WO2024089840A1 - Silver-based metal powder and method for manufacturing silver-based metal powder - Google Patents

Abstract

[Problem] To provide a silver-based metal powder with which stable sintering/burning is possible even in an atmosphere with a low temperature and/or high dew point and which, by having less variation in the strength and dimensions of the sintered bodies, can be used preferably as a raw material powder for powder metallurgy and/or a raw material powder for conductive paste material. [Solution] A silver-based metal powder wherein: the standard free energy of formation of an oxide contained in the silver-based metal powder in a temperature range at or below 600°C is equal to or lower than the standard free energy of formation of silicon dioxide in said temperature range; and the total content of an impurity element having a mixing enthalpy with silver of equal to or lower than aluminum is equal to or lower than 500 ppm.

Description

Silver-based metal powder and method for producing the same

The present invention relates to a silver-based metal powder. More specifically, the silver-based metal powder can be sintered and fired stably even at low temperatures or in an atmosphere with a high dew point, and the strength and dimensional variation of the sintered body is small, so the silver-based metal powder can be suitably used as a raw material powder for powder metallurgy applications or as a raw material powder for conductive paste materials.

Silver has excellent electrical conductivity, thermal conductivity, oxidation resistance, and corrosion resistance, and is popular as a precious metal for use in various accessories, so silver-based metal powders made of silver and silver alloys have traditionally been used as raw powders for metal powder injection molding and silver clay.

In recent years, silver-based metal powders have also been increasingly used in the electronic materials field as raw powder for sintered or thermosetting conductive pastes.

Silver is a precious metal that does not oxidize easily, and silver oxides tend to decompose easily. As a result, as long as the temperature is raised sufficiently, a reducing atmosphere is not required, and sintering can be performed even in an atmosphere with a high oxygen partial pressure, such as air, which is an excellent characteristic.

However, from the perspective of manufacturing costs, it is desirable to have a silver-based metal powder that can be sintered even in adverse conditions such as an atmosphere with a high dew point and a sintering temperature that is as low as possible. However, there is a problem in that stable sintering does not occur even with silver-based metal powder at low temperatures or in an atmosphere with a high dew point, and variations in the dimensions and strength of the sintered body tend to occur.

Therefore, there is a need to develop a silver-based metal powder that can be sintered stably even in low temperatures or in an atmosphere with a high dew point, and that is less susceptible to variations in the dimensions and strength of the sintered body.

JP2001-3101

Patent Document 1 discloses silver powder for silver clay that contains 20 ppm to 900 ppm in total of one or more of tin, lead, copper, bismuth, and iron, and states that the intentional addition of these elements improves sinterability.

However, the sintering temperature of the silver powder in Patent Document 1 is 900°C, and there is no indication of stability when sintered at low temperatures or in an atmosphere with a high dew point.

The inventors set out to solve the above problems as a technical task, and conducted extensive experiments focusing on impurity elements in silver and silver alloys. As a result, they discovered that if the silver-based metal powder contains a standard free energy of formation of oxides in a temperature range of 600°C or less that is less than the standard free energy of formation of silicon dioxide in the same temperature range, and the total content of impurity elements whose mixing enthalpy with silver is less than or equal to aluminum is 500 ppm or less, stable sintering and firing are possible even at low temperatures or in an atmosphere with a high dew point, and variations in the strength and dimensions of the sintered body can be suppressed, which led to the solution of the above technical tasks.

The above technical problems can be solved by the present invention as follows:

The present invention is a silver-based metal powder, which contains an oxide whose standard free energy of formation in a temperature range of 600°C or less is equal to or less than the standard free energy of formation of silicon dioxide in said temperature range, and which contains an impurity element whose mixing enthalpy with silver is equal to or less than that of aluminum, in a total content of 500 ppm or less.

The present invention also relates to the silver-based metal powder, in which the total content of the impurity rare earth elements is 100 ppm or less.

The present invention also relates to the silver-based metal powder, in which the content of the impurity element lithium is 100 ppm or less.

The present invention also relates to the silver-based metal powder, in which the total content of the impurity elements calcium and/or magnesium is 300 ppm or less.

The present invention also relates to the silver-based metal powder, in which the content of the impurity element zirconium is 300 ppm or less.

The present invention also relates to the silver-based metal powder, in which the content of silicon, an impurity element, is 300 ppm or less.

