JPH09256008A - Production of monodispersive silver-palladium multiple powder and the same powder - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a silver-palladium multiple powder good in dispersibility and nallow in size distribution and the obtain silver- palladium multiple powder produced by the same producing method. SOLUTION: This method for producing monodispersive silverpalladium multiple powder includes the followings: (a) a dispersant is previously added (hereinafter referred to as the primary addition of the dispersant) to either an aq. soln. contg. silver salt and palladium salt (hereinafter referred to as a noble metal salt-contg. aq. soln.) or a reducing agent-contg. aq. soln. contg. hydrazine sulfate and hydrazine hydrate so as to regulate the molar ratio of 80wt.% hydrazine hydrate/hydrazine sulfate 0.05 to 20.0, (b) the noble metal salt-contg. aq. soln. and reducing agent-contg. aq. soln. are mixed to precipitate silver-palladium multiple powder by reduction, and (c) after that, before the generation of the secondary flocculation of the grains, the dispersant is furthermore added (denoted as the secondary addition of the dispersant). Silver- palladium multiple powder is produced by the same producing method as well.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for producing a silver-palladium composite powder and a silver-palladium composite powder produced by the production method. More specifically, the present invention relates to a method for producing a silver-palladium composite powder having good dispersibility and a narrow particle size distribution, and a silver-palladium composite powder produced by the method.

[0002]

BACKGROUND OF THE INVENTION In the electronics industry, conductive pastes are used to make thick film circuits. Of these, the silver paste is apt to cause migration and solder cracking.
A silver-palladium paste using a mixed powder was manufactured by adding 0 mol% of palladium powder, putting this in an organic vehicle together with a glass frit, and kneading.

Recently, silver obtained by the coprecipitation reduction method
Palladium alloy powder or silver-palladium composite powder is often used. The coprecipitation reduction method is a method in which a solution containing a reducing agent is added to a solution in which a silver salt and a palladium salt are dissolved, and the solution is reduced to silver and palladium to cause coprecipitation and precipitation. 21968, Japanese Patent Publication No. 1-3
No. 5044, etc.

By the way, in recent years, in order to reduce the cost by downsizing electronic parts such as multilayer capacitors and suppressing the amount of electrode materials used, it is necessary to further reduce the thickness of the electrodes. For this reason, it is necessary to have monodispersed particles having a small particle size, a uniform particle size, excellent printing characteristics and electrode characteristics, and no secondary aggregation.

As a method for obtaining monodispersed particles, a method of adding a dispersant is known.
-55562, JP-A-63-213606, JP-A-4-333504 and JP-A-4-23.
No. 5205, etc. By adding a dispersant, the precipitated metal particles can be dispersed in the solution to form a stable suspension. For this reason, the deposited metal crystals similarly grow in each part of the solution, and therefore the variation in particle size is reduced.

[0006]

With the metal paste using the mixed powder obtained by mixing the silver powder and the palladium powder, it is difficult to obtain a powder in which the respective powders are sufficiently homogeneously mixed, and the powder is produced during firing. Due to the contraction due to the alloying of silver and palladium, the film formed by using the obtained paste has a drawback that the electrical characteristics are inferior.

In the coprecipitation reduction method described in Japanese Patent Publication No. 44-21968 and Japanese Patent Publication No. 1-35044, which are proposed to eliminate this drawback, the particle size of the powder produced is not appropriate. , Secondary aggregation of powder occurs,
The dispersibility of silver and palladium was poor, the amount of oxidation increase was large, and there was a problem in the characteristics as a capacitor electrode.

Japanese Examined Patent Publication No. 61-55562, JP-A-6-56
By the method of adding a dispersant described in JP-A-3-213606, JP-A-4-333504, JP-A-4-235205, etc., although monodisperse particles can be obtained, the particle size can be sufficiently controlled. Can't
The agglomeration of particles could not be escaped completely.

Therefore, an object of the present invention is to provide a method for producing a silver-palladium composite powder having excellent characteristics as a capacitor electrode, excellent printing characteristics, good dispersibility, and a narrow particle size distribution. To do.

[0010]

Means for Solving the Problems The inventors of the present invention have made extensive studies in accordance with the above-mentioned object, and as a result, by specifying the type of reducing agent to be used and the mixing ratio thereof, the above object can be achieved. Heading, the following invention was completed.

