JP4614101B2 - Silver powder, method for producing the same, and conductive paste containing the silver powder - Google Patents

Description

  The present invention relates to silver powder for conductive materials suitable for mounting electronic components and circuit formation, and a method for producing the same.

In recent years, a conductive paste capable of obtaining a low resistance value at low temperature heating has been demanded as a direction of the electronic material market. Here, the conductive paste is a high-temperature firing type conductive paste that decomposes and disperses the solvent and dispersant by heating and sinters the conductive powder, and the conductive powder is mixed in the polymer, and the heating is performed at a lower temperature. Is a heat-curing type conductive paste that obtains conductivity by curing the polymer and bringing the conductive powders into contact with each other. Heat-curing type conductive paste is also used as a conductive adhesive and is being developed as a bonding method similar to solder bonding.
In response to the requirement to obtain a low resistance value by low-temperature heating, the above conductive paste containing metal nanoparticles has been studied. In particular, silver nanoparticles have been studied in various ways because they have a lower silver bulk resistance than other metals and can be sintered at a low temperature due to a melting point drop phenomenon caused by nanometer size.

  Examples of methods for preparing a conductive paste using silver nanoparticles as a raw material to produce a circuit wiring or a conductive thin film include Japanese Patent Laid-Open Nos. 2002-299833, 2002-334618, and 2004-273205. Publications and the like are disclosed.

  By the way, the metal nanoparticles used in the conductive paste are usually provided with an organic dispersant in order to ensure the dispersion stability of the metal nanoparticles. This dispersant makes it possible to maintain the dispersion stability of the metal nanoparticles in a solution without causing particle growth or sintering over a long period of time even at a high concentration.

  However, the dispersant that ensures the stability remains on the surface of the metal nanoparticles or in the metal during low-temperature heating, which hinders sintering or increases the resistance value. If the metal nanoparticles are dispersed and this dispersant is removed by chemical treatment or physical treatment such as heating, the metal nanoparticles will rapidly grow and sinter with other particles, resulting in low-temperature sinterability. The characteristics of metal nanoparticles such as are lost. Therefore, in conductive pastes using metal nanoparticles, especially silver nanoparticles, the dispersion of the dispersing agent quickly disperses and decomposes during low-temperature firing, while taking full advantage of the nanometer feature of melting point depression due to nanometer size. Thus, there is a demand for silver nanoparticles that remain in silver and do not increase resistance or prevent sintering.

JP 2002-299833 A JP 2002-334618 A JP 2004-273205 A JP-A-2005-146408

  There is a demand for conductive pastes using metal nanoparticles, particularly silver nanoparticles, which have a low resistance value at low temperature heating. However, the dispersant that contributes to the dispersion stability described above remains in the silver at the time of low-temperature heating and prevents sintering or increases the resistance value.

Therefore, a method is disclosed in which a supplementary substance having a function of causing a chemical reaction with the dispersant during heating to remove the dispersant is contained in the conductive paste (Patent Document 1: Japanese Patent Laid-Open No. 2002-299833, Patent Document 2: JP-A-2002-334618). This method is excellent in that the dispersant is removed from the surface of the metal nanoparticles by low-temperature heating, but a substance after reaction or an undecomposed supplementary substance remains and becomes a factor for increasing the resistance value.
There is also disclosed a method for improving low-temperature sinterability by adding a high-boiling organic solvent that promotes the detachment of the dispersant without adding a supplementary material to the conductive paste (Patent Document 3: JP-A-2004-273205). Issue gazette). In this case, although it is excellent in terms of removing and scattering the dispersant at a low temperature, since it contains a high boiling point solvent, when used as a heat-curing type conductive paste, after heating, in the paste There is a concern that the high boiling point solvent may remain.

Further, a structure in which silver nanoparticles are deposited on the surface of a large silver powder is disclosed (Patent Document 4: JP-A-2005-146408). This method can greatly reduce the amount of organic matter. However, when this silver powder is used as a conductive paste, the silver powders approach each other during heating, and the deposited silver nanoparticles need to come into contact with other silver powder surfaces over a larger area. In the cross-sectional view, the silver nanoparticles are scattered on the surface of the silver powder serving as the core material, and it has a structure that is not sufficient to expand the contact area and improve the electrical conductivity.
An object of the present invention is to provide a silver powder which is a conductive material suitable as a material for a conductive paste that significantly reduces the amount of organic substances such as a dispersant while taking advantage of the characteristics of silver nanoparticles.

