JP7614381B2 - Method for producing phosphorus-containing silver-coated copper particles, and phosphorus-containing silver-coated copper particles - Google Patents

Description

The present invention relates to a method for producing phosphorus-containing silver-coated copper particles, and phosphorus-containing silver-coated copper particles.

Silver is used as a wiring material for electronic components. As electronic components become more functional every year, there is a growing demand for lower-cost silver wiring. Silver wiring is usually manufactured by sintering silver particles contained in a paste. The reason for using silver particles as a wiring material, which is consumed in large quantities, is that by sintering silver particles in air to form wiring, it is possible to obtain excellent properties such as high oxidation resistance and high conductivity. However, silver is an expensive material, and alternative materials have been sought. From this perspective, in recent years, development of silver-coated copper particles, which are an inexpensive material, has been progressing.

Patent Document 1 discloses the production of silver-coated copper particles using a silver plating solution containing silver, a phosphine compound, a triazole compound, and an acidic substance. This silver plating solution has a silver content of 3 to 10 g/L, a phosphine compound content of 12 to 40 g/L, a triazole compound content of 0.15 to 0.5 g/L, and a pH of 2 to 4.

Patent Document 2 discloses that copper is heated and melted, and the molten metal is dropped while being sprayed with high-pressure water to rapidly cool and solidify the resulting copper particle slurry, which is then held in the presence of a non-oxidizing gas such as nitrogen gas, and then subjected to solid-liquid separation to obtain copper particles, and that these copper particles are coated with silver by a reduction method or a substitution method in a non-oxidizing atmosphere.

Patent document 3 discloses that phytic acid having a phosphorus content of 0.01% by mass or less is coated on the surface of silver-coated copper particles.

JP 2017-128788 A JP 2017-190483 A JP 2015-92017 A

However, the techniques described in Patent Documents 1 and 2 have a problem in that the oxidation resistance of the obtained silver-coated copper particles is insufficient. Also, the technique described in Patent Document 3 has a problem in that the presence of a surface treatment agent such as phytic acid during the formation of the silver coating layer inhibits the silver coating reaction itself, making it difficult to form the silver coating layer.
Therefore, an object of the present invention is to provide silver-coated copper particles which can easily form a silver coating layer on the surface of copper particles and have excellent oxidation resistance.

As a result of intensive research aimed at solving the above-mentioned problems, the inventors discovered that by incorporating phosphorus into the silver coating layer of silver-coated copper particles, a silver coating layer having excellent oxidation resistance can be formed stably on the surface of copper base particles, and thus completed the present invention.

That is, the present invention provides a method for producing phosphorus-containing silver-coated copper particles having a silver coating layer containing phosphorus (P) on at least a portion of the surface of the copper base particles, in which a silver compound and phosphoric acid or a salt thereof are made to coexist in a dispersion liquid of the copper base particles, and a phosphorus-containing silver coating layer is precipitated and formed on the surface of the copper base particles.

The present invention also provides phosphorus-containing silver-coated copper particles having a silver coating layer containing phosphorus (P) element on at least a part of the surface of a copper base particle,
The increase in oxygen (mass%) after 7 days when stored at 85° C. and 85% RH is defined as I ₀ .
The content (mass%) of silver (Ag) element contained in the phosphorus-containing silver-coated copper particles is represented by W _Ag ,
When the content ratio (ppm by mass) of the phosphorus (P) element contained in the phosphorus-containing silver-coated copper particles is W _P ,
The present invention provides phosphorus-containing silver-coated copper particles having a value of I _O ² /( _WAg × _WP )×1000 of 1.9 or less.

The present invention will now be described based on its preferred embodiments.
In the manufacturing method of the present invention, the desired phosphorus-containing silver-coated copper particles can be manufactured by using a substitution method utilizing a substitution reaction between copper and silver caused by the difference in ionization tendency or a reduction method using a reducing agent as a method for coating the surface of the copper base particles with silver. In particular, the substitution method is preferable because it allows phosphorus (P) element to be successfully incorporated into the silver coating layer, and silver-coated copper particles with high oxidation resistance can be obtained.

In this manufacturing method, whether the substitution method or the reduction method is adopted, a dispersion of copper base particles (hereinafter sometimes abbreviated as "dispersion") is prepared.

The average particle size of the copper base particles is preferably 0.1 μm or more and 50 μm or less, and more preferably 1 μm or more and 10 μm or less. By having the particle size of the copper base particles in the above range, it is possible to obtain phosphorus-containing silver-coated copper particles that are excellent in handleability and oxidation resistance. The average particle size refers to the volume cumulative particle size D ₅₀ at a cumulative volume of 50% by volume measured by a laser diffraction scattering type particle size distribution measurement method.

The shape of the copper base particles does not necessarily have to be spherical, and any shape, such as flat, polygonal, or irregular, can be used.
The method for producing the copper base particles is not particularly limited, and they can be produced by any method such as an atomization method, a wet reduction method, or an electrolysis method.

Solvents for preparing the dispersion include, but are not limited to, water, chloroform, methanol, ethanol, propanol, isopropyl alcohol, butanol, ethylene glycol, propylene glycol, glycerin, and other alcohols, dimethyl sulfoxide (DMSO), etc.

