JP2007224422A - Silver powder and paste using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing silver powder where, as shown in the conventional method in which a dispersant is added to a reaction liquid before reduction, there occurs no unreduction reaction in accordance with the additional amount thereof, the kinds and amounts of a dispersant are not limited, and further, silver powder having excellent dispersibility can be obtained, and to provide the silver powder. <P>SOLUTION: The silver powder is produced by adding a reducing agent-containing aqueous solution to an aqueous reaction system comprising either or both of a silver complex salt and silver oxide at a rate of ≥1 equivalent/min, reducing the same, thereafter filtering and water-washing the silver powder-containing slurry, so as to obtain a wet cake having a water content of 20 to 80%, and subjecting the wet cake to mixing and pulverizing in a mixer or to mixing and pulverizing together with a dispersant in a mixer, and has a tap density of ≥2 g/cm<SP>3</SP>, a mean particle diameter of 0.1 to 5 μm according to a laser diffraction process, and a specific surface area of ≤5 m<SP>2</SP>/g. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a method for producing silver powder obtained by a reduction precipitation method and a silver powder produced by the method. More specifically, the present invention relates to a method for producing silver powder suitably used for electronic components such as capacitor internal electrodes or circuit board conductor patterns, and silver powder produced by the method.

Conventionally, a silver paste produced by kneading silver powder in an organic vehicle with glass frit and kneading is used for an internal electrode or a circuit conductor pattern of a multilayer capacitor, and the silver powder for such paste includes particles. It is required that the diameter is appropriately small and the particle size is uniform.
For the production of the above silver powder, alkali is added to the silver salt-containing aqueous solution to form silver oxide, and further a reducing agent is added to produce silver powder, or a complexing agent is added to the silver salt-containing aqueous solution to form a complex. And a method of obtaining a silver powder by adding a reducing agent.

However, the silver powder produced by these methods has a strong agglomeration, and has a disadvantage that it cannot cope with the thinning of the conductive pattern and the internal electrode of the multilayer ceramic capacitor which are becoming finer.
For this reason, improvements have been made so that the resulting silver powder has less agglomeration, and the results of such research have been disclosed in, for example, JP-A-49-113754, JP-A-52-54661, JP-A-54-121270. This is disclosed in Japanese Laid-Open Patent Publication No. 61-243105.

That is, JP-A-49-113754 discloses a method in which a caustic alkali is added to an aqueous silver salt solution to form silver oxide, and a surfactant is added to disperse the silver oxide, followed by reduction. It is described that a homogeneous silver powder with little variation in characteristics can be obtained by adding a surfactant when the slag is formed.
Japanese Patent Application Laid-Open No. 52-54661 discloses a method for adjusting the specific surface area of silver powder by changing the kind and addition amount of amine when a silver salt solution is reduced with hydrazine in the presence of an amine to produce silver powder. It is disclosed that the presence of amines has the effect of reducing the strong reducing power of hydrazine and preventing heterogeneous reactions.

In Japanese Patent Laid-Open No. 54-121270, 0.1 to 5% by weight of a fatty acid is added to a mixed aqueous solution of a silver nitrate solution and formalin with stirring, and an alkaline solution is added thereto to add silver powder. Is disclosed, and when silver fine particles are precipitated by a reduction reaction, it is described that a finely dispersed silver powder is obtained by coating the silver fine particles with a fatty acid.
Japanese Patent Laid-Open No. 61-243105 discloses that a cationic surfactant is added to a reaction solution in a method for producing silver fine particles by reducing an ammoniacal silver nitrate complex solution using a reducing agent in a hydrophobic reaction tank. A method of obtaining monodispersed silver fine particles by adding is disclosed, and the reason for using a hydrophobic reaction vessel is that, in the case of hydrophilicity, the precipitation reaction occurs on the wall surface of the container, so that the aggregated particle size distribution is wide. It describes that only silver particles can be obtained. In addition, the reason for adding a cationic surfactant to the reaction solution is to reduce the ammoniacal silver nitrate complex solution so that the silver fine particles deposited in the aqueous solution do not aggregate and maintain a monodispersed state. ing. In the same manner as this method, JP-A 61-276904 discloses an anionic surfactant, JP-A 61-276905 discloses a nonionic surfactant, and JP-A 61-276906. Describes a method using an amphoteric surfactant and JP-A-61-276907 discloses a method using a protective colloid.
Japanese Patent Laid-Open No. 49-113754 JP 52-54661 A JP 54-121270 A JP 61-243105 A Japanese Patent Laid-Open No. 61-276904 Japanese Patent Laid-Open No. 61-276905 Japanese Patent Laid-Open No. 61-276906 JP-A 61-276907

