JPH10251701A - Manufacturing method of high purity silver powder - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a high-purity silver powder which does not contain an alkali metal impurity such as sodium and is suitable as a raw material for a silver-based paste used in the electronics industry. SOLUTION: Silver powder containing an alkali metal impurity is washed with an aqueous solution of formic acid having a concentration of 0.01% to 10% to be highly purified.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing fine and uniform high-purity silver powder, which is a raw material of a silver paste used in the electronics industry.

[0002]

2. Description of the Related Art With the development of high-density and high-precision electronic components, there has been an increasing demand for higher purity silver powder, which is a raw material of silver paste used in the electronics industry. In particular, alkali metals, chloride ions, and the like cause conduction defects such as migration, and thus reducing the content of these impurities is an important problem.

Conventionally, a method for producing silver powder is to add silver hydroxide by adding caustic soda to a silver nitrate solution or to make silver carbonate by adding sodium carbonate, and then reduce with formalin or hydrazine to obtain a silver powder. Was representative.

However, this conventional method has a problem that high-purity silver powder cannot be obtained because raw materials such as silver nitrate and silver oxide or chemical components such as an intermediate silver compound and caustic soda remain. Particularly, it is difficult to remove caustic alkali. For example, in the method for producing silver powder disclosed in Japanese Patent Application Laid-Open No. 5-156302, an attempt is made to remove impurities by washing with pure water at 80 to 100 ° C. The removal was not sufficient.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for producing a high-purity silver powder suitable for a silver paste raw material used in the electronics industry, which solves the above-mentioned disadvantages.

[0006]

According to the present invention, there is provided a silver powder containing an alkali metal impurity such as sodium (hereinafter referred to as "coarse silver powder") which is washed with an aqueous solution of formic acid to achieve high purity. This is a method for producing high purity silver powder.

The reason that washing with formic acid is effective for purifying silver powder is that formic acid neutralizes hydroxyl groups distributed on the surface of the silver powder, makes it easy to remove alkali components, and dissolves a small amount of the silver powder surface. This facilitates elution of impurities,
As a result, it is considered that impurity removal is promoted.

The formic acid solution used in the method for producing silver powder of the present invention may be dilute as long as silver is dissolved. For example, when the concentration is 0.01% or more, a sufficient washing effect can be exhibited. The upper limit concentration of formic acid is not particularly limited. However, considering that the higher the concentration, the larger the amount of silver dissolved and the progress of the surface modification of silver, the concentration is preferably within 10%.

Although the cleaning temperature with formic acid is sufficiently effective even at room temperature, the cleaning effect is further enhanced by heating.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION As a method for producing a coarse silver powder used in the present invention, a silver hydroxide is formed by adding a caustic alkali solution to a silver nitrate solution, and a silver powder is reduced by adding a formalin solution to reduce silver oxide. Is preferred.

Silver nitrate is generally obtained by dissolving granular or powdered silver bullion in nitric acid. After adding a solution obtained by dissolving a caustic alkali such as caustic soda and caustic to pure water to the obtained silver nitrate solution to generate silver oxide, the temperature is 80 to 104 ° C.
By adding a formalin solution while keeping warm
A silver powder is obtained. The amounts of the formalin solution and the caustic added are preferably 1.4 to 2.2 equivalents of formalin and 1.5 to 2.5 equivalents of caustic with respect to the amount of silver.

[0012]

Embodiments of the present invention and a conventional example will be described below.

