarily pass through the center of the particle, it is considered that there are voids in most of the particles. Further, the average void size determined from the FE-SEM photograph was 0.07 μm, corresponding to 2.3% with respect to the average particle size D ₅₀ , and it was confirmed that the void size was sufficient.

  The impurity element content is as follows: 1.0 g of spherical silver powder is dissolved in 10 mL of (1 + 1) nitric acid, 5 mL of (1 + 1) hydrochloric acid is added to precipitate silver chloride, and pure water is added to the filtrate obtained by filtration. After the volume was fixed, it was determined by quantitative analysis with ICP (iCAP6300 manufactured by Thermo Scientific). As a result, Cr = 1 ppm, Mn <1 ppm, Fe = 7 ppm, Co <5 ppm, Ni <5 ppm, Cu <1 ppm, Zn <5 ppm, Cd <1 ppm, Pb <5 ppm, Sn <10 ppm, Ca <1 ppm, Mg <1 ppm , S <50 ppm, Zr <1 ppm, Bi <10 ppm, Al <10 ppm, Sr <1 ppm, Ba <1 ppm, Li <100 ppm, Na <100 ppm, K <100 ppm, Rb <100 ppm, Cs <100 ppm, any impurity Was also confirmed to be less than 100 ppm.

  The silver content was determined by drying the silver chloride obtained by the above filtration and precisely weighing the weight of the silver chloride. As a result, the silver content was 99.37% by mass.

  Regarding the content of organic components (carbon, nitrogen, oxygen and hydrogen), the carbon content was quantified at a heating temperature of 1350 ° C. using a carbon / sulfur analyzer (EMIA-U510 manufactured by Horiba, Ltd.). However, the content of nitrogen, oxygen, and hydrogen was 745 ppm, 3020 ppm, and 800 ppm, respectively, as determined using a nitrogen / oxygen / hydrogen analyzer (ONH836 manufactured by LECO).

  Next, 83.4% by mass of the spherical silver powder thus obtained, 1.2% by mass of resin (ethyl cellulose manufactured by Wako Pure Chemical Industries, Ltd., 100 cps), and solvent (manufactured by Wako Pure Chemical Industries, Ltd.) Terpineol) 15.4% by mass was mixed twice for 30 seconds using a propellerless self-revolving stirring and deaerator (AR250 manufactured by Sinky Corporation), and then three rolls (manufactured by Otto Herman) No. EXAKT80S) was passed through a roll gap of 100 μm to 20 μm and kneaded to obtain a conductive paste.

  Next, the conductive paste thus obtained is 8 mm × 10 mm at a squeegee pressure of 0.3 MPa on each of two 96% alumina substrates using a screen printing machine (manufactured by Microtech). Screen-printed to form a rectangular film, dried at 200 ° C. for 20 minutes using an air circulation dryer, and then heat-treated each substrate at 400 ° C. and 700 ° C. for 10 minutes using a box furnace. A conductive film was prepared.

About the conductive film thus obtained, the surface of the conductive film on the alumina substrate and the part on which the conductive film is not printed using a surface roughness meter (Surfcoder SE-30D manufactured by Kosaka Laboratory Ltd.) The film thickness of the conductive film is determined by measuring the level difference between the thickness and the surface resistivity of the conductive film is measured by a four-probe method using a resistivity meter (MCP-T410 manufactured by Mitsubishi Chemical Corporation), When the volume resistivity of the conductive film was determined from the surface resistivity based on the volume of the conductive film (= width × length × film thickness), the conductive film fired at 400 ° C. was 5.2 × 10 ⁻⁶ Ω · cm, The conductive film fired at 700 ° C. has a density of 2.6 × 10 ⁻⁶ Ω · cm, and the conductive film fired at 400 ° C. has a conductivity of the order of 10 ⁻⁶ Ω · cm.

[Example 2]
A 1 L beaker containing 753 g of silver nitrate aqueous solution containing 8.63 g of silver was placed in an ultrasonic cleaning machine (US Cleaner USD-4R, output 160 W, manufactured by As One Co., Ltd.) containing water at a water temperature of 20 ° C., and stirring was started. did.

