Silver powder made of silver particles, each to which fine silver particles adhere and process of producing the same

This invention is a silver powder having a low-temperature sintering performance and dispersibility, which allows the powder particles to be agglomerated to a small degree and be nearly in the monodisperse state. Employed is silver powder of fine silver particles each to which fine silver particles adhere, wherein fine silver particles of nano-order particle size are adhered to the surface of each silver powder particle. The powder particles of the silver powder of fine silver particles each to which fine silver particles adhere have excellent dispersibility. In the production of the silver powder of fine silver particles each to which fine silver particles adhere, a process of including the steps of: adding a silver nitrate and a neutralizing agent into a slurry of silver powder in a dispersing medium; dissolving the mixture while stirring to allow fine silver oxide particles to be precipitated on the surface of each silver powder particle; washing the resultant silver powder; and exposing the fine silver oxide particles to UV rays to reduce the same to fine silver particles.

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Description
TECHNICAL FIELD

The present invention relates to a silver powder made of silver particles, each to which fine silver particles adhere and a process of producing the same.

BACKGROUND ART

Silver ink (silver paste) has been conventionally used not only for forming circuit boards by co-firing it with a ceramic substrate at a relatively high temperature but also for forming a wiring circuit on a printed wiring board, via-hole-fillers through such a board and adhesives for mounting electrical parts thereon by mixing and curing the silver ink (silver paste) with various types of resin ingredients, as described in the Patent Document 1. In the latter application, in general, an electrical conductivity has been obtained simply by mutually having the particles of silver powder touched, as a conductive filler without mutually sintering the particles.

However, there have been a demand in recent years for lowering an electrical resistance for conductor portions formed using the silver powder and improving connecting reliability in order to realize lowering of the electrical resistance, and consequently, more demand has occurred for silver ink or silver paste to be formed by sintering a filler itself of the silver powder while resin ingredients are being cured, resulting in that the conductivity is exhibited. It goes without saying that for meeting these demands, size-fining of each of the particles of the silver powder as a conductive filler is necessary in order to lower the sintering temperature.

Conventionally, a wet reduction process where an aqueous solution of a silver/ammine complex using a silver nitrate solution and an aqueous ammonia is produced; and an organic reducing agent is added to the aqueous solution of the complex, as described in the Patent Document 2, has been employed for production of a silver powder, and the obtained silver powder has been made into a silver paste. Additionally, in order to ensure more excellent low-temperature sintering performance than conventional, the silver ink containing silver nanoparticles, as disclosed in the Patent Document 3, has been proposed.

[Patent Document 1]

    • Japanese Patent Laid-Open No. 2001-107101

[Patent Document 2]

    • Japanese Patent Laid-Open No. 2002-334618

[Patent Document 3]

    • Japanese Patent Laid-Open No. 2002-324966

DISCLOSURE OF THE INVENTION

However, in a metal powder such as a silver powder, it is generally said that both the size-refinement of powder particles and the dispersibility meant by that powder particles nearly in a mono-disperse state are hard to become satisfied. For example, in the case of the silver ink containing silver nanoparticles, as disclosed in the above-described Patent Document 1, in order to stabilize a state of the dispersion of the nanoparticles, it is common to add a large amount of dispersant as a protective colloid thereto. In such a case, generally the decomposition temperature of the dispersant is higher than the sintering temperature of the silver nanoparticles, resulting in that the low-temperature sintering performance of the silver nanoparticles themselves cannot be used to the full.

Further, in case of the silver ink containing the silver nanoparticles, because the content of the filler is less than conventional, a thick film is difficult to form while a thin film is easy to form. This makes it difficult to use such silver ink for application to form wiring circuits having a large cross-sectional area sufficient to be usable for power-supply circuit such carries a relatively large amount of current or for application to form a low-resistance circuit. Further, in the use of the silver ink for adhesives for mounting components thereon, not only its conductivity, but also its adhesive strength is required severely. Therefore it is indispensable to add a certain amount or more of resin thereto which exhibits high-adhesive strength when the resin is cured. Thus, there remain many problems that cannot be overcome by conventional silver-nanoparticles-containing silver ink.

In the meantime, it goes without saying that silver powder used for commonly-used silver pastes has had a limitation in terms of its low-temperature sintering performance judging from its particle size. Because in the particle-size of the silver powder obtained by the conventional production processes, the actual condition is that the average particle size between their primary particles DIA is usually more than 0.6 μm, the average particle size D50 measured by a laser diffraction scattering particle size distribution measurement method is more than 1.0 μm, and the degree of agglomeration represented by D50/DIA is more than 1.7 (Note that “DIA” is a particle-size which can be calculated by a particle image from an SEM image analysis.). Thus, the conventional silver powder has been unsuitable when forming recent circuits where fine-pitched wiring patterns are drawn.

Further, in the circuit formation in a case where a silver paste using the conventional silver powder is employed, non-firing type application or low-temperature sintering type one is often used where heating temperature is 300° C. or less. In such circumstance, the use of silver powder having low crystallinity has been considered to be preferable, in order to obtain high sintering performance at a low temperature. However, in order to obtain silver powder of low crystallinity, it is inevitable to employ a reaction system where reduction is rapidly advanced in view of its manufacturing condition. As a result, a silver powder has been merely obtained such that its crystallinity is low, and additionally its agglomeration significantly occurs easily. Thus, in a market, there have been demands for silver powder which has a low-temperature sintering performance, whose particles each is finer than that of particles of any conventional silver powder, and which has an excellent dispersibility such that little agglomerate occurs regarding powder particles.

Also on the other hand, a silver powder has been required to have few impurities. In the production of the silver powder, the above-described wet reduction process has been employed, and the reducing agent, etc. used in the process remains on the surface of the powder particles. Thus, as long as the conventional production process is employed, the problem regarding the impurities is inevitable. And there has occurred another problem that in accompanying with the increase of the amount of impurities on the surface of the silver powder, an electrical resistance of the conductor formed using the silver powder also becomes higher.

Thus, there have been demands in the market for a silver powder which has an excellent low-temperature sintering performance which has not existed until now, whose particles each is very fine and have high dispersibility, and which has little impurities in order to realize a low electrical resistance.

In the light of the above-described problems, the present inventors have made eager research efforts as to any novel process for fining of each of particles of the conventional silver powder particles themselves. However, it is natural that the currently-used technology has some inherent limitations at a level. So then the inventors suppose that when sintering the silver powder particles, an only outer portion of surface of each of silver powder particles may be sufficiently sintered and connected. Further, if so, they suppose that even the above-described conventional silver powder could have a low-temperature sinterable characteristic. Hereinafter, the present invention will be described in terms of two separate points: “silver powder made of silver particles, each to which fine silver particles are adhered” and “process of producing the silver powder”.

<Silver Powder Made of Silver Particles, Each to which Fine Silver Particles Adhere>

The “silver powder made of silver particles whose center part is regarded as a core material, in which silver particles each being finer than the silver particle of the center part adheres to the core material”(i.e., silver powder made of silver particles, each to which fine silver particles adhere) relating to the present invention is in other words expressed by “silver powder made of silver particles each being made adhered to the surface of each of the silver powder particles”. Specifically, the surface of each powder particle of the silver powder 2 as a core material is further coated with much finer silver particles 3, just like the image shown in FIG. 1. Thus, the fine silver particles 3 existing on the surface of each particle of the silver powder 2, each allows the fine silver particles 2 to exhibit a low-temperature sintering performance, independent of the shape and size of the powder particles of the core material, thereby making easier the sintering of the adjacent powder particles of the silver powder made of silver particles 1 made to adhere thereto.