As a result of sincerely examining the process of removing the dispersant for the silver nanoparticles, the present inventors gradually removed the dispersant during the heat treatment in a state where the silver nanoparticles were self-assembled on the surface of the inorganic carrier. Thus, the particles are not agglomerated as in the conventional dispersing agent removal method, but the dispersing agent is removed while leaving the primary particles in the form of crystal growth, and finally the inorganic carrier is chemically treated. It was found that a silver powder having a sheet-like structure in which silver nanoparticles remained in the form of primary particles was obtained by removing the above.
More specifically, a silver nanoparticle dispersion solution having a uniform particle size and a carbonate are mixed, pulverized and then baked, and the carbonate is removed with an acid, which cannot be obtained by conventional silver powder manufacturing technology. The present invention has been completed by finding that a silver powder having a sheet-like structure in which the shape of the primary particles of the nanoparticles is left can be obtained, and that the above object is achieved.

  That is, the present invention is a silver powder composed of silver nanoparticles, wherein the average particle size of primary particles of the silver nanoparticles is 200 nm or less, and the silver powder has a form of primary particles of silver nanoparticles. It is a silver powder characterized by being a structure of the present invention (Invention 1).

  Moreover, this invention is the said silver powder in which a silver nanoparticle has an alloy structure with gold | metal | money, platinum, and a palladium noble metal (invention 2).

  Moreover, this invention is the electrically conductive paste containing the said silver powder (this invention 3).

  In addition, the present invention is to mix and pulverize silver nanoparticles having a uniform particle size and carbonate crystal particles, calcinate, remove the carbonate crystal particles with an acid, and leave a primary particle form of silver nanoparticles. A silver powder having a sheet-like structure is obtained (Invention 4).

  The silver powder having a sheet-like structure in which the shape of the primary particles of the silver nanoparticles obtained in the present invention is retained, while maintaining the characteristics of low-temperature sinterability of the silver nanoparticles, the remaining organic matter is less than the conventional silver nanoparticles. Since there are few, it is suitable as an electroconductive material of an electroconductive paste.

  The configuration of the present invention will be described in more detail as follows.

  The silver powder according to the present invention is a sheet-like silver powder that maintains the shape of primary particles of silver nanoparticles having a particle diameter of 200 nm or less. Sheet-like silver powder may have a laminated structure. In the case of a structure in which sintering proceeds and there is no primary particle shape, the low-temperature sinterability of the silver nanoparticles is lost, so the intended effect of the present invention cannot be obtained. However, since the structure in which the structure of the primary particle size is not seen is the same as ordinary flaky silver powder, there is no problem even if it exists at the same time unless it is an amount that significantly inhibits the intended effect of the present invention.

  The silver powder according to the present invention has a shape of primary particles of silver nanoparticles, but the silver nanoparticles are formed into a sheet by sintering to form primary particles of 200 nm or less.

  Although a structure in which the sheet-like structure collapses during the preparation of the silver powder of the present invention is obtained, it is essentially a silver nanoparticle as long as it has a primary particle form of silver nanoparticles. Therefore, the desired characteristics can be obtained.

  The amount of remaining organic matter (dispersant) is 3% or less with respect to the silver powder. This is preferably 2% or less. More preferably, it is 1% or less.

  Next, a method for producing silver powder according to the present invention will be described.

  When a dispersion solution of silver nanoparticles having a uniform particle size and carbonate crystal particles are mixed while gradually evaporating the solvent, the silver nanoparticles adhere while forming a self-assembled film on the surface of the carbonate crystal particles. To go. When the silver nanoparticle-coated carbonate particles thus produced are heated and fired, a silver film that has been sintered is formed while maintaining the self-assembled film structure. Then, when the carbonate is dissolved with foaming of carbon dioxide gas with inorganic acid such as hydrochloric acid or organic acid such as formic acid and acetic acid, the silver film formed on the surface of the carbonate particles is peeled off and obtained. The silver film has a sheet-like structure that leaves the primary particles of silver nanoparticles.