  Next, 26.2 g of 28 mass% ammonia water (equivalent to 2.7 equivalents with respect to silver) is added to the silver nitrate aqueous solution in the above beaker to form a silver ammine complex salt, 19 minutes after the addition of the ammonia water. Then, ultrasonic irradiation was started at an oscillation frequency of 40 kHz, and after 1 minute, 54.4 g of a 27.4 mass% formaldehyde solution obtained by diluting formalin with pure water (corresponding to 12.4 equivalents with respect to silver) was added, 15 seconds later, 2.16% by mass of a benzotriazole ethanol aqueous solution (1.06 g) was added to obtain a slurry containing silver particles.

  Next, after the ultrasonic irradiation is finished, the slurry obtained by filtering the slurry containing silver particles and washing with water is dried for 10 hours with a vacuum dryer at 75 ° C., and the dried silver powder is dried with a coffee mill for 30 seconds. Crushing to obtain spherical silver powder.

  The spherical silver powder thus obtained was measured for particle size distribution, BET specific surface area, and true specific gravity by the laser diffraction method in the same manner as in Example 1, and the particle cross section was observed. Content of impurity elements and silver And the contents of organic components (carbon, nitrogen, oxygen and hydrogen) were determined.

As a result, the particle size distribution according to the laser diffraction method was D ₁₀ = 1.5 μm, D ₅₀ = 2.8 μm, D ₉₀ = 4.5 μm, and the BET specific surface area was 0.36 m ² / g. Further, the true specific gravity was 9.7 g / cm ³ , and it was confirmed that the density was reduced by 8% with respect to the true specific gravity of 10.5 g / cm ^{3 of} bulk silver.

Further, in the observation of the particle cross section, as shown in the FE-SEM photograph in FIG. 2, it was confirmed that a closed void exists inside the spherical silver powder particles. Moreover, the area of all the particles of the spherical silver powder in the FE-SEM photograph is 11.8 μm ² , the area of the particles having voids is 0.34 μm ² , and the ratio of the area of the particles having voids to the area of all particles is 2.9%. In view of the fact that the cross section of the observed spherical silver powder does not necessarily pass through the center of the particle, it is considered that there are voids in most of the particles. The average void size determined from the FE-SEM photograph is 0.05 .mu.m, equivalent to 1.7% with respect to the average particle diameter _{D 50,} it was confirmed that sufficient void size.

  Furthermore, for the content of impurity elements, Cr = 1 ppm, Mn <1 ppm, Fe = 6 ppm, Co <5 ppm, Ni <5 ppm, Cu <1 ppm, Zn <5 ppm, Cd <1 ppm, Pb <5 ppm, Sn <10 ppm, Ca <1 ppm, Mg <1 ppm, S <50 ppm, Zr <1 ppm, Bi <10 ppm, Al <10 ppm, Sr <1 ppm, Ba <1 ppm, Li <100 ppm, Na <100 ppm, K <100 ppm, Rb <100 ppm, Cs < 100 ppm, and it was confirmed that all impurities were less than 100 ppm. The silver content was 99.21% by mass, and the carbon, nitrogen, oxygen, and hydrogen contents were 2400 ppm, 1710 ppm, 3360 ppm, and 650 ppm, respectively.

Moreover, when the electrically conductive paste was produced and the volume resistivity was calculated | required by the method similar to Example 1 using the obtained spherical silver powder, in the electrically conductive film baked at 400 degreeC, it is 5.7x. The conductive film baked at 10 ⁻⁶ Ω · cm and 700 ° C. is 2.4 × 10 ⁻⁶ Ω · cm, and the conductive film baked at 400 ° C. ensures conductivity on the order of 10 ⁻⁶ Ω · cm. I was able to.

[Example 3]
A 1 L beaker obtained by separating 688 g of an aqueous silver nitrate solution containing 9.00 g of silver is placed in an ultrasonic cleaning machine (US Cleaner USD-4R, output 160 W, manufactured by As One Co., Ltd.) containing water at a water temperature of 30 ° C., and an oscillation frequency of 28 kHz. Then, the ultrasonic irradiation was started and stirring was started.