Whether the substitution method or the reduction method is used, ethylenediaminetetraacetic acid (EDTA), hydrazine, sulfuric acid or hydrochloric acid can be added to remove the oxide film on the surface of the copper base particles.

The dispersion liquid is replaced with the same type of solvent as that contained in it to wash the copper base particles, the oxide film in the dispersion liquid is washed away, and the dispersion liquid with the copper base particles dispersed therein is prepared again.

In either the substitution method or the reduction method, a chelating agent may be added to the dispersion in order to form a more uniform silver coating layer. As the chelating agent, it is preferable to use a chelating agent with a high complex stability constant with respect to copper ions, etc., in order to prevent the reprecipitation of copper ions, etc., which are by-produced by the substitution reaction between silver ions and metallic copper. In particular, since the main component of the copper mother particles, which are the cores of the silver-coated copper particles, is copper, it is preferable to select a chelating agent with attention paid to the complex stability constant with copper. Specifically, a chelating agent selected from the group consisting of ethylenediaminetetraacetic acid (EDTA), iminodiacetic acid, diethylenetriamine, triethylenediamine, and salts thereof can be used as the chelating agent.

When the substitution method is adopted, a solution containing a silver compound and a solution containing phosphoric acid or a salt thereof are separately prepared, and each of them can be introduced simultaneously or separately into the dispersion liquid to allow them to coexist in the dispersion liquid. Alternatively, a plating solution containing a silver compound and phosphoric acid or a salt thereof can be prepared in advance, and this plating solution can be introduced into the dispersion liquid to allow the silver compound and phosphoric acid or a salt thereof to coexist in the dispersion liquid. This causes a substitution reaction between the copper of the copper mother particles in the dispersion liquid and the silver of the silver compound. At the same time, during the substitution reaction, the phosphorus element is incorporated into the silver coating layer that is in the process of being formed. As a result, phosphorus-containing silver-coated copper particles in which a phosphorus-containing silver coating layer is formed can be obtained. In this way, a stable silver coating layer can be obtained, and phosphorus-containing silver-coated copper particles with excellent oxidation resistance can be obtained.

The silver compound may be any water-soluble silver salt that is commonly used, including, but not limited to, silver nitrate, silver oxide, silver sulfate, silver acetate, and silver carbonate.
The phosphoric acid is preferably orthophosphoric acid. Examples of phosphates include, but are not limited to, salts of orthophosphoric acid such as disodium hydrogen phosphate, diammonium hydrogen phosphate, and dipotassium hydrogen phosphate.

When the substitution method is adopted, in the dispersion after the silver compound and phosphoric acid or its salt are allowed to coexist, the ratio of the phosphorus addition amount (mass%) to the silver addition amount (mass%) is preferably 0.1 to 50, more preferably 0.2 to 30. This can improve the oxidation resistance of the silver coating layer, and in turn can improve the oxidation resistance of the intended phosphorus-containing silver-coated copper particles. The "silver addition amount (mass%)" refers to the ratio of the mass of silver element to the mass of copper base particles in the dispersion (for example, if 15 g of silver element is added to 500 g of copper base particles, the silver addition amount is 3 mass%). The "phosphorus addition amount (mass%)" refers to the ratio of the mass of phosphorus element to the mass of silver element in the dispersion (for example, if 2.16 g of phosphorus element is added to 15 g of silver element, the phosphorus addition amount is 14.4 mass%). For example, if the silver addition is 3% by weight and the phosphorus addition is 14.4% by weight, the ratio of the phosphorus addition (% by weight) to the silver addition (% by weight) is 4.80.

The amount of the silver compound in the dispersion after the silver compound and phosphoric acid or a salt thereof are allowed to coexist is preferably 0.1 g/L or more and 50 g/L or less, more preferably 1 g/L or more and 30 g/L or less, and even more preferably 1 g/L or more and 10 g/L or less, calculated as silver.
The total amount of phosphoric acid or a salt thereof in the dispersion after the silver compound and phosphoric acid or a salt thereof are allowed to coexist is preferably 0.01 g/L or more and 50 g/L or less, more preferably 0.01 g/L or more and 20 g/L or less, and even more preferably 0.01 g/L or more and 10 g/L or less, calculated as phosphorus.

The pH of the dispersion after the silver compound and phosphoric acid or a salt thereof are allowed to coexist is preferably from 5 to 10, and more preferably from 6 to 9.5, so that a silver coating layer containing elemental phosphorus is satisfactorily formed on the surface of the copper base particles.
The pH of the dispersion after the silver compound and phosphoric acid or its salt are allowed to coexist is adjusted by appropriately adding an acidic or basic substance to the dispersion. For example, the pH is adjusted appropriately by adding a basic substance such as sodium hydroxide. The pH is the value at the temperature when the dispersion is prepared by allowing the silver compound and phosphoric acid or its salt to coexist.

If the silver coating layer is formed while the copper base particles are in contact with each other, a mass of multiple copper base particles bonded together via the silver coating layer is unintentionally formed. The phosphorus-containing silver-coated copper particles produced in this way are not suitable for use as wiring materials, etc. Therefore, it is preferable to prevent the particles from settling until the silver coating is completed by the substitution reaction. For this purpose, it is preferable to stir and flow the dispersion liquid after the silver compound and phosphoric acid or a salt thereof are allowed to coexist. A stirring blade is generally used for stirring, but other known stirring methods can also be applied.

