JP2008121051A - Method for producing silver powder - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide new silver powder different from the conventional silver powder, and to provide its production method. <P>SOLUTION: To provide a method where powder grains composed of a metal baser than silver are placed in a solution capable of the substitution reaction of silver and are brought into substitution reaction, and ≥99 mass% of the metal is substituted with silver, so as to obtain silver powder grains. By the method, hollow silver powder grains having void parts at the insides of the grains can be stably produced. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a new method for producing silver powder.

  Silver powder is used for forming electrodes and circuits of various electronic components such as conductive pastes, sintering aids, and electrode materials for plasma displays. For example, the conductive paste is applied to circuit formation of printed wiring boards using a screen printing method, various electrical contact portions, and the like, and is used as a material for ensuring electrical continuity.

  As a method for producing silver powder, in addition to an electrolytic method (see Patent Document 1) in which an electrolytic solution containing silver ions is electrolyzed to deposit silver particles on an electrode, as disclosed in Patent Document 2, a silver nitrate solution and ammonia are used. A method of wet reduction in which an aqueous silver ammine complex solution is prepared with water and an organic reducing agent is added thereto, and further, for example, as disclosed in Patent Document 3, for example, sodium phosphinate as a reducing agent in an aqueous silver sulfate solution In addition, a method using a chemical reduction method in which a reaction is performed using one of formaldehyde and hydroquinone and polyvinylpyrrolidone is known.

JP-A-8-209375 JP 2001-107101 A JP-A-6-122905

  The present invention provides a new silver powder different from conventional silver powder and a method for producing the same.

In the present invention, powder particles made of a metal lower than silver (this powder particle is also referred to as “original powder particle” and this powder is also referred to as “original powder”) and a solution capable of replacing silver are provided. A silver powder production method is proposed, characterized in that silver powder particles are obtained by mixing and substitution reaction to replace 99% by mass or more of the metal with silver.
Further, as a preferred example, after adding a chelating agent to an aqueous dispersion slurry in which the original powder particles are dispersed in water, a water-soluble silver salt is added to cause a substitution reaction, and 99% by mass or more of the metal We propose a method for producing silver powder characterized by obtaining silver powder particles by substituting silver with silver.

  According to such a production method of the present invention, hollow silver powder particles having a hollow portion inside the particles can be stably produced. Such hollow silver powder particles cannot be obtained by a conventional manufacturing method and have a low bulk density because they are hollow. For example, if a conductive paste is to be produced, the necessary conductivity can be obtained. The silver (mass) can be reduced, and the cost can be reduced. Furthermore, since it has a cavity inside the particles, it can be easily deformed compared to solid particles, and can be easily processed into flaky particles.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, although the embodiment of the present invention is described in detail, the scope of the present invention is not limited to the following embodiment.
Further, in this specification, when “X to Y” (X and Y are arbitrary numbers) is described, it means “X or more and Y or less” unless otherwise specified, and at the same time, “preferably larger than X, "Less than Y" is included.

  In this embodiment, the surface oxide (oxide film) of the metal powder particles to be the base powder is removed <base powder pretreatment step>, a chelating agent is added to the water dispersion slurry in which the base powder is dispersed in water, Silver powder particles are obtained through a <silver replacement step> in which a silver salt soluble in water is added to replace the metal and silver of the original powder.

<Original flour>
The metal used as the base powder may be a metal that can replace silver, that is, a metal that is baser than silver (a metal that has a higher ionization tendency than silver). For example, Mg, Al, Ti, Zr, Mn, Cr, V, Zn, Cr, Fe, Cd, In, Co, Ni, Sn, Pb, Cu and the like can be mentioned, among which Cu, Fe, Ni and the like are preferable. Of these, Cu is particularly preferable from the viewpoints of being inexpensive and easily available, relatively stable against oxidation, and not significantly detracting from conductivity even when remaining in the final product silver powder particles.
As for the size of the original powder particles, D50 is preferably 1 μm to 50 μm, more preferably 1 μm to 40 μm, and even more preferably 1 μm to 30 μm.
Further, the shape of the original powder particles is not particularly limited, but in producing the hollow silver powder particles, it is not an isotropic particle such as a spherical shape, a hexahedral shape, and an octahedral shape, but a dendrite shape, a needle shape, an indefinite shape. Anisotropic particles such as a shape are preferred, and among these, dendritic particles are particularly preferred. Since ordinary electrolytic copper powder has a dendritic shape, it can be said that electrolytic copper powder is particularly preferable from this viewpoint.

<Original powder pretreatment process>
By removing the oxide film of the original powder particles, the substitution reaction can be promoted in a later step. For this reason, it is preferable to perform a pretreatment for removing the oxide film of the original powder before the silver substitution reaction. However, it is not always necessary to perform pretreatment depending on the state of the original powder.
In addition, when organic substance exists in the surface of the base powder particle | grains, it is preferable to perform a degreasing process previously before performing this pretreatment.

The pretreatment of the base powder is, for example, adding the base powder to water and stirring and mixing, then adding a reducing agent or an acidic solution, stirring and mixing, and then washing the base powder to wash the reducing agent or the acidic solution. Is preferably used after removing from the base powder.
However, other methods can be adopted as long as the oxide film of the original powder can be removed.

  At this time, the temperature of the water into which the raw powder is charged is not particularly limited, but is preferably set to 50 to 60 ° C. When the temperature is lower than 50 ° C, the reaction rate with the reducing agent or the acidic solution may be slow, and even when the temperature is higher than 60 ° C, the reaction rate does not increase.

As the reducing agent or acidic solution to be added, any reducing agent such as hydrazine, potassium borohydride, hypophosphorous acid, etc., may be used as long as it can remove the surface oxide (oxide film) of the metal powder particles as the original powder. Examples include acidic solutions such as sulfuric acid, hydrochloric acid, and phosphoric acid.
The pH of the solution (acid treatment solution) for removing the oxide film is preferably 2 to 5. When the pH is lower than 2, the base powder is dissolved, and the aggregation of the base powder itself easily proceeds. On the other hand, if the pH is higher than 5, the effect of removing the surface oxide is lowered.

  The added reducing agent or acidic solution is preferably washed thoroughly and removed from the original powder. If the reducing agent or the acidic solution remains in the base powder, the reaction efficiency of the substitution reaction in the subsequent process is lowered. In other words, if the reducing agent remains, the substitutional reaction between the original powder and silver becomes difficult, and the silver reacts with the remaining reducing agent and precipitates alone. If the acidic solution remains, the pH will fluctuate, and it will be necessary to adjust the pH separately in a later step.

  In addition, you may make it perform a decantation process after disperse | distributing a base powder in the solution containing a reducing agent or an acidic solution as mentioned above. According to this, since the base powder does not come into contact with the atmosphere, it is possible to proceed to the next step in a state in which reoxidation of the surface of the base powder particles is prevented.

<Silver replacement process>
In the silver replacement step, the base powder is dispersed in water, a chelating agent is added, then a silver salt soluble in water is added to cause a substitution reaction, and silver powder particles are obtained by replacing the base powder metal with silver. .

  At this time, the water in which the base powder is dispersed is not particularly limited in water temperature, but is preferably 20 ° C. or higher, particularly 30 ° C. or higher, and particularly preferably 40 ° C. or higher. If it is 30 degreeC or more, especially 40 degreeC or more, the reaction rate of substitution reaction can be sped up.

Examples of the chelating agent include one or more selected from ethylenediamine tetraacetate (hereinafter referred to as “EDTA”), triethylenediamine, diethylenetriaminepentaacetic acid, and iminodiacetic acid. Is preferably used.
By adding a chelating agent, it is possible to preferentially form a complex of ions (for example, copper ions) of the original powder metal, and to stabilize the original powder metal used for the substitution reaction as an ion. The reaction can be promoted.
The addition amount of the chelating agent is preferably 1 to 10% by mass, particularly 2 to 5% by mass with respect to the original powder. At this time, if it is 1 to 10% by mass, the reaction rate of the substitution reaction is not significantly reduced. In addition, since exceeding 10 mass% does not affect the reaction rate, it is uneconomical.

When adding a silver salt, it is preferable to adjust the pH of the solution, that is, the pH of the solution at the time of the substitution reaction to 3 to 4.
At this time, the substance used for pH adjustment is not particularly limited. For example, phthalates such as potassium phthalate and sodium phthalate can be preferably used. Since phthalates act as a buffering agent, the solution can be stably maintained in the acidic region of pH 3-4.

  Silver salts soluble in water, that is, Ag ion sources include silver nitrate, silver perchlorate, silver acetate, silver oxalate, silver chlorate, silver hexafluorophosphate, silver tetrafluoroborate, and 6 fluorine. One type or two or more types selected from silver arsenate and silver sulfate can be mentioned.

  The addition amount of the silver salt is not particularly limited, but when it is more than the theoretical equivalent, for example, when copper is used as the base powder, it is added so as to be 2 mol or more, particularly 2.1 mol or more of silver with respect to 1 mol of copper. Is preferred. When the amount is less than 2 mol, the substitution is insufficient and a large amount of copper remains in the silver powder particles. However, it is not economical to add 2.5 mol or more.