The present invention also relates to the silver-based metal powder, in which the content of the impurity element aluminum is 300 ppm or less.

The present invention also relates to a method for producing the silver-based metal powder by atomization.

The silver-based metal powder of the present invention has a standard free energy of formation of oxide in a temperature range of 600°C or less that is less than the standard free energy of formation of silicon dioxide in the same temperature range, and the total content of impurity elements whose mixing enthalpy with silver is less than that of aluminum is 500 ppm or less. This allows stable sintering and firing even at low temperatures or in an atmosphere with a high dew point, and there is little variation in the dimensions and strength of the sintered body. Therefore, this silver-based metal powder can be suitably used as a raw material powder for powder metallurgy applications or as a raw material powder for conductive paste materials.

The present invention is a silver-based metal powder made of silver and/or a silver alloy.

The silver-based metal powder of the present invention has a standard free energy of formation of an oxide in a temperature range of 600°C or less that is equal to or less than the standard free energy of formation of silicon dioxide in said temperature range, and the total content of impurity elements (hereinafter sometimes referred to as "specific impurity elements") whose mixing enthalpy with silver is equal to or less than that of aluminum is 500 ppm or less.

　Impurity elements whose standard free energy of formation of oxides in the temperature range below 600°C is equal to or less than the standard free energy of formation of silicon dioxide in said temperature range tend to be easily oxidized in the temperature range below 600°C, and it is believed that even trace amounts of these elements present on the surface or within the powder particles react preferentially with oxygen in the atmosphere to form oxides.

In addition, the mixing enthalpy is a thermodynamic value that indicates the strength of the interaction between elements; the smaller the value, the easier the elements tend to bond with each other. However, impurity elements whose mixing enthalpy with silver is equal to or lower than that of aluminum have a high affinity with silver, and therefore are thought to become oxides by uniformly coating the surface of silver-based metal powder particles or dispersing uniformly within the particles, rather than separating from the silver and forming an oxide locally.

The oxides of the specific impurity elements mentioned above have almost no effect on sintering when the sintering temperature is sufficiently high or under favorable conditions such as a reducing atmosphere or an inert atmosphere with a low dew point.

However, adverse atmospheric conditions such as low temperatures or high dew points prevent the main component, silver, from diffusing into the solid phase on or within the particle surface, slowing down or varying the progress of sintering, which is thought to manifest itself as poor sintering and variations in the dimensions and strength of the sintered body.

The total content of specific impurity elements in this invention is 500 ppm or less.

If the total content of certain impurity elements exceeds 500 ppm, sintering and firing will be unstable, and there is a risk of variation in the dimensions and strength of the sintered body.

In addition, in this invention, "total content" refers to the combined content of all specific impurity elements contained in the silver-based metal powder.

Specific impurity elements include rare earth elements such as yttrium (Y) and neodymium (Nd), lithium (Li), calcium (Ca), magnesium (Mg), zirconium (Zr), silicon (Si), and aluminum (Al).

For the standard free energy of formation of oxides in the temperature range below 600°C and the enthalpy of mixing with silver, values given in the literature can be used.

The literature value for the standard free energy of formation of silicon dioxide at 600° C. is −750 (kJ/mol-O ₂ ), and the literature value for the enthalpy of mixing of aluminum with silver is −4 (kJ/mol).

In the present invention, it is preferable that the total content of rare earth elements, which are specific impurity elements, is 100 ppm or less.

This is because rare earth elements have a large effect of inhibiting stable sintering.

The content of lithium, a specific impurity element, is preferably 100 ppm or less.

This is because lithium is an element that has a large effect of inhibiting stable sintering.

The total content of the specific impurity elements calcium and/or magnesium is preferably 300 ppm or less.

The content of the specific impurity element zirconium is preferably 300 ppm or less.

The content of silicon, a specific impurity element, is preferably 300 ppm or less.

The content of aluminum, a specific impurity element, is preferably 300 ppm or less.

Although calcium, magnesium, zirconium, silicon, and aluminum have less of an effect than rare earths and lithium, they still inhibit stable sintering.

Specific impurity elements can be quantified by dissolving silver-based metal powder in acid to create a solution, then analyzing it using ICP atomic emission spectrometry.