The reaction time for the substitution reaction varies depending on the temperature of the dispersion liquid, but is, for example, from 5 minutes to 60 minutes.
After the phosphorus-containing silver-coated copper particles are produced in this manner, the solvent in the dispersion liquid containing the copper particles is replaced with a solvent such as water, chloroform, alcohol, or dimethyl sulfoxide (DMSO) for washing. Examples of the alcohol that can be used include methanol, ethanol, propanol, isopropyl alcohol, butanol, ethylene glycol, propylene glycol, and glycerin.

When the reduction method is adopted instead of the substitution method, a silver reducing agent is introduced into the dispersion of copper base particles, and then a plating solution containing a silver compound and phosphoric acid or a salt thereof is introduced. The plating solution may contain various plating solution components as necessary. As a result, the silver compound in the dispersion is reduced, and phosphorus element is incorporated into the reduced silver, forming a phosphorus-containing silver coating layer on at least a part of the surface of the copper base particles. As a result, phosphorus-containing silver-coated copper particles having a phosphorus-containing silver coating layer formed thereon can be obtained. In this way, a stable silver coating layer can be obtained, and phosphorus-containing silver-coated copper particles having excellent oxidation resistance can be obtained.

Examples of reducing agents for silver include lithium aluminum hydroxide, sodium amalgam, sodium borohydride, sulfates, sulfites, hydrazine, zinc amalgam, diisobutylaluminum hydride, sodium amalgam, sodium borohydride, and oxalic acid.

When the reduction method is employed, the silver compound and phosphoric acid or a salt thereof can be the same as those described for the substitution method. The solution containing a silver compound and the solution containing phosphoric acid or a salt thereof can also be the same as those described for the substitution method. Furthermore, the solvent constituting the solution containing a silver compound and the plating solution containing phosphoric acid or a salt thereof can also be the same as those described for the substitution method.

The pH of the plating solution is preferably 5 to 10, more preferably 6 to 9.5. This allows a silver coating layer containing elemental phosphorus to be well formed on the surface of the copper base particles. The pH of the plating solution is a value at the temperature when the dispersion and the plating solution are mixed.
The pH of the plating solution containing the reducing agent is adjusted by adding an acidic substance and a basic substance as appropriate, for example, a basic substance such as sodium hydroxide to adjust the pH.

The ratio of the amount of phosphorus added (mass%) to the amount of silver added (mass%) in the dispersion after the plating solution is introduced is preferably 1 to 150, more preferably 1 to 120, even more preferably 1 to 100, and even more preferably 1 to 50. This can improve the oxidation resistance of the silver coating layer, and in turn improve the oxidation resistance of the phosphorus-containing silver-coated copper particles. The "silver addition amount (mass%)" refers to the ratio of the mass of silver element to the mass of copper base particles in the dispersion (for example, if 15 g of silver element is added to 500 g of copper base particles, the silver addition amount is 3 mass%). The "phosphorus addition amount (mass%)" refers to the ratio of the mass of phosphorus element to the mass of silver element in the dispersion (for example, if 2.16 g of phosphorus element is added to 15 g of silver element, the phosphorus addition amount is 14.4 mass%). For example, if the silver addition is 3% by weight and the phosphorus addition is 14.4% by weight, the ratio of the phosphorus addition (% by weight) to the silver addition (% by weight) is 4.80.

Specifically, the amount of silver compounds in the dispersion after the plating solution is introduced is preferably 0.1 g/L or more and 50 g/L or less, more preferably 0.1 g/L or more and 20 g/L or less, and even more preferably 0.1 g/L or more and 10 g/L or less, calculated as silver.
The total amount of phosphoric acid or a salt thereof in the dispersion is preferably 0.1 g/L or more and 50 g/L or less, and more preferably 1 g/L or more and 30 g/L or less, calculated as phosphorus.

As with the substitution method, when the reduction method is used, if the silver coating layer is formed while the copper base particles are in contact with each other, a mass in which multiple copper base particles are bonded via the silver coating layer is unintentionally formed. The phosphorus-containing silver-coated copper particles produced in this way are not suitable for use as wiring materials, etc. Therefore, it is preferable to prevent the particles from settling until the silver coating is completed by the reduction reaction of silver. For this purpose, it is preferable to stir and flow the dispersion liquid after the silver compound and phosphoric acid or a salt thereof are coexisted. A stirring blade is generally used for stirring, but other known stirring methods can also be applied.

The reaction time for the reduction reaction depends on the temperature of the dispersion liquid containing the plating solution, but is, for example, 5 minutes or more and 60 minutes or less.

Whether the substitution method or the reduction method is used, after the phosphorus-containing silver-coated copper particles are obtained, the phosphorus-containing silver-coated copper particles are separated and removed from the dispersion by a solid-liquid separation method such as vacuum dehydration, filter press, centrifugation, or ultrafiltration. The phosphorus-containing silver-coated copper particles are then washed with a solvent. In this way, a phosphorus-containing silver coating layer is precipitated and formed on the surface of the copper mother particles.

