JP2000248303A - Production of silver-coated copper powder - Google Patents

Abstract

PROBLEM TO BE SOLVED: To uniformly and easily form a coating film of silver on the surface of copper powder by bringing metal copper powder into reaction with silver nitrate in an aq. soln. in which a reducing agent is dissolved. SOLUTION: As a reducing agent, an organic reducing agent such as formalin or hydrazine hydrate is preferable. Metal copper powder is suspended into water under the dissolution of the reducing agent, an oxidized film on the surfaces of the copper powder particles is removed, and the state in which an active metallic surface is exposed is held. When silver nitrate is added thereto, it uniformly deposits on the surfaces of the copper powder particles, and its deposition strength is also made better. Therefore, by an extremely thin silver coating film small in the content of silver, its oxidation resistance, thermal stability and electric conductivity can remarkably be improved, and the characteristics as those of an electrically conductive filler can be increased. The reaction between the metal copper powder and silver nitrate under the dissolution of the reducing agent is preferably executed at 40 to 80 deg.C. Moreover, the amt. of the reducing agent to be added is controlled in such a manner that the reduction potential of an aq. soln. before the addition of silver nitrate reaches <=-200 mV.

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 a silver-coated copper powder suitable for a conductive filler or the like.

[0002]

2. Description of the Related Art Conductive pastes and paints are obtained by dispersing metal powder as a conductive filler in a resin binder or vehicle. Copper powder and silver powder are usually used as the conductive filler. Copper powder is less expensive than silver powder, but is inferior in oxidation resistance and has a problem that when the temperature is 110 ° C. or higher, an oxide film is easily generated, which deteriorates the thermal stability of the conductive paint. On the other hand, silver powder has good oxidation resistance and durability, but has problems such as easy migration and high price.

[0003] In view of the above, various methods for coating the surface of copper powder with silver have been proposed. For example, JP-A-53-1
JP-A-34759 and JP-A-60-243277 describe a method of substituting and depositing metallic silver on the surface of copper powder using a silver complex salt solution, and JP-A-1-119602 discloses a method as a chelating agent. Discloses a method in which copper powder is dispersed in EDTA and the surface thereof is reduced and coated with silver.

[0004]

The method of coating silver on a copper powder using a silver complex salt solution has a certain characteristic, but once the copper powder is formed, the copper powder is mixed and dispersed in the silver complex salt solution. In addition, depending on the oxidation state of the surface of the copper powder before mixing and dispersing, the precipitation of silver may vary, and the substitution precipitation may not always be performed uniformly.
It is necessary to control the surface condition of the copper powder to be coated. Also E
In the method using DTA, it is difficult to collect the waste liquid of EDTA. For this reason, some additional equipment is required.
Equipment costs will increase.

An object of the present invention is to solve such a problem, and an object of the present invention is to provide a method capable of forming a silver film uniformly and easily on the surface of metallic copper powder.

[0006]

According to the present invention, as a method of producing a silver-coated copper powder which has solved the above-mentioned problems, a method of preparing a metal copper powder in an aqueous solution in which a reducing agent, particularly hydrazine hydrate or an organic reducing agent is dissolved, is used. Provided is a method for producing silver-coated copper powder by reacting silver nitrate. The reduction potential of the aqueous solution containing the reducing agent is -20.
It is preferably 0 mV or less. The reaction temperature is 40-80.
C. is preferred.

Further, according to the present invention, a reducing agent is added to a suspension obtained by reacting an aqueous solution of a copper salt with an alkali agent to precipitate copper hydroxide, and the resulting mixture is intermediately reduced to cuprous oxide. After the oxygen-containing gas is blown into the suspension and oxidized, or this oxidation is omitted, hydrazine hydrate or an organic reducing agent is added, and the water is finally reduced to copper metal powder in water. A method for producing a silver-coated copper powder, comprising adding silver nitrate to a liquid containing the reduced agent and the copper powder.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION By reacting metallic copper powder and silver nitrate in an aqueous solution in which a reducing agent is dissolved, a silver coating can be formed uniformly on the surface of each particle of metallic copper powder. Hydrazine hydrate or an organic reducing agent is preferably used as the reducing agent. As the reducing agent, any water-soluble reducing agent having a reducing action can be used in principle, but an organic reducing agent such as formalin or hydrazine hydrate is convenient for use. When an inorganic reducing agent containing a metal ion is used, an inorganic substance may be mixed into the silver-coated copper powder. However, when an organic reducing agent or hydrazine hydrate is used, there is no danger.

