TW201442804A - Silver-coated nickel particles and method for producing same - Google Patents

Abstract

Each of silver-coated nickel particles according to the present invention is obtained by coating the surface of a core particle, which contains nickel, with silver. The entire surface of each silver-coated nickel particle is provided with a plurality of projections, and thus the surface has a rough shape. The size of each projection is from 0.05 [mu]m to 1 [mu]m (inclusive) when viewed in plan. The silver coverage of the silver-coated nickel particles is 50% or more. It is preferable that the volume cumulative particle diameter (D50) at the cumulative volume of 50% by volume as determined by laser diffraction/scattering particle size distribution measurement is from 0.5 [mu]m to 100 [mu]m (inclusive).

Description

Silver-coated nickel particles and method of producing the same

The present invention relates to a silver-coated nickel particle and a method of producing the same.

In order to achieve electrical conduction between the conductors, a conductive paste containing a metal powder, a conductive adhesive, or the like is used. As the metal powder, a noble metal such as gold or silver or a base metal such as nickel or copper is used. Since the noble metal is not easily oxidized and has high conductivity, it is preferred as a conductive powder, but it is economically disadvantageous. Therefore, various attempts have been made to reduce the use of precious metals and improve the conductivity of the conductive powder by covering the surface of gold or silver thinly as a metal of inexpensive metal.

For example, a conductive powder coated with silver on the surface of nickel has been proposed (see Patent Documents 1 and 2). In Patent Document 1, silver is precipitated on the surface of the nickel powder while stirring a mixed slurry containing a solution of a slurry containing nickel powder and a binder and silver. The silver precipitation system utilizes a displacement reaction. In Patent Document 2, a solution A containing nickel powder and a reducing agent is reacted with a solution B containing a silver nitrate ammonia complex and a reaction inhibitor, and silver is coated on the nickel powder. The precipitation of silver utilizes a reduction reaction.

[Previous Technical Literature] [Patent Literature]

[Patent Document 1] Japanese Patent Laid-Open Publication No. 2009-84634

[Patent Document 2] Japanese Patent Laid-Open Publication No. 2011-144441

The precipitation of silver by the substitution reaction described in Patent Document 1 is not easily performed uniformly. As a result, there is a problem that it is difficult to improve the conductivity of the silver-coated nickel powder. Further, when the silver is reduced by the displacement reaction, a large number of fine pores are formed in the silver coating layer instead of the nickel eluted by the reduced silver, and the nickel is exposed to the outside through the pores. As a result, oxidation proceeds over time to lower the conductivity of the powder.

In Patent Document 2, silver is coated so that the surface of the precipitated silver is smoothed by the reduction reaction. However, since the surface of the silver is smooth, it is not easy to increase the contact between the silver-coated nickel particles. As a result, there is a problem that it is difficult to improve conductivity.

The present invention provides a silver-coated nickel particle obtained by coating silver on a surface of a core particle containing nickel.

The silver-coated nickel particles form a large number of convex portions throughout the entire surface region thereof, whereby the surface has an uneven shape.

The size of the convex portion in plan view is 0.05 μm or more and 1 μm or less.

The coverage of silver in the silver-coated nickel particles is 50% or more.

Moreover, the present invention provides a method for producing silver-coated nickel particles as a preferred method for producing the silver-coated nickel particles. The silver ions are exchanged with the core particles containing nickel in water to perform displacement plating, and silver is precipitated on the surface of the core particles to obtain precursor particles, and then the precursor particles are obtained. The precursor particles, silver ions, and silver ion reducing agents are brought into contact with water to further precipitate silver on the surface of the precursor particles.

1 is a scanning electron microscope image of silver-coated nickel particles obtained in Example 1.

2 is a scanning electron microscope image of silver-coated nickel particles obtained in Example 2.

3 is a scanning electron microscope of silver-coated copper nickel particles obtained in Example 3.

Figure 4 is a scanning electron micrograph of silver-coated nickel particles obtained in Comparative Example 1. image.

Fig. 5 is a scanning electron microscope image of silver-coated nickel particles obtained in Comparative Example 2.