However, among these conventional techniques, surfactants, fatty acids, amines, and the like have been added to the reaction solution, and therefore, Japanese Patent Laid-Open Nos. 49-113754 and 61-243105 are disclosed. In the methods of JP-A-61-276904 to JP-A-61-276907, depending on the type of surfactant used, the dispersion effect is insufficient or effective depending on the pH of the reaction solution and coexisting ions. In some cases, the type of surfactant that can be used is limited, and it is difficult to control the exact amount of coating on the surface of the silver powder. In the case of Japanese Patent Application Laid-Open No. 52-54661, the ratio is increased by adding an amine. A method of adjusting the area, there is a problem that good dispersibility silver powder can not be obtained.
Therefore, the purpose of the present invention is that, as in the conventional silver powder production method in which a dispersant is added to a reaction solution before reduction, an unreduced reaction does not occur or the type and amount of the dispersant are not limited by the amount added, And it is providing the manufacturing method of the silver powder from which the silver powder excellent in the dispersibility is obtained, and the silver powder obtained by it.

  As a result of diligent research to achieve the above object, the present inventors have developed an aqueous reaction system comprising an aqueous solution or aqueous suspension containing one or both of a silver complex (silver salt) and silver oxide, or a mixture of both. When adding a reducing agent-containing aqueous solution, the addition rate of the reducing agent-containing aqueous solution is increased in order to prevent aggregation of the reduced silver particles, and the resulting silver powder-containing slurry is filtered and washed with water at a rate of 1 equivalent / min or more. If the wet cake has a moisture content of 20 to 80% and this wet cake is mixed and pulverized in a mixer, or mixed and pulverized with a dispersant in the mixer, the above problems are eliminated and excellent dispersibility is obtained. The present inventors have found that silver powder having the above can be obtained and reached the present invention.

That is, the present invention firstly relates to a method for producing silver powder in which a reducing agent-containing aqueous solution is added to an aqueous reaction system containing at least one of a silver salt and silver oxide to reduce and precipitate silver particles. Silver powder-containing aqueous reaction system is filtered to obtain a wet cake, and the wet cake is crushed. Second, the wet cake has a moisture content of 20 to 80%. The method for producing silver powder according to the first aspect, characterized in that thirdly, a dispersant is added to the wet cake, The method for producing silver powder according to either the first aspect or the second aspect, wherein the fourth method is the fourth aspect. In addition, as the dispersing agent, any one or more of fatty acids, fatty acid salts, surfactants, organic metals, and protective colloids are selectively used. The return The method for producing silver powder according to any one of the first to fourth aspects, wherein the addition rate of the agent-containing aqueous solution is 1 equivalent / min or more with respect to the silver content: sixth, the first to first A silver powder produced by the method according to any one of 5 above, wherein the tap density of the silver powder is 2 g / cm ³ or more, the laser diffraction method average particle diameter is 0.1 to 5 μm, and the specific surface area is 5 m ² / g. Silver powder characterized by the following: Seventh, when the silver powder is pasted, the coating film printed on the ceramic substrate has a surface roughness Ra of 2 μm or less and a coating film density of 4.5 g / cm. is ³ or more, and the coating film in the fired film obtained by baking in air, the surface roughness Ra is 2μm or less, of the sixth wherein fired film density leads to 10 g / cm ³ or more characteristic silver powder: the 8 manufactured by the method according to any one of the first to fifth. In the coating film printed on the ceramic substrate of the paste using the silver powder, the surface roughness Ra is 2 μm or less, the coating film density is 4.5 g / cm ³ or more, and the coating film is baked in the air. In the fired film obtained as described above, a paste having a surface roughness Ra of 2 μm or less and a fired film density of 10 g / cm ³ or more is provided.