In consideration of the convenience of handling the powder, the phosphorus-containing silver-coated copper particles may be subjected to a surface treatment as necessary after washing. There are no particular limitations on the surface treatment agent, and it can be appropriately selected depending on the purpose. Examples of the surface treatment agent include fatty acids, fatty acid salts, surfactants, organometallic compounds, chelating agents, and polymer dispersants.

In the phosphorus-containing silver-coated copper particles obtained as described above, due to the manufacturing method, phosphorus elements are contained in the silver coating layer arranged on at least a part of the copper mother particle. The phosphorus-containing silver-coated copper particles of the present invention have high oxidation resistance due to having such a structure. From the viewpoint of cost, it is required that the silver content of general silver-coated copper particles is reduced. However, if the silver content is reduced, the part of the copper particle that is not covered by the silver coating layer increases, thereby exposing copper, which is more easily oxidized than silver, and the oxidation resistance of the silver-coated copper particles tends to decrease. In contrast, according to the above-mentioned phosphorus-containing silver-coated copper particles, although the mechanism is not necessarily clear, it is considered that the coverage of the copper particles by the phosphorus-containing silver coating layer is improved compared to a coating layer consisting only of silver due to the action of the phosphorus element contained in the silver coating layer, and as a result, the oxidation resistance of the phosphorus-containing silver-coated copper particles is excellent.

In contrast, as shown in Comparative Example 2 described below, when a silver coating layer is placed on the surface of copper base particles and phosphorus is present on the silver coating layer (in other words, when phosphorus is not present in the silver coating layer), sufficient oxidation resistance cannot be achieved.

From the viewpoint of improving oxidation resistance, it is preferable that the silver coating layer covers the entire surface of the copper base particle, however, the silver coating layer may cover only a part of the surface of the copper base particle as long as the desired oxidation resistance is exhibited.
There is no particular limitation on the form in which the phosphorus element is present in the silver coating layer. Considering that the inventors analyzed the phosphorus-containing silver-coated copper particles by TOF-SIMS and observed fragments of _PO3 and _PO2 , it is believed that the phosphorus element is present in the form of a ^phosphorus oxide, for example, in the form of _PO43- ions.

The oxidation resistance of the phosphorus-containing silver-coated copper particles of the present invention can be evaluated using the oxidation resistance index OR as an index. The oxidation resistance index OR is defined by the following formula (1).
Oxidation resistance index OR = I _O ² / (W _Ag × W _P ) × 1000 (1)
In formula (1), I ₀ is the oxygen increase (mass%) after 7 days when the phosphorus-containing silver-coated copper particles are stored in an environment of 85° C. and 85% RH. I ₀ is defined as I ₀ =I ₂ -I _1. I ₂ is the oxygen content (mass%) after 7 days when the phosphorus-containing silver-coated copper particles are stored in an environment of 85° C. and 85% RH, and I ₁ is the oxygen content (mass%) before storage.
W _Ag is the content (mass%) of silver (Ag) element contained in the phosphorus-containing silver-coated copper particles.
W _P is the content (ppm by mass) of phosphorus (P) element contained in the phosphorus-containing silver-coated copper particles.

The technical meaning of formula (1) is as follows. The numerator I 2 _O in formula (1) reflects the degree of oxidation of the phosphorus-containing silver-coated copper particles. The reason for squaring I 2 _O is to treat the degree of oxidation as being numerically large even when it is small. Here, I 2 _O is an index of the degree of oxidation of the silver-coated copper particles, and can also be used as an index indirectly indicating the coverage rate of the silver-coated copper particles by the silver coating layer (the area ratio of the surface of the copper particles covered by the silver coating layer). It is difficult to quantitatively observe the coverage rate by the silver coating layer by observing the surface of the silver-coated copper particles, etc. Therefore, the present inventors focused on the characteristics of silver-coated copper particles (i.e., the more the copper particles are not covered by the silver coating layer, the more easily they are oxidized and the value of I 2 _O becomes larger), and decided to use I 2 _O as an index indirectly indicating the coverage rate by the silver coating layer.
The denominator of formula (1), W _Ag × W _P , reflects the amount of silver and phosphorus present in the phosphorus-containing silver-coated copper particles. The greater the amount of silver and phosphorus present in the phosphorus-containing silver-coated copper particles, the higher the coverage of the copper particles with the silver coating layer in the phosphorus-containing silver-coated copper particles, so the oxygen increase I _O tends to decrease. Therefore, the degree of increase in the oxygen increase I _O is standardized by dividing the square of I _O by W _Ag × W _P. Therefore, the smaller the oxidation resistance index OR, the less likely the phosphorus-containing silver-coated copper particles are to be oxidized.
The reason for multiplying by 1000 in formula (1) is that the value of I _O ² /(W _Ag ×W _P ) is very small and therefore difficult to handle, and so multiplying by 1000 is to make it a more easily handleable value.

The phosphorus-containing silver-coated copper particles of the present invention preferably have an oxidation resistance index OR of 1.9 or less, more preferably 1.0 or less, and even more preferably 0.5 or less. Phosphorus-containing silver-coated copper particles having such an oxidation resistance index OR are extremely difficult to oxidize.