That is, according to the present invention, (a) an aqueous solution containing a silver salt and a palladium salt (hereinafter referred to as "noble metal salt-containing aqueous solution") or 80% by weight of hydrazine sulfate and hydrazine hydrate is 0. 05-20.
The dispersant was added in advance to either one of the reducing agent-containing aqueous solutions contained at a ratio of 0 (molar ratio) (hereinafter referred to as “first addition of dispersant”).
A), a step of (b) mixing an aqueous solution containing a noble metal salt and an aqueous solution containing a reducing agent to reduce and precipitate the silver-palladium composite powder, and (c) further dispersant before secondary aggregation of particles occurs. (Hereinafter referred to as "second addition of dispersant")
The present invention relates to a method for producing a monodisperse silver-palladium composite powder and a silver-palladium composite powder produced by the method. By thus specifying the reducing agent to be a mixture of hydrazine sulfate and hydrazine hydrate, a composite powder having good dispersibility can be obtained. Furthermore, by specifying the mixing ratio of hydrazine sulphate and hydrazine hydrate, it is possible to suppress the deposition of palladium that becomes the nucleus in the reaction in the initial stage of the reduction reaction, so that the palladium that becomes the nucleus in the reaction that is continued is the reaction solution. Evenly dispersed throughout.

[0012]

The reducing agent used in the present invention is a mixture of hydrazine sulfate and hydrazine hydrate. By specifying the two types of reducing agents, as in the case of using a substance or a mixture of these substances which has been conventionally used as a reducing agent in the reduction precipitation method,
It is possible to obtain a composite powder having excellent dispersibility without causing problems such as necking between particles in SEM observation of the manufactured powder.

Further, in the present invention, by controlling the mixing ratio of hydrazine sulfate and hydrazine hydrate, it is possible to control the deposition amount of palladium in the initial stage of the reaction. When hydrazine sulfate or hydrazine hydrate is used alone, such control is impossible, and agglomeration occurs between particles, resulting in a composite powder having poor dispersibility. The hydrazine sulfate and the hydrazine hydrate are mixed so that 80% by weight of hydrazine hydrate / hydrazine sulphate have an arbitrary molar ratio of 0.05 to 20.0, preferably 0.10 to 10.0. It If 80% by weight of hydrazine hydrate / hydrazine sulfate is less than 0.05 (molar ratio), the precipitation of palladium in the initial stage of the reaction will be small, and the agglomeration of the particles will occur due to the affinity of silver, and the resulting composite powder will be dispersible. Will be inferior. When 80% by weight of hydrazine hydrate / hydrazine sulfate was more than 20 (molar ratio), palladium was often precipitated in the initial stage of the reaction, resulting in agglomeration of particles, resulting in poor dispersibility of the composite powder. Will be things. The total amount of the reducing agent within the above range is preferably 1 to 10 times equivalent to the total amount of silver and palladium. If it is less than 1, unreacted ones will occur,
Further, even if it is used in excess of 10, no remarkable effect is obtained and it is uneconomical.

In the present invention, the reducing agent-containing solution is a mixture of hydrazine sulphate and hydrazine hydrate. Therefore, it is preferable that the pH of the solution is set to 7 to 12 on the alkaline side. If it is used on the acidic side, unreacted substances are likely to be generated, or agglomerated fine particles are likely to be formed, which is not preferable.

In the present invention, the silver ratio in the precipitated composite powder (70% by weight of silver) at the initial stage of the reduction precipitation reaction, specifically, 10 seconds after mixing the aqueous solution containing a noble metal salt and the aqueous solution containing a reducing agent.
And 30% by weight of palladium, the weight of silver relative to the total weight of silver and palladium is within the range of 73 to 99%. In the above range, in the reaction to be continued, palladium as a nucleus is uniformly dispersed in the reaction solution,
In addition, since the number of palladium nuclei is appropriate, aggregation of particles does not occur and the dispersibility becomes good. Therefore, when printed as an electrode paste, a powder having a high density can be obtained.
When the silver ratio is larger than 99%, the amount of palladium, which is the nucleus, is small, so that the affinities of silver cause the particles to aggregate. When the silver ratio is less than 73%, the amount of palladium deposited in the initial stage of the reaction increases, and as a result, the number of nuclei of palladium nuclei, which are nuclei, increases, and at the same time, the agglutination of the precipitated particles is observed due to the affinity of silver. For this reason, fine particles are aggregated to form a powder having poor dispersibility, and the density when printed as a paste is also low.

