Method Of Production Of High Purity Silver Particles

Abstract

A method for synthesizing high purity silver particles and colloids without requiring the addition of either surfactants or reducing agents thereto, or requiring only a minimal amount thereof. The synthesizing process comprises: (i) a silver oxalate synthesizing process; (ii) a process of dispersing silver oxalate into an appropriate carrier; and {iii) a process of heating said silver oxalate dispersed into said carrier at a temperature of at least 100° C. Silver particles and colloids of various form factor and size may be synthesized depending upon the reaction conditions, the carrier, and the type of surfactant.

Description

This application claims the benefit of U.S. Provisional Application No. 60/618,876, filed Oct. 14, 2004 and entitled “Method for Production of Fine Silver Particles,” which is incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a method of forming silver particles by dispersing silver oxalate into an appropriate carrier and then applying heat at a temperature of 100° C. or higher to decompose the silver oxalate.

BACKGROUND ART

A number of methods have been developed to synthesize silver particles including, but not limited to, chemical reduction, photochemical, sonochemical and gas evaporation methods. Of these methods, the chemical reduction method is widely used due to the ease of production. However, silver powder produced using the chemical reduction method can be contaminated by the reducing agent, the surfactant and impurity ions used during the reaction process, which can serve as a limiting factor in the field of electronics requiring high conductivity or in the field of bacteria resistance requiring high purity.

In order to resolve these problems, it is desirable to have a method of producing high purity silver powder and silver colloids that does not require either surfactants or reducing agents, or only a minimal amount of a surfactant.

DISCLOSURE OF THE INVENTION

The object of the present invention is to synthesize high purity silver particles and colloids in a process that does not require either surfactants or reducing agents, or only a minimal amount of a surfactant. In the present invention, this object is achieved by dispersing silver oxalates into an appropriate carrier and then thermally decomposing the silver oxalates at a temperature of 100° C. or higher to synthesize high purity silver particles and colloids.

The process of synthesizing silver particles and colloids by the method of the present invention comprises: (i) a silver oxalate synthesizing process; (ii) a process of dispersing silver oxalates into an appropriate carrier, for example, water, alcohol or the like, including a combination of more than one carrier; and (iii) a process of heating said silver oxalates dispersed into said carrier at a temperature of 100° C. or higher under a pressure greater than atmospheric pressure.

These and other features, objects and advantages of the present invention will become better understood from a consideration of the following detailed description of the preferred embodiments and appended claims in conjunction with the drawings as described following.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a microphotograph of silver particles obtained under the conditions described in Example 1.

FIG. 2 is a microphotograph of silver particles obtained under the conditions described in Example 2.

FIG. 3 is a microphotograph of silver particles obtained under the conditions described in Example 3.

FIG. 4 is a microphotograph of silver particles obtained under the conditions described in Example 4.

FIG. 5 is a microphotograph of silver particles obtained under the conditions described in Example 5.

FIG. 6 is a microphotograph of silver particles obtained under the conditions described in Example 6.

BEST MODE FOR CARRYING OUT THE INVENTION

With reference to FIGS. 1-6, the preferred embodiments of the present invention may be described as follows.

In the present invention, a method for the production of silver particles and colloids comprises three processes as follows: (i) a silver oxalate (Ag₂C₂o₄) synthesizing process; (ii) a process of dispersing silver oxalate into an appropriate carrier, for example, water, alcohol or the like, including a combination of more than one carrier; and (iii) a process of heating said silver oxalate dispersed into said carrier at a temperature of 100° C. or higher under a pressure greater than atmospheric pressure to form silver particle or colloids from the decomposition of the silver oxalate.

A first solution of a water soluble silver compound and a second solution of an oxalate compound are mixed together to precipitate silver oxalates. The silver compound may be AgNO₃. The oxalate compound may be sodium oxalate or oxalic acid. The present invention is not, however, limited to these specific compounds but may include any two solutions of compounds that form silver oxalates upon mixing. After water cleaning processes, preferably two or more rounds of water cleaning processes, are performed to remove impure ions from the precipitated silver oxalate, the silver oxalate is used as the starting material for synthesizing silver powder or colloids.