The oxidation resistance index OR of the phosphorus-containing silver-coated copper particles of the present invention is as described above, and the oxygen increase I _O after 7 days when stored at 85° C. and 85% RH is preferably 1.0% by mass or less, more preferably 0.8% by mass or less, even more preferably 0.6% by mass or less, even more preferably 0.4% by mass or less, and particularly preferably 0.2% by mass or less. The method for measuring the oxygen increase I _O will be described in the examples below.

The content ratio W _Ag of the silver element contained in the phosphorus-containing silver-coated copper particles of the present invention is preferably 0.1 mass% or more from the viewpoint of enhancing the oxidation resistance of the phosphorus-containing silver-coated copper particles. From the viewpoint of making this advantage more prominent, the content ratio W _Ag of the silver element is more preferably 1 mass% or more, more preferably 2 mass% or more, and even more preferably 3 mass% or more.
The silver content W _Ag is preferably 30% by mass or less, more preferably 20% by mass or less, and even more preferably 10% by mass or less, taking into consideration the balance between improving the oxidation resistance of the phosphorus-containing silver-coated copper particles and the economic efficiency of using silver, which is an expensive element.

The phosphorus content W _P of the phosphorus-containing silver-coated copper particles of the present invention is preferably 50 mass ppm or more from the viewpoint of enhancing the oxidation resistance of the phosphorus-containing silver-coated copper particles. In order to make this advantage more prominent, the phosphorus content W _P is more preferably 70 mass ppm or more, and even more preferably 80 mass ppm or more.
From the viewpoint of a balance between improving the oxidation resistance of the phosphorus-containing silver-coated copper particles and ensuring electrical conductivity, the phosphorus element content _Wp is preferably 5000 ppm by mass or less, more preferably 3000 ppm by mass or less, even more preferably 500 ppm by mass or less, and even more preferably 300 ppm by mass or less.

The method for measuring the silver content W _Ag and the phosphorus content W _P will be described in the examples below.

Since the phosphorus-containing silver-coated copper particles of the present invention have high oxidation resistance as described above, even if the copper particles are placed in an oxidizing environment, the increase in electrical resistance is suppressed. In detail, the phosphorus-containing silver-coated copper particles of the present invention have a volume resistivity increase rate I _R of preferably 532% or less, more preferably 500% or less, even more preferably 300% or less, and even more preferably 100% or less after 7 days when stored in an environment of 85°C and 85% RH.
The increase rate of volume resistivity I _R is defined as (R 2 -R 1 )/R 1 × ₁₀₀ , where R ₁ ( _Ωcm ) is the initial volume resistivity and R ₂ (Ωcm) is the volume resistivity after 7 days of storage at 85 _° C and 85% RH.
The volume resistivity _R2 after storage at 85° C. and 85% RH for 7 days is preferably 9.9×10 ⁻³ Ωcm or less, and more preferably 9.9×10 ⁻⁴ Ωcm or less.
The method for measuring the volume resistivity will be explained in the examples below.

The oxidation resistance of copper particles also depends on their particle size, and the smaller the particle size, the larger the specific surface area, and therefore the more easily the particles are oxidized. From this viewpoint, the phosphorus-containing silver-coated copper particles of the present invention preferably have a volume cumulative particle size _D50 at 50% cumulative volume measured by a laser diffraction scattering particle size distribution measurement method of 0.1 μm or more and 50 μm or less, more preferably 0.5 μm or more and 15 μm or less, and even more preferably 1 μm or more and 10 μm or less.
The method for measuring the particle size _D50 will be described later.

In relation to the above-mentioned particle size _D50 , from the viewpoint of oxidation resistance, the phosphorus-containing silver-coated copper particles of the present invention preferably have a BET specific surface area of 0.1 ^m2 /g or more and 10 ^m2 /g or less, more preferably 0.1 ^m2 /g or more and 5 ^m2 /g or less, and even more preferably 0.1 ^m2 /g or more and 3 ^m2 /g or less.
The method for measuring the BET specific surface area will be described later.

