JP2002266079A - Method for manufacturing silver coated conductive powder, silver coated conductive powder and electroless plating bath for coating conductive powder - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing silver coated conductive powder which is capable of forming a uniform silver plating layer having a high adhesion property on a nickel alloy plating layer formed on the surface of a powdery core material even if a cyano compound is not used. SOLUTION: In this method for manufacturing the silver coated conductive powder by treating the surface of the core material consisting of an organic material or inorganic material with an electroless nickel-phosphorus plating liquid, then treating the surface with an electroless silver plating liquid containing a water-soluble silver salt and a complexing agent, an organic compound containing sulfurous acid or a sulfite or imide group or amide group is used as the complexing agent for the electroless silver plating liquid.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a silver-coated conductive powder having a silver plating layer on the outermost surface, a method for producing the same, and an electroless silver plating bath for coating the conductive powder, which is used as a conductive filling material or the like. About.

[0002]

2. Description of the Related Art Conventionally, there has been known a technique of imparting conductivity to a molded product by molding a kneaded product of a rubber composition such as a silicone rubber composition and a filler material having conductivity. As a conductive filling material used for such a purpose, a resin powder or a core material made of ceramic powder coated with a metal has been developed. An electroless plating method is used. Attempts have also been made to form a coating of a hard-to-oxidize metal such as gold or silver on the outermost surface of the metal coating for the purpose of suppressing a decrease in the conductivity of the filling material due to oxidative deterioration of the metal coating. For example,
(1) A core material such as resin is coated with nickel and then the outermost surface is coated with gold (trade name: Bright, manufactured by Nippon Chemical Industry). (2) A glass bead is plated with silver (Toshiba Barotini Co.) is known.

[0003] Since a filling material having a gold coating is generally expensive, a filling material having a silver coating is desired. However, after a catalyst is attached to an insulator (core material), electroless silver plating is directly performed. The silver coating tends to be non-uniform, including the case where silver plating is applied to the glass beads described above,
There are problems such as weak adhesion of the coating. As a solution to this problem, there is a method of forming a base of another metal coating on the surface of the core material and then performing electroless silver plating thereon. In this case, nickel, copper, and the like can be considered as other metals used for the underlayer, but copper and silver have a property of easily diffusing each other. Particularly, when electroless silver plating is performed on the powder, silver-coated silver is used. Since the thickness is thin, there is a problem that copper is diffused to the surface due to long-term use or heating in a processing step, and as a result, the copper on the surface is oxidized, which tends to cause a decrease in conductivity.

[0004] From such a point, it is preferable to use nickel (nickel alloy), which is less likely to diffuse with silver than copper, as a base metal, but silver plating applied on nickel is generally used. There is a problem that adhesion is low. For this reason, despite the above-mentioned problems,
At present, copper is mainly used as the base metal.

On the other hand, as a method for solving the problem that the adhesion of the silver plating film on the nickel film is low,
Japanese Patent Publication No. 2-47549 discloses a method for obtaining silver-coated conductive powder using silver cyanide in the silver plating step. However, since cyan compounds are generally highly toxic, there are concerns about environmental issues related to wastewater, occupational health issues for workers, and the like. Therefore, there is a demand for a method capable of producing a conductive powder having a silver coating by performing electroless silver plating with high uniformity on a nickel alloy coating without using a cyanide compound.

Among the electroless silver plating methods, those containing no cyanide include (1) a method using a so-called substituted silver plating bath containing silver nitrate and at least one of ammonia and thiosulfate ion as a complexing agent. (2) A method using an autocatalytic electroless silver plating bath in which a tartrate salt, a saccharide, or the like is added as a reducing agent to silver nitrate or ammonia is known.

[0007]

However, when the above-described electroless silver plating method without using a cyanide is used for a powdery core material having a nickel coating, bath decomposition occurs to cause a problem other than the surface of the core material. There has been a problem that silver is deposited or silver deposition is extremely unlikely to occur.
Also, even if silver is uniformly deposited on the surface of the core material,
There was a problem that the adhesion between the silver plating film and the nickel film was significantly lower than that using silver cyanide.

