Specialized metallurgical processes – compositions for use therei – Processes – Free metal or alloy reductant contains magnesium
Reexamination Certificate
2001-10-09
2003-10-14
King, Roy (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Free metal or alloy reductant contains magnesium
C075S343000, C075S362000, C075S370000, C075S371000, C075S374000
Reexamination Certificate
active
06632265
ABSTRACT:
TECHNICAL FIELD
The present invention relates to nickel powder, a method for the production of the nickel powder and a conductive paste for use in making a multilayer ceramic capacitor and more particularly to nickel powder, which has a narrow particle size distribution, which has a low content of the sum of impurities consisting of alkali metals, alkaline earth metals, transition metals belonging to the fourth period of the Periodic Table (except for nickel), sulfur and chlorine and which is, in particular, suitably used as a conductive paste for use in making a thin inner electrode, free of any projection, for a multilayer ceramic capacitor as well as a method for the production of the nickel powder and a conductive paste for use in making a multilayer ceramic capacitor.
BACKGROUND ART
A multilayer ceramic capacitor comprises a plurality of layers of a ceramic dielectric substance and a plurality of inner electrode,layers, which are alternately laminated and united and such a multilayer ceramic capacitor is in general produced by preparing a conductive paste by converting metal fine powder as an inner electrode material into a paste, printing a green sheet of a ceramic dielectric substance with the resulting conductive paste, laminating a plurality of the printed green sheets in such a manner that the ceramic dielectric green sheet and the conductive paste are arranged alternately, pressing the laminated printed green sheets with heating to thus unite them, and then firing the resulting assembly at a high temperature in a reducing atmosphere to thus unify the ceramic dielectric layers and the inner electrode layers.
As the inner electrode material, there has conventionally been used, for instance, platinum, palladium or silver-palladium, but there have recently been developed techniques, which make use of base metals such as nickel in place of precious metals such as platinum, palladium and silver-palladium in order to save the production cost and these techniques have been advanced.
Moreover, electronic parts produced by using conductive pastes such as multilayer ceramic capacitors have recently been more and more miniaturized and the ceramic dielectric layer and the inner electrode layer have correspondingly been more and more thinner and the number of these layers laminated has been increased. Accordingly, there have presently been produced a laminated part such as a multilayer ceramic capacitor in which the thickness of the dielectric layer is not more than 2 &mgr;m, the thickness of the inner electrode layer is not more than 1.5 &mgr;m and the number of layers laminated is not less than 100.
It may be satisfactory to use metal fine powder having a small average particle size balanced with the thickness of the inner electrode layer in order to obtain a thinner inner electrode layer. However, coarse particles may be present in such metal fine powder even if the average particle size of the powder falls within the desired range. Accordingly, if an inner electrode layer is formed by using a conductive paste containing such metal fine powder, the coarse particles present therein may form projections on the resulting inner electrode layer, the projections may in turn break through the thin ceramic dielectric layer to thus form a short-circuit between the neighboring inner electrode layers. Accordingly, to prevent the formation of any short-circuit between inner electrode layers, it is necessary to use metal fine powder having an average particle size substantially smaller than that balanced with the thickness of such a thin inner electrode layer.
For instance, Japanese Un-Examined Patent Publication No. Hei 11-189801 discloses nickel ultra-fine powder whose average particle size ranges from 0.2 to 0.6 &mgr;m and in which the rate of coarse particles having a particle size of not less than 2.5 times the average particle size is not more than 0.1% based on the number of particles and further discloses in the fourth column, lines 21 to 24 that “If the particle size of coarse particles is limited to, for instance, about 1.5 &mgr;m, the average particle size of the ultra-fine nickel powder according to the present invention should accordingly be limited to 0.6 &mgr;m”. Thus, it is necessary to use metal fine powder having a considerably small average particle size in order to produce a thin inner electrode layer. However, problems arise such that the finer the metal fine powder, the higher the viscosity of a paste containing such fine powder and that the heat shrinkage of the printed paste layer and the oxidation of the nickel powder included therein are accelerated upon firing the printed green sheet.
It is thus an object of the present invention to provide nickel powder, which has a narrow particle size distribution, which has a low content of total or overall impurities consisting of alkali metals, alkaline earth metals, transition metals belonging to the fourth period of the Periodic Table (except for nickel), sulfur and chlorine and which can suitably be converted into a conductive paste for forming a thin inner electrode layer free of any projection for use in making a multilayer ceramic capacitor without unnecessarily reducing the particle size of the nickel fine powder, as well as a method for the production of the nickel powder and a conductive paste, which contains such nickel powder and which is used for the manufacture of a multilayer ceramic capacitor.
DISCLOSURE OF THE INVENTION
The inventors of this invention have conducted various studies to achieve the foregoing object, have found that if the content of coarse particles present in nickel powder is reduced to a relatively low level and if the particles are so designed that most of them have a particle size falling within a desired range, an inner electrode layer free of surface projections can be produced without unnecessarily reducing the particle size of the nickel powder, that the resulting multilayer ceramic capacitor hardly forms a short circuit between the neighboring inner electrodes, can inhibit any oxidation of the inner electrodes because of a low content of fine particles and is free of any heat shrinkage, that such nickel powder can be produced if desired conditions are satisfied when producing nickel hydroxide and reducing the same and that a conductive paste containing such nickel powder is particularly suitably used for forming a multilayer ceramic capacitor and thus have completed the present invention.
Accordingly, the nickel powder of the present invention is characterized in that the rate of the nickel particles whose particle size is not less than 1.2 time the average particle size as determined by the observation with an SEM (scanning electron microscope) is not more than 5% of the total number of nickel particles and that the rate of nickel particles whose particle size is not more than 0.8 time the average particle size is not more than 5% of the total number of nickel particles.
Moreover, the conductive paste according to the present invention is characterized in that it comprises the foregoing nickel powder according to the present invention.
In addition, the method for the production of nickel powder according to the present invention comprises the step of bringing a slurry, containing nickel hydroxide, which is prepared by adding an aqueous solution of a nickel salt to an aqueous solution of an alkali metal hydroxide, into contact with a hydrazine reducing agent under the temperature condition of not less than 55° C. to thus reduce the nickel hydroxide.
BEST MODE FOR CARRYING OUT THE INVENTION
The nickel powder of the present invention has a rate of the nickel particles whose particle size is not less than 1.2 time the average particle size as determined by the observation by an SEM with a magnification of ×10000 for the sake of convenience, of not more than 5% and preferably not more than 4% of the total number of nickel particles and a rate of nickel particles whose particle size is not more than 0.8 time the average particle size of not more than 5% and preferably not more than
Araki Takayuki
Mukuno Takashi
Toshima Yoshiharu
King Roy
Mitsui Mining and Smelting Co. Ltd.
Wessman Andrew
Young & Thompson
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