Process for production of nickel powder

Details Process for production of nickel powder Process for production of nickel powder

1999-10-12

2001-05-22

Wyszomierski, George (Department: 1742)

Specialized metallurgical processes, compositions for use therei

Processes

Producing or purifying free metal powder or producing or...

C075S374000, C075S629000

Reexamination Certificate

active

06235077

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a process for production of a nickel powder suitable for various uses such as conductive paste fillers used for electrical parts, bonding material for titanium, and for catalysts, and in particular, relates to a process which can control the particle size in a range of less than 1.0 &mgr;m, which is a suitable particle size for a internal electrode of a multi-layer ceramic capacitor, and which can produce a nickel powder having a spherical shape and a narrow particle size distribution.
BACKGROUND ART
Conductive metallic powders such as those of nickel, copper, and silver are useful for internal electrodes in multi-layer ceramic capacitors; in particular, nickel powder has been recently studied for such purposes. Nickel powder produced by a dry production process is seen as being promising. In particular, an ultra fine powder having a particle size of less than 1.0 &mgr;m is demanded because of requirements to form a thin layer and to have low resistance in accordance with tends toward miniaturization and larger capacity of capacitors.
As one of the process for production of the fine nickel powder, a gas phase reduction process may be mentioned. For instance, JP-A-8-246001 discloses a process in which a reactor is filled with a vapor of nickel chloride by heating and vaporizing (subliming) a solid mass of nickel chloride, hydrogen gas is supplied with an inert gas such as argon gas, and a reducing reaction occurs by contacting and mixing to form a nickel powder. According to this publication, a nickel powder having a 0.1 &mgr;m to 1.0 &mgr;m aver age particle size can be prepared by the process.
Although a nickel powder having a particle size within a desired range (0.1 to 1.0 &mgr;m) can be obtained by the conventional process for production of a nickel powder disclosed in the above publication, it is difficult to control the required particle size more exactly within the range. To form a paste of a nickel powder, an advantageous condition includes an even and uniform particle size, a high smoothability of the particle surfaces, and a high sphericity. However, the conventional production processes cannot satisfy these conditions to a high level.
Objects of the present invention are to provide a process for production of a nickel powder, in which the particle size of the nickel powder may be freely controlled 1.0 &mgr;m, especially within the range of 0.1 to 1.0 &mgr;m, to improve smoothability of particle surfaces and to improve sphericity of the powder.
DISCLOSURE OF THE INVENTION
To solve the foregoing problems, the present inventors have studied various additional factors including additives and volumes of gas supplied, which affect characteristics of particles in nickel powder formed In the process as a basic reduction process to form a nickel powder, wherein a vapor of nickel chloride is supplied to a reduction reactor filled with a reductive gas including hydrogen gas, and thereafter the vapor of nickel chloride is reduced by the reductive gas. As a result, by supplying an appropriate volume of chlorine gas to an atmosphere of a reductive gas with a vapor of nickel chloride, the present Inventors have found that the particle size of the formed nickel powder can be controlled to a desired range, smoothability of particle surfaces, sphericity, and particle size distribution can be improved.
In the present invention, chlorine gas is supplied with a vapor of nickel chloride to an atmosphere of a reductive gas, and nickel chloride is reduced to produce nickel powder.
As a reductive gas used in the present invention, hydrogen gas or hydrogen sulfide gas is used. When effects on particles of a formed nickel powder are considered, hydrogen gas is preferable.
The volume of chlorine gas to be supplied is preferably at a ratio of from 0.01 to 0.5 moles per 1 mole of vapor of nickel chloride, and more preferably, at a ratio of from 0.03 to 0.40 moles, so that a nickel powder having a particle size of 0.1 to 1.0 &mgr;m is stably formed. It was confirmed that the particle size of nickel powder increased in proportion to the mixing volume of chlorine gas. That is the greater the volume of chlorine gas is supplied, the more of the growth of particles of nickel powder is promoted. The formed nickel powder can be controlled to a desired particle size based on the above. It is an important feature of the present invention that particle size can be freely controlled by utilizing the phenomenon of particle size of nickel powder increasing in proportion to volume of chlorine gas supplied, as described above.
In the present invention, chlorine gas is supplied with a vapor of nickel chloride to a reduction rector wherein the atmosphere is a reductive gas. Various methods can be adopted as the supplying method. Specifically, chlorine gas is mixed with a vapor of nickel chloride beforehand, and the mixed gas is then supplied to a reduction reactor. Alternatively, chlorine gas is continuously supplied with a vapor of nickel chloride to a reduction reactor or only chlorine gas is supplied intermittently by installing a supply nozzle for the vapor of nickel chloride and a supply nozzle for chlorine gas separately and positioning the nozzle together. The former method and the latter method can be combined, that is a method in which a mixed gas of a vapor of nickel chloride and chlorine gas and a chlorine gas are respectively supplied from separate nozzles to a reduction reactor can be employed.
Among the above methods, the method in which chlorine gas is supplied continuously from an adjoined nozzle is preferred because smoothability of surfaces of the nickel powder can be improved. The method in which chlorine gas is supplied intermittently from adjoined nozzles is preferable because-growth of icicles of nickel powder can be prevented from forming at the nozzles. In a conventional method, nickel powder formed by reduction adheres to a nozzle jetting a vapor of nickel chloride into a reduction reactor and occasionally grows like an icicle. If this occurs, the supply of the vapor of nickel chloride is affected, and as a result, adversely affects particle characteristics of a nickel powder to be formed. Therefore, solutions to these problems are necessary.
Various methods can be adopted as measures for separately installing the nozzles for the vapor of nickel chloride and chlorine gas and for adjoining the nozzles. Preferably, a nozzle is a double tube in which an internal tube is arranged coaxially with an external tube. By providing a double tube nozzle, a vapor of nickel chlorine gas may be supplied from one of the internal tube and the external tube of the double nozzle, and chlorine gas may be supplied from the other tube to a reduction reactor. In particular, by supplying a vapor of nickel chloride from an internal tube and chlorine gas from an external tube, the chlorine gas surrounds the vapor of nickel chloride, whereby growth of icicles of nickel powder at a supplying nozzle for nickel chloride described above can be prevented and spericity of nickel powder to be formed can be improved.
As a reduction reactor used in a process for production for nickel powder of the present invention, a vertical type reduction reactor, wherein a supply nozzle for a vapor of nickel chloride and chlorine gas is arranged, for instance, as a double tube as mentioned above, is preferably used.
Moreover, as a supply method for the vapor of nickel chloride and chlorine gas in a reduction reactor of the present invention, a method is preferably used wherein a vapor of nickel chloride and chlorine gas are supplied nearly downward and vertically from the nozzle toward the inside of a reduction reactor in a vertical reduction reactor in which the supply nozzle is at the top of the reactor.
As mentioned above, by using a vertical type reduction reactor and adopting a method in which a vapor of nickel chloride and chlorine gas are supplied nearly downward and vertically toward the inside of a reduction reactor, a nickel powder, which can be controlled to a desired particle

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