The present invention has been made in view of such circumstances, and a silver alloy having a uniform and high adhesion on a nickel-phosphorus plating layer formed on a powdery core material surface without using a cyanide compound. An object of the present invention is to provide a method for producing a silver-coated conductive powder capable of forming a plating layer, a silver-coated conductive powder obtained by the method, and an electroless silver plating bath for coating the conductive powder.

[0009]

Means for Solving the Problems and Embodiments of the Invention The present inventors have made intensive studies to achieve the above object, and as a result, after treating the surface of the core material with an electroless nickel-phosphorus plating solution, By treating with a water-soluble silver salt and, as a complexing agent, an electroless silver plating solution containing sulfurous acid or a salt thereof, or an organic compound having an amide group or an imide group, nickel-phosphorus can be obtained without using a cyanide compound. We have found that a uniform and highly adherent electroless silver plating layer can be formed on the alloy plating layer, and that it is possible to obtain a silver-coated conductive powder having a silver plating layer on the surface and excellent conductivity. Thus, the present invention has been completed.

That is, the present invention provides (1) an electroless silver plating solution containing a water-soluble silver salt and a complexing agent after treating the surface of a core material made of an organic material or an inorganic material with an electroless nickel-phosphorus plating solution. Wherein the complexing agent for the electroless silver plating solution is a sulfurous acid or a sulfite, or an organic compound having an imide group or an amide group. (2) a method for producing the silver-coated conductive powder, wherein the electroless silver plating solution contains phosphinic acid, a phosphinate, or hydrazine;
(3) Volume resistivity obtained by the above method, having a nickel-phosphorus plating layer formed on the surface of a core material made of an organic material or an inorganic material, and a silver plating layer formed on the nickel-phosphorus plating layer Is not more than 20 mΩcm, (4) the silver-coated conductive powder is characterized in that the thickness of the silver plating layer is 100 nm or less, (5) a water-soluble silver salt, An electroless silver plating bath for coating a conductive powder comprising a complexing agent, wherein the complexing agent is an organic compound having a sulfite or a sulfite, or an imide group or an amide group. An electroless silver plating bath for powder coating is provided.

Hereinafter, the present invention will be described in more detail. The method for producing a silver-coated conductive powder according to the present invention comprises the steps of: treating a surface of a core material made of an organic material or an inorganic material with an electroless nickel-phosphorus plating solution, and then electroless silver containing a water-soluble silver salt and a complexing agent. A method for producing a silver-coated conductive powder to be treated with a plating solution, wherein the complexing agent is sulfurous acid or a sulfite, or an organic compound having an imide group or an amide group.

Here, the core material is not particularly limited.
The material can be arbitrarily selected from organic materials or inorganic materials such as metals, resins, and inorganic substances. At this time, the most suitable one will be selected in consideration of the use of the filling material, which is the product, the form of use, and the like. For example, when it is important that the specific gravity is small, it is preferable to use a resin or a light element inorganic compound such as silicon oxide or aluminum oxide.
When importance is placed on conductivity, it is preferable to use metal powder. Furthermore, in a filling material used by kneading with a rubber composition or a resin composition, in applications where it is necessary to have a certain degree of elasticity, in the case of a use form, it is preferable to use a resin, while having a rigidity. It is preferable to use an inorganic compound when it is a use or the like in which the specific gravity is required to be small.

The particle size of the above-mentioned core material may be appropriately set according to the intended use, use form and the like. For example, when kneaded with a rubber composition or the like, particles exceeding 150 μm will not fall off from the rubber or resin. The average particle diameter is preferably 150 μm or less, more preferably 5 to 100 μm, because it easily occurs. In addition, the shape of the core material may be appropriately set according to the application, the usage form, and the like. For example, when used as a filling material for a conductive paint, a flat shape is preferable and used as a filling material such as rubber. In this case, a sphere or a shape close to a sphere is preferable in consideration of uniform dispersion during kneading.

