JPS6148586B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing noble metal coated powders.
More particularly, the present invention relates to a method of producing a noble metal coated powder consisting of particles of a substrate material that are substantially completely coated with a coating layer of noble metal.

One of the industrial uses of precious metals such as silver, gold, platinum, and palladium is their application to conductive materials such as conductive pastes (conductive paints) that take advantage of their high conductivity. These conductive materials are compositions consisting of a noble metal powder and a solvent, and are prepared so that a surface layer made of a highly pure noble metal is formed by firing after molding.

On the other hand, since noble metals are expensive, attempts have been made to replace part of the noble metal portion of the conductive material with other materials, such as base metals such as nickel and copper. However, for example, the surface layer of a fired product obtained by molding and firing a conductive paint made of a mixed system of silver powder and copper powder is formed from a solid solution of silver and copper and copper oxide, so the conductivity of the surface layer is lower than that of the silver powder. Compared to a baked product of a conductive paint consisting of a single system, the conductive paint is significantly lowered, resulting in a loss of practicality.

On the other hand, attempts have also been made to use a noble metal coated powder in which a base metal powder is used as the base material powder and each particle of the base material powder is coated with a noble metal instead of the noble metal powder. In principle, noble metal-coated powder forms a layer consisting only of precious metals on the surface of the fired product when fired, and we believe that its conductivity should be at the same level as the fired product obtained using pure noble metal powder. It will be done. However, the fired product obtained by manufacturing the noble metal coated powder using the chemical plating method, which is a common method for producing metal coated powder, and firing the paste using this powder is not the same as the fired product of the paste of the noble metal single powder. It has only a much lower conductivity than that of other materials. When the surface layer of a fired product using the above noble metal-coated powder is observed using an electron microscope, it is found that the surface layer is not formed from a continuous phase of noble metal, and is partially composed of the base material (base metal).
phase is exposed. That is, since the surface layer of this fired product is not formed from a high-purity noble metal, its conductivity is considered to be significantly lower than that of the noble metal phase alone.

As a result of investigating the cause of the base material phase being exposed on the surface layer of the fired paste product despite the use of noble metal coated powder, the inventors discovered that the base material powder was coated with noble metal using the conventional chemical plating method. In this case, for example, a part of the powder of the base material such as copper or nickel was dissolved in the plating solution, and a contamination phenomenon was found when this mixed into the noble metal coating layer during the formation of the coating layer. Therefore, since the coating layer of the noble metal-coated powder obtained by the conventional chemical plating method is contaminated with the base substance, the surface layer of the fired paste using this powder also contains a non-negligible amount of the contaminant. The conclusion was reached that this could lead to a decrease in conductivity. Furthermore, it is extremely difficult to form a precious metal coating layer with sufficient homogeneity, strength, and thickness using the conventional chemical plating method, which also causes the base material phase of the surface layer of the paste fired product to be exposed. I discovered that.

The present invention provides a novel method for producing noble metal coated powder that does not have the various drawbacks associated with the method for producing noble metal coated powder using the conventional chemical plating method as described above.

The present invention is characterized in that a reducing agent is added under stirring to a dispersion liquid in which a base material powder is uniformly dispersed in a gel-like aqueous solution of pH 5-10 containing a noble metal compound, a nonmetallic ammonium salt, and aqueous ammonia. This is a method for producing noble metal coated powder.

The noble metal coated layer of the noble metal coated powder obtained by the production method of the present invention is substantially free from contamination with the base material and has excellent homogeneity, strength and layer thickness. In addition, with the conventional chemical plating method, it was difficult to obtain a uniform metal-coated powder for powder consisting of particles with an average particle size of 1 micron or less, but the method of the present invention makes it difficult to obtain a uniform metal-coated powder of such fine-grained powder. Coating is easily achieved.
Therefore, after the conductive paste using this precious metal-coated powder is molded and fired, it forms a fired product having a surface layer made of a high-purity noble metal, and this fired product is similar to the firing of a conductive paste using a pure noble metal. The fired product obtained by this method shows equivalent conductivity. Therefore, it is possible to reduce the amount of noble metal used in the conductive paste without impairing performance.

Next, the present invention will be explained in detail.

Examples of precious metals used in the present invention include silver, gold,
Examples include, but are not limited to, platinum and palladium, and other platinum group metals can also be used as the coating layer forming material of the present invention. Moreover, the noble metals are not limited to the use of only one type, but may be used in combination of two or more types. The noble metal compound is not particularly limited as long as its aqueous solution (aqueous solution, mineral acid solution, alkaline aqueous solution, etc.) can be prepared. For example, as a silver compound, silver nitrate, silver chloride, or a silver complex compound;
Examples of gold compounds include ferrous chloride, ferric chloride, ferrous cyanide, and ferric cyanide; or complex compounds of gold; platinum compounds include ammonium chloroplatinate, chloroplatinic acid, and platinum chloride. Examples of palladium compounds include palladium cyanide, palladium halide salts, and palladium complex compounds.
Alternatively, a solution in which a noble metal alone is dissolved in a mineral acid can also be used.

