Coating processes – Particles – flakes – or granules coated or encapsulated – Inorganic base
Reexamination Certificate
2001-09-04
2003-01-07
Barr, Michael (Department: 1762)
Coating processes
Particles, flakes, or granules coated or encapsulated
Inorganic base
C427S212000, C427S214000, C427S217000, C428S403000
Reexamination Certificate
active
06503560
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to composite nickel fine powder which has characteristic properties suitably used as a material for preparing an internal electrode of laminated ceramic condensers, which is excellent in resistance to oxidation of metal nickel and resistance to diffusion of nickel oxide, in particular, during binder-removal, as well as in thermal shrinkage properties, which can accordingly prevent the occurrence of delamination and the formation of cracks when manufacturing a large-sized laminated ceramic condenser, and which permits the production of a thin, small-sized multilayered ceramic condenser comprising ceramic dielectric layers and internal electrodes, without impairing the dielectric properties and electric properties thereof, and to a method for preparing the composite fine powder.
2. Description of the Prior Art
A laminated ceramic condenser is produced by alternatively putting ceramic dielectric materials and internal electrodes in layers, attaching them by applying pressure and then unifying them by firing. In this respect, an internal electrode for such a laminated ceramic condenser is in general produced by forming metal fine powder as a material for the internal electrode into a paste, printing a layer of the paste on a ceramic substrate, putting a plurality of these printed substrates in layers, thereafter attaching them to one another with pressing and heating to give a unified product and finally heating and firing in a reducing atmosphere. As materials for such an internal electrode, there have been used, for instance, platinum and palladium, but there has recently been developed and advanced a technique in which base metals such as nickel are used in place of precious metals such as platinum and palladium.
When using metallic nickel fine powder as an internal electrode material, however, it has a tendency to undergo rapid thermal shrinkage at a temperature higher than about 400 to 500° C., although the tendency depends on the particle size of the powder. For this reason, if fine powder of metal nickel is used as an internal electrode material, a unified product such as that described above is liable to cause delamination and/or crack-formation during firing the same because of the difference between the ceramic substrate and the layer of the metal nickel fine powder in thermal shrinkage characteristics and this has become a serious problem.
To solve these problems, the inventors of this invention have developed a technique for improving the thermal shrinkage characteristics of the metal nickel fine powder by incorporating magnesium and/or calcium into metal nickel fine particle in an amount falling within a specific range and already filed a patent application (Japanese Patent Application Serial No. Hei 9-342793). This technique permits the inhibition of the foregoing rapid thermal shrinkage at a temperature higher than about 400 to 500° C., which has been observed for the conventional metal nickel fine powder and can shift the starting temperature of such rapid thermal shrinkage to a higher temperature side in proximity to the range of from about 600 to 700° C.
However, the rapid thermal shrinkage-initiating temperature of the internal electrode material should desirably be shifted to the more higher temperature side in order to prevent the occurrence of delamination and/or the formation of cracks during firing. This is because the firing temperature used for preparing a laminated ceramic condenser may vary depending on the ingredients of ceramic dielectric materials, but that for the barium titanate ceramic dielectrics in general ranges from about 1200 to 1400° C.
Moreover, when firing is carried out while a ceramic substrate is brought into contact with a metal, the metal is in general oxidized and the resulting metal oxide has a diffusion coefficient higher than that of the ceramic substrate. Accordingly, diffusion easily occurs, at the grain boundaries, from the metal oxide phase having a higher diffusion coefficient to the ceramic phase having a lower diffusion coefficient.
More specifically, when using a paste containing conventional metal nickel fine particles, a part of the metal nickel of the fine particle is oxidized and the resulting nickel oxide diffuses into a ceramic dielectric material. As a result, a part of the internal electrode to be formed has a tendency to disappear, defects are formed in the internal electrode and a part of the ceramic dielectric layer has a tendency to damage its dielectric characteristics due to the formation of ferrite. Accordingly, it has been quite difficult to produce a thin, small-sized laminated ceramic condenser which comprises layers of a ceramic dielectric material and internal electrode layers, without impairing the dielectric characteristics and electric properties thereof.
SUMMARY OF THE INVENTION
As has been discussed above in detail, it would be very important that the metal nickel fine powder as a material for preparing a paste used for the preparation of a laminated ceramic condenser must be able to prevent the oxidation of metal nickel of the fine powder and any diffusion of nickel oxide into a ceramic substrate during firing and simultaneously to shift the rapid thermal shrinkage-starting temperature of the metal nickel fine powder to the more higher temperature side so that the thermal shrinkage curve of the nickel fine powder approaches that of the ceramic substrate.
Accordingly, it is an object of the present invention to provide composite nickel fine powder which has characteristic properties suitable for use as a material for preparing an internal electrode of laminated ceramic condensers, which is excellent in resistance to oxidation of metal nickel and resistance to diffusion of nickel oxide, in particular, during the binder-removal stage, which has a thermal shrinkage curve approximately identical to that of the ceramic substrate, which can accordingly prevent the occurrence of delamination and the formation of cracks when manufacturing a large-sized laminated ceramic condenser, and which permits the production of a thin, small-sized multilayered ceramic condenser comprising ceramic dielectric layers and internal electrodes, without impairing the dielectric characteristics and electric properties thereof.
Another object of the present invention is to provide a method for preparing such composite nickel fine powder.
The inventors of this invention have conducted intensive studies to accomplish the foregoing objects, have found that composite nickel fine powder having the foregoing characteristic properties can be obtained by fixing an oxide and/or a complex oxide of a specific metal element to the surface of metal nickel fine particles and that such composite nickel fine powder can be prepared by a wet-carrying process, a dry-carrying process or a semi-dry-carrying process and thus have completed the present invention on the basis of these findings.
According to a first aspect of the present invention, there is provided composite nickel fine powder which is characterized in that at least one member selected from the group consisting of oxides and complex oxides which comprise at least one metal element having an atomic number ranging from 12 to 56 or 82 and belonging to Group 2 to 14 of Periodic Table is carried on the surface of metal nickel fine particles.
According to another aspect of the present invention, there is provided a method for preparing composite nickel fine powder which comprises the steps of adding an aqueous solution containing at least one member selected from the group consisting of water-soluble salts of metal elements each having an atomic number ranging from 12 to 56 or 82 and belonging to Group 2 to 14 of Periodic Table to a slurry containing metal nickel fine particles or metal nickel fine particles whose surface is oxidized; and then adjusting the pH of the mixture with an acid or an alkali to thus carry the metal oxide and/or complex oxide derived from the water-soluble salt on the surface of th
Araki Takayuki
Hayashi Takao
Shimamura Hiroyuki
Toshima Yoshiharu
Yamaguchi Yasuhide
Barr Michael
Blanton Rebecca A.
Mitsui Mining and Smelting Co., Ltd
Young & Thompson
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