  The silver salt is preferably added slowly over time with stirring. When a large amount is added at once, the silver salt becomes excessively large, a large amount of silver ions that do not undergo a substitution reaction with the original powder is generated, and silver is precipitated alone. However, if the time is too long, the base powder is oxidized to form an oxide film, so that the silver salt concentration is adjusted to 0.1 to 10 g / L over an appropriate time, for example, 60 minutes to 120 minutes. Is preferred.

As a measure of completion of the substitution reaction, the silver content of the obtained silver powder particles is 99% by mass or more (the content of the original powder metal is less than 1% by mass), preferably the silver content is 99.5% by mass or more ( The content of the base powder metal is less than 0.5% by mass), and more preferably, when the silver content reaches 99.9% by mass or more (the content of the base powder metal is less than 0.1% by mass). Is preferred.
The silver content in the silver powder particles can be adjusted by the amount of silver salt added, the reaction time, the reaction rate, the amount of chelating agent added, and the like.
In addition, if the content rate of silver of the obtained particle | grains is 99 mass% or more (content of original powder metal is less than 1 mass%), the obtained particle | grains can be recognized as silver powder particle | grains, and manufacture of this embodiment It means that the method can be recognized as a method for producing silver powder or silver powder particles.

  After completion of the substitution reaction, the silver powder particles are preferably thoroughly washed and dried.

Instead of the silver salt, it is considered possible to replace silver by using a silver complex salt solution made of, for example, silver nitrate, ammonium carbonate salt, ethylenediaminetetraacetate, or the like.
Moreover, after adding a silver salt, it is also possible to add a reducing agent and to promote a substitution reaction.

(Silver powder particles)
Most of the silver powder particles obtained in the present embodiment are at least 50% by number or more, preferably 80% by number or more, and particularly preferably 90% by number or more (including 100%) have a cavity inside the particle. Hollow silver powder particles. In other words, silver powder particles obtained by FIB / SEM cross-section processing observation apparatus (FIB: Focus Ion Beam: focused ion beam method: an apparatus that enables fine processing by sputtering the surface with a focused ion beam). Observing the vertical cross section of this particle, it has a feature that it has a cavity inside the particle and a hole communicating the cavity with the outside.

As for the size of the silver powder particles obtained in this embodiment, D50 is preferably 1 μm to 50 μm, more preferably 1 μm to 40 μm, and particularly preferably 1 μm to 30 μm. If D50 is 1 micrometer-50 micrometers, it will become suitable for the use mentioned later.
Further, the silver powder particles obtained in the present embodiment are characterized by a small bulk density, and AD (apparent density) is 0.5 to 2.0 g / cm ³ , particularly 0.7 to 1.4 g / cm ^3. The TD (tap filling density) is preferably 1.0 to 3.0 g / cm ³ , particularly preferably 1.5 to 2.5 g / cm ³ .

(Use)
The use of the silver powder particles obtained in the present embodiment is preferably an application in which the shape of the silver powder particles is maintained, for example, an application that is not used such that sintering progresses and voids in the particles disappear when sintered. For example, it can be used for forming electrodes and circuits of various electronic components such as conductive paste. In particular, when a conductive paste is produced from this silver powder, the amount of silver can be reduced because of the hollow silver powder, and the cost can be reduced. Moreover, since it is a hollow silver powder particle, it is easy to deform | transform compared with a solid particle, and can also be easily processed into a flaky particle.

  Examples of the present invention will be described below, but the present invention is not limited to the following examples.

<Particle size measurement>
Take a measurement sample (copper powder) in a small amount of beaker, add a few drops of 3% Triton X solution (manufactured by Kanto Chemical Co., Ltd.), and blend with the powder. Then 0.1% SN Dispersant 41 solution (manufactured by San Nopco) 50 mL was added, and then a measurement sample was prepared by dispersing for 2 minutes using an ultrasonic disperser TIPφ20 (manufactured by Nippon Seiki Seisakusho, OUTPUT: 8, TUNING: 5).
The volume accumulation standard D50 of this measurement sample was measured using a laser diffraction / scattering particle size distribution analyzer MT3300 (manufactured by Nikkiso).

<Apparent density (AD) measurement method>
Based on JIS Z-2504 (2000), the measurement was performed using a Casa specific gravity measuring instrument manufactured by Kuramochi Scientific Instruments.

<Tap packing density (TD) measurement method>
The sample weight was set to 120 g, and the measurement was performed using a tap measuring instrument KRS-406 manufactured by Kuramochi Scientific Instruments.

<Measurement method of volume resistivity>
The resistivity measurement was performed by forming a measurement sample in which 15 g of a sample was put in a cylindrical container and compression-molded with a press pressure of 40 × 10 6 Pa (408 kgf / cm ² ), and Loresta AP and Loresta PD-41 types (both Mitsubishi Chemical Corporation) The measurement was carried out.

