Specialized metallurgical processes – compositions for use therei – Compositions – Loose particulate mixture containing metal particles
Reexamination Certificate
2001-10-17
2002-12-17
Mai, Ngoclan (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Compositions
Loose particulate mixture containing metal particles
C252S513000
Reexamination Certificate
active
06494931
ABSTRACT:
TECHNICAL FIELD
The present invention relates to 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 and an excellent packing ability in a conductive paste and which is, in particular, suitably used for forming a thin and uniform inner electrode for a multilayer ceramic capacitor without being accompanied by the formation of any crack and the generation of any delamination as well as a conductive paste for use in making a multilayer ceramic capacitor, which comprises the foregoing nickel powder.
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 multilayer 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 sheet 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, a precious metal such as 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. However, if a nickel powder-containing paste is used for forming an inner electrode, problems such as the formation of cracks and/or the generation of delamination arise.
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 multilayer has been increased. Accordingly, there have presently been produced a multilayer 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 multilayer is not less than 100. To produce such a part, however, it is necessary to form a thin and uniform inner electrode without being accompanied by the formation of any crack and/or the generation of any delamination.
To obtain a thinner inner electrode layer, it may be satisfactory to use metal fine powder having a small average particle size balanced with the thickness of the 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.
In addition, to ensure stable conductivity of the resulting electrodes, such nickel fine powder should not only be finer, but also should have a high packing ability in a vehicle used in the production of a conductive paste. However, it may be more and more difficult to improve the packing ability of such fine particle-containing nickel powder in the conductive paste and the viscosity of the resulting conductive paste increases, as the particle size of the fine particles is reduced. In addition, a problem arises such that the heat shrinkage of the conductive 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 an excellent packing ability in a conductive paste and which can particularly suitably be used for forming a thin and uniform inner electrode for use in making a multilayer ceramic capacitor without being accompanied by the formation of any crack and/or the generation of any delamination, as well as a conductive paste, 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 accomplish the foregoing objects, have found that if the rates of coarse particles and fine particles present in nickel powder are reduced to relatively low levels and the tap density of the nickel powder is higher than a predetermined value, an inner electrode layer free of any projection can be formed without unnecessarily reducing the particle size of the nickel powder and the formation of any short circuit between the neighboring inner electrodes of the resulting ceramic capacitor can certainly be inhibited, that the oxidation of nickel particles and the heat shrinkage of the electrode layer can be inhibited because of the low content of fine particles and that the nickel powder has a high packing ability in a conductive paste due to a high tap density of the powder and also permits the formation of a beautiful sintered film and thus have completed the present invention.
Moreover, the inventors have also found that if the particle size of crystallites present in each nickel particle is small in addition to the foregoing characteristics, the sintering of nickel particles gently proceeds, the sintering speed is uniform and as a result, a thin and uniform inner electrode can be formed without being accompanied by the formation of any crack and/or the generation of any delamination.
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 is not more than 5% of the total number of nickel particles, 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 and that the tap density of the nickel powder is not less than 2.5 g/cm
3
.
The present invention also relates to a conductive paste for use in making a multilayer ceramic capacitor, which is characterized by comprising nickel powder having the foregoing characteristic properties.
BEST MODE FOR CARRYING OUT THE INVENTION
It is important in the nickel powder of the present invention that has a rate of the nickel particles whose particle size is not less th
Araki Takayuki
Mukuno Takashi
Toshima Yoshiharu
Mai Ngoclan
Mitsui Mining and Smelting Co. Ltd.
Young & Thompson
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