In the present invention, the dispersant is added plural times in each step in a series of steps. Specifically, a dispersant is added in advance to at least one of the precious metal salt-containing aqueous solution and the reducing agent-containing aqueous solution (first addition of the dispersant), and then the both aqueous solutions are mixed to simultaneously add silver and palladium. After reduction and precipitation, a second addition of a dispersant is performed to produce a silver-palladium composite powder. The first addition of the dispersant makes it possible to control the amount of precipitation nuclei and at the same time control the precipitation rate of subsequently reduced silver / palladium. The second addition of the dispersant can prevent secondary aggregation of the precipitated silver-palladium composite powder particles. Therefore, the second addition needs to be carried out promptly after the precipitation of the primary particles is completed and before the secondary agglomeration occurs.

The dispersant used in the present invention contains a water-soluble polymer compound as a main component. The water-soluble polymer compound has a protective colloid action, and specifically includes celluloses such as carboxymethyl cellulose, polysaccharides such as dextrin and starch, various natural rubbers such as gum arabic, albumin, globulin and prolamin. In addition to simple proteins such as gelatin, various proteins such as gelatin, albumose, peptone, nucleoprotein, glycoprotein, or their derivatives, polyvinyl alcohol, polyvinylamine, polyvinyl polymer compounds such as polyvinylpyrrolidone or polyvinyl alcohol,
Examples thereof include esters such as polyacrylic acid derivatives.
Of these, gum arabic, dextrin, gelatin and starch are particularly preferable.

Regarding the addition form of the first addition, at least one of the noble metal-containing aqueous solution and the reducing agent-containing solution is selected and added according to the kind of the water-soluble polymer compound.
For example, gum arabic, dextrin, and starch are particularly preferably added to a solution containing a noble metal salt, and gelatin is particularly preferably added to an aqueous solution containing a reducing agent.

In the production method of the present invention, as long as the given purpose of the first or second addition can be achieved, the type of water-soluble polymer compound added at each stage may be different.

The preferred amount of the dispersant used in the first addition depends on the type of water-soluble polymer compound selected. For example, gelatin is less than 0.02 g / L, more preferably 0.001 to 0.004 g / L, and gum arabic is less than 0.04 g / L, more preferably 0.01 to 0.02 g / L. , Dextrin 0.002-2.0 g / L, more preferably 0.005
To 0.2 g / L, particularly preferably 0.01 to 0.
It is 02 g / L. The above addition amounts are all concentrations with respect to the reaction solution. If the amount of each dispersant added is more than the above range, the silver salt and the palladium salt in the reaction solution will be precipitated as a large number of fine nuclei, and particles having a uniform composition cannot be obtained. Further, the produced particles are too fine and have poor printing characteristics, particles of a desired size cannot be obtained, and the particles are likely to aggregate. On the other hand, when the amount is less than the above range, the dispersing effect does not appear and it becomes difficult to uniformly deposit nuclei, and spherical particles cannot be obtained.

In the second addition, a necessary amount or more showing the secondary aggregation preventing action is added, and more preferably about 1.2 to 10.0 g / L with respect to the reaction solution.

In addition to the water-soluble polymer compound, a dispersant containing a surfactant can be used for the second addition. The types of surfactants that can be used include anionic, cationic, amphoteric and nonionic surfactants, preferably anionic surfactants. Specifically, stearylamine acetate or the like as a cationic activator, ammonium stearate, ammonium oleate, ammonium linoleate, ammonium linolenate or the like as an anion activator, stearyl betaine or the like as an amphoteric activator, or a nonionic Examples of the system activator include polyoxyethylene stearyl ether and the like. The preferable amount of the surfactant added varies depending on the combination of the type of the water-soluble polymer compound and the type of the surfactant, but is generally in the range of 0.01 to 0.4 g / L with respect to the reaction solution. It is preferable to add it internally.

A dispersant containing glycerin may be used in at least one of the first addition and the second addition. The preferable addition amount is about 1 to 50 g / L with respect to the reaction solution.

The reduction precipitation reaction is performed at 25 ° C. or higher. It is preferable to carry out at 40 ° C or higher while heating, and 40 to 80
Most preferably, it is carried out within the range of ° C. For pH,
The pH of the metal salt-containing aqueous solution is preferably pH 7 to 12, and the pH when mixed with the reducing agent-containing aqueous solution to carry out the metal deposition reaction is preferably in the range of pH 7 to 12.