Examples of non-metallic ammonium salts include ammonium chloride, ammonium acetate, ammonium sulfate, ammonium nitrate, and ammonium phosphate, with ammonium chloride being particularly preferred.

A gel-like aqueous solution with a pH of 5-10 containing a noble metal compound, a nonmetallic ammonium salt, and aqueous ammonia is prepared, for example, as follows. A noble metal or noble metal compound can be dissolved in any aqueous solvent such as water, dilute acid, concentrated acid, mixed acid, or alkaline aqueous solution (a solvent that dissolves the target noble metal or noble metal compound).
This solution is mixed with an ammonium salt such as ammonium chloride (NH ₄ Cl), and the pH of the mixture is adjusted to 5-10 (preferably 6-8) by adding aqueous ammonia to this mixture. do.
By adding ammonium salt to the precious metal solution, or by further adding aqueous ammonia to the mixture,
The mixture is made into a gel by adjusting the pH to 5-10. The amount of ammonium salt such as ammonium chloride added is preferably at least an equivalent amount and no more than three times the equivalent amount of the noble metal in the noble metal compound. If an ammonium salt is added in an amount smaller than this range, it will be difficult to uniformly disperse the powder of the base material into a stable gel-like state, whereas if the amount of ammonium salt is greater than the above range, the gel will form. The viscosity becomes too high, the reaction tends to be non-uniform, and ammonium salts tend to be mixed into the coating layer of the noble metal coated powder after production.

The base material used in the present invention is not particularly limited as long as it does not show substantial solubility in the above-mentioned gel-like aqueous solution. Examples include copper, nickel, cobalt, iron, etc. Periodic transition metals, other base metals, different noble metals forming the coating layer, metallic base materials such as alloys of these metals, and oxides such as silicon oxide, zirconium oxide, titanium dioxide, alumina, barium titanate, etc. Ceramics such as non-oxide ceramics such as titanium carbide, titanium nitride, tungsten carbide, and silicon carbide can be mentioned.

Powders of the base material are usually used having an average particle size of about 30 microns or less, but preferred powders have an average particle size of 10 microns or less, and the particle size does not need to be particularly uniform. Furthermore, as mentioned above, uniform coating of powder with an average particle size of 1 micron or less is also possible.

There is no particular restriction on the ratio of the base substance powder and the noble metal compound contained in the dispersion liquid, but when the noble metal coated powder to be produced is assumed to be used as a conductive paste, the ratio between the base substance and the noble metal in the noble metal compound is The ratio is preferably in the range of 1:9 to 7:3 (weight ratio). A more desirable range is base material:noble metal=2:8-6:4 (weight ratio).

The dispersion of the present invention can be prepared by a method such as adding a base material powder to a gel-like aqueous solution of pH 5-10 containing a noble metal compound, a nonmetallic ammonium salt, and ammonia water prepared as described above while stirring. This method results in a homogeneous dispersion. Stirring can be performed by any method and at any time, and any type of stirring tool or device can be used.

The reducing agent is added to the dispersion while stirring. There are no particular restrictions on the stirring method, but since the dispersion of the gel-like aqueous solution and the base material powder is to be stirred, a method that allows strong stirring is desirable.

The reducing agent is not particularly limited as long as it can reduce the noble metal compound in the gel-like aqueous solution and convert it into a noble metal element. Examples of reducing agents include lithium aluminum hydride, sodium borohydride, etc., but are not limited to these reducing agents. The reducing agent is added in an amount sufficient to reduce the noble metal compound in the gel-like aqueous solution to the noble metal element.

By adding a reducing agent to the stirred dispersion of the gelled aqueous solution and the base substance powder, the gel state of the dispersion is dissolved and at the same time a noble metal coated powder is produced.

The produced noble metal coated powder is then taken out by decantation, removal, etc., washed, dried, and used for various purposes.

Next, examples of the present invention will be shown.

Example 1 Palladium solution (25g of palladium metal was added to aqua regia)
(dissolved in 350ml) ammonium chloride in 70ml
12 g (1.2 times the equivalent of palladium) was added to obtain an orange gel-like solution. By adding 45 ml of ammonia water (28% aqueous solution) to this solution, the solution became a pink gel-like solution, and the pH of the solution was approximately 7. 5 g of barium titanate (BaTiO ₃ ) powder (average particle size 0.013 microns) was added to this solution and thoroughly stirred to obtain a uniform dispersion. While thoroughly stirring this dispersion, 100 ml of an aqueous sodium borohydride solution (NaBH ₄ 0.75 g/100 ml H ₂ O) was added, and the gel state disappeared and a black powder was produced. The liquid phase was removed by decantation, washed 10 times each with water and hot water by the same decantation operation, and then dried at 65°C.
9.9 g of palladium coated barium titanate powder was obtained. Yield 99%. Barium titanate/palladium weight ratio in coating powder: 50/50.