A nickel-phosphorus alloy plating layer is formed on the surface of the core material by an electroless plating method. At this time, the electroless plating reaction is started only by touching an electroless nickel plating solution described later at a predetermined temperature. If it is difficult to carry out the plating, it is preferable to perform a pretreatment such as carrying a trace amount of a metal having catalytic activity such as palladium so that the plating reaction can easily occur. In carrying such a catalyst metal, a known method can be adopted, and a known activation treatment used for the object to be plated may be appropriately selected and performed according to the type of the core material.

[0015] Specifically, when an insulator is used as the core material, (1) a method in which the core material is immersed in a conventionally known tin (II) chloride solution and then immersed in a palladium (II) chloride solution; (2) Tin (II) chloride and palladium chloride (I
A method using the mixed solution of I) and the like can be adopted. Further, as a method for making the above-mentioned catalyst easily adhere to the core material, (1) a method in which the core material is etched for a short time with an appropriate agent, for example, a strong alkali, a mineral acid, or chromic acid; A method of treating with a chemical having both a functional group having an affinity for a catalytic metal and a functional group having an affinity for a core material, for example, a silane coupling agent having an amino group, (3) ) A method of performing a mechanical treatment such as a plasma treatment may be appropriately adopted.

As the electroless nickel plating solution used for forming the nickel-phosphorus alloy plating layer, a known composition containing phosphinic acid or a salt thereof as a reducing agent can be used, and commercially available products are also available. Can be used. Known plating conditions can also be adopted. The phosphorus content in the nickel-phosphorus alloy plating layer obtained from the electroless nickel solution can be arbitrarily set in the range of 0 to 20% by weight, but is preferably 2 to 15% by weight, and is preferably 6 to 15% by weight. More preferably, it is で 14% by weight. The thickness of the nickel-phosphorus alloy plating layer is preferably 50 to 500 nm, more preferably 7 to 500 nm.
5 to 400 nm. When the thickness is less than 50 nm, there is a possibility that a plating layer with sufficient strength may not be obtained. On the other hand, when the thickness is more than 500 nm, there are few advantages and the cost of raw materials increases.

As the electroless silver plating solution of the present invention (electroless plating bath for coating conductive powder), a water-soluble silver salt as a silver ion source and a silver ion are used in the same manner as a general electroless silver plating solution. One containing a complexing agent for stably dissolving is used. Here, the water-soluble silver salt is not particularly limited as long as it shows water solubility, and known silver nitrate, silver sulfate and the like can be used.However, in consideration of solubility in water, silver nitrate is used. Is preferred. The concentration of silver ions is
It is about 0.005 to 0.2 mol / dm ³ , and particularly preferably 0.01 to 0.1 mol / dm ³ . When the silver ion concentration is higher than 0.2 mol / dm ^3, there is a possibility that deposition of metallic silver is likely to occur on the surface of the powder to be plated. On the other hand, when the silver ion concentration is lower than 0.005 mol / dm ³ , the amount of the solution becomes too large, which may cause a decrease in productivity and a decrease in the reaction rate.

As the complexing agent, it is necessary to use sulfurous acid or a sulfite, or an organic compound having an imide group or an amide group. Here, the sulfite serves as a hydrogen sulfite ion source. Alkali sulfites (normal and acidic salts) can be used as the sulfite, and sodium sulfite is particularly preferably used in consideration of ease of handling and cost.

On the other hand, the organic compound having an imide group or an amide group is not particularly limited as long as it functions as a complexing agent. For example, formamide, acetamide, oxamic acid, succinimide and the like can be used. Of these, acetamide and succinimide are preferred, and succinimide is particularly optimal. The amount of the complexing agent is preferably 0.8 to 10 moles, more preferably 2 to 6 moles, per silver ion. When the amount of the complexing agent is less than 0.8 mole, the effect of stably dissolving silver ions may be insufficient. On the other hand, when the amount is more than 10 moles, a further effect can be obtained. Not only is there a possibility that it will not occur, but it will also lead to higher costs and become uneconomical.