  In the conventional method in which a dispersant is added to the reaction solution before the reduction, an unreduced reaction may occur depending on the amount added, and the type and amount of the dispersant are limited. The addition rate of the agent-containing aqueous solution is set to 1 equivalent / min or more, and the resulting silver powder-containing slurry is filtered and washed with water to obtain a wet cake with a water content of 20 to 80%. Since it is mixed and disintegrated together with the dispersant in the machine, a silver powder having excellent dispersibility is obtained, and it can be made into a paste to form a coating film or a fired film suitable for a conductor pattern of a circuit board.

A feature of the production method of the present invention is that an aqueous reducing agent solution is added to an aqueous reaction system containing silver ions (silver salts) or silver oxide in the reduction step at a rate of 1 equivalent / min or more, or silver in the reaction system The reduction reaction, that is, the generation reaction of silver powder should be completed within about 1 minute, and the reduced and precipitated silver powder is washed with filtered water and then disintegrated as a wet cake with a water content of 20 to 80% by a mixer without drying. Furthermore, the addition of a dispersant is to be added during the above-mentioned disintegration.
In the production method of the present invention, the form of silver before being reduced to silver powder is not particularly limited. Specifically, in the case of an aqueous solution containing a silver complex salt, it can be obtained by adding a complexing agent to an aqueous silver nitrate solution, and ammonia water, ammonium salt, chelate compound, etc. can be used as the complexing agent. On the other hand, a silver oxide-containing aqueous suspension can be obtained by adding an alkali such as sodium hydroxide or potassium hydroxide to a silver nitrate aqueous solution. In addition, the silver complex salt-containing aqueous solution or silver oxide-containing aqueous suspension thus obtained may be mixed and then reduced.

Next, as a reducing agent, there is no problem as long as it is a reducing agent used in a known method for producing silver powder using an oxidation-reduction reaction in an aqueous solution. Specific examples include hydrazine, hydrazine compounds, formalin, glucose, sodium borohydride, sodium hypophosphite, sulfite, formic acid, sodium formate, anhydrous sodium sulfite, L (+) tartaric acid, ammonium formate, longgarit, L- Ascorbic acid or a mixture thereof. Of these reducing agents, the solid one is used as an aqueous solution. Therefore, a substance that decomposes when used as an aqueous solution at the time of use requires treatment such as bringing the solution pH to the alkali side. About the temperature at the time of reduction | restoration, it is desirable to carry out in the temperature range of 20-80 degreeC.
Regarding the method of adding the reducing agent, in order to prevent aggregation of the silver powder, the reducing agent-containing aqueous solution is added at a rate of 1 equivalent / min or more or at a rate that the reaction equivalent of the reducing agent reaches the entire reaction system within 1 minute. Like that. As long as the size of the aqueous reaction system to which the reducing agent is added is as large as the reaction tank used in actual operation, it is not necessary to change the addition rate strictly depending on the size of the reaction system, and 1 equivalent / It was confirmed that if it was added at a rate of more than a minute, there was an effect. The reason for this operation is not clear, but by adding the reducing agent in a short time, reduction precipitation on the silver particles occurs all at once, and agglomeration between the nuclei generated because the reduction reaction is completed in a short time occurs. It is difficult to improve dispersibility. In the reduction, the reaction solution is preferably stirred so that the reaction is completed in a shorter time.

The silver powder-containing aqueous solution obtained by reduction and precipitation is made into a wet cake having a water content of 20 to 80% made of silver powder by washing with filtered water. There is no problem if the filtration method is a method used for solid-liquid separation. Examples include centrifugal filters, filter presses, and Buchner funnels.
The reason why the moisture content of the wet cake is limited to 20 to 80% is that the cake obtained when the moisture content is less than 20% becomes harder and the disintegration in the subsequent process becomes less efficient, and the content exceeds 80%. However, the efficiency of the disintegration in the subsequent process is lowered.
Silver powder with excellent dispersibility can be obtained by passing through the step of crushing with the wet cake mixer thus obtained. Although a dispersant may or may not be added, the dispersion effect is increased by adding and mixing the dispersant, and more preferable results can be obtained.