A paste containing the palladium-coated barium titanate powder (Pd-BaTiO ₃ powder) obtained above was prepared according to the following formulation, fired as follows, and the state of the palladium coating was tested.

Pd-BaTiO ₃ powder 2.0 g Single silver powder 8.0 g Borosilicate lead glass frit 0.2 g Ethyl cellulose 1 g Ethyl cellosolub 2.5 g Terpinel 2.5 g The mixture consisting of the above composition is thoroughly kneaded using a three-roll kneading device to obtain a paste.

The resulting paste was printed on a ceramic substrate by screen printing and dried at 150°C for 30 minutes, then placed in a firing furnace, and the temperature of the firing furnace was increased to 850°C over 1 hour. After maintaining this temperature of 850° C. for an additional 10 minutes, the obtained printing plate was cooled.

The metal surface (electrode) obtained in this way has the same fired surface color as a normal silver-palladium electrode, and the fired surface was examined using a scanning electron microscope (JSM-25S manufactured by JEOL Ltd.). ) using a backscattered electron image (composition image)
When observed, no barium titanate phase was observed on the surface, and the entire surface was covered with silver and palladium.

When this fired surface was immersed in a solder bath at 250°C, solder adhered to the entire surface of the metal surface. When the cross section of this soldered sample was analyzed using an X-ray microanalyzer (EMX-SM, manufactured by Shimadzu Corporation), it was found that the adhesion between the electrode and the ceramic substrate was good.
Moreover, the "wetting" between the electrode surface and the solder was also good. These results show that there is no exposed barium titanate phase on the electrode surface in an amount that would pose a practical problem, that is, that palladium is substantially completely covered with barium titanate powder, and that the palladium coating layer was confirmed not to be contaminated with barium titanate.

Example 2 A palladium solution (25 g of palladium metal dissolved in 350 ml of aqua regia) was prepared in the same manner as in Example 1.
ml, ammonium chloride 12g and aqueous ammonia (28
% aqueous solution) to prepare a gel-like solution with a pH of about 7. 5 g of nickel powder (average particle size: 3 microns) was added to this solution and thoroughly stirred to obtain a uniform dispersion. While thoroughly stirring this dispersion, add a sodium borohydride aqueous solution (NaBH ₄ 0.75g/100ml).
When 100ml of H ₂ O) was added, the gel state disappeared.
A black powder was formed. The following 9.82g was obtained by washing and drying the black powder in the same manner as in Example 1.
A palladium-coated nickel powder was obtained. Yield 98.2
%. Nickel/palladium weight ratio in coated powder
50/50.

The same test as in Example 1 was conducted except that the palladium-coated nickel powder obtained above was used instead of the palladium-coated barium titanate powder. As a result, it was found that the palladium coating on the nickel powder was substantially It was confirmed to be complete and that the palladium coating was not contaminated with nickel.

Example 3 3 g of ammonium chloride (1.2 times the equivalent of silver) was added to a solution of 7.87 g of silver nitrate (AgNO ₃ ) dissolved in 150 ml of water, and then aqueous ammonia (28% aqueous solution) was added.
By adding 25 ml, a gel-like solution with a pH of about 7 was obtained. 5 g of nickel powder (average particle size: 3 microns) was added to this solution and thoroughly stirred to obtain a uniform solution. While thoroughly stirring this dispersion, a sodium borohydride aqueous solution (NaBH ₄ 0.2g/20mlH ₂ O) was added.
When 20ml was added, the gel state disappeared and a gray powder was produced. This powder was washed and dried in the same manner as in Example 1 to obtain 9.9 g of silver-coated nickel powder. Yield 99%. 50/50 weight ratio of nickel/silver in the coating powder.

The same test as in Example 1 was conducted except that the above-obtained silver-coated nickel powder (10 g) was used in place of the palladium-coated barium titanate powder and the simple silver powder. It was confirmed that the powder coating was substantially complete and that the silver coating layer was not contaminated with nickel.

Claims (1)

[Claims] 1. A reducing agent is added under stirring to a dispersion in which a base material powder is uniformly dispersed in a gel-like aqueous solution with a pH of 5-10 containing a noble metal compound, a nonmetallic ammonium salt, and aqueous ammonia. A method for producing a noble metal coated powder characterized by the following. 2. The manufacturing method according to claim 1, wherein the nonmetallic ammonium salt is ammonium chloride. 3. The manufacturing method according to claim 1, characterized in that the content of the nonmetallic ammonium salt is at least an equivalent amount and at most three times the equivalent amount of the noble metal in the noble metal compound. 4. The manufacturing method according to claim 1, wherein the base material powder is a powder of an elementary transition metal in the fourth period of the periodic table. 5. The manufacturing method according to claim 1, wherein the base material powder is an oxide-based or non-oxide-based ceramic powder. 6. The manufacturing method according to claim 1, wherein the average particle size of the base material powder is 1 micron or less.