The synthesized silver oxalate is dispersed into an appropriate carrier. The silver oxalate is not dissolved to any substantial extent in the carrier, but is dispersed as solid particles by using ultrasonic treatment. The appropriate carrier may include all types of carriers which can disperse silver oxalate to effectively deliver heat. The carrier is selected to have properties that allow it to behave similarly to a surfactant so as to prevent agglomeration of the silver particles formed from the thermal decomposition of the silver oxalate. For example, alcohols consist of alkyl and hydroxyl groups. Generally, alkyl groups have hydrophobic properties and hydroxyl groups have hydrophilic properties. Organic materials having both hydrophobic and hydrophilic properties can play a role as a surfactant. However, organic materials having higher carbon numbers tend to be dominantly hydrophobic and may therefore tend to lose the ability to act as a surfactant in the process of the present invention. Generally, organic materials having higher numbers of carbon atoms have superior surfactant properties. However, in the present invention, organic materials with a higher number of carbon atoms is observed to agglomerate silver particles. Furthermore, organic materials with a higher number of carbon atoms do not mix well with water. Therefore, the present invention is limited to methyl, ethyl and propyl alcohols, which have a low number of carbon atoms. Water is also effective in the practice of the present invention. The appropriate carrier may therefore consist of ethyl alcohol, methyl alcohol, propyl alcohol, water or a combination of more than one of the preceding.

The carriers selected for the practice of the present invention all have low boiling points: water (100° C.), methyl alcohol (64.65° C.), ethyl alcohol (78.3° C.), and propyl alcohol (82° C.). Accordingly, when the carrier with the dispersed silver oxalate is heated in a container at or above 100° C., the pressure is always above atmospheric pressure. Typical reaction pressures are about 1.86*10⁵ N/m² when using water as the carrier and about 5.31*10⁵ N/m² when using ethyl alcohol as the carrier. During thermal decomposition of silver oxalate, the silver oxalate (Ag₂C₂O₄) decomposes into silver (Ag) and carbon dioxide (CO₂) according to the formula Ag₂C₂O₄=2Ag+2CO₂. The carbon dioxide gas evolved during the thermal decomposition of the silver oxalate and the carrier vapor may be evacuated as necessary but pressure drops of less than about 6.89*10⁴ N/m² do not affect the quality of the silver particles.

The dispersed silver oxalate in the carrier is placed into a closed reactor to heat the dispersed silver oxalate and carrier up to at least 100° C. to synthesize silver powder or colloids of various form factors.

This method may optionally use surfactants in order to prevent coagulation or agglomeration of the silver particles. Surfactants may be added to the water soluble silver or oxalate solutions used to produce silver oxalate, or may be added after the silver oxalate is produced by mixing the two solutions. Surfactants used in this method may include anionic surfactants, cationic surfactants, amphoteric surfactants, nonionic surfactants, fluorochemical surfactants, and polymerizable surfactants, or combinations of the preceding, which may be added to aid in forming silver particles and to break down silver plates or prevent silver plates from coagulation. Surfactants suitable for use in the present invention include PVP (polyvinyl pyrrolidone) and gelatine.

Irrespective of the amount of surfactant added, silver particles or colloids can be obtained by the method of the present invention, however, it is desirable to limit the amount of surfactant to no more than 80% of the weight of the silver. For example, if 10 grams of silver is placed into the reactor, the weight of the surfactant, such as PVP or gelatin, should be no more than 8 grams.

EXAMPLE 1

After 2.8 grams of silver oxalate was placed into 300 cc of distilled water, ten minutes of ultrasonic treatment was performed to disperse the particles. The dispersed silver oxalate was reacted for 15 minutes at 130° C. to obtain a solution containing silver particles as shown in FIG. 1.

EXAMPLE 2

After 28 grams of silver oxalate was placed into 1000 cc of ethyl alcohol, ten minutes of ultrasonic treatment was performed to disperse the particles. The dispersed silver oxalate was reacted for 15 minutes at 134° C. to obtain silver powder as shown in FIG. 2.

EXAMPLE 3

After 70 mg of silver oxalate was placed into 1000 cc of ethyl alcohol, ten minutes of ultrasonic treatment was performed to disperse the particles. The dispersed particles were reacted for 25 minutes at 135° C. to obtain nano-sized silver particles as shown in FIG. 3.