The mixer used here includes a mixer used for solid-liquid mixing or a mixing pulverizer used for solid pulverization.
Any kind of dispersant can be used here, but fatty acid, fatty acid salt, fatty acid or fatty acid salt emulsion, cationic surfactant, anionic surfactant can be preferably used. , An amphoteric surfactant, a nonionic surfactant, an organic metal, or a protective colloid, and one or more can be selected. The addition amount is not particularly limited, and an appropriate amount may be added according to the use of silver powder.
The wet cake crushed by the mixer is a high-concentration silver powder-containing slurry, and is further filtered and washed with water to obtain a silver powder wet cake whose surface is coated with a dispersant. As described above, there is no problem with the filtration and water washing methods if an apparatus used for ordinary solid-liquid separation is used.
The silver powder wet cake thus obtained becomes silver powder through a drying process. As the drying method, known methods and equipment may be used, and the atmosphere is not particularly limited. The drying temperature is preferably 80 ° C. or lower.

The silver powder produced through these steps has physical properties such that the tap density is 2 g / cm ³ or more, the average particle size by laser diffraction method is 0.1 to 5 μm, and the specific surface area is 5 m ² / g or less.
The tap density was measured by a method according to JIS K5101-1991 20.2 tap method. The number of tapping is 1,000 times. The silver powder produced by the conventional method has insufficient dispersibility, and the tap density measured by this method is less than 2 g / cm ³ , whereas the silver powder obtained by the present invention is closer to monodispersion, so the tap density. Is 2 g / cm ³ or more.
About the average particle diameter of silver powder, it measured by the laser diffraction method, and the apparatus used Shimadzu Corporation laser diffraction type particle size measuring machine SALD-1100. As a dispersion medium, sodium hexametaphosphate was dissolved in distilled water and used as a 0.2% aqueous solution. In addition, one drop of household liquid detergent is added per 50 ml of the dispersion medium at the time of measurement. The refractive index at the time of measurement is A3.
When measured by this method, the silver powder obtained by the present invention has an average particle size of 0.1 to 5 μm, whereas the silver powder obtained by the conventional method has a value exceeding 5 μm and is inferior in dispersibility.
The specific surface area was measured by the BET method. The silver powder produced by the method of the present invention is 5 m ² / g or less.
When the silver powder having the above characteristics is evaluated as a paste which is the main application, the excellent surface of the silver powder can be known.

  The pasting method is not particularly problematic as long as it is carried out according to known examples. Here, a thick film silver paste used for forming a conductor pattern such as a hybrid IC is taken as an example. First, the vehicle used for the paste is generally used by dissolving various celluloses, acrylic resins, phenol resins, alkyd resins and the like in solvents such as alcohols, esters, ethers, and hydrocarbons. Various inorganic binders are added to the paste in order to bind the conductor pattern and a ceramic substrate such as an alumina substrate. As the inorganic binder, a metal oxide such as copper oxide or bismuth oxide, or a so-called glass frit obtained by finely pulverizing glass is used.

In the test for evaluation, a paste was prepared with a very general composition from among many combinations of the paste components, and silver powder was evaluated.
As a vehicle, 45 cp of ethyl cellulose was dissolved in terpineol to prepare a 10% solution. This vehicle, Nippon Electric Glass GA-8 glass powder and silver powder are weighed so as to have the following composition.
Vehicle is 37.4%, silver powder 61%, 6A-8 glass powder 1.6%
These paste components are premixed in a beaker and then dispersed by a three roll to obtain a paste.
FIG. 1 is a perspective view showing a printing method after forming a silver powder paste. FIG. 1a shows a state in which the paste is stretched on a substrate with a mending tape and ED printing is performed, and FIG. The pasted coating film is shown.
That is, this paste 3 is placed on a 96% alumina substrate 1 having mending tapes 2 applied in parallel at regular intervals, the paste is stretched with a glass rod 4 and printed, and after peeling off the tape, the paste coating 5 is applied for 10 minutes. After leveling, drying is performed at 150 ° C. for 10 minutes with a hot air circulating dryer.