In the present invention, the oxide may contain an element whose standard free energy of formation in a temperature range of 600°C or less is higher than the standard free energy of formation of silicon dioxide in the same temperature range, or an impurity element whose mixing enthalpy with silver is higher than that of aluminum.

Palladium (Pd) is an example of an element whose standard free energy of formation of an oxide in a temperature range of 600°C or less is higher than the standard free energy of formation of silicon dioxide in the same temperature range, titanium (Ti) is an example of an element whose mixing enthalpy with silver is higher than that of aluminum, and nickel (Ni), copper (Cu), and iron (Fe) are examples of elements whose standard free energy of formation of an oxide in a temperature range of 600°C or less is higher than the standard free energy of formation of silicon dioxide in the same temperature range and whose mixing enthalpy with silver is higher than that of aluminum.

The silver-based metal powder in the present invention may contain silver-copper alloys and silver-palladium alloys, which are used as brazing materials.

The method for producing the silver-based metal powder in the present invention is not particularly limited, but it is preferable to produce it by the atomization method.

The atomization method is relatively inexpensive and efficient, and can produce large quantities of silver-based metal powder.

The atomization method is not limited, and may be any known method such as water atomization, gas atomization, or centrifugal atomization.

In the present invention, the total content of specific impurity elements must be controlled to 500 ppm or less, so care must be taken with the silver bullion used as the raw material. However, since the specific impurity elements in the present invention are basically easily oxidized, when they are melted in an atmosphere containing a certain amount of oxygen, they become oxides and can be separated and removed from the molten silver and silver alloy.
Therefore, the total content of specific impurity elements in the metal before melting does not need to be 500 ppm or less.

The average particle size of the silver-based metal powder in the present invention is not limited and may be determined as desired.

The average particle size of silver-based metal powders used in the field of electronic materials is generally 50 μm or less, and considering the formation of fine circuit patterns, it is more preferable for the average particle size to be 10 μm or less.

To produce fine powder with an average particle size of 10 μm or less, it is recommended to use the water atomization method, which is easy to produce fine particles from the perspective of yield.

The average particle size of silver-based metal powder can be measured using a laser diffraction particle size distribution measuring device.

The following examples and comparative examples are provided, but the present invention is not limited to these.

(Preparation of Powder)
The raw material, high purity silver bullion (99.99% or higher), was heated and melted, and then the impurity elements shown in Tables 1 to 3 were appropriately added to adjust the composition.

The molten metals of the Examples and Comparative Examples in a molten state were allowed to fall freely, and then high-pressure water was sprayed onto them, scattering the molten metal and rapidly cooling and solidifying it, using the water atomization method to obtain each silver-based metal powder.

The resulting silver-based metal powders were sieved through a 100 mesh sieve to remove coarse particles.

(Measurement of average particle size)
The average particle size of each silver-based metal powder was measured using a laser diffraction particle size distribution measuring device MT3000II (manufactured by Microtrack Bell Co., Ltd.).

The "average particle size" in the present invention is a cumulative 50% particle size ( _D50 size) on a volume basis.

(Quantitative determination of impurity elements in silver-based metal powder)
The content of each impurity element was measured by dissolving each of the obtained silver-based metal powders in acid to prepare a solution, and then using an ICP emission spectrometer iCAP7600 (manufactured by Thermo Fisher Scientific Co., Ltd.).

　Blank cells in Tables 1 to 3 indicate levels below the detection limit of 10 ppm.

Qualitative analysis was performed on each of the obtained silver-based metal powders using X-ray fluorescence with a ZSX Primus IV (manufactured by Rigaku Corporation), and it was confirmed that no peaks of elements other than those listed in Tables 1 to 3 were detected.

(Evaluation of sinterability)
Each of the silver-based metal powders of the examples and comparative examples was subjected to die lubrication using zinc stearate as a lubricant, and molded into a cylindrical shape of Φ14-7×5 mm so that the green density was 7.7 g/ ^cm3 . The powder was then held at 400°C for 15 minutes in a pure nitrogen atmosphere with a dew point of 0°C to obtain a sintered body.

The radial crushing strength of each sintered body was measured in accordance with JIS Z 2507 using an Autograph AG-IS 50kN (manufactured by Shimadzu Corporation).

The sinterability of each powder was evaluated with n=5. If all measured radial crushing strengths were 120 MPa or higher, the sinterability was evaluated as ◎. If they were 100 MPa or higher, the sinterability was evaluated as ◯. If even one was less than 100 MPa, the sinterability was evaluated as ×.