The term “fine silver particles” herein used means silver nanoparticles of nanometer-order particle size which exist only on the surface of each particle of the silver powder 2. As above-mentioned, when using the silver nanoparticles themselves for a silver ink, generally a large amount of dispersant whose decomposition temperature is higher than the sintering temperature of silver nano-particles, is added to stabilize the dispersibility of the nanoparticles. As a result, the characteristic of being sintered at low temperatures which silver nanoparticles themselves have cannot be used to the full. However, having much finer silver particles 3 additionally adhered onto the surface of each powder particle of the silver powder 2 enables the characteristic of being sintered at low temperatures which silver nanoparticles have to be sufficiently used, independent of the size and shape of the powder particles of the silver powder as the core material. Accordingly, even if the shape of the silver powder particles is substantially spherical or the shape is flaky having particle size being several tens μm, the silver powder can be used as a core material.

The silver powder used as the core material may be substantially spherical, flaky or flat, and if the production conditions in conventionally-adopted production process are modified, particle size distribution of sharpness and dispersibility can be ensured, to some degree. Thus, once silver powder is used in a form of the silver powder including silver particles to which fine silver particles adhere, it has excellent handleability and does not require any large amount of protective colloid when formed into a paste, though silver nanoparticles have poor dispersibility in themselves. And a silver paste prepared using such silver powder can contain silver particles by an amount being equivalent to an amount of conventional silver pastes, resulting in making it possible to thickening coating when a wiring pattern of a circuit is drawn.

In the silver powder made of silver particles, each to which fine silver particles adhere as above-described, its sinterable temperature is 170° C. or less and it shows extraordinarily excellent sintering characteristics. Thus, if a silver paste (silver ink) is produced using the the silver powder made of silver particles, each to which fine silver particles adhere, and the wiring patterns of a circuit are drawn using such a silver paste (silver ink), a coating thickness can be obtained which is sufficient to provide a circuit that can flow a large amount of current. And what is more, the ease of sintering the powder particles substantially improves the characteristics of the silver paste (silver ink) as a conductor, such as low electric resistance and continuity reliability.

In the silver powder made of silver particles, each to which fine silver particles adhere according to the present invention, the silver powder is used as a core material. Accordingly, the particle size and dispersibility of the powder particles, each to which fine silver particles adhere have a much more tendency to depend on the silver powder as the core material. In other words, it is preferable to select, as the core material, a silver powder which ensures a particle size distribution and dispersibility at a high level. Also it is more preferable to select silver powder containing few impurities, which would be a hindrance when lowering electric resistance. So, the present inventors have made eager research efforts toward the size-refinement of each of particles of silver powder itself and finally obtained the silver powder having the powder characteristics as below. And the inventors have arrived at an idea that excellent products can be obtained by producing the silver powder made of silver particles, each to which fine silver particles adhere using the above-described silver powder as the core material.

The above-described silver powder basically has the following powder characteristics: a. the average particle size of primary particles DIA obtained by the analysis of the images of scanning electron microscope is 0.6 μm or less; b. the degree of agglomeration represented by D50/DIA, where DIA is the above-described average particle size of primary particles and D50 is the average particle size obtained by a laser diffraction/scattering particle size distribution measurement method, is 1.5 or less; and c. the crystallite size is 10 nm or less. And another type of silver powder has not only the above-described powder characteristics a to c, but the following powder characteristic: d. the content of organic purities is 0.25% by weight in terms of amount of carbon. These two types of silver powder are different in content of purities due to their different production conditions. The silver powder having such powder characteristics has dispersibility at a level which cannot be obtained by conventional production processes. In the following, the powder characteristics of the silver powder used as the core material will be described.

The characteristic of a is that the average particle size of primary particle DIA obtained by the analysis of the image of a scanning electron microscope (SEM) is 0.6 μm or less. The term “the average particle size of primary particles DIA obtained by the analysis of the images of scanning electron microscope” is the average particle size of silver powder obtained by analyzing the images observed with a scanning electron microscope (SEM) (preferably the fine silver powder used in the present invention is observed at ×10,000 magnification while conventional silver powder at ×3,000 to ×5,000 magnification). In the image analysis of fine silver powder observed with a SEM, round particle size analysis is conducted by using IP-1000 PC of Asahi Engineering Co., Ltd. and setting the roundness threshold value and the overlap degree at 10 and 20, respectively to obtain the average particle size DIA. The average particle size DIA obtained by analyzing the images of observed fine silver powder will represent a reliable value of the average particle size of primary particles, because it is obtained directly from the observed SEM images. Almost all the DIA values of the fine silver powder used in the present invention fall in the range of 0.01 μm to 0.6 μm as far as the inventors observe; however in reality, silver powder having much smaller particle sizes may also be confirmed. That is why the minimum value is not shown clearly.

The characteristic of b is described by the “degree of agglomeration”, which is an index of the dispersibility of particles, because the fine silver powder used as a core material in the present invention shows excellent dispersibility which conventional silver powder has never had. The “degree of agglomeration” herein used means the value represented by D50/DIA, where DIA is the above-described average particle size of primary particles and D50 is the average particle size obtained by a laser diffraction scattering particle size distribution measurement method. The D50 means that the particle size at volume accumulation of 50% obtained by laser diffraction scattering particle size distribution measurement method and its value is calculated not by directly observing each particle size, but by considering an agglomerated powder particle as a single particle (an agglomerate). This is because, in reality, the particles of silver powder do not lie in the monodisperse state, where particles are completely separated from each other, but typically in state where a plurality of particles agglomerate. However, the less, powder particles agglomerate and the more, they lie closely in the monodisperse state, the smaller the average particle size D50 becomes. The D50 of the fine silver powder used in the present invention is in the range of 0.25 μm to 0.80 μm, which fine silver powder produced by a conventional production process has never had. In the laser diffraction scattering particle size distribution measurement method herein described, the average particle size of fine silver powder is measured with a laser diffraction scattering particle size distribution meter, Micro Trac HRA Model 9320-X100 (by Leeds+Northrup) after mixing 0.1 g of fine silver powder made of ion-exchanged water and dispersing the silver powder made of an ultrasonic homogenizer (Nippon Seiki Co., Ltd.) for five minutes.

On the other hand, “the average particle size of primary particles DIA obtained by the analysis of the images of scanning electron microscope” is the average particle size of silver powder obtained by analyzing the images observed with a scanning electron microscope (SEM), it represents a reliable value of the average particle size of primary particles obtained without considering over the particles in the agglomerated state.

Thus, the inventors take the value calculated by D50/DIA, where DIA is the average particle size obtained by image analysis and D50 is the average particle size obtained by laser diffraction scattering particle size distribution measurement method, as the degree of agglomeration. Specifically, according to the above-described theory, the value D50, which reflects the existence of agglomerates, should be larger than the value DIA, assuming that the values D50 and DIA can be measured with the same accuracy in the fine silver powder of the same lot. And the value D50 becomes infinitely close to the value DIA with decreasing agglomerates of the fine silver powder particles; as a result, the value D50/DIA, the degree of agglomeration, becomes infinitely close to 1. At a stage where the degree of agglomeration becomes 1, it is said that the silver powder is monodisperse powder where no agglomerate of powder particles exists.

Then, the inventors examined the correlation between the degree of agglomeration of the fine silver powder has and the viscosity of the paste produced using the fine silver powder as well as the correlation between the degree of agglomeration and the smoothness of the conductor obtained by sintering the paste, with fine silver powder having different degrees of agglomeration. The examination revealed that there is very excellent correlation between the above factors. This indicates that viscosity of a paste produced using fine silver powder can be freely controlled by controlling the degree of agglomeration the fine silver powder has. The examination also revealed that if the degree of agglomeration is kept 1.5 or less, then the fluctuations in the viscosity of the paste of the fine silver powder and in the smoothness of the conductor obtained by sintering the silver powder can also be kept in a very narrow range. It can also be said that the less, the agglomerates of fine silver powder, the more, the film density of the conductor obtained by sintering the fine silver powder is improved, thereby making it possible to lower an electrical resistance of conductor formed by sintering the silver powder.