The dried coating film was evaluated by the following method. The surface roughness can be measured with a stylus type surface roughness meter, but here, it was measured using Surftest-501 manufactured by Mitutoyo Corporation. The measurement mode was Ra, the measurement range was 80 μm, the cutoff value was 0.3 mm, and the measurement section was 3.
The coating film density was measured by the following method. The thickness of the coating film was measured with a DEKTAKIIA surface roughness meter manufactured by Nippon Vacuum Technology Co., Ltd., but there is no problem as long as the apparatus has a similar measurement mechanism. The width and length of the coating film were measured with calipers. The weight of the coating film was obtained by measuring the weight of the substrate before printing, measuring the weight of the substrate after printing and drying (substrate + coating film), and determining the difference between them. And the coating-film density was calculated | required by following Formula.
D = (w ₂ −w ₁ ) / A × B × T
Here, D is the coating density (g / cm ³ ), w ₁ is the weight (g) of the substrate, w ₂ is the weight (g) of (substrate + coating), and A is the width (cm) of the coating. , B is the length (cm) of the coating film, and T is the thickness (cm) of the coating film.

When silver powder was evaluated by these methods, the following was found.
The coating film of the paste using the silver powder having excellent dispersibility has a Ra of 2 μm or less, the particularly excellent silver powder shows a small value of 0.7 μm or less, and the coating film density D is a value of 4.5 g / cm ³ or more. It becomes a dense coating film. Such a dense and smooth coating film can be obtained from silver powder having a tap density of 2 g / cm ³ or more, an average particle size of 0.1 to 5 μm by laser diffraction method, and a specific surface area of 5 m ² / g or less. Needless to say.
In addition, it has been found that a coating film having such characteristics exhibits excellent characteristics even when a silver conductor pattern is formed by firing.
This coating film is baked at 850 ° C. for 7 minutes in the air using a belt furnace to obtain a baked film. Then, the surface roughness Ra and the fired film density of the fired film were measured by the same method as that for the coating film. As a result, the fired film Ra was 2 μm or less, particularly 0.8 μm or less when silver powder having excellent dispersibility was used. Also, a dense film having a fired film density of 10 g / cm ³ or more can be obtained.

Thus, the use of silver powder having a high coating film density and a high fired film density reduces the shrinkage rate of the pattern by firing or increases the consistency of the shrinkage rate with various substrates and materials, and is obtained as a result. It is a feature of the production method of the present invention that the shrinkage rate of the paste coating film can be controlled by the type and amount of the silver powder dispersant, which greatly contributes to the increase in yield and high performance of applied products. It is also a feature.
Hereinafter, the present invention will be described in more detail with reference to examples. However, the scope of the present invention is not limited by the following examples.

[Example 1]
150 ml of 25% aqueous ammonia was added to 2000 ml of 20 g / l silver nitrate aqueous solution as silver ions to obtain a silver ammine complex aqueous solution. The aqueous solution was brought to a liquid temperature of 20 ° C., and 45 ml of a 37% formalin aqueous solution was added over 40 seconds with stirring to precipitate silver powder to obtain a silver powder-containing slurry. The slurry was filtered and washed with a Buchner funnel to obtain a wet cake having a water content of 55%. The non-ionic anionic surfactant GAFACRA-600 manufactured by Toho Chemical Industry Co., Ltd. for 1% of the weight of the wet cake and silver powder was mixed and disintegrated in a table blender mill for about 60 seconds, and then again. The mixture was filtered with a Buchner funnel, washed with water, and further dried in air at 70 ° C. for 24 hours to obtain silver powder. The obtained silver powder had a tap density of 2.8 g / cm ³ , a laser diffraction method average particle size of 3.0 μm, and a specific surface area of 0.7 m ² / g.
Further, this powder was made into a paste and evaluated. The method is the method described in the preceding section “Embodiments of the Invention”. As a result, the coating film density was 4.7 g / cm ³ , the coating film Ra = 1.7 μm, the fired film density 10.5 g / cm ³ , and the fired film Ra = 1.35 μm, and both the coating film density and the fired film density were good. Results were obtained.