In addition, other complexing agents such as oxycarboxylic acids such as citric acid, malic acid and glycolic acid, water-soluble salts thereof, imidazole and derivatives thereof may be used in combination with the above-mentioned complexing agents. The compounding amount thereof is preferably not more than 10 times mol with respect to silver ions, particularly 0.1 to 6 mol.
Double molar is more preferred. These are effective in moderately and moderately promoting the elution of nickel in the substitutional electroless silver plating, and are effective in stabilizing the eluted nickel to prevent eutectoid.

Further, the electroless silver plating solution preferably contains a phosphinic acid, a phosphinate, or hydrazine as a reducing agent in addition to the water-soluble silver salt and the complexing agent. Here, the phosphinic acid or phosphinate is not particularly limited, but sodium phosphinate is most suitable.
The molar amount is preferably not more than 1 mole, and particularly preferably 0.1 to 3 moles. On the other hand, the amount of hydrazine to be added is preferably 20 times or less, more preferably 0.1 to 15 times, the mole of silver ions. If the amount of these reducing agents exceeds the above upper limit, the plating solution may become unstable. These reducing agents act to reduce silver ions in the solution and facilitate the precipitation of metallic silver under the catalytic action of the underlying nickel-phosphorus alloy, and to uniformly deposit on the nickel-phosphorus alloy plating layer. It is suitable for covering with a silver plating layer. That is, since the nickel-phosphorus alloy has a stronger catalytic effect on the reducing agent than the precipitated metallic silver, a uniform silver plating layer is formed on the nickel-phosphorus alloy plating layer.

The electroless silver plating solution may further contain a known component such as a buffer, if necessary. However, the electroless silver plating of the present invention does not contain a cyanide compound. Further, the pH of the electroless silver plating solution of the present invention
Is preferably 5.0 to 10.0, particularly preferably 6.0 to 9.0. If the pH is too low, the reaction may not easily occur, and if the pH is too high, the plating solution may be unstable.

The temperature at which the electroless silver plating is performed is 0 to 8
0 ° C is preferable, and more preferably 15 to 70 ° C.
If the silver plating temperature is lower than 0 ° C., the deposition rate may be too slow and the productivity may be reduced.
If the temperature is higher than 0 ° C., the reaction may be so vigorous that a uniform silver plating layer may not be obtained. The reaction time at this time is
The time may be set in advance by conducting an experiment or the like to obtain a desired amount of deposition, but is usually about 5 to 60 minutes, preferably 7 to 40 minutes.

The method of plating the powder to be plated (core material plated with nickel-phosphorus alloy) using the above electroless silver plating solution is not particularly limited. For example,
(1) A method of mixing a metal ion, a reducing agent, a complexing agent, a buffering agent, and the like in advance, and directly charging the powder to be plated into an electroless silver plating solution whose pH and temperature have been adjusted; A method in which a slurry obtained by dispersing the powder to be plated in water is added to the adjusted electroless silver plating solution. (3) After dispersing the powder to be plated in the adjusted solution by removing some of the plating components The method of adding the remaining plating components can be appropriately selected.

The silver-coated conductive powder obtained by the above method has excellent volume conductivity of not more than 20 mΩcm, and is excellent in conductivity of various rubbers such as silicone rubber composition and epoxy resin composition, and resin composition. It can be suitably used as a filler or as a dispersoid in a conductive paint.

When the volume resistivity exceeds 20 mΩcm, the conductivity is poor, so that it is not suitable as a conductive powder. The lower limit of the volume resistivity is not particularly limited, and is preferably lower from the viewpoint of conductivity.
mΩcm or more.