(Example 1)
300 g of dendritic electrolytic copper powder (see FIG. 1, purity 99% or more, D50: 15.88 μm) was put into 3000 ml of pure water kept at 50 ° C. and stirred for 5 minutes to obtain a slurry. Next, 27.6 g of 100% hydrazine as a reducing agent was added, and the reduction treatment was performed while maintaining stirring for 30 minutes. Then, after solid-liquid separation with a buchner funnel and washing with 1.8 L of pure water, 0.5 mL of methanol was added to obtain a pretreated copper powder.

  Next, 3000 mL of pure water was heated to 40 ° C., and all of the pretreated copper powder obtained above were added and stirred for 5 minutes to form a slurry. Next, 127.8 g of EDTA was added and stirred for 10 minutes. Then, 4.2 L (1680 g of silver nitrate) prepared in advance was added dropwise with stirring over 2 hours, and the substitution reaction proceeded. Stirring was stopped for a minute and the mixture was allowed to stand for aging treatment. Then, after solid-liquid separation with a buch funnel and washing with 3000 mL of pure water, 500 mL of methanol was added, followed by dehydration with 500 mL of acetone, and the resulting cake was transferred to a stainless steel vat at 100 ° C. Silver powder particles were obtained by drying in an atmosphere for 5 hours.

When the obtained silver powder particles were observed by SEM (2000 times) and FIB / SEM cross-section processing observation (5000 times), each particle had a cavity inside the particle and a hole communicating this cavity with the outside. (Figs. 2 and 3).
Further, D50 was 15.54 μm, the Cu content was 0.13 mass% (silver: 99.87 mass%), and the volume resistivity value was 4.0 × 10 ⁻⁵ mΩ · cm.

(Example 2)
Silver powder particles were obtained in the same manner as in Example 1 except that the dropping time of the silver nitrate solution was changed to 30 minutes.
When the obtained silver powder particles were observed by SEM (2000 times) and FIB / SEM cross-section processing observation (5000 times), each particle had a cavity inside the particle and a hole communicating this cavity with the outside. It was equipped with.
Further, D50 was 14.99 μm, the Cu content was 0.13 mass% (silver: 99.87 mass%), and the volume resistivity value was 5.1 × 10 ⁻⁵ mΩ · cm.

(Example 3)
Silver powder particles were obtained in the same manner as in Example 1 except that the dropping time of the silver nitrate solution was changed to 60 minutes.
When the obtained silver powder particles were observed by SEM (2000 times) and FIB / SEM cross-section processing observation (5000 times), each particle had a cavity inside the particle and a hole communicating this cavity with the outside. It was equipped with.
Further, D50 was 19.47 μm, the Cu content was 0.13 mass% (silver: 99.87 mass%), and the volume resistivity value was 4.0 × 10 ⁻⁵ mΩ · cm.

Example 4
Silver powder particles were produced in the same manner as in Example 1 except that the original powder was changed to spherical copper powder (purity: 99%, D50: 5.51 μm).
D50 of the obtained silver powder particles was 9.85 μm, the Cu content was 0.98% (silver: 99.02%), and the volume resistivity value was 6.2 × 10 ⁻⁵ mΩ · cm.

  From the result that the silver substitution rate of the silver powder of Example 1-3 using the dendritic copper powder as the base powder is higher than that in Example 4 using the spherical copper powder as the base powder, the base powder is spherical copper. It is considered preferable to use anisotropic particles such as dendritic copper powder instead of isotropic particles such as powder.

It is a SEM observation image (magnification 2000 times) of the original powder particle used in Example 1. It is a SEM observation image (magnification 2000 times) of the silver powder obtained in Example 1. It is FIB / SEM cross-section processing observation (5000 times magnification) of the silver powder obtained in Example 1. FIG.

Claims (4)

  It is characterized in that powder particles made of a metal that is baser than silver and a solution capable of replacing silver are mixed and subjected to a substitution reaction, and silver powder particles are obtained by replacing 99% by mass or more of the metal with silver. To produce silver powder.   After adding a chelating agent to a water-dispersed slurry in which powder particles made of a metal lower than silver are dispersed in water, a silver salt soluble in water is added to cause a substitution reaction, and 99 mass of the metal. A method for producing silver powder, characterized in that silver powder particles are obtained by substituting at least% by silver.   The method for producing silver powder according to claim 1 or 2, wherein the oxide film of the powder particles made of a metal lower than silver is previously removed with a reducing agent or acid before the substitution reaction. The silver powder obtained by the manufacturing method in any one of Claims 1 thru | or 3.