Highly crystalline silver powder and production method of highly crystalline silver powder

The present invention relates to highly crystalline silver powder among the silver powders and production method of the highly crystalline silver powder.

Conventionally, highly crystalline silver powder (which has a large crystal diameter) has been widely used for processing into silver ink and silver paste for the reason that it is excellent in thermal shrinkage resistance at the time of sintering. For example, it has been used in wiring circuit of printed wiring board, via hole filling, adhesive for mounting devices and the similar applications in which a silver powder is mixed and cured with various kinds of resin components as well as applications in which a silver powder is sintered at relatively high temperatures such as circuit formation by co-sintering with a ceramic substrate. Silver powder excellent in thermal shrinkage resistance at the time of sintering has been demanded from the point of view for improving accuracy of shape as a conductor particularly in silver powder used for silver ink and silver paste used for wiring and electrodes in circuits.

The crystallinity of silver powder is greatly affected by the production method. For example, as a method for producing silver powder, atomize process as disclosed in Patent Document 1 (Japanese Patent Laid-Open No. 2003-286502) can be used. However, even though silver powder having high crystallinity may be obtained by the atomize process, it is difficult to obtain silver powder consisting of fine particles with a sharp particle size distribution. Although it can be surely supposed that silver powder as a product having a sharp particle size distribution may be obtained by performing classification repeatedly, it is not preferable at all from the point of view of production cost. Therefore, obtaining silver powder by a wet production process described below has been examined.

For example, Patent Document 2 (Japanese Patent Publication No. 57-21001) discloses a method to deposit fine silver powder by adding a fatty acid to an aqueous mixture solution of a silver nitrate and formalin in an amount of 0.1 wt % to 5.0 wt % against to amount of silver to be deposited while stirring followed by adding an alkaline solution into the mixture solution. Patent Document 2 also discloses the production method which gives fine silver powder having an average particle diameter of 0.8 μm to 0.9 μm.

In addition, Patent Document 3 (Japanese Patent Laid-Open No. 04-323310) discloses a method for forming fine spherical particles by dissolving a metal, an alloy or a metal salt in an aqueous solvent, followed by adjusting pH by adding a base chemical thereto and adding a reducing agent to this to deposit fine metal powder. In which, the temperature of the solution is adjusted to a range of 10° C. to 30° C. to obtain spherical fine powder. When the temperature is adjusted to a range of 50° C. or more, fine polyhedron-like metal powder can be obtained. In addition, the particle size distribution of the obtained metal powder is approximately from 0.3 μm to 2.0 μm by the production method.

Since the deposition of crystals in the silver powders obtained by the production methods disclosed in above-mentioned Patent Document 2 and Patent Document 3 are not controlled, thermal shrinkage when sintering process was performed was significant. Therefore, in order to solve such problem, Patent Document 4 (Japanese Patent Laid-Open No. 2000-1706) discloses a method for producing the highly crystalline silver powder wherein UV is irradiated at the time of the reaction when an aqueous solution of silver nitrate and a solution prepared by dissolving acrylic acid monomer in an aqueous solution of L-ascorbic acid are mixed and react simultaneously. The highly crystalline silver powder obtained by the method consists of high crystals single crystals and quasi-crystals whose crystal diameter is 2 μm to 4 μm. In addition, the Patent Document 4 discloses that if the crystal diameter is not more than 2 μm, the shrinkage factor at the time of sintering is large and if the crystal diameter exceeds 4 μm, irregularity of the conductor surface is large to result problems such as increase in losses as an electric circuit. The highly crystalline silver particle as referred to here indicates those having a crystal diameter of 400 Å or more calculated from half-value width of (1.1.1) peak by X-ray diffraction method.