  As long as the silver nanoparticles in the present invention are silver nanoparticles that form a self-assembled film on the surface of the carbonate crystal particles and a dispersion thereof, the structure, the dispersant, and the production method of the silver nanoparticles are not particularly limited. It is preferable to use a dispersion solution of silver nanoparticles in which organic substances such as a solvent and a dispersant are evaporated, decomposed and scattered at 400 ° C. or lower. For example, a 50% toluene solution of silver nanoparticles prepared according to the method described in JP-A-2005-36309 can be used. As the silver nanoparticles are self-assembled as shown in the figure of the publication, the particle diameters are aligned.

  The primary particles of the silver nanoparticles used for the production of the silver powder of the present invention preferably have an average particle size of 3 nm to 50 nm. When the average particle diameter of the primary particles exceeds 50 nm, it is difficult for the silver nanoparticles to sinter at a low temperature in the heating process after self-organization and take a sheet-like structure. Preferably, it is 3 nm to 30 nm.

  Further, the silver nanoparticles may have an alloy structure. In particular, silver alloy nanoparticles with palladium or platinum having migration resistance characteristics can also be used.

  The carbonate of the present invention is not particularly limited as long as it is easily dissolved by an acid. For example, calcium carbonate, magnesium carbonate, sodium carbonate, sodium bicarbonate, ammonium carbonate and the like.

  The carbonate particles are not particularly limited as long as the silver nanoparticles have a specific surface area that is self-assembled by covering the surface of the carbonate particles, but the average particle size of the carbonate particles is 10 μm or more. Preferably, it is 100 μm or more.

  The mixing method of the silver nanoparticles and the carbonate crystal particles is not particularly limited, and it is preferable that the silver nanoparticles and the carbonate crystal particles are mixed under conditions that form a self-organized film on the surface of the carbonate crystal particles. As the solvent, for example, toluene, benzene, hexane and the like are suitable.

  Further, after mixing the silver nanoparticle dispersion solution and the carbonate crystal particles, the solvent may be dried and the silver nanoparticles may be deposited on the surface of the carbonate crystal particles while spraying.

  The weight mixing ratio of the silver nanoparticles to the carbonate crystal particles is 1: 1 to 1: 1000. Preferably it is 1:10 to 1: 1000, More preferably, it is 1:10 to 1: 100.

  The conditions for firing the carbonate crystal particles coated with the silver nanoparticles are such that the silver nanoparticles maintain a self-assembled film to some extent while causing particle growth and particle sintering. Since the optimum conditions for the temperature and firing time differ depending on the silver nanoparticles used, appropriate conditions may be used while confirming the sintered state each time. For example, when a 50% toluene solution of silver nanoparticles prepared according to the method described in JP-A-2005-36309 and sodium hydrogen carbonate are used, heating is performed at 200 ° C. to 300 ° C. in the atmosphere for 30 minutes to 1 hour. Thus, a silver powder having a sheet-like structure in which the shape of primary particles of the target silver nanoparticles is left can be obtained.

  Removal of the carbonate crystal particles after firing can be removed by adding an inorganic acid such as hydrochloric acid or an organic acid such as formic acid or acetic acid. Subsequently, by collecting the silver powder according to a conventional method such as washing, filtration, and drying, it is possible to obtain a silver powder having a sheet-like structure in which the shape of the primary particles of the silver nanoparticles is left.

The silver powder having a sheet-like structure in which the resulting silver nanoparticle primary particle remains is mixed with a commonly used polymer binder and a solvent to form a paste, whereby a conductive paste can be easily prepared.
As the polymer binder, epoxy acrylate, acrylic resin, epoxy resin, phenol resin, polyester resin, polyimide, polyvinyl acetate, or the like can be used. As the solvent, bityl cellosolve acetate, benzyl alcohol, ethyl acetate, methyl ethyl ketone, butyl carbitol and the like can be used.
Further, the sheet-like silver powder that leaves the primary particle shape of the obtained silver nanoparticles can be used by mixing with ordinary spherical silver powder or flaky silver powder. The mixing ratio of the normal spherical silver powder or flaky silver powder and the sheet-like silver powder leaving the primary particles of the silver nanoparticles obtained in the present invention is 0.1 to 1000: 10. Preferably it is 10-100.
When the silver powder of the present invention is made into a conductive paste, it has excellent low-temperature sinterability and also increases the contact area between silver powders, and is less likely to increase the resistance value due to the remaining organic components. is there.

  EXAMPLES Hereinafter, although an Example and a comparative example demonstrate this invention concretely, this invention is not limited to this example.

