JP3074415B2 - Production method of silver-plated copper powder - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for producing silver-plated copper powder. More specifically, the present invention relates to a method for producing silver-plated copper powder used for imparting conductivity to a conductive paste for electronic materials such as conductive paints and conductive adhesives.

[0002]

2. Description of the Related Art Conventionally, conductive polymers,
Carbon powder, carbon fiber, copper powder, as a filler for imparting conductivity to a dispersed composite conductive material such as conductive rubber, conductive plastic, conductive paint, and conductive adhesive.
Silver powder and the like are used. Metal powders such as copper powder and silver powder are mainly used for conductive pastes such as conductive paints and conductive adhesives for electronic materials accompanied by soldering.

[0003] However, copper powder is easily oxidized and thus has unstable conductivity. Silver powder has good conductivity but is expensive and cannot be soldered. Therefore, silver-plated copper powder in which silver is plated on the surface of copper has recently been developed as a filler that compensates for the disadvantages of both.

[0004] Silver plating of copper powder is performed by an electroless displacement plating method. Due to the advancement of the technology and the development of applications, the electroless plating method is not limited to metal as a material to be plated today, but can be applied to organic materials such as plastics and inorganic materials such as ceramics.
It is a plating method that can be applied without limitation on the shape and size of the object to be plated, and also for silver plating of copper powder, disperse metal copper powder in a plating solution containing a reducing agent, silver ion, etc. prepared in advance, After a reaction is performed for a predetermined time, plating is performed according to a conventional general method of stopping the reaction.

That is, copper powder is immersed in a silver ion solution for a certain period of time, and silver is reduced and precipitated on the surface of the copper powder by a substitution reaction that dissolves copper having a lower standard electrode potential (higher ionization tendency) than silver. Things. At this time, according to the following reaction formula, two atoms of silver are precipitated by dissolving one atom of copper.

Embedded image Cu + 2Ag ⁺ → Cu ²⁺ + 2Ag

[0006] In the case of copper powder, the plating reaction speed is high because the specific surface area is remarkably large as compared with other base materials. Therefore, plating is immediately added to the plating solution, and after the reaction, the plating solution is filtered and quenched. Alternatively, a stop such as dilution must be performed. Therefore, the resulting plating film has coarse silver particles, intense but uneven surface, silver is not attached to a part of the surface, it is black, and it is difficult to obtain a uniform plating film. When a silver-plated copper powder is used as a conductive metal powder in a conductive paste, there is a disadvantage that poor conductivity and poor corrosion resistance occur.

Various attempts have been made to obtain a uniform plating film, but none of them are satisfactory. For example, a method of suspending a powder to be plated in an ammoniacal silver nitrate solution and subjecting the powder to silver plating under a strong alkali using formalin, hydrazine, glucose, or the like as a reducing agent is also known (see Japanese Patent Application Laid-Open (JP-A) No. 2002-122,086). No. 57-49632), the silver plating film obtained by this method is poor in uniformity, the yield of silver plating on copper powder is poor, and silver nitride, which is an explosive substance, other than on the copper powder, is used. Precipitates. Accordingly, an object of the present invention is to provide a plating method capable of forming a uniform silver plating film on the surface of copper powder.

[0008]

Means for Solving the Problems In silver ion displacement plating of copper powder, the present inventors have developed a method in which a plating solution is forcibly treated while maintaining an oxidizing atmosphere, so that a copper powder uniformly silver-plated is treated. Was confirmed, and the present invention was completed. That is, the present invention provides a method for producing a silver-plated copper powder by bringing a copper powder into contact with a silver ion solution, wherein the plating is performed while introducing an oxygen-containing gas into the plating solution. It provides a manufacturing method.

[0009]

When copper powder is put in ammoniacal silver nitrate, displacement plating generally proceeds according to the following reaction.

## STR2 ## In the conventional method in which the above reaction is carried out in an atmosphere of air, Cu + 2Ag ⁺ → Cu2 ⁺⁺ 2Ag
While a uniform silver plating film cannot be obtained, the method of the present invention, which is carried out while forcibly introducing an oxygen-containing gas into the plating solution, makes it possible to obtain a uniform silver plating film having good adhesion. I'm not sure why this is possible,
It is considered as follows.