[0013]

Example 1 108 g (1 equivalent) of silver powder was added to 130 ml of nitric acid (62
%), 80 g (2 equivalents) of caustic soda was dissolved in pure water, and the silver nitrate solution was added to a 200 ml solution with stirring for 5 minutes to produce silver oxide. While maintaining the silver oxide-containing solution at 90 ° C, a formalin solution (37%)
74 ml (2 equivalents) were charged in 5 minutes. As a result, a uniform coarse silver powder having a particle size distribution of 0.8 to 4 μm and an average particle size of 2.5 μm was obtained. As a result of analysis, it was found that Na contained 1600 ppm. Next, the thus obtained coarse silver powder 1
00 g was placed in a 1-liter beaker, 200 ml of a 0.1% aqueous solution of formic acid at 25 ° C. was added, and the mixture was stirred, washed, and filtered. The silver powder obtained was analyzed. X-ray diffraction confirmed that no unreduced silver oxide remained. The silver powder was kneaded with an organic vehicle in which ethyl cellulose was dissolved in terpineol using a three-roll machine to prepare a silver paste. As a result of examining the dispersion of the fine powder with a grind gauge according to the method of JIS K5701, the paste was 10 μm
The following fine dispersion was found.

[0014]

Example 2 100 g of the coarse silver powder obtained in Example 1 was heated to 80 ° C.
When silver powder obtained by stirring, washing and filtering with 200 ml of a 0.1% formic acid aqueous solution was analyzed, the Na content was 15 ppm. X-ray diffraction confirmed that no unreduced silver oxide remained. A silver paste was prepared using this silver powder in the same manner as in Example 1, and the dispersion of the fine powder was examined using a crush gauge. As a result, it was found that the dispersion was fine, not more than 10 μm.

[0015]

Example 3 100 g of the coarse silver powder obtained in Example 1 was added at 25 ° C.
After decanting with 200 ml of 0.1% formic acid aqueous solution, the mixture was further stirred at 25 ° C. with 200 ml of 0.1% formic acid aqueous solution, washed,
When the silver powder obtained by filtration was analyzed, the Na content was 4 ppm. X-ray diffraction confirmed that no unreduced silver oxide remained. A silver paste was prepared using this silver powder in the same manner as in Example 1, and the dispersion of the fine powder was examined using a crush gauge. As a result, it was found that the dispersion was fine, not more than 10 μm.

[0016]

Example 4 100 g of the coarse silver powder obtained in Example 1 was added at 25 ° C.
When silver powder obtained by stirring, washing and filtering with 200 ml of a 5% aqueous formic acid solution was analyzed, the Na content was 7 ppm.
X-ray diffraction confirmed that no unreduced silver oxide remained. A silver paste was prepared using this silver powder in the same manner as in Example 1, and the dispersion of the fine powder was examined using a crush gauge. As a result, it was found that the dispersion was fine, not more than 10 μm.

[0017]

Conventional Example 1 100 g of the coarse silver powder obtained in Example 1 was heated at 25 ° C.
When silver powder obtained by stirring, washing and filtering with 200 ml of pure water was analyzed, the Na content was 350 ppm.

[0018]

Conventional Example 2 100 g of the coarse silver powder obtained in Example 1 was
When silver powder obtained by stirring, washing and filtering with 200 ml of pure water was analyzed, the Na content was 120 ppm.

[0019]

Conventional Example 3 100 g of the coarse silver powder obtained in Example 1 was heated at 25 ° C.
After decanting with 200 ml of pure water, 25 ° C pure water
When silver powder obtained by stirring, washing and filtering with 200 ml was analyzed, the Na content was 60 ppm.

[0020]

As described above, the method for producing silver powder according to the present invention comprises washing a coarse silver powder containing a considerable amount of alkali metal impurities with formic acid, thereby obtaining a high-purity silver which has not been obtained conventionally. A powder can be obtained, and when this silver powder is used as a raw material of a silver paste, the occurrence of conduction failure such as migration at the electrode does not occur, which contributes to the improvement of reliability of products for electronic industry. .

Claims (3)

[Claims] 1. A method for producing a high-purity silver powder, comprising washing a silver powder containing an alkali metal impurity with formic acid and purifying the silver powder. 2. The method for producing a high-purity silver powder according to claim 1, wherein the concentration of formic acid is 0.1% to 10%. 3. A silver powder containing an alkali metal impurity produced by adding a caustic solution to a silver nitrate solution to form silver oxide, and then adding a formalin solution to reduce the silver oxide. The method for producing a high-purity silver powder according to claim 1 or 2, wherein