Specialized metallurgical processes – compositions for use therei – Compositions – Loose particulate mixture containing metal particles
Reexamination Certificate
2001-04-09
2002-09-24
Mai, Ngoclan (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Compositions
Loose particulate mixture containing metal particles
Reexamination Certificate
active
06454830
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a nickel powder for multilayer ceramic capacitors suitable as a material of conductive paste fillers for forming internal electrodes in multilayer ceramic capacitors.
BACKGROUND ART
Conductive metal powders such as nickel, copper, silver, etc., are useful for forming internal electrodes in multilayer ceramic capacitors, and in particular, nickel powder has attracted attention since it is less expensive than conventional metal powders such as palladium powder, etc. Additionally, internal electrodes tend to be formed in thin layers having a thickness of 1 to 2 &mgr;m in accordance with miniaturization and capacity increase of capacitors, and therefore, it is required that nickel powder have a particle size of not more than 1 &mgr;m.
As nickel powder which satisfies such properties, fine spherical nickel powder having a purity of 99.5% or more by weight and a particle size of 0.05 to 1.0 &mgr;m is disclosed in Japanese Unexamined Patent Publication No. Hei 3-280304. According to this nickel powder, since filling density in the electrode layer can be increased, specific resistance of the electrode layer after sintering is small and delamination (peeling) and cracking hardly occur.
In addition, as a process for production of nickel powder, a vapor phase reduction in which nickel chloride gas is reduced by hydrogen gas is generally used, recently. This method has an advantage in that spherical nickel powders can be efficiently produced; however, nickel powders having a particle size of 1 &mgr;m or more are often included in the produced nickel powders. The nickel powder having a particle size of 1 &mgr;m or more easily causes problems such as shorting between electrodes or voids in the electrode layer, and it does not have desirable properties. Therefore, high-quality nickel powder having no coarse particles of 1 &mgr;m or more is desired. Since nickel powder particles of 1 to 2 &mgr;m or 5 &mgr;m, depending on the conditions, are included even if an average particle size thereof is 0.4 &mgr;m, a classification technique in which coarse particles of 1 &mgr;m or more can be efficiently removed is desired.
The present invention has been made in consideration of the above circumstances, and objects thereof are as follows.
{circle around (1)} A nickel powder for multilayer ceramic capacitors is provided in which the content of coarse particles is low and the distribution of particle size is narrow.
{circle around (2)} A nickel powder for multilayer ceramic capacitors is provided in which the surface roughness in the paste state is small.
DISCLOSURE OF INVENTION
The present inventors reduced nickel chloride by a vapor phase reduction, then prepared nickel powders in the paste state with respect to each nickel powder in which the contents of nickel powders having a particle size of 2 &mgr;m or more differ, and examined the surface roughness thereof. As a result, it was found that satisfactory surface roughness was not obtained in the case in which nickel powders having a particle size of 2 &mgr;m or more exist at a rate of about 1400 per million; however, superior surface roughness was obtained in the case in which the nickel powders exist at a rate of about 50 per million. It was inferred from this fact that the surface roughness in which there is no problem in practical use would be obtained if nickel powder were classified until nickel powders having a particle size of 2 &mgr;m or more exist at a rate of 700 per million, which is about the intermediate value between 50 per million and 1400 per million, and the present invention therefore was attained. That is, a nickel powder for multilayer ceramic capacitors according to the present invention is characterized in that an average particle size is 0.1 to 1.0 &mgr;m, and the nickel powders having a particle size of 2 &mgr;m or more exist at a rate of not more than 700 per million.
Next, the present invention is more specifically explained.
A. Nickel Powder Characteristics
The nickel powder of the present invention is characterized in that an average particle size is 0.1 to 1.0 &mgr;m, and the number rate of the nickel powders having a particle size of 2 &mgr;m or more is not more than 700 per million, as described above. When such nickel powder is used for the paste, the uniformity of thickness of the internal electrode layer in ceramic capacitors is superior, and shorting between internal electrodes is thereby improved. The content (number rate) of nickel powder having a particle size of 2 &mgr;m or more can be obtained by taking and image-analyzing electron microphotographs of nickel powder, and counting the total number of particles and particles having a particle size of 2 &mgr;m or more.
The number rate of the nickel powders having a particle size of 2 &mgr;m or more must be not more than 700 per million, and it is preferable that the number rate is not more than 300 per million, more preferably that the number rate is not more than 100 per million, and most preferably that the number rate is not more than 50 per million.
It is preferable that the average particle size of the nickel powder be even smaller; however, the nickel powders easily aggregate as they are made finer, and consequently, voids are easily formed in internal electrodes. Therefore, as nickel powder for forming an internal electrode of about 1 &mgr;m, a particle having an average particle size of 0.2 to 0.4 &mgr;m is desirable.
With respect to this nickel powder particle having an average particle size of 0.2 to 0.4 &mgr;m, the content (number rate) of the nickel powders having a particle size of 2 &mgr;m or more is preferably not more than 50 per million, and more preferably that the number rate is not more than 20 per million. Furthermore, the number rate of the nickel powders having a particle size of 1 &mgr;m or more is preferably not more than 100 per million, and more preferably that the number rate is not more than 50 per million.
B. Preparation of Nickel Powder
The above nickel powder of the present invention can be prepared by various methods, and in particular, the vapor phase reduction in which nickel chloride is reduced by hydrogen, etc., is a preferable method from the viewpoint of control of particle size. Specifically, the following processes may be mentioned.
{circle around (1)} Nickel chloride solid as a starting material is vaporized by heating and reacts with hydrogen gas (reduction process), and nickel powder is therefore obtained.
{circle around (2)} Nickel solid as a starting material is chlorinated by contacting chlorine gas (chlorination process), nickel chloride gas is thereby produced and reacts with hydrogen gas (reduction process), and nickel powder is therefore obtained.
In the process {circle around (2)} of the above processes, the partial pressure of nickel chloride gas can be controlled in the reduction process by controlling the feed rate of chlorine gas in the chlorination process or by mixing nickel chloride gas produced in the chlorination process with inert gas. Thus, by controlling the partial pressure of nickel chloride gas, the particle size of forming nickel powder can be controlled, and as a result, the particle size of nickel powder can be stabilized and can be optionally controlled.
Incidentally, the reduction process in the above vapor phase reduction is carried out at a high temperature of about 1000° C. or more. The nickel powder just after production is easily aggregated since it has a high temperature, and it is therefore desirable that it cool rapidly. Specifically, a process in which the produced nickel powder is forced to cool by inert gas such as nitrogen gas, etc., may be mentioned. As a cooling process, a cooling device, etc., can be provided and be used in addition to the reduction reaction system, and in particular, it is desirable that inert gas for cooling be directly contacted with nickel powder just after production from the viewpoint of suppression of the aggregation of nickel powder. In such a cooling process, the cooling rate in which the nickel powder ju
Ito Takayuki
Takatori Hideo
Mai Ngoclan
Oliff & Berridg,e PLC
Toho Titanium Co., Ltd.
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