However, in reality, the calculated value of the agglomeration degree can sometimes be less than 1. This can be possibly because the calculation is made under the assumption that the value DIA used for the calculation of the agglomeration degree is the value of a true sphere. In theory, the degree of agglomeration cannot be 1, but in reality fine silver particles are not true spheres, thereby yielding a calculated value which is less than 1.

The characteristic of c is that the crystallite size is 10 nm or less. The crystallite size and the sinterable temperature mutually have very closely relationship. Specifically, comparing different types of silver powder having the substantially same average particle size, those having smaller crystallite size are more capable of being sintered at low temperatures. Thus, fine silver powder having large surface energy due to its fineness and having a crystallite size being 10 nm or less, just like the fine silver powder according to the present invention, makes it possible to lower the sinterable temperature of the fine silver powder itself, as the core material. The reason why the lower limit of the crystallite size is not provided here, is that certain measuring errors can be produced depending on the measuring device, measuring conditions or the like. In addition, it is difficult to require extreme reliability of measured values in the range where the crystallite size is less than 10 nm. The lower limit dares to be set, the inventors would say, based on their study, that the lower limit would be about 2 nm.

The characteristic of d is that the content of organic impurities in the silver powder is 0.25% or less by weight in terms of amount of carbon. Here the carbon content is used as an index of the content of organic impurities and as a measure of the amount of impurities adhering to the fine silver powder particles. The carbon content was measured with EMIA-320V manufactured by Horiba, Ltd. by a combustion-infrared absorption method in such a manner as to mix 0.5 g of fine silver powder, 1.5 g of tungsten powder 1.5 g and 0.3 g of tin powder and thereafter put the mixture into a porcelain crucible. The carbon content in silver powder obtained by a conventional production process is more than 0.25% by weight even if washing the silver powder is reinforced, too much.

The fine silver powder according to the present invention is directed to a fine silver powder that conventional production process has never produced, in terms of having its powder characteristics: a to c; or a to d. In view of characteristics of sintering temperature, the fine silver powder used as the core material in the present invention is capable of starting a sintering process at as low temperatures as 240° C. or less. The term “sinterable temperature” herein used means the lowest temperature at which a circuit can be drawn on an alumina substrate with a silver paste prepared using silver powder can undergo sintering by a resistor-measurable extend. Although the lower limit of the sinterable temperature has not been particularly specified, either, considering the studies by the present inventors and the common knowledge in this art, it is almost impossible for the fine silver powder, as the core material, itself to be sintered at temperatures lower than 170° C. Thus, 170° C. can be taken as the lower limit of the sinterable temperature.

The effect of the above-described powder characteristics the fine silver powder according to the present invention has is to increase the tap density of the fine silver powder to as high as 4.0 g/cm3. The “tap density” herein used means the density determined in such a manner as to weigh 200 g of fine silver powder accurately, put the weighed powder into a 150-cm3 measuring cylinder, tap the measuring cylinder by repeating the 40 mm-stroke dropping of the cylinder by 1,000 times, and measure the volume of the fine silver powder. Powder has a high tap density when it has a theoretically fine particle size and is in the highly dispersed state where its particles are not agglomerated. Considering the tap density of conventional silver powder is less than 4.0 g/cm3, the above-described tap density value has proved that the fine silver powder according to the present invention is very fine and excels in dispersibility.

The aforementioned fine silver powder, which is used as the core material, is a very fine powder and excels in dispersibility, thereby having low-temperature sintering performance allowing the sinterable temperature of the fine silver powder itself to be 240° C. or lower. And silver powder made of silver particles made to adhere thereto, which is very fine and excels in dispersibility, compared with conventional silver powder, and has a low-temperature sintering performance that allows the sinterable temperature to be 170° C. or lower, is produced by making silver nanoparticles adhere to the surface of the particles of the above fine silver powder. A ratio of a size of each of the silver nanoparticles to a size of the core material is substantially 1/5 to 1/100. The ratio thereof is not limited especially. Namely, the ratio is changed corresponding to various conditions such as pH, temperature, rate of reaction, and the like regarding as used solutions.

<Process for Producing Silver Powder Made of Silver Particles Each to which Fine Silver Particles Adhere>

In the following the process for producing silver powder made of silver particles each to which fine silver particles adhere according to the invention will be described in terms of two major types: production process 1 and production process 2. The fine silver powder suitably used in the production processes 1 and 2 will be described separately in the section “Process for producing fine silver powder”.

Production Process 1:

This production process is “a process for producing silver powder made of silver particles each to which fine silver particles adhere, characterized by bringing silver powder into contact with a solution containing a silver complex which is obtained by mixing silver nitrate and a complexing agent and dissolving the mixture during stirring; and adding a reducing agent to the resultant solution to allow fine silver particles to be precipitated on the surface of each silver powder particle”.

When making the silver powder into the above slurry, the amount of silver powder contained is not particularly limited. However, unless the amount of silver powder in the slurry is defined, the amount of chemicals used in the production cannot be clearly specified. Thus, the production process 1 will be described as a process for producing silver powder made of silver particles each to which fine silver particles adhere, wherein silver nanoparticles are made to adhere to the surface of each silver powder particle in a slurry of 50 g of silver powder dispersed in 1 liter of deionized water. The production process is based on the assumption that the average particle size of primary particles DIA of the silver powder used, which is obtained by the image analysis of the scanning electron microscope, is 1 μm or less.

First, the “solution containing a silver complex which is obtained by mixing silver nitrate and a complexing agent and dissolving the mixture while stirring” will be described. To treat the above-described amount of silver powder, 8 g to 26 g of silver nitrate is used. If the amount of silver nitrate used is less than 8 g, a practically sufficient rate of coating with fine silver particles cannot be obtained, whereas even if the amount of silver nitrate used is more than 26 g, the coating rate cannot be improved. The complexing agent used is a sulfite or an ammonium salt. When using potassium sulfite, the amount used is in the range of 50 g to 150 g. If the amount of potassium sulfite added is less than 50 mg, the complexation of silver does not fully progress, and therefore, a complete silver complex cannot be formed. On the other hand, more than 150 mg of potassium sulfite well exceeds a sufficient amount for forming a silver complex, and adding the excess amount of potassium sulfite does not accelerate the complexiation, resulting in being not economical. The solution containing a silver complex is obtained by dissolving the above-described amount of silver nitrate in 1 liter of deionized water, adding a complexing agent to the solution and fully stirring the mixed solution.

Then, 50 g of silver powder is added to the solution containing a silver complex obtained as above and the slurry of silver powder is fully stirred. A reducing agent is then added to the slurry to cause a reduction reaction, so that fine silver powder of nano-order particle size is allowed to be precipitates uniformly on the surface of each silver powder particle. The reducing agent used here is hydrazine, DMAB, SBH, formalin or hypophosphoric acid. When using hydrazine, 5 g to 50 g of hydrazine is dissolved in 200 ml or less (including 0 ml) of deionized water and the prepared solution is added within 60 minutes (including the case where the solution is added for a very short time). In the present description, it is noted that “an operation that solution is added for a very short time” is meant by an operation of establishing a chemical reaction between different solutions as rapid as possible. If the amount of hydrazine added is less than 5 g, the reduction does not sufficiently progress, and the surface of each of silver powder particles of the silver powder cannot be coated uniformly with fine silver powder. The addition of more than 50 g of hydrazine does not especially accelerate the reduction reaction and is merely uneconomical.