Hereinafter, the present invention will be described based on preferred embodiments thereof. The silver-coated nickel particles of the present invention are formed by coating a surface of a core particle containing nickel with a layer containing silver (hereinafter also referred to as "silver-coated layer"). In the present specification, the silver-coated nickel particles have a case where each particle is represented and a powder composed of an aggregate of particles is used depending on the context. Silver is preferably the entire surface area of the coated nickel particles. In other words, the silver layer is coated with the surface of the nickel particles without any omission, and the nickel is preferably not exposed on the surface of the silver-coated nickel particles.

The core particles comprising nickel consist essentially of only nickel or a metal comprising nickel. In the case where the core particle contains a metal including nickel, the ratio of nickel in the core particle is preferably 3% by mass or more and less than 100% by mass, and the ratio of the metal element other than nickel is preferably more than 0% by mass. It is 97% by mass or less. When the ratio of nickel is less than 3% by mass, there is a case where a convex portion is formed on the surface of the nickel particles which are not easily silver-coated. Examples of the metal element other than nickel include elements such as copper, tin, zinc, iron, chromium, palladium, gold, and silver. These metal elements can be used alone or in combination of two or more. Further, the core particles are inevitably mixed with a small amount of non-metallic elements such as oxygen in the manufacturing process or storage. In the present specification, for the sake of convenience, particles in which silver-containing core particles and core particles containing nickel are coated with silver are collectively referred to as "silver-coated nickel particles".

There is no particular limitation on the shape of the core particles containing nickel. For example, a shape of a spherical shape, a polyhedron, a flat body (flake), a dendritic crystal, or the like can be used as the core particle. Further, since the silver of the surface of the coated core particle has a small coating thickness, the shape of the silver-coated nickel particle is substantially the same as the shape of the core particle.

One of the features of the silver-coated nickel particles of the present invention resides in coating a silver-clad layer comprising the surface of the core particles of nickel. In detail, the silver coating layer contains agglomerates of tiny silver particles. Oxidation of nickel is suppressed as much as possible by coating the surface of the core particles containing nickel with the silver-clad layer of such a structure. As a result, even after long-term storage, the silver-coated nickel particles of the present invention can suppress the decrease in electric resistance as much as possible. On the other hand, in the silver-coated nickel particles described in Patent Document 1 in which the silver-coated layer has a large number of pores, since the surface of the core particle containing nickel is easily contacted with the outside through the pores, nickel is present for long-term storage. The tendency to oxidize, thereby causing the resistance to be easily lowered. A method of forming a silver-clad layer containing an aggregate of minute silver particles will be described below.

One of the characteristics of the silver-coated nickel particles of the present invention is also the shape of the surface of the particles. In detail, the silver-coated nickel particles of the present invention have a large number of convex portions formed on the surface thereof. As a result, the surface of the silver-coated nickel particles of the present invention has a concavo-convex shape caused by the convex portion and the concave portion located between the convex portions. Because of such a concavo-convex shape, the silver-coated nickel particles of the present invention have a larger contact area with the particles than the silver-coated nickel particles having a smooth surface, for example, as described in Patent Document 2. Thus, the silver-coated nickel particles of the present invention have higher conductivity between particles. In particular, since nickel is a hard metal compared with other metals used for conductive particles, for example, copper, it is not easily deformed even when pressure is applied. Therefore, the surface has an uneven shape and is excellent in conductivity between particles.

The size of the convex portion is an element that affects the improvement of the electrical conductivity between the silver-coated nickel particles. From this point of view, the size of each convex portion in plan view is preferably 0.05 μm or more and 1 μm or less, more preferably 0.1 μm or more and 0.8 μm or less, and still more preferably 0.2 μm or more and 0.5 μm or less. By forming the convex portions of such a size, the contact area of the particles with each other can be easily increased.

The size of the convex portion in plan view was obtained by observing the surface of the silver-coated nickel particles with an electron microscope and performing image analysis on the observed image. For image analysis, for example, a scanning electron microscope can be used. Specifically, it is used to measure the area of the silver particles present on the surface of the core particles containing nickel in a plan view, and to calculate the diameter of a circle having the same area as the area. The value of this diameter is set to the size of the convex portion.

The size of the convex portion in plan view is as described above, and the shape of the convex portion in plan view may be, for example, a shape having a small anisotropy such as a substantially circular shape or a polygonal shape. By forming the convex portions having the above shapes, the contact area of the particles with each other can be easily increased. The term "anisotropy is small" means a shape having a long diameter/minor diameter of 5 or less.

The size of the protrusions also depends on the size of the silver-coated nickel particles. From this viewpoint, the particle diameter of the silver-coated nickel particles is preferably 0.05 μm or more and 100 μm or less, more preferably 0.5 μm or more and 50 μm or less, and still more preferably 1 μm or more and 20 μm or less. By setting the particle diameter of the silver-coated nickel particles to the range, the contact area of the particles with each other can be easily increased. Further, in the silver-coated nickel particles, since the silver covering the surface of the core particles containing nickel has a small coating thickness, the particle diameter of the core particles is substantially the same as the shape of the silver-coated nickel particles.

The particle size of the silver-coated nickel particles can be determined by laser diffraction scattering particle size distribution measurement. The volume cumulative particle diameter D _{50 of 50} % by volume of the cumulative volume measured by this method was set as the above particle diameter.

When focusing on one particle, the number of convex portions present in the particle is preferably 2 or more and 500 or less per 1 μm ² , more preferably 5 or more and 500 or less, and furthermore It is preferably 10 or more and 500 or less. By setting the number of the convex portions in this way, the contact area of the particles with each other can be easily increased.

The coverage of the silver-coated layer on the surface of the coated core particle is 50% or more, preferably 60% or more, and more preferably 70% or more. Preferably, the silver-coated layer completely covers the entire surface area of the core particles (i.e., the coverage is 100%). For example, a silver-coated nickel particle is subjected to elemental mapping of silver and an element constituting the core particle using a scanning electron microscope, and the area occupied by the silver and the area constituting the element constituting the core particle are determined. . Based on the area, the coverage ratio is calculated based on the area occupied by {silver/(the area occupied by silver + the area occupied by the elements constituting the core particles)}×100. Or, using a scanning electron microscope, the silver-coated nickel particles are determined based on the difference in contrast of the reflected electron image. The area and area of the elements that make up the core particles. Based on the areas, the coverage ratio is calculated from the area occupied by {silver/(area occupied by silver + area occupied by elements constituting the core particles)}×100. Among the contrasts of the reflected electron image, elements with a large atomic weight are brightly displayed, and smaller elements are darker. For example, in the case where nickel is used for the core particles, the silver is brightly present and the nickel is darker.

Regarding the convex portion, at least the surface thereof contains silver. Preferably, the convex portion is substantially entirely composed of silver. Since the entirety of the convex portion is substantially made of silver, the conductivity between the particles can be improved. For example, elemental analysis of the cross section of the silver-coated nickel particles can be performed to confirm that the entire convex portion is substantially composed of silver. When the ratio of the silver in the convex portion is 80% by mass or more by the elemental analysis, the convex portion is substantially formed of silver.

In the silver-coated nickel particles of the present invention, the ratio of silver is preferably 0.5% by mass or more and 50% by mass or less, and more preferably 5% by mass or more and 20% by mass or less. By containing silver in a ratio of this range, the silver-coated nickel particles of the present invention can achieve a balance between economy and conductivity. The ratio of silver in the silver-coated nickel particles can be measured by, for example, ICP (Inductively Coupled Plasma) emission spectrometry using an acid or the like to dissolve silver-coated nickel particles.

The silver-coated nickel particles of the present invention have a high electrical conductivity between particles by having the convex portions as described above. The conductivity differs depending on the particle size of the silver-coated nickel particles, the content of silver, the coverage, and the like. For example, when the particle size of the silver-coated nickel particles is 6 to 8 μm, the content of silver is 10 to 11% by mass, and the coverage is 50% or more, it is preferable that the powder resistance at a pressure of 102 kgf/cm ² is good. It is 5.0×10 ^-5 Ω. Above cm and 5.0 × 10 ^-3 Ω. Below cm, and further preferably 5.0 × 10 ^-5 Ω. Above cm and 1.0 × 10 ^-3 Ω. High conductivity below cm. On the other hand, in the case where only the core particles and the silver particles are mixed, since the adhesion between the silver particles and the core particles is inferior to that of the silver particles, the contact resistance is increased. Therefore, if only the core particles and the silver particles are mixed, the electric resistance is improved.

The above-mentioned powder resistor can be, for example, a powder manufactured by Mitsubishi Chemical Analytech. The bulk resistance measuring system MCP-PD51 was measured in accordance with the four-terminal four-probe method.