  Next, 27.6 g of 28 mass% ammonia water (corresponding to 2.5 equivalents with respect to silver) is added to the silver nitrate aqueous solution in the above beaker to form a silver ammine complex salt, and 1 minute after the addition of the ammonia water. Then, 2.5 g of a 20 mass% aqueous sodium hydroxide solution was added, and after 20 minutes, 79.4 g of a 27.4 mass% formaldehyde solution obtained by diluting formalin with pure water (corresponding to 13.0 equivalents with respect to silver) 5 seconds later, 2.3 g of a 2.5% by mass stearic acid solution was added to obtain a slurry containing silver particles.

  Next, after the ultrasonic irradiation is finished, the slurry obtained by filtering the slurry containing silver particles and washing with water is dried for 10 hours with a vacuum dryer at 75 ° C., and the dried silver powder is dried with a coffee mill for 30 seconds. Crushing to obtain spherical silver powder.

  The spherical silver powder thus obtained was measured for particle size distribution, BET specific surface area, and true specific gravity by the laser diffraction method in the same manner as in Example 1, and the particle cross section was observed. Content of impurity elements and silver And the contents of organic components (carbon, nitrogen, oxygen and hydrogen) were determined.

As a result, the particle size distribution according to the laser diffraction method was D ₁₀ = 0.7 μm, D ₅₀ = 1.3 μm, D ₉₀ = 2.3 μm, and the BET specific surface area was 0.77 m ² / g. Further, the true specific gravity was 9.3 g / cm ³ , and it was confirmed that the density was reduced by 11% with respect to the true specific gravity of bulk silver of 10.5 g / cm ³ .

Further, in the observation of the particle cross section, as shown in the FE-SEM photograph of FIG. 3, it was confirmed that a closed void exists inside the spherical silver powder particles. Moreover, the area of all the particles of the spherical silver powder in the FE-SEM photograph is 2.08 μm ² , the area of the particles having voids is 0.21 μm ² , and the ratio of the area of the particles having voids to the area of all the particles is 10%. Considering that the observed cross section of the spherical silver powder does not always pass through the center of the particle, it is considered that there are voids in most of the particles. The average void size determined from the FE-SEM photograph is 0.11 .mu.m, corresponds to 8.5% of the average particle diameter _{D 50,} it was confirmed that sufficient void size.

  Furthermore, for the content of impurity elements, Cr = 1 ppm, Mn <1 ppm, Fe = 8 ppm, Co <5 ppm, Ni <5 ppm, Cu = 1 ppm, Zn <5 ppm, Cd <1 ppm, Pb <5 ppm, Sn <10 ppm, Ca <1 ppm, Mg <1 ppm, S <50 ppm, Zr <1 ppm, Bi <10 ppm, Al <10 ppm, Sr <1 ppm, Ba <1 ppm, Li <100 ppm, Na <100 ppm, K <100 ppm, Rb <100 ppm, Cs < 100 ppm, and it was confirmed that all impurities were less than 100 ppm. The silver content was 99.00% by mass, and the carbon, nitrogen, oxygen and hydrogen contents were 3700 ppm, 575 ppm, 3955 ppm and 1300 ppm, respectively.

In addition, using the obtained spherical silver powder, a conductive film was produced from a conductive paste by the same method as in Example 1, and the volume resistivity was determined. The conductive film baked at 10 ⁻⁶ Ω · cm and 700 ° C. is 2.3 × 10 ⁻⁶ Ω · cm, and the conductive film baked at 400 ° C. ensures conductivity on the order of 10 ⁻⁶ Ω · cm. I was able to.

[Comparative Example 1]
A 1 L beaker obtained by separating 28.6 g of silver nitrate aqueous solution containing 8.63 g of silver is placed in an ultrasonic cleaner (US Cleaner USD-4R, output 160 W, manufactured by As One Co., Ltd.) containing water at a water temperature of 35 ° C., and oscillated. Ultrasonic irradiation was started at a frequency of 40 kHz and stirring was started.

  Next, 52.7 g of 28% by mass of ammonia water (corresponding to 5.0 equivalents with respect to silver) is added to the silver nitrate aqueous solution in the above beaker to form a silver ammine complex salt, and 5 minutes after the addition of the ammonia water. Then, 2.2 g of a 0.40 mass% polyethyleneimine (molecular weight 10,000) aqueous solution was added, and 20 minutes after the addition of aqueous ammonia, 19.4 g of a 6.2 mass% hydrous hydrazine aqueous solution (1 with respect to silver). 30 equivalents), 0.77 g of 1.3 mass% stearic acid solution was added to obtain a slurry containing silver particles. In this comparative example, polyethyleneimine is added in order to adjust the particle size which becomes smaller by using hydrazine.