When metallic copper powder is suspended in water in the presence of such a reducing agent, an oxide film on the surface of the copper powder particles is removed,
It is thought that the reason is that the active metal surface is kept exposed, but when silver nitrate is added in this state, it is uniformly deposited on the surface of the copper powder particles, and the adhesion strength is also good. . For this reason, the ultra-thin silver film having a small amount of silver can significantly improve the oxidation resistance, thermal stability and conductivity as compared with ordinary copper powder for conductive filler. Therefore, when silver is coated according to the present invention on ordinary metallic copper powder used as a conductive filler (regardless of its production method), characteristics as a conductive filler can be remarkably enhanced.

When copper metal powder is suspended in water without dissolving a reducing agent and silver nitrate is similarly added, silver coating may occur depending on the surface condition of the metal copper powder. The uniformity is poor and it is difficult to form a silver coating stably.

The reaction between the metallic copper powder and silver nitrate in the presence of the reducing agent is carried out at a temperature of at least 40 ° C., preferably at least 45 ° C., and preferably at most 80 ° C., preferably at most 60 ° C., preferably under stirring. Good. This makes it possible to uniformly coat the entire surface of each particle of the metallic copper powder with silver. If the reaction temperature is lower or higher than this range, a silver film is formed, but the uniformity of the film may not be good. Although the amount of the reducing agent to be added varies depending on the type of the reducing agent used, the reduction potential of the aqueous solution before the addition of silver nitrate is -200 mV or less, preferably -300 mV or less, and more preferably-
What is necessary is just to control it to be 400 mV or less.

Silver nitrate may be added in powder form to an aqueous solution in which a reducing agent is dissolved and metallic copper powder coexists. However, when added as an aqueous solution, a uniform silver film can be formed, and this silver nitrate aqueous solution is added. It is preferable to add the silver nitrate aqueous solution little by little over a period of time while stirring the metal copper powder solution. A more uniform silver film can be formed by adding such a small amount continuously or batchwise than in the case of batch addition.

The amount of silver nitrate to be added may be adjusted according to the intended thickness of the film. However, according to the present invention, a uniform film can be formed on the entire surface of the metallic copper particles. The amount of silver required is small. The amount of silver required to coat the entire particle surface depends on the specific surface area of the metallic copper powder. If the specific surface area of the metallic copper powder is, for example, 1000 cm ² / g or more, Ag of the metallic copper powder relative to the total Cu is used. Silver nitrate may be added so that the weight ratio of Ag / Cu becomes 0.5 / 100 to 10/100, preferably 1.0 / 100 to 5.0 / 100. If this ratio is less than 0.5 / 100, sufficient weather resistance (oxidation resistance) cannot be obtained, and if it exceeds 10/100, electromigration tends to occur.

A typical example is that the specific surface area is 160
1000 g of 0 cm ² / g metallic copper powder was suspended with stirring in 4 liters of an aqueous solution in which 655 g of hydrazine hydrate had been dissolved. If the addition is continued little by little over 60 minutes, a silver-coated copper powder having a silver film having an average film thickness of 2 to 3 nm is produced. This suspension is subjected to solid-liquid separation, and the collected powder is washed with water and dried to obtain a silver-coated copper powder having a uniform silver coating.

The method of the present invention for obtaining silver-coated copper powder by the reaction of copper metal powder and silver nitrate in the presence of a reducing agent is advantageous when applied to the final step of the wet production method of copper metal powder. Industrial production of silver-coated copper powder can be advantageously performed using a copper compound as a starting material. A well-known method for wet production of copper metal powder is to perform a two-stage reduction of copper hydroxide powder in water from a suspension of copper hydroxide in water to copper copper powder. For example, glucose is used as a reducing agent for copper, and hydrazine hydrate or formalin is used as a reducing agent for converting cuprous oxide to metallic copper powder. The liquid at the time when the reduction of cuprous oxide to metallic copper powder, which is the final step, is completed, contains the remaining reducing agent and contains metallic copper powder. Therefore, when silver nitrate is added to this solution as described above, silver-coated metallic copper powder can be obtained.