The thickness of the silver plating layer of the silver-coated conductive powder is a thickness calculated from the particle size and shape of the powder, the silver content by chemical analysis, the components of each plating layer and the specific gravity of the core material. It is preferably 100 nm or less, more preferably 5 to 100 nm, particularly preferably 10 to 100 nm. If the thickness of the silver plating layer exceeds 100 nm, the specific gravity increases and the price must be high, which is disadvantageous in terms of cost. If it is less than 5 nm, a dense and continuous silver plating layer may not be obtained,
As a result, sufficient oxidation resistance and conductivity may not be exhibited.

[0028]

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to Examples and Comparative Examples, but the present invention is not limited to the following Examples.

[Example 1] [1] Attachment of catalyst 30 g of spherical silicon oxide powder (trade name: Silica Ace US-10, manufactured by Mitsubishi Rayon) having an average particle diameter of about 10 µm was weighed,
Aminoalkylsilane coupling agent (brand name KBE9)
03, manufactured by Shin-Etsu Chemical Co., Ltd.)
0 cm ³ and stirred at room temperature for 10 minutes. Here,
Concentrated hydrochloric acid 30 cm ³ , and palladium chloride (PdCl ₂ )
0.09 mol / dm ³ , tin chloride (SnCl ₂ ) 2.6
mol / dm ^3, an aqueous solution 0.15 cm ³ containing hydrogen chloride 3.5 mol / dm ³ was added, stirring was continued for an additional 10 minutes. The powder component was filtered from this mixture with a Buchner funnel,
Dilute hydrochloric acid 150c with a concentration of 1 mol / dm ³ was added to the filtered powder.
After washing by spraying with m ³ , washing was further performed with 100 cm ³ of water to obtain a silicon oxide powder to which a catalyst had adhered.

[2] Nickel plating The catalyst-adhered powder obtained as described above was
m ³ , and dispersed by stirring. Separately, in a 5 liter beaker, 333 cm ^{3 of} electroless nickel plating solution concentrate (trade name: Top Nicolon F-153A, manufactured by Okuno Pharmaceutical), and the concentrate B (trade name: Top Nicolon F-153B, Okuno) 667 cm ³ and pure water 3.0 dm ³ were added and mixed, and the temperature was adjusted to 63 ° C. to form a plating bath. While the plating bath was being stirred, the slurry prepared above was added. Stirring was continued, and the reaction was continued for 20 minutes while maintaining the temperature. After the reaction, the nickel on a Buchner funnel - was filtered off powder phosphorous plating layer is formed, after washing sprinkled water about 1 dm ^3, raking the powder, and stirred dispersion was added again water 105 cm ^3, to form a slurry.

[3] Silver plating To a solution of 4.25 g of silver nitrate dissolved in 1.18 dm ³ of pure water at room temperature, 16.8 g of sodium sulfite was added and dissolved. After confirming that the initially generated precipitate was completely dissolved, 35.3 g of trisodium citrate dihydrate and 2.80 g of citric acid monohydrate were dissolved. The solution was heated with stirring to maintain the temperature at 66 ° C. To this, the slurry obtained in the above [2] was poured, and stirring was continued for 13 minutes. Thereafter, the powder was separated by filtration with a Buchner funnel, and the separated powder was sprinkled with about 1 dm ³ of pure water for washing. After washing, scrape the powder, put it in a petri dish, and keep the inside temperature at 40 ° C.
Dried in a vacuum dryer for 2 hours to obtain a silver-coated conductive powder.
4 g were obtained.

[Example 2] [1] Catalyst deposition and [2] nickel plating were performed in the same manner as in Example 1. [3] Silver plating To a solution of 4.72 g of silver nitrate in 1.16 dm ³ of pure water at room temperature, 19.1 g of sodium sulfite was added and dissolved. It was confirmed that the initially formed precipitate was completely dissolved.
The solution was heated with stirring and maintained at 46 ° C. To this, the slurry obtained in Example 1 [2] was poured, and immediately after the mixture was homogenized, 133 cm ³ of a 3.6 mol / dm ³ hydrazine aqueous solution was added.
Stirring continued for minutes. Thereafter, filtration, washing with water, and vacuum drying were performed in the same manner as in Example 1 to obtain 44.6 g of a silver-coated conductive powder.