[0010] Usually, the following reaction occurs first by the substitution reaction.

## STR3 ## ^{Cu + Ag + → Cu + +} Ag (a) Subsequently, the following reaction takes place.

[Image Omitted] Cu ⁺ + Ag ⁺ → Cu ²⁺ + Ag (b)

At this time, if dissolved oxygen is present, Cu (I) is oxidized to Cu (II) by the oxygen.

[Image Omitted] Cu ⁺ + OH ⁻ + ／ O ₂ → CuO + ／ H ₂ O (c) Accordingly, the precipitation reaction of silver by the reaction of (b) is suppressed by the dissolved oxygen.

In the conventional displacement plating method performed in the atmosphere, dissolved oxygen is present in the plating solution. However, if the plating is performed simply in the atmosphere, the oxygen supply amount is limited, and the amount of silver deposited is large. When this happens, the amount of dissolved oxygen becomes insufficient, and the reaction of (b) tends to occur, the silver deposition rate increases, the crystal grains become coarse, and a high-quality silver plating film cannot be obtained. On the other hand, when the oxygen-containing gas is forcibly introduced from the outside, the dissolved oxygen is maintained at a certain concentration or more, and the reaction of the above (b) is controlled, so that a silver film having a uniform surface and good adhesion is obtained. It is considered that the obtained copper powder is obtained.

[0013]

Configuration of the Invention

[Copper powder] The copper powder used in the plating method of the present invention may be any of an electrolytic powder, a reduced powder, and an atomized powder. The average particle size and particle size distribution are not particularly limited, but the average particle size is preferably 20 µm or less. .

[0014]

[Plating solution] Silver ion-containing plating solution is a silver salt, for example,
It is applied as a solution of a soluble salt in which silver nitrate, silver chloride or silver organic acid is dissolved in an aqueous solution of cyanide such as ammonia water, potassium cyanide, or the like, or an aqueous solution of potassium iodide. The range of the silver concentration in the plating solution is selected according to the particle size of the powder to be plated, the amount of copper powder to be plated, and the like. As the reducing agent, dimethylamine borane, formalin, Rossell salt, hydrazine, glucose and the like can be used. As a specific example of the plating bath composition, for example, a copper powder having an average particle diameter of 10 μm and an Ag coating amount of 10 μm is used.
When silver plating of about weight% is performed, the following plating bath can be used.

Silver plating bath composition-1: silver nitrate 8 g / l,
Aqueous ammonia 75 g / l, sodium thiosulfate 10
5 g / l;

Silver plating bath composition-2: silver nitrate 8 g / l,
Glucose 4 g / l, tartaric acid 4 g / l, aqueous ammonia 75 g / l;

Silver plating bath composition-3: silver nitrate 50 g /
l, potassium iodide 500 g / l;

Silver plating bath composition-4: silver chloride 8 g / l,
Potassium cyanide 38g / l, sodium carbonate 38
g / l, sodium chloride 15g / l, ammonia water
60 g / l.

[0019]

[Introduction of Oxygen-Containing Gas] In the present invention, plating is performed while introducing an oxygen-containing gas, preferably air, into the plating solution into the plating bath. The introduction of air is usually performed continuously so that the dissolved oxygen amount in the plating solution can be maintained at a predetermined level (5 ppm or more), but may be introduced intermittently. Examples of the method of introducing air include a method of introducing a gas into the lower part of the bath while stirring the plating bath and a method of bubbling through a diffuser tube made of a porous material. The bubbling method is preferable.

In order to keep the amount of dissolved oxygen at a constant level and to obtain uniform silver plating, the amount of air introduced is 40 liters or more, preferably 100 to 150 liters, in terms of standard condition, per mol of precipitated silver. It is preferable to perform bubbling through a porous material so that the oxygen absorption efficiency during the plating process is 5% or more. Such a porous material is a molded product made of plastic, sintered metal or ceramic, having an average pore size of 150 μm or less, preferably 5 μm or less.
By attaching a diffuser tube of a porous material of 0 μm or less to the air introduction tube and introducing the above amount of air, the oxygen absorption efficiency becomes 5% or more.