From the above, according to the present invention, a reducing agent is added to a suspension in which copper hydroxide is precipitated by reacting an aqueous solution of a copper salt with an alkali agent, and intermediate reduction to cuprous oxide is carried out. An industrially advantageous method comprising adding an organic reducing agent or hydrazine hydrate to a final reduction to copper metal powder in water, and adding silver nitrate to a liquid containing the obtained reducing agent and copper metal powder. A wet production method of silver-coated copper powder is provided.

The inventors of the present invention provide an industrial process for silver-coated copper powder by providing an oxidation step by blowing oxygen-containing gas after intermediate reduction and before final reduction.
Oxidation treatment by blowing oxygen-containing gas is performed at the stage when the reduction to cuprous oxide is completed, and final reduction is performed after this oxidation treatment. It was found that the particle size distribution of the copper powder after the final reduction was reduced, and that the average particle size could be freely changed by adjusting the degree of oxidation treatment. Therefore, according to the present invention, a reducing agent is added to a suspension obtained by reacting an aqueous solution of a copper salt with an alkali agent to precipitate copper hydroxide, and the resulting mixture is intermediately reduced to cuprous oxide. After the oxygen-containing gas is blown into the suspension to oxidize it, an organic reducing agent or hydrazine hydrate is added to the suspension to make a final reduction in water to copper metal powder, which contains the obtained reducing agent and copper metal powder An industrially more advantageous method for producing a silver-coated copper powder by adding silver nitrate to a liquid is provided.

In the latter method of carrying out the oxidation treatment, when the amount of the oxygen-containing gas blown into the suspension of cuprous oxide is increased, the particle size of the copper powder at the time of final reduction increases. The injection amount of oxygen-containing gas is determined by the flow rate and the injection time.
By adjusting the flow rate and the blowing time, the particle size of the copper powder can be accurately controlled. In addition, when this oxidation treatment is performed, compared with the case where the oxidation treatment is not performed, the width of the particle size distribution is narrowed, and particles having a uniform particle size can be obtained. It turns out that most become spherical. It is preferable to adjust the flow rate and the blowing time so that the oxygen-containing gas is blown at a rate of at least 0.1 mole or more per mole of copper in the solution. However, the effect is saturated even if the blowing amount is too large. Therefore, depending on the blowing method, even if the oxygen amount is 20 moles or less per 1 mole of copper in the liquid, even if the oxygen amount is 10 moles or less in some cases. Good. As the oxygen-containing gas to be blown, the use of air is most convenient. Unless otherwise specified, air at room temperature may be blown into the suspension at room temperature. Of course, oxygen-enriched air or pure oxygen gas can also be used.

In the above-mentioned industrial wet production method of silver-coated copper powder which is subjected to oxidation treatment or is omitted, the steps of depositing copper hydroxide, the intermediate reduction step and the final reduction step may be in accordance with ordinary methods. For example, in the step of depositing copper hydroxide, copper hydroxide may be precipitated by reacting an aqueous solution of copper salt with an alkali agent. As the copper salt aqueous solution, an aqueous solution of copper sulfate is used, and as the alkali agent, an aqueous solution of NaOH is most commonly used. In some cases, the former may be an aqueous solution of copper chloride, copper carbonate, copper nitrate, or the like, and the latter may be an alkaline agent that does not affect the other. For the precipitation reaction of copper hydroxide, a method of separately preparing an aqueous solution of a copper salt having a predetermined concentration and an aqueous solution of an alkali having a predetermined concentration, mixing the two solutions and immediately stirring vigorously, or stirring the aqueous alkali solution in the aqueous copper salt solution with stirring It is preferable to proceed by a method of continuously adding. As a result, a suspension in which granular copper hydroxide is precipitated is obtained.