The solution temperature during the reduction reaction lies in the range of a room temperature to 45° C. If the solution temperature is higher than 45° C., the reduction reaction progresses so rapidly that the precipitation of fine silver powder on the surface of each silver powder particle is likely to be non-uniform, resulting in inferior particle size distribution of the resultant silver powder made of silver particles each to which fine silver particles adhere. Preferably, the time the addition of a reducing agent takes is about 5 minutes to 40 minutes in the above-described reducing agent concentration range. If the reaction time is shorter than 5 minutes, the produced powder particles tend to be agglomerated more strongly, whereas if for the addition of a reducing agent, it takes not shorter than 40 minutes, a satisfactorily uniform coating can be obtained.

After allowing fine silver powder to precipitate on the surface of each silver powder particle through reduction reaction in the above-described manner, the silver powder is separated through a filter, washed, dehydrated, and dried to yield silver powder made of silver particles each to which fine silver particles adhere according to the present invention. The separation through a filter, washing, dehydration, and drying may be carried out by various procedures, and the procedure and conditions employed are not limited to any specific ones.

Production Process 2:

This production process 2 is “a process for producing silver powder made of silver particles each to which fine silver particles adhere, characterized by including: adding silver nitrate and a neutralizing agent into a slurry of silver powder in a dispersing medium; dissolving the slurry mixture while stirring to allow fine silver oxide particles to be precipitated on the surface of each silver powder particle; washing the resultant silver powder; and exposing the silver powder made of silver oxide particles on its surface to UV rays to reduce the fine silver oxide particles to fine silver particles”.

In the above slurry of silver powder, the amount of silver powder contained is not particularly limited. However, unless the amount of silver powder in the slurry is not clearly defined, the amount of chemicals used in the production cannot be specified. Thus, the production process 2 will be described as a process for producing silver powder made of silver particles each to which fine silver particles adhere, wherein silver nanoparticles are made to be adhered to the surface of each of silver powder particles in a slurry of 50 g of silver powder dispersed in 1,500 g of dispersion medium. The production process is based on the assumption that the average particle size of primary particles DIA of the silver powder used, which is obtained by the image analysis of the scanning electron microscope, is 1 μm or less.

First, the “slurry of silver powder added to a dispersing medium” will be described. The dispersion medium used here is ethylene glycol (including monoethylene glycol, diethylene glycol and triethylene glycol), butanediol (including 1,4-butanediol, 1,2-butanediol and 2,3-butanediol) or glycerin. Here, the case where ethylene glycol is used as the dispersion medium will be described. Accordingly, the slurry of the silver powder is prepared by adding 50 g of silver powder to 1,500 g of ethylene glycol and stirring the mixture.

Silver nitrate and a neutralizing agent are added to the slurry of silver powder obtained as above and dissolved while stirring to allow fine silver oxide particles to be precipitated on the surface of each silver powder particle. Preferably, the silver nitrate and the neutralizing agent added are in the form of a solution of silver nitrate or neutralizing agent. The reason is that to do so prevents the maldistribution of chemicals in the slurry of silver powder and allows a neutralization reaction to uniformly occur in the slurry of silver powder. Preferably used is an aqueous solution of silver nitrate prepared by dissolving 2.50 g to 33.34 g of silver nitrate in 500 g of deionized water (equivalent to silver nitrate concentration of 3% by weight to 40% by weight). If the amount of silver nitrate added is less than 2.50 g, a sufficient amount of silver oxide to coat the surface of each particle of the above-described silver powder uniformly will not be precipitated. Even if the amount of silver nitrate is more than 33.34 g, the amount of silver oxide adhering to the surface of each silver powder particle does not change very much, and rather resulting in bringing about inferior particle size distribution and dispersibility of the powder particles.

The neutralizing agent may be an alkali metal salt such as sodium hydroxide and potassium hydroxide. It goes without saying that the amount of the neutralizing agent added depends on the amount of silver nitrate to be neutralized. Assuming that sodium hydroxide is used, the amount is selected from those in the range of 0.588 g to 7.840 g to corresponding amount of silver nitrate. Sodium hydroxide is also preferably used in the form of an aqueous solution. Thus, a solution of sodium hydroxide in 500 g of deionized water is used.

After allowing fine silver oxide particles to be adhered to the surface of each silver powder particle in the above-described manner, the fine silver oxide particles are washed. This washing has to be carried out to remove the solution having been used for the neutralizing reaction and also fully remove water. Accordingly, it is most preferable to employ washing in water and washing in alcohol in combination. In order to wash the silver powder made of fine silver oxide particles adhering thereto obtained under the above-described conditions, at least 500 g or more of water or the largest possible amount of water is used. It is preferably to dehydrate the washed silver powder. This is done to remove impurities as much as possible. In order to ensure that water is removed, washing in alcohol is carried out. For the washing in alcohol, ethyl alcohol, methyl alcohol or isopropyl alcohol may be used. The amount of alcohol used is not particularly limited, as long as the amount used is sufficient to remove water.

After completion of the washing, the silver powder made of fine silver oxide particles adhering thereto is immediately exposed to UV rays without being dried. Silver powder made of silver particles adhering thereto can be obtained by reducing the fine silver oxide particles on the surface of each silver powder particle to fine silver particles. Exposure to UV rays accelerates the conversion of the fine silver oxide particles to fine silver particles and prevents non-uniform reduction from occurring. For UV rays used, their wavelengths are not strictly limited and, for example, UV light used for sterilization can be used. After completion of the exposure to UV rays, the silver powder is fully dried to produce silver powder made of silver particles each to which fine silver particles adhere according to the present invention.

Process for Producing Silver Powder Preferably Used as Core Material:

A process will be described for producing silver powder (of nearly spherical shape) suitably used as the core material for the silver powder made of silver particles each to which finer silver particles adhere according to the present invention. The production process described here is for producing silver powder having the above-described powder characteristics: “a. the average particle size of primary particles DIA obtained by the image analysis of the scanning electron microscope is 0.6 μm or less”; “b. the degree of agglomeration represented by D50/DIA, where DIA is the above-described average particle size of primary particles and D50 is the average particle size obtained by laser diffraction scattering particle size distribution measurement method, is 1.5 or less”; and “c. the crystallite size is 10 nm or less.” Accordingly, the silver powder described here can be said to a fully-fine powder in comparison with conventional silver powder produced using an aqueous solution of silver nitrate.

The process for producing the silver powder is a process including: preparing an aqueous solution of a silver complex by mixing and reacting an aqueous solution of silver nitrate and a complexing agent; making an organic reducing agent contact with the above aqueous solution for reaction to allow silver particles to be precipitated by reduction; filtering the precipitate-containing solution; washing the resultant silver powder; and drying the same, characterized in that the reducing agent, silver nitrate and the complexing agent are added in such amounts that allow each of the above chemicals to be more dilute after the addition. It is conventionally common that a solution of a reducing agent and an aqueous solution of a silver complex are mixed for a very short time in a bath and the concentration of silver is generally kept 10 g/l or more. Thus, silver nitrate, a reducing agent and a complexing agent have needed to be added in large amounts to ensure productivity which balances the scale of facilities. In the following, the production process will be described more specifically, taking an example of the process in which aqueous ammonia is used as a complexing agent.

The most important characteristic of the production process according to the present invention is in that after the contact reaction of an aqueous solution of an ammine complex and an organic reducing agent, the concentration of the organic reducing agent is low, and therefore, the amount of the organic reducing agent remaining adsorbed on the surface of the produced silver powder particle or the amount of the organic reducing agent taken in the inside of each of the powder particles during their growing process can be reduced. Accordingly, in the solution produced by mixing a solution of a reducing agent and an aqueous solution of a silver complex, it is most preferable to keep the concentration of the organic reducing agent be 1 g/l to 3 g/l, while keeping the concentration of silver be 1 g/l to 6 g/l.