In the above-mentioned method for producing a silver-palladium composite powder, the initial precipitation of palladium can be controlled, so that the obtained composite powder is excellent in monodispersity and has a narrow particle size distribution range. It has a uniform particle size. Therefore, when it is made into a paste, it has excellent printing characteristics and electrode characteristics, and enables downsizing and thinning of electronic components.

[0026]

Next, embodiments of the present invention will be described. However, the present invention is not limited to these examples.

<Example 1> 90 g of distilled water in a glass beaker
Add 0 ml, and moderately heat silver nitrate 11.02 g
And 16.02 g of palladium nitrate (palladium metal content 18.73% by weight) were added, and the mixture was stirred and heated to 60 ° C. to dissolve. Further, the pH was adjusted to 11.0 with ammonium hydroxide to obtain a noble metal salt-containing aqueous solution.

Separately, 27.22 g of hydrazine sulfate and 80
Weight% hydrazine hydrate 2.28 g and distilled water 1300 m
The solution was stirred and heated to 1 to 60 ° C. until dissolved.
Adjust the pH to 11.0 with ammonium hydroxide,
0.0023 g of gelatin was added to the solution at 0 ° C. and mixed with stirring (first addition of dispersant) to obtain a reducing agent-containing aqueous solution.

The precious metal salt-containing aqueous solution was added to the reducing agent-containing aqueous solution to reduce and deposit silver-palladium.
Further, after foaming during the reaction, 2.76 g of gelatin was added to the reaction solution and mixed with stirring (second addition of dispersant).

After completion of the reaction, the solution is cooled, filtered through a glass filter, and the silver-palladium composite powder on the filter is repeatedly washed with distilled water until there are no residual salts, then washed with methanol to remove water, and then at room temperature. Dried.

<Example 2> Hydrazine sulfate 16.18
A silver-palladium composite powder was produced in the same manner as in Example 1 except that 7.59 g of 80% by weight of hydrazine hydrate was used.

<Example 3> 27.22 hydrazine sulfate was added.
A silver-palladium composite powder was produced in the same manner as in Example 1 except that 11.38 g of 80% by weight of hydrazine hydrate was used.

<Example 4> Hydrazine sulfate 8.29 was added.
A silver-palladium composite powder was produced in the same manner as in Example 1 except that 11.38 g of 80% by weight of hydrazine hydrate was used.

<Example 5> 4.34 hydrazine sulfate was added.
A silver-palladium composite powder was produced in the same manner as in Example 1 except that 13.28 g of 80% by weight of hydrazine hydrate was used.

Example 6 2.36 of hydrazine sulfate
g, 80 wt% Silver-palladium composite powder was produced in the same manner as in Example 1 except that 17.08 g of hydrazine hydrate was used.

Comparative Example 1 Distilled water 90 in a glass beaker
Add 0 ml, and moderately heat silver nitrate 11.02 g
And 16.02 g of palladium nitrate (palladium metal content 18.73% by weight) were added, and the mixture was stirred and heated to 60 ° C. to dissolve. Further, the pH was adjusted to 11.0 with ammonium hydroxide to obtain a noble metal salt-containing aqueous solution.

Separately, 2.36 g of hydrazine sulfate and 26.18 g of 80 wt% hydrazine hydrate were added to 1300 m of distilled water.
The solution was stirred and heated to 1 to 60 ° C. until dissolved.
Adjust the pH to 11.0 with ammonium hydroxide,
0.0023 g of gelatin was added to the solution at 0 ° C. and mixed with stirring (first addition of dispersant) to obtain a reducing agent-containing aqueous solution.

The aqueous solution containing a noble metal salt was added to the aqueous solution containing a reducing agent to reduce and deposit silver-palladium.
Further, after foaming during the reaction, 2.76 g of gelatin was added to the reaction solution and mixed with stirring (second addition of dispersant).

After the reaction was completed, the solution was cooled and filtered with a glass filter, and the silver-palladium composite powder on the filter was repeatedly washed with distilled water until there were no residual salts, followed by washing with methanol to remove water and room temperature. Dried.