The silver-coated nickel particles of the present invention having various characteristics described above are preferably produced by a silver coating method in which a displacement plating method and a reduction plating method are sequentially combined. In the first displacement plating method, silver ions are exchanged with gold-containing core particles in water to perform displacement plating, and silver is precipitated on the surface of the core particles to obtain precursor particles (step 1). In the second reduction plating method, the precursor particles, silver ions, and silver ion reducing agents are brought into contact with water to further precipitate silver on the surface of the precursor particles (step 2). Hereinafter, each step will be described.

The core particles containing nickel used in the step 1 can be produced by various methods. For example, in the case where the core particles contain nickel, the core particles can obtain the core particles by wet reduction of the nickel compound by using various reducing agents. Alternatively, the core particles may be obtained by atomization using a molten metal of nickel. The preferred particle size or shape of the core particles thus obtained is as previously described. The core particles obtained by the methods are brought into contact with silver ions in water.

Silver ions are produced from a silver compound that becomes a silver source. As the silver compound, for example, a water-soluble silver compound such as silver nitrate can be used. The concentration of silver ions in water is preferably set to be 0.01 mol/L or more and 5 mol/L or less, from the viewpoint of depositing a desired amount of silver on the surface of the core particles, and is preferably set to be 0.05 mol/L or more and 0.5 mol/L or less.

On the other hand, from the viewpoint of the fact that a desired amount of silver can be precipitated on the surface of the core particle, the amount of the core particle in water is preferably 10 g/L or more and 1000 g/L or less. It is preferably set to 50 g/L or more and 500 g/L or less.

The order in which the core particles and the silver ions are added is not particularly limited. For example, core particles can be added to water simultaneously with silver ions. From the viewpoint of controlling the easiness of depositing silver by displacement plating, it is preferred to prepare a slurry by dispersing the core particles in water in advance, and adding a silver compound which is a silver source to the slurry. In this case, the slurry can be 20 ° C ~ 25 ° C Normal temperature, or it can be outside the temperature range of 0 °C ~ 80 °C. Further, before the addition of the silver compound, silver or a mixture of ethylenediaminetetraacetic acid, triethylenediamine, iminodiacetic acid, citric acid or tartaric acid, or a salt thereof may be added to the slurry to control the silver. reduction.

The addition of the silver compound is preferably carried out in the form of an aqueous solution. The aqueous solution may be added to the slurry at one time or may be added continuously or discontinuously for a specific period of time. In terms of the ease of controlling the reaction of the displacement plating, the aqueous solution of the silver compound is preferably added to the slurry for a specific period of time.

The precursor particles are obtained by depositing silver on the surface of the core particles by displacement plating. In terms of forming a dense silver-clad layer having a target convex portion, the amount of silver precipitated in the precursor particles is preferably set to 0.1 to 50% by mass of the amount of silver in the finally obtained silver-coated nickel particles. It is preferably set to 1 to 20% by mass.

In the step 2, a reducing agent of silver ions and silver ions is added to the slurry containing the precursor particles obtained in the step 1. In this case, the precursor particles may be obtained by solid-liquid separation in step 1 and then dispersed in water to form a slurry, or the slurry obtained in step 1 may be directly supplied to step 2. In the latter case, the silver ions added in the step 1 may remain in the slurry or may not remain.

The silver ion added in the step 2 is produced from the water-soluble silver compound in the same manner as in the step 1. The silver compound is preferably added to the slurry in the form of an aqueous solution. The concentration of the silver ions in the aqueous silver solution is preferably 0.01 mol/L or more and 10 mol/L or less, and more preferably 0.1 mol/L or more and 1.0 mol/L or less. In terms of forming a dense silver-clad layer having a target convex portion, the above-mentioned slurry containing the precursor particles of 10 g/L or more and 1000 g/L or less, particularly 50 g/L or more and 500 g/L or less 100 parts by mass of the precursor particles are preferably added in an amount of 1 part by mass or more and 50 parts by mass or less, particularly 5 parts by mass or more and 30 parts by mass or less of a silver aqueous solution having a concentration in the range.