Example 3 [1] Catalyst deposition and [2] Nickel plating were performed in the same manner as in Example 1. [3] Silver plating To a solution of 4.72 g of silver nitrate in 2.14 dm ³ of pure water at room temperature, 13.9 g of succinimide and 9.53 g of imidazole were added and dissolved. It was confirmed that the initially formed precipitate was completely dissolved. The solution was heated with stirring and maintained at 46 ° C. To this, the slurry obtained in Example 1 [2] was poured, and while keeping the temperature,
Stirring continued for minutes. Thereafter, filtration, washing with water and vacuum drying were performed in the same manner as in Example 1 to obtain 38.4 g of silver-coated conductive powder.

Example 4 [1] The catalyst was deposited in the same manner as in Example 1. [2] Nickel Plating The catalyst-adhered silicon oxide powder obtained as described above was added to 135 cm ³ of water and dispersed by stirring to obtain a slurry. Separately, 850 cm ^{3 of} electroless nickel plating solution concentrate (trade name: Schumer S-680, manufactured by Nippon Kanigen) and 3.96 dm ³ of pure water are added to a 5 liter beaker and mixed, and the temperature is adjusted to 43 ° C. To make a plating bath. While the plating bath was being stirred, the slurry prepared above was added. The stirring was continued, and the reaction was performed for 40 minutes while maintaining the temperature. After the reaction, the nickel on a Buchner funnel - was filtered off powder phosphorous plating layer is formed, after washing sprinkled water about 1 dm ^3, raking the powder, and stirred dispersion was added again water 105 cm ^3, to form a slurry.

[3] Silver plating To a solution of 4.59 g of silver nitrate in 1.33 dm ³ of pure water at room temperature, 6.69 g of succinimide was added and dissolved. Separately, a solution prepared by dissolving 2.86 g of sodium phosphinate in 50 cm ³ of pure water was prepared. The silver nitrate solution is heated with stirring to 46 ° C.
Kept. To this, the slurry obtained in the above [2] was poured, and when the mixture was homogenized, a sodium phosphinate solution was immediately added little by little. 30 while maintaining the temperature
Stirring continued for minutes. Thereafter, filtration, washing with water, and vacuum drying were performed in the same manner as in Example 1 to obtain 38.6 g of a silver-coated conductive powder.

Example 5 [1] Catalyst deposition was performed in the same manner as in Example 1, and [2] Nickel plating was performed in the same manner as in Example 4. [3] Silver plating To 1.1 dm ³ of pure water, 4.46 g of succinimide and 6.13 g of imidazole were added, and the temperature was raised to 45 ° C. while stirring.
The mixture was heated until the temperature reached A (liquid A). Further, a liquid (solution B) in which 2.86 g of sodium phosphinate was dissolved in 50 cm ³ of pure water, and a liquid (solution C in which 4.59 g of silver nitrate and 4.46 g of succinimide were dissolved in 200 cm ³ of pure water) ) Was prepared. While stirring the solution A, the slurry obtained in Example 4 [2] was poured and stirred for 3 minutes to obtain a uniform solution. After the solution B was added while the solution was stirred, the solution C was added little by little continuously over about 1 minute. The temperature of the solution at this point was 40 ° C. Stirring was continued for 10 minutes while further heating. The temperature of the solution at this point was 44 ° C. Thereafter, filtration, washing with water, and vacuum drying were performed in the same manner as in Example 1 to obtain 38.1 g of a silver-coated conductive powder.

COMPARATIVE EXAMPLE 1 [1] Catalyst deposition and [2] Nickel plating were performed in the same manner as in Example 1. [3] Silver plated silver nitrate 4.73g to a solution at room temperature in pure water 1.11Dm ^3, we added 28% aqueous ammonia 18.7cm ³ little by little, that the precipitate produced initially completely dissolved confirmed. Thereafter, 2.62 g of Rochelle salt (potassium sodium tartrate tetrahydrate) was added to the solution and dissolved. Example 1 [2] while continuously stirring the above solution
The slurry obtained in was poured, and stirring was continued for 10 minutes. Thereafter, filtration, washing with water, and vacuum drying were performed in the same manner as in Example 1 to obtain 39.1 g of a silver-coated conductive powder.