The concentration of silver has a proportional relationship with relative to the amount of the reducing agent, and it goes without saying that the higher the concentration of silver becomes, the larger amount of silver powder can be obtained. However, if the concentration of silver exceeds 6 g/l, precipitating silver particles tend to be coarser, and the resultant silver powder has a particle size not different from that of conventional silver powder. Thus, fine silver powder having an excellent dispersibility, at which the present invention aims, cannot be obtained. On the other hand, if the concentration of silver is less than 1 g/l, very fine silver powder is certainly obtained; however, too fine silver powder absorb a larger amount of oil and causes the viscosity of its paste to be increased. This in turn requires a larger amount of vehicle to be added, and in the sintered conductor formed as a final product, its film density is low and its electrical resistance tends to be increased. And besides, industrial productivity required cannot be satisfied.

To obtain fine silver powder of the present invention in a good yield, an optimum requirement condition is to keep the concentration of the organic reducing agent be 1 g/l to 3 g/l, while keeping the concentration of silver be 1 g/l to 6 g/l. The concentration of the organic reducing agent in the range of 1 g/l to 3 g/l is selected as an optimum range for obtaining fine silver powder in the relationship with the silver concentration of an aqueous solution of a silver/ammine complex. If the concentration of the organic reducing agent exceeds 3 g/l, the amount of the reducing solution added to the aqueous solution of a silver/ammine complex is certainly decreased; however, the agglomeration of the powder particles of the silver powder precipitated through reduction starts to progress significantly and the amount of impurities (herein the amount of impurities is taken as the carbon content) contained in the powder particles starts to be increased rapidly. If the concentration of the organic reducing agent is less than 1 g/l, the total amount of the reducing solution used increases, thereby increasing the amount of a waste water to be treated. This does not meet the industrial economy.

The term “organic reducing agent” herein used means hydroquinone, ascorbic acid, glucose or the like. Of these organic reducing agents, hydroquinone is preferably selectively used in the present invention. Hydroquinone excels in reactivity compared with other organic reducing agents in the present invention and can bring about an optimum reaction rate for obtaining silver powder of small crystallite size and low crystallizability.

Other additives can also be used in combination with the above-described organic reducing agent. The term “additives” herein used means glues such as gelatin, amine-group polymers, celluloses or the like. Additives are desirably selected which stabilize the process of the precipitation of silver powder through reduction, and at the time, perform a certain function as a dispersant. Any proper additive can be selectively used depending on the type of organic reducing agent, process or the like.

In the process for contact reacting the aqueous solution of a silver/ammine complex and the reducing agent obtained as above to precipitate fine silver powder through reduction, it is desirable to use a process in the present invention in which, as shown in FIG. 2, a certain pass through which an aqueous solution S1 of a silver/ammine complex is flowed through (hereinbefore and hereinafter referred to as “first pass”) and a second pass b which joins the first pass a midway along the pass are provided, an organic reducing agent and optionally additives S2 are flowed into the first pass a through the second pass b so that S1, the organic reducing agent and optionally S2 are contact mixed at the juncture m of the first pass a and the second pass b to precipitate silver particles through reduction (hereinafter referred to as “joining mixing process”).

Employing such joining mixing process makes it possible to complete the mixing of the two solutions for a shortest-mixing time and allow the reaction to progress while keeping the reaction system uniform, which leads to the formation of uniformly shaped powder particles. Further, if the solution after the mixing contains a decreased amount of organic reducing agent, the amount of the organic reducing agent remaining adsorbed on the surface of each particle of the fine silver particle precipitated through reduction is also decreased. This makes it possible to decrease the amount of impurities attached on the fine silver powder obtained through flitration and drying. The decrease in the amount of impurities attached on the fine silver powder in turn makes it possible to lower an electric resistance of the sintered conductor formed of a silver paste which uses the silver powder.

Further, when obtaining an aqueous solution of a silver/ammine complex by contact-reacting an aqueous solution of silver nitrate and aqueous ammonia, it is desirable to use an aqueous solution of silver nitrate whose silver nitrate concentration is 2.6 g/l to 48 g/l and yield an aqueous solution of a silver/ammine complex whose silver concentration is 2 g/l to 12 g/l. Specifying the concentration of an aqueous solution of silver nitrate is, in other words, specifying the amount of the aqueous solution of silver nitrate. Considering over the silver concentration of an aqueous solution of a silver/ammine complex being kept 2 g/l to 12 g/l, the concentration and amount of aqueous ammonia to be added thereto are inevitably determined. Although the technological reasons have not been clarified yet at present, use of an aqueous solution of silver nitrate whose silver nitrate concentration is 2.6 g/l to 48 g/l makes it possible to obtain a fine silver powder having the best production stability and stable quality.

The resultant fine silver powder is then washed and dried to yield silver powder as a core material. The washing may be carried out using washing in water and washing in alcohol in combination or using washing in alcohol alone. Washing process is not particularly limited. The drying may also be carried out by any suitable process.

To provide the fine silver powder not only with the above-described characteristics a to c, which are obtained by the above-described production process, but also with the characteristic d. the content of organic impurities is 0.25% by weight or less in terms of amount of carbon”, the washing process must be changed. In the following, the washing process will be described.

To decrease the amount of impurities contained in the fine silver powder, washing carried out at the final stage is very important. The washing may be carried out either by using washing in water and washing in alcohol in combination or by using washing in alcohol alone; however, the washing is reinforced in washing in alcohol. Thus, washing is carried out for 40 g of fine silver powder by using about 100 ml of deionized water and then using about 50 ml of alcohol. However, in the present invention, when carrying out washing in alcohol, 200 ml or more of alcohol is used for 40 g of fine silver powder, in other words, 1 kg of fine silver powder is washed using an excessive amount of alcohol, that is, 5 L or more of alcohol.

An amount of impurities by reinforcement of washing can be decreased just because, in the contact reaction between an aqueous solution of a silver/ammine complex and a reducing agent, the present invention employs a technique for keeping the amount of organic reducing agent remaining in the solution after mixing small by employing a low-concentration-reaction system of low concentration of organic reducing agent.

The silver powder made of silver particles each to which fine silver particles adhere according to the present invention has a low-temperature sintering performance at a level which conventional silver powder has never had, because it is constructed by making to adhere fine silver particles (silver nanoparticles) to the surface of each of the silver powder particles of the silver powder. Further, it can have a particularly excellent low-temperature sintering performance, because the silver powder is very fine, excellent in dispersibility and has few impurities, which conventional silver powder has never had, as its core material.

In the meantime, the process for producing silver powder made of silver particles each to which fine silver particles adhere according to the present invention is excellent in running stability through the production process, and therefore, it can produce silver powder made of silver particles to each which fine silver particles adhere very effectively. The process can also produce silver powder used as the core material effectively, because it employs a process for producing silver powder using the above-described dilute solution.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of a powder particle of silver powder made of silver particles each to which fine silver particles adhere; and

FIG. 2 is a view showing the concept of mixing of an aqueous solution of a silver/ammine complex and a reducing agent.

BEST MODE FOR CARRYING OUT THE INVENTION

In the following, the preferred embodiment of the present invention will be described in detail, comparing with Comparative Examples. In the following examples, silver powder used as a core material was produced first. Then, silver powder made of silver particles each to which fine silver particles adhere, a silver paste using the above silver powder made of silver particles each to which fine silver particles adhere, and test circuits using the silver paste were produced. The specific resistance and sinterable temperature were measured for the produced test circuits.

EXAMPLE 1

Process for Producing Silver Powder Used as a Core Material:

In this example, first silver powder (whose particle is spherical) used as a core material was produced. The production process was as follows.