[evaluation]
(1) Composition analysis of phosphorus-containing silver-coated copper particles The sample powder was dissolved by wet decomposition with nitric acid, and the concentrations of silver and phosphorus were measured using an ICP emission spectrometer. The silver and phosphorus content ratios W _Ag and W _P of the powder were calculated from the concentrations.
(2) Oxygen increase after 7 days when stored at 85°C and 85% RH _I
The sample was placed in a graphite crucible and heated and melted in a He atmosphere using an EMGA-820ST manufactured by Horiba, Ltd. The carbon monoxide (carbon dioxide) generated by the crucible was measured by a non-dispersive infrared absorption method, and the initial oxygen content I ₁ (mass %) was measured.
The increase in oxygen after 7 days _I0 was calculated by calculating the difference between the initial oxygen content _I1 and the oxygen content _I2 after the measurement sample was kept in a high-temperature humidifier at 85°C and 85% RH for 7 days and then cooled to room temperature. Specifically, the increase in oxygen _I0 was calculated by subtracting the initial oxygen content _I1 from the oxygen content _I2 after 7 days ( _I2 - _I1 ).
(3) Volume resistivity R ₁ , volume resistivity R ₂ after 7 days storage at 85° C. and 85% RH, and increase rate of volume resistivity I _R
After being kept in a high-temperature humidifier at 85°C and 85% RH for 7 days, 5 g of the sample was cooled to room temperature and placed in a cylindrical container and compression molded at a press pressure of 31.83 MPa to form a measurement sample. The volume resistivity _R2 was measured using this measurement sample. Loresta AP and Loresta PD-41 (both manufactured by Mitsubishi Chemical Corporation) were used to measure the volume resistivity _R2 . The volume resistivity _R1 before storage was also measured using the same method. Then, based on the volume resistivity _R1 and the volume resistivity _R2 , the increase rate of volume resistivity I _R was calculated using the method described above.
(4) BET Specific Surface Area The amount of a measurement sample was set to 0.3 g, and the measurement was performed by a BET single-point method using a Monosorb manufactured by Mountech Co., Ltd.
(5) Particle size _D50
0.2 g of the measurement sample was placed in a beaker, and 0.07 g of Triton X-100 (manufactured by Kanto Chemical) was added. Next, the sample was poured into 40 mL of dispersant-added water (dispersant: 0.3% SN-PW-43 solution (manufactured by San Nopco)), and then dispersed by applying ultrasonic waves of 300 watts for 3 minutes using an ultrasonic disperser US-300AT (manufactured by Nippon Seiki Seisakusho) to prepare a measurement sample. The volume cumulative particle size D ₅₀ of this measurement sample was measured using a laser diffraction scattering type particle size distribution measuring device MT3300II (manufactured by Nikkiso).
(6) Antioxidant level OP
The antioxidant degree OP defined by the following formula (1) was calculated using the increase rate of volume resistivity I _R (%) calculated from the volume resistivities R ₁ and R ₂ , the BET specific surface area SSA (m ² /g), and the phosphorus content W _P (ppm by mass).
Degree of oxidation prevention OP=Increase in volume resistivity I _R (%)/(SSA (m ² /g)×W _P (ppm by mass))
The degree of oxidation prevention OP is a value obtained by normalizing the increase rate of volume resistivity I _R by the specific surface area and the phosphorus content, and is a parameter that indicates the resistance of the particles to oxidation. The smaller the value, the more difficult the phosphorus-containing silver-coated copper particles are to be oxidized.

[Example 1]
26 g of ethylenediaminetetraacetic acid (EDTA) was dissolved in 5 L of pure water, and the liquid temperature was adjusted to 40° C., after which 500 g of copper base particles (1) (spherical, average particle diameter D ₅₀ = 1.5 μm, manufactured by Mitsui Mining & Smelting Co., Ltd.) was added and stirred to prepare a copper base particle dispersion. Then, 10 L of pure water was added to the copper base particle dispersion, and the copper base particles were washed by stirring and decantation. A silver salt solution was prepared by dissolving silver nitrate containing 15 g of silver in 1.2 L of pure water, and a phosphate solution was prepared by dissolving disodium hydrogen phosphate dodecahydrate containing 2.2 g of phosphorus in 1.2 L of pure water.

Next, 5 L of the dispersion was kept at 40°C, and the silver salt solution, the phosphate solution, and 35 g of EDTA were added to the dispersion while stirring, and the silver coating reaction was carried out for 30 minutes. Pure water was added to the resulting dispersion containing the phosphorus-containing silver-coated copper particles, and after stirring and washing, the particles were decanted to separate and remove the washing solution, followed by alcohol replacement, followed by fatty acid treatment to prevent aggregation, and then drying to obtain phosphorus-containing silver-coated copper particles (see Table 1).

The composition, initial oxygen increase _I1 and volume resistivity _R1 of the obtained phosphorus-containing silver-coated copper particles, the oxygen content _I2 after 7 days of storage at 85°C and 85% RH, the oxygen increase _I0 , the volume resistivity R2 _, and the increase rate of the volume resistivity _IR were measured according to the above-mentioned evaluation methods. The results are shown in Table 2.

[Example 2]
Phosphorus-containing silver-coated copper particles were produced in the same manner as in Example 1, except that the phosphorus content in the phosphate solution was 6.5 g.

The composition, initial oxygen increase _I1 and volume resistivity _R1 of the obtained phosphorus-containing silver-coated copper particles, the oxygen content _I2 after 7 days of storage at 85°C and 85% RH, the oxygen increase _I0 , the volume resistivity R2 _, and the increase rate of the volume resistivity _IR were measured according to the above-mentioned evaluation methods. The results are shown in Table 2.

[Example 3]
Phosphorus-containing silver-coated copper particles were produced in the same manner as in Example 1, except that copper base particles (2) (dendrite-shaped, average particle diameter D ₅₀ =7 μm, manufactured by Mitsui Mining & Smelting Co., Ltd.) were used instead of copper base particles (1).

The composition, initial oxygen increase _I1 and volume resistivity _R1 of the obtained phosphorus-containing silver-coated copper particles, the oxygen content _I2 after 7 days of storage at 85°C and 85% RH, the oxygen increase _I0 , the volume resistivity R2 _, and the increase rate of the volume resistivity _IR were measured according to the above-mentioned evaluation methods. The results are shown in Table 2.