The plating conditions such as temperature and time depend on the composition of the plating bath and the amount of copper powder to be plated, but the bath temperature is usually from 15 to 35 ° C.
A uniform silver-plated copper powder of about 0.1 to 0.2 μm is obtained.

[0022]

EXAMPLES The present invention will be described below with reference to examples and comparative examples, but the present invention is not limited to the following examples. In the following examples, the oxygen absorption efficiency was calculated from the difference between the amount of introduced air and the amount of discharged air by performing plating in a closed system having an inlet and an outlet for air.

Example 1 A copper powder having an average particle diameter of 10 μm was placed in a 10% by weight aqueous solution of ascorbic acid, stirred at room temperature for 30 minutes, taken out of the liquid, and placed in 1.5% by weight aqueous ammonia.
After stirring for 2 seconds, dry without drying ammonia (5.2 g
/ L), glucose (5.2 g / l), tartaric acid (5.2 g /
1) and ethanol (10 g / l), stirred and homogeneously dispersed.

Next, an aqueous solution of silver nitrate having a concentration of 30 g / l and replenished ammonia water were put into the above solution by a quantitative pump, and passed through a porous ceramic diffuser having an average pore diameter of 15 μm.
Air is introduced into the plating bath at a rate of 40 liters / minute,
The mixture was stirred at room temperature for 2 hours while adjusting the supply amounts of the aqueous silver nitrate solution and the aqueous ammonia so as to maintain the silver ion concentration at 0.8% by weight and the ammonia at 5.2 g / l. I got Table 1 shows the results of observing the hue, electric resistance, surface condition of the obtained silver-plated copper powder by an electron microscope, and the plating film thickness. Also,
When 1 g of the obtained silver-plated copper powder was heated and dissolved in an aqueous nitric acid solution (1: 1) and analyzed by atomic absorption spectrometry, the amount of silver plating was 10% by weight.

Example 2 A plating experiment was performed in the same manner as in Example 1 except that a porous ceramic air-diffusing material having a pore diameter of 50 μm was used and air was introduced into the plating bath at a rate of 100 L / min. . The results are shown in Table 1.

Example 3 A plating experiment was performed in the same manner as in Example 1 except that a porous ceramic air-diffusing material having a pore diameter of 150 μm was used, and air was introduced into the plating bath at a rate of 100 L / min. . The results are shown in Table 1.

Comparative Example 1 A plating experiment was performed in the same manner as in Example 1 except that no air was introduced. The results are shown in Table 1.

Comparative Example 2 Example 1 was repeated except that the amount of air introduced was reduced to 20 l / min.
A plating experiment was performed in the same manner as described above. The results are shown in Table 1.

Comparative Example 3 A plating experiment was carried out in the same manner as in Example 1 except that air was introduced at a rate of 40 L / min using a vinyl chloride pipe having an inner diameter of 9 mm instead of the porous ceramic air-diffusing material having an average pore diameter of 15 μm. Was performed. The results are shown in Table 1.

[0030]

[Table 1]

[0031]

The silver-plated copper powder obtained by the method of the present invention has a silver coating having a uniform surface and good adhesion, and has better conductivity than conventional silver-plated copper powder. It can be suitably used as a filler for imparting conductivity to a conductive paste such as a conductive paint or a conductive adhesive.

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int. Cl. ⁷ , DB name) C23C 18/00-18/54

Claims (3)

(57) [Claims] 1. A method for producing a silver-plated copper powder by bringing a copper powder into contact with a silver ion solution, wherein plating is performed while introducing an oxygen-containing gas into a plating solution. Method. 2. The method for producing silver-plated copper powder according to claim 1, wherein an oxygen-containing gas is introduced by bubbling at a rate of 40 liters or more per mol of precipitated silver. 3. An oxygen-containing gas is introduced by bubbling so as to have an oxygen absorption efficiency of 5% or more.
4. The method for producing a silver-plated copper powder according to the above.