  Next, after the ultrasonic irradiation is finished, the slurry obtained by filtering the slurry containing silver particles and washing with water is dried for 10 hours with a vacuum dryer at 75 ° C., and the dried silver powder is dried with a coffee mill for 30 seconds. Crushing to obtain spherical silver powder.

  The spherical silver powder thus obtained was measured for particle size distribution, BET specific surface area, and true specific gravity by the laser diffraction method in the same manner as in Example 1, and the particle cross section was observed. Content of impurity elements and silver And the contents of organic components (carbon, nitrogen, oxygen and hydrogen) were determined.

As a result, the particle size distribution according to the laser diffraction method was D ₁₀ = 1.8 μm, D ₅₀ = 2.9 μm, D ₉₀ = 4.4 μm, and the BET specific surface area was 0.16 m ² / g. In addition, the true specific gravity was 10.3 g / cm ³ , and it was confirmed that the density was reduced by only 2% with respect to the true specific gravity of 10.5 g / cm ^{3 of} bulk silver.

  Further, in the observation of the particle cross section, as shown in the FE-SEM photograph of FIG. 4, it was confirmed that there was no closed void inside the spherical silver powder particles.

  Further, for impurity elements, Cr = 1 ppm, Mn <1 ppm, Fe = 7 ppm, Co <5 ppm, Ni <5 ppm, Cu = 2 ppm, Zn <5 ppm, Cd <1 ppm, Pb <5 ppm, Sn <10 ppm, Ca <1 ppm Mg <1 ppm, S <50 ppm, Zr <1 ppm, Bi <10 ppm, Al <10 ppm, Sr <1 ppm, Ba <1 ppm, Li <100 ppm, Na <100 ppm, K <100 ppm, Rb <100 ppm, Cs <100 ppm. All of the impurities were confirmed to be less than 100 ppm. The silver content was 99.80% by mass, and the carbon, nitrogen, oxygen and hydrogen contents were 900 ppm, 70 ppm, 320 ppm and 200 ppm, respectively.

Further, using the obtained spherical silver powder, a conductive film was produced from a conductive paste by the same method as in Example 1, and the volume resistivity was determined. In the case of the conductive film fired at 10 ⁻⁵ Ω · cm and 700 ° C., it is 3.0 × 10 ⁻⁶ Ω · cm, and in the conductive film fired at 400 ° C., the order is 10 ⁻⁵ Ω · cm. Compared with ~ 3, the conductivity was inferior.

  For the spherical silver powders obtained in these examples and comparative examples, the particle size distribution, the BET specific surface area and the true specific gravity by the laser diffraction method are shown in Table 1, and the observation results of the particle cross section are shown in Table 2. Table 3 shows the volume resistivity of the conductive films obtained in these Examples and Comparative Examples.

  As can be seen from these examples and comparative examples, particles are produced by reducing and precipitating silver particles by mixing an aqueous reaction system containing silver ions with a reducing agent-containing solution containing aldehyde while generating cavitation. A spherical silver powder having voids closed inside can be produced.

  Since the spherical silver powder according to the present invention has voids closed inside the particles, the spherical silver powder has the same particle size as the spherical silver powder produced by the conventional wet reduction method, and is conductive as a spherical silver powder that can be fired at a lower temperature. It can be used for the production of an adhesive paste.

Claims (5)

A spherical silver powder characterized by having voids closed inside the particles. 2. The spherical silver powder according to claim 1 , wherein the spherical silver powder has an average particle diameter D ₅₀ by a laser diffraction method of 0.1 to 10 μm. The spherical silver powder according to claim 1 or 2 , wherein a true specific gravity of the spherical silver powder is 9.8 g / cm ³ or less. The spherical silver powder according to any one of claims 1 to 3 , wherein the content of impurity elements other than carbon, nitrogen, oxygen and hydrogen contained in the spherical silver powder is less than 100 ppm. A conductive paste using the spherical silver powder according to any one of claims 1 to 4 as a conductor.

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