<Comparative Example 2> 27.22 hydrazine sulfate was added.
g, 80 wt% Silver-palladium composite powder was produced in the same manner as in Comparative Example 1 except that 0.39 g of hydrazine hydrate was used.

<Comparative Example 3> Distilled water 90 in a glass beaker
Add 0 ml, and moderately heat silver nitrate 11.02 g
And 16.02 g of palladium nitrate (palladium metal content 18.73% by weight) were added, and the mixture was dissolved by stirring while heating to 40 ° C. Furthermore, the pH was adjusted to 9.6 with ammonium hydroxide to obtain a precious metal salt-containing aqueous solution. 0.0023 g of gelatin was added to this 40 ° C. solution and mixed by stirring (first addition of dispersant).

Separately, 6.14 g of ammonium formate and 80
Weight% hydrazine hydrate 4.41 g and distilled water 1300 m
It was dissolved by stirring while heating to 1 to 40 ° C.
After adjusting the pH to 6.4 with nitric acid, 50.0 g of ammonium acetate as a buffer was added and stirred to completely dissolve it to obtain a reducing agent-containing aqueous solution.

The aqueous solution containing a noble metal salt was added to the aqueous solution containing a reducing agent to reduce and deposit silver-palladium.
Further, after foaming during the reaction, 2.76 g of gelatin was added to the reaction solution and mixed with stirring (second addition of dispersant).

After the reaction was completed, the solution was cooled and filtered with a glass filter, and the silver-palladium composite powder on the filter was washed with distilled water repeatedly until there were no residual salts. Dried.

The above Examples 1 to 6 and Comparative Examples 1 and 2
And the amount of hydrazine sulphate and 80% by weight hydrazine hydrate used in the manufacture of the silver-palladium composite powder, and
The molar ratio of wt% hydrazine hydrate / hydrazine sulfate is shown in Table 1.
Shown in

[0046]

[Table 1]

The silver-palladium composite powders produced in Examples 1 to 6 all had a particle size of 0.2 to 1.5 μm and were suitable for printing characteristics.

A graph showing the particle size distribution of the silver-palladium composite powder produced in Example 2 is shown in FIG.

From FIG. 1, it is shown that the product of the present invention has a very narrow particle size distribution and a uniform particle size.

Further, in the production of the silver-palladium composite powders of the above Examples and Comparative Examples, the weight ratio of silver and palladium was measured 10 seconds after the precious metal salt-containing aqueous solution and the reducing agent-containing aqueous solution were mixed by quantitative analysis by EPMA. . Further, the tap density and the print dry density of the silver-palladium composite powder produced in each of the examples and the comparative examples were measured. The tap density is 10g of composite powder in a 10ml graduated cylinder.
The density was measured by tapping 1000 times with a tap density measuring device to reduce the apparent bulk. The print dry density is a value obtained by measuring the density of a dry film by making a composite powder into a paste, drying the composite powder into a sheet. 1
The results of measurement of the silver-palladium ratio, the tap density and the dry density after 0 seconds are shown in Tables 2 and 3.

[0051]

[Table 2]

[0052]

[Table 3]

From Tables 2 and 3, Comparative Examples 1 and 2 produced in which the mixing ratio of hydrazine sulfate and hydrazine hydrate was out of the range specified in the present invention, and the conventional reducing agent were used. Compared with the manufactured comparative example 3, it was shown that the ratio of silver-palladium in the initial stage of the reaction of the present invention product was effectively controlled. It was also shown that the product of the present invention has a higher tap density and a higher dry density than the comparative example.

2 to 1 are electron micrographs (x1000 and x5000) showing the shape of the particles of each composite powder.
0 is shown.

[0055]

EFFECTS OF THE INVENTION The production method of the present invention makes it possible to control the amount of palladium deposited in the initial stage of the reaction. It is possible to produce a composite powder in which the particles do not agglomerate to a suitable degree, the monodispersity is excellent, the width of the particle size distribution is narrow, and the particle diameter is uniform.

That is, the silver-palladium composite powder of the present invention is a particle having excellent monodispersity and uniform particle size. Therefore, it can be suitably used as a paste and has excellent printing characteristics and electrical characteristics. When printed as an electrode paste, the unevenness of the printed surface is eliminated, and as a result, when used as an electrode, a thin layer can be realized.

[Brief description of drawings]

FIG. 1 is a graph showing a particle size distribution of a silver-palladium composite powder produced by the method shown in Example 2, which is an embodiment of the production method of the present invention.