[Comparative Example 2] [1] Catalyst deposition and [2] nickel plating were performed in the same manner as in Example 1. [3] Silver plated silver nitrate 4.73g to a solution at room temperature in pure water 1.07Dm ^3, we added 28% aqueous ammonia 18.7cm ³ little by little, that the precipitate produced initially completely dissolved confirmed. Then, the solution was added with sodium thiosulfate 3
A solution in which 0.5 g was dissolved in 1.10 dm ³ of pure water was slowly poured. It was confirmed that no precipitation occurred in the solution. The solution was warmed with stirring and kept at 44 ° C. While continuously stirring the solution, the slurry obtained in Example 1 [2] was poured, and stirring was continued for 20 minutes. Thereafter, filtration, washing with water, and vacuum drying were performed in the same manner as in Example 1 to obtain 37.7 g of a silver-coated conductive powder.

Comparative Example 3 [1] The catalyst was attached in the same manner as in Example 1. [2] Copper plating The catalyst-adhered powder obtained as described above was treated with water 135c.
m ³ , and dispersed by stirring. Separately, in a 5 liter beaker, 400 cm of electroless copper plating solution concentrate (trade name: Sulcup PSY-1A, manufactured by Uemura Kogyo)
³ , 33.6 g of sodium hydroxide, 40 cm ^{3 of} a 37% aqueous formaldehyde solution and 3.30 dm ³ of pure water were added and mixed, and the temperature was adjusted to 33 ° C. to form a plating bath. While the plating bath was being stirred, the slurry prepared above was added. The stirring was continued, and the reaction was performed for 30 minutes while maintaining the temperature. After the reaction, the powder on which the copper plating layer was formed was filtered off with a Buchner funnel, sprinkled and washed with about 1 dm ³ of water, and then the powder was scraped, added again with 105 cm ³ of water, stirred and dispersed to form a slurry. [3] Silver plating Except for using the copper plating powder-containing slurry obtained in [2] above, the same operation as in Comparative Example 2 was performed to obtain 38.5 g of silver-coated conductive powder.

[Composition Analysis] A part of the silver-coated conductive powder obtained in each of the above Examples and Comparative Examples was completely decomposed using hydrofluoric acid and nitric acid, and subjected to chemical analysis to determine the composition of the powder. Was examined. Table 1 shows the results.

[0041]

[Table 1]

[Characteristics and Properties of Powder] With respect to the silver-coated conductive powder obtained in each of the above Examples and Comparative Examples, the particle shape and surface state were observed with an electron microscope. Table 2 shows the results. Also, each powder was filled in a cell, and the resistivity (conductivity) was measured by measuring the potential difference under a constant current by a so-called four-terminal method (current source SMU-257 and nanovoltmeter 2).
000, both manufactured by Keithle Co.). At this time, the resistance to be measured is small, and the contact resistance, the thermal potential difference between the contacts, and the like are non-negligible error factors. Therefore, they are prevented and compensated. Table 2 also shows the volume resistivity (mΩcm) calculated from the measured values.

[0043]

[Table 2]

As shown in Tables 1 and 2, Examples 1 to 5 and Comparative Example 1 had the same core material and substantially the same composition such as the amount of silver. It turns out that there is a big difference in the state. That is, Examples 1 to
In No. 5, silver was not deposited on the surface other than the particle surface and no silver was not precipitated on the particle surface, whereas in Comparative Example 1, the powder was separated away from the core material particles as fine powder and adhered to the inner wall of the container. For example, it can be seen that silver precipitation occurs on the surface other than the particle surface, and the volume resistivity is significantly lower in each of the examples, reflecting the above.

