JPH0892613A - Production of metal powder and device therefor - Google Patents

Abstract

PURPOSE: To obtain a fine metal powder having a uniform particle diameter and reduced in impurities by making the mixed soln. of metallic salt particles and a combustible solvent into a liq. droplet, then successively drying, burning and cooling the droplet. CONSTITUTION: The mixed soln. of a metallic salt (silver nitrate, etc.) soln. and a combustible solvent (methanol, etc.) is atomized into a liq. droplet 1, and the droplet is dried in a high-temp. atmosphere into a semisolid metallic salt and a combustible gas 2. The combustible gas is ignited to initiate combustion, other liq. droplets are ignited by a flame stabilizing means to continue combustion. Consequently, a metallic salt 3 is deposited by the combustion and solidified into a porous solid, and the salt is oxidized to a metal oxide when a specific metal is used. The metallic salt or metal oxide is thermally decomposed by the combustion heat of the combustible gas, and further, the metal 4 is melted into a liq. globule, which is cooled by introducing a cooling gas and so on to form a metal powder 5. The globular metal powder 5 having a smooth surface is obtained 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 and an apparatus for producing a metal powder, which is preferably used for producing an electronically functional material, particularly an electrically conductive paste.

[0002]

2. Description of the Related Art Metal powders are widely used in the field of electronics as materials for conductor pastes and resistance pastes for manufacturing electronic circuits, resistors, capacitors, IC packages and the like. The conductor paste or resistance paste is manufactured by uniformly mixing and dispersing powder of metal, alloy or metal oxide in an organic vehicle together with a vitreous binder and other additives. These pastes are used to form a conductor film or a resistance film on the substrate.

The metal powder used in these pastes is required to have the following properties. 1) The particle size is small and uniform. Particle size affects the dispersibility of the metal powder in the paste. In order to form a dense and uniform film, it is necessary to make the particle size of the metal powder small and uniform and to improve the dispersibility. The particle size is preferably within the range of 0.1 to 10 μm. 2) The shape of the particles is spherical and has a smooth surface. The shape and surface condition of the particles affect the cohesiveness of the metal powder and the dispersibility of the metal powder in the paste. By making the particles spherical and having a smooth surface, dispersibility can be improved and aggregation can be reduced. 3) There is little mixing of impurities. Impurities deteriorate the characteristics of the conductor paste or the resistance paste, and also have an adverse effect on electrical characteristics such as ohmic bonding with semiconductors, corrosion resistance, and environmental resistance.

As a method for producing a metal powder, a method of causing a reducing agent to act on a solution of a metal compound to perform wet reduction (wet reduction method), a method of atomizing a molten metal (atomization method), a method of vacuuming a metal or A method of evaporating in an inert gas to make fine powder (evaporation method) and the like are known. But,
With these methods, there is a limit in obtaining a metal powder having the above-mentioned conditions, and the cost is high depending on the method, which is not preferable.

Therefore, Japanese Patent Application Laid-Open No. 50-137856 discloses thermal decomposition of spray droplets to produce a powder, and Japanese Patent Application Laid-Open No. 62-1807 discloses a high temperature atmosphere of a metal salt solution. There is disclosed a method (spray pyrolysis method) in which a fine powder is obtained by instantaneously causing evaporation of a solvent, thermal decomposition of a metal salt, and melting of a metal by spraying. By using these methods, it is possible to obtain a metal powder having a fine and uniform particle size and containing less impurities.

[0006]

However, the metal powder obtained by the above method becomes porous when the heating temperature is lower than the melting point of the metal and has a low density, so that the surface has depressions and a smooth surface. There is a problem that it is difficult to obtain spherical particles having Also, in this method, the furnace for spraying the metal salt solution is usually heated from the outside, so a large-capacity heater is required, the equipment is expensive and the operating cost is high, and the diameter of the furnace is less than a certain value. There is a problem that it is difficult to scale up because it cannot be increased. Further, there is a problem that it takes time until the temperature inside the furnace becomes stable and the spraying of the metal salt solution can be started.

[0007]

According to the present invention, by heating a droplet consisting of a solution containing a metal salt, the metal salt or the metal oxide formed is decomposed and the metal is melted and then solidified. A method for producing a metal powder, wherein a solution containing a metal salt contains a flammable solvent, and combustion is caused by a combustion reaction between a flammable solvent vaporized from droplets and a combustion gas (also referred to as a combustion-supporting gas). Decomposes the metal salt or metal oxide generated by heat and melts the metal, and heats from the outside, by appropriately adjusting the concentration of the combustible solvent in the solution and the concentration of oxygen in the combustion gas. Provided is a method for producing a metal powder in which the temperature of a flame portion is adjusted and a molten metal is cooled and solidified by using a cooling means. In the above method, it is preferable to ignite the vaporized combustible solvent by using the ignition means and continue the combustion by using the flame holding means, but the ignition from the outside that combines the ignition and flame holding of an electric heater or the like. A flame holding means may be used. Further, the combustion gas contains 21% by volume or more of oxygen, the solution containing the metal salt contains 30% by weight or more of the combustible solvent, and 50% by weight or more of methanol when the combustible solvent is used. The temperature of the flame holding part of the flame holding means is preferably 500 ° C. or higher.

Further, according to the present invention, the furnace, the spraying means provided at one end of the furnace, the ignition means provided near the spraying means, the flame holding means provided on the outer surface of the furnace, and the other end of the furnace are provided. An apparatus for producing metal powder, which comprises a cooling means provided in
By igniting the flammable solvent vaporized from the droplets of the metal salt solution by the ignition means, and the flame holding means sustains the combustion,
Provided is an apparatus for producing a metal powder, which decomposes a metal salt or a produced metal oxide and melts the metal, and solidifies the melted metal by a cooling means. The flame holding means is preferably a heater installed outside the furnace, but an ignition flame holding means in which the ignition means and the flame holding means are integrated may be used. Further, the cooling means is preferably a cooling gas introduced through an inflow hole provided in the furnace.

[0009]

The production of the metal powder according to the present invention is based on the principle shown in FIG. A metal salt solution obtained by mixing particles or a solution of a metal salt with a flammable solvent is a droplet 1 which is atomized in a high temperature atmosphere.
Is sprayed as. The liquid droplets 1 which are liquid spheres are dried in a high temperature atmosphere, and a flammable gas 2 generated by vaporizing a flammable solvent is released. As the solvent evaporates, the droplet 1 of the metal salt solution is concentrated and becomes a semi-solid. The flammable gas 2 is ignited to start combustion, and the flame holding means ignites other droplet particles to continue the combustion. By burning,
The metal salt 3 is precipitated and solidified to become a porous solid. Upon combustion, some metals are oxidized to produce metal oxides. Due to the combustion heat generated by the combustion of combustible gas, metal salt 3
Alternatively, the metal oxide undergoes thermal decomposition, the metal 4 is further melted, and becomes a liquid sphere again. Further, the molten metal 4 is solidified into a metal powder 5 by a cooling means such as introduction of a cooling gas.

In the present invention, the solvent in the droplets of the metal salt solution is removed by heating with the heat of combustion, and the metal is melted after the metal salt is decomposed, so that the liquid metal becomes spherical due to the surface tension and is dense. A spherical powder with a smooth surface can be obtained. On the other hand, in the conventional method, metal particles are formed by heating the inside of the furnace to a high temperature by heating from the outside, which results in high operating cost and expensive equipment.

The combustible solvent is ignited by the ignition means or the ignition flame holding means, and the combustion heat thereof is used to thermally decompose the metal salt to melt the metal. Therefore, when a heater is used as the ignition means or the ignition flame holding means, a small heater that can sustain combustion can be installed outside the furnace, and it is not necessary to install a large capacity heater. or,
There is no problem that it takes time until the temperature in the furnace becomes stable and the spraying of the metal salt solution can be started. Furthermore, in the case of heating with an external heater, it is impossible to make the diameter of the furnace larger than a certain value, but by using the present invention, the apparatus can be made larger and the production efficiency can be improved.

As the metal salt, a target metal by thermal decomposition, for example, a noble metal such as gold, silver, platinum and palladium, a metal such as copper, nickel, cobalt, iron, aluminum, molybdenum and tungsten, or an oxide thereof is used. Any salt may be used as long as it can precipitate, for example, nitrates, sulfates, chlorides, ammonium salts, phosphates, carboxylates, metal alcoholates, resinates of these metals are used. A plurality of different kinds of salts of the same metal may be used in combination, or two or more kinds of salts of different kinds of metals may be mixed and used. Also, double salts or complex salts may be used.

One or more of the above metal salts are dissolved or mixed in a flammable solvent to prepare a metal salt solution.
As the flammable solvent, organic solvents such as methanol, ethanol, isopropyl alcohol, acetone, ethyl methyl ketone, toluene and xylene can be used alone or in admixture of two or more, but the metal salt is supplied as a solution. Therefore, it is preferable to use organic solvents such as alcohols and ketones. Further, one or more of the above metal salts may be dissolved in a solvent such as water and then dissolved or mixed in the above flammable solvent before use. or,
In order to adjust the solubility of the metal salt and the combustion temperature, one or more of the above metal salts may be dissolved and used in a mixture of water and a combustible solvent. For the metal salt solution, use a flammable solvent
The content is preferably 0% by weight or more, and when methanol is used as a flammable solvent, it is preferably 50% by weight or more.

When a salt solution of a single metal is used, a pure metal powder is obtained, and when a solution composed of two or more kinds of metals forming an alloy is used, an alloy powder can be obtained. . Further, when a solution composed of two or more kinds of metals that does not form an alloy is used, mixed powder may be obtained.

As means for spraying the metal salt solution, a two-fluid nozzle using a pressure gas as a spray medium, an ultrasonic nozzle using a combination of an ultrasonic vibrator and a carrier gas, or a high-pressure pump suitable for large-capacity spraying A pressure nozzle or the like using is preferably used. Moreover, the same solvent gas as the metal salt solution can be used as the pressure gas or the carrier gas.

A pilot burner, an ignition coil, or the like is preferably used as the ignition means for the flammable gas, and a flame-holding burner tile, an electric heater, or the like is installed around the furnace as the flame-holding means for continuing the combustion. Or installing an auxiliary burner. In addition, a method of using a small electric heater externally for both ignition and flame holding can be used.

The combustion needs to be performed so that the flame portion has a temperature higher than the decomposition temperature of the metal salt used and the melting point of the metal. If the metal is such that it forms oxides at or below the melting point during or after the decomposition, the combustion is such that the flame part is It is necessary that the temperature be higher than the melting point of the metal. Further, when a solution composed of two or more kinds of metal salts forming an alloy is used, the combustion is carried out at a temperature higher than the decomposition temperature of each metal salt and the melting point of the alloy composed of those metals. Needs to be done. The temperature of the flame part is preferably 50 to 1500 ° C. higher than the melting point of the metal, more preferably 100 to 1000 ° C. higher, and 2
It is more preferable that the temperature is higher by 00 to 800 ° C.

Usually, when air is used as the combustion gas, the temperature in the flame portion when the combustible solvent is ideally burned is around 2000 ° C., but it contains a metal salt,
When alcohol is used as the solvent, the temperature of the flame is generally around 1000 ° C., and the temperature of the particles is further lowered. Therefore, it is difficult to decompose the metal salt or melt the metal unless it is sufficiently heated from the outside with a large-capacity electric heater or the like. Therefore, it is necessary to carry out the reaction at a sufficiently high temperature in the flame portion by using a mixed gas of general industrial oxygen gas and air, or an oxygen-rich gas as a combustion gas. The combustion gas preferably contains 21% by volume or more of oxygen.

Decomposition of the metal salt and melting of the metal are carried out in an oxidizing atmosphere, a reducing atmosphere, or in a reducing atmosphere depending on the kind of the metal and the heating temperature.
It is done in an inert atmosphere. The temperature of the flame part can be adjusted by adjusting the power of the pilot burner, adjusting the output of flame holding means such as an external electric heater, mixing the metal salt in the metal salt solution, the combustible solvent and water, and the combustion gas. It is performed by adjusting the temperature inside the reactor core (reaction zone) by adjusting the oxygen concentration and selecting a flammable solvent.

As a means for cooling the molten metal, a method of introducing a cooling gas into the furnace is generally used, but the furnace wall may be cooled. The type of cooling gas used varies depending on the metal salt solution, but nitrogen gas, air or argon gas is preferably used. The temperature of the cooling gas is 40 to 1
It is desirable to set the temperature to 20 ° C. Furthermore, when installing a solid-gas separator such as a cyclone or bag filter downstream,
Considering the heat resistance of these devices, it is preferable to cool the gas temperature at the furnace outlet in which the cooling gas is accompanied by the powder to 70 to 260 ° C.

The particle size of the metal powder depends on the concentration of the metal salt, the kind and mixing ratio of the solvent, the spraying speed, the size of the sprayed droplets, and the heating temperature, and can be adjusted by appropriately changing these conditions. You can The size of the atomized droplets is directly related to the particle size, and in the case of using a two-fluid nozzle, the particle size can be made smaller by increasing the flow rate of the pressure gas as the atomizing medium. Further, when the boiling point of the solvent is low, it is considered that the droplets are likely to be split due to boiling during heating, so that the droplets become finer and a metal powder having a smaller particle size can be obtained.

FIG. 2 shows an example of an apparatus for producing metal powder. In the schematic view of FIG. 2, the furnace 24 has an atomizer 13 and a pilot burner 14 in its upper part, is connected to a bag filter 18 in its lower part, and has an inflow hole 21 in its upper part. A flame holding burner tile 16 is provided around the sprayer 13, and a heat insulating material 17 is provided on the outer peripheral surface of the body.
The sprayer 13 has a double pipe structure and is connected to the stock solution tank 11 and the spray pressure gas inflow hole 20.

The metal salt solution 19 is sent from the metal salt solution tank 11 to the atomizer 13, and is sprayed into the reaction zone 26 in the furnace 24 by compressed air or pressure gas such as compressed oxygen flowing in from the inflow hole 20. In addition, the fuel gas is introduced into the inflow hole 21.
Supplied by. The pilot burner 14 ignites the flammable gas emitted from the atomized droplets and the combustion is sustained by the flame holding burner tile 16. The metal particles melted by the heat of combustion are sent to the lower part of the furnace 24 and cooled by the air flowing in through the inflow hole 22 to become metal powder. The metal powder is sent to the bag filter 18 provided for collection by air, nitrogen (N ₂ ) or the like flowing in through the inflow hole 23, and the exhaust gas flows out through the outflow hole 25.

FIG. 3 shows another example of an apparatus for producing metal powder. The apparatus shown in the schematic view of FIG. 3 is the same as the apparatus shown in FIG. 2 except that an external electric heater 31 is provided as an ignition / flame holding means in which an ignition means and a flame holding means are integrated.

EXAMPLES The present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples.

Example 1 Using the apparatus shown in the schematic view of FIG. 2, 1 mol of silver nitrate was added to 70% aqueous methanol solution.
It was melted at a rate of 1 / l and supplied to the two-fluid nozzle at a flow rate of 2 kg / hr. At that time, compressed air was flown at a flow rate of 5 kg / hr for atomization and combustion of the two-fluid nozzle, and was sprayed into a quartz tube preheated to 500 ° C. by a pilot burner to ignite and hold flame. At this time, air was introduced as a combustion gas at 5 kg / hr from the outside of the two-fluid nozzle to make up for the shortage of the combustion air, and the melted droplets were sequentially conveyed to the cooling air introduction part. The inside of the quartz tube after combustion was heated to 1300 ° C. by the thermometer 41, and the droplets were thermally decomposed, cooled by cooling air, and collected by a bag filter. The melting point of Ag is 961 ° C.

(Examples 2 to 9) Ag powder was produced in the same manner as in Example 1 except that the apparatus shown in the schematic view of FIG. 3 was used and an external electric heater was used as ignition / flame holding means. did. Table 1 shows the manufacturing conditions and the shape of the obtained Ag powder.
Shown in.

(Comparative Example 1) Ag powder was produced under the same conditions as in Example 1 except that the fire was extinguished immediately after ignition with a pilot burner. If the flame holding means is not used, combustion becomes unstable and becomes intermittent, so the temperature inside the quartz tube is not stable and the temperature rises only up to about 700 ° C.

(Comparative Examples 2 and 3) Ag powder was produced in the same manner as in Example 2. Table 1 shows the production conditions and the shape of the obtained Ag powder.

[0029]

[Table 1]

The powder obtained in Example 1 had a maximum particle size of 3.
0 μm, minimum particle size 0.3 μm, smooth and spherical surface Ag
While the powder was a powder, the powder obtained in Comparative Example 1 was porous and some were crushed, and the maximum particle size was 5.0 μm and the minimum particle size was 0.5 μm. The reason why it could not be spheroidized is considered to be that the combustion was unstable and the temperature inside the quartz tube was lower than the melting point of silver, so the silver particles generated by thermal decomposition did not melt.

As can be seen from the results of Examples 2 to 9 and Comparative Examples 2 and 3, the electric heater part (ignition / flame holding part temperature),
The larger the methanol concentration and the higher the oxygen concentration of the combustion gas, the more favorable the influence on the spheroidization of the particles. Even if one of these values was small, the particle shape could be within the allowable range by increasing the other values.

[0032]

EFFECTS OF THE INVENTION By using the present invention, it is possible to produce a spherical metal powder having a fine and uniform particle size, little contamination of impurities, and a smooth surface. or,
Since the apparatus can be upsized and the time required for starting and stopping the apparatus can be shortened, it is possible to mass-produce metal powder and improve production efficiency.

[Brief description of drawings]

FIG. 1 is a schematic view showing the principle of a method for producing metal powder according to the present invention.

FIG. 2 is a schematic view showing an example of an apparatus for producing metal powder according to the present invention.

FIG. 3 is a schematic view showing another example of an apparatus for producing metal powder according to the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Drop of metal salt solution, 2 ... Combustible gas, 3 ... Metal salt, 4 ... Metal, 5 ... Metal powder, 11 ... Metal salt solution tank, 12 ... ..Liquid supply pump, 13 ... Nebulizer, 14 ... Pilot burner, 15 ... Reactor tube, 16 ... Flame holding burner tile, 17 ... Insulating material, 18 ... Bag filter, 19
... Metal salt solution, 20 ... inflow hole, 21 ... inflow hole, 22 ... inflow hole, 23 ... inflow hole, 24 ... furnace, 25 ...
..Outflow holes, 31 ... Electric heaters, 41 ... Thermometers

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Fujio Iijima 3847 Ikebe-cho, Midori-ku, Yokohama-shi, Kanagawa Okawara Kakoki Co., Ltd.

Claims (8)

[Claims] 1. A method for producing a metal powder, which comprises heating a liquid droplet containing a solution containing a metal salt to decompose the metal salt or the produced metal oxide, melting the metal, and then solidifying the metal. The solution containing a metal salt contains a flammable solvent, and the decomposition of the metal salt or the metal oxide produced by the combustion heat generated by the combustion reaction between the flammable solvent vaporized from the droplet and the combustion gas and the melting of the metal And then
The temperature of the flame portion during combustion is adjusted by externally heating, the concentration of the flammable solvent in the solution and the oxygen concentration in the combustion gas are adjusted, and the molten metal is cooled by a cooling means. A method for producing a metal powder, which comprises cooling and solidifying by using. 2. The method for producing metal powder according to claim 1, wherein the vaporized combustible solvent is ignited by using an ignition means, and the combustion is continued by using a flame holding means. 3. The combustion gas contains oxygen in an amount of 21% by volume or more, the metal salt-containing solution contains a flammable solvent in an amount of 30% by weight or more, and the flame-holding portion of the flame-holding means in combustion is The method for producing a metal powder according to claim 1, wherein the temperature is 500 ° C. or higher. 4. The method for producing a metal powder according to claim 1, 2 or 3, wherein an ignition and flame holding means in which an ignition means and a flame holding means are integrated is used. 5. A furnace, a spraying means provided at one end of the furnace, an ignition means provided near the spraying means, a flame holding means provided at an outer surface of the furnace, and a cooling means provided near the other end of the furnace. A device for producing metal powder, which comprises igniting a combustible solvent that is vaporized from droplets of a metal salt solution by the ignition means, and maintaining the combustion by the flame holding means, thereby producing a metal salt or a metal produced. An apparatus for producing metal powder, characterized in that an oxide is decomposed and a metal is melted, and the melted metal is solidified by the cooling means. 6. The flame holding means is a heater.
The apparatus for producing metal powder according to item 1. 7. The apparatus for producing metal powder according to claim 5, wherein the cooling means is a cooling gas introduced through an inflow hole provided in the furnace. 8. The apparatus for producing a metal powder according to claim 5, further comprising an ignition and flame holding means in which the ignition means and the flame holding means are integrated.