JPH0796681B2 - Method for producing palladium-coated silver powder - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a palladium-coated silver powder suitable for a composition for forming an electrode of a laminated ceramic capacitor, a circuit of a laminated ceramic substrate, or the like.

[Conventional technology]

Laminated ceramic capacitors are mainly composed of a dielectric such as barium titanate, and the laminated ceramic substrate is made of alumina,
Glass, etc. as a main component, these are mixed with a binder and a solvent, and the mixture is made into a green sheet by a doctor blade method, a roll coater method, etc. 900-1400 ° C by applying a composition for forming a protective film and stacking multiple layers of these
It is manufactured by firing at. The firing temperature depends on the sinterability of the main component, and the required firing temperature is determined depending on the main component. The conductive film-forming composition used for the electrodes and circuits must be one that does not melt or evaporate at the firing temperature and that can be baked on the ceramic.

As a conductive material of such a conductive film forming composition,
Palladium or various silver-palladium alloys having different Pd contents depending on the firing temperature are used.

In recent years, it has been possible to sinter the main component in a laminated ceramic as described above at a lower temperature, thereby making the conductive material of the composition for forming a conductive film also cheaper silver having a low Pd content-
Palladium alloys can be used, and cost reductions of both of these are aimed at reducing the price of laminated ceramic products.

By the way, when the conductive material is a silver-palladium alloy,
A silver-palladium coprecipitated powder is used. Add to this silver powder (melting point 960.5 ° C) and palladium powder (melting point 1552 ° C).
This is because the use of the mixture of (3) causes the silver powder to melt before being alloyed with palladium, to become fine spheres or to evaporate during the heating to the firing temperature. The silver-palladium coprecipitated powder is prepared by mixing silver nitrate and palladium nitrate at a desired ratio, adding sodium carbonate and the like to precipitate silver and palladium carbonates, and then applying a reducing agent such as formalin, formic acid, and hydrazine to the mixture. It is obtained at most, and it is recognized from the X-ray diffraction pattern that it is tentatively made into alloy powder.

However, when electrodes, circuits, etc. were formed from the composition for forming a conductive film using the coprecipitated powder, a phenomenon in which silver was locally melted despite the fact that a predetermined amount of Pd was contained Is recognized. Therefore, when this coprecipitated powder is used, silver containing Pd slightly higher than the predetermined Pd content corresponding to the firing temperature is used.
There was no choice but to use a palladium coprecipitated powder.

[Problems to be Solved by the Invention]

The present invention provides a method for producing silver powder in which the temperature at which silver is separated or evaporated when the ratio of silver and palladium is the same is higher than that of coprecipitated powder, and the entire surface of silver powder is covered with palladium without any gaps. This is an issue.

[Means for Solving the Problems]

In order to solve the above problems, in the method for producing a palladium-coated silver powder provided by the present invention, the slurry is a slurry in which silver particles are precipitated by reducing a silver salt with a reducing agent, and the particle shape is almost spherical and average. In a slurry in which silver particles having a particle diameter of 0.1 to 2 μm are suspended at a concentration of 5 to 55 g /, a reducing agent in an amount capable of reducing and precipitating a desired amount of palladium is dissolved,
Then, a dichlorodiaminepalladium solution having a palladium concentration of 10 to 100 g / is added to the slurry, and palladium is reduced and precipitated on the surface of silver particles at a temperature of 20 to 70 ° C.

[Action]

In the present invention, making the shape of the silver particles substantially spherical makes the specific surface area as small as possible and increases the content of the conductive powder in the composition for forming a conductive film by reducing the amount of adsorbed vehicle, thus providing a dense firing. This is so that a film can be obtained. The average particle size of silver particles needs to be 0.1 to 2 μm. If it is less than 0.1 μm, “thickness” tends to occur in the coating due to shrinkage during firing, and if it exceeds 2 μm, the film thickness after firing becomes too thick and it is not economical. Such silver powder includes silver chlorodiamine, silver nitrate, silver carbonate, thiourea dioxide,
It can be obtained by reduction with sodium borohydride, formalin, hydrazine and the like. The surface of the silver particles needs to be coated with 5% by weight or more of palladium. If the amount is less than 5% by weight, the surface of the silver particles may not be completely covered with Pd, and the amount of palladium may be large, but usually up to about 55% by weight. This is because if it exceeds about 55% by weight, the firing temperature becomes high, and the one having a high firing temperature will not be used due to the lowering of the firing temperature.