Example 1
A toluene dispersion solution of silver nanoparticles was prepared with reference to the method described in JP-A-2005-36309.
That is, silver nitrate (4.0 g), toluene (50 mL) and oleylamine (9.4 g) were placed in a beaker and stirred at room temperature. Subsequently, ascorbic acid (8.3 g) was added and stirred for 2 hours. The reaction solution had a blackish yellow color. Subsequently, silver nanoparticles were agglomerated and precipitated using a mixed solution of acetone and methanol-water, and the supernatant solution was removed by decantation to remove excess salts and amines. After the purification operation was repeated three times, the aggregate was dried under reduced pressure. Subsequently, toluene (1.96 g) was added to prepare a 50% dispersion of silver nanoparticles. When observed with a transmission electron microscope (500,000 times), the silver nanoparticles had an average particle diameter of 8 nm.

A 50% toluene solution (0.4 g) of silver nanoparticles obtained by the above method and sodium hydrogen carbonate (2 g) were mixed using a mortar. The solvent toluene was volatilized during mixing. Subsequently, the mixture was put into an electric furnace set at 250 ° C. in the atmosphere, and baked for 30 minutes. After cooling to room temperature, it was poured into a 30% aqueous acetic acid solution to neutralize and dissolve sodium bicarbonate. The obtained powder was washed with normal water and filtered to remove excess salt.
When the obtained powder was observed with a scanning electron microscope, it was found to be a silver powder having a sheet-like structure in which primary particles of silver nanoparticles having a particle diameter of 50 to 200 nm were left (FIGS. 1 and 2). ).

  As a result of thermal analysis, the amount of organic substances was 1% or less. (Figure 3)

Example 2
Silver powder was prepared in the same manner as in Example 1 except that sodium carbonate was used as the carbonate crystal particles. When the obtained powder was observed with a scanning electron microscope, it was found to be a silver powder having a sheet-like structure in which primary particles of silver nanoparticles having a particle diameter of 50 to 200 nm were left (FIG. 4).

Comparative Example 1
Without using the carbonate crystal particles, only the silver nanoparticles were put in an electric furnace set at 250 ° C. in the atmosphere and baked for 30 minutes. When the obtained powder was observed with a scanning electron microscope, silver powder having a sheet-like structure in which the shape of primary particles of silver nanoparticles was left was not obtained (FIG. 5).

Comparative Example 2
Silver powder was prepared in the same manner as in Example 1 using sodium chloride instead of carbonate crystal particles. When the obtained powder was observed with a scanning electron microscope, silver powder having a sheet-like structure in which the shape of primary particles of silver nanoparticles was left was not obtained (FIGS. 6 and 7).

  The silver powder having a sheet-like structure in which the primary particle shape of the silver nanoparticles obtained in the present invention is left has a smaller amount of organic matter (dispersing agent) than conventional silver nanoparticles, and may be mixed with large particles. It contributes to expansion of the conductive path between the particles, and is suitable as a raw material for the conductive paste or the conductive adhesive composition.

4 is an electron micrograph (5000 times magnification) of the silver powder obtained in Example 1. FIG. 2 is an electron micrograph (magnification 50000 times) of the silver powder obtained in Example 1. FIG. 2 is a graph showing the results of thermal analysis of the silver powder obtained in Example 1. 4 is an electron micrograph (magnification 20000 times) of the silver powder obtained in Example 2. FIG. 4 is an electron micrograph (magnification 1000 times) of the silver powder obtained in Comparative Example 1. 2 is an electron micrograph (magnification 10,000 times) of the silver powder obtained in Comparative Example 1. 2 is an electron micrograph (magnification 1000 times) of the silver powder obtained in Comparative Example 2.

Claims (4)

The silver powder is composed of silver nanoparticles, and the average particle diameter of primary particles of the silver nanoparticles is 200 nm or less, and the silver powder is a sheet-like structure having a primary particle form of silver nanoparticles. Silver powder characterized by The silver powder according to claim 1, wherein the silver nanoparticles have an alloy structure with gold, platinum, or palladium noble metal. The electroconductive paste containing the silver powder of Claim 1 or 2. A silver powder with a sheet-like structure in which silver nanoparticles with a uniform particle size and carbonate crystal particles are mixed, pulverized and then baked, and the carbonate crystal particles are removed by acid to leave the primary particles of silver nanoparticles. The method for producing silver powder according to claim 1, wherein: