JPH06340906A - Production of globular silver fine particle - Google Patents

Abstract

PURPOSE:To efficiently mass-produce a globular silver fine particle having a small diameter by supplying a silver oxide fine particle into a high-frequency plasma flame generated by supplying plasma into a high-frequency magnetic field, rapidly heating the particle and then rapidly cooling the particle. CONSTITUTION:A high-frequency induction coil 2 is arranged around a double quartz tube 3, and a high-frequency magnetic field is excited in the tube 3. A plasma gas is introduced from a plasma gas feed pipe 5 placed above to generate a plasma flame F, and a silver oxide fine particle is entrained by the plasma gas through the feed pipe 5 and supplied as a two-phase current. Consequently, the silver oxide fine particle is brought into contact with the tail flame B of plasma, crushed by the decomposition reaction between silver and oxygen, then immediately cooled by cooling water feed pipes 3a and 3b, melted, solidified, globurized, dropped into a recovery vessel 4 and recovered. Globular silver particles having about 0.1-10mum average diameter are efficiently mass-produced in this way.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for efficiently obtaining spherical silver fine particles having an average particle size of about 0.1 to 10 .mu.m, which are used particularly for conductive paste and powder metallurgy.

[0002]

2. Description of the Related Art Conventionally, fine silver particles used for conductive paste, powder metallurgy and the like have been manufactured mainly by a wet method. In this wet method, silver metal is dissolved in nitric acid to form an aqueous solution of silver nitrate, and then sodium hydroxide or the like is added thereto to form silver oxide, which is further reduced with various reducing agents to produce metallic silver fine particles. Is deposited. As the reducing agent, caustic alkali, ammonium, hydrogen peroxide, glucose, hydrazine and the like are used.

However, the fine silver particles thus obtained are incomplete spherical or irregular polyhedrons, and although they can be close to spherical depending on the deposition conditions, sponge-like particles are also obtained in this case. Porous fine particles,
It contains gas inside. Therefore, the conductive paste prepared by directly using the fine silver particles obtained by the wet method leaves room for improvement in the dispersibility, fluidity, screen printability, migration resistance, etc. of the particles. It is desired that the fine silver particles be made finer and have a higher spherical shape.

On the other hand, the applicants of the present application filed the above-mentioned Japanese Patent Laid-Open No. 4-2.
No. 46104 and Japanese Patent Application No. 4-178352, there is disclosed a method for obtaining regular spheroidized particles having a smooth particle surface by supplying the particles into a plasma flame under a predetermined condition using a high frequency plasma generator. is suggesting.

[0005]

For example, by subjecting the silver fine particles obtained by the wet method to the spheroidizing treatment by the plasma generator, it is possible to obtain the regular spheroidized fine particles having a smooth particle surface. However, in the case of the method of depositing silver particles by the wet method, complicated processing steps such as pH adjustment, precipitation generation, and reduction are required, and a batch reactor is used, so mass productivity is poor. Further, since they are often precipitated in the liquid phase and thus aggregated, the silver fine particles obtained by the wet method have drawbacks such as a relatively large particle size and a wide particle size distribution.

Therefore, a main object of the present invention is to provide a method capable of simplifying the manufacturing process, mass-producing spherical silver fine particles, and efficiently manufacturing spherical fine silver particles having a small particle size. is there.

[0007]

[Means for Solving the Problems] The object is to excite a high-frequency magnetic field by a high-frequency induction coil, supply a plasma gas into the high-frequency magnetic field, and supply silver oxide fine particles into a high-frequency plasma frame generated inductively. Then, the silver oxide fine particles are rapidly heated and decomposed in the plasma flame,
It can be solved by crushing and melting and then obtaining spherical silver particles by rapid cooling due to attenuation of a high frequency magnetic field.

In this case, it is desirable that the fine silver oxide particles are supplied in a dispersed state in the range of 500 to 5000 ° C. of the high frequency plasma flame.

[0009]

In the present invention, silver oxide fine particles are used as the spheroidizing substance by high frequency plasma. This silver oxide fine particle manufacturing process is the same as the reduced silver fine particle manufacturing process up to the step of dissolving silver metal in nitric acid to form an aqueous silver nitrate solution and adding an alkali hydroxide, but the process is simple because there is no reduction step. Therefore, it is cheaper to produce the spheroidized silver fine particles at once by using the silver oxide fine particles according to the present invention, than to produce the atypical reduced silver fine particles and then spheroidize them as in the conventional case.

It is generally known that silver oxide easily decomposes at around 190 ° C. to produce silver and oxygen. Therefore, the present inventors have paid attention to this characteristic, and by supplying it into the plasma flame in the state of silver oxide, they are rapidly heated in this plasma flame, decomposed, crushed, and melted at once, and then the high frequency The inventors have found a method for efficiently producing spherical silver fine particles by quenching by magnetic field decay.

More specifically, it is preferable that the silver oxide fine particles are directly treated by high-frequency plasma in a range of 500-500.
It is supplied in a dispersed state in the region of 5000 ° C. Then, the silver oxide fine particles are suddenly subjected to a high temperature heat shock, and the silver oxide fine particles are instantly decomposed into oxygen and silver. At this time, oxygen, which is a gas, is abruptly generated and the volume thereof is expanded, so that the particles are crushed and scattered. Then, the crushed silver fine particles are melted by the heat of the plasma flame and solidified by rapid cooling to become spherical silver fine particles.

As described above, in the case of the method of the present invention, by introducing silver oxide as it is into the plasma flame,
Decomposition, crushing, and melting are performed instantaneously to efficiently obtain spheroidized silver fine particles having an average particle diameter of about 0.1 to 10 μm, and thus are excellent in mass productivity and inexpensive. Also,
When the spheroidized silver fine particles produced by the method of the present invention are used, the dispersibility when formed into a paste becomes good, the amount of added resin and the like can be reduced, and the fluidity is also improved, so that the screen printability is improved. In addition, various advantages are obtained such that the width of the fired film can be made narrow and dense, migration resistance can be increased, and the like.

It is possible to use direct current thermal plasma instead of high frequency plasma. However, high frequency plasma has a larger frame, and since no electrodes are present, it is preferable because impurities are not mixed.

[0014]

EXAMPLES The present invention will be described in detail below based on examples.
FIG. 1 is a vertical cross-sectional view of a high frequency induction plasma reactor used in the method for producing spherical silver fine particles according to the present invention.
FIG. 3 is an overall vertical sectional view of the device. First, referring to FIG. 2, the overall structure of the spheroidized particle production apparatus using a high frequency induction plasma reaction will be described. The spheroidized particle production apparatus includes a plasma reactor 1 at its upper part and an intake / exhaust device (not shown) below the plasma reactor 1. According to the above, a container 4 capable of pressurizing or vacuum pressure operation is provided, and a recovery container 4A for recovering the processed spheroidized particles is provided at the lowermost part thereof. The side portion 4a of the container 4 is a monitoring window, and the reference numerals 4b and 4c are intake / exhaust holes for connecting pipes for installing instruments such as a thermometer and for intake / exhaust.

In the plasma reactor 1, as shown in FIG. 1, high frequency induction coils 2, 2, ... Are arranged outside a quartz tube 3 having a double tube structure, and the quartz tube 3 is provided with an inside thereof. A high-frequency magnetic field is excited, and a plasma gas is supplied from a plasma gas supply pipe 5 installed above the high-frequency magnetic field, and a high-frequency current is passed through the high-frequency coils 2, 2 ... Give rise to. The plasma gas supply pipe 5 has a double pipe structure, and cooling water is supplied from a cooling water supply pipe 7 to this hollow portion. Further, a seal gas such as argon gas is supplied from the seal gas supply pipe 6 to the quartz pipe 3
The turbulence of the gas flow due to the plasma gas sent and supplied therein is suppressed, and the treatment gas is prevented from adhering to the inner surface of the quartz tube by the seal gas flowing along the inner surface of the quartz tube 3.

Cooling water is supplied to the hollow portion of the double tube of the quartz tube 3 by cooling water supply pipes 3a and 3b. As the plasma gas, in addition to argon gas,
It is also possible to use a gas such as helium or nitrogen,
If hydrogen is supplied as an auxiliary gas simultaneously with the argon gas, metal particles with a particularly small amount of oxide can be obtained. The plasma flame F generated by the plasma generator 1 extends further downward from the plasma part A in which the high frequency induction coils 2, 2, ... Are arranged by the force of the inflowing gas to form an elongated plasma flame F. Of the plasma frame F, only the region where the high-frequency coils 2, 2, ... Are not arranged, that is, the energy of the tail frame B that is not directly receiving the high-frequency energy is used for spheroidizing. The tail frame B is about 70
The temperature range is from a high temperature part of 00 ° C. to a low temperature part of about 1000 ° C., and by supplying the processing substance to this region, turbulence of the air flow in the plasma part A is eliminated, so that evaporation and heat convection of the processing substance are eliminated. The problem of clogging of the plasma generating torch portion, the treatment substance supply nozzle 5 and the misfire phenomenon of high frequency plasma due to the shielding are eliminated. On the other hand, the silver oxide fine particles as the processing raw material are supplied in a two-phase flow state on the supplied plasma gas through the plasma gas supply pipe 5. The treatment substance is fed at a low pressure of gravity drop + α of the treatment substance, and is preferably about 1 (l / m).
in), the spheroidizing treatment is performed while adjusting the pressure of the plasma reactor 1, the gas pressure for supplying the processing substance, the position of the processing substance supply nozzle 5 and the like so that the processing substance does not drop into the plasma part A at a flow rate of at least above. I do.

In the case of the above-mentioned plasma device, although the processing amount per unit time is small, the supplied fine particles are
Since it passes through the entire extension range of the tail frame B, the spheroidization rate can be improved. As the carrier gas for supply, an inert gas such as argon gas or helium, or an active gas may be used.

Further, the fine silver oxide particles to be treated are sieved before being fed into the plasma flame, or by a mechanical or non-mechanical method (special nozzle, and change of carrier gas amount for raw material supply). By dispersing and adjusting the amount of aggregation, in addition to the spheroidized particles of 0.1 to 10 μm that are particularly targeted by the present invention, the melting point of the treatment substance,
Alternatively, by appropriately controlling the distance between the plasma flame F and the treatment substance supply nozzle 5, the pressure condition of the plasma reaction portion, the pressure condition of the gas for supplying the treatment substance, etc., according to the thermal conductivity, the viscosity condition, etc. It becomes possible to obtain spheroidized particles having a particle size. By supplying fine particles of silver oxide from the treatment substance supply nozzle 5 using such a plasma generator, the fine particles of silver oxide come into contact with the tail frame B of the plasma, are crushed by the decomposition reaction of silver and oxygen, and are melted immediately thereafter. By being solidified, it is spheroidized and dropped into the collection container 4A of the container 4 to be collected.

EXAMPLES The effects of the present invention will be clarified below based on examples. With the high frequency plasma reactor shown in FIG. 1, polyhedral silver oxide fine particles having an average particle diameter of 0.85 μm are conveyed by argon gas, and 2 g / min of silver oxide fine particles and 5 l / argon gas of argon gas are supplied from the treatment substance supply nozzle 5.
Dispersed and supplied in minutes. When this was treated with a high frequency plasma of 5 KVA, spherical silver fine particles having an average particle diameter of 0.15 μm were obtained.

The composition of the produced silver fine particles was analyzed by an X-ray diffractometer, and the silver content was 100%. 3 and 4 show scanning electron micrographs of silver oxide fine particles and spherical silver fine particles, respectively. From these, the silver fine particles obtained by the method of the present invention are spherical,
It is found that the particle size distribution is also narrow.

Since the present invention can be applied to a substance which decomposes at a relatively low temperature to produce solid fine particles having a melting point and a gaseous decomposition product, when producing spherical gold fine particles from gold oxide. It is also applicable to the case of producing spherical calcium oxide fine particles from calcium carbonate.

[0022]

As described above in detail, according to the present invention, the manufacturing process is simplified, spherical silver fine particles can be mass-produced, and the spherical fine silver particles having a small particle size can be efficiently produced. can do.

[Brief description of drawings]

FIG. 1 is a vertical sectional view of a high-frequency plasma reactor used in the method of the present invention.

FIG. 2 is an overall vertical cross-sectional view of a spheroidized particle manufacturing apparatus.

FIG. 3 is a grain structure photograph of silver oxide fine particles (magnification: × 750).
0).

FIG. 4 is a photograph of a particle structure of spherical silver particles (magnification: × 200
00).

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Plasma generator, 2 ... High frequency induction coil, 3 ... Quartz tube, 4 ... Container, 4A ... Collection container, 5 ... Plasma gas supply tube, F ... Plasma flame, A ... Plasma part, B ...
Tail frame

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hiroyasu Sumitomo 6-22-22 Fujimi, Tsurugashima City, Saitama Fuji Electric Wave Machinery Co., Ltd.

Claims (2)

[Claims] 1. A high frequency magnetic field is excited by a high frequency induction coil, and a plasma gas is supplied into the high frequency magnetic field to supply silver oxide fine particles into a high frequency plasma frame generated inductively. A method for producing spherical silver fine particles, which comprises rapidly heating silver oxide fine particles for decomposition, crushing and melting, and then obtaining spheroidized silver fine particles by rapid cooling due to attenuation of a high frequency magnetic field. 2. The method for producing spherical silver fine particles according to claim 1, wherein the silver oxide fine particles are supplied in a dispersed state in a 500 to 5000 ° C. region of a high frequency plasma flame.