Then, a reducing agent is added to the obtained copper hydroxide suspension to intermediately reduce the copper hydroxide to cuprous oxide, and glucose (glucose) can be used as the reducing agent. This intermediate reduction step is preferably performed while raising the temperature in an inert gas atmosphere. When the oxidation treatment is performed after the completion of the intermediate reduction treatment, the atmosphere gas is replaced with an oxygen-containing gas, and the oxygen-containing gas is bubbled in the liquid. By performing this oxidation treatment, the pH of the solution is 5 to 9
Becomes Then, the suspension was decanted under an inert gas atmosphere, and the supernatant was removed.
Collect the precipitate. When the oxidation treatment is omitted, the suspension after the intermediate reduction treatment is decanted under an inert gas atmosphere, and the supernatant is removed to collect a precipitate.

The precipitate is suspended in fresh water and finally reduced to metallic copper using hydrazine hydrate or formalin as a reducing agent. In this reduction reaction, heat is generated, and the liquid temperature rises.

At that time, since it is usual to add a reducing agent in an amount necessary and sufficient for reducing the metallic copper powder, the reducing agent, ie, hydrazine hydrate or formalin, is added to the liquid which has been reduced to the metallic copper powder. It is dissolved. Therefore, when silver nitrate is added to this final suspension, metallic copper powder and silver nitrate can be reacted in an aqueous solution in which an organic reducing agent is dissolved, whereby a suspension of silver-coated copper powder is obtained. This is separated into solid and liquid and dried to obtain silver metallic copper powder. In the silver coating step, it is preferable that the conditions described above are satisfied.
0 ° C., preferably 45-55 ° C., and the reduction potential before adding silver nitrate is -200 mV or less, preferably -400 mV.
V or less, and the silver nitrate aqueous solution is preferably added little by little continuously or batchwise with stirring over time.

[0023]

[Example 1] A 48% strength NaOH aqueous solution 5
An alkaline aqueous solution at a temperature of 27 ° C. obtained by adding 4158 g of pure water to 39 g, and copper sulfate (CuSO _4.
(5H ₂ O) was mixed with an aqueous solution of copper sulfate at a temperature of 29 ° C. in which 662.5 g of 662.5 g were dissolved (pH was 13.7, and the equivalent ratio of caustic soda to copper in the solution was 1.25). Thus, a suspension in which copper hydroxide is precipitated is obtained.

Glucose 993.
A total amount of an aqueous glucose solution obtained by dissolving 5 g in 4140 g of pure water is added, and after 30 minutes from the addition, the temperature of the solution is raised to 70 ° C., and then maintained for 15 minutes. All the processing operations so far are performed in a nitrogen atmosphere.

Next, air is bubbled through the liquid at a flow rate of 62 ml / min for 200 minutes. As a result, the pH of the solution becomes 6.2.

After the suspension is allowed to stand in a nitrogen atmosphere for 2 days, the supernatant (pH 7.01) is removed, almost all of the precipitate is collected, and 700 g of pure water is added to the precipitate.

Hydrazine hydrate 6 was added to the total amount of the suspension.
Add 5 g. Due to the exothermic reaction, the temperature of the liquid is raised to 50 ° C., and finally to 80 ° C., and the reaction is completed. The solution after the reaction is a solution in which metallic copper powder is contained in an aqueous solution in which hydrazine hydrate is dissolved.

The liquid obtained by suspending the metallic copper powder in the aqueous solution in which hydrazine hydrate was dissolved had a reduction potential of -400 mV, and the metallic copper powder in the liquid was initially copper sulfate. Substantially equal to the molar ratio, approximately 260 g. 4.1 g of silver nitrate is dissolved in 36 g of pure water so that the silver amount corresponds to approximately 1% by weight of the copper amount, and the whole amount of the silver nitrate aqueous solution is continuously reduced in small amounts over 60 minutes using a tube pump. Was added to the suspension of the metallic copper powder maintained at 50 ° C. while stirring.

After the completion of the reaction, the suspension was filtered, washed with water and dried to obtain a silver-coated copper powder. The electrical resistance was measured while applying a load to the powder, and the relationship between the load and the electrical resistance was examined. The result is shown in FIG.