The coating of such silver particles with Pd is carried out by reducing and depositing a desired amount of Pd in a slurry in which silver particles, which are obtained by reducing a silver salt with a reducing agent, are suspended at a concentration of 5 to 55 g / as described above. advance dissolving the amount of reducing agent to obtain, then Pd concentration 10 to 100 g / dichloro diamine palladium _{(Pd (NH 3) 2 Cl} 2) to the slurry was added and reacted at a temperature 20 to 70 ° C. By.

When coating silver particles with Pd, using a slurry in which the silver salt is reduced by reducing the silver salt with a reducing agent to precipitate the silver particles is because when the reduced and precipitated silver particles are filtered out, the surface state changes and The coating is adversely affected, and when silver particles are filtered off and dried, they aggregate into secondary particles, and even if they are redispersed in water, they do not completely return to primary particles, so secondary particles are coated with Pd. This is because a surface not covered with Pd appears during paste kneading, and in any case, it becomes difficult to cover the entire surface of the silver particles with Pd without any gap.

The slurry concentration of silver particles is 5 to 55 g /
If it is less than 55 g /, on the contrary, if it exceeds 55 g /, the concentration of Pd added increases as the number of silver particles increases. As a result, Pd tends to precipitate alone, which is uneconomical.

In the method of the present invention, unless the solution of dichlorodiaminepalladium (Pd (NH ₃ ) ₂ Cl ₂ ) which is a Pd source is added after the reducing agent is added to the slurry in which the silver particles are suspended, the entire surface of the silver particles is Is difficult to coat with Pd. The reason for this is not clear, but since the method uses a slurry in which silver particles have been reduced and precipitated, the reducing agent used for reducing the silver salt remains in the slurry. It is considered that the addition of Pd causes the Pd particles to slowly and locally precipitate due to the small amount of the reducing agent remaining, and to increase the number of Pd particles that precipitate and settle alone without coating the surface of the silver particles.

Dichlorodiamine palladium (Pd (NH ₃ ) ₂ Cl as a Pd source
₂ ) is used because palladium chloride or palladium nitrate causes a rapid reduction reaction so that the precipitated Pd particles have non-uniform particle sizes (palladium black is also generated) and the entire surface of silver particles is made uniform without gaps. This is because it is difficult to coat, but when dichlorodiaminepalladium is used, Pd particles having a uniform particle size are deposited by a mild reduction reaction, and the entire surface of the silver particles can be stably covered without gaps. Also, since dichlorodiamine palladium has a mild reduction reaction,
If the reaction temperature is lower than 20 ° C, the reaction is slow and the production efficiency is low, so it is necessary to react at a temperature of 20 ° C or higher. But,
If the reaction temperature exceeds 70 ° C, the reaction becomes too fast, and the Pd particles may precipitate alone to completely cover the surface of the silver particles, so the reduction reaction is carried out at a temperature in the range of 20 to 70 ° C.

Examples of the Pd reducing agent include thiourea dioxide, sodium borohydride, formalin, hydrazine and the like. Of dichlorodiamine palladium solution to be added to silver particle slurry
If the Pd concentration is less than 10 g /, the production efficiency is poor, and if it exceeds 100 g /, the precipitated particles of Pd may become large or may be precipitated alone, and the silver particle surface may not be completely covered.

The thus obtained palladium-coated silver powder can be kneaded with an organic vehicle in the same manner as conventional palladium and silver-palladium to form a paste-like composition for forming a conductive film. The silver powder is spherical and has a relatively smooth surface and is coated with Pd, so it has a relatively small amount of adsorbed vehicle and can have a higher content in the paste than conventional products, and can be contained in 40 to 90% by weight. . The composition may contain glass powder and inorganic oxide powder in some amounts. As the organic vehicle, it is possible to use a solvent prepared by dissolving ethyl cellulose, a methacrylate resin or the like in a solvent such as terpineol, butyl carbitol or toluene.