As the reducing agent added in the step 2, it is advantageous to use a reducing power having a degree of simultaneous silver plating and reduction plating. By using such a reducing agent, A silver-clad layer having a target convex portion and a dense silver layer is formed smoothly. When a reducing agent having a relatively high reductive property is used, there is a problem that silver is precipitated alone and it is difficult to coat the core particles. On the other hand, when a reducing agent having a weak reducing property is used, there is a problem that it is less likely to cause a reduction reaction of silver by a reducing agent, and a substitution reaction is preferentially generated, and it is difficult to uniformly coat the core particles. From the above viewpoints, as the reducing agent, it is preferred that the standard electrode potential of the aqueous solution of the reducing agent is -1.5 to 0.8 V (NHE, Normal Hydrogen Electrode, standard hydrogen electrode). Specifically, there are formic acid, oxalic acid, L-ascorbic acid, isoascorbic acid, formaldehyde, sodium thiosulfate, hydrazine, sodium borohydride and the like. These organic reducing agents may be used alone or in combination of two or more. Among them, L-ascorbic acid is preferably used.

In terms of forming a dense silver-clad layer having a target convex portion, the amount of the reducing agent added is preferably 0.5 to 5.0 equivalents, more preferably 1.0, with respect to the silver ions in the added silver solution. ~2.0 equivalents.

The order in which the reducing agent and the silver ions are added to the slurry containing the precursor particles is not particularly limited. From the viewpoint of controlling the reduction of silver ions to form a dense silver-clad layer having convex portions, it is preferred to add silver ions after adding a reducing agent to the slurry. The silver compound which becomes a silver source may be added to the slurry at a time, or may be continuously or discontinuously added for a specific period of time. In terms of easy control of the reduction of silver ions, the silver compound is preferably added to the slurry in a state of its aqueous solution for a specific period of time.

In the step 2, the slurry may be brought to a normal temperature of 20 ° C to 25 ° C, or may be heated in a temperature range of 0 to 80 ° C other than the slurry. From the viewpoint of sufficiently performing the reduction reaction of silver ions, it is preferred to continue stirring the slurry for a certain period of time after the addition of the reducing agent.

In step 2, silver-coated nickel particles having a target convex portion can be obtained by appropriately adjusting the reaction time or the concentration of silver ions.

The silver-coated nickel particles thus obtained may contain the state of the conductive composition thereof. Good to use. For example, silver-coated nickel particles may be mixed with a vehicle, a glass frit, or the like to form a conductive paste. Alternatively, the silver-coated copper powder may be mixed with an organic solvent or the like to form an ink. By applying the conductive paste or ink thus obtained to the surface of the applicable object, a conductive film having a desired pattern can be obtained.

[Examples]

Hereinafter, the present invention will be described in further detail by way of examples. However, the scope of the invention is not limited to the embodiment.

[Example 1] (1) Step 1

300 g of nickel powder was placed in 1.5 L of pure water heated to 40 ° C to prepare a slurry. This nickel powder was a volume cumulative particle diameter D ₅₀ = 7 μm using a cumulative volume of 50% by volume. 60 g of sulfuric acid was added thereto, followed by stirring and pickling. Then, the nickel powder is washed by a decantation method using pure water, and is kept in water without drying.

To the nickel powder, 1.5 L of pure water heated to 40 ° C was added, and 12.8 g of disodium edetate was added and dissolved while stirring. Then, 20 g of citric acid was added and dissolved. Further, 24 mL of a 0.4 mol/L silver nitrate aqueous solution was continuously added to the slurry for 1 minute to carry out displacement plating, and silver was precipitated on the surface of the nickel particles to obtain precursor particles.

(2) Step 2

L-ascorbic acid as a reducing agent is added to the slurry and dissolved. The amount added was set to be 1.15 equivalents with respect to the reduction of silver ions. Further, 696 mL of a 0.4 mol/L silver nitrate aqueous solution was continuously added over 29 minutes. Thereby, silver is further precipitated on the surface of the precursor particles to obtain the target silver-coated nickel particles. A scanning electron microscope image of the obtained silver-coated nickel particles is shown in Fig. 1. The ratio of silver in the obtained particles was 10.7% by mass. Further, elemental analysis of the cross section of the obtained particles revealed that the convex portion was formed only of silver.

[Embodiment 2]

Silver-coated nickel particles were obtained by the same procedure as in Example 1 except that the particle diameter of the nickel particles was changed to D ₅₀ = 21 μm. A scanning electron microscope image of the obtained silver-coated nickel particles is shown in Fig. 2 .