Comparative Example A liquid in which metallic copper powder was suspended was obtained in the same manner as in Example 1 until final reduction to metallic copper powder. The powder was filtered, washed with water and dried in a nitrogen atmosphere to obtain metallic copper powder. 493.3 g of this metallic copper powder was
After adding 5 g of EDTA to a 4.8 liter aqueous solution and stirring at 200 rpm for 5 minutes, while continuing the stirring, the entire amount of silver nitrate aqueous solution in which 15.7 g of silver nitrate was dissolved in 200 ml of pure water was added. 10 g of tartaric acid as a reducing agent was added, followed by stirring for 30 minutes, followed by filtration, and washing with water until the filtrate became transparent.

The obtained powder is sucked by a vacuum pump and
It dried at ℃ and obtained 490g of silver covering copper powder. The silver content of this silver-coated copper powder was also 1% by weight, as in Example 1. Similarly to Example 1, the electrical resistance of the silver-coated copper powder was measured while applying a load to the powder, and the relationship between the load and the electrical resistance was examined. The results are also shown in FIG.

From the results shown in FIG. 1, the powder of the example has lower powder resistance (good conductivity) than that of the comparative example.
It can be seen that the powder resistance at low load is particularly low.

The powders of the example and the comparative example were 2
00 mg was loaded into the crucible of the differential thermal analyzer, and the temperature of the powder was raised at 4 ° C./min. In the air atmosphere, and the differential thermal analysis was performed. 473.8 ° C
Indicates a single peak. A peak was observed at 370 ° C. for copper powder without silver coating, and oxidation started at this temperature, but no peak was observed at around 370 ° C. for both powders of the examples and comparative examples. This indicates that the powders of the examples have sufficient oxidation resistance (weather resistance).

[0034]

As described above, according to the present invention,
A silver coating can be formed on the surface of the metal copper powder uniformly and by a simple operation, and since the silver coating is uniform, the amount of silver required to cover the entire surface of the metal copper powder can be reduced. The method of the present invention is also advantageous in that it can be directly applied to the final step of the method for producing wet copper powder, and can produce a high-quality conductive filler industrially and economically.

[Brief description of the drawings]

FIG. 1 is a diagram showing the results of measuring electrical resistance while applying a load to a silver-coated powder obtained by the method of the present invention, in comparison with a silver-coated powder of a comparative example.

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Hiromasa Miyoshi 1-8-2 Marunouchi, Chiyoda-ku, Tokyo F-term in Dowa Mining Co., Ltd. 4K017 AA06 BA02 BA05 BB02 DA01 EH18 EJ01 FB03 FB07 4K018 BA02 BC23 BD04 KA33 4K022 AA02 AA35 AA41 BA01 DA01 DB04 DB05 DB06 DB13 DB14

Claims (6)

[Claims] 1. A method for producing silver-coated copper powder by reacting metallic copper powder and silver nitrate in an aqueous solution in which a reducing agent is dissolved. 2. The method according to claim 1, wherein the reducing agent is hydrazine hydrate or an organic reducing agent. 3. The method for producing a silver-coated copper powder according to claim 1, wherein the reduction potential of the aqueous solution is −200 mV or less. 4. The method for producing a silver-coated copper powder according to claim 1, wherein the reaction temperature is 40 to 80 ° C. 5. A method in which a reducing agent is added to a suspension obtained by reacting an aqueous solution of a copper salt with an alkali agent to precipitate copper hydroxide, intermediate reduction to cuprous oxide is performed, and then hydrazine hydrate or organic reduction is performed. The final reduction in water to the metal copper powder by adding the agent,
A method for producing a silver-coated copper powder, comprising adding silver nitrate to the obtained liquid containing the reducing agent and metallic copper powder. 6. A copper salt aqueous solution and an alkali agent are reacted with each other to cause a copper hydroxide to precipitate, and a reducing agent is added to the suspension for intermediate reduction to cuprous oxide. After oxidizing by blowing an oxygen-containing gas, hydrazine hydrate or an organic reducing agent is added and the final reduction is performed in water to metallic copper powder. A method for producing silver-coated copper powder comprising adding.