EXAMPLE 4

After 4.2 grams of silver oxalate was placed into a mixed solution of water (vol. 50%) and ethyl alcohol (vol. 50%), the solution was reacted for 15 minutes at 130° C. to synthesize 0.5 μm silver particles as shown in FIG. 4.

EXAMPLE 5

30 wt % of PVP (polyvinyl pyrrolidone) was placed into 4.2 grams of silver oxalate in 1 Liter of water and ultrasonic treatment was performed to disperse the particles thereof. The dispersed particles were reacted for 20 minutes at 135° C. to synthesize silver particles of 0.5 μm or smaller in size as shown in FIG. 5.

EXAMPLE 6

10 grams of gelatin was placed into 28 grams of silver oxalate in 1 Liter of water and ultrasonic treatment was performed to disperse the particles thereof. The dispersed particles were reacted for 15 minutes at 135° C. to synthesize silver particles of 50 nm or smaller in size as shown in FIG. 6.

INDUSTRIAL APPLICABILITY

Due to its inherent characteristics of high conductivity and bacteria resistance, silver particles are widely used in the electronics industry as well as in other industries requiring bacteria resistance.

The present invention has been described with reference to certain preferred and alternative embodiments that are intended to be exemplary only and not limiting to the full scope of the present invention as set forth in the appended claims.

Claims

1. A method for production of silver particles, comprising the steps of:

(a) dispersing solid silver oxalate particles in a carrier; and

(b) heating said dispersed silver oxalate to a temperature of at least 100° C. under a pressure that is greater than atmospheric pressure to decompose said silver oxalate into silver particles.

2. The method of claim 1, further comprising the step before step (a) of producing said silver oxalate by mixing a first solution of a silver compound and a second solution of an oxalate compound to form silver oxalate.

3. The method of claims 1 or 2, further comprising the step of adding a surfactant to said dispersed silver oxalate before the heating of step (b).

4. The method of claim 3 wherein said surfactant is selected from the group consisting of anionic surfactants, cationic surfactants, amphoteric surfactants, nonionic surfactants, fluorochemical surfactants, polymerizable surfactants, and any combination of the preceding surfactants.

5. The method of claim 2, further comprising the step of adding a surfactant to said first solution before said mixing step.

6. The method of claim 5 wherein said surfactant is selected from the group consisting of anionic surfactants, cationic surfactants, amphoteric surfactants, nonionic surfactants, fluOrochemical surfactants, polymerizable surfactants, and any combination of the preceeding surfactants.

7. The method of claim 2 further comprising the step of adding a surfactant to said second solution before said mixing step.

8. The method of claim 7 wherein said surfactant is selected from the group consisting of anionic surfactants, cationic surfactants, amphoteric surfactants, nonionic surfactants, fluorochemical surfactants, polymerizable surfactants, and any combination of the preceeding surfactants.

9. The method of claims 1 or 2, wherein the dispersing of step (a) comprises adding silver oxalate to said carrier and subjecting the mixture of silver oxalate and carrier to ultrasonic treatment.

10. The method of claims 1 or 2, wherein said carrier is selected from the group consisting of water, methyl alcohol, ethyl alcohol, propyl alcohol, and a mixture of any of the preceding.

11. The method of claim 2, wherein said silver compound is AgNO3.

12. The method of claim 2, wherein said oxalate compound is selected from the group consisting of sodium oxalate and oxalic acid.

13. The method of claim 2, further comprising the step following the production of silver oxalate of washing said silver oxalate with water to remove impurities.

14. The method of claim 4, wherein said surfactant is selected from the group consisting of PVP (polyvinyl pyrrolidone) and gelatin.

15. The method of claim 6, wherein said surfactant is selected from the group consisting of PVP (polyvinyl pyrrolidone) and gelatin.

16. The method of claim 8, wherein said surfactant is selected from the group consisting of PVP (polyvinyl pyrrolidone) and gelatin.

17. The method of claim 3, wherein said surfactant is no greater than 80% by weight of the silver in step (b).

18. The method of claim 5, wherein said surfactant is no greater than 80% by weight of the silver in step (b).

19. The method of claim 7, wherein said surfactant is no greater than 80% by weight of the silver in step (b).