[Example 4]
Phosphorus-containing silver-coated copper particles were produced in the same manner as in Example 1, except that copper base particles (1) were replaced with copper base particles (3) (spherical, average particle diameter D ₅₀ =2 μm, manufactured by Mitsui Mining & Smelting Co., Ltd.).

The composition, initial oxygen increase _I1 and volume resistivity _R1 of the obtained phosphorus-containing silver-coated copper particles, the oxygen content _I2 after 7 days of storage at 85°C and 85% RH, the oxygen increase _I0 , the volume resistivity R2 _, and the increase rate of the volume resistivity _IR were measured according to the above-mentioned evaluation methods. The results are shown in Table 2.

[Example 5]
47 g of ethylenediaminetetraacetic acid (EDTA) was dissolved in 5 L of pure water, and then 400 g of copper base particles (1) (spherical, average particle diameter _D50 = 1.5 μm, manufactured by Mitsui Mining & Smelting Co., Ltd.) was added and stirred to prepare a copper base particle dispersion. Then, 10 L of pure water was added to the copper base particle dispersion, and the copper base particles were washed by stirring and decantation. 19 g of hydrazine was added as a reducing agent to the dispersion obtained by adding 5 L of pure water to the washed copper base particles.

Separately from this operation, a plating solution was prepared containing silver nitrate containing 16 g of silver, disodium hydrogen phosphate dodecahydrate containing 13 g of phosphorus, 6 g of ethylene glycol as a surfactant, and 108 g of 5,5-dimethylhydantoin as a complex-forming substance in 2 L of pure water, with the pH adjusted to 9.1 using an aqueous sodium hydroxide solution as a pH adjuster.

When the dispersion liquid was added to the plating solution kept at 40° C. while stirring, the white color of the dispersion liquid gradually became stronger. After the change in the color of the solution was sufficiently observed, stirring was stopped to produce phosphorus-containing silver-coated copper particles.
Pure water was added to the dispersion containing the obtained phosphorus-containing silver-coated copper particles, and the dispersion was stirred and washed, then decanted to remove the washing liquid. Then, alcohol substitution was performed. Then, fatty acid treatment was performed to prevent aggregation, and the resulting mixture was dried to obtain phosphorus-containing silver-coated copper particles.

The composition, initial oxygen increase _I1 and volume resistivity _R1 of the obtained phosphorus-containing silver-coated copper particles, the oxygen content _I2 after 7 days of storage at 85°C and 85% RH, the oxygen increase _I0 , the volume resistivity _R2, and the increase rate of the volume resistivity _I3 were measured according to the above-mentioned evaluation methods. The results are shown in Table 2.

[Example 6]
The phosphorus-containing silver-coated copper particles were produced in the same manner as in Example 1, except that the silver coating reaction was carried out for 60 minutes, and the same measurements were carried out as in Example 1. The results are shown in Table 2.

[Example 7]
The phosphorus-containing silver-coated copper particles were produced in the same manner as in Example 4, except that a silver salt solution was prepared by dissolving silver nitrate containing 100 g of silver in 4.8 L of pure water, and a phosphate solution was prepared by dissolving disodium hydrogen phosphate dodecahydrate containing 4.0 g of phosphorus in 1.2 L of pure water. The results are shown in Table 2.

[Example 8]
The phosphorus-containing silver-coated copper particles were produced in the same manner as in Example 4, except that a silver salt solution was prepared by dissolving silver nitrate containing 200 g of silver in 4.8 L of pure water, and a phosphate solution was prepared by dissolving disodium hydrogen phosphate dodecahydrate containing 4.0 g of phosphorus in 1.2 L of pure water. The results are shown in Table 2.

[Example 9]
Except for using copper base particles (4) (flake-shaped, average particle diameter _D50 = 7.5 μm) instead of copper base particles (1), phosphorus-containing silver-coated copper particles were produced in the same manner as in Example 7, and measurements were performed in the same manner as in Example 7. The results are shown in Table 2.

[Example 10]
Except for using copper base particles (4) (flake-shaped, average particle diameter _D50 = 7.5 μm) instead of copper base particles (1), phosphorus-containing silver-coated copper particles were produced in the same manner as in Example 8, and measurements were performed in the same manner as in Example 8. The results are shown in Table 2.

[Comparative Example 1]
Silver-coated copper particles were produced in the same manner as in Example 1, except that the phosphate solution was not added.

The composition, initial oxygen increase _I1 , volume resistivity _R1 , oxygen content _{I2 after 7 days storage at 85°C and 85% RH, oxygen increase I0 ,} volume resistivity _R2 , and volume resistivity increase rate _IR of the obtained silver-coated copper particles were measured according to the above-mentioned evaluation methods. The results are shown in Table 2.

[Comparative Example 2]
A copper base particle dispersion liquid in which copper base particles (spherical, average particle diameter D ₅₀ =1.5 μm, manufactured by Mitsui Mining & Smelting Co., Ltd.) were dispersed in pure water was kept at 40° C. and stirred, while the silver salt solution and EDTA were added to the dispersion liquid, and a silver coating reaction was carried out for 30 minutes. 0.09 g of phytic acid was added to the obtained silver-coated copper particle dispersion liquid and stirred for 5 minutes to produce phytic acid-containing silver-coated copper particles. The phytic acid-containing silver-coated copper particles were washed with 10 L of pure water and decanted to remove the washing liquid. Then, alcohol replacement was performed. Next, fatty acid treatment was performed to prevent aggregation, and then drying was performed to obtain phytic acid-containing silver-coated copper particles.