First, 63.3 g of silver nitrate was dissolved in 9.7 liter of deionized water to prepare an aqueous solution of silver nitrate. Then, 235 ml of 25% by weight aqueous ammonia was added for a very short time to the aqueous solution of silver nitrate and stirred to give an aqueous solution of a silver ammine complex.

The aqueous solution of a silver ammine complex was introduced into a first pass a having an inner diameter 13 mm, shown in FIG. 2, at a flow rate of 1500 ml/sec and a reducing agent was allowed to flow through a second pass b at a flow rate of 1500 ml/sec so that the solution and the agent came into contact with each other at a juncture m while being kept at 20° C. to precipitate fine silver powder through reduction. The reduction agent used here was a solution of 21 g of hydroquinone in 10 liter of deionized water. Accordingly, the concentration of hydroquinone at the time of completion of the mixture was about 1.04 g/l, which is very low concentration.

To superlatively pick up 40 g of the resultant fine silver powder, the solution was filtered using a nutsche, and the separated fine silver powder was washed with 100 ml of water and 600 ml of methanol and dried at 70° C. for 5 hours to yield fine silver powder.

The powder characteristics of the silver powder obtained as above correspond to those of Comparative Example 1, which are shown in Table 1 together with those of the other examples and Comparative Examples. Now the term “sinterable temperature” herein used will be described, because its contents, such as measuring method, has not been clearly defined by the description so far. The term “sinterable temperature” shown in Table 1 means that the lowest temperature at which a silver paste is produced using each of the silver powder and each having been used for drawing a wiring pattern of a circuit on an alumina substrate can be sintered into products to such an extend that the electrical resistance of the sintered products is measurable. The sintering temperature was selected from the temperatures in the rage of 150 to 250° C. Then, specific resistance was measured for the circuits 1 mm width pattern-circuit which was obtained by sintering the silver pastes. To judge whether the sintering was done well or not, the sintered state was also observed with a scanning electron microscope. The composition of silver pastes was: 85 wt % of fine silver powder and 15 wt % of terpineol. FIB analysis was used for measuring the size of the precipitated crystal grains to determine the crystallite size.

Production of Silver Powder Made of Silver Particles Each to which Fine Silver Particles Adhere:

Silver powder made of silver particles each to which fine particles adhere was produced in accordance with the above-described production process 1 using the silver powder, as a core material obtained as above.

First, “a solution containing a silver complex which is obtained by mixing silver nitrate and a complexing agent and dissolving the mixture while stirring” was prepared so as to make fine silver particles adhere onto the surface of each particle of 50 g of the above-described silver powder. The solution containing a silver complex was prepared by first dissolving 17 g of silver nitrate in 1 liter of deionized water and then adding 86 g of potassium sulfite, as a complexing agent, to the solution.

Then, 50 g of the above-described silver powder was added to the solution containing a silver complex obtained as above and stirred for 1 minute. A reducing agent was added to the above silver powder-containing solution so that a reduction reaction was caused to precipitate fine silver powder of nano-order particle size uniformly. The reduction reaction was carried out by adding a solution of 10 g of hydrazine in 90 ml of deionized water, as a reducing agent, for a very short time at a solution temperature of 40° C. for 10 minutes. After fine silver particles were precipitated by reduction on the surface of each powder particle through reduction in the above-described manner, the precipitate was separated through a filter, washed, dehydrated and dried to yield silver powder made of silver particles each to which fine silver particles adhere according to the present invention.

The powder characteristics of the silver powder made of silver particles each to which fine silver particles adhere obtained as above were determined in the same manner as in the case of the silver powder used as a core material. Then, a silver paste was prepared using the above silver powder and a test circuit was formed using the silver paste. Then, the specific resistance and sinterable temperature were measured for the formed test circuit. The resultant characteristics are shown in Table 1 as those of Example 1.

EXAMPLE 2

Process for Producing Silver Powder Used as a Core Material:

In this example, first silver powder (which is substantially spherical) used as a core material was produced. The production conditions were as described below.

Silver powder was produced under production conditions different from those of Example 1 and the powder characteristics of the resultant silver powder were determined. Then, a silver paste was prepared using the above silver powder and a test circuit was formed using the silver paste. Then, the specific resistance of the conductor and sinterable temperature were measured for the formed test circuit.

First, 63.3 g of silver nitrate was dissolved in 3.1 liter of deionized water to prepare an aqueous solution of silver nitrate. Then, 235 ml of 25% by weight aqueous ammonia was added for a very short time to the aqueous solution of silver nitrate and stirred to give an aqueous solution of a silver ammine complex.

The aqueous solution of a silver ammine complex was introduced into a first pass a having an inner diameter of 13 mm, shown in FIG. 2, at a flow rate of 1500 ml/sec and a reducing agent was allowed to flow through a second pass b at a flow rate of 1500 ml/sec so that the solution and the agent came into contact with each other at a juncture m while being kept at 20° C. to precipitate fine silver powder through reduction. The reduction agent used here was a solution of 21 g of hydroquinone in 3.4 liter of deionized water. Accordingly, the concentration of hydroquinone at the time of completion of the mixture was as low as about 3.0 g/l, which is very low concentration.

To superlatively pick up 40 g of the resultant fine silver powder in the same manner as in Example 1, the solution was filtered using a nutsche, and the separated silver powder was washed with 100 ml of water and a large volume of, that is, 600 ml of methanol and dried at 70° C. for 5 hours to yield fine silver powder as a core material. The powder characteristics of the silver powder obtained as above correspond to those of Comparative Example 2, which are shown in Table 1 together with those of the other examples and Comparative Examples.

Production of Silver Powder Made of Silver Particles Each to which Fine Silver Particles Adhere:

Silver powder made of silver particles each to which fine silver particles adhere was produced in accordance with the above-described production process 2 using the silver powder obtained as above as a core material. First, a slurry of silver powder was prepared by adding 50 g of the above silver powder to 1500 g of ethylene glycol, as a dispersion medium, and fully stirring the mixture to disperse the silver powder in the medium.

Then, silver nitrate and a neutralizing agent were added to the resultant slurry of silver powder and dissolved by stirring to precipitate fine silver oxide particles on the surface of each silver powder particle. Silver nitrate was added first using an aqueous solution of 16.67 g of silver nitrate in 500 g of deionized water (equivalent to 30 wt % silver nitrate concentration). Then a neutralizing agent was added and fully stirred. The neutralizing agent used was a solution of 3.92 g of sodium hydroxide in 500 g of deionized water. Fine silver oxide particles were made to adhere to the surface of each silver powder particle in this manner.

The silver powder made of fine silver oxide particles adhering thereto was then separated by filtration and washed. The washing was carried out using washing in water and washing in alcohol in combination. First washing in water was performed. The silver powder made of fine silver oxide particles adhering thereto obtained under the above-described conditions was washed in 500 g of water to remove impurities on the powder as much as possible and dehydrated. Then, to ensure that water is removed, the powder was washed in 500 g of isopropyl alcohol.

Immediately after completion of the washing, the silver powder made of fine silver oxide particles adhering thereto was exposed to UV rays without being dried to reduce the fine silver oxide particles on the surface of each silver powder particle to fine silver particles. The exposure to UV rays was conducted using CL15-A by Toshiba Corporation, which is usually used as a bactericidal lamp, for 3 hours so as to accelerate the rapid conversion of the fine silver oxide particles to fine silver particles and prevent the occurrence of non-uniform reduction. After that, drying was carried out by conventional procedure to yield silver powder made of silver particles each to which fine silver particles adhere according to the present invention on which little impurities were attached.

The powder characteristics of the silver powder made of silver particles each to which fine silver particles adhere obtained as above were determined in the same manner as in the case of the silver powder used as a core material. Then, a silver paste was prepared using the above silver powder and a test circuit was formed using the silver pastes. Then the specific resistance and sinterable temperature were measured for the formed test circuit. The resultant characteristics are shown in Table 2 as those of Example 2.

EXAMPLE 3

Process for Producing Silver Powder Used as a Core Material:

In this example, first silver powder (of nearly spherical shape) having a large crystallite size was produced using the process shown below, and the powder characteristics of the resultant silver powder were determined. Then, a silver paste was prepared using the above silver powder and a test circuit was formed using the silver paste. Then the specific resistance and sinterable temperature were measured for the formed test circuit.

First, 20 g of polyvinyl pyrrolidone was dissolved in 260 ml of deionized water and 50 g of silver nitrate was dissolved to prepare an aqueous solution of silver nitrate. Then, 25 g of nitric acid was added for a very short time to the above solution and stirred to yield a silver-containing nitric acid solution. At the time of completion of the mixing, the concentration of ascorbic acid was about 36.0 g/l.

A reducing solution was prepared by adding and dissolving 35.8 g of ascorbic acid, as a reducing agent, in 500 ml of deionized water.

The silver-containing nitric acid solution was put into a reaction bath and then the above reducing solution was also added for a very short time to the reaction bath. Silver powder was precipitated through reduction by stirring the mixed solution, while keeping the solution temperature 25° C., to cause a reaction.

The resultant fine silver powder was separated by filtration using a nutsche, and the separated silver powder was washed with 100 ml of water and 500 ml of methanol and dried at 70° C. for 5 hours to yield silver powder as a core material. The powder characteristics of the silver powder obtained are shown in Table 1 as those of Comparative Example 3.

Production of Silver Powder Made of Silver Particles Each to which Fine Silver Particles Adhere:

Silver powder made of silver particles each to which fine silver particles adhere was produced in the same manner as in the above-described Example 2 using the silver powder obtained above as a core material. To avoid the repetition, the detailed description of the production process will be omitted here.

The powder characteristics of the resultant silver powder made of silver particles each to which fine silver particles adhere were determined in the same manner as in the case of the silver powder used as a core material. A silver paste was produced using the silver powder made of silver particles each to which fine silver particles adhere and a test circuit was formed using the silver paste. The specific resistance and sinterable temperature were measured for the test circuit. The results are shown in Table 1 as those of Example 3.

EXAMPLE 4

Process for Producing Flake Silver Powder Used as a Core Material:

In this example, flake-shaped silver powder was produced by machining silver powder which is substantially spherical, and the resultant silver powder was used as a core material. The powder characteristics of the flake silver powder are shown in Table 2 as those of Comparative Example 4.

Production of Flake Silver Powder Made of Silver Particles Each to which Fine Silver Particles Adhere:

Flake silver powder made of silver particles each to which fine silver particles adhere was produced in the same manner as in the above-described production process 1 using the flake silver powder obtained above as a core material. The production conditions employed were the same as those of Example 1. To avoid the repetition, the detailed description of the production conditions will be omitted here.

The powder characteristics of the resultant flake silver powder made of silver particles each to which fine silver particles adhere were determined in the same manner as in the case of the flake silver powder used as a core material. A silver paste was produced using the silver powder made of silver particles each to which fine silver particles adhere and a test circuit was formed using the silver paste. The specific resistance and sinterable temperature were measured for the test circuit. The results are shown in Table 2 as those of Example 4.

COMPARATIVE EXAMPLES Comparative Example 1

The silver powder described in Example 1 and used as a core material was also used as Comparative Example. The powder characteristics etc. of the silver powder are shown in Table 1 as those of Comparative Example 1.

Comparative Example 2

The silver powder described in Example 2 and used as a core material was also used as Comparative Example. The powder characteristics etc. of the silver powder are shown in Table 1 as those of Comparative Example 2.

Comparative Example 3

The silver powder described in Example 3 and used as a core material was also used as Comparative Example. The powder characteristics etc. of the silver powder are shown in Table 1 as those of Comparative Example 3.

Comparative Example 4

The silver powder described in Example 4 and used as a core material was also used as Comparative Example. The powder characteristics etc. of the silver powder are shown in Table 1 as those of Comparative Example 4.

<Comparative Examination of Examples and Comparative Examples>

The above-described Examples 1 to 3 and Comparative Examples 1 to 3 are compared while referring to Table 1.

[Table 1]

TABLE I Powder Charactaristics Silver Powder made of Silver Particles, each to Sinterd Conductor Core Material which Fine Silver Charactaristics Tap Crystallite Particles adhere Spesific Sinterable SSA Density D50 Dmax DIA Size Carbon D50 Dmax SSA Resistance Temparature Sample m2/g g/cm3 μm D50/DIA nm Content % μm m2/g μΩ · cm ° C. Example 1 2.54 4.2 0.31 0.97 0.30 1.03 7 0.32 0.29 0.97 3.73 3.4 150 Example 2 1.68 4.7 0.55 1.86 0.49 1.12 7 0.21 0.57 1.85 2.85 5.3 150 Example 3 0.62 4.0 3.03 11.0 1.20 2.53 38 0.22 3.26 11.0 0.99 7.9 150 Comparative 2.54 4.2 0.31 0.97 0.30 1.03 7 0.28 — 7.9 180 Example 1 Comparative 1.68 4.7 0.55 1.86 0.49 1.12 7 0.21 — 5.9 190 Example 2 Comparative 0.62 4.0 3.03 11.0 1.20 2.53 38 0.30 — not 250 Example 3 available

Comparing the powder characteristics of the silver powder as a core material and those of silver powder made of silver particles each to which fine silver particles adhere for the cases of Example 1 and Comparative Example 1, Example 2 and Comparative Example 2, and Example 3 and Comparative Example 3 while referring to Table 1, it is apparent that even if fine silver particles are made to adhere to the core material, the powder characteristics of the core material are hardly changed. Particularly from the fact that there was almost no change in the value Dmax before and after the adhesion of fine silver particles, it is apparent the dispersibility which the silver powder used as the core material has is maintained in the silver powder made of silver particles each to which fine silver particles adhere. This indicates that a silver powder as fine as possible having excellent dispersibility is advantageously used as the core material. The reason of this is that the crystallite size of the silver powder as the core material is kept unchanged, though the crystallite size of the silver powder made of silver particles each to which fine silver particles adhere is not described in the Tables.

Comparing the sintered conductor characteristics for the cases of Example 1 and Comparative Example 1, Example 2 and Comparative Example 2, and Example 3 and Comparative Example 3, it is obvious that sinterable temperature is so decreased by making fine silver particles adhere to silver powder as the core material that conventional knowledge cannot explain. Particularly in the cases of Examples 1 to 3, though the powder characteristics of both core material and silver powder made of silver particles each to which fine silver particles adhere are different from example to example, the sinterable temperature is 150° C. for all the cases. On the other hand, in the cases of Comparative Examples 1 to 3, there exists no fine silver particle layer on the silver powder, the sintered conductor characteristics are largely affected by the powder characteristics, and in the case of Comparative Example 3, it is impossible to measure the specific resistance. The comparison so far confirms that the silver powder made of silver particles each to which fine silver particles adhere according to the present invention is not affected by the powder characteristics of the core material and it can be sintered at low temperatures, because of the fine silver particles adhering to the surface of each of particles of the silver powder as a core material.

Then, the above-described example 4 and Comparative Example 4 will be compared while referring to Table 2. For the flake silver powder as core material and the flake silver powder made of silver particles each to which fine silver particles adhere, the powder characteristics are drawn which are different from those of the nearly sphere-shaped powder particles shown in Table 1. For the flake powder, the measurement of crystallite size and carbon content was omitted because its crystallite size undergoes changes by physical machining and its surface is contaminated by lubricants used in the machining. The value DIA obtained by the observation with a scanning electron microscope is also excluded from measurement items, because of its large fluctuations in a field of view. Instead, the measurements by laser diffraction scattering particle size distribution measurement method are often used.

[Table 2]

TABLE II Powder Charactaristics Flake Silver Powder made of Silver Flake Silver Powder Particles, each to which Sinterd Conductor as Core Material Fine Silver Particles Adhere Charactaristics Tap Tap Spesific Sinterable SSA Density D10 D50 D90 Dmax SSA Density D10 D50 D90 Dmax Resistance Temparature Sample m2/g g/cm3 μm m2/g g/cm3 μm μΩ · cm ° C. Example 4 0.29 2.6 8.91 16.7 28.9 67.9 0.72 2.7 9.17 18.8 29.8 74.0 21 200 Comparative — 109 200 Example 4

Comparing the powder characteristics of flake silver powder as the core material of Comparative Example 4 and those of flake silver powder made of silver particles each to which fine silver particles adhere of Example 4 with reference to Table 2, it is apparent that even if fine silver particles are made to adhere to the core material, the powder characteristics of the core material do not change very much, just like the case of the powder of the substantially spherical particles shown in Table 1. The value Dmax seems to be increased after the adhesion of fine silver particles; however, it cannot be necessarily asserted that there exist very large fluctuations, in view of including any measuring errors.

Comparing over the sintered conductor characteristics of Example 4 with Comparative Example 4, it is apparent that sinterable temperature is decreased by making fine silver powder adhere to the core material. This is the same as in the cases of Examples 1 to 3. The comparison so far confirms that the flake silver powder made of silver particles each to which fine silver particles adhere according to the present invention is not affected by the powder characteristics of the core material and it can be sintered at low temperatures, because of the fine silver particles adhering to the particle surface of the flake silver powder as a core material.

INDUSTRIAL APPLICABILITY

The silver powder made of silver particles each to which fine silver particles adhere according to the present invention is constructed by making fine silver powder (silver nanoparticles) adhere to the surface of each silver powder particle. Such construction enables the silver powder of the present invention to exhibit a low-temperature sintering performance at a level which conventional silver powder has never had. Because of its stable low-temperature sintering performance, which conventional silver powder has never had, a drastic expansion of applications in which silver powder is used is expected and a drastic reduction in energy cost during the sintering process will be made possible. Further, use of the silver powder, which is very fine, excels in dispersibility and contains less impurities compared with any conventional silver powder, as the core material for the silver powder made of silver particles each to which fine silver particles adhere makes it possible to realize an especially excellent low-temperature sintering performance and the formation of a low-resistant sintered conductor.

In the meantime, the process for producing silver powder made of silver particles each to which fine silver particles adhere according to the present invention is excellent in running stability through the production process, and therefore, it can produce silver powder made of silver particles each to which fine silver particles adhere very effectively. Thus, it can provide inexpensive and high-quality silver powder in the market, thereby contributing to the expansion of applications in which the silver powder made of silver particles each to which fine silver particles adhere according to the present invention is used.

Claims

1. Silver powder made of silver particles, each whose center part is regarded as a core material characterized in that:

silver particles each being finer than the silver particle of said center part adheres to said core material.

2. The silver powder made of silver particles each to which fine silver particles adhere according to claim 1, wherein the silver powder is substantially spherical.

3. The silver powder made of silver particles each to which fine silver particles adhere according to claim 2, wherein the silver powder has the following powder characteristics a to c:

a. an average particle size of primary particles DIA obtained by image analysis of a scanning electron microscope is 0.6 μm or less;
b. a degree of agglomeration represented by D50/DIA, where DIA is said average particle size of primary particles and D50 is the average particle size obtained by laser diffraction scattering particle size distribution measurement method, is 1.5 or less; and
c. a crystallite size is 10 nm or less.

4. The silver powder made of silver particles each to which fine silver particles adhere according to claim 2, characterized in that the silver powder has the following powder characteristics a to d:

a. an average particle size of primary particles DIA obtained by image analysis of scanning electron microscope is 0.6 μm or less;
b. a degree of agglomeration represented by D50/DIA, where DIA is said average particle size of primary particles and D50 is the average particle size obtained by laser diffraction scattering particle size distribution measurement method, is 1.5 or less;
c. a crystallite size is 10 nm or less; and
d. a content of organic impurities is 0.25% by weight or less in terms of amount of carbon.

5. The silver powder made of silver particles each to which fine silver particles adhere according to claim 1, wherein each of particles of the silver powder is substantially flat.

6. The silver powder made of silver particles each to which fine silver particles adhere according to claim 1, wherein sinterable temperature is 170° C. or less.

7. A process for producing the silver powder made of silver particles each to which fine silver particles adhere according to claim 1, comprising the steps of:

bringing a silver powder into contact with a solution containing a silver complex, which is obtained by mixing silver nitrate and a complexing agent and dissolving the mixture while stirring; and
adding a reducing agent into the solution to allow fine silver particles to be precipitated on the surface of each silver powder particle.

8. The process for producing the silver powder made of silver particles each to which fine silver particles adhere according to claim 7, wherein the complexing agent is a sulfite salt or an ammonium salt.

9. A process for producing the silver powder made of silver particles each to which fine silver particles adhere according to claim 1, comprising the steps of:

adding a silver nitrate and a neutralizing agent into a slurry of silver powder in a dispersing medium;
dissolving the slurry mixture while stirring to allow fine silver oxide particles to be precipitated on the surface of each silver powder particle;
washing the resultant silver powder; and
irradiating with UV rays to reduce the fine silver oxide particles to fine silver particles.

10. The process for producing the silver powder made of silver particles each to which fine silver particles adhere according to claim 9, wherein the neutralizing agent is any one or two or more selected from the group consisting of sodium hydroxide, potassium hydroxide and aqueous ammonia.

11. A process for producing the silver powder made of silver particles each to which fine silver particles adhere according to claim 1, comprising the step of:

using a silver powder of nearly spherical powder particles which is obtained in the steps of: preparing an aqueous solution of a silver complex by mixing and reacting an aqueous solution of silver nitrate and a complexing agent;
contact-mixing an organic reducing agent with said aqueous solution of a silver complex; allowing silver particles to be precipitated by reduction in the solution after the mixing, while keeping the silver concentration at 1 g/l to 6 g/l and the organic reducing agent concentration at 1 g/l to 3 g/l; separating the precipitated silver particles through a filter; and
washing the silver particles in water and then in an alcoholic solution.

12. A process for producing the silver powder made of silver particles each to which fine silver particles adhere according to claim 1, comprising the steps of:

using silver powder of nearly spherical powder particles which is obtained in the steps of: preparing an aqueous solution of a silver complex by mixing and reacting an aqueous solution of silver nitrate and a complexing agent;
contact-mixing an organic reducing agent with said aqueous solution of a silver complex; allowing silver particles to be precipitated by reduction in the solution after the mixing, while keeping the silver concentration at 1 g/l to 6 g/l and the organic reducing agent concentration at 1 g/l to 3 g/l; separating the precipitated silver particles through a filter; and
washing the silver particles in water and then in an excess amount of alcoholic solution.
Patent History
Publication number: 20050183543
Type: Application
Filed: Jan 19, 2005
Publication Date: Aug 25, 2005
Applicant: Mitsui Mining and Smelting Co., Ltd. (Tokyo)
Inventors: Takuya Sasaki (Shimonoseki-shi), Masashi Kato (Shimonoseki-shi), Taku Fujimoto (Shimonoseki-shi), Takahiko Sakaue (Shimonoseki-shi), Katsuhiko Yoshimaru (Shimonoseki-shi)
Application Number: 11/037,329
Classifications
Current U.S. Class: 75/371.000; 75/255.000