Further, Patent Document 5 (Japanese Patent Laid-Open No. 2003-49202) discloses silver particles having a crystal diameter of 400 Å to 600 Å, tap density of 5 g/cm³ or more and specific surface area of 0.15 m²/g or less. And it is clearly described that the silver particle is obtained by a production method characterized by reacting an alkaline aqueous solution (ammonium hydroxide aqueous solution+one or more kind selected from a group consisting of sodium hydroxide and potassium hydroxide) containing a silver ion and a hydrogen peroxide aqueous solution (added with one or more kind selected from a group consisting of fatty acids, fatty acid salts and derivatives thereof as required). And it is clearly described that the range of the crystal diameter of silver particles obtained is from 400 Å to 600 Å and, if it is less than 400 Å, crystallinity is low for silver particles for high temperature sintering conductive paste, and if it exceeds 600 Å, shape of the silver particles may show deviation. Also, Patent Document 5 discloses that tap density of the silver particles should be 5 g/cm³ or more and specific surface area of the silver particles should be 0.15 m²/g or less. But these are not sufficient as properties to identify the powder because there is no description about particle diameter even though the both conditions should be inherently based on relations with particle diameter.

[Patent Document 1] Japanese Patent Laid-Open No. 2003-286502

[Patent Document 2] Japanese Patent Publication No. 57-21001

[Patent Document 3] Japanese Patent Laid-Open No. 04-323310

[Patent Document 4] Japanese Patent Laid-Open No. 2000-1706

[Patent Document 5] Japanese Patent Laid-Open No. 2003-49202

L ascorbic acid, which is used as a reducing agent in the production method disclosed in above Patent Document 4 relating to high crystalline silver powder, is expensive and may result increasing of the product price. As for the highly crystalline silver particle obtained by substantially using ascorbic acid as a reducing agent and irradiating ultraviolet in the reductive reaction, particle diameter and crystal diameter has an approximately proportional relationship. And products having constantly stable quality can be obtained when large powder particles of 2 μm to 4 μm are aimed. However, when fine silver powder particles having a particle diameter less than 2 μm with a crystal diameter of more than 400 Å are aimed, lack of process stability of the production method may result wide variation in the crystal diameter. Further, it has been confirmed in many case that the fine particles cannot satisfy thermal shrinkage resistance because shrinkage factor after sintering of the fine silver powder particles less than 2 μm is too large.

In addition, the solution used in the production method disclosed in Patent Document 5 consists of ammonia aqueous solution and ammonium nitrate. They are strongly odorous substances, and therefore, it may pollute the working atmosphere as well as causing a problem of accelerated damage on the apparatuses made of copper applied in the facilities. Besides, the method uses a hydrogen peroxide aqueous solution whose ability tends to change largely and lacks in stability of the quality of the solution. Therefore, the average particle diameter of the obtained silver powder deviates significantly and controlling of the particle size and particle size distribution is difficult.

As described above, the highly crystalline silver powder consisting of fine particles has been demanded but there has not been the highly crystalline silver powder which sufficiently satisfies market needs.

Accordingly, the present inventors have conducted intensive studies to solve the problems, and consequently have found that the silver powder obtained with the production method described below has high crystallinity with the fine particle level of which conventional silver powder has never achieved.

Production method of the highly crystalline silver powder: The production method of the highly crystalline silver powder of the present invention is characterized by comprising followings. Preparation of a first aqueous solution in which gelatin, silver nitrate and nitric acid are dissolved in water. Preparation of a second aqueous solution in which L-sorbic acid and a water-soluble organic acid are dissolved. Adding of the second aqueous solution slowly to mix with the first aqueous solution with stirring of the mixture to make the particles to grow to form silver particles after the addition is completed. Keeping of the mixture still to settle the silver particles. Discarding of the supernatant. Filtration and rinsing. Then the highly crystalline silver powder is obtained.

In the production method of the highly crystalline silver powder of the present invention, it is preferable that gelatin concentration in the first aqueous solution is 2g/l to 10 g/l.

In the production method of the highly crystalline silver powder of the present invention, it is preferable that the silver nitrate concentration in the first aqueous solution is 50 g/l to 150 g/l as silver.

In the production method of the highly crystalline silver powder of the present invention, it is preferable that nitric acid is added to arrange the free nitric acid concentration of 40 g/l to 120 g/l in the first aqueous solution by adjusting the nitric acid aqueous solution.