The composition, initial oxygen increase _I1 and volume resistivity _R1 of the obtained phosphorus-containing silver-coated copper particles, the oxygen content _I2 after 7 days of storage at 85°C and 85% RH, the oxygen increase _I0 , the volume resistivity R2 _, and the increase rate of the volume resistivity _IR were measured according to the above-mentioned evaluation methods. The results are shown in Table 2.

[Comparative Example 3]
Silver-coated copper particles were produced in the same manner as in Example 9, except that the phosphate solution was not added, and measurements were carried out in the same manner as in Example 9. The results are shown in Table 2.

[Comparative Example 4]
Silver-coated copper particles were produced in the same manner as in Example 10, except that the phosphate solution was not added, and measurements were carried out in the same manner as in Example 10. The results are shown in Table 2.

From Table 2, it can be seen that in the phosphorus-containing silver-coated copper particles obtained according to the present invention by causing a silver compound and phosphoric acid or a salt thereof to coexist in a dispersion of copper base particles to form a phosphorus-containing silver coating layer on the surface of the copper base particles, the oxidation resistance index OR is suppressed to a low value.

On the other hand, in Comparative Examples 1, 3, and 4, since the silver coating layer formed on the surface of the copper base particle does not contain phosphorus, the oxidation resistance of the silver coating layer and, in turn, the silver-coated copper particles is reduced, and it can be seen that the oxidation resistance index OR is high. In particular, in Comparative Examples 3 and 4, the oxidation resistance index OR was high, despite the silver content W _Ag being high at 10.1% by mass and 20.0% by mass, respectively.

In addition, in Comparative Example 2, the phytic acid-containing silver-coated copper particles are added with phytic acid dispersed in pure water after the silver coating layer is formed, so it is believed that the silver coating layer does not contain phosphorus. Therefore, it can be seen that the oxidation resistance index OR is a high value.

According to the present invention, a silver coating layer can be easily formed on the surface of copper particles, and silver-coated copper particles having excellent oxidation resistance can be provided.

Claims (10)

A phosphorus-containing silver-coated copper particle having a silver coating layer containing phosphorus (P) element on at least a part of the surface of a copper base particle,
The increase in oxygen (mass%) after 7 days when stored at 85° C. and 85% RH is defined as I ₀ .
The content (mass%) of silver (Ag) element contained in the phosphorus-containing silver-coated copper particles is represented by W _Ag ,
When the content ratio (ppm by mass) of the phosphorus (P) element contained in the phosphorus-containing silver-coated copper particles is W _P ,
The value of I _O ² /(W _Ag ×W _P ) ×1000 is 1.9 or less;
Phosphorus-containing silver-coated copper particles having a BET specific surface area of 0.1 m ² /g or more and 10 m ² /g or less . The phosphorus-containing silver-coated copper particles according to claim 1, wherein the phosphorus element content W _P is 50 ppm by mass or more and 5000 ppm by mass or less. The phosphorus-containing silver-coated copper particles according to claim 1, wherein the content ratio W _Ag of silver element is 0.1 mass% or more and 30 mass% or less. The phosphorus-containing silver-coated copper particles according to claim 1, wherein an oxygen increase I _O after 7 days when stored at 85° C. and 85% RH is 1.0 mass % or less. The phosphorus-containing silver-coated copper particles according to claim 1, wherein the increase rate I _R of volume resistivity after 7 days of storage at 85° C. and 85% RH is 532% or less. A method for producing phosphorus-containing silver-coated copper particles having a silver coating layer containing phosphorus (P) element on at least a part of the surface of a copper base particle, comprising:
The method for producing phosphorus-containing silver-coated copper particles includes causing a silver compound and phosphoric acid or a salt thereof to coexist in a dispersion of the copper base particles, and precipitating and forming a phosphorus-containing silver coating layer on the surface of the copper base particles by a substitution method utilizing a substitution reaction between copper and silver resulting from the difference in ionization tendency . The method for producing phosphorus-containing silver-coated copper particles according to claim 6, wherein a plating solution containing the silver compound and the phosphoric acid or a salt thereof is added to the dispersion liquid to form a phosphorus-containing silver coating layer by precipitation. The method for producing phosphorus-containing silver-coated copper particles according to claim 6, wherein the ratio of the amount of phosphorus added (mass%) to the amount of silver added (mass%) in the dispersion in which the plating solution containing the silver compound and the phosphoric acid or a salt thereof coexists is 0.1 or more and 50 or less. 2. The phosphorus-containing silver-coated copper particle according to claim 1, which is obtained by precipitating the silver coating layer containing phosphorus on the surface of the copper base particle by a substitution method utilizing a substitution reaction between copper and silver caused by a difference in ionization tendency in which a silver compound and phosphoric acid or a salt thereof are coexisted in a dispersion of the copper base particle. Phosphorus (P) element is PO ₄ ^3- The phosphorus-containing silver-coated copper particle according to claim 1 , wherein the phosphorus-containing silver-coated copper particle is contained in the silver-coating layer in an ionic state.