By comparing Examples 1 to 9 and Comparative Examples 1 to 3 shown in Table 1, it was confirmed that the sinterability is stable if the combined content of yttrium (Y) and neodymium (Nd), which are classified as rare earth elements, is 500 ppm or less, and that stability decreases if the combined content exceeds 500 ppm. Furthermore, it was confirmed that the sinterability is particularly stable if the combined content is 100 ppm or less.

In addition, by comparing Examples 10 to 12 and Comparative Example 4 shown in Table 1, it was confirmed that the sinterability was stable when the lithium (Li) content was 500 ppm or less, and that the stability decreased when the content exceeded 500 ppm. Furthermore, it was confirmed that the sinterability was particularly stable when the content was 100 ppm or less.

In addition, by comparing Examples 13 to 21 and Comparative Examples 5 to 7 shown in Table 2, it was confirmed that the sinterability is stable if the total content of calcium (Ca) and magnesium (Mg) is 500 ppm or less, and that stability decreases if the total content exceeds 500 ppm. Furthermore, it was confirmed that the sinterability is particularly stable if the total content is 300 ppm or less.

In addition, by comparing Examples 22 to 24 and Comparative Example 8 shown in Table 2, it was confirmed that if zirconium (Zr) is 500 ppm or less, the sinterability is stable, and if it exceeds 500 ppm, the stability decreases. Furthermore, it was confirmed that if it is 300 ppm or less, the sinterability is particularly stable.

In addition, by comparing Examples 25 to 27 and Comparative Example 9 shown in Table 3, it was confirmed that if silicon (Si) is 500 ppm or less, the sinterability is stable, and if it exceeds 500 ppm, the stability decreases. Furthermore, it was confirmed that if it is 300 ppm or less, the sinterability is particularly stable.

In addition, by comparing Examples 28 to 30 and Comparative Example 10 shown in Table 3, it was confirmed that if aluminum (Al) is 500 ppm or less, the sinterability is stable, and if it exceeds 500 ppm, the stability decreases. Furthermore, it was confirmed that if it is 300 ppm or less, the sinterability is particularly stable.

In addition, by comparing Examples 31 to 34 and Comparative Examples 11 to 14 shown in Table 3, it was confirmed that if the standard free energy of formation of an oxide in a temperature range of 600°C or less is equal to or less than the standard free energy of formation of silicon dioxide in the same temperature range, and the total content of impurity elements whose mixing enthalpy with silver is equal to or less than aluminum is 500 ppm or less, the sinterability is stable, and that stability decreases when the content exceeds 500 ppm.

The silver-based metal powder of the present invention has a standard free energy of formation of oxide in a temperature range of 600°C or less that is equal to or less than the standard free energy of formation of silicon dioxide in the same temperature range, and the total content of impurity elements having a mixing enthalpy with silver equal to or less than that of aluminum is 500 ppm or less. Therefore, stable sintering and firing are possible even at low temperatures or in an atmosphere with a high dew point, and there is little variation in the strength and dimensions of the sintered body, and therefore the powder can be suitably used as a raw material powder for powder metallurgy applications or as a raw material powder for conductive paste materials.
Therefore, the present invention has high industrial applicability.

Claims (8)

A silver-based metal powder, the standard free energy of formation of oxides contained in the silver-based metal powder in a temperature range of 600°C or less is equal to or less than the standard free energy of formation of silicon dioxide in the same temperature range, and the total content of impurity elements having an enthalpy of mixing with silver equal to or less than that of aluminum is 500 ppm or less. The silver-based metal powder according to claim 1, in which the total content of the impurity elements, rare earth elements, is 100 ppm or less. The silver-based metal powder according to claim 1, wherein the content of the impurity element lithium is 100 ppm or less. The silver-based metal powder according to claim 1, in which the total content of the impurity elements calcium and/or magnesium is 300 ppm or less. The silver-based metal powder according to claim 1, wherein the content of the impurity element zirconium is 300 ppm or less. The silver-based metal powder according to claim 1, wherein the content of the impurity element silicon is 300 ppm or less. The silver-based metal powder according to claim 1, wherein the content of the impurity element aluminum is 300 ppm or less. A method for producing silver-based metal powder according to claims 1 to 7, which is produced by an atomization method.