〔Example〕

Experiment No. 1 Silver chlorodiamine corresponding to an Ag amount of 80 g was dissolved in pure water 1 and heated to 50 ° C., and 120 g of a 37% formalin solution was added to precipitate silver particles. The silver particles had a substantially spherical shape and the average particle diameter was 0.5 μm. Next, 10 g of thiourea dioxide and pure water were added to the reaction liquid containing the precipitated silver particles to make the total liquid volume 3 and then heated to 50 ° C.
400 ml of a solution of 20 g of dichlorodiaminepalladium in the amount of Pd heated to 50 ° C. was added, and the precipitate was filtered and dried after continuing stirring for 10 minutes. As a result of analysis, the precipitate was Ag 79.1% by weight and Pd 20.4% by weight, and the yield of Pd-coated Ag powder was 97 g. When the Pd-coated silver powder was cracked with a microscope, all of the surfaces were completely coated with Pd.

Experiment Nos. 2 to 5 After obtaining silver particles in the same manner as Experiment No. 1, Pd-coated Ag was variously changed by changing the addition amount of thiourea dioxide, the amount of slurry liquid, the concentration of dichlorodiamine palladium, the addition amount, and the reaction temperature. A flour was prepared.

Experiment Nos. 6 to 7 Pd-coated Ag powder was obtained in the same manner as Experiments 2 to 5 except that sodium borohydride was used as the reducing agent for Pd. When I split these Pd-coated silver powders with a microscope,
All of the surfaces were completely covered with Pd.

Table 1 shows the production conditions and the analytical values of the obtained Pd-coated Ag powder.

Experiment Nos. 8 to 14 Pd-coated silver powder obtained in Experiment No. 1 and the same Ag / Pd as the silver powder.
The Ag-Pd coprecipitated powder prepared so as to have a ratio was prepared by using a terpineol solution of ethyl cellulose as a vehicle to prepare respective pastes having different powder contents, and screen-printed in a predetermined pattern on an alumina substrate. The specific resistance was measured by firing in a belt furnace with a peak temperature of 1000 ° C.

From Table 2, if the content of Pd-coated Ag powder is less than 40% by weight, the fired surface is not dense and the desired resistance value cannot be obtained.
It can be seen that if the content exceeds the weight percentage, the viscosity required for screen printing is exceeded. On the other hand, according to the Ag-Pd co-precipitated powder, the powder content is narrow, 50-85 wt%, the specific resistance ∞ less than 50 wt%, the evaporation of Ag on the fired surface, microspheres are observed, If it exceeds 85% by weight, screen printing cannot be performed.

Experiment Nos. 15 to 24 Pd-coated Ag powders of Experiments Nos. 1 to 7 and paste-like compositions containing 60% by weight of Ag-Pd coprecipitated powders having the same Ag / Pd ratio as those were prepared. Screen printing was performed on an alumina substrate using the same pattern as 8 to 14, and the specific resistance was measured by firing at various temperatures.

From the results in Table 3, it can be seen that the Pd-coated Ag powder is superior in heat resistance to the coprecipitated powder by about 50 ° C. In all samples with a specific resistance of ∞, evaporation of Ag and microsphere formation were observed on the fired surface.

〔The invention's effect〕

According to the present invention, it becomes possible to obtain a silver powder whose surface is completely coated with palladium, and when this palladium-coated silver powder is used, the separation temperature of silver is higher than that of coprecipitated powder even if the Pd content is the same, Therefore, even if the Pd content is lower than before,
It is possible to obtain an economically excellent composition for forming a conductive film, which can be fired at the same temperature as a conventional composition having a large Pd content and can form a conductive film having good conductivity.

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Claims (1)

[Claims] 1. A slurry which is prepared by reducing silver salt with a reducing agent to deposit silver particles, and has a particle shape of approximately spherical and a particle diameter of 0.1 to 2 μm at a concentration of 5 to 55 g /. In the suspended slurry, an amount of a reducing agent capable of reducing and precipitating a desired amount of palladium is previously dissolved, and then a dichlorodiamine palladium solution having a palladium concentration of 10 to 100 g / is added to the slurry at a temperature of 20 to 70. A method for producing a palladium-coated silver powder, which comprises reducing and depositing palladium on the entire surface of the silver particles at 0 ° C.