[Example 2]
The silver nitrate concentration of 2000 ml of 80 g / l silver nitrate aqueous solution was adjusted to a temperature of 25 ° C., and then 800 ml of 100 g / l sodium hydroxide aqueous solution was added with stirring to obtain a silver oxide slurry. To this slurry, 140 ml of 37% formalin solution was added over 50 seconds to reduce to silver particles, and the resulting silver powder-containing slurry was filtered with a Buchner funnel and washed with water to obtain a wet cake with a moisture content of 40%.
Next, 1% by weight of Cellosol 920 obtained by emulsifying stearic acid manufactured by Chukyo Oil & Fat Co., Ltd. was prepared with respect to the amount of silver, and mixed and pulverized in the table blender mill for 60 seconds together with the previous wet cake. The mixture thus obtained was filtered with a Buchner funnel, washed with water, and dried in the atmosphere at 70 ° C. for 24 hours.
The obtained silver powder was excellent in dispersibility with a tap density of 3.2 g / cm ³ , a laser diffraction method average particle diameter of 1.0 μm, and a specific surface area of 1.5 m ² / g.
Furthermore, this powder was made into a paste and evaluated. The method is the method described in the preceding section “Embodiments of the Invention”. As a result, the coating film density was 5.3 g / cm ³ , the coating film Ra = 0.5 μm, the fired film density 10.7 g / cm ³ , and the fired film Ra = 0.4 μm. there were.

[Comparative example]
A silver oxide slurry was obtained by adding 2000 ml of an aqueous silver nitrate solution having a silver ion concentration of 2000 ml at a temperature of 25 ° C. and adding 600 ml of 200 g / l sodium hydroxide. Further, 140 ml of 37% formalin solution was added at an addition rate of 14 ml / min for 10 minutes to reduce the silver oxide particles to silver particles. Thereafter, the mixture was filtered with a Buchner funnel, washed with water, and dried at 70 ° C. for 24 hours to obtain silver powder.
The silver powder thus obtained was inferior in dispersibility with a tap density of 1.0 g / cm ³ , a laser diffraction method average particle size of 15.0 μm and a specific surface area of 0.9 m ² / g.
When this silver powder was pasted and evaluated in the same manner as in the examples, the coating film density was 3.0 g / cm ³ , the coating film Ra = 2.5 μm, the fired film density 7.5 g / cm ³ , and the fired film Ra = 2. It was in a state where the densification was not progressing to 0.1 μm.

The perspective view which showed the printing method after making silver powder paste, The figure a is a state where the paste is extended and printed on the board which pasted the mending tape in parallel at a fixed interval, and the figure b is printed on the board. It is a figure which shows the done coating film.

Explanation of symbols

1 96% alumina substrate 2 Mending tape 3 Paste 4 Glass rod 5 Paste coating

Claims (4)

A reducing agent-containing aqueous solution is added to an aqueous reaction system containing at least one of silver salt and silver oxide at an addition rate of 1 equivalent / minute or more with respect to the amount of silver contained, and silver particles are reduced and precipitated, and then the precipitated silver particle-containing aqueous solution The reaction system is filtered to obtain a wet cake having a water content of 20 to 80%. The wet cake is crushed by adding a dispersant, the tap density is 2 g / cm ³ or more, and the laser diffraction method average particle size is A silver powder having a specific surface area of 0.1 to 5 μm and a specific surface area of 5 m ² / g or less.   2. The silver powder according to claim 1, wherein at least one of a fatty acid, a fatty acid salt, a surfactant, an organic metal, and a protective colloid is selectively used as the dispersant. When the silver powder is pasted, the coating film printed on the ceramic substrate has a surface roughness Ra of 2 μm or less, a coating film density of 4.5 g / cm ³ or more, and firing the coating film in the air. ^3. The silver powder according to claim 1, wherein in the fired film obtained as described above, the surface roughness Ra is 2 μm or less and the fired film density is 10 g / cm ³ or more. The paste using the silver powder according to claim 1 or 2, wherein the paste has a surface roughness Ra of 2 µm or less and a coating film density of 4.5 g / cm ³ or more. And a fired film obtained by firing the coating film in the air, having a surface roughness Ra of 2 μm or less and a fired film density of 10 g / cm ³ or more.

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