FIG. 2 is an electron micrograph showing the particle structure of the silver-palladium composite powder produced in Example 1, which is an embodiment of the present invention. The upper part shows a magnification of 1000, and the lower part shows the white frame part of the upper part. It is further enlarged (x5000).

FIG. 3 is an electron micrograph showing a particle structure of the silver-palladium composite powder produced in Example 2, which is another embodiment of the present invention, the upper part shows a magnification of 1000, and the lower part shows a white frame in the upper part. It is a further enlarged portion (x5000).

FIG. 4 Example 3 which is a further embodiment of the present invention.
2 is an electron micrograph showing the particle structure of the silver-palladium composite powder produced in 1., the upper part is a magnification of x1000, and the lower part is a further enlargement of the white frame part of the upper part (x5000).

FIG. 5: Example 4 which is a further embodiment of the present invention.
2 is an electron micrograph showing the particle structure of the silver-palladium composite powder produced in 1., the upper part is a magnification of x1000, and the lower part is a further enlargement of the white frame part of the upper part (x5000).

FIG. 6 is a further embodiment of the present invention, Example 5.
2 is an electron micrograph showing the particle structure of the silver-palladium composite powder produced in 1., the upper part is a magnification of x1000, and the lower part is a further enlargement of the white frame part of the upper part (x5000).

FIG. 7: Example 6 which is a further embodiment of the present invention.
2 is an electron micrograph showing the particle structure of the silver-palladium composite powder produced in 1., the upper part is a magnification of x1000, and the lower part is a further enlargement of the white frame part of the upper part (x5000).

FIG. 8 is an electron micrograph showing a particle structure of a silver-palladium composite powder produced in Comparative Example 1, which is a comparative product to the present invention, the upper part shows a magnification of x1000, and the lower part further shows a white frame part in the upper part. It is an enlarged image (x5000).

FIG. 9 is an electron micrograph showing a particle structure of a silver-palladium composite powder produced in Comparative Example 2, which is another comparative product to the present invention, the upper part shows a magnification of x1000, and the lower part shows a white frame part in the upper part. Is further expanded (x5000).

FIG. 10 is an electron micrograph showing a particle structure of a silver-palladium composite powder produced in Comparative Example 3, which is a further comparative product to the present invention, the upper part shows a magnification of x1000, and the lower part shows the white frame part of the upper part. Further expansion (x500
0).

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kazane Nakayama 3-1, 1-3 Noritake Shinmachi, Nishi-ku, Nagoya-shi, Aichi Noritake Company Limited Limited (72) Inventor Noriyuki Abe 3-chome, Noritake Shin-cho, Nishi-ku, Aichi No. 1-36 Noritake Co., Ltd. (72) Inventor Yurie Saruki Noritake Shincho, Nishi-ku, Nishi-ku, Nagoya, Aichi Prefecture, No. 1-36 Noritake Co. Ltd. (72) Inventor Akio Takimoto, Noritake Shin-machi, Nishi-ku, Nagoya, Aichi Prefecture No. 3 1-36 Noritake Company Limited Limited

Claims (3)

[Claims] 1. An aqueous solution containing (a) a silver salt and a palladium salt (hereinafter referred to as "precious metal salt-containing aqueous solution") or 80% by weight of hydrazine sulfate and hydrazine hydrate.
A step of adding a dispersant in advance to either one of the reducing agent-containing aqueous solutions containing hydrazine sulfate in a proportion of 0.05 to 20.0 (molar ratio) (hereinafter referred to as "first addition of dispersant"),
(B) a step of mixing an aqueous solution containing a noble metal salt and an aqueous solution containing a reducing agent to reduce and precipitate a silver-palladium composite powder,
(C) a step of further adding a dispersant (hereinafter referred to as "second addition of dispersant") before secondary agglomeration of the particles occurs, and a monodisperse silver-palladium composite powder. Production method. 2. The silver ratio in the precipitated composite powder 10 seconds after mixing the precious metal salt-containing aqueous solution and the reducing agent-containing aqueous solution is 7
The method for producing a monodisperse silver-palladium composite powder according to claim 1, wherein the content is 3 to 99% by weight. 3. A silver-palladium composite powder excellent in monodispersity and having a uniform particle size, which is produced by the method according to claim 1.