In Comparative Example 2, the amount of silver deposited was small and the amount of silver deposited on the surface other than the surface was large, although the same amount of silver was used as in each Example and Comparative Example 1. However, simply increasing the reaction time cannot be expected to improve the precipitation state. Comparative Example 3 using copper as a base metal
It can be seen that immediately after the production, it shows conductivity equal to or higher than that of each example. In addition, it is considered that the silver non-precipitated portion in Comparative Example 3 was generated by the non-precipitation of the underlying copper layer or the separation after the deposition.

[Durability Test] In order to examine the difference in durability of the silver plating layer between the case where the base metal is a nickel alloy and the case where the base metal is copper, the following durability test was performed. (Treatment 1) The sample after production and collection was placed in a petri dish, heated in an oven under an argon flowing atmosphere at an internal temperature of 150 ° C. for 2 hours, and then cooled to room temperature in an argon atmosphere. (Treatment 2) After the treatment 1, the mixture was allowed to stand in the air at room temperature for 10 days. (Treatment 3) The sample after production and collection was placed in a Petri dish and heated in a dryer at 100 ° C. for 5 hours under an air atmosphere. The volume resistivity (mΩcm) of each sample after performing each of the above processes was measured in the same manner as described above. Table 3 shows the results.

[0047]

[Table 3]

As shown in Table 3, it can be seen that the durability of the conductivity in the atmosphere of Comparative Example 3 (Treatments 2 and 3) is considerably inferior to those of Examples 1 to 5. More specifically, in each of the examples, the color of the appearance as seen with the naked eye did not change much, whereas in Comparative Example 3, the color changed to yellow in the processing 1, the color changed to brown in the processing 2, and the processing 3 Discolored to black-brown. In other words, regardless of the presence or absence of oxygen in the atmosphere, the diffusion of copper and silver easily proceeds by heating, and as a result,
By gradually oxidizing copper that has diffused to the surface,
It is considered that the conductivity decreases. On the other hand, in the samples of the respective examples, since the base is made of a nickel-phosphorus alloy, diffusion hardly occurs, and as a result, the surface is kept almost in silver, so that the durability of the conductivity is increased. Conceivable.

[0049]

As described above, according to the present invention,
Electroless silver plating is performed on a nickel alloy using a water-soluble silver salt and a complexing agent comprising sulfurous acid, a sulfite, or an organic compound having an imide group or an amide group. Therefore, it is possible to form a uniform and highly adherent electroless silver plating layer on a nickel alloy without using a cyanide compound, and to obtain a silver-coated conductive powder having excellent durability. This is useful as a method for producing a conductive filler material used for the purpose of kneading rubber, resin, or the like in order to impart.

Claims (5)

[Claims] 1. A silver-coated conductive material in which a surface of a core material made of an organic material or an inorganic material is treated with an electroless nickel-phosphorous plating solution and then treated with an electroless silver plating solution containing a water-soluble silver salt and a complexing agent. A method for producing a powder, wherein the complexing agent of the electroless silver plating solution is sulfurous acid or a sulfite, or an organic compound having an imide group or an amide group. . 2. The method for producing a silver-coated conductive powder according to claim 1, wherein the electroless silver plating solution contains phosphinic acid, a phosphinate, or hydrazine. 3. A nickel-phosphorous plating layer formed on the surface of a core material made of an organic material or an inorganic material, and a silver plating layer formed on the nickel-phosphorous plating layer. 3. A silver-coated conductive powder having a volume resistivity of 20 mΩcm or less obtained by the method described in 2 above. 4. The silver-coated conductive powder according to claim 3, wherein the thickness of the silver plating layer is 100 nm or less. 5. An electroless silver plating bath for coating a conductive powder, comprising a water-soluble silver salt and a complexing agent, wherein the complexing agent is a sulfurous acid or a sulfite, or an organic compound having an imide group or an amide group. An electroless silver plating bath for coating a conductive powder, which is a compound.