[Example 3]

As the core particle, a particle containing a copper-nickel alloy having a D ₅₀ of 27 μm and a nickel content of 10.8 mass% was used. Except for this, the same procedure as in Example 1 was carried out to obtain silver-coated copper-nickel particles. A scanning electron microscope image of the obtained silver-coated copper-nickel particles is shown in Fig. 3 .

[Comparative Example 1]

This comparative example is an example in which the substitution reaction was not carried out in the first step of the first embodiment, and the precursor particles were not produced. That is, in Example 1, a reducing agent was added before the addition of silver nitrate. Except for this, the same procedure as in Example 1 was carried out to obtain silver-coated nickel particles. A scanning electron microscope image of the obtained silver-coated nickel particles is shown in Fig. 4 .

[Comparative Example 2]

This comparative example corresponds to Example 1 of Patent Document 2 (Japanese Laid-Open Patent Publication No. 2011-144441).

0.28 L of pure water was added to a 0.5 L beaker, and nickel powder of D ₅₀ = 21 μm was charged and stirred. 3.6 mL of concentrated nitric acid was added thereto, followed by stirring and pickling. Then, the nickel powder is washed by a decantation method using pure water, and is kept in water without drying.

0.2 L of pure water, 36 mL of aqueous ammonia, and 2.1 g of hydrazine monohydrate were added to the nickel powder, and the mixture was stirred to prepare a solution (A).

In addition to solution (A), 9.45 g of silver nitrate, 0.1 g of a reaction inhibitor (Disperbyk-111 manufactured by BYK Chemie Japan Co., Ltd.), and 72 mL of ammonia water were added to 26 mL of pure water, followed by stirring to prepare a solution ( B).

The solution (B) was added dropwise while stirring the solution (A) to the solution (A) for 10 minutes. Thereafter, the mixture was continuously stirred by Jet Ajiter for 15 minutes, and then the supernatant was removed, and then the silver-coated nickel particles were washed by decantation to carry out filtration and dehydration. Then, drying was carried out at 60 ° C for 15 hours to obtain target silver-coated nickel particles. A scanning electron microscope image of the obtained silver-coated nickel particles is shown in Fig. 5 . The ratio of silver in the obtained particles was 10.1% by mass.

[Evaluation]

With respect to the silver-coated nickel particles obtained in the examples and the comparative examples, the size and shape of the convex portions in plan view were measured by the above method. Further, the particle diameter D _{50 of the} silver-coated nickel particles was measured. Further, the coverage of silver and the powder resistance were measured. These results are shown in Table 1 below.

From the results shown in Table 1, it was found that the silver-coated nickel particles (the present invention) of each of the examples had lower powder resistance than the silver-coated nickel particles of Comparative Example 1. In particular, in comparison with the first embodiment and the comparative example 1, it is understood that even in the comparative example 1 in which the coating ratio is low, the powder resistance is high even when the particle diameter is the same and the size of the convex portion is the same. Moreover, as compared with the comparison between the second embodiment and the comparative example 2, in the comparative example 2 in which the silver-clad layer was not formed in the uneven shape, the powder-pressing resistance was high.

[Industrial availability]

The silver-coated nickel particles of the present invention have higher electrical conductivity between particles.

Claims (6)

A silver-coated nickel particle obtained by coating silver on a surface of a core particle containing nickel, and the silver-coated nickel particle has a large number of convex portions formed over the entire surface region thereof, whereby the surface has a concave-convex shape and is planarly The size of the convex portion below is 0.05 μm or more and 1 μm or less, and the silver coverage in the silver-coated nickel particles is 50% or more. The silver-coated nickel particles of claim 1, wherein the volume cumulative particle diameter D ₅₀ of the cumulative volume of 50% by volume measured by a laser diffraction scattering particle size distribution measurement is 0.5 μm or more and 100 μm or less. The silver-coated nickel particle of claim 1, wherein the convex portion is substantially composed only of silver. The silver-coated nickel particle of claim 1, wherein the core particle comprises nickel or comprises a metal comprising nickel. A conductive composition comprising the silver-coated nickel particles of claim 1. A method for producing silver-coated nickel particles, which is the method for producing silver-coated nickel particles according to claim 1, wherein the silver ions are exchanged with the core particles containing nickel in water to perform displacement plating on the core particles. On the surface, silver is precipitated to obtain precursor particles, and then the precursor particles, silver ions, and silver ion reducing agents are contacted in water, and silver is further precipitated on the surface of the precursor particles.